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EMISSION CONTROL TECHNOLOGIES FOR COAL-FIRED POWER PLANTS


EMISSION CONTROL TECHNOLOGIES FOR COAL-FIRED POWER PLANTS

Ravi K. Srivastava and Frank Princiotta Air Pollution Prevention and Control Division National Risk Management Research La

boratory Office of Research & Development U.S. Environmental Protection Agency January 16, 2004

What We’ll Cover Today . . .
? Electric power industry ? Pollutant emissions ? Control technologies

2

Electric Power Industry
? $250 billion in annual electricity sales in 2002; likely to have annual sales between $250 and $270 billion in 2010 to 2015 ? Industry operates 16,500 units and 5,700 plants ? There are 3,100 electric utilities, 2,800 IPPs, 230 IOUs, and 2,000 publicly owned utilities ? The industry employs 362,000 people ? In the last five years, we have seen industry spend $88 billion in new power plant investments

3

Electricity Generation
Electric Generation in 2002
Hydro 7% Other 3%
5,000

Historical & Projected Electric Generation

4,000

Nuclear 20%
Billion kWh

Other Hydro Nuclear Oil/Gas

3,000

Coal 50%

2,000

Oil & Natural Gas 20%

1,000

Coal
1970 1980 1990 2000 2010 2020

Total Generation = 3,858 billion kWhs

Source: 2002 and historical generation is from EIA’s Annual Energy Review. Projected generation is from EPA’s Integrated Planning Model.

4

5
Source: EEI

Coal-Fired Power Plants
? There are about 530 power plants with 305 GW of capacity that consist of about 1,300 units. ? Coal plants generate the vast majority of power sector emissions: - 100% Hg - 95% SO2 - 90% of NOx

6

Emissions of SO2 & NOx

SO2 Emissions
35 30

NOx Emissions

30

25

25 20

Million Tons

20

All Other Sources

Million Tons

15

All Other Sources

15

10

10

Power Sector

5

5

Power Sector
1980 1990 2000 2010 2020 1970 1980 1990 2000 2010 2020

1970

Power Sector

Power Sector

7
Source: EPA

Emissions of Mercury

1999 Mercury Emissions
Power Sector 40%

2020 Mercury Emissions
w/o further controls
Power Sector 35%

72.4 Tons
Other 60%

47.9 Tons 82.6 Tons 44.9 Tons

Other 65%

8
Source: EPA

Pollutant Reduction
? Emissions reductions possible through:
– – – – Emissions control technologies Advanced power generation technologies Power plant upgrading options Fuel switching

? Focus on emissions control technologies

9

NOx Control Technologies
? Primary – reduce the NOx produced in the primary combustion zone.
– Widely used - low NOx burners (LNBs) and overfire air (OFA)

? Secondary - reduce the NOx already present in the flue gas
– Widely used - reburning, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR)
10

Low NOx Burners
? Limit NOx formation by delaying complete mixing of fuel and air
– Reduced oxygen in primary flame zone – Reduced flame temperature – Reduced residence time at peak temperature

? Can provide reductions in excess of 50%

11

Overfire Air
? 5 to 20% of the total combustion air is injected through ports located downstream of the top burner level
– Burners operate at lower than normal air-to-fuel ratio resulting in NOx control, OFA added to achieve complete combustion – Can be used with LNB to increase NOx reduction by 10 to 25%
12

Reburning
Superheaters

Reburn air Reburn fuel

Flue gas Reburn zone Burning zone

Main fuel and air Main fuel and air

? Reburn fuel (natural gas, coal, other fuels) is injected to provide 15-25% of total heat input ? >50% NOx reduction, mercury and SO2 reduction ? Low capital costs ? Fuels costs, availability of adequate residence time ? Applications: cyclone, wall, tangential; 33-600 MWe

13

SNCR
Superheaters

Reagent

Flue gas

Burning zone

Main fuel and air

? Urea or NH3 injection, generally between 980 to 1150 oC ? 30 to 60 % NOx reduction ? Low capital costs ? Load following, NH3 slip, performance on larger boilers ? Applications: cyclone, wall, tangential; 50-620 MW

14

SCR
? NH3 injection, generally between 350-400 oC ? More than 90 % reduction is possible, especially with LNB ? Capital intensive, space requirements, NH3 slip, SO3 emissions, catalyst deactivation ? Applications:
– More than 75 boilers; cyclone, wall, tangential; 122 - 1300 MW

15

SO2 Control Technologies
Flue Gas Desulfurization Flue Gas Desulfurization
Once-through Regenerable

Wet

Throwaway
Limestone Forced Oxidation Limestone Inhibited Oxidation Jet Bubbling Reactor Lime Magnesium-Enhanced Lime Dual Alkali Seawater

Dry

Wet

Regenerable Dry
Activated Carbon

Wet

Dry

Limestone Forced Oxidation Limestone Inhibited Oxidation Lime Magnesium-Enhanced Lime Seawater

Lime Spray Drying Furnace Sorbent Injection LIFAC Economizer Sorbent Injection Duct Sorbent Injection Duct Spray Drying Circulating Fluidized Bed Hypas Sorbent Injection Lime Spray Drying

Wet

Sodium Sulfite Magnesium Oxide Sodium Carbonate Amine

Dry

Duct Sorbent Injection Furnace Sorbent Injection Circulating Fluidized Bed

Sodium Sulfite Activated Carbon Magnesium Oxide Sodium Carbonate Amine
16

Wet Scrubbers

? State-of-the-art is 95% SO2 removal ? 98 GW (33%) of coal-fired units have scrubbers FGD at Centralia Power Plant ? We project 115 GW to have scrubbers by 2010 for Title IV and State regs

17

Lime Spray Drying
Flue Gas In

Slurry In Recycle Loop Recycle Tank

Flue Gas Out

? State of the art is 90% removal ? More than 14 GW of installation

Disposal

18

Performance
100 Design SO2 Removal Efficiency, % 90 80

70

60 Wet Limestone 50 Spray Drying Median 0 1970s 1980s 1990s
19

Mercury in Coal-fired Boilers
Entrained PM CO2 H2O SO2 NOx HCl N2 300 °F Hg

APCD Inlet

Coal

Hg° ° >2500 °C

Mercury Speciation:
Hgo, Hg2+ compounds, particulate mercury Hg(p)

20

Mercury Speciation
? In general, speciation depends on:
– Coal properties (mercury, chlorine, and ash contents) – Time/temperature profile – Flue gas composition and fly ash characteristics (carbon, calcium, iron, porosity) – Flue gas cleaning conditions

21

Mercury Capture in Existing Equipment
Removal in PM Controls ? Mercury can be adsorbed onto fly ash surfaces; Hg2+ is more readily adsorbed than Hg0 ? Mercury can be physically adsorbed at relatively lower temperatures (hot-side ESP vs. cold-side ESP) Capture in Wet Scrubbers ? Hg2+ capture depends on solubility of each compound; Hg0 is insoluble and cannot be captured ? Capture enhanced by SCR
22

ICR Data
100
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Bituminous Subbituminous

80

Hg Removal (%)

? Bituminous vs subbituminous ? Hg capture for different coal-control technology combinations correlate with coal chlorine content

60

40

20

0

23

Chlorine vs. Mercury Speciation
100%

Cold-side ESP & FF Hot-side ESP
80%

? ICR data for Hg0 at ESP & FF inlet ? Hg0 oxidation appears to be independent of chlorine above 100 ?g/g ? Other important factors

%Hg 0 at PCD Inlet

60%

40%

20%

0% 10 100 1,000 Coal Chlorine, ppm dry 10,000

– Temperature – Fly ash carbon

24

SCR and Mercury Interactions
? Speciation influences emissions control
– Ionic Hg2+ is removed easily by wet scrubbers – Volatile elemental Hg0 is difficult to capture

? SCR units are being used extensively to meet current NOx regulations ? SCR can convert elemental mercury in coal combustion flue gas into the ionic form
– field data in Europe and U.S. reflects increase in Hg2+ across SCR reactor

25

SCR-Mercury R&D
? Tested 4 utility plants in the 2001 and 2 in 2002; retested 2 plants in 2002; total of 8 data points ? Oxidized mercury increase across SCR: bit. - up to 71%; subbit. - 10% (one data point only) ? Removal in PM control and FGD (5 data points) - ~ 85% - 90% ? Results from repeated tests were consistent with previous data; impacts of SCR catalyst aging not apparent ? SCR systems with relatively lower catalyst volumes (space velocity greater than 3500 hr-1) also showed significant oxidation increases ? Data gaps: PRB, blends ? Ongoing EPA bench- and pilot-scale research: HCl provides critical chlorine source for Hg0 oxidation; NOx has a significant promotional effect; SOx has little effect under the conditions of this study

26

PM Control Technologies for Power Plants
? Electrostatic precipitators (ESPs)
– 72% of U.S. coal-fired boilers, total PM up to 99.9%, fine PM 80-95%

? Baghouses
– 14% of U.S. coal-fired boilers, total PM up to 99.9%, fine PM 99-99.8%

27

Sometimes a picture is worth a …

28

How Does an ESP Work?
Particulate Matter (PM) Corona (Negative polarity)

Discharge Electrode (High Voltage Wire) Charged Particles

Particle Flow

Collected Particles Dust Layer

Collection Plate (Positive Polarity)

29

Emerging Technologies

30

Sorbent Injection
? The extent of capture depends on: – Sorbent characteristics (particle size distribution, porosity, capacity at different gas temperatures) – Residence time in the flue gas
Flue Gas Sorbent Injection

ESP or FF

– Type of PM control (FF vs. ESP) – Concentrations of SO3 and other contaminants
Ash and Sorbent

31

Activated Carbon Injection (ACI)
? ACI successfully used to reduce mercury emissions from waste-toenergy facilities. Effort underway to transfer to coal-fired power plants. Not currently installed at any power plant, but short-term testing suggests it may eventually be able to achieve 90% control for all coal types.
Activated carbon injection system Activated carbon storage and feed system

?

32

Carbon Injection Projects
? Alabama Power E.C. Gaston: unit 3, 270-MW, low-sulfur eastern bit. coals (0.14 ppm Hg and 160 ppm Cl); hot-side ESP, COHPAC baghouse; testing on one-half of the gas stream, nominally 135 MW; wet ash to pond WEPCO Pleasant Prairie: unit 2, 600-MW, PRB coal (0.11 ppm Hg and 8 ppm Cl); ESP (468 ft2/kacfm), spray cooling, SO3 conditioning; testing on one ESP chamber (1/4 of the unit); fly ash sold for use in concrete PG&E Brayton Point: unit 1, 245-MW, low-S bit. coal (0.03 ppm Hg and 2000-4000 ppm Cl); SO3 conditioning system; 2 ESPs in series (550 ft2/kacfm); PAC injection between the ESPs PG&E Salem Harbor: 85-MW, low-S bit. coal (0.03-0.08 ppm Hg and 206 ppm Cl); ESP (474 ft2/kacfm); SNCR

?

?

?

33

Mercury Removal Trends with ACI
100 Mercury Removal (%) 80 60 40 20 0 0 5 10 15 20 25 30 Injection Concentration (lb/MMacf)
PPPP Gaston Brayton Point

Source: ADA Environmental Solutions (2003)

34

PG&E Salem Harbor
(w/o PAC Injection)
? 85-MW, low-S bit. coal (0.03-0.08 ppm Hg and 206 ppm Cl); ESP (474 ft2/kacfm); SNCR
Hg Removal (%)
100

17-19% LOI (45 lb/M Macf)
90 80 70 60 50 40 30 20 10 0 270

20-24% LOI (55 lb/M Macf) 25-29% LOI (68 lb/M Macf) >30% LOI 30-35% LOI, C1, High Load 21-27% LOI, C2, High Load LS bitum coal

?

High baseline removal due to high levels of LOI; minimal impact on reducing LOI from 30-35% to 1520% at 300 oF Temperature has greater effect than LOI SNCR has no impact on Hg removal

? ?

290

310

330

350

370

Temperature (oF)

Source: ADA-ES

35

PG&E Salem Harbor
(w/ PAC Injection)
90 80 70
Hg Removal (%)

? At lower temperatures, removal by PAC affected by high baseline removal ? At higher temperatures, linear behavior (similar to that at Brayton Point

60 50 40 30 20 10 0 0 5 10 15 20 25
Injection Concentration (lb/106 acf)

280-290 F 298-306 F 322-327 F 343-347 F

Source: ADA-ES

36

A few more things . . .

37

Wet FGD Modification
? Capable of removing SO2 in excess of 95 % ? Can remove oxidized Hg ? Three routes for NO removal:
– gas phase oxidation to N2O5 – oxidation to NO2 and reduction to N2 in the scrubber via sulfate and bisulfate ions

? Investigate SO2, Hg, NOx removal and SO2 to SO3 conversion
38

ESFF
? ? Electrostatically Stimulated Fabric Filtration (ESFF)--developed by EPA Pulsejet fabric filter with high voltage electrodes centered between groups of four bags Pilot-scale performance data:
– – – – PM2.5 with ESFF=0.14 mg/m3 PM2.5 without ESFF=0.51 mg/m3 PM1 with ESFF=0.05 mg/m3 PM1 without ESFF=0.17 mg/m3

?

?

BHA Group, Inc. licensee has developed preliminary commercial design

39

Development of Multipollutant Sorbents
Capture Capacity of Various Sorbents
160 A-Clay 140 Amount Adsorbed 120 100 80 60 40 20 0 NO2 SO2 Pollutant
NO2 and SO2 capacity given as mg/g adsorbed in 1 hr at 80?C; Hg capacity given as ? g/g adsorbed in 1 hr at 80?C
0

? Sorbent Development
– Synthesis, Characterization, Evaluation & Optimization – Relate structure and chemical nature to adsorption characteristics

K-Clay Act C Lime ox-CSH

Hg

?Types of Sorbents Being Studied
–Sorbents synthesized using industrial by-products –Modified carbon-type sorbents –Surface modified Calcium Silicate Hydrate (C-S-H) –Multipollutant sorbents that also have adsorptive capacity for CO2
40


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