Generally suitable only for small-scale systems, underfeed stokers are a relatively cheap and safe option for biomass combustion. They have the advantage of being easier to control than other technologies, since load changes can be achieved quickly and with relative simplic­ity due to the fuel feed method. Fuel is fed into the furnace from below by a screw conveyor and then forced upward onto the grate where the combustion process begins. Underfeed stokers are limited in terms of fuel type to low ash content fuels such as wood chips. Due to ash removal problems, it is not feasible to burn ash-rich biomass as this can affect the air flow into the chamber and cause combustion conditions to become unstable.