The impact of the fuel feeding mode (continuous or periodic with various stand-by/operation time ratios) on carbon monoxide (CO) and nitrogen oxides (NO, NOx) concentration values in the flue gas was analyzed for coniferous wood pellet firing. Experiments were performed using a 20 kW water boiler equipped with an under-fed (retort) wood pellet furnace located in a full scale heat station simulating real-life conditions. Impact of oxygen concentration and temperature in the combustion chamber on the concentrations of carbon monoxide and nitrogen oxide have been examined. It was concluded that the commonly used periodic fuel supply does not necessarily cause an important increase of the concentrations of carbon monoxide compared to those in the continuous fuel feeding mode. Continuous fuel supply can even imply higher concentrations of carbon monoxide when fuel and air streams are not chosen properly.

The impact of the fuel feeding mode (continuous or periodic with different stand-by/operation time ratios) on carbon monoxide (CO) and nitrogen oxides (NO, NOx) concentration values in the flue gas was analysed for coniferous wood pellet firing. Experiments were performed in a 25 kW water boiler equipped with an over-fed wood pellet furnace located in a full scale heat station simulating real-life conditions. Influence of oxygen concentration and temperature in the combustion chamber on carbon monoxide and nitrogen oxide concentrations was presented in diagrams. Dust and hydrocarbon concentrations were also monitored. It was concluded that the commonly used periodic fuel supply does not necessarily cause a significant increase of carbon monoxide concentration, as compared to the continuous fuel feeding mode. Continuous fuel supply can even induce higher carbon monoxide concentrations when fuel mass stream is not chosen properly. Each time new fuel type is used in a specific furnace, one should perform experiments to determine the adequate settings (stand-by/operation time ratio, fuel mass streams, air stream) to obtain the optimal, lowest possible emission for a certain boiler heat output.

CO, NO, NO 2 and dust concentrations from combustion of deciduous wood (birch, beech, limetree) logs and pellets in two heating boilers (15 and 25 KW), situated in a heat station were investigated. Time dependences of pollutant concentrations as well as the impact of temperature in the combustion chamber and oxygen concentration on pollutant concentrations were presented. Pollutant emission indices have been estimated.

The paper presents the results of numerical and experimental studies aimed at developing a new design of a 10 kW low-emission heating boiler fired with wood pellets. The boiler is to meet stringent requirements in terms of efficiency (η > 90%) and emissions per 10% O2: CO < 500 mg/Nm3, NOx ≤ 200 mg/Nm3, and dust ≤ 20 mg/Nm3; these emission restrictions are as prescribed in the applicable ECODESIGN Directive in the European Union countries. An innovative aspect of the boiler structure (not yet present in domestic boilers) is the circular flow of exhaust gases around the centrally placed combustion chamber. The use of such a solution ensures high-efficiency, low-emission combustion and meeting the requirements of ECODESIGN. The results of the numerical calculations were verified and confirmed experimentally, obtaining average emission values of the limited gases CO = 91 mg/Nm3, and NOx = 197 mg/Nm3. The temperature measured in the furnace is 450–500 °C and in the flue it was 157–197 °C. The determined boiler efficiency was 92%. Numerical calculations were made with the use of an advanced CFD (Computational Fluid Dynamics) workshop in the form of the Ansys programming and a computing environment with the dominant participation of the Fluent module. It was shown that the results obtained in both experiments are sufficiently convergent.

Tests for combustion of hay and sunflower husk pellets mixed with wood pellets were performed in a horizontal-feed as well as under-feed (retort) wood pellet furnace installed in boilers with a nominal heat output of 15 and 20 kW, located in a heat station. During the combustion a slagging phenomenon was observed in the furnaces. In order to lower the temperature in the furnace, fuel feeding rate was reduced with unaltered air stream rate. The higher the proportion of wood pellets in the mixture the lower carbon monoxide concentration. The following results of carbon monoxide concentration (in mg/m 3 presented for 10% O 2 content in flue gas) for different furnaces and fuel mixtures (proportion in wt%) were obtained: horizontal-feed furnace supplied with hay/wood: 0/100 -326; 30/70 -157; 50/50 -301; 100/0 -3300; horizontal-feed furnace supplied with sunflower husk/wood: 50/50 -1062; 67/33 -1721; 100/0 -3775; under-feed (retort) furnace supplied with hay/wood: 0/100 -90; 15/85 -157; 30/70 -135; 50/50 -5179; under-feed furnace supplied with sunflower husk/wood: 67/33 -2498; 100/0 -3128. Boiler heat output and heat efficiency was low: 7 to 13 kW and about 55%, respectively, for the boiler with horizontal-feed furnace and 9 to 14 kW and 64%, respectively, for the boiler with under-feed furnace.