Hydrogen Production Systems

To produce hydrogen, the hydrogen bonds in hydrocarbons or water must be broken and hydrogen must be separated from the reaction mixture. The most efficient method for meeting increasing energy needs could be to convert nuclear power into electricity and hydrogen, thus providing effective and universal energy carriers. Nuclear power plants produce heat that can be used directly or converted into electricity for the production of hydrogen. Four classes of H2 production options are under development (e.g., Forsberg, 2007):

• electrolysis (electricity + H2O [liquid] ^H2 + O2);

• high-temperature electrolysis (electricity + H2O [steam]^H2 + O2);

• hybrid cycles (electricity + heat + H2O ^ [cyclic chemical reactions] ^ H2 + O2);

• thermochemical cycles (heat + H2O^ [cyclic chemical reactions] ^ H2 + O2).

The near-term option is electrolysis. The longer term options involve using heat to convert water to hydrogen and oxygen. Because heat is less expensive than electricity (because the cost is avoided of converting heat to electricity with associated losses), these advanced processes have the long-term potential of lower production costs (Forsberg, 2007).

Estimates from Japan are that the cost of nuclear thermochemical H2 production could be as low as 60% of that for nuclear H2 production by the electrolysis of water (Forsberg et al., 2003). At the most fundamental level, thermochemi-cal H2 production involves the conversion of thermal energy to chemical energy (H2) while electrolysis involves the conversion of thermal energy to electricity and subsequent conversion of electricity to chemical energy.

Nuclear energy provides a source of heat to produce H2. Multiple processes are being investigated to produce H2 from water and heat. If nuclear energy is to be used for H2 production, the nuclear reactor must deliver the heat at conditions that match the requirements imposed by the H2 production process. At this stage of development, it is unclear which chemical processes will be the most economic; thus the major candidate technologies were examined to determine if they impose similar requirements on the reactor (Forsberg, 2003).

Methods for obtaining hydrogen using carbon compounds as the raw material will probably be the main methods in the near future. However, the raw-materials and ecological limitations of steam conversion of methane are stimulating the development of processes to produce hydrogen from water. The most interesting of these methods in the context of nuclear power are electrolysis and thermochemical and thermoelectrochemical cycles. Figure 6.1 presents an overview of nuclear-based hydrogen production technologies. The main processes for hydrogen production include steam reforming of natural gas, catalytic decomposition of natural gas, partial oxidation of heavy oil, coal gasification, water electrolysis, thermochemical cycles, and photo-chemical, electrochemical, and biological processes. The first four processes are based on fossil fuels.

6.3.1 Steam conversion of methane

Large amounts of hydrogen and hydrogen-containing products are now produced primarily by steam conversion of the methane in natural gas. Steam and heat at 750-850°C are required to separate hydrogen from the carbon base in methane; this is what happens on catalytic surfaces in chemical steam reformers. The first step of the reaction splits methane and water vapor into hydrogen and carbon monoxide (the synthesis gas). Next the "shift reaction" converts carbon monoxide and water into carbon dioxide and hydrogen. This reaction occurs at 200-250°C. For the endothermic process, about half of the initial gas is consumed. A model of this system can consist of a nuclear part, which generates the synthesis gas and a process part, where the input gas is used to produce the final product. Figure 6.2 shows the principle of steam methane reforming, and based on this figure its chemical formula is expressed as CH4+2H2O^ 4H2+CO2 -185 kJ/mol. The heat of reaction is 185 kJ/mol, the heat for 1 mol hydrogen is 46.25 kJ/mol, and this reaction is endothermic (Chikazawa et al., 2005).

6.3.2 Thermochemical and thermoelectrochemical cycles

High temperatures (above 2500°C) are required for direct thermal decomposition of water into hydrogen and oxygen. However, water can be decomposed thermally at lower temperatures using a sequence of chemical reactions which perform the following functions: binding water, splitting hydrogen and oxygen from the water, and recovering the reagents. Figures 6.3 and 6.4 show, respectively, the schematics for thermochemical processes that do not require electricity input and electro-thermochemical processes that require electricity input in addition to thermal energy input (Yildiz et al., 2006).

The thermochemical process for producing hydrogen with an efficiency of up to 50% employs a sequence of chemical reactions and requires heat at a tem perature of about 1000°C. A high-temperature reactor serves as the heat source for thermochemical decomposition of water. Electrolysis and plasma can be used, together with heat, for splitting off hydrogen at individual stages of such processes. Many combinations of chemical reactions, where water is split into hydrogen and oxygen in a closed cycle with heat and electricity being absorbed, have been studied. Some examples are as follows as:

Arbre Des Causes Exmples Manufacturing
Fig. 6.1 Technology options for nuclear hydrogen production (modified from Yildiz and Kazimi, 2006).

Sodium Tube

CH4,H2O

Sodium Tube

CH4,H2O

Palladium Membrane

Na Catalyst

Fig. 6.2 Principle of membrane reformer (adapted from Chikazawa et al., 2005).

Palladium Membrane h2

Na Catalyst

Fig. 6.2 Principle of membrane reformer (adapted from Chikazawa et al., 2005).

Reactor

Electricity

Heat

Hybrid Cycle

Fig. 6.3 Simple coupling of an electrothermochemical water-splitting process, a nuclear reactor, and a power conversion system for hydrogen production.

n t \ Thermochemical Heat > Cycle

Reactor

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