Conversion of CO₂ during the DFB biomass gasification process

Abstract

In many industrial processes, the climate-damaging gas CO2 is produced as undesired by-product. The dual fluidized bed biomass gasification technology offers the opportunity to tackle this problem by using the produced CO2 within the process as gasification agent. Therefore, a 100 kWth pilot plant at TU Wien was used to investigate the use of CO2 as gasification agent by converting softwood as fuel and olivine as bed material into high-valuable product gas. A parameter variation was conducted, where the typically used gasification agent steam was substituted stepwise by CO2. Thereby, the amount of CO and CO2 increased and the content of H2 decreased in the product gas. These trends resulted in a declining H2/CO ratio and a decreasing lower heating value when CO2 was increased as gasification agent. In contrast to these declining trends, the carbon utilization efficiency showed an increasing course. As second part of this work, a temperature variation from 740 to 840 °C was conducted to investigate the change of the main product gas components. With increasing temperature, CO and H2 increased and CO2 decreased. To determine the degree of conversion of CO2 in the DFB reactor system, two approaches were selected: (1) a carbon balance and (2) a hydrogen balance. This way, it was found out that a certain amount of CO2 was indeed converted at the investigated process conditions. Furthermore, under certain assumptions, the reverse water-gas shift reaction was identified to be the predominant reaction during CO2 gasification.