On-line detection and kinetic study of selenium release during combustion, gasification and pyrolysis of sawdust

An on-line analysis system was firstly developed to quantitatively measure the temporal concentrations of selenium in the flue gas directly. Then the selenium release during air combustion, CO₂/argon gasification, and argon pyrolysis of sawdust was systematically studied using the on-line analysis system, based on the inductively coupled plasma optical emission spectroscopy. The peak of selenium concentration in the flue gas ranges from 0.38 to 1.76 mg∙Nm^-3 with change of reaction temperature and atmosphere. The overall activation energy for selenium release is 75.3 kJ∙mol^-1 in air combustion, 102.4 kJ∙mol^-1 in CO₂/argon gasification, and 81.9 kJ∙mol^-1 in argon pyrolysis, respectively. The results show that the combustion atmosphere contributes to the selenium release more than that in gasification and pyrolysis. The promotion effect of chlorine on selenium release under combustion environment was one to three times higher than that under gasification and pyrolysis atmosphere. Thermodynamic equilibrium calculation showed that selenium oxides were the main gaseous selenium species in combustion, while the dominant gaseous selenium species were H₂Se (g) and Se (g) under gasification/pyrolysis condition. The selenium release was increased with different degrees by additive chlorine species, mainly because of the formation of SeCl₂ (g). The role of chlorine in selenium transformation has been provided in the proposed reaction pathways of selenium release, based on the new findings using on-line analysis system. The selenium species retained in sawdust can be transformed into selenium oxide (SeO₂, SeO, corresponding to the combustion condition) and selenium hydride (H₂Se, corresponding to the gasification/pyrolysis conditions).