The decreasing effect of ammonia enrichment on the combustion emission of hydrogen, methane, and propane fuels

Abstract

In the present study, non-premixed combustion and NOx emission of H2, NH3, C3H8, and CH4 fuels have been studied in a combustion test unit under lean mixture conditions (λ = 4) at 8.6 kW thermal capacity. Furthermore, the combustion and NOx emission of the H2, C3H8, and CH4 fuels have been investigated for various NH3 enrichment ratios (5, 10, 20, and 50%) and excess air coefficients (λ = 1.1, 2, 3, and 4) at the same thermal capacity. The obtained results have been compared for each fuel. Numerical simulation results show that H2 emits intense energy through the reaction zone despite the lowest fuel consumption in mass, among others, due to its high calorific value. Therefore, it has a higher flame temperature than others. At the same time, C3H8 has the lowest flame temperature. Besides, NH3 has the shortest flame length among others, while C3H8 has the most extended flame form. The highest level of NOx is released from the NH3 flame in the combustion chamber, while the lowest NOx is released from the CH4. However, the lowest NOx emission at the combustion chamber exit is obtained in NH3 combustion, while the highest NOx emission is obtained with H2 combustion. It results from the shortest flame length of NH3, short residence time, and backward NOx reduction to N2 for NH3. As for H2, high flame temperature and relatively long flame, and high residence time of the products trigger NOx formation and keep the NOx level high. On the other hand, excess air coefficient from 1.1 to 2 increases NOx for H2, CH4, and NH3 due to their large flame diameters, unlike propane. Then, NOx emission levels decrease sharply as the excess air coefficient increases to 4 for each fuel. NH3 fuel also emits minimum NOx in other excess air coefficients at the exit, while H2 emits too much emission. With NH3 enrichment, the NOx emissions of H2, CH4, and C3H8 fuels at the combustion chamber exit decrease gradually almost every excess air coefficient apart from λ = 1.1. As a general conclusion, like renewable fuels, H2 appears to be a source of pollution in terms of NOx emissions in combustion applications. In contrast, NH3 appears to be a relatively modest fuel with a low NOx level. In addition, the high amount of NOx emission released from H2 and other fuels during the combustion can be remarkably reduced by NH3 enrichment with an excess air combustion.