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Application of DCL-type sulfur fixation technology in large high-pressure coal-fired boilers Author: Li Guangkeng, Guangzhou Petrochemical Complex Abstract: By optimizing the systems for transporting and spraying the sulfur fixation agent, and by examining the amount of SO2 reduction achieved under different Ca/S molar ratios and various spraying methods, it was confirmed that the DCL-type coal-fired sulfur fixation agent is a new technology that requires less investment, occupies less space, and offers high efficiency; it can be used in 220t/h high-pressure coal-fired boilers. Keywords: sulfur fixative ； Conveying and spraying device ； sulfur dioxide ； The reduction amount: The DCL-type coal-fired sulfur fixation agent desulfurization technology is a research project assigned by the State Environmental Protection Administration to the Chinese Academy of Sciences as part of the “Ninth Five-Year Plan”. It has passed the evaluation by the Science and Technology Commission; industrial trial operations were conducted in September 1999 on a 130 t/h boiler at Ansteel’s First Power Plant, with successful results. When the Ca/S molar ratio was between 2.0 and 2.5, the SO2 emission reduction rate remained stable at 59% to 66%. Sinopec Guangzhou Petrochemical Complex, in collaboration with Guangzhou Yueshou Industrial Co., Ltd., carried out a demonstration project using the DCL type sulfur-fixing agent on Boiler No. 4 of the complex’s own thermal power plant (a WGZ220/100-13 type high-pressure, single-drum, natural-circulation, solid-ash-discharge coal-fired boiler). Advanced technologies were employed in the areas of sulfur-fixing agent storage, transportation, and spraying, and good results were achieved during the trial operation, meeting the SO2 emission reduction targets set for the project evaluation. When the sulfur content in coal was 0.8%, with Ca/S ratios of 2.0 and 2.5, the SO2 emission reduction rates reached 57.9% and 65.5%, respectively. I. Basic principles of sulfur fixation technology Coal is classified as low-sulfur, medium-sulfur, and high-sulfur coal based on its sulfur content. Sulfur in coal exists in various forms such as inorganic sulfur (elemental sulfur, sulfates) and organic sulfur (thiols, sulfur bonds, heterocyclic sulfur), and sulfur oxides SOx are generated during combustion. SOx emissions into the atmosphere undergo photochemical reactions to form sulfuric acid mist, which in turn leads to acid rain, causing severe damage to the ecological environment. Sulfur fixation technology refers to a method of removing sulfur by adding sulfur-fixing agents in certain temperature zones during the boiler combustion process, thereby converting the SOx generated during combustion into MeSOx immediately, which is then fixed in the slag and ash rather than being released into the atmosphere. The principle behind this is: SO2 + MeO + 1/2O2 → MeSO4; however, this reaction equation does not reflect the complex mechanisms involved in the reaction nor various conditional factors. Combustion of combustible sulfur produces SOx; when the excess air coefficient of the boiler is 2 and the temperature at the exit of the combustion chamber is 1100°C, the majority is SO2, with SO3 accounting for only 3%–5%, and this proportion increases as the excess air and the temperature at the exit of the combustion chamber rise. If CaO is used to absorb SO2, the absorption rate is very low. When the excess air is 2%–3%, only a small portion of the generated CaSO3 is oxidized to stable CaSO4, while most of it decomposes into CaO and SO2. CaO, on the other hand, has a much better ability to absorb SO3; its absorption rate depends on the activity of CaO (specific surface area and activation centers). By adding a certain amount of catalyst to CaO to activate it, its absorption rate of SO3 **increases**. The DCL-type sulfur fixation agent utilizes this principle by adding a certain amount of additives to MeO, thereby introducing the concept of catalysis into the sulfur fixation process. It enhances the adsorption surface area and activity of the sulfur fixation agent, converts SO2 into SO3, or transforms CaSO3 into CaSO4, as a result of which the sulfur fixation efficiency of CaO is increased. Unlike traditional sulfur fixation agents that are mixed into raw coal, the DCL-type agent is sprayed above the combustion zone of the boiler; it reacts directly with the SOx in the flue gas, without having any adverse effects on combustion and with minimal impact on the boiler’s efficiency. II. Basic process for in-furnace injection of DCL-type sulfur fixation agent: According to the technical requirements of the DCL-type sulfur fixation agent, the optimal reaction temperature with flue gas is 950°C to 1050°C. The position of the sulfur-fixing agent nozzle was determined at the viewing hole on the front wall at 23.1 m, which is located ahead of the boiler’s turning chamber and 3 m below the center point of the flue gas inlet of the screen superheater. At a boiler load of 170 t/h, infrared thermography using Agima equipment showed that the temperature at this location ranged from 1045°C to 1075°C; thus, no modifications to the boiler were necessary. Physical properties of DCL sulfur-fixing agent: Bulk density: 1.00–1.15 t/m3; Fineness: ≤200 mesh; Appearance: gray powder; Moisture content: ≤5%. Given that this sulfur-fixing agent is prone to moisture absorption and has strong adhesion, when designing the process system we strive to simplify it and optimize the processes, while also ensuring that no secondary pollution occurs throughout the system, as well as maintaining a certain level of automatic control. The entire process flow is shown in Figure 1. Figure 1 The transportation and spraying process flow of the DCL-type sulfur-fixing agent is shown in Figure 1. The 100 m3 storage tank is a steel-structured silo; the silo is equipped with automatic air fluidization facilities based on the physical properties of the DCL-type sulfur-fixing agent, and level sensors for high and low levels are products from the American company DE. A bag filter is installed on the top of the tank to treat the compressed air used to feed materials from bulk vehicles into the tank. The conveyor capacity of the transmitter is 6–8 t/h, with a gas-to-ash ratio of 1:39. The 22-story small silo has dimensions of 2m3×2, and it is equipped with level gauges from the American company DE, as well as heating plates and bag filters. The spraying section includes a Roots blower, a screw feeder (with variable frequency speed control), and nozzles. The conveying system is controlled by PLC: when the level in the small silo is low, the transmitter starts operating; the sulfur-fixing agent is then conveyed to the upper silo. When the level in the upper silo becomes high, the transmitter stops working, and the conveying system is purged. The entire system covers an area of about 50 m2 and includes a space reserved for a second furnace. To prevent the nozzle from clogging, it is arranged that the corresponding feeder will shut down automatically whenever the Roots pump stops under any condition. When the Roots blower is shut down and the system is in programmed mode, the feeder cannot be started. All other parameters are displayed on the dial, and the feeder can have its speed adjusted manually to control the feeding amount. The sulfur-fixing agent nozzles are inserted from two manholes located 23.1 m above the boiler level; the nozzles are cast from heat-resistant alloy steel, with the inner tube made of 1Gr18Ni9Ti steel. The booster wind uses the hot air supplied from the boiler’s hot air duct, with a gas-to-material ratio of <3 kg/m3. To ensure that the sulfur-fixing agent can disperse rapidly and evenly within the furnace, cold tests are conducted on the nozzles prior to shipment, in order to achieve sufficient swirl intensity. This allows the material to disperse quickly as it leaves the nozzle and a recirculation zone to form, thereby ensuring even dispersion of the sulfur-fixing agent. The system incorporates many advanced foreign patent technologies, and the energy consumption of the entire system is shown in Tables 1 and 2. Table 1: Electricity consumption table for conveying and spraying equipment. Table 2: Gas consumption table for conveying and spraying equipment. III. Several key issues regarding the use of sulfur-fixing agents in boilers. 1. Selection of the area within the boiler where the sulfur-fixing agent is injected. Laboratory tests under high temperatures show that temperature plays a crucial role in the efficiency of sulfur fixation; when the temperature exceeds 1200°C, SO2 is released again, and the sulfur fixation rate approaches zero. This is because the diffusion rate of SO2 through the protective layer covering the surface of CaSO4 increases as the temperature rises ； Another possible reason is that when the catalyst in the sulfur-fixing agent becomes ineffective, MeSO3, which is formed from SO2 and MeO, decomposes back into SO2 and MeO. Based on the test results from Furnace No. 4, the second possibility seems more likely. In the first test of Furnace No. 4, the nozzle was installed at a height of 20.8 m, and the temperature in the spraying area was above 1200°C. The data showed that the SO2 level in the exhaust gases was 5% to 10% higher than that in the gases without sulfur fixation agents being used. When the nozzle was installed at 23.1 m, the sulfur fixation efficiency increased significantly (with temperatures measured at 1045°C in all cases). 2. Select an appropriate calcium-sulfur ratio. Different amounts of sulfur-fixing agents yield different sulfur-fixing effects, which depend on the sulfur content in the raw coal and the amount of coal burned. This ratio can be calculated using the formula: M = A × B × X / 0.27. Where: M – amount of sulfur-fixing agent to be injected, in t/h; A – coal consumption per hour by the boiler, in t/h; B – sulfur content in the coal, in%; X – Ca/S molar ratio. Assuming a boiler with a capacity of 220 t/h consumes 28 t/h of coal, with a sulfur content of 0.8%, and requiring a Ca/S molar ratio of 2, then the amount of sulfur-fixing agent to be injected is: M = 28 × 0.8% × 2 / 0.27 = 1.66 t/h. The choice of the appropriate Ca/S molar ratio depends on which amount of sulfur-fixing agent will yield the best sulfur-fixing results for that particular boiler. Factors such as safety, cost, and the requirements for reducing SO2 emissions must also be taken into consideration, and the decision is made through optimization. The SO2 reduction rates measured by the Guangzhou Environmental Protection Science Institute for the 220 t/h boiler of Guangdong Petrochemical during its application trial operation at Ca/S molar ratios of 2 and 2.5 are shown in Table 3. Table 3: Monitoring results of the sulfur fixation efficiency of the DCL-type coal-fired sulfur fixation agent. Note: As stated by the Guangzhou Institute of Environmental Protection Science, after the sulfur fixation agent is injected into the furnace, it ends up being captured by the electrostatic precipitator along with fly ash. The Ca/S molar ratio cannot be increased indefinitely; otherwise, an increase in the dust concentration before electrostatic precipitation will affect the proper operation of the electrostatic precipitator. Even when the electrostatic precipitator is operating at full capacity, the dust emission concentration at its outlet remains above the specified limits. Based on the test results of dust concentration before and after electrostatic precipitation and the dust removal efficiency after introducing a sulfur-fixing agent into Furnace No. 4, it was found that when the Ca/S molar ratio is below 2.5, the dust removal efficiency and the dust concentration at the outlet remain relatively unchanged. 3. Influence of nozzle-assisted air on desulfurization efficiency: Due to the differences in boiler designs, as well as variations in the designed and actual coal types used, the temperature field distribution within boilers also varies. For Guangshi Petrochemical’s Boiler No. 4, the final location determined for injecting the sulfur-fixing agent was at 23.1 meters; the structure of the boiler at this location is shown in Figure 2. Figure 2 Position of the sulfur-fixing agent nozzle. When the secondary air velocity at the nozzle was too high, the SO2 emission reduction rate was less than 35%; however, by adjusting the opening degree of the secondary air and reducing the wind velocity, the SO2 reduction rate increased to over 55%. When the boiler is used in different applications, it is quite important to adjust the wind speed of the booster air (secondary air) at the nozzle. IV. Analysis of desulfurization costs: The sulfur-fixing agents provided by suppliers currently cost 200 yuan per ton. Based on the SO2 reduction rates at Ca/S molar ratios of 2.0 and 2.5, as well as on the sulfur content and coal consumption of the raw coal used in tests conducted on Boiler No. 4 at Guangxi Petrochemical’s Power Division, it can be calculated that when the Ca/S molar ratio is 2.0, the cost of using sulfur-fixing agents to reduce SO2 by one ton is 1509 yuan ； When the Ca/S molar ratio is 2.5, the cost of the sulfur-fixing agent is 1,687 yuan, resulting in an increase in the cost per kW·h of electricity generation of approximately 0.006 to 0.008 yuan. The losses caused by SO2 emissions to the environment are far greater than this figure, and they are also **lower than the cost of reducing one ton of SO2 using desulfurization facilities abroad. The DCL-type coal-fired sulfur fixation agent technology offers advantages such as low investment, small space requirement, high efficiency, and low operating costs. It has little impact on the safety and reliability of boiler operation, making it highly suitable for China’s national conditions. The DCL-type sulfur fixation agent and its associated sulfur fixation technology represent a viable high-tech solution for ensuring environmental protection requirements are met simultaneously during the construction of new industrial boilers, as well as for the renovation and expansion of existing facilities and for reducing the total amount of SO2 emissions generated by enterprises