Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Reduction of CO2 emissions by use of the ZEWA process

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).

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