NO_x Behavior for Lean-Premixed Combustion of Alternative Gaseous Fuels

Gaseous fuels other than pipeline natural gas are of interest in high-intensity premixed combustors (e.g., lean-premixed gas turbine combustors) as a means of broadening the range of potential fuel resources and increasing the utilization of alternative fuel gases. An area of key interest is the change in emissions that accompanies the replacement of a fuel. The work reported here is an experimental and modeling effort aimed at determining the changes in NO_x emission that accompany the use of alternative fuels. Controlling oxides of nitrogen (NO_x) from combustion sources is essential in nonattainment areas. Lean-premixed combustion eliminates most of the thermal NO_x emission but is still subject to small, although significant amounts of NO_x formed by the complexities of free radical chemistry in the turbulent flames of most combustion systems. Understanding these small amounts of NO_x, and how their formation is altered by fuel composition, is the objective of this paper. We explore how NO_x is formed in high-intensity, lean-premixed flames of alternative gaseous fuels. This is based on laboratory experiments and interpretation by chemical reactor modeling. Methane is used as the reference fuel. Combustion temperature is maintained the same for all fuels so that the effect of fuel composition on NO_x can be studied without the complicating influence of changing temperature. Also the combustion reactor residence time is maintained nearly constant. When methane containing nitrogen and carbon dioxide (e.g., landfill gas) is burned, NO_x increases because the fuel/air ratio is enriched to maintain combustion temperature. When fuels of increasing C/H ratio are burned leading to higher levels of carbon monoxide (CO) in the flame, or when the fuel contains CO, the free radicals made as the CO oxidizes cause the NO_x to increase. In these cases, the change from high-methane natural gas to alternative gaseous fuel causes the NO_x to increase. However, when hydrogen is added to the methane, the NO_x may increase or decrease, depending on the combustor wall heat loss. In our work, in which combustor wall heat loss is present, hydrogen addition deceases the NO_x. This observation is compared to the literature. Additionally, minimum NO_x emission is examined by comparing the present results to the findings of Leonard and Stegmaier.