Training Manual on Energy Efficiency - APO Asian Productivity ...

<strong>Training</strong> <strong>Manual</strong> on Energy Efficiency for Small and Medium Enterprises
falling through grate bars or being carried away by the furnace draft.
Blending of coal
When coal lots have excessive fines, it is advisable to blend the predominantly
lumped coal with lots containing excessive fines. Coal blending may thus help
to limit the proportion of fines in coal being fired to 25% or less. Blending of
different qualities of coal may also help to supply a uniform coal feed to the
boiler.
1.3.3 Parameters in coal combustion
Fixed carbon:
Fixed carbon gives a rough estimate of the heating value of coal. Higher
carbon means higher calorific value.
Volatile matter:
Volatile matter includes such substances as methane, hydrocarbons, hydrogen,
carbon monoxide, and incombustible gases like carbon dioxide and nitrogen
found in coal. Thus volatile matter is an index of the gaseous fuels present.
The typical proportion of volatile matter in coal is 20 to 35%. The presence of
volatile matter:
• Proportionately increases flame length and helps in easier ignition of coal;
• Sets minimum limits on the furnace height and volume;
• Influences secondary air requirement and distribution aspects.
Ash content:
Ash is inorganic material that will not burn. The typical proportion of ash
found in coal is 0.5 to 40%, and the ash presence has several detrimental
effects:
• It reduces handling and burning capacity.
• It Increases handling costs.
• It affects flame temperature, heat transfer, combustion efficiency, and
boiler efficiency.
• It causes clinkering and slagging, affecting availability.
• It increases auxiliary power consumption.
Moisture content:
The presence of moisture decreases the heat content per kg of coal, typically
by amounts ranging from 0.5 to 10%. It also increases heat loss through stack
gases, due to evaporation and superheating. A small quantity, however, at
times helps in binding fines together.
Sulphur content:
The typical amount of sulphur found in coal is 0.5 to 5%. An excess of it
affects clinkering and slagging tendencies, corrodes mild steel chimneys and
other equipment such as air heaters and economizers, limits the exit flue gas
temperature due to acid dewpoint, and affects efficiency.
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