Combustion is the act of burning. It is the supplying of oxygen to a substance to form CO2 and other substances. There are many materials that can be combusted such as coal, Bio-fuel and fuels. Coal combustion can be: fixed bed or suspension combustion.
Research has shown the records of fossil combustion rate, that flue gases emitted from fossil fuel is on the increase. This has seriously attracted the world towards emission of any kind and conclusions are made which include: the decrements of fossil fuel combustion, legislation in different countries where it is prevalent and many others.
In fluidization and gasification technology, combustion of coal is common. For fluidized bed, coal is feed continuously and at a maintained temperature of 750-900˚c. This temperature is relatively low but has high convective heat transfer due to bed motion. The rate is higher when compared to radiation of furnace. Fluidized bed combustion provides high heat transfer rate, reduced sulphur control and Nitrogen oxide emission due to low temperature.
The problem associated with this combustion method includes:
- Feeding, removal of coal ash, and unreacted coal for recycling. The reaction is as follows
2SO2 +O2 +CaCO3
The SO2 is produced from SO3
- Combustion temperature: this temperature is used in the fusion of coal and ash in the fluidized bed. It prevents corrosion that can be caused by the alkaline metal, salt, silica and vanadium present in coal. It also limits nitrogen oxide formation.
Gasification of Coal is the process of converting coal through a process to obtain combustible products (gases). The products maybe low or high energy content depending on the coal content. The process is widely applied in the industry for energy supply.
STAGES IN COAL GASIFICATION
For high energy gas, the following reactions are involved:
These reactions are exothermic reactions
The products of gasification contain high amount CO, H and other minute products.
Classes of gasification
Gasification is done based on the reactor design or type with little or no consideration on the pressure of the system. It follows thus:
Coal is fed into the system with the following reactions to produce low energy gas.
Coal and steam rich in H2 is fed into the system to produce intermediate energy gas. It has the following reaction:
Here, coal and hydrogen gas react to give intermediate energy gas.
COMBUSTION CONTROL REGULATION
Combustion of natural gas produces toxic substances and are emitted into the atmosphere. Combustion of petroleum products and coal gives more of the toxic substances than natural gases. Therefore, natural gases are regarded as the cleanest of all the fossil fuels.
Due the complexity of coal and oil in structure, (higher carbon ratio nitrogen and sulphur) they tend to emit more toxic gases. This contribute to pollution of the environment. These emission causes the following:
- Global warming and green house effect
- Acid rain
- Increase in sea level due to high temperature
- Many diseases
Combustion and emission are regulated under EPA program. The overall aim of the regulations is to support environmental protection and prevention of pollution in this balanced ecosystem.
Handbook for natural gas transmission and processing, by Saeid M, William A.P and James G.S, Gulf professional publishing 2006. P 367-382
James G.S. gas processing: environmental aspect and methods. Butterworth Heinemann LTD 1993. P 78-95
G.J. Yates, fundamental of fluidized bed. Chemical processes. Butterworth 19999. P 160-161.