Methane to Ammonia via Pyrolysis
Byon January 27, 2017
Eric McFarland, Professor of Chemical Engineering at the University of California Santa Barbara, likes fossil fuels and nuclear energy and is unimpressed by the menu of renewable energy technologies. But he is worried about climate change and he has an original view on how to modify our current energy system so that we don’t overload the atmosphere with CO2. He believes the key will be to separate fossil hydrocarbons into gaseous hydrogen and solid carbon. The chemistry he is developing in this area involves transferring “electrochemical potential” from hydrocarbons to alternative energy carriers. Ammonia is an energy carrier that McFarland believes is especially promising.
At the center of McFarland’s work is the insight that it is not necessary to oxidize hydrocarbons to extract their energy. In his talk he discussed several chemical pathways that could produce heat, chemical commodities, and/or fuels. One pathway is pyrolysis, which is defined as the breakdown of organic chemicals at elevated temperatures in the absence of oxygen. McFarland discussed at length a pyrolytic approach that involves a bath of molten salt or metal. As an example, he showed a stream of methane bubbling through a “melt” of lithium chloride and potassium chloride. Carbon forms on the surface of the molten material and can be skimmed off mechanically, while hydrogen collects in the space above it.
McFarland and his collaborators have thought about the feasibility of such a system at industrial scale. He sees analogies to steel-making processes, and in fact has conducted a “techno-economic evaluation of methane pyrolysis in molten metals.” The evaluation is based on hydrogen production of 200,000 tonnes per year (the annual consumption of a world-scale ammonia plant) and uses steam methane reforming (SMR) as the baseline technology. Three methods for generating the heat that drives the process are presented: electric arc, methane-firing, and hydrogen-firing. Electric arc pyrolysis, using grid electricity, results in a 59% reduction in CO2 footprint vs. SMR and achieves a 10% internal rate of return (IRR) on the required investment without factoring in a price on avoided CO2 emissions. The methane-fired method results in an 81% reduction in CO2 footprint but requires an avoided CO2 price of $2 per tonne to achieve a 10% IRR. The hydrogen-fired method eliminates CO2 emissions altogether but needs an avoided CO2 price of $22 per tonne to achieve a 10% IRR.
McFarland speculated that pyrolytic hydrogen production could be integrated with Haber-Bosch ammonia synthesis in a way that would allow substantial reduction in CO2 footprint. However, he said that the “grand challenge” for his team is to develop a pyrolytic process for converting methane directly to ammonia. His chemical equation for doing so is as follows:
CH4 + ½ N + nX –> NH3 + C + HX
He indicated that if the “X” is a halogen such as iodine, pyrolytic reaction kinetics should be enhanced and the byproduct hydrogen iodide could be a valuable energy commodity in its own right.
McFarland, saying that he and his team are “keen on ammonia”, devoted a section of his talk to the case in favor:
Ammonia as a fuel is a real interesting target, where it would allow you to make the transfer of that chemical potential to something that fits in well to our infrastructure … It has the same heat of combustion as methanol, so it has plenty of power per weight. When you burn it though, you only make nitrogen and water. There’s ways to get around the issues with NOx and stuff, so this is really a nice fuel. And the wonderful thing about ammonia, if you think about it, if you made it at a price for fuel, it would do something else that’s really important for the poor, which is to lower the price of food …
All of the automobile companies have looked at ammonia as a possible fuel, and it’s one that is certainly worthy of consideration if one wants to think about what we can do about a liquid fuel that fits into our own infrastructure that can be used more or less in our world as we know it.
Among the questions to be answered as the methane pyrolysis method is being brought to industrial scale is what to do with the byproduct carbon. The challenge here is complicated by the likelihood that the carbon would be inadvertently contaminated with material from the molten bath and/or intentionally contaminated with other reaction byproducts. “Ideally you want to precipitate out all of the undesirables and carry those in your waste stream,” McFarland said. “This is ultimately going to be a big waste stream. As much coal as went into the plant before is going to come out.” One idea, he said, would be to “put it back in a coal mine or bury [it] in a landfill.”
Allowing the carbon to emerge from the process as a waste product – and potentially a hazardous one at that – may not be a luxury a pyrolysis commercializer will choose to afford. Phoenix Energy, a developer of biomass gasification plants based in San Francisco, shared some interesting data today during a Webinar on “The Future of Renewable Methane in Today’s Regulatory & Policy Environment.” The company operates two plants in California that together produce 5.5 tonnes/day of byproduct carbon in the form of biochar. They are currently selling all of the biochar they can produce into agricultural markets at a price of $1,700 per tonne.
A video of McFarland’s talk can be found here:
His slides are available here.