Tail-End SCR Technology Cons

EPRI SCR Workshop 2009

Tom Weir Marilynn Martin H
5 bituminous coal-fired units:

Duke Energy’s Allen Steam Station, 

• Units 1 and 2 (165 MW each), 

Units 3 (265 MW), 4 (280 MW) and 5 

( ) 270 MW

). 

• Tangentially-fired CE boilers 

with low-NOx burners and OFA.

• All 5 units combine into 2 wet 

LSFO Alstom FGDs and a wet stack, start-up in 2009.

• SNCRs on all 5 units resulting 

in additional air heater washes.

• M ay requi tre

a grea

ter NO

x

removal efficiency technology such as SCR. 
Duke Energy’s Allen Steam Station, 

Site considerations for a highdust SCR retrofit include:

• No room between the boilers 

and the ESPs or the ESPs and the stacks. 

• Extended downtime of each 

unit if high-dust SCR reactors to be placed on the top of the boiler house. Load issues may make this infeasible.

• Constructability problems 

due to no accessibility of the three center units.

• High specific retrofit costs.
Duke Energy’s Allen Steam Station, 

Other SCR retrofit options for Allen Steam S

• 2 Cold-side low-dust SCRs for all 5 units.

Boiler

ESP

N

Regenerative 

Gas-Gas Heat 

Steam 

Reheater

Boiler

ESP

• 2 tail-end SCRs for all 5 units. 

Air

Air Heater

Steam

Exchanger 

FGD ESP 

Boiler

Air

Air Heater

R

Ga
Duke Energy’s Allen Steam Station, 

Immediately perceived disadvantages of 2 cold-side low-dust or tail-end SCR retrofits compared to 5 highd t SCR t fit i l d dust SCR re

trofit

s inclu

de:

• Additional energy/steam 

consumption for flue gas reheating.

• Additional pressure drop 

from a regenerative gas/gas heat exchanger and steam coil for flue gas reheating. 

• Higher capital investment 

costs due to the need of this 

additional equipment.
Duke Energy’s Allen Steam Station, 

Actually expected advantages of 2 cold-sidend SCR retrofit compared to high-dust SCR units would include:

• Lower expected capital equipment costs 

end SCRs for all 5 units compared to 5 h

• Much lower expected construction costs 

space constraints for easy constructabi

• No interference with current unit operat

construction and tie

-ins.

• Short downtime of the units for tie-ins o

SC s ca be t ed sepa ate y R

s can be

tied in sepa

rately.

• In case of tail-end SCR, dry stack and el

visible wet plume. 
Evonik’s Tail-End SCR Experience – 

Evonik has operated 1 cold-side low-dust afor more than 20 years:Cold-side low dust and tail-end SCRs typica

• one 2-layer SCR reactor filled with 3 h• catalyst with a pitch of 4.2 mm (35 x 

• a regenerative gas/gas heat exchange

gradient of less than 25 – 45 F (15 – 2

• a steam coil for overcoming the GGH

a steam coil for overcoming the GGH
Evonik’s Tail-End SCR Experience – 

Tail-end SCRs typically have a small footpri

• A tail-end SCR 

for a 350 MW unit has an 

actual footprint o

f

only 80 ft x 80 ft including a GGHE and booster fan
Evonik’s Tail-End SCR Experience – C

Tail-end SCRs typically have a small reacto

• Small catalyst volume of only about 

0.75 m

3/MW due to:

• Use of small catalyst pitch 

(4.3 mm, 35 x 35 cells).

• higher tolerable NH

3 slip (5 ppm) 

due to low SO

x concentrations 

l P

Hern

and no NH

3 in fly ash concerns.

• Catalyst deactivation 

is extremely slow. 

Ox potentia

Catalyst life of 150,000 hours of SCR o

peration and 

0 20000 40000 60000 

DeN

p

more is normal.

0 20000 40000 60000 
Tail-end SCRs typically have a higher press

Tail-End SCR Experience – Pressure 

• The higher pressure drop of the smalle

offset by fewer catalyst layers and sho

• Additional pressure drop of 7 

– 8 in. w.c

the GGHE and steam coil for flue gas re
Tail-end SCRs typically have a lower operat

Tail-End SCR Experience – Temperat

• Due to very low SO

2, lower SCR operati

of around 550 F (290 

°C) are suitable. 

• L SCR ti t t 

Lower SCR operating tempera

tures res

• Lower actual flue gas volume 

flow rate

Smaller reactor cross 1.0%

•

sectional area

• Smaller GGHE diameters

• Very low SO

2/SO

3 conversion conversion

rate

• Low SO

3 concentrations 

downstream of the SCR and 

0.5%

in the stac

k

• No SO

3 plume issues
Tail-End SCR Experience – Steam Us

Tail-end SCRs typically have a higher steam 

• Steam consumption for flue gas reheat

by the gradient 

of the GGHE, typically 1 – 3% of unit steam flow de

pendent

on the gradient of the GGHE. 

• Id l f t bi Ideal for turbine

limited units as no reduction in generation is incurred.

• No sootblowers.
Tail-End SCR Experience – Real Life 
High-Dust Versus Tail-End – Qualitat

Allen Steam Plant hi

g

h‐dust cold‐sid

SCR reactor height 3+1 layers 2+1 SCR reactor cross sectional area 100% 1Economizer bypass Economizer bypass for low load for low load needed 

Sootblowers/sonic horns needed LPA screens needed 

Catalyst module transport system needed NOx analyzer maintenance high lRe

generative

g /g g as as heat exchan

ger ‐ ne

Steam coil for flue gas reheating ‐ ne

• Lower SCR reactor height for less require

d t ll t l t it h due t

o smaller ca

t

alyst 

pit

c

h.

• Smaller SCR reactor cross sectional area 

gas temperature and actual flue gas volum
Allen Steam Plant hi

g

h‐dust cold‐sid

High-Dust Versus Tail-End – Qualitat

g

Capital equipment cost $$$ Constructability / installation cost $$$ Cost of lost generation / tie lost generation

/ tie‐in time $$$

Energy consumption (dp, steam) $ Catalyst cost / life $$$ Cost for catalyst exchanges $$$ SO

3 plume issues $$$ 

Visible 

plume

$

LPA issues $$$ NOx analyzer maintenance $$$ 

• In case of Allen Steam Plant, the only tan

remains the higher energy cost due to stefor flue gas reheat in the range of about 2
Cold-side low-dust or tail-end SCR retrofits 

High-Dust Versus Tail-End SCR – Con

for combining several smaller units into one This can very effectively:• Reduce unit downtime and thus lost gene

construction and tie-in.

• R d it l i t t t 

R

e

duce capit

al inves

tment cos

t

s as

a resu

rather than multiple larger SCR reactors. 

• Eliminate space constraints and construc

• Reduce retrofit construction time and min

as a result of ease of construction.

• Eliminate impact of SCR on unit operation
High-Dust Versus Tail-End SCR – Con

Advantages include:• Minimal catalyst deactivation results in a 

life and very little cost for catalyst exchan

• Elimination of operational issues such as 

catalyst erosion, SO

3 plume, NOx analyze

issues catalyst exchange logistics issues issues, catalyst exchange logistics issues

• Virtually fly ash free flue gas results in ve

maintenance requirements and costs.

• High adaptability to site specifics and selOnly really tangible disadvantage in a case the higher energy cost due to steam consureheat in the range of about 2% of unit stea
Questions?