Emission reduction of particulate matter from the combustion of biochar via thermal pre-treatment of torrefaction, slow pyrolysis or hydrothermal carbonisation and its co-combustion with pulverized coal

[Display omitted] •Straw was pretreated via torrefaction, slow pyrolysis and HTC to receive biochar.•Single- and co-firing of straw/biochar and coal were performed to attain PM10.•Thermal pretreatments to straw can highly reduce the ash-based PM1/PM10 emission.•Co-firing of straw/biochar and bituminous coal can obviously reduce PM1 emission.•The emission reduction of PM0.3 is linearly related to the Cl content in biofuels. Emission reduction of PM10 (of an aerodynamic diameter of 10 μm or less) was investigated via the combustion of biochar pre-treated from straw by torrefaction at 300 °C (T-300), slow pyrolysis at 500 °C (S-500), or hydrothermal carbonisation at 240 °C (H-240), and their co-combustion with Ping Ding Shan (PDS) bituminous coal in a drop tube furnace at 1400 °C. The generated PM10 was collected by a Dekati low pressure impactor (DLPI) sample system, and its mass/chemical composition was characterised. During single combustion of the straw and its pre-treated biochar, the emission amount of PM0.3 (aerodynamic diameter of ≤0.3 μm) from the biochar was linearly affected by the release of Cl during various pre-treatments and the formed KCl release into the gas phase during combustion. The emission of PM1-10 still changed linearly, mainly because of the increased ash content after pre-treatment. Co-combustion of biomass fuels and PDS coal presents an obvious reduction in PM10 emission, particularly PM0.3. The higher Cl content in biomass fuels is also linearly correlated with a greater reduction in PM0.3 emission. Aluminosilicates in coal, e.g. kaolinite, are responsible for the capture of gaseous species from biofuels and the subsequent coalescence of sticky minerals, reducing PM0.3 and PM1-10 emissions following co-combustion.