Other Minerals in Coal
Beyond the elements described above, which are provided with every ultimate analysis of coal, it will be found that a substantial part of the periodic table can be shown to be present in coals. These other elements can be divided into macro components, the presence of which is usually given in wt%, and the micro or trace elements that are only present at ppm levels. Values from a sample coal within the typical range are given later in Table 4-7.
The chlorine content in coal is mostly well below 1 wt%. However, in some coals it may be as high as 2.5 wt%. In combination with a low nitrogen content in the coal, this will result in a high caustic consumption in the wash section of a gasifier. Chlorides have three possible detrimental effects in the plant:
1. Chlorides have a melting point in the range 350-800°C; they deposit in the syngas cooler and foul the exchanger surface. The first indication of this is an increase in the syngas cooler outlet temperature.
2. In the reactor chlorides can react with the hydrogen present to form HC1, which will decrease the pH of the wash water or condensate.
3. Chlorides may also form NH4C1 with high nitrogen feeds. With such feedstocks the chloride deposits as NH4C1 in the economizers at temperatures below about 280°C. Further, as an aqueous solution this leads to severe chloride stress corrosion in stainless steels that are used, for example, in burners and instrument lines.
Coals also contain phosphorus, but this has less significance for gasification than, for instance, for the steel industry.
It is of paramount importance that the users of the analytical data of coal are able to convert various types of analysis into each other. Apart from the difference in units used that can readily be transferred into each other, there are problems associated with the basis for the analysis. The proximate analysis and the ultimate analysis may be given based on the so-called as-received (ar) coal; that is, the coal including moisture and ash. When doing so it is also important to record whether the sample was taken direct from storage or air dried before analysis, since the difference between total and inherent moisture cannot be determined. But it is also possible to get these analyses on a moisture-and-ash-free (maf) basis, a moisture-free (mf) basis, or an ash-free (af) basis. Furthermore, the heating value may be the higher heating value (HHV) or the lower heating value (LHV). In relation to the latter it should be kept in mind that the HHV of steam and water are positive and zero, respectively, and that the LHV of steam and water are zero and negative, respectively. A program is included on the companion website to convert given data for proximate and ultimate analysis as well as for the heating value on any basis (ar, af, mf, maf, and HHV or LHV) into any other basis.
