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High grade activated carbons by downstream activation of fast pyrolytic bio-char- production, characterization and applications

Student Name: Ms Sonal Garg
Guide: Dr Piyali Das
Year of completion: 2021

Abstract:

Conventional methods of using Jatropha and Karanja oilseed cakes (JSC and KSC) are in the form of organic manure or biogas generation. Cashewnut shells (CNS) primarily are used as a source of providing process heat while Cotton stalks (CS) are left for open burning in the fields on a large scale. However, fast pyrolysis is a superior method for treatment of such crop and process wastes. Bio-char is obtained as a by-product in a pilot scale (20 kg/h) gas fired auger pyrolysis reactor at 500 ºC under fast pyrolysis conditions (that is originally aimed at maximizing the bio-oil yield under another project in the lab). In the present study, JSC and KSC-based bio-char, CNS and CS-based bio-char have low surface areas (less than 10 m2/g) and negligible porosity. In order to carry out value addition to bio-char, downstream physical and chemical activations are carried out in an externally heated laboratory-scale reactor. JSC and KSC-based activated carbons resulting from CO2 and steam activation is found to have BET surface area reaching maximum up to ~ 200 m2/g, for CNS and CS-based carbons BET surface area is up to 700 m2/g and 300 m2/g respectively with marginal improvement in the porosities. On the other hand, K2CO3 activation enhanced the surface area values to as high as 200- 2400 m2/g along with substantial development of porosities for all the four biomass-based carbons under different activation conditions. JSC and KSC-based carbons predominantly possess small micro and mesopores while CNS and CS-based activated carbons chiefly consist of small sized micropores and ultra-micropores with narrow pore size distribution and limited presence of mesopores. Thus, JSC and KSC-based activated carbons are best suited for liquid phase pollutant removal (MB and I2 sized molecules) while one KSC-based and all CNS and CS-based K2CO3 treated activated carbons prepared at 850 ºC, show high CO2 adsorptivity lying between 4.16-8.3 mmol/g (183.04 mg/g- 365.2 mg/g) at atmospheric pressure and 0 ºC. This study shows the sustainable path of making adsorbents for different applications from the low-cost renewable process waste and biomass precursors. Further, the effect of equilibrium interval (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). These ultra-micropores between 0.46 and 0.8 nm are suitable for CO2 uptake. There is a direct correlation between HK cumulative pore volume and CO2 adsorption.