A remarkable feat of engineering is helping National Grid meet strict emissions standards on its compressors. Project Manager Kye Ettridge explains.
National Grid’s Gas Construction team has just completed one of its biggest structural engineering projects of recent years. It saw us apply innovative technology to a large compressor unit in Aylesbury and drastically reduce the amount of carbon monoxide (CO) it emits.
It’s the first step on a longer journey to bring all our large compressors in line with the Industrial Emissions Directive (IED). This EU regulation aims to reduce air pollutants by providing clear limits on how much nitrous oxide (NOx) and CO industrial installations such as ours are permitted to generate.
Our compressor station in Aylesbury was the first one we looked at. It’s an important site for our business, providing critical back-up for regions in the south of England. It fell under the scope of IED because it has a thermal input of greater than 50MW. This required us to achieve CO levels not exceeding 100mg per cubic metre and NOx not exceeding 75mg by the end of last December.
More than five times more cost-effective
We’ve been working on reducing our environmental impact across the business for a number of years. As part of this earlier work, our compressors in Aylesbury had been fitted with something called a dry low emissions (DLE) system. This meant that NOx output was already low and complied with the IED.
However, we still needed to hit the CO target, and we initially thought we would have to replace the entire compressor unit to achieve this.
As we discussed ideas across our engineering workforce, we realised there was a smarter, more cost-effective solution.
An expert in our Gas Transmission Owner (GTO) team put forward a technology called static catalyst abatement. Rather than replacing the full compressor unit, both exhaust stacks could be removed, with new stacks installed in their place with static catalysts built into them.
After thorough analysis, we established that the solution could cost nearly 5.5 times less than replacing the entire units, reducing the materials used, construction costs and the length of the project dramatically. Savings would run into the millions.
The catalyst technology we used is like a giant metal grid structure made up of lots of individual cartridges. Each of these is covered with a precious metal (in this case platinum), which oxidises with the exhaust gases as they pass through the catalyst. As a result, most of the CO subsides.
Once the solution was agreed by the business, we moved onto design. As this was the first time we’d integrated the technology into one of our compressors, we had to consider how it would affect our GTO operations colleagues who carry out ongoing maintenance work.
We built extensive maintenance access into our designs, putting in a lower and upper platform all the way around the stack to make things like visual inspections and environmental compliance tests as easy as possible.
In addition, we installed a continuous emissions monitoring system (CEMS) on each compressor. These will provide us with ongoing evidence that we are complying with the IED.
With our plans in place, we began the build. Siemens were brought in as the main contractor. We also worked with Cullum Detuners, an expert in exhaust stack replacement and a specialist catalyst supplier.
The construction work itself took around 18 months. During that time we carried out major civil engineering upgrades to either side of the compressor to accommodate the added weight of the new exhaust stack and platforms.
Like a giant Meccano set, a huge number of steel beams and platforms formed the main structure. Once they were built, the new exhaust stacks were lifted into place in sections and coupled together. Finally, the CEMS were added, which required a great deal of cabling and wiring, as well as some additional pipework plumbed in to the outside of each exhaust stack.
Commissioning and performance tests followed, which showed that we were outperforming the IED targets. For example, we needed to ensure the catalyst was 95pc efficient at reducing CO, and ours is operating at 98.5pc. So it’s an incredibly effective solution.
We also anticipate an overall reduction in CO of more than 2,000 tonnes over the 20-year design life of the new system.
With our targets successfully met and all construction complete, we’ve now achieved operational acceptance on Unit B. Our first IED-compliant unit is now a vital part of our compressor fleet and has been delivered with savings in the region of £53m against our Ofgem allowance for entire new units – a fantastic result for Gas Construction and National Grid.
The next phase of innovation
We see this project as a stepping stone to further innovation and want to explore whether this or a similar technology could be used across our compressor fleet. One exciting potential solution is SCR (selective catalytic reduction), a technology that is capable of reducing NOx as well as CO.
Although our final solution may seem relatively simple, it was a complex, multi-discipline project, with everyone from civil to structural engineering, specialist catalyst technology, and internal and external stakeholders closely involved.
I feel proud that our team delivered the project on time and under budget, despite needing to overcome several engineering challenges along the way. Through hard work and collaboration with all parties involved – from our contractors to all our GTO colleagues – we managed it successfully, safely and to good environmental standards.
What’s more, we’ve put important, sustainable measures in place on this installation that will last long into the future.