Bulk Solids Flow: Know What you’re Dealing With

Josh Marion discusses what should go into a proper Basis of Design document for a solids handling system

AS set out by Grant Wellwood in Part 1 of this series (Solid States, issue 945), you’re the lead process engineer on a team assembled to deliver a unique energy-from-waste (EfW) power station. The project is in the FEL-3 stage, having just been authorised for detailed design and delivery, and some of the long lead items have already been ordered.

You know from your colleagues that the feasibility study (FS) team included a number of expert chemical engineers who know their Perry’s Handbook front-to-back, and have calculated the EfW conversion reaction chemistries, thermodynamics, and kinetics within microns of their collective lives – after all, our power station’s main objective is to get useful energy from waste, right? And they’ve gone through the financials to make it all happen.

You realize EfW transformations generate acid gas byproducts (eg SOx, HCl, etc) that need to be scrubbed from the flue gas exhaust prior to releasing to atmosphere. If acid gases are not properly controlled, you could get hit with hefty fines for violating emissions standards. These fines could quickly add up and may ultimately sink the entire project, even if the EfW conversion itself is going really well. Therefore, even though it’s a seemingly unimportant part of the EfW conversion process with no additional energy generation and no apparent monetary value added, the acid gas mitigation system is vital for project success. How is acid gas controlled?

Acid gas control is frequently accomplished with systems that use lime and water slurries to absorb the acid gases, namely, flue gas desulfurisation (FGD) systems. The most common type of FGD system is a wet scrubber system, which typically consists of a lime – either quicklime (CaO) or hydrated lime (Ca(OH)₂) – powder handling system, a weigh feeder for metering out the lime powder dose, a mixing/slaker system for slurrying the lime and water, and a pumping system for pumping the slurry through banks of atomising nozzles into the flue gas scrubber. Lime slurry is sprayed into the flue gas exhaust, and the spent liquor, which is now impregnated with CaSO₄.2H₂O (gypsum), flows down through the absorber. The “waste” slurry is dewatered to produce a gypsum cake for disposal or sale¹. It is critical that the lime powder be metered into the slurry at the proper dosage amount. If the lime concentration in the slurry is too low because of flow stoppages or insufficient flow upstream, emissions excursions may occur, potentially resulting in fines for regulations violations. On the other hand, if the lime concentration in the slurry is too high due to overuse, not only could reagent costs be excessively high, but it can also make it more difficult to sell the waste gypsum cake, damage downstream equipment, and the slaker can quickly boil over due to excess heat of solution (since dissolving lime in water releases heat due to the exothermic reaction).

Your past experience has made you painfully aware of a potential danger lurking here – powder handling. You know that powder handling is regularly overlooked, so that problems with bulk solids/powder handling are all-too-often the fatal flaw and bottleneck in the entire project². Some of the most common flow problems are:

• Arching/bridging: A “no-flow” condition where material forms a stable arch-shaped obstruction over the outlet (Figure 1a).
• Ratholing: A “no-flow” condition where a stable open channel forms above the outlet and stagnant material outside of the channel does not empty (Figure 1b). Ratholing can result in erratic flow and a complete flow stoppage when the channel is emptied, and can reduce the silo’s “live” capacity by as much as 85-90%.
• Limited discharge rate: A “not-enough-flow” condition when interactions between fine powders and air restrict the discharge rate.
• Flooding: A “too-much-flow” condition when the powder is highly aerated and behaves like a liquid, causing it to flow uncontrollably.
• Caking: A condition in which stagnant lime may agglomerate into large clumps, potentially causing flow stoppages and/or altering the particle size fed into the slaker.