Cogasification of Coal-Biomass Mixtures, Syngas Upgrading and Process Optimization as Alternative Transport Fuels for Direct Injection and Compression Ignition Engines

Abstract

Exponential growth in population, demand for fossil fuels, and tricycles in transport sector calls for research in alternative sustainable transport fuel with less environmental pollution. Diesel engines are widely used due to their superior efficiencies, economy and performance but emissions are their shortcomings. Kenya has huge deposits of coal and abundant biomass which are solid, making their application in diesel engines unrealistic. The study focuses on co-gasification of optimized blends of low rank Mui Basin coal (MBC) and selected biomass (rice-husks (RH), Prosopis juliflora (PJ), and Hyphaene compressa (HC)) to generate upgraded-syngas, as an alternative fuel for tri cycles. Samples were air-dried, crushed and blended at various ratios then subjected to calorific value (CV) determination, proximate, and ultimate analysis according to American Society for Testing and Materials standards. Optimized blends were co-gasified to produce upgraded-syngas which was characterized using gas chromatograph and a 3.5kW test engine. Engine loads were varied from 0-100% while data on combustion, performance and emissions were collected. The data collected was presented in tables and analyzed statistically using Microsoft Excel and Statistical package software for social science (SPSS) in terms of mean, and any significant difference at 5% was reported. Analysis of variance (ANOVA) and post hoc analysis using Tukeys’ and Scheffes’ significant difference test were conducted. Origin (2018) was used to present the data graphically. Results reveal that fixed carbon was 29.3±5.51%-MBC, 21.1±1.55%-RH, 31.0±1.00%-HC and 58.7±1.53% for South Africa coal (SAC). Based on fixed carbon (FC), MBC is lignite coal better utilized through co-gasification and SAC is sub-bituminous. Blending improved hydrogen/carbon ratio at a range of 0.12-0.14, while sulphur and nitrogen content decreased implying low emissions. The study reports CV in MJ/kg as; MBC-20.41±0.15, PJ-8.68±0.17, HC-18.69±0.00, RH-12.95±0.1 and 29.51±0.40-SAC. Blending improved CV in a range of 0.4-2.28 MJ/kg for MBC and 5.01-7.21 MJ/kg for SAC. Reduction of 1-2% on moisture content and 33-35% in ash content was noted. Volatile matter increased in a range of 0.2-11%. Ignitability indices and fuel ratio reported an improvement in the blends relative to raw biomass. Upgraded-syngas from optimal blend had higher heating value (HHV) (MJ/m3) of 4.97, 4.78, 4.61 and lower heating value (LHV) (MJ/m3) of 4.92, 4.74 and 4.58 for MBC-PJ, MBC-HC and MBC-RH, respectively. Average H2/CO ratio was observed to be 0.9 MBC-PJ, 0.79 MBC-HC and 0.68 MBC-RH which shows an improvement to moderate ratio fuel. The study shows that at half-load and relative to neat diesel (ND), peak-pressure improved by 31.6% (MBC-PJ), 24.0%(MBC-HC) and 14.6%(MBC-RH). Additionally, peak-pressure increases as load increases and shifts to the right of top-dead-centre with reported increase of 13.1% MBC-PJ, 15.4% MBC-HC, 18.3% MBC-RH and 16.5%-ND. Moreover, net heat release rate (NHRR) in J/degree increased rapidly at 15-25o after/TDC for all loads and also increased as the load increased with values of 33.4 (HC), 26.8 (ND), 28.8 (RH) and 37.8 (PJ) at no load and 35 (HC), 27.8 (ND), 30 (RH), and 38.9 (PJ) at full load reported. The maximum attained diesel replacement rates were 78.5%-HC, 75.5%-PJ and 72.0%-RH at three-quarter load. Brake thermal efficiency (BTHE) increases as brake power (BP), brake load and brake mean effective pressure (BMEP) increases. Maximum BP recorded was 2kW with corresponding optimal efficiencies of 13.97%-PJ, 13.86%-ND, 13.34%-HC and 13.08%- for RH. Optimal BTHE with corresponding optimal BMEP was noted to be 13.97% (at 3.11MPa), 13.86% (at 2.94MPa), 13.34% (at 3.11MPa), and 13.08% (at 3.15MPa) for PJ, ND, HC and RH respectively. At three-quarter load, the maximum observed BTHE were 13.68%-PJ, 12.56%-HC, 12.56%-RH, and 12.88%-ND. Additionally, brake specific fuel consumption (BSFC) decreases sharply as power and load increases. At BP below 0.8kWh, RH reported highest BSFC followed by HC, ND and PJ the least. After 0.8kW and half-load, the BSFC is constant at 0.61-0.68kg/kWh. Air-Fuel ratio decreases as the BP increases with PJ reporting 50.86 (at 1.41kW), ND-50.86 (at 1.46kW), RH-50.85 (at 1.48kW), and HC-48.54 (at 1.51kW). Emissions were noted to be lower and decreased as the load increases, except for sulphur dioxide and NOx. Carbon monoxide and carbon dioxides emissions relative to diesel were higher in a range of 46.5-80.2% and 3.3-18% respectively. Hydrocarbon emissions depends on diesel replacement rate and were noted to be 480 and 1250ppm for ND and upgraded-syngas, respectively. Exhaust temperature were higher in upgraded-syngas and increased as the load increased in the range of 455.83-480.03oC which influenced NOx formation. Diesel NOx were higher, ranging from 32.5-40.5%, compared to those from upgraded-syngas at three-quarter load. Sulphur dioxide were lower in a range of 23.7-57.1% relative to ND at half-load. The optimal blend reduces pollution, ash content, increases CV, and enhances syngas-fuel properties to moderate ratio. In terms of engine characteristics, the novel fuel is able to keep in pace with ND at replacement ratio of 72.0-78% and low emissions. To achieve this, the engine requires minimal modification and retrofitting such as inclusion of T-pipe, advancing injection timing to 25.20 before top dead centre and increasing compression ratio to 17.5. MBC-PJ ranks the best followed by MBC-HC and lastly MBC-RH.