Boiler Tubes Research Articles

CFD analysis of an economizer for heat transfer enhancement using serrated finned tube equipped with variable fin segments

Due to the extensive use of heat exchangers in various daily and industrial applications, the improvement of their performances is of great importance. It is evident from the literature that very limited work has been done in serrated finned tube economizer for heat transfer enhancement and pressure drop performance. This paper deals with the heat transfer improvement and pressure drop performance of helical serrated finned tube bundles in inline bare tube counter cross-flow water tube boiler Economizer, which has larger heat transfer area and higher heat exchange efficiency. Numerical simulation on heat transfer performance is carried out on 116.3 TPH of MCR boiler unit, one tube row with three-row finned tube passes; helically wound fin geometries of partially serrated finned tubes are investigated. A steady-state approach, with two-equation, a k-ε turbulence model is employed to examine the effect of Reynolds number in the range of 8000 to 20000. In this study the conjugate heat transfer is considered for the calculation of the conduction in a solid region is coupled with the calculation of the temperature distribution in the working fluid flowing over or through this solid region. The effect of a number of serrated segmented finned tubes per period on the performance of heat transfer enhancement is also investigated. The computational results fall within the range of values predicted by empirical correlation. The results show that the heat transfer is enhanced by up to 3.53% to 5.14% if the number of segments per period varied from 20 to 50.

Scale law of sCO2 coal fired power plants regarding system performance dependent on power capacities

Once supercritical carbon dioxide (sCO2) cycle is used for coal fired power plant with conventional boiler design (total flow mode TFM), ultra-large pressure drop of sCO2 boiler occurs to suppress system efficiency, thus we proposed partial flow mode (PFM). A CO2 stream is segmented into two parallel lines, each having half flow rate and half length. The effectiveness of PFM was examined at various power capacities (net power output Wnet) covering 50–1000 MWe. A numerical model is established by coupling thermodynamic cycle of system and thermal-hydraulic characteristics of boiler. Scale laws of boiler size, mass flux and pressure drop in boiler tubes are developed, agreeing with numerical simulation results. We show that with increase of Wnet, TFM increases pressure drops to generate additional pressure and load for compressors. This effect not only increases energy loads, but also raises exergy destructions for all components, deteriorating the first and second law efficiencies of the system. Compared with TFM, PFM reduces pressure drops according to the 1/8 criterion, the pressure drop penalty effect is eliminated to have much higher efficiencies than TFM. Because at smaller Wnet such as 100 MWe, sCO2 boiler has larger surface to volume ratio, flow passages of CO2 are sufficient to keep acceptable pressure drop, hence the system performance is similar when using TFM and PFM. We conclude that the flow splitting strategy is necessary for Wnet > 100 MWe, but not recommended only at very small power capacity such as Wnet < 100 MWe. Our work is important to understand the distinct feature of sCO2 cycle driven by different heat sources.

Research on ultrasonic phased array inspection technology of small diameter tube based on Ultravision simulation technology

According to the principle of ultrasonic phased array inspection, the ultrasonic phased array inspection simulation experiment is carried out on the butt weld of small diameter tube of power plant boiler by using the Ultravision software, which mainly includes the simulation of acoustic beam coverage and focused beam distribution. From the simulation test results, it can be concluded that: through the simulation of the acoustic beam coverage, the accessibility and coverage of the ultrasonic phased array acoustic beam can be simulated, and the applicability of the detection process can be verified and optimized; Through the simulation of focusing beam distribution, the distribution of the beam focus in the butt weld of the small diameter tube can be determined, which can guide the selection of focusing mode in the detection process and improve the signal-to-noise ratio of defect detection.

P.191 The impact of physical activity and fitness on resilience to modern-life stress and the role of brain-derived neurotrophic factor (BDNF)

Although zinc is ubiquitous in ambient particulate matter (PM), exposure to size fractionated residual oil fly ash (ROFA) PM is of particular concern due to several studies implicating particle zinc in mortality, adverse respiratory and cardiac effects. The lack of zinc speciation data is an impediment to our understanding of the biological mechanism of Zn induced toxicity. ROFA PM2.5+ samples (i.e. particle with aerodynamic diameter >2.5 μm) are prepared in a small 732 kW practical fire tube boiler by combusting one No. 5 and three No. 6 residual oil(s) of varying sulfur and ash contents. The combined X-ray absorption spectroscopy and selective extraction method is used to determine the Zn speciation in PM2.5+ samples. The Zn speciation in ROFA PM2.5 (i.e. particle with aerodynamic diameter <2.5 μm) samples, reported earlier, is included here for comparison purposes. The data show that zinc sulfate is predominant in both fractions. In addition, an appreciable amount of zinc phosphate was detected in the PM2.5 fraction. The insoluble zinc forms in both the fractions are identified as Zn-sorbed-ferrihydrite and zinc sulphide. The variation of Zn speciation across different size fractions has implication on the bioavailability and toxicity of Zn. Such source specific speciation data are needed for developing source inventories, and for amending existing regulations/framing new regulations for different size fractions.

Corrosion performance of candidate boiler tube alloys under advanced pressurized oxy-fuel combustion conditions

Advanced pressurized oxy-fuel combustion technology has been developed for the significantly improved thermal energy conversion efficiency and reduced greenhouse gas emission. However, significant knowledge gap exists in terms of the selection of appropriate boiler tube materials, which has delayed the commercialization of this promising technology. In this study, the corrosion performance of the three candidate alloys (SS347, Alloy 800AT and Alloy 825) was investigated in the hot flue gas mixtures (60% H2O + 33% CO2 + 2–7% O2) at 600 °C and 15 MPa to simulate typical pressurized oxy-fuel natural gas combustion environments. On all the alloys, Cr2O3 was the main surface oxide and several alloying elements (including Si, and/or Al+Ti) accumulated at the interface of oxide layer/substrate. Among the gas species, O2 acted as the dominant oxidizing agent with a critical content of ∼2%, above which nodular oxidation was suppressed and corrosion rates of alloys were greatly reduced. H2O was likely to enhance the growth and evaporation of oxides, while CO2 did not trigger carburization during the testing period. From a corrosion perspective, three alloys exhibit promising potential for boiler tube constructions while Alloy 825 exhibited the best corrosion performance.

Creep deformation and rupture behaviour of service-exposed Super304H steel boiler tubes

ABSTRACT The determination of creep properties of the components operating at high temperatures in thermal power plants is an enormous concern for safe life assessment. In this study, a series of miniature creep and tension tests were carried out with service-exposed (54,750 h and 68,550 h at 650 °C) and virgin Super304H steel materials. The power law creep behaviour, creep rupture life, and tensile properties were compared among the virgin and service-exposed materials. The creep strength of the 54,750-h-serviced material was comparable to that of the virgin material, while it decreased significantly for the 68,550-h-serviced material. The degradation of the properties of the service-exposed material was discussed based on metallurgical analyses and precipitation mechanisms.

Effect of carbon nanotube (CNT) additions in Cr3C2-25%NiCr coatings on microstructural and mechanical properties

Abstract This research work focuses on Carbon Nanotubes (CNTs) reinforced Cr3C2-25%NiCr coating powders, deposited by the HVOF method on ASTM-SA213 T12 thermal power plant boiler tube alloy steel as substrate. The CNTs mixed with coating powder influence on the microstructure, and mechanical properties. The coating powder Cr3C2-25%NiCr mixed with CNTs of 3, 5 and, 7% by weight are prepared and deposited on the substrate using the HVOF thermal spray process. In this method the product of burnt fuels expands through the nozzle the gas velocities may be supersonic, the composite coating powder fed axially through the nozzle, their particles heated and accelerated powder particles in a molten or semi-molten state to achieve dense, and adhered coatings impinge on the substrate. The deposited coatings achieve dense, uniform microstructure characterized for surface morphology, and the compositions analyzed by Scanning Electron Microscope (SEM) / Energy Dispersive x-ray Spectroscopy (EDS) analysis. The phase identification of a crystalline material was done using X-Ray Diffraction (XRD) techniques. The coatings reinforced with 7% wt. of CNTs in coated samples exhibit low porosity, highest micro-hardness to maintain high-performance surface properties leading to better mechanical strength in the coatings. In HVOF deposition techniques additions of CNTs in the coatings leads to less porous, carbon nanoparticle fills the pores gap between the coatings, and improve the layer of protection to the metal on the contact surface, and other surface degrading conditions.

High temperature solid particle erosion behaviour of SS 316L and thermal sprayed WC-Cr3C2–Ni coatings

The boiler tubes, steam and gas turbines are commonly affected by severe high-temperature erosion wear due to impingement of solid particles entrained in the stream of fluid. The investigation focuses on the applicability of thermal sprayed WC based coatings for turbines and boiler tubes by improving the erosion resistance of the base material. The present work includes high-temperature solid particle erosion behaviour of WC-Cr3C2–Ni coatings deposited by atmospheric plasma spray (APS) and high-velocity oxy-fuel (HVOF) processes. The effects of temperature and impact angle on erosion performance of uncoated and coated specimens were comparatively studied using air-jet erosion tester (ASTM G76). The eroded surface morphology was analysed using a scanning electron microscope (SEM). Uncoated steel exhibit ductile mechanisms; however, both coated specimens follow the mixed mode of material removal under the same operating conditions. Erosion resistance of APS and HVOF coatings is approximately two and three times higher than uncoated specimens at higher temperatures under 30° and 90° impact angles. The HVOF coating offers higher erosion resistance than APS coating due to lower porosity, greater splat adhesion, a lower degree of decarburisation and higher inter-splat sintering at elevated temperatures.

Comparative high temperature oxidation studies of HVOF IN 625 coating on T22 boiler steel at 900 °C and 700 °C

Abstract IN 625 coating was deposited on T22 boiler tube steel by high velocity oxy fuel (HVOF) process to increase high-temperature corrosion resistance. High-temperature oxidation behavior of steel with and without coating has been examined at 700 °C and 900 °C under cyclic conditions. Each cycle consisted 1 h of heating in the silicon carbide tube furnace followed by 20 min of cooling in air. The cyclic oxidation conditions have been chosen as these conditions resembles actual industrial environment conditions where occurrence of shutdown is very frequent. Cyclic studies give the more severe conditions for testing and may be similar to the actual industrial environment of frequent breakdown/shutdown occurrence. Thermo gravimetric technique was implemented to study the kinetics of corrosion by observing weight change after each cycle. The bare steel experienced higher weight gain, which may be credited to the development of un-protective Fe2O3 dominated oxide scales. Evaluation of corrosive products was done using X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDAX) and X-ray mapping techniques. It was observed that the uncoated steel suffered corrosion in the form of spalling and peeling up of its oxide scale. On the converse, the IN 625 coating provided significant resistance to the corrosion. The IN 625 coating was found to be more protective at 700 °C than at 900 °C. The better air oxidation resistance in the coatings may be attributed to the phases discovered in the oxide scale of the coated specimens which mainly consisted of oxides and spinals of nickel and chromium. The information gathering from the investigation will be useful to explore the possibility of the use of the HVOF spray coatings on the boiler tubes working in the range of 700–900 °C.

Long-term degradation behavior of slurry aluminide coating on Super304H stainless steel at 650 °C

An engineering-favored slurry aluminide coating was prepared on the inner wall of a Super304H boiler tube. The degradation behavior of aluminide coatings was investigated at 650 °C in static air for up to 10 kh. The inward diffusion of Al dominated the degradation mechanism. Even though a phase transformation from β to γ' occurred, the Al concentration was sufficient to meet the minimum quantity required for alumina growth after 10 kh. A life prediction model of aluminide coatings with Al content as the main index was proposed according to Fick's second law of a diffusive process.

Study on acoustic time delay localization in boiler tube arrays considering effective sound velocity

Based on the study of sound propagation characteristics and acoustic radiation in periodic tube arrays, we found that the wave front differed from the no-tube arrays at the same frequency. The phenomenon indicated that there was different sound velocity in tube arrays. This research considered tube arrays as an effective medium for sound and studied the effective sound velocity at different frequencies in tube arrays. The numeral calculations and experiments shown similar conclusions that the sound velocity was slower than in no-tube arrays one. Acoustic velocity directly affects the accuracy of time delay localization method, and the time delay localization method in acoustic isotropic media is mature. The velocity of low frequency sound wave is isotropic in the arrays, we investigate the variation of sound velocity theoretically and experimentally. The sound demonstrates anisotropy at high frequency region, the effective sound velocity is related to the band gap in the tube arrays. The research has been found that the same frequency has different sound velocity values in different directions. In different directions of tube arrays, the effective sound velocity is used to conduct an one-dimensional acoustic source location experiment. This study can provide an innovative concept and method for leakage location in boiler tube arrays.

Failure Analysis on U-Bend Header Tube of Boiler Economizer

A water tube boiler is a type of boiler in which water circulates in tubes heated externally by fire. It was found that the water tube economizers are often subject to the serious damage mainly because of the overheating and corrosion. Thus, the metallurgical investigation procedures starts with the visual inspection, stereomicroscopy, metallography and chemical analysis methods. It was identified that the failure of the u-bend tube was due to the cracking caused by bending forces during the procedures. This cracking causes a leak for the tube. The appearance of corrosion on the shell side is due to the presence of small content of sulphur under the boiler environment. Several welding imperfections were also detected, however, both corrosion and welding imperfections were not the primary caused of the tube failure. From the analysis, the use of SUS304 grade stainless steel was discovered to be the appropriate material selection for the tube boiler.

The sensitivity of failure analysis of boiler tubes to the shape of elliptical external erosion flaws

Localized erosion is one of the most common failure mechanisms associated with boiler tubes, driven by impaction of the tube surface by fly ash, soot blowing steam, falling slag or other abrasive substances from the boiler’s combustion chamber. The tubes may experience significant localized reduction in their wall thickness, becoming susceptible to plastic collapse and bursting. The replacement of failed tubes is one of the leading causes of unplanned and forced boiler outages in process and power plants. In this study, geometric functions to enable the accurate modelling of boiler tubes with localized erosion flaws were developed from conceptualized models and finite element analyses were conducted on the modelled flawed tubes. The effect of geometry on the stress concentration in the tubes and on the failure pressure associated with the tubes was investigated. Linear elastic stress analysis is sensitive to the flaw geometry – for a representative problem stresses vary 38% as the flaw shape is varied. However, nonlinear elastic-plastic analysis shows significantly reduced sensitivity to flaw shape. The collapse pressure of the same representative problem now varies only 2.45% as the flaw shape is varied. This finding was demonstrated for various failure thresholds sourced from the literature, based either on plastic strain or Von Mises equivalent stress, which demonstrates that the failure of the tubes is insensitive to the specific choice of shape parameterization, as long as the minimum remaining wall thickness is matched. The outcome of these investigations gives further insight into the behaviour of tubes with localized external flaws while in service.

Experimental and modeling study of H2S formation and evolution in air staged combustion of pulverized coal

High-concentration H2S formed in the reduction zone of pulverized coal air-staged combustion can result into the high temperature corrosion of water wall tube of boiler, so it is of great importance to accurately predict H2S concentration for the safe operation of boilers and burners. H2S formation and evolution depends on two steps: the sulfur release from coal conversion and gas-phase reactions of sulfur species. In this study, the sulfur release characteristics from the pyrolysis of 17 coals, including 5 lignite, 9 bituminous coals and 3 anthracites, are investigated in a drop tube furnace (DTF). Sulfur release model is developed to describe the relationship between sulfur release and coal types. A global gas-phase reaction mechanism of sulfur species composed of ten reactions is used to calculate and predict the formation and evolution of H2S, COS and SO2 in the reduction zone of pulverized coal air-staged combustion. A wide range of air-staged combustion experiments of 17 coals are conducted in the DTF at different temperatures and stoichiometric ratios to validate the developed model. The results show that the prediction errors of sulfur species, including SO2, H2S and COS, are within ± 30%, which indicates that the developed prediction model of sulfur species is of great assistance for CFD modeling of actual engineering application.

High-Temperature Corrosion Behaviour of CNT-reinforced Zirconium Yttrium Coatings on Boiler Tube Steel in Coal-Fired Boiler of Thermal Power Plant

Corrosion behaviour of carbon nanotubes (CNT)-reinforced zirconium yttrium coatings at high temperature under cyclic thermal loading has been investigated in actual coal-fired boiler environment of thermal power plant. The amount of CNT was varied from 0.5 to 6 wt.%, and the coating was developed by the plasma spray technique. The comparative effects of variation in CNT content on hot corrosion behaviour were studied by weight change measurements. The corroded products were then analysed with XRD, FE-SEM with EDAX and X-sectional analysis techniques. The results revealed that the variation in CNT percentile enhances the hot corrosion resistance of boiler steel in actual boiler environment at elevated temperature. The reinforced coatings showed the lower weight gain measurements with the formation of protective scale of oxides during the experiment. Corrosion rate was observed to be reduced with the increased content of CNT in the coating composite.

Creep lifetime prediction of 9Cr-1Mo (grade T91) steel via small punch creep tests and hierarchical multiscale analysis

ABSTRACT In this study, the creep deformation of ferritic/martensitic 9Cr-1Mo (Grade T91) steel boiler tubes that occurs during the operation of thermal power plants is studied using crystal elasto-viscoplasticity finite element analysis (CEV-FEA). The material properties and creep behaviour of Grade T91 were obtained through experimentation and computational analysis at different scales, e.g., molecular dynamics simulation, scanning electron microscopy, small punch (SP) tests. A triple junction of grains with various combinations of crystallographic orientations of polycrystalline Grade T91 was also simulated by CEV-FEA to investigate the stress concentration evolution during creep deformation. The stress concentration at the triple junction was then used as an input for the creep cavitation model, i.e., as the representative stress driving creep cavitation. The combination of CEV-FEA and SP test experiments can improve the evaluation of creep lifetime for as-received boiler tubes with minimum requirements for the amount of material and testing time.

Development of an Automatic Welding System for the Boiler Tube Walls Weld Overlay

Tube walls are an essential part of the thermal power plant boiler. During the operation of the boiler, the heating surface of the tube walls are exposed to furnace particles, intense heat, and chemical components resulting from the combustion reaction. These cause corrosion and wear, which permanently collapse the tubes, and affect the reliability and performance of the boiler. Therefore, a protection layer of heat and corrosion-resistant material is typically welded on the surface of the tube walls. In this study, a dedicated weld overlay automatic system is proposed. The downward welding technique with the pulse gas metal arc welding (GMAW) process is used to accomplish the proposed approach. The system generates and plans beads sequence based on the analysis of the tube walls geometry. The inverse kinematic theory was used to calculate the coordinates and transformations of the welding torch. Then, a mathematical model for the welding torch trajectory was established. A SIMOTION controller was adapted for motion control. A weld-tracking system based on the adaptive neuro-fuzzy inference system (ANFIS) was used to solve the welding distortion and the assembly error. The experiment results show that the proposed design is efficient and reliable compared to previous methods. The degree of automation and the weld overlay quality of the boiler tube walls have been notably improved.

Vibration of the Biomass Boiler Tube Excited with Impact of the Cleaning Device

In boilers with biomass fuel, a significant problem is caused due to the slag layer formed from the unburned particles during combustion. In the paper, a tube cleaning method from slag is proposed. The method is based on the impact effect of the end of the tube with the aim to produce vibration for slag elimination. The tube is modeled as a clamped-free nonlinear oscillatory system. The initial impact of the tube causes vibrations. The mathematical model of the system is a nonlinear partial differential equation with zero initial deflection. To obtain the ordinary differential equations, the Galerkin method is applied. By discretizing the equation into a finite degree of freedom system, using the undamped linear mode shapes of the straight beam as basic functions, the reduced order model, consisting of ordinary differential equations in time, is obtained. The ordinary time equations are analytically solved by adopting the Krylov–Bogoliubov procedure. Special cases of nonlinear differential equations are investigated. In the paper, the influence of the nonlinear parameters and initial conditions on the vibration properties of the tube is obtained. We use the procedure developed in the paper and the analytical results for computation of the impact parameters of the cleaning device.

Thermal Analysis and Cost Evaluation of Boilers Considering Different Insulation Materials and Fuel Oil

The energy cost of heat transferred across the walls of a gas-fired and water tube boiler adopting the fibreglass and calcium silicate as insulating materials is analyzed adopting finite volume numerical scheme. A two-dimensional view of the heat transfer process is analyzed considering varied insulation thickness across the walls where results show from the point of critical thickness that the fibreglass material is a better energy saver compared with the calcium silicate material for varying insulation across the boiler walls. Result reveals the annual energy cost accrued per litre of fuel consumed at critical insulation point are ₦6.630 and ₦8.250 respectively for the gas-fired boiler utilizing LPFO for Fibreglass (FG) and Calcium silicate (CS) respectively. While the annual energy cost accrued per litre of fuel consumed at critical insulation points are ₦17.004 and ₦21.511 respectively utilizing Diesel Oil for FG and CS materials of insulation respectively for a gas-fired boiler. This paper reveals the cost of energy required using the LPFO with fibreglass insulation for gas-fired boiler is more economically viable compared with water tube boilers. However, the pollutants emission produced from LPFO is higher compared to diesel oil leaving a higher footprint in the environment.

Hot Corrosion Performance of Plasma-Sprayed Multiwalled Carbon Nanotube–Al2O3 Composite Coatings in a Coal-Fired Boiler at 900 °C

The present study investigates the high-temperature corrosion behavior of plasma-coated boiler steel for 1000 h in a 110-MW water tube boiler, which had encountered severe corrosion and the tube failure. The coating was carried out by alumina and MWCNTs (multiwalled carbon nanotubes)-reinforced alumina composite powders. The uncoated and coated steel substrates were hung at a platen superheater at 900 °C. After the exposure, thermogravimetric analysis was carried out to evaluate the kinetics of corrosion. XRD, SEM, TEM and EDS analysis was carried out to evaluate the corrosion products at the end of the cyclic high-temperature corrosion. The present research work concludes that CNT-reinforced alumina coatings (4.0 wt.% CNT + Al2O3 coating) have decreased the corrosion rate to 30.72 mpy, which was 113.53 mpy for conventional alumina coatings and 376.96 mpy for bare T11 boiler steel in the boiler environment. This manuscript submits the failure of tube at the platen superheater, exposure conditions and the analysis of the coated and uncoated samples of the steel tubes.