Definition: Steam methane reforming (SMR) is a chemical reaction consisting in extracting dihydrogen from methane using steam at high temperature (700-1000°C) at moderate pressures (15-30 bar).
Steam reforming is a means of producing hydrogen from a light hydrocarbon using high temperature steam (700-1000°C) at moderate pressures (15-30 bars). It is one of the few technology of hydrogen production viable at industrial scale. Although naphtha, methanol or liquefied petroleum gas, fuel oil and diesel can also be used, methane is mostly used. (Garcia 2015, p.99 et seq.) We will therefore only study the latter case. We will see the chemical reaction itself, then its practical dimensions.
Chemical reaction of steam methane reforming
The main principles
The main chemical reaction of steam methane reforming is: CH4 + H2O = CO + 3H2. To occur in the “right” direction (production of H2), the reaction requires an energy of +206kJ/mol. A nickel catalyst is used. This is the steam reforming operation.
However, we will add another important reaction to recover a little more dihydrogen using the carbon monoxide obtained previously: the “Water-gas shift reaction” (= “catalytic conversion” or “vapor conversion of water“): CO + H2O = CO2+H2. This reaction produces some energy ((ΔHθ=−41kJ/mol).
It seems that there are other chemical reactions that can take place. For example
- “dry steam reforming”, using CO2 instead of steam: CH4 + CO2 = 2CO + 2H2 (enthalpy 247.3 kJ/mol)
- The decomposition of methane: CH4 = C + 2H2 (enthalpy 74.9 kJ/Mol)
- The Boudouard reaction, 2CO = C + CO2 (enthalpy – 172.5 kJ/Mol). (Garcia 2015)
To simplify, I won’t talk about it.
The industrial process of steam methane reforming
Pre-reforming is important if it is not pure methane. The steam and hydrocarbon are then brought together in a pre-reforming unit, where “all high-grade hydrocarbons are converted directly to C1 [=1-carbon?] components (methane and carbon oxides) at low temperatures, typically 673 at 823 K [=400 to 550°C]”. The heat can go up to 1073K (=800°C) to reduce the risk of carbon residue forming. (Navarro et al. 2015)] (I put in brackets, since we are talking about methane here)
Desulphurize methane
The first step consists in desulfurizing the methane, the catalyst being very sensitive to sulfur compounds. A “zinc oxide bed” is generally used for this. The “gas leaves this section with a sulfur content of less than 1 ppm and a temperature between 350 and 400°C. (Garcia 2015, p.86)
Reforming
Then we move on to the actual reforming unit. The reactor gets a supply of the natural gas feedstock we cleaned and mix it with water vapor and heated to 800-900°C at a pressure of between 15 and 30 bars: CH4 + H2O = CO + 3H2. To accelerate the reaction, catalysts based on nickel oxide are used, placed in the reformer in the form of a fixed bed. (Garcia 2015) It produces syngas, which is a mixture of hydrogen an carbon monoxide.
Water Gas Shift
Then, it is the water gas shift (=catalytic conversion), which mobilizes two units. The first, at high temperature, is called “HTS” for “high temperature shift” with a catalyst in general Fe2O3-Cr2O3. The gas cools, enters at 350°C and leaves at 400-450°C. For a “typical flow”, between 8 and 10% carbon monoxide (CO), the operation reduces the latter to 4%. (Navarro et al. 2015 write between 350 and 420°C, but I kept the figure from Garcia 2015)
The second unit is called LTS (low temperature shift) and uses a catalyst based on copper, zinc and aluminum. The gas, cooled, returns to 220°C, then is maintained below 260°C. reduces the proportion of CO to 0.4-0.8%. (Navarro et al. 2015 write between 180 and 340°C, but I kept the figure from Garcia 2015)
Pressure Swing Adsorption – PSA
Finally, the last contaminants must be removed (unconverted CH4, CO residues, etc.), which are generally removed by “a pressure swing adsorption unit” (PSA). The result would be a stream of 99.99% pure H2. (Navarro et al. 2015) The process is nevertheless expensive: sometimes more than 10%. (Garcia 2015, p.92) The flue gas is composed of CO2 and a fraction of hydrogen not capted by the device.
Perspectives
There are several ways to improve the process:
- Use catalysts in the form of membranes, which could fluidify the reaction and make a first purification. (Garcia 2015, p. 89 et seq.)
- Sorption-enhanced reforming (= reforming with improved sorption?) mobilizing a substance that absorbs CO2 (ex: CO2(g) + CaO(s) => CaCO3 (s)), which would simplify the device and to avoid purification losses. (Garcia 2015, p. 92 et seq.)
- The use of microreactors (= what have been called units), which would make it possible to intensify the process, better manage heat losses and, by creating smaller installations, a decentralized production of hydrogen. (Garcia 2015, p. 97 et seq.)
Steam methane reforming in practice
When it comes to price, estimates vary. According to Fossil Fuel Hydrogen, Technical, Economic and Environmental Potential, the price of hydrogen production by steam methane reforming, without a carbon capture device, would be between $0.55 and $2.04 /kg of H2 with a median of $1.3 . With carbon capture, the price would be $2. The CEA (2021) estimates its price at €1.5/kg of H2.
These are aspects that I will explore later.
Ecological impact
The SMR plant produces lot of greenhouse gases emissions. Beyond the power generation needed to heat the industrial reactor, the reaction itself produced emissions. Let us recall the initial reactions (CH4+H2O = CO+3H2), then the catalytic conversion (CO+H2O = CO2+H2). So, in total, we have CH4+2H2O= CO2+4H2.
Assuming that there are only these reactions and that they are complete, 4 molecules of H2 are created at the same time as 1 of CO2. The latter has a molar mass of 44g, 2g for dihydrogen. Producing 8g of hydrogen therefore automatically releases 44g of CO2, 1kg of H2 therefore releases 5.5kg of CO2. Estimates are generally that, in total, producing 1kg of H2 by steam methane reforming releases 8-10kg of CO2.
One of the idea to make natural gas reforming “low emissions”, is to capture the carbon from the PSA tail gas (ex: Reddy & Vyas 2009) and other flue gas emitted by the reformer (= carbon capture and stockage). Even if it is often presented as a mature solution, the process still seems experimental. The solution is mentionned by the IPCC:
On the demand side, natural gas can be used to produce hydrogen using steam methane reforming, which is a technologically mature process (Sections 6.4.4 and 6.4.5). When combined with 90% CO2 capture, the costs of producing hydrogen are around USD1.5–2 kg(H 2)–1 (Collodi et al. 2017; Newborough and Cooley 2020), considerably less than hydrogen produced via electrolysis
IPCC, WG III, p.647 (Energy)
The steam reforming of the future, based on the sorption-enhanced reaction?
A study would have shown that steam reforming of methane based on the “sorption-enhanced reaction” (SER) would make it possible to produce already purified dihydrogen (~0.00001% CO) without PSA filtration or catalytic conversion, with very high efficiency (>99 % of hydrogen from CH4 recovered in the form of H2) at lower temperatures (520-590°C). ( 2014, p.9 et seq.)
The difference would be due to
“(a) favorable thermodynamic equilibrium of the highly endothermic SMR reaction at the higher reaction temperature, (b) faster kinetics of the SMR reaction at higher temperatures, (c) favorable removal of CO2 from the reaction zone at lower temperatures, and (d) higher cyclic work capacity for CO2 chemisorption at higher temperature. »
- Xiu G-H., Li P., Rodrigues A.E., (2002) « Sorption-enhanced reaction process with reactive regeneration », Chemical Engineering Science, Volume 57, Issue 18, September 2002, Pages 3893-3908
- Wi Y-J., Li P., Yuo J-G., Cunha A.F., Rodrigues A.E. (2016), « Progress on sorption-enhanced reaction process for hydrogen production », Reviews in Chemical Engineering Volume 32 Issue 3, 2016
- Waldron, W E; Hufton, J R; Sircar, S (2001). « Production of hydrogen by cyclic sorption enhanced reaction process », American Institute of Chemical Engineers. AIChE Journal; New York Vol. 47, N° 6, (Jun 2001): 1477.
- Ortiz A.L. et Harrison D.P. (2001), « Hydrogen Production Using Sorption-Enhanced Reaction », Ind. Eng. Chem. Res. 2001, 40, 23, 5102–5109
Bibliography
- US Energy department, « Hydrogen Production: Natural Gas Reforming »
- Navarro R.M., Guil R., Fierro J.L.G. (2015), Introduction to hydrogen production, in Compendium of hydrogen energy, Vol. 1 Hydrogen production and purification, éd. Woodhead Publishing, Kidlington, 2015
- Garcia L. (2015), Hydrogen production by steam reforming of natural gas and other nonrenewable feedstocks, in Compendium of hydrogen energy, Vol. 1 Hydrogen production and purification, éd. Woodhead Publishing, Kidlington, 2015
- Zhong Zhang J., Li J., Li Y., Zhao Y., « Hydrogen Generation, Storage, and Utilization », éd. Wiley, 2014
- Satish Reddy, Sunil Vyas, Recovery of Carbon Dioxide and Hydrogen from PSA Tail Gas, Energy Procedia, Volume 1, Issue 1, 2009, Pages 149-154, ISSN 1876-6102, https://doi.org/10.1016/j.egypro.2009.01.022.
- psa tail gas
- water gas shift
- combustion natural gas
- methane steam reforming
- hydrogen carbon monoxide
- natural gas reforming
- steam methane reformer
- pressure swing adsorption
- methane reforming smr
- reforming natural gas
- carbon capture storage
- steam carbon ratio
- carbon dioxide hydrogen
- methane reforming process
- gas psa tail
- endothermic reforming reactions
- catalyst filled tubes
- swing adsorption psa
- natural gas feedstock
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