Exergy assessment of a rotary kiln-electric furnace smelting of ferronickel alloy

Accepted Manuscript Exergy assessment of a rotary kiln-electric furnace smelting of ferronickel alloy

W. Rong, B. Li, P. Liu, F. Qi PII:

S0360-5442(17)31293-8

DOI:

10.1016/j.energy.2017.07.119

Reference:

EGY 11307

To appear in:

Energy

Received Date:

30 January 2017

Revised Date:

17 July 2017

Accepted Date:

18 July 2017

Please cite this article as: W. Rong, B. Li, P. Liu, F. Qi, Exergy assessment of a rotary kiln-electric furnace smelting of ferronickel alloy, Energy (2017), doi: 10.1016/j.energy.2017.07.119

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ACCEPTED MANUSCRIPT Exergy assessment of a rotary kiln-electric furnace smelting of ferronickel alloy W. Rong, B. Li*, P. Liu, F. Qi School of Metallurgy, Northeastern University, Shenyang, China, 110819 Abstract The main objective of this paper is to assess the thermal performance of the RKEF process based on the actual operational data by using exergy analysis method. To identify the factors affecting exergy efficiency loss, exergy destruction caused by the main chemical reactions (fuel combustion, reduction reaction, decomposition reaction and slagging reaction) is investigated in detail. The results show that the energy efficiency of the RKEF process is 53.3% whereas the exergy efficiency is 15.7%, indicating a great potential for energy-saving. The overall exergy destruction accounts for 46.4% of the total exergy input, in which 24.8% is caused by chemical reactions. Moreover, the exergy destruction due to chemical reactions of rotary dryer, rotary kiln and electric furnace is 20.4%, 27.1% and 5.1%, respectively. It is also found that combustion is the dominant factor for the efficiency loss of rotary dryer and rotary kiln and slag with high temperature contributes most for the efficiency loss of electric furnace. Several suggestions for improving the thermal performance of RKEF system are proposed. The present work should do helpful effort for further improvement of the RKEF process.

Keywords: rotary kiln-electric furnace; ferronickel smelting; exergy analysis; exergy destruction.

* Corresponding

Author: [email protected]; Tel.: +86-24-83672216; Fax: +86-24-23906316 1

ACCEPTED MANUSCRIPT 1. Introduction About 2/3 of nickel in the world is used in stainless steel production for improving the performance of steel, because nickel alloys are characterized by high strength and ductility as well as excellent corrosion and heat resistances [1]. Since China is a big country in the respect of consumption and production of crude stainless steel, ferronickel smelting has attracted considerable attentions in the past decades. It is identified that 27.8% of the global nickel reserves are nickel sulfide ores, while 72.2% are nickel oxide ores which are categorized into two types - saprolite ores and laterite ores [2]. With the depletion of sulfide nickel resources, the proportion of nickel recovered from nickel oxide ores has increased gradually [3, 4]. For producing nickel by using nickel oxide ores as raw materials, the chemical methods (pyrometallurgical or hydrometallurgical) are more suitable than physical methods due to the complex mineralogy of the ores. The chemical methods mainly consist of pressure acid leaching, Caron process, atmospheric leaching and rotary kiln-electric furnace (RKEF) process [5-9]. Among these methods, the rotary kiln-electric furnace (RKEF) process is widely used in China due to its advantages of good adaptability for various nickel laterite, less harmful elements, high production efficiency and mature process [10]. However, the RKEF process suffers from the high energy consumption and a large amount of by-product (e.g. slag and off-gas). Especially for the lower grade laterite, huge energy consumption is needed for the high amount of water and gangue [11]. Therefore, the energy consumption of the RKEF process needs to be understood and evaluated clearly so as to put forward the corresponding measures to improve the RKEF process. Recently, several research efforts about RKEF process have been demonstrated. Zhao [12] performed pilot plant test on ferronickel alloy production from Myanmar Dagon Hill nickel laterite

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ACCEPTED MANUSCRIPT by RKEF process. The optimum process parameters were obtained and the work provided basis for design and production of a large-scale smelter. Xu et al. [13] proposed an energy-saving project based on the analysis of the waste heat utilization in RKEF process. They concluded that the slag, flue gas of electric furnace, flue gas of rotary kiln and the sensible heat of laterite ores calcined are valuable for recovery. In fact, part of their suggestions have been applied in the present RKEF process. Norgate et al. [14] presented a life cycle assessment for the energy of nickel laterite processing. Waste heat recovery from slag and offgas was identified as potential opportunities to improve the energy efficiency and hence the environmental sustainablility of ferronickel smelting, with the former estimated to give a reduction of approximately 10% in embodied energy over practice. Although the above mentioned researches have been dedicated for RKEF performance, no research has been carried out to assessment the process by exergy analysis. However, in metallurgical industry, exergy analysis is an advanced method for almost all engineering process evaluation [15]. Exergy analysis was applied for metallurgical processes for the first time in 1961 by Szargut et al. [15]. They analyzed the thermodynamic performance of blast furnace, basic oxygen furnace and open hearth furnace. Similarly, Rasul et al. [16] also developed an assessment model for the blast furnace. According to their research, the energy and exergy efficiencies of the blast furnace were found to be of 77.3% and 39.13% respectively. Moreover, Camdali et al. [17] performed an energy and conservation analysis on the production of steel process in an electric arc furnace. The results revealed that the second law efficiency of the system can be increased by using a preheating system. Camdali et al. [18] also investigated the actual and reversible work, irreversibility and exergy efficiency of a ladle furnace. They found that irreversibilities occurring in

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ACCEPTED MANUSCRIPT LF were stemming from chemical reactions and heat transfer to surroundings. Akiyama and Yagi [19] presented exergy analysis of conventional ironmaking, direct iron reduction-electric furnace and smelting reduction systems. The smelting reduction was found to be the most efficient process. Aiming to investigate the induration of iron ore pellet in the grate-kiln-cooler, Zhang et al. [20] presented a combined energy and exergy analysis for the particular balling process of pellet based on the modelling the iron oxide pellet exergy instead of the conventional ideal mixture exergy. Morris et al. [21] made a comparison between heat balance and exergy balance in a typical metallurgical process to emphasize the usefulness of exergy theory. Their study identified that the exergy loss was mainly caused by sulfide oxidation reactions of the sinter plant. The work mentioned above demonstrates how to improve individual processes rather than address the network of processes. Michaelis et al. [22] applied exergy analysis to carry out the research on exergy consumption in the total steel life cycle. They found that exergy consumption of the system would be reduced by increasing the recycling rate. A number of researches of exergy approach are also found in the literatures [23-31], but only two papers focus on exergy of the RKEF process. Khoo et al. [30] presented exergy analysis of two nickel laterite processing technologies (Ferronickel production and high pressure acid leaching). Ferronickel production in their work was very similar with the RKEF process. The exergy efficiency was determined but there was no specific analysis for exergy destruction. Domingues et al. [31] performed exergy accounting for two main routes of nickel production (from laterites and sulfide ores). The exergy accounting was applied to the overall chain of nickel production including mining, drying, roasting, smelting and refining. Nevertheless, only the chemical exergy was taken into account due to insufficient data and the irreversibilities was not considered.

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ACCEPTED MANUSCRIPT Although some efforts have been devoted to evaluate the thermal performance of RKEF process, the identification of the sources of exergy destruction has been not yet discussed. Consequently, not all of the energy-saving potentialities of the process have been highlighted. This paper presents a comprehensive exergy assessment for the RKEF process based on the work of Liu et al. [32]; identifying the energy saving potential and revealing the major sources of losses and exergy destruction. Since many complex chemical reactions are involved in the RKEF process (e.g. combustion, reduction reaction, dissociation and slagging reaction) and chemical reaction is one of the sources of exergy destruction, the exergy destruction caused by chemical reactions are demonstrated in detail. In this regard, the first section includes the introduction. The description of the RKEF process is presented in the second section. Section 3 gives methodology; the exergy analysis equations. And then, the exergy analysis method is employed in the three sub-systems and the overall RKEF process, and the results obtained are discussed. Finally, the conclusions are included in the fifth section. 2. RKEF process description The essential mechanisms of the technological process of RKEF have been interpreted in detail in the work of Liu et al. [32]. Therefore, a simple description about RKEF process is provided in this chapter. There are three production devices used in the RKEF process: rotary dryer, rotary kiln and electrical furnace. The technological process consists of the following stages: 1. The dehydration process in rotary dryer. The wet laterite ores coming from the open yard are firstly broken into small particles in the screening and crushing process and then fed into the rotary dryer. In the rotary dryer, wet laterite ores become semi-dry laterite ores by absorbing both the heat

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ACCEPTED MANUSCRIPT of bituminous coal combustion and the waste heat of flue gas recovery from rotary kiln. 2. The roasting and pre-reduction process in rotary kiln. Semi-dry laterite ores carrying partial free water and all chemically boned water are fed into the rotary kiln together with limestone and anthracite. All the water of semi-dry laterite ores are removed and the majority of the metallic oxide is reduced by carbon monoxide. Meanwhile, limestone as solvent is decomposed. The fuel for rotary kiln is bituminous coal and the flue gas recovery from the electric furnace. 3. The reduction smelting process in electric furnace. The laterite ores calcined transported from rotary kiln undergo the reduction reaction and slagging reaction in the electric furnace. The ferronickel is generated accompanied with slag. In addition, to produce a high nickel grade in ferronickel alloy, the reduction of nickel oxide is completely whereas the reduction of iron oxide is limited. 4. The utilization of waste heat of rotary kiln and electric furnace. As mentioned above, the flue gas of rotary kiln is transported into rotary dryer for drying the wet ores, and the furnace gas of electric furnace is recycled as fuel in rotary kiln. 5. Dust collection and reutilization. The nickeliferous laterite ores particles in dust are usually recovered by using agglomeration technique. The schematic of the RKEF process is shown in Fig. 1. 3. Theoretical analysis Exergy analysis has become an increasingly important tool for the design and analysis of thermal systems, for better understanding exergy analysis method, the concepts about exergy are given first. Exergy is the maximum useful work that could be obtained from the system at a given state in a specified environment. Compared with energy which is a first-law concept, exergy is defined based

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ACCEPTED MANUSCRIPT on both the first and second law of thermodynamics. The exergy analysis allows the quantitative evaluation of thermodynamic imperfections as well as energy degradation and their causes, providing possibilities for process improvements [33]. According to the first principle of thermodynamics, the mass and energy balance of RKEF process were established in the research of Liu et al. [32]. On this basis, the exergy balance of RKEF process is implemented in the present work. Due to the complexity of the RKEF process, the following assumptions are made for the exergy analysis: 1. The RKEF system is a steady-state, steady flow process. 2. The ambient and average surface temperatures are constants throughout the period of the study. 3. The temperature and pressure of dead (environmental) state are taken as T0  298.15 K ,

P0  1atm . 4. The compositions of raw materials are constants. 5. No air enters into the electric furnace during smelting ferronickel. 6. No heat is transferred to the system from the outside. 7. The principle of ideal gas mixture is used for gases in the system. 8. The kinetic and potential energy changes of the input and output flows are negligible. The exergy balance for an ideal system is generally expressed as

 Ex

in

  Ex out   ED

where Exin and

(1)

Exout are the exergy of input flows and output flows, respectively. Due to the

irreversibilities (fuel combustion, reduction reactions and dissociation reaction, etc.) of the RKEF process, exergy output flows are always less than the sum of its inputs, their difference is called exergy destruction, namely ED . Irreversibilities always generate entropy, and anything that

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ACCEPTED MANUSCRIPT generates entropy always destroys exergy. The equation of exergy destruction is given in Eq. (8). The exergy of a flow consists of physical exergy and chemical exergy:

Ex  Ex ph  Exch

(2)

The physical exergy of a system which is related to the environmental pressure and temperature due to the pressure or temperature differences [22], is given by:

Ex ph  H  H 0  T0 ( S  S0 )

(3)

The enthalpy change is calculated based on the first principle of thermodynamics:

H  H 0 =m  c p  T  T0 

(4)

Combining the first and the second principle of thermodynamics, the entropy change of gases is expressed as follows:

 T   P  S  S0 =m  c p  ln    R  ln     T0   P0   

(5)

The pressure term on the right side is not relevant to the calculation of the entropy of liquids and solids [22]. The chemical exergy of the ideal gas and liquid mixtures is given by:

  Exch  m    xk  exch ,k  RT0 xk  ln  xk    k 

(6)

where exch ,k is standard chemical exergy of component which is taken from Ref. [33].

RT0 xk  ln  xk  stands for mixture exergy which is negligible in the calculation of chemical exergy of solids. Thus, in the RKEF process, the components of all the materials should be known. The chemical exergy calculations of solids can refer to the calculation of limestone [34]. Heat transfer is always accompanied by exergy transfer. In the RKEF process, the heat transfer consists of heat transfer through the surfaces (walls and doors of rotary dryer, rotary kiln and

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ACCEPTED MANUSCRIPT electric furnace) as well as heat transfer during material transportation process between two devices (e.g. laterite ores calcined is transported from rotary kiln outlet to electric furnace inlet ). The heat transfer rate Q at a certain location at thermodynamic temperature T is always accompanied by exergy transfer ExQ in the amount of  T  ExQ  1  0  Q  T 

where the 1 

(7)

T0 represents the fraction of energy of a heat source at temperature T that can be T

converted to work [35]. The exergy destruction is proportional to the entropy generated:

Exd  T0  S gen

(8)

where S gen is the entropy generation in the process. Entropy generation is due entirely to the presence of process irreversibilities and it is given by [35-36]:

S gen =S  S f ,Q

(9)

where S is entropy change and S f ,Q is entropy transfer. The entropy change of a system during a process can be determined as:

S  S prod  S r eac

(10)

where the subscript prod and reac indicate products and reactants, respectively. Entropy transfer is identified at the system boundary as it crosses the boundary which represents the entropy gained or lost by a system during a process. Entropy transfer is determined as: S f ,Q 

Qr T

The quantity

(11) Qr represents the entropy transfer accompanied by heat transfer, and the direction T

of entropy transfer is identical to the direction of heat transfer since thermodynamic temperature T is always a positive quantity. 9

ACCEPTED MANUSCRIPT Heat transfer during the steady-flow chemical reaction is determined from the steady-flow energy balance:

Qr   N prod hprod  N reac hreac

(12)

where N is mole number of the substance that participates in the reaction. Exergy efficiency, as a parameter for evaluating thermodynamic performance, has two definitions to be applied usually. One is the general exergy efficiency which is defined as the ratio between exery output and exergy input. The other is exergy efficiency of purpose which is defined as the ratio between exergy gained and exergy paid [37]. Since three sub-systems play different roles in the RKEF process, the exergy efficiency is defined in terms of purpose:

=

Ex pur

(13)

Exin

where exergy paid for three plants are their exergy input. For rotary dryer, the main purpose is dehydration, so the exergy gained is determined as the exergy of evaporation. For rotary kiln, the exergy gained is determined as exergy flows of evaporation and laterite ores calcined since the purpose is dehydration and pre-reduction for semi-dry laterite ores. Moreover, the exergy gained of electric furnace is considered as reduction and melting of ferronickel. 4. Results and discussion Liu et al. [32] established a couple algorithm by VB language so to perform the mass and energy balances, combined the data from their work and the method described in the present paper, the exergy balance is developed by using Excel to evaluate the energy saving potential of RKEF process thoroughly. Firstly, the exergy analysis is applied to individual sub-systems (rotary dryer, rotary kiln and electric furnace). Then, a systematical exergy analysis for the RKEF process is achieved. Additionally, it is worth emphasizing that exergy destruction caused by chemical 10

ACCEPTED MANUSCRIPT reactions in different processes is calculated and analyzed. Finally, according to the calculation results, some suggestions for optimizing the performance of the RKEF process are proposed. 4.1 Exergy analysis of rotary dryer The exergy balance of the rotary dryer includes five input flows, six output flows and exergy destruction. The input and output flows can been observed in the solid line diagram in Fig. 2. The exergy balance is expressed as:

ExD ,bc  ExD , wo  ExD , fm  ExD ,ca  ExR , fg  ExD , fg  ExD , so  ExD , fm  ExD ,e  ExD ,l  ExD ,du  EDD (14) where ExD ,bc is exergy flow of bituminous coal. The bituminous coal works as a fuel to supply heat for the dehydration in rotary dryer and the exergy calculation is given by Eq. (15) [38]. ExD , wo and

ExD , fm are exergy flows of wet laterite ores (without free moisture) and free moisture of input material (include 23% free moisture of wet laterite ores), respectively. ExD ,ca is exergy flow of combustion air and air composition is defined as 0.79 N2 and 0.21 O2. ExR , fg is the exergy flow of flue gas recovery from rotary kiln. ExD , fg is exergy flow of flue gas loss to the surroundings. ExD , so and ExD , fm are exergy flows of semi-dry laterite ores (with 12% chemically bonded water) and residual 7% free moisture, respectively. ExD ,e is exergy flow of evaporation. ExD ,l is exergy flow of heat loss which consists of wall loss of rotary dryer and transfer heat loss of semi-dry laterite ores, free moisture and dust. (to make data comparable, the four heat losses are also put together in energy balance). ExD ,du is exergy flow of dust. EDD is exergy destruction of rotary dryer. The chemical exergy of fuel is given by:

Exch , fuel  m fuel    LHV

(15)

where  is the relation between the chemical exergy and the LHV . LHV is low heat value of fuel.

 for different fuels are tabulated [38]. 11

ACCEPTED MANUSCRIPT The exergy balance of rotary dryer is calculated by using Eqs. (2) - (7) and Eq. (15) and the results are shown in Table 1 together with mass and energy balances. From the exergy balance, it appears that the main input flows are exergy flows of bituminous coal and the flue gas recovery from rotary kiln, accounting for 97.95% of the total exergy input. Moreover, the main output flow is the flue gas of rotary dryer which is more than half of the total exergy output. The second main output flow is evaporation of free moisture (i.e. 14.49%) which defines the exergy efficiency of the rotary dryer. From the results in Table 1, the exergy destruction of rotary dryer is:

EDD = ExD,in  ExD,out  38.159GJ / h

(16)

The exergy destruction of rotary dryer is 47.49% of the total exergy input. As mentioned in Eq. (8), exergy destruction is proportional to entropy generation. In rotary dryer, entropy generation caused by combustion of bituminous coal (C-1) and heat and momentum transfer. The entropy generation caused by combustion is calculated according to Eqs. (9) - (12). Entropy and enthalpy of the substances participating in combustion are listed in Table 5 [39]. The entropy change and heat loss during the combustion can be found as 26.807 kJ/kg∙K and 7.795 MJ/kg, respectively. Finally, the exergy destruction of C-1 is 16.386 GJ/h which accounts for 4.6% of the total exergy input and 42.9% of the total exergy destruction. Fig. 3 (a) shows the exergy balance of rotary dryer. As depicted, the exergy efficiency of rotary dryer is 14.49% which is much lower than the energy efficiency (i.e. 63.59%). Combustion is regarded as the dominant factor of the efficiency loss. On one hand, the flue gas which is mainly composed of combustion products leads to an efficiency loss of 34.41%. Although flue gas is substantial, waste heat recovery value is not tremendous because the flue gas temperature is only 358K. So, for the low-temperature flue gas, reducing quantity instead of waste heat recovery is an

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ACCEPTED MANUSCRIPT executable method to reduce the energy consumption. If the current air excess coefficient of rotary dryer decreases from 1.2 to 1.15, the exergy flow of flue gas would save 229.85kJ/h. On the other hand, the exergy destruction due to combustion causes an efficiency loss of 20.39%. Since the heat sources of rotary dryer are bituminous coal combustion and waste heat of flue gas recycled from rotary kiln, increasing the amount of waste heat recovery can save fuel supply so to decrease the exergy destruction. Thus, the heat loss in the transportation process of flue gas should be minimized by enhancing the insulation performance of pipes. In addition, preheating fuel and combustion air can decrease combustion temperature so to reduce the exergy destruction. Moreover, the exergy destruction caused by the heat and momentum transfer accounts for 27.10% of the total exergy input, and this should be associated with the operation of the rotary dryer. Better homogeneity of wet laterite ores particles obtained in the screening and crushing process help reduce this part of exergy destruction. 4.2 Exergy analysis of rotary kiln The exergy balance of the rotary kiln includes eight input flows, five output flows and exergy destruction. The input and output flows are shown in the dotted line diagram in Fig. 2. The exergy balance is given by:

ExR ,bc  ExE , fg  ExR ,ca  ExD , so  ExR , fm  ExR , L  ExR ,a  ExD ,du  ExR , fg  ExR ,oc  ExR ,e  ExR ,l  ExR , fge  EDR

(17)

where ExR ,bc and ExE , fg are exergy flows of bituminous coal and flue gas recovery from electric furnace and their calculations are given by Eq. (15) with different  . ExR ,ca and ExR , fm are exergy flows of combustion air and free moisture, respectively. ExR , L is exergy flow of limestone which is used as solvent for smelting of ferronickel alloy. ExR ,a is exergy flow of anthracite reaction. The anthracite is used as reducing agent. ExR ,oc and ExR ,e are exergy flows of laterite ores calcined and 13

ACCEPTED MANUSCRIPT moisture evaporation, respectively. ExR ,hl is exergy flow of heat loss which consists of wall loss of rotary kiln and transfer heat loss of laterite ores calcined and flue gas to the surroundings. ExR , fge is exergy flow of flue gas entered in electric furnace. EDR is exergy destruction of rotary kiln. The exergy balance of the rotary kiln is calculated with the same equations as the calculation for the rotary dryer, and the results are shown in Table 2. One of the main exergy input for the rotary kiln comes from the bituminous coal (i.e. 54.33%) which is more than half of the total exergy input, similar to the energy proportion. The other two main exergy inputs are the flue gas recovery from the electric furnace (i.e. 25.48%) and the anthracite (i.e. 19.77%). In addition, the majority of the exergy output is exergy flow of laterite ores calcined (i.e. 21.80%). Meanwhile, the total exergy output is less than 50% of the total exergy input and the exergy destruction of the rotary kiln can be found as:

EDR = ExR,in  ExR,out  118.510GJ / h

(18)

Chemical reactions are significant in the exergy destruction. The chemical reactions occurring in the pre-reduction and roasting processes in the rotary kiln are as follows: C-1: C  O2  CO2

(19)

C-2: 2CO  O2  2CO2

(20)

R-1: NiO  CO  Ni  CO2

(21)

R-2: Fe2O3  CO  2 FeO  CO2

(22)

R-3: FeO  CO  Fe  CO2

(23)

R-6: C  CO2  2CO

(24)

D-1: CaCO3  CaO  CO2

(25)

The entropy and enthalpy of the substances participating in the above reactions are listed in

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ACCEPTED MANUSCRIPT Table 5-7. For entropy generation calculation of chemical reactions, it is assumed that all the reduction reaction temperatures in rotary kiln are 1123K [40] and the limestone decomposition starts at 1173K [41-42]. The exergy balance of the rotary kiln is displayed in Fig. 3 (b). Compared with the rotary dryer, the purpose of the rotary kiln is not only dehydration but also pre-reduction. Correspondingly, the exergy of purpose includes exergy flow of evaporation and laterite ores calcined. Therefore, the exergy efficiency of the rotary kiln is 26.93%. Although the flue gas accounts for 13.23% of total exergy input, the waste heat has been reutilized in rotary dryer. The exergy output proportion of heat loss in rotary kiln is the highest among three sub-systems; wall loss rate is higher than that in rotary dryer since the temperature is higher in pre-reduction process than dehydration process. Therefore, improving the insulation property of the rotary kiln has beneficial impact on enhancing exergy efficiency. Moreover, the exergy destruction is more than half of the total exergy input. It is worth noting that the exergy destruction caused by seven chemical reactions is 63.956GJ/h which is much more than that in rotary dryer and electric furnace. Among all the chemical reactions, the two combustion reactions are the dominant factor of exergy destruction which account for 49.67% of the exergy destruction of the rotary kiln. The similar improvement measures for combustion of rotary dryer can be used in rotary kiln. 4.3 Exergy analysis of electric furnace The exergy balance of the electric furnace includes three input flows, five output flows and exergy destruction. The input and output flows are demonstrated in the dash-dotted line diagram in Fig. 2. The exergy balance is written as:

ExE , J  ExR ,oc  ExR , fge  ExE , Fa  ExE , sl  ExE , fg  ExE ,l  ExE ,lhm  EDE

15

(26)

ACCEPTED MANUSCRIPT where ExE , J is exergy flow of Joule heat. ExE , Fa and ExE , sl are exergy flows of ferronickel alloy and slag, respectively. ExE ,l is exergy flow of heat loss which includes the heat of ferronickel alloy, slag and furnce gas loss to the environment. ExE ,lhm is exergy flow of latent heat of melting. EDE is exergy destruction of electric furnace. The exergy balance calculation results are demonstrated in Table 3. Exergy flow of Joule heat is the main source of exergy input (i. e. 72.20%). The dominant factors of exergy output are exergy flow of slag (i. e. 38.82%) and flue gas (i. e. 32.48%) of the electric furnace. The exergy destruction of the electric furnace is:

EDE = ExE,in  ExE,out  9.765GJ / h

(27)

Exergy destruction of the electric furnace is 5.27% of the total exergy input which is much less compared with both rotary dryer and rotary kiln. Similarly, the chemical reactions are considered in the exergy destruction calculation. On one hand, metallic oxied is reduced by carbon (R-4 and R-5). It is assumed that the temperature of reduction reacton in electric furnace is 1873K [40]. On the other hand, the slagging reaction proceeding in the electric furnace is shown in Eq. (30). Since the thermodynamic data of akermanite (Ca2MgSi2O7) is lacking in the reference materials, a simple estimation method is applied to obtain its Gibbs free energy and enthalpy values [43-44]. Afterward, the entropy can be found. Akermanite is considered as a compound of two alkalin oxide (CaO and MgO) and a acidic oxide (SiO2) and the slagging reaction is divided into two reactions shown in Eq. (31) and Eq. (32). Entropy and enthalpy of the substances participating in slagging reactions are listed in Table 8. Finally, the exergy destruction caused by chemical reactions accounts for 5.08% of the total exergy input and 96.46% of the total exergy destruction. R-4: NiO  C  Ni  CO

(28)

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ACCEPTED MANUSCRIPT R-5: FeO  C  Fe  CO

(29)

2CaO  MgO  2 SiO2  Ca2 MgSi2O7

(30)

S-1: 2CaO  SiO2  Ca2 SiO4

(31)

S-2: MgO  SiO2  MgSiO3

(32)

Fig. 3 (c) illustrates the exergy balance of the electric furnace. The exergy of purpose of the electric furnace is considered as reduction and melting of ferronickel and the exergy efficiency is 17.55%. Consistent with energy analysis results, slag with high temperature is considered as the dominant factor for efficiency loss. Besides, the exergy destruction due to slagging reaction accounts for the highest proportion in the total exergy destruction. Hence, the high-temperature slag as a by-product of the electric furnace with tremendous waste heat should be focused. During practice production, the slag is cooled by water and then used as building materials, so its waste heat is lost. Considering the heat resource for electric furnace is Joule heat, the slag with the exergy flow of 71.958GJ/h has little recycling value for the exergy input of electric furnace, so it can be used to preheating bituminous coals and combustion airs of rotary dryer and rotary kiln. In another words, a new recycle can be created for slag to improve the thermal performance of rotary dryer and rotary kiln. Furthermore, for electric furnace, exergy destruction is the lowest among the three subsystems which shows the advantage of electric energy. 4.4 Exergy analysis of the RKEF process The exergy analyses of rotary dryer, rotary kiln and electric furnace which have been discussed in the previous chapters demonstrate the ways to improve individual processes. Nevertheless, they do not address the network of the whole RKEF process. In order to carry out exergy analysis on the RKEF process, the mass and energy balances are illustrated and all balances can be referred in

17

ACCEPTED MANUSCRIPT Table 4. Firstly, the mass balance of the RKEF process includes the input mass flows of bituminous coal, combustion air, wet laterite ores, limestone and anthracite and the output mass flows of flue gas from rotary dryer, ferronickel alloy and slag:

mS ,bc  mS ,ca  mS , wo  mS , L  mS ,a  mS , fg  mS , Fa  mS , sl

(33)

where, the mass flow of bituminous coal consists of the coals supplied to rotary dryer and rotary kiln, and so does the mass flow of combustion air. The mass balance can be concluded from solid line diagram of Fig. 2. Afterwards, the energy balance of the RKEF process includes the input energy flows of bituminous coal, combustion heat of flue gases, combustion air, wet laterite ores, limestone, anthracite, Joule heat, reduction reaction and slagging reaction and the output flows of flue gas from rotary dryer, ferronickel alloy, moisture evaporations of rotary dryer and rotary kiln, reduction reaction, heat loss, dissociation reaction of limestone, slag and latent heat of melting:

ES ,bc  ES , fgc  ES ,ca  ES , wo  ES , L  ES ,a  ES , J  ES ,rrx  ES , sr  ES , fg  ES , Fa  ES ,e  ES ,rrn  ES ,l  ES ,dr  ES , s  ES ,lhm

(34)

Followed the exergy efficiency definition, energy efficiency of purpose is defined as the ratio of the energy gained and the energy paid. Thus, the energy efficiency of the RKEF process is determined as 53.3%. Finally, the exergy balance of the RKEF process includes the input exergy flows of bituminous coal, combustion air, wet laterite ores, limestone, anthracite and Joule heat and the output flows of flue gas exhausted from rotary dryer, ferronickel alloy, slag, evaporation, latent heat of melting and loss.

ExS ,bc  ExS ,ca  ExS , wo  ExS , L  ExS ,a  ExS , J  ExS , fg  ExS , Fa  ExS , sl  ExS ,e  ExS ,lhm  ExS ,l  EDS

18

(35)

ACCEPTED MANUSCRIPT The exergy efficiency of the RKEF process is 15.7% whereas its energy efficiency is 53.3%, indicating a great potential for energy-saving improvements. The exergy destruction of the RKEF system is the sum of that of three sub-system. The exergy destruction caused by chemical reactions of three sub-systems is listed in Table 9. Since the RKEF process is a multicomponent system, exergy balance can be presented as a pie diagram as shown in Fig. 4. It appears that the exergy output accounts for 53.6% of the total exergy input and the exergy destruction caused by chemical reactions is 23.6%. Moreover, the exergy destruction of rotary dryer, rotary kiln and electric furnace is 10.6%, 33.1% and 2.7%, respectively. Therefore, the rotary kiln is the least efficient one among the three components. In order to gain insight into current analysis results, a comparison is made with a previous study. Khoo et al. [30] carried out exergy analysis on Ferronickel production by using the same method; however, the data source they used was based on simulation results rather than on-site measurements. Although RKEF was not used to describe ferronickel production, their simulation started with drying of wet ore in a rotary dryer and ended with a smelting process in an electric arc furnace which is agreed with the RKEF process. Thus, the comparison could be facilitated. The exergy efficiency in their work was determined as 11%, which is lower than the current result (i.e. 15.7%). There is little difference between two analyses in defining exergy efficiency, but the exergy efficiency of their work based on the current definition is much lower than 11%. This discrepancy is complicated since there are several differences between the two analyses, namely, fuel types (natural gas was used as fuel in their simulation but bituminous coal is the current fuel), insulation situations and raw material properties. However, the main contributor for the current higher efficiency is considered as the flue gas of electric furnace recovered to rotary kiln as fuel.

19

ACCEPTED MANUSCRIPT Hence, decreasing the heat loss of flue gas transportation between electric furnace and rotary kiln would have beneficial influence on exergy efficiency. Despite the little differences in efficiency definitions, both efficiencies are very low, which indicates the significant energy-saving potential in RKEF process. Furthermore, the exergy destruction was 12.43% of the total exergy input which is much lower than that in the current process (i.e. 46.4%). Besides, no specific analysis was carried out for exergy destruction, in this regard, not enough attention was paid to exergy destruction. However, in the current work, exergy destruction caused by chemical reactions (combustion, reduction, dissociation and slagging reactions) are calculated. Since chemical reactions play an important role in the RKEF process, the current practice is worth promoting. Nevertheless, a heat recovery steam generator was suggested and applied in their simulation for improving exergy efficiency which is worth learning in the future work. The results showed that the work recovered helped increase the exergy efficiency of the process from 11% to 15%. In conclusion, the previous work has helped us better understand the results of the current analysis. 5. Conclusion The present paper provides a particularly detailed calculation process for RKEF system based on the exergy analysis method, which includes the exergy analysis of the overall RKEF system and the three sub-systems (rotary dryer, rotary kiln and electric furnace). To identify and quantify the irreversiblities of RKEF process, the exergy destruction caused by chemical reactions is calculated in detail. Chemical reactions involved in the RKEF system which consist of combustion, reduction, dissociation and slagging reactions, are the main source of exergy destruction. The overall exergy destruction accounts for 46.4% of the total exergy input, in which 24.8% is caused by chemical

20

ACCEPTED MANUSCRIPT reactions. In addition, combustion is the dominant factor for the efficiency loss of rotary dryer and rotary kiln and slag with high temperature contributes most for the efficiency loss of electric furnace. Reducing heat loss in transportation process of two flue gas recycles has beneficial impact on waste heat recovery and saves the quantity of bituminous coal so to decrease efficiency loss of combustion. Moreover, preheating bituminous coal and combustion air by using the waste heat of the slag can reducing the exergy destruction caused by combustion. There is also a narrow room for improvement by reducing air excess coefficients. Finally, the energy efficiency of the RKEF process is 53.3%, while the exergy efficiency is 15.7%, indicating that significant improvement potential exists. The exergy destruction of rotary dryer, rotary kiln and electric furnace is 10.6%, 33.1% and 2.7%, respectively. Therefore, the rotary kiln is the least efficient one among the three sub-systems of the RKEF process.

Acknowledgements The authors’ gratitude goes to key program of National Natural Science Foundation of China (Grant No.51210007).

21

ACCEPTED MANUSCRIPT Nomenclature cp specific heat (kJ/(kg·K)) ex specific exergy (GJ/t) energy (GJ/h) E exergy destruction ED Ex exergy (GJ/h) enthalpy (kW) H LHV low heating value (GJ/t) m mass (t/h) N mole number pressure (Pa) P Q heat (GJ/h) universal gas constant (kJ/(kmol·K)) R S entropy (kJ/K) temperature (K) T x mole fraction Greek letters

 

efficiency relation between chemical exergy and LHV

Subscripts a bc ca ch dr du D e E Fa fg fgc fge fm fuel gen in J l lhm L oc out

anthracite bituminous coal combustion air chemical dissociation reaction dust rotary dryer evaporation electric furnace ferronickel alloy flue gas furnace gas combustion flue gas entered into electric furnace free moisture fuel generation input Joule heat loss latent heat of melting limestone laterite ores calcined output 22

ACCEPTED MANUSCRIPT ph prod pur r reac rrx rrn R sl so sr wo 0

physical product purpose reaction reactant exothermic reduction reaction endothermic reduction reaction rotary kiln slag semi-dry laterite ores slagging reaction wet laterite ores ambient

23

ACCEPTED MANUSCRIPT References [1] Rao MJ, Li GH, Jiang T, Luo J, Zhang YB, Fan XH, Carbothermic reduction of nickeliferous laterite ores for nickel pig iron production in China: a review. JOM 2013;65:1573-1583. [2] Lu HB, Thermodynamic research on production of ferronickel alloy by electric furnace reduction from lateritic nickel ore. Chin J. Rare Met. 2012;36:785-790. [3] Butt CW, Cluzel D, Nickel laterite ore deposits: weathered serpentinites. Elements 2013;9:123128. [4] Ashok D，Dalvi W，Gordon B，The past and the future of nickel laterites. PDAC 2004 International Convention, Trade Show ＆ Investors Exchange, 2004;1-27． [5] Guo XY, Shi WT, Li D, Tian QH, Leaching behavior of metals from limonitic laterite ore by high pressure acid leaching. Trans. Nonferr. Metal. Soc. 2011;21:191-195. [6] Nikoloski AN, Nicol MJ, The electrochemistry of the leaching reactions in the Caron process II. Cathodic processes. Hydrometallurgy 2010;105:54-59. [7] McDonald RG, Whittington BI, Atmospheric acid leaching of nickel laterites review. Part I. Sulphuric acid technologies. Hydrometallurgy 2008;91:35-55. [8] McDonald RG, Whittington BI, Atmospheric acid leaching of nickel laterites review. Part II. Chloride and bio-technologies. Hydrometallurgy 2008;91:56-69. [9] Nayak JC, Production of ferronickel from sukinda laterites in rotary kiln-electric furnace. Trans. Indian Inst. Met. 1985;38:241-247. [10] Liu M, Lv XW, Guo EG, Chen P, Yuan QG, Novel Process of Ferronickel Nugget Production from Nickel Laterite by Semi-molten State Reduction. ISIJ International 2014;54:1749-1754. [11] Zevgolis E, Zografidis C, Perraki T, Devlin E, Phase transformations of nickeliferous laterites

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ACCEPTED MANUSCRIPT during preheating and reduction with carbon monoxide. Journal of Thermal Analysis and Calorimetry 2010;100:133-139. [12] Zhao JF, Treatment for Myanmar nickel laterite with RKEF process. Nonferrous Metals (Extractive Metallurgy) 2013;1:8-10. [13] Xu XF, Li M, Analysis of waste heat utilization in RKEF process. World Nonferrous Metals 2014;11:40-43. [14] Norgate T, Jahanshahi S, Assessing the energy and greenhouse gas footprints of nickel laterite processing. Minerals Engineering 2011;24:698-707. [15] Szargut J, Morris DR, Steward FR, Exergy analysis of thermal and metallurgical processes. Hemisphere Publishing Corporation; 1998. [16] Rasul MG, Tanty BS, Mohanty B, Modelling and analysis of blast furnace performance for efficient utilization of energy. Applied Thermal Engineering 2007;27:78-88. [17] Çamdali Ü, Tunc M, A Karakas, Second law analysis of thermodynamics in the electric arc furnace at a steel producing company. Energy Conversion and Management 2003;44:961-973. [18] Çamdali Ü, Tunc M, Dikec F, A thermodynamic analysis of a steel production step carried out in the ladle furnace. Applied Thermal Energy 2001;21:643-655. [19] Akiyama T, Yagi J. Exergy analysis of conventional ironmaking, direct reduction-electric furnace and smelting reduction systems. Tetsu-to-Hagane 1988;74:2270-2277. [20] Zhang Y, Feng JX, Xu JH, Zhang YM, Yang JB, Energy and exergy analyses of a mixed fuelfired grate-kiln for iron ore pellet induration. Energy Conversion and Management 2011;52:2064-2071. [21] Morris DR, Steward FR, Exergy analysis of a chemical metallurgical process. Metallurgical

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ACCEPTED MANUSCRIPT smelting. Applied Thermal Engineering 2016;109:542-559. [33] Morris DR, Szargut J, Standard chemical exergy of some elements and compounds on the planet earth. Energy 1986;11:733–755. [34] Rong WJ, Li BK, Qi FS, Cheung CP, Energy and exergy analysis of an annular shaft kiln with opposite burners. Applied Thermal Engineering 2017;119:629-638. [35] Çengel YA, Boles MA, in: Thermodynamics: An Engineering Approach, seventh ed. McGrawHill; 2011. [36] Gutiérrez AS, Martínez JBC, Vandecasteele C, Energy and exergy assessments of a lime shaft kiln. Applied Thermal Engineering 2013;51:273-280. [37] Zhang Y, Feng JX, Xu JH, Zhang YM, Yang JB, Energy and exergy analyses of a mixed fuelfired grate-kiln for iron ore pellet induration. Energy Conversion and Management 2011;52:2064-2071. [38] Kotas T, in: The Exergy Method of Thermal Plant Analysis, second ed. Krieger Publishing Company, Florida; 1995. [39] Wu YJ, Che YC, Handbook of inorganic thermodynamic data. Northeastern University Press, Shenyang; 1993. [40] Norgate T, Jahanshahi S, Assessing the energy and greenhouse gas footprints of nickel laterite processing. Minerals Engineering 2011;24:698-707. [41] Oates JAH, lime and limestone. Weinheim: Wiley-VCH; 1998. [42] Zuideveld PL, van den Berg PJ, Design of lime shaft kilns. Chemical Engi-neering Science 1971;26:875-883. [43] Li XB, Li YF, Liu XN, A simple method of estimation of Gibbs free energy and enthalpy of

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ACCEPTED MANUSCRIPT complicate silicates. Journal of the Chinese Ceramic Society 2001;29:232-237. [44] Li HL, Liu C, Ji YH, Simple estimation method for thermodynamic data of complex oxysalt minerals. CIESC Journal 2010;21:544-550.

Figure captions Fig. 1.

Schematic of the RKEF process with rotary dryer, rotary kiln and electric furnace. 28

ACCEPTED MANUSCRIPT Fig. 2.

Mass, energy and exergy flows of rotary dryer, rotary kiln and electric furnace.

Fig. 3.

Grassmann diagrams for the RKEF system.

Fig. 4.

Exergy balance in the form of pie diagram for the RKEF system.

Table captions Table 1.

Mass, energy and exergy balances of rotary dryer.

Table 2.

Mass, energy and exergy balances of rotary kiln.

Table 3.

Mass, energy and exergy balances of electric furnace.

Table 4.

Mass, energy and exergy balances of the RKEF system.

Table 5.

Exergy destruction of combustion.

Table 6.

Exergy destruction of reduction reaction.

Table 7.

Exergy destruction of dissociation reaction.

Table 8.

Exergy destruction of slagging reaction.

Table 9.

Exergy destruction of RKEF system.

Table 1 Mass, energy and exergy balances of rotary dryer Mass balance

Energy balance

Exergy balance 29

ACCEPTED MANUSCRIPT mD,in (t/h) mD,bc 1.93 mD,ca 12.63 mR,fg 102.08 mD,wo 119.31

mD,out (t/h) mD,so 80.75 mD,fg 137.13 mD,du 18.03

Total 235.95 Total

235.91

ED,in (GJ/h) ED,bc 44.811 ED,wo 1.447 ED,fm 2.434 ED,ca 0.273 ER,fg 38.139

ED,out (GJ/h) ED,fg 19.918 ED,so 1.163 ED,fm 0.735 ED,e 55.391 ED,l 9.613 ED,du 0.284 Total 87.104 Total 87.104

EXD,in(GJ/h) EXD,bc 47.462 EXD,wo 0.053 EXD,fm 1.1476 EXD,ca 0.113 EXR,fg 31.252 Total

EXD,out (GJ/h) EXD,fg 27.652 EXD,so 0.043 EXD,fm 0.444 EXD,e 11.647 EXD,l 2.401 EXD,du 0.010 80.356 Total 42.197

Table 2 Mass, energy and exergy balances of rotary kiln Mass balance

Energy balance

Exergy balance 30

ACCEPTED MANUSCRIPT mR,in (t/h) mR,bc 5.22 mR,ca 53.69 mR,L 26.13 mE,fg 7.34 mD,so 80.75 mD,du 18.03 mR,a 7.48

mR,out (t/h) mR,oc 96.53 mR,fg 102.08 mR,fge 0.07

Total

Total

198.64

198.68

ER,in (GJ/h) ER,bc 121.347 ER,fgc 74.423 ER,ca 1.141 ED,so 1.163 ER,fm 0.699 ER,L 0.458 ER,a 42.441 ED,du 0.284 ER,rrx 1.023 Total 242.979

ER,out (GJ/h) ER,fg 38.139 ER,oc 38.059 ER,e 50.750 ER,dr 43.378 ER,l 66.375 ER,fge 0.036 ER,rrn 6.243

EXR,in (GJ/h) EXR,bc 128.370 EXR,Fg 60.210 EXR,ca 0.480 EXD,so 0.043 EXD,fm 0.444 EXR,L 0.017 EXR,a 46.700 EXD,du 0.010

EXR,out (GJ/h) ER,fg 31.352 ER,oc 51.511 ER,e 12.116 ER,l 22.857 ER,fge 0.027

Total

Total

Total

242.980

236.274

Table 3 Mass, energy and exergy balances of electric furnace Mass balance

Energy balance

Exergy balance

31

117.763

ACCEPTED MANUSCRIPT mE,in (t/h) mR,oc 96.53 mR,fge 0.07

mE,out (t/h) mE,Fa 12.00 mE,sl 77.27 mE,fg 7.34

EE,in (GJ/h) EE,J 133.833 ER,oc 38.059 EE,sr 51.138 ER,fge 0.036

Total

Total

Total

96.60

96.61

223.066

EE,out (GJ/h) EE,Fa 11.555 EE,sl 106.047 EE,rrn 53.662 EE,fg 1.765 EE,l 18.940 EE,lhm 31.098 Total 223.067

EXE,in (GJ/h) EE,J 133.833 ER,oc 51.511 ER,fge 0.027

EXE,out (GJ/h) EE,Fa 6.356 EE,sl 71.958 EE,fg 60.210 EE,l 10.915 EE,lhm 26.168

Total

Total

185.371

Table 4 Mass, energy and exergy balances of the RKEF system Mass balance

Energy balance

Exergy balance 32

175.607

ACCEPTED MANUSCRIPT mS,in (t/h) mS,bc 7.15 mS,ca 66.32 mS,wo 119.31 mS,L 26.13 mS,a 7.48

mS,out (t/h) mS,fg 137.13 mS,Fa 12.00 mS,sl 77.27

Total

Total

226.39

226.40

ES,in (GJ/h) ES,bc 166.158 ES,fgc 72.658 ES,ca 1.414 ES,wo 3.881 ES,L 0.458 ES,a 42.441 ES,J 133.833 ES,rrx 1.203 ES,sr 51.138 Total 473.184

ES,out (GJ/h) ES,fg 19.918 ES,Fa 11.555 ES,e 106.141 ES,rrn 59.865 ES,l 95.004 ES,dr 43.378 ES,s 106.047 ES,lhm 31.098

ESE,in (GJ/h) ExS,bc 175.832 ExS,ca 0.593 ExS,wo 1.528 ExS,L 0.017 ExS,a 46.700 ExS,J 133.833

ESE,out (GJ/h) ExS,fg 27.652 ExS,Fa 6.356 ExS,sl 71.958 ExS,e 23.763 ExS,lhm 26.168 ExS,l 36.173

Total

Total

Total

473.006

358.503

192.070

Table 5 Exergy destruction of combustion N(kmol)

s(kJ/kmol∙K)

x

-Rln(x∙P) 33

N∙s(kJ/kg∙K)

h(kJ/mol)

ACCEPTED MANUSCRIPT C-1

　

　

　

　

　

　

Reactants

　

　

　

　

　

　

C

0.0692

5.740

-

-

0.397

0.000

H

0.0747

130.680

-

-

9.762

0.000

O

0.0021

205.030

-

-

0.436

0.000

N

0.0024

191.610

-

-

0.454

0.000

S

0.0009

248.200

-

-

0.219

-296.810

　

　

　

　

　

　

　

O2

0.1052

205.030

0.210

12.976

22.933

0.000

N2

0.3957

191.610

0.790

1.960

76.604

0.000

Products

　

　

　

　

　

　

CO2

0.0692

302.670

0.133

16.803

22.097

-313.960

N2

0.3969

248.450

0.761

2.275

99.520

49.400

H2O

0.0374

259.540

0.072

21.926

10.513

-178.630

SO2

0.0009

339.170

0.002

53.072

0.346

-215.130

O2

0.0175

264.730

0.034

28.214

5.136

52.020

S (kJ/kg∙K)

　

　

　

　

26.807

　

Qr (MJ/kg)

　

　

　

　

　

7.795

　

　

　

　

　

　

　

C-2

　

　

　

　

　

　

Reactants

　

　

　

　

　

　

CO

0.0280

212.510

0.758

2.304

4.059

-104.420

H2O

0.0022

206.110

0.061

23.304

0.461

-234.690

O2

0.0004

220.390

0.010

38.202

0.082

6.290

N2

0.0022

206.390

0.059

23.514

0.450

6.080

CO2

0.0023

234.800

0.062

23.173

0.534

-384.770

H2

0.0019

145.140

0.051

24.820

0.271

5.940

　

　

　

　

　

　

　

O2

0.0167

205.030

0.210

12.976

3.646

0.000

N2

0.0629

191.610

0.790

1.960

12.178

0.000

Products

　

　

　

　

　

　

CO2

0.0302

316.410

0.298

10.078

9.872

-285.830

N2

0.0651

256.510

0.641

3.703

16.938

65.880

H2O

0.0041

270.970

0.040

26.688

1.221

-155.230

O2

0.0022

273.200

0.021

31.938

0.666

69.320

S (kJ/kg∙K)

　

　

　

　

4.852

　

Qr (MJ/kg)

　

　

　

　

　

0.547

Table 6 Exergy destruction of reduction reaction 　

N

N∙s

N∙h

N

34

N∙s

N∙h

ACCEPTED MANUSCRIPT (kmol)

(kJ/kmol∙K)

(kJ/mol)

(kmol)

(kJ/kmol∙K)

(kJ/mol)

R-1

　

　

　

　 R-4

　

　

　

Reactants

　

　

　

　 Reactants

　

　

　

NiO

1

109.07

-195.83

　 NiO

1

139.450

-151.250

CO

1

238.26

-84.80

　C

1

39.050

32.200

Products

　

　

　

　 Products

　

　

　

Ni

1

70.41

25.88

　 Ni

1

99.450

71.230

CO2

1

275.03

-353.85

　 CO

1

255.930

-58.900

S (kJ/kmol∙K) 　

-1.89

　

　 S (kJ/kmol∙K)

　

176.880

　

Qr (kJ/mol)

　

　

47.34

　 Qr (kJ/mol)

　

　

-131.380

　

　

　

　

　　

　

　

　

R-2

　

　

　

　 R-5

　

　

　

Reactants

　

　

　

　 Reactants

　

　

　

Fe2O3

1

269.140

-707.280

　 FeO

1

187.590

-153.660

CO

1

238.260

-84.800

　C

1

39.050

32.200

Products

　

　

　

　 Products

　

　

　

FeO

2

266.620

-451.720

　 Fe

1

106.460

76.140

CO2

1

275.030

-353.850

　 CO

1

255.930

-58.900

S (kJ/kmol∙K)

　

34.250

　

　 S (kJ/kmol∙K)

　

135.750

　

Qr (kJ/mol)

　

　

13.490

　 Qr (kJ/mol)

　

　

-138.700

　

　

　

　

R-3

　

　

　

R-6

　

　

　

Reactants

　

　

　

Reactants

　

　

　

FeO

1

133.310

-225.860

CO2

1

275.030

-353.850

CO

1

238.260

-84.800

C

1

27.040

14.510

Products

　

　

　

Products

　

　

　

Fe

1

72.970

31.240

CO

2

238.260

-84.800

CO2

1

275.030

-353.850

S (kJ/kmolK)

　

174.450

　

S (kJ/kmol∙K) 　

23.570

　

Qr (kJ/mol)

　

　

-169.740

Qr (kJ/mol)

　

　

11.950

　

　

　

　

Table 7 Exergy destruction of dissociation reaction

35

ACCEPTED MANUSCRIPT N(kmol)

N∙s(kJ/kmol∙K)

N∙h(kJ/mol)

Reactants

　

　

　

CaCO3

1

214.76

-1107.36

Products

　

　

　

CaO

1

102.67

-589.70

CO2

1

278.03

-351.18

S (kJ/kmol∙K) 　

165.94

　

Qr (kJ/mol)

　

-166.48

　

Table 8 Exergy destruction of slagging reaction 36

ACCEPTED MANUSCRIPT 　

N (kmol)

N∙s(kJ/kmol∙K)

N∙h(kJ/mol)

S-1

　

　

　

Reactants

　

　

　

CaO

2

268.36

-1100.72

SiO2

1

164.57

-797.08

Products

　

　

　

Ca2SiO4

1

452.76

-2001.77

S (kJ/kmol∙K) 　

19.83

　

Qr (kJ/mol)

　

　

103.97

　

　

　

　

S-2

　

　

　

Reactants

　

　

　

MgO

1

115.59

-521.94

SiO2

1

164.57

-797.08

Products

　

　

　

MgSiO3

1

287.00

-1336.06

S (kJ/kmol∙K) 　

6.84

　

Qr (kJ/mol)

　

17.04

　

Table 9 Exergy destruction of RKEF system 37

ACCEPTED MANUSCRIPT Rotary dryer Reactions C-1

total

Rotary kiln Exergy destruction (GJ/h) 16.386

16.386

Reactions

Electric furnace Exergy (GJ/h)

destruction

Reactions

Exergy (GJ/h)

C-1

44.319

R-4

0.226

C-2

10.210

R-5

1.344

R-1

0.199

S

7.849

R-2

2.498

R-3

0.009

R-4

0.839

D

1.543

total

59.617

total

9.419

38

destruction

ACCEPTED MANUSCRIPT

Fig. 1. Schematic of the RKEF process with rotary dryer, rotary kiln and electric furnace.

39

ACCEPTED MANUSCRIPT

Fig. 2. Mass, energy and exergy flows of rotary dryer, rotary kiln and electric furnace.

40

ACCEPTED MANUSCRIPT

(a) Grassmann diagram for rotary dryer

(b) Grassmann diagram for rotary kiln

(c) Grassmann diagram for electric furnace

Fig. 3. Grassmann diagrams for the RKEF system

41

ACCEPTED MANUSCRIPT

Fig. 4. Exergy balance in the form of pie diagram of the RKEF system

42

ACCEPTED MANUSCRIPT Research highlights: 1.

The RKEF system is investigated for exergy performance.

2.

The exergy destruction caused by chemical reactions are calculated in detail.

3.

Energy efficiency of the RKEF is 53.3%, while the exergy efficiency is 15.7%.