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
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
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.