Efficiency of a wood-fired bakery oven - Improvement by Theoretical and Practical

Doctoral thesis, 2014

Combustion of biomass in small-scale furnaces is widely used in many countries and in different applications. The technology used is often “fixed grate” combustion in small batch furnaces. The efficiency of such furnaces is often low, which results in 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 environmentally 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 of the combustion chamber and baking oven, the baking time and the bread quality. The circulation of hot gases within a baking oven was used to describe and predict the behaviour of the heat exchange and the quality of the products produced. A bi-dimensional cold flow model and a CFD model were used to estimate the flow pattern inside the oven by varying the velocity of the air flow in order to simulate changes in the combustion chamber. Experimental measurements to support improvements in the design and performance of the oven were performed on in situ wood-fired bakery ovens in Mozambique in order to evaluate the dependence of the quality of bread produced on different process conditions that originated from design of oven, the temperature profile and the wood used in combustion. 3D CFD model was also used to study the heat transfer process during the baking process to predict the causes of the differences in quality of the bread baked in same batch. The effects of the design of the oven were analysed concerning velocity, temperature distribution and heat transfer during the baking process. As much as 60 tonnes/day of green wood are consumed in the bread baking process in the areas investigated. Two types of bakery ovens are used most commonly: indirect and semi-direct. The specific consumption was found to be 0.55 and 0.90 kg of wood per kg of wheat flour baked for the indirect and semi-direct, respectively. The inlet velocity, the geometry and the mode of the feeding dough into the oven affect the flow pattern in the baking oven. The temperature becomes non-uniform, the velocity varies according to the inlet velocity and consequently the quality of the bread baked in WFBO is not uniform. The analyses of heat transfer from in situ bakery oven shows the dependence on distribution of heat and intensity inside the baking chamber due to the temperature being non-uniform, even with steady temperature conditions in the oven. The heat intensity is affected by burning different wood species. The differences in layers of sand under the oven base give rise to variations of the condition during the baking process. A mathematical model of the transient heat transfer in a wood-fired bakery oven shows a high accuracy with earlier experimental results. Furthermore, it describes the differences in the distribution of heat in the oven. Fifty per cent of hot gases cross the oven without releasing any heat. The variations in the heat distribution are the cause of the differences in the quality of the bread baked in this device. An example of optimisation using CFD model shows good approximation of the optimal geometry of the oven with reasonable heat distribution. However, the number of the samples must be increased to get the realistic heat distribution, which also requires substantial computing resource