# PostCombustion NOx Reductionwith Urea: Theoryand Practice

SeventhAnnual International

PittsburghCoalConference September 10-14, 1990



# William H. Sun, John E. Hofmann Nalco 

# FuelTech Naperville, Illinois

Reinhard Pachaly FuelTech, GmbH Mulheim, FAGPost Combustion NOx Reduction with Urea: Theory and Practice William H. sun, John E. Hofmann

Abstract Nalco Fuel Tech Naperville, Illinois, U.S.A. Reinhard Pachaly

Fuel Tech, GmbH Mulheim, FRG

The NOxOUT Process has ·proven to be effective in controlling NOx emissions from various combustion sources when chemicals are injected at temperatures between 750**~** 1100 

### °c and their distributions optimized. High levels of NOx reduction and low byproduct emissions have been achieved by using urea and other chemicals, along with the use of multi-level injection techniques. The factor which limits the process application at temperatures above 1100 

### °c has been determined through a chemical kinetic model. The model is based on the CHEMKIN subroutine package and uses published reaction parameters and thermodynamic data. This factor, referred to as 'Critical NOx', is affected strongly by temperature but weakly by excess oxygen concentration. It is also affected by baseline NOx concentration at temperatures above 1050 

### °c. Through this study, conditions under which NOx can be reduced at temperatures as high as 1200 °C and beyond have been defined. Experimental results from a laboratory combustor and industrial and utility boilers support this finding.
Introduction The oxides of nitrogen (NOx) contribute to smog, ground level ozone formation, and acid precipitation which can severely damage vegetation, marine life and various man-made structures (1]. Manmade NOX emissions are mostly from both mobile and stationary combustl.on of fossil fuel. To control mobile sources, engine designs were modified, and automobiles have been equipped with catalytic converters. As for stationary sources, tighter regulations have and are being promulgated on utility and industrial boilers and incinerators. Various techniques have been employed to control NOx, and they can be categorized into three areas: (1) Fuel Cleaning or Low Nitrogen Fuel (2) Combustion Modification (3) Post Combustion control The first method lowers NOx by limiting contribution from fuel sources. NO is formed from oxidation of fuel bound nitrogen, termed fuel N6x, and from oxidation of atmospheric nitrogen, termed thermal NOx· By removing fuel bound nitrogen from fuel or burningfuel with low nitrogen,**i.~.** natural gas, fuel NOX is limited. Combustion modification reduces NOx emission by lim1ting thermal and fuel NOx· By lowering combustion temperature through flue gas recirculation and by limiting oxygen for N 2 oxidation through staged combustion, thermal NOx is controlled. Staged combustion also lowers the conversion of fuel bound nitrogen to NOx· Unlike previous methods which limit the formation of NOx, the third method, post combustion control, reduces NOx after the formation is completed. In this method, two categories exist: catalytic and noncatalytic. Catalytic reduction deals with reaction of NOx with ammonia on various metal catalysts at temperatures between 300

- 500 

### °c [2]. The noncatalytic method consists of injecting either ammonia or urea where flue gas temperature ranges between 850

- 1100 

### °c. Ammonia injection is an Exxon process and has been called the Thermal DeNOx Process [3,4] while the urea injection was patented by EPRI [5,6]. Nalco Fuel Tech is EPRI's exclusive licensing agent, and the technology is being marketed as NOxOUT Process. The NO OUT Process has been in development for several years by Fuel

### Tee~, Inc., and several articles have been published dealingwith various facets includingprocess demonstration results [7-11].
NOXOUT Kinetic Model As a part of the development effort, a chemical kinetic model has been developed to understand the basic chemistry, to determine important factors and to define the limits of process capability. This model describes an ideal plug flow, i.e., no temperature or species concentration gradient in radial direction and no backmixing. Species concentrations with this type of flow are described by a set of ordinary differential equations. The number of equations depends on the number of species being modelled, and the equations are interrelated through reaction rates. Enthalpy equations are neglected in the model. Instead, measured or estimated temperature profile is a model input. Reaction rates, density, and thermodynamic information are supplied by a library of gas-phase subroutines called CHEMKIN developed at the Sandia National Laboratories (12]. This package of subroutines compiles user supplied reaction set and thermodynamic data file. The resulting set of differential equations are integrated simultaneously using a numerical integrator called LSODE [13)• This solver was developed at the Lawrence Livermore National Laboratory and has shown to be most efficient in solving combustion problems (14]. Initial condition for the calculation was the equilibrium concentrations of species at temperature and excess 

### o2 as measured. Measured NOx concentration is also an input. Equilibrium concentration was calculated using a program called STANJAN developed at Stanford University [15]. The components of the NOxOUT Kinetic Model are shown in figure 1. The reaction set is adopted from the work of Miller and Bowman (16)• From these elementary radical reactions, only the ones which do not react with hydrocarbons were selected. The wet 

### co oxidationreactions, ammonia oxidationreactions, andHCN oxidationreactions make up the set. Urea decomposition is modelled as a rapid and one step breakdown to NH3 and HNCO. Experimental results by eaton and Siebers [17] show a quantitative conversion of urea to NH3 and HNCO at high temperatures. The final reaction set consists of over 90 reactions describing interactions among 31 species. The major pathway of urea breakdown and reaction with NOx is shown in figure 2. Model Validation With the developed model, results from a pilot scale combustor are compared with model results. A schematic of the pilot combustor and the analytical setup is shown in figure 3. The test zone of the combustor was kept isothermal by electrical heating. The residence time at this zone was approximately 

### o.s second. This is the average residence time between the injection point and the end of the isothermal zone. Temperature was varied from 850 

### °c to 1100 0

### c. Baseline NOx value and treatment rate were kept constant at 200 ppm and at NSR of 2, respectively. NSR is defined as the actual mole ratio of urea to NOx divided by the theoretical stoichiometric ratio, which is 

### o.s for the reaction between urea and NOx. Chemicals were injected with an air atomized nozzle, which creates a spray of fine droplets. The initial condition was the measured baseline NOx and the equilibrium concentration at the measured temperature. Tne comparison of the model and experimental results is shown in figure 4. The available reaction times for injected chemicals are less than the residence time because reaction starts after droplet evaporates. Because of this delay, calculation results from reaction times of 0.1, 0.2, 0.4, and 0.6 seconds are compared to the experimental results. As shown, the trend and the shape of the experimental results are well modelled. In addition, all the experimental points fall within the calculated lines with reaction times of 0.1 and 0.4 seconds. Temperature Window A range of temperature where significant NOx reductions are obtained is called the temperature window as indicated on figure 4. Within this window, the NOx versus temperature curve consists of three zones: left side, right side and plateau. This shape is a result of chemical kinetics. At temperatures lower than this window, reaction initiating hydroxyl radical concentrations and reaction rates are low. These two factors slow the breakdown of NH3 and HNCO. NOx reduction therefore requires longer reaction time than is typically available in most commercial combustion systems. At the left side of the curve, NO decreases with increasing temperature because reaction rates and'oa concentration increase with temperature, which in turn, increases NOx reduction. Residence time limits the NO reduction at this zone. At the plateau, reaction rates and oR concentration are optimum for NOx reduction. A temperature variation at this zone has only a small effect on NOx. A further increase in temperature beyond theplateau zone decreases NOx reduction. At the right side, the oxidation reactions of NH3 and HNCO to NO become a significant path for injected chemical and compete with Aox reduction reactions for reagent. These oxidation reactions increase with temperature and OH concentration and thus NOx reduction decreases with increasing temperature at the right side. A further increase in temperature beyond the right side increases NOx above a baseline NOx. Although the reduction is less than the maximum, operation at the right side is practiced and recommended since byproduct emissions decrease with increasing temperature.
Treatment Rate Increasing NSR has a diminishing return in NOx reduction. In figure 5, model results of NOx concentration as a function of NSR are presented for several temperatures. NOx decreases with increasing NSR for those calculations within the temperature window; at higher temperatures, increasing NSR increases NOx· At temperatures between 900 and 1200 

### °c, NOx reaches a lower limit at an NSR of about 2. A further increase from NSR of 2 increased NOx for 1200 

### °c case but had no effect on other cases. Residence Time The temperature window becomes wider with an increase in residence time. As shown in figure 6, the window is only 150 

### °c wide for a 0.1 second case but the window increases to 300 

### °c for one second residence time case. This widening resulted from shifting of the left side of the NOx versus temperature curve to a lower temperature. At this side, NOx reduction is limited because of slow reaction rates and low OH radical concentrations. Therefore, a longer reaction time increases NOx reduction. The plateau zone where NOx reduction is maximized widened from 50 

### °c at 0.1 second to 150 

### °c at 1 second. At this plateau region, NOx was reduced by only a few ppm when residence time increased from 

### o.a to 1.0 second indicating that the reactions are essentially complete at one second and that the lowest achievable NOx concentration has been reached. The right side is unaffected by residence time. Baseline NOx NOx reductions were calculated at baseline NOx levels of 100, 200, and 

### sao ppm and at temperatures between 750

- 1300 

### °c. In figures 7a, b, and c, NOX concentrations.versus residence times are plotted for three**basel~nes** and at 1000, 1100, and 1200 

### °c, respectively. NSR was kept constant at 2 for all cases. The residence time needed· to reach a steady state condition was shortest at 1200 °C; the baseline values did not affect the time to reach a steady state value at a temperature. As for the controlled NOx level, starting from 100, 200, or 500 ppm resulted in the same reduced NOx of about 30 ppm at 1000 

### °c. The controlled levels were higher for the 500 ppm baseline case than the 200 ppm case by 15 ppm at 1100 °C and 100 ppm at 1200 

### °c.As shown in figure 7c, NOx increases with chemical injection for a baseline of 100 ppm with temperature at 1200 °C. However, when the baseline is 500 ppm, a significant reduction can be achieved. This illustrates that higher NOx baselines shift the right side of the NOx reduction versus temperature curve toward higher temperatures. Aplot of the NOx reduction curve for three baseline NOx levels, as shown in figure 

### a, clearly shows the widening of temperature window.
Critical NOX In figure 6, the curve which represent the reduced NOx concentrations at 1 second residence time and at temperatures between 900 and 1300 

### °c is the lowest achievable NOx concentration curve. As discussed earlier, increases in residence time or NSR do not lower NOx below this curve. At temperatures below 900 °c, calculations with 10 seconds of residence time and several NSRs were performed to determine the lowest achievable concentration. This minimum achievable NOx concentration through urea injection, is termed as 'Critical NO ', as first indicated by Fenimore for the Thermal DeNOx process.

### ~n figure 9, critical NOx concentrations are plotted as a function of temperature for three baseline NO levels. The baseline level affects the reduced concentration, bul only at temperatures above 1050 °C. The critical NOx concentration therefore is a function of baseline NOx only at temperatures above 1050 °c as shown in figure 9. The critical NOx is affected strongly by temperature and baseline NOx. This critical NOx concentration is less than 40 ppm at a temperature of 1050 °C and even lower at temperatures below 1000 0

### c. These low levels usually do not limit the process application. Critical NOx concentration increases rapidly as temperature increases beyond 1050 °c, and therefore, the process application is limited at high temperatures. However, as long as baseline NOx is above the critical NOx, the reduction should be achieved.
Laboratory· and Field Verification The model study has indicated that critical NOx limits the achievable NOx concentration regardless of treatment rate or residence time. The left side of the temperature window shifts to a lower temperature with an increase in residence time while the right side shifts to a higher temperature when starting with a higher baseline NOx concentration. This finding is verified in this section througn experimental results.
case 1. During a process demonstration at an ethylene cracker, NOx reduction was limited regardless of the NOxOUT Process parameters. The cracker unit operated steadily at a temperature ofapproximately 1050 °c. When the unit operated at a higher NO baseline, NOx reduction increased, but the lowest achievable**NO~** concentration remained the same, as shown in figure 10. Increasing NSR or other methods to optimize chemical distribution had no effect on.the minimum controlled NOx. This showed the existence of critical NOx which is unaffected by NSR, chemical distribution, or baseline NOx.

case 2. The NO level decreases with increasing NSR as long as gas

### tempera~ure at the injection point is within the temperature window. At temperatures above 1100 °c, the model, however, calculated a NOx increase when NSR is increased beyond the level where the minimum NOx has been achieved. Testing at a 160 MWe tower boiler firing oil and at a 19 MWe stoker fired boiler burning wood waste verified this phenomenon of NOx increase as indicated by the model. As shown in figure 11, NOx was reduced to the minimum concentration at NSR of 3.5. A further increase was counter productive as NOx increased slightly with NSR. Gas temperature at the injection location was greater than 1150 °c. A testing at a stoker unit also showed an increase in NOx when NSR increased beyond the treatment rate where minimum NOx was achieved, as shown in figure 12. Gas temperature for this case was in excess of 1100 °C at the injection location. This behavior was only seen for those applications where gas temperatures were higher than 1100

## °c.

case 3. The temperature window for NOx reduction widens by shifting the left side of the curve to a lower temperature with an increase in residence time. Testings at municipal solid waste combustors showed that high NOx reductions were achieved as shown in figure 13 [11]. Ammonia emissions were limited to less than 20 ppm. Gas temperatures at injection level for these testings were 900

- 950 0c, and available residence times in this temperature range were in excess of 1 second. This type of performance was not seen from utility boilers where residence time is very limited.

case 4. An increase.in NOx baseline shifts the right side of the curve to a higher temperature. To verify this, a pilot scale combustor was operated at 1200 °c and NOx baseline was increased from 150 to 750 ppm. At baseline NOx below about 200 ppm, NOx increased with urea injection, while at higher than 200 ppm, NOx decreased as shown in figure 14. Acomparison with an ammonia injection showed that urea is more effective 

### in reducing NOx at these high temperatures than ammonia.conclusions The reaction kinetic model has proven to be a valuable tool in the development of the N0¥0UT Process. Model predictions with respect to the temperature w1ndow for reaction, the effect of residence time, and the phenomenon of critical NOx, which limits NOx reduction at the high end of the temperature window, have all been verified in laboratory and field tests. The temperature window has been shown to be defined primarily by residence time at the low temperature (left) side and by baseline NOx at the high temperature (right) side of the window. The model is now used to define temperature/residence time requirements for specific applications and to predict maximum achievable NOx reduction. The model has also shown that the NOxOUT Process can oe applied at higher temperatures than previously thought applicable provided that the initial NOx baseline is sufficiently high. An example of such an application is the utility "cyclone" boiler which is used to burn coals of generally low quality. These boilers typically have furnace exit temperatures of 1200-1250 °C. These temperatures would normally have been said to be outside the temperature window for urea. However, NOx produced in these boilers are high in concentration. This level of NOx which is about 1000 ppm is substantially above the critical NO concentration at 1250 °C. Thus a significant amount of NOx re8uction should be achievable by urea injection into the upper furnace of these boilers.
References 1. Mohnen, V.A., "The Challenge of Acid Rain", Scientific American, Vol. 259, No. 2, p.JO, August, 1988 2. Benson, c.E., Chittick, G.D., and Wilson R.P., Selective catalytic NO Reduction Technology for Cogeneration Plants, Arthur

### o.**Lit~le** Report, Prepared for New England Cogeneration Association, November 1988. J. Lyon, R.K., 

## u.s. Patent 3,900,559, 1975. 4. Lyon, R.K., "Thermal DeNOx: Controlling Nitrogen oxides Emissions by a Noncatalytic Process", Environ. Sci. Techno!., Vol. 21, No. J, p.2Jl, 1987. 5. Arand, J.K., and Muzio, L.J., 

### u.s. Patent 4,208,386, 1980. 6. Arand, J.K., Muzio, L.J., Teixeira, D.P., 

### u.s. Patent 4,325,924, 1982. 7. Epperly, W.R., Broderick, R.G., and Peter-Hoblyn, J.D., "Control of Nitrogenoxide Emissions from Stationary Sources", American Power Conference, April 20, 1988, Chicago, IL a. Epperly, W.R., Hofmann, J.E., "The NOXOUT Process for Reduction of Nitrogen oxides", Presented at the a1st A.P.C.A. Annual Meeting, Dallas, Texas, June, 1988. 9 . Hofmann, J.E., von Bergmann, J., Bokenbrink, D. , and Hein, K. , "NOx Control in a Brown coal-Fired Utility Boiler", EPRI/EPA Symposium on Stationary Combustion NOx Control, March a, 1989 10. Epperly, W.R., Hofmann, J.E., "Control of Ammonia and CarbonMonoxide Emissions in SNCR Technologies", Prepared for AIChE 1989 summer National Meeting, August 20, 1989. 11. Hofmann, J.E., Luftglass, B.K. , sun, 

### w.H. , Ferraro, F.A. , Porter, T.J., Pachaly, R., "NOx Control for Municipal waste Combustors",**Present~d** at Air 

### &Waste Management Association Annual Meeting, Pittsburgh, PA., June, 1990. 12 Kee, R.J., Miller, J .A., Jefferson, T.H., "CHEMKIN: AGeneral Purpose, Problem-Independent, Transportable, Fortran Chemical Kinetics Code Package", Sandia Laboratories Report #SANDS0- 8003, Livermore, CA, 1980. 13 Hindmarsh, A.C., "ODEPACK, A Systematized Collection of ODE Solvers", Scientific Computing, R.S. stepleman et al. (eds.), Vol.l of IMACS Transactions on Scientific computation, p.ss, North-Holland, Amsterdam, 1983. 14 Radhakrishnan, K., "A Comparison of the Efficiency of Numerical Methods for Integrating Chemical Kinetic Rate Equations", 1984. 15 Reynold, 

### w.c., STANJAN, Copyright Stanford University, 1987. 16 Miller, J.A., Bowman, C.T., "Mechanism and Modeling of Nitrogen Chemistry in Combustion", Fall Meeting of the Western States Section/The Combustion Institute, Dana Point, California, October, 1988. 17 Caton, J .A., Siebers, D.L., "Comparison of Nitric Oxide Removal by cyanuric Acid and by Ammonia", Fall Meeting of the Western states Section/The Combustion Institute, Dana Point, California, October, 1988.

# INOxOUTKineticModelj

Thermodynamic Data ReactionSet

# ~ 

# / CHEMKIN

### ~--- 

# + InitialCondition __,...PlugFlowModel •**~** LSOOE 

# i Results Fig. l



## POSTCOMBUSTION NOx REDUCTION with UREA Urea

# /~

# ... HNCO NH3 I I I i 

# ' ' NCO NH2, 

# //'1 I
--------- NO

# ~ I "'---"'-..,_

... /

. ', I**~** 

### NO 

# r**~** NO I 

## N20 !

# ~N2/ Fig. 2

# IPILOTSCALECOMBUSTOR

# I

INJECTOR \CHEMICALBURNER ZONE

ELECTRICALLYHEATED

NOx

- 1 DRYER CHILLER

# ~~**Ji---~=--t_J** N20

Fig. 3


## EffectofTemperatureon NOxReduction UreaInjection,NSA=1.8-2.3,NOxl=,..200ppm Comparl.onofMOdelvs. Experimental**250~--------------------------------------------~**

200



# -**~** 150 

### a. a. 

# - )( 0100 

## z

50 : TEMPERATURE

.._. WINDOW ____... 0.2sec**0~----~--------------------------------~----~** 600 700 800 900 1.000 1,100 1,200 1,300 

### Temperature (C)Fig. 4350

300

250**~200** Q_ !h X 0 150 z

100

50

0 0 

## EffectofNSRonNOxReduction

0

.5 R

.. idenc:aTime 11 1sec,NOxl

### =200ppm NOXOUTKineticModet

15 2 NSA 2

.5
Fig. 5 3 1300C

1200C

800C

1100c

1000C 900C



## EffectofResidenceTimeon NOx Reduction NSR

### =2,NOxl

### =200ppm

NOxOUTKineticModel 3

.5

300**~------------------------------------------------.**
250

-200 

### E a. .3; 150 X 0 

## z 100

50

BOO 900 1

.000 1,100 1,200 1,300 1,400 

### Temperature (C)
0

.1 0

.2 0

.4 0

.6 0

.8 1

.0 ---£:,--
- ...o...... - _.,._- -..----..- _,_.,___ Fig. 6

## {'\ 

## EffectofBaseline NOx NSA

## = 2,Temperatura= 1000C NOxOUTKineticModet**~~--------------------------------------------~**
500 f-

-400f-·**~** 

### a.. 9:,300- x 

### 0 

# z 200 r:

100

## ~-~----~--**--~-~~--~--~~--~----------~~ 0~--------------------------------------------~**  0

600

500**~**

-400**~-. ~**  

### a.. 

## fh300**~** X 0 

## z 200**~**



### ..**~- ~**  100 I--

0 0 0.1 0.2 0.3 

### Time (second) Fig. 7a



## EffectofBaseline NOx NSA::2,Temperatura= 1100C NOXOUTKineticModel

0.05 0.1 

### Time (second) 0.15
Fig. 7b 0.4 0.5

0.2f 

## EffectofBaselineNOx NSR=2.TemperatuN

## = 1200C NOxOUTKineticMOdel**~r----------------------------------------------**
500 f-

........... 400 .__.**~** 

### a.**~300** .... ···. )( ······································ ... . ························································· 

### 0 

## z 200**~.**



### ·--------------------·---·-------------------------------·-------------
100**~ 0~--------------------------------------------~**  0 

# 0.05 0.1 0.15 

# 0.2 Time (second)Fig. 7c



## EffectofBaseline NOxonTemperatureWindow NSA=2, RealdenceTime= 1sec. NOxOUTKineticModel 

# 100-----------------------------------------------

........... 50 .'#, 

# - c: 0 

## 13 ,----------

BaselineNOx

SOOPPM**~ 0~----------------------------~--~--------~**  1:l Q) 

### a: )( 

### 0 ',
• 200PPM 

## z (50)
100PPM (100)**L.-----....:......----__:_----------~---__..:....---------------..~** 700 800 900 1,000 1,100 1,200 1,300 1,400 

### Temperature (C) Fig. 8.l 

## CriticalNOxConcentration 3%ExCHaOXygen NOxOUTKineticModel 

# 500.-----------------------------------------

400**~**

X 

### 0 200 r-- 

## z

100 r-- NOxi::z500PPM 1

(

r ,• 

### r .··

NOxi=100PPM

### oL-------==========~--------------~----~ 700 800 900 1,000 1.100 1.200 1,300 

### Temperature (degreesC)
Fig. 9



## NOx REDUCTION AT ETHYLENECRACKER Temperature
- approximately1050C 1.400**100~----------------------------------------**
-90 N 

### 0 

### ?ft. 80 ('I)**~ ~**  70.**~** 

### a. a. 60 X 

### 0 

## z 50 

### Reduction Limited byCritical NOx IndependentofInitial Baseline**~~------~------------------------------~** 0 5 10 15 20 25 

### NSR. Fig. 10

### f"', ' l 

## NOxReduction atHighTemperature 160MWe011 FiredTowerBoiler

240**~ ~** **[~** GasTemperatureatInjectionPoint:

# ~200l

# ~~-------------------~--------------~B >1150

## c

0 

## z



# -**~** 180**~**

160**~**
0

240

220

200 

# 8: 180 1



# - X 0160 

## z
140

120 2 3 4 5 

### NSR
Fig. 11



## NOxReduction at HighTemperature 19MWeWoodWaeteStokerFiredBoiler

GasTemperatureat Injection Point: >1100C 6**100~------~--------~------~--------~------~** 0 0.5 1.5 2 2.5 

### NSR Pig. 12

## (\ ComparisonofNOxOUTPerformancefromMWC's**so------------------------------------~ ---------------** 

60
- c: 

# j d 40 z 

# j

20 

## c**0~------~------~------~--------~------~------~** 0

60

40

.....-. 

### #. 20



# - c: 0 0 

# n ::l (20) "C Q) 

## a: (40) )( 

### 0 

## z (60)
(80)
(100) 0 0.5 1 15 2 Normattzec:JStoichiometncAatro (NSR) A B C • --·*-·-· ····0·· Fig. 13



### EffectofBaselineon NOxReduction RHultfrom PilotScaleCombuator, NSA

## = 2
GasTemperatureat InjectionPoint: 1200C
UreaInjection-------...



## ------

### ---

## --.--- --- 

# .--**~------**
..•••• '- Ammcnl&lnieclien ,' ,' 

### ...

200 400 600 

### BaselineNOx(PPM) Fig. 14 2.5 3



## -**-~**

800