Effect of heating rate and H3PO4 as catalyst on the pyrolysis of agricultural residues

This study reports the effect of heating rate and the addition of H3PO4 on the pyrolysis of three representative agricultural wastes of different lignocellulosic composition, namely pistachio shell, bitter orange peel, and saffron petal. Pyrolysis was carried out at 500 °C in a fixed-bed, lab scale reactor. Slow pyrolysis provided lower water contents in the liquid fraction. Fast pyrolysis increased the liquid yield for all the feedstocks, promoting the formation of phenolic, ketone/aldehyde compounds. It also enhanced the formation of water for all the agricultural residues. In addition, the energy content in the gas fraction is promoted due to a higher concentration of light hydrocarbons, methane, and hydrogen. However, when high inorganic matter is found in the feedstocks, the formation of CO2 is favored, hindering the energy improvement. The treatment of the biomass with H3PO4 significantly increased the solid fraction, producing a huge porosity development in the char (surface area over 1600 m2/g in pistachio shell product), at the cost of liquid fraction, which is mostly composed of water, with small amounts of acetic acid, phenol and toluene. The results pointed out that pyrolysis of agricultural waste can be targeted to achieve different products by switching pyrolysis conditions such as the heating rate and the treatment of the biomass with H3PO4.