Hydrogen Storage and Transport

Hydrogen provides and excellent decarbonization opportunity for several industries like steel and cement production, power generation, heating, refining amongst others with more and more industries starting to use hydrogen for its environmentally friendly nature. 

This has developed a need for safe storage and transportation of this important material. 

The common practice in the past was producing hydrogen wherever it was needed. However, as the use of hydrogen changes and expands so does the need for transportation. 

While determining the best method for storage and transportation of hydrogen, safety and economics must be considered. A major challenge that needs to be overcome is the extreme pressure and temperature needed for storage and transportation. 

Storage mechanisms for hydrogen in both gas and liquid form

1) High pressure gas cylinders

This is the most common hydrogen gas storage technique. It is done at high pressures (250 to 700 bar). This can be used for on-site storage as well as transportation of hydrogen using trucks. The permeability of the material selected for these tanks should be considered before being used. 

2) Super-insulated low pressure liquid cryogenic storage vessels. 

This is for liquid forms of hydrogen at 1 atm and - 252°C to prevent boil-off and product loss inside insulated tanks. This however has a large energy cost but allows for efficient transportation of large quantities of hydrogen over large distances including via ships.

3) Naturally occurring or specially engineered underground bulk storage. 

4) Alternative molecules.

Hydrogen can also be stored by converting it into ammonia, methanol or ethanol in order to use existing storage systems. These chemicals do not require high pressure or low temperature storage. Thus helps reduce the cost to obtain special storage and transportation vessels. Liquid organic hydrogen carriers (LOHCs) which are compounds that can absorb and release hydrogen through reactions can also be used for effective storage and transportation. LOHC technology enables easy fueling of the fuel cell electric vehicles. 

5) It can also be stored by adsorption on the surface of solids and absorption within solids.

Challenges during the handling of Hydrogen

1) Detecting leaks and managing safety concerns. 

Safety is always of paramount importance for any chemical process for the protection of employees and plant infrastructure. Even though hydrogen is non-toxic, it is extremely flammable. It has a combustible range between 4 - 74% in air as compared to natural gas which has a range between 5 - 20%. 

A safer environment can be ensured by conducting hydrogen leak tests and flames detection. 

2) Minimize product loss

It is vital to measure and verify the amount of hydrogen as it moves through the supply chain through pipelines, loading and offloading sites and storage units. This allows minimization of theft and detection leaks to prevent loss of product and ensure safety of all around the supply chain.

3) Ensure hydrogen purity

The end use of hydrogen determines the purity required. The quality of hydrogen for uses like heavy duty transit is different from the requirements for blending into natural gas. 

Thus it is vital to understand the purity of the hydrogen being produced, moved and used in order to ensure safety and meeting the customers  requirements. 

This can be mitigated by use of gas analyzers to check the purity of hydrogen through every step of storage and transport. 

 

Transportation of Hydrogen

Factors like distance, geography, cost, energy loss and end use are key factors that determine which transportation method is efficient in hydrogen movement to the point of use. These methods include:

1) Pipelines

They offer a way to deliver large volumes of hydrogen. Pipes used to distribute natural gas can also be used to transport hydrogen blended with natural gas.  

2) Trucks

Hydrogen tankers can be used to transport compressed hydrogen in high pressure tube trailers or liquefied hydrogen in a cryogenic storage unit. This method allows for quick deployment and endless flexibility. 

3) Ships

This allows for long distance transportation using the oceans or seas. Cargo vessels can be retrofitted to allow safe shipping. These ships look similar to Liquefied Natural Gas (LNG) tankers. The difference with LNG is the construction materials and instruments used for process optimization and safety monitoring. 

4) Alternative molecules.

Similar to using alternative molecules for hydrogen storage, hydrogen can be converted into an alternative molecules like methanol, ethanol or ammonia or LOHC to allow easy and safe transportation by allowing ambient temperatures and pressures for handling. In this method, the hydrogen should be reconverted back into hydrogen at the consumption point. 

Hydrogen leaks

In order to ensure safety, minimize product loss and ensure compliance with regulations, quick leak detection in pipelines and storage units is extremely important and can be very difficult since hydrogen flames are invisible to the naked eye and conventional flame detection is risky for workers. 

Multi-spectrum Infrared Hydrogen Flame Detectors are effective in detecting any hydrogen leaks even from a long distance and provides fast response and with reduced false alarms. 

Short Notes #1

What is Runaway Reaction

A Runaway Reaction is any reaction system that displays an acceleration in the reaction rate at conditions so great that it becomes very difficult or impossible to control. The accompanying characteristics of a runaway reaction is that it occurs with a rapid increase of temperature and/or pressure.

Runaway reactions are generally extremely exothermic reactions meaning they give off heat to the surroundings as the reaction progresses. Thus, if the heat removal system is not able to remove as much of the heat produced during the reaction then the temperature of the reaction mixture starts increases. If the rapid temperature and/or pressure increase is not properly controlled then there is serious damage done to the equipment and may lead to loss of lives if there is any form of explosion or leak within the reactor.

Risk Assessment and Evaluation of Hazards

Any chemical reaction that has a chance of having a runaway under certain conditions needs to be studied properly and knowledge of the reaction mechanisms need to be properly researched and the risks assessed in order to design proper control systems to prevent any catastrophe. This can be attained by designing adequate hear removal systems or formulating appropriate operational conditions to avoid such conditions from occurring in the reaction i.e. prevention being better than cure. 

The evaluation of any hazard that may occur in a chemical reaction requires adequate knowledge of chemistry and thermodynamics. The amount of reactant, products or intermediate getting decomposed and rate and quantity of gases and heat released during a runaway reaction need to be calculated.

The reaction rate is directly proportional to the volume of a reaction mixture. The rate at which heat is removed decreases with a decrease in the surface area. Thus, an increase in the volume to surface area ratio results in an initial the reaction rate and subsequently increases the difficulty in removing heat from the system. This point is vital when scaling the reaction system.

A full scale test of a runaway reaction requires large sums of money but the safety risks are too high, this means that calculations are done through research from literature, theoretical calculations as well as simulations. Small scale calorimetric studies can be done under controlled conditions to estimate sizes of a full operation. 

A risk assessment involves defining a process and its operating conditions, identifying the risks and assessing the necessary methods needed to deal with the risks. Choosing the best safety and protective measures throughout the operation of the plant is also vital. Safety measures are vital and needed from the planning step.

Safety, Protective and Control measures

The risk assessment helps decide which safety measures are needed. Process control can be used to prevent any hazard from occurring and also installing protective techniques to limit the effects of a runaway reaction.

It is important to try and reduce or eliminate the possibility of a runaway reaction occurring. This can be done by substituting the reactants with safer alternatives, if none are available then very low quantities of the reaction mixture should stay in the reactor through operating using a continuous process instead of a batch process. Semi batch process can be used if a continuous process is not possible as it is easier to control the heat liberated in an exothermic reaction through control of reactant amounts.

Process control procedures should be installed in order to prevent the runaway from occurring by using proper and reliable control systems, actuators, sensors and automatic systems to take actions when they predict the occurrence of such a hazard.

Protective measures are to be used at the time the hazard is occurring. Plant should be designed to contain the pressure generated during the runaway, emergency relief valves should be there to vent the gases produced to a safer place, the reaction should be stopped by either using a reaction inhibitor, quenching by crash cooling or by directly dumping the reaction mixture in to the quenching liquid itself.