| dc.description.abstract | Carbon capture and storage (CCS) technologies allow for the reduction of carbon
dioxide (CO2) emissions from the combustion sources of fossil fuels. One such
technology is based on the use of hydrogen or hydrogen-enriched fuels and allows for
a significant reduction of CO2 emissions. All regulated pollutant emissions, except for
harmful nitrogen oxides (NOx), can also be reduced using hydrogen. Hydrogen’s unique
properties are challenging for burner designers. High hydrogen-air flame temperatures
and flame speeds result in increased NOx emissions and significant changes in flame
shape compared with conventional fuels. A novel burner concept employing partially
premixed combustion and flame stabilization on a conical bluff body is here proposed to
address some of these challenges.
In this study a laboratory-scale burner which can operate with methane, hydrogenenriched
methane and pure hydrogen has been studied. The burner performance and
pollutant emissions were found to depend on factors such as fuel composition, thermal
load, excess air and the burner design parameters. Particular burner operation settings
affected near-burner aerodynamics and consequently NOx emissions.
In more detail, it was found that the flow past the bluff body burner head exhibited
vortex shedding contributing to enhanced fuel-air mixing and formed a recirculation
zone behind the burner head. Flame presence increased turbulent kinetic energy
production near the shear layer of the recirculation zone. The burner operated within the
highly turbulent flow regime and significant turbulent kinetic energy was contained at
the small scales of the flow.
The burner operation was controlled by two parameters: position of the burner head and
the fuel split between primary and secondary fuel ports. When the burner head was
shifted upstream, NOx emissions were reduced for all fuels tested and the position of the
burner head was an important parameter for the reduction of NOx emissions. This was
attributed to shortened residence time in the high-temperature zone, enhanced fuel-air
mixing and an increase in the amount of furnace flue gas entrained into the combustion
zone. The burner head shifted upstream also resulted in lower flame stability and
increased carbon monoxide emissions. Influence of the secondary fuel stream on the flow field and NOx emissions was relatively complex and affected by the remaining
burner operation settings. Generally, NOx emissions increased when more fuel was
provided to the burner through the secondary fuel ports. Some exceptions were
observed and the secondary fuel fraction helped in reduction of NOx emissions from
hydrogen combustion at low burner thermal loads. Depending on the mixing and the
velocities in the burner, the secondary fuel may have penetrated the recirculation zone
and the hot combustion products in this zone, what could have led to flame extinction.
NOx emissions were largely dependent on the fuel composition. More hydrogen in the
methane-hydrogen mixture resulted in higher NOx emissions. This is attributed to higher
peak flame temperatures enhancing NOx formation via the thermal mechanism. Another
factor was the furnace temperature. At higher furnace temperatures the radiative heat
losses from the flame were reduced and consequently NOx emissions increased. Excess
air could be used to suppress increase of NOx emissions, but it was only effective for
methane and methane-hydrogen mixtures containing low fractions of hydrogen. It is
believed that this effect was associated with the flame shape, which changed
significantly between methane and hydrogen at given burner operation settings. Shorter
and wider flames were characteristic for hydrogen combustion.
The burner was capable of firing methane, hydrogen and mixtures of both of these fuels,
but its operational settings had to be carefully chosen for each of these fuels in order to
ensure safe burner operation and low NOx emissions. Empirical models developed on
the basis of statistically cognizant experimental design allowed for NOx emissions
prediction in a wide range of operating conditions. At the most promising burner
operation settings and furnace temperatures of 1050-1150 °C, NOx emissions equivalent
to 26 and 66 ppmvd at 3% O2 were achieved for methane and hydrogen, respectively.
Wide fuel flexibility capacity and the low NOx emissions achieved by the burner make
it worth considering as a technical solution in deployment of CCS technologies or
retroffiting of existing industrial combustion systems. | nb_NO |
