A PSYCHROMETRIC APPROACH TO FIXED BED BIOMASS GASIFIER DESIGN

The classical approach to syngas processing is based on the assumption that the gas stream (composed of H-2, CO2, CO, N-2, CH4 and H2O) carries tars ashes and soot in it. Tars are usually classified as liquid, while the soot and the ashes are evidently considered solid. This approach is undoubtedly valid but needs to be adapted depending on the temperature conditions that continuously change in a typical gasification power plant composed of a reactor, a filtering system and an engine. Particular attention is payed in this work to the water vapor. It is an unavoidable product of the gasification process that origins from the combustion reactions or derives from the biomass moisture. Under the dew point this vapor condensates assisted by the soot and ashes that can act as nuclei for droplets coalescence. The work proposed here uses a chemical equilibrium model for the evaluation of the gas composition varying the biomass moisture content, then the model output is analyzed using the approach typical of psychrometric problems. In this way it was possible to spotlight the limit temperature in the gas line that avoids water condensation. In fact, the temperature cannot be simply increased because, even if this solution prevent condensation, it drastically reduces the engine performance ad even its durability. For this reason, this study takes into account the equations that simplify the engine behavior and, nevertheless, allow us to evaluate the ultimate performance variation produced by the temperature limit imposed by the moisture content in the biomass.