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The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator
Accepted Manuscript The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator Kefang Zhang, Zhongliang Liu, Yanxia Li, Qingfang Li, Jian Zhang, Haili Liu PII:
S1359-4311(13)00699-6
DOI:
10.1016/j.applthermaleng.2013.10.007
Reference:
ATE 5081
To appear in:
Applied Thermal Engineering
Received Date: 30 March 2013 Revised Date:
16 August 2013
Accepted Date: 5 October 2013
Please cite this article as: K. Zhang, Z. Liu, Y. Li, Q. Li, J. Zhang, H. Liu, The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator, Applied Thermal Engineering (2013), doi: 10.1016/j.applthermaleng.2013.10.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator
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Kefang Zhanga,b, Zhongliang Liu a,∗, Yanxia Li a, Qingfang Li c, Jian Zhangc, Haili Liu c (a College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, Shandong Province, China
c
Shengli Engineering & Consulting Co. Ltd, Shengli Oilfield, Dongying 257026, Shandong Province, China)
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b
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Abstract Absorption heat transformer (AHT) and flash evaporator (FE) are used to reduce the heat consumption of CO2 capture processes and an AHT-FE-aided capture system is proposed. Analyses are carried out to verify the effectiveness in reducing heat consumption. Compared with the base CO2 capture system of 3000t/d CO2 capture capacity from a 660MW coal-fired power
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unit, the AHT-FE-aided capture system reduces the heat consumption from 3.873GJ/tCO2 to 3.772GJ/tCO2, and the corresponding energy saving is 2.62%. The economic analysis shows that the annual profit would be 2.94 million RMB Yuan. The payback period of the AHT-FE-aided
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capture system is approximately 2.4 years. Therefore, the AHT-FE-aided capture system is both economically and technically feasible for improving the CO2 capture energy performance.
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Keywords CO2 capture system; heat recovery; absorption heat transformer; flash evaporator; heat consumption
1. Introduction ∗
Corresponding author. [email protected]
Abbreviations: AB, absorber; AHT, absorption heat transformer; CO, condenser; DE, desuperheater; EV, evaporator; FE, flash evaporator; GE, generator; MEA, mono ethanol amine; P, pump; USD,US Dollar. 1
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Solvent-based post-combustion carbon capture is one of the promising technologies for reducing CO2 emissions from existing fossil-fuel power plants due to its easy retrofitting [1].
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However, the solvent regeneration is highly energy intensive and large energy consumption is the main challenge for this technology. Studies [2] have shown that regeneration consumes about 4-6MJ for separation 1kg CO2 from flue gas. Normally, the water steam extracted from steam
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turbine unit is introduced into the reboiler where desorption process is mainly finished and condensed into water, providing the required energy for solvent regeneration. However, the steam
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extraction decreases the power generation and results in an electric efficiency penalty. Many efforts on amine based CO2 capture technology have been made to reduce its energy consumption and the electric efficiency penalty. One of the focuses of current research is on developing novel solvents and packings that are more efficient and energy-saving than mono
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ethanol amine (MEA) [3-5]. Some of the main process parameters including solvent concentration, CO2 loading in lean solutions, operating conditions of absorption and stripper column, stripper height, as well as other parameters such as steam extraction pressure et al. have been discussed
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and optimized [6-8]. In addition, many improved configurations including inter-heated column, split-stream, bi-pressure stripper, multi-pressure stripper, stripper with vapor recompression and
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absorber with inter-cooling have been investigated and are verified to be efficient in reducing energy consumption of CO2 capture [9-12]. Due to the fact that the added CO2 capture system will inevitably influence its mother system, the integration of the added capture system and the mother system has also been studied and optimized to reduce the electric efficiency penalty [13-15]. For a given solvent and a well-designed CO2 capture system that operates at its optimal parameters, the heat required for solvent regeneration in reboiler is fixed and could not be reduced 2
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further. The question is, under this condition, is there any possibility to reduce the steam consumption from the steam turbine unit? The answer is yes, if heat recovery from the waste heat
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of the system is used to replace a part of the heating steam. To the best of our knowledge, a little research work has been carried out on heat recovery of the CO2 capture system that is trying to replace a part of the heating steam. Li et al. studied and simulated CO2 removal processes with
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heat energy supply provided by recovering heat from the lean solvent [16]. The heat recovery from the CO2 removal processes is by a two-stage R600 heat pump and a supercritical CO2 cycle
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heat pump and is simulated by the commercial software APSEN PLUS. Their results show that the energy consumption of CO2 removal process could be reduced by applying heat pump technology，and the best result was achieved by using supercritical CO2 cycle heat pump. Luo introduced the vapor compression heat pump to the CO2 capture system to recover the heat from
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the lean solvent or the solar energy [17]. Simulated results by Aspen Plus showed that the CO2 capture process by solar assisted heat pump had lower desorption energy consumptions than the CO2 capture process by lean solution source heat pump. The above-mentioned R600 heat pump,
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supercritical CO2 cycle heat pump, lean solution source heat pump and solar assisted heat pump all are of vapor compression cycle and need consuming electricity.
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However, as far as the authors could know, no studies have been reported about the reasonable
heat recovery of the condensate using absorption heat transformer in the CO2 capture system. In order to reduce the heat consumption, absorption heat transformer (AHT) and flash evaporator (FE) are introduced into the CO2 capture system to recover the heat of the condensate and in this way an AHT-FE-aided system is proposed in this paper. Detailed process mathematical models are established, calculations are performed and the analysis results are presented of the proposed and 3
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the base capture system. 2. The base capture system
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To make the study comparable, a base capture system for CO2 capture is established. The capture system is to separate CO2 (3000t/d CO2 capture capacity) from a 660MW coal-fired power unit.
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2.1 System description
Fig. 1 shows the base amine absorption CO2 capture system used as the comparison basis of
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this study. The fundamental underlying principle is the exothermic, reversible reaction between a weak acid (e.g., CO2) and a weak base (e.g., MEA) to form a soluble salt. The inlet gas is contacted counter-currently with the lean solvent in the absorber. CO2 is absorbed by the solution. The solution enriched with CO2 is pre-heated before entering the stripper where the reaction is
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reversed by continuous heating. From the bottom of the stripper, the lean solvent exchanges heat with the rich solvent that enters the stripper and is sent back into the absorber. From the top, a high-purity (dry-basis) CO2 is produced.
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For coal-fired power plants, a feasible way to supply the regeneration energy is extracting steam from the turbine steam unit. Considering the fact that the extracted steam is superheated and its
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temperature is usually significantly higher than the MEA safe and stable operation temperature, the extracted steam is first introduced into the desuperheater to regulate its temperature to the value required by the MEA regeneration. The saturated vapor from the desuperheater is then introduced into the reboiler to heat the MEA and is condensed into the condensate.
Fig. 1 The base amine absorption CO2 capture system 4
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For a CO2 capture capacity of 3000t/d from the 660MW unit and the given solvent, the main operation parameters are listed in Table 1. These operation parameters remain unchanged in the
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proposed capture system in this paper. Table 1 Main operation parameters of the base capture system
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For the 660MW coal-fired power plant, the only practical position for the steam extraction is
from the fifth stage turbine. The extracted steam parameters from this stage are 0.418MPa and
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257°C. A desuperheater is used to cool the superheated steam to its corresponding saturation temperature of 144°C at 0.4MPa. The saturated vapor at 0.4MPa and 144°C is introduced into the reboiler and is condensed into sub-cooled water at 128°C, providing the required heat of 134.5MW for solvent regeneration.
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2.2 Model equations
To calculate the mass flow rate of the required extracted steam, the mathematical models in heating process must be established. Therefore, the control volume should be first chosen
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according to thermodynamic theory. The reboiler and the desuperheater in Fig. 1 are selected as the control volume respectively, as is shown in Fig. 2. The mathematical models are established
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for each control volume according to mass and energy conservation principles. The mass flow rate of the required extracted steam can be determined by solving the model equations given in the following sections.
Fig. 2 Mass and energy balance of reboiler and desuperheater 2.2.1 Reboiler 5
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In the reboiler, the rich MEA solvent of a mass flow rate mMEA is heated to vaporize part of the lean amine solution, generating CO2 product stream for stripping. The heating steam of a mass
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flow rate ms (kg/s), temperature Ts and specific enthalpy hs (kJ/kg) enters in the tube side of the reboiler to heat the rich MEA solvent and then is condensed. Neglecting the heat loss to the environment, the mass flow rate of the steam ms can thus be calculated from equation (1).
Qr hs − hco
(1)
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ms =
Where hco is the specific enthalpy of the condensate leaving the reboiler, kJ/kg; Qr is the required
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heat to regenerate the rich EMA solvent in the reboiler in kW. The regeneration heat Qr is 134.5MW as listed in Table 1. 2.2.2 Desuperheater
In the desuperheater, the superheated steam from the steam turbine unit and the desuperheating
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water (here, the condensate from the reboiler is the best choice) are mixed into the saturated vapor that is used to heat the MEA in reboiler. Mass balance in the desuperheater gives,
ms = mext + md
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(2)
And the energy balance in the desuperheater, (3)
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ms hs = mext hext + md hco
Where, mext is the extracted steam flow rate of the base capture system from the steam turbine unit, kg/s; hext is specific enthalpy of the extracted steam, kJ/kg; md is the flow rate of the desuperheating water, kg/s. From the above equations, md and mext can be expressed as,
md =
hext − hs ms hext − hco
mext =
hs − hco ms hext − hco
(4)
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Simple calculation shows that a saturated vapor ms of 220.0t/h at 0.4MPa and 144°C is required and condensed into water at 128°C in the reboiler providing the required heat of 134.5 MW for
turbine unit is 198.4t/h, and the desuperheating water md at 128°C is 21.6t/h. 3. AHT-FE-aided capture system
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solvent desorption. As a result, the required extracted steam at 0.418MPa and 257°C from steam
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In the base system as shown in Fig. 1, the condensate leaves the reboiler at a temperature as
high as 128°C. In order to reduce water consumption, the condensate has to be sent back into the
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mother system. To do so, this high-temperature condensate must be cooled to a temperature lower than 95°C before recycling back to the steam turbine unit. More than that, if the condensate is polluted seriously in the reboiler by MEA leakage which is almost inevitable and could not meet feed-water quality requirement of the power generation system, then it has to be rejected to the
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environment. In this case, the condensate has to be further cooled to a temperature that is lower than 40°C to avoid waste heat pollution. In either case, not only are the additional cooling equipment and medium needed, but also the energy of the high-temperature condensate is wasted.
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To recover the heat of the high-temperature condensate, an absorption heat transformer (AHT) and a flash evaporator (FE) are integrated into the system and thus the AHT-FE-aided capture system
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is proposed.
3.1 System description
The proposed AHT-FE-aided system is shown in Fig. 3. Comparing this new system with the
base system describe above, one can see that the only difference between these two systems is that the AHT-FE-aided system recovers the heat of the condensate. In the AHT-FE-aided capture system, most of the condensate is introduced into AHT, and the upgraded heat is delivered from 7
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AHT. The upgraded heat is then introduced into FE to produce low pressure, low temperature vapor by flash evaporation. The flash-off vapor is introduced into the reboiler to heat the MEA
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solvent. Therefore, in this system, the MEA solvent is not heated by the saturate vapor extracted from the steam turbine, but also heated by the flash-off vapor in the reboiler. Obviously, since the heat required for solvent regeneration in reboiler remains unchanged, the extracted steam flow rate
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is reduced.
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Fig. 3 AHT-FE-aided CO2 capture system 3.2 Model equations
To calculate the mass flow rate of the required extracted steam and the energy saving, the mathematical model for the AHT-FE-aided capture system must be established. The reboiler, the
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desuperheater, the AHT and the FE in Fig. 3 are selected as the control volume respectively, as shown in Fig. 4. Comparing with Fig. 2, the only difference in Fig. 4 is that the reboiler is supplied with the saturated vapor and the flash-off vapor. Therefore, Eq. (2) and Eq. (3) remain unchanged.
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And the mass and energy conservation for the AHT and FE are described by Eq. (5) to Eq. (12),
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on the condition that the heat loss to the environment is neglected.
Fig. 4 Mass and energy conservation of the AHT-FE-aided reboiler
3.2.1 Reboiler
The mass flow rate of the steam ms can be calculated from Eq. (5).
Qr = ms (hs − hco ) + mf (hf − hco )
(5)
Where, mf is the mass flow rate of the flash-off vapor from FE, kg/s; hf is the specific enthalpy 8
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of the flash-off vapor, kJ/kg. The AHT-FE-aided capture system recovers some energy of the high-temperature condensate,
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and this should not change the heat required for MEA regeneration. Therefore, without changing the operation parameters of the system, the regeneration heat of the AHT-FE-aided system remains unchanged, i.e., Qr is still equal to 134.5MW.
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3.2.2 AHT
An absorption heat transformer (AHT) can increase the temperature of low or moderately waste
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heat source to a higher and more useful level [18]. The AHT basically consists of an evaporator (EV), a generator (GE), a condenser (CO), an absorber (AB) and a solution heat exchanger. The evaporator and the generator are supplied with waste heat and the upgraded heat is delivered from the absorber. The coefficient of performance (COP) is used to measure the ability of AHTs to
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transfer heat between different temperature heat sources. The COP is defined as the ratio of the heat released from the AB to the total heat input from the GE and the EV [19]. COP =
QAB QEV + QGE
(6)
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Where, Q is the heat load in kW and the subscripts AB, EV and GE represent evaporator, generator and absorber.
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The calculation of COP defined by Eq. (6) is very complicated and will be discussed later. If
COP is known, then the mathematical model for AHT is simplified and only the mass and energy conservation is considered. The heat loads in the generator and evaporator can be determined by the temperature decrease of the condensate which is introduced into the EV and GE, that is,
QEV + QGE = mext, A cp (tg1 − tg2 ) + mext, A cp (tg2 − tg3 )
(7) 9
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That is,
QEV + QGE = mext, A cp (tg1 − tg3 )
(8)
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Where, mex,A is the extracted steam flow rate of the AHT-FE-aided capture system which is equal to the condensate flow rate entering EV and GE, kg/s; cp is specific heat of the condensate, kJ/(kg.K); tg1 is the inlet temperature of the condensate entering the EV, °C; tg2 is the temperature
°C.
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The absorber heat load is determined by the COP, that is,
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of the condensate leaving the EV, °C; tg3 is the outlet temperature of the condensate leaving the GE,
QAB = (QEV + QGE )COP
(9)
The water leaving the AB is used as the upgraded heat source and heated from th1 to th2 in the absorber. Then the mass flow rate mh of the water leaving the AB can be determined by Eq. (10). (10)
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QAB = mh (hh1 − hh2 )
Where hh1 is the specific enthalpy of the water entering the AB, kJ/kg; hh2 is the specific enthalpy of the water leaving the AB, kJ/kg.
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To produce flash-off vapor in the FE, the water into the AB should be heated until it reaches its boiling point in the absorber. So the enthalpy of the saturation water i.e. hh2 depends on the
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pressure of the water leaving the AB. hh1 can be calculated using the following equation:
mh hh1 = mf hco + (mh − mf )h '
(11)
Where, mf is the mass flow rate of the flash-off vapor, kg/s; h′ is the specific enthalpy of the
leftover water leaving the flash evaporator, kJ/kg. 3.2.3 Flash evaporator The FE pressure should be lower than the water pressure in the AB. Thus the water leaving the 10
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AB produces low-pressure and low-temperature vapor due to flash effects by sudden pressure drop in the FE. Without consideration of the heat loss to the environment, the FE energy balance
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can be simply written as, mh hh2 = mf hf + ( mh − mf ) h′
(12)
Where, mf is the mass flow rate of the flash-off vapor, kg/s; hf is the specific enthalpy of the
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flash-off vapor, kJ/kg; h′ is the specific enthalpy of the leftover water leaving the FE, kJ/kg.
For the flash-off vapor, the optimal option is to introduce the flash-off vapor into the saturated
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vapor from the desuperheater by a pipeline before entering the reboiler. In order to reduce the exergy loss, the temperature and pressure of the flash-off vapor should be equal to those of the saturated vapor from the desuperheater. Therefore, the flash-off vapor temperature and pressure should be 144°C and 0.4MPa.
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From Eq. (10) to Eq. (12), it is easy to find,
QAB = mf (hf − hco )
(13)
Eq. (13) indicates that heat load in reboiler from the flash-off vapor is equal to the absorber
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heat load. That is, the recovery heat in the AB is fully reused in the reboiler. Therefore, no matter what the pressure of the water leaving the AB is, the mass flow rate of the flash-off vapor mf is
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fixed, if only QAB, hf and hco are fixed. 3.3 Performance of the AHT AHT can effectively recover about 50% of the waste heat and reuse it in industrial processes
[20]. For the AHT-FE-aided capture system, it is important to supply the reboiler with flash-off vapor as much as possible at required temperature. Therefore, COP, absorption temperature and heat load in absorber are the main performance parameters of the AHT. For the given 11
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refrigerant-absorbent solution, performance of AHT depends on solution concentration difference between weak solution and strong solution, pressure drop of the refrigerant vapor and temperature
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difference in heat exchanger, the waste heat temperature, the cooling water temperature and so on. The aim of this section is to illustrate the effect of the condensate temperature and the cooling water temperature on COP, the absorption temperature and the absorber heat load using
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water-lithium bromide (water-LiBr) solutions, when other data are fixed. The basic data used in Fig. 5 and Fig. 6 for the AHT performance calculations are summarized in Table 2.
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Table 2 Basic data used in Fig. 5 and Fig. 6 for the AHT performance calculation
3.3.1 COP
Fig. 5 depicts the influence of the condensate temperature and the cooling water temperature
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on COP. It can be seen that the COP of the AHT is insensitive to the variation of the outlet temperature of both the condensate and the cooling water within the calculated region. The COP is about 0.5 and unchanged almost within the range of temperature, which means that the AHT can
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recover about 50% of the condensate sensitive heat and reuse it in the reboiler.
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Fig. 5 Influences of the condensate and the cooling water temperature on COP
3.3.2 Absorption temperature Fig. 6 depicts the effect of the condensate temperature and the cooling water temperature on
the absorption temperature, at a given set of conditions. It can be seen that when the inlet temperature of the condensate and the cooling water is fixed, lowering the outlet temperature of the condensate or increasing the outlet temperature of the cooling water will both reduces the 12
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absorption temperature.
on the absorption temperature 3.3.3 Absorber heat load
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Fig. 6 Influences of the condensate and the cooling water temperature
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The absorber heat load QAB can be determined by Eq. (9). It can be seen from the model
equations that at a given inlet temperature of the condensate, with decreasing the outlet
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temperature of the condensate, the heat loads in the generator and evaporator increase, and the heat load in the absorber increases too. So the outlet temperature of the condensate should be as low as possible, on the condition that the absorption temperature meets the requirement of the upgraded heat source temperature (the water temperature leaving the AB).
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4. Performance of the capture systems
The water leaving the AB produces low-pressure and low-temperature vapor due to flash effects by sudden pressure drop in FE. Thus, the water pressure leaving the AB ought to be higher than
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the flash-off vapor pressure of 0.4MPa. If the temperature difference between the absorption temperature and the water temperature leaving the AB is fixed, then increasing the water pressure
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leaving the AB increases the required absorption temperature. As it is pointed out above, the absorber heat load decreases with the outlet temperature of the condensate. Therefore, in order to recover more heat of the condensate and obtain more flash-off vapor, the water pressure leaving the AB should be as low as possible only if it is higher than 0.4MPa. However, the lower water pressure leaving the AB results in the lower water temperature, a larger mass flow rate of water leaving the AB, more pump work and larger heat transfer area of the AB. 13
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With these considerations, the water pressure leaving the AB is set at 0.5MPa whose saturation temperature is 152°C. Accordingly, as saturated water of 152°C, specific enthalpy of water leaving
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the AB is 640.2kJ/kg. IF the heat transfer temperature difference between the absorption temperature and the water temperature leaving the AB is 5K which is a common practice for
ordinary heat exchanger design, then the absorption temperature is 157°C. From Fig. 6, one can
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find that the outlet temperature of the condensate is 95°C if the outlet temperature of the cooling water is set at 38°C.
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The basic data used for analyzing the AHT-FE-aided capture system are listed in Table 2 and Table 3.
Table 3 Basic data used in the analysis for the AHT-FE-aided capture system
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Based on the data in Table 3, the model equations given above are solved by iteration method and the performance parameters of the AHT-FE-aided capture system obtained are listed in Table 4. If the flash-off vapor temperature is designed lower than 144°C, the absorption temperature
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may be lower than 157°C. From Fig. 6, the outlet temperature of the condensate is lower than 95°C. Therefore, the AHT-FE-aided capture system can recover more heat of the condensate, and
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obtain more flash-off vapor and thus greater energy saving.
Table 4 Performance of the capture systems
In Table 4, the specific heat consumption is an important synthetic indicator that reflects the energy consumption level of the CO2 capture system. The specific heat consumption is defined as the thermal energy consumed for the desorption of 1 ton CO2 in the stripper and is calculated from 14
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the following equation, q=
mext ( hext − hco ) × 3600 DCO2
(14)
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Where, q is the specific heat consumption, kJ/tCO2; DCO2 is the CO2 output, t/h. The energy saving is used as an indicator for the performance improvement of the AHT-FE-aided capture system over the base system. The energy saving is defined by Eq. (15) or
ζ =
mext − mext,A
×100%
q − qA ×100% q
(15)
(16)
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ζ =
mext
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Eq. (16).
Where, q is the specific heat consumption of the base capture system, kJ/tCO2; qA is the specific heat consumption of the AHT-aided capture system, kJ/tCO2.
5. Economic analysis
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A simple economic analysis is made for the AHT-FE-aided capture system. It can be seen from Table 4, the high-parameter steam extracted from the steam turbine flow rate required is 193.2t/h. Thus the reduction in the extracted steam compared with the base system is 5.2t/h and this will
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increase an electricity production of the power generation system by 920kW. Taking an average electricity grid price of China as the electricity price that is approximately 0.4 RMB Yuan/kW⋅h
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and the operation hours is 8000h/year, the annual profit would be 2.94 million RMB Yuan or about 0.48 million USD. It can be seen that, although the energy saving of the AHT-FE-aided capture system is only 2.62%, it can still produce an annual profit of 0.48 million USD. For the AHT-FE-aided capture system, an AHT and a FE were added to the base capture system. The initial equipment cost of the AHT of 3.7MW is about 7.0 million RMB or 1.14 million USD. The payback period for the AHT-FE- aided system is approximately 2.4 years. Compared with the 15
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15 years’ service life of the absorption transformer, a payback period of 2.4 years for the AHT-FE-aided system is very attractive and the technology may produce a net profit of about 6
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million USD during its full service life. 6. Conclusions
Absorption heat transformer and flash evaporation are used to reduce the heat consumption of
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CO2 capture process and the AHT-FE-aided capture system is proposed. To verify its effectiveness, the process models are established and solved. By comparison with the base system, it is found
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that the proposed the system can reduce the energy consumption of the CO2 capture system effectively.
(1) Energy analyses show that the proposed AHT-FE-aided capture system may recover a large portion of the waste heat of the high-temperature condensate. Compared with the base CO2
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capture system of a CO2 capture capacity of 3000t/d from a 660MW unit, it reduces the heat consumption from 3.873GJ/tCO2 to 3.772GJ/tCO2, the resulted energy saving is 2.62%. (2) The economic analysis shows that the annual profit of the new system is 2.94 million RMB
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Yuan or 0.48 million USD. The payback period is approximately 2.4 years. This proves that the proposed system is both economically and technically feasible for improving the CO2 capture
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energy performance.
(3) Within the temperature calculated, it is found that the COP of the AHT is not sensitive to
the variation of the outlet temperature of both the condensate and the cooling water, the COP is about 0.5 and remains almost unchanged within the range of temperature. (4) With the fixed inlet temperature of the condensate and the cooling water, lowering the outlet temperature of the condensate or increasing the outlet temperature of the cooling water will 16
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both reduces the absorption temperature. If the required absorption temperature is 157°C, then the outlet temperature of the condensate is calculated to be 95°C for the example system. The
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flash-off vapor of 6.8t/h is obtained by recovering the heat of the condensate via the absorption heat transformer.
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Acknowledgments
This work is supported by the Key Project No. 2012BAC24B01 of the National Key
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Technologies R & D Program, the Ministry of Science Technology of the People’s Republic of China.
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[11] H. W. Liang, Z. G. Xu, F. Q. Si, Economic analysis of amine based carbon dioxide capture system with bi-pressure stripper in supercritical coal-fired power plant, Int. J. Greenh. Gas Con. 5(2011) 702-709.
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[12] D. H. Van Wagener, G. T. Rochelle. Stripper configurations for CO2 capture by aqueous monoethanolamine, Chem. Eng. Res. Des. 89 (2011) 1639-1646.
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[13] T. Sanpasertparnich, R. Idem, I. Bolea, D. de Montigny, P. Tontiwachwuthikul, Integration of post-combustion capture and storage into a pulverized coal-fired power plant, Int. J. Greenh.
Gas Con. 4(2010) 499-510.
[14] L. M. Romeo, I. Bolea, J. M. Escosa, Integration of power plant and amine scrubbing to reduce CO2 capture costs, Appl. Therm. Eng. 28 (2008) 1039-1046. [15] L. M. Romeo, S. Espatolero, I. Bolea, Designing a supercritical steam cycle to integrate the 18
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energy requirement of CO2 amine scrubbing, Int. J. Greenh. Gas Con. 2(2008) 563-570. [16] Q. Li, Y. S. Yu, J. Jiang, Z. X. Zhang, CO2 capture by chemical absorption method based on
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heat pump technology. Journal of Chemical Engineering of Chinese Universities 24(2010) 29-34. (in Chinese)
[17] Y. G. Luo, Simulation Analysis on CO2 Capture by MEA Method Based on Heat Pump
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Technology, Master Thesis, Shandong University of Science and Technology, China, 2011. (in Chinese)
Energ. 32 (2007) 267-284.
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[18] A. Sozen, H. S. Yucesu, Performance improvement of absorption heat transformer, Renew.
[19] A. Huicochea, W. Rivera, H. Martínez, J. Siqueiros, E. Cadenas, Analysis of the behavior of an experimental absorption heat transformer for water purification for different mass flux
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rates in the generator, Appl. Therm. Eng. 52 (2013) 38-45.
[20] I. Horuz, B. Kurt, Absorption heat transformers and an industrial application, Renew. Energ.
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35 (2010) 2175-2181.
Fig. 1 The base amine absorption CO2 capture system
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Fig. 2 Mass and energy balance of reboiler and desuperheater Fig. 3 AHT-FE-aided CO2 capture system Fig. 4 Mass and energy conservation of the AHT-FE-aided reboiler Fig. 5 Influences of the condensate and the cooling water temperature on COP Fig.6 Influences of the condensate and the cooling water temperature on the absorption temperature 19
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Table 1 Main operation parameters of the base capture system
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Table 2 Basic data used in Fig. 6 and Fig. 7 for the AHT performance calculations Table 3 Basic data used in the analysis for the AHT-FE-aided capture system
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Table 4 Performance of the capture systems
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Table 1 Main operation parameters of the base capture system 40 112 98 134.5
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Regeneration pressure in stripper (kPa) Regeneration temperature in stripper (°C) CO2 product stream temperature (°C) Heat required for solvent regeneration in reboiler (MW)
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Table 2 Basic data used in Fig. 5 and Fig. 6 for the AHT performance calculation 128 32 0.05 66 66 tg2 -3 tg3 -3 tc2+3 3
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Initial temperature of the condensate from the reboiler (°C) Initial temperature of the cooling water (°C) Solution concentration difference between weak solution and strong solution Refrigerant vapor pressure drop between the GE and the CO (Pa) Refrigerant vapor pressure drop between the EV and the AB (Pa) Evaporation temperature (°C) Strong solution outlet temperature in the EG (°C) Condensation temperature (°C) Temperature difference in the solution heat exchanger (K)
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Table 3 Basic data used in the analysis for the AHT-FE-aided capture system 134.5 38 5 144
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Heat load in reboiler (MW) Outlet temperature of cooling water (°C) Heat transfer temperature difference (K) Flash-off vapor temperature (°C)
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Table 4 Performance of the capture systems
198.4 3.873 -
0.5 152 640.2 157 95 193.2 6.8 226.7 0.5 7402 3701 3.772 2.62
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AHT-FE-aided capture system
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Outlet pressure of water leaving the AB (MPa) Outlet temperature of water leaving the AB (°C) Outlet specific enthalpy of water leaving the AB (kJ/kg) Absorption temperature (°C) Outlet temperature of the condensate (°C) Extracted steam required (t/h) Flow rate of the flash-off vapor (t/h) Mass flow rate of water leaving the AB (t/h) COP Heat load of the GE and EV (kW) Heat load of the AB (kW) Specific heat consumption (GJ/tCO2) Energy saving (%)
Base capture system
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Fig. 1 The base amine absorption CO2 capture system
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Fig. 2 Mass and energy balance of reboiler and desuperheater
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Fig. 3 AHT-FE-aided CO2 capture system
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Fig. 4 Mass and energy conservation of the AHT-FE-aided reboiler
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Fig. 5 Influences of the condensate and the cooling water temperature on COP
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Fig. 6 Influences of the condensate and the cooling water temperature on the absorption temperature
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Highlights: 1. Absorption heat transformer (AHT) is introduced into the CO2 capture system. 2. The AHT-FE-aided capture system is proposed. 3. The mathematical models in heating process are established. 4. Detailed analysis and results of the AHT-FE-aided capture system are presented.
S1359-4311(13)00699-6
DOI:
10.1016/j.applthermaleng.2013.10.007
Reference:
ATE 5081
To appear in:
Applied Thermal Engineering
Received Date: 30 March 2013 Revised Date:
16 August 2013
Accepted Date: 5 October 2013
Please cite this article as: K. Zhang, Z. Liu, Y. Li, Q. Li, J. Zhang, H. Liu, The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator, Applied Thermal Engineering (2013), doi: 10.1016/j.applthermaleng.2013.10.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator
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Kefang Zhanga,b, Zhongliang Liu a,∗, Yanxia Li a, Qingfang Li c, Jian Zhangc, Haili Liu c (a College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, Shandong Province, China
c
Shengli Engineering & Consulting Co. Ltd, Shengli Oilfield, Dongying 257026, Shandong Province, China)
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Abstract Absorption heat transformer (AHT) and flash evaporator (FE) are used to reduce the heat consumption of CO2 capture processes and an AHT-FE-aided capture system is proposed. Analyses are carried out to verify the effectiveness in reducing heat consumption. Compared with the base CO2 capture system of 3000t/d CO2 capture capacity from a 660MW coal-fired power
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unit, the AHT-FE-aided capture system reduces the heat consumption from 3.873GJ/tCO2 to 3.772GJ/tCO2, and the corresponding energy saving is 2.62%. The economic analysis shows that the annual profit would be 2.94 million RMB Yuan. The payback period of the AHT-FE-aided
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capture system is approximately 2.4 years. Therefore, the AHT-FE-aided capture system is both economically and technically feasible for improving the CO2 capture energy performance.
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Keywords CO2 capture system; heat recovery; absorption heat transformer; flash evaporator; heat consumption
1. Introduction ∗
Corresponding author. [email protected]
Abbreviations: AB, absorber; AHT, absorption heat transformer; CO, condenser; DE, desuperheater; EV, evaporator; FE, flash evaporator; GE, generator; MEA, mono ethanol amine; P, pump; USD,US Dollar. 1
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Solvent-based post-combustion carbon capture is one of the promising technologies for reducing CO2 emissions from existing fossil-fuel power plants due to its easy retrofitting [1].
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However, the solvent regeneration is highly energy intensive and large energy consumption is the main challenge for this technology. Studies [2] have shown that regeneration consumes about 4-6MJ for separation 1kg CO2 from flue gas. Normally, the water steam extracted from steam
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turbine unit is introduced into the reboiler where desorption process is mainly finished and condensed into water, providing the required energy for solvent regeneration. However, the steam
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extraction decreases the power generation and results in an electric efficiency penalty. Many efforts on amine based CO2 capture technology have been made to reduce its energy consumption and the electric efficiency penalty. One of the focuses of current research is on developing novel solvents and packings that are more efficient and energy-saving than mono
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ethanol amine (MEA) [3-5]. Some of the main process parameters including solvent concentration, CO2 loading in lean solutions, operating conditions of absorption and stripper column, stripper height, as well as other parameters such as steam extraction pressure et al. have been discussed
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and optimized [6-8]. In addition, many improved configurations including inter-heated column, split-stream, bi-pressure stripper, multi-pressure stripper, stripper with vapor recompression and
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absorber with inter-cooling have been investigated and are verified to be efficient in reducing energy consumption of CO2 capture [9-12]. Due to the fact that the added CO2 capture system will inevitably influence its mother system, the integration of the added capture system and the mother system has also been studied and optimized to reduce the electric efficiency penalty [13-15]. For a given solvent and a well-designed CO2 capture system that operates at its optimal parameters, the heat required for solvent regeneration in reboiler is fixed and could not be reduced 2
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further. The question is, under this condition, is there any possibility to reduce the steam consumption from the steam turbine unit? The answer is yes, if heat recovery from the waste heat
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of the system is used to replace a part of the heating steam. To the best of our knowledge, a little research work has been carried out on heat recovery of the CO2 capture system that is trying to replace a part of the heating steam. Li et al. studied and simulated CO2 removal processes with
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heat energy supply provided by recovering heat from the lean solvent [16]. The heat recovery from the CO2 removal processes is by a two-stage R600 heat pump and a supercritical CO2 cycle
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heat pump and is simulated by the commercial software APSEN PLUS. Their results show that the energy consumption of CO2 removal process could be reduced by applying heat pump technology，and the best result was achieved by using supercritical CO2 cycle heat pump. Luo introduced the vapor compression heat pump to the CO2 capture system to recover the heat from
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the lean solvent or the solar energy [17]. Simulated results by Aspen Plus showed that the CO2 capture process by solar assisted heat pump had lower desorption energy consumptions than the CO2 capture process by lean solution source heat pump. The above-mentioned R600 heat pump,
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supercritical CO2 cycle heat pump, lean solution source heat pump and solar assisted heat pump all are of vapor compression cycle and need consuming electricity.
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However, as far as the authors could know, no studies have been reported about the reasonable
heat recovery of the condensate using absorption heat transformer in the CO2 capture system. In order to reduce the heat consumption, absorption heat transformer (AHT) and flash evaporator (FE) are introduced into the CO2 capture system to recover the heat of the condensate and in this way an AHT-FE-aided system is proposed in this paper. Detailed process mathematical models are established, calculations are performed and the analysis results are presented of the proposed and 3
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the base capture system. 2. The base capture system
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To make the study comparable, a base capture system for CO2 capture is established. The capture system is to separate CO2 (3000t/d CO2 capture capacity) from a 660MW coal-fired power unit.
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2.1 System description
Fig. 1 shows the base amine absorption CO2 capture system used as the comparison basis of
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this study. The fundamental underlying principle is the exothermic, reversible reaction between a weak acid (e.g., CO2) and a weak base (e.g., MEA) to form a soluble salt. The inlet gas is contacted counter-currently with the lean solvent in the absorber. CO2 is absorbed by the solution. The solution enriched with CO2 is pre-heated before entering the stripper where the reaction is
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reversed by continuous heating. From the bottom of the stripper, the lean solvent exchanges heat with the rich solvent that enters the stripper and is sent back into the absorber. From the top, a high-purity (dry-basis) CO2 is produced.
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For coal-fired power plants, a feasible way to supply the regeneration energy is extracting steam from the turbine steam unit. Considering the fact that the extracted steam is superheated and its
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temperature is usually significantly higher than the MEA safe and stable operation temperature, the extracted steam is first introduced into the desuperheater to regulate its temperature to the value required by the MEA regeneration. The saturated vapor from the desuperheater is then introduced into the reboiler to heat the MEA and is condensed into the condensate.
Fig. 1 The base amine absorption CO2 capture system 4
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For a CO2 capture capacity of 3000t/d from the 660MW unit and the given solvent, the main operation parameters are listed in Table 1. These operation parameters remain unchanged in the
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proposed capture system in this paper. Table 1 Main operation parameters of the base capture system
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For the 660MW coal-fired power plant, the only practical position for the steam extraction is
from the fifth stage turbine. The extracted steam parameters from this stage are 0.418MPa and
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257°C. A desuperheater is used to cool the superheated steam to its corresponding saturation temperature of 144°C at 0.4MPa. The saturated vapor at 0.4MPa and 144°C is introduced into the reboiler and is condensed into sub-cooled water at 128°C, providing the required heat of 134.5MW for solvent regeneration.
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2.2 Model equations
To calculate the mass flow rate of the required extracted steam, the mathematical models in heating process must be established. Therefore, the control volume should be first chosen
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according to thermodynamic theory. The reboiler and the desuperheater in Fig. 1 are selected as the control volume respectively, as is shown in Fig. 2. The mathematical models are established
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for each control volume according to mass and energy conservation principles. The mass flow rate of the required extracted steam can be determined by solving the model equations given in the following sections.
Fig. 2 Mass and energy balance of reboiler and desuperheater 2.2.1 Reboiler 5
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In the reboiler, the rich MEA solvent of a mass flow rate mMEA is heated to vaporize part of the lean amine solution, generating CO2 product stream for stripping. The heating steam of a mass
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flow rate ms (kg/s), temperature Ts and specific enthalpy hs (kJ/kg) enters in the tube side of the reboiler to heat the rich MEA solvent and then is condensed. Neglecting the heat loss to the environment, the mass flow rate of the steam ms can thus be calculated from equation (1).
Qr hs − hco
(1)
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ms =
Where hco is the specific enthalpy of the condensate leaving the reboiler, kJ/kg; Qr is the required
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heat to regenerate the rich EMA solvent in the reboiler in kW. The regeneration heat Qr is 134.5MW as listed in Table 1. 2.2.2 Desuperheater
In the desuperheater, the superheated steam from the steam turbine unit and the desuperheating
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water (here, the condensate from the reboiler is the best choice) are mixed into the saturated vapor that is used to heat the MEA in reboiler. Mass balance in the desuperheater gives,
ms = mext + md
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(2)
And the energy balance in the desuperheater, (3)
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ms hs = mext hext + md hco
Where, mext is the extracted steam flow rate of the base capture system from the steam turbine unit, kg/s; hext is specific enthalpy of the extracted steam, kJ/kg; md is the flow rate of the desuperheating water, kg/s. From the above equations, md and mext can be expressed as,
md =
hext − hs ms hext − hco
mext =
hs − hco ms hext − hco
(4)
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Simple calculation shows that a saturated vapor ms of 220.0t/h at 0.4MPa and 144°C is required and condensed into water at 128°C in the reboiler providing the required heat of 134.5 MW for
turbine unit is 198.4t/h, and the desuperheating water md at 128°C is 21.6t/h. 3. AHT-FE-aided capture system
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solvent desorption. As a result, the required extracted steam at 0.418MPa and 257°C from steam
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In the base system as shown in Fig. 1, the condensate leaves the reboiler at a temperature as
high as 128°C. In order to reduce water consumption, the condensate has to be sent back into the
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mother system. To do so, this high-temperature condensate must be cooled to a temperature lower than 95°C before recycling back to the steam turbine unit. More than that, if the condensate is polluted seriously in the reboiler by MEA leakage which is almost inevitable and could not meet feed-water quality requirement of the power generation system, then it has to be rejected to the
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environment. In this case, the condensate has to be further cooled to a temperature that is lower than 40°C to avoid waste heat pollution. In either case, not only are the additional cooling equipment and medium needed, but also the energy of the high-temperature condensate is wasted.
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To recover the heat of the high-temperature condensate, an absorption heat transformer (AHT) and a flash evaporator (FE) are integrated into the system and thus the AHT-FE-aided capture system
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is proposed.
3.1 System description
The proposed AHT-FE-aided system is shown in Fig. 3. Comparing this new system with the
base system describe above, one can see that the only difference between these two systems is that the AHT-FE-aided system recovers the heat of the condensate. In the AHT-FE-aided capture system, most of the condensate is introduced into AHT, and the upgraded heat is delivered from 7
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AHT. The upgraded heat is then introduced into FE to produce low pressure, low temperature vapor by flash evaporation. The flash-off vapor is introduced into the reboiler to heat the MEA
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solvent. Therefore, in this system, the MEA solvent is not heated by the saturate vapor extracted from the steam turbine, but also heated by the flash-off vapor in the reboiler. Obviously, since the heat required for solvent regeneration in reboiler remains unchanged, the extracted steam flow rate
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is reduced.
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Fig. 3 AHT-FE-aided CO2 capture system 3.2 Model equations
To calculate the mass flow rate of the required extracted steam and the energy saving, the mathematical model for the AHT-FE-aided capture system must be established. The reboiler, the
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desuperheater, the AHT and the FE in Fig. 3 are selected as the control volume respectively, as shown in Fig. 4. Comparing with Fig. 2, the only difference in Fig. 4 is that the reboiler is supplied with the saturated vapor and the flash-off vapor. Therefore, Eq. (2) and Eq. (3) remain unchanged.
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And the mass and energy conservation for the AHT and FE are described by Eq. (5) to Eq. (12),
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on the condition that the heat loss to the environment is neglected.
Fig. 4 Mass and energy conservation of the AHT-FE-aided reboiler
3.2.1 Reboiler
The mass flow rate of the steam ms can be calculated from Eq. (5).
Qr = ms (hs − hco ) + mf (hf − hco )
(5)
Where, mf is the mass flow rate of the flash-off vapor from FE, kg/s; hf is the specific enthalpy 8
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of the flash-off vapor, kJ/kg. The AHT-FE-aided capture system recovers some energy of the high-temperature condensate,
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and this should not change the heat required for MEA regeneration. Therefore, without changing the operation parameters of the system, the regeneration heat of the AHT-FE-aided system remains unchanged, i.e., Qr is still equal to 134.5MW.
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3.2.2 AHT
An absorption heat transformer (AHT) can increase the temperature of low or moderately waste
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heat source to a higher and more useful level [18]. The AHT basically consists of an evaporator (EV), a generator (GE), a condenser (CO), an absorber (AB) and a solution heat exchanger. The evaporator and the generator are supplied with waste heat and the upgraded heat is delivered from the absorber. The coefficient of performance (COP) is used to measure the ability of AHTs to
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transfer heat between different temperature heat sources. The COP is defined as the ratio of the heat released from the AB to the total heat input from the GE and the EV [19]. COP =
QAB QEV + QGE
(6)
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Where, Q is the heat load in kW and the subscripts AB, EV and GE represent evaporator, generator and absorber.
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The calculation of COP defined by Eq. (6) is very complicated and will be discussed later. If
COP is known, then the mathematical model for AHT is simplified and only the mass and energy conservation is considered. The heat loads in the generator and evaporator can be determined by the temperature decrease of the condensate which is introduced into the EV and GE, that is,
QEV + QGE = mext, A cp (tg1 − tg2 ) + mext, A cp (tg2 − tg3 )
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That is,
QEV + QGE = mext, A cp (tg1 − tg3 )
(8)
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Where, mex,A is the extracted steam flow rate of the AHT-FE-aided capture system which is equal to the condensate flow rate entering EV and GE, kg/s; cp is specific heat of the condensate, kJ/(kg.K); tg1 is the inlet temperature of the condensate entering the EV, °C; tg2 is the temperature
°C.
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The absorber heat load is determined by the COP, that is,
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of the condensate leaving the EV, °C; tg3 is the outlet temperature of the condensate leaving the GE,
QAB = (QEV + QGE )COP
(9)
The water leaving the AB is used as the upgraded heat source and heated from th1 to th2 in the absorber. Then the mass flow rate mh of the water leaving the AB can be determined by Eq. (10). (10)
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QAB = mh (hh1 − hh2 )
Where hh1 is the specific enthalpy of the water entering the AB, kJ/kg; hh2 is the specific enthalpy of the water leaving the AB, kJ/kg.
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To produce flash-off vapor in the FE, the water into the AB should be heated until it reaches its boiling point in the absorber. So the enthalpy of the saturation water i.e. hh2 depends on the
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pressure of the water leaving the AB. hh1 can be calculated using the following equation:
mh hh1 = mf hco + (mh − mf )h '
(11)
Where, mf is the mass flow rate of the flash-off vapor, kg/s; h′ is the specific enthalpy of the
leftover water leaving the flash evaporator, kJ/kg. 3.2.3 Flash evaporator The FE pressure should be lower than the water pressure in the AB. Thus the water leaving the 10
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AB produces low-pressure and low-temperature vapor due to flash effects by sudden pressure drop in the FE. Without consideration of the heat loss to the environment, the FE energy balance
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can be simply written as, mh hh2 = mf hf + ( mh − mf ) h′
(12)
Where, mf is the mass flow rate of the flash-off vapor, kg/s; hf is the specific enthalpy of the
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flash-off vapor, kJ/kg; h′ is the specific enthalpy of the leftover water leaving the FE, kJ/kg.
For the flash-off vapor, the optimal option is to introduce the flash-off vapor into the saturated
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vapor from the desuperheater by a pipeline before entering the reboiler. In order to reduce the exergy loss, the temperature and pressure of the flash-off vapor should be equal to those of the saturated vapor from the desuperheater. Therefore, the flash-off vapor temperature and pressure should be 144°C and 0.4MPa.
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From Eq. (10) to Eq. (12), it is easy to find,
QAB = mf (hf − hco )
(13)
Eq. (13) indicates that heat load in reboiler from the flash-off vapor is equal to the absorber
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heat load. That is, the recovery heat in the AB is fully reused in the reboiler. Therefore, no matter what the pressure of the water leaving the AB is, the mass flow rate of the flash-off vapor mf is
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fixed, if only QAB, hf and hco are fixed. 3.3 Performance of the AHT AHT can effectively recover about 50% of the waste heat and reuse it in industrial processes
[20]. For the AHT-FE-aided capture system, it is important to supply the reboiler with flash-off vapor as much as possible at required temperature. Therefore, COP, absorption temperature and heat load in absorber are the main performance parameters of the AHT. For the given 11
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refrigerant-absorbent solution, performance of AHT depends on solution concentration difference between weak solution and strong solution, pressure drop of the refrigerant vapor and temperature
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difference in heat exchanger, the waste heat temperature, the cooling water temperature and so on. The aim of this section is to illustrate the effect of the condensate temperature and the cooling water temperature on COP, the absorption temperature and the absorber heat load using
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water-lithium bromide (water-LiBr) solutions, when other data are fixed. The basic data used in Fig. 5 and Fig. 6 for the AHT performance calculations are summarized in Table 2.
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Table 2 Basic data used in Fig. 5 and Fig. 6 for the AHT performance calculation
3.3.1 COP
Fig. 5 depicts the influence of the condensate temperature and the cooling water temperature
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on COP. It can be seen that the COP of the AHT is insensitive to the variation of the outlet temperature of both the condensate and the cooling water within the calculated region. The COP is about 0.5 and unchanged almost within the range of temperature, which means that the AHT can
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recover about 50% of the condensate sensitive heat and reuse it in the reboiler.
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Fig. 5 Influences of the condensate and the cooling water temperature on COP
3.3.2 Absorption temperature Fig. 6 depicts the effect of the condensate temperature and the cooling water temperature on
the absorption temperature, at a given set of conditions. It can be seen that when the inlet temperature of the condensate and the cooling water is fixed, lowering the outlet temperature of the condensate or increasing the outlet temperature of the cooling water will both reduces the 12
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absorption temperature.
on the absorption temperature 3.3.3 Absorber heat load
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Fig. 6 Influences of the condensate and the cooling water temperature
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The absorber heat load QAB can be determined by Eq. (9). It can be seen from the model
equations that at a given inlet temperature of the condensate, with decreasing the outlet
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temperature of the condensate, the heat loads in the generator and evaporator increase, and the heat load in the absorber increases too. So the outlet temperature of the condensate should be as low as possible, on the condition that the absorption temperature meets the requirement of the upgraded heat source temperature (the water temperature leaving the AB).
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4. Performance of the capture systems
The water leaving the AB produces low-pressure and low-temperature vapor due to flash effects by sudden pressure drop in FE. Thus, the water pressure leaving the AB ought to be higher than
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the flash-off vapor pressure of 0.4MPa. If the temperature difference between the absorption temperature and the water temperature leaving the AB is fixed, then increasing the water pressure
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leaving the AB increases the required absorption temperature. As it is pointed out above, the absorber heat load decreases with the outlet temperature of the condensate. Therefore, in order to recover more heat of the condensate and obtain more flash-off vapor, the water pressure leaving the AB should be as low as possible only if it is higher than 0.4MPa. However, the lower water pressure leaving the AB results in the lower water temperature, a larger mass flow rate of water leaving the AB, more pump work and larger heat transfer area of the AB. 13
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With these considerations, the water pressure leaving the AB is set at 0.5MPa whose saturation temperature is 152°C. Accordingly, as saturated water of 152°C, specific enthalpy of water leaving
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the AB is 640.2kJ/kg. IF the heat transfer temperature difference between the absorption temperature and the water temperature leaving the AB is 5K which is a common practice for
ordinary heat exchanger design, then the absorption temperature is 157°C. From Fig. 6, one can
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find that the outlet temperature of the condensate is 95°C if the outlet temperature of the cooling water is set at 38°C.
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The basic data used for analyzing the AHT-FE-aided capture system are listed in Table 2 and Table 3.
Table 3 Basic data used in the analysis for the AHT-FE-aided capture system
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Based on the data in Table 3, the model equations given above are solved by iteration method and the performance parameters of the AHT-FE-aided capture system obtained are listed in Table 4. If the flash-off vapor temperature is designed lower than 144°C, the absorption temperature
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may be lower than 157°C. From Fig. 6, the outlet temperature of the condensate is lower than 95°C. Therefore, the AHT-FE-aided capture system can recover more heat of the condensate, and
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obtain more flash-off vapor and thus greater energy saving.
Table 4 Performance of the capture systems
In Table 4, the specific heat consumption is an important synthetic indicator that reflects the energy consumption level of the CO2 capture system. The specific heat consumption is defined as the thermal energy consumed for the desorption of 1 ton CO2 in the stripper and is calculated from 14
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the following equation, q=
mext ( hext − hco ) × 3600 DCO2
(14)
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Where, q is the specific heat consumption, kJ/tCO2; DCO2 is the CO2 output, t/h. The energy saving is used as an indicator for the performance improvement of the AHT-FE-aided capture system over the base system. The energy saving is defined by Eq. (15) or
ζ =
mext − mext,A
×100%
q − qA ×100% q
(15)
(16)
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ζ =
mext
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Eq. (16).
Where, q is the specific heat consumption of the base capture system, kJ/tCO2; qA is the specific heat consumption of the AHT-aided capture system, kJ/tCO2.
5. Economic analysis
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A simple economic analysis is made for the AHT-FE-aided capture system. It can be seen from Table 4, the high-parameter steam extracted from the steam turbine flow rate required is 193.2t/h. Thus the reduction in the extracted steam compared with the base system is 5.2t/h and this will
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increase an electricity production of the power generation system by 920kW. Taking an average electricity grid price of China as the electricity price that is approximately 0.4 RMB Yuan/kW⋅h
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and the operation hours is 8000h/year, the annual profit would be 2.94 million RMB Yuan or about 0.48 million USD. It can be seen that, although the energy saving of the AHT-FE-aided capture system is only 2.62%, it can still produce an annual profit of 0.48 million USD. For the AHT-FE-aided capture system, an AHT and a FE were added to the base capture system. The initial equipment cost of the AHT of 3.7MW is about 7.0 million RMB or 1.14 million USD. The payback period for the AHT-FE- aided system is approximately 2.4 years. Compared with the 15
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15 years’ service life of the absorption transformer, a payback period of 2.4 years for the AHT-FE-aided system is very attractive and the technology may produce a net profit of about 6
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million USD during its full service life. 6. Conclusions
Absorption heat transformer and flash evaporation are used to reduce the heat consumption of
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CO2 capture process and the AHT-FE-aided capture system is proposed. To verify its effectiveness, the process models are established and solved. By comparison with the base system, it is found
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that the proposed the system can reduce the energy consumption of the CO2 capture system effectively.
(1) Energy analyses show that the proposed AHT-FE-aided capture system may recover a large portion of the waste heat of the high-temperature condensate. Compared with the base CO2
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capture system of a CO2 capture capacity of 3000t/d from a 660MW unit, it reduces the heat consumption from 3.873GJ/tCO2 to 3.772GJ/tCO2, the resulted energy saving is 2.62%. (2) The economic analysis shows that the annual profit of the new system is 2.94 million RMB
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Yuan or 0.48 million USD. The payback period is approximately 2.4 years. This proves that the proposed system is both economically and technically feasible for improving the CO2 capture
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energy performance.
(3) Within the temperature calculated, it is found that the COP of the AHT is not sensitive to
the variation of the outlet temperature of both the condensate and the cooling water, the COP is about 0.5 and remains almost unchanged within the range of temperature. (4) With the fixed inlet temperature of the condensate and the cooling water, lowering the outlet temperature of the condensate or increasing the outlet temperature of the cooling water will 16
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both reduces the absorption temperature. If the required absorption temperature is 157°C, then the outlet temperature of the condensate is calculated to be 95°C for the example system. The
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flash-off vapor of 6.8t/h is obtained by recovering the heat of the condensate via the absorption heat transformer.
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Acknowledgments
This work is supported by the Key Project No. 2012BAC24B01 of the National Key
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Technologies R & D Program, the Ministry of Science Technology of the People’s Republic of China.
References
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Fig. 1 The base amine absorption CO2 capture system
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Fig. 2 Mass and energy balance of reboiler and desuperheater Fig. 3 AHT-FE-aided CO2 capture system Fig. 4 Mass and energy conservation of the AHT-FE-aided reboiler Fig. 5 Influences of the condensate and the cooling water temperature on COP Fig.6 Influences of the condensate and the cooling water temperature on the absorption temperature 19
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Table 1 Main operation parameters of the base capture system
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Table 2 Basic data used in Fig. 6 and Fig. 7 for the AHT performance calculations Table 3 Basic data used in the analysis for the AHT-FE-aided capture system
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Table 4 Performance of the capture systems
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Table 1 Main operation parameters of the base capture system 40 112 98 134.5
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Regeneration pressure in stripper (kPa) Regeneration temperature in stripper (°C) CO2 product stream temperature (°C) Heat required for solvent regeneration in reboiler (MW)
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Table 2 Basic data used in Fig. 5 and Fig. 6 for the AHT performance calculation 128 32 0.05 66 66 tg2 -3 tg3 -3 tc2+3 3
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Initial temperature of the condensate from the reboiler (°C) Initial temperature of the cooling water (°C) Solution concentration difference between weak solution and strong solution Refrigerant vapor pressure drop between the GE and the CO (Pa) Refrigerant vapor pressure drop between the EV and the AB (Pa) Evaporation temperature (°C) Strong solution outlet temperature in the EG (°C) Condensation temperature (°C) Temperature difference in the solution heat exchanger (K)
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Table 3 Basic data used in the analysis for the AHT-FE-aided capture system 134.5 38 5 144
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Heat load in reboiler (MW) Outlet temperature of cooling water (°C) Heat transfer temperature difference (K) Flash-off vapor temperature (°C)
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Table 4 Performance of the capture systems
198.4 3.873 -
0.5 152 640.2 157 95 193.2 6.8 226.7 0.5 7402 3701 3.772 2.62
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AHT-FE-aided capture system
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Outlet pressure of water leaving the AB (MPa) Outlet temperature of water leaving the AB (°C) Outlet specific enthalpy of water leaving the AB (kJ/kg) Absorption temperature (°C) Outlet temperature of the condensate (°C) Extracted steam required (t/h) Flow rate of the flash-off vapor (t/h) Mass flow rate of water leaving the AB (t/h) COP Heat load of the GE and EV (kW) Heat load of the AB (kW) Specific heat consumption (GJ/tCO2) Energy saving (%)
Base capture system
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Fig. 1 The base amine absorption CO2 capture system
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Fig. 2 Mass and energy balance of reboiler and desuperheater
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Fig. 3 AHT-FE-aided CO2 capture system
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Fig. 4 Mass and energy conservation of the AHT-FE-aided reboiler
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Fig. 5 Influences of the condensate and the cooling water temperature on COP
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Fig. 6 Influences of the condensate and the cooling water temperature on the absorption temperature
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Highlights: 1. Absorption heat transformer (AHT) is introduced into the CO2 capture system. 2. The AHT-FE-aided capture system is proposed. 3. The mathematical models in heating process are established. 4. Detailed analysis and results of the AHT-FE-aided capture system are presented.