Abstract
Bio-char has the ability to isolate carbon in soils and concurrently improve plant growth and soil quality, high energy density and also it can be used as an adsorbent for water treatment. In the current work, the characteristics of four different types of bio-chars, obtained from slow pyrolysis at 375 °C, produced from hard-, medium-, thin- and paper-shelled walnut residues have been studied. Bio-char properties such as proximate, ultimate analysis, heating values, surface area, pH values, thermal degradation behavior, morphological and crystalline nature and functional characterization using FTIR were determined. The pyrolytic behavior of bio-char is studied using thermogravimetric analysis (TGA) in an oxidizing atmosphere. SEM analysis confirmed morphological change and showed heterogeneous and rough texture structure. Crystalline nature of the bio-chars is established by X-ray powder diffraction (XRD) analysis. The maximum higher heating values (HHV), high fixed carbon content and surface area obtained for walnut shells (WS) samples are found as ~ 18.4 MJ kg−1, >80% and 58 m2/g, respectively. Improvement in HHV and decrease of O/C and H/C ratios lead the bio-char samples to fall into the category of coal and confirmed their hydrophobic, carbonized and aromatized nature. From the Fourier transform infra-red spectroscopy (FTIR), it is observed that there is alteration in functional groups with increase in temperature, and illustrated higher aromaticity. This showed that bio-chars have high potential to be used as solid fuel either for direct combustion or for thermal conversion processes in boilers, kilns and furnace. Further, from surface area and pH analysis of bio-chars, it is found that WS bio-chars have similar characteristics of adsorbents used for water purifications, retention of essential elements in soil and carbon sequestration.
Highlights
This study investigates the physio- and thermochemical characterization of bio-chars obtained from hard-shelled walnut (HSW), medium-shelled walnut (MSW), thin-shelled walnut (TSW) and paper-shelled walnut (PSW)
The results obtained from proximate (on dry ash free basis) and ultimate analysis, and the corresponding hydrogen to carbon (H/C), oxygen to carbon (O/C) and higher heating values (HHV) data, are shown in Tables 1 and 2
The results show that moisture contents in the bio-chars are in the range of 0.2–0.8%, which are comparable with wheat straw [46] but much lower than the moisture contents reported for coconut shell bio-chars (7.1 wt%) [17], lignite (34 wt%) and bituminous coals (11 wt%) [47]
Summary
Continuous environmental issues, ascending prices of petroleum, energy crisis, exhaustion of fossil fuels, increasing application and need for energy are the serious motivations, due to which there is much insistence on substitute sustainable energy sources. Environmental friendliness, sustainability and biodegradability are the important characters which have made the biomass a primary candidate for the generation of bio-energy. The conversion technologies are the possible options to explore the economic potential of bio-resources. Biofuels and bio-chemicals are formed through thermo-chemical conversion, which includes pyrolysis, gasification, liquefaction and combustion [1]. Pyrolysis is the most striking process for converting biomass into bio-fuels [2]. Volatiles and semi-volatiles are discharged from the feedstock residues during pyrolysis of biomass and yields gases, bio-oil and chars. Bio-char may be formed with re-condensing vapors into the bio-char material depending on the residence time of vapor, which increases the bio-char products [3,4]
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