A Miner's Primer on Hydrogen as an Energy Carrier

By Drew D. Troyer, CRE, CEM

Principal Director and Intelligent Asset Management SME

Accenture Industry X

Introduction

Environmental, Social, and Governance (ESG) performance and reducing greenhouse gas (GHG) emissions are top priorities for mining executives. We simply must find ways to mine and process and significantly reduce our GHG impact. Previously, I’ve written extensively on the topic of reducing GHG impacts through easy-to-implement measures to improve energy efficiency. Hydrogen, the most abundant substance in the universe, is getting a lot of attention as an option for green energy transformation. In this primer, I hope to bring miners, and other managers of physical asset-intensive operations, up to speed on some basics of hydrogen production and as an energy carrier, the advantages of hydrogen and some very real challenges associated with hydrogen as a fuel option. I’ll also address some future directions for hydrogen based on cutting-edge research and technology development.

The Hydrogen Rainbow

You’ve probably heard that hydrogen comes in different colors. Sometimes, this is referred to as the hydrogen rainbow (Table 1). It serves as a simple way to describe how hydrogen is produced. White hydrogen is naturally occurring, which is very rare. Black and brown hydrogen is produced by coal gasification and grey carbon is produced by steam reforming of natural gas. Most of the hydrogen produced today is black, brown, and grey, with about 70% of the total being grey. These methods offer no benefit to reducing GHG emissions. Blue hydrogen is also produced using natural gas and employs carbon capture & underground storage (CCUS), which is itself controversial, to reduce GHG emissions. Turquoise hydrogen is produced by the pyrolysis of natural gas. Unlike grey hydrogen, which produces CO2 as a by-product, carbon black is the by-product of pyrolytically produced hydrogen from natural gas.

Table 1 - The "rainbow" of hydrogen production.

Yellow, pink, and green hydrogen represent a major deviation. They all utilize water as the feedstock for hydrogen production and catalyst-assisted electrolysis to “crack” the H2O into H2 and free oxygen. They differ in terms of the energy source used to power the electrolysis. Yellow hydrogen utilizes grid electricity, so the GHG impact is a function of the fuel sources used to generate the electricity. Pink hydrogen is produced utilizing nuclear power. It results in no significant GHG emissions but does present some other environmental challenges – most notably the disposal of radioactive nuclear fission byproducts. Green hydrogen is the holy grail for hydrogen production because it employs solar, wind and other renewable energy sources to power electrolysis. These sources do have their own impact, namely land use, wildlife distribution, but from an overall environmental perspective, green hydrogen is our best option.

Water, electricity, and a catalyst are employed to convert water to hydrogen. It requires 9 liters (9 kg) of water and about 180 MJ, or 50 kWh to produce one kg of hydrogen and eight kg of oxygen as O2. The input electrical energy is used to “crack” H2O into its constituent components of H2 and O, which quickly form O2. One kg of hydrogen has an energy density of 120 MJ or about 33 kWh. So, there is a 33% entropy loss when producing green hydrogen. There are further entropy losses associated with combusting hydrogen for conversion to mechanical energy or converting hydrogen into electrical energy via hydrogen cells. These are discussed later.

Figure 1 - General schematic describing the energy and material balance when producing green hydrogen from water.

 On the Subject of Energy Density

Energy engineers define an energy source in terms of its energy density, which represents the gravimetric or volumetric heating value of the energy source. There are numerous units for energy density, including Megajoule (MJ), kilowatt hour (kWh), and British thermal unit (Btu). These are all interchangeable using simple multipliers. For this article, I’ll discuss energy density in terms of MJ per kilogram (MJ/kg) for gravimetric density, and MJ per litre (MJ/l) for volumetric density. The heating value of an energy carrier is often defined as gross, or high heating value (HHV) or net, or low heating value (LHV). Net energy value, or LHV is simply the gross value minus the latent heat in the water produced, as a byproduct of combustion. For simplicity in this article, I’m only using net or LHV values.

Referring to Table 1, you see that hydrogen has a very high gravimetric energy density. It’s nearly three times the density for liquid hydrocarbon-based energy carriers like diesel fuel. That looks great, but gravimetric energy density is only a part of the story.

Table 2 - The energy density comparison of hydrogen to various liquid energy sources.

For mobile applications like haul trucks, trains, light vehicles, aircraft, etc., volumetric energy density is critical because the vehicle must carry all its fuel on board with the vehicle. As a point of reference, diesel fuel contains a volumetric energy density of nearly 39 MJ/l. Hydrogen is at a disadvantage when it comes to volumetric energy density because of its very low molecular weight. Even in a liquid form, at about -255°C, hydrogen offers only 8.5 MJ/l. So, you’d need about four times the fuel storage to utilize liquid hydrogen as compared to liquid hydrocarbon energy carriers. Liquid hydrogen isn’t really a viable solution for industrial use because it must be cryogenically stored.  More practically, hydrogen is stored under high pressures in the range of 350 bar (H35) to 700 bar (H70). With volumetric energy densities of 3.1 MJ/l and 5 MJ/l for H35 and H70 respectively, storage is even more challenging than it is for liquid hydrogen. In other words, compared to diesel fuel you’d need 12.5 liters of H35 and about eight liters of H70 to equal the energy density of one liter of diesel fuel.

Ammonia (NH3) offers a mechanism by which to transport hydrogen more conveniently. While Ammonia is a gas at atmospheric pressure and temperature, it can be liquified at much lower pressure and much higher temperature than hydrogen. At 11.5 MJ/l, ammonia’s energy density is higher than hydrogen, including liquid hydrogen at 8.5 MJ/l, its energy density is less than one-third of that of diesel. There are toxicity challenges associated with ammonia, it can be very corrosive, and when combusted, produces NOx emissions. So, ammonia offers advantages compared to H35 or H70 in terms of required storage for mobile applications, but it’s still well short of diesel or other liquid hydrocarbon-based fuels and its use introduces several serious challenges. Ammonia can be stored, shipped, combusted and converted back to hydrogen for fuel cells, so it’s likely to continue to be a part of the discussion about transforming to a hydrogen-based economy. In fact, there’s progress to develop fuel cells that convert ammonia to electricity in fuel cells without first converting the ammonia to hydrogen. Ammonia is an energy carrier to watch, but more research and development are required to increase the viability of ammonia as an energy carrier.

Hydrogen Combustion vs. Hydrogen Cells

Hydrogen as a fuel can either be combusted in much the same way diesel fuel, petrol, or jet-a is combusted to produce expansive heating to drive the reciprocation of pistons or the rotation of a turbine, which produces mechanical energy. For example, a reciprocating hydrogen engine produces rotation at the shaft that is transferred to the transmission, differential and the wheels to enable mobility (Figure 2). Unfortunately, hydrogen combustion is only about 25% efficient at the engine (~20% at the wheels). This compares to 40-50% for properly tuned and maintained diesel engines. The poor combustion efficiency of hydrogen exacerbates the volumetric density issues discussed in the previous section. It’s not a very efficient way to accomplish work with heavy mobile equipment (HME) and other mobile machines. Moreover, combusting hydrogen with air is not GHG emissions-free. Any time a fuel is combusted with air, which is 80% nitrogen, nitrous oxides (NOx) are produced as a byproduct. NOx is a very potent GHG. A single metric ton of NOx has the same global warming potential as 298 metric tons of carbon dioxide (CO2). Moreover, NOx emissions produce acid rain that contributes to ocean acidification and produces respiratory and other adverse health effects. In sum, for mobile applications, hydrogen combustion offers significant reductions in GHG emission because no CO2 is produced but because of storage issues, poor combustion efficiency and NOx production, it is not a great overall choice.

Figure 2 - Simple example of a hydrogen combustion-driven vehicle and its associated efficiencies.

 Hydrogen cells, on the other hand, convert hydrogen into electricity. In a hydrogen cell, the hydrogen reacts with oxygen in the presence of a catalyst, which is typically a rough and porous material. Platinum and platinum group metals (PGM) are the most common hydrogen cell catalysts, but research into different catalysts is ongoing and beyond the scope of this article. Fuel cells produce only electricity, water, and a small amount of heat. They are 100% GHG-free. Fuel cells produce DC electricity that can be used to drive DC motors but are more commonly inverted to AC electricity with frequency control to power variable frequency/speed motors (VFD/VSD). The mechanical arrangement can vary. A single motor can be run through a transmission and differential, or individual wheel motors can be utilized. The real benefit of hydrogen cells as compared to direct combustion of hydrogen. Hydrogen cells are about 60% efficient and about 50% efficient at the wheels, depending upon the inverter, control system efficiency and the mechanical drive configuration (Figure 3).

Figure 3 - A simple example of a hydrogen cell-powered vehicle and its associated efficiencies.

 Hydrogen Storage Challenges

Because of its low molecular density, storing hydrogen can be a real challenge. This is especially true for mobile equipment applications. As previously discussed, hydrogen has a high gravimetric energy density of about 120 MJ/kg. However, at atmospheric pressure, it takes about 12,000 litres of hydrogen to add up to one kg (l/kg). Pressure, however, increases the volumetric density of hydrogen (Figure 4). At 700 bar (H70) of pressure, it drops to about 25 l/kg. In its most dense form, liquid hydrogen at about -255°C, requires 14 l/kg. Compare these figures to diesel at about 1.2 l/kg for diesel fuel. Even with hydrogen’s high gravimetric energy density and the increased efficiency of hydrogen cells compared to diesel-fueled internal combustion engines, you’d require at least four times the storage space for liquid hydrogen fuel versus diesel. You’d require about eight times the space for H70 hydrogen. This creates some serious fuel storage and/or refueling frequency challenges.

Figure 4 - Hydrogen - mass (kg/l) versus pressure (bar).

 Storage of hydrogen, an extremely flammable gas at a pressure of 350 (H35) or 700 bar (H70), presents some very real engineering and safety challenges. Safe storage of hydrogen – especially on vehicles – will continue to receive a great deal of attention and scrutiny. The codes for safe storage are apt to be very dynamic for the foreseeable future.

There are other challenges associated with storing hydrogen. Ferritic steel pressure vessels are susceptible to hydrogen-induced mechanisms. While hydrogen is typically found in its molecular form of H2, under conditions of excitation, the hydrogen can temporarily convert to its atomic, or nascent, form H+. In this form, hydrogen is very reactive and can react with the steel to form metal hydrides that embrittle the steel, making it less ductile and more susceptible to fracturing. Likewise, nascent hydrogen can invade the intergranular regions of the steel’s unit cells. If an H+ encounters another H+ atom, they’ll combine to form molecular Hydrogen – H2. The atomic H+ can enter the steel’s metallurgical unit cells, but the molecular H2 can’t exit. Therefore, intergranular pressure builds up causing blistering which also weakens the pressure vessel’s structural integrity.

Alternative materials such as carbon fiber are being explored to reduce the concerns associated with the storge of hydrogen. Carbon fiber offers the benefits of enhanced strength, light weight and it’s impervious to hydrogen-induced degradation mechanisms. Research and development into ideal methods for storing hydrogen will continue.

Green Hydrogen as a Building Block for Green Hydrocarbon Fuel

As previously discussed, hydrogen’s low volumetric energy density presents a challenge for mobile equipment applications because it requires so much space for energy storage. Also, the fuel infrastructure isn’t presently set up and geared towards hydrogen fuel distribution. However, there is another option. If carbon dioxide can be captured at industrial facilities (e.g., flue gas from a power plant) or from the air with direct carbon capture (DCC), it can be combined with green hydrogen and converted into methane (CH4), synthetic diesel, synthetic jet-A, and synthetic petrol (SynFuel) utilising the Sabatier method, the Fischer-Tropsch method, or some combination of the two. Originally developed in Germany in the 1920s to produce liquid fuels from coal, the process can be utilised to convert captured CO2 into carbon monoxide (CO) or SynGas that is hydrogenated with hydrogen and polymerised to form various hydrocarbons. The size and type of hydrocarbons produced depends on what finished product is desired (e.g., SynDiesel or SynJet-A) utilising the gas-to-liquids (GTL) process (Figure 5).

Figure 5 - Summary overview of carbon capture and conversion to SynFuel process.

Synthetic fuels can offer several advantages to conventional fuels that are refined from crude oil utilizing fractional distillation and further processing in refining. Synthetic fuels can offer higher energy density than conventional fuel, which increases the amount of work that can be accomplished for a given volume of fuel. Also, synthetic fuels typically have a higher cetane number (CN). The CN represents the combustion speed of diesel fuel. Optimizing the CN of fuel enables the engine to achieve idealized combustion that approaches stoichiometric combustion. This can significantly reduce NOx and particle matter (PM) emissions (e.g., soot), which are caused by incomplete combustion. The CN is highly dependent upon the distribution of hydrocarbons in diesel fuel. Paraffinic hydrocarbons tend to have consistently high cetane numbers. Aromatic and isoparaffinic hydrocarbons tend to have low and highly variable CN values. Naphthenic hydrocarbons tend to have consistently low CN values (Table 3). The SynFuel manufacturing process offers a much higher degree of control over and consistency of distribution of hydrocarbons than does the process of producing from crude oil utilizing fractional distillation.

Table 3 - Typical cetane numbers for various hydrocarbons found in diesel fuel.

The GHG impact of combusting SynFuel varies depending upon the input materials. SynFuel can significantly reduce GHG emissions, especially CO2, if the inputs themselves are green. The two extreme ends of the SynFuel GHG spectrum are described below. There are numerous scenarios in between:

1. If the CO2 is captured directly from the air or is captured from burning wood chips or other biomatter, the hydrogen is green, turquoise, pink or white, and the energy to run the Fischer-Tropsch plant is green, the resultant SynFuel is carbon neutral and GHG neutral except for the NOx emissions.
2. If the carbon is captured from burning hydrocarbons, the hydrogen is black, brown, or grey and the energy utilized for the Fischer-Tropsch process to produce SynFuel is generated utilizing hydrocarbons, there is little or no reduction in GHG emissions. In fact, the process to energize the Fischer-Tropsch method may make the resultant fuel less environmentally friendly than conventional fuels produced by fractional distillation of crude oil.

Creating liquid SynFuels from captured CO2 and green hydrogen offers some compelling advantages. It can be distributed and combusted in various types for mobile equipment without any infrastructure changes or changes to the machine assets themselves. It also offers the opportunity to significantly reduce GHG emissions – especially under the following conditions:

• The CO2 is captured directly from the air, or from sources where woodchips and other biomatter have combusted.  
• The hydrogen employed for hydrogenation is green, turquoise, or pink.
• Renewable or zero-carbon energy is employed to operate the Fischer-Tropsch production plant.

More research and development are required, but in my opinion, it’s an option that should be aggressively explored.

Future Directions: Other Hydrogen Topics to Explore

Discussing every aspect of hydrogen as an energy carrier is beyond the scope and goal of this primer. There are many other topics regarding hydrogen as and energy carrier that I'll be watching closely. It's not a comprehensive list, but here's a short list of topics that have caught my attention and that I'm looking into more closely.

• Hydrogen from Air – making hydrogen from water through electrolysis is a very water-intensive activity. Molecularly, hydrogen makes up only about 11% of the total mass of an H2O molecule. So, to create a metric ton of hydrogen, one would require more than nine metric tons of water (> 9000 liters). Larg- scale hydrogen production via water electrolysis could create some serious competition for freshwater that’s required for drinking, other domestic uses, agriculture and industry. Researchers at the University of Melbourne in Australia have developed an innovative approach for producing hydrogen from H2O that is dissolved and airborne. If proven at an industrial level, it would solve the problem of competition with other demands for freshwater. Additionally, the water extracted from the air is pure H2O, which is free from electrolytes and contaminants that can upset the hydrogen electrolysis process.
• Hydrogen from Seawater – Seawater offers an unlimited supply of water. Unfortunately, it contains a high concentration of sodium chloride (NaCl), which interferes with the electrolysis process for converting the water to hydrogen when using conventional technology. However, researchers at the University of Adelaide in Australia have recently developed technology that utilises a non-precious cobalt-based catalyst that purportedly does not require desalination, filtration, osmosis or other forms of pre-purification of the seawater. If effective, this technology coupled with green input energy could provide a virtually unlimited supply of green hydrogen.
• Supplemental Hydrogen Injection – While somewhat less futuristic than vehicles that are powered by hydrogen and are the sole energy carrier, direct injection of hydrogen into the combustion chambers of engines’ conventional liquid hydrocarbon fuels, such as diesel, offers some measurable benefits. When directly injected in the combustion chamber of an engine containing compressed air and nebulised diesel fuel, hydrogen increases combustion efficiency and the completeness of combustion. Cleaner and more complete combustion increases energy efficiency and decreases the concentration of soot and other byproducts of incomplete combustion that exit the tailpipe as emissions and/or blowby past the rings to contaminate the lubricating oil. Tests also suggest that these technologies can substantially reduce CO2 emissions.

Conclusions

Pressure for mining & resource, process industry, and manufacturing companies to deliver ESG performance continues to mount and shows no signs of waning. Reducing GHG emissions lies front and center. Hydrogen as a GHG-free (or near-GHG-free) energy carrier option is getting a lot of attention. In this primer, I’ve addressed the hydrogen rainbow, some advantages and disadvantages of hydrogen as an energy carrier, several different embodiments of hydrogen as an energy carrier, including the production of green synthetic diesel and other liquid hydrocarbon fuels and some future directions for employing hydrogen. Hydrogen fuel is a very dynamic field. Research and technology development breakthroughs occur almost daily. I’ll continue to watch these developments closely and will attempt to keep readers up to date and informed.

Drew D. Troyer, CRE, CEM is a Principal Director and Intelligent Asset Management SME with Accenture Industry X. Drew has more than 30-years of experience in the asset-intensive resource, process, and manufacturing industries. His blue-chip client list reads like a Who’s Who list of international industry. A noted thought leader, Drew has published more than 350 books, chapters and articles on various aspects of physical asset management, energy management and sustainable manufacturing. He is a Certified Reliability Engineer (CRE), a Certified Energy Manager (CEM), and he holds master’s degrees in business administration (Oklahoma City University) and environmental sustainability (Harvard University), both in the USA.