Experimental study of ignition, lift-off length and emission characteristics of diesel/hydrogenated catalytic biodiesel blends
Introduction
Due to high thermal efficiency, durability and robust construction of diesel engine, it is widely applied in modern transport systems [1], agricultural machinery [2] and other fields [3], [4]. However, diesel engines are producing high levels of PM and NOx emissions, which bring serious environmental problems and health issues [5]. Hence, the climate deterioration, environmental pollution and stricter emission regulations have promoted the study on clean alternative fuels [6], [7].
Biodiesel can provide a lot of benefits, including the reduction of exhaust gas emissions, national energy security and the development of biodiesel industry especially to the developing countries [8]. The 1st generation biodiesel (fatty acid methyl esters, FAME) which is produced from different kinds of edible oils such as sunflower seed oil, soybean oil, coconut oil and peanut oil via the esterification and transesterification reactions of free fatty acids (FFAs) [9], [10]. But these kinds of materials to produce biodiesel fuel and its application in the engine market are suspended because edible oils are not sufficient to satisfy the local requirements of the food sector in many developing countries.
The 2nd generation biodiesel (SGB) produced from non-edible feedstock like, animal fats, waste cooking oils (WCO) and biomass sources [11], [12] has become more attractive for the sustainable production of biodiesel. However, the cost of production of the traditional SGB increases significantly because of using hydroisomerization reactions to reduce the pour point of SGB which hinder its industrialization [13]. Therefore, finding a new way to improve the economy of biodiesel production is the key issue for commercial production on an industrial scale. Thus the HCB, obtained from hogwash oil which was collected from the waste cooking oil as a raw material using one-step hydrogenation method to remove oxygen and sulfur, has more potential to apply in engines.
The chemical technology producing HCB has the following hydrogenation process; catalytic hydrodesulfurization and catalytic hydrodeoxygenation. Propane, light naphtha and water are produced as by-products. A comparison between FAME and HCB was shown in Table 1. It can be seen that the benefits of these reactions are lower cost of catalyst and higher oxygen removal capabilities. These processes remove the hydroisomerization reaction which results in the cetane number and condensation point are higher than traditional SGB. It also indicates that the HCB needs to blend with diesel before the application of this fuel into the diesel engine directly. However, the cost for producing HCB is cheaper than the traditional SGB which means that the HCB has a great potential for industrialization and commercialization. Moreover, more waste cooking oil is produced into HCB will reduce the hogwash oil into cooking oil which has a significant effect on food safety. The molecular structure of HCB is similar to diesel fuel. Besides, HCB is free from oxygen atom in the molecules which can reduce NOx emission than the traditional biodiesel. Unfortunately, the PM, CO and HC emissions may be increased comparing with traditional biodiesel. Hence, it is necessary to research the auto-ignition, combustion and emissions characteristics of HCB before large proportion application it in diesel engines because the molecular structure and fuel properties of HCB are different from traditional biodiesel.
Since it is difficult to control the environment due to cycle-by-cycle variations and complex engine geometry, studying the ignition and combustion characteristics of fuels using a constant volume combustion chamber (CVCC) is an efficiency choice. The optical setup can provide more fundamental information for understanding combustion process [14], [15]. The optically constant volume combustion chamber is a useful tool to reproduce the engine ambient conditions with good repeatability. Moreover, the parameters such as ambient temperature, ambient gas density and oxygen contents can be adjusted easily [16]. Auto-ignition directly affects the combustion processes which have a significant influence on combustion efficiency and pollutant emissions. The LOL of fuel jets is defined as the distance between the nozzle tip and the stabilized reaction zone after the time of auto-ignition, has a strong effect on downstream soot formation processes [17], [18], [19]. The ignition and combustion characteristics for various biodiesel fuels were researched in a constant volume combustion chamber [20]. Matthias et al. [21] investigated the ignition and combustion behavior of vegetable oils which can be used as fuel in combustion engines using a constant volume combustion chamber. The results show that vegetable oils with a high ignition quality are characterized by a low amount of double bonds of the fatty acids. Huang et al. [22] researched the ignition and combustion characteristics of n-pentanol-WCO biodiesel blends in a constant volume combustion chamber. The results reveal that the biodiesel has a similar or even shorter ignition delay than n-pentanol blends in low oxygen low-temperature conditions. However, the research of ignition and combustion characteristics of HCB which produced from waste cooking oil using hydrogenation process in a constant volume chamber is very limited. It also should be pointed out that the constant volume combustion chamber cannot simulate the engine intake and exhaust processes. Hence, the combustion and emission characteristics for application of biodiesel need to be researched in engines.
Choongsik et al. [23] discussed the combustion and emission characteristics of WCO biodiesel in a compression ignition engine. The results indicate that WCO biodiesel had the benefits in CO, HC and PM reduction at low load and conventional diesel operating conditions while the NOx emissions were increased with WCO biodiesel. Moreover, in the high engine load and early injection timing conditions, the emission characteristics of WCO biodiesel were worse than those of diesel. The performance, combustion and emission characteristics of pure biodiesel derived from WCO and diesel were examined in a Euro IV diesel engine by Yang et al. [24]. The studies have shown that a slightly shorter ignition delay and lower peak heat release rate were found for biodiesel compared to pure diesel and a slight reduction in the major emissions such as HC and NOx were observed with the use of biodiesel. Attia et al. [25] investigated the influence of diesel fuel blended with biodiesel produced from waste cooking oil on diesel engine performance. The results reveal that the most recommended waste cooking oil methyl ester biodiesel blending ratios vary from 30% to 50% for better engine performance and emission characteristics. However, these biodiesel fuels are produced via trans-esterification process with an oxygen atom in its molecular structure which has a significant negative impact on diesel engines. Since HCB, is produced by hydrogenation process, has different molecular structure of chemistry compared to the presently well-known biodiesels (FAME), it is worthwhile to investigate the combustion and emission behaviors of this fuel for future applying it in engines.
In this paper, an extensive research was done to study the differences between diesel/HCB blends and conventional diesel fuel to provide fundamental data for building the simulated model. Two different blends have been researched under different ambient temperatures and oxygen concentrations in CVCC to simulate the in-cylinder conditions with low temperature combustion and EGR. By using two optical techniques (natural flame imaging method and OH chemiluminescence imaging method), the ignition and combustion information were analyzed for HCB blends and pure diesel, such as ignition delay, initial ignition position and flame lift-off length. Then the emission characteristics between B20 and pure diesel were researched for further applying in diesel engines.
Section snippets
Constant volume combustion chamber
Fuel ignition and combustion characteristics were examined in a pre-heated constant volume combustion chamber which was used to simulate the in-cylinder conditions of a diesel engine at the time of fuel injection. This test chamber can maintain the stationary conditions with the maximum ambient temperature of 1000 K and a maximum pressure of 10 MPa. It is equipped with four large windows (100 mm in diameter) placed orthogonally with its thickness of 60 mm. A single-hole Bosch common-rail fuel
Ignition delay
Fig. 7 shows the natural flame images of three tested fuels at the ambient temperature of 820 K, an injection pressure of 100 MPa, the oxygen concentration of 15% and ambient gas density of 21 kg/m3. It can be clearly found from the ignition process that the ignition delay decreases with increasing blend ratio of HCB in the pure diesel and it means that HCB has a positive effect on ignition process. This is because HCB has a high cetane number compared to the pure diesel. In order to further
Conclusion
To clarify the ignition, combustion end emissions characteristics of Hydrogenated Catalytic Biodiesel under different conditions, two optical technologies were applied in a constant volume combustion chamber and the emission characteristics of the Hydrogenated Catalytic Biodiesel blend with diesel fuel (B20) were conducted on a four cylinders diesel engine. The following conclusions can be drawn from this work:
- 1.
The ignition delay and flame lift-off length decrease with increasing ratio of
Acknowledgments
Funding for this research was supported by the National Natural Science Foundation of China (No. 51706088, 51776088, 51876083) and this work also was provided by the China Postdoctoral Science Foundation (2018M632113).
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