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A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger
Applied Energy xxx (2017) xxx–xxx
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A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger q Sheng Shang, Xianting Li ⇑, Wei Chen, Baolong Wang, Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
h i g h l i g h t s A novel non-contact total heat recovery system of gas-fired boiler is proposed. The oxidizing air is heated and humidified by recovering total heat of flue gas. The temperature of boiler flue gas after heat recovery is about 30 °C in winter. The energy saving potential is close to the system using AHP for heat recovery. The initial investment is so low that the payback period is shorter than 1 year.
a r t i c l e
i n f o
Article history: Received 28 January 2017 Received in revised form 17 May 2017 Accepted 26 May 2017 Available online xxxx Keywords: Gas-fired boiler Total heat recovery Flue gas Non-contact heat exchanger Tech-economic analysis
a b s t r a c t Recovering heat from the flue gas of a gas-fired boiler can both improve boiler efficiency and decrease pollutant emissions. To improve the efficiency of the gas-fired boiler in a more cost effective and higher efficient way, a non-contact total heat recovery (NCHR) system is proposed for recovering heat from flue gas for use in heating and humidifying the oxidizing air of the boiler. A mathematical model of a boiler with an NCHR system was established, and the performance of the NCHR system was compared with that of other heat recovery systems. It is shown that the efficiency of a boiler with an NCHR system can reach 103.4% for an inlet oxidizing air temperature of 0 °C, which is 13.4% higher than the efficiency of a traditional boiler. According to the case study, the energy saving potential of a boiler with an NCHR system is 12.97% compared to that of a traditional boiler. As for the economic analysis, the payback period of a boiler with an NCHR system to traditional boiler and the condensing boiler is 1 year and 3 years, respectively. In addition, the operation cost of an NCHR system is less than that of a boiler with an absorption heat pump for heat recovery (AHPB) system, indicating that the NCHR system has obvious economic benefits. Ó 2017 Published by Elsevier Ltd.
1. Introduction Building energy consumption in China has increased rapidly over the past three years, reaching over 7.56 billion tce (tons of coal equivalent) in 2013 [1], which equates to approximately 24% of primary energy consumption [1]. Among all types of energy consumption, coal-fired boilers account for about 85% of the entire heating and power generation process [2]. However, the primary energy efficiency of a coal-fired boiler in terms of heating is approximately 55–75% [3], which is lower than that of a gas-
q The short version of the paper was presented at ICAE2016 on Oct 8–11, Beijing, China. This paper is a substantial extension of the short version of the conference paper. ⇑ Corresponding author. E-mail address: [email protected] (X. Li).
fired boiler which typically has an efficiency of above 90% [4]. Additionally, the coal-fired boiler emits more pollutants (particles and poisonous gas) than the gas-fired boiler [5,6]. Therefore, the use of gas-fired boilers has increased rapidly in recent years because of the environmental protection efforts and policies of the Chinese government. The energy efficiency of a gas-fired boiler is approximately 90%, and the temperature of the emission flue gas is approximately 150–200 °C [7,8]. Significant energy is wasted if the hightemperature flue gas is discharged directly. Therefore, much recent research has focused on recovering heat from the emission flue gas of boilers. The methods for heat recovery of the flue gas can be classified into three types according to the heat recovery depth. The first method is sensible heat recovery, where flue gas is used to preheat oxidizing air and return water [9]. The configuration is simple, but the temperature of the emission fuel gas can reach
http://dx.doi.org/10.1016/j.apenergy.2017.05.169 0306-2619/Ó 2017 Published by Elsevier Ltd.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Nomenclature A C Cp d G Gy h i L LCV m Nu O P PB Pr Q r Re Rg RH T Dtm v w y Z
heat transfer area, m2 initial investment, CNY specific heat capacity, kJ/(kg K) humidity ratio, kg/kg hourly gas consumption, m3/h year-round gas consumption, m3 convective heat transfer coefficient, W/(m2 °C) enthalpy, kJ/kg building load, kW low caloric value, 35,000 kJ/m3 mass flow rate, kg/s Nusselt number operation cost, CNY pressure, kPa payback period Prandtl number heating capacity, kW latent heat of vaporization Reynolds number ideal gas constant relative humidity temperature, °C logic mean temperature difference, °C volume flow rate, m3/s mass transfer rate, kg/s running cost per year running cost each year, CNY/a
100 °C. The boiler efficiency is approximately 93% because only part of the sensible heat of the flue gas is recovered [10]. However, large amounts of latent heat (which can significantly promote boiler efficiency if recovered) will be wasted [11,12]. The second boiler heat recovery method involves a condensing heat recovery system, where the emission flue gas temperature can be decreased below the dew point, and the boiler efficiency can reach 98% [13]. Weber et al. analyzed the feasibility of recovering waste heat from flue gas using a condensing heat exchanger, and the results showed that boiler efficiency could be significantly improved, and CO2 emissions decreased [14]. However, because of acid gas, the heat exchanger can be corroded. Therefore, the anticorrosion properties of the materials used in the heat exchanger are very important for promoting boiler efficiency. Wang et al. investigated the system performance and application of a corrosion resistant heat exchanger. Their results showed that the energy saving ratio of the system could be increased by 10% by using condensing heat recovery [15,16]. Pezzuolo et al. investigated the heat recovery for biomass boiler using the ORC cycle [17,18], and the heat recovery method is similar but the recovered heat is used to generate power. From the abovementioned methods, the emission flue gas temperature is over 55 °C. If the flue temperature can be decreased to 25–35 °C, the boiler efficiency can be increased by over 12% [19]. Therefore, absorption heat pumps (AHPs) have been used for the heat recovery of the flue gas, which is the third heat recovery method. The AHP is driven by natural gas, and the flue gas is the heat source for the evaporator. Simultaneously, the recovered energy is used to heat the return water. The boiler efficiency reaches approximately 104%, and the emission flue gas temperature can be reduced to about 30 °C. Zhu et al. analyzed the performance of AHP heat recovery in a real case and found that boiler efficiency could be increased to 13.6%, with a flue gas temperature of 30 °C [20]. Fu et al. simulated an absorption heat pump heat recovery system and found that its efficiency could be increased by 11% relative to that of the condensing heat recovery system [21]. Qu et al.
Greeks b d
e g k
q
efficiency of the conductive heat transfer thickness, m boiler efficiency rib effect coefficient conduction heat transfer coefficient, W/(m °C) density, kg/m3
Subscripts a air f flue gas g gas gas natural gas i hourly system performance md convective mass transfer rate, kg/s re chemical reaction rhc rated heat capacity sw heat transfer medium tb traditional boiler w water Abbreviations AHP absorption heat pump AHPB boiler using AHP for heat recovery ESR energy saving ratio NCHR non-contact heat recovery system
integrated an AHP with natural gas-fired boilers to improve boiler efficiency; their results showed that efficiency could be increased by over 10% [22]. However, initial investment is high due to the AHP [23]. From the above mentioned methods, the low temperature oxidizing air isn’t well utilized. If the total heat of the flue gas can be recovered by the oxidizing air, in which the oxidizing air is heated and humidified by the flue gas without direct contact, higher heat recovery efficiency with lower costs can be achieved. To improve the boiler efficiency in a more cost effective and higher efficient way, this study proposes a total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. In order to analyze the heat recovery performance of the proposed system, a mathematical model is established. Then, the boiler efficiency of different methods is compared. To investigate the energy saving potential and payback period of this system, technical and economic analyses are conducted for a case study in Beijing.
2. Working principle of total heat recovery of the flue gas A traditional boiler uses a direct heat exchanger. Its flue gas can only be used to increase the temperature of oxidizing air or return water. The heat transfer process is shown as 1–20 in Fig. 1. If the humidity ratio of the oxidizing air can be increased to point 3, the enthalpy difference between point 1 and 3 will be much greater than that between points 1 and 20 for the same temperature difference. In this case, the temperature of the emission flue gas will be close to the ambient temperature. Therefore, this kind of heat transfer process typically saves more energy. As the moisture in the flue gas cannot be directly recovered to the oxidizing air because of corrosion, direct total heat recovery cannot be used in this situation. To achieve the heat transfer process of 1–3 in Fig. 1, a novel non-contact total heat recovery
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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S. Shang et al. / Applied Energy xxx (2017) xxx–xxx
Fig. 1. Psychrometric chart for heat transfer process.
(NCHR) heat exchanger is proposed. Its configuration is shown in Fig. 2. The NCHR system contains three components. The first is the condensing heat exchanger, where the flue gas heats the return water and the vapor in the flue gas will be condensed. The second component is the total heat recovery heat exchanger, which contains the wall type heat exchanger and the spray water heat exchanger. Here, the spray water absorbs energy from the flue gas and the temperature of the water increases. Then, the oxidizing air is heated and humidified by the heated spray water. The spray water has no direct contact with the flue gas, so that the oxidizing air won’t be polluted by the flue gas. A circulation pump is installed in each stage to realize this water cycle. The third component is the sensible heat exchanger. The main function of this heat exchanger is to preheat the oxidizing air when the ambient temperature is too low. The heated and humidified air enters the boiler and is combusted with natural gas, resulting in the increase of the dew point of the flue gas. After the combustion process, the flue gas will then pass through the condensing heat exchanger and heat the return water. Generally, the dew point temperature of the flue gas at this point is high enough for the vapor in the flue gas to be condensed after the heat exchange process with the return water. Then, the return water will be heated in the boiler.
3. Mathematical model of boiler with total heat recovery of flue gas The mathematical model contains three parts, including a gasfired boiler model, a heat and mass transfer model, and the thermal properties of the flue gas. The temperature and mass flow rate of the flue gas are given by the gas-fired boiler model, while the heat and mass transfer model calculates the state point of each heat exchanger inlet and outlet. The thermal properties of the flue gas include the enthalpy and dew point of the flue gas, which considers the humidity ratio of the humidified air and the combustion process. 3.1. Model of gas-fired boiler The model of the gas-fired boiler primarily contains the temperature and mass flow rate of the flue gas. The rated heating capacity is 90 kW, and the temperatures of the return water and supply water are 45 °C and 60 °C, respectively. Gas consumption is calculated by the following equation:
V gas ¼
Q rhc LCV e
ð1Þ
Condensing heat exchanger Wall type heat exchanger
Wall type heat exchanger
Wall type heat exchanger
emission of flue gas
Flue gas
To boiler
Sensible heat exchanger
Return water Spray water heat exchanger
Spray water heat exchanger
Spray water heat exchanger
oxidizing air
Oxidizing air to boiler P12
Non-contact total heat exchanger Fig. 2. Working principle and structure of the novel NCHR system.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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The theoretical chemical oxygen demand can be obtained from the chemical reaction. The excess air coefficient is taken as 1.2 [24]. Therefore, the air mass flow rate can be calculated. The mass flow rate of the flue gas is the sum of the flow rates of natural gas and air. The temperature of the flue gas is set to 150 °C [10]. 3.2. Heat and mass transfer model There are two types of heat exchangers in the proposed device: a wall type heat exchanger and a direct contact heat exchanger. The models for the two types of heat exchangers are developed first, and then the whole model is built by transmitting parameters between the two heat exchangers. The wall type heat exchanger, condensing heat exchanger, and sensible heat exchanger are considered, and the flue gas and air (or water) is regarded as counter flow. A lumped parameter method is used to calculate the performance of the heat exchanger, and the control equations include the energy conservation equation and heat transfer equation. The equation of energy conservation is as follows:
mg ðicgin icgout Þ ¼ mcw C pw ðtcwout t cwin Þ
ð2Þ
The heat transfer equation is as follows:
Q ¼ hADt m
ð3Þ
The calculation of heat transfer coefficient hg considers the convective heat transfer coefficient of the flue gas, the thermal conductivity of the coil, and the convective heat transfer coefficient of the pipe. The detailed calculation formula is as follows:
hg ¼
1 1 hf g
þ bd þ hbw k
ð4Þ
The convective heat transfer coefficients of the inner-wall of the pipe and gas side are calculated by the dimensionless expressions Nuw and Nuf, respectively [25]:
enthalpy after the absolute moisture content changes. This section introduces the calculation method. The saturated vapor pressure is related only to the flue gas temperature at the same atmospheric pressure. The absolute moisture content can be calculated from the chemical reaction of the combustion of the methane and the moisture content in the oxidizing air. The equation is as follows:
mf ¼ mre þ ma
ð12Þ
Methane (CH4) is the main component of natural gas, so the moisture generation during the chemical reaction can be calculated by the following equation:
mre ¼ 2 mg
ð13Þ
The moisture content of air is calculated as follows:
ma ¼ 2V a q da
ð14Þ
Vapor pressure can be calculated using the ideal gas state equation:
P¼
mf Rg T V
ð15Þ
Then, the dew point temperature of the flue gas can be calculated from the vapor pressure, because the saturated vapor pressure is a single value function of the temperature. The enthalpy of the flue gas consists of sensible heat and latent heat, and the equation is as follows:
hf ¼ cpf T þ r mf
ð16Þ
If the temperature of the oxidizing air entering the boiler is 45 °C and the relative humidity ratio is 90%, the relationship of dew point temperature and enthalpy (for different flue gas temperatures) is shown in Fig. 3.
n Nuw ¼ 0:023 Re0:8 w Pr w
ð5Þ
3.4. Model validation
Pr 0:35 ðPr f =Prw Þ0:25 Nuf ¼ 0:71Re0:5 f f
ð6Þ
The accuracy of the entire system is primarily dependent on the accuracy of the wall type heat exchanger model, the spray water heat exchanger, and the thermal properties of the flue gas. The wall type heat exchanger model is relatively simple and very mature. The thermal properties of the flue gas are calculated based on both the thermal properties of components in the flue gas and Dalton’s partial pressure law, which are relatively accurate and whose errors can be ignored. The most complicated part of the whole system is the spray water heat exchanger model, which couples heat and mass transfer. Therefore, the spray water heat exchanger model is validated in this section. The validation of the spray water heat exchanger model is based on an experiment [26]. The outlet air enthalpy and heat transfer rate will be compared in the experiment, and the relative error will be calculated. The experimental conditions are given in Table 1. The relative errors for the outlet air enthalpy and heat transfer rate are calculated, and the results are given in Table 2. According to the results, the maximum relative errors for the outlet air enthalpy and heat transfer rate are within 1%. The simulation results significantly match the experimental results. Therefore, the accuracy of the model is acceptable.
In the spray water heat exchanger, air and water are in direct contact; hence, heat and mass are simultaneously exchanged. Therefore, the equations considered in this process include the energy conservation equation, mass conservation equation, heat transfer equation, and mass transfer equation. The energy conservation equation is as follows:
ma ðisaout isain Þ ¼ msw C pw ðtswin tswout Þ
ð7Þ
The mass conservation equation is as follows:
wr ¼ ma ðdsaout dsain Þ
ð8Þ
The heat transfer equation is as follows:
Q ¼ hmd ADims
ð9Þ
The mass transfer equation is as follows:
w ¼ hmd ADdms
ð10Þ
Similar to the formula for calculating the heat transfer coefficient for the cooling tower, the formula for the mass transfer coefficient is as follows:
hmd ¼
hg C pf
ð11Þ
3.3. Thermal properties of the fuel gas The thermal properties of the flue gas are important for calculations. We are concerned with the dew point temperature and
4. Performance analysis of boiler with NCHR system This section introduces the system performance of the condensing boiler and AHPB system. Then, system performance under typical conditions is analyzed, and a performance comparison between different systems is conducted.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 3. The relationship of flue gas temperature with absolute moisture content and enthalpy.
Table 1 Experimental conditions of the spray water heat exchanger. Inlet air temperature
Inlet air relative humidity
Mass flow of spraying water
Inlet water temperature
Size of filler
31.5 °C
77 ± 3%
0.57 kg/s
36 ± 0.4 °C
580 mm 580 mm 600 mm
Table 2 Model validation of the spray water heat exchanger.
Experiment Simulation Relative error
Inlet water temperature (°C)
Outlet water temperature (°C)
Inlet air temperature (°C)
Inlet air dew point (°C)
Inlet air enthalpy (kJ/kg)
Outlet air enthalpy (kJ/kg)
Heat transfer rate (kW)
36.0 36.0
33.1 32.9
31.5 31.5
28.0 28.0
89.87 89.87
105.82 105.0 0.8%
7.66 7.43 0.3%
4.1. Model of condensing boiler and AHPB system The performance of a boiler with an NCHR system is compared to that of a condensing boiler with an AHPB system. The mathematical model for the condensing boiler is similar to that for the NCHR system, except for the heat and mass transfer model. The oxidizing air is only been heated, and the dew point temperature of the flue gas is influenced by the humidity ratio of the oxidizing air. The vapor in the flue gas will be condensed in the condensing heat exchanger, and the boiler will reach an efficiency of up to 97%. For the AHPB system, the efficiency is considered a constant [21]. The AHP is driven by natural gas and generates chilled water, which is used to recover the flue gas. The flue gas includes emissions from the boiler and AHP, and the recovered heat is used to heat the return water in the AHP. The boiler efficiency is much higher than that of the condensing boiler because the chilled water temperature is much lower than the return water, so the temperature of the emission flue gas is lowered to 30 °C. Additionally, the heat of vapor condensation is utilized, and the boiler efficiency is improved. 4.2. System performance under steady conditions When the inlet oxidizing air temperature is 0 °C and the relative humidity is 40%, the temperature and relative humidity of the oxidizing air and the temperature of the flue gas are shown in Fig. 4. After passing through the sensible heat exchanger, the tempera-
ture rises from 0 °C to 20.8 °C, but the relative humidity decreases, because the humidity ratio is the same. In this process, the temperature of the flue gas decreases from 30.2 °C to 25.7 °C, and vapor is condensed in the wall type heat exchanger. The temperature of the emission flue gas is 25.7 °C. Then, the oxidizing air temperature rises to 28.3 °C, 38.1 °C, and 44.6 °C after the heat exchange process in the spray water heat exchanger, and the outlet relative humidity nears 100%. Since the inlet flue gas is saturated after passing through the condensing heat exchanger, the vapor in the flue gas will be condensed during the heat transfer process in each wall type heat exchanger. After humidifying the oxidizing air, the dew point of the flue gas is 63.5 °C. If the oxidizing air is not humidified in the process, the system behaves like a condensing boiler. The temperature of the oxidizing air entering the condensing boiler will be higher, but the relative humidity will be extremely low. Moreover, the temperature of the emission flue gas will be high, at 50.1 °C. Additionally, the boiler efficiency will be influenced by the decreasing heat recovery from the flue gas. The dew point temperature in the NCHR system is much higher than that in the condensing boiler owing to the humidification process, which is 7.7 °C higher than that of the condensing boiler (the detailed comparison is shown in Table 3). 4.3. System performance in different working conditions The relationship of boiler efficiency with different inlet oxidizing air temperatures and relative humidity is shown in Fig. 5.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 4. System performance under steady conditions.
Table 3 Comparison between the NCHR system and the condensing boiler. Condensing boiler
NCHR system
Flue gas
Inlet temperature (°C) Emission temperature (°C) Dew point temperature (°C)
150 50.1 55.8
150 25.7 63.5
Oxidizing air
Inlet temperature (°C) Outlet temperature (°C) Outlet enthalpy (kJ/kg)
0 45 45.8
0 44.6 207.8
Boiler efficiency
%
97.5
103.4
The boiler efficiency increases with increasing inlet oxidizing air temperature when the relative humidity is the same. The main reason for this is that the humidity ratio is higher when the inlet temperature is higher, so the dew point temperature will be higher. Therefore, the recovered heat in the condensing heat exchanger is higher. When the inlet oxidizing air temperature is the same, the boiler efficiency increases with increases in relative humidity.
When the inlet oxidizing air temperature is 5 °C, the humidity ratio and the humidification amount of oxidizing air after heat exchange between the flue gas and oxidizing air is shown in Fig. 6. It is shown that the dew point temperature increases with increasing relative humidity, although the humidification amount decreases as relative humidity increases. At the same time, the temperature of the emission flue gas changes slightly (shown in Fig. 7), so that the boiler efficiency is higher when more latent heat is released in the condensing boiler. When the temperature and relative humidity are different, the final emission temperature of flue gas is as shown in Fig. 7. It can be seen that the temperature of the emission flue gas increases with increasing inlet oxidizing air temperature when the relative humidity of the inlet oxidizing air is the same. The reason for this is that the temperature difference between the flue gas and inlet oxidizing air decreases when the inlet oxidizing air temperature rises. However, the boiler efficiency increases because the humidity ratio of the flue gas is larger for higher inlet temperatures, and the amount of vapor condensation is larger. When the inlet oxidizing air temperature is the same, the temperature of the emission flue gas increases with increasing relative
Fig. 5. Relationship of boiler efficiency with different oxidizing air temperatures and relative humidity ratio.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 6. Relationship of humidity ratio and humidification amount with the increasing relative humidity of the inlet oxidizing air (T = 5 °C).
Fig. 7. Relationship of flue gas temperature with different inlet oxidizing air temperatures and relative humidity ratio.
humidity. The main reason for this is that the dew point temperature and temperature of the inlet oxidizing air, after humidification, increase with the increasing relative humidity of the inlet oxidizing air, which results in increases to the emission temperature of the flue gas. 4.4. System performance comparison A system performance comparison between the proposed condensing boiler system and the AHPB system is shown in Fig. 8. It can be seen that the efficiency of the boiler with the NCHR system is much higher than the efficiency of the conventional condensing boiler. The efficiency increases with increasing inlet oxidizing air temperature and is 5.3% higher than that of the condensing boiler. The condensing boiler typically uses the return water to recover waste heat from the flue gas, and the heat recovery is affected by the humidity ratio of the inlet oxidizing air. Therefore, the dew point of the flue gas in the condensing boiler will be higher for
higher inlet oxidizing air temperatures when the relative humidity is the same. Therefore, the dew point temperature of the flue gas in the condensing boiler will be much lower than that in the proposed system. Furthermore, the temperature of the emission flue gas will be higher because the heat transfer process is between the flue gas and return water, and the temperature of the emission flue gas is limited to approximately 50 °C (given in Table 3). Compared to the AHPB system, the efficiency of the boiler with the NCHR system is slightly lower when the inlet oxidizing air temperature is lower than 19 °C (RH = 50%). Although the boiler efficiency of this system is lower, it is only 0.3% lower, and is almost the same as that of the AHPB system. When the temperature of the inlet oxidizing air is higher or the relative humidity is higher, the boiler efficiency of the NCHR system is higher than that of the AHPB system. The reason for this is that the humidity ratio in the flue gas will be higher and the vapor condensation heat will increase. The boiler efficiency of the NCHR system will be higher than that of the AHPB system when the vapor condensation heat
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 8. System performance comparison with the condensing boiler.
Li 3600 LCV ei
of the NCHR system is larger than the heat transfer rate in the evaporator of the AHP.
G¼
5. Case study of annual performance
The heating period in Beijing runs from Nov 15 to Mar 15 of the following year. The year-round energy consumption Gy is obtained by the following equation:
5.1. Building load and calculating method 5.1.1. Building load information An office building in Beijing with a total area of 1050 m2 was chosen. The building envelope information and occupant density are listed in Table 4. The building load information is shown in Fig. 9(a). The red portion of the figure represents the heating load, and the blue portion represents the cooling load. The peak heating load in Beijing is 80 kW. The hourly ambient temperature and humidity ratio are given in Fig. 9(b). In this section, we focus on the heating load; the boiler is only used in winter. 5.1.2. Calculating method The return water temperature is taken as 45 °C. The efficiency of the traditional boiler is 90% [27], and the efficiency of the AHPB system is 103.6% [20]. The boiler efficiency for the condensing boiler and this system is calculated considering the hourly ambient temperature and relative humidity. Because of the different dew point temperatures, heat transfer process in the condensing boiler is different, and the boiler efficiency changes every hour. To compare system performance and conduct a tech-economic analysis, four systems are operated in the same building in Beijing: a traditional boiler, condensing boiler, AHPB system, and the novel NCHR system. Then, the energy consumption of each system can be calculated from the hourly building load. The hourly energy consumption G is calculated by the following equation:
Table 4 Building envelope information and occupant density. Wall (W/(m2 °C))
Window (W/(m2 °C))
Floor (W/(m2 °C))
Occupant density (p/m2)
0.622
5.7
3.055
0.15
Gy ¼
1 776 X i¼1
8 760 X Li 3600 Li 3600 þ LCV ei LCV ei i¼6913
ð17Þ
ð18Þ
After calculating the hourly energy consumption and integrating over the whole year, the energy saving ratio can be calculated by
ESR ¼
Gy Gytb Gytb
ð19Þ
The operation cost for the next ten years can be calculated using the following equation:
O¼CþZy
ð20Þ
The payback period is calculated using the following equation [28]:
PB ¼
C C tb Y tb Y
ð21Þ
5.2. Energy saving potential analysis The hourly performance of different systems is shown in Fig. 10. As shown in Fig. 10, the boiler efficiencies of the AHPB and NCHR systems are much higher than those of traditional condensing boilers. The efficiency of the condensing boiler and novel system changes with changing climate parameters. The efficiency of the AHPB system is the highest among all the systems. The average efficiencies of the traditional boiler, condensing boiler, AHPB system, and novel NCHR system are 90%, 97.4%, 103.6% and 103.4%, respectively. Although the efficiency of the AHPB system is the highest among these, the NCHR boiler system has an efficiency very close to that of the AHPB system. The gas consumption for a year of running each system is shown in Fig. 11. The gas consumption of a traditional boiler is
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 9. Building load and hourly climate information.
Fig. 10. Hourly boiler efficiency of each system.
Fig. 11. Year-round gas consumption of each system.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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8697.5 m3. The gas consumption of the condensing boiler is about 600 m3 less than that of the traditional boiler. The AHPB system saved the most energy due owing its high efficiency; the energy saving ratio compared to the traditional boiler is 13.13%. The gas consumption of the NCHR boiler system was close to that of the AHPB system, with an energy saving ratio of 12.97%. Detailed information on gas consumption and energy saving potential is given in Table 5. The boiler with the NCHR system can reduce natural gas consumption and decrease pollutant emissions.
the NCHR system includes three parts, and the total cost is 21634.5 CNY. The highest initial cost is that of the absorption heat pump, whose unit price is 600 CNY/kW [19]. Therefore, the total investment required for the boiler with an AHPB system is 26,160 CNY. The yearly gas consumption of each system is shown in Fig. 11. The yearly operational cost can be obtained from the unit price of natural gas (2.4 CNY/m3) [29]. The operational cost for the next ten years can be calculated from Eq. (20). The results are shown in Fig. 12. It can be seen that the traditional boiler has the lowest initial cost; however, the total operational cost of the NCHR system is lower than that of the traditional boiler after the first year. Although the AHPB system has the highest efficiency, its operational cost after ten years is still higher than that of the NCHR system owing to the higher initial investment. Compared to the traditional boiler, the payback period of the AHPB system is three years, while it is only one year for the NCHR
5.3. Economic analysis The initial investments in each system and unit price of each component are given in Table 6. The rated heating capacity of the boiler is 90 kW, and the total price is 18,000 CNY. The rated heat exchange capacity of the condensation heat exchanger is 7.5 kW, and its unit price is 120 CNY/kW [26]. The initial cost of
Table 5 Year-round gas consumption of each system and the energy saving ratio. Types
Traditional boiler
Condensing boiler
AHPB
NCHR
Year-round gas consumption (m3) Energy saving ratio (%)
8697.51 0
8029.96 7.68
7555.75 13.13
7569.27 12.97
Table 6 Initial investment of each system. Traditional boiler
Condensing boiler
AHP for heat recovery
Non-contact total heat exchanger
Boiler
Capacity (kW) Unit price (CNY/kW)
90 200
90
90
90
AHP
Capacity (kW) Unit price (CNY/kW)
– 600
–
13.6
–
Condensation heat exchanger
Capacity (kW) Unit price (CNY/kW)
– 120
7.5
–
20.93
Spray water heat exchanger
Capacity (m3/h) Unit price (CNY/m3/h)
– 800
–
–
1.4
18,000
18,900
26,160
21634.5
Total price (CNY)
Fig. 12. Ten-year operation cost of each system.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
S. Shang et al. / Applied Energy xxx (2017) xxx–xxx
system. Compared to the condensing boiler, the payback period of the NCHR system is only three years. However, the payback period of the AHPB system is over seven years. The operational cost of the NCHR system for ten years is less than that of the AHPB system; the NCHR system has obvious economic benefits. 6. Conclusion To improve the system performance of a gas-fired boiler, an NCHR system is proposed for heating and humidifying oxidizing air with waste heat from flue gas. A mathematic model is established to calculate system performance. The system performance is compared with other systems. Finally, a case study in Beijing is used to investigate the energy saving potential and economic feasibility. The main conclusions from the above analysis are as follows: 1. Using the NCHR system, boiler efficiency can be promoted to 103.4% for an oxidizing air temperature of 0 °C. Boiler efficiency increases with increasing inlet oxidizing air temperature and relative humidity. 2. The energy saving potential for a year when running the NCHR system is 12.97% higher compared to a traditional boiler. The NCHR and AHPB systems showed very similar results. 3. For a system running continuously, the payback period of the NCHR system is one year relative to the traditional boiler and three years relative to the condensing boiler. The operational cost for ten years is less than that of the AHPB system; therefore, the NCHR system has obvious economic benefits.
Acknowledgments The authors gratefully acknowledge the support of the Innovative Research Groups of the National Natural Science Foundation of China (grant number 51521005). References [1] Tsinghua University Building Energy Saving Research Center. Annual Report on China Building Energy Efficiency. Beijing: China Architecture and Building Press; 2013 [in Chinese]. [2] Jiang Y. Heating schemes analysis for medium and large cities in the North China. Hv & Ac 2000. [3] Han WG, Jiang Y. Energy consumption analysis of several kinds of heating systems. Hv & Ac 2005. [4] Yu CL. Discussions on different distract heating sources. Energy Conserv Technol 2010. [5] Yang Z, Cheng H, Wu X, et al. Research on improving energy efficiency and the annual distributing structure in electricity and gas consumption by extending use of GEHP. Energy Policy 2011;39(9):5192–202.
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[6] Zaporowski Bolesław, Szczerbowski Radosław. Energy analysis of technological systems of natural gas fired combined heat-and-power plants. Appl Energy 2003;75(1–2):43–50. [7] Zhang XP, Lin Q. Discussion on recovery of residual heat from flue gas of boiler. Build Sci 2008. [8] Xu G, Huang S, Yang Y, et al. Techno-economic analysis and optimization of the heat recovery of utility boiler flue gas. Appl Energy 2013;112(4):907–17. [9] Suntivarakorn R, Treedet W. Improvement of boiler’s efficiency using heat recovery and automatic combustion control system. Energy Procedia 2016;100:193–7. [10] Bai CL. Discussion on the gas-fired boiler energy saver -flue gas heat recovery technology. Appl Energy Technol 2012. [11] Li Y, Yan M, Zhang L, et al. Method of flash evaporation and condensation – heat pump for deep cooling of coal-fired power plant flue gas: latent heat and water recovery. Appl Energy 2016;172:107–17. [12] Wei M, Zhao X, Fu L, et al. Performance study and application of new coal-fired boiler flue gas heat recovery system. Appl Energy 2017;188:121–9. [13] Pan Z, Niu J, Lu CP, et al. Study on thermal efficiency of gas-fired vacuum hot water boiler with installing flue gas condensing waste-heat exchanger. In: Bioinformatics and biomedical engineering (iCBBE), 2010 4th international conference on. IEEE; 2010. p. 1–4. [14] Weber C, Gebhardt B, Fahl U. Market transformation for energy efficient technologies – success factors and empirical evidence for gas condensing boilers. Energy 2002;27(3):287–315. [15] Wang SL, Zhang JF, Wen Z, et al. Forced convective heat transfer of flue gas on anticorrosion plane surface with condensation. In: Annals of the assembly for international heat transfer conference; 2006. [16] Wang SL, Wu YY, Pan SY, et al. Impact of the anticorrosion films on the condensing heat transfer of flue gas in fin-tube heat exchangers. In: International heat transfer conference; 2010. p. 153–9. [17] Pezzuolo A, Benato A, Stoppato A, et al. Fluid selection and plant configuration of an ORC-biomass fed system generating heat and/or power. Energy Procedia 2016;101:822–9. [18] Pezzuolo A, Benato A, Stoppato A, et al. The ORC-PD: a versatile tool for fluid selection and Organic Rankine Cycle unit design. Energy 2016;102:605–20. [19] Che D, Liu Y, Gao C. Evaluation of retrofitting a conventional natural gas fired boiler into a condensing boiler. Energy Convers Manage 2004;45(20):3251–66. [20] Zhu K, Xia J, Xie X, et al. Total heat recovery of gas boiler by absorption heat pump and direct-contact heat exchanger. Appl Therm Eng 2014;71(71):213–8. [21] Fu L, Tian G, Sui J, et al. Combining absorption heat pump with gas boiler for exhaust condensing heat recovery in district heating system. Acta Energiae Solaris Sinica 2003;24(5):620–4. [22] Qu M, Abdelaziz O, Yin H. New configurations of a heat recovery absorption heat pump integrated with a natural gas boiler for boiler efficiency improvement. Energy Convers Manage 2014;87:175–84. [23] Brückner S, Liu S, Miró L, et al. Industrial waste heat recovery technologies: an economic analysis of heat transformation technologies. Appl Energy 2015;151 (3Suppl):157–67. [24] Khodabandeh E, Pourramezan M, Pakravan MH. Effects of excess air and preheating on the flow pattern and efficiency of the radiative section of a fired heater. Appl Therm Eng 2016;105:537–48. [25] Kreith F, Manglik RM, Bohn MS. Principles of heat transfer. Principles of heat transfer. Harper & Row; 1986:B100. [26] Huang K. Experimental study on the performance and optimization of packing. Donghua University; 2014. [27] Cheng BW, Qi D, Wei W, et al. Existent problems of gas boiler heating and application of energy saving techniques to boiler plants. Hv & Ac 2005. [28] Wu W, Li X, Wang B, et al. Tech-economy analysis of low temperature hot water system based on air source absorption heat pump. ECM 2013;48 (26):317–24. [29] Li X, Wu W, Zhang X, et al. Energy saving potential of low temperature hot water system based on air source absorption heat pump. Appl Therm Eng 2012;48(26):317–24.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger q Sheng Shang, Xianting Li ⇑, Wei Chen, Baolong Wang, Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
h i g h l i g h t s A novel non-contact total heat recovery system of gas-fired boiler is proposed. The oxidizing air is heated and humidified by recovering total heat of flue gas. The temperature of boiler flue gas after heat recovery is about 30 °C in winter. The energy saving potential is close to the system using AHP for heat recovery. The initial investment is so low that the payback period is shorter than 1 year.
a r t i c l e
i n f o
Article history: Received 28 January 2017 Received in revised form 17 May 2017 Accepted 26 May 2017 Available online xxxx Keywords: Gas-fired boiler Total heat recovery Flue gas Non-contact heat exchanger Tech-economic analysis
a b s t r a c t Recovering heat from the flue gas of a gas-fired boiler can both improve boiler efficiency and decrease pollutant emissions. To improve the efficiency of the gas-fired boiler in a more cost effective and higher efficient way, a non-contact total heat recovery (NCHR) system is proposed for recovering heat from flue gas for use in heating and humidifying the oxidizing air of the boiler. A mathematical model of a boiler with an NCHR system was established, and the performance of the NCHR system was compared with that of other heat recovery systems. It is shown that the efficiency of a boiler with an NCHR system can reach 103.4% for an inlet oxidizing air temperature of 0 °C, which is 13.4% higher than the efficiency of a traditional boiler. According to the case study, the energy saving potential of a boiler with an NCHR system is 12.97% compared to that of a traditional boiler. As for the economic analysis, the payback period of a boiler with an NCHR system to traditional boiler and the condensing boiler is 1 year and 3 years, respectively. In addition, the operation cost of an NCHR system is less than that of a boiler with an absorption heat pump for heat recovery (AHPB) system, indicating that the NCHR system has obvious economic benefits. Ó 2017 Published by Elsevier Ltd.
1. Introduction Building energy consumption in China has increased rapidly over the past three years, reaching over 7.56 billion tce (tons of coal equivalent) in 2013 [1], which equates to approximately 24% of primary energy consumption [1]. Among all types of energy consumption, coal-fired boilers account for about 85% of the entire heating and power generation process [2]. However, the primary energy efficiency of a coal-fired boiler in terms of heating is approximately 55–75% [3], which is lower than that of a gas-
q The short version of the paper was presented at ICAE2016 on Oct 8–11, Beijing, China. This paper is a substantial extension of the short version of the conference paper. ⇑ Corresponding author. E-mail address: [email protected] (X. Li).
fired boiler which typically has an efficiency of above 90% [4]. Additionally, the coal-fired boiler emits more pollutants (particles and poisonous gas) than the gas-fired boiler [5,6]. Therefore, the use of gas-fired boilers has increased rapidly in recent years because of the environmental protection efforts and policies of the Chinese government. The energy efficiency of a gas-fired boiler is approximately 90%, and the temperature of the emission flue gas is approximately 150–200 °C [7,8]. Significant energy is wasted if the hightemperature flue gas is discharged directly. Therefore, much recent research has focused on recovering heat from the emission flue gas of boilers. The methods for heat recovery of the flue gas can be classified into three types according to the heat recovery depth. The first method is sensible heat recovery, where flue gas is used to preheat oxidizing air and return water [9]. The configuration is simple, but the temperature of the emission fuel gas can reach
http://dx.doi.org/10.1016/j.apenergy.2017.05.169 0306-2619/Ó 2017 Published by Elsevier Ltd.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Nomenclature A C Cp d G Gy h i L LCV m Nu O P PB Pr Q r Re Rg RH T Dtm v w y Z
heat transfer area, m2 initial investment, CNY specific heat capacity, kJ/(kg K) humidity ratio, kg/kg hourly gas consumption, m3/h year-round gas consumption, m3 convective heat transfer coefficient, W/(m2 °C) enthalpy, kJ/kg building load, kW low caloric value, 35,000 kJ/m3 mass flow rate, kg/s Nusselt number operation cost, CNY pressure, kPa payback period Prandtl number heating capacity, kW latent heat of vaporization Reynolds number ideal gas constant relative humidity temperature, °C logic mean temperature difference, °C volume flow rate, m3/s mass transfer rate, kg/s running cost per year running cost each year, CNY/a
100 °C. The boiler efficiency is approximately 93% because only part of the sensible heat of the flue gas is recovered [10]. However, large amounts of latent heat (which can significantly promote boiler efficiency if recovered) will be wasted [11,12]. The second boiler heat recovery method involves a condensing heat recovery system, where the emission flue gas temperature can be decreased below the dew point, and the boiler efficiency can reach 98% [13]. Weber et al. analyzed the feasibility of recovering waste heat from flue gas using a condensing heat exchanger, and the results showed that boiler efficiency could be significantly improved, and CO2 emissions decreased [14]. However, because of acid gas, the heat exchanger can be corroded. Therefore, the anticorrosion properties of the materials used in the heat exchanger are very important for promoting boiler efficiency. Wang et al. investigated the system performance and application of a corrosion resistant heat exchanger. Their results showed that the energy saving ratio of the system could be increased by 10% by using condensing heat recovery [15,16]. Pezzuolo et al. investigated the heat recovery for biomass boiler using the ORC cycle [17,18], and the heat recovery method is similar but the recovered heat is used to generate power. From the abovementioned methods, the emission flue gas temperature is over 55 °C. If the flue temperature can be decreased to 25–35 °C, the boiler efficiency can be increased by over 12% [19]. Therefore, absorption heat pumps (AHPs) have been used for the heat recovery of the flue gas, which is the third heat recovery method. The AHP is driven by natural gas, and the flue gas is the heat source for the evaporator. Simultaneously, the recovered energy is used to heat the return water. The boiler efficiency reaches approximately 104%, and the emission flue gas temperature can be reduced to about 30 °C. Zhu et al. analyzed the performance of AHP heat recovery in a real case and found that boiler efficiency could be increased to 13.6%, with a flue gas temperature of 30 °C [20]. Fu et al. simulated an absorption heat pump heat recovery system and found that its efficiency could be increased by 11% relative to that of the condensing heat recovery system [21]. Qu et al.
Greeks b d
e g k
q
efficiency of the conductive heat transfer thickness, m boiler efficiency rib effect coefficient conduction heat transfer coefficient, W/(m °C) density, kg/m3
Subscripts a air f flue gas g gas gas natural gas i hourly system performance md convective mass transfer rate, kg/s re chemical reaction rhc rated heat capacity sw heat transfer medium tb traditional boiler w water Abbreviations AHP absorption heat pump AHPB boiler using AHP for heat recovery ESR energy saving ratio NCHR non-contact heat recovery system
integrated an AHP with natural gas-fired boilers to improve boiler efficiency; their results showed that efficiency could be increased by over 10% [22]. However, initial investment is high due to the AHP [23]. From the above mentioned methods, the low temperature oxidizing air isn’t well utilized. If the total heat of the flue gas can be recovered by the oxidizing air, in which the oxidizing air is heated and humidified by the flue gas without direct contact, higher heat recovery efficiency with lower costs can be achieved. To improve the boiler efficiency in a more cost effective and higher efficient way, this study proposes a total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. In order to analyze the heat recovery performance of the proposed system, a mathematical model is established. Then, the boiler efficiency of different methods is compared. To investigate the energy saving potential and payback period of this system, technical and economic analyses are conducted for a case study in Beijing.
2. Working principle of total heat recovery of the flue gas A traditional boiler uses a direct heat exchanger. Its flue gas can only be used to increase the temperature of oxidizing air or return water. The heat transfer process is shown as 1–20 in Fig. 1. If the humidity ratio of the oxidizing air can be increased to point 3, the enthalpy difference between point 1 and 3 will be much greater than that between points 1 and 20 for the same temperature difference. In this case, the temperature of the emission flue gas will be close to the ambient temperature. Therefore, this kind of heat transfer process typically saves more energy. As the moisture in the flue gas cannot be directly recovered to the oxidizing air because of corrosion, direct total heat recovery cannot be used in this situation. To achieve the heat transfer process of 1–3 in Fig. 1, a novel non-contact total heat recovery
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 1. Psychrometric chart for heat transfer process.
(NCHR) heat exchanger is proposed. Its configuration is shown in Fig. 2. The NCHR system contains three components. The first is the condensing heat exchanger, where the flue gas heats the return water and the vapor in the flue gas will be condensed. The second component is the total heat recovery heat exchanger, which contains the wall type heat exchanger and the spray water heat exchanger. Here, the spray water absorbs energy from the flue gas and the temperature of the water increases. Then, the oxidizing air is heated and humidified by the heated spray water. The spray water has no direct contact with the flue gas, so that the oxidizing air won’t be polluted by the flue gas. A circulation pump is installed in each stage to realize this water cycle. The third component is the sensible heat exchanger. The main function of this heat exchanger is to preheat the oxidizing air when the ambient temperature is too low. The heated and humidified air enters the boiler and is combusted with natural gas, resulting in the increase of the dew point of the flue gas. After the combustion process, the flue gas will then pass through the condensing heat exchanger and heat the return water. Generally, the dew point temperature of the flue gas at this point is high enough for the vapor in the flue gas to be condensed after the heat exchange process with the return water. Then, the return water will be heated in the boiler.
3. Mathematical model of boiler with total heat recovery of flue gas The mathematical model contains three parts, including a gasfired boiler model, a heat and mass transfer model, and the thermal properties of the flue gas. The temperature and mass flow rate of the flue gas are given by the gas-fired boiler model, while the heat and mass transfer model calculates the state point of each heat exchanger inlet and outlet. The thermal properties of the flue gas include the enthalpy and dew point of the flue gas, which considers the humidity ratio of the humidified air and the combustion process. 3.1. Model of gas-fired boiler The model of the gas-fired boiler primarily contains the temperature and mass flow rate of the flue gas. The rated heating capacity is 90 kW, and the temperatures of the return water and supply water are 45 °C and 60 °C, respectively. Gas consumption is calculated by the following equation:
V gas ¼
Q rhc LCV e
ð1Þ
Condensing heat exchanger Wall type heat exchanger
Wall type heat exchanger
Wall type heat exchanger
emission of flue gas
Flue gas
To boiler
Sensible heat exchanger
Return water Spray water heat exchanger
Spray water heat exchanger
Spray water heat exchanger
oxidizing air
Oxidizing air to boiler P12
Non-contact total heat exchanger Fig. 2. Working principle and structure of the novel NCHR system.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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The theoretical chemical oxygen demand can be obtained from the chemical reaction. The excess air coefficient is taken as 1.2 [24]. Therefore, the air mass flow rate can be calculated. The mass flow rate of the flue gas is the sum of the flow rates of natural gas and air. The temperature of the flue gas is set to 150 °C [10]. 3.2. Heat and mass transfer model There are two types of heat exchangers in the proposed device: a wall type heat exchanger and a direct contact heat exchanger. The models for the two types of heat exchangers are developed first, and then the whole model is built by transmitting parameters between the two heat exchangers. The wall type heat exchanger, condensing heat exchanger, and sensible heat exchanger are considered, and the flue gas and air (or water) is regarded as counter flow. A lumped parameter method is used to calculate the performance of the heat exchanger, and the control equations include the energy conservation equation and heat transfer equation. The equation of energy conservation is as follows:
mg ðicgin icgout Þ ¼ mcw C pw ðtcwout t cwin Þ
ð2Þ
The heat transfer equation is as follows:
Q ¼ hADt m
ð3Þ
The calculation of heat transfer coefficient hg considers the convective heat transfer coefficient of the flue gas, the thermal conductivity of the coil, and the convective heat transfer coefficient of the pipe. The detailed calculation formula is as follows:
hg ¼
1 1 hf g
þ bd þ hbw k
ð4Þ
The convective heat transfer coefficients of the inner-wall of the pipe and gas side are calculated by the dimensionless expressions Nuw and Nuf, respectively [25]:
enthalpy after the absolute moisture content changes. This section introduces the calculation method. The saturated vapor pressure is related only to the flue gas temperature at the same atmospheric pressure. The absolute moisture content can be calculated from the chemical reaction of the combustion of the methane and the moisture content in the oxidizing air. The equation is as follows:
mf ¼ mre þ ma
ð12Þ
Methane (CH4) is the main component of natural gas, so the moisture generation during the chemical reaction can be calculated by the following equation:
mre ¼ 2 mg
ð13Þ
The moisture content of air is calculated as follows:
ma ¼ 2V a q da
ð14Þ
Vapor pressure can be calculated using the ideal gas state equation:
P¼
mf Rg T V
ð15Þ
Then, the dew point temperature of the flue gas can be calculated from the vapor pressure, because the saturated vapor pressure is a single value function of the temperature. The enthalpy of the flue gas consists of sensible heat and latent heat, and the equation is as follows:
hf ¼ cpf T þ r mf
ð16Þ
If the temperature of the oxidizing air entering the boiler is 45 °C and the relative humidity ratio is 90%, the relationship of dew point temperature and enthalpy (for different flue gas temperatures) is shown in Fig. 3.
n Nuw ¼ 0:023 Re0:8 w Pr w
ð5Þ
3.4. Model validation
Pr 0:35 ðPr f =Prw Þ0:25 Nuf ¼ 0:71Re0:5 f f
ð6Þ
The accuracy of the entire system is primarily dependent on the accuracy of the wall type heat exchanger model, the spray water heat exchanger, and the thermal properties of the flue gas. The wall type heat exchanger model is relatively simple and very mature. The thermal properties of the flue gas are calculated based on both the thermal properties of components in the flue gas and Dalton’s partial pressure law, which are relatively accurate and whose errors can be ignored. The most complicated part of the whole system is the spray water heat exchanger model, which couples heat and mass transfer. Therefore, the spray water heat exchanger model is validated in this section. The validation of the spray water heat exchanger model is based on an experiment [26]. The outlet air enthalpy and heat transfer rate will be compared in the experiment, and the relative error will be calculated. The experimental conditions are given in Table 1. The relative errors for the outlet air enthalpy and heat transfer rate are calculated, and the results are given in Table 2. According to the results, the maximum relative errors for the outlet air enthalpy and heat transfer rate are within 1%. The simulation results significantly match the experimental results. Therefore, the accuracy of the model is acceptable.
In the spray water heat exchanger, air and water are in direct contact; hence, heat and mass are simultaneously exchanged. Therefore, the equations considered in this process include the energy conservation equation, mass conservation equation, heat transfer equation, and mass transfer equation. The energy conservation equation is as follows:
ma ðisaout isain Þ ¼ msw C pw ðtswin tswout Þ
ð7Þ
The mass conservation equation is as follows:
wr ¼ ma ðdsaout dsain Þ
ð8Þ
The heat transfer equation is as follows:
Q ¼ hmd ADims
ð9Þ
The mass transfer equation is as follows:
w ¼ hmd ADdms
ð10Þ
Similar to the formula for calculating the heat transfer coefficient for the cooling tower, the formula for the mass transfer coefficient is as follows:
hmd ¼
hg C pf
ð11Þ
3.3. Thermal properties of the fuel gas The thermal properties of the flue gas are important for calculations. We are concerned with the dew point temperature and
4. Performance analysis of boiler with NCHR system This section introduces the system performance of the condensing boiler and AHPB system. Then, system performance under typical conditions is analyzed, and a performance comparison between different systems is conducted.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 3. The relationship of flue gas temperature with absolute moisture content and enthalpy.
Table 1 Experimental conditions of the spray water heat exchanger. Inlet air temperature
Inlet air relative humidity
Mass flow of spraying water
Inlet water temperature
Size of filler
31.5 °C
77 ± 3%
0.57 kg/s
36 ± 0.4 °C
580 mm 580 mm 600 mm
Table 2 Model validation of the spray water heat exchanger.
Experiment Simulation Relative error
Inlet water temperature (°C)
Outlet water temperature (°C)
Inlet air temperature (°C)
Inlet air dew point (°C)
Inlet air enthalpy (kJ/kg)
Outlet air enthalpy (kJ/kg)
Heat transfer rate (kW)
36.0 36.0
33.1 32.9
31.5 31.5
28.0 28.0
89.87 89.87
105.82 105.0 0.8%
7.66 7.43 0.3%
4.1. Model of condensing boiler and AHPB system The performance of a boiler with an NCHR system is compared to that of a condensing boiler with an AHPB system. The mathematical model for the condensing boiler is similar to that for the NCHR system, except for the heat and mass transfer model. The oxidizing air is only been heated, and the dew point temperature of the flue gas is influenced by the humidity ratio of the oxidizing air. The vapor in the flue gas will be condensed in the condensing heat exchanger, and the boiler will reach an efficiency of up to 97%. For the AHPB system, the efficiency is considered a constant [21]. The AHP is driven by natural gas and generates chilled water, which is used to recover the flue gas. The flue gas includes emissions from the boiler and AHP, and the recovered heat is used to heat the return water in the AHP. The boiler efficiency is much higher than that of the condensing boiler because the chilled water temperature is much lower than the return water, so the temperature of the emission flue gas is lowered to 30 °C. Additionally, the heat of vapor condensation is utilized, and the boiler efficiency is improved. 4.2. System performance under steady conditions When the inlet oxidizing air temperature is 0 °C and the relative humidity is 40%, the temperature and relative humidity of the oxidizing air and the temperature of the flue gas are shown in Fig. 4. After passing through the sensible heat exchanger, the tempera-
ture rises from 0 °C to 20.8 °C, but the relative humidity decreases, because the humidity ratio is the same. In this process, the temperature of the flue gas decreases from 30.2 °C to 25.7 °C, and vapor is condensed in the wall type heat exchanger. The temperature of the emission flue gas is 25.7 °C. Then, the oxidizing air temperature rises to 28.3 °C, 38.1 °C, and 44.6 °C after the heat exchange process in the spray water heat exchanger, and the outlet relative humidity nears 100%. Since the inlet flue gas is saturated after passing through the condensing heat exchanger, the vapor in the flue gas will be condensed during the heat transfer process in each wall type heat exchanger. After humidifying the oxidizing air, the dew point of the flue gas is 63.5 °C. If the oxidizing air is not humidified in the process, the system behaves like a condensing boiler. The temperature of the oxidizing air entering the condensing boiler will be higher, but the relative humidity will be extremely low. Moreover, the temperature of the emission flue gas will be high, at 50.1 °C. Additionally, the boiler efficiency will be influenced by the decreasing heat recovery from the flue gas. The dew point temperature in the NCHR system is much higher than that in the condensing boiler owing to the humidification process, which is 7.7 °C higher than that of the condensing boiler (the detailed comparison is shown in Table 3). 4.3. System performance in different working conditions The relationship of boiler efficiency with different inlet oxidizing air temperatures and relative humidity is shown in Fig. 5.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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Fig. 4. System performance under steady conditions.
Table 3 Comparison between the NCHR system and the condensing boiler. Condensing boiler
NCHR system
Flue gas
Inlet temperature (°C) Emission temperature (°C) Dew point temperature (°C)
150 50.1 55.8
150 25.7 63.5
Oxidizing air
Inlet temperature (°C) Outlet temperature (°C) Outlet enthalpy (kJ/kg)
0 45 45.8
0 44.6 207.8
Boiler efficiency
%
97.5
103.4
The boiler efficiency increases with increasing inlet oxidizing air temperature when the relative humidity is the same. The main reason for this is that the humidity ratio is higher when the inlet temperature is higher, so the dew point temperature will be higher. Therefore, the recovered heat in the condensing heat exchanger is higher. When the inlet oxidizing air temperature is the same, the boiler efficiency increases with increases in relative humidity.
When the inlet oxidizing air temperature is 5 °C, the humidity ratio and the humidification amount of oxidizing air after heat exchange between the flue gas and oxidizing air is shown in Fig. 6. It is shown that the dew point temperature increases with increasing relative humidity, although the humidification amount decreases as relative humidity increases. At the same time, the temperature of the emission flue gas changes slightly (shown in Fig. 7), so that the boiler efficiency is higher when more latent heat is released in the condensing boiler. When the temperature and relative humidity are different, the final emission temperature of flue gas is as shown in Fig. 7. It can be seen that the temperature of the emission flue gas increases with increasing inlet oxidizing air temperature when the relative humidity of the inlet oxidizing air is the same. The reason for this is that the temperature difference between the flue gas and inlet oxidizing air decreases when the inlet oxidizing air temperature rises. However, the boiler efficiency increases because the humidity ratio of the flue gas is larger for higher inlet temperatures, and the amount of vapor condensation is larger. When the inlet oxidizing air temperature is the same, the temperature of the emission flue gas increases with increasing relative
Fig. 5. Relationship of boiler efficiency with different oxidizing air temperatures and relative humidity ratio.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
S. Shang et al. / Applied Energy xxx (2017) xxx–xxx
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Fig. 6. Relationship of humidity ratio and humidification amount with the increasing relative humidity of the inlet oxidizing air (T = 5 °C).
Fig. 7. Relationship of flue gas temperature with different inlet oxidizing air temperatures and relative humidity ratio.
humidity. The main reason for this is that the dew point temperature and temperature of the inlet oxidizing air, after humidification, increase with the increasing relative humidity of the inlet oxidizing air, which results in increases to the emission temperature of the flue gas. 4.4. System performance comparison A system performance comparison between the proposed condensing boiler system and the AHPB system is shown in Fig. 8. It can be seen that the efficiency of the boiler with the NCHR system is much higher than the efficiency of the conventional condensing boiler. The efficiency increases with increasing inlet oxidizing air temperature and is 5.3% higher than that of the condensing boiler. The condensing boiler typically uses the return water to recover waste heat from the flue gas, and the heat recovery is affected by the humidity ratio of the inlet oxidizing air. Therefore, the dew point of the flue gas in the condensing boiler will be higher for
higher inlet oxidizing air temperatures when the relative humidity is the same. Therefore, the dew point temperature of the flue gas in the condensing boiler will be much lower than that in the proposed system. Furthermore, the temperature of the emission flue gas will be higher because the heat transfer process is between the flue gas and return water, and the temperature of the emission flue gas is limited to approximately 50 °C (given in Table 3). Compared to the AHPB system, the efficiency of the boiler with the NCHR system is slightly lower when the inlet oxidizing air temperature is lower than 19 °C (RH = 50%). Although the boiler efficiency of this system is lower, it is only 0.3% lower, and is almost the same as that of the AHPB system. When the temperature of the inlet oxidizing air is higher or the relative humidity is higher, the boiler efficiency of the NCHR system is higher than that of the AHPB system. The reason for this is that the humidity ratio in the flue gas will be higher and the vapor condensation heat will increase. The boiler efficiency of the NCHR system will be higher than that of the AHPB system when the vapor condensation heat
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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S. Shang et al. / Applied Energy xxx (2017) xxx–xxx
Fig. 8. System performance comparison with the condensing boiler.
Li 3600 LCV ei
of the NCHR system is larger than the heat transfer rate in the evaporator of the AHP.
G¼
5. Case study of annual performance
The heating period in Beijing runs from Nov 15 to Mar 15 of the following year. The year-round energy consumption Gy is obtained by the following equation:
5.1. Building load and calculating method 5.1.1. Building load information An office building in Beijing with a total area of 1050 m2 was chosen. The building envelope information and occupant density are listed in Table 4. The building load information is shown in Fig. 9(a). The red portion of the figure represents the heating load, and the blue portion represents the cooling load. The peak heating load in Beijing is 80 kW. The hourly ambient temperature and humidity ratio are given in Fig. 9(b). In this section, we focus on the heating load; the boiler is only used in winter. 5.1.2. Calculating method The return water temperature is taken as 45 °C. The efficiency of the traditional boiler is 90% [27], and the efficiency of the AHPB system is 103.6% [20]. The boiler efficiency for the condensing boiler and this system is calculated considering the hourly ambient temperature and relative humidity. Because of the different dew point temperatures, heat transfer process in the condensing boiler is different, and the boiler efficiency changes every hour. To compare system performance and conduct a tech-economic analysis, four systems are operated in the same building in Beijing: a traditional boiler, condensing boiler, AHPB system, and the novel NCHR system. Then, the energy consumption of each system can be calculated from the hourly building load. The hourly energy consumption G is calculated by the following equation:
Table 4 Building envelope information and occupant density. Wall (W/(m2 °C))
Window (W/(m2 °C))
Floor (W/(m2 °C))
Occupant density (p/m2)
0.622
5.7
3.055
0.15
Gy ¼
1 776 X i¼1
8 760 X Li 3600 Li 3600 þ LCV ei LCV ei i¼6913
ð17Þ
ð18Þ
After calculating the hourly energy consumption and integrating over the whole year, the energy saving ratio can be calculated by
ESR ¼
Gy Gytb Gytb
ð19Þ
The operation cost for the next ten years can be calculated using the following equation:
O¼CþZy
ð20Þ
The payback period is calculated using the following equation [28]:
PB ¼
C C tb Y tb Y
ð21Þ
5.2. Energy saving potential analysis The hourly performance of different systems is shown in Fig. 10. As shown in Fig. 10, the boiler efficiencies of the AHPB and NCHR systems are much higher than those of traditional condensing boilers. The efficiency of the condensing boiler and novel system changes with changing climate parameters. The efficiency of the AHPB system is the highest among all the systems. The average efficiencies of the traditional boiler, condensing boiler, AHPB system, and novel NCHR system are 90%, 97.4%, 103.6% and 103.4%, respectively. Although the efficiency of the AHPB system is the highest among these, the NCHR boiler system has an efficiency very close to that of the AHPB system. The gas consumption for a year of running each system is shown in Fig. 11. The gas consumption of a traditional boiler is
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
S. Shang et al. / Applied Energy xxx (2017) xxx–xxx
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Fig. 9. Building load and hourly climate information.
Fig. 10. Hourly boiler efficiency of each system.
Fig. 11. Year-round gas consumption of each system.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
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8697.5 m3. The gas consumption of the condensing boiler is about 600 m3 less than that of the traditional boiler. The AHPB system saved the most energy due owing its high efficiency; the energy saving ratio compared to the traditional boiler is 13.13%. The gas consumption of the NCHR boiler system was close to that of the AHPB system, with an energy saving ratio of 12.97%. Detailed information on gas consumption and energy saving potential is given in Table 5. The boiler with the NCHR system can reduce natural gas consumption and decrease pollutant emissions.
the NCHR system includes three parts, and the total cost is 21634.5 CNY. The highest initial cost is that of the absorption heat pump, whose unit price is 600 CNY/kW [19]. Therefore, the total investment required for the boiler with an AHPB system is 26,160 CNY. The yearly gas consumption of each system is shown in Fig. 11. The yearly operational cost can be obtained from the unit price of natural gas (2.4 CNY/m3) [29]. The operational cost for the next ten years can be calculated from Eq. (20). The results are shown in Fig. 12. It can be seen that the traditional boiler has the lowest initial cost; however, the total operational cost of the NCHR system is lower than that of the traditional boiler after the first year. Although the AHPB system has the highest efficiency, its operational cost after ten years is still higher than that of the NCHR system owing to the higher initial investment. Compared to the traditional boiler, the payback period of the AHPB system is three years, while it is only one year for the NCHR
5.3. Economic analysis The initial investments in each system and unit price of each component are given in Table 6. The rated heating capacity of the boiler is 90 kW, and the total price is 18,000 CNY. The rated heat exchange capacity of the condensation heat exchanger is 7.5 kW, and its unit price is 120 CNY/kW [26]. The initial cost of
Table 5 Year-round gas consumption of each system and the energy saving ratio. Types
Traditional boiler
Condensing boiler
AHPB
NCHR
Year-round gas consumption (m3) Energy saving ratio (%)
8697.51 0
8029.96 7.68
7555.75 13.13
7569.27 12.97
Table 6 Initial investment of each system. Traditional boiler
Condensing boiler
AHP for heat recovery
Non-contact total heat exchanger
Boiler
Capacity (kW) Unit price (CNY/kW)
90 200
90
90
90
AHP
Capacity (kW) Unit price (CNY/kW)
– 600
–
13.6
–
Condensation heat exchanger
Capacity (kW) Unit price (CNY/kW)
– 120
7.5
–
20.93
Spray water heat exchanger
Capacity (m3/h) Unit price (CNY/m3/h)
– 800
–
–
1.4
18,000
18,900
26,160
21634.5
Total price (CNY)
Fig. 12. Ten-year operation cost of each system.
Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169
S. Shang et al. / Applied Energy xxx (2017) xxx–xxx
system. Compared to the condensing boiler, the payback period of the NCHR system is only three years. However, the payback period of the AHPB system is over seven years. The operational cost of the NCHR system for ten years is less than that of the AHPB system; the NCHR system has obvious economic benefits. 6. Conclusion To improve the system performance of a gas-fired boiler, an NCHR system is proposed for heating and humidifying oxidizing air with waste heat from flue gas. A mathematic model is established to calculate system performance. The system performance is compared with other systems. Finally, a case study in Beijing is used to investigate the energy saving potential and economic feasibility. The main conclusions from the above analysis are as follows: 1. Using the NCHR system, boiler efficiency can be promoted to 103.4% for an oxidizing air temperature of 0 °C. Boiler efficiency increases with increasing inlet oxidizing air temperature and relative humidity. 2. The energy saving potential for a year when running the NCHR system is 12.97% higher compared to a traditional boiler. The NCHR and AHPB systems showed very similar results. 3. For a system running continuously, the payback period of the NCHR system is one year relative to the traditional boiler and three years relative to the condensing boiler. The operational cost for ten years is less than that of the AHPB system; therefore, the NCHR system has obvious economic benefits.
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Please cite this article in press as: Shang S et al. A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.05.169