Novel Temperature-Controlled Ash Deposition Probe System and Its Application to Oxy-coal Combustion with 50% Inlet O_2

摘要

Prediction of ash deposition characteristics under oxy-firing conditions helps to determine how retrofit to oxy-combustion might affect boiler performance. To obtain data to help achieve this end, a novel temperature-controlled ash deposition probe was designed to collect temporally resolved deposit samples in a 100 kW rated down-flow test furnace system, firing a Powder River Basin coal. The rig was designed to represent practical units in terms of temperatures and particle and gas concentrations yet still be sufficiently well-defined to allow for controlled systematic studies. The deposit probe/furnace system, which is described in detail, was designed to segregate early "inside" deposits from the average deposits gathered over a period of time. Deposits were gathered under controlled conditions for oxidant input conditions of 50% O_2/50% CO_2 (once through, with no flue gas recycle) and air. Effects of the deposit holding time, deposit probe temperature, and flue gas temperature at the probe location were investigated. Temporal segregation of deposits was achieved by physically separating deposits gathered on the horizontal probe surface into "inside" and "outside" deposits, where "inside" deposits approximated the initial deposit layers. Furthermore, results showed that deposits gathered over long times on the vertical surface of the probe were similar with respect to both composition and particle size distribution to the inside layer of the horizontal deposits, but different from the bulk horizontal deposits that have typically been reported in the literature. There were no significant effects of holding times greater than 1 h on bulk deposit compositions, although particle size within the deposit did appear to increase with time. There were also no significant differences between compositions of "outside" deposits from oxy-firing (OXY50) and those from air firing. "Inside" deposits from OXY50, however, contained higher Si and Fe and lower S and Na compared to those from air combustion. These results are interpreted in the light of available mechanisms. Tests in which only the deposit surface temperature was changed showed that the mass of deposits on the vertical surface parallel to the flow, shown to be representative of the "inside" deposits on the horizontal surface, was proportional to the temperature difference between the flue gas and the surface. This supported the hypothesis that the early layer was deposited largely by thermophoresis of small particles and not by Fickian or Brownian diffusion or impaction.