NO X and SO X Flue Gas Treatment System Based on Sulfur-Enriched Organic Oil in Water Emulsion.

Nitrogen (NO X ) and sulfur (SO X ) oxides, the major gaseous pollutants emitted from fossil fuel combustion, have significant health and environmental concerns. Environmental regulations limit these pollutant emissions to tolerable levels. Currently, these pollutants are treated by flue gas desulfurization (SO X removal) and selective catalytic reduction (NO X removal) processes. However, these technologies require large footprints, use expensive catalysts, and operate under high working temperatures. A new catalyst is reported herein, based on sulfur-enriched oil emulsified with water, where the active catalytic species are sulfur-based oxides. The catalyst has been developed using O2 as the oxidation reagent in a low-temperature wet scrubber rather than H2O2 or O3 that are presently used. The catalytically oxidized pollutants are converted to produce ammonium fertilizers by NH4OH addition. As a result of treatment with this novel catalyst, we observed reductions in emissions of SO X and NO X of >85% and 23%, respectively. The catalyst production and the wet scrubbing process are discussed in detail.

Introduction

Nitrogen oxides (NO) and sulfur oxides (SO) are air pollutants emitted in large quantities from fossil fuel industrial plants,[1] including power plants. Sulfur dioxide (SO2) is the predominant form of sulfur oxides. Two of the most common nitrogen oxides are nitric oxide (NO) and nitrogen dioxide (NO2). The nitrogen dioxide is in equilibrium with the colorless gas dinitrogen tetroxide (N2O4), Reaction (2,3)

Currently, in most industrial processes, NO and SO are treated separately.[4,5] Emissions of SO are significantly reduced and completely removed from flue gases by a “wet scrubbing technology,” which uses a slurry of alkaline sorbent (usually limestone or lime), or seawater to scrub gases. In this method, the SO2 wet scrubbing product, calcium sulfite (CaSO3), is further oxidized to produce marketable gypsum (CaSO4·2H2O). This technique is known as forced oxidation, flue gas desulfurization (FGD), or fluidized gypsum desulfurization.[6−9]

Although the flue gas desulfurization process achieves relatively high SO removal efficiency, it is not effective in NO removal. This is due to the fact that nitric oxide (NO) gas, which comprises more than 90% of NO in the flue gas, is relatively insoluble in water.[10−12] Therefore, removing NO is mostly achieved via chemical reduction, in a process termed the “selective catalytic reduction” (SCR). The reduction yield of nitrogen oxides to nitrogen via the SCR is typically high, but this technique is extremely expensive.[13,14] Other options include enhancing the removal efficiency of NO in wet scrubbers by gas-phase oxidation of water-insoluble NO gas to water-soluble NO2, HNO2, and HNO3. This oxidation can be accomplished by using strong oxidation reagents like gaseous hydrogen peroxide (H2O2), ozone (O3), or nonthermal plasma. The oxidized NO species may then be easily removed by caustic water scrubbing.[15] Additional advantages of this process include the fact that both SO and NO can be removed simultaneously. The above process further comprises the step of contacting the oxidized NO and SO, dissolved in a liquid phase, with ammonia to produce an ammonium nitrate NH4NO3 and ammonium sulfate (NH4)2SO4 mixture, which can be used as a nitrogen-contained fertilizer. The drawbacks of gas-phase oxidation are requirements of expensive reagents, corrosion-resistant systems, and specialized safety equipment to handle ozone and hydrogen peroxide safely.[16,17] These additional requirements increase the cost of operations significantly.

The objective of the present work, which is being patented,[18] is to develop an improved method for combining the catalytic oxidation of nitrogen oxides (NO) and sulfur oxides (SO) in flue gases, which eliminates the need for a gaseous ozone/hydrogen peroxide oxidation stage. The novel method can be refurnished in existing wet scrubbing absorption systems like the commonly used spray tower and replaces expensive oxidation reagents,[19] such as H2O2 and ozone, with atmospheric oxygen by catalytic oxidation. To accomplish this objective, we report a new working liquid containing an oxidation catalyst. This liquid comprises a heavy oil–water emulsion, where the organic phase contains active sulfur species in the saturated heavy mineral oil. These active sulfur species catalyze the oxidation of NO and SO via the use of atmospheric O2.

This study reports the selection criteria and optimized procedure for using the appropriate mineral oil as a working liquid organic phase and presents efficient methods for dissolving elemental sulfur in the oil to produce the active catalytic species. The catalyst developed has been tested in bubble column experiments to estimate the efficiency of the new catalytic system for the simultaneous scrubbing of nitrogen oxides (NO) and sulfur oxides (SO) air pollutants and their ultimate conversion to a nitrogen-containing fertilizer mixture.

The active catalytic species likely contain −SO (n = 1, 2), which can oxidize SO2 to SO3 and NO to NO2. The resulting −SO is ultimately oxidized back to −SO, using atmospheric O2 as the terminal oxidizing reagent. The organic phase functions both as the catalyst solvent and as an absorbing liquid (i.e., solvent) for nitric oxide (NO) since NO is only slightly soluble in water.[10−12] The oxidations of nitric oxide to nitrogen dioxide and sulfur dioxide to sulfur trioxide are according to Reactions and 3. The final oxidation products are nitric and sulfuric acids, which are soluble in the water phase and are formed via Reactions and 5.

As a result, the pH of the resulting suspension decreases. In the following stage, ammonia solution (ammonium hydroxide, NH4OH) is injected into the working liquid to increase the pH and to produce ammonium nitrate NH4NO3 and ammonium sulfate (NH4)2SO4 mixture via Reactions –8

The ammonium sulfate and ammonium nitrate are dissolved in the aqueous phase. Upon saturation of the aqueous phase, precipitation of the (NH4)2SO4/NH4NO3 mixture occurs at the bottom of the reactor, and it can be used as a nitrogen-contained fertilizer with substantial economic value. The pH during the entire process must be maintained in the range of 4–7 to keep the reaction going and avoid alkaline solution, from which ammonia gas (NH3) may be emitted. The working temperature range is 50–90 °C after reaching thermal equilibrium of the hot gases in the scrubber.

Experimental Section

The proposed process requires selecting an organic component for the working liquid and a method to integrate the catalyst into the organic phase. The efficiency of each method was examined using a bubble column that has been developed to determine its effectiveness in eliminating the SO/NO contaminations from the flue gases.

Materials

The organic catalyst is based on heavy oil, which contains sulfur compounds as the active species. Essential characteristics for the oil include the fact that it needed to be chemically inert and have a very low vapor pressure, i.e., both a high boiling and a high decomposition point. For use in spray-tower wet scrubbing systems, the oil requires a relatively low viscosity to reduce head losses and increase its fluidity. Furthermore, the oil–water emulsion working liquid should have a relatively short separation time between the two layers, so efficient separation processes for product collection are feasible.

To prepare the catalyst, each oil has been thermally treated with ground elemental sulfur (S8). In the first step, the ground sulfur was added to the oil samples in various concentrations from 0.5 to 5 wt %. The mixture was heated up to 140 °C and stirred for 1 h. The organic solution containing dissolved sulfur was slowly cooled to room temperature and has been visually checked for sulfur precipitation after 24 h. At the second step, oils with the highest sulfur concentration obtained, with no sulfur precipitate, have retreated with solid sulfur. The clearest sulfur-enriched oils without any elemental sulfur precipitate were analyzed for sulfur content.

Various oil types, paraffinic, synthetic, and brominated waste vegetable oils (these oils were tested to increase the sulfur solubility using the bromine heavy atom effect), were chosen for this study. These oils contain no sulfur as reported by the company database[20] and confirmed by X-ray fluorescence (XRF) analysis (e.g., for oil 88, the sulfur content is <0.1%). A summary of the photophysical properties of the oils, including density and viscosity, is presented in Table , and the Fourier transform infrared (FTIR) spectra of these oils are presented in Figure (see the Section ).

Table 1

Photophysical Properties of the Studied Oils and the Enrichment Results for Sulfur Concentrationa

oil type		supplier	density (g/cm3)	viscosity 40 °C (cm2/s)	sulfur wt % (%)	separation time (s)
paraffinic oils	light paraffinic oil – 88	Columbia Petro Chem	0.853	0.866	0.52	80
	light Paraffinic Oil—88 + diphenyl ether 10 wt %		0.861	1.074	0.92	37
	heavy Paraffinic oil—robul B34	Delkol—Delek Oils refinery factory	0.905	5.1	2.73	150
synthetic oils—Synfluid PAO poly(α-olefin) cSt	PAO-6	Chevron Phillips Chemical Company	0.8307	0.305	0.67	75
	PAO-8		0.8307	0.464	0.86	78
brominated waste vegetable oils	brominated vegetable oil 38-1- oxidized palm oil	brominated by bromine compounds—ICL industrial products	0.95	>100	1.5	overspecification
	brominated vegetable oil 39-2 canola oil		0.95	>100	1.5	overspecification

The sulfur was dissolved at 140 °C, and its concentration was determined at room temperature.

Figure 2

FTIR spectra of different oils before sulfur enrichment. Inset: Focus on the discussed region of the spectra.

To increase the solubility of the elemental sulfur in the organic phase, the addition of several solvents to the catalyst was tested.

The solvents were 1,2-dichlorobenzene, decabromodiphenyl ether (99% by Merck), quinoline (98% by Sigma-Aldrich), diphenyl ether, and diglyme (99% by Acros Organics). The elemental sulfur was supplied by Ziv Chemicals, and Frutarom Ltd supplied aqueous ammonia (25 wt %). The NO and SO2 gas cylinders (1% in a nitrogen atmosphere) were supplied by Airgas (an Air Liquide company). The water used throughout this study was ultrapure distilled water (UPDA) with a resistivity of >15 MΩ·cm.

Instrumentation

The sulfur concentrations were measured by X-ray fluorescence (XRF) model AXIOS mAX by Malvern Panalytical with an X-ray power source (4KW), equipped with a gas purge unit for liquid analysis. IR spectra were recorded using a Fourier transform infrared (FTIR) spectrometer model ALPHA 2 by Bruker equipped with a platinum diamond attenuated total reflectance (ATR) module (the liquid samples were dropped on top of the ATR sample platform, and the FTIR spectra were recorded). The spectral resolution was set to 2 cm–1, and 16 scans were taken per measurement. The analyses of NO, NO2, SO2, SO3, O2, and N2 were carried out using an OPTIMA 7—unit by MRU Instruments (based on electrochemical sensors). The accuracies of the sensors are NO ± 5 ppm, NO2 ± 5 ppm, and SO2 ± 10 ppm. The pH values of the solutions were measured with a glass electrode using a CyberScan Bench pH 500 pH meter. The gas flow rates were measured using a digital mass flow meter model M-SERIES V-5lpm-D/5V supplied by Alicat Scientific. The temperature controller was a digital temperature control system model BOX-3216 supplied by MRC.

Sulfur Enrichment Tests

The sulfur enrichment in the oils was studied by using different protocols. A sample of the sulfur-enriched oil was analyzed for the sulfur content by the sulfur reduction method[21,22] and XRF. The acid content of the S-enriched oil was tested by emulsification in water (water/oil = 1:1) and measuring the pH after separation of the organic emulsion’s phases.

Phase Separation Tests

The products of the studied process are ammonium sulfate and ammonium nitrate, which dissolve in aqueous solution. To separate them from the oil–water emulsion, they are first subjected to precipitation, followed by recovery of the ammonium salts from the spray column reactor via separation of the emulsion working liquid into two phases (aqueous solution containing the salts and the organic solution). The oil–water emulsion separation time for each oil was studied at 60 °C for each catalytic oil system.

Generally, in a wet scrubbing pilot facility, the product extraction process is carried out on a bleed stream separated from the fluid in a circulated working liquid. The “mother liquor,” the part of a solution that is left over after crystallization, is being returned to the circulated working liquid.

Bubble Test Column

The studied process and the proposed working catalyst liquid were studied with respect to the absorption efficiency of the two main pollutants, nitric oxide (NO) and sulfur dioxide (SO2), using a laboratory bubble column apparatus (Figure ). The system consists of the main reactor; flow, temperature, and pH control systems; and a gas-composition measurement system. The main reactor is a vertical cylindrical tube with a 5 cm diameter and a gas inlet at the bottom; see the SI for further details. The reactor was filled with 500 mL of working liquid (the liquid level in the column was 30 cm). The working liquid was a catalyst emulsion with a 1:1 volumetric mixture of water and sulfur-enriched oil.

Figure 1

Diagram of the bubble column system. Right: Inlet and gas-sampling units; left: pH control unit.

Gas mixtures were injected at the bottom of the reactor through a sparger to form fine bubbles. A fine thin Teflon wire in a packed formation was placed in the reactor to break the converging bubbles and enlarge the interfacial area. In preliminary tests, the gas flow rates were studied with respect to bubble size, gas hold-up, and pressure drop (Figures S1–S3). The operating parameters were set according to requirements for a 3 s residence time and to maximize the available interfacial area. The gas mixtures of a demonstration synthetic gas mixture: (NO 200–500 ppm/SO2 200–500 ppm in air) were injected into the reactor. The synthetic gas sources were 5 L cylinders of a 1% mass ratio (NO or SO2 balanced with nitrogen). The air and gas flow rates (NO/SO2) were measured using digital mass flow meters. The mixture flow rate and concentration were controlled using needle valves (V1 and V2 in Figure ). The operational flow rate and concentration ranges were 0.5–1.5 LPM and 200–500 ppm, respectively. The gas mixture composition and the tested gas concentration before and after passing the reactor were analyzed using an Optima 7 gas analyzer. The tested gas conversion (Ri[%], i = NO, SO2) for each case was calculated from the ratio between the inlet and outlet concentrations, where Ci,in and Ci,out referred to NO or SO2 gas concentrations, respectively, as shown in eq .

The reactor temperature was controlled by a PID controller digital temperature control system using textile glass heating tape (500 W, 250 °C) and K thermocouple for temperature sensing. The working temperature was kept at 60 °C. The acidity values were monitored by a pH electrode and controlled manually by the addition of 1.0 M ammonia solution to the reactor. Initially, the reactor was charged with a catalyst emulsion at pH 7.0.

Results and Discussion

The results of the sulfur enrichment, phase separation tests, and bubble column tests are presented.

Sulfur Enrichment

Several procedures have been tested to dissolve elemental sulfur in the organic oil to produce the active catalytic species. The most efficient method was the following procedure: The organic oil was heated to 140 °C, elemental ground sulfur (S8) was added to the stirred oil until a clear solution was obtained after ca. 1 h, and the sulfur-enriched oil was cooled slowly to room temperature. A sample of the sulfur-enriched oil was analyzed for sulfur content by the sulfur oxidation method and by XRF. The acidity of the S-enriched oil was tested by emulsification in water (water/oil = 1:1) and measuring the pH after the complete phase separation. The results of the sulfur enrichment are presented in Table .

Table 2

Sulfur Solubility in Organic Solvents at 60 °Ca

organic solvent	wt % sulfur
1,2-dichlorobenzene	8.20
quinoline	11.1
diphenyl ether	12.1
diglyme	12.3

The solubility was measured by dissolving elemental sulfur in the compound at 60 °C and separating the excess undissolved sulfur by filtering the solid.

From the results of Table , it is clear that the best oil to increase the elemental sulfur content is the heavy paraffinic oil—Robul B34, in which a sulfur concentration of 2.73% was obtained. The second best was the brominated vegetable oil 38, which dissolved 1.5% of sulfur. These results are likely due to the fact that dissolved sulfur in the heavy organic oils reacts to form soluble sulfur compounds in the oil, which are the active species in the catalytic process.[23] By contrast, the other oils are less able to provide the environment necessary for these reactions to occur.

The treatment of the heavy organic oils to produce the organic catalytic phase involves chemical changes and the formation of active sulfur species, which are the heart of the catalyst. Thus, it is expected that it will result in changes in the infrared spectrum. To verify it, the spectra of the catalyst compared to the untreated heavy oil have been measured, and indeed new absorption peaks have appeared at 1210–1240 and 1310–1330 cm–1 that are absent in the untreated organic oils (Figure ). The FTIR spectra in Figure relate to the catalyst formed after the dissolution of elemental sulfur in the oil. In the inset, it is clear that two new absorption bands at 1225 and 1320 cm–1 in the sulfur-enriched oil appear in the spectrum. These bands probably stem from the sulfur-active catalytic species, probably R-SO/R-SO2 formed in the oil, which leads to S=O vibrations in the active species.[24]

Figure 3

FTIR spectra of different oils after sulfur enrichment. Inset: Focus on the informative region of the spectra.

One of the advantages of using the heavy organic phase is that the solubility of molecular O2 is much higher than in aqueous solutions.[25] A plausible mechanism that might explain the sulfur-containing organic catalyst activity in the organic/aqueous emulsion is via Reactions –14.

The oxidized products are much soluble in water; thus, they will be transferred at the interphase layer of the emulsion from the organic phase to the aqueous solution, and in the presence of dissolved ammonia in the aqueous phase, a nitrogen sulfur mixture of NH4NO3/(NH4)2SO4 will be produced.

Effect of Solvent Additives on the Sulfur Content

Literature reports indicate that some organic compounds can facilitate the dissolution of elemental sulfur.[26] As the likely active species in the organic catalyst are sulfur-based species, we decided to study the effect of such compounds on the solubility of elemental sulfur by adding them to the heavy oil phase. The compounds chosen were 1,2-dichlorobenzene, quinoline, diphenyl ether, diglyme, and decabromodiphenyl ether; 10 wt % of each additive was added to 50 wt % of water/oil emulsion, and its ability to dissolve elemental sulfur was studied at 60 °C, since some of the compounds are volatile at 140 °C. Elemental ground sulfur was added to the organic solvent at 60 °C until maximum dissolution was obtained, and the organic phase was clear without any precipitated sulfur. The results of the saturated S concentration at 60 °C are given in Table .

As shown in Table , these compounds are efficient solvents for the dissolution of elemental sulfur, with a range of solubilities between 8% and 12.3%. To check the effectiveness of using these compounds, 10 wt % diphenyl ether was dissolved in oil 88 and the dissolution of elemental sulfur in the mixture was checked (Table ). As expected, an appreciable increase (by almost a factor of 2) in the sulfur content of 0.92 wt % was achieved (compared with the pure oil 88 content of 0.52 wt %).

Catalyst Efficiency

Each catalyst oil was checked for its catalytic activity in two reactions: oxidation of NO to NO2 and oxidation of SO2 to SO3 in the bubble column system. The typical NO or SO2 gas concentrations at the inlet point were NO: 400–500 ppm, SO2: 325–370 ppm, at low gas flow rates (for example, mixing 0.5 L/min air with 75 mL/min NO or SO2 resulted in 450 ppm NO, or 350 ppm SO2). Upon operation of the bubble column system, the pH values of the water in the catalytic emulsions were decreased from 7 to 5.5 after 30 min of operation due to the oxidation of NO and SO2 to HNO2, HNO3, H2SO3, and H2SO4, all of which are acidic species that readily dissolve in the aqueous phase.

To determine the effect of the active sulfur species on the oxidation process, reference runs of pure solvent oil were carried out. The results, shown in Table , are presented as percent oxidation of NO or SO2 (eq ). After each run, the separation time of the oil/water emulsion into two phases (lower phase, water; upper phase, catalyst) was measured and is given in Table .

Table 3

Percent Oxidation of NO(g) and SO2(g) in the Pilot System: Wet Scrubber at 60 °C with Different Catalytic Systems.a

	NO(g) oxidation, %		SO2(g) oxidation, %
oil type	pure solvent	S-enriched catalyst	pure solvent	S-enriched catalyst
water (only)	8.8		90	-
light paraffinic oil—88	5.5	14.5	86	91.5
heavy paraffinic oil—Robul B34	9.1	23	76	85
PAO-6	6.7	17.5	88	92
PAO-8	6.9	18.5	87	92
brominated vegetable oil 38-1	9.5	24	70	76

Typical inlet values: NO 450 ppm, NO2 60 ppm, and O2 20.5%; at the outlet of the reaction column: NO 411 ppm, NO2 26 ppm, and O2 18.9%, to get 14.3% oxidation. Typical inlet values: SO2 306 ppm and O2 20.5%; at the outlet of the reaction column: SO2 26 ppm, NO2 and O2 18.8%, to get 91.5% oxidation.

As shown in Table , the formation of the S active species in the oil appreciably increases its catalytic activity in the oxidation of the NO gas to NO2. The best catalyst for this process is the heavy paraffinic oil—Robul B34, which increases the oxidation rate from 9.1% in the pure oil to 23% in S-enriched oil. However, in all S-enriched oils, the conversion of NO to NO2 is improved appreciably by factors of 2–3. In contrast, in SO2 oxidation to SO3, no appreciable improvement is observed and all of the S-enriched oils increased the conversion by only 4–10%. The effect of temperature has been examined, in which increasing the temperature from 60 to 70 °C did not affect the activity. The pH range of the aqueous solution must be maintained at 6–7 since, at acidic pH ≤ 4, the activity of the catalyst is appreciably reduced. Thus, an aqueous ammonia solution is injected to maintain the optimal pH needed. The typical operation period of the catalyst in the experiment was 1 h. To check the stability of the catalyst activity, the test period was increased from 1 to 3 h and no reduction in the activity has been observed. Recall that the requirements for the oils include low volatility, high boiling, and a high decomposition point; indeed, it has been observed that no reduction in the amount of the catalyst occurred during the experiments.

Conclusions

This study aimed to develop a catalyst that will eliminate SO and NO contaminations in flue gases using atmospheric oxygen as an oxidation reagent. The newly developed catalyst is designed as a potential replacement for the existing bulky and expensive wet scrubbing systems operating at a relatively high temperature (more than 350 °C) with much smaller, simpler, and cheaper combination systems operating at 50–90 °C. Another essential feature of the studied system is that the dissolved sulfur species contribute to higher oxidation yields of NO and SO in flue gasses and that higher sulfur concentration results in improved oxidation of the pollutant gases. Therefore, a simple and economic process providing a general method for the oxidation of NO and SO using atmospheric O2 as an oxidation reagent while manufacturing fertilizers (such as ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4), by ammonia injection) is achieved. However, the catalyst developed is very efficient for SO2 oxidation (≥85% reduction) and still should be improved for NO oxidation (∼24% reduction).