Production of Grass Biomethane

What is it about?

The production of grass biomethane is an integrated process which involves numerous stages with numerous permutations. The grass grown can be of numerous species, it can involve numerous cuts. The lignocellulosic content of grass increases with maturity of grass; the first cut offers more methane potential than the later cuts. Water soluble carbohydrates (WSC) are higher (and as such methane potential is higher) for grass cut in the afternoon as opposed to the morning. The method of ensiling has a significant effect on the dry solids content of the grass silage. Pit or clamp silage in southern Germany and Austria has a solids content of about 40%; warm dry summers allow wilting of the grass before ensiling. In temperate oceanic climates like Ireland, pit silage has a solids content of about 21% while bale silage has a solids content of 32%. Biogas production is related to mass of volatile solids rather than mass of silage; typically one ton of volatile solid produces 300m3 of methane. The dry solids content of the silage has a significant impact on the biodigester configuration. Silage with a high solids content would lend itself to a two stage process; a leach bed where volatile solids are converted to a leachate high in chemical oxygen demand (COD), followed by an Upflow Anaerobic Sludge Blanket where the COD can be converted efficiently to CH4. Alternative configurations include for wet continuous processes such as the ubiquitous Continuously Stirred Tank Reactor; this necessitates significant dilution of the feed-stock to effect a solids content of 12%. Various pre-treatment methods may be employed especially if the hydrolytic step is separated from the methanogenic step. Size reduction, thermal and enzymatic methodologies are used. Good digester design is to seek to emulate the cow, thus rumen fluid offers great potential for hydrolysis.

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