Biofuels such as Fischer-Tropsch diesel (FT-diesel), dimethyl ether (DME), biohydrogen (H2) and to a lesser extent, biomethanol, are among the most promising liquid biofuels in the medium-to-long term. The associated production pathways offer the advantage of being based on the conversion of lignocellulosic biomass (wood, grass, agricultural wastes and residues, etc.), available in larger amounts, usually less expensive and with no direct competition with food.
Figure : Synthesis biofuels
These biofuels (often referred to as "synthetis biofuels" or "synfuels") are today limited to pilot scale applications, or even research and development (R&D) activities, and it will most probably take several years before they can play a significant role on the biofuels market. However, several large-scale prR&Dcts are currently under way, aimed at developing the first commercial production plants. One of the most ambitious projects in this field is the RENEW project, with partners such as Volkswagen, BP and Total. One of the production processes evaluated in this project is the Carbo-V® process from the company Choren, ilustrated on the figure below.
Figure : The Carbo-V® process from Choren
The Carbo-V® process (as described by Choren) is a three-stage gasification process involving the following sub-processes:
low temperature gasification;
high temperature gasification;
endothermic entrained bed gasification.
The biomass is first carbonized continually through partial oxidation (low temperature pyrolysis) with air or oxygen at temperatures of 400-500°C, i.e. it is broken down into a gas containing tar (volatile parts) and solid carbon (char). The gas containing tar is then post-oxidized using air and/or oxygen in a combustion chamber operating above the melting point of the fuel’s ash to turn it into a hot gasification medium. The char is ground down into pulverized fuel and is blown into the hot gasification medium. The pulverized fuel and the gasification medium react endothermically in the gasification reactor and are converted into a raw synthesis gas. Once this has been treated in the appropriate manner, it can be used as a combustible gas for generating electricity, steam and heat or as a synthesis gas for producing SunDiesel (Fischer-Tropsch diesel).