Regenerative air heater vs. tubular air heater

[BINGMD]

I am trying to specify the air heater type for an 830,000 lb/hr (378 metric tph) utility PC boiler. The fuel S content will be under 1% and I will set the flue gas exit temperature at air heater outlet to be 20-25F above acid dew point. For this size boiler, is tubular type air heater more economical, vs. regenerative air heater, considering capital cost, maintenance cost and operation cost?

 

[athomas236]

I am not sure of the exact economics but I have only seen a tubular airheater on one utility boiler and that was a 30MWe oil fired unit in East Africa.

On the ten coal fired plant I have worked on, I have never seen a tubular airheater. I have known clients who have toyed with the idea of tubular airheaters for the primary air systems but they were never implemented.

With 1% sulpur coal, the acid dewpoint will be about 132C which adding your 20 to 25F margin will give a airheater outlet gas temperature of about 145C. This more like the temperature for oil-firing rather than coal. The usual temperature for coal is about 130C with Corten cold end elements.

It should be noted that cold end elements are intended to be replaced every 2 - 3 years. It may be worthwhile comparing the cost of replacing the cost of installing cold end elements every 3 years with extra cost of coal over the same period for gas temperatures of 145C compared with 130C.

If does not make the matter clear then more detailed investigations of airheater and associated civil costs may be needed.

Regards,

athomas236

 

[boilerone]

BINGMD,

The boilers of your size (about 110MW, I guess) typically use fixed tubular air heaters. It is higher on initial cost but much lower on maintenance and operation reliability. Larger boilers have to use the rotary, regenerative types mainly because of not having enough spaces for tubular ones plus to avoid high initial costs. Regenerative air heaters belong to mechanical equipment with moving parts so require a lot of maintenances plus sometimes mechanical failures cause forced shut-downs and reduces boiler availability.

As far as corrosion is talked about, under the same gas conditions plus similar materials (such as Corten) used, the tubular ones last much longer because of having much thicker walls vs. regenerative ones which use very thin, corrugated metal sheets as heat-carrying elements which do not last very long and require change-outs more frequently.

Besides, to avoid acid corrosion, it is better to set air heater (no matter what type used) outlet gas temperature even higher than 25F above acid dew point. It is because when gas contacts with the much-colder air heater elements, its temperature might go much lower than dew point so corrosion still happens. Therefore typical and practical designs instead of looking at gas temperatures and acid dew points, they look at cold end element surface temperatures and make sure them to be about 35F above water's dew point but 'somewhat below the acid dew point, where the efficiency gained more than balances the additional maintenance costs.'(ref. B&W 'Steam' book 41Ed. page 20-13). The surface temperatures recommended for particular fuel sulfur content and type of boilers used are given in Fig. 22 and 23 in that 'Steam' book. For your case, it is 160F for tubular air heater or 155F for regenerative ones. When you have this not-to-exceed surface temperature plus having cold air inlet temperature then you can get minimum flue gas temperature by a simple heat-transfer calculation. The formula to show temp. relationships is below:

Twall = Tair + (Tgas - Tair)/(1 + Kair/Kgas)

Kair and Kgas are heat-exchange factors on air and gas sides, respectively. You can easily develop a Tgas formula from above.

If you are dealing with an air heater vendor, they are going to ask you for a full-set of design/operation data and do the similar calculations and recommend your gas outlet temperature.

Hope these help.

Boilerone

 

[rmw]

Here are some facts about each type.

Tubulars will always have a cold corner where the FD air initially enters the APH. (Maybe you should rethink that SCAH you asked about in another thread if you choose this type.)

Regenerative types, (and this comment will be more specific to the Ljungstrom) have constant and guaranteed leakage. A brand new Ljungstrom, if that is what you choose will have a guaranteed 8% leakage rate new out of the box. Over time, that will increase to as high as 25%. (I have seen that much leakage past warped and worn out Ljungstrom seals.)

You then have to elevate your back end temperature even more to account for the temperature dilution caused by the cold FD air leakage past the seals. Remember that the FD air at this point is high pressure and the flue gas side is usually low pressure or negative, so even a little seal opening will result in a healthy leak.

You didn't ask, but this size boiler might lend itself well to a plate type APH. They have been successfully applied to coal and lignite boilers. With a plate type you will still have to deal with the "cold corner" but you can get some really close approach temperatures with these.

rmw

 

[BINGMD]

rmw, what is your recommendation for the approach temperature for plate type AH and Tubular AH? Thanks.

 

[BINGMD]

Thanks for all the helpful recommendation. I am more familiar with regenerative AH and have not worked on the tublular AH before. For a 100 MW boiler, what size of the tubular AH would be? Will it require one AH only? Appreciate your response.

 

[rmw]

Bing,

You are stretching some memory glands and right off the top of my mind I can't come up with an approach temperature. Do you have a copy of B&W Steam? I think you will find a good comparison and some good information on each type there.

The largest tubular I remember seeing (or being in) was on a 500-600,000 lb/hr boiler and I seem to remember it being something like 30 feet tall and about the width of the boiler itself and about 15-20 feet deep, but that has been many years ago so don't trust my memory.

rmw

 

[BINGMD]

Thanks, rmw. I have the steam book and read it. But i do get good info from this forum. Thanks!

 

[boilerone]

RMW,

Yes, I have 41th Edition (the latest?) Steam book. The whole Chapter 20 talks about economizers and air heaters and has good information. I have other text books and resources, too.

The largest PC boiler that still uses tubular air heater and I was involved with was 200MW (about 1.5 Mlb/hr). High leaking rate like you mentioned and high maintenance with the Ljungstrom type was the reason of not using it.

Boilerone