Transformations of sludge waste during combustion in a low-high-low temperature reactor

Abstract

A new method of multizone combustion of a pulp/paper sludge in a low-high-low (LHL) temperature reactor is investigated. Experiments are conducted in a TGA-FTIR (thermogravimetric furnace coupled with Fourier transform infrared spectrometer) facility simulating realistic particle-metal and particle-gas interactions. The influence of time-temperature profiles on morphology, heavy (Cr, Cd), and light (Ca, Si) metal distributions inside ash particles formed during sludge combustion is determined. Sludge samples (70 g) are heated to 1770 K at an average rate of 75 K/min, kept at 1770 K for four different residence times (R0=0 s, R1=60 s, R2=300 s, and R3=900 s), and then suddenly quenched (15 K/s) to simulate LHL temperature regions. In order to obtain time-resolved data on the structural and compositional transformations of the sludge, samples are subjected to a variety of analytical techniques: wave-length-dispersive X-ray spectrometry, scanning electron microscopy, X-ray diffractometry, and atomic force microscopy. Sludge undergoes dramatic morphological transformations from a mixture of complex fibers, skeleton-like structures (45–110 μm), highly porous particles, to dense compact spheres (5–30 μm). Structural changes influence the distribution of metals in the solid residue. Toxic metals (Cr, Cd) are concentrated at the particle core and are successively surrounded by nonporous dense layers rich in Ca, K, Al, Na, and Si. It appears that the multilayered ash particle microstructure can be attributed to internal transformations caused by the LHL combustion. It is found that the residence time of 300 s at 1770 K is optimal to encapsulate Cd inside the particle core. A model of a final ash particle formed at the optimal residence time is designed and a mechanism of the heavy metal microencapsulation is proposed. Calculations show that metal diffusion does not seem to be the dominant process responsible for Cd migration inside the ash particle.