Utility Boiler Research Articles

Fault detection of FWTPs in coal‐fired power plants using K‐WD‐KPCA in consideration of multiple operation conditions

AbstractIn coal‐fired power plants, the feed water treatment processes (FWTPs) supply qualified water to utility boilers. The faults of an FWTP may endanger the whole power plant. The classical PCA‐/KPCA‐based fault detection algorithms are valid mainly under single operation condition. When they are put to a practical FWTP, they cannot deal with the problems as treatment route switching, multiple operation conditions, process fluctuations, and process nonlinearity. In this paper, k‐means, wavelet denoise (WD), and KPCA are integrated together to form a new algorithm as K‐WD‐KPCA. It is expected to deal with the process nonlinearity through KPCA, cope with the multiple operation conditions through k‐means, and relieve the process fluctuations through WD. In the experiments on the data sets collected from a practical FWTP, the multiple operation conditions are classified into three categories; consequently, three KPCA models are trained and correspondingly scheduled for online condition matching. WD is further used to denoise the real‐time T2 and SPE statistics. Results show that the WD part of K‐WD‐KPCA algorithm can indeed lower the false alarm rate without reducing its fault detection performance. Finally, the proposed K‐WD‐KPCA algorithm is coded into a software platform and deployed to a coal‐fired power plant containing 2 × 1000 MW generation units. The effectiveness of the K‐WD‐KPCA algorithm is convinced through field application results.

Failure analysis of Desuperheater

Temperature regulation by desuperheater spray is an important part to ensure the stable operation of utility boiler. However, due to the bad working conditions, the spray pipe of Desuperheater will produce strong vibration, and the connected equipment will have alternating stress. At the same time, due to design or manufacturing reasons, abrasion or cracking occurs at the welding part or the contact part between the water spray pipe and other parts. Taking measures to ensure stable operation, reasonable design and qualified manufacturing can effectively guarantee the safe operation of power plant equipment.

Effects of leaching and additives on the formation of deposits on the heating surface during high-Na/Ca Zhundong coal combustion

Zhundong coal, high-quality coal with large reserves in China, faces great challenges on its large-scale utilization in utility boilers due to the ash-related problems caused by high alkali and alkali earth metals (AAEMs). In this research, to simulate ash deposition behavior on heating surfaces in high-temperature flue gas zone (panel superheater) and that in medium-temperature flue gas zone (superheater or reheater), deposition experiments were carried out during Zhundong coal combustion in a drop tube furnace (DTF) at the deposition temperature of 1200 °C and 900 °C. Meanwhile, water leaching, Na2CO3 additive and kaolin additive were employed to elucidate the formation mechanism of ash deposition and alleviate deposition and slagging. Results showed that leaching reduced the contents of troublesome elements such as AAEMs and S and increased the relative contents of Si and Al, thus alleviating ash deposition. Compared with raw coal, the deposition rate of leached coal decreased from 5.82 mg h−1 cm−2 to 0.88 mg h−1 cm−2 at the deposition temperature of 900 °C. The primary reason for the decreased deposition rate might be that the dominant minerals in leached coal ash were refractory SiO2, CaO and Ca2SiO4. The addition of Na2CO3 would aggravate the slagging and fouling on the deposition surface significantly. The gaseous Na2O decomposed from Na2CO3 reacted with SO2 and SiO2 in the flue gas to form low-melting Na2SO4 and Na2Si3O7, which were easy to cause severe slagging on the deposition surface. The inhibition effect of kaolin addition on the formation of deposition and slagging was similar to that of leaching. After the addition of kaolin, the dominant compounds of fly ash altering to high-melting minerals such as CaSiO3, CaSO4, Ca2MgSi2O7 and NaCaAlSi2O7, resulting in a lower deposition rate than raw coal. Besides, when deposition temperature increased from 900 °C to 1200 °C the slagging degree of deposition surfaces was aggravated, while the deposition rate of four fuels followed different change trends. Specifically, the deposition rate of raw coal and leached coal decreased, whereas that of the case with Na additive and kaolin additive presented an increasing trend. At last, the role of water leaching and additives on the formation of ash deposition during Zhundong coal combustion was clarified.

Behaviors of Chromium in Coal-Fired Power Plants and Associated Atmospheric Emissions in Guizhou, Southwest China

Coal burning is a main concern for a range of atmospheric pollutants, including the environmentally sensitive element chromium (Cr). Cr migrates to the environment through stack emissions and can leach out from solid coal-burning byproducts, thereby causing adverse effects on the ecosystem. In this study, atmospheric emissions of Cr from six coal-fired power plants (CFPPs), as well as the distribution of Cr inside these CFPPs in Guizhou Province, Southwest China, were investigated. Among the six CFPPs, one was a circulating fluidized bed boiler and the others were pulverized coal boilers. The results showed that Cr in the feed fuel of these CFPPs ranged from 39.5 to 101.5 mg·kg−1 (average: 68.0 ± 24.8 mg·kg−1) and was approximately four times higher than the national and global average. Cr in the feed fuel correlated significantly with the ash yield, demonstrating that Cr in coal is closely associated with ash-forming minerals. After the coal combustion and the treatment by different air pollution control devices, most Cr (>92%) in the installation was retained in the captured fly ash and bottom ash, with less as gypsum (0.69–7.94%); eventually, only 0.01–0.03% of Cr was emitted into the atmosphere with a concentration of 1.4–2.2 μg·Nm−3. The atmospheric emission factors of Cr for these utility boilers were as low as 14.86 ± 3.62 mg Cr·t−1 coal, 7.72 ± 2.53 μg Cr (kW·h)−1, and 0.70 ± 0.19 g Cr·TJ−1, respectively. About 981 kg·y−1 of Cr was discharged into the atmosphere from Guizhuo’s CFPPs in 2017, much lower than previous reported values. Most of the Cr in the CFPPs ended up in solid combustion products, identifying the need for the careful disposal of high-Cr-containing ashes (up to 500 mg·kg−1) to prevent possible mobilization into the environment.

A Study on Slag Composition inside Biomass Boiler System

It was found that slagging in high temperature zone is one of the major problems in the refinery, petrochemical, pulp and paper, and steel industries; especially in biomass and coal pulverized power plant. Slagging conditions are critical factors influencing reliability and availability on a biomass utility boiler. In this study, several types of slagging were collected for chemical analysis. It was revealed that the chemical composition and other properties of received slagging showed unique results based on the type of feedstock fuel materials.

Experimental investigation of the characteristics of NOx emissions with multiple deep air-staged combustion of lean coal

Deep air-staged combustion is the most promising low-NOx combustion technology to reduce NOx emissions. Both the single and multiple air-staged combustion were adopted to understand the NOx evolution behaviors and to explore the efficient NOx reduction measures for lean coal. The air-staged combustion experiments of lean coal were carried out in one-dimensional self-sustained pulverized coal combustion experimental furnace with the detailed measurements of flue gas species along the furnace height. And the effects of the excess air coefficient in the primary combustion zone (αM) and the burnout air port position on the NOx emissions and burnout ratio were evaluated. The results indicate that the NOx reduction performance under deep air-staged combustion (αM ≤ 0.85) is much better than that under shallow air-staged combustion (αM = 1.05). And an optimal burnout air port position of Ms = 0.4 exists for the lower NOx emissions under single deep air-staged combustion. The burnout air streams are supplied step by step under multiple air-staged combustion to form a low-oxygen reducing atmosphere in the burnout zone, which is conducive to the heterogeneous reduction between char and NOx on the char surface. Therefore, the NOx abatement under multiple air-staged combustion is greater than that under single air-staged combustion. Triple air-staged combustion presents the best NOx reduction efficiency in the experiments and its burnout ratio has little variation compared with single air-staged combustion. These results will provide useful fundamentals for the optimal control of NOx emissions during lean coal combustion process in the utility boilers.

Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers

The mercury emission was obtained by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and circulating fluidized bed (CFB) utility boilers. The relationship was obtained between the mercury emission and adsorption characteristics of fly ash. The parameters included unburned carbon content, particle size, and pore structure of fly ash. The results showed that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury was enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. As the coal factor increased, the mercury emission to the atmosphere decreased. It was also found that the mercury content of fly ash in the CFB boiler was ten times higher than that in the PC boiler. As the unburned carbon content increased, the mercury adsorbed increased. The capacity of adsorbing mercury by fly ash was directly related to the particle size. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range from 77.5 to 106 µm. The content of mesoporous (4–6 nm) of the fly ash in the particle size of 77.5–106 µm was the highest, which was beneficial to adsorbing the mercury. The specific surface area played a more significant role than specific pore volume in the mercury adsorption process.

AN INTEGRATED MODEL TO STUDY THE EFFECTS OF OPERATIONAL PARAMETERS ON THE PERFORMANCE AND POLLUTANT EMISSIONS IN A UTILITY BOILER

A gas fired tangentially boiler was modeled under full load conditions. Furnace was simulated by CFD and then was joint with two mathematical models to calculate heat transfer in the convective section, and metal temperature of waterwall tubes. Effects of changing the combustion excess air (0 to 20%) and burners tilt angle (-30° to +30°) were studied. Results showed that the boiler efficiency is optimum if excess air= 10% and the burners have a negative angle. However, these optimum settings cannot produce a superheated and reheated steam of 538 °C which is desirable. Indeed, a zero or positive tilt angle with 10% excess air, or a negative burner angle with 15% excess air lead to highest efficiency by considering the potential of generating superheated steam of 538 °C. In addition, CO emission in low excess air values growths by increasing the burner tilt angle. NOx emission in low and high excess air ratios is lower at positive burner angles while a moderate excess air (10%) needs a zero tilt angle to minimize NOx emission. Furthermore, a critical fouling thickness was computed, considering boiler's circulation ratio, in which the metal temperature of the waterwall exceeds the short overheating threshold. With a certain thickness of scale layers inside the tubes, a burner tilting equal to 0° or 30° postpones tube rupture. These results could be utilized by operating engineers to keep their utility boilers in the most efficient state and avoiding overheating and tube rupture.

Deterioration of economizer tube in utility boiler

Steam generation from intermediate petroleum products of different hydrocarbon processes ensures zero waste in oil and gas industries. The generated steam is again used for production of refinery final products. Although, zero waste increases refinery margin, there is downside associated with tube failure in utility boiler. Fire side corrosion occurred on economizer tube and failure was systematically analysed. A series of characterization techniques were conducted to find out the root cause of failure. Visual inspection, mechanical characterization, plant data collection, morphology investigation of scales using SEM, elemental analysis using EDX, and microstructural analysis were employed. Sulfur content was precisely determined on fire side scale deposits. Steam side feed water and steam products were analysed as well. Characterization of fire side deposits revealed iron oxide scales with severe sulfur concentration as well. The results revealed that failure was primarily owing to acid dew point corrosion and sulfur came from intermediate fuel oil (IFO). The failure mechanisms were discussed and good engineering practices were put forward to combat and mitigate acid dew point corrosion.

The influence of intense preliminary mechanical processing on the slag formation in coal-fired utility boilers

Burdukov et al. [1] showed experimentally that enhancement of coal reactivity when micronized in a high-impact disintegrator mill makes it possible to attain self-igniting and self-sustaining (autothermal) compact-flame combustion in a cold environment. But with intensive grinding of coal, the mineral part of the fuel is also being converted, which can lead to both a decrease and an increase in the intensity of the formation of deposits. The effect of grinding on the mineral matter of coal and the formation of deposits in a 5 MW pilot-scale combustor are presented. The experiments showed a decrease in the intensity of formation of deposits after grinding of coal by disintegrating mill as compared to grinding in the ball-tube mill.

Online adjustment of Furnace Exit Gas Temperature field using advanced infrared pyrometry: Case study of a 1500 MWth utility boiler

The Furnace Exit Gas Temperature (FEGT) is a key parameter of the combustion process in utility boilers that needs to be accurately monitored, especially in order to increase their load- and fuel-flexibility. However, neither the average FEGT nor its spatial distribution over the boiler cross section can be measured using standard equipment. Advanced pyrometry provides an accurate and efficient way to monitor the FEGT: by combining the signals from several pyrometers installed around the boiler, the FEGT 2D field can be reconstructed. In this paper, we show the results of a measurement campaign carried out on a 1500 MWth boiler using a system comprising 9 infrared pyrometers. Thanks to online adjustments of the combustion air distribution, the FEGT peak initially observed has been reduced by approximately 100oC, and the flame has been recentered.

PRCI ambient NO2 AERMOD performance assessment and model improvement project: Modeled to observed comparison

ABSTRACT On January 22, 2010, the U.S. Environmental Protection Agency (EPA) established a new one-hour probabilistic National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO2) at 100 parts per billion (ppb) or approximately 188 micrograms per cubic meter (µg/m3). The 1-hr NO2 NAAQS is considerably more stringent than the longstanding annual standard of 53 ppb or approximately 100 µg/m3. AERMOD is the EPA recommended dispersion modeling tool for predicting impacts from emission sources, including compressor station reciprocating engines. Existing datasets were previously reviewed to evaluate and improve AERMOD 1-hr NO2 model performance. However, these datasets were not representative due to either an absence of accurate concurrent hourly emission inventories, being associated with larger electric generating units, shorter duration tracer studies, or were too site-specific (e.g., external influences such as terrain). To address this data gap, an intensive 13-month field data program was undertaken by Pipeline Research Council International (PRCI) to collect a comprehensive dataset including concurrent emissions, meteorological, and ambient ozone and nitrogen oxide measurements. This paper describes the Balko, OK natural gas compressor station host site and the design of the meteorological, ambient NOx and O3 monitors and continuous Parametric Emissions Monitoring System (PEMS). Two comparisons are made between the monitored data and AERMOD results. First, the measured onsite data (ambient air, meteorological, and emissions) were used as input to AERMOD to make comparisons between measured and modeled NOx and NO2. These comparisons showed that NOx was both over and under predicted, sometimes beyond a factor-of-two. The second comparison uses the typical permitting approach where onsite data are not available to contrast how conservative input assumptions for emissions, background O3, NO2, and offsite meteorology cause significant over predictions especially for this site where engines have a low annual utilization rate and operate infrequently throughout the year. Implications: This project resulted following the promulgation of the new 1-hr NO2 NAAQS and the challenges that natural gas pipeline compressor stations have in modeling intermittent emissions using AERMOD to demonstrate compliance with this standard. AERMOD was developed and validated with a primary focus on large sources with tall stacks, such as electric utility boilers. A recently collected dataset at a gas compressor station in Balko, Oklahoma shows that the use of permitted allowable emission values compared to actual hourly emission values causes over predictions using typical regulatory methods. Comparison of model results using actual hourly emission values to coincident ambient air monitoring values at the site shows that AERMOD over predicts near-field impacts due to simplifying assumptions including building downwash, plume dispersion, chemistry, and 1-hr invariant meteorological conditions.

Analysis of the causes of the cracking of the tee joint of the header of the utility boiler

The butt joint of the high temperature superheater inlet tee of 330MW Unit Boiler in a thermal power plant cracked. The causes of the weld cracking were analyzed by means of macroscopic morphology observation, fracture SEM inspection, microstructure inspection, mechanical property inspection and chemical composition inspection. The results show that a large number of small thermal cracks are formed in the weld due to improper welding process or operation during boiler installation. At the same time, the improper post weld heat treatment of the joint results in the high hardness of the weld, the insufficient toughness reserve and the decrease of the crack growth resistance. During the long-term operation of the boiler, under the combined action of the primary stress caused by the medium pressure in the pipe and the secondary stress caused by the expansion and contraction of the pipe system, the small crystal cracks in the weld continue to expand, which eventually leads to the macro cracks.

Experimental and kinetic study on the transformation of coal nitrogen in the preheating stage of preheating-combustion coupling process

Preheating-combustion of pulverized coal has been demonstrated as an effective low NOx combustion technology. This method preheats a coal stream to high temperatures before the coal stream is injected into a utility boiler for combustion. Considering the complex mechanisms during the preheating, the effects of the preheating parameters, including temperature, residence time and excess air ratio are studied in a drop-tube furnace in detail. Meanwhile, a kinetics analysis on the transformation of coal-N in gas phase is also performed by means of Chemkin-Pro coupled with mature NOx reduction mechanisms. The experimental results show that the yield of coal-N converted to NO decreases at first and then increases with increasing excess air ratio. The lowest NO yield after preheating is achieved at excess air ratio of 0.1. To reduce NOx formation effectively, the preheating zone should be under high temperature and long residence time. Kinetic modeling indicates that NO was reduced into N2 by both hydrocarbon radical and HCN. Also, the modeling results show that the increasing excess air ratio weakens the effect of hydrocarbon radical reduction, and the mole ratio of NOHCN/NOHCN+CiHj decreases from 1.99 to 1.17 with the increasing excess air ratio from 0.1 to 0.6.

English

English

Effect of separated over-fire air angle on combustion and NOx emissions in a down-fired utility boiler with a novel combustion system

A novel combustion system, consisting of moving fuel-lean nozzles from the arches to the front/rear walls, rearranging staged air, and introducing separated over-fire air (SOFA), has been proposed and successfully applied in a 600 MWe Foster Wheeler down-fired boilers to reduce high NOx emissions while limiting the carbon content in the fly ash. This study comprehensively evaluates the effect of SOFA angles on flow, combustion characteristics and NOx emissions for the novel combustion system by numerical simulation and in-situ experiment. The numerical simulation shows in acceptable consistence with the in-situ experiment. The simulated results show that significant NOx reduction is obtained for all five SOFA angles compared to the original combustion system. O2 and CO concentrations at the furnace outlet, as well as the carbon content in the fly ash decrease significantly with increasing SOFA angle from 0° to 40°. NOx emissions increase slightly and the average temperature at the re-heater entrance changes slightly with the increase of SOFA angle from 0° to 30°. However, with further increasing SOFA angle from 30° to 40°, NOx emissions increase substantially due to the smaller reducing region below the SOFA and the average temperature at the re-heater entrance falls significantly. Taking combustion, NOx and steam parameters into account, a SOFA angle of 30° is advisable. The actual industrial measurements also show that the steam can achieve the designed values when the SOFA angle is set at 30° instead of 40°. Significant NOx reduction (over 50 %) and good performance are attained after adopting the novel combustion technology of 30 °SOFA angle.

Experimental and numerical investigation of combustion system improvement on 600MW bituminous coal boiler for burning lignite coal

A 600MW supercritical direct-current (DC) utility boiler co-firing bituminous coal with lignite, has the problems from over-large desuperheating water yield in the reheater and over-high NO x emission concentration. In this paper, the causes of the problems are analyzed through, the initial tests of pulverization system and combustion system, the adjustment of combustion, etc. The results show that the combustion in furnace lagged behind to some extent after co-firing the bituminous coal with the lignite and the center of the flame was elevated, which shortened the relative distance between the flame of the main burners and over fire air(OFA), enlarged the absorbed heat amount of the reheater and reduced the residence time of flame in the reduction zone prominently. The much less residence time would increase NO x emission. Based on the calculations of the actual operation conditions of the burners, a series of improvement measures were proposed. After performing numerical simulation and cold test verification for the improvement measures, the corresponding transformations of the power plant were carried out. The desuperheating water yield in the reheater decrease, obviously, and over-high NO x emission is controlled effectively.

Modeling of the Coal Particle Behavior in an Ultra-Supercritical Boiler With Large Eddy Simulation

Abstract Large eddy simulation (LES) is becoming a promising tool for the design and retrofit of utility boilers. It explicitly calculates the large-scale eddies which play an important role in the particle behavior inside the boilers. An ultra-supercritical tangentially fired boiler was simulated under the boiler maximum continuous rate (BMCR) condition by LES. The particle phase was tracked by the simplified direct quadrature method of moments in the Eulerian framework. Five particle sizes were adopted to represent the wide particle size distribution. The predicted gas velocities were in good agreement with the thermal calculation. The LES results showed that the particles were more likely to be concentrated in the main burner zone while quickly dispersed in the over fire air (OFA) zone. A theoretical analysis found that the particle Stokes number based on the sub-grid scale was much smaller than one. The particles would behave as tracers for the eddies resolved by LES. However, some differences between the small and large particles were observed in the particle number density distributed along the vertical and horizontal directions. It meant that the inertial effects on the particle motion cannot be neglected.

Numerical optimization of the influence of multiple deep air-staged combustion on the NOx emission in an opposed firing utility boiler using lean coal

Multiple deep air-staged combustion is a promising technology to significantly reduce NOx emission in the opposed firing utility boilers using low-volatile coal. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of an existing 600 MW coal-fired boiler were numerically simulated to evaluate the influence of excess air coefficient in primary combustion zone (αM), variational air distribution modes and multiple deep air-staged combustion on the NOx formation and destruction process in the furnace. The detailed NOx formation and reduction models were proposed to consider the reduction reaction between hydrocarbon and NO under fuel rich conditions and well validated by the experimental results in the laboratory and field tests. The results show that the αM has an important influence on flue gas temperature distribution and forms a reducing atmosphere in the primary combustion zone to significantly reduce NOx concentration. The NOx concentration at the furnace outlet is greatly decreased when the deep air-staged combustion is adopted with αM of 0.75, and the CO concentration maintains within a lower level. Compared with the balanced air distribution, the pagoda and inverse pagoda air distributions are found to be ineffective to enhance NOx reduction performance under deep air-staged combustion. However, the pagoda air distribution effectively reduces NOx emission under middle air-staged combustion condition. The NOx emission is further reduced by adopting the multiple air-staged combustion due to the higher CO concentration formed in the burnout zone. The results are helpful to the design and operation optimization of the opposed firing utility boilers using lean coal.

Understanding the mechanism for the formation of initial deposition burning Zhundong coal: DFT calculation and experimental study

Zhundong coal as a high quality steam coal has not been widely utilized due to severe slagging and fouling problem during combustion. It is crucial to reveal the slagging and fouling mechanisms in a boiler burning Zhundong coal. In the article, the properties of Ca and Na particles adsorbed on iron oxide film were studied by quantum chemistry. Furthermore, two representative ash deposits at rear water wall and final superheater in a 330 MW utility boiler were also verified the calculation results. These results indicated that α-Fe2O3(1 1 0) is selected as the adsorption substrate. The binding energies of Na or Ca on α-Fe2O3(1 1 0) is −12.44 eV and −13.73 eV, respectively. The charge transfer of Ca on α-Fe2O3(1 1 0) is significantly higher than that of Na, and the orbital hybridization of Na on (1 1 0) surface can be weaker than that of Ca on (1 1 0) surface. Thus, Ca would combine with the oxide film more easily than Na. This indicates that compared with other coals with a high sodium, Ca burning Zhundong coal plays a critical role on severe ash deposition. In addition, the binding energy of Ca adsorbed on Fe or O defect surface for α-Fe2O3 (1 1 0) is −15.13 eV or −4.16 eV, respectively. The amount of charge transfer for Ca adsorption on Fe defect surface may be significantly higher than that on O defect surface, and the orbital hybridization of Ca adsorption on Fe defect surface can be stronger than that on O defect surface. Therefore, Fe defect surface have a higher propensity to Ca. Finally, the result of experiments is in accordance with calculation results.