Effect of inorganic matter on reactivity and kinetics of coal pyrolysis

Two Chinese coals, Shenfu subbituminous coal and Huolingele lignite, were used to investigate the effect of mineral matter in coal on reactivity and kinetic characteristics of coal pyrolysis. The experiments were carried out by using thermogravimetry to check the pyrolysis behavior of raw coal, HCl/HF demineralized coal and demineralized coal with inorganic matter (CaO, K₂CO₃ and Al₂O₃) added, respectively. The results showed that inherent mineral in coal had no evident effect on the reactivity and kinetics of coal pyrolysis. CaO, K₂CO₃ and Al₂O₃ all had a catalytic effect on the reactivity of coal pyrolysis, their effects were closely related to temperature region and coal types. The pyrolysis process of all the samples studied can be described by three independent first order kinetic model. Addition of inorganic matter the activation energy decreased and the characteristic temperature of coal changed.     

Gasification behaviour of Australian coals at high temperature and pressure

This paper presents gasification conversion data generated for a suite of Australian coals reacting with oxygen/nitrogen mixtures at 2.0 MPa pressure and at temperatures up to 1773 K, as part of a wider investigation into the gasification behaviour of Australian coals. The effects of O:C ratio, residence time and coal type on conversion levels and product gas composition were investigated under conditions relevant to those present in entrained-flow gasification systems. At higher temperatures, coal conversion levels are, as expected, higher, whilst product gas compositions continue to reflect the relevant gas phase equilibrium conditions. These gas phase equilibrium concentrations show strong dependence on the amount of carbon in the gas phase (i.e. coal conversion). The increased conversion achieved at high temperatures allows the contribution of coal-specific properties such as char structure and reactivity to be investigated in more detail than previously possible. Furthermore, at higher conversion levels the effects of coal type on product gas composition are more apparent than at lower conversion levels. These high temperature, high pressure gasification conversion data have been reconciled with high pressure bench-scale pyrolysis and char reactivity measurements, highlighting the significance of coal-specific effects of key gasification parameters.     

Effect of demineralization and catalyst addition on N2 formation during coal pyrolysis and on char gasification

Six coals with different ranks and different ash contents have been used to study the effect of demineralization on N₂ formation during coal pyrolysis. Chars obtained after pyrolysis have been also gasified with carbon dioxide at 1000 °C to investigate the influence of the demineralization on char gasification reactivity. The pyrolysis results show that the demineralization by acid washing drastically changes N₂ formation profiles and decreases nitrogen conversion to N₂ for low rank coals; on the other hand, the demineralization has little effect on N₂ formation for high rank coals. Addition of 0.5 wt% Fe promotes N₂ formation from the demineralized coals, but the catalytic effect depends on the coal type. It is found that the Fe remarkably promotes N₂ formation from the demineralized low rank coals, but the effect is much smaller for high rank demineralized coals. These observations suggest that the existing state of Fe-containing minerals and added Fe catalyst is important for catalytic N₂ formation during coal pyrolysis. Gasification results show that the demineralization lowers char gasification reactivity not only for low rank coals but also for high rank coals.     

Linking laboratory data with pilot scale entrained flow coal gasification performance. Part 1: Laboratory characterisation

Pilot scale measurements play an important role in our understanding of the coal gasification process. To gain the most practical benefits from such testing it is important to have a good understanding of the fundamental processes that influence coal behaviour under industrial conditions. In this paper, a suite of Australian coals was characterised in detail at the laboratory scale and preliminary assessments made of their likely performance under practical entrained flow conditions. The same coals were then tested using a 5 MW_th pilot-scale entrained flow gasifier in Part 2. The resulting gasification dataset for a suite of coals at both laboratory and pilot scale provides a unique opportunity to quantify the links between laboratory gasification measurements and coal gasification behaviour under realistic conditions. This paper presents a characterisation of four Australian thermal coals in terms of their slag formation and flow behaviour, coal devolatilisation and reactivity properties, and their gasification conversion behaviour. This work provides the basis for a relative assessment of their potential for use in entrained flow gasification, and identifies possible performance issues which may need consideration for use in larger-scale gasification systems. The second paper discusses the pilot-scale gasification behaviour of these coals, and relates those data with those presented here.     

Distribution of volatile sulphur containing products during fixed bed pyrolysis and gasification of coals

Three sub-bituminous and two bituminous coals from Western Canada were used to study the evolution of H₂S, COS and SO₂ during the pyrolysis and gasification processes in a fixed bed reactor. For all types of coals, most of H₂S and SO₂ were released during the devolatilization stage. COS was formed only during the gasification stage in the presence of CO₂. The mineral matter of coal may have played a role during the gasification stage. Some observations made during this latter stage in CO₂ and/or steam were interpreted in terms of the equilibrium effects.     

Volatilization behavior of fluorine in coal during fluidized-bed pyrolysis and CO2-gasification

The volatilization behavior of fluorine in five Chinese coals was investigated during fluidized-bed pyrolysis and CO₂-gasification at a temperature range of 500-900 °C. The effect of co-existed and added calcium on fluorine volatility during pyrolysis was also determined. With increasing pyrolysis temperature, the volatility of fluorine increases. However, the volatility is greatly dependent on the fluorine chemical forms occurred in coal. Except for Datong and Zhungeer coal, more than 65% of fluorine in other three coals occurs as the steady forms. Fluorapatite is not the major carrier of fluorine in the coals studied. Fluorine volatility is retarded by coexisting calcium during coal pyrolysis, indicating that at least part of the stable forms of fluorine in coal might occur as calcium fluoride or calcium fluoride with complex compounds which are stable even at high pyrolysis temperature. The addition of CaO and limestone can suppress the release of fluorine during pyrolysis. The effect of CaO is better than that of limestone. The volatility of fluorine of coal during CO₂-gasification depends on not only the occurrence mode of fluorine, but also the gasification reactivity of the coal. Compared with N₂ atmosphere, CO₂ is more favorable to the release of fluorine from coal.     

Behaviour of coal mineral matter in sintering and slagging of ash during the gasification process

The mineral matter in typical feed coals used in South African gasification processes and the ash derived from gasifying such coals have been investigated using a variety of mineralogical, chemical and electron microscope techniques. The mineral matter in the feed coals consists mainly of kaolinite, with minor proportions of quartz, illite, dolomite, calcite and pyrite plus traces of rutile and phosphate minerals. The calcite and dolomite occur in veins within the vitrinite macerals, and are concentrated in the floats fraction after density separation. Some Ca and Ti also appear to be present as inorganic elements associated with the organic matter.Electron microscope studies show that the gasification ash is typically made up of partly altered fragments of non-coal rock, bonded together by a slag-like material containing anorthite and mullite crystals and iron oxide particles, with interstitial vesicular glass of calcic to iron-rich composition. Ash formation and characteristics thus appear to be controlled by reactions at the particle scale, allowing the different types of particles within the feed coal to interact with each other in a manner controlled mainly by the modes of mineral occurrence. Integration of such techniques provides an improved basis for evaluating ash-forming processes, based on quantitative phase identification, bulk and particle chemistry, and the geometric forms in which the different phases occur.     

Coal research in Newcastle—past, present and future

Coal research, particularly in the area of coal utilization, has flourished in the University of Newcastle for last several decades. There have been significant developments in the area of furnace modeling and heat transfer—modeling of radiative heat transfer in pulverized coal fired boilers and aerodynamic modeling of swirl burners, blast furnace raceways, coal combustion—kinetics of devolatilisation, combustion and gasification, mineral and ash reactions—thermal behaviour of different minerals, ash formation and their implications on ash deposition and thermal performance. There have been some investigations into in situ gasification, NO_x formation and cofiring with biomass as well. Coal characterization—for organic and inorganic matter and ash has been a strong activity in the past few years.This paper presents a comprehensive review of these activities summarizing the key achievements in each area. The paper also describes possible directions and drivers for future coal research in the current environment.     

Studies of inorganics added to low-rank coals for catalytic gasification

Inorganic complexes were added to low-rank coals by step-wise pH adjustment of a mixture of inorganic solution and brown coal while avoiding the formation of precipitates. The nature and amounts of the added inorganic depend on the pH of the coal/solution mixture and concentration of inorganic salts. The amount of hydroxide added for high loading of iron to coal is consistent with added multinuclear complexes. Computer generated models of brown coal with multi-iron species account for observed OH/Fe ratios. X-ray photoelectron spectroscopy (XPS) data for these samples are consistent with multi-iron species in coal. Computer molecular modelling of two brown coal models with added inorganics shows monodentate carboxyl coordination to metals is sterically favoured. Mononuclear Fe(III) with bidentate carboxyl coordination form distorted structures and are energetically unfavoured. Modelling indicates significant reductions in partial charges on metal centres, consistent with a redistribution of electron density on complexation. Low temperature pyrolysis of brown coals with added inorganics provides increased yields of gases, no detectable tar and lower char, compared with acid washed coals. Gasification of these coals using a nitrogen/oxygen mix at 150–200 °C yields CO, while steam gasification at 250–450 °C yields CO₂, CO and CH₄. Iron oxide/carbonate complexes are postulated during the pyrolysis and gasification.     

Mineral reaction and morphology change during gasification of coal in CO2 at elevated temperatures

Studies of the gasification of char in CO₂ at elevated temperatures are necessary for the development of IGCC technology. Experiments at high heating rates and elevated temperatures revealed that the temperature dependence of gasification reactivity was very different for low compared with high temperature ranges. To elucidate these mechanisms, the reaction of mineral matter and the change in morphology during gasification of a char at elevated temperatures were examined by char characterisation. CO₂ gasification experiments showed a large difference in gasification rate for chars prepared at higher temperatures compared to those prepared at lower temperatures. Changes in char particle morphology and mineral matter during gasification are also quite different. At higher carbonisation temperatures, mineral reactions during pyrolysis, which occurs in addition to ash fusion, appear to be one of the factors accounting for these differences. Certainly, a change of mechanism is involved. Graphite enrichment may also contribute to the decrease in char reactivity.     

Kinetics of mineral matter transformation during coal combustion

The transformation of individual minerals was investigated based on TG and DTG analysis at temperature up to 1700 K in inert and oxidizing atmospheres. The decomposition of minerals in inert atmosphere and the reaction with gaseous atmosphere was described by first order reactions for which the kinetic data were found. The evaluated kinetic parameters were then tested on a complex mineral matter of coals. It has been demonstrated on example of two different compositions that the mass loss during the transformation of coal mineral matter during combustion can be modelled as a mixture of individual minerals.     

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.     

The relationship between excluded mineral matter and the abrasion index of a coal

Predictions of the wear rates of components in grinding mills at pulverised coal-fired power stations are currently made using empirical relationships based on the ash content of the coals. However, modern coal characterisation techniques now allow the mineral inclusions in a coal that are responsible for the abrasive nature of the coal to be accurately characterised. Hence, there is scope to make improved predictions of wear based on a detailed knowledge of the mineral matter in a particular coal. It is first necessary, however, to understand the nature of the minerals and properties of the minerals in a coal that would contribute to abrasive wear. In this study known quantities of quartz, pyrite and slate have been added to a washed coal and the Abrasion Indices of the coal/mineral mixtures have been measured. The results show how the size, shape and hardness of excluded mineral matter contribute to the abrasive properties of a coal.     

Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion

NO-char reaction and char combustion in the presence and absence of mineral matter were studied in a quartz fixed bed reactor. Eight chars were prepared in a fluidized bed at 950 °C from four Chinese coals that were directly carbonized without pretreatment or were first deashed before carbonization. The decomposition of NO over these coal-derived chars was studied in Ar, CO/Ar and O₂/Ar atmospheres, respectively. The results show that NO is more easily reduced on chars from the raw coals than on their corresponding deashed coal chars. Mineral matter affects the enhancement both of CO and O₂ on the reduction of NO over coal chars. Alkali metal Na in mineral matter remarkably catalyzes NO-char reaction, while Fe promotes NO reduction with CO significantly. The effect of mineral matter on the emission of NO during char combustion was also investigated. The results show that the mineral constituents with catalytic activities for NO-char reaction result in the decrease of NO emission, whereas mineral constituents without catalytic activities lead to the increase of NO emission. Correlation between the effects of mineral matter on NO-char reaction and NO emission during char combustion was also discussed.     

Sulfur transfers from pyrolysis and gasification of direct liquefaction residue of Shenhua coal

The sulfur transformation during pyrolysis and gasification of Shenhua direct liquefaction residue was studied and the release of H₂S and COS during the process was examined. For comparison, the sulfur transfer of Shenhua coal during pyrolysis and that of pyrolyzed char during gasification were also studied. The residue was pyrolyzed at 10 °C /min to 950 °C. During pyrolysis about 33.6% of sulfur was removed from the residue, among which 32.1% was formed H₂S in gas and 1.5% was transferred into tar, 66.4% of the sulfur was remained in residue char. Compared with coal, the residue has generated more H₂S due to presence of Fe_1−xS which was enriched in residue during liquefaction process. There is a few COS produced at 400-500 °C during pyrolysis of coal, but it was not detected form pyrolysis of the residue. During CO₂ gasification, compared with pyrolysis and steam gasification, there are more COS and less H₂S formation, because CO could react with sulfide to form COS. During steam gasification only H₂S was produced and no COS detected, because H₂ has stronger reducibility to form H₂S than CO. After steam gasification no sulfur was detected in the gasification residue. The XRD patterns show after steam gasification, only Fe₃O₄ is remained in the gasification residue. This indicates that the catalyst added during the liquefaction of coal completely reacted with steam, resulting in the formation of H₂ and Fe₃O₄.     

Fate of the chlorine and fluorine in a sub-bituminous coal during pyrolysis and gasification

The fate of the chlorine and fluorine present in a sub-bituminous coal from Indonesia during pyrolysis and gasification has been studied with fixed and entrained bed reactors. The rate profile for HCl evolved in the temperature programmed pyrolysis exhibits the main and shoulder peaks at 480 and 600 °C, respectively. Model experiments and subsequent Cl 2p XPS measurements show that HCl reacts with metal impurities and carbon active sites at 500 °C to be retained as inorganic and organic chlorine forms, from which HCl evolves again at elevated temperatures. It is suggested that the HCl observed in the coal pyrolysis may originate from the above-mentioned chlorine functionalities formed by secondary reactions involving the nascent char. In the CO₂ gasification of the 900 °C char at 1000 °C and 2.5 MPa, any measurable amounts of HCl and HF could not be detected even at a high conversion of 75 wt% (daf), suggesting the accumulation of these halogens in the residual char. When the coal is injected into an O₂-blown, entrained bed gasifier at 1200-1400 °C under 2.6 MPa, the partial oxidation proceeds to a larger extent at a higher O₂/coal ratio, whereas the chlorine and fluorine are enriched in the remaining char, and the extent of the enrichment at the latter stage of gasification is larger with the fluorine. The XPS measurements of the chars reveal the presence of the broad F 1 s peak, which can cover a wide range of binding energies attributable to inorganic and organic fluorine. The halogen enrichment during gasification is discussed in terms of secondary reactions of HCl and HF with char.     

Linking laboratory data with pilot scale entrained flow coal gasification performance. Part 2: Pilot scale testing

By exploring the links between laboratory-scale gasification data (e.g. slag viscosity measurements, char reactivity considerations) and the performance of the same coals under realistic pilot-scale conditions, we are provided with a dataset that allow us to refine coal test procedures for entrained flow gasification, understand the entrained flow gasification process in significantly more detail than previously, and gain some new insight into how coal reactivity and slag viscosity properties interact to define operating envelopes for specific gasification technologies. The first paper in this two-part series presented a detailed characterisation of a suite of Australian coals using laboratory-scale gasification facilities. This paper presents gasification data obtained from pilot-scale testing of these same coals, and explores the links between laboratory data, coal assessments made using these data, and the performance of the coals under realistic conditions. The results demonstrate a high degree of consistency between laboratory indications of coal reactivity and coal gasification behaviour at pilot scale. This work also demonstrates the impact of interactions between coal conversion properties and slag formation and flow behaviour, and how these interactions dictate operational parameters for the gasifier.     

Alkylation of coal and peat with alcohols

Modification of coals at low metamorphic stages and peat by alkylation with alcohols in the presence of inorganic acids has been studied. Changes in group and individual compositions of bitumenoid fractions taking place during the alkylation of coal and peat have been examined using IR, NMR, and gas chromatography-mass spectrometry techniques. It has been revealed that the esterification and transesterification reactions prevail in the conversion of the components of the fractions. The alkylation has been shown to have a favorable effect on the yield of the bitumenoid fractions of brown coals and peat.     

Mineral matter-organic matter association characterisation by QEMSCAN and applications in coal utilisation

The association of mineral matter with organic matter is extremely important for coal utilization process such as pf coal combustion. With the development of advanced analytical instruments such as QEMSCAN, it is now possible to measure directly the mineral matter-organic matter association on a particle-by-particle basis. The mineral matter and mineral-organic associations of a suite of fourteen CCSD coal bank coals (as pf) have been determined by QEMSCAN. An interface program was developed to make QEMSCAN data compatible with the CCSEM-based ash formation model developed previously in CCSD. Size and chemistry of flyash was predicted by a partial coalescence sub-model for included mineral grains, and a fragmentation sub-model for excluded mineral grains, respectively. The size and chemistry of predicted flyash was estimated on a particle-by-particle basis, and was used to rank the ash effect on heat transfer reduction for all the CCSD coals using the CCSEM-based model, in which coal property, furnace geometry and operational conditions have been taken into account. Other applications and further developments of the technique are also outlined.