EXPERIMENTAL GAS EMISSIONS EVALUATION AND ENERGY BALANCE ASSESSMENTOF A GASIFYING WOOD STOVE

摘要

Purpose In the last years it has become clear that the combustion processes involving biomass can play an important role to satisfy the increasing energy demand. This work reports the results of an extensive investigation of the combustion process carried out to determine the energy balance and the gaseous products emissions at different process stages of a high efficiency wood stove applying a downdraft gasifying technology. Method. The experimental investigation has been carried on a two staged air gasifying wood stove having a thermal output between 11 and 30 kW. The stove is divided in three sections. In the upper part the feedstock is initially dried during the heating up of the system. Then the temperature grows up to the operating temperature (900-1000 °C) and the pyrolysis and char oxidation processes take place. An electrochemical analyzer has been used to measure the gas composition at the chimney (CO, NO, SO2, O2) and for the evaluation of the excess air supplied. An Hp Agilent 3000 Micro GC able to measure the concentration of CO2, O2, N2, CH4, C2H2, C2H4, C2H6, H2 and CO has been also employed during this experimental activity to measure the gas composition in the combustion chamber, under the grid and at the chimney. Results A considerable number of tests has been carried out to evaluate the performances of the stove supplied with several type of wood. The main gaseous compounds (CO, NO, SO2, O2) produced during the combustion process has been monitored both in the combustion chamber and in the flue stream at the chimney. Furthermore, the gaseous compounds CO2, O2, N2, CH4, C2H2, C2H4, C2H6, H2 and CO of the intermediate phases of the combustion process has been detected. For this stove, the mean efficiency values sets at about 70%, the energy losses at chimney at 15% while the thermal losses due to the heat exchanges through the envelope and to the ash removal are around 15%. Main conclusions The adopted wood stove confirms that gasification technology significantly limits the emission of unburned compounds. The effective excess air, maintained constant around 60% by the stove, is one of the main design variables for the appliance to control both pollutant emissions and performances, independently from the total air flow fed. Considering the increasing biomass demand for energy supply, the use of the investigated biomass combustion systems can be a suitable and enhanced technology to improve the utilization of this renewable energy resource.