Synthesis, modification, and characterization of biochar from agriculture waste streams

Emma Kathryn Snelling

doi:10.17918/00011260

The manufacturing of crop-based food products generates an agricultural waste stream consisting of stems, flesh, skins, seeds, and hulls. In the US alone, 40 million tons are produced annually, less than 1% of which was upcycled into bio-based materials or specialty chemicals. Disposing of agricultural waste is projected to emit 370 kg CO₂ equivalent per ton of greenhouse gases unless better waste management is implemented. Pyrolysis of agricultural waste produces a stable carbon-rich material known as biochar. Like carbon black or activated carbon, biochar's tunable porous structure and high surface impart high adsorption properties. Biochar's condensed aromatic skeleton with various O-containing functional groups can be readily modified and tailored, making it an attractive candidate for various applications including soil remediation, heavy metal removal, and carbon capture and sequestration. Previous biochar research has explored the effects of biomass feedstock, pyrolysis conditions, and chemical and physical modifications with the intentions of improving biochar properties such as surface chemistry, porosity, and adsorption capabilities. Currently, prediction of biochar properties based on biomass feedstock requires chemical analysis on its lignocellulosic components. There is little to no work discussing the impacts of plant anatomy nor is there any study into the effects of industrial processing on biomass and derived biochar. As assessment of plant anatomy and industrial processing can be done immediately without any testing, correlation of these factors to biochar properties would be beneficial. As such four agricultural waste streams with different anatomies and industrial processing were studied -- cocoa bean shells, spent coffee grounds, cranberry pomace, and brewers spent grains. The first two consist of a singular type of plant structure while the other two contain a mix of structures. Pyrolysis occurred for one hour in a 600 °C tube furnace under inert conditions. The derived biochar's surface chemistry, morphology, bulk density, crystallinity, water holding capacity, surface area and porosity were then assessed analytically. It was found that distinct differences exist in biochar synthesized from biomass with a singular plant anatomy versus mixed anatomy. A singular plant anatomy results in a more uniform surface and pore structure while mixed anatomy biochar exhibits a larger surface area. Different industrial processes such as roasting, grinding, and liquid extraction do correspond to variations in biochar's physical and chemical structure. In general, roasting and liquid extraction decrease surface functional groups within biochar while grinding increases surface area and biochar packability. The impact of biomass surface chemistry and plant structures on biochar led to the question, "what about mold?" Biomass is highly susceptible to mold growth yet there has been no consideration regarding mold's effects on biochar. The consumption of biomass by mold involves multiple chemical and physical mechanisms that could alter biochar properties. To explore this idea, all four agricultural waste streams were incubated in a humidity chamber for up to four weeks prior to pyrolysis. The subsequent biochars chemical and physical properties were assessed. It was found that mold significantly altered the chemical and physical properties of biochar due to preferential degradation of hemicellulose and cellulose as well as introduction of N-, K-, and Ca-containing functional groups to biochar's aromatic skeleton. It was also found that pyrolysis of molded biomass results in a core-shell structure consisting of a biomass-derived core covered by a mold-derived shell. This work addresses various gaps in biochar research regarding the influence of plant structure, industrial processing of biomass, and the impacts of mold. Further study of plant anatomical compositions should be conducted to validate these observations regarding singular and mixed anatomy. The industrial processing effects should be expanded to cover other processes as well go more in-depth on the ones studied by exploring factors such as roasting temperature and time. Incubation of other biomass types should be explored as should different types of fungi.

Synthesis, modification, and characterization of biochar from agriculture waste streams

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Synthesis, modification, and characterization of biochar from agriculture waste streams

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