Many coal-fired power plants in operation today do not have a fire-extinguishing medium installed in their coal storage silos. CO2 injection would generally be the preferred method, and is used in many plants, but what do you do when your plant was left without this option after construction?
Wet or just plain “hot” coal making its way into storage is a major contributor to hot spots popping up in your in-plant coal supply. Also, extended silo storage durations due to coal delivery equipment maintenance or unit overhauls can make matters more conducive to a silo fire event.
Most plants have procedures in place to empty silos before beginning planned outages, but many times the unit will come offline unexpectedly before the silos can be emptied. Not taking action could result in a hot spot flare up before the equipment can be placed back in service.
Failing to Plan Is Planning to Fail
A 350-MW coal-fired power plant in the southwestern U.S experienced one of these events without a procedure in place. In this case, operators had run the 1D pulverizer for approximately 36 hours leading up to a weeklong shutdown. While the pulverizer was off, its average temperature was observed to be around 180F.
During normal daily rounds, a plant operator discovered a large, glowing red hot spot at the top of the 1D pulverizer coal silo discharge pipe (Figure 1). The hot spot was underneath the flange where the silo cone and the discharge are married (approximately 10 feet above the stock feeder deck floor).
|1. Red hot. The 1D pulverizer coal silo discharge pipe was glowing when discovered. Courtesy: Rowland (Rusty) Blinston|
Lacking a formal procedure, the decision was made to start the pulverizer and feeder to blow the burning coal from the silo to the boiler. Steam was admitted to the pulverizer, the dampers were set, the pulverizer was started, and then the feeder was started. The flow of fuel did cool the glowing hot spot and the piping metal began to return to its normal color. However, during the four-minute run, flames were noticed in the bottom sight glass of the feeder.
Operators were unable to tell if the fire was above the belt or below it, but seconds later the flame intensity increased significantly inside the feeder, at which time the belt probably broke. A few seconds after that, the order was given to trip the pulverizer, which resulted in the entire unit also tripping.
Substantial damage occurred to the feeder. As noted, the feeder belt was melted, along with all internal and some external feeder wiring. The solid-state control module in the cabinet next to the feeder was also damaged.
The feeder door was opened so maintenance crews could use fire hoses to cool and ultimately extinguish the fire. Several tons of smoking coal had to be drained from the silo onto the feeder deck floor resulting in a huge mess and an expensive clean up requiring use of a vacuum truck.
In the end, the pulverizer and feeder were unavailable for several days due to repairs. Although there were no injuries, plant personnel involved in the incident faced an increased safety risk because no emergency operating procedure had been in place to deal with an event like this.
Does your plant have procedures in place to deal with or prevent this type of incident? If not, now is the time to get them in place.
Developing an Emergency Procedure
One example of an emergency operating procedure for extinguishing a silo hot spot or fire follows.
1. Signage, flashing lights, and/or red barrier tape will be in place to warn personnel to keep out of the area. A responsible operator will be assigned to make sure the area is clear.
2. The control room operator will announce over the radio and loudspeaker that the area around the applicable silo/feeder/pulverizer is off limits for personnel safety and all hands are to stand clear of the area.
3. Tailboard the situation with all employees involved with the job.
4. Do not attempt to put burning coal from the silo into the feeder!
5. Close the feeder inlet gate.
6. If the hot spot is visible on the metal of the silo or transition chute, begin by covering the stock feeder control box with plastic and spraying the hot spot with fire hoses to cool the metal.
7. Contact the coal control room and direct operators not to add any additional coal to the coal silo until further notice.
8. Insert fire hose(s) into the top of the affected silo.
9. Turn on the water for 15 minutes or until water is seen coming from the bottom of the silo area.
10. Use a heat-indicating gun to monitor the hot spot temperature.
11. Wait 15 minutes and repeat step 8 until the silo hot spot temperature is below 120F.
12. At the first opportunity, start the pulverizer and feeder and empty the silo before refilling the silo with coal.
Developing a Standard Operating Procedure
Ideally, plants want to avoid the need to respond to an emergency at all. The best way to do so is through prevention. One example of a standard operating procedure for preventing a silo hot spot or fire follows.
Mitigating silo fires or hot spots:
1. Tailboard the situation with all employees involved with the job.
2. If a pulverizer is going to be down for a week or more, empty the silo and inform the station’s coal control room not to fill the associated pulverizer before shutting it down.
3. Close the pulverizer hot air damper.
4. Close the feeder inlet gate.
5. Run coal off the feeder belt and isolate the outlet gate.
6. If a situation prevents emptying coal from a silo during a lengthy shutdown, the silo should be checked with a heat-indicating gun at least once per shift to identify any developing hot spots.
7. Operators shall record, in the station log, the silo temperatures at least once per shift to allow trending and early detection of hot spot development so action can be taken to mitigate the situation.
Planning for Success
Developing adequate policies and procedures, and training personnel to respond appropriately to emergency situations is important for safe plant operation. Most plants are already equipped with fire hose reels on their coal feeder decks and upper silo areas, so modifying the above procedures to work in your plant should be fairly easy and inexpensive. Don’t leave emergency response to chance, because it can result in serious consequences.
—Rowland (Rusty) Blinston (firstname.lastname@example.org) is a consultant, trainer, and procedure writer with 40-years of experience in operations, maintenance, and management of fossil and hydroelectric generating facilities.