TGA testing of biomass char gasification Antero Moilanen

Transcription

1 TGA testing of biomass char gasification Antero Moilanen Finnish-Swedish Flame Days 2013 Gasification Workshop Paviljonki, Jyväskylä, 17th 18th of April 2013

2 2 Biomass fuel as feedstock Fuel is fed to the reactor Water (moisture) and volatiles are released (pyrolysis) rapidly Residual carbon reacts slower Reactions depend on the characteristics of biomass H 2 O Volatiles Char residue Ash 20 C 100 C C C

3 3 In TG, biomass char gasification is tested The achievement of total carbon conversion is influenced by the char gasification reactivity Char structure formed during pyrolysis: heating rate, temperature gaseous environment (steam, CO2, product gases) porosity Catalyst material (ash forming constituents) Chemical structure Surface area of the catalytic particulate material in contact with carbon Dispersion TG result (mass vs. time) includes all these phenomena as a summary Shape of the reaction rate vs. conversion graph (reactivity profile)

4 4 TG char gasification (cont.) Gasification studies published in literature are mainly directed to char prepared separately In the thermobalance, where fresh biomass is used as sample, the char is formed in situ and it is difficult to define the exact point, where char gasification starts (i.e char conversion = 0) The problematic point is situated in the phase where post pyrolysis and initial char gasification are overlapping.

5 5 TG char gasification (cont.) Heating rate, affects the char reactivity through char formation Various char formation reactions have more time during the slow heating leading to char having low reactivity Also the structure and chemistry of the inorganic catalytically active material is influenced by the heating rate. Heating rate should resemble the feeding to the hot gasifier Feedstock Fast heating rate: small particles and particle surfaces Slow: Cores of large particles ( pieces) In literature, it is often used separate char samples prepared with slow heating rate; the reactivity values obtained may not be usable for reactor design or modelling

6 6 TG char gasification (cont.) Other factors affecting char and its reactivity: Gaseous environment during pyrolysis Pressure Temperature Char formation mechanisms and its influence on reactivity are not actually well known

7 7 Test procedure in VTT s TG - 1 Reactivity measurements in TG are based on the sample weight change as a function of time at constant T (isothermal) and in the desired gas (pure reactants H2O, CO2, their mixture or mixed with product gas. Aiming at (pressurised) fluidised bed gasification Temperature level < 1000 ºC Pressure range 1 20 bar Sample size of mg fresh dried biomass is put into a sample holder having a good gas - solid interaction without mass transfer effects Fast heating rate of the sample to the reaction temperature is essential. In TG tests in VTT, the sample is driven fast into the reactor conditioned with the reaction environment, i.e. temperature, pressure and gas. This input is arranged with a chain winch, which lowers the sample holder fast (within few seconds (abt 7s) into the hot reactor. During the input the sample heats up, pyrolyses and char is formed. Fast heating rate can also be achieved with a special jump mode of the reactor furnace (e.g. TA-instruments at SINTEF)

8 8 Test procedure in VTT s TG - 2 When the sample holder has reached the lowest position the weight recording starts Before the start of the weight monitoring, the sample has pyrolysed partly during the input. In the tests, it has been observed that sample still has raw unpyrolysed biomass at 15 seconds from the initiation of the weight reading. At 30 seconds no fresh biomass is seen, so it takes about seconds for the completion of the pyrolysis to the char During the first 100s., there is overlapping char gasification and post-pyrolysis at 850ºC causing the sample weight loss. For this reason it is difficult read the exact point of the char conversion = 0

9 9 Sample holder Microbalance data acquisition PRESSURIZED THERMOBALANCE Pressure range 1-90 bar Temperature max = 1000 C He-flushing Ø16mm Expansion valve Filter Steam condenser Winch system Sample lock Reactor Thermocouple/ Pyrometer data acquisition Steam generator N 2 CO 2 H2 CO Water pump

10 10 Cylindrical sample holder with wire mesh Ø 16 mm reactor tube centre shaft Local gas circulation wire mesh 24 mm 17 mm Cup for comparison 11 mm Gas flow fuel sample 5 mm 10 mm View from the side View from the top (cross section)

11 11 Fast heating of sample in TG -1 Sample holder - Lowering of the sample to the reactor with a winch (abt 7 seconds) Hot reactor

12 12 Fast heating of sample in TG -2 - Jump mode in the heating of the furnace, example TA Instruments of SINTEF Real mass, mg Mass Temperature T,ºC Time, s 100

13 13 Slow heating rate, usual in TG Spruce bark N2 + CO2, 850ºC K/min Mass, mg T ºC CO 2 on Time, s 0

14 14 TG curve, isothermal test VTT Real mass, mg Pine wood 1 bar H 2 O, 850 C Devolatilization, pyrolysis - In the same gas as the char reactivity is measured Char gasification T, ºC Real mass,mg Spline mass, mg T,C Time, s

15 15 WEIGHT AS A FUNCTION OF TIME, isothermal test Devolatilization - In the same gas as the char reactivity Weight, mg char gasification ash Reaction rate is calculated from the weight-time curve Time

16 16 Fuel conversion Fuel conversion, ash free = 100* Sample weight m2 % Sample weight ash weight Weight, mg m2 Ash Time, seconds

17 17 Reactivity: rate vs. conversion Weight, mg Char gasification rate m1- m2 t2 - t1 mg seconds m1 m2 Instantaneous char gasification rate m1- m2 1 ( t2 - t1) m2 s t1 t2 Time, seconds

18 18 Result:Instantaneous reaction as a function of fuel conversion; reactivity profile Instantaneous reaction rate %/min char gasification pyrolysis Fuel conversion, %

19 19 Char conversion zero, i.e. the starting point for char gasification Bark pellet Mönsterås 1 bar H2O 850 C mass, mg Area of the simultaneous post pyrolysis and char gasification time, s

20 20 Char sample at 15 seconds at 850ºC Fresh biomass 500 m

21 Selection of the point where char conversion = 0 Instantaneous reaction rate, 1/s Time from the beginning 30s 60s 100s Char conversion, -

22 22 TG curve Mass- time curve obtained in TG includes all the phenomena of gasification Development of pore structure Char carbon properties Catalysis Changes in catalytic active material Reactions, evaporation, mobility Characteristic TG curve Different biomass (species) but also plant parts (bark, heart wood, stump)

23 23 Reaction rate vs. char conversion reactivity profile Instantaneous Reaction rate 0 1 Conversion Reactivity profile biomassspecific - increasing and decreasing trends - minimum rate

24 24 Reactivity profile - model Ref.:Umeki, K., Moilanen, A., Gómez-Barea, A., Konttinen, J., A model of biomass char gasification describing the change in catalytic activity of ash. Chemical Engineering Journal vol (2012) works for CO2 gasification, steam needs to be developed

25 25 Gasification reactivity of various biomasses and tree parts Ref: Antero Moilanen & Muhammad Nasrullah. Fundamental studies of synthesis-gas pro-duction based on the fluidized bed gasification of biomass Project UCGFunda. Gasification reactivity and ash behaviour. Espoo VTT Publications 769.

26 26 Conditions in the fluidised bed gasification affecting char conversion and to be tested in TG Temperature C Pressure 1 20 bar (abs) H2 CO CO2 H2O H2 CO CO2 H2O (Fragmented and attrited char) Product gas -H 2,CO, CH 4 - tar -CO 2, H 2 O, N 2 - unreacted char In the reactor, there is char material (particles) at different conversion degree having different gasification reaction rates Fuel O 2 /H 2 O Accumulated char

27 27 Effect of temperature spruce bark, 1 bar, 100% H2O Instantaneous reaction rate, %/min C C C Fuel conversion (Ash free), %

28 28 Temperature dependence of gasification (steam) rates of various fuel chars Ea= ca. 210 kj/mol

29 29 Effect of pressure Spruce bark T = 850 C, 100% steam Instantaneous reaction rate, %/min Fuel conversion (Ash free), % 10 bar 5 bar 1 bar

30 30 Effect of pressure (cont.) on steam and CO2 gasification r" min. (%/min) Pine sawdust Pine bark Forest residue (pine) Salix Wheat straw Barley straw Reed canary grass Miscanthus Sweet sorghum Kenaf 30 bar H2O 1 bar H2O 30 bar CO2 1 bar CO2 Under pressure the reactivity higher in steam Biomass

31 31 Effect of product gas: Different biomasses in gas mixtures T= 875 C & total pressure 1 bar ( 0.3 bar H2O, 0.2 bar H2, 0.25 bar CO2, 0.15 bar Instantaneous reaction rate, %/min CO & 0.1 bar N2) Fuel conversion (Ash free), % = spruce bark 1 bar steam (100%) at 875 C for comparison Bark: Aspen Moilanen, A., Nasrullah, M. Variation in fuel Reactivity and ash characteristics of biomass feedstock for large-scale gasification. Pres. in 17th European Biomass Conference and Exhibition. 29 June - 3 July 2009, Hamburg, Germany Aspen bark (o) Spruce Spruce bark (o) Birch Birch bark (o) Pine Pine bark (o)

32 32 Product gases also inside the char pores char H 2 O H 2 O CO CO CO 2 CO 2 H 2 H 2 H 2 O CO H 2 CO 2

33 Formulas Steam hydrogen : CO 2 - CO: Gas mixture: Temperature dependence H2O 1 1 P H O H P r P r r R CO PCO r P r r R CO2 4 1 P CO CO H O H P r P r P r P r r r R CO2 4 H2O 1 1 P P RT E ke r i i /

34 34 Gasification reactivity of various biomasses steam 850 C 1 bar 30 bar Instantaneous Reaction Rate,%/min 150 Sawdust 100 (pine) Wheat straw Salix Reed canary grass Pine bark Barley straw Miscanthus Sweet sorghum Kenaf Fuel conversion, % Fuel conversion, %, % Fuel conversion, %, % 3 bar H bar H 2 O

35 35 Classification of ash sintering/melting after a TG test under microscope powder o molten *** sintered * - **

36 36 TGA testing of biomass char gasification Summary Fast heating rate In the determination of the kinetic parameters for gasification reactions, the suitable testing conditions shoud be sought for each biomass The effect of pyrolysis conditions on char reactivity Temperature pressure - gaseous environment need to be studied and fixed for testing Char gasification tests in steam, CO 2, and their mixtures and product gas components H 2 and CO

37 37 TGA testing of biomass char gasification Acknowledgements: Financing 1. Nordic Energy Research Top-Level Research Initiative (Nordsyngas) 2. The Academy of Finland (GASIFREAC-project) Collaboration Prof. Kentaro Umeki LTU; Prof. Alberto Gómez-Barea Univ. Seville; Prof. Jukka Konttinen JYU; Mr. Jasob Kramb JYU; Dr. Liang Wang SINTEF; Prof. Rainer Backman UMU; Dr. Nikolai DeMartini ÅA; Mr. Jere Lehtinen VTT; Ms. Mirja Muhola VTT