Energy Fuels 2010, 24, 4445–4453 Published on Web 07/21/2010
: DOI:10.1021/ef900613b
Activated Carbon from Biomass for Mercury Capture: Effect of the Leaching Pretreatment on the Capture Efficiency S. Arvelakis,*,† C. Crocker,‡ B. Folkedahl,‡ J. Pavlish,‡ and H. Spliethoff † † Department of Mechanical Engineers, Chair Energy Systems, Technical University of Munich, Boltzmanstrasse 15, Garching, Munich, Germany, and ‡Energy and Environmental Research Center, University of North Dakota, 15 North 23rd Street, Stop 9018, Grand Forks, North Dakota 58202-9018
Received June 16, 2009. Revised Manuscript Received June 30, 2010
Activated carbons were produced from two European biomasses such as olive residue and wheat straw. The leaching pretreatment is used to study the effect of chlorine and alkali metals on the mercury capture efficiency. The activated carbons were tested for mercury capture in a bench scale reactor against the commercial grade activated carbon NORIT DARCO Hg as well as an activated carbon developed at the Energy and Environmental Research Center (EERC) from lignite. The mercury capture tests showed that the untreated biomass-derived activated carbons performed fair-to-very good compared to the commercial catalyst and better compared to the lignite-derived activated carbon. The leached biomass-derived carbons were seen to have poor performance in the case of the olive residue and similar performance to that of the lignite-derived carbon in the case of the wheat straw.
the importance of mercury emissions from coal combustion relative to total mercury emissions has increased in certain countries. This is because the more concentrated mercury emissions from other sectors (e.g., chlor-alkali plants) are more readily controlled or eliminated. Reducing mercury emissions by coal-fired utilities has, therefore, become more critical.2 On May 18, 2005, EPA issued a nationwide cap-and-trade Clean Air Mercury Rule (CAMR) to regulate mercury emissions from coal-fired power plants. However, the cap-and-trade program was vacated by the D.C. Circuit Court of Appeals in February 2008 before EPA could implement it. EPA was instructed by the court to develop a Maximum Achievable Control Technology (MACT) standard to regulate mercury emissions from coal-fired power plants. As prescribed by the Clean Air Act, the MACT standard requires that mercury emissions from all coal-fired boilers be reduced to the average amount emitted by the best performing 12% of coal-fired boilers. The data gathered by EPA in 1999 show that the 491 U.S. coalfired power plants annually emit 48 tons of mercury into the air. On July 2, 2009, EPA announced that it would conduct another information collection request to update existing emissions data from power plants. This data will serve the basis for development of the MACT standard. Under a court order settlement, EPA is to issue a final MACT standard by end of 2011.3 In Europe, mercury emissions from coal combustion are also becoming a matter of growing interest. A number of European countries such as Germany, Italy, and Switzerland have already adopted national regulations on mercury emissions. Furthermore, in April 2001, the European Commission (EC) approved the protocol on heavy metals to reduce the emissions of metals that are prone to long-range transboundary atmospheric transport and are likely to have adverse effects on human health and the environment. Finally, in January 2005, EC adopted a mercury strategy that envisages a
Introduction Mercury compounds released from human activities are one of the most toxic pollutants to human health and the ecosystem because of their propensity to bioaccumulate by up to a factor of 10 000 within aquatic food chains. This bioaccumulation leads to severe neuron damage in human beings.1 Recent estimates of annual global mercury emissions from all sources (both natural and anthropogenic) range from roughly 4400 to 7500 tons a year. Anthropogenic U.S. mercury emissions are estimated to account for roughly 3% of this global total, and U.S. coal-fired power plants are estimated to account for about 1% of the global total. Mercury emissions from coal-fired power plants contribute about 30% of the anthropogenic sources of mercury. This amount, based on previous results, is estimated at slightly less than 50 tons a year in the United States.2 Coals contain many elements in trace amounts of parts per million (ppmw). Among them is mercury, which is found in concentrations between 0.02 and 1.0 ppm and is readily volatilized during coal combustion.2 Since mercury is highly volatile, major portions of the mercury present in coal avoid capture by existing particulate control devices currently available at coalfired plants. Today, the total Hg emissions in most developed countries have either stabilized or reduced. On the other hand, *To whom correspondence should be addressed. Phone: þ498928916267. Fax:þ498928916271. E-mail:
[email protected]. (1) Cao, Y.; Wang, Q.; Chen, C.-W.; Chen, B.; Cohron, M.; Tseng, Y.-C.; Chiu, C.-C.; Chu, P.; Pan, W.-P. Investigation of Mercury Transformation by HBr Addition in a Slipstream Facility with Real Flue Gas Atmospheres of Bituminous Coal and Powder River Basin Coal. Energy Fuels 2007, 21 (5), 2719–2730. (2) Jones, A. P.; Hoffmann, J. W.; Smith, D. N.; Feeley, T. J.; Murphy, J. T. DOE/NETL’s Phase II Mercury Control Technology Field Testing Program Updated Economic Analysis of Activated Carbon Injection, Prepared for the U.S. Department of Energy (May 2007) http://www.netl.doe.gov/technologies/coalpower/ewr/mercury/pubs/ Phase_II_UPDATED_Hg_Control_Economic_Analysis.pdf. r 2010 American Chemical Society
(3) http://www.epa.gov/camr/.
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have been reported. Many studies show that Hg0 is the dominant species at high temperatures, and kinetic transformations are the controlling mechanisms of mercury reactions. Chemical kinetic models suggest that the chlorine content of the flue gas is the main contributor to mercury oxidation prior to the air pollution control devices (APCDs).15Oxidized mercury is soluble and has a tendency to associate with particulate matter. Therefore, emissions of oxidized mercury may be efficiently controlled by air emission and particulate control equipment, such as a flue gas desulfurization (FGD) scrubber system, electrostatic precipitators (ESPs), and activated carbon injection (ACI) systems. It has been reported that only about 25% of utility boilers have a wet FGD installed for SOx control. In comparison, 80% of utility boilers are equipped with a cold-side ESP to control particulate emissions.15,16 Thus, methods to effectively convert Hg0 to Hg2þ or Hg0 to Hg(p) are welcomed by a majority of utility owners who rely on these APCDs for simultaneous mercury emission control (especially those that use low-sulfur Powder River Basin coal and lignite). On the contrary, elemental mercury is extremely volatile and insoluble. It has a high vapor pressure at typical operating temperatures of air emission and particulate control devices. As a result, effective collection by particulate control devices is highly variable, and elemental mercury emissions are harder to reduce than oxidized mercury emissions.16-18 At present, there is no universally accepted Hg control technology for coal-fired utilities, and the incorporation of the technologies already in use in waste incineration plants could enhance the cost of the process considerably. Several solid materials, such as ACs, calcium-based sorbents, and zeolites, have been considered as sorbents for mercury control in flue gases from coal combustion. Experience in the use of such sorbents has been gained from solid waste incinerators, in which mercury species in gases are typically removed using hydrated lime and ACs.13,19 In addition, in the U.S., the Department of Energy (DOE) through the National Energy Technology Laboratory (NETL) has conducted an extensive field testing program in the previous 10 years with a variety of sorbents such as activated carbons and chemically treated activated carbons, as well as other novel Hg removal concepts.2,20 This extensive field testing program has demonstrated that several ACs as well as other concepts have a great potential of successfully removing Hg from the flue gases at efficiencies as high as 90þ% and at costs that could reach as low as $10 000 per lb of mercury removed.20
number of measures to protect the health of European Union (EU) citizens and of the environment.4-8 Mercury in coal varies with both the coal rank and its origin. Power plants burning lignite coal emit greater proportions of gaseous elemental mercury (Hg0) relative to gaseous oxidized mercury (Hg2þ) than plants burning bituminous coals.9,10 Lignite coals are seen to contain comparable mercury concentrations but significantly lower concentrations of chlorine compared to bituminous coals. Lignite coals are also seen to have much higher calcium contents. These compositional differences affect the quantity and form of mercury emitted from a boiler as well as the capabilities of the various control devices regarding mercury capture.9,10 Bituminous coals contain relatively high concentrations of chlorine and produce a flue gas with Hg2þ as the dominant mercury form. On the contrary, low-chlorine coals, such as lignite, form mainly Hg0, which is substantially more difficult to remove from the flue gas stream than Hg2þ.9-11 The ability of various methods to remove mercury from the flue gas depends largely on the species of the mercury formed upstream of the control devices. Emitted mercury is found in association with particulate matter (Hg[p]), in gaseous elemental form (Hg0) and as various gaseous mercuric compounds (Hg2þ). Every utility power plant utilizes different fuels with different mercury concentrations as well as different plant operating parameters. As a result, the concentrations of specific mercury species cannot be assumed to be the same from plant to plant. A comprehensive understanding of transformations among mercury species is of importance, because control options and transport models for mercury rely heavily on its form or species. Over the past several years, research has provided extensive descriptive information on the physical and chemical factors that govern mercury speciation in combustion flue gas.12-14 The data from full- and laboratory-scale coal combustion vary, and different ratios of the elemental to oxidized fractions (4) Lopez-Anton, M. A.; Dıaz-Somoano, M.; Martınez-Tarazona, M. R. Retention of elemental mercury in fly ashes in different atmospheres. Energy Fuels 2007, 21, 99–103. (5) Onei, B. T.; Tewalt, S. J.; Finkelman, R. B.; Akers, D. J. Mercury concentration in coal-unraveling the puzzle. Fuel 1999, 78, 47–54. (6) U.S. EPA. Mercury, http://www.epa.gov/mercury. (7) http://europa.eu.int/comm/environment/chemicals/mercury/index. htm. (8) Cao, Y.; Chen, B.; Wu, J.; Cui, H.; Smith, J.; Chen, C.-K.; Chu, P.; Pan, W.-P. Study of Mercury Oxidation by a Selective Catalytic Reduction Catalyst in a Pilot-Scale Slipstream Reactor at a Utility Boiler Burning Bituminous Coal. Energy Fuels 2007, 21, 145–156. (9) Pavlish, J. H.; Holmes, M. J.; Benson, S. A.; Crocker, C. R.; Galbreath, K. C. Application of Sorbents for Mercury Control for Utilities Burning Lignite Coal. Fuel Process. Technol. 2004, 85, 563–576. (10) Cao, Y.; Duan, Y.; Kellie, S.; Li, L.; Xu, W.; Riley, J. T.; Pan, W.-P.; Chu, P.; Mehta, A. K.; Carty, R. Impact of Coal Chlorine on Mercury Speciation and Emission from a 100-MW Utility Boiler with Cold-Side Electrostatic Precipitators and Low-NO Burners. Energy Fuels 2005, 19, 842–854. (11) Senior, C. L.; Johnson, S. A. Impact of Carbon-in-ash on Mercury Removal Across Particulate Control Devices in Coal-fired Power Plants. Energy Fuels 2005, 19, 859–863. (12) Zhuang, Y.; Zygarlicke, C. J.; Galbreath, K. C.; Thompson, J. S.; Holmes, M. J.; Pavlish, J. H. Kinetic Transformation of Mercury in Coal Combustion Flue Gas in a Bench-scale Entrained-flow Reactor. Fuel Process. Technol. 2004, 85, 463–472. (13) Lee, S.-H.; Park, Y.-O. Gas-phase Mercury Removal by Carbonbased Sorbents. Fuel Process. Technol. 2003, 84, 197–206. (14) Huggins, F. E.; Yapa, N.; Huffmana, G. P.; Senior, C. L. XAFS Characterization of Mercury Captured from Combustion Gases on Sorbents at Low Temperatures. Fuel Process. Technol. 2003, 82, 167– 196.
(15) Senior, C. L.; Sarofim, A. F.; Zeng, T.; Helble, J. H.; MamaniPaco, R. Gas-phase Transformations of Mercury in Coal-fired Power Plants. Fuel Process. Technol. 2000, 63, 197–213. (16) Nolan, P. S.; Redinger, K. E.; Amrhein, G. T.; Kudlac, G. A. Mercury Emissions Control in Wet FGD Systems. Proceedings of the International Conference on Air Quality: Mercury, Trace Elements, and Particulate Matter, Arlington, VA, September 9-12, 2002. (17) Sjostrom, S.; Bustard, J.; Durham, M.; Chang, R. Mercury Removal Trends and Options for Coal-Fired Power Plants with FullScale ESPs and Fabric Filters. Proceedings of the 19th Annual International Coal Conference, Pittsburgh, PA, September 2002, pp 23-27. (18) Kellie, S.; Cao, Y.; Duan, Y.; Li, L.; Chu, P.; Mehta, A.; Carty, R.; Riley, J. T.; Pan, W.-P. Factors Affecting Mercury Speciation in a 100-MW Coal-fired Boiler With Low-NOx Burners. Energy Fuels 2005, 19, 800–806. (19) Gullett, B. K.; Ragnunathan, K. Reduction of Coal-based Metal Emissions by Furnace Sorbent Injection. Energy Fuels 1994, 8, 1068– 1076. (20) Feeley, T. J., III; Jones, A. P. An Update on DOE/NETL’s Mercury Control Technology Field Testing Program. Prepared for the U.S. Department of Energy (Aug 2008) http://www.netl.doe.gov/ technologies/coalpower/ewr/mercury/pubs/netl%20Hg%20program% 20white%20paper%20FINAL%20Jan2008.pdf.
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Figure 1. Schematic of the EERC fixed bed carbonization reactor.
and emissions such as mercury and, therefore, increase substantially the quality and value of the treated ACs.22,27 In the present study, ACs were prepared from two biomass residues such as wheat straw and olive residue. Both residues were also pretreated using the leaching technique in order to study the effect of chlorine and alkali metals on the efficiency of the ACs prepared from the pretreated biomass samples to capture mercury from the simulated flue gas from coal combustion. The produced results are compared to those derived from ACs prepared from lignite coals as well as to those derived from commercialgrade ACs.
Substances such as HCl and HBr, as well as Cl2, have also been used for mercury removal from the flue gases of coalfired plants in combination with sorbents and/or scrubbers. These substances have shown to have good ability to remove elemental mercury from the flue gases by oxidizing it so it can be removed by the scrubbers, but their effectiveness has sometimes been doubted.1,10,21-25 Commercially available AC typically costs U.S. $1.002.00/kg, and it has been estimated that it would cost approximately U.S. $11 000-U.S. $110 000 (2007 DOE estimates) to remove each kilogram of mercury from flue gas using AC. Consequently, for a 250 MW unit emitting 65 kg of Hg a year, it would cost between U.S. $0.35 and U.S. $3.9 million to remove 50% of the mercury present.2 ACs can be prepared from a variety of raw materials. The most frequently used precursors are hard coal, brown coal, wood, coconut shells, and some polymers. In addition, agricultural byproducts such as fruit stones, various nutshells, and many others can be used as raw materials. Porous carbons are produced by either physical or chemical activation. After carbonization of the base material, development of the internal pore structure is typically achieved by thermal oxidation using steam and CO2 or chemical oxidation using H3PO4, ZnCl2, and alkali metal hydroxides such as KOH and NaOH.8,26-29 As other authors have verified, the metal oxides and hydroxides are very active toward the removal of pollutants
Materials and Methods Two different biomass materials, olive residue and wheat straw, in both untreated and pretreated (leached) form as well as North Dakota lignite were used for the production of activated carbons at the Energy and Environmental Research Center (EERC). Both biomass materials originated from Greece, the olive residue from the Pelloponese and the wheat straw from Thessaly. Both biomass materials were leached following the methodology developed by Arvelakis et al.30 The leaching process removed alkali metals and chlorine as well as sulfur from the treated biomass materials, while the organic material was not affected and was fully recovered at the end of the process. The lignite originated from the North Dakota BNI mine. A commercial activated carbon named NORIT DARCO Hg produced by Norit Americas was also used in order to compare the Hg adsorption capacity of the biomass and the lignite carbons with the capacity of the commercial product. It had a mean diameter by volume of approximately 18 μm and an approximate density of 30 lb/ft3. It was produced using Texas lignite as the raw feedstock. The lignite carbon was prepared in a rotary kiln as part of a larger project for pilot-scale testing of the produced carbon in mercury capture at the EERC. The kiln was a sealed, indirectly electrically heated rotary kiln with a nominal heated zone 15.24 cm diameter and 152.4 cm long, 3-zone temperature control to 1000 °C, inert atmosphere operating conditions, rotation from 1 to 5 rpm, internal screw-fed auger, and variable inclination to 5°. Preparation of Chars and Activated Carbons. Olive residue and wheat straw were first pyrolyzed in a N2 atmosphere using a method developed at the EERC to remove moisture and volatile matter and to increase the surface area of the material by creating mesopores that are thought to facilitate the heterogeneous Hg-C reaction. The process is known as the carbonization process. The carbonization was conducted in batch mode in a fixedbed stainless steel tube reactor (7.62 cm diameter) presented in Figure 1. The olive residue as well as the wheat straw, both
(21) Agarwal, H.; StengerSong, G. H.; Fan, Z. Effects of HO, SO, and NO on Homogeneous Hg Oxidation by Cl. Energy Fuels 2006, 20 (3), 1068–1075. (22) Wu, S.; Ozaki, M.; Azhar Uddin, M. D.; Sasaoka, E. Development of Iron-Based Sorbents for Hg0 Removal from Coal Derived Fuel Gas: Effect of Hydrogen Chloride. Fuel 2008, 87, 467–474. (23) Lei, C.; Yufeng, D.; Yuqun, Z.; Liguo, Y.; Liang, Z.; Xiannghua, Y.; Qiang, Y.; Yiman, J.; Xuchang, X. Mercury Transformation Across Particulate Control Devices in Six Power Plants in China: The Co-Effect of Chlorine and Ash Composition. Fuel 2007, 86, 603–610. (24) Eswaran, S.; Stenger, G. H. Effect of Hallogens on Mercury Conversion in SCR Catalysts. Fuel Process. Technol. 2008, 89, 1153– 1159. (25) Agarwal, H.; Romero, C. E.; Stenger, H. G. Comparing and Interpreting Laboratory Results of Hg Oxidation by a Chlorine Species. Fuel Process. Technol. 2007, 88, 723–730. (26) Clean Air Act, Mercury Control Technologies at Coal-Fired Power Plants Have Achieved Substantial Emissions Reductions Report to the Chairman, Subcommittee on Clean Air and Nuclear Safety, Committee on Environment and Public Works, U.S. Senate United States Government Accountability Office: Washington, D.C., October 2009. (27) Stavropoulos, G. G. Precursor Materials Suitability for Super Activated carbons Production. Fuel Process. Technol. 2005, 86, 1165– 1173. (28) Yan, R.; Ng, Y. L.; Liang, D. T.; Lim, C. S.; Tay, J. H. BenchScale Experimental Study on the Effect of Flue Gas Composition on Mercury Removal by Activated Carbon Adsorption. Energy Fuels 2003, 17 (6), 1528–1535. (29) Yan, R.; Liang, D. T.; Tsen, L.; Wong, Y. P.; Lee, Y. K. BenchScale Experimental Evaluation of Carbon Performance on Mercury Vapour Adsorption. Fuel 2004, 83, 2401–2409.
(30) Arvelakis, S.; Koukios, E. G. Physicochemical Upgrading of Agroresidues as Feedstocks for Energy Production via Thermochemical Conversion Methods. Biomass Bioenergy 2002, 22 (5), 331–348.
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Figure 2. Schematic of the EERC bench-scale apparatus.
and 10.5 cm3/min in the case of wheat straw. The activation of the leached olive residue and the leached wheat straw chars was done following the same pattern. However, in the case of the leached olive residue, the temperature in the activation chamber was seen to reach 910 °C, and in the case of the leached wheat straw, it was 800 °C in the same gentle flow of nitrogen (250 cm3/ min). This was attributed to the higher reactivity of the leached samples compared to the originals and/or the presence of a small amount of air due to inadequate reactor sealing. The lack of resources and materials did not allow us to repeat the activation process for the leached samples. The heating of the chars was done by following the same temperature program and had been followed before during the biomass charring. After reaching the activation temperature, steam was introduced from the bottom of the reactor. The char was then heated in a gentle flow of steam and nitrogen for 30 min in the case of the leached olive residue and 45 min in the case of the leached wheat straw. The steam flow used was 1.2 cm3/min in the case of the leached olive residue and 10.5 cm3/min in the case of the leached wheat straw. At the end of the activation period, the steam was stopped and the reactor was cooled to room temperature in nitrogen flow. Then, the activated carbon was removed from the reactor, weighed, and stored under nitrogen until used in the mercury adsorption tests. The biomass derived activated carbons from all the original as well as the leached samples were ashed at 600 °C according to the ASTM D 1102-84, and the ash was analyzed using the SEMEDX (Scanning Electron Microscopy-Energy Dispersive X-ray) method in order to determine the composition of its surface and assist with the understanding of the adsorption behavior of the specific activated carbons during the mercury capture testing. The ash samples were carbon coated for enhanced electrical conductivity. Ash particles were analyzed in magnifications of at least 600, and three area analyses per sample were performed to produce valid results. The results were automatically converted from counts to % oxides using the LINK ISIS software package for SEM instruments by Oxford. A bench-scale system presented in Figure 2 was used for the mercury capture tests. The system consisted of a 63.5 mm diameter holder supporting a 150 mg fixed-bed of sorbent on a quartz tissue filter; the sorbent under evaluation is vacuum deposited onto this support. The fixed bed and all associated plumbing were held within a temperature-controlled oven at a nominal 135 °C. Flue gas was supplied to the fixed bed via a manifold that combines gases, water, and either Hg0 or HgCl2
untreated and leached samples, were placed in the bench-scale tube reactor. The tube reactor was placed in a vertical electrically heated furnace. The stainless steel tube was also attached to a nitrogen inlet tube. The temperature inside the tube had been checked before the start of the tests using a thermocouple placed into different areas of the tube to check the temperature profile. The analysis had showed that the temperature was uniform in the half upper section of the tube, and this was the area used during the experiments as the other half of the reactor had been filled up with an insulation material. The reactor was heated to a desired temperature (600 °C) in a gentle flow of nitrogen according to the following steps. First, it was heated up to 200 °C and retained at this temperature for 30 min. Then, it was heated up to 600 °C in intervals of 100 °C each time with an isothermal step of 10 min. At 600 °C, the reactor was left to equilibrate for 2 h. This time was considered to be adequate for the tarry material ceased to evolve. The produced char was stored under a nitrogen atmosphere for the production of activated carbons in a second stage. The activated carbons were prepared by steam activation of the char made from the olive residue and the wheat straw samples. Steam activation enhanced the minimal pore structure created during the initial carbonization where volatile matter is released. The diameters of the pores were enlarged, and thus, pore volume was increased. The development of new porosity in the form of micropores during steam activation resulted from the removal of less crystalline carbon by reaction with the steam to form CO and H2 and some thermal rearrangement of the structures to form more graphite sheets. This burn-out of the disordered structure may have been catalyzed by the inorganic material, especially the alkali present.27 In the case of the untreated wheat straw and olive residue samples, the steam activation was conducted in batch mode in the fixed-bed vertical stainless steel tube reactor (7.62 cm diameter) also used for the charring of the biomass samples. The char was placed in the vertical tube reactor, which was heated to the desired temperature (750 °C) in a gentle flow of nitrogen (250 cm3/min). The heating of the char up to 750 °C was done by following the same temperature program and had been followed before during the biomass charring. At 750 °C, steam was introduced from the bottom of the reactor. The char was then heated in a gentle flow of steam and nitrogen for 30 min in the case of olive residue and 45 min in the case of wheat straw. The steam flow used was 1.2 cm3/min in the case of olive residue 4448
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magnesium. The lignite was seen to contain lower amounts of volatiles and higher amounts of fixed carbon compared to the biomass samples. The moisture and ash contents of the lignite were found to be close to those of the wheat straw biomass. The lignite ash was seen to be rich in silicon, aluminum, calcium, and sulfur, while it also contained substantial amounts of sodium, magnesium, and iron. Tables 3 and 4 present the results from the production of activated carbon from the untreated and the leached biomass samples. As it was seen, wheat straw had higher yield compared to olive residue during the charring stage as well as during the steam activation stage. The surface area of the activated carbons was determined by the iodine number, which was used to investigate the effect of the conditions used for preparing the activated carbons, on the surface area of the carbons. The surface area (iodine number) was determined using the ASTM D 4607 standard method. The carbons were first ground to pass through a 200 mesh sieve. The iodine number of the activated carbons produced from the untreated olive residue and wheat straw samples was 766 and 443 mg I2/g, respectively. The iodine number for the activated carbons produced from the leached olive residue and the leached wheat straw biomass was found to be 299.2 and 223 mg I2/g, respectively. The activated carbon made from the untreated olive residue was seen to be superior to NORIT DARCO Hg commercial activated carbon, which has an iodine number in the range of 500-600 mg I2/g, as well as to the activated lignite carbon, which had an iodine number of around 275 mg I2/g. The activated carbon made from the untreated wheat straw was also seen to be superior to the activated lignite carbon. The activated carbons produced from the leached biomass samples were found to have more than 50% lower iodine numbers compared to the carbons derived from the untreated biomass samples. One reason for this could be that the significantly lower amount of alkali metals in the ash of the leached samples may influence the formation and the structure of the pores formed during the activation process, and as a result, the special surface area is also smaller. However, the iodine number for the leached olive residue carbon was still higher compared to the lignite carbon, while the iodine number for the leached wheat straw sample was seen to be lower. All the activated carbons prepared from the untreated and leached olive residue and wheat straw biomass were tested in the bench scale mercury test system to determine the
from permeation sources. These permeation sources (VICI Metronics) were kept under constant temperature and flow conditions. Equipment in use on the bench-scale flue simulator included a PSA conditioning and conversion unit and a PSA Sir Gallahad Online Mercury Analyzer. This analyzer was capable of detecting mercury at levels around 10 ng/m3 and used a gold trap to separate Hg0 from the sample stream, allowing the analysis method (atomic fluorescence spectrometery) to take place in an inert gas.
Results and Discussion Table 1 presents the analysis and characterization of the biomass and the lignite materials, while Table 2 presents the ash elemental analysis. The properties of the biomass materials were determined with the use of standard ASTM methods. The ash analysis was performed using the AAS analysis method. As it can be seen from Table 1, both biomass samples contained a large amount of volatiles and they also had low fixed carbon and ash contents compared to the lignite coals that are normally used for the production of activated carbons for mercury capture. The leached biomass samples were found to contain less ash and more volatiles compared to the untreated biomass samples. The ash elemental analysis in Table 2 showed that both untreated biomass materials contained a large amount of alkali metals as well as silicon, calcium, and chlorine. They were also found to contain average amounts of sulfur and also small amounts of other compounds such as iron, magnesium, aluminum, and titanium. The leached olive residue sample was seen to contain no chlorine and low amounts of alkali metals while the majority of its ash was constituted of silica, calcium, and sulfur. The leached wheat straw sample was seen to contain 34% less chlorine and around 60% less alkali metals compared to the original wheat straw sample. The majority of the leached wheat straw ash was silicon, potassium, and calcium as well as Table 1. Proximate and Elemental Analysis of Wheat Straw, Olive Residue, and Lignite Samples proximate analysis (% dry basis) moisture ash volatiles fixed carbon elemental analysis nitrogen carbon hydrogen sulfur chlorine oxygen gross calorific value (MJ/kg) a
wheat straw
leached olive leached wheat straw residue olive residue lignite
8.1 7.6 76.0 16.4
5.75 5.87 80.65 13.48
9.5 4.6 76.0 19.4
0.79 43.7 5.08 0.43 0.44 41.96 18.91
0.57 46.26 5.31 0.20 0.1 41.68 20.04
1.36 50.7 5.89 0.3 0.18 36.97 21.21
11.13 2.73 78.7 18.57
12.89 9.26 49.26 41.48
1,64 51.1 5.58 0.3 0.01 38.6 21.28
1.24 64.34 4.46 1.31 nda 19.39 24.7
Table 3. % Yield Based on Dry Mass and Conditions of Chars Produced from Untreated and Leached Olive Residue and Wheat Straw
nd: Not determined.
samples
initial mass
dry mass
pyrolysis mass (600 °C)
% yield
olive residue leached olive residue wheat straw leached wheat straw
457.72 936 128.2 113.8
411.95 894 115.38 102.5
128.1 288.7 39.8 36.28
31.1 32.3 34.5 35.4
Table 2. Ash Elemental Analysis of Wheat Straw, Olive Residue, and Lignite Samples samples
K2O
Na2O
CaO
MgO
SiO2
Al2O3
Fe2O3
TiO2
SO3
Cl
olive residue leached olive residue wheat straw leached wheat straw lignite
27.23 10.033 15.86 8.94 0.48
4.17 0.05 4.3 0.33 8.04
10.22 23.94 14.4 2.7 17.6
3.79 0.92 2.67 2.49 6.22
32.6 39.32 39.2 84.7 23.8
2.96 0.93 1.95 0.00 13.5
1.9 2.82 0.38 0.13 9.52
0.1 0.16 0.1 0.01 0.48
4.97 5.85 5.27 1.24 18.91
1.43 0.01 2.73 1.78 nda
a
nd: Not determined.
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Table 4. % Yield and Conditions of Activated Carbons Produced from Untreated and Leached Olive Residue and Wheat Straw samples
initial dry mass
activation mass
% yield
steam rate (cc/min)
activation time (min)
activation temperature (°C)
olive residue leached olive residue wheat straw leached wheat straw
128.1 288.7 39.8 36.28
97.4 225.7 35.4 31.7
23.64 25.3 30.68 30.9
1.2 1.2 10.5 10.5
30 30 45 45
750 910 750 800
Table 5. Composition of the Simulated Flue Gas Used in the Mercury Capture Tests gas components
O2
CO2
NO
NO2
SO2
HCl
H2O
Hg0
concentrations
6%
12%
120 ppm
6 ppm
600 ppm
1 ppm
15%
14 μg/m3
wheat straw carbon was seen to be lower compared to the NORIT DARCO Hg commercial activated carbon but still remained significant capturing more than 50% of the mercury in the flue gas stream during the whole test period. The untreated wheat straw carbon was seen to have a more stable adsorption behavior compared to the untreated olive residue carbon during the Hg capture testing. However, although the untreated wheat straw was seen to have substantially higher chlorine content compared to the untreated olive residue, the activated carbon derived from the untreated olive residue biomass was seen to have higher mercury capture efficiency. This indicated that the higher alkali metal content appeared to have a higher impact on the capture efficiency of the activated carbons compared to chlorine content. The results from the adsorption tests with the carbons derived from the leached biomass samples showed that leaching had a rather negative effect on the capture efficiency and lifetime of the specific activated carbons. The activated carbon derived from the leached olive residue was seen to exhibit higher capture efficiency at the first steps of the test, but this was found to decrease fast down to 50% after 30 min of testing. The capture efficiency remained at the same levels for another 50 min when the specific carbon was seen to break through and to lose all of its capture efficiency. On the other hand, the activated carbon derived from the leached wheat straw biomass was found to give better results. As it is seen from Figure 3, the specific activated carbon was seen to have Hg capture efficiency close to 50% during the whole testing period. It was also seen to have higher lifetime compared to the activated carbon derived from the untreated wheat straw biomass, as well as compared to the lignite activated carbon. The deterioration of the capture efficiency of the activated carbons produced from the leached biomass samples and the limited lifetime in the case of the carbon derived from the leached olive residue biomass was attributed mainly to the conditions applied during the activation process. The very high activation temperatures applied, 910 °C, in the case of the leached olive residue, and 800 °C, in the case of the leached wheat straw samples, led to a substantial reduction of the available alkali metals in the ash of the specific biomass samples after the leaching process to be engaged in catalytic activity. As it is well-known at temperatures higher than 800 °C in the presence of average to large amounts of silica, the alkali carbonates start to melt and decompose reacting partially with the silica to form alkali silicates and producing CO2 and K2O that evaporate, thus, reducing the available alkali for catalytic activity.30,31 In the case of the leached
Figure 3. Results of bench-scale mercury capture screening of carbons made from (a) olive residue, (b) wheat straw, (c) leached olive residue, (d) leached wheat straw, (e) DARCO FGD, and (f) lignite derived carbon (600 °C char, 850 °C activation) under low acid simulated flue gas conditions.
mercury capture capability of the biomass carbons compared to carbons made from coal. Bench-Scale Mercury Capture Tests. The steam-activated carbons were ground to pass through a 400 mesh sieve in preparation for the bench-scale mercury capture screening. The sample was tested at 135 °C under a simulated flue gas with the following composition, as it is seen in Table 5. Nitrogen was used as the carrier gas complementing the flue gas concentration up to 100%. Figure 3 presents the results from the mercury capture tests performed in the bench-scale system at the EERC. The results were given as percentage of inlet mercury for the different activated carbon during the different mercury capture tests. Each testing period lasted for at least 2 h or until the tested carbon showed signs of complete deactivation. The results from Figure 3 showed that outlet total mercury concentration dropped to a minimum of approximately 18% after 30 min during the test with the NORIT DARCO Hg commercial activated carbon. The NORIT DARCO Hg continued to adsorb mercury at the same level during the whole testing period. The activated carbon from the untreated olive residue biomass was seen to capture mercury as good as the commercial grade NORIT DARCO Hg after 30 min of testing. After that point, the capture efficiency of the untreated olive residue carbon was seen to slowly decrease while it remained comparable to that of the NORIT DARCO Hg until the completion of 1 h of testing. The untreated olive residue carbon was seen to capture more than 78% of the total mercury in the flue gas stream after one hour of operation, while its capture efficiency dropped slowly down to 50% by the end of the 2 h testing period. The mercury capture efficiency of the untreated
(31) Arvelakis, S.; Jensen, P.-A.; Dam-Johansen, K. Simultaneous Thermal Analysis (STA) on Ash from High Alkali Biomass. Energy Fuels 2004, 18 (4), 1066–1076.
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of the ash particles derived after the ashing of the leached material at 600 °C contains mainly calcium and the amount of alkali metals there was seen to increase only after heat treatment at temperatures higher than 900 °C that initiated the alkali melting/decomposition reactions and transferred alkali metals from the interior of the ash particles to the surface.30,31 As it was seen from Figure 4, the morphology of the ash samples analyzed with the SEM method supported the previous statements. The morphology of the ash sample derived from the untreated olive residue carbon activated at 750 °C was seen to have a looser texture, and the ash particles were seen to be loose and not agglomerated according to Figure 4a. On the contrary, the ash particles derived from the ashing of the leached olive residue carbon activated at 910 °C are seen to be agglomerated and to have a much denser structure due to the high temperature exposure, as it was seen in Figure 4b. Figure 4c showed that the ash particles derived from the untreated wheat straw carbon activated at 750 °C had started to agglomerate due to the high amounts of alkali metals, chlorine, and silica available in this particular sample.31 On the contrary, the structure of the ash derived from the leached wheat straw activated carbon at 800 °C was seen to have a much looser structure and the ash particles were seen not to be agglomerated due to the substantially lower alkali metals and chlorine content of the sample now. In addition, the mercury capture efficiency of the activated carbon derived from the leached wheat straw sample was seen to be affected marginally mainly due to the lower temperature of activation compared to the case of the leached olive residue, which allowed the alkali metals to remain in the sample, while the carbon lifetime was seen to be substantially improved compared to the other biomass and lignite activated carbons. The authors understand that, although there is evidence supporting that the elimination of the chlorine from the biomass samples during the leaching process may benefit the mercury capture efficiency of the derived carbons and although the amount of alkali metals contained in the specific carbons is also seen to decrease, the limited number of experiments performed in combination with the problems occurring during the experimental procedure and resulting in higher and variable activation temperatures compared to the original biomass creates some doubts and controversial results regarding the effect of the leaching process. We plan to repeat the production of the activated carbons using the leached biomass materials under the same conditions used in the case of the activated carbons produced from the original biomass and the mercury adsorption testing in order to verify the effect of the leaching process in the adsorption behavior of the biomass activated carbons when the appropriate funding is allocated. In addition, determination of the pore structure in the case of the biomass activated carbons will also assist us to determine the effect of alkali metals in the mercury adsorption behavior. The mercury capture efficiency of the activated carbon made from the lignite coal activated at 850 °C at the EERC was found to be lower compared to those of the olive residue and the wheat straw as well as the NORIT DARCO Hg carbons during the whole duration of the mercury capture testing. Its capture efficiency was seen to be close to that of the wheat straw carbon as well as the leached wheat straw carbon. According to Figure 3, the capture efficiency of the lignite carbon was seen to remain steady during the whole
Table 6. SEM-EDX Analysis of Ash from the Biomass-Derived Activated Carbons oxides
olive residue
leached olive residue
wheat straw
leached wheat straw
Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO FeO Cl
1.07 3.50 6.01 9.46 10.36 21.05 9.83 33.53 4.12 1.06
1.36 4.49 1.68 6.97 4.94 9.33 19.17 48.31 2.87 0.89
8.09 4.49 5.56 43.12 6.59 2.81 15.26 8.95 1.58 3.56
0.73 1.27 nda 84.03 nd 3.86 6.54 3.57 nd nd
a
nd: Not detected.
olive residue, the substantially higher activation temperature compared to the case of the leached wheat straw resulted in a substantially lower amount of alkali metals available for catalytic activity after the end of the activation process. In the case that the activation temperature had not exceeded the set temperature of 750 °C in the case of the leached biomass samples, then, a substantially higher amount of alkali metals would have been available during the mercury adsorption testing to act as a capturing agent. This was more evident in the case of the leached olive residue where the activation temperature reached a maximum of 910 °C, while in the case of the leached wheat straw, the substantially lower maximum temperature reached during the activation process allowed for a higher amount of alkali metals to be available for mercury capture, and while the capture efficiency was seen to be a little lower compared to the untreated wheat straw, the lifetime of the leached wheat straw carbon was seen to be substantially higher now. The averaged results from the SEM-EDX analysis of the activated carbons ash are presented in Table 6. As it was seen from Table 6, the alkali content in the ash of the activated carbon was seen to decrease by more than 20% compared to the amount in the ash of the leached wheat straw biomass. It also decreased more than 60% compared with the amount of the alkali metals found on the surface of the ash derived from the activated carbon from the untreated wheat straw, while the amount of chlorine there was seen to be eliminated. However, this did not affect significantly the capture efficiency of the carbon derived from the leached wheat straw while its lifetime was seen to be considerably higher now, as it was seen from Figure 3. This showed that the elimination of chlorine due to the higher activation temperature of 800 °C versus 750 °C in the case of the untreated wheat straw, potassium chloride melts at 770 °C and starts to evaporate,31 had a positive effect on the behavior of the carbon derived from the leaching wheat straw despite the lower alkali concentration compared to the carbon derived from the original wheat straw. In the case of the leached olive residue, the alkali % on the surface of the ash particles analyzed in the case of the activated carbon from the leached olive residue was seen to increase considerably compared to the case of the activated carbon deriving from the untreated olive residue. This was attributed to the high activation temperature of 910 °C that initiated the reaction of alkali carbonates with the silica in the ash as well as the decomposition and evaporation of the alkali carbonates and shifted alkali to the external surface of the ash particles. As it has been found during previous research in the case of the leached olive residue, the surface 4451
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Figure 4. SEM of ash derived from activated carbons: (a) olive residue, (b) leached olive residue, (c) wheat straw, and (d) leached wheat straw.
test. The lifetime of the specific carbon was seen to be close to that of the carbon from the untreated wheat straw, improved compared to the carbon from the leached olive residue sample and smaller compared to the carbon from the leached wheat straw biomass.
Optimization of the activated carbon production process such as a higher activation period is anticipated to further improve the capture efficiency as well as the lifetime and capture capacity of the activated carbons derived from biomass compared to the commercial catalysts. There is some evidence that the amount of chlorine in the activated carbon is not so important regarding its mercury capture efficiency as is the amount of the alkali metals. In some cases, such as in the case of the wheat straw deriving carbons, there is evidence that the removal of chlorine can actually increase the lifetime of the activated carbons while the capture efficiency remains similar. The effect of leaching on the mercury capture efficiency of the activated carbons produced from the leached biomass samples was not well understood mainly because of the high activation temperature applied that led to an alkali reaction and subsequent loss of efficiency in the produced activated carbons. The effect was seen to be higher in the case of the olive residue due to the significantly higher activation temperature applied. In
Conclusions The produced activated carbon from the European untreated olive residue and wheat straw biomass was comparable or superior to commercial grade activated carbon (NORIT DARCO Hg) for surface area, and the activated carbon from the untreated olive residue was comparable to the NORIT DARCO Hg for mercury removal during the first hour of operation, as it was seen in Figure 3. Both the untreated biomass activated carbons had higher mercury capture efficiency as well as longer lifetime compared to the lignite activated carbon prepared at 850 °C at EERC, though they were prepared at lower temperatures and using shorter activation periods. 4452
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the case of the wheat straw, the leaching pretreatment shows improvement of the activated carbon lifetime despite the small decrease of the capture efficiency mainly due to the high activation temperature applied. This provides evidence that the leaching pretreatment may be beneficial regarding the preparation of high quality activated carbons for mercury capture from biomass in the case that the activation temperature is kept low to avoid initiation of alkali melting/decomposing reactions. The amount of available alkali in the ash of the activated carbons as well as its chemical form is believed to be the critical factor regarding the capture efficiency and capacity of the biomass activated carbons. The special surface area of the activated carbons does not seem to have a significant effect on the capture efficiency and lifetime of the carbons. The
activated carbon produced from olive residue and wheat straw may be a viable and competitive product in the market and a cheaper source of raw material compared to the commercially available lignite-based carbons. Acknowledgment. Although the research described in this article has been funded wholly or in part by the United States Environmental Protection Agency through Grant CR-83092901 to the University of North Dakota Energy and Environmental Research Center, it has not been subjected to the Agency’s required peer and policy review and, therefore, does not necessarily reflect the views of the Agency and no official endorsement should be inferred. The financial support from the European Union through the MEIF-CT-2005-025711 BIO-PRETREAT Marie Curie research grant is also acknowledged.
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