Energy of Hydrogen

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The flow of a reacting gas mixture in small channels is used in microchemical reactors to intensify the heat and mass transfer at chemical conversions. The advantages of small channels are the large specific surface of the reactor and small external diffusion resistance. This makes possible chemical conversions of gas mixtures at small residence times in the reaction zone and reactions under conditions of considerable thermodynamic nonequilibrium. Microchannel reactors are used, for instance, to convert hydrocarbons and alcohols into a synthesis gas and hydrogen and for a number of other energy-intense chemical  processes. The advantages of such reactors are most evident in the case of strongly exothermal or endothermal reactions; for instance, at partial oxidation of methane into a synthesis gas or steam reforming of methane. The considerable interest in these reactions is due to the fact that they are used in fuel processors to obtain a synthesis gas (hydrogen and carbon oxide) for fuel cells. In such processors, a catalyst is usually used to increase the reaction rate at low temperatures. The main reactions of methane steam reforming are the following:

CH4 + 2H2O = CO2+4H2     H298 K ° = 165 (kJ/mol);

CH4 + H2O = CO + 3H2        H298 K ° = 206 (kJ/mol).

These reactions are strongly endothermal, and an external heat supply is needed to sustain them. One thinks that this requirement prevents the development of methane vapor conversion reactors due to a decrease in their effectiveness. However, the use of the heat of the chemical reactions in the neighboring reactor channels proceeding in parallel can make steam reforming an attractive alternative of other methods to produce hydrogen.

The methane steam reforming is a complex multistage process including not only the exchange of reactants and reaction products between the gas phase and the reaction zone. It also consists of a series of reactions proceeding on the catalyst. Xu J. G. and Froment G. F. proposed a three-step mechanism of methane steam reforming, and above mentioned reactions are complemented by chemical conversions of their products:

CO + H2O = CO2 + H2      H298 K ° = –41.2 (kJ/mol).

A peculiarity of methane steam reforming is the need to supply heat to the reaction zone. At small heat flux density, the methane conversion degree is small and increases with increasing heat flux density. It has been found that the steam reforming reaction is more intensive at the reactor inlet and a heat supply is most effective at the reactor inlet. The heat supply at the reactor outlet mostly heats the gas mixture, thereby sustaining endothermal reactions. Thus, not only the external heat flux density but also the way it is supplied along the channel length affects the degree of methane conversion. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for producing a synthesis gas.

rather more details http://www.springerlink.com/content/j634065qq85w1064/

         As I noted before chemical reactions in fuel cells take place on special porous electrodes (anode and cathode) activated by palladium (or other platinum group metals), where chemical energy of hydrogen and oxygen is efficiently converted into electricity. Hydrogen is oxidized on the anode and oxygen  is reduced on the cathode. Catalyst on the anode speeds up the oxidation of hydrogen molecules into hydrogen ions (Н+) and electrons. Hydrogen ions (protons) pass through the membrane to the cathode where the catalyst stimulates the formation of water out of protons, electrons and oxygen. Free electrons are conducted through the external circuit to produce direct current for various applications. Interruption of the circuit brings fuel cell to a standstill.

         Voltage in a separate fuel cell doesn’t exceed 1,1V. To achieve the required voltage fuel cells are consequently combined in stacks, and this stacks are connected in parallel to reach the required capacity. Every fuel cell produces heat as a by-product but one can use co-generation to recapture it.
          There are several types of fuel cells. They are usually differentiated by the type of fuel used, operating pressure and temperature, area of application. In the most wide-spread classification they are distinguished by the type of electrolyte material used as a medium for the internal transfer of ions (protons). The type of electrolyte determines the operating temperature on which the type of catalyst depends. The choice of fuel and oxidant for any fuel cell depends on their electrochemical activity (that is, the speed of electrode reaction), cost, and easiness of fuel and oxidant delivery and removal of reaction by-products. The main source of fuel for fuel cells is hydrogen, but fuel conversion process allows recovering hydrogen from other materials like methanol, natural gas, oil, etc.

• Alkaline Electrolyte Fuel Cells. The electrolyte in this fuel cell is concentrated (85 wt.) potassium hydroxide (KOH) in high temperature cells (~250ºC), or less concentrated (35-50 wt.) KOH for lower temperature (<120ºC) operation. In mid-1960s they were used for the Buran and Shuttle space vehicles. However, they have had relatively little success in terrestrial applications due to the high cost of producing high purity fuel and oxidiser streams, plus corrosion problems. Typical efficiency is 60%.
• Proton Exchange Membrane Fuel Cells. The electrolyte in this fuel cell is a solid polymer membrane (thin plastic film) that is an excellent ion (proton) conductor. High current density in these cells means low weight, volume and cost. Solid electrolyte makes easier the process of sealing in the fuel cells production, reduces corrosion and provides longer service life. Low operating temperature (below 100˚C) facilitates start-up and reaction to power requirements. These fuel cells are ideal for transport vehicles and small-scale stationary applications.
• Phosphoric Acid Electrolyte Fuel Cells. The electrolyte in this fuel cell is 100% concentrated phosphoric acid retained in a matrix which is usually silicon carbide. These fuel cells were the first to reach commercialization. Applications: stationary power plants in houses, hotels, hospitals, airports. Their efficiency exceeds 40% and may reach 85% when the by-product steam is.
• Molten Carbonate Electrolyte Fuel Cells. The electrolyte in this fuel cell is usually a combination of alkali carbonates, such as Na and K, which is retained in a ceramic matrix of LiAlO2. The fuel cell operates at about 600 to 700ºC thus allowing to use fuel directly, without any additional processing, and Ni may be used as a catalyst. These fuel cells offer higher electrical efficiencies than phosphoric acid fuel cells at around 60% plus the possibility of cogeneration (water heating) which makes overall efficiencies of 80% feasible. Reaction to any changes in the power requirement is slow; this is why they are suitable for applications where high power is needed constantly. At present there are numerous demonstration plants in the U.S.A. and Japan. One of American plants has a capacity of 1.8 MW.
• Solid Oxide Electrolyte Fuel Cells. The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3-stabilised ZrO2. Cells operate at 650 to 1000ºC where efficient conduction of anode seeking oxygen ions takes place. Operating temperatures are high enough to allow internal reforming and promote rapid kinetics with non precious materials. They are suitable for use in stationary power plants of large and very large scale. Overall efficiency is about 60%.

Since hydrogen doesn’t exist on earth as a gas, we must separate it from other elements. We can separate hydrogen atoms from water, biomass, or natural gas molecules. Hydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via biological production (scientists have even discovered that some algae and bacteria give off hydrogen), by electrolysis, by chemical reduction or by thermolysis.Hydrogen can be produced at large central facilities or at small plants for local use. Every region of the country (and the world) has some resource that can be used to make hydrogen. Its flexibility is one of its main advantages.

• Steam reforming is currently the least expensive method of producing hydrogen. The production of hydrogen by reforming is a well-established process with large-scale production (330,000 kg hydrogen/day) and large facilities that produce hydrogen at costs that approach the DOE target ($2.00−$3.00/gge) (gge is gasoline gallon equivalent on an energy basis). Steam methane reforming accounts for 80% of the hydrogen produced. The remaining 20% is a by-product of chemical processes such as chlor-alkali production.
• Electrolysis is a process that splits hydrogen from water. It results in no emissions but it is currently a very expensive process. New technologies are being developed all the time. Water electrolysis represents only a niche segment of the merchant hydrogen market. There are two ambient temperature electrolysis processes for producing hydrogen: alkaline electrolysis, which uses concentrated potassium hydroxide (KOH) as the electrolyte, and PEM
  electrolysis, which uses the ionomer Nafion™ as the electrolyte. The membrane for PEM electrolysis is similar to that used in PEM fuel cell. Alkaline electrolysis stacks can be either monopolar or bipolar; PEM electrolysis stacks are bipolar.
• Coal gasification breaks down the coal into smaller molecular weight molecules, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of syngas, a mixture mainly consisting of carbon monoxide (CO) and hydrogen (H₂).

Today, approximately 9 million tons (~9 billion kg) of hydrogen are produced annually. More than 95% of the merchant hydrogen is captive for industrial applications―chemical, metals, electronics, and space. The cost of hydrogen produced safely and efficiently from on-site hydrogen generators must be lowered enough to be competitive with gasoline on a cost per mile driven basis, without adverse environmental impacts. Today the cost of high-volume hydrogen production and delivery is two to three times the DOE target of $2.00−$3.00/gge untaxed.

This figure depicts the reductions in hydrogen production costs that need to be achieved for distributed steam methane reforming and electrolysis to be competitive with gasoline.