Cost Effective Strategy
Implementation of efficiency enhancement
schemes in captive power plants in India
This article discusses the approach, methodology of the energy efficiency enhancement and benchmarking study conducted in the coal based power plants '<100 MW' in India and highlights the result. Benchmarking analysis with energy performance in the industry sector level have been performed in the past, but some recent studies on power plant equipment performance and benchmarking are also available.
- Dr Debashis Pramanik
Furthermore benchmarks for 80-100 MW range power plants (including captive power units) are not available. The median size of individual power generating unit is 100 MW.
Following steps are followed to benchmark the figures on boiler efficiency/ η, specific coal/ energy consumption (SCC/ SEC for equipment) in a power plant:
- Collection of design data and selective test reports/ characteristics curves
- Extensive field measurement of operating parameter/ data using the portable or on-line instruments/ data logger
- Analysis of data and performance (η, SEC/ SCC) evaluation of the boiler and other equipment
- Comparison of operating parameter and design data
- Comparison of analysed data with the design performance data
- Selection of most consistent benchmark level in boiler/ equipment and evaluation of energy saving possibility
- Consultation of energy audit reports of other equivalent capacity power plants
- Comparison of power plant parameter/ data with the data of other power plants.
The efficiency of the boiler and turbine has a marked effect on Auxiliary Power Consumption. Heat consumption reduces with an increase in boiler efficiency. Some cases the power plant equipment revamping is done for 5-10% additional power generation and improve the performance level. The study covering 6-7 states reveals the components/ systems where modifications have to be made to conserve energy. It presents generic recommendations to be implemented to make power plant energy efficient. The generation rate would be improved by adopting these measures. The benchmarks developed based on such equipment basis data are considered to have acceptable accuracy and reliability.Control of energy related parameters and their benchmarking enable the system/ equipment/ plant operation at highest efficiency.
The operating range of specific coal consumption (SCC), heat rate (HR), APC in some of the 80-100 MW thermal power plants is 0.64-0.85 kg/ kWh, 2150-2700 kCal/ kWh and 9.12-12.27% respectively. The present focus of power plants is to achieve improvement in operating HR by about 10-12% and APC to 7.5-8.5% respectively. The design SCC, HR is in the range of 0.77-1.03 kg/ kWh and 2330-2520 kCal/kWh respectively. The operating HR is either equivalent (close level) to or higher than the operating HR for northern region (weighted average opn. HR 2603.2 kCal/ kWh) 110-500 MW power stations also. The heat rate of 14 Indian power plants (Central Electricity Authority document source) & world’s 9 power plants (International Energy Agency document source) is between 2279-2393 kCal/ kWh and 2261-3607 kCal/kWh respectively. However, the DTPS, Dahanu HR is observed to be lowest (2261 kCal/ kWh in 2004/05) throughout all the previous years. Now, the DTPS is the first ever utility in the World to be certified for ISO 50001:2011 for its strong commitment for energy use and conservation.
The present methodology with CEA/ Central Electricity Regulatory Commission (CERC) guideline is acceptable/ adopted to most of the public/ private sector efficient power plants and norms are being met by them. The norms must, therefore, reflect higher levels of efficiency to induce a sense of inter-plants competition and promote efficient operation. Essentially, a World Class Energy Efficient power generating unit will be operating with the world’s lowest SEC/ SCC (electrical & thermal) for similar power plants. More efficient operation would also lead to less CO2 emission which is the current focus of global efforts for lower GHG emissions.
The worldwide highly competitive energy market is demanding the power generation companies/ plants for a critical look on their business policy. Re-orientation of the thermal power plants/ captive power sub-sector has progressed and has put pressure on heat and power production cost with the additional power generation from the existing systems with some modification possible or sale the surplus electricity. Nowadays, power plants (PPs) generally operate as commercial enterprises/ units that need to create a profit. This means that a positive margin should result from the gap between the benefits from the electricity despatched/ sold on one hand and the expenses for running the power plant on the other. In this context, many PPs and captive power plants (CPPs) have been taking up several projects/ programs on efficiency and performance improvement (including strategic review for cost reduction and benchmarking).
In today’s generation business, optimizing the operation of boiler, steam turbine and other equipment is no longer a goal but, rather, a necessity for power producers to remain competitive. The benchmarking also plays a role in PP regulation in several jurisdictions. It is undertaken by the competitive PPs/ utilities/ regulators for improving operational efficiency and cost control/cutting. It means technically the cost efficient power and heat generation. In many CPPs, steam extractions during turbine operation are taken in the production process stages/ equipment. The aim of steam turbine and turbine generator (TG) design is an optimum efficiency operation characterizing an optimal energy conversion. Moreover, the operational performance of modern generating equipment has to match the rated value, under almost all circumstances. Thus any improvement in boiler, turbine & associated power generating equipment/ system, however slight, can increase power availability, decrease equipment/ component operating costs, and generate significant cost savings. Consequently, keeping a PP in optimum condition requires the professional maintenance & operation approach.
The term "Benchmark" originally meant a surveyor's mark-out in a target used as a reference point. It has come to mean any parameter taken as a point of reference or comparison. Benchmarking of PP/ CPP involves development of essential technical and organizational elements for the long-term commercial sustainability of the plant operation. "Benchmarking" provides a chance to compare a generation unit at maximum/ full (or beyond peak level) load with another or a "Best Practice"- PP / unit or CPP/system, to find out its own indicators and ranking in a competitive market. The own position in the competitive market will be given by means of indicators, which will be defined at the beginning of the process. In today's business environment, it is recognised as an effective approach towards improvement in efficiency, productivity, quality and other dimensions of performance that are determinants of competitiveness.
The generating capacity in India can be divided into utility power sector and captive power sector. The utility sector capacity is 170 GW of which nearly 50% comes from the state share. The private sector share which is presently 20% is like to go up in near future because of the active major dependence on coal based generation (about 54%). Individual CPPs have sizes in the range of 1 MW to 1 GW with the median size of individual unit of 100 MW. Power generation in CPPs would be either from external fuels like coal or from process generated fuels like bagasse or rice husk. Following subjects are preconditions in thermal power plants: Low operation and maintenance cost, High technical efficiency in PP operation, Effective management organization, and Compliance with surrounding regulations.
Basis of Energy Efficiency Benchmarking
The PP performance can be expressed through some common factors as heat rate (HR/ energy efficiency); thermal efficiency; capacity factor; load factor; economic efficiency and operational efficiency. HR is an important index for efficiency assessment of a PP. The PP efficiency mainly centers around temperature (heat derived through coal combustion) & pressure (steam derived through water) optimization for a given technology. Superior technologies provide better efficiency over subcritical pulverized coal power generation.
Unit operation efficiency (or HR) is a function of unit technology, equipment, design, size, capacity factor, fuel quality fired, maintenance condition of the unit, and operating and ambient conditions. As is the case with optimizing the plant HR at peak/ maximum load, enhancing the PP output can be best approached by balancing boiler and steam turbine performance. The factors that affect operating HR and are beyond the control of PPs are coal quality, grid frequency, cooling water temperature and unit loading or dispatch instructions. The detailed energy performance (DEP) analysis in PP is the first step for the study of efficiency and equipment performance. Attempt is made to explore the best operating PPs equipment performance, HR and compare with HR of the target plant. Consequent to review of performance of all the equipment, energy benchmarking is undertaken.
In order to be useful in its underlying aim of encouraging best practice in equipment efficient performance, fuel use, power generation and understanding the potential for further improvement, an agreed view of participating PPs on best practice performance and maximum generation is required by the plants, to become one of the 10-15 most efficient plants in the country or among the top 20% of the most energy-efficient PPs in the world. After the thorough study, plant management is normally given the key data (including specific energy consumption/ SEC) of a typical/ reference PP/ CPP using similar capacity range boilers, turbines and associated equipment. The best performance key data of any efficiently operating typical, reference plant is also used as comparative levels & benchmarks, providing a basis for units/ plants to consult and determine appropriate future development strategies that consider the plant level constraints.
Separate benchmarking indicators are calculated for energy efficiency of PP equipment, based on Weighted Average Energy Efficiencies. It can be categorised in many different ways, according to various criteria:
- Result benchmarking (inter plant comparison)
- Product benchmarking (quality improving),
- Process benchmarking, and
- Strategic benchmarking (Business processes).
Statistical benchmarking has in recent years become an accepted tool in the PP/ CPP performance assessment. Energy conservation (ECS) through benchmarking can be broadly categorised as "process benchmarking". ECS measures for a better economic efficiency of the PP focused on, are to be derived from the results.
Power generating process benchmarking involves the following basic steps:
- Identification of the equipment/ control mechanism for the plant/ unit
- Collection of information to thoroughly understand the process and identification of key/ controlling parameters
- Determination of energy performance
- Analysis of the gap between the existing and the bench marked efficiency levels.
Benchmarking of EC internally (trend analysis) and externally (across similar, equivalent generating units) are two powerful tools for performance assessment and logical evolution of avenues for improvement. Historical data well documented helps to bring out EC and month-wise / day-wise cost trends. The analysis of EC, cost, relevant generation systems/ features, SEC, help to understand effects of capacity utilization on energy use efficiency and costs in regular basis.
External benchmarking relates to inter-unit comparison across a group of similar units. However, it would be important to ascertain similarities, as otherwise findings can not be appropriate. Few comparative factors, which need to be looked into while benchmarking externally are: Scale of operation, Vintage of technology, Fuel specifications and quality, and Steam supply configuration and quality.
Energy performance benchmarking permits the following parameters.
- Quantification of fixed and variable EC trends vis-à-vis generation levels
- Comparison of Penergy performance (PPEP) with respect to various generation levels (capacity utilization)
- Identification of best practices (based on the external benchmarking data)
- Scope and margin available for EC and cost reduction
- Basis for monitoring and target setting exercises.
The benchmark parameters can be:
- Electricity generation related e.g. % thermal efficiency of a boiler e.g. Sp. Coal consumption (kg/kWh) e.g. Auxiliary power consumption (%) e.g. % cooling tower effectiveness in a cooling tower e.g. kWh/Nm3 of compressed air generated.
While such benchmarks are referred to, related crucial process parameters need mentioning for meaningful comparison among participating PPs. For instance, in the above case:
- For a CPP – fuel quality, type, steam pressure, temperature, flow, are useful comparators alongside thermal efficiency and more importantly, whether thermal efficiency is on GCV/ NCV basis or whether the computation is by direct method or indirect heat loss method, may mean a lot in benchmarking exercise for meaningful comparison.
- Cooling tower effectiveness – ambient air wet/dry bulb temperature, relative humidity, air and circulating water flows are required to be reported to make meaningful sense.
- Compressed air specific power consumption – is to be compared at similar inlet air temperature and pressure of generation.
Equipment Energy Performance and Efficiency Degradation
PPEP is the measure of whether a PP equipment is now using more or less energy to generate electricity than it did in the past. It is a measure of how well the energy management programme is doing. It indicates the year-wise change in EC considering generation output. Power plant performance monitoring compares plant energy use at a reference year with the subsequent years to determine the improvement achieved. However, the PP’s year-wise generation output would vary and the output has a significant bearing on its energy use. For a meaningful comparison, it is necessary to determine the energy that would have been required to produce this year production output, if the PP had operated in the same way as it did during the reference year. This calculated performance level/ value can then be compared with the actual value to determine the improvement or deterioration that has taken place since the reference year based on the available documented data from plant.
Following equipment tests are conducted in the energy performance/ audit study in PPs/ CPPs:
- Boiler efficiency test
- Air heater leakage test
- Mill performance test
- Furnace efficiency test
- Turbine performance test
- Condenser performance test
- HP cylinder efficiency test
- Regenerative system performance test
- Lighting illumination test
- DM water flow study
- Fans and pumps (BFP, CEP & CWP) performance test
- Tests in air compressors & compressed air distribution systems
- Ash / coal handling plant study (optimum coal path)
- Cooling tower performance test
- Transformers/ motors performance test
- Test of electro-static precipitator (ESP).
The efficiency of the boiler and turbine has a marked effect on auxiliary power consumption (APC). Heat consumption reduces with an increase in boiler efficiency. The benchmarks developed based on such equipment-wise data are considered to have acceptable accuracy & reliability. However, the efficiency degradation of the equipment/ plant is a normal process, but a continuous challenge to minimise the degree of degradation. State of the art of power plant/ equipment operation and maintenance is to keep the degradation level between the ranges of +/- 0.1% net efficiency. This impressive figure is the outcome of a well equipped energy efficient plant in developed country in terms of monitoring and maintenance system. The backbone of these systems is a state of the art process control and instrumentation system allowing the record of any relevant data.
The targeted system indicators are heat transfer decrease in the boiler caused by ash deposits, condenser caused by fouling effects,air control of the boiler system, avoidance of corrosion in the boiler and pipe system by the well suited operation of the ESP and turbine system – vibration and frequency control.
Thermal Power Plants Up-gradation and Benchmarking
The weighted average efficiency of the coal fired PPs is about 35%, against 45% for natural gas and 38% for oil fired. The greatest attention has been steadily changing the HR and total operating cost in PPs/ CPPs.
Following are the key points attended by many PPs management for improving HR:
- Preparation and execution of the detailed plan listing specific activities those are to be undertaken to make improvements every year, & review it periodically (quarterly) to ensure that the plan/ scheme is implemented.
- Heat rate improvement is a continuing process in the PP. It must become a part of the normal work activities. It must be considered along with, and at par with, reliability, safety, environmental concern, etc, when operating the unit, scheduling maintenance, and all other routine activities.
The present focus of PPs is to achieve improvement in operating HR by about 10-12% and APC to 7.5-8.5% respectively. However, the typical thirteen (1977-1990) years historical trend of design HR in Indian power plants is also given in the Table 1.
It has been also observed that the APC on % basis would be high, when the PPs generation is on the lower side. Thus it can be reduced simply by generating at maximum power and beyond the peak level up to the permissible level. Low load operation is also avoided as the efficiency of main equipment and associated auxiliaries undergo variations, vibration increase.
Many PPs have been revamping the steam turbine (its earlier capacity and steam flow regulation), addition of 1st stage 5% generation & 2nd another 4-5% by increasing the steam input to the turbine. Addition of power generation required existing valves new opening/ level with manufacturers/ suppliers assistance for additional/ extra amount of HP steam flow with the existing turbine rating. Overall efficiency of a steam turbine and PP, however, strongly depends on the turbines’ performance. Conventionally, the design, operation limit of the turbine was basically efficiency and reliability centric investment and SEC/ benchmarking was not the top most priority.
Generally, the turbines design margin remains unused with the test for performance guarantee (PG) and Boiler Maximum Continuous Rating (BMCR) before revamping. First level key changes are the requirement for greater heat input and regulatory pressure to maintain or optimize emissions (within limits). These issues are economically analysed and addressed through improved performance of the existing fans and fuel preparation systems. In the boiler next change from the normal operation is also typically encountered. Reheat changes are more complex and typically would have involved new tube surface to increase heat absorption with minimal effect on system pressure drop. In most cases, electrical generators also has sufficient design margin to accommodate the extra MW generation. The methods of increasing the capacity by fitting a new design of coil to remove the extra heat generated. Similarly the generator transformer also has adequate design margin to accommodate a minor increase in output.
Numerous other measures to improve efficiency and generation cost is also to be described,evaluated, compared and finally best options need to be introduced. The total cost is divided into different groups of cost and then analyzed and compared with other, similar PPs/ CPPs. This gives the advantage to the PPs to judge and benchmark key indicators versus the background of practical operations and approach to become an energy efficient plant. Essentially, a World Class Energy Efficient power plant/unit will be operating with the world’s lowest SEC (electrical & thermal) for similar PPs.
A World Class Energy Efficient Power Generating Unit is:
- A trend setter in SEC norms – the lowest in the world for similar industries
- A leader in implementing the latest technologies
- Unit has practically “nil” Energy Wastage
- Unit has made ENCON an “On-going activity” and incorporated as a part of the management system.
Benchmarking Methodology and Guideline/ Norm
Benchmarking process uses the design characteristics and operational factors of the target unit equipment group as its starting point. The methodology involves the analysis of equipment monitored/ collected data and discussion with the plant personnel in PP followed by the consultation of the following documents published by Central Electricity Authority (CEA) & Central Electricity Regulatory Commission (CERC) to explore the best performing plants benchmark data/ level:
- Performance Review of Thermal Power Stations/ Plants 2007-2008
- Recommendations on Operation Norms for Thermal Power Stations/ Plants for Tariff Period beginning 1stApril 2009.
For the energy performance evaluation, the efficiency of individual equipment is calculated, which broadly covers the energy efficiency comparison of operating and design/ guideline.
In order to supplement the plant data monitored and information collected (including design/ logbook data) on the equipment, calculated efficiency and SEC, the CEA and CERC documents are consulted after the site measurements. This methodology with guideline is acceptable/ adopted to most of the PPs and norms are being met by them.
Collected data/ information and characteristics curves would cover various aspects related to steam generation at steady load, turbine loading and power generation, their fluctuation, associated equipment performance and maintenance quality, reliability besides seeking information on energy loss incurred due to efficiency related problems. The plant information/ data is broadly divided in four different sections, viz.
- general information (equipment used, production, energy cost/ consumption, process, energy/ fuel usage);
- details on equipment peak load performance and its impact on the PP;
- problems due to maintenance/ quality, reliability and its impacts on plant and generation; and
- issues related to coal handling/ feeding.
The CEA document based available data/ information would be divided in four
- all India WA design heat rate (ADHR) and average operating HR (AOHR);
- region-wise WA of design HR (RDHR) and region-wise operating HR (ROHR); and
- most efficiently operating power plants/ units WA design HR (EDHR) and operating HR (EOHR).
The CERC document based data/ information would be categorized for the prevailing norms and “Normative value for units of equivalent capacity and above 110 MW units”. The tables are mentioning the data/ practices of coal fired PPs, which are always in line and maintain the trend for wide capacity range.
It is pertinent to note that the issues related to quantity/ supply of coal available to the Indian power industry is not directly related to quality of power, but poor coal quality does impact the generation efficiency of boilers, steam quality and associated power plant equipment.
Norms specifying approach of CERC
Utilities performance benchmarking is facilitated by the extensive data that they report to regulating commission. Possible approaches for specifying operation norms could be
- Uniform single value norm for all stations
- Norm in terms of % of design value.
Uniform single value norm for all power plants
CERC has prescribed the single value norms for power station/ plant HR and APC. The single value may be expressed as either as absolute number as done in case of station HR or as a % as done for APC. Such norms are appropriate for APC data which do not vary significantly with the unit size or other technological parameters. However, the single value concept has limitations when applied to operating parameters like the unit HR.
A large variation in HR exists due to different equipment design, steam parameters, and design fuel quality etc. Thus, considerations are invariably required to be made to accommodate the worst combinations of turbine cycle HR, boiler efficiency. This leads to considerable variation in the margin available to different PPs between the operating HR and design HR. Even for same unit size & steam parameters, the HRs vary due to improvements affected by the suppliers progress over time and therefore, considerable HR variations offered by different manufacturers for same unit size/ steam parameters.
Also even with the same TG, the unit HR could vary significantly at two different sites due to large variations in coal quality, cooling water temperature, etc. Thus even with the same equipment efficiency, a PP/ CPP could have considerably higher design HR due to site specific factors beyond his control and the normative HR based on single value concept would provide much lower operational margin to such a plant.
This concept provides very high cushion for operational variation (or to high savings to units with lower design HRs) and leads to undue penalty to those with higher design HRs which could be for reasons beyond the plant/ utility control, like coal quality and cooling water temperature. Thus, instead of rewarding operational efficiency, which should be the aim of any good benchmarking exercise, it rewards better designs or better site inputs where the operator reaps the benefits of intrinsic advantages of the equipment or site environment or coal quality without major operational efforts. However, this approach provides incentive to the PP management to go for more efficient design and technologies which may result in higher capital cost. In the cost plus approach this would result in higher fixed charges for such PPs which would be passed on to the beneficiaries of the project. However, the benefit of higher efficiency in operation may not be passed on to the beneficiaries and may be retained by the management.
Norms in terms of % of design value
The other approach could be to specify the normative parameter as a certain percentage above the design parameters of the unit. The design HR indicates the intrinsic capability or the best achievable efficiency of any generating unit. Such an approach automatically provides for consideration of variations in design/technology, ambient conditions and fuel quality in the norms and thus provides more rational basis for operation norms specially in the developing scenario with large variations in design of the PPs/ units. It also provides for incentive to the plant management members to achieve better operational efficiencies. However, in this approach there is no incentive to the management to adopt more efficient designs/technologies as the entire benefit of having more efficient designs/technologies is passed on to the beneficiaries. Thus, it is suggested that while single value approach may be continued for specifying norms for APC, the % over design approach may be followed for specifying Unit HR with some benchmark values for different unit sizes to ensure minimum efficiency standards in the future units by the management.
Recommended normative HR and APC
There is a strong case for change over from the conventional single value norm system for PPsHR to % margin over the design HR system with a view to accommodate large range of unit HRs likely to be seen in future for reasons discussed in the CERC document. Thus, the normative HR would remain at the low level with the operating margin practically available, notwithstanding numerous technological developments in equipment design & operation.
The present norms of APC are comfortable being met by most of the PPs. Further, there seems to be no major technical development in recent past leading to significant lowering of APC barring minor/ slight reduction in APC for units using superior/ imported coal. However, most of the stations with 200-500 MW rating have shown appreciably low power consumption as compared to the normative APC and PPs and with 100-200 MW rating shown slightly high consumption as compared to the normative APC.
Degree of accuracy of benchmarking
- Each PP adopts packages for equipment procurement based on prevailing conditions and considers the package and procedure most suited for the project.
- Degree of accuracy and benchmarks reliability rests on data relied upon and stage-wise methodology followed.
- Data relied upon is from the sources of central technical office and division/ state power plants/ units.
- Data, documents, records and registers available with the above PPs/ CPPs/utilities are maintained as per applicable rules, regulations, accounting standards and are subject to audit as per those rules and regulations.
- The benchmarks developed based on such available data are considered to have acceptable accuracy and reliability.
Energy benchmarking and effect of variable/ over load on equipment design/ operating performance in power plant
The time series data on generation and coal quality of the individual PP is consulted. Moreover, the personnel indicate that weather the plant had undergone through the fluctuation of power generation load. In each PP, the turbine is to generate base load electricity, operate with over capacity due to high and fluctuating/ de-rated load or steam generation in the complex. Therefore, the design and operating performance of all the equipment get severely effected due to variable load and benchmark data happened not to be matched with the occasionally minimum achieved level.
The benchmarking data (including SEC) of a typical coal fired 100 MW PP’s boiler efficiency, turbine HR, condenser/ cooling tower effectiveness, BFP/ CEP/ CW/ ACW pumps efficiency, forced draft (FD)/ primary air (PA)/ induced draft (ID) fans efficiency, reciprocating compressors free air delivery & ESP efficiency are given in the Table 2. At present the operating range of specific coal consumption (SCC), HR, APC in some of the 80-100 MW thermal power plants is observed to be 0.64-0.85 kg/ kWh, 2150-2165-2610-2700 kCal/ kWh and 9.12-12.27% respectively. The design SCC, HR is in the range of 0.77-1.03 kg/ kWh and 2330-2520 kCal/ kWh respectively. TERI 2011, TERI 2010, TERI 2009 Report on “Energy Performance and Benchmarking Study” in 4 states is the source of this data. The operating HR is either equivalent (close level) to or higher than the operating HR for northern region (WA operating HR 2603.2 kCal/ kWh) 110-500 MW power stations/ plants also.
The public/ private/ Govt. industry sector and Bureau of Energy Efficiency (BEE) in India would jointly work out on the capacity-wise applicability and compile the successful case studies of energy efficient motors (about 100 models), boilers, fans and other equipment in power plants, update the India’s energy efficient products directory and recommend for the extensive use.
The characteristics and method of use of large/ medium size boiler (including FBC type) and large size steam turbine is largely influenced by the extent of steam over load (above design level) on the generation load in PP. Most of the time, the steam load on the turbine would be higher (above design level) or increasing.
Effect of variable over load on power plant equipment design performance
In the present situation, the governor comes into action operating a steam valve and increases steam flow (admitting above the peak level) to turbine and increase the turbine speed well above the normal level. The governor response from the load to turbine is quite prompt, but further above the optimum point, the governing response would be slower. The reason is explained as given below:
- In most automatic combustion control systems, steam pressure variation is the primary signal used. The boiler must operate with unbalance between heat transfer and steam demand long enough to suffer a slight but definite increase in steam pressure. The controller must then increase fuel, air and water flow in the proper amount. This will affect the operation of practically every component of auxiliary equipment. Thus, there is a certain time lag element present in combustion control. Due to this, the control components should be of most efficient design so that they are quick to cope with the variable demand around peak load.
- The peak level/ progressively fluctuating generation load results in higher (above peak level) and fluctuating steam demand. Due to this it becomes, very difficult to secure excellent combustion since it requires the co-ordination of so many various services/ activities. However, the efficient combustion is readily attained under steady steaming conditions.
- The higher level variable load requirements also modify the operating characteristics built into equipment. Due to non-steady overload on the plant, the overloaded equipment cannot operate smoothly at the designed load points. Hence for the equipment, a flat-topped load efficiency curve is more desirable than a peaked one.
- Regarding the retrofitted PPs, since their number and sizes are selected to fit a known or a correctly predicted load curve, therefore, it would be possible to operate them at or near the point of maximum efficiency (considering revamped condition). However, to follow the variable load curve very closely, the total PP capacity is usually sub-divided into several PPs of same/ different sizes. Many times, the total PP capacity would more nearly coincide with the variable load curve, if some similar/ higher size units (boiler, TG) are employed than a few units of slightly smaller unit size. Also, it will be possible to load the similar/revamped units somewhere near their most efficient operating points. However, the same size/ duplicate units and equally balanced steam load may not fit the load curve as closely as units of unequal power generation capacities or unbalanced steam load. If identical units are installed, there is a benefit in the operational advantage at maximum/ optimum efficiency level and saving in first cost because of the duplication of sizes/ pipes dimensions, foundations, wires insulations etc, and also because spare parts required are less.
When the load on the PP decreases, the situation would increase the rotational speed of the TG. The governor would come into action operating a steam valve & restricting steam and decreasing the turbine speed to its higher value. In non-revamped units, the governor response from the load to turbine is quite prompt, but after this point (beyond maximum level), the governing response would be quite slower.
Effect of variable over load on power plant equipment operating performance
In addition to the effect of progressively higher/ variable load on PP design performance level, the higher/ variable load conditions impose operation problems also, when the PP is stabilized and continuously operated. In a PP, the variable load on electric generation plant ultimately gets reflected on the variable steam demand on the boiler and on electricity consumption in other auxiliary equipment. The operation characteristics of such equipment are not linear with load, so, their operation becomes quite complicated. As the load on electrical supply systems fluctuate, a number of PPs are interconnected to meet the load. The load would be divided among various power generating units to achieve the utmost economy in the whole system. When the system consists of one base load plant and one or more peak load plants, the load in excess of base load plant capacity is dispatched to the best peak system, all of which are nearly equally efficient, the best load distribution needs detailed study.
Optimum operating efficiency, generic ECMs & implementing criteria
Power plant optimum operational efficiency is dependent on the following:
- Proper air and fuel distribution, temperature and mixing with O2 (air) and adequate fuel quantity and velocity, i.e. 3 Ts (time, temperature & turbulence) help swings,
- Use of low-excess air burners to save in NOx and unburnt O2 and optimum heat recovery in the air preheater & economizer, as a 4.5OC rise in flue gas temperature would reduce boiler efficiency by about 1%,
- Minimization of unburnt carbon loss in flue gas and bottom ash by a stipulated fineness of coal, which is not to be off-set by “coal mill/ handling unit & fan power” consumption
- Minimization of radiation and unaccounted loss like casing radiation, sensible heat in refuse, ash hopper evaporation & bottom water seal evaporation.
Following are the basis for computing the energy efficiency schemes implementation and benchmark level.
- Estimates are based on design/ rated operating parameter and EC level.
- Payback period 3 years for cost benefit analysis.
Following generic Energy Conservation Measures are recommended to improve the operational efficiency of systems in the CPP.
Excess air reduction
The amount of excess air used depends on the type of boiler and on the nature/quality of the fuel. Typically, 12-20 % excess air is used for a pulverised coal-fired boiler with a dry bottom. For reasons of combustion quality (related to CO and unburned carbon formation), and for corrosion and safety reasons (e.g. risk of explosion in the boiler) it is often not possible to reduce the excess air levels further.
Flue gas temperature reduction
The flue-gas temperature leaving the boiler (depends on the fuel type) is traditionally between 120-135OC, due to risks of acid corrosion by the condensation of sulphuric acid. However, some designs sometimes incorporate a second stage of air heaters to lower this temperature below 100OC, but with special claddings on the air heater and the stack, which makes this reduction economically unprofitable.
Optimum level combustion
It is not possible to obtain an ideal mix between the fuel/ coal and air, and therefore, more air than is necessary for stoichiometric combustion is supplied to the boiler. Furthermore, a small percentage of the fuel does not fully combust. The flue-gas temperature must be kept high enough to prevent condensation of acid substances on the heating surfaces.
Reduction in unburnt carbon in ash
Optimization of the combustion leads to less unburned carbon-in-ash. Increased unburned carbon could also worsen and harm the quality of the fly ash and make it difficult, with the risk that they may not comply with the relevant national standards.
Minimization of vacuum in condenser
After leaving the low-pressure section of the steam turbine, the steam is condensed in surface condensers and the heat released into the cooling water. In order to ensure the maximum pressure drop over the steam turbines, it is desirable to reduce the vacuum to a minimum. In general, the temperature of the cooling water dictates the vacuum, which is lower with once-through cooling systems than with a cooling tower. Installation of properly designed inserts also on the tube side (i.e. water side) of an existing steam condenser sometime increases the turbine’s power output between 0.5-2.0%. The best electrical efficiency is possible by water-cooling and a condenser-pressure at optimum level. The thermal energy saving thumb rule for a PP (up to 100 MW) is that the saving of 0.5 kCal/ kWh is achievable for every 1 mm Hg vacuum reduction in condenser.
The dry cooling air cooled steam condenser (ACSC) system has made it possible to build the few CPPs without adequate cooling water resources also in some of the northern states.For dry cooling in ACSC, the sensible heat transfer is the only form of heat rejection, so performance depends on the ambient dry-bulb temperature (DBT). For ACSC’s optimum performance, power generation, air flow through fans and initial temperature difference (ITD, difference between condensing steam temperature and inlet air temperature/ DBT) across modules should be as high and low as possible and pressure drop should be as low as possible.
Variable pressure and fixed pressure operation
In fixed pressure operations, the pressure before the turbines at all load levels is kept more or less constant by changes in the flow cross-section at the turbine inlet. In variable pressure operations with the turbine inlet cross-section at its maximum, the power output is regulated by changes in the pressure before the turbines.
Condensate and feed water preheating
The condensate coming out of the condenser and the boiler feed-water are heated by steam to just under the saturation temperature of the extracted steam. The thermal energy from the condensing process thus feeds back into the system, reducing the amount of heat otherwise released from the condenser, therefore improving the efficiency.
Replacing high speed impactor/ ring granulator with a pair of low speed roll crusher
Installation of a pair of low speed roll crushers (primary & secondary) in series replacing one high speed impactor/ ring granulator crusher would result in reduced combustibles in fly ash (or coal fines generation) in coal handling plant (CHP). Installation of the low speed roll crushers result in reducing fines generation thus improves crusher efficiency and reduces combustibles in fly ash and motor input power in ESP.
Coal mills appropriate in number run by reviewing the requirement of steam/ plant output, furnace temperature, and the condition of the mills. The critical parameters are to be monitored with change in load.
Development & best practices in Indian power plants
Success in this endeavour requires a comprehensive plan to implement key technology advancements (including selective design improvements) which have occurred over last many decades, while successfully accounting for the complex interaction of the various system elements. Industry ‘best practices’ often associate performance with various types of ranking.
Recent experience with a number of units upgrade also indicates that the greatest value can frequently be obtained by component upgrade instead of replacement where availability, performance, or cost can be improved by the use of cutting edge technology to gain a cost advantage.
Huge thermal capacity additions/ expansions/ modifications are envisaged in the next decade. The existing thermal capacity in public/ private sector and CPPs is expected to be more than doubled in the next 10 years. Thus the implication of norms becomes even more important at this stage as the CERC norms would either directly or indirectly be applicable to this huge capacity and number of plants being inducted. Thus it is imperative that better efficiency norms are adopted with a view to conserve scarce fuel resource & infuse efficiency in power generation.
The CERC norms would also be applicable to the private sector stations, albeit indirectly and must, therefore, reflect the reasonable efficiency levels achievable. The sector-wise APC in India PPs for the year 2009-10 reported by CEA in April 2011 is given in Fig. 1.
Fig. 1: Sector-wise auxiliary power consumption 2009-10 (%)
The norms must, therefore, reflect higher levels of efficiency to induce a sense of inter-plants competition and promote efficient operation. More efficient operation would also lead to less CO2 emission which is the current focus of global efforts for lower GHG emissions.
The top five power stations/ plants (above 110 MW) in India, whose operating PLF and operating HR falls in the range of 96.64-102.33% and 2288-2389 kCal/kWh are given in Table 5 for the years 2009-2010 and 2008-09 respectively.
The HR of 14 power plants (CEA document source for national level data) & 9 power plants (International Energy Agency document source for international level data) is between 2279-2393 kCal/ kWh & 2261-3607 kCal/kWh respectively. Some examples of systems/components implemented in Indian power plants between 2004-2006 are in Table 6.
Table 7 gives the energy efficient product-wise technology source being used in India for more than a decade.
The EC in thermal power plants, as in any PP constitutes a major operating cost component. Targeting and benchmarking are recognised attributes globally employed to remain more competitive and can also be used as management tools for improvement in performance and criteria for technology selection for new plants.
Major constrains in PPs in improving EC and benchmarking are:
- Comparatively small size of power generating units, vintage equipment especially boilers, turbines, condensers, cooling towers, and compressors etc.
- Old plants having very high energy consumption.
- Available space constraints for envisaged modifications.
- Boilers and TGs are loaded to the normal level and operated for moderate electricity generation, which is much lower than the optimum level. It leads to the higher SEC.
- Suggested approach for optimizing EC and benchmarking - Compare key parameters/ identify potential areas throughout the plant and improve them to the design level. Irrespective of the number of boilers, TGs in operation, it must be noted that the boilers, TG sets should be loaded to the optimum level (95%). This will ensure the best operating condition of the boilers, TG sets with the maximum electricity generation and minimum SEC. Follow the gradation of potential areas in terms of savings and payback. Implement the economically viable listed schemes.
The public/ private industry sector and BEE would jointly work out on the applicability and compile the successful case studies of energy efficient motors, boilers, fans & other equipment in power plants, update the India’s energy efficient products directory and recommend for the extensive use. The report fixing target for individual equipment and benchmark for the steam generating boiler, turbine, condenser, cooling tower, and power systems and energy targets for individual equipment should be written in details for plants’ further perusal.
The HR data of some of the 80-100 MW Units is either less than or equivalent (similar level) to the HR for northern region (WA operating SHR 2603.2 kCal/ kWh) PPs in 2007-08 as given in CEA document. However, the Dahanu TPS heat rate is observed to be lowest throughout all these years. The DTPS achieved the lowest HR 2261 kCal/ kWh in 2004-05 against CERC norm of 2500 KCal/kwh in eight years. Now, the DTPS is the first ever utility in the World to be certified for ISO 50001:2011 for its strong commitment for energy use and conservation.
It is always recommended that all the EC measures identified be implemented in PP and operate the boiler, TG and associated equipment at the maximum load (as per revamped condition). The EE motors should also be installed selectively, wherever the VFDs not commercially viable and its long term benefit should also be derived. The on-going heat rate of the many inefficient PPs would be further reduced after implementation of EC measures. The present methodology with CEA/ CERC guideline is acceptable/ adopted to most of the power plants in India and norms are being met by them. The targets and benchmark should also be regularly updated which will help the individual plant management to judge their performance as well as to achieve higher profitability level and remain competitive locally
Author is presently working as Fellow in the industrial energy efficiency and division of the The Energy and Resources Institute (TERI).
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