Hydrogen Utilization: Combustion - Hydrogen Generation, Storage, and Utilization - page 141

FIGURE 8.2 Branching ratio of rate constants of the chain branching reaction (H + O 2 = O + OH)
and the chain termination reaction (H + O 2 + M = HO 2 + M) as a function of temperature for three
different pressures based on two different models: solid lines [2] and dashed lines [1]. Source : Reproduced
with permission from Li et al. [2].
8.3 MAJOR FACTORS AFFECTING COMBUSTION
One of the most important factors affecting H 2 combustion is presence of
other gases such as O 2 and CH 4 that participate in combustion or other gases
such as N 2 or Ar that affect combustion. The presence of O 2 is critical for
combustion, and the ratio between H 2 and O 2 is thus a key parameter. Mixing
with other active gases such as CH 4 has major consequences on the outcome
and mechanism of combustion, including emission products. For example,
mixtures of hydrogen and methane as fuel allow a substantial reduction of
CO 2 emissions at affordable costs [4].
When hydrogen combustion is carried in air or with N 2 present, nitrogen
species, particularly NO x (x indicating various ratios between N and O), play
a critical role in the combustion process [5]. Several important reactions
involving NO x are summarized in Table 8.2.
The two main reaction paths forming NO x relevant in H 2 combustion are
thermal NO (reactions 3-5 in Table 8.2) and the nitrous oxide mechanism
(N 2 O) (reaction 1). The NNH mechanism (reaction 2) is always important at
low temperatures while only relevant at low residence times for rich mixtures
at high temperature. Figure 8.3 show a dependence of maximum [NO] on
maximum temperature [6]. It is clear that the overall maximum [NO] increases
with the maximum temperature in the range of about 1850-2150 K.