Considering Reaction only: Per Methane mole basis
Steam methane reforming (SMR) is a process used to produce hydrogen from natural gas (primarily methane) by reacting it with steam at high temperatures. The byproduct of this reaction is carbon dioxide (CO2).
The main reactions in the SMR process are:
- Methane + steam (heat) → carbon monoxide + hydrogen CH4 + H2O (heat) → CO + 3H2
- Carbon monoxide + steam → carbon dioxide + hydrogen CO + H2O → CO2 + H2
In the first reaction, one molecule of methane (CH4) produces one molecule of carbon monoxide (CO). In the second reaction, one molecule of carbon monoxide (CO) produces one molecule of carbon dioxide (CO2).
Considering these reactions, for every molecule of methane that undergoes steam methane reforming, one molecule of CO2 is produced.
To determine the amount of CO2 produced from SMR, we need to know the mass of methane being processed. Methane has a molar mass of 16 g/mol, and carbon dioxide has a molar mass of 44 g/mol.Using the stoichiometry of the reaction, for every mole of methane processed, one mole of CO2 is produced. Therefore, for every 16 g of methane, 44 g of CO2 is produced:
CO2 produced = (mass of methane) * (44 g CO2 / 16 g CH4)
In practice, CO2 emissions from SMR can vary depending on the efficiency of the process and the purity of the natural gas being used. Generally, steam methane reforming produces around 9-12 kg of CO2 per kg of hydrogen produced.
Considering Reaction only: Per Hydrogen mole basis
To calculate the CO2 emissions per unit mass of hydrogen produced, we can use the stoichiometry of the steam methane reforming process.
From the reactions:
- CH4 + H2O (heat) → CO + 3H2
- CO + H2O → CO2 + H2
Overall, the balanced reaction is:
CH4 + 2H2O → CO2 + 4H2
For every molecule of methane, 4 molecules of hydrogen are produced. Using the molar mass of each component, we can calculate the mass ratio of CO2 produced per mass of hydrogen:
- Molar mass of CH4 (methane): 16 g/mol
- Molar mass of H2 (hydrogen): 2 g/mol
- Molar mass of CO2 (carbon dioxide): 44 g/mol
Now, let's calculate the mass of CO2 produced per mass of hydrogen:
For every 16 g of methane (CH4) reacted, 44 g of CO2 is produced, and 4 * 2 g (8 g) of hydrogen is produced.
So, CO2 produced per hydrogen mass = (44 g CO2 / 8 g H2) = 5.5 g CO2/g H2
This means that for every gram of hydrogen produced from steam methane reforming, 5.5 grams of CO2 are emitted. This value, however, may vary in practice depending on the efficiency of the process and the purity of the natural gas being used.
Considering Life Cycle of emissions from natural gas extraction, transportation, and processing ...etc
A life cycle assessment (LCA) considers the environmental impacts of a product or process throughout its entire life cycle, from resource extraction to disposal or recycling. For hydrogen production through steam methane reforming (SMR), this would include CO2 emissions from natural gas extraction, transportation, and processing, as well as the actual steam methane reforming process.
Estimating the exact amount of CO2 emitted during the life cycle of hydrogen production through SMR is challenging due to the variability in factors like natural gas sources, transportation distances, and specific process efficiencies. However, a general range can be provided.
According to various studies, life cycle greenhouse gas (GHG) emissions for hydrogen production from natural gas (primarily through SMR) range from 5 to 13 kg CO2-equivalents per kg of hydrogen produced. This range accounts for variations in natural gas extraction, transportation, and processing efficiencies, as well as the specific SMR process.
It's important to note that these figures are not just for CO2 emissions but also include other greenhouse gases (such as methane) emitted throughout the life cycle, expressed in CO2-equivalents. However, the majority of the emissions are likely to be CO2.