Exploring the sustainable production of ammonia by recycling N and H in biological residues: Evolution of fuel-N during glutamic acid gasification

In this work, the recycling of nitrogen and hydrogen from nitrogen-rich (N-rich) biological residues via autothermal gasification has been proposed as a process suitable for the sustainable production of ammonia (NH₃). Two N-rich biological residues, sewage sludge and meat and bone meal, were used and analyzed in this work and glutamic acid was selected as the model compound of the protein-fuel-N in these residues. Glutamic acid gasification experiments were carried out in order to study the effect of temperature (800–900 °C) and steam-to-carbon ratio (0.5–1.0 g g⁻¹) on the conversion of fuel-N into the most typical N-containing gasification products: NH₃, molecular nitrogen (N₂), hydrogen cyanide (HCN), nitrogen monoxide (NO), tar-N and char-N. Sewage sludge and meat and bone meal were also gasified under selected operating conditions with the main aim of assessing the NH₃ production. The most abundant N-containing compounds obtained in the gasification of glutamic acid were NH₃ (35–51% over fuel-N) and N₂ (45–63% over fuel-N). The highest conversion of fuel-N to NH₃–N in the glutamic acid gasification experiments (51%) was obtained at the lowest temperature (800 °C) and the lowest S/C ratio (0.5 g g⁻¹). The increase in the temperature caused a decrease in the yield of NH₃, as a consequence of its decomposition into N₂. A similar fuel-N distribution was found when sewage sludge and meat and bone meal were gasified, obtaining joint yields of HCN–N, NO–N, tar-N and char-N lower than 5%, and being NH₃–N (30–67%) and N₂–N (28–68%) the majority products. The yields of NH₃–N obtained from glutamic acid (51%), sewage sludge (30%), and meat and bone meal (67%) under the same gasification operating conditions were significantly different. These differences were attributed to the catalytic effect of the metals present in these residues and point to the need to optimize the operating conditions specifically for each residue. In summary, gasification of sewage sludge and meat and bone meal may be able to produce around 10% of the NH₃ produced annually in Europe and between 102 and 262 GJ·ton⁻¹ NH₃ thanks to the combustion of the syngas generated.