Circulating Fluid Bed Boiler

Application

Circulating fluid bed boiler. Two off-line pulse-jet baghouses, each filtering 90,000 cfm.

Optimization Potential

An air-pollution control system with low capital costs that would access a non-attainment area (i.e., emissions less than 0.003 gr/acf for particulate) and recover from process upsets during boiler debugging/shakedown. The combination of high grain loading, small particle size, and high filter-cake drag associated with fluid-bed-boiler fly ash and the low emission requirement would usually require an air-to-cloth ratio of 3.0 cfm/ft or less with conventional filter bags.

Solution

GORE® membrane/Teflon® B fiberglass fabric filter bags were specified at an air-to-cloth ratio of 5.3 net/4.0 gross cfm/ft. Gore guaranteed performance, provided representatives be permitted to assist during bag installation, fluorescent powder leak testing, and start-up.

Results

The filter bags were successfully installed with assistance from Gore representatives, and several tubesheet-weld leaks were identified and corrected. Gore representatives worked closely with start-up personnel and determined that several episodes of high DP were the result of higher-than-design airflow and overloading of the fly-ash evacuating system. Airflows as high as 52 percent above design were measured, resulting in a net air-to-cloth ratio of 8.0 cfm/ft. Several hoppers filled up to the baghouse gas inlet, resulting in reentrainment of fly ash. After corrections, the baghouse recovered to a flange-to-flange DP of 4.5" w.g. after cleaning. It was noted that conventional filter media would have become blinded due to fly-ash penetration caused by high filtration rates and high pressure drops. Excessive airflows may be a long-term condition, and the ability of GORE-TEX membrane filter bags to operate at higher air-to-cloth ratios permits further boiler debugging until permanent solutions can be implemented.