Convective drying behavior of sawdust/sludge mixtures in a fixed bed

This work is part of a project aiming at developing a renewable fuel for gasification purposes, through the convective drying of sludge/wood mixtures. The first step consists in characterizing the drying behaviour of sludge/sawdust mixtures, in a convective fixed bed dryer. The influence of the mixing step (no mixing against 30 s at 40 rpm) and the sawdust/sludge ratio (1/9, 2/8, 3/7 and 4/6 on a dry basis) was investigated. X-ray tomography, a non-invasive imaging technique, was used to assess changes in volume and exchange surface during drying. The sawdust addition was shown to have a positive impact on the drying process from mass ratio of 2/8. Results showed that the volume and exchange surface increased with higher sawdust addition during the whole drying process. Moreover, a linear decrease of the exchange surface is observed with sludge, while a plateau is reached after a linear phase for sludge/sawdust mixtures. As expected, the drying rate of sludges or sludge/sawdust mixtures decreased nearly linearly with the exchange surface. For sawdust/sludge mixtures, the drying rate continued to decrease after the end of shrinkage, due to internal transfer limitations. These promising results open the way to new valorisation ways, especially for very pasty sludge.