(490g) Combustion Characteristics of Alternative Fuels: Butanol Isomers

Introduction & Motivation

            Nearly 85% of the United States' energy needs,
including essentially all of our transportation fuel requirements, are met with
the burning of fossil fuels.  Furthermore, due to our high consumption, much of
our fossil fuels are imported.  In an effort to decrease our dependency on
foreign oil, innumerable resources have been dedicated to finding an
alternative, carbon-neutral, renewable energy source.

            Deciding which alternative fuel to pursue is an
arduous task.  Besides addressing the political questions of ?food vs. fuel,?
questions such as ?does this fuel exhibit the desired combustion properties?
and ?are the fuel's emissions within the government's standards? must be
answered.  Normally, the solutions to these questions are obtained by running
numerous experiments, which come at the expense of the fuel.  Even once a
candidate alternative fuel is identified, there is usually little insight gained
into why this particular fuel exhibits the desired combustion characteristics
while countless others do not.  Moreover, it is impossible to predict what
engines will be used in the future or how emission standards will change; the
results of either could render the alternative fuel source of today as the infeasible
fuel source of tomorrow.

            Another means of accessing an alternative fuels'
feasibility is by constructing a kinetic model for the fuel.  If the fuel
chemistry can be validated against experiments, i.e. the model can be
shown to capture the important combustion characteristics, e.g., the
ignition delay and laminar flame speeds, one could then extend the chemistry of
the base fuel to different molecules.  The chemistry model for the new species
could then dictate whether experimental efforts should be devoted to this
potential alternative fuel.  For example, if one had a validated chemistry
model for the butanol isomers ? the smallest alcohol system to contain a
primary-, secondary-, and tertiary alcohol ? it would be relatively
straightforward to extend the kinetic mechanism to larger alcohols that would otherwise
be computationally and experimentally expensive to study.

Computational Methodology

            One means of constructing these chemical
mechanisms, in an automated fashion, is the open-source software package,
Reaction Mechanism Generator (RMG).[1] 
The software requires the user to enter the following information: the
temperature and pressure of interest; the initial species and their
concentrations; a termination criterion, either the desired conversion or
reaction time; and an error tolerance, which controls how detailed (large) of a
chemistry model is generated.  Using group additivity contributions to estimate
a species' thermochemistry,[2]
and 42 unique reaction families to estimate kinetics, the RMG software can readily
construct a detailed mechanism for the combustion of any oxygenated hydrocarbon. 
Additionally, the RMG software can compute pressure-dependent reaction rate
coefficients using the steady-state master equation method[3]
or the modified strong collision approximation.[4]

Results & Discussion

            Herein we present a reaction mechanism for the
combustion and pyrolysis of normal-, secondary-, and tertiary-butanol; the
mechanism was constructed using the RMG software.  This mechanism has been
validated against data previously published in the literature, including: a doped
methane diffusion flame,[5]
ignition delay measurements at 1 bar obtained in shock tube experiments,[6]
and laminar flame speed calculations.[7]^,[8] 
Flux and sensitivity analysis revealed important species and pathways for which
more accurate thermochemistry parameters were computed using statistical
mechanics and quantum chemistry.  The dominant decomposition pathways for each
isomer, in each reactor model, will be discussed.  The effect of the location
of the hydroxyl group, i.e. whether the alcohol is primary, secondary,
or tertiary, will also be discussed.