Solar pyrolysis of waste biomass: A comparative study of products distribution, in situ heating behavior, and application of model-free kinetic predictions

A fixed-bed based solar pyrolysis of three waste biomass types: waste wood (WW), waste straw (WS), and sewage sludge (SS) was performed with emphasis on heating behaviour description, products yields, and quality. Laboratory solar pyrolysis reactor setup consists of a copper block indirectly heated by research-grade 1.6 kW xenon lamp radiation. Actual heating rates observed for 90, 95 and 100% xenon lamp power are 4.51, 5.32, and 5.49 K/min for WS; 5.10, 5.19, and 5.37 K/min for WW; and 3.95, 4.48, and 5.25 K/min for SS, respectively. Solar pyrolysis product yields (bio-oil, gas, char) were 51.68, 9.53, and 38.79 wt% for SS, 69.23, 9.18, and 21.59 wt% for WW, 62.30, 10.64, and 27.06 wt% for WS, denoted for the specific heating rates in ascending order. The highest quality bio-oil was obtained from WW pyrolysis with the lowest oxygen to carbon fraction and higher heating value (HHV) of 21.99 MJ/kg. Obtained chars, especially after solar pyrolysis of lignocellulosic biomass, presented HHV of 27.09 MJ/kg and 22.02 MJ/kg for WW and WS respectively, showed significant potential for renewable solid fuel. The increase of the lamp power, and long process time, caused degradation of porous structures within the biomass, where among the samples the highest specific surface area (BET) was denoted for WW samples at 5.2 K/min with 145.09 m²/g. Correlation between model-free kinetic predictions and gaseous species evolution was found, proving the value of apparent isoconversional activation energy profiles in describing actual solar pyrolysis gas compound formation at the laboratory scale.