Coal

“Big Musky”

Fossil Fuels: Coal

Outline:Brief history of coalOrigin and geographical distributionChemical composition of coalChemistry of coal use

--gas, liquid, coke, tarEnvironmental impact of coal combustion

References:On Reserve

Coal: The Energy Source of the Past and Future, Harold H. SchobertAn Introduction to Coal Technology, N. Berkowitz

On the webhttp://chemistry.anl.gov/carbon/coal-tutorial/http://www.eia.doe.gov/emeu/iea/contents.html

Coal in History

Coal Timeline:200 BC - first published record of coal used for heating (Greece)300 AD - Coal adopted as heat source in China900 AD - Coal mining begun in western Europe1285 AD - Coal burning begins to pollute Londonearly 1600’s - Great Britain runs out of wood, switches to coal,

beginning of the industrial revolution1800’s - US becomes leading producer of coal1945 - coal is leading energy source in US1950 - USSR becomes leading producer of coal - coal provides 60% of world energy sources2000 - coal provides 30% of world energy sources as dependence on petroleum grows

The rise of western industry was tied to coal,until the discovery of petroleum

Global Coal Use--25% of US primary energy consumption is coal--Coal-fire technology supplies >50% of US electricity production

http://www.iea.org/statist/keyworld2002/key2002/keystats.htm

Global Total Primary Energy Supply Outlook

Mill

ion

tons

of o

il eq

uiva

lent

(Mto

e)

0

1.0

2.0

3.0

4.0

5.0

6.0

Energy usage (x 1020 J)

Geographical Distribution of Coal Reserves

http://www.eia.doe.gov/emeu/iea/table82.html

By country:

Country Total RecoverableCoal (x106 tons)

United StatesRussiaChinaIndiaAustraliaGermanySouth AfricaUkraineKazakhstanPoland

273,656173,074126,21593,03190,48972,75354,58637,64737,47924,427

By region:

Region Total RecoverableCoal (x106 tons)

Asia & OceaniaEastern EuropeNorth AmericaWestern EuropeAfricaCentral & South AmericaMiddle East

322,394290,183282,444101,34361,03223,977

1,885

The geopolitics of coal is very different from the geopolitics of petroleum Coal mining is labor-intensive and dangerous work

~2.1x107 BTU/short ton (2000 lbs);1 BTU = 1.055x103 JHence 2.1x1022 JConsume 1x1020 J/yr; so 200 yrs of coal reserves

% World Oil/Coal Reserves By Region:

C./S. America

Asia & OceaniaMiddle East

57

North America

W. EuropeEastern Europe

Africa

2618

2866

9

27

7

30

3

Source: EIA, International Energy Outlook, 2002

Oil

Coal

http://www.ket.org/Trips/Coal/AGSMM/agsmmwhere.html

US Distribution of Coal Resources

State Coal Resources (x 106 tons)

MontanaIllinoisWyomingWest VirginiaKentuckyPennsylvaniaOhioColoradoTexasIndianaOther

12078683730291917131051

Formation of CoalThe origin and formation of coal is better understood than the origin ofpetroleum, largely because it has 3-dimensional structure that can be traced tothe original organic material that formed it

Marks left bylignin source (ietree brances/twigs)

10 µm

Remnants ofcellular structureof source material

Coal deposits are formed from plant material that died and was deposited in aswampy environment - low in O2 Anaerobic bacteria convert the organic material until environment becomes tooacidic and the bacterial die - decomposition stops when the plants have been convertedto peat Peat becomes buried at bottom of swamp Peat is transformed at high pressure and low temperature (< 200 ˚C) over ~300million years to coalhttp://chemistry.anl.gov/carbon/coal-tutorial/

Peat Swamp

The first step in coal formation is accumulation of organic debris in a peat swamp. In most environments, such as the forest floor, plant material decays as fast as it is produced, so it does not accumulate. However, in a peat swamp, stagnant waterthat does not contain oxygen inhibits the decay of organic material allowing it to accumulate and form peat. Burying the peat with sediment further inhibits the decay of peat.

Delta Deposits

The coal deposits in the Appalachian Basin are associated with delta deposits. This outcrop from NE Kentucky shows alternating layers of sandstone, siltstone, shale and coal which are characteristic of deltas.

Glacial Eustatic Sea Level Fluctuations

Another model for cyclothems is glacial eustatic sea level fluctuations. In this model the land surface remains stationary and sea level rises and falls.

Geologic Time Scales

The majority of earth’s history isrepresented by the Precambrian,which consist of unfossiliferous rocksthat were deposited before there wasabundant life on earth.

The remainder of geologic time isdivided into the Paleozoic, Mesozoicand Cenozoic Eras which representancient life, middle life and recentlife, respectively.

The last three eras are further dividedinto the Periods of the Geologic TimeScale.

Age of Coal Deposits

Majority of the coal deposits are Pennsylvanian (P) in age; no coal deposits that are older than theDevonian (D). Land plants had not fully evolved prior to Devonian therefore coal deposits which are theremains of plants could not have formed. The Pennsylvanian Period which corresponds to the majority ofcoal deposits including the Appalachian Basin was a time when plant life flourished in North America. It isalso a time in geologic history when major continental glaciers occurred in the southern continents. Thecoal deposits in the western states, such as Wyoming, are younger than those in the Appalachian Basin andare mostly Jurassic (J), Cretaceous (C) and Tertiary (T) in age.

Changes After Burial

Burial of peat by overlying sediments resultsin an increase in the temperature andpressure. One change that happens iscompaction.

It is estimated that it takes 20 feet of peat toform a one foot coal bed.

In addition to compaction there is a loss ofmoisture and volatiles. Much of the waterthat is lost was trapped in pore spaces and isexpelled during compaction. Some of thewater, plus the volatiles (gases) are releaseddue to chemical changes in the peat.

Peat vs Coal

Continuous Mining

Coal is cut from face and loaded intoa truck/rail car that brings it to the surface

Roam and Pillar Mining

Longwall Mining

Surface Mining

Formation of CoalThe different organic constituents that form coal can be distinguished in the coal material

-- called macerals

MaceralType

Origin

ResiniteSporiniteBituminiteAlginiteVitriniteFusiniteSclerotinite

Plant resinsSpores, pollensDegraded algaeAlgaeWoody tissuesCarbonized woody tissuesFungal hyphae

Study of macerals is necessarybecause different startingmaterials will give differentC/H ratios in the coal product,which is important in coal use

Energy Value of Coal Ranks

The higher the coal rank the higher the temperature and pressure of coal formation. The higher coal ranks have a higher percent carbon and less water.As moisture and volatiles are driven off during coal maturation, carbon is left behind. The increased carbon content increases the heat content (Btu/lb) of the coal.

kJ/g12.516.622.827.735.8

Oil:43.6Gas: 51.6

US Coal Reserves

The majority of U.S. coal is bituminous. The highest rank coal anthracite only makes up 2 percent of coal reserves. This coal is too valuable to be burned as a fuel and is used mainly for coking steel.

Chemical Composition of CoalObservations:Chemical composition of coal was very difficult to solve

unlike petroleum, coal cannot be separated into its components easily

Heat coal to 100 ˚C, loses weight from H2O evaporation

Heat coal to higher temperatures, organic constituents begin breaking down, releaselow FW hydrocarbon gasses

Organic matter remaining can be combusted, products are H2O and CO2a small amount of inorganic ash is left behind

Instrumental analysis:X-ray crystallography - composed of clusters of benzene rings, but this is difficultbecause coal is not highly crystalline

NMR - as C/H ratio increases, aromaticity increases

Chemical Composition of Coal

Unlike petroleum, coal cannot be separatedinto individual products. It must bereformed into smaller FW useful material(synthetic fuels) or combusted to capturethe heat.

http://chemistry.anl.gov/carbon/coal-tutorial/

Conclusion: Coal is a 3D cross-linked polymer of aromatic rings and alkane linkerswith a small amount of inorganic contaminants

Model of coal, although even this is toospecific for some coal scientists

Inorganic constituents are from originalplant material, plus minerals leachedinto the coal from surroundingsediments

Na, Ca, Mg, K salts,Al, Si, Fe, S oxides

Coal also contains trace amounts of Gaand Ge, both of which are important forthe modern electronics industry

Graphite is “perfect” coal

Chemistry of Coal

COAL Coke Oven

CokeAsh

Condensationand Separation

Gasses

Tar

Distillationand Separation

Heavy OilsWash oils

OH

OH

OH

OH

CH4

H2 + CO

Fischer-TropschProcess

SyntheticGasoline

Light Oils

Creosote,Pitch

Coal GasCoal carbonization: coal is heated at moderate temperature, thermaldecomposition of the organic material releases small amounts of flammablegas

CmHn CH4 + (m-n)/4 C

Carbonization carried out at ~400 ˚C, products are H2 and CH4Important historically because this was the first large source of CH4 which was

used for lighting in the early 20th centuryNot a useful process anymore because of the large known reserves of CH4

C + 2 H2 = CH4 + 74.9 kJBut “hydrogasification” requires high temps, 800 CAlso is inefficient because is exothermic and driven to left at high T

Coal Gas

Coal gasification: conversion of coal into methane:

• More efficient route to methane:

CO + 3 H2 = CH4 + H2O + 206.3 kJ

Reaction is more exothermic but operates at 400 C with Ni catalyst

• Or if wanted, can produce liquids by Fischer-Tropsch chemistry:

n CO + (2n+1)H2 = CnH2n+2 + n H2O

• Or can even make methanol:

CO + 2 H2 = CH3OH

• But, where do the CO and H2 come from?

Coal GasSteam reforming:

C + H2O = CO + H2 - 131.4 kJ (1)Produces equal amounts of CO and H2

If need extra H2, run “water-gas shift” reaction:CO + H2O = CO2 + H2 + 41.4 kJ (2)

This makes the needed CO and H2

So overall, use two equiv. of (1), add to (2) and methanation to get:2 C + 2 H2O = 2 CO + 2 H2 - 262.8 kJCO + H2O = CO2 + H2 + 41.4 kJCO + 3 H2 = CH4 + H2O + 206.3 kJ2 C + 2 H2O = CH4 + CO2 - 15.1 kJ

Hence in theory can convert coal to methane with energy of only 15.1 kJBut exothermic reaction goes at 400 C, endothermic one at 900 CNeed to supply heat to endothermic one, to do so burn more coalRequires 262.8 kJ/mole of methane, about 32% of energy content of methaneHence upper limit is 68%, lower in practice due to other losses

Coal LiquidificationIndirect liquidification: Product of coal gasification (CO and H2 mixture, also calledsynthetic gas or syngas) is reacted to form larger FW hydrocarbons

Fischer-Tropsch Process: react CO and H2 at ~400-500 ˚C and ~100-150 atm overFe or Co catalysts

products vary depending on reaction conditions

CO + H2 Fischer-TropschReactor

OH

O

O

OH

alkanes

alkenes

alcohols

ketones

carboxylic acids

Indirect liquidification can produce syntheticgasolines (and was an important source of gasfor oil-poor Germany and Japan duringWWII), but it is not yet economically viable.

Production and Use of CokeGasification breaks apart hydrocarbons - products are volatile gases and carbonized coalCarbonized coal is called coke - can be burned at high temperatures up to 1100 ˚C

If coal carbonization is carried out at high temperature:Coke is used in blast furnaces in the steel industry

Although coke can support high combustion temperatures, some unburned cokeis carried out of the furnace as small particulate matter called smoke

Smoke is a major contributor to air pollution

If coal carbonization is carried out at low temperature:Coke burned at temperatures lower than 750 ˚C combusts completely (no smoke)This is commonly used for small-scale industries that cannot afford the scrubbers used to

clean smoke

Production and Use of TarCoal carbonization also produces a small amount of sticky, black liquid - called tarTar contains low FW aromatic compounds

distilled to produce benzene, toluene, xylenes, and their corresponding alcohols,which are important petrochemical feedstocks

Secondary products that can be made from tars are synthetic dyes, antibiotics andanesthetics, flavoring agents, and perfumes

Integrated Gasification Combined Cycle (IGCC)

--High efficiency (~60%)--Already near zero emission, could be made zero emission--Products are electricity, H2 (sold as fuel), and CO2 (which can be sequestered)--IGCC plants currently producing 1500 MW electricity; 2200 MW more online soon

Figure from Julio Friedmann, U. of Maryland

Integrated Gasification Combined Cycle (IGCC)Combine gas turbine generators with steam turbine generators powered by

steam from the waste heat of the gas turbine generator

800-300 800

= 62.5%

= 43.7%

800-450 800

450-300 450 = 33.3%

43.7+33.3=77.0%

Mass Balance During Coal Combustion

Numbers in Joules

Environmental IssuesSmokeSmoke is particulate matter containing either uncombusted coke or coal tars, or ash

Particulate matter eventually falls to the ground or is inhaledPittsburgh, 1911 - 1031 tons/mi2 of soot deposited (highest in the world)

death rate from pneumonia highest in the worldUncombusted coke can be eliminated by a well-designed furnaceAsh (called fly ash) must be removed from the emission vapors in the furnace

Electrostatic Precipitators: Smoke passes between two electrode plates,charged ash particals are attracted to the plates where they are collectedCyclone Collectors: Furnace gas sent into a vortex, heavy ash particles arecentrifuged down to collectors

Collected ash is used as raw materialCurrently ash is used in concreteIn the future it may become an important source of Al, Ga, and Ge

Environmental IssuesSulfurCoal is < 3% S, has no effect on the combustion of coal and coke

Sources of S are FeS2 (pyrite), organic sulfides (R-S-R’), and sulfates (SO42-)

About 15% of S is left behind in ash, the rest is emitted as SO2 and SO3 (SOx)Major environmental pollutant

SO2 deposits on surfaces (ie buildings, skin)SO3 forms SO4

2-, which dissolves in H2O and is washed from the air as acid rain

S Emission Reduction Strategies:Out of sight, out of mind: Early solutions emitted the SOx out of very tall stacks to disperse

it, carry it away from the point of origin, make it someone else’s problemBurn less S: Use coals with lower S content (most coal in the US is ~ 2.5% S)

Clean coal before combusting (this is expensive)Scrubbing: Vapor emission is run over an aqueous limestone (Ca) slurry, forms CaSO4

sludge

Environmental Issues

NitrogenN2 in air reacts with O2 at high temperatures to form NO, NO2, NO3 (NOx)NOx contributes to smog formationCoal combustion emissions account for only ~5% of NOx formation, and there is

currently no commercial process that removes NOx from vapor streams