Thermodynamic equilibrium study of trace element mobilisation under air blown gasification conditions

Thermodynamic equilibria have been examined for several trace elements at levels found in coal that might be used in the air blown gasifier air (ABG). The results obtained with the factwin suite of programs were used to test the degree to which predictions are consistent with observed partitioning. Conditions of lime-free gasification, gasification with the design limestone addition and limestone addition at a higher fuel-lime ratio have been examined for 20 atm. pressure over a range of temperatures. The effects of varied sulphur and chlorine levels have also been examined. Curtailment of the ABG program means that the range of experimental data does not allow deep comparison of predictions with observations but in general there is broad agreement. It is concluded that the results form the basis of a method of representing the potential for mobilisation (low to standard limestone) from coal particles followed by reincorporation into condensed phases (standard to high limestone) during the in-bed processes. Simulation of cooling of the equilibrium gas phase at 1273 K in isolation by 10 K steps predicts incorporation of a range of elements into a matte, and others into an alkali-based chloride melt.     

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.     

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.     

Devolatilisation behaviour of petroleum coke under pulverised fuel combustion conditions

The combustion of petroleum coke in large scale facilities has been limited due to its high sulphur content, but the increasing installation of flue-gas desulphurisation units makes possible the firing of petroleum coke either as a primary fuel or blended with coals. This study focuses on the behaviour of three fuel-grade petroleum cokes of different provenance under pulverised fuel combustion conditions. These cokes, ground and sieved 125–20 μm were fed to a drop tube reactor operating at 1300 °C under different atmospheres to produce chars with different combustion degrees. Char reactivity assessment was performed isothermally in a thermobalance at 550 °C and morphology and optical texture of the chars were studied by optical and scanning electron microscopy. Petroleum coke chars are composed of two main types of particles: (i) porous anisotropic particles that passed through a plastic stage and generated either cenospheric or network-like chars and (ii) angular particles with fine-mosaic optical texture that did not swell and show abundant contraction cleats. The relative proportions of both types of particles were very different in the three petroleum coke chars indicating significant differences in their devolatilisation patterns. The morphology and optical texture of the petroleum coke chars were related to their reactivity (as measured in a thermobalance) and to the characteristics (chemical composition and optical texture) of the parent petroleum cokes, in an attempt to understand the implications of their different devolatilisation behaviours on the combustion efficiency.     

Thermodynamic equilibrium study of trace element transformation during underground coal gasification

In order to provide theoretical basis for gas cleaning and pollution control thermodynamic equilibrium calculations were performed to predict the partitioning of trace element species under special conditions for underground coal gasification, including both oxygen-steam gasification and air blown gasification under elevated pressures. The trace elements studied include As, Se, Pb, Ni, Cd, Cr, Sb. The results indicate, in the condition of large-section UCG process with oxygen-steam injection, all the elements studied present in the gas phase during gasification stage. Ni and Cr are hardly volatile and tend to condense below 1000 °C. Most of them will be enriched in bottom ash. As, Pb, Cd, Sb totally or partially occur in gas phase in underground gas cleaning system. In cold cleaning system, they exist in condensed phases and tend to be enriched in fly ash, which is beneficial to trace element removal. Se presents in gas phase in the form of H₂Se(g) even in ground cold gas cleaning system. The presence of potassium makes arsenic less volatile due to the formation of K₃AsO₄ and selenium is not affected. Also the amount of gaseous antimony chloride is reduced because of prior formation of alkali metal chloride. Pressure shows a remarkable effect on equilibrium partitioning of As, Se, Sb. With rising pressure, increasing quantities of hydrides of these trace elements are generated due to the enhancement of the reducing atmosphere. At the same time, the condensation points of all the trace elements sharply increase with pressure. It is found that for underground air blown gasification, the gaseous species of trace element sulfide can be easily formed, and the trace elements have lower condensation points than those for oxygen-steam gasification.     

Evidence of PAH production under lean combustion conditions

While it is common knowledge that PAH formation can be expected at rich combustion conditions, this paper presents experimental and computational evidence that PAH formation can also be expected under lean, or cool, combustion conditions. Experimental evidence for diesel engine operation at lean conditions, as well as the literature references related to waste tire and coal combustion, are cited as supportive evidence of conclusions drawn here. A computational model based on reaction mechanisms for three surrogate fuels: C₂H₂, C₇H₁₆ and C₁₄H₂₈, was validated against the available diesel engine data, and projected to both lean and rich combustion conditions. The finding was that for the adiabatic case, minimum production of PAHs occurs at equivalence of unity and increases as equivalence becomes either lean or rich; the latter observation being consistent with the common knowledge.     

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.     

Biodiesel combustion in an optical HSDI diesel engine under low load premixed combustion conditions

An optically accessible single-cylinder high-speed direct-injection (HSDI) diesel engine was used to investigate the spray and combustion processes for biodiesel blends under different injection strategies. The experimental results indicated that the heat release rate was dominated by a premixed combustion pattern and the heat release rate peak became smaller with injection timing retardation. The ignition and heat release rate peak occurred later with increasing biodiesel content. Fuel impingement on the wall was observed for all test conditions. The liquid penetration became longer and the fuel impingement was stronger with the increase of biodiesel content. Early and late injection timings result in lower flame luminosity due to improved mixing with longer ignition delay. For all the injection timings, lower soot luminosity was seen for biodiesel blends than pure diesel fuel. Furthermore, NO_x emissions were dramatically reduced for premixed combustion mode with retarded post-TDC injection strategies.     

Partitioning of trace elements during the combustion of coal and biomass in a suspension-firing reactor

A novel, quartz ‘suspension-firing’ reactor is described for monitoring trace element release during solid fuel combustion under conditions relevant to fluidised bed combustors. The new design allows the examination of fuel particle combustion in the absence of bed solids. Experiments have been conducted using two coals, a sample of wood bark and one of straw. Ash from the reactor walls and base have been analysed separately from ash collected on a sintered disc in the path of exit gas. Trace element concentrations in these samples were analysed by Inductively Coupled Plasma (ICP)-mass spectrometry and ICP-atomic emission spectrometry (AES). The fractions of original trace elements retained by the ash have been reported; relative enrichment in the ‘sinter-ash’ was calculated by comparing with ‘bottom ash’. Mercury was almost completely volatilised from all fuels, as was selenium for all except wood-bark. Chromium, manganese and thallium were partially volatilised and nickel mostly retained in all samples. The behaviour of beryllium, lead, molybdenum, vanadium and zinc varied, depending on the fuel sample. Beryllium was released to a greater extent from coal/straw than the other fuels. Vanadium was partially volatilised from wood-bark and coal/straw, while the largest proportion of the zinc released was from the wood-bark. Lead and molybdenum were retained to a greater extent by ‘Colombian coal’ and wood-bark, respectively. Evidence of the enrichment of certain trace elements on the finer ‘sinter-ash’ particles has also been observed, e.g. for As, Cd, Pb and Tl during the combustion of the ‘Colombian-coal’.     

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.     

中微量元素肥料的生产与应用研究进展

阐述微量元素肥料在农业生产中的重要作用,分析我国中微量元素生产和应用的现状及发展前景.概述中微量元素生产的原料来源及工艺技术.对发展我国中微量元素肥料的生产和应用提出了建议.     

Numerical and experimental study of water/oil emulsified fuel combustion in a diesel engine

Numerical and experimental studies were made on some of the chemical and physical properties of water/oil emulsified fuel (W/OEF) combustion characteristics. Numerical investigations of W/OEF combustion's chemical kinetic aspects have been performed by simulation of water/n-heptane mixture combustion, assuming a model of a homogenous reactor's concentric shells. The injection and fuel spray characteristics are analyzed numerically also in order to study indirectly the physical effects of water present in diesel fuel during the combustion process. The experimental results of W/OEF combustion in the DI diesel engine are also presented and discussed. The results of engine testing in a broad field of engine loads and speeds have shown a significant pollutant emission reduction with no worsening of specific fuel consumption.     

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.     

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.     

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.     

Effect of coal liquefaction conditions on the trace element content of the soluble non-volatile product

Analysis of 18 trace elements via inductively coupled plasma atomic emission spectrometry has been performed on ‘in-house’ coal-derived non-volatile products. Analyses were conducted in a pyridine matrix to determine the effect of various conversion parameters on metal content. Four subbituminous coals (Wyodak 1–4) and one bituminous coal (Indiana V) were employed in conjunction with both non-basic (tetralin) and basic (1,2,3,4-tetrahydroquinoline) model process solvents. Trace metal data on solvent-refined coals as a function of feed coal, process solvent, reaction time, reaction temperature and extraction solvent are reported. Few trends in metal concentration are apparent on changing various liquefaction parameters. Metal concentrations are, however, approximately one to two orders of magnitude higher in pyridine soluble SRC relative to toluene soluble SRC. The majority of soluble metals, it is therefore suggested, are in the form of coordination complexes rather than true organometallics in SRC. Information regarding the effective molecular size of metal-containing species has been obtained via size exclusion chromatography with specific metal detection. Subtle changes are observed in the effective molecular size of metal-containing materials using different liquefaction parameters. For example, a greater fraction of each metal appears to be bonded to larger ‘sized’ molecules in pyridine soluble fractions than in toluene soluble fractions.     

Estimation of trace element release and accumulation in the sand bed during bubbling fluidised bed co-combustion of biomass, peat, and refuse-derived fuels

The experimental study and thermodynamic equilibrium calculations were performed to investigate the interactions of fuel-based lead, copper, zinc, and manganese with the bed material of the bubbling fluidised bed boiler (BFB) during the co-combustion of sawdust, peat, and refuse-derived fuel. Flue gas trace element measurements, a chemical analysis of process streams, and mass balance calculations over the boiler reveal that the bed material captures substantial amounts of Pb, Cu, Zn and Mn, but also that these metals are released from the bed when the fuel characteristics or the process parameters are changed. The study shows that the trace metal emissions of a BFB boiler are not necessarily directly related to metal concentrations of the fuel but are rather a result of a complex process combining the release of trace metals from the fuel, the interaction between the fuel and fuel ash particles within the bed material, and the release of trace metals from the bed. The EDS analysis of the bed material particles shows that the original sand particles are covered with a Ca- and K- rich layer. In combustion temperatures, the layer is assumably in a melt form and has an important role in the trace metal accumulation/release process. The thermodynamic multi-phase multi-component equilibrium calculation for zinc suggests that the release of zinc from the bed material is strongly increased with an increasing Cl-concentration of the fuel, due to the conversion of the bed-bound zinc to ZnCl₂.     

Mineral and trace element composition of the Lokpanta oil shales in the Lower Benue Trough, Nigeria

The concentrations of minerals and trace elements in the Lokpanta oil shale from the Lower Benue Trough, Nigeria have been determined by X-ray diffraction (XRD) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), respectively. X-ray diffraction data were evaluated using the SIROQUANT™ interactive data processing system based on Rietveld interpretation methods. A new method of trace element determination in oil shale, involving LA-ICP-MS analysis of glass beads prepared by fusing oil shale ash on an iridium strip heater was used, and the accuracy of the method was assessed by including a standard shale reference material (SGR-1b) in the analysis program.The minerals in the raw oil shales are mainly quartz, calcite and clay minerals, with the latter being represented by kaolinite and interstratified illite/smectite. Ashes of the oil shale samples prepared at 815 °C have quartz and (in some cases) illite as the dominant mineral phases, along with a significant proportion of amorphous materials. The Lokpanta oil shales are highly enriched in some potentially hazardous trace elements, including V, Cr and Ni, when compared with oil shales from other deposits around the world. The results obtained for the trace elements in the reference material show that the LA-ICP-MS method described in this study is very accurate and precise for the determination of a wide range of trace elements in oil shales.     

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.