Geology Reference
In-Depth Information
the identification and differentiation of gasoline and light solvents; however, these hydrocarbons are less stable in
the environment and less useful for the study of source signatures of coal and coal-derived by-products. Similarly,
nonvolatile hydrocarbons eluting after
-C 44 comprise a small and poorly characterized portion of the solvent
extractable fraction of coal and coal-derived by-products that are less useful for observing the thermal processes
associated with coal fires.
n
The purpose of this chapter is to describe and provide examples of the current analytical methods used for the
characterization of the semivolatile hydrocarbon source signatures of coal and coal-derived by-products. A
particular focus is given to the thermally derived by-products of coal formed from its thermal decomposition by
industrial carbonization (coking), which mirror the by-products formed during natural coal fires. The similarity
between these natural and industrial processes is relevant for environmental assessments because many of the
environmental impacts derived from coal-derived tars associated with anthropogenic sources (e.g., coke and
industrially-manufactured gas), such as elevated carcinogens like benzene and polycyclic aromatic hydrocarbons
(PAHs), may also exist around natural sources such as coal-fire sites.
Fossil Fuels
F ossil fuels form from prehistoric biomass that accumulated at the bottom of water bodies faster than ambient
microbes could remineralize and recycle it. Most coal formed from the residues of terrestrial plants and trees that
accumulated in marshes and swamps while most petroleum formed from algae residues that accumulated in marine
or lacustrine areas. The biomass mixed with varying amounts of inorganic materials (sediment) before burial over
geological time. The weight of the overburden and heat from the earth over long periods of time transformed the
terrestrial and marine biomass into fossil fuels. Although sometimes described differently by coal and petroleum
scientists, the geochemical transformations that convert biomass into fossil fuels occurred in three general stages,
often termed diagenesis, catagenesis, and metagenesis (Tissot and Welte, 1984).
Coal and petroleum theoretically exhibit formation-specific source signatures resulting from the accumulation of
characteristics associated with the prehistoric biomass, depositional environment, and formation conditions.
Prehistoric biomass generally consisted of the dominant photosynthetic and microbial communities at the time
of accumulation. Biological evolution dictated that the biomass sources changed significantly over time. Once in
the sediment, some biomass experienced different degrees of microbial alteration before burial. The suboxic or
anoxic nature of the depositional waterbodies significantly influenced the properties of the biomass. Differences
also occurred during fossil fuel formation; that is, biomass rarely experienced the same degree or duration of
exposure to temperature and pressure regimes. Consequently, the chemical composition and therefore source
signatures of fossil fuels varies considerably around the world.
Carbonization
C arbonization is the complex process of concentrating and purifying carbon by denaturing organic matter with heat
in the presence of little to no oxygen. In the context of coal, carbonization consists of four coincident and partly
competing steps. First, heat distills the mobile and freely moving volatile and semivolatile hydrocarbons into a
vapor phase. Second, surplus heat pyrolyzes the hydrocarbon vapors; that is, some to most of the carbon
-
carbon
bonds break or
forming increasingly lighter and smaller hydrocarbon gases. This process is often catalyzed
by the presence of metallic surfaces. Pyrolysis also cleaves thermally unstable molecules (e.g., hydrocarbons and
heteroatomic moieties) off of polymeric coal matrix, which can subsequently crack. Third, surplus heat helps
pyrosynthesize ever larger molecules from smaller, cracked reactive molecular fragments, thereby forming
increasingly condensed polymeric networks of aromatized carbon (coke). Fourth, the volatile and semivolatile
hydrocarbons in the vapor phase condense into stable gases, PAH-rich liquid coal tar, and particulate soot as the
vapor phase cools. Collectively, carbonization of coal produces four organic-dominated by-products: coke (ther-
mally stable coal minerals bound by graphitic carbon), coal tar (liquid pyrolytic condensate), soot (solid pyrolytic
condensate), and hydrocarbon gases (methane, ethane, acetylene, carbon monoxide, and others).
crack
By contrast, combustion is the thermal decomposition of organic matter in the presence of oxygen. The processes of
distillation, pyrolysis, pyrosynthesis, and condensation (described above) are largely the same during the combustion
of coal, except oxygen acts powerfully to transform most of the hydrocarbon vapors into nonhydrocarbon gases
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