Test of a small domestic boiler using different pellets

This paper presents results from an experimental study performed on a 13 kWth commercial domestic boiler using pellets as fuel. Four different types of pellets were used and, for each one, the boiler was tested as a function of its capacity and the fan regulation affecting excess air. Measurements were performed for boiler heat load, pellets consumption rate, flue-gas temperature and composition. Mass balances allowed the calculation of the flue-gas flow rate and associated heat losses. Losses from incomplete combustion have also been quantified. Under boiler steady-state conditions the flue-gas O2 concentration changes with boiler load and ventilation due to the regulation scheme of the boiler. Flue-gas CO shows a minimum for values of O2 in the flue-gases of about 13%. NOx emissions are independent of excess air for low values of nitrogen in the fuel whereas, for larger values, NOx emissions increase with the O2 present in the combustion products. The fractional conversion of the pellets nitrogen into NOx is in line with literature data. The boiler start-up was characterised by the temperature evolution inside and above the bed showing the propagation of combustion in the bed during about 10 min. During boiler start-up, a maximum in CO emissions was observed which is associated with the maximum combustion intensity, as typified by the flue-gas O2 concentration and temperature, regardless the pellets type.