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20.1.8 NOx Reduction by SNCR

The selective noncatalytic reduction of NOx (SNCR), first described by Lyon [ 226], is a method to reduce the emission of NOx from combustion by injecting a selective reductant such as ammonia (NH $_3$) or urea (CO(NH $_2$) $_2$) into the furnace, where it can react with NO in the flue gas to form N $_2$. However, the reductant can be oxidized as well to form NOx. The selectivity for the reductive reactions decreases with increasing temperature [ 242] while the rate of the initiation reaction simultaneously increases. This limits the SNCR process to a narrow temperature interval, or window, where the lower temperature limit for the interval is determined by the residence time.



Ammonia Injection


Several investigators have modeled the process using a large number of elementary reactions. A simple empirical model has been proposed by Fenimore [ 103], which is based on experimental measurements. However, the model was found to be unsuitable for practical applications. Ostberg and Dam-Johansen [ 273] proposed a two-step scheme describing the SNCR process as shown in Figure  20.1.2, which is a single initiation step followed by two parallel reaction pathways: one leading to NO reduction, and the other to NO formation.

Figure 20.1.2: Simplified Reaction Mechanism for the SNCR Process
figure


$\displaystyle \mbox{NO} + \mbox{NH$_3$} + \frac{1}{4}\mbox{O$_2$}$ $\textstyle \longrightarrow$ $\displaystyle \mbox{N$_2$} + \frac{3}{2}\mbox{H$_2$O}$ (20.1-87)
       
$\displaystyle \mbox{NH$_3$} + \frac{5}{4}\mbox{O$_2$}$ $\textstyle \longrightarrow$ $\displaystyle \mbox{NO} + \frac{3}{2}\mbox{H$_2$O}$ (20.1-88)

The reaction orders of NO and NH $_3$ at 4% volume O $_2$ and the empirical rate constants $k_r$ and $k_{\rm ox}$ for Equations  20.1-87 and 20.1-88, respectively, have been estimated from work done by Brouwer et al. [ 44]. The reaction order of NO was found to be 1 for Equation  20.1-87 and the order of NH $_3$ was found to be 1 for both reactions. As such, the following reaction rates for NO and NH $_3$, at 4% volume O $_2$, were proposed:


$\displaystyle {\cal R}_{\rm NO}$ $\textstyle =$ $\displaystyle -k_r[\mbox{NO}][\mbox{NH$_3$}] + k_{\rm ox}[\mbox{NH$_3$}][\mbox{O$_2$}]$ (20.1-89)
$\displaystyle {\cal R}_{\rm NH_3}$ $\textstyle =$ $\displaystyle -k_r [\mbox{NO}][\mbox{NH$_3$}] - k_{\rm ox}[\mbox{NH$_3$}][\mbox{O$_2$}]$ (20.1-90)

The rate constants $k_r$ and $k_{\rm ox}$ have units of m $^3$/gmol-s, and are defined as


 k_r = 4.24 \times 10^2 T^{5.30} e^{-E_r/RT}; \;\;\; k_{\rm ox} = 3.50 \times 10^{-1} T^{7.65} e^{-E_{\rm ox}/RT}

where $E_r = 349937.06$ J/gmol and $E_{\rm ox} = 524487.005$ J/gmol.

This model has been shown to give reasonable predictions of the SNCR process in pulverized coal and fluidized bed combustion applications. The model also captures the influence of the most significant parameters for SNCR, which are the temperature of the flue gas at the injection position, the residence time in the relevant temperature interval, the NH $_3$ to NO molar ratio, and the effect of combustible additives. This model overestimates the NO reduction for temperatures above the optimum temperature by an amount similar to that of the detailed kinetic model of Miller and Bowman [ 242].

figure   

The SNCR process naturally occurs when NH $_3$ is present in the flame as a fuel N intermediate. For this reason, even if the SNCR model is not activated and there is no reagent injection, the natural SNCR process may still occur in the flame. The temperature range or "window" at which SNCR may occur is 1073 K $<$ T $<$ 1373 K. To model your case without using the natural SNCR process, please contact your support engineer for information on how to deactivate it.



Urea Injection


Urea as a reagent for the SNCR process is similar to that of injecting ammonia and has been used in the power station combustors to reduce NO emissions successfully. However, both reagents, ammonia and urea, have major limitations as a NOx reducing agent. The narrow temperature "window" of effectiveness and mixing limitations are difficult factors to handle in a large combustor. The use of urea instead of ammonia as the reducing agent is attractive because of the ease of storage and handling of the reagent.

The SNCR process using urea is a combination of Thermal DeNOx (SNCR with ammonia) and RAPRENOx (SNCR using cyanuric acid that, under heating, sublimes and decomposes into isocyanic acid) since urea most probably decomposes into ammonia and isocyanic acid [ 242].

One problem of SNCR processes using urea is that slow decay of HNCO as well as the reaction channels leading to N2O and CO can significantly increase the emission of pollutants other than NO. Urea seems to involve a significant emission of carbon-containing pollutants, such as CO and HNCO.

Also, some experimental observations [ 311] show that SNCR using urea is effective in a narrow temperature window that is shifted toward higher temperatures when compared to Thermal DeNOx processes at the same value of the ratio of nitrogen in the reducing agent and in NO in the feed, $\beta$. The effect of increasing the $\beta$ value is to increase the efficiency of abatement, while the effect of increasing $O_2$ concentration depends on the temperature considered.

The model described here is proposed by Brouwer et al. [ 44] and is a seven-step reduced kinetic mechanism. Brouwer et al. [ 44] assumes that the breakdown of urea is instantaneous and 1 mole of urea is assumed to produce 1.1 moles of $NH_3$ and 0.9 moles of HNCO. The work of Rota et al. [ 311] proposed a finite rate two-step mechanism for the breakdown of urea into ammonia and HNCO.

The seven-step reduced mechanism is given in Table  20.1.2 and the two-step urea breakdown mechanism is given in Table  20.1.3.


Table 20.1.2: Seven-Step Reduced Mechanism for SNCR with Urea
Reaction A b E
$NH_3 + NO \rightarrow N_2 + H{_2}O + H$ 4.24E+02 5.30 349937.06
$NH_3 + O_2 \rightarrow NO + H{_2}O + H$ 3.500E-01 7.65 524487.005
$HNCO + M \rightarrow H + NCO + M$ 2.400E+08 0.85 284637.8
$NCO + NO \rightarrow N{_2}O + CO$ 1.000E+07 0.00 -1632.4815
$NCO + OH \rightarrow NO + CO + H$ 1.000E+07 0.00 0
$N{_2}O + OH \rightarrow N_2 + O_2 + H$ 2.000E+06 0.00 41858.5
$N{_2}O + M \rightarrow N_2 + O + M$ 6.900E+17 -2.5 271075.646


Table 20.1.3: Two-Step Urea Breakdown Process
Reaction A b E
$CO{(NH{_2})}_2 \rightarrow NH_3 + HNCO$ 1.27E-02 0 65048.109
$CO{(NH{_2})}_2 +H{_2}O \rightarrow 2NH_3 + CO_2$ 6.13E+04 0 87819.133

where the units of A, in Tables  20.1.2 and 20.1.3, are m-gmol-sec and E units are J/gmol.



Transport Equations for Urea, HNCO, and NCO


When the SNCR model with urea injection is employed, in addition to the usual transport equations, FLUENT solves the following three additional mass transport equations for urea, HNCO and NCO species.


 \frac{\partial}{\partial t} (\rho Y_{\rm CO{(NH_2)}_2})+ \na... ...ot (\rho {\cal D} Y_{\rm CO{(NH_2)}_2}) + S_{\rm CO{(NH_2)}_2} (20.1-91)


 \frac{\partial}{\partial t} (\rho Y_{\rm HNCO})+ \nabla \cdo... ...O}) = \nabla \cdot (\rho {\cal D} Y_{\rm HNCO}) + S_{\rm HNCO} (20.1-92)


 \frac{\partial}{\partial t} (\rho Y_{\rm NCO})+ \nabla \cdot... ...NCO}) = \nabla \cdot (\rho {\cal D} Y_{\rm NCO}) + S_{\rm NCO} (20.1-93)

where $Y_{\rm CO{(NH_2)_2}}$, $Y_{\rm HNCO}$ and $Y_{\rm NCO}$ are mass fractions of urea, HNCO and NCO in the gas phase. Source terms $S_{\rm CO{(NH_2)}_2}$, $S_{\rm HNCO}$ and $S_{\rm NCO}$ are determined according to the rate equations given in Tables  20.1.2 and 20.1.3 and the additional source terms due to reagent injection. These additional source terms are determined next. The source terms in the transport equations can be written as follows:


 S_{\rm CO{(NH_2)}_2} = S_{\rm pl,CO{(NH_2)}_2} + S_{\rm CO{(NH_2)}_2 - reac} (20.1-94)


 S_{\rm HNCO} = S_{\rm pl,HNCO} + S_{\rm HNCO - reac} (20.1-95)


 S_{\rm NCO} = S_{\rm NCO-reac} (20.1-96)

Apart from the source terms for the above three species, additional source terms for $NO$, $NH_3$ and $N{_2}O$ are also determined as follows, which should be added to the previously calculated sources due to fuel NOx:


 S_{\rm NO} = S_{\rm NO-reac} (20.1-97)


 S_{\rm NH_3} = S_{\rm pl,NH_3} + S_{\rm NH_3 - reac} (20.1-98)


 S_{\rm N{_2}O} = S_{\rm N{_2}O-reac} (20.1-99)

Source terms $S_{\rm i - reac}$ for $i^{\rm th}$ species are determined from the rate equations given in Tables  20.1.2 and 20.1.3.



Urea Production due to Reagent Injection


The rate of urea production is equivalent to the rate of reagent release into the gas phase through droplet evaporation:

 S_{\rm pl,CO{(NH_2)}_2} = \frac{S_{reagent}}{V} (20.1-100)

where $S_{reagent}$ is the rate of reagent release from the liquid droplets to the gas phase (kg/s) and $V$ is the cell volume ( $m^3$).



$NH_3$ Production due to Reagent Injection


If the urea decomposition model is set to the user-specified option, then the rate of $NH_3$ production is proportional to the rate of reagent release into the gas phase through droplet evaporation:


 S_{\rm pl,NH_3} = MCF_{NH_3} \frac{S_{reagent}}{V} (20.1-101)

where $S_{reagent}$ is the rate of reagent release from the liquid droplets to the gas phase (kg/s), $MCF_{NH_3}$ is the mole fraction of $NH_3$ in the $NH_3$/ $HNCO$ mixture created from urea decomposition and $V$ is the cell volume ( $m^3$).



HNCO Production due to Reagent Injection


If the urea decomposition model is set to the user-specified option, then the rate of HNCO production is proportional to the rate of reagent release into the gas phase through droplet evaporation:


 S_{\rm pl,HNCO} = MCF_{HNCO} \frac{S_{reagent}}{V} (20.1-102)

where $S_{reagent}$, the injection source term, is the rate of reagent release from the liquid droplets to the gas phase (kg/s), $MCF_{NH_3}$ is the mole fraction of HNCO in the $NH_3$/ $HNCO$ mixture created from urea decomposition and $V$ is the cell volume ( $m^3$).

figure   

The mole conversion fractions (MCF) for species $NH_3$ and HNCO are determined through the user species values such that if one mole of urea decomposes into 1.1 moles of $NH_3$ and 0.9 moles of HNCO, then $MCF_{NH_3}$ = 0.55 and $MCF_{HNCO}$ = 0.45. When the user-specified option is used for urea decomposition, then $S_{\rm pl,CO{(NH_2)}_2} = 0$.

However, the default option for urea decomposition is through rate limiting reactions given in Table  20.1.3 and the source terms are calculated accordingly. In this case, both values of $S_{pl,NH_3}$ and $S_{pl,HNCO}$ are zero.


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