Low-Temperature Combustion Technology on Biodiesel Combustion
Introduction
The modern world carries out its daily energy activities in different forms and the demand for energy continues to increase for obvious reasons such as demographic growth and the introduction of new technologies, which once again depend on the external energy source. Most of the energy used in day-to-day activities is extracted from fossil fuels (Nanthagopal et al. 2019). The energy extracted from fossil resources is non-renewable and anticipated to last for only a few years depending on the current use; also fossil fuels emit pollutants such as carbon dioxide. C02 and other toxic gases are the major causes of pollution and global warming. According to forecast statistics, there will be an increase in C02 emissions of 1.7% per year until 2030 (Laha and Chakraborty 2017).
In a developing country like India, most of the energy extracted from fossil fuel sources is through diesel engine combustion. Major fossil fuel resources are used in off-road and on-road vehicles, power generators, the agricultural sector, the industrial sector, and marine transportation (Vigneswaran et al. 2018). Diesel engines are the most prominent users of fossil fuels due to their better brake thermal efficiency, more affordable fuel consumption, lower brake-specific energy consumption, more rigid structure, and higher power delivery compared to gasoline engines (Dhinesh et al. 2016). Continuous increases in the diesel engine population and in the quantum of C02 emissions from diesel engines are primary reasons for global warming, acid rain, ozone layer depletion, and nitrogen oxides emission (Thiyagarajan et al. 2019). As per the Paris agreement, India as one of the parties has agreed to reduce its carbon footprint in order to maintain the global surface temperature rise below 2°C from the pre-industrial level (Schleussneret al. 2016). Hence, carbon neutral biofuels can be used in Cl engine to reduce engine emissions and decrease fossil fuel usage.
A biofuel is a different alternative to crude oil-based fuels, and it can be extracted from many sources such as vegetable oils, diluted oils, and bio fats. In fact, its biodegradable, sulfur-free, and non-toxic properties may serve beneficial roles in engine combustion. Particular attention has been paid to adjusting and replacing the conventional fuel resources in different countries (Elumalai, Annamalai, and Dhinesh 2019). Biofuels derived from biomass release exhaust gas that contains lower carbon emissions compared to fossil fuels, and it also produces lower hydrocarbons emission and particulate emissions due to its non-toxic and sulfur-free constituents. Despite the known benefits of using biofuels instead of fossil fuels, it is important to evaluate possible negative impacts on the environment during biofuel production (Naik et al. 2010). During the research and development (RD) stage of processes, evaluation of the biofuel synthesis pathways is essential to eliminate the undesired processes according to predetermined criteria and objectives (Jayswal et al. 2011). To date, several analyses on the production of biofuels have been carried out around the world based on different aspects, such as economic performance, environmental impact, and social development (Jaeger and Egelkraut 2011). That information provides an in-depth understanding of the present research gap in biofuel production.
In addition, extensive research has started to continually improve technologies and assessment tools for cleaner biofuel production (Klein-Marcuschamer et al. 2010). During the evaluation of biofuel synthesis processes, some fuel upgrade processes have been identified as increasing production costs and global w'arming potential due to higher energy and catalyst consumption at the production and processing stage (Sims et al. 2010). Due to this fact, some biofuels are used in Cl engines without further refinement and processing, which results in poor physicochemical properties and combustion behavior. Poor kinematic viscosity and higher density were identified in biodiesel due to greater molecular strength between the bonds and higher molecular weight. These factors cause poor atomization as well as deposition and formation of coke in the throat of the piston, piston segments, and injectors (Hoang et al. 2019).
Different research has reported that the use of highly viscous biofuels like straight vegetable oils and biodiesel in a diesel engine emits higher smoke, CO, and HC emissions due to improper atomization (Che Mat et al. 2018). In general, fuel modification with diesel or cetane improver blending is the most common method for reducing viscosity. Previous w'orks have reported that biofuel blended w'ith diesel up to 20% decreased engine emissions, but beyond that limit, this type of blending affects fuel injection systems and increases engine emissions (Ramalingam, Rajendran, and Ganesan 2017; Abed et al. 2018). Advanced high-pressure injection systems like the CRDI system have been adopted for highly viscous fuel to improve the combustion efficiency and decrease the smoke and hydrocarbon emissions. The CRDI system improves the atomization rate and decreases the surface tension of the biofuel but also produces higher NOx emissions (Babu and Anand 2019). Diesel engine combustion is generally a lean-burn process. However, the diffused combustion phase is mainly stoichiometric combustion microscopically due to flame initiate and tends to propagate to towards the downstream of spray. Diffused phase combustion improves the combustion temperature and heterogeneous nature of the biofuel combustion in diesel engines but also increases NO emissions (Zheng, Reader, and Hawley 2004).
Different research has reported use of different additives, water injection (WI), emulsification technology (ET), exhaust gas recirculation (EGR), fuel injection pattern modification, geometrical modification in the combustion chamber, and low- temperature combustion strategy (LTC) for simultaneous reduction of smoke and NOx emissions for biofuel combustion. Among these various technologies, LTC mode is most efficient to decrease NOx and smoke emissions by 95% and 98%, respectively (Jiaqiang et al. 2017). Hence, this review elaborates on the highly viscous biofuels used in Cl engines with different emission reduction techniques previously used for emission reduction. Also, biofuels used in Cl engines under low-temperature combustion are evaluated in terms of the NOx and smoke trade-off.
History and Different Methods Used for Emission Reduction
Researchers have attempted biofuel synthesis from lignocellulose biomass (second- generation biofuels) using different techniques like catalyst cracking, hydrotreating, and pyrolysis for biofuel production. The second-generation biofuels have decreased emission percentages compared to first-generation biofuels (Sims et al.
2010) . However, these second-generation biofuels do not meet the essential fuel properties like viscosity and density and also contain water during the pyrolysis (Lee and Lavoie 2013). Hydrocracking and hydrotreating of vegetable oil, which produce comparable viscosity to diesel, are better compared to the other methods. However, consumption of catalyst and hydrogen increases the production cost (Kohli et al. 2019). The higher viscosity of the raw vegetable oil restricted the diesel engine operation to within certain limits. Incomplete combustion and higher smoke emissions were identified in the combustion of biofuel, due its higher kinematic viscosity and surface tension (Hellier, Ladommatos, and Yusaf 2015).
In current practice, raw vegetable oil has undergone a transesterification process to synthesize biodiesel, and the viscosity of the fuel is closer to that of diesel. In some respects, this transesterification process is time-consuming and expensive. Moreover, the major drawback for biodiesel is fuel stability. Fuel stability is influenced by different factors—temperature, light, and oxidation reactions—and biodiesel is more hygroscopic and highly corrosive compared to diesel. The maximum storage stability of the biodiesel was reported to be 6 months; after that it lost stability (Jiaqiang et al. 2017). Also, biodiesel production forms undesirable by-products like glycerol. Hence, preheating the raw oil before introducing it into the diesel engine will decrease the production cost and increase energy recovery. With preheating, biodiesel was found to be comparable to diesel fuel (Ndayishimiye and Tazerout
2011) . Similarly, different additives and combustion chamber modifications are also done improving biofuel combustion efficiency and reducing emissions (Cheung, Zhu, and Huang 2009). In recent days, the CRDI system has been adopted for improving the combustion efficiency of biofuels (Jaliliantabar et al. 2020). Detailed emission reduction technologies commonly used in Cl engine combustion are given in Sections 6.2.1 to 6.2.7.
Blending with Diesel
The main disadvantage of using straight vegetable oil on an existing diesel engine it that it decreases engine performance because of higher kinematic viscosity, which leads to improper atomization. The raw vegetable oil is blended with diesel to decrease the kinematic viscosity. Twenty percent raw oil mixed with 80% diesel may be the best option for running Cl engines without any major modifications (Chauhan et al. 2016). However, increased raw oil percentages in the blends have also been studied by several researchers. Ramadhas, Jayaraj, and Muraleedharan (2005) conducted an engine study using 20%, 40%, 60%, and 80% rubber seed oil blends with diesel. They reported a blending of 20% and 40% rubber seed oil performed well and comparable with diesel. The brake thermal efficiency was highest for 80% rubber seed oil. The smoke density of the 40% and 60% blends were similar to that of the diesel and the 20% blend was highest.
Another engine study reported that a Cl engine was operated with preheated and unpreheated raw rapeseed oil blended with diesel at 20% and 50%. They reported raw fuel and preheated fuel show' lower brake power compared to diesel. The unpreheated blend shows higher smoke density compared to the baseline value due to higher viscosity. In contrast, preheated oil emitted lower smoke compared with diesel because the viscosity decreased w'ith increasing preheated temperature (Hazar and Aydin 2010). The earlier works prove that the brake-specific fuel consumption (BSFC) of raw oil-diesel blends indicates the incremented BSFC compared to diesel. This is because the lower heating value of raw oil-diesel blends increases the fuel consumption to maintain the same power output. Also, brake thermal efficiency (BTE) decreased due to the increase in BSFC. The blend of 50% raw oil shows higher BTE at partial load and on par with diesel at full load (Hazar and Aydin 2010). Some research exhibits higher exhaust gas temperature due to higher viscosity and poor atomization, which undergoes delayed combustion. The emission characteristics of raw oil-biodiesel blends show increased CO and HC emissions due to a rich air- fuel mixture because of undesirable spray characteristics. The blend w'ith less than 50% exhibits lower smoke value; however, smoke increases with increase in blend ratio due to poor atomization (Agarwal, Kumar, and Agarwal 2008). The increase in blend ratio decreases the NOx emission significantly due to the decrease in peak cylinder temperature and the lower calorific value of the raw' oil-diesel blend.
