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,
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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.
