IMPROVEMENT OF THE PERFORMANCE OF THERMAL POWER SYSTEMS THROUGH ENERGY AND EXERGY ANALYSIS

ABSTRACT

This research work is aimed at using the energy and exergy analysis with thermodynamic data to suggest improvements in the performance of steam and gas turbine power plants. In this regard, specific data from Egbin steam power plant and Geregu I gas turbine power plant were used for the analysis.   In the analysis, scientific tools such as Engineering Equation Solver  (EES)  programme with  built-in  functions  for  most  thermodynamic  and  transport properties was used to calculate the enthalpy and entropy at various nodal points, while EXCEL spreadsheet and SCILAB software code were used to analyze both the energetic and exergetic efficiencies of the individual components, thermal efficiencies, gross station heat rate etc. These software were also used to calculate   the exegetic performance coefficient and exegetic sustainability indicators of the power plants. The results of the analysis at both design and operating conditions show that exergy destruction occur more in the boiler/steam generator of Egbin steam power plant and combustion chamber of Geregu I   gas turbine power plant than in other major components of each plant. The normal operating conditions of the steam boiler exit pressure and temperature are 125.70/540.72 and condenser pressure

and temperature are 0.0872bar and 42.950Crespectively for Egbin steam power plant in the

year 2009. From the study, the maximum exergy loss was found in the boiler/steam generator with a value of 55.32% in the year. Changing the boiler exit pressure and temperature from the normal operating conditions to 165.70/560.72 (ie, in step of 10 bar and 50C), the exergy loss reduced to 53.99%.The cycle thermal energy and exergy efficiencies at  the normal operating conditions were 41.03% and 39.94 % respectively.   Improvement in the cycle thermal energy and exergy efficiencies with the same steps from normal operating conditions to 165.70/560.72 were 41.23% and 40.12% respectively.   The improvement increased the

power output from 197593.8KW to 199358.57kW showing power increase of 1764.77kW or

1.765MW. The gross station heat rate decreased from 8775kJ/kWh to 8732kJ/kWh which is good for the life of the plant. Also, the improvement increased the exergetic performance coefficient   from   0.6133   to   0.6188.      The   exergy   sustainability   indicators  such   as environmental effect factor decreased from the value 1.0412 to 1.0230 showing about 1.75% reduction in hazardous gaseous emissions to the environment. Another exergy indicator, the sustainability  index  factor  increased  from the  value  0.9604 to  0.9775 indicating  1.78% resource utilization and sustainability. For Geregu I gas turbine plant, the operating condition

of the combustion chamber are pressure of 11 bar and 10600C Turbine Inlet Temperature (TIT). The study showed that maximum exergy loss was found in the combustion chamber at a value of 26.30%. It is higher than any other major component at the operating conditions in the year 2009.Adjusting the normal operating pressure and temperature to 15 bar and 10800C reduced the exergy destruction ratio to 26.06%. The gas turbine    cycle energy and exergy efficiencies    increased from 32.77% to 34.85% and30.44% to 32.34% respectively.   The power output in the year was increased from 133456.01kW to 145013.13kW showing power increase of 11557.12kW or 11.56 MW due to the improvement. For exergetic sustainability indicators, the environmental effect factor decreased from the value 0.7132 to 0.6522 indicating 8.55% reduction in greenhouse gas emission to the environment and ecology. The sustainability index factor was increased from the value of 1.4022 to1.5332 showing 9.34% energy resource utilization for societal development.  In conclusion, any increase in exergy efficiency has effect on the environmental effect factor and sustainability index factor for both power plants. Therefore, increase in exergy efficiency improves the exergetic sustainability index. However, any increase  in  environmental effect  factor decreases the sustainability index factor. These parameters are expected to quantify how thermal power plants become more environmentally benign and sustainable.

CHAPTER ONE

1.0 Background

INTRODUCTION

Thermal power plants are widely utilized throughout the world for electricity generation. They include steam power plants, gas turbine power plants, nuclear power plants, internal combustion engines. There are numerous aged and new thermal power plants that are in service throughout the world today, for example, about 1,300 steam power plants have been in service for more than 30 years in the USA, [1].  In recent years, global warming has been a major issue due to continuous growth of greenhouse gas emissions from different sources. The contributors to greenhouse effects are carbon dioxide (CO2), nitrogen dioxide (NO2) and sulphur dioxide (SO2). Carbon dioxide is a major greenhouse gas which is mainly blamed for global warming.

Different industrial processes such as power plants, oil refineries, fertilizer plants, cement and steel plants are the main contributors of CO2  emission. Fossil fuels such as coal, oil and natural gas are the main energy sources for power generation and will continue to generate power due to large reserves and affordability. Demirbas, [2] reported that about 98% of CO2 emission results from fossil fuel combustion. Many power companies have investigated and undertaken measures to improve the efficiencies of such power plants in order to minimize their environmental impacts(e.g. by reducing emissions of CO2, NO2 and SO2), and to make them more competitive, as deregulation of the power industry proceeds. Such investigations have been based on energy consideration. It has also sparked interest in the scientific community to take a closer  look at  the energy conversion devices  and to  develop  new techniques to better utilize the existing transfer and energy change.

The most commonly used method for analysis of an energy conversion system is the first law of thermodynamics.  Engineers and scientist have been traditionally applying the first law of thermodynamics to calculate the enthalpy balances for more than a century to quantify the loss  of  efficiency  in  a  process  due  to  loss  of  energy.  However,  the  first  law  of thermodynamics deals with the quantity of energy and asserts that energy cannot be created or destroyed, [3]. This law serves as a necessary tool for accounting for energy during a process and offers no challenges to the engineer. However, in recent years the second law analysis,

also known as exergy analysis of energy systems has more and more drawn the interest of energy engineers and scientific community. Exergy analysis provides an effective technique for measuring and optimizing performance of a thermal system by accounting for energy quality. It can also be used to assess the sustainability level of energy systems. Sustainability means a supply of energy resources that is sustainably available at reasonable cost and causes no  minimal  negative  effects.  Sustainability  is  necessary to  overcome  current  ecological, economic, and developmental problems. The exergy sustainability indicators include exergy efficiency, waste exergy ratio, recoverable exergy rate, exergy destruction factor, environmental effect factor and exergetic sustainability index, [4].

For power plants, exergy analysis allows one to determine the maximum potential for electricity  production associated  with  the  incoming  fuel  or  any  flow  in  the  plant.  This maximum is achieved if the fuel or flow is utilized in processes that ultimately bring it to complete thermodynamic equilibrium with the environment, while generating electricity reversibly. Thus, exergy analysis provides the theoretical efficiency limitations upon any power plant. Losses in the potential for electricity generation occur due to irreversibilities and determined directly with exergy analysis. The exergy concept has gained considerable interest in the thermodynamic analysis of thermal processes and plant systems since it has been seen that the first law analysis has been insufficient from an energy performance point of view. Based on the second law of thermodynamics, the exergy analysis represents the third step in the plant system analysis, following the mass and the energy balances. The aim of the exergy analysis is to identify the magnitudes and the locations of exergy losses, in order to improve the existing systems, processes or components, or to develop new processes or systems, [5]. The method of exergy analysis is particularly suited for furthering the goal of more efficient resource utilization, since it enables the location, and time magnitudes of wastes and losses to be determined.  Improved resource utilization can be realized by reducing exergy destruction within a system. The objective in exergy analysis is to identify sites where exergy destructions and losses occur and rank them for significance.   Exergy losses include the exergy flowing to the surroundings, whereas exergy destruction indicates the loss of exergy within the system boundary due to irreversibility. This allows attention to be centered on the aspects of system operation thatoffer the greatest opportunities for improvement, [6]. Exergy analysis which is the combined first and second law analysis gives much more meaningful evaluation indicating the association of irreversibilities or exergy destruction with combustion and heat transfer

processes. This allows thermodynamic evaluation of energy conservation option    in power and refrigeration cycles, thereby provides an indicator that points the direction in which engineers should concentrate their efforts to improve the performance of thermal systems. The second law of thermodynamics has proved to be a very powerful tool in the optimization of complex thermodynamic systems, [7],[8],[9].

1.1 EnergySources in Nigeria

The country is endowed with both the conventional and the non-conventional energy resources. The conventional comprises mostly of the non-renewable resources such as crude petroleum oil, natural gas, coal, tar sand and uranium, [10]. The country has the tenth largest oil and gas reserves in the world. The various non-conventional energy resources available in the country that  can be  harnessed  for power generation are  nuclear, solar, wind power, biomass energy, wave and tidal energy and geothermal energy. Nigeria’s near equatorial location, extensive and diverge vegetation, prevailing trade winds and many rivers endow her with large quantities and quality of renewable energy sources,[11].These include solar radiation, hydro  power, wind and  biomass energy. Nigeria’s coal reserves are  large and estimated at 2.7 billion metric tonnes of which 650 million tonnes are proven reserves.  About

95% of the Nigerian coal production in late 1950s and early 1960s was consumed locally, mainly for railway transportation, electricity production and  industrial heating  in cement production. Nigeria has abundant reserves of natural gas.   The quantity of natural gas is at least twice as much as the oil, and the horizon for the availability of natural gas is definitely longer than that of oil.  In energy terms,  the quantity of natural gas used  for electricity generation is very significant. The known reserves of natural gas have been estimated at about

187.44 trillion standard  cubic  feet  or  5.30  x 1012   standard  cubic  meters  as  at  the  year

2007,[12].

The third major source of energy, oil, is Nigeria’s major source of revenue used for development. As at January 2005, Nigeria’s proven crude reserve stood at 35.2 billion barrels. The majority of the reserves are found along the country’s coastal Niger Delta. As at 2007, Nigeria’s energy resource availability expressed in barrels (bbls) and standard cubic feet (scf) and other units showed that crude oil availability in Nigeria stood at 36.5 billion barrels. Other energy resources include natural gas whose availability is 187.44 trillion standard cubic feet, coal and lignite estimated at 2.7 billion tonnes as shown in Table 1.1

Table 1.1:      Energy Resource Availability in Nigeria

RESOURCES	AVAILABILITY
Crude oil	36.5 billion bbl
Natural gas	187.44 trillion scf
Coal and lignite	2.7 billion tones
Tar sands	31 billion bbl oil equivalent
Hydropower (large scale)	11,250mw
Hydropower (small scale)	3,500mw (estimate)
Solar radiation	3.5 – 7.0kwh/m2-day
Wind	2 – 4m/s annual average
Fuel wood	13.1 million ha of forest/wood land
Animal waste	Very significant   Quantity not available
Crop residue
Tidal energy
Uranium

Source: Energy Commission of Nigeria, 2007

Solar radiation intensity varies  in a quasi-predictable way. It varies with day and night, location, weather and climate. It increases with altitude and solar altitude angle. For instance, at an altitude of 3,000m and solar altitude angle of 900  (i.e. overhead) it gets as high as

1.18KW/m2, while at sea levels it is < 1.0 KW/m2. It is reduced by cloudiness, atmospheric

gases, atmospheric particles (aerosols) and obstructions.

1.2 Electricity Generation in Nigeria

Generation of electricity is a very complex process involving many sub-processes and has multiple  critical  parameters.  A  decline  in  thermal  efficiency  leads  to  a  higher  cost  of electricity generation due to more fuel usage and also will result in much higher carbon deposits.  Therefore,  it  is  very  important  to  stress  on  the  performance  of power  plants. Electricity generation is the conversion of other kinds of energy, mainly primary energy into electrical  energy.  Generally,  the  process  of  generating  electricity  goes  through  several

transformations from primary energy directly into electricity. For instance, in a thermal power station, the primary energy is converted to a high temperature steam, as an intermediate heat source, then into mechanical energy in the turbine physically connected with the generators where the electrical energy is produced.

Power generation in Nigeria is mainly from three technologies only which include hydro- electric power stations, steam and gas thermal stations. Most of these facilities are being managed by the Power Holding Company of Nigeria (PHCN); a government owned utility company that coordinates all activities of the power sector such as generation, transmission, distribution  and  marketing  before  they  were  privatized.  Since  inception  of  PHCN,  the authority expands annually in order to meet the ever increasing demand. Unfortunately, the majority of Nigerians have no access to electricity and the supply to those provided is not regular. In a bid to make the power sector more functional, the PHCN was unbundled into 18 successor companies (1 Transmission, 11 Distribution and 6 Generation companies). This was done due to current privatization in the sector [13].

Prior to 1960s, energy supply and consumption consisted predominantly of non-commercial energy, viz-fuel wood, charcoal, solar radiation, agricultural waste and residues. Major commercial fuel was coal used in railway engines and for power generation. Contributions to commercial energy came frompetroleum products (petrol and diesel) for road vehicles and from electricity (from coal and diesel generators). Up till 2005, the grid electricity supply industry was predominantly the vertically integrated public utility-National Electric Power Authority (NEPA), which owned about 98% generating capacity and 100% of transmission and distribution capacity. In consequence and in particular through former President OlusegunObasanjo’s power project and President Goodluck  Jonathan’s power road map for power sector reform of August 2010, actual maximum peak generation has now more than doubled (4300MW) since the start of the reform in 2000 and installed generation is now above 10109.5MW, [13].

At present, the installed capacities in power stations in Nigeria are shown in Tables 1.2, 1.3 and 1.4 for pre-1999 power stations and other power stations as contained in a document prepared by Energy Commission of Nigeriain 2007.

Table 1 .2: Pre- 1999 Power Stations

Station	Capacity (MW)
Kainji Hydro	760
JebbaHydro	578
Shiroro Hydro	600
Egbin Thermal	1320
Sapale Thermal	1020
Ijora Thermal	60
Delta Thermal	912
Afam Thermal	711
Orji River Thermal	30
NESCO	30
Total	6,021MW

Source: Energy Commission of Nigeia,2007

Other  power  generating  stations  include  eight  National Integrated  Power Project  (NIPP) which were built in some states of the country. These are  Gbarain Integrated Power Project in Bayelsa State, Egbema Integrated Power Project located in Imo State, Ibom Integrated Power Project in AkwaIbom State.

Table1.3:  National Integrated Power Project (NIPP)

Station	Capacity(MW)
Gbarain, Bayelsa	225
Ihubor, Edo	451
Omoku, Rivers	230
Sapela,Delta	451
Egbema, Imo	338
Calabar, Cross Rivers	561
IkotAbasi, AkwaIbom	300
Ibom Power, AkwaIbom	188
Total	2,744MW

Source: Energy Commission of Nigeria, 2007

Another  milestone  in the  power  sector  for  electricity generation  is  the  establishment  of Independent Power Producer (IPP) in different parts of the country. These include the AES power station in Lagos State, Alaoji power station in Abia State, Papalanto power station in Ogun State and others.

Table 1.4:  Independent Power Producers (IPP)

Station	Capacity(MW)
Geregu, Kogi	414
Omotosho, Ondo	335
Papalanto,Ogun	335
Alaoji, Abia	346
AES, Lagos	270
Geometric, Aba	140
Agip JV, Okpai/Kwale, Delta	480
Chevron JV, Agura,Igbin, Lagos	750
Total Fina, Obite, Rivers	500
Exxon Mobil Bonny, Rivers	500
Total	4070MW

Source: Energy Commission of Nigeria,2007

These National Integrated Power Projects (NIPPs) and Independent Power Producers (IPPs) will augment the power generated by these power generating stations to meet the electricity demand of the country.

1.3 Statement ofProblem

The global power sector is facing a number of issues, but the most fundamental challenge is meeting the rapidly growing demand for energy services in a sustainable way. This challenge is   further   compounded   by   the   today’s   volatile   market-rising   fuel   costs,   increased environmental regulations etc. Thermal power plants are one of the most important elements of energy sector and they are masterworks that enable production of electrical energy which can be thought as one of the basic needs after food and water. Preference of the thermal power

plant type in electricity production is a big dilemma and prior discussion subject to related parties in recent years. For instance, environmentalist act against fossil-fuelled thermal power plants or nuclear power plants and they try to warn decision makers about environmental pollution, global warming, carbon emission etc. The primary energy source possibilities of countries are one of the basic factors that determine the preferences of a thermal power plant. Namely, USA, Germany, India and China produce more than 50% of their electrical energy by coal-fired thermal power plant, while most of the thermal power plants, in countries that have abundance of natural gas such as Qatar, are gas fired. The choice is directly related to the reserve capabilities of the primary energy sources which are one of the main issues for government policies and preferences.

Today, many generating utilities are striving to improve the efficiency of their existing power generating stations. The problem of low power generation output from these plants is as a result of defective plant components and improper fuel utilization in the systems.

1.4 Aims and Objectives of the Study

The aim of this research is to

(i)   Carry out energetic and exergetic performance analysis, at the design and actual operating conditions for the existing unit 5 (220MW) of the 1320MW Egbin steam power plant and unit 11(138MW) of the 414MW Geregu I gas turbine power plant in order to identify the components that needs improvement.

The objectives of the study are to determine:

(i)       the quantity of energy and exergy flows and location of losses. (ii)      the energy efficiency of the plant and its components.

(iii)     plant performance parameters such as heat rate, specific fuel consumption   and thermal discharge index.

(iv)      theexergy efficiency of the plant and its components. (v)      theexergy destructionswithin the system components. (vi)     exergetic performance coefficient.

(vii)    exergetic sustainability indicators- exergy destruction ratio,   waste exergy ratio, environmental effect factor and exergetic sustainability index and

(viii)   systems that have potential for significant performance improvement.

To achieve these objectives, we summarize thermodynamic models for the considered   power plants on mass, energy and exergy balance equations.

1.5 Scope of the Study

The scope of this work involves

   analysis of thermal power systems.

   determining the irreversibility rates in the plant components.

   comparative  performance  of  the  power   plants  at   both  design  and  operating conditions,and

   performing sensitivity analysis on the variation of thermodynamic intensive properties like temperature and pressure in improving the plants performance.

1.6 Significance of the Study

The growth, prosperity and national security of any country are critically dependent upon the adequacy of its electricity supply industry.  Over the past two decades, the stalled expansion of Nigeria’s grid capacity, combined with the high cost of diesel and petrol has crippled the growth of the country’s productive and commercial industries. It has stifled the creation of jobs which are urgently needed in a country with a large and rapidly growing population; and the erratic and unpredictable nature of electricity supply has engendered a deep and bitter sense of frustration that is felt across the country as a whole and in its urban centers in particular. Electricity consumers and the citizenry as a whole demand a fundamental reversal of the long and debilitating malaise which has blighted the industry and, in doing so, bridled the tremendous energy and creativity of this great and populous nation. More particularly they demand real and immediate improvements in service levels, [14].

Nigeria needs over 10,000 MW of electricity for her domestic and industrial demands of which about 4000MW is currently being generated from power plant locations across the country. The quantity generated are transmitted and distributed through the national grid to primary energy consumers. As a result of inefficient operation of some of these plants owing to long age in service, the need to identify the location of the inefficiency in the plant becomes imperative. Generally, the performance of thermal power plants is evaluated through energetic performance criteria based on first law of thermodynamics, including electrical power and thermal efficiency. In recent decades, the exergetic performance based on the second law of

thermodynamics has been found to be a useful method in the design, evaluation, optimization and improvement of thermal power plants. The exergetic performance analysis can not only determine magnitudes, location and causes of irreversibilities in the plants, but also provides more meaningful assessment of plant individual components efficiency, [15]. The use of exergy analysis becomes the answer as a tool for pin-pointing inefficiencies. The features of this technique make it valuable in the thermodynamic analysis aiming at the improvement of the efficiency of existing thermal plants through an adjustment of their operating parameters or in the design of efficient new thermal plants.

1.7 Study Area

Nigeria is the most populous African country with the total population estimate of over 152 million people. She has over ten power generating stations (both thermal and hydro power stations) established before the year 1999.

Besides  having  these  power  stations,  there  are  eight  National Integrated Power  Projects (NIPPs) established after the year 1999 and many Independent Power Producers (IPPs).  For the purpose of this study, the Egbinpower station and the Geregu power station will be considered because the former uses steam and water as working fluid and the later uses air and combustion products as working fluid where the boiler/steam generator and combustion chamber are fired by natural gas. These power plants contribute good percentages of over 15.9 million MW of electricity consumed for both domestic and industrial use by the populace annually.

Egbin thermal plant is located atIjede area of Ikorodu, a suburb of Lagos State. The plant was commissioned in 1985 and consists of 6 units of 220 (6X220) MW (Reheat – Regenerative). They are dual fired (gas and heavy oil) system with modern control equipment, single reheat; six stages of regenerative feed heating.  Natural gas is supplied to the plant directly from the Nigerian Gas Company (NGC) Lagos operations department, which is annexed to the thermal plant. Since Egbin thermal plant is located on the shores of the lagoon, cooling water for the plant’s condensers is pumped from the lagoon into the water treatment plant en route to the condensers.

The Geregu I gas thermal power station located in Ajaokuta, Kogi State of Nigeria was commissionedin 2006 and it consists of three independent units, each being rated at 138MW. The total installed capacity is 414MW. Natural gas is the fuel used for the plant.

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