Monday, June 13, 2011

Electric and Hydrogen-Powered Cars

Charging a Chevy Volt
This topic is intimately related to West Virginia and the health of the environment. Everyone has seen these slick commercials about the Chevy Volt and other electric cars on TV. They look very clean and they plug right into the wall. No pollution, nothing dirty, no messy toxic gasoline, no toxic emissions or carbon dioxide. Very nice. A simple solution to a complex problem. It's fantastic that battery technology has developed to the extent that electric cars are feasible and available at reasonable prices. The chemistry behind modern battery technology is exciting and highly innovative. I have some very smart colleagues who work on battery materials.

However, no one discusses the obvious question: where exactly does the electricity that comes out of this nice clean charging station come from? In the United States, there are more than 100 coal-fired electric power plants, and they provide 57% of the total electricity generated. These plants are responsible for 93% of the sulfur dioxide (think acid rain) and 80% of the nitrogen oxides (think acid rain and the greenhouse effect) generated by the electric power industry. Coal-fired power plants emit 48 tons of mercury per year into the environment, more than 40% of the total from anthropogenic sources. The fly ash produced by burning coal contains significant amounts of uranium and thorium, among other heavy metals. More importantly, the vast majority of coal mining in the U.S. is accomplished by strip mining or mountain top removal, both of which are less than environmentally friendly.

Strip Mine in WV
Strip Mine in AZ
The energetic cost of extracting the coal, transporting it thousands of miles, burning it to produce heat to generate steam to run turbines to generate electricity is unbelievably inefficient. Transforming AC current into DC battery power is also inefficient. Running an electric motor from a battery is inefficient. The whole process is inefficient, it generates a great deal of pollution, and it contributes to carbon dioxide emissions at each step of the process, except when driving the Chevy Volt. The nice environmentally conscious consumer drives away feeling self-righteous, except the whole idea simply does not make sense. You cannot have a "clean" environmentally friendly electric car when the electricity necessary to power that car is generated in an unbelievably "dirty" and environmentally unfriendly manner.

The only thing more nonsensical than an electric car is a car that runs on hydrogen. It is absolutely true that hydrogen burns without producing any waste. The reaction only produces water and heat, and thus is the most environmentally friendly chemical reaction:

 2 H2 + O2 → 2 H2O + heat

However, hydrogen (H2) is generated either by electrolysis of water or from steam refining of hydrocarbons (petroleum). The balanced chemical equation for the combined syngas production/water gas shift reaction for methane (natural gas) is:

CH4 + 2 H2O → CO2 + 4 H2

Natural gas is heated at high temperature (700-1100 °C) with water in a highly endothermic reaction to produce CO (carbon monoxide) and 3H2; additional hydrogen is produced by subsequent reaction of CO with H2O to give CO2 (carbon dioxide) and H2. These reactions are moderately efficient, in the 55-65% range. Chemically, methane or carbon monoxide reduces water to hydrogen, becoming oxidized in the process to carbon monoxide or carbon dioxide, respectively. (Most of the hydrogen produced by these two processes is used for the synthesis of ammonia from nitrogen via the Haber reaction. The ammonia is used for fertilizer.)

Wouldn't it be more logical to burn the methane to power the car in the first place?  Methane (natural gas) burns cleanly and quantitatively to produce carbon dioxide and water, exactly the same amount of carbon dioxide that is generated in the production of hydrogen, without the need for large energy inputs to produce it. If we take the first two equations, for hydrogen formation from methane and for hydrogen combusion, and sum them:

 CH4 + 2 H2O → CO2 + 4 H2 (syngas production/water gas shift reaction)

 4 H2 + 2 O2 → 4 H2O + heat (hydrogen combustion in car engine)

 We get:

CH4 + 2 O2 → CO2 + 2 H2O + heat

This is the exact same balanced chemical equation for the combustion of methane!

So instead of huge energy inputs for steam refining of petroleum, chemical separation of the resulting carbon dioxide from hydrogen, shipping the hydrogen, storing the hydrogen, distributing the hydrogen, and completely reengineering a car engine to burn hydrogen, we could just use the existing natural gas distribution system to power cars with methane, with no significant reengineering or manufacturing changes. Internal combustion engines can be easily altered to burn methane rather than gasoline. Both are hydrocarbons.

For the down-side of using electricity to produce hydrogen from water, see above.

Nice clean simple solutions to a complex problem...that are both wrong.

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