Myth 2: Particulate traps can work with low sulphur diesel


The Tata Engineering Locomotive Company (TELCO) in its submission to Environment Pollution (Prevention and Control) Authority (EPCA) says, “The equipment manufacturers have stated that their filters can be used up to 350 ppm sulphur diesel. But they admit, “filter efficiency will be low at higher sulphur content and will improve substantially as the sulphur level goes down (SIAM 2001, Note on information given by TELCO during the SIAM meeting with EPCA on April 16, 2001, TELCO’s submission to EPCA, mimeo).


Fact: Trap efficiency depends on the sulphur levels in fuel

Ensuring particulate trap efficiency is critical to make diesel vehicles dramatically cleaner than what they are today. But for advanced aftertreatment systems like CRT to be effective, diesel with minimal sulphur content, if not totally sulphur free, is essential. Moreover, this application is still limited and very expensive.

It is misleading to claim that low cost filters on high sulphur fuel will be as effective as CNG in reducing emissions.

Neither the industry nor their experts explain adequately that simple soot or particulate filters are grossly inefficient when used along with high sulphur fuel and that advanced filters like the CRTS do not even work on high sulphur fuel.

The future of diesel vehicles depends to a large extent on the effective application of exhaust emission control devices along with engine development and fuel quality improvements. But these devices will work only if sulphur level in the fuel is minimal — below 30 ppm (0.003 per cent). This is very different from saying that even low cost particulate filters or soot filters along with low sulphur diesel with 500 ppm sulphur can be as effective as CNG.

Information from Hong Kong shows that fitting diesel vehicles with low cost particulate traps has had very little impact. Fitting low cost particulate traps to 66,400 diesel vehicles weighing lower than four tonnes and run on 500 ppm (0.05 per cent) sulphur diesel, has cut particulate emissions by only 7.5 per cent. Fitting catalysts into 83,000 diesel vehicles weighing more than four tonnes has lowered particulate emissions by only 13.2 per cent.11

Air quality regulators worldwide are therefore looking into the possible development and application of more advanced exhaust emission control devices such as CRTs. But these are extremely sulphur-sensitive. USEPA has gained experience with its voluntary diesel retrofit programme. For this programme it has considered a number of aftertreatment systems including two types of diesel particulate filters — base metal oxidising filter and highly oxidising precious metal particulate matter filter. USEPA informs that the base metal oxidising particulate matter filter, which has a potential of reducing particulate matter by 80 per cent, can operate only when diesel with sulphur content much lower than 500 ppm is used12 and a CRT will work only on 15 ppm sulphur diesel.13 The highly oxidising precious metal particulate filter, which can reduce particulate emissions by more than 90 per cent needs diesel with lower than 15 ppm sulphur.14

Some particulate filter manufacturers, Engelhard for instance, claim that their filters can work even on 500 ppm (0.05 per cent) sulphur diesel fuel. But no data on any test having been conducted is available from these manufacturers to substantiate their claims.15 Moreover, most of these traps have been used in off-road and stationary engines, which are very different from heavy-duty vehicles like buses and trucks, and are still in the demonstration stage for vehicles.10 The overwhelming evidence provided by regulatory agencies across the world contradicts any claim of sulphur-neutral traps.

According to the California Air Resources Board (CARB), “Although catalyst-based diesel particulate filters can be used with diesel fuel of varying sulphur content, the greatest reductions come from using very low sulphur fuels. Used with very low sulphur (less than 15 ppm) diesel fuel, catalyst-based diesel particulate filters have been reported to reduce diesel PM emissions by over 85 per cent.”16

Emission test results from USEPA show that when a heavy-duty diesel engine fitted with CRT and diesel sulphur level is reduced from 150 ppm to 3 ppm, particulate matter dips by 96 per cent17 (see table 4: Sulphur poisons).

Test results from UK also show that lower the level of sulphur in diesel, greater is the efficiency of CRT to reduce particulate matter emissions (see table 5: Similar results).

Though the use of CRTs can help heavy-duty diesel vehicles meet Tier 2 emission standards, according to the USEPA, they will need another aftertreatment device called NOx-adsorber to meet the emission standards set for nitrogen oxides.18 While USEPA says that none of these technologies will operate efficiently enough to help heavy-duty vehicles meet the emission standards if the sulphur content of diesel is more than 15 ppm, the US-based Engine Manufacturers’ Association has demanded diesel containing no more than 5 ppm (0.005 per cent) sulphur for these technologies to be able to function.19