[katieHaylor (OP)]

Paul asks:

If the exhaust gases from a ship were passed through a contained amount of salt water, could the emissions be filtered out, solidified (with some processing) and then disposed of at the next port?

What do you think?

[Kryptid]

Water has a fairly limited solubility for carbon dioxide, so I doubt this would be practical. Maybe I'll actually do some math for it later in this thread.

[evan_au]

(Oops! overlap with Kryptid...)
The exhaust gases from a ship include Carbon dioxide, sulphates, nitrates, particulates and partially-burnt hydrocarbons, among others.
- Container ships use a semi-solid fuel called bunker fuel which generates a lot of pollutants; some countries ban its use within territorial waters.

There is a very limited amount of these substances which can be dissolved in water; after the water is saturated with these substances, the exhaust gas will just bubble through to the atmosphere anyway.
- It is possible to capture some of these (like sulphates) as a powder by using an electrostatic precipitator
- Even though this technology has been available for use on power stations for many years, still many power stations just pump it straight into the atmosphere
- I understand that in Australia, coal-fired power stations choose to pay a "pollution tax" of a few cents on every Megawatt hour, rather than install the electrostatic precipitators. This tax is just a few percent of the health costs that are caused by emitting the sulphates.

If our governments can't enforce sensible pollution rules on land-based power stations within their own territory, what hope do we have of controlling emissions in international waters, with ships registered under "flags of convenience", and all aiming for the lowest cost per delivered container?

[Kryptid]

Here we go. I'll use octane as the fuel of choice. The balanced equations for the combustion of octane are shown below.

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O

We can see that one mole of octane produces 8 moles of carbon dioxide. Now to figure out how many grams of carbon dioxide are produced by the combustion of 1 gram of octane. We'll multiply the number of moles by their respective molar masses:

1 mole of C₈H₁₈ x 114.22852 grams per mole = 114.22852 grams
8 moles of CO₂ x 44.0095 grams per mole = 352.076 grams

Now divide the two:

352.076 grams / 114.22852 grams = 3.0822

So for every gram of octane we burn, we get 3.0822 grams of carbon dioxide (or for each kilogram of octane, it's 3.0822 kilograms of carbon dioxide).

1 liter (equal to 1 kilogram) of water can dissolve up to 1.45 grams of carbon dioxide at about standard pressure and temperature. How many liters of water would be needed to dissolve all of the carbon dioxide given off by burning one kilogram of octane?

3,082.2 grams of CO₂ x 1.45 grams per liter = 4,469.19 liters of water needed

So it seems that, if we wanted to dissolve all of our CO₂ emissions in water, we would need to carry 4,469.19 liters of water for every liter of octane we carry with us. Not exactly practical.

[Kryptid]

As an alternative, perhaps we can react the carbon dioxide with magnesium instead.

CO₂ + 2Mg → 2MgO + C

1 mole of carbon dioxide can react with 2 moles of magnesium to produce 2 moles of magnesium oxide and 1 mole of carbon. In terms of total masses, this is equal to:

1 mole CO₂ x 44.095 grams per mole = 44.095 grams
2 moles Mg x 24.305 grams per mole = 48.61 grams
2 moles MgO x 40.3044 grams per mole = 80.6088 grams
1 mole C x 12.0107 grams per mole = 12.0107 grams

So for each given mass of CO₂ produced:

Mg: 48.6 grams/44.095 grams = 1.102
MgO: 80.6088 grams/44.095 = 1.828
C: 12.0107 grams/44.095 = 0.272

Since we already know that octane produces 3.0822 times its weight in carbon dioxide when burned, we can calculate the needed mass of magnesium and resulting mass of magnesium oxide and carbon per kilogram of octane burned (which will produce 3.09822 kilograms of CO₂):

3.09822 kilograms x 1.102 = 3.414 kilograms of magnesium are required.
3.09822 kilograms x 1.828 = 5.664 kilograms of magnesium oxide are produced.
3.09822 kilograms x 0.272 = 0.843 kilograms of carbon are produced.

So for each kilogram of octane you bring with you on your ship, you would also need to bring 3.414 kilograms of magnesium. You would also produce a total 6.507 kilograms of waste material per kilogram of octane burned that you would have to carry with you back to port.

Volume would be a good measure in addition to weight. Going from densities:

1,000 grams of octane / 0.703 grams per milliliter = 1,422 milliliters = 1.422 liters
3,414 grams of magnesium / 1.738 grams per milliliter = 1,964 milliliters = 1.964 liters
5,664 grams of magnesium oxide / 3.6 grams per milliliter = 1,573 milliliters = 1.573 liters
843 grams of carbon / 2.1 grams per milliliter = 401 milliliters = 0.401 liters

So per liter of octane burned:

1.964 liters of magnesium x 0.703 = 1.38 liters of magnesium needed.
1.573 liters of magnesium oxide x 0.703 = 1.106 liters of magnesium oxide produced.
0.401 liters of carbon x 0.703 = 0.282 liters of carbon produced.

It may be practical from a technical standpoint, but I'm not so sure about it from the economic standpoint. It would no doubt be more expensive than normal ships. You now have to use some of the space on the ship which could have originally been used for cargo or octane to store magnesium and waste products. The overall design of the ship would be more complicated as well, making costs go up further.

[alancalverd]

It's an interesting idea.

Carbon dioxide is not a pollutant. It is an essential component of the atmosphere, without which there would be no plant growth and life would cease to exist. If we grew more trees (or at least stopped deforestation) Mother Nature would take care of the CO2.

The other stuff can generally be precipitated electrostatically (unburned hydrocarbons and solid particulates) or dissolved in water (sulphur oxides, nitrogen dioxide) to some extent.

But whilst rocket science is very simple, rocket engineering is a lot more difficult. Whatever system you add to your ship/rocket/car/aeroplane  will add cost and weight, increase maintenance costs, and reduce performance. You might devise a scrubber that removes 90% of particulates etc at "high-seas cruise" power, but that stuff would eventually settle in the sea anyway and be eaten by some bug or other. The real problem occurs close to land when you are using full power to accelerate or manouver, belching more black smoke than your scrubber can remove. If you increase the scrubber capacity, you will be carrying extra deadweight and capital during the cruise phase.....

[Bored chemist]

In principle, it is possible to trap the CO2
https://en.wikipedia.org/wiki/Amine_gas_treating
but it would be impractical on a ship, simply because of the weight of chemicals involved.

[wolfekeeper]

You'd probably do better to use a biofuel, use plants to just suck the equivalent amount of CO2 out of the air when the biofuel was growing.

[evan_au]

Can you solidify exhaust gas emissions from a ship?

Submarines and aircraft carriers have been doing it for years.

Fuel=Uranium
Exhaust gas = nil
Retained gases = small amounts of radioactive Radon, Iodine, Xenon, Krypton etc (but almost all waste products are solid)

...it's just that most countries haven't yet figured out what to do with the solid waste...

[Bored chemist]

it's just that most countries haven't yet figured out what to do with the solid waste

That implies that some countries have.
Would you like to say which?

[evan_au]

That implies that some countries have (figured out what to do with the solid nuclear waste)

A lot of countries are just storing nuclear waste in above-ground water tanks. Not a long-term solution.
- Some countries have been dumping nuclear waste in the oceans, but that is now forbidden by international treaties.
- France planned to reprocess fuel, and feed the plutonium into fast breeder reactors. However, fast breeder reactors have proven very difficult to operate reliably
- A lot of countries are talking about long-term nuclear waste disposal, but none have an operating solution, as far as I can see...

https://en.wikipedia.org/wiki/High-level_radioactive_waste_management

At least the volume of radioactive waste is a lot lower than if you tried to capture carbon dioxide by reacting it with Magnesium...
- but radioactive waste is also a lot more dangerous than magnesium oxide...
- magnesium metal does not occur in nature, so you have to refine it from ores like magnesium oxide. And how do you do this without producing a lot of CO₂?