Journal of Environmental Sciences 2011, 23(Supplement) S149–S152
Disposal of low concentration fume with solid waste modiﬁed by microwave Zhijun He1,∗, Yonglong Jin2 , Junhong Zhang1 , Jihui Liu1 , Zhigang Guan1 1. School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China. E-mail: [email protected]
2. Tangshan Iron and Steel Group Co., Ltd., Tangshan 063016, China
Abstract The feasibilities of two solid waste, smelting slag and ﬂy ash, as desulfurization and denitrogenation absorbents and additives by microwave were investigated. The inﬂuences of operating parameters were also studied. Under the optimal operating conditions, the removal eﬃciencies of 65.9% and 65.0% were achieved for SO2 and NOx respectively. Scanning Electron Microscope and energy dispersive spectroscopy were employed to study the micro-area characteristics of ﬂy ash, smelting slag absorbent and the spent absorbent. In addition, the mechanisms of simultaneous removal of SO2 and NOx were investigated. The microwave-modiﬁed absorbent in fact played the role of solid catalyst in the process of SO2 and NOx treatment from fume. SO2 and NOx were oxidized because of microwave-modiﬁed absorbent catalytic eﬀect. Key words: microwave; solid waste; low concentration; desulfurization and denitrogenation
Introduction In China, the main energy source conﬁguration was coal, which is consumed 1.2–1.3 billion tons, and 80% is used to combust directly as raw coal. SO2 and NOx that come from the fume of combustion coal are the main gas pollutant sources. As far there are many technologies about the desulfurization and denitrogenation in the high concentration fume. But there are two problems that are diﬃcult to solve in the low concentration fume. First are the characteristics in the low concentration fume, such as the wide distributing, great harm and disposing diﬃcultly. Second is the higher cost and less stabilization by utilizing the former methods of desulfurization and denitrogenation. The microwave as a new technology and research area to dispose the fume is paid widespread attentions. How to dispose the SO2 and NOx in the combusted coal boiler by high frequency electromagnetic wave has been studied (Kong and Cha, 1995, 1996; Cha, 1994, 1998; Cha and Kong, 1995; Cha and Carlisle, 2001). And the wave-carbon reduction technology without catalyst to remove SO2 was studied under the conditions of laboratory by Zhang et al. (1997). The research showed that the removal eﬃciency of desulfurization and denitrogenation was very high when the microwave was used continuously. Comparing with the traditional methods such as assimilating, adsorbing, catalyzing and other gas processing technology, treating ﬂue gas by microwave has a lot of distinct advantages, such as gas puriﬁcation eﬃciency is high without secondary pollution, processes and the equipment are simple. Because of the various technical advantages, removal of SO2 and NOx from ﬂue gas by microwave has become a * Corresponding author. E-mail: [email protected]
new breakthrough to treat the fume and the most promising new technology of ﬂue gas treatment. In this article, the method for low concentration fume (SO2 < 100 ppm, NOx < 100 ppm) is disposed with solid waste modiﬁed by microwave. The experiment conditions of aﬀecting the eﬃciency of desulphurization and denitrogenation are discussed and the reaction mechanism has been analyzed primarily.
1 Experiment 1.1 Absorbent preparation Smelting slag and ﬂy coal ash were mixed according to diﬀerent mass ratios, it was stirred evenly with water, made into balls with balling disc, then ﬁltrated by metal sieve with the diﬀerent particle diameters. The sample was put into the industry microwave and radiated for a certain time. 1.2 Equipment The experimental system was designed to dispose the low concentration fume by microwave-modiﬁed waste solid. The absorbent was used in experiment. The process is shown in Fig. 1. 1.3 Experiment On the condition of using microwave, the low SO2 and NOx concentration produced in the resistance furnace from the combustion of the coal was reacted and adsorbed with the absorbent in the microwave furnace. The concentrations of SO2 and NOx were measured at the entrance and exit by fume analysis equipment.
Journal of Environmental Sciences 2011, 23(Supplement) S149–S152 / Zhijun He et al.
Fig. 1 Schematic diagram of experiment progress. (1) blast system, (2) rotaty valve, (3) simulation gas system, (4) cooling device, (5) ﬂow control system, (6) gas analysis system, (7) industrial microwave furnace, (8) absorbent.
1.4 Calculation of disposing eﬃciency The desulfurization or denitrogenation eﬃciency (η, %) was calculated according to following equation.
C – C × 100% C
where, C (ppm) and C (ppm) are the gas concentrations of the reactor entrance and exit, respectively.
2 Results and discussion 2.1 Inﬂuence of gas ﬂow on the desulfurization and denitrogenation
70 NOx SO2
Removal efficiency (%)
2.2 Inﬂuence of the microwave power on the desulfurization and denitrogenation The experimental conditions are as follows: gas ﬂow 0.13 L/hr, mass ratio of smelting slag and ﬂy coal ash 2 to 1, mass of absorbent 0.5 kg, the diameter of absorbent particle between 4 and 8 mm. When the microwave power was changed only, the experimental results are as shown in Fig. 3. The desulfurization and denitrogenation eﬃciencies were raised with increasing microwave power. The reaction rate of the gas was very low when the microwave power was 132 W, from which the air and absorbent could not be reacted very well with the low microwave power. The disposal eﬃciencies of SO2 and NOx were reached 60% at the microwave power 528 W. Thus, the gas disposal eﬃciency can be raised when the microwave power was increased. Because the maximum power is 528 W in the experiment, the impact on desulphurization rate and denitrogenation rate of extensions to improve microwave power should be further investigated. 2.3 Inﬂuence of mass ratio of smelting slag and ﬂy coal ash on the desulfurization and denitrogenation The experimental conditions are as follows: gas ﬂow 0.13 L/hr, microwave power 528 W, mass of absorbent 0.5 kg, diameter of absorbent particle between 4 and 8 mm. When mass ratio smelting slag and ﬂy coal ash was changed only, the experimental results are as shown in Fig. 4. The desulfurization and denitrogenation eﬃciencies were raised with increasing smelting slag percent. The possible reasons were that the CaO content was high in the absorbent with the smelting slag percent increasing, and which has the better adsorbing ability to the SO2 and NOx. Figure 4 also shows that the disposal eﬃciencies of SO2 and NOx were changed a little when the mass ratio of smelting slag and ﬂy coal ash was 2 to 1. 2.4 Inﬂuence of absorbent particle diameter on the desulfurization and denitrogenation The experimental conditions are as follows: gas ﬂow 0.13 L/hr, microwave power 528 W, mass of absorbent 70 65 Removal efficiency (%)
In order to determine the inﬂuence of gas ﬂow on the desulfurization and denitrogenation, the experimental conditions were as follows: microwave power 528 W, the mass ratio of smelting slag and ﬂy coal ash 2 to 1, absorbent 0.5 kg, the diameter of absorbent particle varied from 4 to 8 mm. When the gas ﬂow was changed only, the experimental results are as shown in Fig. 2. Figure 2 shows that the disposal eﬃciencies of SO2 and NOx was very high when the gas ﬂow was very low (< 0.13 L/hr). The disposal eﬃciency was reduced as the gas ﬂow increasing, because the gas was kept longer time in the lays of the absorbent. In this way, it contracted suﬃciently with each other and reaction absolutely, gas reaction was very high on the condition of low gas ﬂow. But the gas was kept shorter time in the lays of the absorbent and a lot of gas was not reacted with absorbent in time and removed away. As a result, the disposal eﬃciency was reduced when the gas ﬂow was very high.
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Gas flow (L/hr) Inﬂuence of gas ﬂow on desulfurization and denitrogenation.
300 400 500 600 Microwave power(W) Fig. 3 Inﬂuence of microwave power on desulfurization and denitrogenation.
Disposal of low concentration fume with solid waste modiﬁed by microwave
Removal efficiency (%)
particle was from 4 to 8 mm. When absorbent particles mass was changed only, the experiment results are shown in Fig. 6. With increasing amount of absorbent, the gas disposal eﬃciencies were increased a little. The possible reason is that the surface area was raised with increasing absorbent mass, but it was restricted by the cubage of the reactor (the maximal load was 1.5 kg), thus the chance that the gas and the absorbent contracted was not increased greatly resulting in the disposal eﬃciencies were not increased apparently.
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1:1 2:1 3:1 Absorbent mixture ratio Fig. 4 Inﬂuence of absorbent mixture ratio on desulfurization and denitrogenation.
0.5 kg, mass ratio of smelting slag and ﬂy coal ash 2 to 1. When absorbent particle diameter was changed only, the experiment results are as shown in Fig. 5. The curve shows that the disposal eﬃciencies of the SO2 and NOx were changed little when the diameter of absorbent particle was both less than 4 mm and more than 8 mm. The possible reasons are the smaller particles diameter, the larger surface areas of the absorbent on the condition of certain mass. But the permeability was worse when the particles diameter changed smaller. As a result, the gas could not be contracted with absorbent well. While the particle diameter was in the range of 4 to 8 mm, the surface decreased respectively, but the permeability can be improved apparently, and the available surface areas of the reaction were not less than the particle with diameter less than 4 mm. The absorbent surface area that was touched with gas reduced a lot, therefore the desulfurization and denitrogenation eﬃciencies were decreased when the particle diameter was between 8 to 12 mm.
Figure 7 is the magniﬁed microstructure picture of the absorbent. Figure 7a shows that the surface of the particle was very smooth. Some holes of diﬀerent diameters were disturbed on the surface which did not go deeply into the inner. The ratio surface area of the absorbent was very small on this surface condition. The magniﬁed microstructure picture of the absorbent that had been microwave-modiﬁed is shown in Fig. 7b. The surface was coarse and uneven, lots of the closed holes was opened, which was changed into narrow sew and extend to inside, then the surface shape with larger the ratio surface area was formed under the condition of microwave-modiﬁed. The advantages were created for diﬀusing and chemistry adsorption owing to the larger surface.
2.5 Inﬂuence of absorbent mass on the desulfurization and denitrogenation The experimental conditions are as follows: gas ﬂow 0.13 L/hr, microwave power 528 W, mass ratio of smelting slag and ﬂy coal ash 2 to 1. The diameter of the absorbent
3.1 Microstructure change of microwave-modiﬁed absorbent
3.2 Microstructure change of microwave-modiﬁed absorbent reacted with fume Figure 8a is the magniﬁed microstructure picture of the absorbent reacted with fume which had not been microwave-modiﬁed. From the picture it can be seen that the particles was joined one another with few holes and narrow sew. There were glue curdle shape matter that was collected on the surface of the particles, and the holes of the absorbent surface had been covered by the reaction production, which prevented the SO2 and NOx to diﬀuse into the inner of the absorbent.
70 70 65
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