Experimental and modeling investigation on pyrolysis of agricultural biomass residues: Khat stem and coffee husk for bio-oil application

Khat stem and coffee husk are commonly available agricultural residues in Ethiopia and pose serious environmental hazards through the eutrophication of water streams and greenhouse gas emissions. To alleviate these impacts, thermochemical conversion of these residues through fast pyrolysis can be employed. The samples are characterized for their composition, experimental investigation has been carried out using thermogravimetric analysis at different heating rates (10, 20, 30, 40, and 50 °C/min) in the temperature range 30–800 °C under an inert atmosphere to scrutinize the decomposition characteristics. Estimation of pyrolysis kinetic parameters for these materials, which are not widely available in the literature, has been done using different model-free approaches: Kissinger, Flynn–Wall–Ozawa, and Kissinger–Akahira–Sunose. Empirical models were developed and employed for comparison of experimental data and error analysis. Validation of experimental data has been done and evolved pyrolysis product yields are predicted using the CRECK-S-B model. The effect of biomass characterization parameters and heating rate are examined. From the results, it is possible to perceive that the Kissinger, single first order, and 4-component models can predict with good accuracy but can not predict the residual char evolution that occurs at higher temperatures (> 450 °C) and the derivative of thermogravimetric shoulders, correspondent to the hemicellulose decomposition. The CRECK-S-B model revealed good agreement with the experimental data for both samples, while being capable of capturing the hemicellulose shoulder and the residual decomposition of char at higher temperatures. The yields and composition of gas, bio-oil and char fractions are estimated, for which the biomass characterization and heating rates have influenced the distribution of the products. This study reveals the potential of these residues to promote local sustainable economies and also gives a possible solution to the environmental issue caused by their disposal.