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H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289 It was discovered that major heat recovery equipment are mainly categorised based on the temperature range and the type of fluid being recovered in the process and each has a different usage. For instance, it was studied, regenerative and recuperative burners optimise energy efficiency by incorporating heat exchanger surfaces to capture and use the waste heat from the hot flue gas from the combustion process, whereas, economisers recover low – medium waste heat and are mainly used for heating liquids. On the other hand, it was explored that waste heat boilers are suitable to recover heat from medium – high tem- perature exhaust gases and are mainly used to generate steam for power generation or energy recovery. Systems such as air preheaters were found to be useful for exhaust- to-air heat recovery and for low to medium temperature applications. This system was revealed to be particularly useful where cross con- tamination in the process must be prevented. Nonetheless systems that incorporate several heat recovery systems such as the heat recovery steam generator was also reviewed and it was discovered that this complex system recovers the waste heat and employs a thermodynamic cycle to generate power and improve the efficiency of a power or manufacturing plant. The working principles of thermodynamic cycles that are mainly used for waste heat recovery such as the Organic Rankine Cycle and the Kalina cycle were also studied and it was determined that the Kalina cycle offers a better result when the recovered heat is the medium-high grade. However, the ORC was a competitor when the recovered heat was in the low-medium range. Nevertheless, the functionality of plate heat exchanger and heat pipe systems were also looked at and it was learned that these heat exchangers can be used to transfer heat from any temperature range and from one source to another when cross contamination needs to be avoided. The functionalities of direct electrical conversion devices were also explored and it was discovered that these systems produce electricity directly from waste heat and eliminate the need for converting heat to mechanical energy to produce electrical energy. Nonetheless, due to the limitations these technologies offer, they are not widely used in in- dustry. The working principles of heat pumps were also studies and it was learned that this technology is in particular good for low-tem- perature waste heat recovery, as it gives the capability to upgrade waste heat to a higher temperature and quality. By considering the heat recovery opportunities for energy optimi- sation in the steel and iron, food, and ceramic industries, current practices and procedures were assessed and reviewed. For instance, by looking into the waste heat recovery potential for the iron and steel industry, it was revealed that the sources of waste heat are mainly within the range of medium-high temperature but challenges and lim- itations related to recovery methods exist due to the presence of dirty and low quality waste heat. In this regard, new and innovative tech- nologies and techniques are employed to recover the waste heat from different sources in iron and steel production processes. The investigated technologies and techniques include, the use of Heat Pipes to recover heat from the cooling line; regenerators to recover the waste heat from the exits of coke ovens, oven chambers, and blast furnaces; semi wet and wet open combustion and supressed combustion techniques for recovery from the basic oxygen furnaces; capturing and using flammable by-product gases and waste heat through preheaters in electrical arc furnaces; heat recovery through coke dry quenching and wet quenching and recovery as hot air steam or conversion of the waste heat to fuel through chemical reactions from hot slag. Waste heat recovery opportunities in food industry were also in- vestigated and it was discovered that the potential sources of waste heat in this industry are mainly associated with heating and refrigeration systems, hot streams of water or air and heat from processing opera- tions. Heat recovery from these sources must be studied on an in- dividual case basis as some waste heat sources such as waste water and cooker exhaust recovery may be difficult and uneconomical to utilise because of grease and food waste products in the exhaust. Having said that, technologies such as thermoacoustic heat engines (TAHEs), economisers, automatic scalding chambers, heat pumps and de-superheaters are considered to be alternative methods of optimising energy management in the processes of the food industry. In the ceramics industry, the primary sources of available waste heat come from kilns and furnaces mainly through the drying and firing operations. The available techniques for waste heat recovery in this industry include recovery of excess heat from roller kilns though co- generation or combined heat and power, the Organic Rankine Cycles to generate electricity and heat pipe systems. The functionality of each system has been analysed and it has been shown that recovery of excess heat from roller kilns can be carried out by accumulating the waste heat from the cooling zones of the kiln tunnel and using that as a heat source to preheat the dryer or through the means of other mentioned methods such as CHP or ORC to generate heat and electricity for the plant. To sum up, this paper indicates that improving energy efficiency by utilising waste heat recovery in industrial processes is achievable based on different approaches and with the use of different state of the art technologies. However, in order to obtain the most optimum efficiency for a system through waste heat recovery, the type of process in ques- tion should be always examined and analysed and then a method of waste heat recovery for optimising energy efficiency should be as- signed. Conflict of interest None. Acknowledgements The presented work is the result and contribution of the following Fig. 31. Schematic of a Ceramic Kiln [228]. 285PDF Image | Waste Heat Recovery Technologies and Applications
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