Suppressed CNG facts

Pollution watchdog’s study on fuels is full of loopholes

By Arushi Mittal
Published: Saturday 15 January 2011

imageTHE Central Pollution Control Board (CPCB) has fumbled in answering a critical question on carbonyl and methane emissions from vehicle exhausts: which pollutes less—petrol, diesel, CNG or LPG? The pollution watchdog’s study on emissions provides a database that could be used to give a policy message. It declared CNG to be the worst fuel for four-wheeled light and heavy commercial vehicles. But the finding is flawed as it compared factory-fitted petrol and diesel engines with retro-fitted CNG and LPG engines to test the fuel. In retro-fitted vehicles, engine is installed at local workshops.

CPCB experimented on two-, threeand four-wheeled vehicles, which operated on CNG, LPG and various combinations of Ethanol, biodiesel, Bharat Stage (BS) II & III petrol and BS II & III diesel. It tested carbonyl and methane emissions from these vehicles.

CPCB does not mention the vintage of the vehicles used, claiming “unavailability of dedicated CNG engine vehicles”. It is surprising because automobile majors such as Maruti, Hyundai, Mahindra and Chevrolet have already launched dedicated CNG vehicles. Delhi and many other Indian cities have banned retrofitted CNG buses.

“The methodology the board used to brand fuels as best or worst is flawed “as retro-fitted CNG and LPG vehicles can never achieve the same results as factory-fitted vehicles; all the aspects cannot be optimised,” said P M V Subbarao, who teaches mechanical engineering at IIT-Delhi.

Petrol vehicles converted to run on natural gas suffer because of low compression ratio of their engines, while diesel engine conversions result in compression ratio more than necessary. Also the ratio of air and fuel is changed in retro-fitted vehicles.

A stringent quality assurance policy on retro-fitting is necessary and it can be based only on scientific evidence on emissions from factory-fitted vehicles, Subbarao said.

Spurious alternate fuel kits fitted at ill-equipped service centres account for 32 per cent of the Indian market. The illegal business thrives as the kits come 30 per cent cheaper.

The criticism of CNG on its high methane emissions does not have sound basis. Methane exhaust from CNG engines is 12 per cent lower than that from diesel engines. Methane emission from CNG is lower even if one compares the life cycles of the two fuels.

“Unregulated emission cannot be ignored. A reliable database is required to facilitate policy action and promote after-treatment systems in Indian vehicles,” said Anumita Roychowdhury, who heads the air pollution and urban mobility unit at the research and advocacy organisation, Centre for Science and Environment, in New Delhi. Aftertreatment of exhaust gases is an effective technique to reduce toxic emissions. Equipment like air pumps, catalysts and re-circulation technologies need to be promoted.

Health hazards

Carbonyls are produced due to partial oxidation of hydrocarbons that escape from the vehicle’s combustion chamber to the exhaust system. Through repeated exposure, carbonyls like formaldehyde, acetaldehyde, acrolein and acetone can cause irritation to eyes, respiratory tract, mucous and skin. Acetaldehyde and formaldehyde are suspected human carcinogens.

The study fails to take note of the cumulative effect of suspended particulate matter (SPM) and carbonyl emissions on human body. Most of these toxic substances enter human body only through air-borne particles. So, reduction in SPM emission, as done by CNG, reduces human intake of toxic substances.

According to California Air Resource Board, the risk from formaldehyde is 2 per cent of that from SPM. Formaldehyde emission standards have been proposed in many parts of the world such as California (0.015g/mile) and Europe (0.016-0.021 g/mile). The study recommends that India should follow suit.

The CPCB study also neglects the higher formaldehyde emission from diesel vehicles. For four-wheelers, formaldehyde emissions from diesel vehicles are 20 per cent more than from petrol vehicles.

When confronted, CPCB seemed to know little about the content of its own publication and denied to comment without a formal procedure.

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