Ultra-low NOx Research Articles

Dual fuel combustion and hybrid electric powertrains as potential solution to achieve 2025 emissions targets in medium duty trucks sector

The European commission is targeting a 15% reduction in CO2 emissions for medium and heavy-duty transportation starting in 2025. Moreover, the next European normative (EU VII) will impose a decrease of 50% for NOx and particulate matter emissions with respect to the current EUVI normative. Meeting these requirements pose a significant challenge to truck and bus manufacturers. Several proposals appeared in the last few years as improve the cabin aerodynamics, decrease the friction losses and improve the powertrain efficiency. The last point involves improving the current combustion systems as well as the transmission and energy management. This work proposes to couple two potential technologies to reduce at the same time the global (CO2) and local pollution (NOx and soot). For this, two truck platforms representative of medium-duty applications (18 ton and 25 ton) are tested using the reactivity controlled compression ignition (RCCI) combustion mode with diesel and gasoline as fuels. In addition, the trucks are electrified to full hybrid technology in a parallel pre-transmission (P2) architecture. A 0D vehicle numerical model is used to evaluate the trucks under four different driving cycles representative of homologation and real driving conditions. The numerical model is validated against on road measurements. The RCCI combustion is modeled by means of a map-based approach with 54 points measured in steady-state conditions. This work presents a complete engine map calibration with measurements up to 350 hp using two combustion modes inside the map (so-called dual-mode dual-fuel). As a baseline, the commercial diesel no-hybrid trucks and the dual-fuel no-hybrid trucks are used. The results show the potential of the dual-mode dual-fuel combustion to achieve ultra-low NOx and soot emissions. In addition, the CO2 target reduction is achieved for several truck platforms and driving conditions due to the hybridization of the driveline. The cycles with large phases of urban driving are the most favorable due to the ability of recovering energy by means of the regenerative braking system and the possibility to avoid large idling phases with respect to the no-hybrid versions. In addition, the decrease of the payload improves the CO2 reduction with respect to the baseline cases.

NOx emission and its reduction mechanism investigation in one diffusion-like vortex-tube combustor

In order to overcome the instability of ultra-lean combustion, a novel vortex-tube combustor with axial fuel injection was proposed. The stability limit, flame configuration, NOx emissions, and burn-off rate of the combustor were investigated experimentally under various global equivalence ratios (ϕg) and fuel flow rates (qf). Results show that the combustor exhibits a large stability limit with ϕg and qf as low as 0.01 and 6 × 10−5 m3/s respectively. Complete combustion and ultra-low NOx emissions of less than 3 ppm can be achieved at ϕg of 0.3 and qf of more than 30 × 10−5 m3/s, indicating that the combustor has a good potential for ultra-lean combustion and low NOx emission. The pressure fluctuation amplitude is always less than 1300 Pa during the entire experiments. The vortex-induced flame has a diffusion-like flame structure, which provides a suitable equivalence ratio zone under ultra-lean conditions, whilst the high peak combustion temperature indicates an intensified combustion, which is responsible for the large stability limit and low-pressure fluctuation amplitude. Subsequently, the enhanced stabilization can enable the ultra-lean combustion and the ensued low temperature to conduct, whilst the vortex-flow can decrease the local flow velocity and enable the turbulent diffusion velocity of NO to be dominant, which can make the NO emission reduced further. The decreased area of high-temperature region and the appearance of the negative reaction rate region of NO is the essential reason for the decrease of the NO emission at lean cases.

CO <sub>2</sub> Well-to-Wheel Abatement with Plug-In Hybrid Electric Vehicles Running under Low Temperature Combustion Mode with Green Fuels

<div class="section abstract"><div class="htmlview paragraph">Plug-in Hybrid Electric Vehicles (PHEVs) can be considered as the most promising technology to achieve the European CO<sub>2</sub> targets together with a moderate infrastructure modification. However, the real benefits, in terms of CO<sub>2</sub> emissions, depend on a great extent on the energy source (fuel and electricity mix), user responsibility, and vehicle design. Moreover, the electrification of the powertrain does not reduce other emissions as NOx and particulate matter (mainly soot). In the last years, low temperature combustion (LTC) modes as the reactivity controlled compression ignition (RCCI) have shown to achieve ultra-low NOx and soot emissions simultaneously due to the use of two fuels with different reactivity together with high exhaust gas recirculation (EGR) rates. Therefore, the aim of this work is to assess, through numerical simulations fed with experimental results, the effects of different energy sources on the performance and emissions of a series RCCI PHEV. The dual-fuel engine was fueled with diesel as high reactivity fuel and bioethanol as low reactivity fuel. The powertrains are optimized to meet the European homologation legislation (WLTP) for PHEVs. The tailpipe emissions and fuel consumption of the series RCCI PHEV is analyzed and compared to the OEM no-hybrid passenger vehicle running under conventional diesel combustion and RCCI. A life-cycle analysis (LCA) is performed to evaluate the potential of the different energy sources to reduce the overall CO<sub>2</sub> emissions. Different electricity sources are investigated as well as different liquid fuels: e-diesel and bio-ethanol. The series RCCI PHEV technology achieved the tank-to-wheel 50 g/km CO<sub>2</sub> emissions target with medium battery size (13 kWh). In addition, thanks to the RCCI technology, the vehicle is able to meet the Euro 6 NOx and soot limits. The LCA shows that, depending on the fuel source, the total CO<sub>2</sub> reduction benefits variate. Also, the final use and the charging events of the series PHEV have a great impact in this sense.</div></div>

Ion current–based homogeneous charge compression ignition combustion control using direct water injection

Homogeneous charge compression ignition has proven to be both highly efficient and to operate with ultra-low NOx raw emissions. However, homogeneous charge compression ignition combustion is a dynamic process due to strong cycle-to-cycle coupling effects caused mainly by the residual gas. Compared to conventional spark-ignited and diesel engines, the lack of direct mixture composition and ignition control increases the challenge of combustion instabilities, especially at boundary conditions. To stabilize the combustion process, real-time in-cylinder combustion diagnostics and control are often used. In this study, for the first time, ion current detection technology and direct water injection are combined for homogeneous charge compression ignition combustion control. By analyzing the return map of the crank angle at 50% cumulative heat release under unstable conditions, it was identified that a misfire or incomplete combustion is usually followed by knocking-like early combustion with high cylinder pressure gradients. Through the correlation analysis between ion current and combustion, a cycle-to-cycle closed-loop control strategy was developed and implemented on a rapid control prototyping engine control unit. Real-time calculated ion current parameters were used to predict the 50% cumulative heat release position of the next cycle and prevent early combustion by direct water injection. The calculation results and controller performance were validated on a single-cylinder research engine. With the controller activated, the standard deviation of 50% cumulative heat release and dynamic programming to the max could be reduced by 19% and 11%, respectively. The coefficient of variation of indicated mean effective pressure was reduced by 12%. A slight increase in indicated mean effective pressure after activating the controller also shows the potential for efficiency improvement. Moreover, not only early combustion is controlled, but also late combustion is significantly reduced.

The ultra-low NOx emission characteristics of pulverized coal combustion after high temperature preheating

Preheating combustion is a promising low-nitrogen combustion technology. The article mainly describes the ultra-low NOx emission characteristics during preheating combustion, including preheating characteristics, combustion characteristics, and fuel nitrogen migration and transformation characteristics. Under the condition of fluidized bed equivalence ratio of 0.63 and preheating temperature of 980 °C, the calorific value of high-temperature coal gas is about 1.75 MJ/Nm3. Compared with raw coal, preheated char is modified and upgraded with a smaller particle size, larger specific surface area, and more pore structure. The combustion temperature is about 1045 °C, and the main form of NOx (NO2 or NO) along the DFC changes with the intensity of combustion. During the preheating process, 76.1% of the fuel nitrogen is converted to N2. Only 5.9% of the fuel nitrogen is converted to NH3, and NH3 is the only precursor of NOx. The average NOx emission is 31 mg/m3 (@6% O2), and corresponding conversion ratio of fuel nitrogen to NOx is 0.85%. This experimental results will play a vital role in the development of efficient and clean coal combustion technology.

OMEx-diesel blends as high reactivity fuel for ultra-low NOx and soot emissions in the dual-mode dual-fuel combustion strategy

Previous works demonstrated that the use of Oxymethylene ether (OMEx) in advanced combustion modes, as the dual-mode dual-fuel combustion, leads to a notable reduction of the lifecycle CO2 emissions while promoting lower NOx and soot emissions than those from conventional diesel combustion. Nonetheless, the low heating value of OMEx results in a fuel consumption increase. A possible solution to avoid this drawback is by blending OMEx with diesel fuel. This will help to introduce the OMEx in the market with minimum changes in the infrastructure. In this context, this work evaluates the impact of using OMEx-diesel blends in different mass percentages (50% and 70% of OMEx in diesel), compared to the reference net fuels (net diesel and OMEx) in a multi-cylinder compression ignition engine operating under dual-mode dual-fuel combustion at different engine loads (25%, 50%, 80% and 100%) and 1800 rpm. At each condition, an air mass sweep was performed to assess the limiting operating conditions with each fuel due to either excessive pressure gradients and soot production, or low combustion efficiency. The results suggested that the OMEx-diesel blends allow to reduce the soot emissions compared to net diesel for all the conditions tested. In addition, blends having an OMEx mass content greater than 70% allowed to fulfill the EUVI limits for NOx with ultra-low soot levels (<0.01 g/kWh) up to 80% engine load. Nonetheless, the unique fuel able to achieve the EUVI limit for NOx with zero soot emissions simultaneously at 100% of engine load was found to be the pure OMEx.

Lower-arch location effect on the flow field, coal combustion, and NOx formation characteristics in a cascade-arch, down-fired furnace

For a cascade-arch low-NOx and high-burnout configuration designated as a solution to the currently incompatible problem of the ultra-low NOx combustion and high burnout in down-fired furnaces, numerical simulations, which were verified by real-furnace measurements of a 600 MWe down-fired furnace with a multiple-injection and multiple-staging combustion technology, were performed to evaluate its in-furnace flow, coal combustion, and NOx formation at various lower-arch location setups of CH = 0.65, 0.6, 0.55, and 0.5 (CH representing the ratio of the lower arch height to the front/rear wall height in the lower furnace). The aim was to evaluate its lower-arch location effect on the furnace performance and confirm its superiority compared with the prior art. With asymmetric combustion developing at extreme settings of CH = 0.65 and 0.5, the combustion symmetry and furnace performance originally improved but then worsened with decreasing CH, resulting in the exhaust gas loss, carbon in fly ash, and CO and NOx emissions all decreasing first and then increasing. Consequently, the CH = 0.6 location setup achieved the optimal furnace performance characterized as NOx emissions of 707 mg/m3 at 6% O2 and carbon content in fly ash of about 5.50%. In comparison to the prior art, the new combustion configuration owned an improved symmetrical combustion pattern, strengthened deep-air-staging conditions, and lengthened flame travel by positioning hopper air, applying flue gas recirculation, and introducing fine pulverized-coal reburning, thereby achieving the apparently improved low-NOx and high-burnout performance with a NOx reduction of 22% without affecting burnout.

Experimental study on NOx formation and burnout characteristics of pulverized coal in oxygen enriched and deep-staging combustion

Deep-staging combustion is an effective technology for reduction of NOx, but is often accompanied by the combustion worsening. Oxygen enriched and deep-staging combustion is a promising technology to realize both low NOx formation and high burnout rate. NOx reduction and burnout characteristics of pulverized coal in oxygen enriched combustion were experimentally investigated in this study with a drop-tube reactor. Results show that the NOx reduction is significantly promoted by elevated O2 concentration under deep-staging condition. The NOx formation of coal under 40% oxygen concentration condition is 40.7% lower than those in air combustion as the stoichiometric ratio in the fuel-rich zone (SR1) is 0.5. The thermal and fuel NOx formations were separately investigated. The fuel NOx reduction with increase of O2 concentration makes a major contribution to the total NOx reduction under deep-staging conditions. The elevated O2 concentration facilitates the release of fuel nitrogen in the fuel-rich zone, avoiding its oxidation in the fuel-lean zone. Moreover, the negative correlation between NOx formation and unburned carbon (UBC) in coal ash is evidently weakened with the increase of O2 concentration. Ultra-low NOx formation and high combustion efficiency could be concurrently achieved in oxygen enriched combustion under deep-staging condition. Furthermore, the comprehensive comparison of the oxygen enriched combustion, air combustion and oxy-fuel combustion was conducted. Low NOx formation can be reached in oxy-fuel combustion under non-staging condition, while oxygen enriched combustion performs much better in NOx reduction under deep-staging conditions.

Study on ultra-low NOX emission technology of 660 MW ultra-supercritical circulating fluidized bed boiler

For the first 660MW ultra-supercritical circulating fluidized bed boiler in China, the original NOX emission concentration would be controlled and maintained below 50mg/Nm3, by improving the uniformity of bed temperature and bed pressure, strengthening the secondary air fractional combustion, optimizing the combustion temperature and operating oxygen and so on. Meanwhile, selective non-catalytic reduction (SNCR) technology with urea as reducing agent is selected as the auxiliary method to ensure the boiler achieves ultra-low NOX emission under full load conditions.

Influence of feeding position and post-combustion air arrangement on NOx emission from circulating fluidized bed combustion with post-combustion

To explore the influences of different feeding position and post-combustion air (PCA) arrangement on combustion and original NOx emission characteristics of Shenmu semi-coke, the experiments were carried out on a new circulating fluidized bed (CFB) test platform, which was composed of a CFB and a post-combustion chamber (PCC). The experimental results showed that the feeding position of loop seal could effectively reduce original NOx emission and CO emission concentration in comparison to the conventional coal feeding position of furnace. The NOx emission concentration was 60.3 mg/m3 with single stage PCA. As the grading number of PCA increased, NOx emission can be gradually reduced, eventually falling to 49.6 mg/m3 with triple stages PCA. However, for CO emission, the double stages PCA were able to minimize CO emission. In addition to grading number of PCA, an appropriate injection position of PCA was also helpful for CO burnout. For double stages PCA, the two PCA nozzles should be arranged in the middle of the PCC. According to the above research results, the best feeding position and PCA distribution mode to achieve ultra-low original NOx emission (<50 mg/m3) were obtained.

Prospect analysis of high temperature air combustion technology for low calorific value coalbed methane in Liupanshui area of Guizhou province

China’s coalbed methane resource is abundant, which ranks third in the world. The total amount of coalbed methane resources in Guizhou Province accounts for about 10% of China, which ranks second in the country. However, the utilization rate of coalbed methane in Guizhou Province is not only low, but also relatively single, especially the low calorific value coalbed methane extracted from underground mines which has a methane concentration of only 10-30%. And the main method to deal with low calorific value coalbed methane is discharging directly, which caused a huge waste of energy. High temperature air combustion technology (HTAC), which expands the utilization range of low calorific value fuels, has attracted wide attention due to its characteristics of extreme recovery of waste heat and ultra-low NOx emission. In this paper, the characteristics of high temperature air combustion of low calorific value coalbed methane are analyzed by theoretical and numerical calculation based on combustion theory and CFD method.

Partially premixed combustion optimization using double injection strategy in transient operation

Partially Premixed Combustion (PPC) has been proved a high efficiency combustion concept, along with ultra-low soot and NOx emissions. However, the nature of high pressure rise rate prevents PPC from operating range extension and further practical use. This paper aims to optimize mixture preparation to achieve the benefits of PPC with the constraints of engine states through injection strategy during transient operation. A control-oriented model (COM) is developed based on double-Wiebe function to predict combustion process, where a new linear algorithm is proposed to identify the model parameters. The root mean square error (RMSE) of the predicted cylinder pressure is less than 2.58 bar and peak error is less than 5% against experimental measurements of steady states. A constrained model predictive controller (MPC) is designed and implemented in a PPC engine. Simulation and experiment results show that the proposed controller manipulates injection events to optimize premixing period and fuel distribution towards more benefits of PPC concept. In the testing scenario, soot, NOx and pressure rise rate are regulated within 0.15 mg/m3, 400 ppm and 8 bar/deg, respectively. Consequently, cumulative soot and NOx emissions are reduced by 43.2% and 6.8% in the whole transient cycle.

The comparison of combustion, engine performance and emission characteristics of ethanol, methanol, fusel oil, butanol, isopropanol and naphtha with n-heptane blends on HCCI engine

Recently, there has been increased emphasis on the homogenous charge compression ignition (HCCI) engine which offers higher thermal efficiency and ultra-low NOx and soot emissions. The central thesis of this paper is that the effects of different alternative fuels with different physical and chemical properties on combustion, performance and emissions in a HCCI engine. Ethanol (E25), methanol (M25), fusel oil (F25), butanol (B25), isopropanol (IP25) and naphtha (N25) were used alternative fuels blended with n-heptane 25% by volume. The experiments were performed at 333 K intake air temperature (IAT) and with various excess air coefficient in a single cylinder SI-HCCI test engine. The parameters such as in-cylinder pressure, heat release rate (HRR), start of combustion (SOC), combustion duration, CA50, indicated thermal efficiency (ITE), COVIMEP, maximum pressure rise rate (MPRR), HC and CO emissions were investigated. In addition, operating range of the fuels were also defined. The results showed that N25 has the largest operating range among all the test fuels. The maximum indicated mean effective pressure (IMEP) value was obtained for E25 as 5.71 bar at 800 rpm engine speed and λ = 1.3. Knocking tendency were determined for all test fuels but it decreased with increasing lambda value. The highest HC and CO emissions were obtained for F25 due to the water content of the fuel. On the contrary, the lowest HC and CO emissions were obtained for N25.

Design and optimization of distribution structure of fully premixed surface burner

In response to China’s energy policy and the increasingly stringent NOx emission standards, premixed surface burners have gradually entered the field of vision as an ultra-low NOx emission gas burner. In this paper, the numerical simulation software ANSYS FLUENT was used to analyze the airflow distribution structure of a certain type of premixed surface burner, and the distribution characteristics of the premixed gas flow in the distribution structure during actual operation were also obtained. At the same time, the parameters of the distribution structure were optimized according to the problems appearing in the distribution of airflow by numerical simulation. The results showed that the design and optimization of the distribution structure of the premixed surface burner were reasonable and effective.

Diesel Passenger Car with Ultra-low NOx Emissions in Real Driving Conditions

Diesel Passenger Car with Ultra-low NOx Emissions in Real Driving Conditions

An experimental assessment on semi-low temperature combustion using waste oil biodiesel/C3-C5 alcohol blends in a diesel engine

Biodiesel fuel properties should be improved in order to be able to use such alternative fuels at high mixing ratios in diesel engines. Higher alcohols such as propanol (C3), n-butanol (C4) and 1-pentanol (C5) are competitive alternative fuels for diesel engines due to their liquid nature, high oxygen contents, high cetane number (CN), lower latent heat of evaporation (LHE) and their production from renewable biomass. Oxygen-containing fuels that can be used in diesel engines reduce the combustion chamber temperature and harmful emissions. At this point, the addition of higher alcohols to biodiesel also has the potential to improve both low temperature combustion and fuel properties. The purpose of this work is to investigate the fuel properties of binary blends of waste oil methyl ester (WB) and the higher alcohols of C3, C4 and C5, and their effects on engine characteristics to achieve semi-low temperature combustion (S-LTC) and obtain lower regulated emissions in comparison to WB. Three different test fuel blends were prepared by volume: 80%WB-20%C3, 80%WB-20%C4, and 80%WB-20%C5. In order to determine engine performance and exhaust emissions, tests were performed at four engine loads (0, 3, 6, 9 kW) with a constant engine speed (1800 rpm). According to engine performance and exhaust emissions results, the addition of C3, C4 and C5 to WB had the effect of increasing brake specific fuel consumption (BSFC) and exhaust gas temperatures (EGT), while the brake thermal efficiency (BTE) decreased. The overall WB-C3/C4/C5 blends lean combustion of the diesel engine provided low carbon monoxide (CO) and oxides of nitrogen (NOx) emissions compared to WB, alone. Use of binary blends as S-LTC providers in diesel engines offer the benefits of ultra-low NOx and CO emissions but suffers from a lowered energy efficiency and higher hydrocarbon (HC) emission due to the high oxygen and volatility of higher alcohols. It can be concluded that C3, C4 and C5 are capable of reducing harmful emissions, however, this is accomplished at the expense of lower engine performance characteristics.

Review paper on thermo-acoustic instabilities in a gas turbine burners – Flashback avoidance

Next generation modern aircraft gas turbine engines (GTE) should provide ultra-low emissions, higher operating efficiencies and cost-effective production and use of energy with decreased emissions at local and global levels. However, the combustors particularly developed for ultra-low NOX emission combustors being developed for aircraft gas turbine engines are more susceptible to combustion instabilities. Premixed combustion systems have the capability to meet future regulations on NOx emissions. However, premixed systems always involve the risk of flame flashback into the premixing section. From a gas turbine manufacturer’s point of view, it is required to enlarge the safe operating range, in particular with respect to flame flashback. The significance of these flashback phenomena is a strong function of fuel composition and operating conditions. In the literature, flashback along the wall boundary layer shows the most critical failure mechanism for many burner configurations using hydrogen-rich fuels. Therefore, in the past few years, Government, academia and several industries conducting extensive research in this area for the introduction of green technologies as lean fuel combustion and premixed burners in aero-engines. Therefore, this review paper focus on flashback propensity mechanisms in more detail and the conclusions are drawn as to mitigation technologies.

NOx Minimization in Staged Combustion Using Rich Premixed Flame in Porous Media

ABSTRACT Two-stage combustion of methane/air is studied experimentally and numerically with a focus on achieving ultralow NOx emissions. The primary flame is a rich premixed flame in the porous medium. The flame is stabilized in the range of equivalence ratios from 1.1 to 1.7 using upstream heat extraction. The products of the primary flame are rich in the partial oxidation and reforming products such as CO, H2, and CH4. Due to the self-regulated heat losses from the flame the maximum flame temperature of the primary flame remains close to 1700 K. The rich-flame environment and low-flame temperatures limit NOx formation in the primary flame. The NOx emission index of the primary flame shows a maximum at the equivalence ratio of 1.1 and reduces for richer mixtures. The products of the primary flame are burned in the secondary nonpremixed flame. The products could be intercooled to reduce temperature and minimize NOx formation in the secondary nonpremixed flame. The emission index of the secondary flame increases with the equivalence ratio. Variation of combined emission index shows a minimum value at the equivalence ratio of 1.2. The trend remains consistent with the intercooling of the primary flame products.

Optimization of the parallel and mild hybrid vehicle platforms operating under conventional and advanced combustion modes

Abstract The stringent regulations, increased global temperature and customer demand for high fuel economy have led to rapid developments of different alternative propulsion solutions in the last decade, with special attention to the electrified vehicles. The combination of electric machines with conventional powertrains allows to diversify the powertrain architectures. In addition, alternative combustion modes as reactivity controlled compression ignition (RCCI) have been shown to provide simultaneous ultra-low NOx and soot emissions with similar or better thermal efficiency than conventional diesel combustion (CDC). Therefore, the combination of both technologies creates a promising horizon to be implemented in commercial vehicles of the near future. In this work, experimental and numerical simulations were combined to study the potential of the parallel full hybrid electric vehicle (P2-FHEV) and mild hybrid vehicle (MHEV) to obtain lower fuel consumption and NOx emissions than a conventional powertrain in the Worldwide Harmonized Light Vehicles Cycle (WLTC). The hybrid vehicles are simulated with both CDC and diesel-gasoline RCCI combustion engines as power source. Each powertrain was optimized in terms of components (battery, electric motors…) capacity, internal combustion engine operative points, energy management strategy and gear ratios. The results show a significant fuel consumption reduction as the complexity of the hybrid system increases. The parallel architecture, which represents the most complex hybrid system tested in this work, allows obtaining a fuel consumption reduction of around 20% as compared to CDC. The dual-mode CDC-RCCI concept showed improvements in NOx and soot emissions with comparable values in terms of energy consumption and CO2 emissions than CDC. Additionally, the mild hybrid technology with the functionality of start-stop, torque assist and regenerative braking showed an acceptable balance between complexity and fuel consumption gain.

Investigation on high temperature corrosion of water-cooled wall tubes at a 300 MW boiler

Because of the updated requirement on ultra-low NOx emission (<50 mg/Nm3), most of Chinese coal-fired boilers have to be operated at a low NOx combustion mode. However, for high-sulfur coal, water-cooled wall tubes probably suffer severe corrosion in such a strong reduction atmosphere. This work aims to investigate the high temperature corrosion behavior of water-cooled wall tubes inside a 300 MW boiler unit. A short length of corroded water-cooled wall tube was cut down and was analyzed by various characterization methods to further figure out the detailed corrosion mechanism. The typical corrosion products can be distinguished by blue, black and pale-green. Results showed that blue and black color products were mainly consisted of iron sulfides and iron oxides while the pale-green ones were identified as zinc sulfide. Along the radial direction, a layered structure of corrosion products can be observed. The formation of inner layer resulted from the reaction between iron oxide and hydrogen sulfide. The sulfur element displays a gradual increase trend while the Fe element gives out an opposite trend along the radially outward direction. The intermediate layer comes from the fly ash deposition and the outer layer is formed via condensation and deposition of ferrous sulfide gas on the water-cooled wall. The corrosion in this power plant is typical sulfide type for large amounts of Fe and S element were found in the corrosion products.