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 Gasifier
- Direct Gasifier
- 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.
Figure 2: Direct Gasification Process
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
Gasification is an efficient, commercially-proven process which converts low-value feedstocks into the building blocks of high-value fuel products. Gasification is a flexible technology that converts hydrocarbons such as coal, petroleum coke, and biomass into syngas, which, in turn, can be converted into hydrogen, electricity, and a variety of chemicals, fertilizers, and liquid fuels. Gasification occurs from the endothermic reaction of hydrocarbons with steam at high temperatures. The product of gasification is a gaseous mixture of hydrogen, carbon monoxide, carbon dioxide, and light hydrocarbons.
The gasification process is the same for coal, oil, biomass, oils or any hydrocarbon or carbon based material. The fundamental difference is the O:C and H:C ratios within the matrix. Gasification produces CO and H2. As biomass contains a significant amount of oxygen, more CO2 is generated relative to CO. Because coal contains small amounts of O, it makes more CO.
Gasification is not a complete oxidation (combustion) process, but rather a partial oxidation process. In contrast, combustion processes produce primarily thermal energy, solid waste, criteria air pollutants (NOX and SO2), and carbon dioxide (CO2). In contrast, the gasification process produces syngas. See Figure 1.
Gasification is the reaction of carbon-containing material with steam in order to produce CO and H2 (syngas). Pyrolysis (chemical decomposition) is an unwanted reaction pathway as it produces tars and other soot-like materials that require aggressive conditions to gasify.
SGT’s high pressure gasification system allows operation at higher pressure and temperatures compared with low pressure gasifiers. SGT’s process eliminates the need for a costly compressor, increases carbon efficiency, while costing significantly less than do other gasifiers. SGT’s High Pressure Gasifier offers a number of advantages over inferior technologies.
In other high pressure gasifiers a significant fraction of the inorganic constituents, known as slag, melts at elevated temperatures and flows to the bottom of the high-temperature reactor. The slag requires a costly system to continuously remove the runoff. SGT technology maximizes conversion of carbon, eliminating the need for the costly slag removal system.
SGT's new technology is a directly heated (auto-thermal) gasifier capable of generating syngas at elevated pressures (>150 psig). SGT’s technology enables operation at elevated temperature and pressure without the use of a continuous flame region and the associated elevated temperatures, cost, and operability issues associated with existing high-pressure technologies.
SGT’s proposed new technology significantly reduces cost through use of a second fluid bed vessel which operates at more aggressive conditions. The second vessel operates at higher temperatures (1700 F +) and provides additional residence time to maximize the conversion of all carbon.
SGT’s new technology utilizes a pulsed oxygen system that intermittently adds oxygen. As collapsing solids around the oxygen entry point promote mixing, and as a flame shroud will not develop, the average temperature in the flame region is lower. This oxygen delivery system will greatly reduce the local high-temperature regions and will allow the reactor to operate at higher overall temperatures.
Gasification requires relatively high temperatures (greater than 1400 F). However, pyrolysis (destructive distillation using heat) of cellulose occurs at much lower temperatures (700-900 F). As a result, most biomass gasifier designs require high gasification temperatures (greater than 1600 F) in order to gasify the pyrolysis products formed during the initial stages of the process.
Most operating and planned biomass gasification systems currently operate at low pressure with high levels of CO2. The product syngas must be compressed when used in turbine or catalytic processing. The removal of CO2 reduces compression cost and improves the heating value of the syngas.
The syngas generated from biomass contains a high concentration of CO2 (typically greater than 20%), which negatively impacts downstream catalytic processing. The CO2 level must be reduced when the desired products are oxygenated products such as methanol and DME. When producing diesel, jet and other hydrocarbons the high CO2 acts as a dilutant requiring the use of higher larger reactor volumes (more catalyst), and the CO2 can only be partially offset through operations at higher pressures. Most existing and all planned coal-derived syngas facilities utilize expensive CO2 removal systems such as Rectisol® or amine-based absorption systems that are not economically viable for the smaller biomass systems.
Biomass is introduced into the gasifier’s bottom reactor, which can be heated directly or indirectly, and gasified at modest temperatures in the range of 1500-1700 F. This is consistent with many gasification technologies in use by Front Line, Prime, TRI, etc. During this initial gasification process, the biomass also undergoes pyrolysis, yielding difficult-to-gasify products of tar, soot, and carbon-rich solids. Most gasifier systems use a long residence time (large reactor volume) to gasify as much solids as they can in an attempt to maximize the conversion of the solids at temperatures sufficiently low enough to prevent melting (slagging) of the inorganic components.
Several commercial gasifier designs (TRI, Choren) utilize a multi-stage design approach in which the biomass is converted into two types of materials: syngas and solids. The solids contain both the inorganic constituents contained within the biomass as well as the resultant tars and carbon deposits from the pyrolysis reactions. The solids are separated from the gases and treated at higher temperatures in order to gasify the less reactive carbon compounds.