Coal Feed Research Articles

Emission and Migration Characteristics of Mercury in a 0.3 MWth CFB Boiler with Ammonium Bromide-Modified Rice Husk Char Injection into Flue

The emission and migration characteristics of flue gas mercury in a pilot-scale 0.3 MWth circulating fluidized bed combustion boiler system by injecting the adsorbent of ammonium bromide (NH4Br)-modified rice husk char (NBr-RHC) into the flue duct before a fabric filter (FF) were investigated in this study. Mercury concentration in the flue gas was sampled by the Ontario Hydro Method. Meanwhile, the feeding coal, lime, process water, fly ashes both in the flue gas and FF hopper, bottom ash, gypsum, and wet flue gas desulfurization (WFGD) effluent were also collected during the flue gas mercury sampling. The temperature-programmed desorption (TPD) method was used to identify the mercury species in the sample of fly ashes. The results show that the combination system of the selective catalytic reduction (SCR) + adsorbent injection (AI) + FF + WFGD obtains a total flue gas mercury removal efficiency of 94.62%, which is 13.42% higher than that without the AI of NBr-RHC. A total of 93.83% of mercury exists in byproducts of the solid and liquid, and the amount of mercury collected by FF accounts for 97.28% due to the physisorption and chemisorption of gaseous mercury by NBr-RHC injection. The TPD analysis for mercury compound in the particulate matters from the AI device indicates that HgBr2 formation is the key reason for NBr-RHC having high mercury removal efficiency. Elemental mercury is found to oxidize across the SCR, with 70.23% converted to oxidized mercury. WFGD exhibits a good oxidized mercury absorption rate of 86.67%, but elemental mercury increases from 0.19 to 0.26 μg/m3 due to the oxidized mercury reduction, and more attention should be given on this increase.

An Unsupervised Classification Method for Flame Image of Pulverized Coal Combustion Based on Convolutional Auto-Encoder and Hidden Markov Model

Combustion condition monitoring is a fundamental and critical issue that needs to be addressed in the wide-load operation of coal-fired boilers. In this paper, an unsupervised classification framework based on the convolutional auto-encoder (CAE), the principal component analysis (PCA), and the hidden Markov model (HMM) is proposed to monitor the combustion condition with the uniformly spaced flame images, which are collected from the furnace combustion monitoring system. First, CAE is adopted to extract the features from the flame images, which obtain the sparse representations in the images. Then, PCA is applied to project the feature vectors into the orthogonal space for robustness and computation efficiency. Finally, a HMM is built to calculate the corresponding optimal states by learning the temporal behaviors in the compressed representations. A coal combustion adjustment experiment was conducted in a 660 MW opposed-firing boiler, and the sequential 14,400 flame images with three different combustion states were obtained to evaluate the effectiveness of the proposed approach. We tested six different compression dimensions of the latent variable z in the CAE model and ensured that the appropriate compress parameter was 1024. The proposed framework is compared with five other methods: the CAE + Gaussian mixture model (GMM), CAE + Kmean, the CAE + fuzzy c-mean method, CAE + HMM, and the traditional handcraft feature extraction method (TH) + HMM. The results show that the proposed framework has the highest classification accuracy (95.25% for the training samples and 97.36% for the testing samples) and has the best performance in recognizing the semi-stable state (85.67% for the training samples and 77.60% for the testing samples), indicating that the proposed framework is capable of identifying the combustion condition, changing when the combustion deteriorates as the coal feed rate falls.

A study on the chemical state of carbon present in fine ash from gasification

AbstractTaking feed coal as a reference, the chemical states, content changes, microstructure, distribution, and graphitizable extent of carbon present in gasification fine ash were investigated in detail. The results show that the fine ash includes a significant amount of unreacted carbon. The C1s spectrum is composed of four components in which the contents of C–C and C–H decrease dramatically and graphitized carbon (39.1%) presents. The elemental carbon primarily distributes on the layer structure, whereas elemental silicon and aluminum are mainly found on spherical particles embedded in the layer structure. After gasification, carbon atoms in gasification fine ash are arranged in an orderly manner, and the extent of graphitization of carbon increases, which indicates that carbon in feed coal may be partly graphitized and deposited in fine ash during gasification process.

Evolution of four-decade atmospheric mercury release from a coal-fired power plant in North China

Coal-fired power plants (CFPPs) are an important anthropogenic emission source of atmospheric mercury (Hg). Understanding the co-benefits of conventional air pollution control devices (APCDs) on Hg reduction is crucial for reducing Hg emission from this source. In this study, atmospheric Hg emission from a pulverized coal (PC) utility boiler in North China was investigated along the emission control stage of selective catalytic reduction (SCR), electrostatic precipitator (ESP) and fabric filter (FF) and lime stone-gypsum wet flue gas desulfurization (WFGD). Historical atmospheric Hg emission from this CFPP with total 1550 MW capacity since its operation in 1976 was estimated based on the implementation of different configurations of APCDs. The results showed that a combination of SCR + ESP-FF + WFGD leads to a co-beneficial Hg control of 96%, with the in-stack Hg concentration (0.87 μg m−3) much lower than the Chinese national standard for CFPPs (30 μg m−3), and the stack Hg speciation (Hg0:Hg2+:Hgp) was in 86%:13%:1%. Mass balance analysis indicates that Hg from the input of feed coal and limestone was mainly ended up in fly ash (89%), followed by gypsum (5%), stack gas (5%) and bottom ash (1%), respectively. During the past 42 years of operation (1976–2017), the total air Hg emission from this CFPP summed to 17.62 tonnes, in which Hg0 make up 14.02 tonnes (79.5%), Hg2+ make up 3.15 tonnes (17.9%) and Hgp 0.45 tonne (2.6%). Although the speciation of emitted Hg into the atmosphere changed with respect to the configuration of APCDs, Hg0 was the most dominated emission species (64–98%) for all time. Nearly 90% of total Hg emissions occurred in the first three decades when venturi scrubber (VS) and ESP were the primary APCDs. The installation of WFGD and SCR substantially decreased the Hg emission factors by 17 times compared to the peak values during 1998–2004. This study demonstrates the combination of SCR + ESP-FF + WFGD as a best available technology (BAT) for Hg control in CFPPs in China.

Correction to: Characterization of feed coal and combustion by-products from Kemerkoy power plant in Turkey

Correction to: Characterization of feed coal and combustion by-products from Kemerkoy power plant in Turkey

The Role of Mineral Matter in Concentrating Uranium and Thorium in Coal and Combustion Residues from Power Plants in Poland

Based on the results of tests on feed coal from the Lublin Coal and Upper Silesian Coal Basin and its fly ash and slag carried out using X-ray diffraction and X-ray fluorescence analysis, atomic emission spectroscopy, and scanning electron microscopy, it was found that in feeds, coal Th is associated with phosphates and U with mineral matter. The highest Th content was found in anhedral grains of monazite and in Al-Si porous particles of fly ash of <0.05 mm size; whereas in the slag, Th is concentrated in the massive Al-Si grains and in ferrospheres. U is mainly concentrated in the Al-Si surface of porous grains, which form a part of fly ash of <0.05 mm size. In the slag, U is to be found in the Al-Si massive grains or in a dispersed form in non-magnetic and magnetic grains. Groups of mineral phase particles have been identified that have the greatest impact on the content of Th and U in whole fly ash and slag. The research results contained in this article may be important for predicting the efficiency of Th and U leaching from furnace waste storage sites and from falling dusts to soils and waters.

The ash formation and attrition characteristics of an Indonesia lignite coal ash for a 550 MWe ultra supercritical CFB boiler

The Samcheok CFBC (circulating fluidized bed combustion) power generation plant, the world largest, consists of 4 units of 550 MWe USC (ultra-supercritical) CFB boilers, started operation since the end of 2016. The years’ operation proves that there lacks circulating material and low solid hold-up in the upper furnace when burning Indonesia coal with low ash content of 4.5%. To solve this problem, the present study studied the mass balance of the Samcheok CFB boiler by investigating the ash formation and attrition characteristics of the feed coal in the lab. The ash formation PSD (particle size distribution) proves that the burnout-produced fly ash and fragmentation-produced fly ash are not the main sources of the production of fly ash. The heterogeneity of specific chemical compositions of particles in terms of particle sizes is proved to be one of the key factors for determining the attrition rate. For large particles, SiO2 and Al2O3 compositions dominate and results in low attrition rate, while for small particles, Fe2O3 and CaO compositions dominate and results in high attrition rate. The fine particles produced due to attrition have very high Fe2O3 contents, indicating that the heterogeneity of chemical composition is enhanced due to attrition. These findings show good agreement with the phenomenon of the high content of Fe2O3 in fly ash in the CFB boiler.

Mineralogy and geochemistry of feed coals and combustion residues of the Kangal power plant (Sivas, Turkey)

This study focuses on mineralogical and geochemical compositions of feed coal (FC) and combustion residues, namely fly ash (FA) and bottom ash (BA) samples, obtained from the Kangal coal-fired power plant in central Turkey. The X-ray powder diffraction data indicate that carbonate and clay minerals are dominant phases in the FC samples. In the FA samples, quartz, hematite, anhydrite, lime, and feldspar are generally dominant and abundant phases, whereas calcite, ettringite, and portlandite are generally more abundant in the BA samples. The elements Mo, Cs, and U are significantly enriched in the studied FC, FA, and BA samples. The statistical analysis and SEM-EDX data show that Ca, Ti, and the vast majority of trace elements are inorganically affiliated, and only Tl and U have prob-able organic affinity in the FC. In addition, the redox conditions in the paleomires presumably resulting in Mo and U enrichment in FC, whereas their enrichment in FA and BA is most likely related to retention by CaO and Ca-sulfate. The Cs enrichment in FA is due to retention by glass. The elements in the FA and BA are distinguished into four groups according to their volatility during combustion. The elements As, Mo, Cd, Tl, and U (Group I) are the most volatile elements during combustion and condensation in the FA. The elements Li, Zn, Ga, Rb, Nb, Cs, Ba, La, and Pb (Group IV) did not become more volatile or less volatile during combustion and are located in BA. Nevertheless, Zn and Pb in the BA seem to be related to the presence of unaffected pyrite and sphalerite, and are due to low combustion efficiency of the boiler during the sampling period. Overall, enriched elements and minerals in FA and BA suggest that their disposal should be undertaken with caution.

Optimization of particle size distribution in circulating fluidized beds via adjustment of crushers and tuning parameters of two-toothed roll crusher

Properties of coal gangue feed, especially its particle size distribution (PSD) and variable hardness, result in considerable impacts on both the efficiencies of the circulating fluidized bed (CFB) boiler and wastage of crushers. These two aspects are addressed in this study. First, the Aspen Plus software was used to optimize the coal gangue crushing system for the CFB boiler; it facilitated in predicting the PSD in with excellent accuracy, and optimized the PSD of the crushing system for the CFB. The optimized PSD for the CFB was approximately 80% within 2.3–7.5 mm. The applied PSD of coal feeding in the CFB improved the combustion efficiency of the CFB significantly. Furthermore, the two-toothed roll crusher was found to be the most vulnerable part in the optimized crushing system. Next, the two-toothed roll crusher was optimized based on the framework proposed by the Whiten model. The reasonable rolls gap and teeth length should be 75 mm and 23 mm, respectively. Further, a physical model of a two-toothed roll crusher was established to obtain the optimized roll diameter; it was later found to be >660 mm. The results were applied in optimizing an industrial-scale crushing system and two-toothed roll crusher of a coal gangue-fired power plant. Investigations demonstrated that the PSD can improve the combustion efficiencies of the CFB significantly by 3%, as well as a 3× extension of the original service life of the two-toothed roll crusher.

Effect of air pollution control devices on mercury isotopic fractionation in coal-fired power plants

Coal combustion is an important source of anthropogenic mercury (Hg) emissions. Air pollution control devices (APCDs) are widely used in coal-fired power plants (CFPPs) to reduce pollutant emissions including Hg. To gain insight into the Hg isotopic fractionation produced by APCDs, the Hg isotopic compositions of feed coal (FC) samples and combustion byproducts from 7 CFPP utility boilers in Hebei Province of China were studied. Significant mass-dependent fractionation (MDF) was observed in combustion byproducts when comparing with FC samples, while mass-independent fractionation (MIF) was not identified. Compared to FC samples, no significant difference in bottom ash (BA, δ202HgBA-FC = 0.08 ± 0.16‰, 1SD, n = 3) samples while light isotopes were enriched in fly ash (FA, δ202HgFA-FC = −0.38 ± 0.75‰, 1SD, n = 7) samples. Boilers lacking of a selective catalytic reducer (SCR) unit produced gypsum (GY) samples with negative MDF signatures compared to FC samples (δ202HgGY-FC, no SCR = −0.22 ± 0.10‰, 1SD, n = 3). In contrast, boilers equipped with a SCR unit produced GY samples with higher MDF signatures (δ202HgGY-FC, SCR = 0.54 ± 0.40‰, 1SD, n = 3), probably as a result of the oxidation of Hg0 in the SCR unit. Calculated isotopic signatures of gaseous Hg in the stack emission (SE, δ202HgSE-FC = 0.03 ± 0.20‰, 1SD, n = 7) had no significant difference from that of FC samples. The results suggested distinct isotopic compositions should be fully considered during tracing Hg pollution via Hg isotopic signatures, especially when solid produced by CFPPs were involved.

Fate of mercury in two CFB utility boilers with different fueled coals and air pollution control devices

Mercury (Hg) emitted from two circulating fluidized bed (CFB) utility boilers (150 MW, P#1 and 300 MW, P#2) in Guizhou province, Southwest China, was characterized. P#1 used anthracite coal and was equipped with in-furnace desulfurization (IFD) and electrostatic precipitator (ESP). P#2 co-burned gangue and coal slime and was equipped with selective non-catalytic reduction (SNCR), ESP, fabric filter (FF) and limestone gypsum wet flue gas desulfurization (WFGD). Flue gas samples from the inlets and outlets of air pollution control devices (APCDs) in the two plants were analyzed using the Ontario Hydro Method (OHM). Solid samples including feed coal, limestone, bottom ash, fly ash and gypsum were also collected and analyzed. Hg removal efficiency by the APCDs and Hg emission factors (MEFs) were estimated. Total Hg (THg) in the flue gas at the inlet of ESP are 32.3 µg·m−3 and 15.3 µg·m−3 for P#1 and P#2, respectively, both dominated (95.11–99.26%) by the particulate form (Hgp). THg decreased significantly to 0.19–0.24 µg·m−3 at the final APCDs outlets of these two power plants, yielding an overall Hg removal efficiency of 98.53–99.41% for THg. Mass balance analysis indicated that >97% of Hg in coals ended up in captured fly ash. MEFs of the two tested power plants are relatively lower than the values typically found in CFB and pulverized coal (PC) utility boiler power plants. The emission data of Hg from CFB coal-fired power plants should be updated and attention should be directed to the secondary emission/release of Hg from the captured fly ash.

Experimental study of pilot-scale CFB gasification: Effect of gasifying agent and coal feeding modes on the gasification performance

A series of pilot-scale CFB gasification tests were carried out to investigate the effect of gasifying agent staging, including the injecting height and direction, and coal feeding modes on gasification performance. Some important indexes such as the gas composition, total gas yield, effective gas yield, cold gas efficiency and carbon conversion are selected to evaluate the gasification performance. The results show that the high/tangential injection of auxiliary gasification agent (AGA) and coal feeding by loop seal are beneficial to the improvement of gasification performance. If we take effective gas yield as an example, the maximum could significantly increase 8.3%, 4.9% and 13.6% in comparison with those of low-height and radical injection of AGA, traditional coal furnace feeding, respectively. Moreover, the distribution of temperatures along the furnace height becomes more balanced with the rise of AGA injecting height and coal feeding by loop seal. The results of this research could provide some useful guidance to the design optimization of CFB gasifier.

Study on distribution, chemical states and binding energy shifts of elements on the surface of gasification fine ash

Using feed coal as a reference, a series of modern advanced analysis and measurement technologies such as X-ray photoelectron spectroscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy and X-ray diffraction have been used to analyze the gasification fine ash obtained from a pulverized coal gasification unit. The results show that the fine ash mainly consists of elemental carbon, oxygen, silicon, and aluminum. The elemental carbon distributes primarily on the foams structure, while the spherical particles mainly consisted of silicon and aluminum are embedded in the foams structure. The C1s spectrum is composed of five components in which the content of graphitized carbon is up to 38.36%, and the content of aromatic C–C or C–H, the main existing form on coal surface, is only 25.75%. The 67.31% of elemental silicon is combined with bridging oxygen (Si–O) and 32.69% of that connected to non-bridging oxygen (Si–O2). The existence of aluminum is in the form of aluminum oxides with two coordinated modes ([AlO6] and [AlO4]), and the content of [AlO6] group is nearly double that of [AlO4]. Simultaneously, the binding energies of silicon and aluminum increase by approximately 3 eV, while that of carbon almost no change because of the number of carbon atoms is significantly higher than that of other elements. The silicon atoms and aluminum atoms are surrounded by masses of carbon atoms for the special microstructure of FA. The different chemical states of carbon with higher electronegativity along with the role of high temperature and pressure make the binding energies of silicon and aluminum changed dramatically.

Distribution Characteristics of Valuable Elements, Al, Li, and Ga, and Rare Earth Elements in Feed Coal, Fly Ash, and Bottom Ash from a 300 MW Circulating Fluidized Bed Boiler.

Feed coal, fly ash (FA), and bottom ash (BA) were collected from a 300 MW circulating fluidized bed boiler. A mechanical screen classifier was used to separate and obtain particles with different sizes. The distribution of valuable elements, including aluminum (Al), lithium (Li), and gallium (Ga), and rare earth elements (REE) in the samples was investigated. Results indicate that the contents of SiO2 and Al2O3 in sized ash particles decreased as the particle size decreased; meanwhile, the contents of CaO, SO3, and Fe2O3 apparently increased. The sulfur-fixing product anhydrite tended to be distributed in the fine ash particles. The valuable elements were more enriched in FA than in BA. The Li, Ga, and REE were evenly distributed in FA particles with different sizes. Separating ash particles with a superhigh concentration of these rare elements was difficult using the mechanical screen, but a part of the anhydrite or CaO in circulating fluidized bed ash could be removed easily. The Li and REE in the feed coal were highly associated with quartz, kaolinite, and other fractions containing SiO2 and Al2O3. No definitive relationship between the Ga concentration and Al2O3 content in the feed coal and ash samples was observed.

Separation characteristics of 100-mm dual-cone water-only cyclone to wash high NGM coal

ABSTRACTExperiments were carried out on a 100-mm laboratory water-only cyclone with dual-type cone to treat −3.0-mm Indian coal. Separation behavior of the individual size fractions (within −3 + 0 mm size) feed coal during separation was analyzed critically and their cut densities (SG50) were estimated. A minimum ash content of down up to 18% from a feed ash of 28.7% is possible with an appreciable yield of 49.3% for the coal below 3 mm size range. While analyzing the results of yield and ash percentages for separation of bulk coal fraction (−3.0 + 0.075 mm size) and the fines fraction below (0.075 mm size), it was found that the ash reduction achieved for the coarse coal were very high when compared to the fines. Further, attempts made to predict the performance of the cyclone using the concept of generalized distribution curve revealed that the Plitt’s equation can be conveniently adopted. This equation can be given as for separation of bulk coal. Further, it was established that, besides the outlet diameters of cyclone, the presence of fines below 0.075 mm in the feed influences the throughput of slurry of the cyclone significantly given as .

Characterization of feed coal and combustion by-products from Kemerkoy power plant in Turkey

Kemerkoy power plant is a coal-fired power station located in Mugla, Turkey. The plant burns 21,600 tons per day of low-quality lignite to produce electricity, which induce severe slagging and fouling problems in boilers and generates large amounts of coal combustion by-products. In this study, feed coal, fly ash, bottom ash, and boiler slag samples from Kemerkoy power plant were subjected to characterization studies. The chemical, mineralogical, and morphological characteristics of the samples were measured using different techniques including chemical analyses, XRD, SEM, and EDS. The results of the characterization studies were interpreted to reveal ash deposit formation mechanism and the relationship between the naturally existing mineral phases in feed coal and mineral phases generated by coal combustion process.

Differentiation of rare earth elements and yttrium in different size and density fractions of the Reshuihe coal, Yunnan Province, China

Rare earth elements and yttrium (REY) in coal, fly ash as well as coal preparation products have attracted much attention due to the increasing demand worldwide. In this paper, the distribution of REY in different size and density fractions, their mode of occurrence, and the insight they provide on the sediment source region of the Reshuihe coal in southern China are studied. The results show that the physical separation method caused light-REY (LREY) to be enriched in the low-density and large-sized coal particles, whereas medium-REY (MREY) and heavy-REY (HREY) are enriched in small-sized and high-density fractions. In addition, LREY are more closely associated with organic matter, while MREY and HREY occur more often in aluminosilicate minerals. Moreover, the sediment source of the Reshuihe coal was mainly derived from the felsic–intermediate rocks at the top of the Emeishan basalt sequence. Further, REY in the feed coal are generally not enriched, though REY are enriched in density fractions of 1.4 kg/L. The combustion products of coal with a density of 1.4 kg/L obtained by gravity separation have a potential economic value as promising raw materials for REY.

Fate of potentially hazardous trace elements during the entrained-flow coal gasification processes in China

With the rapid and wide application of the coal gasification technology in China, the corresponding environmental issues require increasing attention. In this study, the feed coal, coal gasification wastewater samples at key technical links, and the coal gasification residues were collected from three entrained-flow coal gasification plants in Northwest China and were characterized to understand the migration and partition of 21 potentially hazardous trace elements (PHTEs) in coal gasification process and their potential environmental impacts.Compared with Chinese coal, the concentrations of the PHTEs in the feed coals are mainly at the level of “depleted” and “normal”. Undergoing the heating process in the gasifier, purification procedure for the syngas, and the residue-wastewater separation, the PHTEs in the feed coals are transferred into gaseous phase, wastewater, and gasification residues. The PHTEs are easier to be released into the gaseous phase during the GSP gasification process compared with the OMB and GE gasification processes. The chalcophile elements Zn, As, Se, Cd, Sb, Tl, Pb, and Hg tend to be enriched in the fine residues, and the retention capability of Hg by the residues is the lowest. With the grey water circulation and the use of additives, the black water samples have higher PHTEs concentrations than other water samples. Except for Sr, Mo, Sb, and Cs, most of the elements in the original water samples can be reduced effectively by removing the particulate matters. In some cases, Cr6+, Zn, As, Se, Cd, and Pb in the original wastewater samples exceed the limit for industrial use as indicated in the Chinese Environmental Quality Standards for Surface Water and the Emission Standard for Pollutants from Coal Industry. The potential environmental impacts of As and Se in the water cannot be reduced by filtration. The environmental impacts of the coal gasification residues should be further evaluated.

Experimental Research on NOx Emission Characteristics During Coal Combustion in the Stoker-fired Boiler

In this paper, the combustion stability and nitrogen oxides (NOx) emissions test were made in two coal-fired chain grate boilers with hot water supplies of 29MW and 46MW. The investigations aim to clarify the key characteristics of NOx emission, e.g.instability of NOx emission, temperature and gaseous components profiles and the unsteady variation of NOx emission with power level changing. Main conclusions include: 1) In the process of stable combustion, NOx emissions still have±5% fluctuations, it is not stable. 2) The furnace temperature distribution show double temperature peaks. One peak (~1500°C) is located on the burning solid-fuel surface on the chain grate, and the other (~1050°C) is ~5.6m above the arch. 3) With increasing the thermal power, NOx emission shows a dramatic increase, higher around 1.25 times, as due to the rapid adjustment of coal feeding in grate furnace. Those above show the NOx control difficulty and the proper injector location of SNCR (selective non-catalytic reaction), which is key data collected for further NOx reduction retrofits.

Effect of gas bypassing and operating parameters on coal gasification in internally circulating fluidised bed

ABSTRACTIn internally circulating fluidised bed, there occurs gas bypassing from draught to annulus region and vice versa. The effect of gas bypassing on product gas composition in both annulus and draught tube is studied using a model proposed in literature. The same modelling is applied to estimate the number of circulations a particle should undergo to achieve a specified conversion and the total gas yield and solid circulation rate inside the reactor are calculated at different temperatures for different conversion levels. The effect of coal feed rate, particle size, oxygen to coal mass ratio and steam to coal mass ratio on the number of circulations, total gas yield and solid circulation rate is studied. The optimum conditions to achieve 95% conversion with minimum number of circulations and maximum gas yield per kg of coal per pass are obtained. The conversion and product gas composition are studied for a mixture of particles and compared with uniform size particles. The product gas composition depends strongly on operating parameters such as oxygen/coal mass ratio and steam/coal mass ratio and gas bypassing.