Combustion of polyethylene and polypropylene in the fluidized bed with a variable vertical density profile

Abstract

A new type of fluidized bed with a variable vertical density profile was developed. A mixture of sand (Ønominal = 100–140 μm, 2.6 g/cm3) and cenospheres (Ønominal = 140–160 μm, 0.85 g/cm3) was used to form a binary fluidized bed. The minimum fluidization velocity of the binary fluidized bed was determined experimentally. Two points of inflection on the fluidization curve were observed. Additionally, a measurement of the pressure as a function of height was used to determine the profile of fluidized bed density. It was proven that the fluidized bed made out of cenospheres and sand can be characterized by a variable vertical density profile. The binary fluidized bed was used for the combustion of polyolefins. The results were compared to combustion in a conventional fluidized bed made out of sand. The surface of the fluidized bed was video recorded and the changes in the image brightness were used to identify diffusion combustion in the freeboard. A Fourier Transform Infrared (FTIR) analyzer with high spectral resolution (1 cm−1) was used to record the IR spectra of flue gases. In-house software was used to deconvolve complex IR spectra. In the process performed in the fluidized bed made out of sand, regardless of temperature, around 30 % of the polymeric carbon left the reactor in the form of soot. The lack of dependence on the temperature results from the fact that in such an organized process combustion did not take place inside the fluidized bed but on its surface. The binary fluidized bed eliminated the problem of the presence of diffusion flames and soot formation. High fuel conversion to CO2 (above 90 %) was observed at temperatures above over 800 °C during polyolefin combustion in the binary fluidized bed.