Drying, devolatilization & char oxidation of solid fuel

Transcription

1 Drying, devolatilization & char oxidation of solid fuel Oskar Karlström Dr. Sc. Åbo Akademi 2017: Chemistry in Combustion Processes

2 Solid fuel combustion

3 Solid fuel combustion fuel In pulverized fuel combustion, size of fuel pieces also called particles are around 0.1 mm

4 mass Solid fuel combustion What happens with the mass of one 0.1 mm single fuel particle in this furnace? 100% drying devolatilization fuel 50% char oxidation 0.2 s 0.4 s 0.6 s time

5 Terms and concepts Small vs large fuel particle Devolatilization/pyrolysis of small and large particles Char oxidation of small and large particles Kinetic regime Mass transfer regime Chemical kinetics control Mass transfer control Char gasification Gasification Coal types Difference in burning between coal and biomass Oxy-combustion

6 H 2 O Volatile matter Char Ash Drying Devolatilization Oxidation Ash

7 Typical proximate analysis of wood: Typical proximate analysis of a coal: H2O vol char ash H2O vol char ash less volatile matter, and less moisture in coal than in biomass coal forms more char than biomass

8 Drying Fuel + heat water vapor (endothermal: ΔH >0)

9 Energy balance during drying of a small (<0.1 mm) fuel particle Temp. Convective heat transfer Energy consumption Radiation heat transfer m p c p dt dt p hs p p 4 4 T T H S T T gas p dm dt p p R p Mass Specific heat Vaporization enthalpy h k gas d Nu External surface area Surface emissivity

10 Devolatilization - pyrolysis Fuel + heat volatiles + char (ΔH 0) Volatiles CO CO 2 H 2 H 2 O Hydrocarbons Tars Exact volatile composition depends on the heating rate

11 Devolatilization - pyrolysis As the volatiles are oxidized a flame can be observed

12 Devolatilization of wood Hemicellulose start pyrolyzing at C Cellulose start pyrolyzing at C Lignin start pyrolyzing at C Certain extractives such as terpenes (few % in wood) start pyrolyzing at less than 225 C

13 Hemicellulose Total Cellulose Lignin Di Blasi 2008

14 Devolatilization of coal Fossil coals can be divided into anthracite, bituminous coal and lignite Volatile matter very low for anthracites Volatile content higher for lignite than for bituminous coals Anthracite Bituminous coal Lignite

15 Devolatilization of lignite Lignite: starts to pyrolyse at C releasing CO and CO 2 At around C also hydrocarbon vapors, tars and H 2 are produced

16 Devolatilization of bituminous coals Bituminous coals becomes plastic and can swell remarkably during pyrolysis CO and CO 2 is formed but to a less extent than for lignite, due to lower oxygen content Tars, hydrocarbons, and soot are formed at temperatures higher than 400 C

17 Devolatilization of anthracite Low volatile content of anthracite

18 What happens with T for a 0.05 mm wood particle if we put in 1000 C? What happens with T for a 6 mm wood particle if we put in 1000 C? External surface of particle External surface of particle 1000 C 1000 C 600 C Center of particle 600 C Center of particle 200 C 200 C devolatilization time (s) Time (s)

19 Drying and devolatilization: in sequences or simultaneously? For small particles (< 0.1 mm), drying and devolatilization occurs in sequences For large particles, drying and devolatilization occurs partly simultaneously due to temperature gradients inside the particle

20 Drying and devolatilization: in sequences or simultaneously? How do we determine this? Radius Bi k rh eff c,particle Heat transfer coefficient High Bi temperature gradient significant Low Bi temperature gradient negligible Thermal conductivity

21 Char Char yield of wood around 15% on dry basis Char yield of coal up to more than 90% on dry basis Devolatilization Char Oxidation

22 Biomass char (Di Blasi 2011) Biomass char yield slightly dependent on heating rate

23 Coal char Coal char yield strongly dependent on heating rate (Kobayashi et al. 1976)

24 Char Biomass Lignite Bituminous coal Anthracite HHV (MJ/kg) db Char yield % (dry basis) Aging

25 Biomass Lignite Bituminous coal Anthracite HHV (MJ/kg) db C % daf Char yield % db Char yield % db 8 60 H % daf O % daf Char yield % db Char yield % db

26 Char Char Oxidation generally significantly slower than Devolatilization: important to consider in the design of combustion units Devolatilization Char Oxidation

27 Char Organic and inorganic part Organic part consists of C, N, O, S, H Of the organic part, almost 100% on mass basis is C Oxidation of char refers to heterogeneous oxidation of solid carbon, C(s) Heterogeneous combustion: C(s) + O 2 (g) CO 2 (g) Homogeneous combustion: 2CO(g) + O 2 (g) 2CO 2 (g)

28 Char oxidation O 2 C(s)+ O 2 (g) CO/CO 2 (g) (ΔH<0) C(s) m p c p dt dt CO/CO 2 p hs p p 4 4 T T H S T T gas Tp T gas p dm dt < 0 p p R p

29 Char oxidation Char is porous Oxidation reactions occurs inside char particle and at the external surface of the char particle

30 Char oxidation O 2 C(s) CO/CO 2 The faster the reaction (kinetics), the shorter way the oxygen will penetrate inside the char If the reactions are very fast, all the reactions take place at the external surface

31 O 2 conc. O 2 concentration inside and outside char particle 0.6 Inside particle Outside particle RI RII RIII Particle Radius

32 Char oxidation limited by mass transfer if particles are large (>1 mm) limited by chemical kinetics if particles are small (<0.05 mm) Otherwise limited by the combined effects of chemical kinetics and mass transfer Regime III Regime I Regime II

33 2000 Temp. ( o C) RII RIII 500 RI particle diameter (m) (For a typical bituminous coal char, note that the diagram is fuel dependent)

34 Chemical kinetics control (Regime I) Fuel type and mass limits the char oxidation rate Char oxidation rate is strongly temperature dependent Rate not limited by external mass transfer Mass transfer control (Regime III) External surface limits char oxidation rate Char oxidation rate is not strongly temperature dependent 2000 Temp. ( o C) RII RIII 500 RI particle diameter (m)

35 Chemical kinetics control (Regime I & Regime II) High internal surface area gives high kinetic rate Biomass chars have higher internal surface area (up to 1000 m 2 /g) than coal chars ( m 2 /g) Catalytic elements such as Na, K, Mg, Ca can reduce activation energy and enhance rate Biomass char has higher fraction of catalytically important elements than coal char Biomass chars significantly more reactive than coal chars with respect to O 2

36 Internal surface area Mass Efficiency factor Concentration Activation energy dm dt A m i c O 2 Ae E / RT p Temperature RIII => 0 RI => 1 Low E gives high rate High T p gives high rate For coal chars E is around 160 kj/mol for char oxidation by O 2 For biomass chars E is generally around 100 kj/mol for char oxidation by O 2

37 Char oxidation and char gasification In combustion, char is always oxidized to some extent by CO 2 and H 2 O Char oxidation by CO 2 and H 2 O is called char gasification Char gasification reactions are important in combustion systems and gasification systems Note: char gasification and gasification have different meaning

38 Char oxidation Char gasification O 2 CO 2 H 2 O Char particle Char particle Char particle CO exothermic 2CO endothermic H 2 CO

39 Combustion Gasification N 2, CO 2 electricity heat Some CO 2 CO, H 2 Combustion Gasification Fuel Air Fuel Insufficient amount of air

40 Gasification In combustion the carbon and hydrogen reacts to CO 2 and H 2 O In gasification, the goal is to produce a gas rich in CO and H 2 that for example can be burned in a gas boiler In gasification systems an insufficient amount of air is used, so that the oxidation of gases is incomplete high levels of CO and H 2

41 Waste gasification in Lahti Fuel handling 2.FB gasifier 3.Gas cooling 4.Gas filter 5.Gas fired boiler 2.

42 Oxy-combustion A combustion technique for the future? Fuels are combusted in O 2 instead of air To avoid extremely high temperatures, the CO 2 is partly recirculated The goal is to produce electricity, but so that the formed CO 2 is stored

43 Combustion Oxy-combustion Storage electricity electricity N 2, CO 2 heat CO 2 heat Combustion Oxy- Combustion Coal Coal Air O 2

44 Terms and concepts Small vs large fuel particle Devolatilization/pyrolysis of small and large particles Char oxidation of small and large particles Kinetic regime Mass transfer regime Char gasification Gasification Coal types Difference in burning between coal and biomass Mass transfer control Chemical kinetics control Oxy-combustion

45 ...stay tuned for part II of this course