The ever increasing demand for energy, uncertainties associated with resources of petroleum and natural gas, inherent problems with nuclear power plants and current unfavourable economics of solar energy and biomass utilization have been primary contributory factors for the renaissance of coal as a sustainable energy resource for the next decade and beyond.
The U.S. reserve of coal is about 3 trillion tons. Although the most abundant (80%) fossil fuel in America is coal, the consumption pattern in the United States of America is quite a reversal of form in terms of utilization, with coal representing only 17% and oil and gas about 78%.
The demand for all fossil fuels combined is expected to double by the year 2000, even with increasing the use of nuclear power. While the domestic supply of crude oil and natural gas is not likely to keep pace with the energy demand, coal can play an important role in filling such a gap, and thus reduce the requirements for imported supplies of oil and gas. If this vast coal reserve can be converted to clean fuel, it can supply most of the energy needs of the United States for the next three centuries. Petroleum and natural gas would be utilized for other essential uses, especially as a fuel stock for the synthetic, organic chemical, resin and rubber industries.
However, utilization of coal for power generation and process heat is beset with environmental problems. The major problem with coal combustion units is that associated with sulfur dioxide emissions, although emissions of nitrogen oxides, particulates and trace elements also contribute to environmental degradation. In the last decade several alternatives for controlling sulfur dioxide emissions from coal combustion units have been proposed. These can be broadly classified as:
A. Use of low sulfur content coals.
B. Pre-combustion physical and chemical coal cleaning.
C. Retention of sulfur in the ash during combustion.
D. Post-combustion flue-gas cleanup.
Reserves of coal which contain sufficiently low concentrations of sulfur to enable them to meet the present emission standard of 1.2 lb. SO.sub.2 /10.sup.6 Btu (which corresponds to 0.7 wt. % sulfur in coal with a heating value of 12,000 Btu/lb.) are both limited and restricted to specific geographical locations. In fact, only 12.3% of U.S. coal reserves are within this compliance level. The major recoverable fractions of Eastern and Midwestern coals contain more than 2 wt. % sulfur.
Sulfur in coal exists primarily in two forms--inorganic and organic in almost equal proportions. The average sulfur content in coals generally varies from 0.5-7% depending on the source and location. The major constituent of inorganic sulfur is iron sulfide, FeS.sub.2, commonly known as pyrite. The other forms of inorganic sulfur in coal are sulfate sulfur and elemental sulfur which are normally present in very low concentrations. The low concentration of sulfate sulfur together with its solubility in water make it of little consequence during coal cleaning. The concentration of elemental sulfur in coal is also very small. Pyrite in general is believed to be present as a discrete phase in coal, which incidentally facilitates its removal by float-sink methods. However, with very fine particles even complete pyritic sulfur removal is not possible.
Organic sulfur in coal is thought to be uniformly distributed and firmly bound to the coal matrix. Precombustion physical coal cleaning to remove mineral matter is widely practiced in the coal industry. By the conventional float-sink methods as much as 60% of the pyritic sulfur in coal is also removed. However, a significant portion of coal is also rejected along with the high density material of high sulfur content. In addition, physical methods are not effective in removing organic sulfur content of coal which in certain cases may constitute 50% of the sulfur in coal. During the last decade several chemical coal cleaning methods have been proposed. However, a majority of these are applicable for the removal of only pyritic sulfur and no chemical coal desulfurization process uniformly applicable for the removal of both inorganic and organic sulfur fractions in coal is as yet available commercially.
Retention of sulfur during combustion is studied widely employing dolomite, limestone, etc. in fluidized bed combustion units. Chemical modification of coal and incorporation of alkaline earth metals into the coal matrix as a means of retaining sulfur in the ash have also been proposed. Amongst the postcombustion gas-cleaning methods, the most widely adopted one is flue gas desulfurization (FGD) employing wet scrubbers. However, scrubbers generate large quantities of sludge which has to be disposed of in an economical manner. Also in many instances scrubbers were found to be unreliable requiring excessive maintenance.
Amongst the various methods that have been proposed for controlling the SO.sub.2 emissions from coal fired power plants, precombustion coal desulfurization offers several potential advantages over flue gas desulfurization. In the past decade, several processes have been proposed for extracting pyritic and organic sulfur from coal. Most of these processes can be classified into a few groups based on the chemistry of the reactions involved in the process.
Exposure of coal to air results in a slow oxidation of pyrite to the sulfate which is water soluble. A majority of the processes reported for the removal of pyritic sulfur in coal are aimed at enhancing this natural process of oxidation. Oxidants ranging from metal ions (Fe.sup.3+) to strong acids (HNO.sub.3), oxygen, air, SO.sub.2, Cl.sub.2, H.sub.2 O.sub.2, NO.sub.2, etc. have been employed for this purpose. The PETC oxydesulfurization process, AMES wet oxidation process, LEDGEMONT Oxygen Leaching Process, ARCO promoted oxydesulfurization process, TRW Meyers desulfurization process, and JPL chlorinolysis process amongst others, all involve oxidizing the sulfur fraction in coal to sulfuric acid or to a soluble sulfate. There is a wide variability in processing conditions and the removal efficiencies amongst the various processes.
Processes based on the displacement of sulfur such as the Battelle Hydrothermal process, TRW Gravimelt process and the General Electric Microwave process involve heating coal with sodium hydroxide to remove the sulfur in the form of sulfides and polysulfides. The TRW Gravimelt process in addition to removing sulfur also removes substantial quantities of mineral matter from coal. However, one major disadvantage of using caustic is that the excess sodium retained in coal may cause severe ash slagging problems in the boiler.
Amongst the processes based on reduction, mention may be made of the IGT flash desuIfurization process for producing chars. The process involves preliminary air oxidation of coal to facilitate sulfur removal in the subsequent hydrodesulfurization step. A sulfur acceptor such as calcium oxide or iron oxide was found to limit the hydrogen consumption during the latter step.
The JPL Low Temperature Chlorinolysis process is one of the few processes capable of removing both inorganic and organic sulfur from coal. There are two basic variations of the process, although both are based on the oxidation of sulfur by chlorine. The original version (U.S. Pat. No. 4,081,250) employed methyl chloroform as the reaction medium during chlorination which was later substituted by water (U.S. Pat. No. 4,325,707) or methanol (U.S. Pat. No. 4,334,888). A more recent version of the process consists of:
(i) chlorination of an aqueous coal slurry (water:coal 2/1) at 60.degree. C. for 45 min. (Cl.sub.2 /S 8/1 by wt.)
(ii) filtration--wash of chlorinated coal (coal:water 1/2)
(iii) dechlorination of dry coal with N.sub.2 at 400.degree. C. for 1 hr., and/or
(iv) advanced dechlorination with H.sub.2 at 650.degree. C. for 1 hr.
The last step was found to further enchance the total sulfur removal to the level of 90%. The chemistry of the process is somewhat complex, but is based on the sulfur bond scission in organic compounds. The reactions are exothermic and proceed favorably at low temperature.
Almost all of the precombustion desulfurization processes have been practiced in the liquid phase. There are very few processes in which coal has been desulfurized by treatment as a solid with a gas phase reagent.
Coal desulfurization by treatment with different gases at elevated temperatures was reported by several investigators. Early interest in such treatments was mainly for the production of metallurgical coke. Sulfur removal during carbonization was studied in both inert and reactive environments such as oxygen, hydrogen, steam, etc. Iron pyrites decompose on heating, releasing half of its sulfur, while 1/4-1/3 of the organic sulfur is converted to hydrogen sulfide.
One investigator treated coal in various reactive gases and found hydrogen to be most effective. However, hydrodesulfurization of coal is strongly inhibited by the presence of hydrogen sulfide in the gas-phase. Treating with hydrogen at high temperatures (&gt;900.degree. C.) was found to be very effective in the removal of organic sulfur but the accompanying coal losses were found to be substantial.
Desulfurization of coal with oxygen and oxygen carriers was studied by several investigators. However, it was found that mainly pyritic sulfur was removed under the oxidizing atmosphere. One exception is the KVB or Guth process where the oxidation of sulfur compounds is brought about in the solid phase by employing NO.sub.2 followed by a caustic wash to remove up to 40% of organic sulfur. NO.sub.2 is reported to selectively oxidize part of the pyritic and organic sulfur in coal.
Coal has also been desulfurized or treated in various other processes. Long et al (U.S. Pat. No. 3,878,051) utilize a mixture of CO and Cl.sub.2 (forms phosgene in situ) to desulfurize coke. Sauer (U.S. Pat. No. 1,052,592) teaches decolorizing carbon with heat and an active gas such as steam, CO.sub.2, producer gas, CO, air or Cl.sub.2. Hartwick (U.S. Pat. No. 2,698,777) purifies anthracite or coke with Cl.sub.2 at elevated temperature to volatize metal impurities. Use of hydrogen to desulfurize coal or coke is taught by McKinley (U.S. Pat. No. 2,726,148) and Loevenstein (U.S. Pat. No. 3,130,133). Fluidized bed desulfurization is disclosed by Whitten (U.S. Pat. No. 3,759,673) who suspends coal in recycled reducing gas (H.sub.2 plus methane) and then contacts coal with this gas mixture in a multi-stage contactor. Kreusi (U.S. Pat. No. 4,118,200) desulfurized coal in a liquid salt bath in the presence of chlorine.
Thus, while numerous chemical coal cleaning processes have been proposed in the past decade, none are being practiced on a commercial scale at this time. There are inherent technical and economic problems still to be overcome. Most of these processes besides being effective for the removal of only pyritic sulfur involve severe operating conditions, long retention times and multiple processing steps. In addition, a majority of these processes are carried out in the liquid phase, thus necessitating a phase change at the beginning and end of the process.