Influence of oxygen content of the certain types of biodiesels on particulate oxidative potential
Graphical abstract
Introduction
Biodiesel is made of vegetable oils and animal fats and therefore is well known as a renewable source. It has become increasingly important in recent years due to its nature, and because it is more environmental friendly than fossil diesel (Lapuerta et al., 2008a, Lapuerta et al., 2008b). Furthermore, it has a high combustion efficiency of 90–100%, depending on the chemical structure. Biodiesel can be used as either the neat fuel or blended with petroleum diesel. Engine performance and emissions generated using biodiesel is dependent on many factors including: engine load condition; engine type; and engine operation temperature. The majority of published studies have reported lower production of hydrocarbon (HC), PM and CO emission, but higher NOx emission (Lapuerta et al., 2008a, Lapuerta et al., 2008b). It has also been found that, in diesel/biodiesel blend, particle emissions reduce consistently with fuel oxygen content (Rahman et al., 2014). The presence of oxygen in biodiesel molecules causes more complete combustion, promoting the oxidation of the formed soot (Flynn et al., 1999). Study of Ullman et al. (1994) shows that 1% increase in oxygen content leads to 6–7% PM reduction (Ullman et al., 1994). On the other hand, application of biodiesel leads to an increase in emitted NOX and oxygenated hydrocarbons (Staat and Gateau, 1995, Lapuerta et al., 2008a, Lapuerta et al., 2008b). The production costs for biodiesel synthesis can also be very high (Bender, 1999).
A number of studies have investigated the toxicological potential of biodiesel and given warning about replacement of diesel with biodiesel (de Brito et al., 2010, Kooter et al., 2011). The chemical composition of the PM coming from diesel engine running on biodiesel is highly linked to the chemical structure of the fuel. The presence of oxygen atoms in the molecular structure of the fuel plays an important role in changing the levels of the oxygenated toxic species (Lapuerta et al., 2008a, Lapuerta et al., 2008b).
Among the various types of biodiesels, waste cooking biodiesel is the most economical as it is a waste product. As the oxygenated fuel, it is being explored as a potential replacement for diesel in compression ignition engines (Lapuerta et al., 2008a, Lapuerta et al., 2008b, Lin et al., 2011). Lapuerta et al., 2008a, Lapuerta et al., 2008b observed a significant reduction in particle mass emissions while using different blends of waste cooking biodiesel with neat diesel.
Furthermore, methanol and ethanol (Song et al., 2008, Surawski et al., 2009, Sayin, 2010, Yilmaz, 2012) are often used as diesel fuel alternatives in diesel engines. The major problem in ethanol/diesel blends is that it is immiscible with diesel at room temperature. Increasing the amount of ethanol causes a reduction in cetane number and lubricity of these fuel blends (Fernando and Hanna, 2004). Hence, butanol has been drawing the attention of researchers (Szwaja and Naber, 2010, Altun et al., 2011) due to its ability to blend with diesel. Butanol has a higher energy, lower auto ignition temperature and higher cetane number than methanol and ethanol, and thus, is recognised as a more suitable alternative for diesel fuel. Furthermore, butanol is reported as a very promising biofuel as it emits less CO, NOx and soot (Rakopoulos et al., 2010a, Rakopoulos et al., 2010b, Arslan, 2013).
Additives can affect biodiesel production, performance and emission characteristics (Rao and Rao, 2012). Compounds such as triacetin (T) perform as an anti-knocking agent in biodiesel. Triacetin is also able to increase the density and viscosity of the fuel, and to some extent improve the cold flow properties and oxidation stability (Saka and Isayama, 2009) while it can cause decreasing of the flash point, heating value, and cetane number (Casas et al., 2010). 10% of triacetin blended with 90% of coconut oil biodiesel resulted in significant improvement in engine performance (Rao and Rao, 2012).
This study investigates the outcome of using different biodiesels blends, with various ranges of oxygen content, by monitoring the reactive oxygen species (ROS) concentration. To investigate this, two ROS assays were employed. Dithiothreitol (DTT) assay basically allows the determination of particulate ability to catalyse the electron transfer to the oxygen resulting in the superoxide generation. This chemical based assay has been used for measuring the oxidative potential of the ambient PM (Li et al., 2003), diesel exhaust (Kumagai et al., 2002) and ageing process of PM (Li et al., 2009). The second assay, 9,10-bis (phenylethynyl) anthracene nitroxide (BPEA-nit), is a poorly fluorescent compound which turns highly fluorescent upon exposure to Reactive Oxygen species (ROS). ROS is a collective term for various free radicals, molecules and ions. In this case the most important ROS are the superoxide radical (O2), hydrogen peroxide (H2O2), hydroxyl radical (OH), alkoxyl radical (RO), carbon-cantered radicals (R) and singlet oxygen (O2). This assay has been previously applied to detect the ROS related: to combustion-generated aerosols including cigarette smoke (Miljevic et al., 2010a); biomass combustion in a pellet boiler and a logwood stove (Miljevic et al., 2010b); diesel exhaust (Surawski et al., 2009, Stevanovic et al., 2013); and biodiesel exhaust (Stevanovic et al., 2013, Pourkhesalian et al., 2014).
This study has investigated the relationship between the oxygen content of different biodiesels and the oxidative potential (OP) of the created PM2.5. In the scope of this work, we aim to use two well-established ROS assays in order to evaluate their performances in measurements of particulate OP and also their dependency on the oxygen content of the fuel. This study makes an overall assessment for the application of each probe and does not focus on the chemical details of each probes detection procedure.
Section snippets
Engine
A Euro III Cummins engine was operated to generate aerosols. Details on the engine specification are similar as the study by Stevanovic et al. (2013). It was run in the European Stationary Cycle, ESC (DieselNet, 2015). Fig. 1 shows the schematic of the experimental setup.
The ESC test takes 28 min in total. In this cycle, the engine must work for the specified time in every single mode, in order to complete engine speed and load changes in the first 20 s. The particular speed needed to be held to
Results and discussion
The use of biodiesel either neat or blended with diesel can provide reductions in particulate matter (PM) emissions. This reduction has been reported by a number of studies (de Brito et al., 2010, Jalava et al., 2010, Kooter et al., 2011) which investigated the influence of biodiesel on the engine emissions. The reason for this is found in the shorter carbon chain length of these fuels which reduce the amount of the soot formed (Graboski and McCormick, 1998). Soot formation is favoured by high
Conclusion
The results obtained in this study emphasise that biodiesel can potentially cause more toxic emissions than neat diesel. The measured potential is mostly related to the presence and quantity of oxygenated organic compounds in PM. Despite having a lower total mass emission, tested biodiesels caused emissions with higher OP expressed per unit mass of particles. It was found that this effect is directly related to the chemical composition of tested fuels. The presence and quantity of oxygen in
Conflict of interest
The authors declare no competing financial interest.
Acknowledgement
The authors would like to acknowledge the support from the Australian Research Council Discovery Grant (DP120100126). The contribution of Mr. MD Farhad Hossein in the campaign and laboratory assistance from Mr. Noel Hartnett was also appreciated.
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