Yi Xiaolu, Liu Zhenxian, Guo Dongyan, Xu Min, Sun Li
Abstract: The rice husk was taken as an example to investigate the effect of different rice husk particle size on pyrolysis and combustion characteristics, heat transfer coefficient, and coke combustion. The TG/DTG6200 thermogravimetric differential thermal analyzer was used to heat rice husk powder with different particle sizes. After solving the experiment, different TG, DTG, and DTA curves were obtained. The results show that the smaller the grain size of rice husk, the lower the temperature at which the volatilization is analyzed, the shorter the precipitation time, the higher the heat transfer coefficient, and the higher the heating rate. Burning, but too small particle size increases heat loss at the same time. When the particle size is lower than the critical dimension, flameout occurs easily. The coke combustion process is affected by particle size, particle size decreases, combustion reaction rate increases, combustion The time is shortened and it is conducive to burning.
Biomass is an ideal renewable energy source and attracts people's attention due to its extensiveness, reproducibility, and cleanliness. [1-8]There are many types of biomass, and China is a large agricultural country. The annual output of biomass straw is about 600 million tons, mainly concentrated in corn stalk (28.7%), wheat straw (25.4%) and rice straw (14.3%).[9-10]Therefore, the following research on biomass is also for crop residues.
At present, there are many studies on pyrolysis of biomass, and there are few reports on the characteristics of biomass combustion. The shape of biomass is complex, the density is small, and the calorific value is low, which makes its application difficult. Coal and biomass doping Burning technology, biomass granulation and combustion technology have been reported, and there are few reports on the direct combustion of biomass. The direct combustion of biomass is mainly stove burning and boiler burning. The use of stove burning is inefficient, resulting in energy waste; Boiler combustion technology greatly improves the combustion efficiency. The main boiler combustion methods include fluidized bed boilers and layered burners, and the fluidized bed is more adaptable to biomass fuels.
The biggest difference between fluidized bed combustion and ordinary combustion lies in the state of the combustion of raw material particles. The fluidized bed particles are in the process of fluidized combustion reaction and heat exchange. This combustion method is very suitable for direct combustion of biomass. Taking rice hulls as the research object, the effects of biomass particle size on combustion were summarized and provided for the design of fluidized bed. At the same time, this provided a favorable reference for the utilization of biomass energy.
1 rice hull characteristics
1.1 Physical Characteristics
Rice hull is a hollow material with a rough surface and small burrs. Its length is generally about 10mm, and its maximum diameter is 2~3mm. Its main physical properties and its industrial element analysis are shown in Table 1 and Table 2 (Industrial Analysis The rice husks were taken from the outskirts of Jinan City. The characteristics of rice hulls and other straws are very similar. The main difference is that the components in the husk ash are basically SiO. 2, so rice husk is also extracted SiO 2One of the best biomass raw materials.
1.2 Chemical properties
It can be seen from the data in Table 1 and Table 2 that the rice hull density is small, and the N and S content in the rice husk composition is very low. The combustion and flue gas need not consider the problem of desulfurization and denitrification. The volatile content of rice hull is as high as More than 70% indicates that it is easy to catch fire. It also shows that the heat released from the combustion of rice husk is mainly derived from volatile combustion. Therefore, for rice husks and other biomass, the combustion status of volatile matter directly affects the combustion efficiency. This series of characteristics of rice husk requires special attention when using it.
2 Effect of Particle Size on Combustion Characteristics
2.1 Effect of particle size on pyrolysis
Take three types of rice husks with different grain sizes. The length of the rice hull is used to distinguish the grain size. The length of rice husk a is 8~10mm, the length of rice hull b is 0.5~2mm, and the length of rice hull c is 0.01~0.05mm. DTG6200 thermogravimetric differential thermal analyzer, temperature range: room temperature ~ 1100°C, mass sensitivity: 0.2μg, heating rate 50°C/min, TG, DTA and DTG curves of rice husks a, b, c are obtained, eg As shown in Fig. 1. It can be seen from the curves that the husk combustion is roughly divided into three stages.
The water analysis phase (AB), this phase is mainly the water analysis phase, and it can be seen that there is a clear peak between the AB segment, which represents the maximum rate of water analysis. Volatilization analysis and combustion stage (CD) Generally speaking, when the temperature at which DTG=0.1mg/min is taken is the temperature at which the volatiles are analyzed, the volatiles precipitated in the oxygen atmosphere will burn very quickly. The coke burnout phase (DE) can be seen from the figure. The change of TG or DTG curve in the FG segment is relatively weak, especially the DTG curve is more obvious. Therefore, the burning of coke in rice husk is relatively slow.
From the data in Table 3, it can be seen that the temperature required for the analysis of the volatilization of three kinds of rice hulls and the required temperature for the maximum precipitation of rice hulls are Ta>Tb>Tc. That is, as the particle size decreases, the rice husk volatilization is analyzed. The temperature decreases, and the required time also decreases. The combustion of volatiles can be divided into two stages of precipitation and combustion. The gas combustion rate is fast, and the combustion time is much longer than its precipitation time, so the combustion time can be approximately ignored here. From the precipitation data in Table 3, it can be seen that the volatilization analysis time of rice husk a is 1.61 times that of rice hull b and 1.64 times that of rice hull c, which shows that the rice husk particle size has a greater impact on the time of volatilization analysis. As the particles decrease volatilization, the analysis time is reduced; the temperature of rice hull c is the highest, also due to its small particle size.
The parameters of rice husks b and c are basically similar, and the parameters of rice hull a are quite different. On the one hand, because the grain size of rice husk a is relatively large, it is not conducive to combustion; on the other hand, it can be seen that when the rice husk is granular To a certain extent, the effect of reducing grain size of rice husks on pyrolysis or combustion gradually weakens. The smaller the grain size of rice husks, the greater the power consumption will be. Now, the consumption of biomass that is 10-20 mm in size is reduced. The power is about 5kW/t. If the treatment is less than 0.1mm, the required power is about 20kW/t. Therefore, the particle size of rice husk combustion is not unlimited, and a reasonable grain size is selected by comprehensive economic evaluation.
In order to fully evaluate the combustion of biomass, the '11-12' combustion characteristic index P was introduced to describe:
The combustion index P is a comprehensive index reflecting the ignition and burnout of rice husks. The larger the P value, the better the combustion characteristics of the rice husk. The temperature increase rate, the change of the sample particle size and the sample quality are certain for the combustion characteristics of rice hulls. The effect of the combustion characteristics index increases with the decrease of the particle size. [9]The combustion characteristics index P was calculated for each of three types of rice hulls and the result was Pa.[13].
2.2 Effect of particle size on heat transfer
From the aspect of heat absorption of raw materials, the particle size decreases and the heat exchange area of the raw material increases, which facilitates rapid heat absorption and temperature increase. Generally speaking, the particle size is small and the heat transfer coefficient is large. When the difference between the particle temperature and the ambient temperature is reduced to When the initial value is 1%, that is, the time required for the particle temperature and the ambient temperature to substantially reach thermal equilibrium has the following relationship with the particle size:
Therefore, the particle size decreases and the temperature rise time of the particles is reduced. At the same time, the particle size decreases and the heat transfer coefficient increases, according to Zheng Qiayu. [14]In the study of circulating fluidized bed, it was found that the heat transfer coefficient increased with the decrease of the diameter of the ball probe. Therefore, the decrease of the particle size was beneficial to the combustion of raw materials.
From the aspect of raw materials in natural conditions (ie, thermal ignition conditions), the reduction of particle size has unfavorable factors for combustion of raw materials. Particle size has no effect on heat release, but convection heat dissipation decreases with diameter. Inversely increased [15]Therefore, the temperature of the raw material is lowered, which is not conducive to combustion. For burning the same kind of raw material in a fixed bed, the particle size decreases, the bulk density increases, the void ratio decreases, the resistance increases, and the raw material is in an anoxic state, which is not conducive to combustion. The principle is very simple, but there are many discussions. The influence of convective heat dissipation on the combustion is mainly discussed here. Usually the convection heat transfer coefficient and the Nusselt criterion have the following relationship:
When the particle size d of the raw material decreases, the criterion Nu decreases a little, λ increases the α when the physical property parameter does not change, and the particle size decreases, the specific surface area increases, the number of particles increases, and the probability of collision between the particles increases. Enhance the heat exchange, so when the particle size decreases, on the one hand is conducive to the combustion of raw materials, the release of heat Qf; On the other hand, the raw material itself and the surrounding environment heat Qs, When Qf> Qs, the raw material combustion to release a lot of heat, temperature Rising; When Qf = Qs, the combustion of the raw material releases heat and heat balance of the raw material, which is in a relatively stable state at this time.
But when the convection heat transfer coefficient is large enough, there is Qf[15].
The calorific value of biomass is low, and the mass of released heat per unit mass is much smaller than that of coal. Under the same particle size, the heat stored by biomass is only half that of standard coal. When the amount of heat is large, the biomass is more likely to be extinguished. Fig. 2 shows the combustion temperature curves of rice hull a and rice husk c in the fluidized bed. It can be seen from the figure that the temperature change of rice hull a is relatively small and relatively stable; while the rice husk c reaches a certain instant temperature 1000 °C, exceeds the maximum temperature of rice husk a, but its temperature change is large, the minimum temperature is lower than 200 °C, has been in the extinguishing state. From the perspective of pyrolysis, the pyrolysis rate of rice husk c is greater than that of rice husk a, volatilization analysis rate Soon, the rice husk c has a short-term high temperature. Taken together, the rice husk a's combustion condition is better than that of the rice husk c. Therefore, in the actual combustion equipment, the raw material enters the high-temperature combustion furnace hall from the cold state. The high-temperature flue gas heats the raw material, and the smaller the granularity at this time, the shorter the heating time, which is favorable for the raw material to quickly reach the combustion temperature; but when the raw material combustion continues to increase the temperature, it becomes the raw material to dissipate heat to the flue gas. Smaller, more heat dissipation, is not conducive to complete combustion of raw materials, Add the loss of mechanical incomplete combustion starting material.
2.3 Particle size coke combustion effect
The combustion process of rice husk can be divided into two processes: volatile combustion and coke combustion. The gas combustion rate is much greater than the solid combustion rate, so the length of the rice husk combustion mainly depends on the burning time of coke. The rice husk volatile content is high, precipitation Fast, complete combustion requires a large amount of oxygen for a short period of time. The volatile coke is encapsulated in the middle and retards its reaction with oxygen. Therefore, the combustion of rice husk coke is slow. There are 4 possible reactions of carbon and oxygen as follows: :
For these four reaction mechanisms, Wicke and Wurzbacher proposed the conclusion that the combustion of carbon follows the reaction mechanism of (4). Especially at temperatures above 1100°C, reaction 4 is more pronounced. At lower temperatures, it was introduced into the literature '15'. The formula for the relationship between the two product ratios is as follows. When the temperature is 457 to 897°C:
Here, the average value was 677°C, and no carbon reduction occurred at this temperature. The ratio was calculated to be 3.4 and the proportion of CO was larger. From this, it appears that the carbon combustion reaction was mainly carried out according to reaction (3). The ash melting point is relatively low. In practical applications, the combustion temperature is generally controlled at 800-900°C. The combustion reaction is mainly based on reaction 3.
The raw material particle size is reduced and the specific surface area is large, and the mass reaction rate of carbon per unit mass is large. [16]The increase in specific surface area, the mixing of oxygen per unit volume and the increase in the contact area increases the combustion reaction of carbon. The diffusion of oxygen on the carbon surface is hampered by the combustible gas on the one hand, and on the other hand by the combustion of the carbon surface. The retardation of ash, the reduction of raw material particles, the formation of ash retardation when carbon is burned is small, which is conducive to the diffusion of oxygen. Refinement of carbon particles can increase the mass transfer coefficient of oxygen diffusion to the carbon surface. [17]Therefore, the reduction of raw material particle size is beneficial to the combustion of coke.
3 conclusions
(1) Through the thermogravimetric analysis of rice hulls, it can be seen that the smaller the particle size, the lower the temperature at which the volatilization is analyzed, the shorter the time to complete the same amount of volatilization analysis, and the faster the burning rate; for coke, the greater the particle size is Smaller, the faster the burning rate, the shorter the burning time of the raw material; the smaller the granularity, the larger the heat transfer coefficient, the faster the raw material production rate, and the shorter the time required to reach the combustion; but the particle size decreases. The same will increase the amount of raw material heat dissipation, when the particle size is reduced below a certain threshold d, it will be extinguished due to excessive heat dissipation.
(2) The choice of raw material particle size should also consider the economy. When the raw material needs to be processed to a very small particle size, the power it needs to consume is also very large, and the power consumption of biomass treated to 10-20 mm is 5 kW/liter. About t, if the treatment is less than 0.1mm, the required power is about 20kW/t. Therefore, the particle size treatment of the raw materials must be determined by the selection of the critical value and the economy.
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