Causes: 

1. Before ignition, there are sundries burning in the hearth or the coke occurring  during ignition has not been cleaned. 

2. The hole of the ventilation hood is blocked or damaged, thus causing local coking. 

3. During operation, the bed temperature is controlled too high, thus causing coking. 

4. The refractory bricks inside the inside wall of the hearth fall off locally. 

5. There are larger particles of coal entering into the hearth. 

Treatment: 

1. When slag-discharging pipe is blocked, it shall be reported to the shift chief- operator to properly increase the negative pressure of combustion chamber and special person shall be assigned to poke the slag. 

2. The person poking the slag should wear helmet and special mask, wear protective clothing, use special tools to poke the sundries, crush and discharge the sundries.

3. When there is ash slag ejecting outward, it is forbidden to poke the slag. If measures have been taken and slag-discharging cannot be ensured, the load shall be appropriately reduced and shall be treated after the instructions have been asked for banking up or shutdown the boiler.