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Pulverised Coal Pyrolysis and Char Combustion Characteristics In 1st Oxy-fuel Combustion Conference, Cottbus, Germany - 09 September 2009 Pulverised Coal Pyrolysis and Char Combustion Characteristics in Simulated Air (O2/N2) and Oxy-fuel (O2/CO2) 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 2308, Australia. Priority Research Centre for Energy 2 OUTLINE OF PRESENTATION . Oxy‐fuel (O2/CO2) Combustion . Pulverised Coal Reactivity in Oxy‐fuel Conditions . Hypothesis & Objectives of the Project . Experimental . Results and discussion . Conclusions Priority Research Centre for Energy 3 OXY-FUEL (O2/CO2) COMBUSTION N2 COAL FURNACE H2O AIR O CO + ASU 2 2 gases WET DRY RECYCLED FLUE GAS “High CO2 concentrations are encountered during oxy-fuel combustion” Properties of CO2 in comparison to N2 2 . Higher emissivity – affects radiative 1.8 heat transfer 1.6 (no unit) (no 2 1.4 . Higher molar heat capacity – affects for N 1.2 ee 1 gas and partilicle temperatures / Valu / 0.8 2 0.6 . Lower O2 diffusivity – affects char 0.4 combustion reactivity 0.2 lue for CO Va 0 Thermal Molar Heat Density Mass Diffusivity . Ignition and flame stability Conductivity Capacity of O2 Properties of CO2 are significantly different when compared to N2 Priority Research Centre for Energy 4 PULVERISED COAL REACTIVITY IN OXY-FUEL CONDITIONS . Pyrolysis characteristics O2/CO2 pg . Char formation . Char reactivity Tp . Ash characteristics . Emissions Tg ps Lower O2 diffusivity ‐ O2 availability on the surface of the particle (ps) Higher specific heat ‐ gas & particle temperatures (Tg & Tp) Char‐CO2 gasification reaction ‐ chemical reactivity of char Priority Research Centre for Energy 5 OBJECTIVES HthiHypothesis “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 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 o To identify the conditions under which the reactivity is significantly different in air and oxy conditions o To obtain the intrinsic and apparent kinetic parameters (activation energy, pre‐ exponential factor, reaction ord)der) 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 O2 usage Priority Research Centre for Energy 6 EXPERIMENTAL TECHNIQUES 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 Pyrolysis and char reactivity measurements Thermogravimetric Analyzer (TGA) Drop Tube Furnace (DTF) o 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 Priority Research Centre for Energy 7 COAL PROPERTIES & RANKING 100 Carbon 91.40 95 90 83.91 Volatile matter 81.60 81.86 78.40 79.10 90 Coal H 80 75.00 70 67.51 85 57.19 60 80 wt. % daf basis) daf % wt. % daf basis) basis) daf % 51.46 (( 50 75 37.93 40 37.01 35.51 36.51 32.61 70 30 Coal A 20 65 C content in coal coal in content C 9.83 Coal property (wt. 10 60 0 0 0.5 1 1.5 2 2.5 3 Coal A Coal E Coal G Coal D Coal F Coal C Coal B Coal H Vitrinite Reflectance (%) Wt. % air dried basis Coal Proximate analysis Ultimate analysis Moisture Ash V M F C C H N S O Coal A 11.20 5.40 47.70 35.70 56.30 4.16 0.55 0.96 21.43 Coal B 2.00 17.20 29.50 51.30 67.80 4.08 1.19 0.68 7.05 Coal C 3803.80 22. 90 23. 90 49. 40 60. 00 3293.29 1461.46 1941.94 6616.61 Coal D 8.00 19.90 25.60 46.50 57.03 3.25 0.84 0.17 10.81 Coal E 5.20 28.60 24.50 41.70 49.65 3.08 0.80 0.22 12.45 Coal F 1.70 19.60 40.50 38.20 64.22 5.38 0.99 0.50 7.60 Coal G 5.90 5.00 33.80 55.30 69.85 4.57 1.91 0.46 12.30 Coal H 1.7 9.8 8.7 79.8 80.89 3.34 1.66 0.67 1.95 Priority Research Centre for Energy 8 PYROLYSIS & CHAR REACTIVITY MEASUREMENTS Air Oxy-fuel Volatiles N2 , CO2 CO2 H2O O2 , CO2 O2 , N2 coal char carbon + unburnt carbon + mineral matter ash drying pyrolysis char oxidation + ash formation gasification Coal Pyrolysis reactivity Coal Char O2 Reactivity in N2 & CO2 in O2/N2 & O2/CO2 atmospheres atmospheres TGA – coal slow heating TGA- coal slow heating DTF – coal rapid heating DTF – coal rapid heating TGA – char isothermal Priority Research Centre for Energy 9 THERMOGRAVIMETRIC ANALYSIS 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 NON-ISOTHERMAL (HEATING) SETARAM Setsys Evolution TGA Priority Research Centre for Energy 10 ISOTHERMAL TGA EXPERIMENTS EXPERIMENTAL PROCEDURE 1200 Step Atmosphere Process Temperature Program 5 6 1000 1 N2 Flow stabilization C) oo 2 N Heatinggygp to drying temp. 800 2 3 N2 Drying 4 N Heating to T 600 2 f 4 5 No gas Gas change (if required) 6 N , CO , Reactivity measurement 400 2 2 emperature ( O2/N2, TT O /CO 200 2 2 3 1 2 0 0 50 100 150 200 Time (()min) EXPERIMENTAL CONDITIONS Parameter Condition Initial sample (char) mass 3 to 5 mg Temperature range (isothermal) 800 to 1000 oC Total gas flow rate (carrier + 50 mL/min auxiliary) Oxygen concentration in N2/CO2 0 to 21 % v/v basis Priority Research Centre for Energy 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. Coal Feeder • Volatile yields in N2 and CO2 atmospheres at various temperatures Coal • Coal burnouts at various O2 levels and Secondary gas temperatures Primary gas o •Char formation in N2 atmosphere at 1400 C for TGA experiments Coal feeder probe EXPERIMENTAL CONDITIONS Vertical furnace Parameter Condition Coal feed rate 3to5g/h Total gas flow rate 5.2 LPM (primary + secondary) Collector probe Quench gas flow rate 3LPM Curtain gas flow rate 3LPM To vacuum O2 concentration in N2/CO2 0.5 to 30% v/v basis pump Furnace temperature 900 to 1400 oC Cyclone Aerosol filter Priority Research Drop Tube Furnace (DTF) Centre for Energy 12 EXPERIMENTAL – DROP TUBE FURNACE Coal particles + Primary gas Cooling water in Cooling water out Secondary gas Feeder probe Rotameter Primary gas - top bed O2 O2 Gas MFC in Mixing Rotameter Coil N2 / CO2 N2 / CO2 Fluidised bed coal feeder MFC in Fluidised coal particles Hot zone Primaryyg gas - bottom bed Rotameter Curtain gas (N2) in Curtain gas (N2) out Collection probe Cooling water out Cooling water in Quench gas (N2) in Filter paper Exhaust gas to atmosphere Pump ClCyclone arrangemen t for collecting char particles Two-way valve Water bottle Schematic of the DTF Experimental Facility at the University of Newcastle Priority Research Centre for Energy 13 PYROLYSIS OF COAL D IN N2 & CO2 ATMOSPHERES IN TGA EXPERIMENTS 0.0016 0.0014 Coal 100% N2 Coal 100% CO2 ) 000120.0012 CO2 reactiv ity -1 with char (s 0.001 m,p Pyrolysis R reactivities are , 0.0008 yy similar CO2 Char gasification 0.0006 begins Reactivit 0.0004 0.0002 N2 0 500 600 700 800 900 1000 1100 1200 1300 Temperature, T p (K) Pyrolysis reactivities are similar in both N2 and CO2 atmospheres. Char-CO2 gasification reaction is clearly evident in a CO2 atmosphere. Priority Research Centre for Energy 14 COMPARISON OF APPARENT VOLATILE YIELDS OF TEST COALS o IN N2 & CO2 ATMOSPHERES IN DTF EXPERIMENTS AT 1400 C 100 90 N2 80 CO2 daf basis) basis) daf Proximate Analysis %% 70 60 50 yield (wt. .45 66 77 40 8 76.3 74.3 70.2 30 66.6 60.0 59.4 57.19 50.9 51.46 48.7 49.19 46.7 20 48.57 37.93 37.01 36.51 36.18 35.80 35.51 ent volatile 32.61 rr 10 0 Appa 67.51 75.00 78.40 79.10 81.60 81.86 83.91 C in Coal (wt. %, daf basis) Higher apparent volatile yield at higher temperatures and heating rates in DTF at 1400 oC compared to the proximate analysis volatile matter. Higher apparent volatile yield in CO 2 atmosphere - attributed to the char-CO2 gasification reaction. Priority Research Centre for Energy 15 CHAR SWELLING IN N2 & CO2 ATMOSPHERES Chars were formed in the DTF at 1400 oC 100 100 90 Coal 90 Coal N2 Char 80 N2 Char 80 CO2 Char CO2 Char ercentage ercentage 70 70 pp pp 60 60 50 50 40 40 Larger CO2 30 30 chars 20 20 volume ulative ulative volume volume ulative 10 Coal A mm 10 Coal B Cu Cum 0 0 1 10 100 1000 1 10 100 1000 Particle diameter (μm) Particle diameter (μm) ClACoal A 100 ClBCoal B 90 Coal 80 N2 Char 70 CO2 Char Similar N e percentage 60 Swollen 2 mm 50 chars and CO2 40 chars 30 20 10 Coal C umulative volu umulative CC 0 1 10 100 1000 Coal C Particle diameter (μm) Priority Research Centre for Energy 16 CHAR SWELLING IN N2 & CO2 ATMOSPHERES Chars were formed in the DTF at 1400 oC 4.5 4.0 N2 Char CO2 Char 3.5 3.0 2.5 g factor (-) g factor nn 2.0 1.5 Swelli 1.0 0.5 0.0 60 65 70 75 80 85 90 C content in coal (wt.
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