Motor vehicles are the largest contributors to air pollution in the United States. Many efforts to reduce transportation emissions are underway, including improvements in mass transit options, improvements in vehicle fuel economy, and use of alternative fuel vehicles. Because fuel cells operate with extremely low emissions, they are the most attractive power source for future transportation applications.


 
There are a number of chemicals and materials classified as air pollutants. These include:

Carbon dioxide (CO₂), although not considered an air pollutant, is a chemical that contributes to the greenhouse effect and is therefore monitored.


 
Nitrogen Oxides

The oxides of nitrogen (NO and NO₂), collectively referred to as NO_x, are typical emissions produced by motor vehicles. NO is a colorless, tasteless, and odorless gas that gradually transforms into NO₂ in air. NO₂ is a reddish-brown poisonous gas with a strong odor that can cause lung irritation and respiratory problems. NO₂ has a major role in the formation of ground-level ozone (the primary component of smog), and it significantly contributes to the formation of acid rain.




 
Hydrocarbons

In motor vehicles, hydrocarbons (in various forms) are present in emissions when a fuel is not combusted or only partially combusted in an engine. HCs react with NO₂ to produce ground-level ozone. Some hydrocarbons are also considered toxic and carcinogenic.




 
Carbon Monoxide

Carbon monoxide is a colorless, tasteless, odorless gas that can significantly reduce the flow of oxygen in the bloodstream once inhaled, and can cause serious health problems and, in extreme exposures, death. In motor vehicles, CO is produced by the incomplete combustion of a fuel.

In a methanol-fueled fuel cell, CO is produced during the initial reforming process. Since CO can 'poison' the catalyst in the fuel cell, most of it is removed by selective oxidation or by conversion in a low-temperature shift converter. However, there are still trace amounts emitted.




 
Particulate Material

Particulate material refers to any solid or liquid particles in the air, including dust, soot, and smoke. Most particles are too small to be seen, but they contribute to haze in both urban and rural areas. When inhaled, particulates can cause respiratory problems and lung disease or tissue damage; certain people such as the children, the elderly, and people suffering from chronic lung disease, asthma, or bronchitis are particularly vulnerable to the health effects of particulates.




 
Carbon Dioxide

Carbon dioxide, a product of complete fuel combustion in motor vehicles, is not classified as a toxic air pollutant; it does not directly affect human health. However, it is a greenhouse gas, which helps to trap heat from the sun, so it is a concern when evaluating emissions.

In a methanol-fueled fuel cell, CO₂ is produced during the reforming process.

Because they rely on chemical processes rather than combustion, fuel cells have much lower emissions than internal combustion engines (ICEs). In fact, a fuel cell that is fed pure hydrogen will only have water vapor as an emission. In Georgetown's methanol-fueled fuel cell buses, the emissions are primarily by-products of the fuel reforming process.

This graph depicts representative emissions of internal combustion vehicles using various fuels as compared to the established 1998 EPA Standards for transit buses. Fuel cell data is based upon vendor projections derived from stationary fuel cell power plant experience. Note that the fuel cell emits no particulate matter, trace amounts of hydrocarbons and NOx and very little carbon monoxide. Because fuel cell emissions are so low, there has been a rapid worldwide escalation of activity in the development of transportation fuel cells in the past several years.

GU's fuel cell buses have emission levels well below any projected clean air standards. In fact, the systems now being developed are classified as “ultra-low emission” and the buses are very nearly “zero emission” vehicles. This can be seen in the emission test results performed at West Virginia University below. Emission testing from representative existing transit vehicles is compared to the Generation I 30-foot bus (line 6), the UTC Power test results for the Generation II PAFC bus (line 7), and the Ballard test results for the Generation II PEMFC bus (line 8). Line 1 is the 1998 standards; all units are in grams/brake horsepower-hour.

* with catalytic converter

	Fuel	Power Plant	HC	CO	NOx	PM
1	1998 EPA Transit Bus Standards		1.30	15.50	4	0.05
2	Diesel	DD Series 50*	0.1	0.9	4.7	0.04
3	CNG	DD Series 50	0.8	2.6	1.9	0.03
4	Diesel	Cummins C8.3	0.2	0.5	4.9	0.06
5	CNG	Cummins C8.3	0.1	1	2.6	0.01
6	Methanol	94 Fuji Fuel Cell	0.09	2.87	0.03	0.01
7	Methanol	98 UTC Fuel Cell	<0.01	<0.02	0.0	0.0
8	Methanol	99 Ballard Fuel Cell	<0.02	0.0	0.0	0.0

Vehicle Power Plant	Bus Test Weight (lb)	CO2 (g/mi)	Corrected CO2 (g/mi/1000lb)
TBB Fuel Cell	27,475	1876	68.3
Diesel ICE	32,843	2611	79.5
CNG ICE	35,091	2607	74.3
Methanol ICE	31,779	2960	93.1