Trace element behaviour in the Sasol-Lurgi MK IV FBDB gasifier. Part 1 - The volatile elements: Hg, As, Se, Cd and Pb

Coal-fired power and heat production are the largest single source of Hg in the atmosphere, and in March 2005, the US-EPA ruled regarding Hg reduction from coal utilization in the USA. Appropriate Hg pollution control of technology, as well as reductions in the uses of Hg and coal-containing Hg can readily reduce the releases of Hg from coal utilities. Integrated multi-pollutant (SOx, NOx, particulate matter and Hg) control technologies may be a cost-effective approach. Prior to considering mitigation technologies, it is necessary to understand the quantity of mercury in the feed coal, its mode of occurrence (i.e. mineral or organic associations), its partitioning behaviour during the process, and the volume and species in which it is being emitted via stacks. These factors have all been investigated up to the point of release for the Sasol gasification and steam-raising plants, including other trace elements.The focus of this paper is to discuss the more recent environmental research developments by Sasol, where trace element simulation and validation of model predictions have been undertaken for the Sasol-Lurgi gasification process operating on lump coal. Fact-Sage thermodynamic equilibrium modeling was used to simulate the trace elements: Hg, As, Se, Cd and Pb gas phase and ash phase partitioning and speciation behaviour occurring in a fixed-bed pressurised gasifier. A Sasol-Lurgi Mark IV (MK IV) fixed-bed dry bottom (FBDB) gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, findings being used to validate the modeled results. This paper will focus on the behaviour of the volatile Class I trace elements: Hg, As, Se, Cd and Pb within the Sasol-Lurgi MK IV FBDB gasifier as function of coal quality. This study excludes the downstream gas cleaning partitioning and speciation behaviour of these elements, which will form the basis of a future paper.Good agreement between model-predictions and measurements have been attained in this study, with the exception of As. Hg, Cd, Pb, As and Se were all found to be highly volatile, partitioning into the gas phase. Hg was found to be the most volatile element during fixed-bed gasification and is present in the gas phase in the form of elemental Hg (g). As, Se, Cd and Pb have lower volatilities when compared to Hg, and they vary in an order: Hg > Se > Cd > Pb > As. Speciation predictions showed that: Hg, AsH₃, H₂Se, PbSe, Cd, CdS, and PbS/Pb/PbCl, species could potentially exist in the raw gas phase.     

Trace elements (Mn,Cr, Pb,Se, Zn,Cd and Hg) in emissions from a pulverized coal boiler

The concentrations of seven trace elements (Mn, Cr, Pb, Se, Zn, Cd, Hg) in raw coal, bottom ash and fly ash were measured quantitatively in a 220 tons/h pulverized coal boiler. Factors affecting distribution of trace elements were investigated, including fly ash diameter, furnace temperature, oxygen concentration and trace elements' characteristics. Modified enrichment factors show more directly element enrichment in combustion products. The studied elements may be classified into three groups according to their emission features: Group 1: Hg, which is very volatile. Group 2: Pb, Zn, Cd, which are partially volatile. Group 3: Mn, which is hardly volatile. Se may be located between groups 1 and 2. Cr has properties of both Groups 1 and 3. The smaller the diameter of fly ash, the higher is the relative enrichment of trace elements (except Mn). Fly ash shows different adsorption mechanisms of trace elements and the volatilization of trace elements rises with furnace temperature. Relative enrichments of trace elements (except Mn and Cr) in fly ash are larger than that in bottom ash. Low oxygen concentration will not always improve the volatilization of trace elements. Pb forms chloride more easily than Cd during coal combustion.     

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.     

Entrained‐flow gasifier fuel blending studies at pilot scale

For the foreseeable future, coal and petroleum‐based materials, such as petroleum Coke, residuals, and high‐sulphur fuel oil, are being adopted as the feedstocks of choice for gasification projects. Of particular interest from a Canadian perspective is Coke generated from the thermal cracking of the oil sands in Western Canada. Oil sand Coke contains high sulphur (5–6%), and also typically has a low volatile content, and lower reactivity than most coals. Experimental runs have recently been conducted on the pilot‐scale entrained‐flow gasifier at CETC‐Ottawa, blending oil sand Coke with sub‐bituminous and lignite coals, to try and enhance the gasification potential of these materials. Blending Genesee sub‐bituminous coal with the delayed oil sands Coke was found to alleviate problems encountered with slag plugging the reactor when running with Genesee coal alone. Blends of Genesee sub‐bituminous and Boundary Dam lignite coals with Coke achieved higher carbon conversions and cold gas efficiencies than runs completed with the Coke by itself. While using CO₂ as the conveying gas into the gasifier was not found to significantly affect the conversion obtained, steam addition was found to have a marked effect on CO and H₂ concentrations in the syngas.     

Volatilisation of trace elements for coal–sewage sludge blends during their combustion☆

The influence of sewage sludge addition on the volatility of 37 trace elements (Ag, As, Au, B, Ba, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, Hf, Hg, La, Li, Mn, Mo, Nb, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Tl, U, V, Y, Zn, Zr) during coal combustion was studied. For this purpose, a bituminous coal from the Asturian Central Basin and sewage sludge treated with Ca(OH)₂ and FeCl₃, as well as 10 and 50 wt% sludge–coal blends were used. Combustion experiments were performed in a laboratory electric furnace at 800, 900, 1000 and 1100 °C. The results have confirmed that the high Cl contents of the sludge can produce a pronounced effect on the volatilisation of some trace elements (Ag, Cd, Cs, Cu, Li, Pb, Rb and Tl) due to the probable formation of volatile chlorides, while the high CaO concentrations increase the retention of some elements in ash as As, Se and Te. The above opposite effects of Cl and CaO on trace element volatilisation were generally inappreciable for the 10 wt% blend, while they were more significant, but not as noticeable as expected, for the 50 wt% blend.     

The fate of trace elements in fluidised bed combustion of sewage sludge and wood

Combustion tests have been carried out in a fluidised bed boiler to investigate the fate of trace elements during co-combustion of wood and municipal sewage sludge. The approach was to collect fuel and ash samples and to perform thermodynamic equilibrium calculations for gasification (reducing) and combustion (oxidising) conditions. Trace elements are found in the ash. Even most of the highly volatile Hg is captured in the bag filter ash. The bag filter ash offers higher surface area than the secondary cyclone ash and enhances the capture of Hg. There is no obvious correlation between capture and parameters investigated (sludge precipitation agent and lime addition). As, Cd, Hg, Pb, Se, Sb and Tl are predicted by equilibrium calculations to be volatile in the combustion chamber under oxidising conditions and Hg even at the filter temperature (150 °C). Reducing conditions promote, in some case more than others, the volatility of As, Cd, Pb, Sb, Se, Tl and Zn. The opposite effect was observed for Cu and Ni. Data points to the necessity of including bag filter in the gas cleaning system in order to achieve good removal of toxic trace elements.     

Trace element behaviour in the Sasol-Lurgi MK IV FBDB gasifier. Part 2 - The semi-volatile elements: Cu, Mo, Ni and Zn

Gasification is a coal conversion process that could be considered to be more amenable with regards to environmental impact factors when compared to combustion, as it provides minimum direct emission to the atmosphere due to the opportunity to apply a series of gas cleaning processes. Emissions could be in the form of the well known trace elements labelled as toxic present in feed coal. Due to the minimal literature available on coal gasification when compared to coal combustion, a large amount of inference to coal combustion has been applied in discussing the partitioning behaviour of trace elements during coal utilization. Conducting mass balance calculations of trace elements around gasification processes have proven to be a challenging task. This is due to the limitation of the analytical techniques employed to quantify at the parts per million levels at which trace elements exist. The other challenge is analyzing for trace elements in all the different stream phases that occur after gasification. The availability of thermodynamic equilibrium packages i.e. Fact-Sage to perform high temperature calculations, at the same time handling all phases of material involved has simplified the challenges. Results obtained from such calculations have also proved to be close to reality, but have not been related to the fixed-bed counter-current gasification reactor operating on lump coal.The focus of this paper is to discuss more recent environmentally-focused research developments by Sasol, where trace element simulation and validation of model predictions have been undertaken for the gasification process. Fact-Sage thermodynamic equilibrium modelling was used to simulate the semi-volatile trace elements (Cu, Mo, Ni and Zn) gas phase and ash phase partitioning and speciation behaviour occurring in a fixed-bed pressurized gasifier. A Sasol-Lurgi Mark IV FBDB gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, results being used to validate the modelled results.The semi-volatile elements: Cu, Mo, Ni and Zn all showed limited (5% in the case of Zn) de-volatilization behaviour in the drying and pyrolysis zone of the fixed-bed gasifier. Predictions revealed that within the reduction zone of the fixed-bed gasifier that they are all highly volatile, producing gaseous species with an increase in temperature, varying in the order: Zn > Mo > Cu > Ni, which is contrary to what was found from the experimental results. This could imply that thermodynamic equilibrium conditions do not necessarily prevail in a fixed-bed gasifier operating on lump coal, since in reality mass and heat transfer limitations across coarse coal particles apply and the reactions are therefore more kinetically limited. Over-balances of Ni and Mo partitioning to the solid ash fraction, was found for the measured results. This anomaly was found to not be caused by erosion of the gasifier internals, but rather possibly be ascribed to accumulation and contamination caused by likely condensation and vaporisation of these species during the gasifier sampling campaign, as well as by the particle size reduction processes utilized prior to elemental analyses. Leaching tests conducted on the bottom ash collected from the gasifier have shown that the trace elements studied are firmly bound into the ash matrix and therefore would not be released during later disposal. The relative enrichment in trace element content observed for Ni and Mo within the gasifier should be further investigated.     

Volatility of fly ash and coal

The volatilization of fly ash has been examined by a number of techniques including TGA—DTA, Knudsen cell mass spectrometry, volatilization of neutron-activated fly ash, and X-ray fluorescence analysis of sized fly ash, low-temperature ash, and the parent coal. At low temperatures, H₂O, CO₂, SO₂, and a number of organic compounds are the primary volatile species as determined by mass spectrometry. Analysis of the volatiles collected from activated fly ash heated to temperatures up to 1400 °C shows that Hg, Se, As, Br, and I are nearly completely volatilized. The analysis of the bulk and size fractions of fly ash, and parent coal, is consistent with this and provides evidence for volatilization of 15 elements during coal combustion. Comparison of coal and fly ash compositions also shows that significant amounts of Se are still present in the gas phase at the precipitators and more than 50 wt % of the Se is contained in the stack emissions. The results are consistent with present models for fly ash formation and trace element enrichment.     

Identification of reaction zones in a commercial Sasol-Lurgi fixed bed dry bottom gasifier operating on North Dakota lignite

The Sasol-Lurgi fixed bed dry bottom gasification technology has the biggest market share in the world with 101 gasifiers in operation. To be able to further improve the technology and also to optimise the operating plants, it is important that the fundamentals of the process are understood. The main objective of this study was to determine the reaction zones occurring in the Sasol-Lurgi fixed bed dry bottom (S-L FBDB) gasifier operating on North Dakota lignite. A Turn-Out sampling method and subsequent chemical analyses of the gasifier fuel bed samples was used to determine the reaction zones occurring in the commercial MK IV, S-L FBDB gasifier operating on North Dakota lignite. The reaction zones were further compared with the same reactor operating on bituminous coal.Based on the results obtained from this study it was found that about two thirds of the gasifier volume was used for drying and de-volatilising the lignite thus leaving only about a third of the reactor volume for gasification and combustion. Nonetheless, due to the high reactivity of the lignite, the char was consumed within a third of the remaining gasifier volume. Clear overlaps between the reaction zones were observed in the gasifiers thus confirming the gradual transition from one reaction zone to another as reported in literature. Due to the high moisture content in the lignite, the pyrolysis zone in the gasifiers operating on North Dakota lignite occurred lower/deeper in the gasifier fuel bed as compared to the same gasifier operating on South African bituminous coal from the Highveld coalfield. All the other reaction zones in the gasifier operating on bituminous coal were also higher in the bed compared to the lignite operation. This can therefore explain the higher gas outlet temperatures for the S-L FBDB gasifiers operating on higher rank coals when compared to the gasifiers operating on lignite. The fact that the entire reactor volume was utilized for drying, de-volatilisation, gasification and combustion with carbon conversion of >98% makes the S-L FBDB gasifier very suitable for lignite gasification.     

Volatilization of mercury,arsenic and selenium during underground coal gasification

Mercury, arsenic and selenium are trace elements well-known for their high volatility in underground coal gasification (UCG) which can lead to environmental and technical problems during gas utilization. In this paper, the volatilization of mercury, arsenic and selenium from coal in a seam during the process of UCG were investigated, based on comparison of their volatility during the transformation of coal to char and the conversion of char to ash. Three types of coal were involved in this study. The results indicate that the volatility of mercury, arsenic and selenium during UCG in the seam follows the sequence of Hg>Se>As. Mercury and selenium show volatility higher than 90% from coal to ash. The volatility of arsenic is lower than 60% as confirmed by arsenic enrichment in UCG ash. Arsenic volatilization during UCG is also enhanced by increasing temperature, which is different from the result during the combustion of crushed coal. Coal type has obvious effect on element volatilization. The higher the coal reactivity, the easier is the evaporation of the elements from coal in the seam. At the same time, thermodynamic equilibrium calculations using MTDATA program were performed to predict the possible species in UCG gas. With regard to UCG gas at the production well, mercury presents as Hg(g), H₂Se(g) is the main gaseous species of selenium, whereas arsenic occurs in condensed phase as As₂S₃ and As. The effect of the pressure on the equilibrium composition of the gas results in major changes of the proportions of the species. High pressure leads to the formation and enhancement of the reduced species and increases the condensation temperature of the volatile elements.     

Distribution of potentially hazardous trace elements in coals from Shanxi province, China

Shanxi province, located in the center of China, is the biggest coal base of China. There are five coal-forming periods in Shanxi province: Late Carboniferous (Taiyuan Formation), Early Permian (Shanxi Formation), Middle Jurassic (Datong Formation), Tertiary (Taxigou Formation), and Quaternary. Hundred and ten coal samples and a peat sample from Shanxi province were collected and the contents of 20 potentially hazardous trace elements (PHTEs) (As, B, Ba, Cd, Cl, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, Pb, Sb, Se, Th, U, V and Zn) in these samples were determined by instrumental neutron activation analysis, atomic absorption spectrometry, cold-vapor atomic absorption spectrometry, ion chromatography spectrometry, and wet chemical analysis. The result shows that the brown coals are enriched in As, Ba, Cd, Cr, Cu, F and Zn compared with the bituminous coals and anthracite, whereas the bituminous coals are enriched in B, Cl, Hg, and the anthracite is enriched in Cl, Hg, U and V. A comparison with world averages and crustal abundances (Clarke values) shows that the Quaternary peat is highly enriched in As and Mo, Tertiary brown coals are highly enriched in Cd, Middle Jurassic coals, Early Permian coals and Late Carboniferous coals are enriched in Hg. According to the coal ranks, the bituminous coals are highly enriched in Hg, whereas Cd, F and Th show low enrichments, and the anthracite is also highly enriched in Hg and low enrichment in Th. The concentrations of Cd, F, Hg and Th in Shanxi coals are more than world arithmetic means of concentrations for the corresponding elements. Comparing with the United States coals, Shanxi coals show higher concentrations of Cd, Hg, Pb, Se and Th. Most of Shanxi coals contain lower concentrations of PHTEs.     

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.     

Laboratory-scale coal reactivity testing for entrained gasification

A relatively simple and rapid micro-gasification test has been developed for measuring gasification reactivities of carbonaceous materials under conditions which are more or less representative of an entrained gasification process, such as the Shell coal gasification process. Coal particles of < 100 μm are heated within a few seconds to a predetermined temperature level of 1000–2000 °C, which is subsequently maintained. Gasification is carried out with either CO₂ or H₂O. It is shown that gasification reactivity increases with decreasing coal rank. The CO₂ and H₂O gasification reactions of lignite, bituminous coal and fluid petroleum coke are probably controlled by diffusion at temperatures 1300–1400 °C. Below these temperatures, the CO₂ gasification reaction has an activation energy of about 100 kJ mol^?1 for lignite and 220–230 kJ mol^?1 for bituminous coals and fluid petroleum coke. The activation energies for H₂O gasification are about 100 kJ mol^?1 for lignite, 290–360 kJ mol^?1 for bituminous coals and about 200 kJ mol^?1 for fluid petroleum coke. Relative ranking of feedstocks with the micro-gasification test is in general agreement with 6 t/d plant results.     

Trace element behaviour in the Sasol-Lurgi fixed-bed dry-bottom gasifier. Part 3 - The non-volatile elements: Ba, Co, Cr, Mn, and V

Coal contains most of the naturally occurring chemical elements in (at least) trace amounts, with specific elements and their concentrations dependent on the rank of the coal and its geological origins. The focus of this paper is to discuss more recent environmentally-focused research developments by Sasol, where trace element simulation and validation of model predictions have been undertaken for the gasification process operating on low-rank bituminous Highveld coal. A Sasol-Lurgi fixed-bed dry-bottom (FBDB) gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, results being used for comparison with Fact-Sage modelled data for the non-volatile trace elements Ba, Co, Cr, Mn and V.Considering the experimental error, good agreement between measured results and model predictions in terms of ash phase partitioning behaviour was obtained for Ba, Co, Mn and V. On the contrary, rather poor agreement between model predicted and measured results were obtained for Cr partitioning to the solid ash fraction, which yielded a large overbalance (outside of experimental error) in the case of the measured results. This anomaly was found to not be caused by erosion of the gasifier internals, but rather possibly be ascribed to accumulation and contamination caused by likely condensation and vaporisation of this species during the gasifier sampling campaign, as well as by the particle size reduction processes utilized prior to elemental analyses. When considering the predicted speciation behaviour of the elements studied, the model output in some cases needs to be treated with some caution when validating findings with standard text book data for the elements studied, but was found to correctly model the elemental ash phase partitioning behaviour during fixed-bed gasification. Leaching tests have been conducted on the bottom ash collected from the gasifier and results have shown that the trace elements studied are firmly bound into the ash matrix and therefore would not be released during later disposal. The relative enrichment in trace element content observed for Cr within the gasifier should be further investigated.     

A study of trace element behaviour in two modern coal-fired power plants: II. Trace element balances in two plants equipped with semi-dry flue gas desulphurisation facilities

Two Finnish coal-fired power plants were experimentally investigated with regard to the distribution of environmentally harmful trace elements in the process. The plants were equipped with low NO_x burners, an electrostatic precipitator and a semi-dry flue gas desulphurisation unit consisting of a spray-dryer-type reactor and a fabric filter. All the in-going and out-going mass streams were analysed for As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se and Tl in the first plant, and for As, Be, Cd, Cr, Hg, Mn, Ni, Pb, Tl and Zn in the other plant. Total atmospheric emissions of trace elements from the plants studied were very low; most of them even orders of magnitudes smaller than 10 μg/m³ (NTP). The vaporous fraction of the trace elements was found to play a predominant role in the atmospheric emissions. However, the total atmospheric emissions of even the strongly volatile Hg were very low, which can be ascribed to the applied plant technology. Enrichment of the elements into the various ash streams—from the bottom ash up to the particulates downstream of the electrostatic precipitator—was found to be in satisfactory compliance with literature data. Element balances were calculated over the whole process and over the desulphurisation unit for both plants: for the whole process the closure (i.e., ratio output/input) of element balances was within ±30% for all elements studied, except for chromium in plant HB, and for the vast majority of elements it was even within ±20%. The results imply that the sampling techniques and analytical methods developed for this work can be well applied to quantitative mass balance studies in this kind of processes.     

Investigation of the remaining major and trace elements in clean coal generated by organic solvent extraction

A sub-bituminous Wyodak coal (WD coal) and a bituminous Illinois No. 6 coal (IL coal) were thermally extracted with 1-methylnaphthalene (1-MN) and N-methyl-2-pyrrolidone (NMP) to produce clean extract. A mild pretreatment with acetic acid was also carried out. Major and trace inorganic elements in the raw coals and resultant extracts were determined by means of inductively coupled plasma optical emission spectrometry (ICP-OES), flow injection inductively coupled plasma mass spectrometry (FI-ICP-MS), and cold vapor atomic adsorption spectrometry (CV-AAS). It was found that the extraction with 1-MN resulted in 73–100% reductions in the concentration of Li, Be, V, Ga, As, Se, Sr, Cd, Ba, Hg, and Pb. The extraction with NMP yielded more extract than that with 1-MN, but it retained more organically associated major and trace metals in the extracts. In the extraction of WD coal with NMP, the acid pretreatment not only significantly enhanced the extraction yield but also significantly reduced the concentrations of alkaline earth elements such as Be, Ca, Mg, Sr, and Ba in the extract. In addition, the modes of occurrence of trace elements in the coals were discussed according to their extraction behaviors.     

The characteristics of inorganic elements in ashes from a 1 MW CFB biomass gasification power generation plant

This paper investigated the characteristics of inorganic elements in ashes from biomass gasification power generation (BGPG) plant. The ash samples of the gasifier ash, separator ash and wet scrubber ash were collected in a 1 MW circulating fluidized bed (CFB) wood gasification power generation plant. Particle size distribution of ashes was determined by gravimetric measurement and super probe analyzer. The concentrations of trace elements and major ash-forming elements, such as As, Al, Ca, Cd, Cr, Cu, K, Mg, Na, Ni, Pb, Ti in different ashes as a function of particle size were determined by Inductive Coupled Plasma Spectrometer. The concentrations and distribution coefficient and enrichment factors of the inorganic elements in ashes were studied. X-ray fluorescence spectrometer and X-ray powder diffraction were used to provide information on the characteristics of the ashes. The results showed that most of the trace elements had an enrichment tendency in the finer size particles. A considerable amount of the ashes was residual carbon. Most of the volatile e.g. halogen elements and alkali elements existed mainly in wet scrubber ash and enriched in fly ash. Most of the Si, Ni, Pb, Zn, Cr, Cd were found in separator ash, indicating an enrichment of heavy metal elements in separator ash. K, S, Mn, Cu mainly existed in gasifier ash.     

西部煤中环境敏感性痕量元素的燃烧迁移行为

应用仪器中子活化( INAA)、电感耦合等离子体原子发射光谱( ICP- AES)和原子吸收光谱( AAS)对我国西北部五个电厂原煤、底灰和飞灰中环境敏感性痕量元素的含量进行了系统测定,通过不同电厂原煤与燃烧产物中痕量元素的含量变化特征,揭示了痕量元素在不同燃烧产物中的相对富集规律.以痕量元素在不同燃烧产物中的相对富集系数为评价标准,建立了燃烧产物中痕量元素的分配模型.结合痕量元素的原始赋存状态,总结了痕量元素燃烧的迁移富集机理和环境效应.     

Characterization of unburned carbon from ash after bituminous coal and lignite combustion in CFBs

This paper deals with the characterisation of carbon (UC) from bottom ash (BA) and fly ash (FA) samples from two fluidised-bed power stations burning bituminous coal and lignite. The laboratory results for the carbon determinations and its mass balances are evaluated. Chemical and mineral analyses and textural characteristics (specific surface area and pore-size distribution) are presented. Depletion/enrichment of selected elements (S, Cl, V, Cr, Ni, Cu, Zn, As, Se, Sb, Hg, and Pb) in carbon from the bottom ash are compared with both ash compostions. The strong positive relationships between the concentrations of some trace element contents (Hg, Se, As, Cu, Ni, V and Cl) in fly ash with the content of carbon and the specific surface area of FA are presented and expressed by regression equations with very high correlation coefficients. Laser ablation-ICP-MS has been used to obtain an insight into element distributions within carbon grains from the bottom ash.     

CO2 gasification kinetics of two alberta coal chars

CO₂ gasification kinetics of chars from two Alberta coals (Obed Mountain, high volatile bituminous and Highvale, subbituminous) have been studied using a thermogravimetric analyzer (TGA) and a fixed bed reactor. Charification and gasification reactions were performed sequentially in both the TGA instrument and in the fixed bed reactor to simulate real gasifier operating conditions. TGA and fixed bed data were processed numerically to evaluate the kinetic rate of CO₂ gasification of the chars. Calculated gasification kinetics could be correlated using both the volume reaction and the grain models. Activation energies of the kinetic rate constants were near 200 kJ/mol for both Highvale and Obed Mountain coal chars using the TGA data. The activation energies calculated for the Obed Mountain coal char using the fixed bed reactor were about 250 kJ/mol. For all the cases studied the calculated activation energies were nearly the same for both the volume and grain reaction models.