FUEL CONVERSION IN FLUIDIZED DUAL-REACTOR SYSTEMS

Transcription

1 61st IEA-FBC Meeting, Salerno, October 2010 FUEL CONVERSION IN FLUIDIZED DUAL-REACTOR SYSTEMS Bo Leckner Department of Energy & Environment Chalmers University of Technology Göteborg, Sweden

2 THIS PRESENTATION WILL GIVE EXAMPLES ON REACTORS AND MODELLING

3 REACTORS

4 EXAMPLES OF DUAL-REACTOR FUEL-CONVERSION SYSTEMS Fluidized catalytic crackers Chemical looping conversion Pressurised FBC for coal conversion. Conversion of biomass to high-(medium) value gas. Pyrolysis-combustion plants for waste fuels.

5 ARRANGEMENTS: The coupled reactors could be A. Circulating B. Sequential C. Gravitational

6 A. CIRCULATING SYSTEMS: VARIOUS PROPOSALS HAVE BEEN MADE FOR BIMASS GASIFICATION

7 ADD-ON GASIFIER/CFB BOILER FOR BIOMASS GASIFICATION (The Chalmers unit [6]) Heat, Electricity, Steam Flue gas Heat, Electricity, Steam Flue gas Bio Product Gas Fuel Hot bed material Fuel Hot bed material Biomass Air Air Fluidization gas (Steam or Bio Producer Gas or )

8 Principle scheme: Chemical Looping Combustion (CLC) N 2, (O 2 ) CO 2, H 2 O MeO Air reactor bbb Fuel reactor Me Air Fuel Particulate material and heat are brought from one reactor to the other

9 CLC WITH LIME FOR BIOMASS GASIFICATION [8] Reactor 1: CaCO 3 +heat CaO+CO 2 High-temperature combustion and calcination CO 2 release Reactor 2: CaO+CO 2 CaCO 3 +heat Low-temperature gasification. CO 2 is bound by CaO H 2 because the water-gas shift reaction is dominant in biomass gasification CO+H 2 O CO 2 +H 2

10 DEVELOPMENTS FOR COAL GASIFICATION To improve conversion efficiency dual-reactor systems were proposed (for coal gasification in fluidised bed). There were many proposals, for instance, the Cogas gasifier (1974)

11 B. SEQUENTIAL REACTORS FOR WASTE CONVERSION Many arrangements follow the general layout: Air Flue gas cleaning Prepared fuel, waste Devolatilisation o C Combustion o C Heat transfer Char Coarse ash separation Metals, glass Fine ash (molten? Alternatively Steam process Oxygen Product gas Gasification o C Ash (molten?)

12 Example: WASTE COMBUSTION, EBARA

13 C. GRAVITY TYPE OF GAS GENERATORS Blauer Turm, Muehlen Biomass Heat pipe Reformer, J. Karl Hydrogasification of coal for methane production 1974 IGT [9]

14 THE HEAT-PIPE REFORMER

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16 MODELLING The simplest possible model presented in a survey comprising 400 references will be explained. (A. Gómez-Barea and B. Leckner, Modeling of biomass gasification in fluidized bed, Progress in Energy and Combustion Science 36 (2010) ).

17 COMPARISON BETWEEN DIRECT AND INDIRECT GAS GENERATORS BY HEAT AND MASS BALANCES Combustion gas Product gas Product gas Combustion air Gas generator Ash Fuel Reactant gas (H2O or CO2) Combustion air Heat genera tor Char Heat Gas generator Ash Fuel Reactant gas (H2O or CO2) Autothermal or direct Allothermal or indirect Bo Leckner CTH 17

18 COMPOSITION OF SOLID FUELS =b+w+a w b a x c x v B. Leckner CTH 18

19 METHOD OF ANALYSIS The comparison is based on heat and mass balances of the entire reactors plus a few assumptions. The fuel analysis is given. ASSUMPTIONS Devolatilisation x v and drying take place in the gasifier The char x c =1-x v is gasified to an assumed extent ϕ gas. The autothermal case: remaining char is a loss and volatiles are burnt for heating The allothermal case: remaining char is burnt in the combustor and volatiles are only burnt if the char is completely consumed

20 mf, in = mf (1 + ξu + ξb) THE FUEL ξ u is the loss of fuel kg/kg fuel converted due to incomplete conversion ξ b is fuel consumed to produce heat, expressed in kg/kg fuel converted. (4) This gives m f, in = m f a(1 + ξu + ξb) + ashes + m w(1 + ξ + ξ ) + f u b + m b(1 + ξ + ξ ) f u b moisture combustibles (5) The combustible part is m f b gas from volatiles and gasification of char m f bξ u conversion loss, mostly unreacted char m f bξ b consumption to maintain reactor temperature The combustible part consists of char x c and volatiles x v (from fuel analysis). Then the heating value of the volatiles is H = ( H xh )/ x (3) uv, ub, c uc, v Bo Leckner CTH 20

21 THE GAS PRODUCED The amount of gas produced (kg gas/s) is m gas = mb+ f + m w+ f, in + mbξ g f = b 0 Volatiles+gas from gasified char, m f bx c ϕ gas. Fuel moisture Combustion gas in the autothermal reactor Additional assumption for the autothermal reactor: the flue gas g 0 and air demand l 0 are those of combustion of fuel with char withdrawn.

22 HEAT AND MASS BALANCES mbξ H f b hb, = { ( ) } = m c T T + wh f, in pmf b 0 w { ( ) } + m bx ϕ c T T + H f c gas pm, H 2O b 0 C, H + mbξ c ( T T) f b 0 pm, air b 0 Input of energy with fuel burnt Heating of fuel + evaporation of moisture Heating of gasification vapour + heat for production of gas Heating of combustion air + radiation losses from reactor (neglected here) where H u,b = H u,v in an autothermal reactor and H u,b = H u,c in an allothermal one and T b is the bed temperature Bo Leckner CTH 22

23 PERFORMANCE CHARACTERISTICS The heating value of the gas m gas H u,gas = m f b H u,v (1- x c )+ H C,H x c φ gas The cold gas efficiency of gasification η g = m gas H u,gas /(bm f,in H u,f ) ξ b The fraction of the combustibles burnt = m w( c T + H ) + m bxϕ ( c T + H ) f, in pmf w f c gas pmh 2 O C, H m bh ( c T) f u, b o pmair

24 GASIFIER EFFICIENCY AND HEATING VALUE OF EXIT GAS VS REACTOR TEMPERATURE Zero moisture (w=0) and no gasification of char (φ gas =0) in autoand allothermal gas generators. Efficiency Allothermal Autothermal f =0 w=0 Heating value MJ/kg gas Allothermal Autothermal f =0 w= Temperature deg C Temperature deg C

25 EFFICIENCY AND HEATING VALUE OF EXIT GAS 1) Various moisture contents in the fuel (w varies) and no gasification of char (φ gas =0). 2) No moisture (w=0) and various φ gas are also shown. Gasifier efficiency Allothermal Auto thermal j =0 j =0 w= Fraction of moisture (w) or of char gasification (j ) Heating value MJ/kg gas Allothermal Auto thermal j =0 j =0 w=0 w= Fraction of moisture (w ) or of char gasification (j )

26 FRACTION OF FUEL NEEDED TO ATTAIN A CERTAIN GASIFIER TEMPERATURE, m f b ξ b,. Degree of gasification Moisture content Autothermal w=0 Fraction burnt f =0.0 Autothermal Temperature deg C 0.25 Allothermal w=0 Allothermal f = Fraction burnt Fraction burnt Allothermal 0.05 f = w= Temperature deg C Temperature deg C

27 AIR RATIO BASED ON FUEL ADDED. λ =m f bξ b l o /m f,in b l o =ξ b /(1+ ξ u + ξ b ) 1 Autothermal w=0 1 Autothermal f = Air ratio Air ratio w= f = Temperature deg C 0.2 w= Temperature deg C

28 THE COUPLED REACTORS Flue gas, F fgas F s m gas T 1 T 2 Fuel air F s steam With fuel bunt m f bξ b =B and flue gas B f g v =F f,gas, the heat transferred between the two reactors 1 and 2 is BH = Fc ( T T ) + F c ( T T ) u, b s pms 1 2 f, gas pmg 1 o With the adiabatic temperature T = H /( F c ) + T ad u, b f, gas pmg 0 The flow of solids between the reactors F = F c ( T T) /( T T ) c s f, gas pmg ad pms

29 ENERGY TRANSPORT BETWEEN COUPLED REATORS

30 CONCLUSIONS A simple balance model can be used for performance analysis The limitations are: 1. The amount of char gasification ϕ gas has to be estimated by more advanced modelling. 2. The gas composition has to be predicted by additional models, e.g. equilibrium models in combination with species balances. However, the formulation gives this information with a low resolution: produced gas+water vapour from moisture + combustion gas. 3. Fluidisation conditions and reactor dimensions have to be determined

31 CONCLUSION REGARDING THE PERFORMANCE The energy in the char is about equal to the quantity of heat required for the gasifier. So, no gasification is really needed, only devolatilisation. There is an effort to design autothermal reactors to avoid the predominant combustion of volatiles and instead burn char. The location of the control surface for balance has to be considered (what is to be included/excluded). In the allothernal case just one point of operation balances the fuel burnt and the heat requirement. In all other points there is either too much char (loss) or too little char (additional fuel is needed).

32 REFERENCES 1. D. Kunii, O. Levenspiel, Fluidization Engineering, Butterworth-Heinemann, ISBN , H. Leion, T. Mattisson, A. Lyngfelt, Solid fuels in chemical-looping combustion, Int. J. Greenhouse Gas Control 2 (2008) K. Svoboda, S. Kalisz, F. Miccio, K. Wieczorek, M. Pohorelý, Simplified modeling of circulating flow of solids between a fluidized bed and a vertical pneumatic transport tube reactor connected by orifices, Powder Technology 192 (2009) J. Corella, J.M Toledo, G. Molina, A review on dual fluidized biomass gasifiers, Ind. Eng. Chem. Res. 46 (2007) W.K. Lewis, E.R. Gilliland, Production of pure carbon dioxide, US Pat (1954). 6. H. Thunman, L.-E. Åmand, B. Leckner, F. Johnsson, A cost effective concept for generation of heat, electricity and transport fuel from biomass in fluidized bed boilers using existing energy infrastructure, 15th European Biomass Conf., Berlin, CM. van der Meijden, A. van der Drift, BJ. Vreuugdenhil, Experimental results from the allothermal biomass gasifier Milena, 15th European Biomass Conf., Berlin, NH. Florin AT. Harris, Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents, Chem. Eng. Sci. (2008) A. Gómez-Barea and B. Leckner, Modeling of biomass gasification in fluidized bed, Progress in Energy and Combustion Science 36 (2010)