Improvement on Hybrid SNCR-SCR Process for NO Control : a Bench Scale Experiment

The reducing agent [ammonia ( 3 NH )] injection procedure was improved for the hybrid process of selective non-catalytic reduction followed by selective catalytic reduction (hybrid SNCR-SCR) to remove nitric oxide (NO) through a bench-scale experiment. Instead of injecting all of the 3 NH from the SNCR inlet, part of it was injected from the SNCR inlet and part from the SCR inlet, to react with NO in the flue gas. The experiment resulted in the significant reduction of NO. The effects of the operational conditions such as the SNCR reaction temperature, the SCR reaction temperature, and the initial concentration ratio of 3 NH to NO were also investigated. Under the initial NO concentration of 300 ppm (dry, 6% 2 O ), the space velocities of SNCR 5100-6300 1 hr , the space velocities of SCR 7100-10000 1 hr , and with the initial concentration ratios of 3 NH to NO 1.0-1.5, the best operational temperatures were discovered to be SNCR reaction of 850°C and SCR reaction of 350°C for the improved hybrid SNCR-SCR process. In addition, a correlation equation has been developed of the maximum NO reduction under the above bestoperational temperatures for the hybrid SNCR-SCR process, and closely fits with the experiment results.


INTRODUCTION
Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are two major nitric oxide (NO) reduction technologies for post-combustion treatment of flue gas from industrial or utility boilers.The SNCR technology using ammonia ( 3 NH ) as reducing agent to react with NO selectively to yield 2 N and O H 2 in flue gas, is named the Thermal De-NO x Process (Lyon, 1975).The optimal reaction temperature range for Thermal Dex NO is around 900-950℃ (Muzio et al., 1977).Furthermore, to avoid ammonia slip in the exit flue gas, it was recommended that the reaction temperature higher than 930℃ and the resident time of reaction more than 0.25 s should be controlled (Pohl et al., 1993).Miller and Bowman (1989) present a SNCR reaction mechanism, composed from several chain reactions, with the help of a self-generated active radical (OH, 2 NH ) to reduce NO.Two important reactions were identified: Because of possible toxicity and safety problems while handling 3 NH , alternative SNCR processes were developed.These processes use agents, including urea (Arand et al., 1980) and cyanuric acid (Perry and Siebers, 1986).These alternative agents decompose to form 3 NH and HNCO under high temperatures and reduce NO to 2 N or O N 2 .The primary equations for HNCO to react with NO to form O N 2 are: O N 2 is an environmentally harmful gas.It contributes to ozone layer destruction and increased global warming.When the SNCR processes were applied to stationary industrial pollution sites, the NO reduction range was around 30-60% (Himes et al., 1995).
Selective catalytic reduction (SCR) is a process that also uses TiO catalyst on the monolithic surface.The application of SCR in industrial or utility boilers can reach a higher NO reduction rate than the SNCR process.In applying the SCR process, European studies indicated that the lower the space velocity in catalyst bed the higher the NO reduction.The space velocity decreased from 3,400 1 hr  to 1,000 1 hr  , causing the resident time of the flue gas to increase from 1.1 s to 3.6 s, and facilitating increased NO reduction from 70% to 90% (Cichanowicz, 1987).
When the flue gas contained residual oxygen, the overall chemical reaction of SCR has the stoichiometry: Equation ( 5) shows that 3 NH and NO, with the same chemical stoichiometry, cooperate with the higher NO reduction of SCR.It shows why the 3 NH slip of SCR is lower than that of SNCR.
A hybrid technology combining SNCR with SCR (hybrid SNCR-SCR) was developed in order to achieve higher NO reduction or to reduce costs.Gullett et al. (1994), and Groff and Gullett (1997) used urea as reducing agent, injecting it through the SNCR inlet, partially reducing NO and forming 3 NH from the decomposition of excessive urea.The process was then transferred to SCR for further NO reduction.This resulted in a space velocity of 10,000 1 hr  for SCR and a total 85% NO reduction for a 590 kw package boiler.Under the stoichiometric ratio of nitrogen from urea, initial NO was controlled by 2:1.Wendt et al. (2002) studied a pilot-scale SNCR-SCR hybrid process using 3 NH as agent, where the total agent was also injected into the SNCR inlet.These results were used to develop a combined SNCR-SCR model, which was extended to operational coal-fired utility boilers to explore the economic benefits of the combined SNCR-SCR process under certain conditions.
While some high-efficiency SCR technologies are under development, minus SNCR, to achieve the same NO reduction (Zhu et al., 1999;Hsu and Teng, 2001), this study attempted to improve the 3 NH injection technique of a hybrid SNCR-SCR process.Through a bench-scale hybrid process, 3 NH was injected separately, a part from the SNCR inlet and a part from the SCR inlet, to react with NO in the flue gas.NO reduction and 3 NH slip of the different agent injection procedures were measured and compared.Moreover, the effects of operational conditions to the hybrid SNCR-SCR process such as the SNCR reaction temperature, the SCR reaction temperature, and the initial concentration ratio of 3 NH to NO were also investigated.The optimal operational conditions to reach higher NO reduction and lower 3 NH slip were also revealed by the experiment' s results.tube with a 1.5 cm inner diameter and 23 cm long was put in the SNCR furnace.Another quartz tube the same diameter and length, but with a catalyst bed, was added to the SCR furnace.The 2 5 2 TiO / O V monolithic catalyst 1.0 cm in diameter supplied by a domestic SCR company, as shown in Fig. 2, was lined in SCR quartz tube with a total length of 11.5 cm.A total flue-gas flow rate was fixed at 1,500 ml/min (1atm, 300 K).Based on the fixed gas flow rate and the operation temperatures of SNCR and SCR, the space velocities of SNCR were calculated to be 5,100-6,300 1 hr  (for SNCR temperature of 700-850 ℃ ) and the space velocities of SCR were calculated to be 7,100-10,000 1 hr  (for SCR temperature 200-450℃).

MEASUREMENT OF NO AND NH 3
The measuring point for NO and To measure the 3 NH in the flue gas, an electrical conductivity method was used.A sampling pump was employed to extract a fixed amount of flue gas (0.7 l/min), which was bubbling out and in contact with a fixed volume of distilled water (120 ml) in a sampling bottle for 10 min.After sampling, the water solution was taken from the sampling bottle and conductivity was measured.The concentration of 3 NH was then calculated by the following calibration equation:

EXPERIMENTAL SCOPE
During the hybrid SNCR-SCR experiment, some of the reducing agent 3 NH was injected through the inlet for SNCR and some through the inlet for SCR.The coal-fired boiler.Also, the total flow rate of the dry flue gas (i.e., Q) was fixed at 1,500 ml/min (1 atm, 300K) and was preheated to 500℃ before input into the SNCR furnace.Other experimental conditions include reaction temperature of SNCR (i.e., SNCR temperature), reaction temperature of SCR (i.e., SCR temperature), and stoichiometry ratio of initial total 3 NH to i NO (i.e., NH 3 :NO).The ranges of all experimental conditions are summarized in Table 1.

SCR REACTION ALONE
An SNCR reaction does not occur when set at a low temperature of 550℃. Figure 5 shows the effects of SCR temperature and a NH 3 :NO ratio on NO reduction (i.e., the percentage of NO reduced from i NO ) of SCR reaction alone.NO reduction is increased as SCR temperature increases until an optimal temperature of 350℃ is reached.Above 350℃, some 3 NH will decompose and cause the NO reduction to be lessened.In addition, optimal NO reduction occurs when NH 3 :NO is at a 1:1 ratio.Furthermore, reduction improved from 77.5% at 1:1 to 85% 1.5:1.However, increasing NH 3 :NO increases the unreacted 3 NH contained in the ventilated flue gas (i.e., 3 NH slip) (Fig. 6).

EFFECT OF SCR TEMPERATURE ON HYBRID SNCR-SCR
The effects of SCR temperature and 3 NH ratio on the hybrid SNCR-SCR process, at the SNCR temperature of 850℃ and NH 3 :NO of 1:1 is shown in Fig. 7.This figure depicts the optimal operating temperature (with the maximum NO reduction) of SCR in the hybrid SNCR-SCR is the same as the SCR reaction alone; i.e., 350℃.Figure 7 also shows that by reducing the 3 NH ratio (within range of HN 3 ratio shown in Table 1), part of the agent will input directly to SCR, avoiding the over consumption of 3 NH in SNCR, thus maximizing NO reduction.Comparing Fig. 7 with Fig. 5, the maximum NO reductions at the optimal SCR temperature of 350℃ increases from 77.5% of the SCR reaction alone to 87% of the hybrid SNCR-SCR when the 3 NH ratio is 50:50.In addition, 3 NH slip is 11 ppm of SCR reaction alone comparing to 0 ppm of the hybrid SNCR-SCR, as indicated in Fig. 6 and Table 2.  NH slip of hybrid SNCR-SCR process at the optimal SCR temperature 350℃ and NH 3 :NO = 1.5:1.0.NO reduction begins to rise from the SNCR temperature of 750℃ to the maximum value when the SNCR temperature reaches 850℃ (Fig. 8).When the SNCR temperature is higher than 850℃, the over consumption of to 92% when the 3 NH ratio reduces from 100:0 to 50:50 (Fig. 8).It is believed that an optimal 3 NH ratio exists for the hybrid SNCR-SCR, because when reducing 3 NH ratio to 0:100, the NO reductions of the hybrid SNCR-SCR will be the same as SCR reaction alone.The SNCR is unnecessary in such a condition.Figure 9 shows the effect of SNCR temperature on 3 NH slip of the hybrid SNCR-SCR.At a SCR temperature of 350℃, NH 3 :NO = 1.5:1.0,and SNCR temperature above 850℃, 3 NH slip of hybrid SNCR-SCR is obviously lower than the SCR reaction alone (see Fig. 6).

EFFECT OF NH 3 :NO ON HYBRID SNCR-SCR
The optimal SNCR temperature of 850℃ and SCR temperature of 350℃, the effects of NH 3 :NO on NO reduction, and 3 NH slip of the hybrid SNCR-SCR process are shown in Figs.10-11.Figure 10 shows that NO reduction increases as the stoichiometry ratio NH 3 :NO increases.This figure also demonstrates that when the ratio of NH 3 :NO is higher than 1.5, the effect on NO reduction is insignificant.In Fig. 11 it can be seen that the 3 NH slip becomes serious when NH 3 :NO is higher than 1.5.Therefore, it is not recommended to operate NH 3 :NO any higher than 1.5 for the improved hybrid SNCR-SCR process.

COMPARING NO REDUCTION AND NH 3 SLIP UNDER OPTIMAL TEMPERATURES
Maximum NO reductions are reached under the optimal SNCR and SCR temperatures with 3 NH slips for comparing the performance of hybrid SNCR-SCR and SCR reaction alone.Table 2 shows that the performance of hybrid SNCR-SCR is superior to that of the SCR reaction alone and NO reduction can be further improved for hybrid SNCR-SCR if 3 NH is injected partly from the inlet of SNCR and partly from the inlet of SCR.
A correlation equation of maximum NO reduction for hybrid SNCR-SCR from the experimental results of Table 2 is shown as:

3
NH as an agent, but with the help of a catalyst to reduce NO to 2 N in the lower temperature range.A SCR catalyst is commonly fabricated into a monolith with the shape of honeycomb or flat-plate, when applied to an industrial site.

Figure 1
Figure 1 is a schematic diagram of the experimental apparatus.The apparatus is composed of highpressure gases, flow rate controllers, SNCR furnace, SCR furnace, and the NO, NH 3 3 NH measuring

Figure 1 .
Figure 1.Schematic diagram of the experimental apparatus.

3Figure 2 .
Figure 2. Cross section diagram of the SCR catalyst.
electric conductivity differences after sampling an d b e f o r e s a mp l i n g ( μ s / c m) This equation was prepared by sampling flue gas with a known 3 NH concentration, following the same steps to measure the conductivity difference of the solution after and before sampling.The calibration diagram is shown in Fig. 4. It is suggested that the suitable application range of the 3 NH measurement is 0-400 ppm.

Figure 5 .Figure 6 .
Figure 5. Effects of SCR temperature and NH 3 :NO on NO reduction, SCR reaction alone.

Figure 7 .
Figure 7. Effects of SCR temperature and within 100:0 to 50:50, the maximum NO reduction increases from 88

Figure 11 .
Figure 11.Effects of NH 3 :NO and the hybrid SNCR-SCR.The comparison of the calculated NO R from equation (7) and the measured NO R from Table2for hybrid SNCR-SCR process is shown in Fig.12.Figure12indicates that the calculated values from the correlation equation are qualitatively confirmed with the experimental results.The correlation equation identifies the conditions of SNCR temperature (850℃), SCR temperature (350℃), NH 3 :NO = 1:1-1.5:1,and SNCR R = 0.5-1.0.It is suggested to apply equation (7) within the identified conditions to avoid the misguided error for NO R calculations.

Table 1 .
The ranges of all experimental conditions.

Table 2 .
The maximum NO reduction and