process analysis of the effective utilization of molten

ISIJ International, Vol. 50 (2010), No. 9, pp. 1319–1325

1. Introduction

In the last decade, energy and environmental problems,especially pertaining to greenhouse effect gases resultingfrom the utilization of fossil fuel energy, have attractedworldwide attention. In 2008, Japan dealed with this chal-lenge to reduce CO2 emission by 20% at 2020 in compari-son to that in 1990; it was in accordance with the Kyotoprotocol.1) However, the amount of emission exhibits an op-posite trend, i.e., some increase was seen in 2007. To ensurethe effectiveness of actions to cope with the global warm-ing, it is essential to evaluate the energy flow in the presentenergy-consuming industry. This will promote the effectiveutilization of energy, identification of the most energy-con-suming industries, and formulation of strategies for the fu-ture.

Cement production is well-known as an energy-consum-ing industry involving high-temperature processes such as arotary kiln in which the calcination and burnining of the ce-ment materials occur at a high temperature over 1 780 K.Needless to say, the calcination process (CaCO3�CaO�CO2) emits CO2, which is generated from both the thermaldecomposition of limestone and combustion of the fossiland other carbonaceous fuels. In the cement industry, alarge amount of blast furnace slag has been used as a rawmaterial and its product is often called as ‘blast furnace ce-

ment’ (BFC). The BFC is particularly effective in the re-ducing of CO2 generation because decomposition processof limestone can be omitted. BFC is a mixture of cementclinker and water-granulated BF slag. The granulated BFslag is produced by quenching the molten BF slag by im-pinging water without heat recovery. Therefore, althoughBFC is well known as an environmental-friendly material,its production process provides scope to save energy furtherby recovering a significant amount of waste energy at1 780 K.

Based on the abovementioned viewpoints, energy evalua-tion of the cement production process is essential for en-ergy saving and CO2 emission reduction.2) Exergy is an es-sential thermodynamic concept that has been widely ap-plied in the design/evaluation of thermal plants, and it con-tributes significantly toward improving the system effi-ciency and thermo-economy.3–9) However, to the best of ourknowledge, a comparative exergy analysis of the two ce-ment production systems—portland cement (PC) andBFC—is not yet reported. Therefore, the purpose of thisstudy is to evaluate the exergy of PC cement against the ex-ergy of BFC without heat recovery from molten slag. Theresults will identify the areas of energy degradation,thereby promoting the effective use of energy and enablethe proposal of energy saving and reduction of CO2 emis-sion.

Process Analysis of the Effective Utilization of Molten Slag Heatby Direct Blast Furnace Cement Production System

Hadi PURWANTO,1) Eiki KASAI2) and Tomohiro AKIYAMA3)

1) Faculty of Engineering, International Islamic University Malaysia, Jalan Gombak 53100 Kuala Lumpur, Malaysia.2) Institute of Multidiciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577 Japan.3) Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628 Japan. E-mail: [email protected]

(Received on December 10, 2009; accepted on March 17, 2010 )

This paper principally presents a process analysis of the production systems of blast furnace cement(BFC) based on exergy analysis and its carbon dioxide emission. The analysis was first carried out by usingexergy balances of actual operating data in the cement industry. The results revealed that a large sum of netexergy losses was found on the conventional BFC production; this was contrary to the preliminary expecta-tions. In the BFC production, the recovery of the thermal exergy of the molten slag should reduce the totalexergy losses by up to 20%. In contrast, the emission of CO2–488.2 kg/ton–in BFC production was owerthan 797.5 kg/ton emission in portland cement production; this was because portland cement consumesmore carbonaceous fuels such as coal. In conclusion, to reduce exergy loss, save energy and minimize CO2emission, it is imperative that the BFC production process should be improved with the recovery of the ther-mal exergy of the molten slag, e.g. the direct mixing of raw material of limestone with molten slag is an in-novative and attractive solution because limestone can be easily decomposed by sensible heat of themolten slag. This means thermal combination of portland cement and BFC for the effective use of wasteheat, in which waste heat in the conventional BFC process is recovered and used for limestone decomposi-tion in the clinker production to produce BFC.

KEY WORDS: CO2 emission; waste heat recovery; exergy; blast furnace slag; cement production.

1319 © 2010 ISIJ

2. Method

The conventional enthalpy method is not suited for draw-ing enthalpy flow diagram. Generally, while calculating en-thalpy, we use the standard conditions of 298.15 K (T0) and0.101325 MPa (P0) and define the enthalpy of all elementsas zero. This is termed the standard formation of enthalpyor standard heat formation and is denoted as DH0 (kJ/mol).Based on this definition, the DH0 values of compounds arealways negative. For example, in the case of carbon, the en-thalpy of carbon element is zero and that of CO2 is negative(�393.5 kJ/mol). However, in the enthalpy balance sheet ofthermal/chemical plants, the use of negative values for en-thalpy inflow and outflow is rather inconvenient. Therefore,in this study, we employed the so-called ‘modified en-thalpy’ for the analysis of the cement process by changingthe definition of a standard substance.3)

For calculating the modified enthalpy, we consider T0 andP0 as the standard conditions and the most stable materialsin the atmosphere, such as CO2, as standard substances.Based on this definition, the enthalpy of element C is393.5 kJ/mol and that of CO2 is zero at T0 and P0. The en-thalpies of the most stable substances such as O2, N2, H2O,Fe2O3, CaCO3, and Al2O3 can also be determined by fol-lowing this method.

The above mentioned method yields the modified stan-dard enthalpy of any material; hence, the enthalpy of allmaterials is either positive or zero. When the molar specificheat at constant pressure is denoted as Cp, the enthalpy isgiven by the following equations:

.......(1)

.............(2)

where H is enthalpy (kJ), T is temperature (K) and C�av,p isdefined as the average specific heat at constant pressure(kJ/mol K) and obtained by modifying the following basicequation of Cp:

..................(3)

...............(4)

where A, B, C, and D are the constants of the average spe-cific heat modified from a, b, c, and d, respectively. Thethermodynamic database of the thermochemical propertiesof inorganic substance provides the values of a, b, c, and d,as functions of temperature.10) Therefore, the modificationof the original data on specific heats into corresponding av-erage specific heat values will be useful in calculating theenthalpy balance of substances. The values of the constantsin the modified specific heat of some substances involved incement production are given in Appendix 1.

Prior to the calculation of the exergy flow, it is imperativeto verify the material balances of operating data in the ce-ment process. The exergy flow in the process is defined asthe sum of four types of exergy; chemical exergy, thermal

exergy, pressure exergy, and mixing exergy, follows theequation:

............................(5)

where, e is exergy (kJ), Cp is specific heat constant(kJ/mol K), T is temperature (K), P is pressure (kg/m s2)and R is gas constant (J/mol K).

The numerical calculation is performed by using a com-puter program that considers the temperature dependencyof the average specific heat of each substance. Before cal-culating exergy, a closed system is set up by creating aboundary surface for a combination of processes pertainingto cement production. After the heat and mass balances ofthe operating data are verified, the net exergy loss (EXL) isevaluated by the following equation:

......(6)

In this equation, the fuel and raw materials account for theexergy input, whereas the product and waste, such as wastegas and dust, account for the exergy output. Exergy losswill always be equal to zero or negative. The net EXL indi-cates the degradation of energy in the process. Thermody-namically, the exergy in the process is not always conservedeven if the heat and mass are perfectly conserved, becausethe entropy always increases in an irreversible process. Thenet EXL in the cement process is unique and extremely im-portant for evaluating the cement production process com-paratively. The net EXL in each process within a cementproduction system is calculated and summed up for evaluat-ing the system. Moreover, the exergy efficiency of the sys-tem is calculated by using the following equation:

...........................................(7)

In addition, the CO2 gas emission in the two cement pro-duction systems is comparatively evaluated from both thefuel consumed and raw materials used. Limestone, which isthe main raw material for cement production, releases CO2

due to thermal decomposition. The fossil fuel, which isused as a heat source in this process, also causes CO2 emis-sion during its combustion.

3. Process Descriptions

3.1. Portland Cement Production

The portland cement production process comprises threeunit processes: material preparation (grinding, drying, andmixing), clinker production, and clinker crushing. Theprocess of clinker production consumes the largest amountof energy since it involves the high-temperature processesof firing and decomposition of limestone. Figure 1 illus-trates a new suspension pre-heater (NSP) kiln, which is industrially used for cement production at one cement com-

exergy efficiency (

exergy out ( exergy waste (

exergy input (

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exergy loss (EXL)

exergy output ( exergy input (out in� �ε ε) )

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n n C T T T T T

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′C T A BT CT DTav,p ( )� � � ��2 2

C T a bT cT dTp ( )� � � ��2 2

′ ∫C C T dT TT

av,p p� �298 15

298 15.

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298 15298 15∫ ′p av,p

ISIJ International, Vol. 50 (2010), No. 9

1320© 2010 ISIJ

pany, Japan.11) The NSP kiln has four cyclones in its uppersection to recover heat from the hot gas, heat the raw mate-rial to approximately 1 173 K, and calcinate limestone intolime before it enters the rotary kiln. Within the rotary kiln,the calcinated material is transformed into clinker at a tem-perature of 1 773 K through several chemical reactions. Theclinker is then quenched by air blowing and stored in a silo.Among the three unit processes, clinker production is themost exergy consuming step in PC production; it accountsfor approximately 94.3% of the total exergy loss in theoverall system in comparison to the material preparationand clinker crushing processes.12) In this study, a practi-cally-operated clinker process of the scale of 100-tonclinker production per hour is analyzed. Note that the mainraw materials are limestone and coal, the secondary rawmaterials are fly ash, silica, iron ore, dust, sludge, and boneas well as used tire. The detailed compositions of the rawmaterials are provided in Table 1.

3.2. Blast Furnace Cement Production

In Japan, the most common treatment of molten BF slaginvolves the use of a considerable amount of water to pro-duce glassy granulated slag. The molten BF slag at 1 773 Kis quenched by water impingement and immersion. Theproduct is a granular slag with a size of 3–5 mm; it exhibits

very limited crystal formation since it is highly cementi-tious in nature. The slag obtained is then dried, mixed withPC, and used as commercially available BFC. The draw-back of this process is that a considerable amount of water,approximately 10 ton water per ton molten slag, is required.In addition, after the granulation, the water temperature isvery low; therefore, its sensible heat cannot be recovered.The chemical composition of the BF slag used for produc-ing the BFC is as follows: 33.7% SiO2, 41.7% CaO, 14.4%Al2O3, 0.41% Fe2O3, 6.4% MgO, 0.98% S, 0.5% MnO,and 1.1% TiO2.

The BFC is produced by mixing the water-quenched slagwith the PC clinker. The blending ratio of the cementclinker to the granulated slag in the commercial BFC atNittetsu Cement Co. Ltd. is 60 to 40 mass%. Thus, the ex-ergy analysis of BFC production is conducted by consider-ing the EXL in the two processes of clinker production andwater-cooling of molten slag

4. Results and Discussion

4.1. Material and Heat Balances of Clinker Produc-tion

The operating data on clinker production at a capacity of100 ton/h at the Muroran Works of Nittetsu Cement Co.,Ltd. are employed. The material balance of clinker produc-tion based on chemical compounds is given in Table 2. Theelement balances for the detailed calculation of exergy areprovided in Appendix 2. The input and output elements arein excellent agreement; in other words, the difference be-tween them is enough negligible for starting enthalpy flowanalysis.

Enthalpy flow in clinker production is shown in Fig. 2. Itis found that approximately 70% of the input enthalpy inclinker production is effectively used in the process. It com-prises the enthalpy input for the calcination (45%) and thatfor the preheating of a raw material (25%). As much as30% of the input enthalpy is mostly wasted in the form ofoff-gas and clinker. Coal and other carbon containing mate-rials are combusted in the rotary kiln, and limestone ther-mally decomposes and undergoes calcination, after which itis fired to yield clinker. The NSP system emits dust and hot


1321 © 2010 ISIJ

Fig. 1. Schematic diagram of the NSP clinker production systemconsisting of a preheater and a rotary kiln. Here, note thattire, bone, and sludge are recycled as fuel or raw materi-als.

Table 1. Chemical compositions of raw materials for clinker.

gas at 473 K from the rotary kiln; this hot gas is recycledand reused for preheating the raw materials. The clinker isfinally discharged as a main product at 473 K and cooled toatmospheric conditions.

4.2. Enthalpy and Exergy Analysis in Cement Produc-tion Processes

Figure 3 illustrates a process-system diagram of theclinker production process at the production capacity of100 ton/h. The detailed description of this diagram; so-called ‘thermodynamic compass’, is reported elsewhere.12)

Thus, only the principle of this method is explained here. Inthe diagram, a solid circle indicates a process; solid arrow,a material; white arrow, an intermediate energy; and dottedcircle, a system boundary. The delta value is the differencebetween the output and input values. The input enthalpyand exergy are thermodynamically calculated based on theconditions of pressure, temperature, and compositions ofthe input materials. The input materials include coal and airunder atmospheric conditions. The output enthalpy is ob-tained from the enthalpy calculations of clinker, off-gas,and dust as byproducts. For calculating the modified en-thalpy of the products, we assume the atmospheric tempera-

ture as 298.15 K, although the actual temperature of the dis-charged material is slightly different.

In the production of BFC, cement clinker and water-granulated BFC are mixed with a mass ratio of 60% : 40%.The sum of the enthalpy and exergy in BFC production iscalculated based on this mixing ratio. Process-system dia-gram of BFC production is shown in Fig. 4. Heat in themolten slag is assumed to be heat source in the processanalysis. For simplicity, the calculation is performed byconsidering the production of 100 ton clinker as the basisso that the amount of slag can be adjusted. The molten BFslag and water at temperatures of 1 773 and 298 K, respec-tively, are employed as input materials for the water granu-lation process to produce granulated slag as the main prod-uct and water as the by product at atmospheric temperature.The enthalpy is balanced in the overall process of BFC pro-duction.

4.3. Comparison of Exergy Losses

EXL in cement production is usually caused by severalreasons such as heat loss through walls, waste heat contentin products, chemical reactions, cooling, and mixing. EXL


1322© 2010 ISIJ

Table 2. Material balances for clinker production.

Fig. 2. Heat flow in the NSP clinker production process. Fig. 3. Process-system diagram of the clinker production at a ca-pacity of 100 ton/h.

through walls is caused by imperfect thermal insulation andmaterial transportation during the process. Clinker, dust,and off-gas are the final products of the process dischargedat over 473 K before finally cooling to atmospheric condi-tions. In fact, the clinker temperature is 1 573 K before dis-charge and excess air is introduced for cooling it to 473 K.EXL also results from the transportation of hot air to theNSP for preheating the raw materials. The waste heat con-tent in the product also causes EXL. Similar to other high-temperature industries, which involve chemical processes,the largest EXL in the cement process results from chemi-cal reactions; in this case, the reaction is combustion. It isnoteworthy that the clinker processing requires a high tem-perature of over 1 773 K, which is obtained by the combus-tion of carbon from coal with air.

Figure 5 shows the overall net exergy losses in the pro-duction of PC and BFC. The net EXL in the production ofclinker for PC is approximately 1.7 GJ/ton-cement, and it ismainly in the form of chemical exergy. In the case of BFC,a large temperature exergy exists at the input; however, it isabsent at the output. The water-cooling process for themolten slag from 1 773 K to atmospheric temperaturecauses temperature EXL, without any remainder in theproduct. Since this process involves the mixing of clinkerand slag, the EXL caused by mixing is extremely small and

therefore, negligible. The overall net EXL in BF cement isapproximately 2 GJ/ton-cement or around 57% of the totalinput exergy (BF Cement A). This value is higher than thatof the EXL for PC 45%. In other words, the exergy effi-ciencies for PC and BFC are 55% and 43%, respectively.The percentage efficiency of the PC process is 12 pointshigher than that of the BFC process. This is because a con-siderable amount of exergy is wasted during the granulationof molten slag using water. Although heat and enthalpy areconserved, chemical reaction and cooling undoubtedly de-grade the energy. It is noteworthy that the extremely largenet EXL in BFC is caused by the cooling process. There-fore, it is desirable to consider a high-efficiency techniquefor exergy recovery from molten slag. Therefore, it is possi-ble to increase the efficiency of BFC by recovering the ex-ergy of the molten slag.

An alternative method to produce blast furnace cement isproposed by mixing the limestone with molten slag directly,so called direct blast furnace cement production. The imageof the direct blast furnace cement production is given inFig. 6. The limestone will be decomposed with support ofheat source in the molten slag as its temperature is over1 773 K. Fly ash and limestone are introduced into themolten slag before its mixture is granulated using such as a


1323 © 2010 ISIJ

Fig. 4. Process-system diagram of BFC production, in whichcold clinker and cold slag are mixed in a ratio of60 mass% clinker and 40 mass% BF slag.

Fig. 5. Net exergy losses (EXL) in the PC production (left) andthe BFC one with the mixing ratio of 60 mass% clinkerand 40 mass% BF slag (right). The BFC production hassmaller input exergy, but larger EXL when the thermalexergy is not recovered.

Fig. 6. Schematic diagram of the proposed process for the Direct blast furnace cement production.

rotating cup atomizer without water impingement to pro-duce glassy slag.13) A rotary cup atomizer is set betweencalcination and rotary kiln furnace. Since granulationprocess is carried out in a closed system, there is no heat re-lease from the slag outside the system. The remained heatof the slag at above 1 673 K is also used to support calcina-tion process and clinker production. The glassy slag pro-duced is directly fed into rotary kiln together with othermaterials. When the proposed process is applied and thethermal exergy of the molten slag is completely recovered,the total EXL for the production of BFC will be 1.6 GJ/tonor 20% less EXL compared to the present BFC method asshown in Fig. 5 (BF Cement B). It can be understood thatthe proposed direct blast furnace cement process will pro-mote energy recovery and avoid exergy loss by utilize boththermal energy and glassy slag.

4.4. Energy Requirement and CO2 Gas Emission

The energy consumption in PC and BFC production areevaluated based on the amount of coal-based energy. Theanalysis is carried out using common basic-energy con-sumption in Japanese industries.14) One ton of PC con-sumes 105 kg coal, equivalent to approximately 2 985 MJenergy. The total consumption of electricity for the grindingprocess is 99 kW h/ton-cement; the individual consump-tions are 28.6, 2.4, and 68 kW h/ton for raw material, coal,and clinker, respectively. The overall energy requirementfor cement production is shown in Table 3. The resultsshow that the energy requirements are 4 036.2 MJ/ton forPC and 2 666.0 MJ/ton for BFC using present technology(refer to Fig. 5–BF Cement A). Coal is the main energysource for the calcination process in the rotary kiln andelectricity is mainly used for grinding. On comparing theresults we observe that PC and BFC consume 75% and67% of the energy from coal, respectively, and the remain-ing energy is derived from the electricity that is mainlyused for grinding the raw material and clinker. In otherwords, BFC production consumes 34% lesser energy thanPC. This difference can be accounted for by the fact that apart of the raw material in BFC is the glassy slag that is di-rectly mixed with the clinker.

With regard to the environmental aspect, reduction in en-ergy consumption leads to reduction in carbon gas emis-sion. The comparison between the CO2 gas emissions in thetwo cement processes is shown in Fig. 7. The CO2 gasemitted by the cement industry originates from threesources: coal combustion, electricity, and limestone decom-position. In the case of coal and electricity, a conversionfactor is used for calculating CO2 emission; this factor isestimated based on the guidelines provided by the Japanesegovernment.15) The results show that carbon gas is mainlyemitted from the decomposition of limestone rather thanfrom coal combustion and electricity. In BFC production(refer to Fig. 5–BF Cement A), where slag replaces a partof the limestone, the carbon gas emission reduces by about25% from 797.5 to 488.2 kg. The existing slag also con-tributes toward the reduction of carbon gas because of thereduction in the consumptions of coal and limestone.

5. Conclusions

The analysis first showed that a large amount of net ex-ergy losses (EXL) was found on the conventional blast fur-nace cement (BFC) production. The BFC production, therecovery of the thermal exergy of the molten slag should re-duce the total EXL by up to 20%. Most of the EXL in theBFC production is caused by the water granulation ofmolten slag.

Secondly, the CO2 emissions in portland cement andBFC productions are approximately 800 and 490 kg/ton, re-spectively; the large CO2 emission for portland cement isattributed to the utilization of more carbonaceous fuels forcalcination process.

Next, the proposed direct blast-furnace-cement processby utilization of waste heat in the molten slag as an energysource for limestone decomposition will promote energysaving in the BFC production. The recovery of thermal ex-ergy from the molten slag will reduce the total EXL in BFCproduction by around 20% of CO2 emission.

In conclusion, the recovery of thermal energy in themolten slag is essential to prevent the degradation of energyin conventional BFC production. By effectively utilizingboth thermal energy of molten slag and glassy slag pro-duced from direct cement production process will promoteenergy recovery and reduce not only exergy loss but alsoCO2 emission. When the BFC manufacturing plant locatesnext to an ironmaking plant, both ironmaking and cementproduction industries cooperate with the effective use of en-ergy easily.

Acknowledgments

The authors gratefully acknowledge the Japan Societyfor Promotion of Science (P04150) for the support to thepresent study, The Ministry of Economic Trade and Indus-try of Japan for their financial support and Nittetsu CementCo., Ltd. (Muroran Works), for providing the operatingdata.


1324© 2010 ISIJ

Fig. 7. Comparison between CO2 emissions in PC and BFCprocesses.

Table 3. Energy requirements for cement production.

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Appendix 1. Values of chemical enthalpy and exergy for major raw materials along with the information for calculating the averagespecific heat (M: molecular weight; A, B, C, D: constants; T1, T2: temperatures; L: latent heat).

Appendix 2. Element balances for clinker production.

process analysis of the effective utilization of molten

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