Experimental Study On The Inhibition Of C-Cl Bond By H₂ During The Gasification Of MSW

In the thermochemical conversion process of MSW, the formation of C-Cl bond is the precondition of dioxin generation, and also determines the toxicity of dioxins. In view of the fact that C-Cl has the lowest bond energy in dioxin molecules, the author suggests to control the dioxin emission by inhibiting the formation of C-Cl bond and promoting its destruction. The C-Cl bond in the products can be in those chlorinated hydrocarbons that survive the reactions, and also can be formed through the chlorination reactions by active chlorines. If all Cl atoms in the C-Cl bonds and active chlorines can be directed transferred into HCl, C-Cl bond will not be formed.During gasification, the oxygen-lean condition ensures the abundance of H for Cl to form HCl. It was reported that C-Cl bonds and active chlorines are mainly released during pyrolysis. On the other hand, the gas above the pyrolysis zone contains all the gasification products, which means a high level of H₂. Thus, the author suggests the addition of a homogeneous conversion section above the pyrolysis zone for the sake of transferring all the Cl into HCl, taking advantage of the abundant H₂ in the syngas. Then the dioxin formation can be restrained.Firstly, in order to verify that H₂ has the ability to transfer the Cl atoms from active chlorine and C-Cl bond into HCl, chlorination and dechlorination experiments were carried out. It is important to ensure the representative of the hydrocarbon molecular used in the experiments. Thus, experiments on the TG-FTIR were carried out to study the pyrolysis products of plastics, one of the main sources of hydrocarbons and Cl. The vibration of-CH₂- showed in the pyrolysis products of all the plastics. Therefore, C2H4, the simplest form of olefin with-CH₂- group and the monomer of PE, was used in the chlorination and dechlorination experiments as a typical hydrocarbon.Experiments on the chlorination of the hydrocarbon（C2H4） by the active chlorine（Cl2） were carried out on the homogeneous flow reactor. The C-Cl products were observed by GCMS, and gases were measured by FTIR. With the increase in temperature, chlorination reaction was strengthened and the main C-Cl product gradually changed from C2H3Cl（400℃） to C2Cl4（700℃）. High temperature also promoted polymerization and C6Cl6 reached its maximum at 900℃. After H₂ was added into the reaction system, the chlorination and polymerization were inhibited. At 700℃, almost all the Cl atoms were in the form of HCl. C2H3 Cl was still observed in the experiments under 400-700℃ and declined with the rise of temperature. The participation of O2 contributed to the inhibition of C-Cl formation under low temperatures, and the concentration of HCl rose as a result.To study the crack of C-Cl bond that originally exists, C2H3 Cl was selected as the model. Organochlorides tend to polymerize under inert conditions, while the polymerization of C2H3 Cl did not happened with the addition of H₂. The increase of temperature strengthened the conversion of C2H3 Cl, especially in the temperature range of 500-700℃. At 700℃, almost all C2H3 Cl was converted to HCl, C2H₂ and C2H4. The improvement caused by the addition of O2 was a higher conversion ratio of C2H3 Cl at 500-700℃, and the generation of HCl was enhanced as well.The results gained from the experiments on the homogeneous flow reactor indicated that the transfer of Cl into HCl can be achieved with sufficient H₂ at temperatures higher than 700℃. Considering that pyrolysis zone is the main district C-Cl bond forms, the homogeneous conversion of pyrolysis products were studied on the Hotrod system, using PVC as the fuel. The transfer of Cl was very sensitive to the residence time. With a short residence time, the C-Cl formed in the process increased, even more than the pyrolysis product. With a residence time longer than 1.5 second, the formation of C-Cl can be avoided. The inhibition of C-Cl formation can also be enhanced by the rise of temperature and the increase in H₂ concentration. The results indicated that the directed transfer of Cl into HCl can be achieved by the homogeneous conversion of the pyrolysis product with the H₂ contained in it.In the end, an updraft fix-bed gasification rig was built to study the homogenous conversion of gasification products. C-Cl products, such as CH3 Cl, were generated during gasification, and dioxins were also found in the products. After the homogenous reaction section was introduced into the process, no C-Cl was found in the GCMS results, and a sharp fall of the amount of dioxin was observed, from 1.66 ng to 0.0893ng（TEQ）. The conversion ratio of fuel（Cl content 11.36%） to dioxin was 7.1pg/g. With the H₂ contained in the gasification syngas itself, a clear inhibition effect on the formation of dioxins was observed by the introduction of homogeneous conversion.