Industrially relevant pyrolysis of diverse contaminated organic wastes: Gas compositions and emissions to air

Pyrolysis is a promising waste handling technique compared to incineration, especially due to its potential for greenhouse gas reduction through biochar carbon removal. This study investigated greenhouse gas and air pollutant emissions and emission factors from waste feedstocks and a reference clean wood pyrolyzed at 500–800 °C in an industrially relevant small version Biogreen® unit with condensation prior to pyrolysis gas combustion. Emissions were generally lower than literature values, except for nitrogen oxides (NOX) and sulfur dioxide (SO2). Methane (CH4) and nitrous oxide (N2O) emissions ranged 30–570 mg CH4 and 0–32 mg N2O per kg biochar produced. Waste lignocellulosics (waste timber and garden waste) had comparable emissions to clean wood, except for higher NOX emissions. All waste feedstocks exceeded the EU NOX emission limit value for waste incineration (>200 mg NOX/Nm3 at 11% oxygen, O2), while no carbon monoxide (CO) was measured (<0.6 mg/Nm3) suggesting possible compliance with EU emission limit values for waste incineration with simple pollution control measures such as air-to-fuel ratio regulation, or other NOX reducing measures such as flue gas recirculation or selective (non-)catalytic reduction. Sludges and food waste reject also exceeded the SO2 EU emission limit value (50 mg SO2/Nm3 at 11% O2) for waste incineration, emissions ranging 61–298 mg SO2/Nm3 at 11% O2, indicating the potential need for SO2 pollution control. In conclusion, this study shows continuous pyrolysis with condensation as a promising alternative for waste management with potential for simplified air pollution control compared to incineration. Future work should focus on optimized combustion systems for waste pyrolysis and emissions from waste pyrolysis without condensation.