Energy Saving Opportunities through Heat Recovery from Cement Processing Kilns: A Case Study

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Margaret Rodgers
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1 Energy Saving Opportunities through Heat Reovery from Cement Proessing Kilns: A Case Study I. I. AL-HINTI a, A. AL-GHANDOOR b, A. AL-NAJI, M. ABU-KHASHABEH, M. JOUDEH, M. AL-HATTAB a Department of Mehanial Engineering, b Department of Industrial Engineering The Hashemite University Zarqa, JORDAN Abstrat: - This paper proposes a system for the utilization of dissipated heat from the surfaes of ement proessing kilns at the Jordan Cement Fatories in heating heavy fuel oil used in the burning proess of these kilns. It is proposed that this an be ahieved by irulating thermal oil inside large oil-shaped tubing whih is plaed around the kiln s shell. The heated thermal oil then transfers the reovered heat to the heavy fuel oil inside a heat exhanger. This is intended to effetively redue the ost of energy (both eletriity and diesel fuel) urrently used to heat the heavy fuel oil to failitate its pumping to the proessing kilns. The proposed system an also be onsidered as an environment-friendly system sine it utilizes available wasted energy and redues the resulting emissions of ement prodution proesses. Key-Words: - Energy effiieny, Heat reovery, Cement, Kilns, Jordan. 1 Introdution The ement industry is one of the most energy intensive industries where the energy ost typially aounts for 30-40% of the total prodution osts [1]. Over the past years, there has been an inreasing number of studies performing energy analysis of ement manufaturing proesses in order to identify potential opportunities for energy savings [2, 3]. Some of these studies have foused on ement proessing kilns [4, 5], whih onstitute the largest omponents in any ement prodution faility. Jordan is a lower-middle inome Middle Eastern ountry, of about 5.8 million inhabitants, that suffers from a hroni lak of adequate supplies of onventional energy resoures. As a result, Jordan depends heavily on imports of oil from neighboring ountries as the main soure of energy. Its urrent imports of around 100,000 barrels of rude oil per day are plaing the ountry under extreme eonomi pressures. The annual fuel bill has been rapidly inreasing over the past few years due to population and eonomi growth ombined with the onseutive inreases in oil pries [6]. This was diretly refleted on loal energy pries where both fuel and eletriity tariffs have reently witnessed onseutive inreases. These inreases have adversely affeted the ement industry and resulted in unpreedented inreases in the loal pries of ement produts. 2 Bakground Information Cement proessing kilns are the worlds largest moving manufaturing mahines. They are inlined, rotating furnaes of ylindrial shape with a shell inside diameter up to 5 m, and a length up to 80 m. A rotary kiln is normally fired by pulverized oal, petroleum oke or heavy fuel oil at its lower end. Raw materials of limestone, lay, and shale are preheated, normally by the kiln exhaust gases, before entering the upper end of the kiln at 900 C. The inlination and rotation of the kiln gradually onveys the raw mixture towards the firing zone. As this mixture ontinues to tumble towards the 1800 C flame, it undergoes hanges until linker is finally produed at the kiln outlet. The produed linker is later ground with gypsum to form ement. Jordan Cement is onsidered as the main ement manufaturing ompany in Jordan. It has two prodution plants: Rashadiaya plant loated 200 km south of Amman (the apital of Jordan), and Fuhais plant loated 25 km west of Amman. The urrent analysis will be limited to the latter whih has a prodution rate of over 1.5 million tons of ement. At this plant there are two similar rotary kilns that use heavy fuel oil to omplete the ement burning proesses. The speifiations of those two kilns are shown in table 1. Due to the high visosity of the fuel oil, preheating is essential to enable its pumping to the kilns. The fuel oil is usually heated to temperatures ISSN: Page 79 ISBN:

in the range of 150 C. This heating is done on several stages: The main heavy fuel oil tanks, whih have a total apaity of 3800 m 3, ontain eletrial heaters used to pre-heat the heavy fuel oil to 50 C.

2 in the range of 150 C. This heating is done on several stages: The main heavy fuel oil tanks, whih have a total apaity of 3800 m 3, ontain eletrial heaters used to pre-heat the heavy fuel oil to 50 C. These tanks feed two daily tanks, one for eah kiln. Eah daily tank has a apaity of 80 m 3, with an eletrial heater used to inrease the heavy fuel temperature from 50 C to 60 C. Two 195 kw eletrial heaters are used to heat the heavy fuel to 150 C before pumping it to the first kiln. For the seond kiln, the heating system onsists of a dieselfired boiler whih heats a speial thermal oil that irulates in a losed loop and transfers this heat to the heavy fuel inside a heat exhanger. This boiler onsumes diesel fuel at a daily rate of 2.7 m 3. Table1: Speifiations of kilns Type Dry proess, straight shell type kiln Number of supports 3 Slope 35/1000 Rotating speed (rpm) Length (m) 70 Inside Diameter (m) 4.4 Outside Diameter (m) 4.9 In 2006, the ombined ost of onsumed eletriity and diesel fuel was JD 1. The annual ost for both heating systems over the period between 2002 and 2006 is shown in figure 1. The lear inreasing trend is due to a ombination of inreasing energy pries and inreasing prodution. It should be noted that the surge in the ost in 2005 is mainly due to the implementation of the liberalization sheme of energy pries whih will be ompleted in Available Energy and System Desription Heat is transferred from the inside of the kiln where the burning proess takes plae to the kiln s outside shell surfae. As a result, a large temperature differential exists between the kiln s shell and the ambient, resulting in signifiant heat losses through onvetion and radiation. It is estimated that 15% of the input energy is lost in the form of onvetion and radiation heat transfer from the kiln s shell [4]. Heat transfer alulations based on natural onvetion and radiation indiate that the heat lost to the ambient from the kiln s shell is around 5 kw/m 2. Considering that the effetive outside surfae area of the kiln is around 1000 m 2, it an be seen that a total of 5 MW of heat is available at kiln s surfae. This result is onsistent with the alulations presented by a reent study on a similar proess kiln with similar temperature distribution [7]. The temperature distribution at the outside shell of the kiln is shown in Fig. 2. The highest temperature is at the flame zone, while the lowest is at the other side (the inlet side of the kiln). A Sudden temperature drop ours around the support zones of the kiln. This is beause they are exposed to ontinuous ooling to avoid their deformation. Heating Cost (x10 5 JD) Year Figure 1: Total annual ost of heating heavy fuel oil over the period A Jordanian Dinar (JD) is equal to $1.41. Figure 2: Temperature distribution ( C) along the length of the kiln s shell In order to utilize the available heat at the kiln s shell for a useful appliation, a liquid medium is needed. Liquids are onsidered the most appropriate option, beause they have higher speifi heats in omparison with gases and their visosity is inversely proportional to the temperature. The hosen liquid has to remain stable under high temperatures and should have high speifi heat and thermal ondutivity. The heat transfer thermal oil whih is already used in the existing heating system for the seond kiln ombines the previous harateristis and an be used in the proposed system. ISSN: Page 80 ISBN:

3 The proposed system onsists of a losed loop of oil-shaped, 50 mm-diameter, high ondutivity steel tube in whih the thermal oil is irulated. The oil is arranged to pass around the kiln s shell to absorb the available heat and then through a ounter flow heat exhanger (similar to the one used in the existing system for the seond kiln) to transfer the absorbed heat to the heavy fuel oil. This oil onfiguration has a half irular shape that engulfs the kiln s shell from both sides, as shown in figure 3. The distane between eah oil turn and the other is 50 mm, while the distane between the oil and the kiln s surfae is 300 mm. The whole oil onfiguration stands on a moving rail and has flexible ends. This is intended to failitate its motion to and from the kiln s surfae in order to ensure easy installation and maintenane without the need to stop the kiln. Thermoouples are used at different loations at the oil to monitor the temperature of the thermal oil and to ensure that it remains within the required range. Figure 3: Outline of the oil arrangement around the kiln s shell The number of oil loops depends on the required heat gain whih is haraterized by the temperature and the mass flow rate of the thermal oil. The whole arrangement is overed with a light sheet metal shield to limit the effet of environmental fators suh as air veloity, ambient temperature, and humidity. The effet of these fators an also be negleted due to the large amount of heat leaving the kiln s shell. The oil will be plaed between the seond and third supports at a distane of around 40 to 50 meters from the firing hood. The reason for hoosing this region (outlined by a square on figure 2) in partiular is that the temperature of the kiln s shell in this region is stable around 260 C. 4 Design Calulations Applying simple energy balane in the heat exhanger between the thermal oil and the heavy fuel and assuming negligible heat losses in the exhanger, the following is appliable: m T = m ΔT (1) o p o Δ, o f p, f f Knowing the mass flow rate of the fuel, its speifi heat and the required temperature rise (from 60 C to 150 C), the amount of heat that needs to be transferred by the thermal oil an be alulated. If a similar energy balane is applied to the ontrol volume representing the oil passing next to the kiln s shell, it an be shown that under steady state onditions, and assuming negligible temperature differene between the oil and the oil inside it, the following is appliable: m o p, o ΔTo = ( qonv + qrad )L (2) Free onvetion and radiation heat transfer rate between the oil and the kiln s surfae an be alulated by treating the oil surfae as a shield around the kiln. Therefore, the problem resembles that of long, horizontal, onentri ylinders, whih is a well-treated problem in relevant heat transfer literature. The rate of heat transfer per unit length by free onvetion an be alulated as follows [8]: 2 π k eff q ( ) ( ) onv = Ts T (3) ln D Ds where the effetive ondutivity is found by the following orrelation: 1 4 Pr 1 4 k eff = k ( Ra ) (4) Pr where 4 [ ln( D Ds )] Ra = Ra (5) x x ( Ds + D ) The Rayleigh number here is alulated depending on the distane between the shell surfae and the oil as follows: g β ( Ts T ) x Rax = (6) υ α All properties in equations 4 and 6 are evaluated at the average temperature of the oil and the kiln s shell. On the other hand, the radiation heat transfer rate per unit length an be alulated aording to the following equation: 4 4 π Ds σ ( Ts T ) qrad = (7) 1 1 Ds + s D ISSN: Page 81 ISBN:

4 Substituting the values of properties and parameters in equations 3-7, the amount of available heat transfer per unit length between the kiln s shell and the oil an be found. Therefore, equation 2 an be used to find the effetive length of the kiln that needs to be overed by the oil. Based on these alulations it was found that the required length is equal to 20 m whih translates to 200 loops and an approximate total tube length of 3300 m. In order to minimise heat losses from the oil tube surfae to the ambient on its route between the kiln and the heat exhanger, the tube surfae an be overed with a 20 mm-thik layer of glass fibre insulation. The alulated drop in the temperature of the tube surfae after it leaves the kiln s surfae until it reahes the heat exhanger is only 2 C. 5 System Eonomi Feasibility In order to examine the feasibility of the proposed system, the total ost of this system needs to be estimated. As detailed in table 2, the total ost of the proposed system was estimated to be around JD for both kilns. Assuming that the proposed system will be used for only 80% of the fatory s operational time and on the basis of the alulated ost for 2006, the net annual energy ost savings will be equal to JD. This signifiant saving is expeted to inrease depending on future hanges in a number of variables suh as energy pries, inreased prodution...et. However, on the basis of urrent onditions, the paybak period is only equal to 200 days. This relatively short paybak period, ombined with the simpliity of the proposed system, makes it very attrative and feasible for industrial implementation. Table 2: Estimated ost of the proposed system for one kiln Equipment Cost (JD) Commerial steel tubing 40,000 Thermal oil pump 6,000 Heat exhanger 20,000 Cost of onstrution 10,000 Others (Fittings, motors, valves,..et.) 30,000 Total ost 106,000 6 Conlusion This study reveals that there are signifiant energy saving potentials in the ement industry, espeially through the reovery of heat from the surfaes of rotary kilns whih normally dissipate large amounts of heat to the ambient. This study proposed a simple, tehnially and eonomially feasible system for reovering heat from kilns surfaes and utilizing this heat in the preheating proess of the heavy fuel oil. The resulting annual savings are estimated to be around $0.54 million for the Jordan Cement Fatory. Similar systems an also be used for spae and water heating appliations in ement proessing plants. Nomenlature: : Speifi heat (kj/kg.k) p D : Diameter (m) k : Condutivity (W/m.K) L : Length of kiln overed with the oil (m) m : Mass flow rate (kg/s) q : Convetive heat transfer rate per unit onv length (W/m) q : Radiative heat transfer rate per unit length rad (W/m) Pr : Prandtl number Ra : Rayleigh number Ra : Correted Rayleigh number T : Temperature (K) x : Distane between kiln s shell and oil (m) Greek Symbols: α : Thermal diffusivity (m 2 /s) β : Volumetri thermal expansion oeffiient (K -1 ) : Emissivity υ : Kinemati visosity (m 2 /s) σ : Stefan-Boltzmann onstant Subsripts: : Coil f : Fuel o : Thermal oil s : Kiln s outside shell Referenes: [1] L. Szabo, I. Hidalgo, J.C. Cisar, A. Soria, P. Russ, Energy onsumption and CO 2 emissions from the world ement industry, European Commission Joint Researh Centre, Report EUR EN, [2] E. Worell, N. Martin, L. Prye, Potentials for energy effiieny improvements in the US ement industry, Energy, 25, 2000, pp [3] Z. Utlu, Z. Sogut, A. Hepbasli, Z. Oktay, Energy and exergy analyses of a raw mill in a ement ISSN: Page 82 ISBN:

5 prodution, Applied Thermal Engineering, 26, 2006, pp [4] T. Engin, V. Ari, Energy auditing and reovery for dry type ement rotary kiln systems A ase study, Energy Conversion and Management, 46, 2005, pp [5] M.G. Rasul, W. Widianto, B. Mohanty, Assessment of the thermal performane and energy onservation opportunities of a ement industry in Indonesia, Applied Thermal Engineering, 25, 2005, pp [6] I. Al-Hinti, Energy saving potentials in Jordan through the introdution of diesel passenger ars, WSEAS Transations on Environment and Development, 2 (5), 2006, pp [7] B.K. Chakrabarti, Investigation on heat loss through the kiln shell in magnesite dead burning proess: a ase study, Applied Thermal Engineering, 22, 2002, pp [8] F. P. Inropera, D. P. DeWitt, Fundamentals of heat and mass transfer, Wiley & Sons, 5 th edition, ISSN: Page 83 ISBN:

Effect of Variation of Fuel Composition on Gas Turbine Off-Design Performance

Effect of Variation of Fuel Composition on Gas Turbine Off-Design Performance Applied Mehanis and Materials Submitted: 2014-08-28 ISSN: 1662-7482, Vol. 666, pp 194-198 Aepted: 2014-08-28 doi:10.4028/www.sientifi.net/amm.666.194 Online: 2014-10-20 2014 Trans Teh Publiations, Switzerland