Wang Xu Tao 1Zhang Bailiang2
(1. Henan Urban Construction Institute, Pingdingshan 467036, Henan, China; 2. Key Laboratory of Renewable Energy, Ministry of Agriculture, Henan Agricultural University, Zhengzhou 450002, China)
Abstract: As a kind of renewable clean energy transformed by straw, biomass straw fuel has many advantages.In this paper, the resource characteristics, physicochemical properties, combustion characteristics and technical and economic characteristics of biomass straw fuel are analyzed. Has rich resources, after forming the density increases, so that the transport of storage superior to the original biomass; without losing the original biomass characteristics of coal and similar, easy to ignite, complete combustion, CO 2, SO 2, NO xThe concentration is lower than that of coal; however, the higher alkali metal and Cl content will lead to corrosion and deposition during combustion. At present, the technical economy is not prominent, and comprehensive environmental benefit is a good alternative energy option.
1 straw molding fuel status
Biomass energy refers to the chemical energy contained in biomass, which is the energy stored by green plants after they convert solar energy into chemical energy by photosynthesis. It is the only carbon source on the earth that can be regenerated and can be processed and transformed into the conventional Solid, liquid and gaseous fuels [1]With the accelerating process of industrialization in China, the issue of energy shortage and environmental pollution has become increasingly prominent, and the demand for clean combustion technologies for the development of clean energy and conventional energy sources such as coal is becoming more and more urgent.The environment is severely damaged by the straw burning every year during the crop harvest season Pollution, but also increase the community's concerns about the disposal of abandoned straw, continue to promote the conversion of straw resources from another perspective.
China's research and development of biomass as a renewable and clean source of energy began in the 1980s [2], The output has been increased from 300,000 t before 5a to over 3 million t at present, and has developed rapidly, but there is still a big gap compared with developed countries. There is a lack of corresponding industry standards for straw molding fuel production machinery and product categories, therefore, products Uneven, relative to the industrialization there is a big gap.
2 straw molding fuel characteristics
2.1 resource characteristics
As an agricultural country, China has abundant crop straw resources. Based on the current grain yield and grass-to-grain ratio, it is estimated that the annual straw production in China is about 600 million tons, calculated as 25% [3]About 150 million tons of straw can be used to produce shaped fuel.
At the same time, it should also be noticed that farmland in China is still under scattered planting and is not conducive to large-scale mechanized operations. Therefore, the straw resources are relatively dispersed. In addition, the large straw, low density and unit calorific values are affected by the harvesting time, resulting in the collection of straw resources And storage of fuel than coal and other difficulties.
2.2 Physical and chemical properties
Straw-shaped fuel is produced by dewatering the collected loose stalks and crushing them into rod-shaped, block-shaped, spherical or granular solid fuel with a regular shape and a relatively large density, which is common in sticks and pellets. Fuel physical and chemical properties have been different from the straw, to solve the straw bulk density, low energy density, not easy to store and transport shortcomings, density, calorific value with the compression method, the production process varies, the main parameters in Table 1.
2.3 combustion characteristics
The combustion performance of straw-shaped fuel is much improved compared with that of bulk straw, and the combustion efficiency can be increased from 10% -15% of direct straw burning to 30% -40%, while it has the advantages of high volatility, easy ignition and low ash content Advantages, only a trace amount of SO in the flue gas 2, As a renewable energy can achieve CO 2Zero emissions, environmental pollution is small[4].
2.3.1 straw molding fuel ignition performance
The compact structure of straw molding fuel will inhibit the rate of outward volatilization and inward diffusion of air during the combustion of the molding fuel while reducing the heat transfer from the outside to the inside. The ignition performance of the straw molding fuel is higher than that of the bulk biomass straw Significant decline[5- 6].
Experiments show that the ignition time is about 2 ~ 3min, the main factors affecting the ignition performance of the biomass fuel structure density (density), volatile content and fixed carbon content of biomass molding fuel ignition temperature, ignition time Increasing and increasing the density of its extension, the density of 1.2g / cm 3The straw molding fuel ignition time than the density of 0.8g / cm 3More than 1 time longer than the ignition temperature of 300 ~ 400 ℃, than the biomass straw about 50 ℃ [7]; The higher the volatile content of the straw molding fuel, the more likely to ignite the fire, the overall point of view, the straw molding fuel ignition characteristics tend to the original straw ignition characteristics and better than the briquette[8].
2.3.2 straw molding fuel combustion mechanism
When the straw-shaped fuel burns in the hearth, the combustion process of the similar coal is static osmotic diffusive combustion without losing the characteristics of biomass straw burning. The whole combustion process can be roughly divided into the ignition (volatiles Combustion), diffusion combustion (mixed combustion of combustible gas and carbon), smooth combustion (coke combustion), burning ash (ash, residual coke) 4 stages[9- 11].
(1) Combustion of flammable volatiles on the surface of the stratified fuel, ignition of flammable gas and oxygen, due to its compact structure, non-porous seams, small thermal conductivity and slow release of combustible volatile gases, Long and weak.With the slow heating of the straw molding fuel internal heat, sustained precipitation, combustion lasted about 10min.
(2) Diffusion combustion As the combustion temperature continues to rise, the combustion begins to enter the transitional combustion phase of the surface coke. The carbon in the surface layer of the biomass-shaped fuel begins to burn and a small amount of combustible gas escaping from the interior is generated.
Steady combustion As the combustion continues, the outer blue flame becomes thinner, the flame becomes shorter and stronger, the combustion begins to become violent, and the combustion penetrates into the deeper coke layer. At this stage, CO is burned on the fuel surface and the fuel The inner layer is charcoal burning [12]Due to the relative diffusion of air and flue gas fields, there is not enough air inside the biomass-forming fuel, and the cellulose and lignin are gasified at high temperatures to form CO, H 2, CH 4And other combustible gases, they continue to spread out combustion to ensure smooth combustion until the flame becomes shorter and stronger, which is mainly the combustion of coke.
(4) The residual carbon in the burning embers continues to burn, the combustibles are basically exhausted, the flame is shortened, the color starts to change from light to dark and disappears, and the fuel turns into dark red and gray as the combustion process lasts about 60 minutes. Burning temperature curve shown in Figure 1.
In the initial stage of combustion, the temperature rose rapidly, and the rate of temperature rise in the diffusion combustion and steady combustion phase slowed down gradually. When the temperature was higher, the temperature in the combustion embankment began to decrease gradually.
2.3.3 straw molding fuel dust, sedimentation corrosion
Biomass straw molding fuel in the production process does not occur chemical changes, composition and biomass straw or less, and the number of fumes emitted in addition to combustion (not higher than 200mg / m 3) Has decreased [13- 14], Which has no significant difference with the biomass straw dust composition, the main components of soot as shown in Table 2. Biomass straw molding fuel Cl content is higher, will lead to severe corrosion inside the furnace, while Burning ash easily appear similar to the end of the coal The phenomenon of slag deposition and corrosion is that the straw contains more than 1% potassium and other metal elements. The volatile alkali metal salts in the combustion process enter the gas phase, volatile ash and other fine particles in the furnace at a high temperature of about 700 DEG C Part of the condensing surface condensation to form the initial deposition layer.With the temperature increase the surface will appear fusion bonding surface, the larger particle soot impacted the initial deposition surface to form a new deposition layer [15- 16], When the furnace temperature is higher than 780 ℃, these deposits form a glassy slag (complex mixture), resulting in serious slagging boilers can not operate normally large biomass boiler every 10 to 12 months to stop the furnace for a week Clean up around.
Cl content in biomass straw fuels is as high as 0.2% to 3.0%, which is the most significant feature of biomass straw unlike coal and other mineral fuels. In biomass combustion furnaces, Cl 2And various forms of chloride on the water wall erosion of the greatest role, because of HCl and Cl 2Has the ability to penetrate the metal oxide protective film and reacts with iron to form iron chloride, which is easily re-oxidized to form a thick iron oxide flake in the furnace at high temperature and high oxidizing atmosphere. In the SO 2, SO 3Cl is formed by reacting with iron chloride in the deposit and generating (subscript) iron sulphate 2, Will repeat the process of gaseous corrosion, aggravate the furnace corrosion.In addition, KCl, CrCl 2And FeCl 2Alkali metal chlorides react with metal surfaces or metal oxides to form eutectic compounds with a low melting point (350-450 ° C), resulting in slag formation problems with straw molding fuels[15].
2.4 Technical and economic characteristics
The purchase price of biomass straw is 400 yuan / t, plus transportation costs, storage costs, labor and fuel costs of processing, equipment depreciation and other costs, the cost price of 500 ~ 550 yuan / t. This price and the current dynamic coal sales Price 500 ~ 600 yuan / t compared to no advantage, but if coupled with the government subsidies for clean energy, basically be able to be flat or have a surplus in the coal price is higher than 600 yuan / t, plus straw molding fuel not Compared with the environmental benefits, which have the advantages of technical and economic began to show, especially when used as a small boiler, blast furnace, cooking and other rural energy is even more apparent.
3 Conclusion
1) Biomass briquette has relatively abundant resources as a clean and renewable energy resource, overcoming the problems of low heat density, difficult storage and transportation of straw, etc., but the production equipment and products need to be standardized in order to promote and promote biomass forming Fuel industrialization process.
2) The physical characteristics of biomass briquette are similar to that of biomass because of its ignition characteristics close to that of biomass.Combustion process belongs to static infiltration diffusion combustion, similar briquette, which can be divided into volatile combustion, diffusion combustion, stationary combustion and combustion ash 4 stages, the combustion emissions of soot concentration is low, CO, CO 2, SO 2, NO xConcentrations are lower than coal, but higher levels of alkali and Cl can lead to corrosion of the heated face metals and deposition of fumes.
3) Regardless of the environmental benefits, the current economy of biomass briquette production has no advantage over coal and requires government policies to achieve significant growth.
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