Valorisation of plastic waste in the steel sector

AUTHORS NOTE

polynSPIRE is heading to the last period of the project with the aim to show real technology in pilot plants, but also recycle and reuse of plastics materials in real trials approaching industrial environments.

polynSPIRE’s work has continued to progress towards technical requirements for future technology development. Overcoming the difficulties inherent to new technological developments, selecting specific materials and the current complexity in the raw material flows have been the main challenges to face during the last stage of the project.

One of the innovation pillars where polynSPIRE project is focused is valorisation of plastic residues of low-grade plastic wastes by using them as carbon source in steel industry. To achieve this objective, our project partners CSM, FENO, HTT and I.blu have worked in developing this concept, studying and characterising the material properties to be used in a safety process and developing specific pneumatic injector to be assembled into an existing Electric Arc Furnace as a feeding system in the process of the steel foundry.

Industrial trials have been performed successfully, by achieving an initial rate of coal (fossil fuel) replacement of 30-50%. Thus, this feasible and promising technology will allow the steel sector to decarbonise some processes and even to reduce the costs of raw materials.

A complementary work after completion of the industrial trials consisted in the collection and processing of all process data via application of machine learning techniques. These techniques permit to fine tune and analyse the best process configuration considering several scenarios in terms of material waste and processed product used in and obtained from the Electric Arc Furnace.

Results are very positive, and we are very proud to bring new advances to the industry to facilitate a technological progress that cares for the environment and use of resources. However, the solution to the plastic future cannot be found by a single action, working on improving regulations and waste flows is crucial. Follow us to learn more about plastic recycling and reuse.

Injection system

LATEST DEVELOPMENTS
IN THE POLYNSPIRE PROJECT

Plastic utilisation for steel production from Electric Arc Furnace (EAF)

Plastics materials are used in a wide range of applications because of their properties, versatility, lightweight and price. Due to these advantages, plastic production has been growing continuously in the last years. The plastics demand is continuously increasing, so does the fossil sources demand, which implies a high dependency on the oil markets’ price fluctuations. Moreover, this extensive use leads to huge waste streams. These materials are normally heterogeneous materials such as composites or reinforced plastic materials (most of them having fibres, mineral fillers or other additives to improve performance) with complex structures to fulfil specific requirements. Because of these structures, the materials are difficult to re-process in an efficient way, while maintaining the quality requirements. The production of multilayer plastics has been especially increasing for packaging applications, the sorting of which is particularly challenging with available technologies. For these reasons, in their post-consumer phase, the streams often end up being incinerated or landfilled.

EAF uses fossil coal as a reducing agent, re-carburiser of the steel in order to promote slag foaming due to a reaction with oxygen and subsequent CO formation. The foaming improves the energetic and general environmental performances of the furnace. Thus, coal also acts as carbon supply for the steel liquid bath and for reduction of iron oxide, to obtain steel. In this framework, carbon bearing materials (as biomass, rubber and plastic) have been previously tested as fossil substitute. Plastic is an interesting material due to high heating value, high carbon content and large availability. Its hydrogen content also further contributes to its reducing function. Due to low-demanding application of the steel furnace, hard-to-recycle plastics waste, such as mixed multilayer plastics from packaging waste can be used for this purpose without previous sorting operations. Thus, these materials can be introduced in the EAF after specific pre-treatment and valorisation, and the activities carried out in Work Package 6 of polynSPIRE project are centred around the use of a Secondary Raw Material deriving from post-consumer plastic packaging waste (largely heterogeneous polyolefin-based multilayer streams).

The concept of slag foaming

Despite the name, Electric Arc Furnace uses a relevant amount of fossil (natural gas and coal) used in separate phases and for different purposes: first, to provide further energy for scrap melting and increase process efficiency and, once the scrap is melted into liquid steel, to provide foaming and reducing agents to the process and to increase the carbon content of the steel bath. In particular, pulverised coal is traditionally injected by purposely designed lances, and the EAF has the role to melt the steel scrap. Melting energy is provided by the electric arc between the graphite electrode and the metallic scrap and by the energy of the burners, fed with natural gas. Once obtained a melted steel bath, additional fossil materials are added to the process through the injection system to provide the elements necessary for the following oxides reduction and foaming reactions. The steel liquid bath is protected by a layer of liquid oxide phase addressed as a “slag”. The EAF’s injection system has the aim to inject the pulverised coal into the system steel/slag in order to:

• penetrate the slag layer
• promote the formation of the emulsion steel slag
• make the injected coal react efficiently with the iron oxide of the slag, forming gaseous species promoting slag foaming.

The foaming of the slag by CO/CO2 gas bubbles occurs naturally in the process by the oxidation of the carbon in the molten steel by oxides in the slag (reactions can be found below). This foaming process is intensified and maintained by the injection of coal into the slag. The injected carbon can directly react with the iron oxide according to reaction (2), or reduce the iron oxide indirectly by reaction (3) and (4) via an intermediate step of gasification.

(FeO) + [C] ® Fe + CO                                                                                 (1)

(FeO) + Cs ® Fe + CO                                                                                 (2)

(FeO) + (CO) ® Fe + CO2                                                                            (3)

Cs + CO2 ® 2CO                                                                                          (4)

The entire scheme of the foaming process can be seen below.

The formed foam submerges the electrodes and has several beneficial effects on the entire process:

• Reduces heat losses (by radiation) of the molten steel bath and consequently improves the process efficiency
• Lowers noise emissions
• Decreases NOx formation

The current injection systems are designed for pulverised coal, but since plastic is a new material for this type of application, specific activities were necessary in order to:

• Design a new injection system tailored for plastics
• Install the new injection systems
• Improve the plastic grain product for injection
• Long term industrial trials

Design of a new injection system tailored for plastics

Computational Fluid Modelling (CFD) has been developed in ANSYS-Fluent frame aimed at simulating new injectors’ flow behaviour for EAF metallurgical operations. They were designed specifically for plastic injection while maintaining the same metallurgical performance of the standard injectors.

Starting from industrial plant reference conditions, the basic new injectors’ design, and their performance has been evaluated in presence of separate injectors for particles and oxygen; and they have operated with a blend of plastic and fossil coal. The possibility of multi-fold injectors for coal and plastic was investigated in this work package of the polynSPIRE project, with the aim to achieve the best foaming behaviour. The fluid-dynamic quantities involved in the EAF bath level allowed to calculate, by semi-empirical correlations, the penetration of the gas jet and coal (and plastic) particle into slag and steel. A modelling operations  included simulation of different powder material flows, transport gas flows and secondary boosting air nozzle flows.

The CFD simulations results were assessed with the EAF steelmaking partner, and the performance of the new plastic injection systems has been compared to the reference coal case.

New developed injector and CFD simulation of plastic grains jet

Installation of the new injection systems

The injection and pneumatic system were designed and installed during the polynSPIRE project, as a prototype representing an industrial system. It is composed by following sections:

• Plastic grains charging station, directly from silo trucks
• Silos of 100 m3, equipped with vibro fluidification and screw extractor to avoid grains packing
• Weighting system
• Dispenser
• Injector; it has been purposely designed within this project and it is not a commercial component

Even if not originally foreseen, the system is equipped with ATEX components (protection from explosion), in order to guarantee the highest safety working conditions.

Below is an installation process of a new injection system on a site of polynSPIRE partner FENO in Italy.

Improvement of plastic grain product for injection

In order to be suitable for the injection, the above-described hard-to-recycle plastic packaging streams must be reprocessed into a secondary raw material, in compliance with the technical standard UNI 10667-17. The final product consists out of the plastic grains with good mechanical resistance to handling and charging, including a controlled grain size distribution.

Within the polynSPIRE project the technology of production of grains has been vastly improved. The most relevant interventions to achieve the desired goals were:

• The adjustment of the combination of grinding/milling/sieving grids with the objective of pushing down the grain size distribution curve and obtaining several technical and economic benefits.
• The further cleaning/selection of relevant mixed plastic streams through cutting-edge technologies and aspiration devices both at the stage of previous waste separation/sorting, and during the recycling site’s preliminary treatments, to improve the quality of the input stream.
• The monitoring, testing and modulation of the densification settings and material mixes, in order to produce an optimal densification degree in the material.
• The implementation of a well-structured monitoring plan to ensure compliance with the Italian technical standard UNI 10667-17:2021

Industrial testing phase

After the installation of the whole injection system and some preliminary trials, a first industrial testing has been carried out, ranging in the amount of injected plastics, up to 50% of replacement on the 1:1 rate. The largest number of trials were carried out with a replacement rate of about 30%.

Around two thousand tests were performed, while monitoring the energetic performances of the furnace and steel, including slag analysis. The whole process parameters have not showed any significant variations respecting standard practice.

The pneumatic and injection system worked as expected, without any significant problems. Finally, no significant changes have been observed in the final product, in the emission trends, nor in the slag.

EVENTS & POLYNSPIRE

The 2nd International Online Conference on Polymer Science—Polymers and Nanotechnology for Industry 4.0.

polynSPIRE project took part in the The 2nd International Online Conference on Polymer Science—Polymers and Nanotechnology for Industry 4.0., which took place between 1 – 15 November 2021. The conference established the Best Paper Awards to acknowledge the support of conference participants and to recognise their outstanding scientific accomplishments.

3th Annual Plastic Recycling Online Conference 2021

Our project was presented during the 3th Annual Plastic Recycling Online Conference, which took place on 1 December 2021. The event offered a meeting place for top manufacturers, researchers, academics, decision-makers and other business professionals who are keen to share their knowledge and experience in the most recent trends of plastics recycling.

1st International Workshop of BlackCycle project

polynSPIRE project took part in the 1st International Workshop of BlackCycle project | Black Cycle (blackcycle-project.eu) that was organized in France. The BlackCycle project is led by Michelin, the world’s leading French tyre manufacturer, and it aims to create a European value chain for the recycling of end-of-life tyres in order to produce highly technical secondary raw materials for the production of new tyre ranges. Our partner, Circe (Spain), presented polynSPIRE project and its results to the BlackCycle consortium and other stakeholders.

NEXT STEPS FOR POLYNSPIRE

• On March 25th, polynSPIRE consortium will organize next Steering Committee Meeting (online) where the partners will discuss the results and agree on the next steps in developing our innovative solutions.
• polynSPIRE partners are also planning new exploitation workshop, with an idea to have a face-to-face event in November, if the COVID-19 situation allows it. Participation of stakeholders/technology potential users is foreseen, so polynSPIRE team has started preparing the event already.
• CIRCE, one of polynSPIRE partners, is set to start polyurethane tests with proprietary mid-scale microwave reactor, including a validation of corresponding samples in Work Package 7 - Validation of recycled products.
• Participation of polynSPIRE partners in the technical standardization Committee, CEN/TC 249 Plastics, has been ongoing and it includes a contribution to any current and future standardization developments from the results of the project.

STAY TUNED!