Torrefaction of Pulp Industry Sludge to Enhance Its Fuel Characteristics

Recently, under COP26, several countries agreed to phase-out coal from their energy systems. The torrefaction industry can take advantage of this, as the fuel characteristics of torrefied biomasses are comparable to those of coal. However, in terms of economic feasibility, torrefied biomass pellets are not yet competitive with coal without subsidies because of the high price of woody biomass. Thus, there is a need to produce torrefied pellets from low-cost feedstock; pulp industry sludge is one such feedstock. In this context, this study was focused on the torrefaction of pulp industry sludge. Torrefaction experiments were carried out using a continuous reactor, at temperatures of 250, 275, and 300 °C. The heating value of the sludge increased from 19 to 22 MJ/kg after torrefaction at 300 °C. The fixed carbon content increased from 16 wt.% for dried pulp sludge to 30 wt.% for torrefied pulp sludge. The fuel ratio was in the range of 0.27 to 0.61. The ash content of the pulp sludge was comparable to that of agricultural waste, i.e., around 12 wt.% (dry basis). The cellulose content in the sludge was reduced from 35 to 12 wt.% at 300 °C. Ash related issues such as slagging, fouling, and bed agglomeration tendency of the sludge were moderate. This study shows that torrefaction treatment can improve the fuel properties of pulp industry sludge to a level comparable to that of low-rank coal.