Low-calorific ammonia containing off-gas mixture: Modelling the conversion in HCCI engines

Abstract

Industrial waste gases from chemical production plants often contain toxic products that usually have to be removed or destroyed by waste gas purification or thermal afterburning. These off-gases are still interesting from an energetic and chemical perspective because they contain products with low-calorific but non-negligible heating values and carbon. This work addresses the conversion of typical ammonia and amine containing off-gases from chemical plants in fuel-rich operated HCCI engines with respect to carbon-recovery and simultaneous output of work and heat. To test this concept and to overcome technical limitations and avoid pollutant formation, the HCCI engine process is simulated. Two off-gas mixtures were chosen exemplarily, containing ammonia, dimethylamine, methanol, and acetaldehyde diluted in nitrogen at equivalence ratios of 9.6 (mixture 1) and 3.2 (mixture 2). The investigation showed that without further oxygen addition, mixture 2 ignites, but the ammonia conversion remains below 42%, while the heat release from mixture 1 is small. Oxygen addition up to equivalence ratios of 2 – 2.5 leads to higher ammonia conversion, entailing maximum yields for H2 and CO of 60% and 80%, respectively. The NOx and HCN formation are reasonably low at equivalence ratios between 1 and 2 for both mixtures. The equivalence ratio strongly affects the ammonia decomposition and product gas formation, while a variation of the inlet temperature can be used to achieve a stable combustion phasing. Considering all findings, good conditions are inlet temperatures of 348 K (Mixture 1) and 423 K (Mixture 2), and an equivalence ratio of 1.5. High exergetic efficiencies of up to 76% are predicted for these conditions. Since off-gas mixtures may have a varying composition, the sensitivity of the ignition in relation to single species in the mixture was assessed. Methanol and acetaldehyde mainly affect the ignition because they lead to the highest radical concentrations, whereas dimethylamine mainly consumes radicals. The reactions that mostly change the outcome of the ammonia conversion are decomposition reactions of dimethylamine. The theoretical analysis revealed a feasible process awaiting an experimental validation regarding kinetics and engine operation.