Other hydrogen-production methods

As well as the previously described methods of hydrogen production, there are other commercial processes whose application is restricted to specialised production conditions. These include the partial oxidation of heavy hydrocarbons, autothermal reforming and the Kvcerner process. In addition, there are numerous production processes that are still at the basic research stage, but show promising potential. These primarily include thermochemical hydrogen production, photochemical and biological processes. The main characteristics of these methods are outlined below. For a more detailed discussion, please refer to the relevant specialist literature. 

The catalytic conversion of heavy hydrocarbons, such as heavy oil or sulphurous organic residues, from the oil industry via steam reforming is not feasible because solid carbon starts to be deposited at temperatures above 800 °C, which renders the catalyst inactive in a short period of time and, furthermore, blocks the gas flow in the reactor. Heavy hydrocarbons are, therefore, converted to hydrogen using partial oxidation (POX). Note that in refineries the term gasification is more commonly used partial oxidation is the scientific terminology. 

Partial oxidation is mainly used in refineries, since the raw materials, i.e., refining residues, are available here at low cost. As far as the hydrogen produced is concerned, it can be assumed that this is primarily used by the refineries themselves, since the availability of lighter crudes is decreasing and hydrogen is being increasingly used to process heavier oils.9 

9 Since EU Directive 2005/33/EC bans high-sulphur heavy oil (bunker fuel) as fuel on ships from 2010, this source may become relevant as a cheap feedstock for hydrogen production in the future. 

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Two other hydrogen production methods, pyrolysis and aqueous reforming, have been explored for use in microreactors. Pyrolysis is the decomposition of hydrocarbons into hydrogen and carbon in water-free and air-free environments. ° If no water or air is present, no carbon oxides (e.g., CO or CO2) are... 

Hydrogen is used in a large number of chemical processes, and may be used as a fuel itself or as a reactant in the production of synthetic fuels such as in the Fischer-Tropsch hydrocarbon synthesis process, for example. In applications where hydrogen purification is required, membranes can be used for hydrogen separation. Other hydrogen purification methods include pressure swing adsorption and cryogenic separation. 

Other potential synthetic methods include fermentation (qv) of certain carbohydrates (qv), oxidation of propane, hydrogenation of acetone, and hydrolysis of isopropyl acetate. The hydrogenation of by-product acetone is the only method practiced commercially. 

Deuterioboration of 5a-cholest-2-ene (171), followed by oxidation of the alkylborane intermediate with hydrogen peroxide in the presence of sodium hydroxide, illustrates the application of this method for the preparation of c/5-deuterium labeled alcohols.(For the preparation of tra 5 -deuterium labeled alcohols see section VII-A.) The predominant reaction product is 2a-di-5a-cholestan-3a-ol (172, 1.03 D/mole) which is accompanied by 3a-di-5a-cholestan-2a-ol (173) and other minor products." ... 

The steam reforming of natural gas process is the most economic near-term process among the conventional processes. On the other hand, the steam reforming natural gas process consists of reacting methane with steam to produce CO and H2. The CO is further reacted or shifted with steam to form additional hydrogen and CO2. The CO2 is then removed from the gas mixture to produce a clean stream of hydrogen. Normally the CO2 is vented into the atmosphere. For decarbonization, the CO2 must be sequestered[l,2]. The alternative method for hydrogen production with sequestration of carbon is the thermal decomposition of methane. 

Hydrogen production by SIP can be accomplished through direct and indirect employment of hydrocarbon feedstocks (e.g., NG). In the direct employment method, iron oxide directly reacts with methane or other hydrocarbons to produce the reduced form of iron oxide and methane oxidation products, according to the following generic reaction ... 

Hydrogen will possibly play a major role among prospective energy carriers, and the most suitable method for industrial hydrogen production is water electrolysis. Membrane cells provide much better efficiency in comparison with other methods.27... 

In the four components, i.e., TiOj/MV /electron donor/bacterial cells, each of the last three components has its own specific function and each facilitates the other s role, thereby enhancing the yield of hydrogen production. It was found that with sensitized TiO, there is a higher amount of hydrogen production than with the naked TiO. Among the sensitizers used, Rhodamine B and Ru(bpy)3 " exhibited higher efficiencies compared with other sensitizers, as well as other methods of sensitization (2 and 3) (Gumnathan, 2000). 

The presence of HMX as an impurity in RDX is not a problem when the product is used as an explosive. However, the need for an analytical sample of RDX makes other more indirect methods feasible. One such method involves the oxidation of 1,3,5-trinitroso-1,3,5-triazacyclohexane (109) ( R-salt ) with a mixture of hydrogen peroxide in nitric acid at subambient temperature and yields analytical pure RDX (74%) free from HMX." The same conversion has been reported in 32 % yield with three equivalents of a 25 % solution of dinitrogen pentoxide in absolute nitric acid. l,3,5-Trinitroso-l,3,5-triazacyclohexane (109) is conveniently prepared from the reaction of hexamine with nitrous acid at high acidity. ... 

CO is derived from a variety of feedstocks such as petroleum gas, fuel oil, coal, and biomass. The industrial scale production of PO starts from propylene, which is mainly obtained from crude oil. However, due to the high importance of this compound, many pathways from renewable sources have additionally been developed [54]. PP is converted to PO by either hydrochlorination or oxidation [55]. The use of chlorine leads to large amounts of salts as by-products, therefore oxidation methods are more important, such as the co-oxidation of PP using ethylbenzene or isobutene in the presence of air and a catalyst. However, this process is economically dependent on the market share of these by-products, thus new procedures without significant amounts of other side-products have been developed, such as the HPPO (hydrogen peroxide to propylene oxide) process in which propylene is oxidized with hydrogen peroxide to give PO and water [56, 57] (Fig. 14). 

In addition to the three major techniques discussed above other methods, that we now briefly consider, have been studied for hydrogen production via water splitting. 

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