Py-MS Study of Sulfur Behavior during Pyrolysis of High-sulfur Coals under Different Atmospheres

In this study sulfur pyrolysis behavior of two Chinese high sulfur coals and their treated coal samples was investigated by Py-MS at a heating rate of 5 °C/min from room temperature to 1025 °C under hydrogen, helium and 2% O₂-He. It is found that the internal and external hydrogen do not show hydrogenation ability at temperature below 400 °C, due to no H₂S formation at this temperature region for all the coal samples. At temperature higher than 400 °C, not only the indigenous hydrogen but also indigenous oxygen can react with sulfur-containing radicals to form H₂S or SO₂. The evolution of H₂S and SO₂ displays the same profiles in pyrolysis of ZY pyrite-free coal under He, further revealing that after the breakage of C-S bond in the organic sulfur structure in coal to form sulfur-containing radicals, which can equally react with indigenous hydrogen and oxygen. The similar tendency between evolution of CO₂ and SO₂ and the same ending temperature also shows that not only C-S but also C-C bond can be broken in pyrolysis of ZY coals under 2% O₂-He atmosphere. However, unlike SO₂ evolution, CO₂ emission increases in the temperature ranging from 500 °C to 800 °C in LZ raw and deashed coals, implying the breakage of C-C bond at high temperature, which might be related to their low coal rank and high pyrite content.     

Elucidation of hydrogen transfer behavior of coal with tritiated gaseous hydrogen in the absence and the presence of a catalyst using a fixed-bed reactor

Hydrogen transfer behaviors of four Argonne coals with gas phase tritiated H₂ in thermal (non-catalytic) and catalytic reactions (in the presence of 3 wt% Pt/Al₂O₃) in temperature range from 250 to 400 °C have been investigated using a fixed-bed reactor. The result showed that the efficiency of hydrogen transfer reaction of coal with the gas phase in the thermal run increased with increasing oxygen functionalities, providing the indication that some oxygen functional groups have a role for effective promotion of hydrogen transfer reaction with the gas phase H₂ under mild condition. When the reaction was run in the presence of the catalyst, the efficiencies of hydrogen transfer reaction for the all coals, except for the POC coal, were significantly enhanced even at temperature as low as 250 °C. The results for the catalytic reaction have provided indications into the reactive sites (formation of free-radicals) in coal and patterns for coal matrix degradation reactions.     

NH3 and HCN formation during the gasification of three rank-ordered coals in steam and oxygen

A Victorian brown coal (68.5% C), a Chinese high-volatile Shenmu bituminous coal (82.3% C) and a Chinese low-volatile Dongshan bituminous coal (90% C) were gasified in a fluidised-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H₂O, 2000 ppm O₂ or 15% H₂O + 2000 ppm O₂. While the gasification of these coals in 2000 ppm O₂ converted less than 27% of coal-N into NH₃, the introduction of steam played a vital role in converting a large proportion of coal-N into NH₃ by providing H on char surface. The importance of the roles of steam in the formation of NH₃ in atmospheres containing 15% H₂O + 2000 ppm O₂ decreased with increasing coal rank. This is largely due to the slow gasification of high-rank coal chars, resulting in low availability of H on char surface. The gasification of chars from the high-rank coal appears to produce higher yields of HCN than that of lower rank coals, probably as a result of the decomposition of partially hydrogenated/broken/activated char-N structures during gasification at high temperature. The alkali and alkaline earth metallic species in brown coal tend to favour the release of coal-N as tar-N but have limited effects on char-N conversion during gasification.     

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.     

Adsorption behaviors of mercury from aqueous solution using sulfur-impregnated adsorbent developed from coal

Adsorption of mercury from aqueous solution on sulfur-impregnated adsorbent has been studied. Raw coal was mixed with K₂S powder, and then heated at 800-1000 °C for 30 min in nitrogen to produce sulfur-impregnated adsorbent. The sulfur content and specific surface area of the adsorbent were determined, and the ability of the adsorbent to adsorb mercury in aqueous solution was examined. With increasing temperature of sulfur-impregnation, specific surface area of the adsorbent increases, while sulfur content of the adsorbent is almost constant. The adsorbent obtained at 900 °C shows the highest and fastest adsorption of mercury from aqueous solution at 25 °C, and the elution extents of adsorbed mercury are negligible in distilled water and 10% in 0.1 M HCl solution, respectively. Adsorption kinetics was tested for pseudo-first order and pseudo-second order reactions, and the rate constants of adsorption for these kinetic models were calculated. Adsorption experiments demonstrate that the adsorption process corresponds to pseudo-second-order kinetic model than pseudo-first-order model. With increasing temperature of aqueous solution, the kinetics of adsorption becomes faster and the amount of mercury adsorbed on the adsorbent increases. The thermodynamic values, ΔG⁰, ΔH⁰ and ΔS⁰, indicated that adsorption was an endothermic and spontaneous process.     

Removal of sulfur at high temperatures using iron-based sorbents supported on fine coal ash

This paper deals with the simultaneous removal of H₂S and COS in the temperature range of 400-650 °C at 1 bar by using iron-based sorbents. The iron-based sorbents were prepared using iron oxide and cerium oxide with coal fine ash as the support. Simulated coal gas was used in the sulfidation experiments and 5% O₂ in N₂ gas was used for regeneration of sorbents. Both sulfidation and regeneration experiments have been carried out using a fixed-bed quartz reactor. The product gases were analyzed using a GC equipped with a TCD and a FPD. The results demonstrated that both H₂S and COS can be effectively reduced using the iron-based sorbents supported on fine coal ash. XRD analysis shows that Fe_1−xS phase has formed during sulfidation indicating a high sulfur capacity of the sorbent. The mechanism of the removal of COS simultaneously with H₂S is also discussed.     

Catalytic oxidative desulfurization of diesel utilizing hydrogen peroxide and functionalized-activated carbon in a biphasic diesel-acetonitrile system

This paper presents the development of granular functionalized-activated carbon as catalysts in the catalytic oxidative desulfurization (Cat-ODS) of commercial Malaysian diesel using hydrogen peroxide as oxidant. Granular functionalized-activated carbon was prepared from oil palm shell using phosphoric acid activation method and carbonized at 500 °C and 700 °C for 1 h. The activated carbons were characterized using various analytical techniques to study the chemistry underlying the preparation and calcination treatment. Nitrogen adsorption/desorption isotherms exhibited the characteristic of microporous structure with some contribution of mesopore property. The Fourier Transform Infrared Spectroscopy results showed that higher activation temperature leads to fewer surface functional groups due to thermal decomposition. Micrograph from Field Emission Scanning Electron Microscope showed that activation at 700 °C creates orderly and well developed pores. Furthermore, X-ray Diffraction patterns revealed that pyrolysis has converted crystalline cellulose structure of oil palm shell to amorphous carbon structure. The influence of the reaction temperature, the oxidation duration, the solvent, and the oxidant/sulfur molar ratio were examined. The rates of the catalytic oxidative desulfurization reaction were found to increase with the temperature, and H₂O₂/S molar ratio. Under the best operating condition for the catalytic oxidative desulfurization: temperature 50 °C, atmospheric pressure, 0.5 g activated carbon, 3 mol ratio of hydrogen peroxide to sulfur, 2 mol ratio of acetic acid to sulfur, 3 oxidation cycles with 1 h for each cycle using acetonitrile as extraction solvent, the sulfur content in diesel was reduced from 2189 ppm to 190 ppm with 91.3% of total sulfur removed.     

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.     

Fischer-Tropsch catalyst testing in a continuous bench-scale coal gasification system

A bench-scale oxygen-blown fluid-bed gasifier was coupled to a modular fixed-bed Fischer-Tropsch (FT) reactor system for testing an FT catalyst under syngas. Various blends of subbituminous coal, torrefied biomass, and untreated biomass were gasified at 22 bar absolute, 800°-860 °C, and 4 kg/h. Syngas exiting the fluid bed passed through a cyclone, candle filter, and sulfur sorbent to reduce fine particulate and H₂S to levels well below 1 ppmv. The syngas was cooled to condense out moisture and volatiles and then reheated to temperatures required for FT synthesis. The clean syngas then flowed into the FT reactor with a 5:1 ratio of recycled FT product gas-to-fresh syngas feed. A 70% overall conversion of CO and H₂ was achieved at 269 °C and 18.9 bar over an iron-based catalyst supported on gamma-alumina pellets.     

Sulfurization of carbon surface for vapor phase mercury removal - I: Effect of temperature and sulfurization protocol

The uptake of hydrogen sulfide by carbon materials (ACFs and BPL) under dry and anoxic conditions was tested using a fixed bed reactor system to determine the effects of sorbent properties, temperature (200-800 °C) and sulfurization protocols on the sulfur content, sulfur stability, sulfur distribution, and to elucidate possible reaction mechanisms for the formation of sulfur species. Sorbents with higher surface areas showed higher uptake capacity, indicating that active sites for sulfur bonding are formed during the formation of the pore structure. The sulfur content and stability generally increased with the increase in temperature due to a shift in the reaction mechanism. The sulfurization process is associated with the decomposition of surface functionalities, which creates active sites for sulfur bonding. The presence of H₂S during the cooling process increased the sulfur content by increasing the presence of less stable sulfur forms. Sulfurized sorbents produced at high temperatures have pore structure similar to that of the virgin carbons.     

Thermodynamic investigation into carbon deposition and sulfur evolution in a Ca-based chemical-looping combustion system

Chemical-looping combustion is a promising technology that concentrates CO₂ and separates it during combustion. In this study, both the carbon deposition and sulfur evolution in the reduction of a calcium sulfate (CaSO₄) oxygen carrier with a typical syngas were investigated using thermodynamic simulations. The effects of reaction temperature, operating pressure and the oxygen ratio number (defined in this paper) on the amount of deposited carbon and released sulfurous gases are discussed. A reaction temperature from 750 to 950 °C, an operating pressure from 1 to 15 bars and an oxygen ratio number between 0.4 and 0.8 were determined to be the most favorable operating conditions. In addition, the amounts of released sulfurous gases were found to be largely dependent on the partial pressures of H₂ and CO based on the thermo-gravimetric analyzer (TGA) tests. When the partial pressure of H₂ or CO was above 40 kPa, the release of sulfurous gases could be prevented in the reaction between CaSO₄ and syngas, even if the reaction temperature was as high as 1000 °C. The XRD profiles of the products also demonstrated that the mole fraction of CaS in the products increased gradually with an increasing partial pressure of H₂ or CO, until the products were almost pure CaS.     

Desulfurization matching with coal poly-generation system based on dual gas resources

The coal poly-generation system for the production of alcohol and ether fuels as well as power is one of advanced coal utilization techniques. The team leaded by Professor Xie Kechang is carrying out the research on the poly-generation system to produce the syngas from the combination of gasified and pyrolyzed coal gas (dual gas resources) for the alcohol ether synthesis. Gas desulfurization is one of the key technologies for this system. The desulfurization matching with dual gas resources based poly-generation system for the production of alcohol and ether fuels as well as power is presented according to gas components, sulfur content, sulfur species and desulfurization accuracy in this technology. This matching desulfurization is classified into hot gas desulfurization, normal gas desulfurization, warm gas desulfurization and organic sulfur catalytic conversion. The preparation of H₂S removal sorbents, organic sulfur hydrolysis catalyst and the evaluation of their activities involved in the system were investigated. The H₂S removal efficiencies of the crude and fine desulfurization sorbents prepared for hot gas desulfurization are 90% and 99% at 500 °C in simulating coal gas, and their sulfur capacities are 21.85 wt.% and 24.91 wt.%, respectively. The organic sulfur catalyst shows the high hydrolysis activity, and the hydrolysis conversion of COS is more than that of CS₂ on the same catalyst. The research will provide necessary information for the matching desulfurization technology in the demonstration project on dual gas resources coal poly-generation system.     

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.     

Vaporization behavior of boron from standard coals in the early stage of combustion

Boron-containing compounds have been listed as one of environmentally hazardous substances in Japan since 2001, and known to condense in coal fly ash particles during coal combustion and coal fly ash formation in coal-fired electric power stations. So far, the authors have revealed that the speciation of boron-containing compounds in coal fly ash particles is mostly a calcium orthoborate or pyroborate. In this research, the speciation of boron compounds in standard coals and their char generated by laboratory-scale combustion test has been investigated by using a microwave-assisted acid digestion method and a Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) in order to reveal the vaporization behavior of boron in standard coals during combustion at relatively low temperature. Three isolated peaks are observed in ¹¹B MAS-NMR spectra of standard coals, and all of them are attributed to four-oxygen-coordinated boron atom. Around 50% of boron vaporizes even though heating condition is 200 °C and O₂ = 25%, and the percentage of vaporization reaches higher value than 80% at 400 °C and O₂ = 25%. The remaining boron contents in ash components are relatively small, and it suggests that most of boron in standard coals exist with relatively volatile carbon contents, and they volatilize in the very early stage of coal combustion.     

Sulfur removal in coal tar pitch by oxidation with hydrogen peroxide catalyzed by trichloroacetic acid and ultrasonic waves

A procedure for the desulfurization of coal tar pitch (CTP) by oxidation with hydrogen peroxide (H₂O₂) was developed, in which trichloroacetic acid (TCA) was used as a catalyst combined with ultrasonic waves. For comparison, the effects of H₂O₂ combined with different catalysts on sulfur removal were also investigated. The oxidative system composed of H₂O₂ and TCA is highly effective for sulfur removal from CTP. The reaction conditions such as type of solvent used, temperature, and CTP-to-TCA ratio considerably influence sulfur removal when the same oxidant is used. The desulfurization efficiency for CTP with 0.9 wt.% sulfur content reaches 91.1 wt.% at a xylene-to-CTP volume ratio of 2.5, a CTP-to-TCA mass ratio of 0.5, an ultrasonic treatment duration of 60 min, a reaction temperature of 70 °C, and with an extraction liquid containing methanol and sodium hydroxyl. The experiment confirms that the addition of surface active agent has no beneficial effect on sulfur removal.     

Characteristics of the removal of mercury vapor in coal derived fuel gas over iron oxide sorbents

The characteristics of a novel method for Hg removal using H₂S and sorbents containing iron oxide were studied. Previously, we have suggested that this method is based on the reaction of Hg and H₂S over the sorbents to form HgS. However, the reaction mechanism is not well understood. In this work, the characteristics of the Hg removal were studied to clarify the reaction mechanism. In laboratory made sorbents containing iron oxide were used as the sorbent to remove mercury vapor from simulated coal derived fuel gases having a composition of Hg (4.8 ppb), H₂S (400 ppm), CO (30%), H₂ (20%), H₂O (8%), and N₂ (balance gas). The following results were obtained: (1) The presence of H₂S was indispensable for the removal of Hg from coal derived fuel gas; (2) Hg was removed effectively by the sorbents containing iron oxide in the temperature range of 60-100 °C; (3) The presence of H₂O suppressed the Hg removal activity; (4) The presence of oxygen may play very important role in the Hg removal and; (5) Formation of elemental sulfur was observed upon heating of the used sample.     

Sulfur distribution in coke and sulfur removal during pyrolysis

In order to investigate the mechanisms of coke desulfurization by blowing additional gas into coking chamber during pyrolysis process, some experiments were conducted to study the effects of some factors on the distribution of sulfur in coke. These factors include pyrolysis temperatures, the kinds of the blowing gases and the diameters of coking chamber. It was found that sulfur was mainly released at the range of 300–600 °C. Under this temperature range, the sulfur content in coke can be reduced by 0.05–0.06% by blowing N₂, CO or CH₄, and by 0.14% by blowing H₂ at a space velocity of 1.2 mm/s compared with the absolute sulfur content of 0.92% in the case without gas feeding. Obviously, H₂ is more effective on sulfur removal than N₂, CO or CH₄. The total, organic and inorganic sulfur contents in coke increase with increasing the diameter of coking chamber under identical pyrolysis conditions. Both organic and inorganic sulfur contents in coke increase regularly from the center to brim at identical height of a coke column for all the cases. In addition, the organic and inorganic sulfur contents at the cranny surface are higher than those in interior at the same sampling position. X-ray photoelectron spectroscopy (XPS) analyses suggest that the main contributions to the increase of inorganic and organic sulfur contents are due to the formation of negative bivalent sulfur and thiophenic compounds, respectively.     

Co-gasification of meat and bone meal with coal in a fluidised bed reactor

After the Bovine Spongiform Encephalopathy illness appeared, the meat and bone meat (MBM) produced from animal residues became an important waste. In spite of being a possible fuel due to its heating value (around 21.4 MJ/kg), an important fraction of the meat and bone meal is being sent to landfills. The aim of this work is to evaluate the co-gasification of low percentages of meat and bone meal with coal in a fluidised bed reactor as a potential waste management alternative. The effect of the bed temperature (800-900 °C), the equivalence ratio (0.25-0.35) and the percentage of MBM in the solid fed (0-1 wt.%) on the co-gasification product yields and properties is evaluated. The results show the addition of 1 wt.% of MBM in a coal gasification process increases the gas and the liquid yield and decreases the solid yield at 900 °C and 0.35 of temperature and equivalence ratio operational conditions. At operational conditions of 900 °C and equivalence ratio of 0.35, the specific yield to gas (y_gas) increases from 3.18 m³(STP)/kg to 4.47 m³(STP)/kg. The gas energy yield decreased 24.1% and the lower heating value of the gas decreases from 3.36 MJ/m³(STP) to 2.16 MJ/m³(STP). The concentration of the main gas components (H₂, CO and CO₂) hardly varies with the addition of MBM, however the light hydrocarbon concentrations decrease and the H₂S concentration increases at the higher temperature (900 °C).     

Hydrogasification characteristics of bituminous coals in an entrained-flow hydrogasifier

The characteristics of hydrogasification to generate substitute natural gas (SNG) by using various bituminous coals such as Alaska, Cyprus, Curragh, and Datong have been determined in an entrained-flow hydrogasifier (0.025 m I.D.×1 m high) with high pressure coal feeder and data acquisition system. The effects of reaction pressure (60–80 atm), reaction temperature (600–800 °C) and H₂/coal ratio (0.3–0.5) on composition of product gas and carbon conversion have been determined. The concentration of SNG and carbon conversion increased with an increasing of reaction pressure and temperature, but the carbon conversion and concentration of each bituminous coal were quite different because of different coal properties. Also the H₂/coal ratio affected the carbon conversion and the concentration of SNG.