Pulverised Coal Pyrolysis and Char Combustion Characteristics in Simulated Air (O 2 /N 2 ) and Oxy-fuel (O 2 /CO 2 ) Conditions

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1 1 st Oxy-fuel Combustion Conference, Cottbus, Germany - 9 September 29 Pulverised Coal Pyrolysis and Char Combustion Characteristics in Simulated Air (O 2 /N 2 ) and Oxy-fuel (O 2 /CO 2 ) Conditions Renu K Rathnam Terry F Wall Liza K Elliott Yinghui Liu Behdad Moghtaderi Presented by Rohan Stanger Oxy-fuel Working Group, Chemical Engineering, The University of Newcastle, Callaghan, NSW 238, Australia.

2 OUTLINE OF PRESENTATION 2 Oxy fuel (O 2 /CO 2 ) Combustion Pulverised Coal Reactivity in Oxy fuel Conditions Hypothesis & Objectives of the Project Experimental Results and discussion Conclusions

3 OXY-FUEL (O 2 /CO 2 ) COMBUSTION 3 AIR N 2 COAL FURNACE H 2 O O2 CO ASU 2 + gases WET DRY RECYCLED FLUE GAS High CO 2 concentrations are encountered during oxy-fuel combustion lue for CO2 / Value for N2 (no unit) Va Thermal Conductivity Properties of CO2 in comparison to N2 Molar Heat Capacity Density Mass Diffusivity of O2 Higher emissivity affects radiative heat transfer Higher molar heat capacity affects gas and particle il temperatures Lower O 2 diffusivity affects char combustion reactivity Ignition and flame stability Properties of CO 2 are significantly different when compared to N 2

4 PULVERISED COAL REACTIVITY IN OXY-FUEL CONDITIONS 4 Pyrolysis characteristics O 2 /CO 2 p g Char formation Char reactivity T p Ash characteristics Emissions T g p s Lower O 2 diffusivity O 2 availability on the surface of the particle (p s ) Higher specific heat gas & particle temperatures (T g & T p) Char CO 2 gasification reaction chemical reactivity of char

5 OBJECTIVES 5 Hypothesis The reactivity of pulverised coal is significantly different during oxy-fuel combustion when compared to air combustion under high temperature and low oxygen partial pressure conditions. o o o To measure and compare the reactivity of a range of coals in air and oxy fuel conditions and evaluate the impact of these differences on the design and operation of oxy fuel technology To identify the conditions under which the reactivity is significantly different in air and oxy conditions To obtain the intrinsic and apparent kinetic parameters (activation energy, pre exponential factor, reaction order) of char combustion Practical applications o Estimate differences in furnace operation and performance between air and oxy fuel for a range of coals o Propose modifications to obtain better burnout in an oxy fuel combustor or a reduction in O 2 usage

6 EXPERIMENTAL TECHNIQUES 6 Coals selected for study A suite of coals of varying ranks o Three international coals (Coals A, B, C) o Five Australian coals (Coals D, E, F, G, H) Techniques and Methodology o o Pyrolysis and char reactivity measurements Thermogravimetric Analyzer (TGA) Drop Tube Furnace (DTF) Coal and Char characterisation Proximate and ultimate analysis, ash analysis, maceral content analysis Scanning Electron Microscope (SEM) analysis char structure characterisation Internal Surface Area Measurements char internal surface area Particle Size Analysis coal and char particle size distribution

7 COAL PROPERTIES & RANKING 7 Coal property (wt. % daf basis) Carbon Volatile matter Coal A Coal E Coal G Coal D Coal F Coal C Coal B Coal H (wt. % daf basis) C content in coal ( Coal H Coal A Vitrinite Reflectance (%) Wt. % air dried basis Coal Proximate analysis Ultimate analysis Moisture Ash V M F C C H N S O Coal A Coal B Coal C Coal D Coal E Coal F Coal G Coal H

8 PYROLYSIS & CHAR REACTIVITY MEASUREMENTS 8 Air H 2 O Volatiles N 2, CO 2 Oxy-fuel CO 2 O O 2, N 2, CO 2 2 coal char carbon + mineral matter drying pyrolysis char oxidation + gasification unburnt carbon + ash ash formation Coal Pyrolysis reactivity in N 2 & CO 2 atmospheres TGA coal slow heating DTF coal rapid heating Coal Char O 2 Reactivity in O 2 /N 2 & O 2 /CO 2 atmospheres TGA- coal slow heating DTF coal rapid heating TGA char isothermal

9 THERMOGRAVIMETRIC ANALYSIS 9 Thermo gravimetric analysis is an useful tool to perform reactivity measurements It involves the measurement of the mass loss/gain of a substance subjected to a controlled temperature program under controlled conditions ISOTHERMAL SETARAM Setsys Evolution TGA NON-ISOTHERMAL (HEATING)

10 ISOTHERMAL TGA EXPERIMENTS 1 EXPERIMENTAL PROCEDURE Temperature ( o C) Temperature Program Time (min) Step Atmosphere Process N 2 N 2 N 2 N 2 No gas N 2, CO 2, O 2 /N 2, O 2 /CO 2 Flow stabilization Heating to drying temp. Drying Heating to T f Gas change (if required) Reactivity measurement EXPERIMENTAL CONDITIONS Parameter Initial sample (char) mass Temperature range (isothermal) Total gas flow rate (carrier + auxiliary) Oxygen concentration in N 2/ CO 2 Condition 3 to 5 mg 8 to 1 o C 5 ml/min to 21 % v/v basis

11 DROP TUBE FURNACE (DTF) EXPERIMENTS A drop tube furnace (DTF) was used to measure volatile yield and char burnout, and form char for TGA experiments. 11 Coal Feeder Coal Secondary gas Primary gas Coal feeder probe Volatile yields in N 2 and CO 2 atmospheres at various temperatures Coal burnouts at various O 2 levels and temperatures Char formation in N 2 atmosphere at 14 o C for TGA experiments EXPERIMENTAL CONDITIONS Vertical furnace Parameter Condition Collector probe To vacuum pump Coal feed rate Total gas flow rate (primary + secondary) Quench gas flow rate Curtain gas flow rate O 2 concentration in N 2 /CO 2 Furnace temperature 3to5g/h 5.2 LPM 3LPM 3LPM.5 to 3% v/v basis 9 to 14 o C Cyclone Aerosol filter Drop Tube Furnace (DTF)

12 EXPERIMENTAL DROP TUBE FURNACE 12 Coal particles + Primary gas Cooling water in Cooling water out Secondary gas Feeder probe Primary gas - top bed Fluidised bed coal feeder Rotameter Rotameter Gas Mixing Coil O2 MFC N2 / CO2 MFC O2 in N2 / CO2 in Fluidised coal particles Hot zone Primary ygas - bottom bed Curtain gas (N2) in Rotameter Curtain gas (N2) out Collection probe Cooling water out Cooling water in Quench gas (N2) in Filter paper Cyclone arrangement for collecting char particles Pump Exhaust gas to atmosphere Water bottle Two-way valve Schematic of the DTF Experimental Facility at the University of Newcastle

13 PYROLYSIS OF COAL D IN N 2 & CO 2 ATMOSPHERES IN TGA EXPERIMENTS Reactivity y, R m,p (s -1 ) Coal 1% N2 Coal 1% CO2 Pyrolysis reactivities are similar CO 2 reactivity it with char Char gasification begins CO 2.2 N Temperature, T p (K) Pyrolysis reactivities are similar in both N 2 and CO 2 atmospheres. Char-CO 2 gasification reaction is clearly evident in a CO 2 atmosphere.

14 COMPARISON OF APPARENT VOLATILE YIELDS OF TEST COALS IN N 2 & CO 2 ATMOSPHERES IN DTF EXPERIMENTS AT 14 o C 14 yield (wt. % daf basis) Appar rent volatile N2 CO2 Proximate Analysis C in Coal (wt. %, daf basis) Higher apparent volatile yield at higher temperatures and heating rates in DTF at 14 o C compared to the proximate analysis volatile matter. Higher apparent volatile yield in CO 2 atmosphere - attributed to the char-co 2 gasification reaction.

15 CHAR SWELLING IN N 2 & CO 2 ATMOSPHERES 15 Chars were formed in the DTF at 14 o C ulative volume percentage Cum Coal N2 Char CO2 Char Coal A Particle diameter (μm) Cum mulative volume percentage Coal N2 Char CO2 Char Larger CO 2 chars Coal B Particle diameter (μm) Coal la 1 Coal B Similar N 2 and CO 2 chars Coal C Cumulative volum me percentage Coal N2 Char CO2 Char Coal C Particle diameter (μm) Swollen chars

16 CHAR SWELLING IN N 2 & CO 2 ATMOSPHERES 16 Chars were formed in the DTF at 14 o C Swellin ng factor (-) N2 Char CO2 Char C content in coal (wt. % daf basis) Swelling factor is the ratio of the median diameter of the char particle to the corresponding coal particle diameter d char / d coal

17 SEM PICTURES OF CROSS SECTIONS OF COAL A N 2 & CO 2 CHARS Chars were formed in the DTF at 14 o C 17 Coal A - N 2 CHAR Coal A - CO 2 CHAR

18 SEM PICTURES OF CROSS SECTIONS OF COAL B N 2 & CO 2 CHARS 18 Chars were formed in the DTF at 14 o C Coal B - N 2 CHAR Coal B - CO 2 CHAR

19 SEM PICTURES OF CROSS SECTIONS OF COAL C N 2 & CO 2 CHARS 19 Chars were formed in the DTF at 14 o C Coal C - N 2 CHAR Coal C - CO 2 CHAR

20 CHAR MICROPORE SURFACE AREAS OF N 2 & CO 2 CHARS OF TEST COALS 2 Chars were formed in the DTF at 14 o C 5 Microp ore surface area (m 2 /g) N2 Char CO2 Char C content in coal (wt. % daf basis)

21 COAL A REACTIVITY IN O 2 /N 2 & O 2 /CO 2 IN SLOW HEATING TGA EXPERIMENTS Constant Heating Rate 25C/min.6 - Lower reactivities & heat flows at low.5 temperatures in O 2 /CO 2 combustion - Endothermic gasification reduces heat flows and increases reactivities at high temperatures -1 ) Reactivity (min Coal A REACTIVITY 21% O 2 1% O 2 3% O 2 N 2 CO Coal A HEAT FLOW 21% O Temperature ( o C) low (mw) Heat Fl % O 2 3% O CO 2 N 2 Endothermic gasification -1 Temperature ( o C)

22 COAL A BURNOUT/REACTIVITY IN O 2 /N 2 & O 2 /CO 2 ATMOSPHERES 22 DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS s) Coal burnout (wt. %, daf basi Coal A O2/N2 O2/CO Reactivity (min -1 ) COAL A 14N2 CHAR 1 O C, 5% CONVERSION Air - O2/N2 Oxy - O2/CO Oxygen concentration (atm) Oxygen Concentration (atm) Coal burnout (wt. %, daf basis) Coal A O2/N Furnace temperature ( o C) O2/CO2 Re eactivity (min -1 ) Air - O2/N2 Oxy - O2/CO2 COAL A 14N2 CHAR 1% O 2, 5% CONVERSION Temperature ( o C)

23 COAL B BURNOUT/REACTIVITY IN O 2 /N 2 & O 2 /CO 2 ATMOSPHERES 23 DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS s) Coal burnout (wt. %, daf basi Coal B O2/N2 O2/CO Oxygen concentration (atm) Reactivity (min -1 ) COAL B 14N2 CHAR 1 O C, 5% CONVERSION Air - O2/N2 Oxy - O2/CO Oxygen Concentration (atm) Coal burnout (w wt. %, daf basis) Coal B O2/N Furnace temperature ( o C) O2/CO2 Re eactivity (min -1 ) COAL B 14N2 CHAR 1% O 2, 5% CONVERSION Air - O2/N2.1 Oxy - O2/CO Temperature ( o C)

24 COAL C BURNOUT/REACTIVITY IN O 2 /N 2 & O 2 /CO 2 ATMOSPHERES 24 DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS Coal burnout (w wt. %, daf basis s) Coal C O2/N2 O2/CO Oxygen concentration (atm) Reactivity (min -1 ) COAL C 14N2 CHAR 1 O C, 1% CONVERSION Air - O2/N2 Oxy - O2/CO Oxygen Concentration (atm) Co oal burnout (wt. %, daf basis) O2/N2 O2/CO Furnace temperature ( o C) Coal C Reac ctivity (min -1 ) COAL C 14N2 CHAR 1 % O 2, 1% CONVERSION.25 Air - O2/N2 Oxy - O2/CO Temperature ( o C)

25 25 SUMMARY OF RESULTS Variable Technique Difference in O 2 /CO 2 or CO 2 atmosphere Apparent Volatile Yield DTF Higher Pyrolysis Rate TGA Similar Char Appearance DTF/SEM Similar Char Swelling DTF/PSD Similar for most coals Micropore Surface Area DTF/BET Slightly higher for CO 2 chars of most coals Coal Burnout DTF Similar or slightly higher in O 2 /CO 2 conditions Char Reactivity TGA Higher in O 2 /CO 2 conditions

26 CONCLUSIONS There are significant differences in pulverised coal reactivity in air and oxy fuel conditions. The extent of the differences depend on the coal type (rank). 26 The pyrolysis reactivity of coal in N 2 and CO 2 is similar as shown by TGA results. The apparent volatile yield appears to be higher in CO 2 due to mass loss during the char CO 2 gasification reaction. Depending on the coal type, there are also significant differences in the characteristics of char formed in N 2 and CO 2 atmospheres. Some coals exhibit greater swelling in a CO 2 atmosphere. Higher char reactivity in O 2 /CO 2 conditions, especially at high temperatures and low O 2 levels, is attributed to the char CO 2 gasification reaction. TGA and DTF results complement one another and are consistent with literature data. Reactivity parameters for char combustion need to be estimated separately for oxyfuel conditions including the char CO 2 gasification reaction and other relevant differences need to be accounted for when modelling pulverised coal combustion in O 2 /CO 2 conditions.

27 ACKNOWLEDGEMENTS 27 Cooperative Research Centre for Coal in Sustainable Development (CCSD) for financial support Vattenfall AB, Stockholm, Sweden, for financial ial support The University of Newcastle for PhD scholarships (Renu Rathnam) Ms. Jenny Martin, Ms. Jane Hamson, and Dr. Artur Ziolkowski, for technical support in the DTF and TGA experiments The University of Newcastle Electron Microscope & X ray Unit for support in the SEM analysis of char samples SRC for Multi phase Processes for help with the particle size measurements Mr. Rohan Stanger for presenting this work

28 28 THANK YOU! Oxy fuel Working Group The University of Newcastle

29 COMPARISON OF BURNOUTS OF TEST COALS IN N 2 & CO 2 ATMOSPHERES IN DTF EXPERIMENTS AT 14 o C 29 Coa al burnout (w wt. % daf bas sis) % O2 1% O % O % O2 2% O % O % O % O % O2 1% O O2/N2 O2/CO C in Coal (wt. %, daf basis) 1% O % O % O % O2 Similar or even higher burnout in O 2 /CO 2 conditions at similar or lower O 2 levels for all the test coals at 14 o C in the DTF. Higher char reactivity in O 2 /CO 2 conditions for high rank coals.