The combustion behaviour of forced-draught industrial burners when fired within the EASEE-gas range of Wobbe Index

Abstract In Europe, a common business practice for the cross-border exchange of natural gases has been formulated by EASEE-gas. The proposed range of Wobbe Index for this purpose is 48.96–56.92 MJ/Nm 3 (25 °C/0 °C, 1013.25 hPa). However, it is unclear whether the entire range of end-use equipment can accept the suggested range and still function as desired. Research is currently underway tot assess whether this range of gases can be accepted by domestic appliances. To our knowledge, there has been little systematic investigation of the acceptable range of gas quality for large-scale, forced-draught industrial burners. Although natural gas is predominantly methane with lesser amounts of higher hydrocarbons and inerts, the variation in gas composition can have significant effects on burner performance. The vast majority of forced-draught industrial burners features a flow of air independent of the gas composition, therefore the air factor is (inversely) proportional to the Wobbe Index of the gas. In this paper we present the results of experiments on a number of generic large-scale industrial burners for boilers and industrial processes in which the Wobbe Index is varied across the EASEE-gas range. The combustion effects focused mainly on flame stability (burner overheating/blow-off and vibrations) and the pollutant emissions, such as CO, NO x and unburned hydrocarbons over the desired range of turndown ratio. The experimental results showed that, for the burners studied the measured oxygen concentration was the predominant factor governing burner behaviour; between 0.5 and 7% O 2, all burners showed a stable flame while emitting less than 400 ppmv (dry, air free) CO. Outside this O 2 range, the low-NO x boiler burners, being the most sensitive, showed decreasing flame stability and increasing CO production. One burner even exhibited blow-off, causing the safety system to shut down burner operation, an undesirable event in practice. In the burners studied, the NO x emission can increase significantly with increasing Wobbe Index. Even a relatively small increase can cause the burner to exceed the locally enforced NO x limit. The visible flame length increased with increasing Wobbe Index, due to the concomitant decrease in air factor and increase in thermal input. This can lead to a higher thermal load on furnaces, which may be unacceptable in some cases. The experimental results indicate that to widen the acceptable range some form of active control is necessary for the burners for boilers. The merits of using a feed-back control system, using oxygen measurement in the flue gas, compared to a feed-forward control using the Wobbe Index of the fuel gas are discussed. In particular, aspects regarding safety in the event of sensor malfunction are seen to be important when choosing a system. In this case, the Wobbe method seems technically preferable.