Microwave-assisted pyrolysis and activation of pulp mill sludge

This work analyses the effects of microwaves on the production of char - the solid product of pyrolysis. Using pulp mill sludge and a 1200 W microwave oven, the pyrolysis of the sludge was analyzed and compared with that of conventional heating. Alkali metal hydroxide is an effective microwave absorber; adding a mass fraction of 5% of KOH to dried sludge enabled a complete pyrolysis in a few minutes, compared to hours in conventional pyrolysis. Microwave-assisted pyrolysis, however, retained 20% less carbon from the sludge, likely caused by the faster release of volatiles. The secondary sludge yielded more char than the primary sludge, which may be attributed to its greater protein content. Microwave-assisted combined pyrolysis and activation was feasible, due to the dual roles of alkali metal hydroxides as activating agent and microwave absorber. The combined one-step process generated an activated carbon with a specific surface area of 660 m2 g−1 from the sludge in three minutes.