Quality improvement of fast pyrolysis bio-oil through alkali removal from feedstock

As the pressure to reduce greenhouse gas (GHG) emission grows, more interest towards sustainable bio-fuels emerges. Utilizable, non-edible and renewable waste streams are one of the most promising feedstocks to maximize the emission reduction. Fast pyrolysis is robust and rather simple liquefaction technique which can turn these solid feedstocks to high energy density liquid bio-oil. Although utilization of waste streams is one of the most potential ways to reduce GHG emissions, reduced quality of feedstock is problematic from the scope of the processing. Inorganic, ash forming elements of biomass, cause several problems during the fast pyrolysis process. Especially alkali and alkaline earth metals (AAEM) potassium, sodium, calcium and magnesium have been shown to possess catalytic activity, which results in decreased product yield and altered chemical composition of produced biooil. In addition, larger inorganic content in feedstock results in higher inorganic concertation in bio-oil, which is problematic from the scope of the further processing. Thus, pretreatment of low quality feeds, i.e. feeds with high ash content, has awaken interest. Water and dilute nitric acid washing of biomass were studied with pine forest residues, eucalyptus and wheat straw grinded to particle size 0.55 - 0.98 mm. The effect of several parameters to total ash and AAEM removal were investigated. Although a large part of potassium and sodium were removable with water, the acidic washing liquid was more efficient regarding the total ash and AAEM removal. Especially removal of ion-exchangeable cations, such as calcium, was enhanced when acid washing was used. Changing parameters had varying effects within the raw materials. In the case of eucalyptus, elevated temperature and higher acid concentration were more efficient, whereas in the case of forest residues and especially with wheat straw, milder washing conditions were as efficient. Regarding the raw materials, wheat straw was identified as the most promising feedstock to washing due to its high AAEM content and mildness of treatment conditions. Especially washing of the whole straws without grinding is interesting option which needs to be studied further. In addition, suitable AAEM levels in feeds should be clarified from the scope of bio-oil yield and quality maximization so that appropriate washing method could be chosen.