By-Products of Bioenergy Systems (Anaerobic Digestion and Gasification): Generation and Prospects of Utilization

The producer gas and biogas are biomass-based renewable energy sources drawing increased attention worldwide. Substantial quantities of by-products are also generated during the production of these fuels from biomass. While primary emphasis is on main energy form, management and utilization of by-products are also of concern, both from economic and environmental points of view, as these are the inevitable commodities. There have been some reports on utilization of these by-products, mostly traditionally, to supplement soil nutrients. However, lack of proper management could lead to their nonoptimal use through loss of nutrients. The present investigation considers the by-products of two bioenergy systems, viz. digested slurry from anaerobic digestion unit and tar from bio-gasification unit, available in food industry through case studies to understand the rate of by-product generation, characterization and prospective utilization. From the total slurry generated using cow dung as feedstock, an estimated amount of 3.68 kg of dry mass (solid digestate) and 36.28 l of liquid (liquid digestate) per cubic metre of biogas were obtained. While analysing the digested slurry in terms of thermal and nutritional characteristics, it is known that recovery of second-stage thermal energy without losing essential nutrients could be possible if appropriate management strategy is taken. It has been estimated that 54.56 MJ energy could be made available per cubic metre of biogas, considering second-stage energy recovery route through briquetting technology using solid fraction of digestate. While estimating gasification tar, for generating 1 m3 of producer gas using bamboo, an average of 0.041 g tar could be obtained. The prospects of utilization of gasification tar could be possible from the presence of a number of useful chemicals as indicated by characterization study.