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Physical activation of biomass with H 2 O or CO 2 is an important method for producing activated carbon (AC). Activation of biomass with CO 2 or H 2 O follows varied mechanisms and the use of mixed CO 2 /H 2 O might show potential synergistic effects for creating pores. This was investigated by activation of lignin-derived biochar with H 2 O, CO 2 and their mixture. The results demonstrated that H 2 O as an activator was much more effective than CO 2 for generation of pores (804.1 m2g−1 versus 516.1 m2g−1in CO 2), especially mesopores (40% versus 14% in CO 2). This was achieved via enhancing cracking/gasification reactions of the biochar with H 2 O. The synergistic effect of CO 2 and H 2 O for activation of the lignin-derived biochar was insignificant as cracking/gasification reactions were not enhanced, leading to remarkably lower specific surface area of the AC (674.8 m2g−1) than that with H 2 O as an activator alone. Competitive adsorption of CO 2 and H 2 O existed and preferable adsorption of CO 2 hindered deoxygenation of the biochar with H 2 O. The in-situ IR characterization indicated that CO 2 presence suppressed decarbonylation reactions, retaining more oxygen species in form of C O, while it was the opposite in H 2 O. Additionally, activation with H 2 O formed numerous round-shape dents on surface of activated carbon, while the activation with CO 2 formed the characteristic round edges. The AC activated with H 2 O showed superior capability for adsorption of phenol than that activated with CO 2 or mixed H 2 O/CO 2. [Display omitted] • H 2 O is more effective than CO 2 for cracking, forming less activated carbon (AC). • Synergistic effect of CO 2 /H 2 O is insignificant as creating pores are not enhanced via cracking. • Competitive while preferable adsorption of CO 2 hinders activation with H 2 O. • CO 2 is not as active as H 2 O for cracking of C O in intermediates and for creating pores. • AC activated with H 2 O is superior for adsorbing phenol. [ABSTRACT FROM AUTHOR]

Pyrolysis of biomass typically involves intensive dehydration, cracking, secondary condensation, etc. It is known that these reactions could be accelerated or suppressed with externally added acidic/basic sites presenting on surface of a biomass feedstock, which is expected to impact evolution of pyrolytic products and pyrolysis kinetics. This was investigated herein by pyrolysis of the poplar sawdust loaded with H 2 SO 4 or NaOH at 600 °C. The results indicated that H 2 SO 4 presence enhanced charring of the volatiles including sugars, sugar-derivatives and some lignin-derivatives via dehydration, polymerization, etc. This enhanced biochar production while diminished bio-oil formation and abundance of most of the organics in bio-oil including carboxylic acids, heavy tar (i.e. π-conjugated aromatics), etc. Nevertheless, H 2 SO 4 did not lead to substantial removal of hydrogen and oxygen in the biochar, but increased the activation energy (Ea) of the pyrolysis from 162.6 kJ/mol in pyrolysis of unloaded sawdust to 194.7 kJ/mol. In converse, NaOH promoted cracking/gasification of biochar and the volatiles including heavy tar, forming dominantly gases. NaOH presence also made structures of the semi-char (biochar precursor) more fragmented, reducing the Ea for pyrolysis to 135.7 kJ/mol. The in-situ IR characterization of the pyrolysis process showed that H 2 SO 4 promoted removal of -OH via dehydration or transformation to C O and the further condensation of C O, but not dehydrogenation of -C-H. NaOH promoted mainly deoxygenation and cracking of the sugary structures but not dehydration of -OH to carbonyls. [Display omitted] • H 2 SO 4 enhances charring to form more biochar, while NaOH promotes cracking to gas. • Both H 2 SO 4 and NaOH diminish tar formation via condensation to solids or cracking. • Charring-H 2 SO 4 reserves H and O in biochar, while cracking-NaOH enhances aromatization. • Activation energy in pyrolysis is enhanced with H 2 SO 4 , while decreased with NaOH. • DRIFTS shows H 2 SO 4 does not promote dehydrogenation, while NaOH suppresses dehydration. [ABSTRACT FROM AUTHOR]

Hydrothermal carbonization (HTC) is a process for enhancing energy content of biomass through deoxygenation reactions, effectiveness of which depends on composition of biomass. Thermal pretreatment could induce structural change of biomass and thus affects the chemical inertness/reactivity in HTC. This was investigated herein by HTC of the pine needles, thermally treated at 250 or 350 °C with/without presence of 11.5% O 2 , at 250 °C for 10 h. The results showed that pretreatment at 250 °C could increase overall yield of hydrochar in subsequent HTC, due to the enhanced aromatization and the resistance towards fragmentation in HTC. Nonetheless, the HTC worked on biochar formed at 250 °C, facilitating deoxygenation to remarkably decrease O/C ratio. In comparison, the pretreatment at 350 °C removed substantial amount of volatile fractions, significantly increasing chemical inertness of resulting char-350 during the followed HTC. Oxygen presence induced oxidation reactions in pretreatment, forming more bio-oil/gases and making resulting hydrochar more hydrophilic. Especially, oxidation at 250 °C formed more aliphatic structures that could be easier removed during followed HTC, enhancing thermal stability of resulting hydrochar. Although a portion of cellulose retained in the biochar obtained from pretreatment at 250 °C, the elimination of aliphatics suppressed formation of spheric structures of hydrochar, while retained original fibrous structures from pine needles. [Display omitted] • Pine needle pretreated at 250°C is more inert in HTC, generating more hydrochar. • HTC barely works the sample pretreated at 350°C, due to removing most of volatiles. • Pretreatment at 250°C retains cellulose crystals, which are removed at 350°C. • O2-pretreatment makes hydrochar thermally more stable via removing more aliphatics. • Pretreatment at even 250°C suppresses formation of hydrochar of spheric structures. [ABSTRACT FROM AUTHOR]