Knowledge Brief

How is fuel preparation done in a Zigzag kiln?

In a Zigzag kiln, solid fuels or their appropriate mixtures are fed, which include coal, sawdust, rice husk, wheat straw, mustard stalk, wood shavings etc. Coal is the most commonly used fuel in Zigzag kiln. For efficient combustion, fuel to be fed must be small sized. Fuel is dropped through the feedholes (situated at the top) into the kiln using small hand shovel.

What is the size requirement of coal to be fed in a Zigzag kiln?

To feed in a Zigzag kiln, the heap must contain coal sizes ranging from fine particles to ½ inch. When coal of recommended size grading is fed little and often through the feedholes, the finer particles are ignited and burnt as it falls through the setting and larger pieces fall to the floor where they are burnt, thereby obtaining uniformity in firing across the height of the setting. Therefore the quantity and quality of top sizes and fines are important.

How is coal prepared to the size as recommended?

Coal crushers are used to crush “as received” coal to control the top-size of the fuel as required by the combustion process. In a brick kiln, the most commonly used equipment for size reduction of coal are – roll crushers and hammer mills.

Roll crushers most commonly used have either one or two rolls. Single-roll crusher does the size reduction by squeezing the coal between a smooth/toothed/corrugated cylindrical roll and a stationary breaker plate (provided with renewable wear plates) whereas, in double-roll crushers, two toothed cylindrical rolls, meshed together, rotate in opposite direction to grind the coal. Double-roll crushers have higher capacity and are able to crush more finely than single-roll crushers; but they are more expensive and difficult to maintain.

Hammer mills reduce coal size in two steps. The material size reduction is initially done by dynamic impact from the rotating hammers and then due to attrition and shear caused by rubbing of the coal pieces between hammer and the grid or screen plates. When the material does not pass through the screen plates, the hammers continue to strike the particles, further reducing them to fragments until they are small enough to pass through the grid plates. The hammer mill usually produces a good product size uniformity; the fineness obtained can be varied by adjustments of rotational speed, feed rate, grid opening size or the spacing between the hammer tips and the grate bars.

What is the appropriate way of fuel storage and why it is important to follow that?

Solid fuels are prone to quality deterioration and self-combustion during storage. Therefore, they must be stored inside a shed with proper ventilation, temperature and moisture control. The storage platform should preferably be made of raised concrete flooring with proper drainage system. As an example, improper storage of coal leads to following problems:

1. Mixing of soil/ash with coal causes deterioration in calorific value and combustion efficiency of fuel.
2. High moisture content results in undesirable cohering of coal particles, thereby when hand fed, fall in lump form to the kiln floor where difficulty will be met in burning it. Moisture in the coal also leads to increase in fuel consumption since it requires heat for its removal when fired in the kiln.
3. Coal dust is considered as highly explosive product. It reacts with ambient oxygen, even at ambient temperatures and heat is liberated during the process. If the storage area is not adequately ventilated, heat gets accumulated and the subsequent rise in temperature makes the coal oxidation reaction self-sustaining resulting in spontaneous combustion and coal is destroyed by fire. The critical safe temperature for Lignites is 50°C whereas for Bituminous coals it is 80°C.
   It is recommended to store only large sizes of coal by screening out fractions below 3 mm. Maximum safe size of individual piles for Bituminous coals is 200 ton and maximum depth 2.5 m. Maximum safe size for Lignites is 50 ton and depth 1.0 m.
4. The other ill effects of low temperature oxidation of coal are – decrease in calorific value and decrease in carbon and hydrogen content in the coal.