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Application of DCL type sulfur fixation technology in large-scale high-pressure pulverized coal boilers Author: Summary by Li Guangkeng of Guangzhou Petrochemical Plant: Through the optimization of sulfur-fixing agent delivery and injection facilities and the reduction of boiler SO2 under different Ca/S molar ratios and different injection methods, it is confirmed that the DCL coal-fired sulfur-fixing agent is a new technology with low investment, small footprint and high efficiency, and can be used in 220t/h high-pressure pulverized coal boilers. keywords: Sulfur fixing agent ； Conveying and injection device ； sulfur dioxide ； The amount reduction DCL type coal-fired sulfur-fixing agent desulfurization technology is * * The “Ninth Five-Year Plan” research project assigned by the State Environmental Protection Administration to the Chinese Academy of Sciences has been approved * * According to the results of the Science and Technology Commission's appraisal, an industrial test operation was carried out on the 130t/h boiler of Angang No. 1 Power Plant in September 1999 and was successful. When the Ca/S molar ratio is between 2.0 and 2.5, the SO2 emission reduction is stable between 59% and 66%. Sinopec Guangzhou Petrochemical General Plant and Guangzhou Yueshou Industrial Co., Ltd. cooperated to carry out a DCL type sulfur-fixing agent demonstration project in the 4# boiler of the Guangzhou Petrochemical General Plant's self-owned thermal power station (WGZ220/100-13 high-pressure, single drum, natural circulation, solid slagging pulverized coal furnace). Advanced technologies were adopted from the sulfur-fixing agent storage, transportation, and injection processes. Good results were achieved in the trial operation and the SO2 emission reduction rate was achieved for the results appraisal. When the sulfur content of coal combustion is 0.8% and Ca/S is 2.0 and 2.5, the SO2 emission reduction rates reach 57.9% and 65.5% respectively. 1. Basic principles of sulfur fixation technology Coal is called low, medium and high sulfur coal according to different sulfur content. Sulfur exists in coal in different forms such as inorganic sulfur (elementary sulfur, sulfate), organic sulfur (mercaptans, sulfur bonds, heterocyclic sulfur), etc., and generates sulfur oxides SOx during the combustion process. SOx is discharged into the atmosphere and undergoes photochemical reactions to form sulfuric acid mist, which then forms acid rain, seriously harming the ecological environment. Sulfur fixing technology refers to the method of adding sulfur fixing agents to desulfurize in a certain temperature area of the boiler combustion process, so that the SOx generated during the combustion process is immediately converted into MeSOx and fixed in the slag and coal ash without being discharged into the atmosphere. The principle is as follows: SO2＋MeO＋1/20 → MeSO4 However, this reaction equation does not express the complex mechanism and various conditional factors of the reaction. Combustible sulfur generates SOx after combustion. When the boiler excess air coefficient = 2 and the combustion chamber outlet temperature = 1100°C, most of it is SO2, and SO3 is only 3% to 5%, which increases as the excess air and combustion chamber outlet temperature increase. If CaO is used to absorb SO2, the absorption rate is very low. When the excess air is 2% to 3%, only a small part of the generated CaSO3 is oxidized into stable CaSO4, and most of it is decomposed into CaO and SO2. CaO absorbs SO3 much better. Its absorption rate depends on the activity of CaO (specific surface and activation center). For example, if a certain amount of catalyst is added to CaO for activation treatment, the absorption rate of SO3 by CaO will be * * Improved. The DCL type sulfur fixing agent uses this principle to add a certain amount of additives to MeO, introducing the concept of catalysis into sulfur fixing. By improving the adsorption surface and activity of the sulfur fixing agent, it simultaneously converts SO2 into SO3, or converts CaSO3 into CaSO4, thereby increasing the sulfur fixation rate of CaO. * * To improve the efficiency, the DCL type sulfur fixing agent is different from the traditional sulfur fixing agent that is mixed into raw coal, but is sprayed onto the upper part of the boiler combustion center. The sulfur fixing agent directly reacts with SOx in the flue gas, causing no adverse effects on combustion and little impact on boiler efficiency. 2. The basic process of injecting DCL type sulfur-fixing agent into the furnace is in accordance with the technical requirements of DCL-type sulfur-fixing agent. The optimal reaction temperature with flue gas is 950℃~1050℃. The position of the sulfur-fixing agent nozzle is determined at the 23.1m front wall fire hole, which is in front of the boiler steering chamber and 3m below the center point of the screen superheater flue gas inlet. When the boiler load is 170t/h, the temperature there measured by Ajima infrared thermal imaging is 1045℃~1075℃, so there is no need to modify the boiler at all. DCL sulfur fixing agent physical properties: bulk density: 1.00～1.15t/m3 fineness: ≤200 mesh appearance: Moisture content of gray powder: ≤5% According to the characteristics of the sulfur-fixing agent that is susceptible to moisture and has strong adhesion, when considering the design of the process system, we try to simplify the system and optimize the process, and consider that no secondary pollution will occur in the entire system, and there must be a certain degree of self-control. The entire process flow is shown in Figure 1. Figure 1 The DCL type sulfur-fixing agent delivery and injection process flow is shown in Figure 1. The 100m3 storage tank is a steel structure storage silo. The storage silo is equipped with automatically heated air fluidization facilities based on the physical properties of the DCL-type sulfur-fixing agent. The high and low material levels are made of American DE company products. A bag dust collector is installed on the top of the tank to handle the power air source for bulk vehicle materials entering the tank. The conveying capacity of the transmitter is 6~8t/h, and the gas gray level is 1: 39. The 22-story small silo is 2m3×2, and is also equipped with material level gauges, heating plates and bag dust collectors from DE Company of the United States. The injection part includes Roots blower, screw feeder (with frequency conversion speed regulation), and nozzle. The sending system adopts PLC program control: The material level of the small silo is low → the transmitter is started → the sulfur-fixing agent is transported to the upper silo → the material level of the upper silo is high → the transmitter is stopped → the purge conveying system is performed. The entire system occupies an area of about 50m2, and includes a reserved location for the second furnace. In order to ensure that the nozzle will not be clogged, it is set to jump to the corresponding feeder when the Roots machine is stopped under any condition. The Roots blower is out of service, the system is set in program-controlled state, and the feeder cannot be started. All other parameters can be displayed on the dial, and the feeder speed can be adjusted manually to adjust the feeding amount. The sulfur-fixing agent nozzle is inserted from the two hand holes at the boiler elevation of 23.1m. The nozzle is made of heat-resistant alloy steel and the inner tube steel is 1Gr18Ni9Ti. The boosting air uses the hot air sent from the boiler's hot air duct, and the air-to-material ratio is <3kg/m3. In order to ensure that the sulfur-fixing agent can spread quickly and evenly in the furnace, the nozzle has been tested in a cold state before leaving the factory to ensure sufficient swirl intensity, so that the material leaving the nozzle can quickly spread and form a reflow area to ensure that the sulfur-fixing agent spreads evenly. Many foreign advanced patented technologies are used in the system. The energy consumption of the entire system is shown in Tables 1 and 2. Table 1 Electricity consumption table of conveying and injection equipment Table 2 Gas consumption of conveying and injection equipment Table 3. Several key issues in the application of sulfur-fixing agents in boilers 1. Selecting the area where the sulfur-fixing agent is injected into the furnace Laboratory high-temperature sulfur fixation shows that temperature is very important for sulfur fixation efficiency. When the temperature exceeds 1200°C, SO2 is re-released, and the sulfur fixation rate is close to zero. This is because: The diffusion rate of SO2 through the protective layer covering the surface of CaSO4 increases with increasing temperature ； Another reason may be that when the catalyst in the sulfur fixing agent fails, the MeSO3 generated by SO2 and MeO decomposes into SO2 and MeO. Judging from the test results of furnace #4, the second type may be larger. In the first test of furnace 4, the nozzle was installed at 20.8m, and the temperature of the injection area was higher than 1200°C. The data showed that the SO2 in the outlet flue gas was 5% to 10% higher than the SO2 in the flue gas without spraying the sulfur fixing agent. When the nozzle is installed to 23.1m, (the measured temperatures are all 1045°C) the sulfur fixation efficiency is significantly improved. 2. Choose the appropriate calcium-sulfur ratio. Different sulfur-fixing agent additions have different sulfur-fixing effects, which depend on the sulfur content of raw coal and the amount of coal burned. It can be obtained by the calcium-sulfur molar ratio.: M=A×B×X/0.27 where: M--Injection amount of sulfur-fixing agent t/h A--Coal consumption of boiler per hour t/h B--Sulfur content in coal %: M=28×0.8%×2/0.27=1.66t/h Setting different Ca/S molar ratios depends on which sulfur-fixing agent is injected into the furnace to achieve the best sulfur-fixing effect. It must take into account safety, economy, requirements for reducing SO2 emissions, etc., and is determined by an optimization method. When the Ca/S molar ratio of Guangzhou Petrochemical's 220t/h boiler was used in trial operation at 2 and 2.5, the SO2 reduction rate measured by the Guangzhou Environmental Protection Science Institute is shown in Table 3. Table 3 Monitoring results of sulfur-fixing effect of DCL type coal-fired sulfur-fixing agent Note: Monitoring by the Guangzhou Institute of Environmental Protection Sciences needs to explain that after the sulfur-fixing agent is sprayed into the furnace, it is eventually captured by the electrostatic precipitator together with the fly ash. The Ca/S molar ratio cannot be increased indefinitely, otherwise, the increase in smoke concentration before electrostatic precipitator will affect the normal operation of electrostatic precipitator. When the electrostatic precipitator is operating at full load, the smoke and dust emission concentration at the electrostatic precipitator outlet still exceeds the standard. According to the test results of smoke concentration and dust removal efficiency before and after electrostatic precipitation after the sulfur fixing agent was put into the 4# furnace, when the Ca/S molar ratio is below 2.5, the dust removal efficiency and outlet smoke concentration change little. 3. The influence of nozzle boosting air on desulfurization efficiency. Due to the different structures of various boilers, the designed coal type and the actual coal type are different, and the temperature field distribution of the boiler is different. The final determined injection position of the sulfur fixing agent for Guangzhou Petrochemical No. 4 furnace is 23.1m. The structure inside the furnace is shown in Figure 2. Figure 2 Position of the sulfur fixing agent nozzle. When the secondary air speed of the nozzle was too high, the SO2 emission reduction rate was less than 35%. When the secondary air opening was adjusted and the wind speed was reduced, the SO2 reduction rate rose to more than 55%. In different boiler applications, the adjustment of the wind speed of the nozzle's boost air (secondary air) is very important. 4. Desulfurization Cost Analysis The current sulfur-fixing agent provided by the supplier is 200 yuan per ton. According to the SO2 reduction rate when the Ca/S molar ratio is 2.0 and 2.5, and based on the theoretical calculation of the sulfur content and coal consumption of raw coal during the test of the 4# boiler of Guangzhou Petrochemical Power Division, when the Ca/S molar ratio is 2.0, the cost of reducing the SO2 sulfur-fixing agent per ton is 1,509 yuan. ； When the Ca/S molar ratio is 2.5, the cost of the sulfur fixing agent is 1,687 yuan, and the power generation cost per kW·h increases by approximately 0.006 yuan to 0.008 yuan. The losses caused by SO2 emissions to the environment are far greater than this number, and * * The cost of reducing one ton of SO2 is lower than that of foreign desulfurization facilities. DCL coal-fired sulfur-fixing agent technology has the advantages of low investment, small footprint, high efficiency, and low operating costs. It has little impact on the safety and reliability of boiler operation and is very suitable for my country's national conditions. DCL-type sulfur-fixing agent sulfur-fixing technology is a feasible high-tech solution for the "three simultaneous" environmental protection of boilers in newly built enterprises, the reconstruction and expansion of existing enterprises, and the reduction of the total SO2 emissions of enterprises.