Comparison of the Slow Pyrolysis Behavior and Kinetics of Coal, Wood and Algae at High Heating Rates
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Original Paper
Comparison of the Slow Pyrolysis Behavior and Kinetics of Coal, Wood and Algae at High Heating Rates Bothwell Nyoni ,1,3 Sifundo Duma,2 Shaka V. Shabangu,2 and Shanganyane P. Hlangothi1 Received 22 August 2019; accepted 29 April 2020
Coal, like any other nonrenewable energy resource, will, by exploitation, deplete in the near future. In light of this, biomass is fast becoming an alternative material that can be used in the existing coal-to-energy conversion processes like combustion, gasification and pyrolysis. Specifically, woody and algal biomasses have a huge potential in pyrolysis processes because of their renewability and high volatile matter content. A thermogravimetric and differential scanning calorimetry study for the comparison of the slow pyrolysis behavior of wood and algal biomass with that of coal is presented in this work. At the investigated heating rates, the three materials show a significant change in pyrolysis behavior when the heating rate is increased fivefold or more. It is shown via mass loss curves that the primary pyrolysis of the three materials is characterized by multi-peak and single-peak decompositions at low and high heating rates, respectively. Wood and algae generally display high instantaneous reactivity, due to the decomposition of cellulose and protein, respectively. Heat flow curves reveal that at low heating rates (10 °C/min) heat must be constantly supplied in order to sustain the pyrolysis processes for wood and algae except for coal. At high heating rates (50 °C/min), the pyrolysis of the three materials is dominated by exothermic events, which greatly reduce the magnitude of external heating. Kinetic results based on the Coats–Redfern integral method and corroborated using the distributed activation energy model method reveal that the primary pyrolysis stage of coal at high heating rates is a multistep reaction with an activation energy range of 399.3–420.8 kJ/mol. Furthermore, results show that decomposition of wood and algae follows second- and first-order kinetics with activation energy range of 148.4–204.9 and 108.9–130.3 kJ/mol, respectively. KEY WORDS: Algae, Coal, Heat flow, Pyrolysis, Thermogravimetry, Wood.
INTRODUCTION Pyrolysis is a process that was developed for the devolatilization of coal, oil shale and other carbonaceous materials through destructive distillation 1
Department of Chemistry, Nelson Mandela University, P.O. Box 77000 Port Elizabeth 6031, South Africa. 2 Innoventon, Downstream Chemicals Technology Station, Nelson Mandela University, 1-12 Gomery Ave, Summerstrand, Port Elizabeth 6031, South Africa. 3 To whom correspondence should be addressed; e-mail: [email protected], [email protected]
to form solid chars, oily liquid and gaseous products (Karrick 1934). There are several technologies available for extracting energy from coal. Due to the negative environmental impact and the change in energy demand, the technologies have been further developed and improved in the course of the years. Coal is a major primary source of
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