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Of coal and bioprocessing
The Organization for Economic Cooperation and Development describes biotechnology as “the application of scientific and engineering principles to the processing of materials by biological agents to provide goods and services”.
Biotechnology is quite complex due to its multidisciplinary approach, its roots being in the biological, chemical and engineering sciences. It arises from the combination of a number of specialist subjects, such as molecular genetics, microbial physiology and biochemical engineering.
Because of the many advances in the field of biotechnology, these techniques are applied in a number of novel areas. One of these is coal bioprocessing.
Coal does not appear to be an ideal material for biological attack. It is a highly variable material, being mainly a carbonaceous one with mineral inclusions. In fact, it is such a heterogeneous substance that microorganisms may be able to attack, degrade or use a number of different constituents present in it.
Bioprocesses may have significant advantages over conventional technologies in use today, and may open up the possibility of viable production of new products. This is because the reaction conditions involved would be a lot ‘milder’ than those required for the equivalent chemical transformations, besides increasing the removal of certain impurities.
The use of biotechnology can provide alternative process routes for coal conversion and/or provide alternate and viable pathways for the cleaning of coal (desulphurization).
The vast amount of coal in this world exists because the carbon present in the precursor of coal was not degraded. This was the result of imposed temperature and pressure and the limited amount of oxygen, due to which complete oxidation was not possible. Coal has biological origins and is largely insoluble both in water and many organic solvents, except under extreme conditions.
The small size of the porous structure in most coals presents an obstacle to the ingress of microorganisms and the outer surface is the main area for biological attack. In addition, there are substantial quantities of inorganic impurities present in the form of clays, quartz and sulphur compounds. There may be trace elements which can be toxic for various microorganisms.
Several routes to microbial coal ‘beneficiation’ are possible. Microorganism may attack either carbonaceous coal or the interspersed inorganic materials. One route is the depolymerization of coal polymer and the breaking of various key links. This could provide the basis for the liquefaction processes.
Bioprocesses may have significant advantages over conventional technologies in use today, and may open up the possibility of viable production of new products
The second route can be the reduction in oxygen content, either through reduction of C=O to CH2 or by decarboxylation to CO2. This can improve the heating value.
The third route can be the removal of sulphur, nitrogen or metals from coal before combustion, which will lead to the reduction of unwanted emissions and residues.
High sulphur-containing coal presents many environmental problems. When coal is burnt, its sulphur content combines with oxygen to produce sulphur oxides, which contribute to both pollution and acid rain. The best possible way to save our environment from sulphur oxides is to reduce the amount of sulphur in coal before combustion.
Different techniques are used for the desulphurization of coal, which includes physical, chemical and biological ones. Biological processes are based on the degradation of coal using microorganisms.
These processes are performed under mild conditions with no harmful products. Moreover, the value of coal is not affected.In coal, sulphur is present in both organic and inorganic forms. The inorganic sulphur is predominantly in the form of metal sulfides and sulfates. Pyrite is generally the most abundant inorganic sulphur in coal. The crystals of pyrite are randomly distributed throughout the coal matrix.
The organic sulphur in coal is covalently bound in its large complex structure, which is difficult to remove from the coal structure. The organic sulphur in coal exists in both forms, that is, aliphatic and aromatic or heterocyclic. These can be classified into four groups:
— Aliphatic or aromatic thiols (mercaptans, thiophenols);
— Aliphatic, aromatic or mixed sulfides (thioethers);
— Aliphatic, aromatic or mixed disulfides (dithioethers), and;
— Heterocyclic compounds or the thiophene type (Dibenzothiophene).
Microbial desulphurization of coal is a wet process. Therefore, the process also provides a cleaning step in preparation of coal slurries for combustion.
Seventy per cent of coal slurry has approximately the same bulk calorific value as dry coal. Under supercritical conditions such slurry hardens and it behaves as a sold. Water provides hydroxyl radicals that react with carbon monoxide formed during burning, thereby decreasing the amount of carbon monoxide for reaction with oxygen and increasing the amount of oxygen that can burn the carbon.
Certain microorganisms oxidize pyrite and marcasite and have been considered as potential agents for a coal desulphurization process. Microbial desulphurization of coal has shown various advantages, including a higher pyrite removal efficiency and lower coal wastage as compared to the physical methods.
Costs are reduced as compared to chemical methods because microbial methods operate at ambient conditions with fewer reagents. However, microbial processes are slower, requiring days to complete.
The best known of the pyrite-oxidizing bacteria is Thiobacillus ferrooxidans, a gram-negative iron, sulphur and metal sulphide-oxidizing bacterium. This organism is acidophilic and chemoautotrophic, and is a common inhabitant of acidic environments associated with metal sulphide weathering. Indeed, this organism is important in the production of acidic coalmine drainage that results from uncontrolled pyrite oxidation during and following mining.
Certain thermophilic bacteria also remove pyritic sulphur from coal. These include Sulpholobus acidocaldarius. Sulpholobus, a member of archaebacteria, oxidizes pyrite, elemental sulphur, certain metal sulphides and organic compounds at temperature of up to 85ºC. Thermophilic bacterial processes may result in faster coal desulphurization rates as associated chemical reactions are accelerated at elevated temperatures. However, this has yet to be demonstrated conclusively.
Two mechanisms have been proposed for biologically catalysed oxidation of pyrite by Thiobacillus ferrooxidans: direct and indirect mechanism. In direct mechanism, the pyrite is oxidized biologically and it requires physical contact between the bacterium and pyrite particles.
Several attempts have been made to demonstrate the direct attack of T. ferrooxidans on metal sulphides. It can be considered as a heterogeneous process, in which the bacterial cell attaches itself to the sulphide crystal surface and corrosion takes place in a thin film located in the space between the bacterial outer membrane and the sulphide surface.
In indirect mechanism, pyrite slowly oxidizes with exposure to the air and water to produce acid and ferrous ion. Thiobacillus ferrooxidans oxidizes soluble ferrous ions to ferric ions at low pH as a source of metabolic energy.
This reaction is considered as the rate-limiting step in pyrite dissolution and bacteria accelerate the dissolution by a factor of up to 106. Under acidic conditions, ferrous ion is relative stable to chemical oxidation. However, bacteria catalyse this reaction, generating ferric ions that react with additional pyrite.
In the indirect mechanism, the role of bacteria is considered not to attack the pyrite directly but to catalyse aerobic oxidation of ferrous ion in solution to the ferric state. The ferric ion in solution then oxidizes pyrite to the ferrous state and additional acidity is produced due to the protons released.
Experimental results indicate that the removal of pyritic sulphur from coal in batch systems is governed by first-order reaction kinetics. The relative ease of coal desulphurization follows the order bituminous>subbituminous>lignite depending upon the neutralizing capacity of coals.
Several different microorganisms have been suggested for the process, and each of these behaves differently. Advancement in genetic engineering could perhaps fulfil the need for microbial cultures that present more complete and rapid sulphur removal activities.
Actually, a wide range of studies on coal biodesulphurization process is required. Key engineering issues that need to be dealt with include reactor design, separation processes, by-product deposition and product quality. Scientists and engineers, therefore, need to cooperate if the processes are to be improved.
The writer < to_alvi@hotmail. com > is a freelance contributor
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