Slow Pyrolysis of the Sewage Sludge With Additives: Calcium Oxide and Lignite

Sewage sludge is a waste from the water treatment installations. It is used in agriculture. However, due to various environmental restrictions, not all of the sewage sludge can be utilized within that sector. Using this resource as a sustainable energy source might be an interesting alternative to the landfilling. Some of the fuel-related properties of sewage sludge make it difficult to be used as a fuel without preprocessing. Torrefaction is a promising pretreatment technique that could prove itself suitable to be used for improving sewage sludge. Additives might be used for obtaining some further improvements, either during the torrefaction stage or further at the final energy conversion stage (combustion, gasification, etc.). This paper presents the results of torrefaction experiments performed with sewage sludge from the local water treatment facility. Torrefaction was performed with laboratory-scale rotary reactor at three different temperatures (250 °C, 275 °C, and 300 °C). Cotorrefaction of sewage sludge with lignite was also performed. Torrefaction tests with quicklime (CaO) as an additive were also performed. Fuel-related properties of products of torrefaction and feedstock were determined. By-product of torrefaction, called torgas, was also a subject of the analysis. Propensity of the torrefied product to absorb moisture was assessed. Thermogravimetric analysis (TGA) of raw and torrefied samples was performed in order to compare the behavior of raw and torrefied materials during subsequent pyrolysis.