SO 2 concentrations, high acidic deposition to the forest canopy) plays a dominant role in driving forest health. Retrospective analysis of Norway spruce ( Picea abies) tree rings in the Czech Republic shows that air pollution (e.g. However, the fine particulate can also cause problems with the generation of effective corona by the DEs due to an effect called space charge.The forests of central Europe have undergone remarkable transitions in the past 40 years as air quality has improved dramatically. The resistivity of the particulate is low, so it is collected easily in the ESP. A recovery boiler is a unique application for a precipitator due to the small particulate size and the tendency for the cohesive ash particles to stick together. For recovery boilers, precipitators are used to collect the residual salt cake in the flue gas. The power boiler particulate emissions requirements are the same as those for the industrial units using the same fuels. In the pulp and paper industry, precipitators are used on power boilers and chemical process recovery boilers. For these applications, the ash in the flue gas is typically more easily collected than coal fly ash so an ESP of modest size will easily collect the particulate. In addition to coal, industrial steam generating units where ESPs are successfully applied include municipal refuse incinerators, and wood or bark-fired boilers. ESPs have also been installed on boilers that fire oil as their principal fuel and operate at emission levels similar to ESPs operating on coal-fired units. The result will be a collector that can continuously operate to meet the particulate outlet emissions requirements. To meet the particulate control regulations for utility units, as well as the required high collection efficiency, special attention must be given to the details of precipitator sizing, powering, electrical controls, rapping, flow distribution and gas bypass around the collector plates. See our article on ash removal basics in the Learning Center.īecause coal is a common fuel for steam generation, collection of the coal ash particles via an ESP is historically the most commonly used collection system. Once the particulate has reached the hopper, it is important to ensure that it remains there in bulk form with minimal re-entrainment until the hopper is emptied. The dislodged sheets fall from the collection surface into hoppers. A separate rapping system is therefore used to remove deposits from the DEs and maintain proper operation. If allowed to accumulate, the ash layer would suppress corona generation. While most of the particles are driven to the CEs, particles in close proximity to the DEs receive a positive charge and are therefore attracted to the DEs. This sheeting is important to prevent the re-entrainment of individual particles into the flue gas stream, requiring additional recharging and recollection downstream. It is important that the rapping frequency allows an adequate thickness of dust to collect on the plates so that the accumulated ash can be removed in sheets. The most common removal method is rapping which involves mechanically striking the collection surface to dislodge the ash. The ash layer must be periodically removed. These forces also tend to make the individual particles agglomerate, or cling together. The particles remain on the collection surface due to the forces from the electric field as well as the cohesive forces between particles. The ash particles form an ash layer as they accumulate on the collection plates. A series of CE and DE sections is generally necessary to achieve overall particulate collection requirements. Gas velocity between the plates is also an important factor in the collection process since lower velocities permit more time for the charged particles to move to the CEs and reduce the likelihood of migrating back into the gas stream (re-entrainment). Lower resistivity indicates improved ability to accept a charge and be collected in an ESP. Resistivity is an inverse measure of a particle’s ability to accept and hold a charge. Other particles are charged easily and driven toward the plates, but also may lose the charge easily after contacting the grounded CE, requiring recharging and recollection. Some particles are difficult to charge, requiring a longer residence time. The negatively charged particles are attracted toward the grounded collecting electrodes and migrate across the gas flow.
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