Oil industry’s last-ditch stand on sulphur

New research from Concawe, the oil industry’s research body, that questions the environmental benefits of zero-sulphur road fuels has been greeted with scepticism by the auto industry.1 The research appears to be a bid to influence the European Commission’s decision on when zero-sulphur diesel must be supplied across the EU.

The "zero sulphur" Directive, adopted in 2002 (ENDS Report 336, p 51 ), requires all petrol and diesel sold in the EU to have a sulphur content of less than 10mg/kg of sulphur from January 2009. It builds on an earlier requirement for all fuels to contain less than 50mg/kg of sulphur from 2005. The reduction in sulphur is intended to allow the introduction of cleaner, more fuel efficient vehicles.

However, the Directive contains a loophole - it requires the Commission to confirm the 2009 deadline for the switch to zero-sulphur diesel by the end of this year. One concern is that switching too early may increase emissions of CO2 as greater energy use in refining will not be offset by savings from road transport until fuel-efficient vehicles make up more of the fleet.

The level of sulphur in fuel is also relevant to the prospects for reducing urban air pollution - where concentrations of the most challenging pollutants, oxides of nitrogen and particulate matter, remain stubbornly high. Heavy duty vehicles will have to meet tougher Euro V limits from 2008, and later this year the Commission will propose Euro V standards for 2010 and beyond for cars and light goods vehicles.

However, Concawe’s report suggest that further lowering of sulphur in fuels will have only a limited impact on vehicle emissions. The findings contradict the auto industry’s position that sulphur-free fuels are essential for the next generation of fuel-efficient engines and advanced pollution control equipment like particulate traps and de-NOx catalysts which are "poisoned" by sulphur (ENDS Report 276, p 9 ).

Concawe measured the emissions of a range of pollutants from three HGV engines and two light duty vehicles (LDVs). The HGVs ranged from a Euro III compliant engine with no pollution control, to a Euro IV system fitted with a continuously regenerating trap (CRT) particulate filter combined with exhaust gas recirculation (EGR), and a Euro V system with a urea-based selective catalytic reduction system for NOx control.

The LDVs were both direct injection diesel engines with EGR. One was fitted with a standard oxidation catalyst, while the other was fitted with a diesel particulate filter (DPF).

Concawe ran the engines on fuels with sulphur concentrations ranging from 300mg/kg to less than 5mg/kg.

Unsurprisingly, the tests showed that the lowest sulphur diesel consistently produced lower emissions than the high-sulphur fuels. But they also showed only very small differences in emissions between fuels containing 50mg/kg of sulphur - the current limit - and those with less than 10mg/kg. Differences in pollution control methods and driving test cycles were far more significant than the fuels used.

In the case of the Euro III standard LDVs, the vehicle with a catalyst was most successful at controlling NOx emissions. The vehicle with a DPF was best at cutting out particulates, with emissions more than an order of magnitude below the Euro IV limit. However, the catalyst-equipped vehicle failed to meet the particulate limit on all test cycles with all but the lowest sulphur fuels. Given that catalysts are the dominant pollution control systems in use at the moment, this would suggest that very low sulphur fuels would have some small impact in reducing emissions from the existing fleet.

The picture was similar for the HGVs. The Euro IV system using EGR and a CRT was more successful at reducing particle emissions than the Euro V system using SCR with all fuels. However, it was less effective when it came to cutting NOx emissions. Once again, however, the effect of the fuels was small - though importantly the SCR system only attained the Euro V NOx standard with the lowest sulphur fuels.

Ian Godwin of pollution control company Johnson Matthey expressed surprise at the findings. "There is no question that sulphur can rapidly deactivate catalysts," he said, "and if you’re going to use particulate trap or NOx adsorber technology, you need sulphur levels to be as low as possible." This is still the case, he said, despite catalyst manufacturers’ progress in making systems less sensitive to sulphur.

"You can control NOx with SCR, and this is one of the major approaches being used by heavy duty diesel vehicle manufacturers to meet Euro IV emissions legislation," Mr Godwin said. "However, you need to use a reducing agent like urea, and this requires a separate tank - it bumps up the cost and makes the system more complicated so in the light duty diesel area there is a good deal of interest in NOx adsorber traps. The problem is that they are more sensitive to sulphur."

Controlling both NOx and particulates is tricky as engine technologies (such as EGR) which are effective at lowering one tend to push up the other, requiring a mix of technologies and delicate engine management.

This could spell trouble for CO2 emissions. Concawe found that the HGV system which combined EGR with a catalyst was a lot less fuel efficient than the other systems. For example, it used 10-13% more fuel than the system fitted with SCR.

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