Influence of Liming on Kinetics of Sewage Sludge Pyrolysis

Abstract

Thermogravimetry (TG) is the fast and reliable method for characterization of thermal decomposition of any material and in particular to determine the kinetics of pyrolytic decomposition of sewage sludge. Two types of sewage sludge with and without addition of lime were investigated from kinetic point of view. For TG analysis samples of selected sewage sludge were heated under the inert atmosphere of argon with constant heating rate from 303 to 1273 K; the three heating rate β = 5, 10 and 20 K/min were chosen. The iso-conversion methods of Friedman and Ozawa-Flynn-Wall were employed for analysis of TG results. As the sewage sludge decomposition is very complex process it cannot be described by a simple stoichiometric equation, therefore the so called lumping of reactions in the selected temperature ranges were used with detailed principles arising from visual analysis of DTG curve. The deconvolution of DTG curves performed according to Fraser-Suzuki asymmetric profile allowed the identification of number of lumps and their contribution to the overall mass loss. So the decomposition of sewage sludge with lime addition could be described with five groups of reactions while the one without lime by means of six lumps. The thermal decomposition of sewage sludge was assumed to proceed according to the scheme of parallel concurrent independent reactions of n-th order. The values of the apparent activation energies at different constant values of conversion degrees were determined by the iso-conversion analysis. To estimate the kinetic parameters the non-linear regression with Levenberg-Marquart optimization procedure was used. The kinetic parameters such as activation energy, pre-exponential factor, reaction order and fraction of total mass loss associated with a given reaction were determined. The impact of sewage sludge liming revealed in essential differences of pyrolysis products and pyrolysis kinetics of limed sludge and without lime one was highlighted.

Thermogravimetric Kinetics of Selected Energy Crops Pyrolysis

“…Both Friedmann's and Ozawa-Flynn-Wall's analyses are very insensitive for high conversion degrees [44]. Figures 3 and 4 indicate that the values of activation energy and pre-exponential factors for conversion degree α ≥ 0.8 were much higher than those for the lower values of the conversion degree; therefore, they were excluded from further analysis.…”

“…Those two methods are useful tools in seeing if the process proceeds via single or multiple reactions. If the values of activation energy and pre-exponential factor obtained by these methods are not constant as a function of conversion degree in its entire range, this means that the process should be described by multiple reactions [44]. In order to determine kinetic parameters by the isoconversional method, the assumption of the known reaction model f (α) is necessary.…”

“…Apparent activation energy E a is obtained from the slope of the curve in plot ln(β i ) versus 1/T α,i [21,44]. Subscripts α and i in Equations ( 5) and ( 6) stand for the given values of conversion and heating rate, respectively.…”

“…In order to estimate the kinetic parameters of biomass pyrolysis under nonisothermal conditions, a minimum of three heating rates are required [44]. In this study, for each energy crop, three heating rates of β = 5, 10, and 20 • C/min were applied.…”

Co-Pyrolysis of Beet Pulp and Defecation Lime in TG-MS System

“…This process was investigated in the thermobalance [20] and in the fixed bed reactor [21]. The effect of the CaCO 3 addition on the co-pyrolysis process was studied with the sewage sludge [22,23] and biomass pellets [24]. Calcium carbonate only slightly influenced the decomposition rate of biomass pellets at temperatures below 400 • C [24].…”