| dc.description.abstract | In recent years, syngas has gained research interest as the focus shifts from fossil to renewable fuels. Syngas generated from biomass such as wood, agricultural residue, and paper is a promising fuel to achieve net-zero carbon emissions. It is mainly composed of CO, H2 and diluents such as CO2 and N2. The combustion characteristics of biomass-derived syngas significantly differ from the well-studied fuels in terms of calorific value, stoichiometric fuel-to-air ratio, ignition delay, and flammability limits. In the present study, a combination of catalytic and swirl combustion is explored using a two-stage combustor to achieve low emissions without losing combustion stability. The first stage is the catalytic stage, and the second stage is the swirl stage. A preheated and premixed syngas-air mixture (Фfirst = 4) is supplied to the first stage, where syngas is consumed partially. The remaining unburnt fuel is combusted using an oxidizer in the second stage. In this stage, the flame is stabilized by two co-centric swirling streams where the inner stream is the unburnt gases coming from the first stage, and the outer stream is the oxidizer. By changing the swirling direction of the streams, co and counter-swirl flames are obtained.
In this thesis, the experimental results are described in six parts organized as follows. The first part of the study investigates the catalytic combustion of syngas in the first stage at an equivalence ratio of 4. The transient temperature measurements show three distinct regions of catalytic combustion: kinetically controlled, light-off, and steady-state regions. It is found that the start-up time depends on the conductivity of the monolith placed inside the first stage. The NOx emission is found to be low (< 1 ppm) at the exit.
The second part of the study explores the lean unconfined co/counter-swirl flame stabilized in the second stage. Both flames can be stabilized at a higher flow rate, maintaining a constant overall equivalence ratio. Co-swirl creates a ‘U’-shaped flame, and counter-swirl creates a more distributed and compact flame. A low NOx concentration (< 20 ppm) is found in the flame; however, the CO emission is more than 10000 ppm.
In the third part, the emission characteristics are studied, with the second stage being confined. The emission, temperature, flame shape and noise are investigated at three overall equivalence ratios (Фoverall). The CO emission reduces drastically in the presence of confinement, and the emission is found to be ~10 when Фoverall = 0.55. The NOx emission is below 2 ppm for all cases. The shape of co and counter-swirl flames behave differently when Фoverall changes. Planar laser-induced fluorescence (PLIF) shows the radicals are well distributed over the combustor for co/counter-swirl flames at a higher equivalence ratio, unlike existing studies. Particle image velocimetry (PIV) indicates that the flame shape is related to the shape of the inner recirculation zone. The sound pressure level (SPL) for co/counter-swirl flames is ~110 dB. The frequency domain contains a dominating peak at all Фoverall. The high-speed OH* chemiluminescence (5 kHz) confirms that the combustion is exciting the axial mode of the combustor, which is captured in the noise data.
The sound pressure level can be reduced by changing the O2 concentration in the oxidiser stream. Thus, in the fourth part of the thesis, we investigate the effect of O2 concentration in the oxidizer stream on swirling flames, which is discussed in the fourth part. The OH* chemiluminescence intensity reduces, and flame height increases when O2 concentration decreases. The sound pressure level (SPL) also shows a decreasing trend with O2. The global luminosity calculated from high-speed chemiluminescence indicates that the peaks corresponding to axial mode and PVC are suppressed at low O2 concentrations. In terms of emissions, CO emission increases with a reduction in O2. The feasibility of achieving distributed combustion is assessed from emission, chemiluminescence intensity, SPL, and combustion stability. It is found that the co-swirl configuration is suitable for studying distributed combustion.
In the fifth part, we investigated the combustion dynamics for co and counter-swirl flames. The noise emitted by the combustor shows signatures of intermittency. Pressure signals can be divided into periodic and aperiodic zones, and aperiodic epochs randomly appear between periodic epochs. The high-speed OH* chemiluminescence unravels two motions of the combustion. One oscillates in the axial direction, and the other represents the precessing vortex core (PVC) motion. Axial fluctuation of heat release decreases with an increase in momentum ratio; however, the PVC motion remains prevalent. Interestingly, both fluctuations are suppressed when the oxygen percentage is reduced to 13.13%, indicating that low-oxygen dilution can be an effective strategy to reduce combustion instability.
Finally, in the last part of the study, the effect of momentum ratio (M), density ratio and heat-release rate on the flow field is studied in detail. For counter-swirl configuration, the flow field shows a drastic change at M = 1.7 as the vortex breakdown mode changes from bubble to conical form. The vortex breakdown mode influences the shape of the flame. In the bubble mode, the flame is distributed, whereas ‘V’-shaped for the conical mode.
Thus, the current study has led to an improved understanding of fuel-rich catalytic combustion and shape, dynamics, and flow field of co/counter-swirl flames under various conditions. The present study also establishes that the two-stage rich-catalytic lean-swirl stabilized combustor is suitable for stationary power production applications with ultra-low NOx and low CO emissions. | en_US |
