Design of a cost effective controlling technology for generation of electricity in standalone biogas plant in Bangladesh

Abstract

Biogas technology, which can provide pollution free, efficient energy for cooking and cleaner environment, is one of the best means for providing natural gas to a large number of rural people. According to International Finance Corporation (IFC), the number of poultry farms in Bangladesh is 215000, can generate thirty thousand tons of droppings per day. Poultry droppings, with total solid (TS) content of 20%, produce 0.4m3 of biogas per kg of TS. The whole amount of dropping, if can be collected; can produce 82 million CFT of biogas everyday or 30 billion CFT of biogas per year; which is good enough to produce 3.65 GWH of electricity per day or 1.33 TWH a year; for 14 hours of operation this biogas can run a 260MW generation unit. Despite all these prospects, only a limited number of biogas plants are deployed in the poultry farms of Bangladesh. The study shows that, poultry size with population of 5000 or more are viable for commercial production of electricity and fertilizer under present condition. But if the capital and operational costs can be reduced by 40% for the range of 2000 to 5000 and 60% for the range of 0 to 2000 then they become economically viable. There are different factors which influence the production of biogas e.g.-temperature, pH, bacterial population in digester, mixture of different substrate, hydraulic retention time, total solid (TS), periodic agitation, periodic loading and unloading of substrate etc. Most of the biogas plants don‟t have the schemes for controlling, apart from few of the biogas plants which are imported, which are well equipped for controlling these parameters, hence they produce up to 75% more biogas than the locally produced biogas plant. But for locally produced biogas plants additional controlling schemes can be provided making an additional investment. Although providing controlling schemes incurs additional costs but the increment in gas production suppresses these costs and the cost for per unit of electricity generation decreases by up to 36%. The biogas plants in poultry farms in Bangladesh either use imported or locally converted generators, which are not provided with automatic voltage regulator (AVR) or governors in most of the cases. The voltage and frequency can be regulated by AVR, governor and PFI. But as they are costly for smaller plants, voltage can be regulated using cost effective field excitation control and managing load strategically. The developed automatic voltage regulators are up to 42% less costly than the available AVR in the market. The strategic load management module costs only about 6000BDT and is not suitable for very small size biogas plant e.g.-biogas plants smaller than 100m3.