Overview of biodiesel

Thread Content

I. What is biodiesel? Biodiesel is a form of biomass energy; its main components are mixtures of low-carbon alkyl esters of long-chain fatty acids derived from animal and plant oils. It is named for its physical and chemical properties, which are similar to those of petroleum diesel, and for the fact that it can directly replace petroleum diesel or be mixed with conventional petroleum diesel in any proportion to serve as a substitute for it. The main parameters of biodiesel are basically the same as those of diesel produced from petroleum; its carbon content ranges from 18 to 22, similar to that of diesel (16–18). Its color is clear and transparent just like that of diesel, and it has excellent compatibility with diesel. It can be used as a substitute fuel for diesel on its own or mixed with diesel in any proportion. It can be widely applied in various diesel internal combustion engines such as trucks, trains, locomotives, ships, construction machinery, agricultural machinery, and generator sets. Additionally, it can also be used as a non-power fuel in industrial furnaces, boilers, power plants, as well as in cooking appliances in hotels, guesthouses, and institutional canteens. Diesel molecules are composed of about 15 carbon chains, while studies have shown that plant oil molecules generally consist of 14–18 carbon chains, a number similar to that in diesel molecules. Therefore, biodiesel is a new type of fuel produced by processing renewable plant oils such as rapeseed oil. Based on chemical composition analysis, biodiesel fuel is a type of fatty acid methyl ester obtained through the decomposition of glycerides whose main component is the unsaturated oleic acid C18. < II. Properties of Biodiesel The main specifications of biodiesel are basically the same as those of petroleum-based diesel; the key technical parameters are as follows: Sequence Parameter Name Unit Refined High-Quality Product Crude Product 1 Color Light yellow and clear Reddish-bright color 2 Cetane number 46 46 3 Ash content, % (m/m) ≤0.005 ≤0.05 4 Pour point, °C ≤-5 ≥0 5 Cold filter plugging point ≤-1 ≤4 6 Flash point (closed cup) ≥55 ≥55 7 Carbon residue from 10% distillate, % (m/m) ≤0.5 ≤0.5 8 Copper sheet corrosion grade ≤1 ≤1 9 Moisture, (m/m) Trace Trace 10 Mechanical impurities None None Compared with conventional diesel, biodiesel possesses the following unparalleled advantages: 1. It has excellent environmental benefits. This is mainly reflected in the low sulfur content in biodiesel, which results in low emissions of sulfur dioxide and sulfides – a reduction of about 30% (70% when a catalyst is used) ； Biodiesel does not contain aromatic alkanes that can pollute the environment, so the harm to human health caused by its exhaust fumes is less than that of diesel. Tests have shown that, compared to conventional diesel, the use of biodiesel can reduce air toxicity by 90% and the risk of cancer by 94% ； Due to its high oxygen content, biodiesel produces less smoke when burned, with carbon monoxide emissions reduced by about 10% compared to diesel (95% when a catalyst is used) ； Biodiesel has high biodegradability. 　　2. It has good low-temperature engine starting performance. The cold filter point without additives is -20°C. 　　3. It has good lubricating properties. It reduces the wear rate of the fuel injection pump, engine cylinder block, and connecting rods, thereby extending their service life. 　　4. It has good security performance. Due to its high flash point, biodiesel is not considered a hazardous material. Therefore, the safety in terms of transportation, storage, and use is obvious. 　　5. It has good fuel performance. Its high cetane number improves its combustion performance compared to diesel, and the combustion residues are slightly acidic, which extends the lifespan of catalysts and engine oils. 　　6. It has renewable properties. As a renewable resource, unlike oil reserves, its supply will not run out thanks to the efforts of agricultural and biological scientists. 　　7. No modifications to the diesel engine are required; it can be used directly, and there is no need for additional fueling equipment, storage facilities, or specialized technical training for personnel. 　　8. When blended with petrochemical diesel in a certain proportion, biodiesel can reduce fuel consumption, improve performance, and decrease exhaust pollution. The excellent properties of biodiesel ensure that the exhaust emission standards for engines using biodiesel not only meet the current European Standard II, but also satisfy the even stricter European Standard III, which is set to be implemented in Europe soon. Moreover, since the carbon dioxide emitted during the combustion of biodiesel is much lower than the amount absorbed by the plants during their growth, this helps to address global warming, a serious environmental problem that poses a threat to humanity due to carbon dioxide emissions. Therefore, biodiesel is a true green diesel. III. Its advantages over petrochemical diesel: Compared with conventional diesel, biodiesel has two main advantages: 1. Better flow properties and combustion performance at low temperatures. The product contains no paraffin; it has a carbon atom count of 16–22 in its molecules, a freezing point of -6°C, a cold filtration point of -1°C, and its flowability at low temperatures ranges from 0# to -10#. Biodiesel has a cetane number of 50, indicating a low auto-ignition point and good combustion properties. 2. It has a low smoke emission, helping to protect the atmospheric environment. The main raw material for biodiesel is the waste from plant oils, which is processed through a series of refining and purification steps to produce it. It burns completely, resulting in low smoke emissions; the exhaust gases produced after combustion contain very low levels of sulfur dioxide. Therefore, **it reduces air pollution and helps to protect the atmospheric environment. According to tests conducted by the motor vehicle inspection center, biodiesel produces low levels of smoke emissions in diesel engines, with an emission value of only 0.7Rb, which meets the standard range of 0.5–1.2Rb. This indicates complete combustion, absence of impurities, and makes it an environmentally friendly fuel. 3. It has the characteristic of being non-corrosive. It has a low sulfur content, even lower than the requirement for high-quality diesel specified in the standards issued in 2000. Biofuel with low sulfur content is less corrosive, which helps to extend the service life of diesel engines. Additionally, it is free of moisture and mechanical impurities. 4. It contains no heavy metals harmful to the human body. Since biodiesel is derived from plant oils rather than ordinary mineral oils, and is purified through high-vacuum distillation without the use of any additives, it essentially contains no heavy metals that can be harmful to human health. 5. Good lubricity. Its average molecular weight is slightly higher than that of petrochemical diesel, and it possesses good viscosity-temperature properties; as a result, it has a higher dynamic viscosity without compromising the fogging and evaporation characteristics of diesel. The dynamic viscosity of petrochemical diesel is generally 4.5 mm2/s, whereas that of biodiesel is 6.9 mm2/s. Due to its high kinematic viscosity, not only does the diesel engine operate smoothly and start easily, but it also possesses good lubricating properties that significantly reduce wear on the engine components, thereby extending the engine’s service life. 6. Renewable raw materials: Plant oils are an inexhaustible and renewable resource. Using them as an energy source reduces dependence on and consumption of petroleum, thereby protecting the atmospheric environment. At the same time, the biodiesel production of this group utilizes large amounts of oil residues remaining from the refining process of edible oils as raw materials, turning waste into value and eliminating the environmental pollution caused by these residues; it is thus a green and environmentally friendly industry. IV. Raw materials for biodiesel production • Plant-based: Oils extracted from oil-rich plants such as rapeseed, cotton, soybeans, sesame, peanuts, Chinese tallow tree, castor bean, palm trees, coconut trees, tung trees, flax, wild rue, eucalyptus, camellia oleifera, jatropha, and Chinese honey locust. • Gutter oil from restaurants in various cities • Animal fats (such as various fats derived from pigs, cows, sheep, etc.) • By-products from various fat processing plants, as well as acidified oil • Recycled used engine oil, gasoline, diesel, etc. • Diesel produced from recycled plastic and rubber. V. Current status of biodiesel technology: Through years of research and development, the production and utilization technologies for biodiesel have reached a fairly advanced level. Early technologies for producing gasoline and diesel through the high-temperature pyrolysis of oils were abandoned due to low conversion rates, high energy consumption, and poor economic viability. Today, biodiesel production technologies can be mainly categorized into two aspects: one is physical methods for biodiesel production ； The first is the chemical method for biodiesel production. Physical method biodiesel production technology: In terms of physical method biodiesel production technology, it mainly makes use of the high energy density and combustibility of animal and plant fats and oils as alternatives to diesel fuel. Due to the high viscosity of animal and vegetable oils, one method to enable their use in internal combustion engines is to directly mix vegetable oils with petrochemical diesel as a diesel substitute. In 1983, Amans et al. mixed soybean oil with No. 2 diesel and tested it in a directly injected turbine engine. The results showed that soybean oil and No. 2 diesel could be blended well in a 1:2 ratio, which reduced the viscosity of the fuel and enabled its use as a substitute fuel for agricultural machinery. Typically, a mixture of vegetable oil and petrochemical diesel in a ratio of 5 to 30% is used, and its performance is very similar to that of No. 2 petroleum diesel. Another method is to convert vegetable and animal oils into microemulsions in order to address the high viscosity of these oils. Georing et al. prepared microemulsions using an ethanol-water solution and soybean oil, Ziejewski et al. prepared emulsions using winterized sunflower oil, methanol, and 1-butanol, while Neuma et al. created diesel alternatives in the form of microemulsions using surfactants (mainly soybean oil soaps, sodium dodecyl sulfate, and ethyl fatty acid amine), co-surfactants (mainly ethyl, propyl, and isopentyl alcohols), water, petroleum diesel, and soybean oil. Jiangsu University of Science and Technology in our country, in collaboration with the German company ELSBETT, has successfully developed a small-bore, high-speed direct-injection internal combustion engine that can burn vegetable oils. Application research using vegetable oils as fuel was carried out on the developed automotive internal combustion engines, and it was successful to burn various types of vegetable oils. Chemical biodiesel production technology: Unlike physical methods, which do not alter the composition and properties of fats and oils, chemical biodiesel technology involves chemically transforming animal and plant fats and oils by changing their molecular structure. This transformation converts fats and oils, whose main component are fatty acid glycerides, into fatty acid low-carbon alkyl esters whose molecular weight is only one-third that of the original compounds. As a result, the flow properties and viscosity of these new substances are significantly improved, making them suitable as fuel for diesel internal combustion engines. Esterification and transesterification are the main methods for producing biodiesel; these involve carrying out esterification or transesterification reactions using animal and plant fats and oils (with or without free fatty acids) along with low-carbon monohydric alcohols such as methanol (usually C1-4 alcohols) to produce the corresponding low-carbon alkyl esters of fatty acids. After appropriate post-treatment processes such as glycerin separation, washing, and drying, biodiesel is obtained. The low-carbon alkyl esters of fatty acids obtained through chemical conversion possess flow properties and viscosity ranges almost identical to those of petrochemical diesel, as well as complete miscibility with it, making them an excellent fuel for diesel internal combustion engines. After years of development, the chemical production technology for biodiesel has developed into a relatively complete set of techniques and methods. The core of this technology lies in using various esterification or transesterification catalysts And different process conditions to convert animal and plant oils into biodiesel. A large number of patent technologies and published research studies exist in these areas, covering various biodiesel production methods such as those using chemical catalysts, biological enzyme catalysts, catalyst-free methods (performed under high temperature and pressure), normal-pressure methods, and pressurized methods. The raw materials used for production include animal and plant oils as well as waste edible oils. Among these technologies, biodiesel production methods such as chemical continuous transesterification under atmospheric pressure and pressurized continuous transesterification have been adopted on a large scale in industrial production in developed countries like Europe and the United States. They represent the current mainstream biodiesel technologies, and these techniques are still evolving. Biodiesel utilization technologies are closely related to the biodiesel industry; research on these utilization technologies is a necessary prerequisite for their development on an industrial scale. Meanwhile, the growth of the industry contributes to the establishment and improvement of norms and standards for such utilization technologies. Biodiesel produced by physical methods is developed specifically for certain diesel engines, or the structure of diesel internal combustion engines is modified to make them suitable for using biodiesel; as a result, biodiesel obtained through physical methods is used directly, and its application technology is inherently included in the production technology. However, biodiesel produced by physical methods is a dispersed multiphase system that has always faced stability issues, and its physicochemical properties are difficult to control. Therefore, physical method biodiesel accounts for a very small proportion in the biodiesel industry. Biodiesel produced by chemical methods completely alters the physical properties of biodiesel produced by physical methods, resulting in a fully homogeneous liquid product. Its viscosity is also significantly reduced, and it can mix with petrochemical diesel in any ratio to form a single homogeneous mixture, which makes its use much more convenient. Its application technology is more reflected in the standards and specifications for biodiesel products. The United States makes more use of B20 blended biodiesel, which involves adding biodiesel in a 20% proportion to petroleum diesel, and has built numerous B20 biodiesel gas stations; currently, over 400,000 tons of B20 blended biodiesel is put on the market each year. In some Western European countries with well-developed biodiesel industries, such as Germany and Austria, biodiesel is cheaper than petrochemical diesel due to policy reasons; as a result, 100% biodiesel can be used directly, and this type of fuel is available for purchase at gas stations. Considering factors such as climate change, biodiesel is subject to similar classification and regulation rules as petrochemical diesel, in order to ensure its safe use under various climate conditions. 1.1.3 Methods for preparing biodiesel There are mainly 5 methods for producing biodiesel; among them, the direct mixing method and the microemulsion method belong to physical methods, while high-temperature pyrolysis is a chemical method. The transesterification method can be further divided into chemical transesterification and bio-enzymatic transesterification. Using physical methods can reduce the viscosity of animal and vegetable oils, but issues such as carbon buildup and lubricant contamination are difficult to resolve ； The main product of high-temperature thermal cracking is biofuel gasoline, with biodiesel being only a by-product. In comparison, the transesterification method is a better preparation method. 1.1.3.1 Preparation of biodiesel by direct mixing method In the early stages of biodiesel research, researchers envisioned mixing natural fats and oils with diesel, solvents, or alcohols in order to reduce viscosity and increase volatility. In 1 degummed soybean oil and No. 2 diesel were mixed in ratios of 1:1 and 1:2 respectively, and tests were conducted for 600 hours in a direct-injection turbine engine by Amans et al. in 1983. When the two types of oil are mixed in a 1:1 ratio, the lubricant becomes cloudy and gels; this phenomenon does not occur at a 1:2 ratio, allowing it to be used as a substitute fuel for agricultural machinery. Ziejewski et al. mixed sunflower oil with diesel in a volume ratio of 1:3, and measured the viscosity of this mixture at 40°C to be 4.88×10-6 m2/s. The maximum viscosity specified by ASTM (American Society for Testing and Materials) is below 4.00×10-6 m2/s; therefore, this blended fuel is not suitable for long-term use in direct-injection diesel engines. Tests on a mixture of safflower oil and diesel yielded satisfactory results. However, over time, this mixture can still cause the lubricant to become cloudy. 1.1.3.2 Preparation of biodiesel via microemulsion method: Mixing vegetable and animal oils with a solvent to form a microemulsion is also one of the methods to address the high viscosity of such oils. A microemulsion is a transparent, thermodynamically stable colloidal dispersion, which is a colloidal equilibrium system with a diameter of 1–150 nm formed by mixing two immiscible liquids with ionic or non-ionic amphoteric molecules. In 1982, Georing et al. prepared microemulsions using an ethanol-water solution and soybean oil; aside from having a lower cetane number, these microemulsions had properties similar to those of diesel fuel No. 2. Ziejewski et al. prepared an emulsion using 53.3% winterized sunflower oil, 13.3% methanol, and 33.4% 1-butanol; no significant deterioration was observed during a 200-hour laboratory durability test, but issues such as carbon deposition and an increase in the viscosity of the lubricant did occur. Neuma et al. used surfactants (mainly soybean oil soap, sodium dodecyl sulfate, and fatty acid ethylethanolamine), co-surfactants (composed of ethyl, propyl, and isopentyl alcohols), water, refined diesel, and soybean oil as raw materials to develop new microemulsion systems that can replace diesel. Among these, the microemulsion system with a composition of 3.160 g of diesel, 0.790 g of soybean oil, 0.050 g of water, 0.338 g of isopentyl alcohol, and 0.676 g of sodium dodecyl carbonate had properties most similar to those of diesel.  1.1.3.3 Production of biodiesel via high-temperature pyrolysis The initial purpose of pyrolyzing vegetable oils was to synthesize petroleum. Schwab et al. analyzed the products of the thermal cracking of soybean oil and found that the contents of alkanes and alkenes were very high, accounting for 60% of the total mass. It was also found that the viscosity of the pyrolysis products was more than 3 times lower than that of ordinary soybean oil, but this viscosity value was still much higher than that of ordinary diesel. In terms of cetane number and calorific value, the pyrolysis products of soybean oil are similar to conventional diesel. In 1993, Pioch and others conducted research on the production of biodiesel from vegetable oils through catalytic cracking. Coco oil and palm oil were pyrolyzed at 450°C using SiO2/Al2O3 as a catalyst. The products obtained from pyrolysis are divided into three phases: gas, liquid, and solid, among which the liquid phase consists of biogasoline and biodiesel. Analysis shows that this biodiesel has properties very similar to those of conventional diesel. 1.1.3.4 Production of biodiesel by chemical transesterification Various natural vegetable oils and animal fats, as well as waste oils from the food industry, can be used as raw materials for producing biodiesel through transesterification. Alcohols that can be used for transesterification include methanol, ethanol, propanol, butanol, and pentanol. The most commonly used one is methanol, as it is inexpensive, has a short carbon chain and high polarity, allowing it to react rapidly with fatty acid glycerides; moreover, basic catalysts are soluble in methanol. This reaction can be catalyzed by acids, bases, or enzymes. Alkaline catalysts include NaOH, KOH, various carbonates, as well as alkoxides of sodium and potassium, while common acidic catalysts are sulfuric acid, phosphoric acid, or hydrochloric acid. The more methanol, the higher the yield, but it also makes separation more difficult. Through transesterification, the molecular weight of natural fats (triglycerides) can be reduced to one-third of its original value, the viscosity can be decreased by a factor of 8, and at the same time the volatility of the fuel is increased. The viscosity of the produced biodiesel is similar to that of diesel, with a cetane number of 50. 1.1.3.5 Production of biodiesel via bioenzymatic transesterification New research indicates that lipases are excellent catalysts for promoting the transesterification reaction between alcohols and fatty acid glycerides. As a biological catalyst, enzymes are attracting increasing attention due to their high catalytic efficiency and cost-effectiveness. The catalysts currently used in the chemical production of biodiesel suffer from problems such as difficulty in separation and high energy consumption, all of which can be addressed by using enzyme catalysts. For example, fixed enzyme catalysts using porous kaolinite as a carrier not only have a longer lifespan and do not require frequent replacement compared to other catalysts, but also exhibit high activity and are easy to separate; they represent a new type of catalyst that is competitive both in terms of performance and cost. Ban et al. carried out an enzymatic catalytic reaction using olive oil and oleic acid, resulting in a methyl ester content of 90% in the product. The biological method has the following limitations: first, biocatalysts are expensive ； Second, methanol is toxic to biocatalysts ； Third, the viscosity of the reaction system is too high ； Fourth, the methanol conversion rate is not high. Existing experiments have shown that it is necessary to remove the glycerol produced during the reaction in a timely manner; otherwise, this glycerol can easily clog the pores of the particulate immobilized enzyme, thereby reducing its lifespan. Therefore, it is necessary to develop new methods for immobilizing lipases and esterification processes in order to produce high-quality, low-cost biodiesel.