Challenges in pulverized coal combustion: oxyfuel and low-rank fuel. Prof. Shuiqing Li

Transcription

1 Challenges in pulverized coal combustion: oxyfuel and low-rank fuel Prof. Shuiqing Li Clean Utilization of Coal Workshop, WUSTL, 19 March 2014

2 Part 1. Oxy-coal combustion (future) In collaboration between Tsinghua & WUSTL Part 2. Low-rank coal combustion (current)

3 Part 1. Oxy-coal combustion (future) Oxy-fuel combustion is one of the most promising carbon capture technologies for pulverized coal power plants. Courtesy of Vattenfall Courtesy of IHI(Japan)

4 1. Introduction to oxy-fuel combustion National 973 Program led by Tsinghua l The characteristics of fine particulate (PM2.5) formation and ash deposition play an important role in retrofitting the conventional airfired coal power plant into the recycled oxy-fuel plant. l Recently, several researches about fine particles formation and ash deposition were conducted on drop tube furnaces or once-through entrained flow furnaces. But little literature focuses on the real flue gas recycle systems.

5 2. Experimental Set-up: lab-scale Tsinghua 25kW one-dimensional, self-sustained, down-fired furnace ~4 meters tall, 5kg/h coal Furnace temperature profile: Keep same thermal condition (~30% O 2 in oxy-coal mode) Zhuo JK, Li SQ, et al. Proc. Combust. Inst., 2009, 32, 2059

6 2. Experimental Set-up: lab-scale Tsinghua 25kW one-dimensional, self-sustained, down-fired furnace ~4 meters tall, 5kg/h coal Furnace temperature profile: Keep same thermal condition (~30% O 2 in oxy-coal mode) Zhuo JK, Li SQ, et al. Proc. Combust. Inst., 2009, 32, 2059

7 Coal properties and operating conditions Coal properties One typical Chinese Bituminous Coal Proximate analysis (wt.%, dry basis) Fixed Carbon Volatile Matter Ash HHV(MJ/kg) Ultimate analysis (wt %, dry and ash free basis) C H 4.39 O N 0.89 S total 0.93 Cl 0.02 LTA analysis (wt.%, ash basis) SiO Al 2 O Fe 2 O CaO 2.68 MgO 0.7 TiO SO P 2 O K 2 O 1.02 Na 2 O 0.2 Operating conditions Compositions Concentration in flue gas Air Oxy O 2 4.7±0.3 % 8.4±0.4 % CO ±0.2 % 79.2±2.2 % CO 226±45 ppm 309±10 ppm SO 2 573±55 ppm 877±90 ppm NO x 750±45 ppm 1590±40 ppm Emissions amount from stack, mg/mj CO 90±20 20±1 SO 2 520±50 150±30 NO x 320±20 100±5 Burnout 91.8±1.1 % 92.4±1.3 % Residence time 1.3 s 1.75 s Stoichiometric ratio

8 3. Two sampling techniques mp Coal feeder Air Stages Flame zone Char combustion Residence time 0.23 s 0.53 s Tempt K 1300 K Position 350 mm 1120 mm 2660 mm P1 P2 P3 Burner T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 Flowmeter autovalve Eletric heater Flowmeter Control sysytem Auto valve Secondary Air Fan I/O Primary Air Fan Char burnout Gas cooling 1.11 s 1100 K 500 K 5970 mm P4 T12 T13 T14 T15 P7 P6 P5 P8 P9 P10 P5 P10 ID Fan Bag Filter 1st Heat exchanger 2st Heat exchanger

9 (1) PM1 sample and collection system A two-stage nitrogen-aspirated, isokinetic sampling probe for PM1 was developed. Sample Cooling Second Dilution Downflow combustor CO2,CO,NOX, O2,SO2 analyzer Water Inlet Outlet Water cooled Isokinetic Probe P1 T1 PM 2.5 Cyclone ELPI First Dilution Flow meters Corona charger Impactor with insulators and contact needles Vacuum pump Electrometers RS232 The sample after the probe was firstly pre-separated by PM2.5 cyclone, and then directed to a 13-stage ELPI for size-segregated collection. Valve Filter Nitrogen quench gas 30, 63, 109, 173, 267, 407, nm

10 (2) Ash deposit sample system Deposit Baxter LL. Biomass Bioenergy (1993 Aho M et al., Fuel (2005) Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 Xu XG, Li SQ, et al. Energy & Fuels (2010)

11 4. Results and Discussions Part 1. Comparison of fine particle formation (PM 1-10 ) Per gas volume Per energy input The mass concentration PSDs produced in the oxy-fuel mode is higher than that produced in the air mode under similar temperature profiles. The more violent oxidation and thermal crack of the coal char at the relatively high O 2 concentration (30%). Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 Li GD, LI SQ, Axelbaum RL et al. Fuel, 2013,106:

12 Results and Discussions Part 1. Comparison of fine particle formation (PM 1 ) Per gas volume Per energy input The mass concentration PSDs produced in the oxy-fuel mode is higher than that produced in the air mode under similar temperature profiles. Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 Li GD, LI SQ, Axelbaum RL et al. Fuel, 2013,106:

13 Results and Discussions Part 2. Comparison of bulk ash particles BET-SSA: m 2 /g (a) Air mode l The bulk ash particle size distribution of conventional combustion is quite similar to oxy mode. l No big differences of PSDs are found but BET- SSA result shows a little higher value under oxy mode. Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 BET-SSA: m 2 /g (b) Oxy mode

14 5 Mechanisms: from particle formation to ash deposition Clean Utilization of Coal Workshop, WUSTL, 19 March Submicron particle: gas-to-particle conversion Sticky inner layer: Thermophoretic depositio 2.Supermicron particle: Intraparticle reaction Inertial impaction

15 Results and Discussions Part 3. Comparison of ash deposition characteristics m d = Im pact Adhesion + Thermophoresis This is in contrast to previous findings in a bench-scale Drop Tube Furnace! Fryda et al. Fuel, 2012, 89:

16 Results and Discussions Part 3. Inertial impaction efficiency (L=R cyl ) ρ d ρ St = = = 18µ L L τ U St = 2 p p 2 pdu p 0 18µ τ relax 1 18χ d p L 0 2 Re Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 f cov Smaller St number derived from lower flue gas Velocity makes less ash deposition under oxy mode combustion. η(stk) = [ ( Stk 0.125) ( Stk 0.125) ( Stk 0.125) ] 3 1 Li G, Li SQ, et al. Fuel,

17 Part 2. Low-rank fuels (current needs) 55% 70% AQI Bejing Marathon, Oct. 18, 2014 China energy construction Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 China EPA US Embassy

18 Clean coal utilization technology Clean coal technology: Supercritical coal boiler SCR+ Advanced ESP+ FGD+ WESP (future) PM: <15 mg/nm 3 SO 2 : ~20 mg/nm 3 Nox: ~25 mg/nm 3 gross coal consumption rate: 280 g/kwh Competitive to gas turbine plant Reason for Haze day: Distributed car/truck +distributed coal

19 Part 2. Low-rank fuels Within the past decade, the huge untapped reserve of Zhundong lignite coal, up to 390 billion tons (200 years of domestic use), has stimulated lots of fundamental coal studies in China. However, this coal is rich in a significant amount of alkali and alkaline earth metals (AAEM) in the coal structures. Victoria Brown coal

20 1. Very sticky particles from low-rank coals AAEM (Na 2 SO 4 ) Slagging Slagging

21 2. Particle size distributions 1000 dm/dlogdp (mg/g ash) Zhundong lignite Hulunbuir lignite HAF bituminous Aerodynamic Diameter (um)

22 3. Ash deposition tendency CE(%)= ma & my & A d c F ash p Why? Clean Utilization of Coal Workshop, WUSTL, 19 March 2014 Li GD, Li SQ, Fuel, In press

23 3. Evaluation of deposition mechanisms Percentage % c + V c = ( D c) ( VT c) t Deposition time /min AAEM (Na) vapor Condensation PM 1 (fine PM) Thermophoresis Bulk ash particles Inert impact Inert impaction Thermophoresis Bituminous (SH) Percentage % Φ = Φ= V + c c VT c D x Inert impaction Condensation Thermophoresis min V c T T Φ c = k cg ln 1+ Zhundong Lignite Deposition time /min xs x 1 x s b

24 5. Conclusions l For coal utilization as fuels, the interplay between fine particle formation and ash deposition is of importance. Oxy-fuel combustion l For high-rank bituminous coal, the high amounts of fine particles (PM1) formation in Oxy-coal model doesn t result in the serious ash deposition, because the changes of aerodynamics conditions. Low-rank fuel l For lignite coal containing more AAEM species, the high amounts of fine particles (PM1) formation causes the serious ash deposition and slagging problems. The basic research greatly help us for a clean coal utilization!

25 Thanks! Ministry of Science and Technology of China National Natural Science Fundation of China Acknowledge: Ø The students (particularly Gengdali, Ming dong, Qian Huang, Ye Yuan) Ø all collaborators: Prof. Qiang YAO at Tsinghua (12-years) Prof. Axelbaum/Biswas at WUSTL (4-years)

26 Thanks and Question? Clean Utilization of Coal Workshop, WUSTL, 19 March 2014