Improvement of the Performance of Thermal Power Systems Through Energy and Exergy Analysis

Improvement of the Performance of Thermal Power Systems Through Energy and Exergy Analysis

Chapter One

Aims and Objectives of the Study

The aim of this research is to

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:

the quantity of energy and exergy flows and location of losses.
the energy efficiency of the plant and its components.
plant performance parameters such as heat rate, specific fuel consumption and thermal discharge index.
the exergy efficiency of the plant and its components.
the exergy destructions within the system components.
exergetic performance coefficient.
exergetic sustainability indicators- exergy destruction ratio, waste exergy ratio, environmental effect factor and exergetic sustainability index and
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.

CHAPTER TWO

LITERATURE REVIEW

Energy Demand and Supply

The world’s energy demand is projected to grow significantly over the next 20 years. This increase will result to economic growth, industrial expansion, high population growth, increases in energy demand and urbanization, especially in the developing countries. By extending past trends of energy consumption into the future, the International Energy Agency(IEA), projects that the global primary energy consumption is going to increase from about 110,000TWh ( or 9400 Mtoe) in 1996 to 160,000TWh(or 13700Mtoe) in 2020. This implies substantial growth in the energy demand and CO₂ emissions. The main sources of energy today are fossil fuels like oil, coal and natural gas. These fuels are thought to continue to supply the major part of the world’s energy demand in the foreseeable future with an increase in the use of natural gas, IEA [16]. In the utilization of energy, IEA projects that the demand for electricity and transport will continue with upward trend. The growth in industrialization and population in Nigeria has brought about increase in demand for electrical energy.The current power generation in Nigeria is 4,167MW, [17].According to World’s Electricity production statistics in 2010 and 2012, Nigeria, Kenya, Ghana and Guatemala electricity production in 2010 stood at 24,872GWh; 7,330GWh; 8,213GWh and 8,624GWh respectively[18]. Other nations like United States, China, South Africa, Japan, United Kingdom and Russian in 2012 had electricity production stood at 4,250,100GWh ; 4,937,800GWh; 251,724GWh;1,101,500GWh; 363,187GWh and1,066,400GWh respectively [19]. Even though the importance of power to socio-economic development cannot be overemphasized, Nigeria has not being able to generate the maximum amount of power required for her population of over 150 million people. Unfortunately, majority of Nigerians have no access to electricity and the supply to those provided is not regular, [20].The main source of energy, fossil fuels is non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed. They are excellent sources of energy for transportation needs; however, they are also the primary source of electrical energy in the world today. Fuel is an organic substance used for its energy content. The energy content of a fossil fuel, before any treatment or conversion, corresponds to primary energy. Fossil fuel or mineral fuels are fuels from hydrocarbon found within the top layer of the Earth’s crust. These range from volatile materials with low carbon to hydrogen ratio like methane, to liquid petroleum, to non-volatile material composed of almost pure carbon, like coal.The hydrocarbon, methane can be found in hydrocarbon fields, alone, associated with oil, or in the form of methane dathrates. It is generally accepted that they are formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the earth’s crust over hundreds of millions of years.

The main point of this is that all of these fossil fuels are made of hydrocarbons. It is important to note that what makes hydrocarbons valuable to our society is the stored energy within them. This energy is contained in the numerous bonds of the hydrocarbons. The original source of this energy is all the solar energy the prehistoric organisms trapped in their bodies long ago. The energy contained in the atomic bonds is broken down in the power of combustion to release energy in the form of light and heat.

In a fossil fuel power plant, the chemical energy stored in fossil fuel such as coal, fuel oil, natural gas or oil shale is converted successively into thermal energy, mechanical energy and finally, electrical energy for continuous use and distribution across a wide geographic area. Most thermal power stations use fossil fuel, outnumbering nuclear, geothermal, biomass, or solar thermal plants. The complete combustion of fossil fuel using air as the oxygen source is summarized in the following chemical reaction.

O₂+3.76 N₂ → aCO₂+ H₂O + 3.76 N₂ (2.1)

Where a andb in equation (2.1) represent number of carbon and hydrogen atoms in the fuel. In simple word, equation (2.1) for this chemical reaction is fuel + air →heat + carbon dioxide + water + nitrogen.

In all energy supply systems, primary energy from fossil fuels, nuclear fuels and renewable energy is converted to electricity to end users. This energy conversion process from primary to consumer energy involves unavoidable transformation and distribution losses that decrease the overall efficiency of the energy system.

Overview of Thermal Power Plants

The global utilization of different types of thermal power plants for electricity generation cannot be over emphasized. In this research work, attention would be centered on steam and gas turbine power plants. The Rankine cycle is the ideal cycle for steam or vapourpower plants. A steam power plant continuously converts the energy stored in fossil fuels(coal, oil, natural gas) or fissile fuels(uranium) into shaft work and ultimately into electricity. All the energy from steam cannot be utilized for running the system because of losses due to friction, viscosity; bend-on-blade. The working fluid is water which is sometimes in the liquid phase and sometimes in the vapour phase (steam) during its cycle of operations as functions of temperature, pressure, enthalpy and entropy. The basic components of a steampower plant include the steam generator (boiler), turbine, condenser, pump, and their accessories. Energy released by the burning of fuel is transferred to water in the boiler to generate steam at a high pressure and temperature, which then expands inthe turbine to a low pressure to produce shaft work. The steamleaving the turbine is condensed into water in the condenser where cooling water from a river, a lake or sea circulates carrying away the heat released during condensation. The water (condensate) that passes through series of heaters is then fed back to the boiler by the pump and the cycle goes on repeating itself. The number of stages of feedwater heating is determined by an economic balance of increased first cost against the return on the investment by fuel savings and by operating reliability[21].

The current drive for low emissions has resulted in the preference forthermal power systems based on gas turbine cycles. The Brayton cycle is the ideal cycle for gas turbine engines. A gas turbine is a device designed to convert the heat energy of fuel into useful work using the combination of internal fuel combustion and a gas turbine power unit. A simple gas turbine consists of a compressor, a combustion chamber and a turbine. In this device, fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The compressed air is then passed into the combustion chamber where it is burned at constant pressure. The hot combustion product gas is then made to flow over the moving blades of the gas turbine, which imparts rotational motion in the runner. During the process, the hot gas gets expanded and finally, it is exhausted into the atmosphere. Gas turbines tend to be lighter and more compact than the vapour power plants. In oil and gas industries, the gas turbine is widely employed to drive auxiliaries like compressors, blowers and pumps.

CHAPTER THREE

METHODOLOGY

Conceptual Framework

This study is based on the concept that for a system that undergoes a process under steady or quasi-steady-state conditions, the exergetic efficiency (second law efficiency, effectiveness or rational efficiency) is a valid measure of performance of the system from a thermodynamic point of view. Thus, an exergy analysis of a thermal power plant used in conjunction with an energy analysis enables the location, types and true magnitudes of wastes and losses to be determined. More revealing insights can be made if the analysis is conducted using varying reference environments and compared using the same reference environment.

General Approach

Exergy analysis is a methodology that uses the conservation of energy principle (embodied in the first law of thermodynamics) together with non-conservation of entropy principle (embodied in the second law) for the analysis, design and improvement of energy and other systems. The principles are applied to individual components such as boiler, turbines, pumps, heat exchangers, compressor, combustion chamber as well as the most complicated overall power plant. The system was simplified into control volumes with distinct exergy inflows and outflows from each control volume representing the different process flows. The processes were approximated to steady or quasi-steady state flow conditions. The desired exergy output was compared to the necessary exergy input (rational efficiency).

CHAPTER FOUR

DATA PRESENTATION AND ANALYSIS

Combustion Equation of Fuel used for Egbin Steam Power Plant

The combustion equation used for the plants can be determined using equation(3.1)

CHAPTER FIVE

RESULTS AND DISCUSSIONS

Presentation of result of Egbin Steam Power Plant

The power plant was analyzed using energy and exergy analysis noting that the environment reference temperature and pressure were 25^o and 1 bar respectively. The thermodynamic properties of water and steam at indicated nodes shown in the process diagram were calculated using EES software and summarized in Tables A1, A2, A3 and A4 of the Appendix. The energy and exergy model equations for the components of the power plant were formulatedas an array of a matrix and analyzed using SCILAB software code and the result shown in Table 5.1.

From Table 5.30 and Figure 5.52, it was observed that the thermal efficiency of the steam or vapour power plant was 40. 24% higher than the thermal efficiency of gas turbine power plant which was 31.36%. The exergy efficiencies of the two plants were 39.07% and 29.06% respectively. The back work ratio of gas turbine plant was 57.38% higher than that of the steam or vapour power plant which was 1.70% For the same pressure rise, a gas turbine compressor would require a much greater work input per unit of mass flow than the pump of a steam or vapour power plant because average specific volume of the gas flowing through the compressor would be many times greater than that of the liquid passing through the pump. Hence a relatively large portion of the work developed by the turbine is required to drive the compressor. The change in specific enthalpy for the expansion of vapour through the turbine is many times greater than the increase in enthalpy for the liquid passing through the pump. Hence the back work ratio is characteristically low for vapour power plants. Typical back work ratio of gas turbines range from 40 to 80%. In comparison, the back work ratios of the vapour are normally only 1 to 2% [7]. Also, in Table 5.30, the exergetic performance coefficient and the exergetic sustainability indicators of the two plants were compared. The exergetic sustainability indicators considered include waste exergy ratio, environmental effect factor and sustainability index factor. It was observed that the exergetic performance coefficient of Egbin power plant was0.5854 which is more than twice that of Geregu plant which was 0.2752 at design conditions for both plants. The waste exergy, environmental effect factor and the sustainability index factor of Egbin plant were 0.4251, 1.0853 and 0.9213 respectively. For the Geregu I gas turbineplant, the values of waste exergy, environmental effect factor and the sustainability index factor were 0.2180, 0.7485 and 1.3361 respectively. From the above result, the choice of location of gas turbine power plant is environmentally friendly more than the choice of the location of steam power plant.

Research contribution to knowledge

This work has shown that increasing thermodynamic properties such as pressure and temperature, increases the exergy efficiency of boiler of steam power plant and combustion chamber of gas turbine plants
The cycle energy and exergy efficiencies of Egbin power plant is greater than that of Geregu I gas power plant by 8.76% and 0.94% respectively.
Comparing the values of waste exergy ratio and environmental effect factor, itwas observed that these two parameters were higher in Egbin steam power plantthan Geregu 1 gas power plant by 0.207 and 0.3386which makes steam power plants operation more harmful to the ecosystem, aquatic life and the environment than gas turbine plants.
As combustion contributes greatly to irreversibility in the process, the research has helped us to know measures that would be taken to reduce irreversibility and the required theoretical amount of air needed for complete combustion of the fuel used in the power plants. This will provide a suitable guide for fuel loss minimization that will lead to correct conclusion in terms of fuel economy.
Improvements on the efficiency of the two power plants were done through an adjustment of their operating parameters such as temperature and pressure or in the design of efficient new plants.
This study has revealed that exergy is the nucleus of energy, environment and sustainability.
As federal government is trying to find a lasting solution to the energy and power problem in Nigeria. The result of this research work will provide insight to the performance of power plants in Nigeria which will help in energy policy formulation and contribution to reducing global warming.

Recommendation

Since the scope of this study does not involve exergy costing, there is need for future studies in thermoeconomics and exergoeconomics. This will provide the opportunity for power plant owners/operators to determine the cost implication of any improvement made in the power plants.
The study also recommends the redesign of the installed capacity of each unit of Geregu 1 power plant. This is because the power outputs from the last three steps taken were greater than each unit installed capacity of 138MW.
As a result of gaseous emissions especially from the exhaust of the plants which contribute to global warming, the study advocates cogeneration plant that will utilize the waste heat from exhausts for other processes.

Conclusion

In this study, the energy and exergy analysis of Egbin and Geregu thermal power plants were performed. The primary objectives of this study were to analyze the system components separately and identify the sites having thelargest exergy losses at design and operating conditions. The maximum exergy loss was found in the boiler/steam generator of Egbin steam power plant for the design and operating years of 2008, 2009 and 2010 to be 56.22%, 59.02%, 55.33% and 57.47% respectively. Improvement performance at 5⁰C and 10 bar temperature and pressure increase showed reduction in exergy loss to 55.82%, 58.69%, 54.87% and 57.14%respectively. Further improvement performance at 20⁰C and 40 bar temperature and pressure incrementshowed reduction in exergy loss to 54.87%, 57.78%, 53.99% and 56.31%respectively. The variation in ambient temperature with the exergy efficiency of the boiler/steam generator showed that decrease in ambient temperature increased slightly exergy efficiencies in the range 0.03% to0.04%.The cycle thermal energy and exergy efficiencies of the plant at design and operating years were 40.24% and 39.18%; 41.64% and 40.53%; 41.03% and 39.94%; 37.45% and 36.45% respectively. The reasons why operational efficiencies in the year 2008 and 2009 were higher than the efficiency at design condition was because of maintenance activities done in the plant in those years and also higher turbine inlet temperature and pressure at the operating years being higher than those parameters at the design condition.The plant gross station heat rate at design condition and operating years were 8945KJ/kwh, 8645 KJ/kwh, 8775 KJ/kwh, and 9614 KJ/kwh respectively. Improvement performance at 5⁰C and 10 bar temperature and pressure increase respectively revealed increase in cycle thermal energy and exergy efficiencies to be 40.28% and 39.21%; 41.70% and 40.58.%; 41.08% and 39.98%; 37.50% and 36.50% respectively. The overall power plant energy and exergy shows similar increment. The gross station heat rates were decreased to 8937KJ/kwh, 8634KJ/kwh, 8763KJ/kwh and 9599 KJ/kwh respectively. Further improvement performance to20⁰C and 40 bar temperature and pressure respectivelyshowed more improvement on the cycle thermal energy and exergy efficiencies. It was evident that the energy and exergy efficiencies were 40.45% and 39.36%; 41.86% and40.73%; 41.23% and 40.12%; 37.66% and 36.65% respectively. Also, overall power plant energy and exergy efficiencies increased with such increase in temperature and pressure. The gross station heat rates decreased further to 8901KJ/kwh, 8600 KJ/kwh, 8732 KJ/kwh, and 9560 KJ/kwh respectively.There were also improvements in other second law parameters like exergetic performance coefficient and exergetic sustainability indicators. The increase in temperature and pressure from the initial normal operating temperature and pressure for the design and actual conditions to 20^oC and 40 bar temperature and pressure respectively increased the exergetic performance coefficients from 0.5855, 0.5987, 0.6133 and 0.5228 to 0.5907, 0.6042, 0.6188 and 0.5277 respectively. For the exergetic sustainability indicators such as waste exergy, environmental effect factor and sustainability index factor, the improvement performance reduced the waste exergy ratios from 0.4251, 0.4465, 0.4159and 0.4421 to 0.4195, 0.4409, 0.4104 and 0.4368 respectively. The environmental effect factors were reduced from 1.0851, 1.1015, 1.0412 and 1.2128 to 1.0659, 1.0824, 1.0230 and 1.1920respectively. Again, the improvement performance approach increased the sustainability index factor from 0.9216, 0.9078, 0.9604 and 0.8245to 0.9381, 0.9238, 0.9773 and 0.8390 respectively

For Geregu I gas power plant, the maximum exergy destruction ratios at the initial design and operating years of 2007, 2008, 2009, 2010 and 2011 was found in the combustion chamber to be 25.71%, 26.48%, 26.31 %, 26.30 %, 26.34% and 26.34 % respectively. Improvement on the performance of the combustion chamber with both 5^oC and 1 bar increase in turbine inlet temperature (TIT) and pressure showed reduction in exergy destruction ratios to 25.65%, 26.41%, 26.24%, 26.23%, 26.27% and 26.27% respectively. Again, further increase to 20^oC turbine inlet temperature and pressure of 4bar reduced the exergy destruction ratiosto 25.49%, 26.24%.26.07%, 26.06%, 26.09% and 26.10% respectively. The variation in ambient temperature with the exergy efficiency of the combustion chamber indicates that decrease in ambient temperature increased slightly exergy efficiencies in the range 0.01%to 0.03%. The cycle thermal and exergy efficiencies of the plant at design and operating years were 31.36% and 29.13%; 32.60% and 30.28%; 33.04% and 30.68%; 32.77% and 30.44%; 32.76% and 30.43% ; 32.61% and 30.29% respectively. The reason why the efficiencies of the plant were higher in the operating years than the design condition was because of the inlet temperature to the compressor being higher in operating years than at design. The plant gross station heat rate at design and operating years were11480KJ/kwh, 11043 KJ/kwh, 10897 KJ/kwh, 10986 KJ/kwh, 10990 KJ/kwh and 11041 KJ/kwh respectively.

The improvement on the performance of plant with both increase in turbine inlet temperature (TIT) and pressure to 5⁰C and 1 bar increased the cycle thermal and exergy efficiencies of the plant to 31.84% and 29.57%; 33.08% and 30.72%; 33.51% and 31.12%; 33.29% and 30.92%; 33.24% and 30.87% and 33.09% and 30.73% respectively. The overall cycle energy and exergy efficiency shows similar improvement changes. The first law parameter such as gross station heat rate decreased to 11305KJ/kwh, 10883 KJ/kwh, 10742 KJ/kwh, 10813 KJ/kwh, 10831 KJ/kwh and 10880 KJ/kwh respectively. Further increase in the turbine inlet temperature(TIT) and pressure to 20⁰C and 4 bar respectively increased the energy and exergy efficiencies of the plant to 33.30% and 30.90%; 34.77% and 32.26%; 35.20% and 32.67%; 34.85% and 32.34%; 34.92% and 32.41%; 34.78% and 32.27% respectively. The gross station heat rate was further reduced to 10811 KJ/kwh, 10355 KJ/kwh, 10227 KJ/kwh, 10331 KJ/kwh, 10308 KJ/kwh and 10351 KJ/kwh, respectively.

Similarly, for the Geregu I power plant, increase in temperature and pressure from the initial normal operating temperature and pressure for the design and actual conditions to 20⁰C and 4 bar temperature and pressure respectively increased the exergetic performance coefficients from 0.2752, 0.2877, 0.2926, 0.2894, 0.2896, and 0.2877 to the range 0.2919 , 0.3058 , 0.3108, 0.3065, 0.3077 and 0.3058 respectively .The improvement performance reduced the waste exergy ratios from 0.2180, 0.2187, 0.2164, 0.2171, 0.2177 and 0.2180 to 0.2126, 0.2123, 0.2102, 0.2109, 0.2114, 0.2117 respectively. The environmental effect factor of the gas turbine plantwas reduced from0.7485, 0.7222, 0.7054, 0.7132, 0.7154,and 0.7199respectively to 0.6879, 0.6580, 0.6435, 0.6522, 0.6522, and 0.6559 respectively. Also, the improvement performance approach increases the sustainability index factors from 1.336, 1.3847, 1.4177, 1.4022, 1.3978 and 1.3892 to1. 4537, 1.5197, 1.5541, 1.5332, 1.5333, 1.5245 respectively.

Finally, some useful expressions were developed in the present work for the study of first and second law analysis of thermal power systems. The second law analysis identified the components that have the highest exergy destruction in each of the power plant. Improvement performance at some conditions improved the exergy efficiency of the concerned component in each plant.

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