There are two methods available in solid fuel firing.
Hand firing technique can handle the boiler of small output. Here, we feed coal into the furnace with shovels. The primary and secondary air for combustion we control with dampers. The grate of such type of boilers consists of vertical iron bars over which we feed the coal.
Mechanical Stoker Firing
Here the feeding of fuel is done by mechanical stoker. There are many advantages of mechanical stocker firing over hand firing. We can feed the coal fuel uniformly into the furnace. We can control the combustion much easily. We can manage the fluctuation of load demand by controlling the coal combustion. We can quickly increase the output of the boiler by feeding more coal for burning. By controlling primary and secondary air, we can burn poor-graded coal efficiently as well. This method reduces labour cost much as there is no need for additional human workers to feed fuel to the boiler.
There are two types of mechanical stoker firing available. These are under feed stokers and travel grate stockers.
Under Feed Stokers
Here, coal gets burnt on the grate. The primary combustion air is fed to the grate. The secondary air is allowed at the top. Here the burnt out fuel is pushed forward by fresh fuel with the help of rams or screws on the fuel bed. The ignition occurs downwards against the primary air flow. The volatile matters get filtered through the bed, and it is totally burnt out. If the rate of the combustion of fuel is high enough, the ash contents are picked up by primary air along with the combustion gases and get delivered to the atmosphere. When fresh fuel is fed into the bed, most of the ashes move along the grate and fall into the ash pit.
Travel Grate Stockers
Here, the coal fuel is not pushed away by fresh fuel instead the coal fuel rests on a movable chain grate and the chain grate moves slowly forward. During travelling of the great, the coal gets wholly burnt. The ignition takes place below the stocker and burning of the field occurs at the top of the grate. Ash contents are lifted up along with the combustion gases to the atmosphere unless it is checked. Ash particles content in the flue gas is about one third less than that of the underfeed stockers.