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Calliandra calothyrsus and Gliricidia sepium ash characteristics as co-firing fuel to high-sulfur and -iron coal were investigated through theoretical prediction, FactSage modelling, and combustion experiment in drop tube furnace. The results show that the addition of Calliandra and Gliricidia were able to mitigate the slagging risk according to theoretical prediction and FactSage modelling. Slag formations were predicted to occur at higher combustion temperature compared to coal which was reflected in higher ash fusion temperature. The results of combustion experiment showed an agreement with less ash deposition and cleaner probe surface. The addition of Calliandra and Gliricidia increased the high-melting minerals and decreased Fe-based minerals. However, domination of Ca 2 SO 4 in addition of 25 wt% Gliricidia can be detrimental with presence of strongly adhered ash and material degradation. This research provides an important insight of Calliandra and Gliricidia utilization as co-firing fuel that improve the ash characteristics of high-sulfur and -iron coal., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Calliandra calothyrsus and Gliricidia sepium are developed as wood energy forest plantations in degraded land. They have good survivability, are fast to be harvested, and have good calorific value to be utilized as biomass fuel. This study aims to investigate their combustion characteristics by thermogravimetric analysis and their ash-related issues by combustion experiments in drop tube furnace and ash observation. The results show that G. sepium has a better combustion performance with higher values of ignition index, comprehensive combustion index, and flammability index, while C. calothyrsus shows better ash-related issues with less risk in theoretical slagging prediction, clearer metal surface in the fouling area, and less ash deposit formation. The domination of K, Ca, and S elements in the ash deposits of both biomasses results in several sticky aggregate particles and low-melting minerals. This study provides an in-depth understanding of C. calothyrsus and G. sepium combustion for broader utilization., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)