CIRED

17th International Conference on Electricity Distribution

Barcelona, 12-15 May 2003

POWER GENERATION USING RECOVERED ENERGY FROM NATURAL GAS NETWORKS Mohamed Salah ELSOBKI (Jr.)* and Hafez Abdelaal EL-SALMAWY** * Egyptian Electric Utility and Consumer Protection Regulatory Agency - Egypt ** Department of Mechanical Power Engineering, Zagazig University - Egypt [email protected] , [email protected]

SUMMARY This paper presents an approach of using Turbo-expanders for energy recovery to generate electricity from the natural gas distribution networks. This approach is an alternative to the standard pressure reduction valves normally used. Based on the current level of gas consumption in Egypt and suitable sites to install these units over the gas network, an economical reachable potential to generate 120 MW of electricity is identified. The implementation of this potential has a sounding impact on the abatement of greenhouse gases. It enables an annual abatement of 325,977, 7 and 205 tons of CO2, CO and NOx respectively. To optimize the application of Turbo-expander within gas networks for electric generation, an integrated resource planning-operational scheme is introduced. The location and sizing of the turboexpanders to be installed in a gas distribution network are determined according to a developed optimal formulation. Regarding location optimization the formulation accounts for predominant levels of gas pressure, accessibility to the electricity grid, gas flow rate and it steadiness. For the sizing, the formulation accounts for the by-pass ratio, unit load factor, preheating requirement of gas and economical viability. Both the technical and financial feasibility of the introduced scheme are presented and the associated environmental impacts are evaluated. Implementation of the introduced scheme has been simulated for a case covering a power plant in Egypt. This enables the achievement a competitive electricity selling price at 1.6 US cent/kWh. Better figures can be achieved if waste heat is used for gas preheating. Discussion is underway to implement a pilot project in a cement plant as a start for a large-scale implementation program over the Egyptian gas network. BACKGROUND Egypt enjoys good reserves of natural gas. These reserves reached 55 Trillion cubic feet (1.56 Trillion cubic meters) in year 2001 [1]. It is expected that these reserves will exceed 100 Trillion cubic feet in the coming few years based on the current consumption level and its rate of development. The Egyptian energy sector is becoming more relying on natural gas as a prime source of energy. The consumption of natural gas has increased by 25.89% in the year 2000/2001 compared with the year before [1]. It reached 19.77 million T.O.E in the year 2000/2001. This represents a monatomic trend

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where the rate of increase in consumption was 22.7% in the year 1999/2000 compared with the year before. Electricity generation represents the main consuming sector of natural gas where its consumption represents 62.54% of the total gas consumption. More than 80% of electricity generation power plants have been converted to burn natural gas [1, 2]. This tendency toward using natural gas not only attributed to its availability, but also due to its higher combustion efficiency and positive environmental impact. Natural gas mixes easily with combustion air at lower excess air ratio [3]. This enables high combustion efficiency due to complete combustion as well as less flue gas losses. Also natural gas has the advantage of low CO2 emissions, which is about 40% less than the liquid fuel for the same quantity of heat release. Furthermore it is free of Sulfur and accordingly switching to natural gas automatically means abatement of SOx emissions. Natural gas cannot be used unless a distribution network becomes available to connect both the supply and the demand centers. Currently a distribution network of a capacity of 98 Million Nm3 per day is available. This represents around 1.5 times the current daily gas consumption. This network is covering the delta area (northern part of Egypt), with some extension to east and west. Southern Egypt still has no gas network. It is expected that the gas network will extend to cover a part of southern Egypt by the year 2005 [1]. Similar to the electricity distribution networks, the gas networks are working at different potential (pressure) levels. The pressure level of 70 bars is used to connect the production centers with the main distribution stations, at which the pressure is reduced to 42 bars. Large consumers such as power, fertilizers, cement and metallurgical plants as well as industrial districts are supplied at that pressure. Pressure is reduced in these distribution stations to 7 bars. For the medium size industrial and residential districts gas is being distributed at 7 bars. Inside the facilities gas is distributed at 4 bars till the consuming equipment at which the pressure is reduced to the equipment operating pressure. The working pressure of normal combustion system is around 20 mbar. It is important to mention that all the above pressures are gauge pressures. Gas network is equipped with gas compressors, which are used to collect and pressurize the gas into the network. These compressors are normally driven by gas turbines. Considering the current consumption, efficiency and pressure ratio of these compressors the average power demand of these compressors is 771 MW. The gas consumed in the driving

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gas turbines is equivalent to 11.7% of the gas compressed in the network. The gas pressure is being reduced at the reduction station using throttling valves. The throttling process is a constant enthalpy process, which is almost constant temperature. This process is thermodynamically irreversible. Accordingly all the energy in the pressurized gas is consumed in friction losses. This represents loss of the pressure energy of the gas. The annual lost energy in the gas network at the current consumption rate is calculated to be 6751 GWh. Figure 1 shows schematic diagram for the predominant pressure levels in the gas network of Egypt. Supply

Main Distribution Gas Turbine Power Plant Based Distribution pressure for industrial and id ti l Distribution Pressure within Facilities Working Pressure

70 barg

42 barg

13 barg

Barcelona, 12-15 May 2003

TURBO-EXPANDERS FOR PRESSURE REDUCTION IN GAS NETWORKS Turbo-expanders have been used for a long time in natural gas and air liquefaction plants. In these plants the produced power from these expanders are used to provide part of the energy necessary to drive the gas or air compressors respectively. A Turbo-expander is basically a radial or axial gas turbine which produces power due to gas expansion from high pressure at its upstream to a low down stream pressure. If the supplied gas is at the ambient temperature the exhaust gas temperature will be at a lower temperature depending on the expansion pressure ratio, type of the gas and expansion efficiency. In cases when the Turbo-expander is not being used for condensation and consequently it is not required to have low down stream temperature, suitable upstream gas preheating is used. This is preferably to be achieved by recovering the waste heat from any available heat source in the plant. If this is not possible, a dedicate gas pre-heater is used. It is important to mention that gas preheating boosts the power of the Turbo-expander due to the increase in the enthalpy of the upstream gas.

7 barg 4 barg

20 mbarg

Figure 1. Predominant Pressure Levels in the Gas Network

An experimental project has been implemented to use Turboexpander for power generation in San Diego Gas and Electric company in 1983 [4]. A Turbo-expander unit of a capacity 260 kW had been installed parallel to the reducing valve in a pressure reduction station. Pressure reduction was primary achieved by the Turbo-expander. The reducing valve is only used during maintenance time of the expander. The Turboexpander has been used to drive an electric generator, which is interconnected with the electricity grid. The unit was designed for unattended operation. The use of the Turboexpander showed high degree of reliability. Also it did not affect the performance of the pressure reducing station under different operating conditions. Figure 2 shows a schematic diagram for a pressure reducing station, which uses a parallel turbo-expander for pressure reduction with gas pre-heater. Pressure Reducer

Gear Box

G

Gas Pre-Heater Turboexpander

Figure 2-Schematic Diagram for a Turbo-expander Installed Parallel to the Existing Pressure Reducing Valve

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17th International Conference on Electricity Distribution

POTENTIALS OF ENERGY RECOVERY FROM THE GAS NETWORK IN EGYPT The category of reducing stations which include the 70/42, 42/7 and 42/13 are the most attractive sites to install Turboexpander units. These stations enjoy steady, high-pressure ratio as well as high flow rate. Furthermore most of them are supplying large industrial companies as well as power plants and accordingly they are close to the electricity grid. Down stream reduction stations are not attractive, as they are highly dispersed and consequently supplying limited number of equipment. Accordingly they suffer from low diversity factor, which makes the gas flow less steady. Also they have limited flow rates as well as low-pressure ratios. All these factors limit the potential recovered power per site as well as reduce the unit loading ratio. Accordingly these micro units will have higher cost and less financial attractiveness. The identified pressure reduction levels between which Turboexpanders are recommended to be installed are shown in Figure 3. To calculate the potential electrical energy, which can be recovered from these gas reduction stations a thermodynamic model have been developed. An overall efficiency of 73% has been considered based on the manufacturer supply data [5]. According to this model 28.2, 82.7 and 9 MW can be recovered from the 70/42, 42/7 and 42/13 reducing gas reducing stations respectively. Gas is preheated to ensure

Barcelona, 12-15 May 2003

down stream temperature above the dew point of the heavy hydrocarbons in the gas. Considering units availability of 92% [5], Table 1 shows the potential annual electricity to be generated as well as the net equivalent fuel saved. Also Table 1 shows the corresponding abatement of green house gases.

Supply

70 barg

Main Distribution

42 barg

Gas Turbine Based Power Plant

13 barg

Distribution Pressure for industrial and residential Districts Distribution Pressure within Facilities

7 barg 4 barg

20 mbarg

Working Pressure

Figure 3 The Recommended Pressure Reduction Levels for Turboexpander Installation

TABLE 1-Emission Reduction Due to the Potential Fuel Saving Pressure Ratio

70/42

42/7

42/13

Total

Units

Annual Electricity Generated

226,838,429

666,483,694

72,037,242

965,359,365

kWh

Equivalent Annual Gas Saved

66,078,317

194,147,531

20,984,538

281,210,387

m3

5,422,265

51,507,152

3,403,063

60,332,479

m3

60,656,053

142,640,380

17,581,476

220,877,908

m3

102,790

241,723

29,794

374,307

Ton/year

Gas Consumption for Preheating Net Annual Gas Saved Emission Reduction CO2 CO

2

5

1

8

Ton/year

Nox

64

150

19

233

Ton/year

INTEGRATEDRESOURCE PLANNING/OPERATION OPTIMAL FORMULATION A twofold optimal formulation is introduced to account for the proposed locations of Turbo-expanders and their capacities under a regulated electricity market [6]. The first stage of the optimal formulation is the determination process of the best locations for the installation of Turbo-expenders in the gas distribution network (planning phase). The selection process is executed through a combinatorial optimization process, where pressure ratio “Rp” across the reduction station is maximum, the flow rate “G” is maximum, the variation in the flow rate “∆G” is minimum and interconnection requirements with the electricity grid are minimum. EEU_ElSobki_B1

Session 4 Paper No 4

The second stage of the optimal formulation is to determine the optimum rating of the Turbo-expander and the corresponding operational conditions. This is highly dependent on the bypass ratio “bs”, which determines the amounts of gas flow through the Turbo-expender “GT” and that through the reduction valve. The target is to reach the maximum possible electric energy “E”, as well as minimum fuel consumption for preheating, with a target upper limit of the cost per kWh generated. This is expressed as: Maximize E

(1)

Where,

E = PR * LFS * AS ∗ 8760

(2)

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17th International Conference on Electricity Distribution

Where,

Barcelona, 12-15 May 2003

Both the Turbo-expander efficiency “ζT” and the generator

PR : rated electric output of the Turbo-expander system LFS : load factor of the Turbo-expander system AS : Availability of the Turbo-expander system

efficiency “ζG” are functions in the load factor. Figure 4 shows the dependence of the expander efficiency on the load factor (design flow percent).

The rated electric power “PR” is expressed in terms of the rated flow capacity “GR” of Turbo-expander, the specific heat “CP” of the gas, the density of the gas at normal conditions

The Turbo-expander inlet temperature “Tin” is related to the down stream temperature “Td” of the gas reduction station as:

“ρGN”, the inlet temperature “Tin” to the Turbo-expender, the pressure ratio “Rp”, the overall efficiency of the Turboexpender system “ζs”, and gas specific heat ratio “k”. Gas specific heat is considered as a function in temperature. The

Td

Tin =

1 − ζ T ∗ [1 − (

electric power “PR” is expressed as:

1 ( PR = GR ∗ CP * ρGN * Tin ∗ [1 − ( ) RP

k −1 ) k

]∗ζ S

1 ( ) RP

k −1 ) k

(8)

]

(3)

The Turbo-expender load factor “LFs” is expressed in terms of the reduction station gas flow rate “G”, the rated flow capacity of the Turbo-expender “GR” and the system bypass ratio “bs” as:

LFS =

G ∗ (1 − bs ) GR

(4)

The bypass ratio “bs” at the gas reduction station is expressed

Figure 4-Variation of Turbo-expander Efficiency with Gas Flow Rate

in terms of the maximum gas demand “Gmax”, as:

G bs = 1 − R Gmax

(5)

The availability “AS” of the Turbo-expender system is normally a manufacturer based value and is around 92% [5]. The pressure ratio “Rp”, is expressed in terms of the upstream and down stream pressure levels “Pu” and “Pd” at the gas reduction station and is expressed as:

P RP = u Pu

(6)

The overall efficiency “ζs”, represents all the sub systems efficiencies including the Turbo-expander efficiency “ζT”, the transmission efficiency “ζTran” (gearbox) and the generator efficiency “ζG”, and is expressed as:

ζ S = ζ T ∗ ζ Tran ∗ ζ G

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The down stream temperature “Td” is limited by the possible liquid condensation, if the down stream temperature went below the dew point “Tdp” of the heavy constituents of the gas. Condensation increases the erosion of expander blades as well as reduces its efficiency [6]. Since the concentration of the heavy constituents can vary, then a safety margin “∆Ts” should be considered. Therefore inlet gas preheating is needed to control the down stream temperature. Additional heating of the inlet gas will lead to an increase in the turbine output yet this will take place on the expense of the system efficiency. The gas flow rate for preheating “Gpre”, is expressed as follows:

G pre =

GT ∗ C p ∗ (Tin − Tu )

(9)

QHV ∗ ζ B ∗ ρGN

Where: Qhv: Natural gas heating value ζB: Thermal efficiency of the heating boiler The maximization process of the electric output energy will be subject to a number of constrains which simulate the operational conditions of the Turbo-expander. This can be expressed as follows: -4-

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17th International Conference on Electricity Distribution

Gmin ≤ GR < Gmax

(10)

Gmin ≤ G < Gmax

(11)

(1 −

Gmin ) ≥ bS ≥ 0 Gmax

Gmin ≤ LFS ≤ 1 GR Td ≥ Tdp + ∆TS

AnnulaizedLCC E

corresponding dew temperature of butane according to its concentration in gas at a pressure of 7 barg is (-34oC). This represents a limiting down stream exhaust temperature of the expander. Considering the margin at which butane concentration in the gas can vary, an additional safety margin

(12)

“∆Ts” of 14oC is considered. Hence the down stream temperature is preset in the optimal model at (-20oC).

(13)

Considering the above shown load curve as well as the aforementioned operating conditions the optimization program has been implemented and its outcome is shown in Table 2. Gas cost is 0.141 LE/Nm3 in Egypt. The cost of basic expander and generator unit is frame size dependent. An additional cost for the plant balance as well as the installation is considered at 17.5% of the cost of the basic unit. Considering the custom duties and taxes in Egypt an additional 20% of the commodity cost is considered. The finance was assumed to be at 25% equity and 75% commercial loan at a rate of 14% and five years loan life time and an exchange rage of 4.645 LE/US $. A sensitivity analysis has been conducted and the output represents the most probable scenario [7].

(14)

The price per kWh “PR” is determined by calculating the Annualized Life Cycle Cost “LCC” of the Turbo-expender system divided by the generated kWh as:

PR =

Barcelona, 12-15 May 2003

(15)

The LCC depends on the cost of the system, running costs (fixed and variable) and financing parameters. Both the cost of the system (size dependent) and running cost (fuel for preheating), depends on the bypass ratio “bs” and consequently “LCC”. To ensure best cost of service a TABLE - 2 shows the output of the optimization scheme Turbo- Capacity (PR) limiting value for “PR” is set. CASE STUDY

Max Gas Flow Rate (GR)

20034Nm3/hr

Average Load Factor (LFav)

83.3%

By-pass Ratio (bS) Annual Electricity Generated (E)

Cairo West power plant is a steam power plant, which consists of four units of capacity 87.5 MW each as well as two units of capacity 330 MW each. Figure 5 shows the load duration curve of the gas flow to one of the 87.5 MW units. The optimization scheme have been implemented on the load curve of this unit to identify the optimum bypass ratio which indicates the expander size and ensure highest load factor as well as minimum possible cost per kWh.

716kW

Limit of the Expander Exhaust Temperature (Td) Annual Gas Consumption in Preheating Annual Cost of Gas Consumed Maintenance Cost Net Annual Savings Total Application Cost

10.0% 4,660,155kWh -20 342,847Nm3 48,341L.E. 29,001L.E. -77,342L.E. 1,542,804L.E.

Environmental Impact Annual CO2 Abatement

1,456.6Ton

Annual CO Abatement

0.0302Ton

Annual NOx Abatement Cost per kWh (PR)

Figure 5 Load Duration Curve of Gas Flow to Cairo West Steam Power Plant

The supply gas station operates between 42 and 7 barg. The average Natural gas composition in the network includes butane at a concentration of 1.37% by volume. The

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0.9Ton 0.074 LE/kWh

As shown in the table the optimum Turbo-expander rating is 716 kW. This corresponds to a bypass ratio of 10%. The corresponding average load factor is 83.3. The optimization scheme enables sizing at the limiting size of the frame range through controlling the bypass ratio. This ensures the lowest cost per kWh. As shown in the table the optimum cost per kWh generated is 0.074 LE/kWh (1.6 US cent/kWh). The impact of the unit on greenhouse gas abatement is also presented in the table and is highly attractive. CONCLUSIONS

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17th International Conference on Electricity Distribution

Use of Turbo-expender as an alternative approach for pressure reduction in gas distribution networks its technically, financially and environmentally attractive. The current reachable potential for electricity generation form the Egyptian gas distribution network is 110 MW. This potential will increase at the gas consumption expands. An optimization scheme has been developed. It proved to be an effective tool at the planning as well as the sizing/operational stages. Implementing the developed operational scheme has resulted in achieving a competitive cost of 0.074 LE /kWh (1.6 US cent/kWh). Discussion is underway to implement a pilot project in a cement plant as a start for a large-scale implementation program over the Egyptian gas network. REFERENCES [1] Organization for Energy Planning, 2000/2001, "Energy in Egypt", http://www.ritsec.com.eg/govern/oecp

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[2] Ministry of Electricity and Energy,2000/2001, "Annual Statistical Report" [3] Energy Conservation and Efficiency Project, 1992, “Improving Combustion Efficiency” handbook. [4] Michael R. Hale, American Gas Association, Distribution and Transmission Conference, “Generating Electricity with A Letdown Gas Compressor”, May 4-5, 1987, Las Vegas, Nevada. [5] Rotoflow turbo-expanders for hydrocarbon Applications, GE power systems-Gas and Oil, 2002. [6] X.Y. Chao,X.M. Feng, D.J. Slump, “Impact of Deregulation on Power Delivery Planning”, Proc. IEEE Transmission and Distribution Conference, New Orleans, 1999 [7] E.O. Crousillat, P. Dorfner, P.Alvarado, H.M.Merrill, “Conflicting Objectives and Risk in Power System Planning”, IEEE Trans. Power Systems, vol.8. no.3, pp887-893, 1993.

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