Kinetics of catalytic oxidation of benzene, n-hexane, and emission gas from a refinery oil/water separator over a chromium oxide catalyst

With the advances made in the past decade, catalytic incineration of volatile organic compounds (VOCs) has become the technology of choice in a wide range of pollution abatement strategies. In this study, a test was undertaken for the catalytic incineration, over a chromium oxide (Cr2O3) catalyst, of n-hexane, benzene, and an emission air/vapor mixture collected from an oil/water separator of a refinery. Reactions were carried out by controlling the feed stream to constant VOC concentrations and temperatures, in the ranges of 1300-14,700 mg/m3 and 240-400 degrees C, respectively. The destruction efficiency for each of the three VOCs as a function of influent gas temperature and empty bed gas residence time was obtained. Results indicate that n-hexane and the oil vapor with a composition of straight- and branch-chain aliphatic hydrocarbons exhibited similar catalytic incineration effects, while benzene required a higher incineration temperature or longer gas retention time to achieve comparable results. In the range of the VOC concentrations studied, at a given gas residence time, increasing the operating temperature of the catalyst bed increased the destruction efficiency. However, the much higher temperatures required for a destruction efficiency of over 99% may be not cost-effective and are not suggested. A first-order kinetics with respect to VOC concentration and an Arrhenius temperature dependence of the kinetic constant appeared to be an adequate representation for the catalytic oxidation of these volatile organics. Activation energy and kinetic constants were estimated for each of the VOCs. Low-temperature destruction of the target volatile organics could be achieved by using the Cr2O3 catalyst.     

Kinetics of Catalytic Oxidation of Benzene,n-Hexane,and Emission Gas from a Refinery Oil/Water Separator over a Chromium Oxide Catalyst

ABSTRACT

With the advances made in the past decade, catalytic incineration of volatile organic compounds (VOCs) has become the technology of choice in a wide range of pollution abatement strategies. In this study, a test was undertaken for the catalytic incineration, over a chromium oxide (Cr₂O₃) catalyst, of n-hexane, benzene, and an emission air/vapor mixture collected from an oil/water separator of a refinery. Reactions were carried out by controlling the feed stream to constant VOC concentrations and temperatures, in the ranges of 1300–14,700 mg/m³ and 240–400 ° C, respectively. The destruction efficiency for each of the three VOCs as a function of influent gas temperature and empty bed gas residence time was obtained.

Results indicate that n-hexane and the oil vapor with a composition of straight- and branch-chain aliphatic hydrocarbons exhibited similar catalytic incineration effects, while benzene required a higher incineration temperature or longer gas retention time to achieve comparable results.

In the range of the VOC concentrations studied, at a given gas residence time, increasing the operating temperature of the catalyst bed increased the destruction efficiency. However, the much higher temperatures required for a destruction efficiency of over 99% may be not cost-effective and are not suggested. A first-order kinetics with respect to VOC concentration and an Arrhenius temperature dependence of the kinetic constant appeared to be an adequate representation for the catalytic oxidation of these volatile organics. Activation energy and kinetic constants were estimated for each of the VOCs. Low-temperature destruction of the target volatile organics could be achieved by using the Cr₂O₃ catalyst.     

Use of regenerated ferric oxide for CO destruction and suppressing dioxin formation in flue gas in a pilot-scale incinerator

Catalytic destruction of chlorinated compounds is one of the key methods in reducing pollutant emissions. For the purpose of utilizing waste materials, a catalyst was regenerated from ferric ion sludge, obtained from the addition of iron salts to precipitate heavy metals. The sludge was dewatered, heated (800 degrees C for 4 h), and ground into smaller particles. The regenerated ferric oxide particles were then used as the oxidation catalyst to destroy CO formation during the combustion of three chlorinated solvents and to suppress dioxin formation in flue gas in a real waste solvent. In the presence of catalyst, the combustion efficiency (ratio of CO(2) to the sum of CO(2) and CO) for chlorobenzene was more than 98% at 850 degrees C in a pilot-scale incinerator. The destruction and removal efficiencies of chlorobenzene, 2,4-dichlorophenol and trichlorofluoroethane were more than three nines. In the absence of catalysts, the flue gas emission from a real waste could not meet the regulatory dioxin standard of 0.1 ng-TEQ/Nm(3) even with the powdered activated carbon injection. The use of catalyst at either 100 or 300 g/h, however, was able to meet the emission standard.     

The Relationship between the Quantity of Heavy Metal and PAHs in Fly Ash

ABSTRACT

Heavy metal and polycyclic aromatic hydrocarbons (PAHs) in flue gas have received considerable attention in recent years due to their mutagenic or carcinogenic properties. The present study is carried out to investigate the influence of the quantity of heavy metals on PAHs formation in fly ash.

A fluidized bed incinerator was used in this experiment to obtain fly ash of chemical similarity by incinerating various compositions of waste. The obtained fly ash, both with and without heavy metal, were used to adsorb the PAHs in the flue gas and to investigate the formation of PAHs in fly ash. The results indicate that carbon and heavy metals most greatly influence the formation of PAHs in the fly ash. Carbon is absorptive; heavy metals encourage not only absorption of PAHs but also catalyze PAHs formation.     

Removal of dioxins and related aromatic hydrocarbons from flue gas streams by adsorption and catalytic destruction

The dioxin removing capacity of the shell dedioxin system (SDDS-a Ti/V oxidative type catalyst) has been tested using the Ume? lab-scale incinerator over the temperature range 100-230 degrees C and at space velocities of 8000 and 40,000 h(-1). Other analogous organic compounds, such as PCBs, PAHs, chlorobenzenes and chlorophenols have also been investigated. Results show a high degree of dioxin removal already at 100 degrees C (82%), which occurs mainly by adsorption. When the temperature is raised a transition towards destruction is seen and at 150 degrees C, gas hour space velocity (GHSV) 8000 and at 230 degrees C, GHSV 40,000 virtually all removal is by destruction. High PCDD/F destruction efficiencies are reported (> 99.9%, based on I-TEQ); the other dioxin-related species and PAHs are also removed and destroyed to a significant extent. The SDDS has proved to be an effective means of destroying organic compounds in the gas phase, particularly dioxins, at temperatures as low as 150 degrees C.     

Simultaneous removal of sulfur dioxide and polycyclic aromatic hydrocarbons from incineration flue gas using activated carbon fibers

Incineration flue gas contains polycyclic aromatic hydrocarbons (PAHs) and sulfur dioxide (SO₂). The effects of SO₂ concentration (0, 350, 750, and 1000 ppm), reaction temperature (160, 200, and 280 °C), and the type of activated carbon fibers (ACFs) on the removal of SO₂ and PAHs by ACFs were examined in this study. A fluidized bed incinerator was used to simulate practical incineration flue gas. It was found that the presence of SO₂ in the incineration flue gas could drastically decrease removal of PAHs because of competitive adsorption. The effect of rise in the reaction temperature from 160 to 280 °C on removal of PAHs was greater than that on SO₂ removal at an SO₂ concentration of 750 ppm. Among the three ACFs studied, ACF-B, with the highest microporous volume, highest O content, and the tightest structure, was the best adsorbent for removing SO₂ and PAHs when these gases coexisted in the incineration flue gas.

ImplicationsSimultaneous adsorption of sulfur dioxide (SO₂) and polycyclic aromatic hydrocarbons (PAHs) emitted from incineration flue gas onto activated carbon fibers (ACFs) meant to devise a new technique showed that the presence of SO₂ in the incineration flue gas leads to a drastic decrease in removal of PAHs because of competitive adsorption. Reaction temperature had a greater influence on PAHs removal than on SO₂ removal. ACF-B, with the highest microporous volume, highest O content, and tightest structure among the three studied ACFs, was found to be the best adsorbent for removing SO₂ and PAHs.     

Oxidation of mercury across selective catalytic reduction catalysts in coal-fired power plants

A kinetic model for predicting the amount of mercury (Hg) oxidation across selective catalytic reduction (SCR) systems in coal-fired power plants was developed and tested. The model incorporated the effects of diffusion within the porous SCR catalyst and the competition between ammonia and Hg for active sites on the catalyst. Laboratory data on Hg oxidation in simulated flue gas and slipstream data on Hg oxidation in flue gas from power plants were modeled. The model provided good fits to the data for eight different catalysts, both plate and monolith, across a temperature range of 280-420 degrees C, with space velocities varying from 1900 to 5000 hr(-1). Space velocity, temperature, hydrochloric acid content of the flue gas, ratio of ammonia to nitric oxide, and catalyst design all affected Hg oxidation across the SCR catalyst. The model can be used to predict the impact of coal properties, catalyst design, and operating conditions on Hg oxidation across SCRs.     

Experimental study on the removal of PAHs using in-duct activated carbon injection

This paper presents the incineration tests of municipal solid waste (MSW) in a fluidized bed and the adsorption of activated carbon (AC) on polycyclic aromatic hydrocarbons (PAHs). An extraction and high performance liquid chromatography (HPLC) technique was used to analyze the concentrations of the 16 US EPA specified PAHs contained in raw MSW, flue gas, fly ash, and bottom ash. The aim of this work was to decide the influence of AC on the distribution of PAHs during the incineration of MSW. Experimental researches show that there were a few PAHs in MSW and bottom ash. With the increase of AC feeding rate, the concentrations of three- to six-ring PAHs in fly ash increased, and the concentration of two-ring PAH decreased. The total-PAHs in flue gas were dominated by three-, and four-ring PAHs, but a few two-, five-ring PAHs and no six-ring PAHs were found. PAHs could be removed effectively from flue gas by using in-duct AC injection and the removal efficiencies of PAHs were about 76-91%. In addition, the total toxic equivalent (TEQ) concentrations of PAH in raw MSW, bottom ash, fly ash, and flue gas were 1.24 mg TEQ kg-1, 0.25 mg TEQ kg-1, 6.89-9.67 mg TEQ kg-1, and 0.36-1.50 microg TEQ Nm-3, respectively.     

A Study of Catalyst Support Systems for Fume-Abatement of Hydrocarbon Solvents

In a stainless steel pilot plant system, operating with combined thermal incineration and catalytic oxidation, studies have been made of the influence of catalyst-support geometry on abatement efficiency. Catalyst supports tested were ceramic spherical pellets, metal foils, and ceramic honeycomb structures, all with precious metal catalyst. Data obtained cover a range of space velocities from 30,000 to 175,000 scf/hr— cu ft bed and a range of catalytic reactor temperatures from 150 to 450°C. Results show that optimum fume-abatement performance is obtained by combining incineration and catalytic oxidation and that catalyst support geometry has a significant effect on performance.     

Catalytic NOx reduction with simultaneous dioxin and furan oxidation

The engineering, construction, performance and running costs of a catalytic flue gas cleaning component in the low dust area of a municipal waste incinerator is discussed. For this purpose, the case study of a Flemish incineration plant is presented, covering the history, the design procedure of the catalyst, relevant process data and the financial aspects. A reliable PCDD/F-destruction by means of oxidation by the catalyst to typical values of 0.001 ng TEQ/Nm3 has been demonstrated. At the same time, NOx- and CO-emissions are reduced by 90% and 20% to about 50 mg/Nm3 and below 10 mg/Nm3, respectively.     

Catalytic NO(chi) reduction with simultaneous dioxin and furan oxidation

The engineering, construction, performance and running costs of a catalytic flue gas cleaning component in the low dust area of a municipal waste incinerator is discussed. For this purpose, the case study of a Flemish incineration plant is presented, covering the history, the design procedure of the catalyst, relevant process data and the financial aspects. A reliable PCDD/F-destruction by means of oxidation by the catalyst to typical values of 0.001 ng TEQ/Nm3 has been demonstrated. At the same time, NO(chi)- and CO-emissions are reduced by 90% and 20% to about 50 mg/N m3 andbelow 10 mg/N m3, respectively.     

Performance of the supported copper oxide catalysts for the catalytic incineration of aromatic hydrocarbons

A fixed bed reactor was used to assess the catalytic incineration of toluene by various transition-metal oxide species supported on gamma-Al(2)O(3). CuO/gamma-Al(2)O(3) was found to be the most active of seven catalysts investigated. The CuO species, with a Cu content of 5% (wt), was hence used with four different supports (CeO(2), gamma-Al(2)O(3), TiO(2) and V(2)O(5)) in order to define the optimal combination. Results of the catalytic incineration of toluene, X-ray diffraction (XRD) analysis, oxygen-temperature programmed desorption (O(2)-TPD), toluene-temperature programmed desorption (toluene-TPD) and hydrogen-temperature programmed reduction (H(2)-TPR) showed that CuO/CeO(2) was the most active catalyst, followed by CuO/gamma-Al(2)O(3). The activity of CuO/CeO(2) with respect to the VOC molecule was observed to follow this sequence: toluen>p-xylene>benzene. The addition of water vapor or CO(2) significantly inhibited the activity of the CuO/CeO(2) and CuO/gamma-Al(2)O(3) catalysts. The inhibiting effect of both was reversible for CuO/gamma-Al(2)O(3). For CuO/CeO(2), the inhibiting effect of CO(2) was reversible and even insignificant at a higher temperature (220 degrees C), but the effect of H(2)O vapor was somewhat irreversible at lower incineration temperatures (220 degrees C). For complete oxidation of toluene, the required reaction temperature increased with gas hourly space velocity (GHSV) and toluene inlet concentration.     

Catalytic oxidation of gaseous PCDD/Fs with ozone over iron oxide catalysts

Catalytic oxidation of PCDD/Fs (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans) with ozone (catalytic ozonation) over nano-sized iron oxides (denoted as FexOy) was carried out at temperature of 120-180 degrees C. The effects of operating temperature, ozone concentration, space velocity (SV) and water vapor contents on PCDD/F removal and destruction efficiencies via catalytic ozonation were investigated. High activity of the iron oxide catalyst towards PCDD/F decomposition was observed even at low temperatures with the aid of ozone. The PCDD/F removal and destruction efficiencies achieved with FexOy/O3 at 180 degrees C reach 94% and 91%, respectively. In the absence of ozone, the destruction efficiencies of all PCDD/F congeners are below 20% and decrease with increasing chlorination level of PCDD/F congener at lower temperature (120 degrees C). However, in the presence of ozone, the destruction efficiencies of all PCDD/F congeners are over 80% on FexOy/O3 at 180 degrees C. Higher temperature and ozone addition increase the activity of iron oxide for the decomposition of PCDD/Fs. Additionally, in the presence of 5% water vapor, the destruction efficiency of the PCDD/Fs is above 90% even at lower operating temperature (150 degrees C). It indicates that the presence of appropriate amount of water vapor enhances the catalytic activity for the decomposition of gas-phase PCDD/Fs.     

Biofiltration of gasoline vapor by compost media

Gasoline vapor was treated using a compost biofilter operated in upflow mode over 4 months. The gas velocity was 6 m/h, yielding an empty bed retention time (EBRT) of 10 min. Benzene, toluene, ethylbenzene and xylene (BTEX) and total petroleum hydrocarbon (TPH) removal efficiencies remained fairly stable approximately 15 days after biofilter start-up. The average removal efficiencies of TPH and BTEX were 80 and 85%, respectively, during 4 months of stable operation. Biodegradation portions of the treated TPH and BTEX were 60 and 64%, respectively. When the influent concentration of TPH was less than 7800 mg TPH/m3, approximately 50% of TPH in the gas stream was removed in the lower half of the biofilter. When the influent concentration of BTEX was less than 720 mg BTEX/m3, over 75% of BTEX in the gas stream was removed in the lower half of the biofilter. Benzene removal efficiency was the lowest among BTEX. A pressure drop could not be detected over a 1-m bed height at a gas velocity of 6 m/h after approximately 4 months of operation. Results demonstrated that BTEX in gasoline vapor could be treated effectively using a compost biofilter.     

Catalytic oxidation for air pollution control

Bench-scale experiments have been conducted to evaluate a series of titania-supported Pt-Pd (as oxides) catalysts in the presence and absence of MoO₃ and Fe₂O₃ additives for their effectiveness in the complete catalytic oxidation of volatile organic compounds (VOCs) in air likely to be found in waste gases. Under oxidizing conditions, all of the catalysts promoted the complete oxidation of VOCs to CO₂ and H₂O. 99 % Conversion was achieved with a C₂H₄-C₂H₆ gas mixture in air at temperatures between about 160–450 °C and at a space velocity of 20,000 h^?1. Oxidation activity for the titania supported catalysts were found to decrease in the order Pt-Pd-Mo-Fe > Pt-Pd-Mo > Pt-Pd-Fe > Pt-Pd. However, the addition of MoO₃ and Fe₂O₃ increase the catalyst activity and reduce the reaction temperature for the complete destruction. Ageing was also performed in order to study the stability of the most active catalyst. Pt-Pd-Mo-Fe (as oxides) on titania catalyst is effective in oxidizing a wide range of volatile organic compounds at relatively low temperatures (220–405 °C) and and at a space velocity of 40,000 h^?1 and is resistant to poisoning by halogenated and amine volatile organic compounds.     

The utilization of catalyst sorbent in scrubbing acid gases from incineration flue gas

Catalyst sorbents based on alumina-supported CuO, CeO2, and CuO-CeO2 were applied to a dry scrubber to clean up the SO2/HCl/NO simultaneously from pilot-scale fluidized-bed incineration flue gas. In the presence of organic compounds, CO and the submicron particles SO2 and HCI removed by the fresh catalyst sorbents and NO reduced to N2 by NH3 under the catalysis of fresh and spent desulfurization/dechloridization (DeSO2/DeHCl) catalyst sorbents (copper compounds, Cu, CuO, and CuSO4) were evaluated in this paper. The fresh and spent catalyst sorbents were characterized by the Brunner-Emmett-Teller method (BET), X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), inductively coupled plasma-mass spectrometry (ICP-MS), and the elemental analyzer (EA). The study showed that the performances of CuO, CeO2, and CuO-CeO2/gamma-Al2O3 were better than that of Ca(OH)2. The removal efficiency of SO2 and HCl was 80-95% in the dry scrubber system. Under NH3/NO = 1, NO could not be reduced to N2 because it was difficult to control the ratio of air/fuel in the flue gas. For estimating the feasibility of regenerating the spent catalyst sorbents, BET and EA analyses were used. They indicated that the pore structures were nearly maintained and a small amount of carbon accumulated on their surface.     

Mercury oxidation promoted by a selective catalytic reduction catalyst under simulated Powder River Basin coal combustion conditions

A bench-scale reactor consisting of a natural gas burner and an electrically heated reactor housing a selective catalytic reduction (SCR) catalyst was constructed for studying elemental mercury (Hg(o)) oxidation under SCR conditions. A low sulfur Powder River Basin (PRB) subbituminous coal combustion fly ash was injected into the entrained-flow reactor along with sulfur dioxide (SO2), nitrogen oxides (NOx), hydrogen chloride (HCl), and trace Hg(o). Concentrations of Hg(o) and total mercury (Hg) upstream and downstream of the SCR catalyst were measured using a Hg monitor. The effects of HCl concentration, SCR operating temperature, catalyst space velocity, and feed rate of PRB fly ash on Hg(o) oxidation were evaluated. It was observed that HCl provides the source of chlorine for Hg(o) oxidation under simulated PRB coal-fired SCR conditions. The decrease in Hg mass balance closure across the catalyst with decreasing HCl concentration suggests that transient Hg capture on the SCR catalyst occurred during the short test exposure periods and that the outlet speciation observed may not be representative of steady-state operation at longer exposure times. Increasing the space velocity and operating temperature of the SCR led to less Hg(o) oxidized. Introduction of PRB coal fly ash resulted in slightly decreased outlet oxidized mercury (Hg2+) as a percentage of total inlet Hg and correspondingly resulted in an incremental increase in Hg capture. The injection of ammonia (NH3) for NOx reduction by SCR was found to have a strong effect to decrease Hg oxidation. The observations suggest that Hg(o) oxidation may occur near the exit region of commercial SCR reactors. Passage of flue gas through SCR systems without NH3 injection, such as during the low-ozone season, may also impact Hg speciation and capture in the flue gas.     

Relationship between pressure fluctuations and generation of organic pollutants with different particle size distributions in a fluidized bed incinerator

The hydrodynamic behaviors of fluidization perhaps significantly influence the uniformity of fluidization in fluidized bed incinerator. Good uniformity of fluidization expressed the air across uniformly through the bed and the particles being distributed well in the fluid stream. The aggregates, flocs and channels of particles do not happen during fluidization. The Good uniformity will maintain high heat and mass distribution to improve reaction efficiency. These parameters include the height of static bed, gas velocity, mixing and distribution of bed particle, which have rarely been studied in previous investigations. Consequently, this study examines how the hydrodynamic parameters affect the generation of organic pollutants (BTEXs and PAHs) during incineration. The statistical and power spectral analysis of the measured pressure fluctuation during incineration are used to elucidate the relationship between behaviors of fluidization and generation of pollutants during incineration. Experimental results show the organic concentration does not increase with uniformity of fluidization decreasing. The reason may be the explosion of the gas and the consequent thermal shock destroy the coalescent bubbles to form small bubbles again and enhance the efficiency of transfer of oxygen to increase combustion efficiency. Additionally, the mean amplitude and fluidized index of pressure fluctuation similarly vary with the concentration of organic pollutants. These two indices can be used to assess the efficiency of combustion. The four particle size distributions could be divided into two groups by statistical analysis. The Gaussian and narrow distributions belong to one group and the binary and flat the other. The organic concentration of the Gaussian and narrow distributions are lower than that of the other distributions. Consequently, the bed materials should maintain narrow or Gaussian distributions to maintain a good combustion efficiency during incineration.     

Catalytic destruction of 1,2-dichlorobenzene on V2O5-WO3/Al2O3-TiO2 catalyst

Studies on the catalytic destruction of 1,2-dichlorobenzene were carried out on a specially constructed semi-technical equipment whose most important element was a catalytic reactor with a monolithic catalyst in the form of 150 x 150 x 100 mm cubes. A catalyst made from cordierite with an active layer composed of Al2O3 - 64 wt%, TiO2 - 26 wt%, V2O5 - 6.6 wt% and WO3 - 3.4 wt% was used. The reactor made it possible to carry out the process in the temperature range 150-350 degrees C, at variable catalyst loading and different velocities of gas flow through the reactor. The content of 1,2-dichlorobenzene in the air was analysed by a chromatographic method. A significant effect of catalyst loading and temperature on 1,2-dichlorobenzene destruction efficiency was observed and no effect of the linear flow velocity through the catalyst on o-dichlorobenzene destruction efficiency was reported. The applied vanadium-tungsten catalyst on a monolithic carrier made from TiO2/gamma-Al2O3 revealed very good activity that resulted in an over 80% efficiency of 1,2-dichlorobenzene destruction at the temperature around 250 degrees C at a very high catalyst loading reaching ca. 8200 h(-1). Additionally, in this study the kinetics of 1,2-dichlorobenzene decomposition was determined, specifying the order of reaction and dependence of the decomposition rate constant on temperature, using a simple power-rate law model.     

Suppressing effect of goethite on PCDD/F and HCB emissions from plastic materials incineration

Polyethylene (PE) and polyvinyl chloride (PVC) are the leading plastics in total production in the world. The incineration of plastic-based materials forms many chlorinated compounds, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). In this study the addition of goethite (alpha-FeOOH) was investigated to determine its suppressing effect on the emission of PCDD/Fs and hexachlorobenzene (HCB) during the combustion of wastes containing PE and PVC. Goethite was being considered since it acts as a dioxin-suppressing catalyst during incineration. Results showed that incorporation of goethite greatly reduced the generation of PCDD/Fs and HCB in the exhaust gas and fly ash. The concentration of PCDD/Fs in flue gas decreased by 45% for lab-scale and 52% for small incinerator combustion experiments, where the goethite ratios in feed samples were 0.54% and 0.34%, respectively. Under the same conditions, the concentration of HCB in flue gas decreased by 88% and 62%, respectively. The present study showed a possible mechanism of the suppressing effect of the goethite for PCDD/F formation. It is likely that iron chlorides react with particulate carbon to form organo-chlorine compounds and promote PCDD/F formation in the gas phase. XRD analysis of combustion ash revealed that the goethite was partially dehydrated and converted to alpha-Fe(2)O(3) and Fe(3)O(4) but no iron chlorides formation. Therefore the goethite impregnated plastics can contribute the reduction of PCDD/Fs and HCB in the exhaust gas during incineration of MSW.