Slow-rate devolatilization of municipal sewage sludge and texture of residual solids

Ash-rich sludge samples originating in four large plants were analyzed and employed to explore primarily the kinetics and the chemistry of devolatilization. A gravimetric, slowly increasing-temperature method was used in the range 298–1,123 K in a milieu of nitrogen. As an intricate combination of numerous (bio)organic and inorganic compounds, the dry sludge commences devolatilizing at approximately 418 K. The bulk of organic matter is released up to 823 K, at the rate becoming very slow thereafter. Basic constituents of the product gas are CO2, CO, H2, and CH4 with undesired nitrogenous, sulfurous, and chloro compounds. The residual solids contain significant amounts of organic matter/carbon and, on account of their favorable textural characteristics, they can be viewed as promising sorbents or catalysts. Kinetic triad was inferred from the experimental data: the model is well-capable of simulating the process of devolatilization and can be used for design considerations. An explicit equation, based upon a tractable approximation to the temperature integral (for [E/(RT)]≥0.1), has been verified and proposed for predicting the maximum reaction rate temperature. Remarkable differences in thermal behavior were explored in detail between the sludge and the alkali bicarbonates.