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Heat and mass transfer inside fixed bed coal gasifiers

Heat and mass transfer inside fixed bed coal gasification


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INTRODUCTION: In order to facilitate a more rational right atmospheric fixed bed gasifier internal structure design, we are very necessary for the gasifier inside physical and chemical processes in depth understanding, including the mass inside heat and mass transfer, the gas phase reaction, gas phase with the movement of the solid, the solid phase reactions of each stage. This article on a certain level of atmospheric fixed bed gasifier internal heat and mass transfer processes are briefly reviewed.

Keywords: atmospheric fixed bed gasifier, heat and mass transfer, coal gasifier, coal gasification

Atmospheric fixed bed gasifier internal heat and mass transfer process is very complicated. On mass transfer in terms of not only their own gas and solid body movement, as well as gas-solid white, solid particles inside the particle external mass transfer process. On the heat, there are gas-solid phase, gas-solid phase furnace walls, solid-phase heat transfer between the various different aspects of the process. Speaking from the mechanism of mass transfer processes are diffusion mass transfer and convective mass transfer, heat transfer process are conduction, convection and radiation, etc.. The heat transfer process comprises the following steps:
(1) conduction within the particles;
(2) contact between the conductive particles;
(3) inter-particle radiation;
(4) inter-particle fluid convection;
(5) the fluid particle radiation;
(6) the fluid conduction;
(7) flow in vivo radiation;
(8) fluid mixing;
(9) conductive particles furnace walls;
(10) the furnace walls particle radiation;
(11) the fluid convection furnace walls;
(12) Fluid radiant furnace walls.
Relatively speaking, the mass transfer process is necessary to simpler, the following three reasons:
(A) particle diffusion can often be ignored;
(2) No mass of the furnace wall;
(3) does not correspond with the radiation heat transfer and mass transfer method.
Heat and mass transfer process may be accompanied by a chemical reaction, or may not be accompanied by chemical reaction. Here for atmospheric fixed bed gasification reactor heat and mass transfer inside a simple induction.
Many features of gases and solids (such as heat capacity, viscosity, mass transfer coefficients, etc.) are a function of temperature and pressure, when the temperature range is small, can be used to simplify the way the average heat and mass transfer mathematical model. However, in the coal gasification reactor, the temperature variation along the height of the bed is large, and therefore must be determined for various properties as a function of temperature.
In a simple model of one-dimensional homogeneous, the bed of the heat transfer to the furnace wall can be used to represent the overall heat transfer coefficient. Typically used in the literature value is generally 15 ~ 35W / (m2 • K), namely 54 ~ 126kJ / (m2 • h • k). Such values are forced convection ranges. And some higher values used in the model, such as Biba model mentioned later, is 217kJ / (m2 • h • k). The heat loss through the furnace wall mainly from gas, that is, the rate of movement of gas in the furnace is high, so its radial effective thermal conductivity coefficient is high.
The wall of the furnace bed overall heat transfer coefficient can have different calculation methods, in addition to selecting experience, but also can obtain the formula. Example, using the formula given Li and Finlayson or by Hobbs et formulas given. As the coal particles in the size and shape variability, plus porosity bed different layers of different height, the overall heat transfer coefficient is difficult to precisely obtain the existing theoretical value calculated as the deviation between the experimental values of 20 % or less when they can be considered accurate enough.
In addition to the overall heat transfer coefficient outside the gas and the heat transfer coefficient between the solid phase is a very important parameter. This factor is calculated to be more difficult, gas-solid phase heat transfer in the furnace disturbances, the existence of chemical reaction, the irregular shape of the coal particles are likely to bring the deviation calculation results, and sometimes even up to this bias a few times. 1963, Gupta and Thodos formula gives a better, Bhattacharya et al, 1986 to establish a fixed bed coal gasifier on the use of a mathematical model of the formula [16]. Hobbs et al, 1992, the coefficient calculation process is simplified, it is assumed along the whole of the bed of coal particles are homogeneous.
1971 DeWasch and Froment are given a set of mathematical formulas, you can calculate the furnace wall bed effective heat transfer coefficient and the coefficient in the gas phase and solid phase respective contributions, Yagi also conducted research in this area, in addition to Hobbs et who also gives a complex formula, the above brief review of the calculation of the heat transfer process, now to discuss the flow gasification reactor, heating rate and the bed voidage. Industrialized Lurgi coal furnace residence time in hours and gas residence time only seconds.
On atmospheric fixed bed gasification reactor, and its gas-solid phase linear velocity of motion were generally less than 3m / s and 0.1m / s, high pressure atmospheric fixed bed gasification reactor, the gas-solid phase velocity generally was lower than 0.3m / s and 0.15m / s. Obviously, the gas linear velocity decreases rapidly due to the presence of stress is greatly reduced in the gasification reactor gas volume flow rate and velocity increased solidus is because the high pressure operation of the coal gasification reactor to increase processing capacity . The line speed value given above is only an estimate of the average linear velocity, in fact, gas and solid phase linear velocity in the furnace is constantly changing. Gas lines of movement velocity impact factors: ① As the reaction proceeds, the amount of gas constantly increasing; ② pressure variation along the bed; ③ changes in temperature along the bed; ④ change of bed voidage. Bottom as the gas movement in the combustion zone while after the bed temperature is decreased continuously, but the amount of gas increases, the pressure drop and the bed voidage will cause the decrease of the gas linear velocity increases. Factors affecting the speed of the solid lines are: ① With continued loss of solid mass flow rate caused by the reduction of the solid; ② increasing bed voidage. From the viewpoint of material balance analysis, the unit time into the gasification reactor and ash out of the gasification reactor are equal, and the entrance to the ash content of the solid stream only represent up to 25% solids stream at the outlet the ash content has to account for 95% or more, obviously the entrance of the solid stream mass flow and volume flow rate have to be greatly reduced, resulting gray partitions low speed movement.
Heating rate and the gas-solid temperature distribution inside the reactor and the residence time for gas-solid phase. In general, solid-vapor phase at a heating rate higher than four orders of magnitude. The combustion zone, as heterogeneous oxidation reaction vigorously proceeds, heating rate is large, and in the gasification zone, gas-solid phase to a slow temperature change much.
On the gasification reactor pressure drop, the operation of the gasification reactor pressure, such as Wellman and Galusha furnace, generally only about 1.1kPa (~ 100mmH2O); high-pressure gasification reactor operation, such as Lurgi furnace, Hobbs et al kPa simulation results are also orders of magnitude. Therefore, in the model calculation, in addition to the calculation of conservation of momentum, the gasification reaction furnace is assumed to be constant constant pressure can be obtained sufficiently accurate results.
Bed bed voidage is the void volume and the total volume of the bed. Generally, the bed porosity of the top layer is 0.3, and even up to 0.7 at the bottom of the porosity. Top of the bed can be substantially void of coal bulk density and particle density obtained. Obviously, the greater the porosity, the smaller the resistance to gas flow, the pressure drop along the smaller bed.


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