For any design, specification of the plant is very important. The input includes the specification of the fuel, gasification medium, and product gas. A typical fuel specification will include proximate and ultimate analysis, operating tem­peratures, and ash properties. The specification of the gasifying medium is based on the selection of steam, oxygen, and/or air and their proportions. These parameters could influence the design of the gasifier, as follows:

• The desired heating value of the product gas dictates the choice of gasifica­tion medium. Table 6.4 gives typical ranges of heating value for different mediums.

• Hydrogen can be maximized with steam, but if it is not a priority, oxygen or air is a better option, as it reduces the energy used in generating steam and the energy lost through unutilized steam.

• If nitrogen in the product gas is not acceptable, air cannot be chosen.

• Capital cost is lower for air, followed by steam. A much larger investment is needed for an oxygen plant, which also consumes a large amount of auxiliary power.

• Equivalence ratio

For the product gas, the specification includes:

• Desired gas composition

• Desired heating value

• Desired production rate (Nm3/s or MWth produced)

• Yield of the product gas per unit fuel consumed

• Required power output of the gasifier, Q

The design outputs of process design include geometry and operating and per­formance parameters.

Basic size includes reactor configuration, cross-section area, and height (hardware design). Important operating parameters are: (1) reactor temperature; (2) preheat temperature of the steam, air, or oxygen; and (3) amount (i. e., steam/ biomass ratio) and relative proportion of the gasifying medium (i. e., steam/ oxygen ratio). Performance parameters of a gasifier include carbon conversion and cold-gas efficiency.

A typical process design starts with a mass balance followed by an energy balance. The following subsections describe the calculation procedures for these.