A Study of Biomass Combustion Problems and the Selective Catalytic Oxidation of Ammonia

Sammanfattning: Increasing awareness of the need of reducing CO2, NOx and SOx emissions from combustion systems has led to a strong interest in biomass as a fuel source. Using it in combination with fluidised bed combustion technology can lead to a lowering of NOx emissions to the atmosphere. However, this is not a problem-free technique, since the flue gas can contain ammonia, NOx and tar. The fluidised bed can also cease to fluidise if the bed agglomerates. Ammonia can be removed from the flue gas by catalytic methods either by degradation to hydrogen and nitrogen or by selective catalytic oxidation (SCO) to nitrogen and water. Choosing a proper combination of fuel, bed material, NOx repressing additive and an alkali-capturing additive can lower both the NOx emissions and the degree of agglomeration. The optimal combination of fuel, bed material, alkali-capturing additive and NOx-repressing additive for minimising agglomeration in the bed involves use of sawdust, mullite, calcite and ammonia bicarbonate. The optimal combination for minimising NO emission would be use sawdust, bone ash, mullite and urea, whereas the optimal combination for minimising N2O involves the choice of straw, bone ash, clay and Na2CO3. Agglomeration can be either of a homogeneous type, involving particles of uniform size less than 2 mm in diameter, or of a heterogeneous type, in which particles are up to 60 mm in diameter. In the case of heterogeneous agglomeration, there is a severe risk of the bed ceasing to fluidise. Catalysts were tested for ammonia abatement in biogas and flue gas. An Ni-based catalyst was tested for the degradation of ammonia to hydrogen and nitrogen. Pt/CuO/Al2O3 catalysts containing 20 wt% CuO and 0.5-4 wt% Pt that were prepared were used for the selective catalytic oxidation of ammonia in wet and dry biogas/waste gas and in synthetic biogas containing CO, hydrogen and methane. For a “real” biogas, the Ni-based catalyst was able to provide a 35-90% ammonia removal and 90-95 % removal of LHCs at 790-880°C. Activity measurements showed Pt/CuO/Al2O3, to be active and selective for the oxidation of ammonia to form nitrogen. A selectivity of about 90% for the oxidation of ammonia to nitrogen was achieved at full conversion. The catalyst showed a higher selectivity to nitrogen when the feed was wet than when it was dry. Pretreatment of the catalyst by SO2 containing gas provided an improved catalyst with a selectivity of about 97-98% to nitrogen at complete ammonia conversion. Although Pt/CuO/Al2O3 showed good performance for the oxidation of ammonia to nitrogen in the presence of biogas components under wet conditions, performance was less good under dry conditions. However, under both wet and dry conditions the catalyst was not selective for ammonia oxidation but oxidised CO and hydrogen as well. The reaction mechanism for the selective catalytic oxidation of ammonia on Pt/CuO/Al2O3 to yield nitrogen was investigated. The results obtained exclude the possibility of NO being a reaction intermediate in nitrogen formation. Most of the nitrogen is formed by direct oxidation and the combining of two NHx. The data obtained suggest the reaction to take place at the Pt/CuO phase boundary between the NHx species being adsorbed on the CuO and the oxygen at the Pt sites.