PUMPS, VALVES & ACTUATORS TAKING ON ABRASIVE LIME SLURRY


Wilfried Staijen, regional account manager at Bredel Hose Pumps, says peristaltic hose pumps outperform centrifugal units in flue gas desulphurisation


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n order to stay within strict emissions thresholds, coal-fired and energy-from- waste (EfW), steel plants and oil and gas


refineries, use lime to remove sulphur dioxide (SO2) from their flue gas output. Although effective, the abrasive nature of lime presents a problem to the dosing pumps employed for this purpose. Many facilities using centrifugal pumps, for example, have experienced recurring problems with seal failures, leading to excessive maintenance and repair costs. The solution to this issue for many energy plants is peristaltic hose pumps, such as the Bredel range from Watson-Marlow Fluid Technology Group (WMFTG). A recent case in point has seen the


adoption of eight Bredel pumps as part of a flue gas desulphurisation (FGD) process at a European EfW plant. Used for dosing abrasive limestone slurry, the Bredel pumps are helping the facility gain greater control over its processes and reduce OPEX (operational expenditure) costs. The use of lime in all three main flue gas


treatment processes - dry, semi-dry and wet - shows its flexibility and adaptability as a global flue gas treatment. Calcium oxide (CaO - quicklime), calcium hydroxide (Ca(OH)2 - hydrated lime) and calcium carbonate (CaCO3 - limestone or chalk), can all be used to neutralise acidic gases and remove SO2 from flue gases. This process helps to ensure that plants comply with both local and international environmental legislation for emissions. In short, lime is the most cost effective


and versatile alkali that can be used for flue gas treatment, and mineral lime reagents are used in abatement techniques at more than 85% of UK sites that treat flue gas. The number of EfW plants in the UK is


increasing, as the government targets a reduction in landfill, and looks to encourage more recycling and use of EfW. Such developments are reflected


around the world. In all locations, lime- based products can provide a cost-effective, efficient solution to the treatment of flue gases generated from the energy recovery process, in turn reducing the volume of waste sent to landfill. A further benefit of FGD is the creation of high grade by-products, such as gypsum, which is sold commercially for use in plasterboard manufacture and other industrial processes.


10 MARCH 2021 | PROCESS & CONTROL A variety of flue gas abatement


techniques are in use today, each designed to suit particular applications. Dry scrubbing, for example, can be achieved using low-temperature dry injection. Here, hydrated lime is fluidised in air and injected directly into the exhaust ducting. High temperature dry injection is


another type of dry scrubbing technique where hydrated lime is injected into the kiln at temperatures in excess of 850˚C. The hydrated lime decomposes within 30 milliseconds to produce a porous and very reactive form of quicklime. This process can reduce reagent consumption if combined with traditional, lower temperature alkali scrubbing processes. When it comes to semi-dry scrubbing,


calcium hydroxide in water (called milk of lime or liquid lime) is atomised at the top of a spray dryer chamber into hot flue gases of approximately 220˚C. The water in the milk of lime evaporates, cooling the gases (SO2 and SO3, together with any HCl/HF present), which dissolve and react with the lime. However, it is wet scrubbing that is


considered the principal flue gas abatement technique, where crushed limestone is added to water before the resulting alkaline reagent slurry is sprayed into a flue gas scrubber or tower. In a typical system, the gas to be cleaned enters the bottom of a cylinder-like tower and flows upwards through the limestone slurry spray. An important aspect is the volume ratio of


reagent slurry to flue gas, which is known as the L/G ratio. L/G ratios are usually 1:1 (litres of slurry to 1000Nm3


applications. It is in this application that reliable, high-performance pumps have a fundamental role to play. During a site visit to a major EfW plant,


running a 15-hour wet scrubbing operation, it was noted that centrifugal pumps were frequently failing when transferring abrasive lime slurry reagent. In such cases, the principal problem is that slurry entering the scrubbers is both high in temperature and contains up to 25% dry solid content. Adding to these challenging characteristics, the slurry crystallises as it cools. Emissions from incinerators are under


scrutiny, so they need to perform without any unpredictable downtime. Realising the


of flue gas) in wet scrubbing


requirement to change its pumps in the wet scrubbing process to meet stringent industry demands, the European EfW plant trialled Bredel hose pumps. The successful trialled resulted in the installation of eight Bredel pumps. In Bredel hose pumps, which are both dry


running and self-priming, the actual pumping principle is based on alternating compression and relaxation of the reinforced rubber hose, drawing content in and propelling product away from the pump. As a result, the fluid being transported is only in contact with the hose, so it suits abrasive chemicals, such as lime. Unlike centrifugal pumps or progressive cavity


pumps, Bredel pumps are virtually maintenance-free as there are no expensive seals, valves, diaphragms, glands, rotors, stators or pistons to leak, clog, corrode or replace, leading to lower OPEX costs. Bredel pumps also include a rotor design


which does not rely on the gearbox shaft. This protects the bearings of the gearbox from overloads which might occur in other hose pumps, guaranteeing trouble-free and long lasting operation, even in heavy duty operations. In addition, despite the lime slurry being both


high in temperature and solid content, Bredel pump hose life is repeatable, and is not impacted by abrasion or crystallisation. Following the change from centrifugal pumps


to peristaltic hose pumps, the EfW plant reported a more controlled operation and improved OPEX costs. On a typical basis, some 255,000 tonnes of waste are incinerated at the facility from a total of 1 million tonnes collected. This process leads to the generation of 139,000 MWh of electricity, around 35,000 MWh of which is used by the plant, with the remaining 104,000 MWh sent to the grid. The Bredel pumps are helping the plant continue these operations while maintaining its commitment to the environment.


Watson-Marlow Fluid Technology Group www.wmftg.com


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