PC boiler at 30% MCR load, how many PA fans will be in operation?

[BINGMD]

I am working on a PC boiler (100MW) unit and trying to calculate aux load at 30% MCR load. We have 2 PA fans, 2 FD fans, and 2 ID fan. 3 pulverizers.
I knew that at 30% load, one pulverizer will be put into operation. At that time, the ID fans and FD fans will be up and running. However, I am not sure if one PA fan will be in operation or both PA fans will be in service? What percentage of the total PA air flow will be needed for the 30% MCR load?
Thanks in advance for your help.
Bing

 

[DRWeig]

Hi Bing,

Lots of really good boiler folks on this board, but your acronyms might not be apparent to everyone. I think FD (forced draft) and ID (induced draft) are pretty universal, but you might enlighten us as to the others:

PC? MCR? PA?

Different terms get used in different parts of the world, too...

Thanks!

Goober Dave

 

[rmw]

PA is primary air. PC is pulverized coal. MCR I don't really know, but I understand the question.

Bing,

I don't know that your question can be answered without knowing the burner and pulverizer arrangement. I would think that it would depend on which burners each mill was supplying coal to and which PA fans were required for those burners based on that arrangement.

Is this a wall fired unit, or a corner fired furnace?

rmw

 

[boilerone]

Bing,

I would normally say "one down, one in service" but there are still many other cases that might affect the right answer to your question.

1. Are the PA fans each designed with capacity of 50% (both work at MCR) or 100% (one work, one spare, rare but possible). For the later case you already have the answer.
2. Are ther PA fans centrifugal types or axial types (rare for 100MW units). Axial fans have much wider adjustable ranges so they might still be able to run both at low loads and still have good efficiencies.
3. If have centrifugal fans, is each of the fans sigle-speed type, dual-speed or even variable frequency drive type? Again, if they can adjust the speed, they might go this way without turning one fan off.
4. Same as rmw is asking, does each fan feed to a dedicated pulverizer or share the common PA fan outlet duct and the duct split after passing through the air heater? If feed to dedicated pulverizers, of course, then one fan has to shut down when its pulverizer is off-line.

These are all related to how the air system is designed and how the system tries to maximize fan efficiencies to reduce electric power consumption. Actually you should be able to quickly get answers from the boiler's operation manual or call the plant engineer/operator directly also hoping he/she could tell you exactly why the control philosophy or DCS is set the certain way.

For your 2nd question, at 30% MCR load the PA flow should be a few percentages higher than 30% also means higher excess air at lower loads. This is for ensuring enough air velocity for the combustion and it is different for wall fired vs. corner fired plus very much related to how the burners are designed. The exact answer also come from the operation instructions or the control settings.

Boilerone

 

[BINGMD]

Thanks to all. I knew I should ask my boiler supplier about this question, but it is too early to do so. I have read some documents and figure it out. As Boilerone said, it is really depending on the air system design.

For a hot PA fan system (one fan per mill, located downstream of Air heater), I will have one PA fan for one mill at 30% MCR (maximum continous rating), so olnly one PA fan will be running. For a cold PA fan system, I have 2 fans (my current design 2X50% fans) which will both operate to provide around 40-45% MCR air flow.

Now I have another question - for such a small unit, is hot PA fan sytem better or cold PA fan system better? Thanks a lot.

 

[athomas236]

If it is an existing boiler, why not ask the operator?

regards,

athomas236

 

[BINGMD]

I have not bought this boiler yet. I hope I knew a power plant operator so I can ask. Thanks.

 

[dougarthur42]

Good question, looks like youve got some good direction now. As for Acronyms FD = Forced Draft
PA = Primary air
ID = induced draft
MCR = maximum combustion rating
You said you had 2 PA fans and 2 Forced draft fans. Where the PA primary air is actually a forced draft fan and you still have 2 more FD fans, usually called SA secondary air fans. Primary air is what is says, the primary supply of air to the boiler. Next are the secondary air fans which are fuel feeder air systems and air trim.(or vice versa) Usually on boiler systems where 2 PA, 2 ID and 2 FD fans are used, they are each rated to give you 75% of MCR , load. This is so when one of them are down you can still operate at above 50 % load. As was previous mentioned when 30% mcr is needed slightly high than 30% air is needed. You could run 2 fans at 15% or 1 at 30 %. At 15 % you are on the lower end of you fan curves and they will not operate as well as at high load. This is just general info, my take on it. Your situation my not be the same. Hope it helps

 

[athomas236]

my understanding is that MCR = maximum continuous rating.

Regards,

athomas236