NOx Behavior for Lean-Premixed Combustion of Alternative Gaseous Fuels

Abstract

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 NOx emission that accompany the use of alternative fuels. Controlling oxides of nitrogen (NOx) from combustion sources is essential in nonattainment areas. Lean-premixed combustion eliminates most of the thermal NOx emission but is still subject to small, although significant amounts of NOx formed by the complexities of free radical chemistry in the turbulent flames of most combustion systems. Understanding these small amounts of NOx, and how their formation is altered by fuel composition, is the objective of this paper. We explore how NOx 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 NOx 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, NOx 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 NOx to increase. In these cases, the change from high-methane natural gas to alternative gaseous fuel causes the NOx to increase. However, when hydrogen is added to the methane, the NOx 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 NOx. This observation is compared to the literature. Additionally, minimum NOx emission is examined by comparing the present results to the findings of Leonard and Stegmaier.

Analysis of the performance of a low-power atmospheric burner for gas appliances for households and their impact on the emission and stability of the burner

“…This onedimensional model of the reactor allows the calculation of: temperature profile, concentrations of the main components, intermediate elements and then flame propagation ratio as a function of distance. The flame structure and burner emissions with a laminar premixed methane/air flame [12] were calculated at an initial temperature of 298 K and at a pressure of 1.013 x 10 5 Pa. [6] The calculation was performed in the domain of 10 cm, which is significantly more than the thickness of the flame. [9] The coefficient of primary excess air λ' varied from 1.0 to 1.7.…”

Effect of fuel composition on NOxformation in high‐pressure syngas/air combustion

“…Of the two major constituents of NO x (NO + NO 2 ), nitric oxide (NO) is known to be an ozone destroyer and a precursor of acid rain . It could be formed through four main mechanisms: (1) The extended Zeldovich mechanism at high temperatures, (2) Fenimore (or prompt NO) mechanism through the reactions of CH radicals with N 2 (3) nitrous oxide mechanism and (4) NNH mechanism . Nitrogen dioxide (NO 2 ) is more toxic and visible in brown color at relatively low concentrations.…”

“…Several studies have previously been carried out at different pressure ranges in order to obtain an in‐depth understanding of the fundamental combustion characteristics of syngas flames focusing on measurements of burning rates, laminar flame speeds, ignition delay times and auto‐ignition properties, flame‐front structures and intrinsic instabilities; however, there has been a limited amount of research on NO x formation during syngas combustion at elevated pressure . While numerous researchers have experimentally studied combustion of syngas fuel mixtures under atmospheric pressure investigating the effect of temperature, equivalence ratio, H 2 /CO ratio and diluents on NO x formation, NO x emission data at relevant high pressure conditions are sparse. An experimental study conducted with low calorific value gas (LCVG) flames have revealed that increasing the pressure results in elevated NO emissions at constant methane concentration.…”

“…Moreover, experimental efforts have also been devoted to study NO x formation from syngas combustion in presence of hydrocarbons . In a study reporting measurements of NO concentration in low calorific value gas (17.5%H 2 /21.2%CO/12.2%CO 2 /49.1%N 2 ) flames with the equivalence ratio of 0.7–1.3 and fuel‐CH 4 of 0–14.9%, it has been shown that addition of methane can influence NO concentration in several possible ways.…”

“…Although the addition of CH 4 has been found to decrease the flame temperature, hence the thermal NO formation, overall NO concentration increases due to a larger increase in prompt NO formation. Other experimental investigations have compared the emissions from methane as the reference fuel with syngas blends . Increasing H 2 /CO content in the fuel decreases NO x emissions for a given adiabatic flame temperature .…”