Filter backwashing unit, char recovery unit, method of filter backwashing, and integrated gasification combined cycle

A filter backwashing unit disposed in a path in which process gas flows to remove at least a part of trapped dust included in a process gas by backwashing an element of a filter device that traps the dust when a process gas passes, the filter backwashing unit comprising:

• a gas injection device disposed on a downstream side of the element in a flow direction of a process gas to inject backwashing gas toward the element from the downstream side;
• a parameter detection device configured to detect a parameter used for determination of a state of dust adhering to the element;
• and
• a control device configured to estimate a thickness of dust deposited on a surface of the element on an upstream side of a process gas based on a result of detection obtained by the parameter detection device, and determine an interval at which the element is backwashed based on the estimated thickness of the dust, wherein
• the parameter detection device includes a deposition amount detection device configured to detect an amount of dust deposited in a lock hopper temporarily storing therein dust exhausted from the filter device, and
• the control device estimates the thickness of the dust based on a timing when the gas injection device injects the backwashing gas and based on a change in the amount of deposition detected by the deposition amount detection device.