A gas-particle processor comprising: a chamber having a gas inlet, a gas outlet and one or more particle inlets; a gas flow arrangement operable to flow gas through the chamber from the gas inlet to the gas outlet at a first controlled mass flow-rate; and a particle flow arrangement operable to introduce particles in one or more streams into the chamber at a second controlled mass flow-rate, each particle stream flowing through respective processing regions in the chamber, wherein the processor is operable to control the first and/or second controlled mass flow-rates to provide a gas-particle mixture porosity in a substantial portion of each processing region of 0.900-0.995.