Catalytic effects of ion-exchangeable K⁺ and Ca²⁺ on rice husk pyrolysis behavior and its gas–liquid–solid product properties

The effects of ion-exchangeable K⁺ and Ca²⁺ on rice husk pyrolysis, with a focus on the yields and properties of the gas–liquid–solid products, and their pyrolysis kinetic characteristics, were investigated by thermogravimetry/Fourier-transform infrared spectroscopy and a laboratory-scale fixed-bed reactor. The results indicated that the pyrolysis gas and char yields increased, and the tar yield decreased, with increasing concentrations of K⁺/Ca²⁺ in the rice husks. Compared with that of 340 °C for H-form rice husks, the maximum weight loss temperature decreased by more than 25 °C for the 2.0 wt% K-loaded sample, and increased to 360 °C for the 0.2 wt% Ca-loaded one. K⁺ (2.0 wt%) lowered the first-order activation energy by 18 kJ/mol, and Ca²⁺ (<0.1 wt%) was more reactive in pyrolysis. The K⁺/Ca²⁺ ratio affected the amounts of pyrolysis gases, but not the species, and their presence increased the amount of aromatic C[dbnd]C structures and surface C[dbnd]O groups in the biochar. Catalysis with K⁺/Ca²⁺ transformed heavy pyrolysis tar compounds into small-molecule ones.