The propellant, for example air or nitrogen, will typically be at2-2.5 bar A.By way of comparison current pulverised material flow transport systems used with air-fuel burners deliver between 20 and 30 kg of pulverised material per 50 kg of propellant.Advantageously, the pulverised fuel will be between 100 and 200 mesh (150-75 .mu.m).Preferably, 60% to 80%, advantageously 70 to 75%, and more advan