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At present, foreign countries have carried out research on the use of adsorbents to trap heavy metals such as lead, Uberoi et al. have studied the long-term adsorption reaction between several adsorbents and lead, and proposed the chemical reaction equation between kaolin and PbCl2; Owens et al. studied the capture of lead by spraying silicon-based vapor adsorbent precursors into small tubular reactors, but the particle size of the product particles is small, which is not conducive to being removed by dust removal equipment; Scotto, Linak, Daivs et al. studied the trapping of heavy metals such as lead by spraying solid-phase granular adsorbents into large combustion furnaces, and the results showed that the particle size distribution of lead will be significantly shifted from the sub-micron range to the micron range.
Domestic research on the use of adsorbents to control heavy metals such as lead has also been carried out, and such research has several characteristics: (1) mainly focuses on the coal-burning process; (2) the adsorbent is added in a mixed way with fuel; (3) the redistribution of heavy metals after adding adsorbents is mostly expressed in bottom ash and fly ash. Since fly ash contains a wide range of particle sizes, and at present, people are most concerned about air emission pollution PM10, and the detailed distribution information of heavy metals in PM10 after adding adsorbents is more important. In addition, coal contains many minerals and heavy metal elements, and these influencing factors are complex and interacting, and it is difficult to judge the effect of specific adsorbents in capturing specific heavy metal elements.
In this study, under the condition of combustion gas fuel without the influence of other minerals, the capture of lead by three solid-phase granular adsorbents on the typical semi-volatile heavy metal was investigated, and the adsorbent was directly sprayed from the side of the furnace into the high-temperature region of the furnace to react with lead. Based on the sampling of the product particles according to the aerodynamic particle size classification, the redistribution of lead elements with particle size after spraying into the adsorbent is given. The effect of three adsorbents on lead capture, the effect of different feed amounts in kaolin, and the influence of chlorine on lead capture in kaolin were studied.
Experimental system
The experimental study used a one-dimensional furnace test bench with an effective height of 3.4 m and an inner diameter of 150 mm. In the experiment, an adsorbent injection system was added, which was driven by the motor to push the adsorbent in the needle tube up, and at the same time the compressed air entered the needle tube to locally fluidize the upper particles, and the fluidized adsorbent was sprayed into the one-dimensional furnace furnace with the compressed air through the pipeline. In the experiment, the motor speed was adjusted to keep the distance between the upper surface of the adsorbent and the needle inlet constant, and at the same time the needle tube was in a vibrating state, so that the adsorbent entered the furnace stably and evenly.
Two classes of adsorbents were used in the experiment: the first was two analytically pure chemical reagents alumina (Al2O3) and calcium hydroxide (Ca(OH)2); The second category is a natural mineral kaolin (Al2O3·2Si2O·2H2O). The main chemical composition of the three sorbents was measured using X-ray fluorescence spectroscopy (XRF) and the results are shown in Table 1. The average particle size of the adsorbent was measured by laser particle size analyzer, and the specific surface area of the samples after the experimental high temperature of the three adsorbents was measured by the nitrogen automatic adsorption analyzer (BET), and the results are shown in Table 2. The average particle size of the three adsorbents used in the experiment is 6~9μm, so the choice is mainly because the small particle size particles of the same mass will provide more adsorption surface to enhance the adsorption effect, but the particle size can not be too small, otherwise the product particles are still submicron and are not easy to be captured by dust removal equipment.
Table 1
Table 2
After the one-dimensional furnace test bench is started, after 6h operation adjustment to reach the experimental working conditions, the flow rate of liquefied petroleum gas is 0.8m3/h (standard state), the negative pressure of the furnace outlet is 100Pa, and the oxygen content is 3.5%. The flow rate of lead acetate solution is 10mL/min, the feeding amount of lead element is 0.017mol/h, and the feeding amount of chlorine element is 0.116mol/h. In the experiment, the adsorbent was sprayed into the furnace from the side hole of the first section of the furnace body of the one-dimensional furnace, and the temperature of the spray point was 1473K.
After the experimental conditions were stabilized, the ThermoAndersen 8-stage particle impactor was used to conduct momentum sampling of particles at the lower end of the first-stage casing cooler in strict accordance with the US Environmental Protection Agency (EPA) standard method for fixed-source particle sampling. The sampling membrane is a dedicated glass fiber filter supplied by ThermoAndersen. The sampling flow rate is 19.8L/min, and the sampling port temperature is 403K. At this time, the aerodynamic cutting diameter d50 of the 8-stage impactor stage is 12.63, 7.88, 5.32, 3.65, 2.37, 1.18, 0.72 and 0.48 μm.
Sample analysis
Samples collected at each stage of the Andersen impactor are weighed using an electronic balance with a resolution of 0.1 mg. In order to obtain the redistribution of lead elements at all stages of the impactor after spraying the adsorbent, the sampling membrane and particulate matter were digested by the four-stage gradient temperature method of the MARS5 microwave digester with reference to EPAMethod3051 in the United States, and the solution to be measured was entered into an inductively coupled plasma emission spectrometer (ICP-AES) to measure the content of lead. It should be noted that the lead contained in the adsorbent itself and the lead contained in the glass fiber membrane may affect the results, and researchers have taken this into account and given measurement data for trace elements in the adsorbent. In this experiment, the content of lead in adsorbent and glass fiber was measured by the same method, and the results showed that the lead content was below 8μg/g, and the blank content accounted for up to a few thousandths of the total sample lead content, so the influence of lead content in the background could be ignored.
Capture of lead by different sorbents
Three adsorbents, alumina, calcium hydroxide and kaolin, were sprayed into the furnace at 60g/h from 1473K to capture lead, and the lead elements were redistributed at all levels of the Andersen impactor under the influence of the adsorbent. Figure 1 shows the distribution of lead elements in three groups of particle sizes, that is, the part less than PM0.5, the part larger than PM0.5 less than PM1, and the part larger than PM1 less than PM13, it can be clearly seen from the figure that after adding the adsorbent, the three adsorbents all make the lead element transfer from the submicron range to the ultra-micron range, which indicates that the three adsorbents are effective for lead trapping, and the effect of kaolin is better than that of alumina and calcium hydroxide.
Figure 1
For the redistribution of lead, the mass concentration distribution of lead shown in dM/dlogDP-DP coordinates shown in Figure 2 is used. After adding alumina and calcium hydroxide, the mass concentration of lead element showed a clear bimodal distribution, with two peaks roughly bounded by 1 μm, one in the submicron range and one in the micron range. The formation of these two peaks has different mechanisms: for submicron peaks, according to the direct weighing mass of particles collected by the impactor 6~8 (submicron range) and the lead element mass measured by the impactor 6~8 ICP-AES, it can be confirmed that almost all the particles collected by the impactor 6~8 (submicron range) are lead particles.
This part of lead is not captured by the adsorbent, but is formed by the gas phase lead directly through nucleation, coagulation and agglomeration, and still exists in the form of PbO particle clusters; The micron peak does not exist before the adsorbent is sprayed, and it appears due to the capture of fumed lead by alumina and calcium hydroxide with a particle size of the micron range, which will exist on the adsorbent in the form of physical adsorption and chemical adsorption. After the addition of kaolin, the lead changes from a submicron unimodal distribution to a micron unimodal distribution, and this part of the lead exists on kaolin in the form of physical adsorption and chemical adsorption. From the adsorption effect, alumina is close to calcium hydroxide, while kaolin is significantly better than the first two.
Researchers believe that the physical adsorption and chemical adsorption of metal vapors by general adsorbents exist simultaneously. Physical adsorption relies on the van der Waals force between the surface of the adsorbent and the lead vapor molecules, and has the characteristics of non-selectivity, fast adsorption speed, and the number of adsorption layers can be single or multi-layer; Chemical adsorption relies on the chemical bond force between the adsorbent surface and lead vapor molecules, which has the characteristics of high selectivity and the number of adsorption layers as a single layer, and the metal vapor reacts chemically with the active sites (SiO2, Al2O3, CaO, etc.) on the outer surface and internal pore surface of the adsorbent at high temperature to form stable crystals or vitreous bodies.
Table 2 shows that the specific surface area of kaolin is smaller than that of the other two adsorbents when adsorbing lead vapor. Alumina and calcium hydroxide can provide more physical adsorption surface and chemisorption active site to trap lead, but their adsorption effect is much lower than that of kaolin with a small specific surface area, which indicates that in the process of adsorption and capture of lead vapor, it is more important to provide an adsorbent with the appropriate type of active site than to provide an adsorbent with a larger specific surface area, that is, chemical adsorption is more important than physical adsorption.
Figure 2
Kaolin exhibits good performance in capturing lead, providing the right active site for chemical reaction with lead vapor despite its small specific surface area. Uberoi et al. conducted XRD analysis of the adsorption reaction products of kaolin and PbCl2 vapor on a thermogravimetric reactor for 2h, found PbAl2Si2O8 substances, and proposed the reaction pathway Al2O3⋅2SiO2+PbCl2+H2O→ PbO⋅Al2O3⋅2SiO2+2HCl(1), of which Al2O3·2Si2O is variable kaolin, that is, the phase transition product of kaolin after dehydration at high temperature. Since lead will exist in the form of PbO vapor at high temperatures, granular kaolin sprayed into the high temperature region will chemically react with PbO vapor in a short period of time, assuming that the product generated by the reaction is PbAl2Si2O8 proposed by Uberoi et al
Al2O3·2Si2O produced by the phase transition of kaolin dehydration provides an efficient active site for the capture of PbO vapor molecules, which makes the capture of PbO by kaolin better than that of alumina and calcium hydroxide.
Effect of kaolin feed amount on lead capture
Kaolin is injected from 1473K into the furnace at three feed volumes of 10, 20 and 60 g/h to capture lead. Figure 3 shows the distribution of lead elements in the three groups of particle sizes under different feed amounts of kaolin, and when the feed amount is 10g/h, there are very few lead elements transferred to the ultra-micron range (impactor grade 1~5). When the feed amount is 20g/h, about 60% of the lead element is still in the submicron range; At a feed volume of 60 g/h, most of the lead trap is transferred to the micron range.
Figure 3
Figure 4 shows the mass concentration distribution of lead in dM/dlogDP-DP coordinates. According to the previous analysis, lead in the submicron range is not captured by the adsorbent; Lead in the micron range is captured by adsorbents. Therefore, the three feed amounts of kaolin reflect three typical capture conditions: (1) lead is not captured, mainly in the submicron range; (2) some lead is trapped, with a bimodal distribution of submicron and micron; (3) most lead is trapped, mainly in the micron range. When the feed amount was 10g/h, only a small amount of lead was adsorbed by kaolin, which may be related to the small number of Al2O3·2Si2O active sites. At a feeding rate of 60g/h, the number of Al2O3·2Si2O active sites increased greatly, so that kaolin could efficiently capture most of the lead elements and transfer them to the micron range.
Figure 4
Effect of chlorine on lead capture in kaolin
The effect of chlorine on lead capture in kaolin was also examined. Kaolin is injected into the furnace from 1473K at a feed volume of 60g/h, and the concentration of chlorine is 100μL/L. The distribution of lead in the three groups of particle sizes at this time is shown in Figure 5. In the presence of chlorine, kaolin reduces the mass fraction of lead in the submicron range (impactor grade 6~8), while the mass fraction of lead in the ultramicron range (impactor grade 1~5) increases, which has a certain effect on lead capture, but compared with the capture of lead in kaolin without chlorine, more lead is retained in the submicron range, that is, it is not effectively captured by kaolin.
Figure 5
Figure 6 shows the mass concentration distribution of lead in dM/dlogDP-DP coordinates. Due to the presence of chlorine, the mass concentration of lead has changed from the original micron single-peak distribution to an obvious bimodal distribution, and the two peaks are also roughly bounded by 1 μm, one in the submicron range and one in the micron range. Submicron peaks are formed by direct nucleation, condensation and agglomeration of gas-phase lead that has not been captured by kaolin, and this part of the lead will exist in the form of PbCl2 particle clusters. The micron peak is formed by lead captured by kaolin, but this part of the lead is less than the lead captured by kaolin without chlorine, indicating that the addition of chlorine has a great inhibitory effect on lead capture in kaolin.
Since lead mainly exists in the form of gas phase PbCl2 in the presence of chlorine, the chemical adsorption reaction between Al2O3·2Si2O and PbCl2 vapor in kaolin active site Al2O3·2Si2O in equation (1) will be enhanced, while the chemical adsorption reaction between Al2O3·2Si2O and PbO vapor in formula (2) will be greatly weakened, but the experimental results show that the reactivity of Al2O3·2Si2O and PbCl2 is weaker than that of Al2O3·2Si2O and PbO, so the formula ( 2) The chemisorption reaction represented is the key to effective lead trapping in kaolin.
Figure 6
Based on the aerodynamic particle size classification method, the capture of lead by alumina, calcium hydroxide and kaolin was investigated without the influence of other minerals, and the redistribution of lead elements according to aerodynamic particle size was obtained. (2) The experimental data show that the lead distributed in the submicron range is not captured by the adsorbent, and the gaseous lead is directly formed by nucleation, condensation and agglomeration; Lead distributed in the micron range is formed by the capture of micron-sized adsorbents, which are present on the adsorbent in the form of physical adsorption and chemical adsorption.
(3) Alumina, calcium hydroxide and kaolin are all effective in the capture of lead, but the effect of kaolin is significantly better than the first two. Although physical adsorption and chemical adsorption exist at the same time when the adsorbent captures lead, chemical adsorption is more important than physical adsorption, and kaolin has a stronger chemical adsorption effect on lead than alumina and calcium hydroxide. (4) Different feed amounts of kaolin have a great influence on the redistribution of lead. When the feeding amount is 10g/h, most of the lead is distributed in the submicron range, when the feeding amount is 20g/h, the lead shows an obvious bimodal distribution of submicron and micron, and most of the lead capture is transferred to the micron range when the feeding amount is 60g/h.
(5) When chlorine is present, kaolin has a strong inhibitory effect on lead capture, and lead changes from micron unimodal distribution to submicron and micron bimodal distribution, which is likely to be because the reactivity of Al2O3·2Si2O and PbCl2 at the kaolin active site is much lower than that of Al2O3·2Si2O and PbO.