The data were obtained in industrial testing with manufacturing batches of about 1,750 t of brick. The daily production capacity of the plant where the tests were performed is, depending on the format and characteristics of the brick manufac­tured, approximately 750 t/day. Such large test runs are necessary to assess with sufficient accuracy, considering the latency of the tunnel kiln, the effects of addi­tions or substitutions on the firing process, emissions into air, and the energy consumption.

The use of ashes as an addition to the brick feed must take into account that in the clay free calcium ions are available for a pozzolanic reaction with some compo­nents of the biomass ash. This reaction is not immediate but rather slow and certainly accelerated by temperature. This reaction can cause problems in extru­sion: stiffening associated with a loss of plasticity and hence denting of the extruded strand. It is hence necessary to hinder or delay this behavior either by adding the ashes, as done for polystyrene, for example, directly prior to extrusion or by hydrophobizing the ashes at least partially with a suitable agent. In brick making
the two most suitable agents are waste glycerine from the production of biodiesel and waste antifreeze from the car industry. So far, no industrial tests with waste antifreeze compounds have been carried out. The trials were carried out with glycerine-treated ash.

The fruit and the wood combustion ashes were mixed in a 30:70 ratio and then subjected to hydrophobization with waste glycerine. The ashes were added to the raw material feed prior to its storage in the silo for souring. The raw material characteristics are indicated in Table 9.2. The mix was made up of 20% by volume of ashes and 80% by volume of clay. The use of ashes resulted in a noticeable reduction of the density and hence substantial improvement of the thermal perfor­mance of the finished product, as summarized in Table 9.3.

The reduction in unit weight is tied to a lower setting density. In the tests carried out, a reduction of about 8% of fuel was observed. A certain percentage can certainly be attributed to the energy content, body fuel, of the biomass ash.

Extrusion data were collected automatically and show that the power require­ment of the extruder was lowered by 8%, whereas the density achieved is lower. The overall savings in production costs, considering that the ash is delivered free of charge to the brick plant, is estimated to be about 7.5%.

Reactive swelling was not observed, at least at the percentages used in the tests. The bricks manufactured with the addition of the biomass ashes are of good quality. The analytical data for the raw ashes used and the brick manufactured are given in Table 9.4. No significant changes to the leaching values were found between a standard brick and a brick formed from the addition of biomass combustion ashes.

9.2 Conclusion

When hydrophobized biomass ashes were not used, the extrusion power require­ments increased rapidly to a point where the tests had to be stopped because of excessive power requirement.

The results of the tests with hydrophobized ashes are as follows:

• Lower power requirement of the extruder at almost the same cutting frequency but at the same time a higher extrusion pressure

• Reduction of the water content of the brick by 2.5-3.0% (wt)

• Reduction of drying cracks

• Reduction of fired sulfate visible on the surface of the brick

• Substantial modifications to the firing curve

These results are positive. A problem, at least with the ashes used in the test, is that they are not delivered in a way that allows the brickyard to accept them, without any major and costly modifications to the plant. Most are delivered in big bags. This requires extra effort and expense on the part of the brickyard. If the ashes were delivered in trucks, an appropriate silo, similar to one used for coal or pet coke, could be installed. Another unresolved problem is the seasonality of the ashes. A storage bunker to overcome this problem usually cannot be accommodated in a brickyard. Once the producer of the ash has resolved these problems, a brickyard can certainly become an ideal recycler for this type of waste.