4.8.1 Safety of the hydrogen plant
Safety of the process is an essential part of the production. The most severe hazards in the hydrogen plant are material over-stressing, fire and explosion.
Over-stressing is caused by incorrect operation in the plant, or inadequate maintenance or repair work. Incorrect operation means exposing process equipment to conditions for which it is not designed, such as pressure, temperature, corrosion, erosion, mechanical forces, vibrations, alternating stress or thermal expansion.
In order to maintain the safety of the plant, hydrogen’s flammable properties have to be taken into account. Hydrogen burns above 2000 ˚C with colourless flames that are extremely dangerous and difficult to detect in time. Hydrogen also has a rather low ignition temperature of 500-571 °C. The risk of hydrogen auto-igniting is considerable. Simultaneous monitoring of UV and IR radiation at two wavelengths could be used to detect the fire.
In an enclosed area, small leaks of hydrogen pose a danger of exposure to hydrogen, fire and even explosion since hydrogen diffuses quickly to fill the volume. According to Press et al. [20], exposure to hydrogen can cause oxygen
deficiency in the human body, the effects of which may include rapid breathing, diminished mental alertness, impaired muscular co-ordination, faulty judgment, depression of all sensations, emotional instability and fatigue.
The safety of the raw material also has to be taken into account. Natural gas does not have a characteristic smell, so leakages cannot be detected from smell without adding sulphur-containing compounds. Sulphur cannot be used because it acts as a catalyst poison. Natural gas is also lighter than air. Natural gas fuel may cause a risk of water formation in reforming furnaces. Burning hydrocarbon-based fuel can form water if the equipment materials or catalysts are not suitable. The formation of water causes risks of weathering and freeze-up of the furnaces.
Incomplete burning of natural gas caused by temperature and pressure changes may cause the formation of toxic carbon monoxide. This can be avoided by ensuring that sufficient combustion air is fed to the furnace and that the flue gases are successfully removed from the furnaces by a closed system, vent system or a combination thereof.
The equipment safety determines the maximum and minimum values for process parameters. These are taken into account in the equipment design parameters.
These alarm values should be avoided in order to ensure plant safety and minimize the risks caused by process condition changes. Possible risks caused by the use of incorrect process conditions are process shut-down, low quality of product, equipment breakage and even an explosion in the plant.
4.8.2 Reformer bottlenecks and failure mechanisms
The steam reformer is the most important and expensive part of the hydrogen plant. Its tubes have a certain lifetime and the replacement is expensive. The right timing of the tube change and correct operation are therefore essential.
The main bottlenecks with reformers are usually the reformer tubes, radiant box, convection section, fuel cell (FC) fans and burners. The most common steam reformer tube failure mechanisms are normal ‘end-of-life’ failures and accelerated normal ‘end-of-life’ by overheating and thermal cycling. One of the most
dominant damage mechanisms in reformer tubes is creep damage. Creep damage is a slow, sustained increase in the diameter of the tube caused by stress at elevated temperatures.
Less common steam reformer tube failure mechanisms are unidense loading, burner firing, thermal shock, stress corrosion cracking, dissimilar weld cracking and the tube support system. Consequences of these kinds of failure mechanisms include flames from the furnace burners accidentally impinging directly on the outside surface of one or two tubes, the activity of the catalyst in the odd tube becoming impaired by carbon formation, reducing the reaction rate and creep damage on the reformer tubes.
In order to replace the tubes in time, reformer tubes have to be inspected regularly.
For this kind of inspection, two kinds of testing methods are used: non-destructive testing (NDT) and destructive testing. In non-destructive testing, reformer tubes are tested without removing the tubes from the process by visual examination, radiography or tube outer diameter measurement. The most used non-destructive testing methods are the detailed mapping and the Laser optical tube inspection system (LOTIS). In destructive testing methods, in which the tubes are removed from the process, the testing is mainly based on metallurgical examination.
Monitoring and testing of the reformer tubes is necessary in order to maximize the tube lifetime and maintain the safety of the reformer. The tube life can also be maximized by temperature control by maintaining the temperature as low as possible. Using improved metallurgy at the investment stage of the plant also maximizes the tube lifetime.
5 OTHER MANUFACTURING TECHNOLOGIES
Besides steam reforming, multiple other technologies are used for hydrogen production. These methods differ from steam reforming in their structure and principle, raw materials or efficiencies. Chemical processes, in which hydrogen is produced by a chemical reaction or reactions, are called synthesis processes.
Hydrogen can be produced from water via electrolysis. Another method is to produce hydrogen from a gas mixture of hydrocarbons, as in partial oxidation and auto-thermal reforming. Hydrogen can also be produced from solid and liquid materials as in coal/biomass gasification and biomass pyrolysis. The combination of gas and liquid hydrogen production technology is called thermo-catalytic cracking of methane and ammonia.