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Heat recovery and heat pumping opportunities in a slaughterhouse
Energy xxx (2015) 1e13
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Heat recovery and heat pumping opportunities in a slaughterhouse dard a, *, Bahador Bakhtiari b, Bruno Poulin a Omid Ashrafi a, Serge Be a b
Natural Resources Canada, CanmetENERGY, 1615 Lionel-Boulet Blvd., Varennes, QC J3X 1S6, Canada American Process Inc., 750 Piedmont Avenue N.E., Atlanta, GA 30308, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2014 Received in revised form 11 May 2015 Accepted 23 May 2015 Available online xxx
Pinch analysis was employed to recover and reuse a part of the waste energy during the production and cleaning periods of a slaughterhouse in Canada. The INTEGRATION software was used to find possible options for HR (heat recovery). Several HR opportunities were identified in various systems of the slaughterhouse including boilers, hot water production, singeing, refrigeration, and barn air heating. A recommendation was also made to store more energy in the form of hot water. Opportunities for using heat pumps for additional HR were also investigated. The proposed projects can significantly reduce natural gas consumption in the steam and hot water boilers as well as in the air heating systems. It is shown that the proposed projects can reduce natural gas use by approximately half a million dollars per year. © 2015 Crown Copyright and ELsevier Ltd. Published by Elsevier Ltd. All rights reserved.
Keywords: Slaughterhouse Process integration Heat recovery Heat pump Pinch analysis
1. Introduction Higher energy prices and the global climate change have increased the need for efficient energy use in the food industry. Energy consumption represents a significant proportion of the production costs in this industry. In 2010, food processing establishments in Canada consumed approximately 102.4 PJ of energy, of which 30.3% was electricity and 69.7% was other purchased fuels [1]. Significant energy-demanding sectors in the food industry are slaughterhouse and meat processing plants that use approximately 18% of the total energy consumption in the Canadian food industry [2]. The most important energy consumers in a slaughterhouse are the refrigeration system, the singeing process [3], the steam production system, the hot water production system and the compressed air system [4]. In slaughterhouses, an issue that makes optimal energy recovery difficult is the presence of discontinuous heat sources, normally available during the production periods, and semi-continuous heat sinks. The most important semi-continuous heat sink is the production of hot water which is needed during both production and cleaning periods [5,6]. The Canadian food industry also contributes to the GHGs (generation of greenhouse gases) from three direct sources of emission: combustion of fossil fuels in steam and hot water boilers, loss of HFCs (hydro-
* Corresponding author. dard). E-mail address: [email protected] (S. Be
fluorocarbons) from refrigeration systems, and methane generation in wastewater treatment systems. This industry generates approximately 3500 kt/y of CO2 [7]. Energy recovery projects are not only beneficial for food companies to improve their bottom line, but also for the environment as they decrease GHG emissions. PI (process integration) is a method that can be used during the design and retrofit of many application and industrial processes [8] in order to optimize the use of energy and other resources [9]. PI allows defining the best opportunities for HR (heat recovery) in a process and focuses on the efficient use of energy. Pinch analysis, a PI technique, uses graphical tools for the calculation of energy targets and applies heuristics to achieve those targets [10]. Recently, a new methodology was proposed by Bakhtiari and dard [11] for retrofitting heat exchanger network in industrial Be plants. Their methodology includes the capability to handle some practical constraints more systematically than other pinch analysis methods while offering a systematic step-by-step approach. Heat pumps are commonly used when a certain amount of low grade energy is available to be used effectively at an insufficient temperature level [12]. This technology can significantly increase HR in a process when it is appropriately located and designed [13]. PI can also be used to identify opportunities for the application of heat pumps in a process [14]. In slaughterhouses and meat processing plants, a considerable amount of low temperature heat is rejected through evaporative condensers and cooling towers. Some of this heat could be used for air and process water heating purposes through the use of heat pumps [15].
http://dx.doi.org/10.1016/j.energy.2015.05.129 0360-5442/© 2015 Crown Copyright and ELsevier Ltd. Published by Elsevier Ltd. All rights reserved.
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Nomenclature HU CP eff HR m NG Q YOH DH DT
hot utility requirement heat capacity (kJ/kg C) efficiency heat recovery mass flow rate (kg/s) natural gas energy (kJ/s) yearly operating hours (h/y) enthalpy of vaporization/condensation (kJ/kg) temperature difference ( C)
PI was used to identify HR potential in various food processing industries, mainly dairies [16] and breweries [17]. Fritzson and Berntsson [5,6] used pinch analysis techniques in a slaughterhouse in Sweden and found significant potential for electricity and fuel savings. This slaughterhouse was already quite efficient, notably due to the use of a heat pump for process water heating. The refrigeration plant was modified in this study by changing the temperature levels of the refrigeration system, resulting in a 10% reduction of the electricity consumption. In addition, HR in the slaughterhouse was increased by reducing the DT of existing heat exchangers to 5 C. It was reported in another study by MAYEKAWA Europe [18] that the use of two ammonia heat pumps for hot water generation at 52 C in a Norwegian slaughterhouse saved approximately 1.6 GW of natural gas consumption per year (based on lower heating value). In this paper, the results of a PI study performed in a slaughterhouse located in Canada are presented. In addition to HR projects, the potential for implementing heat pumps is also investigated. The INTEGRATION software, a process integration software developed by CanmetENERGY, was used to identify the HR projects proposed in this work. INTEGRATION uses a sequential interactive approach originally proposed by Asante and Zhu [19], called Network Pinch, and extended by Varbanov and Klemes [20].
Additional improvement to the algorithm was made by Bakhtiari dard [11] to reduce the number of retrofit options that are and Be considered using the Match Penalty concept. In addition, the software includes models to calculate the amount of waste heat, and the temperature levels, that can be recovered from industrial boilers, air compressors and refrigeration systems [21].
2. Pork slaughterhouse Fig. 1 presents the main processing steps in the pork slaughterhouse, with the main departments being the barn, the kill floor, the evisceration department, the clean floor, the cold warehouses and the powerhouse. Animals are received by truck in the barn where a large amount of fresh air is needed. On the kill floor, animals are put through different processing steps such as slaughtering, bleeding, scalding, hair removal, and singeing. The hogs are then going to the evisceration and head removal processes. The kill floor requires hot water at 56 C and 87 C. The hot water heating and the singeing processes are the two largest natural gas users in the slaughterhouse. The singeing process generates flue gas with a temperature of approximately 500 C. After final washing, hogs are sent in a blast tunnel to rapidly freeze their surface to prevent water loss. They are then sent to cold storages so that they can entirely cool down to the temperature of approximately 4 C. Following the cool down, the hogs are sent to the cut floor to be processed. The cut floor requires hot water at 56 C and 87 C for cleaning and sterilization purposes. The temperature of the cut floor is maintained at about 4 C. The refrigeration system is a two-stage ammonia system with screw compressors, having a refrigerant discharge temperature of 71 C in average. The low-stage system consists of booster compressors with a suction temperature of 40 C and is used mainly for the blast tunnel and the freezers. The suction temperature of the high-stage system is maintained at 10 C and is used mainly for the cut floor and the cold storages. The superheated ammonia from the compressors is cooled in a HR desuperheater from 71 C to 28 C, which corresponds to the condensing temperature at the
Fig. 1. Simplified diagram of the slaughterhouse.
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compressor discharge pressure. A thermosiphon is used on each compressor to reduce compressor oil temperature from 71 C to 54 C. The desuperheater is used to preheat the process hot water from approximately 10 Ce30 C using available energy from the superheated ammonia. Afterward, the temperature of the process hot water is increased to 56 C using a direct contact water heater and is then stored in a hot water tank. The part of the hot water that needs to be used at 87 C is further heated using steam in a heat exchanger. Approximately 18 L/s (280 usgpm) hot water is required for the slaughterhouse. Two 4900 kW boilers are used in the slaughterhouse with a maximum steam production capacity of 7250 kg/h. The steam production system also includes a heat exchanger to preheat boiler make-up water with the boiler blowdown, a condensate tank to store the condensate return and the boiler make-up, and a deaerator. Feedwater is sent to the boilers after the deaeration process at a temperature of 107 C. Steam is mainly used to produce hot water, to heat the boiler feedwater, to heat some processed meat in cookers, for sterilization processes, and for some space heating in cold days. 3. Methodology
4.1. Heat sources and demands The available energy sources for HR include superheated ammonia, ammonia condensation and subcooling, compressor oil, flue gas from boilers and the singeing process, and boilers blowdown. The INTEGRATION software was first used to analyze the refrigeration system (e.g. available energy, COP (coefficient of performance) of the system, refrigerant flow rate, etc.) and the boiler system (e.g. boiler efficiency, energy available in the boiler flue gas and blowdown, etc.) and to calculate available waste heat for HR from these systems. The analysis was performed for four different scenarios: a) Winter conditions 1. Production period (day-time) 2. Cleaning period (night-time þ weekends) b) Summer conditions 1. Production period (day-time) 2. Cleaning period (night-time þ weekends) The duration of the production period is 16 h per day, 5 days a week, while the cleaning period lasts 8 h per day and 16 h during the weekends. There are significant differences between these scenarios in terms of energy demands and waste heat availability: Process water at 87 C is only needed during the production periods; A greater amount of process water at 56 C is required during the production periods than for the cleaning periods; The singeing process and its associated flue gas are only available during the production periods; The refrigeration system load is much higher during the production periods as the blast tunnel is in operation and more cooling is required to cool the hogs.
This analysis was conducted following these steps: 1. Construction of a simplified process flow diagram containing the equipment and processes where heat is being used, recovered or lost 2. For each major operating scenario, development of a heat and mass balance to add every relevant temperatures, flow rates and duties to the process flow diagram 3. Realization of a pinch analysis comprising: a. Data extraction and construction of the composite curves b. Identification of the existing heat transfer inefficiencies c. Identification of new/improved HR opportunities d. Quantification of the potential benefits of each opportunities and evaluation of the cross-effect between these opportunities 4. Selection of the most promising HR opportunities with the mill personnel, considering operating constrains and plant layout 5. Development of HR projects for most promising opportunities, including the sizing of equipment.
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Table 1 presents the heat sources from the refrigeration system, while Table 2 presents some other heat sources and all heat sinks in this process. Energy requirements for warm and hot water production as well as air heating were calculated using the associated flow rates and water/air temperature provided by the plant using eqs. (1) and (2). Q ¼ m$Cp$DT
(1)
Q ¼ m$DHlatent
(2)
where
4. Pinch analysis
Cp, Cp, Cp, Cp,
There are currently only two heat exchangers used to recover waste heat in this slaughterhouse, the ammonia desuperheater and the boiler blowdown heat exchanger. Several other sources were available and could be used to perform additional HR.
¼ 4.18 kJ/kg C, ¼ 1.01 kJ/kg C, air flue gas ¼ 1.13 kJ/kg C, ¼ 2.00 kJ/kg C, cooling oil water
Table 1 Available energy from the refrigeration system. Summer day Energy (kW) high stage Compressor oil (71 Ce54 C) Superheated NH3 (71 Ce28 C) Condensation of NH3 Subcooled NH3 (28 C toa) a
1270 1328 12,531 1657
Summer night Energy (kW) booster 324 520 e e
Energy (kW) high stage
Winter day Energy (kW) booster
Energy (kW) high stage
476 498
51 81
953 996
4699 571
e e
9398 1594
Winter night Energy (kW) booster 259 416 e e
Energy (kW) high stage 445 465 4386 744
Energy (kW) booster 44.2 71 e e
The final temperature of ammonia to calculate the energy from subcooling, is considered to be 18 C and 10 C during summer and winter, respectively.
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Table 2 Heat sources and heat sinks for winter day scenario.a Heat sources
Energy (kW)
Tinitial ( C)
Tfinal ( C)
Heat sink
Energy (kW)
Tinitial ( C)
Tfinal ( C)
Boiler flue gas Singeing flue gas Boiler blowdown
1045 3000 50
185 500 107
20 20 15
Boiler make-up Condensate return Boiler feedwater Warm water at 56 C Hot water at 87 C Barn air make-up Barn air heating
170 455 180 3500 1275 810 600
4 51 107 4 56 4 20
25 107 135 56 87 20 30
a The energy available in the boiler flue gas and blowdown was corrected to take into account the steam savings that will be achieved by implementing the energy saving projects. Otherwise, savings could have been double counted. Similarly, the energy needed to heat the boiler feedwater and the boiler make-up was also adjusted.
Cp; superheated NH3 ¼ 2.54 kJ/kg C, Cp; subcooled NH3 ¼ 4.67 kJ/kg C, DHlatent; NH3 ¼ 1171.03 kJ/kg
4.2. Excluded heat sources and demands from the analysis The effluents from the slaughterhouse also contain a considerable amount of energy but were considered too dirty to be sent to a heat exchanger for HR. Effluent filtration and a CIP (cleaning-inplace) system could have been installed but the plant personnel did not want to consider these options. Also, the energy contained in the hog themselves was not considered as a heat source as no heat can be recovered from the hogs. The singeing process is an important natural gas user that generates a flame for hair burning and hog sterilization. Since it is not possible to replace the natural gas by HR, the energy demand for this process was therefore not considered in the PI study. Other steam use for cooking and sterilization were also not considered in the analysis as the steam usage in these processes cannot be replaced by HR. 4.3. Analysis of the existing network and construction of the composite curves The existing heat exchanger network of this slaughterhouse is shown in Fig. 2 in the form of the grid diagram traditionally used in PI studies. The red lines in the grid diagram represent hot streams that should be cooled down for process requirements. The green lines represent the liquid and gaseous effluent streams. An effluent stream is a particular type of hot stream that does not absolutely need to be cooled for process reasons. For example, the boiler flue gas is an effluent stream as it is not absolutely necessary to cool it down to 20 C. The blue lines represent cold streams that should be heated for process reasons. The energy required by cold streams could be provided by hot utility or by HR from hot or effluent streams. Fig. 3 presents the composite curves of the slaughterhouse for two different scenarios: winter day (Fig. 3a) and summer night (Fig. 3b). The composite curves show the temperature profile of the heat to be supplied to the process and the heat to be evacuated from the process. The hot composite curve (upper red line) is the combination of hot and effluent streams from which heat can be recovered, while the cold composite curve (bottom blue line) is the combination of all process heat demands. The flue gas from the singeing process is included in the composite curves during the production periods as HR is possible from this heat source. As shown in Fig. 3a, the composite curves for winter days are divided into two zones. In the HR zone, heat from hot streams (including effluents) can be used to heat cold streams. In the cold utility zone, hot streams require cooling to reach their final temperature. The DTmin represents the minimum approach
temperature that is acceptable in a given heat exchanger. The DTmin is set using several factors including the shape of the composite curves, types of process streams, type of heat exchangers and acceptable investment costs [22]. DTmin of 5e15 C is common in refrigeration systems and in the food industry in general [23]. In this study, a DTmin of 5 C was chosen for liquideliquid heat exchangers and 20 C for gaseliquid heat exchangers. Plate and frame heat exchangers were considered for liquideliquid HR and tubes and fines heat exchangers were used for gaseliquid HR. It can be seen in Fig. 3a that all the energy required by the cold streams can be provided by the hot streams when a DTmin lower than 20 C is used. This means that no hot utility is therefore needed to satisfy the heat demands considered in this analysis, providing that the HR system is optimally designed. In pinch analysis, this is normally referred to as a threshold problem. In such a situation, there is no need for heat pump as the cold streams can be fully satisfied by HR. A similar conclusion can be drawn when analyzing the summer day composite curves. Fig. 3b shows the composite curves for the summer cleaning period. During this period, a significant amount of hot water is needed while the singeing flue gas is not available. In this condition, the heat demands cannot be fully satisfied by HR and hot utility is required. For the summer cleaning period, it would be beneficial to implement a heat pump to eliminate the need for hot utilities. The analysis of composite curves during the winter night scenario results in a similar conclusion.
5. Review of the proposed heat recovery projects To facilitate the design of the most promising HR system, a few assumptions were made: Two storage alternatives were investigated for hot water production. Instead of storing water at 56 C in a water tank and further heating only a part of the process water to 87 C, it was decided to investigate an alternative approach. It consists in storing water at 87 C during the production periods and producing the required 56 C water by mixing cold city water with water at 87 C. The benefit of this approach is to store more energy in the existing hot water tank for the cleaning periods; The initial temperature of process water during summer and winter is assumed to be 18 C and 4 C, respectively; An average water flow rate is considered during both production and cleaning periods. Even if the flow rate varies in reality, this assumption is acceptable considering that a large hot water tank is used to stabilize the operations; Barn air make-up heating is required only during winter. The HR projects identified using the INTEGRATION software are as follows:
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Fig. 2. Heat exchanger network before heat recovery.
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Fig. 3. Composite curves: a) Winter day b) Summer night.
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1. 2. 3. 4.
HR in steam production system; Preheat process water using the energy from the ammonia; Heat process water using the energy from the compressor oil; Heat process water using the energy from the singeing stack flue gas; 5. Heat barn air make-up using the energy from the ammonia.
5.1. HR in the steam production system In the current heat exchanger network presented in Fig. 2, boiler make-up is preheated with the blowdown, is mixed with the returned condensate and is then heated in the deaerator using live steam injection. The boiler make-up and the boiler feedwater can be further heated by adding a new boiler economizer and a new condensing economizer as presented in Fig. 4. This project significantly reduces the amount of steam consumption in the deaerator (almost no steam is then required) and considerably increase the efficiency of the boiler. This results in a significant decrease in the amount of energy available in the boiler flue gas and boiler blowdown which was taken into account in the calculations. In order to reach the maximum HR from the boiler flue gas, a part of the process water (6.5%) would be sent to the condensing economizer (second stage) as presented in Fig. 4. Depending on the specific plant context and its size, the second stage economizer may or may not be cost effective. Since flue gas is cooled below its dew point temperature, the impact of water condensation was considered to evaluate the amount of energy that can be recovered in the second stage economizer. 5.2. Preheat process water using the energy from the ammonia A significant amount of energy is available at the condensing temperature of the ammonia in the evaporative condenser, approximately 28 C (see long plateau on the hot composite curve). Recovering a part of this energy to preheat process water is required in order to achieve an optimal HR design. Fig. 5 presents the two alternatives of preheating the process water with the energy from the ammonia. The first alternative, option (A) in Fig. 5, is
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to use a new HR condenser before the evaporative condenser. This can unload the evaporative condenser and reduce the power usage to drive the condenser fans. The second alternative, option (B) in Fig. 5, requires a new HR subcooler after the evaporative condenser. The use of a subcooler increases the efficiency of the system and allows more heat to be removed from the process for a given amount of refrigerant circulating in the refrigeration cycle. If the liquid ammonia is cooled to about 10e15 C before being depressurized in the low temperature ammonia receiver, a smaller amount of flash gas would be sent to the compressors. Therefore, this option reduces both the power requirement at the compressor and the power requirement to drive the condenser fans as less refrigerant would circulate in the refrigeration system. Approximately 93% of the process water is preheated to about 22e23 C in either the new condenser or the new subcooler, corresponding to HR of approximately 1100 kW in winter day scenario. The temperature of the liquid ammonia can be reduced to 10 C in the subcooler for cold winter months when the water temperature is low. The maximum amount of energy available from ammonia condensation and subcooling is presented in Table 1. In areas where the electricity price is high, using a subcooler is generally a better option. When the electricity price is relatively low, the benefit of the subcooler is not as high and the simplicity of the installation of the condenser generally becomes the main decision making factor.
5.3. Heat process water using energy from the compressor oil As shown in Fig. 5, superheated ammonia is collected from the refrigeration compressors and is sent to the existing desuperheater to heat process water. The compressors oil that contains a considerable amount of energy is currently cool down from 71 C to 54 C using a thermosiphon on each compressor. The ammonia gas coming out of the thermosiphon is sent to the evaporative condenser. A new HR oil cooler could be installed before the thermosiphon (Fig. 5) to reduce the oil temperature and transfer its energy to the process water. This would eliminate the use of natural gas in the direct contact water heater and would also unload the
Fig. 4. Scheme of boiler heat recovery e winter day scenario.
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Fig. 5. Heat recovery projects in the refrigeration system e winter day scenario.
evaporative condenser by reducing the amount of ammonia gas returning to it. Seeing as it is not easy to retrofit the screw compressor oil cooling system to perform HR, three scenarios were considered to determine the impact of this project on the overall plant energy use. In all cases, it is assumed that the energy from the superheated ammonia is recovered in the HR desuperheater. These scenarios are: 1. 100% HR; 2. 50% HR (corresponding approximately to the amount of energy available from the largest compressors in the plant under study); 3. No HR. The energy available from the compressor oil and in the superheated ammonia was calculated and presented in Table 1 for summer and winter scenarios. The INTEGRATION software showed that the process water temperature increase is equal to 17 C when all the energy available from the oil is recovered during the winter day. This temperature increase is equal to 12 C, 10 C, and 13 C during winter night, summer day and summer night scenarios, respectively. When internal HR cannot fully satisfy process water heating requirements, the direct contact water heater will be used to complete the water heating process. When too much energy is available and the water
temperature exceeds its target temperature, a by-pass will be open to prevent overheating the water (not shown in Fig. 5). For this slaughterhouse, HR from the compressor oil was considered cost-effective only for the large compressors corresponding to approximately 50% of the available energy. Recovering the energy from the compressor oil would be too expensive for a compressor having a cooling capacity below approximately 250 kW. It should be noted that installing a HR system from the compressor oil is not an easy sale. The plant personnel and the compressor manufacturer may be reluctant to implement such a project even if it is, from an engineering standpoint, a relatively simple undertaking for large compressors. Because the amount of energy available from the refrigeration system and the energy required to heat the process hot water are fluctuating over time, the temperature of the preheated hot water is changing. This is not an issue from an operating standpoint as the direct contact water heater and the steam heat exchangers will complete the water heating. This mismatch only affects the amount of energy that can be recovered. 5.4. Heat process water using the energy from the singeing stack flue gas The singeing process uses burners that work at maximum firing rate most of the time during the production periods. The temperature of the singeing stack flue gas is about 500 C with a heat load
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over 3000 kW. This is a high grade heat source with a considerable amount of energy that can be recovered to produce hot water. Two options are considered to recover this energy as presented in Fig. 6. In the first option, the process water is heated up to 56 C using the waste heat from the refrigeration system and heated further with the direct contact water heater if needed. The water is then stored at 56 C in the hot water tank as is currently done in the plant. As presented in Fig. 6a, 9.5 L/s of hot water is heated up by receiving 1172 kW from the singeing flue gas (winter day) to increase its temperature to 87 C. In this step, the flue gas temperature is reduced to 270 C and the steam usage for hot water production can be completely eliminated during production hours. Hot water at 56 C is taken from the hot water tank for process requirements.
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The second option is to heat all of the process water to 87 C with the singeing flue gas (winter day) and then to store it in the hot water tank, as shown in Fig. 6b. In this option, 1783 kW is recovered from the flue gas and its temperature is decreased to about 180 C. To produce water at 56 C, hot water from the storage tank is sent to a mixing station and mixed with city water. This option extends the effective energy storage capacity of the hot water tank, reduces the overall energy usage and reduces the energy use for hot water production during the cleaning period. The amount of energy recovered from the singeing flue gas strongly depends on how much energy can be recovered from the compressor oil; the greater the amount of energy recovered, the higher the inlet water temperature to the singeing flue gas heat
Fig. 6. Scenarios for the heat recovery from the singeing stack flue gas: a) 56 C hot water tank b) 87 C hot water tank.
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exchanger will be. It should be mentioned that water would continue to be stored at 56 C during the cleaning period as the singeing process is not in operation during that time. The energy requirement to heat water to 56 C during winter cleaning periods is equal to 498 kW (430 kW for the summer scenario) considering 50% HR from the compressor oil. After discussion with the plant personnel, it was decided to store hot water at 87 C to increase the effective energy storage capacity of the existing tank.
5.5. Heat barn air make-up using the energy from ammonia condensation During winter, the barn air make-up needs to be heated. This is currently done using a significant amount of natural gas in several air heaters. Three scenarios were considered for air heating during the winter period using waste energy from the refrigeration system. Scenario 1: As presented in Fig. 7a, a new condenser is installed with a glycol loop to transfer energy from the ammonia coming out of the desuperheater to preheat the barn air make-up. The energy is transferred to the fresh air by installing new coils at the inlet of the existing air make-up units. Air is then further heated by natural gas air heaters to reach the desired temperature. The use of natural gas would be significantly reduced using this approach. It would also be possible to further increase the glycol temperature by sending it to the second stage of the boiler condensing economizer. In this case, the use of natural gas for air heating could be completely eliminated, except for very cold days. Using the second stage condensing economizer for air heating can be cost-effective only for relatively large slaughterhouses. If the refrigeration system was located in a single mechanical room near the barn, the glycol loop could be eliminated and the fresh air stream could be directly heated by the new HR condenser. This was however not the case for the slaughterhouse studied. Scenario 2In this scenario, a commercial heat pump is used to heat the air make-up. The natural gas units are kept only for back-up and to provide additional heat when the temperature is very low. The glycol loop preheats fresh air and the heat pump is used to further heat up the air to reach its target temperature. An alternative option would be to use only the commercial heat pump without using the glycol loop to heat air. This may require less capital investment but would require more electricity to run the heat pump. Scenario 3: In this scenario, a supercharger is installed to boost the pressure and consequently the temperature of a part of the ammonia coming out of the desuperheater (Fig. 7b). This higher grade energy is used to heat the glycol loop at a temperature of approximately 35e40 C. The glycol loop preheats the fresh air make-up and the natural gas heater is used to further heat up the air if required. Seeing as the supercharger only has to upgrade the NH3 temperature from 28 C to 45 C, it has a very high COP of 10e12. Scenarios 2 and 3 are relatively similar in terms of net energy savings. The commercial heat pump analyzed in Scenario 2 has a much lower COP than the supercharger presented in Scenario 3. However, the supercharger must boost the temperature level of all the energy transferred to the make-up air while the commercial heat pump needs to heat only the part of the load that cannot be provided by the HR. Overall, net savings are similar but in the end, the supercharger system was recommended as it provides slightly higher savings and is considered easier to operate and maintain.
6. Heat upgrading opportunities for hot water production Five HR projects were proposed with different alternatives for each project. Table 3 presents the impact of the different alternatives suggested for HR from the compressor oil when hot water is stored at 87 C and the energy from the singeing flue gas is properly recovered. These results were calculated by comparing the total energy requirements to heat all the process streams and the energy available for HR for different scenarios (eq. (3)). The opportunity to implement a heat pump for process water heating will be analyzed while assuming that the hot water will be stored at 87 C in the hot water tank during production periods and at 56 C during cleaning periods. QHU ¼ Qtotal e QHR
(3)
The results presented in Table 3 indicate that the slaughterhouse would require natural gas to heat water only during cleaning periods when more than 50% of the compressor oil energy is recovered. As a result, the opportunity to use heat pumps to produce hot water with this scenario (only for 8 h per day) would be uneconomical in the studied slaughterhouse. On the other hand, when no energy recovery from the compressor oil is cost-effective or desirable, the slaughterhouse would require energy to heat water even during the winter production period. In that case, the use of a heat pump would become cost-effective when the electricity cost is low (e.g. in the provinces of Quebec, British Colombia and Manitoba). For plants that have several mechanical rooms for the refrigeration system and/or many small compressors, it would be uneconomical to recover the energy from the compressor oil and it may be more cost-effective to use a heat pump for hot water production. If the plant management had been willing to accept a longer payback period, the heat pump would have brought additional energy savings. This heat pump would have been made of a supercharger such as the one presented in Fig. 7 for air heating. Note that if the energy usage in the singeing process (and therefore the energy rejected in the flue gas) could be considerably reduced by a process modification, a heat pump for hot water heating would then be more profitable. It should be noted that the benefit of reducing the ammonia condensing pressure/temperature in the refrigeration system during winter months is considerably reduced when a desuperheater, a HR condenser and a supercharger are used for HR. Some electricity savings for the operation of the compressors could be achieved but these savings were not analyzed by the authors. Reducing the condenser temperature from 28 C to about 22 C during winter months would not change the conclusions of this study but would affect the magnitude of savings for certain projects. It would not be possible to reduce the condenser temperature below 22 C due to some constraints on the operation of the existing compressors (e.g. size of the oil separators, defrosts, etc.). Fig. 8 presents the heat exchanger network for the modified slaughterhouse, considering that the hot water will be stored at 87 C. The combination of all projects would increase the internal HR in the process from 1431 to 5564 kW during the winter production periods (see Fig. 2 for original design). Table 4 presents the HR and the associated natural gas saving for different scenarios when 50% of the energy available from the compressor oil is recovered. The natural gas saving was calculated with eq. (4), considering 7 $/GJ for natural gas price, 90% for the efficiency of the direct contact water heater and 70% for the efficiency of boilers.
NGsaving ¼ 3.6$(Qafter
HR
e Qbefore
HR)$NGprice$YOH/eff/1000
(4)
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11
Fig. 7. Barn air heating during winter scenario: a) Installation of a glycol loop b) Installation of a supercharger with a glycol loop.
In addition to reducing energy costs, these projects also reduce greenhouse gas (GHG) emissions. The implementation of the proposed projects would reduce GHG emissions in the winter production period by 3235 t/y of CO2. The reduction of natural gas consumption would also have the benefit of reducing NOx emissions. GHG emission reductions will bring additional financial benefits when CO2 credits can be exchanged on the market (already in place in Quebec and California).
were considered for the study: production and cleaning periods for both winter and summer months. The first recommended project was the use of boiler economizers to recover heat from the boiler flue gas to heat the boiler make-up and the boiler feedwater. The next recommended projects consisted in using a larger proportion of the waste energy from the refrigeration system to preheat process water by installing: A new HR condenser or a new subcooler (these are competing projects) A compressor oil HR system
7. Conclusion A PI study was performed for a relatively large slaughterhouse in Canada using the INTEGRATION software, and various projects were suggested to improve the plant's HR system. Four periods
A new HR condenser was recommended as it was easier to install for this specific slaughterhouse.
Table 3 Hot utility requirements to produce hot water with different oil heat recovery scenarios (Assuming the hot water tank is maintained at 87 C). Energy recovery from oil
100% recovery 50% recovery 0% recovery
Winter
Summer
Production period (kW)
Cleaning period (kW)
Production period (kW)
Cleaning period (kW)
0 0 585
254 498 742
0 0 0
166 430 693
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Fig. 8. Modified heat exchanger network e winter day scenario.
12
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O. Ashrafi et al. / Energy xxx (2015) 1e13 Table 4 Internal heat recovery in the slaughterhouse for different scenariosa. Different scenarios
Heat recovery before process integration (kW)
Heat recovery after process integration (kW)
Natural gas savings ($/y)
Winter day Winter night Summer day Summer night Total
1431 541 1861 586
5564 1881 4918 1366
225,000 40,000 167,000 24,000 456,000
a
In winter scenarios, a supercharger was selected to preheat air make-up stream.
Three scenarios were analyzed to recover heat from the compressor oil (100% recovery, 50% recovery, and no recovery) and the impacts of using these scenarios on the overall energy usage were investigated. The scenario where 50% of the energy is recovered was considered the most cost-effective for this slaughterhouse since it would be expensive to recover the energy from the smallest compressors. A HR project from the singeing flue gas was recommended for hot water production. The amount of high grade energy available from the singeing flue gas is sufficient to produce the required hot water during production periods when this project is implemented together with the HR projects from the refrigeration system. Two scenarios were considered to determine what would be the optimal hot water tank temperature (56 C or 87 C). The 87 C scenario was preferred seeing as it would result in additional energy savings from the singeing flue gas and because it would allow more energy to be stored in the tank and reused during the cleaning period. During cleaning periods, the available energy is not sufficient to heat the process water to 56 C. Natural gas is needed in the water heater except in the case where a heat pump would be used. However, such a heat pump would not be cost-effective if more than 50% of the energy from the compressor oil is recovered and if the energy from the ammonia is recovered (superheat and condensation). Three options were investigated to provide the energy required to heat the barn air make-up during winter. The most cost-effective option for the slaughterhouse studied was the use of a supercharger to boost the ammonia pressure/temperature. Implementation of the proposed options significantly reduces steam consumption and natural gas use in the slaughterhouse and as presented in Table 4 result in savings of approximately $456,000 in operating costs per year considering the savings of the four different scenarios. An additional reduction of natural gas could be achieved by using heat pumps to heat the process water. The implementation of suggested projects would decrease the amount of GHG emissions associated with natural gas combustion. In the winter production period, this reduction would be equal to 3235 t/y of CO2.
13
References [1] Industrial consumption of energy (ICE) survey e summary report of energy use in Canadian manufacturing sector (1995e2010). Natural Resources Canada. oee.nrcan.gc.ca/publications/statistics/ice10/ [access date 25.02.15]. [2] Sectoral economic and energy profiles. Natural Resources Canada. oee.nrcan. gc.ca/17838 [access date 25.02.15]. [3] Agriculture Canada. Heat recovery for Canadian food and beverage industries. Ottawa, Canada: Agriculture Canada; 1984. [4] Fritzson A, Berntsson T, Vamling L. Energy conservation in the food industry using process integration e methodologies and case study. In: International conference engineering and food, Montpellier, France; 2004. ID no. 1186. [5] Fritzson A, Berntsson T. Energy efficiency in the slaughter and meat processing industry - opportunities for improvements in future energy markets. J Food Eng 2006;77:792e802. [6] Fritzson A, Berntsson T. Efficient energy use in a slaughter and meat processing plant e opportunities for process integration. J Food Eng 2006;76: 594e604. [7] Eilers W, MacKay R, Graham L, Lefebvre A. Environmental sustainability of Canadian agriculture: agri-environmental indicator report series e report #3. Ottawa, Canada: Agriculture and Agri-Food Canada; 2010. publications.gc.ca/ collections/collection_2011/agr/A22-201-2010-eng.pdf [access date 13.07.15]. [8] Gadalla MA. A new graphical method for pinch analysis application: heat exchanger network retrofit and energy integration. Energy 2015;81:159e74. [9] Kemp IC. Pinch Analysis and Process Integration. A user guide on process integration for the efficient use of energy. 2nd ed. Great Britain: IChemE; 2007. [10] Du S, Wang RZ. Optimal ammonia water absorption refrigeration cycle with maximum internal heat recovery derived from pinch technology. Energy 2014;68:862e9. [11] Bakhtiari B, Bedard S. Retrofitting heat exchanger networks using a modified network pinch approach. Appl Therm Eng 2013;51:973e9. chal F, Vuillermoz A. Process integration and opportunities for [12] Becker H, Mare heat pumps in industrial processes. Int J Thermodyn 2011;14:59e70. [13] Bakhtiari B, Fradette L, Legros R, Paris J. Opportunities for the integration of absorption heat pumps in the pulp and paper process. Energy 2009;35: 4600e6. [14] Ranade SM. New insights on optimal integration of heat pumps in industrial sites. Heat Recovery Sys CHP 1988;8:255e63. [15] Kapustenko PO, Ulyev LM, Boldyryev SA, Garev AO. Integration of a heat pump into the heat supply system of a cheese production plant. Energy 2008;33: 882e9. € yleme MS. Optimum heat pump in dairy systems with waste heat recovery. [16] So J Food Eng 2006;74:292e8. [17] Dumbliauskaite M, Becker H, Marechal F. Utility optimization in a brewery process based on energy integration methodology. In: Proceeding of ECOS, Lausanne, Switzerland; 2010. p. 91e8. [18] Boone J. Natural refrigerants in different industrial heat pump plants in Norway. In: ATMOsphere Europe, Brussels, Belgium; 2012. DOC.2012-368 R5a. [19] Asante DK, Zhu XX. An automated approach for heat exchanger network retrofit featuring minimal topology modifications. Comput Chem Eng 1996;20:S7e12. [20] Varbanov PS, Klemes J. Rules for path construction for HENs debottlenecking. Appl Therm Eng 2000;20:1409e20. [21] INTEGRATION software. Natural Resources Canada. nrcan.gc.ca/energy/ efficiency/industry/processes/systems-optimization/process-integration/ products-services/5529 [access date 30.02.15]. [22] Klemes J. Handbook of process integration. Philadelphia, USA: Woodhead Publishing; 2013. [23] Introduction to pinch technology. Cheshire, England: LinoffMarch; 1998 [access date 30.02.15], ou.edu/class/che-design/a-design/Introduction to Pinch Technology-LinhoffMarch.pdf.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Heat recovery and heat pumping opportunities in a slaughterhouse dard a, *, Bahador Bakhtiari b, Bruno Poulin a Omid Ashrafi a, Serge Be a b
Natural Resources Canada, CanmetENERGY, 1615 Lionel-Boulet Blvd., Varennes, QC J3X 1S6, Canada American Process Inc., 750 Piedmont Avenue N.E., Atlanta, GA 30308, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2014 Received in revised form 11 May 2015 Accepted 23 May 2015 Available online xxx
Pinch analysis was employed to recover and reuse a part of the waste energy during the production and cleaning periods of a slaughterhouse in Canada. The INTEGRATION software was used to find possible options for HR (heat recovery). Several HR opportunities were identified in various systems of the slaughterhouse including boilers, hot water production, singeing, refrigeration, and barn air heating. A recommendation was also made to store more energy in the form of hot water. Opportunities for using heat pumps for additional HR were also investigated. The proposed projects can significantly reduce natural gas consumption in the steam and hot water boilers as well as in the air heating systems. It is shown that the proposed projects can reduce natural gas use by approximately half a million dollars per year. © 2015 Crown Copyright and ELsevier Ltd. Published by Elsevier Ltd. All rights reserved.
Keywords: Slaughterhouse Process integration Heat recovery Heat pump Pinch analysis
1. Introduction Higher energy prices and the global climate change have increased the need for efficient energy use in the food industry. Energy consumption represents a significant proportion of the production costs in this industry. In 2010, food processing establishments in Canada consumed approximately 102.4 PJ of energy, of which 30.3% was electricity and 69.7% was other purchased fuels [1]. Significant energy-demanding sectors in the food industry are slaughterhouse and meat processing plants that use approximately 18% of the total energy consumption in the Canadian food industry [2]. The most important energy consumers in a slaughterhouse are the refrigeration system, the singeing process [3], the steam production system, the hot water production system and the compressed air system [4]. In slaughterhouses, an issue that makes optimal energy recovery difficult is the presence of discontinuous heat sources, normally available during the production periods, and semi-continuous heat sinks. The most important semi-continuous heat sink is the production of hot water which is needed during both production and cleaning periods [5,6]. The Canadian food industry also contributes to the GHGs (generation of greenhouse gases) from three direct sources of emission: combustion of fossil fuels in steam and hot water boilers, loss of HFCs (hydro-
* Corresponding author. dard). E-mail address: [email protected] (S. Be
fluorocarbons) from refrigeration systems, and methane generation in wastewater treatment systems. This industry generates approximately 3500 kt/y of CO2 [7]. Energy recovery projects are not only beneficial for food companies to improve their bottom line, but also for the environment as they decrease GHG emissions. PI (process integration) is a method that can be used during the design and retrofit of many application and industrial processes [8] in order to optimize the use of energy and other resources [9]. PI allows defining the best opportunities for HR (heat recovery) in a process and focuses on the efficient use of energy. Pinch analysis, a PI technique, uses graphical tools for the calculation of energy targets and applies heuristics to achieve those targets [10]. Recently, a new methodology was proposed by Bakhtiari and dard [11] for retrofitting heat exchanger network in industrial Be plants. Their methodology includes the capability to handle some practical constraints more systematically than other pinch analysis methods while offering a systematic step-by-step approach. Heat pumps are commonly used when a certain amount of low grade energy is available to be used effectively at an insufficient temperature level [12]. This technology can significantly increase HR in a process when it is appropriately located and designed [13]. PI can also be used to identify opportunities for the application of heat pumps in a process [14]. In slaughterhouses and meat processing plants, a considerable amount of low temperature heat is rejected through evaporative condensers and cooling towers. Some of this heat could be used for air and process water heating purposes through the use of heat pumps [15].
http://dx.doi.org/10.1016/j.energy.2015.05.129 0360-5442/© 2015 Crown Copyright and ELsevier Ltd. Published by Elsevier Ltd. All rights reserved.
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Nomenclature HU CP eff HR m NG Q YOH DH DT
hot utility requirement heat capacity (kJ/kg C) efficiency heat recovery mass flow rate (kg/s) natural gas energy (kJ/s) yearly operating hours (h/y) enthalpy of vaporization/condensation (kJ/kg) temperature difference ( C)
PI was used to identify HR potential in various food processing industries, mainly dairies [16] and breweries [17]. Fritzson and Berntsson [5,6] used pinch analysis techniques in a slaughterhouse in Sweden and found significant potential for electricity and fuel savings. This slaughterhouse was already quite efficient, notably due to the use of a heat pump for process water heating. The refrigeration plant was modified in this study by changing the temperature levels of the refrigeration system, resulting in a 10% reduction of the electricity consumption. In addition, HR in the slaughterhouse was increased by reducing the DT of existing heat exchangers to 5 C. It was reported in another study by MAYEKAWA Europe [18] that the use of two ammonia heat pumps for hot water generation at 52 C in a Norwegian slaughterhouse saved approximately 1.6 GW of natural gas consumption per year (based on lower heating value). In this paper, the results of a PI study performed in a slaughterhouse located in Canada are presented. In addition to HR projects, the potential for implementing heat pumps is also investigated. The INTEGRATION software, a process integration software developed by CanmetENERGY, was used to identify the HR projects proposed in this work. INTEGRATION uses a sequential interactive approach originally proposed by Asante and Zhu [19], called Network Pinch, and extended by Varbanov and Klemes [20].
Additional improvement to the algorithm was made by Bakhtiari dard [11] to reduce the number of retrofit options that are and Be considered using the Match Penalty concept. In addition, the software includes models to calculate the amount of waste heat, and the temperature levels, that can be recovered from industrial boilers, air compressors and refrigeration systems [21].
2. Pork slaughterhouse Fig. 1 presents the main processing steps in the pork slaughterhouse, with the main departments being the barn, the kill floor, the evisceration department, the clean floor, the cold warehouses and the powerhouse. Animals are received by truck in the barn where a large amount of fresh air is needed. On the kill floor, animals are put through different processing steps such as slaughtering, bleeding, scalding, hair removal, and singeing. The hogs are then going to the evisceration and head removal processes. The kill floor requires hot water at 56 C and 87 C. The hot water heating and the singeing processes are the two largest natural gas users in the slaughterhouse. The singeing process generates flue gas with a temperature of approximately 500 C. After final washing, hogs are sent in a blast tunnel to rapidly freeze their surface to prevent water loss. They are then sent to cold storages so that they can entirely cool down to the temperature of approximately 4 C. Following the cool down, the hogs are sent to the cut floor to be processed. The cut floor requires hot water at 56 C and 87 C for cleaning and sterilization purposes. The temperature of the cut floor is maintained at about 4 C. The refrigeration system is a two-stage ammonia system with screw compressors, having a refrigerant discharge temperature of 71 C in average. The low-stage system consists of booster compressors with a suction temperature of 40 C and is used mainly for the blast tunnel and the freezers. The suction temperature of the high-stage system is maintained at 10 C and is used mainly for the cut floor and the cold storages. The superheated ammonia from the compressors is cooled in a HR desuperheater from 71 C to 28 C, which corresponds to the condensing temperature at the
Fig. 1. Simplified diagram of the slaughterhouse.
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compressor discharge pressure. A thermosiphon is used on each compressor to reduce compressor oil temperature from 71 C to 54 C. The desuperheater is used to preheat the process hot water from approximately 10 Ce30 C using available energy from the superheated ammonia. Afterward, the temperature of the process hot water is increased to 56 C using a direct contact water heater and is then stored in a hot water tank. The part of the hot water that needs to be used at 87 C is further heated using steam in a heat exchanger. Approximately 18 L/s (280 usgpm) hot water is required for the slaughterhouse. Two 4900 kW boilers are used in the slaughterhouse with a maximum steam production capacity of 7250 kg/h. The steam production system also includes a heat exchanger to preheat boiler make-up water with the boiler blowdown, a condensate tank to store the condensate return and the boiler make-up, and a deaerator. Feedwater is sent to the boilers after the deaeration process at a temperature of 107 C. Steam is mainly used to produce hot water, to heat the boiler feedwater, to heat some processed meat in cookers, for sterilization processes, and for some space heating in cold days. 3. Methodology
4.1. Heat sources and demands The available energy sources for HR include superheated ammonia, ammonia condensation and subcooling, compressor oil, flue gas from boilers and the singeing process, and boilers blowdown. The INTEGRATION software was first used to analyze the refrigeration system (e.g. available energy, COP (coefficient of performance) of the system, refrigerant flow rate, etc.) and the boiler system (e.g. boiler efficiency, energy available in the boiler flue gas and blowdown, etc.) and to calculate available waste heat for HR from these systems. The analysis was performed for four different scenarios: a) Winter conditions 1. Production period (day-time) 2. Cleaning period (night-time þ weekends) b) Summer conditions 1. Production period (day-time) 2. Cleaning period (night-time þ weekends) The duration of the production period is 16 h per day, 5 days a week, while the cleaning period lasts 8 h per day and 16 h during the weekends. There are significant differences between these scenarios in terms of energy demands and waste heat availability: Process water at 87 C is only needed during the production periods; A greater amount of process water at 56 C is required during the production periods than for the cleaning periods; The singeing process and its associated flue gas are only available during the production periods; The refrigeration system load is much higher during the production periods as the blast tunnel is in operation and more cooling is required to cool the hogs.
This analysis was conducted following these steps: 1. Construction of a simplified process flow diagram containing the equipment and processes where heat is being used, recovered or lost 2. For each major operating scenario, development of a heat and mass balance to add every relevant temperatures, flow rates and duties to the process flow diagram 3. Realization of a pinch analysis comprising: a. Data extraction and construction of the composite curves b. Identification of the existing heat transfer inefficiencies c. Identification of new/improved HR opportunities d. Quantification of the potential benefits of each opportunities and evaluation of the cross-effect between these opportunities 4. Selection of the most promising HR opportunities with the mill personnel, considering operating constrains and plant layout 5. Development of HR projects for most promising opportunities, including the sizing of equipment.
3
Table 1 presents the heat sources from the refrigeration system, while Table 2 presents some other heat sources and all heat sinks in this process. Energy requirements for warm and hot water production as well as air heating were calculated using the associated flow rates and water/air temperature provided by the plant using eqs. (1) and (2). Q ¼ m$Cp$DT
(1)
Q ¼ m$DHlatent
(2)
where
4. Pinch analysis
Cp, Cp, Cp, Cp,
There are currently only two heat exchangers used to recover waste heat in this slaughterhouse, the ammonia desuperheater and the boiler blowdown heat exchanger. Several other sources were available and could be used to perform additional HR.
¼ 4.18 kJ/kg C, ¼ 1.01 kJ/kg C, air flue gas ¼ 1.13 kJ/kg C, ¼ 2.00 kJ/kg C, cooling oil water
Table 1 Available energy from the refrigeration system. Summer day Energy (kW) high stage Compressor oil (71 Ce54 C) Superheated NH3 (71 Ce28 C) Condensation of NH3 Subcooled NH3 (28 C toa) a
1270 1328 12,531 1657
Summer night Energy (kW) booster 324 520 e e
Energy (kW) high stage
Winter day Energy (kW) booster
Energy (kW) high stage
476 498
51 81
953 996
4699 571
e e
9398 1594
Winter night Energy (kW) booster 259 416 e e
Energy (kW) high stage 445 465 4386 744
Energy (kW) booster 44.2 71 e e
The final temperature of ammonia to calculate the energy from subcooling, is considered to be 18 C and 10 C during summer and winter, respectively.
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Table 2 Heat sources and heat sinks for winter day scenario.a Heat sources
Energy (kW)
Tinitial ( C)
Tfinal ( C)
Heat sink
Energy (kW)
Tinitial ( C)
Tfinal ( C)
Boiler flue gas Singeing flue gas Boiler blowdown
1045 3000 50
185 500 107
20 20 15
Boiler make-up Condensate return Boiler feedwater Warm water at 56 C Hot water at 87 C Barn air make-up Barn air heating
170 455 180 3500 1275 810 600
4 51 107 4 56 4 20
25 107 135 56 87 20 30
a The energy available in the boiler flue gas and blowdown was corrected to take into account the steam savings that will be achieved by implementing the energy saving projects. Otherwise, savings could have been double counted. Similarly, the energy needed to heat the boiler feedwater and the boiler make-up was also adjusted.
Cp; superheated NH3 ¼ 2.54 kJ/kg C, Cp; subcooled NH3 ¼ 4.67 kJ/kg C, DHlatent; NH3 ¼ 1171.03 kJ/kg
4.2. Excluded heat sources and demands from the analysis The effluents from the slaughterhouse also contain a considerable amount of energy but were considered too dirty to be sent to a heat exchanger for HR. Effluent filtration and a CIP (cleaning-inplace) system could have been installed but the plant personnel did not want to consider these options. Also, the energy contained in the hog themselves was not considered as a heat source as no heat can be recovered from the hogs. The singeing process is an important natural gas user that generates a flame for hair burning and hog sterilization. Since it is not possible to replace the natural gas by HR, the energy demand for this process was therefore not considered in the PI study. Other steam use for cooking and sterilization were also not considered in the analysis as the steam usage in these processes cannot be replaced by HR. 4.3. Analysis of the existing network and construction of the composite curves The existing heat exchanger network of this slaughterhouse is shown in Fig. 2 in the form of the grid diagram traditionally used in PI studies. The red lines in the grid diagram represent hot streams that should be cooled down for process requirements. The green lines represent the liquid and gaseous effluent streams. An effluent stream is a particular type of hot stream that does not absolutely need to be cooled for process reasons. For example, the boiler flue gas is an effluent stream as it is not absolutely necessary to cool it down to 20 C. The blue lines represent cold streams that should be heated for process reasons. The energy required by cold streams could be provided by hot utility or by HR from hot or effluent streams. Fig. 3 presents the composite curves of the slaughterhouse for two different scenarios: winter day (Fig. 3a) and summer night (Fig. 3b). The composite curves show the temperature profile of the heat to be supplied to the process and the heat to be evacuated from the process. The hot composite curve (upper red line) is the combination of hot and effluent streams from which heat can be recovered, while the cold composite curve (bottom blue line) is the combination of all process heat demands. The flue gas from the singeing process is included in the composite curves during the production periods as HR is possible from this heat source. As shown in Fig. 3a, the composite curves for winter days are divided into two zones. In the HR zone, heat from hot streams (including effluents) can be used to heat cold streams. In the cold utility zone, hot streams require cooling to reach their final temperature. The DTmin represents the minimum approach
temperature that is acceptable in a given heat exchanger. The DTmin is set using several factors including the shape of the composite curves, types of process streams, type of heat exchangers and acceptable investment costs [22]. DTmin of 5e15 C is common in refrigeration systems and in the food industry in general [23]. In this study, a DTmin of 5 C was chosen for liquideliquid heat exchangers and 20 C for gaseliquid heat exchangers. Plate and frame heat exchangers were considered for liquideliquid HR and tubes and fines heat exchangers were used for gaseliquid HR. It can be seen in Fig. 3a that all the energy required by the cold streams can be provided by the hot streams when a DTmin lower than 20 C is used. This means that no hot utility is therefore needed to satisfy the heat demands considered in this analysis, providing that the HR system is optimally designed. In pinch analysis, this is normally referred to as a threshold problem. In such a situation, there is no need for heat pump as the cold streams can be fully satisfied by HR. A similar conclusion can be drawn when analyzing the summer day composite curves. Fig. 3b shows the composite curves for the summer cleaning period. During this period, a significant amount of hot water is needed while the singeing flue gas is not available. In this condition, the heat demands cannot be fully satisfied by HR and hot utility is required. For the summer cleaning period, it would be beneficial to implement a heat pump to eliminate the need for hot utilities. The analysis of composite curves during the winter night scenario results in a similar conclusion.
5. Review of the proposed heat recovery projects To facilitate the design of the most promising HR system, a few assumptions were made: Two storage alternatives were investigated for hot water production. Instead of storing water at 56 C in a water tank and further heating only a part of the process water to 87 C, it was decided to investigate an alternative approach. It consists in storing water at 87 C during the production periods and producing the required 56 C water by mixing cold city water with water at 87 C. The benefit of this approach is to store more energy in the existing hot water tank for the cleaning periods; The initial temperature of process water during summer and winter is assumed to be 18 C and 4 C, respectively; An average water flow rate is considered during both production and cleaning periods. Even if the flow rate varies in reality, this assumption is acceptable considering that a large hot water tank is used to stabilize the operations; Barn air make-up heating is required only during winter. The HR projects identified using the INTEGRATION software are as follows:
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Fig. 2. Heat exchanger network before heat recovery.
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Fig. 3. Composite curves: a) Winter day b) Summer night.
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1. 2. 3. 4.
HR in steam production system; Preheat process water using the energy from the ammonia; Heat process water using the energy from the compressor oil; Heat process water using the energy from the singeing stack flue gas; 5. Heat barn air make-up using the energy from the ammonia.
5.1. HR in the steam production system In the current heat exchanger network presented in Fig. 2, boiler make-up is preheated with the blowdown, is mixed with the returned condensate and is then heated in the deaerator using live steam injection. The boiler make-up and the boiler feedwater can be further heated by adding a new boiler economizer and a new condensing economizer as presented in Fig. 4. This project significantly reduces the amount of steam consumption in the deaerator (almost no steam is then required) and considerably increase the efficiency of the boiler. This results in a significant decrease in the amount of energy available in the boiler flue gas and boiler blowdown which was taken into account in the calculations. In order to reach the maximum HR from the boiler flue gas, a part of the process water (6.5%) would be sent to the condensing economizer (second stage) as presented in Fig. 4. Depending on the specific plant context and its size, the second stage economizer may or may not be cost effective. Since flue gas is cooled below its dew point temperature, the impact of water condensation was considered to evaluate the amount of energy that can be recovered in the second stage economizer. 5.2. Preheat process water using the energy from the ammonia A significant amount of energy is available at the condensing temperature of the ammonia in the evaporative condenser, approximately 28 C (see long plateau on the hot composite curve). Recovering a part of this energy to preheat process water is required in order to achieve an optimal HR design. Fig. 5 presents the two alternatives of preheating the process water with the energy from the ammonia. The first alternative, option (A) in Fig. 5, is
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to use a new HR condenser before the evaporative condenser. This can unload the evaporative condenser and reduce the power usage to drive the condenser fans. The second alternative, option (B) in Fig. 5, requires a new HR subcooler after the evaporative condenser. The use of a subcooler increases the efficiency of the system and allows more heat to be removed from the process for a given amount of refrigerant circulating in the refrigeration cycle. If the liquid ammonia is cooled to about 10e15 C before being depressurized in the low temperature ammonia receiver, a smaller amount of flash gas would be sent to the compressors. Therefore, this option reduces both the power requirement at the compressor and the power requirement to drive the condenser fans as less refrigerant would circulate in the refrigeration system. Approximately 93% of the process water is preheated to about 22e23 C in either the new condenser or the new subcooler, corresponding to HR of approximately 1100 kW in winter day scenario. The temperature of the liquid ammonia can be reduced to 10 C in the subcooler for cold winter months when the water temperature is low. The maximum amount of energy available from ammonia condensation and subcooling is presented in Table 1. In areas where the electricity price is high, using a subcooler is generally a better option. When the electricity price is relatively low, the benefit of the subcooler is not as high and the simplicity of the installation of the condenser generally becomes the main decision making factor.
5.3. Heat process water using energy from the compressor oil As shown in Fig. 5, superheated ammonia is collected from the refrigeration compressors and is sent to the existing desuperheater to heat process water. The compressors oil that contains a considerable amount of energy is currently cool down from 71 C to 54 C using a thermosiphon on each compressor. The ammonia gas coming out of the thermosiphon is sent to the evaporative condenser. A new HR oil cooler could be installed before the thermosiphon (Fig. 5) to reduce the oil temperature and transfer its energy to the process water. This would eliminate the use of natural gas in the direct contact water heater and would also unload the
Fig. 4. Scheme of boiler heat recovery e winter day scenario.
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Fig. 5. Heat recovery projects in the refrigeration system e winter day scenario.
evaporative condenser by reducing the amount of ammonia gas returning to it. Seeing as it is not easy to retrofit the screw compressor oil cooling system to perform HR, three scenarios were considered to determine the impact of this project on the overall plant energy use. In all cases, it is assumed that the energy from the superheated ammonia is recovered in the HR desuperheater. These scenarios are: 1. 100% HR; 2. 50% HR (corresponding approximately to the amount of energy available from the largest compressors in the plant under study); 3. No HR. The energy available from the compressor oil and in the superheated ammonia was calculated and presented in Table 1 for summer and winter scenarios. The INTEGRATION software showed that the process water temperature increase is equal to 17 C when all the energy available from the oil is recovered during the winter day. This temperature increase is equal to 12 C, 10 C, and 13 C during winter night, summer day and summer night scenarios, respectively. When internal HR cannot fully satisfy process water heating requirements, the direct contact water heater will be used to complete the water heating process. When too much energy is available and the water
temperature exceeds its target temperature, a by-pass will be open to prevent overheating the water (not shown in Fig. 5). For this slaughterhouse, HR from the compressor oil was considered cost-effective only for the large compressors corresponding to approximately 50% of the available energy. Recovering the energy from the compressor oil would be too expensive for a compressor having a cooling capacity below approximately 250 kW. It should be noted that installing a HR system from the compressor oil is not an easy sale. The plant personnel and the compressor manufacturer may be reluctant to implement such a project even if it is, from an engineering standpoint, a relatively simple undertaking for large compressors. Because the amount of energy available from the refrigeration system and the energy required to heat the process hot water are fluctuating over time, the temperature of the preheated hot water is changing. This is not an issue from an operating standpoint as the direct contact water heater and the steam heat exchangers will complete the water heating. This mismatch only affects the amount of energy that can be recovered. 5.4. Heat process water using the energy from the singeing stack flue gas The singeing process uses burners that work at maximum firing rate most of the time during the production periods. The temperature of the singeing stack flue gas is about 500 C with a heat load
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over 3000 kW. This is a high grade heat source with a considerable amount of energy that can be recovered to produce hot water. Two options are considered to recover this energy as presented in Fig. 6. In the first option, the process water is heated up to 56 C using the waste heat from the refrigeration system and heated further with the direct contact water heater if needed. The water is then stored at 56 C in the hot water tank as is currently done in the plant. As presented in Fig. 6a, 9.5 L/s of hot water is heated up by receiving 1172 kW from the singeing flue gas (winter day) to increase its temperature to 87 C. In this step, the flue gas temperature is reduced to 270 C and the steam usage for hot water production can be completely eliminated during production hours. Hot water at 56 C is taken from the hot water tank for process requirements.
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The second option is to heat all of the process water to 87 C with the singeing flue gas (winter day) and then to store it in the hot water tank, as shown in Fig. 6b. In this option, 1783 kW is recovered from the flue gas and its temperature is decreased to about 180 C. To produce water at 56 C, hot water from the storage tank is sent to a mixing station and mixed with city water. This option extends the effective energy storage capacity of the hot water tank, reduces the overall energy usage and reduces the energy use for hot water production during the cleaning period. The amount of energy recovered from the singeing flue gas strongly depends on how much energy can be recovered from the compressor oil; the greater the amount of energy recovered, the higher the inlet water temperature to the singeing flue gas heat
Fig. 6. Scenarios for the heat recovery from the singeing stack flue gas: a) 56 C hot water tank b) 87 C hot water tank.
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exchanger will be. It should be mentioned that water would continue to be stored at 56 C during the cleaning period as the singeing process is not in operation during that time. The energy requirement to heat water to 56 C during winter cleaning periods is equal to 498 kW (430 kW for the summer scenario) considering 50% HR from the compressor oil. After discussion with the plant personnel, it was decided to store hot water at 87 C to increase the effective energy storage capacity of the existing tank.
5.5. Heat barn air make-up using the energy from ammonia condensation During winter, the barn air make-up needs to be heated. This is currently done using a significant amount of natural gas in several air heaters. Three scenarios were considered for air heating during the winter period using waste energy from the refrigeration system. Scenario 1: As presented in Fig. 7a, a new condenser is installed with a glycol loop to transfer energy from the ammonia coming out of the desuperheater to preheat the barn air make-up. The energy is transferred to the fresh air by installing new coils at the inlet of the existing air make-up units. Air is then further heated by natural gas air heaters to reach the desired temperature. The use of natural gas would be significantly reduced using this approach. It would also be possible to further increase the glycol temperature by sending it to the second stage of the boiler condensing economizer. In this case, the use of natural gas for air heating could be completely eliminated, except for very cold days. Using the second stage condensing economizer for air heating can be cost-effective only for relatively large slaughterhouses. If the refrigeration system was located in a single mechanical room near the barn, the glycol loop could be eliminated and the fresh air stream could be directly heated by the new HR condenser. This was however not the case for the slaughterhouse studied. Scenario 2In this scenario, a commercial heat pump is used to heat the air make-up. The natural gas units are kept only for back-up and to provide additional heat when the temperature is very low. The glycol loop preheats fresh air and the heat pump is used to further heat up the air to reach its target temperature. An alternative option would be to use only the commercial heat pump without using the glycol loop to heat air. This may require less capital investment but would require more electricity to run the heat pump. Scenario 3: In this scenario, a supercharger is installed to boost the pressure and consequently the temperature of a part of the ammonia coming out of the desuperheater (Fig. 7b). This higher grade energy is used to heat the glycol loop at a temperature of approximately 35e40 C. The glycol loop preheats the fresh air make-up and the natural gas heater is used to further heat up the air if required. Seeing as the supercharger only has to upgrade the NH3 temperature from 28 C to 45 C, it has a very high COP of 10e12. Scenarios 2 and 3 are relatively similar in terms of net energy savings. The commercial heat pump analyzed in Scenario 2 has a much lower COP than the supercharger presented in Scenario 3. However, the supercharger must boost the temperature level of all the energy transferred to the make-up air while the commercial heat pump needs to heat only the part of the load that cannot be provided by the HR. Overall, net savings are similar but in the end, the supercharger system was recommended as it provides slightly higher savings and is considered easier to operate and maintain.
6. Heat upgrading opportunities for hot water production Five HR projects were proposed with different alternatives for each project. Table 3 presents the impact of the different alternatives suggested for HR from the compressor oil when hot water is stored at 87 C and the energy from the singeing flue gas is properly recovered. These results were calculated by comparing the total energy requirements to heat all the process streams and the energy available for HR for different scenarios (eq. (3)). The opportunity to implement a heat pump for process water heating will be analyzed while assuming that the hot water will be stored at 87 C in the hot water tank during production periods and at 56 C during cleaning periods. QHU ¼ Qtotal e QHR
(3)
The results presented in Table 3 indicate that the slaughterhouse would require natural gas to heat water only during cleaning periods when more than 50% of the compressor oil energy is recovered. As a result, the opportunity to use heat pumps to produce hot water with this scenario (only for 8 h per day) would be uneconomical in the studied slaughterhouse. On the other hand, when no energy recovery from the compressor oil is cost-effective or desirable, the slaughterhouse would require energy to heat water even during the winter production period. In that case, the use of a heat pump would become cost-effective when the electricity cost is low (e.g. in the provinces of Quebec, British Colombia and Manitoba). For plants that have several mechanical rooms for the refrigeration system and/or many small compressors, it would be uneconomical to recover the energy from the compressor oil and it may be more cost-effective to use a heat pump for hot water production. If the plant management had been willing to accept a longer payback period, the heat pump would have brought additional energy savings. This heat pump would have been made of a supercharger such as the one presented in Fig. 7 for air heating. Note that if the energy usage in the singeing process (and therefore the energy rejected in the flue gas) could be considerably reduced by a process modification, a heat pump for hot water heating would then be more profitable. It should be noted that the benefit of reducing the ammonia condensing pressure/temperature in the refrigeration system during winter months is considerably reduced when a desuperheater, a HR condenser and a supercharger are used for HR. Some electricity savings for the operation of the compressors could be achieved but these savings were not analyzed by the authors. Reducing the condenser temperature from 28 C to about 22 C during winter months would not change the conclusions of this study but would affect the magnitude of savings for certain projects. It would not be possible to reduce the condenser temperature below 22 C due to some constraints on the operation of the existing compressors (e.g. size of the oil separators, defrosts, etc.). Fig. 8 presents the heat exchanger network for the modified slaughterhouse, considering that the hot water will be stored at 87 C. The combination of all projects would increase the internal HR in the process from 1431 to 5564 kW during the winter production periods (see Fig. 2 for original design). Table 4 presents the HR and the associated natural gas saving for different scenarios when 50% of the energy available from the compressor oil is recovered. The natural gas saving was calculated with eq. (4), considering 7 $/GJ for natural gas price, 90% for the efficiency of the direct contact water heater and 70% for the efficiency of boilers.
NGsaving ¼ 3.6$(Qafter
HR
e Qbefore
HR)$NGprice$YOH/eff/1000
(4)
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Fig. 7. Barn air heating during winter scenario: a) Installation of a glycol loop b) Installation of a supercharger with a glycol loop.
In addition to reducing energy costs, these projects also reduce greenhouse gas (GHG) emissions. The implementation of the proposed projects would reduce GHG emissions in the winter production period by 3235 t/y of CO2. The reduction of natural gas consumption would also have the benefit of reducing NOx emissions. GHG emission reductions will bring additional financial benefits when CO2 credits can be exchanged on the market (already in place in Quebec and California).
were considered for the study: production and cleaning periods for both winter and summer months. The first recommended project was the use of boiler economizers to recover heat from the boiler flue gas to heat the boiler make-up and the boiler feedwater. The next recommended projects consisted in using a larger proportion of the waste energy from the refrigeration system to preheat process water by installing: A new HR condenser or a new subcooler (these are competing projects) A compressor oil HR system
7. Conclusion A PI study was performed for a relatively large slaughterhouse in Canada using the INTEGRATION software, and various projects were suggested to improve the plant's HR system. Four periods
A new HR condenser was recommended as it was easier to install for this specific slaughterhouse.
Table 3 Hot utility requirements to produce hot water with different oil heat recovery scenarios (Assuming the hot water tank is maintained at 87 C). Energy recovery from oil
100% recovery 50% recovery 0% recovery
Winter
Summer
Production period (kW)
Cleaning period (kW)
Production period (kW)
Cleaning period (kW)
0 0 585
254 498 742
0 0 0
166 430 693
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Fig. 8. Modified heat exchanger network e winter day scenario.
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O. Ashrafi et al. / Energy xxx (2015) 1e13 Table 4 Internal heat recovery in the slaughterhouse for different scenariosa. Different scenarios
Heat recovery before process integration (kW)
Heat recovery after process integration (kW)
Natural gas savings ($/y)
Winter day Winter night Summer day Summer night Total
1431 541 1861 586
5564 1881 4918 1366
225,000 40,000 167,000 24,000 456,000
a
In winter scenarios, a supercharger was selected to preheat air make-up stream.
Three scenarios were analyzed to recover heat from the compressor oil (100% recovery, 50% recovery, and no recovery) and the impacts of using these scenarios on the overall energy usage were investigated. The scenario where 50% of the energy is recovered was considered the most cost-effective for this slaughterhouse since it would be expensive to recover the energy from the smallest compressors. A HR project from the singeing flue gas was recommended for hot water production. The amount of high grade energy available from the singeing flue gas is sufficient to produce the required hot water during production periods when this project is implemented together with the HR projects from the refrigeration system. Two scenarios were considered to determine what would be the optimal hot water tank temperature (56 C or 87 C). The 87 C scenario was preferred seeing as it would result in additional energy savings from the singeing flue gas and because it would allow more energy to be stored in the tank and reused during the cleaning period. During cleaning periods, the available energy is not sufficient to heat the process water to 56 C. Natural gas is needed in the water heater except in the case where a heat pump would be used. However, such a heat pump would not be cost-effective if more than 50% of the energy from the compressor oil is recovered and if the energy from the ammonia is recovered (superheat and condensation). Three options were investigated to provide the energy required to heat the barn air make-up during winter. The most cost-effective option for the slaughterhouse studied was the use of a supercharger to boost the ammonia pressure/temperature. Implementation of the proposed options significantly reduces steam consumption and natural gas use in the slaughterhouse and as presented in Table 4 result in savings of approximately $456,000 in operating costs per year considering the savings of the four different scenarios. An additional reduction of natural gas could be achieved by using heat pumps to heat the process water. The implementation of suggested projects would decrease the amount of GHG emissions associated with natural gas combustion. In the winter production period, this reduction would be equal to 3235 t/y of CO2.
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