10
Water for Energy and Fuel Production
The chapter first examines the changes in the properties of water as temperature
and pressure increase. One of the most important transformations that occur is that
water becomes nonpolar at higher temperatures. For example, the properties of water
at 370°C are similar to those of acetone at 25°C. This transformation allows many
hydrocarbon reactions to occur in the aqueous phase.
It is well known that fossil fuels, such as coal, oil, and gas, are the results of slow
geological transformations of biomass waste and human and animal remains buried
underground. These transformations have taken millions of years, resulting in fossil
fuels such as coal and oil with higher carbon content and lower hydrogen and oxygen
content than the original feedstock of biological nature.
Even within coal and oil, there are gradations of properties. For example, the highest ranking (i.e., longest geological age) anthracite coal contains the highest amount
of carbon and the lowest amount of hydrogen and oxygen compared to younger coals
such as bituminous, subbituminous, and lignite. The youngest lignite coal contains
hydrogen/carbon ratio (H/C) and oxygen/carbon ratio (O/C) similar to that of biomass and peat. Just like biomass, lignite coal contains high oxygen concentration
and high moisture content. All this is well illustrated by the famous Van Krevelen’s
plot [2], which shows H/C versus O/C for various types of fossil fuels and biomass.
The plot shows that, in general, fossil fuels contain lower hydrogen and oxygen and
higher carbon contents compared to those found in the biomass.
The chapter illustrates that the aging and geological transformation of biomass can
be accelerated by the hydrothermal conversion processes. High thermal and pressure
forces exerted during hydrothermal conversion processes rapidly convert biomass
into more coal (hydrochar), oil (biocrude), or syngas similar to natural gas. Biomass
is easily transformed to these products in high-temperature and high-pressure water
because of strong thermochemical interactions between the biomass and the water.
Hydrothermal carbonization brings solid biomass properties closer to that of
coal [3]. The hydrochar produced during this process exhibits properties that are closer
to a subbituminous coal. Biomass produces cleaner hydrochar, which is an important
raw material in the production of numerous types of gaseous and liquid fuels.
At higher temperatures and pressures under subcritical conditions, biomass can
also be converted to oils. This process of hydrothermal liquefaction produces biocrude, which is similar to crude oil, and just like crude oil, it can also be upgraded.
Finally, hydrothermal gasification produces methane, hydrogen, or syngas. The
hydrothermal carbonization, liquefaction, and upgrading of biocrude as well as
hydrothermal gasification processes have been successfully examined in the recent
years, and some new commercial processes have been evolved based on these concepts. Chapter 5 briefly illustrates these topics.
The chapter also evaluates the coal–water chemistry in three different areas:
(1) the effect of pretreatment of coal by water on coal liquefaction, (2) the liquefaction of coal in high-temperature and high-pressure water, and (3) the use of coal–
water slurry in various combustion processes. The low-rank lignite coal is amenable
to liquefaction in high-temperature water; however, the liquid product that is generated is of poor quality and requires significant upgrading. Coal–water slurry can
be used as a fuel for combustion in boilers, gas turbines, and diesel engines. While
the affinity of coal for water is not as pronounced as that of biomass, the chapter