Performance analysis of updraft coal gasifiers fed by oxygen with steam, CO2 or recirculated syngas mixtures

Small-scale fixed bed coal and biomass gasifiers represent an attractive solution for distributed power and CHP generation and, in a midterm perspective, for hydrogen production. Coal gasification leads to a reduction in dependence on oil and gas, to enhancement of industrial waste disposal, to conversion of agricultural products into renewable fuels and to reinforcement of the development of advanced energy systems with low pollutant and CO2 emissions. Air-blown fixed bed gasifiers, as a strengthened technology, generate a syngas with a low LHV (about 6-7 MJ/kg) and a high N2 concentration (40-45%). The syngas LHV can be enhanced by using oxygen (or oxygen enriched air) instead of air. However, oxygen use requires that a suitable amount of diluent agent as additional steam, CO2 or recirculated syngas be fed to the gasifier to control gasification temperature. In this paper the main performance of conventional air-blown updraft gasifiers and oxygen-blown gasifiers using mixtures of steam, CO2 and recirculated syngas are compared. The analysis was carried out by means of a non-dimensional simulation model specifically developed for predicting performance of small-scale updraft gasifiers