Carbon In Fly Ash Research Articles

Effect of fly ash composition on the retention of mercury in coal-combustion flue gas

Fly ash from coal-fired power plant has been reported to be a promising sorbent for mercury capture. However, the nature of the Hg–fly ash interaction is still unknown. In this work, the thermal decomposition method is used to evaluate the effect of the fly ash composition on the retention of mercury in coal-combustion flue gas. Eight fly ash samples were collected from the electrostatic precipitators of eight full-scale pulverized coal-fired power plants. The high-temperature treatment is performed to gradually decompose the carbon and mercury species in the fly ashes, and each fly ash are divided into four groups. The content of the carbon and mercury present in all the examined samples was measured. The results show that the most mercury species are decomposed from the fly ash at 300°C. The quantity of carbon in the fly ash alone does not determine the amount of mercury retained at any temperature. The decomposed mercury content is related to the mercury content of the fly ash. The carbon particle with ultra-strong mercury capture capacity cannot be found in the fly ash. Furthermore, X-ray fluorescence spectrometry (XRF) is used to identify the elemental composition of inorganic components of all the native fly ashes. Multivariate linear regression method is used to assess the dominated components that determine the mercury retention capacity of these fly ashes. The results indicate that the carbon, silicon, iron, sulfur, and magnesium in fly ash all play a major role in mercury retention. Hence, both carbon and inorganic components influence the retention of mercury in the fly ash.

Chemical forms of the fluorine, chlorine, oxygen and carbon in coal fly ash and their correlations with mercury retention

Fly ashes recovered from the particulate control devices at six pulverized coal boiler unites of China, are studied using an X-ray photoelectron spectroscopy (XPS) with a particular focus on the functionalities of fluorine (F), chlorine (Cl), carbon and oxygen on fly ash. It is found that the inorganic forms of F and Cl are predominant on the ash surface in comparison with their organics, and the proportion of organic Cl is relatively higher than that of organic F. Similar results are also obtained in the bulk by correlating the F and Cl contents with those of the unburnt carbon and other compositions in ash. Strong correlations of mercury retention with surface carbon–oxygen functional groups indicate that the CO, OH/CO and (OCO)O on surface are of significant importance for mercury retention in fly ash. Their surface concentrations are related to coal type. The presence of Cl in fly ash helps with mercury retention. No obvious effect of F is observed.

Novel Mineral and Organic Materials from Agro-Industrial Residues as Fillers for Natural Rubber

Bio-based materials are being developed, increasingly, because of the need for sustainable, environmentally friendly, processes and products. We have made natural rubber composites containing low cost fillers derived from agro-industrial wastes, namely, eggshells, carbon fly ash, processing tomato peels and guayule bagasse. Composites manufactured with 35 phr (parts per hundred rubber) of carbon black (particle size 108 ± 31 nm) were used as a reference filled material. The amount of carbon black was gradually replaced by a specific waste-derived macro (<300 µm diameter) or micro (<38 µm diameter) filler. Despite differences in particle size distribution and surface chemistry, the tensile properties of micro sized tomato peels composite were similar to those of the carbon black reference composite. Composites manufactured using micro sized particles at low loadings (5 and 10 phr) of carbon fly ash and eggshells, as co-fillers with carbon black, also demonstrated comparable tensile results to those of composites manufactured solely with carbon black. Hierarchical cluster analysis partitioned the data into five groups of composites having statistically similar mechanical properties. Composites containing micro sized particles at low loadings (5 and 10 phr) were grouped in clusters 1, 2 and 5. These clusters gathered composites with the highest tensile strength and 300 % modulus. As the amount of non-carbon black filler increased above 10 phr and up to 35 phr (maximum filler loading used), the composites became weaker than the carbon black filled composites and modulus decreased.

Improved cement mortars by addition of carbonated fly ash from solid waste incinerators

This article presents the results of a research developing high performance cement mortars with the addition of municipal solid waste incineration fly ash (MSWIFA) stabilized as insoluble carbonates. The encapsulation of hazardous wastes in mortar matrixes has also been achieved. The ashes present high concentrations of chlorides, Zn and Pb. A stabilization process with NaHCO 3 has been developed reducing 99% the content of chlorides. Developed mortars replace 10% per weight of the aggregates by treated MSWIFA. Physical/mechanical properties of these mortars have been studied. Presence of Zn, Pb, Cu and Cd has been also analyzed confirming that leaching of these heavy metal ions is mitigated. Conclusions prove better behavior of CAC and CSA mortars than those of CEM-I and CEM-II cement. Results are remarkable for the CAC mortars, improving reference strengths in more than 25%, which make them a fast-curing product suitable for the repair of structures or industrial pavements.

CFD modelling of co-firing of biomass with coal under oxy-fuel combustion in a large scale power plant

Abstract Co-firing biomass is the principal means of mitigating the future energy crisis by expanding the use of renewable energy. Oxy-fuel combustion is the most capable technologies for carbon capture and storage (CCS) system. This paper presents a 3D numerical study considering co-firing concepts in a 550 MW tangentially fired furnace using a commercial CFD code AVL Fire ver.2009.2. Necessary subroutines were written and coupled with the code to account for chemical reactions, heat transfer, fluid and particle flow fields and turbulence. Due to irregularities of the biomass particle shape, a special drag effect was considered. Three different co-firing cases (20% biomass with 80% coal, 40% biomass with 60% coal and 60% biomass with 40% coal) were considered. All the co-firing cases were simulated under air-firing and three different oxy-firing cases (25% O 2 /75% CO 2 , 27% O 2 /73% CO 2 and 29% O 2 /71% CO 2 ). Level of confidence has been achieved by conducting a study on co-firing of biomass with coal in a 0.5 MW small scale furnace under air and oxy-fuel conditions. Similar findings have been observed in the present study which indicates the model can be used to aid in design and optimization of large-scale biomass co-firing under oxy-fuel conditions. This study enables the calculation of species transport and mixing phenomena and the simulation of ignition, combustion and emission formation in industrial furnace. Results were presented by the aerodynamics of burner flow, temperature distributions, gaseous emissions such as O 2 and CO 2 distributions. With the increase of biomass sharing, peak flame temperature reduced significantly. The dominant effect of the lower calorific value of biomass dampens the effect of volatile content contributing to lower temperature. Comparatively, improved burnout is observed for the improved oxy-fuel cases. But, the CFD model predicted a significant increase in unburned carbon in fly ash for the increase of biomass co-firing sharing. Overall, this study highlights the possible impact of changing the fuel ratio and combustion atmosphere on the boiler performance, underlining that minor redesign may be necessary when converting to biomass co-firing under air and oxy-fuel conditions.

Direct mineral carbonation of coal fly ash for CO2 sequestration

Developing countries face the projected scenario of increased CO2 emissions in the coming years as their major dependence for power generation relies on the coal reserves. Mineral carbon sequestration using industrial alkaline solid residues is a promising technology for carbon capture and storage. In this laboratory work, direct mineral carbonation of coal fly ash (CFA) through gas–solid as well as aqueous route has been studied with 100% CO2 gas in a stainless steel reactor at low pressure (<10 bar) and room temperature. The dry route demonstrated to be a slow process, wherein the maximum sequestration capacity was found to be 26.3 g of CO2/kg of CFA at 10 bar for 1 h. In the case of wet carbonation, the maximum capacity achieved was 50.3 g of CO2/kg of waste with the CFA water slurry of 15 as L/S ratio, stirred at 900 rpm at 4 bar for 2 h. The carbonation process was evidenced by the characterization of the carbonated CFA using FT-IR, XRD, SEM and TGA. The results show that CFA can be utilized for carbon capture through the simple, direct carbonation approach and the process parameters play vital role in effective sequestration.

Electrical properties of fly ash and its decarbonization by electrostatic separation

The basic principle of fly ash triboelectrification is analysed. The mineral electrical index and test method are introduced. The electric difference of different mineral composition of fly ash is discussed by analysis of chemical and mineral composition of fly ash in Xinwen power plant. The dielectric constant and charge–mass ratio of carbon and ash of fly ash are tested. Combined with the experimental study on rotary triboelectrostatic separation, the charged characteristic of fly ash particles with different size is gained. The results show that the dielectric constant of fly ash with different grain size decreased with the decrease of particle size, which lead to the poor electrical conductivity. Thus it can be seen that particle size plays a leading role in conductivity. The charge of carbon and ash with each size increased with the decreased of particle size; and the charge–mass ratio between carbon and ash with the same size larger with the decrease of size, which indicated that the finer particle size, the more favorable for triboelectrification separation. In the same conditions, the best decarburization effect is realized when the particle size ranges from 0.038 to 0.074mm, whose decarbonization rate and efficiency index reached 38.93% and 120.83% respectively.

Using a genetic algorithm and CFD to identify low NOx configurations in an industrial boiler

Abstract This paper focuses on a computational intelligence approach used for minimizing NOx emissions in a 600 MW tangentially-fired pulverized-coal boiler. Genetic Algorithms (GA) were used to correlate operating parameters to significant output data predicted by CFD simulations of the boiler. The operating parameters include the opening or closing of air dampers, changing the coal distribution through mill selection and feed rate and vertical tilting of the burners. A target function was introduced to estimate for each boiler settings defined by given operating parameters, the costs associated with corrosion on the water-wall tubes, heterogeneous heat flux distribution along the walls, unburned carbon in fly ash and NOx emissions. The GA was able to automatically generate innovative boiler configurations among thousands of CFD calculations performed. The target function allowed the search space to be explored to establish configurations offering a good compromise between NOx reduction and the cost associated with corrosion in particular. Moreover, the predicted NOx emissions from the GA model are consistent with the NOx levels measured during test campaigns.

Petrological and biological studies on some fly and bottom ashes collected at different times from an Indian coal-based captive power plant

Abstract India has about a tenth of the world’s coal reserves, much of it with high mineral content. These coals produce a large amount of fly ash, which can affect human health and environmental quality aspects during utilization. In this paper, the petrological and biological aspects of some industrially important Indian coal fly ash (CFA) from a coal-based captive power plant are addressed. The petrology of the CFAs is also studied for the samples collected in different times. The study has revealed that the CFAs contain mainly glass fragments, spinel, quartz, and other minerals in lesser quantities. Fly ash carbons were present as chars, possibly from the incomplete combustion of the coals (bituminous and/or subbituminous) used in the power plant. The deposition of CFAs over the leaves of different plant species reduces the photosynthesis rate by about 95% within a period of 2 h. The CFAs also show minor effects to some test microbes. This investigation will be useful in assessment of the environmental impact of a coal-based power plant.

Activated carbon monoliths by pressureless technique for environmental applications

Carbon monoliths have been prepared through simple procedure by carbonization in inert atmosphere at 550°C from commercial activated carbon and from unburned carbon in waste bagasse fly ash, using two different kinds of phenolic resin binders. The bagasse fly ash was collected from sugar mills where bagasse is used as a biomass based fuel for cogeneration. Commercial activated carbon based monoliths where 5 and 10 wt % of the activated carbon had been replaced by iron oxide (hematite) were also prepared. Results indicate that BET surface area decreases upon carbonization and loading of hematite. Scanning electron microscopy studies shows that the iron oxide is well distributed over the monoliths and X‐ray diffraction shows that it is reduced to magnetite during carbonization. Temperature programmed reduction experiments show that the iron oxides on the monoliths are redox active. The monoliths based on commercial activated carbon show 80% removal of phenol in diluted phenol based water solutions whereas unburned carbon derived monoliths showed 51% removal in similar solution. © 2015 American Institute of Chemical Engineers Environ Prog, 34: 1420–1426, 2015

NO Emissions and Combustion Efficiency during Biomass Co-firing and Air-staging

Experiments were carried out in a drop tube furnace to investigate the effects of biomass/coal co-firing and air staging on NO emission and combustion efficiency. NO and CO emissions along the height of the furnace were monitored by a gas analyzer, and the content of unburned carbon (UBC) in fly ash was also tested. Results showed that NO emission from straw or wood combustion only account for 1/3 or 1/2 that from coal combustion, respectively. Under the conditions of biomass co-firing, the increase in blending ratio had a positive effect on the reduction of NO emission and combustion efficiency. Moreover, results of air-staging combustion showed that for coal combustion, air staging notably reduced NO emission and combustion efficiency. For biomass combustion, the effect was slight. Synergetic analysis indicated that there was an optimum biomass co-firing ratio around 0.4, when the positive synergetic effects on reducing NO emission and UBC were the most significant. When the co-firing ratio exceeded this optimum value, further increasing the co-firing ratio had little influence on NO emission and combustion efficiency. After air staging was adopted, the degree of synergetic effect on NO emissions was reduced while that of UBC was increased.

Chemical characterization of unburned carbon in coal fly ashes by use of TPD/TPO and LRS methods.

Functional forms of the unburned carbon present in six kinds of coal fly ashes have been examined mainly by the temperature-programmed desorption (TPD)/temperature-programmed oxidation (TPO) and laser Raman spectroscopy (LRS) methods. The carbon contents of the ash samples range from 0.4 to 4.1 mass%. The LRS analysis shows that the C consists of both amorphous and crystallized forms, and the proportion of the former is as large as 50-65 C%. Further, the TPD measurement exhibits that the C contains several types of surface oxygen species, such as carboxyl and lactone/acid anhydride groups, which can readily be decomposed into CO2 up to 700 °C to provide active carbon sites. The results of the TPD also indicate that the ashes have surface CaCO3, and most of this species can be converted to CaO and CO2 around 600-700 °C. Interestingly, there is a significant correlation between organic fluorine concentrations and carboxyl/lactone/acid anhydride groups or surface CaCO3 contents in the ash samples. It might thus be possible that the formation of organic F forms proceeds through gas-solid-solid interactions among HF (and/or F2) in flue gas, active carbon sites and surface Ca species produced around 600-700 °C downstream of coal-fired boilers.

Geochemical modeling of brine remediation using accelerated carbonation of fly ash

A protocol is proposed, where waste fly ash and CO2 emissions from coal-fired power plants are utilized in remediating brine waste. The untreated brine sample was made up of Na+, SO42-, Cl−, Ca2+, Mg2+, and K+ as the major ions with trace concentrations of other ions. The brine can thus be classified as Na2SO4--rich water with respect to major cations and anions. Following carbonation, over 99% removal of NO3− was achieved while B3+, V2+, MO2+, and Cl− concentrations increased. Major components removal from brine upon carbonation was as follows: Na+ (15–29%), Mg2+ (53–87%), K+ (70–88%), Ca2+ (40–73%), and SO42- (12–36%). Speciation modeling of the major components present in brine showed that Na+, K+, and Cl− exist mainly as free ions, while Mg2+ and Ca2+ are associated with SO42- as well as being in their free forms. SO42- ions on the other hand were present in its free form to a great extent as well as associated with Na+, Ca2+, Mg2+, and K+, respectively, in a decreasing order. The carbonated brine effluents are therefore depleted with regards to major and trace elements concentration, when compared with the untreated brine. Mineral carbonation may therefore be a potent brine remediation protocol, which can be optimized for maximum removal of various elements.

Adsorbents/catalysts from forest biomass fly ash. Influence of alkaline activating agent

The aim of this research work is to study the influence of an alkaline activating agent on the preparation of adsorbent/catalyst materials from fly ashes derived from the combustion of Eucalyptus Globulus bark in the energy plant of a pulp mill located in Northern Spain. Different fly ash pre-treatments carried out in previous studies had provided a range of precursors with a high content in unburned carbon and good textural properties. The fraction with a grain size larger than 500 μm was selected as it seemed the best precursor for obtaining adsorbent/catalysts materials due to its higher unburned carbon content and better textural properties. This precursor was chemically activated with several alkaline hydroxides and carbonates at different activating agent/precursor weight ratios and under diverse experimental conditions of activation: an activation temperature of from 600 to 900 °C, and a nitrogen flow of 150 and 500 ml min−1. The materials obtained were characterized both chemically and texturally. The results show that it is possible to obtain adsorbent/catalyst materials by chemical activation using a rich-unburned carbon fly ash fraction as precursor (>500 microns). A BET specific surface area of up to 2108 m2 g−1 and total pore volume of 1.120 cm3 g−1 were achieved. KOH and NaOH were found to be the best activating agents. The materials obtained were mainly microporous. In general, the development of mesoporosity was promoted by NaOH chemical activation. The parameters that had the greatest influence on the textural development of the materials were the activating agent/precursor ratio and the final activating temperature.

Rapidly Measuring Unburned Carbon in Fly Ash Using Molecular CN by Laser-Induced Breakdown Spectroscopy

In coal-fired plants, the balance between unburned carbon and NOx emissions stresses the need for rapid and accurate methods for the measurement of unburned carbon. In this paper, molecular CN was adopted to rapidly measure unburned carbon in fly ashes by laser-induced breakdown spectroscopy technique for the first time. The use of molecular CN overcame the interference between the Fe 247.98 and C 247.86 nm lines and the strong diminishing of the C 193.09 nm line intensity in air. Especially, the multivariate regression method combined with the correction of plasma temperature and self-absorption was used to construct calibration model. The performance of the calibration model was evaluated by the quantitative analysis of unkown fly ashes from different types of coal. The results show that the averaged relative error of prediction and the limit of detection are 0.26% and 0.16 wt %, respectively, while the averaged relative standard deviation is less than 5%. The performance of the quantitative analysis of unburned carbon meets the requirement of PRC power industry standard and the most of commercial instruments for online or rapidly analysis of unburned carbon.

Field test of coal type adaptability on a 300 MW CFB boiler

Coal type adaptability for CFB boilers is of great importance because it influences whether the unit can operate safely, economically and environmentally when the coal type is changed. Research is carried out on a 300MW CFB boiler located in Longyan power plant, Fujian province by burning three types of coal, i.e. Fujian anthracite, Shenhua bituminous coal and Indonesia lignite. The results show that all the three coals are capable to meet the basic requirement of bed temperature. But the fly ash carbon content and exhaust gas temperature depend on the coal type. Compared with Fujian anthracite, Shenhua bituminous coal and Indonesia lignite tend to possess lower fly ash carbon content and lower exhaust gas temperature under high boiler load, which helps to increase the boiler efficiency. The auxiliary power consumption rates under all the three types of coal are all lower than 4.3% with the successful utilization of lower energy consumption technology based on state specification design theory. Besides, when the fuel is changed from Fujian anthracite to Shenhua bituminous coal and Indonesia lignite, SO2 emission and dust emission decreases, while the NOx emission increases.

ICOPE-15-C083 Study of Numerical Simulation and Field Test for Co-Combustion of Sewage Sludge on a 100 MW Coal Fired Boiler

A series of field test and numerical simulations was carried out to evaluate the characteristics of coal combustion and coal combined firing with sewage sludge in a 100 MW tangentially coal fired utility boiler. The simulation results was highly consistent with the experimental data when the moisture content of sewage sludge was 56% with mass blending ratio less than 10%: the flue gas temperature decreased, char burnout rate fell, unburned carbon in fly ash and NOx emissions, but the differences were small. Both the experimental and simulation results indicated that small mass ratio of sewage sludge co-combustion has little effect on the operation of boiler. Additionally, the influence of combustion characteristics and pollutant emissions when co-combustion high blending ratio of sewage sludge was further studied. Results showed that the decrease of sewage sludge moisture content was helpful in improvement of the boiler combustion characteristics, besides, the NOx emission also reduced. The NOx emission increases and the flue gas temperature and the burnout rate of char decreases when increasing the sewage sludge blending ratio up to 30%. And the changes become more and more significant. In addition, the NOx reduction strategy and the slagging tendency caused by the co-combustion of sewage sludge were also studied.

Structural Properties and Gasification Reactivity of Shenmu Fly Ash Obtained from a 5t/d Circulating Fluidized Bed Gasifier

Coal gasification is one of the key technologies for the high efficiency and clean use of coal resources. Circulating fluidized bed gasification is attracting considerable interest worldwide. The advantages of circulating fluidized bed gasification are that it can use bigger particle sizes, has a wider adaptability to a range of solid fuels, can operate at moderate temperatures and therefore emit lower amounts of nitrogen and sulfur compounds. However, the lower operating temperature can also result in the problem of high content of residual carbon in fly ash. Gasification fly ash is a by-product of the coal gasification process. To achieve “zero emission” from coal gasification technology, the environmentally safe utilization of fly ash by-products from gasifiers must be addressed and development. In this study, work has been carried out to investigate the structural properties and gasification reactivity of Shenmu fly ashes, which were obtained from a 5t/d circulating fluidized bed gasifier with different operating conditions. The structural properties of fly ashes were characterized with SEM, XRD and BET study. The gasification reactivity of fly ashes with CO2 was analyzed by isothermal and non-isothermal thermogravimetric technology. A comparative investigation of fly ashes and fast pyrolyzed Shenmu char was also conducted. The results proved fly ashes to be more active than Shenmu char, which could be attributed to the lower graphitization degrees, the smaller particle sizes and higher specific surface areas of fly ashes.

Leaching characteristics of toxic constituents from coal fly ash mixed soils under the influence of pH

Leaching behaviors of Arsenic (As), Barium (Ba), Calcium (Ca), Cadmium (Cd), Magnesium (Mg), Selenium (Se), and Strontium (Sr) from soil alone, coal fly ash alone, and soil-coal fly ash mixtures, were studied at a pH range of 2–14 via pH-dependent leaching tests. Seven different types of soils and coal fly ashes were tested. Results of this study indicated that Ca, Cd, Mg, and Sr showed cationic leaching pattern while As and Se generally follows an oxyanionic leaching pattern. On the other hand, leaching of Ba presented amphoteric-like leaching pattern but less pH-dependent. In spite of different types and composition of soil and coal fly ash investigated, the study reveals the similarity in leaching behavior as a function of pH for a given element from soil, coal fly ash, and soil-coal fly ash mixtures. The similarity is most likely due to similar controlling mechanisms (e.g., solubility, sorption, and solid-solution formation) and similar controlling factors (e.g., leachate pH and redox conditions). This offers the opportunity to transfer knowledge of coal fly ash that has been extensively characterized and studied to soil stabilized with coal fly ash. It is speculated that unburned carbon in off-specification coal fly ashes may provide sorption sites for Cd resulting in a reduction in concentration of these elements in leachate from soil-coal fly ash mixture. Class C fly ash provides sufficient CaO to initiate the pozzolanic reaction yielding hydrated cement products that oxyanions, including As and Se, can be incorporated into.

Improvement on Quantitative Measurement of Fly Ash Contents Using Laser-Induced Breakdown Spectroscopy

Contents of fly ash are important factors for the operation of coal-fired plants. Real-time monitoring of coal and fly ash such as unburned carbon in fly ash can be an indicator of the combustion conditions. Because of the strong signal intensity and the relative simplicity of the LIBS (Laser- Induced Breakdown Spectroscopy) technique, LIBS can be applicable for real-time composition measurement of coal and fly ash. This research presented here focused on the clarification of the effects of plasma temperature and coexisting materials on quantitative measurement of fly ash contents. Quantitative capability of LIBS was improved using the proposed plasma temperature correction method. The CO2 effect was also discussed to accurately evaluate unburned carbon in fly ash in exhausts. Using the results shown in this study, quantitative measurement of fly ash contents has been improved for wider applications of LIBS to practical fields.