Acid Dew Point Research Articles

Failure Analysis of a Welded 316L Stainless-Steel Stack with Premature Damage Due to Stress-Corrosion Cracking

AbstractThis study analyzed the premature failure of a stainless-steel stack obtained from a mining company. The stack was manufactured using ASTM A240 Type 316L stainless steel. After one year of its service, massive leakages and cracks were observed near the welds. Following a visual examination of the stack, a representative sample was extracted and analyzed using visual and radiographic inspection, metallographic examination, scanning electron microscopy, energy-dispersive spectrometry, optical emission spectrometry, ferrite number analysis, and leak testing. Furthermore, corrosion mechanisms and the sequence of events leading to failure were determined. Two different mechanisms were identified: acid dew point corrosion caused by the condensation of H2SO4 and HCl at the inner walls of the chimney, followed by stress-corrosion cracking near the welded joints.

Optimization study on acid condensation and anti-corrosion performance of 3-D finned tube heat exchangers

Low-temperature acid corrosion on flue gas heat exchanger surfaces seriously reduces the stability, safety, and economy of equipment operation. Efficient strategies are urgently required to improve the anti-corrosion performance of heat exchangers. In present paper, a three-dimensional heat and mass transfer model integrating the acid dew point, acid condensation rate, and solution concentration was developed to predict the corrosion characteristics on the surfaces of 3-D finned tube heat exchangers. Firstly, a comparative analysis of acid condensate characteristics for 3-D finned tubes with different configurations was performed. The results showed that the elliptical tube effectively improves the distribution uniformity of acid condensate rate and promotes anti-corrosion performance. Then, the effects of five operating parameters (gas temperature, gas velocity, water vapor concentration, acid vapor concentration and wall temperature) on overall corrosion characteristics were systematically examined in terms of acid dew point, condensation rate and solution concentration using single-factor and orthogonal analysis, and the effect order of each factor was determined. Furthermore, optimization analysis of anti-corrosion performance was conducted based on the orthogonal results, and the optimum working conditions (water vapor concentration below 11.8% and wall temperature of 347 K to 355 K) are recommended. This research can provide theoretical guidance for the anti-corrosion designation and optimization of 3-D finned tube exchangers.

Experimental study on the dew point corrosion behavior in atmospheric tower system

Atmospheric towers are large pieces of equipment used in the petroleum refining process. Hydrochloric acid dew point corrosion often occurs at low-temperatures in the tower. In order to study the dew point corrosion problem of atmospheric tower in a petrochemical enterprise, the process simulation method was used to determine the logistics parameters in the high risk area of dew point corrosion of atmospheric tower system. Based on this, the experimental conditions were set up to closely integrate laboratory research with engineering practice. In view of this special and complex corrosion process, a dew point corrosion experiment was designed to realize the dew point condensation process. The influence of concentration and temperature on the dew point corrosion rate was explored by an electrochemical test and a corrosion weight loss method. The results show that the initial dew point temperature of the tower is 89 ℃, and the risk area of dew point corrosion is located at the top of the tower and in the position of the volatile line, which is consistent with the actual corrosion area of the equipment. In this logistics environment, the dew point corrosion rate of Q345R steel increases with the increase in HCl concentration and temperature in the atmospheric tower system.

Removal characteristics of SO3 in the low-low temperature electrostatic precipitator

ABSTRACT As SO3 endangered the industrial equipment and environment in the coal-fired power plants, it was necessary to give full play to the SO3 removal effect in a low-low-temperature electrostatic precipitator (LLT-ESP). The relationship between SO3 and fly ash particles was investigated during the cooling process. In addition, the SO3 removal performance in the LLT-ESP was further analyzed. The migration and deposition of SO3 into fly ash particles were promoted with the decrease in the gas temperature and the fly ash particle size, as well as the increase in the SO3 concentration. As the temperature of the gas decreased below the acid dew point (ADP), the H2SO4 vapor tended to migrate into fly ash particles with smaller sizes. The SO3 mass concentration increased significantly in fly ash particles whose sizes ranged from 0.03 to 0.26 μm. When the operating voltage turned up, the SO3 removal efficiency was increased from 43.2% to 60.1%. The increase in the inlet fly ash concentration promoted the SO3 removal, and the removal efficiency increased by nearly 20%. When the gas temperature was below the ADP, it had little influence on the SO3 removal efficiency.

Determination of Acid Dew Point Based on the Calculation of Thermodynamic Equilibrium of Chemical Reactions of the Sulfuric Acid Formation Given the Condensed State

Determination of Acid Dew Point Based on the Calculation of Thermodynamic Equilibrium of Chemical Reactions of the Sulfuric Acid Formation Given the Condensed State

EFFECTS OF EXCESS AIR AND COAL SULFUR CONTENT IN PERFORMANCE IMPROVEMENT OF 660 MW SUPERCRITICAL BOILERS IN TERMS OF ACID DEW POINT TEMPERATURE

The objective of this study was to select an appropriate fuel having different sulfur contents in various coal fuels and optimize the excess air in order to reduce dry flue gas loss, avoid equipment damage due to sulfur dew point corrosion, and reduce maintenance and equipment installation costs. A tri-sector rotary air preheater was employed to exchange heat between dry flue gases and preheat the air (both primary and secondary air). The results showed that coal with 0.6165% sulfur (fuel 2) caused acid dew point temperatures (ADPTs) of 122.38°C and 124.947°C with 15% and 20% excess air, respectively, in a rotary air preheater set at a 30.5° angle, whereas coal with 1.627% sulfur (fuel 1) caused ADPTs of 131.299°C and 133.602°C with 15% and 20% excess air, respectively, in a rotary air preheater set at a 35.99° angle. The experimental results indicate that the use of fuel 2 coal consumption can reduce carbon dioxide (CO<sub>2</sub>) emission by nearly 4 metric tons (MT)/hour, which is equivalent to a reduction of CO<sub>2</sub> emission of 3.8 MT/hour.

Feasibility Analysis of Coupling Hydrogen-Derived Fuel on a Coal-Fired Boiler for Power Generation

As an alternative to hydrogen, hydrogen-derived fuel (NH3, CH3OH, or CH3OCH3) is considered to be an ideal secondary energy source. The effects of coupling hydrogen-derived fuels with coal on the thermodynamic parameters of a boiler are analyzed through thermal calculations according to the principles of mass and energy conservation. A 300 MW tangentially fired boiler is selected for a case study. The results show that the variations of boiler thermodynamic parameters depend on the properties of hydrogen-derived fuels and coupling mass percentage. After the coupling, the furnace exit flue gas temperature changes, which results in alterations in heat distribution of radiation and convection for the water-cooled walls and the convective heating surfaces. The furnace exit flue gas temperature drops for coupling H2 or NH3 but rises for coupling CH3OH or CH3OCH3 with the growth of the coupling mass percentage. The variations of boiler thermal efficiency with different coupling mass percentages under BMCR load are 0.55% to 0.90% for H2, −0.32% to −1.16% for NH3, −0.22% to −2.03% for CH3OH, and −0.08% to −0.59% for CH3OCH3. The acid dew point rises and then drops as the coupling mass percentage grows. The flow rate of the total flue gas increases at the outlet of the air preheater. Thus, the current forced draft fan could not meet the requirements, and its retrofit should be performed. After the large proportion of cofiring, the coal consumption rate drops. The annual emissions reductions of CO2 are up to 0.734 million tons for H2, 0.231 million tons for NH3, 0.062 million tons for CH3OH, and 0.101 million tons for CH3OCH3 under BMCR load at a 20% coupling mass percentage. The lifecycle CO2 reduction potential should be considered for better illustrating the benefits of cocombustion in future studies.

Performance Analysis of an Absorption Heat Pump System for Waste Heat and Moisture Cascade Recovery from Flue Gas.

The typical flue gas exhaust temperature of coal-fired boilers is up to 150 °C. There is a considerable amount of waste heat that cannot be efficiently recovered. This study describes a zero-energy consumption absorption heat pump system that can improve the thermal efficiency and recover moisture. In the proposed system, lithium bromide solution serves as the recyclable heat-transfer medium, which absorbs sensible heat at the outlet of the air preheater by a generator. The latent heat of the flue gas is absorbed by an evaporator, and the condensate water appears. Theoretical investigation and mathematical models are built. Meanwhile, the operating parameters of the system are displayed. The results showed that the acid dew point of flue gas is related to the water content. The sorption heat pump efficiency (coefficient of performance) increased to 1.64. Compared with the two-stage heat exchanger system, 24.4 t/h condensate water and 47.21 MW waste heat were recovered. The flue gas temperature at the chimney entrance was reduced by 4.18 °C.

Electrochemical Behavior and Passive Film Properties of Hastelloy C22 Alloy, Laser-Cladding C22 Coating, and Ti–6Al–4V Alloy in Sulfuric Acid Dew-Point Corrosion Environment

The electrochemical behavior and passive film properties of Hastelloy C22 alloy, laser-cladding C22 coating, and Ti–6Al–4V alloy in sulfuric acid dew-point corrosion environment were investigated through a combination of electrochemical measurements and surface analyses. The C22 alloy and laser-cladding C22 coating exhibited similar passivation and repassivation behavior without pitting corrosion, resulting from a similar passive film with a bilayer structure consisting of a Cr2O3-dominated compact inner layer and a porous outer layer containing oxides of Mo and hydroxides of Ni and Cr. The slightly poorer corrosion resistance and higher sensitivity to localized corrosion exhibited by the C22 coating were attributed to the microscale heterogeneity of the passive film resulting from the element segregation in the microstructure introduced by the laser-cladding process. The corrosion of the TC4 alloy performed as the preference dissolution of the β phase. Compared to the C22 alloy and C22 coating, the TC4 alloy exhibited more stable passivation behavior but poorer corrosion resistance, which is attributed to a compact but less protective single-layer passive film consisting of oxides of Ti and Al. An increase in temperature degrades passive film stability and accelerates the charge transfer process.

Effect of Cr on Corrosion Behavior of Laser Cladding Ni-Cr-Mo Alloy Coatings in Sulfuric Acid Dew Point Corrosion Environment

Ni-Cr-Mo alloy coatings with different content of Cr element were produced on carbon structural steel by laser cladding. Coatings exhibited similar phase composition containing γ-Ni solid solution and similar microstructure composed of primary grains and eutectics with segregated Mo. Corrosion behavior of coatings in a sulfuric acid dew point corrosion environment were investigated through the combination of immersion test, electrochemical measurements, and surface analysis. Coatings exhibited similar passive behavior and passive film with a bilayer structure consisting of a compact inner layer and a porous outer layer. An increase in Cr resulted in a more protective passive film with thicker and more compacting inner layer. However, the corrosion resistance was firstly degraded as the content of Cr increased from 18 wt.% to 22 wt.%, and then improved as the content of Cr increased from 22 wt.% to 26 wt.%. This phenomenon was attributed to the competition effect between the increase in Cr and relative decrease in Ni.

Electrochemical Behavior and Passive Film Properties of Hastelloy C22 Alloy, Laser Cladding C22 Coating, and Ti–6al–4v Alloy in Sulfuric Acid Dew Point Corrosion Environment

Electrochemical Behavior and Passive Film Properties of Hastelloy C22 Alloy, Laser Cladding C22 Coating, and Ti–6al–4v Alloy in Sulfuric Acid Dew Point Corrosion Environment

Influence of thermal stability on organic Rankine cycle systems using siloxanes as working fluids

• An off-design model with fluid decomposition parameters was established. • The influence of MM thermal decomposition on the ORC system was evaluated. • MM decomposition reduced the net power and efficiency of ORC system obviously. • MM decomposition may lead to incomplete evaporation and evaporator acid corrosion. The organic Rankine cycle (ORC) is an efficient power generation technology that has been widely used in renewable energy utilization and industrial waste heat recovery. The thermal stability of working fluids has a significant effect on the fluid selection and system design of ORC systems. In this study, an off-design model of an ORC system was established with hexamethyldisiloxane (MM) as the working fluid, and the effects of the MM thermal stability on the system were analyzed. The results showed that the effect of MM thermal decomposition on the cold source and working fluid pump was limited. The outlet temperature of the evaporator decreased with MM decomposition, which might lead to incomplete evaporation of working fluids and possible damage to the expander. The outlet temperature of the heat source also decreased with MM decomposition, which led to lower outlet temperatures than the acid dew point limit temperature. Both of these results can affect the safe operation of ORC systems. The net power and thermal efficiency of the system decreased with increasing thermal decomposition ratios of MM. The net power and thermal efficiency decreased by 7.48% and 10.72% respectively in the model of this study with a 10% decomposition ratio.

Analysis and monitoring of the combustion performance in a biomass power plant

Combustion air systems are an important aspect of the industrial boiler's performance, and they are being used increasingly to reduce emissions. Generally, improvements in boiler performance and emissions reduction can be achieved by modifying the air system. In the specific case of primary air, its temperature, distribution, and vibration of the grate significantly affect the conversion and mixing of biomass. Consequently, in this work, a comprehensive analysis of a biomass power plant operation, which is a result of an experimental campaign developed for 12 days is presented. In this regard, since the fuel has highly variable physical and chemical properties, disturbances causing an unsteady conversion process and corrosion risks were verified. Furthermore, the influence of the primary air distribution and the grate vibration cycles was evaluated through different tests. The grate vibration periods revealed that increasing the interval without vibrations results in a significant reduction of CO emissions. However, increasing this period may lead to some agglomeration problems in the grate. Concerning the primary air supply, in the three different sections of the grate, the supply of a more significant amount of air in the first two sections of the grate, in the central region of the grate, decreases the CO emissions. Additionally, the possibility to reduce the flue gas temperature using empirical correlations for a quick estimation of acid dew point during flue gas cooling for heat recovery to preheat the primary air was studied. The results revealed a large amount of energy that can be recovered without low-temperature corrosion problems, and 7% of the total input energy can be saved.

Particle charging in electric field under simulated SO3-containing flue gas at low temperature

Particle and SO3 emitting from the processes of fuel combustion considerably deteriorate environment. Charging of particle in electric field was widely used for achieving effective removal of particle from SO3-containing flue gas in various industries. This study provided an insight into particle charging in electric field with presence of SO3 in terms of particle charging simulation and experiments under various conditions. Results illustrated that once temperature was below acid dew point, particle charge increased and then declined with the increasing concentration of SO3. Besides, sulfuric acid aerosol transformed from gaseous SO3 was enhanced by higher electric field strength and temperature, but this increase seemed to be limited when reached maximum. Meanwhile, electrode optimization could enhance charging by improving ion density, which charge of particle with 0.04 μm and 1.22 μm was increased by 3.53 and 1.16. Moreover, improving dielectric constant and electric field strength mainly acted on charging of larger particle, whereas improving ion density determined small particle charging. As for medium-sized particle, simultaneously combining electric field strength and ion density adjustment to enhance particle charging was effective. Find suggested that different parameter adjustment methods should be selected according to particle diameter to enhance particle charging and reduce particle emission of SO3-containing flue gas.

Performance Analysis of the Technology of High-Temperature Boiler Feed Water to Recover the Waste Heat of Mid-Low-Temperature Flue Gas.

In coal-fired power plants, most of the working fluids used in a mid–low-temperature flue gas waste heat recovery system (FGWHRS) are low-temperature boiler supply air or condensate water in the flue gas condenser. This is prone to cause low-temperature corrosion, as the system temperature is lower than the acid dew point of the flue gas. In this study, an experimental apparatus was set up at the entrance of the desulfurization tower of a 330 MW unit in Xinjiang, China, which uses the technology of high-temperature boiler feed water (above 80 °C) to recover the waste heat of mid–low-temperature flue gas. The heat exchange performance of the mid–low-temperature FGWHRS was evaluated under different working conditions, and the optimal input parameters of the system for each considered working condition are given based on the analysis. It was found that the low-temperature corrosion in the system could be avoided using this technology. To eliminate low-temperature corrosion, the lowest temperature for the inlet water was predicted to be 69 °C in our study via curve fitting based on the experimental data. The results could provide a theoretical basis and engineering guidance for determining the best heat recovery strategy of mid–low-temperature FGWHRS.

Numerical study on sulfur-bearing natural gas oxy-fuel combustion power plant

• A sulfur-bearing natural gas power plant adopting transcritical CO 2 cycle is built. • Oxy-fuel combustion technology is employed to reach near zero CO 2 emission. • The effect of sulfur content of fuel and combustion diluent are studied. • The efficiency of the proposed systems with recycled flue gas can reach 67.59%. Waste heat recovery technology can significantly improve the efficiency of oxy-fuel combustion system. The restriction of the acid dew point of the flue gas is essential to be considered due to the low-temperature corrosion during the waste heat recovery process. In this paper, a sulfur-bearing natural gas oxy-fuel combustion power plant is established to explore the effect of sulfur content on the system performance. Sulfur content (taking H 2 S as an example) in natural gas is set to 1 ppmv-1 vol%. Three different diluents of recycled flue gas, H 2 O and CO 2 are adopted to moderate the combustion temperature. Transcritical CO 2 cycle is introduced to efficiently utilize flue gas waste heat. As H 2 S content increases from 1 ppmv to 1 vol%, the power generation efficiency of recycled flue gas system, H 2 O system and CO 2 system decreases by 0.61%, 1.15% and 1.65%, respectively. When H 2 S is 1 ppmv, the energy efficiency can reach 67.59% in recycled flue gas system, 18.74% and 12.09% respectively higher than H 2 O and CO 2 system. The recycled flue gas system is preferred to be applied in the oxy-fuel power generation system driven by sulfur-bearing natural gas, which can improve energy efficiency and reduce environmental impact.

Determination of optimum organic Rankine cycle parameters and configuration for utilizing waste heat in the steel industry as a driver of receive osmosis system

The existence of the waste heat from the exhaust of steel industry with the temperature of 227 °C and water evaporation in the cooling process, the limitation of the water sources with high quality, the sharp decline of the groundwater, and access to the saline water are the reasons to study and analyze the ORC-RO system in this paper. This investigation is based on energy analysis (1E), exergy analysis (2E), economic analysis (3E), and environmental impacts (4E). The environmental analysis is conducted considering three aspects: (1) The effects of the ORC system depending on the selection of the working fluid. (2) The impacts of RO system on the brine, which depends on the brine stream density (introduced by the non-dimensional function Ω), showing the span of the RO system brine dilution. (3) The decline in the removal of the groundwater happens when the fresh water increases. In this comparison, four types of cycles consist of basic configuration, basic configuration with recuperator, basic configuration with regenerative, and basic configuration with the combination of regenerative and recuperator are investigated. The trade-off between exergy and thermal efficiency parameters and TAC (total annual cost) are the reason for using the Genetic Algorithm (GA) as the optimization algorithm. The main optimization parameters are: (1) configuration of ORC, (2) the optimized design of RO system, (3) Rankine cycle working fluid based on thermal source, (4) membrane selection among company membranes DOW, (5) determining the approach and pinch temperatures of WHR for maximum recovery, according to the limitation of acid dew point. These parameters are obtained based on two objectives; reducing the price of produced water and increasing the system’s exergy efficiency. Production of freshwater reduces water withdrawal from groundwater resources, and the working fluid with low ODP and GWP reduces the environmental impacts of the cycle. The results show that the cycle with basic configuration and the cycle with recuperator are the two optimum cycles in the Pareto curve. The working fluid R245ca (GWP = 640; ODP = 0) is chosen as optimum fluid in each of the two cycles. The exergy efficiency, total cost and Ω in basic cycle and the cycle with recuperator are 32.51%, 2.84 $/m3, 16.94% and 32.49%, 2.64 $/m3, 12.74% respectively. Also, the production of freshwater reduces 2.5% of water withdrawal from groundwater resources.

Experimental study on novel waste heat recovery system for sulfide-containing flue gas

In order to further recover the waste heat from sulfide-containing flue gas, improve the recovery efficiency and reduce corrosion of heat exchanger before flue gas desulfurizer (FGD), a novel system based on the phase change heat transfer theory is proposed to recover the waste heat from sulfide-containing flue gas. The performance of novel waste heat recovery system is investigated by the experimental method. The results show that the heat recovery coefficient will increase with the heat load ratio while decrease with the circulation ratio. To ensure the high-efficiency operation of the novel system, the mass flow rate of lithium bromide solution should be adjusted in terms of the heat load ratio and circulation ratio in range of 48–178. Otherwise, it will be only adjusted in terms of the heat load ratio and increase with that. The exergy efficiency of the novel system will reduce with the heat load ratio because of the influence of energy efficiency of FGHE. But the exergy efficiency is about 25%–34% and lower. The heat transfer coefficient of FGHE is relatively higher for the heat load ratio in range of 0.7–1.1. The novel system is more advantageous in waste heat recovered from flue gas, the CO2 emission reduction and economic performance when the acid dew point temperature in range of 110–140 °C is changed under different operation conditions. The above conclusions will provide a theoretical basis for the operation of novel waste heat recovery system for sulfide-containing flue gas.

Numerical study on acid condensation and corrosion characteristics of three-dimensional finned tube surface

Low temperature corrosion is a key factor affecting the performance and safety of heat exchanger for industrial waste heat recovery system. Acid dew point temperature and vapor condensation characteristics are two major parameters to evaluate corrosion behavior. However, previous studies focused solely on acid dew point temperature or condensation characteristics. In present paper, a numerical model was established to investigate the heat and mass transfer process of flue gas on three-dimensional finned tube surface and predict the acid dew point temperature and condensation characteristics simultaneously. Firstly, the effects of operation parameters were examined both by single variable method and orthogonal method. The results show that the flue gas inlet velocity and acid vapor concentration have the most significant effects on acid dew point temperature; the tube wall temperature and water vapor concentration have the most significant effects on the condensed acid solution concentration. Then, the corrosion characteristics was discussed and the optimization working parameters (the water vapor concentration in flue gas below 10.12% and the tube wall temperature above 349.3 K) are recommended.

Failure analysis of air preheater tubes in a circulating fluidized bed boiler

A tubular air preheater (APH) is located at the cold end in the back pass region of a circulating fluidized bed (CFB) boiler. After five years of operation, a significant number of tubes at the air inlet section of the tube were found to have a leak. Some samples were taken from the site for laboratory analysis. Optical microscopy, scanning electron microscopy-energy dispersive spectroscopy, and other standard laboratory tests were used to analyze the samples. A computational fluid dynamics (CFD) simulation was performed to simulate the temperature distribution in the APH using typical operating parameters of the circulating fluidized bed boiler. Chemical analysis of the tube shows high phosphorus content in the steel above that allowable by the standard. Analytical calculation of the acid dew point temperature indicates that sulfuric acid condenses at 143 °C. The CFD analysis showed that the tube surface temperature has a skin temperature of less than 143 °C. This low skin temperature combined with high sulfur content in the coal triggers high condensation of sulfuric acid, which led to severe corrosion and leaks. The high phosphorus content of the steel amplifies the corrosion rate under sulfuric acid deposition. The present work shows how a high phosphorous content in steel accelerates corrosion by sulfuric acid in power plant equipment.