It is not all that easy to make money from electricity production by waste incineration. If they feed electricity generated from such plants into the grid, often the operators of waste incineration plants (WIPs) must pay for the privilege: This happens whenever the sun and wind supply a great deal of energy and the stock exchange electricity prices in negative figures. ‘This will happen even more often in the future’, says Bernd Epple of the Institute of Energy Systems and Energy Technology at the Technische Universität Darmstadt.
But the mechanical engineering professor immediately offers a way out: Use the electricity itself to convert the carbon dioxide produced at waste incineration into methanol. The chemical product can be further processed to the diesel substitute OME (oxymethylenether) or used as raw material for the chemical industry, says Epple. This way, the process benefits the environment. This method is driving the energy transition forward, says Epple, by strengthening so-called sector coupling and networking the pillars of energy, transport and industry. Secondly, the process removes the CO2 from the environment, because WIPs also burn wood. Because the resultant methanol partially ends up in plastics, it binds the CO2 collected for an extended period. ‘This is plastic without crude oil,’ points out Epple.
Two reactors, several floors
At a pilot plant on the TU Lichtwiese campus, scientists have tested their methods in an industrial scale. Two reactors climb several floors into the sky. In one of them, carbon dioxide reacts with calcium oxide to form calcium carbonate, also known as limestone. ‘The limestone is solid and can be easily removed from the off-gas stream,“ explains Epple. In the second reactor, the CO2 is released again in that calcium is heated. The heat for this – and this is what is new about the process – comes from the combustion of waste materials. ‘We can handle many types of waste,’ says Epple. He is confident that there will always be fuel for the new process in WIPs.
Once pure CO2 is present, the surplus electricity comes into play. It helps convert the climate killer into methanol. Electrolysis splits the stream of water. This produces oxygen, which fuels the combustion of waste, and hydrogen. This binds with the CO2 to form methanol. The research group has optimized operation of the waste-fuelled plant so that, for example, no lumps arise. ‘The system is very stable,’ says Epple. ‘We can remove over 90 percent of the CO2 from the tested waste flue gases.’
Now the method is to be tested in a demonstration plant. A project partner, the Cologne waste management company ‘Suez Deutschland GmbH’, is investigating the integration of this process into its waste incineration plant. Epple hopes to kick off working there in early 2021. The researcher sees a bright future for the process. The plant is designed such that it can easily be retrofitted. ‘The great thing is that the existing plant remains unchanged.’
Not only with WIPss, but with all types of incineration plants. Economically, carbon capture and storage with the new method is superior, says Epple. ‘Compared to all other CO2 capture processes, we are more than 50 percent cheaper.’ Only the electrolysis is still expensive. But more favourable processes are being developed worldwide, emphasizes Epple.
The development described is part of the project MONIKA (‘Methanol from electricity and CO2 of a waste incineration plant – Studies of CO2 capture with limestone’), which is funded by the Federal Ministry of Economics and Technology with around 370,000 euros. Contact person is the scientific assistant Martin Haaf: firstname.lastname@example.org-…