CANDU and Coal-fossil power plant heat rejection

[Koros]

I would like to compare the heat rejection of a CANDU nuclear plant(700 MW) with a coal-fired fossil plant(700MW) both rejection via the main condenser to a cooling tower which one has overally more heat rejection than the otherone and why?

 

[IRstuff]

double posted here: thread404-286598


TTFN

FAQ731-376

 

[racookpe1978]

Your thermodynamics prof should have covered this is great detail. Also your heat exchange class prof and plant design prof and (possibly) your fluid flow too. 8<)

The typical nuke pant cannot generate superheated steam, but a coal plant - of the same electric power output! - is expected to generate power using superheated steam. Thus, you need more steam flow at lower (saturated) pressure and temperature into the HP turbine with a generic nuke plant. Phrased differently, there is more energy in a lb of steam in a coal plant than in a nuke plant. (Caution: I don't know the CANDU reactor characteristics: use the following AFTER you get the real spec's).

Energy across the turbine is the difference of inlet conditions - outlet conditions, but a typical clean turbine condenser is going to create just about the same outlet condensed water conditions regardless of type of plant.

Therefore, the nuke plant is slightly less efficient than the coal plant and must reject more heat energy (for the same electric power generated) into the raw cooling water flow. Because of this extra rejected heat in the cooling water, and because most US nuke plants are larger than most US coal plants, the outlet raw water at many (most?) nuclear plants must be cooled with the cooling towers in addition to the river water or lake water in order that the raw water outlet stays inside enviro limits during the hottest part of the year..

 

[davefitz]

The above responses are correct.

A modern coal plant ( if any more are to be built) would use supercritcal conditions and advanced steam temperatures to 1050 F HP main steam and 1100 F 1st and 2nd reheat. The latest innovation in steam cycles also includes a "tuning turbine " ( refer to the "Master Cycle" from the EU ), which may add another 3% in cycle efficiency for cycles with advanced reheat steam temperatures. And finally, more units will be proposed with incorporation of induction steam from either solar thermal or biomass fired LP boilers. So, a steam turbine chycle efficiency for such a unit may be on the order of 50%, while a conventional nuclear might be on the order of 35% efficient. The balance msut of course be lost to the cooling water to the cooling tower.

 

[racookpe1978]

Best - from a thermal efficiency viewpoint (but NOT a nuclear cost justification viewpoint, considering today's low nat gas costs (and probably future costs too!)) - is a triple plant: 2x large gas turbines burning natural gas at 300-500 MWatt electrical, each passing its hot outlet gasses through a heat-recovery steam generator sized to power a third steam plant also 300 - 400 MWatt.

You can go on-line with that arrangement in 15 months from start of digging, and only use thirty acres. With no noise pollution or cooling tower or coal trains/coal barges or coal storage yard or ash disposal or SOx cleanup systems.

Net thermal efficiency is 65 - 68%.

 

[davefitz]

racookpe1978:
68% ? I thought todays max net plant efficiency was about 61% ( LHV basis).

Anyway , a better thermodynamic combo would be to have the HRSG's configured only to provide superheat and economizer duty , and have the evaporator duty provided by a nuclear steam generator. But the mismatch in construction time and the avoidance of using nat gas at a nuclear site rules it out as a practical alternative. Although an oil fired superheater was used at the con ed plant in the 1960's.

 

[gmax137]

For completeness, should note that the B&W units with once-through SGs do produce some superheat, though not so much as a fossil boiler.

 

[rmw]

Another factor that causes a hit on Nuclear Power plant steam turbine efficiency is speed. The larger units are typically 1800 RPM. Turbines love speed and the faster they rotate, the more efficient they are. Therefore MW for MW, for identical steam conditions, a 1800 RPM unit would be less efficient than a 3600 rpm unit which would be less efficient than a 5000 RPM unit (neglecting gear losses to turn a synchronous generator), etc.

rmw

 

[davefitz]

rmw:

As I recall, the stage efficiency is strongly related to the relative velcoity betweeen the steam and the rotating balde- the approx ideal differntial speed is 1/2 of soundspeed.

This differential speed can be accomplished by whatever rpm x blade diameter ( with corrections for pitch etc) which is found to be practical ; for low temp/ low pressure steam as is common with nuclear cycles, this would be an 1800 rpm turbine ( for 60 Hz systems). The same 1800 rpm steam turbine has been used on some double reheat supercritical cycles for the LP turbine.

 

[rmw]

Good point Davefitz, but one couldn't help but notice that the HP and IP sections of that same unit weren't 1800 RPM unlike the Nuclear turbines which are 1800 from inlet valve to the condenser neck.

All my old turbine training stuff is at the office and I won't see it again until next week, but this will make for some interesting reading when I get back.

rmw