Kinetic parameters study for the slow pyrolysis of coffee residues based on thermogravimetric analysis

Thermal decomposition of coffee husks was investigated by thermogravimetric analyses. The proximate, ultimate and composition analyses were performed. Thermogravimetric tests were realized, the material was heated to 1173 K using five heating rates: 5, 10, 15, 20, 25 K min-1. The kinetic parameters were estimated using the methods of Kissinger-Akahira-Sunose and Friedman, the distributed activation energy model and the independent parallel reactions model. The isoconversional models of Kissinger-Akahira-Sunose and Friedman showed the dependence between determined values of activation energy and mass conversion, the activation energy values varied from 1437.39 to 199.22 kJ mol-1 for Kissinger-Akahira-Sunose and from 127.81 to 230.35 kJ mol-1 for Friedman model. The values of activation energy were determined for Miura-Maki method; varying from 137.39 to 199.22 kJ mol-1. The model of parallel and independent reactions showed the presence of six different reactions (with activation energy values varying from 42.0 to 214.2 kJ mol-1) occurring during coffee husks pyrolysis, indicating a complex reaction. Currently, works regarding the determination of kinetic parameters for coffee husks pyrolysis are not common. The present work is the first report using the model of parallel and independent reactions to estimate kinetic parameters for pyrolysis of coffee husks, a residue widely generated worldwide.

本研究通过热重分析法（thermogravimetric analyses）探究咖啡果壳的热分解过程，并完成了工业分析（proximate analysis）、元素分析（ultimate analysis）与组分组成分析（composition analysis）。开展了热重测试实验，以5、10、15、20、25 K·min⁻¹的五种升温速率将样品加热至1173 K。本研究采用Kissinger-Akahira-Sunose法、Friedman法、分布活化能模型（distributed activation energy model）以及独立平行反应模型（independent parallel reactions model）对动力学参数进行了估算。Kissinger-Akahira-Sunose法与Friedman法对应的等转化率模型（isoconversional models）表明，活化能测定值与质量转化率之间存在关联；其中Kissinger-Akahira-Sunose法得到的活化能取值范围为1437.39~199.22 kJ·mol⁻¹，Friedman法则为127.81~230.35 kJ·mol⁻¹。采用Miura-Maki法（Miura-Maki method）测定的活化能取值范围为137.39~199.22 kJ·mol⁻¹。独立平行反应模型显示，咖啡果壳热解过程中共存在6个不同的反应步骤，其活化能取值范围为42.0~214.2 kJ·mol⁻¹，表明该热解过程为复杂反应体系。目前，针对咖啡果壳热解动力学参数测定的相关研究较为稀缺。本研究首次采用独立平行反应模型，对全球广泛产生的农业废弃物——咖啡果壳的热解动力学参数进行估算。