Release of nitrogen precursors from coal and biomass residues in a bubbling fluidized bed

This work was undertaken with the aim of quantifing the relative amounts of NH3 and HCN released from different residues, which included sewage sludge, refuse derived fuel (RDF), and sawdust, during their devolatilization under fluidized bed conditions. The results were compared with data collected for bituminous coals of different origin (USA, South Africa, Colombia, and UK). The relation between amounts of HCN and NH3 released and the levels of NOx and N2O formed during cocombustion was also addressed. The partitioning of nitrogen between volatiles and char was also quantified. The pyrolysis studies were undertaken in a small fluidized bed reactor of 80 turn of ID and 500 turn high using an inert atmosphere (N-2). The HCN and NH3 were quantified by bubbling the pyrolysis gases in absorbing solutions which were subsequently analyzed with selective electrodes. The combustion studies were carried out on a pilot installation. The fluidized bed combustor is square in cross section with each side being 300 turn long. There is secondary air supply to the freeboard at different heights to deal with high volatile fuels as almost all waste materials are. The temperatures in the bed and in the freeboard and that of the flue gases leaving the reactor were continuously monitored. The results obtained suggest that, while coal releases nitrogen mostly as HCN, residues like RDF and sewage sludge give out fuel-N in greater quantities as NH3. Residues at fluidized bed combustion (FBC) temperatures release more than 80% of the fuel-N with the volatiles. The NH3 evolved during pyrolysis acted as a reducing agent on NOx emissions. The presence of calcium significantly reduces the emission of N2O probably by interfering with HCN chemistry. With high amounts of residues in the fuel mixture, the relative importance of char on the nitrogen chemistry substantially decreases. By using cocombustion, it is possible to reduce fuel-N conversion to NOx and N2O, by tuning the amounts of coal and residue in the mixture.