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Procedia Engineering 205 (2017) 477–484
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China
Optimization of solution heat exchanger of AHP in flue gas waste heat recovery Yang Niua, Jing Huab, *, Hongming Fana a a
Department of Beijing Key Laboratory of Green Built Environment and Energy Efficient Technology, Beijing University of Technology, Beijing100124, China b b Department of Building Science, Tsinghua University, Beijing100084, China
Abstract Latent heat recovery is a significant issue in heat recovery of nature gas, and it can improve thermal efficiency of boilers greatly. A scheme involving AHPs (absorption heat pumps) to generate cold water to recover latent heat in flue gas is proposed and applied to northern China. COP of AHPs is greatly affected by solution HXs (heat exchangers), so performance of the HX cannot be too low. In the article, a new system is put forward, investment of solution heat exchanges can be decreased greatly. Afterwards, key parameters and investment saving ratio are discussed. Optimal flue gas temperature drop in flue gas-solution HXs is calculated and maximum of investment saving ratio is put forward. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Conditioning. Air Conditioning. Keywords: Absorption heat pump; Flue gas waste heat recovery; District heating; Solution heat exchanger
1. Introduction Flue gas from gas boilers often contains over 15% of fuels’ heat content. This heat is available over the range of temperature from 30 ℃ to 150℃. With the energy structure adjustment, usage of nature gas is increasing rapidly in China. Methane is the primary component of natural gas, and it contains much hydrogen. Therefore, compared to *Corresponding author. Tel.:+86-135-8150-4155. E-mail address:
[email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.398
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coals, nature gas combustion produces more water vapor in the flue gas, which carries a large amount of latent heat [1]. Latent heat recovery is a significant issue in heat recovery of nature gas, and it can improve boiler’s thermal efficiency greatly. In Fig. 1, with different excess air ratios (λ), when flue gas temperature decreases to lower than the dew point, thermal efficiency would rise rapidly. Therefore, with the help of latent heat recovery, boiler’s thermal efficiency can exceed 100%, referenced to the lower heat value of the fuel. With the increase of λ, the vapor partial pressure in flue gas is decreasing and its dew point will be lower. At present, economizers and air preheaters are two major devices used in flue gas waste heat recovery. Main of them are indirect HXs, such as shell-and-tube HXs and plate heat HXs. Now, many studies focus on mechanism study and design of HXs[2-4] and waste heat recovery effects[5-10]. When a plant is a heat source of DH (district heating), there are two cold sources to recover waste heat from flue gas. One is return water from DH systems, and the other is fresh air. Dew point temperature of flue gas is about 50-60℃, and return water temperature of DH systems is about 50℃, so latent heat of flue gas recovery is very limited. Because of heat capacity of fresh air is much smaller than heat capacity of flue gas with vapor condensation, so the waste heat recovered by fresh air is also very small. Therefore, both of them cannot recover latent heat of flue gas effectively.
Fig 1. Theoretical efficiency of a boiler changing with exit flue gas temperature.
Considering that cold source temperature is too high[11] proposed a scheme involving AHPs to generate cold water at 20℃ to recover latent heat in flue gas. This system has been applied to northern China since 2011. Afterwards, there are a few researches about the system[12], and an open-cycle AHP scheme is put forward[13-15]. Because of the need of cold sources in systems, these systems are usually used in DH systems. Generators of AHPs used in heat recovery systems have two heat sources. When they are used in natural gas boilers, generators are usually driven by nature gas, and when they are used in power plants, they are usually driven by extraction steam from LP (low pressure cylinder). Cold sources of absorbers and condensers are usually return water from DH systems. Flue gas is cooled by the cool water from evaporators, and the temperature in evaporators is controlled by the temperature in generators. In AHP systems, COP of AHPs is greatly affected by solution HXs. Therefore, in AHPs design, the decision of minimum temperature difference in solution HXs is very important. If the temperature difference is too small, investment of solution HXs will become too expensive, and if the temperature difference is too large, COP of AHPs will be too low. If the generator of a AHP is driven by nature gas in a DH gas boiler plant, efficiency of the system will not be affected by COP of the AHP, because the heat from nature gas is just used for DH. While if the generator of a AHP is driven by extraction steam in a CCPP (Combined Cycle Power Plant), the system can recover the same amount of waste heat from flue gas by using less extraction steam, when COP of the AHP is higher. The less extraction steam usage means more electric power generated in CCPP. Therefore, solution HX design is very important when
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the system is used in CCPP systems. Because of a heat source (flue gas) and a cold source (fresh air) in flue gas waste heat recovery systems, there is a little difference from other AHP systems. However, if we make full use of the heat source and the cold source, investment of solution heat exchanges will decrease greatly. In the article, a new system is put forward, and key parameters and investment saving ratio is discussed. 2. Methods 2.1. Traditional heat recovery system A traditional heat recovery system is introduced as a reference system, see Fig. 2. In this system, the temperature of flue gas from the waste heat boiler is 150 ℃. An air preheater is set in the system and flue gas temperature decreases to 90℃ when the flue gas passes through the air preheater, and fresh air is heated from 0℃ to 74.2℃. Then, flue gas is cooled by a cooling tower, and flue gas exhaust temperature is 28℃. AHP I is driven by 110℃ extraction steam from low pressure cylinder and return water from DH system primary network is heated by AHP I from 50℃ to 55℃. Cooling water of the cooling tower is cooled by AHP I, and the temperature decreases from 28℃ to 25℃. Then return water is heated by AHP II and a steam-water HX. AHP II is also driven by extraction steam from LP and waste heat of exhausted steam from LP is recovered in this heat pump. Because of AHP II and the steam-water HX is not involved in the article, they will not be introduced and analyzed here.
Extraction steam
Steam–water HX
Exhausted steam Absorption heat pump II
Fresh air Flue gas
Cooling tower
Nature gas
~ ~
Waste heat HP/IP boiler
Absorption heat pump I
Supply water
Return water
LP
Fig 2. Schematic diagram of CCPP with heat recovery by absorption heat pump.
Detailed structure involved the air preheater and AHP I is shown in Fig. 3 (a). Return water from DH system is heated by absorber firstly, and then heated by condenser. The heat exchange between dilute LiBr solution and concentrated LiBr solution is occurred in the solution HX. In this case, the temperature of concentrated solution from generator is 107℃, and the temperature of concentrated solution to absorber is 54.5℃. The temperature of dilute solution from absorber is 53℃, and the temperature of dilute solution to generator is 101.8℃. The temperature difference of concentrated solution in the solution HX is bigger than dilute solution. It is because that the mass flow rate of the concentrated solution is lower than dilute solution. In this system, minimum heat transfer temperature difference of the solution HX is 1.5℃ and the LMTD (logarithmic mean temperature difference) of the solution HX is 3℃. Therefore, the performance of the solution HX is well.
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Extraction steam
Flue gas
Flue gasair HX
Extraction steam
G
C
Flue gassolution HX
Solution HX
Return water Fresh air
Flue gas
Cooling tower
A
E
Return water
flue gasair HX
C
Solution HX
Airsolution HX
(a)
G
Cooling tower
Fresh air
A
E
`
(b)
Fig 3. Schematic diagram of the air preheater and absorption heat pump: (a) traditional system, (b) new system
2.2. New heat recovery system Based on the traditional heat recovery system introduced above, a new system is put forward, see Fig. 3 (b). In the new system, a flue gas-solution HX and an air-solution HX are added to the system. Because flue gas temperature is much higher than extraction steam temperature, and fresh air temperature is much lower than the temperature of return water from DH system, with the help of the two HXs, LMTD of the solution HX can be enlarged greatly. Also, LMTD in the two new added HXs are also very large. Therefore, the new system can reduce HX’s areas and remain other parameters unchanged. In the new system, according to the circulation ratio, which amount is 13, the temperature drop relationship between flue gas and solution in the flue gas-solution HX can be calculated, as well as fresh air and solution in the air-solution HX. According to the flue gas temperature at the outlet of flue gas-solution HX, dilute solution temperature at the outlet of solution HX can be calculated. Then, in the flue gas-air HX, fresh air temperature flow in this HX is determined. In the air-solution HX, concentrated solution temperature flow in this HX is determined. For each HX, LMTD can be calculated by Eq. (1)
Δtm =
( th, in − tc, out ) − ( th, out − tc, in ) ln
th, in − tc, out
(1)
th, out − tc, in
in which, th,in is inlet temperature of hot fluid; tc,out is outlet temperature of cold fluid; th,out is outlet temperature of hot fluid; tc,in is inlet temperature of cold fluid. Then, area of each HX can be calculated by Eq. (2) F = Q / ( K Δt m )
(2)
in which, F is area of HXs; Q is heat transfer flux; K is heat transfer coefficient of HX. For air (flue gas)-solution HX, K is 80W/(m2·K). For air-flue gas HX, K is 40W/(m2·K). For solution HX, K is 1800W/(m2·K). In the article, flue gas is considered as unit mass flow, 1kg/s. Therefore, Q can be calculated by mass flow and temperature drop of flue gas. Investment of each HX can be calculated by Eq. (3) I HX = CHX F
(3)
in which, IHX is investment of HX; CHX is investment of HXs per area. For air (flue gas)-solution and air-flue gas HXs CHX is 1500 ￥/m2, and for solution HXs CHX is 1200 ￥/m2. Investment of AHPs is 0.7￥/W(cooling capacity).
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3. Discussion In the traditional system, investment of the air preheater, the solution HX and the AHP is a fixed cost. The cost is regarded as a reference investment. According to the reference investment, investment saving ratio of the investment of changed HXs in the new system can be calculated. For the new system, HXs’ areas are affected by flue gas temperature at the outlet of the flue gas-solution HX, see Fig. 4. With the temperature decreasing, solution HX’s area is decrease rapidly at first and decrease slowly later, air-flue gas HX’s area is decreased slowly and steadily and air (flue gas)-solution HX’s area is increased steadily. Therefore, there is an optimal temperature of flue gas at the outlet of flue gas-solution HX. In the system, there are three main parameters affect the optimal point and the investment saving ratio.
Fig 4. Area of HXs changing with flue gas temperature at outlet of gas - solution HX.
3.1. Effects of flue gas temperature from waste heat boiler In the new system, the key point is the high temperature difference usage. Therefore, as a high-temperature level heat source, the temperature of the flue gas from waste boiler can affect the advantage of the system greatly. In this section, fresh air temperature is 0℃ and return water temperature is 50℃, and flue gas temperature varies from 115℃ to 150℃. From the results (see Fig. 5(a)), the optimal temperature drop increases with the flue gas temperature increasing. The investment saving ratio increases with the flue gas temperature increasing. When the flue gas temperature is 150 ℃, the optimal temperature drop is 11.3℃ and corresponding investment saving ratio is 6.6%. Fig.5 (b) shows the flue gas temperature drop in the HX changing with the flue gas temperature. The temperature drop increases with the flue gas temperature increasing, while increase rate decreases with the flue gas temperature increasing. It is caused by the inversely proportional relationship between the LMTD and area of HXs, when the heat flux is fixed. Fig. 5 (c) shows that the investment saving ratio increases with the flue gas temperature increasing, while the increase rate decreases with the flue gas temperature increasing. It is also caused by the reason mentioned in the analysis of optimal flue gas temperature drop. 3.2. Effects of fresh air temperature As a low-temperature level cold source, the temperature of the fresh air from environment can also affect the advantage of the system greatly. In this section, flue gas temperature is 150℃ and return water temperature is 50℃, and fresh air temperature varies from -10℃ to 5℃. The results (see Fig. 6(a)), the optimal temperature drop decreases
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with the fresh air temperature increasing. The investment saving ratio decreases with the fresh air temperature increasing. From Fig. 6 (b), the temperature drop increases steadily with the fresh air temperature increasing. From Fig. 6 (c), the investment saving ratio decreases steadily with the flue gas temperature increasing. It is caused by the small temperature variation range of fresh air.
Fig 5.The influence of flue gas temperature: (a) Investment saving ratio, (b) Flue gas temperature drop, (c) Maximum of investment saving ratio
Fig 6. The influence of environment temperature: (a) Investment saving ratio (b) Flue gas temperature drop (c) Maximum of investment saving ratio.
3.3. Effects of return water temperature of DH system Return water temperature is a middle temperature in the system, and the heat transfer assignment is affect by it. Therefore, it can also affect the advantage of the system greatly. In this section, flue gas temperature is 150℃ and fresh air temperature is -10℃, and return water temperature varies from 39℃ to 60℃. From Fig.7(a), in general, the optimal temperature increases at first and decreases later with the fresh air temperature increasing. The investment saving ratio increases at first and decreases with the fresh air temperature increasing. From Fig.7 (b), the temperature drop increases at first and decreases later with the return water temperature increasing. From Fig. 7 (c), the investment saving ratio increases at first and decreases later with the flue gas temperature increasing. It shows that there is a balance of heat transfer assignment in the system.
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Fig 7. The influence of return water temperature: (a) Investment saving ratio, (b) Flue gas temperature drop, (c) Maximum of investment saving ratio.
4. Conclusions • • •
A new AHP flue gas waste heat recovery system is put forward via making full use of the heat source and the cold source, and the investment of the solution heat exchange will decrease greatly. Then a thermal model of the new system is established and three main parameters are analyzed. In a typical case, the investment saving ratio is 6.6%. Maximum of the investment saving ratio is 7.1% in the parameter ranges in the article.
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