Emission control and phase migration of PCDD/Fs in a rotary kiln incinerator: hazardous vs medical waste incineration

Abstract

This study was carried out in a full-scale (50 t/d) rotary kiln incinerator to explore the removal characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) by different units of air pollution control devices (APCDs), and special interest was focused on the “memory effect” phenomenon of PCDD/Fs in the wet scrubber (WS), which usually caused an undesirable rise in PCDD/F emission concentrations. The general removal efficiency of PCDD/Fs by APCDs was 99.4% (from 14.11 at exhaust heat boiler (EHB) outlet to 0.09 ng I-TEQ/Nm³ at stack) under medical waste (MW) incineration condition, and 99.2% (from 19.91 to 0.16 ng I-TEQ/Nm³) under hazardous waste (HW) incineration condition. The PCDD/F concentrations in flue gas decreased along the APCDs except for WS, in which the “memory effect” was observed. In detail, WS largely increased the I-TEQ concentration of gas-phase PCDD/Fs from 0.047 to 0.188 ng I-TEQ/Nm³ in the flue gas, and the concentration of particulate-phase PCDD/Fs increased from 0.003 to 0.030 ng I-TEQ/Nm³. In addition, this study found that phase migration promoted the accumulation of PCDD/Fs in scrubbing water, and the flow entrainment phenomenon played a great role in causing the “memory effect”. The PCDD/F concentrations of fly ash collected from cyclone and fabric filter (FF) were as high as 4.23 and 6.99 ng I-TEQ/g, respectively, which had exceeded the national landfill limitation (3 ng I-TEQ/g) in China. The system balance calculations revealed that APCDs promoted the migration of PCDD/Fs from the gas-phase to the particulate-phase, which caused fly ash to be the main carrier of PCDD/Fs and led to excessive emissions. The results of this study can contribute to the optimized design of combustion conditions and system cleaning for controlling PCDD/F emissions from rotary kiln incinerators.