Abstract
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.
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Abbreviations
- a:
-
coefficient in integral approximation given by Eq. (6) and in Table 4
- A:
-
frequency/pre-exponential factor, fitted parameter [s−1]
- A′:
-
mass fraction of inert inorganic matter in the moisture-free sample of digested sludge
- b:
-
coefficient in integral approximation given by Eq. (6) and in Table 4
- dX/dT:
-
rate of an nth order reaction in the linearly increasing-temperature mode=(dX/dτ)/β=k(T)(1−X)n/β [K−1]
- (dX/dT)p :
-
maximum (peak) rate of reaction corresponding to the inflection point of sigmoidal curve X vs T [K−1]
- e:
-
fractional porosity of solid=(ρHe-ρHg)/ρHe
- E:
-
apparent (effective) activation energy [J mol−1]
- E/R:
-
apparent activation energy term, fitted parameter [K]
- I(T):
-
approximation to the Arrhenius integral=(E/R) p(u) [K]
- k:
-
apparent (effective) reaction rate constant=A·exp(− u) [s−1]
- n:
-
apparent order of reaction, fitted parameter
- p(u):
-
dimensionless approximation function
- R:
-
ideal gas constant=8.31441 [J mol−1 K−1]
- T:
-
thermodynamic temperature [K]
- Td :
-
temperature at which the pressure of gaseous reaction product(s) is equal to 101.325 kPa [K]
- Tp :
-
peak temperature corresponding to the inflection point of sigmoidal curve X vs T [K]
- u:
-
normalized temperature=E/(RT)
- v:
-
relative amount of organic matter remaining in the sample= residual amount of organic matter at any moment of time/initial amount of organic matter=(w′- A′)/(1−A′)=1−Xt= A′y/[(1−A′)(1−y)]
- Vp :
-
pore volume=(1/ρHg)-(1{vn/}ρHe)=e/ρHg [cm3 g−1]
- wo :
-
initial mass of moisture-free sample [g]
- w(τ):
-
mass of the sample at any moment of time [g]
- w′:
-
relative mass of the sample at any moment of time = \({\rm{w}}(\tau)/{{\rm{w}}_0} = {{\rm{A}}^\prime} + (1 - {{\rm{A}}^\prime}){\rm{v}} = {{\rm{A}}^\prime}/(1 - {\rm{y}}) = 1 - (1 - {\rm{w}}_f^\prime){\rm{X}} = 1 - (1 - {{\rm{A}}^\prime}){{\rm{X}}_t}\)
- \({\rm{w}}_f^\prime \) :
-
relative mass of the sample at the end of the process=w (τf)/w0
- X:
-
apparent fractional conversion of organic matter from a solid state to a gaseous/vapor state=amount of organic matter released from the sample at any moment of time/amount of organic matter released at the end of the process = \((1 - {{\rm{w}}^\prime})/(1 - {\rm{w}}_f^\prime) = {{\rm{X}}_t}(1 - {{\rm{A}}^\prime})/(1 - {\rm{w}}_f^\prime)\)
- Xt :
-
true/overall fractional conversion of organic matter from a solid state to a gaseous (vapor) state=amount of organic matter released from the sample at any moment of time/initial total amount of organic matter in the sample = \((1 - {{\rm{w}}^\prime})/(1 - {{\rm{A}}^\prime}) = 1 - {\rm{v}} = (1 - {\rm{y}} - {{\rm{A}}^\prime})/[(1 - {{\rm{A}}^\prime})(1 - {\rm{y}})] = {\rm{X}}(1 - {\rm{w}}_f^\prime)/(1 - {{\rm{A}}^\prime})\)
- y:
-
mass fraction of organic matter remaining in the sample at any moment of time=(w′- A′)/w′=v(1−A′)/[A′+(1−A′)v]=(1−A′)(1−Xt)/[1−(1−A′) Xt]
- Y(T):
-
dimensionless integral temperature function = \(({\rm{A}}/\beta)\,\int_0^T {[\exp (- {\rm{u}})]} \,{\rm{dT}} \cong ({\rm{A}}/\beta){\rm{I}}({\rm{T}})\)
- Y(X):
-
dimensionless integral conversion function for an nth order reaction = \(\int_0^X {{\rm{dX}}/{{(1 - {\rm{X}})}^n} = [1 - {{(1 - {\rm{X}})}^{1 - n}}]/(1 - {\rm{n}})} \)
- β :
-
rate of heating [deg s−1]
- ρ He :
-
true solid density [g cm−3]
- ρ Hg :
-
apparent (particle) density [g cm−3]
- τ :
-
elapsed time of reaction/exposure [s]
- τ f :
-
final elapsed time of reaction [s]
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Acknowledgements
This research was supported by the Ministry of Education, Youth, and Sports of the Czech Republic under OP RDE grant No. CZ.02.1.01/0.0/0.0/16_019/0000753 “Research Center for Low-Carbon Energy Technologies” and by the Ministry of Agriculture of the Czech Republic in the 2017–2025 EARTH program under Grant No. QK 21020022 “Complex Evaluation of the Application of Sewage Sludge in Agriculture with Respect to Emerging Pollutants”. Mrs. Eva Fišerová is thanked for her aid with the manuscript.
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Hartman, M., Čech, B., Pohořelý, M. et al. Slow-rate devolatilization of municipal sewage sludge and texture of residual solids. Korean J. Chem. Eng. 38, 2072–2081 (2021). https://doi.org/10.1007/s11814-021-0847-8
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DOI: https://doi.org/10.1007/s11814-021-0847-8