Two density-separated Chinese coals and steel slag were prepared and then combusted in a modification furnace with different excess air ratio. Non-staged and Fuel-staged combustion experiments were compared under different pulverized coal mixing ratios. Thus, in the fuel-staged combustion experiments, the concentration of NOx in the flue gas was found to greatly decrease under a bitumite: anthracite mixing ratio of 1:1 and an air excess coefficient of 1,2 under the fuel-staged ratio is 15:85. The flue gas temperature was as high as 1 615 °C, while the NOx concentration in the flue gas was as low as 320 mg/m3. Compared with the other types of burners, the experimental combustion device designed herein efficiently reduced the NOx emissions ca. 80%. Fuel-staged combustion has a significant effect on reducing particulate matter (PM) emissions, and a suitable coal ratio was also beneficial for reducing particulate matter emissions

D. Q. Cang

J. X. Guo

L. L. Zhang

L. Y. Qi

W. B. Dai

English

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47METALURGIJA 57 (2018) 1-2, 47-50J. X. GUO, L. L. ZHANG, W. B. DAI, L. Y. QI, D. Q. CANGNOx AND ULTRAFINE PARTICLE EMISSION CHARACTERISTICS IN A NEW STEEL SLAG MODIFICATION DEVICE Received – Primljeno: 2017-09-24Accepted – Prihvaćeno: 2017-11-20Original Scientific Paper – Izvorni znanstveni radJ. X. Guo (e-mail:q.qguo@163.com), D. Q. Cang, University of Science and Technology Beijing, School of Metallurgical and Ecological Engi-neering, China. L. L. Zhang, University of Science and Technology Beijing, School of Energy and Environment Engineering, China. W. B. Dai, Beijing Building Materials Academy of Sciences Research (BBMA), State Key Laboratory of Solid Waste Reuse for Building Ma-terials (SKL-SWR), China. L. Y. Qi, Hebei University of Engineering, College of Energy and Envi-ronmental Engineering, China.Two density-separated Chinese coals and steel slag were prepared and then combusted in a modification furnace with different excess air ratio. Non-staged and Fuel-staged combustion experiments were compared under different pulverized coal mixing ratios. Thus, in the fuel-staged combustion experiments, the concentration of NOx in the flue gas was found to greatly decrease under a bitumite: anthracite mixing ratio of 1:1 and an air excess coefficient of 1,2 under the fuel-staged ratio is 15:85. The flue gas temperature was as high as 1 615ºC, while the NOx concentration in the flue gas was as low as 320 mg/m3. Compared with the other types of burners, the experimental combustion device designed herein efficiently reduced the NOx emissions ca. 80 %. Fuel-staged combustion has a significant effect on reducing particulate matter (PM) emissions, and a suitable coal ratio was also beneficial for reducing par-ticulate matter emissions. Key words: steel slag modification, NOx emission, particulate matter (PM), coal swirl combustionINTRODUCTIONSteel slag has become the main solid waste of the steel industry in China, with an annual production as high as 107 million tons (2016). However, since the content of free CaO (f-CaO) in steel slag is relative high, poor volume stability of steel slag will be caused, and not qualified [1, 2]. In addition, high content of me-tallic iron which leads to the poor grindability of steel slag, and its volume stability is not qualified [3]. Coal-based swirl combustion reforming furnace is a new type of devices allowing steel slag modification [4, 5]. The operation flow is as follows: pulverized coal and steel slag are first mixed, and then the amount of the mixed are controlled by the screw feeder, which enter into the furnace combustion from the primary outlet with air as a carrier. Currently, most of the researches in domestic and international on thermal modification of steel slag focus on using waste heat of steel slag to melt additive or modifying agent, in order to adjust the physical and chemical properties of steel slag. However, for other previously reported similar steel slag reforming devic-es, their thermal efficiency, NOx emission and the par-ticulate size distribution are not systematically reported up to now. It increasing its thermal efficiency and re-ISSN 0543-5846METABK 57(1-2) 47-50 (2018)UDC – UDK 669.14.666.88.168.2:661.99:669.661.183.2=111ducing NOx emissions are important prerequisites for its industrialization [6-8].EXPERIMENTAL FACILITY AND MATERIALSThe proximate and ultimate analysis of bitumite and anthracite are presented in Table 1. The composition of slag steel is also shown in Table 1.Table 1  Physical properties of the steel slag, bitumite, and anthracite materialsThe test projectBitumite AnthraciteProximate analysis /wt.%, as receivedMoisture 2 2,04Ash 12,02 15,48Volatile matter 32,23 8,5Fixed carbon 53,75 73,98Net calorifi c value /kJ/kg 29620 29440Ultimate analysis /wt.%, dry ash freeC 70,7 73,86H 4,43 3,01S 2,06 2,35N 1,39 1,1O 7,4 2,16Composition /wt. %, dry basis Steel slagCaO 36,8SiO2 15,4MgO 7,56Al2O3 0,87Fe2O3 17,43FeO 9,48MFe 3,43TFe 2348J. X. GUO et al.: NOx AND ULTRAFINE PARTICLE EMISSION CHARACTERISTICS IN A NEW STEEL SLAG... METALURGIJA 57 (2018) 1-2, 47-50ficient. When the ratio of bitumite to anthracite was 1:1 and the air excess coefficient was 1,2, the NOx concen-tration in the flue gas was relatively low. In the case of 1,2 and the fuel-staged ratio of 15:85, the flue gas tem-perature was 1 615 ºC, and the minimum NOx concen-tration in the flue gas was 320 mg/m3 (Figure 2b). The NOx concentration in the flue gas was minimum in the measured hole 6. The above results demonstrated that flue gas NOx concentration obtained herein was signifi-cantly lower than those obtained by other similar com-bustion devices.The burnout ratios of the non-staged experiments and the average values of the different fuel-staged burn-out ratios with different coal ratios are shown in Figure 3. As can be seen, the burnout ratios of the fuel-staged experiments were higher than those of their non-staged counterparts under various ratios. The burnout ratios obtained for both non-staged and fuel-staged experi-ments were equal or better than those achieved by other burners. Our previous studies have indicated that the emis-sion concentration of NOx, SO2 and particulate matter will have the lowest value when the fuel-staged ratio is 15:85. Therefore, the particulate matter is the object of the following part under the condition of different coal ratios and fuel-staged ratio of 15:85.It can be seen from Figure 4 that the concentration of particulate matter Experiments were conducted in a swirl one-dimen-sional, self-designed, down-fired coal combustor, with 240 mm ID (Inner Diameter) and 3,5 m height. The de-tailed descriptions of this facility as well as operation procedures were shown in Figure 1 and Table 2. The coal feed style was divided into a non-staged and fuel-staged. Non-staged feed means that all the coal and steel slag enter from the primary air outlet into the fur-nace. But for the fuel-staged feed, the coal and steel slag with a certain percentage separately enter from the pri-mary air outlet and the secondary outlet into the furnace combustion.Each gas-sampling hole was equipped with a sam-pling tube, and all sampling tubes were connected with the main sampling tube connected to a gas analyzer. By switching the valve between each sampling tube and the sampling main tube, the gas coming out each sampling hole can be individually analyzed. The measurements of the species present in the flue gas were carried out by a NOVA PLUS-S enhanced flue gas analyzer (German MRU Company). The aerosol monitor equipment (DustTRAKTM II) was used to measure the particle size of PM1, PM2.5, PM4, PM10 and inhalable particles produced during the combustion process.Table 2 The coal ratio of non-staged and fuel-stagedQuantity 100 % 3:2 1:1 2:3Non-staged a b c a b c a b c a b cFuel-staged a b c a b c a b c a b cNote:  a, b, c, represents the air excess coefficient of 1,4, 1,2, 1,0, re-spectively. 100 %, 3:2, 1:1, and 2:3 represent the ratio of bitumite and anthracite.RESULTS AND DISCUSSIONFigure 2 suggested that the concentration of NOx increased gradually with the decreased air excess coef-Figure 1 Schematic diagram of the combustion systemFigure 2  Variation of the concentration of NOx for each measured hole under different coal ratios and fuel-staged is 15:85. a) Air excess coefficient of 1,4. b) Air excess coefficient of 1,2. c) Air excess coefficient of 1,0.49J. X. GUO et al.: NOx AND ULTRAFINE PARTICLE EMISSION CHARACTERISTICS IN A NEW STEEL SLAG...METALURGIJA 57 (2018) 1-2, 47-50(PM1, PM2,5, PM4, and PM10) is not at the highest value under 100 % bitumite and fuel-staged combustion. At the same time, the concentration of particulate matter released with the fuel-staged combustion is lower than that of non-staged combustion under different air ex-cess coefficients. Visible fuel-staged combustion has a significant effect on reducing particulate emissions. On the one hand, the fuel-staged combustion can make the furnace temperature higher than the non-staged com-bustion, and high temperature can make the particles better melt and gather in the ash, and finally reduce fly ash particulate matter emissions. On the other hand, un-der high temperature condition, alkali metals and other elements are more volatile, form more submicron parti-cles smaller than 1 mm, and thus reduce the proportion of PM1, PM2.5, PM4, and PM10. When the coal ratio is 3:2, the emission concentration of each particle size is in low level, which indicates that the appropriate coal ratio is also beneficial to reduce the emission of particu-late matter.In the device between the Z1 and Z2 cross section, the main existence of the above two air flow state. As shown in Figure 5, air flow state is more conducive to hot experiments in mixing coal and prolonging burning time. This is beneficial to improve combustion efficien-cy and reduce pollutant emissions. At the same time, it can reduce the particulate matter emission.CONCLUSIONSWhen the ratio of bitumite to anthracite was 1:1 and the air excess coefficients were 1,4  and 1,2  under fuel-staged combustions, the NOx concentration in the flue gas was low. Under air excess coefficient of 1,2 and a fuel-staged ratio of 15:85, the flue gas temperature reached 1 615 ºC, while the minimum NOx concentra-tion in the flue gas was 320 mg/m3.The concentration of particulate matter under the condition of three excess air coefficients (1,4, 1,2, and 1,0) has the following relationship: PM10 > PM4 > PM2.5 > PM1, indicating coal particles are more likely to pro-duce large particulate matter during the swirling com-bustion process. 100 % bitumite combustion can be clearly seen PM10, PM4, PM2.5, PM1 emission concentra-tion higher than other coal ratio combustion particulate matter emission concentration. The difference of the particulate matter emission concentration under the oth-er three kinds of coal ratio (3:2, 1:1, and 2:3) and non-staged combustion conditions is small.REFERENCES[1] D. Q. Cang, Y. Li, W. B. Dai. An on - line modification device for hot slag [P]. Beijing, 2014, CN203794916U. [2] G. Z. Zhao, L. Yu, W. B. Dai, Preparation and processing parameter research of high basicity steel slag-based glass-ceramics with one-step sintering process, Chinese Journal of Engineering 38(2016)2, 207 - 212.Figure 3  Burnout ratios for the different coal ratios under non-staged and fuel-staged conditions Figure 4  Particulate matter concentrations under different coal ratios and fuel-staged ratio of 15:85 conditions. a) Air excess coefficient of 1,4. b) Air excess coefficient of 1,2. c) Air excess coefficient of 1,0.Figure 5  Air flow field from Z1 (measured hole 1) to Z2 (measured hole 4) cross-section50J. X. GUO et al.: NOx AND ULTRAFINE PARTICLE EMISSION CHARACTERISTICS IN A NEW STEEL SLAG... METALURGIJA 57 (2018) 1-2, 47-50[3] D. Peter, E. Andreas, K. Michael, Recent Development in Slag Treatment and Dust Recycling, Steel Research Inter-national 80 (2010)10, 737-745. [4] Q. Wang, P. Yan, J. Feng. A discussion on improving hydration activity of steel slag by altering its mineral com-positions, Journal of Hazardous Materials 186(2011) 2-3, 1070-1075.[5] X. B. Ai, H. Bai, L. H. Zhao, Thermodynamic analysis and formula optimization of steel slag-based ceramic materials by FACTsage software, International Journal of Minerals Metallurgy and Materials 20(2013), 379-385.[6] W. D. Fan, Y. Li, Q. H. Guo, Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal, Energy 125(2017), 417-426.[7] F. Ren, Z. Q. Li, G. K. Liu, Combustion and NOx emis-sions characteristics of a down-fired 660-MWe utility boi-ler retro-fitted with air surrounding fuel concept, Energy 36(2011), 70-77.[8] S. Zellagui, G. Trouve, C. Schonnenbeck, Parametric stu-dy on the particulate matter emissions during solid fuel combustion in a drop tube furnace, Fuel 189(2017), 358-368.Note: The responsibile for English language is the lecturer from University of Science and Technology, Beijing

NOx and ultrafine particle emission characteristics in a new steel slag modification device

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