“Coal Dust Explosion” in Coal fired Cement plant – Its prevention is necessary for safety and enhancement of environment standards.
The numbers of coal fired industrial systems, particularly cement plants, is rapidly increasing, because of high cost and uncertainty in availability of fuel oil and natural gas. Many of the countries do not have access to sufficient quantity of petroleum products; there by, worldwide growth in coal-fired cement plant is tremendous. There is inherent risk associated with coal pulverizing, drying, blending, transportation and storing.
To understand fully the hazard potential of using pulverized coal as a fuel in a cement plant, one should be familiar with the factors responsible for development of coal dust explosion. A typical pulverized coal fuel system process bulk coal into a form that can be efficiently utilized as a fuel to heat the kiln for calcining the raw material of cement, i.e., clay, limestone, etc., into clinker. This is usually accomplished by grinding and drying the bulk feed in a pulverizer so that coal emerging from pulverizer consist 70 to 80% particulate that passes through 200 mesh screen. High temperature air from clinker cooler is often used to dry the coal and convey it from pulverizer to the burning pipe of the kiln. Coal pulverizing is one of the most hazardous jobs from fire and explosion point of view, as both fuel and oxygen are present at the pulverizer. A coal dust explosion is described as rapid burning of combustible particulate within a confined area; which generates a considerable heat and corresponding pressure rise. The factors responsible for coal dust getting exploded:
- presence of dust in suspension at a concentration above flammability limit,
- presence of sufficient oxygen to enable the combustion,
- source of ignition of the coal dust air mixture,
- a certain degree of confinement to the mixture.
Most coal used in firing system have tendency for spontaneous ignition (Spontaneous heating is an inherent property of coal to get heated on oxidation, when it is kept for prolonged time with air or oxygen. Oxidation of coal is an exothermic process, hence the self-ignition) in the pile. Therefore, care should be taken in storage, handling within firing system and prevention of accumulation of coal dust during system operation.Special precautions are necessary to ensure safe operation:
- use oxygen-deficient air in the pulverizers under normal operating condition,
- use of rock dust, carbon dioxide, water systems in the pulverizers and dust collectors when shutdown occurs,
- inerting with water sprays or steam when over-temperature conditions are observed,
- care must be taken to prevent development of coal dust cloud,
- use of magnet or metal detection to remove foreign iron substance in the system to prevent occurrences of sparks,
- cutting & welding operation should be carried out as per the safety norms,
- electric components used in the system should be of non-inflammable type,
- hot coal, if any, should be avoided to charge into pulverizers,
- proper control measures should be adopted to prevent spontaneous ignition of coal,
- prevention of static electricity discharge into the system should be adopted by grounding dust collector bags,
- by properly designing of several coal handling equipments in order to prevent accumulation coal dust at various idle spots of locations.
As coal dust explosion in coal-fired industrial systems (whether in a cement plant or in a thermal power plant) is very much serious in nature and may cause damage to the property and personnel, utmost care should be taken to prevent such occurrences. Last but not least, proper training the personnel is most important to handle such situation most effectively.