The great red bus upgrade02 October 2017

London mayor Sadiq Khan wants the capital’s bus fleets to clean up their act, so more than half of the existing vehicles are being upgraded to Euro VI in an £86.1m Transport for London (TfL) retrofit programme. Steve Banner reports

Sadiq Khan, mayor of London, is on a mission to improve the capital’s air quality: he has called for bus operators to cut nitroden oxide (NOx) and particulate emissions. The aim is to cut harmful pollutants from some 5,000 buses by up to 95% and bring the capital’s entire fleet to at least Euro VI levels by September 2020. This exercise will cost an average of £17,000-plus per vehicle, with the work carried out for the most part in operators’ garages. The bill looks steep.

“But it shouldn’t be compared with the £3,000 to £4,000 it costs to retrofit a diesel particulate filter,” says Kathye Vicente, marketing manager at Eminox. “What we’re talking about here is a sophisticated emission control system.”

Five approved suppliers of retrofit exhaust systems have been appointed by TfL after a competitive tender process – Eminox is one of them alongside Amminex, Baumot Twintec, HJS and Proventia.

In most cases (Eminox; Proventia; HJS) the retrofit package favoured combines a continuously regenerating particulate trap with selective catalytic reduction that relies on AdBlue (aqueous urea) to bring down NOx by converting it to water and nitrogen gas, which is prevalent in the atmosphere; see diagram (below). “The one we offer is delivering a 99% cut in NOx and NO₂,” says Vicente.

The key challenge for system manufacturers is to design one that functions effectively at the low speeds typical of London’s congested traffic. Low speeds mean low exhaust temperatures, and AdBlue usually requires catalysts to reach at least 220ºC.

“Dealing with the temperature issue is a challenge,” agrees Vicente. “One way we’re doing so is to position our system as close to the engine and the particulate filter – the latter is a lovely heat sink – as we possibly can.

“We’ve also done a lot of work on developing highly responsive control and calibration systems, and we’ve got access to the most advanced catalyst technology available, because we work so closely with Johnson Matthey,” she adds.

Baumot Twintec’s answer is BNOx, says Alan Martin, a consultant to the company.

It uses AdBlue, but does not inject it into the exhaust in liquid form. It uses hydrolysis and a small generator to create ammonia gas from it, and injects that instead.

“This approach enables the system to operate at temperatures as low as 120ºC and it has achieved a 99.6% cut in NOx ,” he says.

“We’ve installed it on 24 vehicles for Transport for Greater Manchester.”

Cities outside London are equally eager to cut atmospheric pollution, with clean air zones proposed for Birmingham, Leeds, Nottingham, Derby and Southampton by 2020.

There are no significant safety concerns with BNOx, Martin stresses. “You are not carrying large quantities of ammonia around with you,” he explains.

Amminex’s response is ASDS – Ammonia Storage and Delivery System. It uses gaseous ammonia, too, but stores it in solid form in removable and replaceable cartridges about the size of a standard fire extinguisher under the ‘AdAmmine’ banner.

When the cartridges are heated to 55ºC, a process that uses around 150W of power, the gas is released. The ASDS system is usually only switched on when the catalyst and NOx sensors reach around 140ºC, however, says Amminex.

“The amount of gaseous ammonia used at a time is minimal and we’re setting up arrangements that will allow operators to swap empty cartridges for full ones,” says Amminex product line manager Lars Tinggaard Johannesen.

More familiar with AdBlue, bus fleets may understandably be wary of having to rely on cartridges, especially given that Amminex’s AdBlue–reliant rivals meet TfL’s requirements.

Johannesen admits that using cartridges makes ASDS more expensive than what is on offer from most of the competition. He contends that it requires less maintenance however, with only one moving part.

“It needs an annual check; the NOx sensors will have to be replaced after five years; and the small start–up cartridge may have to be replaced after three years,” he says.

It heats up quickly and doses ammonia until the main cartridge, which feeds it, is ready to take over. Swapping a main cartridge – two are fitted – takes no more than a minute, reckons Amminex, and usually needs to be done once a week on a city bus.

A warning on the dashboard tells the driver when a cartridge needs changing.

Not that systems that rely on AdBlue need a lot of maintenance, other than flushing with distilled water once in a while to get rid of any crystallisation.

ASDS has been retrofitted to 300 buses in Copenhagen, the Danish capital, achieving NOx and NO₂ reductions of up to 99%, says Amminex. London fleets using it include Metroline, which has it installed on 55 of its vehicles.

How long does it take to install a retrofit package? Fitting times vary, with some suppliers quoting anywhere from 24 to 36 hours depending on the vehicle and whether it is the first time they have installed one on that particular make and model. “We can fit two in a day,” reckons Vicente at Eminox.

However, remote monitoring of the technology to ensure it is working properly is vitally important.

For example, Finland’s Proventia, represented in the UK by Excalibre Technologies, offers Procare Drive, which keeps an eye on its NoxBuster City system and provides 24/7 updates on NOx emissions from the buses equipped with it.

Amminex offers its Live NOx Tracker app. Data from the app shows that the Copenhagen buses it retrofitted removed more than 300 tonnes of NOx from the city’s atmosphere in a year, says the company, without increasing fuel consumption and CO₂ emissions.

Such figures are not produced simply to satisfy idle curiosity. Bodies such as TfL have to be reassured that the systems retrofitted work satisfactorily in service – not just on a lab test bench.


According to TfL, some 6,900 buses do not meet Euro VI criteria. Since2,000 of those will be replaced as contracts run out in 2020, the project aims to convert Euro IV and V buses that would remain. TfL estimates suggest that the retrofit would save 8g of NOx per km (from 8.56g before), and 0.03g of particulate matter (from 0.04g before), over a test cycle keyed to the 159 bus route. That corresponds to 95% reductions in NOx and 80% in PM.

The first tranche of funding covers 1,150 retrofits to spring 2018, and will create over 40 new apprenticeships with the suppliers, says TfL (see also

This is in fact the second bus retrofit; with DfT match–funding, TfL carried out a similar programme in 2013, upgrading some 2,000 buses from Euro III to Euro IV.

Steve Banner

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Related Companies
Amminex Emissions Technology A/S
Baumot UK Ltd
Eminox Ltd
HJS Emission Technology GmbH & Co KG
Johnson Matthey plc
Proventia Oy

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