Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: evan_au on 07/03/2019 20:15:00

Title: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: evan_au on 07/03/2019 20:15:00
A recent story on TNS described a process to turn carbon dioxide (eg from a coal-fired power station) back into low-grade coal.

The scientists described a process using electrolysis with a gallium and cerium liquid metal electrode. They commented that the efficiency is quite good, not taking much more than the minimum energy required for this conversion.

However, the efficiency of burning coal to produce electricity is quite low (around 30%)
- coal mining is quite an energy-intensive industry, being a major user of explosives
- extracting CO2 from the atmosphere is quite energy intensive, since it is a small fraction of the air (around 1 part in 2000)

So why would you bother?
You can achieve even higher efficiency by just leaving the coal in the ground.

See, for example: https://e360.yale.edu/digest/scientists-turn-atmospheric-co2-into-coal
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: chris on 07/03/2019 20:28:44
This is the story in question: https://www.thenakedscientists.com/articles/interviews/technique-turns-carbon-dioxide-back-coal

Actually, @evan_au you are jumping to a conclusion; if you read the transcript you'll see that what the RMIT researchers (Torben Daeneke and his colleagues) propose is that their system can be used as a means to claw back released CO2 and sequester it in a static, stable form (akin to brown coal) so that it may be safely stored. Nowhere do they propose that we burn coal with the aim of turning the evolved CO2 back into coal. This is a strategy for remediation purposes in order to offset progressive damage caused by atmospheric CO2.

The point they are making when referencing coal is that other, existing proposals for long-term sequestration of CO2 involve approaches like pumping the gas into old gas wells and other geological traps. These are not guaranteed to remain stable and could leave to a convulsive release of massive volumes of stored carbon dioxide if, say, an earthquake affects the integrity of the geology.

So the story is about recovery of CO2 released by burning and returning that gas to a reduced carbon state that is long-lived and self-stable.
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: chiralSPO on 07/03/2019 21:12:53
As far as I can tell evan_au's objection stands.

It doesn't really matter what is done with the "coal." I have worked on a few carbon sequestration projects myself--it really only works out if (a) the energy being used to capture and convert the CO2 is 100% carbon free, and (b) if the product has value. There may be use in products from electrocatalytically reduced CO2, but if you think of the carbon footprint required to generate the raw materials to build a CO2-reducing plant on the scale needed to have any effect on the atmospheric composition, it is unlikely to pay for itself (in terms of net CO2) before it breaks!

Photosynthetic organisms already capture and convert CO2 far faster than we could hope to do it (in the next few decades, at least). Just look at the slope of the jags on the Keeling curve compared to the slope of the overall trend:

* maunaloa_2004.gif (23.92 kB . 540x404 - viewed 4475 times)
We just need to focus on not burning the fossil fuels in the first place, and allowing the photoautotrophs to do the work (ie preventing deforestation, promoting reforestation, and promoting healthy oceans). As long as we can allow the downward parts of the jags to extend a little bit, and the upward parts of the hags to be a little less intense, we can return to a reasonable equilibrium pretty quickly.

There might be a tiny little nudge we can do technologically to speed the process along, but focusing money and attention on these sorts of "band-aid fixes" merely diverts it from the "severed jugular" that the actual problem is.
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: alancalverd on 07/03/2019 23:02:44
I've often wondered whether the whole process of extracting and burning coal was ever energy-efficient, but that isn't always the prime concern. Carbon fuels of all kinds have a very high energy density and handling convenience, unmatched by any other substance. Hydrogen and plutonium beat coal for energy density but not for practicality, and a power station fed with coal slurry or methane can be fired up from cold or completely shut down in minutes, unlike a reactor or a windmill. You could run a steam engine on green wood, but it turns out to be more convenient and practicable to use the tree as a pit prop and fire the engine with coal.

So if you have a free but unreliable source of electricity, it makes sense to use it to manufacture liquid or solid fuel from carbon dioxide, almost regardless of the overall efficiency.
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: alancalverd on 07/03/2019 23:06:08
PS the jags on the Keeling curve are very interesting. The maxima occur in summer, when human use of fossil fuels is least. That's the kind of anomaly that distinguishes scientific truth from political convenience. 
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: chiralSPO on 07/03/2019 23:33:04
Actually, as I understand it, the maxima are related to the summer/winter cycle of deciduous trees.

https://www.aps.org/programs/outreach/history/historicsites/keeling.cfm

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Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: alancalverd on 07/03/2019 23:56:24
Now my limited understanding of plant growth was that they absorbed carbon dioxide when they were growing, which is why we have coal. And they grow fastest in spring and summer, and are dormant in winter. So why the CO2 peak in May? But just to make it more interesting, the abscissa labels don't correspond with the labelled data points, nor do they seem to be equally spaced.

My reading of other versions of the Mauna Loa data was that CO2 levels were driven by the activity of cold-blooded creatures eating plants, which would produce a plausible maximum just before caterpillars and the like stop eating leaves and pupate.
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: evan_au on 08/03/2019 07:53:02
In Australia (and I assume, in other countries), the traditional coal industry is promoting "clean coal", saying they can capture the CO2, and save it away, so it will be as clean as renewables.

This leaves out the:
- Spoil from the mines
- Soot and heavy metals from the power station
- Ash from the power station
- The huge loss in efficiency when you capture the CO2, and do something permanent with it

I am concerned that some segments of the community will seize on this "efficient" form of carbon capture as an excuse to build new coal-fired power stations.

If and when we get an effective fusion reactor, maybe we could use this to capture some CO2 from the air and turn it into a solid (after everyone has gone to bed, and we have finished charging their home batteries and the electric taxi car fleet).
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: alancalverd on 08/03/2019 08:38:57
Mine spoil can be used to modify landscapes or even to backfill exhausted shafts. There isn't much from modern coal mining anyway - you only dig the coal seam and then use about 90% of what you extract, compared with say copper mining where less than 10% of the extracted rock is useful. My brother-in-law was employed to count worms in the topsoil of playing fields built on coalmine tailings, and one of my sons is now employed refilling deep mines with hard rock dross.

Soot has been pretty well eliminated from modern power stations since the 1950s: if it's combustible, why waste it? The source of London smog was actually inefficient domestic open fires, and appears in literature long before we had electricity!

Heavy metals are recovered by electrostatic precipitators. Much less of a problem with coal (trees can only extract minerals from their immediate environment, and don't go seeking lead) than with metal smelting.

Ash is a very useful building material (PFA or "cinder" block is ideal for Aussie housing as it is a better insulator than brick and lasts well in dry conditions)

The energy efficiency of carbon capture is indeed problematic unless you turn it into another fuel, but it does at least provide a use for all the surplus wind electricity that we now pay people not to generate.

Electric cars are not a solution. Diesel trucks and buses are the so-called problem.

Fusion power seems to be subject to a Schwarzchild effect: every year, it gets 18 months further away!
Title: Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?
Post by: chris on 08/03/2019 08:44:55
As far as I can tell evan_au's objection stands.

It doesn't really matter what is done with the "coal." I have worked on a few carbon sequestration projects myself--it really only works out if (a) the energy being used to capture and convert the CO2 is 100% carbon free, and (b) if the product has value. There may be use in products from electrocatalytically reduced CO2, but if you think of the carbon footprint required to generate the raw materials to build a CO2-reducing plant on the scale needed to have any effect on the atmospheric composition, it is unlikely to pay for itself (in terms of net CO2) before it breaks!

Photosynthetic organisms already capture and convert CO2 far faster than we could hope to do it (in the next few decades, at least). Just look at the slope of the jags on the Keeling curve compared to the slope of the overall trend:

* maunaloa_2004.gif (23.92 kB . 540x404 - viewed 4475 times)
We just need to focus on not burning the fossil fuels in the first place, and allowing the photoautotrophs to do the work (ie preventing deforestation, promoting reforestation, and promoting healthy oceans). As long as we can allow the downward parts of the jags to extend a little bit, and the upward parts of the hags to be a little less intense, we can return to a reasonable equilibrium pretty quickly.

There might be a tiny little nudge we can do technologically to speed the process along, but focusing money and attention on these sorts of "band-aid fixes" merely diverts it from the "severed jugular" that the actual problem is.

Actually, Torben Daeneke was at pains to impress on me the point that their approach was only of value if the energy supplied is carbon neutral; so you could envisage a huge solar array across the Sahara, for instance, feeding juice into a CO2 draw-down facility that would churn out solid carbon that would be shipped off and buried.

If you can site the plant near an existing open-cast facility (like in Australia) so you can dump the coal back in there and bury it again, more's the better.