Abstract
Gasification processes allow the energy recovery of solid biomasses by their transformation in a synthesis gas with high content in H2 and CO, which can be used for energy production. Syngas also contains many pollutants and the gas cleaning is one of the major bottleneck to the industrial development of this technology. Among these pollutants, tars are organic compounds that condense below 350°C and clog the equipment downstream of the gasifier. Their condensation impacts the reliability of these processes due to recurrent maintenance and the reduction of the lifespan of some equipment. The use of char for syngas cleaning has been extensively studied on laboratory scale model molecules, but far less on real tars coming directly from a gasifier. This work aims at studying the conversion of tars and syngas over a char bed.An experimental study was performed based on an original tar conversion reactor developed and designed in the framework of this thesis. This catalytic reactor was coupled to a commercial gasification unit.In the reference operating conditions for tar conversion (800°C; 2s), and for a tar content of the order of 9 g/Nm3, tars conversion rate is 51%. Part of thermal cracking in tars conversion is 11%.Tests were also performed to investigate the influence of air and water content, residence time and temperature on tars and syngas conversion. Tars conversion is improved by increasing theses parameters, except steam content : a water content of 19% decreases the conversion rate of tars to 40%, compared to 50% for a water content of 12%. The addition of 11.2% air to the syngas increases tars conversion rate to 70% compared to 40% without air at 800°C and 2s. In this case, syngas LHV decreases by 12.6% despite a H2 production increases by 31.9% over the char bed. For a low tar content of 5.7 g/Nm3 and a residence time of 1.82 s at 800°C, a syngas composed mainly of benzene, toluene, ethylbenzene, xylenes (BTEX) is obtained. Under these conditions, the tar content (excluding BTEX) of 151 mg/Nm3 and the tar dew point of 40°C after cracking make it possible to envisage direct coupling with an engine. For residence times of less than 0.6 s at 800°C, PAH compounds are formed despite a tars conversion rate of 40.5%, resulting in an increase in the tar dew point over the char bed.Finally, a set of operating parameters allowing the elimination of all problematic tars was proposed and tested in readiness for an industrial scale-up.