OCR Text |
Show - 4 - therefore, several tests can be conducted on as little as 50 grams of coal. This allows for testing of coals to determine the effects of certain mineral enrichment achieved by gravity fractionation, ion-exchange effects, or additives on deposit formation. The advantage of such a small-scale test is that many tests can be performed at a much lower cost under closely controlled conditions. The disadvantage is that it is an accelerated test which may not give results completely comparable to the relatively slower growth of deposits in a utility boiler furnace. A medium cleaned Pittsburgh #8 was prepared and tested to determine the effect of size and specific gravity fractions on slagging and fouling behavior. Pittsburgh #8 coal was fractionated by gravity into 1.4 float, 1.4x1.8, 1.8x2.5, and 2.5 sink fractions. Each gravity fraction was sized into <44, 44-75, and > 75 microns using a sonic sieve. Each of the sized gravity fractions was combusted and deposits were formed in the drop-tube furnace. A typical deposit formed in the drop-tube furnace consists of several layers. Starting at the steel substrate surface, these layers consist of a dust layer, a base layer, and a sintered mass growing in the direction of the oncoming gas stream. The deposits are weighed after removal from the furnace. Deposits are formed over timed intervals to determine deposit growth rates in terms of ash sticking coefficient. The sticking coefficient is defined as the ash deposit formation rate divided by the rate of firing of ASTM ash. The sticking coefficient is determined by differentiating the weight versus time curve and dividing by the rate of ash fired. The deposits produced from the Pittsburgh #8 coal in the drop-tube furnace and the CE combustor were characterized using the scanning electron microscope point count routine (SEMPC). This technique was developed to quantitatively determine the relative amount of phases present in ashes and deposits. Extensive details of the technique can be found elsewhere (9). However, a brief description will be given here. The method involves microprobe analysis of a large number of random points in a polished crosssection of a deposit. The quantitative analysis of each point is transferred to a Lotus 123 spreadsheet for data base analysis. The software is used to calculate molar and weight ratios for each point. Using these ratios, the points which have compositions of known phases (common to ashes and coal minerals) are identified and counted. The software then finds the relative percents of all of the identified points as well as the percent number of unknown phases. The unknown phases are those for which there is no corresponding chemical composition. For this study, we have assumed that these points are amorphous. In addition, the average chemical composition of all the points in the sample is calculated. Previous unpublished work at UNDEMRC has shown that the SEMPC average composition corresponds very well to the bulk chemical analysis. Advantage of analyzing the microprobe data by spreadsheet makes it convenient to further analyze the compositional data to determine the chemical interactions that have taken place. The quantitative ability of the SEMPC allows for detailed comparisons to be made between different samples. |