APES 6.5-6.6: Fossil Fuels and Nuclear Power

Coal

Coal is a combustible sedimentary rock formed from plant material and composed mostly of carbon, with variable amounts of hydrogen, oxygen, sulfur, and nitrogen. It is a fossil fuel because its carbon originated from ancient organisms.

How does dead vegetation become coal?

Dead plant matter accumulates in oxygen-poor wetlands and is converted to peat. Deep burial then subjects the peat to heat and pressure for millions of years, causing coalification.

Coalification

Coalification is the geologic conversion of plant remains, especially peat, into progressively higher-rank coal through burial, heating, and compression.

Why must plant material be protected from oxidation during the early stages of coal formation?

Protection by mud or acidic water limits decomposition and oxidation, allowing carbon-rich organic matter to accumulate as peat instead of being converted primarily to carbon dioxide and water.

What chemical changes generally occur as coalification proceeds?

Coalification removes water, carbon dioxide, methane, and other volatile substances, increasing the relative proportion of carbon. The organic structure becomes more aromatic and contains fewer hydrogen and oxygen atoms.

Dehydration during coalification

Dehydration removes water from the maturing organic material; a simplified reaction is $2\,\mathrm{R{-}OH} \rightarrow \mathrm{R{-}O{-}R} + \mathrm{H_2O}$.

Decarboxylation during coalification

Decarboxylation removes carboxyl groups as carbon dioxide. A simplified reaction is $\mathrm{RCOOH} \rightarrow \mathrm{RH} + \mathrm{CO_2}$.

Demethanation during coalification

Demethanation removes methyl-derived groups as methane, for example $2\,\mathrm{R{-}CH_3} \rightarrow \mathrm{R{-}CH_2{-}R} + \mathrm{CH_4}$. It becomes important after the coal has reached approximately bituminous rank.

How do temperature, pressure, and burial time affect coal rank?

Increasing temperature and pressure generally increase coal rank, but temperature has a stronger effect than pressure or burial time. Higher rank corresponds to greater carbon content and generally greater fuel value.

Coal rank sequence

The usual progression is peat $\rightarrow$ lignite $\rightarrow$ sub-bituminous coal $\rightarrow$ bituminous coal $\rightarrow$ anthracite. Graphite is a difficult-to-ignite, highly carbon-rich material associated with the highest degree of carbonization.

Lignite

Lignite, or brown coal, is the lowest coal rank and forms under relatively mild coalification conditions. It is used mainly as fuel for electricity generation and generally produces especially harmful emissions when burned.

Sub-bituminous coal

Sub-bituminous coal has properties intermediate between lignite and bituminous coal and is used primarily as fuel in steam-electric power generation.

Bituminous coal

Bituminous coal is a dense, commonly banded coal used mainly for electricity generation and coke production. It is also called thermal or steam coal when used as a power-station fuel.

Anthracite

Anthracite is the highest common coal rank: a hard, glossy, carbon-rich coal formed under the greatest temperatures and pressures. It is used primarily for residential and commercial heating.

How can coal rank vary independently of depth?

Although deeper coal is usually higher rank within a small area, contact with an igneous intrusion can locally metamorphose coal into a higher rank, such as anthracite, without increasing its burial depth.

Thermal coal versus metallurgical coal

Thermal coal is burned to generate heat and electricity. Metallurgical, or coking, coal is heated without oxygen to produce coke, which is used in conventional iron and steel production.

Macerals

Macerals are coalified plant components that retain features of their original biological material. Examples include vitrinite from woody tissue, lipinite from spores and algae, and inertite from previously burned woody matter.

How does the elemental composition of bituminous coal illustrate carbonization?

On a dry, ash-free basis, bituminous coal is approximately $84.4\%$ carbon, $5.4\%$ hydrogen, $6.7\%$ oxygen, $1.7\%$ nitrogen, and $1.8\%$ sulfur by mass. Its high carbon and low oxygen content reflect loss of oxygen-containing groups during coalification.

Why is coal not considered a pure substance?

Coal is a heterogeneous mixture of many organic compounds and polymers, along with inorganic minerals that form ash. Its elemental composition and physical properties vary among deposits and coal ranks.

Ash in coal

Ash is the noncombustible inorganic residue left after coal burns. It commonly contains compounds represented as oxides such as $\mathrm{SiO_2}$, $\mathrm{Al_2O_3}$, $\mathrm{Fe_2O_3}$, $\mathrm{CaO}$, and $\mathrm{MgO}$.

How can inorganic sulfur in coal be removed before combustion?

Some inorganic sulfur occurs as iron pyrite, $\mathrm{FeS_2}$, a relatively dense mineral that can be separated mechanically, such as by washing or froth flotation. Organically bound sulfur is much more difficult to remove economically before combustion.

What pollutants can sulfur and nitrogen in coal produce during combustion?

Sulfur compounds can be oxidized to sulfur dioxide, $\mathrm{SO_2}$, while nitrogen-containing compounds can produce nitrogen oxides, collectively $\mathrm{NO_x}$. These gases contribute to air pollution and, in relevant atmospheric processes, acid deposition and smog.

Why can trace elements in coal become environmental contaminants?

Trace elements such as mercury, arsenic, and selenium are relatively immobile in unburned mineral matter but can become volatile or water-soluble during combustion. This increases their potential to enter air, soil, or water.

Coal combustion and climate change

Combustion oxidizes carbon in coal primarily to carbon dioxide: $\mathrm{C + O_2 \rightarrow CO_2}$. Coal combustion is a major source of anthropogenic carbon dioxide and therefore contributes substantially to the enhanced greenhouse effect.

How does incomplete combustion of coal differ from complete combustion?

Complete combustion produces carbon dioxide when sufficient oxygen is available. Incomplete combustion can produce carbon monoxide, $\mathrm{CO}$, and unburned carbon, releasing less energy per mole of carbon and creating additional health hazards.

Coal energy density

The approximate energy density of coal is $24\,\mathrm{MJ\,kg^{-1}}$, or about $6.7\,\mathrm{kWh\,kg^{-1}}$. The energy released by burning a sample can be estimated with $q=m\times$ energy density.

How is electricity generated in a conventional coal-fired power plant?

Pulverized coal is burned to heat water in a boiler. The resulting steam spins a turbine connected to a generator, converting chemical energy into thermal, mechanical, and finally electrical energy.

How can the efficiency of a coal power plant be calculated?

Efficiency is $\eta=\dfrac{\text{useful energy output}}{\text{energy input}}\times100\%$. Because much heat is lost to the surroundings and exhaust, conventional coal plants convert only a fraction of the coal's chemical energy into electricity.

Integrated gasification combined cycle (IGCC)

IGCC first converts coal into synthesis gas, or syngas, rather than burning the solid coal directly. The syngas drives a gas turbine, and hot exhaust produces steam for a second turbine, allowing combined-cycle operation.

Coke

Coke is a porous, carbon-rich solid produced by heating metallurgical coal to about $1000\,^{\circ}\mathrm{C}$ without oxygen. It serves as both a fuel and a reducing agent in blast-furnace iron production.

What redox reaction does carbon monoxide perform in a blast furnace?

Carbon monoxide reduces hematite to iron: $\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$. Iron is reduced from oxidation state $+3$ to elemental iron, while carbon in CO is oxidized from $+2$ to $+4$.

Why must metallurgical coke be mechanically strong?

Coke must support the overlying material in a blast furnace while remaining sufficiently porous for gases to flow. Weak coke would collapse and interfere with furnace operation and the reduction of iron ore.

Coal gasification

Coal gasification converts coal into syngas, a mixture containing mainly carbon monoxide and hydrogen. Syngas can be burned for energy or used as a feedstock for producing chemicals.

Why is syngas chemically useful?

Carbon monoxide and hydrogen are versatile building blocks. They can be converted into methanol, hydrogen-based products, ammonia, urea, acetic acid, formaldehyde, and other chemicals.

Coal liquefaction

Coal liquefaction converts coal into liquid fuels chemically similar to gasoline or diesel. It can occur through processes such as hydrogenation or carbonization.

Why does coal-to-liquid fuel production generally have a substantial carbon footprint?

The process requires converting solid carbon-rich material into liquid hydrocarbons and typically releases more carbon dioxide than producing liquid fuel from crude oil. Adding biomass and carbon capture can potentially reduce, but not eliminate, these emissions.

How do coal-fired power plants affect human health and the environment?

Coal mining and combustion can cause illness and premature death through particulate matter, toxic metals, sulfur oxides, nitrogen oxides, and other pollutants. Mining also disturbs land, while combustion releases greenhouse gases and produces coal ash.

Why did coal use expand during the Industrial Revolution?

Coal provided a concentrated, transportable energy source for steam engines and industrial heating. It allowed industry to expand beyond locations with suitable flowing water for water wheels.