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H. Jouhara et al. Thermal Science and Engineering Progress 6 (2018) 268–289 chamber and by passing an inert gas over the coke to recover and de- liver the waste heat loss to a waste heat boiler [169]. This process can also be conducted through wet quenching where heat is transferred to cool water that is sprayed over the hot coke. Wet quenching is also used to recover waste heat from hot slag, however as Shan et al. [170] re- ports, this process is found to be an inefficient method of waste heat recovery as it consumes a large amount of water, fails to recover the sensible heat and is less environmental friendly. Having said that, it is reported that other technologies that employ chemical techniques have also been developed that offer more efficient waste heat recovery. As Sun [171] explains, the recovery from slag is possible in three different forms: recovery as hot air or from steam, conversion of the waste heat to fuel through chemical reaction, and the use of thermo- electric power generation. In regard to thermal energy recovery, Zhang [172] explains that recovery is conducted through dry granulation in- cluding mechanical crushing methods such as the solid slag impinge- ment process, the mechanical stirring process and the rotating drum process. Other techniques such as the air blast method, the centrifugal granulated method such as the spinning disk (SDA) and Rotating cup (RCA) atomiser processes are also available and have been studied as shown in Table 1 below. Chemical methods on the other hand include the use of the methane reforming reaction process and direct use for making high value-added products (see Table 2). 5.2. Waste heat in food industry The food industry is estimated to account for about 26% of the EU’s total energy consumption and to be the UK’s fourth highest industrial energy user [192,193]. Most of the waste heat produced in the food industry is classified as low-medium temperature [194]. Having said that, the amount of available waste heat in the food industry depends largely on the type of process in question and widely varies from sector to sector. This is mainly due to the fact that different industries use different processes for production and this indicates that the actual amount of useful waste heat can only be determined by conducting a comprehensive audit for the energy usage of processes. Based on the study conducted by Feldman [39], it is claimed that there are, however, general opportunities for waste heat recovery in the food industry that can be discussed in this paper. It is estimated that depending on the process, energy wastage is between 10% and 45%. Potentially, the main sources of the waste heat are associated with heating and refrigeration systems, hot streams of water or air used in production and heat from processing operations [195]. In the red meat processing industry, for example, the source of waste heat can be classified into recovery from refrigeration systems, meat processing and by-product rendering [196]. For instance, in a slaughter house, the refrigeration of carcasses is the most energy in- tensive process. On the other hand, if by-product rendering takes place, this can be a major energy user. The clean-up operation that uses large amount of hot water can also be nominated as a major energy consumer and also processes such as scalding, singeing and hair removal can use an extensive amount of energy [197]. For instance, in hog singeing operations where heat is used to dry out the hog carcasses, the majority of heat is released into the atmo- sphere. Waste heat recovery can be used largely for this operation to provide a more efficient production by supplying the energy require- ment for the dehairing and scalding processes. For instance, based on the study conducted by Ashrafi et al. [198] and Environment Agency [199] it is explained that singeing operations produce a waste flue gas of up to 800 °C that through the use of waste heat recovery equipment such as economisers can be utilised for boilers and to pre-heat the feedwater to produce hot water. On the other hand, recovery of heat from overflow hot water from the hot scalding process with the use of automatically operated scalding chambers is also achievable [197,200]. The production of processed meat is more energy demanding than slaughtering meat as it involves operations such as cooking, cooling, smoking, etc. As Fritzson and Berntsson [201] reports, the majority of heat loss in food processing operations is associated with refrigeration and curing of the product. Based on the type of operation, waste heat sources can include heat from condensers, waste water, smoking vents and cooker exhaust [202]. Again, recovery from these sources must be studied based on individual cases as for some waste heat sources, such as waste water and cooker exhaust, recovery may be difficult and not economical because of the grease and food waste products in the ex- haust. On the other hand, it has been shown that in poultry processing, where bird meat is prepared and processed, the largest quantity of energy and energy loss is associated with the scalding, cooling and freezing processes. Shupe and Whitehead [203] reports that, for instance, when overflow from scalders and chillers occur, heat recovery can ea- sily be conducted by collecting and returning the energy back to the scalder or chiller systems. Heat can be recovered from refrigeration condenser systems and be used to preheat the boiler that is used for processing wash water [204]. The operation of obtaining heat from a refrigerant condenser can be conducted through the use of a de-su- perheater, which can be installed between the compressor and con- denser to recover heat in a temperature range of 60–90 °C [205]. Heat rejected by the pasteurisation process and refrigeration con- densers on the other hand are the main source of waste heat in dairy processing plants [206,207]. Nevertheless, the waste heat from the dryer exhaust can also be a potential waste heat source that can be used to preheat supply air for the spray dryers [208]. Having mentioned that, the heat from the pasteurisation and milk cooling processes can be recovered and be used to preheat cold milk in the regenerator through the use of heat exchangers such as economisers or CO2 heat pumps [209]. The heat from refrigeration condensers is used to produce hot water for clean-up, preheat boiler feed water, or heat culture tanks for some operations [207,209]. Singh and Dasgupta [209] showed that with the use of a heat pump with an internal heat exchanger for combined heat recovery and hot water generation, the total fuel cost for pro- duction can be reduced by nearly 46% with a payback period of ap- proximately 40 months. In another study conducted by Bowater [210] and with the use of heat pumps, the energy efficiency of a large meat production plant was improved. In this study, heat pumps were used to recover heat from the refrigeration condensers of the plant to produce heating and hot water to a temperature of 65 °C, indicating a possible daily energy saving of up to £530. Similar to the dairy industry, in the egg processing industry waste heat can be recovered from the pasteurisation process and can be used through a regenerator to preheat the cold egg product. The waste heat from the refrigeration systems, hot waste water from egg washing and exhaust air from egg drying process can also be recovered to preheat boiler feed water, heat egg wash water and preheat the dryer air [211]. In freezing and canning processes the main operations are conducted in fruit and vegetable processing [39]. In freezing operations, the major waste heat is dissipated from the refrigeration system condenser. The waste heat is derived from hot refrigerant and is easily recoverable [212]. On the other hand, in canning operations the major waste heat is reported to come from wastewater and retort vents. Waste heat re- covered from these operations in fruit and vegetable processing can be used for water heating, can washing, blancher makeup water, plant clean up and boiler feed water [195]. In biscuit manufacture and bakeries the major waste heat sources are from flue gases coming from the cooking ovens, fryers, pan washers and boilers. Hot water can then be produced from the recovered waste heat for use in clean-up. The recovery of heat from the cooking oven exhausts is also a possibility for other uses [207,208]. For instance, [213] used a thermo-acoustic heat engine (TAHE) to recover the low grade waste heat that is dissipated from the exhaust gas of cooking ovens in biscuit manufacture. The technology works with a prime 283PDF Image | Waste Heat Recovery Technologies and Applications
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