Abstract
The process of slow pyrolysis of seven nut shell samples, in a nitrogen-purged atmosphere, has been studied, as well as characteristics of biochar obtained. The heat carrier with a temperature of 400–600 °C (with a step of 100 °C) was supplied indirectly using a double-walled reactor. The heating rate was 60 °C/min. At increased temperature of the heating medium, a decrease in the amount of the resulting carbon residue averaged 6.2 wt%. The release of non-condensable combustible gas-phase compounds CO, CH4, and H2, with maximum concentrations of 12.7, 14.0, and 0.7 vol%, respectively, was registered. The features of the obtained biochar sample conversions were studied using thermal analysis in inert (nitrogen) and oxidative (air) mediums at 10 °C/min heating rate. Kinetic analysis was performed using Coats–Redfern method. Thermal analysis showed that the main weight loss (Δm = 32.8–43.0 wt%) occurs at temperatures ranging between 290 °C and 400 °C, which is due to cellulose decomposition. The maximum carbon content and, hence, heat value were obtained for biochars made from macadamia nut and walnut shells. An increased degree of coalification of the biochar samples affected their reactivity and, in particular, caused an increase in the initial temperature of intense oxidation (on average, by 73 °C). While technical and elemental composition of nut shell samples studied were quite similar, the morphology of obtained biochar was different. The morphology of particles was also observed to change as the heating medium temperature increased, which was expressed in the increased inhomogeneity of particle surface. The activation energy values, for biochar conversion in an inert medium, were found to vary in the range of 10–35 kJ/mol and, in an oxidative medium—50–80 kJ/mol. According to literature data, these values were characteristic for lignin fibers decomposition and oxidation, respectively.
Highlights
The pyrolysis is a useful technology for the valorization of biomass of different origins [1,2]
A good catalytic activity towards oxygen reduction was observed for olive and pistachio nut shell biochars, obtained at 600 ◦C with 30 min exposure time and activated in CO2 at 800 ◦C for 45 min [7]
This study reports the results of the parametric study into the effect of pyrolysis temperature on the characteristics of biochars derived from seven different nut shell samples
Summary
The pyrolysis is a useful technology for the valorization of biomass of different origins [1,2]. Biochar obtained could be used as a feedstock for production of supercapacitors, batteries, and as fuel, catalyst, or adsorbent [1]. The biochar, obtained via pyrolysis of nut shells, is known to be a suitable carbon-neutral fuel [3] and a feedstock for many other applications. A common issue, in this area, is varying properties of different biomass feedstocks, making obtaining products with stable characteristics problematic. The walnut shell biochar, produced via pyrolysis in a muffle furnace at 700 ◦C for 2 h at 5 ◦C/min heating rate, was found to be a very strong adsorbent for metolachlor removal [4]. The chichá-do-cerrado shell biochar, produced in a stainless reactor at 500 ◦C with 1 h exposure at 10 ◦C/min heating rate, showed good adsorption properties after activation by KOH [5]. A good catalytic activity towards oxygen reduction was observed for olive and pistachio nut shell biochars, obtained at 600 ◦C with 30 min exposure time and activated in CO2 at 800 ◦C for 45 min [7]
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