Development of clean coal technology by using the feature of oxy-fuel combustion

Transcription

1 Development of clean coal technology by using the feature of oxy-fuel combustion Hirotatsu Watanabe Department of mechanical and control engineering, graduate school of science and engineering, Tokyo Institute of Technology 4th oxy-fuel capacity building course, 2/3 September, 2012 Tokyo Institute of Technology, Japan

2 Introduction 2 CO 2 has attracted unfavorable attention as one of the greenhouse gases, which is the main cause of global warming. O 2 /CO 2 combustion (oxy-fuel combustoin) is seen as one of the major options for CO 2 capture for future clean technologies. Caprock CO 2 transport Coal firing plant Saline aquifer CO 2 Fig. Schematic diagram of CCS

3 Introduction 3 Air Coal O 2 ASU (Air Separation Unit) Over 95% of CO 2 Boiler Flue gas treatment Recycled gas Fig. Exhaust gas concentration Fig. Oxy-fuel combustion and CCS system CO 2 capture Direct CO 2 recovery becomes possible without additional energy consumption. Another approach for reducing CO 2 emission is the use of renewable fuels such as biomass CO 2 N 2 O 2

4 Introduction 4 CO 2 Coal/Biomass or Biomass Air ASU (Air Separation Unit) O 2 Over 95% of CO 2 Boiler Recycled gas Fig. Exhaust gas concentration Flue gas treatment Fig. Oxy-fuel combustion and CCS system CO 2 capture O 2 /CO 2 biomass combustion with CCS can be used as a sink for CO 2. O 2 /CO 2 coal or biomass combustion is a promising technology for reducing CO 2 emission N 2 O 2

5 5 Introduction Small scale facilities are useful for fundamental mechanism clarification of O 2 /CO 2 combustion Demonstration plant Small experimental facilities Scaledown Cylinder of CO 2 Cylinder of Ar Vacuum pump Flow meter Reaction tube Infrared furnace Valve Valve Valve PC To tar trap Biomass sample Thermometric point P Pressure gauge

6 6 Introduction What are differences between O 2 /N 2 and O 2 /CO 2 combustion? Recycling process High CO 2 concentration Heat transfer characteristics (Thermophysical Properties of CO 2 ) CO 2 chemical reactivity CO 2 is not inert but participates in chemical reactions primarily through the reaction (CO 2 + H = CO + OH)

7 Introduction 7 By using CO 2 chemical reactivity, clean coal technology for oxy-fuel combustion is potentially developed Mechanism clarification through fundamental research is required High CO 2 concentration Minerals (Na,,) Coal or biomass CO 2 chemical reactivity affects gas/solid phase reaction Gas phase reaction Volatile-N NO or N 2 Solid phase reaction Carbonate formation (Na 2 CO 3 )

8 8 Introduction Our laboratory has used different experimental facilities and calculation for mechanism clarification of O 2 /CO 2 coal and biomass combustion Flat flame reactor, Drop tube furnace, TGA Detailed chemical reaction kinetics Flat flame Cylinder of CO 2 Flow meter Reaction tube Infrared furnace Valve To tar trap Biomass sample Valve Cylinder of Ar Thermometric point Primary gas (CH 4, O 2, CO 2, NH 3 /Ar) Vacuum pump Valve PC P Pressure gauge Fig. Flat flame reactor Fig. Drop tube furnace Fig. Thermobalance

9 9 Table of contents Effect of CO 2 on gas phase reactions Ultra-low NO x emission by using CO 2 chemical reactivity Effect of CO 2 on solid phase reaction Salt formation during biomass pyrolysis Summary

10 10 Table of contents Effect of CO 2 on gas phase reactions Ultra-low NO x emission by using CO 2 chemical reactivity Effect of CO 2 on solid phase reaction Salt formation during biomass pyrolysis Summary

11 11 Ultra-low NO x emission Uniform field Low NO x emission Recycles gas (Mainly CO 2 including NO x ) Fig. NO x conversion ratio in O 2 /CO 2 combustion [1] NO x emission decreased to 1/7 owing to recycling process when equivalent ratio is assumed to be uniform O 2 /CO 2 combustion has potential for reducing further NO x emission by combined with staged combustion [1] Liu and Okazaki, Fuel 2003

12 Ultra-low NO x emission 12 NO x, HCN and NH 3 formation are inhibited in fuel-rich region Primary gases Coal with gases Secondary gases Fuelrich Fuellean Air ratio (excess O 2 ratio) High Low High conc. of CO 2 Fig. Staged combustion (without recycling NO x ) The effect of high CO 2 concentration on NO x formation and reduction mechanisms under staged combustion is discussed

13 13 Contents This research NO x formation and reduction mechanism in staged O 2 /CO 2 combustion and air combustion were investigated. A flat CH 4 flame doped with NH 3 for fuel-n was used, and measurements were performed. CHEMKIN-PRO was used to investigate a detailed NO x reduction mechanism

14 Flat flame reactor 14 A large part of the fuel conversion in the combustion process occurs in the gas phase Flat flame Honeycomb Quartz tube Primary gas (CH 4, O 2, CO 2, NH 3 /Ar) Fig. Flat flame reactor Flat flame is very useful for one-dimensional analysis and mechanism clarification of gas phase reactions

15 Exhaust Flow Meter Chemiluminescent NO x detector Primary combustion GC Pump Silica-gel Cold Trap Exhaust Insulator Quarts Tube Flow Controllers Flat flame Primary gas (Air, CH 4, NH 3 or O 2, CO 2, CH 4, NH 3 ) Air CH 4 CO 2 O 2 comp- NH 3 ressor (Ar) Fig. Schematic diagram of experimental apparatus Pre-heater Ceramic honeycomb NO x, HCN, NH 3 emissions in air or O 2 /CO 2 combustion were investigated under primary combustion

16 Primary combustion 16 Flat flame Table 1 Experimental conditions (primary combustion) Initial O 2 conc. [vol. %] Initial NH 3 conc. [vol. %-CH 4 ] primary Primary gas (CH 4, Air, NH 3 /Ar.or. CH 4, O 2, CO 2, NH 3 /Ar) Air Oxy-fuel Primary combustion characteristics are important to discuss NO x emission in staged combustion. HCN, NH 3 (gas detector) and NO x emission of primary combustion were measured.

17 OH radical measurement 17 OH radicals are relevant with NH 3 and HCN formation and decompositon. OH* chemiluminescence images of flat flame was acquired by ICCD camera Interference filter (306.3 nm) 26 p = 0.70 CCD camera Lens Flat flame SUS mesh (a) CCD camera with (b) Flat flame interference filter Fig. Schematic diagram of OH * measurement system Primary gas (CH 4, O 2, CO 2, NH 3 /Ar)

18 Exhaust Flow Meter Chemiluminescent NO x detector Flow Controllers Staged combustion GC Pump Silica-gel Cold Trap Secondary gas (Air or O 2 +CO 2 ) Exhaust Insulator Flat flame Primary gas (Air, CH 4, NH 3 or O 2, CO 2, CH 4, NH 3 ) Air CH 4 CO 2 O 2 comp- NH 3 ressor (Ar) Fig. Schematic diagram of experimental apparatus 26 Quarts Tube Pre-heater Ceramic honeycomb NO x reduction by staged combustion was investigated 18

19 Staged combustion 19 a Air ratio (excess O 2 ratio) F FO O 2 2 F F CH CH 4 4 st F : flow rate [l min -1 ] Secondary gas (Air or O 2, CO 2 ) 26 secondary a Secondary gas Flat flame primary 30 mm SUS mesh Ceramic honeycomb Primary gas (CH 4, O 2, CO 2, NH 3 /Ar) Fig. Mixing part Primary gas (CH 4, O 2, NH 3 /Ar) Fig. Flame photograph

20 Experiment conditions 20 Table 1 Experimental conditions (staged combustion) Air Oxy-fuel Initial O 2 conc. [vol. %] Initial NH 3 conc. [vol. %-CH 4 ] s = 1.2 s = 1.2 s = 1.2 p = 0.60 Secondary gas (Air or O 2, CO 2 ) Primary gas (CH 4, Air, NH 3 /Ar, or CH 4, O 2, CO 2, NH 3 /Ar) Nomenclature : : Air ratio or O 2 /CH 4 stoic. ratio Flat flame p = 0.65 Primary gas Flat flame p = 0.70 Primary gas Staged combustion experiments were performed by changing primary air ratio (excess O 2 ratio)

21 21 Results (staged combustion) s = 1.2 Secondary gas (Air or O 2, CO 2 ) primary 40 % decreased due to CO 2 reactivity Primary gas Fig. Experimental apparatus Primary O 2 /CH 4 stoic. ratio [-] Fig. NO x CR (Conversion Ratio) [2] [2] H. Watanabe et al. Combustion and Flame 2011 The lowest NO x CR of O 2 /CO 2 combustion is lower than that of air combustion by 40 %

22 Results (primary combustion) N-min = 0.7 N-min = O 2 /CH 4 stoic. ratio [-] Fig. 1 Air-fuel combustion O 2 /CH 4 stoic. ratio [-] Fig. 2 O 2 /CO 2 combustion HCN and NH 3 concentrations in O 2 /CO 2 combustion are quite low compared with those of air combustion [2] H. Watanabe et al. Combustion and Flame 2011

23 23 Results x 10 0 (a) Air combustion (b) O 2 /CO 2 combustion Fig. Measured OH * images at O 2 /CH 4 stoich. ratio stoich. of 0.7 OH* concentration is higher in O 2 /CO 2 combustion than in air combustion Fig. Measured profiles of OH * chemiluminescence at O 2 /CH 4 ratio of 0.7 The following reaction is progressed in O 2 /CO 2 combustion CO 2 + H CO + OH

24 Reactions related with OH 24 Reaction paths relevant with NH 3 and OH NH 3 + OH NH 2 + H 2 O NH 2 + OH NH + H 2 O NH + NO N 2 + OH Reaction paths relevant with HCN and OH HCN + OH HNCO + H HNCO + H NH 2 + CO OH radicals progress the decomposition of NH 3 and HCN

25 CHEMKIN-PRO calculation 25 Primary gas Mesh Flat flame One-dimensional plug flow reactor Temperature [K] [mm] Air Oxy Air Oxy Distance from mesh [mm] Fig. Temperature distribution NO x formation mechanisms in primary combustion were investigated with detailed chemical reaction kinetics (GRI-Mech 3.0) Table Calculation conditions O 2 /CH 4 stoich. ratio [-] 0.7 NH 3 conc. [vol. %-CH 4 ] 1.0

26 Fig. Predicted OH concentration distribution in O 2 /CO 2 combustion Calculation results (OH) 26 Mesh Flat flame Primary gas [mm] Calculation also shows that OH radicals in O 2 /CO 2 combustion is higher than in air combustion because of the reaction: CO 2 +H CO + OH

27 Results (primary combustion) 27 (a) Experiment (b) Calculation Fig. Exhaust NH 3, HCN, NO concentration ( = 0.7) [2] NH 3 and HCN are decomposed in O 2 /CO 2 combustion due to OH radical [2] H. Watanabe et al. Combustion and Flame 2011

28 Results (primary combustion) 28 (NO+HCN+NH 3 ) exhaust / NH 3,inlet [-] Exp. Total nitrogen-compounds emission (NO, HCN, NH 3 ) of O 2 /CO 2 combustion is lower than that of air combustion (NO+HCN+NH 3 ) exhaust / NH 3,inlet [-] Calc. 0 Air Oxy 0 Air Oxy Fig. The sum of exhaust NO, HCN and NH 3 concentration N 2 is easily formed during O 2 /CO 2 combustion

29 Reaction pathways 29 Calculation shows reaction pathways from NH 3 to N 2. NH formation by OH radical is important in N 2 formation Fig. Reaction pathways at primary = 0.6, total = 0.8 (X CO2,inlet = 0.70) [3] [3] H. Watanabe et al. Energy and Fuels 2012

30 Effect of OH radical 30 +OH HNO +O +OH +H NH 3 NH 2 NH +OH +H +NO +O +OH +NO N 2 O +H NO +OH N +NO N 2 Fig. Reaction paths related with N 2 formation Under fuel-rich condition HNO +OH +OH +O NO +OH +H NH 3 NH 2 NH N +OH +H +NO +NO +NO +O +OH N 2 O N +H 2 Fig. Reaction paths related with N 2 formation OH radical oxidizes NH, and converts to HNO OH radical contributes to NO formation when NH 3 does not remain OH radical produces NH, which is NO reduction agency OH radical contributes to N 2 formation when an amount of NH 3 remains

31 31 Conclusion (ultra-low NO x ) NO x formation mechanism in O 2 /CO 2 and air combustion was investigated experimentally and numerically. CO 2 chemical reactivity produced OH radical through (CO 2 + H CO + OH) The lowest NO x conversion ratio in O 2 /CO 2 staged combustion was lower than it in air staged combustion by 40 % due to CO 2 reactivity Reaction pathways from NH 3 to N 2 were revealed, and it was shown that OH radical contributed to N 2 formation when an amount of NH 3 remains

32 32 Table of contents Effect of CO 2 on gas phase reactions Ultra-low NO x emission by using CO 2 chemical reactivity Effect of CO 2 on solid phase reaction Salt formation during biomass pyrolysis Summary

33 33 Introduction Pyrolysis of solid fuel Pyrolysis occurs as the first step in solid combustion. Pyrolysis has been generally investigated under an inert gas such as N 2, Ar, He. Understanding pyrolysis under CO 2 is important for the design of O 2 /CO 2 biomass combustors Gas Solid fuel Tar Char Fig. Pyrolysis

34 34 Introduction What is pyrolysis difference between under inert gas and CO 2? Heat transfer characteristics (Themophysical properties of CO 2 ) Reaction of CO 2 with minerals (Ca, K, Na, Mg) Salt (carbonate) formation is expected. Heat CO 2 atomosphere Na, K,Ca Solid fuel Gas Tar Biomass ex.) Na 2 CO 3 Fig. Pyrolysis Char

35 35 Introduction This work Effect of CO 2 on pyrolysis process through mineral reactions is studied. Cellulose and lignin which are the main components of biomass are heated under CO 2, or Argon atmosphere Metal-depleted lignin is also used to investigate the effect of CO 2 on minerals in lignin The chemical composition of char is characterized by FT-IR (Fourier Transform Infrared Spectroscopy).

36 Experiment 36 Table 1 Ultimate analysis of sample (wt%) Sample C H N S Ash Cellulose Lignin (Alkali lignin) Metal depleted lignin When the metal-depleted lignin was prepared, metal was removed from the lignin by stirring a mixture of lignin, water, and ion-exchange resin Lignin stirring Separate, dry process Ion-exchange resin Metal depleted lignin

37 Experiment 37 Table 2 Ash composition [mg/g] Sample Na K Lignin (Alkali lignin) Metal depleted lignin Ash is removed by 80 % by ionexchange Lignin and metal depleted lignin were analyzed by using FT-IR No significant difference was observed between both samples except for -OH: -ONa Ion exchange -OH The difference is mineral content

38 Experiment 38 Cylinder of CO 2 Cylinder of Ar Vacuum pump Flow meter Reaction tube Infrared furnace Valve Valve Valve PC To tar trap Biomass sample Thermometric point P Pressure gauge Fig. Schematic diagram of thermobalance Table Experiment Surrounding conditions gas CO 2 or Ar Gas flow rate [l min -1 ] Heating rate [K s -1 ] 0.8 1, 10, 60 Thermogravimetric curve is measured. The surface chemistry of the char was investigated by FTIR

39 Results 39 [4] H. Watanabe et al. Proc. Combust. Inst. 2012, in press Weight fraction [mg/mg d.a.f.] Cellulose (Ash: 0 wt%) Weight fraction [mg/mg d.a.f.] Lignin (Ash: 18.2 wt%) Char-CO 2 reaction (> 1100 K) Temperature [K] Fig. Pyrolysis curves of cellulose (Ash: 0 wt%, Heating rate : 1 Ks -1 ) Temperature [K] Fig. Pyrolysis curves of lignin (Ash: 18.2 wt%, Heating rate : 1 Ks -1 ) Contrary to expectations, the weight of the lignin chars formed under CO 2 increased by about 10 % above 873 K

40 40 Results Weight fraction [mg/mg d.a.f.] Why increased? Lignin (Ash: 18.2 wt%) Temperature [K] Fig. Pyrolysis curves of lignin (Ash: 18.2 wt%, Heating rate : 1 Ks -1 ) CO 2 physical adsorption Carbonate formation have the potential to cause an increase in the weight fraction due to CO 2 Char weight does not change by a degassing procedure (5 kpa for 1 h). CO 2 physical adsorption is insignificant Carbonate is expected to be formed during pyrolysis

41 Results Weight fraction = Sample weight Initial weight of CaO 41 Minerals can react with CO 2, and salt form. (Ⅰ) CaO + CO 2 CaO CO 2 (Ⅱ) CaO CO 2 CaCO 3 (Ⅲ) CaCO 3 CaO + CO 2 Weight fraction [-] CaO heating under CO 2 (Ⅰ) (Ⅰ+Ⅱ) (Ⅲ) There is a possibility that Na or K in lignin reacts with CO 2, and Na 2 CO 3 and K 2 CO 3 are formed Temperature [K] Fig. Thermogravimetric curves of CaO (Heating rate : 1 Ks -1 )

42 Results 42 [4] H. Watanabe et al. Proc. Combust. Inst. 2012, in press Weight fraction [mg/mg d.a.f.] Lignin (Ash: 18.2 wt%) Metal-depleted Lignin (Ash: 3.2 wt%) Temperature [K] Temperature [K] Fig. Pyrolysis curves (Heating rate : 1 Ks -1 ) Mineral components in lignin react with CO 2, and carbonate is expected to be formed

43 FT-IR analysis 43 Weight fraction [mg/mg d.a.f.] Lignin (Ash: 18.2 wt%) Temperature [K] Fig. Pyrolysis curves of lignin (Ash: 18.2 wt%, Heating rate : 1 Ks -1 ) Surface chemistry of chars derived under CO 2 or Ar at 1073 K were investigated by FTIR to investigate carbonate formation. Na 2 CO 3 was also characterized by FTIR as reference

44 FT-IR 1450 cm -1 Aromatic C-C Carbonate 880 cm -1 Carbonate 44.] ụ [a ity s n te In Na 2 CO 3.] ụ [a ity s n te In Char derived from lignin (at 1073 K) Ar CO2 CO 2 Ar Wave number [cm -1 ] Fig. FTIR spectra of Na 2 CO 3 and char derived from lignin

45 FT-IR [4] H. Watanabe et al. Proc. Combust. Inst. 2012, in press 45.] ụ [a ity s n te In Ar CO cm -1 Table The atomic group and structures Ar CO 2 Wave number (cm -1 ) 1450 Atomic group and structures Aromatic C-C Carbonate Peak area corresponding to carbonate under CO 2 is almost twice of that under Ar Wave number [cm -1 ] Fig. FTIR spectra of chars focusing on 1450 cm -1 A salt such as Na 2 CO 3 or K 2 CO 3 is formed during lignin pyrolysis under CO 2

46 FT-IR 1730 cm -1 The difference of peaks for C=O appeared (1730 cm -1 ). 840 cm ] ụ [a ity s n te In Na 2 CO 3.] ụ [a ity s n te In Char derived from lignin (1073 K) Ar CO2 CO 2 Ar Wave number [cm -1 ] Fig. FTIR spectra of char derived from lignin and Na 2 CO

47 .] ụ [a s ity n te In Ar CO2 Results Ar CO Wave number [cm -1 ] Fig. FTIR of spectra focusing on C=O group (Char, 1073 K) Table The atomic structures Wave number (cm -1 ) Atomic group and structures C=O Over 1700 Below 1700 C=O groups which are not in conjunction with aromatic ring are only found in chars formed under CO 2 [4] H. Watanabe et al. Proc. Combust. Inst. 2012, in press C C O C O 47

48 Heating rate 48 Char yield [mg/mg d.a.f.] % 8 % Ar CO 2 7 % 1 Ks Ks Ks -1 Fig. Char yield derived from lignin of 1073 K Although, an increase in char yield under CO 2 declines with increasing heating rate, carbonate is formed at various heating rate

49 49 Mechanism Alkaline compounds highly favor the carbonization, dehydration, decarboxylation, and demethoxylation reactions, leading to a modified carboneous structures. Sodium ion is very small and it can penetrate into the biomass textures and break intermolecular hydrogen bridges under heating. Breaking the hydrogen bridges by carbonate compound seems to form C=O group not associated with an aromatic ring. Futher investigations are required to clarify more detailed catalytic mechanisms

50 Conclusion (carbonate) 50 In this study, the effect of CO 2 on pyrolysis was investigated. Cellulose, lignin, and metal-depleted lignin pyrolysis experiment were performed. Pyrolysis of lignin, but not that of cellulose and metaldepleted lignin, was affected by CO 2. The salts such as Na 2 CO 3 or K 2 CO 3 were formed during lignin pyrolysis under CO 2 It was suggested that these salts affected the char formation reaction, in that, char formed during lignin pyrolysis under CO 2 had unique chemical bands

51 51 Table of contents Effect of CO 2 on gas phase reactions Ultra-low NO x emission by using CO 2 chemical reactivity Effect of CO 2 on solid phase reaction Salt formation during biomass pyrolysis Summary

52 52 Summary In this presentation, unique CO 2 characteristics such as OH radical and carbonate formation were presented. Under specific condition, OH radical formed by CO 2 reactivity can be used for low-no x emission. Carbonate was found during lignin pyrolysis under CO 2, while it was not found in air combustion.

53 Thank you very much for your kind attention