Efficient and environment-friendly combustion of biomass in bakery oven furnaces

Licentiatavhandling, 2011

Abstract Combustion of biomass in small-scale furnaces is widely used in many countries and in different applications. Often, the technology used is fixed grate combustion in small batch furnaces. The efficiency of such furnace is often low which results in a high environmental impact due to the poorly controlled combustion of the wood logs that are largely used as the heat source for baking bread in wood-fired bakery ovens. This work has been undertaken in order to develop an efficient and environment friendly bakery oven furnace fired by biomass. The work was performed in Mozambique, and a survey was used to evaluate the consumption of wood and the technology used in the process of bread baking in two selected townships. The data collected from the 104 bakeries consisted of the dimensions of the oven, the temperature profiles within the combustion chamber and baking oven and, finally, the baking time. The circulation of hot gases within the baking oven was used to describe and predict the behaviour of the heat exchange and the quality of the products produced. Bi-dimensional cold flow equipment and a CFD model were used in conjunction to estimate the flow pattern inside the oven, which is one of the key factors in determining the baking efficiency of such ovens. Experiments were conducted in a physical model; various velocities were used to simulate changes in the air flow within the combustion chamber of the bakery ovens. Mathematical modelling was used to describe the flow using a standard k-ω model, which took the effects of the feeding door into account. As much as 60 tons/day of green wood are consumed in the bread baking process in the investigated area. The bread baking process in a semi-direct bakery oven occurs when contact is made between the bread/dough and the flue gases as they exit the combustion chamber. The relationship between the mass of firewood consumed and mass of wheat processed (specific consumption) was found to be 0.55 in indirect ovens and 0.90 in semi-direct. The temperature profile in a semi-direct oven beyond the inlet airflow is affected negatively by the feeding door, an effect that is magnified with manual feeding of the dough causing lack of uniformity on the quality of the bread produced. Surrounding air rushes in via the bottom edge of the feeding door when it is opened, whilst hot gases exit rapidly, flowing oven the top edge of the door, and thus affects the overall heat transfer by changing the effective thermal conductivity. Urgent improvement to this system is recommended not only to minimize this phenomenon but also to improve the efficiency of such ovens and the quality of the bread produced therein. Comparisons made between the flow pattern achieved by the bi-dimensional cold flow model and the CFD model show acceptable agreement. The flow pattern inside the baking oven varies with variations in the hot gases leaving the combustion chamber. The increment in velocity directly improves the baking conditions in the temperature range desired by increasing the circulation and effective thermal conductivity of the hot gases within the baking oven.