Energy & Fuels 2003, 17, 369-377
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Comparison of High-Unburned-Carbon Fly Ashes from Different Combustor Types and Their Steam Activated Products Yinzhi Zhang, Zhe Lu, M. Mercedes Maroto-Valer,* John M. Andre´sen, and Harold H. Schobert The Energy Institute and The Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, 405 Academic Activities Building, University Park, Pennsylvania 16802 Received August 12, 2002. Revised Manuscript Received December 16, 2002
Fly ashes of high-unburned-carbon content derived from coal-fired combustors are an increasing problem for the utility industry, since they cannot be marketed as a cement extender, and therefore, have to be disposed. A series of six unburned carbon samples from different combustors was collected and characterized by elemental analysis, petrographic composition, nitrogen adsorption isotherms, and thermogravimetric analysis. The elemental analyses show that all the unburned carbon samples consist mainly of carbon with very little hydrogen, nitrogen, sulfur, and oxygen. There appears to be a correlation between the C/H atomic ratio and the porous texture properties, where the lowest C/H ratio corresponds to the highest specific surface area and pore volume, and the smallest pore size. In addition, the potential use of unburned carbon as a precursor for activated carbon (AC) was investigated. Activated carbons with specific surface area up to 1270 m2/g were produced from the unburned carbon. The porosity of the resultant activated carbons was related to the properties of the unburned carbon feedstock and the activation conditions used. It was found that not all the unburned carbon samples are equally suited for activation, and furthermore, their potential as AC precursors could be inferred from their physical and chemical properties. The developed porosity of the activated carbon was a function of the oxygen content, porosity, and H/C ratio of the parent unburned carbon feedstock. It was observed that extended activation times and high activation temperatures increased the porosity of the produced activated carbon at the expense of the solid yield.
Introduction With the ever-expanding market for activated carbons (AC) due to their widespread number of environmental applications, manufacturers of AC are constantly seeking cost-effective and abundant feedstocks. Such feedstocks include industrial byproducts1,2 and wastes from agricultural and municipal sources.3-6 These byproducts are competitive precursors for AC manufacturers and simultaneously reduce waste disposal costs for other industries. One industrial byproduct of particular interest is fly ashes of high-unburned-carbon content from coal-fired combustors. The U.S. electric power industry relies heavily on the use of coal as the main energy source, where coal-fired units generate over 53% of the total electricity produced annually.7 Nevertheless, for coal to continue being a * Corresponding author. E-mail:
[email protected]. (1) Haghseresht, F.; Lu, G. Q. Energy Fuels 1998, 12, 1100-1107. (2) You, S. Y.; Park, Y. H.; Park, C. R. Carbon 2000, 38, 1453-1460. (3) Moreno-Castilla, C.; Carrasco-Marin, F.; Lopez-Ramon, M. V.; Alvarez-Merino, M. A. Carbon 2001, 39, 1415-1420. (4) Sanchez, A. R.; Elguezabal, A. A.; Saenz, L. D. Carbon 2001, 39, 1367-1377. (5) Nagano, S.; Tamon, N.; Adzumi, T.; Nakagawa, K.; Suzuki, T. Carbon 2000, 38, 915-920. (6) Cunliffe, A. M.; Williams, P. T. Energy Fuels 1999, 13, 166175. (7) Hong, B. D. Mining Eng. 1998, 50 (5), 60-68.
long-term energy source, it must meet current environmental challenges, such as emissions of pollutants such as NOx and nondisposal of waste streams, including high-unburned-carbon fly ashes. The implementation of Title IV of the 1990 Clean Air Act Amendments regarding NOx emissions is mainly being addressed in coal combustion furnaces by a combination of low-NOx burners and selective catalytic and noncatalytic reduction systems.8 Although low-NOx burner technologies effectively decrease NOx emissions by lowering the temperature of combustion, they also reduce the combustion efficiency with a corresponding increase in the concentration of uncombusted coal in the fly ash, generally referred to as unburned carbon.9 This results in the ash being unsuitable for the cement industry, since the unburned carbon tends to adsorb the air-entrainment reagents that are added to the cement to prevent crack formation and propagation.10 Consequently, the carbonrich ash is nowadays regarded as a waste product and its fate is mainly disposal.11 In 2000, about 57 million tons of fly ash was generated by the electric utilities in (8) The U.S. Department of Energy. Reducing emissions of nitrogen oxides via low-NOx burner technologies; Topic Report No. 5, 1996. (9) Tyson, S. S. Proceedings of Third Annual Conference on Unburned Carbon on Utility Fly Ash 1997, Pittsburgh, pp 3-5. (10) Hill, R. L.; Sarkar, S. L.; Rathbone, R. F.; Hower, J. C. Cem. Concr. Res. 1997, 27 (2), 193-204.
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the United States and only about 32% was used. Applications in the cement and concrete industries utilized over 50% of the total ash marketed, followed by structural fills and waste stabilization applications.12-14 An increasing role of coal as a source of energy in the 21st century will demand environmental and cost-effective strategies for the use of these carbonaceous waste products from coal combustion. There are already various commercial technologies to separate the unburned carbon from the ash.15,16 These separation techniques can generate ashes with carbon contents below 6% that are suitable for use in the cement industry, as well as concentrate the unburned carbon that could subsequently be used as feedstock for the production of AC. However, there are only a few studies that have looked into the activation of unburned carbon from coal-fired fly ash.17-20 The treatment of flue gases by activated carbon from fly ash has been previously reported.21 However, this work focused on oil-fired fly ash from the combustion of heavy oils, and did not include samples from coal-fired combustors. The composition and properties of the unburned carbon inherently depend on the type and origin of the source coal. Fly ashes are classified as class C and class F, which are generally generated from lignite/subbituminous coals and anthracite/bituminous coals, respectively.12 Furthermore, the combustion system in which the unburned carbon was produced and devolatilized may also affect its properties as a precursor for AC. There are three main types of combustion systems used in power plants: (1) fixed-bed firing, such as stokers; (2) suspension firing, such as cyclones and pulverized coal combustors (PCC); and (3) fluidized-bed combustion (FBC). Fluidized-beds are relatively new technologies, which have recently entered commercial markets,22 and therefore, are not yet widely used. Suspension-firing technologies, and particularly PCC, are the most widely used by electric utilities in the United States. In PCC, the coal is finely ground to less than 75 µm (80%) and air is blown into the combustor, where the residence time for the coal particles is only about 1-2 s, and the combustion temperature is around 1300-1400 °C when using low-NOx burners, and 1700-1800 °C without lowNOx burners.22 The amount of unburned carbon in the fly ash is generally a function of the combustion temperature as well as the rank of the coal utilized, where (11) Maroto-Valer, M. M.; Taulbee, D. N.; Hower, J. C. Energy Fuels 1999, 13, 947-953. (12) Sloss, L. L. Trends in the use of coal ash IEA coal research, CCC/22, 1999. (13) Giovando, C. Power 2001, 145 (1), 33-44. (14) Kalyoncu, R. S. Coal combustion products, U.S. Geological Survey Mineral Yearbook, 2000. (15) Ban, H.; Li, T. X.; Hower, J. C.; Schaefer, J. L.; Stencel, J. M. Fuel 1997, 76 (8), 801-805. (16) Groppo, J. G.; Robl, T. L.; Lewis, W. M.; McCormick, C. J. Miner. Metall. Process. 1999, 16 (3), 34-36. (17) Hwang, J. Y. Proceedings of Third Annual Conference on Unburned Carbon on Utility Fly Ash 1997, pp 47-49. (18) Graham, U. M.; Robl, T. L.; Rathbone, R. F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (1), 265-269. (19) Maroto-Valer, M. M.; Taulbee, D. N.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1999, 44 (1), 101-105. (20) Maroto-Valer, M. M.; Andre´sen, J. M.; Zhang, Y. In Advancing Sustainability through Green Chemistry and Engineering; Lankey, R. L., Anastas, P. T., Eds.; American Chemical Society Symposium Series, 2002; Vol. 823, Chapter 15, pp 225-241. (21) Davini, P. Carbon 2002, 40, 1973-1979. (22) Boram, G. L.; Ragland, K. W. Combustion Engineering; WCB McGraw-Hill: New York, 1998.
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high temperatures and low rank generate very low unburned carbon concentrations, while high ranks and low temperatures decrease the combustion efficiency.12 Mixtures of unburned carbon and fly ash are blown upward out of the PCC combustor and collected in hoppers. Cyclones are also used by the electric utilities, and they can be operated at temperatures as high as 2150 °C.22 This allows most of the ash to melt and flow along the inclined wall of the furnace and to be removed as a liquid slag, and therefore, only 20-30% of the ash is converted to fly ash, compared to over 80% for PCC.22 In the work reported here, a series of unburned carbon samples from different combustion units (PCC and cyclone) was collected and characterized. In addition, the suite of samples collected was activated and the produced ACs were compared. Our previous studies have shown that unburned carbon only requires a onestep activation process, since it has already gone through a devolatilization process while in the combustor.19,20 This is an important advantage compared to the conventional two-step activation process that includes a devolatilization of the raw materials, followed by an activation step. Therefore, only direct steam activation was performed in this study. The physical and chemical properties of the different unburned carbons were compared and correlated with the resultant steamgenerated ACs. Finally, a parametric study was conducted to ascertain the effect of activation time and temperatures on the porosity of the AC produced. Experimental Section Study Samples. Six unburned carbon samples, named FA1, PO, SH, DA, WE, and CFA, were collected from different combustors. The first five unburned carbons were from PCC units, while the last, CFA, was from a cyclone unit. FA1 was collected from The Penn State University pulverized coal-fired suspension firing research boiler (2 MM Btu/h) that uses a high volatile bituminous coal from the Middle Kittanning seam. PO, SH, DA, and WE were obtained from PCC located in Pennsylvania, Kentucky, and Wisconsin with a net capacity of 243 MW, 180 MW, 70 MW, and 136 MW, respectively. All the units use high-volatile bituminous coals from different seams. PO, SH, and WE were collected from the electrostatic precipitators, while DA was collected from the mechanical precipitators. CFA was obtained from a utility cyclone unit, and the sample was crushed, ground, and sieved, and the fraction between 100 and 200 mesh was collected and used for AC generation studies. Characterization of Unburned Carbon Samples. The loss-on-ignition (LOI) contents of the unburned carbon samples were determined according to the ASTM C311 procedure.23 Around 1 g of sample was oxidized in air for 3 h at 800 °C to constant weight in a muffle furnace. The LOI content was then calculated from the weight loss of the sample after oxidation. The LOI analyses were conducted in duplicate. Elemental analyses were conducted using a Leco CHN-600 analyzer, Leco Sulfur Determinator SC-132 and Leco Mac-400 Thermogravimetric Determinator, where the C, H, N, and S content (dry basis) of the calibration standards used were 62.79% ( 0.89%, 2.11% ( 0.06%, 0.95% ( 0.17%, and 1.00% ( 0.02% respectively. The petrographic analyses were performed on epoxybound polished pellets under polarized reflected white light at 625× magnification and oil-immersion using a Zeiss Uni(23) American Society for Testing and Materials, C311-00, Book of Standards Volume: 04.02, Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland-Cement Concrete.
Activation of High-Unburned-Carbon Fly Ashes from Combustors
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Table 1. LOI and Elemental Analysis (%daf) of the Unburned Carbon Samples Investigated sample
LOI
C
H
N
S
Oa
C/H atomic ratio
DA WE PO SH FA1 CFA
50.0 32.0 50.8 45.8 58.9 86.6
97.2 ( 0.1 97.3 ( 0.1 96.5 ( 0.7 98.8 ( 0.9 94.9 ( 0.4 97.7 ( 0.2
0.17 ( 0.02 0.23 ( 0.02 0.18 ( 0.06 0.13 ( 0.09 0.70 ( 0.16 0.02 ( 0.02
1.77 ( 0.02 1.25 ( 0.02 1.30 ( 0.01 1.30 ( 0.04 1.27 ( 0.01 1.4 ( 0.01
ndb 1.23 ( 0.02 1.16 ( 0.01 1.01 ( 0.02 0.63 ( 0.63 0.94 ( 0.01
0.86
