Types of Gasifiers
Gasification is an efficient, commercially-proven process that converts low-value feedstocks into the building blocks of high-value fuel products.
There is a multitude of gasifier designs that incorporate a variety of mechanical configurations for managing solids and providing the thermal energy required to drive the reaction. Most gasifiers can be classified in terms of the direction of the biomass flow and the method of which heat is introduced. Gasifiers can be divided into these general categories:
- Indirect Gasification
- Direct Gasification
- Hybrid Gasifier
- Gasifier with Reformer
1. Indirect Gasification (shown in Figure 1) refers to the use of pipes, or heat exchangers, typically heated to 1700 F or higher within the reactor. Hot gas is passed inside of the pipes, which maintains the reactor at temperatures of approximately 1500-1600 F. In this design, the amount of thermal energy that can be transferred into the gasification reactor is limited by both the surface area of the hot surfaces and the temperature of the gas supplying the heat. These gasifiers cannot operate at elevated pressure (greater than 60 psig) due to the amount of thermal energy required at higher pressures. These gasifiers are also relatively expensive since special metallurgy is required for the high-temperature heat transfer tubes and the vessel size due to operations at the lower pressure. Since the temperature of indirect gasifiers is typically limited to less than 1800 F, a relatively long residence time (large reactor volume) is needed to convert at least a portion of the tars.
Figure 1: Indirect Gasification Process
2. Direct Gasification (shown in Figure 2) involves the addition of an oxidant (oxygen or air) with the biomass in order to internally generate the required thermal energy for gasification. This class of gasifier is generally significantly less expensive than the indirect type, providing one has a sound design for the oxidant injection nozzle and is able to control the temperature within the constraints of the mechanical design. The oxidant reacts rapidly within a shroud or flame region arising from the injection points. Within this shroud the temperatures can be very high (about 2600 F). These elevated temperatures readily gasify biomass but creates slagging and related solids problems previously discussed.
3. Hybrid Gasifier (see Figure 3): Several commercial gasifiers utilize a Hybrid Gasifier style system that performs the primary gasification using either indirect heat and/or a relatively small amount of oxidant. The solid carbon residues are then passed to a second vessel in which they are partially oxidized to complete the gasification process. The hybrid process allows the carbon residues (soot) to be treated at an elevated temperature. However, these systems have to balance the temperature within the partial oxidation reactor to avoid the solids issues.
Figure 3 – Hybrid Gasifier
4. Inert Solids Gasifier with Reformer[EB1] (Figure 12) is another configuration involving the use of solids transferring between the gasification and partial oxidation reactions. This technology has the advantage of better temperature control through the use of a high solids flux. There are several mechanical configurations that optimize the solids management between the two reactors. A critical component of the design involves the separation of solids within the gasification zone. By converting the excess methane and other hydrocarbons produced during biomass gasification into CO, a reforming catalyst increases the total syngas yield of the process.
Figure 4 – Inert Solids Gasifier with Reformer