BEST PRACTICES

Best practices for auxiliary power reduction in thermal power stations   

Improved power generation efficiency, reduction in auxiliary power consumption, reduction in CO2 emissions and cost savings are ensured when easy to implement best practices are adopted in different sub-systems in thermal power stations

Introduction
India has an installed capacity of 1,12,581 MW power stations (as on 1st May 2004) of which the thermal share is 77,931 MW (69%). It’s worth considering that even a 1% reduction in auxiliary power consumption from the existing levels, would yield 5000 MU of energy annually, worth Rs. 1,000 crores (@ Rs. 2/kWh).

Realizing this need and opportunity, several power stations (like NPTC) have already initiated voluntary energy audits. As per the Energy Conservation (EC) Act 2001, it is now mandatory for all power generating stations to get their facilities audited by an accredited energy auditor. The process of implementing the provisions contained in the EC Act becomes effective from March 2007 onwards.

This study is based on several energy audit & conservation studies undertaken by National Productivity Council (NPC), India, in various thermal & super thermal power stations. The authors in this paper present a comparison of auxiliary power consumption trend and it’s break-up, amongst different capacity units ranging from 500 MW, 210 MW to 110 MW units. The auxiliary power consumption (APC) varies from 6-14% (depending on the size of the plant), use of Turbine Driven Boiler Feed Water Pumps (TDBFP) and age of the plant etc. The 500 MW units register the least APC, largely due to the incorporation of TDBFP. In some of the old 110 MW plants APC consumption of 14% is also observed. Energy audit in a vast thermal power station (TPS) is better tackled when the thermal power plant operations are segregated into different sub-areas like: main plant auxiliaries, draft system (consisting of ID/FD/PA fans), feed water system [consisting of Boiler Fed Pumps (BFPs)/ Condensate Extraction Pumps (CEPs), Circulating Water (CW) system-including Cooling Tower (CTs)], and off sites (consisting of coal handling plants, ash handling plants, air compressors, AC plants, station lightings etc.).

Table 1 presents a comparison of the typical break-up of APC for different capacity ratings, all of which have cooling towers with fans.

As may be clear from figure 1, BFPs constitute 33%, the single largest contributor to the APC (excepting in the case of 500 MW units where TDBFPs are used) followed by induced air (ID) fans, primary air (PA) fans and CW pumps. All of these are generally HT drives

Best Practices
Based on the experience gained from the vast number of NPC energy audit studies in thermal power plants and also drawing from several other industrial studies, an attempt has been made to bring out few worthwhile and easily adoptable best practices in APC, in different sub-systems in thermal power stations.

Draft System
The draft system comprises of forced draft (FD) fans, PA fans and ID fans which together account for more than 30% of the auxiliary power consumption (around 12-13 MW of power in 500 MW units).

Interestingly, ample scope for conservation in the draft system exists. NPC’s experience suggests scope for improvement, potentially for at least 15-20% energy savings.

The audit of the above fans would involve measurement of duty parameters like air flow, head developed and motor input power drawl.

The analysis, based on comparison of as-run combined (motor and fan) efficiency and specific energy consumption (SEC) with corresponding rated values, would indicate the margins available for improvement in performance. The typical bench mark SEC values for the fans are given below:

PA fan = 3.75 - 4kWh per ton air
FD fan = 1.2 - 1.3kWh per ton air
ID fan = 2.3 - 2.4kWh per ton flue gas

NPC studies have been able to demonstrate up to 1100 kWh power savings (in all FD, ID and PA fans alone) simply by arresting air ingress in the flue gas path.

Power plant O&M personnel are certain to be familiar with the phenomena of sizeable power reduction in fans systems immediately after an overhaul of the fans and it’s associated ducting system. It is therefore vital to periodically assess percentage of O2 levels at different locations in the flue gas path, which reflect the extent of stray air ingress through the air pre-heater and ESP system.

Simple corrective measures to arrest and identify stray air in leaks bring about huge energy savings. Very often, these fans are required to operate below their rated discharged capacity and head, which impose by default an intrinsic inefficiency in these equipments.

The situation offers an opportunity for power saving in the draft systems by judicious incorporation of VFDs for PA and ID fans. In the case of FD fans, the operating duty parameters are so low that one could easily justify replacement of existing oversize fans with more efficient properly matched smaller fans.

Recent genre of power stations are incorporating VFDs at the design stage itself for PA and ID fans. For a typical 210 MW unit, the difference in power consumption in ID fans alone (with and without VFDs) is 500 kWh/unit, a reduction of 24% in power consumption worth around Rs. 75 lakhs per annum. HT VFDs (3.3, 6.6 kV) VFDs, of course are very expensive. To reduce their initial investment, some smart industries have adopted less expensive route by employing LT VFDs with step down and step up transformers back to back.

Feed Water System
The feed water circuit in a thermal power plant consists of the following key equipments which make significant impact to auxiliary power consumption and heat rate:

• Condensate extraction pumps (CEPs),
• LP heaters,
• Deaerator,
• Boiler feed water pumps (BFPs),
• HP heaters and
• Economisers.

A detailed energy audit and analysis of energy performance parameters of LP and HP heaters and deareators often brings out scope for heat rate reduction (extraction steam use reduction) in power plants. As we are now dealing with auxiliary power consumption, this paper restricts its discussion to CEPs and BFPs.

The audit of BFPs and CEPs, involves the assessment (through field measurement of duty parameters) of their efficiencies and specific energy consumption.

The 500 MW units in India are usually provided with a steam Turbine Driven Boiler Feed Pump (TDBFP) and hence the APC in 500 MW units is as low as 5% whereas in 210 MW units and 110 MW units it is in the range of 34% and 25% respectively. This gives an indication of the impact that an electric driven BFP has on APC, and hence the need for a closer look and observation of these pumps from time to time.

The specific power energy consumption of BFPs ranges between 8-9 kWh/M3 feed water and CEPs ranges between 0.8–1.0 kWh/M3 of condensate. Due to their criticality, the BFPs always operate with a standby, hence, it is easy to perform overhaul of the spare BFPs without disturbing the main stream activity and thus ensures that the BFPs are always at peak efficiency and performance.

It would be worthwhile to remember that even a 1% power reduction in BFPs could mean a huge savings in terms of energy [around 60-70 million units (MU) annually in a 500 MW unit].

Similarly the CEPs also always have a stand-by, and frequent performance assessment to determine margins for saving and prompt overhaul will help CEP energy conservation. Some of the best practices and energy conservation scope areas in BFPs and CEPs are:

• Replacement of inefficient BFPs as a part of renovation and modernization in some of the old plants.
• Clipping of one stage from the multi stage BFPs to balance the pressure drop requirements between HP heaters, economiser and boiler drum etc.
• Use of higher pressure in the deareator to commensurately reduce BFP power consumption (reduced head developed)
• Running of two CEPs instead of 3 CEPs (3 CEPs are run to avoid tripping due to lower frequency in some of the power plants)
• Application of variable speed drivers
• Installation of hydraulic turbine instead of feed regulating section to avoid pressure drops and to generate additional power.

Milling system
Mills or pulverizers in thermal power plants by itself contribute to around 6-7% of auxiliary power consumption. Typically, in 500 MW units, 8 mills are operated, while in 210 MW units, 4-5 mills are normally operated.

The key indicators for assessing the performance of coal mills are, specific energy consumption (SEC), air to fuel ratio (A/F), mill fineness, pressure drop across the mill (dp) and mill rejects.

The specific power consumption varies from 8-9 kWh/ton of coal for bowl mills. This value is slightly higher for tubular mills ranging from 10-12 kWh/ton of coal.

The key contributing factors towards higher SEC are:

• Low coal output (part load operations)
• High ash in coal
• Condition of grinding rolls and bull rings
• Grind-ability index
• Wetness of coal
• Quantity and temperature of primary air
• Classifier settings
• Rejects etc.

The designed air to fuel ratio would normally be in the range of 1.5 - 2.0 tons of air/ton of coal. This ratio largely depends on the moisture content in the coal (quality of coal), and temperature of the primary air. One can measure coal flows and air flows by employing a dirty pitot tube, for measurements.

The desired air to fuel ratio for the as-run operating conditions can be assessed by drawing up a heat balance across the mill. The pulverized fuel distribution in each of the outlet coal pipes has a profound influence on air to flue ratio and is important when attempting to maintain proper combustion.

Mill fineness has a direct impact on the specific energy consumption. Normally fineness of 75% passing through 200 mesh is desirable. Over grinding drastically increases the power consumption.

By systematic energy audit of the milling system, one can optimize power consumption by identifying those mills having higher specific power consumption.

In one 500 MW unit for instance, normal overhauling and periodic maintenance of the badly performing mills yielded energy savings to the tune of 2.77 MU per annum worth Rs. 50 lakhs/annum.

Similarly, in another instance, optimizing air to fuel ratios yielded a reduction of 3.1 MU per annum in the energy consumption of PA fans.

One of commonly employed ‘best practice’ for reducing auxiliary power consumption in milling system is by operating only 4 mills instead of 5 mills in 210 MW units and modification/retrofitting of the existing XRP type of mills.

Circulating Water (CW) System
The CW system consists of cooling water pumps and cooling towers. In some of the thermal power stations, one can find cooling tower (CT) pumps too, in addition to CW pumps.

The contribution of APC in CW system ranges from 9-17%, depending on whether it is a once-through system or a re-circulating cooling tower system, or a combination of both CW pumps and CT pumps.

By conducting an energy audit of CW system, the performance of CW pumps can be evaluated. The measurements of water flow by online flow meters wherever feasible or by ultrasonic flow meters and simultaneous head and motor input power measurements need to be made at site.

The specific energy consumption, along with combined (motor and pump) efficiency of the pumps gives clues about the margins and scope for improvement.

The specific energy consumption figures typically vary from 0.06 – 0.1kWh/M3 CW water. The variation in specific energy consumption, largely depends on factors like:

• the fore bay level
• bowl condition
• profile condition of impeller and casing
• availability of suction lift
• throttling
• bends and scaling effects
• discharge side lifts

The design efficiency levels of most of the vertical centrifugal CW pumps would be in the range of 85-89%. Inter-se comparison of the battery of pumps in the CW pump house is strongly recommended, mainly because it would be profitable and energy efficient to work with more efficient pumps for longer periods than with inefficient ones. In the intervening period the inefficient pumps could be examined closely and necessary corrective measures could be incorporated. It is possible to achieve substantial energy saving by judiciously switching off one or more of the pumps, based on favorable condenser vacuum and favorable weather conditions.

Optimized cleaning schedules for condenser tube help overall performance of the station, particularly in CW water flow optimization (reduction in CW pumping energy consumption).

Depending on the needed quantum of cooling water flow, the weather condition, condenser cleanliness and as forebay level variation, it is possible to reduce CW pump energy consumption by incorporating two speed motors. By refurbishing the casing and impellers and alternatively going in for application of special coatings to improve the impeller and the casing profile condition, it is possible to increase the efficiency of the CW pumps by 3-4%, thus lowering SEC and realizing energy savings.

The contribution of cooling tower alone (with fans) to APC would be around 3-4%. Periodic assessment of CT performance would enable timely intervention of corrective action that would further result in cooler water for better condensor vacuum & heat rate.

It is worth remembering that priority should always be given for improvement in condensor back pressure (through any means) in preference to reduction in fans’ power consumption in CTs (by shutting off fans during favorable weather). A useful performance indicator applicable in thermal power plants would be the specific circulation flow rate i.e. CW flow/MW. This value ought to be around 120-150 M3/MW in 500 MW and 210 MW plants.

Reduction of energy consumption in CTs could be achieved by blade angle reduction, shutting of CTs’ cells in conjunction with favorable weather conditions and replacing existing aluminum cast and GRP blades with FRP blades. Also incorporation of efficient nozzles for efficient water spare/atomization, incorporation of efficient fill material for providing mass and heat transfer areas are other option that can be considered.

Coal Handling Plant (CHP)
The connected load of the CHP is huge and the actual operating load will be surprisingly low. The contribution of CHP in overall APC ranges between 1.5-2.5%.

The specific energy consumption is a key indicator that reflects the performance of CHP. The typical specific energy consumption figures would be in the order of 1-1.2kWh/ton of coal handled. Again this figure varies depending on the type of operations i.e. direct bunkering or stacking & reclaiming; wagon tippling or track hopper system etc.

By simultaneous measurement of power and coal flow for different coal handling equipment like, conveyors, paddle feeders, crushers, stacker/reclaimer etc. one can determine the specific energy consumption, depending on the conveyor route followed. The CHP is a critical component of any power plant and it is also a neglected area due to the harsh working environment. Several NPC energy audit studies of CHP indicate energy saving potential of 30-40%. Some of the good practices in CHP area are:

• increasing the plant utilization factor (PUF) throughout
• incorporating PLC controllers
• avoiding idle running of conveyors & crushers
• incorporating soft starter - energy savers etc.

Ash Handling System
The ash handling plant (AHP) consists of ash water pumps and ash slurry series pumps.

Many recent genre plants have designed their AHP on dry ash handling mode using transport air compressors for movement of ash. Some of these plants also supply fine fly ash collected from ESPs on a continuous basis to cement plants.

The contribution of wet AHP in APC varies between 1.5 to 2%. In some of the NTPC plants like Dadri, where dry AHPs are prevalent, the contribution of AHP in APC is around 3.5%.

Some of the best practices (for energy conservation) applicable to AHPs are:

• Constant monitoring of ash to water ratio. The designed ash water ratios are around 1:5 for fly ash and 1:8 for bottom ash. But in reality one encounters ash to water ratios as high as 1:20. Reducing ash to water ratios directly results in pump power savings and also water savings. NPC studies have shown savings to the tune of 0.2 MU/annum for every 1% reduction in ash water ratio
• Changing of worn out pump internals based on periodic pump efficiency assessment
• Variation of scoop angle (control position) of ash slurry disposal pumps based on slurry level in the pit etc
• Replacement of inefficient pumps with high efficiency pumps
• Optimization of pump operations

Compressed Air System
Compressed air system consists of instrument air compressors (IAC) and process air compressors (PAC) and air drying units. Transport air compressors (TAC) are not included in this system.

The contribution of air compressors system in APC would range between 1-1.5%. The best way to assess the performance of air compressors is by generating specific energy consumption figures for each of the bank of compressors. An inter-se comparison of SECs would help decide which air compressor to operate for longer duration of time to achieve reduced energy consumption. Simultaneous measurement of power and flow delivered (by FAD test method), would enable calculation of specific power consumption (SPC) values. These can be compared with either design or with performance guarantee (PG) values to assess margins for improvement.

For typical reciprocating air compressors or screw compressors, the SPC’s values vary between from 7.5-8.5kWh/Nm3. The present genre of modern modular screw compressor has SPC’s as low as 6.7kWh/Nm3.

A selection of best practices for reducing power consumption in the compressed air system is presented as under:

• Reduction of air leaks. Conducting leakage tests would be difficult in continuous running plants (a plant shutdown is essential for evaluating leakage by this method). However by physically identifying leaks, one can quantify compressed air leakage to a fair extent (tables showing leakage from different hole sizes are available)
• Optimizing discharge pressure by toning down as per needs. This can be affected through pressure settings alterations
• Regular assessment of inter cooler/after cooler performance and periodic cleaning of tubes
• Adoption of heat of compression (HOC) dryers for air drying units
• Use of demand controller for optimal pressure setting
• Use of transvector nozzles for cleaning application etc

Based on NPCs energy audit experience in compressed air systems it is possible to save 25-30% of energy in compressed air systems.

A recent compressed air system follow-up study at one of the thermal power stations by NPC has indicated savings to the tune of 28% worth Rs. 73 lakhs annually.

Air Conditioning System
It is normal practice to provide unit control rooms (UCBs) in thermal power plants with centralized air conditioning. A typical UCB of a 200 MW unit has a connected load of 150 TR. The normal operating electrical load is around 150kWh (including chilled water pumps, condensor water pumps and cooling tower fans).

The AC system contribution in APC would be around 0.5-1% (The AC load of administration building and ESP control rooms are not included in this).

The ideal way to evaluate the performance of AC system is by measuring specific power consumption in terms of kWh/TR. Typical kWh/TR values for centralized reciprocating machines ranges between 1.0-1.1kWh/TR. A list of a few of the best practices for energy conservation in AC system is:

• Incorporation of variable speed devices for AHU fans
• Periodic cleaning of condensors and improving its performance
• Periodic cleaning of evaporator and improving its performance
• Installation of absorption refrigeration system instead of present vapour compression system
• Incorporation of SCADA/building management system (BMS)
• Improving CT performance
• Improving pump performance
• Incorporation of 3 way valves in AHU’s

Lightning System
The station lightning system consists of main plant lighting, off sites lighting and service buildings lighting. A typical connected load of lightning system in a super thermal power project of 2000 MW would be around 2 MW, but the operating load including day and night time would be around 1.2 MW.

Though the lighting loads are wide spread, the contribution of lighting consumption in APC is merely around 0.8-1 %. Despite the low consumption by lighting system (<1%), there exists a good scope for reducing energy consumption to the tune of 20-30%, by adopting energy efficient practices.

A recent energy audit study of station lighting system at a thermal power plant yielded savings of 27%, equivalent to 2.9 MU worth Rs. 43 lakhs annually (@ Rs. 1.5/kWh). Some of the best practices for conserving energy in lighting are:

• Reducing single phase voltage to 220-230 V by transformer tap setting (in most of the generating stations the single phase voltage has been observed to be in the range of 250-260V)
• Use of electronic ballasts
• Incorporation of CFL’s
• Incorporation of lighting energy savers
• Use of metal halide lamps by replacing HPSV lamps
• Incorporation of timers
• Incorporation of photo sensors etc.

Conclusion
It is evident that thermal power stations offer tremendous scope for reducing auxiliary power consumption. In fact the auxiliary power consumption of 200 MW units is close to the connected load of a large cement plant. Detailed energy audit and analysis can help in identifying a number of energy conservation options.

NPC has brought out energy audit procedures for all these systems and identified some of the best practices that are applicable in thermal power stations.

It is immaterial whether energy audit is mandatory or not. Ample scope exists for energy conservation and one needs to continuously identify ways and means to conserve energy and retain one’s competitive edge in this fiercely competitive industry.

References
NPC Energy Audit Reports of various Thermal Power Stations

Back