Peer-Reviewed Journal Details
Mandatory Fields
Thamsiriroj, T; Murphy, JD;
Energy & Fuels
Difficulties Associated with Monodigestion of Grass as Exemplified by Commissioning a Pilot-Scale Digester
Optional Fields

Grass is potentially a crop that in temperate oceanic climates (such as Ireland) offers an excellent yield of gaseous biofuel per hectare. The biogas/biomethane industry in Ireland is in its infancy as compared to its neighbors in continental Europe; Germany, in particular, has a thriving biogas industry. As a result, there is a tendency for digesters that were developed in continental Europe based on high solids content feedstocks, such as maize and the organic fraction of municipal solid waste, to be imported to Ireland for use as grass digesters. This paper examines the difficulties associated with monodigestion of grass through the design and commissioning of a two-stage wet continuously stirred tank reactor. The functional specific gravity of grass silage in a digester remains below 1 for a period of 24 h. This results in accumulations of undigested floating grass, which can be problematic. The agitation system for a grass digester must be designed to overcome this tendency for grass to float. Methane yields for Irish grass silage were found to be particularly high in the experimental period. Expected values for the design of 300 L of CH4 kg(-1) of volatile solids added (VSadded) were based on analysis of the literature and with functional full-scale grass digesters. The value of 455 L of CH4 kg(-1) of VSadded was measured, equating to 91\% destruction of volatile solids. The solids content of digestate was estimated at 5\% dry solids (from a feedstock with 30.7\% dry solids), which would suggest handling difficulties if used in a vertical garage-door-type dry batch process.

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