Boilers In China Research Articles

Numerical Study and Structural Optimization of Water-Wall Temperature-Measurement Device for Ultra-Supercritical Boiler.

The temperature of the water wall in the furnace chamber is extremely important for the daily operation of a boiler. Considering the high temperature and dusty environment in the furnace, a temperature measurement device mainly composed of four parts (armored temperature sensor, in-furnace heat-collecting block, out-furnace fixing base, and protective cannula) was designed in this study, which could be used to directly obtain the temperature of the in-furnace water-wall. Numerical simulations of temperature measurement devices with different heat-collecting block structures were carried out using the computer fluid dynamics method. After comparing the measurement accuracy and considering the practical application scenarios, the optimized heat-collecting block structure with a specific expansion gap (0.5 mm wide and 4 mm deep) was selected for practical application. Such a temperature measurement device was then applied to a 1000 MW ultra-supercritical coal-fired boiler in China, and the tested in-furnace water-wall temperature data were in good agreement with relevant research. Compared with the conventional temperature measurement device arranged outside the furnace, the in-furnace water-wall temperature-measurement device adopted in this study has a more sensitive response characteristic and can directly reflect the temperature of the water wall inside the furnace. In addition, it can also reflect the local slag formation state of the water wall and has a long service life.

Leveraging long short‐term memory networks and transfer learning for the soft‐measurement of flue gas flowrate from coal‐fired boilers

AbstractThe dynamic operation and deep peak‐shaving of power‐generating units cause significant fluctuations in flue gas flowrate, thus affecting the accuracy of CO2 emissions measured by continuous emission monitoring systems (CEMS). This study established a long short‐term memory network with an attention mechanism (LSTM‐AM) for the soft measurement of the flue gas flowrate in real‐time. First, flue gas flowrate data and continuous operation parameters over 25 days were sampled from a typical 660 MW coal‐fired boiler in China. Then, a carbon balance model was established to verify the data reliability. The LSTM‐AM model was trained and testified at the 660 MW coal‐fired boiler. Results show that the LSTM‐AM model significantly surpassed the pristine LSTM model without attention, the convolutional neural network (CNN) with LSTM, and the static support vector regression (SVR) model in the real‐time prediction of flue gas flowrate. Finally, the LSTM‐AM model was generalized to a 630 MW coal‐fired power unit via transfer learning, which was further demonstrated to outperform the model re‐trained from scratch. This work manifests the feasibility of deep learning for the soft measurement of flue gas flowrate, which is promising to solve data‐lagging issues when measuring CO2 emissions from coal‐fired power plants.

Emission and distribution characteristics of PCDD/Fs during the co-processing of various solid wastes in coal-fired boilers in China

The advancement of co-processing solid wastes in coal-fired boilers is significant for waste recycling and contributes to the sustainable development of the coal-fired power industry. However, concerns over the emission of dioxins during co-processing have prompted a comprehensive investigation into the dioxin emission properties. In this study, the PCDD/F emission concentrations of seven coal-fired boilers, including three pulverized coal boilers and four circulating fluidized bed boilers were examined. The results indicate that co-processing solid wastes in coal-fired boilers did not lead to an increase in the mass concentration of dioxins in either the flue gas or solid samples, and the international toxic equivalents (I-TEQ) of dioxins in the flue gas complied with prevailing emission standards (0.1 ng I-TEQ/Nm3) in China, proving that coal-fired boilers co-processing would not raise the emission risk of dioxins. The types of waste during co-processing had minimal effect on the I-TEQ of dioxins. A significant proportion of PCDD/Fs was observed in the ash samples, while only 13.0–25.7% and 0.7–6.8% of dioxins were distributed in the boiler slag and the flue gas, respectively. The emission factor of dioxins under the blank conditions ranged from 0.009 to 0.327 ng I-TEQ/kg-coal, while it ranged from 0.015 to 0.129 ng I-TEQ/kg-(coal+waste) under the co-processing conditions. The reduction of emission factor under co-processing condition could be attributed to the significant decrease of dioxins I-TEQ.

Type IV cracking of P92 steel welded joint used in China power plants

ABSTRACT P92 (9Cr-1.8W–0.5Mo-NbV) martensite steel is widely used in high-temperature pipes of ultra-supercritical boilers in China. During high-temperature operation, microstructural evolution of the heat affected zone (HAZ) across welded joint leads to type IV cracking failure. In this article, microstructural evolution and type IV cracking of P92 steel welded joint in an actual 605℃ supercritical boiler were analysed. The results show the cracking appeared in the fine-grained heat affected zone (FGHAZ). Laves phase and M23C6 phase clustered around the cavity. Their precipitation and coarsening promote the nucleation and growth of cavities. The Z phase was also observed in the heated affected zone. Compared with other zones, FGHAZ has lower dislocation density and more degeneration of tempered martensite lath, the coarsening of Laves phase and M23C6 phase is more severer in FGHAZ. These factors led to the deterioration of the creep properties and type IV cracking.

Research on Pollutant Emission Characteristics of Typical Biomass-fired Boilers

The combustion of biomass fuel boilers not only produces inorganic pollutants such as carbon oxides, nitrogen oxides, sulfur compounds, but also organic pollutants such as VOCs and PAHs, which directly affect the global biogeochemical cycle and atmospheric changes. Compared with foreign countries, there is relatively less research and related standards on the emission of organic pollutants from biomass fuel boilers in China. Therefore, it is necessary to study the emission characteristics of organic pollutants from biomass boilers, which can provide reference for the establishment of organic pollutant emission standards for biomass fuel boilers in China.The emission characteristics of volatile organic compounds(VOCs) and polycyclic aromatic hydrocarbons(PAHs) organic pollutants in the flue gas of three typical biomass-fired boilers in Chongqing were studied, and the emission concentrations of particulate matter, sulfur dioxides(SO2), and nitrogen oxides(NOX) were measured.The results show that the sulfur dioxide emissions from the three types of biomass-fired boilersrs are lower than the local standards in Chongqing, while nitrogen oxides and particulate matter have a phenomenon of exceeding the emission limits of the local standards in Chongqing. The main components of VOCs emitted from biomass powder and biomass particle combustion are OVOCs and aromatic hydrocarbons, while the main components of VOCs emitted from wood biomass combustion are alkynes. The gas-solid distribution pattern of PAHs during the combustion of three types of biomass is basically consistent. Low ring PAHs are mainly enriched in the gas phase, while medium and high ring PAHs are mainly enriched in particulate matter.

Analysis of emission characteristics and driving forces of air pollutants and GHG from coal-fired industrial boilers in China

Coal burning is considered to be the main anthropogenic source of air pollutants, which can cause serious environmental pollution. As one of the important coal burning sources, coal-fired boilers have become the important challenges of the policy of “carbon peaking” and “carbon neutrality” of Chinese government. In this paper, the emission inventory of air pollutants (TSP, SO2, NOX, VOCs), and greenhouse gases (GHG, including CO2, CH4, N2O) from coal-fired industrial boilers (CFIB) from 2006 to 2020 are established by using the “bottom-up” emission factor method. Besides, Kaya identical equation and Logarithmic mean Divisia index (LMDI) model are used to quantify the driving forces of the changing tendency in pollutants emission based on five driving factors, which include pollution production coefficient (f), control technology level (β), economy (GOV), industrial energy intensity (e) and coal consumption structure of industrial boilers (s). Combined with existing relevant policies, the future emission tendency of air pollutants and GHG in 2025 and 2030 is projected. The result shows that policy adjustment has a significant impact on pollutant emission trends. In 2020, emissions of TSP, SO2, NOX, VOCs and GHG from CFIB in China are 679.5 kt, 644.7 kt, 603.9 kt, 40.7 kt and 4.4 × 105 kt CO2e, respectively. Control technology level is the main factor to the emission reduction of SO2 and TSP, which has caused a decrease of 4278.6 kt and 3337.3 kt in emissions of SO2 and TSP compared to those of 2006 respectively, while the influencing effect on NOX, GHG and VOCs was relatively weak. In addition, in the scenario of improved control technology, pollutant emissions could be reduced by 62.6%–94.9% until 2030.

CPFD modeling of hydrodynamics, combustion and NOx emissions in an industrial CFB boiler

The ultra-low NOx emission requirement (50 mg/m3) brings great challenge to CFB boilers in China. To further tap the NOx abatement potential, full understanding the fundamentals behind CFB boilers is needed. To achieve this, a comprehensive CPFD model is established and verified; gas-solid flow, combustion, and NOx emission behavior in an industrial CFB boiler are elaborated; influences of primary air volume and coal particle size on furnace performance are evaluated. Simulation results indicate that there exists a typical core-annular flow structure in the boiler furnace. Furnace temperature is highest in the bottom dense-phase zone (about 950 °C) and decreases gradually along the furnace height. Oxygen-deficient combustion results in high CO concentration and strong reducing atmosphere in the lower furnace. NOx concentration gradually increases in the bottom furnace, reaches maximum at the elevation of secondary air inlet, and then decreases slightly in the upper furnace. Appropriate decreasing the primary air volume and coal particle size would increase the CO concentration and intensify the in-furnace reducing atmosphere, which favors for NOx reduction and low NOx emission from CFB boilers.

Technical-economic-environmental analysis of biomass direct and indirect co-firing in pulverized coal boiler in China

An analysis was conducted on two integrated models involving direct co-firing and pyrolysis co-firing systems within a typical 600 MW coal-fired power generation plant. The study assessed the performance of various biomass sources (sawdust, corn straw, and rice husk) using distinct co-firing ratios (5%, 10%, 15%, 20%) across technological, environmental, and economic dimensions. The findings revealed that in terms of thermal and exergy efficiency, the pyrolysis co-firing approach exhibited lower results compared to direct co-firing. Notably, among the biomasses tested, sawdust demonstrated the least impact on boiler efficiency. The CO2 emission was estimated by the Life Cycle Assessment method. The emission of the conventional boiler, along with direct and pyrolysis co-firing of corn straw at a 20% ratio, resulted in CO2 emissions of 1.075 kg, 0.954 kg, and 0.942 kg per 1kWh of electricity, respectively. The different feeding methods were also discussed, providing technically and economically rational options for coal-fired power plants. The economic feasibility of both co-firing systems was also proposed. At carbon tax rates of 410 CNY/t and 464 CNY/t for the direct co-firing and pyrolysis co-firing models respectively, the operational cost of the co-firing systems becomes comparable to that of a coal-fired boiler. However, when the by-product price exceeds 3700 CNY/t, the pyrolysis co-firing model becomes the more economically favorable option. This study primarily concentrates on investigating the real-world application and effective utilization of biomass co-firing systems within the specific framework of China's energy scenario.

Facility-Level Emissions and Synergistic Control of Energy-Related Air Pollutants and Carbon Dioxide in China.

Boilers involve ∼60% of primary energy consumption in China and emit more air pollutants and CO2 than any other infrastructures. Here, we established a nationwide, facility-level emission data set considering over 185,000 active boilers in China by fusing multiple data sources and jointly using various technical means. The emission uncertainties and spatial allocations were significantly improved. We found that coal-fired power plant boilers were not the most emission-intensive boilers with regard to SO2, NOx, PM, and mercury but emitted the highest CO2. However, biomass- and municipal waste-fired combustion, regarded as zero-carbon technologies, emitted a large fraction of SO2, NOx, and PM. Future biomass or municipal waste mixing in coal-fired power plant boilers can make full use of the advantages of zero-carbon fuel and the pollution control devices of coal-fired power plants. We identified small-size boilers, medium-size boilers using circulating fluidized bed boilers, and large-size boilers located in China's coal mine bases as the main high emitters. Future focuses on high-emitter control can substantially mitigate the emissions of SO2 by 66%, NOx by 49%, PM by 90%, mercury by 51%, and CO2 by 46% at the most. Our study sheds light on other countries wishing to reduce their energy-related emissions and thus the related impacts on humans, ecosystems, and climates.

Distribution, chemical fractionation, and potential environmental risks of Hg, Cr, Cd, Pb, and As in wastes from ultra-low emission coal-fired industrial boilers in China

This study conducted a comprehensive investigation of the distribution, chemical fractionation, and potential environmental risks of Hg, Cd, Cr, Pb, and As in waste based on new data from five ultra-low emission (ULE) coal-fired industrial boilers (CFIBs). The results showed that fly ash was enriched with Cd, Pb, As, and Hg, while its Cr contents were not invariably higher than those of slag. Fly ash was the predominant output flow for Hg, Cd, Cr, Pb, and As in the tested ULE boilers, with higher proportions of HTEs in the fly ash and lower proportions of HTEs in the flue gas than in the non-ULE boilers. The average proportions of residual Hg, Cd, Cr, Pb, and As in wastes revealed the following order: slag > fly ash > flue gas desulfurization (FGD) by-products. The potential environmental risks of Hg, Cd, Cr, Pb, and As in the fly ash, slag, and FGD by-products of CFIBs at the county level in the Beijing-Tianjin-Hebei Air Pollution Transmission Channel Cities (“2 +26 cities”) region showed spatial heterogeneity. It is predicted that the potential release of Pb, Cr, and Cd in the fly ash would increase and that of the FGD by-products would decrease after the implementation of the ULE retrofitting of all CFIBs.

Key technologies for CO2 capture and recycling after combustion: CO2 enrichment in oxygen enriched combustion of converter gas

With the progress of industrial development, global total CO2 emissions have increased exponentially. For example, although iron and steel industries have played an important role in the process of economic development, their CO2 emission intensities have reached 2.0 tCO2/t steel. Therefore, accomplishing CO2 emission reduction in iron and steel industries will have a great impact on the overall CO2 emission reduction. Devising a method for capturing CO2, with low energy consumption and high efficiency, will be of great significance to the large-scale capture of CO2 by iron and steel enterprises. In this study, numerical simulation and experimental research are used to analyze the influence of the parameters of oxygen-enriched combustion and conditions of converter gas on the concentration of CO2 in flue gas and the generation of NOx. The variation laws for the temperature field of oxygen-enriched combustion and flue gas composition of converter gas are explored. A front-stage combustion scheme suitable for CO2 capture after combustion is then defined. Herein, the CO2 content of flue gas was 74.32%, whereas the generation of NOx was 113.3 ppm, which fulfilled the NOx emission standard for new gas-fired boilers in China. The influence of the CO2 content of flue gas on the capture efficiency and energy consumption was further studied. The capture time and power consumption were reduced by 37.6% and 38.8%, respectively. The results of this study can provide a new idea and theoretical data reference for large-scale CO2 capture and utilization in iron and steel industries.

Integrated assessment of volatile organic compounds from industrial biomass boilers in China: emission characteristics, influencing factors, and ozone formation potential.

Industrial biomass boilers (IBBs) are widely promoted in China as a type of clean energy. However, they emit large amount of volatile organic compounds (VOCs) and the emission characteristics and the underlying factors are largely unknown due to the sampling difficulties. In this study, three wood pellet-fueled and two wood residue-fueled IBBs were selected to investigate the characteristics of VOC emissions and to discover their underlying impacting factors. The emission factor of VOCs varied from 21.6 ± 2.8mg/kg to 286.2 ± 10.8mg/kg for the IBBs. Oxygenated VOCs (OVOCs) were the largest group, contributing to 30.3 - 73.6% of the VOC emissions. Significant differences were revealed in the VOC source profiles between wood pellet-fueled and wood residue-fueled IBBs. Operating load, excess air, furnace temperature, and fuel type were identified as the primary factors influencing VOC emissions. The excess air coefficient should be limited below 3.5, roughly corresponding to the operating load of 62% and furnace temperature of 630°C, to effectively reduce VOC emissions. VOC emissions also showed great differences in different combustion phases, with the ignition phase having much greater VOC emissions than the stable combustion and the ember phases. The ozone formation potential (OFP) ranged from 4.3 to 31.2mg/m3 for the IBBs, and the wood residue-fueled IBBs yielded higher OFP than the wood pellet-fueled ones. This study underscored the importance of OVOCs in IBB emissions, and reducing OVOC emissions should be prioritized in formulating control measures to mitigate their impacts on the atmospheric environment and human health.

Research and Application of Double-Reheat Boiler in China

The ultra-supercritical (USC) double-reheat technology is an immediate area of research focus in China, since the thermal efficiency of USC double-reheat unit is higher than the USC single-reheat unit. The parameters and capacity of USC double-reheat unit in China have made major breakthroughs, and the thermal efficiency of the units are as high as 47–48%, which is the highest in the world. USC double-reheat boiler is one of the most important devices for USC double-reheat unit, and the design concept and precision have a vital effect on the operation of boiler and power station. The most difficult and important factors in the design of double-reheat boiler are the layout of heating surface and the steam temperature regulation method. Therefore, this work summarizes the arrangement of heating surface and temperature regulation method of existing double-reheat boiler in China. It is hoped that the work will set a benchmark for the development of double-reheat boiler and the future 700 °C power generation unit in the world.

Industrial-scale Investigations on Combustion Characteristics and NO x Emissions of a 300-MWe Down-fired Boiler: Bituminous Coal Combustion and Coal Varieties Comparison

ABSTRACT Number of down-fired boilers burning bituminous coal or anthracite blended with bituminous coal has exceeded 10% of the total down-fired boilers in China. However, lacking of the industrial-scale research on the performance of down-fired boiler burning bituminous coal or blended coal, which limits the actual operation adjustment. Industrial-scale experiments on a 300-MWe down-fired boiler fueled with bituminous coal and blended coal were performed, which boiler was originally designed to burn low-volatile coal. Without the over-fire air, air-staging combustion can still be realized when the down-fired boiler burning bituminous coal. The combustion performance in the down-fired boiler under different coal varieties was compared. Compared with the anthracite case, flue gas temperature is higher, coal flame fullness is greater, O2 consumption occurs faster and NO x concentration is lower in the primary combustion zone when burning bituminous coal. Strong reducing atmosphere is formed in the primary combustion zone. The NO x emissions are low at a level of 410 mg/m3 at 6% O2 at furnace exit and boiler thermal efficiency reaches 92.590% when burning bituminous coal. The NO x emissions cannot be further reduced by burning blended coal. In addition, the boiler thermal efficiency reduced by 1.108%. Without the over-fire air, compared with burning anthracite or lean coal, the NO x emissions and carbon in fly ash significant reduction by 70% and 90%, respectively, and the boiler thermal efficiency increased when burning bituminous coal.

Measure-specific environmental benefits of air pollution control for coal-fired industrial boilers in China from 2015 to 2017.

From 2015 to 2017, China took strong air pollution control measures (APCMs) for coal-fired industrial boilers (CFIBs), including eliminating CFIBs, promoting clean fuels, and updating air pollution control devices (APCDs). Based on the industrial boiler's emission inventory of air pollutants, measure-specific emission reductions from 2015 to 2017 was estimated in this study. Besides, the measure-specific environmental benefits of unit emission reduction on concentration and deposition flux were systematically evaluated by WRF-CMAQ model. The total emission reductions for CFIBs of PM10, PM2.5, SO2, NOx, Hg, As, Cd, Cr and Pb from 2015 to 2017 were 1.2Tg, 0.53Tg, 2.06Tg, 0.65Tg, 37.6 tons, 179.5 tons, 17.9 tons, 1029.3 tons and 676.0 tons, respectively. Based on meteorological fields in 2017, their corresponding national population-weighted mitigated concentration was 1.8μgm-3, 1.3μgm-3, 3.6μgm-3, 0.6μgm-3 (NO2), 0.076ngm-3, 0.37ngm-3, 0.04ngm-3, 1.83ngm-3 and 2.3ngm-3, respectively. Updating APCDs was identified as the major measure to reduce air pollutants (except NOx), accounting for more than 35% of emission reductions and mitigated concentration. Moreover, elimination was the major NOx reduction method, contributing to 55% of NOx emission reductions. The promoting of fuels, including replacement of CFIBs with gas-fired and biomass-fired industrial boilers, had higher environmental benefits for unit emission reductions. Furthermore, there were still more than 43,000 CFIBs with the capacity <10th-1, accounting for 14%, 21%, and 11% of total PM2.5, SO2, and NOX emissions for CFIBs in 2017; meanwhile, 20% and 59% of CFIBs did not install flue gas desulfurization and denitrification devices, respectively. Therefore, it is recommended to give priority to phase out CFIBs with capacity <10th-1 and APCDs updating for larger capacity CFIBs in the future.

Air‐Source Heat Pump for Distributed Steam Generation: A New and Sustainable Solution to Replace Coal‐Fired Boilers in China

AbstractWith the ratification of the Paris Climate Change Agreement, the use of coal‐fired boilers in China are being gradually restricted and removed from use to control air pollution and reduce CO2 emissions. However, current alternatives to coal‐fired boilers such as gas‐fired boilers, electric boilers, biomass boilers, and solar boilers have obvious limitations and drawbacks that can limit the spread of their use. An air‐source heat pump boiler that can extract thermal energy from air and generate high‐temperature steam is proposed, developed, and verified. The air‐source heat pump boiler shows a stable system output with ambient temperature variations from 5 to 35 °C, with an energy efficiency as high as 150–210% to generate steam at more than 120 °C. Furthermore, by taking technical and economic analysis and carbon trading into consideration, air‐source heat pump boiler has shown significant application advantages than other boilers. This research shows that air‐source heat pump steam generation is feasible for replacing coal‐fired boilers in China and could be a sustainable steam generation method.

How does urbanization affect CO2 emissions of central heating systems in China? An assessment of natural gas transition policy based on nighttime light data

Understanding the different impacts of urbanization on sectorial carbon dioxide (CO2) emissions at different spatial scales is of great importance for the evaluation of energy transition policies and reduction of environmental inequality. However, how urbanization affects the CO2 emissions of central heating systems at high spatial resolution in China has not been fully studied before. Based on satellite-observed NPP-VIIRS nighttime light (NTL) data, we develop a 5 km × 5 km annual CO2 emission inventory for coal boilers, thermal power plants (TPPs), and natural gas boilers in China’s central heating systems for the period 2012–2017 by using the geographical and temporally weighted regression (GTWR) model. It is observed that nonurban areas generated 2–4 times the CO2 emissions of coal boilers in urban areas. The largest increments of CO2 emissions of gas boilers are observed in urban areas of the eastern (6.80 times) and central regions (2.86 times) in 2013–2014, due to the clean heating policy in the “2 + 26” cities in China. The effects of urbanization on CO2 emissions from natural gas boilers are approximately 2–3 times those of coal boilers, and the differences are largest in western cities with only minor differences in northeastern cities. Our results will aid in designing low-carbon development goals and provide micro-level information on central heating facilities in urbanized and less developed regions.

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NOx combustion of blended coals is widely used in coal-fired boilers in China to control NOx emission; thus, it is necessary to understand the formation mechanism of NOx and H2S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NOx and H2S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H2S, and NOx in the fuelrich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H2S as NO reduction accelerated. Both the CO and H2S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H2S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NOx, and H2S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.

Development and technical progress in large-scale circulating fluidized bed boiler in China

Circulating fluidized bed (CFB) boilers has realized the clean and efficient utilization of inferior coal like gangue and coal slime, high sulfur coal, anthracite, petroleum coke, oil shale and other resources. As a country with the largest amount of CFB boilers and the largest installed capacity in the world, China has 440 100–600 MWe CFB boilers with a total capacity of 82.29 GWe, including 227 units of 135 MWe, 95 units of 300 MWe, and 24 supercritical units. The statistics of typical 100–300 MWe CFB boilers showed that the average number of unplanned shut-down was only 0.37 times per year, among which the 135 MWe was 0.26 times per year and 300 MWe was 0.46 times per year. The auxiliary power ratio of some 300 MWe CFB boilers based on flow-pattern reconstruction can be reduced to about 4%, which is closed to the same level of pulverized coal (PC) boilers. This paper summarizes the development process and application status of China’s large-scale CFB boilers, analyzes the characteristics and technical performance of the iconic units, and introduces solutions to the problems such as water wall wear and bottom ash cooling.

Industrial-scale investigations on effects of tertiary-air declination angle on combustion and steam temperature characteristics in a 350-MW supercritical down-fired boiler

Industrial-scale experiments were conducted to study the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion (MIMSC) technology at medium and high loads. The experimental results indicated that as the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In near-burner region, the concentration of O2 decreased while the concentrations of CO and NO increased. The concentration of NO decreased in near-tertiary-air region. The carbon in fly ash decreased significantly from 8.40% to 6.45% at a load of 260 MW. At a TDA of 15°, the ignition distances were the shortest (2.07 m and 1.73 m) at a load of 210 MW and 260 MW, respectively. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest (4.83%) at a load of 210 MW. On changing the TDA from 15° to 25°, the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously. The change of TDA has little effect on NOx emissions(660–681 mg/m3 at 6% O2). In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic performance, an optimal TDA of 15° is recommended.