effectiveness of nox removal

CHEMICAL AND PROCESS ENGINEERING

2010, 31, 699–709

MACIEJ JAKUBIAK , WŁODZIMIERZ KORDYLEWSKI*

EFFECTIVENESS OF NOX REMOVAL FROM GAS VIA PREOXIDATION OF NO WITH OZONE AND ABSORPTION

IN ALKALINE SOLUTIONS

Wrocław University of Technology, Institute of Power Engineering and Fluid Mechanics, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

The process of NO removal from the carrier gas (air) applying preoxidation of NO with ozone and absorption of higher oxides in alkaline aqueous solutions was studied in a laboratory apparatus. The molar ratio of O3/NO was in the range of 0.5–1.5. The main purpose of the experiments was to determine the absorption performance of NO oxidation products in aqueous solutions of NaOH, KOH, Ca(OH)2, CaCO3 and in water. Additionally, the influence of the initial concentration of NO in the carrier gas and the NaOH concentration in a solution on the effectiveness of NOx removal via the preoxidation–absorption process was studied.

Wykonano badania w skali laboratoryjnej procesu usuwania NOx z gazu nośnego (powietrza) przez wstępne utlenianie NO ozonem oraz absorpcję wyższych tlenków w roztworach alkalicznych. Stosunek molowy O3/NO wynosił 0,5–1,5. Głównym celem eksperymentu było określenie skuteczności absorpcji produktów utleniania NO w roztworach wodnych: NaOH, KOH, Ca(OH)2, CaCO3 i w wodzie. Zbadano również wpływ początkowej koncentracji NO oraz stężenia NaOH w roztworze sorpcyjnym na efektyw-ność procesu usuwania NOx.

1. INTRODUCTION

Denitrification processes of flue gases from coal-fired boilers can be classified in-to three main groups: low NOx combustion systems, reduction of NOx to N2 using ammonia or methane and preliminary oxidation–absorption processes in alkaline aqueous solutions [1]. The former two groups have got a commercial status and they are in common use in coal-fired power plants. Actually, low NOx combustion systems have found an application near in all pulverized coal-fired boilers, as a basic or first stage measures for the abatement of NOx emission under reasonable costs.

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*Corresponding autor, e-mail: [email protected]

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M. JAKUBIAK , W. KORDYLEWSKI 700

In the EU, selective catalytic reduction (SCR) is regarded the best available tech-nology (BAT) because the efficiencies of NOx removal range from 80% to 90%. Also a selective non-catalytic reduction (SNCR) method in which ammonia or urea is being used for reduction of NOx is under development. However, ammonia applied directly or indirectly from urea frequently is not completely reacted and ammonia leakage is a serious environmental problem in these methods.

The methods of denitrification of flue based on preliminary oxidation–absorption processes are under vigorous development and recently many wet processes for simul-taneous removal of NOx and SO2 have been examined [2]. They could be especially efficient by combining with the installation of wet flue gas desulphurization (FGD), particularly with limestone-based wet scrubbers [2]. Because wet FGD technologies are in common use for control of SO2 emissions in the EU, including Poland, they could be successfully adopted for control of NOx and Hg emissions by the removal of their oxidized forms.

On account of low solubility of nitric oxide its preliminary oxidation (preoxida-tion) to higher forms is an important step in the absorption of NOx in caustic scrub-bers. The most often studied oxidants of NO are: ozone, hydrogen peroxide and chlo-rine compounds like ClO2, NaClO and NaClO2 [3]. Depending on the type of oxidant, the oxidation process occurs in a gas or liquid phase. The oxidation of NO in the gas phase with ozone or chlorine dioxide could proceed directly in a flue gas channel, the liquid phase oxidation requires an additional absorption vessel. Ozone is a strong oxi-dant which converts NO rapidly to NO2 in the gas phase. It cannot be stored and trans-ported, therefore ozone have to be generated in the place of use, which is an advan-tage, e.g. in power plants. It is also important that the use of ozone does not produce wastes.

The main objective of the study was to examine the effectiveness of NOx removal from gas via the oxidation–absorption process using ozone pretreatment and caustic washing. The results of the experimental research conducted on the oxidation of nitric oxide with ozone for the molar ratio O3/NO ≈ 0.5–1.5 and the absorption of the oxida-tion products into selected alkaline aqueous solutions are presented in the paper.

2. NOX OXIDATION WITH OZONE AND ABSORPTION IN AQUEOUS SOLUTIONS

Ozone is a strong oxidant, which plays an important role in atmospheric chemi-stry, where a basic chemical reaction of ozone destruction assumes [4]:

NO + O3 = NO2 + O2 (1)

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NOx removal from flue gases 701

When the molar ratio X = O3/NO takes sub-stoichiometric values (X < 1), NO2 is practically the only product of NO oxidation [5]. Nitrogen dioxide and nitric oxide participate in a relatively rapid equilibrium reaction to dinitrogen trioxide [6]:

NO + NO2 = N2O3 (2)

In liquid phase (l) both, NO2 and N2O3, when absorbed react with water to form nitrous and nitric acids:

2NO2(l) + H2O → HNO3(l) + HNO2(l) (3)

N2O3(l) + H2O → 2HNO2(l) (4)

The nitrous acid being unstable in the presence of any strong acid disproportion-ates to form nitric acid and nitrogen oxide [7]:

3HNO2 → HNO3 + 2NO + H2O (5)

This is the main reason for NO2 absorption in water solutions being limited ap-proximately to 65%, which is a serious obstacle to attain high performance of the me-thods of NOx removal based on the oxidation–absorption processes.

Ozone can also react with NO2 to form nitrogen trioxide:

NO2 + O3 → NO3 + O2 (6)

This reaction, important for intensive ozonation (O3/NO >> 1), could lead to for-mation of N2O5 [8]:

NO2 + NO3 = N2O5 (7)

Extremely water soluble, N2O5 is effectively scrubbed to form nitric acid:

N2O5(l) + H2O → 2HNO3(l) (8)

In the case of ozone excess, the yield of nitric acid is increased due to the probable direct reaction in water [9]:

HNO2(l) + O3(l) → HNO3(l) + O2(g) (9)

3. EXPERIMENTAL

The scheme of the laboratory apparatus used in the experiments is shown in Fig. 1. The oxidation of NO by O3 proceeded in a tubular reactor (9) of a glass tube 0.72 m long with the inner diameter of 6 mm. Dried air was used as the carrier gas contained approximately 200 ppm of NO, doped from the gas cylinder (8). Ozone was produced from air at the rate of 0.3–3 mg O3/min by the DBD generator (13) described else-where [10]. The oxidant (5–7 g of O3 per m3 of air) was injected into the oxidizing

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reactor (9). The flow conditions in the reactor (9) were maintained similar like in [1]: the volumetric flow rate V was 125 dm3/h, the flow velocity – 1.23 m/s, the residence time was τres = 0.59 s and the Reynolds number Re = 492.

Fig. 1. Scheme of the laboratory apparatus: 1, 7 – pressure reductors, 2, 4, 12, 18 – T connectors, 3, 5 – mass flow controllers, 6, 11, 14 – valves;

8 – NO cylinder, 9 – oxidizing reactor, 10, 15 – rotameters, 13 – ozone generator, 16 – ozone analyser, 17 – absorber, 19 – ozone destructor, 20 – drier, 21 – gas analyser

The gas carrying the products of NO oxidation flew from the oxidizing reactor (9) into the Dreshler washer (bubble column absorber) charged with alkaline aqueous solution (100 cm3) (17). The oxidation–absorption processes were conducted at 25 °C.

The NO and NO2 concentrations were measured with the gas analyser (21) after the gas dryer (20), when the ozone generator (13) was switched off (ref) and switched on (out), respectively. In order to determine the effectiveness of NO oxidation, the oxidation ratio OR (%) was introduced:

[ ][ ]

out

ref

NO1 100%

NOOR

= − ×

(10)

The effectiveness of NOx removal was determined by:

,out

out,ref

NO1 100%

NOx

x

η = − ×

(11)

The volumetric flow rates of the carrier gas (ca. 125 dm3/h) and NO diluted in N2 (ca. 1.6 dm3/h) were controlled by Aalborg’s Instruments & Controls Inc. mass flow controllers GFC17 (3, 5). The concentrations of NO and NO2 were measured using a gas analyser Testo 350 XL based on electrochemical sensors (21). The electrochemi-cal sensors were protected against the residual ozone by a thermal ozone destructor

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(19). The flowrate of ozone was controlled by two rotameters: one (10) was used for the measurement of ozone/air flow rate into the oxidizing reactor (9) and the other (15) to measure flow rate of ozone/air into the ozone analyzer BMT 964 BT model, BMT Messtechnik GmBH. Nitric oxide diluted in N2 was delivered from a gas cylind-er (2.5% of NO from Messer). Calcium carbonate water suspension was prepared from CaCO3 (Merck) and distilled water (Chempur). Hydroxides of sodium (NaOH), potas-sium (KOH) and calcium (Ca(OH)2) were purchased from POCH S.A.

4. RESULTS AND DISCUSSION

4.1. ABSORPTION IN AQUEOUS SOLUTIONS OF SODIUM HYDROXIDE

Most of effort has been assigned to the absorption of NOx into NaOH aqueous so-lutions because of its application for the removal of NOx from flue gas, especially as an prospective sorbent for flue gas conditioning in the CCS installations.

Fig. 2. Time dependences of NO and NO2 concentrations as a function of time after ozone injection into the oxidizing reactor for X = O3/NOref = 0.5 and 1.5

Histories of the NO and NO2 concentrations measured in the gas after ozone injec-tion into the oxidizing reactor (9) for two selected molar ratios X = O3/NOref = 0.5 and 1.5 are shown in Fig. 2. The delay period, accounted for approximately 40 s, is mainly attributed to the transitional processes in the bubble column absorber charged with 100 cm3 of 0.1 M NaOH solution (17).

Figure 3 presents the effect of ozone addition on the oxidation ratio OR and the ef-fectiveness of NOx removal ηout calculated from Eqs. (10) and (11). The molar ratio O3/NOref was varied in the range of 0.5–1.5. Other conditions in the absorber were the same like before.

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Although nitric oxide was almost completely oxidized (OR ≈ 100%) when the mo-lar ratio O3/NOref approached 1, the amount of removed NOx from the gas was only 65%. However, when the molar ratio O3/NOref increased approximately to 1.5, the effectiveness of NOx removal attained almost 95%.

Fig. 3. Oxidation ratio OR and the effectiveness of NOx removal ηout vs. molar ratio X = O3/NOref

Usually, the effectiveness of flue gas denitrification is worse when the concentra-tion of NOx assumes small values. In order to examine this effect, the initial concentra-tion of NO (ref) in the carrying gas was varied between 50 and 400 ppm for the molar ratio O3/NOref = 1.5 and the absorber was charged of 100 cm3 of 0.1 M NaOH solution (Fig. 4). It is very promising that the method remains effective also for low NOx con-centrations.

Fig. 4. Dependence of the effectiveness of NOx removal ηout on the initial concentration of NO (ref) for the molar ratio O3/NOref = 1.5

The influence of NaOH concentration in water solution on the absorption perfor-mance was studied by measurement of the effectiveness of NOx removal vs. the molar ratio O3/NOref for the two chosen values: 0.01 and 0.1 M (Fig. 5).

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Fig. 5. Effectiveness of NOx removal ηout vs. molar ratio O3/NOref for 0.1 and 0.01 M solutions of NaOH

No distinct differences between the absorption performances were noticed, per-haps due to a considerable excess of NaOH in the washer charge.

4.2. ABSORPTION IN WATER

Due to excellent solubility of NO2 and higher nitrogen oxides, water is an efficient absorbent and could serve for the removal of NOx [2, 10]. It was examined for the pretreatment of NO with ozone, when the absorber (17) was charged with 100 cm3 of distilled water. The effectiveness of NOx removal with water was not much worse than that with NaOH solution (Fig. 6). Unfortunately, water cannot attain environmental acceptance as an absorbent because contaminated by other pollutants in a scrubber is difficult to disposal.

Fig. 6. Variation of the effectiveness of NOx removal ηout in water as a function of time after ozone injection for selected molar ratios O3/NOref

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4.3. ABSORPTION IN AQUEOUS SOLUTIONS OF OTHER ALKALI METALS COMPOUNDS

A series of experiments were performed using aqueous solutions of potassium and calcium hydroxides and water suspension of calcium carbonate as absorbents of the products of NO oxidation. The molar ratio O3/NOref in the oxidizing reactor was in the range of 0.5–1.5 and the absorber was charged with 100 cm3 of an alkaline aqueous solution. The measurement data, including the results obtained for NaOH and water, are collected in Fig. 7.

Fig. 7. Effectiveness of NOx removal ηout using various alkaline absorbents for the molar ratio X = O3/NOref = 0.5–1.5

The differences between the effectiveness of NOx removal ηout for the particular alkalies were in the range of the experimental scatter, especially for O3/NOref > 1. Even CaCO3 suspension appeared to be a very effective absorbent.

5. DISCUSSION

Absorption of NOx is an extremely complex process, hence an interpretation of the experimental results could be sometimes difficult [10]. Fortunately, the mechanism of NO2 absorption and chemical reactions in water (1)–(4) are fairly understood [7]. The overall absorption effectiveness of NOx in water is reduced because aqueous nitrous acid is unstable and decomposes with releasing of NO. Therefore, even if the degree of NO oxidation OR in the gas phase was almost 100%, the effectiveness of NOx re-moval was limited by the liquid phase processes.

However, if ozone was added into the gas in a considerable excess, the absorption performance of nitrogen oxides into water attained 90% (Figs. 6 and 7). This effect could result from the fact that the increase of the molar ratio O3/NO >> 1 favours con-

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version of nitrogen dioxide to nitrogen trioxide, and leads to N2O5 formation. Addi-tionally, it is probable that the ozone surplus directly oxidizes nitrous acid in water increasing the nitric acid yield.

Scrubbing of NO2 into alkaline aqueous solutions proceeds first via reactions (3), (4), and next, nitric and nitrous acids react with an alkaline solution to form nitrates and nitrites, like for sodium hydroxide:

HNO2 + NaOH → NaNO2 + H2O (12)

HNO3 + NaOH → NaNO3 + H2O (13)

Reaction (12) is important because it improves the effectiveness of NOx removal from the gas by the increase of the absorption performance into sodium hydroxide and prevention HNO2 decomposition with release of NO [12].

6. CONCLUSIONS

Experimental studies were performed on the removal of NOx from gas applying the preliminary oxidation of NO with ozone and the absorption of the oxidation prod-ucts in water and alkaline aqueous solutions in order to determine the optimum ozone excess and the most effective absorbent.

An evident effect of the increase of the molar ratio O3/NOref was noticed from ap-proximately 1 to 1.5 on the absorption performance in alkaline aqueous solutions, namely the effectiveness of NOx removal increased from 65% to 95%, respectively. The observation can be correlated with the ability of oxidation of NO2 with ozone and further formation of N2O5. Perhaps, also direct oxidation of nitrous acid to nitric acid in aqueous solutions with ozone surplus improves the effectiveness of the oxidation –absorption process.

Absorption tests of oxidized NOx gases into aqueous solutions of hydroxides: Ca(OH)2, KOH and NaOH showed their similar performance, especially for O3/NOref = 1.5. Aqueous suspension of calcium carbonate also appeared to be an efficient ab-sorbent, which is promising for its application for the removal of NOx in wet FGD limestone-based scrubbers.

When O3/NOref ≤ 1 water was less effective absorbent of NO2 than aqueous NaOH solution, only when O3/NOref ≥ 1.5 the effectiveness of NOx removal attained 90%. Unfortunately, water cannot be considered as a commercial absorbent because of problems with disposal of nitrite and nitrate contaminated scrubber wastewater.

It is important for the industrial applications that the method of NOx removal via NO oxidation with ozone and absorption of the resulting NOx species appeared to be efficient also for small, initial concentrations of NOx.

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ACKNOWLEDGEMENT

The results presented in this paper were obtained from a research work co-financed by the National Centre of Research and Development in the framework of Contract SP/E/1/67484/10 – Strategic Research Programme – Advanced technologies for obtaining energy: Development of a technology for highly efficient zero-emission coal-fired power units integrated with CO2 capture.

SYMBOLS

[NOout] – output NO concentration, ppm [NOref] – reference NO concentration, ppm [NOx,out] – output NOx concentration, ppm [NOx,ref] – reference NOx concentration, ppm ηout – effectiveness of NOx removal, % OR – oxidation ratio of NO, % τres – residence time in the oxidizing reactor, s X – molar ratio, mol/mol

REFERENCES

[1] WILK R., Low-emission combustion, Gliwice, Wyd. Politechniki Śląskiej, 2002. [2] ELLISON W., Chemical process design alternatives to gain simultaneous NOx removal in scrubbers,

POWER_GEN International, Las Vegas , December 9–11, 2003. [3] HUTSON N.D., KRZYŻYŃSKA R., SRIVASTAVA R.K., Ind. Eng. Chem. Res., 2008, 47, 5825. [4] PRATHER M.J., LOGAN J.A., Combustion’s impact on the global atmosphere, 25th Symp. (Int.) on

Comb., The Combustion Institute, Pittsbourgh, 1994, 1513. [5] NELO S.K., LESKELA K.M., SOHLO J.J.K., Chem. Eng. Technol. 1997, 20, 40. [6] GŁOWIŃSKI J., BISKUPSKI A., SŁONKA T., TYLUS W., Chem. Proc. Eng., 2009, 30, 217. [7] THIELMANN M., SCHEIBLER E., WIEGAND K.W., Nitric Acid, Nitrous Acid and Nitrogen Oxides,

Ullman’s Encyclopedia of Industrial Chemistry, Weinheim, Wiley-VCH Verlag GmbH&Co., 2002, 23, 8.

[8] NELO S.K., LESKELA K.M., SOHLO J.J.K., Chem. Eng. Technol. 1997, 20, 40. [9] CHACUK A., MILLER J.S., WILK M., LEDAKOWICZ S., Chem. Eng. Sci., 2007, 62, 7446.

[10] DORA J., GOSTOMCZYK M.A., JAKUBIAK M., KORDYLEWSKI W., Chem. Process Eng., 2009, 30, 621. [11] JOSHI J.B., MAHAJANI V.V., JUVEKAR V.A., Chem. Eng. Commun., 1985, 33, 1. [12] THOMAS D., VADERSCHUREN J., Sep. Purif. Techn., 2000, 18, 37.

SKUTECZNOŚĆ USUWANIA NOX Z GAZU PRZEZ WSTĘPNE UTLENIANIE NO OZONEM I ABSORPCJĘ W ROZTWORACH ALKALICZNYCH

Badano możliwość ograniczania emisji tlenków azotu z instalacji spalającej paliwa opartą na utle-nianiu NO do wyższych tlenków azotu, a następnie ich usuwaniu ze spalin przez absorpcję w roztworach alkalicznych. Ta skuteczna metoda może stanowić alternatywę dla powszechnie stosowanych, komercyj-nych metod pierwotnych i wtórnych ograniczania emisji NOx z kotłów. Wyższe tlenki azotu są bardzo dobrze rozpuszczalne, co powoduje, że mogą być skutecznie wymywane ze spalin, w przeciwieństwie do trudno rozpuszczalnego NO, który wymaga utlenienia przed absorberem. Najczęściej badane typy utle-niaczy NO to ozon, nadtlenek wodoru i związki chloru (np. ClO2, NaClO, NaClO2). Badana metoda jest szczególnie obiecująca również dlatego, że umożliwia jednoczesne usuwanie NOx, SO2, a także Hg

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z zastosowaniem mokrej instalacji odsiarczania spalin (IOS). Szczególnym celem badań było sprawdze-nie efektywności utleniania NO do wyższych tlenków dla dużego nadmiaru ozonu, czyli dla stosunku molowego O3/NO > 1.

Badania przeprowadzono w skali laboratoryjnej, gazem nośnym było powietrze o temperaturze oko-ło 25 °C, do którego dodawano NO z butli w ilości 200 ppm. Utlenianie NO zachodziło w szklanym reaktorze rurowym typu przepływowego. Produkty utleniania NO były absorbowane z gazu nośnego za reaktorem utleniającym w płuczce Dreshlera zawierającej 0,1 M wodny roztwór alkaliczny lub wodę. Jako alkaliów użyto w badaniach wodorotlenki: NaOH, KOH, Ca(OH)2 oraz węglan wapnia CaCO3. Ozon wytwarzano z powietrza w generatorze ozonu typu DBD. Efektywność utleniania NO i usuwania NOx określano na podstawie wskazań analizatora składu gazu pobieranego za absorberem.

Zwiększenie stosunku molowego O3/NO w reaktorze utleniającym przed absorberem od około 1 do 1,5 powodowało znaczny wzrost skuteczności usuwania NOx od 65% do 95%. Poprawę skuteczności usuwania NOx w wyniku bardziej intensywnej ozonizacji tłumaczono zwiększeniem efektywności ab-sorpcji produktów utleniania NO, na skutek pojawienia się, obok NO2, także NO3 i N2O5, gdy O3/NO > 1. Wskazano także na możliwość bezpośredniego utleniania HNO2 nadmiarowym ozonem w roztworze wodnym do stabilnego kwasu azotowego.

Stwierdzono dużą skuteczność absorpcji wyższych tlenków azotu w alkalicznych roztworach wod-nych. Wykazano, że skuteczność absorpcji produktów utleniania NO w roztworach wodnych NaOH, KOH i Ca(OH)2 była bardzo zbliżona, zwłaszcza dla O3/NO = 1,5. Podkreślono pozytywną rolę wodoro-tlenków w absorpcji NO2 w roztworach wodnych i zapobieganiu uwalniania NO. Również zawiesina wodna CaCO3 okazała się skutecznym absorbentem NO2, co jest obiecujące ze względu na możliwość zastosowania wstępnego utleniania NO do usuwania NOx w instalacjach mokrego odsiarczania spalin wykorzystujących kamień wapienny. Woda okazała się również efektywnym absorbentem NO2, jednakże kiedy O3/NO ≤ 1, proces usuwania NOx był mniej efektywny w porównaniu do roztworu NaOH, dopiero kiedy O3/NO = 1,5 efektywność procesu osiągała 90%. Niestety woda nie może być rozpatrywana jako komercyjny absorbent z powodu trudności w jej zagospodarowaniu po zakwaszeniu (HNO2 i HNO3) oraz zanieczyszczeniu innymi substancjami w skruberze.

Zmieniając koncentrację NO w gazie przed reaktorem utleniającym wykazano, że metoda usuwania NOx z gazu z zastosowaniem wstępnego utleniania i absorpcji zachowuje dużą skuteczność także dla małych, początkowych koncentracji NOx. Stwierdzono także, że zmiana stężenia NaOH w roztworze wodnym w zakresie 0,01–0,1 M nie wpływa na efektywność absorpcji NOx.

Received 28 September 2010

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effectiveness of nox removal

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