This chapter was published on “Inuitech – Intuitech Technologies for Sustainability”
on February 6, 2012.
Nuclear heat for hydrogen production is another advanced application of nuclear energy.
Hydrogen is the most abundant element in the universe and the third most abundant on Earth. Hydrogen gas does not exist on the earth or in our atmosphere in significant quantities. Instead, it reacts quickly with other elements to form more stable compounds. Hydrogen compounds are abundant in water and fossil fuels; its supply is effectively limitless. Because pure hydrogen is not as readily available as fossil fuels, hydrogen is not considered to be a source of energy but an energy carrier. Like electricity, hydrogen is “Manufactured”. Energy carriers are a convenient medium to store, transport, and use energy. But the convenience comes at a price, that is, efficiency.
There are several methods for producing hydrogen. All involve splitting compounds that contain hydrogen and capturing the hydrogen gas that results. To split water directly with heat (Thermolysis) requires temperatures in excess of 2500°C for significant hydrogen generation. This method is not currently practical for industrial production, as those temperatures cannot be sustained. Instead, thermochemical cycles or electrical drivers allow the splitting to occur at lower temperatures.
Hydrogen has been researched as an energy transport medium since the 1960s. Two recent technological developments have piqued the interest in hydrogen:
- Fuel cell technology to cleanly make electricity on location (or perhaps in vehicles); and
- The direct use of hydrogen as a fuel, such as in rocket fuel or military aircraft.
Currently, hydrogen production is a major area of research throughout the world, especially in the US, Europe, and Japan. Burning hydrogen with oxygen, as is done in the space shuttle, creates no pollution. The only by product of that combustion is water. Burning hydrogen with air does form some pollutants, such as NOX, but in much smaller quantities than when burning fossil fuels. Therefore, there are significant potential environmental benefits to the use of hydrogen as an energy carrier.
Here is a graph which illustrates the process of using nuclear heat for hydrogen production:
A number of thermo-chemical water splitting cycles have been identified in recent years. These cycles essentially split water into hydrogen and oxygen through a series of heat-driven chemical reactions. Early progress — including bench-scale testing of the leading cycles best suited for the high temperature gas-cooled reactor — is under development in the U.S., Japan, France and other countries. In the thermo-chemical processes, only water, heat and electricity (as a utility) are needed to produce hydrogen and oxygen. Although many of these cycles have been identified, most of the current development work is focused on the sulfur-iodine (SI) process.
Use of the high-temperature nuclear reactor as the heat source would eliminate carbon dioxide emissions and result in efficiencies approaching 80 percent. Areas of commercial interest in hydrogen include: oil refining, ammonia manufacturing (fertilizer), and methanol production. Hydrogen can be combined with gasoline, ethanol, methanol, or natural gas to increase engine performance and reduce pollution. This increasing demand for hydrogen in the refining sector is driven by the need to produce cleaner transportation fuel for meeting environmental regulations. Hydrogen can be added in the refining process to create a cleaner-burning fuel.
A fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and as a result, it produces electrical power efficiently, without producing any CO2. The by-products of an operating fuel cell are heat and water. In principle, a fuel cell operates like a battery. However, unlike a battery, a fuel cell does not run down or require recharging. With a fuel cell, chemicals constantly flow into the cell so it never goes dead – as long as there is a flow of chemicals into the cell, the electricity flows out of the cell. Most fuel cells in use today use hydrogen and oxygen as the chemicals.
Looking at the graph, there are six types of fuel cells, connected to two different types of applications – Stationary and Transport. Here is a brief description of each type of fuel cell:
- Proton Exchange Membrane Fuel Cells (PEMFC): This type also known as Poly Electrolyte Membrane. These fuel cells deliver high power density and offer the advantages of low weight and volume compared to other fuel cells. PEMFC are particularly suited to powering passenger cars and buses due to their fast start-up time, favourable power density, and power-to-weight ratio;
- Phosphoric Acid Fuel Cells (PAFC): Phosphoric acid fuel cells (PAFC) use phosphoric acid as an electrolyte and porous carbon electrodes containing a platinum catalyst. They were the first fuel cells ever used commercially and over 200 units are currently in use. Primarily used in stationary power applications, as well as for powering buses;
- Direct Methanol Fuel Cells (DMFC): Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. DMFC, however, are powered by pure methanol. DMFC fuel cell technology is relatively new, compared to that of fuel cells powered by pure hydrogen, and research and development are roughly 3-4 years behind that of other fuel cell types;
- Alkaline Fuel Cells (AFC): Alkaline fuel cells (AFC) were the first fuel cell technology ever developed and used in the United States’ space programme. They use a potassium hydroxide solution as the electrolyte and a variety of non-precious metals as a catalyst at the anode and cathode. AFC typically operate at between 100-250 °C, but recent versions operate at between 23-70 °C. AFC are high-performance devices that achieve an efficiency of 60 percent, but they are vulnerable to poisoning by even small amounts of carbon dioxide;
- Molten Carbonate Fuel Cells (MCFC): Molten Carbonate Fuel Cells (MCFC) are being developed to be fuelled by natural gas. These fuel cells cannot be fuelled by pure hydrogen. MCFC use a molten-carbonate-salt electrolyte suspended in a porous, inert ceramic matrix. They do not need an external reformer, because they operate at high temperatures (>650 °C). In addition, they do not use precious-metal catalysts, further reducing their cost; and
- Solid Oxide (SOFC): Solid Oxide Fuel Cells (SOFC) used a non-porous ceramic electrolyte and appeared to be the most promising technology for electricity generation. When combined with a gas turbine, SOFC, expected to achieve an electrical efficiency of 70 percent and up to 80-85 percent efficiency in cogeneration. High operating temperatures of 800-1000 °C mean precious-metal catalysts and external reformers are unnecessary, helping to reduce the cost of SOFC.
The applications have been divided into the following two categories:
1. STATIONARY APPLICATIONS:
Here are some facts about the fuel cell stationary applications:
- MPR Associates Inc. – Hydrogen Production by Nuclear Heat;
- Thermochemical Water Splitting Cycles;
- 2009 Annual Report of the Hydrogen & Fuel Cell Technical Advisory Committee;
- Hydrogen Fuel Cells;
- Energy Efficiency – Fuel Cells;
- Hydrogen Fuel Cell: Stationary Applications;
- Wikipedia: Internal Combustion Engine;
- Wikipedia: Fuel Cell;
- Ballard Power Systems Inc.;
- Nuclear Energy: Hydrogen Cars;
- Hydrogen Fueling Stations; and
- Fuel Cells 2000.