Making a virtue out of necessity

Cement with potential

CO₂ is found naturally in limestone, which is the main component of cement. The limestone is heated in large rotary kilns at high temperatures to produce Portland clinker, an intermediate product. During this process, the limestone is broken down and the carbon dioxide escapes into the air. It is therefore possible to substitute the heated cement clinker in the cement or concrete with alternative materials, resulting in a significant decrease of potential greenhouse gases. For example, approximately 30% of CO₂ emissions can be reduced in each ton of cement by substituting calcium clays. Currently, there is no complete replacement for the raw material, as the clinker which is derived from the limestone is responsible for the strength of the concrete.

The industry has begun developing solutions which could reduce this output to almost zero through the targeted deployment of concentration and separation procedures. Carbon capture and storage (CCS) and carbon capture and utilisation (CCU) are two processes which separate the CO₂ produced during the manufacture of cement, enabling it to be stored or used for subsequent chemical processes.

CO₂ as a reusable raw material

The ‘Low-Carbon Transition in the Cement Industry’¹ technology roadmap from the OECD/International Energy Agency has calculated that using new technologies such as carbon capture and storage or carbon capture and utilisation would result in a substantial reduction of CO₂ emissions. “Innovative technologies including carbon capture (CO₂ emissions reduction of 48%) and reduction of the clinker to cement ratio (CO₂ emissions reduction of 37%) lead the way in cumulative CO₂ emissions reductions in cement making in the roadmap vision compared to the RTS by 2050.”²

Global cumulative CO₂ emissions reductions by applying the roadmap vision (2DS – 2 Degrees Scenario) compared to the RTS2. Note: Cumulative CO₂ emissions reductions refer to the period from 2020 to 2050 and are based on the low-variability case of the scenarios.²

A new combustion process

An alternative solution for reducing emissions must be found. To this end, thyssenkrupp has been researching a new oxyfuel combustion process in which the combustion air is replaced by pure oxygen. The emissions would then almost completely consist of pure CO₂ and steam, thus radically simplifying the complicated separation process and enabling the CO₂ to be stored or processed. The first experimental plants for the cement industry in the USA and Europe were introduced from 2010, but the project has not yet moved beyond the experimental phase.

Operators can retrofit their existing plants to the oxyfuel process. For older concepts (from around 2005 onwards), exhaust gas recirculation systems can be retrofitted to existing plants. This requires additional equipment, which in turn significantly increases the complexity and the operating costs. Engineers at the research centre of thyssenkrupp Industrial Solutions AG are therefore working on an improved process, and success is within reach. The new polysius^® pure oxyfuel procedure uses pure oxygen as a combustion gas and does not require exhaust gas recirculation, thus significantly reducing the effort required to separate CO₂. For all known CCS or CCU procedures, retrofitting represents a notable change in plant operation.

thyssenkrupp Industrial Solutions is also researching processes to convert the separated CO₂ into reusable materials such as methane or methanol. Methane can be fed into the natural gas network, while methanol is a base for synthetic fuels such as kerosene. This process enables CO2 to be used in a sensible way while also reducing the demand for fossil fuels.

Exhaust gas cleaning through Calcium Looping

Alongside oxyfuel combustion and solvent-based separation after combustion, Calcium Looping is regarded as a further promising new technology for CO₂ separation in cement plants as it enables the utilisation of numerous energy and material synergies. Calcium Looping is a regenerative process that uses the capacity of sorbents based on calcium oxide to separate CO₂ at high temperatures.

The process is divided in two basic steps:

• The capture of CO₂ by ‘carbonation’ of CaO to form CaCO₃ in a reactor operating around 650°C: CaO + CO₂ —› CaCO₃ + heat; the heat development is negligible here.
• oxyfuel calcination in a reactor operating above 900 – 920°C, which makes the CaO available again and releases a gas stream of nearly pure CO₂: CaCO₃ + heat —› CaO + CO₂.⁴

In the highly integrated Calcium Looping process configuration, the carbonator is used to capture the CO₂ contained in the kiln flue gases and the pre-calciner of the cement plant is oxy-fuelled, coinciding with the calciner of the CaL process. The CaO-rich pre-calcined raw meal produced in the calciner is broken down into two streams: one goes to the rotary kiln for clinker production, the other to the carbonator, to act as the sorbent of the CO₂ released in the kiln. The solid stream with re-carbonated raw meal is returned to the calciner to be regenerated, together with the raw meal coming from the pre-heater tower. CO₂ from fresh raw meal calcination, from kiln fuel combustion and from calciner fuel combustion is collected in the single flue gas stream exiting the calciner. In order to demonstrate the technical and economic feasibility of the integrated CaL process in large, retrofitted cement plants under realistic conditions, a test plant is currently being built in Italy within the scope of the EU Horizon 2020 CLEANKER project. As a technology partner, the IKN GmbH engineering firm from Neustadt is one of 13 international consortium members in this project, which is financed through €8.9 million of EU funds. The main objective is to demonstrate the technology in an industrial environment, laying the foundation for its exploitation by the companies in the EU on industrial basis. What is missing, however, is the infrastructure required to transport the carbon dioxide and an approval framework covering how it can be reused or stored. In the view of VDMA Construction – Equipment and Plant Engineering, it is conducive to promote the standardisation of alternative cements, as well as the approach of the BMU of developing a sales market for cements produced using greenhouse gas-neutral procedures. It is, however, necessary to create an approval framework quickly, and therefore also the requisite infrastructure for transporting and reusing the separated CO₂.

Implementation of an integrated CaL concept in a cement plant. Source: Politechnico di Milano.

Alternative fuels

Alongside the possibilities for separating and reusing CO₂, there are also ways to prevent it from being generated in the first place. Various non-fossil fuels are options here, including biomass such as wood chips, rice husks, sewage sludge or refuse-derived fuel (RDF). In order to ensure that the clinker is of a sufficient quality, cement manufacturers must keep the temperature of the kiln at a constant level. They do this by feeding the fuels at an even rate. Although alternatives are cheaper, they are more difficult to dose as they can vary significantly in terms of their composition, shape and size – even when using the same type. Powders, flakes and fibres have a complicated discharge behaviour and a low bulk density, and are difficult to feed into the combustion process evenly. Some of them are unhygienic or explosive, presenting an additional challenge.

With its PFISTER^® rotor weighfeeders, FLSmidth in Augsburg offers solutions for plant operators that wish to switch their production to alternative fuels with a minimum of effort and expense. The system is closed from the silo to the kiln and is designed in such a way that it is resistant to pressure surges. The patented rotor weighfeeders utilise the functional principle of horizontally positioned rotary valve feeders (type DRW, URW, SRW) for pulverised secondary fuels, or rotors designed for fibrous, flaky, pelletised and fragmented secondary fuels (type TRW). The simple and closed design enables direct silo discharge, weighing, dosing and direct feeding of the bulk material using a single device.³

As a general rule, it is prudent for cement manufacturers to use fuels that are available in their local area. This reduces the CO₂ emissions produced when transporting the material.

Cementos Molins is a cement manufacturer in Sant Vicenç dels Horts near Barcelona. The company operates a kiln around the clock, which is fired using a main burner and a precalciner. Both are fed with petroleum coke and around 40% alternative fuels. The company uses various FLSmidth PFISTER^® rotor weighfeeders in order to gravimetrically dose solid fuels continuously. Three different dosing stations were installed between 2011 and 2014. The patented predictive dosing control (ProsCon^®) feeds the fuels precisely and controls the burning process in a stable manner, resulting in lower CO₂ emissions while at the same time saving costs. Another advantage is that a single system can be used for all fuels. The company is planning to use more alternative fuels in the future.

PFISTER^® Alternative Fuel Installation at Cementos Molins.

What to do with the CO₂?

A key technology in this scenario is power-to-X, with which both synthetic gas and liquid fuels can be manufactured. Alternative energy sources can be used as seasonal storage in electricity or transport applications, such as in heavy-duty transport or in shipping and aviation. One significant benefit is that the infrastructure which is in place to transport and fill fossil fuels can continue to be used for the synthetic gases and fuels. As research projects have shown, these synthetic fuels can also be admixed to fossil fuels in almost any ratio, thus contributing to the quick reduction of greenhouse gases. VDMA views power-to-X as a guarantor of the energy transition. As CO₂ is required to convert hydrogen into alternative energy sources, cement plants could therefore contribute to the reliable and permanent generation of energy.

Becoming active

The work by VDMA and its member companies makes a significant contribution towards achieving the goals set out in the Paris Agreement, while simultaneously safeguarding Germany’s standing as an industrial location, its jobs, its leading technological position and its social cohesion and prosperity. This is a task which requires all involved parties to act. Companies that wish to contribute are invited to join VDMA and take on an active role here.

References

1. The roadmap is the result of a collaborative effort between the International Energy Agency (IEA) and the World Business Council on Sustainable Development (WBCSD) Cement Sustainability Initiative (CSI).

2. IEA (2018). Technology Roadmap: Low-Carbon Transition in the Cement industry. All rights reserved. S.22

3. HÄFNER, H. W., KUDORFER, G. A., ‘Dosierte Aufgabe von Sekundärbrennstoffen für den Klinkerbrennprozess’, Augsburg/Germany; (Volume 53) No. 4/2000 – ZKG INTERNATIONAL, S.206-207

4. FANTINI, M., ‘Clean and green’, CLEANKER, Piacenza/Italy; World Cement no. 09/2019, pp 66 – 70.

Read the article online at: https://www.worldcement.com/special-reports/22042020/making-a-virtue-out-of-necessity/