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Nitric or Carbonic acid in condensate?

steamfitter
steamfitter Member Posts: 156
Regarding the condensate produced in Condensing Boilers, I understand that the pH is low making the condensate acidic and therefore corrosive. A Neutralizer (limestone) is supposed to help raise the level of the pH. However, I am reading discussions, in other forums, call the general make-up of this condensate, Nitric Acid (a strong acid-pH 1-3), while others say it is, Carbonic Acid (a weaker acid-pH 3-5). I was under the impression that Carbonic Acid was found in steam condensate and it's pH was relative to the steaming process, which could be either weak or strong.

"Nitiric Acid" is a new term for me. Does anybody have information on this terminology? Can it be either one or the other when we're talking about condensate formed from condensing hot flue gases in a Condensing Boiler?


Also, I have seen some photos online that show corrosion of floor drains and ductwork, but very little on piping. PVC and other plastic piping is recommended. Does anybody have any photos that show corroded copper tubing drain lines as a result of NOT using a Neutralizer? Just curious!

Thanks guys!!!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,165
    It's both. Nitric acid, HNO3, is a strong acid (to put it mildly). As you say, pH in the concentrations in flue gas will probably be around 1 to 3 -- but it can go much lower. It attacks almost any metal. The nitric acid is formed in any combustion process which uses air as the oxidizer, because air contains 80% nitrogen. The heat of combustion is enough to oxidize that nitrogen to one of a variety of nitrogen oxides, which then react with the water from the combustion to form the nitric acid.

    Carbonic acid, H2CO3, is also found in flue gas. It is a weaker acid, but that doesn't mean that it isn't less corrosive -- and it's only slightly less corrosive than nitric. It is formed by the reaction of the water of combustion with the carbon dioxide from combustion. The concentration in the flue gas from a natural gas fired appliance is lower, as there is less carbon in natural gas than there is in LP gas or fuel oil.

    Carbonic acid can be found in small quantities in steam condensate, but shouldn't be a problem in most installations. It can be a problem if distilled water is used as the boiler fill. In that situation, as distinct from the flue gas situation, it comes from the natural solution of the carbon dioxide in the air in the pipes dissolving in the condensate.

    If the boiler doesn't condense, you obviously don't have a liquid solution of either one. If it does, you do, and it is the liquid solutions which are corrosive.

    I'm afraid I don't have any pictures to show you, but I can assure you that carbonic will go through copper, but the amounts in steam condensate shouldn't do significant damage in any reasonable time frame (like centuries).

    The carbonic in flue gas will, and the nitric will go through metals a lot faster. A neutralizer, such as limestone, is a must before the condensate from flue gas is allowed to get into any metal at all. Plastic piping of any sort is pretty much immune from acid corrosion.

    Hope this helps...

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Solid_Fuel_Mansteamfitter
  • Leonard
    Leonard Member Posts: 903
    edited February 2017
    When hydrocarbon fuel burns among other things you get water (steam), CO, CO2, NOx (nitrogen oxides), SO2( sulfur dioxide). The CO , NOx , SO2 and maybe CO2 combine with steam to make the different acids.

    I would expect the:
    CO, CO2 to make carbonic acids,
    NOx to make nitric acid,
    SO2 to make sulfuric acid (batery acid).

    Pure acids of nitric and sulfuric acids can be very strong ( low ph), but if have a lot more water in the mix they can be weaker acids ( closer to 7 ph). All depends how on how much water (condensate) is in the mix to dilute the acid. (or how little of these gases are in the exhaust compared to the water)
    Solid_Fuel_Mansteamfitter
  • Jamie Hall
    Jamie Hall Member Posts: 17,165
    I didn't mention the sulphuric acid; it won't be a problem with either natural gas or LP -- unless you have the misfortune to have a raw sour gas feed which is unlikely. It will be a problem with some fuel oils; low sulphur diesel wouldn't be, but some heating oils (never mind heavier oils) can be.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Leonard
    Leonard Member Posts: 903
    The nitric acid comes from flame heating the nitrogen and oxygen in the air to make NOx ( various oxides of nitrogen) . Then the NOx reacting with steam in exhaust to make some strength of nitric acid.

    Most any acid can attack cements, which is generally an alkaline material.
    steamfitter
  • steamfitter
    steamfitter Member Posts: 156
    Great info!!! Thank you all!
    So, JH, is it safe to say that, generally speaking, more nitric acid is formed in condensed flue gases than carbonic acid, while carbonic acid is formed in steam condensate?

    I was suprised to hear that carbonic acid is almost as corrosive as the nitric acid. Interesting!

    I have seen pictures of clear neutralizers. What would a service tech look for there? Discoloration?
    Should the limestone be changed annually? or is there a possibility of a need to change it more or less frequently?

    Thanks again!
  • NY_Rob
    NY_Rob Member Posts: 1,370

    Great info!!! Thank you all!

    ... I have seen pictures of clear neutralizers. What would a service tech look for there? Discoloration?

    Should the limestone be changed annually? or is there a possibility of a need to change it more or less frequently?



    Thanks again!

    I have one of the clear neutralizer tubes on my mod-con...
    The limestone or marble chips just wear down to sand grain size and wash out with the condensate over time. You start with a full tube of rocks, and over time you see less an less rocks in the tube. I went six months (three months with just using the boiler for heating the DHW tank, and the last three months for DHW and spaceheating) without touching my condensate tube. In Jan it was looking a little funky in there, so I emptied it into a bucket, rinsed the (now noticeably smaller) stones and put it back in service with probably about 75-80% of it's original volume of stones inside.
    By the end of June, it's one year anniversary- it will be ready for a new load of marble chips ($2 bag from Home Depot).

  • Jamie Hall
    Jamie Hall Member Posts: 17,165
    You will get more nitric than carbonic in the flue gas, but you will inevitably get some of each. You'd get some carbonic in the steam condensate, but no nitric at all.

    And yes, it is surprising that the corrosivity of an acid isn't so much a function of the pH, although that certainly makes a difference, as it is of the type of acid.

    As @NY_Rob said, it isn't so much that one changes the limestone (or marble or dolomite -- depends on just what you are neutralizing) chips as you renew them. As they work to neutralize the acid, they themselves are dissolved in the water -- chemically speaking, you are substituting Calcium or Magnesium ions for hydrogen ions. And the chips do get smaller; some people find it necessary to put a particulate filter after the neutralizer.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hydro_newbie
    hydro_newbie Member Posts: 37
    Just wondering, how much do these neutralizers bring up the pH of the condensate? Is the goal to get it all the way to 7, or is something a bit lower sufficient? Not sure what limits this...I guess the amount of neutralizing material and amount of time the liquid stays in there).
  • JUGHNE
    JUGHNE Member Posts: 9,001
    The first Modcon I installed did not have any condensate neutralizer. The drain line ran to a cast iron floor drain thru a garden hose. The male brass fitting was left on the end of the garden hose. After maybe 2 heating seasons the bottom of the brass fitting was eaten away. I have since added a neutralizer, cut off the brass fitting and inserted a scrap of 5/8" OD copper tubing. The copper scrap is my monitor of the effectiveness of the neutralizer.
    If using a neutralizer before a condensate pump I stick a scrap of 3/8" OD copper in the discharge tube for the same reasons.

    My first home boiler was a high 80% near condensing cast iron WM Gold GV model. The plastic drain hose was about 6" short of the PVC floor drain. The first years of use eroded/etched the concrete floor, cutting a groove. FWIW
  • steamfitter
    steamfitter Member Posts: 156
    Thank you JH for the clarification and addt'l info!
    Thank you NY_Rob for your personal example and vital info!
    And thanks to hydro_newbie and JUGHNE as well for your input! I like the idea of installing a sacrificial copper pipe so you can easily tell if the neutralizer is working.
    You guys are GREAT!
    The way you answer questions and provide detailed and concise answers, backed by experience and tremendous knowledge is so incredibly valuable!
    I feel like I learn so much with each post.
    Wishing you all the BEST!
    RLuck
  • Steve Minnich
    Steve Minnich Member Posts: 2,680
    We just started making our own at a small fraction of the cost we were buying them for. We used a screwed cap on one end so we could clean and refill them annually.
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
    GrallertMilanD
  • rick in Alaska
    rick in Alaska Member Posts: 1,246
    My supplier charges me a lot for them, so building them, even with the time involved, makes for a good investment.
    Rick
    GrallertSteve Minnich
  • Grallert
    Grallert Member Posts: 524
    I've been using 3" drum traps to make neutralizers.
    Solid_Fuel_Man