CONCLUSIONS AND FUTURE SCOPE
9.3 Scope for Future Work
In this investigation, the independence to foreign petroleum diesel fuel is demonstrated by using two alternative gaseous fuels, biogas and syngas, in a compression-ignition diesel engine under dual fuel mode. Also, the important CDM potential of these fuels has been verified. Most of the important findings have been highlighted in the section 9.1. However, the shortcoming of dual fuel operations can be mastered by for further developments in this area. In this connection, some scope and suggestions for further studies are discussed here.
Dual fuel mode showed poor thermal efficiency due to (mainly) less energy conversion during the operation. Thus, a waste exhaust gas heat recovery system can be added with the dual fuel engine operation. In this way, a dual fuel mode converts to a combined heat and production (CHP) mode (also called cogeneration) where the recovered heat may be used for the heating purpose. The dual fuel operations produced higher heat losses because the bowl-in combustion chamber which is originally meant for diesel operation with inherent swirl. Hence, the engine combustion chamber modifications can be looked upon to reduce the heat losses.
At low loads, dual fuel operations revealed a maximum reduction in efficiency as compared to diesel mode. This is because of the poor combustion of high self ignition temperature bearer gaseous fuel under low temperature domain. Hence, a gas to gas heat exchanger may be incorporated in between exhaust gas muffler and inlet manifold where the required heat transfers to the sucked inlet air-gas mixture to provide the high temperature environment for an improved combustion of fuel-gas. In a different way to heat exchanger system, a spark plug or a glow plug may be merged with the inbuilt fuel injection system in which some extra ignition energy can assist the combustion of gaseous fuels.
The biogas dual fuel modes reached their cycle peak pressure shifted by about 140 CA further towards expansion stroke than the diesel mode. While, the attainment peak pressures lag by about 6 to 8° CA for 100 and 75% H2 content syngas modes, and about 10 to 15°CA for other modes as compared to diesel mode. This affects lower peak cylinder pressure and temperature, and hence, resulted poor performance of dual fuel operations. Therefore, the ignition timing of dual fuel modes should be adjusted as per the primary fuel-gas composition and fuel-gas mass share to derive maximum shaft output and efficiency for an equal energy input.
The lower calorific value and the lower (less than one) product to reactant mole ratio for gaseous fuels contribute to de-rating of dual fuel engine output. in addition, the poor combustion of gaseous fuels under low temperature domain leads to poor thermal efficiency at part-loads. However, it might be possible to reduce the de-rating of dual fuel modes by working with even higher compression ratio (CR) to that with base diesel engine. The engine CR for dual fuel gas operation can be limited by presuming the knock occurred during the higher CR operation.
During dual fuel combustion, a smaller amount of pilot fuel is surrounded by much more volume of air-gas mixture, and hence, ignition delay is increased for the pilot fuel and the dual fuel mode as well. Therefore, some ignition improvers like amyl nitrate and iso-propyl nitrate can added to the pilot diesel which has the effect of further lowering the self-ignition temperature of the fuel but more importantly reducing the lag time for ignition to commence once this temperature is reached. In another means, some oxygenated compounds such as, diethyl maleate, dibutyl maleate and diglyme can be blended with pilot fuel which enhances the oxygen concentration for a better combustion environment.
For the biogas dual fuel modes, in particular, the presence of high volume of CO2 in the fuel composition reduces the engine performance along-with higher emissions of CO, CO2 and HC levels. An online technology like CO2 sequestration from the pre- combustion biogas can be fit to this problem. In this method, the biogas may be passed through CO2 absorbing elements like; 4A (pore size 4 angstroms (0.4nm)) molecular sieves, potassium hydroxide (KOH) solution or calcium hydroxide (Ca(OH)2) solution before supplying to the gas carburetor for mixing with sucked air.
The syngas used in this work is simulated by adding H2 and CO from two individual high pressure gas cylinders. Still, using real syngas as primary fuel, the dual fuel operations may be tried in diesel engines for the comparison of its engine performance behavior to that of simulated syngas case. Again, during the dual fuel operations, the measurement and control of syngas was performed by flow meters of rotameter type. Although this was done very precisely and carefully, still there may be chances of H2 leakage. Hence, mass flow controller may be used in place of rotameter for a better measurement and control of syngas, and accurate estimation of flow data.
It has been seen that higher H2 content dual fuel operations produce higher NOx levels as compared to diesel mode. Reformed exhaust gas recirculation (REGR) is a technique in which of a catalytic fuel reformer is incorporated in the EGR loop to produce energy-rich gaseous fuels such as H2, CO and CH4 in addition to CO2 gas. It can be achieved by reacting engine fuel with engine exhaust gas in the presence of solid catalyst. This type of dual fuel operation can improve the engine performance and fuel economy from energy-rich gases. In addition, there will be lower NOx values due to feeding of CO2 gas through REGR process.
The most critical emissions from diesel engines are particulate matter and smoke. In
unavailability of equipments. Therefore, these two all important emissions should be considered between diesel and dual fuel modes for a complete emission comparison.
In this present experimental investigation, all the completed dual fuel operations performed well in short-term tests. However, the problems are anticipated when these engines operations are performed for long term commercial scale. These problems may include corrosion of engine components, carbon deposits on engine surfaces, engine lubrication, and excessive wear etc. Thus, before adopting usage of biogas and syngas in diesel engines under dual fuel mode as successful technology, long-term dual fuel tests of these fuels are essential.
In order to promote the application of biogas and syngas as the alternative fuels for diesel engines, a comprehensive analysis of the economic assessment of their dual fuel operations is required from a social viewpoint. Therefore, a cost analysis of the dual fuel operations can be executed by considering various economic terms such as, the fuel cost per kW power production, environmental externalities and other costs.
The performance and emissions study merged with the economic assessment can provide a perfect competitive characteristic of the dual fuel modes as the alternative fuels for diesel engines.