The effects of fly ash particle size, furnace temperature, oxygen content and trace element properties on the occurrence and volatilization characteristics of trace elements were analyzed. According to the occurrence and volatilization characteristics of trace elements, the elements are divided into three categories: the first type of elements are Hg; the second type of elements are Pb, Zn and Cd; the third type of elements are Mn.Se are between the first type of elements and Between the second type of elements; Cr simultaneously exhibits the properties of the first and third types of elements. It is found that except for Mn, the enrichment of other elements in fly ash increases with the decrease of fly ash particle size; fly ash has different adsorption mechanism for various trace elements; the increase of furnace temperature can promote the volatilization of some trace elements. In addition to Mn and Cr, the relative enrichment coefficient of other elements in fly ash is greater than that of bottom slag; low oxygen content does not promote the volatilization of all trace elements; the classification of trace elements should be determined on a case-by-case basis.
Coal is a complex mixture of aromatic clusters of fused ring organic compounds and mineral impurities. To date, humans have discovered 84 elements from coal, including many trace elements enriched in organic matter and minerals. Although these trace elements are rare (generally less than 100 g/g), environmental pollution caused by them has become an objective fact. There is information that the amount of heavy metals in the soil far exceeds the limits set by the US Environmental Protection Agency (EPA) within a few kilometers or even hundreds of kilometers around a US power plant.
Using data from a joint investigation conducted by Western Europe, the United States, Canada, and the former Soviet Union in 1983, it can be found that the annual total emissions of As caused by the burning of As from the coal-burning factor in the coal-burning process have been polluting the trace elements in the coal-burning process. Control has become a new frontier. In 1990, the Clean Air Amendment issued by the United States required estimates of toxic element emissions from fossil fuel power plants and the development of viable pollution reduction measures. Domestic emission standards for pollutants in agricultural fly ash (GB8173-87) and pollution control standards for domestic waste incineration (GB18485-2001) have also included trace element emission standards. With the maturity of acid gas control methods, the control of trace elements is also on the agenda. Many scholars in China have conducted research on this. However, from the two aspects of bottom slag and fly ash, the complete research on the trace element storage and volatilization characteristics and the classification of trace elements in industrial pulverized coal fired boilers is rare. In this paper, the content of 8 trace elements in raw coal, bottom slag and fly ash was quantitatively determined on a 220t/h pulverized coal fired furnace. The volatility of trace elements themselves, fly ash particle size and combustion conditions were analyzed. The influence of trace elements on the occurrence and volatilization characteristics, and the distribution of trace elements in the combustion products were calculated by means of mass balance. Finally, the elements were classified according to the trace element storage and volatilization characteristics.
1 Test part 1.1 Boiler equipment specification and coal burning characteristics No. 6 boiler of Yangzi Petrochemical Thermal Power Plant is a 220t/h high-pressure natural circulation solid-state slag pulverized coal boiler, and the furnace is square. The burner is a DC oscillating burner with a regular four-corner distribution. The imaginary cut circle size is 800mm and rotates counterclockwise. 8 primary air outlets and 12 secondary air outlets. The secondary air adopts the customary waist-shaped air distribution method, that is, the upper and lower layers have a large amount of air, especially the upper layer has the largest amount of air, and the middle layer has a small amount of air. Under the rated load, the coal consumption coal test coal is Jiangsu Xuzhou bituminous coal, and the particle size distribution of the incoming coal is measured by HYDAO2000 laser particle size analyzer, and the average particle size is 35.1 m.
1.2 Before the test condition test, the primary and secondary wind speeds were calibrated by standard pitot tube. After checking, the actual wind speed was higher than the meter wind speed by 3m/s. Industrial analysis from the surrounding of the furnace wall, elemental analysis, full moisture and dry basis Moisture air dry basis volatiles fixed carbon ash coke slag characteristics high heat generation low heat generation 7.55 non-melt bonding RAYNGERMX4TM optical meter to calibrate the furnace temperature, adjust the air supply volume and the feeder speed to make the furnace horizontal temperature roughly balanced. Simultaneous calibration of the online zirconia oxygen analyzer between the superheater and the economizer using the austenometer and the TESTO360 flue gas precision analyzer from TESTO, Germany. After checking, the online oxygen analyzer was found to be lower than the actual value. In the test, first adjust the feeder speed and secondary wind speed, keep the furnace temperature substantially unchanged (up and down floating does not exceed 20), get 3 different excess air coefficients; secondly, adjust the feeder speed and secondary wind speed, keep the superheater The excess air ratio between the economizer and the economizer is constant (the upper and lower errors are not more than 0.01), and three different furnace temperatures are obtained. The test was carried out at a common load of 200220 t/h. The exhaust gas temperature is 1391.3. The sample collection time is 23h for each working condition. To ensure the consistency of the sample, the coal sample, bottom slag and fly ash sample are each retained for 1 hour. The coal sample is taken from the sampling port below the pulverized coal separator. The bottom slag sample is obtained by leaching the ash water to be precipitated and then removing and removing the slag. The fly ash sampler is used to collect three electric field fly ash at the bottom of the electrostatic precipitator. The electric field mixed ash is mixed by the electric field 1, 2, 3 ash according to a certain ratio, and then obtained by the quarter method. According to the design parameters of the original electrostatic precipitator and some empirical values â€‹â€‹during the operation, the weight ratio of the ash collected by the electric fields 1, 2, and 3 is about 80155. After taking the sample, it is stored in a sealed container to prevent water dispersion and dust intrusion.
1.4 Sample test methods and results Trace elements were measured: Se and Cd were determined by graphite furnace atomic absorption spectrometry; Hg was determined by cold atomic absorption spectrophotometer; Mn, Cr, Pb, As, and Zn were determined by X-ray fluorescence spectrometer. At the same time, the ash content in the coal, fly ash and bottom slag was measured.
2.1 The availability of fly ash resources can be found that some trace elements in fly ash (such as Pb, Se, Hg) are much larger than the average Taylor abundance of the element in the earth's crust, especially the abundance of Se.Se in the earth's crust. Small, belonging to discrete elements, without separate minerals. Therefore, in addition to the potential hazard, the fly ash of the dust collector can also be used as a material source for extracting trace elements. The magnetic elements and particle separation can be used to further enrich the trace elements to achieve commercial application. In Japan, the extraction of elements such as Al, Pb, and Zn from waste incineration ash has attracted great attention.
2.2 Relative Enrichment Coefficient In order to express the occurrence and volatilization tendency of trace elements in fly ash and bottom slag, different scholars have proposed their respective relative enrichment coefficients. The geochemical enrichment factor K is used to express the state of distribution of the elements. The expression is: C and C respectively represent the i element and Fe content in the ash; C respectively represents the content of i element and Fe element in the coal. The representation method is based on the content of Fe element in coal and ash as a reference standard. Cenni et al. used the relative enrichment factor B relative to the bottom ash to represent the i element content in the fly ash and bottom slag, respectively. This representation avoids errors due to the difficulty in determining trace elements in coal. The relative enrichment coefficient (E) proposed by Meij is E, which means the hollow coal ash of raw coal. The relative enrichment factor only considers the influence of ash content in the raw coal on the enrichment factor, and does not consider the ash in the fly ash and bottom slag. The effect of changes on the enrichment factor.
In this paper, the improved Meij relative enrichment coefficient is used. The expression is that the gray-like hollow dry ash =1 means that the trace elements are neither enriched nor depleted in the fly ash or bottom slag; E>1 means that the trace elements are There is a tendency for enrichment in fly ash or bottom slag; E < 1 indicates a tendency for trace elements to be depleted in fly ash or bottom slag.
Relative Enrichment Coefficient of Trace Elements in Bottom Slag and Relative Enrichment Coefficient of Trace Elements in Fly Ash E Jin Baosheng et al.: Part of trace elements in pulverized coal furnace characteristics sample sample number furnace temperature excess air coefficient (empty After pre-measurement) trace element type bottom slag relative enrichment coefficient E electric field mixed ash relative enrichment factor E Note: The content of As in most samples is lower than the limit value measured by X-ray fluorescence spectrometer (1g/g), ignoring As Relative enrichment factor.
2.3 The influence of fly ash particle size on enrichment factor, except for Mn, other trace elements have a tendency to enrich in small particles, and the smaller the fly ash particle size, the larger the enrichment coefficient. The authors believe that Mn may be present in some stable high melting point (melting point 1200) coarse crystalline particles in this test, showing a tendency to enrich in large particles.
The average particle size of the collected fly ash is getting smaller and smaller, and the specific surface area is getting larger and larger. According to the principle of physical adsorption, most heavy metal elements are enriched on fine ash particles. The adsorption mechanism of trace elements in the combustion process is studied. It is considered that the volatile trace elements are difficult to chemically react with the minerals in the ash to form stable compounds, and the adsorption is mainly physical adsorption. Since mercury is mainly present in the form of elemental mercury, mercuric chloride, mercurous chloride, and oxidized mercury at the exhaust gas temperature, and the partial pressure of these substances is lower than the pressure required for condensation in the flue gas, it exists in a gaseous form. In flue gas, gas-solid physical adsorption is different from solid-solid physical adsorption, and the content of Hg in fly ash is not as obvious as other volatile components. Cenni believes that Hg in fly ash is due to the unburned charcoal in the fly ash and the fly ash with high specific surface activity adsorbing Hg. The content of element Cr in fly ash does not change significantly with particle size, indicating that chemisorption dominates. When the temperature of smoke is lower than 500K, Cr mainly exists in Cr and reacts with CaO in ash: adsorption of Cr, physical adsorption and chemical adsorption Very important. When the smoke temperature is lower than 800K, the main form of Pb is PbCl: PbCl believes that the following reaction exists in the flue gas: 3CaO As and Ca content are consistent.
2.4 The effect of temperature on the relative enrichment factor. Under the condition that the excess air coefficient is basically unchanged, the relative enrichment coefficients of Cr, Zn and Hg in the electric field ash increase with the increase of the furnace temperature. Due to the limitations of combustion conditions science and technology, the range of furnace temperature variation is too narrow, and the influence of furnace temperature on bottom slag is not obvious.
2.5 Comparison of the influence of excess air coefficient on the relative enrichment factor It can be seen that when the furnace temperature is basically constant, the reduction of oxygen will promote the volatilization of some trace elements (such as Pb, Se and Hg). The lower the excess air coefficient, the higher the relative enrichment coefficient of Pb, Se and Hg in the fly ash, and the lower the relative enrichment coefficient in the bottom slag, while other elements do not follow this rule.
Therefore, hypoxic conditions do not increase the volatilization of all trace elements, and the mechanism remains to be further studied. YanRong et al. believe that the reducing atmosphere can increase the volatilization of minerals in coal, but the volatilization of trace elements depends on the nature of trace elements and on the characteristics of coal. Not all elements can be reduced in a reducing atmosphere. Increase the amount of evaporation. Ouyang Zhonghua also believes that high temperature and reducing atmosphere will promote the evaporation of some elements, but not all elements. This phenomenon is related to the nature of specific elements and substances in the surrounding environment, and it is difficult to explain only from the total amount of elements.
2.6 The effect of the element's own properties on the relative enrichment factor. Comparing the relative enrichment coefficients of trace elements in the bottom slag and fly ash, it can be found that the relative enrichment coefficients of other elements in fly ash are greater than Mn and Cr. Bottom slag. Some of the volatile minerals (such as K and Na salts) in the bottom slag are re-condensed in the fly ash, resulting in a slight enrichment of Mn and Cr in the bottom slag relative to the fly ash. However, the relative enrichment coefficients of the two are different, the relative enrichment coefficient of Mn is close to 1, and the relative enrichment coefficient of Cr is much less than 1. The reason is that some Cr in the coal exists in the metal organic state (such as Cr), this part of organic Cr It precipitates along with the volatiles, and then in the process of cooling with the flue gas, the same kind of nucleation occurs, becoming submicron particles, and discharging directly from the chimney in the form of an aerosol. Both Constance and Stanislav believe that some of the Cr in the coal studied is related to organic matter. It can be found from Table 3 that although the boiling point of Pb is 1750 higher than the boiling point of Cd (769), the relative enrichment factor of Pb in fly ash is Higher than Cd, the relative enrichment factor of Pb in the bottom slag is lower than Cd.
This can be explained by the fact that the melting point of Pb to the active atom Cl in the flue gas is greater than the melting point (950) is much smaller than the melting point of CdO (1500), so Pb is more volatile than Cd under the same combustion conditions. Both Stanislav and YanRong have found that Pb is easier to form chloride than Cd.
2.7 Classification of trace elements The coal-burning products are divided into bottom slag, dust collector fly ash and into the atmosphere. The trace elements follow the conservation of mass: where M is the total amount of i elements in coal; M is the fly ash i The amount of element; the amount of element i in the bottom slag; M is the amount of element i entering the atmosphere.
Because it is difficult to obtain the distribution law of total ash content in large-scale power plants, based on the original boiler design value and some empirical values, it is assumed that the fly ash in the bottom slag, dust collector fly ash and flue gas is distributed into the boiler according to the mass ratio of 10891. Ash. When studying the distribution characteristics of Hg in the combustion products of 300MW pulverized coal boilers, the selected fly ash and bottom slag shares are basically the same as the assumptions in this paper. When studying the dust removal efficiency of the electrostatic precipitator, it is recommended that the ratio of the fly ash entering the dust collector to the total ash content should be 0.850.95. Use R to indicate the fly ash, bottom slag and discharge into the atmosphere. The i element accounts for the total i element in the coal. The percentage of the amount.
Now select typical samples 0-1, 1-4 and 2-4, and calculate the distribution of each trace element in the combustion products as follows: Some trace elements are negatively distributed in the smoke, which may be present in the elemental measurement. The error, or the calculation, exaggerated the amount of fly ash from the dust collector, ignoring other ash distribution (such as the bottom of the economizer, the dust on the equipment wall, the chimney dust, etc.). All trace elements are mainly distributed in the fly ash and gas phase of the electrostatic precipitator. Since the amount of fly ash in the pulverized coal furnace is much larger than the amount of bottom slag, the fraction of trace elements in the fly ash is much larger than the fraction of trace elements in the bottom slag.
1) Elements that are extremely volatile and are mainly emitted in a gaseous state (such as Hg), exhibiting E and R > 80%; 2) elements that are volatile and mostly condensed in fly ash (such as Pb, Zn, Cd), E and R>80%; 3) elements that are less volatile and have a relative enrichment coefficient in the bottom slag slightly larger than the relative enrichment factor in the fly ash (such as Mn), showing that Eb.Se is between the first element and the first Between the two types of elements. Both Cenni and YanRong classify Cr as the third element, but in this study, part of Cr is related to organic matter in coal, and exhibits special properties in terms of relative enrichment factor and percentage distribution of combustion products. The nature of the elemental elements (R>60%) also has the third type of elemental properties. Therefore, the author believes that the classification of Cr should be based on its existence in coal. If most of the Cr is related to inorganic minerals, then Cr belongs to the first For three types of elements, if a considerable part of Cr is related to organic matter, then Cr should belong to a special type of element.
Occurrence characteristics of some trace elements in pulverized coal furnaces Because As is low in coal and ash samples, the data obtained cannot be derived from which type of elements. Clements classifies As in the second type of element between the second type of element and the third type of element, which also indicates that the element classification should be determined by the specific characteristics of the element, combustion conditions, coal type and furnace type.
3 Conclusions 1) All elements except Mn have a tendency to be enriched in fine fly ash particles, and the smaller the fly ash particle size, the larger the relative enrichment factor. Some trace elements (such as Pb, Se) have a large enrichment in the fly ash relative to the average Taylor abundance of the crust, indicating the potential hazard of fly ash and the resource availability of fly ash.
2) The adsorption of Cd, Zn, Se and Hg by fly ash is mainly physical adsorption, and the adsorption of Cr is mainly chemical adsorption, while the adsorption of Pb and As, physical adsorption and chemical adsorption are all important.
3) The excess air coefficient is basically unchanged. The higher the furnace temperature, the higher the relative enrichment coefficient of Cr, Zn and Hg in fly ash; the furnace temperature is basically unchanged, the excess air coefficient is lower, Pb, Se and Hg are in fly ash. The higher the relative enrichment coefficient, the lower the relative enrichment coefficient in the bottom slag, but this rule is not true for all trace elements.
4) As some volatile minerals re-condense in the fly ash, the relative enrichment coefficient of Mn and Cr in the bottom slag is higher than the relative enrichment factor of the elements in the fly ash. The relative enrichment coefficients of other elements in the fly ash are higher than the relative enrichment coefficients of the elements in the bottom slag.
5) Although the boiling point of Pb is higher than Cd, the relative enrichment coefficient of Pb in the bottom slag is lower than Cd due to the affinity of Pb to the active atom Cl in the flue gas. The relative enrichment coefficient of Pb in fly ash is higher than Cd.
6) 8 kinds of elements except As can not get enough data due to low content, the other 7 elements can be divided into 3 categories according to their volatility: the first type of elements: Hg; the second type of elements: Pb, Zn and Cd; Class element: Mn; Se is between the first type element and the second type element, and Cr exhibits the first class and the third type element property at the same time. There is no absolute limit to the classification of elements, and it should be determined by the specific characteristics of the element characteristics, combustion conditions, coal type and furnace type.
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