Solid Fuels

1
Solid Fuels

• Storage of Coal

2
Storage of Coal

• The storage of coal has many disadvantage
• Blockage of money
• Blockage of space
• Deterioration in quality
• Fire hazards
• It is necessary to store coal in sufficient
  quantities to counter the uncertainties in
  availability and transportation

3
Storage of Coal

• Freshly mined coal when stored in bulk undergoes
  low temperature atmospheric oxidation due to the
  presence of methane and other volatile matter
• This exothermic oxidation causes rise in
  temperature
• If heat is not removed coal begins to burn
• This is called spontaneous combustion

4
Storage of Coal

• Low temperature oxidation of coal causes
• decrease in its calorific value
• Decreases in carbon and hydrogen content
• Increase in its oxygen content
• Decrease in its gas yields on its carbonization
• Fire if heat not removed and temperature exceeds
  the critical value i.e. 70 C

5
Storage of Coal

• Oxidation of high volatile matter containing coal
  is more as compared to low volatile coals
• Low temperature oxidation is accelerated by
• Rise in temperature
• Access to diffused air
• Presence of small particles
• Storage in large heaps with small surface to
  volume ratio

6
Storage of Coal

• Low temperature oxidation can be retarded by
• Cooling either by ventilation or water spray
• Reducing access to air
• Reducing the size of the storage piles
• Lower the rank more shallow and smaller the pile
• Anthracite is stored with less risk than
  bituminous

7
Storage of Coal

• Conditions for Safe Storage

Factors favourable to safe storage Not favourable to safe storage
Size grading Exclusion of fine coal Run of mine coal having fines
Size of pile lt200 tons for bituminous and higher ranks lt50 tons for lignite gt200 tons gt50 tons
Height of pile lt 3 metres for bituminous lt 1 metre for lignites gt3 metres gt1 metre
Storage period after mining gt 6 months Upto 4 months for bituminous, upto 1 month for lignite
Coal Type Anthracite lignite
Site of storage Open positions on clean firm ground In contact with hot surfaces
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Storage of Coal

• Losses of Coal during storage
• Oxidation Loss (Weathering of coal)
• The coal starts absorbing oxygen from the
  atmosphere when it is mined out. Absorption of
  oxygen by coal results in deterioration of coal
  quality ( C.V. is lowered)
• Wind Loss
• 0.5 1 of coal is lost due to blowing of wind
• Carpet Loss The mixing of ground clay with the
  bottom most layer of coal. This may be avoided by
  paving the storage floor
• Spontaneous combustion loss Due to spontaneous
  combustion considerable amount of coal is lost
• For a given coal there is a critical temperature
  below which spontaneous combustion can not take
  place

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Storage of Coal

• Guideline for Safe storage
• Coal Dust dust coal should be separated and used
  or disposed off as soon as possible
• Ventilation Ventilation must be reduced e.g.
  three sided wall storage
• Location Storage area should be away from the
  heat source
• Height of Heap height of heap should not be more
  than 3.0 metres
• Conical Heap conical heap should be avoided
  because it increases the exposed area
• Smaller heap On one hand air supplies oxygen
  for auto ignition, on the other hand produces
  cooling effect by heat dissipation. The coal is
  ignited inside the heap and burns without flame
  turning into ash. Heat dissipation on smaller
  heaps is faster. So smaller heaps should be
  preferred against one big heap
• Flow of Stock storage of coal for longer period
  should be avoided
• Temperature monitoring the temp. at different
  places of the stock should be observed at least
  once a week

10
Selection of Coal

• There are various factors which are considered to
  select coal for a particular purpose
• Coal for making coke
• Have low ash, sulphur and phosphorous content
• Be strong and hard
• Be of uniform quality
• Coal for producing gas Coal should have
• High volatile content
• Low ash contenr (around 8 )
• Low sulphur content
• High ash fusion temp. to avoid clinker formation
• Weakly coking to avoid coal bed fusion

11
Selection of Coal

• Coal for steam raising Almost all types of coal
  which are not suitable for carbonization or
  gasification can be used for steam raising. The
  design of plant and the method of operation
  during combustion should be satisfactory
• Coal for combustion practices Coal should have
• High calorific value
• Low ash content
• Low sulphur content
• Low moisture content
• Uniform size

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Reading Assignment

• Comparative study of Solid , Liquid and Gaseous
  Fuels