Assessment of oxygenated fuels for lowering NOx emissions of a diesel engine

The supply of petroleum sources is finite, non-renewable and, at the current rate of consumption, it will become severely depleted by 2050. Furthermore, the use of petroleum fuel increases greenhouse gas (GHG) emissions, leading to global warming which is harmful. Thus, there is an urgent need to find alternative sources of energy that are renewable, cost effective and can be produced in a sustainable manner. Non-edible feedstock biodiesels are a promising alternative fuel for reducing most petroleum fuel related environmental problems. They are attracting increasing attention due to their abundant availability and similar physicochemical properties as petroleum-derived diesel. This study carefully investigated six major non-edible vegetable oils (papaya seed oil, stone fruit kernel oil, jatropha oil, rapeseed oil, beauty leaf tree oil and waste cooking oil), that are locally available, out of 350 oil-bearing crops that could be potentially used to produce biodiesel. Four multiple criteria decision analysis (MCDA) methods with twelve physicochemical properties of biodiesel feedstocks and three different weightage (%) determination methods were used to rank these six feedstocks, with the view to find the best performing biodiesel feedstocks. The overall results show that the stone fruit kernel oil (SFO) was ranked as the best performing feedstock on the basis of engine performance amongst the six locally available feedstocks examined, papaya seed oil (PSO) came out as the second best, and the waste cooking oil was the worst performing biodiesel. Alkali catalysed transesterification reaction is the most widely used method for producing biodiesel from oil/animal fats due to its higher conversion efficiency in a short reaction time (30-60 min). The current study was undertaken to optimise the transesterification process for PSO and SFO with the view to increasing the efficiency of biodiesel conversion. A response surface method (RSM) based Box-Behnken design was employed to optimise biodiesel conversion processes for both PSO and SFO. Biodiesel conversion efficiencies of 96.5% and 95.8% were found for PSO and SFO at their respective optimum operating conditions. These PSO and SFO biodiesels were evaluated using a 4-cylinder, 4-stroke Kubota diesel engine. In general, both PSO and SFO blends decreased engine performance slightly compared to diesel as expected, however, SFO biodiesel blends gave about 3% better performance compared to PSO blends. On the other hand, PSO blends (20%) decreased most of the engine emissions by up to 34% except for an increase of about 5% in nitrogen oxide (NOx) compared to diesel. These emission performances are up to 14% better than the corresponding SFO emissions. Although the SFO biodiesel blends have slightly better engine performance than PSO biodiesel blends, the PSO biodiesel blends proved to be a better overall choice due to their excellent environmentally friendly attributes as they can reduce exhaust emissions to a great extent. Therefore, PSO was chosen subsequently to develop interactive relationships between three operating parameters of PSO, namely biodiesel blends, engine load, and engine speed and four responses of brake power (BP), torque, brake specific fuels consumption (BSFC), and brake thermal efficiency (BTE) for engine testing and emissions behaviour. Analysis of variance (ANOVA) and a statistical regression model show that load and speed were the two most important parameters that affect all four responses. The biodiesel blends parameter had a significant effect on BSFC. The engine load and engine speed were the two most important parameters that affect four of the responses (NOx, hydrocarbon (HC), particulate matter (PM) and carbon monoxide (CO)). In-cylinder peak pressures for PSO biodiesel blends were higher than for diesel irrespective of engine speed. Heat release rates of PSO biodiesel blends were found to be lower than for diesel due to lower ignition delays and lower caloric values of biodiesel. The maximum cylinder temperatures of PSO biodiesel blends were higher (3.73%) than that of diesel. To minimise the exhaust emissions, PSO biodiesel blends were mixed with two oxygenated additives, namely diethylene glycol dimethyl ether (diglyme) and n-butanol, to make ternary blends. These blends were tested for both engine performance and emissions. The addition of oxygenated additives increased the BP, torque and BTE values of PSO biodiesel ternary blends and it lowered the average BSFC by 0.5% and 17.7% compared with diesel and PSO blends (20%), respectively. PSO-diglyme-diesel ternary blend performed better than all other binary blends as well as the PSO-n-butanol-diesel ternary blend. The average reductions of HC, CO, NOx and PM of PSO-diglyme-diesel ternary blends compared with diesel were 32.4%, 61%, 0.64% and 47.4% respectively, whereas a 2.8% increase in carbon dioxide (CO2) emission was observed. The average increase of NOx, and CO2 for PSO blends (20%) compared with diesel were 4.1% and 4.5%, respectively. In conclusion, this study provided a solid base of new knowledge regarding biodiesel feedstock selection and optimisation techniques for PSO and SFO, assessed the suitability of PSO and SFO as alternatives to petroleum diesel and analysed how the emissions from these biodiesels could be reduced. These are very useful information for engine manufacturers, Government, stakeholders and policy makers to eliminate the lack of awareness of using second-generation biodiesel in Australia