Equipping the public transport fleet with exhaust filters and selective catalytic reducers

Summary

The introduction of new filter technology was seen as a way to reduce emissions and increase the attractiveness of public transport use.

Implementing sustainable mobility

At the start of the TELLUS project, the bus fleet in Rotterdam comprised 181 buses conforming to Euro I standard and 42 buses with CRT (continuously regenerating technology) filters. Although emissions of particulates and nitrogen oxide (NOx) had decreased, traffic remained a major polluter, with air-quality levels in Rotterdam only just compliant with national directives. The conversion of the whole bus fleet to Euro IV/V standard using DNOx filter technique would be a step towards decreasing emissions and improving urban air quality.

Progress

Under the TELLUS project, seven buses were fitted with a complete DNOx filter system, with the goal of:

• conforming to at least Euro III emissions standards;
• verifying the manufacturers’ claim that the system would meet Euro IV standards; and
• testing the technical viability of the system in daily public transport use.

If the trials on the seven buses proved successful, the system would be fitted on the entire bus fleet (that is, a further 205 vehicles) of the public transport operator RET.

Unfortunately, the test results were very disappointing. All seven DNOx filters broke down in the first testing period. The main cause of the failure was that the system could not function properly with the exhaust gas temperatures from the buses in the RET fleet, a problem that was almost impossible to solve. The supplier did not succeed in getting the DNOx system to work, so offered a newly developed retrofit SCR (selective catalytic reduction) system instead, certified to reach Euro V emissions standard.

Outcomes

Based on the views of the manufacturers and the non-satisfactory experiences in the earlier project period, the municipality of Rotterdam decided:

• not to retrofit the rest of the RET bus fleet by equipping them with filter technologies such as CRT and DNOx;
• to allocate the budget to the installation of SCR systems in 90 new buses; and
• to test the seven retrofitted buses again to investigate the failure in these systems.

The originally selected DNOx filters clearly had a negative impact due to the large number of breakdowns. The SCR filters had a clear environmental impact, reducing emissions of NOx and particulates, although leading to higher emissions of carbon dioxide.