Dynamics of trace elements release in a coal pyrolysis process

Samples of coal and solid carbonization product obtained at four temperatures: 400, 600, 850 and 1000 °C were tested on account of the contents of trace elements. The following hazardous trace elements were considered: arsenic, beryl, cadmium, manganese, nickel, lead, mercury and selenium. The release curves for the elements tested were determined.     

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.     

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’.     

Volatility and chemistry of trace elements in a coal combustor

The volatility of 16 trace elements (TEs) (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, Te, Tl, V, Zn) during coal combustion has been studied depending on the combustion conditions (reducing or oxidizing) and type of coal (high- or low-ash coal), together with their affinities for several active gaseous atoms: Cl, F, H, O, and S.

The elements can be divided into three groups according to their tendencies to appear either in the flue gases or in the fly ashes from a coal combustor:

Group 1: Hg and Tl, which are volatile and emitted almost totally in the vapor phase.

Group 2: As, Cd, Cu, Pb and Zn, which are vaporized at intermediate temperature and are emitted mostly in fly ashes.

Group 3: Co, Cr, Mn and V, which are hardly vaporized and so are equally distributed between bottom ashes and fly ashes. In addition, Sb, Sn, Se and Te may be located between Groups 1 and 2, and Ni between 2 and 3.

At 400 and 1200 K, the 16 TEs behave differently in competitive reactions with Cl, F, H, O and S in a coal combustor.     

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.     

Speciation of major and selected trace elements in IGCC fly ash

The speciation of Ga, Ge, Ni, V, S and Fe in fly ash from IGCC power plant were investigated for possible further extraction process by combining conventional mineral and chemical analysis, leaching tests, wet sequential extraction, Mössbauer and XAFS spectroscopies. The results shown that Ge occurs mainly as water-soluble species, GeS and/or GeS₂ and hexagonal GeO₂. Ga is present as an oxide, Ni occurs mainly as nickeline (NiAs), with minor proportions of Ni arsenates and vanadium as V(III) with minor amounts of V(IV) in the aluminosilicate glass matrix. Pyrrhotite and wurtzite-sphalerite are sulfide species containing Fe and Zn, but an important fraction of iron is also present in the aluminosilicate glass. These clear differences between the speciation of the above elements in this material and those reported for fly ash from conventional PC combustion.     

Fundamentals of coal topping gasification: Characterization of pyrolysis topping in a fluidized bed reactor

Coal topping gasification refers to a process that extracts the volatiles contained in coal into gas and tar rich in chemical structures in advance of gasification. The technology can be implemented in a reactor system coupling a fluidized bed pyrolyzer and a transport bed gasifier in which coal is first pyrolyzed in the fluidized bed before being forwarded into the transport bed for gasification. The present article is devoted to investigating the pyrolysis of lignite and bituminite in a fluidized bed reactor. The results showed that the highest tar yield appeared at 823 to 923 K for both coals. When coal ash from CFB boiler was used as the bed material, obvious decreases in the yields of tar and pyrolysis gas were observed. Pyrolysis in a reaction atmosphere simulating the pyrolysis gas composition of coal resulted in a higher production of tar. Under the conditions of using CFB boiler ash as the bed material and the simulated pyrolysis gas as the reaction atmosphere, the tar yields for pyrolytic topping in a fluidized bed reactor was about 11.4 wt.% for bituminite and 6.5 wt.% for lignite in dry ash-free coal base.     

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.     

Suppressing secondary reactions of coal pyrolysis by reducing pressure and mounting internals in fixed-bed reactor

Pyrolysis of Shenmu coal was performed in fixed-bed reactors indirectly heated by reducing operating pressure and mounting internals in the reactor to explore their synergetic effects on coal pyrolysis. Mounting internals particularly designed greatly improved the heat transfer inside coal bed and raised the yield of tar production.Reducing pressure further facilitated the production of tar through its suppression of secondary reactions occurring in the reactor. The absolute increase in tar yield reached 3.33 wt% in comparison with the pyrolysis in the reactor without internals under atmospheric pressure. The obtained tar yield in the reactor with internals under reduced pressure was even higher than the yield of Gray–King assay. Through experiments in a laboratory fixed bed reactor, it was also clarified that the effect of reducing pressure is related to volatile release rate in pyrolysis. It did not obviously vary tar yield at pyrolysis temperatures below 600 °C, while the effect was evident at 650 and 700 °C but became limited again above 800 °C. Under reduced pressure the produced tar contained more aliphatics and phenols but less aromatics.     

Modeling changes of fractal pore structures in coal pyrolysis

Coal pyrolysis processes are numerically investigated in mathematically produced coal pore models which simulate real coal pores in the parameters of the porosity and fractal dimension. The simulations include FG-DVC chemical reaction model, gas molecular diffusion in pores, energy conservation model and coal swelling model. Numerical results are verified by experimental results qualitatively, and they revealed that both the porosity and volatile contents of the parent coal can affect the fractal dimension of the final char pores after pyrolysis linearly. A formula to predict the fractal dimension of char pores from its parent coal properties is obtained by curve fitting in numerous results.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

The physical character of coal char formed during rapid pyrolysis at high pressure

Rapid pyrolysis was conducted in a drop tube reactor using seven coals under various operating conditions. In addition to dense char, porous chars (network char and cenospheric char) were formed by the rapid pyrolysis under certain conditions. Porous char was mainly composed of film-like carbon and skeleton carbon. The pyrolyzed coal char particles were characterized in detail. Morphology and bulk density of porous char were quite different from the dense char formed under the same conditions, but elemental composition and BET surface area were similar to each other. CO₂ gasification reactivity of porous char was lower than dense char in the later gasification stage, and this was ascribed to the low reactivity of skeleton carbon.     

The reactions of formation of selected gas products during coal pyrolysis

Pyrolysis examinations conducted under non-isothermal conditions as well at low heating rate can show that the processes of hydrogen and methane formation are the result of several constituent reactions. In the presented paper a number of these reactions has been determined separately for each of the above mentioned gaseous products. The kinetic parameters of the reactions as well as the yields of products have also been calculated. It has been found that, during coal pyrolysis, methane is formed as a result of six constituent reactions and hydrogen is produced as a result of five constituent reactions. The values of activation energy and frequency factor for the reactions in question were determined. These values fall within the range, which is typical of chemical reactions.     

A comparison of basket willow and coal hydrogasification and pyrolysis

The presented work aimed at investigating the course of basket willow hydrogasification and comparing that process to the hydrogasification of bituminous coal. The examinations focussed on basket willow (Salix viminalis), bituminous coal of low degree of metamorphism and their blends at the mass ratio 1:1. Measurements of evolution kinetics of gaseous hydrocarbons during hydrogasification of those materials were conducted in the atmosphere of hydrogen under the pressure of 2.5 MPa. In the investigations, the non-isothermal method was employed. The examined samples were heated from ambient temperature up to 1200 K at the rate of 3 K/min.     

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.     

Gas yields from coal devolatilization in a bench-scale fluidized bed reactor

Devolatilization behavior of Australian bituminous coal-gasification was determined in a 0.1 m diameter fluidized bed at 650-900 °C. To predict gas yields from devolatilization, several correlations reported for gas yields were evaluated with the present experimental data. The correlation of Goyal and Rehmat [1993] was found to be good one. Also, a correlation for the product gas yields has been proposed from devolatilization of bituminous coals as a linear function of temperature with constants from the experiment. The experimental yields of product gas show good agreement with the values calculated by the proposed correlation. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Tar destruction and coke formation during rapid pyrolysis and gasification of biomass in a drop-tube furnace

This paper describes tar destruction and coke (or soot) formation of biomass in three different conversion processes: pyrolysis (in a pure nitrogen stream), steam gasification (in a mixture stream of steam and nitrogen), and partial oxidation (in a mixture stream of oxygen and nitrogen), over a wide temperature range from 600 to 1400 °C. A woody waste, hinoki cypress sawdust (HCS), was used as a feedstock, and an entrained drop-tube furnace (DTF) was applied to all experimental tests. It is found that raising the temperature remarkably decreases tar evolution. Steam and oxygen also have a positive effect on tar destruction. Benzene and toluene are the most difficult condensable tar species to destroy. The achievement of their complete destruction in the product gas requires extremely high temperatures above 1200 °C, regardless of the gasifying agents. The coke deposits from 900 °C and reaches a maximum formation at 1000 or 1100 °C. The results obtained in this study suggest that competition occurs between the secondary decomposition of hydrocarbon species and gasification reactions of the produced char and/or coke with gasifying agents in the temperature range of 900-1100 °C.     

Change of nitrogen functionality of N-enriched condensation products during pyrolysis

Changes in the nitrogen functionality of ¹⁵N-enriched condensation products prepared from glucose and ¹⁵N-glycine were investigated during pyrolysis at 600–1000 °C. The structural changes in the condensation products were studied by means of solid-state ¹³C and ¹⁵N NMR spectroscopies. During pyrolysis, the aliphatic moieties of the condensation products decomposed and evolved as gas and tar. At pyrolysis temperatures above 600 °C, almost all the carbon in the chars were converted to aromatic carbon. After pyrolysis, large amounts of nitrogen remained in the chars as char nitrogen (char-N), and about 30% of the nitrogen was eliminated from the chars as HCN and NH₃. With increasing temperature, the production of HCN and NH₃ increased and the amount of char-N decreased. By combining X-ray photoelectron spectroscopy and NMR results, detailed results for nitrogen fractions in chars were obtained. During pyrolysis, the fraction of unsubstituted pyrrole-N decreased and the fraction of quaternary-N increased. The fraction of pyridine-N remained almost constant at temperatures below 800 °C, but at 900 °C and above, the fraction of pyridine-N decreased. The fraction of substituted pyrrole-N showed minimum at 800 °C. On the basis of these results, structural changes of nitrogen functional groups during pyrolysis are discussed.     

Methods for detecting the mobility of trace elements during medium-temperature pyrolysis

The mobility (volatility) of trace elements in coal during pyrolysis has been studied for distances of up to 40 cm between the coal and the trace element collector, which was graphite or a baffled solvent trap. Nineteen elements not previously recorded as mobile were detected.