Can batch lab-scale studies of slow pyrolysis describe biochar yields and properties upon upscaling to a continuous auger kiln?

The literature is replete with laboratory scale demonstrations of the thermochemical conversion of agroindustrial residues intended to provide industrial scale information for renewable biofuels and sustainable material production. Yet little work exists to translate laboratory findings to pilot scale and beyond. This study investigates the impact of upscaling on the pyrolysis of rice husk from batch laboratory reactors to a continuous pilot scale kiln. The same rice husk material was pyrolyzed to generate biochar across four scales: (1) milligrams at a time in a thermogravimetric analyzer (TGA); (2) grams at a time in a fixed bed tube furnace; (3) hundreds of grams at once in an agitated drum charcolator; (4) in a continuous pyrolysis kiln yielding kg/hr of biochar. The TGA and kiln biochars had the most similar level of devolatilization and carbonization (as reflected via relative loss in volatile matter and increase in ash and elemental carbon content and changes in surface functional groups). Biochar hydroxyl surface groups - indicative of cellulosic links breaking - were strongly dependent on temperature for the tube furnace and charcolator. The TGA-produced biochars developed thin-walled pores and high surface areas with the kiln having opposite morphology. Overall, the TGA - despite its small sample size - best mimics the kiln due to their similar sweep gas rates and reduced transport limitations, whereas the packed biomass particles in constant contact in the tube furnace and charcolator devolatilize less matter and perhaps condense devolatilized compounds on the biochar surface. We do not find evidence of any statistically significant differences in the H/C or O/C ratios as a function of reactor configuration, only pyrolysis temperature. However, the auger screw of the kiln appears to compromise the structural integrity of the biomass, leading to collapsed pores and lower surface areas than any laboratory-produced biochar.