Coal rank and thoughts on EROEI
Posted by Heading Out on February 6, 2006 - 1:50am
For those who wonder what is going on, this is a weekend tech talk, where some underlying aspect of fossil fuel energy is discussed. References to earlier posts are given at the end of this one, and the subjects are usually simplified to get across the basic ideas, within a reasonable amount of space.
Back when my grandfather was mining coal, with a pick and a shovel, he would very carefully separate rock from the coal as he picked lumps out from the solid. The reason for this relates to how he was paid. Before he began to fill a tub with coal, he would place a holder and a token with his mark on it at the bottom of the tub. Thus when it got to the surface, the teller would check that the tub only contained coal, and then would give him credit for the tub. But if the tub contained much stone, from the roof, or from layers of rock within the coal, then the tub would not be counted and my grandfather got no credit for it.
Because it is now much more difficult to separate the rock and coal (which is usually crushed to less than half-an-inch in size) both rock and coal are carried to the surface, and fed into a coal preparation plant. Here, using a combination of methods including gravity separation in a liquid that has a density between that of coal and the rock, the coal is separated from the rock. The waste rock, historically was stored in the large coal tips that dotted the landscape of the Eastern United States, and Europe, while the coal was screened into different sizes and sold.
As coal is used more and more in power stations it is usually further crushed in additional mills to a much smaller size so that it will burn more efficiently in these plants. This is more costly in the energy that it takes to prepare the coal. To understand why consider that there is a physical property called surface energy. Simply it is the strength of the bond that holds two surfaces together. Let us, to work an example, say that it takes 10 units of energy to break the bonds over 1 sq-inch of surface, So that when I split a four-inch block of coal into two pieces then I am creating an additional 16 sq-inches of surface, and to do that I have to put in 16 x 10 = 160 units of energy to make that change. Now if I want to break the piece of coal into quarter-inch pieces, then it will take 15 x 15 x 15 cuts, and take therefore a total of 540,000 units of energy. Breaking the original four-inch piece from the solid would have taken 5 x 16 x 10 = 800 units of energy. In this way you can see that the finer the coal is ground, the more energy that is used in the process. Unfortunately grinding systems are not highly efficient so that there are additional energy costs over and above those needed for the simple surface separation.
The energy balance is then even more complicated when we add the fact that there are a variety of different types of coal. These are generally given a rank based on their carbon content. Thus, for example the lowest ranking coal is brown coal or lignite, and this may only have a carbon content of around 60% and contain a high percentage of water. Thus it has to be dried before it can be effectively used. (Peat, it's historical ancestor is even lower in energy and higher in water content). Moving up the scale, sub-bituminous coal has a carbon content of perhaps 75% and 10% moisture; bituminous coal can go up to 90% carbon and perhaps 5% moisture. The highest rank coal is known as anthracite and this has a carbon content above 90%.
As a result of the difference in carbon content, the heating value of the coal also changes. While numbers and definitions vary somewhat as one moves around the world as a rough guide a ton of Lignite is around 7 million Btu; sub-bituminous 17 - 18 million Btu; Bituminous 21 to 30 million Btu; and Anthracite around 16 million Btu. These values are if the coal has been cleaned of other rocks. The quality of the coal also affects the price. (It should be noted that current spot prices of coal may now be quite different.
There are other issues, however, that also control the price of the coal. These include the sulphur content, since this, in turn, has controlled the amount of scrubbing of the flue gases from power station that has been required to remove the resulting compounds. Since this is an expensive process, in the past it has driven some coal production areas to close, while purchases of lower sulphur coal have increased.
This is a part of a series of talks that has, most recently, dealt with coal mining. Earlier talks in that series dealt with three forms of mining;
As usual any concerns, corrections, or questions, should be addressed in comments.
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machines used to mine coal are electic, then
there are the huge losses in the system that
provides the electricity.
And if the machines are oil powered, then
peak oil is going to make something of a mess
of the economics of coal mining. It was
reported that North Korean coal mining declined
substantially after the USSR fell because of
the shortage of oil.
But my main concern at this point of time is
that the media are finally starting to report
peak oil as a serious and imminent problem, but
all sorts of biomass solutions are being
touted as a fait acomplis, even though EROEI
of many of them have been determined to be
marginally positive or actually negative.
Currently ethanol is being touted as a magic
bullet, yet to grow the sugar can or beet
consumes energy (and fertiliser), harvesting
the crop requires energy, all the filtering and
transfering require energy and most
importantly, distillation requires a lot of
energy.
Yet in most feature items there is no mention
of any of this.
Seems to me it is all about calming the markets,
rather than providing genuine solutions, as
always.
Distallation to useable ethanol is also a relatively low energy process (perhaps 1% or 2% I would guess). Solar distillation is certainly technically feasible.
Unlike corn ethanol*, sugar ethanol is certainly net energy positive. Only problem is it can be grown only in Louisiana, Florida and Hawaii inside US and Louisiana is only major producer left.
*If one assigns an energy value to the cattle feed created by corn ethanol byproduct, corn ethanol is also net positive. One can feed cows is either corn "straight" or extract the ethanol and feed them the mash. Extracting the ethanol is very much a net positive process.
then we must assume that over a period of time
their yeilds will fall, since it is not possible
to remove nutrients from soil (by way of removing
crops to another location) and still maintain
fertility.
You say that transportation is minimal, but most
fuel is used along way from sugar plantations.
Although pure ethanol is a good fuel, it still
does not have as good an energy content as oil
or petrol and carting ethanol across the
country does amount carting a certain amount of
'water' [O-H bonds versus C-H bonds] across the
country.
I have not got any figures in front of me, but
the distillation process is not insignificant.
However it might me possible to use dried stalks
to fire the distillation apparatus.
Nevertheless, a large amount of work and capital
investment would be required to produce
relatively small quantities of fuel. There is
no way that the world can produce 84 million
barrels of ethanol a day, or even a quareter of
that quantity, from any sustainable agricultural
system.
All the hype about Brazil running cars on a 15%
ethanol blend is just hype. How many cars are
there in Brazil? 10 million? 20 million? And
in the world? 600 million?
Biofuels will never be more than a small and
very local contribution to future fuels.
Louisiana persists as sugar cane producer in large part due to the fertility of the soil. Depletion after two centuries has not been noted AFAIK. (And let some flood water onto the fields and they will be fertile for millenium. See Nile Delta).
The bagasse (stalks after pressing) has been used for fuel for small electrical plants elsewhere (3 to 5 MW from memory from waste from a single sugar mill).
Water and pipeline transportation are both extremely energy efficient. Blending ethanol with local gasoline and pipelining or barging it out will use little energy.
The limit is not capital or operating expense but land. Relatively little of Louisiana is suitable for sugar cane production. Best case, we could supply most of our own needs for liquid fuels with a bit left over for export.
ethanol blend is just hype. How many cars are
there in Brazil? 10 million? 20 million? And
in the world? 600 million?
A 15% ethanol bland simply translates into saying that cars can run completely on ethanol if they were, on average, seven times more efficient. As we know, that is easily achievable. A hybrid like the Toyota Prius is 3 times as efficient as the average American road park. Achieving just a bit over twice that efficiency is actually quite easy. Most electric cars achieve that efficiency already, which is of course the main reason why the auto companies are buying them back and destroying them.
A hybrid usually has something like a 15 to 25 Hp electrical motor for city use, and a 80 to 120 Hp combustion engine for highway use. Considering this, it should be painfully obvious that combining the requirements for a SUV into a hybrid is plain stupid: combining a 25 Hp electrical motor with a 250 Hp engine just because mom likes to make the beast roar when driving the kids to school, borders on insanity.
Just consider this: A car with only 25% of it's current engine power will still get you where you want at each and every legal speed. What good is a family car that can do 150 Mph and drive off-road when you live in down-town New York and your car gets impounded if you are caught doing above 90?
One word: Marketing.
http://www.coal.org/facts/types.htm
Also, when the BP report quotes production in tons, is that as it comes from the mine, or dry stuff without rocks that is ready to use?
Do you have any idea of the energy involved in mining one tone of coal?
In today's world how important is Coal transportation in the final process?
Subkommander Dred
Excellent post in that it indirectly shows the difference between a LIQUID fuel (Oil) and a SOLID fuel (Coal).
Folk who think the switch back from oil to coal is just a matter using different noises from our English language for designating different parts of the generic idea for "fossil fuels" often miss these finer particulate points. The chemical/ physical bonds between solid fuel molecules are harder to break. We need to break them to get the oxygen in between so that the fuel will burn (combust). EROEI goes down as the interparticle bonding strength increases.
As I understand it, petroleum is largely the result of deposited marine biota being subjected to high pressure and heat over a long period time. It is usually encountered in current or former coastal regions and at a considerable depth. Coal on the other hand is largely the result of deposited terrestrial biota and is generally encounterd inland and at a much more shallow depth.
Now, while the morphology of the millions of different plant life forms can vary tremendously, for the most part their respective chemical compositions lie with a relatively narrow range.
Question 1: If they both have a more or less similar biological origin, then why do coal and petroleum have such vastly different chemical compositions and such a vastly different range of physical properties?
Question 2: Is coal considered to be just a younger form of petroleum in the making? In other words, if I heated the coal and squeezed it real, real hard for the next several million years, would I get petroleum? Or are there other things at work here?
A specific example I gathered from a plant tour of Fayette Power Plant (outside LaGrange Texas, half owned by City of Austin & half by LCRA). A 550 MW plant (net from memory) used 10 MW for grinding coal to a powder, i.e. about 2%.
In addition, energy to mine (low) in WY & MO and rail transport to plant (medium, but trains could be electrified, significantly increasing energy efficiency and going to coal/wind power).
Why?
Coal is a complex set of hydrocarbons with sulfer & ash mixed in. Some coal is pure carbon, but not much.
Coke, used for steel production, is much purer carbon, but it is produced from coal by partial oxidation, burning off much of the H.
I see this CO2 process as a relatively low energy way to seperate ash, sulfer and high carbon/low hydrogen compounds from medium carbon/higher hydrogen compounds. If the high carbon fraction can be used for steel production, then there is a good demand.
The economics and usefulness of this would vary (IMHO) dramatically from one coal source to another.
Older boiler only NG plants would have heat rates in the high 9,000s BTU/kWh (perfect efficiency is 3,412 BTU/kWh). Combined cycle plants have manufacturer nameplate heat rates of high 5,000s to low 6,000s and "real world" #'s in high 6,000s to low 7,000s. GE is the dominant player in combined cycle NG plants. (Combined cycel olanst are also cheap and quick to build as well. In the 1990s about 85% to 90% of all new US power plants were NG, with the high majority being combined cycle and the rest simple peak only turbines with heat rates ~10,000 BTU/kWh).
All coal fired power plants are boiler operations (like old NG plants) and 10,000 BTU/kWh is the "gold standard" for constant output coal fired plants (higher for load following plants). GE says that they are working on a coal gasification process that would feed into a coal gas combined cycle plant.
Given that gasification is not a free process, my SWAG is that this process could give us 15% to 20% more electricity per unit of coal when developed.
If anyone has any good stats on what coal plants are in the works, that would make a good post/comment.