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H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289 mover that does not have any mechanical parts and the engine consists of two heat exchangers and a stack of parallel plates that are contained in a cylindrical casing and it converts thermal energy to acoustic en- ergy. The investigation concluded that by recovering waste heat with a temperature of 150 °C, an output of 1030 W of acoustic power with a thermal engine efficiency of 5.4% can be obtained. In another study Mukherjee et al. [214]demonstrated that with the use of air-preheaters, 4% savings can be achieved in the oven fuel consumption. In an experiment, heat was recovered from the exhaust flow of an industrial baking oven and the primary air supply of the burner was increased to 105 °C. This study indicated that the increase in the primary air temperature can result to a saving of at least £4,200 of running cost. 5.3. Waste heat in the ceramic industry The ceramic industry is one of the most energy-intensive industries in the UK [215]. As stated by the British Ceramic Confederation Energy Policy, ceramic manufacturers should be concerned about the en- vironmental and economical impacts of their businesses and take steps in that regard to protect the world’s resources and reduce their carbon footprint [216]. With respect to this, the use of waste heat recovery units in the ceramic industry has been identified as an effective way of achieving this goal and improving industrial energy efficiency [217]. In order to discover the waste heat potential and identify how the recovered waste heat can be used, it is important to identify the sources of available heat in the production process and investigate the effec- tiveness of waste heat recovery technologies with these sources [215]. As UNIDO & ECC [218] reports, electrical energy and chemical energy in the form of fuel are the main types of energy used in the ceramic industry. The electrical energy is used to power the motors of the production equipment and machines and the chemical energy in the form of fuel is used to provide thermal energy to heat the kilns and furnaces. The ceramic production process typically consists of five stages. In the first stage, the raw material and additives are ground and mixed to produce material slurry. The material slurry is then then fed into a drying tower where it gets dried and converted to powder so it can be pressed to a shape and form unfired ceramic. This then passes to another drying operation through a hot chamber where controlled heat allows the product water content to be reduced before the material enters a kiln and is fired to form blank ceramic. The product is then sent to a polisher to achieve a smooth surface. As Peng et al. [219] explains, the two energy consuming operations producing the most emissions are the drying and firing operations. It is reported by Delpech et al. [220] that the firing stage is the largest consumer of energy in the ceramic production process and this contributes to almost 50% of energy loss through the flue gas and the cooling gas exhaust. In this stage of production, the ceramic structural integrity such as mechanical strength, abrasion resistance, dimensional stability and re- sistance to water, chemical and heat is increased by heating up the product to a temperature between 750 °C and 1800 °C. The chart below illustrates a breakdown of the main thermal energy consumption in ceramic manufacture [221] (see Fig. 30). Many different waste heat recovery technologies in this regard have been investigated and introduced to accommodate and recover heat from the drying and firing processes. For instance, as Delpech et al. [220] explains, the best available techniques for recovery in the ceramic industry include recovery of excess heat from roller kilns by the use of cogeneration (or combined heat and power), Organic Rankine Cycles to generate electricity and the use of heat pipe systems. 5.3.1. Recovery of excess heat from roller kilns It is investigate that for the recovery of excess heat from roller kilns, some processes employ heat exchangers to recover heat from the kiln exhaust and preheat the combustion air entering the system [222]. Nonetheless, it is noted that because the combustion gases, possible corrosion problems for the heat exchanger can occur. Having said that, the heat from the cooling zones of tunnel kilns can be recovered to preheat the dryer or used through the mean of other methods men- tioned such as CHP or ORC to generate heat and electricity for the process and plant. Through this method, the generated electricity can be utilised in the oven to power the air and exhaust fans and the generated heat from the process can be used to heat equipment that dries the ceramics [223,224]. For instance in an experimental study of an ORC (Organic Rankine Cycle) for low grade waste heat recovery in a ceramic industry [225] proved that by recovering 200–300°C from Kiln gases, the maximum cycle efficiencies can reach a gross electrical efficiency of 12.5% with a net electrical efficiency of 11%. Nonetheless, on the other hand, Mezquita [226] demonstrated in a study that through the re- covery and diluting the flue gas stacks by using an oxidiser with am- bient air, the working fluid temperature can be raised to 105 °C, esti- mating a potential energy savings up to 17.3%. The technique was discovered to lead to an energy saving of 685kW and annual cost savings of more than 190 k€ without the requirement of any special investment. Nevertheless it is argued that transporting the heat from the source to use may be a challenge and in this regard suitable heat insulation maybe required. Having mentioned that, significant energy savings have been achieved with the use of new technologies such as the use of thermal oil to transfer heat from the afterburner to the dryer [227] (see Fig. 31). 6. Conclusion In conclusion, industrial waste heat is the energy lost in industrial processes to the environment. Waste heat recovery in industry covers methods of collection and re-use of the lost heat of industrial processes that can then be used to provide useful energy and reduce the overall energy consumption. Heat loss is mainly classified into high tempera- ture, medium temperature and low temperature grades and waste heat recovery systems are correspondingly introduced for each range of waste heat. The selection of heat recovery methods and techniques largely depends on key factors such as the quality, quantity and the nature of heat source in terms of suitability and effectiveness. The identification of the waste sources is an important aspect when looking into waste heat recovery methods for industrial processes in order to achieve optimum results and efficiency. In this regard, a comprehensive review is presented for waste heat recovery methodologies and state of the art technologies used in industrial processes. It was investigated that, there are many different heat recovery technologies available for capturing the waste heat and they mainly consist of energy recovery heat exchangers in the form of a waste heat recovery unit. These units mainly comprise common waste heat re- covery systems and all work by the same principle to capture, recover and exchange heat with a potential energy content in a process. Fig. 30. Thermal energy sources in ceramic industry. 284PDF Image | Waste Heat Recovery Technologies and Applications
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