Characterization of bio-oil, syn-gas and bio-char from switchgrass pyrolysis at various temperatures

Pyrolitic conversion of lignocellulosic biomass, such as switchgrass and other agricultural residues, to bio-fuels is being considered for national energy security and for environmental advantages. Bio-oil, syn-gas and bio-char were produced and characterized from switchgrass at 400, 500 and 600 °C by pyrolysis. Bio-oil yield increased from 22 to 37%, syn-gas yield increased from 8 to 26%, and bio-char yield decreased from 48 to 25% with increases of pyrolysis temperatures from 400 to 600 °C. Bio-oil heating value was 36.3 MJ/kg, density was 920 kg/m3 and viscosity was 10 cST. GC–MS study indicated that the bio-oil contained 37% oxygenates that can be upgraded to transportation fuel in future research. Syn-gas compositional analysis shows that, with increasing pyrolysis temperature, CO2, CO, C2H4 and C2H6 contents increased, whereas H2 and CH4 contents decreased. Part of the syn-gas consisting of H2, CO and CO2, when converted to syn-fuel, can be beneficial to the environment; sulfur free, presence of oxygenates results in less CO emissions and ozone to the atmosphere. Bio-char may be used as a co-product to enhance soil quality, and for carbon sequestration. Analysis of elemental composition and physical properties of bio-char show increase in carbon content, decrease in oxygen, hydrogen, and nitrogen content, and increase in surface area and pore volume with increases of pyrolysis temperature. The optimized pyrolysis process for bio-oil production in this study will help meet future goals of oil upgrading to produce transportation fuel.

► Bio-oil and syn-gas yields increase, whereas bio-char yield decreases with increasing temperature of pyrolysis. From pyrolysis at 600 °C, product yield was 37% bio-oil, 26% syn-gas and 25% bio-char. However, at 400 °C pyrolysis, product yield was 22% bio-oil, 8% syn-gas and 56% bio-char. ► Efficiency of pyrolysis improved with the pyrolysis temperature; product yield increased from 78% at 400 °C to 88% at 600 °C. ► The bio-oil was highly oxygenated (37%). It had a heating value of 36.3 MJ/kg. Viscosity of the bio-oil was 10 cST, which is comparatively higher than viscosities of gasoline (0.12) or diesel (2.1). ► The oil phase is a complex mixture of hydrocarbons; alkanes, phenols, aromatics, acids, alcohols, and ketones, and the aqueous phase is comprised mainly of branched ketones and acetic acid. ► For syn-gas, heating values of CO, C2H4, C2H6, and C to CO2 conversion increase, whereas heating values of H2 and CH4 decrease at higher temperature, owing to decrease in the volumes of the latter products produced at higher temperatures. The fixed carbon increased and volatile matter content of bio-char decreased with increasing temperature of pyrolysis. Bio-char surface area increased from 0.1 m2/g at 400 °C to 1.0 m2/g at 600 °C pyrolysis. ► From pyrolysis at 400 °C, energy distribution was 33% from bio-oil, 11% from syngas and 56% from bio-char; energy distribution from pyrolysis at 600 °C was 47% from bio-oil, 28% from syn-gas, and 25% from bio-char.