Physical and chemical properties

of coal and its products

Lecture no.L-02-1

Dr hab. inż. Marek Ściążko

Prof. nadzw.

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Chemical properties of coal

• Coal comes in four main types or ranks:

lignite or brown coal, bituminous coal or

black coal, anthracite and graphite. Each

type of coal has a certain set of physical

parameters which are mostly controlled by

moisture, volatile content (in terms of

aliphatic or aromatic hydrocarbons) and

carbon content.

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Proximate analysis

• The objective of coal proximate analysis is to determine the amount of fixed carbon (FC), volatile matters (VM), moisture, and ash within the coal sample.

• The variables are measured in weight percent (wt. %) and are calculated in several different bases. AR (as-received) basis is the most widely used basis in industrial applications. AR basis puts all variables into consideration and uses the total weight as the basis of measurement.

• AD (air-dried) basis neglect the presence of moistures other than inherent moisture while DB (dry-basis) leaves out all moistures, including surface moisture, inherent moisture, and other moistures. DAF (dry, ash free) basis neglect all moisture and ash constituent in coal while DMMF (dry, mineral-matter-free) basis leaves out the presence of moisture and mineral matters in coal, for example: quartz, pyrite, calcite, etc. Mineral matter is not directly measured but may be obtained by one of a number of empirical formula based on the ultimate and proximate analysis.

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Moisture

• Moisture is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analysed quantitatively. Moisture may occur in four possible forms within coal:

– Surface moisture: water held on the surface of coal particles or macerals

– Hydroscopic moisture: water held by capillary action within the microfractures of the coal

– Decomposition moisture: water held within the coal's decomposed organic compounds

– Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays

• Total moisture is analysed by loss of mass between an untreated sample and the sample once analysed. This is achieved by any of the following methods;

– Heating the coal with toluene

– Drying in a minimum free-space oven at within a nitrogen atmosphere

– Drying in air at 100 to 105 °C (212 to 221 °F) and relative loss of mass determined

Source: http://en.wikipedia.org/wiki/Coal_assay#Chemical_properties_of_coal Copyright-MS-2013 4

Volatile matter

• Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur. The volatile matter of coal is determined under rigidly controlled standards. In Australian and British laboratories this involves heating the coal sample to 900 ± 5 °C (1650 ±10 °F) for 7 minutes in a cylindrical silica crucible in a muffle furnace. American Standard procedures involve heating to 950 ± 25 °C (1740 ± 45 °F) in a vertical platinum crucible.

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Ash

• Ash content of coal is the non-combustible

residue left after coal is burnt. It represents

the bulk mineral matter after carbon,

oxygen, sulfur and water (including from

clays) has been driven off during

combustion. Analysis is fairly

straightforward, with the coal thoroughly

burnt and the ash material expressed as a

percentage of the original weight.

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Detailed ash analysis

• An analysis of coal ash may also be carried out to determine not only the content of coal ash, but also to determine the levels at which trace elements occur in ash. These data are useful for environmental impact modeling.

• Beside composition of coal ash, ash fusion point is also one significant parameter in ash analysis. The optimum operating temperature of coal processing will depend on the gas temperature and also the ash fusion point. Melting of the ashes may cause them to stick to the walls of the reactor and result in a build-up.

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Fixed carbon

• The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass of volatiles determined by the volatility test, above, from the original mass of the coal sample.

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Conversion of the data basis

Proximate Analysis unit (ar) (ad) (db) (daf)

Moisture (wt. %) 3.3 2.7

Ash (wt. %) 22.1 22.2 22.8

Volatile Matter (wt. %) 27.3 27.5 28.3 36.6

Fixed Carbon (wt. %) 47.3 47.6 48.9 63.4

Gross Calorific Value (MJ/kg) 24.73 24.88 25.57 33.13

(ar) – as received

(ad) – air dried

(db) – dry basis

(daf) – dry ash free

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Ultimate analysis

• Ultimate analysis is the determination of

the percentage composition in terms of

carbon, hydrogen, sulfur, nitrogen, and

ash, and the calculation of the oxygen

content the difference.

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Conversion of the data basis

Ultimate Analysis unit (ar) (ad) (db) (daf)

Carbon (C) (wt. %) 61.1 61.5 63.2 81.9

Hydrogen (H) (wt. %) 3.00 3.02 3.10 4.02

Nitrogen (N) (wt. %) 1.35 1.36 1.40 1.81

Total Sulfur (S) (wt. %) 0.4 0.39 0.39

Oxygen (O) (wt. %) 8.8 8.8 9.1

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Conversion table

Desired

Basis >

Given Basis

As Received Air Dried Dry Dry, ash

free

Dry, mineral

matter free

As Received 100-Mad

100-Mar

100

100-

Mar

100

100-

(Mar+Aar)

100

100-

(Mar+MMar)

Air Dried 100-Mar

100-Mad

100

100-

Mad

100

100-

(Mad+Aad)

100

100-

(Mad+MMad)

Dry 100-Mar

100

100-Mad

100

100

100-Ad

100

100-MMd

Dry, ash

free

100-

(Mar+Aar)

100

100-

(Mad+Aad)

100

100-

Ad

100

100-Ad

100-MMd

Dry,

mineral

matter free

100-

(Mar+MMar)

100

100-

(Mad+MMad)

100

100-

MMd

100

100-MMd

100-Ad

Definition of Variables

M = moisture (%)

A = ash (%)

MM = mineral matter (%)

Definition of Subscripts

ar = as received basis

ad = air dried basis

d = dry basis

Specific Energy Conversions

For conversions between units:

1 Btu/lb = 0.002326 MJ/kg

1 kcal/kg = 0.0041868 MJ/kg

1 Metric Ton (tonne) = 1.10231

short tons

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Chemical constitution of coal

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Atomic composition of different fossil fuels

(kmol at. in 100 kg of org. substance)

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7Type of fuel

Nu

mb

er

of

ato

ms

1 Peat

2 Lignite3 Flame coal, type 314 Gas coal, type 33

5 Ortho-coking coal, type 356 Semi-coking coal, type 377 Anthracite

No. Type of fuelHydrogen Oxygen Carbon

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Atomic ratio of (C/O)at, (C/H)at, (H/O)at in different

fossil fuels

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7

Typ paliwa

Sto

su

nk

i a

tom

ow

e

C/O C/H*10 H/O

The critical change

is observed in oxygen

content

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• On the average in a bituminous coal hydrogen content equals to ca. 5% w/w daf., and carbon content ca. 84%. It means, that statistically every 7 atoms of carbon meets 5 atoms of hydrogen.

• The next substantial element is oxygen, that can be found in a coal at the average concentration 10% w/w. It means that for every 7 atoms of carbon acounts for only 0.5 atoms of oxygen, however what will be shown its content substantially effects the coal properties.

• The remaining elements like sulfur - S and nitrogen - N can be found in coal on the average in amount of 0.5 i 1% and due to its concentration does not influence the coal gross properties very much.

Atomic structure of coal

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Atomic composition of different fossil fuels

(kmol at. in 100 kg of org. substance)

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7Type of fuel

Nu

mb

er

of

ato

ms

1 Peat

2 Lignite3 Flame coal, type 314 Gas coal, type 33

5 Ortho-coking coal, type 356 Semi-coking coal, type 377 Anthracite

No. Type of fuelHydrogen Oxygen Carbon

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Atomic ratio of (C/O)at, (C/H)at, (H/O)at in different

fossil fuels

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7

Typ paliwa

Sto

su

nk

i a

tom

ow

e

C/O C/H*10 H/O

The critical change

is observed in oxygen

content

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• On the average in a bituminous coal hydrogen content equals to ca. 5% w/w daf., and carbon content ca. 84%. It means, that statistically every 7 atoms of carbon meets 5 atoms of hydrogen.

• The next substantial element is oxygen, that can be found in a coal at the average concentration 10% w/w. It means that for every 7 atoms of carbon acounts for only 0.5 atoms of oxygen, however what will be shown its content substantially effects the coal properties.

• The remaining elements like sulfur - S and nitrogen - N can be found in coal on the average in amount of 0.5 i 1% and due to its concentration does not influence the coal gross properties very much.

Atomic structure of coal

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High heating value vs. carbon

content in a fuel (daf)

0

5000

10000

15000

20000

25000

30000

35000

40000

60 65 70 75 80 85 90 95 100 105

Zawartość pierwiastka C [%]

Cie

pło

sp

ala

nia

[kJ/k

g]

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Coal heating - devolatilization

Heating rate, K/min

Source: Journal of Analytical and Applied Pyrolysis

58–59 (2001) 685–701 Copyright-MS-2013 21

5,860,171,173,9369,5730,216,82,5

OdaS

caNaHaCaVaAaWa

OxygenSulphurNitrogenHydrogenCarbonVolatileAshMoisture

Content, % mas.

5,860,171,173,9369,5730,216,82,5

OdaS

caNaHaCaVaAaWa

OxygenSulphurNitrogenHydrogenCarbonVolatileAshMoisture

Content, % mas.

Heating valueEnthalpy

of formation

Activation

energy

Frequency

index

Qsdaf, MJ/kg

fHo

, kJ/kg E, kJ/kmol ln (k8

)

34,7 -463,2 11181,2 -5,97

0

0,1

0,2

0,3

0,4

500 700 900 1100 1300

Temperature, K

Yie

ldo

f p

rod

uc

ts,

kg

/kg

H2 CO

CH4 CO2

C2H6 H2O

V Tar

0

0,1

0,2

0,3

0,4

500 700 900 1100 1300

Temperature, K

Yie

ldo

f p

rod

uc

ts,

kg

/kg

H2 CO

CH4 CO2

C2H6 H2O

V Tar

0

0,1

0,2

0,3

0,4

0,5

0,6

500 700 900 1100 1300

Temperature, K

Ga

s c

om

po

sit

ion

, km

ol/k

mo

l

0

0,1

0,2

0,3

0,4

0,5

0,6

500 700 900 1100 1300

Temperature, K

Ga

s c

om

po

sit

ion

, km

ol/k

mo

l

Biuminous coal

- Wieczorek -

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Coke

• Coke is the solid carbonaceous material

derived from destructive distillation of low-

ash, low-sulfur bituminous coal. Cokes

from coal are grey, hard, and porous.

• Coke is used as a fuel and as a reducing

agent in smelting iron ore in a blast

furnace. It is there to reduce the iron oxide

(haematite) in order to collect iron.

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Coal tar

• Coal tar is a brown or black liquid of extremely high viscosity, which smells of naphthalene and aromatic hydrocarbons.

• Coal tar is among the by-products when coal is carbonized to make coke or gasified to make coal gas.

• Coal tars are complex and variable mixtures of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds, about 200 substances in all.

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Coke gas

• Coal gas (also town gas and illumination gas) is a flammable gaseous fuel made by the destructive distillation of coal containing a variety of calorific gases including hydrogen, carbon monoxide, methane and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen.

• It was the primary source of gaseous fuel both the United States and Great Britain until the widespread adoption of natural gas during the 1940s and 1950s. It was used for lighting, cooking and heating and was often supplied to households via a municipally-owned piped distribution system.

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