Study of thermal conductivity in organic solid wastes before composting
Résumé
In France, like in all developed countries, the amount of solid wastes generated per year has increased continuously since the 1960’s. To hold back this trend, waste policies have been set up, as illustrated by current EU waste policy and its five main priorities: prevention, reuse, recycling, recovery and disposal. Composting can be defined as the process whereby aerobic micro-organisms convert organic substrates into compost: a hygienic, biostable product that can be beneficially applied to land (Haug, 1993; Mohee & Mudhoo, 2005). Therefore, it fits perfectly with the fourth priority, recovery, which fosters extraction of useful material or energy from wastes. In this context, interest towards composting has increased continuously during the last few years. During treatment, micro-organisms breakdown organic matter and produce carbon dioxide, water and heat. Heat generated by biological activity modifies moisture content and temperature conditions. These changes result in the appearance of a temperature peak in first days of treatment and most pathogens are killed by the high temperatures reached (around 70-80°C), turning waste into a hygienic product. Among the various physical parameters taking part in the composting process, thermal conductivity seems to be of major importance, and could be used as an indicator to follow heat transfers within the organic matrix. However, as all physical parameters involved in the process, the initial preparation of the substrate (adjustment of moisture content or C/N ratio, addition and mixing with bulking agent …) has an influence on the physical parameters involved in the process (such as bulk density, Free Air Space or air permeability) and thus, on thermal conductivity. Moreover, difficulties often occur in composting experiments because the effects of compaction on physical properties are ignored, or information about these effects is lacking. As soon as the pile of waste is built, the settlement of the composting matrix begins. This settlement, called primary settlement or physical compressive settlement (Gourc et al., 2010; Yue et al., 2008) is related to the vertical load and leads to compaction. Despite its major importance, until now little has been written on thermal conductivity in composting, in particular about its link with compaction. That’s why this study focus on it and aims to investigate how it evolves with (i) compaction or depth within the pile of waste, (ii) preparation parameters of the substrate and (iii) temperature. This investigation was carried out on mixtures of urban sludge and wooden palettes used as bulking agents. To understand how preparation parameters of the mixtures would affect thermal conductivity, two moisture content (50 and 65%), two types (fresh and recycled) and two meshes of bulking agent (< and > 20 mm) were tested. The influence of compaction (or depth) was evaluated in two steps. First, a “Schaub-Szabo” device (strongly inspired by the apparatus described in Schaub-Szabo and Leonard (Schaub-Szabo & Leonard, 1999)) was used to get depth-bulk density profiles in the different substrates. Then, these bulk densities were re-created in a modified air pycnometer where thermal conductivity was measured with a thermal probe directly embedded in the composting sample. Therefore, a link between thermal conductivity and compaction (or depth) could be established. On the other hand, the study of the impact of temperature on thermal conductivity was carried out in 10 liter cells where biological activity was prevented by a nitrogen atmosphere. The cells were filled with the same eight sludge-wooden palettes mixtures as before, and a thermal probe was put directly inside each sample. Then, they were put in different constant-temperature baths with target temperature from 5 to 75°C. In this study, thermal conductivity increased with depth and a statistical analysis highlighted the fact that it was only significantly impacted by moisture content (among the three preparation parameters cited above). Moreover, the impact of temperature on thermal conductivity was clear and a linear relationship between these two parameters could be established. Each correlation was specific to the substrate but with a similar slope. These results are interesting in two ways: first, until now little has been written on thermal conductivity in organic solid wastes, and in particular about its link with compaction. Besides this originality, the data obtained can now be used in numerical modeling to get a more thorough and accurate way to model heat transfers, an essential part of modeling composting systems.
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