So far as efficient running of the unit is concerned, condenser vacuum is probably the most sensitive terminal conditions. The heat rejected in the thermal cycle depends upon the vacuum as it is equal to the product of absolute temperature (which depends upon mvacuum) and change in entropy (during condensation). For higher cycle efficiency higher vacuum is required, but the increase in vacuum leads to increase in leaving and exhaust losses. Thus, there are two conflicting requirements and an optimum value for vacuum may exist as shown in Figure-17.
But in practice, particularly, in Indian due to atmospheric conditions, the vacuum conditions where diminishing returns start do not prevail and hence operator is always required to maintain good vacuum.
The usual reasons for departure of condenser conditions from optimum are one or more of the following ;
Cooling water inlet temperature different from design. Cooling water quantity following through condenser incorrect.
Fouled tubes and plates.
Air ingress into system under vacuum.
The air ingress into the -system can be checked by starting stand by ejector, if the performance improves it shows the existence of leak in the system. Under this condition the condensate will also the under cooled. It may be remembered that running of additional ejector constitutes loss and hence air leak points shall be detected and sealed.
Steam Pressure at Turbine Stop Valve
The effect of variation in pressure can be studied in the following way for a throttle governed turbine (Figure-15).
Supposing the unit is running at full load and the pressure at TSV increases. The effect of this will be that control valves will tend to close to suite the new pressure load condition thus the steam will be throttled. This will result in lower temperature, lower is entropic drop and higher wetness at exhaust, all factors leading to lower efficiency.
In case the pressure falls when the unit is on part load, to meet the new conditions control valves may further open thereby reducing the throttle, hence the effect will be opposite to that given in above para i.e. an increase in efficiency. Thus the operation on low pressure may look to be advantageous in the first instance. But further, considerations will show that reduced pressure may look to be advantageous in the first instance. But, further, considerations will show that reduced pressure w41 effect the .regenerative system which will require more steam at extractions. Similarly when the unit is on full load the fall in pressure will require more steam both in terms of weight and volume to be admitted , provided that control valves have the capacity, otherwise load will fall. It may also be remembered that cycle efficiency is higher at higher cycle conditions.
Final Feed Temperature
The lower final (i.e. at economizer inlet) feed water temperature denotes the inefficient functioning of feed water heater(s) or non-functioning of some from the chain. The introduction of regenerative system has been done to increase the efficiency of cycle and thus if its most important component vis. Heaters do not work effectively upto the designed parameters, the desired efficiency of cycle and hence of plant will not be achieved. Lowering of final feed temperature will also result in increased boiler firing rates. This will upset the heat absorption balance between various surfaces making steam temperature control difficult and resulting in high steam temperatures at outlet of exit super heater.
To check the performance of a feed water heater it is necessary to compare the following figures with the design values for the particular load on turbine.
a. Bleed steam inlet pressure and temperature. b. Drain water outlet temperature.
C Feed water inlet temperature. d. Feed water outlet temperature.
e. Feed water temperature at economizer inlet or feed regulating station.
The Excess Air
The excess air which is the quantity of air required to be fed to the boiler over the theoretically correct quantity of air needed for complete combustion of fuel, determines the extent of this loss. If too little air is supplied, the fuel is not completely burnt if too great quantity of air is supplied the heat by being carried up in the stack ion greater quantities than normal. It must be remembered that nitrogen which forms 79% of air is merely a passenger, requires fan power and carried away heat. The % oxygen at air heater inlet is directly proportional to the excess air quantity and is used as guide to combustion conditions. We cannot get rid of excess air because it is impossible to feed right quantity of air at right time to the fuel particles in suspension.
Air infiltration is another factor which should be controlled to limit this loss. This factor in addition to lowering of boiler efficiency due to cooling effect on gas also affects the performance of ESP and increases fan loading. Moreover boiler auto control does not give any allowance to this air which is infiltrating from hopper seals, inspection doors and ducts joining.
Air Heater Gas Outlet Temperature
The last heat exchanger in a gas circuit is the air heater. On one hand it will be desirable in the interest of overall efficiency that these gases leave air heater etc. the lowest temperature on the other hand this temperature is required to be high on account of corrosion problems.
The flue gas in addition to carbon dioxide contains water, vapour, sulphur-dioxide and chlorides. In case the metal temperature is below the dew point, water vapour will be formed which will combine with constituents to form sulphuric acid. Due to the fact that partial pressure of vapor in flue gases is less than atmospheric its dew point may be below 100°C. Most coal burning boilers have specified air heater gas outlet temperature of the order of 130°C as being the
minimum practical temperature which is consistent with minimizing air heater corrosion.
The air heater gas outlet temperature if higher the optimum value will lead to increased heat loss. For every 2°C above optimum each Kilogram of dry flue gas carries approx. 2.5 K3 of extra heat up the stack. A rise in air heater outlet temperature of approx. 22°C will reduce the boiler efficiency by about 1%.
Excess air, low feed water temperature, deposits on boiler and air heater heat transfer surfaces, shortage of air heater plates (in rotary type), defective soot blower operation and using higher tier burner on low load etc. are some of the causes for higher gas temperature at outlet of the air heaters.
Combustible Material in Ash
The main loss is due to the unburnt carbon in the ash and is expressed as a x c x 33940 K3 / Kg. of fuel
100
Where a = Weight of ash/Kg, of fuel.
c = % combustible matter (i.e. carbon in the ash).
A small amount of combustible material amounting to about 1.5% of ash can usually be tolerated. It may be uneconomical even to remove all the combustible as mill grinding power may have to be increased or its output reduced. The grinding is considered to be in order if the PF through 200 mesh is 75% for bituminous coal or 85% for low volatile coal. The various causes of high carbon in ash are :-
Coarse grinding.
Mal adjustment of flame.
Unequal loading of different mills particularly in direct firing system.
condensation and higher temperature carrying both resulting in blocking of coal pipes. Suitable coal temperature range in 65 - 82°C at mill outlet.
Make Up Water
The amount of make up depends upon the leakage and the quantity of heat lost depends also on the point of leakage. Naturally leakage of steam is more costly as compared to leakage of water in the system. The normal sources of leakages are due to soot blowing and blow down operations. The amounts of heat lost at different locations are as under ;-
Leakage from condensate pumps entails a loss of 45 to 70 K3/Kg.
For non reheat turbine leakage from TSV will mean a loss of 3025 to 3260 K3/Kg. The % loss of steam at full enthalpy will cause a heat loss of 1 .2%. In case of reheat units it may be of the order of 1.0 to 1.5%.
1% loss of water after final feed heater cause about 0.4% heat loss. 1% of soot blowing causes 0.8% of heat loss.
1% Blowing down may cause 0.25 to 0.5% heat loss.
During starting metal and silica pick up is higher. The amount of Blow down has to be increased under the advice of Chemist, thus heat loss may also be more.
The parameters being also low at start thus the heat loss per Kg. will be a bit less, thus it is the quantity of blow down which is the cause of increased loss.
Works Power
Every Megawatt that can be saved on works power becomes available to the system thereby improving the efficiency of the station and the grid. As the load demand falls the works power also falls, but not in the same proportion i.e. percentage of works power will be more on low station running load Figure-18.
We can take a number of steps to improve the situation few of which are given below:
The station loading must be such that sufficient load is available to justify the operation of second auxiliary.
Wherever variables speed drives are available, attempt should be TO run these at the lower speeds commensurate with the load.
In the milling plant attempt should not be made to over grind the coal. The fineness of PF should be kept to optimum value. Higher fineness means more grinding power mills to be run.
Pumps which are liable to accumulate air and have no automatic venting arrangements should be vented regularly. Trapped air increases power consumption, a case in point is the performance of cooling water pumps.
The Unit Loading
The efficiency of a plant depends upon the load. Every machine is designed for a maximum efficiency at a particular load coiled 'Maximum Economic Continuous Rating (MECR)' which is quite different from 'Maximum Continuous Rating (MCR)'. Now the difference between M£CR and MCR is not sharp as the turbines give lowest heat rate of unit maximum loading. The curve of Figure -19 shows variation of unit efficiency with loading. A lot of heat consumption VS load is Willan’s line and gives incremental heat as it’s slope Figure 20.
The efficiency of most boilers reaches a peak at about 80% MCR and then falls. This means boiler shall be limited to 80% MCR, but in practice it is not possible, keeping in view the load demands and efficiency of turbine being high at higher loads. The boilers have some over capacity to meet demands when one of HP heater is off, thus even when unit is on full load, the boiler may be nearer to MCR.
7. Summary
1. The cycle efficiency is the theoretical ideal efficiency for a given set cf terminal conditions, i.e. initial steam pressure and temperature, reheat pressure and temperature, vacuum and final feed temperature.
2. Cycle efficiency (percent) = available energy x 100
heat added
3. Available energy = work that could be obtained from the cycle = heat = heat added - heat rejected
4. The heat content of water is called sensible heat. The heat required to evaporate water is called latent heat. The heat required to raise the temperature of steam above its evaporation temperature is called superheat.
5. Regenerative feed-water heating improves the efficiency of the cycle. After expanding part of the way down the turbine some steam is bleed off to heat the teed water returning to the boiler. This bleed steam does some work in the turbine and then rejects its heat to the feed water so reducing the amount of heat which would have been rejected from the cycle in the condenser, thus improving the efficiency.
6. The Carnot cycle gives the maximum efficiency that can be obtained by any heat engine working between an upper and a lower temperature.
T1 – T2
Carnot cycle efficiency
Where T, = upper temperature in "Kabsolute. T2 = lower temperature in "Kabsolute.
7. Providing it does not jeopardize boiler availability, optimum boiler efficiency occurs when the sum of the auxiliary power and the heat losses are at minimum.
8. Increasing excess air should reduce the unburnt carbon loss, but increases the dry gas loss.
9. As the efficiency of a given, boiler is dependent on the fuel being burnt, the best assessment of fuel cost is on the basis of heat to the turbine, i.e. heat cost of fuel divided by the boiler efficiency.
10. A fundamentally better combustion process is achieved .by burning finely pulverized fuel suspended in air.
11. Short-time turbulent burners are usually used when the firing is from the wall. Corner firing uses long-flame burners firing tangentially to a vortex in the center of the furnace. Down-shot burners giving a long U-shaped flame are sometimes used for low-volatile coal.
12. Primary air fans for use in pressure-type mills and more efficient than exhausters on suction-type mills so that milling power will be less with pressure-type mills. The tub a mill is much heavier on power consumption than either the ring ball mill or the bowl mill.
13. Low primary air quantity reduces velocity through the mill so tending to increase the quantity of rejects and increase the fineness of product.
14. The differential air pressure across a mill is a measure of the coal in the mill.
15. Classifiers can be adjusted to give the fineness of product required but should not be used to increase mill output as this will increase the unburnt
carbon loss.
16. Spring loading affects the wear of the grinding elements; it may be beneficial to relax spring pressure when milling abnormally soft coal.
17. Equal distributions of fuel and air to the burners, particularly for wall firing, will give the most efficient combustion.
18. Coal pipe temperature should be maintained within the range 65°C to 85°C. Tempering air causes a loss of efficiency because it by-passes the air heater and results in a higher flue-gas temperature.
19. High ash content coal increases mill power and wear for the same through output of heat.
20. Mill. output and power consumption depends on. the hardness of the coal. Milling hard coal with a Hardgrove Index of 40 would reduce mill output to about 50 or 60 percent of output achieved with coal having a Hardgrove Index of 100.
21. Low volatile coals need finer grinding than high volatile coals to keep down the unburnt carbon loss.
22. Mills should not be lightly loaded as this increases milling power; only the minimum number of mills necessary to produce the required output should be in .service.
23. Tests should be carried out on large pulverized fuel boilers to determine the optimum fineness of grinding and the optimum percentage of CO,. 24. Oil-fired boilers can now be operated with less than 1 percent oxygen in
the flue gases primarily to prevent the formation of SO- but with beneficial effects on the efficiency.
25. Soot-blowing causes an appreciable heat loss. The heat loss for a 120 MW reheat boiler is about 0.8 to 0.9 percent when 1 percent of the steam
raised is used for soot-blowing.
26. The heat loss which is due to blow-down for a 120 MW boiler is about 0.4 percent for 1 percent blow-down.
27. There is little change in boiler efficiency over the normal loading range. 28. Methods of shutting-down, banking and starting-up should be investigated
with a view to reducing off-load heat losses.
29. Turbine efficiency is the mechanical work obtained from the turbine expressed as a percentage of the heat available lor conversion into work according to the steam cycle.
30. Overall turbo-alternator efficiency is the electrical output expressed as a percentage of the heat added to the working fluid. It is the product of the cycle efficiency, turbine efficiency and alternator efficiency.
31. Heat rate is a more usual way of expressing overall turbo alternator efficiency; it is the heat to be added to the working fluid per kilowatt hour generated. Dividing the heat equivalent of one KWH by the heat rate in Btu/KWH gives the overall turbo-alternator efficiency.
32. Stage efficiency is the work done on the shaft by the combination of one fixed and one moving row of blades expressed as a percentage of the stage available energy.
33. The fixed blades or nozzles convert heat drop in to kinetic energy. Because of friction and eddies the velocity is actually lower than the theoretical value. Similar losses occur in the moving blades or buckets. 34. Short blades are less efficient than long blades because of interference
caused by roots and tips.
35. Wind age, disc friction, or rotational losses occur because the rotors are revolving in an atmosphere of steam.
36. Nozzle control governed turbines suffer a partial admission loss owing to steam being admitted around part of the periphery only.
37. Interstage glands and tip clearances cause internal leakage losses. 38. External gland leak-offs result in a loss of available energy.
39. Wet steam causes a loss of stage efficiency of 1 percent for every 1 percent of water.
40. Leaving loss in the kinetic energy of the high-velocity exhaust steam.
41. The efficiency of any stage except the last (and the first stage with nozzle control governing) remains sensibly constant over a very wide range of steam lows.
42. Pressure drops anywhere in the steam path cause a loss of available energy.
43. Mechanical losses are bearing losses plus the power required to drive the oil pumps and governor. They are constant in terms of kilowatts irrespective of load.
44. Alternator losses vary with load and power factor Windage and friction losses are constant but copper, iron, and stray losses vary with load and power factor.
45. Terminal conditions are all important in obtaining best efficiency, particularly the initial steam and reheat steam temperatures and the vacuum,
46. Unless some change occurs in the area of the steam flow paths through the turbine, stage pressures are proportional to the rate of flow of steam to the following stages.
47. Simple throttle governing means that all the first stage nozzles are in a common annulus and are subject to the same amount of throttling. More
than one throttle valve may be used but they would have the same effect as only one valve.
48. Nozzle control governing consists of separate groups of first-stage nozzles each controlled by its own throttle valve. This method of governing minimizes throttling losses at part load.
49. Throttling steam at the turbine inlet causes a loss of available energy. 50. The loss of available energy which is due to throttling is not as great for a
reheat set as it is for a non-reheat set.
51. With nozzle control governing the first stage operates with a pressure ratio which varies with load, as a result the efficiency on the first stage is