Figure 16.13 is a schematic diagram of the entrained bed coal gasifier when the solid fuel and gaseous stream are flowing in same direction. Entrained bed gasifiers are generally modeled as plug flow reactors. Now, as the solid fuel flows along with gaseous stream, it undergoes several reaction intensities depending on different operating parameters (like temperature, pressure, concentration, and time). During the whole process, there is continuous exchange of mass and energy between the gaseous and solid fuels.

Development of the model for the entrained bed gasifier is divided into three parts: (a) mass balance, (b) determination of the equation for calculating reaction rates, and (c) heat balance. Thus, the iteration of the equations developed from these three parts will give the desired output.

Figure 16.13 shows the model chart for the entrained flow reactor. Here it is assumed that the solid particles and gas are in one-dimensional plug flow in the axial direction and radially well mixed. For this case, the mass balance equation can be written as:

Mass balance for solid component

^ = — NArATs, L [16.38]

^ tv » /■

W = W (1 — X),

5 soK s

where Ws = flow rate of the solid (g/s), Wso = initial solid (g), NV = number of coal particles per unit volume, A = cross sectional area of the gasifier (cm2), Ts = temperature of the solid (K), L = gasifier length (cm), r = rate of solid gas reaction (gs-1), Xs = solid conversion.

Mass balance for the gas component

where Fgl = flow rate of gas (mol s-1), vlk = stoichiometric coefficient for the th gaseous components in kth solid gas reaction, | = extent of reaction (mol s-1).

Now the diffusion of the gaseous component into the solid component is governed by the reaction rates. Thus, the overall reaction rates can be defined as:

5

,</./. sJ’Kl. lul,. I 16.421

A=l

where, rk(Ts, L) = xsk(T)(Pyi)n4nr2ps, k = 1,. . .,5, N = number of coal particles per sec (s-1), x = surface reaction rate coefficient (gs-1 cm-2 atm-1), y = mole fraction of the gaseous component.

Using the above equation the coal conversion can be predicted and the size of the particles can be known from the relation:

" 5

Х0«г*(7’.’

_ *=i

where a = amount of gaseous reactant required to react with the unit mass of coal (mol g-1), D = diffusion coefficient of the gas (cm2 s-1), R = universal gas constant (KJ mol-1 K-1).

The expression for the rate of energy transfer (considering conduction and radiation) between the solid and the gas phases is as follows.

-еиа(г;-Т„4)^о,-л.(т.-Г„>о,

where cps = specific heat capacity of the solid (J g-1 K-1), DH = heat of the reaction (cal g-1), Я = thermal conductivity (cal s-1 cm-1 K-1), є = emissivity, о = Stefan — boltzman constant (cal s-1 cm-2 K-4), Di = internal diameter of the gasifier (cm).

Thus, the system of non-linear equations is developed by conducting mass and energy balance. By solving this system the composition of different product gases can be obtained.

The relation of the dimension (length, diameter, and thickness of the entrained reactor and the primary and secondary nozzle diameter) with coal capacity has been developed by Kim and Kim (1996). The results are shown in Fig. 16.14.

16.14 Design graphs for entrained bed gasifiers (Kim and Kim, 1996).

16.5 Conclusions

To secure a quality of life for current and future generations, sufficient water, land, and energy must be available. It is generally recognized that human development cannot continue to depend on fossil fuels in the present manner forever. Therefore, the issue is not whether renewable biofuels will play a role in providing energy but to what extent, and what the implications of their use will be for the economy, for the environment, and for the global security. What is seldom mentioned is that even in a ‘sustainable world’ not only energy but also carbon for organic chemicals, including plastics, is required.

Over the years, we have seen that the principal roles of syngas have shifted from domestic heating fuel, to feedstock for Fischer-Tropsch (F-T), to petrochemical feedstocks, to starting materials for alternative fuels, to IGCC, and to hydrogen sources. The only way to produce these useful resources from waste and biomass is to first gasify them in order to make syngas. In electric power generation, IGCC has contributed tremendously to improvement of power generation efficiency, thus keeping the cost of electric power competitive against all other forms of energy. Interest in methanol and dimethylether is revived due to the ever-rising cost of conventional clean liquid fuel. With the advent of hydrogen economy, there is no doubt that the use of hydrogen in combination with fuel cells as a transport fuel will improve the climate by eliminating CO2, NOx, CO, hydrocarbon, and soot emissions — and this is a prospect that could become reality within two decades.

The issue here is how to produce this hydrogen and make it available in a useable form. Gasification, coupled with water-gas shift, is the most widely practiced process route for biomass to hydrogen; however, it needs to be refined further. It is our opinion that gasification can and will have an important role to play in the coming decades. Therefore, more advances are expected in the areas of product gas cleaning, separation and purification, feedstock flexibility and feeding, disposition of ash/slag, plant availability, economics of scale, and integrated or combined process concepts.