Thursday, February 22, 2007

Pebble bed reactors can supply hydrogen for power

The hydrogen economy is a futuristic vision of our society using hydrogen for vehicle fuel. Hydrogen can burn in an internal combustion engine creating only water vapor, H2O, which is nonpolluting and does not contribute to global warming.

However, hydrogen is a very reactive element and on earth hydrogen is bound up in molecules, principally water, H2O. To create pure H2 the hydrogen must be separated from the molecule, requiring energy. Today hydrogen is produced by electrolysis, using electrical energy. Hydrogen is best viewed as an energy storage system; energy is absorbed to create hydrogen and later released when the hydrogen is burned.

The pebble bed reactor is a high temperature helium gas cooled nuclear reactor. The 950 degree Celsius heat can be used to disassociate water into hydrogen and oxygen. This is substantially hotter than the temperatures reached by today's boiling water and pressurized water nuclear reactors. Two chemical processes show promise for commercial scale production of hydrogen: (1) high temperature electrolysis, and (2) the sulfur-iodine cycle, which is pictured in the schematic diagram above.

So the pebble bed reactor, with high temperature disassociation of water, can be a safe, clean source of power for the hydrogen economy.

But one of the problems of the hydrogen economy is that the storage and transportation of highly reactive hydrogen is extremely difficult. Hydrogen makes steel tanks brittle. Liquefied hydrogen must be kept cold (-253 degrees C), and the liquefaction process is energy intensive. Hydrogen can be compressed and stored in strong tanks at room temperature, but the pressures must be very high and the pressurization process is energy intensive. The H2 hydrogen molecule is so small it permeates containers and leaks out.

Indy 500 race cars run on methanol

Nobel prize winning chemist George Olah has proposed a more practical system he terms the methanol economy. Methanol can be used as a vehicle fuel. Methanol, H3COH, is readily created by combining H2 with recycled CO2 captured from existing coal-fired power plants. Methanol burns to form H2O and CO2, but the process is carbon-neutral because the CO2 would have been released into the atmosphere at the coal-fired power plant.

There will be enough CO2. US coal-fired power plants will continue to produce CO2 for a century even if they are replaced by one 100 megawatt PBR per week. CO2 comprises 0.06% of air. Later on in this century we will be able to glean CO2 from the atmosphere, perhaps using nanotechnology to create advanced membrane filters. In this way methanol fuel can remain carbon-neutral.

The great advantage of methanol over hydrogen is that methanol can be transported and stored using the existing pipelines, storage tanks, tanker trucks, and fuel stations used for gasoline, with minor modifications. Methanol can be burned as fuel in an internal combustion engine. Indianapolis 500 race cars have used methanol since 1964 because it is safer than gasoline in an accident; methanol is not as explosive as gasoline.

Beyond Oil and Gas: The Methanol Economy
, by George A. Olah, Alain Goeppert, and G.K. Surya Prakash, also discuses related concepts, such as the direct methanol fuel cell that could replace the internal combustion engine. For example, dimethyl ether, CH3OCH3, is a nontoxic, noncorrosive chemical that can be used a fuel for diesel engines.

Chemists and chemical engineers can develop processes to produce all the common hydrocarbons we now derive from petroleum. These chemical processes rely on an inexpensive, plentiful supply of hydrogen, which can be created from water using the high temperatures of the pebble bed reactor.

The pebble bed reactor can be a source of carbon-neutral fuel for vehicles.

1 comment:

Stewart Peterson said...

More conventionally, they can obviously also displace coal for Fischer-Tropsch synthesis of existing motor fuels.