Coal trace elements

Acid-treated coal, trace element removal, 65... 

There is a good deal of information on the inorganic forms of several elements in many rocks, soils and coals, but much remains to be done on the organic associations of trace elements. For example copper, lead and zinc are associated with humic acids, probably through carboxyl or phenolic groups (Saxby, 1969 Nissenbaum and Swaine, 1976). Vanadium porphyrins occur in petroleum (Davis, 1967), but the form of vanadium in coal has not been established. In most shales and coals, trace elements probably occur partly inorganically and partly organically bound. 

Gallium is often found as a trace element in diaspore, sphalerite, germanite, bauxite, and coal. Some flue dusts from burning coal have been shown to contain as much 1.5 percent gallium. 

Zinc, like most metals, is found in all natural waters and soils as well as the atmosphere and is an important trace element in plant and animal life (see Mineral nutrients). Rocks of various kinds contain 20—200 ppm zinc and normal soils 10—30 ppm (average ca 50 ppm) in uncontaminated areas. The average zinc content of coal is 33 ppm. Seawater contains 1—27 )-lg/L (median ca 8 p.g/L), and uncontaminated freshwater usually <10 / g/L. 

Minor elements contribute >1 wt % to the ash trace elements contribute <0.1 wt %. The degree of de-ashing achievable by physical cleaning depends on the distribution of mineral matter in the coal. In some cases, a considerable amount of the mineral matter can be removed in other cases, especially where the mineral matter is distributed throughout the coal as microscopic particles, deashing by physical cleaning is not practical. 

Concern over the release of hazardous trace elements from the burning of coal has been highlighted by the 1990 Clean Air Act Amendments. Most toxic elements are associated with ash-forming minerals in coal (5). As shown in Table 1, levels of many of these toxic metals can be significantly reduced by physical coal cleaning (6). 

Table 1. Effect of Coal Cleaning on Trace Elements ...

Trace Elements in Coal—An Intedaboratory Study of Analytical Techniques,", Technical Bulletin ConsoHdation Coal Co., Oct. 1987. 

Trace elements such as sulfur and nitrogen are also involved in the gasification reactions. Sulfur in coal is converted primarily to H2S under the reducing conditions of gasification. Approximately 5 to 15% of the sulfur is converted to COS, whereas the coal nitrogen is converted primarily to N, trace amounts of NH and HCN ate also formed. 

Group 3 elements which are not mostly vaporized in the boiler (1423 K) V, Cr, Mn, Co, Ni Referring to the classification, we investigated the temperature dependency of release of trace metals in coal combustion. We already reported the behavior of these three types of elements during high temperature coal processing and reported elsewhere . So in this paper, we investigated the effect of atmosphere for the emission behavior of trace elements. 

L.B.Clarke and L. L.Sloss, Trace Elements-emission from Coal Combustion and Gasification, IEACR/49, lEA Coal Resereh (1992)... 

Von Lehmden DJ, Jungers RH, and Lee Jr RE (1974) Determination of trace elements in coal, fly ash, fuel oil, and gasoline - a preliminary comparison of selected analytical techniques. 

White DH, King KA, Mitchell CA, et al. 1986. Trace elements in sediments, water, and American coots (Fulica americans) at a coal-fired power plant in Texas, 1979-1982. Bull Environ Contam Toxicol 36 376-383. 

Clarke L.B., Sloss L.L. Trace Elements - emissions from coal combustion and gasification. London IEA Coal Research, 1992. 

The coprocessing of coal in gasification processes together with biomass (C02 separation and sequestration, sulfur species removal, and trace element emissions)... 

Birge, W.J. 1978. Aquatic toxicology of trace elements of coal and fly ash. Pages 219-240 in J.H. Thorp and J.W. Gibbons (eds.). Energy and Environmental Stress in Aquatic Systems. U.S. Dep. Energy, Sympos. Ser. 48, CONF-771114. 

Hatcher, C.O., R.E. Ogawa, T.P Poe, and J.R.P. French, III. 1992. Trace elements in lake sediment, macrozoobenthos, and fish near a coal ash disposal basin. Jour. Freshwater Ecol. 7 257-269. 

Grady, W. (compiler). 2002, Trace Elements in West Virginia Coals, West Virginia Geological and Economic Survey Website http //www.wvgs.wvnet.edU/www/datastat/te/i ndex.htm... 

The principal advantage of the ion microprobe (as opposed to the Auger microprobe) is the ability to obtain depth profiles for trace elemental species present in the analytical volume. The characterization of coal fly ash clearly illustrates this point (11-14). Auger detection limits are comparable to BSCA, and thus only elements with bulk concentrations greater than 1% by weight in fly ash (Si, Al, Fe, Ca, S, Na, K) can be... 

One of the aims of the work carried out in this laboratory was to investigate the deposition of trace elements on the hydrocracking catalyst. To this end, a continuous laboratory hydrocracker was installed and, to provide feed material for this, a 2-1 autoclave and pressure filter were also installed. It was noticed on the initial runs with the autoclave that the ash level of the filtered extract solution was considerably lower, at 100-200 ppm, than that normally obtained by British Coal, at 500-600 ppm. The reason for this difference was not known, so an investigation was carried out which found that the lower ash levels could be produced by a higher digestion pressure. Table 1 shows some of the results obtained, which have b n previously published (10). Note that throughout all the experiments described in this paper the coals used were Point of Ayr and Calverton, whose analyses are given in Table 2, and that the HAO to coal ratio us was always 2 1. Hence, ash analyses in the coal extract solution are comparable. 

So far, no mention has been made of the individual trace elements. At the beginning of this work, it was found that the proportions of the trace elements in the ash vary from original coal to filtered coal extract solution. Some of the original results for Point of Ayr coal are shown in Table 5 (9). 

The results in Table 5 show a large increase in proportion of Mn and Ti, and increases for Mg and Ca from coal to extract. Fe stays about the same and all the others decrease. Previous work Q) has also shown that the elements which increase in proportion also deposit readily and steadily on the hydrocracking catalyst. Thus, any method which reduces the ash level in the filtered extract solution needs to be, if at all posible, selective towards Mn, Ti, Ca, Mg, and Fe. In the case of digestion pressure, as shown by the results above, most trace elements do not significantly change their proportion as the ash level decreases, except for Mn where the situation becomes worse and Mg where there is some improvement. Liquefaction of different-sized coal fractions has little effect on the distribution of elements in the ash as the overall ash level decreases. However, with the use of HAO with different saturates levels, there is a distinct increase in the levels of Ti with decreasing ash level, but a significant decrease in Mn. 

Trace element analysis of coal and extract ashes... 

As indicated previously, it is important to look at the individual trace elements in the coal extract solution. Table 9 shows the concentration of trace elements in the ash from the original extract and the filtrate produced after toluene extraction. This was for an extract with an ash level of 350 ppm, extracted with toluene at 165 C to give a filtrate with an ash level of 100 ppm. The results show that there are some differences, with Ca reducing and Fe and Ti increasing their proportions. However, the changes are not great and it would be expected that if the filtrate were fed to the hydrocracker it would deposit trace elements as observed previously Q), except, of course, there would be a smaller quantity overall. 

Several options have been outlined for the reduction of mineral matter to low levels in filtered extract solution. These are mainly applicable to the British Coal process, but have implications for other 2-stage processes. By running the digestion at a pressure above about 30 bar and reducing the filtration temperature as required, low levels of trace elements have been obtained with a series of different coals and HAO solvents. Discarding the fine fraction of the coal from the feed to the digestor will also result in lower levels of trace elements in the extract solution, as will increasing the saturates content of the HAO. However, in the latter case this may also lead to reduced overall conversion. 

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