Quality assessment of bio-oil and biochar from microwave-assisted pyrolysis of corn stover using different adsorbents

Abstract

Biofuels in the form of bio-oil and biochar have been produced by microwave heating of corn stover. Different adsorbents such as silicon carbide (SiC) and activated carbon (AC) with different ratios (10%, 20%, and 30%) to corn stover are investigated with the aims of enhancing the absorption of the microwave, maximizing the yields of the desirable products i.e. bio-oil followed by biochar as well as improving their qualities. Use of both the adsorbents improved the heating performance through increase in the maximal temperature and heating rate by 170 °C and 36 °C/min respectively compared to without adsorbent. Use of AC increased the yield of bio-oil up to 45 wt% while SiC increased the gas yields up to a maximum value of 48 wt%. The better physical properties such as 37%, 5.3, 2.9 mPa s, and 1 g/mL for water content, pH, viscosity and density respectively of bio-oil were obtained when using AC as an adsorbent. Also, use of AC helped in producing bio-oils which contained higher carbon and hydrogen contents as well as high calorific value (17 MJ/kg). Similarly, use of AC increased the contents of hydrocarbons and phenols in the bio-oil up to 36% and 28% respectively with low oxygen-containing compounds (2%). Also, use of AC produced the biochar having cleanest pores that were almost free of carbon-like impurities with a higher surface area of the pore (105.2 m2/g). In addition, biochar having low ash content, high carbon content, and calorific value (28 MJ/kg) were achieved. The net energy recovery with respect to raw corn stover was calculated to be of maximum value i.e., 55% when using AC. The AC and SiC were reused for further runs of the experiments (5 times) with few changes in the heating behavior and the yields of the pyrolytic products.