Use of alternative bed materials to inhibit bed agglomeration in fluidized bed combustion of biomass fuels
Mäntynen, Leo (2020)
2020
Ympäristö- ja energiatekniikan DI-ohjelma - Programme in Environmental and Energy Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2020-11-09
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202010267509
https://urn.fi/URN:NBN:fi:tuni-202010267509
Tiivistelmä
Bed agglomeration is a phenomenon, in which particles in a fluidized bed attach to each other and form larger clusters called agglomerates. It is a significant problem in fluidized bed combustion of certain biomass fuels that contain large amounts of alkali metals. Without countermeasures the agglomerates increase in number and size and may cause full defluidization, which refers to the collapse of the fluidized bed. This outcome necessitates boiler shutdown and leads to significant costs, which is why a lot of research is dedicated to the mechanisms and prevention of bed agglomeration.
The general consensus among researchers is that bed agglomeration is caused by an adhesive, silicate-dominated melt forming in the bed. Two main mechanisms of melt-formation have been recognized. In coating-induced agglomeration, bed particles are covered in ash, and chemical reactions in the ash coatings lead to melt formation. In melt-induced agglomeration, the melt is formed from ash without the presence of coatings. One possible way to prevent bed agglomeration is using quartz-free bed materials instead of natural sand. However, it is not clear how the usefulness of alternative bed materials is related to the different agglomeration mechanisms.
In this thesis, a literary review was conducted to summarize the current understanding of bed agglomeration mechanisms, its dependence on operational parameters, as well as methods to monitor and predict its occurrence. Additionally, combustion experiments were conducted in a labscale bubbling fluidized bed reactor to investigate the effect of fuel and bed material composition on agglomeration. Pine bark, tropical wood, oil palm empty fruit bunch (EFB) and wheat straw were used as fuels, while quartz, natural sand, GR Granule and olivine diabase were used as bed materials. The duration of each test was measured and bed samples were taken at regular intervals. The samples were analyzed using methods such as scanning electron microscopy and x-ray diffraction.
The results indicate that the benefits of using alternative bed materials depend on the composition of the fuel. Based on literature, tropical wood and pine bark were assumed to follow the coating-induced mechanism. For tropical wood, using alternative bed materials was effective. Defluidization was not achieved for bark, so bed materials could not be compared based on test durations. For EFB the choice of bed material did not affect agglomeration tendency, which could be explained by melt-induced agglomeration being the primary mechanism of agglomeration. Straw tests were unsuccessful, because of the excessive slagging of the reactor walls. Based on the test results and relevant literature, it seems that alternative bed materials are useful for fuels following the coating-induced agglomeration mechanism, but not for those obeying the melt-induced mechanism.
The general consensus among researchers is that bed agglomeration is caused by an adhesive, silicate-dominated melt forming in the bed. Two main mechanisms of melt-formation have been recognized. In coating-induced agglomeration, bed particles are covered in ash, and chemical reactions in the ash coatings lead to melt formation. In melt-induced agglomeration, the melt is formed from ash without the presence of coatings. One possible way to prevent bed agglomeration is using quartz-free bed materials instead of natural sand. However, it is not clear how the usefulness of alternative bed materials is related to the different agglomeration mechanisms.
In this thesis, a literary review was conducted to summarize the current understanding of bed agglomeration mechanisms, its dependence on operational parameters, as well as methods to monitor and predict its occurrence. Additionally, combustion experiments were conducted in a labscale bubbling fluidized bed reactor to investigate the effect of fuel and bed material composition on agglomeration. Pine bark, tropical wood, oil palm empty fruit bunch (EFB) and wheat straw were used as fuels, while quartz, natural sand, GR Granule and olivine diabase were used as bed materials. The duration of each test was measured and bed samples were taken at regular intervals. The samples were analyzed using methods such as scanning electron microscopy and x-ray diffraction.
The results indicate that the benefits of using alternative bed materials depend on the composition of the fuel. Based on literature, tropical wood and pine bark were assumed to follow the coating-induced mechanism. For tropical wood, using alternative bed materials was effective. Defluidization was not achieved for bark, so bed materials could not be compared based on test durations. For EFB the choice of bed material did not affect agglomeration tendency, which could be explained by melt-induced agglomeration being the primary mechanism of agglomeration. Straw tests were unsuccessful, because of the excessive slagging of the reactor walls. Based on the test results and relevant literature, it seems that alternative bed materials are useful for fuels following the coating-induced agglomeration mechanism, but not for those obeying the melt-induced mechanism.