Depending on the company’s coal, sometimes the smooth flow towards the coal mill can be disrupted. The mill will then trip on flow protection, upsetting boiler parameters and demanding immediate operator corrective action.
Bowl mills are used for pulversing coal in pulverized coal fired boilers. The coal to the mill is fed with a coal feeder in the coal bunker. There are lots of kind of feeders useful for coal feeding like the belt feeder which is often in volumetric or gravimetric mode, the chain link feeder, The drag feeder, etc. Row coal in the yard is sized in crushers and stored in coal bunkers. It is sometimes experienced, based upon coal quality, that the smooth flow to the coal mill can get disrupted. The mill will trip on no coal flow protection, and such a mill trip can upset boiler parameters and require corrective action.
For taking corrective action throughout a coal hang-up in bowl mills, the operator will have to be aware of basis for such a hang up the phone, how the plant will respond, what he has to reply to, and exactly what the local operator will have to do. This may lessen the potential for mill trip and convey the boiler normal again condition as soon as possible.
The reasons for coal hang-up to mills could be many, like large raw coal size, jamming of the feeder, jamming of the coal chute from the bunker due to high moisture inside the coal, foreign material at raw coal inlet to feeder, etc.
The boiler fuel input coming down as a result of coal flow hang-up the mill slowly gets unloaded. This is often inferred from your response with the boiler.
- Mill differential pressure comes down
- Mill outlet temperature will rise
- Boiler steam pressure will begin falling
- No coal flow alarm will appear
- Coal feeder and mill will trip if you do time
- Mill outlet temperature high alarm will appear
On seeing adjustments inside the operating parameters from the mill the operator will have to take corrective action.
- Decrease the strain on the boiler and inform the steam consumer accordingly
- Maintain boiler pressure and temperature of steam
- Start reserve mill and stabilize, if reserve mill is on maintenance, then ask local operator for that reason for hang-up and the action plan
- Keep steam consumers informed of your action
- Once coal flow is restored through the reserve mill or even the same mill, steam consumer may be asked to boost load
Local operator responses
In this case the local operator must play a major role in helping the control room boiler operator by performing these checks and deciding further plan of action.
- Check whether the heat and cold air gates are closed, or even then close both
- Check heat to raw coal and feeder is closed, if not close this
- Close the raw coal feeder inlet gate if not already closed
- After making certain every one of the above are closed, open the inspection door of the mill
- See if the coal pipe from feeder to mill is apparent and be sure they are made clear
- Check raw coal feeder and if chocked then have it cleaned
- Look at the raw coal feeder shear pin and replace if needed
- Now operate on no load and appearance for smooth running
- Look at the coal chute from bunker to feeder is obvious, that is from your closed feeder inlet gate onwards
- Look at the bunker for coal, if empty organize for filling immediately
- If mill has any remnant materials then clean the mill and restart the mill and check for smooth operation
- Now clear the mill for resuming operation