Coal desulfurization, importance

Emissions control systems play an important role at most coal-fired power plants. For example, PC-fired plants sited in the United States require some type of sulfur dioxide control system to meet the regulations set forth in the Clean Air Act Amendments of 1990, unless the boiler bums low sulfur coal or benefits from offsets from other highly controlled boilers within a given utiUty system. Flue-gas desulfurization (FGD) is most commonly accomphshed by the appHcation of either dry- or wet-limestone systems. Wet FGD systems, also referred to as wet scmbbers, are the most effective solution for large faciUties. Modem scmbbers can typically produce a saleable waUboard-quaUty gypsum as a by-product of the SO2 control process (see SULFURREMOVAL AND RECOVERY). 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

Desulfurization of fossil fuels was the subject of an authoritative review by J. B. Hyne (Alberta Sulphur Research Institute). This is a topic of increasing importance as Canada relies more and more on sulfur-containing fuels such as tar sands and heavy oils. Hyne reviewed the present state of the chemistry and technology for both precombustion desulfurization of natural gas and crude oils and postcombustion tailgas clean up of coals and cokes. He clearly identified areas of possible future research such as the high temperature-high pressure chemistry pertaining to in-situ desulfurization processes. 

Metal sulfides play an important role in catalyzing a wide variety of hydrogenations (e.g., of fats, coal, or olefins) and also desulfurization reactions, which are used in pretreatment of fossil fuels to reduce the emission of sulfur oxides during combustion (Section 8.5). Molybdenum disulfide, an important defect catalyst, can be made to function as an n-type (Moi+xS2) or p-type (Mo1 xS2) semiconductor by exposure to an appropriate mixture of H2S and hydrogen at temperatures on the order of 600 °C. The equilibrium... 

Because of the importance of the promotion effect and because many of the central questions surrounding TMS catalysis are about promotion, it is valuable to review a history of the effect. The first reference to a catalyst based on molybdenum and cobalt sulfides capable of desulfurizing coal oils in the presence of hydrogen was a patent from I. G. Farben Industrie dated May 24, 1928 (5). Before this, M. Pier and his team at BASF (1924-1925)... 

As mentioned, the aim of the study is to develop desulfurization equipment of industrial interest so understanding its general performance is important. Several sets of typical operation data measured under stable operations are listed in Table 7.5. The comparable data are depending on coal type, S02 content in flue gas ranges from 1400 to 11400 mg/m while the permitted discharge level in China is normally 1200 mg/m3. The data show that the designed equipment exhibits satisfactory global performance and meets the requirements for desulfurization by wet process. Under moderate operation conditions, the content of S02 in the cleaned gas can achieve a much lower level than that permitted. Even if the mole ratio of Ca/S is as low as 1.0, a sulfur-removal efficiency of nearly 90% can be achieved (see the fourth row in Table 7.5) while the pressure drop across the reactor is very small, ca. 400 Pa only. 

Although the selective oxidation of coal has been extensively studied (31-33, surprisingly little has been reported about sulfur species in the oxidation products. Even less is known about the distribution of organic sulfur species between different coal macerals despite the fact that this information is important for the development of any future desulfurization technology. 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

Application of these A/E values to the 41 eastern AQCRs exceeding the primary standard indicates several important factors. First, although electric utilities consume about 80% of the coal and 60% of the residual oil, even complete desulfurization of this consumption without attention to the other source categories would only bring 21 of these problem AQCRs within the primary air quality standard in 1975. 

GRP is a heat insulator. This means that when transporting hot gases, a reduction in the added outer insulation can be considered, and in certain conditions, no insulation at all may be required. One important area for this application is in flue gas desulfurization for coal fired electrical power generation plant. In this process, sulfur dioxide-containing gas is fed into an absorber tower, where it is washed with a limestone slurry. The sulfur dioxide in the gas stream forms calcium sulfite, which is subsequently oxidized to calcium sulfate (gypsum). 

With coal and oil as the most Important fuels in the years to come it is obvious that a large and costly effort will have to take place in the field of desulfurization in order to prevent the acidification described in this paper. Recently, however, our Government Issued a note of more than 500 paiges about a very detailed study on the economic consequences of NOx and SO2 abatement in The Netherlands. In this note the expectations about the SO2 emission in the next 20 years are more optimistic. At different categories of control they expect an SO2 emission between 300,000 and 700,000 of tons for 1985, hardly increasing in the years after 1985. I hope it will come true, but I doubt it very much. Because most... 

The change of feedstock from coal to naphtha or natural gas and the improved feedstock desulfurization units have been cmcial steps for the industrial use of Cu-based catalysts to conduct the WGS reaction at low temperature. On the basis of new processes, an important reduction of the amount of chlorine and sulfur (as poisons for the active copper phase) was achieved. Consequently, the lifetime of Cu-based catalysts was increased until long periods without deactivation (2—4 years, depending on the operating conditions (Twigg Spencer, 2001)). Moreover, ZnO protects the Cu from sulfur poisoning in the case of inefficient removal in the upstream units. 

The sulfur content is an important, but undesirable property of coals. If the coal is to be used for the production of metallurgical coke, its sulfur content has to be limited because of the deleterious effect of sulfur on the properties of steel. The sulfur content of coal used as fuel is also undesirable because of the resultant SO2 in the flue gases. At present, low-sulfur coals are at a premium and much research is being done on the desulfurization of coal. 

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