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Burning High Ash Coals in Power Plants - A Need of the Hour

written by: Dr V T Sathyanathan • edited by: Lamar Stonecypher • updated: 5/8/2011

Optimization of combustion in high ash coal fired boilers is of special interest due to the organic and inorganic mix and the large amount of variation in the organics. One such experience with Indian low reactive coal in a tangential fired steam generator of 670 T/hr capacity is given here.

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    Indian coals, by virtue of their formation, are in some characteristics different from the European and American coals. Indian coal formation due to the drift origin gives more chances for variations in property and a high amount of inorganic inclusions resulting in high ash content. Petrographic analysis of Indian coals indicates the presence of very high percentages of low reactive constituents like semifusinites, fusinites, etc. apart from subjugation to oxidation/weathering. The above variations have their impact on combustion performance in large capacity boiler furnaces.

    Pulverised coal combustion involves two main sequel, but possibly overlapping, stages: (1) rapid heating and devolatilisation as a result of pyrolysis followed by (2) combustion of char residue emanating from the devolatilisation stage. The first stage is fast, taking times of the order of 0.1 sec. only. The second stage is slow, requiring time of the order of 1 sec. to 2.5 sec. and therefore, thus has a major effect on the size of combustion chamber.

    Optimization of combustion in Indian high ash coal fired boilers is of special interest due to the organic and inorganic mix up and the large amount of variation in the organics. It is found that the high percentage of ash, the low reactives in the organics of coal, the encapsulation of organics in inorganics, the presence of oxidized coals in many cases, and the blending of many types of coals are some of the reasons for varying behavior of coals during combustion.

    Detailed measurements of furnace flame temperature profile in the field were carried out while varying the air distribution pattern. The results of the study shows that the proper understanding of the fuel’s combustion characteristics could help in tuning the combustion regime whereby the flat furnace gas temperature profile could be changed to match closely the normal profile experienced with reactive coals. The temperature in the burner zone could be increased by about 50 Degree C and the furnace outlet temperature could be reduced by about 90 Degree C.

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    This can be achieved by keeping in mind:

    • Indian high ash coals result in high primary air requirements- primary combustion dilution. As an indirect effect, the high percentage of ash in coal makes it necessary to use a higher amount of primary air than required for actual combustion, as the transportation velocity requirement limits the minimum primary air.
    • Secondary air distribution at required elevation is very important.
    • Avoid/reduce all unwanted secondary air at any location and divert them to another needy elevation.
    • Keep mill air flow just above settling velocity.
    • Keep total air flow - 20% excess air @ eco out.
    • Close all fuel air dampers if VM less than 20 - 22% - look at the flame front - decide for higher VM coal.
    • Keep wind box pr. 100 - 150 mm - better distribution across elevation.
    • Adopting these steps has given a large benefit in combustion optimization in Indian power plants.

    Understanding the Coal is Key

    The combustion behavior of Indian coals can vary widely, hence combustion optimization is a must for Indian boilers, and that for low reactive coals, the air distribution plays a very important role. The height at which the maximum quantity of the hydrocarbon is to be burnt will depend upon the reactivity, the petrographic characteristic, and the burning profile of the coal being fired. Understanding the type of coal being fired and correspondingly making proper operational adjustments/modifications will help in combustion optimization and the reduction of unburned carbon in bottom ash/fly ash. The Indian high ash coals are found to give large variation in properties due to its virtue of formation which also affects the performance of the boiler and needs a regular watch and tuning of the operational regime.

    The low reaction rate of the organic in Indian coals makes it necessary to provide a higher residence time. This means that firing coals significantly different from design coal can drastically vary the amount of unburned. In boilers with low residence time, the percentage unburned can be very high both in fly ash and bottom ash if coals of high fuel ratio are used. Oxidized coals need much higher residence time for complete combustion and are not amenable for accurate prediction of unburned carbon loss percentage.

    Related Reading

    You can learn more about operating thermal power plants at Bright Hub. Coal may be defined as a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. In situ and drift are the two major theories of coal formation. Petrographic analysis can give useful information about the combustion of pulverized coal. Petrographic analysis of coal has been used to evaluate the efficiency of pulverized coal fired boilers since 1968. Combustion tuning in high ash pulverized coal fired boilers having direct tangential firing system will lead to optimizing the performance in high ash coal fired boilers. In high ash coal fired boilers, proper combustion will ensure minimum water wall deposits and thus an optimal furnace outlet temperature

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