Dinesh Mohan of Delhi IIT cites a European study that has revealed that CNG emits even finer particles than diesel which have greater propensity to enter the lungs thereby making the CNG option that much more dangerous (Business Standard, May 21,2001).


Fact

The detractors of CNG pull out extremely limited and yet unproven data to claim that CNG vehicles emit more ultrafine particles.

While particles come from all kinds of combustion sources it is the toxicity of the particulate emissions that help to prioritise the control of emissions. Particulate emissions from diesel vehicles are tiny and are coated with extremely toxic chemicals called polycyclic aromatic hydrocarbon (PAH) some of which are known to be the most potent carcinogens.

Numerous studies are now available that establish that CNG is a cleaner fuel compared to diesel.
     

Issues get murkier when the problems associated with diesel are hurled back at CNG. So if diesel emits high amount of fine particles so does CNG — is now a common refrain. The limited evidence the CNG detractors pull out of their bag is a “study” done by the US-based Harvard Centre for Risk Analysis, which contends that CNG vehicles emit more ultra fine particles (also called nanoparticles) than diesel vehicles.20 In 2000 this “Harvard” study had mysteriously made its way to the tables of all top decision-makers in the Delhi government. The Lieutenant Governor of Delhi went on record to the media arguing that CNG was a problem because of “nanoparticles”. TERI researchers use this study to support their claim as well.

The “Harvard” study when examined was found to be a six-page pamphlet, which was a literature survey with no references to the information cited. Moreover, the study was funded by the world’s largest truck manufacturer — Navistar International. Michael Walsh, a highly respected air pollution expert and former official of the USEPA says of the study, “Any undergraduate who turned such a report in to his professor would surely get a very poor grade.”

The lobbies at work completely ignore the fact that while particles come from all kind of combustion sources, it is the toxicity of the particulate emissions that should guide prioritising the control of emissions. Across the world, scientific studies have established that particulate matter from diesel exhaust is extremely toxic. It comprises tiny particles coated with extremely toxic chemicals called polycyclic aromatic hydrocarbons (PAH), some of which are known to be the most potent carcinogens. Compared with diesel vehicles, CNG vehicles emit negligible amount of particles. Moreover, even the little particles that are emitted by CNG vehicles are not as toxic as particles emitted by diesel vehicles as CNG is composed of mainly methane gas.

While particles come from all kinds of combustion sources it is the toxicity of the particulate emissions that help to prioritise the strategies for control of emissions.

More studies are now available to confirm that diesel vehicles emit more ultra-fine particles than CNG vehicles. The Harvard study was first countered by the US Department of Energy (DOE), when it came out with a paper called Separating Myth from Fact, in April 2000. The paper said CNG buses consistently emit dramatically less particulate matter than diesel buses. Emissions testing of on-road buses in Boulder,Colorado, on the central business district (CBD) driving cycle of the US demonstrated a 97 per cent reduction in particulate matter emission and a 58 per cent reduction in nitrogen oxide when compared to diesel buses.21

The trace amount of particulate matter associated with CNG is attributed to crankcase lubricating oil consumption (which also occurs in diesel engines). The DOE report said, “Some tests have shown that CNG actually produces much fewer ultrafine particles than diesel fuel. However, the study of particle size distribution measurement and ultra fine particle counting are developing technologies, and initial data is mixed. New diesel engines have been observed to emit more ultrafine particles while at the same time emitting less total particulate matter mass than older diesel engines.”

In one of his papers published in 2000, by the US-based Society of Automotive Engineers, Christopher Weaver, president of the California-based Engine, Fuel, and Emissions Engineering, Inc. says that even deterioration of the natural gas engine does not have significant effect on particulate emissions. “Particulate matter emissions from natural gas engines are unlikely to increase substantially due to wear or inadequate maintenance — at least until the piston rings, valve seals, or turbocharger oil seals are so worn that oil control is lost,” says Weaver. According to him, this is not surprising, since particulate matter emissions from natural gas engines are derived from lubricating oil rather than fuel combustion.22

When the Department of Environment and Transport for the Regions (DETR) of the Government of UK tested emissions from buses run on CNG and diesel, the results showed that the mass of ultrafine particles smaller than 0.056 micron (a micron is a millionth of a metre) emitted by CNG bus was just marginally higher than a Euro III diesel bus run on Swedish Class 1 diesel fuel (which has a sulphur content of 10 ppm). But diesel buses were found to emit more particles, bigger than 0.056 micron, than the CNG buses23 (see graph 2: Ultrafine emissions).

But when the number of the ultrafine particles was considered, the Euro III bus on the Swedish Class 1 diesel was found to emit many more particles both smaller and larger than 0.05 micron compared to the CNG bus24 (see graph 3: More from diesel). This clearly shows that ultrafine particle emissions remain a problem even with advanced diesel engines.

Again, when the Sweden-based Motor Test Centre (MTC) tested emissions from diesel and CNG buses, it observed similar results. The buses were tested on two driving cycles — the European and Santiago driving cycles. On both driving cycles the number of ultrafine particles (smaller than 0.1 micron in this study) emitted by the diesel engine was much higher than emissions from CNG buses25 (see graph 4: Ultrafine emissions: European driving cycle, and graph 5: Ultrafine emissions: Santiago driving cycle).

More studies prove that CNG is a much cleaner option.

The trace amount of particulate matter associated with CNG is attributed to crankcase lubricating oil consumption which also occurs in diesel engines.

It is surprising how CNG detractors ignore a large number of studies that show CNG is a much cleaner fuel. Even the basic science of clean fuels eludes them. In its report on clean fuel, submitted to the Supreme Court, EPCA clearly states that no hydrocarbon fuel can be treated as a clean fuel. But based on the nature and structure of hydrocarbons it is possible to classify some of them as environmentally acceptable fuels. So the report states, “The pollution potential of the hydrocarbon fuels depends on the ratio of carbon to hydrogen atoms. Petrol and diesel belong to long-chain hydrocarbons with a larger number of carbon atoms forming the chain with hydrogen atoms. On the other hand, fuels like CNG, LPG and propane belong to the group of short-chain hydrocarbons having lesser number of carbon atoms. Hence, the latter are

less polluting. This factor, together with the combined effect of fuel characteristics, fuel additives and exhaust treatment systems in automobiles as well as secondary pollutants generated through atmospheric reactions, is the reason for air pollution and its health effect caused by automobile emissions.”26

Emissions results of CNG vehicles available from different countries confirm that CNG vehicles are inherently cleaner than diesel vehicles. A report from Canada, published in 2000, compiles emissions test results from across the US and Canada which show that CNG buses emit up to 43 times less particles than comparable diesel engines fitted with particulate traps27 (see table 6: Clean chit).

CNG also wins the race for cleaner fuels because of lower emissions of other gaseous pollutants. Swedish test results show that a CNG bus emits lower non-methane hydrocarbon and oxides of nitrogen compared to Euro II diesel bus with particulate trap.28 For a Euro II diesel bus running on diesel with 10 ppm sulphur (0.001 per cent) to achieve emission levels comparable to that of a CNG bus, would need exhaust gas recirculation system in addition to a particulate filter29 (see graph 6: Package deal).

When the DETR, government of UK, conducted emissions test on diesel buses running on 0.005 per cent (50 ppm) and 0.001 per cent (10 ppm) sulphur diesel fitted with particulate traps, and CNG buses, it found that particulate emissions from Euro I bus fitted with CRT using 10 ppm sulphur diesel was close to that from a CNG bus. But running the CRT-fitted Euro I diesel bus on 50 ppm (0.005 per cent) sulphur produced greater particulate emissions than the CNG bus.30

The CNG bus gave better results despite the fact that it was tested on a transient cycle while the Euro I bus with CRT was tested on a steady state cycle. A transient test cycle is more representative of real driving conditions, emits more pollutants than a steady state cycle. Moreover, the CNG bus tested had a stoichiometric engine and only an oxidation catalyst which is not the state of the art technology. A three-way catalyst, would have given even better results.31

A package of advanced emission control systems is needed along with the best quality diesel to reduce particulate emissions from diesel to the level of CNG engines.