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The Electricidade de Moçambique, E.P.
The Mozambique power utility, « Electricidade de Moçambique, E.P. » (EDM), manages the bulk of the electricity distribution in Mozambique, it is also responsible for power generation facilities and transport infrastructures along the country, Figure 2 shows the electrical infrastructure. EDM is a government-owned public company that operates under a performance contract with the Government of Mozambique.
EDM’s total installed generation capacity is nominally about 310 MW, of which the main part is diesel fuelled generation. Installed capacity of hydro power is the vicinity of 100 MW, of which 75 MW is currently available, distributed among the stations of Mavuzi (25), Chicamba (38) both in the central part of Mozambique and Corumana 12 MW in the south. Earlier there were small diesel/gas power stations scattered all over the country for provincial headquarters and other important towns. They are now replaced to a great extent by grid electricity through an extensive electrification program in addition to its own generation capacity.
EDM has three gas turbine units installed at Central Térmica Maputo (CTM), i.e. Maputo Power Station (71 MW) and one gas turbine installed in Beira main substation (14 MW). All units are fuelled by diesel and it is intended to carry out the CTM gas turbines conversion to burn natural gas.
It is planned to convey natural gas to Maputo city from an off take point (Ressano Garcia) in the Temane – South Africa gas pipeline.
Objectives
This assignment focuses on gas conversion of Maputo power plant, comprised of the following main equipment:
• Unit #1: Rolls Royce jet engine, with electrical output of 17000 kW.
• Unit #2: Sulzer BBC 11-B Gas Turbine, with electrical output of 29473 kW.
• Unit #3: GE-Alsthom Frame 5 Gas Turbine, with electrical output of 24504 kW.
This work focuses only on units #2 and #3, provided that the Rolls-Royce unit is out of order due to generator failure after the heavy floods of year 2000 that devastated the southern part of Mozambique.
The thesis general objectives are:
• Review gas turbines’ cycles;
• Assess gas turbines’ performance with different fuels.
The specific objectives are:
• Evaluate the conversion of Maputo Power Plant to burn natural gas;
• Propose modifications of the current power turbine cycles to combined cycle and steam-injected gas turbine;
• Select optimum cycle and propose its implementation.
Method of attack
The study is conducted as follows:
assessment of current plant condition;
plant performance analysis with diesel oil and conversion to natural gas;
modification of current gas turbine cycle to steam injected gas turbine and combined cycle gas turbine;
• sizing of HRSG;
• sizing of steam turbine(s);
• steam-injected gas turbine system sizing;
overall efficiency analysis for all proposed power generation cycles; and
economic analysis for all proposed power generation cycles.
Figure 2 – Map of Mozambique with electrical network (EDM, 2008)
Thesis Structure
The report is divided in eight chapters, beginning with Chapter one, of the introduction, where the thesis project description and objectives are presented.
The treatise of the power plant facility is addressed in Chapter two.
Chapter three analyses the performance of the installed units in current operation mode, i.e., whilst fuelled with diesel oil.
Chapter four proposes converting the installed units to be fuelled by natural gas and presents performance analyses of the units on this mode of operation.
Chapter five proposes converting the installed units to be fuelled by natural gas, changes of the gas turbine cycle into steam injected gas turbine and presents performance analysis of the units with these assumptions.
Chapter six proposes converting the installed units to be fuelled by natural gas, changes the gas turbine cycle into combined cycle gas turbine and presents performance analyses of the units for the new cycle.
Chapter seven does the overall evaluation of proposed modifications, outlines the operability issues, assesses environmental impact and presents economic and financial analysis for each cycle.
Chapter eight discusses the study results and presents project conclusion and recommendations.
Unit #1: Bristol Siddeley / GEC
The gas turbine of this unit was manufactured by Bristol Siddeley Engines Ltd (BSEL), a former British aero engine manufacturer, purchased by Rolls -Royce Limited in 1966, and the generator was built by The General Electric Company (GEC), a former major British-based industrial conglomerate, dedicated to defence electronics, communications and engineering.
The unit was installed in 1967 with a nominal capacity of 17.5 MW burning Jet Fuel. Due to engine failure, a new gas generator of this unit was installed in 1990 and operated only 60 running hours. The summary of the main data is presented in Table 1 and the schematic diagram of the gas turbine cycle is Figure 4.
Current status of the unit
The Bristol Siddeley / GEC unit was originally installed in 1967 with Olympus gas generator serial number 200618 and in 1990, due to low pressure compressor failure; it was replaced with Olympus gas generator serial number 200616. The installed gas generator recorded sixty two total operating hours since its installation.
The last time that the unit was in operation was in 1995. This is approximately seventeen years of a non-operation, what can be considered corresponding to 36,000 equivalent operating hours, when no corrosion inhibition is provided. Although the Olympus gas generator has only 62 operational running hours, the fact that the engine has been inactive for long period is a significant factor, too.
When Vattenfall Power Consultants AB inspected the unit in June 2009, the following findings have been reported (the author also took part in the team as project owner’s engineer):
Gas generator: minor corrosion was found on the inlet bell mouth, inlet guide vane and the first row of blades;
Air intake system: the weather louvers are corroded; the static filter was dirty and damaged; the air channel is of a wrong design according to Alba inspection;
Generator and exciter: damaged, flooded with water;
Rotor: removed from stator and corroded;
The following components are considered old fashioned (out of date) and should benefit from upgrading to today’s standards:
• Gas detection system;
• Fire extinguishing system;
• High voltage cubicle;
• Generator circuit breaker;
• Voltage regulator and protections;
• Instrumentation: measuring equipment, indicators, recorders, printers;
• Governing and control system; and
• Low voltage equipment.
Unit #2: BBC Type 11-B
This unit was manufactured by Brown Boveri & Cie (BBC) a former Swiss group of electrical engineering companies. It was installed in 1974 with a nominal capacity of 36 MW burning diesel oil. The summary of the main data are presented in Table 2 and Figure 5 illustrates the schematic diagram of the gas turbine cycle.
Current status of the unit
This unit has lain dormant for 12 years without no-corrosion inhibitors. When Vattenfall Power Consultants AB inspected the unit in June 2009, the following findings were reported:
Compressor: the air intake duct had been flooded with water. Rust was observed on the inlet duct, bars, baffles, diffuser and guide vanes. The water level has been so high that the compressor has been submerged in water up to almost half the height of the first impellor row. The compressor must have been submerged to the same water level for a long period since the sign of the water level is marked as a deep as the rust line. Foreign objects were observed at the inlet of the compressor;
Compressor bleed valves: water flushing through the valve from the compressor during start up(s);
Compressor rotor blading: rust was observed on the first stage of blading below the water level mark. Rust was observed on the rotor;
Exhaust and silencer: exhaust damper only functioning on half the stack area. Rust on the man hole hatch was observed;
Measuring equipment: thermocouples exhaust dust corroded and difficult to remove;
The following components are considered old fashioned (out of date) and should benefit from upgrading to today’s standards:
• Voltage regulator and protections;
• Instrumentation: indicators, recorders;
• Governing and control system; and
• Low voltage equipment.
The unit #3 was manufactured by Alsthom in France, under the licence of General Electric Company (GE), an American multinational conglomerate corporation operating through four branches: Energy, Technology Infrastructure, Capital Finance and Consumer & Industrial.
It was installed in 1990 with a nominal capacity of 25 MW burning diesel oil. The summary of main data are presented in Table 3 and the schematic diagram of the gas turbine cycle is illustrated in Figure 6. Figure 8 shows the partial view of the installation.
Current status of the unit
This is the unique unit available in the plant and is regarded as backup unit. When Vattenfall Power Consultants AB inspected the unit in June 2009, the following findings were reported:
Compressor: small amount of oil detected at the compressor inlet;
Air intake system/air channel: corroded enclosure roof structure;
Combustion chamber: extension corrosion observed through external inspection(s);
Building structure: frame basement flooded with rain water from corroded enclosure structure;
Generator circuit breaker: corroded enclosure.
Plant Performance Analysis with Diesel Oil
The gas turbine cycle applicable for both units is depicted in Figure 6, chapter 2. A Microsoft Excel spread sheet was developed to calculate the plant performance.
Unit # 2, BBC Type 11-B
The following parameters were used to undertake the plant performance analysis with diesel oil:
• The fuel is diesel oil, with LHV of 42,330 kJ/kg and HHV of 45,467 kJ/kg at 25 °C. The calculations will approximate these figures.
• Compressor outlet temperature, T2, is 308 °C, average from operating reading records,
• Compressor mass flow is 205.2 kg/s, as given by the manufacturer
• Compressor pressure ratio, π c , is 7.5, as given by the manufacturer
• Turbine inlet temperature, T3, is 840 °C, average from operating reading records
• Turbine exhaust temperature, T4, is 445 °C, average from operating reading records
• Compressor inlet temperature, T1, is 24 °C, operational readings;
• Compressor inlet pressure, p1, is 1.013 bar, operational readings from site ambient barometric pressure;
• Combustion chamber pressure loss of 3 %, assumed;
• The overall mechanical efficiency, ηm, is 95%. Assumed figure, it includes gearbox and other mechanical losses;
• The overall electrical efficiency, ηel, is 95%. Assumed figure, it includes loss in generator, power consumption for cooling, lubrication and control systems.
Unit # 3, GE Frame 5
The performance analysis for unit # 3, GE Frame 5, uses the same philosophy and equations used for unit # 2, BBC Type 11-B. The following parameters were used to undertake the plant performance analysis with diesel oil:
• The fuel is diesel oil, with LHV of 42,330 kJ/kg and HHV of 45,467 at 25 °C. The calculations will approximate these figures;
• Compressor outlet temperature, T2, is 325 °C, average from operating reading records;
• Compressor pressure ration, πc, is 10.2, as given by manufacturer;
• Combustion chamber fuel mass flow is 2.12 kg/s, as given by manufacturer;
• Turbine inlet temperature, T3, is 946 °C, average from operating reading records;
• Turbine exhaust temperature, T4, is 487°C, average from operating reading records;
• Gas turbine cycle flue gas mass flow is 122.22 kg/s, as given by manufacturer.
• Compressor inlet temperature, T1, is 25.5 °C, unit operational readings inlet temperature;
• Compressor inlet pressure, p1, 1.013 bar, unit operational readings inlet pressure, site ambient site barometric pressure
• Combustion chamber pressure loss of 3 %, assumed;
• The overall mechanical efficiency, ηm, is 95 %. Assumed. It includes gearbox losses and other mechanical losses;
• The overall electrical efficiency, ηel, is 95 %. Assumed, it includes loss in generator, power consumption for cooling, lubrication and control systems.
Expected Plant Performance Analysis with Natural Gas
As the gas conversion project targets unit #2 and unit #3, the performance analysis with natural gas will focus on these two units, only.
Converting the installed units to run with natural gas requires installing new combustion chambers and new gas fuel nozzles. Other major equipment such as compressor, gas turbine and generator would remain the same. Gas piping hardware interface with the existing units is dealt in details in this report.
The gas turbine cycle used for calculations and applicable for both units is depicted in Figure 6 (chapter 2) and data from plant performance analysis with diesel oil are used (chapter 3). A Microsoft Excel spread sheet (Appendix II) was created to calculate the plant performance.
The calculations are approximated with the tables and diagrams that are used for gases from light oil combustion.
The following general parameters were used to undertake the plant performance analysis with natural gas:
• The fuel is natural gas, with LHV of 47,587 kJ/kg and HHV of 50,496 kJ/kg at 25 °C. The calculations will, again, approximate these figures;
• Combustion chamber pressure loss of 2%, assumed;
• The overall mechanical and electrical efficiencies are same as for diesel oil calculations; and
• The isentropic exponent for flue gas as well as the isentropic efficiency for turbines are the same as for diesel oil calculations; and
• Turbine output power for natural gas is the same as for diesel oil results.
Table of contents :
Abstract
1 Introduction
1.1 Energy overview in Mozambique
1.2 The Electricidade de Moçambique, E.P.
1.3 Objectives
1.4 Method of attack
1.5 Thesis Structure
2 Plant description
2.1 Unit #1: Bristol Siddeley / GEC
2.2 Current status of the unit
2.3 Unit #2: BBC Type 11-B
2.4 Current status of the unit
2.5 Unit #3: GE Frame 5
2.6 Current status of the unit
3 Plant Performance Analysis with Diesel Oil
3.1 Unit # 2, BBC Type 11-B
3.2 Unit # 3, GE Frame 5
4 Expected Plant Performance Analysis with Natural Gas
4.1 Unit # 2, Sulzer BBC 11-B
4.2 Unit #3, GE Frame 5
5 Expected Plant Performance Analysis with Steam Injection Gas Turbine (STIG)
5.1 Discussions on Steam Injection Gas Turbine Cycle
6 Expected Plant Performance Analysis with Combined Cycle Gas Turbine
6.1 The Combined Cycle Gas Turbine
6.2 Unit # 2, BBC Type 11-B
6.3 Unit #3, GE Frame 5
7 Plant Operability Analysis
7.1 Plant Re-commissioning with current fuel
7.1.1 Re-commissioning of unit #1 with Jet fuel
7.1.2 Re-commissioning of unit #2 with diesel oil
7.1.3 Re-commissioning of unit #3 with diesel oil
7.2 Plant Conversion to Natural Gas
7.2.1 Conversion to natural gas for unit #2
7.2.2 Conversion to natural gas for unit #3
7.3 Plant Re-commissioning with Combined Gas and Steam Cycles
7.3.1 Combined Cycle for unit #2 and unit #3
7.4 Technical and Emissions Analysis
7.5 Financial Assessment
8 Discussions and Recommendations
8.1 Discussions
8.2 Conclusions and Recommendations
Bibliography
