Effect of acid pretreatment and process temperature on characteristics and yields of pyrolysis products of peanut shells

Abstract

Pyrolysis of acid pretreated peanut (Arachis hypogaea) shells was examined in order to improve the yield of liquids (bio-oils) and the characteristics of the three kinds of pyrolysis products. Also, pyrolysis of the pristine shells was comparatively investigated. The acid pretreatment was carried out employing a dilute HCl solution and it successfully diminished the ash content of the shells. Pyrolysis assays were performed in a fixed-bed reactor at different process temperatures (400 °C, 500 °C, and 600 °C). The maximum bio-oil yield was obtained at a temperature of 500 °C for both the pretreated and the pristine shells, but pyrolysis of the formers yielded more bio-oils than the untreated ones (42 wt% vs. 33 wt%). The increase of the process temperature resulted in a reduction of the solid (bio-char) generation for both samples. Demineralization also led to a further reduction of the bio-char yield. Regarding the products characteristics, neither the pretreatment nor the temperature had a noticeable influence on the elemental composition of the bio-oils. However, water content of the bio-oils was lower for the ones arising from pyrolysis of the demineralized shells although it increased with growing process temperature. Likewise, pyrolysis of the demineralized shells resulted in bio-chars with less ash, improving their potentialities as bio-fuels. Also, the bio-chars arising from the treated shells at the higher temperatures (500 °C and 600 °C) resulted in higher BET surface areas (up to 300 m2/g), pointing to their possible use as rough adsorbents or for further upgrading to activated carbons.