The design methods for each type of gasifier would be different, as they differ in their operation. We know every reaction is governed by three main parameters: time, temperature, and turbulence. So before going into detail in designing we must be clear about which parameters among these three are most important for each type of the gasifier.

• Fixed bed gasifier: residence time

• Fluidized bed gasifier: turbulence (better mixing)

• Entrained bed gasifier: temperature.

Being that fixed bed gasifiers are governed by residence time does not mean that other factors do not play a role. In fixed bed gasifiers, it is very difficult to create turbulence and it is even more difficult to maintain a uniform temperature difference. Thus, by maintaining higher residence time in the reactor one can get higher conversion. Similarly, in the case of the fluidized bed gasifier, better mixing of the fuel with the bed material can result in higher conversion as the residence time is very small. In the case of the entrained bed gasifier, a very high temperature is maintained, as the residence time is low and mixing is poor. Thus, innovation in entrained bed gasifiers is always focused on how to create better mixing/turbulence or increase residence time in the reactor, while in fluidized bed gasifiers it is focused mainly on increasing the residence time and maintaining uniform temperature. However, in the case of fixed bed gasifiers, new ideas are being researched to develop uniform temperature distribution. In the design process, these key points should always be kept in mind.

There are different approaches to design but every design method consists of two major sections:

1 Mass balance: This section identifies the amount of mass that is going into and out of the system.

2 Energy balance: This section identifies the amount of energy that is going into and out of the system.

The idea is to first define the system and then identify the correct mass and energy balance for that system.

446 Handbook of biofuels production