Find at least two solution recommendation to reduce space rocket effect on the environment

Received: 27 February 2019 Revised: 20 June 2019 Accepted: 21 June 2019

R E V I E W P A P E R

DOI: 10.1002/er.4730

Frontiers in combustion techniques and burner designs for emissions control and CO2 capture: A review

Medhat A. Nemitallah | Ahmed A. Abdelhafez | Asif Ali | Ibrahim Mansir |

Mohamed A. Habib

TIC on CCS and Mechanical Engineering Department, Faculty of Engineering, KFUPM, Dhahran 31261, Saudi Arabia

Correspondence M.A. Nemitallah, TIC on CCS and Mechanical Engineering Department, Faculty of Engineering, KFUPM, Dhahran 31261, Saudi Arabia. Email: [email protected]

© 2019 John Wiley & Sons, Ltd.7790

Summary

The use of fossil fuel is expected to increase significantly by midcentury because

of the large rise in the world energy demand despite the effective integration of

renewable energies in the energy production sector. This increase, alongside

with the development of stricter emission regulations, forced the manufacturers

of combustion systems, especially gas turbines, to develop novel combustion

techniques for the control of NOx and CO2 emissions, the latter being a green-

house gas responsible for more than 60% to the global warming problem. The

present review addresses different burner designs and combustion techniques

for clean power production in gas turbines. Combustion and emission character-

istics, flame instabilities, and solution techniques are presented, such as lean

premixed air‐fuel (LPM) and premixed oxy‐fuel combustion techniques, and

the combustor performance is compared for both cases. The fuel flexibility

approach is also reviewed, as one of the combustion techniques for controlling

emissions and reducing flame instabilities, focusing on the hydrogen‐

enrichment and the integrated fuel‐flexible premixed oxy‐combustion

approaches. State‐of‐the‐art burner designs for gas turbine combustion applica-

tions are reviewed in this study, including stagnation point reverse flow (SPRF)

burner, dry low NOx (DLN) and dry low‐emission (DLE) burners, EnVironmen-

tal burners (including EV, AEV, and SEV burners), perforated plate (PP) burner,

and micromixer (MM) burner. Special emphasis is made on the MM combustor

technology, as one of the most recent advances in gas turbines for stable

premixed flame operation with wide turndown and effective control of NOx emissions. Since the generation of pure oxygen is prerequisite to oxy‐

combustion, oxygen‐separation membranes became of immense importance

either for air separation for clean oxy‐combustion applications or for

conversion/splitting of the effluent CO2 into useful chemical and energy prod-

ucts. The different carbon‐capture technologies, along with the most recent

carbon‐utilization approaches towards CO2 emissions control, are also reviewed.

Highlights

• Different burner designs and techniques for clean power production are

reviewed.

Int J Energy Res. 2019;43:7790–7822.wileyonlinelibrary.com/journal/er

7791NEMITALLAH ET AL.

• Combustion and emission characteristics of LPM air‐ and oxy‐combustion

are presented.

• Fuel flexible premixed oxy‐combustion for reducing emissions and instabil-

ities is discussed.

• State‐of‐the‐art burner designs for gas turbine combustion applications are

presented.

• Different carbon capture/utilization approaches are reviewed and compared.

KEYWORDS

carbon capture/utilization, fuel flexibility, high‐temperature membrane reactors (HTMRs), lean

premixed combustion (LPM), micromixer (MM) combustion technology, oxy‐combustion

1 | INTRODUCTION

The increasingly stricter environmental regulations encouraged researchers to develop combustion systems that can meet such restrictions. Older gas turbine engines for power generation used diffusion‐flame combustors thanks to their superior stability characteristics.1-3 How- ever, these combustors are no longer used because they generate unacceptably high concentrations of NOx pollut- ant emissions.4 An approach for controlling NOx emissions is flue gas recirculation (FGR), where reactants are diluted with inert (cooled) exhaust gases. However, mixing the reactants with inert exhaust gases results in lower flame burning velocity and, consequently, lower combustion effi- ciency, and the combustor is prone to loss of stability.5

Modern technologies and concepts have been introduced in the last five decades to the gas‐turbine power‐generation industry, such as lean premixed combustion (LPM) and catalytic combustion.6 The latter was found to be expen- sive, unsafe, and less durable.7 In LPM combustors, fuel and air are homogenously mixed in lean proportions upstream of the combustor to control the flame tempera- ture and, as a result, thermal NOx emissions are signifi- cantly reduced. Nowadays, most manufacturers adopt the LPM technology in novel gas turbine combustors to replace the nonpremixed technology for the control of combustion temperature and NOx emissions. However, switching from the nonpremixed (diffusion) combustion mode to the lean premixed one causes undesired combus- tion characteristics, including increased levels of flame instabilities, autoignition, and flashback. Furthermore, operation under extra lean conditions at low loads adversely affects flame stability through excessive CO emissions and increased tendency to blowout.8 These chal- lenges necessitate the development of novel combustor designs that can operate stably under extra lean conditions while keeping emissions at low levels.

In this study, novel burner designs adopting premixed combustion are introduced for the sake of improving the combustor stability and performance in gas turbine appli- cations under different operating conditions. Such burner designs include stagnation point reverse flow (SPRF) with internal FGR (IFGR),9 dry low NOx (DLN) or dry low emission (DLE) combustors,10 EV‐burners, and micromixers (MMs).11 In addition to these novel burner designs, perforated plate (PP) burner is considered as one of the oldest and important techniques used in many combustion applications. The technology of PP burner is reviewed carefully in this study as a basis for the develop- ment of the novel MM combustion technology. The development of syngas‐fueled micromixing gas‐turbine burner is also reviewed in this study.

Despite reaching very low levels of NOx and CO emis- sions considering the above‐mentioned techniques, the complete control of all emissions has not been achieved yet considering the lean premixed air combustion tech- nique. Additional technologies should also be applied, alongside with premixed combustion, for the capture of CO2 emissions. CO2 is one of the primary greenhouse gases and contributes by over 60% to the global warming problem, because anthropogenic activities release over 30 Gt of CO2 per year into the atmosphere. CO2 capture technologies have thus been introduced to capture these vast amounts of CO2 emissions.

12 There are three main categories for CO2 capture technologies, namely, precom- bustion, oxy‐fuel combustion, and postcombustion. Among these technologies, oxy‐fuel combustion is very promising for the capture of CO2 in gas turbine applica- tions. In this technology, fuel is burned with pure oxygen to result in CO2 and H2O as main combustion products, which makes the process of CO2 separation simple and economic after H2O condensation.

13 However, oxy‐fuel combustion brings its own challenges in terms of com- bustion stability and combustor efficiency.14 Different

7792 NEMITALLAH ET AL.

fuels have different combustion and emission characteris- tics, which reflects on combustor stability at the different loading conditions. This necessitates the development of fuel‐flexible burners that can handle different fuels based on the loading conditions to sustain stable flame. The fuel flexibility approach, considering mainly hydrogen‐ enriched combustion of hydrocarbons, is addressed in detail in this review work.

Oxy‐fuel combustion is a very promising technology for the successful deployment of zero emissions power plants (ZEPPs). The pure oxygen required for the oxy‐fuel combustion process is currently obtained via cryogenic distillation in air‐separation units (ASU), which con- sumes copious amounts of energy and hinders the eco- nomic application of oxy‐fuel combustion technology. The thermodynamic and economic penalties incurred in cryogenic air separation can easily offset any advantage gained by oxy‐fuel combustion. Such shortcomings prompted many researchers to develop and explore alter- native membrane‐based air‐separation systems for oxy‐ fuel combustion applications. The application of mem- branes for power production in high‐temperature mem- brane reactors (HTMRs) is reviewed in this study.

The main objective of this article is to perform an extensive review of the state‐of‐the‐art advances in com- bustion techniques and burner designs for clean combus- tion in gas turbines. The review is extended to link such environment‐friendly burners with carbon‐capture tech- nologies, targeting full control of emissions out of gas tur- bines. In particular, oxy‐fuel combustion, as a promising carbon‐capture technology, is investigated for clean energy production while capturing the effluent CO2. The performance of modern gas turbine combustors is evaluated under oxy‐combustion conditions and com- pared with air‐combustion in terms of flame stability and the associated emissions.

FIGURE 1 NOx and CO formation as function of equivalence ratio15

2 | LEAN PREMIXED AIR COMBUSTION

2.1 | Combustion and emissions characteristics

NOx is a generic term for the nitrogen oxides, primarily NO and NO2, which contribute to the formation of smog and acid rain, as well as depletion of the tropospheric ozone. CO, as a product in the cases of incomplete com- bustion, is a toxic, colorless, odorless, and tasteless gas. NOx is formed within the combustor at elevated firing temperatures. The dominant mechanism for NOx forma- tion is the thermal (Zeldovich) mechanism.10 Controlling the temperature within the combustor can result in

significant control of NOx emissions. For example, when the temperature is raised from 1800 K to 2100 K, the amount of NOx produced in milliseconds at 2100 K is of the same order as the amount produced in few seconds at 1800 K for the same oxygen fraction.15 NOx formation is thus highly dependent on flame temperature. More- over, the equivalence ratio plays a key role in forming both NOx and CO as illustrated in Figure 1. NOx increases exponentially near stoichiometric firing, whereas CO emissions become excessive near the lean blowout limit. The typical window of operation for methane fuel, for example, is 0.45 to 0.70 in premixed combustors. Beyond this window, special flame stabilization means are neces- sary, such as pilot flames or subchambers.15,16

Developers are seeking technologies to expand the oper- ation range with low NOx emissions. One solution is to manipulate the geometry of the combustor. Staged com- bustion is another solution to reduce the NOx emissions while maintaining the design requirements.17 Rich‐ Quick‐Lean (RQL) or Over‐Fire‐Air (OFA) implement staging of combustion air, where the primary flame zone is maintained fuel‐rich to enjoy excellent stability, then the reacting flow is diluted rapidly with the remaining combustion air to jump from rich to lean with minimum residence time near stoichiometry.18,19 This method is more reliable and economical way than changing the geometry.20 OFA has been extensively employed in air staging technique.19 Zhou et al21 studied the NOx emis- sions and overall performance of a staged‐air combustion system using a large‐scale horizontal laboratory furnace, which was equipped with a swirl burner and fired with heavy oil. They investigated the effects of conditions of air staging (ie, OFA ratio and position) on the emissions of NOx and CO both experimentally and numerically. It was reported from their experimental results that in heavy‐oil combustion, both the location and ratio of OFA

7793NEMITALLAH ET AL.

affected the emissions of NOx; the larger the distance between burner outlet and OFA location, the lower the NOx emissions with a negligible rise in the burnout loss. Furthermore, a 22.22% reduction in NOx emissions was reported when a ratio of 20% OFA was implemented in an industrial furnace. Liu et al22 numerically investigated the impact of OFA location and ratio on the combustion of coal and, particularly, on the NOx emissions. They reported that by increasing the OFA ratio, the burner‐zone temperature and fly‐ash carbon content increased while NOx emissions decreased. Similarly, when the OFA loca- tion was increased, reduction in NOx emissions and an increase in fly‐ash carbon content were also observed. Hodzic et al23 also reported that both the ratio and distance between the OFA nozzle and burner outlet considerably reduced the NOx emissions. Yang et al

24 performed a numerical study on a fuel‐oil industrial‐scale combustor to establish the impact of staged combustion on NOx emis- sions. They reported that with greater staging, there is a decrease in total NOx emissions, since higher OFA ratio causes lower flame temperature and oxygen concentra- tion, which leads to a lower rate of thermal NOx formation. They also reported that deviation from stoichiometric pri- mary flame zone decreases NOx emissions and that the rate of reduction is proportional to the residence time. They also reported that air staging has no significant influence on the rate of burnout.

Staging can also be done on the fuel side by varying the equivalence ratio of burners.25 This approach enhances the annular combustor efficiency.26 Yang et al27 focused on fuel staging to reduce the NOx emis- sions from low ranked coal. The influence of several parameters (such as reburn fuel fraction, residence time, and stoichiometric ratio) on NOx emissions was investi- gated. They reported reduction in emissions NOx when reducing the stoichiometric ratio within the combustion zone down to 0.8. Zhao et al28 investigated the effect of equivalence ratio and other parameters on fuel staging in wood‐powder cyclone gasification. They reported that there is no obvious effect of fuel staging on the reduction of the tar content in the produced gas. The range of opti- mum equivalence ratio was 0.26 to 0.29, and the ratio of staged fuel was not greater than 20%. Samarasinghe et al29 investigated the impact of fuel staging on flame instability and structure in a swirl‐stabilized can combus- tor. They reported that fuel staging effectively curbs the instability when the overall equivalence ratio was either increased by staging or remained constant during staging. Lin et al30 studied a premixer and prevaporizer multihole tube burner with a radially in‐line dual‐stages fuel arrangement. They reported that the premixer and prevaporizer multihole had a significant impact on the reduction of NOx emissions.

2.2 | Combustion instabilities and solution techniques

The conversion of the flame from the nonpremixed (diffu- sion) type to the premixed one results in increased levels of flame instabilities, which brings its own challenges to the gas turbine manufacturers. In premixed combustion systems, premixing of fuel and oxidizer as well as flame stabilization is usually achieved through swirl. Creation of a swirl‐induced recirculation zone downstream of the burner is an efficient way of stabilizing the flame when a physical flame holder is not available.31 However, premixing of the reactants increases the homogeneity of the temperature and species‐concentration fields, which makes the combustor more responsive to swirl‐induced oscillations in equivalence ratio and/or pressure.32-34 Per- manent damages may occur when the frequencies of oscillations interfere constructively with the cold‐flow fre- quencies and/or natural frequencies of the combustion‐ system geometry, a situation that is known as resonance and characterized by high amplitude pressure pulsations.32

Combustion instabilities can be categorized into static and dynamic instabilities. The mechanisms for static instabilities are flame blowout and flashback. Such mechanisms are mainly function of the operating condi- tions. The flame blows out when the flow speed exceeds the flame speed, whereas flashback occurs when the flame speed exceeds the flow speed. On the other hand, dynamic instabilities, or thermos‐acoustics, are gener- ated because of coupling between the heat release and the acoustic fluctuations of pressure resulting in unwanted pressure waves.35,36 At a certain level, such pressure waves could be destructive and may result in damage of the combustor hardware.37,38 Premixed com- bustion is considered stable when the amplitude of pres- sure fluctuation is less than 5% of the mean chamber pressure. Beyond 5% is unstable or oscillatory combus- tion.37-40

Combustion instability can be developed because of two reasons. The first reason is that instability can be initiated by any artificial perturbation outside the com- bustion chamber. The second reason is that instability can be generated within the system. For the first reason, the instabilities are initiated by an external perturbation that is stable to small disturbances.41,42 The second reason is the instabilities arising from the oscillations triggered by a small fluctuation that grows out due to the system noise.6,43,44 The pressure fluctuations lead to instability depending on the excitation mechanism and the driving forces of these fluctuations and their corresponding damping process.6 Combustion instabilities in gas turbines can be classified into the

7794 NEMITALLAH ET AL.

following categories: low, intermediate, and high fre- quency instabilities.45-47 Many researchers have devel- oped methods and techniques for controlling these instabilities.25,48,49 Rashwan et al48 defined two methods for damping combustion oscillations in gas turbines, namely, active and passive control. In the passive con- trol technique, some changes are made to limit the rate of heat release or to increasse the rate of energy loss through changing the fuel/oxidizer compositions, fuel‐ injection device, design of combustion chamber, or through installing acoustic dampers.49 In the active control technique, on the other hand, an energy source is introduced into the combustion system to interrupt the coupling between heat release and the acoustic waves.25

Recently, several techniques are being proposed to avert the circumstances leading to oscillations in combus- tion systems while maintaining wide ranges of operabil- ity. These techniques include strategies to control the mixing profile. Two active control strategies stand out, namely, pulsing injection of pilot fuel at the frequency of instability50 and fuel staging.51 The latter technique allows for decreasing NOx emissions and suppressing instabilities simultaneously without the sophistication of high‐frequency actuation of the pulsing‐injection tech- nique. Lacarelle et al52 studied the effect of different fuel injection strategies on the residence time delays and qual- ity of mixing in a premixed system. They reported that for a given distribution of fuel, an increasing quality of mixing is achieved when the fuel injection line is formed of two rotating jets to cross‐flow angles (25° and 55°) as compared with a 45° single jet angle. Lacarelle and Paschereit53 designed a fluidics oscillator to avert the drawbacks of mechanically triggered flows. The mixing efficiency of four of such injectors, applied in a generic jet‐in‐crossflow arrangement, was compared with that of a typical nonoscillating jet. It was reported that the fluid- ics injectors performed better than the normal jets and that, for a given quality of mixing, about 50% decrease in mixing length could be achieved by using fluidics injectors. Lee et al54 investigated the concepts of both axial and radial flow micromixing using experimental and numerical (ie, CFD) analyses. They reported that axial micromixing increases the tendency for flashback, while radial micromixing offers lower emissions perfor- mance. The utilization of MM method, therefore, seems promising in achieving low emissions and eliminating flashback. Estefanos55 investigated the influences of air‐ fuel mixing on NOx emissions and combustion instabil- ities in a lean premixed system. He reported that injecting fuel axially into a region of strong unsteady mixing tends to eliminate the combustion dynamics by damping the unsteady mixing.

3 | OXY‐COMBUSTION FOR CARBON CAPTURE

3.1 | Oxy‐fuel combustion technology

Burning fuel in pure oxygen instead of air, ie, oxy‐fuel combustion, serves primarily to facilitate simple and eco- nomic capture of the effluent CO2 emissions. Since CO2 and H2O are the main combustion products, the former can be separated and captured in high‐purity form after condensing the latter. The process of oxy‐fuel combus- tion, however, is characterized by high flame tempera- tures, which necessitates recycling part of the flue gases (mainly CO2) back into the combustor to dilute the reac- tants and control the flame temperature.56 There has been considerable progress in the development of oxy‐ combustion technology after the International Panel for Climate Change (IPCC, 2005) published its special report on CO2 capture and storage.

57,58 However, for stable oper- ation under oxy‐combustion conditions, the oxygen frac- tion in the oxidizer mixture (O2 plus CO2) should not fall below a certain level.59-63 CO2 addition affects the combustion characteristics, in terms of adiabatic flame temperature, radiative heat transfer, transport properties, chemical kinetics, and flame macrostructure. Therefore, oxy‐combustion in highly CO2 concentrated environ- ments has its own challenges and opportunities.64-68

3.2 | Comparison of air‐fuel vs oxy‐fuel combustion concepts

The concepts of air‐fuel and oxy‐fuel combustion differ significantly in terms of combustor operability and flame characteristics. The main control parameter in the opera- tion of air‐fuel combustors in gas turbines is the equiva- lence ratio, which is used to control the flame temperature and prevent/mitigate combustion instabil- ities. The overall equivalence ratio is always kept lean, especially in premixed combustors, to reduce NOx emis- sions and tune the combustor‐exit temperature to the cycle requirements and the metallurgical limits of the tur- bine first stage. The fact that air is an abundant low‐cost resource allows for lean operation. The composition of air as an O2/N2 oxidizer remains constant throughout all modes of gas‐turbine operation. Flue‐gas recirculation (FGR) is optional and serves primarily to reduce NOx emissions and mitigate combustion instabilities. Oxy‐fuel combustors are, however, quite different. The generation of pure oxygen for combustion is an energy‐consuming process, which makes this oxygen a limited precious resource (as compared with air). Oxy‐fuel combustors are, thus, typically operated near stoichiometry to

7795NEMITALLAH ET AL.

conserve the oxygen. FGR becomes a must in this case to replace the equivalence ratio as the main control param- eter. Part of the effluent CO2 is recirculated in precise proportions to dilute the primary reactants for controlling the flame temperature69 and tuning the combustor‐exit temperature to the cycle requirements and the metallur- gical limits of the turbine first stage.70 The composition of the O2/CO2 oxidizer is, thus, variable based on the FGR ratio. Air and oxy‐combustors are thus significantly different in design and operability. Nonetheless, current gas‐turbine users will not be easily motivated to switch to oxy‐combustion, unless their current LPM air‐based gas turbines are retrofitted to implement oxy‐combustion with minimal hardware changes. Converting a traditional air‐combustion system to utilize oxy‐combustion thus requires careful investigation of combustor operability and flexibility.

From the perspective of combustion physics and reaction kinetics, air‐fuel and oxy‐fuel combustion are again quite different because of replacing the air‐based N2 with CO2. The difference in physical properties of N2 and CO2 significantly affects the physical properties of the reacting mixture,71-73 ie, density, volumetric heat capacity, and transport properties.74 The transport prop- erties comprise mass diffusivity, dynamic viscosity, and thermal conductivity. CO2 has higher density than N2, which affects flame shape, bulk velocity, and the pressure drop across combustor headend, as well as the total mass flowing through the combustor and turbine. The CO2/O2 oxidizer also has higher volumetric heat capacity compared with air, which in turn reduces the flame temperature, thereby reducing flame speed and stability. Since the rate of chemical kinetics is directly proportional to the laminar flame speed and combustion efficiency, the presence of CO2 in increased concentrations in oxy‐fuel combustion, results in reduc- tion in both laminar flame speed and combustion effi- ciency, because retarded chemical kinetic rates have been observed in oxy‐fuel flames.75,76 An additional challenge in oxy‐fuel flames is CO2 dissociation, which is an endothermic reaction that consumes energy and affects the reaction kinetics,77-80 especially at high CO2 concentrations. It can thus be deduced that if an air‐fuel combustor is converted to oxy‐fuel operation at the same equivalence ratio and without any geometry changes, a 21% oxygen fraction (by vol.) in the O2/CO2 oxidizer will not be enough to sustain stable flame,81,82

ie, the minimum oxygen fraction is greater than 21%. Ditaranto et al83,84 have shown that the oxy‐combustion process requires at least 30% oxygen fraction to achieve stable combustion as compared with air‐based combus- tion. The higher oxygen concentration is required to compensate for the higher volumetric heat capacity of

CO2 compared with N2. Such challenges necessitate the modification of burner designs to be able to hold stable flames under oxy‐fuel combustion conditions.

3.3 | Premixed oxy‐fuel combustion

Speth et al85 investigated the dynamic steadiness of premixed stoichiometric CH4/CO2/O2 flames. The adia- batic flame temperature (Tad) of the reactant mixture was adjusted through the oxygen fraction. Although air‐ and oxy‐flames have different flame speeds at the same Tad, similarity was found between the two flames in the fact that Tad primarily rules the transformation from one stability mode to another. The above finding was sup- ported by high‐speed images that showed similar vortex collapse modes. Since Tad changes the turbulent flame geometries, so these modes, which are symptomatic of related turbulent flame geometries, are thereby representing a change in Tad. Kutne et al

86 also studied and compared oxy and air‐methane flames but under par- tially premixed conditions. They considered equivalence ratios of 0.5 to 1.0 and oxygen fractions of 20% to 40%. OH*‐chemiluminescence imaging and Raman spectros- copy were used to quantify the flame structure. It was reported that the oxygen fraction has a major effect on flame characteristics, whereas the equivalence ratio has a little effect. It was also observed in air‐flames that the inner recirculation zone is located further upstream, as compared with oxy‐flames, indicating inferior stability of the latter. The authors also confirmed that the 21% oxygen fraction of air is unattainable in oxy‐flames, which have a higher minimum threshold. The inferior combustion temperatures and flame speeds of oxy‐flames were attributed to the higher volumetric heat capacity of CO2 compared with N2. Numerous other researches also reinforced these observations.66,78,87-90 Temperature pro- files were found to be matched in CH4/air and CH4/ 36%O2/64%CO2 flames, according to Song et al.

66 Compa- rable values were reported in the literature.59,63,91,92

Rashwan et al93 reported similar findings; an oxygen frac- tion of 29% to 40% is needed for stable oxy‐combustion with FGR. Moreover, oxy‐combustion limits the combus- tor stability window, because of the negative impact of adding CO2 on the combustion kinetics. Abdelhafez et al94 showed that no stable oxy‐flames could be obtained with oxygen fractions below 22%, even at stoi- chiometric conditions, in a swirl‐stabilized premixed gas‐turbine model combustor. They added that the effect of oxygen fraction is significantly greater than that of equivalence ratio, ie, stable flames can be obtained at much leaner conditions with higher oxygen fractions. Jerzak and Kuznia79 compared the blowout mechanisms

7796 NEMITALLAH ET AL.

of premixed CH4/O2/N2 and CH4/O2/CO2 flames in a swirl combustor. They reported that oxy‐flames with FGR have significantly narrower operability windows compared with air‐flames. Hu et al95 examined the lami- nar flame speeds of CH4/O2/N2 and CH4/O2/CO2 flames at atmospheric conditions, targeting the effects of oxygen fraction, equivalence ratio, and dilution (CO2 vs N2). The laminar flame speed was reported as a quadratic function of oxygen fraction. Moreover, the measured flame speed of an N2‐diluted flame was about five times that of a CO2‐diluted flame, keeping other parameters constant. Oh et al87 supplemented carbon monoxide (CO) as a sub- ordinate fuel along with CH4 in a nonpremixed oxy‐fuel gas turbine combustor; as the CO mole fraction was increased, the experiments indicated longer flame lengths and inferior stability.

Saanum and Ditaranto96 studied a new high‐pressure oxy‐fuel burner designed at SINTEF (The Foundation for Scientific and Industrial Research in Norway) with a 100‐kW input load and pressure of 10 bar. The swirl‐ stabilized oxy‐burner uses a mixture of O2 and CO2 as oxidizer and generates a flame in a cylindrical double‐ wall combustor. It was observed that as the pressure increases, the CO level decreases radically. The flame also becomes more compact at higher pressures. It was found that stable flames are obtained at oxygen concentrations above 36.5% at 20 kW and above 40% at 40 kW. An exper- imental study was performed by Rashwan et al90 on par- tially premixed methane‐oxygen and methane‐air flames over a PP burner. The results showed that as the premixing ratio increases, the emissions of CO and NOx decrease. The flame color changed to pure blue at higher premixing ratios from reddish color at lower ratios; high levels of soot formation were observed in diffusion flames. The results also indicated that as the oxygen frac- tion increases in the oxidizer mixture, the stability of the oxy‐fuel flames increases due to the associated increase in flame speed. Ramadan et al78 experimentally investigated different nonpremixed flames involving air‐fuel, O2‐ enriched air‐fuel and oxy‐fuel flames stabilized over a bluff‐body of various blockage ratios. The results showed that flame stability increases with increasing fuel momentum for the same blockage ratio, caused by the corresponding increase in Reynolds number, turbulence, and mixing.

FIGURE 2 Modified Wobbe index (MWI) map of all known gaseous fuels [from www.ge.com] [Colour figure can be viewed at

wileyonlinelibrary.com]

4 | FUEL‐FLEXIBLE COMBUSTION APPROACH

The nonrenewable nature of fossil fuels and the stringent emissions regulations have pushed combustion researchers to look for efficient utilization of all available

fossil fuels and develop alterations in the existing gas tur- bine technologies for utilizing nonconventional fuels. Original gas turbine manufacturers (OEMs) have been facing this challenge of fuel flexibility because most of the traditional gas turbines have been designed and manufactured for standard natural gas only. Nonconven- tional fuels pose quite a different set of chemical and transport properties, as compared with standard fuels, which causes operability issues in traditional gas tur- bines. This created a wider need for research in fuel‐ flexible combustors.

4.1 | Fuel flexibility

The Wobbe index (WI) is a common indicator for inter- changeability and fuel characteristics in the gas turbine industry. It applies to gaseous fuels and is defined as the ratio of lower heating value (LHV) per unit mass of fuel to the square root of fuel specific gravity, where the spe- cific gravity is the ratio of fuel density to air density; see Equation (1). The Modified WI (MWI) was introduced to account also for the fuel temperature. Figure 2 shows the MWI map of all known gaseous fuels. Standard natu- ral gas (mostly CH4) lies at the center of the map. The larger the content of higher hydrocarbons (C2+), the higher the MWI is at constant LHV, eg, liquefied petro- leum gas (LPG). Hydrogen enrichment increases LHV at constant MWI. Increasing the inert content (N2, CO2, H2O) decreases both LHV and MWI, eg, syngas and coke‐oven gas (COG). Table 1 complements Figure 2 by categorizing the different fuels.

TABLE 1 Wobbe index ranges of different fuels97

Fuel Category Wobbe Index Range, MJ/Nm3

Syngas type 24‐29

Natural gas type 48‐53

LPG type 72‐87

Methane 47‐53

Hydrogen 40‐48

Carbon monoxide 13

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WI ¼ LHVfuelffiffiffiffiffiffiffiffi ρfuel ρair

r and MWI ¼ LHVfuelffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Tfuel ×

ρfuel ρair

r : (1)

Justification and validation of using the WI has been widely discussed in the literature.98 Different gaseous fuels exist for potential use in gas turbines. Even for nat- ural gas itself, different compositions are found from dif- ferent geographical locations. Some applications dictate running the gas turbine on high H2 fuels or low‐quality ones (such as COG). The MWI was thus developed to characterize these different fuels and baseline them to standard natural gas. The power input to a combustor is given as the product of LHV per unit volume of fuel and the volumetric fuel flow rate. This implies higher vol- umetric fuel flow rate for lesser LHV per unit volume, for providing the same input power to the combustor. The fuel density is the link between LHVs per unit volume and per unit mass. The MWI thus accounts for the com- monly used LHV per unit mass and the fuel density.

An empirical relation was developed to link the input power to combustor pressure drop and fuel WI, as fol- lows:

Pinput ∝ ffiffiffiffiffiffiffi ΔP

p :WI: (2)

The combustor pressure drop represents the air flow rate through the engine and, thus, its load. For a charac- teristic equivalence ratio (flame temperature), the fuel flow rate can also be linked to the combustor pressure drop. Hence, the input power is proportional to the prod- uct of pressure drop and WI, as the latter involves heating value.

Gas‐turbine manufacturers have established standards for combustor design on natural gas and use the WI or MWI for gaseous fuel interchangeability and characteris- tics with fixed pressure drops and geometry of the combustor.99,100 Combustors are typically designed to operate within narrow ranges of WI.

4.2 | Fuel‐flexible combustion approaches

4.2.1 | Hydrogen enrichment

Hydrogen (H2) is one of the simplest chemical elements available and is a promising fuel for carbon emissions– free operation. As H2 has a higher heating value per unit mass compared with other hydrocarbon fuels, it has been used as rocket propellant since the 1950s for space‐ exploration uses. It is an abundant, clean energy carrier, confined in water, different hydrocarbons, and various organic compounds. Its efficient extraction in bulk quan- tities from these resources is a challenging task. Mean- while, natural gas reforming with steam is currently being used to produce hydrogen. It is also the by‐product of many industrial and chemical processes. In precom- bustion carbon‐capture techniques, it is viable to use this by‐product H2 as fuel, either along with natural gas or, in some potential cases, alone. Since traditional gas turbines have been historically designed for standard natural gas, the variations in fuel composition and/or switching to a different fuel are challenges being faced by the gas tur- bine designers.101,102 Even hydrogen enrichment poses a challenge, especially in premixed combustors, because the chemical and transport properties of hydrogen are quite different from standard fuels. Moreover, the storage and cost of hydrogen are also major concerns. The use of hydrogen as pure fuel in gas turbines is rather limited. There are quite a few of such power plants that use pure hydrogen as fuel. A 12‐MW power plant (nonpremixed combustion system), opened in Fusina, Italy, in 2010, is the first of its kind to operate completely on pure hydrogen.103

Hydrogen is typically used to enrich hydrocarbon fuels, especially natural gas (CH4), in gas turbines. Since the adiabatic flame temperature, heating value, and laminar flame speed are quite larger for hydrogen as com- pared with methane, hydrogen enrichment caters for better combustion quality by increasing both the combus- tion efficiency and fuel economy.104 Patil et al105 reported that the power of a LPM engine fueled by compressed natural gas increased by 11% and the fuel consumption decreased by 8% with hydrogen enrichment, as compared with operation of the same engine with compressed natural gas only.105 Biogas combustion is also enhanced with hydrogen enrichment, especially the lean blowout limit.106

Switching from standard natural gas to C2+ fuels (higher WI) dictates the use of smaller fuel‐injection ori- fices, whereas switching to low‐quality fuels (lower WI) has to be accompanied by enlargement in fuel orifices. Hydrogen enrichment, on the other hand, induces mini- mal changes in WI (see Figure 2) so a gas turbine can

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be hydrogen‐enriched to a certain extent without geome- try changes. If WI is within the range of 30 to 50 MJ/Nm3, it has been reported that hydrogen can be used with methane in combustors without any considerable geome- try modification.107 The heating value of hydrogen is far greater than that of methane, which implies that a little amount of hydrogen can replace a bigger amount of methane, thereby posing no significant geometry‐change requirements in a combustor designed for methane. It should be noted, however, that despite its common use in the gas‐turbine industry, the WI does not account for changes in flame speed and ignition and chemical delay times102; therefore, high H2 require special burner designs to create a safe margin to flashback.

One of the primary reasons for using hydrogen with methane is its capability to reduce CO2 emissions; pure hydrogen results in no CO2 emissions, just H2O. The con- centration of H2O in the flue gases increases with hydro- gen enrichment. However, since the adiabatic flame temperature of hydrogen combustion is considerably higher than that of methane, the generation of thermal NOx increases.

104 It has been reported that with 28% to 36% hydrogen enrichment, very low NOx emissions can be achieved108 at the expense of moderate increase in the concentration of unburned hydrocarbons. To com- bine the benefits of increased power output, fuel econ- omy, and reduced NOx emissions, an optimal hydrogen fraction of 20% has been reported for compressed natural gas engines.109-113

4.2.2 | Syngas combustion

Syngas is a fuel gas mixture consisting primarily of H2, CO, and some CO2. It is produced from the gasification of virtually any hydrocarbon fuel, feedstock, or waste. Some industries also generate H2‐rich gas mixtures as by‐products, eg, coke‐oven gas, which can fuel gas tur- bines for power generation. Syngas (including H2) can also be intentionally produced by reforming fossil fuels. Integrated gasification combined cycle (IGCC) plants have been developed over a number of decades to utilize this principle, and they are gaining significant interest in recent years for precombustion capture of CO2 to meet the stringent environmental regulations governing greenhouse‐gas emissions. Implementing the water‐gas shift reaction allows for converting most CO to CO2; the latter is captured again prior to combustion, so the gas turbine combustor runs on very H2‐rich syngas fuel.

The concept of precombustion carbon capture has trig- gered fuel reforming to generate syngas, which motivated research to evaluate syngas combustion under LPM con- ditions for further control of emissions without affecting

the efficiency. However, the conversion of current natural‐gas fueled combustors to work with high‐H2 syn- gas is not feasible yet. This may be attributed to the higher tendency of syngas for flashback and to the higher syngas flow rates required to maintain the same power obtained from natural gas,114 since syngas has lower WI. Without any enlargement of fuel orifices, this increased flow rate will induce higher velocities, which might also cause early blowout of the flame as well. In addition, it might result in unstable operation of the com- pressor and ineffective cooling of the turbine blades.114,115

This necessitates more research to evaluate properly the combustion and emissions characteristics of syngas for safe application in gas turbines.

As per the open literature, a great number of studies have been performed on syngas combustion considering mixtures of H2 and CO with different concentrations. On the other hand, there is a limited number of studies on the combustion and emission characteristics of syngas fuel mixtures containing other gases such as H2O and CO2 as well. There is also lack of experimental data on the laminar burning velocity of syngas diluted with NH3 or mixed with CH4. Actually, the composition of syngas mixture depends on the adopted gasification method and the utilized feedstock. Typical compositions of syngas mixtures, considering different gasification approaches, ranges from 0% to 9.3% CH4, 0% to 20.4% H2O, 1.3% to 29.6% CO2, 8.1% to 60.5% CO, and 6.8% to 50.4% H2, in addition to other impurities.116 Both CO and H2 gases have different burning characteristics than conventional hydrocarbons. A number of recent investigations on syn- gas combustion of different hydrogen concentrations reported adverse effects on flame structure and stability as well as flashback and blowout limits.117-120 Fu et al120

reported significant changes in flame structure using syn- gas fuels of different hydrogen concentrations, even for the same operating pressure and equivalence ratio. The presence of CO2 and H2O in syngas also affects its com- bustion characteristics significantly. As both are highly radiative species, high concentrations of CO2 and/or H2O in syngas may have drastic effects on the tempera- ture and radiation characteristics of the flame, in addition to undesired flame dynamics.121 Lee et al122 showed that CO2 has insignificant effect on the ignition delay time in syngas combustion, based on experiments on syngas mix- tures of different hydrogen contents with CO2 concentra- tions above 14% under higher operating pressure.

4.2.3 | Ammonia combustion

The use of ammonia (NH3) either for carbon‐free com- bustion or for the reduction of NOx emissions in

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conventional combustion systems has recently received lot of interest despite the safety concerns associated with NH3 combustion. In this section, a condensed review is presented of the performance and combustion character- istics of NH3 as a fuel for gas turbine applications. NH3 enrichment of hydrocarbon fuels can result in better con- trol of combustion temperature and emissions. Although the idea is not new, there is a limited number of studies on the performance of NH3 as a fuel for power production in gas turbines. Actually, the idea was initiated during the 1960s.123-125 Such studies reported higher ignition energy for NH3 due to its low reactivity as compared with hydro- carbon fuels. NH3‐air combustion under stoichiometric conditions has longer quenching distance, 3.5 times that of propane fuel. Furthermore, the combustion of NH3 showed limited operability range based on equivalence ratio as compared with propane fuel.124 Moreover, the amount of air supplied to a combustor for burning NH3 has to be lower to increase the residence time for com- plete combustion. This reduces the inlet Reynolds num- ber, and hence, flow mixing and turbulence are adversely affected, resulting in reduced combustion effi- ciency.125 NH3 flames also have low laminar burning velocity in the range of 0.06 to 0.08 m/s.126 However, NH3 combustion showed some good characteristics in terms of faster dissociation rate, which may result in faster flames of comparable characteristics to some hydrocarbon fuels. It was reported that 28% dissociation of NH3 could be utilized as enrichment fuel for gas tur- bine applications designed to work on hydrocarbons. Other studies considered NH3 as an engine fuel and showed limited success in terms of reduced NOx emissions.127,128

Recently, a number of studies have been performed considering NH3 a flexible fuel for gas turbine applica- tions.129,130 They reported that the associated emissions with the combustion of ammonia limits the wide applica- tion of such fuel technology. The upper and lower con- centrations of NH3 by volume in air, corresponding to the upper and lower flammability limits under atmo- spheric conditions, were found to be 33.5% and 14.8%, respectively.131 Preheating of the reactants resulted in wider operability limits, whereas the presence of water vapor restricts the operability limits. SPG Advanced Pro- pulsion and Energy129,130 is one of few companies that tried to produce commercial systems for NH3 combus- tion. They presented a series of reports summarizing the challenges involved in the development of a reliable NH3 combustion technology, including (a) slower reactiv- ity and, accordingly, lower combustion temperature, (b) combustion instabilities, (c) vaporization approach of NH3 for improved combustion, and (d) cracking of NH3 upstream of the burner for enhanced burning velocity.

The research is ongoing, trying to improve the perfor- mance of NH3 as fuel for gas turbine applications, sup- ported by the global trend of decarbonizing fuels for reduced carbon emissions out of combustion systems. Focus is made on understanding the slower reactivity of NH3, particularly in fuel blends with hydrogen and meth- ane. Overproduction of OH radicals and enhanced reac- tivity were reported at operating equivalence ratios in the range of 1.05 to 1.25.131 Such results are promising if NH3 is blended with other light hydrocarbons for gas turbine applications. More research is required in this field to reduce the time required for the development of reliable NH3 combustion systems on the industrial scale.

4.3 | Fuel‐flexible premixed oxy‐fuel combustion

The use of low‐carbon or carbon‐free fuels (eg, H2) seems to be a satisfactory solution for controlling CO2 emis- sions. However, hydrogen production and storage on the industrial scale is expensive, and more research work is required on this subject. Moreover, the world still depends on fossil fuels for meeting the increasing energy demand because of their competitive price. Therefore, combining oxy‐combustion of low‐carbon fuels with H2‐ enrichment provides a good solution for controlling both NOx and CO2 emissions at reasonable cost. H2‐enriched fuels have better combustion and emissions characteris- tics compared with nonenriched fuels.132-134 The use of premixed combustion technique also maintains the flame temperature inside the gas turbine combustor within safe limits, especially under H2‐enriched oxy‐fuel conditions. However, the application of premixed combustion with H2‐enrichment brings its own challenges in terms of flame stabilization.

Mazas et al135 conducted experimental and numerical analyses of the laminar flame velocity of premixed oxygen‐enhanced methane combustion. They measured the flame velocity using the conical flame method, while the calculations were carried out using the detailed kinetic mechanism GRI‐Mech. 3.0. The study was con- ducted at different equivalence ratios from 0.6 to 1.5 with the molar oxygen‐enrichment ratio [O2/(O2 + N2)] rang- ing from 0.21 to 0.5, and the effect of adding water vapor was also analyzed with molar fraction varying from 0 to 4.5. The results indicated linear decrease of the laminar burning velocity with the addition of water vapor. For a given O2‐enrichment level, the laminar burning velocity was independent of the operating equivalence ratio. They concluded that the effect of adding water vapor on the laminar flame velocity is mainly a thermodynamic one and does not affect the reaction kinetics. A numerical

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study on the combustion characteristics of premixed oxy‐ methane combustion at various flue‐gas dilution ratios was carried out by Li et al.67 As shown in Figure 3, the laminar burning velocity with CO2‐dilution is restrained, as compared with that with N2‐dilution. The burning velocity with CO2‐dilution was found to be close to that of air/methane stoichiometric combustion at a diluent/oxygen ratio of 200%. The calculations showed small differences in adiabatic flame temperature for vari- ous diluents (N2/H2O/flue gas/CO2) at lower diluent/oxygen ratios, but such differences increase as the diluent/oxygen ratio increases.

Using H2‐enrichment and syngas in combined cycles is attracting the interest of research community as an alter- native to natural gas.101,136,137 Syngas and H2‐enriched fuels are regarded as the prominent alternative fuels for the future energy mix.138-140 Recycling of the industrial by‐product hydrogen gas and the use of H2‐rich fuels after precombustion carbon removal can be integrated with combined cycle plants. It has been reported that the addi- tion of H2 to hydrocarbons can reduce the soot formation and CO emissions drastically.141-145 Moreover, the addi- tion of even a small amount of H2 to the fuel mixture can affect both the instantaneous and average character- istics of a turbulent premixed flame,146 necessitating rig- orous research on the combustion dynamics of such flames with varied fuel compositions.

Gersen et al147 investigated the ignition delay time of hydrogen/methane mixtures at various compositions in

FIGURE 3 Effect of recirculation ratio on adiabatic flame temperature and laminar burning velocity of premixed methane

flames in different oxidizer environments67 [Colour figure can be

viewed at wileyonlinelibrary.com]

a rapid compression machine. They found that the igni- tion delay time decreases with the increase in H2 content and can be correlated quantitatively to the hydrogen frac- tion of the fuel in accordance with the mixing law pro- posed by Cheng and Oppenheim.148 The laminar burning velocity tends to increase linearly with the addi- tion of hydrogen.149,150 The unstretched flame‐ propagation speed and the unstretched laminar burning velocity were reported to increase with fuel hydrogen fraction in a study conducted by Tang et al.151 They also found that this dependence becomes even more remark- able at higher hydrogen fractions. Sankaran and Im152

concluded that the high laminar burning velocity of H2 is attributed to its higher molecular diffusivity. Further- more, the addition of H2 to hydrocarbon fuels in lean flames has been found to increase the turbulent burning velocity monotonically.153 In rich flames, however, H2 addition had no apparent effect on the burning velocity but was reported to depend on the type of hydrocarbon fuel. Halter et al146 reported similar dependence of the turbulent burning velocity on hydrogen fraction in premixed H2‐enriched methane/air flames. They further concluded that the combustion intensity increases with hydrogen addition, implying that the turbulent burning velocity increases faster than the laminar one with hydro- gen addition. This was attributed to the fact that the flame‐front curvature distribution tends to enlarge slightly with hydrogen addition, which enhances the smaller‐scale flame wrinkling, resulting in higher turbu- lent burning velocity. Similar findings were reported by Daniele et al.154,155

Hydrogen enrichment enhances the reaction rate and burning velocity due to the increased H, O, and OH con- centrations in the flame, as reported by Hu et al.149 Sim- ilar observations were reported by Wang et al156 from their numerical analysis of the effect of H2‐enrichment on methane/air combustion using the GRI‐Mech 3.0 mechanism. Moreover, they suggested that H2‐enrich- ment reduces the formation of CH2O and CH3CHO spe- cies, consequently reducing the aldehyde emissions in methane combustion. An increase of approximately 20% in OH mole fraction was reported by Schefer157 when blending 20% hydrogen with methane. Due to the high combustibility of hydrogen, flames with higher hydrogen fractions are characterized by higher reaction‐zone tem- peratures, although the adiabatic flame temperature of the fuel blend may decrease with hydrogen addition. The reaction‐zone expands faster because of its higher temperature, consequently inhibiting the cooler recircula- tion flow of combustion products into the reaction zone.158

Schefer157 conducted an experimental study of flame stability with hydrogen addition. He found that the lean

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blowout limit is maximum at stoichiometric conditions and reduces for both the lean and rich conditions. More- over, he reported that the lean blowout limit could be reduced by around 15% by the addition of up to 20% hydrogen to the fuel. A laboratory‐scale investigation of a low‐swirl injector (LSI) was conducted by Cheng and Littlejohn159 to study the air‐combustion characteristics of pure hydrogen and N2‐diluted hydrogen with the aim of implementing this in the gas turbines of the IGCC. They found an optimum swirl number of 0.51 for H2 in the various LSI configurations examined, as compared with 0.54 for hydrocarbon LSI. The lean blowout (LBO) limit was found to occur at a nearly constant equivalence ratio of 0.17 at different bulk velocities, indicating that the LBO of LSI hardly depends on Reynolds number. Another parameter that has been found to predict the LBO limit successfully is the Damkohler number. Zhang et al160 reported an interesting finding that the LBO limit occurs at a Damkohler number of 0.4 for various fuel blends of H2/CO/CH4. Taamallah et al

74 presented a com- prehensive review of syngas use in gas‐turbine combus- tion. They stated that the composition of syngas is a vital parameter that determines the flame characteristics, as the physical and chemical properties of H2, CO, and CH4 are significantly different, which affects the combus- tor performance. A higher percentage of H2 in the fuel blend results in minor reduction in adiabatic flame tem- perature. However, the addition of hydrogen has been observed to lower the LBO limit, allowing the combustor to remain lit at lower equivalence ratios. Nonetheless, the higher turbulent flame speed, characteristic of H2‐ enriched flames, increases the risk of early flashback even at lower equivalence ratios.

Lounici et al161 studied the improvement of a natural‐ gas engine by hydrogen enrichment. They reported a reduction in break specific fuel consumption as well as the emissions of hydrocarbon, CO, and CO2. Schefer et al157,162 revealed that adding an amount of H2 to the combustion process results in significant reduction in CO concentration and a slight increase in NOx emissions, as presented in Figure 4, in addition to improved flame

stability and shorter flame length. Tuncer163 showed experimentally that hydrogen enrichment considerably extends the lean blow‐off limit. However, despite the above‐mentioned enhancements, the implementation of hydrogen‐enriched combustion is still limited because of the associated phenomena, including flashback, auto ignition, and combustion dynamics.

5 | GAS TURBINE COMBUSTION SYSTEMS

5.1 | SPRF burners

As previously reviewed, diffusion flames induce excessive thermal NOx emissions because of the existence of high‐ temperature stoichiometric zones within the flame. How- ever, diffusion flames can still be utilized with reduced NOx emissions, if mixing is enhanced within the combus- tor to minimize the stoichiometric zones. This can be achieved through careful design of the combustor to con- trol the mixing process and minimize the residence time of combustion products within the flame core.164 This approach is most effective in cases of jet flames with high coaxial air velocity. The high‐velocity flow improves the shear and, accordingly, enhances the mixing of air and fuel before combustion takes place, which reduces the tendency of creation of stoichiometric zones. The high‐ velocity jet also reduces the residence time of combustion products within flame core, resulting in lower NOx emissions.

Another approach to controlling NOx emissions is FGR, where the reactants are diluted with inert (cooled) exhaust gases to lower the concentrations of oxygen before the combustion process and, consequently, extend the ignition delay period.165-167 Longer ignition delay allows air and fuel to mix within the combustor before the onset of combustion, even in the diffusion‐flame mode. Besides, recirculating the exhaust gases at a rea- sonable rate spreads the combustion zone out throughout the whole mixing zone, rather than over a thin front.

FIGURE 4 Effect of hydrogen enrichment on CO and NOx emissions

162

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Consequently, a uniform temperature distribution is obtained within the combustion zone and, accordingly, NOx emissions are reduced to lower levels. However, mixing the reactants with inert exhaust gases results in lower flame burning velocity and, consequently, lower combustion efficiency, and the combustor is prone to loss of static stability.

Recently, the idea of a novel gas‐turbine combustor design, called stagnation‐point reverse‐flow (SPRF) com- bustor, has been demonstrated to meet such conflicting requirements of stable flame and low emission levels.168

The size of SPRF combustor is small, which promotes internal gas recirculation, because the outflow ports are on the same geometrical plane as the inflow, while the opposing end is closed.169 This design creates a low‐ velocity region near the closed end, as shown in Figure 5, which helps to stabilize the combustion process. As they leave the combustor, the hot products interact with the incoming reactants to form a more reactive mix- ture. The reactants are diluted with substantial amounts of burnt gases laden with radicals, which lowers the igni- tion temperature, resulting in lower NOx and CO emis- sions as well as enhanced blowout limit.169-171

A few recent studies were performed to evaluate the performance of SPRF combustor considering ranges of operating parameters. Gopalakrishnan et al172 investi- gated experimentally the mechanisms for reducing NOx emissions in a nonswirling SPRF combustor in nonpremixed mode. Lifted flames were obtained away from the injector, which resulted in better mixing of air and fuel before the onset of combustion and, conse- quently lower NOx emissions. Bobba et al

173 studied

FIGURE 5 Schematic diagram presenting the key features of the SPRF gas turbine combustor169 [Colour figure can be viewed at

wileyonlinelibrary.com]

experimentally the stabilization mechanisms and flame structure of premixed methane‐air flames in a nonswirling SPRF combustor. The results showed two stabilization zones within the combustor, a primary zone located downstream of the inlet section, where strong tur- bulence and low velocities are obtained. The increased turbulence strength in the shear layer resulted in more entrainment of the recirculated exhaust gases, thus improved reaction rates, and enhanced flame stability. The secondary stabilization zone is located near the stag- nation zone beside the wall. The analysis of the turbulent flame structure revealed that the flame is mainly in the thin reaction layer regime throughout the entire combus- tor. Castela et al174 conducted an experimental study to understand the combustion regimes occurring in an SPRF burner. For a given preheat inlet temperature of air, the recorded data showed a considerable effect of increasing the air inlet velocity on the reduction of NOx emissions, as presented in Figure 6. Furthermore, increasing the air inlet velocity resulted in stable flame operation at leaner conditions. Undapalli et al175 per- formed large eddy simulations (LES) of an SPRF combus- tor, considering both diffusion and premixed combustion modes. The premixed flame was attached to the injector, whereas the diffusion flame was lifted away. The flame lift‐off in the nonpremixed mode enabled longer ignition delay for better mixing of the reactants. This may justify the similar NOx emission levels obtained in both the dif- fusion and premixed combustion modes.

Moderate or intense low‐oxygen dilution (MILD), aka flameless combustion, is one of the combustion tech- niques that also uses reverse flow burner configurations and can exhibit very low NOx emissions and wider flam- mability limits.176-178 Flameless combustion can be obtained within a combustor by raising the inlet temper- ature of reactants above the mixture auto‐ignition tem- perature while entraining enough inert exhaust gases to increase the ignition delay. Such conditions result in no visible flame while keeping emissions at very low levels.

FIGURE 6 Effect of inlet air velocity on NOx emissions for different inlet temperatures and excess air factors174

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Despite being used in a number of industrial applications, the implementation of MILD combustion in gas turbines is still in the research and development phase.179-181 A number of studies were performed considering MILD combustion for gas turbine applications. Luckerath et al182 studied flameless combustion for gas turbine applications at an operating pressure 20 bar. Levy et al183 proposed a novel gas turbine combustor design that can adopt MILD combustion by creating a large recirculation zone within the combustor. In another study, Lammel et al184 were able to achieve low CO and NOx emissions using a combustor with high power den- sity. In addition, Arghode et al185 discussed the concept of flameless combustion for gas turbine applications and were able to achieve ultra‐low NOx emissions.

FIGURE 7 Schematic diagram comparing a typical DLE combustor with a conventional one189 [Colour figure can be

viewed at wileyonlinelibrary.com]

FIGURE 8 Solar Low NOx burner system for natural gas 190

5.2 | DLN/DLE burners

The recent advances in the development of future gas tur- bine technologies largely target efficiency improvements and nitrogen oxides (NOx) emission reduction. Several methods of lowering NOx emissions have been established. NOx formation during combustion is a strong function of flame temperature.186 Lowering the tempera- ture using diluents like water or steam apparently is the most promising solution for reducing NOx emissions, par- ticularly in stationary power‐generation systems. How- ever, this usually results in lower overall engine efficiency, possible corrosion due to water impurities, and reduction or quenching of CO burnout, which makes it counterproductive.6 Solutions not involving steam or water injection include LPM combustors implementing DLN or DLE technologies.187,188 Such combustors in gas turbine applications can reduce NOx emissions down to the single digit level.189 Figure 7 shows a schematic dia- gram comparing the DLE concept with a conventional burner. Both burners are swirl stabilized. The size of fuel injector in the DLE burner is larger to account for the larger quantity of airflow in the premixing zone.

The major design requirements for optimum perfor- mance of DLE burners are (a) conforming to emission standards at peak load and (b) regulating the emissions across the range of engine load. Other important param- eters to ensure combustion stability and wide operability range under all conditions include the system ability to respond rapidly to load changes, maintaining minimum combustions acoustics, as well as smooth switching from one fuel to another, especially in dual‐engine engines.190

Different manufacturers have different design approaches to the DLE combustor concept with the sole aim of reduc- ing NOx emissions without utilizing steam or water injection.

Figure 8 shows the cross‐section of a Solar Low NOx burner designed for the multiple‐can combustors devel- oped by the Solar Turbine Company. It is a fuel‐injec- tor/swirl‐air assembly that contains an 18‐vain air swirler with radial flow orientation to provide the required swirl to the primary air, thereby aiding air‐fuel mixing by ensuring recirculation in the primary combus- tion zone. This burner concept allows three modes of fuel injection: pilot, channel, and 18‐spoke swirler injection. The later mode provides better mixing. This design was found to reduce NOx emission levels to less than 10 ppmv and also lowered both CO and unburned hydrocarbon

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emissions when fueled by natural gas at pressures of up to 11 bar.191 The schematic diagram shown in Figure 9 represents the design concept of the premixed two‐staged DLN‐1 combustor that is capable of using both natural gas and liquid fuels.192 The burner comprises four main parts: fuel injectors, liner, venturi, and centerbody/cap assembly. These components provide three core zones: primary, secondary, and dilution zones.

The burner operates in four distinct modes, namely, Primary, Lean‐Lean, Secondary, and Premix, as shown in Figure 10. In the primary mode, fuel is supplied through the primary nozzle only, and the primary air through the swirlers. Initial flame ignition and low loads of up to 20% are conducted in this mode. The lean‐lean mode is selected at intermediate engine loads by supplying fuel to both the primary and secondary zones. The fuel mixes with air and passes through the swirler at the exit of centerbody, thereby forming the secondary combustion

FIGURE 9 General Electric DLN combustor192

FIGURE 10 Fuel‐staged DLN modes of operation192

zone. The secondary mode of operation is the transition mode between lean‐lean and premix modes, with the flame being sustained in the secondary zone only. The fuel supplied to the primary nozzle is slowly reduced while that of the secondary nozzle is increased, which extinguishes the primary flame. In the premix mode, some fuel is still supplied to the primary nozzle, but the flame is sustained in the secondary flame zone only. This mode provides minimum emissions at the exhaust. When this combustor operates using natural gas at base load, the levels of CO and NOx can be as low as 25 ppmv and 9.0 ppmv, respectively.190

5.3 | EnVironmental (EV/AEV/SEV) burners

Although the market of LPM gas turbines has many com- peting combustion technologies, some stand out based on their ultralow emissions and remarkable flexibility. EV‐ burners, for example, compete strongly with MMs (to be discussed in the next section), especially that EV‐burners offer significantly effective flame stabilization over a wide range of operation, which enhances engine turndown. Both MMs and EV‐burners have never been examined under oxy‐combustion conditions; thus, the following condensed review is dedicated to EV‐burners to discuss their potential for implementation in zero‐emission gas turbines.

The acronym EV‐burner stands for Enhanced‐Vortex burner. The ability of this technology to achieve ultralow NOx emissions in LPM gas turbines has triggered the name EnVironmental (EV) burner,193 which also abbreviates to

FIGURE 12 Axial velocity distribution inside an EV burner194

[Colour figure can be viewed at wileyonlinelibrary.com]

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EV‐burner. Its geometry consists of two half‐cone shells, where one is slightly offset in the radial direction with respect to the other while maintaining parallel axes; see Figure 11. Two tangential slots of constant width are thus created between the shells. Combustion air enters the burner tangentially through these slots, creating swirling core flow with vortex breakdown near burner exit.195 The swirl strength increases in the axial direction because of the increasing diameter; the divergence angle is designed carefully to position the vortex breakdown point close to burner exit (see Figure 12). Premix gaseous fuel is injected transversely into the combustion air through a row of holes just upstream of each air slot; this transverse injection boosts mixing effectiveness, which is a key prerequisite for ultralow NOx emissions. EV‐burners have dual‐fuel capability, ie, gaseous fuel (DLN technology) and liquid fuel (with water injection).193

Vortex breakdown, which is one of the main charac- teristics of EV‐burners, is an abrupt and drastic change in the structure of strongly swirling flows. Some studies reported that the onset of vortex breakdown is indepen- dent of the flow Reynolds number but depends mainly on the swirl number.196 Vortex breakdown occurs in two different forms, depending on the burner design, namely, the axisymmetric bubble‐like and the spiral‐ shaped forms. The axial flow decelerates along the vortex axis, and the velocity gradients within the vortex get weaker as flow recirculation is distributed over a large area within the combustor.197 This behavior results in perfect mixing of the reactants and higher flame stability, even at startup and under ultra‐low load conditions. The perfect mixing prevents the creation of high‐temperature spots within the flame, and, consequently, NOx emissions can be kept at the minimum level. The flow core acceler- ates downstream, preventing flame flashback. Stable operation can thus be achieved under very lean condi- tions, resulting in complete combustion at controlled temperature as well as lower CO and NOx emissions.

Several features194 distinguish the EV‐burner design from other DLN premix hardware198:

1. Strong swirl is created merely by tangential air entry; no sophisticated swirlers are needed.

2. The distribution of air entry in the axial direction strongly accelerates the core flow, which creates a natural barrier against flashback.

3. An inner or central recirculation zone exists down- stream of vortex breakdown point, which effectively stabilizes the flame.

4. The vortex breakdown point is aerodynamically fixed in the free flow without the need for extra stabiliza- tion hardware (such as bluff‐bodies, ramps, and dump expansions) that could potentially be damaged due to flame impingement.

5. No pilot (nonpremixed) fuel injection is needed to assist the stabilization of premixed flame at medium to high loads; the burner can operate in 100% premixed mode, which allows for achieving ultralow NOx emissions in the absence of diffusion combustion.

FIGURE 11 EV‐burner principle194

[Colour figure can be viewed at

wileyonlinelibrary.com]

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At low loads, where the burner equivalence ratio is below the premix extinction limit, a separate pilot circuit injects fuel through a lance at burner centerline. The core flow becomes fuel‐rich, and the resulting flame is again stabilized at the vortex breakdown point. Similarly, if the burner is operated with liquid fuel, it is injected through a central plain jet nozzle in the cone head. The jet disintegrates into small droplets that disperse over the entire flow field inside the burner cone. The resulting flame is stabilized with the help of inner recirculation zone.

The need to enhance the evaporation and mixing of liquid fuel for further reduction of the associated NOx emissions, led to the development of the Advanced EV (AEV) burner199-202; see Figure 13. Four inlet air slots are used instead of two, which reduces the tangential var- iation of radial velocity component and thus prevents the impingement of fuel droplets on cone walls. An addi- tional mixing tube is affixed to the burner, in order to accommodate full evaporation of liquid fuel and boost mixing in gaseous phase. The axial and tangential veloc- ity profiles inside conical section (Figure 14) show a jet‐ like core flow with a distinct small body vortex. At cone

FIGURE 13 Design features of AEV burner194

FIGURE 14 Axial (top) and tangential (bottom) velocity profiles in AEV

burner194

exit, the peak axial velocity at centerline exceeds twice its average.194 This high axial velocity of core flow persists throughout the mixing tube, thus increasing the margin to flashback. Even the lowest velocity near the walls of mixing tube can be increased by proper admission of a film of air (less than 10% of total airflow). This film signif- icantly reduces the flashback risk at the wall and dilutes any local fuel‐rich residues.

The EV‐burner was first developed and implemented by the former Alstom Company in its GT11 gas turbine, which hosted 36 burners plus a central ignition burner in a hexagonal three‐ring arrangement.203 All burners were fired in premixed mode only because that initial design had not been equipped with central pilot injection yet. Overall combustor load and turndown were achieved by switching individual burners on and off. The same hexagonal arrangement was also used in the Alstom GT8, GT9, GT13, and GT11N2 gas turbines.204 The afore- mentioned EV‐burner design with pilot‐fuel injection was first implemented in the annular combustors of the Alstom GT13E2 and GT10 gas turbines, and later in the GT8C2, GT24, and GT26 engines.11 Today, all Alstom gas turbines have low‐NOx combustion systems based

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on EV and AEV‐burners. Although Alstom has been recently acquired by General Electric, the latter believes in the strong potential of EV‐burner technology and invests in further developing it.

Multiple studies highlighted the wide flexibility of EV‐ burners. Paschereit et al205 examined the effect of extend- ing the central fuel lance closer to the vortex breakdown point. Such simple design modification allowed for fixing the axial location of breakdown point over wide ranges of operating conditions and for better control of the combus- tion instabilities. Even without central fuel injection, extending the central lance changed the burner aerody- namics and the resulting flame. Flow visualizations were performed to quantify the effect of extended lance on combustion instabilities (pressure and heat release oscil- lations) and on emissions of NOx, CO, and unburned hydrocarbons. The authors pursued the optimization of axial lance position and amount of central fuel at differ- ent burner equivalence ratios and output powers.

Zajadatz et al195 investigated controlled splits of gas- eous fuel between the main premix circuit and the central pilot lance to create a staged premixed burner that gener- ates even lower NOx emissions at lower part loads com- pared with the traditional operation of EV‐burner. This smart fuel staging required no major design changes to the burner or its fuel‐supply system, which makes the concept applicable to most existing operational EV gas turbines. In a similar study, Cho et al206 examined numerically the effects of fuel staging and fuel‐air unmixedness on NOx emissions. The authors recom- mended fuel splitting between the premix circuit and the central pilot lance, in order to reduce unmixedness and the resulting NOx. Another design enhancement was also proposed to boost fuel‐air mixing, namely, by increasing the interval between fuel holes in the premix circuit to give fuel more time to mix with air.

Güthe et al207 also examined fuel staging by comparing three modes of burner operation, namely, perfectly premixed (fuel premixed with air in the air supply line upstream of the burner), standard (fuel injected through the premix circuit of the burner), and staged (fuel split between the premix circuit and the central pilot lance). The first mode was taken as a reference case that is free of any spatial variation in equivalence ratio, in order to decouple the effects of unmixedness and fluctuating turbu- lence intensity on combustion instabilities. The third mode of operation utilized the inherent flexibility of EV‐burners to optimize the fuel split for controlling both instabilities and emissions. It was reported that allowing the flame anchoring point (vortex breakdown) to move further inside the burner cone increases the pressure drop across the burner and deteriorates the control of combustion instabilities. The staged mode was thus optimized to

control the flame position, fluctuations, and emissions. Biagioli et al208 confirmed that one of the main drivers of instabilities in EV‐burners is the unsteady displacement in flame anchoring point, which induces unsteady varia- tion in flame surface and the corresponding heat release. The authors highlighted two aspects that make EV‐burners more challenging than other technologies, namely, varia- tions in turbulent burning rate in such strongly swirled flows and the jump in sensible enthalpy across the flame. Both aspects were analyzed to prove a strong correlation between the tendency of the flame to be displaced and the measured amplitude of pressure pulsations. Döbbeling et al209 implemented closed‐loop‐controlled adaptive fuel staging that monitors the pulsations in flame anchoring point and adjusts the fuel split between the premix circuit and the central pilot lance to maintain flame stability. NOx was also observed to improve with the implementa- tion of this closed‐loop control.

Since the EV‐burner enjoys efficient flame‐ stabilization (based on vortex breakdown), its use in a combustor extends the operational flexibility of high loads to lower loads and colder flame temperatures, which enhances engine turndown. Magni et al210

discussed an upgrade that can be retrofitted to existing AEV‐based GT13E2 engines to extend the minimum environmental part load from 60% down to 30%, which allows for full operational flexibility with high efficiency within the 30% to 100% load range. By closing the inlet guide vanes (to reduce airflow inside combustor) and redistributing the fuel to fewer lit burners, the flame tem- perature can be maintained at reasonably high levels, which significantly reduces CO emissions. Reiss et al211

fueled the GT13E2 engine successfully with syngas (45% H2, 48% CO, and 7% CO2) of a lower heating value of 13.9 MJ/kg. To overcome the problems of high burning velocity and large fuel volume flux, minor design changes were implemented by concentrating the fuel injection near the burner end. Fuel dilution by nitrogen was also examined down to 6.8 MJ/kg. The recorded emissions were 20 to 25 ppm NOx and less than 5 ppm CO indepen- dent of load, which indicates remarkable flexibility. Wind et al212 tested fueling the Alstom GT26 engine with natu- ral gas containing variable amounts of hydrogen and higher hydrocarbons (C2+), targeting wider fuel flexibil- ity. The authors examined the primary EV‐burners as well as the Sequential ones (SEV‐burners) of the reheat combustor. Hydrogen and higher hydrocarbons not only affect the burning velocity but also the auto‐ignition delay time, which is a critical design parameter of the SEV‐ burners. The standard operating procedure of the engine had to be only slightly modified to accommodate those variations in fuel composition without any hardware modifications.

7808 NEMITALLAH ET AL.

Based on this condensed review, EV‐burners offer supreme flexibility, stability, and turndown, which makes them a promising option for oxy‐fuel combustors to over- come the deteriorated performance associated with CO2‐ diluted oxy‐combustion (as compared with traditional air‐fuel combustion). Since EV‐burners have never been examined under oxy‐combustion conditions before, extensive investigation is needed to quantify the applica- bility of this technology in zero‐emission gas turbines.

5.4 | MM combustion technology

MM burners are a successful emerging LPM combustion technology that is already implemented on the utility scale (eg, General Electric's F‐class gas turbines).213-215

The MM technology is based on the concept of mixing of fuel and oxidizer at the microscale. A MM consists of a two‐dimensional array of straight, parallel, equally‐ spaced tubes (each of a diameter of few millimeters) welded to two faceplates; one forms the inlet section of the burner, while the other forms the exit section. A fuel gas plenum is thus created around the tubes between the plates. The incoming fresh oxidizer stream is distributed among these tubes, and fuel is supplied from the gas ple- num and injected radially (cross‐flow) through side holes (each less than 1 mm in diameter) in the tube walls. The fuel holes are located axially near the inlet faceplate; a premixing region is created inside each tube, and the length of the tube is calculated to guarantee a fully‐ developed and fully‐premixed flow at the exit section of each tube. Fuel is injected at the same axial location from different holes distributed equally along the circumfer- ence of the tube, in order to minimize the length required for generating fully premixed flow. The faceplate geome- try, in terms of diameter and number of tubes, is opti- mized to minimize the emissions and widen the operability limits. The operating equivalence ratio and the pressure drop within the tubes are controlled to pre- vent flashback.

Fluid mixing plays a major role in combustion and the resulting emissions. As the global trend is shifting towards microsystem technology development, this vital fluid‐mixing process is also in search of mixing the fuel and oxidizer at the micro level. Mixing the reactants on the macroscopic scale only gives rise to turbulent mix- tures, whereas flows at the micro level are streamlined and characteristically laminar. The molecular diffusion process is also involved under certain standardized condi- tions, though such a mixing strategy is generally quite inefficient due to very low molecular diffusivities. For preventing this inefficiency and increasing the rate of fluid mixing, distinctive devices like MMs are used.216

The mixing rate can be increased by any of the active or passive mixing methods.217-219 The active ones include the use of numerous external fields like electric, mag- netic, acoustic, etc. for changing the fluid flow rate. Pas- sive approaches, on the other hand, involve changes in the tube geometry by using different types of inserts. Con- sideration of the fluid stream structure and mixing mech- anism are required for the optimization of the mixing process. Therefore, mathematical modeling plays a major role in the investigation of these flows. The design of MMs has been broadly examined with the aid of compu- tational fluid dynamics (see previous studies220-227). Conlisk219 has elaborated on the general working princi- ples of different MMs. Research was mostly conducted at constant bulk jet velocity of the reactant mixture, as this represents real operating conditions of LPM DLE combustors. These units are normally operated at fixed pressure drop across the headend, resulting in nearly con- stant inlet velocity of the combustible mixture. This assumption is applicable if flow compressibility effects are neglected, which is a common exercise among gas turbine manufacturers.

MMs are reviewed in this section, starting from basic PP burners up to the development of MM technology.

5.4.1 | PP burners

PP burners are widely used in household and industrial applications such as boilers and furnaces. PP burners are the precursor of MMs. A PP burner comprises a sim- ple PP that receives a premixed or partially premixed flow of fuel and oxidizer on one side and distributes this flow among its array of holes to exit as jets on the other side. Conical‐shaped flames are formed downstream of the PP. These flames are susceptible to various instabilities like flashback, quenching, and blow off. Therefore, it is very important to study the stabilization mechanism of such flames. Kedia and Ghoniem228 studied laminar, premixed, methane‐air flames stabilized on a heat‐ conducting PP. They investigated the conditions of flame stability, which led to blow off. Unsteady, 2D simulations were performed with thorough chemical kinetics for methane‐air species combustion. The joint impact of heat transfer and flame stretch is credited for flame blow off principle. Jithin et al229 numerically studied the charac- teristics of premixed propane‐air flames stabilized over a PP using 3D simulations. They reported that increasing the inlet velocity increased the flame stand‐off distance. For adiabatic plates, this distance was zero, ie, the flame rested on the plate. However, by increasing the plate ther- mal conductivity, the stand‐off distance increased again. Higher equivalence ratios made the flames move closer

7809NEMITALLAH ET AL.

to the plate, which increased the heat flux transferred to the plate. Similar findings were reported by Altay et al,230 who investigated the stability characteristics of PP‐stabilized methane‐air flames under various operating conditions using a 2D numerical model. Conductive plates were considered with thermal conductivities up to 10 W/m·K only. Jamal et al,231 however, considered con- ductivity values as high as 385 W/m·K and proved that if considerable heat is transferred through the plate to pre- heat the incoming combustible mixture, the flames become shorter and more stable. The authors examined the effects of plate material, thickness, and hole diameter and showed that heat transfer through the plate is the hidden primary control parameter behind all these effects. Kedia and Ghoniem232 numerically studied the blow‐off conditions and flame stabilization of PP‐ stabilized flames for different inlet velocities. They reported that the dynamic stability of the flame is highly dependent on the heat transfer from the flame to burner plate; this transfer was, however, considered as heat loss because the thermal conductivity was limited to 1.5 W/m·K only. Rashwan et al93 experimentally studied par- tially premixed oxy‐fuel flames anchored over a PP burner. At fixed equivalence ratio, stable flames were attainable only over the oxygen fraction range of 29% to 42% by volume in the O2/CO2 oxidizer. The premixing ratio of fuel/air mixture was also found to affect flame stabilization. Edacheri Veetil et al233 numerically studied the effect of hole geometry in PP burner on flame struc- ture under rich and lean conditions. They reported that, with reduction in hole spacing, the curvature of flame base increases and the flame thickness decreases. Hindasageri et al234 experimentally and numerically stud- ied the heat transfer distribution of impinging flame from PP burner for three different configurations (inline, stag- gered, and star) over range of Reynolds number (50 to 600) and at three different pitch to diameter (p/d) ratios (1.67, 2.33, 3.33) but 7 mm pitch is considered optimal for the entire study. Oh, Shin, and Kim235 reported that the oscillations induced by combustion in a PP burner can be well stabilized by decreasing the hole diameter or by reducing the plate thickness. Wang and Wen236

numerically studied laminar hydrogen flames in a PP installed channel and observed M‐shaped flames. They found that the overall effect of PP was that flame speed decreases downstream of it. Li et al237 experimentally studied flame stabilization on a PP burner of a square cross‐sectional channel and reported four phases of flame front, ie, laminar, jet, turbulent, and secondary flame front. The flame shapes for three hole configurations of p/d = 2.33 with varying Reynolds number are illustrated in Figure 15. Oh et al235 performed a numerical study on combustion instability in PP burners. The results showed

that plates with smaller holes are better for stabilizing the oscillations caused by combustion.

5.4.2 | MM combustion characteristics

The main features of the MM technology are staging capability, scalability, and fuel flexibility. York et al238

conducted a full‐can durability test of MM nozzles at the combustion conditions of General Electric's F‐class gas‐turbine at full load conditions considering air com- bustion of fuel blends of hydrogen, nitrogen, and natural gas. Using a fuel blend of hydrogen and nitrogen (hydro- gen is more than 90% by volume) and after 100 hours of firing, NOx emissions were at the level of single‐digit ppm. Funke et al239 performed a combined experimental and numerical investigation on the effect of momentum‐ flux ratio on NOx emissions and flame stabilization mech- anism in a MM nozzle considering air combustion of hydrogen. The results showed significant effect of flame stabilization mechanism on NOx emissions. Dodo et al240 tested MM air‐combustion of IGCC‐syngas fuel simulants containing hydrogen, methane, and nitrogen with a hydrogen content of 40% to 65%. Three fuel blends were considered with 0%, 30%, and 50% carbon‐capture rate. Stable combustion was recorded for all fuel blends with NOx emissions at the single‐digit ppm level. Later, the same group240 evaluated the Mitsubishi Hitachi Power Systems MM performance in an IGCC plant with focus on the effect of flame shape; slenderer flames were reported to show superior performance in terms of NOx emissions and combustor component temperatures.

Flashback, and consequent hardware failure, is the primary challenge associated with hydrogen combustion because of very high flame speeds. Lean Direct Injection (LDI) was found to drastically reduce the risk of flash- back in H2 combustors.

241 LDI designs are based on achieving rapid mixing through lean fuel injection from numerous locations within the combustion zone. Miniature‐scale distributed premixer arrays, aka MMs, were developed and tested in several studies, in order to reduce NOx emissions safely with H2‐rich fuel. The LDI approach was mimicked in these MMs but with the addi- tion of a small‐scale premixing region. If carefully designed, MMs could approach the LDI operability char- acteristics. Some of the examined configurations were jet‐ in‐crossflow mixing,242 co‐flow mixing,243 and swirl‐ induced mixing in miniature cups with axial and radial inflow.244

The combustion of high‐H2 fuels using air in MMs has been examined in past studies,238-240 but never under oxy‐ combustion conditions. Hernandez et al245 indicated vast differences in the characteristics of H2/air flames vs.

FIGURE 15 Flame shapes corresponding to three hole configurations for different Reynolds numbers at a pitch to diameter (p/d) ratio of 2.33234 [Colour figure can be viewed at wileyonlinelibrary.com]

7810 NEMITALLAH ET AL.

natural gas/air ones in MMs. The same group243 later suc- cessfully developed and tested a flashback‐free MM fueled by a variety of fuels ranging from 100% H2 to 100% natural gas. Best operability was attained at a firing temperature below 1470 K using a fuel blend of 87% H2 and 13% natural gas. It was concluded that MMs are excellent candidates for future gas turbines fueled by hydrogen or a wide variety of syngas compositions. Zhang et al246,247 proved the effectiveness of MM combustion technology in reducing NOx in a syngas‐fueled combus- tor. They combined CO2 dilution with MM combustion to reduce NOx and lower the faceplate temperature at MM exit. It was reported that low CO and NOx emissions can be obtained simultaneously, which is typically a chal- lenge in most swirl‐based dry‐low‐emissions (DLE) com- bustors. By adjusting the parameters of operation, the researchers managed to achieve 2 ppm NOx15 and 3

ppm CO15. The use of CO2 dilution was reported to lower NOx at the expense of CO emissions. The effect of syngas composition was analyzed through the corresponding C/H ratio. Lower ratios showed promising results in terms of lower faceplate temperature and lower NOx and CO emissions.

Based on the above discussion of the different burner designs and combustion techniques for clean combustion in gas turbines, Table 2 summarizes the main features of the different technologies in terms of their advantages and the disadvantages.

6 | HIGH‐TEMPERATURE MEM- BRANE REACTORS

Retrofitting an existing power plant to work under oxy‐ combustion conditions requires the integration of an

TABLE 2 Advantages and disadvantages of the different burners for clean combustion in gas turbines

Burner Design Advantages Disadvantages

Stagnation point reverse flow (SPRF) burners

• Small size. • Promotes internal gas recirculation. • Excellent flame stability. • The reactants are diluted with part of the burnt gases laden with radicals, which lowers the ignition temperature, resulting in lower NOx and CO emissions as well as enhanced blowout limit

• Suitable for flameless combustion.

• Lower combustion efficiency. • Lower flame burning velocity.

Dry Low‐NOx (DLN) and Dry Low‐Emission (DLE) burners

• Acceptable NOx and CO emissions. • Achieve flame stability using swirlers. • Less compact. • Flame instabilities are encountered under low load condition

Environmental burners (EV) (including AEV, and SEV)

• Superior flame stability over wide range of operations based on vortex breakdown.

• Achieve ultra‐low NOx emissions. • Dual fuel capability (gaseous and liquid). • No sophisticated swirlers are needed (strong swirl is created merely by tangential air entry).

• Accelerated core flow that creates a natural barrier against flashback.

• No need for extra stabilization hardware. • No pilot (nonpremixed) fuel injection is needed to assist the stabilization of premixed flame.

• Enhances engine turndown time. • Excellent candidate for oxy‐fuel combustors.

• Imperfect fuel‐air mixing in fuel staging.

• Unsteady variation in flame surface. • Variations in turbulent burning rate.

Perforated plate (PP) burners

• Suitable for household and industrial applications. • Flame symmetry.

• Susceptible to various flame instabilities like flashback, quenching, blow off, etc.

Micromixer (MM) burners • Staging flexibility. • Fuel flexibility. • Can achieve single digit NOx emission level. • Scalability. • Excellent candidate for oxy‐fuel combustors.

• Combustion dynamics near stoichiometric condition.

• Still under the development phase for gas turbine applications.

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ASU with the conventional combustion system, where the ASU is used to produce the required amount of oxy- gen for combustion. At the present state, ASU technology based on cryogenic distillation can meet the flow and purity requirements of large‐scale boilers. In this unit, air is compressed, cooled, and cleaned before introducing it to the distillation column to make the separation pro- cess. Air is separated inside the ASU into a nitrogen‐rich stream and an oxygen‐rich one.248 Cryogenic air separa- tion consumes about 0.24 kWh/kg‐O2 at a purity of 95%,249,250 which corresponds to more than 15% of the total plant output power.251-253

In HTMRs, however, both oxygen separation and oxy‐ fuel combustion processes occur within the same unit. If successful, this should allow for the replacement of con- ventional combustors by HTMRs. In such membrane reactors, as shown in Figure 16, air is passed on the feed side of the membrane, and a mixture of fuel plus

recirculated CO2 is passed on the permeate side. The membranes are selective to oxygen and separate the feed- ing oxygen to the permeate side, where it reacts with the fuel within the same unit. The oxygen permeation flux depends mainly on the membrane temperature and the difference in oxygen partial pressure on both sides of the membrane. Part of the heat of combustion is used to heat‐up the membrane (to temperatures above 650°C) to be activated for oxygen separation. Numerical investiga- tions of the HTMR concept are abundant,72,73,255-257 but experimental studies are very rare.254

Tan et al258 fabricated hollow fiber membrane reactors (HFMRs) based on LSCF‐6428 crystals to examine the catalytic combustion of methane. A simple mathematical model was developed that combines the local oxygen per- meation rate with approximate catalytic reaction kinetics and can be used to predict the reactor performance for methane combustion. A mixture of CH4 and He was used

FIGURE 16 Schematic representation of a stagnation flow high temperature membrane reactor254 [Colour figure can be viewed at

wileyonlinelibrary.com]

7812 NEMITALLAH ET AL.

as sweep gas. The effects of operating temperature and methane and feed‐air flow rates were studied both exper- imentally and theoretically. It was concluded that coating platinum on the feed side can improve both methane conversion and oxygen permeation. Wei et al259 success- fully investigated CO2‐diluted methane oxy‐combustion in a U‐shaped hollow fiber membrane; a study that is reviewed here in detail due to the importance of its findings. The membrane was based on (Pr0.9La0.1)2(Ni0.74Cu0.21Ga0.05)O4‐δ (PLNCG) with an inner diameter of 0.5 mm. Complete combustion was achieved at 975°C for 450 hours. The membrane was swept with a mixture of CH4 and CO2. For an air flow rate of 150 mL/min on the feed side and a CH4 + CO2 flow rate of 5.34 mL/min on the sweep side with 10% CH4 (≡ 0.534 mL/min CH4), the associated oxygen per- meation flux was 0.7 mL min−1 cm−2. No details on the equivalence ratio were reported, but if at least stoichio- metric combustion is assumed, the flow rate of permeated oxygen would have to be greater than or equal to 2 × 0.534 = 1.068 mL/min, from which one can deduce that the inner surface area of hollow membrane was at least 1.068/0.7 = 1.53 cm2; this area was not reported. The cor- responding minimum membrane length would thus have to be 1.53/(π × 0.05) = 9.71 cm.

The results of Wei et al259 showed excellent reaction performance and good stability under oxy‐fuel reaction conditions. Partial and total oxidation of methane took place simultaneously at the U‐shaped PLNCG membrane. The operation showed that complete combustion can only occur at temperatures higher than 975°C; see Figure 17A. Partial oxidation of methane commenced well below 875°C. Between 900 and 925°C, the oxygen flux increased gradually with temperature, which increased the CO concentration due to partial oxidation. Between 925 and 950°C, total oxidation commenced as the temperature and oxygen flux increased, which kept the CO concentration fairly constant (independent of temperature), because a balance was reached between CO generation (partial oxidation) and CO consumption (total oxidation). Beyond 950°C, the total‐oxidation reac- tions gradually dominated as the temperature and oxygen flux further increased, until complete combustion was achieved at 975°C. Figure 17B explains why a flow rate of 5.34 mL/min was chosen for the CH4 + CO2 sweep gas. The oxygen permeation flux was found to increase with sweep‐gas flow rate up to 0.7 at 5.34 mL/min, beyond which no significant flux enhancement was observed if the sweep‐gas flow rate was further increased. Moreover, complete combustion (total oxidation) was sacrificed beyond 5.34 mL/min, which can be attributed to a Damkohler number below unity (chemical time scale exceeds flow residence time inside the membrane). Figure 17C explains the choice of CH4 concentration. A peak in oxygen flux exists at 7.6% CH4; for a CH4 + CO2 flow rate of 7.33 mL/min, the corresponding CH4 flow is 0.557 mL/min, which is almost the same optimum value of 0.534 mL/min used in the temperature analysis of Figure 17A. Complete combustion was not fully attained beyond 0.557 mL/min CH4, which proves that this oxy‐combustion process is extremely sensitive to CH4 flow rate. Figure 17D finally analyzes the effect of increasing the sweep‐gas flow rate well beyond the opti- mum value. The CH4 flow rate was limited to 0.733 mL/min to avoid rich combustion. While the oxygen per- meation flux increases to 1.5 mL min−1 cm−2 at 56 mL/min, the CH4 conversion (partial oxidation) was sacrificed substantially (drops down to 10%). Since com- bustion commences by partial oxidation, it can be con- cluded that 90% of CH4 exits the membrane as is, ie, without reaction.

Recently, many studies investigated the integration of oxygen separation membranes in power cycles for clean energy production.256,260-266 Mancini and Mitsos267 pro- posed a monolith‐structure design of an oxygen transport reactor for power production in the range of 300 to 500 MWe. Based on 3‐D modeling, Nemitallah et al256

designed an oxygen transport reactor for replacement of

FIGURE 17 A, Effect of temperature on CH4 conversion, CO2 selectivity, and oxygen permeation flux in the oxy‐fuel combustion of methane.259 Conditions: Fair = 150 mL/min, FCH4 + CO2 = 5.34 mL/min, CCH4 = 10%. B, Effect of sweep‐gas flow rate (low range) on CH4 conversion, CO2 selectivity, and oxygen permeation flux in the oxy‐fuel combustion of methane at 975°C.

259 Conditions: Fair = 150 mL/min,

CCH4 = 10%. C, Effect of CH4 concentration in the sweep gas on the CH4 conversion, CO2 selectivity, and oxygen permeation flux in the oxy‐

fuel combustion of methane at 975°C.259 Conditions: Fair = 150 mL/min, FCH4 + CO2 = 7.33 mL/min. D, Effect of sweep‐gas flow rate (high

range) on CH4 conversion, CO2 selectivity, and oxygen permeation flux in the oxy‐fuel combustion of methane at 975°C. 259 Conditions: Fair =

150 mL/min, FCH4 = 0.733 mL/min [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 18 Design of a two‐path fire tube boiler incorporating multi membrane reactors255 [Colour figure can be viewed at wileyonlinelibrary.com]

7813NEMITALLAH ET AL.

a gas turbine combustor to produce 5 MWe of power. Habib and Nemitallah255 designed multi‐separated mem- brane reactors for the application in fire‐tube boilers (see Figure 18) to produce about 5 MWe of power. The results are promising for future application of membrane reac- tors for power production; however, the size of the mem- brane reactor is much bigger than normal combustors for the same output power.

7 | CONCLUSIONS

In this extensive literature review, the most recent com- bustion technologies have been discussed, including lean premixed air‐fuel combustion, premixed oxy‐fuel com- bustion, fuel‐flexible combustion, and the integrated fuel‐flexible premixed oxy‐combustion. The latter has proven to facilitate effective control of both flame stability

7814 NEMITALLAH ET AL.

and emissions under the different loading conditions. Hydrogen‐enriched combustion has shown better com- bustion efficiency, wider flame stability limits, and better fuel economy. The state‐of‐the‐art burner designs for gas turbine applications have been addressed in detail in this study with their technology readiness level for the differ- ent applications. Different burner designs have been reviewed including SPRF, dry low‐NOx (DLN) and dry low‐emission (DLE), EnVironmental (EV, AEV, and SEV), PP, and MM burners. Reverse flow burners exhibit very low NOx emission with wider flammability limits; however, this technology is still in the research and devel- opment phase for application in gas turbines. The advanced EnVironmental (AEV) burner resulted in the widest flame stability limits with the lowest level of NOx emissions among the considered burner designs in this review. Alstom invented such burner technology and implemented it in several industrial gas turbines. The novel MM burner design seems to have also wide stability limits, especially near the blowout limit. MM technology is a well‐developed technology with a technology‐ readiness level of 9.0, and it is already implemented on the utility scale (eg, General Electric's F‐class gas tur- bines). The application of oxy‐fuel combustion technol- ogy is a very promising approach towards ZEPP application while capturing CO2. However, the cost of oxygen separation stands as the main barrier towards the wide application of this technology. Compact high temperature membrane reactors (HTMRs) seems to be a promising solution towards the reduction of oxygen‐ separation cost through the application of in‐situ oxy‐ combustion inside the membrane separation unit. How- ever, more development is required for the membrane material to overcome the low oxygen permeation flux and membrane contamination after short period of operation.

ACKNOWLEDGEMENTS

The authors like to appreciate the support from King Fahd University of Petroleum and Minerals (KFUPM) to perform this work through the deanship of research on project number IN181028. The support from the Research and Development Office (RDO) of the Ministry of Educa- tion (MOE) through the high‐impact research publication grant is highly appreciated.

ORCID

Medhat A. Nemitallah https://orcid.org/0000-0001-9075- 2844 Mohamed A. Habib https://orcid.org/0000-0003-3459- 1462

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How to cite this article: Nemitallah MA, Abdelhafez AA, Ali A, Mansir I, Habib MA. Frontiers in combustion techniques and burner designs for emissions control and CO2 capture: A review. Int J Energy Res. 2019;43:7790–7822. https://doi.org/10.1002/er.4730