Technology Focus: Pumps, mixers and pre-treatment technology sand which accumulate in the tank. Not only does this incur a large initial cost to remove – often manually and by cutting a hole in the side of the tank – it also brings long-term financial implications through operational downtime and the subsequent restart of the AD process.”
Some of the grit, glass and bone removed from the grit washer installed by Huber
For over 50 years, the process of using a vortex principle to remove grit from pulp has existed within the paper industry. Cellwood has developed a solution for the problem of grit in digesters by adapting its high-density cleaner used in the paper industry for use in AD. The technology has shown outstanding results and has been successfully installed in sites throughout Scandinavia. It can remove particles up to 250 µm and can be installed as part of an upgrade or within a complete pre-treatment system. “Most importantly, the removal of this grit means that build up in the digester is no longer a problem and the extra space due to lack of sediment means that biogas production is optimised. Not only are tank maintenance costs reduced, but less servicing is required on other machinery such as pumps and agitators, as they are subjected to reduced wear and tear,” adds Martin. Also providing solutions to the problem of grit in digesters is AD treatment specialist Huber Technology. The company was on hand when a competitor’s grit trap caused problems during the commissioning of a new AD plant in Northern Ireland. “The site operators were entering the original settlement tank up to three times a week to clear excess grit, as the system would block
The Bioextruder is brought to the UK by Rika Biofuels
and abrade pumps and tanks,” explains James Tucker, Industrial Business Development Manager. “The material removed was high in organics and therefore expensive to landfill. On top of this, the organic material removed with the grit resulted in lost biogas yield.” The plant takes in a mixture of green waste and food waste and puts this through a pair of hammer mills. This material contains a higher than normal amount of grit, which Huber had experience of following a similar project in Germany. The initial design request was for a circular tank for grit settlement. However, after reviewing the situation, Huber recommended its longitudinal grit trap and grit washer. The performance and reliability of the AD plant is now considerably improved – the stoppages experienced previously have been dramatically reduced, downstream equipment is experiencing less wear as the abrasive grit and glass have been removed, and the gas yield has increased due to the returned organics. For some AD operators, however, the issue is not just mixing before digestion takes place, it’s about mixing after the AD process, too. When David and Philip Hardy at Slipperlow Farm in Matlock installed a new AD plant, they built a suitably large lagoon to store the digestate and were keen to find a machine capable of mixing such a construction. Fortunately, they discovered GEA’s range of very long lagoon mixers that are built and used in the US and Canada. They selected the www.adbioresources.org adbioresources.org
15.85m Trailed SuperPump machine, which uses a powerful jetting device to break up crusts and fire down through the liquid to break up any sand dunes. Philip had some concerns about whether the machine would go through the thick crust, but within a matter of minutes it was mixing the lagoon. “I was pleasantly surprised at the ease it did this,’’ says David. “The jetting device got the whole crust moving in a circular motion and as the heavy lumps came to the 28" manure inlet, it gobbled them up without any issue.’’ The machine is capable of jetting 48.77m and in a 320-degree arc. In fact, with its 15.85m length on top, it made the lagoon look quite small. The ability to top-fill slurry tankers or ‘force feed’ umbilical pumps was also a feature that appealed to David and Philip: “Umbilical pumping was our intention, but since we have experienced the quick turnaround time with tankers, due to the filling speed, we now think a trailing shoe on the tankers would be a better option,” adds David. “Because the pump is so efficient at mixing the lagoon, we may consider mixing it once or twice in the winter, which should further reduce the thickness of crust.” www.futurebiogas.com www.vogelsang.info/en/vogelsang-uk www.rikabiofuels.com www.salvtech.com www.huber.co.uk www.gea.com autumn 2017 | AD & Bioresources News
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