General System of Pulverizer
It is a positive pressure direct air swept pulverizing system with 5 HP963 medium speed coal pulverizers per boiler. Each pulverizer is equipped with one gravimetric coal feeder and one coal bunker. There are five independent pulverizing systems for each boiler. Four pulverizers operating together can satisfy the requirements of boiler maximum continuous rating and have a storage coefficient of more than 1.1 for design coal.
Coal from the bunker enters the coal feeder which conveys the coal to the pulverizer for grinding and drying. The processed pulverized coal is passed through a classifier by the drying agent (primary air). There are four pulverized coal pipes (φ590×10) connecting each classifier to supply coal to burners on the same layer in the four corners of the furnace (see pulverized coal flow chart). Five coal pulverizers correspond to five burner levels. This arrangement ensures even heat distribution along left and right side of front and rear wall when any one coal pulverizer is shutdown during boiler operation, thus effectively preventing coking and reducing the flue gas temperature difference and flow difference at furnace outlet. A pneumatic shutoff gate is installed in each pulverized coal pipe. It can be closed within ten seconds and provides a tight air seal. The shutoff gate is opened when coal pulverizer starts running and closed quickly during pulverizer shutdown or accidents to prevent the flue gas in the furnace returning to the pulverizers and to prevent cold air entering the furnace via pulverizers. For enhanced protection, an electric shutoff gate is installed at the burner inlet. Due to the different resistance in different coal supply pipes, adjustable dampers are located in the pipes. This makes the resistance across all the pipes similar, which is beneficial for combustion control.
One gravimetric coal feeder with 100 percent capacity is matched to each pulverizer. With electric shutoff gates located at inlets and outlets of pulverizer, it has good air tightness and self-cleaning functions. A sealing air port is located on the housing of the feeder to ensure its pressure is higher than pulverizer inlet air pressure to prevent hot air backflow. The coal feeder is equipped with coal supply cutoff alarm, pluggage alarm and coal flow monitor, etc., to assist with troubleshooting and safe operation.
The coal pulverizer output is 6 - 60t/h. The coal flow can be regulated by changing the rotating speed either at local or in remote control (manual/auto) as required by the boiler combustion control system to coordinate coal supply with boiler load.
Each boiler is equipped with 5 coal bunkers which connect to 5 coal feeders respectively. Total effective coal storage of four bunkers can satisfy over 8 hours coal consumption under boiler MCR working condition.
Pulverized-coal system is a significant auxiliary system for the unit. Its purpose is to grind coal into qualified pulverized coal as is required by combustion; its operation has direct influence on the safety and efficiency of the boiler.