Production Technologies for Hydrogen: History
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Rishabh Agarwal

Hydrogen is a frontrunner in the race to net-zero carbon because it can be produced using a diversity of feedstocks, has versatile use cases, and can help ensure energy security.

  • hydrogen
  • green hydrogen
  • low-carbon hydrogen

1. Overview of Production Technologies

A key benefit of hydrogen is that it can be produced using a large number of inputs that include both renewable sources and fossil fuels. Until recently, hydrogen was produced using the most economic process (steam methane reforming) with little or no consideration of the effect it had on the environment. As alternative and environment-friendly technologies are developed, it is important that these be sufficiently efficient and economical to spur mass industrial adoption [1]. Table 1 provides an overview of the major hydrogen-production pathways.
Table 1. Overview of hydrogen-production pathways.
Production Pathways Energy Sources
Electrolysis  
  Electrolysis of water Solar, wind, nuclear, microbial
  Photolytic splitting of water Solar
Biological  
  Fermentation Biomass
Thermochemical  
  Thermal splitting of water Solar
  Gasification Biomass, oil, coal
  Pyrolysis Biomass, natural gas
  Steam reforming Natural gas
  Plasma reforming Natural gas
  Partial oxidation Natural gas, oil, coal

2. Fossil-Fuel-Based Hydrogen Production

2.1. Steam Methane Reforming (SMR)

The current most widely used process for hydrogen production can convert natural gas to hydrogen through a two-stage process. In the first stage, desulfurized raw hydrocarbons are mixed with high-temperature steam (700–1000 °C) to produce syngas, a mixture of H2 and CO. Then, the carbon monoxide reacts with steam to produce carbon dioxide and hydrogen [2]. This is also called a water–gas shift reaction. Finally, carbon dioxide and other impurities are removed from the gas stream, leaving behind pure hydrogen. The process typically uses nickel as a catalyst. The reactions involved are given below [3].
Steam-methane-reforming reaction:
CH4 + H2O (+ heat) → CO + 3H2
Water–gas shift reaction:
CO + H2O → CO2 + H2 (+ small amount of heat)
The steam reforming process can also be used to produce hydrogen from other lighter hydrocarbons such as ethane and propane or oxygenated hydrocarbons such as methanol or ethanol. While the process helps to achieve a relatively high heat efficiency, it produces significant carbon emissions.

2.2. Partial Oxidation (POX)

In partial oxidation, natural gas is made to react with a limited amount of air at temperatures ranging from 1300 to 1500 °C and pressures ranging from 3 to 8 MPa [1]. The partial oxidation converts natural gas to a mixture of primarily carbon monoxide, hydrogen, and nitrogen. The carbon monoxide further undergoes the water–gas shift reaction in which it reacts with steam to produce hydrogen and carbon dioxide. The partial oxidation reaction is given below [3].
Partial oxidation of methane reaction:
CH4 + ½O2 → CO + 2H2 (+ heat)
The process is exothermic; i.e., it gives off heat. Further, it is much faster than steam reforming and requires a smaller reactor vessel. Additionally, the process can handle sulfur in the feedstock because it does not require catalysts. However, catalysts can be used to lower the reaction temperature and make thermal management easier if the feedstock has a low sulfur content.

2.3. Autothermal Reforming (ATR)

Autothermal reforming combines the steam reforming and partial oxidation processes. Steam reforming generates high hydrogen and low carbon monoxide yields but is exothermic and requires external energy. On the other hand, partial oxidation produces less hydrogen and more carbon monoxide but is endothermic. The hydrogen-to-carbon-monoxide ratio can thus be varied based on the process requirements. The generated carbon monoxide then undergoes a water–gas shift reaction in the presence of steam to produce hydrogen and carbon dioxide. The process can be shut down and started very rapidly compared to steam reforming [1] and achieves a higher efficiency compared to SMR and POX [4].

2.4. Coal Gasification

Coal is heated in a pyrolysis process that vaporizes the volatile component of the feedstock. Then, a sub-stoichiometric amount of oxygen is added to the combustion chamber so the char undergoes gasification at high temperatures, which produces syngas composed of hydrogen and carbon monoxide along with solid residues [4]. Next, the carbon monoxide undergoes the water–gas shift reaction in which it is converted to carbon dioxide and hydrogen. Finally, the gas stream is purified to remove the carbon dioxide and other impurities to produce pure hydrogen. The process can also be adjusted to use biomass or other hydrocarbons that are abundantly available.

3. Renewable Processes for Hydrogen Production

3.1. Electrolysis

In electrolysis, a direct current is passed through two electrodes, which results in the breaking of chemical bonds present in water molecules into hydrogen and oxygen. The electrodes are immersed in an electrolyte and separated by a membrane through which ions can move. Hydrogen ions move toward the cathode to form hydrogen gas. The reactions involved are [1]:
Cathode: 2H2O (l) + 2e− → H2 (g) + 2OH− (aq)
Anode: 4OH− (aq) → O2 (g) + 2 H2O (l) + 4e−
Overall: 2H2O → 2H2 + O2
While the process is ecologically clean because no greenhouse gases are formed, it is extremely energy-intensive. Electrolysis requires 50–60 kWh of electricity to produce 1 kg of hydrogen and 8 kg of oxygen. The three types of electrolysis technologies that are used most commonly are described in Table 2.
Table 2. Electrolysis production technologies [4][5].
Alkaline Electrolysis Proton-Exchange Membrane (PEM) Solid Oxide Electrolysis Cell (SOEC)
Description Alkaline technology is used extensively in the chlorine industry; a strong base such as potassium hydroxide is generally used as the electrolyte due to its high conductivity. PEM uses a ionically conductive solid polymer; hydrogen ions travel through the polymer membrane toward the cathode. PEM has a very short reponse time of less than 2 s. SOEC is based on steam water electrolysis at high temperatures, thereby reducing need for electrical power. Heat is only needed to vaporize water and can be obtained from waste industrial heat.
Capital Costs(stack-only, >1 MW, USD/kWe) 270; <100 expected 400; <100 expected >2000; <200 expected
Efficiency(%, LHV) 52–69% 60–77% 74–81% excluding heat to vaporize water)
Typical Plant Size(tpd H2) 60; 100 expected 50–80; 100–120 expected <20; 80 expected
Stack Lifetime(in thousands of hours) 60; 100 expected 50–80; 100–120 expected <20; 80 expected
Operating Temperature (°C) 60–80 50–80 650–1000
Operating Pressure (bar) 1–30 20–50 1
Expected R&D Improvements Scaling benefits to reduce costs; improvement in lifetime; improved heat exchangers. Scaling benefits to reduce costs; improvement in material and component lifetimes. Improvement in component lifetime by improving the resistance to high temperatures and improving the response to fluctuating energy inputs.
Pros and Cons Most mature technology; has the lowest capital cost but also the lowest efficiency. Highly efficient but more expensive than alakaline electrolysis. High future potential but still in the developmental stage.
Electrolysis requires fresh distilled water as a feedstock to produce hydrogen. From a stoichiometric perspective, 1 kg of hydrogen requires 9 kg of water as input, but the actual consumption could be between 18 and 24 kg due to process inefficiencies and requirements for water demineralization [5]. The IEA estimates that around 61.7 Mt of H2 will be produced using electrolysis by 2030 [6]. This implies a total water demand of 1.1–1.5 billion m3, which is less than 0.05% of the current global freshwater consumption [7]. Additionally, in places with water stress, electrolysis can be integrated with salt water desalination at minimal additional costs of USD 0.01–0.02/kg H2 [8]. Thus, direct water use during electrolysis is not expected to be a barrier to scaling up electrolysis.

3.2. Microbial Processes

In addition to conventional chemical and electrical processes, biological approaches could also help to increase the production of low-carbon hydrogen [4]. The first approach, called dark fermentation, uses anaerobic bacteria and microalgae in a dark tank to convert biomass and water into hydrogen and carbon dioxide. The operating temperature is typically maintained between 25 and 40 °C to prevent thermal inactivation of the microbes. The second approach, called microbial fermentation, combines electrical energy with microorganism activation to produce hydrogen with low energy inputs. Bacteria are attached to the anode and fed acetic acid to release hydrogen ions. This reduces the amount of electrical energy required from renewable sources and leads to low-carbon generation of hydrogen. However, both of these technologies are currently at the pilot/lab scale and need to be further developed prior to their applications in industry.

4. Cost of Production

A key challenge in producing hydrogen, especially from renewable resources, is offering hydrogen at economical price points. From the standpoint of fuel cells used in transportation, hydrogen needs to be provided at a price that is competitive with prevalent vehicle fuels and conventional technologies when compared on a per-mile basis. This translates to a cost of hydrogen lower than USD 4.7/gallon of gasoline equivalent [9] irrespective of the technology used for hydrogen production. To reduce the levelized cost of hydrogen (LCOH), research has focused on improving hydrogen production efficiency and reducing the costs involved in operations, capital equipment, and maintenance [10].
The key cost factors in producing blue hydrogen include the underlying capital expenditure and the cost of the fossil fuel. For green hydrogen, the key cost components are the electrolyzers and the renewable energy. While these factors vary significantly from region to region, Table 3 below provides the high and low ranges of the global average LCOHs based on different production sources. It is expected that by 2050, increasing CO2 prices will disincentivize grey hydrogen production. Further, green hydrogen is expected to achieve cost parity with blue hydrogen as the price of electrolyzers and renewable energy comes down significantly while that of natural gas increases.
Table 3. Global average LCOH by production source in 2019 and 2050 [11].
Production Source LCOH-Low (USD/kg H2) LCOH-High (USD/kg H2) CAPEX (USD/kWe) OPEX
(% of CAPEX)		 Efficiency Capacity Factor Fuel Price
2019              
Natural gas 0.7 1.6 910 4.7% 76% 95% USD 1.5–6.6/MMbtu
Natural gas with carbon capture and sequestration (CCS) (95% capture) 1.2 2.1 1580 3.0% 69% 95%
Coal gasification 1.9 2.5 2670 5.0% 60% 95% USD 50–250/ton
Coal gasification with CCS
(90% capture)		 2.1 2.6 2780 5.0% 58% 95%
Electrolysis with dedicated renewables supply 3.2 7.7 870 2.2% 64% 35% USD 35/kWh
2050              
Natural gas with CCS 1.2 2.1 1280 3.0% 69% 95% USD 1.8–7.4/MMBtu
Coal with CCS 2.2 2.5 2780 5.0% 58% 95% USD 30–65/ton
Electrolysis with dedicated supply of renewables 1.3 3.3 270 1.5% 74% 45% USD 20/kWh

5. Current Challenges for Green Hydrogen

Green hydrogen refers to the hydrogen produced by splitting water via electrolysis by employing an electric current in the hydrogen and oxygen in an electrolyzer using renewable electricity [12]. Green hydrogen and e-fuels (synthetic fuels produced from hydrogen and carbon dioxide) have much lower carbon emission levels compared to grey hydrogen and fossil fuels. However, to be truly carbon-free, the electricity used in this process must be renewable. The production of hydrogen and e-fuels by using a regular power grid as the source defeats the purpose because, depending on the location, a significant portion of the electricity is generated from coal, oil, and natural gas, which does not benefit the environment [13].
Developing a fully “green” hydrogen facility often requires a dedicated wind farm or solar plant to provide reliable zero-carbon electricity. This limits the capacity factor of the hydrogen facility to that of the underlying renewable resource, which leads to significant additional capital expenditures for electrolyzers and compressed hydrogen storage. Furthermore, the relatively nascent state of the green hydrogen market leads to many other challenges as described below. It must be noted that these challenges provide potential business development opportunities for both established and emerging companies operating in the green hydrogen space.
  • Limited knowledge of optimum design, thus limiting profitability and stability: Fulfilling market demand will make it necessary for organizations to augment and enhance the designs of their plants for green hydrogen generation. However, optimizing plant designs and end-to-end green hydrogen systems can be a complicated affair and extremely expensive due to the dearth of market data. When green hydrogen generation plants are built within existing industrial clusters, designing and scaling up become even more complicated because care has to be taken to minimize any adverse commercial impact of the transition to green hydrogen on existing operations during the transition phase [14].
  • Elevated operational costs and inadequacy of dedicated workforce: The hydrogen economy will create many new employment opportunities, but a slow rate of technical learning and lack of necessary skill sets has led to the inadequacy of the specialized labor required to support the hydrogen economy. This will be a significant impediment to the development and maturity of the industry. In addition, storing and transporting a highly inflammable and explosive gas such as hydrogen requires substantial investments in specialized pipelines and carriers. Astronomical expenses and uncertainties accompanying the infrastructure adaptation and transfiguration for generation, distribution, and storage systems are among the key issues [15].
  • Significant energy losses: Green hydrogen loses a substantial amount of energy throughout the supply chain. About 30–35% of the energy is lost during hydrogen production through the electrolysis process. Additionally, liquifying hydrogen or converting it to carriers such as ammonia causes a 13–25% energy loss. Furthermore, transporting hydrogen incurs another 10–12% loss [16]. Lastly, the application of hydrogen in fuel cells will give rise to an additional 40–50% energy loss. Unless these inefficiencies are addressed and improved, a substantial volume of renewable energy will be required to feed green hydrogen electrolyzers that are capable of competing with end-use electrification [14].
  • Green hydrogen procurers and value: Monetizing green hydrogen is a crucial challenge due to the exigence of storage and distribution. Green hydrogen can be produced economically in places that receive copious amounts of sunlight, such as Spain, Portugal, Australia, and Tunisia, but the industrial procurers are usually not located in close proximity. This makes it essential to install a dedicated transportation infrastructure, thereby increasing costs and lead times. Moreover, green hydrogen valuation presupposes “Guarantee of Origin” certification and carbon credit convertibility. Both of these schemes are still in the developmental stage and are constantly subjected to intense debates [17].

6. Green vs. Blue Hydrogen

There are various propositions in favor of “blue hydrogen”; i.e., employing carbon capture and storage to reduce GHG emissions from fossil-fuel-based production of hydrogen, which is often endorsed as a low carbon emitter [18]. The process typically employs steam methane reforming or auto thermal reforming and captures carbon dioxide emissions, which are either used in other chemical processes or stored in underground reservoirs. However, production of the natural gas feedstock invariably involves some methane emission, which could add to significant GHG emissions [19]. Most studies overlook this fugitive methane emission [20].
On the other hand, green hydrogen is experiencing a global resurgence as an alternative fuel generated using clean energy, which will help bring the world into a net-zero emission regime [21]. Green hydrogen currently makes up a very meager percentage of global hydrogen production [22]. Nevertheless, it can act as a formidable tool for resolving the problems of the intermittency of renewable energy sources and the decarbonization of heavy industry. Augmenting the production of green hydrogen presents significant challenges. However, contemporary state-of-the-art technology can provide solutions to a certain extent.
At present, the cost of generation of green hydrogen is relatively high (Table 3). Even though carbon capture and storage of blue hydrogen is an expensive proposition, nevertheless, the process is able to generate low-carbon fuel at a much lower cost in comparison to green hydrogen. The scientific community is of the opinion that only green hydrogen generated using renewable energy can be considered truly clean, while blue hydrogen is only a stopgap measure until green hydrogen production is scaled up sufficiently. However, blue hydrogen remains relevant in the transition because it is synonymous with high volumes, an affordable cost, and thus competitiveness, which is particularly pertinent for countries with a large energy export market such as Norway [23]. The IEA estimated that nearly 18% of hydrogen production in 2030 will be “blue”, while 34% will use “green” sources.

This entry is adapted from the peer-reviewed paper 10.3390/su142315975

References

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