How much gas can we get from grass? A.S. Nizami a,b , A. Orozco c , E. Groom c , B. Dieterich d , J.D. Murphy a,b,⇑ a Department of Civil and Environmental Engineering, University College Cork, Cork, Ireland b Biofuels Research Group, Environmental Research Institute, University College Cork, Cork, Ireland c School of Chemistry and Chemical Engineering, Queen’s University Belfast, Ireland d School of Biology & Environmental Sciences, University College Dublin, Ireland article info Article history: Received 22 June 2011 Received in revised form 17 August 2011 Accepted 18 August 2011 Available online 17 September 2011 Keywords: Biofuel Biomethane Grass silage UASB abstract Grass biomethane has been shown to be a sustainable gaseous transport biofuel, with a good energy balance, and significant potential for economic viability. Of issue for the designer is the variation in characteristics of the grass depending on location of source, time of cut and species. Further confusion arises from the biomethane potential tests (BMP) which have a tendency to give varying results. This paper has dual ambitions. One of these is to highlight the various results for biomethane potential that may be obtained from the same grass silage. The results indicated that methane potential from the same grass silage varied from 350 to 493 L CH 4 kg 1 VS added for three different BMP procedures. The second ambition is to attempt to compare two distinct digestion systems again using the same grass: a two stage continuously stirred tank reactor (CSTR); and a sequentially fed leach bed reactor connected to an upflow anaerobic sludge blanket (SLBR–UASB). The two engineered systems were designed, fabricated, commis- sioned and operated at small pilot scale until stable optimal operating conditions were reached. The CSTR system achieved 451 L CH 4 kg 1 VS added over a 50 day retention period. The SLBR–UASB achieved 341 L CH 4 kg 1 VS added at a 30 day retention time. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Grass biomethane has been shown to be a sustainable gaseous transport biofuel [1]. It has an excellent energy balance; superior to first generation liquid biofuels from temperate climates and similar to tropical biofuel systems [2]. It is also shown to allow eco- nomic viability both to the producer and the consumer [3]. Of issue with this paper is the variable nature of grass, the variable data relating to gas production from grass, and the preferred technology to maximize gas production from grass. 1.1. Difficulties in comparing digester configurations from scientific literature Data from scientific press on biogas production from grass si- lage using a variety of systems (dry batch, wet batch, one stage and multi stage systems) are numerous [4–8]. Of issue however is that the characteristic of grass varies with location, soil type, time of harvesting (both date and time of day) and grass species. For example, the level of water soluble carbohydrates is higher in the afternoon than the morning; higher levels of water soluble car- bohydrates increases production of methane [9]. The structural and chemical composition of grass changes as the plant matures; grass cut later in the growing season has a higher lignin content and the potential for methane decreases [9]. Furthermore different grass species have different biomethane potentials. Grass may be ensiled using different methodologies: field wilting in continental Europe to obtain 40% dry solids as compared to temperate Europe where pit or clamp silage may be limited to 22% dry solids; baling of silage in plastic as opposed to pit or clamp silage; use or other- wise of silage additives [10]. Thus in comparing biogas or biome- thane production of grass silage from different digester designs as assessed from a review of the scientific literature, it is possible to draw inconclusive or incorrect results. To assess the preferable digester configuration there is a need to compare different digester configurations using the same grass silage, cut from the same field, at the same time, ensiled in the same manner. 1.2. Comparability of different biomethane potential assays The biomethane potential (BMP) assay has a significant role in digester design, including for economic and management issues. The importance of BMP is highlighted with the significant number of papers in the scientific press relating to biodegradability of different substrates [11]. Despite this there are issues relating to 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.08.033 ⇑ Corresponding author at: Department of Civil and Environmental Engineering, University College Cork, Cork, Ireland. Tel.: +353 21 4902286; fax: +353 21 4276648. E-mail address: jerry.murphy@ucc.ie (J.D. Murphy). Applied Energy 92 (2012) 783–790 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy