Characteristics of O3 Oxidation for Simultaneous Desulfurization and

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Characteristics of O3 Oxidation for Simultaneous Desulfurization and Denitration with Limestone−Gypsum Wet Scrubbing: Application in a Carbon Black Drying Kiln Furnace Qiang Ma,† Zhihua Wang,*,† Fawei Lin,† Min Kuang,‡ Ronald Whiddon,† Yong He,† and Jianzhong Liu† †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zhejiang, Hangzhou 310027, P. R. China Institute of Thermal Engineering, China Jiliang University, Hangzhou 310018, P. R. China

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ABSTRACT: The special process for producing carbon black precludes the application of traditional, mature flue-gas treatments (e.g., SNCR and SCR) to reduce NOx emissions. However, simultaneous removal of sulfur and nitrogen by the combination of ozone oxidation with limestone−gypsum wet scrubbing is suitable for the conditions found in carbon black flue gas. This article presents the first industrial-scale deployment of this technology in a 100000 t/year carbon black production line (flue-gas volume of 60000 Nm3/h). Optimizations were performed by studying the effectiveness of desulfurization and denitration for various operating parameters such as O3/NOx molar ratio, spray-tower liquid/gas ratio, initial NOx concentration, and forced oxidation. The results showed that (1) the O3/NOx molar ratio is the determining factor in denitration efficiency, (2) both forced oxidation and increasing initial NOx concentration improve the denitration efficiency but reduce the desulfurization efficiency, and (3) increasing the liquid/gas ratio can improve the desulfurization efficiency but has little effect on denitration. The maximum desulfurization and denitration efficiencies of the system can be up to 98% and 95%, respectively, with SO2 and NOx levels of 1000 mg/Nm3 (6% O2) and 900 mg/Nm3 (6% O2) dropping to 20 mg/Nm3 (6% O2) and 45 mg/Nm3 (6% O2), respectively.

1. INTRODUCTION The tire production industry uses carbon black as a rubber reinforcing agent. Largely because of the growth of car ownership, world carbon black consumption is expected to maintain a 4% annual growth rate between 2014 and 2020. In the typical industrial process, carbon black is generated by the incomplete combustion of fossil fuels quenched by cold water. After drying and granulation in a drying rotary klin furnace, carbon black product can be obtained. However, this process yields large volumes of flue gas with high pollutant concentrations. Average emissions for the production of 1 ton of carbon black are 16.8 kg of sulfur dioxide and 10 kg nitrogen oxides,1 with flue-gas concentrations of 600 mg/m3 (6% O2) for NOx and 800 mg/m3 (6% O2) for SO2. According to 2010 worldwide statistics, emissions of sulfur dioxide and nitrogen oxide from the carbon black production industry account for 2.67% and 1.47%, respectively, of the total emissions of these pollutants.2 In accordance with the European Union’s air pollutant emission standards (NOx < 200 mg/Nm3, SO2 < 200 mg/Nm3) and China’s most stringent air pollutant emission standards for boilers (NOx < 100 mg/ Nm3, SO2 < 50 mg/Nm3), desulfurization and denitration are urgently needed for flue gas generated by the carbon black industry. At this point, the mature technologies for flue-gas denitration are usually selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). Because of limitations inherent to the carbon black production process, these two technologies are not suitable for use in this application because of the unfitted temperature range. The typical carbon black production process (shown in Figure 1) is as follows: High-temperature (1700−1900 °C) flue gas containing SO2 and NOx flows into a cracking furnace © XXXX American Chemical Society

Figure 1. Schematic diagram of the carbon black production process.

where high-temperature pyrolysis of the feedstock oil occurs. Cooling water is injected at end of the cracking furnace to quickly quench the cracking process. The partially pyrolyzed oil has, at this point, been converted to carbon black. The flue gas, which contains the carbon black, is drawn through an air preheater (flue-gas temperature from 750 to 400 °C) and a feedstock-oil heater (flue-gas temperature from 400 to 220 °C). The flue gas/carbon black passes into a baghouse where bag filters separate the carbon black from the flue gas. Large amounts of combustible gases are produced by the pyrolysis of the feedstock oil. Some of this gas is fed to a gas-fired boiler for the generation of high-pressure steam to be used in a steam turbine for power generation. The remaining combustible gas is burned in a carbon black drying kiln where the wet clumps of carbon black are dried. Mature technologies for exhaust-gas denitration include selective noncatalytic reduction (SNCR) and selective catalytic Received: November 17, 2015 Revised: January 20, 2016

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DOI: 10.1021/acs.energyfuels.5b02717 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

small molecular compounds. Wang et al.3 injected O3 into flue gas that was then passed through an alkaline spray tower to study the removal efficiencies of SO2, NOx, and Hg with O3 assistance. Asif and Kim14 used numerical simulations to study both the reaction between O3 and NOx and the following wet scrubbing. They found that this technology could achieve 100% desulfurization efficiency, 97% denitration efficiency, and 90% mercury removal efficiency. Although the simultaneous removal of pollutants by wet gas scrubbers with ozone preoxidation has been successful at the laboratory scale,7−13 its industrial-scale application on real furnaces has not been publicly reported, and the true desulfurization and denitration efficiencies obtainable under the complicated industrial-scale conditions have not been confirmed. To evaluate the technology in an actual complex environment, the application of O3 oxidation combined with wet scrubbing in a 100000 t/year carbon black production line (flue-gas volume of 60000 Nm3/h) for simultaneous desulfurization and denitration was tested. The O3-oxidation stage was added to the carbon black production line together with a limestone−gypsum wet desulfurization system. Accordingly, the effects of various operating parameters of this technology15−17 (such as the O3/NOx molar ratio, spray-tower liquid/ gas ratio, initial NOx concentration, and forced oxidation) on the NOx and SO2 removal efficiencies were determined and optimized. The results reported herein will help researchers further understand the simultaneous desulfurization and denitration performance of O3 oxidation combined with wet washing technology in industrial-scale environments.

reduction (SCR). The optimal temperature window for SNCR is between 850 and 1100 °C, temperature conditions found at the exit of the pyrolysis furnace. SNCR requires the injection of urea or ammonia to reduce the NOx, but if these materials are injected at the exit of the pyrolysis furnace, they will be absorbed by the carbon black. This would require an increased feed rate of the reductant (urea/ammonia), and the resulting carbon black would be of diminished quality. Thus, SNCR cannot be used in this application for flue-gas denitration. The temperature of flue gas after pyrolysis between the air preheater and feedstock-oil heat exchanger is between 220 and 400 °C, which meets SCR operating temperature range (300−400 °C). However, two factors make SCR inapplicable: (i) At the tail end of the cracking furnace, a large volume of quench water is injected, which makes the moisture content of flue gas very high (volume fraction of ∼45%). If SCR catalyst were used in highly moist flue gas, its denitration efficiency would be severely diminished. (ii) There is a high level of partially pyrolyzed viscous oil in the flue gas at this location, which would quickly clog the SCR catalyst, resulting in decreased SCR efficiency and a shortened service life of the SCR catalyst. Combustible gases in the flue-gas flow that are burned in the carbon black drying kiln also produce NOx; however, the operating conditions and equipment space in the carbon black drying kiln furnace are also not suitable for SNCR or SCR. Finally, flue gas flows out of the carbon black drying kiln furnace, and its temperature is lower than 180 °C, which is also not suitable for SNCR or SCR. Thus, neither of the current mature technologies SNCR and SCR is practically applicable to the denitration of flue gas in carbon black production. NO, which has a low solublility in water, accounts for 95% of the nitrogen oxides in flue gas.3 Meanwhile, trace heavy metals in the flue gas are also present in forms that have low solublilities in water (e.g., Hg as Hg0).4 By pretreating flue gas with a strong oxidizing agent to oxidize NO and Hg0 into their highly water-soluble species (i.e., NO → NO2, NO3, and N2O5; Hg0 → Hg2+), these generated species can be removed simultaneously with SO2 by wet-flue-gas desulfurization (WFGD) equipment. Because of the strong oxidation potential and long lifetime of ozone (standard electrode potential = 2.07 mV; half-life = 19.2 s at 150 °C),5 some researchers have proposed using ozone oxidation combined with water washing to achieve integrated control of flue-gas pollutants at low temperature (1.75, most of the NOx has been oxidized to be N2O5, which is absorbed by the slurry. With increasing O3/NOx molar ratio, the reaction will be more thorough, decreasing the amounts of NO and NO2 remaining. The denitration efficiency will thus be increased slightly with increasing O3/NOx molar ratio. Asif and Kim14 found that N2O5 began to appear at O3/NOx molar ratios of >1 and remained stable at O3/NOx molar ratios of >1.5. Finally, one can see that the denitration efficiency was almost consistent with the formation regularity of N2O5. Accordingly, the desulfurization curve in Figure 3 can also be divided into three stages: (i) At O3/NOx molar ratios of 1.75, the liquid/gas ratio has almost no effect on denitrification. The denitration efficiencies at the two liquid/gas ratios (i.e., all three layers in service and the lowermost layer turned off, Figure 2) are both above 90% when the O3/NOx molar ratio is greater than 2.5. According to panel b of Figure 6, the higher the liquid/gas ratio, the higher the desulfurization efficiency. Because the reaction of SO2 with water is slow in comparison to that of N2O5 with water, the mass-transfer rate is controlled by the area of the gas−liquid interface. The desulfurization efficiency is therefore more sensitive than the denitration efficiency to the liquid/gas ratio. At O3/NOx molar ratios of >1.2, we found that the increase of the liquid/gas ratio from 8 to 12 L/Nm3 improved the desulfurization efficiency to a certain extent. There are two reasons for this result: (i) A larger liquid/gas ratio increases the contact area between SO2 and the slurry, promoting mass transfer and thus increasing the desulfurization efficiency. (ii) As mentioned above, N2O5 dissolved in water will ionize H+, inhibiting SO2 dissolution in the water, which reduces the desulfurization efficiency. The increase of the liquid/gas ratio weakens the acidification of N2O5 to the spray zone droplets and thus weakens the inhibition of SO 2 dissolution in water to a certain extent, thereby increasing the desulfurization efficiency. In this case, in addition to SO2, N2O5, which is the anhydride of HNO3, is also water-soluble in flue gas. That is, the concentration of acidic gas in the flue gas increases. Therefore, the original liquid/gas ratio for desulfurization will be too small. As a result, the liquid/gas ratio requires an appropriate increase to weaken the inhibition of N2O5 to desulfurization, and the higher the concentration of N2O5, the greater the increase. We also found that the inhibitory effect of N2O5 on desulfurization can be completely offset by increasing liquid-to-gas ratio, which confirms the accuracy of our analysis of the reason that the desulfurization efficiency declines with increasing O3/NOx molar ratio.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-571 8795 3162. Fax: +86-571 8795 1616. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51422605), National Basic Research Program of China (2009CB219802), and Zhejiang Key Discipline of Instrument Science and Technology.

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REFERENCES

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4. CONCLUSIONS In summary, O3 oxidation combined with wet scrubbing for simultaneous desulfurization and denitration was used for the pollution emission reduction in the carbon black production process. Industrial-scale experiments and measurements were carried out on the system to evaluate the effects of various operating parameters (i.e., O3/NOx molar ratio, liquid/gas ratio in spray tower, initial NOx concentration, and forced oxidation) on the desulfurization and denitrification efficiencies. The results suggest the following conclusions: (i) In terms of denitration, NO oxidation to N2O5 is the key step, and NO2 plays a secondary role. The O3/NOx molar ratio is the decisive F

DOI: 10.1021/acs.energyfuels.5b02717 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.5b02717 Energy Fuels XXXX, XXX, XXX−XXX