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A Gas Based Power Plant or a Combined Cycle Gas Turbine Power Plant, frequently identified by the abbreviation CCGT, is essentially an electrical power plant in which a Gas Turbine and a Steam Turbine are used in combination to achieve greater efficiency than would be possible independently. The Gas Turbine drives on electrical Generator. The Gas Turbine exhaust is then used to produce steam in a heat exchanger (called a heat recovery steam generator) to supply a Steam Turbine whose output provides the means to generate more electricity. If the steam is used for heat (e.g. heating buildings), the unit would be called a cogeneration plant or a CHP (Combined Heat and Power) plant. Following figure is a simplified representation of a CCGT and shows it to be two heat engines coupled in series. The "upper" engine is the Gas Turbine. It expels heat as the input to the "lower" engine (the Steam Turbine). The Steam Turbine then rejects heat by means of a Steam Condenser.
The combined
cycle efficiency (ncc) can be derived fairly simply by the equation
ncc = nB + nR + -nB nR ……. In other words, the sum of the individual
efficiencies, minus their product. This remarkable equation gives insight
as to why CCGTs are so successful. Suppose nB = 40%, which is a reasonable
upper value for current high performance Brayton cycle Gas Turbines. A reasonable
value for a Rankine Cycle Steam Turbine operating at typical CCGT conditions
would be nR = 30% thus, the sum minus the product of the individual efficiencies
becomes
ncc
= 0.40 + 0.30 - 0.12
= 0.58
= 58%One
can see that the combined efficiency (ncc = 58%) is greater than the efficiency
of either of the component engines taken separately ……. 40%, 30%.
The value given by the equation, however, represents an upper limit on the
actual CCGT efficiency because there are losses in the system.
Actual
efficiency values as high as 52 - 58% have been attained with CCGT units during
the last few years. These units are particularly popular for gas turbine power
plants constructed in developing countries.
Control Systems
The Control System / Equipments is the nervous system of a Power Plant. Its tasks are the control and protection of the installation and data acquisition. It must provide assurance of safe and reliable operation. Because the standard Gas Turbine is supplied as a fully automated machine, the steam process should likewise be correspondingly automated so as to achieve a certain degree of standardization in operation of the plant as a whole, thereby reducing the risk of operator error. For this reason, the control and automation systems of a Gas based Combined Cycle Power Plants form a relatively complex system even though the process itself is fairly simple. Modern Combined-Cycle Plants generally have electronic Control Systems.
A hierarchic and decentralized structure for the open and closed loop control systems is best adopted to the logic of the whole process. It simplifies planning, makes it possible to commission the plant in stages, and raised the availability of the Power Station.
The Gas based Combined Cycle Power Plant instrumentation is categorized as closed-control loops and open-control loops.
The closed-loop controls for a power station can be grouped into
The open-loops are either the loops for indication or can be grouped as protection circuits. The protection circuits (interlocks) are the most important for any Gas based Combined Cycle Power Plant. The way the Combined Cycle Plant responds to the output power demand so is the emergencies of the Plant. As the Gas Turbines operate at very high speeds and uses normally high calorific value fuels, the safety standards have to be given a very high importance.
The Gas based Combined Cycle Plant instrumentation consists of mainly
a) distributed Control Systems for monitoring and control of Plant process parameters and also sometimes used for interlock logics (protection systems).
b) Safety shutdown systems like PLC's for any abnormal behaviour of Plant process parameters going beyond the extreme limits.
c) The instrumentation of a Combined Cycle Plant is broadly divided as shown in the figure.
Gas TurbineInstrumentationControls
Heat RecoveryBoiler Controls Steam Turbine Instrumentation Controls
GTG HRSG's STG
Balance ofPlantInstrumen-tation
DM PlantGas Treatment Plant Etc.
Gas Turbine Instrumentation
CONTROL CIRCUITS
1. Fuel Control: The fuel flow to the Gas Turbine is always determined by the lowest value of one of the following algorithms:
Data collected by the Control System for above controls are Turbine Speed, Compressor Discharge Pressure, Exhaust Temperature etc.
2. Inlet
Guide Vane Control
3. Steam/Fuel ratio Control for Nox reduction.
Protection Circuits
HEAT RECOVERY
STEAM GENERATOR (HRSG)CONTROLS
Control
Circuits
Protection Circuits
STEAM TURBINE GENERATOR CONTROLS
Control Circuits
Protection Circuits
Generators
AUTHOR INFORMATION
After Completing M.Sc. (Engineering Instrumentation) from Osmania University, started career as a Graduate Engineer Trainee (Instrumentation) at M/s. National Fertilisers Limited, Panipat. Held the position of NFL-Captive Power Project, Instrumentation incharge as Assistant Engineer and visited Japan for Inspection/Training. Thereafter resigned and joined M/s. Nagarjuna Fertilisers and Chemicals Limited, Kakinada as Engineer (Instrumentation) and roseup to the position of Manager-Department Head (Instrumentation). During the stay at NFCL, visited Germany, Italy and Netherlands as a part of training/Inspections. Presently working as Manager (Control & Instrumentation) for M/s. Spectrum Power Generation Limited, Kakinada.
He hold a regular membership for ISA, America (Instrument Society of America) and in course of carrying out M.S. in Control Systems Engineering from Okhlahama State University,USA. Back
Instrumentation Engineering in Agricultural Research and Environmental sciences, adaptation of technologies & practices in achieving sustained development
N S S Prasad and Y V Srirama
International Crops Research Institute for the Semi-Arid Tropics, Patancheru, 502 324, Andhra Pradesh, India.
Natural Resource management is an important issue that affects all sections of people and all sectors of economy. Everyone's livelihoods, well being and future prosperity depends directly and indirectly on natural resources (Paroda 2000). While adapting to improved cultural and crop management practices, land and water management systems coupled with Instrumentation engineering techniques in association with sensors/transducers and custom built instruments, we are able to monitor, evaluate and control different processes in micro and macro levels. Instrument serves as an extension of human faculties and in many cases enables a person to determine the value of an unknown quantity which unaided human faculties could not measure. An instrument, may be defined as a device for determining the value or magnitude of a quantity, variable or process. While adapting to new technologies and practices derived from Research in different spheres of science, we the Instrumentation engineers are supporting in enhancing and sustaining crop productivity, reducing soil degradation and increasing farmers' incomes through better management of natural resources such as land, water and environment. This is been coined as "Science with a Human face" by the renowned scientist Dr.William Dar, Director General of International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).
Water (Hydrology)
Hydrology is the science concerned with the study of the different forms of water, as they exist in the natural environment. Its central focus is the circulation and distribution of water as it is expressed by water balance and Hydrological cycle. Hydrology embraces not only the study of water quantity and movement but also the degree to which these are affected by man's activities, including deliberate management of water resources and the inadvertent effects of man on hydrological processes. It is often subdivided into physical and applied hydrology. Physical hydrology includes the detailed measurement and analysis of information on hydrological processes to improve understanding of the functioning of the hydrological system and also the refinement of statistical and mathematical methods of predicting and modeling these physical processes. Applied hydrology is concerned with the application of the understanding of hydrological processes to their modification and management. Water resources and pollution on the one hand and flooding and erosion on the other, are the chief concerns of the hydrologist.
Climate (Climatology)
Climate
physically limits the geographical extent of crop growth and largely determines
the level of crop yields. The primary climatic variables governing the geographic
distribution and productivity of agricultural crops are solar radiation, surface
air temperature and precipitation. Soil, nutrients, environmental variables
are important for plant growth, and are in turn influenced by climate, as
are infestations of the major crop pests. Crop responses to these environmental
factors are often interactive; for example, nutrient uptake is temperature
dependent, and soil moisture often governs crop responses to atmospheric evaporative
demand. Atmospheric carbon dioxide is an additional factor critical to crop
growth
(Rosenberg 1974). Weather parameters are monitored using various instruments/systems
for carrying out various farm activities as well as to know the potential
of an environment for agricultural production. To study climatic effects on
agricultural productivity, dynamic crop growth models are used to simulate
principal physiological, morphological, and physical processes involving the
transfer of energy and mass within the crop and between the crop and its environment.
In watersheds under rain-fed agriculture, use of such model results in prediction
of integrated crop performance under various climatic, soil and management
conditions.
With increasing population and decreasing farm size, pressure on land has been increasing to support the expanding population. In semi-arid areas, less fertile land under cultivation has not been able to feed the population resulting in both malnutrition and under nutrition. The rural livelihood systems in the semi-arid regions are increasingly becoming unsustainable. The combinations of crop and livestock enterprises based on agro-climatic conditions and resource endowments in different socio-economic groups contribute to livelihood sustainability in several ways and at the same time, it will also result in environment friendly management systems. For the majority of the world's farmers, self-reliance in food production depends on adapting technologies and germplasm to a wide range of poor production environments. Ultimately, farming communities hold the key to conservation and use of agricultural biodiversity, and food security for millions of the world's poor. Success depends on integrated approaches that combine the best of traditional knowledge and institutional technologies. This success is through technologies developed in ensuring conservation of bio-diversity of both plant and animal species, which are humane, gender sensitive, equitable, sustainable and environmentally friendly (Kurien 2000). Poverty, water and air pollution, soil degradation, extinctions of species, global warming and many other forms of environmental degradation have raised doubts about the wisdom of the pattern of development which is being currently pursued. The World Commission on Environment and Development hence stressed the importance of ensuring that today's economic progress is not at the expense of tomorrow's developmental prospects. Development, to lead to lasting benefits, must be sustainable over time. Sustainability has several dimensions - ecological, economic, social and cultural. In agriculture, concepts of ecological and economic sustainability have implications for those engaged in developing for farmers' packages of technology, services, training and public policies.
Sustainability can be achieved only through the application of the best in modern technologies, such as biotechnology, space technology, information technology embedded with instrumentation engineering and coupled with management technology, to promote the growth of conservation-based farming techniques. For this purpose there is need for appropriate research, extension and training efforts in instrumentation engineering which helps in monitoring different physical, chemical and biological processes in nature in micro (Ex. crop canopy temperature, transpiration from leaf, chlorophyll content, leaf lengh, soil moisture etc.) and macro level (Ex. Eavapotranspiration, photosynthesis, climate, runoff, crop cover, yield etc.), leading to cause and effect analysis. Recently, one of the post-graduate colleges of science (PGCS) of Osmania University and ICRISAT signed an agreement to collaborate on teaching a university degree course where in various scientific processes are explained with the help of appropriate instrumentation program (ICRISAT 2001).
Our aim should be to develop and disseminate "green" on environmentally friendly agricultural technologies which can help to optimise employment, income and agricultural productivity from units of land, water, energy, labour and credit (Swaminathan 1990).
Soil (Edaphic factors)
The physical environment of the soil plays an important role in crop production through its influence on soil physical, chemical, and biological processes. Soil structure has tremendous influence on a soil's physical environment. Some important processes influenced by changes in soil structure are infiltration, water storage, runoff, erosion, nutrient cycling, and soil floral and faunal activity. Many soil and crop management practices play a significant role in altering soil structure. Soil management aims at manipulating soil structure in such a way that the soil environment will provide optimum temperature, water, air, and nutrients for the growth of roots and beneficial soil organisms. Judicious soil management requires an understanding of the physical, chemical, and biological processes that influence soil structural stability. It also requires an understanding of how structural stability in turn affects processes involved in the water balance of the root zone, e.g., infiltration, percolation, evaporation, runoff, and erosion (Laryea et. al. 1997).
Productivity of soils can be much improved by the application of improved understanding of climatic constraints to develop effective soil and water management practices. Sustainability of agriculture in the future is linked to our ability to understand the limits imported by environmental constraints on soil productivity and appropriately manage these soils while ensuring long-term stability (Sivakumar et. al. 1992).
Soil properties are functions of various soil development processes over geological time and soil management practices during the recent history of land use. Soils vary according to their parent material, position in the landscape, variability of water and heat regimes, biological activity, and time over which various processes have acted on the parent material. In other words, soils in the field and their properties vary with time and space. Soil bodies and soil surfaces provide water, nutrients, and support for crop plants and a multitude of other flora and fauna. Interactions of edaphic, atmospheric, and other environmental elements influence the distribution of energy, precipitation and wind, which ultimately affect near-surface energy and hydrologic balances. Information on soil properties is needed to understand processes in agricultural activities and to apply such knowledge to solve practical problems in agricultural environmental management. Soil physical properties such as particle size, particle density, bulk density, soil pore, soil water, soil water potential, vapor potential of soil air, hydraulic conductivity, infiltration, air pressure, air permeability, gas diffusivity, soil heat, heat capacity, thermal conductivity, soil strength, aggregate stability, compaction and penetrability, consistency and trafficability; and soil chemical properties such as soil pH, fertility, nitrogen, phosphorus, potassium, organic matter, cation exchange capacity and electrical conductivity are monitored in research fields with the help of instruments attached with appropriate sensors/transducers.
Plants cover much of the earth's land surface, and they play a significant role in exchanges of heat, mass, and momentum between the surface and the atmosphere. Measurement of these exchanges on the local scale and proper understanding of their linkage to synoptic and global-scale phenomena requires measurements and modeling of vegetative properties and processes. As a consequence, it is important to develop instrumentation required to measure and monitor relevant plant properties, parameters and processes such as leaf extinction, leaf area, root length, leaf and canopy reflectance and transmittance, foliar temperature, chlorophyll content, leaf water potential, transpiration, photosynthesis, growth and yield, total dry matter accumulation and carbon allocation.
Agricultural meteorology deals in quantifying conditions in and above foliage, in the soil, and in the lower portion of the planetary boundary layer. It has a closer affinity to the interface with soil physics and plant science, higher resolution at the earth's surface (Unwin 1980). The rapid advances in microelectronics and microcomputing have a salutary effect on agricultural meteorological measuring instruments. The advances in measurement technology and the need to critically plan a measuring instrument emphasize the need for training in the measurement of environmental variables. Agricultural meteorologists are seldom involved in selection of materials to be used in sensors or in the development of new sensors. In today's environment, that work usually falls to instrumentation engineers with science background (Griffiths 1994). The role of the instrumentation engineer is to select sensors and sensor interfaces that will produce data of sufficient accuracy and relevance to meet the goals of the experimental design. New techniques are used to improve the reliability and relevance of measurements (Massey 1986). Proper instruments along with sensors having known accuracy, precision, linearity, range, repeatability, resolution, sensitivity, stability, hysteresis, offset and drift are used to monitor different scientific processes/parameters (Monteith 1972). They are: Soil characteristics (physical, chemical and biological), soil heat flux, soil water interactions, soil water balance, runoff and soil loss; Global Solar radiation, sunshine duration, light quality, diffused radiation, radiation balance, radiation use efficiency, photosynthesis, reflectivity (Albedo), net radiation, and relative humidity ; Rain distribution, dew, evaporation, transpiration, evapotranspiration, water use efficiency, water balance and deep drainage; Wind, turbulent transfer, and wind direction.
FESP at ICRISAT has designed and fabricated many instruments/systems utilizing various types of transducers and sensors viz, strain gauge , semiconductor, piezo-resistive, piezo-electric, ultrasonic, resistance, capacitance, thermal, acoustic, fibre-optic, optical, electro-mechanical, biological and radioactive to monitor and quantify different processes/phenomena in Systeme' International (SI) units in agricultural research activities. Some of the instruments/systems are:
Geographic information system (GIS) is used as one of the decision support tool while using the data collected by the efficient instrumentation in solving increasingly complex urban and environmental problems, forestry and wildlife tracking, wasteland developments, agriculture and groundwater resource exploration, water resource management, site suitability analysis and demographic studies. Modeling, programming, and interfacing models with GIS provide vital research tools. Integration of GIS and remote sensing supported with specific instrumentation favor better natural resources and environmental management.
In the coming years, as perceived by eminent engineer Dr.K.L.Rao, conservation and appropriate use of natural resources would be a key in sustainable development. Ecologists, Agrometeorologists and Instrumentation engineers as a team should be empowered to play an active role in eco-technological innovations and introduction of improved practices for sustainable agriculture (Fritschen and Gay 1979). Through efficient and effective instrumentation, different processes in the nature are monitored, and analyzed for cause and effect analysis. This in turn systematizes the best of farmers' practices, NGO-led innovations and practical research that emphasize locally available resources, crop diversification, animal integration, natural plant protection and systems of soil, water and genetic resource conservation.
In spite of the political will and public understanding generated at Rio de Janeiro in 1992, the world is still witnessing continued damage to basic life support systems of land, water, forests, biodiversity, oceans and the atmosphere. It is our considered view that the Instrumentation engineer community can effectively undergird a science-led approach to solve many of the inter-linking natural resource related problems basic to life support systems.
References:
1. Fritschen,
L.J., and Gay, L.W. 1979. Environmental Instrumentation. Published by Springer-Verlag,
New York, U.S.A.
2. Griffiths, John F. 1994. Editor 'Handbook of Agricultural Meteorology',
published by Oxford University Press.
3. ICRISAT Happenings. Number 942, 27th April 2001, ICRISAT, Patancheru, India.
4. Kurien, V. 2000. International Symposium on 'Value based Crop-Livestock
production systems for the future in Semi-Arid Tropics' on 14th November 2000
at ICRISAT, Patancheru,India.
5. Laryea, K.B., Pathak, P., and Katyal, J.C. 1997. Editors of technical manual
no. 3, 1997, ICRISAT titled 'Measuring soil processes in agricultural research'.
6. Massey, B.S. 1986. Measures in science and engineering: their expression,
relation and interpretation. Publisherd by Ellis Horwood Limited, U.K.
7. Monteith, J.L. 1972. Survey of instruments for micrometeorology. Oxford
Blackwell Publishers, U.K.
8. Paroda, R.S. 1998. Forward in 'Fifty years of natural resource management
research in India' an assessment & management report edited by G.B.Singh
and B.R.Sharma and published by CSSRI, Karnal, India.
9. Rosenberg, N.J. 1974. Microclimate: 'The biological environment' published
by Wiley-Interscience.
10. Sivakumar, M.V.K., Manu, A., Virmani, S.M., and Kanemasu, E.T. 1992. Myths
and Science of Soils of the Tropics. SSSA Special Publication no.29, Chapter
6., pp 91-119.
11. Swaminathan, M.S. 1990. Key note address in an international seminar on
'Sustainable land use systems' , New Delhi, 12th Februrary.
12. Unwin, D.M. 1980. Microclimate measurement for ecologists. Biological
techniques series published by Academic press. Back