Variation of Auxiliary Power Demand of a Typical Coal Based Steam Plant from Start up to Full Load

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Thread Starter

pikachoo99

Can anybody please explain how the auxiliary power demand varies for a typical 500 MW steam plant from start up to full load. If it start with say, 2% of its max capacity as start up power. and they go up to 8% of max capacity at the time of providing full load,in what steps do this auxiliary power demand increase from 2% to 8%? is it continuously vary with load? like if i load my unit say by 50 MW, then 60 MW and so on at each step? will the aux power increase by 8% of 50 MW,then 8% of 60 MW and so on? or is it varying with discrete steps? i know a unit having five coal mills engages each coal mill at some discrete time intervals like at reaching 100 MW, 200 MW, 300 MW loadings and so on up to 500 Mw? i Know feedwater systems (BFP), draught systems, cooling systems, fuel handling and firing systems, and condensate pumps, circulating pumps all demand power, but i want to know how this demand varies with loading of the unit? how this transition occurs?
 
Your aux load will not "step" up. It is a continuous demand as opposed to a discrete one. As the turbine/generator load increases, the aux loads will increase as a function of mostly heat load.eg. For example- As the turbine load increases, the cooling requirements do the same. As turbine load goes up, lube oil temp will rise, the cooling system will increase flow continuously to maintain the oil temp set point. As the cooling regulators open up to supply more water, the cooling pumps will draw more current to maintain required flow. This does not occur in "steps".

As you have a coal plant, the pulverisors, etc. will come on-line at necessary times, and increase output as the demand requires(these do represent a jump in aux load, however)--maybe that is what you are referring to?
 
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Bruce Durdle

The auxiliary power demand will start before the steam plant is fired up. You have to run forced-draft FD and induced-draft ID fans, and boiler feedwater BFW pumps. For each of these, the power demand is proportional to the pressure difference developed x the volumetric flow.

Until the boiler pressure is raised by firing, the pressure needed to force water into the drum will be small, and the flow will also be small - but you may have to develop full pressure at the pump and throw that pressure away in a control valve. Variable speed control of the pump will reduce the power needed considerably.

For the fans, you may need to run the fans at full flow for a specified time as part of the purge sequence. So the power required will be about 100% for this period. Flow for lighting off will usually be reduced - maybe to somewhere between 10 and 20 %. Again, if the fans are single-speed, the developed pressure will not change but the flow will, so power will be approximately proportional to the air flow required. If fan speed is reduced as well, the power needed will be reduced.

Once you are running, the same relationship applies.

Hope that helps,
Bruce.
 
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Thanks sggoat and bruce durdle!

To sggoat:

Yes, obviously the cooling requirement increase as a function of heat developed. but the question is, what percentage of total auxiliary demand is suitable allotted to the cooling power demand in a typical coal based turbogenerator plant?

http://www.eecpowerindia.com/codelibrary/ckeditor/ckfinder/userfiles/files/APC-EEC workshop.pdf

From the above document, i saw that cooling requirements are 15%, combustion air and flue gas requirements are 16.4% (that is possibly ID and FD fans and ESP requirements), but feedwater systems take 41% load of the total aux power demand at full load. and this load is around 8 % for a 500 MW plant at full load. At part load, logically the requirements are bound to be low. I even heard that turbine driven BFPs take less power than Motor driven BFPs.

Also i am quoting from another document: "The design aux consumption is a percentage figure for unit operating at rated load. The fans and pumps are designed for high performance at full load. In the unit operating at partial load these equipments consume power disproportionately at higher levels, so aux consumption will be higher."

So disproportionate means the initial jump in aux demand, say from 20% to 50% loading of unit, will be a lot more than the increase in aux demand from 80% to 100% unit loading. So the aux demand do not increase uniformly with load.

Lastly, what u guessed is absolutely correct. Yes, Since the coal pulverisors/coal mill are engaged only at specific times depending upon loading of unit, say at 20%, 40%, 60% and last at 80% of a 500 MW plant, i am also deducing that BFP ,air flow and duty on FD, ID fans and even cooling tower requirements also discretely increase with opening of coal mills at these intervals. as with more coal to burn all other needs are bound to increase though they may be disproportionate with unit loading. So am i correct in deducing the given simplifications?

To Bruce

Please see my above comments.

Yes, we have to run BFP, ID, FD, PA fans some circulating pumps before synchronising as start up procedure. And one mill is to be engaged while raising temp and pressure to turbine conditions, after firing, with starting of say 2nd set of ID FD and PA fans. So if there is a 500 MW unit, i am taking this accumulated initial aux demand as 2.5% i.e., 12.5 MW and the full load aux demand will be 40 MW or say 8%. As u said Variable speed control of the pump (BFP) will reduce initial power demand until firing. So i think i am right in deducing the bfp demand is less in this period than during when each new mill is engaged during further loading after synchronizing.

For fans u said power is 100% at purging sequence and 20% at lighting off or boiler firing instant, and its dependance on air flow and fan speed is present. So once again we see that aux demand is not linearly increasing. its a complex summation of several variables.
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Now my question is that after initial minimum block load, when further mills and pulverisors are engaged at say 100 mw, 200 mw, 300 mw, i assume that the Bfp, air handling ,cooling, circulating, condensing demand all increase at least by some with respect to the initial demand of 2.5% to cater to extra coal burning. Now is this percentage of demand increase very distinguishable with say loading unit by 20-30 MW more? or can i use a step increase in all demands at given intervals of 100 mw, 200 mw, 300 mw, 400 mw mark. i.e., demand equals 12.5MW aux demand at zero loading, 22.5 at 100 MW, 30 at 200 MW, 36 at 300 and 40 at 400 MW, remaining constant up to 500 MW full loading.disproportionate increase in aux with part loads?

I want to use this approximation for ease of my academic analysis purpose. is this justifiable? Obviously, if my claim is too out of step with actual happening, i may have to reject it and may have to account for some increase in aux demand for each small increase in unit loading. but i somehow feel that change in aux demand for a small loading increase is not distinguishable. That's why i take above simplification.

Plz comment.
 
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