Iron-Catalyzed Cocurrent Flow Destruction and Dechlorination of

chlorobenzene destruction in the ChemChar thermal waste gasification process ... in the total system including condensation of condensable matter from...
0 downloads 0 Views 389KB Size
Download PDF
Environ. Sci. Technol. 1997, 31, 409-415

Iron-Catalyzed Cocurrent Flow Destruction and Dechlorination of Chlorobenzene During Gasification REBECCA MORLANDO AND STANLEY E. MANAHAN* University of MissourisColumbia, 123 Chemistry Building, Columbia, Missouri 65211 DAVID W. LARSEN University of MissourisSt. Louis, 8001 Natural Bridge Road, St. Louis, Missouri 63121

Iron-treated char was used in an attempt to catalyze chlorobenzene destruction in the ChemChar thermal waste gasification process. The contribution to the destruction removal efficiency (DRE) of chlorobenzene from gasification per se was increased from 99.75% when char was used in the gasifier bed to >99.99% when char treated with iron was used. This clearly satisfies the EPA requirement of 99.99% (4-nines) destruction. Furthermore, other operations in the total system including condensation of condensable matter from the gas stream, filtration of the dried gas over a char filter, and secondary combustion of the product gas contribute to additional removal of unreacted parent compound and byproducts, so that the DRE should easily be increased to 99.9999% (6-nines). The primary mechanism for chlorobenzene dechlorination was determined to involve the reduction of chlorobenzene by reaction with nascent hydrogen. Nascent hydrogen is produced through the water-gas shift reaction, which is believed to be catalyzed by FeO on the iron-treated char surface. Direct electron transfer from the reductive iron surface is also believed to contribute to dechlorination, however, to a lesser extent.

Introduction Chlorobenzene (CB) is considered to be one of the most difficult to incinerate of the EPA’s principle organic hazardous constituents (POHCs). This is due primarily to the strength of the C-Cl bond, which is 95 kcal/mol as compared to more typical values around 85 kcal/mol (1). For this reason, chlorobenzene is on Dellinger’s list of the most difficult to incinerate compounds and has been investigated as a compound for measuring incinerator performance (2). A simplistic way of viewing chlorobenzene destruction is in terms of either a unimolecular or bimolecular mechanism (1, 3). Unimolecular decomposition, which involves a simple rupture of the weakest bond within the molecule, is unlikely because the lowest bond dissociation energy for CB at 95 kcal/mol is prohibitively large. Destruction by bimolecular reactions involves addition or abstraction of functional groups or atoms. Addition reactions may involve free radical species, such as hydroxyl radicals (OH•) or hydrogen radicals (H•) to unsaturated sites within the molecule. A typical abstraction would be the removal of H from a molecule by OH•. It is expected that the primary mechanism for thermal decomposition of CB is bimolecular. * Corresponding author phone: 573-882-6429; fax: 573-882-2754; e-mail: [email protected].

S0013-936X(96)00226-X CCC: $14.00

 1997 American Chemical Society

Gasification is a thermal process that involves the reaction of carbonaceous solids with a substoichiometric amount of an oxidizing or reducing gas to produce combustible gases and vapors (4) and can be used to effectively destroy organohalide compounds, such as chlorobenzene. Unlike incineration, gasification produces a combustible reduced gas product that does not contain oxidized contaminants, such as SO2 or NOx. Of particular importance is the fact that the reductive conditions of gasification prevent generation of chlorinated dibenzodioxins and dibenzofurans, which are a major concern in the incineration of organochlorine compounds. Although cocurrent flow gasification, ChemChar gasification, does achieve good organohalide destruction, previous studies of cocurrent flow gasification have suggested that the introduction of a catalyst into the system may be necessary to significantly enhance organohalide destruction (5). In this study, it was observed that gasification of chlorobenzene on char coated with iron species increases the destruction of chlorobenzene. This waste gasification system is very complex with numerous chemical species, reactive intermediates, and solid phases present along with sharp gradations in temperature and abrupt changes in conditions with time during the gasification process. Therefore, it is unrealistic to claim knowledge of the exact mechanism and all the reactions involved during gasification. Known gasification reactions (4, 6) are used here to provide insight into the processes responsible for the products observed. The primary heat-yielding reaction in the gasification of coal or organic wastes is the reaction of oxygen with carbon to produce carbon dioxide:

C + O2 f CO2 + heat

∆H ) -94.05 kcal/mol (1)

[The thermodynamic data was taken from ref 4.] The partial oxidation of carbon is the primary overall reaction that yields combustible gas during the gasification of carbonaceous solids:

C + 1/2O2 f CO + heat

∆H ) -26.42 kcal/mol (2)

This reaction is exothermic; however, the reaction does not stop at CO. Any free oxygen present will react with CO to produce CO2. Therefore, for a fuel-rich system the slower endothermic reaction

C(s) + CO2 + heat f 2CO

∆H ) 41.2 kcal/mol (3)

also occurs. The carbon monoxide produced in the above reactions is one of the two major reducing gases produced by gasification; the other is hydrogen, which is produced by the oxidation of carbon by steam (H2O-gas shift reaction):

H2O + C + heat f H2 + CO

∆H ) 31.4 kcal/mol (4)

An intermediate step in this reaction is the production of nascent hydrogen (atomic hydrogen), which then combines to produce hydrogen gas:

H2O + C + heat f 2{H•} (nascent hydrogen) + CO (5) 2{H•} f H2

(6)

The production of nascent hydrogen (4, 6) is extremely important in the gasification of hazardous waste because the highly reactive {H•} is extremely effective in destroying refractory organic compounds while preventing the production of undesirable byproducts such as dioxins.

VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

409

Methane is also a product gas of gasification and is produced by the reaction of carbon with hydrogen:

C + 2H2 f CH4

(7)

The production of methane by this reaction is increased by high pressures. Of particular importance to the dechlorination of chlorobenzene in the presence of iron is the H2O-gas shift reaction (reaction 4) due to the intermediate step that involves the formation of nascent hydrogen. As explained by Kirk and Othmer (6), the presence of a “reduced iron” surface catalyzes the H2O-gas shift reaction causing an increase in nascent hydrogen production. As is discussed in the mechanistic evaluation, it is the catalysis of the H2O-gas shift reaction by the presence of the reduced iron surface on the char that is believed to promote the destruction of chlorobenzene during ChemChar gasification. The X-ray diffraction data on the reduced iron surface produced by ChemChar gasification does not indicate the presence of Fe0; however, other studies involving gasification with iron salts have indicated the production of Fe0 (7). It may be possible that Fe0 is produced on the char surface but is too finely dispersed to be observed with X-ray diffraction. If iron metal is produced by gasification, it could also promote the dechlorination of chlorobenzene via a different mechanism. As explained by Matheson and Tratnyek (8), in aqueous systems the redox reaction involving zero-valent iron metal, Fe0, and the iron(II) ion, Fe2+

Fe2+ + 2e- a Fe0

(8)

has a standard reduction potential of -0.440 V; therefore, Fe0 is a reducing agent relative to many redox-labile compounds, such as organohalides, RX. There are three basic mechanisms through which the dechlorination process may occur: (a) direct electrolytic reduction, (b) reduction by hydrogen produced from water during the corrosion of iron, and (c) reduction by dissolved iron(II) ions (9):

Fe + 2RX a Fe2+ + 2R• + 2X-

(9)

Fe + 2H2O a Fe2+ + 2OH- + H2

(10)

H2 + RX a RH + HX

(4)

2Fe2+ + RX + H+ a 2Fe3+ + RH + X-

(11)

In reaction 10, H2 alone is not a good reductant of chlorinated organics; however, in the presence of a catalyst, especially one with an irregular surface, it can cause rapid dechlorination. Excessive accumulation of H2 on the iron surface has in fact been shown to inhibit corrosion and the dechlorination process (8). Although the study mentioned involved ironcatalyzed dechlorination in aqueous systems, it is possible that the dechlorination reaction during gasification could also be enhanced by the presence of Fe on the char surface via a similar mechanism.

Cocurrent Flow Gasification The ChemChar gasification process (10, 12-15) used in these studies is a cocurrent flow gasification system, meaning that the oxygen, steam, and char are introduced through the top of the reactor and the products exit through the bottom. In a batch mode of operation, the thermochemical gasification zone is initiated at the bottom of the reactor and travels upward until it reaches the top. This configuration is best suited for the treatment of hazardous waste because it forces the waste and pyrolysis products to pass through the hightemperature thermal gasification zone in the middle of the

410

9

FIGURE 1. Illustration of a cocurrent flow gasification system describing the various reaction zones within the high-temperature, flame front.

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997

FIGURE 2. Electron micrograph of TRB char taken at 1000× illustrating the macroporous nature of the char surface. The scale bar represents 10 µm. reactor before exiting from the bottom of the reactor as shown in Figure 1. As summarized in the literature (13), the major wastes that have been studied with this system are refractory organic waste sludges, including those containing both PCBs and heavy metals, mixed wastes containing organic substances, and radioactive surrogate materials, sewage sludge, spent activated carbon, and contaminated soils. A key constituent of the gasification system described here is the matrix of macroporous char shown in the electron micrograph in Figure 2. This char is produced from gasification of subbituminous coal from Hanna, WY, in a batch gasifier (see Experimental Section for char preparation). Figure 1 illustrates a typical batch ChemChar gasification system and describes the different portions of the thermochemical reaction zone. The thermochemical reaction zone is a hightemperature (>1500 °C) zone that moves through the column of coal toward the incoming oxidant. This reactive zone is variously called a thermochemical reaction zone, incandescent thermal zone, or flame front. The “reverse-burn” process in which the thermochemical gasification zone moves counter to the flow of oxygen is repeated three times to produce a “triple reverse-burn” solid, TRB char, which has a number of useful characteristics for the handling and treatment of wastes, including the following (10): (a) Highly macroporous with a surface area of 600 m2/g. (b) Inexpensive to produce (∼$0.25/lb). (c) Sorbs a wide variety of wastes; both organic and inorganic. (d) Ideal matrix for mixing, drying, and retaining sludges.

FIGURE 3. Schematic of gasification reactor used for the dechlorination studies. The offset illustrates the steam/oxygen mixing chamber added to the apparatus for steam gasification. (e) Carbonaceous surface that provides a matrix on which thermochemical reactions can occur under controlled conditions. Wastes to be treated by cocurrent flow gasification are first mixed with the char, which is used as the waste carrier for gasification and as a matrix to retain non-gaseous products. Part of the char may be consumed as a fuel, although organic constituents in the wastes are the primary fuel. The introduction of iron compounds onto the char surface for treatment of chlorinated organics has very little effect on the operational parameters in the overall gasification process, so that a direct comparison with results obtained with noniron-containing char can be made. Char that contains iron compounds on the surface will collectively be referred to as Fe-char.

Experimental Section Chemicals. All chemicals used were at least reagent-grade chemicals. The chlorobenzene was not purified before use; no benzene contamination was detected in it by GC/MS analysis. Char Preparation. The TRB char was prepared by three consecutive cocurrent flow gasifications of 20-60 mesh Hanna Basin subbituminous coal in a batch gasifier, similar to the one illustrated in Figure 3; however, the oxygen flow was through the bottom of the reactor. The oxygen flow was approximately 1.4 L/min. The process for TRB char production involves grinding of subbituminous coal to the appropriate mesh size followed by sieving to remove the unwanted size fractions. The ground coal is then gasified in a 2-in. o.d. steel pipe. The pipe filled with char is mounted vertically, and oxygen is introduced into the bottom of the pipe. The char at the top of the reactor is heated to incandescence with a propane torch, oxygen flow is started, and the thermochemical gasification zone is allowed to travel down the column of char until it reaches the bottom, at which point the oxygen flow is stopped. This process is repeated twice more to achieve good porosity with

minimal mass loss (10). Prior to the second and third gasification runs, the char was loaded with 5% water to aid in the gasification. The Fe-char is prepared by treating TRB char with 0.75 M Fe(NO3)3 to achieve a 5% loading of Fe on the char. The iron(III) nitrate solution was added to the char in a beaker, and the char was stirred to achieve even distribution of the iron throughout the char. The char was then dried overnight at approximately 120 °C. After drying, the char was stirred and subjected to a single cocurrent flow gasification treatment in the gasifier used for TRB char preparation. The Fe-char was stored in a sealed container. Gasification Studies. The reactor used for the gasification experiments is shown in Figure 3. The reactor consisted of a 2 ft long 1.5 in. i.d. Vycor tube that holds approximately 74 g of char. The reactor was connected to an oxygen tank with rubber stoppers using a glass wool plug to keep the char in the reactor. The oxygen was passed through the reactor at approximately 1.4 L/min. Steam was introduced into the oxygen stream from a steam line using a mixing flask to allow for condensation of liquid water, see Figure 3. A region of untreated char was added to the bottom of the reactor to allow for good flame front development before contacting the waste-treated char. The waste-treated char samples were prepared by loading 58 g of char with 5% by mass chlorobenzene. The liquids were added to the char, and the char was stirred with a glass stirring rod to ensure even distribution and mixing of the waste with the char. Two initially empty traps immersed in ice water were attached to the end of the reactor to trap volatile components as they exited the reactor; these were tested and shown to be sufficient for trapping the effluent of interest. Condensed column effluent in the traps was extracted with an equal volume of methylene chloride and transferred to a vial. Gas samples were also taken during the gasification process using 2-L Teflon gas sampling bags. Two gas samples were taken for each run after the incandescent thermal zone had reached the waste-treated char region within the reactor. Pyrolysis Studies. The pyrolysis studies were performed using the reactor setup outlined in Figure 4, consisting of a 2 ft. long 1.5 in. i.d. Vycor tube reactor inserted into a tube furnace. Only the portion of the reactor inside the furnace contained char (approximately 30 g). For these studies, the char was preheated to remove all adsorbed water vapor from the char and brought to the desired reaction temperature before introducing the chlorobenzene. A total of 20 mL of chlorobenzene was evaporated slowly, and the vapors passed into the reactor with the gas stream. To produce the chlorobenzene vapors, liquid chlorobenzene was added to an external flask, which was heated with a heating mantle. The vapors were carried through the reactor with helium as the carrier gas. For the steam studies, water was added to a separate flask located before the chlorobenzene flask, and the water vapor was carried through the chlorobenzene flask and into the reactor, again with He as the carrier gas. Two ice water traps were added to collect the column effluent, which was then analyzed directly. Analysis of the gas products from the pyrolysis studies was performed by the same methods used for the gasification gas analysis. Extraction of the char in the reactor after pyrolysis was not performed because in an operating waste treatment system the char would be recirculated through the gasifier. Instrumentation. Analysis of the pyrolysis organic condensate was performed on an HP-5890 gas chromatograph equipped with a J&W DB5-30W capillary column and a Model HP-3390A integrator. The analysis of the gasification condensate and char extract was performed on a Shimadzu QP5000 gas chromatograph/mass spectrometer equipped with an Alltech, Rtx-5 capillary column. The quantitative analysis

VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

411

FIGURE 4. Schematic of pyrolysis apparatus. was performed using toluene as an internal standard. Gas analysis was performed on a Shimadzu GC-14A gas chromatograph using a Carbosphere 80/100 molecular sieve column with helium as the carrier gas. Analysis of the aqueous condensate was performed on a Shimadzu LC-10A equipped with a conductivity detector and an ion-exchange column. The X-ray diffraction studies were performed in the MU Department of Geology using a Scintag Model PAD-V instrument.

Results and Discussion X-ray Diffraction. The X-ray diffraction studies performed on the Fe-char are illustrated in Figure 5. Pattern a is for the Fe(NO3)3-loaded char prior to gasification and b and c are patterns for the Fe-char and Fe-char after gasification with chlorobenzene, respectively. The peaks associated with Fe(NO3)3 are very broad and ambiguous indicating that the Fe(NO3)3 is most likely not crystalline. After gasification of the Fe(NO3)3-loaded char, the iron compounds present on the surface of the char are primarily FeO, Fe3O4, and γ-Fe2O3. The production of these compounds on the char surface can be explained through the following reactions (11):

2Fe(NO3)3 a 2FeO + 6NO2 + 2O2

(12)

FeO + H2O a Fe(OH)2

(13)

6Fe(OH)2 + O2 a 2Fe3O4 + 6H2O

(14)

4Fe3O4 + O2 a 6γ-Fe2O3

(15)

It is important to note that the Fe2O3 produced (reaction 16) is γ-Fe2O3 and not R-Fe2O3. The γ-Fe2O3 form is an oxidation product of Fe3O4, to which it is more structurally similar than R-Fe2O3. The production of Fe2O3 is believed to be due to exposure to air before analysis. The Fe3O4 is ferromagnetic and produced from decomposition of Fe(OH)2, to which it is structurally similar. The magnetic properties of Fe3O4 were observed by the response of the char to a magnet after gasification, and the char has a metallic appearance after gasification. [The optical photos are not shown here but can be obtained through personal correspondence if desired.] The X-ray pattern of the Fe-char after gasification in the presence of chlorobenzene shows a decrease in the presence of Fe3O4 and γ-Fe2O3 along with an increase in the presence of FeO. The presence of FeO after gasification with chlorobenzene is due to a further reduction of Fe3O4 and γ-Fe2O3 to FeO in the high-temperature reduction zone of the gasifier.

412

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997

FIGURE 5. X-ray diffraction patterns for (a) iron(III) nitrate on char, (b) iron char after one gasification in the absence of CB, and (c) iron char after one gasification in the absence of CB and one gasification in the presence of CB. It is possible to sequester FeO on the char surface if the char surface is cooled rapidly (11). Under the conditions of gasification, the char is cooled rapidly from approximately 1200 °C to approximately 200 °C in the matter of a few seconds. It is the presence of the FeO on the char surface that we believe catalyzes the chlorobenzene destruction during gasification, see Mechanistic Evaluation. Reaction 12 shows the production of NO2 during the gasification of Fe(NO3)3. Although NO2 may be produced by this reaction, any NO2 produced will be reduced to elemental nitrogen during the gasification process (12).

TABLE 1. Chlorobenzene Gasification Results char type

loading of CB (g)

CB in traps (g)

% DRE

error ((%)

TRB char Fe-char Fe-char + steam

2.5000 2.2521 2.5022

6.241 × 10-3 3.205 × 10-5 2.180 × 10-5

99.7504 99.9987 99.9991

0.060 0.045 0.001

Gasification Studies. It is important to note at this time that the results indicated here are for the primary gasification step. In an actual unit process application, the only product of interest would be the final gas product, the combustible gas. Condensate collection, particulate removal, and activated carbon filtration of the gas stream are performed after the primary gasification step, and the DRE in a regulatory sense applies to the entire process. The purpose of this study, however, has been the improvement of the primary step (gasification), so that the DREs were determined for the gases coming directly off the reactor without the benefit of subsequent steps, which will increase the DRE. Additional pollution control measures can also be used to further increase the % DRE, if necessary. The results of the gasification studies are summarized in Table 1. These indicate that the use of Fe-char increases the destruction of chlorobenzene during the gasification process. Examination of the chlorobenzene gasification results indicate that the addition of FeO on the surface of the TRB char increases the CB % DRE from ∼99.75% to >99.99% and that the addition of steam increases the destruction even further. The quantity of chlorobenzene remaining on the char in the reactor is not reported here because the primary interest is the gases coming off of the reactor. Typically, however, there is less than 0.1% of the original chlorobenzene remaining on the char surface. This amount of chlorobenzene is insignificant considering that the char will be recycled several times, which will eliminate essentially all of the chlorobenzene remaining on the char. These results are extremely promising in that they indicate that 4-nines destruction of CB is possible solely in the gasification step without the addition of effluent pollution control measures. Therefore, with the addition of such postcolumn treatment processes mentioned above, the DRE for chlorobenzene should easily reach 99.9999% (6-nines). Pyrolysis Studies. The pyrolysis studies under controlled temperature conditions were performed to help elucidate the mechanism of destruction and to provide insight into the mechanism of CB destruction in the presence of the reductive iron surface. The results of the pyrolysis studies are shown in Figure 6, which illustrates the percent conversion of CB to benzene for different char types and under different reaction conditions. This plot shows that the presence of the reductive iron surface on the char clearly promotes the dechlorination of chlorobenzene under pyrolysis conditions in the presence of steam. Comparison of the Fe-char curve and the TRB char curve indicates very little or no promotion of the dechlorination reaction, in the absence of steam, especially at low temperatures. Near 600 °C, in the absence of steam, however, the dechlorination increases rapidly in the presence of the reductive iron surface. The increase in percent conversion around 600 °C suggests that the presence of an FeO reductive surface acts to decrease the activation energy of the dechlorination, reduction pathway, thus resulting in higher conversion. The slight dechlorination observed with TRB char in the absence of iron is due to the naturally reductive nature of the char surface (4). Introduction of steam into the reactor enhances dechlorination to an even greater extent, see Figure 6. The introduction of steam into the system should increase the likelihood of dechlorination via reaction of water with the

FIGURE 6. Results of the pyrolysis studies showing the percent conversion of chlorobenzene for different char types under controlled temperature conditions. The error bars represent one standard deviation from triplicate analysis. char or reductive iron surface to produce reactive intermediates that promote dechlorination, see Mechanistic Evaluation. The addition of steam to TRB char without iron does not appear to enhance the dechlorination properties of TRB char. This is because the temperatures employed during the pyrolysis studies were too low to produce nascent hydrogen by the reaction of water with the TRB char surface without the addition of a catalyst. Gasification Gas Analysis. Figure 7 illustrates the gas composition for chlorobenzene gasification with TRB char and Fe-char. Analysis of the gas products from both pyrolysis and gasification indicates an increase in the production of CO2 in the presence of the reductive iron surface. The observed increase in CO2 can be explained by the catalysis of the water-gas shift reaction (reaction 16) by the presence of FeO in the reducing zone of the gasifier (6):

CO + H2O a H2 + CO2

∆G298 K ) -28.64 kJ (16)

The water-gas shift reaction is a well-known gasification reaction believed to be responsible for the production of hydrogen through a mechanism that involves nascent hydrogen as an intermediate (6). We do not observe an increase in hydrogen during gasification along with the increase in carbon dioxide; however, the method used for hydrogen determination was not extremely accurate because He was used as a carrier gas rather than Ar. It is also likely that a large portion of the hydrogen is consumed during the dechlorination of chlorobenzene. The increase in hydrogen

VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

413

FIGURE 8. Illustration of the two possible reaction mechanisms for FeO catalyzed dechlorination during chlorobenzene gasification: (1) the reaction of chlorobenzene with H• produced during the watergas shift reaction, (2) direct electron transfer. The H• produced can also combine with itself to produce H2.

FIGURE 7. Final gas composition for chlorobenzene gasification. was observed, however, in the pyrolysis studies where much more steam was utilized. The water-gas shift reaction (reaction 16) is significant in this study for two reasons. The first of these is that it explains the elevated CO2 levels in the presence of a reduced iron surface on the char, in that the carbon monoxide reacts with water to produce carbon dioxide (4). The water-gas shift reaction occurs in the downstream reducing region of the thermal gasification zone in the gasifier (see Figure 1). In this region, temperatures are too low for the following reactions of carbon to occur:

H2O + C f H2 + CO

(17)

CO2 + C f 2CO

(18)

These reactions are responsible for the production of combustible synthesis gas during the gasification of char. However, since it is too cool for these reactions to occur, the reducing region of the thermal gasification zone is no longer in equilibrium with elemental carbon; therefore, in the presence of catalytic iron, the water-gas shift reaction becomes predominant in determining the product gas composition. The second reason that the water-gas shift reaction is significant in this study is that it generates hydrogen, H2, through the production of nascent hydrogen, H•. Nascent hydrogen is a highly reactive reducing species that is particularly effective in dehydrohalogenating organohalide compounds, such as chlorobenzene (1). The higher DRE for chlorobenzene in the gasifier during gasification with Fechar as compared to untreated char is consistent with the production of nascent hydrogen on the catalytic iron surface (see below). Mechanistic Evaluation. Although a detailed knowledge of the dehalogenation mechanism during gasification of chlorobenzene cannot be clearly established from this study,

414

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997

the major reactions can be inferred from the results of gasification, pyrolysis, and X-ray analysis studies. Figure 8 illustrates what we believe to be the primary reaction pathway for dehalogenation in the presence of a reductive iron oxide surface and water during gasification and a possible secondary pathway. The primary mechanism for chlorobenzene destruction is believed to be related to the catalysis of the water-gas shift reaction in the presence of the FeO, which is produced on the iron surface by the reduction of Fe3O4 and γ-Fe2O3 in the reduction zone of the gasifier. As mentioned previously, catalysis of the water-gas shift reaction by iron compounds results in an increase in hydrogen via a mechanism that involves the formation of nascent hydrogen (6). The nascent hydrogen is extremely reactive and should react with the chlorobenzene to form a carbon-centered radical and the acid gas HCl. Cl + H•

• + HCl

(19)

Ion chromatographic analysis of the condensate does not show an increase in Cl- after gasification with iron, although X-ray microanalysis studies confirm an increase in chloride concentration on the char surface. Attempts to identify the chloride compound on the char surface by XRD were unsuccessful, probably due to the fine dispersion of the chloride over the surface of the char. Figure 8 also shows the FeO surface acting as an electron donor for direct reduction of the chlorobenzene. This reaction most likely does not occur to any extent; however, the pyrolysis studies suggest that the Fe-char surface may be more reductive than the char surface and could directly increase dechlorination in the absence of water at high temperatures. The formation of intermediate phenyl radical during the gasification and pyrolysis processes is supported by the observation of certain byproducts in the effluents from gasification and pyrolysis. GC/MS analysis of the gasification effluent showed the presence of several byproducts of CB destruction: benzene, benzenethiol, benzonitrile, phenol, and biphenyl. Although benzene, benzenethiol, benzonitrile, phenol, and biphenyl are formed during the gasification process, they constitute only about 0.002%, 0.002%, 0.001%, 0.005%, and 0.001%, respectively, of the chlorobenzene introduced into the reactor. Also, these byproducts can be captured and burned, whereas incineration of chlorobenzene typically results in the formation of chlorinated dioxins and furans upon combustion. The formation of the observed

byproducts can be explained based on the following reactions between the phenyl radical and several common products of gasification (4): • + H2S

SH + 1/2H2

(20)

• + H2O

OH + 1/2H2

(21)

• + HCN

CN + 1/2H2

(22)

• + • • + Cl

(23)

+ Cl–

(24)

Although it is possible that benzenethiol, benzonitrile, and phenol could be produced by nucleophilic substitution of SH-, CN-, and OH-, respectively, for Cl- on chlorobenzene, the formation of biphenyl can be attributed only to the production of the phenyl radical.

Acknowledgments We wish to thank Mr. Naiyu Zhao and Mr. Louis Ross Jr. in the MU Department of Geology for their assistance with the X-ray diffraction analysis, SEM analysis, and X-ray microanalysis. The financial support of ChemChar Research,

Inc., Columbia, MO, which holds rights to the ChemChar waste gasification process, is acknowledged.

Literature Cited (1) Graham, J. L.; Hall D. L.; Dellinger, B. Environ. Sci Technol. 1986, 20, 703-710. (2) Thurnau, R. C. Waste Manage. 1990, 10, 185-195. (3) Tsang, W. Waste Manage. 1990, 10, 217-225. (4) Johnson, J. L. In Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley: New York, 1981; Chapter 23. (5) Gorman, M. Ph.D. Dissertation, University of MissourisColumbia, 1995. (6) Kirk, R. E.; Othmer, D. F. in Encyclopedia of Chemical Technology; Grayson, M. Ed.; Wiley: New York, 1978. (7) Tanaka, S.; et al. Energy Fuels 1995, 9, 45-52. (8) Matheson, L. J.; Tratnyek, P. G Environ. Sci Technol. 1994, 28, 2045-2053. (9) Sweeny, K. H. AIChE Symp. Ser. 1981, No. 77, 72-78. (10) Kinner, L. Ph.D. Dissertation, University of MissourisColumbia, 1995. (11) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 1254-55. (12) Medcalf, B. D.; Manahan, S. E., Larsen, D. W. Environ. Sci. Technol. 1997, 31, 194-197. (13) Kinner, L.; McGowin, A. Environ. Sci Technol. 1993, 27, 482488. (14) Kinner, L.; Larsen, D. J. Environ. Sci Health, Part A 1993, A28, 697-727. (15) Kinner, L.; McGowin, A. Chemosphere 1991, 22 (12), 1197-1209.

Received for review March 12, 1996. Revised manuscript received October 1, 1996. Accepted October 8, 1996.X ES960226V X

Abstract published in Advance ACS Abstracts, January 1, 1997.

VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

415