Investigations into the variability and control of dioxin formation and emissions from coastal power boilers

Stack dioxin emissions from power boilers burning salt-laden hog fuel in British Columbia have decreased by over 68% since 1995, from 10.5 grams of NATO TEQ per year in 1995 to 3.3 - 3.4 grams of TEQ in both 2001 and 2002. These reductions have been realized through capital expenditures, boiler shutdowns, and ongoing efforts to improve power boiler combustion efficiency and decrease particulate emissions. However, the large variations in individual test results present difficulties for many of the mills in ensuring compliance with the current Canada-wide standard limits for existing facilities (0.5 ng TEQ/m ³ @ 11% O2). Detailed analysis of the results of over 100 stack dioxin emission tests on eight different power boilers indicates that most of the emissions variations can be attributed to variations in combustion efficiency, particulate removal efficiency in the ESP or scrubber, hog fuel salt content, and ESP or stack operating temperatures. The co-firing of high calorific value, solid fuels, such as coal or tire derived fuel (TDF), with salt-laden hog fuel was found to increase combustion efficiency, facilitate the burning of more hog fuel and decrease dioxin emissions particularly when hog fuel quality was poor. Sulphur introduced into the boiler, particularly with tire derived fuel, also helps to reduce dioxins formation and emissions although it can aggravate boiler plugging and corrosion problems.