He Fang, Ji Jianwen, Wang Lihong, Gao Zhenqiang, Li Yongjun
Abstract: Calculation of biomass pellet combustion process is the basis of biomass energy utilization, fire process analysis and urban solid waste incineration technology.The accuracy of the calculation results depends on whether the mathematical model of the physical problems studied is correct or not.The biomass Particle combustion model.The surface reaction model and the volume reaction model in the reaction zone model are compared.The surface reaction model is suitable for the calculation of the transmission control process.The volumetric reaction model is suitable for the calculation of the common control process of the power control or the power transmission.It is summarized that the drying, Pyrolysis, carbon oxidation and other physical and chemical changes as well as momentum, heat and mass transfer of the main equations and equations involved in the parameters.The results show that these equations and parameters significant differences, the experimental model still need to select the auxiliary model.
0 Preface
At present, direct combustion accounts for more than 90% of all biomass applications. Because of the difficulty of biomass crushing of '1' and the large-size application of particulate pollutants, Large size (2mm-150mm, particle size when burning coal<75μm)原料｡垃圾焚烧中也有大量大颗粒生物质物料, 我国城市固体垃圾中与生物质相关的成分(竹木, 纸张, 布与纤维, 动植物垃圾)占20%-50%[3]｡同样, 大颗粒生物质的燃烧也存在于各种火灾, 特别是森林火灾中｡因此, 大颗粒生物质燃烧过程的理论计算是生物质能利用和城市固体垃圾焚烧设备设计, 操作, 更新以及火灾分析控制的基础｡
Large particle biomass combustion process is very complicated, including the internal thermochemical conversion of materials and external combustible gas oxidation, involving biomass drying, pyrolysis, carbon gasification and oxidation, volatile combustion and other physical and chemical processes and porous media momentum, heat, Mass transfer, etc. In addition, it also involves the shrinkage of particles, the development of internal cracks, the melting and detachment of ash, etc. Mathematical analysis is very difficult, and simulation is an effective method for theoretical calculation. The mathematical model describing the above processes is a simulation The key to the accuracy of the results.This paper will summarize and analyze the main mathematical models involved in the combustion of biomass particles and provide references for the theoretical calculation and model development of the combustion process.
1 biomass particle combustion process involved in the calculation of the model
However, due to the complexity of the process, most literatures adopt a one-dimensional model, except for the few models that describe particle combustion in three-dimensional and two-dimensional models, if they are not devoted to studying the characteristics of cracks or ash during engineering simulation. For the irregular particles, the calculation unit (control body) in the one-dimensional model is divided as follows: 4) Take the outer surface of the particle as the base surface and take the shell with the thickness of dr as the outermost unit. The surface of the base surface, take the thickness of the dr shell to do the next unit, followed by until the central entity size is less than or equal to the thickness of the unit, the entity is the last calculation unit. For example, cylinder particle control body partition method shown in Figure 1 For the sake of simplification, the ones discussed later mainly refer to one - dimensional models if they involve space.
1.1 Reaction Area Model
Large-particle biomass combustion involves physical and chemical processes such as drying, pyrolysis, and carbon oxidation, and studies have shown that the drying in the particles is controlled by heat transfer, with carbon mass controlled by mass transfer and pyrolysis / carbon gasification controlled by power and transport. The process of transmission control generally occurs in small areas because the rapid transfer of heat or matter to the reaction area, which is no more than diffusion, can be reduced to surface reactions. However, reactions controlled jointly by power or power and transmission are generally Occurs in a wide area, and is a typical volumetric reaction. Due to this characteristic, a frontreaction model or a volume reaction model is commonly used for particle combustion calculation.
1.1.1 surface reaction model
The surface reaction model assumes that the chemical reaction / physical change takes place on an infinitely thin reaction surface that moves as the reaction progresses. This model is commonly used for the calculation of drying and carbon oxidation processes. In the early Caram'5 ' The membrane, two-membrane model assumes that the carbon reaction occurs only on a solid surface and that the Cano'6 'calculation of the fouling particles assumes a combustion front between the carbon and ash shells. Gupta'7' assumes carbon oxidation Occurred on the surface of carbon particles.He Fang '8' and others in the calculation of the accumulation of carbon powder natural smoldering down (similar to the internal combustion of large particles), it is also assumed that the front of the oxidation reaction occurred between the ash layer and the carbon layer. The entire biomass combustion process (drying, pyrolysis, and carbon oxidation) is modeled using a surface reaction model. For example, Ouedrago'9 'assumes that pyrolysis occurs at a reaction temperature of 773 K and carbon oxidation occurs on the surface of the block. Assuming that the particles are divided into wet material zones, pyrolysis zones, residual carbon zones and ash zones when the particles are burned, the source terms produced by dry pyrolysis and carbon oxidation occur at the interface of the zones. Galgano'2 also assumes that the dry and hot Solution takes place in the very thin On, and apply the method to calculate the integral of propagation drying and pyrolysis in the particle surface.
1.1.2 Volume reaction model
The volumereaction model assumes that the reaction takes place throughout the material and the rate of reaction at a given point is determined by the temperature, gas-solid composition, etc. Calculations are often used in pyrolysis processes such as Groni'11 ', Janse'12' and Yu Chunjiang The pyrolysis process was simulated using the mass, momentum, conservation of energy, chemical reaction and heat and mass transfer equations throughout the biomass 13. Many researchers also used the volumetric reaction model to directly calculate the combustion of the entire biomass particle The process, for example, Porteiro'4 'is also a conservation and kinetic equation for the entire block.
It is assumed that drying takes place only in one calculation unit and the drying rate is determined by the rate of heat transfer to this unit; the rate of carbon oxidation is described by the Arrehnius equation, adding innersurface to the equation for adsorption and desorption. A more detailed mathematical model is used to describe the combustion of wood blocks, taking into account not only the conservation, transport and kinetic equations, but also the tar cracking, the combustion of volatiles, etc. Lautenberger'15 'also uses volume reaction models to analyze wood blocks Oxidation and pyrolysis process, the drying process using Arreh-nius equation described.
1.1.3 Comparison of the two models
Because only complex chemical reactions need to be calculated on the reaction surface, the surface reaction model can greatly shorten the calculation time of the program. However, this method is only suitable for the transmission control process and can not accurately quantify the volume reaction. Similarly, the volume reaction model, Calculation is also very difficult, the equation in the reaction area of the serious and strange can easily lead to non-convergence calculation, even with the optimization method also requires a small space, the time step, the calculation is very time-consuming, 10mm bio-particle combustion using volume reaction method to calculate Dozens of hours.
Peters '16 studied the pyrolysis of 8-17 mm particles at 900 ° C and concluded that under these conditions, drying is controlled by heat transfer and pyrolysis is controlled by both transmission and power. '8' calculates the natural smoldering charcoal downward smoldering, concluded that carbon oxidation is mainly controlled by mass transport. Large particle biomass combustion control reaction and power transmission co-exist in the reaction, the current need to study two Model combining the calculation method, He Fang '17 'and others have conducted a preliminary exploration in this regard.
1.2 Physical Chemistry Process
At present, there are a large number of equations describing the physical and chemical processes such as drying, pyrolysis, carbon oxidation and volatile combustion, and the complexity is quite different. Taking pyrolysis for example, from a simple Arrehnius equation to a series of reactions involving various types of reactions Of the complex equation is conducive to the process of in-depth understanding, but from the engineering application point of view, the equation is too complicated and easily lead to difficult to solve the problem, so engineering calculations generally do not use particularly complex equations. The equation describing the physicochemical process.
1.2.1 Drying
Drying under biomass burning conditions occurs at very high temperatures (500 ° C) and in general dry<200℃)差别较大, 这方面的专门的理论和实验较少｡目前, 干燥速率的方程主要有两类: 一是假设干燥速率由热传输决定[4, 14, 16], 按其物理意义, 蒸发速率为传到干燥前沿净热量除以水分蒸发潜热, 即:
The concept of transmission control theory is clear, and it is believed that the reaction takes place on a very thin surface, consistent with some studies.2,16 The kinetic equation describes the drying process for numerical calculation, but some high-temperature drying kinetic parameters are currently lacking Experimental basis.
1.2.2 pyrolysis
Pyrolysis kinetic equations involved in the literature have obvious differences in terms of form and parameters, and are mainly divided into the following three types from the equation form: The first assumption is that biomass is directly pyrolyzed to generate volatile matter and carbon, Level Arrehnius equation to describe the reaction rate, such as Yang'14 'using the formula:
1.2.5 carbon oxidation
It should be pointed out that the oxidation of carbon is very important in the process of biomass combustion, which determines the important parameters such as the burn-out time, the primary and secondary air ratio, etc. The research on carbon oxidation has a history of nearly one hundred years. Typical models include The single-membrane model and the double-membrane model assume that carbon is directly oxidized to carbon dioxide on a solid surface. The double-membrane model assumes that the carbon is oxidized to carbon monoxide on the surface, CO diffuses outwards and rapidly oxidizes to CO2 on a very thin flame surface. Later, Amundson et al. Proposed a continuum membrane model that considers the oxidation of CO to occur throughout the boundary layer. The continuous membrane model is complex and difficult to use in the calculation. Zhang et al. Proposed the use of a mobile flame frontier model for CO Burning in the boundary layer.
Carbon monoxide and carbon dioxide mixtures are often considered as primary products of carbon oxidation in engineering calculations and engineering simulations. Carbon monoxide in the primary product will be re-oxidised and burned in a gas-phase flame in large spaces where conditions permit. Primary oxidation reactions and products The CO / CO2 molar ratio x is expressed as the equation:
1.3 transmission process
There are many transmission models of momentum, mass and heat in porous media. The main assumptions, calculation methods, calculation equations and coefficients of the transmission process in the literature for calculating the particle combustion process are shown in Table 1.
1.3.1 Momentum transfer
Biomass combustion involves the movement of multiple gases in porous media, especially the steam produced by the drying process, and the forced flow of volatiles from the pyrolysis process in the particles. The flow rate of gas within a particle is often calculated using two methods : First, assume that the gas flows out immediately (immediateoutflow) '4, 10, 16', and the other assumes that the flow agrees with Darcy's theorem.
1.3.3 mass transfer
Oxygen transport in the particles determines the carbon consumption rate, therefore, in the calculation of mass transfer, the general consideration of oxygen diffusion and convection caused by gas flow, and some literature to consider the diffusion of other multiple components. Diffusion coefficient multiplier rms (Product of diffusion coefficient and porosity):
Of course, there are many differences in the diffusion coefficient values or calculation methods of the remaining gases in the literature, which are not described in detail here.
2 Conclusion
1) In the reaction area model, the surface reaction model is suitable for the calculation of transport-controlled reactions such as drying, carbon oxidation, etc. The volumetric reaction model is suitable for the calculation of kinetic or kinetic and transport jointly controlled reactions such as pyrolysis reactions.
2) There are usually two types of models describing physicochemical processes for drying - the heat transfer control model and the first-order Arrehnius formula. There are three types of pyrolysis reactions - single reaction, three parallel reaction and three parallel reaction considering tar cracking (A total of five reactions); carbon consumption of simple oxidation of carbon to consider the reaction of carbon and water vapor, etc .; some literature assumes that volatilization in the gas phase combustion does not affect the combustion of particles, part of the consideration of hydrocarbon volatiles, carbon monoxide And hydrogen combustion.The reaction mechanism and reaction kinetic parameters in different literatures differed significantly.
3) There are two models for the calculation of momentum transfer: rapid outflow model and Darcy's theorem transfer model. The heat transfer adopts the assumption of thermal equilibrium of gas-solid phase, and generally considers the heat conduction and convection. A few consider the radiation Radiation method). The mass transfer mainly considers the convection and the diffusion of oxygen, while the minority considers the diffusion of all gases. The difference of thermal mass transfer coefficient is obvious.
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