16.6.1 Designing of fixed bed gasifier

One-dimensional modeling is generally employed to study the fixed bed gasifier. It is not only simple but also provides a better understanding of the engineering design and process optimization for the fixed bed gasifier. The modeling of updraft gasifier explained below is taken from De Souza (2004).

Figure 16.12 shows the model chart for the updraft gasifier. Here the gasifier is divided into two segments: gas and solid, flowing in a counter current direction.

There is continuous exchange of mass and heat along the interface of these two phases in the radial direction. The flow of each phase is assumed to be in plug flow mode. Also, there is no momentum transfer between the phases, which means the velocity profile of one phase is not affected by another. Thus, for the model chart developed, the following equation can be written to represent the mass and energy balance.

Mass balance for gas

.. dP,

Mass balance for solid:

where RMGj and RMSj is the rate of production if positive and the rate of consumption if negative. p represents the density of the respective phase. u represents the velocity of the respective phase.

The mass flux of chemical species j in gas in the z or axial direction is given by

And the mass flux of chemical species j in the solid is given by

where SS and SG are the fraction of area available for the flow of the solid and the gas phases, respectively, and can be defined in terms of the voidage of the reactor,

e=lk, A-I, A = _L.

V ’ Sa є Ss~ 1-е

where VG is the volume occupied by the gas and V is the total bed volume.

The energy balance equation for the gaseous phase and the solid phase can be written as:

Fccc dz — £S(f<j. g + Rc. c + R*.C + R«.c)’

Fscs dz — 0 £^{Rq, s + Rc„s + R/ij + R/u)’

where Rq is the energy source or sink, RC is the convective heat transfer, Rh is the enthalpy addition (or subtraction) from one phase to another due to mass transfer between the phases, Rr is the radiation heat transfer between the phases. Thus, solving equations [16.28] to [16.34], one can get the composition of the gas and also the conversion of the solid fuel.