Sunday, July 03, 2016

Reduction of Oxidation: Not So Easy


The historical root of chemistry is metallurgical, the creation of metals from ores.  This process is called reduction of the ore, and the trick to doing it was discovered early on, to heat the ore as high as possible after mixing it with charcoal.  The achievable final temperature at the time pretty much determined whether the locals were living in the copper, bronze or iron age.  There are nice video demonstrations on line. 

The charcoal is transformed into carbon monoxide or carbon dioxide depending on the conditions in the furnace.  This is pretty much how most metals are produced today, except that instead of charcoal, coke is used.

Coke is what happens to coal when you heat it but do not provide enough oxygen to form CO2 or CO.  This drives a conversion of the carbonaceous components of the coal to carbon and dries out any water content.  A detailed description of the process can be found at steel. org, but for practical reasons coke can be thought of as plain carbon or if you wish plain carbon with a bit of stone and other seasonings.

http://image.slidesharecdn.com/productionofironandsteel-150427080734-conversion-gate01/95/production-of-iron-and-steel-17-638.jpg?cb=1430122364
In any case, the point is that bunnies need coal to make iron and just about any metal they can think of, and that reduction of the ore to metal requires oxidation of the coke to CO2.  Even electrolysis of bauxite (Al2O3) to aluminum requires using graphite electrodes, which, guess what, during the electrolysis process, are oxidized to CO2.

Which, in the usual Rabett Run way brings Eli to the point of all this.  According to the EIA, 2012 production of coal is about 8.7 billion short tons. In real units that is 7.9 x 1012 kg or, if a bunny prefers 7.9 109 tonne.

The amount of metallurgical coke produced in 2012 was about 0.7 billion short tons, but given the wastage from coke production, 2/3 of a ton of coke is produced from a ton of coal.  Multiplying, dividing and so forth (hopefully correctly, this comes out to about a billion short tons or  0.9 109 tonne.

Bottom line is that about 1/8th of the current coal mined is used for metal refining and this is still quite a bit, making it harder to reach zero CO2 emissions. 

Eli apologizes for this Sunday sadness.

13 comments:

William Connolley said...

As time goes by, and more steel is made, more steel is made from recycled iron and requires less CO2. So there should be some tendency for the amount of CO2 directly attributable to steel production to go down, perhaps.

EliRabett said...

Already happened in the US, look up mini mill,but there is a limit and the developing world will need more steel

http://www.nytimes.com/1981/09/23/business/the-rise-of-mini-steel-mills.html

Tom Curtis said...

You ignore the possibility of reversing the transition from charcoal to coal.

I agree however that net zero emissions will require some active sequestration of CO2.

EliRabett said...

Going back to charcoal requires a much smaller world. There are also some interesting technical issues which limit substitution, for example, charcoal is softer than coke which means that blast furnaces can only be partially charged with charcoal at best

http://www.iea-coal.org.uk/documents/82861/8363/CO2-abatement-in-the-iron-and-steel-industry,-CCC/193

Tom Curtis said...

The limitations of charcoal are only relevant to the corex process, or large blast furnaces. Using a HIsmelt furnace, or small blast furnaces, charcoal is an acceptable substitute.

Further, based on the 400 km^2 per 500 Megatonnes of steel estimate from your source, global steel production could be based on charcoal using a 1300 km^2 footprint. That is 0.0035% of global forest area.

I am not saying it will not be without costs, most notably from replacing current blast furnaces with newer furnace types; but it is doable.

Tom Curtis said...

I should also note this possible game changer:
http://www.scientificamerican.com/article/cleaner-cheaper-way-to-make-steel-uses-electricity/

Arthur said...

My father is a (retired) electrochemist and worked on a speculative project for iron electrolysis in the 1970's. Coal/coke isn't required, it's just cheaper than alternatives. Price it properly and the alternatives will win.

Russell Seitz said...

The starting point for many performance superaloys has long been 99+% pure Armco iron, prepared without carbon by hydrogen reduction of iron oxide.

Andrew said...

It's just another of those problems for which the solution starts with 'Given sufficient cheap zero-CO2 electricity..'.

What we should be doing is looking seriously at processes that can use variable amounts of electricity to power the big industrial energy users. So.. Iron by electrolysis, Ammonia by electrolysis and heat, Methanol by H2-reduction of CO2.. and more. If we have a situation where electric grids are typically over supplied then it makes sense to base your industry around 'free' surplus electricity. A lot more sense than trying to store the stuff to balance supply and demand.


Howard said...

Andrew: That only works if nukes are used, which means that the CAGW coffee klatch will have to cure themselves of China syndrome.

Russell Seitz said...

By CAGW, Andrew obviously means Commonplace Anthropocene Global warming

Jumper said...

H2 is a fine and adequate reduction agent. One reason CH4 is also one. See the Midrex process.
http://www.midrex.com/assets/user/media/MIDREX_NG.pdf
Too bad H2 is expensive. That is, any without generation of CO2 in its preparation, which is the point.

EliRabett said...

AFAEK there are no naturally occurring hydrogen mines. Methane might be interesting, but it is also a greenhouse gas.

An interesting tactic might be to reduce methane to hydrogen and carbon. The carbon can be sequestered and the hydrogen used for reduction of metals.

Of course, controlling the metallurgy of the final products is a complex issue and most of these suggestions require a low cost of energy or a high cost of carbon.