Lappeenranta University of Technology From the SelectedWorks of Kari Myöhänen December, 2011 Lectio Praecursoria Kari Myöhänen, Lappeenranta University of Technology Available at: https://works.bepress.com/kari_myohanen/7/
Lectio praecursoria Modelling of combustion and sorbent reactions in three-dimensional flow environment of a circulating fluidized bed furnace Kari Myöhänen Lappeenranta University of Technology 2.12.2011
Circulating fluidized bed boiler Videoclip from http://www.fwc.com/globalpowergroup/steamgenerators/animation_cfb_steam_gen_071608.wmv
Development of CFB unit capacities 600 Samcheok, KR Power capacity (MWe) 500 400 300 200 100 Tri-State, Nucla, US Duisburg, DE Kauttua, FI Pihlava, FI Lünen, DE Provence/Gardanne, FR Turow, PL Nova Scotia, CA NPS, Tha Toom, TH Emile Huchet, FR Kajaani, FI Jacksonville, US Ebensburg, US Baima, CN Seward, US Alholmen, FI EC Tychy, PL agisza, PL 0 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 Year CN China DE Germany FI Finland FR France KR South-Korea PL Poland TH Thailand US United States
Fuel selection chart Lower heat value (MJ/kg, as rec.) 35 20 10 5 Petroleum coke ANTRACITE Bituminous coal Peat Anthracite Lignite Bark Polyolefin plastics (PE,PP,PC) Chipboard Colored or printed plastics, clean Plywood Wood biomass Bio & fiber sludge Colored or printed mixed plastics REF I Commercial and industrial Consumer REF II - III REF PELLETS Demolition wood Peat w/ high Ca,Cl,Br Deinking sludge REF pellets Sewage sludge Mixed plastics Oil shale Estonian Mid-East/ N. African Agro biomass Wood & plastics Paper & wood RDF PVC MSW Standard design Some challenges REF = recovered fuel RDF = refuse derived fuel MSW = municipal solid waste Multiple challenges
Oxygen-fired combustion Oxygen Carbon diox. Water vapour Nitrogen Other Oxygen Carbon diox. Water vapour Nitrogen Other Flue gas Flue gas Fuel CFB boiler Fuel CFB boiler Air Oxygen Air-fired Oxygen-fired
Control volume method w v u + v + u w +
Challenges of modelling circulating fluidized beds 1) Small flow structures -> demands for calculation mesh. "Accurate" modelling of the whole furnace would require billions of calculation cells. 2) Time dependent (transient) flow. Time step small (1 ms) -> long calculation time. 3) Large number of phases and species. Gaseous species (O 2, CO 2, CO,...). Solids (burning fuel, ash, limestone components, sand, different particle size fractions). 4) Different phenomena and their dependence on each other. Flow and mixing of gases and solids. Combustion reactions, formation of emissions Comminution of particles. Heat transfer. Image of circulating fluidized bed in a two-dimensional test reactor (Åbo Akademi). Reactor width 1 m. Mesh spacing 0.2 m. Conclusion: the comprehensive modelling of circulating fluidized beds is very challenging.
Challenge of large furnaces agisza CFB 460 MWe
Furnace phenomena Heat transfer Temperature field Heat recovery External parameters Reactions Gas and solid sources/sinks Heat generation Comminution Solid sources/sinks Particle size External parameters Fluid dynamics Flow and mixing of solids and gas species
Comprehensive 3D-models 1) Model by Technical University Hamburg-Harburg First published in 1999. 2) Model by Chalmers University of Technology First published in 2008. 3) Model presented in this work First version in 1989. In this work, the model frame has been completely updated. Development of the combustion model. New model for limestone reactions.
Model frame Flue gas, fly ash Heat transfer to walls and internal surfaces Gas, solids Separator(s) - separation eff. - heat transfer - reactions Combustion, gasification & other reactions Inlet sources - sec. gas - fuel - limestone - sand Fluidization gas Bottom ash Exchange of gas / solids Solids Gas Solids to furnace Solids External heat exchangers - heat transfer - reactions Fluidization gas Recirculation of flue gas / fly ash
Combustion model Inert Evaporation H 2 O Example modelling results from bottom of a furnace Ash Moisture Devolatilization Volatiles Char Char combustion +O 2 Devolatilization Char combustion HCN, NH 3 H 2 S Char gasification +H 2 O, +CO 2 CO H 2 NO, N 2 O H 2 O CO, CO 2 Max CO, CO 2 CH 4, C 2 H 4 H 2 S SO 2 N 2 H 2 H 2 CO Gas combustion reactions CO + 0.5O 2 CO 2 Shift conversion CO + H 2 O CO 2 + H 2 H 2 + 0.5O 2 H 2 O CH 4 + 2O 2 CO 2 + 2H 2 O Min C 2 H 4 + 3O 2 2CO 2 + 2H 2 O H 2 S + 1.5O 2 H 2 O + SO 2
Sorbent model for calcitic limestone CaCO 3 CO 2 Modelled sulphur dioxide profile in oxygen-fired combustion CO 2 CaO CaSO 4 Carbonation SO 2 +CO 2 Desulphation CaCO 3 CaO
Example of model validation by field measurements Gas Concentration (%-dry). Modelled vs. measured profiles 10 9 8 7 6 5 4 3 2 1 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative Width O2 (measured) CO (measured) CH4 (measured) T (measured) O2 (calculated) CO (calculated) CH4 (calculated) T (calculated) Mesh O2 (0-16%) CO (0-10%) T (500-1000 C) 1000 900 800 700 600 500 400 300 200 100 0 Temperature T ( C) Max Measurement probe Min
Calculation example: Compostilla 300 MWe A planned circulating fluidized bed boiler for flexible operation with air-fired and oxygen-fired combustion. Temperature Sulphur capture Air-fired Oxygen-fired Air-fired Oxygen-fired
Summary The circulating fluidized bed combustion is a complex process, thus, the modelling is challenging. Only a few comprehensive process models exist, which can model large circulating fluidized bed furnaces three-dimensionally. The main achievements of this thesis work: A three-dimensional model frame, which can be applied for comprehensive calculation of large CFB furnaces and for further development of submodels. A combustion model, which can be applied for several fuel types. A sorbent model, which considers all the essential limestone reactions in air-fired and oxygen-fired combustion. The development of the model is a continuous process: the different sub-models can be always improved as more knowledge is achieved. This work sets ground for the future development.