INVESTIGATION OF FLAME STRUCTURE AND STABILITY FOR OXYGEN- ENHANCED COMBUSTION OF LOW CALORIFIC VALUE GASES By Mr. BISRAT YOSEPH (2011MEZ8167)

The gaseous by-product from natural or industrial processes which contain a small concentration of combustible gases is called a low-grade fuel or low calorific value gases (LCVG). Identifying the best strategy for efficient burning of such gases, where there is a bulk production, becomes inevitable to improve the efficiency of a system and reduction of pollutants emission. LCVGs have calorific value below 7 MJ/m and often contain impurities that need special care to enable stable and clean combustion. In the present work, the flame structure and stability of LCVG modeled as nitrogen-diluted methane (keeping CH4 concentration ≤ 20 % by vol.) fuel with co-flowing oxygen-enhanced oxidizer (O2 concentration ≥ 21 % by vol.) in laminar, non-premixed combustion have been investigated. Various flame properties were measured and predicted experimentally and numerically respectively. The influence of varying the oxygen enrichment level, fuel dilution level, jet velocity and co-flow velocity were studied extensively. The length of LCVG-OEC flame was very sensitive to the given change of jet velocity and oxygen enrichment level, but less sensitive to the given change of fuel dilution levels (reduction of methane concentration level below 20% by vol.) and high range of co-flow velocities. The flame length shows an increasing trend with reduction of co-flow velocity in the higher global equivalence ratio regime. The lift-off height was responsive to the given change of oxygen and methane concentration levels in the fuel and oxidizer streams respectively. However, it did not show appreciable change with jet and co-flow velocity variation for wider laminar flow regions as long as the CH4 and O2 concentration levels are not close to the blow-out limit. The flame showed liftoff tendency with increasing co-flow velocity when the O2 enrichment level close to the blow-out limit. Most of LCVG-OEC flames have exhibited less than 2 mm lift-off height. Numerically, the iso-contour of maximum has been identified as a marker of the flame base. With this approach, the predicted flame lift-off height showed a good agreement with experimental results. This method also enables distinguishing the lift-off height with that of dead space where wall proximity effect dominates. The LCVG-OEC flame width becomes narrow as the oxygen enhancement and fuel dilution levels are increased, since the stoichiometric mixture fraction, increases. But it remains constant for a wider laminar range of jet and co-flow velocities except a few changes at very low jet velocities. When the co-flow velocity was relatively higher for highly diluted fuel or low oxygen-enriched oxidizer, the flame width was less. The critical oxygen enhancement level showed an increasing trend when the fuel dilution level was increased (CH4 concentration level was decreased). But the critical fuel dilution level showed an increasing trend with rise of oxygen enrichment. An attempt has been made to investigate numerically the edge of LCVG-OEC flame structure. Mostly such flames showed tri-brachial and bi-brachial flame structures. To conclude, the present work contributes to the fundamental knowledge in the context of efficient burning of LCVG in oxygen-enhanced combustion mode in a non-premixed burner.