Water use in nuclear & coal-fired power generation 4

[eurocopper]

Dear Colleagues:

Could somebody give me some ballpark figures about water use in large nuclear & coal-fired power generation plants. Are there any benchmark figures, such as tonnes of water use per GW and hour of operation? What temperature increase can be expected in water temperature between water intake and after the cooling tower?

Thanks,
Hans



Hans De Keulenaer
Manager - Leonardo ENERGY initiative

http://www.leonardo-energy.org

 

[davefitz]

ballpark figures for plants built in the late 1970's:

coal , subcritical , steam turbine cycle heat rate = 7961 btu/kwhr ( 42.86% effcient, 57.14% of steam energy discharged to cooling tower)

nuclear turbine cycle heat rate about 10,623 btu/kw hr (32.12% efficient, 67.88% of steam energy discharged to cooling tower).

The above are at full load , original equipment. As the unit ages, the heat rate gest worse. Some upgrades are possible ,espescially better LP tubine blades.

Newer plants will have better efficiencies, but they have to be built first.

the colig tower does not discharge directly to a river- the small blowdown may need to be treated first before disharge, so there is no relevance to the cooling tower DT- but for estimating purposes, the circ water may have a 20F DT.

 

[athomas236]

eurocopper,

For your information guesses based on 400MWe lignite fired unit:

1. Cooling Tower Water Consumption
CW flow 10000kg/s
Cooling range 12C
Approach temp 8C
DBT 30C, WBT 23.87C
Low tech (4 concentrations, 0% reclaim of purge) 690tonnes/h
High tech (8 concentrations, 60% reclaim) 640 tonnes/hour

2. Boiler and demin
Boiler steam flow 316kg/h
Boiler blowdown 3% of steam flow
Demin plant 34 tonnes/h with loss of 15%
Low tech 0% blowdown recovery 53 tonnes/h
High tech 50% blowdown recovery 35 tonnes/h

3. Ash handling
13kg/s ash
20% bottom ash, 80% fly ash
Bottom ash water 8kg/kg ash
Fly ash water 3kg/kg ash
Low tech (60% reclaim of ash water) 75 tonne/h
High tech (100% dry ash removal) 0 tonnes/h

Miscellaneous
FGD loss 0.15 tonne/h per MW output
Domestic loss 0.01 tonne/h per person with 126 people
Service water 5 tonnes/h
Low/High tech 66 tonne/h

No information on nuclear

Regards,

athomas236

 

[EdStainless]

euro,
There are many plants, both coal and Nuke that run with cooling towers and zero discharge of water. They recycle and reuse the water until all that is left is solid waste. They have to make up what is lost to evaporation in the cooling towers, but that is it.
There is a cooling tower institute that has information on typical water loss rates for various sizes/usage.

In plants that use once-through flow to cool the condensers have large numbers. Typical temperature rise is between 12F and 30F. Flow rates x temp rise will be based on plant capacity.
For example 1,000MW nuke plant is running 33,000kg/sec at 16C temp rise.
Or a 325MW coal fired plant is running 4,800 kg/sec at a 14C temp rise.

The range between plnats is very large. It depends on a lot of factors.

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[rmw]

Davefitz,

You are one of my heros on these fora, but I have to question you on this one. Do all the losses or steam turbine cycle inefficiencies go to the cooling tower (heat sink)?

rmw

 

[davefitz]

rmw:
there are a couple of different methods of calculating turbine cycle efficiency, but overall:

some of the steam turbine losses are frictional losses in the turbine bearings, and this is transferred as heat to the lube oil then to its cooler.

some turbine losses are related to the efficiency of the generator, again these magnetic losses result in heat that may convect to the air in the turbine building or other cooling systems ( H2 cooled) not related to the cooling tower.

As I recall, these above 2 losses may be as high as 2% of electrical output on large units, but the rest of the steam turbine cycle losses relate to turbine exhaust loss, seal leakages, thermodynamic limits, and end up in the circ water going to the cooling tower.

Overall station heat rate, of course, has many other losses, including boiler efficiency, boiler aux power ( mills, fans ) air pollution control devices ( SCR, FGD scruber, precipitator), then we finally need to deal with transmission losses. To crown it all off, the incandescent light bulb we ust to light our homes has a very low efficiency all by itself.

Rough figures for cooling tower water demands are as explained by athomas , above. The circ water enters the cooling tower about 20F hotter than it leaves the cool tower. The cold circ water from the cooling tower ( to the steam surface condenser) may be 10 F hotter than the current wet bulb temp of the ambient air. Most of the heat transferred at the cooling tower is by virtue of evaporation of makeup water, but some of the heat is due to sensible heat change of the air flowing thru the tower. Add'l makeup water is related to the required blowdown water flow used to control the "cycles of concentration" of the circ water's salinity.

 

[ccfowler]

eurocopper,

The intended duty of the plant can have a significant effect on steam cycle efficiency. For a base load plant where efficiency is most important, the above discussion is applicable, but some quite large (as well as smaller) coal, oil, or gas fired steam plants are designed for mid-range or peaking duty where rapid load changes and frequent stop-start cycles are the controlling considerations. Some mid-range duty plants are in the range of 500 Mwe or even larger, but most are smaller.

In order to deal with rapid load changes or frequent stop-start cycles, the steam temperatures and pressures must be significantly lower, and all other "burdens" are proportionately greater. (The steam flow to the turbine is proportionately greater, the heat "wasted" at the condenser is proportionately greater, and the auxiliary power requirements are proportionately greater.) Where the more efficient base load units will have "station" heat rates in the general range of 9k to 10k Btu/kwhr, the mid-range and peaking plants will have "station" heat rates in range of 13k to 20k Btu/kwhr. The mid-range and peaking units tend to achieve their highest efficiency somewhere in the range of 60% to 80% of their full load rating (where they tend to spend most of their operating time). They normally operate from around 25% to 30% of full load to 100% of full load. The large, high efficiency coal fired units can operate at decent efficiency over a fairly wide range, but they don't handle rapid load changes without thermal fatigue problems.

When the entire generation system is considered, the lower efficiency of the mid-range and peaking units is not a bad thing. The overall cost and efficiency of the entire generation system is improved by allowing the more efficient plants to operate at their higher efficiency for a greater portion of the total generation with the larger and more rapid power demand variations being handled by the mid-range and peaking units.

 

[davefitz]

ccfowler:
Your observations are correct for coal fired units, but do not accurately summarize the load scheduling for the most efficient units in the US.

The most efficient units in the US are the gas fired combined cycle plants (plant heat rate = 6800 btu/kwe-hr), but due to the high fuel cost, are normally operated in a 2-shift cycling mode of operation ( shutdown at night, restart every morning). Also, their cooling tower thermal loads are much lower, as only 40% of the elect output is via the Rankine steam cycle. The coal fired units are base loaded, more or less. While this approach minimizes system wide fuel costs, it maximizes CO2 production per MWE hr generated.

If it becomes neccesary to limit CO2 emissions, the load cycles of the combined cycle units would need to switch with the coal units; base lod the combined cycle units and load cycle the coal fied units. This would imply the need to import a lot of LNG , and also modify the 40 yr old coal units for variable pressure operation and add thermal stress monitoring devices.

In the meantime, the combined cyle units are taking a real beating interms of fatigue damage.

 

[rmw]

Davefitz,

I read your earlier post very carefully, and I took it that your 7961 heat rate was turbine cycle heat rate as stated and not station heat rate-you had referenced coal and sub-critical, after all.

My previous question was to raise the point that you then verified, that while small, there are other heat losses that didn't go to the condenser heat sink be it a cooling tower (wet or dry) or surface water. Granted most do, but not all of them.

Once you get into the station heat rate, of course you have to consider the Btu's that go up the stack, out with the ash, other boiler losses et al.

But you answered my question and made the point I wanted made.

rmw

 

[ccfowler]

This discussion has "blossomed" a bit from the original question, as well it should, since there is no simple answer. For the sake of additional clarification, it should be noted that some of the comparisons between the various types of power generating facilities may not be as direct and simple as they may appear.

The basis of expressing station heat rate tends to vary and should be recoginzed in useful comparisions. The traditional steam plants most commonly are evaluated on the basis of the higher heating value of the fuel whether the fuel is coal, oil, or gas. The highly efficient combined cycle gas turbine plants and many newer reciprocating engine plants are commonly evaluated on the basis of the lower heating value of their fuel. This can lead to the differences in apparent efficiency being somewhat greater than they actually are. (I have always thought that the lower heating value is a better measure since none of the plants ever include any provision to recover energy from the portions of the fuel reflected in the difference between the lower and higher heating value.)

Another consideration that is often not fully recognized is the relatively poor efficiency of gas turbine plants when operating at reduced power. They are only at their best efficiency at full load. Because of the relatively large compressor load, these machines consume huge amounts of fuel at low loads. For estimating purposes, they can be expected to consume nearly 90% of their full load fuel consumption rate when operating at synchronous speed and no net power output (from the gas turbine). Thus, they do not have to drop very far down from their full plant power output before they are down in the neighborhood of conventional steam plants with respect to efficiency. Due to fuel costs, it is easy to see why they are often best dispatched in nearly on-off duties.

In comparison, conventional steam plants can usually retain fairly stable plant heat rates well below 50% of their full load rating. The actual profile of plant heat rate vs. load for a steam plant can vary greatly due to the details of the design of the steam generator and the steam cycle.

Rather than the classic issue of comparing "apples and oranges," comparing different power plants is more a matter of comparing apples, oranges, onions, peppers, pumpkins, .... Including concerns for CO2 emissions and water usage complicates the comparisions even further. The only thing that is certain is that there are no simple answers or simple comparisions.