Interview with Tim Schulzke and Stefan Schlüter

The second phase of the joint project Carbon2Chem^® is currently underway. One of its aims is to validate the large-scale implementation of sustainable methanol production based on metallurgical gases. What are the next steps? What is the significance of simulation for real plant operation? Tim Schulzke and Stefan Schlüter in an interview.

First of all, please give us a brief overview: What is the aim of the joint project Carbon2Chem^®?

Stefan Schlüter: The core objective of the project launched in 2016 is to make steel production at thyssenkrupp greener. To this end, the carbon monoxide and carbon dioxide produced during steel production is recycled chemically. Specifically, this involves the product paths methanol, higher alcohols, urea and polymers. Carbon2Chem^® is intended to make a large proportion of Germany's annual carbon dioxide emissions economically recoverable from the steel industry. The approach is a kind of bridge on the way to genuine green steel, for the production of which sustainable hydrogen will be used instead of coking coal in the future.

Fraunhofer UMSICHT is very experienced in methanol synthesis and contributes its expertise accordingly.

Tim Schulzke: That is correct. At the institute, we have been dealing with synthesis gas chemistry for a long lasting period, specifically with the production of methanol and dimethyl ether from synthesis gases. Fraunhofer UMSICHT has several test facilities in different sizes and with different focus areas of investigation, which we use within the framework of Carbon2Chem^®.

What specifically distinguishes the individual plants?

Tim Schulzke: Catalyst behavior is tested on the smallest experimental plant. With which gas compositions does a catalyst deliver reasonable product quantities and product qualities? Under which conditions are there more by-products, and under which are there fewer? This gives us detailed information about the function of gas purification in the steel mill. The second experimental plant is somewhat larger in size and is used to study reactor behavior. We measure the temperature control, the pressure and the temperature profile in the reactor. Finally, we have the demonstration plant in container format. Here we look at the plant behavior, i. e. the interaction of catalyst, reactor, separation of the liquid product and loop gas control. For example, we detect negative effects due to the enrichment of trace gases that are not observed in the single-pass operation of the smaller experimental plants. We are investigating these relationships first with bottled gases and later with real gases from the smelter.

How does the circular gas flow work in the demonstration plant?

Tim Schulzke: At the beginning, the so-called make-up gas enters the plant and is fed through the reactor. When the gas is cooled at the end, methanol and water vapor condense and can be separated from the rest of the gas. The result is methanol with some water. However, for thermodynamic reasons, the makeup gas can only be partially converted in the reaction, leaving a certain amount of hydrogen with a little carbon monoxide and carbon dioxide. These are recycled as recycle gas and mixed with the makeup gas, and the resulting feed gas goes back into the reactor. The mixing ratios are such that there is always slightly more hydrogen than is needed for the reaction. As a result, the hydrogen concentration in the recycled gas is very high - which in turn leads to a high conversion of carbon. The excess hydrogen and inert components are released into the environment via the so-called purge gas in order to avoid an increase in pressure. If a gas mixture containing only hydrogen and carbon carriers is used for the reaction, the purge gas volume flow is relatively small. In the case of blast furnace gas, on the other hand, it is significantly greater, since here the makeup gas contains up to 22 percent nitrogen, which has to be released again.

In addition to practical tests, Fraunhofer UMSICHT also deals with the topic of simulation.

Stefan Schlüter: We simulate the entire network. This includes the steel mill gas purification stages, the compressors, the actual methanol plant, and other technical equipment such as separators. In principle, everything that is needed to produce a saleable product. We work on two tracks: Detailed models for the methanol process are used to recalculate previous practical experiments. Simplified models, based on simpler assumptions, are then parameterized with this calculated data. The simplified models are necessary for the overall simulation, in which we simulate all the necessary units simultaneously over a longer period of time - e.g., a whole production year. We have to keep the computing times within a range that is still manageable for us. Our goal is that a simulation of the entire process over a year should take about one day.

Tim Schulzke: Thanks to the simulations, we can define a technically possible framework. Whereas in practice there are limits to physics, in the simulation we are relatively free, for example, in terms of maximum pressures, temperatures or flow rates. And we can represent considerably more points than would be possible in a limited test time.

Which main parameters influence the simulation?

Stefan Schlüter: One main parameter is the gas composition, more precisely what type of metallurgical gases are used: Blast furnace gas or basic oxygen furnace gas? Currently, we mainly use blast furnace gas, as it is expected to be available at the site for a longer period of time. The question arises as to how the purification works. We are unable to simulate this with sufficient accuracy and are therefore dependent on the results of laboratory tests. The structure of the actual methanol process is also central. Here, there are a number of free variables that are also used in classic methanol plants to optimize the operation. Since the steel mill gas flows and also the gas composition are not constant, our models are set up in such a way that they can handle fluctuating operation. A working group at Fraunhofer UMSICHT considers the entire process over a longer period of time with a focus on economic aspects. When does it make sense, for example, to utilize the methanol plant to a greater extent because, for example, the electricity for the production of hydrogen is cheap or methanol can be sold at a higher price?

How are the results from the simulations verified?

Stefan Schlüter: On the one hand, we compare our simulations with other calculations and with data provided to us by the steel mill and by the working groups at thyssenkrupp involved in the project. On the other hand, we have the possibility of comparison with the pilot plant tests.

In the first half of 2022, the demonstration plant is to be transferred from Fraunhofer UMSICHT in Oberhausen to the steel mill in Duisburg. Are there any further plans beyond that?

Stefan Schlüter: In the next phase, we will test methanol production in the pilot plant at the steel mill using real gases. By the end of the project period in 2024, one process for methanol and one for ammonia/urea should have been worked out in detail to such an extent that technical demonstration plants can then be built. As things stand at present, we are confident that we can achieve this goal.

Can the processes initially developed for a metallurgical plant also be adapted to other industries?

Tim Schulzke: Alternative carbon sources are part of a lighthouse project within Carbon2Chem^® and there are already two exposed partners. One is GMVA Gemeinschafts-Müll-Verbrennungsanlage Niederrhein GmbH, part of REMONDIS Assets & Services GmbH & Co. KG, where the flue gas from waste incineration is cleaned and carbon dioxide is obtained for methanol synthesis and the manufacture of other products. On the other hand, there is the lime burner Lhoist Germany Rheinkalk GmbH. In this case, carbon dioxide is inherent in the process, i. e. it cannot be avoided. It is inevitably produced during the conversion of limestone (calcium carbonate) by heat into burnt lime (calcium oxide). The next objective is therefore to chemically bind the carbon dioxide from the lime kilns with electrolytic hydrogen as methanol and to use this, for example, as an e-fuel or chemical raw material.

Tim Schulzke (Low Carbon Technologies department) is adapting the existing demonstration plant to the local conditions at the steel mill site. He is also conducting test campaigns to obtain basic data and test the limits of use.

Stefan Schlüter (Low Carbon Technologies department) is responsible for simulating the overall composite using mathematical methods and appropriately developed programs.