Abstract
Biomass-derived bio-oil cannot be applied directly in diesel engines due to several poor fuel properties such high viscosity and instability. Emulsification could overcome the disadvantages of bio-oil. In this study, bio-oil emulsified with diesel was combusted in a DI diesel engine, and the effects of bio-oil compositions on the engine performance and emissions were investigated. Crude bio-oil has aging and instability problems, which could cause inconsistencies in the results. Synthetic bio-oil was used to avoid the limitations posed by crude bio-oil. Synthetic bio-oil was fractioned into three categories (hydroxyl compounds, aldehydes and sugars), and the corresponding emulsions were prepared. Compared with diesel, the brake-specific fuel consumption (BSFC) and brake-specific energy consumption (BSEC) were increased, and the brake power remained almost unchanged for the emulsions. For the exhaust emissions, CO and smoke emissions were decreased, while CO2 emissions were increased. The emulsions derived from the aldehydes and hydroxyl compounds reduced NOx emissions, whereas emulsions derived from sugars and the whole bio-oil increased NOx emissions. The results indicated that the aldehydes and hydroxyl compounds in bio-oil were more desirable for clean combustion than sugars. The hydroxyl compounds could result in the largest emissions reductions due to the presence of OH radicals.
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•Bio-oil fractions were classified into sugars, aldehydes and hydroxyl compounds.•Synthetic bio-oil and bio-oil fractions-based emulsion fuels were tested in diesel engines.•Hydroxyl compounds facilitated clean combustion.•The presence of sugars increased NOx emissions.•The results gave insights into the production of bio-oil with desirable fuel properties.