Solid Fuels
Storage of Coal
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
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
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
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
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
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
<200 tons for bituminous and
higher ranks
<50 tons for lignite
>200 tons
Height of pile
< 3 metres for bituminous
< 1 metre for lignites
>3 metres
>1 metre
Storage period after
mining
> 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
>50 tons
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
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
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
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
Reading Assignment
• Comparative study of Solid , Liquid and
Gaseous Fuels