Tuesday, September 4, 2007

PMT Buses and Pollution

Part 2 of a six part series on Pune City pollution and environment. Updated every Tuesday.

New Delhi, Asian Games 1982. I have memories of broad, clean avenues and clean air. When I returned after a long gap in 1999, New Delhi was urban hell. My friend pointed out a particularly busy intersection and said, "that's the most polluted intersection in Asia". I went back in 2004, and something strange had happened. Something we Indians are not used to. Pollution had reduced dramatically. Air quality was better. The reason experts tell us, was the conversion of almost the entire public transport fleet (15-20 thousand vehicles by various estimates) of rickshaws, taxis and buses from diesel to CNG (Edit: Compressed Natural Gas).

How much will Pune Municipal Transport (PMT) benefit from a conversion from Diesel to CNG? Graph below shows the potential in reducing emissions (tons/year) by converting the current Pune Bus Fleet of 990 buses (PMT plus rental) from Diesel to CNG. (for the purpose of the calculation I have assumed all 990 buses are on the road everyday, which given the state of disrepair of PMT buses may not be the case).

Source: Emission factors for CO2,PM10 and SOx from World Bank: A Simple Model for Better Air Quality (2005); N.Harshadeep and S. Guttikunda. Bus operating kms per day, PMC Env. Status Report, 2005.

A CNG fleet will lower emissions of PM10 (particulate matter) from 135 tons/year to about 40 tons/year (70% reduction), of SOx from 80 tons/yr to about 25 tons/yr (68% reduction) and CO2 from about 65 thousand tons/yr to about 35 thousand tons/yr (46% reduction). There are considerable (about 50%) reductions in nitrogen compounds as well, but I did not have the emission factors available to make any specific calculations. But what about methane? CNG is about 80% methane, which is 20 times more potent a greenhouse gas than CO2. Won't the higher methane emissions offset the lower CO2 emissions? Apparently if one considers the total fuel cycles of diesel and CNG including emissions during fuel production, CNG buses have total greenhouse gas emissions similar to diesel buses. Diesel also emits toxic compounds like polyaromatic hydrocarbons, benzene and 1,3 butadiene that CNG does not.

How does bus pollution compare with other vehicles? Although Diesel or CNG buses may emit more pollutants than a 2 wheeler or car in an absolute sense, are they at least more efficient on a per passenger km basis considering they carry more passengers per trip? Graph below shows the emission of pollutants per passenger km, if a commuter makes a daily trip of 30 kms to and from work. The calculations show that even on a per passenger km basis Diesel buses emit more particulate matter than 2 wheelers. CNG however on a passenger km basis is much cleaner than petrol and diesel vehicles.

Source: Emission factors for CO2, PM10 and SOx from World Bank: A Simple Model for Better Air Quality (2005); N.Harshadeep and S. Guttikunda.
Assumed Occupancy: 2 wheels-1, Car-2, Bus- 40

To summarize, conversion of PMT buses to CNG will definitely benefit the city. An even bigger reduction in emissions would be achieved through the conversion of several thousand private buses. There is a move to convert the fleet of rickshaws to LPG/CNG. More on that in another blog.

As fuel costs keep increasing, public transport will increasingly be the more cost effective means of transport. Below graph shows Per Km Petrol Cost for Two Wheelers vs Bus Fare Cost per passenger km

Source: Central Institute of Road Transport

The trend indicates that due to increase in petrol prices, public transport per passenger km is increasingly a more efficient mode of transport than private vehicles.

Be sure to read about CNG facts and myths in this fact sheet from the Centre for Science and Environment in Delhi and also about CNG buses and how they compare with Diesel in this fact sheet from the U.S. Dept. Of Energy.

Part 1. Idling and Pollution


  1. Silly question, but I didn't see a definition of "CNG" on that fact sheet.

    Compressed Natural Gas?

  2. sorry about that lapse. Yes, it is Compressed Natural Gas. I will post an edit. thanks for pointing that out!