Feasibility of a dutch process for microbial desulphurization of coal

Abstract

Abstract The technical and economical feasibility of microbial coal desulphurization has been studied with attention on the removal of finely dispersed inorganic sulphidic minerals which cannot be removed by sink-float processes. Data have been collected on the suitability of various low-sulphur coals representative of future Dutch imports. It was possible to remove 90% of the pyrite present within 8 to 10 days by using a mesophilic, pyrite-oxidizing, microbial system. No removal of organic sulphur was observed. Depending on the type of coal, a considerable reduction in the contents of ash-forming minerals and heavy metals was achieved. To determine the optimal dimensions of a reactor system, the kinetics of bacterial growth and pyrite oxidation were studied. The kinetics of bacterial pyrite oxidation appear to be first order in the amount of pyrite present, indicating a plug flow reactor is most appropriate. However, because almost all of the bacterial biomass is attached to the solids, incoming coal in a continuous working system can be inoculated only by intensive contact with particles already loaded with bacteria. For this purpose, a plug flow reactor is not adequate, and a configuration consisting of a mixed flow reactor followed by a plug flow is proposed. To prevent biomass limitation, the residence time in the mixed flow reactor must be longer than the reciprocal value of the specific growth rate of the pyrite oxidizing bacteria, 0.05 h −1 at pH 1.8. The slow kinetics and the enormous amounts of coal to be treated require huge reactors. To make the process economical, the reactors must be simple and cheap. The choice is a cascade of Pachuca tank reactors. The first tank functions as the mixed flow reactor. The following tanks, in series, approximate a plug flow. Studies on oxygen mass transfer, mixing and settling in laboratory scale Pachuca tank reactors revealed that the air flow required for the oxygen supply of the pyrite-oxidizing microflora is enough for adequate suspension and mixing of the coal in the water and to prevent settling. From the kinetic and technological studies a process design has been derived. This has been adopted by an engineering consultancy firm to calculate the cost of the process. It appears that, depending on the coal types to be cleaned, the location and governmental environmental regulations, microbial desulphurization might be a realistic option.