Structural Characteristics Analysis of Biomass Pellet Fuel Boilers

The proportion of coal in the national energy production structure is always 67% and above. Coal is the mainstay of China's energy. China consumes about 1.2 billion coals a year. 1.5 billion tons. Most of them are consumed directly as fuel. Coal-based energy China's energy production structure has always been 67% or more. Coal is the mainstay of China's energy. China consumes about 1.2 billion coals a year. 1.5 billion tons. Most of them are consumed directly as fuel. The coal-based energy structure directly causes energy activities to cause great harm to environmental quality and public health. Biomass solid molding fuel (referred to as biomass fuel) is a modern clean fuel that uses new technology and special equipment to compress crop straw, wood chips, sawdust, peanut shells, twigs, hay and so on. Without any additives and binders, it can solve the basic living energy of rural areas, and can also be used directly in the city's traditional coal-fired boiler equipment instead of coal. The biomass forming fuel has a crushing rate of less than 1.5% to 2.0%. The dry basis moisture content is less than 15%. The ash content is less than 1.5%, especially the sulfur and chlorine content are generally less than 0.07%. The nitrogen content is less than 0.5%, and biomass fuel is a renewable energy resource advocated by China.

1. Characteristics of biomass molding fuel

We will analyze the biomass fuel that is pressed and formed with pine as the main raw material. Compared with the traditional fossil fuel, the composition and combustion characteristics of the biomass forming fuel are very different.

1.1 Composition characteristics of biomass molding fuel

The chemical composition of biomass fuel is a very complex polymer material. In engineering applications as a fuel. It can be roughly divided into two parts, organic (flammable) and inorganic (flammable). The organic matter is mainly volatile matter (a gaseous substance composed of elements such as C, H, O, N, S) and fixed carbon (a solid substance composed of C element), volatile matter in the fuel and its calorific value to the biomass. Both the fire and the burning situation have a great influence. The more volatile the fuel has, the more likely it is to catch fire. The more stable the combustion.

Volatility and calorific value of biomass and coal

Another parameter that is closely related to the ignition and combustion conditions is the calorific value of the fuel. Different biomass types have different main components and different calorific values. The elemental composition and calorific value of several major biomasses are shown in Table 2. Can be seen from Table 1 and Table 2. The biomass fuel has a higher volatile content than coal, and the ash, nitrogen and sulfur content is much smaller than that of coal, and its calorific value is also lower than that of coal.

1.2 Combustion characteristics of biomass forming fuel

Biomass shaped pellet fuel is formed by high pressure. Its density is much greater than the original biomass. The structural and structural characteristics of the shaped fuel determine that the rate of precipitation of volatiles and the rate of heat transfer are both low. The combustion process of biomass molding fuel can be divided into several stages: dry dehydration, volatilization analysis, volatile combustion, coke combustion and combustion. The initial stage of heating. The water in the biomass pellet fuel evaporates, and the fuel is dried and dehydrated; as the temperature of the pellet fuel increases, the volatile matter begins to precipitate, which can be considered as a gasification process; as the fuel continues to be heated, the temperature of the volatiles is also It will increase accordingly. When the combustibles in the volatile matter reach a certain temperature and concentration, the volatile matter begins to ignite; at the same time, the volatile matter does not burn. Charcoal can only be heated and cannot be burned. Only when the volatiles are burned out. Oxygen can diffuse to the charcoal surface. Charcoal began to catch fire.

Several typical biomass elemental analysis, industrial analysis and calorific value of calorific value

2. Structural design of biomass forming fuel boiler

Starting from the analysis of the characteristics of biomass molding fuel. The boiler designed in this project is a chain grate furnace. Boiler rated evaporation D = 2 t / h rated working pressure only = 0.8 MPa, rated steam temperature t = 175 ° C, feed water temperature k. = 20 ° C.

2.1 Overall design of biomass forming fuel boiler

The boiler's combustion technology uses layer-fired technology, similar to a conventional chain grate coal-fired steam boiler. According to the combustion characteristics of biomass fuel. The air supply system and the furnace arch of the chain grate are modified to be consistent with the conventional coal-fired boiler use method. Biomass-forming particulate fuel combustion process: biomass-forming particulate fuel is first introduced into the hopper 1. Adjust the coal gate according to the boiler load and combustion conditions. To control the thickness of the fuel layer. The primary air enters the furnace from the air supply 3 through the grate 4 and the fuel layer 5 to provide the primary air required for combustion. The fuel is slowly moved toward the back of the furnace by the grate. The shaped particulate fuel is dehydrated, vaporized, and combusted by the action of the heat source and the air, and the generated high-temperature flue gas containing the combustible gas quickly enters the furnace. Radiation heat exchange is carried out; combustibles that have not burned enter the combustion chamber 7 and continue to burn. Then the high temperature flue gas enters the first and second return pipe 8 and the boiler economizer 10 to perform convective heat transfer. The dust blower 11 is sent to the chimney 13 by the action of the induced draft fan 12. Discharged to the atmosphere. The ash formed after the fuel is burned is transported from the grate to the slag El by the slag remover 9 to discharge the furnace body (Fig. 1). According to the burning characteristics of biomass particles. There is a large amount of volatiles in the front arch. Therefore, a secondary air is arranged at the exit of the front arch to supplement the oxygen to assist combustion and disturb the smoke: the design of the secondary air should have a certain flow rate and a certain effective range. Considering that the volatile matter of biomass fuel is particularly high, it is easy to catch fire, and the ignition point of the boiler should be moved backwards, and the coal gate is changed to water cooling. To avoid fuel in the fuel hopper.

Biomass boiler structure

2.2 Biomass forming fuel boiler furnace design

Biomass forming fuel boiler furnace design, furnace volume and grate area are the two main parameters of boiler furnace design. According to the standard recommendation, the heat load of the grate area of ​​the fuel is 600,850 kW/m2; the furnace volumetric thermal strength Qy230- 350kW/m3. According to the heat balance calculation, the calculated fuel consumption of the fuel selected from the boiler is 341.1 kg/h, and the low calorific value of biomass-forming particulate fuel Qnet, ar = 18040 kJ/kg; The boiler furnace volume is

Biomass fuel boiler calculation formula

2.3 Design of Radiant Heating Surface of Biomass Forming Fuel Boiler

The radiant heating surface consists of a drum and a water wall tube. The water wall tube adopts a single row of seamless steel tubes with Φ=63.5 mm, and the spacing is s=100 mm. The area of ​​radiation heating is = 5.26 m2. According to China's layered combustion and boiling combustion, the boiler thermal calculation method and calibration calculation, the thermal calculation of the radiant heating surface of the boiler was carried out. The calculated outlet temperature of the furnace outlet is Q=1027.120°C, and the heat transfer amount of the radiation heating surface is 1700.25kcal/kg. The thermal surface of the radiation heating surface is checked, and the check result meets the design requirements.

2.4 Design of convective heating surface of biomass forming fuel boiler

Design calculation of convection tube. The convection heating surface is divided into two regions. The first zone consists of 61 cigarette tubes of Φ57×3.5 mm and L=3220 mm. The designed flue gas flow rate is 19.22 m/s; the second zone is composed of 41 cigarette tubes of Φ57x3.5mm and L=3220 mm, and the designed flue gas flow rate is 16.54 m/s, which is in line with the standard recommended value. At the same time, in order to meet the purpose of reducing the boiler exhaust temperature. In addition to the boiler body, a cast iron economizer is installed with a heating area of ​​21.12 m2. In order to improve the heat transfer effect, the first and second convection pipes adopt advanced threaded pipe technology. The heat transfer amount of the convection tube bundle is checked according to the relevant calibration method, and the results meet the design requirements.

3. Summary of thermal calculation of biomass forming fuel boiler

For the thermal calculation of biomass-forming fuel boilers, the fuel composition, thermal efficiency and heat balance of biomass-formed pellet fuel boilers are shown in Tables 3 and 4, respectively. The relevant technical parameters of the furnace are: theoretical combustion temperature (Watt): 1643.95 °C; furnace exit flue temperature (mine): 1027.12 °C; Radiation heat absorption (Qr): 1700.25 kcal/kg; (2): 2253.77 kcal/kg; average heat capacity (V.) 2.85 kcal/kg. °C. See Table 5 for the calculation of the heating area and smoke parameters of each part of the boiler.

Biomass fuel composition and thermal efficiency calculation of heating area and smoke parameters of biomass boiler

4, boiler fan selection design

The ventilation is the breathing organ of the boiler. Ventilation is the means to adjust the output of the boiler. Only the proper design of the ventilation system and the selection of ventilation equipment can ensure the normal combustion and heat transfer process of the boiler. There are three types of mechanical ventilation commonly used: negative pressure ventilation, balanced ventilation and positive pressure ventilation. The boiler designed in this paper uses balanced ventilation. That is, the blower and the draft fan are installed in the boiler equipment, and the blower is used to overcome the air passage resistance. Depending on the amount of air required for combustion and the appropriate excess air ratio and total pressure drop across the duct. When selecting a blower, certain reserves should be considered for safety reasons and corrected by the reserve factor. The induced draft fan is used to overcome the flue resistance, the amount of flue gas generated by combustion and the total pressure drop of the flue and a certain reserve factor. To make a choice. The air volume and air pressure of the air blower and the draft fan can be determined by calculation, and the fan model is selected according to the calculation result and the product catalogue provided by the boiler auxiliary machine manufacturer. This design selects the blower and induced draft fan produced by Fortis New Energy Company. The blower model is 4-72 No 3.6A, and the induced draft fan model is Y6-30No-7.5C.

5 Conclusion

Through the analysis of the combustion characteristics of biomass-forming fuels, the structural characteristics of the burn-off coal boilers are compared, the structure of the burn-off coal boilers is improved, and the high and extended furnaces are used. The gasification effect of the particulate fuel is enhanced to make the combustion more sufficient, thereby improving the boiler thermal efficiency; the multi-point equilibrium air distribution method is adopted to reduce the excessive air coefficient of the furnace and reduce the smoke loss: the technology of the reciprocating grate is applied to the biomass forming fuel On the boiler. This increases the combustion intensity of the furnace and improves the ignition of the fuel. The fuel burn rate is high.

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