from vertical to a 14-inch OD outlet. Below the silo was a slide gate and a double flap valve that discharged ash into a “conditioner” before it was unloaded into trucks. The desired fly ash discharge rate from the silo was about 6,000 pounds per hour. As a result of its temperature, consolidation pressure in the silo, storage time without discharge, ash moisture content and the presence of reagents, the ash frequently set up in the silo. It also arched and ratholed over the silo outlet. This had been a persistent problem at all three power stations. Dominion needed to rely on air cannons and air lances to break the arches and restart flow. While these flow aids provided some help, they were not able to resolve the issue. These ash flow problems were resulting in frequent manual intervention, inconsistent flow to the ash conditioner and long waiting times for truck unloading. In addition, during periods of high fuel moisture, the unburned carbon content of fly ash increased. Coupled with the air leaks to the baghouse hoppers and pneumatic conveying system of ash, extremely high-temperature (greater than 1,000 degrees F) clinkers would form in the baghouse hoppers and the fly ash silos, further complicating the material buildup issue. Dominion recognized the need for a scientific approach to solve this problem, and in 2015, contracted Jenike & Johanson Inc. to address the flow problems in the ash unloading silos at these three stations. Riad Dandan, balance of plant systems engineer from Dominion’s headquarters in Richmond, Virginia, was involved throughout the project and was instrumental in identifying the problems and providing project direction, as were numerous personnel from the three power stations.
Bulk Solids Handling Fundamentals
Before presenting analysis of the problems with the fly ash silo, it is helpful to understand some of the fundamentals of material flow in a silo. There are two primary flow patterns that can develop in a silo during discharge: funnel flow and mass flow. In funnel flow, an active flow channel forms above the hopper outlet, with stagnant material at the periphery. As the level
of material in the silo decreases, material from stagnant regions may or may not slide into the flowing channel, depending on the bulk solid’s cohesive strength. When the bulk solid has sufficient cohesive strength, the stagnant material does not slide into the flow channel, which results in the formation of a stable rathole. In addition to flow stoppages that occur as a consequence of ratholing, funnel flow results in a first-in, last-out flow sequence, and allows flooding if fine powders such as the biomass fly ash are filled into a rathole, or if a rathole collapses. In mass flow, the material is in motion whenever any is withdrawn from the hopper. Material from the center, as well as the periphery, moves toward the outlet. Mass flow hoppers provide a first-in, firstout flow sequence, eliminate stagnant material and provide a steady discharge with a consistent bulk density and flow that is uniform and well controlled. Requirements for achieving mass flow include sizing the outlet large enough to prevent arching and ensuring the hopper walls have sufficiently low (material/surface boundary) friction and are steep enough to achieve flow at the walls. It is critical to note that the flow pattern in a hopper is strongly influenced by the feeder below the hopper. Ultimately, a feeder should accomplish the following: 1. Provide reliable and uninterrupted flow of material from the silo above. 2. Control discharge rate from a silo, achieving the required rate while preventing flooding. 3. Remove material from the entire cross section of the hopper outlet to avoid interfering with mass flow in the silo above. If a screw feeder has constant pitch (Figure 2, top image), the first pitch will fill up and subsequent pitches will be unable to accept more material. As a result, material will flow through one small preferential channel above the first pitch, creating the potential for ratholing, increasing arching potential and decreasing the maximum flow rate for fine powders. If the screw feeder is properly designed with increasing capacity in the direction of flow (Figure 2, bottom image), it will remove material from the entire cross section of the hopper, and the flow channel will open up.
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