Hot Water

[Maine farmer]

We just installed a new and ridiculously expensive propane water heater.  This heater brings the water temperature up to 185 degrees Fahrenheit for cleaning our milking system and bulk milk tank.  There is a drain for the condensate, which we were advised should not be drained  onto the concrete due to it's acidity.  Can anyone tell me what the acid is that is produced from the combustion of propane?

[exchemist]

We just installed a new and ridiculously expensive propane water heater.  This heater brings the water temperature up to 185 degrees Fahrenheit for cleaning our milking system and bulk milk tank.  There is a drain for the condensate, which we were advised should not be drained  onto the concrete due to it's acidity.  Can anyone tell me what the acid is that is produced from the combustion of propane?

That's a strange one. If the burner works properly,  and the propane is pure, I would have expected the products of combustion to be just CO2 and water. Normally I would associate acid exhaust products with the presence of sulphur in the fuel (sulphur acids), or in the case of very high temperature combustion, nitrous and nitric acids from reaction of a bit of the nitrogen in the air. But not in a water heater. 

 

Conceivably what they mean is that the condensate will have carbonic acid, i.e. dissolved CO2, in it. As concrete is chemically related to limestone, which is slowly attacked by carbonic acid, possibly over time this would be enough to erode your concrete. 

 

P.S. Found this link, which explains a bit more about acid attack on concrete, though not why you get acids in propane exhaust: http://www.concrete-experts.com/pages/acid.htm

Edited September 7, 2017 by exchemist

[DrKrettin]

Is it possible that the propane is not completely pure, and that impurities cause the acidity?

[OceanBreeze]

Not being a chemist, (or an exchemist) but being curious, I admit I had to do a search online:

 

The hot fire in a gas furnace or boiler is a reaction between natural gas (or sometimes propane) and oxygen — the units burn methane (CH4) or propane (C3H8) and oxygen (O2) to produce H20 (water) and CO2 (carbon dioxide). But the hot flame also sucks in atmospheric nitrogen, which makes up 70% of the atmosphere, and the fire is hot enough to force the nitrogen to react with oxygen (even though that is an "endothermic" uphill reaction that absorbs heat rather than releasing it). The resulting NO2 (nitrogen dioxide) and NO (nitrous oxide) dissolve into the water vapor in the exhaust stream to form nitric acid.

 

In old-fashioned low-efficiency gas burners, all that gas — the CO2, the water vapor, and the nitrogen products (known collectively as "NOX"), went up the hot chimney into the atmosphere. They polluted the air, but they didn't damage the house. But that was a waste of energy: gas burners that didn't condense the water out of the exhaust stream can't rate much higher than 80% efficiency at best, and about 65% was more typical. To squeeze 90% or more of the heat energy out of a gas flame, you have to condense the water vapor out of the exhaust into liquid form and recover the heat of evaporation from it.

 

It turns out, however, that when you condense out the water vapor, the nitrogen products — the NOX — come along with it. What you end up with isn't clean water — it's a dilute solution of nitric acid.

 

And it looks nasty!

 

[Maine farmer]

Thanks for the replies!  

 

Not being a chemist, (or an exchemist) but being curious, I admit I had to do a search online:

 

The hot fire in a gas furnace or boiler is a reaction between natural gas (or sometimes propane) and oxygen — the units burn methane (CH4) or propane (C3H8) and oxygen (O2) to produce H20 (water) and CO2 (carbon dioxide). But the hot flame also sucks in atmospheric nitrogen, which makes up 70% of the atmosphere, and the fire is hot enough to force the nitrogen to react with oxygen (even though that is an "endothermic" uphill reaction that absorbs heat rather than releasing it). The resulting NO2 (nitrogen dioxide) and NO (nitrous oxide) dissolve into the water vapor in the exhaust stream to form nitric acid.

 

In old-fashioned low-efficiency gas burners, all that gas — the CO2, the water vapor, and the nitrogen products (known collectively as "NOX"), went up the hot chimney into the atmosphere. They polluted the air, but they didn't damage the house. But that was a waste of energy: gas burners that didn't condense the water out of the exhaust stream can't rate much higher than 80% efficiency at best, and about 65% was more typical. To squeeze 90% or more of the heat energy out of a gas flame, you have to condense the water vapor out of the exhaust into liquid form and recover the heat of evaporation from it.

 

It turns out, however, that when you condense out the water vapor, the nitrogen products — the NOX — come along with it. What you end up with isn't clean water — it's a dilute solution of nitric acid.

 

And it looks nasty!

 

I don't know why none of us thought about the nitrogen in the equation.  Very interesting.

 

Now a follow up question.  

 

We use limestone in our bedding in the stalls, and the drain where the condensate drains ultimately ends up in our manure pit along with the used bedding.  So what will result when the limestone reacts with the nitric acid?

[exchemist]

Not being a chemist, (or an exchemist) but being curious, I admit I had to do a search online:

 

The hot fire in a gas furnace or boiler is a reaction between natural gas (or sometimes propane) and oxygen — the units burn methane (CH4) or propane (C3H8) and oxygen (O2) to produce H20 (water) and CO2 (carbon dioxide). But the hot flame also sucks in atmospheric nitrogen, which makes up 70% of the atmosphere, and the fire is hot enough to force the nitrogen to react with oxygen (even though that is an "endothermic" uphill reaction that absorbs heat rather than releasing it). The resulting NO2 (nitrogen dioxide) and NO (nitrous oxide) dissolve into the water vapor in the exhaust stream to form nitric acid.

 

In old-fashioned low-efficiency gas burners, all that gas — the CO2, the water vapor, and the nitrogen products (known collectively as "NOX"), went up the hot chimney into the atmosphere. They polluted the air, but they didn't damage the house. But that was a waste of energy: gas burners that didn't condense the water out of the exhaust stream can't rate much higher than 80% efficiency at best, and about 65% was more typical. To squeeze 90% or more of the heat energy out of a gas flame, you have to condense the water vapor out of the exhaust into liquid form and recover the heat of evaporation from it.

 

It turns out, however, that when you condense out the water vapor, the nitrogen products — the NOX — come along with it. What you end up with isn't clean water — it's a dilute solution of nitric acid.

 

And it looks nasty!

 

Thanks for this. Evidently I was wrong to assume that a heating burner would not be hot enough to crack nitrogen in the air. We're all familiar enough with this happening in internal combustion engines but I never thought it would be significant in a domestic boiler.  Now we know. So it's the old NOx story. 

[exchemist]

Thanks for the replies!  

 

I don't know why none of us thought about the nitrogen in the equation.  Very interesting.

 

Now a follow up question.  

 

We use limestone in our bedding in the stalls, and the drain where the condensate drains ultimately ends up in our manure pit along with the used bedding.  So what will result when the limestone reacts with the nitric acid?

Do you mean that limestone dust goes into the manure pit? If so all it will do is tend to neutralise the acid: 2HNO3 + CaCO3 ->  Ca(NO3)2 + H2O + CO2. CaNO3 is quite soluble - it is even used in fertilisers.

[Maine farmer]

. Evidently I was wrong to assume that a heating burner would not be hot enough to crack nitrogen in the air. We're all familiar enough with this happening in internal combustion engines but I never thought it would be significant in a domestic boiler.  Now we know. So it's the old NOx story. 

Well, it is an industrial water heater, and the exhaust gasses are cool enough to vent with pvc.

 

Do you mean that limestone dust goes into the manure pit? If so all it will do is tend to neutralise the acid: 2HNO3 + CaCO3 ->  Ca(NO3)2 + H2O + CO2. CaNO3 is quite soluble - it is even used in fertilisers.

No wonder our grass is so green.

[exchemist]

Well, it is an industrial water heater, and the exhaust gasses are cool enough to vent with pvc.

 

No wonder our grass is so green.

I had a look at the temperature dependence of thermal NOx formation and various sources indicate formation can become significant at temperatures above about 1500C. A lot depends on the residence time at that temperature however. Evidently boiler burners get hotter than I had realised.

 

There can also be contributions from organic nitrogen-containing impurities in the fuel, which would burn to produce NOx at much lower temperatures. However I don't think I'd expect much of that in propane, as most organic nitrogen compounds in crude oil will have molecular weights quite a bit higher than propane and would not distill over with it.  

[Maine farmer]

I had a look at the temperature dependence of thermal NOx formation and various sources indicate formation can become significant at temperatures above about 1500C. A lot depends on the residence time at that temperature however. Evidently boiler burners get hotter than I had realised.

 

These new high efficiency heaters are an impressive design.  They are claiming 96% efficiency, and given the cool temperature of the exhaust gasses, I tend to believe it.

 

It's a far cry from the original heater we had that had a pilot light and just had a flame under the tank.  We went from that to using two of the tankless heaters, which could put out the hot water at high volume, but we were at the high end of their design capabilities,and as they aged, we were spending too much time and money on repairs, so we figured if we were going to have to spend the big money on a replacement, we may as well get the latest and greatest

Edited September 8, 2017 by Farming guy