Investigation on the Use of Intermediate Crops for Anaerobic Digestion as a Renewable Source of Energy

Abstract: The eminent dilemma around the relationship between the increasing world population, the need of new fuel sources and environment issues has raised great awareness in the last decades. The European Parliament has set common goals such as the reduction of the greenhouse gases by 20 % by the year of 2020, followed by the requirement of at least 10 % of the fuel has to be biofuel. An adjacent issue is the source of the biofuel, which are currently most of agricultural crop nature. However these energy crops should not compete with food crops and favourably have other features that promotes its usage. Intermediate crops are for example a promising resource since it may reduce the risk of the nutrients leaching since the crop can take up nutrients in the soil. The aim of this thesis was to investigate the potential of six different intermediate crops (hemp, oilseed radish, white mustard, phacelia, sudangrass and hairy vetch) as energy resources under anaerobic digestion condition after 30 days. Hairy vetch showed the highest methane yield (343 m3/ t VS) followed by sudangrass (316 m3/ t VS). Sudangrass showed a slight potential to increase the methane yield (97 % of theoretical yield based on the component analysis for the sudangrass) if steam pretreatment is for instance applied. This relatively high yield result could be attributed to the relatively low lignin content of sudangrass compared to other crops (16 % of TS). An important factor taken into consideration when selecting which crop to investigate for pretreatment was also the methane yield per hectare for sudangrass which had the second highest value (995 Nm3/ ha) after oilseed radish (1217 Nm3/ ha). Other factors were also taken into account in the decision such as: total solids content, how easily the crop is managed in agriculture for example due to water content and the negative impact of shared diseases with other crops. Based on the results and factors discussed above, the crop which had the greatest potential for methane yield improvement but also availability of ensiled material, sudangrass, was further investigated for the pretreatment effects on methane production. The ensiled sudangrass was pressed into a liquid fraction and a solid fraction, where the latter was taken forward to the pretreatment step. The pretreatment conditions studied were steam pretreatment with added catalyst (1 % acetic acid or 2 % sodium hydroxide weight percentage base on total solid of sudangrass, sprayed in the crop) and it was compared to steam pretreatment alone. Also different temperatures (180 °C, 190 °C, 200 °C and 210 °C) and retention times (5 and 10 min) were studied. The sodium hydroxide impregnated crop did not show better yield than the ensiled sudangrass. Quite the contrary: the alkaline catalyst showed in the best case scenario (190 °C and 10 min) a methane production decrease of 12 % compared to the solid fraction of ensiled sudangrass (325 m3/ t VS). On the other hand the acid treatment at 190 °C and 5 min residence time showed an 11 % increase in the same context. The acid treatment at 190 °C and 5 min showed the highest final methane yield (362 m3/ t VS). Conclusively it could be said that all the intermediate crops have a potential for usage in methane production, assuming other aspects are optimized for its usage. Pretreatment can be with advantage be used to improve the methane yield, where there is room for improvement. The question is rather what specific combination of pretreatment conditions will yield the best enhancement and careful investigations should be made before determining the ultimate pretreatment for a specific crop