remarkable suppression of the formation of pollutants : no x , co and uhc , in the flameless combustion of gaseous fuel with excess air is achieved by passing the fuel and air through a porous surface combustor directly into an adiabatic zone , and by controlling the amount of excess air to be greater than about 50 % but not greater than about 150 % in excess of the stoichiometric requirement . the furnace with the adiabatic zone for the suppression of pollutants will preferably have a refractory body as part of the adiabatic zone .