Chemical coal cleaning, problems

The detrimental effects of acid rain are a major reason why legislation such as the Clean Air Act places strict limitations on sulfur and nitrogen emissions. It is also a reason why low sulfur coal is preferred over high sulfur coal. To reduce sulfur dioxide emissions, industry also uses a technique call scrubbing. Industrial scrubbers employ a variety of physical and chemical processes to remove sulfur dioxide from emissions. Another technique used to combat acidification of lakes is to treat these systems with lime. The lime acts to neutralize the acid, but such techniques are usually costly and are only a temporary remedy for combating the problem. 

More recently, there has been much concern about the possible effects of the mineral matter in coal on processes used to convert coal to other fuels such as gasification, liquefaction, and production of clean solid fuels. Not only is removing and disposing of the mineral matter a problem, but also the possible chemical effects such as catalyst poisoning, which might be expected in the methanation of gas from coal, should be considered. 

In this section we focus on four specific environmental problems coal combustion aerosol formation, dynamics of atmospheric aerosols, the chemical characterization of particles, and the role of aerosols in clean room technology or so-called microcontamination control. For the reader interested in an introduction to aerosol science, we recommend three texts [6-8]. 

The above problems can be avoided by the application of an active compound onto a thermally and chemically stable support material. Since desulfurization has to be performed on a large scale, the absorbent must be cheap. Expensive supports or active materials are not acceptable. The only generally used support that is inert towards coal gas and SO2 is silica. However, silica is not therm ly stable at high temperatures. Especially in the presence of steam sintering proceeds rapidly and even volatilization may occur. Until now a supported absorbent appropriate for high-temperature desulfurization is therefore not available. Therefore our research was aimed at the development of an absorbent that fulfills all the specifications for employment in hot-gas clean up. 

PVC has a special position in the recycling of plastics [1]. As a result of the relatively weak bonding of chlorine to the polymer chain, the chlorine atom is split off at low temperatures of about 150 °C and hence complicates the material recycling of plastics wastes containing PVC. Corrosion, which could be initiated by the released chlorine or hydrochloric acid produced from it is less of a problem than the coal like polymer residue, which remains suspended in the machine or is distributed in the melt and further in the product. Many times it produces breakdown of the equipment and necessitates cleaning of the screw, barrel and dies. Therefore as a rule, PVC must be carefully removed from mixed plastic waste before most types of recycling - material or chemical. Because the significant difference between the density of PVC and that of other thermoplastics separation with one of the well known methods is possible. It is, however relatively expensive. 

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