The pulverization of coal to be used as fuel for cement kilns is common. Processing of coal includes unit operations or equipment such as size reduction, storage, pneumatic conveyances, and dust collection. When the coal is in a finely divided state (powder), a dust explosion can occur. Explosion-protection measures are designed to prevent physical damage or injury by releasing the pressure brought on by coal pulverization by suppressing the incipient explosion or by isolating the explosion.

Coal pulverization to improve burning efficiency and maximize energy output is a method that has been used for more than 75 years. Pulverized coal processing and storage systems are typically found in industries that employ injection furnaces.

Compliance with the Clean Air Act Amendments (CAAA) of 1990 requires reduced emissions of sulfur dioxide and nitrogen oxides. Compliance with Phase I of the CAAA was required by Jan. 1, 1995, whereas Phase II compliance was required Jan. 1, 2000.

Many facility operators were able to accomplish compliance by simply switching fuels from high-sulfur bituminous coals to low-sulfur sub-bituminous coals of the Powder River Basin (PRB). However, considerable re-engineering of these facilities must occur along with the switch to PRB coal. One area of concern is the explosion protection system. While switching to PRB coal addresses the environmental concerns, the risk of an explosion can increase, and the degree of the explosion also can increase due to hazard characteristics, when compared to bituminous coals.

First, the possibility of spontaneous combustion is more likely due to lower auto-ignition temperatures and short auto-ignition times (Table 1).

Second, the explosibility index, KST, is usually higher. This means if a deflagration (explosion) occurs, PRB coal will experience faster rates of pressure rise and possibly require enhanced explosion protection systems to achieve the same level of safety compared with bituminous coal (Table 2).

Protection solutions Explosion-protection strategies include both preventative and responsive methods. Preventative methods, such as inerting and carbon-monoxide (CO) monitoring, are primarily directed at decreasing the probability of an explosion. Preventative techniques seek to eliminate one of the ingredients of an explosion (suspended fuel, air, or ignition source). Preventative methods are not 100% effective. In the time required for preventative systems to detect, interpret, and adjust process conditions, an explosion can occur. Explosions typically result from a process upset condition (loss of airflow), equipment failure (incomplete fuel combustion, overheated bearings), or human error.

Responsive methods come into effect after a deflagration has started and are designed to minimize or eliminate damage due to deflagration pressures.

There are two principle systems for processing, distributing, and burning pulverized coal: direct-firing systems and storage-firing systems. In a direct-firing system, the coal is gravity fed into the pulverizer from the coal bunker, where it is dried and pulverized, then pneumatically conveyed to the burners in a single continuous operation. A storage-firing system also includes a cyclone separator, a dust filter, and a storage bin between the pulverizer and burners. In either system, the fly ash from the furnace/boiler is removed downstream by baghouses, electrostatic precipitators, or scrubbers.

The process equipment most susceptible to explosions are the pulverizers, cyclones, dust collectors, storage bins, and conveying lines between these enclosures (Figure 1).

Pulverizer Explosion venting of the pulverizer is not allowed per NFPA 8503, so the available responsive techniques are pressure containment or explosion suppression.

For containment, the current version of NFPA 8503 clarifies the 50-psig design requirement says that the pulverizer components be tested to 200 psig. The air supply ducts to the pulverizer are not included in this requirement, but NFPA 8503 states, "Consideration shall be given to the fact that this ductwork can be exposed to explosion pressures from the pulverizer." Explosion venting can be effectively utilized to prevent the rupturing of this ductwork and protect the primary air fan.

Although containment is designed to prevent rupturing of the pulverizer, internal components, such as the classifier, can be damaged. Explosion suppression can be applied to prevent internal damage as well as extinguish the ensuing fire. Explosion suppression utilizes pressure sensors to detect the initial pressure wave and release dry powder suppressant agent, within milliseconds, into the pulverizer. The suppression system controller interfaces with other process controls to initiate feed and fan shutdown and other similar process changes.

Conveyance piping Deflagration propagation through interconnecting ductwork not only spreads the fire hazard but also can cause "pressure piling." This occurs when the pressure is built up in adjoining vessels prior to the flame arriving. The ensuing secondary deflagration in the connected vessels now starts at an increased pressure with correspondingly more serious consequences-both in terms of the rate of pressure rise and the final pressure. To prevent deflagration propagation, explosion isolation is employed. This is accomplished by either a mechanical, fast-acting valve system or a chemical suppression system on the piping. Typically, mechanical isolation systems are used on piping of 24-in.-diam and smaller. Ductwork and pipe diameters greater than 24 in. require the dry powder chemical isolation systems.

Cyclone/dust collector/pulverized fuel bins The cyclone, baghouses, and pulverized fuel bins can employ containment, explosion suppression, or explosion venting as the responsive strategy. Generally, due to the large sizes of these vessels, explosion venting tends to be the most economical solution. NFPA 68 (1998 Edition) recommends the use of lightweight rupture diaphragms because hinged venting devices are less efficient. NFPA 68 also provides vent sizing equations to determine the necessary relief area.

The basic equations are based on the enclosures being located outdoors so they can be vented to a safe area. Indoor equipment would require discharge ducts, but this is usually impractical due to the significant increase to the vented pressure, which the enclosure will be exposed to.

If venting is impractical because equipment is located indoors, then explosion suppression is utilized. An advantage of explosion suppression systems, compared with venting, is the elimination of the fire hazard.

Summary Companies that process pulverized coal need to be aware that a greater potential for explosion may be present when switching grades of coal. A process safety and hazard review should be conducted to determine the effects that switching coal may have in each system. In general, low-sulfur, sub-bituminous coals of the Power River Basin are much more likely to ignite and produce a greater explosion severity than are high-sulfur, bituminous coals.

The systems that safely processed bituminous coal in the past may require design changes and a higher level of explosion protection to achieve the same level of safety with sub-bituminous coal.