The myth of carbonic acid in flue gas condensate

BernardCuzzillo

After reading here and elsewhere that the corrodant in flue gas condensate was carbonic acid, it didn't track with my understanding. So I dug out the original research from the early 1980s by Battelle-Columbus Laboratories for the Gas Research Institute (Report No. GRI-85/0282, Technology Development for Corrosion-Resistant Condensing Heat Exchangers). They did an elaborate study of the composition of condensate an found no carbonic acid (at least none worth their mentioning it--the word "carbonic" is not in it). The main acids are sulfuric (derived from odorants in fuel gas), nitric (from gaseous NOx produced in the flame), hydrochloric (usually from household chemicals like bleach), and hydrofluoric (from release of chlorofluorocarbons like refrigerants, probably obsolete today). They categorically dismiss organic substances generally: "Trace amounts of organics that are found in flue-gas condensate were judged to have no significant effect on its corrosivity." (And carbonic acid is usually not considered organic in spite of its carbon content.)

Further, carbonic acid is weak, forming only a fraction of a percent of the CO2 dissolved in water, and only a fraction of a percent of that dissociates into ions which are the aggressive materials. And the solubility of CO2 in condensate is low because the temperature is relatively high and the partial pressure of CO2 is so low. In contrast, to get a lot of CO2 in carbonated beverages, the water is cold and the pressure is several atmospheres of pure CO2.

The idea of carbonic acid as a corrodant may have come from the water side of steam boilers where it IS important. In that case it comes from carbonates in the feedwater.

Flue gas condensate is certainly aggressive which is why the inner wall of metal vent pipes for condensing appliances are made of superferritic stainless steel. But it has nothing to do with carbonic acid.

Leonard

I think in past sulfur was present in small amounts in fuel oil. When burned that made SO2 which combined with steam in the exhaust to make H2SO4 ( sulfuric acid) . That attacks cement between fuel tiles. At top of chimney I can stick my hand where cement used to be.

I remember reading years ago ( may 10 ) they removed sulfer from diesel for EPA reasons and it caused all kinds of trouble with diesel gelling in the school bus lines in cold weather. Apparently sulfur depress the gell point of diesel, which is essentially #2 heating fuel.

So maybe not much sulfur in # 2 these days , I don't know.

nicholas bonham-carter

The condensate from the flue would be Sulphuric acid, while condensate from the steam side would be Carbonic acid-see “condensate grooving”—NBC

Jamie Hall

Nice commentary -- except for one error. Carbonic acid is not classed as an organic compound. The Batelle research is quite correct regarding organic acids (they are indeed very weak acids, and rarely a problem). Carbonic acid is a strong acid, and classed as a mineral acid, but as the Batelle research also noted -- correctly -- the concentration in flue gas is very small, and thus it also is much less of a problem than either sulphuric (now much reduced by using ULSD fuels) or nitric and nitrous (which remain a problem unless assisted catalytic conversion or flue gas recycling is used).

BernardCuzzillo

@Jamie Hall: Thank you. Clarification made.

But carbonic acid is weak, not strong, because very little of it ionizes.

STEVEusaPA

The initial 'problem' with ULSD fuels was lubricity, now remedied with an additive. Pour point and gelling issues came from bio fuel, which gels at a higher temperature.

mxfrank

I suspect the Battelle research was with regard to natural gas boilers only, not fuel oil. I had a Hydropulse condensing boiler during that era, and I can tell you that it experienced extreme corrosion problems. It was a problem, regardless of the chemistry.

BernardCuzzillo

@mxfrank: Yes. That whole project was natural gas only, although probably the same applies to LP. But fuel oil is another matter because 1.) Different impurities, and 2.) Oil burns with a diffusion flame, unlike the partial-premix (i.e. Bunsen) burners of that era.

JUGHNE

So then do we know where the "coffee grounds" in Mod Cons comes from.

RayWohlfarth

My research shows that for each cubic foot of gas burned, the flue gases will contain one cubic foot of carbon dioxide, eight cubic feet of nitrogen, and two cubic feet of water vapor. When the water vapor mixes with the carbon dioxide, carbonic acid is formed. The pH of the flue gas is slightly acidic, between 3-4. This could be acidic enough to damage to pipes, sewage treatment facilities, and septic systems. Hope this helps.