|Hoosier Energy’s Merom Generating Station in Indiana, US
Credit: Ametek Land
Risks of unwanted combustion – potentially causing injury, damage and/or downtime – occur everywhere coal is handled, processed or stored.
Safe coal handling practices are designed to ensure that the fuel remains intact throughout its journey from the mine until the point at which it is ignited in the boiler.
It takes as little as 1.4 kg of pulverized coal in 28.3 m3 of air to form an explosive mixture. Since a large boiler burns 45.4 kg or more of coal per second, the safe burning of pulverized coal necessitates strict adherence to planned operating sequences.
All coals oxidize during storage, but sub-bituminous coals are especially prone to self-ignition. The increasing use of sub-bituminous coals throughout Asia has increased the risks of silo fires.
Good operating procedures are designed to ensure that coal is used before it has time to self-ignite, and many operators use a nitrogen blanket to minimize the scope for oxidation. Even with appropriate precautions, silo fires can still occur and appropriate monitoring is needed to prevent oxidation from developing into a silo fire.
The greatest risk of fire occurs when the mill is shut down under load, as this leaves a large amount of pulverized fuel inside a hot mill. The large surface area of the pulverized coal and the high temperature inside the mill lead to rapid oxidation of the coal. This results in further heat buildup and the potential for a fire. If the mill is restarted without first removing the hot coal, an explosion can occur when particles are suspended and exposed to the air.
Even in routine mill shutdowns, there is a danger that any residual coal left within the mill will oxidize, and may explode as the mill is restarted. To prevent a coal fire, the mills can be made inert with a steam deluge when an unexpected shutdown occurs, or when there is a high risk of a coal fire.
Several methods are available to detect the presence of oxidization within the mill or silo:
- Thermocouples are widely used to detect the heat buildup from oxidation or an early-stage mill fire, but they have limited sensitivity and discrete sensors have difficulty monitoring the whole volume of the mill. It also takes time for sufficient heat to build up within the mill to give a detectable increase in temperature. Experience shows that thermocouples do not provide a reliable indication that a hazardous condition is developing.
- Carbon monoxide (CO) gas detection offers a fast and sensitive means to detect the presence of oxidizing coal, as the oxidation inevitably produces large amounts of CO. The most important reasons to choose CO measurement for this application are the availability of sensitive CO sensors able to detect a few parts per million (ppm) of CO, and the ability to sample a large portion of the mill using a probe mounted at the classifier outlet. CO monitoring is fast, sensitive, specific and can be calibrated to determine alarm levels that reliably identify a potentially hazardous condition while minimizing the occurrence of false alarms.
Once a fire has started, optical detectors respond to sparks and flames within the mill. By the time flames are visible, it is too late to take preventative actions because the mill is already in a very hazardous condition.
One of the biggest challenges in configuring a Millwatch system is the determination of suitable alarm levels. A carbon monoxide concentration greater than 250 ppm can be seen during mill startup, but in normal operation the CO concentration is in the region of 10 ppm.
Millwatch analyzers offer two independent alarm points, so alarm levels were set at 300 ppm during startup and 50 ppm in normal operation. Although the startup alarm seemed robust, there were occasional spikes above 50 ppm CO in normal operation, so the alarm level was increased several times with a final figure of 125 ppm. This avoided nuisance alarms, while providing good sensitivity and response to abnormal operating condition when the mill may need to be inerted.
|Inside view of Millwatch analyzer
Credit: Ametek Land
Monitoring in China and the US
HouShiPower is a 4200 MW electricity generating plant in China’s Fujian province, operated by the Huayang Group. It supplies electricity to the city of ZhangZhou and the surrounding area.
There are seven electricity generating units at the site, each of which is rated for 600 MW. In 2011, the plant operators decided to add CO monitors to the five coal mills in Unit 1, supplementing their existing temperature and fire sensors. They determined that Ametek Land’s Millwatch analyzers were best suited to the task. The analyzers have a long track record, with hundreds of installations worldwide, and include a number of desirable features:
- Rugged sample probes with automatic blowback to maintain a good sample flow;
- Automatic calibration verifies correct operation of the analyzers, confirming that they respond correctly to CO;
- Continuous measurement of each sample point, with no multiplexing and response time less than 60 seconds.
This last feature is especially important as a hazardous condition can develop within a few minutes, and a multiplexed system sampling six measurement points will typically sample each point only once every 10-15 minutes.
The system proved its value in 2013 when the Millwatch system showed rapidly rising CO levels in the outlet of one of the coal mills. It would have taken at least 15 minutes for the temperature and fire detection systems to respond and show an indication of a problem, so the Millwatch analyzers allowed corrective action to be taken significantly earlier than would otherwise have been possible.
Hoosier Energy’s Merom Generating Station in Indiana, US, is a coal-fired baseload plant with two 535 MW generating units. It went into commercial operation in 1982 and provides power to electric distribution co-operatives in the midwestern US. At full load it uses 10,000 tonnes of coal per day, with the supply coming from mines by road and rail.
Hoosier Energy has a strong commitment to safety and maintains a robust safety programme, endeavouring to operate with the utmost regard for the health and safety of its employees and the public.
Each generating unit at Merom Station has three Riley Power double-ended ball tube mills. The mills can each provide 65 tonnes per hour of pulverized coal to the boiler, a total of 195 tonnes per hour per boiler. The boilers at Merom station burn 54 kg of coal per second.
Because the ball-tube mills have outlets at each end with a classifier on each outlet, two sample points were needed on each mill. For enhanced reliability, a redundant configuration was chosen with two sample points on each classifier, giving four samples per mill. With three mills per generating unit, a total of 12 sample points were needed for each unit or six twin-stream analyzers. Redundant measurements reduce the likelihood of a nuisance alarm, as a high CO measurement is unlikely to be detected on one coal pipe while the others continue to show normal readings.
The initial proposal was to mount the sample probes directly on the classifiers. Even though the inside of the classifier is a hazardous area, the sample probes are simple devices with no electrical connection and so no special precautions were needed. Although this would have provided a representative sample, the probes have an abrasion shield which prevents the stainless steel filter from being damaged by the high concentration of coal dust. An installation location at the classifier outlet was preferred, since this allowed the abrasion shield to face the flow of coal dust and protect the filter. Blowback controllers were installed close to the classifiers, but outside the hazardous area.
Along with the CO monitors, an in situ oxygen probe was installed on each classifier, to determine the oxygen concentration while the mills are steam-inerted. The CO analyzers were installed at the same level as the classifiers. This meant that the sample lines could be kept short and the response time minimized. The chosen location also gave easy access for maintenance. Commissioning took place during an outage in May 2011.
In the three years since the Millwatch analyzers were installed at Merom Station, there have been a number of high-CO alarms, but no mill explosions — an impressive achievement for a baseload station. During that time, the Millwatch analyzers have proven to be reliable, requiring no more than routine maintenance and providing enhanced safety.
On 8 December 2013, the Millwatch analyzers demonstrated their value. With Unit 2 running at full load, one of the mills tripped and the operators observed a rapid increase in CO readings even though there was no indication of a temperature rise. Within a few minutes, the CO level was above the alarm threshold, and the operators made the decision to activate the deluge system. The boiler continued to operate using coal from the remaining mills, with output dropping to 60 per cent of its rated value.
The CO level in the affected mill started dropping after 15 minutes, and within 45 minutes it was below 10 ppm. The mill was restarted two hours after the high CO alarm was detected, and was returned to full operation in three and a half hours.
The Millwatch CO analyzers detected a potentially dangerous condition and allowed it to be dealt with quickly with no damage to plant or personnel.
In conclusion, CO monitoring provides a rapid and reliable method for detection of potentially dangerous coal oxidation within a mill so that action can be taken to reduce the risk of a fire or explosion.
At both HouShiPower Plant and Hoosier Energy Merom Station, Millwatch analyzers from Ametek Land have provided good reliability and a high level of safety coverage with no explosions in the mill since they were installed more than three years ago.
Derek Stuart is Product Manager at Ametek Land, based in Pittsburgh, Pennsylvania, US
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