Issue Date: April 25, 2016
Dry reforming puts CO2 to work
It’s an industrial chemist’s dream to transform carbon dioxide into fuels and chemicals. A few processes do use the abundant waste gas as a starting point for polyols and specialty polymers. Other companies are looking to make chemicals by reacting CO with hydrogen acquired from water electrolysis. But multi-million-metric-ton use of CO2 as a chemical reagent is relegated mostly to old-school production of urea and sodium bicarbonate.
The German industrial gas and engineering giant Linde is looking to change that.
Company officials claim to have made a breakthrough in dry reforming, a process that reacts CO2, instead of steam or oxygen, with methane to yield the mixture of CO and H2 known as synthesis gas.
Dry reforming may be the way to introduce CO2 into the manufacture of large-scale chemicals such as methanol, acetic acid, and the diesel substitute dimethyl ether (DME), according to Nicole Schoedel, head of chemical development and services at Linde Engineering.
“It is one of the few options where you can use CO2 without an additional H2 source,” she says. She says that tapping into methane’s hydrogen simplifies the incorporation of CO2 in a large-scale chemical manufacturing process.
Dry reforming would be a welcome addition to the slate of methane reforming processes for producing synthesis gas. Companies employ different technologies depending on how much CO and H2 they need to make a specific downstream chemical.
Partial oxidation is the exothermic reaction of methane with oxygen to yield syngas with a 2:1 ratio of H2 to CO, ideal for making methanol.
Steam reforming reacts methane and steam under additional heat to make syngas in a 3:1 ratio of H2 to CO. Often the CO will be reacted with water in a so-called water-gas shift to yield CO2 and even more H2. Ammonia plants and oil refineries, both thirsty for H2, use such setups.
Autothermal reforming combines partial oxidation and steam reforming in a single heat-releasing process that results in a 5:2 ratio of H2 to CO.
Linde’s process isn’t pure dry reforming. The company does use some steam in the reaction to boost the amount of H2 in the final syngas. Otherwise, dry reforming would yield a 1:1 H2-to-CO ratio, too low to make chemicals such as methanol, Schoedel acknowledges. The amount of steam used in Linde’s process can be adjusted to hit the desired H2-to-CO ratio.
Dry reforming has been the subject of academic interest for a long time, but the Linde process would be a commercial first. According to James J. Spivey, a professor of chemical engineering at Louisiana State University, the biggest challenge in dry reforming is carbon deposition on the catalyst, also known as “coking.” Under the 800–1000 °C temperatures of the process, the methane breaks up into hydrogen and carbon, which often accumulates on the catalyst.
Steam, present in Linde’s process, alleviates the coking problem because it is a stronger oxidizer than CO2. But too much water can oxidize the active metal site on the catalyst. “Linde may have solved that problem,” Spivey observes.
Linde is testing two catalysts, both of which it developed during a four-year collaboration with BASF, BASF’s high-throughput experimentation subsidiary HTE, and Karlsruhe Institute of Technology. One, a nickel-based catalyst, is similar to those widely used in steam reforming. The other, cobalt-based, has less of a propensity for coking than nickel catalysts.
Linde says it has tested the catalysts in the lab for more than 1,000 hours. Last October, the company inaugurated a pilot plant near Munich where it plans to conduct longer-term and larger-scale runs to gather the data needed for commercial reactor design.
Spivey points out that dry reforming is no magic environmental bullet. The reaction is endothermic, and, like steam reforming, it is powered by a furnace that burns natural gas and emits CO2. “Because of the laws of thermodynamics, you’re never going to consume more CO2 than you used to drive that reaction,” he says.
However, dry reforming can reduce the carbon footprint of an integrated process, Schoedel claims. For example, she says that a dry reformer integrated with a DME plant can offer 30% CO2 emissions savings versus a “state of the art” setup where a combined steam and autothermal reformer feed a plant making methanol that is then used to make DME.
Schoedel says dry reforming has the potential to use larger amounts of CO2 than specialty polymer production, which she calls a “niche” application of the gas. “It is one of the only options where you can really make bulk chemicals with a relevant amount of CO2,” she says.
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