Why don't methanol fuel cells kill people with CO2?

[jonjo]

First please verify my basic science.

• A small fuel cell might consume 1 litre of methanol in 24 hours while generating about 90 amp hours.
• I understand basic chemistry dictates that 1.6kg of CO2 will be generated from the litre of methanol as the fuel cell consumes it.
• The volume of that CO2 will be about 3/4 of a cubic meter.
• Gaseous CO2 is 1.6 times heavier than air.
• Butane known to collect in bilges is 2.4 heavier than air.

The suppliers of fuel cells have a relaxed approach to installation they can have an exhaust pipe, but just stick em unvented in a cockpit locker is considered ok practice.

Now here is the situation that concerns me. I leave the boat for a few days with the fridge running during which the fuel cell works at say 50% capacity and it generates a few cubic meters of CO2. I return to the boat, sit down in the cabin in a pool of CO2, get breathless and then pass out before realizing what has happened. Even if death by fuelcell is a remote possibility I understand that small % concentrations of CO2 can trigger headaches. Are these concerns reasonable?

 

[AntarcticPilot]

First please verify my basic science.

• A small fuel cell might consume 1 litre of methanol in 24 hours while generating about 90 amp hours.
• I under basic chemistry dictates that 1.6kg of CO2 will be generated from the litre of methanol as the fuel cell consumes it.
• The volume of that CO2 will be about 3/4 of a cubic meter.
• Gaseous CO2 is 1.6 times heavier than air.
• Butane known to collect in bilges is 2.4 heavier than air.

The suppliers of fuel cells have a relaxed approach to installation they can have an exhaust pipe, but just stick em unvented in a cockpit locker is considered ok practice.

Now here is the situation that concerns me. I leave the boat for a few days with the fridge running during which the fuel cell works at say 50% capacity and it generates a few cubic meters of CO2. I return to the boat, sit down in the cabin in a pool of CO2, get breathless and then pass out before realizing what has happened. Even if death by fuelcell is a remote possibility I understand that small % concentrations of CO2 can trigger headaches. Are these concerns reasonable?

I think you may be confusing Carbon DIOXIDE and Carbon MONOXIDE. The former - which is what is produced by a fuel cell - is not toxic except at extreme concentration, but is not breathable, and could feasibly cause asphyxiation. It also triggers the breathing reflex so you would start gasping for breath in a sub-lethal concentration, which would tell you to get out and ventilate the place. However, the rate of production from a fuel cell isn't high, and diffusion in a normally ventilated boat will almost certainly dissipate it sufficiently for it not to be a problem for humans - a dog on the cabin sole might have problems, but even that is doubtful. Even if you had pure CO2 on the cabin floor, 3/4 of a cubic metre resting on the cabin sole would still be a thin layer - maybe 10 cm or so.

Carbon MONOXIDE is extremely toxic, and would give the symptoms you describe, along with a red complexion. It is also about the same density as air. However, it is not produced by fuel cells, so it isn't a problem in this case.

 

[prv]

Some very rough figures from google suggest that we each exhale about a kg of CO₂ per day, or roughly the same rate as your fuel cell. We don't worry about suffocating in our own exhalations unless in a submarine, so I think your normal cabin ventilation should cover it.

Pete

 

[knuterikt]

I have been looking at the Efoy fuel cell

This is from the install maual http://www.efoy-comfort.com/manuals

CAUTION!
The device generates waste heat and waste gases.
The waste heat and gases must be released into the open air.

 

[jonjo]

Some very rough figures from google suggest that we each exhale about a kg of CO₂ per day, or roughly the same rate as your fuel cell. We don't worry about suffocating in our own exhalations unless in a submarine, so I think your normal cabin ventilation should cover it.

Hmm this is a highly persuasive and reassuring fact. Thankyou.

Many years ago I raised a related question about a family I saw closing up a small 23ft yacht for the night. I asked if there was a danger and after much ridicule someone worked the basic science and concluded that with poor ventilation they might wake up with a CO2 headache in the morning but no chance of death.

 

[jonjo]

I have been looking at the Efoy fuel cell

This is from the install maual http://www.efoy-comfort.com/manuals

I have been reading the same I think from an Aus site. Read on further and it seems that a cockpit or stern locker counts as open air.

For peace of mind I think I would still fit the optional exhaust pipe which is a small bore tube and route that to a cockpit coaming. I hope these guys are at SIBS with a show discount.

 

[jonjo]

I think you may be confusing Carbon DIOXIDE and Carbon MONOXIDE.

I should have clarified I was aware that CO is the proven killer but at the end of the day plants like to breath CO2 and not humans. Maybe this is more of a quality of life issue.

 

[knuterikt]

I should have clarified I was aware that CO is the proven killer but at the end of the day plants like to breath CO2 and not humans. Maybe this is more of a quality of life issue.

Normal air has 0.0314% CO2
seems that consecrations of more than 10% cause problem for humans.

 

[nigelmercier]

seems that consecrations of more than 10% cause problem for humans.

I'd agree with that

 

[prv]

For peace of mind I think I would still fit the optional exhaust pipe which is a small bore tube and route that to a cockpit coaming.

I think I would too - it just seems neater that way.

Doesn't it emit water vapour too? I'd definitely prefer to have that going outside the boat.

Pete

 

[jonjo]

Doesn't it emit water vapour too? I'd definitely prefer to have that going outside the boat.

Just a cup a day they claim. However these same fuel cell suppliers view 3/4 of a cubic metre daily as a small mount of CO2.

 

[VicS]

Just a cup a day they claim. However these same fuel cell suppliers view 3/4 of a cubic metre daily as a small mount of CO2.

1 litre, or 791g, of methanol will lead to 890g water vapour being produced. Even if it all condenses and drains into the bilges it's going to make the locker a rather damp space.


Incidentally I don't agree with your calculations in #1.

1 litre (791g) of methanol will produce 1088g of CO₂, or approx 618 litres (0.618 m³) at ambient temperatures.

I dont agree with your figures for the relative densities of air, CO₂ and butane

 

[vyv_cox]

Once when touring a distillery I was persuaded to look inside a manhole on a vessel that contained the brew, with a high concentration of carbon dioxide above it. It was unmistakeably not air! The gas caused a burning sensation in my throat and lungs and I withdrew my head very quickly, which I am sure is a natural reflex action. I doubt that high concentrations of carbon dioxide in a locker or cockpit would be any different

 

[Hydrozoan]

... It was unmistakeably not air! The gas caused a burning sensation in my throat and lungs and I withdrew my head very quickly, which I am sure is a natural reflex action. I doubt that high concentrations of carbon dioxide in a locker or cockpit would be any different

I would expect the headspace gas above a distillery brewing vat to contain all sorts of volatile compounds, not just CO2, and some of these might affect the throat and lungs. This site http://www.analox.net/carbon-dioxide-dangers.php refers to a choking sensation after 30 minutes at 4-5% CO2, and first refers to an olfactory effect at 5-10% CO2, describing it as "... a pungent but stimulating smell like fresh, carbonated water".

For comparison - and to inform the wider discussion - the same site says the normal international safety limit is 0.5%, and the normal short term exposure limit 1.5%. It links to this site https://www.eiga.eu/fileadmin/docs_...siological_Hazards_Not_just_an_asphyxiant.pdf which says that the average exposure of a healthy employee during an eight hour working shift should not exceed 0.5%, so I presume that is what is meant by "normal international safety limit".