Dioxin formation and minimization in municipal solid waste incinerators

• Main Authors: Edward Ma, 馬志宏
• Other Authors: Yong Chien Ling
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/83361868328957043403

博士 === 國立清華大學 === 化學系 === 91 === Dioxins were first discovered in the stack gas of municipal solid waste incinerators in 1977. Thereafter, a considerable amount of research has been carried out to elucidate dioxin related issues with various approaches. Management of municipa waste by incineration is a worldwide consensus. The Taiwan Government has issued a policy of constructing more incinerators for municipal waste treatment in 1990. An estimation of twenty one large-scale municipal solid waste incinerators will be completed before 2005. However, conflicts between government and public arose from this policy due to public fear and awareness of adverse health effects caused by pollutants emitted from incineration process. Government has, therefore, promulgated stringent emission regulations, implemented the development and validation of national standard methods for sampling and analysis of dioxins, and funding dioxin related research and monitoring projects. The thesis focuses on dioxins in the municipal solid waste incinerators, to elucidate the formation behavior of dioxins and dishcarge from different matrices, to understand the relationship between formation and combustion conditions and to reduce dioxins production, emissions and preventing environmental impact. Dioxins, heavy metals and relevant information were collected and formulated to establish the basic database of Peitou municipal solid waste incinerator. The formation of dioxins and heavy metals in the incineration process was considered a continuous, dynamic and complex process. A macroscopic view from feeding control, incineration control and pollution control revealed an apparent effect on operational conditions. The estimated annual dioxin production of the incineration facility from stack, fly ash and slag are 0.243, 9.24 and 0.48 g, respectively. Contribute to 6.5% of annual dioxin production of 3.7g when 21 municipal solid waste incinerators are all put into operation. The emission factor is estimated as 468 ng-TEQ/ton, about 20% lower than average emission factor 570 ng-TEQ/ton estimated by Taiwan EPA. The annual emission of Cr, Cu, As, Cd, Hg and Pb from stack are 4.06, 8.11, 0.81, 3.65, 24.33 and 56.77 kg; from fly ash are 2235, 9064, 361, 2570, 264, 33880 kg and from slag are 48787, 280922, 2376, 2218, 175 and 167904 kg, respectively. The results showed that the average dioxin content of the five samples in first sampling period exceeded the 0.1 ng-TEQ/m3 regulatory permission level. Other three periods had an average of 0.066 ng-TEQ/m3, which is nearly the same emission level as the trial burn when the incinerator was first set up. For good operating practice, MSWI should be more cautious about combustion conditions, such as temperature in the process, thermal capacity, and APCD operating conditions during start-up and shutdown cycles. Under normal operational conditions, the dioxin removal efficiency fits the designed specifications. In addition, AC injection promotes the reduction of Hg and dioxin content in stack emission. Fabric filter is the major pollution control device for elimination of dioxin and heavy metals emission. The research analyzes dioxin content in slag and fly ash from Mucha municipal solid waste incinerator after the completion of dioxin abatement work. The results are used to establish the database of dioxin content in slag and fly ash, to understand the effects of incineration and operation conditions on dioxin formation, to investigate the dioxin’s distribution and formation behavior, and to reveal dioxin’s relationship with operating parameters. The results indicate the selective catalytic reactor’s (SCR’s) dioxin removal efficiency is 93% and 98%, respectively. The average dioxins content in the fly ash were 4689, 9698 and 15330 pg-TEQ/g dm, respectively, at Aug 6th~7th, Sep 4th, and Sep 5th. The dioxins content in the fly ash from electrostatic precipitator (ESP) correlates well with the temperature, which related well to the de novo synthesis. The annual dioxin amount in the fly ash is ca. 8.02 g-TEQ/year. Although ESP efficiently removes most PM, however the formation occurred within the EPS could result in a net increase of PCDD/Fs emission. Operating at the lowest practical temperature is critical in minimizing dioxin emission, and it may be necessary to scrub the acid gas from the post-combustion flue gas to permit lowering ESP operating temperature to certain level where corrosion is not a problem. After retrofitting, the findings verified the capability of newly installed SCR to successfully comply with minimum cost of space and requirements demanded and the dioxin emission below regulatory permitted level. The above two typical municipal solid waste incinerators in Taiwan were compared based on the aforementioned analytical results; Facility A (Mucha) consisted of an ESP, a wet scrubber (WS) and a SCR; while facility B (Peitou) employed lime plus activated carbon (AC) injection in a semi-dry scrubber (SDS) and a baghouse fabric filter (FF). The ESP operating temperature is the most important factor affecting PCDD/Fs formation in the gas cleaning system. FF is more effective than ESP in controlling PCDD/Fs because of higher particulate matter (PM) control efficiency. Good combustion practice coupled with air pollution control device (APCD) operating temperature maintained in the region of 200 oC or below, will permit consistent achievement of PCDD/F emissions < 0.1 ng TEQ/m3. The dioxin concentrations in the slag from both MSWIs are similar. The dioxin concentration in fly ash in facility A is 3.1 and 2.9 times to that in facility B during normal operation and start-up period. It can be ascribed to different characteristics of the APCD and operating parameters used in the facility. FF is more effective than ESP in controlling PCDD/Fs because of higher PM control efficiency. Any PM not collected may contain PCDD/Fs. PM control devices collection efficiencies > 99% are probably necessary to control adequately PCDD/Fs emissions.