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The world's thermal power is mainly dependent on coal as its fuel. When designing a boiler, fuel analysis plays a major role. The performance of the boiler, and ultimately the entire unit, can change considerably if the coal being used is substantially different from that for which the boiler was designed.
What is fuel?
- Any combination of organics and inorganic material which during chemical reaction or transformation gives out large amount of heat is called fuel
- Fuel can be hydrocarbon fuel and non-hydrocarbon
- Industrial fuels have heat values from as low as 500 kcal/kg to as high as 11000 kcals/kg
Heat generated by fuel is used a boiler to generate steam for process, power generation, and a variety of other applications. The chemical characteristics of the fuel decide many aspects of boiler design. The boiler designed for gas fuel will have the smallest furnace size, and the boiler designed for coal will have the biggest size.
Why is coal different?
- All fuels are hydrocarbons
- Gas and oil have defined hydrocarbons and structure, which means C & H in fuel does not vary much
- Coal is a heterogeneous fuel and has only an assumed structure – C & H vary highly
- C & H in Coal can be only be known if you do an ultimate analysis, and the way in which these hydrocarbon behave can be different from one coal to the other. This will depend on its reactivity and formation.
- Formation of coal has vast impact on boiler design; the Gondwana coals are different from American and European. (Coal formation theories).
Why is consistency in hydrocarbon important for boiler performance?
- It starts from combustion air calculation; the carbon hydrogen ratio decides the quantity of combustion air.
- Flame temperature is dictated by the chemical composition of the fuel, and this changes the furnace behavior.
- The completion of combustion is another very important factor in boiler design. This will depend on how the hydrocarbon rings are formed and bonded. If the coal burns slowly requiring more residence time then the SH and RH (superheater and reheater) behavior changes when compared to the fast burning coal. This will lead to large injection in SH and/or RH which will reduce the life of the tubes and also reduce boiler efficiency.
- The amount of unburned material in fly ash and bottom ash of the boiler can change, and, if it increases will increase fuel requirement, maybe only marginally.
- Lack of consistency can lead to loss of the capability to generate when the ash level increases beyond a threshold of the fuel preparation system capacity. This can also lead to limitations on the fuel handling system.
Major effects of fuel on boiler design
- A wide range of coal properties can lead to oversized pulverizing/firing equipment which will in turn lead to a limitation on turn down and a possible increase in auxiliary power consumption. This will also increase the furnace size and the SH and RH will need to be sized properly, which can be difficult at times, to get the rated outlet temperature.
- Oversizing of pulverizers and air heaters to handle occasionally high moisture coals will require excessive tempering air when handling normal moisture coals. Ultimately all these will result in higher gas outlet temperature, reducing the boiler efficiency.
- The behavior of the ash during the combustion process and managing the coal-ash are the major issues in boiler design. Deposition and slagging, heat transfer upsets, and fouling in convection passages are some of the major issues to be dealt during operation of the boiler.
Steam generator performance is greatly influenced by the coal/ash properties. Changes in coal property result in efficiency variation and the designer may not be able to demonstrate the guarantees. This can also result in changes in power consumption, load limitations, boiler availability loss due to slagging and fouling, tube failure increases, and plant heat rate reduction. There are known and unknowns with coal during combustion. The known are the coal Proximate, Ultimate, Calorific Value, HGI, and Ash analysis. The unknown are Petrographic analysis, Reactivity, Slagging and fouling analysis, etc. All unknown become known only after the combustion of coal in the boiler furnace and at the cost of boiler performance.