Optimization study of small scale biomass combustion appliances

Introduction. Biomasses are recognized as clean and renewable fuels, but to lower their environmental impact there is need to develop suitable technologies to improve thermochemical conversion processes both in terms of efficiency and pollutant emissions. This is particularly important in small scale plants designed to residential heating, where only a minimum control of combustion conditions can be ensured and where clean smoke devices are generally absent. Domestic heating represents about the 30% of the Italian energy consumption, so a further exploitation of these applications is desired to replace electrical or fossil-fed appliances. Our activity is aimed at optimizing small scale plants for packed bed biomass combustion. Experimental and numerical methods. Several approaches are combined to evaluate both process parameters and geometry modifications on the performance of the appliance. 1) Experimental measures of process parameters (T, draught, wood weight) and emissions from a fireplace wood heater. Also an experimental apparatus is developing for the study of particles emitted from steady state combustion of a single wood pellet. 2) Numerical CFD mathematical models were defined to simulate the fluid-dynamic conditions inside the plant supporting the optimization analysis. A commercial code (CFD-ACE+ by ESI Software) was used. 3) A theoretical approach conceived from the combination of the two previous activities, useful at the definition of a simple procedure able to summarize the more relevant parameters for an improved efficiency. Results and discussion. The proposed approaches were applied to a specific configuration of closed fireplace wood heater. The analysis showed the great influence of fluid-dynamic on the performance of the plant, strongly affected by geometrical design and air feeding conditions. The theoretical model allowed to point out and quantify the actions aimed at improving the plant performance. A strong interdependence between chemical and heat exchange phenomena was recognized. The important role of the amount of comburent fed to the plant both for chemical combustion and heat exchange efficiencies was noticed and deepen analyzed, concluding that air excess must be controlled and possibly reduced compatibly with the chemical process. Further study of the combustion process is directed to the study of particle emissions that represents one of the major problems related to wood combustion. Conclusions. A complete optimization study on a small scale plant for wood combustion using experimental measures and CFD analysis is shown, furnishing some widening on the role of different parameters in the performance of wood combustion technologies. A general presentation of further activity is furnished, presenting the major issues related to particulate emissions from wood combustion.