A gas-particle processing method comprising: introducing gas into a chamber through a gas inlet; flowing the gas through the chamber from the gas inlet to the gas outlet at a first controlled mass flowrate; introducing at least one particle stream into the chamber through one or more particle inlets of the chamber at a second controlled mass flowrate; flowing each particle stream through a respective processing region in the chamber; and controlling the first and/or second mass flowrates, such that the gas-particle mixture porosity in a substantial portion of each processing region is 0.900-0.995.