Coupling scales for modelling heavy metal vaporization from municipal solid waste incineration in a fluid bed by CFD

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).

Temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust

The temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust was studied using an on-line analysis system. The arsenic release amount in combustion atmosphere was higher than that in CO₂ gasification and argon pyrolysis. The derived values of activation energy followed the order: combustion < gasification < pyrolysis. Furthermore, the enhancement effect of chlorine species on arsenic release percentage in air combustion was also higher than that in gasification and pyrolysis conditions. The total proportion of arsenic release in combustion with additive chlorine is bigger than the case in gasification and pyrolysis, especially when 20% chlorine is added. According to equilibrium analysis, arsenic oxides were identified as the main gaseous arsenic species and their formation were decreased in the oxygen-deficient environment, mainly accounting for lesser arsenic release proportion in gasification and pyrolysis than combustion. The release of arsenic was promoted to a different extent with additive chlorine, mainly caused by the AsCl₃ (g) formation. By the findings of the experiments and theoretical analyses, the possible reaction pathways and release mechanisms of arsenic species were proposed.

Thermodynamic equilibrium predictions of zinc volatilization, migration, and transformation during sludge co-incineration

The effects of interactions between and among chlorine (Cl), sulfur (S), phosphorus (P), and minerals on migration, transformation, and volatilization of zinc (Zn) were numerically simulated in sludge co-incineration using the chemical thermodynamic equilibrium method. Our results showed that all the minerals of Fe₂ O₃ , Al₂ O₃ , Fe₂ O₃ , and TiO₂ except for CaO in the sludge co-incineration system reacted with Zn which inhibited the Zn volatilization. The presence of S and P was beneficial to the formation of ZnSO₄ (s) and Zn₃ (PO₄ )₂ (s). Cl weakened the chemical reactions between the minerals and Zn, thus increasing the Zn volatilization. Changes in Zn transformation and migration induced by the coupling of Cl + S were mainly controlled by Cl, S, and the minerals, while those induced by Cl + P and S + P were mainly controlled by P and S + P. The presence of P + Cl, S + Cl, S + P, S + Cl + P, Cl, and Al₂ O₃ in the coexisting mineral system controlled the reactions between the minerals and Zn.