A chemical kinetic analysis: Influence of post-flame chemistry, combustion pressure, premixing degree (fully premixed to non-premixed), and secondary air supply on NOx emissions from NH3/CH4-air combustion

Abstract

Numerous studies assessed the viability of ammonia blends for the wide-scale application of ammonia in carbon-free energy systems. NOx emissions from ammonia combustion are the primary concern constraining the adoption of NH3 as a fuel. The present study explores the effect of post-flame chemistry, combustion pressure, reactant-mixing (premixing degree), and secondary air supply on NOx emissions from NH3/CH4 swirl flame. Furthermore, an extensive chemical kinetic analysis is carried out using the rate of production and sensitivity analyses to study NOx formation and destruction with the above-discussed techniques. The present study adopts the NH3/CH4 reaction model developed by Okafor et al. and a PSR-PFR chemical reactor network for predicting the NOx formation chemistry in NH3/CH4-air combustion. The analyses are conducted for the values of ammonia heat fraction in the fuel mixture (ENH3) = 0.1–0.3 and equivalence ratio (ϕ) = 0.6–1.4. The present work suggests that providing a post-flame zone is advantageous in reducing both NOx and NH3 emissions, especially for rich NH3/CH4 flames. Increasing the combustion pressure to achieve low NOx without significant ammonia slip appears to be a viable method for NH3/CH4 flames with ϕ < 1.2. Adopting a non-premixed injection mode is a feasible low NOx combustion technique for 0.9 < ϕ < 1.2, especially for higher ammonia fractions in the fuel mixture. An excess air supply of 10% reduces the NOx emissions for fuel-lean conditions (ϕ < 0.9) with a negligible rise in unburnt NH3 emissions under atmospheric conditions.