Sylvie Willis
Hydrogen is the most abundant chemical substance in the universe, yet it is rarely found in its pure form on Earth. This means that hydrogen fuel production is a complex and energy-intensive process. The environmental impact of hydrogen production varies depending on the method used, but it is estimated that 830 million tonnes of CO2 are emitted each year to produce 74 million tonnes of hydrogen. The two most common methods for producing hydrogen are steam-methane reforming and electrolysis (splitting water with electricity). Steam-methane reforming is the most common method of hydrogen production, and it typically results in CO2 emissions of around 8-10 kg of CO2 per kg of hydrogen produced. However, other methods such as electrolysis of water using renewable energy or photolysis can produce much lower CO2 emissions of around 1-2 kg of CO2 per kg of hydrogen produced.
| Characteristics | Values |
|---|---|
| CO2 emissions per kg of hydrogen produced | 8-10 kg CO2 per kg of hydrogen produced for Steam Methane Reforming (SMR) 14-15 kg CO2 per kg of hydrogen produced for Coal Gasification 2-3 kg CO2 per kg of hydrogen produced for Biomass Gasification 1-2 kg CO2 per kg of hydrogen produced for Electrolysis of water 0 kg CO2 per kg of hydrogen produced for Photolysis |
| Carbon footprint of hydrogen production via SMR | 100% of the carbon in the incoming methane is converted to CO2. Per 1 million standard cubic feet (SCF) of hydrogen produced from methane, 250,000 SCF of CO2 is produced. There are 19,253 SCF of carbon dioxide in one metric ton, so 1 million SCF of hydrogen will produce 13 metric tons of carbon dioxide. |
| Carbon footprint of hydrogen production in terms of energy | 1 million SCF of hydrogen contains 79,100 kilowatt hours of energy, which converts to 0.28 kg of carbon dioxide emissions associated with one kilowatt-hour of hydrogen production. |
| CO2 emissions when hydrogen is burned | Hydrogen does not produce CO2 when burned, instead it yields water. |
| Hydrogen production methods | Steam-methane reforming Electrolysis Thermochemical processes Photolytic processes Biological processes Biomass-Derived Liquid Reforming Microbial Biomass Conversion Thermochemical Water Splitting Photobiological Water Splitting Photoelectrochemical Water Splitting Water splitting Carbon assisted electrolysis Biological processes using microbes Biomass Gasification |
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What You'll Learn
Steam methane reforming
In the SMR process, natural gas is reacted with steam at an elevated temperature to produce carbon monoxide and hydrogen. The reaction is conducted in multitubular packed bed reactors, a subtype of the plug flow reactor category. These reactors consist of an array of long and narrow tubes situated within the combustion chamber of a large industrial furnace, providing the necessary energy to keep the reactor at a constant temperature during operation. The steam is heated to 700–1,100 °C (1,300–2,000 °F) in the presence of a nickel catalyst. The resulting endothermic reaction forms carbon monoxide and molecular hydrogen (H2). The carbon monoxide then reacts with steam to obtain further quantities of H2 in a process known as the water-gas shift reaction (WGSR), which also requires a catalyst, typically iron oxide or other oxides. The byproduct of this reaction is CO2.
The carbon footprint of hydrogen production via SMR can be broken down into two parts. First, as indicated by the SMR and WGSR reactions, 100% of the carbon in the incoming methane is ultimately converted to CO2. In the process of producing one molecule of CO2, four molecules of hydrogen (H2) are produced, with the steam contributing the additional hydrogen. Thus, per 1 million standard cubic feet (SCF) of hydrogen produced from methane, 250,000 SCF of CO2 will be produced. This converts to 9.3 kilograms (kg) of CO2 produced per kg of hydrogen production.
SMR is not a zero-carbon process, but the carbon footprint can be reduced by capturing and storing the carbon dioxide produced. Hydrogen produced by SMR is termed 'grey' hydrogen when the waste carbon dioxide is released into the atmosphere and 'blue' hydrogen when the carbon dioxide is captured and stored geologically.
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Electrolysis of water
Hydrogen fuel can be produced through the electrolysis of water. This is a process that uses electricity to split water into hydrogen and oxygen gas. The reaction takes place in a unit called an electrolyser, which contains two electrodes and an electrolyte separating them. When an electric current is applied between the electrodes, water molecules are separated into hydrogen and oxygen gases.
There are different types of electrolysis methods, including alkaline, solid oxide, and proton exchange membrane (PEM) electrolysis. PEM electrolysis uses a proton-exchange membrane or polymer-electrolyte membrane (PEM) to allow the passage of protons while preventing the mixing of hydrogen and oxygen gases. This improves the efficiency of hydrogen production. Solid oxide electrolysis uses a solid oxide electrolyte to split water into hydrogen and oxygen at elevated temperatures. This process is similar to solid oxide fuel cells but operates in reverse to generate hydrogen instead of electricity.
While electrolysis of water can produce hydrogen fuel, it is generally a more expensive approach compared to steam methane reforming (SMR). Over 95% of the world's hydrogen is produced using SMR, which involves reacting natural gas with steam at elevated temperatures to produce carbon monoxide and hydrogen. The carbon monoxide then reacts with more steam to produce additional hydrogen and carbon dioxide. This process emits carbon dioxide, a greenhouse gas, and is known as gray hydrogen. If the carbon dioxide is captured and stored, the hydrogen produced is known as blue hydrogen.
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Coal gasification
During coal gasification, the coal is reacted with steam (water vapour) to produce hydrogen and CO2. This process typically results in CO2 emissions of around 14-15 kg of CO2 per kg of hydrogen produced. This makes it a highly polluting and energy-intensive method of hydrogen production.
The carbon footprint of coal-based hydrogen production without carbon capture and storage (CCS) technology is significant. In China, for example, the carbon footprint of coal-to-hydrogen without CCS is estimated to be 19.37-22.87 kg of CO2 equivalent per kg of hydrogen (CO2eq/kg H2). With CCS technology, the carbon footprint can be reduced by 52.34%-74.59%, resulting in a carbon footprint of 4.92-10.90 kg CO2eq/kg H2.
Some coal gasification plants, such as the Kemper Project in Mississippi, have implemented carbon capture and storage technologies to reduce their CO2 emissions. The Kemper Project captures over 65% of the CO2 generated using the Selexol process, and the CO2 is sent by pipeline to depleted oil fields for enhanced oil recovery operations. Another example is the Texas Clean Energy Project (TCEP), which will capture 90% of its CO2 emissions using the Rectisol process. The captured CO2 will be used for enhanced oil recovery in the West Texas Permian Basin.
While coal gasification for hydrogen production can result in significant CO2 emissions, advancements in carbon capture and storage technologies can help mitigate these emissions. However, the use of coal as a feedstock for hydrogen production is still considered highly polluting and energy-intensive compared to other methods, such as biomass gasification or electrolysis of water using renewable energy.
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Biomass gasification
The production of hydrogen fuel often involves the use of fossil fuels, resulting in carbon dioxide emissions. Biomass gasification is a mature, controlled process that offers a promising pathway for producing hydrogen-rich syngas while addressing environmental concerns. This process involves converting biomass, such as wood, crops, and agricultural residues, into hydrogen and other products using heat, steam, and oxygen without combustion.
The gasification process typically involves an extra step to reform hydrocarbons with a catalyst, yielding a syngas mixture of hydrogen, carbon monoxide, and carbon dioxide. The carbon monoxide is then converted into additional hydrogen and carbon dioxide through a shift reaction with steam. The hydrogen produced is separated and purified.
The use of catalysts in biomass gasification can further enhance sustainability and efficiency, contributing to reduced CO2 emissions. Additionally, biomass gasification can utilize wastes as feedstock, minimizing environmental problems associated with landfill disposal. However, key challenges include reducing costs associated with capital equipment and improving agricultural practices to stabilize feedstock expenses.
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Fermentative hydrogen production
The process of fermentative hydrogen production can be integrated with CO2 mitigation, wastewater treatment, and the production of high-value chemicals. Microalgae, for example, can be used as feedstock for fermentative hydrogen production. They have a high growth rate, superior CO2 fixation capacity, and do not require arable land to grow. Additionally, microalgae can be cultivated in freshwater, seawater, and wastewater.
The use of synthetic biology allows for the genetic alteration of bacteria to enhance the hydrogen production reaction. This process is known as photofermentation, where light energy is required for the reaction to proceed.
The carbon footprint of hydrogen production through steam methane reforming (SMR) can be significant. The SMR process converts all incoming methane into CO2, resulting in the production of 13 metric tons of carbon dioxide for every million standard cubic feet (SCF) of hydrogen produced. This equates to approximately 9.3 kilograms of CO2 produced for every kilogram of hydrogen generated.
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Frequently asked questions
The amount of CO2 produced during hydrogen fuel production varies depending on the method of production. Steam Methane Reforming (SMR), the most common method, produces 8-10 kg of CO2 per kg of hydrogen. Other methods include Coal Gasification, which emits 14-15 kg of CO2 per kg of hydrogen, and Biomass Gasification, which emits 2-3 kg of CO2 per kg of hydrogen. Electrolysis of water produces 1-2 kg of CO2 per kg of hydrogen, while photolysis produces none.
The cleanest methods of hydrogen production in terms of CO2 emissions per kg of hydrogen are electrolysis of water using renewable energy and photolysis. Photolysis uses solar energy to split water into hydrogen and oxygen without producing any CO2 emissions.
Hydrogen fuel production can contribute to climate change depending on the method of production. Currently, around 95% of hydrogen is produced from fossil fuels, resulting in 830 million tonnes of CO2 emissions annually. However, hydrogen itself does not produce CO2 when burned, making it a potential tool for fighting climate change by replacing fossil fuels.
