Experimental Study On Carbon Conversion In The De Novo Synthesis Of Dioxin-like Compunds

In recent years, haze has become a big problem plagueing China’s environment, black carbon, an improtant component in haze particles, will cause some serious environmental problems, including harming peolple’s health and impacting climate change. Incineration has become the most cost-efficient and effective disposal method for the growing amount of municipal and industrial wastes, but dioxin emissions have gradually become a problem restricting waste incineration technology. Black carbon and dioxins are both important pollutants in common combustion processes, carbon is an important raw material for dioxins formation, there is a very close relationship between the two pollutants. But there’re few published studies aiming at black and dioxins simultaneously. To explore the relationship between black carbon and dioxins and the transformation of carbon in the de novo synthesis and decomposing of dioxin-like organic pollutants, this paper carried out a lot of research work, and got several valuable conclusions:1) Based on the caculation of this paper, the estimated black carbon emission is 963.4Gg in the mainland of China in 2007. Rural residents fuel consumption is the largest contributor and accounts for 39.6% black carbon emissions. The emissions of coking and other industrial process together account for 41.4%. According to fuel type, coal contributes 46% of black carbon emission, biomass and contributes 37% and oil 17%. The emission factors of 14 main black carbon and dioxin sources are highly related, the correlation coefficiency is 0.64.2) The influences to de novo synthes of CBz, PCP and PCDD/Fs of different carbon materials in model fly ash have a similar tendency, but the discrepancies of the products influenced by black carbon are CBz< PCP< PCDD/Fs. Fly ash contain carbon materials produced in reducing atmosphere have higher activity than that contain carbon produced in oxidizing atmosphere, the amount of dioxin-like products generated by fly ash contain reducing carbon increases at first, then decreases, then increase, then decrease with the increase of temperature, it shows a two-peak curve. Moisture and oxygen have a synergy effect for the de novo process of dioxin-like products. Chlorobenzenes have a better correlation with dioxins compared to pentachlorophenol, but the correlation is effected by temperature, moisture and oxygen.3) Different fly ahses with different elemental distributions show almost the same chromatogram in the PY-GC/MS experiment. Fly ash generates more kinds and amounts of organic compounds at 500℃ than at 350℃. PY-GC/TOFMS are more suitable for fly ash analysis than PY-GC/MS, all chlorobenzenes, biphenyl, naphthalene chain hydrocarbon and polycyclic aromatic organics are detected in the pyrolysis gas of fly ash, which is not detected in PY-GC/MS system. It is speculted that the carbon in fly ash exists in the following forms, large amount of organic compounds absorbed on the surface of fly ash which can be desorpted completely below 250℃, carbon matrix exist in the whole body of fly ash particles which is macromoleculars has polycyclic armatic structure with side chains.4) Fly ash prticles from a typical fluidized bed MS W incinerator is agglomerated by tiny particles, fly ash mainly concentrates in the particle size of 37～74μm and 74～149μm, they account for 93% of the total mass. The element distribution of fly ashes divided by different particle size have little difference, but the fly ash with smallest particle size has the highest dioxin content. Fly ash with smallest particles pruduces the most amount of dioxins in de novo synthesis, fly ash accounts for the largest proportion with the size of 37～74 μm have the weakest de novo synthesis ability. SO2 can effectively inhibit the generation of dioxin in de novo synthesis and has equal inhibitory capacity for both PCDDs and PCDFs. SO2 have stronger inhibitory capacity for lower chlorinated PCDD/Fs.5) Ball milling with CaO can effectively degrade 2,4,6-TCP,6h milling leads to 96.6% degradation. The degradation process includes a series of reactions, including dechlorination, broking of aromatic rings and forming of macromolecular graphite carbon by condensation. Ball milling of fly ash sampled from behind the filter bag has good degradation effect of dioxins,6h milling leads to 41.7% degradation rate of total dioxins and 30.7% degradation rate of dioxin I-TEQ. Dechlronination take place and macromolecular graphite carbon are formed in the milling process. The capacity of de novo synthesis of dioxins by the milled fly ash decresed greatly. After ball milling, the fly ash generates little PCDDs compared with PCDFs, the generated dioxins have a much higher content of low chlorinated dioxins.