One of Alden’s key business areas is designing, analyzing, and improving air pollution control equipment for fossil power plants. Over the last decade, circulating dry scrubbers (CDS) have become a popular way to remove SO2 and other acids from the flue gas of coal-burning power plants. A few of the benefits of CDS technology include a relatively low cost of installation, high removal efficiency, low water usage, and no wastewater that needs to be treated – in fact it can be a means to dispose of other wastewater from the plant.
The technology works by injecting lime powder into a reaction vessel, and also spraying in a small amount of water (Figure 1). The water coats the lime particles and eventually evaporates, which helps to improve acid removal. The powder is suspended in the reaction vessel and mixed using a high velocity stream of flue gas entering from the bottom of the reactor. Once mixed and reacted, the gas carries the solids out through the top of the reactor into a fabric filter, which acts like a vacuum bag to separate the dust from the flue gas. The dust gets recycled back into the reactor, where in can circulate up to 100 times before being disposed of. The clean flue gas is then routed to the stack.
Figure 1: Schematic of a typical CDS system
One of the design goals of a CDS is to minimize the gas flow pressure loss of the system, which means keeping the gas velocity needed to suspend the bed in the bottom of the reactor as low as possible. The complimentary goal is to keep the bed of solids continuously suspended, even at low plant load when the flue gas flow rate is very low. Both of these need to happen while also maintaining a dense enough bed of solids to remove upwards of 98% of all the acids in the flue gas.
Alden has worked with several CDS manufacturers over the past several years - one of them being Babcock Power Environmental, Inc. (BPEI). Alden has worked closely with BPEI to significantly improve the typical venturi design they use for solids entrainment – which might someday be the subject of another post – but along the way we have also designed and patented a new entrainment device (Figure 2).
Figure 2: Gas Distributors for Circulating Fluidized Bed Reactors, U.S. Patent 8,883,082 B1
The annular venturi shape works as a two-stage entrainment device, whereas a typical venturi only uses one stage for solids entrainment. The second entrainment stage makes it so that the annular venturi can entrain the same solids flow at less than half the gas pressure loss of a normal CDS venturi.
The annular venturi was designed and tested at Alden using a two-phase gas/solids physical flow model at reduced scale. Glicksman’s scaling laws for fluidized beds were used to match the dynamic similarity between the scale model and the full size scrubber, which has shown excellent results in other model studies. Best of all, it was a lot of fun to test!
Here is a video of the final design with a solids flow rate a little higher than the maximum design condition. Gas flows upward from the bottom of the screen, and solids are injected through a conveying trough at the upper right of the video. Most of the solids are entrained from the upper lip of the venturi, and those that aren’t are entrained off of the top of the central hub. Every once in a while, a little cluster of solids falls out through the bottom of the throat, but that too gets entrained before it has a chance to build up in the bottom of the inlet duct.
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