ENERGY INNOVATION
ENERGY INNOVATION Unlocking the potential of industrial waste heat recovery and thermal energy storage Realise your business's energy potential with accessible technologies that slash costs, boost productivity and drive sustainability in the race towards a greener future. The South African industrial sector relies heavily on thermal-energy systems, accounting for approximately 70% of its energy consumption. Historically, South African industries have been nurtured under the wing of low-cost coal and electricity. This environment has led to a typically inefficient and carbon-intensive industrial sector which is now suffering under the burden of failing energy availability and rising energy costs. These factors, together with justified international pressure to reduce greenhouse gas emissions has had a compounding negative impact on industrial competitiveness both locally and internationally. To demonstrate the effectiveness of waste-heat recovery technologies in the local industrial sector, the Council for Scientific and Industrial Research (CSIR) launched a pilot project with CERadvance, an advanced industrial ceramics manufacturing company in Pretoria. Opportunity in WHR and TES Even though waste-heat recovery within industry has the potential to significantly reduce operating costs by reducing peak energy demands and improving energy efficiency, especially in batchwise manufacturing processes, it is unfortunately not widely deployed. This is due, in part, to the technical difficulties involved in predicting and optimising new system performance to quantify the feasibility of planned projects. The available scale, capacity, technology and efficiency of both thermal energy capture and storage systems can play an important role in presenting a positive investment case. This techno-economic feasibility goes beyond the obvious operational benefits of systems which can convert waste heat into a readily available energy source that easily aligns with batch cycle demands. Over and above the complexities of these unknowns, there is often a need to develop bespoke technical solutions to serve unique industrial cases, which leads to the development of novel thermal components and systems. A ceramics company as a pilot site The pilot project with CERadvance investigated the recovery of waste heat from the ceramic-sintering process and the medium-term storage and reuse of that energy in the manufacturing processes. The company operates a series of high-temperature electric sintering furnaces which run at temperatures above 1 000°C. Once the sintering cycle is completed, both natural and forced convection cooling is used to reduce the temperature of the kiln's contents. This results in a waste hot-air stream which peaks at more than 800°C and 4kW per kiln. The temperature of this waste-heat stream decays gradually until the furnace contents are cooled to a temperature sufficiently near ambient. While the total waste energy is a function of the furnace contents, this is typically in the order of 160kWh per batch. During the manufacturing process, a ceramic slurry needs to be cast into prepared Plaster-of-Paris moulds. The moulds absorb water from this slurry during the product-casting process and subsequently require drying before further processing. Before adoption of waste-heat recovery, CERadvance used a couple of 6kW direct-heating electric ovens to dry their moulds at temperatures around 60°C. The concept of this pilot was to demonstrate the capture and reuse of kiln heat to run the drying ovens. Zeolite master class The unique thermal energy-storage capacity of Zeolites is based on adsorption, which involves the attachment of gas molecules to the surface of a solid. This process differs from absorption, which involves the bonding of gas molecules to a liquid. The substance that is adsorbed is called the adsorbate. The adsorbate enters the pores of the adsorbent, and a chemical bond is formed which releases energy as a result. When the pores are saturated, no further heat is released. The system can then be regenerated by using the waste heat from hightemperature kilns. The heat breaks the bonds between the adsorbent and adsorbate. This is an endothermic process and can also be referred to as desorption. Once the system has been regenerated, the adsorption process can be repeated. 28 | www.opportunityonline.co.za
ENERGY INNOVATION 1D Simulation model of System using Flownex®. Introducing thermo-chemical heat storage In this context, a thermo-chemical heat storage system was developed. The system includes two fixed-bed regenerators that are connected to the waste hot-air stream of the high-temperature ceramic kilns. This hot air is diverted through the packed bed reactors for thermal capture and storage. This is where it gets exciting, as the heat storage material selected for this project was Zeolite 13X. Zeolites are aluminosilicate materials which consist of orderly distributed micropores. Zeolites can store heat in sensible and thermo-chemical form, with the thermo-chemical heat energy being retained independently of the temperature of the packed bed. The thermo-chemical storage capacity within Zeolites is typically four times the usable sensible heat storage capacity, making this thermal-storage material a highly attractive option. As reference, using stored thermo-chemical heat alone, just 500g of charged Zeolite 13X is required to heat 100L of water by 1°C. The application The regenerators are connected to a drying chamber to use the stored heat to dry the casting moulds. The team had to design a low-cost high-temperature ceramic valve to enable cycling between the charging and discharging of the two storage vessels, so that one reactor can charge while the other discharges the heat to a drying chamber. The regenerators were designed to store more than 150kWh which is sufficient for heating two days’ worth of drying air. CFD model of packed bed reactor for thermal storage showing thermal striations. Benefits and impact The reuse of waste heat from the kiln significantly reduces the demand for electrical drying. This not only lowers manufacturing costs, but also greenhouse gas emissions. If the Zeolite bed is kept dry, standing losses are theoretically capped to the sensible-heat portion only, which amounts to a 20% loss no matter how long the heat is stored. This implies that a major portion of the waste heat can be stored even during prolonged shutdowns. Zeolites like these can store heat for much longer with minimal losses compared to conventional sensible-heat storage systems, making them ideal for intermittent or batch production processes. The system not only provides hot-air, but also has the added advantage of chemically drying the process air. The steady supply of hot-air with ultra-low moisture content to the drying chambers significantly increases the control, speed and repeatability of drying processes. The performance seen in this pilot implementation at CERadvance demonstrates the tremendous potential of thermo-chemical heat storage systems. By capturing waste heat and optimising its utilisation, businesses can unlock substantial economic and environmental benefits. Contact the CSIR to explore how these technologies can propel your business forward. Authors: Tobias van Reenen: tvreenen@csir.co.za Muhammad Sheik: msheik@csir.co.za Packed bed reactors for thermal capture and storage.
