Operational modes for effective recovery of energy from ryegrass using anaerobic digestion

Abstract

In the United Kingdom a large proportion of agricultural land is laid to grass which is used for grazing and also harvested for animal feed. Grass is also potentially the crop most suited to energy production in the UK because of its high yield, low maintenance and suitability for growing under the climatic conditions. Anaerobic digestion is a potential technology for conversion of grass to energy and the current work looks at the design and operation of digester types that could be used to maximise the energy yield per hectare of crop and take advantage of the requirement to store harvested material over the winter period. Initial experiments established the methane potential of ryegrass (Lolium perenne) to be 0.245 m3 kg-1 VS added. This was determined in a series of conventional batch digestion studies at different inoculum to substrate ratios using an anaerobic sludge taken from a municipal wastewater digester. The research then went on to examine potential energy losses through the use of conventional continuous stirred tank reactor (CSTR) digester design and from this began to focus on plug flow designs that could be simulated through a batch digestion model. Experimental work used a batch feed cycle to simulate a continuous fed plug flow reactor, although the results are equally applicable to a cyclic batch feeding regime. The minimum feed cycle length to gain 70% of the methane potential was found to be six days at an initial substrate loading rate (ISLR) of 10 g VS L-1 and twelve days at an ISLR of 20 g VS L-1; in both cases this was equivalent to an Organic Loading Rate (OLR) of 1.7 g VS L-1 day-1. In a batch or plug flow system it is necessary to add an inoculum, and experiments were designed to show the advantages and disadvantages associated with using the liquid or solid fractions derived from separated digestate material for this purpose. Both proved to be suitable as an inoculum at a 10 g VS L-1 batch loading, but a higher gas yield was achieved from the separated solids inoculum due to the capturing of residual VS by increasing the solids retention time of the system. Results from a number of experiments indicated that in a ryegrass digestion system mechanical stirring could be problematic, and there were indications that this type of mixing might not be necessary for optimal performance. At an ISLR of 20 g VS L-1 some small advantages were found as a result of stirring during acclimation of the inoculum to the feedstock but this could be compensated for by the adoption of once per day liquid recirculation around the digester. This mixing strategy was therefore adopted in subsequent experiments. 30L digesters were used to test a digester operating mode in which solids were allowed to accumulate over a number of feed cycles, achieved by removing only the liquor which passed a 1 mm mesh at the end of each cycle. The solids accumulation rate for ISLR of 10 g VS L-1 loading on a seven-day cycle would allow the digester to operate for 30 weeks if no solids were broken down. In practice the rate of VS destruction measured extended this by between ~24-67% depending on the initial solids make up of the digester. In a subsequent smaller-scale solids accumulation experiment a specific methane yield of 0.415 L CH4 gVS-1 was achieved over 10 feed cycles (weeks) and showed this reached an optimum at an I:S ratio of 3 – 3.5 on a VS basis

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Identifiers

ORCID for C.J. Banks: orcid.org/0000-0001-6795-814X

ORCID for S. Heaven: orcid.org/0000-0001-7798-4683

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