So does anyone really know about coal and CO2 emissions?

So, that last little blog, looking at emissions from pet coke.

I\’m getting extremely confused here. And if anyone really knows this stuff then please do tell the rest of us.

Looking around it\’s easy enough to see that lignite produces more CO2 per unit of energy (it\’s usually in BTUs) than does sub-bituminous coal. Sub-bituminous more than bituminous.

This makes logical sense: there\’s more C in the material as you progress. As the C is what your source of energy is a higher concentration of this and a lower concentration of everything else that you\’re heating up at the same time will lead to more useful energy per unit of CO2 produced.

However, the stats that I can see claim that this reverses when we get to anthracite. Which is weird, for anthracite has more C in it than bituminous coal. We would expect (or at least I would) there to be a straight line relationship with C rather than a U shaped one.

So, it\’s possible that this is true: maybe the \”volatiles\” in the lower grade coals more than make up for the higher C in the highest grade one.

It\’s also possible that there\’s something hinky with the numbers. Anthracite isn\’t actually mined in the US any more (and all the stats I\’ve found are US ones). What is used is anthracite culm: that\’s the waste left over in slag piles from previous anthracite mining. And what makes me suspect this is that the numbers for emissions from anthracite culm appear to be the same as those quoted for anthracite. Which really doesn\’t sound right at all.

And then we get pet coke numbers equated to anthracite ones. And I can imagine that that\’s true: high C level after all. But what I can\’t work out is why a higher C level should lead to greater emissions per BTU. Sure, higher emissions per tonne fuel used: but why per useful energy?

Anyone know?

14 comments on “So does anyone really know about coal and CO2 emissions?

  1. “slag piles”: do you mean spoil tips? Surely you of all people should know what slag is?

  2. @ dearieme
    Depends where you come from. When I was very young the locals referred to the small artificial hills of mining waste as “slag heaps”
    @ Tim
    If you get some hydrocarbons mixed in then heat is generated by converting them into CO2 + H2O with more BTU per unit of CO2. Low-quality coal also contains some sulphur so you get some heat released by conversion of S into SO3. Normal human beings prefer to burn sulphur-free or low-sulphur coal than to generate sulphuric acid (which is what you get when that SO3 in the atmosphere reacts with rain) even if it generates marginally more CO2 per BTU.

  3. http://www.eia.gov/tools/faqs/faq.cfm?id=73&t=11

    Tim,

    Take a look at the chart from the EIA. I don’t know where your stats came from, but this shows that bitumous coal has the least CO2 per BTU of usable energy. As was correctly pointed out, the total energy produced is from more than than the carbon content, but also sulphur, hydrogen, and other volatile components.

  4. Lotus 51 @ #4,

    No – although the C density is a fairly good approximation for the H density. C approximately equals 2H.

    C + O2 -> CO2 is exothermic @ 393kJ.

    2H +1/2O2 -> H2O is exothermic @ 286kJ.

    So, on average, the C contributes about 15% more to the total any release from the hydrocarbon element than H does (I haven’t allowed for the energy cost of breaking the C-H bonds).

  5. the locals referred to the small artificial hills of mining waste as slag heaps – well, they probably referred to pretty girls as witches and cats as Beelzebub.

  6. Dearieme

    are your posts, normally devoid of information, and often full of condescension, one reason why the greenies are in power? Your kind gave them a motivation to ruin the world?

  7. Diogenes>

    One is reminded of your namesake, who, being inquired of whether it wouldn’t be easier to flatter the Tyrant than live on beans in a barrel, replied that for the small price of living in a barrel on beans, one does not have to flatter the Tyrant.

    Similarly, you, for the small price of saying noting, might not appear to be a humourless git.

  8. @ dearieme
    Your willingness to pronounce on things about which you know nothing does little for your credibility when there are subjects about which you know something.

  9. @Alasdair, your link confirms Tim’s research, but doesn’t answer the question of why. Theoretically, anthracite should have a higher calorific content. The EIA appears to see sulfur as reducing heat values, so that’s not it. Perhaps other volatiles contribute, but that should be outweighed by higher moisture content and other impurities?

    If I had to guess, I would suggest that perhaps the denser, more compressed carbon in anthracite takes more energy to ignite, so more of the internal heat content is consumed in keeping the combustion process going. Lignite has too high a water content. Bituminous perhaps exists in a sweet spot between the two?

  10. Hi Tim,

    I think that Surreptitious Evil is going the right way about it. Look at the balance between the chemical processes involved and also look into the relative temperature and pressure conditions in plant at point of use. The chemical equations can probably be found straightforwardly.

    Every atom has to go somewhere in the equation and perhaps in the u-shaped case you discuss there is a carbon-rich waste term. Not my area though!

  11. Paddy, if anything SE’s approach only confirms the dilemma. A more exothermic C reaction should mean higher C fuels produce more energy per unit of CO2. They don’t apparently.
    You’re right that looking into the temp and pressure conditions matters, probably way more than you think. The chemical equations are not all that straightforward, not once you start worrying about phase changes and latent heat absorption and release. I doubt there is a carbon rich waste term – anthracite burns hot and clean, without the soot associated with coal in general. An energy sucking phase transition sounds more likely to me.

    Not my area either though.

  12. For instance, the transition from ice to liquid water takes significant energy, without any change in temperature (the definition of latent heat). No chemical change is involved – it’s still H2O – but energy went somewhere. Keeping track of atoms won’t help there.
    Refrigeration works the same way – the heat transfer is mostly by evaporation and condensation of the working fluid – temp changes within the same phase have miniscule effect.

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