Evolution of the mass-loss rate during atmospheric and pressurized slow pyrolysis of wheat straw in a bench-scale reactor

Abstract

HIGHLIGHTS Higher pressure led to higher devolatilization rates in a narrower period of time  Using a mixture of CO2 and N2 at 0.1 MPa favored the thermal cracking of volatiles  Under CO2/N2 an increased pressure led to a decrease in the yields of CO and CH4 Biochar produced at 0.1 MPa under CO2/N2 had the highest specific surface area ABSTRACT In the present study, the effects of the absolute pressure (0.1 or 0.5 MPa) and the reactor atmosphere (pure N2 or a mixture of CO2/N2) on the pyrolysis behavior of wheat straw pellets (at 500 °C) were investigated. The most interesting aspect of this work was the use of a weighing platform (with a maximum capacity of 100 kg and a resolution of 0.5 g) to monitor the real-time mass-loss data for the biomass sample (with an initial mass of 400 g). It was observed that an increased pressure considerably affects the mass-loss profiles during the pyrolysis process, leading to higher devolatilization rates in a shorter period of time. Regardless of the pyrolysis atmosphere, an increase in the absolute pressure led to higher yields of gas at the expense of produced water and condensable organic compounds. This finding could be due to the fact that an increased pressure favors the exothermic secondary reactions of the intermediate volatile organic compounds in both liquid and vapor phases. The switch from pure N2 to a mixture of CO2 and N2 at 0.1 MPa also led to a remarkable increase in the yield of produced gas at the expense of the total liquid. This could be mainly due to the promotion of the thermal cracking of the volatile organic compounds at a high partial pressure of CO2, which is also consistent with the measured higher yields of CH4 and CO. The increased yield of CO can also be seen as a direct result of the enhanced reverse Boudouard reaction, which can also explain the much higher specific surface area (and ultra-micropore volume) measured for the biochar produced under the same operating conditions (0.1 MPa and a mixture CO2/N2 as pyrolysis medium).3

Effects of Biochar Application in a Sorghum Crop under Greenhouse Conditions: Growth Parameters and Physicochemical Fertility

“…The temperature inside of the bed was measured using four thermocouples placed in a thermowell at different heights. More details about the configuration of the reactor are available in previous publications [18,48]. A total of 13 experiments were carried out at two different final pyrolysis temperatures at atmospheric pressure.…”

Physically activated wheat straw-derived biochar for biomass pyrolysis vapors upgrading with high resistance against coke deactivation

“…The biochar employed in this work was obtained from binder-free wheat straw pellets (9 mm OD and 10-13 mm long), which were provided by a Belgian company. Biochar was produced via slow pyrolysis (at an average heating rate of 5 °C min -1 ) using a bench-scale fixed-bed reactor (the description of which is available in previous studies [24,39]), at a highest temperature of 500 °C, a soaking time (at 500 °C) for the solid fraction of 60 min and a residence time of the carrier gas (N2) within the reactor of 100 s. We chose these operating conditions in light of a previous study [24], where they were suggested as appropriate conditions to reach a reasonable compromise between the yield of biochar and its properties in terms of potential stability. The produced biochar (referred as "RW") was crushed and sieved to obtain particle sizes within the range of 0.212-1.41 mm.…”

“…Indeed, an appropriate value of surface area guarantees a high level of interaction between the reactants and the active sites. In any case, pristine biochar has a relatively low specific surface area, which is generally dominated by narrow micropore contributions [23,24]. Therefore, an activation step is required to expand the initial porosity of biochar and thus, facilitating high mass transfer fluxes and high active loadings.…”

Study of the Influence of Pyrolysis Conditions on the Textural Properties of Physically Activated Biochars

“…Wheat straw-derived biochar was produced using a bench-scale fixed bed reactor. Details about the device are available elsewhere [6]. Slow pyrolysis (at an average heating rate of 5 °C min -1 ) was conducted under different combinations of absolute pressure, peak temperature and type of gas atmosphere during the process (see Table 1).…”