Agro-residue bio-oil emulsions in CI engines: A critical review of combustion and emission characteristics

Abstract:

Compression ignition (CI) engines are widely used in transport, industry, and agriculture due to their high thermal efficiency and strong torque generation capability. With depleting fossil fuel reserves and stringent emission regulations, interest in alternative fuels is increasing. Non-conventional fuels offer environmental benefits and improved combustion characteristics and are therefore considered essential for future CI engine development. Second-generation liquid fuels could be produced as second yield of bio-oil from pyrolysis of agricultural residues. Moreover, pyrolysis also generates two other by-products: biochar and syngas. The conversion of these biomass residues into bio-oils, biochar and syngas, therefore, supports the principles of the circular bioeconomy and also zero-waste biomass valorization. Raw bio-oil cannot be directly utilised as a substitute to diesel fuel for CI engine application due to several inherent barriers. Although bio-oil diesel emulsion provides several benefits in terms of inherent oxygen in bio-oil, reduction in smoke and PM emissions; several challenges including extended ignition delay, lower BTE and greater corrosiveness and unknown effects on NOx levels limits the bio-oil diesel emulsion’s prospective for a mass scale implementation. The influences of bio-oil emulsion on NOx levels primarily depend on many parameters such as bio-oil properties, diesel to bio-oil ratios, emulsifier concentration and engine operating conditions. A comprehensive review is presented, covering the combustion characteristics, engine performance and emission features of CI engines running on agro-residue bio-oil emulsions. Benefits and limitations of this emerging fuel are highlighted and research priorities identified.