Improvement of the performance of thermal power systems through energy and exergy analysis

Thesis Overview

<p> </p><p>INTRODUCTION<br>1.0 Background<br>Thermal power plants are widely utilized throughout the world for electricity generation.<br>They include steam power plants, gas turbine power plants, nuclear power plants, internal<br>combustion engines. There are numerous aged and new thermal power plants that are in<br>service throughout the world today, for example, about 1,300 steam power plants have been in<br>service for more than 30 years in the USA, [1]. In recent years, global warming has been a<br>major issue due to continuous growth of greenhouse gas emissions from different sources.<br>The contributors to greenhouse effects are carbon dioxide (CO2), nitrogen dioxide (NO2) and<br>sulphur dioxide (SO2). Carbon dioxide is a major greenhouse gas which is mainly blamed for<br>global warming.<br>Different industrial processes such as power plants, oil refineries, fertilizer plants, cement and<br>steel plants are the main contributors of CO2 emission. Fossil fuels such as coal, oil and<br>natural gas are the main energy sources for power generation and will continue to generate<br>power due to large reserves and affordability. Demirbas, [2] reported that about 98% of CO2<br>emission results from fossil fuel combustion. Many power companies have investigated and<br>undertaken measures to improve the efficiencies of such power plants in order to minimize<br>their environmental impacts(e.g. by reducing emissions of CO2, NO2 and SO2), and to make<br>them more competitive, as deregulation of the power industry proceeds. Such investigations<br>have been based on energy consideration. It has also sparked interest in the scientific<br>community to take a closer look at the energy conversion devices and to develop new<br>techniques to better utilize the existing transfer and energy change.<br>The most commonly used method for analysis of an energy conversion system is the first law<br>of thermodynamics. Engineers and scientist have been traditionally applying the first law of<br>thermodynamics to calculate the enthalpy balances for more than a century to quantify the<br>loss of efficiency in a process due to loss of energy. However, the first law of<br>thermodynamics deals with the quantity of energy and asserts that energy cannot be created or<br>destroyed, [3]. This law serves as a necessary tool for accounting for energy during a process<br>and offers no challenges to the engineer. However, in recent years the second law analysis,<br>2<br>also known as exergy analysis of energy systems has more and more drawn the interest of<br>energy engineers and scientific community. Exergy analysis provides an effective technique<br>for measuring and optimizing performance of a thermal system by accounting for energy<br>quality. It can also be used to assess the sustainability level of energy systems. Sustainability<br>means a supply of energy resources that is sustainably available at reasonable cost and causes<br>no minimal negative effects. Sustainability is necessary to overcome current ecological,<br>economic, and developmental problems. The exergy sustainability indicators include exergy<br>efficiency, waste exergy ratio, recoverable exergy rate, exergy destruction factor,<br>environmental effect factor and exergetic sustainability index, [4].<br>For power plants, exergy analysis allows one to determine the maximum potential for<br>electricity production associated with the incoming fuel or any flow in the plant. This<br>maximum is achieved if the fuel or flow is utilized in processes that ultimately bring it to<br>complete thermodynamic equilibrium with the environment, while generating electricity<br>reversibly. Thus, exergy analysis provides the theoretical efficiency limitations upon any<br>power plant. Losses in the potential for electricity generation occur due to irreversibilities and<br>determined directly with exergy analysis. The exergy concept has gained considerable interest<br>in the thermodynamic analysis of thermal processes and plant systems since it has been seen<br>that the first law analysis has been insufficient from an energy performance point of view.<br>Based on the second law of thermodynamics, the exergy analysis represents the third step in<br>the plant system analysis, following the mass and the energy balances. The aim of the exergy<br>analysis is to identify the magnitudes and the locations of exergy losses, in order to improve<br>the existing systems, processes or components, or to develop new processes or systems, [5].<br>The method of exergy analysis is particularly suited for furthering the goal of more efficient<br>resource utilization, since it enables the location, and time magnitudes of wastes and losses to<br>be determined. Improved resource utilization can be realized by reducing exergy destruction<br>within a system. The objective in exergy analysis is to identify sites where exergy destructions<br>and losses occur and rank them for significance. Exergy losses include the exergy flowing to<br>the surroundings, whereas exergy destruction indicates the loss of exergy within the system<br>boundary due to irreversibility. This allows attention to be centered on the aspects of system<br>operation thatoffer the greatest opportunities for improvement, [6]. Exergy analysis which is<br>the combined first and second law analysis gives much more meaningful evaluation indicating<br>the association of irreversibilities or exergy destruction with combustion and heat transfer<br>3<br>processes. This allows thermodynamic evaluation of energy conservation option in power<br>and refrigeration cycles, thereby provides an indicator that points the direction in which<br>engineers should concentrate their efforts to improve the performance of thermal systems. The<br>second law of thermodynamics has proved to be a very powerful tool in the optimization of<br>complex thermodynamic systems, [7],[8],[9].<br>1.1 EnergySources in Nigeria<br>The country is endowed with both the conventional and the non-conventional energy<br>resources. The conventional comprises mostly of the non-renewable resources such as crude<br>petroleum oil, natural gas, coal, tar sand and uranium, [10]. The country has the tenth largest<br>oil and gas reserves in the world. The various non-conventional energy resources available in<br>the country that can be harnessed for power generation are nuclear, solar, wind power,<br>biomass energy, wave and tidal energy and geothermal energy. Nigeria’s near equatorial<br>location, extensive and diverge vegetation, prevailing trade winds and many rivers endow her<br>with large quantities and quality of renewable energy sources,[11].These include solar<br>radiation, hydro power, wind and biomass energy. Nigeria’s coal reserves are large and<br>estimated at 2.7 billion metric tonnes of which 650 million tonnes are proven reserves. About<br>95% of the Nigerian coal production in late 1950s and early 1960s was consumed locally,<br>mainly for railway transportation, electricity production and industrial heating in cement<br>production. Nigeria has abundant reserves of natural gas. The quantity of natural gas is at<br>least twice as much as the oil, and the horizon for the availability of natural gas is definitely<br>longer than that of oil. In energy terms, the quantity of natural gas used for electricity<br>generation is very significant. The known reserves of natural gas have been estimated at about<br>187.44 trillion standard cubic feet or 5.30 x 1012 standard cubic meters as at the year<br>2007,[12].<br>The third major source of energy, oil, is Nigeria’s major source of revenue used for<br>development. As at January 2005, Nigeria’s proven crude reserve stood at 35.2 billion barrels.<br>The majority of the reserves are found along the country’s coastal Niger Delta. As at 2007,<br>Nigeria’s energy resource availability expressed in barrels (bbls) and standard cubic feet (scf)<br>and other units showed that crude oil availability in Nigeria stood at 36.5 billion barrels. Other<br>energy resources include natural gas whose availability is 187.44 trillion standard cubic feet,<br>coal and lignite estimated at 2.7 billion tonnes as shown in Table 1.1<br>4<br>Table 1.1: Energy Resource Availability in Nigeria<br>RESOURCES AVAILABILITY<br>Crude oil 36.5 billion bbl<br>Natural gas 187.44 trillion scf<br>Coal and lignite 2.7 billion tones<br>Tar sands 31 billion bbl oil equivalent<br>Hydropower (large scale) 11,250mw<br>Hydropower (small scale) 3,500mw (estimate)<br>Solar radiation 3.5 – 7.0kwh/m2-day<br>Wind 2 – 4m/s annual average<br>Fuel wood 13.1 million ha of forest/wood land<br>Animal waste<br>Very significant<br>Quantity not available<br>Crop residue<br>Tidal energy<br>Uranium<br>Source: Energy Commission of Nigeria, 2007<br>Solar radiation intensity varies in a quasi-predictable way. It varies with day and night,<br>location, weather and climate. It increases with altitude and solar altitude angle. For instance,<br>at an altitude of 3,000m and solar altitude angle of 900 (i.e. overhead) it gets as high as<br>1.18KW/m2, while at sea levels it is &lt; 1.0 KW/m2. It is reduced by cloudiness, atmospheric<br>gases, atmospheric particles (aerosols) and obstructions.<br>1.2 Electricity Generation in Nigeria<br>Generation of electricity is a very complex process involving many sub-processes and has<br>multiple critical parameters. A decline in thermal efficiency leads to a higher cost of<br>electricity generation due to more fuel usage and also will result in much higher carbon<br>deposits. Therefore, it is very important to stress on the performance of power plants.<br>Electricity generation is the conversion of other kinds of energy, mainly primary energy into<br>electrical energy. Generally, the process of generating electricity goes through several<br>5<br>transformations from primary energy directly into electricity. For instance, in a thermal power<br>station, the primary energy is converted to a high temperature steam, as an intermediate heat<br>source, then into mechanical energy in the turbine physically connected with the generators<br>where the electrical energy is produced.<br>Power generation in Nigeria is mainly from three technologies only which include hydroelectric<br>power stations, steam and gas thermal stations. Most of these facilities are being<br>managed by the Power Holding Company of Nigeria (PHCN); a government owned utility<br>company that coordinates all activities of the power sector such as generation, transmission,<br>distribution and marketing before they were privatized. Since inception of PHCN, the<br>authority expands annually in order to meet the ever increasing demand. Unfortunately, the<br>majority of Nigerians have no access to electricity and the supply to those provided is not<br>regular. In a bid to make the power sector more functional, the PHCN was unbundled into 18<br>successor companies (1 Transmission, 11 Distribution and 6 Generation companies). This was<br>done due to current privatization in the sector [13].<br>Prior to 1960s, energy supply and consumption consisted predominantly of non-commercial<br>energy, viz-fuel wood, charcoal, solar radiation, agricultural waste and residues. Major<br>commercial fuel was coal used in railway engines and for power generation. Contributions to<br>commercial energy came frompetroleum products (petrol and diesel) for road vehicles and<br>from electricity (from coal and diesel generators). Up till 2005, the grid electricity supply<br>industry was predominantly the vertically integrated public utility-National Electric Power<br>Authority (NEPA), which owned about 98% generating capacity and 100% of transmission<br>and distribution capacity. In consequence and in particular through former President<br>OlusegunObasanjo’s power project and President Goodluck Jonathan’s power road map for<br>power sector reform of August 2010, actual maximum peak generation has now more than<br>doubled (4300MW) since the start of the reform in 2000 and installed generation is now<br>above 10109.5MW, [13].<br>At present, the installed capacities in power stations in Nigeria are shown in Tables 1.2, 1.3<br>and 1.4 for pre-1999 power stations and other power stations as contained in a document<br>prepared by Energy Commission of Nigeriain 2007.<br>6<br>Table 1 .2: Pre- 1999 Power Stations<br>Station Capacity (MW)<br>Kainji Hydro 760<br>JebbaHydro 578<br>Shiroro Hydro 600<br>Egbin Thermal 1320<br>Sapale Thermal 1020<br>Ijora Thermal 60<br>Delta Thermal 912<br>Afam Thermal 711<br>Orji River Thermal 30<br>NESCO 30<br>Total 6,021MW<br>Source: Energy Commission of Nigeia,2007<br>Other power generating stations include eight National Integrated Power Project (NIPP)<br>which were built in some states of the country. These are Gbarain Integrated Power Project in<br>Bayelsa State, Egbema Integrated Power Project located in Imo State, Ibom Integrated Power<br>Project in AkwaIbom State.<br>Table1.3: National Integrated Power Project (NIPP)<br>Station Capacity(MW)<br>Gbarain, Bayelsa 225<br>Ihubor, Edo 451<br>Omoku, Rivers 230<br>Sapela,Delta 451<br>Egbema, Imo 338<br>Calabar, Cross Rivers 561<br>IkotAbasi, AkwaIbom 300<br>Ibom Power, AkwaIbom 188<br>Total 2,744MW<br>Source: Energy Commission of Nigeria, 2007<br>7<br>Another milestone in the power sector for electricity generation is the establishment of<br>Independent Power Producer (IPP) in different parts of the country. These include the AES<br>power station in Lagos State, Alaoji power station in Abia State, Papalanto power station in<br>Ogun State and others.<br>Table 1.4: Independent Power Producers (IPP)<br>Station Capacity(MW)<br>Geregu, Kogi 414<br>Omotosho, Ondo 335<br>Papalanto,Ogun 335<br>Alaoji, Abia 346<br>AES, Lagos 270<br>Geometric, Aba 140<br>Agip JV, Okpai/Kwale, Delta 480<br>Chevron JV, Agura,Igbin, Lagos 750<br>Total Fina, Obite, Rivers 500<br>Exxon Mobil Bonny, Rivers 500<br>Total 4070MW<br>Source: Energy Commission of Nigeria,2007<br>These National Integrated Power Projects (NIPPs) and Independent Power Producers (IPPs)<br>will augment the power generated by these power generating stations to meet the electricity<br>demand of the country.<br>1.3 Statement ofProblem<br>The global power sector is facing a number of issues, but the most fundamental challenge is<br>meeting the rapidly growing demand for energy services in a sustainable way. This challenge<br>is further compounded by the today’s volatile market-rising fuel costs, increased<br>environmental regulations etc. Thermal power plants are one of the most important elements<br>of energy sector and they are masterworks that enable production of electrical energy which<br>can be thought as one of the basic needs after food and water. Preference of the thermal power<br>8<br>plant type in electricity production is a big dilemma and prior discussion subject to related<br>parties in recent years. For instance, environmentalist act against fossil-fuelled thermal power<br>plants or nuclear power plants and they try to warn decision makers about environmental<br>pollution, global warming, carbon emission etc. The primary energy source possibilities of<br>countries are one of the basic factors that determine the preferences of a thermal power plant.<br>Namely, USA, Germany, India and China produce more than 50% of their electrical energy<br>by coal-fired thermal power plant, while most of the thermal power plants, in countries that<br>have abundance of natural gas such as Qatar, are gas fired. The choice is directly related to the<br>reserve capabilities of the primary energy sources which are one of the main issues for<br>government policies and preferences.<br>Today, many generating utilities are striving to improve the efficiency of their existing power<br>generating stations. The problem of low power generation output from these plants is as a<br>result of defective plant components and improper fuel utilization in the systems.<br>1.4 Aims and Objectives of the Study<br>The aim of this research is to<br>(i) Carry out energetic and exergetic performance analysis, at the design and actual<br>operating conditions for the existing unit 5 (220MW) of the 1320MW Egbin steam<br>power plant and unit 11(138MW) of the 414MW Geregu I gas turbine power plant in<br>order to identify the components that needs improvement.<br>The objectives of the study are to determine:<br>(i) the quantity of energy and exergy flows and location of losses.<br>(ii) the energy efficiency of the plant and its components.<br>(iii) plant performance parameters such as heat rate, specific fuel consumption and<br>thermal discharge index.<br>(iv) theexergy efficiency of the plant and its components.<br>(v) theexergy destructionswithin the system components.<br>(vi) exergetic performance coefficient.<br>(vii) exergetic sustainability indicators- exergy destruction ratio, waste exergy ratio,<br>environmental effect factor and exergetic sustainability index and<br>(viii) systems that have potential for significant performance improvement.<br>9<br>To achieve these objectives, we summarize thermodynamic models for the considered power<br>plants on mass, energy and exergy balance equations.<br>1.5 Scope of the Study<br>The scope of this work involves<br>Ø analysis of thermal power systems.<br>Ø determining the irreversibility rates in the plant components.<br>Ø comparative performance of the power plants at both design and operating<br>conditions,and<br>Ø performing sensitivity analysis on the variation of thermodynamic intensive properties<br>like temperature and pressure in improving the plants performance.<br>1.6 Significance of the Study<br>The growth, prosperity and national security of any country are critically dependent upon the<br>adequacy of its electricity supply industry. Over the past two decades, the stalled expansion<br>of Nigeria’s grid capacity, combined with the high cost of diesel and petrol has crippled the<br>growth of the country’s productive and commercial industries. It has stifled the creation of<br>jobs which are urgently needed in a country with a large and rapidly growing population; and<br>the erratic and unpredictable nature of electricity supply has engendered a deep and bitter<br>sense of frustration that is felt across the country as a whole and in its urban centers in<br>particular. Electricity consumers and the citizenry as a whole demand a fundamental reversal<br>of the long and debilitating malaise which has blighted the industry and, in doing so, bridled<br>the tremendous energy and creativity of this great and populous nation. More particularly they<br>demand real and immediate improvements in service levels, [14].<br>Nigeria needs over 10,000 MW of electricity for her domestic and industrial demands of<br>which about 4000MW is currently being generated from power plant locations across the<br>country. The quantity generated are transmitted and distributed through the national grid to<br>primary energy consumers. As a result of inefficient operation of some of these plants owing<br>to long age in service, the need to identify the location of the inefficiency in the plant becomes<br>imperative. Generally, the performance of thermal power plants is evaluated through energetic<br>performance criteria based on first law of thermodynamics, including electrical power and<br>thermal efficiency. In recent decades, the exergetic performance based on the second law of<br>10<br>thermodynamics has been found to be a useful method in the design, evaluation, optimization<br>and improvement of thermal power plants. The exergetic performance analysis can not only<br>determine magnitudes, location and causes of irreversibilities in the plants, but also provides<br>more meaningful assessment of plant individual components efficiency, [15]. The use of<br>exergy analysis becomes the answer as a tool for pin-pointing inefficiencies. The features of<br>this technique make it valuable in the thermodynamic analysis aiming at the improvement of<br>the efficiency of existing thermal plants through an adjustment of their operating parameters<br>or in the design of efficient new thermal plants.<br>1.7 Study Area<br>Nigeria is the most populous African country with the total population estimate of over 152<br>million people. She has over ten power generating stations (both thermal and hydro power<br>stations) established before the year 1999.<br>Besides having these power stations, there are eight National Integrated Power Projects<br>(NIPPs) established after the year 1999 and many Independent Power Producers (IPPs). For<br>the purpose of this study, the Egbinpower station and the Geregu power station will be<br>considered because the former uses steam and water as working fluid and the later uses air<br>and combustion products as working fluid where the boiler/steam generator and combustion<br>chamber are fired by natural gas. These power plants contribute good percentages of over 15.9<br>million MW of electricity consumed for both domestic and industrial use by the populace<br>annually.<br>Egbin thermal plant is located atIjede area of Ikorodu, a suburb of Lagos State. The plant was<br>commissioned in 1985 and consists of 6 units of 220 (6X220) MW (Reheat – Regenerative).<br>They are dual fired (gas and heavy oil) system with modern control equipment, single reheat;<br>six stages of regenerative feed heating. Natural gas is supplied to the plant directly from the<br>Nigerian Gas Company (NGC) Lagos operations department, which is annexed to the thermal<br>plant. Since Egbin thermal plant is located on the shores of the lagoon, cooling water for the<br>plant’s condensers is pumped from the lagoon into the water treatment plant en route to the<br>condensers.<br>The Geregu I gas thermal power station located in Ajaokuta, Kogi State of Nigeria was<br>commissionedin 2006 and it consists of three independent units, each being rated</p><p>&nbsp;</p> <br><p></p>