Effect of hydrothermal and hydrothermal oxidation pretreatment on the physicochemical properties of biochar pellet and activated carbon prepared from biomass wastes

Abstract

Hydrothermal (HT) pretreatment and hydrothermal oxidation (HTO) pretreatment can change the internal chemical components of biomass wastes to effectively promote the diversified utilization of the biomass resource and upgrade the quality of terminal productions. In this paper, cotton stalk (CS) and Fir wood sawdust (FS) were pretreated firstly at 160–260 °C to prepare the biochar pellets and activated carbon respectively. The effect of evolution behavior of the three components in CS/FS during HT/HTO process on the physicochemical properties of biochar pellets and activated carbon is explored. The results indicate that HT and HTO pretreatment are beneficial to the quality of biochar pellets and activated carbon. Compared with HT pretreatment, HTO process can effectively alleviate the pretreatment intensity of the preparation of high-quality biochar pellets and activated carbon. HTO pretreatment can promotes the significantly decompositions of cellulose and hemicellulose and aromatization growth of CS/FS. The biochar pellets with higher physical properties can be prepared at the HT temperature of 230 °C and at the HTO temperature of 200 °C respectively, and the physical properties of the samples prepared from FS basically higher than CS. Crystalline cellulose is the main contributor to the physical property of biochar pellets. The HHV of biochar pellets prepared from pretreated FS is basically higher than CS, and the cellulose is responsible for the increasing HHV. HT and HTO pretreatment can significantly improve the energy density of CS/FS samples, and the apparent density has the crucial effect on the energy density than HHV. The increasing of cellulose content is conductive to the improvement of combustion performance of biochar pellets, while recondensed lignin in FS prepared by HTO process also has the higher HHV and combustion performance. The total yield of resulted samples was affected by the combination of pretreatment yield and activation yield. Compared with the direct activation, the total yield of pretreated CS activated carbon prepared by HT and HTO pretreatment could up to 166.6 % and 118.4 % respectively, and 189.8 % and 118.9 % for FS samples. CS is the more excellent precursor to prepare high-quality activated carbon than FS. Compared with HT pretreatment, the higher specific surface area and adsorption capacity of resulted activated carbon can be obtained by HTO process, in which the iodine adsorption value of CS-HTO180-A and CS-HTO200-A meet the China standard of activated carbon for water purification (GB/T 13803.2–1999).