Tim Lieuwen

There is significant interest in utilizing hydrogen as an energy carrier in a net-zero carbon energy economy. A variety of fundamental and practical issues associated with premixed hydrogen/methane combustion must be addressed, particularly flame flashback, combustion instabilities, and NO_x emissions. Significantly less attention has been given to CO emissions from hydrogen blending. However, recent H₂ blending engine tests have clearly noted major reductions in CO emissions, reductions which substantially exceed what would be expected based upon C atom mass balances in the fuel. Moreover, NO_x, CO, and combustor operability always tradeoff with each other, and so for example, these CO reductions can also enable operational or design modifications to reduce NO_x emissions and/or wider operability windows. This paper addresses H₂ blending effects on CO emissions, addressing both the direct influences on CO emissions, as well as how it influences NO_x-CO tradeoffs. The paper presents computations that differentiate between the kinetic and equilibrium effects of H₂ blending, as well as how H₂ blending can lead to reductions in both NO_x and CO by moving the Pareto front downward. Finally, this work suggests that H₂ blending, in addition to its CO2 reductions, can also increase plant operational flexibility and so provide additional value propositions to an evolving electricity market.

The turbulent burning velocity (S_T) is one of the most important combustion properties controlling combustor operability limits, directly influencing blowoff, flashback, and combustion instabilities. Hydrogen has particularly significant influences on the turbulent flame speed. This paper presents new H₂/CH₄ data of high pressure, high hydrogen turbulent burning velocities. The data sets were designed to address fundamental questions as well as provide engineering/design relevant insights. It presents new scaling analysis of fuel composition, pressure, and preheat temperature effects on turbulent burning velocity. We also discuss the importance of considering what is being held constant (temperature, flame speed, Reynolds number, etc.) when one is analyzing these sensitivities. Data show that hydrogen fraction and pressure causes an increase in turbulent flame speed; whether quantified as raw S_T,GC, or normalized as S_T,GC/S_L,0 or S_T,GC/S_L,max, (where S_L,0 and S_L,max are the unstretched and stretched laminar flame speed respectively). We also propose that observed increases in S_T,GC with pressure are due to increases in Reynolds number, and not a kinetics/stretch sensitivity effect. With increasing preheat temperature, S_T,GC increases while its normalized value (S_T,GC/S_L,0 and S_T,GC/S_L,max) can either increase or decrease, depending upon fuel composition. We also show how these sensitivities vary, depending on whether these comparisons are made at constant raw turbulence intensity urms, or at constant normalized urms/S_L,0 or u_rms/S_L,max.

Lean, premixed combustor designs have almost completely replaced nonpremixed combustors for industrial gas turbine applications where NOx emissions are regulated. Nonetheless, these designs have also introduced turndown and fuel flexibility constraints and made combustion instabilities and flashback more problematic. Future gas turbine applications will require combustors to accommodate a range of alternative fuels, provide operational flexibility, and compete for services with a host of new technologies, including energy storage and fuel cells. The purpose of this paper is to propose nonpremixed, multistage designs for the next generation of high turndown, high fuel flexibility, low NOx combustion designs—referred to here as a Nonpremixed, Rich, Relaxation, Lean (NRRL) combustor. The key concept, we explore is nonpremixed combustion, followed by additional fuel mixing to locally fuel-rich conditions, a relaxation stage, and then a lean stage. This nonpremixed approach can handle essentially any fuel composition, including pure hydrogen, liquid fuels, pure methane, pure ammonia, and any combination in between while breaking the NOx-CO tradeoff and reducing combustion instability risk. This paper provides chemical reactor network modeling calculations to identify key kinetic processes and time scales required for such a concept. This concept has completely inverted sensitivities from lean, premixed systems which prefer short residence times, low pressures, and low temperatures to minimize NO formation. This concept prefers long residence times, high pressures, and high temperatures, indicating a very different set of design trades for part load and off-design performance.

Ammonia (NH3) is currently being actively investigated as a carbon-free alternative fuel, however, the fuel-bound nitrogen in NH3 causes significant increase in NO_x emissions during ammonia combustion. Different from conventional combustors designed for hydrocarbon fuels, it is still unclear what strategy and structure to use for ammonia-based combustor design therefore combustor suitable for ammonia combustion still does not exist. A useful benchmark and guideline to have when designing ammonia-based combustors is the theoretical minimum NO_x emissions which could be achieved under ideal conditions for ammonia combustion. In this work, we will first evaluate the performance of different kinetic models in literature and compare the results from simulation with existing experimental results. Then one kinetic model will be selected for further calculation. We will then employ reduced order modeling (chemical reaction network) to conduct simulation to model different conceptual combustors. In this way, we can find the theoretical minimum NO_x emissions for NH3 combustion with constraints on unburnt NH3 in the exhaust gas. Different combustion strategies, such as fuel-staging, will be evaluated under ideal conditions for NO_x emission. The simulation results will provide theoretical guidance on ammonia combustor design with minimized NO_x emission.

To limit climate change and promote energy security, there is widespread interest toward transitioning existing fossil fueled combustion systems to sustainable, alternative fuels such as hydrogen (H2) and ammonia (NH3) without negatively impacting air quality. However, quantifying the emission rate of air pollutants such as nitrogen oxides (NOx) is a nuanced process when comparing pollutant emissions across different fuels, as discussed in our paper GT2022-80971 presented last year. That study indicated that the standardized approach for measuring combustion emissions in terms of dry, oxygen-referenced volumetric concentrations (i.e., dry ppmv at the reference O2 concentration (ppmvdr)) inflates reported pollutant emissions by up to 40% for hydrogen combustion relative to natural gas. In this paper, we extend our prior analysis of these so-called “indirect effects” on emissions values to ammonia (NH3) and cracked ammonia (i.e., molecular hydrogen and nitrogen, 3H2 per N2) fuel blends. The results reveal that ppmvdr-based pollutant reporting approaches have a less prominent influence on emissions interpretations for molecular ammonia–methane blends than for hydrogen–methane blends. Nonetheless, we still find that ppmvdr reporting induces up to a 10% relative increase in apparent emissions when comparing 100% NH3 and 100% methane (CH4) fuels at an equal mass-per-work emission rate. Cracking the ammonia is shown to increase this relative bias up to 21% in comparison to a methane system. Further analysis shows how drying, dilution, thermodynamic, and performance effects each influence the relationship between ppmvdr and mass-per-work emissions across the spectrum of fuels and fuel blends. Following discussion of these findings, we conclude that quantifying combustion emissions using ppmvdr is generally inappropriate for emissions comparisons and advise the combustion community to shift toward robust mass-per-energy metrics when quantifying pollutant emissions.

In addition to their lifecycle carbon emissions, another important issue with decarbonized energy pathways is their air quality, water, or land use implications. This paper considers the air quality issue for ammonia combustion. When directly combusting ammonia, reactions of its N atom with atmospheric oxygen lead to NO_x emissions that are O(10³) ppm, 2 orders of magnitude higher than EPA limits or the amount emitted by current natural-gas-fired technologies. In order to provide guidance to policymakers and technologists on what is fundamentally possible, this Perspective analyzes the fundamental minimum NO_x emissions that can be produced from ammonia combustion. The analysis shows that it is possible to achieve quite low NO_x emission levels of O(10) ppm, but these designs differ markedly from those used in today's lean, premixed combustion systems.