SIMULTANEOUS REMOVAL OF SO2 AND NOx WITH AMMONIA ...

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Chemical Industry & Chemical Engineering Quarterly

Chem. Ind. Chem. Eng. Q. 21 (2) 305−310 (2015)

SHAOPENG GUO1 LINA LV1 JIA ZHANG1 XIN CHEN2 MING TONG2 WANZHONG KANG2 YANBO ZHOU1 JUN LU1 1

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science & Technology, Shanghai, P. R. China 2 SINOPEC Ningbo Engineering Co., Ltd., Ningbo, P. R. China SCIENTIFIC PAPER UDC 66.081.2:546.214:54 DOI 10.2298/CICEQ140618029G

CI&CEQ

SIMULTANEOUS REMOVAL OF SO2 AND NOx WITH AMMONIA COMBINED WITH GAS-PHASE OXIDATION OF NO USING OZONE Article Highlights • Simultaneous removal of SO2 and NOx was achieved by oxidation of NO with O3 and ammonia absorption • The appearance of SO2 in fuel gas has little impact on the oxidation of NO • The O3/NO molar ratio is the most important factor in the ozone oxidation process • Increasing of O3/NO mole ratio and SO2 concentration are favorable to recycling the byproducts Abstract

A process for simultaneous desulfurization and denitrification is proposed, consisting of ozone as the oxidizing agent for NO and ammonia solution as the absorbent. The results showed that the presence of SO2 and the concentration changes of NO and SO2 have little impact on the oxidation of NO, the oxidation efficiency of NO can achieve over 90% when the molar ratio of O3/NO is 1.0. The presence of NOx had little effect on the absorption of SO2, while an appropriate increase of SO2 concentration favorably affected NOx absorption. The removal efficiency of SO2 and NOx reached 99.34 and 90.01% at pH 10, flow rate 0.95 Nm3/h, n[O3]/n[NO] 1.0, initial SO2 concentration 2000 mg/Nm3, initial NO concentration 200 mg/Nm3, ammonia concentration 0.3%, oxygen content of the simulated flue gas 12%, oxidation reaction temperature 423 K and absorption reaction temperature 298 K in the experimental system. Keywords: ozone, nitrogen oxides, sulfur dioxide, ammonia, simultaneous absorption.

Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the most abundant air pollutants during coal combustion. These pollutants have brought about significant effects on both the environment and human health in China [1,2]. Many technologies have been used to reduce the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx), among which wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) are regarded as the most effective technologies for SO2 and NOx removal, respectively. However, the individual treatment technology may lead to high investment and operating costs. To overcome Correspondence: J. Lu, Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science & Technology, No. 130 Meilong Road, Shanghai 200237, P. R. China. E-mail: [email protected] Paper received: 18 June, 2014 Paper revised: 16 August, 2014 Paper accepted: 25 August, 2014

this problem, many technologies such as nonthermal plasma, electron beam irradiation [3-5] and adsorption process [6-15] have been developed for simultaneous removal of SO2 and NOx, but only few commercial applications have been reported until now. As an efficient gas phase oxidant with adventages of selectivity, high oxidation efficiency, fast oxidation speed and non-pollution decomposition products, ozone can easily oxidize NO into high order nitrogen species such as NO2, NO3 and N2O5 etc., which are highly soluble in water [16,17]. Wang et al. [18] have confirmed that it is possible to achieve about 97% of NO removal efficiency and nearly 100% of SO2 removal efficiency through ozone oxidation and Ca(OH)2 absorption. Young et al. [19] obtained NOx removal efficiency of about 95% and SO2 removal efficiency of 100% with an ozonizing chamber and an absorber containing Na2S solution. In addition, the ammonia-based wet flue gas desulfurization pro-

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cess has attracted much attention in China recently due to its high desulfurization efficiency, useful byproducts, no secondary pollution and lower costs [20-22]. Therefore, ammonia-based wet flue gas desulfurization process combined with ozone oxidation is a promising technology for simultaneously desulfurization and denitrification and it is worthy of further research. In this paper, a flue gas treatment process by utilizing ozone as oxidant and ammonia solution as absorbent was established to achieve simultaneous removal of NOx and SO2. The oxidation of NO by ozone was studied and the performance of this combined treatment process was investigated under different operation parameters.

The concentration of O3 was measured according to iodometric method (CJ/T 3028.2-1994). The concentrations of NO, NO2 and SO2 were detected by flue gas analyzer Optima 7 (MRU, Germany). The flow rate and oxygen content of the simulated flue gas were fixed to 0.95 Nm3/h and 12%, respectively. The mass fraction of ammonia in the absorption solution was 0.3%. The oxidation of NO was conducted at 150 °C. The NO oxidation efficiency was investigated with the mole ratios of O3/NO changing from 0.3 to 1.5, the initial concentration of NO ranging from 200 to 800 mg/N m3 and initial SO2 concentration varying from 1000 to 3000 mg/Nm3 in the oxidation process. In the absorption process, the removal efficiency of SO2, NOx were analyzed in different experimental conditions which are mentioned above.

EXPERIMENTAL As shown in Figure 1, the experimental apparatus for simultaneously desulfurization and denitrification is a set of self-made equipment. Ozone was produced by the ozone generator (Qingdao Guolin Industry Co., Ltd, Qingdao, China). The gas reactor was made of stainless steel, 32 mm in inner diameter and 700 mm in length, which was inserted into a tube type resistance furnace with temperature control. The bubbling reactor was a glass cylindrical vessel with a total volume of 1.4 L. The simulated flue gas was supplied by air and compressed gas cylinders filled with N2, NO and SO2. The flow rates of all gases were controlled by gas volume flow meters. The mixture of air, N2, NO and SO2 reacted with O3 in the reactor, then the gas mixture went through the cooling pipe and went into the bubbling reactor filled with ammonia solution as absorbent. When the absorption was completed, the gas reactor was purged with nitrogen for 10 min.

RESULTS AND DISCUSSIONS Influence of O3/NO molar ratio In this section, the oxidation efficiency of NO, SO2 and NOx removal efficiency were studied in different O3/NO mole ratio. The initial concentrations of NO and SO2 were fixed at 500 and 2000 mg/N m3. Figure 2 shows the oxidation efficiency of NO both in presence and absence of SO2 under different O3/NO mole ratios. From Figure 2 it can be seen that the NO oxidation efficiency gets higher with the increase of O3/NO mole ratio up to 1.0, the oxidation reaction happens quickly as following reaction (1):

NO + O3 → NO2 + O2

(1)

The oxidation efficiency exceeds 90% and many other reactions take place at the same time when the O3/NO mole ratio gets higher than 1.0, mainly including reactions (2)–(5):

Figure 1. Schematic of the experimental apparatus; 1 - air pump; 2 - gas volume flow meter; 3 - gas reactor; 4 - cooling pipe; 5 - ozone generator; 6 - bubbling reactor; 7 - magnetic stirring constant-temperature water bath; 8 - rubber stopple; 9 - aerator; 10 - magnetic stirrer; 11 - Ph meter; a, b, c, d - sample connection.

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NO2 + O3 → NO3 + O2

(2)

2NO2 +SO32- +H2O → 2H+ + 2NO2- +SO4 2-

(9)

NO2 + NO3 → N2O5

(3)

2NO2 +HSO3- +H2O → 3H+ + 2NO2- +SO42-

(10)

N2O5 → NO2 + NO3

(4)

NO + NO3 → 2NO2

(5)

Figure 2. The influence of O3/NO mole ratio on NO oxidation efficiency (flow rate: 0.95 N m3/h; oxygen content of the simulated flue gas: 12%; reaction temperature: 423 K).

The oxidation efficiencies of NO are slightly lower when NO coexists with SO2, the reason is that SO2 can be oxidized by O3 as the following reaction: SO2 + O3 → O2 + SO3

(6)

However, the decreases are limited and the NO oxidation efficiency can still reach 90% in presence of SO2 when n[O3]/n[NO] is 1.0. Besides, the SO2 oxidation efficiency is only about 5% during the experiment, it can be concluded that the present ozonizing method can successfully be used for the oxidation of NO, the coexistence of SO2 has little impact on oxidation of NO. A similar result was reported by Wang et al. at a lower temperature 373 K [18]. It can be seen from Figure 3 that the SO2 removal efficiency is always nearly 99%, which indicates that the SO2 absorption in to ammonia solution is almost unaffected by the existing of NOx. NOx removal efficiency increases from 62.1 to 86.7% as O3/NO mole ratio increases from 0.5 to 1.5. Obviously, the increasing of O3/NO molar ratio is favorable to the absorption of NOx. The increasing molar ratio of O3/NO increased NOx oxidation rate and accelerated NOx dissolution, which was helpful for the NOx removal [23,24]: 3NO2 +H2O + 2NH3 → 2NH4NO3 + NO

(7)

NO2 + NO + H2O +2NH3 → 2NH4NO2

(8)

Figure 3. The influence of O3/NO molar ratio on SO2 and NOx removal efficiency (flow rate: 0.95 Nm3/h; initial SO2 concentration: 2000 mg/Nm3; initial NO concentration: 500 mg/Nm3; ammonia concentration: 0.3%; oxygen content of the simulated flue gas: 12%; oxidation reactor temperature: 423 K; absorption reaction temperature: 298 K; absorption solution pH value: 10.0).

Additionally, the concentration of SO42– and NO3– in the solution was tested at pH 5.5. The content of SO42– goes up from 53.3 to 76.4% as the O3/NO mole ratio increases from 0.5 to 1.5 and the content of NO3– keeps a relatively stable level around 10%. More NO2 can be generated as the molar ratio of O3/NO grow, the reactions (9) and (10) will be promoted to produce more SO42–. In ammonia-based wet flue gas desulfurization process, a great amount of air is needed to oxidize (NH4)2SO3 and NH4HSO3 into (NH4)2SO4, the byproduct. Due to this cause, increasing the O3/NO molar ratio, the energy consumption of air supply will be greatly reduced in the ammonia-based simultaneously desulfurization and denitrification process. Influence of initial NO concentration Figure 4 shows the effect of initial NO concentration on the ozonation process. The increase of the initial NO concentration results in increasing NO oxidation efficiency, because higher initial NO concentration can speed up the oxidation rate when the reactions begin. When the mole ratios of O3/NO are 1.0 and 1.2, the NO oxidation efficiency rises to over 90% and keeps steady in spite of initial NO concentration changing. As long as controlling the mole ratio of O3/NO above 1.0, a favorable oxidation effect can be obtained regardless of initial NO concentration. This

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result is very helpful for practical engineering applications.

The content of SO42- in absorption solution increases from 60.69 to 73.01% at pH 5.5 as the NO initial concentration increases from 200 to 800 mg/N m3. Though the denitrification efficiency declines with the increase of NO concentration, more amount of NOx in quantity is absorbed into solution which can boost the reactions 9 and 10, resulting in the increases of the SO42– content in absorption solutions.

Figure 4. The influence of initial NO concentration on NO oxidation efficiency (flow rate: 0.95 N m3/h; initial SO2 concentration: 2000 mg/Nm3; oxygen content of the simulated flue gas: 12%; reaction temperature: 423 K).

The experimental results indicating the influence of initial NO concentration on SO2 and NOx removal efficiency are shown in Figure 5. The SO2 removal efficiency increases from 99.34 to 99.86% when initial NO concentration increases from 200 to 800 mg/N m3 and the pH value of absorption solution was 10.0. The possible reason is that an appropriate increase in initial NO concentration promotes the reaction between NO2 and SO32–, but the increase of desulfurization efficiency is very limited, which indicates the reactions (11)-(12) are the main reactions for SO2 removal. As NOx concentration increases, the reaction contact time and the chemical equivalent of absorbent gradually cannot meet the requirements of NOx absorption, the NOx removal efficiency declines form 90.01 to 71.95%. However, Wei et al. [5] draw a conclusion that NOx concentration has little effect on denitrification when they used microwave reactor with NH4HCO3 and zeolite to study the simultaneous removal of SO2 and NOx from flue gas. The possible reason may be that the chemical equivalent of absorbent has little change in the range of initial NO concentration from 120 to 200 mg/N m3. Jin et al. [6] used aqueous chlorine dioxide solution as absorbent and NOx removal efficiency only reached 66-72%, which proves ozone is more efficient than chlorine dioxide: 2NH3 + H2O + SO2 → (NH4 )2SO3 (NH4 )2SO3 + SO2 +H2O → 2NH4HSO3

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(11) (12)

Figure 5. The influence of initial NO concentration on SO2 and NOx removal efficiency (flow rate: 0.95 Nm3/h; n[O3]/n[NO]: 1.0; initial SO2 concentration: 2000 mg/Nm3; ammonia concentration: 0.3%; oxygen content of the simulated flue gas: 12%; oxidation reactor temperature: 423 K; absorption reaction temperature: 298 K; absorption solution pH value:10.0).

Influence of initial SO2 concentration The influence of initial SO2 concentration on NO oxidation efficiency was investigated by varying initial SO2 concentration from 1000 to 3000 mg/N m3. As can be seen in Figure 6, the increasing of initial SO2 concentration has a slight inhibition on the oxidation of NO with the NO oxidation efficiencies declining from 96.4 to 90.5%. The oxidation efficiencies of SO2 always maintain at around 5% under different initial SO2 concentrations, which means that the oxidation reaction between SO2 and O3 is extremely weak in comparison with the oxidation of NO by ozone in system. Combined with the previous results, we can draw a conclusion that the appearance of SO2 and the concentration changes of NO and SO2 have little impact on the oxidation of NO, the O3/NO mole ratio is the most important factor in the ozone oxidation process for desulfurization and denitrification, keeping the O3/NO mole ratio above 1.0, the oxidation efficiency of NO can achieve over 90% within the experiment conditions. Figure 7 illustrates the influence of initial SO2 concentration on SO2 and NOx removal efficiency. It is evident that the desulfurization efficiencies under dif-


ferent initial SO2 concentrations are all close to 100%, the denitrification efficiency increases with rising initial SO2 concentrations. Fang et al. [10] draw an opposite conclusion that NOx removal efficiency sharply decrease with the increment of SO2 concentration. The reason may be that the increase of initial SO2 concentration can generate more SO32– in absorption solution, reactions (9) and (10) are promoted and more NO2 will be assimilated. An appropriate increase of SO2 concentration does not cause the absorption capacity to decline sharply in our experimental conditions. This is why an appropriate increase of initial SO2 concentration can contribute to the NOx absorption.


CONCLUSIONS In this paper, simultaneous removal of SO2 and NOx with ozone as oxidant and ammonia as absorber was investigated. In the ozonation part, the appearance of SO2 and the concentration changes of NO and SO2 have little impact on the oxidation of NO, the O3/NO mole ratio is the most important factor in the ozone oxidation process. The oxidation efficiency of NO can achieve over 90% within the experiment conditions and the conversion of NO is almost complete when the mole ratio of O3/NO is 1.0. In the absorption part, the SO2 absorption into ammonia is almost unaffected by the existing of NOx, an appropriate increase of initial SO2 concentration can contribute to the NOx absorption. The removal efficiency of SO2 and NOx was about 99.34 and 90.01% at pH 10, flow rate 0.95 N m3/h, n[O3]/n[NO] 1.0, initial SO2 concentration 2000 mg/N m3, initial NO concentration 200 mg/N m3, ammonia concentration 0.3%, oxygen content of the simulated flue gas 12%, oxidation reactor temperature 423 K and absorption reaction temperature 298 K. Increases of O3/NO mole ratio and initial SO2 concentration could elevate the content of SO42– in absorption solutions, the energy consumption of air supply for oxidation of SO32– will be greatly reduced. Acknowledgement The work was supported by Sinopec Ningbo Engineering Co. Ltd.

Figure 6. The influence of initial SO2 concentration on NO oxidation efficiency (flow rate: 0.95 Nm3/h; initial NO concentration: 500 mg/N m3; n[O3]/n[NO]: 1.0; oxygen content of the simulated flue gas: 12%; reaction temperature: 423 K).

Figure 7. The influence of initial SO2 concentration on SO2 and NOx removal efficiency (flow rate: 0.95 Nm3/h; n[O3]/n[NO]: 1.0; initial NO concentration: 500 mg/Nm3; ammonia concentration: 0.3%; oxygen content of the simulated flue gas: 12%; oxidation reactor temperature: 423 K; absorption reaction temperature: 298 K; absorption solution pH value: 10.0).

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SHAOPENG GUO1 LINA LV1 JIA ZHANG1 XIN CHEN2 MING TONG2 WANZHONG KANG2 YANBO ZHOU1 JUN LU1 1

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science & Technology, Shanghai, P. R. China 2 SINOPEC Ningbo Engineering Co., Ltd., Ningbo, P. R. China NAUČNI RAD

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ISTOVREMENO UKLANJANJE SO2 I NOX POMOĆU AMONIJAKA I OKSIDACIJE NO U GASNOJ FAZI KORIŠĆENJEM OZONA Predložen je proces za istovremeno odsumporavanje i denitrifikaciju sa ozonom kao oksidacionim sredstvom za NO i amonijačnim rastvorom kao apsorbentom. Rezultati pokazuju da prisustvo SO2 i promene koncentracija NO i SO2 malo utiču na oksidacije NO. Efikasnost oksidacije NO može iznositi i preko 90 % pri molskom odnosu O3/NO = 1,0. Prisustvo NOx neznatno utiče na na apsorpciju SO2. Odgovarajuće povećanje koncentracije SO2 dovodi do bolje apsorpcije NOx. U eksperimentalnom sistemu efikasnost uklanjanja SO2 i NOx dostiže vrednost 99,34 i 90,01%, redom, pri pH 10, protoku 0,95 N m3/h, n[O3]/n[NO] 1,0, početnoj koncentraciji SO2 2000 mg/N m3, početnoj koncentraciji NO 200 mg/N m3, koncentraciji amonijaka 0,3%, sadržaju kiseonika u modelu dimnog gasa 12%, temperaturi reakcije oksidacije 423 K i temperaturi reakcije apsorpcije 298 K. Ključne reči: Ozon, azotovi oksidi, amonijak, istovremena apsorpcija.