## Reactions¶

A description of how reactions are defined in CTI input files

## Basic Reactions¶

Cantera supports a number of different types of reactions, including several types of homogeneous reactions, surface reactions, and electrochemical reactions. For each, there is a corresponding entry type. The simplest entry type is reaction(), which can be used for any homogeneous reaction that has a rate expression that obeys the law of mass action, with a rate coefficient that depends only on temperature.

## Common Attributes¶

All of the entry types that define reactions share some common features. These are described first, followed by descriptions of the individual reaction types in the following sections.

### The Reaction Equation¶

The reaction equation determines the reactant and product stoichiometry. A relatively simple parsing strategy is currently used, which assumes that all coefficient and species symbols on either side of the equation are delimited by spaces:

2 CH2 <=> CH + CH3  # OK
2 CH2<=>CH + CH3  # OK
2CH2 <=> CH + CH3  # error
CH2 + CH2 <=> CH + CH3  # OK
2 CH2 <=> CH+CH3  # error


The incorrect versions here would generate “undeclared species” errors and would halt processing of the input file. In the first case, the error would be that the species 2CH2 is undeclared, and in the second case it would be species CH+CH3.

Whether the reaction is reversible or not is determined by the form of the equality sign in the reaction equation. If either <=> or = is found, then the reaction is regarded as reversible, and the reverse rate will be computed from detailed balance. If, on the other hand, => is found, the reaction will be treated as irreversible.

The rate coefficient is specified with an embedded entry corresponding to the rate coefficient type. At present, the only implemented type is the modified Arrhenius function

\begin{equation*} k_f(T) = A T^b \exp(-E/\bar{R}T) \end{equation*}

which is defined with an Arrhenius() entry:

rate_coeff=Arrhenius(A=1.0e13, b=0, E=(7.3, 'kcal/mol'))
rate_coeff=Arrhenius(1.0e13, 0, (7.3, 'kcal/mol'))


As a shorthand, if the rate_coeff field is assigned a sequence of three numbers, these are assumed to be $$(A, b, E)$$ in the modified Arrhenius function:

rate_coeff=[1.0e13, 0, (7.3, 'kcal/mol')]  # equivalent to above


The units of the pre-exponential factor $$A$$ can be specified explicitly if desired. If not specified, they will be constructed using the quantity, length, and time units specified in the units() directive. Since the units of $$A$$ depend on the reaction order, the units of each reactant concentration (different for bulk species in solution, surface species, and pure condensed-phase species), and the units of the rate of progress (different for homogeneous and heterogeneous reactions), it is usually best not to specify units for $$A$$, in which case they will be computed taking all of these factors into account.

Note: if $$b \ne 0$$, then the term $$T^b$$ should have units of $$K^b$$, which would change the units of $$A$$. This is not done, however, so the units associated with $$A$$ are really the units for $$k_f$$ . One way to formally express this is to replace $$T^b$$ by the non-dimensional quantity $$[T/(1 K)]^b$$.

#### The ID String¶

An optional identifying string can be entered in the ID field, which can then be used in the reactions field of a phase() or interface entry to identify this reaction. If omitted, the reactions are assigned ID strings as they are read in, beginning with '0001', '0002', etc.

Note that the ID string is only used when selectively importing reactions. If all reactions in the local file or in an external one are imported into a phase or interface, then the reaction ID field is not used.

#### Options¶

Certain conditions are normally flagged as errors by Cantera. In some cases, they may not be errors, and the options field can be used to specify how they should be handled.

skip

The 'skip' option can be used to temporarily remove this reaction from the phase or interface that imports it, just as if the reaction entry were commented out. The advantage of using skip instead of commenting it out is that a warning message is printed each time a phase or interface definition tries to import it. This serves as a reminder that this reaction is not included, which can easily be forgotten when a reaction is “temporarily” commented out of an input file.

duplicate

Normally, when a reaction is imported into a phase, it is checked to see that it is not a duplicate of another reaction already present in the phase, and an error results if a duplicate is found. But in some cases, it may be appropriate to include duplicate reactions, for example if a reaction can proceed through two distinctly different pathways, each with its own rate expression. Another case where duplicate reactions can be used is if it is desired to implement a reaction rate coefficient of the form:

\begin{equation*} k_f(T) = \sum_{n=1}^{N} A_n T^{b_n} exp(-E_n/\hat{R}T) \end{equation*}

While Cantera does not provide such a form for reaction rates, it can be implemented by defining N duplicate reactions, and assigning one rate coefficient in the sum to each reaction. If the 'duplicate' option is specified, then the reaction not only may have a duplicate, it must. Any reaction that specifies that it is a duplicate, but cannot be paired with another reaction in the phase that qualifies as its duplicate generates an error.

negative_A

If some of the terms in the above sum have negative $$A_n$$, this scheme fails, since Cantera normally does not allow negative pre-exponential factors. But if there are duplicate reactions such that the total rate is positive, then negative $$A$$ parameters are acceptable, as long as the 'negative_A' option is specified.

negative_orders

Reaction orders are normally required to be non-negative, since negative orders are non-physical and undefined at zero concentration. Cantera allows negative orders for a global reaction only if the negative_orders override option is specified for the reaction.

#### Reaction Orders¶

Explicit reaction orders different from the stoichiometric coefficients are sometimes used for non-elementary reactions. For example, consider the global reaction:

\begin{equation*} \mathrm{C_8H_{18} + 12.5 O_2 \rightarrow 8 CO_2 + 9 H_2O} \end{equation*}

the forward rate constant might be given as 6:

\begin{equation*} k_f = 4.6 \times 10^{11} [\mathrm{C_8H_{18}}]^{0.25} [\mathrm{O_2}]^{1.5} \exp\left(\frac{30.0\,\mathrm{kcal/mol}}{RT}\right) \end{equation*}

This reaction could be defined as:

reaction("C8H18 + 12.5 O2 => 8 CO2 + 9 H2O", [4.6e11, 0.0, 30.0],
order="C8H18:0.25 O2:1.5")


Special care is required in this case since the units of the pre-exponential factor depend on the sum of the reaction orders, which may not be an integer.

Note that you can change reaction orders only for irreversible reactions.

Normally, reaction orders are required to be positive. However, in some cases negative reaction orders are found to be better fits for experimental data. In these cases, the default behavior may be overridden by adding negative_orders to the reaction options, e.g.:

reaction("C8H18 + 12.5 O2 => 8 CO2 + 9 H2O", [4.6e11, 0.0, 30.0],
order="C8H18:-0.25 O2:1.75", options=['negative_orders'])


Some global reactions could have reactions orders for non-reactant species. One should add nonreactant_orders to the reaction options to use this feature:

reaction("C8H18 + 12.5 O2 => 8 CO2 + 9 H2O", [4.6e11, 0.0, 30.0],
order="C8H18:-0.25 CO:0.15",
options=['negative_orders', 'nonreactant_orders'])


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P. H. Stewart, C. W. Larson, and D. Golden. Combustion and Flame, 75:25, 1989.

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R. J. Kee, F. M. Rupley, and J. A. Miller. Chemkin-II: A Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics. Technical Report SAND89-8009, Sandia National Laboratories, 1989.

6

C. K. Westbrook and F. L. Dryer. Simplified reaction mechanisms for the oxidation of hydrocarbon fuels in flames. Combustion Science and Technology 27, pp. 31—43. 1981.