Pressurized fluidized-bed gasification experiments with wood, peat and coal at VTT in 1991-1992: Part 1. Test facilities and gasification experiments with sawdust

Fluidized-bed air gasification of Finnish pine saw dust was studied in the PDU-scale test facilities of VTT to support the development of simplified integrated gasification combined-cycle processes by providing new information on the formation and behaviour of different gas impurities in wood gasification. The gasifier was operated at 4 - 5 bar pressure and at 880 - 1020 oC. Product gas was cleaned by ceramic candle filters operated at 490 - 715 oC. Concentrations of tars, fixed nitrogen species and vapour-phase alkali metals were determined in different operating conditions. Carbon conversion exceeded 95 % in all test periods although the gasifier was operated without recycling the cyclone or filter fines back to the reactor. However, at the gasification temperature of 880 - 900 oC more than 5 % of the wood carbon was converted to tars. The tar concentrations seemed to be clearly dependent on temperature at most of the set points. The total concentration of tars (compounds heavier than benzene) was reduced from 6 000 to 3 000 mg/m3n by increasing the gasification temperature from 880 oC to 1 000 oC. The expected catalytic effects of calcium on tar decomposition could not be achieved in these experiments by feeding coarse dolomite into the bed. The use of sand or aluminium oxide as an inert bed material did neither lead to any decrease in tar concentrations. However, the tar concentrations were dramaticly reduced in the cogasification experiments, when a mixture of approximately 50%/50% wood and coal was used as the feedstock. Moreover, the concentrations of the most problematic high-molecular weight polyaromatics were <50 mg/m3n in the cogasification experiments, while >1 000 mg/m3 concentrations were typical of sawdust gasification alone. Wood nitrogen was mainly converted into ammonia, while the concentrations of HCN and organic nitrogen containing compounds were very low. The vapour-phase concentrations of alkali metals were determined both for the hot raw gas before gas filtration and for the cooled and filtered product gas. The total concentration of vapour-phase alkalis was reduced close to the target level of 0.1 ppm-wt by carrying out the filtration at <500 oC. The operation of the ceramic filter unit with saw dust derived product gas was, however, more problematic than the filtration of peat or coal derived gas. In the early experiments, where the filter unit was operated at 700 oC, the pressure drop was increased very rapidly resulting in almost complete blinding of the filters in less than 5 hours. Later tests were carried out at <600 oC, with face velocities of 1.9 - 2.6 cm/s and either dolomite or coal was added to the gasifier. In these conditions the pressure drop of the filter remained more stable and longer test periods could be operated without filter blinding.