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Most combustion related problems in coal-fired fire-tube boilers are caused by too much air

Most combustion related problems in coal-fired fire-tube boilers are caused by too much air. Common problems with coal combustion. Clinker Forms at the trailing edge of the base of the ignition arch; removal often damages the arch refractory

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Most combustion related problems in coal-fired fire-tube boilers are caused by too much air

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  1. Most combustion related problems in coal-fired fire-tube boilers are caused by too much air

  2. Common problems with coal combustion • Clinker • Forms at the trailing edge of the base of the ignition arch; removal often damages the arch refractory • Forms large sheets on the stoker which bridge the chain grate and block the ash port • Fireside fouling • “Bird nesting” in entrance to first tube pass • Excessive fly ash causing blockages and equipment erosion

  3. Clinker

  4. Location of “bird nesting”

  5. “Bird nesting” and fouling

  6. Proximate analysis(Engineer’s analysis) • Moisture 2% • Ash 13% • Volatiles 25% • Fixed carbon 60% • Gross calorific value 27 MJ/kg

  7. Ash analysis • Percentage composition of oxides of • silica • aluminium • iron • titanium • phosphorous • calcium • magnesium • sodium • potassium • sulphur • Ash fusion temperatures • from 1000degC upwards • Iron in ash reduces AFTs

  8. What happens when temps reach AFTs?

  9. Temperature and colour

  10. How much air is ideal? • Generally, 50% excess of stoichiometric requirement is considered ideal • How do you know that 50% is left over?

  11. Excess air • Air contains 21% oxygen (approx.) by volume • At 50% excess air, 14% O2 used in combustion, 7% O2 remains in flue gases • If flue gas at 14%O2, only 7%O2 used in combustion - 200% excess air!! • Why are boilers run with too much excess air?

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