Effects of slow-pyrolysis conditions on the products yields and properties and on exergy efficiency: A comprehensive assessment for wheat straw

In the present work, the effects of the peak temperature (400–550 °C), absolute pressure (0.2–0.9 MPa), gas residence time (100–200 s) and reactor atmosphere (pure N2 or a mixture of CO2/N2) on the pyrolysis behavior of wheat straw pellets were investigated. A factorial design of experiments was adopted to assess the effects of the above-mentioned factors on the pyrolysis products, the exergy efficiencies related to them and to the overall process, and the heat required. The pyrolysis energy/exergy assessment is nowadays of great interest, for the scaling of the installations from lab-scale to commercial-scale. Results showed that, as expected, the peak temperature was the most influential factor on the yields and distributions of all the pyrolysis products as well as the char properties related to its potential stability and pore size distribution. However, an increased pressure enhanced the release of the gas species at the expense of the liquid products, without altering the final char yield. The char exergy efficiency was negatively affected by an increase in peak temperature, whereas its effect on the exergy efficiency of the produced gas resulted to be positive. It was also found that pressurized pyrolysis favored the exergy efficiency of the process, even at relatively high pyrolysis peak temperature. For the biomass feedstock and the range of operating conditions studied here, thermodynamic irreversibilities of the pyrolysis system were considerably lowered when the process was conducted at 550 °C, 0.9 MPa and using a mixture of CO2 and N2 as carrier gas at relatively short residence times.