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Deliverables
An evaluation of potential reduction of CO2 emissions through the implementation of the ZEWA process has been performed in the frame of the project. Relevant partners were involved: CRM as project co-ordinator, ARCELOR as steelmaker, VAI as ZEWA data supplier and LAFARGE as slag user in cement plants.
The CO2 balance of the process can be calculated in different ways. First, an overall balance can be estimated for the combined steel mill and cement works system, considering all emissions and savings linked to the ZEWA process and the recycling of its products in the steel and cement plants. However, this overall balance is not relevant to CO2 allocations and CO2-trading scheme for steel making and cement making companies. Therefore, separate calculations have also been performed.
We considered for calculations that a ZEWA plant should preferably be installed inside a steel plant (for limiting transportation of by-products and taking advantage of liquid slag charging, if any). Process CO2 emissions are direct emissions from the metallurgical process (use of C-reductants, to the release of volatiles and to the consumption of graphite electrodes) and indirect emissions caused by the production of the necessary electricity, the transportation of input materials and products, the production of refractory materials and electrodes, etc. On the other hand, CO2 savings arise from the use of slag in cement kilns (substitute for clinker) and the recycling of hot metal at the converter to replace virgin hot metal from the blast furnace.
Detailed calculations (with necessary assumptions, scenarios and simplifications) show that a typical overall CO2 balance is -1 t CO2 emitted per tonne of steelmaking slag processed, with values ranging from 0 down to -1.8 t CO2 per tonne of input slag, depending on scenarios.
For the steel plant only, a typical saving of about 1.15 t CO2 per tonne of produced hot metal is obtained, provided that the blast furnace throughput is reduced by the recovered hot metal amount (i.e. no increase in the steel production of the plant). In the CO2 allocations and trading scheme, such a reduction in emissions directly contributes to the economic viability of the process by the value of the saved emission certificates.
It is worthy to note that the indirect CO2 emissions linked to the production of the electrical power consumed in the ZEWA process have not been considered in this steel plant calculation. These emissions have indeed to be covered by the power company, as per European emission certificate rules.
For the cement plant only, clinker substitution by blast furnace slag saves about 0.72 t CO2 per tonne of slag. The various ZEWA slags produced during pilot plant campaigns have been fully characterised by LAFARGE. Regarding hydraulic properties, some were shown to be better than a reference blast furnace slag, and some to be worse. Because product consistency (regularity with time) could not be demonstrated during pilot test campaigns, the exact substitution coefficient of ZEWA slag cannot be given. It can however be stated that the reduction of CO2 emissions at the cement plant would be in the range 0.55 to 0.8 t CO2 per tonne of ZEWA slag used, depending on slag average quality.
Considering that the investment for a slag granulator is high and that the benefit of recycling granulated slag is gained by the cement producer only (savings in CO2 certificates), the ZEWA plant operator (i.e. the steel maker or a sub-contracting company operating the ZEWA plant inside the steel shop) should be willing either to share this benefit with the cement producer (through higher price given for the granulated slag) or to ask the cement company to invest in the slag granulator (joint venture).
The study of the availability of complementary materials and by-products as input materials for the ZEWA process gives an overview of considered materials (mainly acidic by-products from other activity sectors), including availability and economic aspects (mainly cost).
Fly ash is considered as the main acidic input material, with the advantages of availability in large quantities at various locations, of generally poor valorisation. An additional benefit is found in the use of fly ash qualities with high carbon contents, which are unusable for cement production but beneficial for the ZEWA process.
Lafarge provided a detailed information on typical fly ash qualities in several European countries were Lafarge is present (Austria, Spain, Turkey, etc), at various locations (power stations using different coal qualities), including variability with time. ICPC also supplied information, mainly for Eastern European countries. This contributed to assess the feasibility of the process (case studies for process implementation) and to evaluate the interest of the other acidic alternatives (silica carriers). These alternatives include the processed scrap residues (see the first project result), processed shredder residues, bottom incinerator ash and various sands (including foundry sand). Processed shredder residue stands for the mineral fraction of the light residue from car dismantling supplied by Cometsambre. This material does not justify erecting a dedicated ZEWA plant, but could be supplied for processing in any such plant erected in reasonable proximity. Other materials (bottom incinerator ash, foundry sands, etc) can also be used as silica-carriers, depending on local availability and costs.
This study also included other secondary complementary materials such as alumina dross (used as alternative strong reductant), ferrous bauxite (used when necessary to increase the alumina content of the produced slag).
The evaluation of needs / potentialities for processing by-products from the steel industry through the ZEWA process integrates an overview of concerned by-products, including annual production figures, economic aspects (cost for land filling and/or processing by alternative routes) and takes into account local and European regulations (current and future).
The ZEWA process features various charging systems: deep pneumatic injection for dried powdery materials, top chute for coarse materials and a dedicated pouring system for liquid slags. From a technical point of view, a great variety of by-products and materials can thus be charged. This feature was demonstrated during test campaigns were various fine materials (including several types of dusts), sludges, solid slags and scales were processed.
Taking now into account localisation, regulation and economical aspects, first potential sites for process implementation have been selected (C-steel and stainless steel plants), and several processing scenarios have been considered taking into account site constraints.
Former project partners USINOR, ALZ and Profilarbed have been integrated during the course of the project into the ARCELOR group, which is a major player in steel making with an annual production of about 45 Mt of steel.
Regarding the integrated C-steel production route (former partner USINOR), various processing options are under development for slags (especially converter slag), but huge stockpiles and excess production remains to be tackled: current situation is non-sustainable. Regarding the various dusts and sludges, current internal recycling will have to be replaced to a large extent with external processing. Several processes are under development. The ZEWA process allows processing all by-products in only one installation. One drawback is the high P content of the recovered hot metal, which must be re-processed before recycling to the converter. Detailed scenarios have been produced for ZEWA implementation in a Belgian plant (SIDMAR, located in Ghent), including converter slag, various dusts, sludges and scales. Other locations are also considered. Comparison with other alternative processing routes is under way.
For the stainless steel production route (former partner ALZ), the industrial context has profoundly evolved during the course of the project. A plant in France was shut down (Lardoise), another one will be closed (Isbergues), while the capacity of ALZ steel plant (located in Genk, Belgium) has been doubled (with an additional change in the process itself), and a new plant is under construction in Charleroi, Belgium. This led company management to re-consider the by-products problem as a whole. Regarding slags, possible applications (re-use as secondary raw material) are largely dependent on specific regional legislation and, even when allowed, the market is uncertain. As a consequence, stockpiles accumulate and the situation is non-sustainable. For dusts, only one external treatment facility exists in Europe, resulting in high treatment costs and uncertainty for the future. Moreover, hot rolling slam is less and less recyclable due to the increase of its chromium content. Detailed scenario calculations have been performed for the implementation of the ZEWA process in Belgian plants (Genk and Charleroi) with very good economic feasibility when processing slags, dusts, sludges and scales (slam). The ZEWA process is generally seen by stainless steel producers as a very good solution for turning their slags into a sellable product and recovering valuable metals (Cr, Ni, Mo). Therefore, a new project was started within ARCELOR - UGINE & ALZ which aims at re-defining by-products policy. This project notably includes a benchmark review of existing valorisation possibilities and treatment processes. This project started at the beginning of 2005. A meeting is notably planned in June 2005 with VAI representatives for examining the ZEWA alternative. Results / conclusions are expected for the end of the year and decisions for future investment(s) should take place in 2006.
For the EAF C-steel production route (former partner ProfilARBED), the processing of main by-products (slags and dusts) was not considered in the project because of rather low technical and economical incentives. The considered by-product (processed scrap residue, a.k.a. silica-containing aggregates) does not justify erecting a dedicated ZEWA plant, but could be supplied for processing in any such plant erected in reasonable proximity.
Similar studies should be set-up in the future for CORUS plants, first through CRM, then with VAI.
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