Current status and future prospects of bioenergy development

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Current Status and Future Prospects of Bioenergy Development Wang Li (Southwest Chemical Engineering Research and Design Institute, Chengdu, Sichuan 610225) Chemical Abstracts, Issue 2, 2009. Bioenergy, along with solar and wind energy, is considered a green energy source. It is an energy form that uses biomass such as crop straws, livestock manure, organic waste, and other agricultural and forestry residues as well as environmental pollutants as raw materials, which are then converted into solid, liquid, or gaseous fuels through modern technologies. Bioenergy boasts advantages such as renewability and being clean and pollution-free, making it a valuable supplement to conventional fossil fuels like coal, oil, and natural gas. According to experts’ predictions, at the current rate of consumption, coal can be mined for 100 years; oil will be depleted in 100 years, while natural gas can be extracted for at most 50–60 years. Biological energy, on the other hand, is recyclable; it contains no sulfur, and its carbon cycle is dynamic. As a result, countries are paying increasing attention to the development and research of bioenergy, and have formulated corresponding development strategies, such as the United States’ \"Energy Farm Program.\" It is estimated that by 2016 and 2017, the amount of corn used for producing alcohol will account for more than 30% of the total corn production. The use of soybean oil in biodiesel will account for 2/3 of the total soybean oil production, and the extraction of oils from fibers will also gradually be incorporated into energy renewal programs. Brazil’s \"Alcohol Energy Program\" calls for converting land in the southern region, where grains and oil crops are currently grown, into sugarcane plantations; land in the central and western regions, where soybeans are produced, will also be converted to grow sugarcane. As a result, the total production of biofuels will increase from the current level of about 52 million gallons to 92 million gallons by 2016. There are also Japan’s “Sunshine Program” and India’s “Green Energy Project”, among others. Our country also attaches great importance to the development of bioenergy; as early as the 1950s, biogas was promoted in rural areas, with excellent results. At the beginning of this century, it was decided to develop fuel ethanol; large-scale production facilities for fuel ethanol were established in the Northeast, Central China, and East China, and relevant standards were set. Currently, vehicles in some provinces and cities are already using ethanol-blended gasoline. In recent years, China has also invested considerable effort in organizing research and development in bioenergy, achieving remarkable results in areas such as the study of microorganisms for production, bioconversion research, and process and equipment development. The main forms of bioenergy at present are fuel ethanol, biodiesel, biohydrogen, and biogas. 1 Current status and future prospects of fuel ethanol. Fuel ethanol refers to anhydrous ethanol that is added to gasoline or diesel in a certain proportion. It can increase the octane rating of gasoline, enabling more complete combustion, while simultaneously reducing the emissions of substances such as tar, carbon dioxide, and nitrogen oxides in vehicle exhaust. It is the bioenergy with the largest production scale at present. The biomass feedstocks used for fuel ethanol mainly fall into the following four categories: (1) Starchy feedstocks: primarily cassava, sweet potatoes, corn, potatoes, wheat, rice, and sorghum. (2) Carbohydrate raw materials: mainly sugarcane, molasses, and beetroot. (3) Cellulose raw materials: Cellulose raw materials are the most promising sources for producing fuel ethanol on Earth, including crop straws, residues from logging and wood processing, firewood, waste materials containing cellulose from paper mills and sugar factories, as well as household waste. (4) Other raw materials: such as sulfite pulp waste from paper mills, sweet potato starch residue and potato starch residue from starch factories, and by-products from the cheese industry. Biomass-based synthetic fuel ethanol mainly falls into two categories: biological methods and chemical methods. The process flow of the biological method involves converting cellulose and hemicellulose into monosaccharides (pentose and hexose sugars) through acid hydrolysis, enzymatic hydrolysis, or fermentation; the resulting products are then transformed into ethanol via chemical methods, enzymatic catalysis, and microbial fermentation. Biological methods have advantages such as high selectivity, good activity, and mild reaction conditions; however, they suffer from low utilization of raw materials, long reaction times, low product concentrations. Moreover, the activity of enzymes and microorganisms is easily affected, and the enzymes and microorganisms required for cellulose degradation and monosaccharide conversion are adapted to different reaction conditions, making it difficult to integrate them effectively. The chemical process involves converting cellulose, hemicellulose, lignin, and other organic substances from organisms into syngas containing H2 and CO through pyrolysis, and then producing ethanol via chemical reactions, enzyme catalysis, or microbial fermentation. The chemical method has advantages such as high raw material utilization, short reaction time, simple catalyst composition, and no strict requirements regarding reaction conditions; however, its operation involves high temperatures and pressures, which imposes high demands on the equipment. Currently, 95% of the world’s ethanol production is carried out through fermentation, with the technology for fermenting sugar crops being the most mature. However, in the long term, converting fiber plants into fuel ethanol is the effective way to reduce costs, address the issue of limited raw materials, and thus compete with fossil fuels. Table 1 lists the background and processes for producing fuel ethanol from three typical biomass resources, respectively. Table 1 presents the background and processes for producing ethanol from corn, molasses, and straw. Corn raw materials: (1) Corn germ is an important source of fats and proteins ； (2) The slag-containing fermentation process results in a high load for treating waste liquor and high energy consumption ； (3) The distillers’ grain liquor resulting from the fermentation of concentrated corn mash is rich in nutrients ； (4) Reduce the average time required for continuous fermentation, increase the production capacity of the equipment, and minimize pollution. Process: (1) Supercritical extraction process ； (2) Study thick mash fermentation, high-proportion direct recycling of waste mash liquid technology, or solid-state fermentation ； (3) Used in the production of chemical and pharmaceutical products, enzyme preparations, etc ； (4) Clarified fermentation via yeast cell self-flocculation; Molasses raw material: (1) Increasing wine yield ； (2) Improve the quality of molasses pretreatment ； (3) Reducing emissions pollution from molasses ； (4) Shorten the fermentation cycle and increase the fermentation efficiency. Process: (1) Flocculated yeast continuous fermentation technology ； (2) Biostil process ； (3) Biostil process ； (4) Biostill process ； (5) Fixed yeast production technology – Straw raw material: (1) Degradation of straw ； (2) Cellulase is expensive ； (3) Improvement of cellulase properties ； (4) In the natural environment of straw, only some microorganisms are capable of using pentoses for fermentation or to produce ethanol ； (5) Removal of product inhibition by enzymes: Process: (1) Pretreatment with dilute acid + cellulase hydrolysis ； (2) Use liquid fermentation method ； (3) Rational design of enzymes and directed evolution techniques ； (4) Develop genetically engineered bacteria with various advantages ； (5) Meanwhile, with the outbreak of the two oil crises, countries around the world have conducted increasingly in-depth research on fuel ethanol through the simultaneous saccharification and fermentation process (SSF). Among them, Brazil is a pioneer in the development of fuel ethanol, producing it mainly from sugarcane. The United States is the largest country in the world that produces fuel ethanol from grains; its annual production capacity reaches 5 million tons. In addition, it has developed a comprehensive research and development system for producing fuel ethanol from decomposed cellulose and hemicellulose used as fertilizers. China began research and development on projects to produce fuel ethanol using corn as a raw material in the year 2000; currently, Henan Tianguan Group Company and Heilongjiang China Resources Jinyu Industrial Co., Ltd. both produce fuel ethanol from starchy raw materials such as old wheat and corn... 2 Current Status and Future Prospects of Biodiesel Development. Biodiesel refers to methyl esters of fatty acids that can replace petrochemical diesel; it is produced by converting raw materials such as oil crops, aquatic plants like engineered microalgae, animal fats, or waste oils from food services into such biodiesel through biological or chemical methods. Studies have shown that in vehicles with 20% biodiesel blended in, diesel particulate emissions are reduced by 14%, total carbon oxide emissions are reduced by 13%, and sulfur compound emissions are reduced by over 70%. Currently, vegetable oils are the main raw materials used for producing biodiesel. Brazil uses castor oil and genetically modified soybean oil as primary raw materials, the United States relies on genetically modified soybean oil, while the European Union, Canada, and other countries use canola oil with low erucic acid content. Malaysia and Indonesia, on the other hand, utilize their abundant palm oil to produce biodiesel. There are mainly 4 methods for synthesizing biodiesel from biomass: (1) Direct mixing method: Natural oils are directly mixed with petroleum diesel, solvents, or alcohols in various proportions to serve as fuel. This method is simple to operate, but the product has a low cetane number and is prone to deterioration. (2) Microemulsion method: Animal and vegetable oils are mixed with solvents such as methanol, ethanol, and 1-butanol to form microemulsions for direct use. This method is simple to operate, but the cetane number of the product is low, and it can easily cause contamination of lubricants. (3) High-temperature pyrolysis method: Triglycerides are broken down in an inert gas stream into mixtures containing alkanes, alkenes, dienes, aromatics, and carboxylic acids. This method has a simple process, but the equipment is expensive and the reaction is difficult to control. (4) Transesterification method: Biodiesel is produced by using triglycerides and lower alcohols in the presence of a catalyst. This is the main method for lipid modification at present. Depending on the catalyst used, it is further divided into chemical transesterification and enzymatic transesterification. The former mainly involves transesterification reactions using animal and vegetable oils along with low-carbon alcohols such as methanol or ethanol, in the presence of acid/base catalysts and at high temperatures (230–250°C), to produce the corresponding fatty acid methyl esters or ethyl esters; biofuel is then obtained through washing and drying. The latter involves the transesterification of animal fats with low-carbon alcohols using lipase to produce the corresponding fatty acid methyl esters and ethyl esters. In addition, research institutions such as Tsinghua University have also carried out studies on the production of biodiesel using engineered microalgae. Europe is currently the region where biodiesel is used the most, accounting for 5% of the refined oil market. In 2004, the EU’s **biodiesel production exceeded 200,000 tons. In addition, the United States, Canada, Brazil, Japan, and others are also actively developing biodiesel. Japan began researching biodiesel in 1995, and its current annual production capacity reaches 400,000 tons. Although the development of biodiesel projects in our country started late, the progress has been rapid. Many domestic research institutes and some universities have carried out research on biodiesel and achieved phased results. 3 Current Status and Future Prospects of Biohydrogen Production Biohydrogen production refers to the method of using carbohydrates as a hydrogen source, along with photosynthetic or anaerobic bacteria, to generate hydrogen gas. The biological hydrogen production process not only provides hydrogen needed for daily life but also opens up new avenues for waste recycling. The raw materials for biological hydrogen production are relatively abundant. To date, hydrogen has been produced from agricultural solid wastes such as cow manure, refined sugar wastewater, soy product wastewater, dairy product wastewater, starch wastewater, brewing wastewater, wheat bran, distillers’ grains, and corn straw, as well as kitchen waste. The most extensive research has been conducted on using glucose, sewage, and cellulose as raw materials, but all of this is still at the laboratory stage. There are mainly three methods for biohydrogen production: (1) Hydrogen production by photosynthetic organisms: that is, using photosynthetic bacteria or microalgae to convert solar energy into hydrogen. The microorganisms known to be capable of photosynthetic hydrogen production include photobacteria and microalgae. Among them, photosynthetic bacteria belong to photosynthetic heterotrophic microorganisms; those that have been extensively studied include Rhodospirillum rubrum, Pseudomonas spheroides, Rhodopseudomonas palustris, Pseudomonas capsulata, Microbacterium sphaeroides, and Thiospirillum extravacuolatum. Microalgae are photosynthetic autotrophic microorganisms, including cyanobacteria, green algae, red algae, and brown algae; among them, green algae are the ones that have received the most research attention to date. Photosynthetic organisms for hydrogen production have advantages such as high light conversion efficiency, but since the hydrogen production process involves the release of oxygen, it can easily lead to the inactivation of hydrogenases. (2) Hydrogen production by fermenting bacteria: That is, using heterotrophic anaerobic bacteria or nitrogen-fixing bacteria to break down small organic molecules to produce hydrogen. Bacteria that have been identified as capable of fermenting organic matter to produce hydrogen include obligate anaerobes and facultative anaerobes, such as Clostridium butyricum, Escherichia coli, Enterobacter aerogenes, Azotobacter brunneus, Oenococcus oeni, and rhizobia. Fermenting bacteria have advantages such as a wide usable spectrum for hydrogen production, but the fermentation broth can easily cause water pollution. (3) Hybrid hydrogen production by photosynthetic organisms and fermenting bacteria: That is, utilizing both photosynthetic bacteria and fermenting bacteria to produce hydrogen from complex substrates. It is reported that Yokoi H and others used Clostridium butyricum, Escherichia aerogenes, and Rhodococcus to produce hydrogen from sweet potato starch residues through co-cultivation. This technology has the advantage of a wide range of substrates, but it also requires stricter cultivation conditions. As one of the development directions for future energy production technologies, biohydrogen production has attracted widespread attention from countries around the world. In recent years, the United States has spent an average of several million dollars per year on research related to biohydrogen technology, while Japan invests about five times more in this area each year; it has also established development plans for biohydrogen. 4 Current Status and Future Prospects of Biogas Development. Biogas is a combustible gas produced through the fermentation process carried out by anaerobic microorganisms, using materials such as human and animal waste, remains of plants and animals, as well as organic waste and liquid residues from industry and agriculture, under conditions of specific temperature, humidity, acidity, and oxygen deficiency. It consists of methane, carbon dioxide, nitrogen, along with small amounts of hydrogen, oxygen, hydrogen sulfide, and other gases. It is a clean energy source with a high calorific value and good explosion resistance. There are a wide variety of materials that can be used as feedstock for biogas fermentation, such as livestock manure, crop straws, food processing waste, wastewater, and alcohol waste. Their main chemical components are polysaccharides, proteins, and lipids, among which polysaccharides are the primary components involved in the fermentation process. Generally speaking, biogas fermentation is roughly divided into three stages: In the first stage, microorganisms secrete extracellular enzymes to hydrolyze biomass into water-soluble substances ； In the second stage, the water-soluble substances that enter the microbial cells are further broken down and metabolized by various intracellular enzymes into volatile fatty acids, alcohols, as well as a certain amount of hydrogen and carbon dioxide ； In the third stage, methane is ultimately produced under the action of methanogens. Currently, many developed **countries and energy-deficient developing** countries are actively developing and utilizing biogas. Chicago, United States, has built underground pipelines that connect various waste dumps in the city; the large amount of biogas generated as waste decomposes is delivered to households through these interconnected underground pipelines. Biogas use in our country is primarily in rural areas, where household biogas digesters can meet the daily energy needs for heating and cooking. 5 Conclusions and Recommendations China has achieved remarkable results in the research and utilization of bioenergy, but it has not yet reached the level of large-scale production. The main reasons for this are as follows: biomass-based synthetic ethanol poses the issue of competing with food crops for land; biomass-based synthetic biodiesel has the drawback of causing significant pollution; biohydrogen production suffers from low hydrogen yield and difficulties in hydrogen collection; while biogas fermentation has problems such as low fermentation efficiency and poor ability to operate continuously. It is recommended that, while developing bioenergy in a manner suited to local conditions, all regions should fully leverage its environmental benefits; they should choose appropriate non-grain feedstocks for the production of fuel ethanol, seek more efficient biological enzymes to improve the efficiency of hydrogen and biogas production, and explore more sustainable processes to reduce the environmental impact associated with biofuel production. References (omitted)