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Show rebuming zone. Furthermore, Bose and Wendt were unable to predict their NH3 profiles during staged combustion, while Mereb and Wendt were able to do so for their reburning results. It therefore seems appropriate to attempt to reconcile the previous work, and to determinme whether, in fact, there is not a common fundamentally based mechanism that is equally valid for both air staging and reburning processes. In attempting this reconciliation, we recognize that some changes in our previously published conclusions might be required, and we think it fitting that this laboratory should now be in the position of being able to correct these inconsistencies and errors. EXPERIMENTAL APPROACH Figure 1 shows a schematic of the 15 cm ID laboratory combustor that was used in this study. A more detailed description of the combustor is given by Bose et al.5 • The original design of the fire tube (air staging experiments) consisted of three layers of va urn formed alumina cylinders. In the reburning experiments, the inner layer of alumina cylinders was replaced by a more durable layer of silicon carbide refractory (Figure 1). The combustor allowed self sustaining combustion of 1-2 kg/h coal, with no external heating, in a configuration that was representative of practical units in terms of characteristic times and temperatures. The unit was designed to bridge the gap between the more 1----80.4 c",---I CoallTronsport Air ~ ~ 5.0c", Secondary Air I I ! . -Exhaust fundamental flat flame and drop tube Figure 1: Laboratory Downflow Combustor. experiments and pilot and full scale coal combustion tests. The burner was designed to produce a premixed flame, as described by Bose et al.( 1988). In the reburning experiments, the first stage was operated fuel lean and the reburning fuel (natural gas or coal) was injected downstream. The reburning fuel injector was designed to produce adequate mixing in the reburn zone within 0.18 seconds, as indicated by temperature and species profiles that were independent of radius at the first mea rement downstream of the injection point. The injector tip was supported by a water cooled holder and consisted of 8 holes, 0.14 cm ID eacr nd allowed the reburning fuel to flow radially with respect to the probe axis. Nitrogen gas was injected with the reburning fuel to produce jet exit velocities that were at least 70 times the bulk flue gas velocity (1 second). The fuel rich air staged combustion data were extracted from the work of Bosell . These tests were conducted under a wide range of conditions, with fuel rich stoichiometries ranging from 0.6 to 0.8 and involved two coals, Utah Bituminous coal and German Brown coal. In some experiments, temperature changes were achieved by N2 dilution and O2 enrichment of the primary flame. 2 |