If biofuel is burnt, for example wood, and the stove I am building is made of metal, what is the most appropriate metal material I should use to avoid excessive corrosion in the exhaust pipe of the stove?
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$\begingroup$ 'for example wood' What other biofuels do you have in mind? Manure? $\endgroup$– martNov 13, 2015 at 6:33
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3 Answers
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It depends on flue gas temperature.
- High temperature $(>250\ \textrm{C})$: use galvanized steel.
- Low temperature $(<250\ \textrm{C})$, use stainless steel or PVC as appropriate.
See below for an explanation.
Flues conveying higher temperature flue gas are typically made from galvanized, rolled, plain steel, as sulfuric acid condensation doesn't occur, so acid damage risk is low. The galvanization also protects plain steel from atmospheric humidity.
Flues conveying lower temperature flue gas, such as in a high-efficiency furnace, are typically made from PVC or stainless steel depending on the expected operating temperatures. If the temperature is near or above the glass-transition temperature of the PVC used (approximately $80\ \textrm{C}$), then stainless steel should be used instead.
The breakpoint between higher and lower temperatures here is the point at which condensation of sulfuric acid occurs. The reasoning is that combustion fuel contains non-negligible quantities of sulfur, which combines with oxygen and water to form sulfuric acid. Note that water is a byproduct of combustion of organic materials, so ambient humidity is irrelevant as to whether the process can occur, it only affects the temperature at which it occurs. See this link for more details. The image below (from the link) can be used to estimate the sulfuric acid dew point from the gaseous sulfur trioxide concentration and the water concentration.
Based only on the provided chart, a safe minimum dew-point is likely $250\ \textrm{C}$. I chose this temperature as sulfur trioxide and water both appear to have a logarithmic relationship with dew point, and we don't know the concentration of either, so must assume both are high to be safe. If your flue gases are below this temperature, or the flue inner wall is below this temperature, then you should use at least something as corrosion resistant as stainless steel. Zinc (galvanization) reacts readily with sulfuric acid.
Engineering Note: Chlorine compounds, including hydrochloric acid, may also be necessary to examine depending on the specifics of the fuel composition. I have not considered these, but stainless steel and PVC should both be resistant to these compounds. Zinc reacts readily with hydrochloric acid.
SAFETY NOTE: It is important to be safe when dealing with flue gases, as carbon monoxide poisoning is a serious danger. The gas is colorless and odorless, and you will not notice symptoms until you are already severely poisoned. Most commercially available CO alarms are of questionable quality. Be sure to have a licensed, qualified installer help with any installation involving flue gas venting. See this EPA link for more information on carbon monoxide.
I am not a qualified HVAC engineer. Only a materials engineer. Please consult a qualified HVAC engineer, or at least licensed, qualified HVAC contractor, before attempting to install a home-built system. For your safety.
answered Nov 12, 2015 at 21:37
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$\begingroup$ Nice answer. You clearly have the engineering background on this subject that I lack (most of my building are designed to not have fire in them!) $\endgroup$– grfrazeeNov 12, 2015 at 22:07
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$\begingroup$ Thanks! Part of this is materials engineering, part is some hvac/plumbing knowledge from working in that industry with residential and light commercial. $\endgroup$ Nov 12, 2015 at 23:08
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Just to add some unique engineering solutions that are used, depending upon applications:
- Brick works wonders - it resists the acid in the smoke well and can withstand the heat. Not metal, but well worth it if the application is right.
- For power plants, typical material use is acid brick lined steel.
- For industry, such as fume exhaust of potential violently reactive chemicals (not just limited to combustion), typically Fiberglass Ducting is utilized, or fluoropolymer coated steel ducting. Both have excellent corrosion resistance, and the flouropolymer coating can withstand 150C.
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$\begingroup$ Building codes may prohibit a masonry flue, if this is in a home or commercial building. In my area, at least, the requirement is for any brick chimney to have internal metal ducting - if memory serves. $\endgroup$– AirDec 11, 2015 at 18:37
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what is the most appropriate metal material i should use to avoid excessive corrosion in the exhaust pipe of the stove?
If you're looking for corrosion protection, stainless steel is hard to beat. However, its corrosion resistance comes at a price. You will likely find that a stainless exhaust pipe will be MUCH more expensive than a regular mild steel pipe.
Stoves have been made for hundreds of years using much cheaper plain mild steel, wrought iron, or cast iron for fittings and piping. There's no reason that these should not work for really any kind of biofuel. If you want to help mitigate corrosion (on the outside of the pipe), you can paint it with a heat-resistant paint.
