The PCDD/Fs Emission Characteristics And Their Key Control Technology In Incinerators

In recent years, more and more municipal solid waste, hazardous waste and medical waste incinerators continue to put into operation. However, excessive emissions of highly toxic substance of dioxin during combustion can’t be ignored in all incinerators, and also raises some social problems. The problems during operation of incinerators nowadays in China were studied in the thesis, the dioxin emission characteristics in municipal solid waste, hazardous waste and medical waste incinerators was investigated, and the main factors to reduce the dioxin emission related to control methods were discussed in target. In addition, new equipment and technology for dioxin emission reduction in fly ash and flue gas were developed, and related experiments were carried out to demonstrate the feasibility of the device and the control effect. The main findings are as follows:(1) The dioxin emission characteristics in typical fluidized bed municipal solid waste incinerators were surveyed and analyzed, and found that the emissions of toxic PCDD/Fs when activated carbon was not sprayed in the testing conditions was between0.073-14.8ng I-TEQ/Nm3. and between0.064-4.70ng I-TEQ/Nm3was found when was sprayed, the PCDD/Fs in fly ash was between254-8200ng I-TEQ/kg, and the newly installed bag filter was much better than aged one in relate to removal efficiency; heterogeneous catalysis formation of de novo synthesis already existed in boiler, the increase of combustion temperature in the furnace was important in the formation of PCDD/Fs, but not the main reason affecting the PCDD/Fs emissions, the cooling process of flue gas passing through air pollution control advices was the main reason for excessive emissions of PCDD/Fs; activated carbon spraying can significantly improve the removal efficiency of bag filter, and newly installed bag filter was more useful than aged one, however, due to "memory effect" existed in aged bag filter, the activated carbon spraying condition can’t even guarantee that PCDD/Fs emission was lower than standard required; the PCDD/Fs formation ability under low combustion load was higher than the high combustion load, the different combustion loads could lead to the change of heavy metals emission, which was found that all concentrations of tested heavy metals except for Cu in fly ash increased with the increase of combustion load;(2) The emission of PCDD/Fs among three experimented medical waste incinerators varied huge different, the toxic concentration PCDD/Fs in slag was1-2orders of magnitude lower than that in fly ash. the PCDD/Fs emission factors in three medical waste incinerators indicates that the emission factor in medical waste incinerators is generally higher than in municipal solid waste incinerators; if long-term spraying condition was operated in a hazardous waste incinerator, a brief suspension of spraying does not result in the increase of PCDD/Fs in stack, but for long-term non-activated carbon spraying condition, the spraying condition is obviously very important for the removal of PCDD/Fs. incinerator, and the removal efficiency increase with enhanced spraying amount; the PCDD/Fs emission during start-up process of the hazardous waste incinerator was most important part of total emission, the results in this study showed that the PCDD/Fs emissions capacity in per unit time during start-up process was56times higher than normal operating condition, the amout of PCDD/Fs emission in start-up stage was about55.5%of total annual emission, and the amout of I-TEQ emission in start-up stage was about50.5%of total annual emission; similar emission characteristics of chlorobenzenes under start-up and normal operating conditions was seen, but the concentration was two to three orders of magnitude higher than PCDD/Fs. The correlation of chlorobenzenes and PCDD/Fs was also studied, and found that the higher chlorinated chlorobenzenes such as HxCBz were much correlated with PCDD/Fs concentration; however, the best correlated chlorobenzenes with the I-TEQ was1,2,4.5-TeCBz, which turns out to be the best candidate of PCDD/Fs indicators at stack of real hazardous waste incinerator.(3) A simulated fly ash was used to study the inhibition mechanism of SO2on dioxin, and found that when the reaction temperature was300C could generated most PCDD/Fs; SO2could obviously inhibit the PCDD/Fs formation in the fly ash, and the inhibition increased with the increase of reaction temperature; it was not very clear of inhibition effect when SO2reacted after fly ash reaction, which suggests that the inhibition mechanism of SO2on PCDD/Fs formation is mainly inactivating of catalyst, such as CuCl2, rather than inhibiting of gas phase formation or reacting with PCDD/Fs itself; rely on simulation of inorganic substances translation during the reaction, it was found that CuCl2could remain stable when temperature was lower than300C without SO2reaction, but it could decompose when the temperature continued to increase, which resulted in the reduction of catalytic activity, the simulation results confirms the phenomena in the experiment that maximum PCDD/Fs formation happened in300C very well; when SO2existed, the simulation results show that the PCDD/Fs inhibition mechanism is that, first to translate CuCl2with catalytic activity into non-catalytic activity of CuSO4. second to reduce the formation of Cl2which is the major element during reaction, and to transfer the optimum formation temperature from low to high temperature.(4) Hazardous waste incinerator fly ash (HFA) and municipal solid waste incinerator fly ash (MFA) were compared and found that the specific surface area of the HFA is much larger than the MFA, HFA has a smaller particle diameter distribution, and the distribution which is less than10μm is much more than WFA; The analysis of thermal desorption chemical kinetics showed that the reaction of fly ash at low temperature with low activation energy, at high temperature with high activation energy, which is about two orders of magnitude higher than in the low-temperature condition, and possibly due to its gasification chemical reaction of residual carbon. IR spectra analysis showed that the main products at low temperature were H2O and CO2, which might be caused by the physical reaction of volatile gases; the main products at high temperature of residual carbon gasification were H2O and CO2and when the carbon content was high enough could generate partial CO; the PCDD/Fs removal efficiency in fly ash under N2atmosphere treated with thermal process shows that300C was able to meet the requirement of substantial degradation, and the removal efficiency elevated significantly with the increase of temperature, which mainly due to effects of dechlorination and degradation processes; the increase of flow rate could enhance the removal efficiency of PCDD/Fs in fly ash, but in the desorbed flue gas, when the flow rate increased to4cm/s, PCDD/Fs and I-TEQ concentrations in gas phase increased significantly; and with different types of fly ash, in spite of the different PCDD/Fs concentrations level in original fly ashes. PCDD/Fs concentrations could be reduced into low level after thermal treatment, but with obviously different PCDD/Fs concentrations which desorbed into the flue gas:the thermal treatment of fly ash in pilot scale plant showed that the removal efficiency of PCDD/Fs and I-TEQ concentration were66.9%and54.3%respectively, because the PCDD/Fs are mainly distributed in fly ash, it is feasible to thermal treat with fly ash with high concentration of PCDD/Fs.(5) Sulfur-based inhibitors was used in a real hazardous waste incinerator, pyrite and ammonium sulfate as inhibitors co-combusted with the hazardous waste could increase the concentration of SO2in the flue gas. which could improve the inhibitor atmosphere. Co-combustion of sulfur-based inhibitors was relatively small effect to the PCDD/Fs emission reduction in flue gas, but the overall emission factor reduction efficiency could be larger than50%; the application of sulfur-based recycling and fly ash thermal treatment combination platform indicated a certain accumulation of SO2and the addition of exogenous sulfur-based inhibitors could increase the SO2concentration within the system, pyrite used as an exogenous sulfur could accumulate significantly more SO2in the system than gas SO2Although the debugging run under the new system test platform could reduce the PCDD/Fs emission in a certain extent but with limited extent. The thermal treatment capacity of fly ash increased with the increase of SO2accumulation capacity, at the same time, the whole PCDD/Fs emission factor reduced correspondingly; after long run, the PCDD/Fs emission in the outlet of pilot system increase significantly, which possibly due to "memory effect", but it was more effective of PCDD/Fs emission control for the long-term operating condition, the overall emission factor reduction efficiency could reach more than80%, and when pyrite was used as exogenous sulfur-based inhibitor could ensure more than90%reduction of PCDD/Fs emission factors and when the pyrite was10%to total treated fly ash, the PCDD/Fs emission could guarantee to lower than0.1ng I-TEQ/Nm3.