Minimum PF Fineness required in 250 MW Tangential Fired Boiler

[wildpete]

Hi, I have recently started as an asset engineer responsible for Mills,Fans,Air Heaters,Burners and ductwork etc at a thermal power station. This station predominantly burnt gas in the first 25 years of life so not much time and effort was put into keeping milling plant at its optimum. Thinks have changed in the last 5 or more years and we are now burning alot more coal. More time/money has been put into our 863RP Pulverisers (vanewheels,tiling classifiers, hardfacing rollers and tables) etc yet it has become the norm to except PF fineness at anything over 60% thru 200 mesh. All Epri and other documentation I have read suggest minimum of 70% thru 200 mesh yet all my predecessors say that its a waiste of kW's grinding any finer. Would their be a significant Heat Rate benefit in grinding our coal finer and what should we be aiming for. Thanks

 

[rmw]

Do you have other combustion or post combustion issues like LOI, smoking, carbon in flyash, etc that would make you want to grind to the finer degree?

rmw

 

[wildpete]

On the surface there appears to be no problems but in my opinion we are not really looking very hard for them either. I spoke to the station chemist and it seems that he checks the cegrit samplers about once per week and we have two of them on each boiler located above the secondary air heaters. He empties the contents out and spreads a portion out on a table for a visual inspection. If he sees large unburnt particles or if the ash is generally darker than usual he sends the sample away to a lab for futher analysis as we do not have that equipment. We do not sample bottom ash or electrostatic precipitator hopper ash. He is still getting back to me on the results of some of these test so I cannot give you any of the results yet. Does this sort of practise sound normal for a coal fired power station and what is considered to be max acceptable level of carbon in ash content. Thankyou

 

[rmw]

Do you sell your flyash? If you do, the purchaser will let you know in a heartbeat if there is too much indication of LOI in it. Typically flyash is sold to those who add it to concrete. Too much carbon in it makes it unsuitable for that purpose and they typically scream (and want to get out of having to pay for it) when that happens.

You can expect to find unburned carbon in bottom ash, and it isn't sold - you can't give it away.

rmw

 

[wildpete]

We give the ash away to a cement manufacturer and they do grumble on the odd occasion about the carbon in ash content. Unfortunately we are not ever likely to be able to sell it to them so I guess them taking it off our hands for nothing saves us about $75 ton in disposal costs. If its completely acceptable to have less than 70% thru a 200 mesh and there are no combustion or heat rate efficiencies to be gained should we be counting ourselves lucky?

 

[davefitz]

The fineness affects the burner -to-burner coal flow unbalance. Too much coarse, and some burners will get 30% over the average coal flow, and some will get 30% under the average coal flow. This adversely affects NOx, CO, furnace slagging, unburned carbon, and ash salability.

If you are happy with your current values of the above parameters, the forget about the fineness problem and go on your merry way.

If you want to improve the above parameters, then the modern fix is to update the mill with dynamic rotating classifiers. This ensures zero coarse product in the steady state, and allows for low stochiometry in the lower furnace , staged combustion, low low low NOx, ash carbon below 5%, low CO, and may also cure the common cold.

 

[rmw]

DaveF, I thought it was the rotating vanes that cured the common cold not the classifier.

WildPete, do you have a copy of or access to B&W Steam? I recommend something past the 1889 edition being discussed in another thread.

If you do, there should be some good insightful reading there. In the 43rd edition, the newest I have, see page 12-11.

rmw

 

[wildpete]

DaveFitz,
As far as I'm aware the burner to burner coal flow balance has not been checked since the sation was comissioned nearly 30 years ago. We have never replaced a Mill P.F. orifice but instead the fitters told me that they have turned the orifice plates upside down to extend their life?

We have recently purchased $100 000 worth of new Rotorprobe P.F.sampling gear with dirty air velocity pitot included so once I get this new equipment installed it will interesting to see what imbalance we have. At this stage there are no issues with Nox,CO production and we give away our ash so thats acceptable as well.

We do now and again have issues with boiler slagging/fouling in the secondary superheater but thats usually when operating at high loads for extended periods of time and burning 100% local coal (approx 8% ash content)instead of a 50/50 blend of local and imported coal (Indonesian approx 4% ash).

Both our coals are ranked as 'Subbituminous C'and according to the Book "Combustion: Fossil Power" from CE this requires a fineness of 60-70% passing a 200 mesh(which we generally achieve) and less than 2.0% on a 50 mesh (which we always acheive(unless something has gone wrong inside a mill).

Interestingly we have just at the start of this week commissioned some new O2 probes (8 of them)that were installed a couple of years ago just before the economiser on one of our units. Seems that we are now saving approx 380kW of auxilliary power on the FD/ID fans to maintain the same excess 02 setpoint. I wonder if this lower effective excess air will increase our slagging problem?

rmw, thanks for the book reference, we have the 41st edition of Steam in our station library. I will have a good read when I can pry it out of the hands of our engineering services manager.

 

[davefitz]

sounds like you already have everything under control.

good luck!

 

[Tmoose]

My copy of the 41st edition of steam has a PDF copy of itself on a CD hidden in a pocket on the back cover.

Dan T