Enthusiasm for CNG Misplaced
By Peter Samuel
Enthusiasm for replacing diesel engines with compressed natural gas (CNG) is misplaced, according to the Harvard Center for Risk Analysis (HCRA), a branch of its school of public health. While diesels generate more fine (over 2.5 microns) particulates, natural gas combustion generates more ultra-fine (sub-micron) particulates, a study says.
Several studies indicate that ultrafine particles may have an even more dramatic impact on health than those in the fine category.
So natural gas may be worse in particulates emissions than diesel fuel. Diesel engines, especially when poorly maintained, emit black clouds of larger particulates, which are highly visible and unpleasant but probably not very harmful, because the respiratory system can intercept many of them. By contrast the ultrafines of CNG are unseen and may be ingested deep into the lungs more easily.
CNG being largely methane has the advantage that it combusts more completely than diesel fuel. In particular it is less liable to produce nitrous oxides (NOx) than diesel, or for that matter gasoline. NOx along with hydrocarbons from unburnt fuels produce ozone (smog) in hot sunny weather. High ozone levels reduce lung function temporarily.
Diesel on the other hand is more efficient a fuel than CNG, in the sense that it converts a larger proportion of available chemical energy into mechanical work. For those who believe in the theory of manmade global warning and want to reduce greenhouse gas emissions Diesel-to-CNG conversion is unhelpful, the Harvard Center says. More greenhouse gas emissions will be generated with CNG than if diesel engines continue to be the major propulsion plant for heavy vehicles.
Diesel is much safer than CNG, which is highly flammable. This is wellknown in the Washington DC metro area where there have been a rash of fires among CNG vehicles. CNG has cost disadvantages compared with diesel but the Harvard study thinks this might be overcome in time. (Contacts: Edmond Toy 617-432-1566, firstname.lastname@example.org www.hsph.harvard.edu/press/releases)