Study of alkali metal migration during corn straw baking and the co-pulpability of torrefied charcoal and coal

Biomass resources are abundant, carbon neutral and a sustainable alternative to fossil fuels. The development and use of biomass resources is an important part of the revolution in energy production and consumption. Low-temperature carbonization is an effective biomass pre-treatment, which can achieve efficient biomass utilization. Corn straw was selected as the experimental raw material, and initially pretreated by hydrothermal carbonization and dry torrefaction in the temperature range of 180−240 ℃. Subsequently, a comprehensive analysis of alkali metal migration in the raw material and torrefied charcoal was conducted through the use of microwave digestion and chemical fractionation. The findings indicated that the total potassium content decreases with increasing torrefaction temperature, accompanied by a shift in the potassium's form from water-soluble to ion-exchangeable. The effect of the baking process on the pulping performance of biomass particles was examined following the mixing of torrefied charcoal with coal for pulping, and the correlation between alkali metal migration and pulping performance was further analysed. The results showed that the multifunctional slurry, prepared from torrefied charcoal and coal by dry torrefaction, exhibits superior flowability and lower apparent viscosity compared to hydrothermal carbonization. The lowest apparent viscosity of the slurry was observed at 469.31 mPa·s, at a concentration of 56% and a torrefied charcoal blend of 10%. This was due to the reaction of potassium in the water-soluble state with the carboxyl functional groups on the surface of the particles, resulting in the formation of potassium carboxylate salts. This phenomenon resulted in a reduction in the number of hydrophilic groups and a relative increase in the hydrophobicity of the particles, which in turn enhanced the fluidity of the slurry and reduced the viscosity.