Any power plant will have fuel storage and handling equipment and systems, the fuel preparation system, water treatment system, the boiler, the ash handling system, compressed air system, lighting system, firefighting system, turbine, generator and electric power gridding system. The coal power plant is no exception. The pros and cons of these systems individually has to be looked into to understand the total value of a coal fired power plant. Here the discussion will be mainly on the fuel handling, fuel preparation, ash handling system, and boiler point of view.
Fuel storage and handling
- The calorific value of coal varies very widely across the globe. It can be as low as 2800 kcals/kg to as high as 7000 kcals/kg on an air dried basis. This means the quantity to be stored, and handled per million kilo calories, will also vary widely.
- Oil and gas, which are classified under fossil fuels, do not have such large variation. Hence storage and handling does not differ much.
- Coal is stored in the open generally; however closed storage is also adopted in a limited basis. Storage of oil and gas need special tankers and equipment.
- Coal requires large and heavy equipment to handle, however oil and gas requires much simpler and compact equipment to handle.
- Coal can result in lot of dust problems, but gas and oil can result in leakage which can explode.
- Coal requires large bunkers near boiler area; oil can be stored in day tanks and gas can be pressure reduced and needs, in most cases, no storage.
Fuel preparation system
The fuel preparation system in the case of oil is a simple heating and pumping unit. In the case of gas it is also a simple pressure reducing and water removing system. In the case of coal fired boiler it is an elaborate system starting from a crushing unit to a grinding unit with a rejects handling system, if the mills in the grinding system have rejects. Optimizing the milling system for optimum performance requires knowledge about the coal being fired and the combustion characteristics, which involves large laboratory tests. In the case of oil, simple laboratory tests estimating the temperature for firing viscosity will be enough. The need to know the chemical composition is for all fuels.
Ash handling system
This is unique to the coal fired boiler. There are two ash handling systems to be provided: the fly ash and bottom ash handling system. This size of this system will depend upon the ash percentage in the coal. However no such system is required for an oil or gas fired boiler.
Boiler design depends on the type of fuel being fired; gas fired boilers have the smallest furnace and the coal fired the biggest. The coal fired boiler furnace size further changes with respect to the amount and type of inorganics (ash) the coal contains. When we compare two boilers firing with the same percentage ash coal- one slagging type and the other non-slagging- the one with the slagging nature will have to be designed with a larger furnace size. Except for the economics of coal fired boiler for steam generation, in all other aspects it has a lesser preference with the users. Coal fired boilers have many more auxiliaries, are cumbersome to operate, the uncertainties in fuel characteristics are high, higher pollutants have to be addressed, etc. There are many types of design available for all fuels, and more so for coal firing with combinations of auxiliaries. The amount of auxiliary power consumption is the highest for coal fired boilers.
All other systems like the compressed air system, lighting system, firefighting system, turbine, generator and electric power gridding system can have the same features for any type of boiler used with any fuel as they all are independent of boiler type and fuel.
Boiler classification can be based on many factors like usage, fuel fired, fuel firing system, type of arrangement etc. Commonly known types are pulverized coal fired boilers, fluidized bed boilers, super critical boilers, oil and gas fired boilers. All cater to industrial and power generation. Hydrocarbon fuels are the major source of energy for power and process steam generation, and coal takes a major share in this. Boiler furnace design will depend more on fuel characteristics, and further heat transfer surface sizing will depend on furnace outlet temperature. Furnace sizing is a very important for a successful boiler design. The variation in boiler furnace size can upset the performance drastically. Oil and gas fired units have more uniform sizes, but the coal fired units have large variation. Optimization of combustion in high ash Indian coal fired boilers is of special interest due to the organic and inorganic mix up and the large amount of variation in the organics.