Coal/transport air outlet velocity at burner tip here typically may be chosen as low as possible,as no demand of the pneumatic transport system is existing (due to vertical arrangment).Outlet velocity of 6m/s for petcoke is a common value.
Combustion in a rotary kiln is completly different as it has to take care for the process demands.The heat release of the burner flame needs to meet the process requirements.
In rotary kiln burners the coal transport velocity (due to horizontal arrangment) plays a more important role.In order to avoid pulsations of transport air pressure the velocity in the transport piping system must not decrease below approx.30 m/s. The same is valid for the long annular duct of the rotary kiln burner where a velocity of 25 m/s is common.
The optimum outlet velocity of the pulverized fuel in a rotary kiln burner depends on a number of influencing factors(volatile release behaviour,kiln lenght,secondary air temperature,etc.) which means that adaption of coal outlet geometry is a good instrument enabling burner operator for optimising operation.