Trace element evaporation during coal gasification based on a thermodynamic equilibrium calculation approach

Thermodynamic equilibrium calculations using the HSC-Chemistry program were performed to determine the distribution and mode of occurrence of potentially toxic and corrosive trace elements in gases from coal gasification processes. The influence of temperature, pressure and gas atmospheres on equilibrium composition was evaluated. In these reducing conditions, the behaviour of the trace elements is complex, but some form of organization can be attempted. Elements were classified into three groups. Group A includes those elements that, according to thermodynamic data at equilibrium, could probably be condensed in coal gasification. Mn is classified in this group. Group B contains those elements that could be totally or partially in gas phase in gas cleaning conditions, and can be divided into two subgroups, depending on whether the cleaning conditions are hot or cold. Co, Be, Sb, As, Cd, Pb, Zn, Ni, V, Cr are elements in this group. Group C contains those elements that could be totally in gas phase in all the possible conditions, including flue gas emissions. Se, Hg and B are the elements that make up this group.     

A thermodynamic equilibrium comparison of the mobilities of trace elements when washed and unwashed coals are burnt under pf firing conditions

Thermodynamic equilibrium predictions of trace element partitioning between the gaseous and condensed products of combustion have been made for four coals as supplied and after washing to remove further mineral. The calculations modelled the products of combustion of the whole coal with excess oxygen (1% excess for each coal with additional calculations at 3% for Harworth coal and 5% for Gascoigne Wood); the trace elements examined were As, Cr, Cu, Mn, Ni, Pb and Zn. The calculations indicated that almost all thermodynamic equilibrium distributions included a greater percentage of the trace element in the gas phase at 1300–2100 K for feedstock with extra washing as compared with the standard feedstock. When the amounts volatilised were converted to moles per 100 g of coal burnt it was found that in some cases the relative amount of some elements volatilised from washed compared with unwashed coal was lower, in other cases the relative amount volatilised increased.Step cooling calculations were carried out using as initial concentrations the amounts of each element in the gas phase at 1300 K for washed and unwashed Gascoigne Wood coal. The results indicated deposition of sulphates in the lower part of the temperature range 1300–600 K, with a greater tendency to form alkali sulphate based melts for the washed coal.     

Speciation of As, Cr, Se and Hg under coal fired power station conditions

Coal combustion from power stations is an important anthropogenic contributor of toxic trace elements to the environment. Some trace elements may be emitted in range of valencies, often with varying toxicity and bioavailability. Hence, determination of trace element speciation in coals and their combustion products is important for conducting comprehensive risk assessments of the emissions from coal-fired power stations. This study focuses on speciation of selected trace elements, As, Cr, and Se, in coal combustion products and Hg in flue gas, which were sampled at one Australian power station. Different analytical methods such as secondary ion mass spectrometry (SIMS), ion chromatography-inductively coupled plasma mass spectrometry (IC-ICPMS) and X-ray absorption near edge structure spectrometry (XANES) were used to determine trace element speciation in coal and ash samples. Results showed that As, Cr and Se are all present in a range of valency states in coal. Concentrations of As and Se in the bottom ash as well as the more toxic hexavalent chromium were less than the detection limits. The more toxic As³⁺ form in fly ash was at 10% of the total arsenic, while selenium was mainly found in Se⁴⁺ form. Hexavalent chromium (Cr⁶⁺) in fly ash was 2.7% of the total fly ash chromium. Mercury speciation in flue gas was determined using the Ontario Hydro sampling train and analysis technique. Approximately 58% of the total mercury in flue gas was released in the elemental form (Hg⁰), which, among all mercury species, has the highest residence time in the environment due to lower solubility. This work summarises the performance of the selected analytical techniques for speciation of trace elements.     

The composition of size-fractionated pulverised coal and the trace element associations

Major and trace element analyses have been performed on size fractions of a pulverised coal from Eggborough Power Station (UK). Minerals are concentrated in the fractions less than 10 μm in size and there is relative enrichment of pyrite in the fractions greater than 50 μm. Because of the compositional variation with size it is possible to proportion statistically the elements between, in this case, organic matter, silicates and pyrite. Germanium, Br and V are dominantly organic associated and Cr, Cu, Ni, Zn, Sr, Ba and Pb are also present in the organic matter, although concentrations are lower than in other fractions of the coal. These elements are either in the organic structures or contained within pore fluids. Chromium, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Sm, Th and U are dominantly associated with the silicate fraction, as are V, Ni and Zn, but other coal fractions contribute more to the total coal composition. Concentrated in pyrite are Mo, Se, As, Pb, Sb, and to a lesser extent Ni, Cu and Zn in that these elements are sufficiently concentrated in other fractions that pyrite is not the major location in the coal. Validation for the method is achieved by summing element concentrations in the three fractions and comparing with the bulk composition. Previous calculations on a related coal have been extended and close agreement observed for the composition of the three fractions. The calculated values for the fractions apply specifically to one coalfield, although some of the values may have more general application.     

Mobilisation of trace elements from as-supplied and additionally cleaned coal: Predictions for Ba, Be, Cd, Co, Mo, Nb, Sb, V and W

The partitioning of the elements antimony, barium, beryllium, cadmium, cobalt, molybdenum and vanadium between the products of combustion of coals containing them burnt as pulverised fuel in excess air has been modelled using the MTDATA thermodynamic equilibrium package with data from the MTOX silicate melt model added to the standard database and trace element data added where necessary. The coals examined were Gascoigne Wood, ElCerrejon and Harworth coals as normally supplied (washed) and after additional washing, and Binungan low ash coal only as normally supplied, represented by the analyses for coal, coal mineral and trace elements obtained in a study of the partitioning carried out in a pilot scale pf combustor by PwerGen on behalf of the United Kingdom DTI. Excess air levels were 1% for all coals and 3% in addition for Harworth. The equilibrium amount of silicate melt was predicted to fall more rapidly with falling temperature for additionally washed than for normally washed coals. It was also predicted that Ba and Co would be almost immobile, Be and V would be relatively immobile, and Sb, Cd and Mo would be mobile. Additional calculations were carried out for niobium and tungsten as trace elements in the coals, and Nb was found to be relatively immobile and W mobile. The mobilities of Ba, Be, Co, Cd, Mo, Sb and V were in agreement with those implied by the ratio of bottom ash to fly ash concentrations found in experimental investigation.     

Trace elements in combustion residues from a UK power station

Trace element analyses of combustion residues from Eggborough Power Station (UK) are re-examined and augmented with new data, which has enabled original conclusions [Fuel Process Technol 47 (1996) 79] to be extended in two areas and developed in a third. These are: (a) potential emissions through mass closure with the conclusion that of the elements analysed only As appears to have been lost from the system (mass closures for Se and Hg were not determined but these elements are also probably lost); (b) volatility of elements based on ratios of element concentrations in fly ash to furnace bottom ash and element concentrations in a fine ash fraction to a coarse fraction with the latter appearing to be the more sensitive indicator and (c) the percentage surface association based on trace element analyses of size-fractionated fly ash. From the latter it is concluded that As and Mo are predominantly surface associated with little contained within the glass or other phases, Cu, Zn and Pb have a surface association of about 50%, V, Cr and Ni a lower surface association of between 30 and 20% and Ba and Sr a minor association of about 10%. The same conclusion was also reached for As, Mo and Pb using LA-ICP-MS [Fuel 2004] with, in addition, a similar association suggested for U, Tl and Se.     

几种潮州传统小吃中微量元素及维生素分析

本文用国内外公认的标准方法对5种潮州传统小吃中的无机元素和维生素进行测定。结果表明,被测的5种潮州传统小吃中含有较丰富的无机元素和维生素,为食物资源的开发提供科学依据。     

Trace element geochemistry of the Obed Mountain deposit coals, Alberta, Canada

This study reports on the elemental concentrations and vertical variation of coal seams from the Obed Mountain deposit, Alberta Foothills, Canada. Results from two sections of Seam 1 show that the major elements (i.e. Al, Fe, Mg, K, Na, Ti, and Si) have high concentrations in intervals having high ash content, with the only exception of Ca. Similarities are apparent, in both sections, in the vertical variations of Th, U, Se, and Zn; Rb, Cs, and K; Sb, Mo, and W; Mn and Sr; and Ba, Cr, Co, Hf, and Sc. These similarities are also evident among the REEs, notably between Ce and La; also between Dy, Eu, and Sm. Most elements, with the exception of Ba and Sr are slightly more concentrated in Section 2 of Seam 1, located approximately 1.5Fig. km away from Section 1. Compared to Seam 1, Seam 2 has lower mean concentrations of elements. Boron in the coal ranges from 27 to 100 ppmw, though most values are less than 50 ppmw. Boron concentrations suggest a freshwater depositional environment. The element is depleted in the sedimentary partings (12–29 ppmw only) and is enriched in the coal interval near the roof and immediately beneath the partings. This enrichment shows possible downward mobilization of boron. Vertical variations of elements are helpful in delineating the boundaries between coal and sedimentary partings in the succession. The Obed Mountain coals are “clean” by world standards and their elemental concentrations are comparable with those in coals of a lower rank from the same coal formation used for power generation in Alberta. All sedimentary partings have low concentrations of Ba, Hf, Sc, Sr, Ta, Th, U, and REEs; this, along with the absence of an Eu negative anomaly suggests a non-volcanic origin for the partings.     

Behavior of trace elements during pyrolysis of coal in a simulated drop-tube reactor

Release behavior and chemical form distribution of As, Pb, Cr, Cd and Mn in Datong coal during pyrolysis was studied in a simulated drop-tube reactor at a heating rate of about 1000 °C/s, including effects of temperature (300-1000 °C), atmosphere (N₂ and H₂), and holding time (0.3-10 min). Results show that the bleeding ratios of As, Pb, Cr, Cd and Mn increase with increasing pyrolysis temperature and holding time. Reductive environment results in higher emission of the elements. Among the five trace elements, As, Pb and Cd show similar behavior with volatilities higher than that of Cr and Mn at 1000 °C. The five trace elements in the coal and coal-derived chars are separated into five fractions through an extraction procedure. Ion exchangeable form of the elements is not found in the coal and the chars, and the elements remained in the residue fraction is the most dominant occurrence form in the coal and the chars for As, Pb, Cd and Cr. All the forms for all the elements undergo transformation in the pyrolysis resulting in reduced content in the chars.     

Fate of trace element haps when applying mercury control technologies

During the past several years, and particularly since the Clean Air Mercury Rule (CAMR) was promulgated in June of 2005, the electric utility industry, product vendors, and the research community have been working diligently to develop and test Hg control strategies for a variety of coal types and plant configurations. Some of these strategies include sorbent injection and chemical additives designed to increase mercury capture efficiency in particulate control devices. These strategies have the potential to impact the fate of other inorganic hazardous air pollutants (HAPs), which typically include As, Be, Cd, Cr, Co, Mn, Ni, Pb, Se, and Sb. To evaluate this impact, flue gas samples using EPA Method 29, along with representative coal and ash samples, were collected during recent pilot-scale and field test projects that were evaluating Hg control technologies. These test programs included a range of fuel types with varying trace element concentrations, along with different combustion systems and particulate control devices. The results show that the majority of the trace element HAPs are associated with the particulate matter in the flue gas, except for Se. However, for five of the six projects, Se partitioning was shifted to the particulate phase and total emissions reduced when Hg control technologies were applied.     

Cleaning of Indian coals by agglomeration with xylene and hexane

A laboratory scale agglomeration process has been undertaken for cleaning Indian coals using oils namely, xylene and hexane. Maximum organic matter recovery for xylene has been found to be 91.9% whereas with hexane, the value is 54.7% on a dry basis. The highest ash rejection values with xylene (90.7%) and with hexane (89.7%) are almost same. Promising results for rejection of metals (Fe, Mg and Zn) have been observed. It has been found that xylene is more selective than hexane for the agglomeration process. Knowledge gained from this study will be helpful for technological advancement of this kind of work.     

Leaching of ashes and chars for examining transformations of trace elements during coal combustion and pyrolysis

One sub-bituminous coal and two bituminous coals were subjected to the combustion and pyrolysis by slow heating to a temperature ranging 550-1150 °C. Leaching of raw coals, ashes and chars with dilute HCl and HNO₃ was carried out, and leachate concentrations of major and trace elements were determined. Such a comparative leaching method was validated for characterizing the modes of occurrence of trace elements in coal and their transformations upon heating. Leaching results suggested that Be, V, Co, Cr and Ni were partially associated with organic matter, and As was partially associated with pyrite. During the ashing at 550-750 °C, the organically associated trace elements in coal formed some acid-soluble species. After the ashing at 1150 °C, Be, Co, Cr and Ni, together with Mn, Zn, and Pb, were immobilized in ash against leaching, whereas As was not immobilized. After pyrolysis, the organically associated trace elements in chars remained insoluble in both acids, and some HNO₃-soluble As in coal turned to a HNO₃-insoluble species.     

Prediction of unburned carbon and NOx in a tangentially fired power station using single coals and blends

Coal blends are now widely used by the power generation industry and the general characteristics are well known. Attention is still directed to the emission of NO_x, which is subject to more stringent regulation, and to the amount of carbon in ash. The latter is increased when low NO_x burners are employed, which is the norm now. It is also increased as a result of additional air staging when over-fire air is added in furnaces, especially tangential fired systems. Such a furnace is studied here. Two approaches can be employed for prediction of NO_x and unburned carbon. The first approach uses global models such as the ‘slice’ model which requires the combustor reaction conditions as an input but which has a detailed coal combustion mechanism. The second involves a computational fluid dynamic model that in principle can give detailed information about all aspects of combustion, but usually is restricted in the detail of the combustion model because of the heavy computational demands. The slice model approach can be seen to be complimentary to the CFD approach since the NO_x and carbon burnout is computed using the slice model as a post-processor to the CFD model computation. The slice model that has been used previously by our group is applied to a commercial tangentially fired combustor operated in Spain and using a range of Spanish coals and imported coals, some of which are fired as blends. The computed results are compared with experimental measurements, and the accuracy of the approach assessed. The CFD model applied to this case is one of the commercial codes modified to use a number of coal combustion sub-models developed by our group. In particular it can use two independent streams of coal and as such it can be used for the combustion of coal blends. The results show that both model approaches can give good predictions of the NO_x and carbon in ash despite the fact that certain parts of the coal combustion models are not exactly the same. However, if a detailed insight into the combustor behaviour is required then the CFD model must be used.     

Chemometric analysis of trace elements distribution in raw and thermally treated high sulphur coals

In the present study, chemometric analysis is applied as a tool to evaluate the release behaviour of trace elements (TEs) during coal utilization processes. Principal component analysis (PCA) and linear discriminant Analysis (LDA) were applied on the TE concentrations of raw and thermally treated coals. PCA and LDA successfully predicted the association of 21 trace elements (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sb, Te, Pb) contained in coal and their thermal behavior at various temperatures. Application of chemometric on thermally treated coals shows that at temperature 450 °C, elements like Na, P, K, Fe, Ca, Mg, Al and Si have affinity with mineral matter and therefore have low volatility. Elements like Te, Sb and Ti may form their chlorides, which enhance the volatilities of these elements, while Co and Pb may form sulfides like Co₂S₄ and PbS. In the temperature range of 600-850 °C, either coal undergones an intense degradation of its structure during pyrolysis and the elements released may be adsorbed on coal surface or be volatile. The elements Cr, Co, V, Ni may react with sulphurous gases evolved during pyrolysis. At temperature 1000 °C, wide dispersion in data elements interact with carbon and sulphur compounds of coals. The formation of compounds like Si carbide, bassanite, gehlenite, anarthite may also be responsible for low volatilities of the elements Si, Al and Ca at higher temperatures. Predictive capabilities of PCA and LDA were evaluated in terms of TEs volatilities at different temperatures. The results of chemometric analysis are not only in good agreement with volatilities of TEs present in coals at various temperatures but also with FTIR analysis.     

Biodesulphurization of coal: behaviour of trace elements

The purpose of this study is to show how the different size fractions of coal are affected during a process of biodesulphurization in a packed column and to examine the repercussions of the process on the elimination of certain heavy and trace elements. The total desulphurization obtained in 51 days is about 25 wt%, as only pyritic sulphur is attacked. The greatest reduction in sulphur content was for the 1-0.5 mm fraction although the lowest actual sulphur content was found in particles of under 0.125 mm, where it dropped from 1.76 to 1.17 wt%. The most important changes in the metal content were decreases in Ni, Cu, Zn, As, Cr, Co and Sr, mainly in the smaller sizes.     

Characterization and re-use potential of by-products generated from the Ohio State Carbonation and Ash Reactivation (OSCAR) process

Two novel sorbents (i.e. “regenerated sorbent” and “supersorbent”) for dry flue gas desulfurization were tested, and by-products characterized, using a pilot-scale version of the Ohio State Carbonation and Ash Reactivation (OSCAR) process. The main elements of the process consisted of sorbent production, a riser reactor, cyclone and baghouse. Trace elements, including As, Se and Hg, were found at higher levels in the OSCAR solid by-products (in both the cyclone and baghouse) compared to traditional lime spray dryer (LSD) ash. Polycyclic aromatic hydrocarbons (PAHs) detected on solid by-products were primarily small molecular weight compounds at low concentration (e.g., μg/kg). Small particulates (?3 μm) that escaped from the cyclone and were captured by the baghouse showed higher trace element concentrations, possibly due to the lower operating temperature and greater specific surface area of solids in the baghouse. Operating conditions including flue gas flow rate and sorbent injection rate influenced the levels of trace elements and PAHs in OSCAR by-product material. Capture of PAHs was observed to increase with Ca concentration in experiments using supersorbent injection. However, possible release of PAHs occurred with regenerated sorbent injection. The concentrations of trace elements in leachate for all OSCAR cyclone samples tested were below Resource Conservation and Recovery Act limits. The concentrations of most trace elements in OSCAR by-product were also below the limits regulated in the EPA 503 Rule except As and Se. The similarity in the physical and engineering properties of OSCAR cyclone samples to natural cohesive soils suggests that this material can be utilized in a variety of construction, reclamation, and agricultural applications.     

Trace elements in coal: Associations with coal and minerals and their behavior during coal utilization - A review

Trace elements such as mercury, arsenic and selenium present in coal are known to be of concern for public health. Coal-fired power plants have resulted in emission of several tons of TEs in environment. These elements mostly evaporate during combustion and condense either homogeneously as sub-micron ash or heterogeneously onto already existing fine ash. The coal-mineral and mineral-mineral associations play an important role in the formation of fine particles and in subsequent condensation of trace elements in various phases. Any retention of these elements in fly ash particles is strongly influenced by their association with other minerals in individual coal and mineral grains. Clean coal technology development is, therefore, a priority area for research and needs continuous improvements in increased efficiency and decreased pollutant emission. The paper will include trace elements in different coals from around the world. It will consider different modes of occurrences present in coals, the ash formation and evaporation of trace elements and emissions. The typical emissions from typical power stations will be presented. The paper will also review different approaches adopted in estimating the deportment of these elements. The paper at the end would discuss control strategies for reducing emissions and future directions.     

Trace element concentration reduction by beneficiation of Witbank Coalfield no. 4 Seam

South Africa remains the 5th largest producer and joint 4th largest exporter of coal in the world. It is also a major supplier of coal to the European Union. This is significant as the European Union has recently supported the environmental lobby that threatens the combined full scale use of coal in Europe and other first world countries. This promotes the development of clean coal technologies in order to counter the ever increasing number of environmental constraints threatening the export market. One critical development in clean coal technologies is coal beneficiation, which allows the reduction of mineral content. Permian coals from South Africa have characteristically high ash and inertinite contents and therefore require further beneficiation. The no. 4 Seam in the Witbank Coalfield is no exception, and it can be described as containing higher inertinite content and minerals compared to the no. 2 Seam in the same Coalfield. Beneficiation, therefore, is an important requirement for improving the quality of the coal, especially for export purposes.With the increase in environmental legislation and the drive towards “clean coal” a concern is raised in terms of the performance and marketability of export coal produced from the no. 4 Seam in the Witbank Coalfield. This seam is economically significant and remains an important source of export steam coal.Due to the nature and composition of the no. 4 Seam, coal beneficiation is essential to reduce the mineral content to be in line with export quality specification levels. This paper focuses on the association of the trace elements within the seam with the organic and inorganic components and possible methods of trace element reduction by removal using coal beneficiation techniques. The techniques investigated include reduction by dense medium beneficiation and flotation. The associated mineral reducibility investigations included mineralogical distribution and liberation analysis. By studying the relationship between coal mineralogy, petrography and trace element distribution, methods of optimum trace element reduction established. Furthermore, the distribution of mineralogical and organic components of sulphur in the Witbank Coalfield No. 4 seam was found to bear unique relationships with trace elements of special concern. By assessing the distribution and occurrence of trace element concentrations during washability and floatability tests, data was produced which could now be used in the promotion of saleable coal products from no. 4 seam in the Witbank Coalfield.     

A three-step metal fractionation scheme for fly ashes collected in an Argentine thermal power plant

A new three-step fractionation scheme was applied to study the distribution of Al, As, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, S, Sb, Ti, V and Zn in fly ashes collected in the electrostatic precipitator of a thermal power plant in the city of San Nicolás (Argentina). Seven samples were collected during one week of operation in 2005. For the fractionation, the scheme applied consisted of extracting the elements in three fractions: (i) soluble and exchangeable elements, (ii) carbonates, oxides and reducible elements and (iii) residual elements. Metals and metalloids at μg g⁻¹ level were determined in each fraction by plasma based techniques namely, inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry (ICP-MS). For validation, a certified reference materials NIST SRM 2711 (Montana soil) was subject to the same chemical sequential extraction procedure. X-ray diffraction powder (XRD) analysis and scanning electron microscopy (SEM) were used to characterize the major minerals present in the matrix. The predominant phases found in the total samples were mullite, quartz, iron oxides and lime. Total analyte concentration varied (in μg g⁻¹) from 1.54 for Cd to 30 600 for Al. The leachability of the 15 elements under study proved to be different. All the elements (except Cd and Pb) were detected in the soluble fraction in the order: Cu (0.10%) ∼ Mn (0.13%) < Ni (0.17%) ∼ Ti (0.19%) ∼ Fe (0.20%) ∼ As (0.21%) < Zn (0.86%) < Al (1.3%) < Cr (2.9%) < V (3.9%) < Sb (6.9%) < Mo (45.1%) < S (58.0%). Percentages higher than 20% of S (24.1%) < V (27.5%) < Mn (29.0%) were detected in the second fraction. Al, As, Cr, Cd, Cu, Fe, Ni, Pb, Sb and Zn were mostly associated to the residual fraction. Recoveries of the overall procedure varied between 106% (Mo) and 72% (Cr).     

Environmental impacts of coal combustion: A risk approach to assessment of emissions

This paper summarises some of the work performed in the Cooperative Research Centre for Coal in Sustainable Development (CCSD) on emissions from current power generation. A comprehensive approach was taken in the CCSD program to assessing environmental issues of concern for the power, and by implication the coal, industries. Here results of sampling on full scale operating plants are described, and detailed data on emission fluxes, particle size distributions, trace element concentrations as a function of particle size, and speciation of the trace elements are illustrated. The results show that particle capture in electrostatic precipitators (ESPs) is significantly less efficient than in fabric filters (FFs), particularly for submicron material, and that significant enrichment is observed in the finer particle sizes emitted from both ESPs and FFs. Results for the speciation of chromium, arsenic and selenium in coals, bottom ash and fly ash are also presented. The majority of chromium in fly ash is present in the less toxic Cr³⁺ form. Speciation of arsenic in feed coals is variable but the dominant form of As in fly ash is the less toxic As⁵⁺.