Comparison of Inorganic Constituents in Three Low-Rank Coals

with tungsten carbide. The pulverized coal was screened with a wedge wire screen to determine that 85% of the resultant coal is less than 200 mesh. The coals were treated by a modification of the procedure reported by Miller and Given (1977). Figure 1shows a flow chart of the procedure used to examine the various coals. Each coal was ground to approximately 400 mesh in an alumina grinding apparatus. The ground coal was freeze-dried for at least 24 h by using a freeze dryer to reduce the possibility of oxidation. Oxidation of low-rank coals is discussed by Durie and Sternhell (1958). Elemental analyses of the dried coal and residues were performed by X-ray fluorescence (XRF), neutron activation analysis (NAA), and by inductively coupled argon plasma emission spectroscopy (ICP) of digested Ca samples. The coals were subjected to proximate, ultimate, heating value, sulfur forms, and mineral analyses which are listed in Table 11. Each sample was run in duplicate by treating 15-20 g of coal in a plastic beaker with 100 mL of 1M ammonium acetate at 70 OC with stirring for 20 h. Each sample was filtered and the residue was washed with an additional 100 mL of 1 M ammonium acetate and 5-20-mL portions of deionized water. After final washing, the residue was freeze-dried for at least 24 h. The filtrate was transferred to a 500-mL volumetric flask, made up to volume, and analyzed by ICP. After drying, samples of norganic materials present in low-rank coals can have adverse effects on process performance. During combustion processes sodium is liberated and becomes a major cause of boiler tube fouling problems (Rindt et al., 1981). If concentrations of sodium and sulfur in the coal are low, the ash has a high resistivity which makes it difficult to collect in an electrostatic precipitator (Selle et al., 1975). In fluidized bed combustion alkaline components, such as calcium, can be useful for retention of sulfur (Goblirsch and Sondreal, 1979). On the other hand, bed agglomeration is encountered when the coal has high concentrations of sodium (Benson et al., 1982). In coal conversion processes some inorganic species, such as iron, can serve as a catalyst for some processes but act as a catalyst poison in other conversion processes (Abdel-Baset et al., 1978). Inorganic components are associated with coal in several different ways. They may be present as discrete mineral phases, as ions in ion-pair bonds with carboxylate ions of the coal structure, as ions associated with ionic sites of clays, or coordinated to other nitrogen- or oxygen-containing functional groups of the coal structure (Miller and Given, 1979). As a result, the distribution of these components is very complex. Knowledge of the distribution of inorganic material within the coal is useful in predicting the effects of a particular metal on a given process. The coals chosen for study were used in pilot plant projects at the Energy Research Center. These coals are lignites from the Beulah mine in North Dakota, the Bryan mine in Texas, and the subbituminous coal from the Rosebud mine in Montana. Experimental Methods and Materials The coals used in this study were tipple or test pit samples from the Fort Union, Powder River, and Gulf Coast coal-bearing regions. A description of the coals is summarized in Table I. The coals were primarily used for combustion testing at UNDERC. The procedure used to prepare coals a t UNDERC involves crushing the coal with a hammer-mill type crusher equipped with hardened steel hammers. The crushed coal is further reduced in size using a pulverizer equipped with stirrup hammers tipped
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 145-149
145
Comparison of Inorganic Constituents in Three Low-Rank Coals
Steven A. Benson
University of North Dakota, Energy Research Center, University Station, Grand Forks. North Dakota 58202
Paul. L. Holm"
Department of Chemistry, University of Minnesota -Crookston, Crookston, Minnesota 567 16
Inorganic components in low-rank coals can be incorporated as discrete mineral particles, ion-exchangeablecations, or coordination complexes. The mode of incorporation may influence the way in which the metals behave in utilization processes. This work describes the mode in which AI, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Si, Sr, and Ti were found in a North Dakota lignite, a Texas lignite, and a Montana subbtuminous coal. Elements extracted by 1 M ammonium acetate include Ba, Ca, Mg, Mn, Na, and Sr. These elements likely occur as ion-exchangeable ions associated with carboxylate groups. Extraction with 1 M hydrochloric acid removes substantial amounts of AI, Ba, Cr, Cu, Fe, Ni, Sr, and Ti. These are present either as coordination complexes or as acid-decomposable minerals. The inorganics remaining in the coals after both extractions are present as quartz, clays, and sulfides. The three coals studied show significant differences not only in amounts of the elements present but also in how they occur in the coal.