The production of gas in large-scale use is meticulously regulated, and coal serves as the primary feedstock for most gas production facilities. Different grades of coal can be used to produce gas, but each with associated requirements and producing gas of a different quality. For example, one grade of coal may produce a gas that is more beneficial for use in a chemical reaction, while another grade may produce something more suitable for equipment operation.
Gasification is the process of producing gas in which coal or a different feedstock is converted into a combustible gas to be used for a myriad of purposes. The quality of gas produced greatly depends upon the quality of the coal and other feedstock used and the type of gasifier operating. There are a number of specific requirements for coal in order to carry out gasification operations effectively, these are laid out below.
Firstly, the coal must have sufficient porosity, as this influences its ability to exchange gases, provide an endothermic reaction and hold ash. The porosity of fuel is most commonly assessed using an analytical technique, such as nitrogen adsorption or mercury porosimetry, to determine the total pore volume of the coal in cm3/g. Coal with porosity levels of 0.2 to 0.6 cm3/g are ideal for gasification operations, however as the gasifier continues to operate, the porosity of the coal can become further increased by the gasification process itself.
The feedstock coal must also contain sufficient volatile matter, as this is pertinent to the quality of the generated gas and calorific value of the coal. The volatile matter content of coal is determined during the coal analyses process and can vary dramatically between different grades of coal. For gasification operations, a sufficient volatile matter content of 78-88% is preferable as it ensures a higher product gas yield.
The moisture content of the feedstock coal is also important, as too high a content can lead to a decrease in the calorific value of the coal, not to mention operational problems. Generally, the moisture content should be below 20%, although it is much more constrained depending on the gasifier design and operational conditions.
The ash content of the coal is also of importance. The lower the ash content the more efficient gasification operations will be, as the ash will cause a decrease in the calorific value of the coal, thus operating at lower efficiency. In addition, the ash can also form a bed in the gasifier, leading to operational issues and reduce the thermal conductivity of the system.
The grain size of the coal is final characteristic that must be ascertained. Generally, the grain size is within a range of 0.5-3 mm, although the specific range can be further determined by the design of the gasification system. If the coal is too large it may not enter the gasifier, whereas if it is too small the reaction of the transformation of the coal may be too quick, leading to operational issues.
The chemical composition of the feedstock and the chemical reactions that take place during the gasification process are two additional areas of consideration. An understanding of the chemical makeup of the coal will allow necessary corrections of acid gases, contents of combustibles and tar, as these three components have an effect on the quality of the generated gas.
To sum up, there are a number of specific requirements for coal when it comes to gasification operations. Firstly, the coal must have sufficient porosity and volatile matter content, with an acceptable moisture and ash content, for efficient use within the system. Additionally, the chemical makeup of the coal and grain size must also be considered. If all these requirements are met, then successful gasification operations can be attained.
