Title

Preliminary study on the influence of pyrolysis process conditions on the textural properties of activated carbons from wheat straw-derived biochars

Conference Dates

September 15-20, 2019

Abstract

It is widely known that biochar can be used in a wide range of applications including —but not limited to— soil amendment, energy production, adsorption and catalysis [1]. For the last two final uses, however, the pristine biochar produced through pyrolysis processes does not have an appropriate porosity development. In fact, the porosity of biochar is mainly due to narrow micropores[2]. Therefore, an activation process is needed to develop a more hierarchical porous structure in order to facilitate the diffusion path of the corresponding adsorbates and/or reactants. To date, there is almost no research on how the pyrolysis process conditions, at which the biochar precursors are produced, can affect the porosity of the subsequent activated biochars.

The aim of this work is to evaluate how the slow pyrolysis process conditions adopted for the production of biochars from wheat straw pellets can affect the textural properties of the final activated carbons, which are obtained via physical activation with CO2 at 800 °C of the raw biochars. For this purpose, the following operating parameters were selected: the absolute pressure (from 0.2 to 0.9 MPa), the pyrolysis peak temperature (from 400 to 550 °C), and the type of gas atmosphere during pyrolysis (pure N2 or a binary mixture of CO2 and N2 60:40 v/v). Not only the main effect of these parameters individually but also the possible effects derived from the interactions between them have been considered. To this aim, an unreplicated 2-level factorial design has been adopted to objectively analyze the influence of these parameters on the response variables (i.e., the BET specific surface area, pore volumes, and pore size distribution of the physically activated biochars).

The wheat straw-derived biochars were produced through slow pyrolysis in a bench-scale fixed-bed reactor, made of stainless steel and electrically heated. More details regarding the reactor configuration are available in a previous publication [3]. Then, the produced raw pellet-shaped biochars were crushed and sieved to obtain particle sizes within the range of 0.212–1.41 mm. For the subsequent physical activation, around 20 g of pristine biochar was heated up to 800 °C in a fixed-bed reactor placed within a tubular furnace (model EVA 12/300 from Carbolite Gero, UK) under pure N2 atmosphere and at atmospheric pressure. Once this temperature was reached, the inlet gas stream was switched from N2 to pure CO2. During the reaction step, both the soaking time and gas-hourly space velocity were kept constant at 1 h and 7000 h–1, respectively. Under these conditions, the degree of burn-off of the produced activated biochar could be dependent on the intrinsic reactivity and porous structure of the precursor. The textural properties of both raw and activated biochars have been determined from the adsorption isotherms of N2 at –196 °C and CO2 at 0° C. The activation tests are almost finished and the results obtained will be presented during the course of the conference.

References

[1] Liu, W.-J., Jiang, H.; Yu, H.-Q. Chem. Rev. 2015, 115, 12251–12285.

[2] Manyà, J. J., González, B., Azuara, M.; Arner, G. Chem. Eng. J. 2018, 345, 631–639.

[3] Greco, G., Videgain, M., Di Stasi, C., González, B.; Manyà, J. J. J. Anal. Appl. Pyrolysis 2018, 136, 18–26.

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