Selecting nitrogen carriers used for chemical looping ammonia generation of biomass and H2O by thermodynamic method

Abstract

The demand for NH3 as a new energy carrier will lead to a gradual increase in its production. The Haber-Bosch process, commonly used today, has high energy consumption, high operating pressure and high CO2 emissions due to hydrogen extraction from fossil fuels. For efficient and environmentally friendly preparation of NH3, chemical looping ammonia generation (CLAG) of biomass and H2O is proposed. The selection of suitable nitrogen carriers is the key in CLAG. Since the variety of available substances is large, a systematic method is needed to perform screening efficiently. In this paper, 28 different oxide/nitride pairs were investigated. We mainly investigated the Gibbs free energy change and the equilibrium compositions in the N-absorption and N-desorption reactions. Suitable nitrogen carriers were selected by the minimum N-absorption temperature, nitrogen transport capacity, sintering resistance, price and by-product. When Al2O3/AlN was used as the nitrogen carrier, the minimum N-absorption temperature was 1371 °C, and the nitrogen transfer capacity was high. When TiO2/TiN was used as a nitrogen carrier, the minimum N-absorption temperature was 966 °C, the nitrogen transfer capacity was lower than Al2O3/AlN, and NH3 and H2 were generated simultaneously in the N-desorption step. Compared with SiO2/Si3N4, Cr2O3/Cr2N, Cr2O3/CrN, MnO/Mn5N2, Fe2O3/Fe4N, ZrO2/ZrN, and MoO2/Mo2N, Al2O3/AlN and TiO2/TiN produced less carbide in N-absorption step and had less mass decay in the cycle, making them more suitable as nitrogen carriers. Appropriately increasing the molar ratio of nitrogen carrier to carbon can improve feedstock utilization and the efficiency of CLAG.