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.     

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.     

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.     

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.     

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.     

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).     

A comparative study of a fluidised bed reactor and a gas lift loop reactor for the ibe process: Part III. Reactor performances and scale Up

Continuous fermentations using Clostridium spp. DSM 2152 immobilised in calcium alginate beads to produce butanol and isopropanol from glucose were carried out in a fluidised bed reactor with liquid recycle (FBR, 10.9 dm³ working volume, 41 % solids) and in a gas lift loop reactor (GLR, 11.4 dm³ working volume, 32% solids). In the FBR in-situ produced non-coalescing gas bubbles had a negligible influence on the fluidisation pattern and the steady state results of the fermentation were in accordance with those predicted by a reactor model. The FBR was operated reliably for 5 weeks without decrease of activity. The GLR behaved as a three phase reactor because of the recycled fermentation gas. The steady state fermentation results were as predicted by the GLR model. Scale up to a 50 m³ FBR and a 65 m³ GLR led to development of a plug flow with recycle model for the FBR and a stirred tank model for the GLR. On the basis of overall reactor performance and ease of integration with a simultaneous product recovery the FBR is preferred to the GLR for application in a large scale butanol/isopropanol process using immobilised Clostridia spp.     

Fluidized bed gasification of Kingston coal and marine microalgae in a spouted bed reactor

Steam gasification experiments were performed using a low-rank coal from South Australia, a marine microalga, and a blend of leached microalgal biomass and coal, in a spouted, fluidized bed reactor. The effect of different operating conditions – air-to-fuel ratio (A/F), steam-to-fuel ratio (S/F) and bed temperature (T_b) – on the producer gas composition was investigated. Producer gas compositions were analyzed and samples of bed material were also examined to identify ash components formed during each experiment. The optimum operating conditions for coal gasification, in this system, were identified to occur with A/F = 1.82, S/F = 0.75 and T_b = 850 °C. These conditions resulted in a producer gas with the highest heating value (per mass of fuel fed), the highest extent of carbon conversion and the optimum H₂:CO ratio for Fischer–Tropsch synthesis. In addition, preliminary attempts to gasify a sun-dried marine microalga are reported. The dried biomass, sieved to 1.0–3.35 mm, was gasified with air and steam. Preliminary experiments, utilizing the as-received biomass, proved unsuccessful due to rapid bed sintering. Leaching of the algal biomass to remove the extra-cellular salt and co-gasification of the resultant biomass (10 wt%) with low-rank coal also proved unsuccessful due primarily to blockages of the downstream product lines most likely due to attrition of the algae feed in the screw feeder and elutriation from the bed.     

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.     

Effect of reactor internal properties on the performance of a flow reversal catalytic reactor for methane combustion

This paper describes the use of a flow reversal reactor for the destruction of lean emissions of methane with ambient temperature feed. The reactor consisted of two parallel sections, each containing a packed bed reactor and inert sections to act as heat traps. In this paper, the effect on reactor performance of the inert properties is illustrated. Three different inert types are described. These are a ceramic monolith of 100 cells per square inch cell density, a metal monolith and a packed bed of Denstone balls. Use of monolith inserts reduces the reactor pressure drop. Inert sections with lower thermal mass give rise to greater movement in temperature fronts, requiring the use of shorter cycles.     

The redistribution of trace and minor elements during coal liquefaction

Radioactivation analysis of four sets of coal liquefaction feed and product materials provided information on the concentrations of forty trace and minor elements. Coal-derived oil was found to have low levels of most elements, comparable to crude petroleum. Aqueous process effluents were shown to have potentially troublesome levels, especially of Cr and Ba. The concentration data derived in this study and relevant process data were combined to construct elemental mass balances around the liquefaction process; these varied widely between about 80 and 140%, depending mostly on analytical errors. Enrichment of chromium and other transition metals in the conversion residues indicated a possible source through equipment erosion while consistent depletion of barium suggested a loss to aqueous stream particulate matter or possible reactor deposits.     

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.     

Certification of the contents of some trace elements in a coal

The preparation of a coal reference material to be used in trace element analysis is described, as well as the tests to ascertain the homogeneity and stability of the material. Several analytical methods were applied by several laboratories to certify the contents of As, Cd, Cr, Co, F, Mn, Hg, Ni, Pb and Zn. The procedure followed for certification is explained.     

Modeling of biomass devolatilization in a fluidized bed reactor

A simple model that simulates a single biomass particle devolatilization is described. The model takes into account the main physical and chemical factors influencing the phenomenon at high temperatures (>700 K), where the production of gaseous components far outweighs that of liquids. The predictions of the model are shown to be in good agreement with published data. The model is then applied to the devolatilization of biomass in a fluidized bed, in which attention is focused on heat transfer, particle mixing and elutriation, and gas production. Predictions on the overall devolatilization time for a biomass particle are compared with experimental results obtained in a fluidized bed reactor in which the process was monitored by continuous measurement of the bed pressure. Good correspondence of predicted with calculated values was obtained, supporting the validity of the many approximations made in the derivation of the governing relationships for the pyrolysis process.     

Partitioning of elements in a entrained flow IGCC plant: Influence of selected operational conditions

The partitioning of trace elements and the influence of the feed conditions (50:50 coal/pet-coke feed blend and limestone addition) was investigated in this study. To this end feed fuel, fly ash and slag samples were collected under different operational conditions at the 335 MW Puertollano IGCC power plant (Spain) and subsequently analysed. The partitioning of elements in this IGCC plant may be summarised as follows: (a) high volatile elements (70->99% in gas phase): Hg, Br, I, Cl and S; (b) moderately volatile elements (up to 40% in gas phase and ?60% in fly ash): As, Sb, Se, B, F, Cd, Tl, Zn and Sn; (c) elements with high condensation potential: (>90% in fly ash): Pb, Ge, Ga and Bi; (d) elements enriched similarly in fly ash and slag 30-60% in fly ash: Cu, W, (P), Mo, Ni and Na; and (e) low volatile elements (>70% in slag): Cs, Rb, Co, K, Cr, V, Nb, Be, Hf, Ta, Fe, U, Ti, Al, Si, Y, Sr, Th, Zr, Mg, Ba, Mn, REEs, Ca and Li. The volatility of As, Sb, and Tl and the slagging of S, B, Cl, Cd and low volatile elements are highly influenced by the fuel geochemistry and limestone dosages, respectively.     

中国煤中微量元素的研究

论述了中国煤中微量元素地球化学研究的现状、分析方法、发展趋势和存在的不足,提出了相应的建议。认为应加强微量元素的富集机理、赋存状态、环境影响、富集元素的综合利用等方面的工作。     

Production of synthetic natural gas (SNG) from coal and dry biomass - A technology review from 1950 to 2009

SNG production from coal or biomass is considered again due to rising prices for natural gas, the wish for less dependency from natural gas imports and the opportunity of reducing green house gases by CO₂ capture and sequestration. Coal and solid dry biomass (e.g., wood and straw) have to be converted to SNG by thermo-chemical processes (gasification followed by gas cleaning, conditioning, methanation of the producer gas and subsequent gas upgrading). During the 1970s, a number of methanation processes have been developed comprising both fixed bed and fluidised bed methanation. Meanwhile several new processes are under development, especially with a focus on the conversion of biomass. While coal based systems usually involve high pressure cold gas cleaning steps, biomass based systems require, due to the smaller unit size, different gas cleaning strategies. Moreover, the ethylene content of a few percent, typical for methane-rich producer gas from biomass gasifiers, is a challenge for the long-term catalyst stability in adiabatic fixed bed methanation due to the inherent high temperatures.This paper reviews the processes developed for the production of SNG from coal during the sixties and seventies and the recent developments for SNG production from coal and from dry biomass.     

The fragmentation of coal particles during the coal combustion in a fluidized bed

This paper presents an experimental method for studying the fragmentation of coal particles during coal combustion in a fluidized bed and the quantitative fragmentation indexes of 10 typical Chinese coal ranks. The influences of a variety of factors such as the bed temperature, the size of coal particles, the coal rank and the fluidizing medium on the fragmentation index of coal particles are also studied. The research results show that the main reason for the fragmentation of coal particles is the primary fragmentation, and that the volatile matter can drastically influence the degree of fragmentation of coal particles.     

双级燃料反应器的煤化学链燃烧特性

在5 kW_th双级燃料反应器的化学链燃烧装置上,开展煤化学链燃烧特性研究,重点考察反应温度和气化介质对燃烧补偿率、碳增补率、出口气体组分浓度、额外耗氧率以及碳捕集效率的影响规律。实验结果表明,较高的反应温度能显著提高燃烧效率,900℃时出口烟气中CO₂浓度可达92.1%;随着反应温度升高,碳捕集率和燃烧补偿率分别上升至99.6%和83.4%,额外耗氧率和碳增补率下降至12.1%和4.8%。以CO₂为气化介质时,整体反应效率有大幅下降,额外耗氧率提高至23%。此外,在Ⅰ级FR反应器内发现有少量的团聚颗粒,但并未对流化产生影响。     

Thermodynamic equilibrium study of trace element mobilisation under pulverised fuel combustion conditions

The thermodynamic equilibrium distributions of the trace elements lead, arsenic, zinc, copper, nickel, chromium, manganese and boron have been examined using the factwin computational software and associated databases. It is found that the facility for simulation of these elements in a comprehensive model of the oxide melt formed by the major coal mineral elements and (in the early stages of combustion) in an extended sulphide melt model enhances the value of the predictions over those for earlier restricted fact melt models. This is emphasised by comparison with the limitations of predictions for strontium, barium and vanadium, for which no solution models are available in factwin. The influence of sulphur and chlorine concentration variations on the mobilities of the elements varies, and can differ markedly between oxidising and reducing conditions. The predictions are extensively compared with published partitioning results calculated from experimental observations on large combustors. Given the uncertainties involved in both observations and predictions, the degree of agreement is considered satisfactory. The condensation sequence from the equilibrium gas phase at 1300 K has been predicted on cooling, in isolation, by 10 K steps. An alkali sulphate-based melt is predicted to form, and the majority of the other elements are predicted to form sulphates, implying development of a complex sulphate melt, which cannot at present be modelled.