Classifications

• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
  • C10L9/00—Treating solid fuels to improve their combustion
  • C10L9/02—Treating solid fuels to improve their combustion by chemical means

Definitions

• the present invention relates to a process for the removal of pyritic sulfur from coal.
• coals which are availaible for use in this country contain sulfur in several different forms, i.e., sulfate sulfur, organic sulfur and pyritic sulfur.
• the organic sulfur is chemically bonded within the organic molecular framework of the coal while the pyritic sulfur consists of sulfur in the form of iron pyrite, which is disseminated as a separate mineral phase throughout the body of the coal.
• sulfate sulfur constitutes a minor fraction of the sulfur content in coals, i.e. less than about 0.2 weight % of the coal.
• the organic sulfur and pyritic sulfur constitute a major fraction of sulfur in the coal and together they constitute up to about 5 - 8 weight % of the coal.
• the pyritic sulfur constitutes between about 40-60% of the total sulfur content in the coal. Therefore, the removal of the pyritic sulfur alone can significantly reduce the sulfur content and, therefore, the sulfur emissions which occur upon combustion of coal.
• the pyritic sulfur exists as a distinct phase within the body of the coal. It is therefore possible to liberate the pyrite from the coal physically and by means of chemical techniques to remove the liberated pyrite from the coal, without altering in any significant way-the properties of the coal.
• pyritic sulfur can be removed from coal by chemical oxidation to a species which is soluble in water. Such processes are preferably carried out by employing a basic solution as the reaction medium.
• a basic solution as the reaction medium.
• an aqueous slurry containing crushed coal is treated with ammonium hydroxide in the presence of oxygen.
• the pyritic sulfur in the coal is converted to a soluble species, e.g. ferric or ferrous sulfate, along with ammonium sulfate as a by-product.
• the soluble sulfate may be removed by a conventional solids-liquid separation.
• Ammonium sulfate may then be recycled to generate additional ammonium hydroxide.
• elevated temperatures and pressures e.g. about 300 psig. This of course necessitates the use of expensive equipment which increases the cost of desulfurization process.
• the present invention is directed to a process for the removal of pyritic sulfur from coal by chemical oxidation utilizing a basic solution as the reaction medium under mild conditions, i.e. temperatures only slightly above ambient and atmospheric pressure.
• this process comprises preparing an aqueous slurry containing finely-divided coal particles, adding to the slurry an hydroxide or carbonate of an alkali or alkaline earth metal selected from the group consisting of sodium, lithium, potassium and magnesium, as well as mixtures of these hydroxides and carbonates, in an amount sufficient to continuously maintain the pH of the slurry between about 8 and 12 and then agitating the slurry while treating the slurry with oxygen or an oxygen containing gas such as air.
• the process may be advantageously carried out at temperatures of between about 40 and 70°C and at atmospheric pressure.
• the treated slurry may be subjected to a conventional solids-liquid separation to recover the desulfurized solid coal.
• the process of the -present invention is broadly applicable to the treatment of various types of coal.
• the process is directed to the desulfurization of bituminous coals which are combusted to generate steam in electric utility plants or industrial boilers.
• Coals that may be treated in accordance with the present invention are the medium and high volatile bituminous coals such as for example Ohio No. 6 coal. It will be understood of course that the present invention is not limited to the treatment of the above-mentioned coals alone and that coals other than bituminous coals such as anthracite and lignite coal may be treated as well.
• the coals that are treated in accordance with the present invention will contain a pyritic sulfur concentration in the range of from about 0.5 to about 4% by weight of the coal.
• the raw coal which is obtained from mines in chunk size is first reduced to a finely-divided particle size.
• the particle size of the coal should be sufficient to expose a substantial fraction of the total surface of the pyrite that is contained in the coal. Generally speaking, the coal is reduced to a particle size smaller than about 200 mesh.
• formed into aqueous slurry for example, by mixing the coal particles together with water in a suitable reaction vessel.
• the coal slurry should preferably possess a solids concentration in the range of between about 4 and 40% by weight coal.
• the pH of the slurry is adjusted to a value of between about 8 and 12 and preferably about 10 or 11, by the addition of a caustic, such as sodium hydroxide or a mixture of sodium hydroxide and sodium carbonate.
• a caustic such as sodium hydroxide or a mixture of sodium hydroxide and sodium carbonate.
• Other alkali and alkaline earth metal hydroxides and carbonates can also be used as shall be described further hereinafter.
• the coal slurry is then agitated and subjected to an oxidizing medium such as oxygen or an oxygen-containing gas e.g. air.
• the oxygen or air should be introduced in intimate contact with the coal slurry. This may be accomplished by bubbling oxygen through the slurry or by aerating the slurry in the reaction vessel. It may also be necessary to periodically add caustic to the slurry in order to continuously maintain the pH of the slurry within the desired range as indicated above.
• the desulfurization process may be carried out at temperatures of between about 40 and
• sodium hydroxide or a mixture of sodium hydroxide and sodium carbonate as the caustic reagent in the practice of the present invention
• ether alkali and alkaline earth metal hydroxides and carbonates as well as mixtures of these hydroxides and carbonates can also be employed.
• sodium hydroxide lithium and potassium hydroxide as well as lithium and potassium carbonate or mixtures thereof.
• magnesium hydroxide or magnesium carbonate would be useful.
• Calcium hydroxide for example, will impede the oxidation process at these pH values and should not therefore be employed.
• the duration of the process will vary depending upon the caustic material used and the pH of the coal slurry. It may be noted for example that the process of desulfurization can be carried out to virtual completion with 90% or more of the pyrite removed in one or two days when sodium hydroxide is employed and the pH is continuously maintained at about 10 or 11. The duration of the desulfurization process can be shortened by employing pH values greater than 12 but unwanted side reactions will occur that destroy valuable constituents in the coal. In contrast, at low pH values of about 8, the reaction -proceeds at a much slower rate and may take as long as about one week to completition.
• FeS 2 leaching was considered to begin when the desired reaction pH, e.g. pH 11, was achieved.
• the initial NaOH required to achieve the desired pH was consumed in the hydrolysis of any iron sulfates in the coal.
• the NaOH consumption was monitored, and slurry samples were withdrawn. The samples were filtered and the coal collected and washed with water and then air dried.
• Total sulfur was determined on all coal samples. Proximate analyses on starting and finishing coals were obtained on a number of samples. Solution samples were tested for soluble Na and Fe by atomic absorption and soluble SO- 4by gravimetric analysis. The Na content in the product coal was determined as HCl soluble Na. Where .full proximate analyses were not available, the percent pyritic sulfur leached was calculated from initial and final values for total sulfur, assuming all non-sulfate sulfur leached to be pyritic.
• Example II Basically the same procedure as outlined in Example I above was followed except that in this experiment Na 2 C0 3 in an amount sufficient to produce a concentration of 0.2 molar was added to the percolator immediately following addition of the coal-and prior to addition of any NaOH .
• the rate of pyritic sulfur removal at pH 11 and 50°C depends on concentration and the presence of Na 2 CO 3 .
• the initial rate is faster with pure 0 2 than with air when Na 2 CO 3 is present. There is no benefit to using Na 2 CO 3 when air is the oxidant.
• the amount of pyritic sulfur leached is the same within experimental error after 48 hours for-all four samples, although this amount is leached after only 24 hours when 0 2 and Na 2 C0 3 are used.
• a number of product coals were analyzed for Na content.
• the Na content was found to increase with increased pH and the extent of leaching. Where necessary, the Na content in the coal can be reduced to acceptable levels by treatment with acid.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Combustion & Propulsion (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Organic Chemistry (AREA)
• Solid Fuels And Fuel-Associated Substances (AREA)

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Citations (3)

• 1979
  • 1979-10-03 ZA ZA00795273A patent/ZA795273B/xx unknown
  • 1979-10-15 AU AU51786/79A patent/AU5178679A/en not_active Abandoned
  • 1979-10-16 EP EP79103981A patent/EP0010289A1/fr not_active Withdrawn
  • 1979-10-17 JP JP13302179A patent/JPS5556191A/ja active Pending

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