Table B-20 Composition of synthesis gas from biomass gasification.
| Constituents | % v/v (dry, nitrogen free |
|---|---|
| Carbon monoxide (CO) | 28-36 |
| Hydrogen (H2) | 22-32 |
| Carbon dioxide (CO2) | 21-30 |
| Methane (CH4) | 8-11 |
| Ethylene (C2H4) | 2-4 |
Gasification is the thermochemical conversion of biomass into gaseous fuels by means of partial oxidation of the biomass at high temperatures. The low calorific gas (low heat content gas) that is produced can be fired directly or be used for firing engines and gas turbine cycles and can also serve as syngas in the production of chemicals (e.g., methanol). Gasification is not a new technology; however, its use for the conversion of biomass into a viable fuel has only been investigated for the past thirty years. Production of synthesis gas, or syngas, from biomass can be accomplished by two broad categories of the chemical and thermal processes, namely catalytic routes and noncatalytic processes. Typically, noncatalytic processes require a high temperature of operation – as high as 1,300°C (2,370°F) – whereas catalytic processes can be operated at substantially lower temperatures. With advances in catalysis, the temperature requirement is expected to be reduced even further.
Two reactor types exist: (i) the fluidized bed reactor, which is used with large-scale gasification system, and (ii) the fixed bed reactor, which is employed for small-scale producer gas systems. There are three varieties of fixed bed reactor: (i) the updraft reactor, (ii) the downdraft reactor, and (iii) the crossdraft reactor. Each reactor type produces a different ratio of gases at different temperatures.
In the fluidized bed system, fluidizing gas and heat for the gasification can be supplied by the combustion of a hydrocarbon such as natural gas (methane) or propane in the absence of air. The particulates and char are removed using a high temperature cyclone.
The most common method of gasifying biomass is using an air-blown circulating fluidized bed gasifier with a catalytic reformer, even though there are many different variations. Most fluidized bed gasification processes use closed-coupled combustion with little or no intermediate gas cleaning. This type of process is typically operated at around 900°C (1,650°F), and the product gas from the gasifier contains hydrogen, carbon monoxide, carbon dioxide, water, methane, olefins, benzene, and tar. Gasification uses oxygen (or air) and steam to help the process conversion, just as coal gasification. While the effluent gas from the fluidized bed gasifier contains a decent amount of syngas compositions, the hydrocarbon contents are also quite substantial. Therefore, the gasifier effluent gas cannot be directly used as syngas for further processing for other liquid fuels or chemicals without major purification steps.
Indirect gasification is another gasification process technology that takes advantage of the unique properties associated with biomass. As such, indirect gasification of biomass is substantially different from most coal-based gasification process technologies. For example, biomass is low in sulfur, low in ash, highly reactive, and highly volatile. In an indirect gasification process, biomass is heated indirectly using an external means such as heated sands and a typical gaseous product from an indirect gasifier is close to a medium-Btu gas.
A promising concept is the biomass integrated gasification gasification/combined cycle (BIG/CC) system. Gas turbines can convert gaseous fuel to electricity with a high efficiency. An important advantage of BIG/CC systems is that the gas is cleaned before being combusted in the turbine which means that more compact and less costly gas cleaning equipment is required. The integration ensures high conversion efficiency; on a scale of 30-60 MWe, net efficiencies of 40-50% (LHV basis of the incoming fuel) can be expected. BIG/CC systems offer a relatively high efficiency on a modest scale. This is important to limit transportation distances for the biomass delivered. A variety of biomass gasification processes are commercially applied.
Another process option for biomass gasification for syngas production involves the use of an entrained flow reactor. This type of process is operated at a high temperature, around 1,300°C (2,370°F), and without use of catalyst. The high temperature is necessary due to the fast reaction rate required for an entrained reactor whose reactor residence time is inherently short.
If a specific biomass feed has high ash content, which is not very typical for biomass, slag can be formed at such a high temperature. Learning from the research developments in coal gasification, a slagging entrained flow gasifier may be adopted for high-ash biomass conversion. Another important process requirement, in addition to high temperature and short residence time, is the particle size of solid feed that must be fine for efficient entrainment as well as for better conversion without mass transfer limit. However, pulverization or milling of biomass is energy-intensive and costly, in general. To facilitate an efficient size reduction of biomass feed, two options are most commonly adopted, viz., torrefaction and pyrolysis.
Production of synthesis gas from biomass by means of gasification also allows for the production of methanol or hydrogen, each of which may have a good future as transportation fuels. For the production of methanol or hydrogen, indirect or oxygen-blown gasification processes are favored because of the medium calorific gas that is produced by such processes.
See Biomass – Pyrolysis, Gasification, Torrefaction.
Biomass – Gasification Principles
The biomass gasification is complex and proceeds primarily through a two-step process, pyrolysis followed by gasifi-cation. Pyrolysis is the decomposition of the biomass feedstock by heat. This step, also known as devolatilization, is endothermic and produces 75 to 90% volatile materials in the form of gaseous and liquid hydrocarbon derivatives. The remaining non-volatile material, containing high carbon content is referred to as char. Volatile hydrocarbon and char are subsequently converted to syngas in the second step, gasification.
While pyrolysis begins at temperature in excess of 500°C (930°F), gasification reactions are predominating at temperature nearing and excess of 1,000°C (1,830°F). The following are the details of the reactions during gasification process.
Exothermic Reactions
Combustion: C + O2 → CO2
Partial Oxidation: 2C+ O2 → CO
Methanation: C + 2H2 → CH4
Water Gas Shift: CO + H2O → CO2 + H2
Carbon Monoxide Methanation: CO + 3H2 → CH4 + H2O
Endothermic Reactions
Steam Carbon Reaction: C + H2O → CO + H2
Boudouard Reaction C + CO2 → 2CO
See also: Biomass – Gasification.
Biomass – Gasification Process
Biomass gasification is the complete conversion of biomass to gaseous products consisting of carbon monoxide (CO), hydrogen (H2), and methane (CH4), with traces of other gases depending on the character