Simultaneous Determinations of Sulfur and Heat Content of Coal Revitalization of a Traditional Physical Chemistry Experiment Michael R. ~ueller'and Kent L. McCorkle Rose-Hulman Institute of Technology, Terre Haute, IN 47803 Determining the heat of combustion of a substance using a Parr oxygen bomb calorimeter is a traditional expen: ment performed in many undergraduate physical chemistrv laboratories. The techniaue involves first determinine the heat capacity of the calorimeter by combustion of a nure substance with a known heat of combustion such as benzoic acld and then determ~ntngthe heat of combustion of another substance such as naohthalene r l . 21. The onlv real laboratory challenge for the student iB to obtain H value close to that listed in the literature. Since the student is asked to measure a quantity that is well established. there is little sense of discoverv. Coal is the product of the partial decomposition of prehistoric terrestrial flora via numerous biochemical and physical processes. Sulfur in coal (typically 1-5% by mass of d m coal) is found in the form of organic and inorganic compounds. It is reasonable to assume that much of the organic and some of the inorganic forms of sulfur in coal originates from the original plant tissues and bacterial reduction of dissolved sulfates. Some inorganic forms of sulfur such as FeSl (write) and ZnS (sohalerite) in the form of fissures w i t h h &e coal are usualiy the result of hydmthermal deposition. These last forms are routinely removed a t the mine site by pulverizing and sifting the coal. I n the following experiment, the student is asked to measure the sulfur and heat content of a coal sample of approximately 1-5% sulfur by mass. The sulfur content refers to the sulfur that is ..free% form sulfur oxides upon combustion and any sulfate forms already present in the conl such as CaSO,.2H,O (&ypsumi.Sulfates arc, however, typicnlly less than lrr of the, total amount of sulfur present in coal (datum obtained from thc Illinois State Geolo~lcalSur;eyZ) which will have a negligible effect on the r k u l t s obtained herein. Since the composition of coal is dependent
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on the original flora and geological processes fmm which it was formed, there is no "set" value for the combustible sulfur content or heat value in the literature. This type of measurement is nerformed bv industrial chemists for coal burning industries, such a s in electric generation, in comnliance with local and federal sulfur oxide emission controls, and hence increases a student's sense of accomplishment. The kchniaue used bv these chemists is soecified bv the American society for Testing and ~ a t e r i a l s ' ~ u b l i s h e d under ASTM Dcsimat~onsD3176-89 13, and D'3177-89 (41. Though the ASTM technique is a more accurate analysis, the proposed experiment is more amenable to an undergraduate laboratory. The heat canacitv of the calorimeter is first determined with a standard suih as benzoic acid. The combustion of a coal pellet pmceeds with a small amount of water added to the bottom of the bomb. The heat content is determined in the usual fashion ( 1 . 2). The sulfur content is determined in the solution formed following combustion of the coal by standard gravimetric analysis with BaClz (5) selectively precipitating sulfur oxides in the form of BaSOa (KT for BaS04 a t 25 OC is 1.07 x 10-lo (6)) over the nitro en oxldes (the solubility of Ba(N0J2 a t 20 'Cis 8.7 g mL- (6)).
B
Procedure The method of determining the heat capacity of the bomb calorimeter and the heat of combustion of a sample is already well-described in many physical chemistry laboratory textbooks ( I , 2);hence, it will not be detailed here. The net temperature rise and radiation correction calculations 'Author to whom correspondenceshould be addressed. 2~~~inois Basin Coal Sample Program, State Geological Survey,615 East Peabody Drive. Champaign, 11 61820.
Volume 71
Number 2 February 1994
169
Table 1. Results from a Low-Sulfur Coal Sample
Trial
Heat of Combustion
%Sulfurby Mass
kg SO2iGJ
Table 2. Results from a High-Sulfur Coal Sample
Trial
Heat of Combustion (kJ g-'1
%Sulfur by Mass
kg SO?/GJ
added to the BaClz solution. The precipitant is filtered, washed and dried in a filtering crucible or with ashless filter paper (5). Results and Discussion There is essentially no interference in the sulfur determination from the co-precipitation of BaC03 because the resulting solution is highly acidic (pH < 1)making COz essentially insoluble (8).Hence, the massing of the final precipitant corresponds directly to the amount of combustible s&r in the &a1 sample. ~ i h l e 1 s and 2 portray the reproducibility obtained usinx this technique on low and high sulfur coal samples, respectively. Because coal is a n energy source, i t is practical to discuss the ratlo of'sulfur oxldes generated to heat energy generated. Thc pcrwnt sulfur and hcot content ofthe coal can he converted readily into mass of sulfur dioxide per heat unit by assuming that all of the sulfur oxides are in the form of SO?. The results of this calculation are shown in Tahles 1 and 2. The air quality regulations require that less than 0.517 kaSOdGJ (9)are allowed to be emittedfmm the flue gas w i t 6 o u t h i z i n g some type of desulfurization of either the coal prior to comhustion andlor the flue gas. The method described is both a n accurate and convenient means of simultaneouslv determinina the heat and combustible sulfur content of coal. This experiment can be utilized in the physical and environmental chemistnr lab. . oratory. Coal samples along with data on its composition can be obtained from a number of different s ~ u r c e sThe . ~ coal samples are mined from different locations in the United States and around the world. There is typically a nominal fee for obtaining coal samples with composition data. ~~~
for bomb calorimetry are specified under ASTM Designation D240-87 (7). Vials wntaining ground coal (approximatelv 10 a) are distributed to the students: thevmust further gknd'the coal into a powder with a mortar"and pestle makina the s a m ~ l homoeeneous. e Coal el lets are made of no less-than 1;and no more than 1.5; total mass. This ensures adequate sulfur content in the sample for a n accurate weighing. The Parr bomb calorimeter is assembled in the usual fashion except approximately 1mL of water is added to the bomb prior to sealing. This ensures the bomb is saturated with water vapor tomaximize condensation of the sulfur oxides. Following comhustion of the coal pellet, a 114-in. Tygon tube is attached to the prcssure release valve of the hornb, and the gas trapped in the bomb is slowly buhhled throueh aooroxirnatelv 10 mLof0.5 M B a C l solution acidified ro P H = 1 with "kNo3. The water in t h e bomb and quantitative washing thereof are first filtered to remove flyash and other inorganics that do not combust and then
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Acknowledgment The authors would like to thank PSI Energy, Wahash River Generating Station, West Terre Haute, Indiana, for the donation of the ground coal samples. Literature Cited
24.
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3. ASTM D3176-89. A n n u d Book o,fASTM Smndonis; Philadelphia, 1991: Vol. 05.05,
3Coal ana Organ c Pelrology Laboratory. Pennsy vanla Stare Un verslly 205 Researcn Bdg E, Un verslly Parn, PA 16802. 'Dutch Control for Coal Specirnan, Rinburgerleg 2, post bus 151, 6470 ED Eygelshoven, Netherlands. 'Coal Analysis Chemistry Laboratory, Chemistry Division. Building 211 Aroonne National Laboratow. Aroonne. IL60439. 6~eiterfor~pplied Energy fleieaGh, ~njversit~ of Kentucky, 3572 Ironworks Pike, Lexington. KY 40511.
170
Journal of Chemical Education
pp 332-334. 4. ASTM 03177-89, Annual Bmk ofASTM Standards: Philadelphia. 1991; Vol. 05.05,
7. ASTM D240-87,AnnuolBoak ofASTM Standards;Philadelphia, 1991: Vol. 05.01, pp 114-118.
8.
Manahan.5. E.EnuimnmrnfolCk~mialry,3rd ed.:W3lardGrant:Boston. 1979: p 22.
9. Moore. J. W ; Moore. 1976: p 97.
E. A. Enuimnmnf.1 Chmistry; Academic Press: New York
