Solid Fuels 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 Factorsfavourable to safe storageNot favourable to safe storage Size gradingExclusion of fine coalRun of mine coal having fines Size of pile<200 tons for bituminous and higher ranks <50 tons for lignite >200 tons >50 tons Height of pile< 3 metres for bituminous < 1 metre for lignites >3 metres >1 metre Storage period after mining > 6 monthsUpto 4 months for bituminous, upto 1 month for lignite Coal TypeAnthracitelignite Site of storageOpen positions on clean firm ground In contact with hot surfaces

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