OCR Text |
Show gases. Other elements may partially volatilize during combustion. Non-volatile species such as cadmium will be retained by the fly ash particles and subsequently captured by air pollution control equipment. For most of the eleven Title III Air Toxics metals, variations in behavior have been observed in combustion field data (3-6). Once an element has escaped to the vapor phase, it may interact with the fly ash particle through condensation or gas-solid reaction. The extent of interaction is driven by the surface area of the fly ash, suggesting that the more volatile elements will associate preferentially with the higher surface area or smallest ash particles. Because particles between approximately 0.1 and 1 /xm are more difficult to remove from combustion stack gases, elements enriched in these size fractions may escape to the atmosphere. Under this situation, control strategies to reduce trace element emissions may be required. Many of the observed differences in partitioning have been attributed to differences in coal type. More likely, the observed differences in behavior are the result of differences in the form of an element in coals of differing rank. This difference is clearly observed with the major element calcium. In bituminous coals, calcium is most often present as the mineral calcium carbonate; in low-rank coals, it is generally present as an organically bound element (7). By analogy, trace elements can be present in a variety of forms (1,5). During combustion, elements associated with the organic matrix are more likely to be released, either through vaporization or through escape with the gases of devolatilization. The determination of elemental form is thus a key factor in determining the combustion and gasification behavior of an element. Several studies have been conducted at PSIT during the past few years to investigate the combustion transformations of trace element species. Limited exploratory work to assess the feasibility of removing some of these species from the vapor phase with selected sorbents has also been conducted. A n overview of these efforts is given in this presentation. For more detail, publications describing individual aspects of the process are available (8-11). EXPERIMENTAL Two bituminous coals from the Illinois basin, a physically beneficiated sample of one of the Illinois coals, and a Wyoming sub-bituminous coal were considered in the studies reported herein. Combustion experiments were conducted in the PSIT laboratory reactor facility shown in Figure 1. This facility is described elsewhere (12). Briefly, utility grind samples of pulverized coal (nominally 7 0 % through a 200 mesh screen) are mechanically injected into the top of the furnace. Combustion gases enter through a separate port. Combustion occurs as the gases and particles flow downward through a three zone electrically heated furnace with a maximum wall temperature of 1773 K. Upon completion of combustion, and after approximately 2.5-3 seconds residence time in the furnace, particles are sampled extractively with a water cooled nitrogen quenched sampling probe. Nitrogen is transpired radially through the inner porous walls of the probe to minimize particle loss during sampling. Particles are then size segregated aerodynamically with a Pollution Control 2 11-14 |