Battery Switch Technology


Monday, January 18, 2010

Production Technologies of Hydrogen

Current LWR technology can make electricity to produce hydrogen through electrolysis at an overall efficiency of about 25%. However, proposed advanced HTGRs operate at higher temperatures, producing electricity and hydrogen much more efficiently (up to 50%). These advanced reactors potentially have many advantages over the current LWRs used in the U.S. today.

There are two main categories of hydrogen production technologies using HTGRs:
• Thermochemical water-splitting cycles 
• High-temperature electrolysis
Like conventional electrolysis, both technologies separate water into hydrogen and oxygen. Both technologies also use high temperature heat for economical, emission-free hydrogen.

Thermochemical (TC) Water-splitting Cycles
Thermochemical production of hydrogen involves the separation of water into hydrogen and oxygen through chemical reactions at high temperatures (450-1000 °C). A TC water-splitting cycle involves a series of chemical reactions, some at higher temperatures than others. Engineers carefully choose chemicals to create a closed loop system that reacts with water to release oxygen and hydrogen gases.  All reactants and compounds are regenerated and and recycled. Studies conducted through the Nuclear Energy Research Initiative have identified more than 100 different TC water-splitting cycles. A few of the most promising cycles have been selected for further research and development based on the simplicity of the cycle, the efficiency of the process and the ability to separate a pure hydrogen product. The biggest challenge with TC processes today is corrosion of process reactors and system materials.

Of the identified processes, the sulfur family, including the sulfuriodine (S-I) cycle and the Hybrid Sulfur (HyS) cycle, has shown the most promise for hydrogen production The S-I cycle uses iodine (I2) and sulfur dioxide (SO2) as chemical reactants to split water. First, water reacts with I2 and SO2 to form hydrogen iodide (HI) and sulfuric acid (H2SO4). The HI and H2SO4 are separated from each other. H2SO4 and HI are decomposed in separate thermal decomposition steps into SO2 and O2 , and I2 and hydrogen (H2) respectively. The SOand I2 are recycled and used again and again. The H2 and Ogases are available as products. The reaction that requires the greatest heat input is the thermal decomposition of H2SO4, typically at temperatures in the range of 800°C. Higher temperatures tend to favor greater efficiency.

The Hybrid Cycle uses the same high temperature decomposition of H2SO4 into SO2 and O2, but substitutes electrolysis of SO2 and H2O into H2SO4 and H2, for the HI reaction and decomposition step. This avoids the use of iodine and potentially simplifies the process.

High-temperature Electrolysis (HTE)

HTE, or steam electrolysis, involves the separation of water into hydrogen and oxygen through electrolysis at high temperatures (up to 1100°C). Conceptually, HTE is the same as conventional low-temperature (<100°C) electrolysis. However, HTE uses heat from the reactor to replace some of the premium electricity required in conventional low temperature electrolysis. How much extra heat is needed? To produce 1 kilogram of hydrogen at 100°C, the system needs about 350 megajoules of heat energy. At 850°C, only about 225 megajoules are needed—a potential savings of more than 35% at the higher temperature.

Nuclear energy can help provide the hydrogen needed for a Hydrogen Economy. Today's LWRs produce hydrogen by conventional low temperature electrolysis, while advanced reactors can potentially improve electricity production, economically producing emissions-free hydrogen. Together with fossil and renewable resources, nuclear energy and its companion technologies can produce hydrogen for our portable, stationary and transportation needs.

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