Participation of alkali and sulfur in ammonia combustion chemistry: investigation for ammonia/solid fuel co-firing applications

Ammonia (NH₃) is a promising carbon-free energy carrier. Co-firing of ammonia in solid fuel-fired facilities is a feasible solution to reduce carbon dioxide (CO₂) emissions. Solid fuels, such as coal and biomass, contain various trace elements, such as alkali metals and sulfur, which are released to the gas phase during combustion. Experimental characterization and modeling are used to study the participation of alkali and sulfur species in ammonia conversion in a post-flame environment, focusing on the characteristics of NO emissions and NH₃ slip. The combustion environment was provided by a laminar flame burner with a temperature decreasing from about 2000 K in reaction zone to 1500 or 1100 K in flue gas zone and an equivalence ratio of around 0.65 or 1.3. Known amounts of ammonia (up to 20,000 ppm), potassium hydroxide (KOH, representative of alkaline substances, up to 25 ppm), and sulfur dioxide (SO₂, up to 1500 ppm) were uniformly introduced into the burner for high-temperature thermochemical research. The concentrations of NH₃, nitric oxide (NO), KOH, SO₂, and hydroxyl radicals (OH) downstream of the burner were measured quantitatively in situ using broadband UV (ultraviolet) absorption spectroscopy. In the oxidizing reaction environments, the influence of SO₂ on the NO formation was negligible, while KOH significantly reduced the concentration of NO, and even led to residual ammonia in the low temperature case. Under reducing conditions, both SO₂ and KOH significantly inhibited the decomposition of ammonia, especially at relatively low temperature. Meanwhile, consumption of KOH/K was observed after the mixing with ammonia, possibly due to a direct reaction of KOH/K with ammonia. One dimensional modeling using a detailed mechanism containing N/S/K chemistry qualitatively predicted the impact of S/K on ammonia oxidation and decomposition. The effect was mainly contributed to the enhanced radical consumption by SO₂ and KOH. However, the model could not describe the observed consumption KOH/K by ammonia. Potassium amide (KNH₂) can be generated through KOH + NH₃ = KNH₂ + H₂O. However, according to quantum chemistry calculations for KNH₂, this reaction is endothermic by 80 kJ mol⁻¹, shifting the equilibrium strongly towards KOH + NH₃, and more work is required to clarify the mechanism of removal of potassium by NH₃.