By Chris Douglas1, Rob Steele2, Tom Martz2, Bobby Noble2, Ben Emerson3, and Tim Lieuwen3,4
1École Polytechnique, 2EPRI, 3Georgia Institute of Technology, 4Strategic Energy Institute

Hydrogen-fueled gas turbines are coming. Adverse climate impacts from carbon dioxide and other greenhouse gasses have spurred substantial momentum from a variety of policy and industry groups to leverage hydrogen as a renewable energy carrier. Hydrogen fuel can be sustainably generated using excess renewable power and optionally further processed into other compounds such as ammonia (NH3).

Like fossil fuels, such “green” fuels are chemically stable and have high energy densities that allow for efficient transportation, storage and on-demand use. Yet, unlike fossil fuels, the chemical energy from hydrogen-based fuels can be released by combustion without contributing to CO2 emissions.

Nonetheless, combustion generally produces other pollutants besides CO2 that impact environmental and human health. Important examples include carbon monoxide (CO), particular matter (PMx), and nitrogen oxides (NOx), whose emissions are all tightly regulated. While neither hydrogen nor ammonia contain the carbon atoms necessary to generate CO or PMx emissions, they still generate NOx emissions when burned via reactions of nitrogen (N2) and oxygen (O2) from ambient air. In ammonia-burning systems, fuel-bound nitrogen atoms are also partially converted to NOx during combustion. This means that minimizing NOx emissions will remain a concern in the coming era of H2– and NH3-based combustion systems.

Lately, there has been a significant buzz in the energy community after some studies reported higher NOx emissions rates from hydrogen and ammonia combustion systems based on comparisons of adjusted NOx concentrations in the exhaust gasses, reported in dry parts per million (ppm) at 15% O2. In the combustion field, this concentration adjustment procedure is a standardized process that involves drying an exhaust gas sample (i.e., removing its water/steam) and diluting it to a reference O2 concentration.

Historically, this dry-and-dilute adjustment technique was used to remove equivalence and bypass ratio dependencies on pollutant concentrations measured in different systems. However, as detailed in our recent series of papers [1-3], such concentration adjustments turn out to introduce a subtle but decisive detail when comparing combustion emissions from conventional and alternative fuels.

While such comparisons of NOx emissions rates remain an active area of research, it is now clear that several of these earlier reports have misinterpreted their NOx emissions in a manner that significantly overestimates the true emissions of high-hydrogen fuel blends. Our primary aim in this article is to illustrate the reason for such discrepancies, as well as to explain how to avoid such pitfalls – namely, to abandon concentration-based emissions reporting and instead utilize mass-based metrics (i.e., pollutant mass per unit of plant output, quantified as ng/J or lbs/MMBTU).

When comparing combustion emissions from different fuels, two crucial influences must be considered.

  1. First, variations in the actual NOx production rate of systems firing different fuels directly contributes to differences in adjusted NOx concentration values. Such direct effects are not the focus here, but follow from fuel-specific changes in NOx formation pathways and from differences in combustor designs and operating conditions. It is precisely the direct effects of fuels on NOx emissions that environmental and air quality communities seek to understand. Readers are encouraged to see recent work on this topic from our group here [4].
  2. Second, variations in the combustion characteristics of systems burning different fuels can induce apparent differences in adjusted NOx emissions values independently from true NOx production rates. These indirect effects primarily reflect how the standardized emissions measurements and adjustments depend on the fuel composition and operating parameters. Having no relation to actual pollutant production rates, indirect effects act only to confound interpretations of NOx emissions.

To better illustrate how plants with the same mass production of NOx lead to different concentration (ppm) based measurements when hydrogen is substituted for natural gas, example calculations were performed for a range of different fuels at representative F-class gas turbine conditions. Based on these calculations, the individual and combined relative effects of sample drying and dilution compared to the case of 100% methane fuel are shown in Figure 1.

The data in Figure 1 indicate that, for hydrocarbon and ammonia fuels, these drying and dilution effects have a tendency to counterbalance each other, resulting in only minor indirect effects on emissions adjustments. However, for hydrogen fuel, drying and dilution have a compounding effect that results in substantial indirect emissions adjustments. More precisely, these influences manifest almost a 40% apparent increase in the adjusted NOx concentration for hydrogen fuel relative to methane fuel even though the actual emission rate in this analysis is identical for all fuels.

Thus, interpreting adjusted NOx concentration measurements (dry NOx concentration at a reference O2 concentration) as a surrogate for actual NOx production rates (emitted NOx mass per unit of work output) introduces a significant, but entirely artificial, penalty for high-hydrogen fuel blends in comparison to conventional hydrocarbon fuels. To avoid such confusion, we recommend following the example of the vehicle and aviation transport sectors, and compare the emitted NOx mass per unit of energy instead of adjusted NOx concentrations.

Figure 1: Emissions adjustments for drying, dilution, and drying × dilution for various fuels relative to the same adjustments for methane
at F-class gas turbine combustion conditions.

References:

  1. Douglas, C. M., Emerson, B. L., Lieuwen, T. C., Martz, T., Steele, R., and Noble, B., 2022, “NOx Emissions From Hydrogen-Methane Fuel Blends,” Georgia Institute of Technology, Strategic Energy Institute, Atlanta, GA, Report. https://doi.org/10.35090/gatech/65963
  2. Douglas, C. M., Shaw, S. L., Martz, T. D., Steele, R. C., Noble, D. R., Emerson, B. L., and Lieuwen, T. C. (July 28, 2022). “Pollutant Emissions Reporting and Performance Considerations for Hydrogen–Hydrocarbon Fuels in Gas Turbines.” ASME. J. Eng. Gas Turbines Power. September 2022; 144(9): 091003. https://doi.org/10.1115/1.4054949
  3. Douglas, C. M., Emerson, B. L., Lieuwen, T. C., Martz, T., Steele, R., and Noble, B., 2022, “Pollutant Emissions Reporting for Ammonia Fuel Blends,” Georgia Institute of Technology, Strategic Energy Institute, Atlanta, GA, Report. https://doi.org/10.35090/gatech/67542
  4. Breer, B., Rajagopalan, H., Godbold, C., Johnson, H., II, Emerson, B., Acharya, V., Sun, W., Lieuwen, T., and Noble, D., 2022, “NOx Production From Hydrogen Methane Blends,” In Spring Technical Meeting of the Eastern States Section of the Combustion Institute, Orlando, FL, Mar. 6–9, Paper No. 149RKF-0019.

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