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Coal combustion

The most common example of solid fuel combustion is pulverised coal combustionRelated applications are: fluidised-bed combustion, coal gasification, biomass combustion and gasification, and the burning of refuse and woodThese processes all involve the initial liberation of volatile material (devol

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Coal combustion

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    1. Coal combustion

    2. The most common example of solid fuel combustion is pulverised coal combustion Related applications are: fluidised-bed combustion, coal gasification, biomass combustion and gasification, and the burning of refuse and wood These processes all involve the initial liberation of volatile material (devolatilisation), which reacts in the gas phase, followed by the subsequent burnout of the remaining char with any inert material remaining as ash

    3. Coal Classification Coals are classified based on their rank from lignite (lowest rank) through subbituminous, bituminous and anthracites (highest rank) The ASTM system is based on the fraction of fixed carbon (ie. combustible material in the coal) and on heating value Large deviations in behaviour still exist within a given rank and many properties of coal (e.g. N-content) are largely independent of rank

    6. Structure of Coal

    7. Although the structure of coal is very random, it is highly planar and layered with a pore volume of approx. 8-20% A fragment of a hypothetical coal molecule is shown in the previous slide In addition to aromatic and aliphatic carbon, coals contain C, H, 0, S and N atoms in a number of different structural groups Coals also contain moisture which may be free water or water which is physically bound within the coal matrix Coals contain a diverse range of mineral matter as ash

    8. Combustion of Coal

    9. Outline for Combustion of Coal: Devolatilisation Devolatilisation Models Particle Heat-up during Devolatilisation The Char Char Burnout Global Reaction Rate Burnout Time Char Surface Temperature

    10. Devolatilisation: It occurs as coal is heated in inert or oxidising- environments Moisture is evolved early during heating At higher temp, gases and heavy tars (volatiles) are emitted Particle may soften and become plastic Extent of pyrolysis varies from a few percent up to 70-80% Both the pyrolysis time and the extent of pyrolysis depends on particle size, coal type and pyrolysis temperature

    11. Devolatilisation Models: Smoot (1991) discusses a number of empirical and semi-empirical models for predicting coal devolatilisation rates Badzioch and Hawksjey (1970) proposed a simple first-order model Postulate that the devolatilisation rate is proportional to the amount of volatile material remaining in the coal: with k = A exp(-E / RT) "Total" volatile matter (v?) is determined from proximate analyses

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