: Qualitative Characterization of Biodiesel Fuels: Basics and Beyond

Niraj Kumar Md. Abul Kalam

Introduction

Energy is essential for the existence of mankind. The past few decades have witnessed a multifold upsurge in energy requirements globally. This increase is essentially driven by exponential growth in technology and intensive economic growth as well as an unrestricted rise in the population. Energy demand is expected to rise further with each passing year. It has been estimated by the US ‘Energy Information Administration' (EIA) hat the global energy requirement will increase by 28% from 2015 to 2040. Most of this growth is expected to come from the two Asian giants, China and India (EIA, 2017).

Conventional energy resources are under severe pressure to meet demand. Fossil fuels are a major source of energy. The most widely used fossil fuels by the industrialized and developing countries are crude oil, coal, and natural gas. Among these, crude oil is the most utilized for the conversion of energy, while the consumption of coal is second, follow'ed by natural gas. The prime consumer of petroleum fuels is world surface transport. The limited known petroleum reserves and the concentration of such reserves in certain regions of the world have created an undesirable situation. Among petroleum fuels, diesel is used in transportation and power generation. The overbearing dependence on fossil fuels has created a disquieting situation particularly related to trade and the environment. The unwanted pollutants from ‘compression engine’ (CI) engines are the oxides of carbon (CO_S), the oxides of nitrogen (NO_X), soot, and ‘unburned hydrocarbons’ (UHC). The impact of these undesirable emissions can have a wider impact, including global warming, degraded air quality, acid rain, ozone layer exhaustion, and deforestation. These events are aggravated by the emissions generated by diesel engines, mainly smoke and NO_X.

Biodiesel has evolved as a potential alternative to diesel due to its ability to mitigate environmental degradation. Biodiesel can be produced from different feedstocks depending on crops that are compliant to a local climate. Intense research in recent years has showm the viability of biodiesel as a technical and commercial alternative to petrodiesel due to its inherent characteristics, such as renew'ability, biodegradability, enhanced lubricating ability, high cetane number, and availability in domestic regions, as well as having an environmentally friendly emission profile. The acceptability of biodiesel may look straightforward from the perspective of its properties, but that perception is somewhat deceptive since its suitability is hindered by some shortcomings, including oxidative instability, inferior cold flow properties, unsaturation, higher cost, lower volatility, water absorbency, and lower energy content (Mahmudul et al., 2017; Ong et al., 2013). The performance of an engine in many ways depends upon the properties of the fuel. The properties of biodiesel are a function of many factors, including the feedstock quality, the fatty acid composition of the feedstock, the process of conversion, as well as the catalyst and alcohol used, the post-production parameters, and the storage condition and duration. This implies that a probable variation in the properties of biodiesel is always possible and which may be anticipated by a number of variables. This chapter is an attempt to analyze and summarize these issues with the help of relevant data and technical information.

Characteristics and Properties of Biodiesel

Combustion Properties

Cetane Number

The ‘cetane number’ (CN) is a non-linear dimensionless parameter, which is a measure of the ignition quality of diesel engine fuel and which bears an inverse relation to ignition delay (ID). A high CN of a fuel is a most desirable quality which is a factor in the smoother and longer life of an engine and is directly or indirectly related to engine starting ability, emission profile, combustion, etc. A poor CN of a fuel may lead to diesel knock, inferior operation of engines - such as misfiring, engine deposits, piston tarnishing, and hard starting especially in cold climatic conditions - and poor tail pipe emissions.

Any new fuel, in order to qualify as diesel engine fuel, must possess a minimum CN set by different authorities. The minimum value of the CN for neat biodiesel is prescribed as 47 and 51 in the ‘American Society for Testing and Materials’ (ASTM) and European standard, respectively. The CN of biodiesel varies with the variation in the feedstock source and is much higher than that of diesel (Mahmudul et al., 2017; Ong et al., 2013). However, some values can be higher or even lower, as shown in Table 15.1. This large variation can be due to one effect or the cumulative effect of several factors, such as chemical composition, oil extraction and processing methodology, and climate condition of the feedstock cultivation area. The CN of saturated compounds such as myristic acid (04:0), palmitic acid (C16:0), and stearic acid (08:0) is much higher than those of unsaturated compounds and increases with chain length (Kumar et al., 2013).

Flash Point

The ‘flash point’ (FP) of a fuel is a measure of the minimum temperature at which the air-fuel mixture can exhale vapor to form an ignitable mixture in the vicinity of the surface of the fluid which will burn when it comes in contact with a spark or flame. In this process, a visible flame, either unsustainable or sustainable, can be detected. This is the property which shall be considered in assessing the overall flammability and safety of a fuel.

The FP of biodiesel is measured with ASTM D93 or EN 14214 standards which limit the flash point at min 100 and 120, respectively. The higher FP set for biodiesel is to ensure the removal of excess methanol during the production process as very small amounts of residue can reduce the FP significantly and may affect the durability of some of the constituents. The FP of biodiesel is significantly higher (159°C) than for petrodiesel fuel (58°C), which largely reflects the boiling points of the individual constituents present. In the case of petrodiesel, the branched and lower molecular weight components, which possess lower boiling points, lead to a reduction of the FP. Biodiesel, due to its higher boiling point (BP), can be considered a combustible fuel rather than a flammable fuel and hence a safer fuel to transport. However, the FP is a function of the storage as it can deteriorate as a consequence of the poor stability of esters (Kumar, 2017).

TABLE 15.1

Properties of Different Biodiesel

Source: Mahmudul ct al. (2017); Ong et al. (2013). ME: Methyl ester.

302 Biodiesel Fuels

Physical Properties

Specific Gravity

‘Specific gravity’ (SG) can be defined by ASTM D4439 as the ratio of the density of a substance to the density of a standard substance in a prespecified condition. It is notable that the properties of fuel which are injection characteristics have to be precisely optimized in order to achieve proper combustion. Further, they can be efficiently applied as a precursor for estimating some of the important properties, whose direct measurement is difficult as well as having poor repeatability and reproducibility.

The SG of biodiesel is marginally higher than that of petrodiesel due to factors such as molecular weight, the free fatty acid (FFA) content, and the presence of unsaturation and water content. Among biodiesel fuels, the one having a lower molecular weight and a high degree of unsaturation exhibits a higher SG. Further, biodiesels with longer chain constituents have smaller values of density (Kumar et al., 2013).

Heat of Combustion

The ‘heat of combustion’ or ‘calorific value’ of a fuel is an indication of the energy chemically bound in it. Further, it signifies the amount of heat generated by combustion of fuel inside an engine that provides power to perform useful work. It is largely acknowledged that consumption of fuel is directly related to its volumetric calorific value.

Biodiesel has a lower calorific value due to the presence of 10-12% oxygen by weight. The degree of unsaturation in biodiesel obtained from different sources causes a difference in the carbomhydrogen ratio. Hence, greater levels of unsaturation result in lower calorific values despite having a similar chain length. The energy content of biodiesel is directly related to chain length, since longer chain esters have more carbon but a similar number of oxygen atoms. Further, biodiesel fuels with larger ester head groups (such as ethyl, propyl, or butyl) generally possess a higher heat of combustion as a consequence of their larger carbon to oxygen ratios (Moser, 2009).

Distillation Curve

The ‘distillation curve’ for different fuels is unique. This curve is obtained using the ASTM D86 and helps to estimate fractional composition based on boiling points. A specific volume of fuel (0, 10, 50, 90, and 100%) boils off at a certain temperature. These temperatures help to establish the distillation curve. The volatility of a fuel has a great impact on the performance of diesel engines. A lower volatility indicates high distillation end points, longer combustion duration, as well as poor combustion, while ‘vapor lock’ is likely to occur with a highly volatile fuel.

As mixtures of a few comparatively similar compounds, fat and oil esters have a narrower as well as a higher boiling range relative to those of diesel. The boiling range of diesel is between 159°C and 336°C, while biodiesel exhibits a range of 293°C to 356°C. Thus, the distillation range for diesel and biodiesel is approximately 177°C and 63°C, respectively (Yang, 2008).

Flow Properties

Low Temperature Flow Properties

The ‘cloud point' (CP), ‘pour point’ (PP), and ‘cold filter plugging point’ (CFPP) are the key parameters that largely determine the low temperature characteristic of esters. The CP refers to the temperature at which the separation of wax starts and the diameter of crystal forms is larger than 0.5 pm. while the PP is the minimum temperature below which the fluid fails to flow. The CFPP is the lowest temperature at which a certain volume of fuel entirely flows under certain conditions through a standardized filtration device within a specified interval. Biodiesel fuels show inferior ‘cold flow properties’. The low-temperature performance of biodiesel is largely determined by the molecular structure as well as the nature of the feedstock and strongly depends upon the degree of saturation.

The unsaturated fatty esters, as a consequence of their much lower melting points, behave as solvents. The saturated esters which dissolve in a solvent, crystallize upon cooling at a comparatively higher temperature than that of an unsaturated compound. Unfortunately, the CP of a biodiesel derived from something of a highly saturated nature can rise up to an undesirable level and severely limit its blending with diesel (Lopes et al., 2008).

Viscosity and Surface Tension

The ‘viscosity’ of a fluid is a measure of its resistance to flow. Viscosities of biodiesels can be determined with the help of standards such as ASTM D445 or ISO 3104. The viscosity and surface tension are key properties, and their values vary in the range of 3.9-5.8 mm²/s and 25-30 mN/rn, respectively. These properties are higher in comparison to petrodiesel fuels and have pronounced effects on injection characteristics as well as altering the combustion pattern. The spray penetration is deeper, faster, and has a narrow spray plume angle along with a larger ‘Sauter mean diameter’ for biodiesel injection compared to those of petrodiesel. The variation in these properties is largely a function of the ester chain length, its nature (cis or trans), the degree of unsaturation, and alcohol moiety. Further, FFAs or compounds with hydroxy groups exhibit considerably higher viscosity (Knothe and Steidley, 2005b).

Storage and Stability

Oxidative Stability

A standard specification ASTM D6751 for Bl00 biodiesel and ASTM D7467 for biodiesel blends have been developed for the commercial distribution of biodiesel. Additionally, the standard for diesel D975 has been revised to incorporate 5% biodiesel mixing to meet the D6751 specification. However, EN 14112 is the currently followed standard for the stability of biodiesel. An inferior oxidative susceptibility of biodiesel and its blends restricts the wide applicability of biodiesel, which eventuates on aerobic contact during storage and handling.

Due to the environmental concerns, it is necessary to degrade fuels. However, degradation process also degrades the vital properties of biodiesel and seriously diminishes its acceptability.

‘Biodiesel oxidation’ involves a multi-step reaction. The primary products formed during the primary reactions are conjugated diene and hydroperoxides, which further chemically react to produce several secondary oxidation compounds, including shorter chain fatty acids, aldehydes, aliphatic alcohols, formic acid, formate esters, and species with higher molecular weights. Free radicals are formed through hydrogen abstraction and these reactions are accelerated when exposed to propitious conditions such as presence of light, heat, peroxides, and transition metal. These radicals in the latter stage produce peroxides after reacting with oxygen (Moser, 2009).

A wide variation in stability is reported among different biodiesels, depending upon the inbuilt natural antioxidant and its ‘fatty acid methyl esters’ (FAME) composition, which include a number of double bonds, the orientation of double bonds, the length of the carbon chains, and the types of the ester head groups. In addition, purification of biodiesel by distillation, which removes some of the natural antioxidants such as tocopherol, is also one of the reasons for lowering its stability. The oxidation of biodiesel can adversely alter fuel quality by affecting its properties and may lead to some operative issues.

Iodine Number

The ‘iodine number’ (IN) is a measure of the amount of unsaturation of an oil, fat. or wax. measured in grams of iodine absorbed by 100 g of a sample when formally adding iodine to the sample. The IN for biodiesel is higher than that of petrodiesel. It indicates the susceptibility of the oil or fat to undergo polymerization and the ability to form engine deposits. A limiting value of IN is specified as 120, 130. and 140 in EN 14214, EN 14213, and the South African standard, respectively, while it is not addressed in American and Australian standards. It decreases when higher alcohol is used in the production of biodiesel since the IN is the molecular weight dependent. The IN in some cases can be misleading as the same IN can be obtained from an infinite number of fatty acid profiles as well as different fatty acid structures, despite the propensity for oxidation to have extreme values (Knothe. 2002; Knothe et al., 2004).

Chemical Properties

Biodiesel can be derived from a number of feedstocks depending upon its domestic availability. Animal fats and plant oils are comprised mostly of fatty esters of glycerol (triacylglycerides or triglycerides). The triacylglycerides can be mono-, di-, or triesters of glycerol. Fatty acids may be classified as saturated (no carbon-carbon double bonds), monounsaturated (one C=C double bond), and polyunsaturated (two or more double bonds). FFA profiles of different biodiesel fuels are shown in Figure 15.1. The amount of each fatty acid in triglyceride and biodiesel largely determines their characteristics. In general, fatty acids or carboxylic acids are straightchain compounds with carbon atoms ranging from three to eighteen. However, the chain length can be more or less particular for tropical oils, which is supplemented with lauric acid. The chain length and amount of unsaturation largely determine the chemical composition of fat and oil esters.

FIGURE 15.1 FFA profiles of different biodiesels.

Source: Mahmudul et al. (2017); Ong et al. (2013).

Biodiesel contains nearly 10-12% of oxygen by weight, which lowers its heat of combustion. The carbon content of biodiesel is nearly 15% lower and nearly the same as hydrogen in comparison to petrodiesel fuel on a weight basis. Biodiesel is essentially sulfur free. The amount of sulfur prescribed in ASTM D5453 is as low as 0.00011% by mass (1 ppm), whereas petrodiesel contains around 0.02% (200 ppm). This is advantageous for after-treatment devices and helps in reducing SO_X emission. Unlike petrodiesel, biodiesel is essentially non-aromatic, which helps to reduce emissions. Most of the biodiesel fuels possess a substantial amount of saturated long chain fatty acids which are quite comparable to long chain paraffins. Further, the amount of unsaturation/saturation affects some vital properties.