Behaviour of fuel-bound nitrogen in gasification and in high-temperature NH3 removal processes: Dissertation

Factors affecting the release and reactions of fuel-bound nitrogen in air gasification conditions were studied in order to be able to minimise their production during gasification. Another objective was to find new means to remove nitrogenous compounds from hot gasification gas. In fluidised-bed and fixed-bed types of gasifier NH3 is the predominant nitrogenous compound in addition to N2. In thermodynamic equilibrium the NH3 content of gasification gas is low. The equilibrium content ranges from 1 to 350 ppms depending on the temperature, pressure and stoichiometric ratio. The highest equilibrium content is usually encountered at high pressure. The real NH3 content of gasifier product gas is usually at least an order of magnitude higher. The fraction of fuel nitrogen that is converted to NH3 in gasification is affected by several factors. During pyrolysis young biomass fuels tend to release part of the fuel nitrogen directly as NH3, whereas coals seem to release fuel nitrogen primarily from structures that favour HCN formation. A low heating rate and factors that increase the extent of secondary reactions of gases and char during pyrolysis also increase the conversion of HCN to NH3. In fluidised-bed (and fixed-bed) gasifiers the conditions seem to favour NH3 formation but in entrained-bed gasifiers a high heating rate may promote HCN formation. Factors affecting the release of char nitrogen in gasifiers are poorly known. In this work, a statistical model was developed, which can predict the conversion of fuel nitrogen to NH3 in a laboratory-scale fluidised-bed gasifier and a larger entrained-bed gasifier. It was found that the formation of NH3 was mostly dependent on the freeboard temperature of the gasifier, and on the iron, calcium, ash and volatile matter content of fuel. Increasing the iron content of the fuel resulted in a strong decreasing effect in the NH3 formation in the fluidised-bed gasifier, but it had a smaller effect in the entrained-bed gasifier. The decomposition rate of NH3 may be increased by adding or creating suitable reactive gas species into the hot gas. In selective catalytic oxidation oxidisers like O2, NO or a mixture of these are added to the gasification gas before flowing the gas through the catalyst bed. Aluminium oxide and aluminium silicate catalysts proved to be the most effective of the materials tested in promoting NH3 decomposition in the presence of oxidisers. The temperature of optimum NH3 reduction was tightly dependent on the composition of the oxidiser used. If NO was allowed to convert to NO2 in the oxidiser mixture before feeding the oxidiser to the reactor, the best conversion was achieved at temperatures below 500 °C. If pure NO and O2 were used, the best conversion was achieved at 500 - 600 °C. An important finding in this work was the ability of aluminium oxide to catalyse the reaction between added O2 and NH3. No nitrogen oxides were formed during this type of NH3 oxidation on the aluminium oxide catalyst. The reaction product was probably N2. When O2/NH3 ratio was 4 in the experiments with synthetic gasification gas, 75% of the NH3 was decomposed in the aluminium oxide bed at the temperature range of 550 - 700 °C. The development of a gas cleaning process based on these results of selective catalytic oxidation still leaves several questions unanswered. The most important questions are connected with optimising the process, with the effect of gas impurities on the reactions and with the alternatives of oxidising HCN to N2 simultaneously with NH3.