Understanding hydrogen colors beyond green

With the potential to revolutionize sectors like transportation and power generation, hydrogen is a developing and largely untapped source for harnessing energy. While hydrogen combustion produces no greenhouse gas (GHG) emissions, its lifecycle environmental footprint is a result of all the processes leading up to its production and consumption, which varies dramatically based on energy sources used along the way.

These variances gave rise to the color-coded classification system, enabling differentiation among the various shades of hydrogen and their implications for net sustainability. Green hydrogen is the net-zero global standard, but its scalability faces considerable infrastructure and efficiency challenges. Nuclear-powered water electrolysis creates pink hydrogen and maintains a carbon-neutral lifecycle, but its dependence on nuclear energy creates other concerns. Blue and turquoise production methods toe the tradeoff line between economic viability and sustainability. Then black, brown and grey hydrogen tip the scale in favor of affordability, presenting a cost-effective alternative to eco-friendlier options.

Black and brown hydrogen are predominantly derived from coal and both are produced via coal gasification, a multistep process of reacting coal with oxygen and steam at high temperatures to produce syngas. This is a mixture of gases, one of which is hydrogen. 

Black hydrogen is created by combusting bituminous coal, which is dense and considered high-grade. Brown hydrogen, by contrast, is produced from lignite, a younger and less compacted version, with a higher moisture content and lower energy density. 

In terms of environmental impact, black and brown hydrogen production are quite similar and both methods are relatively efficient. However, their reliance on fossil fuels without carbon capture can offset some of the sustainability benefits associated with hydrogen energy.

Coal gasification commences with the pulverization and treatment of coal to remove impurities, and then the following steps.

1. Drying and pyrolysis (devolatilization)

This initial stage requires heating the coal to remove moisture and volatile matter, and to separate the coal into other elements and materials. Drying is conducted at around 200 °C (392 °F), followed by pyrolysis at temperatures between 300 °C (572 °F) and 700 °C (1,292 °F). During pyrolysis, larger coal molecules decompose into smaller gaseous products - primarily methane (CH₄), hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂) - and tar.

2. Combustion

A portion of the coal (C) is char, the solid residue from pyrolysis, and it reacts with volatile gases and oxygen (O₂) in a controlled combustion reaction. This exothermic reaction provides the heat required for the subsequent gasification reactions. Both complete and partial combustion occur, creating carbon dioxide and carbon monoxide gases.

C + O₂ → CO₂ (Complete combustion)
2C + O₂ → CO (Partial combustion)

3. Gasification reactions

Gasification reacts the remaining char with steam (H₂O) and oxygen at high temperatures of 1,200-1,500 °C (2,192-2,732 °F) in a reducing environment, generating hydrogen and other gases. The primary gasification reactions are: 

Water-gas reaction: C + H₂O ⇌ CO + H₂ (Endothermic)
Boudouard reaction: C + CO₂ ⇌ 2CO (Endothermic)   

These reactions produce syngas, a mixture primarily composed of carbon monoxide and hydrogen, along with carbon dioxide and other trace gases.

4. Methanation

In some cases, an additional step called methanation is employed to increase the methane content of the syngas. Sometimes this is referred to as E-Methane if green hydrogen and recycled CO₂ or CO₂ from Direct Air Capture (DAC) are used. This requires reacting carbon monoxide with hydrogen in the presence of a catalyst:

CO + 3H₂ ⇌ CH₄ + H₂O (Exothermic)

5. Syngas cleaning and upgrading

The raw syngas contains impurities that must be removed before further use. This cleaning process typically entails:

• Dust removal, where physical separation techniques are employed to remove particulate matter. 
• Sulfur removal, where compounds like hydrogen sulfide (H₂S) are removed through amine scrubbing or similar processes. 
• Carbon dioxide removal, where the CO₂ is captured and stored or utilized in other industrial processes.

6. Hydrogen separation and treatment

The final step is separating the hydrogen from the purified syngas mixture. This can be achieved by various methods, the two most common being:

• Pressure swing adsorption, which uses adsorbent materials to selectively capture carbon monoxide, leaving behind purified hydrogen. 
• Membrane separation, which employs specialized membranes that allow hydrogen to pass through, while retaining other gases.

Grey hydrogen is the most common type currently encountered in industry, which is produced either via steam methane reforming (SMR) or autothermal reforming (ATR). Both of these methods require a hydrocarbon feedstock consisting primarily of methane, with natural gas the most frequently used source.

Obtaining natural gas and extracting hydrogen

Natural gas is an odorless and colorless gas, predominantly found beneath the Earth's surface in proximity to petroleum deposits. Formed over millions of years from the decomposition of organic matter under intense heat and pressure, this versatile energy source is a cornerstone of modern society, used to fuel homes, power industries and produce electricity. Additionally, it is a raw material in several compounds that are eventually formed into products, such as synthetic fabrics, antifreeze, paints, packaging materials, shampoos, lotions and fertilizers.

This gas is found in porous and permeable rock formations called reservoirs, often trapped beneath layers of impermeable rock that prevent its escape. These reservoirs can be located beneath dry land onshore or offshore under the seabed. Natural gas exploration requires sophisticated geological surveys, seismic imaging and exploratory drilling to pinpoint hidden reserves. Once a potential reservoir is identified, the extraction process begins, often by a combination of technologies tailored to the specific geological conditions.

The most common extraction method entails drilling a wellbore into the reservoir, which creates a conduit for the trapped gas to flow to the surface. This flow is often driven by the natural pressure of the reservoir itself. As the gas is extracted, pressure typically declines, causing the addition of artificial lift techniques to maintain production, such as pumps or compressors.

Upon reaching the surface, the extracted natural gas - often accompanied by impurities like water vapor, sand and other gases - undergoes a series of processing steps. These steps are crucial for removing impurities, separating valuable components and preparing the gas itself for use. The processed natural gas is then transported via pipeline, or in compressed (CNG) or liquified (LNG) format by specialized tankers, to consumers all over the world. 

Grey hydrogen’s prevalence lies primarily in the global abundance of natural gas. Additionally, SMR and ATR are less carbon-intensive than coal gasification, prompting preference over black and brown hydrogen. Blue hydrogen takes SMR and ATR a step further by incorporating carbon capture, transport and storage, but this requires significant operating expense. 

While transitioning to fully renewable energy systems is the ultimate goal in the hydrogen value chain, blending hydrogen with natural gas in existing power plants presents an interim solution. Success requires high-precision flow measurement and real-time gas analysis instrumentation, ensuring a consistent gas mixture. Hydrogen can also be blended into residential and commercial natural gas supplies at concentrations up to 20% depending on the country regulations.

This reduces emissions because hydrogen burns cleaner than natural gas. While home appliances can only burn up to a 20 % blend of hydrogen with natural gas, the gas turbines used in power plants can burn a much higher blend, up to 100 % hydrogen with newer models.

Blending approaches enable a gradual transition to cleaner energy sources without requiring immediate and complete replacement of existing infrastructure, easing the need for significant capital investments in new power plants and pipelines.

The success of this strategy largely depends on the hydrogen source. While blending grey, brown or black hydrogen has limited environmental impact due to emissions from production, using green hydrogen produced from renewable sources significantly lowers overall GHG emissions, supporting net-zero goals.

As the hydrogen energy revolution unfolds, understanding the various types - including production methods and environmental impacts - is crucial. While green hydrogen is the net-zero goal, low-carbon hydrogen, along with black, brown and grey, are essential for advancing infrastructure, research and energy diversification.

As humanity navigates climate change in the coming decades, ensuring power continuity and sustainability will require varied energy sources. While there remain several hurdles to overcome, hydrogen holds promise as a clean, versatile and sustainable carrier.