Modelling of kraft recovery boiler flue gas temperature with auxiliary fuels
Shemeikka, Venla (2022)
2022
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
2022-03-29
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202203102449
https://urn.fi/URN:NBN:fi:tuni-202203102449
Tiivistelmä
In this Master’s thesis heat transfer and its modelling in a kraft recovery boiler furnace are studied. The main objective was to develop a calculation tool to estimate flue gas temperature at bullnose level in the furnace during auxiliary fuel combustion. The studied auxiliary fuels were natural gas, fuel oil, and black liquor.
The work focused on radiative heat transfer modelling in a furnace. The modelled fuels and their flue gases have different radiative properties. The weighted-sum-of-grey-gases model was used when modelling the radiative properties of flue gas and soot, and a simple correlation between black liquor fume emissivity and temperature was used to model the radiative properties of particles in black liquor flue gas.
Radiative heat transfer in the furnace was modelled with the Zone method, where the furnace is divided into isothermal zones, each with uniform gas properties. The zones are divided into smaller volume and surface elements and direct exchange areas between each surface-surface, surface-gas, and gas-gas element is calculated. The exchange areas of the elements are then summed up according to the zones, and energy balances over each zone is solved to obtain a flue gas temperature for each zone.
An Excel calculation tool was created to estimate flue gas temperature at bullnose level. The tool was validated by comparing results obtained with it to measured bullnose temperature during fuel oil firing in a boiler start-up, and CFD calculations. The developed calculation tool seemed to accurately predict the flue gas bullnose temperature. It did not seem to accurately predict the temperature of the main combustion area in the lower furnace. However, the lower furnace temperature profile was not particularly interesting for the applications that motivated this work, as the main interest was to estimate the flue gas bullnose temperature.
Besides developing the Excel calculation tool, the effects of for example furnace geometry, auxiliary burner capacity, and tertiary air temperature have on flue gas bullnose temperature were studied. As burner capacity increased in larger boilers, higher flue gas temperatures at bullnose level could be reached. Tertiary air temperature on the other hand had only a small effect on flue gas temperature on bullnose level.
Areas for future developments were also presented. More research should be done relating to furnace wall temperature and emissivity, well as developing black liquor combustion modelling, if more accurate results are wanted. Moreover, since the developed Excel calculation tool was compared to field measurements and CFD calculations during fuel oil combustion, more studies should be conducted to confirm the calculation in natural gas and black liquor combustion too.
The work focused on radiative heat transfer modelling in a furnace. The modelled fuels and their flue gases have different radiative properties. The weighted-sum-of-grey-gases model was used when modelling the radiative properties of flue gas and soot, and a simple correlation between black liquor fume emissivity and temperature was used to model the radiative properties of particles in black liquor flue gas.
Radiative heat transfer in the furnace was modelled with the Zone method, where the furnace is divided into isothermal zones, each with uniform gas properties. The zones are divided into smaller volume and surface elements and direct exchange areas between each surface-surface, surface-gas, and gas-gas element is calculated. The exchange areas of the elements are then summed up according to the zones, and energy balances over each zone is solved to obtain a flue gas temperature for each zone.
An Excel calculation tool was created to estimate flue gas temperature at bullnose level. The tool was validated by comparing results obtained with it to measured bullnose temperature during fuel oil firing in a boiler start-up, and CFD calculations. The developed calculation tool seemed to accurately predict the flue gas bullnose temperature. It did not seem to accurately predict the temperature of the main combustion area in the lower furnace. However, the lower furnace temperature profile was not particularly interesting for the applications that motivated this work, as the main interest was to estimate the flue gas bullnose temperature.
Besides developing the Excel calculation tool, the effects of for example furnace geometry, auxiliary burner capacity, and tertiary air temperature have on flue gas bullnose temperature were studied. As burner capacity increased in larger boilers, higher flue gas temperatures at bullnose level could be reached. Tertiary air temperature on the other hand had only a small effect on flue gas temperature on bullnose level.
Areas for future developments were also presented. More research should be done relating to furnace wall temperature and emissivity, well as developing black liquor combustion modelling, if more accurate results are wanted. Moreover, since the developed Excel calculation tool was compared to field measurements and CFD calculations during fuel oil combustion, more studies should be conducted to confirm the calculation in natural gas and black liquor combustion too.