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Effects of Varying Combustion Conditions on PCDD/F Formation
Umeå University, Faculty of Science and Technology, Chemistry.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are by-products emitted from combustion sources such as municipal solid waste (MSW) incineration plants. These organic compounds are recognized as toxic, bioaccumulative and persistent in the environment. PCDD/Fs are removed from flue gases before released from MSW incineration. However, the PCDD/Fs are not destroyed but retained in the residues, thus in the environment. Understanding the pathways that lead to their formation is important in order to develop ways to suppress their formation and prevent their release into the environment. Suppressing the formation can also allow less expensive air pollution control system to be used, and/or the costs of thermally treating the residues to be reduced. The main objective of the studies underlying this thesis was to elucidate process, combustion and fuel parameters that substantially affect the emission levels and formation of PCDD/Fs in flue gases from MSW incineration. The experiments were conducted under controllable, realistic combustion conditions using a laboratory-scale reactor combusting artificial MSW.

The parameter found to most strongly reduce the PCDD/F emissions, was prolonging the flue gas residence time at a relatively high temperature (460°C). Increasing the sulfur dioxide (SO2) to hydrogen chloride (HCl) ratio to 1.6 in the flue gas was also found to reduce the PCDF levels, but not the PCDD levels. Fluctuations in the combustion process (carbon monoxide peaks), high chlorine levels in the waste (1.7%) and low temperatures in the secondary combustion zone (660°C) all tended to increase the emission levels. The PCDD/PCDF ratio in the flue gas was found to depend on the chlorine level in the waste, fluctuations in the combustion process and the SO2:HCl ratio in the flue gas. The formation pathways were found to be affected by the quench time profiles in the post-combustion zone, fluctuations in the combustion process and addition of sulfur. In addition, increased levels of chlorine in the waste increased the chlorination degrees of both PCDDs and PCDFs. A tendency for increased SO2 levels in the flue gas to increase levels of polychlorinated dibenzothiophenes (sulfur analogues of PCDFs) was also detected, however the increases were much less significant than the reduction in PCDF levels.

Place, publisher, year, edition, pages
Umeå: Kemi , 2008. , 74 p.
Keyword [en]
dioxin, formation, PCDD, PCDF, MSW incineration, transient combustion, sulfur, memory effects, quench time, chlorine, combustion, polychlorinated dibenzothiophenes, NO, water
National Category
Environmental Sciences
Identifiers
URN: urn:nbn:se:umu:diva-1795ISBN: 978-91-7264-617-9 (print)OAI: oai:DiVA.org:umu-1795DiVA: diva2:141996
Public defence
2008-09-22, KB3A9, KBC, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2008-09-01 Created: 2008-09-01 Last updated: 2009-06-18Bibliographically approved
List of papers
1. Effects of varying combustion conditions on PCDD/F emissions and formation during MSW incineration
Open this publication in new window or tab >>Effects of varying combustion conditions on PCDD/F emissions and formation during MSW incineration
2009 (English)In: Chemosphere, ISSN 0045-6535, E-ISSN 1879-1298, Vol. 75, no 5, 667-673 p.Article in journal (Refereed) Published
Abstract [en]

Process, combustion and fuel parameters were varied to elucidate factors that substantially affect the formation and emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) arising from municipal solid waste (MSW) incineration. The combustion conditions were varied by changing the: freeboard temperatures; quench time in the post-combustion zone; fuel load; chlorine and copper levels in the fuel; and the water, sulfur dioxide, carbon monoxide and oxygen levels in the combustion gases. The study was performed using a 5 kW laboratory-scale fluidized-bed reactor and PCDD/Fs were sampled at a point at 300 °C in the post-combustion zone. The results showed that increasing the SO2 level (from 0 to 130 ppm) substantially reduced the PCDD/F emissions, by up to 60%. In contrast, increasing the CO levels (due to transient combustion conditions), raising the Cl level (from 0.7% to 1.7%) and reducing the freeboard temperature (from 800 °C to 660 °C) all substantially increased the emission levels (more than 3-fold). Changes in PCDD/F profiles associated with increases in Cl, SO2 or CO levels and increasing the freeboard temperature (from 800 °C to 950 °C) indicate that the PCDFs were mainly formed by chlorination. In addition, increasing the Cl level increased the chlorination activity in the formation of PCDDs. Increasing the SO2 level appeared to be less effective in reducing the amount of PCDDs formed via the precursor pathway. While increased CO levels induced PCDD formation via the precursor pathway, although this was found to depend on the O2 level in the flue gas.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
PCDD/F, SO2, Chlorine, Quench time, NO, N2O
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-18474 (URN)10.1016/j.chemosphere.2008.12.038 (DOI)
Available from: 2009-02-10 Created: 2009-02-10 Last updated: 2017-12-14Bibliographically approved
2. Effects of quench time profiles on PCDD/F formation in the postcombustion zone during municipal solid waste incineration
Open this publication in new window or tab >>Effects of quench time profiles on PCDD/F formation in the postcombustion zone during municipal solid waste incineration
2009 (English)In: Environmental Engineering Science, ISSN 1092-8758, E-ISSN 1557-9018, Vol. 26, no 3, 541-550 p.Article in journal (Refereed) Published
Abstract [en]

Effects of postcombustion zone temperatures and flue gas residence times on monochlorinated to octachlorinated dibenzo-p-dioxins and -furans (PCDD/F) in a laboratory-scale reactor combusting municipal solid waste were investigated. Four different quench time profiles were investigated, falling from 400°C to 200°C, 300°C to 100°C, 450°C to 200°C and 460°C to 260°C, with corresponding residence times of 1.4 to 4.4sec, 1.4 to 5.4sec, 1.3 to 4.3sec, and 1.3 to 4.0sec, respectively. Flue gas samples were collected simultaneously at three fixed sampling points, with an additional sampling of flue gas entering the postcombustion zone (640°C). The quench time profiles resulted in different total PCDD/F yields, as well as differences in homologue profiles and congener patterns. Thus, the formation pathways seem to be dependent on sufficient residence time within a specific temperature region. Formation of PCDD/F was shown to be rapid and mainly located to the 640°C to 400°C temperature region, with non-detectable levels at 640°C and concentrations at 400°C that did not increase or decrease significantly downstream. Furthermore, a prolonged residence time at and above 450°C/ 460°C showed low yields even downstream the postcombustion zone compared to the 400°C to 200°C and 300°C to 100°C profiles. The 460°C to 260°C profile resulted in increased abundance at the last two sampling points of PCDD congeners known to correlate with 2,4,6-trichlorophenols, suggesting that chlorophenol condensation reactions were promoted by this quench time profile.

Place, publisher, year, edition, pages
Mary Ann Liebert, 2009
Keyword
temperature, quench time, PCDD/F, waste incineration
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-21689 (URN)10.1089/ees.2008.0044 (DOI)
Available from: 2009-04-16 Created: 2009-04-16 Last updated: 2017-12-13Bibliographically approved
3. Effects of transient combustion conditions on the formation of polychlorinated dibenzo-p-dioxins, dibenzofurans, and benzenes, and polycyclic aromatic hydrocarbons during municipal solid waste incineration
Open this publication in new window or tab >>Effects of transient combustion conditions on the formation of polychlorinated dibenzo-p-dioxins, dibenzofurans, and benzenes, and polycyclic aromatic hydrocarbons during municipal solid waste incineration
2009 (English)In: Environmental Engineering Science, ISSN 1092-8758, E-ISSN 1557-9018, Vol. 26, no 3, 509-520 p.Article in journal (Refereed) Published
Abstract [en]

This study investigated the effects of transient combustion conditions on formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) along the postcombustion zone. Polychlorinated benzenes (PCBz) and polycyclic aromatic hydrocarbons (PAH) were also studied. The study was conducted in a laboratory fluidized-bed reactor fed with an artificial municipal solid waste (MSW) under controllable but realistic combustion conditions. PCDD/Fs, PCBz, and PAHs were monitored under normal, transient, and posttransient combustion conditions and simultaneously sampled at three different sampling points/temperatures (400°C, 300°C, and 200°C). Substantially higher PCDD/F, PCBz, and PAH concentrations were found during transient combustion than during normal combustion. Elevated concentrations were found to decrease with time. PCDD/F concentrations were similar at all points during normal and transient combustion conditions, but were found in higher concentrations at 200°C than 400°C under the posttransient combustion periods. Higher concentrations of the sum PAH and PCBz were also found at 200°C than 400°C in the posttransient combustion periods. Transient combustion conditions induced changes in both PCDD/F homologue profile and PCDD/F congener patterns compared to normal combustion. PCDD/PCDF ratios indicated an increase of the de novo synthesis during transient combustion conditions. Although, the PCDD/F congeners found to be most strongly affected by the transient combustion conditions indicated different reactions pathways active for formation of PCDF and PCDD, de novo synthesis and precursors, respectively. The most strongly affected PCDD/F congeners of transient combustion were identified and are presented.

Place, publisher, year, edition, pages
Mary Ann Liebert, 2009
Keyword
waste incineration, PCDD/F, PCBz, PAH, transient combustion, congener pattern
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-21690 (URN)10.1089/ees.2008.0032 (DOI)
Available from: 2009-04-16 Created: 2009-04-16 Last updated: 2017-12-13Bibliographically approved
4. Effects of sulfur on PCDD/F formation under stable and transient combustion conditions during MSW incineration
Open this publication in new window or tab >>Effects of sulfur on PCDD/F formation under stable and transient combustion conditions during MSW incineration
2009 (English)In: Chemosphere, ISSN 0045-6535, E-ISSN 1879-1298, Vol. 76, no 6, 767-773 p.Article in journal (Refereed) Published
Abstract [en]

SO2 levels in the flue gas from a laboratory-scale fluidized bed reactor combusting artificial municipal solid waste (MSW) were varied (resulting in four different SO2:HCl ratios 0, 0.2, 0.7 and 2.7 (by mass)) to study the effects of sulfur on the formation of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzothiophenes (PCDTs). Sampling was performed simultaneously at three fixed points in the post-combustion zone with temperatures of 400, 300 and 200 °C, under normal combustion conditions and both during and after transient combustion conditions. The findings indicate that sulfur has a greater inhibitory effect on PCDF formation than on PCDD formation and that the PCDD/PCDF ratio in the flue gas depends on both the SO2:HCl ratio in the flue gas and memory effects arising from transient combustion conditions. The results also indicate that the relative importance of different pathways shifts in the post-combustion zone; condensation products increasing with reductions in temperature and increases in residence time. However, these changes appear to depend on the SO2:HCl ratio in the flue gas and combustion conditions. Sulfur seems to inhibit the chlorination of PCDFs. A tendency for increased SO2 levels in the flue gas to increase levels of PCDTs was also detected, but the increases were much less significant than the reductions in PCDF levels.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
PCDD/F, PCDT, Waste incineration, SO2
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-3338 (URN)10.1016/j.chemosphere.2009.04.065 (DOI)
Available from: 2008-09-01 Created: 2008-09-01 Last updated: 2017-12-14Bibliographically approved

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