“Capturing CO₂ in power stations is a good thing but does cost extra money and extra energy. In order to make CO₂ removal feasible, affordable technology should be made available and the energy loss should be restricted as much as possible. Our calculation models help bring that a little closer”, says Jan Wilco Dijkstra, researcher of the ECN unit Hydrogen & Clean Fossil Fuels.

With its research into the capture of CO₂ ECN is participating in CATO (see www.co2-cato.nl/), a consortium of universities, businesses and knowledge institutions that research the various options for the capture, transport and storage of CO₂ generated, for example, by fossil-fired power stations. Dozens of researchers are involved in this project.
Mr Dijkstra: “There are three ways of capturing CO₂. The first is to capture the CO₂ from the flue gases released during the combustion of natural gas and coal. As CO₂ only constitutes 5-15% of the flue gases, this is actually quite difficult. The second option is to burn the natural gas and coal with pure oxygen as opposed to air. The combustion gas will then not contain any nitrogen, just CO₂ and water. Condensing the water will then result in pure CO₂ which can be stored. The third option, which we primarily focus on at ECN, is the pre-combustion removal of CO₂. This is done as follows: with the aid of steam, natural gas and coal can be converted into hydrogen and carbon monoxide and, in turn, into hydrogen and CO₂. The CO₂ can then be separated from the hydrogen.”

Models bring CO₂-free power generation a step closer

Membrane or sorbent
“You could, in fact, separate the CO₂ in a single stage if you used a membrane or sorbent”, Mr Dijkstra continues. “With the aid of steam and a catalyst in a tubular chemical reactor the natural gas, or syngas generated from natural gas or coal, is then converted directly into hydrogen and CO₂ (carbon dioxide). With a membrane reactor, the hydrogen is simultaneously removed by means of a membrane that filters the other substances. In a so-called sorbent reactor, the CO₂ is combined with a special adsorption agent and the other substances remain in the process stream. We have two experimental setups where these reactors work quite well.”
The process after the separation is, in effect, little different. Mr Dijkstra explains: “In a power station you then burn the hydrogen in a gas turbine that drives an electricity generator. In addition, the heat released is used to make steam for a steam turbine that drives the same generator. The CO₂ can then be separated from the adsorption agent again using steam, after which it can be compressed, transported and stored.”

Calculating the process from natural gas to electricity
Besides these two test reactors ECN has also developed calculation models. These simulate not only the reactors but also the entire process from natural gas to electricity. Mr Dijkstra is convinced of the indispensability of these models. “Without models you can, in a laboratory, design a reactor that will optimally convert natural gas to hydrogen and CO₂ at a certain temperature. But that doesn’t mean that it will be the most efficient solution in practice. What works well for the reactor, such as a mild temperature and pressure, will not necessarily work for the entire process of electricity generation. Our models are a bridge between the laboratory and the real world.”

Examining financial feasibility
“With our models we also examine the economic aspects of the processes continuously. Because it is not just a matter of the technical feasibility of CO₂ capture, it’s especially about the financial feasibility”, Mr Dijkstra emphasises. “At the end of the day it’s about how much a electricity company would have to pay for the power plant and what this would mean for the cost of generating electricity. A CCS installation would easily cost several tens of percent of extra investments; that’s hundreds of millions of euros.”
Whether or not CO₂ capturing actually gets off the ground will therefore depend primarily on political choices. Mr Dijkstra: “Whatever happens, it is important to build demonstration installations in Europe in the coming years to acquire practical experience of capturing and storing CO₂.”

Slicing the tube reactor
ECN uses commercial software packages for its modelling. “For the membrane and adsorption reactors we have developed and even partially integrated our own highly advanced models”, Mr Dijkstra says, not without pride. “We cut the tubular reactor into slices, so to speak, and then calculate what happens in each slice. This gives us an idea of what happens along the entire length of the reactor. We can also vary the temperature, pressure and other things to see what effect this has on the efficiency and costs. This enables us to identify problem areas sooner and to concentrate on improving the technology.”
Mr Dijkstra can combine the reactor simulation with gas turbine and heat exchanger simulations to have a reliable picture of a complete power station. Mr Dijkstra: “Our models enable us to predict fairly accurately the scale of equipment needed to generate a particular capacity in megawatts, what the appropriate process flows would be, and the efficiency the power station would be able to realise.”

Supporting organisations
The experimental reactors are now working well. “We will use these to gauge how reliable the real-world predictions of our simulations are. Our models will enable us to help companies realise projects to capture CO₂”, Jan Wilco Dijkstra concludes.

Text: Erik te Roller

Contact
Jan Wilco Dijkstra
ECN Hydrogen & Clean Fossil Fuels
Phone: +31(0)22 456 8238
E-mail: Jan Wilco Dijkstra 

Info
To view or download the report on ECN modelling tools for CO₂ capture, click here.