Powerplant "start-up" boiler - recommendations ? 3

[MJCronin]

To all,

We are evaluating the design, schedule,installation costs and other requirements for a new packaged auxiliary (start-up) boiler for a fossil fired power plant. It will operate 400-500 hrs/year and be of conventional packaged "D" tube design

The boiler will be used only during the start-up phase of operation to provide DA pegging steam, gland steam for the main turbine and to get the boiler feed pump turbine rolling. Most of the time the new aux boiler will be in "hot standby" condition.

What design features should be included in the new aux boiler ?

We are going to include:

- Warming coil in the lower mud drum

- Stack damper to keep heat "in the box"

Any other suggestions based on experience ?

Regards

-MJC

 

[insult2injury]

I assume this is for a cycling coal plant?
What is the required ramp time from "hot standby"?

Options to consider:
1. Flue Gas Recirculation w/ separate or combined fans
2. Combustion air preheating (if required)
3. Economizer bypass (if desired)
4. Purchase with Aux boiler feed pump and deaerator as a separate skid
5. Continuous and intermittent blowdown tank
6. Fan details (speed, bearing type, lubrication type, lube oil cooling)
7. Control valve stations for make-up water, aux DA pegging, aux DA overflow, etc.
8. Insulation requirements
9. Materials of construction for tubes, etc. (may depend on manufacturer)
10. Refractory or membrane tube design
11. Steam separation (chevron/cyclone etc.) features in steam drum
12. Access requirements
13. Emissions/CEMS
14. Sootblowers if req'd

Just a few for you to think about.

I2I

 

[rmw]

Where will this boiler exhaust its flue gas to and what kind of damper do you intend to use for "stack damper to keep 'heat in the box'".

If you keep the boiler hot its stack will tend to draw a draft through the boiler. This is especially pronounced when the Aux boiler flue gas ductwork ties into a tall chimney.

THe draft draws in cold air and if the heating system isn't up to par, parts of the boiler can 'sweat' and literally rust away. I've seen several Aux boilers that literally rusted down every so many years no matter how much precaution or drum heating that they did. Heaters or no, it was impossible to keep down condensation when the leakage air is really cold.

Is this a supercritical unit-not that it makes a difference.

I recommend the use of a zero leakage guilliotine damper in the boiler stack. That will "keep heat in the box" and will prevent draft caused inleakage.

What is the fuel going to be? If it is oil, double beware.

rmw

 

[metengr]

Yes, do not specify rolled tube joints. Because of cycling duty, the tube rolls will tend to break loose and you will have lower drum leaks.

 

[MJCronin]

Perhaps I have not made the boiler duty clear......

This is a small "start-up" or "light-off" boiler that is operated only when the plant starts from an all cold condition.... it is not in cycling duty. Most of the time it will not be in use.

This small packaged, D-style boiler, (200,000 lbs/hr) will only be used for the purposes stated above, during startup and sometimes shutdown of the main boiler. It will not be supercritical nor will it be equipped with a regenerative air heater

The boiler will be fired by fuel oil only.

The utility wants the boiler to be available in 1-2 hrs, and wants to know what design features are necssary. The older units that this one will replace had trouble with the lower mud-drum heating coil

Does anyone have experience with this specific kind of "quick start" utility boiler ?

 

[toothless]

We have a 250 kpph boiler that we have for back-up purposes only. And as such, we require a 5 minute ramp-up time. To accomplish this, the 800 hp fan motor is equipped with a VFD, and we installed a center-fire pilot burner that operates continuously keeping the boiler hot and at pressure ready to go in the event we need it. If/when we need it, the centerfire burner acts as the pilot for the main (big) burner therefore we do not have the delay of waiting on a purge. On pilot we consume about 5 MMBTU/hr to keep the boiler warm and pressured up.

 

[ApC2Kp]

MJCronin,

Your proposed D-style package boiler is probably similar to four of Nebraska boilers supplied to a university power house that has a total of 6 boilers for 200 psi steam heat to the campus buildings. There is a heating coil in each bottom drum to maintain boilers in 'hot' standby, when any other operating boiler might go down, or if weather changes to require more steam than the three or four that are on-line typically. The off-line boilers are shut-in by the outlet dampers at FD fans and open to stack breeching duct. A superheater section on outlet of each boiler made 650F steam to main header. You should include non-return valve for the auxiliary boiler outlet, as well as block valve at main header (double block + bleed vent between).

The warming coil could be bayonet type U-shape, or line that runs length of drum. Some offset or dog-leg in full length line would provide flexibility for differential expansion of otherwise straight line 'warming coil' vs drum. If the warming coil were to be stainless steel for corrosion resistance, then flexibility would be very important. An alternate technique to warm the bottom drum would attach Contro-trace heating 'rectangular tubes' to the outside of drum. See site for Controls Southeast at

http://www.csiheat.com

for description of their bolt-on external tube products that replace jacket style heating on vessels and piping. This would avoid failure problems with an inserted bayonet or line inside drum.

Your 200,000 lb/hr of steam might be size that could fit a fire tube type boiler. The straight tubes could be more easily maintained or replaced, compared to D-style boiler. It will depend what steam pressure you need for startup. Many firetube boilers are used in waste heat recovery for 600 psi steam and higher. The 1 to 2 hour startup required might be a problem for a large firetube boiler.

 

[insult2injury]

I realized that it was an aux boiler. No one mentioned a regenerative air heater and I think rmw was referring to the main power plant boiler. The points I listed are all options for an aux boiler package.

Why would you keep the boiler in hot standby when its only planned use is for cold plant startup purposes which would imply that the plant has been shut down for at least 12 hours. Hot standby usually corresponds to a ramp time of 30 minutes or less, beyond that it's warm standby which I believe is what you want for the 1-2 hour ramp time with a lower drum heating coil.

I2I

 

[rmw]

insult2injury has correctly understood my question as referring to the main boiler plant. Aux boilers are used for the start up of supercrital boilers until a certain point in the start up process when the steam generation of the once through boiler is capable of sustaining itself. If not supercritical, is this a once through boiler?

My experience with those aux boilers and the grief they suffer from being left in 'hot standby' (they have to be ready to go if the unit trips) with respect to the air leakage and infiltration due to 'stack effect' from their heating coils is sufficient to say that I stand by my recommendation above especially in light of the revelation that the fuel is to be oil with some sulphur in it. "Sweating" (condensation) and sulfur residue on the tubes and casing are a recipe for disaster.

The guilliotine damper I recommended is the "stack damper" that you ask about. With that type of damper then you have truly 'bottled up' the heat input of the drum heaters.

rmw

PS: can someone send me to a link for a 200K pph firetube boiler?

PPS: are there other main plant boilers from which start up steam can be piped over for this unit which would then allow this aux boiler to be left cold for truly cold start ups.

 

[davefitz]

back to the original question.


some add'l features to be included in the aux boiler spec include :

steam purity- most large steam turbine vendors have stringent steam purity requirements, including steam used for seal steam and pre-warming operations

- may need an electric superheater if the steam is to be used for seal steam , as during hot restarts.

The cost of the aux boiler is often a major issue for EPC vendors- the lowest cost aux boiler which can also be started up very quickly is a packaged once thru unit, much less costly than a D-type unit. Must add an elect superheater to meet some steam turbine requirements for seal steam.

For gas fired combined cycle plants, the aux boiler can greatly assist in reducing the plant cold startup times- in order to meet the environmental permit requirement to not exceed permit emmissions for more than 4 hrs during a cold startup, it is neccesary to pre-warm the steam turbine and HP main steam line with aux steam.

For a large coal fired supercritical unit that uses a recirculaton pump in the furnace waterwall circuit, the boiler startup time can be greatly reduced if you pre-warm the boiler waterwalls etc by injecting aux steam into the suction pipe of the furnace outlet steam/water separators. One would first fill the boiler waterwall with demin water, start the recirc pump, inject aux steam into spargers installed in the separators inlet pipe, and bring the entire waterwall up to ( 200 C). This reqires vents be closed after air is ejected , and steam traps used on all superheater piping drains.

 

[ApC2Kp]

to rmw,

The Technotherm website has a photo of firetube WHB for sulfuric acid plant (think of firing molten sulfur in place of fuel oil) claiming 400,000 pph steam,

http://www.technotherm-usa.com/SulfuricAcidPlantBoilers.htm

 

[rmw]

AP...

Thanks for the site. I have not seen this manufacturer's product in the limited number of sulphuric acid plants I have any experience in but I have seen others (ABCO for example). I am not going to go look up the definition of 'fire-tube' boiler but nothing I saw on that site fit the description of what I have always considered to be the connotative use of the term of 'fire-tube' boiler. Granted the products of the combustion of the sulfur are on the tube-side of those Hx's but the "fire tube" that gives the fire-tube boiler its name was nowhere to be seen.

Really interesting link though. Some very nice looking equipment.

rmw

 

[rmw]

AP....

To add to the above, yes, yes, yes, I have plenty of experience with wood fired "fire-tube" boilers that had the furnace under the belly of the beast so I know....don't go there.

Somehow still, the mental picture that pops into my mind when the term 'fire-tubed' boiler has been used over the years, and especially in the context of the OP is the package style fire tube boiler where the furnace tube is in the same tube sheet as the fire tubes some in the center others offset from the center, some 2 pass, some 3 pass, some 4 pass.

It was not meant to quibble with your use of the word. I just haven't seen any of those that got anywhere near 200K PPH hence my question.

And yes, too, I have seen that type package fire tube boiler (Cleaver Brooks to name one-Kewanee another) used as Aux boilers for Supercritical units in Utility Power plants.

I come here to learn too and I did from your link. Thanks again.

rmw

 

[kennyc71]

MJC,
One installation I am familiar with also added a circulation pump for the water side. The chem feed system wasn't able to get an even distribution thruout the boiler without it & they continually had DO2 corrosion. They ended up circulating some boiler water, chem feed rates dropped significantly, & the corrosion disappeared.
Either way, good luck!