From 2GreenEnergy Intern Fabio Porcu: Gasification
Gasification is a thermochemical process which allows the conversion of materials that are rich in carbon, such as coal, petroleum, or biomass, into carbon monoxide, hydrogen and other gaseous compounds.
The thermal degradation process requires high temperatures (above 700-800°C) in the presence of a sub-stoichiometric proportion of an oxidizing agent, usually air (oxygen) or steam. The resulting gaseous mixture constitutes what is called syngas and is itself a fuel. Gasification is a method of extracting energy from different types of organic materials, and also provides the added benefit of the treatment of waste.
The use of the gasification process for the production of heat presents some advantages compared to direct combustion; however, it introduces some complications too. Syngas can be burned directly in internal combustion engines, or used to make hydrocarbons like methanol, ethanol, gasoline and diesel by the Fischer-Tropsch process. Today, plants that generate synthetic fuels via the gasification of biomass are very few in number, and plants primarily use coal as feedstock. Gasification can also use raw materials that otherwise are not very useful as fuels, such as organic waste. Furthermore, the gasification process allows the elimination of problematic elements from combustion such as ashes, chlorine and potassium; it produces a very clean gas.
Considering that almost any type of organic material can be used as feedstock for gasification, such as wood, biomass, or even plastic, this can be a useful technology as we wish to increase the contribution that renewable energy makes to our overall energy mix.
Thermoelectric power plants built around coal gasification allow us to reduce pollutants and to obtain high efficiency. In addition, gasified coal is highly flexible, and can be used, in addition to the production of electricity, for transport and in various chemical industries.
These systems are called IGCC, or Integrated Gasification Combined Cycle. Coal is not burned directly, as it is in coal power traditional (that is, PCC, Pulverised Coal Combustion), but reacts with oxygen and steam to form the syngas (mainly hydrogen and carbon monoxide). After being purified a second time, it is then burned in a gas turbine to produce energy, then re-used to produce steam to power a steam turbine.
Gasification is based on chemical processes that take place at temperatures above 700°C, which differentiates it from biological processes such as anaerobic digestion that produce biogas at temperatures just above ambient.
In a gasifier, the carbonaceous material undergoes several different processes:
- The pyrolysis process: this is done by heating the material in the absence of oxygen. In this process, hydrogen and methane are released and a carbonization occurs. The result is a weight loss greater than 70% for coal. In addition to the gases, it is also produces various forms of solid carbon, e.g., carbon black. The process depends on the characteristics of the carbonaceous material.
- The combustion process: this occurs when the volatile products and part of the carbon react with oxygen to form carbon dioxide and carbon monoxide (partial oxidation), releasing heat for the subsequent gasification reactions.
- The gasification process: this occurs when coal reacts with carbon dioxide and with water vapor, producing carbon monoxide and hydrogen:
C + CO2 → 2 CO
C + H2O → CO + H2
the carbon monoxide produced reacts with water vapor giving rise to a balance reaction
CO + H2O ⇄ CO2 + H2
After the initial pyrolysis is introduced into the reactor with a limited amount of oxygen, part of the organic material burns producing carbon monoxide and energy, useful for the subsequent reaction that converts further organic material into hydrogen and more carbon monoxide.
Gasifiers differ according to some design parameters. The most common are:
- Fixed-bed gasifiers
- Fluid-bed gasifiers
- Entrained bed gasifiers
As shown in the graphics below, in fixed-bed gasifiers, the fuel is positioned in such a way as to form a bed which can be crossed by the combustion air in the direction of:
- co-current (down-draft configuration), in which both the oxidizing agent and the fuel follow a downward motion,
- counter (up-draft configuration) in which the oxidizing agent rises to the top while the fuel drops down the reactor.
In Entrained flow gasifiers, gasification is performed using spray reagents and oxygen (less frequently air) under the co-current. High temperatures (> 2000°C) and pressure of this technology give it a greater productivity; however, the thermal efficiency is much lower and the gas must be cooled before being subjected to cleaning. At the operating temperatures tars and methane are not present in the product gas; however, the amount of oxygen required is higher than the other types of gasifier.
Gasification is also used for the production of electricity using integrated gasification combined cycle (IGCC). IGCC is a more efficient method for the capture of CO2 compared to conventional technologies. Demonstration IGCC plants have been operating since the 1970s and some of the plants built in the 1990s have now become commercial. It is possible to exploit gasification products as raw materials for the production of ammonia and liquid fuels; there is also the possibility to produce methane and hydrogen for fuel cells.
Over the last years, various gasification technologies have been developed that use plastic waste. In Germany, a plant adopts such technology on a large scale to convert plastic waste into methanol, upon production of syngas.
Small scale gasifiers, fueled by agricultural biomass, are widespread in India, especially in the State of Tamil Nadu. Most of the applications are under 9 kW, and are used to pump drinking water (with a saving of around 70% of the cost compared to the classical electrical network) and for street lighting.
Entrained bed and fluid bed gasifiers are robust and versatile but more difficult to design, build, and operate; they are also expensive and not suitable for installations of small scale (less than 1 MW). The fluid bed gasifiers also require fuels, such as wood biomass, with a high melting point of the ashes (> 1000°C), while they are inadvisable with herbaceous biomass, with a melting point of the ashes sometimes less than 700°C, this value therefore represents a limit for the operating temperature. Fixed bed gasifiers are the most common, especially in developing countries, due to the simplicity of design and construction, and with the low cost of initial investment, management and maintenance.
