Ammonia: Managing the risks associated with cracking

Alternatively, a cracking process can be employed to break ammonia back down into its constituent elements, nitrogen and hydrogen; this hydrogen can then be used in fuel cells or directly as fuel. Though not as refined as the reverse process, the method for decomposing ammonia is also tried and tested: Gaseous ammonia is conducted into a cracking reactor, where it is cracked at temperatures of between 600 and 900 °C in the presence of a catalyst. The product is then cooled. Byproducts can be removed to obtain pure hydrogen. At present, there are numerous plants, built to various scales, dotted all across the world – and their numbers continue to grow. Europe, in particular, has a fair few large-scale ammonia crackers in the works, some of which are still at the planning stage and some of which are already under construction.

Ammonia is classed as a basic, building block chemical in the industrial sector. It is also used for cooling processes. The industrial sector has a good grasp of the risks associated with its use and is able to implement appropriate mitigation measures. Other applications involving the use of ammonia as an energy carrier and the decentralised conversion of ammonia to hydrogen for non-industrial purposes require careful consideration of the associated dangers and risks. This is the only way to ensure that the necessary safety precautions are in place when manufacturing, storing, transporting and using the chemical. Additional risks arise notably when decomposing it to obtain hydrogen.

The biggest danger posed by direct exposure to ammonia is down to its water solubility. When it combines with water, ammonia is both corrosive and toxic. It is highly irritating to the skin, eyes and respiratory tract and can have serious health implications, including death. However, even at very low concentrations, this gas has an incredibly pungent odour. As a result, this will normally compel people to evacuate the area long before any corrosive damage or toxicity can take hold. But in cases where people are not able to evacuate or they do not notice due to an impaired sense of smell, ammonia leaks can present a very serious risk to people's wellbeing. Working with ammonia necessitates the use of extraction or ventilation systems in combination with PPE comprising a respirator and protective suit.

While there are certain risks associated with ammonia's intrinsic combustion properties, it is clear when you compare hydrogen and ammonia (see table) that ammonia is far less flammable. Both the ignition temperature and the required ignition energy are significantly higher. Moreover, only oxygen/ammonia mixtures with an ammonia content of between 14 and 32.5 vol% are flammable. The flammability range for hydrogen mixed with air is much wider. This means that ammonia is far less susceptible to explosion than hydrogen – in fact, it is less susceptible even than fuel oil. If escaped ammonia starts to burn, the fire will quickly go out provided there is no pilot light or other heat source that might sustain it.

Since liquid ammonia is usually stored and transported at a temperature of -33 °C at atmospheric pressure, the most important safety precautions during storage and transportation are the provision of adequate cooling and leaktight containers. Well insulated, leaktight tanks protected by safety valves are generally able to prevent gaseous ammonia from escaping. Unfortunately, we cannot entirely eliminate the possibility of unwanted incidents, e.g. as a result of an accident that occurs during transportation by sea or by road. Such incidents can be particularly damaging to aquatic life.

The elevated risk level when converting ammonia back to hydrogen can be attributed to hydrogen's properties. During the cracking process and subsequent transportation and storage, it is absolutely vital that the equipment, lines and containers are checked for leaks. This is important because hydrogen/air mixtures are flammable at a wide range of concentrations, from 4 Vol% to 77 Vol%. If exposed to an ignition source, this could result in an explosion. As you can see from the table, it requires very little ignition energy to set this in motion.

Unlike ammonia, hydrogen has no telltale pungent odour that allows for easy detection in the event of a leak. Besides ensuring that equipment and containers are leaktight, we therefore cannot proceed without implementing a multitude of additional safety measures such as automatic shut-off valves and pressure relief devices. Ignition sources such as naked flames must be eliminated at all costs. In addition, electrical explosion protection measures must be implemented in accordance with the applicable explosion protection directives, e.g. ATEX (Europe) and IECEx (worldwide). Only approved, explosion-protected devices may be used in the vicinity of hydrogen plants, which therefore applies to ammonia crackers too. Looking at the different zones enables us to determine which devices are needed. The zone classifications tell us whether the respective electric device must be suitable for Zone 0 (highest risk of explosion), Zone 1 or Zone 2 (hazardous explosive atmosphere not likely to occur in normal operation and, if it occurs, will only exist for a short time).