Reactions of fuel bound nitrogen in gasification processes: Licentiate thesis

The object of this licenthiate thesis was the nitrogen chemistry of gasifica- tion. The formation of nitrogen compounds in fixed-bed and fluidized-bed gasifiers, their removal from hot gas and the formation of NOx from nitrogen compounds were studied. The thesis consists of five papers. Paper 1 discusses conversion of peat and wood nitrogen in the fixed-bed gasifier and formation of NOx in combustion of gasification gas. In peat gasification, 19 - 30 wt% of fuel nitrogen was converted to NH3 but only 4 wt% of wood nitrogen formed NH3. HCN conversion was 3 - 5 wt% for peat and 9 - 14 wt% for wood. The proportion of organic nitrogen in tars was 13 - 19 wt%. The source of NOx in gas combustion proved to be almost entirely the gaseous nitrogen compounds. In papers 2 and 3, catalytic decomposition of nitrogen compounds and tars was studied. The activity of several catalytic materials was tested using a side stream of a commercially operated fixed-bed gasifier. The materials tested were iron sinter, iron pellet, ferrous dolomite, dolomite and lime-stone and, as reference materials, inert silicon carbide and a commercial nickel catalysts. The ferrous materials and the commercial nickel catalyst proved to be the most effective agents for decomposing NH3. Limestone and dolomite did not exhibit any essential catalytic capacity for decomposing NH3 although they reduced the HCN content of the gas.The carbonate rocks were instead effective in tar cracking. The effect of process parameters on the formation of nitrogen compounds and tars in fluidized-bed air gasification of peat was studied in paper 4. Benzene was the main component of the tars, representing 50 - 75 wt% of the total tar. Addition of both secondary air and dolomite reduced the tar content of the gas. With no secondary air, 30 wt% of fuel nitrogen formed NH3 and 3 - 5 wt% formed HCN. A rise in the freeboard temperature, caused by the addition of secondary air, had no significant effect on the conversion of nitrogen compounds. On the other hand, the presence of dolomite increased the NH3 concentration and reduced the HCN concentration of the gas. In paper 5, the possibility of converting NH3 to N2 by adding small amounts of O2, NO or both to hot gasification product gases was studied by means of gas-phase kinetic modelling. Significant N2 yields were obtained by introducing O2 or NO to the gas mixture of CO, CO2, H2, H2O and N2 containing 5 000 ppmv NH3. At temperatures above 1 000 oC, higher than 90 % total NH3 reduction percentages were reached. When adding O2 the reduction occurred typically in less than 10 ms. In addition of NO, the reduction was characterized by a reaction time exceeding 1 s. The presence of small amounts of CH4 reduced the total NH3 reduction. However, by adding NO reduction percentages of about 50 % were still obtainable in a residence time of 5 s.