A study on the fast pyrolysis of millet and sorghum straws sourced from arid and semi-arid regions of Nigeria in a twin-screw mixing reactor

In this study, fast pyrolysis of sundried sorghum straw (Sorghum bicolor, SS) and millet straws (Pennisetum glaucum (L). R. Brown, MS), was conducted in a 10 kg.h−1 pyrolysis pilot plant. The aim was to compare their pyrolytic products with products derived from other widely used straw biomasses. Gas chromatography with flame ionization detection (GC–MS/FID), as well as proximate and ultimate analyses, were conducted to characterize the feedstocks and fast pyrolysis products. Compared to other ash-rich feedstocks (such as wheat straw), SS and MS yields recorded lower yields of organic-rich condensate (ORC) and aqueous condensates (AC); but higher yields of pyrochar and pyrolysis gas. The mass balance results showed that the ORC yields from pyrolysis of SS and MS were within a close range of 24–25 wt%, db. The ORC derived from SS and MS retained up to 34 wt% and 32 wt% of renewable carbon from their respective raw biomass. Furthermore, O:C atomic ratio slightly decreased in ORCs from 0.8 to 0.7 for SS, and from 0.9 to 0.7 for MS. However, the H:C atomic ratio increased significantly resulting in an elevated heating value from approx. 18 MJ.kg−1 in feedstocks to approx. 24 MJ.kg−1 in both ORCs. GC–MS/FID analysis showed that these condensates contain substantial quantities of acids, non-aromatic ketones, lignin-derived phenols, and levoglucosan. The ORCs obtained from these straw biomasses (SS and MS) exhibited similar characteristics to ORCs from the commonly valorized straws of wheat and miscanthus except that the concentrations of vital organic compounds are considerably low. This in effect also makes them promising for fuel applications and as precursors for the production of chemical platforms.