Nitrogen transformation during gasification of livestock compost over transition metal and Ca-based catalysts

Catalytic gasification of a pig compost (PC) was investigated over transition metal catalysts (TMCs, including limonite, CoMo/Al2O3, Ni/Al2O3, and nickel loaded on lignite char) and Ca-based catalysts (dolomite and CaO) in a two-stage fixed-bed reactor to understand the effects of catalyst, temperature, and steam on nitrogen distributions. Non-catalytic thermal decomposition (TD) of PC volatiles below 750 °C is not effective for decomposing the entire volatile nitrogen species (VNSs) to N2. NH3 was found to be the predominant nitrogenous gas under inert conditions used in this investigation, and its yield increased with raising TD temperature. The N yield in HCN is lower than 5% below 550 °C, and sharply increased to 13.9% at 750 °C due to TD of volatiles. Most of VNSs were converted to N2 over TMCs, especially over Ni-based ones. The TMCs proved to be quite active not only for tar reduction, but also for VNSs decomposition at 450-650 °C. On the contrary, CaO-based catalysts, especially dolomite, significantly promoted the conversion of VNSs to NH3. Ni/Al2O3 effectively promoted the conversion of NH3 and HCN to N2 at 550 °C. Steam introduced mainly prevented HCN decomposition over dolomite and coke deposition over Ni/Al2O3. This study provides a basic insight into the nitrogen transformations during catalytic gasification of PC, which would benefit the clean utilization of PC as an energy source.