Assessment of rice husk biomass from different agro-ecological zones of Tanzania for biofuels feasibility via supercritical water gasification

In this work, rice husks (RH) from 4 agro-ecological zones of Tanzania were characterised for their potential use via supercritical water gasification (SCWG). The physical and chemical characteristics of different varieties of RH were evaluated, and the results were correlated with their geographical factors. The SCWG was conducted at a temperature of 492 °C, feed concentration of 9.5 wt%, residence time of 120 min, and catalyst loading of 15%. The catalyst was prepared by impregnating calcined limestone with 5% iron from the iron (III) ammonium citrate precursor. The RH samples exhibit a moisture content of 6.3–6.9 wt%, volatile matter 56.8–62.8% dry basis (%db), fixed carbon 11.64–19.5%db, ash content 22.2–27.3%db, and carbon content 29.97–3.4%db. The higher heating value ranged between 12.0 and 12.8 MJkg−1, extractives from 10.93 to 18.0%db, cellulose from 34.4 to 38.5%db, lignin from 16.3 to 17.5%db, and hemicellulose from 17.0 to 21.0%db. The results from noncatalytic SCWG showed that the gasification efficiency (GE) varied between 59.3 and 62.7%, gas volume between 278.5 and 305.2 mLg−1 biomass, gravimetric tar between 24.3 and 31.6 mgg−1 biomass, and char yield between 34.6 and 38.6 wt%. Phase characterisation of the fabricated catalyst showed that the main reactive sites are Ca2Fe2O5, CaO and Fe2O3, which promote char gasification and gas and tar formation reactions. When 5%Fe/limestone is employed, the GE ranges between 69.3 and 73.6%, gas volume is between 401.8 and 427.3 mLg−1, tar yield ranges between 57.5 and 62.8 mgg−1, and char between 20.1 and 24.9 wt%. The findings show that the fabricated catalyst is suitable for the simultaneous production of gaseous and liquid biofuels from rice husks via the SCWG process.