Negative emissions despite enormous energy input

Carbon and hydrogen from methane: Researchers at Empa are working on a pyrolysis
Carbon and hydrogen from methane: Researchers at Empa are working on a pyrolysis process that is to be used in a demonstration plant in the Tech Cluster Zug. Image: Empa
From the desert to Swiss industry: As part of the new Mining the Atmosphere initiative, researchers are pursuing the goal of harvesting renewable energy in the Earth’s sun belt, converting it several times and transporting it over long distances to where it is needed. A look at the energy and greenhouse gas balances shows: Although the concept requires a lot of energy, it can lead to negative CO2 emissions overall.

Industry is Switzerland’s third-largest energy consumer, alongside buildings and mobility. In particular, high-temperature processes in metal processing and the chemical industry, which are often operated with natural gas, lead to an overall energy consumption in this sector of around 22 terawatt hours per year. Together with the Tech Cluster Zug, the Canton of Zug and over a dozen additional partners, Empa has joined forces in 2022 to form the Association for the Decarbonization of Industry (AfDI). Within this framework, researchers want to contribute to the decarbonization of high-temperature process heat. "We take decarbonization literally," says Christian Bach, Head of Empa’s Automotive Powertrain Technologies lab. "We use a pyrolysis process to separate the carbon in natural gas before combustion." What remains is pure hydrogen, with which the industrial high-temperature processes can be operated, and the separated carbon as a powder, which is to be further processed for applications in construction and agriculture. A demonstration plant is currently in the design phase and will be set up in Zug over the next two years. The hydrogen will then be used in the enameling furnace at V-Zug AG.

If synthetic methane is used instead of natural gas, it is even possible to achieve negative greenhouse gas emissions over the entire process. This is because, for the production of synthetic methane, CO2 is extracted from the atmosphere, which is then no longer emitted but is available in the form of solid carbon in the end. "It is not realistic, however, to think that we will be able to cover the huge energy needs of our industry through the domestic production of renewable hydrogen or synthetic methane," says Bach. The focus is therefore on desert regions of the world - areas where the solar radiation per square meter is twice as high as in Switzerland.

However, the production of synthetic methane in the desert, its transport to Europe and the subsequent pyrolysis are processes that reduce the overall efficiency. Accordingly, the energy and greenhouse gas balances of the entire process must be closely scrutinized. Christian Bach and his team analyzed the entire value chain with partners from AfDI and compared it with other processes. One megawatt hour (MWh) of high-temperature heat for industry serves as a comparative value. If natural gas is to be used - as is currently the case - 1.2 MWh of primary energy is required and 288 kg of CO2 (resp. CO2 equivalents) are emitted. The primary energy also includes the energy used to extract the gas - for example in the Middle East - and transport it, and also takes into account the losses due to methane slip. Around one-fifth of the emissions are generated during the production of natural gas, the rest during its use.

If natural gas is decarbonized by pyrolysis beforehand and only the resulting hydrogen is used to generate high-temperature heat, CO2 emissions can be reduced by 40% to 178 kg. At the same time, however, the primary energy requirement increases because more natural gas is required and because additional electricity is needed for pyrolysis. In this scenario, 1 MWh of high-temperature heat requires 2.6 MWh of primary energy.

If renewable synthetic methane is used instead of natural gas, CO2 emissions actually fall into the negative range, but the primary energy requirement continues to rise. The calculation is based on the assumption that the CO2 required to produce synthetic methane is extracted directly from the atmosphere using a direct air capture plant. "This requires a great deal of energy," explains Bach, which is also the reason why he can only imagine such plants being operated in desert regions. Moreover, the construction of solar and wind power plants is also associated with emissions. If all these factors are taken into account, the direct use of synthetic methane to generate 1 MWh of high-temperature heat results in a primary energy requirement of 3.5 MWh and greenhouse gas emissions of 126 kg of CO2. If, however, the carbon is again separated from hydrogen using pyrolysis, the emission balance turns negative: The entire process leads to emissions of -77 kg CO2 - but with an even higher primary energy requirement of 6.2 MWh per MWh of high-temperature process heat.

"Of course, the primary energy requirement of this concept is high - around 2.5 to three times higher than the most efficient hydrogen production in Switzerland," admits Bach. "But since two to 2.5 times more electricity can be generated per square meter of photovoltaics in desert regions compared to here, this approach hardly needs any more photovoltaic area." One challenge is costs. However, Bach is convinced that if it were possible to market the carbon as a raw material for non-energy applications, the entire process could certainly be economically viable.