MANUFACTURING OF HYDROGEN
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MANUFACTURING OF HYDROGEN
Introduction:

Hydrogen is a chemical element which normally exists in the gaseous state. Hydrogen has the second lowest boiling point and melting points of all substances, second only to helium. Hydrogen is a liquid below its boiling point of 20 K (–423 ºF; –253 ºC) and a solid below its melting point of 14 K (–434 ºF; –259 ºC) and atmospheric pressure [1]. Temperatures below –150 ºC (123 K; –238 ºF) are collectively known as cryogenic temperatures, and liquids at these temperatures are known as cryogenic liquids [2].
Production of Hydrogen
Hydrogen can be produced in many ways, the most common of which are through fossil fuels. Some of the methods of hydrogen production are;
a) Gasification of coal
Gasification of coal is the oldest method of producing hydrogen. Typically, the coal is heated up to 900ºC where it turns into a gaseous form and is then mixed with steam. It is then fed over a catalyst – usually nickel. There are also other more complex methods of gasifying coal. The common factor is that they turn coal, treated with steam and oxygen at high temperatures, into H2, CO and CO2. In addition, sulphur is released from the raw material and creates sulphur and nitrogen compounds [3].
b) Steam reforming of natural gas
Steam reforming of natural gas is currently the cheapest way to produce hydrogen, and accounts for about half of the world’s hydrogen production. Steam, at a temperature of 700-1000 ºC, is fed methane gas in a reactor with a catalyst, at 3-25 bar pressure.
CO-shift - The processes described above produce gas with a high content of carbon monoxide – CO. It is therefore necessary to put the gas through the CO-shift process to increase the content of hydrogen. The shift reaction is a two-step process to achieve the most complete reaction between CO and steam. Initially steam is added in a high-temperature step (300-500ºC), followed by a a low-temperature step (200ºC), with different catalysts in the two steps.
Separation of CO2 - Each of the processes described above produces CO2 in addition to H2. To separate hydrogen and CO2, it is common to use amine based absorption processes. This is conventional technology. Methods based on selective membranes or sorbents are under development.
Depositing - To avoid having the fossil CO2 released into the atmosphere, it must be deposited permanently. Possible depositories include empty oil and gas reservoirs, or underground water reservoirs, called aquifers [3].
c) Thermal dissociation
By heating up hydrocarbon compounds without oxygen at very high temperatures, it is possible to separate the hydrogen and carbon. The formula for this process using methane as fuel is:
CH4 -> C + 2H2
1 mol methane -> 2 mol hydrogen [3]
d) Carbon Black & Hydrogen Process (CB&H)
Carbon Black is a super pure carbon (soot) which is used in car tire production, and as a reducing material in metallurgic industries. Kvaerner developed a process called the “Kvaerner Carbon Black & Hydrogen Process “ (KCB&H). The first commercial plant based on this process started production in June of 1999. The Kvaerner process is emission-free, while the traditional production methods for Carbon Black are extremely polluting. The by-product from this process is hydrogen. In a high temperature reactor, the correct amount of heat for splitting the hydrogen compounds is supplied by a plasma burner, which utilises recycled hydrogen from the process as plasma gas. A heat exchange system heats up the process flow [4].
e) Plasmatron
At the Massachusetts Institute of Technology (MIT), researches are developing a reformer which uses plasma for reformation of hydrocarbons. The advantage of a plasma reformer is that it can use all forms of hydrocarbons, including heavy oil fractions. MIT’s “Plasmatron” operates at temperatures of over 2,000º C. Hydrogen yield is 80-90%. The main disadvantage of plasma reformation in general, is its dependency on electrical power [5].
f) Electrolysis of water
Water electrolysis is splitting water into hydrogen and oxygen. An electrolyser is a device for electrolysis. Water is subjected to electrical power and the result is hydrogen and oxygen.
2H2O + energy -> 2H2 + O2
Alkaline electrolysers - In alkaline electrolysers a liquid electrolyte is used – typically a 25% potassium hydroxide solution.
Polymer electrolyte membrane (PEM) electrolysers - Another type of electrolyser utilises polymer membranes as electrolytes (PEM). Efficiency factors for PEM electrolysers up to 94% are predicted, but this is only theoretical at this time.
Steam electrolysers - A third type of electrolysers is the so-called steam electrolysers. These use a ceramic ion-conducting electrolyte. Steam electrolysers can reach a very high efficiency factor, but are currently not commercially feasible.[6]
g) Photoelectrolysis
Instead of first converting sunlight to electricity and then using an electrolyser to produce hydrogen from water, it is possible to combine these two steps.
The photovoltaic cell combines with a catalyst, which acts as an electrolyser and splits hydrogen and oxygen directly from the surface of the cell. This can quite realistically be a commercially viable means of producing hydrogen. The advantage with these systems is that they eliminate the cost of electrolysers and increase the systems’ efficiency. Tests performed outdoors with silicon based cells have shown an efficiency of 7.8% in natural sunlight. Research is being done to increase the efficiency factor and the life span for such cells [7].
2.2.2.3. Thermal decomposition of water
In a thermal solar power plant with a central collector such as Solar Two, a 10 MW power plant in California, the temperatures can reach over 3,000ºC. By heating water to over 2,000ºC, it is broken down into hydrogen and oxygen. This is considered to be an interesting and inexpensive method of producing hydrogen directly from solar energy. Research is also being done on the use of catalysts to reduce the temperature for dissociation. One central problem is the separation of gases at high temperatures to avoid recombining. The efficiency factor is uncertain.
Storage of hydrogen
If hydrogen is to be used on a large scale basis, storage is a key problem. In vehicles for instance, it must be possible to store enough hydrogen to allow for the same driving distance as today’s cars.
In the energy sector the ability to store the hydrogen effectively, quickly and inexpensively is most important. This chapter will take a look at hydrogen storage with special focus on storage in vehicles.
Hydrogen is a substance with high energy content compared to its weight. This is the reason that hydrogen is naturally the first choice in space travel and very well suited for air travel. On the other hand, the energy content compared to volume is rather low. This poses greater challenges with respect to storage compared to storage of gasoline which is a liquid.
The US DOE has determined that an energy density of 6.5 weight percent hydrogen and 62 kg hydrogen per m3 must be achieved, in order for a hydrogen storage system of appropriate weight and size to facilitate a fuel cell vehicle driving distance of 560 kilometres.
There are basically three options:
• hydrogen may be compressed and stored in a pressure tank
• hydrogen may be cooled to a liquid state and kept cold in a properly insulated tank
• hydrogen may be stored in a solid compound
Various strategies for storage are described in the following section.
2.4.2. Liquid hydrogen
Hydrogen can be stored as a liquid (LH2) at 20 K (-253º C) in super insulated tanks.
LH2 is particularly interesting for long distance transportation purposes and as fuel in spacecraft and airplanes. A great deal of experience has been accumulated over the years when it comes to the usage and handling of LH2. In order to cool the hydrogen down, energy equalling 30-40% of that in the fuel is needed. Development of a new cooling process that would cut the energy use in half is considered feasible. [Nytek 2000] LH2 is especially well suited for use in air and space travel, where its characteristics rate it higher than any other fuel. Today, LH2 is the most frequently used fuel within space travel.
BMW has studied use of liquid hydrogen in combustion engines in cars for over 20 years and says that using liquid hydrogen in automobiles is a good alternative. The German company Linde has developed a tank for liquid hydrogen where the cold from some of the liquid hydrogen is used to cool down the insulation surrounding the tank; this is done with cooling elements. This way the tank keeps the hydrogen in a liquid state for up to 12 days.[Hyweb, 2000] This type of tank is now being tested and will probably be installed in BMW’s hydrogen cars among others.
2.5.2. Transport of liquid hydrogen
Liquid hydrogen (LH2) is hydrogen which has been cooled below -253ºC. The cooling process requires a great deal of energy, but for long-distance transportation and as fuel in certain applications used in air and space travel, LH2 still has obvious advantages over other fuels
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