Hydrogen Production

Although hydrogen is the most abundant element in the Universe, it does not exist in free state in any significant amount on the Earth. It is found almost always chemically bound to other elements such as water, biomass, or fossil fuels. Molecular hydrogen, therefore, must thus be extracted from compounds such as water or organic molecules. Various methods of production have unique needs in terms of energy sources (such as heat, light, electricity) and generate specific by-products. Figure 2.4 shows a pathway of hydrogen production from different resources and technologies. One can distinguish between productions using a primary energy carrier and productions using a secondary energy carrier. Primary energy production presently means hydrogen production from fossil fuels via natural gas reforming as well as the partial oxidation of heavy fuel oil (or Diesel) and coal. Along with these further processes are in the research and development phases. The leader among these is the gasification of biomass, but also worth mentioning is the direct production of hydrogen from algae subjected to solar radiation. It is, however, only the biomass gasification process whose development phase is so developed, that its transformation into a market competitive product within the next few years can be expected.

Electricity is presently the only secondary energy carrier used to produce hydrogen, either by the electrolysis of water or as a by-product resulting from the chlorine-alkaline electrolysis. Water electrolysis is independent of primary energy use and as such is seen as the essential element of hydrogen based energy sector. As another secondary energy based production method, the reforming of methanol in mobile applications could play a role in the near future. About 95% of today's hydrogen is produced from fossil fuels using high-temperature chemical reactions that convert hydrocarbons into a synthetic gas, which is then processed to make hydrogen [96]. In many areas of the world, including Germany, large-scale natural gas reforming is currently the lowest cost method for hydrogen production. Hydrogen could also be produced at large scale by the gasification of feedstock such as coal, heavy oils, biomass, wastes or petroleum coke. In regions with plentiful, low-cost biomass resources, biomass gasification could become an economically attractive method of hydrogen production. Limiting factors are likely to be land availability and competing uses for low-cost biomass feedstock in the electricity sector.

Primary energy sources

Secondary energy

Chemical energy sources gasification

Coke, Heavy hydrocarbons, Light hydrocarbons, Refinery gas

Primary energy sources

Secondary energy gasification

Coke, Heavy hydrocarbons, Light hydrocarbons, Refinery gas

Methanol Production Energy Sources

metabolism "

"water electrolysis-^"

Fig. 2.4 Hydrogen resources and production technologies [96]

= Partial Oxydation

Synthesis

Reforminc metabolism "

Purification gas, or ammonia synthesis/cracking, or methanol synthesis/cracking, or reforming

"water electrolysis-^"

Hydrogen a s

Fig. 2.4 Hydrogen resources and production technologies [96]

Hydrogen production by the above processes (e.g. electrolysis, reforming or else) is a process of energy transformation. Electrical energy or chemical energy of hydrocarbons is transferred to chemical energy of hydrogen. Unfortunately, the process of hydrogen production is always associated with energy losses. This section discussed briefly the two methods of hydrogen production mostly commercially today. Description of several hydrogen production technologies are presented in the Appendix B.

2.3.1.1 Electrolysis

In electrolysis, electricity [46, 211] is used to decompose water into its elemental components: hydrogen and oxygen. Electrolysis is often considered as a preferred method of hydrogen production as it has high product purity, and is feasible of small and large scales. Electrolysis can operate over a wide range of electrical energy capacities, for example, taking advantages of more abundant electricity at night. At the heart of electrolysis is an electrolyzer. An electrolyzer is a series of electrolysis cells (Figure 2.5) each with a positive and negative electrode. The electrodes are immersed in water that has been made electrically conductive, achieved by adding hydrogen or hydroxyl ions, usually in the form of alkaline potassium hydroxide (KOH).

The rate of hydrogen generation is related to the current density (the amount of current divided by the electrode area measured in amps per area). In general, the higher the current density, the higher the source voltage required, and the higher the power cost per unit of hydrogen. However, higher voltages decrease the overall size of the electrolyzer and therefore result in a lower capital cost. State-of-the-art electrolyzers are reliable, have energy efficiencies of 65 to 80% and operate at current densities of about 2000 A/m2 [46].

For electrolysis, the amount of electrical energy required can be somewhat offset by adding heat energy to the reaction. The minimum amount of voltage required to decompose water is 1.23 V at 25 °C. At this voltage, the reaction requires heat energy from the outside to proceed. At 1.47 V (and same temperature) no input heat is required. At greater voltages (and same temperature) heat is released into the surroundings during water decomposition. To be truly clean, the electrical power stored during electrolysis must derive from non-polluting, renewable sources. If the power is derived from natural gas or coal, the pollution has not been eliminated, only pushed upstream. In addition, every energy transformation has an associated energy loss. Consequently, fossil fuels may be used with greater efficiency by means other than by driving the electrolysis of hydrogen. Furthermore, the cost of burning fossil fuels to generate electricity for electrolysis is three to five times that of reforming the hydrogen directly from the fossil fuel.

Fossil Fuel Reforming
Fig. 2.5 Work principles of a typical electrolysis cell [46]

2.3.1.2 Reforming

Reforming [46, 211] is a chemical process with the reaction of hydrogen-containing fuels in the presence of steam, oxygen, or both in a hydrogen-rich gas stream. When applied to solid fuels the reforming process is called gasification. The resulting hydrogen-rich gas mixture is called reformate. The equipment used to produce reformate is known as a reformer or fuel processor. The specific composition of the reformate depends on the source fuel and the process used, but it always contains other compounds such as nitrogen, carbon dioxide, carbon monoxide and some of the unreacted source fuel. When hydrogen is removed from the reformate, the remaining gas mixture is called raffinate. In essence, reforming a fossil fuel consists of the following steps: (1) Feedstock purification (including sulfur removal); (2) Steam reforming or oxidation of feedstock to form hydrogen and carbon oxides; (3) Primary purification—conversion of carbon monoxide to carbon dioxide; (4) Secondary purification— further reduction of carbon monoxide.

Any hydrocarbon or alcohol fuel can serve as a feedstock to the reforming process [46, 211]. Naturally, fuels with existing distribution infrastructures are the most commonly used. For example, natural gas has a well-established infrastructure and is the most economical of all reforming feedstock. Natural gas contains low levels of sulfur compounds that must be removed, as they would block active catalyst sites in the reformer and fuel cells. These sulfur compounds require fuel purification prior to reforming.

At the heart of reforming is a reformer. There are three basic types of reformers: steam reformers, partial oxidation reactors and thermal decomposition reactors. A fourth type results from the combination of partial oxidation and steam reforming in a single reactor, called an auto-thermal reformer. The steam reformers are currently the most efficient, economical and widely used technique of hydrogen production [46] Steam reforming is based on the principle that hydrogen-containing fuels decompose in the presence of steam over nickel-based catalysts to produce a mixture of hydrogen and carbon monoxide. The steam reforming process is illustrated schematically in Figure 2.6.

Fuel Steam

Fuel Steam

Reformate

Reformate

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  • menegilda
    How to make fosile fuel reformer?
    6 years ago

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