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Thermodynamic analysis of waste heat recovery of molten blast furnace slag
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Thermodynamic analysis of waste heat recovery of molten blast furnace slag Peng Li a,b,* a
CISDI R&D CO., LTD., Chongqing, No.11, Huijin Road, New North Zone, Chongqing, 401122, PR China Chongqing CISDI Thermal & Environmental Engineering Co., Ltd., No. 1, Saidi Road, New North Zone, Chongqing, 401122, PR China b
article info
abstract
Article history:
Based on the first law of thermodynamics and second law of thermodynamics, using
Received 7 November 2016
enthalpyeexergy compass, the thermodynamic analysis of the waste heat recovery system
Received in revised form
using different methods to recover the sensible heat of molten BF slag was studied. The
26 December 2016
results show that the heat efficiency of physical methods is 76.9%, and the exergy effi-
Accepted 28 December 2016
ciency of recovery as steam is 34.2%; the heat efficiency of combined methods is 92.2%, and
Available online xxx
the exergy efficiency is above 60%. The heat efficiency and exergy efficiency of combined methods are higher than that of physical methods. On the promise of making compre-
Keywords:
hensive consideration for the exergy efficiency and the reaction condition, whether or not
Heat recovery
to consider the use of catalyst, the consumption of reactant and the amount of product, the
Molten slag
C-CO2/H2O reaction are selected as the best reaction.
Thermodynamic analysis
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Blast furnace
Introduction Molten BF (Blast Furnace) slag is the byproduct of iron-making process at the high temperature ranging from 1723 K to 1923 K. Because of its low thermal conductivity, ranging from 1 to 3 W m1 K1 for solid to 0.1e0.3 W m1 K1 for liquid [1], the waste heat recovery of molten BF slag is difficult. Now, the treatment of molten BF slag is water quenching. However, using this method, we can not recover the waste heat of slag, and during the water quenching process, it polluted environment, consumed water resources. Thus in order to resolve these problems, many researchers proposed several methods. One is physical method [2e9], i.e. transferring the heat of slag to hot air, steam or molten salts. Another is chemical method [10e21], i.e. using the waste heat of molten slag to produce fuel gas. Akiyama et al. [12e14] pointed out that decomposition of
limestone, reforming of methane and gasification of carbon are the more suitable for the energy recovery from molten slag, as they present the least exergy loss among all the reactions studied. The concept of reforming of methane using molten slag has been proposed by Kasai et al. [15]. Shimada et al. [10] studied the effects of slag composition on the reaction rates. Kasai et al. [15] studied the rates of methane-steam reaction on the slag surface and. It was shown that larger CaO content in the slag gives rise to higher reaction rates while FeO and sulphur have inhibiting effects. The apparent rate of reforming was found to be 103e101 mol cm2 s1. Maruoka et al. [11] designed a new system in which the slag is first granulated using a rotary cup and is then accumulated in a packed bed. Mizuochi et al. [16,17] reported the industrial design based on the RCA (Rotary Cup Atomizer) technology. Purwanto and Akiyama [18] studied the possibility of hydrogen production from biogas using hot slag, in which decomposition rate of
* Corresponding author. Chongqing CISDI Thermal & Environmental Engineering Co., Ltd., No. 1, Saidi Road, New North Zone, Chongqing, 401122, PR China. Fax: 86 023 63545842. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2016.12.135 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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CO2-CH4 in a pack bed of granulated slag was measured at a constant flow-rate and pressure. Li et al. [19e21] proposed a new route to effectively utilize molten BF slag by using molten BF slag as heat carrier of coal gasification. Liu et al. [23,24] conducted a series of experiments about the RCA method to recover the molten blast furnace slag waste heat and obtained some constructive and instructional results. In the more recent experimental works, the focuses of these researches were only on the mechanism, conversion kinetics, and reaction rate; no information may be obtained regarding the system design. Accordingly, the purpose of this work is to investigate the system design using enthalpyeexergy compass.
Method Based on the first law of thermodynamic and second law of thermodynamic, we use enthalpyeexergy compass proposed by Ishida [22] to analyze the waste heat recovery system of molten BF slag. In the process of energy conversion, the enthalpy is constant, but the exergy is changed which can be expressed by Equations (1) and (2). X
DHj ¼ 0
(1)
Dεj 0
(2)
j
X j
In the waste heat recovery system of molten BF slag, there are not only the exergy of physical energy, but also the exergy of chemical energy. The exergy of thermal energy can be calculated by Equation (3). Dε ¼ DH
T T0 T
(3)
where T is the temperature of energy carrier; T0 is the temperature of environment. The exergy of chemical energy can be expressed as Equation (4). Dε ¼ DH T0 DS
(4)
where Tinitial is the initial reaction temperature. And when the temperature is not initial reaction temperature, the exergy of chemical energy can be calculated by Equation (8). Dε ¼ DH0
0 T T0 1 T0 P DG' 0 þ DH0 0 0 DH þ Ci DT T T 0
(8)
0
where T is the reaction temperature, K; DH is the absorbed 0 heat of reaction, kJ mol1; DG is the Gibbs free energy at the reaction temperature, kJ mol1; DT is the difference between reaction temperature and environmental temperature, K;Ci is the specific heat of product, kJ$(mol K)1. Thus using Equations (3), (7) and (8), we can calculate the enthalpy and exergy before and after energy conversion, based on that, we can get Fig. 1. From Fig. 1, we can see the exergy loss in the process of energy transformation. Thus we can get the energy efficiency and exergy efficiency of the system.
System model Physical method Thermal energy method use hot air, steam and so on as heat carrier to recover the waste heat of molten BF slag. Fig. 2 is a typical waste heat recovery system using physical method. In this system, molten slag at the temperature of 1773 K was granulated by RCA (Rotary Cup Atomizer). The temperature of slag granules is about 1473 K. And then the slag granules enter into the energy recovery equipment. Finally, the temperature of slag granules discharged from the energy recovery equipment is about 473 K. In this system, the amount of energy recovered during RCA process is 0. Using this system, we can produce hot water and steam. In this paper, we consider using this system to produce hot water at the temperature of 353 K and steam at the temperature of 473 K, the equations are expressed as: waste heat
H2 Oð303KÞ ! H2 Oð353KÞ waste heat
H2 Oð303KÞ ! H2 Oð473KÞ
(9) (10)
where DS is the entropy of energy carrier, it can be calculated by Equation (5). DG ¼ DH TDS
(5)
where DG is the change value of Gibbs free energy. Based on Equations (4) and (5), the exergy of chemical energy can be expressed as: Dε ¼ DH T0
DH DG T T0 T0 ¼ DH þ DG T T T
(6)
According to Equation (6), if the temperature is the initial reaction temperature, DG ¼ 0, thus we can get Equation (7). When the temperature is initial reaction temperature, the exergy of chemical energy can be expressed as: Dε ¼ DH
Tinitial T0 Tinitial
(7) Fig. 1 e Enthalpyeexergy diagram for the process of heat transformation.
Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Fig. 2 e The system of waste heat recovery from molten BF slag by physical method. Using this model, we can get the heat efficiency and exergy efficiency of the system and the amount of steam produced using this system. The heat efficiency of the system can be calculated by Equation (11). h1 ¼ DH1 =DH 100%
(11)
where DH is the enthalpy of energy entered into system, MJ; DH1 is the enthalpy of energy recovered in this system, MJ. The exergy efficiency of energy recovery equipment can be calculated by Equation (12). hexi ¼ Dε'i Dεi 100%
(12)
where Dεi is the exergy of energy entered into the energy recovery equipment, MJ; Dε0i is the exergy of energy recovered in energy recovery equipment, MJ. The exergy efficiency of the system can be calculated by Equation (13). hex ¼ Dε0 =Dε 100%
(13)
where Dε is the exergy of energy entered into the system, MJ; 0 Dε is the exergy of energy recovered in the system, MJ. The amount of hot water or steam can be calculated by Equation (14). Wwater ¼ Q=ðihot icool Þ
(14)
where Q is the energy recovered using boiler, MJ; ihot is the enthalpy of hot water or steam, MJ kg1, icool is the enthalpy of cold water entered into boiler, MJ kg1.
Combined method of physical and chemical Chemical methods use chemical reaction to recover the waste heat of molten slag. Due to restrictions of initial reaction temperature, the chemical method can not recover the waste heat of slag granules at low temperature. Thus in this section, we build the combined model of physical and chemical method directly, and do not build the chemical model alone. Fig. 3 shows a typical waste heat recovery system using combined method of physical and chemical. In this system, the process consists of four steps. In the first step, the sensible heat of molten slag is recovered using chemical method, and the molten slag is cooled rapidly to the temperature 1573 K. The still molten slag (~1573 K) is then transferred to the second step, in which the slag is granulated
using a rotary cup. In the third step, the waste heat of slag granules is recovered using chemical method until the temperature of slag granules decrease to the initial reaction temperature. And then the waste heat of slag granules is recovered using physical method. When the slag granules leave the system, the temperature of slag granules is lower than 373 K. The chemical reaction used in this system must meet two requirements, one is endothermic reaction, and another is widely used in industry field. Based on the abovementioned two points, several chemical reactions were selected to recover the waste heat of molten slag as shown in Table 1. Reaction 1 is the chemical reaction of hydrogen generation by water electrolysis. Reaction 2 is the chemical reaction of decomposing of calcium carbonate. Reaction 3 and 4 are chemical reaction of coal gasification. Reaction 5, 6, and 7 are the chemical reaction of methane integration. Reaction 8 is the chemical reaction of hydrogen production from propane. Using this model, we can calculate the heat efficiency and exergy efficiency of the system using different chemical reaction to recover the sensible heat of molten slag, and evaluate which one is the best chemical reaction. The heat efficiency and exergy efficiency of energy recovery equipment using different reactions can be calculated by Equations (11) and (12). The amount of reactants can be calculated by Equation (15). Wi ¼ Q
. X DHi þ Ci Dti Mi
(15)
where Q is the energy recovered by energy recovery equipment, MJ; DHi is the changes of enthalpy of chemical reaction I, MJ; Ci is the specific heat of reactants I, MJ$(mol K)1; Dti is the temperature difference, K; Mi is the molar mass of reactants i, g mol1. The amount of product can be calculated by Equation (16). Wi0 ¼ Q
. X DHi þ Ci Dti a M0i
(16)
where Q is the energy recovered by energy recovery equipment, MJ; DHi is the changes of enthalpy of chemical reaction I, MJ; Ci is the specific heat of reactants I, MJ$(mol K)1; Dti is the temperature difference, K; Mi is the molar mass of product I, g mol1; a is the stoichiometric number.
Fig. 3 e The system of waste heat recovery from molten BF slag by combined method of physical and chemical. Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Table 1 e The enthalpy, exergy and initial reaction temperature of the reactions which were used in the system of waste heat recovery from molten BF slag. No. R1 R2 R3 R4 R5 R6 R7 R8
Chemical reaction
Enthalpy DH (kJ)
DGT¼1573 K (kJ)
Exergy Dε (kJ)
Tinitial (K)
A ¼ (Dε/DH)
H2O / 0.5O2 þ H2 CaCO3 / CaO þ CO2 C þ CO2 / 2CO C þ H2O / CO þ H2 CH4 þ CO2 / 2CO þ 2H2 CH4 þ H2O / CO þ 3H2 CH4 þ 2H2O / CO2 þ 4H2 C3H8 þ 3H2O / 3CO þ 7H2
242 166 171 136 260 223 187 498
194 58 104 90 186 171 157 379
229 123 119 93 175 149 122 298
5400 1160 974 948 916 894 865 742
0.945 0.743 0.694 0.686 0.675 0.667 0.655 0.598
The amount of hot water or steam can be calculated by Equation (14).
Model assumption In this paper, the following regulations and assumptions are applied to evaluate the waste heat recovery system. (1) One ton BF slag was studied as study object. (2) The chemical composition of BF slag is 41.21 mass% CaO, 34.38 mass% SiO2, 11.05 mass% Al2O3, 8.22 mass% MgO, 0.35 mass% TiO2. (3) The average specific heat is 1.21 kJ$(kg K)1, which means the enthalpy and exergy of molten BF slag flowed into system are 1573 MJ and 1308.7 MJ separately. (4) In the energy recovery equipment, the heat efficiency is 1, which means the heat loss is 0. (5) There is no energy recovered during the process of slag granulation. (6) The temperature of slag granules discharged from unit 2 is initial reaction temperature. (7) Without regard to heat loss of BF slag transporting between equipment. (8) The temperature of cold water flowed into boiler is 303 K.
Results and discussion Physical method Fig. 4 shows the enthalpyeexergy diagram for heat recovery from BF slag to produce hot. It is found that the enthalpy and
exergy entered into this system is 1573 MJ and 1309 MJ separately. The enthalpy and exergy entered into boiler is 1210 MJ and 965 MJ separately. The enthalpy of energy recovered using boiler is 1210 MJ, but the exergy is only 188 MJ, the exergy loss of boiler is 777 MJ. The exergy loss of this system is 1121 MJ. The heat efficiency of this system is 76.9%, the exergy efficiency of this system is only 14.4%. The hot water yield of per ton molten slag can attain 5781 kg. Thus we can see that using the waste heat of molten BF slag to produce hot water is not suitable. Fig. 5 shows the enthalpyeexergy diagram for heat recovery from BF slag to produce steam. It is found that the enthalpy and exergy entered into this system is also 1573 MJ and 1309 MJ separately. The enthalpy and exergy of energy recovered using this system is 1210 MJ and 448 MJ separately. The exergy loss of this system is 861 MJ. The heat efficiency of this system is 76.9%, the exergy efficiency of this system is 34.2%. The steam yield of per ton molten slag can attain 453 kg.
Combined method of physical and chemical Fig. 6 shows enthalpy and exergy diagram using combined method of physical and chemical to recover sensible heat of molten slag. In this system, we use chemical method to recover the waste heat of molten slag at high temperature in the unit 1 and unit 2, and use physical method to recover the waste heat of slag granules at low temperature. In Fig. 6, we use R1 (H2O ¼ H2 þ 0.5O2) as endothermic reaction to recover the waste heat of BF slag. It is found that the exergy of energy recovered using this system is more than that entered into this system. Based on the first law of thermodynamic and second law of thermodynamic, we know this is impossible.
Fig. 4 e Enthalpyeexergy diagram for heat recovery from BF slag to produce hot water. Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Fig. 5 e Enthalpyeexergy diagram for heat recovery from BF slag to produce steam.
Fig. 6 e Enthalpyeexergy diagram for heat recovery from BF slag by using the reaction: H2O ¼ H2 þ 0.5O2.
And from Table 1, we can see that the initial reaction temperature is 5400 K which is higher than the temperature of molten slag. Thus the reaction was not happened. Fig. 7 shows the enthalpyeexergy diagram for heat recovery from BF slag by using decomposing of calcium carbonate. It is found that the exergy of energy entered into unit 1, unit 2 and unit 3 are 201 MJ, 302 MJ and 618 MJ separately. And the exergy of energy recovered from unit 1 unit 2 and unit 3 are 187 MJ, 281 MJ and 308 MJ separately. Besides, the exergy loss of unit 1 is only 14 MJ. The exergy loss of unit 2 is only 21 MJ. The enthalpy and exergy of energy entered into this system is 1573 MJ and 1309 MJ. The enthalpy and exergy of energy recovered using this system is 1451 MJ and 776 MJ. The exergy efficiency of unit 1 and unit 2 is 93%. The exergy efficiency of unit 3 is 49.8%. The heat efficiency of this system is 92.2%, and the exergy efficiency of this system is 59.3%. Those both of the heat efficiency and the exergy efficiency are higher than that using physical method to recover sensible heat of molten slag. The CaCO3 consumption of per ton BF slag is 230 kg, and the steam yield of per ton molten slag can attain 312 kg.
Fig. 8 shows the enthalpyeexergy diagram for heat recovery from BF slag by using coal gasification. It is found that the exergy of energy entered into unit 1, unit 2 and unit 3 are 201 MJ, 482 MJ and 421 MJ separately. And the exergy of energy recovered from unit 1 unit 2 and unit 3 are 177 MJ, 419 MJ and 224 MJ separately. Besides, the exergy loss of unit 1 is only 24 MJ. The exergy loss of unit 2 is 63 MJ. The enthalpy and exergy of energy entered into this system is 1573 MJ and 1309 MJ. The enthalpy and exergy of energy recovered using this system is 1451 MJ and 821 MJ. The exergy efficiency of unit 1 is 88.1%. The exergy efficiency of unit 2 is 86.9%. The exergy efficiency of unit 3 is 53.2%. The heat efficiency of this system is 92.2%, and the exergy efficiency of this system is 62.7%. The carbon consumption of per ton BF slag is 45 kg, and the steam yield of per ton molten slag can attain 227 kg. Besides, using this system, per ton BF slag can consume 166 kg CO2 which is benefit to reduce carbon emission. Using this method, we can also get the heat efficiency and exergy efficiency of the other system using R3, R4, R5, R6, R7, or R8 as endothermic reaction to recover the waste heat of BF slag. Table 2 shows the heat efficiency and exergy efficiency of
Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Fig. 7 e Enthalpyeexergy diagram for heat recovery from BF slag by using the reaction: CaCO3 ¼ CaO þ CO2.
Fig. 8 e Enthalpyeexergy diagram for heat recovery from BF slag by using the reaction: C þ CO2 ¼ 2CO.
Table 2 e The enthalpy and exergy efficiency of the system of waste heat recovery from molten BF slag. No.
Method
Unit 1
Unit 2
Unit 3
Consumption of reactant
The yield of steam
hex1
hex2
hex3
hheat
System hex
Wi (kg t1)
Wsteam (kg t1)
e 93.0 88.1 87.9 87.4 87.2 88.6 87.8
e 93.1 86.9 86.0 84.6 83.5 82.1 74.9
e 49.8 53.2 54.0 54.8 55.3 56.3 61.5
e 92.2 92.2 92.2 92.2 92.2 92.2 92.2
e 59.3 62.7 63.1 63.3 63.5 63.9 63.1
e WCaCO3 ¼ 230 WC ¼ 45; WCO2 ¼ 165 WC ¼ 56; WH2 O ¼ 85 WCH4 ¼ 41; WCO2 ¼ 113 WCH4 ¼ 48; WH2 O ¼ 54 WCH4 ¼ 51; WH2 O ¼ 114 WC3 H8 ¼ 73; WH2 O ¼ 90
e Wstream Wstream Wstream Wstream Wstream Wstream Wstream
e e
e e
19.5 46.4
76.9 76.9
14.4 34.2
WH2 O ¼ 5781 WH2 O ¼ 453
Wstream ¼ 5781 Wstream ¼ 453
Combined of chemical and physical method R1 R2 R3 R4 R5 R6 R7 R8
H2O ¼ H2 þ 0.5O2 CaCO3 ¼ CaO þ CO2 C þ CO2 ¼ 2CO C þ H2O ¼ CO þ H2 CH4 þ CO2 ¼ 2CO þ 2H2 CH4 þ H2O ¼ CO þ 3H2 CH4 þ 2H2O ¼ CO2 þ 4H2 C3H8 þ 3H2O ¼ 3CO þ 7H2
¼ 312 ¼ 227 ¼ 216 ¼ 201 ¼ 191 ¼ 178 ¼ 122
Physical method 1 2
H2O(303 K) ¼ H2O(353 K) H2O(303 K) ¼ H2O(473 K)
Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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the system using different chemical reactions or different methods. It is found that both of the heat efficiency and the exergy efficiency of the system using combined method is higher than that of the system using physical method. The heat efficiency of the system using physical method is 76.9%; however the heat efficiency of the system using combined method is 92.2%. The exergy efficiency of the system using the sensible heat of molten slag to produce hot water is only 14.4%; and that of the system using the sensible heat of molten slag to produce steam is only 34.2%. Both of them are lower than that of the system using combined methods, which is around 60%. Besides that, it is found that the exergy efficiency of the system using different chemical reaction is different. The exergy efficiency of the system using R2 is lower than that using the other chemical reactions. The difference of the system using R3, R4, R5, R6, R7, and R8 as endothermic reaction is little. But we found that the reaction of CH4 and H2O or CO2 needs catalyst. Thus the separation of catalyst and slag granules is difficult. And in our other studies [19e21], we found the BF slag can catalyze the coal gasification reaction. Thus we think that R3 and R4 are the suitable reaction used to recover the waste heat of molten slag. Besides we can also see that using R3 and R4 as chemical reaction can reduce the carbon emission.
Conclusions The heat efficiency and exergy efficiency of the system using different method to recover waste heat of molten slag was calculated using enthalpyeexergy diagram. The heat efficiency and exergy efficiency of combined method is higher than that of physical method. The heat efficiency of combined method is 92.2%, and that of physical method is only 76.9%. The exergy of combined method is higher than 59%, and that of physical method is lower than 35%. Although these technologies belong to the same method, the exergy efficiencies are different. The exergy efficiency of the system using the waste heat of molten slag to produce steam is 34.2% which is more than that of the system to produce hot water. And the exergy efficiencies of the system using different reaction to recover the waste heat of molten slag are different. The exergy efficiency of the system using R2 is 59.3%, and the others are around 63%. Considering the reaction condition, reactant, whether or not to use catalyst, and so on, we think that R3 and R4 were the best reaction to recover the waste heat of molten BF slag.
Acknowledgement This research was supposed by National High-tech R&D Program (2006AA05Z209), National Natural Science Fund-Joint Fund of Iron and Steel Research (50574021), Key Technologies R&D Program (2013BAA03B03), Fundamental Research Funds for the Central Universities (N110602002) and Fundamental Research Funds of Chongqing (cstc2014jcyja90002).
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Thermodynamic analysis of waste heat recovery of molten blast furnace slag Peng Li a,b,* a
CISDI R&D CO., LTD., Chongqing, No.11, Huijin Road, New North Zone, Chongqing, 401122, PR China Chongqing CISDI Thermal & Environmental Engineering Co., Ltd., No. 1, Saidi Road, New North Zone, Chongqing, 401122, PR China b
article info
abstract
Article history:
Based on the first law of thermodynamics and second law of thermodynamics, using
Received 7 November 2016
enthalpyeexergy compass, the thermodynamic analysis of the waste heat recovery system
Received in revised form
using different methods to recover the sensible heat of molten BF slag was studied. The
26 December 2016
results show that the heat efficiency of physical methods is 76.9%, and the exergy effi-
Accepted 28 December 2016
ciency of recovery as steam is 34.2%; the heat efficiency of combined methods is 92.2%, and
Available online xxx
the exergy efficiency is above 60%. The heat efficiency and exergy efficiency of combined methods are higher than that of physical methods. On the promise of making compre-
Keywords:
hensive consideration for the exergy efficiency and the reaction condition, whether or not
Heat recovery
to consider the use of catalyst, the consumption of reactant and the amount of product, the
Molten slag
C-CO2/H2O reaction are selected as the best reaction.
Thermodynamic analysis
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Blast furnace
Introduction Molten BF (Blast Furnace) slag is the byproduct of iron-making process at the high temperature ranging from 1723 K to 1923 K. Because of its low thermal conductivity, ranging from 1 to 3 W m1 K1 for solid to 0.1e0.3 W m1 K1 for liquid [1], the waste heat recovery of molten BF slag is difficult. Now, the treatment of molten BF slag is water quenching. However, using this method, we can not recover the waste heat of slag, and during the water quenching process, it polluted environment, consumed water resources. Thus in order to resolve these problems, many researchers proposed several methods. One is physical method [2e9], i.e. transferring the heat of slag to hot air, steam or molten salts. Another is chemical method [10e21], i.e. using the waste heat of molten slag to produce fuel gas. Akiyama et al. [12e14] pointed out that decomposition of
limestone, reforming of methane and gasification of carbon are the more suitable for the energy recovery from molten slag, as they present the least exergy loss among all the reactions studied. The concept of reforming of methane using molten slag has been proposed by Kasai et al. [15]. Shimada et al. [10] studied the effects of slag composition on the reaction rates. Kasai et al. [15] studied the rates of methane-steam reaction on the slag surface and. It was shown that larger CaO content in the slag gives rise to higher reaction rates while FeO and sulphur have inhibiting effects. The apparent rate of reforming was found to be 103e101 mol cm2 s1. Maruoka et al. [11] designed a new system in which the slag is first granulated using a rotary cup and is then accumulated in a packed bed. Mizuochi et al. [16,17] reported the industrial design based on the RCA (Rotary Cup Atomizer) technology. Purwanto and Akiyama [18] studied the possibility of hydrogen production from biogas using hot slag, in which decomposition rate of
* Corresponding author. Chongqing CISDI Thermal & Environmental Engineering Co., Ltd., No. 1, Saidi Road, New North Zone, Chongqing, 401122, PR China. Fax: 86 023 63545842. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2016.12.135 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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CO2-CH4 in a pack bed of granulated slag was measured at a constant flow-rate and pressure. Li et al. [19e21] proposed a new route to effectively utilize molten BF slag by using molten BF slag as heat carrier of coal gasification. Liu et al. [23,24] conducted a series of experiments about the RCA method to recover the molten blast furnace slag waste heat and obtained some constructive and instructional results. In the more recent experimental works, the focuses of these researches were only on the mechanism, conversion kinetics, and reaction rate; no information may be obtained regarding the system design. Accordingly, the purpose of this work is to investigate the system design using enthalpyeexergy compass.
Method Based on the first law of thermodynamic and second law of thermodynamic, we use enthalpyeexergy compass proposed by Ishida [22] to analyze the waste heat recovery system of molten BF slag. In the process of energy conversion, the enthalpy is constant, but the exergy is changed which can be expressed by Equations (1) and (2). X
DHj ¼ 0
(1)
Dεj 0
(2)
j
X j
In the waste heat recovery system of molten BF slag, there are not only the exergy of physical energy, but also the exergy of chemical energy. The exergy of thermal energy can be calculated by Equation (3). Dε ¼ DH
T T0 T
(3)
where T is the temperature of energy carrier; T0 is the temperature of environment. The exergy of chemical energy can be expressed as Equation (4). Dε ¼ DH T0 DS
(4)
where Tinitial is the initial reaction temperature. And when the temperature is not initial reaction temperature, the exergy of chemical energy can be calculated by Equation (8). Dε ¼ DH0
0 T T0 1 T0 P DG' 0 þ DH0 0 0 DH þ Ci DT T T 0
(8)
0
where T is the reaction temperature, K; DH is the absorbed 0 heat of reaction, kJ mol1; DG is the Gibbs free energy at the reaction temperature, kJ mol1; DT is the difference between reaction temperature and environmental temperature, K;Ci is the specific heat of product, kJ$(mol K)1. Thus using Equations (3), (7) and (8), we can calculate the enthalpy and exergy before and after energy conversion, based on that, we can get Fig. 1. From Fig. 1, we can see the exergy loss in the process of energy transformation. Thus we can get the energy efficiency and exergy efficiency of the system.
System model Physical method Thermal energy method use hot air, steam and so on as heat carrier to recover the waste heat of molten BF slag. Fig. 2 is a typical waste heat recovery system using physical method. In this system, molten slag at the temperature of 1773 K was granulated by RCA (Rotary Cup Atomizer). The temperature of slag granules is about 1473 K. And then the slag granules enter into the energy recovery equipment. Finally, the temperature of slag granules discharged from the energy recovery equipment is about 473 K. In this system, the amount of energy recovered during RCA process is 0. Using this system, we can produce hot water and steam. In this paper, we consider using this system to produce hot water at the temperature of 353 K and steam at the temperature of 473 K, the equations are expressed as: waste heat
H2 Oð303KÞ ! H2 Oð353KÞ waste heat
H2 Oð303KÞ ! H2 Oð473KÞ
(9) (10)
where DS is the entropy of energy carrier, it can be calculated by Equation (5). DG ¼ DH TDS
(5)
where DG is the change value of Gibbs free energy. Based on Equations (4) and (5), the exergy of chemical energy can be expressed as: Dε ¼ DH T0
DH DG T T0 T0 ¼ DH þ DG T T T
(6)
According to Equation (6), if the temperature is the initial reaction temperature, DG ¼ 0, thus we can get Equation (7). When the temperature is initial reaction temperature, the exergy of chemical energy can be expressed as: Dε ¼ DH
Tinitial T0 Tinitial
(7) Fig. 1 e Enthalpyeexergy diagram for the process of heat transformation.
Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Fig. 2 e The system of waste heat recovery from molten BF slag by physical method. Using this model, we can get the heat efficiency and exergy efficiency of the system and the amount of steam produced using this system. The heat efficiency of the system can be calculated by Equation (11). h1 ¼ DH1 =DH 100%
(11)
where DH is the enthalpy of energy entered into system, MJ; DH1 is the enthalpy of energy recovered in this system, MJ. The exergy efficiency of energy recovery equipment can be calculated by Equation (12). hexi ¼ Dε'i Dεi 100%
(12)
where Dεi is the exergy of energy entered into the energy recovery equipment, MJ; Dε0i is the exergy of energy recovered in energy recovery equipment, MJ. The exergy efficiency of the system can be calculated by Equation (13). hex ¼ Dε0 =Dε 100%
(13)
where Dε is the exergy of energy entered into the system, MJ; 0 Dε is the exergy of energy recovered in the system, MJ. The amount of hot water or steam can be calculated by Equation (14). Wwater ¼ Q=ðihot icool Þ
(14)
where Q is the energy recovered using boiler, MJ; ihot is the enthalpy of hot water or steam, MJ kg1, icool is the enthalpy of cold water entered into boiler, MJ kg1.
Combined method of physical and chemical Chemical methods use chemical reaction to recover the waste heat of molten slag. Due to restrictions of initial reaction temperature, the chemical method can not recover the waste heat of slag granules at low temperature. Thus in this section, we build the combined model of physical and chemical method directly, and do not build the chemical model alone. Fig. 3 shows a typical waste heat recovery system using combined method of physical and chemical. In this system, the process consists of four steps. In the first step, the sensible heat of molten slag is recovered using chemical method, and the molten slag is cooled rapidly to the temperature 1573 K. The still molten slag (~1573 K) is then transferred to the second step, in which the slag is granulated
using a rotary cup. In the third step, the waste heat of slag granules is recovered using chemical method until the temperature of slag granules decrease to the initial reaction temperature. And then the waste heat of slag granules is recovered using physical method. When the slag granules leave the system, the temperature of slag granules is lower than 373 K. The chemical reaction used in this system must meet two requirements, one is endothermic reaction, and another is widely used in industry field. Based on the abovementioned two points, several chemical reactions were selected to recover the waste heat of molten slag as shown in Table 1. Reaction 1 is the chemical reaction of hydrogen generation by water electrolysis. Reaction 2 is the chemical reaction of decomposing of calcium carbonate. Reaction 3 and 4 are chemical reaction of coal gasification. Reaction 5, 6, and 7 are the chemical reaction of methane integration. Reaction 8 is the chemical reaction of hydrogen production from propane. Using this model, we can calculate the heat efficiency and exergy efficiency of the system using different chemical reaction to recover the sensible heat of molten slag, and evaluate which one is the best chemical reaction. The heat efficiency and exergy efficiency of energy recovery equipment using different reactions can be calculated by Equations (11) and (12). The amount of reactants can be calculated by Equation (15). Wi ¼ Q
. X DHi þ Ci Dti Mi
(15)
where Q is the energy recovered by energy recovery equipment, MJ; DHi is the changes of enthalpy of chemical reaction I, MJ; Ci is the specific heat of reactants I, MJ$(mol K)1; Dti is the temperature difference, K; Mi is the molar mass of reactants i, g mol1. The amount of product can be calculated by Equation (16). Wi0 ¼ Q
. X DHi þ Ci Dti a M0i
(16)
where Q is the energy recovered by energy recovery equipment, MJ; DHi is the changes of enthalpy of chemical reaction I, MJ; Ci is the specific heat of reactants I, MJ$(mol K)1; Dti is the temperature difference, K; Mi is the molar mass of product I, g mol1; a is the stoichiometric number.
Fig. 3 e The system of waste heat recovery from molten BF slag by combined method of physical and chemical. Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Table 1 e The enthalpy, exergy and initial reaction temperature of the reactions which were used in the system of waste heat recovery from molten BF slag. No. R1 R2 R3 R4 R5 R6 R7 R8
Chemical reaction
Enthalpy DH (kJ)
DGT¼1573 K (kJ)
Exergy Dε (kJ)
Tinitial (K)
A ¼ (Dε/DH)
H2O / 0.5O2 þ H2 CaCO3 / CaO þ CO2 C þ CO2 / 2CO C þ H2O / CO þ H2 CH4 þ CO2 / 2CO þ 2H2 CH4 þ H2O / CO þ 3H2 CH4 þ 2H2O / CO2 þ 4H2 C3H8 þ 3H2O / 3CO þ 7H2
242 166 171 136 260 223 187 498
194 58 104 90 186 171 157 379
229 123 119 93 175 149 122 298
5400 1160 974 948 916 894 865 742
0.945 0.743 0.694 0.686 0.675 0.667 0.655 0.598
The amount of hot water or steam can be calculated by Equation (14).
Model assumption In this paper, the following regulations and assumptions are applied to evaluate the waste heat recovery system. (1) One ton BF slag was studied as study object. (2) The chemical composition of BF slag is 41.21 mass% CaO, 34.38 mass% SiO2, 11.05 mass% Al2O3, 8.22 mass% MgO, 0.35 mass% TiO2. (3) The average specific heat is 1.21 kJ$(kg K)1, which means the enthalpy and exergy of molten BF slag flowed into system are 1573 MJ and 1308.7 MJ separately. (4) In the energy recovery equipment, the heat efficiency is 1, which means the heat loss is 0. (5) There is no energy recovered during the process of slag granulation. (6) The temperature of slag granules discharged from unit 2 is initial reaction temperature. (7) Without regard to heat loss of BF slag transporting between equipment. (8) The temperature of cold water flowed into boiler is 303 K.
Results and discussion Physical method Fig. 4 shows the enthalpyeexergy diagram for heat recovery from BF slag to produce hot. It is found that the enthalpy and
exergy entered into this system is 1573 MJ and 1309 MJ separately. The enthalpy and exergy entered into boiler is 1210 MJ and 965 MJ separately. The enthalpy of energy recovered using boiler is 1210 MJ, but the exergy is only 188 MJ, the exergy loss of boiler is 777 MJ. The exergy loss of this system is 1121 MJ. The heat efficiency of this system is 76.9%, the exergy efficiency of this system is only 14.4%. The hot water yield of per ton molten slag can attain 5781 kg. Thus we can see that using the waste heat of molten BF slag to produce hot water is not suitable. Fig. 5 shows the enthalpyeexergy diagram for heat recovery from BF slag to produce steam. It is found that the enthalpy and exergy entered into this system is also 1573 MJ and 1309 MJ separately. The enthalpy and exergy of energy recovered using this system is 1210 MJ and 448 MJ separately. The exergy loss of this system is 861 MJ. The heat efficiency of this system is 76.9%, the exergy efficiency of this system is 34.2%. The steam yield of per ton molten slag can attain 453 kg.
Combined method of physical and chemical Fig. 6 shows enthalpy and exergy diagram using combined method of physical and chemical to recover sensible heat of molten slag. In this system, we use chemical method to recover the waste heat of molten slag at high temperature in the unit 1 and unit 2, and use physical method to recover the waste heat of slag granules at low temperature. In Fig. 6, we use R1 (H2O ¼ H2 þ 0.5O2) as endothermic reaction to recover the waste heat of BF slag. It is found that the exergy of energy recovered using this system is more than that entered into this system. Based on the first law of thermodynamic and second law of thermodynamic, we know this is impossible.
Fig. 4 e Enthalpyeexergy diagram for heat recovery from BF slag to produce hot water. Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135
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Fig. 5 e Enthalpyeexergy diagram for heat recovery from BF slag to produce steam.
Fig. 6 e Enthalpyeexergy diagram for heat recovery from BF slag by using the reaction: H2O ¼ H2 þ 0.5O2.
And from Table 1, we can see that the initial reaction temperature is 5400 K which is higher than the temperature of molten slag. Thus the reaction was not happened. Fig. 7 shows the enthalpyeexergy diagram for heat recovery from BF slag by using decomposing of calcium carbonate. It is found that the exergy of energy entered into unit 1, unit 2 and unit 3 are 201 MJ, 302 MJ and 618 MJ separately. And the exergy of energy recovered from unit 1 unit 2 and unit 3 are 187 MJ, 281 MJ and 308 MJ separately. Besides, the exergy loss of unit 1 is only 14 MJ. The exergy loss of unit 2 is only 21 MJ. The enthalpy and exergy of energy entered into this system is 1573 MJ and 1309 MJ. The enthalpy and exergy of energy recovered using this system is 1451 MJ and 776 MJ. The exergy efficiency of unit 1 and unit 2 is 93%. The exergy efficiency of unit 3 is 49.8%. The heat efficiency of this system is 92.2%, and the exergy efficiency of this system is 59.3%. Those both of the heat efficiency and the exergy efficiency are higher than that using physical method to recover sensible heat of molten slag. The CaCO3 consumption of per ton BF slag is 230 kg, and the steam yield of per ton molten slag can attain 312 kg.
Fig. 8 shows the enthalpyeexergy diagram for heat recovery from BF slag by using coal gasification. It is found that the exergy of energy entered into unit 1, unit 2 and unit 3 are 201 MJ, 482 MJ and 421 MJ separately. And the exergy of energy recovered from unit 1 unit 2 and unit 3 are 177 MJ, 419 MJ and 224 MJ separately. Besides, the exergy loss of unit 1 is only 24 MJ. The exergy loss of unit 2 is 63 MJ. The enthalpy and exergy of energy entered into this system is 1573 MJ and 1309 MJ. The enthalpy and exergy of energy recovered using this system is 1451 MJ and 821 MJ. The exergy efficiency of unit 1 is 88.1%. The exergy efficiency of unit 2 is 86.9%. The exergy efficiency of unit 3 is 53.2%. The heat efficiency of this system is 92.2%, and the exergy efficiency of this system is 62.7%. The carbon consumption of per ton BF slag is 45 kg, and the steam yield of per ton molten slag can attain 227 kg. Besides, using this system, per ton BF slag can consume 166 kg CO2 which is benefit to reduce carbon emission. Using this method, we can also get the heat efficiency and exergy efficiency of the other system using R3, R4, R5, R6, R7, or R8 as endothermic reaction to recover the waste heat of BF slag. Table 2 shows the heat efficiency and exergy efficiency of
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Fig. 7 e Enthalpyeexergy diagram for heat recovery from BF slag by using the reaction: CaCO3 ¼ CaO þ CO2.
Fig. 8 e Enthalpyeexergy diagram for heat recovery from BF slag by using the reaction: C þ CO2 ¼ 2CO.
Table 2 e The enthalpy and exergy efficiency of the system of waste heat recovery from molten BF slag. No.
Method
Unit 1
Unit 2
Unit 3
Consumption of reactant
The yield of steam
hex1
hex2
hex3
hheat
System hex
Wi (kg t1)
Wsteam (kg t1)
e 93.0 88.1 87.9 87.4 87.2 88.6 87.8
e 93.1 86.9 86.0 84.6 83.5 82.1 74.9
e 49.8 53.2 54.0 54.8 55.3 56.3 61.5
e 92.2 92.2 92.2 92.2 92.2 92.2 92.2
e 59.3 62.7 63.1 63.3 63.5 63.9 63.1
e WCaCO3 ¼ 230 WC ¼ 45; WCO2 ¼ 165 WC ¼ 56; WH2 O ¼ 85 WCH4 ¼ 41; WCO2 ¼ 113 WCH4 ¼ 48; WH2 O ¼ 54 WCH4 ¼ 51; WH2 O ¼ 114 WC3 H8 ¼ 73; WH2 O ¼ 90
e Wstream Wstream Wstream Wstream Wstream Wstream Wstream
e e
e e
19.5 46.4
76.9 76.9
14.4 34.2
WH2 O ¼ 5781 WH2 O ¼ 453
Wstream ¼ 5781 Wstream ¼ 453
Combined of chemical and physical method R1 R2 R3 R4 R5 R6 R7 R8
H2O ¼ H2 þ 0.5O2 CaCO3 ¼ CaO þ CO2 C þ CO2 ¼ 2CO C þ H2O ¼ CO þ H2 CH4 þ CO2 ¼ 2CO þ 2H2 CH4 þ H2O ¼ CO þ 3H2 CH4 þ 2H2O ¼ CO2 þ 4H2 C3H8 þ 3H2O ¼ 3CO þ 7H2
¼ 312 ¼ 227 ¼ 216 ¼ 201 ¼ 191 ¼ 178 ¼ 122
Physical method 1 2
H2O(303 K) ¼ H2O(353 K) H2O(303 K) ¼ H2O(473 K)
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the system using different chemical reactions or different methods. It is found that both of the heat efficiency and the exergy efficiency of the system using combined method is higher than that of the system using physical method. The heat efficiency of the system using physical method is 76.9%; however the heat efficiency of the system using combined method is 92.2%. The exergy efficiency of the system using the sensible heat of molten slag to produce hot water is only 14.4%; and that of the system using the sensible heat of molten slag to produce steam is only 34.2%. Both of them are lower than that of the system using combined methods, which is around 60%. Besides that, it is found that the exergy efficiency of the system using different chemical reaction is different. The exergy efficiency of the system using R2 is lower than that using the other chemical reactions. The difference of the system using R3, R4, R5, R6, R7, and R8 as endothermic reaction is little. But we found that the reaction of CH4 and H2O or CO2 needs catalyst. Thus the separation of catalyst and slag granules is difficult. And in our other studies [19e21], we found the BF slag can catalyze the coal gasification reaction. Thus we think that R3 and R4 are the suitable reaction used to recover the waste heat of molten slag. Besides we can also see that using R3 and R4 as chemical reaction can reduce the carbon emission.
Conclusions The heat efficiency and exergy efficiency of the system using different method to recover waste heat of molten slag was calculated using enthalpyeexergy diagram. The heat efficiency and exergy efficiency of combined method is higher than that of physical method. The heat efficiency of combined method is 92.2%, and that of physical method is only 76.9%. The exergy of combined method is higher than 59%, and that of physical method is lower than 35%. Although these technologies belong to the same method, the exergy efficiencies are different. The exergy efficiency of the system using the waste heat of molten slag to produce steam is 34.2% which is more than that of the system to produce hot water. And the exergy efficiencies of the system using different reaction to recover the waste heat of molten slag are different. The exergy efficiency of the system using R2 is 59.3%, and the others are around 63%. Considering the reaction condition, reactant, whether or not to use catalyst, and so on, we think that R3 and R4 were the best reaction to recover the waste heat of molten BF slag.
Acknowledgement This research was supposed by National High-tech R&D Program (2006AA05Z209), National Natural Science Fund-Joint Fund of Iron and Steel Research (50574021), Key Technologies R&D Program (2013BAA03B03), Fundamental Research Funds for the Central Universities (N110602002) and Fundamental Research Funds of Chongqing (cstc2014jcyja90002).
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Please cite this article in press as: Li P, Thermodynamic analysis of waste heat recovery of molten blast furnace slag, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.135