76 Energy zyxwvut & Fuels 1987,1, zyxwvu 76-79 Use of X-ray Computed Tomography To Examine Microbial Desulfurization of Lump Coal Clifford L. Spiro,* David S. Holmes, John Lobos, and Donald H. Maylotte General Electric Corporate Research and Development Laboratory, Schenectady, New York 12301 Received June 5, zyxwvutsr 1986. Revised Manuscript Received September 25, 1986 The bacterial removal of pyritic zyxwvu sulfur from in situ deposits of coal deserves attention. The process has the advantages of economy and extended periods of time to carry out the sulfur removal. For example, some projected coal reserves are not expected to be mined for another 25-50 years. Possibly a culture and appropriate medium could be admitted to the seam and be allowed to slowly permeate the coal for sufficient time that significant desulfurization occurs. A fair assessment of the in situ desulfurization kinetics and efficiency has not been made for wont of an adequate assay. Therefore, as a preliminary step to examine the feasibility of in situ pyrite removal we have investigated the bacterial removal of the pyrite from lump coal by using X-ray computed tomography to follow the dissolution of pyrite inclusions. It is concluded that X-ray computed tomography is an effective tool to assay in situ phenomena. In particular, mixed cultures of Thiobacillus ferrooxidans and associated acidophilic bacteria were used to treat a Lower Kittanning seam high-volatile A bituminous coal. It was demonstrated that pyrite inclusions can be removed at a depth of 17 mm within the lump over a 4-month period. Introduction The removal of pyritic sulfur from pulverized suspen- sions of coal by microorganisms such as Thiobacillus ferrooxidans is well established.’P2 In the laboratory, up to 90% of the pyrite can be removed by bacteria under optimal conditions of pH, nutrient supply, and tempera- ture.’-* Reaction rates can be enhanced by the use of finely ground coal, generally 200 mesh or less, and a maximum of 20% pulp density.’-4 Such reaction condi- tions are compatible with the emerging technology of coal-water slurries; hence, this has increased interest in the use of microorganisms as an alternative to the existing methods of pyritic sulfur removal. Another approach involves the use of microorganisms to remove or at least reduce pyritic sulfur from in situ deposits of coal. The rate of pyritic sulfur removal from in situ deposits does not need to be extremely fast since certain extensive coal reserves are not expected to be ex- ploited for another 25-50 years. The crucial questions are how fast and to what extent the pyritic sulfur can be re- moved during this period and what factors affect these parameters. Obviously, the accessibility of the coal to aqueous solutions and the removal of the resulting acid mine drainage are of prime importance. In order to fa- cilitate a feasibility study of in situ pyrite removal, we have explored the use of X-ray computed tomography as a po- tential candidate assay. A program to explore the use of X-ray computed to- mography (CT) as a diagnostic tool for fossil fuels has been initiated in our laboratory. CT is of profound importance to the field of medical diagnostics, representing a marriage of advanced radiology with computing technology. With this noninvasive technique, a two-dimensional map of X-ray attenuation coefficients is obtained for a given plane (1) Dugan, P. R.; Appel, W. A. Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomea; Murr, L. E., Torma, A. E., Brierley, J. A., Eds.; Academic: New York, 1978; pp 223-250. (2) Andrews, G. F.; Maczuga, J. Biotechnol. Bioeng. Symp. zyxwvuts 1982, zyxwvutsrq No. 12, 337. (3) Detz, C. M.; Barvinchak G. Min. Congr. J. 1979, 65, 75. (4) Hoffman, M. R.; Faust, B. C.; Panda, F. A.; Koo, H. H.; Tsuchiya, H. M. Appl. Environ, Microbiol. 1981, 42, 259. 0887-0624/87/2501-0076$01.50/0 within the object of interest. This is in contrast to a conventional radiograph which is a two-dimensional shadow graph obtained by projection through a three-di- mensional object. In CT, images are not degraded by planes adjacent to the plane of interest, and therefore, higher resolution and low-contrast details are re~overed.~ The experiment consists of passing a flat, 1.5-mm-thick plane of X-rays through the object of interest. The X-rays are attenuated by the object, the extent of which is mea- sured by a linear array of 517 gas-filled detectors. The entire array of detectors and the X-ray source rotate about the stationary object, and a second series of readings are made, each reading referred to as a “view”. Within 2-9 s, 576 such views are taken, each consisting of 517 indi- vidual X-ray attenuation data points. The images are reconstructed by tomographic mathematics by using a complex procedure whose description is beyond the scope of this text.6 The resultant image consists of an arry of “pixels”. Each 0.25 X 0.25 X 1.5 mm pixel has an x,y coordinate corre- sponding to its actual location within the plane of the object, as well as a 12-bit CT number, which measures the net X-ray attenuation coefficient within that specific region in space. A CT number of 0 means that all X-rays from every angle passed through that region of space unim- peded, while a CT value of 4095 represents a maximum X-ray attenuation. For reference, distilled water has a value of 1025. The X-ray tube spectrum ranges from 20 to 80 keV, so that the dominant mode of X-ray attenuation in coal is from Compton scattering. Because Compton scattering is dependent on average electron density and electron density is nearly a linear function of mass density for low atomic number elements, the CT technique rep- resents a measure of mass density in coal to a high degree of accuracy. When CT numbers are plotted as a function of mass density for a broad range of organic liquids, a nearly straight line is ~btained.~ (5) Newton, T. H.; Potts, D. G. Radiology of the Skull and Brain; C. V. Mosby: St. Louis, MO, 1981; Vol. 5. (6) Swindell, W.; Barett, H. H. Phys. Today 1977, 30, 32. (7) Maylotte, D. H.; Kosky, P. G.; Lamby, E. J.; Spiro, C. L. DOE Annual Report, Contract DE1AC21-82MC19210, in press. 1987 American Chemical Society