Effects of Combustion Temperatureon PCDD/Fs Formation inLaboratory-Scale Fluidized-BedIncinerationT A K E S H I H A T A N A K A , *T A K A S H I I M A G A W A ,A K I O K I T A J I M A , A N D M A S A O T A K E U C H I

Institute for Energy Utilization, National Institute ofAdvanced Industrial Science and Technology,AIST Tsukuba West, 16-1 Onogawa, Tsukuba-shi,Ibaraki-ken, 305-8569 Japan

Combustion experiments in a laboratory-scale fluidized-bed reactor were performed to elucidate the effects ofcombustion temperature on PCDD/Fs formation duringincineration of model wastes with poly(vinyl chloride) orsodium chloride as a chlorine source and copper chlorideas a catalyst. Each temperature of primary and secondarycombustion zones in the reactor was set independently to700, 800, and 900 °C using external electric heaters. ThePCDD/Fs concentration is reduced as the temperature of thesecondary combustion zone increases. It is effective tokeep the temperature of the secondary combustion zonehigh enough to reduce their release during the wasteincineration. On the other hand, as the temperature of theprimary combustion zone rises, the PCDD/Fs concentrationalso increases. Lower temperature of the primary combustionzone results in less PCDD/Fs concentration in theseexperimental conditions. This result is probably related tothe devolatilization rate of the solid waste in the primarycombustion zone. The temperature decrease slows thedevolatilization rate and promotes mixing of oxygen andvolatile matters from the solid waste. This contributes tocompleting combustion reactions, resulting in reducing thePCDD/Fs concentration.

IntroductionMunicipal solid waste incinerators are a major emissionsource of polychlorinated dibenzo-p-dioxins and dibenzo-furans (PCDD/Fs) in Japan (1). Although the recent develop-ments of dioxin control technologies on incineration systemshave made it possible to significantly reduce their release,such emission is still a serious problem. The dioxin controltechnologies in waste incineration systems are roughlydivided into two groups. One is measures to decompose orremove PCDD/Fs in flue gas before their release to theenvironment. For example, an addition of exhaust gasprocessing equipment that decomposes them by catalyticreaction is effective to reduce their concentration of flue gas(2). Another is to reduce their formation itself. It is said thatPCDD/Fs are mainly formed via two pathways (3). One is thecondensation of precursors such as chlorobenzenes andchlorophenols (4, 5). Another is the breakdown of carbon/

polycyclic aromatic hydrocarbons (PAHs) (6). As thesecompounds are products of incomplete combustion (PICs),it is certain that the combustion condition affects the PCDD/Fs formation during waste incineration. Therefore, to reducetheir emission, for example, an advanced operating systemis applied to the municipal solid waste incinerator to keepthe combustion condition in the furnace optimum (7).

Furnace temperature is one of the key variables thatdetermine the combustion condition in a furnace, and itexerts a large influence on PCDD/Fs formation in a wasteincineration system (8, 9). High furnace temperature, whichintends to promote combustion reactions, is generallyrecommended to reduce their release. To decrease the PCDD/Fs emission effectively, it is necessary to investigate the effectsof the furnace temperature on their formation during wasteincineration. However, the furnace temperature is deter-mined by operating conditions such as the amount of airand waste supplied, making it difficult to examine thoseeffects in detail in actual waste incineration facilities (10).Additionally, in many cases, flue gas temperature at the outletof the combustion furnace is controlled as the combustiontemperature in the actual facilities and the temperature ofeach part in the furnace, such as primary and secondarycombustion zones, are not examined separately. However,it might exert a different influence on the PCDD/Fs formation.

The aim of this study is to elucidate the effects of thetemperature in the furnace on the PCDD/Fs formation duringwaste incineration. Most of the waste incineration systemsconsist of a primary combustion zone, a secondary combus-tion zone, and other facilities. The reactor used in this studyhas a similar configuration. The temperature of each part inthe reactor is controlled by an electric heater independently,and it can be set to a constant value without changing otheroperating conditions. We made two series of experiments toevaluate the effects of temperature of each part in the reactoron the PCDD/Fs formation. One was a series of experimentsto change the temperature of the secondary combustion zone,and another was to change the temperature of the primarycombustion zone.

Experimental SectionFigure 1 shows the schematic diagram of the experimentalsetup. A main combustion section is divided into primaryand secondary combustion zones. The primary combustionzone was the laboratory-scale fluidized-bed reactor, whichhad a diameter of 60 mm and a height of 300 mm. Thissmallness could suppress radial distribution of combustionintensity in the reactor. The fluidized material was silica sand

* Corresponding author phone: +81-298-61-8228; fax: +81-298-61-8229; e-mail: t.hatanaka@aist.go.jp.

FIGURE 1. Schematic diagram of experimental setup.

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of 100-140 µm, and the static bed height was set to 100 mm.The secondary combustion zone was a freeboard section 30mm in diameter and 1450 mm in height. All parts of the maincombustion section coming in contact with flue gas weremade of quartz. As a postcombustion section to examine thePCDD/Fs formation at low temperature, three glass tubes,30 mm in diameter and 300 mm in height, were used. Air wassupplied to the primary and secondary combustion zonesfrom a compressor. The excess air ratio (λ) was set to 1.3 atthe primary and 0.7 at the secondary; in total, the air wassupplied twice the stoichiometric ratio.

All operating conditions except the temperatures, forexample, the fuel feed rate and the amount of air supplied,were kept constant in all experiments. Temperatures of theprimary and secondary combustion zones and the post-combustion section were controlled to be the target tem-peratures independently using electric heaters. The targettemperatures were 700, 800, and 900 °C in the primary andsecondary combustion zones and 350 °C in the postcom-bustion section. In practical experiments, the electric heaterswere controlled to follow the set points of the heaters duringcombustion. The set point of each heater was decided inadvance so that the temperature measured by a thermocoupleinserted into the reactor was equal to each target temperature.In the secondary combustion zone, the electric heater wascontrolled by four separate sections. The set point of eachheater section was decided so that the measured temperaturebecame the target temperature as closely as possible. In theprimary combustion zone, at first, the set point was obtainedin the same manner. In fact, the set point was equal to thetarget temperature. Next, the temperature rise at thethermocouple for heater control was measured when sup-plying model waste to the reactor. The actual set point wasdecided by subtracting the temperature rise from the setpoint obtained beforehand.

A model waste was used to define the waste compositionstrictly. The base ingredients of the model waste were 45%unbleached pulp powder, 40% unbleached flour, and 15%wood powder. Properties and elementary composition ofthe model waste containing only the base ingredients aregiven in Table 1. The composition adds up to 54%. Since thewaste has only 0.43% ash, it is estimated that most of the restof it is oxygen. In addition to the base ingredients, poly(vinylchloride) (PVC, degree of polymerization n = 1100, purchasedfrom Wako Pure Chemical Industries, Ltd.) or sodium chloride(NaCl, Wako) was mixed as a chlorine source. Copper chloride(CuCl2‚2H2O, 0.25%, Wako) was also mixed as a catalyst ofthe PCDD/Fs formation. All ingredients were ground sepa-rately, mixed mechanically, and then pelletized into particlesin the range of 1-3 mm in diameter. Analyzed Cl contentswere 0.45% in the waste containing PVC and 0.48% in thatcontaining NaCl. The former waste was used in the series ofexperiments on the primary combustion zone, and the latterwas used on the secondary combustion zone. The same wastewas used in the series of experiments to precisely examinethe effects of only the temperatures of the primary and

secondary combustion zones on the PCDD/Fs formation. Ina previous report on the combustion experiments at 900 °Cin the primary and secondary combustion zones, it wasconcluded that there is no significant difference betweenthe effects of PVC and those of NaCl in the model wastes aschlorine sources on the PCDD/Fs formation (11). However,the results in both series of experiments were not compareddirectly in this study because of the difference of chlorinesources in the wastes.

The setup was assembled with new sand for the fluidizedmaterial in each experiment, because the least amount ofresidue such as chlorine and catalyst in the fluidized materialand on the inner surface of the reactor affected the PCDD/Fsformation strongly (12). After the experiment, the quartzsurface in contact with flue gas was washed out to avoid theeffects of experimental order. In case the inner surface didnot become clear enough, the contaminated quartz part wasreplaced with new material. Actually, the reactor was remadefrequently because copper compounds adhered to it byreacting with quartz surface of the reactor. The experimentalconditions are listed in Table 2. Sampling for the PCDD/Fsanalysis was carried out for 4 h or more at the top of the maincombustion section (indicated as point A in Figure 1) andafter the postcombustion section (point B in Figure 1). Awater-cooled probe was used at sampling point A. Solids inthe flue gas were trapped by a glass wool filter and Soxhletextracted with toluene for 24 h. The flue gas sample was alsocollected in an ice-cooled water trap and a Florisil (60-100mesh, Wako) trap and was extracted with hexane. Theextracted samples were cleaned up by silica gel and activatedcarbon columns. PCDD/Fs in the samples were analyzed bygas chromatography-mass spectrometry (GC-MS) HitachiM-80B or JEOL JMS-700. Details of analysis and identificationof PCDD/Fs have been described elsewhere (13).

Results and DiscussionFigure 2 shows the homologue profiles after the postcom-bustion section in the experiment of the model wastecontaining NaCl. The ordinate is the concentration of eachhomologue, which is calculated by dividing the amountdetected in the sample by the normalized sampling gas

TABLE 1. Properties and Elementary Composition of ModelWaste, Based on Analysis of Dry Material

Propertiesmoisture (%) 3.24ash (%) 0.43calorific heat value (MJ/kg) 17.7

Elementary Composition (%)C 47.62H 6.07N 0.68S 0.07

TABLE 2. Experimental Conditions

temperature (°C)primary combustion zone 700-900secondary combustion zone 700-900postcombustion section 350

flow rate (Nm3/h)primary air 0.46 (λ ) 1.3)secondary air 0.26 (λ ) 0.7)

fuel feed rate (g/h) 100

FIGURE 2. Homologue profiles of PCDD/Fs in the experiment on thetemperature of the secondary combustion zone. The temperaturesof the primary and secondary combustion zones are 700 and 900 °C.

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volume. The temperatures of the primary and secondarycombustion zones were 700 and 900 °C. PCDFs are formedmore than PCDDs. Higher chlorinated homologues domi-nate, especially in PCDDs. The similar trends were observedin all the experiments conducted in this study. The homo-logue profiles such as in Figure 2 are reported in an actualfluidized-bed incinerator (14, 15). It is the typical dioxin“fingerprint” from combustion sources that the PCDFs toPCDDs ratio is larger than 1 (16). Wikstrom and Marklund(17) made a detailed report of the homologue profiles ofPCDD/Fs and other organic micropollutants using a labora-tory-scale fluidized-bed reactor fed with an artificial mu-nicipal solid waste fuel. PCDFs are predominant over PCDDsin their results. However, as compared with their homologueprofiles, the degree of chlorination in Figure 2 is likely to betoo high. Copper chloride is known as a very potentchlorinating agency (18). It increases the amount of PCDD/Fs formed and makes the homologue profiles shifted towardthe highly chlorinated species (19). Also in the presentexperiments, it seems that CuCl2 strongly affected the PCDD/Fs formation. Wikstrom et al. (20) have investigated theinfluence of variation in combustion conditions on theprimary formation of organic micropollutants. They showedthat variations in the combustion efficiency defined by theconcentrations of carbon monoxide (CO) and carbon dioxide(CO2) influence the degree of chlorination of PCDD/Fs andother organic micropollutants. However, in this study, it islikely that the homologue profiles such as in Figure 2 wereobtained in all the experiments because the strong chlorina-tion catalyst CuCl2 was too effective.

Effects of Furnace Temperature. Figure 3 shows theresults of the temperature change of the secondary combus-tion zone. The PCDD/Fs concentration after the postcom-bustion section is plotted for the series of experiments usingthe waste containing NaCl. The ordinate is the total con-centrations of PCDDs (tetra- to octa-chlorinated dibenzo-p-dioxins, T4CDDs to O8CDD) and PCDFs (tetra- to octa-chlorinated dibenzofurans, T4CDFs to O8CDF). The tempera-ture of the primary combustion zone was fixed to 700 °C,and that of the secondary combustion zone was changed to700, 800, and 900 °C. Figure 3 clearly shows that the PCDD/Fs concentration decreases rapidly as the temperature of thesecondary combustion zone is elevated. It is certainly effectiveto keep the temperature of the secondary combustion zonehigh enough to reduce the PCDD/Fs release.

The results of the temperature change of the primarycombustion zone are shown in Figure 4. The PCDD/Fsconcentration after the postcombustion section is plottedfor the series of experiments using the waste containing PVC.The temperature of the secondary combustion zone was fixedto 900 °C, and that of the primary combustion zone waschanged to 700, 800, and 900 °C. As the temperature of theprimary combustion zone rises, the PCDD/Fs concentration

increases. The results of the temperature change of theprimary combustion zone are opposite to those of thesecondary combustion zone. Lower temperature of theprimary combustion zone contributes to decreasing thePCDD/Fs concentration. This trend is definite and veryinteresting. It is contrary to the common expectation thatPCDD/Fs are reduced with a temperature rise. What causesthe interesting effects is discussed below.

In the series of experiments on the secondary combustionzone, gas composition that enters into the secondarycombustion zone does not change. Combustion in thesecondary combustion zone at a different temperature makesa large difference in the amount of PCDD/Fs detected afterthe postcombustion section. High temperature promotescombustion of unburnt hydrocarbon and degradation ofPCDD/Fs related compounds in the secondary combustionzone, which decreases the PCDD/Fs concentration as shownin Figure 3. On the contrary, in the series of experiments onthe primary combustion zone, gas composition coming fromthe primary combustion zone certainly changes due to thetemperature change of the primary combustion zone.Although the combustion and the degradation reactionsproceed under the same operating conditions in the sec-ondary combustion zone in each experiment, the differenceof the gas composition coming from the primary combustionzone is not completely canceled and makes the differenceof the PCDD/Fs concentrations as shown in Figure 4.

Now consider what changes of gas composition in thefurnace happen due to the temperature change of the primarycombustion zone. It is expected that the temperature changecauses the differences of PICs composition and concentra-tion, hydrogen chloride (HCl) concentration, and so on. PICsis used here as organic matter that is directly or indirectlyrelated to PCDD/Fs formation such as carbon/polycyclicaromatic hydrocarbons (PAHs), chlorobenzene, and chlo-rophenol. HCl concentration is mentioned as a measurableparameter that represents the concentration of chlorinerelated to chlorination. In the series of experiments on thetemperature effects of the primary combustion zone, themajor factors that affect the amount of PCDD/Fs are probablyPICs composition and concentration in the furnace. Blu-menstock et al. (21) and Zimmermann et al. (22) have showedat a pilot-scale waste incinerator that the conditions in thepostcombustion chamber (650-900 °C) are strongly influ-encing the formation of chlorinated and nonchlorinatedaromatics. They reported that a malfunction at the post-combustion chamber caused drastic changes in the emissionpatterns of PAH and PCDD/F. Wikstrom et al. (20) havementioned the importance of the primary formation oforganic micropollutants at high temperature in PCDD/Fsformation. Also in this study, the temperature change of theprimary combustion zone could make the different com-position and concentration of PICs. In fact, CO concentration

FIGURE 3. Effects of temperature of the secondary combustionzone on PCDD/Fs formation. The temperature of the primarycombustion zone is 700 °C. Cl source is sodium chloride.

FIGURE 4. Effects of temperature of the primary combustion zoneon PCDD/Fs formation. The temperature of the secondary combustionzone is 900 °C. Cl source is poly(vinyl chloride).

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of flue gas decreased with the temperature, i.e., 62.3 ppm at900 °C, 45.8 ppm at 800 °C, and 14.1 ppm at 700 °C. Acorrelation between CO and PICs has been reported by others(20, 21). A similar tendency has also been found by Iwata etal. (23). They indicated that less CO and total organic carbon(TOC) were formed at 600 °C than at 800 °C in newspapercombustion experiments using an electric furnace. As forHCl concentration, since the same waste containing PVC asa chlorine source was used in the series of experiments, thetemperature change of the primary combustion zone wouldmake little difference in the effects of the HCl concentrationon the PCDD/Fs formation. In view of kinetics, the resultsof the temperature change of the secondary combustion zoneindicate that the temperature rise promotes the combustionand degradation reactions more than the PCDD/Fs formationreactions. It is possible that chlorine is more available athigher temperature (20, 24). However, the results of thetemperature change of the secondary combustion zone andthe CO concentrations mentioned above lead us to theconclusion that kinetics has a smaller influence on the PCDD/Fs formation than flue gas composition in our experiments.

Considering why combustion at lower temperature de-creases PICs in the primary combustion zone, it could beconnected to the devolatilization rate of solid waste. Fueldevolatilization has long been recognized to have a profoundinfluence in combustion reactions. Kobayashi et al. (25)reported devolatilization rates of lignite and bituminous coalat temperatures of 1000-2100 K under rapid heating condi-tions using a laminar flow furnace system. They showed arapid drop of the devolatilization rate as the furnacetemperature decreases, especially at high temperature.Similarly, in our experiments, the devolatilization rate of themodel waste probably slows down with the temperaturedecrease. Under the rapid devolatilization condition, mixingof volatile matters from the solid waste with oxygen bydiffusion and convection is slow in comparison with thedevolatilization rate, and enough oxygen is not suppliedlocally. This causes combustion reactions in a lack of oxygen,giving rise to the PICs formation. As the temperature becomeslower, slower devolatilization could cause mild combustionfollowed by mixing of volatile matters with enough oxygenand complete combustion reactions, resulting in reducingPICs. This means that the devolatilization rate has a largerinfluence on combustion than the factors that promotecombustion and degradation reactions with the temperatureincrease. Sakurai et al. (7) reported that the PCDD/Fs emissionis reduced mainly by decreasing the temperature of theprimary combustion zone and a change in supply of primarycombustion air in the fluidized-bed incinerators combustingmunicipal solid waste. They also connected less PCDD/Fsformation with soft burning conditions that avoided suddengas expansion.

PCDD/Fs Formation in the Main Combustion Section.The PCDD/Fs formation in the main combustion section,i.e., their concentration at the top of the secondary combus-tion zone, was investigated to elucidate the temperatureeffects of the primary and secondary combustion zones inmore detail. The PCDD/Fs concentration after the post-combustion section includes the effects of both the maincombustion section at more than 700 °C and the postcom-bustion section at 350 °C. Some studies have shown that theformation at a lower temperature range (650-200 °C) is veryimportant for their formation (18, 26-29). On the other hand,the formation of PCDD/Fs and other PICs at high temperature(>650 °C) has become of major interest lately (17, 20-22,30). Ghorishi and Altwicker (30) reported the rapid PCDD/Fsformation in the heterogeneous bed region of a spouted bedcombustor within fraction of seconds (0.1-0.2 s) usingchlorobenzene and chlorophenol. Wikstrom et al. (20)indicated that, at high temperature, PCDDs are mainly formed

by condensation of chlorophenols, while PCDFs are formedthrough a non- or low-chlorinated precursor followed byfurther chlorination reactions. In addition, Wikstrom andMarklund (17) showed that most of the dibenzofuran, thedibenzo-p-dioxin, and the biphenyl are formed at temper-atures higher than 650 °C, and further chlorination reactionsof organic micropollutants formed at higher temperaturereactions are more important in the lower temperature rangethan formation through the elements C, H, O, and Cl. In thissection, we inquired into the PCDD/Fs concentration at thetop of the main combustion section and compared the effectsof the main combustion section with the postcombustionsection. Sampling at the top of the main combustion sectionand after the postcombustion section was conducted at thesame time.

Figure 5 shows the ratio of the PCDD/Fs concentrationat the top of the main combustion section (point A in Figure1) to that after the postcombustion section (point B in Figure1) in the series of experiments on the temperature effects ofthe secondary combustion zone. The concentration ratio isplotted with the intention of making the effects of the maincombustion section more obvious. The value of the ordinatemeans the ratio of the amount of PCDD/Fs formed in themain combustion section to that detected after the post-combustion section. Therefore, superficially, one minus thevalue means the ratio of the amount of PCDD/Fs formed inthe postcombustion section to that detected after thepostcombustion section. An apparent tendency of theconcentration ratio cannot be found in Figure 5. On the whole,the ratio of PCDFs is slightly higher than that of PCDDs,which could mean that PCDFs are formed more in the hightemperature range and/or that PCDFs are more easilydecomposed than PCDDs after the main combustion section.It is likely related to the different formation mechanisms ofPCDDs and PCDFs (20).

Figure 6 shows the ratio of the PCDD/Fs concentrationsat both sampling points in the experiments on the primarycombustion zone. The concentration ratio is evidentlydependent on the temperature of the primary combustionzone. It is interesting that the dependency of the ratio on thetemperature is different in the primary and secondarycombustion zones. The ratio tends to become smaller at lowertemperature in the primary combustion zone. The tendencythat lower levels of PCDD/Fs are detected at lower temper-atures of the primary combustion zone appears moredefinitely at the top of the main combustion section thanafter the postcombustion section. In the experiments at 700and 800 °C, since a very small amount of PCDD/Fs is detectedin the main combustion section, the amount of PCDD/Fsformed in the postcombustion section becomes relativelylarge although the amount detected is considerably small.

FIGURE 5. Ratios of PCDD/Fs concentrations at the top of the maincombustion section (point A) to those after the postcombustionsection (point B) in the experiments on the temperature of thesecondary combustion zone. Stacked bars show PCDD/Fs concen-trations at points A and B.

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Otherwise in the experiment at 900 °C, a large amount ofPCDD/Fs is formed in the main combustion section. It shouldbe noticed that this does not mean that there is a smallamount of PCDD/Fs formed in the postcombustion section.Although a large amount of PCDD/Fs is formed in thepostcombustion section, the amount of PCDD/Fs decom-posed in the postcombustion section would also increasesince enough PCDD/Fs are already formed in the maincombustion section, resulting in the concentration ratio at900 °C becoming relatively high as shown in Figure 6.

Lowering the temperature of the primary combustion zonecontributes to the decrease of the PCDD/Fs concentration,especially by reducing them in the main combustion section.This is probably connected to the devolatilization rate of themodel waste as mentioned above. The results in the primarycombustion zone are contrary to those in the secondarycombustion zone on both the PCDD/Fs concentration (asseen in Figures 3 and 4) and their ratio (as seen in Figures5 and 6). This makes the data obtained in our experimentsvery precious. The change of the devolatilization rate canalso be the reason for the difference in the dependency ofthe concentration ratio on the temperatures of the primaryand secondary combustion zones shown in Figures 5 and 6.Decreasing the temperature of the primary combustion zoneis an important means of reducing the PCDD/Fs emissionas well as keeping the temperature of the secondarycombustion zone high enough.

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Received for review April 16, 2001. Revised manuscript re-ceived September 13, 2001. Accepted October 9, 2001.

ES015506B

FIGURE 6. Ratios of PCDD/Fs concentrations at the top of the maincombustion section (point A) to those after the postcombustionsection (point B) in the experiments on the temperature of the primarycombustion zone. Stacked bars show PCDD/Fs concentrations atpoints A and B.

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