CONTROLLING DIOXINS EMISSIONS IN COASTAL POWER BOILERS - A STATUS-REPORT

Dioxins formation in power boilers fuelled with salt-laden wood waste was recently evaluated at four Canadian coastal mill operations. At th e four sites, stack dioxins emissions were found to range from 0.064 t o 218 ng/m(3) (in terms of toxicity equivalents (TEQs), while net diox ins production ranged from 23 to 330 mu g TEQ/t of hog fuel. Stack TEQ emissions (i) not always correlate with overall dioxins formation in the power boiler. The mill scale test program also provided further in sights on potential control strategies: (i) While some dioxins appear to have been generated within the particulate collection devices, the excellent TEQ removal efficiency of these units confirms their essenti al role in controlling dioxins stack missions. (ii) The poor correlati on between stack particulate emissions suggests that particulate emiss ions cannot function as a surrogate parameter for the corresponding di oxins emissions. (iii) Regarding minimization of dioxins formation at source, good combustion control was shown to be-an integral component of any effective control strategy. Large variations in the organic con tent of precipitator ash, were generally an indication of correspondin g changes in combustion conditions and hence dioxins formation rates i n the power boiler. A preliminary assessment of the possible beneficia l effect of a reduced chlorine:sulphur ratio was encouraging. As well, temperature quenching as a means of minimizing dioxins formation is p redicted to be relatively effective.