Fixed bed gasification of wood char: Thermal, chemical and mechanical characterisation

Fixed bed gasification is one of the most promising technologies in the area of biomass gasification to be introduced into the market of low and medium power ranges due to the higher electricity output expected compared to combustion. Moreover, downdraft or twostage gasifiers are known to produce low tar content in fuel gas. Objectives In this work, we focused on the step of the char bed gasification in fixed bed or two-stage gasification processes. We propose here a fine characterisation of the char bed during its gasification by H2O and CO2. The global objective is to provide key information to improve both the char conversion in char bed reactor and the control of fixed bed gasifiers, which remain today a brake to industrial development. Approach The characterisation of char bed gasification has been carried out thanks to the Continuous Fixed Bed reactor (CFiB) at Cirad. This reactor replicates the gasification zone apart from the rest of the fixed bed process. It is specifically instrumented to allow the measurement of thermal and chemical profiles all along the bed. Indeed, measurements of temperature, pressure and gas composition each 10 cm in the bed are performed. The non condensable gases are on-line analysed by gas-chromatography analysis. The condensable products (water + residual tar) are collected to be analysed in the lab by Karl Fisher, gas- and liquid- chromatography analyses. At the reactor outlet, ash removed is continuously weighed and gases are quantified to establish a precise mass balance of each experiment. Scientific innovation and relevance This CFiB reactor is an innovative tool to study wood char gasification in continuous fixed beds. The accuracy of the measurements carried out inside the bed is a step forward in the understanding of fixed bed gasification processes. Such profiles of temperature, gas composition, char conversion, and bed morphology provide original and relevant data to localise the zones where char oxidation by O2 and char gasification by H2O and CO2 effectively occur. Results In this paper, full characterisation of the fixed bed is presented, including axial and radial profiles inside the 700 mm height bed. Influence of steam partial pressure (from 0,1 to 0,4 atm) and initial oxygen content (2 to 5 %) on char gasification has also been investigated with the proposed methodology.