Sep 23 2007
Hydrogen Production via Thermochemical Method
The worldwide demand for hydrogen (H2) is ~50 million tons per year and growing rapidly. Hydrogen is used primarily for production of ammonia for fertilizer and conversion of heavy crude oils into cleaner liquid fuels. The international cooperation has effort to deliver H2 as a replacement fuel for transport vehicles. The future, we would like use hydrogen as fuel for transport vehicles than gasoline/diesel oil/natural gas as fuel.
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There are many way to produce hydrogen, but not all of the methods can be compromise with hydrogen economy and safe for our environmental. We need reach the hydrogen economy so that could enough compete to conventional fuel (gasoline, diesel oil, natural gas). A Hydrogen Economy only makes sense if hydrogen is produced with sustainable, non-fossil, non-greenhouse gas energy (Schultz, 2003).
Recently, there is new method to produce hydrogen that show big potential on hydrogen ecomony, that is thermochemical method. On this method, we only use high temperature heat as energy input to break molecular binding of water to get hydrogen. The electric power not use as energy input. I believe if just heat that use as energy input, it will be cheaper compared when electric power as energy input. That because, nowdays the conventional electric power generation is using thermal power to moving steam turbine which couple with electric generator.
The explanation of electric power generation nowdays are like this: chemical energy on fossil fuel convert to thermal power by burning fossil fuel. Thermal power then convert to kinetic power by using steam/gas turbine. Because turbine couple with electric generator, the moving of turbine also will make electric generator move. The moving of electric generator will produce the AC electric power. Then AC phase convert to DC phase for water electrolysis process. We can see there are 4 step energy conversion and every step has system efficiency. An overall efficiency ~30%.
Let think if we can cut energy conversion by just using thermal power. Chemical energy on fossil fuel convert to thermal power by burning fossil fuel. Then thermal power direct use to produce hydrogen via thermochemical method. You can look just one step energy conversion. This is the new idea, but I will bring you to use the better energy source. Thermochemical process will perform by nuclear power, not fossil fuel, because if we continue burning fossil fuel we will destroy our environmental and there no useful we produce hydrogen.
The research collaboration of General Atomics (GA), Sandia National Laboratory (SNL) and University of Kentucky (UK) found 115 unique thermochemical cycles, but just 25 top thermochemical cycles selected to screeing (suitability for coupling to a nuclear or solar heat source) and evaluated (perform on chemical thermodynamics analysis and build engineering block flow diagrams) (Schultz, 2003). Several of favour thermochemical cycle which intensive studied are Sulfur-Iodine, UT-3 Univ. of Tokyo, Westinghouse, Ispra Mark. On this article, I just explain the Sulfur-Iodine Cycle.
Sulfur-Iodine Cycle invented at General Atomics in 1970s which has serious purposed for nuclear and solar energy application. When we use S-I cycle to produce hydrogen, the overall system efficiency can get high enough, ~ 52% at 900°C (Brown, et al AIChE, 2003), as you can see at graph of S-I efficiency. We can say that inefficiencies process increase rapidly with decreasing temperature (incomplete reactions), so the process can not produce hydrogen at temperature below 700 C. But when we can make process temperature high enough, the overall efficiency very high ~ 60% at 1000 C.
The S-I cycle has three main chemical reaction:
- Prime or Bunsen reaction (exothermic) : 2H2O + SO2 + I2 –> H2SO4 + 2HI
- Sulfuric acid decomposition (endothermic) : H2SO4 –> SO2 + H2O + 1/2O2
- Hydrogen iodine decomposition (endothermic) : 2HI –> I2 + H2
The sulfuric acid decomposition need high temperatures heat ~ 830 C (highly endothermic process). This condition presents a major challenge. If the chemical process requires 830 C heat, the nuclear reactor must operate at significantly higher temperatures. This condition allow heat transfer from reactor core to chemical process. The high temperature heat has limits of practical on current material. We should use suitable ceramic which can operate at high temperature and hold the chemical corrosion of high temperature sulfur acid.
Then on hydrogen iodine decomposition section, the HI must be heating until 320 C to release iodine and hydrogen gas. Hydrogen gas will go out from process. After sulfuric acid and hydrogen iodine decomposition reaction, all of the chemical products must sent to bunsen section to colling down near 120 C to form H2SO4, HI and release oxygen from process. Then the mixture of H2SO4 and HI will separate and sent to dissociation reaction.
Unless the chemical reactions go almost to completion, the energy losses in separations and the heat exchangers to heat and cool all the unreacted reagents (H2SO4) result in a very inefficient and uneconomical process.
On S-I cycle, all reagents (H2SO4, SO2, HI, I2) returned within the cycle and recycled. So only high temperature heat and water are input and only low temperature heat, H2 and O2 as product. The simple concept of S-I cycle will difference with system plant design which consist three section and use just heat exchanger for heating and cooling. You can see how much complicate system plant design of S-I cycle.
The promising heat source from nuclear power technology is Very High Temperature Reactor (VHTR). The reactor supplies heat with core outlet temperatures up to 1,000 degrees Celsius, which very suitable for hydrogen production via thermochemical cycle. This technology is being advanced through near or medium term projects lead by several plant vendors and national laboratories, such as: PBMR, GT-HTR300C, ANTARES, NHDD, GT-MHR and NGNP in South Africa, Japan, France, South Korea and the United States. Experimental reactors: HTTR (Japan, 30 MWth) and HTR-10 (China, 10 MWth) support the advanced concept development, and the cogeneration of electricity and nuclear heat application (www.gen-4.org, 2006).
When S-I cycle powered by VHTR, hydrogen cost can reach $1.40/kg (Schultz, 2003) - this cost could compete with H2 made from natural gas by steam reformation if O2 can be sold.
At current natural gas cost ~$6.50/MBtu then H2 from steam reformation ~$1.50/kg or $11/MBtu (Campbell, 2004).
But when S-I cycle driven by solar power tower which can deliver higher temperature than nuclear, the hydrogen cost become $3.45/kg. We need optimize system design and reduce maintenance cost of solar power tower.
We can conclude that there are big barier on thermochemical cycle, such as high temperature reactor and material problem on S-I process. Intensive research on this method must be performed to make hydrogen economic to compete with the conventional fuel and safe our environmental.
References:
- Schultz Ken, 2003, Thermochemical Production of Hydrogen from Solar and Nuclear Energy, General Atomics, San Diego, CA, Accessed August 26 2006.
- Paul M. Mathias and Lloyd C. Brown, 2003, Thermodynamics of the Sulfur-Iodine Cycle for Thermochemical Hydrogen Production, The University of Tokyo, Japan, Accessed August 26 2006.
- Campbell Mike and Schultz Ken, 2004, Fusion as a Source for Hydrogen Production, General Atomics, Accessed September 5 2006.
- Forsberg Charles, 2003, Lowering Peak Temperatures for Nuclear Thermochemical Production of Hydrogen, Oak Ridge National Laboratory, United States of America, Accessed August 26 2006.
- The Very-High-Temperature Reactor (VHTR) is a graphite-moderated, helium-cooled reactor with a thermal neutron spectrum, Accessed September 9 2006.
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