Coal Age

JUL-AUG 2017

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40 www.coalage.com July/August 2017 operating ideas A Better and Safer Way to Clean Coal Ash Silos by jimmy peck MPW Industrial Services borrowed con- cepts from the oil field, window-washing and confined space rescue trades to de- velop a better and safer way to clean coal ash silos, which receive as much as 100 million tons of material in the United States each year. The ash is generated when coal is combusted and the resulting flue gas is captured by electrostatic precipitators or baghouses before entering smoke stacks. The ash, which has a fine particle size dis- tribution with most less than 100 microns, is typically collected in pyramid hoppers then transferred to silos. Silos require cleaning when moisture bonds with the coal ash inside, restricting the downward flow of material to trail- er trucks that transport it to landfills or to other facilities for a myriad of reuses. Sources of moisture include condensation from duct work and water seeping through cracks in the silo's concrete structure. Rat holing and arching are the two most icommon conditions that require the cleaning of coal ash silos. Rat holing occurs when the ash sticks to the sides of the silo, narrowing the pathway for the downward flow of material. Arch- ing, meanwhile, occurs when ash forms an arch or bridge above the outlet at the bottom of the silo, effectively clogging the vessel and preventing it from receiv- ing material. As a result of these and other condi- tions, silos must be cleaned, which typical- ly happens once a year during outages in early spring or late fall, when the demand for energy is traditionally low. The cleaning process begins by allow- ing time for the coal ash to cool, which can reach as much as 2,800°F during combus- tion. The traditional cleaning method then requires a worker to enter the silo and vac- uum the residual material while descend- ing a ladder built into one side of the struc- ture's interior. Given the ease with which the hose collects the ash and the limited range for maneuvering it while holding onto the ladder, workers can effectively dig in a hole in the fly ash. As a result, they of- ten find themselves with unstable material overhead, which can engulf them. To minimize the risk, MPW Industrial Services recently devel- oped a process in which workers remain above the coal ash at all times thanks to ideas borrowed from other industries. Using a tri- pod situated at the top of the silo, just like the ones used for con- fined space rescues, MPW lowers a worker in a full-body harness and seat approximately 5 ft at a time down the middle of the silo, similar to the process oil compa- nies use to raise and lower work- ers servicing derricks. In addition, the worker sits on a Bosun's chair, which is typi- cally used in the washing of win- dows on skyscrapers and other tall buildings. This enables the worker to vacuum fly ash comfortably be- neath him or her and across the entire diameter of the building until reaching the bottom. MPW and its novel approach to the cleaning of coal ash si- los were recently requested by a coal-fired power plant when a prior contractor required two confined space rescues. With four silos still to be cleaned, MPW dispatched an eight-per- son team consisting of one su- pervisor, two workers to vacuum the ash, two hole watchers, two confined space rescuers and one parts person or runner. Workers who entered the silos wore safety shoes, hard hats and full-face respirators with P-95 car- tridges. Each tripod was outfitted with two wenches. The primary wench raised and lowered work- ers. The secondary wenches were solely for bringing the workers back to the top of the silo if the primary wench failed. Confined space rescuers, meanwhile, had their own tripods and wenches. Using a tripod situated at the top of the silo, MPW lowers a worker in a full-body harness and seat ap- proximately 5 ft at a time down the middle of the silo to vacuum coal ash that has bonded with moisture.

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