Tailored production and application of biochar for tar removal

Abstract

The impact of biochar modification on its behavior towards the adsorption and cracking of naphthalene (in a mixture naphthalene/toluene) as tar model compound has been investigated. Both tar adsorption and cracking may be considered alternative tar cleaning technologies which enable its further energetic use. Biochars were produced using different feedstocks and process conditions to tune their physico-chemical and morphological properties. Beech wood, representative of woody biomass, and almond shell and rice husk, representative of agricultural residues, were used. Furthermore, beech wood was also washed with deionized water and doped with KCl and FeSO4. Biochars were produced at 700 °C under several atmospheres (pure N2 and mixtures of N2, CO2, H2O, and CO), resulting in different physico-chemical properties and thus naphthalene adsorption and cracking behavior. Naphthalene adsorption experiments were performed at 250 °C and naphthalene cracking experiments were carried out at 750 °C and 850 °C. The performance of the investigated biochars towards naphthalene adsorption and cracking depended strongly on the production conditions, with biochars produced under reactive atmosphere (including N2, CO2, and H2O) yielding the best behavior. For naphthalene adsorption, an easily accessible high surface area seemed to be the dominant parameter determining the biochar adsorption capacity. Wood doping with FeSO4 resulted in biochars with higher adsorption capacity due to the formation of mesopores and Fe clusters within these mesopores that could likely enhance the adsorption capacity. In the case of naphthalene cracking, biochars produced under reactive atmosphere (G1) with untreated wood and H2O-washed wood showed the highest naphthalene conversion. In this case, the specific surface area was not the most relevant parameter determining the biochars performance. Wood doping with KCl had a negative impact on naphthalene cracking, while wood doping with FeSO4 impacted positively when the biochar was produced in N2 atmosphere, but negatively when produced in reactive atmosphere (G1), likely due to the oxidation state of Fe. Biochars from agricultural residues and woody biochars, produced under the same conditions, performed similarly.