What Is Gasification?
Gasification is the process of converting carbon based products such as coal, biomass, and petrol into carbon monoxide and hydrogen. This is done by causing a chemical reaction between the product and an amount of oxygen in a gasifier to produces a gas mixture known as synthetic gas, or syngas. Gasification is a highly efficient method for obtaining energy from organic materials, and can be used as a method for disposing of waste. Gasification was developed in the 1800s and was previously used to produce gas for lighting and cooking. However, shortly after, town gas and electricity become more widespread and so gasification was less important.
Syngas is more efficient than the original organic material, as more of the energy in the fuel is extracted. Syngas produced by gasification can be used in vehicle engines, and can also be used to produce methanol and hydrogen. Gasification of fossil fuels to create electricity is commonplace, but almost any type of organic material can be used. In fact, syngas from gasification was used to fuel cars during WW2.
Gasification allows organic products that are not naturally thought of as fuel to be used as fuel. Biomass and other organic waste can be converted into fuel using gasification. The gasification process also cleans the fuel and reduces chloride and potassium. Syngas from gasification burns cleanly and without any harmful emissions. In fact, the only emissions from gasification are water vapour and carbon dioxide.
There are currently four types of commercial gasifier available, counter current fixed bed, co- current fixed bed, fluidised bed and entrained flow.
The Counter Current Fixed Bed (up draft) Gasifier has a fixed bed of carbon fuel such as coal or biomass, through which steam, oxygen or air flows through in a counter clockwise direction. The throughput of this method of gasification is quite low, but thermal efficiency is high. Tar and methane production is quite considerable at normal operating temperatures, meaning that the syngas must be cleaned before it can be used.
The Co-current Fixed Bed (down draft) Gasifier is similar to the Counter Current Fixed Bed Gasifier, except that oxygen flows downwards. Gas produced by this method of gasification is at a higher temperature and has similar energy efficiency to the Counter Current method. Tar levels are much lower than Counter Current gasification.
Fuel is fluidised in oxygen, steam or air in the Fluidised Bed Gasifier. The operating temperature is much lower, meaning that the fuels need to be highly reactive. Fluidised Bed Gasifiers are mainly used for fuels that form a highly corrosive ash.
In the Entrained Flow Gasifier, a solid fuel or a liquid fuel is gasified with oxygen. The gasification reactions occur in a dense cloud of very small particles. Using high temperatures and pressures means that a high throughput can be achieved, but the thermal efficiency is reduced, as the gas must be cooled before it can be cleaned. The high temperatures used in this form of gasification mean that tar and methane are not produced, but more oxygen is required.
Gasification has many environmental benefits as it is a form of renewable energy. Biomass gasification could be a significant factor in terms of the environment, and bioenergy, as biomass removes CO2 from the atmosphere. Biofuels produced by gasification such as biogas and biodiesel are carbon neutral, and the gasification process allows a much more varied range of organic products to be used. Looking to the future, biomass gasification is currently one of the most convincing forms of renewable energy in the quest to become carbon neutral. There is currently very little gasification carried out on a large or commercial scale, but, as the use of biomass and gasification become more widespread, the more popular and important gasification will become.
There are several additional gasification processes under development, as gasification has distinct advantages over incineration. Gasification allows waste products to be recycled and so reduces the reliance on traditional fossil fuels for energy production. Gasification also reduces the amount of landfill as more waste is reused.
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