ABSTRACT
This research appraised the performance improvements of gas turbine engines using compressor online and offline water washing optimization techniques in the Niger Delta area of Nigeria from the stand point of thermodynamics. This was achieved by collation of data from service records at Sapele power generation station over a period of twenty four months (two yrs) from two gas turbine engines “GT1 & GT2”, of the same capacity, at the same location, commissioned the same time (Date). GT1 uses both compressor online and off-line water washing optimization technique, while GT2 uses only off-line water washing optimization technique. The data collated was anaylsed using Scilab programming language, the result of the analysis shows that with the use of compressor online water washing on GT1 yielded a compressor efficiency of 81.6% and overall operational efficiency of 28.3%. On the other hand, GT2 yielded a compressor efficiency of 78.5% and overall operational efficiency of 27.7% with the application of compressor offline water washing only. Thus, gas turbines to be imported into the country should be equipped with compressor online and off-line water washing skid.
TABLE OF CONTENTS
Title page………………………………………………………………………… i
Certification……………………………………………………………………… ii
Acknowledgement………………………………………………………………. iii
Abstract …………………………………………………………………………. iv
Table of Contents………………………………………………………………… v
Nomenclature…………………………………………………………………… vi
List of Figures…………………………………………………………………….. vii
List of Tables…………………………………………………………………….. iix
Introduction
Introduction…….. …………………………………………………… 1
1.1 Gas turbine & Power generation in Nigeria…………..…………………… 1
1.2 Background of the Study…………………………………………..…… 3
1.3 Axial Compressor Performance Deterioration……………………..…….. 4
1.4 Underlying causes of Compressor fouling ………… ……………. …… 4
1.5 Humidity effect on the fouling of Axial compressors …… ………… . 5
1.6 Nature of Compressor Foulants ……… ……………………………….. 5
1.7 Detection of fouling …………………………………………… ………… 5
1.8 Overview of other major factors affecting GT performance………..….. 6
1.9 Problem Statement……………………………………………… 8
1.10 Aim and Objective…………………………………………………….. 8
1.11 Significant of Study…………………………………………………… 9
Literature Review 10
2.1 Control of fouling by Compressor Washing…………………….. 15
2.2 Offline Washing……………………………….………………… 15
2.3 Online Washing…………………… …………………………… 15
2.4 Washing Parameters…………………………………………….. 17
Methodology
3.1 Description of the GT under investigation…………………………… 19
3.2 Manufacturer’s Technical Specification …………………………….. 25
3.3 Design Thermodynamic Cycle of GE Frame 9E …………………………… 27
3.4 Overview of the GT Maintenance Instructions ……………………… 29
3.5 Climatic & Environmental Conditions of Sapele Power Station ……. 30
Data analysis and results
4.1 Climatic Parameters Analysis ………………………………………… 31
4.2 The Upper & Lower limits of Climatic parameters ………………….. 31
4.3 Climatic Parameters Variation … ………………………………….. 33
4.4 Analysis of Performance Parameters ………………………………… 34
4.5 Discussion of Results ………………………………………………… 42
Conclusion and recommendatioN 43
References …………………………………………………………………… 45
CHAPTER ONE
Introduction
Power generation is an important issue today, prompt by the zeal to have an efficient energy sector which is generally regarded as a pre-requisite for the realization of goals and objectives of economic reforms such as wealth creation, employment generation, poverty reduction and eradication. In the face of these challenges, government after government have not lacked in the amount of pious resolves to steer the nation on the path of reform that would dramatically cure the apparently intractable energy problem. The growth in electricity demand being experienced in Nigeria has resulted in the need to build power plants that generate maximum power output overtime, with a reduction in down-time. Due to less installation time, low installation cost and availability of natural gas in the country, many states of the country and private establishments are currently building gas turbine power plants to meet this demand. However, one major disadvantage that penalizes the gas turbine performing is the adverse effect of the environment on the gas turbine power output and efficiency. Gas turbines designed to operate at maximum efficiency at standard ambient temperatures and relative humidity may tend to reduce in performance due to adaptation problems resulting from variation in weather conditions as they are installed at different locations.
The ability to predict the behaviour of a gas turbine engine and optimize its performance is critical in economic, thermal and condition monitoring studies. To utilize the high economic and energy saving potential of a gas turbine engine in their simple and combined cycles, it is important to identify their optimal design parameters and determine the impact of the deviation of these parameters from the standard conditions, on the overall performance of the engine. Compressor fouling has been identified as one of the major sources of this deviation and gas- turbine degradation. Thus compressor online water washing optimization technique is often applied to recover gas turbine performance loss.
1.1 Gas turbine and power generation in Nigeria:
Gas Turbine (GT) sets were first used for electricity generation in Nigeria in 1963 when four units of installed capacity of 10 mega watts (MW) each were commissioned at the Afam Generating Station, about 50 Kilometers (Km) North East of Port Harcourt, by the then Electricity Corporation of Nigeria (ECN). The ECN was merged with the Niger Dam Authority (NDA) in 1969 to form the National Electric Power Authority (NEPA). Between 1963 and 1990, NEPA installed several more GTs at Afam, Ijora Power Station in Lagos State, Delta Generating Station in Ughelli Delta State and Sapele Generating Station in Sapele Delta State. All these stations except Ijora are situated in the Niger Delta area of Nigeria. The installed capacities of these sets were between 10MW and 100MW.
Several sets installed during the early period proved to be inefficient and troublesome to run to the extent that those at Ijora had to be abandoned in 1971 after they had been in use for only a few years. It was thought then that the difficulty associated with running the sets was due mostly to the fact that gas turbine technology was new in the country and the operators had not acquired the requisite experience. However, after over forty (40) years from the time the first sets were installed the problem of inefficient and troublesome performance of GTs especially the large power generating ones has persisted.
In spite of these drawbacks, GTs continues to be preferred for generation of electricity by NEPA. Gas turbine constitute close to 60% of the total number of generating sets in the facility of NEPA, the other major means of electricity generation being by the use of hydro and steam turbines. The major problem of the hydro-turbine Power generating system in Nigeria is the unreliability of the hydrological potential of the rivers of the country to produce large power on a continuous basis (Hart, 1993). The most prominent river in Nigeria which is the River Niger washes other countries most of which are in the dry Sahel region before finally entering the country. These other countries depend largely on the River Niger for their numerous water needs. Already there are two hydro-turbine generating stations on the River Niger at Kainji, Jebba and another on the Shiroro gorge on a tributary of the river. It is therefore not feasible to build more stations on that River. Reports of hydrological surveys of other rivers in Nigeria have only encouraged the siting of small scale hydro-turbine stations along them (Aliyu, 1990). On the other hand, Steam turbines have not fared better in Nigeria mainly as a result of their bulky and intricate auxiliary systems which require not only a high level of trained and experienced personnel to man them but also a large stock of spare parts for their maintenance. Moreover, steam turbine plants are better located only in places where there is sufficient supply of water for cooling the condensers and other equipment of the sets among other uses. With the foregoing and the level of technological know-how, gas turbine seems to be the best option for large scale generation of power in Nigeria.
For several years, despite consistent perceived cash investment by the Federal Government, power outages have been the standard for the Nigerian populace, however citizens of the country still do not see this as normal. On April 15, 2005, the Bureau of Public Enterprises (BPE) named the PHCN as the new government incorporated body that will take over all assets and liabilities of the National Electric Power Authority (NEPA), preparatory to the full privatization of electricity generation and distribution in the country.
On the other hand, the tariff has been criticized as being too low compared to the cost of generating power. The Federal government of Nigeria has increased the tariff to attract foreign investors since the 1st of July, 2010 in order to meet the growing concern for foreign investors into the electricity sector. Presently, the average available capacity of Power for distribution is below 4,000MW against the projected estimation (projected requirement) of 60,000 MW.
1.2 Background of the study:
All gas turbines experience loss of performance overtime. Performance deterioration is caused by many different factors such as erosion of blade surfaces due to particle ingestion, fouling due to air borne pollution or oil vapor, loss of mass flow rate of compressed air due to ambient air temperature etc. Deterioration is generally referred to as either recoverable by routine maintenance actions or non-recoverable expect by replacement of degraded engine components. Recoverable performance loss is associated with loss of axial compressor performance due to the fouling of internal surfaces by air borne contaminant. These contaminants which vary on site locations are deposited on the front surfaces of the inlet shroud, first stage of compressor blades and the internals gas path of the axial compressor, causing an overall reduction in the power capacity, an increase in the specific fuel consumption and emissions production, and a reduction in time between overhaul.
Historically, compressor cleaning was first attempted by injecting solid particles such as nutshell or rice. Since the introduction of coated compressor blades, this method has been avoided due to concerns of pitting corrosion.
The state-of-the art method for removing fouling is a liquid wash in which a wash fluid is injected at the front end of the gas turbine. The wash fluid penetrates the gas path, where it dissolves and removes the fouling. There are presently two washing methods in use for liquid washing: Offline wash and online wash.
In offline washing, the gas turbine is run at sub-idle shaft speeds while a cleaning solution is injected into the engine. This method is well-proven and effective in removing deposits not only in the axial compressor but also on the interior surfaces of the entire gas path. Before an offline wash, the gas turbine unit must be shut down and cooled to avoid excessive thermal loading of the internal gas turbine components. This causes loss of availability and possibly production losses.
Online washing is done during gas turbine operation by injecting the cleaning solution into the compressor section while the engine is running in normal operation, hence avoiding the associated down-time cost. Online washing is often combined with offline washing to optimize uptime while maintaining an acceptable thermal efficiency. Cost of fuel and lost production are the predominant economic factors in determining the time between offline washes. Available information provides no consensus for online washing of gas turbines. Systems properties such as pressure, temperature and fluid injection rate vary from one system to another. Each system’s benefits are backed up with reference to sites where the particular system works.
1.3 Axial compressor performance deterioration:
Axial compressors are an integrated part of the gas turbine. The compressor consists of several stages of air foils circumferentially positioned on a rotor driven by a turbine. The purpose of the compressor is to increase the total pressure of the gas stream with minimum power absorbed. Axial compressors consume up to 60% of the produced turbine power, therefore maintaining the compressor at its optimum performance during operation is of major importance. During operation, the axial compressor will be deteriorated by airborne particles adhering to the internal surfaces. The first stage of an axial compressor will normally be the ones most heavily fouled, although deposits will have different characteristic depending on the nature of contamination. Dry particles in dry atmospheres are likely to deposit in different areas depending on the location of sticky material and oily compounds. At high inlet humidity, the drop in static pressure during acceleration of the air through the compressor will increase dust adhesion on the blades because of the condensing water. An axial compressor is a machine where the aerodynamic performance of each stage depends on that of the earlier stages. Thus, when fouling occurs in the inlet guide vanes and the first few stages, there may be a dramatic drop in compressor performance. The fouling of axial flow compressors is a serious operating problem and its control is of supreme importance to gas turbine operators especially in the deregulated and highly competitive power market. Loss in power output reduces revenues for the plant owner and increase in fuel consumption increases operating cost & emissions. Estimates have placed fouling as being responsible for 70 to 75 percent of all gas turbine performance losses accumulated during operation.
By means of suitable software, data available in the gas turbine instrumentation system can often be used to monitor compressor deterioration.
1.4 Underlying causes of compressor fouling:
Experience has shown that axial compressors will foul in most operating environments, be they industrial, rural or marine. There are a wide range of industrial pollutants and a range of environmental conditions (fog, rain, humidity) that play a part in the fouling process. Compressor fouling is typically caused by:
*Industrial Pollution: hydrocarbons, fly ash, smog, exhaust emissions matter.
*Internal Gas Turbine Oil Leaks – Leakage from the front bearing of axial compressor is a common cause. Oil leaks combined with dirt ingestion causes heavy fouling problems.
*Airborne salt in marine environment.
*Insects – this can be a serious problem in tropical environments.
*Ingestion of gas turbine exhausts or lubrication oil tank vapors.
In general, particles up to 10µm (microns) cause fouling, but not erosion. Particles above 10 to 20 microns cause blading erosion. The importance of climatic conditions, rain showers, relative humidity, etc, cannot be over-emphasized. Several operators have reported dramatic drops in gas turbine output coincident with rain showers. Often air filters will exhibit a sudden growth in differential pressure as the filters get saturated with water due to high humidity. Under certain conditions, the filter may suddenly unload into the airflow cause rapid compressor fouling.
1.5 Humidity effect on the fouling of axial compressors:
As air passes through the intake and filtration system, it proceeds at a very low velocity with filter face velocities being typically around 3m/sec. As it approaches the compressor face, the air accelerates to a high velocity. This results in a static temperature reduction of about 100˚C to 150˚C. The saturation air temperature also drops. If the relative humidity is high enough, it is possible that the static air temperature falls below the saturation air temperature. This causes condensation of water vapor, which is a common occurrence in most gas turbines when ambient relative humidity is high. On the other hand, due to drop in static pressure at the bell mouth caused by the acceleration of air near the IGVs, the humidity in the air will start to condense.
1.6 Nature of compressor foulants:
Compressor foulants are often classified as being “oil soluble”, “water soluble” or “water wettable”, but experience has shown that they typically are a combination of these types. For example, although sea salt is essentially water soluble, its retention within the compressor may be significantly influenced by trace quantities of oil and grease. In this case, the use of water alone for washing may not be sufficient, and a chemical detergent would be required for effective compressor cleaning.
1.7 Detection of fouling:
Gas turbine manufacturers and operators usually develop guidelines to define when fouling deterioration calls for corrective action. This is based on a combination of load and exhaust gas temperatures. Users also monitor compressor discharge pressure and compressor efficiency. However, it is the opinion of some operators that the only way to detect a fouled compressor is by visual inspection. Unfortunately, though, with most turbine designs this means shutting down the unit, removing the inlet plenum hatch, and visually inspecting the compressor inlet, bell mouth inlet guide and visible early stage blading. The following factors can be used as indicators of fouling:
- Drop in compressor mass flow rate.
- Drop in compressor efficiency and discharge pressure.
The most sensitive parameter of the above factors is the mass flow rate.
The real problem is to detect fouling in time to prevent a significant power drop and before a fuel penalty cost has been incurred. Some operators believe in regular periodic washing of the machine, whereas others base the washing requirement on a certain set of performance parameters. Once a judicious schedule for online and offline washing has been established, it is important to monitor the performance of the gas turbine and track for unexpected events and monitor the efficacy of washing.
Fig 1. Fouled axial flow compressor. Fig 2. Fouled compressor inlet Bell mouth and Blading.
1.8 Overview of other major factors affecting gas turbine performance:
- Ambient air temperature and relative humidity.
- Inlet and exhaust pressure losses.
- Air extraction
- Type and quality of fuel.
- Ageing of engine.
1.8.1 Ambient air temperature and relative humidity:
In gas turbine, since the combustion air is taken directly from the environment, their performance is strongly affected by weather conditions (Mohmoudi et al, 2009). A rise of 1˚C temperature of inlet air decreases the power output by 1%. Thus, power rating can drop by as much as 20 to 30%, with respect to international standard organization (ISO) design condition, when ambient temperature reaches 35 to 45˚C. The performance of gas turbine power plant is sensible to the ambient condition. As the ambient air temperature rises, air becomes less dense, and less air can be compressed by the compressor. One way of restoring operating conditions is to add an air cooler at the compressor inlet (Sadrameli and Goswani, 2007). The air cooling system serves to raise the turbine performance to peak power levels during the warmer months when the high atmospheric temperature cause the turbine to work at off-design condition, with reduced power output (Kakaras et al, 2004). The techniques for cooling of inlet air temperature include mechanical chillers, media type evaporative coolers and absorption chillers.
It is found that the power consumption of the cool inlet air is of considerable concern since it decrease the net power output of gas turbine. The mechanical chiller auxillary power consumption is very high compared to media type evaporative coolers. The efficiency of cooler largely depends on moisture present in the air.
1.8.2 Inlet and exhaust pressure losses:
Inserting air filtration, silencing, evaporative coolers or chillers into the inlet or heat recovery devices in the exhaust causes pressure losses in the system. The effects of these pressure losses are unique to each. Output reduces because of pressure drop and not getting the required pressure for combustion.
1.8.3 Air Extractions:
In some gas turbine applications, it may be desirable to extract air from the compressor for sealing and cooling of bearing, blades, among other uses. So less air is left for combustion, thus output will decrease. Generally, up to 5% of the compressor airflow can be extracted from the compressor discharge casing. As a role of thumb, every 1% in air extraction results in a 2% loss in power.
1.8.4 Type and quality of fuel used:
Selection of fuel depends on several factors such as fuel availability, fuel cost and cleanliness of fuel. Gas turbines are capable of burning a range of fuels including:
- Natural gas. 2. Naphthalene. 3. Distillate oil. 4. Crude oil (HDS, Heavy oil). 5. Coal gasification.
Natural Gas: Natural gas is of low sulphur content, this can increase the combustion temperature and hence more output. The natural gas (methane) produce nearly 2% more output than does distillate oil because of its high calorific value. Natural gas is an ideal fuel because it provides high thermal efficiency and reliability with a low operation and maintenance cost.
The Quality of Natural Gas includes:
* Lighter than air, so it rises and dissipates rapidly in case of a leak.
* Flammability Range of 5% – 15% and Ignition Temperature of 650˚C.
* It takes less air for combustion and produces less CO compared to LPG.
* It is colourless, odourless, non-corrosive & non-toxic.
Liquid Fuels: Particularly heavy oils usually contain contaminants, which cause corrosion and fouling in the gas turbine. Contaminants which cannot be removed from the fuel may leave deposits in the gas turbine, which reduce performance and add maintenance costs.
Dual fuel systems are commonly used, enabling the gas turbine to burn back-up fuels when the primary fuel source is not available.
1.9 Problem Statement:
Fouling restricts flow and causes increased boundary layer thickness both on the blades and along the end walls of the annulus and hub. Blockage of the air path and increased frictional losses reduce the compressor head and flow, causing an overall reduction in the power output and an increase in the specific fuel consumption.
Fouling is traditionally seen in the following performance parameters:-
- Reduced compressor discharge pressure.
- Reduced compressor efficiency
iii. Increased compressor discharge temperature
- Reduced power output.
- Increased vibration in the compressor due to unequal distribution of deposit along the circumference.
- Onset of compressor stall or surge.
1.10 Aim and Objectives of the research:
The purpose of this work is to appraise the performance of “Power generating gas turbine sets using compressor water washing optimization technique, installed in the Niger Delta area of Nigeria from the basis of thermodynamics.
Specific Objectives of this study are:
- To determine the compressor discharged pressure variation.
- To determine the compressor discharge temperature variation.
iii. To determine the compressor efficiency variation.
- To determine the specific fuel consumption variation.
- To determine the turbine inlet temperature variation.
- To determine the overall power output variation.
1.11 Significance of the research:
In view of the gas turbine performance degradation challenges faced by Power generating firms in Niger Delta due to climatic and environmental factors, there is need for proper investigation and analysis of the potential savings that can be gained from gas turbine using online compressor washing optimization technique in this region.
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