You have moved the fuel, burned it, generated the steam to drive the generator and sent the power on its way. You might think you are in good shape if you have addressed the fire hazards associated with production, but you still have some hazards associated with the flue gas discharge – the AQCS.
The exhaust discharge has to be cleaned up to meet environmental regulation for discharge to atmosphere. These systems have a variety of hazards and a fire could result in the need for permission to discharge out of specification for the short term or even worse, an extended shutdown of generation while repairs are made. In the case of major damage to large key pieces of equipment you may be shut down for a lengthy lead-time replacement.
One such discrete piece of equipment is the Air Heater – notably the regenerative types. For this, we refer you to our “What’s New” archives for a discussion on this specific subject in January 2003.
Another major element is particulate removal. This can involve a dust collector or an electrostatic precipitator. These units can be damaged by over temperature or fire from burning particulate carry over during incomplete combustion or by ignition of deposits built up over periods of over rich firing. Whichever type of unit you use, there are several common features you should consider:
– Thermal detection and unit bypass for over temperature flue gas could limit the fire development and reduce losses.
– An additional consideration might be a flue gas cooling water spray in the duct between the boiler and the unit.
– Thermal detection in the discharge stream can provide a warning that something is burning in the unit.
– In any event, should a fire occur, automatic or manual fire fighting will result in the application of considerable quantities of water and you have to accommodate for the increased weight loads with adequate drainage capability or structural stability.
Bag type dust collectors should be subdivided into compartments so that a fire will be isolated and permit compartment maintenance and repair without limiting boiler load. Each compartment should have heat detection set reasonably above normal operating temperature. The partitions should extend from the hopper area up through the bag area to the clean air plenum. The partitions should be at least non combustible and preferably fire resistance rated – especially if high temperature bags are not used or the unit is not provided with automatic fire sprinkler protection. Automatic sprinkler protection should be considered for the collector housing and extending to cover the bag area.
If you use an electrostatic precipitator be sure to arrange for automatic emergency shutdown initiated by the fire detectors. The interior atmosphere is oxygen poor and any attempt to access the unit for manual firefighting, or continuing to run fans after a boiler trip, can provide an enriched atmosphere capable of faster fire development or deflagration.
In addition to combustible deposits on precipitator plates, the interior space contains transformer-rectifiers which present a fire hazard. While dry transformers are preferred, larger units will likely have oil insulated transformers – typically mineral oil. These transformers should be considered the same as any other indoor oil filled transformers with respect to fire protection. The protection requirements vary with the type and quantity of oil, the transformer power rating, as well as the number and spacing of the transformers. Some of the typical protective features include spacing, fire rated barriers between transformers, curbs and drainage for oil confinement, and automatic water spray or fire suppression sprinkler systems.
Once particulates have been dealt with, there is the removal of gasses from the discharge, as needed. This is done with scrubbers which can be stand alone outdoor units or units within a building. When in a building, the entire facility should be protected with automatic fire sprinklers in addition to requirements for the units. Because of the process, scrubbers often use considerable plastic or rubber materials for ducts, liners, packing and piping. This introduces a considerable combustible load. When the units are in operation, water spray is continuously being applied to many of the surfaces, which mitigates the fire hazard. However, when shut down for maintenance, the possibility of a serious fire exists – the maintenance work itself is often the fire initiating factor. For this reason, consideration should be given to a water fire suppression system. This can be an independent system or one that utilizes the scrubber spray system. For the latter case, be sure the portions of the scrubber spray systems used for fire protection are non combustible inside the scrubber. Protection should extend to cover combustible linings, packing and plastic ducts. The water supply for the system should be reliable during outages.
The major individual blocks have been discussed here. Keep in mind that the AQCS can typically include support elements such as conveyor systems, fan systems with lube oil lines to bearings and oil reservoirs, stacks with combustible liners. These elements need to be considered in the fire hazards analysis as well.
A number of items discussed have been treated in a general sense due to the wide variety of physical and practical applications in the field. Risk Logic, Inc. has the capability to evaluate your specific site to develop and recommend appropriate fire protection. We are available to assist you with your fire protection needs and look forward to that opportunity.