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248 8 Pre-combustion Air Emission Control

the solids can be used as alternative fuel for combustion too. The rest of the water
goes to a water treatment facility.

8.4.3.4 Bioethanol Combustion
Bioethanol is mainly mixed with petroleum gasoline for combustion in light
engines. The overall stoichiometric combustion formula can be described as fol-
lows, although the actual chemical reactions are much more complicated.

C 2 H 5 OH þ 3O 2 ! 2CO 2 þ 3H 2 O ð8:21Þ

On a volumetric base, bioethanol (or ethanol) has only 2/3 of the heating value
of petroleum gasoline [6]. There is a great power reduction if pure ethanol (E100) is
used to power an automobile. Therefore, bioethanol is mixed with gasoline for sale.
Water contamination remains a challenge to the bioethanol industry. The chal-
lenge to bioethanol-gasoline mixed fuels is the phase separation induced by water
contamination. Ethanol absorbs water readily in a storage tank or transport pipeline.
This resultant water in the fuel negative affects the engine operation.

8.4.4 Hydrothermal Conversion of Biomass to Biofuels

8.4.4.1 Properties of Hot Compressed Water

Before we introduce hydrothermal conversion of biomass to biofuel, we have to
start with the phase behavior of hot compressed water. Water exhibits unique
properties at a critical point of 374 °C, 22 MPa. Above this critical point, water is in
a homogeneous phase known as a supercritical fluid. Supercritical water exists
simultaneously as both a liquid and a gas. Subcritical water is marked by a higher
density and the presence of two phases, one being liquid and the other being vapor
[29].
The first special property of hot compressed water is its ionic constant. The ionic

þ
products are H and OH ions:
þ
H 2 O ! H þ OH ð8:22Þ
The ionic constant of subcritical water is considerably higher than those of
supercritical water and regular water. As such, subcritical water is a special reaction
medium due to its dual acid-base catalysis nature, which enables rapid conversion
of biomass compounds to biofuels.
Another special property of hot compressed water is its density. Under low
density conditions, diffusion of radicals through the solution is enhanced; under
high-density conditions, ionic reaction mechanisms are promoted.