Case study: peristaltic pumps win converts at Danish site

Manufacturer Dong Energy began work last year on converting its combined heat and power plant in Denmark from coal to biomass.

Its goal was for its Studstrup CHP site – one of the largest in the country – to supply sustainable energy to Denmark’s second city, Aarhus, and provide green electricity to the national grid.

The switch necessitated construction of a plant facility for firing ash from coal combustion to ensure optimum binding of alkali in the flue gas – which would also protect boiler walls from scale/slag accumulation, corrosion and reduce poisoning of the deNOx catalyst.

Dong’s lead contractor Eurocon had to determine the most appropriate form of pump to employ for the process, which involved handling high density, inhomogeneous mass, abrasive particles and high pressure.

[The company] had to determine the most appropriate form of pump to employ for the process, which involved handling high density, inhomogeneous mass, abrasive particles and high pressure

Progressive cavity pumps had been considered but successive trails resulted in the selection of peristaltic hose pumps. This was a first use for Eurocon, which turned to Watson-Marlow Fluid Technology Group (WMFTG) for the necessary equipment, in the shape of its range of Bredel hose pumps.

For the plant facility, fly ash is mixed with 35% water, fed to the dosing tanks (each with a recirculation line) through a 650m-long pipeline and finally injected to the pellet mills.

Inside, to avoid settlement, four Bredel 80 hose pumps are deployed to circulate the fly ash slurry, which is then dosed from the recirculation line into a Babcock boiler.

The ash slurry is dosed via four wood pellet mills, each mounted with a Bredel 40 pump. In total, the installation involves 21 Bredel pumps of mixed sizes, some of which run continuously.

Before construction it was necessary to determine the viscosity of the ash slurry, as well as measure the pressure drop of the pipes at a given water percentage so that pumps and pipe runs could be dimensioned, explains Edo van der Meulen, training and application manager at Watson-Marlow Bredel BV.

“From this, we calculated that the mixing plant could be sized to 50m3/h, with the supporting dosing system to feed in at a maximum of 280m3 per day. WMFTG participated fully in the dimensioning of the pumps and with the pressure calculations,” he says.

The effect of abrasiveness on hose life is low; the effect of factors such as dry solid content and pump speed is much higher

Edo van der Meulen, training and application manager, Watson-Marlow

Fly ash is traditionally handled by double-acting hydraulic pumps at £0.5 million apiece, but collaboration between the three companies produced a cheaper alternative.

This focused on maximising natural rubber hose life, adjusting line velocity, and use of pulsation dampeners to reduce system back pressure to 50%.

“The effect of abrasiveness on hose life is low; the effect of factors such as dry solid content and pump speed is much higher,” says van der Meulen.

“At Studstrup, the dry solids content of the fly ash entering the system is controlled automatically – when it exceeds the limit at which the slurry can be successfully pumped, a Bredel 40 is activated to add water until the B100 [used to transfer the prepared slurry from a buffer tank to the storage tanks] is able to pump again.

“As for pump speed, this has an almost linear effect on hose life: half the speed is double the hose life.”

A second installation for Dong Energy is under discussion.