Effects of Air Distribution on Bubbling Fluidized Bed Furnace
Virtanen, Ville Henrik (2015)
2015
Ympäristö- ja energiatekniikan koulutusohjelma
Luonnontieteiden tiedekunta - Faculty of Natural Sciences
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Hyväksymispäivämäärä
2015-11-04
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201510201669
https://urn.fi/URN:NBN:fi:tty-201510201669
Tiivistelmä
Bubbling fluidized bed combustion is a combustion process that can be used with variety of different kind of solid fuels, commonly biomass. Combustion process is based on a fluidized bed of inert material, usually sand, through which air blown to provide oxygen for combustion. Large amount of bed mateial, with the help of overfire air, stabilizes the combustion process in fluctuations. Inside the furnace, temperatures are that high that combustion is restricted mostly by mixing, not by reaction kinetics. In emission formation though, kinetics play an important role.
About half of the oxygen needed for stoichiometric combustion is fed through the bed and rest through the overfire air. Overfire air is usually fed in three different levels and from the opposite walls. There are different air configurations. Different configurations suit different fuels better than others.
Jet momentum describes how far the jet penetrates and it can also be used to indicate mixing. If momentum is high and there is staggered array of jets, the jet can hit the opposite wall. When the jet hits the wall stagnation area is born. Stagnation area can have high local heat transfer points, but usually the average heat transfer is more constant even with different momentum ratios.
Jet slot spacing has effect on mixing. There are certain optimum spacing for single side, opposed rows of staggered jets and for opposed row of inline jets. Later on the flow, the difference of mixing between the inline and staggered array configuration is quite small.
Nozzle shape has almost none effect on mixing. Near the slot region there are some differences but after the trailing edge, differences in mixing are little. There aren’t also much difference in heat transfer between different shapes if the jet is able to reach the opposite wall.
Jet impingement affects emissions and slagging. It can enhance or weaken those, but because slagging is affected by many things, it is hard to foretell what certain air configuration causes. Air staging reduces NOx emissions because overfire air produces a complete mixing between the rich and lean zones and eliminates hot, near stoichiometric reactant pockets that leads to NOx formation. SNCR system is another way to further reduce NOx emissions. In SNCR, additive is blown inside the furnace to chemically change NOx to its elemental form.
Reactive turbulent jets in a crossflow is a complex process. Modelling is a cheap way to study different scenarios and to do combustion optimizing. By using momentum equations and correlations, a simple model was developed to predict jet path, velocity profile and jet mixing. Validation showed that the result received could be used to make simple predictions about the jet behavior. Further study to see the effects on heat transfer and to see the capability to use model in SNCR is still needed.
About half of the oxygen needed for stoichiometric combustion is fed through the bed and rest through the overfire air. Overfire air is usually fed in three different levels and from the opposite walls. There are different air configurations. Different configurations suit different fuels better than others.
Jet momentum describes how far the jet penetrates and it can also be used to indicate mixing. If momentum is high and there is staggered array of jets, the jet can hit the opposite wall. When the jet hits the wall stagnation area is born. Stagnation area can have high local heat transfer points, but usually the average heat transfer is more constant even with different momentum ratios.
Jet slot spacing has effect on mixing. There are certain optimum spacing for single side, opposed rows of staggered jets and for opposed row of inline jets. Later on the flow, the difference of mixing between the inline and staggered array configuration is quite small.
Nozzle shape has almost none effect on mixing. Near the slot region there are some differences but after the trailing edge, differences in mixing are little. There aren’t also much difference in heat transfer between different shapes if the jet is able to reach the opposite wall.
Jet impingement affects emissions and slagging. It can enhance or weaken those, but because slagging is affected by many things, it is hard to foretell what certain air configuration causes. Air staging reduces NOx emissions because overfire air produces a complete mixing between the rich and lean zones and eliminates hot, near stoichiometric reactant pockets that leads to NOx formation. SNCR system is another way to further reduce NOx emissions. In SNCR, additive is blown inside the furnace to chemically change NOx to its elemental form.
Reactive turbulent jets in a crossflow is a complex process. Modelling is a cheap way to study different scenarios and to do combustion optimizing. By using momentum equations and correlations, a simple model was developed to predict jet path, velocity profile and jet mixing. Validation showed that the result received could be used to make simple predictions about the jet behavior. Further study to see the effects on heat transfer and to see the capability to use model in SNCR is still needed.