Heat transfer through anaerobic digester concrete tank walls

Master Thesis


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University of Cape Town

This dissertation is a study of the heat transfer through concrete walls in anaerobic digester facilities. In the biogas industry, the term “heat loss” is synonymous with heat transfer. The dissertation identifies the reasons why heat is critical in the operations of these facilities. Concrete has traditionally been a material used for the retention of liquid-based products and additionally provides good thermal insulating properties. It combines the benefits of being relatively cost effective for the construction of large tanks and requiring low maintenance during the operational life span. The research focuses on the thermal properties of the various constituents of concrete and the influence these have on the overall thermal properties of the concrete tank. The constituents forming part of the study are cement, corex slag, water, fine and coarse aggregate. The study showed that the aggregates have a greater influence on the thermal conductivity than the other constituents. It also showed that the mineral composition of the aggregates has a greater effect on the thermal conductivity than the porosity of the aggregates. The study also looked at the influence of the interfacial transition zone around the aggregate and this was found to be not significant and generally can be ignored as a contributor to the thermal conductivity of normal / structural concrete. The effects of the porosity of the binder paste does affect the thermal conductivity specifically when aqueous solutions are being retained. The capillary pores of the paste can be filled with liquid (mainly water) and less with air. Due to water having a higher thermal conductivity than air, the thermal conductivity of the binder paste is significantly increased. The effects of reinforcement on the thermal conductivity of concrete was also investigated. Different types and arrangements of reinforcement could have a big influence. Steel fibres and reinforcement if aligned in the direction of the thermal gradient will greatly increase the thermal conductivity. However, it was found that the reinforcement used in the sample wall did not increase the thermal conductivity significantly as it was mainly aligned perpendicularly to the direction of the thermal gradient. Similarly, no steel fibres were used in the concrete. Once the thermal conductivities of the constituent materials were determined, the effective thermal conductivity of the concrete could be calculated using the effective medium theory. The subsequent heat losses, which are a function of the thermal conductivity, the temperature gradient between the internal and external faces of the concrete wall, the contact surface area and the heat transfer coefficient, could be calculated as a function of time. The New Horizons Waste to Energy Project in Cape Town was used as a reference project and the research was based on the materials used in the construction of the concrete anaerobic tanks. The project was also used for the measurement of the temperature gradients and subsequent calculation of actual heat losses at various points along the concrete walls. Furthermore, a computational model was developed using Abaqus to compare the results with those derived from the theoretical model. The heat loss from the computational model compares very well with that of the theoretical model.