Continuous Slow Pyrolysis of Wheat Straw in a Screw Reactor and Use of the Char for Water Decontamination

Abstract

There is a need to replace fossil fuels and fossil fuel-derived products, and pyrolysis can contribute to both production of energy and production of useful bioproducts such as biofertilisers and bioadsorbents. At the same time, there is increasing demand for solutions to polluted air and water, due to population and consumption growth, and more stringent limits on pollutants. Adsorption is one of the main solutions for water decontamination, and activated carbon is the most used adsorbent due to its high performance and stability. However, production of activated carbon is usually time and resource-demanding, using high temperatures, coal or coconut shells as feedstock, and activating agents which can be strong acids and bases that can harm the environment. There is thus a need to develop novel adsorbents, with comparable performance to activated carbon, and be produced through more sustainable processes from greener and abundant feedstocks such as biowastes. To address this, slow pyrolysis was used on a wheat straw feedstock to produce the solid product char and also the liquid and gaseous products. The char was tested as an adsorbent for water contaminants, using a cationic dye as model compound for organic pollutants. The liquid and gaseous products were characterised and useful to use as fuel. Both the feedstock and the slow pyrolysis process conditions were modified in order to evaluate how product properties and adsorption performance were affected. The adsorption performance with the chars was evaluated mainly in terms of maximum removal of the dye, and comparable values to a commercial activated carbon (≈99% removal) were achieved. The adsorption capacity was also determined, and for the tested adsorption conditions a maximum of 98 mg/g was achieved, which was even higher than the commercial activated carbon. The best performing chars were the ones produced at lower pyrolysis temperature (400 °C), and feedstock modifications such as increased moisture content, and impregnation with a diluted KOH solution allowed to improve the adsorption results. The chars were valuable for water decontamination even though the surface area was relatively low (maximum 8.8 m2/g), which signified that adsorption of the cationic dye with the chars occurred mainly due to interactions with the surface chemical functionalities of the chars. For the commercial activated carbon, on the other hand, the relatively high surface area (615 m2/g) was the main cause for its high adsorption performance. The chars as bioadsorbents were produced in a continuous process, requiring relatively low temperature, and with additional treatment not being strictly necessary, although some improvements were verified when mild modifications were implemented on the feedstock or the process conditions. This contributes to the possibility of replacing or improving traditional activated carbon production processes, which are time- and resource-consuming. Furthermore, the feedstock used was wheat straw, which is a relatively abundant agricultural by-product frequently considered low value.