Stability limits and combustion measurements in low calorific value gas flames
A Hilton combustor was substantially modified to a suitable symmetrical configuration for research purposes. Provisions for swirl, preheat and injection of LCV gases were incorporated with appropriate burner management systems for safe operation. Instrumentation for temperature, velocity and concentration measurements was developed and fully automated by interfacing to a microprocessor for rapid data acquisition. Flame stability limits were determined over a wide range of operating conditions by varying swirl, secondary air temperature and excess air levels while maintaining the burner momentum constant. Addition of swirl up to a limit of O~ 69 generally improved stability. Preheating secondary air alone was beneficial only if the temperature was raised to at least 250 oC. A combination of intermediate swirl and moderate preheat of the secondary air resulted in satisfactory flame stability over a wide range of calorific values of the fuel. Thus, existing concentric pipe burner systems may be easily modified at low cost to burn LCV gases of variable compositions. With LCV gas flames the excess air factor (EAF) had a major influence on values of temperature, species concentration and velocity. Unburnt hydrocarbon (UHC) and CO not surprisingly increased in concentration close to the blow-off limits under the majority of operating conditions. This indicated incomplete combustion probably resulting from the lowered flame temperatures and partial flame lift-off. On the other hand, burnout efficiencies at the exhaust were reasonably high for most operating conditions involving LCV gases. The combustion data were analysed to extract the characteristic mixing and chemical reaction times the ratio of which gave the parameter epsilon, originally proposed for unconfined flames. Close to the blow-off limit epsilon took the value 4.9 compared with 6.2 for fully stable flames. This finding showed that the criterion was also valid for confined flames, supporting the extinction mechanism proposed by Peters and Williams, and providing an important basis for predicting stability limits and burner design parameters.