Synergetic effect on the combustion of lignite blended with humus: Thermochemical characterization and kinetics

Highlights: • Co-combustion behavior of humic matter and lignite was explored. • A novel synergy index was developed to explore the degree of synergetic effect. • The catalytic effect and non-catalytic effect were distinguished by synergy index. • The best kinetic models for combustion were determined by Master-plots method. -- Abstract: The co-combustion of lignite with humus was investigated as an effective way to mitigate carbon emission. The thermal characteristics of the co-combustion were successfully evaluated, which was the prerequisite for predicting synergy and blending optimization of this process. A novel synergy index (SI) was proposed to quantify the synergetic effect and differentiate the effect of minerals and organics in humus during co-combustion. The non-catalytic synergy of organics (SI = 1.76) was stronger than the catalytic synergy of minerals (SI = 1.55) for the 20 wt% humus blend. However, catalytic synergy was more dominant at high humus blending ratios. The kinetic parameters were estimated through two model-free methods. The activation energy of the blend by Kissinger-Akahira-Sunose and Flynn-Wall-Ozawa methods was 104.99 and 110.71 kJ/mol, respectively. The more precise kinetic model of the blend was determined by master-plots method. This study also demonstrated that humus could be used as a potential fuel in co-combustion system.