Cement manufacturing is a vital industrial activity across the continent of Africa and in many other countries throughout the world. Growth in demand for cement continues to remain strong as government and private sectors focus on infrastructure and housing development.
Fire and explosion hazards arise due to the nature of several processes occurring during cement manufacturing and the raw materials being used. Beside property damage resulting from fires and explosions, these accidents can cause injuries and fatalities to workers. Therefore, understanding hazard prevention is important in these industries.
This article outlines the various fire and explosion hazards present in cement manufacturing industries. A description of the manufacturing process is given followed by five typical fire hazards. Lastly, five explosion hazards are also reviewed.
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Cement Manufacturing Process
The cement manufacturing process typically involves quarrying, drying & grinding, kiln burning & cooling, finish grinding, and packaging. Each of these process steps are described in the following sections.
Step #1: Quarrying
Querying involves the mining of limestone. This includes crushing the raw material and transporting to the factory through conveyors or roads. Approximately 1.6 tonnes of limestone are needed for every 1 tonne of cement, and therefore the availability of limestone is an important parameter in locating the cement plant.
Step #2: Drying & Grinding
The crushed rock obtained from quarries is taken to a mix bed plant where the raw material components are pre-homogenized, dried and ground.
Raw materials generally contain moisture and need to be dried before or during the grinding process. The heat for drying is typically is supplied by waste gases from the rotary kiln or clinker cooler. For grinding, large cement plants use high speed roller mills or bowl type grinding mills, and smaller plants use tube mills packed with paper bags filled with iron grinding balls.
Step #3: Kiln Burning & Cooling
Raw mix is transported hydraulically to an air separator. Here the mix is separated into coarse and fine particles. Coarse material is returned to the grinding mill and fine particles (raw mix) are blended in homogenizing silos.
Homogenization takes places when air entering the bottom of the kiln passes through the raw mix fed from above. Raw mix is heated up to 800C by kiln waste gases in a multi-stage cyclone pre-heater before it enters the kiln. The pre-calciner in lower part of the pre-heater almost entirely calcinates the raw mix. The raw mix passes through various reaction zones due to rotation of the inclined kiln.
The various reaction zones in the inclined kiln include: the inlet or preheating zone, the calcinating zone where the raw mix gets split up at a temperature between 1050C – 1150C, and the burning zone where lime reacts with clay and sand at a temperature between 1250C – 1450C. This results in partial melting and formation of granular clinker. The length of each zone depends on the composition of raw material, the type of rotary kiln, and the cooling system employed. The clinker formed in the kiln drops into the clinker cooler where it gets cooled to a temperature between 120C to 200C. From here clinker is shifted to an intermediate storage silo through conveyors.
Step #4: Finish Grinding
Clinker granules from the storage silos are conveyed to the cement mill where they are ground with additives like gypsum to make cement. Grinding is most frequently carried out in tube mills through rolling and grinding elements which reduce the grinding stock to powder. Ground cement is then transferred to silos for storage through conveyors.
Step #5: Packaging & Loading
Packaging and loading stations with weighing devices are used in bagging, packing, and transferring the cement to trucks. These are dispatched to the end destination in individual bags or as bulk transfer.
Five fire hazards associated with the cement manufacturing process are outlined in the following sections.
1) Coal Storage
On average 0.2 – 0.3 tonnes of coal are consumed in the kiln per kilogram of clinker cement production. As such coal storage during cement manufacturing is an important component to consider in hazard analysis.
Among the various causes of coal fires, spontaneous combustion during storage results in a large number of losses. Spontaneous combustion is caused by adsorption of oxygen at the outer and inner surface of coal which results in oxidation. Oxidation is an exothermic reaction which causes the temperature of the coal stack to rise gradually. As the pile exceeds approximately 800C ignition can occur. Spontaneous combustion of the coal depends on many factors and critical among these is the type, age and composition of coal.
2) Coal Conveying
The combustible nature of crushed coal poses a fire hazard in the conveying system. Frictional resistance between idlers and rollers, insufficient lubrication, belt misalignment, and faulty bearings can generate sparks or heat, which can ignite the crushed coal.
3) On-Site Power Plants
Cement manufacturing is a very energy intensive process. As such, on-site power plants are increasingly being used to meet energy demands of the cement plant.
Some of the fire hazards associated with on-site power plants include fire in power house buildings due to ignition of lube oil. Fuel oil fires and fire in the power plant boiler can also occur, as well as cable gallery fires.
4) Electrical Equipment & Cables
Cement plants are energy intensive industries and have a large number of heavy duty electrical equipment used for distribution, control, and electric power utilization. Large quantities of insulation oil used in equipment like switchgears, transformers, and capacitors are combustible in nature and pose a major fire hazard. Fire hazards also arise from large-scale electrical cabling carried out between various substations and equipment.
Various modifications and capacity enhancements may occur in the cement plant over time, requiring installation of new cable layouts. This can give rise to multiple cable joints which decreases fire safety. Since most of these substations are unmanned this can increase the fire risk.
5) Empty Bag Storage
Lastly, storage of empty bags are a large fire hazard due to the combustible nature of the material used. Fires in bag storage areas can spread to adjoining areas in the plant like the cement bagging station, increasing the potential loss of these fires.
In addition to fire hazards there are several explosion hazards that can arise in cement manufacturing. Five of these potential explosion hazards are listed in the following.
1) Coal Pulverizing
Coal is often the main fuel source used to heat the cement kiln. Coal pulverizers are required to grind the coal from the bulk storage size to a smaller particle diameter to use in the kiln.
Coal pulverizers significantly reduce the mean particle size of the coal, drastically increasing the fire/explosion hazard risk. This risk is present inside the pulverizing equipment and during transport to the kiln for burning.
2) Bag Filters
Fire and explosion hazards in bag filters can arise due to any of the following factors: spontaneous combustion, dust explosion, static electricity, and high temperature materials passing through the filters. Bag filters used for the coal mill rank as one of the highest fire and explosion hazards in cement plants, due to the small particle size of the crushed coal.
3) Electrostatic Precipitators
Build-up of explosive mixtures like finely dispersed coal dust in air, or carbon monoxide in air can result in an explosion hazard in Electrostatic Precipitators. As such, special care is required at these locations to prevent such a build-up.
4) Storage and Handling of Explosives
Most cement plants own limestone quarries that are located nearby. Limestone quarrying involves the use of explosives and as such, it is vital that they are stored and handled in a safe manner to avoid unexpected detonation.
5) Power Plant Boilers
Lastly, the boilers used in the on-site power plants require special care to avoid explosion hazards. Routine inspection and maintenance should be used when operating these systems.
This article outlined five fire hazards and five explosion hazards present during cement manufacturing and processing operations. The activities of process safety experts include analysing, designing, and implementing measures to prevent and mitigate these hazards for safe operation of cement manufacturing facilities.
Another issue that was not mentioned in this article, but is important to consider is respiratory hazards. Dusty environments likely to be present in cement plants can expose workers to various health complications. Of these, inhalation of excess amounts of cement dust is a critical concern. It can cause lung function impairment, chronic obtrusive lung disease, restrictive lung disease, pneumoconiosis and carcinoma of the lungs, stomach and colon. Studies show that cement dust can enter into systemic circulation and reach various body organs including the heart, liver, spleen, bone, muscles and hair, which may affect their micro-structure and physiological performance.