Biomass properties such as higher volatile content, higher moisture contents, and complex reaction kinetics create challenges in predicting its performance as a fuel for gasification and make it equally difficult to design a gasifier to obtain the desired output. A number of methods have been proposed and used to predict the performance of fluidized bed biomass gasifier. They are zero dimensional, 1D, 2D, and 3D. Very limited work has been done on 2D and 3D modeling. The most frequently used method is equilibrium modeling, as it is easy and gives a quicker prediction of gasifier performance. Equilibrium modeling is the zero dimensional space independent modeling method and is helpful in identifying the maximum possible conversion of biomass and the theoretical efficiency (Huang and Ramaswamy, 2009). Ramayya et al. (2006) have used a stoichiometric equilibrium model to carry out a feasibility study of the coffee husk fluidized bed gasifier. Adhikari et al. (2007) have used a non-stoichiometric equilibrium model for studying the hydrogen production from steam reformation of glycerine and the optimum condition for higher hydrogen yields was at temperatures higher than 900 K, with a water to glycerine ratio of 9 at atmospheric pressure. Non­stoichiometric equilibrium models have been used for modeling steam gasification of coal and pure carbon fuel to predict production of hydrogen from ethanol in the presence of CaO (Florin and Harris, 2008). Jarungthammachote and Dutta (2007 and 2008) used the stoichiometric model to study a downdraft waste gasifier and non-stoichiometric model for both spout bed and spout fluid bed gasifiers. Thus, equilibrium modeling can be very helpful in modeling fluidized bed gasifiers for use with non-convectional biomass like coffee husks, glycerine, ethanol, etc., whose reaction kinetics are not identified correctly. The equilibrium model considers only the mass and energy balance and does not take into account the kinetics of the reaction, so the results obtained may differ a lot from the practical results.

Gasification consists of homogenous and heterogeneous reactions, whose reaction kinetics, as well as mass and energy transfer phenomenon, depends on the operating conditions, and accordingly the product gas composition and yield changes. To overcome this disadvantage and to predict the gasification performance more closely to reality, different simulators were developed. Nikoo and Mahinpey (2008) have used ASPEN PLUS for simulating an atmospheric bubbling fluidized bed biomass gasifier. The AS PEN PLUS simulator uses Gibbs free energy for simulation of product from homogenous reaction and reaction kinetics for char gasification. A shrinking core model has been used for kinetic modeling. Enden and Lora (2004) used a CSFB simulator to predict the performance of fluidized bed biomass gasifiers in terms of maximum char conversion obtained, as well as the amount of tar present in the gas produced, its hot and cold gas efficiency and the heating value of the gas produced, considering point to point mass and energy balance, chemical reactions kinetics and fluidization dynamics. So, once the preliminary sizing is done, one can use this simulator to evaluate whether the designed gasifier will give the desired performance or not.

Guo et al. (2001) have used a hybrid neural network model for simulating a steam fluidized bed gasifier to predicting the gas yield and composition of the gas. Because of its complex nature, it is rarely used for modeling the gasification process.

Other mathematical models were also developed to simulate the fluidized bed biomass gasifier. Raman et al. (1981) developed a one dimensional model to study the gasification of feedlot manure. Their work does not consider the devolatilization step and considers only the char gasification and water gas shift reaction. Ji et al. (2009) have used a 1D non-isothermal model to study steam gasification of biomass in the fluidized bed. Ergudenler et al. (1997) have developed a kinetic — free homogeneous equilibrium model for predicting the steady state performance of a fluidized bed straw gasifier. The Department of Energy has developed a design chart, but this is for the gasification of coal.

The first step in the design of a gasifier is to define the input and expected output. Depending upon the gasification process choice and its configuration, the input and output parameters can also vary, but in general the input and output could be listed as below:

Design input:

1 Fuel

(a) Proximate and ultimate analysis

(b) Feedstock temperature

2 Gasifying medium

(a) Choice of the medium steam, oxygen, air, or a mixture in suitable proportion.

(b) The gasifying medium may be chosen based on the following criteria:

(i) The desired heating value of the product gas.

(ii) Hydrogen content of the product gas can be maximized with steam, but if it is not a design priority, oxygen, or air could be a better option.

(iii) If an nexpensive source of external heat, such as waste recovery is available, steam is a good choice.

(iv) If N2 in product is not acceptable, steam or oxygen are to be chosen.

(v) Capital investment is lowest for air as the medium, followed by that for steam. Much larger investment is needed for an oxygen plant, which consumes a large amount of auxiliary power as well.

3 Product

(a) Desired composition of product gas

(b) Desired heating value

(c) Desired output of the gasifier (Nm3/s or MWth produced)

Design outputs:

1 Geometry

(a) Reactor configuration, its cross-section area and height

2 Operating parameters (a) Reactor temperature

(b) Input temperature of the gasifying medium

(c) Amount and relative proportion of the gasifying medium

3 Product

(a) Yield of product gas

(b) Composition of product gas

4 Performance parameters

(a) Carbon conversion efficiency

(b) Cold gas efficiency