Wu Jianfei
Abstract: The rationality of the design and improvement of the biomass boiler system in Wuhan Branch of Sinopec Lubricating Oil Co., Ltd. was analyzed. Based on the experimental data of several boiler reforms, the rationalization of the use of biomass pellet fuel for the chain grate boiler was proposed. Design proposal. The results show that: When the chain grate burning biomass particles must take appropriate measures to improve thermal efficiency, prevent safety accidents, reduce the impact of alkali metals.
Biomass fuel is recognized as clean energy in the world. It is an inevitable trend to use it as a substitute for fossil fuels such as coal and oil. Currently, biomass fuels are used in small and medium-sized industrial boilers in many countries. In the starting stage, the main type of chain grate boilers is used. Due to the difference in the combustion characteristics between biomass pellets and coal, the traditional chain grate boilers must be improved in the furnace structure and operation process. This article is from Sinopec Lubricating Oil Co., Ltd. The Wuhan branch used to blend the heated biomass-grilled fuel grate grate boilers in the process of no-accounting and use of the system. The measured data and structural transformation were analyzed and discussed in the following aspects.
1 Biomass boiler system design
1.1 Design of biomass particle fuel feeding system
Biomass pellet fuel is a kind of crop straw, which is processed from wood scraps. It has a certain crushing rate. It will inevitably produce a certain amount of debris in the process of transportation and transportation. This kind of debris is easy to disperse due to its small density. In the design of the feeding system, an automatic feeding system was adopted, and the feeding system was completely closed to avoid debris and personnel contact. The boiler hopper was added to reduce the operating frequency of the feeding system. Material, use the flap and coal gate to isolate the boiler furnace from the hopper to prevent the hopper from catching fire due to high temperature. The flap adopts variable frequency control to precisely control the feeding amount.
1.2 dust removal system design
Due to the deteriorating domestic environment and severe haze this year, the Ministry of Environmental Protection has repeatedly raised the boiler emission standards. Among current technologies, the most reliable technology is the use of bag dust removal, which can control soot emissions at 30 mg/m. 3, To meet the domestic emission standards. However, the bag requirements for Mars, temperature, oxygen content, etc., in the dust removal system design, the following measures must be taken: (1) The use of multi-cyclone dust removal between the boiler and the bag filter, Mars enters the bag filter; (2) The material of the bag is made of three protective fabrics, the bag peritoneum raises the temperature, the oxygen tolerance; (3) increase the economizer heat exchange area, reduce the boiler exhaust temperature; (4) Increase the boiler induced draft fan, increase the total pressure of the fan, use frequency control to offset the two-stage dust removal, increase the heat transfer area of the economizer, and increase the resistance caused by the material requirement of the bag.
1.3 Water Supply and Other System Design
Biomass fuel boilers are basically similar to coal-fired boilers in the design of water supply, air supply, smoke extraction, sampling, etc. They can be designed completely in accordance with the auxiliary system of coal-fired boilers, but due to the combustion of biomass pellets The density is smaller than the cinder, the slag removal system can only use the scraper deslagging device.
2 Combustion characteristics of biomass pellets
Biomass-forming fuels have the following characteristics in the combustion process: (1) About 80% of the volatiles are precipitated at 350 degrees Celsius, and the precipitation and burning time are short, which only accounts for about 10% of the entire combustion process time. (2) Only suitable The air supply method strictly controls the air supply volume, supply area, and supply method so that the fuel can be fully burned; (3) Compared to the lower density particulate fuel of high density granular fuel, the combustion speed in the combustion process is relatively stable. The particle size is uniform. Granular fuel, relatively speaking, the combustion conditions are also more stable and easier to control; (4) The combustion of particulate fuel is mainly the combustion of volatiles, the ventilation furnace enthalpy temperature is lowered, the volatile precipitation rate is relatively stable, and the lower Furnace temperature can reduce the emission of nitrogen oxides; (5) The biomass ash contains Ca, Na, K and other elements. These elements easily form a slag layer in the combustion process, and the softening temperature of ash is low. Compared with coal, the ash melting point of biomass pellets is low, and it is easy to slag.
3Improvement of Furnace Structure of Biomass Chain Grate Boiler
3.1 Furnace volume
The chain grate boiler is a stratum-fired boiler. After it is used to burn biomass biomass pellet fuel, it will become a room-fired boiler to a certain extent due to the relatively large amount of volatile matter in the biomass fuel. According to China's industrial boiler experts The recommendation is that the volumetric heat load of the slag-fired boiler is generally 230~350KW/m. 3, And the volumetric heat load of a small chamber boiler burning furnace is generally 140~260KW/m 3In view of this, the furnace volume heat load should be set to 180 KW/m at design time. 3Therefore, the furnace volume needs to increase by 30%.
3.2 furnace structure
Because of the flammable and flammable characteristics of biomass, the following improvements have to be made to the internal structure of the furnace: (1) The front arch is depressed, and the radiant heating is preheated. The most ideal way is to basically not use the front arch, and use the front wall of the thick furnace. In place of the front arch, this not only minimizes radiation and heat, but also keeps the fire site away from the hopper as much as possible, preventing the flame from backfire to the hopper and causing accidents. (2) The rear arch extends to the furnace 1/2, reducing excess air without burning Spill, reduce the emission concentration; (3) increase the height of the furnace, increase the height of 30%, but also increase the area of radiation heating; (4) reduce the grate length to reduce unnecessary grate mechanical failure; (5) increase the convection heat Area, increase thermal efficiency.
4 Analysis and Improvement Scheme for Heat Loss of Biomass-fueled Boiler
4.1 Incomplete combustion heat loss
Under normal circumstances, the heat loss due to incomplete combustion of solids decreases gradually with the increase of the excess air ratio of the furnace. When a certain value is reached, the heat loss due to incomplete combustion of solids increases with the increase of the excess air coefficient. Large. The excess air coefficient should be controlled at around 1.5. The heat loss of the incomplete combustion of the solid is about 3%.
4.2 Incomplete combustion gas heat loss
The heat loss of incomplete gas combustion is similar to the heat loss of solid incomplete combustion. When the excess air coefficient is around 1.5, it is only about 1%.
4.3 heat loss
The heat loss mainly depends on the size of the heat-dissipating surface of the boiler and the surface temperature. In the normal use of the biomass boiler, the temperature of the furnace should be controlled at 900-1000 degrees Celsius. At the same time, the boiler is kept warm, and the external surface temperature does not exceed 45 degrees Celsius.
4.4 Smoke heat loss
The heat loss of exhaust fumes mainly depends on the amount of smoke exhausted and the temperature of the exhaust. Reducing the amount of smoke discharged and reducing the temperature of the exhaust fumes can help reduce the heat loss from exhaust fumes.
4.5 improvement program
(1) Blower and induced draft fan adopt variable frequency control to control the excess air ratio of the boiler to about 1.5; (2) Increase the convective heating area and the economizer heat exchange area and reduce the exhaust temperature; (3) Make the boiler insulation, reduce Heat loss
5 Analysis of Potential Safety Hazards and Improvement Measures for Biomass Pellet Fuel Boiler
5.1 Safety Hazards Caused by Change in Thermal Load on Furnace Passengers
In industrial boilers, one of the main characteristic parameters of the combustion equipment when the furnace volume is hot-loaded. For the laminar burner, the furnace volume heat load is only a controllable indicator. However, for the chamber burner, the furnace volume heat load Affect the residence time of the fuel in the furnace and the furnace outlet temperature. When the furnace volume heat load is too high, that is, the furnace volume is designed too small, the fuel will be discharged from the furnace before it burns. The too small furnace volume will also make the heating surface layout. Difficulties arise, reduce the amount of radiation heat transfer in the furnace, resulting in too high furnace outlet temperature. Furnace outlet temperature is too high, the amount of flue gas increases, the flow rate is too fast, heat exchange is strengthened when the chain grate boiler burning biomass fuel The main factors that cause safety hazards. Due to the fact that the actual fuel consumption of biomass fuel and the amount of flue gas produced is greater than that of ore fuel, flushing and perturbation of the heating surface of the boiler enhance the heat exchange of the local heating surface. When the chain grate boiler is running, The rear tube plate undergoes radiation of high-temperature smoke, the complex process of convection and heat conduction, the heat exchange of the rear tube plate is strengthened, and the temperature is too high. If the rear tube plate is processed, the artist will be dissatisfied. Requirements, i.e. the high temperature side tube sheet causes the boiler to produce over-temperature leakage accident.
5.2 Prevention Improvement Measures
(1) Increase the boiler volume and reduce the boiler volume heat load. It is recommended that the design take 180KW/m. 3(2) Increase the area of radiant heating and convective heating in the furnace, it is recommended to increase by 30% compared with coal-fired boilers; (3) Increase the secondary air and reduce the amount of one blast.
6 Study on the Problem of Alkali Metal in the Process of Biomass Combustion
6.1 Features of Biomass Fuel Composition
The general characteristics of biomass fuels: Dry ash-free volatiles (60% ~ 80%), low ash. Biomass dry matter contains elemental carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, potassium, calcium, Magnesium, sodium, silicon, chlorine, etc. Among them, the metal elements potassium, calcium, magnesium and sodium are high in content and active in nature. These elements easily generate KCl, NaCI, and NO at high temperatures. x, HCI and other substances, are harmful to the combustion of harmful factors, is in the thermochemical conversion and utilization of biomass leading to slagging, dust and corrosion of the main material.
6.2 Related Elements Conversion Process and Influencing Factors
In the process of biomass burning or gasification, only a small amount of alkali metals in the biomass remain in the ash. They exist in the form of aluminosilicates in the alkali metal compounds and have a very high melting point; water-soluble or ion-soluble. Forms of alkali metals are generally volatile from biomass.
At different temperatures, potassium, sodium, chlorine, and sulfur may react with each other to form different substances. The reaction temperature, pressure, and excess air ratio will have an effect on the precipitation of alkali metals.
6.3 Problems Caused by Alkali Metals in Biomass
Biomass alkali metal precipitation will have the following effects on the boiler: (1) increase corrosion; (2) increase slagging amount, resulting in increased ash content and lower thermal efficiency.
6.4 Measures to reduce the adverse effects of alkali metals
Due to the national environmental protection policy, the method of blending biomass fuels is not in conformity with the requirements of national policies. Measures to reduce the adverse effects of alkali metals can only reduce the combustion temperature and control the excess air coefficient at around 1.5.
7 Conclusion
(1) The feeding system of the biomass chain grate boiler should adopt full-closed automatic feeding to prevent dust damage to the workers; (2) Adopt the cyclone pipe dust removal + bag dust removal system to meet the new boiler emission standards, Reduce the probability of bag breakage; (3) Biomass boiler deslagger should use upper scraper deslagging device; (4) Biomass chain grate boiler recommended design uses furnace volume heat load 180KW/m 3Increase the furnace height by 30%, increase the radiation heating area by 30%, and increase the convective heating area. (5) Blower, induced draft fan using variable frequency control, ensure that the excess air coefficient is around 1.5 during operation; (6) Control furnace temperature About 900 degrees Celsius, to prevent leakage accidents after the tube plate and reduce the impact of alkali metal on the boiler operation.
references
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'3' Zhang Xiaowen, Zhao Gabin, Yang Renquan, et al. Comprehensive Utilization of Crop Straw in Circular Economy 'J' Journal of Agricultural Engineering, 2006, 22(Supplement): 107-109.
'4' Yuan Chao, Zhang Ming, Qin Lichen, et al. Heat loss test and analysis of straw shaped fuel boiler 'J'. Journal of Henan Agricultural University, 2005(3): 345-348.
