Combustion Problem--What am I doing wrong?

Mike T., Swampeast MO

Was writing a reply to someone who wrote me asking me to explain the efficiency numbers coming out of condensing boilers...

This is part of the reply and I had to stop. PLEASE TELL ME WHAT I HAVE DONE THAT IS WRONG!!!!!

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A pound of natural gas requires 17½ pounds of "standard" air for combustion. Even in the 1951 book excess air in natural gas combustion was considered to be quite low--with 12% to 15% the normal range and 15% used for computation.

18.3 pounds of air was the 1951 number for burning a pound of natural gas. I do though believe that modern condensing boilers reduce excess air quite considerably and some come extremely close to "perfect" combustion of the fuel.

I'm going to work "backwards" from a conventional boiler...

So, say you have 40° incoming air and 440° stack temp. Each pound of air is taking away 400 * 0.24 = 96 btu. Since it was 18.3 pounds of air for burning a pound of natural gas total loss is about 96 * 18.3 = 1,757 btu. (The 0.24 number is number of BTUs required to raise 1# of air 1°.)

This was NOT easy to find, but one pound of natural gas has about 21,000 BTUs. So in this simple example the stack loss was 1,757/21,000 or about 8%.

Now it get "jiggy."

That pound of natural gas produced something else in that combustion air--water--LOTS of water. About 8.94 pounds! But, what is the temperature of that water? Is it the same as the stack temperature of 440°? I don't believe so, because it is STEAM at zero pounds of pressure so it is about 212°.

Each pound of steam contains 970 BTUs of latent heat--the heat required to make it "steam" instead of "water".

SO...in the latent heat ALONE there are 8,672 BTUs of heat just waiting to be recovered!!!!

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WHAT HAVE I DONE WRONG????

R. Kalia

maybe...

> That pound of natural gas produced

> something else in that combustion

> air--water--LOTS of water. About 8.94 pounds!


My estimate, based on nat gas being mostly methane (CH4), suggests that the correct amount will be around 2 lb water. This is based on the fact that CH4 (nat gas) and H20 (water) molecules weigh almost the same (CH4 is 12+4=16, H2O is 2+16=18), and two of the latter are produced per one of the former during combustion:

CH4 + 2 O2 --> 2 H20 + CO2

Notice that 2lb water happens to lead to a ballpark reasonable upper limit (2*970/21000 =~10%) for the efficiency increase due to condensing.

JimGPE_3

It ain't saturated.

The vapor is stack temperature.

You are correct, steam at 0psig is 212 IF it is saturated, which this steam is not. This is superheated steam. In addition, the pressure of the water vapor is much less than 0psig, because if you want to look at the steam tables, you need the PARTIAL pressure of the steam, not the pressure of the steam/air mixture.

Lastly, don't forget - pressures in steam tables are normally absolute pressures - 0psig = 14.696 psia.

Mike T., Swampeast MO

Will search again but [think] I was looking at tables where everything was in pounds...

Mike T., Swampeast MO

Thank you!

Please forgive my ignorance of steam and vapor pressure, but what I [think] you are saying is that since it's not "just" steam and it's not "contained" that it's vapor that hasn't picked up much? any? indeterminate? latent heat. Correct?

Mike T., Swampeast MO

Must be wrong table...

THANK YOU!!!!!

Just did some in-depth searches with all sorts of permutations and STILL couldn't find: burn x amount of natural gas and get y amount of water!

Your molecular weight estimate is CERTAINTLY more accurate!

scrook_2

back of the envelope review...

The water is at 440°F as is the rest of the constituents of the flue gas, mostly N2, CO2, O2, (and a trace of argon), the 440°F water is *superheated*, not saturated vapor (gas) (or saturated liquid, or in between -- part vapor, part liquid -- either for that matter), it heats just as the other gasses (which are also well above their boiling -- or sublimation for the CO2 -- temperatures.

The heat to warm that water vapor to 440°F AFTER it is vaporized is somewhere around and 1 BTU/lb°F (about 0.8 at 130° and about 1.4 at 440° if I read the tables and do the metric to english conversion right), but you are correct, after removing the superheat the process of condensation releases 970 BTU/lb with no temperature change, just a phase change.

However if you look up the Higher Heating Value (HHV or gross) for Methane (the major ingredient in NG) you find it is 1027 BTU/cu ft (102,700 per CCF) and if you look up the Lower Heating Value (LHV or net) it is 930 BTU/cu ft, a difference of only 97 BTU/cu ft. Methane at 60°F and 1 atm has a density of 0.68kg/ cu meter or about 0.042 Lb/cu ft so the HHV and LHV are about 24,450 and 22,140 BTU/lb a differance of only 2310 BTU/lb, or about 10.4% more (still not a trivial amount, and on top of that is the ~0.24BTU/lb (is this a good number over the temperature range in question, or is it a little higher at the higher temps?) recovered getting from 440°F to 100% humidity (perhaps 130°F gas temp) -- call it 75 BTU/lb -- whereupon the water starts condensing releasing the 970 BTU/lb of water (not per lb of flue gas, a lot of which isn't water). If you condense all the water (this all assumes ideal conditions) you're up to 2385 BTU/lb more, a 10.8% improvement, so 83% efficient (we're living right on the edge of condensation) becomes 92% efficient, in the ballpark of combustion efficiency numbers.

Mark Hunt

uhhhhhhhh


Can you actually "super-heat" steam?

I was told that you can't by a Nuclear Engineer.

You can super-heat water, but not steam. Meaning, you can "super heat" water (liquid) beyond it's boiling point and still have a liquid. There is no "super heating" of steam(a gas).

Mark H

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bob_25

latent

scrook, xlnt! When the flue gas mixture starts to condense and some water drops out, doesn't that lower the dew point temperature of the mixture stoping condensation? bob

Mark Eatherton1

Jeeze...

if that what you do on the back of an envelope, I'd HATE to see what you could do on an 8-1/2 X 11 Big Chief tablet...:-)

I'm impressed!

ME