Kinetic Study and Characterization of Sewage Sludge for Its Incineration

By Rodriguez, R.; Gauthier, D. et al. | Journal of Environmental Engineering and Science, May 2008 | Go to article overview

Kinetic Study and Characterization of Sewage Sludge for Its Incineration


Rodriguez, R., Gauthier, D., Udaquiola, S., Mazza, G., Martinez, O., Flamant, G., Journal of Environmental Engineering and Science


Abstract: The sewage sludge from San Juan, Argentina, was characterized in an attempt to improve the thermal treatment efficiency and to reduce the environmental impact of incineration. The ash content is about 50% dry basis, and its weight loss at 105 [degrees]C was high. Taking into account the moisture content, it required drying for an autothermal combustion, thus imposing an additional combustible for incineration. The sludge contains high concentrations of several trace elements, and all but Hg are two to fivefold more concentrated in its ash. Thermogravimetric analyses were carried out on dry samples of sludge in inert and oxidative atmosphere. Three peaks can be observed in all differential thermogravimetric curves during the organic matter decomposition. The pyrolysis curve is over the combustion curve and parallel to it up to high temperatures. A kinetic model of the sludge weight loss during its incineration, based on these studies and combining the multi-step pyrolysis and combustion processes, is proposed and discussed.

Key words: sewage sludge, incineration, thermal analysis, pseudo-mechanistic models.

Resume : Les bones d'epuration de San Juan, en Argentine, ont ete caracterisees afin d'ameliorer l'efficacite du traitement thermique et de reduire l'impact environnemental de l'incineration. Les cendres contiennent environ 50 % d'humidite sur sec et sa perte de poids a 105 [degre]C etait elevee. En tenant compte du taux d'humidite, ces bones doivent etre sechees pour une combustion adiabatique, exigeant ainsi l'usage de combustible additionnel pour l'incineration. Les bones contiennent de fortes concentrations de plusieurs elements traces, et tons, sauf le Hg, sont de deux a cinq fois plus concentres dans les cendres. Les analyses thermogravimetriques, sous atmosphere inerte et oxydante, ont ete effectuees sur des echantillons secs de bones. Durant la decomposition, toutes les courbes thermogravimetriques presentaient trois pics de la matiere organique. La courbe de pyrolyse se situe au-dessus de la courbe de combustion et elle y est parallele jusqu'a des temperatures elevees. Nous proposons et analysons un modele cinetique de la perte de poids des bones durant leur incineration, base sur ces etudes et combinant la pyrolyse a etapes successives et les procedes de combustion.

Mots-cles : bones d'epuration, incineration, analyse thermale, modeles pseudo mecanistes.

[Traduit par la Redaction]

Introduction

The treatment of municipal sewage sludge is becoming a concern because its production has noticeably increased and is expected to keep increasing. This increase is due to both world population growth and the development of wastewater treatment, which has become a necessity (Gomez-Rico et al. 2005). Most municipalities in Argentina are currently adopting land filling and biocomposting as main alternatives for the final disposition of sewage sludge, but these conventional options are being progressively restricted because sewage sludge contains different toxic substances (both organic and inorganic). The current trend indicates an increasing interest in thermal processes, particularly incineration, due to its many advantages: reduction of volume, destruction of organic and pathogenic micropollutants, and energy production (Thipkhunthod et al. 2005). While producing thermal energy, incineration also generates gases and solid residues (powders, ash). Heavy metals (HM) are the main pollutants in the solid residues and their concentration and leachability determines the final disposal (Yao et al. 2004). The gas pollutants are nitrogen and sulphur oxides, highly toxic organic compounds such as dioxins and furans, several halogenic acids and also several vaporized heavy metals. The sludge composition varies greatly from one place to another. For instance, the heavy metals' concentration depends strongly on the local industrial activity, which has a tremendous effect on the potential hazard of the incineration residues. …

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