Combustion Efficiency & Flue Gas Condensation

Mike T., Swampeast MO

Can someone point me to a good MODERN reference on how this is computed? Web or otherwise--layman or technical--doesn't matter.

My old references (pre 1970s) tend to make me believe that a condensing boiler would be better rated on some type of dual scale, like 93-13, meaning 93% combustion efficiency and 13% condensation recovery efficiency...

Call me dense, but despite repeated, patient explanations I'm still not understanding how the energy of state change of water produced by burning hydrogen fits into the equation. If it's not considered this seems the only explanation for 100+% efficiency claims in boilers. If it is considered I'm at a loss for any explanation beyond another "bonus" of the state change between plasma and gas.

pr_malk

Heat of condensation

I think you're right, the boilers should be rated with both numbers. Actually I would like to see ratings of a great many other things as well (electrical consumtion, emissions, etc).

Are you unsure because you aren't making the assumption that immetiatly after combustion, the water is in the gas state? In most all chemical reations, it is assumed the phase is constant across the reaction.

Mike T., Swampeast MO

Hard to assume that water over 1000° by any scale exists as a liquid.

When I read references in my library regarding how natural gas is combusted, the gain to be had from condensing this vapor into a liquid does not seem to be considered a loss.

If it is not a loss, it cannot be part of the fuel.

The BTU content of a cubic foot of gas seems to based on these same equations of combustion. That's what REALLY confuses me!

Glen

If you consider that about 970 btus are "recovered" each time a pound of water condenses (and that is the change of state not temperature) - the total efficiency equation accounts for much more than the consumption of fuel. I bet that Timmie can give you good stats re exact condensate production per therm -

scrook_2

does it help...

to know that there are 2 specifications for the heating value of a fuel, the "Higher Heating Value" or HHV and the Lower Heating Value" or LHV. The HHV is the amount of heat energy available if the fuel is completely combusted and ALL the water that results is condensed to liquid phase, the LHV is the energy available if the fuel is completely burned but ALL the water is still in the gas phase. The difference is exactly the heat of vaproization of the water produced by burning the fuel.

You see #2 oil described as 140,000 BTU/gallon or natural gas as 100,000 BTU per 100 cubic feet, etc., these are all HHV's. If you were to *completely* burn 100 cu. ft of NG, condense ALL the water in the combustion products AND cool the liquid water and gasious CO2 ALL the way to the original temperature of the unburned NG and th ecombustion air (assume the NG and the air were at the same temperature initially) then you would have 100% combustion efficiency.

If the liquid water and gassious CO2 is still warmer than the initial gas and air. and/or if there is ANY gas phase water present, then your efficiency will be less than 100%.
If most or all the water stays as gas phase then your efficiency will be quite a but lower than 100% (here's where using the LHV could be useful (though it is little used), it would tell you how close to ideal you were given the limitation that you were not allowed to condense any of the water).

In the real world, condensing or not, matters become even worse since you need at least a little excess air and it in addition to the CO2 and H2O must be at least a little warmer than they were when they came in, though you can, yith well designed and set up equipment, get into the mid 90's% combustion efficiency.

Mike T., Swampeast MO

YES! Thank you!

Are the HHV and LHV relatively new distinctions?

scrook_2

1000°F water

Note that energy is liberated by burning the fuel, but as you are near atmospheric pressure and at 1000's of degree's F the water component is indeed in gas phase. Letting it revert to liqud phase releases an additional 970 BTU/lb that was taken from the energy released when the fuel was oxidized -- the energy to vaporize it as it is formed from the HC fuel comes from energy formerly stored in the fuel. The BTU content of the gas reflects all the energy stored in it before it is burned, but that energy doesn't know or care whether it will be become heat or become the energy to vaporize water --it's just energy. You can store or release or restore it simply by vaporizing, condensing or revaporizing that water, but initially it was stored in the chemical bonds of the fuel.

Glen

HHV & LHV

To what is the difference credited to? More air entrained in fuel gas, more impurities? And would you then make adjustments to combustion and ventilation air? To which value? Got to ask - just use 1000 btus/cf here in B.C.

jim sokolovic

Lower heating value of fuel...

I understand that the Europeans use the LHV of the fuel in the input side of their efficiency calculation, and the actual thermal energy produced in the output side of the calculation, thus the misleading efficiencies that can exceed 100 % are derived. As far as I know, only the HHV is used in the efficiency calculations for I=B=R in U.S.A..