In-Situ Suppression of Dioxin Formation in Waste Incinerators by SO2 and NH3/Urea

Lead Research Organisation: University of Leeds
Department Name: Energy Resources Research Unit

Abstract

The issue of dioxins (PCDD- polychlorinated dibenzodioxins) and furans (PCDF- polychlorinated dibenzofurans) in the environment are of considerable public concern. Their association with emissions from municipal solid waste incineration contributing significantly to the negativity of the debate surrounding the public acceptability of the technology. This research proposal has been developed following two recent EPSRC funded research grants investigating the reactions of PCDD/PCDF under conditions simulating the post combustion zone of municipal solid waste incinerators. Our research involves detailed, analyses of PCDD/PCDF congeners and isomers in relation to the process conditions influencing PCDD/PCDF formation, reactions and destruction. The work has developed mechanisms to describe the formation of PCDD/PCDF on flyash and their subsequent desorption into flue gases. Although there has been extensive and intense investigation of PCDD/PCDF formation in waste incinerator flyash and air pollution control residues, the exact mechanisms still remain unclear. The aim of this present research proposal is to investigate the influence of particular flue gases (SO2) and NOx control gases (NH3/urea) on the formation of PCDD/PCDF in flyash and air pollution control residues. This data is important in understanding the formation mechanisms of PCDD/PCDF and the suppressive effect of SO2 and NH3/urea. It could provide a key advance for a step change control strategy for the elimination or reduction of PCDD/PCDF formation in flyash and air pollution control residues from waste incinerators by using inherent, in-situ gases to control PCDD/PCDF. It would also result in a significant cost reduction due to savings in the use of activated carbon, which is currently used for control. In addition, the detailed understanding of the suppressive effect of SO2 and ammonium compounds on PCDD/PCDF formation could have wider implications for reducing PCDD/PCDF emissions from other combustion sources, such as coal-fired power plant and metal sinter plant at low cost. This is an ambitious proposal involving difficult experimentation with toxic materials and difficult analysis at extremely low levels of PCDD/PCDF concentration to fundamentally understand the important role of SO2 and NH3/urea in PCDD/PCDF formation reaction mechanisms.
 
Description The aim of this research was to investigate the influence of SO2 and NH3, which are found in the flue gases from waste incineration, and their role in the formation of PCDD/PCDF in flyash and air pollution control residues. This data is important in understanding the formation mechanisms of PCDD/PCDF and the suppressive effect of SO2 and NH3. This was an experimental project involving experiments on waste incinerator flyash from a municipal waste incinerator using laboratory scale reactors. The flyash samples were reacted under a simulated flue gas stream at a range of temperatures over different periods of time up to 96 hours. The temperatures studied were in the range of the de-novo formation window of dioxins and furans i.e. between 225 and 375 C. After reaction, the PCDD/PCDF content of the reacted flyash and the PCDD/PCDF released into the exhaust gas, and subsequently trapped by XAD-II resin in a down-stream condensation system were analysed.

Research into PCDD/PCDF involves difficult experimentation with toxic materials and difficult analysis at extremely low levels of concentration and usually involves expensive high resolution GC/MS. However, in this work, we developed a methodology to use a lower cost bench-top gas chromatograph-triple quadrupole mass spectrometer which was demonstrated to be effective for the analysis of PCDD/PCDF using a CP-Sil 88 column at 240 C. The five-point calibration curve for a range of congeners showed very good linearity with R2 values greater than 0.999 and excellent recovery of labelled PCDD/PCDF compounds. Analytical results from a reference incinerator flyash (BCR-490) compared very well with the certified values.

The suppressive effect of NH3 in relation to PCDD/PCDF formation was demonstrated, showing a decrease in the concentration of both PCDD and PCDF by 34-75% from the solid phase and by 21-40% from the gas phase depending on the particular PCDD/PCDF congener. The application of SO2 led to 99% and 93% reductions in the PCDD and PCDF average congener concentrations, respectively in the solid phase. In the gas phase, the total reductions were 89% and 76% for PCDD and PCDF, respectively. Therefore, sulphur dioxide was more effective than ammonia in suppressing PCDD/PCDF formation in flyash under the conditions investigated.

Further experiments to understand the formation mechanism of PCDD/PCDF were undertaken using oxygenated PAH (dibenzofuran and benzo[b]naphtho[2,3-d]furan) in waste incinerator flyash. The results showed significant formation of PCDF, but less so for PCDD. Benzonaphthofuran was significantly more reactive than dibenzofuran in PCDD/PCDF formation, in spite of the fact that dibenzofuran is structurally more similar to that of PCDD/PCDF. Thus, there was no clear attribution between the chemical structure of PAH used and the formation of PCDD/PCDF.

Additional experiments, investigated the use of waste derived activated carbons for their potential in downstream control of PCDD/PCDF. Activated carbons are the most commonly used emission control protocol for waste incinerators. Activated carbons derived from the pyrolysis of refuse derived fuel, textile waste and scrap tyre were compared with commercial activated carbon, and the results showed that the total PCDD/PCDF toxic equivalent removal efficiencies in the exhaust gas stream were 52%, 57%, 64% and 58% respectively. There was also some formation of PCDD/PCDF on the reacted activated carbons. The activated carbons derived from the pyrolysis of waste therefore showed good potential as a control material for PCDD/PCDF emissions in waste incinerator flue gases.

Overall the research found that SO2 and NH3 have a suppressive effect on PCDD/PCDF formation which has significance for PCDD/PCDF emissions from waste incinerators. In addition, the results have wider implications for reducing PCDD/PCDF emissions from other combustion sources, such as coal-fired power plant and metal sinter plant.
Exploitation Route Control of dioxins in waste incineration
Sectors Energy,Environment