Development prospect of domestic biomass power generation technology and equipment



1 Introduction to biomass energy

With the rapid growth of the global population and the rapid development of The Times, the problem of energy shortage has become a problem that the world needs to face together, and at the same time, energy security is also one of the problems restricting the peaceful and stable development of a country. Therefore, how to reasonably and effectively develop and use new energy, research and develop new equipment, explore new technology is particularly important.

China is a large agricultural production country. As far as Fujian Province [1] is concerned, the biomass resources such as various agricultural and forestry wastes, forestry wastes and forest product processing wastes produced every year contain huge wealth and value, which not only provides material conditions for the development of low-carbon economy in China, but also provides a reliable development basis and improvement space for biomass power generation technology in China. Biomass clean energy mainly refers to biomass gas, biomass solid fuel and biomass liquid fuel [2]. Compared with traditional fossil energy, biomass energy is a new type of clean energy, which has the advantages of renewable, low pollution and high added value. The rational use of biomass power generation can not only reduce the harmful waste gas caused by the use of traditional fossil energy, but also greatly reduce the emission of carbon dioxide.

In 2016, the National Energy Administration of the National Development and Reform Commission jointly announced the 13th Five-Year Plan (2016-2020) for China's power industry, according to the 13th Five-Year Plan for power development [3]. China will launch a large area of biomass power generation pilot demonstration projects, the key task is to solve the biomass resources generated from agriculture and forestry, while ensuring that biomass power generation and grid-connected installed capacity in quantity and quality increase. By the end of 2016, China's grid-connected biomass power generation capacity was about 12.2GW[4]. By 2020, it will reach a total installed capacity of 15GW of biomass power generation, as well as 900,000 GWH of annual electricity generation. Therefore, the full use of biomass clean resources to help sustainable development of biomass power plants will become an important strategy.

2 Characteristics of biomass energy

Biomass refers to the organic matter containing energy left behind by animals and plants and for microorganisms after death, and the energy contained in these residues and the metabolites produced by living organisms comes from solidified solar energy. Although fossil energy such as coal, oil and natural gas is also converted from biomass energy, it is biomass that is different from traditional fossil energy [5]. Energy has the following characteristics:

① Recycling and regeneration. Biomass can be regenerated through the photosynthesis of plants, and it is a renewable energy source like wind, solar and tidal energy, and its variety is more abundant, which can ensure the sustainable use of energy and sustainable economic development.

② Energy saving and emission reduction. Because the amount of carbon dioxide required for the growth of organisms is basically the same as the amount of carbon dioxide emitted by them. Therefore, in the production of biomass energy, the emissions of greenhouse gases in the atmosphere are basically zero.

(3) The content of sulfur and nitrogen and ash in biomass is low, especially after sulfur combustion, the emissions of sulfur oxides and dust are very low compared with fossil fuels, and it is a very clean fuel.

④ The distribution is not only very wide, but also the reserves of biomass resources in our country are very large.

3. Biomass power generation technology

Biomass power generation technology is mainly divided into three categories, respectively, biomass direct combustion power generation, biomass gasification power generation, biomass coupled power generation (biomass and other fuel mixed power generation).

3.1 Direct-fired power generation technology

The technical principle of biomass direct-fired power generation is actually to convert the thermal energy of the steam into mechanical energy and electrical energy in the steam turbine by burning the biomass. This is basically the same as the principle of coal-fired power generation. During combustion, complex and numerous chemical reactions and physical changes occur, as well as heat transfer between fuel and air. The gas produced in the steam boiler mainly runs the steam turbine and thus generates electricity from the generator. The core of biomass direct combustion power generation technology is to increase the thermal efficiency of biomass fuel, and boiler equipment with extremely high thermal efficiency has become a crucial step. Biomass combustion equipment is divided into large boilers, small boilers and cogeneration boilers according to their size [6]. According to its combustion characteristics can be classified into beam furnace, granular layer furnace, plate boiler and so on.

3.2 Gasification power generation technology

The basic principle of biomass gasification power generation technology is to convert biomass into combustible gases (carbon monoxide, hydrogen, etc.), and then use the heat generated by the combustion of combustible gas to convert into electricity through power generation equipment. To some extent, it makes up for the shortcomings of biomass which is difficult to burn due to its own characteristics. The process flow of biomass gasification power generation mainly includes the following steps: The biomass raw materials after raw processing are sent to the gasifier through the feeding equipment or conveyor belt. Due to the oxygen content is limited by the volume of the furnace body, incomplete reaction occurs in the biomass combustion. At this time, the commonly used process is to recover and preheat the gasification gas, that is, heat exchange between the high-temperature gas and the material to heat the biomass raw materials, and then filter the gas through the cooling system and purification device. The purified gas goes into steam turbines and internal combustion engines to generate electricity. Compared with traditional direct combustion power generation technology, gasification power generation is cleaner and more reliable. In addition, it is the flexibility of gasification technology, which can combine the waste steam generated in the process with the thermal boiler power generation system, and the addition of steam turbine ensures efficiency, small scale, small investment, low cost and high economic efficiency.

3.3 Biomass coupled power generation technology

In 1970, many developed countries in the world began to put forward the theory of biomass power generation technology. It was precisely because of the emergence of the global fossil energy crisis that Denmark [7] took the lead in vigorously promoting the power generation technology based on straw and other biomass in order to ensure energy security and alleviate energy pressure. After the 1990s, biomass power generation technology and theory began to spread and develop rapidly in many developed countries [8]. In the traditional technology of biomass power generation, in fact, it also includes the combined power generation technology of biomass and coal, oil and natural gas, but in the industrialization and application, it is often dominated by small equipment, which is also directly related to the development and national conditions of the power industry in Western countries.

In fact, in developed countries more than 300MW of power generation equipment and technology is actually rare. In China 300 ~ 600MW equipment will be the main application equipment, the development of small biomass generator set can adapt to the international leading biomass power generation technology trend, but also to adapt to the actual national conditions of domestic power development. This will not only solve the problem of burning more coal, but actually solve the technical scope of the transition from coal to clean energy.

4 Biomass power generation process

After the processing of biomass raw materials such as agricultural and forestry wastes and forestry wastes, they are transported to the gasifier, where they begin thermal cracking and carbonization under the dual action of high pressure and high temperature, and a large amount of mixture gas is produced in this process. The main components of these gases are high concentrations of carbon monoxide, hydrogen and methane. When these mixtures with calorific value come out of the furnace, the gas temperature is basically in a high temperature state (800~900 ° C), in order to make full use of this part of the gas, easy to calculate the calorific value and scientific measurement, so that it can be reduced to a low temperature below 400 ° C, after cooling the biomass mixture is transmitted through the transport system, part of it is sent into the boiler for combustion. Another part of the waste gas with the calorific value is returned to the pretreatment stage for heat reuse. According to this step, the biomass power generation process is divided into the pre-treatment of biomass raw materials, the heating of circulating fluidized bed gasifier, the cooling and pressure reduction of mixed gas, the pressure of combustible gas, the detection and accounting and measurement of biomass mixed gas combustion. In the above links also need to purge and circuit protection and security system. The core factor in these links is the equipment and technology of biomass hot gas coupling.

5 Main process equipment of biomass power generation technology

5.1 Circulating Fluidized Bed (CFB) gasifier

The circulating fluidized bed (CFB) [9] gasifier is mainly composed of an upper part, a lower part and a tail part. Each link and each part of this has high temperature and high pressure resistance protection treatment. The two-stage suspension is the key part to ensure the adequate reaction of biomass gasification. The lower part of the gasifier is mainly composed of the main air chamber, the wind baffle, the cap body and the sealing phase. In fact, the first suspension part is the reaction zone of the main chemical reaction process, and the second suspension part only ensures the smooth progress of the gasification process. The tail of the furnace body is equipped with cyclone and electrostatic dust removal equipment. In order to make the exhaust gas does not cause secondary pollution at the same time can simply remove the gas and solid impurity particles.

In fact, the core of biomass power generation technology is the choice of circulating gasifier. The gasifier uses diesel ignition equipment, so that the temperature in the gasifier is always maintained at about 800 ° C, and the furnace is in an internal environment of approximately hypoxia, which is more conducive to the adequate pyrolysis of biomass fuel, and then the biomass mixture of hydrogen, carbon dioxide and carbon monoxide is produced. Only when the mixture stays fully and completely in the furnace body can it ensure efficient gasification yield and combustion efficiency.

5.2 Mixture cooling system

The core equipment of the gas cooling system [10] is the high-pressure heat exchanger, whose function is to reduce the high temperature (about 650℃ above) gas generated by the gasifier to the medium temperature or low temperature, where the moderate and low temperature refers to less than 400℃. Because of the special working nature of the high pressure heat exchanger, the heat transfer oil that needs to be selected should be selected at 500 ° C or above, so as to ensure high efficiency conversion and ensure production safety. This heat transfer oil in the country is basically customized, according to the specific situation to choose. Heat transfer oil in the high pressure heat exchanger is placed behind the biomass gasifier, the heat transfer oil through the circulating oil pump to the heat collection equipment, the heat transfer equipment under high pressure through the transfer of heat transfer oil, the heat collection and then the heat transfer oil through the return pump re-injection collector heat, through such equipment circulation to achieve the purpose of temperature control.

5.3 mixture pressure conveying equipment

After experiencing the cooling system, the mixed biomass gas is basically in the middle temperature (300~500 ° C), so for the safety of the pressurized equipment, the equipment used for pressurized hot air should be able to meet the temperature of at least 500 ° C. When the mixture enters the furnace from the outside, the air pressure and gas amount will change due to the different position and quantity of the air intake. Therefore, it is necessary to depressor or boost the pressure to the standard degree before the mixture enters the furnace body, so that it is not affected. When the air pressure reaches stability, the mixture is sent to the tube body, and the tube body will finally equalize the gas and then send it to the furnace through the branch line for reaction. In the actual design of the pipeline, the gas backsuction and backflow caused by unstable air pressure and uneven heat should be considered, and the corresponding anti-backsuction and backflow valves should be set.

5.4 Coupling burner

The biomass gas burned in the coupling burner basically does not contain ash and volatiles, so the rotary convection device assembled in the front or back section of the furnace in the burner can continue to use the hot steam as a secondary energy source. The selection and installation of coupled burners requires a more scientific design based on factors such as the amount of biomass gas produced and the change in the combustion zone of the furnace. In China, if the coupling rate of biomass gas is less than 5% of the pulverized coal boiler [11], it can basically ensure that there is no need to treat the water wall of the boiler.

6 Development prospects of biomass power generation in China

(1) China's biomass power generation industry is extremely affected by government support, and has a particularly obvious embedment in the current industrial chain. China's agricultural production mode and current situation is very different from that of developed countries. Developed countries' agricultural economy, which is mainly based on farms, has a complete upstream and downstream system. Whether it is from transportation to recycling or to sales, each link and each chain has a clear division of labor and accurate positioning. Looking at China's agricultural model and policy conditions, this leads to the collection and transportation of biomass raw materials will inevitably be limited by local governments and farmers, resulting in the contradiction of raw material supply. In addition, incomplete acquisition channels, transportation and other industries will also restrict the normal operation of power plants.

(2) For the current power plant, the installed capacity of the equipment is basically 200 ~ 600MW, but for large thermal power generation units, it is mainly more than 600MW. Therefore, the characteristics of the equipment make the coupling rate of biomass power generation in China less than 10%. This shows that if China's biomass power generation industry wants to develop rapidly and on a large scale, it must use small boilers and do not need to carry out a comprehensive modification of the original coal-fired boilers. In fact, the uneven distribution of agricultural resources under the smallholder economy and the difference in production methods are bound to be the most fatal bottleneck among the many adverse effects on biomass power generation in China. From the perspective of the operation of domestic biomass power generation enterprises, the source and quality of raw materials are unstable, which is the main reason for the poor economic benefits of enterprises, and even the operating losses of power plants and power stations.

(3) In the measurement and accounting of biomass actual power generation, it is necessary to further improve the calculation method, accounting norms, calculation technology and accounting standards, which is also an important step to promote the stable development of biomass power plants and improve efficiency.