A flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier and its preparation method and application

Abstract

The present invention relates to the technical field of solid waste resource utilization, and specifically to an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash, a preparation method and an application thereof. Calculated by dry basis mass percentage, the raw materials of the components of the modifier include 90% of steel slag, 1%-3% of rare earth tailings and 7%-9% of electric furnace phosphorus slag. The modifier prepared by the present invention can realize in-furnace modification of flue gas desulfurization circulating fluidized bed boiler ash, and the obtained modified ash has low water demand, SO3 content of less than 3.0%, and a strength activity index increased to more than 90%. The modifier of the present invention also has combustion-supporting and desulfurization and denitrification functions, can reduce combustion temperature, improve desulfurization and denitrification effects, and reduce the load of the flue gas desulfurization system. The raw materials of the present invention are all bulk solid waste and do not contain carbonate. The modifier itself does not decompose during use and does not emit CO2. It is green, low-carbon and environmentally friendly, and helps to promote ultra-low emissions of circulating fluidized bed boiler coal-fired power generation technology with significant economic effects.

Description

Technical Field

[0001] The present invention relates to the technical field of solid waste resource utilization, and in particular to a flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier, a preparation method and application thereof. Background Art

[0002] Circulating fluidized bed boiler coal-fired power generation technology is considered a clean coal-fired power generation technology. Currently, circulating fluidized bed boiler coal-fired power generation mostly adopts a desulfurization process that incorporates limestone desulfurizer into the furnace, resulting in the discharged ash having high CaO and SO3 contents. The CaO and SO3 in the ash generally exist in the form of undecomposed limestone, active CaO and anhydrite (CaSO4), making the circulating fluidized bed boiler coal-fired ash generally water-absorbent, self-hardening and expansive, which is significantly different from traditional coal-fired ash.

[0003] Therefore, the use of circulating fluidized bed (CFB) boiler ash in cement concrete can lead to problems such as high water demand, low activity, and poor long-term volume stability. Currently, annual emissions of CFB boiler ash (sulfur-binding ash) are estimated to exceed 200 million tons, and its comprehensive utilization rate is far lower than that of fly ash and bottom ash from conventional pulverized coal furnaces. This has led to CFB boiler ash becoming the third major industrial solid waste after tailings and fly ash. To address the challenges of comprehensive utilization of CFB boiler ash, many power companies have begun to phase out desulfurization processes that use limestone desulfurizers in separate furnaces. Instead, they are opting for a dual-process approach combining furnace desulfurization with flue gas desulfurization (FGD), or directly using FGD. This overcomes the heat absorption caused by the decomposition of traditional limestone desulfurizers in the furnace, further reducing combustion temperatures. Furthermore, the active CaO and anhydrite content in the ash are significantly reduced, preventing long-term volume stability problems when used in cement and concrete.

[0004] However, due to the decrease in combustion temperature, the pulverized coal burnout rate decreases, the ash water absorption and ignition loss both increase, and the activity decreases; in addition, due to the lack of in-furnace desulfurization, the SO2 content in the flue gas emitted after combustion increases significantly, the desulfurization system load increases, the desulfurization efficiency decreases, and the amount of desulfurization gypsum produced increases significantly; therefore, it is considered to design an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash, which can increase the pulverized coal burnout rate and reduce the SO2 content in the flue gas without the need for the desulfurizer to decompose and absorb heat and produce active CaO, without increasing the combustion temperature and under the premise that the SO3 in the ash is controllable, thereby achieving the purpose of reducing the ash water demand and ignition loss and improving the activity; in view of this, we propose an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash, as well as its preparation method and application. Summary of the Invention

[0005] The purpose of the present invention is to solve the deficiencies mentioned in the above background technology and to provide a flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier and its preparation method and application.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier, wherein the raw materials of the modifier components include 90% steel slag, 1%-3% rare earth tailings and 7%-9% electric furnace phosphorus slag, calculated by dry basis mass percentage.

[0008] Preferably, the mass percentage of CaO in the modifier is ≥40%, and the total mass percentage of CaO and MgO is ≥50%.

[0009] Preferably, the sum of the mass percentages of the four oxides CaO, MgO, Fe2O3 and SiO2 in the steel slag is ≥90%;

[0010] The SiO2 mass percentage is less than 12%;

[0011] The CaO mass percentage is greater than 45%, and the total mass percentage of CaO and MgO is greater than 52%;

[0012] The mass percentage of Fe2O3 is greater than 15% and less than 20%.

[0013] Preferably, the rare earth tailings are feldspar rare earth ore grinding tailings, wherein the K2O mass percentage is 3%-5%, F - The mass percentage is 0.2%-0.3%, and the mass percentage of rare earth oxide La2O3 is 0.2%-0.3%.

[0014] Preferably, the total mass percentage of the four oxides CaO, MgO, Al2O3 and SiO2 in the electric furnace phosphorus slag is ≥90%;

[0015] The SiO2 mass percentage is less than 35%;

[0016] The CaO mass percentage is greater than 48%, and the total mass percentage of CaO and MgO is greater than 50%;

[0017] The mass percentage of P2O5 is greater than 2.0% and less than 3.0%.

[0018] A method for preparing an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash, the preparation method specifically comprising the following steps:

[0019] Step 1: Weigh various raw materials according to mass percentage;

[0020] Step 2: Mix the weighed raw materials and transfer them to a tubular ball mill;

[0021] Step 3: Spray the activation agent and start grinding;

[0022] Step 4: Grind until the ash reaches the standard and then store it in the warehouse to obtain the flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier.

[0023] Preferably, the amount of the activation aid accounts for 1% of the total mass of the ground raw material;

[0024] Calculated by mass percentage, the activation aid includes: 25%-30% phosphoric acid, 15%-20% ethanol, 50% water and 3%-5% triethanolamine.

[0025] Preferably, the compliance condition in step 4 includes: grinding the raw material until the particles pass through a 300 μm sieve.

[0026] Preferably, the modifier is mixed with pulverized coal and then sprayed into a circulating fluidized bed boiler for combustion.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The raw materials used in the modifier of the present invention, steel slag, rare earth tailings and electric furnace phosphorus slag, are all bulk solid wastes. They can overcome the problem of heat absorption caused by the decomposition of limestone desulfurizer in traditional circulating fluidized bed boilers. At the same time, the desulfurization process in the furnace basically does not emit CO2, practicing the green and low-carbon concept.

[0029] 2. The modifier contains F, P2O5, rare earth elements, etc. brought into the raw materials, which have combustion catalytic and denitrification catalytic effects. It reduces the combustion temperature of pulverized coal and improves the burnout rate, thereby reducing the ash loss on ignition and the reduction of unburned carbon particles. It will also reduce the water demand of the ash, making it easier for SO2 to enter the silicate mineral lattice, fixing sulfur and improving the activity of the ash;

[0030] 3. The pulverized coal of the present invention can maintain a high burnout rate at a relatively low temperature, reduce the SO2 content in the flue gas, reduce the load of the flue gas desulfurization system, reduce the ignition loss and water demand of the ash, but greatly improve the activity, which is conducive to further improving the application value and usage of the ash in the field of cement and other building materials.

[0031] 4. The present invention provides a practical in-furnace modification technology and means for circulating fluidized bed boiler ash, which will greatly improve the value and comprehensive utilization rate of the ash; the boiler operating temperature is low, and the ash modification can also fix part of SO2 and catalyze the conversion of nitrogen oxides into nitrogen, so that the modifier has multiple functions such as ash activation, sulfur fixation, and denitrification, and is a green and low-carbon technology suitable for flue gas desulfurization circulating fluidized bed boilers. DETAILED DESCRIPTION

[0032] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. In the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0033] The present invention describes the above technical solution in detail through the following embodiments:

[0034] Example 1

[0035] This embodiment provides a modifier, which is prepared based on the flue gas desulfurization circulating fluidized bed boiler ash furnace. The modifier is mixed with multiple raw materials. The multiple raw materials, calculated by weight percentage on a dry basis, include 90% steel slag, 1% rare earth tailings, and 9% electric furnace phosphorus slag. The CaO mass percentage of the modifier prepared by the present invention is ≥40%, and the CaO+MgO mass percentage is ≥50%.

[0036] In this embodiment, steel slag is solid waste generated in the steelmaking process, in which the total mass percentage of the four oxides CaO, MgO, Fe2O3, and SiO2 accounts for ≥90%; the total mass percentage of CaO+MgO is greater than 52%, and the mass percentage of CaO is greater than 45%; it should be explained that the higher the CaO and MgO content in the steel slag, the higher the theoretical absorption and fixation of SO2, the greater the amount of SO2 entering the silicate mineral and iron phase solid solution, and the higher the activity of the ash.

[0037] The total content of CaO and MgO in the steel slag of the present invention is higher than the CaO content of limestone, a desulfurizer in a conventional circulating fluidized bed boiler furnace. The CaO and MgO content in the steel slag is high and basically exists in the form of silicate, iron phase solid solution, and free calcium oxide. It is strongly alkaline and can quickly react with acidic SO2 gas to generate calcium sulfate. At the same time, the silicate minerals and iron phase solid solution in the steel slag are cement clinker-like minerals, which can dissolve sulfur in the mineral lattice, achieving desulfurization on the one hand and increasing the clinker mineral lattice defects on the other hand, achieving mineral activation and improving the slag activity. In addition, after desulfurization, steel slag forms anhydrite and solid solution minerals, and the ash basically does not contain active CaO, which reduces the water demand of the ash; the molecular weight of MgO is lower than that of CaO, that is, for the same mass content of CaO and MgO, MgO can absorb more mass of SO2; the mass percentage of SiO2 in steel slag is less than 12%; due to the presence of SiO2, CaO and MgO in steel slag will combine with SiO2 to form dicalcium silicate, tricalcium silicate and magnesium silicate minerals. The above minerals are relatively stable at high temperatures, and the barrier of SiO2 entering the mineral lattice is increased; if the SiO2 content is too high, the content of free calcium oxide and iron phase solid solution with higher desulfurization activity will be reduced, the steel slag minerals cannot be activated, and the ash activity is reduced.

[0038] The mass percentage of Fe2O3 in steel slag is greater than 15% and less than 20%; if the iron content is too low, the iron phase solid solution content will decrease, and the reaction product of the modifier and SO2 will mainly be anhydrite, which has extremely low activity; if the iron content is too high, the amount of SO2 entering the mineral lattice will increase, and the SO3 in the ash will exceed the limit, making it difficult to utilize the ash.

[0039] In this embodiment, the rare earth tailings are feldspar-type rare earth ore grinding tailings, containing potassium, F, and rare earth elements. The phosphorus slag in the rare earth tailings contains phosphorus oxides. Rare earth elements generally exist in feldspar, carbonate, and granite minerals in the form of fluorides. Fluorine is a commonly used flux and mineralizer in the high-temperature kiln industry. In cement clinker production, F can lower the low eutectic point of tricalcium silicate minerals, allowing cement clinker to be fired at a lower temperature. At the same time, when F and P coexist, they can also activate clinker minerals and enhance hydration activity. Rare earth elements have stronger combustion-supporting, catalytic, fluxing, and mineralizing effects, and can improve combustion efficiency and promote the formation of target minerals in the trace range. Therefore, the introduction of rare earth tailings can provide F and trace rare earth elements, which, on the one hand, promotes combustion; on the other hand, it can promote the absorption of SO2 by free calcium oxide, calcium silicate minerals, and iron phase solid solution; thirdly, it can also activate coal ash and steel slag minerals and enhance ash activity.

[0040] In this embodiment, electric furnace phosphorus slag is solid waste discharged from the process of preparing yellow phosphorus by electric furnace method using phosphate rock, wherein the total mass percentage of the four oxides CaO, MgO, Al2O3, and SO2 is ≥90%; the mass percentage of SO2 is <35%; the mass percentage of CaO+MgO is >50%, and the mass percentage of CaO is >48%; the mass percentage of P2O5 is >2.0% and <3.0%; the phosphorus slag has a high CaO content and mostly forms a glass with SiO2, mostly existing in the form of calcium silicate glass, with a low softening point, and has a desulfurization effect and can absorb SO2. Phosphorus slag is also a commonly used sintering aid in the cement clinker production industry; generally, when F and P coexist, cement clinker minerals can be formed at low temperature, and the clinker mineral lattice is activated and the activity is enhanced; the present invention mainly utilizes the phosphorus content of electric furnace phosphorus slag, and cooperates with F and rare earth in rare earth tailings to achieve the effect of improving coal powder combustion and ash activation.

[0041] Specifically, the preparation method of the flue gas desulfurization circulating fluidized bed boiler ash furnace modifier in this embodiment includes the following steps:

[0042] First, various raw materials are weighed according to the percentage of material mass, and 90% of steel slag, 1% of rare earth tailings and 9% of electric furnace phosphorus slag are weighed for use; the raw materials weighed according to the ratio are mixed and ground, and transported to the grinding system for mixed grinding. During the grinding, an activation auxiliary agent accounting for 1% of the total mass of the ground raw materials is sprayed. After grinding until the standards are met, the raw materials are discharged from the mill and stored to obtain a flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier.

[0043] It should be noted that the activation aid in this application comprises, by mass percentage, 25% phosphoric acid, 20% ethanol, 50% water and 5% triethanolamine; phosphoric acid is a strong acid, and the steel slag and phosphorus slag particles of the present invention are relatively dense, and it is difficult for the solid phase of calcium-rich mineral CaO inside the particles to melt out, so the addition of phosphoric acid can erode the particles and promote the dissolution of CaO, while making the dense particles loose and porous, increasing the contact area between the particles and SO2 and SO2 adsorption, thereby improving the sulfur fixation efficiency; triethanolamine and ethanol have a grinding aid effect, which can improve the grinding efficiency of the modifier and the particle dispersion effect, and both can also burn to provide heat; ethanol and water are both diluents and dispersants, which can buffer the strong acidity of phosphoric acid and reduce the corrosion of phosphoric acid on the grinding equipment.

[0044] In this embodiment, the mixed raw materials are ground until all the particles pass through a 300 μm sieve and then discharged from the mill and stored to obtain a flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier; the limestone particle size of the circulating fluidized bed boiler in-furnace desulfurizer is generally required to be 150 μm-500 μm, and the limestone of this particle size level can be suspended in the boiler furnace and has a suitable residence time; the modifier of the present invention is different from the in-furnace desulfurizer. The modifier has desulfurization ability, but its core purpose is to modify the ash, so the modifier of the present invention only limits the maximum particle size of the particles to no more than 300 μm; in addition, steel slag particles are dense and their density is much higher than that of limestone. If the particle size is too high, most of the modifier particles will easily settle to the boiler roadbed, and the fly ash modification effect will be poor.

[0045] During the use of the modifier of the present invention, no decomposition endothermic reaction occurs, and a solid phase reaction mainly occurs, which is mostly an exothermic process, and can further improve the combustion efficiency and thermal efficiency of the circulating fluidized bed boiler.

[0046] The application scheme of this embodiment is specifically as follows: testing the sulfur content of pulverized coal, calculating the amount of modifier according to a Ca / S molar ratio of 0.5:1; weighing pulverized coal and modifier according to the calculated amount ratio, mixing and spraying them into a circulating fluidized bed boiler for combustion.

[0047] Example 2

[0048] 90% of steel slag, 2% of rare earth tailings and 8% of electric furnace phosphorus slag are weighed according to the dry basis weight percentage of the materials; after passing through the iron removal device, they are transported to the grinding system for mixed grinding, and an activation aid accounting for 1% of the weight of the raw materials entering the mill is sprayed during grinding. The materials are ground until all particles pass through a 300μm sieve and then discharged from the mill and stored to obtain an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash.

[0049] Example 3

[0050] 90% of steel slag, 3% of rare earth tailings and 7% of electric furnace phosphorus slag are weighed according to the dry basis weight percentage of the materials; after passing through the iron removal device, they are transported to the grinding system for mixed grinding, and an activation aid accounting for 1% of the weight of the raw materials entering the mill is sprayed during grinding. The materials are ground until all particles pass through a 300μm sieve and then discharged from the mill and stored to obtain an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash.

[0051] Comparative Example 1

[0052] In this comparative example, no modifier was added, and only pulverized coal with a sulfur content of 1.4% was used as fuel.

[0053] Specifically, the modifiers prepared in Examples 1 to 3 were set aside, pulverized coal with a sulfur content of 1.4% was selected as fuel, and the amount of the modifier was calculated according to a Ca / S molar ratio of 0.5:1; pulverized coal and the flue gas desulfurization circulating fluidized bed boiler ash furnace modifier prepared in Examples 1 to 3 were weighed according to the calculated dosage ratio, and the mixture was sprayed into a circulating fluidized bed boiler equipped with a flue gas desulfurization system, and the combustion temperature was controlled at 750°C-820°C.

[0054] Comparing the SO2 content data of flue gas with and without modifier, the SO2 concentration of flue gas before entering the desulfurization system is shown in Table 1.

[0055] Table 1:

[0056]

[0057] The physical and chemical properties of the circulating fluidized bed boiler ash after the modifier was applied to Examples 1-3 were tested. The collected circulating fluidized bed boiler fly ash and bottom ash were ground separately to a fineness (45 μm square mesh residue) of ≤12%. The ash was then analyzed for SO₃ content, free calcium oxide content, loss on ignition, water requirement ratio, and 28-day activity index, in accordance with GB / T1596 "Fly Ash for Cement and Concrete." The SO₃ content, free calcium oxide content, loss on ignition, and strength activity index are shown in Table 2.

[0058] Table 2: Properties of modified circulating fluidized bed boiler ash

[0059]

[0060] As can be seen from the data in Table 1 above, the flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier prepared by the present invention has a certain desulfurization and sulfur fixation effect. Compared with the control group without the modifier, the SO2 content in the flue gas is reduced by more than 15%, which will greatly reduce the pressure on the flue gas desulfurization system.

[0061] As shown in Table 2, the addition of the modifier increases the sulfur trioxide content in both fly ash and bottom ash. This is due to the modifier's desulfurization function. The free calcium oxide content in the ash remains relatively low because the modifier contains no limestone and the amount added is below the Ca / S molar ratio required for complete desulfurization. The loss on ignition of the ash is significantly lower than that of the ash without the modifier. This is due to the modifier's combustion-supporting and combustion-catalytic effects, resulting in a high burnout rate for the pulverized coal. The water requirement of the modified ash is significantly reduced, primarily due to lower carbon residue and the lack of active CaO production, which reduces water demand. The addition of the modifier significantly increases the ash strength activity index. Firstly, SO2 enters the aluminosilicate lattice, activating the minerals and increasing hydration activity. Second, the high burnout rate of the pulverized coal and the relatively low combustion temperature ensure sufficient activation and decomposition of the aluminosilicate minerals in the pulverized coal.

[0062] According to this scheme, after using the modifier, the water demand ratio of the modified ash is ≤105%, the SO3 content is <3.0%, and the strength activity index is >90%.

[0063] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier, characterized by: Calculated by mass percentage, the raw materials of the components of the modifier include 90% steel slag, 1%-3% rare earth tailings and 7%-9% electric furnace phosphorus slag; The CaO content of the modifier is ≥40% by mass, and the total content of CaO and MgO is ≥50% by mass; The sum of the mass percentages of the four oxides CaO, MgO, Fe2O3, and SiO2 in the steel slag is ≥90%; the mass percentage of SiO2 is <12%, the mass percentage of CaO is >45%, the total mass percentage of CaO and MgO is >52%, and the mass percentage of Fe2O3 is >15% and <20%; The rare earth tailings are made from feldspar rare earth ore grinding tailings, in which the K2O mass percentage is 3%-5%, F - Mass percentage content 0.2%-0.3%, rare earth oxide La2O3 mass percentage content 0.2%-0.3%; The total mass percentage of the four oxides CaO, MgO, Al2O3, and SiO2 in the electric furnace phosphorus slag is ≥90%, the mass percentage of P2O5 is >2.0% and <3.0%; the mass percentage of SiO2 is <35%, the mass percentage of CaO is >48%, and the total mass percentage of CaO and MgO is >50%.

2. A method for preparing an in-furnace modifier for flue gas desulfurization circulating fluidized bed boiler ash, suitable for the modifier described in claim 1, characterized in that: The preparation method specifically comprises the following steps: Step 1: Weigh various raw materials according to mass percentage; Step 2: Mix the weighed raw materials and transfer them to a tubular ball mill; Step 3: Spray the activation agent and start grinding; Step 4: Grind until the ash reaches the standard and then store it in the warehouse to obtain the flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier.

3. The method for preparing the flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier according to claim 2, characterized in that: The amount of the activation aid is 1% of the total mass of the ground raw materials; Calculated by mass percentage, the activation aid includes: 25%-30% phosphoric acid, 15%-20% ethanol, 50% water and 3%-5% triethanolamine.

4. The method for preparing the flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier according to claim 2, characterized in that: The compliance conditions in step 4 include: grinding the raw materials until the particles pass through a 300 μm sieve.

5. An application of a flue gas desulfurization circulating fluidized bed boiler ash in-furnace modifier, applicable to the modifier described in claim 1, characterized in that: The modifier is used to be mixed with coal powder and then sprayed into a circulating fluidized bed boiler for combustion.

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