Sunday, December 07, 2008

An idea??

Eli has been thinking about clean coal. Not being an absolutist, the bunny will gladly settle for half a carrot (or even a third, below that we can negotiate), and his idle thoughts went along the following track. . . Coal gasification is an interesting technology, certainly an old one with town gas being around for going on 200 years. You start by burning coal

C(s) + O2(g) --> CO2(g), ΔH0 = -394 kJ/mole

The - means that that much energy is released per mole of carbon burned. Coal is mostly carbon. If you keep the oxygen availability low the CO2 reacts with hot coal to give

CO2(g) + C(s) --> 2CO(g), ΔH0 = +172 kJ/mole

so you get carbon monoxide and now you add superheated steam to the system

C(g) + H2O(g) <--> CO(g) + H2(g), ΔH0 = +131 kJ/mole

You can also play with the water gas shift reaction

CO(g) + H2O(g) --> CO2(g) + H2(g) ΔH0 = -41 kJ/mole

usually using some catalyst to produce more hydrogen or you can drive coal gasification at high pressure by

C + 2H2 <--> CH4, ΔH0 = -75 kJ/mole or

CO + 3H2 <--> CH4 + H2O(g), ΔH0 = -206 kJ/mole

Town gas has a varied composition depending on how the process was tuned (temperatures, pressure, etc. Typically 50% H2, 35% methane, 10% CO and 5% ethylene, but pretty much any variation is possible depending on end use. The CO is why old movies have people killing themselves by doing a turkey imitation and sticking their heads into an oven. It doesn't work real well with natural gas, although you can blow your house up by leaving the gas full on for a couple of hours and lighting a match.

But to get to the point, combining a concentrating solar power plant with a coal mine, or come to think about it an oil shale or tar sand extraction facility might be a viable way to get to lower carbon emissions for producing liquid fuels and chemicals. If nothing else we need lots of hydrocarbons for the chemical industry. While the net energy output would be lower than from separate coal + solar electricity generation (there will be inefficiencies), it could be greater than either separately and the CO2 emissions would be lower than from a coal burning plant alone. In other words, this is a strategy for less polluting coal, not clean coal. We talked about that earlier.

Now this is but an idle thought, and whether it is worth anything depends on the details, but let Eli throw it out their for others to chew on. He suspects others have thought about this too and would appreciate if somebunny pointed it out



Arthur said...

Hi Eli - I'm not sure I get it - your idea is to use the solar energy to heat water so you can run the C +H2O reaction? What's the advantage of that rather than just using some of the output hot steam from the coal burrning itself? I believe that's done now in some "clean coal" demonstration projects. You can in principle extract more energy from gasified coal thanks to advanced gas turbine technology, and it's easier to limit at least some of the pollutants with gasification. But you still end up with exactly as much CO2 from each ton of coal, as would your approach, unless I'm missing something?

EliRabett said...

Part of the point is to get hydrocarbons for the chemical industry which is a serious issue. Also gas turbines can run at over 60% efficiency vs. something like 40% for coal (and you can use some of the waste heat to run the process at the other end raising overall efficiency).

Might be the more interesting idea is to use solar to cook tar sands and oil shale where you can use 100% of the generated thermal.

Admittedly this was a doodle.

Anonymous said...

I think the idea of clean coal is to produce a fairly pure CO2 stream on the back end. You need an air separation plant to run the gasification reaction, so just make the O2/N2 separation plant a bit bigger and you can do an oxyfuel combustion, and have a pure CO2 stream for subsurface sequestration.

Petrochemicals are just a pimple on the arse of the oil industry, and most years are a small fraction of their profit (but on good years [once every 7-10 years], they are very lucrative indeed]). It gives them something to do with the low-value ethane, propane, and butane by-products. Less than 10% of refinery product goes to petrochemicals.

Before the ethylene based petrochemical industry, we used acetylene from coal tar and fischer-tropsch on syngas: we could go back to that. Or we could use all those phenyl rings in the lignin in lignocellulosic biomass to make hydrocarbons: or dehydrate sugars to furfurals using ionic liquids. There's several options, but replacements for petrochemicals is a small, small problem compared to the energy issue.

Anonymous said...

I think you need to restart your analysis from the beginning. Coke is all carbon. Coal contains hydrogen as well. The rules of thumb are:
Coal: CH
Oil: CH2
Gas: CH4

cynthia said...

This picture of a coal-gas plant outlined in fluorescent blue against an eerie night sky looks like something straight out of a really bad sci-fi film.

Anonymous said...

Producer gas/Water gas. That takes me back.This was of course common furnace technology for the 19th and a good part of the 20th century but in the UK it was not the method for making Town gas. It was as you say a mixture of CO and H2 but it was made by the destructive distillation of coal and it was a highly noxious and locally polluting process.It was probably a very inefficient way of producing useful energy but it did produce as a by-product a very rich mixture of organic feedstocks such as phenols etc which were used by the developing chemical industries in the 19C.

But in the end if you want the energy you have to burn the carbon and I have to say I become increasingly uneasy about the chances of achieving any real reduction in carbon consumption in any civilised fashion.


Anonymous said...


The gasometer you mention held blast furnace gas, but is no longer operative. Standing 117 meters high, and 65 meters in diameter it was the biggest one in Europe and the second largest in the world.

It has been turned into an exhibition hall: Gasometer Oberhausen

Apologies for this side track.


EliRabett said...

Thanks blue. Eli used to live about 10 km from the gasometer

Anonymous said...

There are so many reasons for not going the coal route (visit Appalachia, if you need one) that it is actually counterproductive to even talk about "clean coal", which is an oxymoron if ever there was one.

If we were not burning up the oil and gas in cars and electric plants at the current phenomenal rate, there would be plenty of oil and gas left for the chemical industry for many many years to come. (some scientists even believe that there is oil and gas deep within the earth's crust that dwarfs that found near the surface to date).

The focus should not be on "coal as a way out" at this point -- (or tar sands or oil shale, whose extraction has the same environmental drawbacks as coal).

Douglas Watts said...

Mr. Rabbett,

I like the feel of this doodle. If we look at the principal value of coal as a chemical raw material, rather than an energy source via combusting, then we avoid the whole Either/Or dichotomy, which to me is distasteful. In the same sense that we don't look at a beautiful piece of black walnut as firewood.

Anonymous said...

@Satan's sock puppet

Unfortunately, oxyfuel is not the utopia you imagine. In principle a great idea, stoichiometric combustion all the way to water and CO2, condense the water out, compress the CO2 and pump down to where it's not going to come out again (unless we need to use it in a few million years to reheat the atmosphere, because all the fossil fuels are gone, and the the renewable guys still haven't got there act together..).

However, as with most technologies there are big obstacles to be overcome. The separation of O2 from N2 in the air doesn't produce pure O2 (if you want to be cheap), and you have to go higher than stoichiometric anyway to ensure complete combustion. As a result even Vattenvall with their 30MW oxyfuel demonstration expect something like 15% inerts in the CO2 rich gas. Which means you have to clean it up anyway, just like you have to do in any pre- or post-combustion "CO2-cleaner" technologies. As with everything there are work-a-rounds but these cost more money and makes a level-playing-field comparison even more difficult.

I'd be very surprised if oxyfuel didn't get used somewhere in the world for CO2-cleaner technology, but I would be very surprised if it turned out to be the leading technology without some major breakthrough. (I am not anti-oxyfuel or anti any other carbon capture technologies, just trying to be realistic)

A slight nuance in your numbers may be of order. State-of-the-art NGCC will get you 58% efficiency, and state-of-the-art IGCC 48%. However, if you look at most coal-fired power plant operating today, you'd be lucky to find ones operating above 40%. Power plant are built with 40 years operations in mind, coal plant are generally older using older technology than gas-powered plant most of which have had combined-cycles from the beginning.

Ignoring arguments about continuity. When combining say a solar thermal array with a plant to either produce electricity from coal or as you suggest to produce chemicals from coal. The question to ask is where can I use that energy most effectively in my process. Am I better off producing electricity for all the utilities in my plant so that I (or more probably someone else) don't (doesn't) need to burn coal to produce that electricity, or am I better of using the heat energy in any reactions that I want to take place instead of burning coal to produce that heat. If it for electricity then there is no reason for my factory to produce it, dedicated facilities will be more efficient and electricity isn't my core-business. Using the heat energy is a better idea if I want to decrease my carbon footprint. Even more so if it can be used directly in any carbon capture I want to do. So then the question is what are the temperature levels available and how can I feed that into my process. The solar array you linked to produces a 400°C fluid, if this was transfered to steam then there are lots of chemical processes which could use this. However, the refinery business seems to be moving more and more towards power export. They generally have heat over, which is then used to make steam, a steam turbine and I'm also a power exporter. But hang on, didn't I just decide to let some else make electricity? Of course if I have to start decreasing my CO2 emissions then maybe this heat can be put to better use.


Anonymous said...

Florifulgurator thinks:

What about wood gas?!

I hope everybody meanwhile heard of biochar, the only serious looking carbon sequestration method we got. Last time employed large-scale in pre-Columbian Amazon. Time to dig it out ähm IN again and grow some carrots with it.,9171,1864279,00.html

Craig Allen said...

I'd be a lot more excited if someone could come up with a way to take air (H2, O2, CO2 etc.) and water then use the heat and/or electricity from a solar power station, to combine it all to produce hydrocarbons.

David B. Benson said...

My previous attempt to comment was eaten by TFSM or something. Anyway, not being a chemist, I'm interested in knowing just how exothermic the reaction

Mg2SiO4 + 4 CO2 + 4 H2O --> 2 Mg2+ + 4 HCO3- + H4SiO4

is. Thanks for assistance.

Anonymous said...

See p. 6 on exothermicity of magnesium silicates and CO2.

Cymraeg llygoden

David B. Benson said...

Cymraeg llygoden --- Thank you for your prompt reply.

EliRabett said...

Thanks to the Welsh mouse. Also, it is worth bookmarking for tables of thermodynamic parameters and other stuff.

Douglas Watts said...

What is fascinating is how well the map of worldwide magnesium silicate deposits tracks plate boundaries and subduction zones.

Anonymous said...

Florifulgurator asks:

What about wood gas?!

Call me a purist if you like, but actually we could potentially supply ALL of our hydro-carbon chemical needs with plants.

In many cases, the materials developed from plants (eg, bags made with cellulose) are much more environmentally benign (and recyclable) than the petrochemical versions.

There is no need for coal at all. After all, coal is just old plant material anyway.

The argument that we "need" coal (or any other fossil fuel) to supply our chemical needs is not just mistaken, but actually lends (possibly unintended) support for those who are calling for "clean coal."

Anonymous said...

we could potentially supply ALL of our hydro-carbon chemical needs with plants." combination with bacteria or yeasts feeding on plants (eg to produce ethanol) -- preferably using non-food source organic material.

EliRabett said...

Rocco, the efficiencies were for a gas turbine with a combined cycle and co-generation I'll accept that it is optimistic, but 80% can be done.

As you say the game is to use every part of the pig without being very squealish, so if you are generating electricity, you want to use the rejected heat to do chemistry or processes, and visa versa. I would not be very wedded tho to the operating temperature of a solar plant optimized for a steam/electricity cycle.

This sort of thinking needed is motivated by carbon taxes, which is what Eli is for.

David B. Benson said...

Not being a chemist, I need to ask.

I'm interested in the enhanced weathering of silicates, mainly magnesium and iron silicates, olivine and peridotite. Schuiling states that grnding these to 1000 micron (0.1 mm)
rock flour and then spreading these in the tropics will result in complete weathering within a year. I suspect that the centers of the particles are likely to remain unweathered. Opinions are welcomed.

Rather than just speading in the wild, so to speak, I am imagining a reactor which keeps the rosk flour well supplied
with water and air, but also controlled heating to around 80 degrees Celsius, since I suppose that the reaction will go faster in the hotter conditions. Is the heating worth it?

Since I suppose that the supply of carbon dioxide is now a
limiting factor, I am thinking about supplying extra (along with heat) by fermenting biomass to supply a little heat and biogasse. The biogasse is a mixture of carbon monoxide, carbon dioxide, methane and some micro-organisms. The biogasse is burnt in air to produce considerable heat and
flue gas. The flue gas, together with the air, is introduced into the reactor, providing a richer supply of carbon
dioxide. Is this worth it? Also, what about the NOx and carbon monoxide in the flue gas, will this poison the
weathering reaction or are these harmless (to these reactions, that is)?

One year of college shemistry 50 years ago isn't enough for me to even know how to begin. Assistance is appreciated.

David B. Benson said...

Oops. 100 micron (0.1 mm)

Anonymous said...


You could always try starting here for some starting information. Although it doesn't have the reaction ending in aqueous Mg2+. The nice thing about this reaction is the solution that comes out of it is slightly basic. This solution could end up in the ocean, which is being acidified by CO2. Two birds, one stone kind of thing. This is also probably the reason that in doesn't matter to much that the particles are so "big". The outer layer is a solution and is easily penetrated if not washed away/redistributed in the soil. Which is another reason for using it in the tropics instead of at some direct source. It should help in fertilisation of the ground with minerals. If it really works fast enough in the tropic (which means within a few years of having been distributed), then there's no need to heat it, accelerating the reaction. NOx would probably help to accelerate this reaction.


Anonymous said...

I'm all for carbon taxes too - get these things moving. Companies almost always choose for the cheapest option, even when they are of the "Do no evil" type. Not that that's applicable to any energy company outside of renewables.

Rocco Bocco.

David B. Benson said...

Rocco Bocco --- Thank you for the reference.

I'm interested in a reactor for the weathering due to the scale problem. An efficient size for a olivine mine is 100 million tons of rock per year, enough to remove 1.25 million tonnes of CO2; cost to mine and grind is $8 per tonne of CO2 removed. However, spreading the rock flour around to a depth of 1+ mm would require all the land around the mine within 100 km; the transportation costs will be very large.

Further, the open pit mine is then not being filled up again; about 75% of the reacted 'tailings' should go back in the deep hole. So about 75% of the rock flour needs to stay close to the mine; reaction towers may prove to be a feasible method if the reaction can be made to go a bit faster. Even so, many thousands of hectares around the mine would be needed for the reaction towers.

I'm in the process of attempting some cost estimates for the latter part of the entire exercise.