Hydrogen Utilization: Combustion - Hydrogen Generation, Storage, and Utilization - page 139

groups: (1) H 2 /O 2 chain reactions, (2) H 2 /O 2 dissociation/recombination reac-
tions, (3) formation and consumption of HO 2 , and (4) formation and con-
sumption of H 2 O 2 .
Thermodynamics data and rate coefficients for these reactions are given
in Mueller et al. [1] and updated in Li et al. [2] for reactions 1 to 3. Most of
the early profile measurements of the H 2 O 2 reaction have been obtained using
a variable pressure flow reactor (VPFR) over pressure ranges of 0.3-15.7
atm and temperature range of 850-1040 K, respectively [1]. These data span
the explosion limit behavior of the system and place significant emphasis on
HO 2 and H 2 O 2 kinetics. The reaction mechanism proposed was later updated
with newer experimental results and validated against a wide range of experi-
mental conditions, including those in shock tubes, flow reactors, and laminar
premixed flames.[2] Overall, excellent agreement was found between the
model predictions and experimental observations, demonstrating good pre-
dictive capabilities of the proposed reaction mechanism for different experi-
mental systems. Good agreement between different models has also been
found. For example, Figure 8.1 shows the temperature and pressure depen-
dence of the rate constant of the reaction H + O 2 (+M) → HO 2 (+M) (for
M = N 2 ) predicted by two different models [2, 3], which agree reasonably
well (within 20%) with each other over 300-3000 K and from low to high
pressure range.
The reaction rate constant for each of the reaction can be expressed in
Arrhenius form as follows:

E
RT

n a

k = AT exp − ,

(8.2)

where A is a prefactor, T is temperature ( K ), n is a number between −2.00
and 2.67 depending the specific reaction and is zero for many of the reac-
tions, especially reactions involving the formation and consumption of HO 2
and H 2 O 2 , E a is the activation energy, and R is the gas constant. The different
parameters and related thermodynamics data have been obtained from mod-
eling of experimental results.[1, 2] For example, the low-pressure-limit rate
constant for the chain termination reaction (H + O 2 + M = HO 2 + M) for
M=N 2 or Ar has been found to be (in unit of cm 6 mol −2 s −1 ):

264

N
0

20 − 1 72 .

k = 6 37 10 . × T exp −

(8.3)

2

T

248

Ar
0

19 − 1 50 .

k = 9 04 10 . × T exp − .

(8.4)

T