OCR Text |
Show Unlike other incineration systems, the M5FB process does not require a downstream scrubber for acid gas control. Limestone is fed directly into the combustor under optimum temperature conditions to remove acid gases such as 502' Hel, HF, and P20S. In addition to acid gas control, it has also recently been reported that recirculated unreacted lime can also serve as a catalyst in other emission reduction reactions such as the destruction of phenolic compounds (4). As in the case of fuel, the dense bed permits a wide range of ll:mestone feed sizes. The dense bed also provides turbulent mixing and additional residence time for sorbent calcination and reaction. Because the limestone subsequently becomes part of the entrained bed, the high solid recirculation rate - inherent in circulating fluidized bed systems - results in high limestone utilization. NOx control is achieved through staged combustion. The lower dense combustion zone can be operated under fuel rich conditions while secondary air is introduced to the upper entrained bed region to complete combustion. Depending on the level of control required, the lower combustion zone can be operated at stoichiometries of 40-90% of theoretical air. Since there are no heat exchange tubes immersed in the dense bed, erosion and corrosion of steam generation tubes is avoided. As in all staged combustion systems, proper injection and effective mixing of secondary air is required to prevent the formation of unmixed combusion zones. The design of the secondary air ports is particularly important in incineration systems to insure that unburned products of combustion and trace organics leaving the first combustion stage are efficiently consumed and destroyed. The mixing of secondary overfire air with gases and entrained solids rising from lower combustion zone have been analyzed using FLUENT, a three dimensional turbulent flow simulation code. A typical analysis for an MSFB entrainment zone is illustrated in Figure 3. COMMERCIAL M5FB DESIGNS The evolution of industrial MSFB boiler designs are shown in Figures 4-6. Figure 4 describes one of two lS0,000 lb/hr MSFB boilers designed and constructed by Riley Stoker for the General Motors Truck and Bus Group in Fort Wayne, Indiana. This unit is designed to burn both coal and waste paint sludges in the presence of limestone injection for S02 control. The system design utilizes a circular refractory lined combustor. Flue gases leaving the cyclone are cooled in a conventional convective boiler. A 37S,000 lb/hr coal fired system currently under construction at A.E. Staley's food processing plant in Decatur, Illinois is illustrated in Figure S. Unlike the GM design, this unit includes waterwall heat transfer surface in the upper oxidizing zone of the combustor. Both the combustor heat transfer surface and an EHE are used to control combustor temperatures. A 660,000 1b/hr MSFB boiler plant recently commissioned at an Idemitsu oil refinery in Japan is illustrated by the general arrangement shown in Figure 6. This represents the largest MSFB system designed and built to date. The Idemitsu unit is also designed to provide superheated steam at 1004°F. 3 |