Energy & Fuels 1994,8, 399-401
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Behavior of Chlorine during Coal Pyrolysis Dakang Shao, Erik J. Hutchinson, Haibin Cao, and Wei-Ping Pan* Center for Coal Science, Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky 42101
Chen-Lin Chou Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820 Received August 18,1993. Revised Manuscript Received November 5, 199P The behavior of chlorine in Illinois coals during pyrolysis was evaluated by combined thermogravimetry-Fourier transform infrared spectroscopy-ion chromatography (TG-FTIR-IC) techniques. It was found that more than 90% of chlorine in Illinois coals (IBC-103, 105, 106, and 109) was liberated as HC1 gas during pyrolysis from 300 to 600 "C, with the rate reaching a maximum at 440 OC. Similarity of the HCland NHarelease profiles during pyrolysis of IBC-109supports the hypothesis that the chlorine in coal may be associated with nitrogen and the chlorine is probably bonded to the basic nitrogen sites on the inner walls of coal micropores.
Introduction The main chemical forms of chlorine present in coal that have been proposed are as follows:l (1) inorganic chlorides; (2) chlorine ions in brine and other water associated with the coal. In 1991, Chou investigated the distribution of chlorine in Illinois coals.2 The chlorine content in coal lithotypes of a column of Illinois coal varies from 0.13 to O H % , and two forms of chlorine occur in coal: (1)chloride ions (Cl-1; (2) chlorine enriched in organic matter, most likely as chloride ions adsorbed on the inner surfaces of the micropores in macerals. The adsorbed chloride ions probably occur as hydrogen chloride (HC1)associated with positively charged nitrogen functional groups and may be held in the diffuse electrical double layer. The charge of adsorbed chloride ions may be balanced by hydrogen ions, not by sodium ions. Few studies have been done on the behavior of chlorine in coal during pyrolysis and combustion. Edgcombe investigated the evolution of chlorine as hydrogen chloride on heating 29 British coals (0.2-1.0% C1) in dry air at 200 "C for 24 h.8 He concluded that more than half of the chlorine in the coals was liberated as HC1 in air (combustion) at 200 OC, but not in nitrogen (pyrolysis) at the same temperature. On the contrary, Daybell indicated that chlorine in some British coals is liberated as HC1 in oxygen-free nitrogen (pyrolysis) at 200 OC.4 Gibbb concluded, however, that British coals give off 97% of its chlorine as HC1 in oxygen-free nitrogen (pyrolysis) at 258 OC. In 1992,Muchmore et al. investigated chlorine removal from an Illinois high-chlorine coal, IBC-109 (0.42% Cl), with thermal treatment followed by analysis using an electrode technique? They concluded that 84.3 % of the Abstract ubLhed in Advance ACS Abstracts, January 1, 1994. (1) holey, Halogen Emissions from Coal Combwtion; IEACRI46, IEA Coal Reaearch London, Feb. 1992. (2) Chou, C. L. Distribution and Forms of Chlorine in Illinoh Coale. In Chlorinein Coal; Stringer, J., Banerjee, D. Do,Ede.;Coal Science and Technology 17; Elaevirr: New York, 1991; pp 11-29. (3) Edgcombe, L. J. Fuel 1966, 36,38-48, (4)Daybell, G. N. J. Inst. Fuel 1967,40,3-6. (5) Gibb, W. H.; Angus, J. G. J. Inst. Energy 1988,63, 109.
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Table 1. Compositions of Illinois Basin Coal Sampler comDosition/coal IBC-103 IBC-105 IBC-106 IBC-109 % moisture 6.70 9.40 10.40 9.20 8.70 % ash 18.60 9.00 8.20 36.10 36.80 % vol matter 39.70 35.00 74.48 63.60 % carbon 71.86 75.05 5.04 4.58 % hydrogen 4.93 4.89 1.73 1.22 % nitrogen 1.67 1.74 2.30 4.55 % sulfur 3.77 1.13 7.56 7.39 % oxygen 8.76 8.63 0.18 0.10 % chlorine 0.02 0.42
total chlorine was removed from the coal by preheating the coal in a nitrogen gas flow at lower temperatures prior to a six minute reaction at 385 OC. There is, however, a lack of data concerning the dynamic behavior of chlorine evolution during pyrolysis and combustion of Illinois coals. The purpose of this study is to identify gaseous chlorine species from the mixtures of volatile species evolved and the kinetics of their release during coal pyrolysis. The results should lead to a better understanding of the relationship between the level of chlorine in coal and boiler corrosion. Chlorine evolution during pyrolysis of Illinois coals was studied using simultaneous TG-FTIR and TGIC techniques.
Experimental Section Four coal samples from the IllinoisBasin Coal Sample Program were used in the experiments of coal pyrolysis. The coal eamples (calledIBC coale for short) contain 0.02-0.42% chlorine on a dry basis. The compositions of the coal eamples are shown in Table 1. By using the combined TG-FTIR instrumental system, the volatile species produced on a DuPont 961 TG during pyrolysis of a coal eample were analyzed by a Perkin Elmer 1660 FTIR spectrometer. The system is able to continuously measure the mass change of the coal sample with the temperature increase at a heating rate of 10 W m i n (by TG) as well as to identify qualitativelyand determine quantitativelythe individual gaseous species such as HC1, NHa, COS, SOs, HsS, HsO, CHI, Cn&, C& (6) Muchmore, C. B.;Hesketh, H. E.; Chen, H. L. ThermalTreatment for Chlorine Removal from Coal. Technical Report, Center for Raaesrch on Sulfur in Coal Carterville, IL,August 1992; 20 pages.
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etc. (by FTIR) emitted during coal pyrolysis. The sample chamber in the TG furnace is connected to the 100 X 24 mm gas cell in the FTIR epectrometer through a 1mm diameter Teflon tube. Both the Teflon tube and the gas cell can be heated by heating coils up to 160 O C , and the temperaturecan be controlled continuously by temperaturecontrollers. The time lag between the sample chamber and the gae cell is 1min with a oxygen-free nitrogen flow at a rate of 60 mL/min during pyrolysis.' The FTIR spectrometer is also able to automatically scan the pyrolysis gas mixture, and the spectra can be taken automatically every minute or every 10O C during heatingby using GalacticIndustries Corp.'s LabCalc software.8 Calibrationof the FTIR spectra was made by scanningindividual pure gases or prepared gas mixtures. The TG-IC technique is based on a combined analytical method, in which thermogravimetry (TG)is able to monitor the mass change of a coal sample with temperatureduring pyrolysis and combustion, and ion chromatography (IC) quantitatively determines the concentration of gaseouschlorine speciesemitted in specific temperatureranges. The sample chamber on the TG furnace was connected with a series of buffered traps. Carried by a nitrogen flow during pyrolysis, the volatile species produced on TG was collected in three buffered traps connected in a series every 60 "C within the temperature range from ambient to 860 "C. Thus, the gaseous chlorine was trapped as chloride ions in approximately90 mL of the buffer solution (pH = 9.6) containing sodium bicarbonate (70 ppm)-sodium carbonate (16 ppm) and 1.6% HaOa. Then, each trapped buffered solution was diluted exactly to 100 mL, filtered through a membrane filter, and analyzed for chloride using the Shimadzu HIC-6A ion chromatograph system. The Shimadzu HIC-6A system consists of a liquid chromatograph (LC-600,a pump), a conductivitydetector (CDD-GA),an ion chromatograph (HIC-GA), a column with a guard column (Shim-pack IC-All, and a data processor for chromatography (Chromatopac CR601). Before a trapped buffered solution was analyzed for chloride concentration,the ShimadzuHIC.6A system was calibratedby analyzing a standardbuffer solution containing 1.00ppm of chloride and 1.00 ppm sulfate. The standard Cl/S buffer solution was prepared with sodium chloride (reagent, A.C.S., crystals; Matheson Coleman & Bell), sodium sulfate (Fisher Laboratory Chemical, crystals, Cat. No. 5-420; Fisher Scientific Co.) and pure water. The calibration wae conducted by the absolute calibration curve method (made automatically by CR601). During the experiment, each trapped buffer solution (unknown) was analyzed by IC for chlorine content emitted in each 60 O C range. As a conmquence, the releaee profiles of gaseous chlorine during coal pyrolysis were establishedquantitatively by plotting the chlorine content vs. temperature (from ambient to 860 "C).
Results and Discussion
TO Results. The four coal samples have similar behavior of thermal decomposition during pyrolysis, but the amount of the weight loss varies from one to another, with different maximum weight loss rates and TmU's. The Tmuis the temperature at which the maximum rate of the weight loss occurs. Table 2 outlines the pyrolysis results of the TG/DTG runs of the four Illinois Basin coals from 22 to 960 "C. It is interesting to note that, within the temperature range of 22-940 OC, the observed total weight loss for the four coals increases in the order of IBC-109 (33.24%)
