Sing along now… ♫ ♪ Jimmy, meanwhile, is still on a corn ethanol binge…
At the Coaltek facility near Paducah, Kentucky (halfway between Possum Trot & Monkey’s Eyebrow), where the conveyor of coal goes through an enormous microwave oven, which, like the one in your kitchen, selectively vibrates the H2O. This drives the moisture out of the coal – cracking it – and energy density (BTU) of the coal increases by up to 50%
Sorry, Steve, I have to cry foul on this one. It doesn’t pass the sniff test, not by a Kentucky mile.
So let’s see, you’re using high-grade energy (electricity) to convert to microwaves, to vapourize water to drive it out of damp coal, so you don’t have to burn coal to heat the coal.
1 kg of burning coal can drive off 100010 (edit oops) kg of water, heating it directly. After inefficiencies of converting to electricity (40%), then microwaves (80%, on a good day), you’ve just increased the coal required to 3 kg.
I can see how you’re increasing the calorific value of the coal (like making coke), and potentially decrease smokestack emissions, but I can’t see how this does anything but *decrease* net plant efficiency.
(now, using plant waste heat to bulk pre-heat the coal, that makes sense)
Smells like the industrial equivalent of magnets on a fuel line.
(btw: the physicist pedant in me has to point out that microwave energy does not selectively vibrate water bonds: its electric field interacts directly with the highly-polar water molecule itself. The molecule flips around at gigahertz rates in response, and bumping into the surrounding moleculaes and atoms converts that flipping into bulk kinetic energy we see as heat. Water molecules in ice are literally frozen in place, and can not move, so don’t absorb much microwave energy, and don’t transfer it to the bulk material)
Continuing the sniff test: The heat value of dry coal is about 30 MJ/kg. If we have wet coal coming in at 20 MJ/kg, and want to raise it 50% to the "dry coal" level, we are assuming that 10 MJ deficiency is because of water we are removing. 10 MJ represents about 4 kg of water evaporated off. It’s pretty tough to believe we have 4 kg of water hiding in 1 kg of coal.
P^2, the mechanics are intriguing, indeed.
From an economic viewpoint, I guess one could take a very close look at the costs — capital equipment depreciation, electricity bills etc. — and the expected revenue, which would be a function of the differential in market prices between the lower and improved grades of coal. Parameters such as BTU density and crackability/grindability might have a quite large influence on the coal price. I have no idea.
There might also be a financial arbitrage and options play involved here, between a (subsidized ? stable ?) electricity price and (volatile ?) market prices for various grades of coal.
Ignoring the economics for a while, and looking at the chemistry, I think the hydrogen factor should also be considered.
Coal is not pure carbon, and its thermal output is dependent on the ratio of volatile compounds it contains. Hydrocarbons have a much higher thermal output than pure carbon — e.g. methane has about 4 times the energy content of pure carbon per unit mass.
An interesting question is whether there would be, say, enough hydrogen-containing molecules — e.g. water moisture and impurity hydroxides — in the input coal that could be separated, using e.g. microwave radiation, into highly reactive H+ and OH- free radicals.
OTOH, one could always spray the coal with water so that enough steam is generated in the microwave reaction chamber, but perhaps Coaltek’s process depends on the water molecules to be already deeply embedded in the coal interstices so that specific pressure and temperature conditions are achieved.
It’s not unthinkable that under some conditions, H+ and OH- radicals, reacting with heated air and C, would transform some portion of the low-grade coal into CO, and then form long-chain hydrocarbons and alcohols, with a resulting increase in BTU density. The syngas and Fischer-Tropsch processes come to mind, but one then wonders what, if any, catalysis mechanism is used.
P.S.
A back of the envelope calculation indicates that wet carbon, consisting of about 68% of C and 32% of H2O by mass, would release a net thermal output of about 21MJ/kg when burned.
C without any moisture would release about 32MJ/kg.
Hence, a ~50% improvement in BTU density would seem achievable on paper, just by thoroughly drying the coal, without any kind of hydrogenation chemical reactions involved.
No idea whether the economics would make sense, though.
Happy Tinfoil: Yes, the first of its kind.
P^2: not sure why they would go through all the trouble if that hypothesis were correct. =)
But I asked them:
"Steve -Water has no heat value, so it is removed either by a pre-combustion process like CoalTek or during combustion. Since the average coal-fired power plant (existing) is about 35% efficient, the process of vaporizing the water during combustion is highly inefficient. Conversely, our microwave process is 90%+ efficient. It does, however, require power to run it. The good news is that our effluent is low grade steam. Steam has a large capacity for storing energy. We can recapture this waste energy through a simple heat exchanger, recovering up to 2/3 of the energy used during the process. That heat is then used to offset the microwave power required to process the coal. So the CoalTek process is significantly more efficient than combustion. Drying using thermal heat is also very inefficient. Simply crushing it and letting it air dry isn’t practical because the volumes of coal required to power even a small plant are massive, making handling and logistics of a crushed product impossible. More simply, microwave processing is the most efficient means of vaporizing water available. Waste heat recovery makes it even more so. But the economics are certainly influenced by the price of power, therefore site selection is important."
I fear this has grave potential of devolving into a usenet-style debate, so I’ll try not to go there.
So the gains in calorific yield (BTU/ton, MJ/kg, whichever) are because driving off the moisture reduces the mass of the coal. If you drive off 33% of the mass of a lump of wet coal by drying it, you’ve just increased its calorific yield 50%. No mystery there.
Discounting other potentially valuable uses of microwave pre-treatment (improved grindability and handling, faster processing time, decreased smokestack emissions, product quality uniformity), I seriously question the claim that using microwaves for drying is more efficient than simple heating.
Using CoalTek’s numbers from their response above, you’re burning coal to produce electricity at 35% efficiency, then converting to microwaves at (an unbelievable) 90% efficiency, to heat coal at a net 35*90=31.5% efficiency. (Recovering 2/3 of it as low-grade heat is useless at a coal plant: it’s just more that needs to be thrown away in the cooling towers.)
I’m sure there’s some legitimate value in microwave preprocessing of coal but to present it as a more efficient way of drying coal or, worse, appearing to claim to increase the gross heat value of a trainload of coal, is misleading (at best).
OK, I’ve said my piece. I didn’t mean to hijack a perfectly good photo. Thanks for the forum, and I’ll get off the soapbox now.
Yeah, probably best to take the deep dive offline. Happy to connect you with the company.
P.S. in my photo caption, I do not claim a free lunch at the thermodynamic buffet, but an improved energy density. Yes, it takes energy to get there, but it can lead to various economic benefits, some of which are site specific, and others are more general, like the benefits of pre-processing that you mention.
Adding fuel to the P^2 fire:
Remember leaving your tricyle out in the rain when you were a kid? You come back on a sunny day, helmet on, ready to ride. And just as you put the foot to the pedal. Creeeeek. The darn thing is rusty as an old wheel barrel. Shoot!
Its no news that moisture is no friend to metal.
Coaltek can speak to this. Mositure creates wear and tear on the machines which ultimately decreases efficiency. Should also mention that moisture increases parasitic load for peripheral systems like crushers, pulverizers, and air handling systems. All of the above results in lower plant yield, decreased efficiency and wasted BTUs.
It could be the coal still clouding my vision, but I think this issue has some place in the BTU argument. No?
Hi, I’m an admin for a group called Ugg Power, and we’d love to have this added to the group!
Look at all that carbon before it gets put into the air! I believe Al Gore has stated that there is no such thing as clean coal technology.
Save the mircowaves for the popcorn and lets use cleaner burning natural gas for electricity generation. Better still – solar, hydro, and wind. Sure it may cost a little more, but high cost is the only proven way to get most Americans to change their habits in a significant way…
Just the other day I heard people putting down more efficient HVAC by claiming that the payback period was 25 years. They’re argument – its cheaper overall to use the less expensive and less efficient equipment than to spend more on the more efficient systems.
An excellent free book:
http://www.withouthotair.com/
It’s pretty UK-centric for data, but explains energy concepts REALLY well in a very readable way. It shows, for instance, why alternative technologies will have a really tough time replacing coal. Highly recommended.
Leave a Reply to Vanita Cancel reply