Methods of Improving Limestone Utilization in FBC

摘要

Increasing the calcium utilization of spent limestone from 20 to 50% in a typical 600-MW AFBC would eliminate the need to quarry and transport approximately 500,000 tons per year of fresh limestone, would decrease waste production by a corresponding amount, and would save a total of approximately $6,000,000 per year in direct costs. The techniques that have been proposed to improve limestone utilization can be divided into three broad categories: (1) pre-sulfation treatment, (2) modification of sulfation parameters, and (3) post-sulfation treatment to allow reuse of the limestone. Pre-sulfation treatments include both heat treatment (pre-calcination) of the limestone and addition of small quantities (0.5 to 2.0%) of various salts to the limestone. Both of these methods alter the pore structure of the limestone so that more of the calcium is accessible for sulfation. Operational parameters having significant effects on limestone utilization are gas residence time, limestone particle size, and bed temperature. Gas residence time can be increased (for increased limestone utilization) by utilizing deeper beds or lower superficial gas velocities. Also, decreasing the limestone particle size generally increases limestone reactivity but also results in more elutriation. Elutriated particles must then be captured in downstream cyclones and recycled to the bed in order to improve limestone utilization. Post-sulfation treatments to allow the reuse of limestone include (1) grinding and subsequent pelletizing, (2) hydration, and (3) regeneration. These methods are compared as to their mechanisms, effectiveness, difficulty of scale-up, potential side effects, retrofit potential, possible problems with materials of construction, energy efficiency, and cost. (ERA citation 81:024955)