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VTT and Andritz Oy measure recovery boilers using high‐speed infrared imaging


VTT has developed a high‐speed infrared imaging technology for use in recovery boiler monitoring. VTT’s technology enables an in‐line method for combustion measurement in a recovery boiler. Measurements help to optimize combustion processes and fuel particle trajectories. VTT and its partners, Andritz Oy and Aalto University, are evaluating this high‐speed infrared camera system performance in the challenging conditions of operating recovery boilers.

VTT and Andritz Oy have been developing measurement equipment that consists of a high performance infrared focal plane array camera with an indium antimonide photodetector cooled to 70 K temperature and a suitable optical bandpass filter between the optics and detector to reduce the level of radiation. The aims of the development have been to detect the shape and angle of black liquor spray characteristics in difficult furnace environment where flying particles in flue gases prevent their visualization.

“Our long-span co‐operation with VTT has been productive”, say sales manager Pasi Miikkulainen and development engineer Niko Metsämuuronen from Andritz Oy.

Black liquor is among the most important fuels in the world. Kraft Recovery boilers convert the spent liquor from the pulping process into energy and regenerate used chemicals. The spent liquor, known as black liquor, consists of lignin, pulping chemicals and water. The dry solids content of black liquor is 60–85%. Chemicals are recovered into the char bed, which is formed on the bottom of the furnace. Energy from flue gases is recovered by heat exchangers in the upper parts of the furnace and utilized as steam and electricity. The boilers are very large; the cross‐section of a modern boiler is e.g. 16.5 metres x 18 metres, and they may be up to 70 metres high.

Black liquor is sprayed into the furnace of a recovery boiler through liquor sprayers. It is important to be able to describe the drop size distribution of spray exactly. Inaccurate description leads to a reduced chance of optimizing combustion. In the case of a recovery boiler, small black liquor droplets are carried up by the gas flows from the lower furnace into the convective heat transfer section. This phenomenon is known as a carry‐over, and it leads to the fouling of heat transfer surfaces, while simultaneously reducing chemical recovery through the char bed process. Large drops cool down and impede the process by increasing the size of the char bed.

The results of the study were published in the Proceedings of SPIE 8354 (2012). Andritz is one of the largest manufacturers of pulp mill recovery boilers in the world. Research and development co‐operation with VTT continues and is helping to optimize combustion and reduce pulp mill maintenance costs in the long run.