There is a need to develop improved designs of environmentally acceptable thermal plant for use in process industries in developing countries. Such plant may take advantage of the underfeed combustion principle which exhibits a tolerance towards smoke emission in the burning of high volatile matter biomass fuels. This paper describes an experimental investigation into the conversion of a small, 30 kW underfeed stoker from coal to wood chip firing. Two methods of air staging are considered for NOx abatement, including a novel air deflection technique. Comparisons are made between plant performances using the two methods. The results are also compared against representative values for coal firing. It was concluded that the air deflection technique showed potential as a means of NOx minimisation in small, underfeed stokers taking into account other requirements of adequate plant thermal efficiency and exhaust gas combustion quality. Copyright (C) 2000 John Wiley & Sons, Ltd.