Cost of Biofuel Production

What is it about?

In a wet digestion process, it is necessary to dilute high solid content feedstocks, such as grass silage. However, grass silage tends to be a problematic feedstock for wet digestion due to its tendency to float, to wrap around moving parts, and to cause inhibition to the microbial process due to production of ammonia. Grass silage may be better suited to batch digestion. However, in a batch process, half the feedstock is left behind after each cycle to provide innoculum for the next batch of feedstock. This reduces the effective reactor volume and increases capital costs. A solution is to combine the leach beds with a high-rate reactor. The system employed in this paper is termed a SLBR-UASB and is a two-phase process. The leach beds are the conduit for hydrolysis, and the methane production takes place in the UASB. The leach beds may be emptied at each cycle, reducing the size requirement of the leach beds to 67% of a pure batch system. This paper documents the problems in designing and commissioning a small pilot-scale SLBR-UASB system. The SLBR-UASB showed itself to be a reliable system when the commissioning was completed. A batch test suggested the upper limit for methane production of 350 L CH4 kg−1 VS added. The recorded gas production when the system was operated as designed was 305 L CH4 kg−1 VS added (87% of gas production from batch test) at a retention time of 42 days, effecting a volatile solid reduction of 68%. The first 5 days of the 7-day cycle resulted in 86% of CH4 production.

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