Behavior of selective oxygen functional groups upon hydrothermal carbonization and pyrolysis of biomass and their roles on selective applications
Hydrothermal carbonization (HTC) is a thermochemical process where biomass is treated in water under high temperature and corresponding vapor pressure. During HTC, biomass undergoes a series of reactions including hydrolysis, decarboxylation, dehydration, aromatization, and polymerization. As a result, a solid product is produced from biomass after HTC process, which is widely known as hydrochar. Depending on the HTC process conditions, a wide-variety of oxygen-containing groups formed on hydrochar surfaces. Many of these groups could play critical roles in applications such as adsorption, and energy and gas storage, etc. However, not all functional groups are exposed on the surface due to the low surface area and the formation of a condensate layer (secondary char) on the hydrochar surface. Pyrolysis could be a possible pathway to expose additional functional groups by removing the volatile condensate layer. Therefore, the overall objective of this dissertation is to study HTC following by the physical activation of a model compound and real biomass (pine, paper mill sludge, winery waste, and citrus waste) to study the effects of these treatments on functional groups. HTC was primarily performed from 180-420 °C for 30 min in a batch reactor. Produced hydrochar was pyrolyzed at 400, 500, and 600 °C for 1h in a muffle furnace. Several analytical techniques such as boehm titration, Brunauer, Emmett, and Teller (BET) surface area analysis, attenuated total reflector-fourier transform infrared spectroscopy (ART-FTIR), thermogravimetricanalysis (TAG), scanning electron microscope (SEM), and X-ray diffraction (XRD) were conducted to characterize the hydrochars and pyrolyzed hydrochars. Various applications of the hydrochars and pyrolyzed hydrochars, such as alternative fuel, adsorbent, and electron storage material were investigated. The results showed that the higher heating value of paper mill sludges derived hydrocharscould be increased up to 1.5 times compared to the raw feedstock and that can be used as solid fuel in the existing coal-fired power plant. However, the findings suggested that the fuel properties are highly dependent on both feedstock and the HTC treatment conditions, which led me to investigate additional applications of the hydrochar. Herein, the hydrochar’s potential as a solid adsorbent was investigated, where the maximum dye adsorption capacities of citrus waste and winery waste derived hydrochars were measured 66.23 and 36.23 mg/g, respectively at 4 °C. The thermodynamic properties, such as Gibbs energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were studied to understand the adsorption mechanism. The results revealed that the adsorption phenomenon of both citrus and winery derived hydrochars occurred spontaneously, as the ΔG values at all temperatures are negative. On the other hand, all positive ΔH values indicate the adsorption process was endothermic in nature and not governed by the enthalpy. It was concluded that the hydrochars produced at low HTC temperatures showed highest dye adsorption capacity and the adsorption capacity highly depended on the density of the surface functional groups. To further enhance the surface functional groups, pyrolysis of the hydrochar was conducted. Results showed that pyrolysis of hydrocharat lower temperatures exposed additional surface functionality; however, high-temperature pyrolysis removed the functional groups from the hydrochar surface. Similar to the functional groups, electron storage capacity was highest (3.25 and 2.43 mmol/g for pine and cellulose, respectively) at low pyrolysis temperature. A positive correlation between the electron storage capacity and concentration of lactonic groups on hydrochar was observed. The findings of this study are encouraging and the hydrochar could be of further use for other applications, such as gas storage materials, catalysts, etc.). As both the HTC and pyrolysis treatments are energy intensive, the operational variables (i.e., temperature, residence time) need to be optimized, and a techno-economic analysis must be conducted prior to scale up these processes.