Coal pulverization mill is an important component of a coal based cement plant. Generally, hot flue gases from the kiln are introduced into the pulverization plant to dry the coal. The flue gas thus introduced, after carrying additional pollution loads from coal processing, leaves from the pulverization mill stack. The air flow used to dry the coal can not be reintroduced into the kiln due to the relatively high moisture and sulfur contents.
It implies that in case of coal as kiln fuel, the emissions originating from kiln are divided into two parallel streams, one emitting from the kiln stack and the other from the coal pulverization mill stack. Continued use of electrostatic precipitators or fabric filters on kiln systems as well as coal pulverization mills, shall be necessary to control Particulate Matter emissions and it is necessary to keep them well within Environmental Quality Standard.
For our case from a coal pulverization mill equipped with fabric filter, the emission of stuck from the firing system when coal is used as a fuel is expected to be with in the standard.
6.2 Spontaneous dust fires and coal dust explosion
One potential hazard of the coal, depending upon its characteristics and ambient conditions, is its propensity to spontaneous ignition and combustion at rather low temperatures, which leads to outbreak of fire in stored coal. This spontaneous combustion may occur naturally or the combustion process may be triggered by other causes, like lightening, electric or mechanical sparks.
Three necessary elements of fire triangle, which must occur simultaneously to cause a fire, are Fuel, Heat and Oxygen. Removing any one of these requirements would prevent fire.
Coal reacts with atmospheric oxygen even at ambient temperatures and this reaction is exothermic. If the heat liberated during the process is allowed to accumulate, the rate of the above reaction increases exponentially and there is a further rise in temperature. When this temperature reaches the ignition temperature of coal, the coal starts to burn.
For conditions favoring spontaneous combustion the air supply needs to be high enough to support the oxidation, but too small for a sufficient cooling. The temperature, at which the coal oxidation reaction becomes self - sustaining and at which spontaneous combustion occurs, varies depending on the type (nature and rank) of coal and surrounding conditions of heat dissipation. In poor quality coal and where the heat retention is high the coal and carbonaceous material may start burning at temperatures as low as 30-40 0C.
Besides burning away the important nonrenewable energy resource and causing financial losses, these fires pose danger to man and machine; raise the temperature of the area; and may cause damage to the adjacent buildings and structures. Excessive levels of air pollution of the occupational and surrounding atmosphere would result from such fires, for as long as the fire continues.
Under all circumstances, coal dust would continuously be present in the occupational atmosphere of the plant. Its entry and accumulation in the electrical panels, equipment and machinery, not duly protected, and on power cables, laid in open or in trenches, may damage them otherwise or as a result of the spontaneous ignition, due to self heating or else triggered by some electric spark. Such ignitions may lead to the outbreak of fire and cause more damage.
Due to the high moisture and high content of volatile matter, the tendency and propensity of Yayu coals to spontaneous ignition and combustion is high, and thus accordingly the required level of precautions in their storage, for prevention of spontaneous fires shall be high.
Pulverized coal is potentially explosive and can be set off by spontaneous ignition or by proximity of hot elements, static charges or flames. Pulverized coal is present in the coal pulverization mill and in the elements the coal feeding and firing system. Like fire triangle, there is explosion pentagon of five necessary elements of Fuel, Heat, Oxygen, Suspension and Confinement, which must occur simultaneously, to cause and propagate an explosion.
Like the fire triangle, removing any one of these requirements would prevent an explosion from propagating. For example, if fuel, heat, oxygen, and confinement occurred together in proper quantities, an explosion would still not be possible without the suspension of the fuel. However, in this case, a fire could occur. If the burning fuel were then placed in suspension by a sudden blast of air, all five sides of the explosion pentagon would be satisfied and an explosion would be imminent.
The principal impact of any coal dust explosion is consequent potential loss of the human life and limb. Other major impacts include loss of fuel, destruction of the machinery, equipment and surrounding structures and loss of production due to possible closure of plant for some time. Socio-psychological impacts of such an explosion, if it ever occurs unfortunately, on the surrounding communities and the plant staff itself, owing to their persistent fears in future, and their reaction to such an incidence, may turn out to be more disastrous, than the incidence itself. The general measures for prevention and control of pulverized coal explosions should be exercised.
6.3 Other related impacts
Open storage of coal on unpaved soil surface, with no proper drainage system, may lead to contamination of soil and groundwater, particularly, consequent to rainfall. The rainwater passing through stacks of coal may leach certain dissolved organic constituents like phenols and carry them to soil and ground water.
Coal crushers and pulverization mill are two main additional sources of noise pollution, resulting from fuel switchover from oil to coal.