OCR Text |
Show The distinguishing feature of MSFBC is its entrained bed of fine ash and limestone particles superimposed on a dense bed of large - 1/8 to 3/4 inch diameter - particles. This lower dense zone of large heavy particles along with an external heat exchange (EHE) system and a two stage design give the MSFB process broad fuel flexibility. These features are illustrated in Figure 1. The lower dense bed of large particles, which do not leave the combustor, serves to promote mixing and increase the residence time of the entrained ash, fuel and limestone particles. The effect of this dense zone in increasing the residence time of the entrained bed is shown in Figure 2. The dense bed essentially IIholds Upll fine particles in the lower combustor at superficial velocities higher than their entrainment velocity. To achieve comparable residence times without a dense bed the system would have to operate at much lower velocities, or at much higher recycle rates. The residence time provided by the dense bed allows MSFB combustors to operate at superficial velocities of 30 to 35 ft/sec. The recycle ratio of returned solids to fresh fuel feed is typically 60:1 for coal-fired MSFB systems. A variety of naturally occurring and synthetic materials from gravel to alumina beads have been evaluated as dense bed materials. Since combustion zone residence time is crucial to combustion efficiency and thermal destruction of hazardous chemical species, the size of this dense zone can be designed to accommodate fuels with differing combustion characteristics and to meet various emission requirements. Because of the intense back mixing in the dense bed region and the attrition produced by the heavy stones, coarse fuels up to 2 inch in size can be accommodated. Most of the entrained particles leaving the combustor are captured by a hot cyclone and recycled to the combustor through an external heat exchanger (EHE). The EHE consists of heat exchange tubes immersed in a conventional fluidized bed operating at one to two feet per second. This design allows for all or some of the recycle solids to cool before being injected back into the combustor. Non-mechanical "L" valves ·are used to transport the recycled solids. Combustor temperatures can be controlled to 1500° to 1700°F by the recycling of solids through the EHE. The EHE combined with the ability to operate at high combustion velocities give MSFBC flexible load-following and high turndown capabilities. The decoupling of combustion and heat transfer allows each process to be optimized separately. This important feature of MSFBC can be used to advantage in achieving destruction of hazardous organic species, as well as in controlling criteria pollutants such as sulfur dioxide, S02, nitrogen oxides, NOx and carbon monoxide, CO. 2 |