Commercial hydrogen production facilities have been around since the early 1900’s. Up until recently, hydrogen produced from these facilities was primarily used for industrial processes such as oil refining, ammonia production and methanol production. Now, hydrogen is finding new uses such as a transportation fuel and as a means of energy storage. To meet the growing demand, a significant number of new hydrogen production facilities are in various stages of planning and construction. Because of the explosive nature of hydrogen, one of the keys for the successful design, construction and operations of a hydrogen production facility is getting the ventilation system right. In this article, I’ll cover some of the design challenges for ventilating hydrogen production facilities.
Types of Hydrogen Production
There are many ways to generate hydrogen. These are currently the most common methods:
- Electrolysis of Water. Electrolysis is a process in which an electric current is used to split water into hydrogen and oxygen. This method is emission-free if the electricity used is from
renewable sources (Green Hydrogen). There are different types of electrolysis including alkaline electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis, each with its own advantages and disadvantages. - Steam Methane Reforming (SMR). Steam Methane Reforming is currently the most common method of producing hydrogen on a large scale. In this process, natural gas, which is primarily composed of methane, is reacted with steam at high temperatures (around 700-1000°C) in the presence of a catalyst to produce hydrogen and carbon monoxide. Further reactions can convert the carbon monoxide to carbon dioxide and additional hydrogen.
- Partial Oxidation of Hydrocarbons. In this process, hydrocarbons such as oil or gas are partially oxidized by reacting them with a controlled amount of oxygen, typically under high pressures and temperatures. This produces a synthesis gas (syngas), which is a mixture of carbon monoxide and hydrogen. The syngas can then undergo further processing to extract hydrogen.
- Coal Gasification. Coal can also be used to produce hydrogen through a process called gasification. In this process, coal is reacted with oxygen and steam under high pressures and temperatures to form syngas, which is then processed to extract hydrogen and carbon dioxide.
- Biological Methods. Biological processes such as fermentation and microbial electrolysis can be used to produce hydrogen. Certain microorganisms produce hydrogen as a byproduct when breaking down organic materials. This process is currently not as efficient or scalable as other methods but is an area of active research.
- Photoelectrochemical (PEC) Water Splitting. Photoelectrochemical cells use semiconductor materials to absorb sunlight and generate the voltage needed for water electrolysis. This method is currently in the research and development phase but has the potential to produce hydrogen using only sunlight and water.
- Plasma Reforming. This relatively new method involves the use of electric plasma to reform hydrocarbon fuels into hydrogen. This process can occur at lower temperatures than conventional steam reforming and has been suggested as a way to produce hydrogen with lower greenhouse gas emissions.
Risks of Hydrogen Production
Regardless of the method used to produce hydrogen, the process is inherently risky because of the characteristics that make hydrogen a dangerous gas:
- Wide Flammability Range. Hydrogen has a wide flammability range when mixed with air. The concentration of hydrogen in air can be as low as 4% and as high as 75% by volume, and still be ignitable. This makes hydrogen more dangerous compared to many other fuels, as even a small leak can create an ignitable mixture in a large volume of air.
- Low Ignition Energy. Hydrogen required very little ignition energy. That means hydrogen can be ignited by very weak sparks, small amounts of static electricity, or even some electromagnetic radiation. It means that in the presence of an air-hydrogen mixture within the flammable range, the risk of accidental ignition is very high.
- Buoyancy and Diffusion. Hydrogen is 14 times lighter than air and thus tends to rise and diffuse rapidly. While this can sometimes be a safety advantage, as leaks tend not to accumulate at ground level, it can also be a disadvantage. Hydrogen can accumulate at high points, such as roof peaks, leading to potentially explosive mixtures in unexpected areas.
- Small Molecular Size. Hydrogen has the smallest atoms of any element, which allows it to diffuse through materials much more easily than other gases. This makes hydrogen capable of escaping through very small openings, micro-cracks, or even the molecular matrix of certain materials.
- Hydrogen Embrittlement: Hydrogen can cause certain materials to become brittle. This can lead to the development of micro-cracks in storage containers and pipelines. These micro-cracks can then provide pathways for hydrogen to escape.
Design Scenarios
The design of a ventilation system for any type of hydrogen production facility is extremely challenging for one reason: Hydrogen is very difficult to contain. Therefore, it is not a matter of if a leak while happen but when. That means that the ventilation system for a hydrogen production facility will need to have two primary operating modes: normal and emergency mode.
Air flow in the normal operating mode should remove internal and solar heat loads on the facility. The size of the space, maximum ambient air temperature, amount of insulation, process heat generation and allowable temperature rise (delta T) are factors that will determine the number of air changes per hour that are needed. An additional volume of air may be needed in normal operating mode depending on the sensitivity of the hydrogen leak detectors. The presumption is that hydrogen gas is always present but not at a detectible level.
In the emergency operating mode, a hydrogen leak has been detected and all hydrogen production processes are suspended. The preferred method for reducing the combustion risk for hydrogen is to diffuse the concentration below 4%. The volume of air needed to do this will be determined by two factors. The first is the size of the space that needs to ventilated in the event of a leak. The send is more difficult to determine. That would be the estimated amount of hydrogen gas that has leaked before it was detected.
Additional Design Considerations
When the normal and operating mode air volumes are determined, there are other considerations that need to be incorporated into the ventilation system design:
- Winter Heating: Hydrogen production facilities operating in colder climates need to consider heating in the wintertime. The equipment used for some of the hydrogen processes is sensitive to low temperatures. In normal operating mode, the hydrogen production process may give off sufficient heat in the wintertime. That means do additional heat is required. However, in emergency mode the process stops, and no heat is generated. The air volume moving through the space will be significantly higher in emergency mode. Therefore, the amount of heat required to keep equipment above the minimum temperature may be significant.
- Exhaust Locations: Leaked hydrogen will rise rapidly. Exhaust locations should be located at the highest points in the facility to prevent accumulation.
- No Dead Zones: Ventilation equipment should be strategically placed to prevent creating dead air zones. That will prevent hydrogen gas accumulating to combustible levels. This may mean using a higher number of smaller fans to create a more even distribution of air.
- Filtration: Hydrogen production processes are susceptible to particulate contamination. Supply air should be filtered to an acceptable fines level. Filter houses should be sized to minimize pressure drops. Also, they should have filter gauges to indicate when filters need to be changed.
- Isolation: It may be desirable to isolate certain sections of the facility in the event of leak detection. Using dampers with rated actuators should be used to isolate the space. Also, negative pressure air flow in the isolated space will prevent hydrogen disbursement into adjoining spaces.
Fluid Flow Analysis
The ventilation system is a highly critical component of a hydrogen production facility in the event of a hydrogen leak. It is important that the ventilation system design be analyzed using a fluid flow analysis. Most emergency operating modes will have powered supply and powered exhaust. A fluid flow analysis will determine where the supply and exhaust fans are operating on their fan curves. This means that the proper selection of each fan in the system can be verified. Also, fluid flow analysis is a good way to model filter loading to determine the regular cycle for filter replacement.
Conclusion
For a hydrogen generation facility, an effective ventilation system design is as important as an efficient hydrogen production process design. That is because the ventilation system needs to protect the people, equipment, and structures in the event of an emergency. When you have a critical ventilation systems design need, trust the experts at Eldridge. Our customers have been doing so for over 77 years. We know what is required to create a safe and successful environment for the operations of a highly critical process like hydrogen production.