Combustion control in domestic boilers using an oxygen sensor

This paper deals with the problem of controlling the combustion process in a domestic boiler fed with natural gas. Combustion control is of particular interest because of its implications with respect to safety, energy saving and reduction of pollution. Traditionally, the control system of a domestic boiler is conceived in such a way to regulate simply the water temperature by acting on the gas flow. Actually, in order to improve the quality of the combustion process, it would be necessary to regulate also the quantity of air, and hence of oxygen, in relation to the gas flow. Using a suitable sensor for measuring the percentage of oxygen in the combustion chamber, the actual value of the lambda ratio can be calculated, as well as the excess of air with respect to a reference value depending on the gas used. It is shown that this information can be used efficiently in a feedback control loop in order to regulate, by means of a variable-speed fan, the quantity of air involved in the combustion process. Simulation and experiments are discussed and the results obtained by means of the proposed control architecture are illustrated