Effects of moisture release and radiation properties in pulverized fuel combustion: A CFD modelling study

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Abstract

Pulverized fuels (PF) prepared and fired in utility boilers always contain some moisture. For some fuels with high moisture contents (e.g., brown coals), the share of the evaporation enthalpy is quite significant compared to the heat released during combustion, which often needs to be reclaimed to improve the plant efficiency and is also expected to affect the combustion process. Thermal radiation is the principal mode of heat transfer in combustion. In PF furnaces, radiation consists of contribution from both participating gases and solid particles, in which gas and particle radiation properties play an important role. There are different methods or models in the literature to address fuel moisture release and radiation properties, some of which may be inappropriate and can produce misleading results. This paper compares the different methods and models and demonstrates their implementation and impacts via a computational fluid dynamics (CFD) study of a 609 MWe pulverized coal-fired utility boiler. Overall speaking, it is suggested to add the free moisture in the fuel to the primary air stream while lump the bound moisture with volatiles in PF combustion modelling, although different methods for fuel moisture release may not induce distinct difference in combustion of PF with relatively low moisture content. For radiation, it has to be emphasized that particle radiation largely overwhelms gas radiation in PF combustion. The current tide of radiation research that over-focuses on gas radiation while over-simplifies particle radiation or even neglects particle radiation needs to be turned. Even for gaseous fuel combustion in which particle radiation is negligible, more generic model for gas radiative properties that can naturally and correctly accommodate the changes in combustion condition (e.g., oxy-fuel or air–fuel), account for the variations in CO2 and H2O concentrations in a flame, and include the impacts of other participating gases (e.g., CO and hydrocarbons) needs to be derived for combustion CFD community.
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Pulverized fuels (PF) prepared and fired in utility boilers always contain some moisture. For some fuels with high moisture contents (e.g., brown coals), the share of the evaporation enthalpy is quite significant compared to the heat released during combustion, which often needs to be reclaimed to improve the plant efficiency and is also expected to affect the combustion process. Thermal radiation is the principal mode of heat transfer in combustion. In PF furnaces, radiation consists of contribution from both participating gases and solid particles, in which gas and particle radiation properties play an important role. There are different methods or models in the literature to address fuel moisture release and radiation properties, some of which may be inappropriate and can produce misleading results. This paper compares the different methods and models and demonstrates their implementation and impacts via a computational fluid dynamics (CFD) study of a 609 MWe pulverized coal-fired utility boiler. Overall speaking, it is suggested to add the free moisture in the fuel to the primary air stream while lump the bound moisture with volatiles in PF combustion modelling, although different methods for fuel moisture release may not induce distinct difference in combustion of PF with relatively low moisture content. For radiation, it has to be emphasized that particle radiation largely overwhelms gas radiation in PF combustion. The current tide of radiation research that over-focuses on gas radiation while over-simplifies particle radiation or even neglects particle radiation needs to be turned. Even for gaseous fuel combustion in which particle radiation is negligible, more generic model for gas radiative properties that can naturally and correctly accommodate the changes in combustion condition (e.g., oxy-fuel or air–fuel), account for the variations in CO2 and H2O concentrations in a flame, and include the impacts of other participating gases (e.g., CO and hydrocarbons) needs to be derived for combustion CFD community.
Original languageEnglish
JournalFuel
Volume165
Pages (from-to)252-259
Number of pages8
ISSN0016-2361
DOI
StatePublished - Feb 2016
Publication categoryResearch
Peer-reviewedYes

    Research areas

  • CFD, Combustion, Fuel moisture, Particle radiation, Pulverized fuel, Utility boiler
ID: 220823123