Oxy-fuel combustion Prof. Luis I. Díez University of Zaragoza

Transcription

1 Oxy-fuel combustion Prof. Luis I. Díez University of Zaragoza Napoli (Italy), April 2017

2 Oxy-fuel combustion 1. CCS 2. Oxy-fuel combustion 3. Application to FB units 4. Control of pollutants (SO 2,NO x ) 5. Conclusions

3 1. CCS

4 1. CCS

5 1. CCS

6 1. CCS

7 1. CCS Air Flue gas N 2 H 2 O Air Power plant Power plant (CCS) Coal Natural gas CO 2 (supercritical) Coal Natural gas

8 1. CCS Air Flue gas N 2 H 2 O Air Power plant Power plant (CCS) Coal Natural gas CO 2 CO 2 (supercritical) Coal Natural gas

9 1. CCS CO 2 phase diagram

10 1. CCS Best choice for CCS? Capture Low cost ( per tone of CO 2 avoided) Storage Low cost, and Long term High capacity Safety Public acceptance Legislation? 80% of the total cost can be allocated to the capture

11 1. CCS 1.4 Avoided Emitted Reference Clean

12 1. CCS EU SET Plan vision for 2050

13 1. CCS In most scenarios for stabilization of atmospheric greenhouse gas concentrations in a leastcost portfolio of mitigation options, CCS contributes 15 55% to the cumulative mitigation effort worldwide until 2100

14 1. CCS CO 2 capture Post-combustion: once conventional combustion has taken place in a boiler, CO 2 is separated from the flue gas (typically by chemical absorption). It is a well-know alternative, suitable for PF and FB power plants, new and existing. Pre-combustion: carbon is separated before the combustion process. CO 2 is taken from a syngas produced by fuel gasification. H 2 -enriched gas stream is fired in a gas turbine. This scheme is suitable for IGCC plants. Oxy-fuel combustion: combustion is carried out without supplying nitrogen (N 2 ). Pure O 2 is supplied, along with CO 2 recycled from the flue gases. In this way, a very high CO 2 concentration is found at the stack, that can be directly post-processed. Difficult to apply to large-scale retrofitting, then more suitable for new plants.

15 1. CCS CO 2 capture Post-combustion Coal NG Biomass Combustion Separation of CO 2 CO 2 N 2 Pre-combustion Coal Biomass Air Gasification Shift + CO 2 separation H 2 Combustion CO 2 N 2 Oxy-combustion Coal Gas Biomass Air NG HC s H 2 O + O 2 Combustion O 2 Separation CO 2 Air N 2 CO 2 CPU

16 1. CCS Post-combustion Air Boiler STEAM TURBINE Coal Steam POWER CO 2 + N 2 + H 2 O Steam Capture N 2 + H 2 O CPU Transport

17 1. CCS Post-combustion

18 1. CCS Post-combustion: chemical absorption with amines Amina pobre Lean amine Cooler Enfriador CO 2 50ºC Clean gas 60ºC Gas limpio 75ºC Absorbedor Gas de Combustión pre-tratado Pre cleaned flue gas Intercambiador de calor 60ºC Heat exchanger 95ºC 110ºC Regenerador Steam Vapor 130ºC Rich amine Amina rica Lean amine Amina pobre

19 1. CCS Post-combustion: chemical absorption with amines Advantages Well know technology, commercially available Hundreds of plants worldwide Drawbacks Very sensitive to flue gas impurities Very high demand of heat for regeneration (2.7 GJ/tCO 2 ) Amines manageability (very corrosive) Toxicity of residues

20 1. CCS Pre-combustion

21 1. CCS Pre-combustion

22 1. CCS Pre-combustion: physical absorption with Selexol (among others)

23 1. CCS Pre-combustion: physical absorption Advantages Well know technology, but for low sizes Production of pressurized CO 2 Drawbacks Not scaled up: uncertainties about efficiency and costs Requirements for high CO 2 concentrations (then not suitable for conventional flue gases)

24 1. CCS Oxy-fuel combustion

25 1. CCS Oxy-fuel combustion

26 1. CCS Oxy-fuel combustion Advantages Not limited to 21% O 2 : increase of efficiency and reduction of specific size CO 2 separation is not required Drawbacks Not scaled up (current largest units around 30 MW) Need for large scale ASU: increase of cost and energy penalty Redesign of heat transfer sections in boilers

27 1. CCS Emerging technologies: calcium looping Flue gas without CO 2 CO 2 EXISTING POWER PLANT Flue gas CARBONATOR CaCO 3 CaO OXY-FIRED CFBC CALCINER O 2 Coal Air CaCO 3 CaO Purge Air Coal ASU N 2

28 1. CCS gases without CO 2 CO 2 Calciner Carbonator CO 2 by Abanades, INCAR HEAT (high T) HEAT (high T) CaO CaCO 3

29 1. CCS Emerging technologies: chemical looping Me + Air MeO + N 2 MeO + CH 4 CO 2 + H 2 O + Me

30 1. CCS

31 1. CCS Capture costs Cost of Electricity ($ / MWh) New Plants w ith Capture Pulverized Coal Combustion (PC) Coal Gasification Combined Cycle (IGCC) Natural Gas Combined Cycle (NGCC) New Gas and Coal Plants without Capture CO 2 Emission Rate (tons CO 2 / MWh)

32 1. CCS Capture costs

33 1. CCS Storage

34 1. CCS Storage

35 1. CCS Enhanced Oil Recovery

36 1. CCS Enhanced Coal Bed Methane (ECBM)

37 Oxy-fuel combustion 1. CCS 2. Oxy-fuel combustion 3. Application to FB units 4. Control of pollutants (SO 2,NO x ) 5. Conclusions

38 2. Oxy-fuel combustion Oxy-fuel combustion Already used in high temperature ovens in metallurgical industries, also with flue gas recycling (control of temperature + emissions) Combustion in a O 2 /CO 2 atmosphere (free of N 2 ), instead of the conventional O 2 /N 2 atmosphere Flue gas recycling: Main goal: control of temperature (and combustion stability) Wet or dry? Recycling of pollutants (SO 2, NO x )? Challenges: Optimum operating conditions for large-scale boilers: size and price Energy integration: ASU-PP-CPU, to reduce penalties

39 2. Oxy-fuel combustion ASU Large-scale plants are based on cryogenic distillation Air is firstly compressed and then cooled down below the O 2 critical point (by regenerative heat exchangers + Joule-Thompson effect) Liquid oxygen is after re-boiled in a distillation column + heat exchangers

40 2. Oxy-fuel combustion ASU

41 2. Oxy-fuel combustion ASU Energy requirements: kwh/tone O 2 O 2 purity: 93-99% Largest unit in the world: 5000 tone O 2 /day (roughly, the equivalent to a 300 MW e oxy-fired coal plant) Suppliers: Air Liquide, Linde, Praxair

42 2. Oxy-fuel combustion CPU After removal of H 2 O and other trace pollutants, CO 2 is compressed in several stages: High capacity compressors Intercoolers to reduce consumptions (useful heat to oxidizer preheating) Pressure is increased up to 200 bar and conveyed by CO 2 -ducts High impact on PP efficiency

43 2. Oxy-fuel combustion Balance of plant (modern unit) FUEL 100% NET POWER 44.3% ACTUAL NET POWER 34% LOSSES 53.5% Auxiliar Power 2.2% ASU 6.8% CPU 3.5%

44 2. Oxy-fuel combustion Oxy-fuel combustion: economy Estimated impact in large-scale power stations: around 25 per avoided ton of CO 2 European Trading System for CO 2 Emissions:

45 2. Oxy-fuel combustion Oxy-combustion vs. conventional Carbon dioxide is a participative gas in radiative heat transfer, but nitrogen is not. This issue brings relevant differences in the heat transfer processes inside furnaces/boilers. Oxy-fuel combustion units can be operated with O 2 concentrations over 21%: the larger the oxygen concentration, the larger the flame temperature.

46 2. Oxy-fuel combustion Oxy-combustion vs. conventional Density of CO 2 is higher than N 2 (molar masses respectively are: 44 kg/kmol vs. 28 kg/kmol) Heat transfer is also modified by the different flow rates inside the boilers (due to the higher density, but also the higher O 2 concentration). Thermo-physical properties of CO 2 are also different from N 2 (some figures at 800ºC, N 2 vs. CO 2 ): Specific heat, c p (kj/kg K): 1,17 vs. 1,25 Viscosity, (kg/m s): 4, vs. 4, Thermal conductivity, k (W/ m K): 0,068 vs. 0,073 Fuel conversion, NO x production and SO 2 control are also affected in some extent if the atmosphere is shifted from O 2 /N 2 to O 2 /CO 2

47 2. Oxy-fuel combustion Oxy-combustion vs. conventional Retrofit of an existing large-scale unit to oxy-fuel combustion is quite complex, and it does not seem competitive in comparison to available post-combustion techniques: High investment and operation costs related to ASU and CPU New condensing system for H 2 O removal and new flue gas recycling, wet or dry: interactions with existing gas cleaning systems %O 2 in oxidizer: temperature control Resizing of heat exchangers High CO 2 concentration in gases: acid damages on materials

48 2. Oxy-fuel combustion Oxy-fuel industrial-scale (demo) units Schwarze Pumpe, Germany (30 MW e ): pulverized coal; CO 2 storage not solved (experiments at 800 km), large-volume tanks Lacq, France (30 MW t ): natural gas; useful heat; storage in natural gas fields CIUDEN, Spain (30 MW t ): pulverized fuel and fluidized bed units; plant for experimentation, not for industrial production, neither electricity nor useful heat; CO 2 storage not solved (experiments at 200 km) Yingcheng, China (35 MW t ): pulverized coal; retrofit of small-scale unit; CHP; CO 2 storage not addressed Callide, Australia (30 MW e ): pulverized coal; retrofit of small-scale unit; storage in natural gas fields [Projects under consideration in UK, Australia and China, in a range about 300 Mw e but not in the short-term!]

49 2. Oxy-fuel combustion Oxy-fuel combustion: research There is a lot of research worldwide, mainly in pilot-scale units, in order to increase the knowledge about some not well-known processes and contribute to the oxy-fuel combustion development and technology scale-up: Solid fuel conversion Gas-fired flames Heat transfer Fouling and corrosion Pollutants: NO x, SO 2

50 2. Oxy-fuel combustion Solid fuel conversion Volatiles release is barely affected by the atmosphere changes: if conventional combustion is compared to oxy-combustion, differences in mass loss are almost negligible even for coarse particles

51 2. Oxy-fuel combustion Solid fuel conversion Char conversion is different under oxy-combustion: Depending on the particle size: under pulverized fuel combustion, the process is governed by the reaction kinetics and the O 2 concentration; under fluidized bed combustion, internal diffusion also plays a role (not being the same for N 2 than for CO 2 ) Depending on the CO 2 concentration: gasification is enhanced, increasing the CO production

52 2. Oxy-fuel combustion Gas-fired flames Ignition: the higher the O 2 concentration, the lower the self-ignition temperature Propagation velocity: it significantly increases with O 2 concentration (ten times, if air is compared to pure oxygen). Nevertheless, under the same O 2 concentration, velocity is lower for O 2 /CO 2 mixtures (in comparison to O 2 /N 2 mixtures) Flame stability: if conventional burners are used under oxy-fuel conditions, the CO 2 density and specific heat can promote flame pulsation and instability; this can be balanced by enriching the O 2 concentration

53 2. Oxy-fuel combustion Heat transfer Increase of radiative heat transfer Strongly dependent on temperature, then much more relevant in pulverized fuel boilers than in fluidized bed boilers Furnace could be re-sized Decrease of convective heat transfer Due to a lower temperature level in gases (leaving the furnace) and a lower flow rate Heat recovery sections could be re-sized

54 2. Oxy-fuel combustion Heat transfer reheater drum mix superheater radiative superheater convective superheater economizer downcomers furnace wind box air preheater burners mills

55 2. Oxy-fuel combustion Heat transfer

56 2. Oxy-fuel combustion Heat transfer

57 2. Oxy-fuel combustion Fouling/corrosion Fouling ratio remains almost the same under air-firing and oxyfiring: it is much more dependent on the fuel type and chemical composition However, deposits can be of different composition: Enrichment on iron Enrichment on potassium sulfates and calcium sulfates Enrichment on potassium and sodium silicates (fluidized beds) The higher the temperature and the higher the O 2 concentration, the higher the long-term corrosion risk

58 2. Oxy-fuel combustion SO 2 emissions SO 2 emissions are barely affected for pulverized fuel conditions, while SO 3 is slightly increased due to the higher O 2 concentrations (anyway, SO 3 is very low in comparison to SO 2 ) In fluidized bed combustion, SO 2 emissions can be significantly increased under oxy-combustion due to the CO 2 concentration, affecting: Desulfurization efficiency Limestone activity Ca/S molar ratios (More details later)

59 2. Oxy-fuel combustion NO x emissions NO emissions are much more dependent on operating conditions, then very sensitive to the O 2 concentration and the temperature At least, thermal-no x is avoided (no N 2 supplied) For pulverized fuel conditions, N-fuel conversion to NO x is reduced if O 2 concentration is remained the same (but increases with %O 2 ) For fluidized bed combustion, NO x is not only affected by the %O 2 in the atmosphere, but also by the CO/CO 2 ratios (depending on the fuel) and the CaCO 3 /CaO ratios (depending on the sulfur capture) (More details later)

60 2. Oxy-fuel combustion Some international pilot-experiences Energy and Environmental Research Corporation (EERC) 3 MW Pioneering studies on coal oxy-combustion Target: Results: Characterization of oxy-fuel combustion operative Guidelines for a further scale-up of the technology Wet recycling: burners with 23.8% O 2 Dry recycling: burners with 27% O 2 Good temperature control and recycling stability. Combustion efficiencies comparable to air combustion. Emissions: Less NO x (50%), similar SO x No relevant operating issues.

61 2. Oxy-fuel combustion Some international pilot-experiences International Flame Research Foundation MW Conclusions: Suitable for swirl coal-fired burners Characterization of radiative/convective heat transfer shares Comparable results to air combustion as concerns: Combustion efficiency Flame size and stability Recycling ratio: 40% Largest CO 2 concentration in flue gases : 91.4%

62 2. Oxy-fuel combustion Some international pilot-experiences International Flame Research Foundation MW

63 2. Oxy-fuel combustion Some international pilot-experiences Air Liquide & Babcock & Wilcox (B&W) Company 1.5 MW Experimental facility: Small Boiler Simulator B&W, to fire US coals under O 2 /CO 2 atmospheres Conclusions: No operational issues during the switch from air combustion to oxy-coal combustion Low NO x emissions Good performance of wet scrubbers for SO 2 removal Reduction of Hg emissions (50%) Increase of combustion efficiency (lower carbon content in residues)

64 2. Oxy-fuel combustion Some international pilot-experiences Air Liquide & Babcock & Wilcox (B&W) Company 1.5 MW PO = Primary O2 SO = Secondary O2 TO = Tertiary O2

65 2. Oxy-fuel combustion Some international pilot-experiences CANMET, Canada Energy Research Oxy-fuel combustion facilities: natural gas (0,3 MW), pulverized coal and fluidized bed reactor (0,8-1,5 MW) Conclusions: High CO 2 in flue gases (over 90%), low air in-leakage Good control of combustion temperatures for atmospheres with O 2 concentration up to 35% NO x emissions were reduced by 1/3 using recycling CO was not a concern SO 2 capture is worsened in fluidized bed boilers

66 2. Oxy-fuel combustion Some international pilot-experiences

67 Oxy-fuel combustion 1. CCS 2. Oxy-fuel combustion 3. Application to FB units 4. Control of pollutants (SO 2,NO x ) 5. Conclusions

68 3. Application to FB units Fluidized beds Concept of fluidized bed: a volume of solid particles behaving as a fluid This is achieved by injecting a gas stream by the bed bottom, using a multi-perforated plate or with a lot of small tuyeres (nozzles) Depending of the gas velocity: Fixed bed No movement of the particles Bubbling bed Volume regions with low particles concentration, moving in the direction of the gas (bubbles) Circulating bed In-between bubbling bed and pneumatic transporting; significant elutriation of solids Oxy-combustion: solid phase, fuel + others (inert, sorbent) gas phase, O 2 /CO 2

69 3. Application to FB units Fluidized beds Aire Aire Aire Aire Lecho Fijo Fluidización Incipiente Lecho Burbujeante velocidad del aire de fluidización Lecho Circulante Fixed Incipient Bubbling Circulating fluidization velocity

70 3. Application to FB units Fluidized beds BUBBLING Evaporator inside the bed zone. Heat recovery after free-board, without solids separation. SH Sobrecalentador secundario SH Sobrecalentador primario EZ Economizador Gases Bed temperature 750 ºC 950 ºC Freeboard temperature 650 ºC 850 ºC Paredes de agua Paredes de agua Waterwalls Límite del lecho Splash zone Lecho burbujeante Dense zone Gases Hogar Ash hopper Tolva de cenizas Evaporador Evaporator Bed pressure 1bar Particle size 0,5 mm 2 mm Fluidization velocity 1 m/s 3 m/s Bed height 0,3 m 1 m Heat transfer coefficient 275 W/m 2 K (tubes inside the bed) Toberas de entrada Nozzles de aire Aire Caja de vientos Windbox Air

71 3. Application to FB units Fluidized beds CIRCULATING Water-walls act as evaporation section. Gases Heat recovery sections after solids separation. Solids recycling to keep the inventory in the reactor. Water Paredes de walls agua Hogar Sobrecalentadores SH Paneles separadores de partículas Solids separation Solids recycling Recirculación de sólidos Bed temperature 800 ºC 950 ºC Freeboard temperature 750 ºC 900 ºC Secondary air Aire secundario Bed pressure 1bar Particle size 1 mm 1 cm Fluidization velocity 5 m/s 15 m/s Heat transfer coefficient 150 W/m 2 K (water walls) Toberas de entrada Nozzles de aire Primary Aire primario air Caja de vientos Windbox

72 3. Application to FB units Fluidized bed oxy-combustion Fuel flexibility: low rank fuels, fuel blends Lower recycling rates, due to high concentration of solid particles Smooth temperature profile inside the boiler Better temperature control than PF units Balance O 2 concentration vs. fluidization conditions Not so affected by the CO 2 participative effect on thermal radiation Thermal NO x emissions are avoided (no N 2 + low temperature) In-situ sulfur retention: but different from air-fired units!

73 3. Application to FB units Thermoenergy Engineering Research Institute, Southeast University (China) 50 kw th CFB

74 3. Application to FB units Czestochowa University of Technology (Poland) 100 kw th CFB

75 3. Application to FB units CIUDEN (Spain) 30 MW th (largest in the world) CFB

76 Oxy-fuel combustion 1. CCS 2. Oxy-fuel combustion 3. Application to FB units 4. Control of pollutants (SO 2,NO x ) 5. Conclusions

77 4. Control of pollutants CIRCE-University of Zaragoza 90 kw th bubbling fluidized bed 2.5 m height, 20 cm ID Water-cooled: probes inserted on-load litres hoppers for fuel feeding (coal, sorbent, biomass) CO 2 /O 2 mixer (from bottles) and wet flue gas recirculation Gas cleaning: baffle chamber, cyclone and fabric filter Recycling ratio: from 0% to 60% O 2 in the mixture: from 20% to 60% On-line flue gas analysis: O 2, CO 2, CO, SO 2, NO

78 4. Control of pollutants CIRCE-University of Zaragoza

79 4. Control of pollutants CIRCE-University of Zaragoza

80 4. Control of pollutants SO 2 emissions SO 2 concentration in flue gases increases under oxy-fuel combustion in comparison to air combustion, but this is only a consequence of the reduction of the total gas flow rate. However, sulfur conversion to SO 2 is seldom modified by the oxycombustion conditions (very small increment of the SO 3 production if %O 2 is raised) A relevant issue: control of SO 2 emissions can be significantly affected under oxy-combustion. Typical sulfur capture under air combustion (by indirect limestone calcination) is replaced by direct limestone sulfation, leading to a decrease in the efficiency.

81 4. Control of pollutants SO 2 emissions

82 4. Control of pollutants Test # Fluidizing gas SO 2 emissions Coal to biomass ratio Ca:S Chlorine in corn (%wt.) A1 Air 80/ OXY1 30/70% O2/CO2 80/ OXY2 30/70% O2/CO2 80/ OXY3 30/70% O2/CO2 90/ OXY4 30/70% O2/CO2 80/ OXY5 30/70% O2/CO2 80/ A1 OXY1 OXY2 OXY3 OXY4 OXY5 Tbed (ºC) Tfb (ºC) O2 (%) CO (mg/nm 3 ) SO2 (mg/nm 3 ) SO2 (mg/mj) Desulfurization eff. (%) HCl (mg/nm 3 )

83 4. Control of pollutants SO 2 emissions /40 O2/CO2 600 SO2 (mg/mj) Tb (ºC) De Diego et al. (Fuel, 2013): optimum temperature ºC, for 35/65 % O 2 /CO 2

84 4. Control of pollutants Desulphurization mechanism A G6I Air Granicarb ºC Calcining O G6D 35/65% O 2 /CO 2 Granicarb ºC Non calcining O G6I 35/65% O 2 /CO 2 Granicarb ºC Calcining O G2I 35/65% O 2 /CO 2 Granicarb ºC Calcining O B6D 35/65% O 2 /CO 2 Bahoto ºC Non calcining O B6I 35/65% O 2 /CO 2 Bahoto ºC Calcining Test # Fluidizing gas Limestone Ca:S Bed temperature SO 2 emissions A G6I O G6D O G6I O G2I O B6D O B6I T fb (ºC) O 2 (%) CO (mg/nm 3 ) NO (mg/nm 3 ) SO 2 (mg/nm 3 ) NO (mg/mj) SO 2 (mg/mj) Eff. Desulph (%) HCl (mg/nm 3 )

85 4. Control of pollutants SO 2 emissions

86 4. Control of pollutants SO 2 emissions Indirect sulfation of limestone yields higher desulphurization efficiencies than direct sulfation. Direct sulfation conditions would require an increase of Ca:S molar ratios. In comparison to air-firing, optimum temperature for indirect sulfur capture is higher under oxy-firing. Limestone fragmentation largely influences the desulphurization efficiency.

87 4. Control of pollutants NO x emissions NO x concentration in flue gases increases under oxy-fuel combustion in comparison to air combustion, but this is only a consequence of the reduction of the total gas flow rate. Nitrogen conversion to NO x may be increased under oxycombustion conditions due to the O 2 concentration and the temperature Gasification induced by the high CO 2 concentration has a positive side-effect: CO inhibits nitrogen oxidation The trend is also very dependent on the fuel fired: the higher the volatile content, the higher the NO x increase

88 4. Control of pollutants NO x emissions

89 4. Control of pollutants NO x emissions

90 4. Control of pollutants NO x emissions

91 4. Control of pollutants NO x emissions

92 4. Control of pollutants NO x emissions

93 4. Control of pollutants NO x emissions Under oxy-firing, the higher the O 2 concentration, the lower the normalized emissions. NO x emissions are largely determined by the operating conditions (in FB, mainly oxygen excess); behaviour is also very different between high and low rank fuels. Free CaO in the reactor (indirect sulfation conditions) promotes the formation of NO. The extent of gas staging to control NO x emissions is attenuated for the richest O 2 atmospheres.

94 Oxy-fuel combustion 1. CCS 2. Oxy-fuel combustion 3. Application to FB units 4. Control of pollutants (SO 2,NO x ) 5. Conclusions

95 5. Conclusions Conclusions Sooner or later, CCS techniques will be demanded to face the challenge of the CO 2 concentration in atmosphere Oxy-fuel combustion is a promising alternative, which deployment will mostly depend on economic/environmental constrains rather than technical issues Oxy-fuel combustion in FB is an outstanding solution to increase efficiency and control other pollutants but industrial-scale plants are already to be commissioned to gain knowledge and experience