Use of Mixed Plastic Waste in Blast Furnaces

As indicated, pig iron production requires input of a reducing agent. Stahlwerke Bremen uses plastic waste as a substitute for fuel oil. Plastics are injected into the blast furnace in a similar way to coal powder or fuel oil. In order to remove fibres and metal particles a separation takes place. Large particles are separated via a screen of 18 mm. The smaller plastic waste particles ( 18 mm) go to the injection vessel. There, an injection pressure of about 0.5 MPa is built up. Via a pneumatic process the plastics can be dosed and discharged into the blast furnace. The bulk density of the plastics has to be 0.3 tonnes/m.  

As indicated, the reliability of the process is crucial and hence the input has to be controlled. MPW has the advantage that it contains relatively low amounts of sulfur, but the chlorine content has to be limited. 

Concern has been expressed about the possible formation of dioxins and furans. However, measurements during experiments indicated that the emissions of dioxins and furans were not significantly elevated. Dioxin emissions with or without plastic input appeared to be about a factor of 100 below the standard of 0.1 ng/Nm TEQ TCCD (toxicity equivalent in relation to the toxic dioxin TCCD) (a.7). This might be due to the benefit of the strongly reducing atmosphere and the high temperature of 2100 °C. In total, until now the conclusion has been that at current PVC levels in MSW, pretreatment for chlorine removal is unnecessary. 

However, the PVC throughput in the blast furnace kiln is just a fraction of the total material throughput. This is comparable to MSWIs, where PVC in general forms less than 1% of the input. Under such circumstances, the relation between PVC input and dioxin formation appears quite difficult to asses. Most research reports claim that there is no clear relation (e.g., a.8, a.9). However, Greenpeace has published a number of reports that suggest otherwise (e.g., a.l0). Eurthermore, it has to be noted that the off-gas of blast furnaces is generally used as an energy carrier in other processes. Checks on dioxin formation are desirable there as well. 

On top of this, PVC is by no means the only chlorine source. Other raw materials and (particularly for blast furnaces close to the sea) even the air used in incineration processes may have significant contributions to the chlorine throughput too. 

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