Carbon Content In Fly Ash Research Articles

Experimental study on multi‐fuel combustion of lean coal and by‐product gas in a 75t/h tangentially‐fired boiler

AbstractBy‐product gas is used as an alternative energy for steel mills to relieve the pressure of the fossil fuel crisis. In order to study the influence of blending by‐product gas on boiler operation, the multi‐fuel combustion experiments on the mixture of coke oven gas (COG), blast furnace gas (BFG), and lean coal were carried out in a 75t/h multi‐fuel combustion boiler so as to determine the most balanced fuel formula. The results indicated that the mixed combustion of COG and lean coal could improve the boiler efficiency. However, the balance of by‐product gas utilization in steel mills was destroyed for not using BFG. Although mixed combustion of BFG and COG improved the boiler efficiency, the boiler could only be operated under low load because of lower furnace temperature. After applying the most balanced flue ratio of BFG, COG, and lean coal, with heat value rations of 35%, 35%, and 30%, respectively, the efficiency of the boiler became 77.21%. The flue gas temperature lowered 4.5°C and the carbon content of fly ash and boiler slag were reduced by 1.06% and 1.09%, respectively. As a result, the stand coal consumption of the boiler decreased by 226.1 kg/h with the ratio of coal saving being 3.3%.

In-furnace flow field, coal combustion and NOx emission characteristics regarding the staged-air location in a cascade-arch down-fired furnace

A cascade-arch-firing low-NOx and high-burnout configuration (CLHC) is developed to strengthen low-NOx combustion and maintain high burnout in down-fired furnaces. Numerical simulations of the in-furnace flow field, coal combustion, and NOx formation are performed in a 600 MWe down-fired furnace to evaluate the staged-air location's effect on the low-NOx and high-burnout performance. The furnace, which is in service with a deep-air-staging multi-injection and multi-stage combustion technology (MIMSCT), was modified with the CLHC combustion system for the present simulations. Various staged-air location coefficients of CH = 0.65, 0.70, 0.75 and 0.80 were analysed. Moving downward staged-air to enlarge CH initially improves but then deteriorates the in-furnace flow-field symmetry. Similar changes in the gas temperature and species distribution patterns in the furnace are evident. As CH increases from 0.65 to 0.75, levels of the residual O2 concentration, carbon content in fly ash, and NOx emissions at the furnace outlet decline continuously. The highest setting CH = 0.80 generates severely asymmetric combustion accompanied by poor performance indexes, raising the unburnt rate and NOx emissions by 20–30%. The staged-air location of CH = 0.75 achieves a symmetric flow-field pattern, satisfactory low NOx emissions of ~670 mg/m3 at 6% O2 and high-burnout performance with carbon in fly ash of 5%. NOx emissions are reduced by 26% without affecting burnout in comparison to the currently advanced deep-air-staging MIMSCT.

Improving circular economy by the valorization of non-conventional coal fly ashes in composite cement manufacturing

Spreader Stoker thermal power plants are initially used to burn bagasse during the sugarcane harvest, but also coal when the season is over. The drawback is that, compared to standard pulverized coal fly ashes, the coal fly ashes resulting from this process contain many more unburned particles. Therefore, Spreader Stoker Coal Fly Ash (SSCFA) is sent to landfill despite its potential for valorisation in construction. The main objective of this work was to evaluate the potential of laboratory manufactured composite cements incorporating SSCFA by using a variety of manufacturing methods, such as co-grinding/separate grinding of the components, and grinding of laboratory cements to equivalent global fineness. The behaviour in the fresh, hardening and hardened states was investigated to bring out the influence of Blaine specific surface and type of addition in different combinations of composite cements. Despite the high carbon content of the coal fly ash studied, the mechanical behaviour of SSCFA-cements was better than that of cement with the local pozzolan, which is the reference in Reunion Island. In addition, the use of slag and the pozzolan/SSCFA association in composite cements showed good results. An artificial neural network approach was employed to predict compressive strength in mortars with parameters such as amounts of clinker, gypsum, pozzolana, SSCFA and slag, and the Blaine specific surface. The results also enabled compressive strength response curves to be developed for mixes according to the amount of clinker in the cements.

Quantitative analysis of carbon content in fly ash using LIBS based on support vector regression

Laser-induced breakdown spectroscopy (LIBS) has been proved as an on-line detection technology to measure the carbon content in fly ash, which is beneficial for immediate assessment of the boiler combustion efficiency. Support vector regression (SVR) was adopted as the quantitative model for the carbon content measurement in fly ash in this study. Ash species was one of the key factors affecting quantitative accuracy. Experiments have proven that, the index of plasma temperature and the electron density among different species could be similar, while the partition function ratios and the temperature correction factor showed obvious differences among different ash species. Based on the partition function ratios, the Matrix Effect Correction Factor (MECF) was defined. SVR model was optimized by MECF and the analysis results showed that the correlation coefficient of calibration (R2) increased from 0.989 to 0.991, the root-mean-square error of calibration (RMSEC) decreased from 2.02% to 0.850%, the root-mean-square error of prediction (RMSEP) decreased from 2.13% to 1.07%, and averaged relative standard deviation (ARSD) decreased from 8.62% to 1.89%. The results showed that SVR combined with MECF was an effective method to improve the accuracy of LIBS quantitative analysis of the carbon content in fly ash.

Impact of the upper/lower furnace depth ratio on the strengthened low‐NOx combustion performance in a two‐stage W‐shaped flame furnace

AbstractFor achieving the strengthened low‐NOx combustion plus high burnout in W‐shaped flame furnaces, a cascade‐arch low‐NOx and high‐burnout configuration (CLHC) is realized by positioning two burner layers in a 600 MWe furnace. The gas/particle flow, combustion, and NOx emission characteristics in the furnace are numerically evaluated at four upper/lower furnace depth ratios of CW = 0.458, 0.500, 0.529, and 0.558 (CW signifying the furnace throat shrinkage level) so as to disclose the impact of the furnace throat shrinkage level on the overall furnace performance and meanwhile establish an available CW setup. Increasing CW initially updates but then deteriorates the low‐NOx combustion performance. At the narrowest CW = 0.485 setup, the gained slightly asymmetric combustion shows relatively lower burnout loss but higher NOx emissions, while the widest CW = 0.558 setup forms severely asymmetric combustion with apparently raised carbon in fly ash but relatively lower NOx emissions. At moderate setups of CW = 0.500 and 0.529, a symmetrical combustion pattern develops, showing the low‐NOx and high‐burnout performance characterized as NOx emissions of 700–707 mg/m3 (6% O2) and carbon content in fly ash of 5.4–5.5%. In comparison to CW = 0.500, the larger downward flame penetration, higher flame fullness, and lower CO emission at CW = 0.529 suggest that a reasonable upper/lower furnace depth ratio prefers CW = 0.529. Compared with a currently advanced low‐NOx combustion technology, the CLHC lowers again NOx emissions by about 200 mg/m3 (6% O2) while maintaining high burnout, which confirms the CLHC's availability.

Combined operation mode of sub-critical W-flame boiler and coal mill optimized numerical simulation

The flow, combustion, heat transfer and NOx emission characteristics of a 600 MW subcritical W-flame boiler were numerically simulated under different combined operation modes of coal mills, and compared with the measured results. The results show that the combustion, average residence time, burnout rate, NOx emission characteristics and temperature distribution near the side wall of pulverized coal particles in the furnace have different effects on the combined operation mode of pulverized coal. In the combustion efficiency of give attention to two or more things, screen superheater section of fly ash carbon content and flue gas temperature of entrance at the same time, compared with six coal mill run at the same time, 5 coal mill run, shut down near the side wall of the coal mill is beneficial to reduce NOx emission concentration, to achieve the emission reduction, at the same time under the wing wall and side wall area chamber of a stove or furnace slagging significantly reduce.

Upgrading the strengthened low-NOx and high-burnout combustion performance by staging the hopper air in a down-fired furnace

With respect to the unexpected hopper-air function of strengthening the deep-air-staging conditions in a 600 MWe down-fired furnace equipped with a cascade-arch low-NOx and high-burnout configuration (CLHC), the in-furnace flow field, coal combustion, and NOx formation are numerically evaluated at four hopper-air layout setups of the elevated hopper-air scheme (EHS), conventional hopper-air scheme (CHS), descended hopper-air scheme (DHS), and staged hopper-air scheme (SHS). The aim is to determine the hopper-air layout impact and establish an available hopper-air layout setup for upgrading the strengthened low-NOx and high-burnout combustion performance. At EHS, the combustion symmetry seems to be a little imperfect, with the shallow flame penetration and involuntarily strengthened combustion in the upper furnace incurring high NOx emissions and burnout loss. DHS forms asymmetric combustion with a poor combustion status appearing in the furnace's front-half side, corresponding to high levels of residual O2, poor burnout, and low NOx emissions at the furnace outlet. Both CHS and SHS achieve symmetrical combustion to improve the furnace performance. In comparison to CHS, SHS partitions the flux-increased hopper air into a staged form for achieving a perfect combination of (i) strengthening air staging to suppress NOx generation and (ii) extending the downward flame travel to promote burnout. Consequently, the furnace performance is upgraded further at SHS, showing the best indexes of the calculated NOx emissions of 632 mg/m3 at 6% O2 (amended to 604 mg/m3 at 6% O2 according to the NOx calculation error) and carbon content in fly ash of 5.4%. The generated further NOx reduction rate of 10.5% without affecting burnout (compared with CHS) thus confirms the availability of SHS in upgrading the CLHC’s strengthened low-NOx and high-burnout combustion performance. Compared with a currently used low-NOx combustion technology, CLHC with SHS reduces NOx emissions by 30% without affecting burnout.

Dried sludge reburning blended with calcium magnesium acetate addition in a fluidized bed combustor

The combustion and pollutant emission characteristics of the dried sludge in a fluidized bed combustor (FBC) were studied. The sludge reburning technology was employed for the first time in FBC. A scheme for using the dried sludge reburning blended with calcium magnesium acetate (CMA) to reduce pollutants simultaneously was also proposed, and the effect of the CMA addition was investigated. Results showed that fluidized bed reburning technology can be used to effectively treat the sewage sludge with lower calorific values. As the secondary fuel mass percentage increases, the unburned carbon content in fly ash and the CO emission increase, resulting in lower combustion efficiency. However, the de-NO ratio increases from 37.1% to 53.7%, and the de-SO2 ratio only increased from 1.6% to 7.6% as the secondary fuel mass percentage increase from 10% of 25%. For dried sludge blended with CMA, the minimum emissions of SO2 and NO significantly decrease to 254 mg/Nm3 and 70.9 mg/Nm3 at Ca/S ratio of 2, respectively. Results of main gaseous pollutant emissions from the sludge combustion in an FBC using different pollutant removal methods were compared under well-defined conditions. Fuel reburning blended with CMA addition has the smallest SO2 and NO emissions.

Development and application of thermal calculation software for boiler furnace

Aiming at the thermal calculation of supercritical boiler furnace check, the article is based on the divisional calculation method in the thermal calculation standard of the Soviet Union in 1973, combined with the 1-D block flow model of pulverized coal combustion, and proposes a new divisional 1-D furnace exchange thermal calculation model, and compiled the corresponding thermal calculation software. The software is easy to use, not only can obtain the average temperature distribution of flue gas along the height of the furnace and the heat load of the section water wall, but also the carbon content of fly ash. The article takes a 600 MW supercritical boiler as an example and uses software to calculate the heat transfer in the furnace.

Quantitative analysis method of unburned carbon content of fly ash by laser-induced breakdown spectroscopy

Quantitative analysis method of unburned carbon content of fly ash by laser-induced breakdown spectroscopy

Prediction of Carbon Content in Fly Ash in Ultra Supercritical Generation Unit with Long Short-term Memory Neural Network

The accurate measurement of carbon content in fly ash is the basis of improving the thermal conversion efficiency of boiler and reducing the coal consumption of power generation. Based on the operation data of 1000MW Ultra-supercritical Unit in a thermal power plant, the input parameters of the model were selected by correlation analysis. The Long Short-Term Memory neural network (LSTM) was used to establish a prediction model with the boiler operation parameters as the input and the carbon content of the boiler fly ash as the output. According to the actual operation data of the unit, the prediction performance of the model was trained and tested. The simulation results show that the average prediction error of the LSTM model is 2.8136%, and 86% of the total samples are data with error less than 4.5%, which means it has high accuracy and strong generalization ability.

Trends of the low-NOx and high-burnout combustion characteristics in a cascade-arch, W-shaped flame furnace regarding with the staged-air angle

For a cascade-arch-firing low-NOx and high-burnout configuration (CLHC) designated for solving the incompatible problem of strengthened low NOx combustion and good burnout in W-shaped flame furnaces, numerical simulations verified by real-furnace results of a 600 MWe furnace with the multiple-injection multiple-staging combustion technology (MIMSCT), were performed to evaluate trends of the CLHC’s in-furnace flow field, combustion, and NOx production at different staged-air angles of θ = 15°, 20°, 25°, and 30°. The flow-field symmetry and combustion performance first improved and then worsened with θ, generating a same change trend in the residual O2 at the furnace outlet, carbon content in fly ash, and emission levels of CO and NOx. The 20° setting finally established the optimal performance indexes labeled as NOx emissions of 669 mg/m3 at 6% O2 and carbon content in fly ash of 5.1%. In the NOx formation aspect along the flame travel in the CLHC, it was found that NOx was initially inhibited obviously in the preceding combustion stage but then surged in the primary combustion stage before finally reduced sharply by the reburning process. As a replacement of the prior MIMSCT, the CLHC achieved a strengthened low-NOx combustion performance with NOx reduced by 26% without affecting burnout. The prominent improvement was attributed to the combination of improving symmetrical combustion pattern, strengthening deep-air-staging conditions, lengthening flame travel by positioning hopper air, and introducing the flue gas recirculation and reburning process.

Fuzzy modeling of boiler efficiency in power plants

Boiler efficiency, an important index that participates in adjusting the combustion process in the boiler, usually cannot be evaluated in real time due to the time-consuming detection process of unburned carbon content in fly ash and cinder. In this paper, we adopt a data-driven soft computing method for measuring the boiler efficiency by incorporating offline fuzzy modeling and online operations. The uncertainty and the non-real-time property of the indirect method are analyzed quantitatively and qualitatively. An improved fuzzy rule extraction method is applied in a 600 MW power plant for boiler efficiency evaluation and real-time control.

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.

Combustibility analysis of high-carbon fine slags from an entrained flow gasifier

The fine slag produced from the entrained flow gasifier in coal chemical industry contains a high amount of unburned carbon content, which can reach more than 40%. The coal gasification fine slag is dissipated just by land filling which occupies a lot of land. Consequently, it causes the pollution of soil, water and wastes the combustible carbon in coal gasification fine slag. It is crucial to develop an environmental friendly and economical scheme for the utilization of coal gasification fine slag. To achieve this aim, it is significant to investigate the combustibility of coal gasification fine slag and then propose a comprehensive utilization technology. In this study, the physical and chemical properties of the raw bituminous coal and the produced coal gasification fine slag, including proximate and ultimate analysis, particle size distribution, ash composition, morphology, and specific area were investigated. The combustion and co-combustion characteristics of coal gasification fine slag were analyzed by a thermo-gravimetric analyzer. A drop tube furnace and a fluidized bed reactor were employed to test the combustibility of coal gasification fine slag in a pulverized furnace and a fluidized bed furnace, respectively. Results show that the carbon content in dried coal gasification fine slag is >40% with a heating value > 16 MJ kg−1. Further, thermo-gravimetric analyzer test showed that the combustion property of coal gasification fine slag is worse than that of anthracite and close to that of high ash coal, and there is a non-negligible synergistic effect for raw bituminous coal and coal gasification fine slag co-firing. The combustibility test in drop tube furnace and fluidized bed reactor showed that coal gasification fine slag can be well burned in a pulverized furnace requiring combustion temperature >900 °C and oxygen concentration >10 vol%. However, the fluidized bed furnace was not appropriate for high efficiency coal gasification fine slag burning, because the unburned carbon content of fly ash after coal gasification fine slag combustion is still >14%, even at 900 °C, 21% oxygen concentration and a low fluidization number. It is suggested that coal gasification fine slag will be better to burned it in a pulverized furnace rather than fluidized furnace.

Assessment of the compressive strength and strength activity index of cement incorporating fly ash

In recent years, the utilizations of industrial by-product as substitute for non-renewable material in cement and concrete production has received significant attention for sustainable development goals. In this study, fly ash blended cement was produced by replacing 5%, 10%, 15%, 20%, 25%, 30% and 35% by weight of Portland Cement with fly ash. In addition, the loss on ignition (LOI) content of fly ash, which used for the 20% replacement ratio, varies from 5 to 20% to assess the effect of unburnt carbon content in fly ash on the strength activity index of fly ash blended cement. The experimental results show that the addition of fly ash reduces the compressive strength of the mortar and strength activity index of fly ash blended cement at early ages. However, the presence of fly ash increases the strength of the mortar and the strength activity index of fly ash blended cement at longer ages when the fly ash replacement ratio is lower than 25%. The unburnt carbon in fly ash does not reduce the strength of the mortar and strength activity index of fly ash blended cement when its content does not exceed 20%.

Zinc Oxide Recovery from Solid Waste of Electric Arc Furnace Dust (EAFD) Using Hydrometallurgical Method

Indonesia coal ash is predicted to reach 10.8 million tons in the year 2020 but its utilization is still limited. In the last decade, coal ash has become a promising REY source candidate. To determine the potency of REY in Indonesia coal ash, information about element concentration and mineralogy of the ash is essential. In this study, coal ash samples were taken from Paiton-2, Pacitan, Rembang, and Tanjung Jati coal-fired power plants. Element content and mineralogy were analyzed using Inductive Couple Plasma Mass Spectroscopy/Atomic Emission Spectroscopy (ICP-MS/AES), X-Ray Diffractometer (XRD) and petrographic. The results showed that coal fly ash and bottom ash contains critical REY in the range of 38% to 41% with Coutlook larger than one. XRD analysis showed that both fly ash and bottom ash have similar mineral phases with slightly different concentrations. The mineral phase is dominated by amorphous glass, quartz, Fe-bearing minerals, and unburned carbon. The amorphous glass phase in fly ash is in the range of 23 to 34% while in bottom ash between 14 and 34%. Unburned carbon content in fly ash and bottom ashes are 7-13% and 7-19%, respectively. Fe-bearing mineral content in fly ash is 15-20% and bottom ash is 13-20%. In addition, Indonesia coal ash has a higher Heavy-REY enrichment factor than Light-REY. The Enrichment Factor of HREY in fly ash is as much as 1.3 times (in average) of the bottom ash.

Potency of Rare Earth Elements and Yttrium in Indonesia Coal Ash

Indonesia coal ash is predicted to reach 10.8 million tons in the year 2020 but its utilization is still limited. In the last decade, coal ash has become a promising REY source candidate. To determine the potency of REY in Indonesia coal ash, information about element concentration and mineralogy of the ash is essential. In this study, coal ash samples were taken from Paiton-2, Pacitan, Rembang, and Tanjung Jati coal-fired power plants. Element content and mineralogy were analyzed using Inductive Couple Plasma Mass Spectroscopy/Atomic Emission Spectroscopy (ICP-MS/AES), X-Ray Diffractometer (XRD) and petrographic. The results showed that coal fly ash and bottom ash contains critical REY in the range of 38% to 41% with Coutlook larger than one. XRD analysis showed that both fly ash and bottom ash have similar mineral phases with slightly different concentrations. The mineral phase is dominated by amorphous glass, quartz, Fe-bearing minerals, and unburned carbon. The amorphous glass phase in fly ash is in the range of 23 to 34% while in bottom ash between 14 and 34%. Unburned carbon content in fly ash and bottom ashes are 7-13% and 7-19%, respectively. Fe-bearing mineral content in fly ash is 15-20% and bottom ash is 13-20%. In addition, Indonesia coal ash has a higher Heavy-REY enrichment factor than Light-REY. The Enrichment Factor of HREY in fly ash is as much as 1.3 times (in average) of the bottom ash.

Characteristics of ash and slag from four biomass-fired power plants: Ash/slag ratio, unburned carbon, leaching of major and trace elements

The utilization of biomass energy is attracting worldwide attention due to the worsening energy crisis and the concerns on carbon dioxide emission. However, the rapidly development of biomass-fired power plants generate enormous amount of slag and ash. At present, the main treatment method of biomass slag and ash is landfill, which not only requires high cost, but also causes a series of environmental issues. Aiming at this problem, the slag and fly ash from four biomass power plants were sampled and characterized. The major/trace element composition and leaching characteristics of slag and fly ash are analyzed, and the effect of volatile mineral in these solid residues on furnace efficiency is also evaluated. The results indicate that for biomass-fired grate furnaces, in the generated solid residues, the slag accounts 60 ~ 70% while the fly ash accounts for 30 ~ 40%. The volatile elements in slag are much lower than those in fly ash, and the unburned carbon content of fly ash is generally lower than that of slag. Due to the enrichment of volatile minerals in fly ash, instead of 815 °C, 550 °C is suggested to measure the unburned carbon content in fly ash. When the measuring temperature of unburned carbon decreases from 815 °C to 550 °C, the measured energy loss from the incomplete combustion of solid fuels decreases by one third, which affords a more reasonable evaluation on combustion efficiency. The contents of cadmium and lead in certain fly ash samples exceed the standard, however, all fly ash samples have a high leaching rate of potassium but low leaching rates of copper, zinc, cadmium, lead, chromium and arsenic. It indicates that the fly ash from biomass-fired grate furnace is suitable for producing potassium fertilizer by leaching instead of direct soil fertilization or landfill. The leaching rate of most minerals in slag is much lower than that in fly ash and the contents of heavy metals are far below the prescribed upper limit in standard, which indicates that the slag from biomass-fired grate boiler is more suitable for direct use in soil improvement due to the low contents of hazardous elements.

Improving the LIBS Quantitative Analysis of Unburned Carbon in Fly Ash Based on the Optimization of Reference Value

The unburned carbon in fly ash is an important indicator of the coal-fired boiler combustion efficiency. Laser-induced breakdown spectroscopy (LIBS) has been applied for measurement of unburned carbon for its fast response. Traditionally, the unburned carbon content determined by the loss-on-ignition (LOI) method was used as the reference value of the LIBS calibration model. However, the reference value of unburned carbon content would not correspond to the LIBS spectral information because the result measured by the LOI method represents not only unburned carbon but also decomposition of some mineral phases in fly ash during ignition. In this paper, different values of unburned carbon content determined by the loss-on-ignition method and elemental analysis technique, respectively, were compared as the reference of LIBS calibration model. The analysis results showed that there is a better correspondence between the total carbon content determined by elemental analysis technique and the LIBS spectral information on fly ash. Compared with the reference value determined by the LOI method, the regression coefficient (R2) of the calibration model by the reference value determined by elemental analysis technique improved from 0.980 to 0.992, and the root-mean-square error of prediction decreased from 0.702% to 0.111%. This research demonstrates that the total carbon content is more suitable to be used as reference value to establish a calibration model of unburned carbon content in fly ash by LIBS analysis.