Commercial Application of 

Urea SNCR for NOx RACT 

Compliance on a 112 MWe 

Pulverized Coal Boiler 
COMMERCIAL APPLICATION OF UREA SNCR FOR NOx RACT 

COMPLIANCE ON A 112 ~We PULVERIZED COAL BOILER1 

James E. Staudt, Ph.D. 

Robert P. Cas ill, P .E. 

Research-Cottrell 

ABSTRACT 

Montaup Electric Company, a subsidiary of 

Eastern Utilities Associates, operates a 

tangentially-fired pulverized coal bdiler at 

its Somerset, MA generating station. NOx 

emissions from the 112 MWe Montaup 

boiler #8 must be reduced from their 

uncontrolled levels in order to comply with 

the Reasonably Available Control 

Technology (RACT) requirements 

promulgated by the Commonwealth of 

Massachusetts. According to RACT, NOx 

emissions from the unit must not exceed 

0.38 lb/MMBtu when firing coal or 0.25 

lb/MMBtu when firing oil. 

For reduction of NOx emissions from 

Montaup boiler #8, combustion controls 

and flue gas treatment were considered. 

Montaup Electric chose Selective NonCatalytic Reduction (SNCR) technology as 

its primary means of reducing NOx from 

baseline levels to the levels required for 

RACT compliance. The SNCR technology 

operates by injection of controlled amounts 

of aqueous urea into the furnace to reduce 

the NOx to nitrogen, water and carbon 

by 

Thomas S. Tsai 

Leonard J. Ariagno 

Eastern Utilities 

dioxide. Total project scope included design 

engineering, equipment supply, installation, 

and system startup. Optimization and 

startup of the system were completed in 

early March. Startup testing demonstrated 

that the system can achieve compliance 

levels across the load range 35% to 100% 

MCR while maintaining ammonia slip at 

about 1 0 ppm or less. This paper will 

describe the system provided to Montaup 

Electric and will discuss the results of 

startup and optimization testing. 

BACKGROUND 

Montaup Electric Company operates a 11 2 

MWe boiler at its Somerset, MA Generating 

Station. Boiler #8 began operation in 1959. 

It is a natural circulation, reheat, singledrum radiant boiler with a dry bottom that 

is rated at 800,000 lbs/hr steam flow at 

the superheater outlet at 1,925 psig and 

1 000 ° F. The furnace has four corners and 

four levels of tilting, tangentially fired coal 

buckets. Between the four coal buckets in 

each corner are three oil guns. The fuel for 

the boiler was changed from coal to oil in 

the early 1970's, and in 1984 the boiler 

1

Submitted to EPRI/EPA 1995 Joint Symposium on Stationary Combustion NOx Control, Kansas City, MO, 

May 16-19,1995 

1 
was retrofit to operate primarily on coal. A 

Continuous Emission Monitoring System 

(CEMS) was recently installed on the boiler, 

and a new Distributed Control System 

(DCS) is planned for installation in April and 

May of this year. Plans are to maintain 

boiler #8 in operation well beyond the year 

2000. 

Boiler #8 operates on dispatch typically in 

a load following mode. During the day and 

during other high demand periods the boiler 

operates at full capacity.ln the evening and 

on weekends the boiler load is frequently 

reduced to 35% to 55% MCR. Maintaining 

boiler #8 within environmental compliance 

under this wide range of operating 

conditions is, therefore, of critical 

importance. 

In the Commonwealth of Massachusetts, 

Reasonably Available Control Technology 

(RACT) for control of NOx from 

tangentially-fired utility boilers is defined as 

compliance with the following emissions 

performance levels: 

Coal 

Oil 

Coal +Oil 

0.381b/MMBtu 

0.25 lb/MMBtu 

Weighted average 

of above two 

values 

Under some conditions, particularly at part 

loads, boiler #8 cofires oil and coal. On an 

annual basis about 90% of the thermal 

input is from coal. Uncontrolled NOx levels 

measured from boiler #8 during 

temperature mapping and more recently 

during SNCR system initial optimization 

testing are shown in Figure 1 as a function 

of load. As shown in Figure 1, at part load 

conditions NOx levels can be much greater 

than those at full load conditions due to the 

increased excess air levels that are 

necessary to maintain reheat and superheat 

temperatures at their optimum level. 

Manipulation of furnace air distribution 

enables some reduction of uncontrolled 

NOx. Simulated Over Fire Air (SOFA), 

performed during temperature testing in 

December 1993 by feeding coal through 

only the lower three coal buckets, 

produced the lowest NOx levels. However, 

the NOx reduction possible in this manner 

alone is not adequate to achieve the NOx 

emissions levels required in Massachusetts 

for compliance with RACT. Additional NOx 

control measures are necessary. 

To comply with RACT, Montaup Electric 

evaluated several technologies, including 

low NOx burners, Selective Non-Catalytic 

Reduction (SNCR) and Selective Catalytic 

Reduction (SCR). Capital and operating 

cost, and performance guarantees over the 

entire load range were the key issues in 

selecting a technology· for RACT 

compliance. SNCR using aqueous urea 

solution was selected by Eastern Utilities 

for RACT compliance on Montaup Electric's 

boiler #8. Research-Cottrell supplied all 

engineering, equipment, installation, and 

startup services needed for implementation 

of the SNCR system, including initial 

furnace temperature mapping. 

SYSTEM DESCRIPTION 

The SNCR system provided to Montaup 

Electric included reagent storage, all 

pumping systems, and twenty eight wallmounted injectors arranged at four 

elevations in the furnace as in Figure 2. 

The four zones of injection are necessary in 

order to achieve NOx reduction over the 

operating range of 35% to 100% MCR 

when firing coal and/or No. 6 fuel oil. Of 

2 
Figure 1, Montaup Electric 

Baseline NOx vs. Load 

1.10 --------------------------------------------------------

1.00 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -[j - - - - - - - - - - - - - - - - - - - - - - -

0.90 

c 0 

~ 0.80 

+ + 

I 

~ 8 

= 

~ 

= 

0.50 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - '=-F - - - - - - - - -

0.40 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7 - - - - - - - ..± S..QF.A - - - - -

0.30 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

0.20 --------------------------------·------------------------

QlO --------------------------------------------------------

0 30 40 so 60 70 80 90 100 110 120 

Load(MWe) 

I+ 12193 I 

Figure2 

Boiler #8 and Urea Injector Locations 

3 
the twenty eight injectors, nineteen utilized 

existing furnace penetrations. Nine new 

penetrations were installed to 

accommodate the injectors. Major pieces of 

equipment are schematically displayed in 

Figure 3 and include: 

• a 20,000 gallon storage tank for 

outdoor storage of 50% by 

weight aqueous urea solution 

• a circulation loop with 

circulation pumps and in-line 

heater to maintain the 

temperature of the concentrated 

urea solution above the urea 

crystallization temperature 

• a chemical metering and mixing 

system that provides a 

measured amount of reagent to 

the injection levels at a 

prescribed pressure 

• distribution modules that 

modulate liquid flowrate and 

atomizing air pressure to the 

individual injectors 

• two-fluid, air-atomized injectors 

that automatically retract on 

zones 1 and 2 when these zones 

are not in operation 

• a control system composed of a 

Programmable Logic Controller, 

computer interface, local control 

panels that enable local manual 

control of equipment, and 

associated valves and 

instrumentation 

Total project duration, from receipt of 

order to completion of startup and 

optimization, was about eleven months. 

Installation of the boiler penetrations was 

achieved during an outage of about two 

weeks. 

Control of the system is feed-forward 

based upon inputs of steam flow (or oil 

flow) and mill configuration. From these 

signals the PLC will determine which 

injectors are operated and the maximum 

and minimum reagent that can be pumped 

to each injection level. A NOx feedback 

signal from the CEMS to the PLC enables 

control of the reagent flowrate within this 

range to m1n1m1ze overcontrol or 

undercontrol. The operator can adjust the 

setpoints in the control system as he sees 

fit. 

Results Of Initial Start Up and 

Optimization Testing 

Initial startup and optimization testing was 

performed to confirm the operability of the 

SNCR system and to establish the setpoints 

for all system components, including 

injector atomization parameters and the 

control system look-up table set points. 

During this period the system performance 

was characterized by: 

• NOx reduction 

• ammonia slip 

• reagent consumption 

NOx was measured by the plant's CEMS. 

Ammonia slip was determined by wet 

chemical means. Flue gas samples from 

downstream of the economizer were drawn 

through a known volume of sulfuric acid 

solution. The solution was analyzed for 

concentration of ammonia ion using an 

ammonia ion selective electrode. By this 

method ammonia concentration in the flue 

gas could be determined. Samples were 

taken by Research-Cottrell and an 

independent company contracted by 

Montaup Electric. Correspondence between 

4 
Figure3 

Process Flow Diagram of SNCR System 

~--------------------------------~ All 

..... ~--------------------------~ 

.......... 

Module 

To-.chZonel 

~~-L--~------~~ 

To II8Ch Zone. 

~~-1----~----~~·~ 

To II8Ch Zone ID 

~+-~----~----~~ 

Dletrtbullon 

MaduiH 

5 
independent measurements was generally 

very good. 

Reagent treatment rate in this paper is 

expressed in terms of gallons per hour of 

50% aqueous urea solution and in terms of 

Normalized Stoichiometric Ratio (NSR). NSR 

is the ratio of the actual stoichiometric ratio 

(of urea to uncontrolled NOx> to the 

stoichiometric ratio for theoretically 1 00% 

NOx reduction and 1 00% chemical 

utilization. For urea, NSR is equal to: 

NSR = moles of urea X 2 

moles of uncontrolled NOx 

Full Load Testing 

At 100% MCR conditions, NOx reduction 

was achieved through injection of reagent 

into the two uppermost injection zones 

(zones #3 and #4). Full load optimization 

testing results, shown in Figure 4 as 

ammonia slip versus NOx reduction, 

indicate that at full load conditions NOx 

reductions in excess of 50% may result in 

ammonia slip in excess of 1 0 ppm. Figure 

5 shows the results of testing at full load 

with variation of treatment rate and a 

constant 1 to 2 bias in reagent flow 

between zones 3 and 4, respectively. The 

testing demonstrated that NOx compliance 

(at or below 0.38 lb/MMBtu) could be 

achieved with less than 1 0 ppm of 

ammonia slip. 

Mid-Load Testing 

Boiler #8 operates periodically between 85 

and 1 05 MWe and very little between 60 

and 85 MWe. Normally the boiler passes 

through these load levels during early 

morning load ramp up to full load or during 

evening load ramp down to about 35-60 

MWe (net). Although the boiler operates 

less often in these middle lof:~ds, the high 

baseline NOx levels under these load 

conditions determined some of the design 

limitations of the SNCR system provided to 

Montaup Electric. 

At 95 MWe, uncontrolled NOx levels can 

exceed 0.90 lb/MMBtu. In terms of total 

NOx emissions, this is the worst case 

condition for the boiler. However, baseline 

NOx levels can be reduced by adjustment 

of furnace air distribution. Figure 6 shows 

the results of testing at 95 MWe with an 

initial NOx of 0.63 lb/MMBtu. Testing 

demonstrated that NOx can be reduced to 

compliance levels with roughly 1 0 ppm or 

less of ammonia slip. 

In order to test the limits of the SNCR 

system and to demonstrate the economic 

benefits of reducing baseline NOx through 

combustion air adjustment, testing of the 

SNCR system was performed at high NOx 

levels (0.94 lb/MMBtu) and 95 MWe. 

Testing showed that under these conditions 

NOx could not be reduced by the SNCR 

system to below 0.38 due to the high 

chemical flowrates required, which 

exceeded pump capacity. These results 

were expected since the system provided 

to Montaup Electric was not designed to 

accommodate this extreme case. In any 

event, operation in this manner is 

uneconomical due to the very high 

chemical consumption. Reagent chemical 

costs ranged about $0.80-$1.00 per gallon 

during this testing. However, as shown in 

Figure 7, at the lower baseline NOx level of 

0.63 lb/MMBtu, chemical consumptions 

can be greatly reduced and compliance 

more economically achieved. 

At 85 MWe, baseline NOx emissions during 

testing ranged from about 0.90-0.98 

lb/MMBtu. This is the lowest load condition 

on Boiler #8 where only coal is fired. Below 

85 MWe, Montaup Electric usually co-fires 

6 
0.90 

0.80 

0.70 

-

I 0.60 

0.50 

0.40 

..c 

0 0.30 z 

0.20 

0.10 

11 

10 

9 

8 

Figure 4, Montaup Electric 

Full Load 

NH3 slip vs NOx Reduction 

--------------------------------------11----------------

-------------------------------------------- --o--------

0 ------------------------------------------- ------------

7 ------------------------------------------ -------------

6 

s 

4 --------------------------------------- -----------------

3 

2 

I 

-----------------------------~---- 0 

-------------------------------- -!1-~----------------

0 1K 

Figure S, Montaup Electric 

Boiler#8 

Full Load Test 

---~--------------------------------------------------------------80 I 

I 

---~-------------------------------------------------------------- 70 il ------------------------------------------------------------ 60 

-----------------------------------------------------so dt fn 

________________________________ ii _____ 40 2~ 

---~--------------------------------------------------------- 30 

----:-------------------------------------------------------------. 20 

---~------------------------------------------------------- ~--. 10 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

NSR 

I+ NH3(ppm) 

7 
Figure 6, Montaup Electric 

Boiler #8, 95 MWe 

NOx NH3 slip vs NSR 

0.70 t-- -- ~- -- --- ------ ------- -- ---------- -- --- --- --- ------ --- --- - - - - - - -- - - - --170 I 

I 

0.60 t- - - - ~-• ~ - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --160 I 

~ o.so --- ~--------- ~----------------------------------------------------- i so ... 

~ I ~ ~ 0.40 - - - -:- - - - - - - - - - --- --- - - - --- - - 40 ~ 

- : ~ ";;' 0.30 - - - ~- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -.-- - - - 30 

0 I z I 

I 

0.20 ----~-------------------------------------------------------------~----~20 

I + I 

I 

0.10 ---~--------------------------------------------- 1 

I 

o.oo• o 

0 

1.00 

0.90 

0.80 

0.70 

NO X (lbiMMB1U) 0.60 

0.50 

0.40 

0.30 

0.20 

0.10 

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 

I 

NSR 

1 • NOx ab1MMB1U)J I+ NH3 slip --~ 

Figure 7, Montaup Electric 

NOx vs. Reagent Flowrate 

95 MWe, NOx Baseline 0.93, 0.63 

---,------~------------------------------------------------------ 1 

I 

---~-------------~-----------------------------------------------

--- _,_- I .-..---------------- -~--------------------------------------- I 

-------------------aone.if·-- I 

----.------------------ ~----- ---------------- - ---------------- -- -- I 

I 

---~----------------------------- 1 ----.. ft .. ~4------------------ I 

I 

---~-------------------------------------------------------------- 1 

I 

---~-------------------------------------------------------------- 1 

I 

0 so 100 ISO 200 2SO 300 350 

Reagent Flowrate (gph) 8 

1 • o.93 1 1 + o.63 ·I 
some oil with the coal. The results of 

testing at 85 MWe are shown in Figure 8. 

Testing showed that NOx emissions below 

0.381b/MMBtu could be achieved with less 

than 5 ppm ammonia slip by injection 

through zone 3 and 4 injectors. Injection of 

reagent through zone 2 and 3 injectors 

could not achieve adequate NOx reduction 

because of the higher temperatures in 

these zones at this load condition. 

Low Load Testing 

The results of SNCR testing at 40 MWe are 

shown in Figure 9. NOx emissions were 

reduced to nearly 0.30 lb/MMBtu while 

maintaining below 1 0 ppm ammonia slip 

with reagent injection through zones 1 and 

2. Reduction to below 0.30 lb/MMBtu was 

possible; however, ammonia slip increased 

above 10 ppm. Had NOx levels below 0.30 

lb/MMBtu been necessary under this load 

condition, it could have been provided for 

by design of a different injector 

configuration. Figure 8 also demonstrates 

the benefit of independent zone control 

with injection into two zones. Injection 

through zone 2 injectors alone produced 

high ammonia slip (over 20 ppm - the 

highest ammonia slip measured during the 

test period) with no advantage in NOx 

reduction. Injection into zones 1 and 2 

produced lower ammonia slip with good 

NOx reduction. 

Load Following 

Because Montaup Electric boiler #8 

operates over a wide load range, the SNCR 

system was designed to follow load and 

maintain the unit in compliance over the full 

load range. The boiler, in fact, spends very 

little time at intermediate loads. Typically 

daytime operation is at or near 1 00% MCR. 

At times that the unit is not operating at or 

near full load, it is operating in the range of 

about 35%-55% MCR. Since the unit 

normally passes through intermediate loads 

and these are the loads where NOx is most 

difficult to control on this unit, this is a 

challenging application for a control 

system. The control system must very 

quickly respond to a situation that initially 

requires increasing reagent injection rates 

while changing injection levels, and then 

requires decreasing injection rates while 

injection levels continue to change. 

In the short time that Montaup Electric has 

operated the SNCR system, the SNCR 

system's response to load changes has 

been observed. From this limited 

experience, the system seems to take 

about 30 minutes to fully adjust to a major 

load transient, as shown in Figure 1 0. This, 

however, is expected to be improved 

through adjustment of control system time 

constants and could be further improved, if 

needed, by use of in-duct NOx analyzers for 

feedback. Several minutes pass before the 

stack CEMS will indicate a change in NOx 

within the furnace, which results in a lag in 

the SNCR system response. In-duct NOx 

analyzers would reduce that lag time to 

seconds. Nonetheless, although NOx may 

pass above the NOx setpoint for a short 

time during the transient, the system 

consistently stabilizes at or below 

compliance levels. Figure 1 0 also shows a 

transient in the SNCR system while the 

CEMS was taken out of service, which 

exacerbated the NOx excursion during the 

load transient. The SNCR control system 

provided to Montaup Electric has a feature 

to properly accommodate for such 

situations; however, it will not be in 

operation until after Montaup Electric 

completes its DCS installation this spring. 

With this feature enabled and with 

additional operating experience, Montaup 

Electric operators can adjust the SNCR 

system controls to minimize any NOx 

excursions during transients. 

9 
Figure 8, Montaup Electric 

Boiler #8, 85 MWe 

NOx and NH3 slipvs NSR, vary injection zones 

1.00 - - - I - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7 

I 

0.90 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

S' 

E- 0.80 I 0.70 

c 

~ 0.60 

0.40 

___ .J I _________ _ 

I 

I 

I 

-------------------------------------------------- s 

3 --------- -----.---------------------------------- 4 

2&3 

0.30 -------'------------11---------+-------' 0 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 

NSR 

I • NO, (IJMMB1U) I J + NH3 sHp (ppm) I 

Figure 9, Montaup Electric 

Boiler #8, 40 MWe 

NOx and NH3 vs NSR, zones 1&2 or 2 only 

0.80 - - - ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 80 I 

0. 70 - - - ., - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 70 I 

I ! ::: ~ ~~t~~ ~~ ~ ~- ~~~~~- -~~~~ ~ ~~~ ~ ~~~ ~~~~~ ~~~~~~~~~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ : ~ ~ o.40 - - - ~- - - - - - - - - - - - - - - - - - - - - - _IJ!I_ - - - - - - - -1

- ._-2

------------- 40 !2"' 

~ 0.30 - - - - ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -t A 2.. - 30 

2 

0.20 - - - ..: - - - - - - - - - - - - - - - - - - - - - - - - -+- ------------------------- 20 I 

: 1&2 

Q10 ---4---------------------------------- -- ------------- 10 I 

1&2 

o.oo L.....t..~====------_-_-_-_--_--_-_--_-_--_-_--_-_--_-_--_-_--_--_-..J- o 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

NSR 10 

• NOx I+ NH3s6p I 
100 

90 

80 

70 

• 60 

at 

ftl 50 ... 

1: :40 

.. 

• A. 30 

20 

10 

0 

Figure 10, SNCR System Response to Load Change 

Steam 

Pumro 1 

Fow 

Flow 

"-Pum~3 Fow 

CEM Off CEMOn SNCROff 

.---

90 80 70 60 50 40 30 20 

Time (in Minutes) 

10 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 

11 
Carbon Monoxide Emissions 

Carbon monoxide (CO) emissions can 

potentially increase as a result of SNCR 

with urea. However, noticeable increases in 

CO emissions were not observed during the 

start up and optimization of the SNCR 

system. 

Economics 

The fully installed capital cost of this SNCR 

system is in the range of $1 5-$1 6 per 

KWe. This cost includes a technology 

license fee which is a significant portion of 

this total cost. Were Montaup Electric a 

member of E.P.R.I., the license fee would 

have been substantially less. 

Under full load conditions it was 

demonstrated that compliance could be 

achieved with roughly 1 50 gallons per hour 

of reagent. Using a price of $0.80-$1.00 

per gallon, this equates to a reagent cost of 

$700-$880 per ton of NOx removed, or 

about 1.1 to 1.3 mils per KW-hr. 

SUMMARY 

Start up and optimization of the SNCR 

system at Eastern Utilities 112 MWe 

Montaup Electric Boiler #8 demonstrated 

that the system is capable of reducing NOx 

to below Massachusetts RACT compliance 

levels over the load range of 35% to 100% 

MCR. Over this load range ammonia slip 

remained at or below 1 0 ppm. Operation of 

the system also demonstrated the 

economic benefits of minimizing baseline 

NOx to minimize reagent consumption. 

Montaup Electric plans to incorporate 

automated combustion controls that will 

reduce their baseline NOx and enable 

operation of the SNCR system under the 

most economical conditions. 

While experience with SNCR operation 

during load changes is limited at this time, 

the control system provided has 

demonstrated that it can follow a load 

change and adjust SNCR system operation 

to bring the NOx emissions in compliance at 

the new load. Although a transient in NOx 

emissions may occur for a few minutes 

while the SNCR system adjusts to the new 

condition, the system consistently brings 

NOx emissions into compliance levels. 

System response of the SNCR system will 

be improved through operator experience 

and adjustment of system control 

parameters, thereby minimizing these 

transients. 

12