(April 1, 1996 through June 30, 1996)

Transcription

1 TECHNICAL PROGRESS REPORT (April 1, 1996 through June 30, 1996) Prepared for the Project CONTROL OF TRACE METAL EMISSIONS DURING COAL COMBUSTION Thomas C. Ho Department of Chemical Engineering Lamar University Beaumont, Texas July 1996 Prepared by LAMAR UNIVERSITY Beaumont, Texas for the U.S. DEPARTMENT OF ENERGY PITTSBURGH ENERGY TECHNOLOGY CENTER under Grant No. DE-FG22-94PC94221 PATENT CLEARANCE REQUIRED PRIOR TO PUBLICATION OF THIS REPORT 4

2 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

3 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, rccommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

4 TECHNICAL PROGRESS REPORT April 1, 1996 through June 30, 1996 Project Title: CONTROL OF TRACE METAL EMISSIONS DURING COAL COMBUSTION DOE Grant Number: Principal Investigator: DOE Project Officer: DE-FG22-94PC Thomas C. Ho, Lamar University Mike Baird, PETC ABSTRACT Emissions of toxic trace metals in the form of metal fumes or submicron particulates from a coal-fired combustion source have received greater environmental and regulatory concern over the past years. Current practice of controlling these emissions is to collect them at the cold-end of the process by air-pollution control devices (APCDs) such as electrostatic precipitators and baghouses. However, trace metal fumes may not always be effectively collected by these devices because the formed fumes are extremely small. The proposed research is to explore the opportunities for improved control of toxic trace metal emissions, alternatively, at the hot-end of the coal combustion process, i.e., in the combustion chamber. The technology proposed is to prevent the metal fumes from forming during the process, which would effectively eliminate the metal emission problems. Specifically, the technology is to employ suitable sorbents to (1) reduce the amount of metal volatilization during combustion and (2) capture volatilized metal vapors. The objectives of the project are to demonstrate the technology and to characterize the metal capture process during coal combustion in a fluidized bed combustor. The project was started on July 1, 1994 and this is the eighth quarterly technical progress report. Specifically, the following progress has been made during this performance period from April 1, 1996 through June 30, 1996: 1. Combustion Experiments Continued - Additional combustion experiments involving both coal and wood pellets were carried out in the constructed quartz fluidized bed combustor New Atomic Absorption Spectrophotometer Involved - A new Buck Scientific Model 210VGP Atomic Absorption Spectrophotometer equipped with a continuous flow hydride generator was involved in the project especially for arsenic and selenium measurement. UCR Contractor's Review Conference Attended - A presentation summarizing the project accomplishments was delivered at the Annual UCR Contractor's Review Conference held in Pittsburgh on June 4 and 5, Presentation Accepted - A paper, entitled "Trace Metal Capture by Various Sorbents during Fluidized Bed Coal Combustion," was accepted for presentation at the 212th American Chemical Society National Meeting to be held in Orlando, Florida, August 25-29,

5 EXECUTIVE SUMMARY Toxic (or potentially toxic) trace metallic elements such as barium, beryllium, boron, cadmium, chromium, lead, mercury, nickel, selenium, strontium, vanadium, zinc and zirconium are usually contained in coal in various forms. These metals will either stay in the ash or be vaporized during high temperature combustion. Portions of the vaporized metals may eventually be emitted from a combustion system. Most of the emitted metals will be in the form of metal fumes or particulates with diameters less than 1 micron and are potentially hazardous to the environment. The U.S. EPA has reported that metals account for almost all of the identified risks from waste incineration systems. Concern over toxic trace metal emissions from coal-fired combustion sources is growing, especially as the result of the passage of the 1990 Clean Air Act Amendments (CAAA). To address the concern, the U.S. DOE has recently co-sponsored a workshop jointly with the Electric Power Research Institute (EPRI) and the Energy and Environmental Research Center (EERC) on Trace Elements Transformations in Coal-Fired Power Plants. The objective of the workshop was to evaluate the current level of understanding on metal behavior during coal combustion and to identify potential technologies for improved metal emission control. Current practice of controlling trace metal emissions during coal combustion employs conventional air pollution control devices (APCDs), e.g., venturi scrubbers, electrostatic precipitators, baghouses etc., to collect fly ash and metal fumes. This type of control is essentially a cold-end control because metals are allowed to vaporize and condense before being controlled. The control may not always be effective on metal fumes due to their extremely fine sizes. An alternative technology for metal emission control is to minimize the formation of metal fumes at the hot-end of the coal combustion process, Le., in the combustion chamber. The technology proposed is to prevent the metal fumes from forming during the process, which would effectively eliminate the metal emission problems. Specifically, the technology is to employ suitable sorbents to (1) reduce the amount of metal volatilization during combustion and (2) capture volatilized metal vapors. The objectives of the project are to demonstrate the technology and to characterize the metal capture process during coal combustion in a fluidized bed combustor. The project was started on July 1, 1994 and this is the eighth quarterly technical progress report. Spifically, the following progress has been made during this performance period from April 1, 1996 through June 30, 1996: 1. Combustion Experiments Continued - Additional combustion experiments involving both coal and wood pellets were carried out in the constructed quartz fluidized bed combustor. 2. New Atomic Absorption Spectrophotometer Involved - A new Buck Scientific Model 210VGP Atomic Absorption Spectrophotometer equipped with a continuous flow hydride generator was involved in the project especially for arsenic and selenium measurement.

6 3. UCR Contractor's Review Conference Attended - A presentation summarizing the project accomplishments was delivered at the Annual UCR Contractor's Review Conference held in Pittsburgh on June 4 and 5, Presentation Accepted - A papr, entitled "Trace Metal Capture by Various Sorbents during Fluidized Bed Coal Combustion," was accepted for presentation at the 212th American Chemical Society National Meeting to be held in Orlando, Florida, August 25-29,

7 DETAILED PROGRESS REPORT 1. Combustion Experiments Continued Additional combustion experiments involving both coal and wood pellets were conducted in the constructed quartz fluidized bed combustor during this performance period. In an experiment, a coal (or wood) sample was burned in the bed with a sorbent under a specific set of combustion conditions and the amount of metal capture by the sorbent was determined. Seven different coal samples from the Illinois Basin Coal Sample Bank along with a wood sample were tested in the study. The metals involved were cadmium, lead and chromium, and the sorbents tested included bauxite, zeolite and lime. Typical current experimental results are summarized in Tables 1, 2 and 3 in ATTACHMENT 1, 2 and 3, respectively. The observed experimental results shown in Tables 1, 2 and 3 indicate that metal capture by sorbent can be as high as 91 % depending on the metal species and sorbent involved. All three sorbents are observed to be capable of capturing lead and cadmium in various degree, and zeolite and lime are able to capture chromium. Bauxite, however, is not capable of capturing chromium. It should be pointed out that additional experiments are much needed to provide more statistically representative data. Note that the detailed description of the corresponding experimental conditions was given in a manuscript presented in ATTACHMENT 4 in the sixth TECHNICAL PROGRESS REPORT. 2. New Atomic Absorption Spectrophotometer Involved A new Buck Scientific Model 210VGP Atomic Absorption Spectrophotometer (AAS) equipped with a continuous flow hydride generator was involved in the project. The equipped hydride generator provides much needed detectability in the measuring of trace metal concentrations, especially for arsenic and selenium. A description of the Buck Scientific Continuous Flow HydrideKold Vapor System for parts-per-trillion level detectability was included in the fifth TECHNICAL PROGRESS REPORT. Measurements of arsenic and selenium capture by various sorbents are on-going. These results will be reported in the next progress report. 3. UCR Contractor's Review Conference Attended A presentation describing the project and summarizing its accomplishments was delivered at the Annual UCR Contractor's Review Conference held in Pittsburgh on June 6 and 7,1996. The conference was well-organized, well-attended, and was well-worth attending. 4. Presentation Accepted A paper, entitled "Trace Metal Capture by Various Sorbents during Fluidized Bed Coal Combustion, was accepted for presentation at the American Chemical Society National Meeting to be held in Orlando, Florida from August 25 to 30, A copy of the paper is included in ATTACHMENT 4. 'I 4

8 FUTURE WORK PLANNED The work planned for the next quarter will be to continue metal capture experiments in the quartz fluidized bed combustor to provide statistically representative results. In addition to lead, cadmium and chromium, arsenic and selenium will also be involved in the experiments. The newly tested Buck Scientific Model 2 lovgp Atomic Absorption Spectrophotometer will mainly be used to measure arsenic and selenium concentrations. The experiments will continue to use both wood pellets and coal as the testing material to characterize the metal capture process. 5

9 1. Table 1. Percentage Lead Capture by Sorbents (%) 2. Table 2. Percentage Cadmium Capture by Sorbents (%) 3. Table 3. Percentage Chromium Capture by Sorbents (%) 4. "Trace Metal Capture by Various Sorbents during Fluidized Bed Coal Combustion," paper accepted for presentation at the American Chemical Society National Meeting to be held in Orlando, Florida from August 25 to 30,

10 ATTACHMENT 1 Table 1. Percentage Lead Capture by Sorbents (%) Fuel Type Bauxite Zeolite Lime Coal (IBC-101) Coal (IBC-102) Coal (IBC-106) Coal (IBC-109) Coal (IBC-110) Coal (IBC-111) Coal (IBC-112) Wood Pellets

11 ATTACHMENT 2 Table 2. Percentage Cadmium Capture by Sorbents (%) Fuel Type Bauxite Zeolite Lime coal (IBC-101) coal (IBC-102) Coal (IBC-106) Coal (IBC-109) Coal (IBC-110) Coal (IBC ) Coal (IBC-112) Wood Pellets

12 ATTACHMENT 3 Table 3. Percentage Chromium Capture by Sorbents (%) Fuel Type Bauxite Zeolite Lime Coal (IBC-101) Coal (IBC- 102) Coal (IBC-106) Coal (IBC-109) Coal (IBC- 110) Coal (IBC-111) Coal (IBC-112) Wood Pellets

13 ATTACHhZENT 4 TRACE METAL CAPTURE BY VARIOUS SORBENTS DURING FLUIDIZED BED COAL COMBUSTION* T. C. Ho, A. N. Ghebremeskel and J. R. Hopper Department of Chemical Engineering Lamar University Beaumont, TX Keywords: Coal Combustion, Fluidized Bed, Trace Metals INTRODUCTION Toxic trace metallic elements such as arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, and selenium are usually contained in coal in various forms and trace amounts. These metals will either stay in the ash or be vaporized during high temperature combustion. Portions of the vaporized metals may eventually be emitted from a combustion system in the form of metal fumes or particulates with diameters less than 1 micron, which are potentially hazardous to the environment (1). Current practice of controlling trace metal emissions during coal combustion employs conventional air pollution control devices (APCDs), such as electrostatic precipitators and baghouses, to collect fly ash and metal fumes. The control may not always be effective on metal fumes due to their extremely fine sizes (2). This study is to explore the opportunities for improved control of toxic trace metal emissions from coal-fired combustion systems. Specifically, the technology proposed is to employ suitable sorbents to (1) reduce the amount of metal volatilization and (2) capture volatilized metal vapors during fluidized bed coal combustion. The objective of the study was to investigate experimentally and theoretically the metal capture process. POTENTIAL B$ETAL-SORBENT REACTIONS The following reactions between metals and sorbent constituents have been confirmed both theoretically and experimentally (3,4,5): PbO SiO, ---->Pb2Si04(s) CdO SiO, ---->CdSiO,(s) CdO A > CdA1204(s) PbC1, A1203:2Si02 H >PbO:A1203:2Si0,(s) CdC12 A1203 H2O ---->CdAl,O4(s) 2 HCl(g) (A) + 2 HCl(g) (B) (C) (F) (G) For presentation at the 212th ACS National Meeting to be held in Orlando, Florida, August 24-29,

14 EQUILIBRIUM CALCULATION In this study, combustion equilibrium was calculated using a PC-based computer software package (6) especially developed for predicting equilibrium compositions during fuel or waste combustion. The simulation would reveal potential metal-sorbent reactions for the proposed metal capture process. EXPERIMENTAL Metal capture experiments were Carried out semi-batchwise in a 25.4 mm (1") OD quartz fluidized bed coal combustor enclosed in an electric furnace. The metals involved in the study were cadmium, chromium and lead. Three coal samples from the Illinois Basin Coal Sample Bank (IBCSB) and an artificially prepared metal-containing wood sample were tested in the experiments. The corresponding concentration of chlorine, sulfur, and the target metals in each sample was summarized in Table 1. The sorbents tested included bauxite, zeolite and lime. The chemical composition and fluidization properties of the sorbents are listed in Table 2. For an experimental run, a bed of sorbent was preheated to the desired temperature under the designed operating conditions. A predetermined amount of coal or wood pellets was then charged in the bed at a constant feed rate for combustion. After the combustion was completed, the bed residue was discharged for analysis of metal concentration. The experimental parameters and operating conditions associated with the experiments are shown in Table 3. Metal concentration in the coal, wood pellets, original sorbent, and combustor residue was determined by an atomic absorption spectrophotometer. An HF modified EPA Method 3050 was used to digest metals from the sorbent, which involves the use of HNO,, HCl and HF acids (7). RESULTS AND DISCUSSION Simulation Results Equilibrium calculations were performed to identify thermodynamically preferred metal speciation in a combustion system. A typical set of simulation results indicating potential leadsorbent reactions and the effect of sulfur on metal capture by sorbents are shown in Table 4. The corresponding elemental composition and combustion conditions used in the simulations were: carbon wt%, hydrogen wt%, nitrogen wt%, oxygen to 7.8 wt%, sulfur - 0 to 4.6 wt%, lead concentration - 50 ppm, ash wt%, combustion temperature 90O0C, and percent excess air - 50%. Experimental Results Typical experimental results indicating the effectiveness of metal capture by various sorbents are shown in Tables 5, 6 and 7 for lead, cadmium, and chromium, respectively. The combustible materials tested were three coal samples, i.e., IBC-110, IBC-111 and IBC-112, and a wood sample. The sorbents used were bauxite, zeolite and lime. It is essential to point out that, due to the non-uniformity and trace-quantity nature associated with the process, it is still difficult to discuss the effect of fuel type, coal type, and coal properties such as chlorine, sulfur, ash and metal contents on the metal capture process based on the current results. 11

15 CONCLUSIONS This study is to explore the possibility of employing suitable sorbents to capture toxic trace metals during fluidized bed coal combustion. The observed experimental results indicated that metal capture by sorbents can be as high as 91 % depending on the metal species and sorbent involved. All three sorbents tested, i.e., bauxite, zeolite and lime, were observed to be capable of capturing lead and cadmium in a various degree. Zeolite and lime were able to capture chromium. Results from thermodynamic equilibrium simulations suggested the formation of metal-sorbent compounds such as Pb,SiO,(s), CdAl,O,(s) and CdSiO,(s) under the combustion conditions. Additional experiments are being carried out to provide more statistically representative results for better understanding the metal capture process. ACKNOWLEDGEMENTS The authors are grateful for the fmancial support of this study by the USDOE Pittsburgh Energy Technology Center through the 1994 University Coal Research Program (Grant No. DEFG22-94PC94221). REFERENCES 1. Davidson, R. L., Natush, D. F. S., Wallace, J. R., and Evans, C. A., "Trace Elements in Fly Ash Dependence of Concentration on Particle Size," Environmental Science & Technology, 8, 1107 (1974). 2. Oppelt, E. T., "Incineration of Hazardous Waste - A Critical Review," JAPCA, 37,558 (1987). 3. Uberol, M. and F. Shadman, "Sorbents for Removal of Lead Compounds from Hot Flue Gases," AIChE J., 36, 307 (1990). 4. Uberol, M. and F. Shadman, "High-Temperature Removal of Cadmium Compounds Using Solid Sorbents," Environmental Science & Technology, 25, 1285 (1991). 5. Ho, T. C., R. Ramanarayan, J. R. Hopper, W. D. Bostick, and D. P. Hoffman, "Lead and Cadmium Capture by Various Sorbents during Fluidized Bed combustion/ Incineration," Proceedings of Fluidization VIII, p. 899, held in Tours, France May 1419, Ho, T. C., Incineration Equilibrium IECP, software listed in CEP Software Directory, 68 (1996). 7. Gao, D. and Silcox, G. D., "The Effect of Treatment temperature on Metal Recovery from a Porous Silica Sorbent by EPA Method 3050 and by An HF-Based Method," Air and Waste, 43, 1004 (1993). 12

16 Table 4.1. Concentration of Chlorine, Sulfur and Target Metals in Tested Coal Samples and Wood pellets (Units: ppm for metals, % for C1 and S) Species IBC-110 IBC IBC-112 Wood' Cd < 0.4 < 0.4 < Cr Pb Cl 0.0% 0.0% 0.2% 0.0% S 4.6% 2.0% 2.8% 0.0% *Spiked Metals: Metal Nitrates 13

17 Table 4.2. Major Composition, Trace Metal Concentration and Fluidization Properties of the Three Tested Sorbents Composition or property Bauxite Zeolite Lime

18 Table 4.3. Experimental Parameters and Operating Conditions Parameter Range Fuel Type coal, wood Coal Size mm Wood Size 4.8 mm Fuel Amount 6og Fuel Feed Rate 0.22 g/min Sorbent Type Sorbent Size Bauxite, Zeolite, Lime mm Sorbent Amount g Static Sorbent Height 6 cm Air Flow Rate 3 U, of Sorbent Combustor Temperature 900 C Combustion Duration 4.5 hrs 15

19 Table 4.4. Equilibrium Simulation Results for Lead with or without Sulfur Sorbent Constituent Metal With or Without Sulfur Sulfur-Metal-Sorbent Compound SiO, Pb Without S Pb,SiO,(s) < 1OOO"C PbO(g) > 1OOo"C With S PbSO,(s) < 950 C Pb,SiO,(s) < 1OOo"C PWg) > 1OOo"C Pb Without S With S < 900 C > 900 C < 950 C > 950 C CaO Pb Without S With S < 900 C > 900 C > 500 C < 950 C > 950 C 16

20 Table 4.5. Percentage Lead Capture by Sorbents (%) Fuel Type Bauxite Zeolite Lime Coal (IBC-110) coal (IBC-111) Coal (IBC-112) Wood Pellets

21 ~~ ~ ~ Table 4.6. Percentage Cadmium Capture by Sorbents (%) Fuel Type Bauxite Zeolite Lime Coal (IBC-110) Coal (IBC-111) coal (IBC-112) Wood Pellets

22 Table 4.7. Percentage Chromium Capture by Sorbents (%) Fuel Type Bauxite Zeolite Lime Coal (IBC-110) coal (IBC-111) Coal (IBC-112) Wood Pellets