Pressurized fluidized-bed combustion ash-letdown systems are plagued with several reliability and operational problems. These system deficiencies include mechanical lockups resulting from expansive thermally-induced forces, high wear rates due to the abrasive nature of fast-moving ash particles and significant difficulty in maintaining system pressure resulting from excessive gas leakage across existing lock-hopper systems. A very simple system concept has been conceived as an alternative to the unreliable high-pressure lock-hoppers systems, rotary valves or solid let-down pumps presently utilized in todays commercial pressurized fluidized-bed combustors. The approach involves a single ash quench pressure vessel operated at system pressure. The quench vessel receives ash flowing by gravity from the attached high-pressure coal combustor. This hot-ash is directly water quenched as gravity-fed solids particles are fed from the main combustor vessel. The density of the ash-water slurry is controlled by recycled make-up water fed to the quench drum. The slurry mixture flows across a single control valve to lower the pressure of the stream. A quench drum level controller maintains a constant liquid head to prevent any high-pressure gas from escaping, thus providing a leak-tight seal on the coal combustor. The byproduct liquid water-ash slurry is processed in conventional sludge handling and separation equipment. The proposed Phase I SBIR research effort is to develop a prototype design of an Advanced Slurry-Phase High-Pressure Let-Down and Transport System for Coal Ash. A detailed thermal, mass transport and fluid flow analysis of the system is planned, a representative design basis will be selected to establish representative capacity requirements, two-phase density and pressure vessel level control algorithms shall be developed, equipment-component specifications including materials and sizes will be generated. Additionally, specific recommendations for the Phase II SBIR program shall be formulated. The detailed analytical results of an ash-water-steam fluid flow and transport model of the high-pressure let-down system shall be reported upon.
Commercial Applications and Other Benefits as described by the awardee: Typical applications for the slurry-phase ash transport system include commercial use and installation in both new and existing pressurized fluidized-bed combustors and high-pressure coal gasification complexes. The benefits of this system concept are its design simplicity, low cost, few moving parts, ash metering capability and zero gas leakage from the adjacent high-pressure combustor vessel. The market for this type of system would principally be the United States, but would also have commercial potential in world-wide energy sectors.
