• Cement manufacturing is one of the largest mineral commodity industries in the United States, with an estimated production capacity of greater than 73 million metric tons annually.
• Essentially the heart and soul of the cement manufacturing process, the kiln is a horizontal steel cylinder, lined with firebrick and sloped slightly downward, from the raw material feed to the fuel feed supply and burner pipe. Typically, kilns will rotate at one to three revolutions per minute, heating the raw material mix to a red-hot, 2,500-plus degree molten form that emerges from the lower end of the kiln in the form of very hot, marble-sized chunks known as “clinker”. Once out of the kiln, the clinker goes through a cooling process. When cool, the clinker is ready to pass through a series of grinding and milling processes that result in the gray powder we know as cement.
• The cement industry is very capital intensive. Some of the major pieces of equipment include pulverizers, dust collectors, electrostatic precipitators, and rotary kilns.
• Energy costs can account for up to 40 percent of the total cost of cement manufacturing. Currently, over 90 percent of the installed capacity uses coal as the primary fuel. Conversion to coal was started in the 1970’s. In 1972, only 39 percent of the industry’s energy was supplied by coal.
• Inland Cement, “We are also concerned about the potential fire and explosion hazard of working with coal and are confident that we can use coal safely. Coal has been the fuel of choice for the cement industry around the world for many years and the technology is well developed to prevent fires and explosions.”
Explosions in Cement Plants
• Coal is fed into a pulverizer where it is crushed and dried using hot air at approximately 300°F. The pulverized coal is then pneumatically transported to the dust collector where the pulverized coal is collected and either fed directly to the burner at the kiln or to a storage bin.
• During normal operation the coal stream is inerted with nitrogen or similar gas. The problem typically occurs during startup, shutdown or failure of the inerting system.
• It is not uncommon for tramp metal, railroad spikes, or similar metal objects to be fed into the grinder and thus create sparks. These sparks can travel from the pulverizer through the transport pipe and into the dust collector.
• Another source of ignition in the dust collector is static electricity. The atmosphere in the dust collector is typically at ~200°F with the dry coal dust suspended. These conditions increase the risk of explosions in the dust collector.
• The other common ignition source is pyrites that are collected below the pulverizing bed. If there is a failure of the inert gas source then air is introduced and the pyrites can ignite the coal dust.
• Coal is milled to a fine powder in a pulverizer – this increases the surface area of the coal and hence the rate of
combustion. The powdered coal is blown into the combustion chamber of the burner nozzle where it is burnt at around 1400°C.
• The Mine Safety and Health Administration (MSHA) a branch of U.S. Department of Labor reports that during the past 5 months, eight explosions have occurred, resulting in one fatality and nine nonfatal injuries. Six of these explosions
occurred in cement plants.
Cement Plant Equipment with Highest Explosion Potential
As reported by Factory Mutual, the following equipment have experienced losses of greater than $26,000,000 over the last ten years due to explosions. Coal dust explosions were the cause of twenty-seven losses. Forty-seven percent of kiln losses occurred in the cement industry. A typical plant layout of a coal burning cement plant is shown in the next diagram. The equipment shown in red are the ones most likely to have explosions and the need for explosion protection. Each of these pieces of equipment is described in greater detail in the remainder of the document.