: Use of Biodiesel Fuels in Diesel Engines
Table of Contents:
Anh Tuan Hoang
Van Viet Pham
Xu an Phuong Nguyen
Owing to their continuous depletion, fossil fuels are currently an unsustainable source of energy. They also trigger problems for the environment such as global warming and climate change. Therefore, biobased fuels with renewable, nontoxic, biodegradative characteristics should be an ecofriendly choice to replace fossil-based fuels. Biobased fuels could be considered bioproducts generated from biosources either for transportation or for combustion, which can be extracted from agricultural or forest products and which can even originate from the reusable and biodegradable portion of industrial and municipal wastes. Based on an analysis made at the end of 2017, around 50% of total renewable energy came from renewable bioenergy, which delivered four times the overall amount from the sun. solar cells, and wind power. Biofuels are forecast to lead the growth in the consumption of renewable energy in the period 2018-2023. Indeed, nearly 30% of the increase in bioenergy is bound to come from advanced biofuel generation, in liquid, solid, or gaseous form, which is believed to be suitable for use as fuel in internal combustion engines.
Biofuels are viable renewable energy supply sources because of their ecofriendly properties. Biodiesel is divided into four categories depending on the type of input feedstocks, according to the ‘European Academies Science Advisory Council' (EASAC). Biodiesel is derived from first-generation edible oils, second-generation nonedible oils, and third-generation waste oils, which are naturally renewable and also locally found. Biodiesel of the fourth generation requires synthetic biology technology, though the research is still at the infancy level. As shown by the ‘American Society for Testing and Materials’ (ASTM), monoalkyl esters of long-chain lipids are labeled ‘biodiesel’ and classified as Bl00 and are generated using methanol and a catalyst through the process of transesterification of triglycerides, in which edible oils, nonedible oils, and waste oils or animal fats are considered as suitable feedstocks for producing ‘fatty acid alkyl esters’ (FAAE). In a transesterification reaction, there are three categories of catalysts, namely strong alkaline, strong acid, and enzyme, which are usable, as well as methanol, a type of alkanol, which is used in most cases to manufacture biodiesel. Therefore, biodiesel is also referred to as ‘fatty acid methyl ester’ (FAME). In the biodiesel production process, glycerol (glycerin) is generated as a by-product.
Regarding the use of biodiesel for engines, there are a few drawbacks associated with higher ‘nitrogen oxides’ (NOX) levels, low-temperature cold start issues, inferior calorific value, considerable copper strip degradation, as well as fuel piping challenges caused by high viscosity. For cold climates, the main concerns when using biodiesel regards cold starting and quality issues. Several other drawbacks relating to the use of biodiesel for diesel engines are shown in Figure 16.1. The aim of this chapter is,
Poor low temperature fluidity
Auto-oxidaton of fuel, Swelling of rubber by oxygen
A uto-oxidation offuel
Deposit of glycerin and alkali catalysts in fuel
FIGURE 16.1 Drawbacks relating to the use of biodiesel for diesel engines.
therefore, to offer a review of using biodiesel as an alternative fuel regarding engine efficiency, engine performance, combustion and emission characteristics, corrosiveness, and the accumulation of deposits in engines.
Advances in the quality of biodiesel are growing at a global level. Since biodiesel is made from various plants, its properties and qualities are different. Due to this reason, the standardization of biodiesel quality is essential to maintain the best possible efficiency of the engine. Some of the critical chemical and physical attributes of the fuel are kinematic viscosity (mm2/s), oxidation stability, glycerin (% mass), cetane number (CN), sulfur content (% mass), pour point (°C), acid number (mg KOH/g oil), boiling point (°C), cloud point (°C), flash point (°C), density (kg/m3), heating value (MJ/kg), ash content (% mass), water and sediment quality, copper corrosion, distillation rate, and carbon residues. The physicochemical properties of biodiesel fuels with standard limitations according to the test methods are described in
Physicochemical Properties of Biodiesel Fuels with Standard Limitations
Viscosity is the most crucial fuel parameter because it signifies the capacity of a liquid substance to circulate; hence it involves the function of the fuel injection devices as well as spray atomization, especially at low temperatures, because viscosity is inversely proportional to temperature. The viscosity of biodiesel is two to four times higher than that of petrodiesel. This is because of its massive molecular mass and complex chemical structure. The standards ASTM D445, EN ISO 3104, or ISO 3104/ P25 are used for assessing the kinematic viscosity of biodiesel fuel, which varies from
1.9 mm2/s up to 6 mm2/s as shown by ASTM requirements, and scales from 3.5 mm2/s to 5 mm2/s (Yusuf et al., 2011) under the ‘European standards’ (EN) requirements.
Density is a physical property that is used to measure the exact quantity of fuel required to ensure sufficient combustion. Biodiesel is typically denser and less compressible than petrodiesel, which thus poses one of the most significant barriers toward widespread biodiesel use. Biodiesel density can be determined according to the specifications ASTM DI298 and EN ISO 3675. Based on such standards.
biodiesel density is tested in a temperature range of 15°C to 20°C (Torres-Jimenez et al., 2011). The variation of densities for biodiesel generated from various feedstocks is typically between 832 kg/m3 and 982 kg/m3.
The ‘flash point’ (FP) is the temperature at which, when exposed to flames, the fuel will fire; the volatility of the fuel is inversely proportional to the FP. This is the method defined in ASTM D93, EN ISO 3679, and P21 for assessing the FP. Biodiesel has an FP higher than l50°C while petrodiesel fuel has it at 55-66°C. Biodiesel’s FP is greater than the permissible petrodiesel limit for safe transport, processing, and storing purposes. However, the FPs of biodiesel are considerably smaller than those of vegetable oils. The FP reaches a maximum in ASTM D93 at 90°C, and is 120°C in EN ISO 3679 (Antolin et al., 2002).
The boiling point (BP) is the temperature at which the vapor pressure of the element equals the surrounding pressure. If any element has a greater BP than this it is a sign of that element's lower volatility. The BP is dependent on the type of bond between the element's molecules. Gas chromatography is applied with the assistance of the ASTM D7398 standard to figure out the range of the BP, which is 100-615°C.
Cloud Point, Pour Point, and Cold Filter Plugging Point
An important quality criterion is the attributes of biodiesel at low temperatures. Partial or entire solidification of the fuel can trigger fuel lines and filters to be blocked, resulting in fuel starvation, difficulties starting, and damage to the engine because of inadequate lubrication.
Biodiesel's ‘cloud point’ (CP) is the degree at which wax crystals become observable when the biodiesel is chilled. Different feedstocks are used in the manufacture of biodiesel, and these have various fatty acid compositions, resulting in CP differences in the generated biodiesel. In ASTM D2500, the standard framework for assessing the CP (for biodiesel) is defined within the range of -3°C to 12°C (Moser and Vaughn, 2010a).
The ‘cold filter plugging point’ (CFPP) is used for testing the fuel's usability in cold flow. The CFPP is the minimum temperature at which a normal, precise filter discharges the sample fuel. The CFPP and CP are used for defining the filterability limit; the sample fuel CFPP value is less than the CP. The standards of ASTM D6371 and EN-14214 are applied to the biodiesel CFPP.
The ‘pour point’ (PP) is the temperature at which the solution's total wax is adequate to gel the liquid, hence it is thus considered as the minimum temperature at which the fuel can flow at. ASTM D2500. EN ISO 23015, and D97 processes are used for calculating the PP. Compared with petrodiesel, biodiesel has a greater PP.
The ‘cetane number’ (CN) is one of the essential parameters identified during the selection process of biodiesel as an alternative fuel. The CN is the element that directly affects the delay in ignition. Normally, the superior quality of the ignition is always linked to an increased CN value. The higher CN indicates the fuel's capacity for self-igniting more quickly upon delivery into the combustion chamber. Biodiesel has a greater CN than petrodiesel, suggesting greater efficiency in combustion. The CN of fuels defined by ASTM D613 and EN ISO 5165 is 47 min and 51 min, respectively (Lapuerta et al., 2008).
The ‘heating value’ or ‘calorific value’ (CV) of the fuel represents the amount of energy that is released when the fuel quantity is burnt. Fuel with a higher CV is desirable for an internal combustion engine. Biodiesel CV has been reported to be lower than that of petrodiesel fuel because of the 10-11% oxygen content in it (Moser, 2009). The CV is not designated in the biodiesel specifications ASTM D6751 and EN-14214, but it is recommended in EN-14213 (for heating purposes) with a minimum value of 35 MJ/kg (Skagerlind et al., 1995).
Stability of Oxidation
The stability of oxidation is a vital parameter for measuring the extent of the biodiesel response to air and the oxidation level. The quantity of bis-allylic sites found in unsaturated biodiesel substances affects the fuel's ‘oxidation stability’, which is determined by the biodiesel period, the composition of FAME, and the storage conditions. The molecular structure of biodiesel fuels makes it more degradable than petrodiesel fuel to oxidative depletion. Adding additives into biodiesel enhances and improves its quality. The ‘Rancimat method’ (EN ISO 14112) is the specification for oxidation stability in standards ASTM D6751 and EN-14214. For ASTM D6751, a minimum stability of oxidation at 110°C is 3 h. while in EN-14214 a more stringent limit of 6 h or more is set (Moser and Vaughn, 2010b).
The ‘acid number’ (AN) is characterized by the amount of ‘free fatty acid" (FFA) contained in a fuel sample. The FFA would be the saturated or unsaturated monocarboxylic acid normally found in fats, oils, or greases. The greater acid value corresponds to a larger quantity of FFA. A higher AN causes the problem of corrosion in the engine's fuel injection system. The acid value is measured by mg ‘potassium hydroxide’ (KOH) essential to neutralize I g of FAME. Acid values are measured using ASTM D664 and EN-14104. Both criteria accept a total acid content of 0.50 mg of KOH/g for biodiesel (Agarwal, 2007).
The ‘iodine number’ (IN) is an indicator of the number of double bonds present in biodiesel, and is used to determine the extent of ‘biodiesel unsaturation'. This IN index greatly affects the oxidation stability and the polymerization of glycerides as well as deposit development in diesel engine injectors. The IN has been found to be strongly associated with the viscosity of biodiesel, the CN, and the CFPP. ASTM D6751 does not have an IN criterion, although EN-14111 specifies a total IN of 120 mg 1,/g (Atabani et al., 2013).
The ‘carbon residue' (CR) of the fuel reflects the amount of carbon depositing fuel after burning. The CR is associated with FFA, more unsaturated fatty acids, glycerides, polymers, and other inorganic impurities. The limited range of the CR standard on the basis of ASTM D4530 is a maximum of 0.05 % (m/m) and of EN ISO 10370 is a maximum of 0.30% (m/m) (Murugesan et al., 2009).
Sulfate Ash Content
The ‘sulfate ash content’ comprises synthetic pollutants such as abrasive solids, catalyst residues, and the concentration of soluble metal soaps present in a fuel. The amount of sulfate ash in biodiesel is lower than that of petrodiesel (0.02% m/m). Biodiesel would thus not pose any challenges when used as a fuel in diesel engines. Additionally, the level of sulfur oxide emitted during combustion is dependent on the level of sulfur in the fuel available. Biodiesel fuels derived from vegetable oils have less sulfur in comparison to petrodiesel. The maximum sulfate ash content standard based on ASTM D874is 0.002% mass and EN ISO 3987 is 0.02% mass.
Water and Sediment Content
The purity of the biodiesel is reflected by the content of water and the sediments available in it. Biodiesel is generally regarded as insoluble in water absorbing far more water than petrodiesel fuel. Biodiesel can include up to 1,500 ppm of soluble water while petrodiesel fuel typically absorbs only around 50 ppm (Atabani et al., 2012). With the occurrence of water in biodiesel, its CV is reduced; the existence of water is considered as one of causes of corroding engine components. The content of water and sediments is evaluated on the basis of the standards ASTM D2709 and EN ISO 12937. The water content and the sediment for biodiesel requirements are a total of 0.05% volume under both mentioned standards (van Gerpen, 2005).
Copper Strip Corrosion
The assessment of the corrosion characteristics of copper strips is a qualitative approach to assessing ‘biodiesel corrosiveness’, which might be caused by some sulfur compounds; this parameter is thus correlated with AN. In a fuel bath, a copper strip is warmed up to 50°C for 3 h to evaluate the rate of corrosion as compared to standard strips (Atabani et al., 2012). ASTM DI30 and EN ISO 2160 standards are used to determine the corrosiveness of biodiesel to copper strips, the levels being class 3 and class 1, respectively.