Experimental investigation of flame structure of CO2-diluted syngas and biogas mixtures by laser diagnostics

Abstract

The primary objective of this research project is to experimentally investigate the laminar flame structure of syngas and biogas mixtures through Raman laser spectroscopy. The gaseous fuel mixtures have been predetermined by an industrial partner and are composed of varying concentrations of H2, CO and CH4 with CO2 dilution. The laminar flame structure was characterized through measurements of flame temperature as well as major species concentration (H2, CO, H2O, CO2, O2, N2 and CH4 where applicable) at standard temperature and pressure conditions. The target operating conditions were set at an equivalence ratio of 3 and a Reynolds number of 1400. In total, four different groups of fuel mixtures are represented in this study: 1) one biogas fuel with 40% CO2 dilution; 2) four syngas fuels with CO2 dilution; 3) three syngas fuels with 5% CH4 and 20% CO2 dilution; 4) two syngasmethane mixtures with CO2 dilution. The analysis of the experimental results is divided into four sections, each one corresponding to a fuel group. Concerning the biogas with 40% CO2 dilution, it was seen that CH4 depletion occured at a radial distance which corresponds to the the maximum concentration of H2O and the minimum concentration of O2. The maximum temperature was located at the flame’s reaction boundary whereas much of the central axis of the flame was occupied by unburned reactants. The syngas mixtures with 25% CO2 dilution demonstrated that a decrease in H2/CO ratio causes a decrease in flame temperature due to an increase in radiative heat loss stemming from the additional CO2 production. An increase in flame cone length, or a decrease in laminar burning velocity, was noted in conjunction with decreasing H2/CO ratio. Conversely, increasing H2/CO ratios coincide with higher levels of H2O production and shorter flame cones. CO2 addition causes a decrease in flame size as well as a decrease in flame temperature. Regarding syngas mixtures with 5% CH4 and 20% CO2 dilution, the experimental results suggest that the CH4 reacts and/or dissociates early, within the first 10% of the flame’s visible height. It was shown that the height of flame cones decreased in conjunction with increasing H2/CO ratio, suggesting an increase in laminar burning velocity. The flames of methane-syngas mixtures with CO2 dilution provided evidence of CH4 dissociation early in the flame’s development since measured H2 and CO concentrations increased slightly. It was noted that the maximum concentration of H2O occurred at the same radial location as the maximum temperature. In general, reaction boundaries of laminar, partially premixed flames of all syngas and biogas mixtures could be identified by a decrease in the concentrations of the unburned reactants and an increase in the concentrations of H2O and CO2. The reaction boundary is also characterized by an increase in O2 and N2 concentrations. The flame temperature reduces to room temperature beyond this reaction boundary.

Date	9 Jul 2013
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Patrice Seers (Supervisor)