China is rich in biomass energy resources. According to estimates, China's theoretical biomass energy resources are about 50× 108t, which is four times of China's total energy consumption at present. According to the chemical properties of raw materials, biomass energy is mainly sugar, starch and lignocellulose. According to the source of raw materials, it mainly includes the following categories: (1) Agricultural production wastes, mainly crop straws. (2) firewood, twig firewood, firewood. (3) Agricultural and forestry processing wastes, sawdust, chaff and chaff. (4) Human and animal excrement and domestic organic garbage, etc. (5) Industrial organic waste, organic wastewater and waste residue, etc. (six) energy plants, including all crops, trees and aquatic plant resources that can be used as energy. Among them, agricultural biomass and forestry biomass have the widest sources, the largest reserves and the most promising utilization prospects.
1) agricultural biomass
Agricultural biomass resources include agricultural product processing wastes and crop straws, as shown in Figure 7. 13. Agricultural product processing wastes include peanut shells, corncobs, rice husks and bagasse. Crop straw includes rice straw, wheat straw and corn straw. According to statistics, the total amount of agricultural biomass available in various regions of China is about 5.6× 108t, and the top three regions are Shandong, Henan and Hebei. The main utilization direction of straw agricultural biomass resources is that 24% is used as feed, 15% is returned to the field, 2.3% is used for industry, and the remaining 60% is used for open burning or firewood. Therefore, the application potential of agricultural biomass resources in China is great.
Fig. 7. 13 agricultural biomass
2) Forest biomass
The existing forest area in China is about 1.95× 108hm2, and the total forest biomass exceeds 180× 108t, among which there are three kinds of forest biomass resources that can be used: first, woody starch resources, such as oak, fruit and acorn; The second category is woody oil resources, such as tung oil, camellia oleifera, Pistacia chinensis, Xanthoceras sorbifolia, Jatropha curcas and so on. The third category is wood fuel resources, and the "three leftovers" of shrub forest, firewood forest and forestry. Moreover, there are nearly 4000× 104hm2 of barren hills and wasteland suitable for planting energy forests in China, 600× 104hm2 of sparse woodlands with canopy density below 0.4, and 5000× 104hm2 of low-yield woodlands.
At present, more than 20 countries in the world are planting "diesel trees". On the rolling green hills of ma jia zhuang xiang, Wu 'an City, Hebei Province, China, there are luxuriant Pistacia chinensis growing all over the mountains. The fruit of this tree can be used to extract diesel oil, and the locals call it "diesel tree". Now there are 65,438+10,000 mu of such "diesel trees" in Wu 'an City, which can produce refined diesel oil 1 1,000×1.04 kg every year. According to reports, by 20 12, Wu' an plans to develop the "diesel tree" to 200,000 mu, with an annual output of 2,000×104 kg of diesel.
2. Utilization of biomass energy resources
Mainly used in bioethanol, biodiesel, biomass solid fuel and biomass energy power generation industry.
1) application of bioethanol
Bioethanol refers to the conversion of various biomass into fuel ethanol through microbial fermentation. It can be used alone or mixed with gasoline to make ethanol gasoline as automobile fuel. The raw materials for bioethanol production in China are sugarcane, sweet sorghum, cassava and other high-energy varieties, and an industrial demonstration device with an annual output of 5,000 tons of sweet sorghum stalks has been built. Due to the high price of producing ethanol from traditional grain, in order to reduce the production cost, China turned to the research and development of microbial mixed fermentation. According to the national development and reform commission, by 2020, 15% of China's biomass fuel will be used in automobile, ship and other industries.
2) Application of biodiesel
Biodiesel can be extracted from animal and vegetable oils, such as soybean, rape, animal fat and restaurant garbage. Because of its good environmental protection, lubricity and safety, it can be mixed with petrochemical diesel oil as fuel. In June, 2005, China produced biodiesel by using self-developed biological enzyme method, and the technical indicators reached the European and American biodiesel standards, marking a breakthrough in biodiesel research in China. The production capacity of biodiesel in 20 10 year reaches 300× 104t/ year, which is mainly used in transportation industry. China put forward the goal of biodiesel production capacity reaching 200× 104t in 2020, and Hainan has built a 6× 104t/ year plant, with the output ranking first in China.
3) Application of Biomass Solid Molding Fuel
Biomass solid briquette fuel is a combustible substance that compacts municipal waste or agricultural and forestry waste by external force to increase its density, which has the advantages of high efficiency, cleanliness and no pollution. Fig. 7. 14 is a schematic diagram of biomass packing compression. The production equipment of biomass briquette in China includes screw extrusion, piston stamping and die roller rolling, and the fuel shapes are mainly block, rod and particle. The annual output of biomass solid molding fuel of Beijing Aoke Ruifeng Company is 60× 104t, ranking first in China, which is mainly used for direct-fired heating and industrial boilers.
Figure 7. 14 Biomass Bundling Compression
4) Application of biomass power generation
Biomass energy power generation is a kind of renewable energy power generation, including direct combustion power generation of agricultural and forestry wastes, gasification power generation of agricultural and forestry wastes, garbage incineration power generation, landfill gas power generation and biogas power generation. In order to promote the development of biomass power generation technology, since 2003, the state has successively approved three demonstration projects of straw power generation in Jinzhou, Hebei, Shanxian, Shandong and Rudong, Jiangsu, promulgated the Renewable Energy Law of People's Republic of China (PRC), and implemented relevant supporting policies such as preferential on-grid tariffs for biomass power generation, which has enabled biomass power generation, especially straw power generation, to develop rapidly.
In 2008, Mengniu built the world's largest biomass biogas power plant, which was strongly supported by the Environmental Protection Fund of the United Nations Development Programme. Fig. 7. 15 shows Mengniu biomass biogas power plant.
Fig. 7. 15 Mengniu's largest biomass biogas power plant in the world.
3. The main technologies of biomass energy development and utilization
At present, there are three main technologies for the development and utilization of biomass energy: physical transformation, chemical transformation and biological transformation. It involves specific technologies such as compression molding, gasification, liquefaction, pyrolysis, fermentation and hydrolysis, as shown in Figure 7. 16.
1) physical transformation
The physical transformation of biomass is to dry agricultural and forestry wastes, such as straw, sawdust, rice husk and bagasse, under a certain pressure and press them into rod-shaped, granular and blocky fuels or feeds. Agricultural and forestry wastes are mainly composed of cellulose, hemicellulose and lignin, and the compression molding of biomass mainly depends on the cementation of lignin. Lignin is a natural polymer formed by photosynthesis, and it has a complex three-dimensional structure. It is a polymer substance, and its content in plants is about 15% ~ 30%. When the temperature reaches 70 ~ 100℃, lignin begins to soften and has a certain viscosity. When the temperature reaches 200 ~ 300℃, lignin is in a molten state and its viscosity becomes larger. At this time, applying a certain pressure can combine lignin with cellulose, so that the plant volume is greatly reduced and the density is significantly increased. After the external force is removed, the given shape can still be maintained due to the intertwining of inelastic fiber molecules.
Fig. 7. 16 Main technologies of biomass energy development and utilization
2) chemical transformation
Chemical conversion of biomass includes gasification, liquefaction and pyrolysis.
(1) gasification:
Biomass gasification refers to the process that highly polymerized biomass undergoes pyrolysis, oxidation, reduction and other reactions at a certain temperature with the help of oxygen or water vapor, and is finally converted into combustible gases such as carbon monoxide, H2 and low molecular hydrocarbons. In China, the most widely used field of biomass gasification technology is biomass gasification power generation (BGPG). The cost of biomass gasification power generation is about 0.2 ~ 0.3 yuan/(kW h), which is close to or better than conventional power generation. Its unit investment is about 3500 ~ 4000 yuan /kW, which is only 60% ~ 70% of coal-fired power generation. With the conditions to enter the market competition, the development prospect is very broad.
(2) Liquefaction:
Biomass liquefaction technology refers to the process of thermochemical transformation of biomass at high temperature and high pressure. Through liquefaction, biomass can be transformed into liquid products with high calorific value, that is, solid macromolecular organic polymers can be transformed into liquid micromolecule organic substances. Biodiesel is a renewable fuel produced by biomass liquefaction technology. Oil crops, such as soybean, rape, palm, etc. Under the action of acidic or alkaline catalyst, transesterification reaction is carried out at high temperature to generate corresponding fatty acid methyl ester or ethyl ester, and biodiesel is obtained after washing and drying. Compared with traditional petrochemical energy, it has low sulfur and aromatic hydrocarbon content, high cetane number, high flash point and good lubricity, and can be added to fossil diesel oil.
(3) Pyrolysis:
Biomass pyrolysis refers to the process that the macromolecules of biomass are interrupted by thermal energy and then converted into low-molecular-weight compounds with less carbon atoms, that is, biomass is finally converted into gas, liquid and solid products with high energy density after heating or incomplete combustion under the condition of complete hypoxia. Charcoal is an important product produced by biomass pyrolysis technology. Charcoal products include white charcoal, black charcoal, activated carbon and machine-made charcoal, among which activated carbon is the most widely used. Activated carbon is charcoal and has a series of advantages, such as developed pore structure, strong adsorption force and huge specific surface area. In China, activated carbon is widely used in the production of glucose, monosodium glutamate and medicine.
3) biotransformation
Bioconversion technology refers to the technology of bioconversion of biomass through microbial fermentation or enzymatic hydrolysis to produce liquid or gaseous fuels such as ethanol, hydrogen and methane. Biomass raw materials for biotransformation include starch and lignocellulose. Starch crops such as corn, cassava and wheat are the main body of biotransformation, but the products transformed by crops are expensive, easily restricted by land and population factors, and the yield cannot be greatly improved. Therefore, the bioconversion technology using cheap lignocellulose such as crop waste is an effective way to solve the energy crisis. However, the structure and composition of lignocellulose are very different from starch raw materials, and solving the problem of degrading lignocellulose raw materials with high efficiency and low cost is the fundamental way to replace fossil fuels with lignocellulose conversion products.