The effect of using upgraded biogas on generator performance
Mpungu, Ibrahim Luqman
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Uganda is experiencing an energy crisis especially in the agro processing subsector of the economy. This is partly due to the small distribution network of the generated hydroelectric power which makes it inaccessible by the many scattered farmers. Organic wastes on farms have therefore been turned to as a solution through biogas production. Biogas is an excellent fuel with a number of applications but is commonly used for cooking and lighting in Uganda. These do not require biogas upgrading which is expensive and exclusively designed for large scale operations. This research aimed at assessing the effect of using upgraded biogas on the performance of a petrol generator. It was carried out at Okweru Poultry Farm using biogas from chicken droppings. In the study, the characteristics of raw biogas were determined and a biogas upgrading system for small scale operations was locally fabricated. Experiments to determine the performance of the generator on upgraded biogas were done and lastly an economic assessment carried out. A Geotech GA 2000 plus gas analyser and an Omega HHF11A airflow meter were used for biogas characterization. For biogas upgrading, a plastic tank was mounted on a metallic stand and connected to a fabricated column packed with marbles. A biogas inflow line was added to the carburetor of the generator, parallel to the petrol line, as a modification to conduct the generator performance experiments using both raw and upgraded biogas. The payback period and the life cycle cost were used to determine the economic viability of using upgraded biogas. The findings established that biogas was flowing at 15 litres/minute. It majorly comprised of 57% CH4 and 41% CO2 with small amounts of H2S, O2 and NH4. Biogas upgrading was done using three solvents that is NaOH, KOH and water. At the highest flow rate of 0.80, NaOH excelled with 93.0% CO2 reduction followed by KOH at 82.5% and lastly water at 63.0%. Using upgraded biogas improved the generator’s brake thermal efficiency to 17.6% from 14.2% for raw biogas and reduced the brake specific fuel consumption from 1.27 kg/kWh to 0.84 kg/kWh at maximum percentage load. Economically, raw biogas had a shorter payback period of 5.6 years compared to 8.7 years for upgraded biogas. Although, using raw biogas increased the frequency of generator maintenance and replacement of specific parts which increased operational costs, its life cycle cost for the 15 years period was Ushs. 4,231,400 lesser than that for upgraded biogas. Hence, using raw biogas technology was recommended for adaptation through aggressive sensitization, where need be, soft loan schemes be established.