Characteristics of slow pyrolysis biochars produced from rhodes grass and fronds of edible date palm

•Rhodes grass biochar has higher fixed carbon (56.6 wt%), lower volatile (11.8 wt%) and higher ash (28.8 wt%) compared to date palm biochar (fixed carbon 45 wt%; volatile 43.2 wt%; ash 7.1 wt%).•Biochars have a similar H/C ratio (0.46 for rhodes grass; 0.49 for date palm), but different O/C and (O + N)/C ratios - they are lower in rhodes grass (0.11 and 0.12, respectively) compared to date palm (0.32 and 0.33, respectively).•Rhodes grass biochar has higher BET surface area compared to date palm biochar (16.78 m2 g−1 vs. 1.99 m2 g−1).•Analysis of pore size diameter in the two biomass before and after pyrolysis suggests date palm is heat sensitive in comparison to rhodes grass.•Water sorption analysis suggests biochars behave as macroporous (>50 nm diameter) materials because both show low water uptake at low percent of relative humidity.

Biochars from two different biomass (rhodes grass and fronds of date palm) were produced using the same pyrolysis condition (1 atm, 400 °C, heating rate of 5 °C min−1, duration of 11 h) and compared in terms of chemical composition, thermal stability, and respective microstructures. Proximate analysis of the biochars showed rhodes grass has higher fixed carbon (56.6 wt%), lower volatile (11.8 wt%) and higher ash (28.8 wt%) compared to date palm biochar (fixed carbon 45 wt%; volatile 43.2 wt%; ash 7.1 wt%). The pH of the two biochars were similar (∼9.6). Elemental analysis showed that carbon becomes enriched, whereas hydrogen, oxygen and sulfur become depleted after pyrolysis. The elemental O/C and (O + N)/C ratios are lower in rhodes grass biochar (0.11 and 0.12 respectively) compared to date palm biochar (0.32 and 0.33 respectively), but both biochars have a similar H/C ratio (0.46 for rhodes grass and 0.49 for date palm). Thermogravimetric Analysis (TGA) performed in inert gas (N2) showed considerable differences in the thermal degradation profile of the two biochars. Only one degradation event occurring over a wide temperature range (120–1000 °C; no obvious degradation maximum) was observed in rhodes grass, whereas two degradation events were observed in date palm – the first occurring rapidly (280–380 °C; maximum at 340 °C) and the second event occurring slowly over a wider temperature range (380–1000 °C; no temperature maximum detected). The Fourier transform infrared (FTIR) spectra of both biochars are featureless in comparison to raw biomass, but bands assigned to O–H stretching (3200–3000 cm−1) and C–H stretching (3100–3000 cm−1) while markedly decreased suggest cellulose might not have been completely decomposed during pyrolysis. The BET surface area of rhodes grass biochar is higher (16.78 m2 g−1) compared to date palm biochar (1.99 m2 g−1). This is in agreement with scanning electron microscopy investigations (SEM) which show that the average pore size diameter is smaller in rhodes grass biochar (3.1 μm + 0.3 μm) compared to the one of date palm biochar (7.2 μm ± 2.9 μm). Changes in the distribution of pore size diameter in the two biomass before and after pyrolysis suggests date palm is relatively heat sensitive in comparison to rhodes grass. Overall, our results demonstrate that both biochars are substantially different despite being produced under the same pyrolysis condition.