A case study of cogeneration for Jeddah and Yanbu petromin refineries in Saudi Arabia

Heat Recovery Srstems & CHP Vol. 9, No. 5, pp. 485-491, 1989

0890-4332/89 $3.00 + .00 Pergamon Press plc

Printed in Great Britain

CASE STUDY A CASE STUDY OF COGENERATION FOR JEDDAH AND YANBU PETROMIN REFINERIES IN SAUDI ARABIA

ESSAMFAQEEH,A. M. KHALIFAand A. M. RADHWAN Thermal Engineering Department, King Abdulaziz University, P.O. Box 9027, Jeddah 21413, Saudi Arabia (Received 3 January 1989)

Almtraet--The Petromin refineries in Jeddah and Yanbu, Saudi Arabia produce power and process steam separately. The Jeddah refinery gas turbines that have an installed capacity of 88 MW, run at less than 50% utilization factor. The refinery demand of steam is 70-120 tons h-L The electric power demand of Yanbu refinery is supplied by The Royal Commission of Yanbu at a rate of 16--25 MW. The steam consumption is 68 tonsh-L Data were cotlected for the performance and requirement for both plants. An integrated system for cogeneration is proposed which consists of a gas turbine, heat recovery steam generator equipped with a supplementary duct burner and economizer. The thermal and economical analyses have proved the feasibility of the proposed system with payback periods of 23 and 36 months for Jeddah and Yanbu refineries, respect"ively. However, the payhack period for Jeddah refinery can be reduced to 15 months if the utilization factor is improved, and the excess power generated by the gas turbines is connected to the public utility grid. In fact, it is worth mentioning that the present study is considered the fast to be carried out in Saudi Arabia; more studies and investigations could lead to tremendous saving in the fuel consumption in this country.

NOMENCLATURE BD

Cf C0j Co: Cps CT Ah, hs.o.t h..i. kw.,., m, ms n P Ps

QAB Qs R Rev TE T,,, T~,oh

Continuous blowdown rate (%) Capital cost of equipment (million Saudi Riyals) Operating cost of gas turbines (million Saudi Riyals) Operating cost of heat recovery steam generator equipment (million Saudi Riyals) Specific heat of exhaust gases (kJ kg- ' K - ' ) Total capital cost (million Saudi Riyals) Energy required to evaporate I kg of steam in the heat recovery steam generator (kJ kg- ~) Specific enthalpy of steam leaving boiler (kJ kg- t ) Specific enthalpy of water entering boiler (kJ kg- ~) Specific enthalpy of water at boiler saturation temperature (kJ kg- z) Steam mass flow rate (kg s -~) Exhaust gases mass flow rate (kg s -t ) Payback period (months) Capital cost (million Saudi Riyals) Saturation pressure of the steam in the boiler (bar) Heat absorbed by the steam in the heat recovery steam generator (kW) Heat loss by the exhaust gases in the heat recovery steam generator (kW) Fixed charge rate (%) Revenues (million Saudi Riyals) Exhaust gas temperature (°C) Steam saturation temperature (°C) Pinch point temperature (°C)

INTRODUCTION Cogeneration is the process of simultaneous production of electric power and useful heat. In the 1970's, when inexpensive sources of energy were threatened, interest in cogeneration was revived as a result of the new attention paid to conservation techniques and nontraditional fuels. Today many institutions and industries have built, or plan to build cogeneration facilities. With commercially available heat recovery equipment, the cycle efficiency of a gas turbine can reach as high as 75% [3]. Such a high efficiency makes the application of gas turbines for continuous or base load feasible and attractive. Two heavy fuel oil fired gas turbines each having 37.4 MW and two 19.5 kg s-t supplementary fired heat recovery steam generators are being installed in the India 485

486

ESSAM FAQEEH et al.

Petrochemical Corporation Limited at Barada, India [1]. It is expected that when the cogeneration plant with the integrated electrical system is commissioned, it will provide a reliable source of electrical power and process steam to the petrochemical complex. On the other hand, two identical heat recovery steam generators were installed in the Gulf Coast Plant of a major chemical company. They were to replace existing boilers which were scrapped. Each heat recovery steam generator recovers energy from the exhaust of a 15 MW Westinghouse W-191 gas turbine. Design steam conditions from the heat recovery steam generator are 68 tons h-~ at 43.1 bars, superheated to 399°C [2]. However, the design of the heat recovery steam generator is based upon the pinch point concept. Ericksen et al. [3] outlined the types of heat recovery boilers available for use with gas turbines in the power range of (1-100 MW). Also they discuss the design and performance criteria for both unfired and supplementary fired gas turbine exhaust heat recovery boilers of single and multiple pressure levels. In Saudi Arabia more than 6000 MW are generated by gas turbines. The refineries and the petrochemical plants use gas turbines to generate electricity and use conventional boilers to produce steam. Thus, the cogeneration in Saudi Arabia is a very important and viable alternative for the refineries and petrochemical industries, which can lead to large savings. In spite of the vital importance of cogeneration, this study represents the first of its kind for Saudi Arabia. P O W E R A N D S T E A M D E M A N D FOR J E D D A H A N D Y A N B U PETROMIN REFINERIES Jeddah Petromin Refinery generates its electric power demand using four General Electric industrial gas turbines for a total of 88 MW. Two of the gas turbines are running at base load, the third is kept as standby while the fourth is scheduled for maintenance. The load in the refinery rarely exceeds 50% of total capacity of all units, thus the utilization factor is less than 0.5. The exhaust temperature, which is an important factor in designing the heat recovery steam generator, is dependent on the load. It ranges from 300°C at 25% of full load to 420°C at 50% of full load. The process steam is produced by four Mitsubishi water tube boilers with a capacity of 50 tons heach. Usually three of the boilers are on duty and the fourth is kept as a standby. The steam demand in the refinery fluctuates between 70 and 120 tons h-~. Yanbu Petromin refinery receives its electric power demand from the Yanbu Royal Commission. In the winter season the electric power demand reaches a low value of 16 MW. In the summer it increases to 25.5 MW. The process steam is generated by three Mitsubishi water tube boilers with a capacity of 68 tons h- ~ each. Two boilers are running and the third one is on a standby status. The steam condition is 12 bars pressure and 188°C saturation temperature. DATA COLLECTION For the purposes of the feasibility study, data were collected from Jeddah and Yanbu Petromin Refineries. For each refinery the data are divided into two parts; electric power and process steam. In the following a presentation of the collected data is given. Jeddah Petromin Refinery data 1. Electric power. The electric power demand in the refinery is provided by four gas turbines. Gas turbines numbers 3 and 4 are running at base load, while gas turbine number 2 is kept as standby and gas turbine number I is scheduled for maintenance. Table l shows the specifications of the four gas turbines. The power output listed in Table l is at ISO conditions (l 5°C ambient temperature and l bar). It decreases as the ambient temperature increases. For gas turbines numbers 2, 3 and 4 the power output decreases from 24 MW to Table I. Specification of gas turbines numbers 1,2,3 and 4 in the Jeddah refinery Gas turbine number

Type

I 2 3 4

BBC General Electric General Electric General Electric

Model

Power output (MW)

Heat rate (kJ k w - Lh - i )

Exh temp. (C)

Exh flow (kg s- t )

PG5211 MSS001m PG5341 PG5341

24 16 24 24

13,145 13,663 13,145 13,145

497 481 497 497

123 90 123 123

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Fig. 1. The monthly minimum and maximum load for gas turbine number 3.

MONTH

Fig. 2. The monthly minimum and maximum exhaust temperature for gas turbine number 3.

approximately 21 MW as the ambient temperature increases to 35°C. However, gas turbines numbers 3 and 4 are installed in series and share a common control room. Figure 1 shows the monthly minimum and maximum load of gas turbine number 3 during the year 1987. The lowest load is 4.8 MW which occurred in March, while the highest load is 12 MW which occurred in August. Figure 3 displays the monthly minimum and maximum load of gas turbine number 4. For this turbine the lowest load is 5.6 MW, which occurred in December while the maximum load is 11.4 MW, which occurred in July. Figures 2 and 4 show the monthly minimum and maximum exhaust temperature for gas turbines numbers 3 and 4. For gas turbine number 3 the exhaust temperature ranges from 300°C to 434°C, while for gas turbine number 4 it ranges from 300°C to 410°C. 2. Steam. Figure 5 shows the demand in Jeddah Petromin Refinery for different days in December 1984. However, Fig. 6 compares the demand in December 1984 with that in December 1987. The demand in December 1987 reaches a high value of 110 tons h-~, while it reaches only 95 tons h -~ in December 1984. Figure 7 depicts the fluctuation of the monthly steam demand in Jeddah Petromin Refinery during the period 1985--1987. The average hourly demand for 1985, 1986 and

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Fig. 5. Steam demand for Joddah Refinery in December 1984.

Fig. 6. Comparison between the steam demand in December 1984 and 1987, in Jeddah Refinery.

1987 are 104, 97 and 100 tons, respectively. In fact,the demand increases rapidly in the summer season. The steam required in the refinery must be at pressure of 12 bars and saturation temperature of 188°C. Yanbu Petromin Refinery data 1. Electric power. The electric power demand of Yanbu Refinery is supplied by The Royal Commision of Yanbu at a rate of 16-25 MW. To make the refinery self-dependent in electric power, two industrial gas turbines have to be installed with a capacity of 25 MW each at ISO condition (15°C ambient temperature and 1.0 bar), plus a third one with a capacity of 9.0 MW. Table 2 shows the specifications of the proposed gas turbines for the Yanbu Refinery. Figure 8 shows the daily average electric power in Yanbu Petromin Refinery during four months of the year 1987: There is no significant variation except in March, where the daily average electric power ranges from 16 to 26 MW. The Royal Commission of Yanbu supplies the refinery with the required load at a rate of 57.5 Saudi Riyals per MW h -~. Unit number 1 is to be operated at baseload, while unit number 2 will be kept as standby. If the load exceeds the full capacity of unit number 1, unit number 3 will be turned on.

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Fig. 7. Fluctuation of the steam demand in Jcddah Refinery during the period 1985-1987.

A case study of cogeneration for Petromin refineries

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Table 2. Specification of proposed gas turbines in Yanbu Refinery Power Heat Exhaust Exhaust Unit Model output rate flow rate temp. number number (MW) (kJ k W - J h -i ) (ks s- i ) (°C)

I

MOD POD

25

12,502

109

553

2

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3

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37

459

9.0

2. Steam. Figure 9 presents the variation of steam consumption in Yanbu Petromin Refinery. The demand in the refinery varies from 58 to 67 tons h -~ in March 1987. The quality of steam required must be at pressure of 12 bars and saturation temperatures of 188°C.

THERMAL ANALYSIS Figure 10 shows the proposed system, which consists of a boiler, economizer and an auxiliary duct burner. The amount of steam generated by a heat recovery boiler is calculated using the energy balance equations. This energy balance incorporates the concept of "pinch point" which is defined as the difference between the boiler outlet gas temperature and the boiler steam saturation temperature:

r .ch =

r

oo,-

T.,

(l)

The heat transferred from the exhaust gas to generate steam is given by: Qi = M s C r ~ [ T s . i . - T~o.t].

(2)

The heat loss from the heat recovery boiler is approximately 2% according to Ericksen et al. [3]. Thus the energy absorbed by the steam is given by: Q^B = 0.98Qg.

(3)

However, the energy required to generate one unit mass of steam is determined by: Ah. : h,. o., - hw,~. + B D (hw,., - h.,~.).

(4)

Consequently, the steam flow rate recovered by the heat recovery steam generator is calculated as follows: ms = QAe IAh~.

(5)

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Fig. 8. The daily average electric power demand in Yanbu Refinery.

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Fig. 9. Variation of steam consumption in Yanbu Refinery in 1987.

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Fig. 10. Heat recovery steam generator diagram. According to Ericksen et al. [3] continuous blowdown is required to maintain the concentration of solids at a tolerable level in the boiler and is usually in the range 2-5%. Pinch point temperature is often in the range of I 1-32°C. The specific heat of exhaust gases is taken as I. 13 kJ kg- ] K - J. The thermal analysis was applied to both Jeddah and Yanbu Refineries. The results are given in the following: I. Jeddah Petromin Refinery. At 5 MW load and 303°C exhaust temperature the heat recovery steam generator recovers ll.7 tons h -~ of steam (12 bars and 188°C). At full load it recovers 63 tons h- ~of steam. In the worst case the refinery needs twice this amount. However, this amount will be available to the refinery if gas turbines numbers 3 and 4 are running at full load. But the excess power which sometimes reaches as high as 30 MW must be connected to the grid in order to make this alternative attractive. The second alternative is to run gas turbines numbers 3 and 4 at baseload and add a supplementary duct burner to the heat recovery steam generator to boost the exhaust temperature. This alternative is much more attractive than the first alternative if there is no possibility of connecting the excess power to the grid. 2. Yanbu Petromin Refinery. If gas turbine number 1 (Table 2) runs at full load, the heat recovery steam generator will generate 67 tons h -] of steam. Figure 9 represents the variation of steam demand in the refinery with a peak of 67 tons h-~. Therefore, the steam generated by the heat recovery steam generator will meet the demand in the refinery. The difference between electric power demand and the power generated is connected to the public utility grid.

ECONOMICAL ANALYSIS The economical feasibility of any cogeneration plant is dependent on the payback period. The shorter the period the better the result. According to Grant et al. [4] and Gramo [5] the total annual cost is calculated as follows: CT = Co, + Cf,

(6)

where Cf is the capital cost of equipments multiplied by the fixed charge rate and Co] is the operating cost of the gas turbine. The payback period: n = CT/Rev -- C02.

(7)

A case study of cogeneration for Petromin refineries

491

Table 3, Summary of results for thermal and economical analysis Jeddah Petromin Refinery Yanbu Petromin Case I Case II Refinery Capital cost plus operating cost 32 31.6 25.4 of gas turbines (million S.R.) Operating cost of heat recovery steam generator (million S.R.) Revenues from steam (million S.R.)

4.3

4.0

21.4

21.4 7.65

11.2

23

15

36

Revenues from excess power (million S. R.) Payback period (months)

2.71

The fixed charge rate, R, is often 15% according to Strotzki et al. [6]. In Saudi Arabia the cost of l kW h is 0.03 Saudi Riyal and the cost of 1 ton of steam is 20 Saudi Riyal. Table 3 lists the summary of the results for the economical analysis for both Jeddah and Yanbu Petromin Refineries. It seems that the cogeneration solution for both refineries is attractive since the payback period is much shorter than the normal life of the heat recovery boilers (about 15 years). CONCLUSION .~ A survey of power and steam demand for Jeddah and Yanbu Petromin Refineries was done for the purpose of studying the feasibility of cogeneration for these plants. A heat recovery boiler equipped with auxiliary burner was proposed. Thermal and economical analyses have proven the feasibility of the proposed system with a payback period of 23 and 36 months for Jeddah and Yanbu Refineries, respectively. However, the payback period for the Jeddah plant can be reduced to 15 months if both gas turbines numbers 3 and 4 are running at full load and the excess power is connected to the utility grid. It is concluded that the cogeneration option for these refinery plants is feasible and can lead to a considerable saving in fuel consumption. In addition to the improvement of steam and power production economy, there is a reduction of air pollution caused by the extra amount of fuel burned when the cogeneration is not implemented. REFERENCES I. O. K. Muthappa, Gas Turbine cogeneration plant for Indian Petrochemical Corporation Limited, Baroda, India, ASME Cogan-Turbo Conference (1987). 2. D. R. Logeais, Design of a heat recovery steam generator, ASME conference on gas turbines, paper No. 160, pp. 1-8 0984). 3. V. L. Ericksen, J. M. Frcomming and M. R. Carroll', Design of gas turbine exhaust heat recovery boiler system, ASME conference of gas turbines, paper No. 126, pp. I-7 (1984). 4. L. E. Grant, W. Ireson Grant and R. S. Leavenworth, Principles of Engineering Economy, Ronald Press, New York 0976). 5. E. E. P. DeGarmo and J. R. Canada, Engineering Economy, Macmillan, New York (1973). 6. G. Bernhardt, A. Strotzki and W. A. Vapot, Power Station Engineering and Economy. McGraw-Hill (1960).