Re: Coal Gasification
- From: William.Mook@xxxxxxxxx
- Date: 3 Oct 2006 02:36:44 -0700
Coal gasification takes hydrogen from water and carbon from coal and
makes simple hydrocarbons like CH4 methane, which is the principal
component of natural gas! One way of doing this is by use of a Lurgi
reactor, whose operation is described here;
http://www.dieter-ulber.de/gtc.htm
What my explanation above the URL and what tomcat's explanation below
avoid discussing is how much CO2 is produced in creating natural gas in
this way.
They hydrogen comes from the water-shift reaction. That is; a
measured amount of carbon (coal) and oxygen and water are made to flow
through a reactor, and heat is applied to the system by the burning of
additional carbon. The burning of carbon to heat the water to
superheated steam is the first source of carbon-dioxide in the process.
The carbon and oxygen being heated are in a proportion so that
carbon-monoxide and steam is produced;
CO + H2O
This is called synthesis gas, or 'syngas'. This gas is passed through
a catalytic converter, similar to the converter in your car exhaust.
The catalyst is also a pollution source as well as a large cost center
in a practical system, and the availability of the rare metals in the
catalyst is a limit on the scale of the process as well. Nevertheless,
the system does work! In the presence of the right catalyst and the
right conditions, nearly all the syngas gets converted through the
water shift reaction to carbon dioxide and hydrogen gas. Here's how
that works
CO + H2O --> CO2 + H2
The carbon-monoxide scavenges the oxygen from the water to form CO2
leaving the hydrogen behind. The CO2 can then be removed by passing it
through a cryogenic separator. These are usually run by electricity -
sort of like a refrigerator or air conditioner. And in at a coal mine
guess where this electricity comes from? That's right, a coal fired
generator. And guess what that produces? A third source of CO2 for
process energy! lol.
Well, you can see that this source of hydrogen is very costly in terms
of producing CO2. We have the CO2 produced by heating the coal, oxygen
and water. We have the CO2 produced in the shift reaction itself (by
far the largest component) and we have the CO2 produced to power the
process to get our hydrogen. About 2/3 of the carbon comes from the
shift reaction and 1/3 from the other sources. So, we can estimate the
shift reaciton easily and from that figure the total Here's the net
balanced chemical equation
C + O + H2O ---> CO2 + H2
And C = 12 amu, O = 16 amu, H = 1 amu
12 + 16 + 18 ---> 44 + 2
So, to produce a kg of hydrogen in a shift reaction I've got to react 6
kg of carbon and 9 kg of water and 8 kg of oxygen to produce 22 kg of
CO2!!
Add another 1/3 to this total to account for process heat and power and
you have 9 kg of carbon and 33 kg of CO2 for every kg of hydrogen!
Now, coal is typically 80% carbon and 20% other stuff. And if that
other stuff happens to be poisonous, well,then you have that too!
Also, you get NOx out the wazoo with this reaction (sure you can make
some of it into ammonia! haha.. but not ALL of it! which is the
problem. But we divide the 9 kg by .8 to obtain 11.25 kg of COAL to
run this reaction, which produces 33 kg of CO2 and 2.25 kg of OTHER
WASTE PRODUCTS to make one stinkin' kg of hydrogen!
You can quickly see this is a dirty source of hydrogen.
Okay, now NEXT the Lurgi reactor efficiently makes methane gas from the
hydrogen and yet MORE carbonl! haha.. This is the net reaction
C + 2 H2 ---> CH4
12 + 4 ---> 16
Now this means that each kg of hydrogen requires the addition of 4 MORE
kg of carbon,which means 5 MORE kg of COAL, and when the methane is
burned it produces 11 MORE kg of CO2 and 1 MORE kg of other waste
products!
So, for each kg of hydrogen processed this energy cycle produces an
astounding 44 kg of carbon dioxide and 3.25 kg of other waste products.
At $50 per ton for coal and $18 per ton of CO2 (carbon credit cost)
this is a total cost of $1.50 per kg of hydrogen. The cost of
sequestering the other waste products can add another $1.00 per kg of
hydrogen. The volumes of waste are so large relative to the volume of
hydrogen, the availability of sequestration locations is a problem.
So, this is what the Lurgi reactor has cost us. Its taken a relatively
clean burning fuel, methane gas, that produces only 2.75 kg of
carbon-dioxide for every kg of it burned and burdened it with the
production of 11 kg of carbon-dioxide! Nuts!
This is why some have said that the Lurgi reactor is a carbon-dioxide
production system that produces fuels as a byproduct!
With coal costing $50 per metric ton, we can see that the production of
1 ton of methane gas by this process costs $140 per ton of methane gas
in coal costs alone, and another $140 in capital costs, and at $18 per
ton for carbon credits, would pay another $200 per ton of methane gas
produced this way for those. A whopping $480 per ton direct costs. Add
another 15% for labor and maintenance and you can see that the profits
of this sort of process are nil at best.
Which explains why these systems are not in wide use.
Another approach is to develop very low cost solar collectors to make
DC electricity when the sun is shining. The cost of the collectors
here is key. Making solar collectors for 7cents per peak watt means I
can make energy when the sun shines at 1/3 cent per kWh. Since it
takes 50 kWh to make a kg of hydrogen from 9 kg of water, producing 8
kg of oxygen as a byproduct. This means that a kg of hydrogen costs 16
cents.
What can we do with the hydrogen? Use the Sabatier reaction to take a
source of CO2 and make methane gas out of it;
CO2 + 4H2 ---> CH4 + 2 H2O
44 + 8 ---> 16 + 36
Water is a byproduct, and the process ABSORBS 2.75 kg of CO2 for each
kg of methane made this way! This EARNS the process $50 per ton in
carbon credits for each ton of methane produced this way, AND the cost
of the hydrogen is only $320 per ton of methane produced. So, the NET
cost is $270 per ton of methane produced. The cost of the equipment to
run this simple process is about $80 per ton - and the maintenance
costs add another 5% (lot less than coal since only gases, no solids,
are handled) So, we're at about $375 per ton - HALF the cost of the
Lurgi reactor!
A clearly superior system all around - because we're NOT burning coal
and we're using SUNLIGHT to power everything cheaply.
Of course, we could also use hydrogen directly or the solar electricity
directly, which is a benefit to this system.
tomcat wrote:
Coal gasification is a process that converts coal from a solid to a
gaseous state. The gas that is created is very similar to natural gas
and can be used to make chemicals, fertilizer, and/or electric power.
Cleanest of all coal-based electric power technologies, gasification
has significantly lower levels of air emissions (including volatile
mercury), solid wastes, and wastewater.
Due to its high efficiencies, gasification also uses less coal to
produce the same amount of energy, resulting in lower carbon dioxide
(CO2) emissions. Some scientists believe that CO2 in the atmosphere
contributes to a "greenhouse effect" that will lead to global warming.
Coal gasification has proven technology for capturing CO2 at a fraction
of the cost required for coal combustion technologies. The United
States is debating whether CO2 abatement should be required for all new
and existing coal power plants.
Gasification provides the only economically feasible route for
production of hydrogen from coal. Because hydrogen-powered vehicles are
nearly pollution-free, many people favor a long-term program to convert
America's cars, trucks, and buses to hydrogen.
In summary, coal gasification is a proven technology that can utilize
America's 250+ years of coal reserves to produce clean electricity,
fuels, and chemicals, and to provide a bridge to a future
hydrogen-based economy.
.
- References:
- Coal Gasification
- From: tomcat
- Coal Gasification
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