Technology Application of District Heating System with Co-Generation Based on Absorption Heat Exchange: a Renovation Project in Datong City of North China

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014 November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-37355

TECHNOLOGY APPLICATION OF DISTRICT HEATING SYSTEM WITH CO-GENERATION BASED ON ABSORPTION HEAT EXCHANGE: A RENOVATION PROJECT IN DATONG CITY OF NORTH CHINA

Yan Li1 Lin Fu2 Shuyan Zhang1 Xiling Zhao2 1 College of Civil Engineering&Mechanics, Yanshan University, Qinhuangdao, PR China 2 School of Architecture, Tsinghua University, Beijing, PR China a corresponding construction inconvenience. This limited the ABSTRACT availability of DH technique although the heat supply capacity

To explore the energy saving potential of Co-generation and of plants is huge [4-5]. enhance the feasibility of district heating, Tsinghua University (2) Waste heat discharged by the cooling tower: To ensure proposed a new type of district heating system with Co-generation safe operation of the large Co-generation units, the amount of based on absorption heat exchange (Co-ah system). It can improve the exhausted steam from the low-pressure cylinder cannot be less heating capacity and the energy efficiency by recycling the exhausted than the given minimum. The exhausted steam is cooled and the steam waste heat, and enhance the delivery capacity of the heat waste heat is discharged by the cooling tower. The amount of network by increasing the temperature difference between the the waste heat is enormous, but the temperature is too low (35℃ supplying water and the returning water. This paper focuses on the first or so) to be directly utilized for building heating. It will be large-scale industrial application project built in the Datong City First CHP Plant. We increase the delivery capacity of the heat network by significantly to energy saving and emission reduction if these 40% after adopting the Co-ah technology, and both the heating capacity exhausted waste heat can be recycled [6]. and the energy efficiency of Co-generation had increased 50%. This (3) In addition, the traditional system itself has obvious paper provides an overview of this project. Both the performance of deficiencies in the thermodynamics view: In the design two kinds of absorption equipments (absorption heat pumps in the plant condition, the turbine extraction steam (0.3~1.0MPa) is used to and absorption heat exchanger units in the substations) and the heat in sequence: the primary heat network (130℃/70℃), then operation effect of the system were obtained through the experiment, ℃ ℃ and thus the feasibility and reliability of Co-ah technology were the secondary heat network (70 /50 ), and finally supply the verified. heat energy to various consumers. The larger temperature-difference is bound to result in enormous irreversible exergy (available energy) losses in two INTRODUCTIOND heat-exchange processes, namely, a steam-water heat exchange Currently, the large-scale district heating systems (DH in the plant and a water-water heat exchange in the substations. system) has already occupied a dominant position of building Therefore, one of the emphasis tasks of energy-saving is to heating in Northern China. Many cities in this area have explore the potential of Co-generation and DH system in developed the district heat networks supplied by the CHP Northern China. Tsinghua University proposed a new type of (combined heating and power) plants or the district boilers. By district heating system with Co-generation based on absorption 2012, the DH systems supplied by the large Co-generation units heat exchange (Co-ah System) [7-10]. (above 300MW) made up about 40% of total building heating due to its significant advantages in both energy-saving and CO 2 2 THE DESCRIPTION OF THE CO-AH SYSTEM emission-reduction [1-3]. However, the traditional DH system together with 2.1 The Absorption Heat-Exchange Method Co-generation (Fig1) has been limited by the following Considering the irreversible exergy losses in the exchange problem: between the primary heat network and the secondary heat (1) Limitation of poor delivery capacity of the heat network: network in the DH system substations, we can find way to lower Generally, the CHP plants were far away from the urban greatly the return water temperature — the technique of heat-load concentrations, there are huge investments on absorption heat-exchanger (Fig2a). extension or reconstruction of the pipe network, not to mention

Fig.1. Technological Process of Traditional DH System

(a) The System Technological Process (b) The Contrast Fig.2. Process Chart of Absorption Heat-Exchanger

The absorption heat-exchanger unit composes of a the secondary heat network remains unchanged. conventional water-water heat exchanger and an absorption heat The application of absorption heat-exchange units will pump (absorption refrigeration). The high-temperature heat flux obtain the following impacts: Qe is used as the driving force to recover low-temperature heat 1) Because the supply/ temperature of the primary heat flux Qg, and eventually the total heat Qg+Qe is released at an network is changed from 130 ℃ /70 ℃ to 130/25 ℃ , the intermediate temperature. The hot water of the primary heat temperature drop is increased correspondingly from 60℃ to network release heat successively to the high-temperature side A 105℃, which means the delivery capacity mostly doubles. For a of the absorption heat pump, the heat-exchanger 2 and the newly-built heat network, the diameter of the pipe can be small low-temperature side B of the absorption heat pump, and finally and significantly reducing the investment. return the Co-generation plant at a lower temperature (about 25 2) Because of a lower temperature, the return water is ℃). The hot water of the secondary heat network is heated easier to be heated by exhausted waste heat through respectively by the intermediate-temperature side C of the exchanger or heat pump, as a result, the low-grade exhausted absorption heat-pump and the heat exchanger 2, and finally heat is easier to be recovered and improve significantly the piped to the consumers. As a result, the return water temperature overall energy efficiency of the CHP plants. of the primary heat network can be lowered to about 25℃ with the supply water temperature 130℃, while the temperature of

Fig.3. Technological Process of the Co-ah system In 2010 winter, the heating demand of this residential area was 2.2 The Co-ah System 2 In the exchange link between the extraction steam and the increased to about 6.4 million m . However, the heating capacity primary heat network in the CHP plant, lots of irreversible heat of the CHP plant has already reached limitation, and it is also transfer losses can be avoided by the introduction of absorption difficult to develop more large heat production units (district heat pumps technology. The return water of the primary heat boilers etc.) due to the environmental limitation. In view of the network was gradually heated by the exhausted steam-water urgent heating demand, a renovation project of Co-ah exchanger, the absorption heat pump, and the extraction technology was put forward. The technological process of the steam-water exchanger. Therefore, a whole Co-ah system is project was shown in the Fig.4. proposed. (Fig3) This project including: Obviously, the Co-ah system uses the extraction steam and 1) Transform the 2×135MW air cooled turbine units, and the exhausted steam as heat sources, and the energy efficiency set two waste heat recovery units (composed of the absorption can be greatly improved because of reducing the extraction heat pump and the exhausted steam-water exchanger) for steam consumption. The exhausted waste heat can supply above recovering 132MW waste heat from 2×100t/h exhausted steam 30% of heating power in the design condition and accounts for (Fig.5a). Therefore, the heating capacity can be increased to 400MW, which can supply the building heating of 6.4 million above 40% of total heating output in the whole heating period, 2 as a result, the heating capacity can be increased above 30% and m . the energy efficiency can be increased above 40%. 2) Transform fourteen district heating system substations (the heating areas were about 2.8 million m2), and set eighteen absorption heat-exchanger units. The return water temperature 3 THE APPLICATION DEMONSTRATION PROJECT of the primary side was reduced to about 25℃, while other 3.1 Engineering Situation substations were about 52℃. Therefore, the total return water This paper focuses on the technology application of the temperature in the plant is about 37℃ (Fig.5b). Co-ah system in Datong City. This application project is to meet 3.2 System Description the building heating demand of the newly added 2.0 million m2 residential area of Datong Coal-Mine Group Co. by recovering 3.2.1 The Gateway Station of the CHP Plant the exhausted steam waste heat from the 2×135MW air cooled Co-generation units in the Datong City first CHP plant. The parameters in the rated condition of the 135MW air The maximum heating capacity of the 2×135MW cooled Co-generation unit are shown in the Tab.1. The flow rate Co-generation units are about 268MW (the maximum flow rate of extraction steam (the fifth segment adjustments) is 200t/h, of the extraction steam is 2×200t/h), this CHP plant had been and the pressure is 0.245MPa. The flow rate of exhausted steam responsible for about 4.4 million m2 building heating until 2010. from the low pressure cylinder is 161t/h, and the back pressure in winter is 15kPa.

Absorption Absorption Heat Heat Plate Heat Exchanger Exchanger Exchanger Waste Heat Substations Recovery Units 25 25 Substations Extraction Steam 120 0.245MPa Primary Heat 400MW 2×200t/h network 37 4000t/h 268MW Condensate Main Steam Water Gateway Station

Exhausted Steam 2×100t/h 132MW 2×135MW Steam Turbine Exhausted Steam Air Cooled Condenser Condensate Water Fig.4 Technological Process of the Demonstration Project

a) In the Gateway Station of the CHP Plant b) In the Substations

Fig.5 Scene Photographs of Renovation Project

Tab.1 Main Thermal Parameters in the Rated Condition Tab.2 Design Parameters of the Waste Heat Recovery Unit

CKZ135-13.24/535/535/0.245 Item Parameter Value Type Ultra high pressure, Intermediate reheat, Direct air cooled Inlet Temperature (℃ ) 37 Hot Water of Pressure of Main Pressure of Outlet Temperature (℃ ) 73 13.24MPa 2.48MPa heat network Steam Reheat Steam Flow Rate (t/h) 2000 Flow Rate of Main Flow Rate of 480 t/h 411 t/h Steam Reheat Steam Flow Rate (t/h) 100 Flow Rate of Pressure of 200t/h 0.245MPa Exhausted steam Pressure (kPa) 20 Extraction Steam Extraction Steam Heat quantity (MW) 66 Flow Rate of 161 t/h Back Pressure 15 kPa Exhausted steam Flow Rate (t/h) 28 The length of last Heat 450mm Extraction Steam Pressure (MPa) 0.245 stage blade Consumption 6535kJ/kWh It needs recycle 100t/h exhausted steam of every turbine Heat quantity (MW) 23 unit according to the new heating demand of 2.0 millions. In the In addition, the renovation of part pipes in the thermal design condition, the return water temperature of the primary system is shown as follow (Fig.6): main heat network should be drop to 37℃, and the design 1) Install a switching valve V1 on the original return water pressure of exhausted steam of the turbine is 20kPa according to pipe of the heat network, and add the introduced pipe、valve the parameter optimization method of Co-ah system. V2、 valve V3. The return water can be introduced to the waste Considering the actual construction conditions, the heat recovery units when the valve V1 closes. renovation scheme in the CHP plant is to set two HRU-85 waste 2) Set a introduced pipe on the original exhausted steam heat recovery units (the total heating output was about 85MW). riser pipe, and install the switching valve V8 and V9. The The design parameters of the HRU-85 waste heat recovery units exhausted steam can be introduced to the waste heat recovery are shown in Tab.2. units when the switching valves are open.

Fig.6 Schematic Diagram of Renovation Gateway Station in Plant 3) Set a introduced pipe on the original extraction steam pipe, and install the switching valve V17 and V18. The 4 THE EFFECT AND PERFORMANCE OF SYSTEM extraction steam can be introduced to the waste heat recovery units when the switching valves are open. 4.1 The Measure of the Heat Network Operation 4) Set a introduced pipe on the original condensate water Some measures were taken to test the operation pipe, and install the switching valve V11 and V12. The performance of the absorption heat exchanger units, and the condensate water of extraction steam and exhausted steam in the measure data were taken from the PLC data acquisition system waste heat recovery units can be introduced to the condensate of the units. The effect on two substations and the gateway water tank when the switching valves are open. station was shown in Fig.7 and Fig.8. The test time before 5) Set a introduced pipe on the original vacuum pipe, and renovation was from December 1 to December 15, 2010, the install the switching valve V14 and V15. The exhausted steam daily average outdoor temperature was -15.5~-4.9 ℃. The test side of the waste heat recovery units can maintain the vacuum time after renovation was from January 20 to February 4, 2011, by the original water ring vacuum pump when the switching the daily average outdoor temperature was -15.4~-6.2℃. The valves are open. outdoor temperature of two time phases was similar. 1) The return water temperature of the primary side in the 3.2.2 In the substations substations was 42~62℃, and the return water temperature of There are forty-eight substations in the district heating the primary heat network in the gateway station was 40~60℃ system of the Dating Coal-Mine Group Company residential (average 52℃) before renovation. After installing the absorption area. Eighteen absorption heat exchanger units in fourteen heat exchanger units, the return water temperature of the substations have been installed, and the total area of building primary side in the substations was 19~33℃, and the return heating amount to about 2.8 million m2. The supply water water temperature in the gateway station was drop to 34~42℃ temperature of the primary heat network is 120℃, and the hot (average 37℃). It verified that the absorption heat exchanger ℃ ℃ water temperature of the secondary heat network is 60 /45 units can operate as the expected effect. in the design condition, the returning water temperature of the 2) The supply water temperature of the primary heat ℃ primary side could drop to 20~30 by these absorption heat network in the gateway station was only 68~95℃ (average 85 ℃ exchanger units, while other substations maintain about 52 . ℃), which cannot meet the heating demands before renovation. Therefore, the total return water temperature in the plant can The supply water temperature of the primary heat network can ℃ drop to about 37 . be increased to 93~114℃ (average 102℃) after renovation, because the heating capacity of the CHP plant was increased.

5 Copyright © 2014 by ASME 4.2 The Measure of the Plant Operation Some measures were taken to test the operation performance of the waste heat recovery units. The comparison of the heating capacity of before and after renovation is shown in Fig.9. The heating capacity was deficient because of bad coal quality before renovation. The supply water temperature can only reach 95℃ when the flow rate of the primary heat network is 3000t/h. The heating capacity can be increased about 120MW due to the recovery of waste heat from exhausted steam after renovation. The supply water temperature can reach above 110 ℃ when the flow rate of the primary heat network is 4000t/h. From Fig.9, we can see that both the heating capacity and the energy efficiency of Co-generation were increased about (a) C# substation 50% because the exhausted steam waste heat account for half of the total heat output.

(b) School# Substation Fig.7 Measure Data of the Temperature of the Heat Fig.9 Measure Data of the Composition of the Heat Network in Two Substations Output in the CHP Plant 4.3 The Measure of the Consumers Ten residential consumers were selected as the test object. The daily average indoor temperature were recorded for 16 days prior to renovation and 16 days post renovation, and the comparison of the daily average indoor temperature is shown in Fig.10. Results showed that the heating quality was improved due to the heating capacity of district heating system increasing after renovation.

Fig.8 Measure Data of the Temperature of the Primary Heat Network in the Gateway Station The delivery capacity of the heat network is defined as: ∆⋅⋅= p tGcQ

Where, cp is the specific heat capacity of the delivery medium, G is the flow, and ∆t is the temperature difference between the supplying water and the returning water. According to the equation, the delivery capacity of the heat network can be increased about 40% because of the temperature difference ∆t increasing about 40% (from average Fig.10 Measure Data of Indoor Temperature of the 60℃ to 83℃). Residential Consumers

6 Copyright © 2014 by ASME based on Co-ah cycles in combined heating and power systems. 6 THE CONCLUSION Tsinghua Univ(Sci & Tech) 2008; 48(2):1377-1380 (Language: This paper introduces the fundamental and the process Chinese) [9] Peng Hu, Lin Fu, Shigang Zhang, etc. Study on a Newly design of the application project of Co-ah technology. The Absorption Heat Pump District Heating System. ASME 3rd performance of two types of absorption equipments (absorption International Conference on Energy Sustainability 2009, San heat pumps in the plant and absorption heat exchanger units in the substations) and the operation effect of the system were Francisco, California verified to be satisfied. The main conclusions were as follows: [10] Fangtian Sun, Lin Fu, Shigang Zhang, etc. New waste heat 1) In the Co-ah system, the heating capacity and the energy district heating system with combined heat and power based on absorption heat exchange cycle in China. Applied Thermal efficiency can be significantly increased by recycling the Engineering 2012; 37(05):136-144 exhausted steam waste heat in the plant, and the delivery capacity of the primary heat network can be significantly improved by increasing the temperature difference between the supplying water and the returning water. 2) The renovation project in Datong city first CHP plant is the first large-scale application project the of Co-ah technology. 3) The operation effect of this system was provided to be good by experiment. The delivery capacity of the primary heat network can be increased about 40%, due to the return water temperature of the primary heat network can be drop to about 37 ℃. Both the heating capacity and the energy efficiency can be increased about 50% because the exhausted steam waste heat account for half of the total heat output. Therefore, the heating quality of consumers can be improved.

Project Supported by: The Natural Science Foundation of Hebei Province, China (E2013203181)

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