PERFORMANCE AND EMISSION CHARACTERISTICS OF SPARK IGNITION ENGINE OPERATING WITH PURIFIED BIOGAS

Abstract

The Spark Ignition (SI) engine remains a global prime mover in the agricultural and transportation sectors as well as in electricity generation. However, its low thermal efficiency and consequential high emissions as related to the use of fossil fuels continue to be major concerns and necessitates the search for new fuels, such as purified biogas. Literature is sparse on the impact of purified biogas on the performance and emission characteristics of SI engines. This study was designed to evaluate the performance and emission characteristics of SI engine operating with purified biogas. Cattle dung was obtained and tested for pH, total solids, carbon-nitrogen ratio and Biological Oxygen Demand (BOD) using standard procedures. A floating gas cap digester was designed and fabricated using standard principles. The cattle dung was fed into the digester and biogas was generated. The biogas was purified using single and double pass water scrubber to obtain Single-stage Water Scrubbed Biogas (SWSB) and Double-stage Water Scrubbed Biogas (DWSB), respectively. Both SWSB and DWSB were each compressed to 375.8 kPa. The methane content of Raw Biogas (RB), SWSB, DWSB were determined using Liquid Displacement Method (LDM), while the scrubber efficiencies were evaluated using established procedure. The Brake Power (BP), Brake Specific Fuel Consumption (BSFC), Brake Thermal Efficiency (BTE) and emissions (O2, SO2 and CO) from a 4.125 kW 4-stroke air cooled SI engine operating with Liquefied Petroleum Gas (LPG), RB, SWSB and DWSB were obtained and compared using established procedure. Data were analysed using ANOVA at α0.05. The pH, total solids, carbon-nitrogen ratio and BOD of the substrate were 7.20, 17533.33 mg/L, 17.72 and 14956.66 mg/L, respectively. Methane content in RB, SWSB and DWSB were 73.47, 88.57 and 96.67% by volume, respectively. The capacity of the fabricated digester was 1.12 m3 . Scrubber efficiencies were 56.92 and 70.87% for SWSB and DWSB, respectively. Engine BP, BSFC and BTE for LPG at full load were 2.04±0.06 kW, 730.38±20.93 g/kWh and 10.70%, respectively and RB corresponding values were 1.03±0.03 kW, 672.37±25.72 g/kWh and 20.57%, respectively. BP, BSFC and BTE when SWSB was used were 1.26±0.09 kW, iii 551.53±40.20 g/kWh and 20.87%, respectively and corresponding DWSB values were 1.5±0.08 kW, 461.63±18.17 g/kWh and 22.78%, respectively. Engine O2, SO2 and CO emission characteristics for LPG at full load were 20.5±0.18, 51.8±24.42 and 4200 ±330 ppm, respectively and RB corresponding values were 20.88±0.04, 73.6±27.66 and 3100.00±265 ppm, respectively. Engine O2, SO2 and CO emission characteristics when operating with SWSB were 20.73±0.46, 71.33±18.9 and 2246.33±355.09 ppm, respectively and 20.6±0.12, 41.67±3.51 and 657.67±115.15 ppm, respectively when DWSB was used. Mean performance of engine run on SWSB and DWSB were better than RB. Mean performance of DWSB variables were significantly higher than corresponding means of SWSB indices. Operating a spark ignition engine with double stage water scrubbed biogas gave better performance and lower emissions compared to liquefied petroleum gas and raw biogas. Thus, purified biogas is an alternative fuel for spark ignition engines.