Feature: Reliable measurements pave the way for eco-friendly aggregate

Many countries in Europe have seen a rapid growth in energy from waste as a means of dealing with municipal waste and producing green energy. This process produces a small amount of residue from the flue gas treatment (APCr) that has historically ended up in landfills.

Instead of that environmentally-unfriendly outcome, UK company Carbon8 Aggregates has come up with a green option. It uses a patented process known as Accelerated Carbonation Technology to transform the APCr into a product called C8Agg. This is a carbon-negative aggregate that has many uses including in the manufacture of dense and medium-dense masonry blocks.

Tens of thousands of tons of C8Agg is produced annually at two plants that Carbon8 operates in the UK. Each site has a number of silos for reception and storage of the incoming residue. The site at Avonmouth, for example, has eight 130 cubic meter silos receiving between seven and eight tanker loads of APCr per day.

Driven by the environmental urgency to both divert landfill and reduce carbon dioxide in the environment, a third site in Leeds is currently being built, with other plants expected to come online in the coming years.

A uniquely difficult situation

The recipe for C8Agg requires the treatment of a precise mixture of APCr’s from a number of sources which vary widely in density, with the need for reliable and accurate inventory control being fundamental to the efficient running of the plant.

Unfortunately, getting accurate measurements turned out to be easier said than done with the technology Carbon8 had been using.

The grades of APCr not only vary in density but also have a very low dielectric constant, making the material hard to detect with conventional level solutions. Carbon8 engineers considered a number of options including silo weighing, but this was discounted because of the density variations meaning the weight is not a direct representation of the capacity of the silo.

Desperate for a solution, Carbon8 turned to their partner Underwoods Electrical Distributors, who brought in Siemens technology.

A high-frequency SITRANS LR560 radar level transmitter for solids was installed in each silo. The first high-frequency device on the market for solids level measurement, SITRANS LR560’s narrow four-degree beam immediately proved that it could deliver consistently accurate, repeatable and reliable measurements of the vexing low-density APCr ” and do it all without any recalibration.

Installation was a challenge, where the ideal mounting and sighting position in the silo for the transmitter was occupied by a filter unit already in that position. But because of the SITRANS LR560’s narrow beam and compact size, it could be placed under the filter and still perform perfectly.

With their reliable measurement package in place, staff at Carbon8 can now view the accurate level readings they need right on their control system’s monitor. The technology is free of problems and there’s no longer any need for manual intervention, while maintenance, cleaning and upkeep are minimal.

“It was vitally important that we have a reliable level measure,”says Tom Bissett, National Engineer and Projects Manager for Carbon8 Aggregates, admitting that after all the technological problems he and his team had been through, he was initially skeptical that Underwoods and Siemens could deliver, only to end up with a very pleasant surprise.

“The SITRANS LR560 solved a problem we thought could not be achieved within our budget,” he explains.”Siemens and Underwood worked closely together through the initial presentation, feasibility trial, extended plant trial and final installation and commissioning. I am completely satisfied with the results of their combined effort.

“Other manufacturers and distributors did not have the confidence to offer a no obligation trial. We will spread the word in the energy-from-waste world if we hear of any of our customers and partners having similar difficulties to what we had before using the Siemens technology.”

Denise Flanagan is Business Development Manager at Siemens.

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