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| Fig.1: Burning coal can produce flame, smoke AND smelly sulphurous fumes - even when well-supplied with oxygen. It's the result of DESTRUCTIVE DISTILLATION of coal aka pyrolysis, in addition to simpler chemical combustion. Expect volatile sulphur compounds that are smellier than sulphur dioxide (SO2) e.g. CS2, COS, H2S etc. |
I shall keep this posting brief, it merely flagging up this
blogger’s latest research project, with much reading and research still to be
done.
The title is prompted by the experience in recent years of
walking into town – a distance of a mile or so – and being hit once in a while at one or even two places by an
overpowering stench of coal smoke, not just any old coal smoke, but sulphurous
smoke, smelling of bad eggs or worse.
How can that be, one may ask? Even if one does not live in a
town with the strictest restrictions on what kind of solid fuel can be burned –
in a boiler or open fire – there are still regulations on what can and cannot
be burned. Typically they are that the coal has to be:(a) smokeless and (b) low
sulphur – max.2% of dry weight.
Here’s where it gets interesting – or should that be ominous?
On every occasion I’ve walked into a sulphurous fug, I’ve looked around
carefully at all the chimney pots, and never seen a trace of smoke!
How can that be? Has the fug come in from somewhere distant,
and then suddenly dropped to ground level?
Unlikely, because already I’m finding certain locations tend to have the fug,
not others, suggesting the source of pollution is local, despite the absence of
visible smoke.
That leaves us with another disturbing possibility, namely
that coal is being marketed as smokeless, as indeed it may be, and meeting the
regulations on maximum sulphur content, but is putting smelly, noxious sulphur
compounds out into the atmosphere that are invisible. How likely is that? If
likely, is it a result of simplistic chemical thinking that assumes that
because a particular coal is smokeless and has low sulphur content, it cannot
be a pollution hazard in the sense that it its combustion products are odourless
or largely so?
Let’s look closely at what is meant, or rather assumed by “combustion”,
because what happens in the real world may be a far cry from what happens in a
chemistry laboratory under ideal conditions.
First, when one looks at official documents on coal for
domestic use, one sees time and again the assumption that the sulphur in coal
ends up in the atmosphere as sulphur dioxide, SO2. In other words the sulphur
has become highly oxidized through reaction with oxygen.
[S] in coal + O2 (from air) -> SO2
Sulphur dioxide, especially from coal-fired power stations, one may recall is the gas held responsible
for acid rain and acidification of distant lakes (dissolving in water to form sulphurous acid, and later oxidizing
to the much stronger sulphuric acid). Sulphur dioxide gas also irritates the
airways, causes asthma attacks, and was a major contributor to the 4000 or so premature deaths
in the 50s from the killer smogs due to domestic and other coal fires. It was that experience that led
to the Clean Air Acts, the most recent in 1993, specifiying the maximum sulphur
content of coal and other domestic solid fuels derived therefrom.
OK, so there’s always going to be some SO2 when one burns a “house
coal” right, and one has to live with that. End of story as regards sulphur?
No, far from it, and anyone who says otherwise is either chemically naive or if
not is failing to tell the whole story.
The fact of the matter is that when one sets light to coal,
say with kindling, one does not get immediate combustion with complete
oxidation (C to CO2, S to SO2). That may be true for the surface layers, where
there is plenty of oxygen and a flame (from wood) to ensure that gases oxidize
completely. But there’s another chemical process that goes on called PYROLYSIS,
aka destructive distillation ( a more descriptive term).
When coal and other organic materials (paper, wood etc) are
heated, the first thing to happen is that the complex molecules (cellulose, lignin) break down into fragments that are liquid or gaseous. That’s the process
called “pyrolysis” – splitting into smaller more volatile fragments.
Here’s where it gets interesting. If there’s a naked flame
and oxygen, then those pyrolysis gases ignite, and one gets a bigger and better
flame. The fire takes hold as we say; the kindling does its job of setting the
coal alight (or rather, not the solid coal initially, but the pyrolysis gases).
But what if those pyrolysis fumes manage to evade the flame
and/or cool before they have had been able to mix with air and oxygen? What
then? What happens to them? Answer: they go up the chimney, and out into the
neighbourhood, and no, the sulphur will not be in the form of SO2, with its hopefully
limited odour and toxicity. It goes out in the form of pyrolysis products of
the sulphur in the coal, the presence and nature of which is rarely if ever acknowledged
in the official literature. Indeed, one has to search the chemical literature
on coal combustion, complete versus incomplete and especially “destructive
distillation” to find out what they are. It would be no exaggeration to say one
hits a brick wall, and starts to wonder why.
Fortunately there’s a near obsolete literature to which one
can turn to get some clues. It’s to do with the manufacture of “town gas” aka
coal gas that occurred prior to the introduction of North Sea natural gas in
the 60s onwards. One is referring to gas works chemistry (and oh boy, were gas
works smelly places, as the writer can personally attest).
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Fig. 2 and acknowledgements:
Making gas from coal (by pyrolysis, aka destructive distillation)
Graphic
and caption from the National Gas Museum Site. This is a description of
destructive distillation of coal, aka pyrolysis, NOT combustion. Note
the need for a purifier to remove the volatile sulphur compounds from
the gas stream before the latter could be piped to people's homes. IMPORTANT:
we are not talking about sulphur dioxide, SO2, but MUCH SMELLIER SULPHUR
COMPOUNDS (see earlier). That distinction between destructive distillation - which can take place in the absence or presence of air - and combustion, complete or otherwise, is crucial if one is to understand how the"cleanest" of coals can pollute if still containing sulphur and burned under non-ideal conditions.
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In gas works, the coal was not “burned”. It was placed in
retorts, open at one end only, and heated where it underwent pyrolysis, i.e.
destructive distillation. The impure coal gas was mainly hydrogen and methane,
but had smelly sulphur compounds too.
But here's the difference with the domestic coal fire, lacking oxygen. The coal
gas was cleaned up to remove the sulphur impurities. It was called flue gas
desulphurisation and used “scrubbers” that contained alkali (slaked lime etc) to take out the sulphur compounds. Domestic fireplaces do not have
scrubbers needless to say. Expect chimney smoke from coal fires to contain sulphur compounds that are NOT entirely sulphur dioxide as seems to be the assumption in the UK's Clean Air legislation and regulation.
Expect to see an appendix at the end for more detailed chemical information in a day or two for some
of the objectionable sulphur compounds. For now, let’s list the main culprits,
which were likely to be one or more of the following in varying proportions: hydrogen
sulphide (H2S), carbon disulphide (CS2) and carbonyl sulphide (COS). The hydrogen sulphide could be a primary product, but is also formed as a secondary product when carbonyl sulphide reacts with water in moist air (of which there's no shortage in the UK) to form H2S and CO2.
So what’s all this to do with the “clean green” house coal
that folks are having delivered to their homes, often brought thousands of
miles (naming no countries of origin!) on account of its impeccable credentials
(easy to light, burns with bright flame, burns longer and hotter, low ash, and
yes, the all-important tags: smokeless and low sulphur).
Answer: not a definitive one, at least not yet, more a suspicion,
a hunch, that there’s been a terrible oversight on the part of the regulatory
authorities: they have assumed that the low sulphur of these super-smokeless fuels
goes up the chimney and into the neighbourhood as sulphur dioxide.
But does it, especially when the solid fuel is burning under
less than ideal conditions, and indeed is not burning in the true sense, but
undergoing pyrolysis/destructive distillation?
Might it be possible that a coal that is smokeless and
possessing other desirable characteristics in the grate does NOT combust to
SO2. Might it be predisposed to become less oxidized sulphur compounds like the
ones listed – H2S, CS2, COS etc?
Have these much-touted super-smokeless fuels ever been tested
to see what happens to their intrinsic sulphur when ‘burned’ under real-life
conditions? Have their emissions from chimneys ever been monitored and analysed
for SO2 and OTHER SULPHUR COMPOUNDS?
Do we need to worry about them? Yes. H2S, despite being
easily dismissed as a “bad egg smell” is in fact highly toxic. Weight for
weight it’s said to be more toxic than hydrogen cyanide. Carbon disulphide? So
far I’ve not been able to find much except that it can (details to follow). Carbonyl sulphide
– again , hard to track down detailed information, but such as exists is not reassuring
– it produces ‘neurological damage’. What, one might ask: like Parkinson’s disease, Alzheimers and other dementia conditions of
that nature?
No, I don’t wish to appear alarmist, but neither should one
be complacent when dealing with a class of compounds that are notorious for their
toxicity at low levels (reminder: mustard gas of WW1 was an easily-vaporisible sulphur
compound - C4H8Cl2S, bis(2-chloroethyl) sulphide).
So what are the next steps, being a responsible citizen, or
so I like to think?
First, one needs to build up a stock of anecdotal evidence
that certain 'hotspots' in this blogger's home neighbourhood are regularly afflicted with the
particular ‘sulphurous stink’ as distinct from that of ordinary coal smoke. By "ordinary" I mean
the kind one encountered in one’s childhood with the sharp but not disgusting SO2 smell, NOT the
gaswork's "rotten egg" type.
Secondly, the jackpot,
attempt to spot smoke from a particular chimney, even if the merest wisp, that
would identify the property as burning particular varieties of house coal.
Thirdly:
inform the local authority, urge them to pay a call and see whether (as I
suspect) the coal being burned IS approved for use.
Fourthly: press for proper and
thorough analytical tests be run on the combustion/pyrolysis characteristics of
allegedly ‘clean’ solid fuels marketed as approved house coal to check them out for possible release of noxious
sulphur compounds under realistic conditions – NOT just those that are designed to ensure complete
combustion under ideal laboratory conditions.
I shall of course be sending my local Council a link to this
posting. What would be nice would be for the Council to become proactive in
checking out the hypothesis presented here. I shall also send a link to my
contact at a national newspaper, one whom I recently assisted with a different
chemically-related issue.
Chemical appendix (more to follow):
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| The red box contains some chemical reactions that can take place in a bed of hot coal to produce (a) sulphur dioxide (b) hydrogen sulphide (c) carbon disulphide and (d) carbonyl sulphide. Irrelevant reactions to the present discussion scored out. |
These data were taken from the following recent and splendidly detailed paper:
 |
| 2012 Paper by J. Preciado et al. |
Also from this blogger:
1. A solution to the enigmatic Turin Shroud?
See most recent posting on specialist Turin Shroud site:
2. A solution to the enigmatic Silbury Hill?
See preview in
comments under this posting on the ancient-origins site:
SUMMARY OF POSTING
So what’s the nub of the hypothesis? How is it different
from what has appeared before, and is it grounded in authentic, dare one say
authoritative chemistry?
1. “Smokeless” coals have an internal structure that allows
the coal to be heated without releasing “puffs” of microparticulate carbon,
read soot or smoke.
2. But that predilection to ‘hang onto’ carbon makes the
sulphur of the coal behave differently on heating. In ordinary smoke-releasing coal,
the sulphur is liberated in a form that is easily oxidized to sulphur dioxide.
3. In smokeless coal, the sulphur is NOT easily liberated in
a form that oxidizes to sulphur dioxide. Maybe it is retained, in a manner
similar to the carbon, and NOT released in puffs that are easily oxidized to
SO2.
4. Instead of being easily released and oxidized to SO2, it
is retained, at least for a bit longer, and instead of being oxidized to SO2,
it participates in entirely different chemical reactions that convert it to
other smellier sulphur compounds, notably carbon disulphide, carbonyl sulphide,
hydrogen sulphide etc.
In other words, the properties of being “smokeless” and “releasing
sulphur as SO2” are not mutually exclusive, as seems to be assumed by the regulatory
authorities. One cannot go selecting coals for their tendency to burn without
smoking, while assuming that the sulphur burns off normally and conventionally to SO2 as if an entirely
independent process.
It was not sufficient
to select for smokeless property and (merely) low sulphur content. The fate of
that sulphur in the domestic fireplace should have been investigated
experimentally. It was wrong to assume that the sulphur in a smokeless fuel
burns off as sulphur dioxide.
Alternative chemical pathways, not requiring oxygen, could
predominate in smokeless fuels, giving rise to volatile sulphur end-products
that go up the chimney and may well proceed to stink out the immediate neighbourhood far
more than would be the case if SO2 had been the main endproduct.
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