Hydrogen Utilization: Combustion - Hydrogen Generation, Storage, and Utilization - page 143

combustion regime applied to methane/hydrogen mixtures in a laboratory-
scale pilot furnace with or without air preheating, results show that mild
flameless combustion regime is achieved from pure methane to pure hydro-
gen whatever the CH 4 /H 2 proportion [7]. The main reaction zone remains
lifted from the burner exit, in the mixing layer of fuel and air jets, ensuring
a large dilution correlated to low NO x emissions, whereas CO 2 concentrations
obviously decrease with hydrogen proportion. A decrease of NO x emissions
is measured for larger quantity of hydrogen due mainly to the decrease of
prompt NO formation. Without air preheating, a slight increase of the excess
air ratio is required to control CO emissions. For pure hydrogen fuel without
air preheating, mild flameless combustion regime leads to operating condi-
tions close to a “zero emission furnace,” with ultralow NO x emissions and
without any carbonated species emissions.
Another important factor in hydrogen combustion is pressure. For example,
a chemical kinetic model for high pressure combustion of H 2 /O 2 mixtures
has been developed recently by updating some of the rate constants important
under high pressure conditions without any diluents [8]. The revised mecha-
nism is validated against experimental shock-tube ignition delay times and
laminar flame speeds, with predictions of the model compared with those by
several other kinetic models. While predictions of the different models agree
well with each other and with the experimental data of ignition delay times
and flame speeds at pressures lower than 10 atm, substantial differences are
observed between experimental data of high pressure mass burning rates and
model predictions, as well as among the model predictions themselves. Dif-
ferent pressure dependencies of mass burning rates above 10 atm in different
kinetic models result from using different rate constants in these models for
HO 2 reactions, especially for H + HO 2 and OH + HO 2 reactions. The rate
constants for the reaction H + HO 2 involving different product pathways
were found to be very important for predicting high pressure combustion
properties.
Temperature is another important factor in hydrogen combustion. In par-
ticular, high temperature hydrogen combustion is of interest to nuclear
reactor safety [9]. In a combined experimental and simulation study, it has
been found that the mass flux or burning velocity increases exponentially
with increasing temperature in a wide temperature range, as shown in Figure
8.4 [10]. In this example, the experimentally measured temperature was
based on coherent anti-Stokes Raman scattering (CARS). The calculations
were carried out using different standard models. By comparing the com-
puted variation of flame temperature with mass flux in burner-stabilized flat
flames with those obtained experimentally, the predictive power of a chemi-
cal mechanism was tested at constant equivalence ratio over a range of more