Microporous carbon from cashew nutshell pyrolytic biochar and its potential application as CO2 adsorbent

Abstract

Cashew nutshell (CNS)–based biochar is obtained as a by-product in a pilot scale (20 kg/h) gas-fired auger pyrolysis reactor at 500 °C during bio oil production. This pyrolytic biochar has low BET surface area (BET SA 0.80 m2/g) and poor porosity as the fast pyrolysis conditions are set to augment the bio oil yield over other products. To value add to this carbon, downstream activation is performed in an externally heated lab scale reactor. Formation of activated carbon with BET SA between 300 and 700 m2/g with moderate improvement in porosity ensued from CO2 and steam activation, while chemical activation with K2CO3 enhanced the BET SA to 1225 m2/g and Langmuir surface area to 1707 m2/g in addition to significant enhancement of porosity. CNS-based activated carbons predominantly possess narrow pore size distribution with small-sized micropores and ultra-micropores limiting the presence of mesopores. The effect of equilibrium time (10 s and 45 s) on N2 adsorption is extensively studied, and it is found to have a significant role in detection of ultra-micropores below 0.8 nm (at very low pressures). The CO2 absorptivity of K2CO3-activated CNS carbon is found to be between 4.16 and 6.22 mmol/g (i.e. 183.04–273.6 mg/g) at atmospheric pressure and 0 °C. These activated CNS carbons possess ultra-micropores between 0.46 and 0.8 nm suitable for CO2 uptake. This study shows the sustainable path of making CO2 adsorbent from a low-cost renewable biomass precursor like CNS.