Techniques and industrial means of hydrogen production.
Key words: hydrogen generation, industry, electrolysis, pyrolysis, reforming, metal catalysts, costs, conditions, operation.
Very fashionable today and considered, perhaps wrongly, as an energy solution for future generations, hydrogen nevertheless does not exist in its native state on Earth.
He can not not be considered a source of energy (unlike fossil or renewable energies) but simply as an energy vector, ie a means of transporting or transferring energy. Unfortunately, the constraints linked to the use of energetic hydrogen are numerous, so that liquid petroleum fuels still have good years ahead of them.
But, in addition to these considerations related to the use of hydrogen, let us come to the subject of this article. Indeed, since hydrogen does not exist in natural form on earth, it was necessary (and above all it will be necessary) to develop ecologically profitable production methods. Here is an overview of current methods.
For information, currently hydrogen energy (in addition to marginal fuel cells vehicles running on pure H2) is only used in one area: space launchers.
1) Raw materials
Mainly hydrocarbons (natural gas) and water.
2) Industrial manufacture.
Principle of reduction of H2O by:
(a) hydrocarbons, mainly natural gas,
3) Natural gas reforming: the main source of dihydrogen.
Since 1970, the reforming of naphtha is, in general, replaced by that of natural gas.
The synthesis gas is produced by steam reforming, at 800 - 900 ° C and 3,3 MPa, in the presence of a catalyst based on nickel oxide on alumina rings impregnated with 10 to 16% by mass of Ni ( lifetime 8 to 10 years) and depending on the reaction:
CH4 + H2O <====> CO + 3 H2 Enthalpy of reaction at 298 ° K = + 206,1 kJ / mole
The reaction, very endothermic, requires a continuous supply of energy. The gas mixture circulates in tubes, heated externally, containing the catalyst. In the order of ten to a few hundred tubes (up to 500) 10 cm in diameter and 11 m long are placed in an oven. After reforming, the synthesis gas contains 5 to 11% by volume of unconverted methane.
The catalyst is very sensitive to the presence of sulfur which gives NiS: less than 1 S atom for 1000 Ni atom is sufficient to poison the catalyst. Natural gas must be desulphurized to less than 0,1 ppm S
After a pre-desulfurization obtained by catalytic hydrogenation followed by absorption in an aqueous solution of diethanolamine (see the treatment of Lacq gas in the sulfur chapter), a new hydrogenation carried out at around 350 - 400 ° C, allows, in the presence of molybdenum catalysts -cobalt or molybdenum-nickel, to transform all sulfur compounds into hydrogen sulphide. Hydrogen sulfide is fixed at around 380 - 400 ° C on zinc oxide depending on the reaction:
H2S + ZnO –––> ZnS + H2O
b) Use of synthesis gas to produce ammonia (without CO recovery):
A secondary reforming is carried out by adding air in an amount such that the nitrogen content is, with H2, in the stoichiometric proportions of the NH3 formation reaction. O2 from the air oxidizes the remaining CH4. The catalyst used is based on nickel oxide.
The CO of the synthesis gas is then converted, by conversion, into CO2 with additional production of H2, into 2 steps. A gas containing 70% of H2 is thus obtained.
CO + H2O <====> CO2 + H2 DrH ° 298 = - 41 kJ / mole
- at 320 - 370 ° C with a catalyst based on iron oxide (Fe3O4) and chromium oxide (Cr2O3) with metallic addition based on copper. The catalyst is in the form of pellets obtained from powdered oxides or spinels, its lifespan of 4 to 10 years and more. The 2 to 3% by volume of residual CO are converted in a second step,
- at 205 - 240 ° C with a catalyst based on copper oxide (15 to 30% by mass) and chromium and zinc oxides on alumina, lifespan 1 to 5 years. After conversion: residual CO of about 0,2% by volume.
- CO2 is eliminated by dissolving in a solution of amines at 35 bar or in a solution of potassium carbonate. By expansion to atmospheric pressure, CO2 is released, and the solution recycled.
- Dihydrogen is then used to synthesize ammonia
c) Use of synthesis gas with recovery of CO and H2.
Reforming is an interesting source of CO raw material for the manufacture of acetic acid, formic acid, acrylic acid, phosgene and isocyanates.
After removal of the carbon dioxide present and drying, the dihydrogen and the carbon monoxide are separated. Air Liquide uses two cryogenic processes:
- By cooling in exchangers and condensation of CO: CO has a purity of 97-98% and H2 contains 2 to 5% of CO.
- By cooling by washing with liquid methane: CO has a purity of 98-99%, and H2 contains only a few ppm of CO.
For example, the Rhône-Poulenc acetic acid unit in Pardies (64) (14 m800 / h of CO and 3 m32 / h of H290) taken over by Acetex (Canada) in 3 and that of phosgene from SNPE in Toulouse use these processes.
d) Obtaining high purity H2
Applications such as electronics, food, space propulsion require very high purity hydrogen. This is purified by adsorption of the impurities on activated carbon (PSA process). The purity obtained can be greater than 99,9999%.
- NaCl: H2 co-produced (28 kg of H2 per tonne of Cl2) gives 3% of the world's H2. In Europe, more than half of the hydrogen distributed by industrial gas producers comes from this source.
- H2O: not currently profitable. Profitability is linked to the cost of electricity, consumption is around 4,5 kWh / m3 H2. The global installed capacities, ie 33 m000 of H3 / h, give about 2% of the global H1.
The electrolysis is carried out using an aqueous solution of KOH (25 to 40% concentration), using the purest possible water (filtration on activated carbon and total demineralization by ion exchange resins). The resistivity must be greater than 2 W.cm. The cathode is made of mild steel activated by the formation of a surface deposit based on Ni. The anode is made of nickel-plated steel or solid nickel. The most used diaphragm is asbestos (chrysotile). The voltage is between 104 and 1,8 V. The power per electrolyser can reach 2,2 to 2,2 MW.
5) Pyrolysis of coal which contains about 5% H2.
The production of coke (by removing volatile matter from coal, at 1100-1400 ° C) gives a gas at 60% H2 - 25% CH4 (1 t of coal gives 300 m3 of gas). Since the use of natural gas to produce H2, the coking gas is burned and the energy released is recovered (see the natural gas chapter).
6) Coal gasification
Main source of H2 before natural gas is used. It is no longer used at present except in South Africa (Sasol company) which thus produces synthesis gas intended to manufacture synthetic fuel. This technique is currently not profitable except for a few production units of: NH3 (Japan), methanol (Germany), acetic anhydride (United States, by Eastmann-Kodak).
- Principle: gas formation with water or syngas, at 1000 ° C.
C + H2O <====> CO + H2
Enthalpy of reaction at 298 ° K = + 131 kJ / mole
Endothermic reaction that requires an O2 blast to maintain the temperature by burning carbon. Gas composition: 50% H2 - 40% CO.
Improvement of H2 production by CO conversion, see above.
- Technique used: gasification in gasifiers (Lurgi).
In the future, underground gasification could be used.
7) Other sources
- Reforming and catalytic cracking of petroleum products.
- Steam cracking of naphtha (production of ethylene).
- By-product of the manufacture of styrene (Elf Atochem, Dow): important source.
- Methanol cracking (Grande Paroisse process): used in Kourou in French Guiana, by Air Liquide, to produce liquid hydrogen (10 million L / year) intended for Ariane flights.
- Partial oxidation of petroleum cuts (Shell and Texaco processes).
- Purge gas from ammonia production units.
- Microorganisms by biochemical reactions. For example with a micro-alga: Chlamydomonas the yields are still quite low but current research is promising. More information, click here. But beware: genetic modifications to organisms at the base of the oceanic food chain are not without risks ...