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Lecture 10

Use of coal

• Power generation à electricity

• Reduction of metal ores à smelting (e.g. Fe; Cu)

• Minor uses:

– Liquefaction of coal à liquid fuel (gasoline)

– Gasification of coal à coal gas (CO; CH4)

Burning of coal

• Main reaction:

– C + O2 à CO2

– Exogenic reaction à release of heat and CO2

• Byproducts:

– Ashes: remnants of sediments and impurities (trace metals)

– Soot and Fly-ash: solid particles small enough to be carried by released gases

– Gases: volatiles formed from impurities: SO2; Hg; etc.

Environmental concerns

• Impurities in coal:

• Coal contains trace quantities of all the elements found in living organisms (e.g. S, P, N)

• In addition, elements deposited together are also present: S, U, Hg, Pb etc

Sources of Sulfur

• Two sources:

– Organic S, bound in organic molecules

– Inorganic S, deposited together with organic source material

• Level of S depends on conditions under which coal formed

– Coastal locations with brackish water have high content of inorganic S à eastern coal in the USA

– Inland areas with fresh water conditions have low content of inorganic S à western coal in the USA

Photosynthesis

• Basic process in photosynthesis:

• 6H₂O + 6 CO₂ à C₆H₁₂O₆ + 6O₂

• Net effect: removal of CO₂ from and release of O₂ into the atmosphere

• Fixation of CO₂ and build-up of O₂ (à BIF!!)

• Burning or respiration:

• C₆H₁₂O₆ + 6O₂ à 6H₂O + 6 CO₂ + Q

• Net effect: Release of CO₂ into the atmosphere

• Part of carbon cycle

Formation of Oil and Gas

• Oil: complicated mixture of hydrocarbons

• Composition:

– 83à87 % C

– 11à14 % H

– Minor amounts of O, N, S

• à low on ash

Hydrocarbon compounds

• Aliphatic Hydrocarbons:

– Alkanes: C_n H_2n+2

– à methane CH₄

– Cycloalkanes (naphthenes): C_n H_2n

– à cyclobutane C₄H₈

• Aromatic Hydrocarbons: C_n H_2n-6 (n>6)

– à benzene C₆H₆

Refining process

• Crude oil is separated essentially based on evaporation temperature

– Evaporation T depends on molecular weight of molecules

– Simple molecules are components of natural gas: methane CH4 à butane C8H18

– Molecules with between 5 and 12 C atoms and low H/C ratios are liquid at room temperature: Benzene etc.

– The specific composition of gasoline depends on type of crude oil and refining process

– Heavy oils etc. have low H/C ratio à diesel to bitumen

Oil terminology:

• Sour – sweet refers to high or low content of S

• Composition of oil characteristic for origin

– North Sea oil low in S

– Middle East high in S

– Venezuelan dominantly naphthenic with high V and N

• Vitrinite: organic ‘minerals’; increasing abundance with increasing T à measure of maturation T

• Porphyrines: oil-soluble organometallic compounds à derived from coloring pigments in plants à organic origin; unstable at T> 200^oC

Formation process for Oil (and gas) (Fig. 1)

1. Fine grained muds rich in planktonic remains are deposited offshore in a low-energy, oxygen poor environment (e.g. Black Sea)
2. Slow decomposition by anaerobic bacteria turn planktonic remains into amorphous material à sapropel
3. Muds are buried and compressed by sediment accumulation à sapropel converted to petroleum compounds
4. Increased pressure and temperature brake down organic molecules
5. Specific Temperature range for formation of liquid hydrocarbons à Liquid Window
6. Migration: Sediments are compressed by pressure of burial; Water and hydrocarbons move to regions of lower pressure à lateral movement shoreward along gentle inclinations

• Held in host rock à Oil Shale
• Migration to surface à Tar Sands

7. Accumulation: Migration stops at ‘traps’ (Fig. 2) à formation of oil reservoir