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Design of a Pure Waste Heat Boiler for a 2500 T/D Cement Kiln at Weifang Shanshui Cement Company I. System Concept This project aims to generate electricity by utilizing the waste heat from the exhaust gases at the beginning and end of the cement kiln. To make full use of the waste heat from the exhaust gas emitted by the kiln head coolers, separate low-temperature superheaters for the ASH kiln heads, a waste heat boiler for the AQC kiln heads, and a waste heat boiler for the SP kiln tails have been installed. The exhaust gas from the cement kiln clinker cooler passes through an ASH low-temperature waste heat superheater before entering the kiln head AQC boiler. The role of the ASH is to superheat the saturated steam produced by the AQC and SP boilers at 2.5 Mpa to 380°C superheated steam for use in generating electricity via turbines. Due to layout and thermal efficiency requirements, a horizontal layout is adopted structurally, with the exhaust gas temperature at the superheater outlet controlled within the range of approximately 300°C to 340°C. During design, the severe wear effect of the exhaust gas from cement kiln clinker coolers on the waste heat superheater must be taken into account, while also paying attention to measures such as air leakage prevention, wear protection, and blockage prevention. The exhaust gas from the cement kiln clinker cooler passes through an AQC low-temperature waste heat boiler before entering the kiln head dust collector. The function of the AQC boiler is to produce 2.5Mpa saturated steam, which is then superheated in a superheater before being supplied to the turbine for power generation ； Producing 0.25MPa saturated steam for deoxidizing boiler feed water and supplying make-up steam to turbines ； The hot water produced enters the deaerator for deoxygenation (serving simultaneously as feedwater for the 0.25 MPa steam section); the deoxygenated water is then supplied to the SP boiler and AQC boiler’s 2.5 MPa steam sections by the boiler feed pump. Due to space constraints and requirements regarding boiler thermal efficiency, a modular vertical layout is adopted; the temperature of the exhaust gas at the boiler outlet is controlled within the range of 90°C to 100°C. When designing boilers, account should be taken of the severe wear caused by the exhaust gases from cement kiln clinker coolers on the waste heat boiler, while also paying attention to measures such as air leakage prevention, wear protection, and clogging prevention. The exhaust gas at the tail end of the cement kiln passes through an SP low-temperature waste heat boiler before entering the dust collection system at the kiln’s tail end. The function of the SP boiler is to produce saturated steam at 2.5 Mpa, which is then superheated using the low-temperature waste heat superheater in the clinker cooler at the kiln head, before being supplied to the turbine for power generation. Due to space constraints and requirements regarding boiler thermal efficiency, a vertical layout is adopted; the temperature of the exhaust gas at the boiler outlet is controlled within the range of approximately 195°C to 210°C. When designing boilers, the characteristics of cement kiln waste heat boilers should be taken into account, with attention paid to measures such as air leakage prevention, wear protection, and blockage prevention. II. Design of the kiln head boiler 1. Design parameters of the kiln head boiler: Flue gas volume at the boiler inlet: Vr = 92512 Nm3/h; Negative pressure of the exhaust gas at the boiler inlet: P = -800 Pa; Designed flue gas temperature at the boiler inlet: t = 327°C; Dust content: μg = 50 g/Nm3; Total air leakage of the boiler: ≤2%; Total resistance to exhaust gas flow in the boiler: ≤480 Pa. Steam generation section of the boiler at 2.5 MPa: Rated steam pressure: 2.5 MPa (absolute pressure); Rated steam temperature: Saturated temperature; Rated evaporation capacity: ≥5.5 t/h. Steam generation section of the boiler at 0.25 MPa: Rated steam pressure: 0.25 MPa (absolute pressure); Rated steam temperature: 160°C; Rated evaporation capacity: 3.0 t/h. Hot water generation section of the boiler: Rated outlet water pressure: 0.5 MPa (absolute pressure); Rated outlet water temperature: 100°C; Rated water output volume: 21 t/h; Maximum water output volume: ≤28 t/h; Feedwater temperature: 30°C; Designed flue gas temperature at the boiler outlet: ≤98°C. 2. Key points in the design of the kiln head boiler: 2.1 The ash particles in the flue gas from the kiln head are hard, posing a wear protection issue for the heating surfaces ； 2.2 When designing the boiler, it is necessary to take into account the significant fluctuations in flue gas at the kiln head ； 2.3 An acceptable amount of dust accumulation ensures effective heat exchange ； 2.4 A reliable sealing structure to minimize air leakage, reduce heat loss, and lessen the impact on cement kiln operation ； 2.5 Appropriate flue gas side resistance, acceptable power consumption. 3. Structural features of the kiln head boiler 3.1 Vertical natural circulation; the 2.5MPa evaporation tubes and the 2.5MPa economizer are arranged from top to bottom ； 0.25MPa section evaporation tube ； Water heater. 3.2 The evaporation tubes, economizers, and water heaters all form separate tube banks together with the frame, and are assembled before leaving the factory. 3.3 The heating surface tubes are spiral finned tubes. 3.4 The internal shield sealing structure of the boiler reduces air leakage. 4. Anti-wear design of the kiln head boiler 4.1 The anti-wear design of the AQC boiler is crucial. 4.2 During design, a lower flue gas flow velocity should be selected, below 5 m/s. 4.3 The flue at the flue gas inlet has a variable diameter and is equipped with a flue gas flow equalization device. 4.4 Small-pitch, highly coiled finned tubes are used. 4.5 The tube bank is equipped with partitions and guide plates to reduce uneven flue gas flow. 4.6 Anti-wear devices are installed at the headers and elbow joints of the tube bank inside the tube box. 5. Sealing design of the preheater boiler 5.1 The operating conditions of the waste heat boiler in cement kilns place higher demands on the sealing design of the boiler. 5.2 Given the high negative pressure, a reliable structural seal must be implemented. 5.3 A high air leakage rate can cause unstable operation of the cement kiln. 5.4 A high air leakage rate will reduce the efficiency of the waste heat boiler. 5.5 It is assembled in a tube box and shipped out, ensuring excellent manufacturing quality. 5.6 Internal shielding plates are used for sealing, and all sealings are subjected to sealing test inspections in the factory. 5.7 The header box is of the built-in type. 5.8 Ensure the air leakage rate is below 1%. 6. Insulation design of the kiln head boiler 6.1 Cement kiln waste heat boilers need to make full use of waste heat, which imposes higher requirements on the insulation design of such boilers ； 6.2 Use lightweight shielded furnace walls ； 6.3 The material used is aluminum silicate fiber board ； 6.4 The thickness of the insulation layer is 160 mm ； 6.5 At an ambient temperature of 25°C, the surface temperature of the protective cover is below 40°C. III. Design of the kiln tail boiler 1. Design parameters of the kiln tail boiler: Flue gas volume at the boiler inlet: Vr = 180,000 Nm3/h; Negative pressure of the exhaust gas at the boiler inlet: P = -6,480 Pa; Designed flue gas temperature at the boiler inlet: t = 330°C; Dust content: μg = 120 g/Nm3; Designed flue gas temperature at the boiler outlet: t ≤ 209°C; Total air leakage of the boiler: ≤ 3%; Total resistance to exhaust gas flow: ≤ 800 Pa; Rated steam pressure: 2.5 MPa (absolute pressure); Rated steam temperature: saturated temperature; Rated evaporation capacity: ≥ 12.5 t/h; Feedwater temperature: 100°C. 2. Key points in the design of the kiln tail boiler: 2.1 The dust concentration in the flue gas at the kiln tail is high; therefore, reliable measures must be taken to prevent dust accumulation. 2.2 Due to the high negative pressure, a reliable sealing structure must be used to minimize air leakage, reduce heat loss, and lessen the impact on the operation of the cement kiln. 2.3 Appropriate flue gas side resistance, acceptable power consumption. 3. Structural features of the boiler at the kiln exit 3.1 Vertical natural circulation, with the evaporation tubes and economizer arranged from top to bottom. 3.2 Both the evaporation tubes and the economizer adopt a suspended structure. 3.3 The structure of the light-tube serpentine tube bundle addresses the thermal expansion of the tube bundle. 3.4 The internal shield sealing structure of the boiler reduces air leakage. 3.5 The use of built-in headers **reduces the number of pipes passing through walls. 3.6 Mechanical rapping is employed, an energy-saving and continuous dust removal method. 4. Sealing design of the boiler at the kiln exit 4.1 The operating conditions of the waste heat boiler in cement kilns place higher demands on the sealing design of such boilers. 4.2 Higher negative pressure requires reliable structural sealing. 4.3 A high air leakage rate can cause instability in the operation of the cement kiln. 4.4 A high air leakage rate will result in a reduced efficiency of the waste heat boiler. 4.5 The use of built-in manifolds **reduces air leakage from pipes passing through walls. 4.6 Ventilation beams, duct boxes, etc. are sealed using metal expansion joints. 4.7 Flexible sealing shall be used at the points where the mechanical arm penetrates the wall. 4.8 Ensure the air leakage rate is below 2%. 5. Hydrodynamic characteristics of the boiler at the kiln exit 5.1 Vertical boilers at the kiln exit adopt a structure with naturally circulating horizontal evaporation heating surfaces. 5.2 Hydrodynamic calculations are performed for all parameters, taking into account economic factors such as boiler steel consumption, space required, and investment in the boiler foundation, to ensure that the circulation height provides sufficient driving force for natural water circulation. 5.3 The diameter of the horizontal evaporation tube and a safe and reliable 5.4 circulation ratio are determined based on reasonable vapor content and mass flow rate; a high circulation ratio can enhance heat transfer. 5.5 For equivalent heating surfaces, increasing the number of tube circuits as much as possible depending on the available space can reduce the total length of the tubes along the flow path and lower the vapor content, thereby decreasing flow resistance. 5.6 At the lowest circulation height, the evaporation tubes are arranged at an inclined angle to prevent heat transfer problems and vapor-liquid stratification on the evaporation surface, thereby enhancing heat transfer and improving thermal efficiency. 5.7 Large-diameter downcomers and outlet pipes are used to reduce their own flow resistance, with the ratio of the total cross-sectional area fxj of the downcomers and the total cross-sectional area fyc of the outlet pipes to the total cross-sectional area fs of the evaporation tubes being greater than 0.4. 5.8 The circulation flow rate in the evaporation tube shall be such that it is sufficient to remove the fouling inside the tube, with Wo>0.4 m/s, to ensure the safety and reliability of the boiler. IV. Design of the ASH superheater 1. Design parameters of the ASH superheater: Flue gas volume at the boiler inlet: Vr=36000 Nm3/h; Negative pressure of the exhaust gas at the boiler inlet: P=-800 Pa; Designed flue gas temperature at the boiler inlet: t=486°C; Dust content: μg=50 g/Nm3; Rated inlet steam pressure: 2.45 MPa (absolute pressure); Rated inlet steam temperature: saturated temperature; Rated outlet steam pressure: 2.4 MPa (absolute pressure); Rated outlet steam temperature: 380°C; Rated steam flow rate: 18 t/h. 2. Key points in the design of the ASH superheater: 2.1 The ash particles in the flue gas coming from the kiln are hard, which poses a problem regarding wear protection for the heating surfaces ； 2.2 When designing the boiler, it is necessary to take into account the significant fluctuations in flue gas at the kiln head ； 2.3 An acceptable amount of dust accumulation ensures effective heat exchange ； 2.4 A reliable sealing structure to minimize air leakage, reduce heat loss, and lessen the impact on cement kiln operation ； 2.5 Appropriate flue gas side resistance, acceptable power consumption ； 3. Structural features of the ASH superheater 3.1 The ASH superheater is arranged in a horizontal configuration. 3.2 The heating surface uses alloy spiral finned tubes. 3.3 The pipe elbows and headers are located outside the flue, so they will not get worn out. 3.4 Overall assembly and shipment from the factory. V. Issues to Consider in Boiler Design 1. The impact of air leakage: Theoretically, when the air leakage rate increases from 2% to 3%, the boiler’s evaporation capacity decreases by about 0.8%. In practice, after the boiler was put into operation, due to factors such as the location of air leakage points and the concentration of such leaks, when the air leakage rate increased from 2% to 3%, the boiler’s evaporation capacity dropped by far more than 0.8%. 2. Influence of ash concentration on the boiler’s evaporation capacity: A high ash concentration can cause ash to accumulate on the heating surfaces, affecting heat transfer. After the boiler starts operating, this results in a decrease in evaporation capacity and an increase in flue gas temperature. The higher the ash concentration, the more heat the ash carries. For the boiler at the kiln exit, mechanical rapping is used for ash removal, which reduces the impact of high ash levels on heat transfer; for every 20 g/Nm3 increase in ash concentration, the evaporation capacity of the boiler increases by 0.9%~1%. 3. Impact of changes in pressure parameters during boiler operation: For a waste heat boiler with a rated steam pressure of 2.45 MPa, if it operates at a pressure of 2.0 MPa, the steam production will increase by about 2%, while the temperature of the superheated steam will decrease by approximately 1–2%. At the same time, the average flow velocity of the fluid in both the superheater and the economizer increases significantly. To adapt to changes in operating pressure parameters, the arrangement of the heated surfaces should follow the principle of more at the top and less at the bottom; that is: ① Sufficient superheaters ； ②Appropriate evaporator ； ③Fewer economizers or no economizers at all. 4. Impact of temperature parameter variations on boiler component design: The flue gas temperature at the cement kiln inlet varies significantly and frequently. When the flue gas temperature rises, the steam production capacity of the waste heat boiler increases as well. To keep the steam-water resistance at a reasonable level, a relatively low average flow velocity of the medium should be considered when designing the superheater and economizer (this approach is also beneficial for operating the boiler at reduced pressure). A large-diameter drum is used to increase the water capacity and reduce fluctuations in water level. Select appropriate materials based on the possible highest temperature of the incoming flue gas, and determine components such as the boiler safety valve according to the possible maximum evaporation rate. 5. Comparison of system design pressures: Currently, the representative design pressures for pure low-temperature waste heat boilers in cement kilns are 2.45 MPa, 1.25 MPa, and 0.8 MPa, with 1.25 MPa being the most common. Reheat boilers with different design pressures do not differ significantly in terms of structure; however, due to variations in the height of the narrow sections, there are differences in the arrangement of the economizers in the steam section: boilers at 2.45 MPa have more economizers arranged ； Boilers at 1.25 MPa have few economizers or none at all ； Boilers at 0.8 MPa generally do not have economizers installed. The steel consumption per ton of steel produced varies among waste heat boilers with different design pressures; higher pressure results in slightly higher steel consumption. For waste heat boilers with the same inlet and outlet flue gas conditions but different design pressures, the main steam enthalpy values remain fairly similar. However, the higher the pressure, the greater the work efficiency from a thermodynamic perspective; to determine appropriate steam pressure parameters, factors such as system configuration and investment costs also need to be taken into account. VI. Comparison of different waste heat power generation thermal systems: The commonly used methods include single-pressure, flash, and double-pressure waste heat power generation ； A single-pressure system refers to a situation where the waste heat boiler at the kiln head and the waste heat boiler at the kiln tail generate main steam with similar parameters, which then mix together before entering the turbine ； The hot water produced by the pre-heater waste heat boiler is supplied to the steam section of the pre-heater waste heat boiler and the tail-end waste heat boiler ； A flash vaporization system refers to the process by which a boiler generates main steam and hot water at a certain pressure; the main steam enters the high-pressure inlet of the turbine, while the hot water undergoes flash vaporization to produce low-pressure saturated steam, which is then supplied to the low-pressure inlet of the steam-replenishing type turbine. A dual-pressure system refers to a waste heat boiler that generates steam at higher and lower pressures, which then enter the high- and low-pressure steam inlets of the turbine, respectively. Basis for selection: the flue gas volume and temperature determined by the inherent characteristics of the cement kiln, as well as the temperature at which the flue gas is used for drying materials. The amount of heat absorbed by the boiler depends on the exhaust temperature of the boiler, the heat loss from the boiler, and the air leakage in the boiler. Heat absorption: The two-pressure system is higher than the flash system, and the flash system is higher than the single-pressure system. Power generation: The two-pressure system is higher than the flash system, which in turn is higher than the single-pressure system. VII. Features of Weifang Shanshui Cement’s waste heat power generation system: A dual-pressure power generation system designed to achieve maximum power output ； The air intake at the kiln head is located in an area with higher temperatures; the system operates at a pressure of 2.45 MPa. The boiler has a high heat absorption capacity, resulting in high power generation efficiency ； The dual-pressure system exhibits good adaptability to changes in the waste heat conditions of cement kilns ； An independent superheater is used, facilitating the adjustment of the superheated steam temperature. VIII. Design calculations for boilers: Based on years of experimental and theoretical research, as well as extensive experience in boiler design and operation, a set of design calculation methods suitable for waste heat boilers used in cement kilns has been developed. Thermal calculation programs for single-pressure and double-pressure systems have been created, and practical applications on multiple boilers have shown that the operating parameters and design parameters of these boilers are in good agreement.