An assessment of acid wash and bioleaching pre-treating options to remove mercury from coal

The United States Environmental Protection Agency is expected to begin regulating the release of vapor-phase mercury from coal-fired power plants in the year 2007. Chemical pre-treatment methods were investigated for mercury removal effectiveness from pulverized low-sulfur North Dakota lignite coal. More limited results were obtained for a pulverized high-sulfur Blacksville bituminous coal. A two-step acid wash treatment showed removal rates of 60-90%, compared to one-step treatments with concentrated HCl, which yielded removals of 30-38%. Removal effectiveness is similar for first step solvents of water, pH 5.0 acid, or pH 2.0 acid followed by concentrated HCl as the second step solvent, and is independent of first step incubation time. Neither of two bacterial strains, Thiobacillus ferrooxidans and T. thiooxidans, was found effective for mercury removal.     

Mineralogy, elemental composition and modes of occurrence of elements in Canadian feed-coals

Feed-coals used in some Canadian power plants were examined for their mineralogical and elemental composition. The mode of occurrence of elements (organic/minerals) in these samples were determined using sequential leaching by H₂O, NH₄OAc and HCl.Canadian feed-coals examined in this study are subbituminous to bituminous rank (calorific values: 19.62-29.88 MJ/kg). The sulphur content of these coals is 0.32-3.55%.Quartz and aluminosilicates (clay minerals and feldspar) dominate the mineralogy of these samples. The accessory minerals consists of pyrite, sphalerite, barite, calcite, anhydrite, chromite, zircon, biotite, and monazite, which occur as both primary and secondary cell filling types. An interesting mineral found in one of the subbituminous feed-coals is the gorceixite that is normally associated with degraded volcanic ash.The sulphur content of the feed-coals is an indication of their geological setting with feed-coal formed in a fresh water setting containing the least amount of S and those associated with evaporites having the highest S content.The concentrations of Cd, Cr, Cu, Hg, Ni, Se, V, and Zn in these Canadian feed-coals are low compared to world coals. Mercury content of these feed-coals ranges from 0.04 to 0.16 ppm. Mercury is mostly associated with the pyretic portion of coal and has direct correlation to As and S content in these coals. Arsenic in low sulphur coals is mostly associated with coal macerals and in higher sulphur coals with pyrite. About 47% of As in high sulphur coal is removed after leaching by HCl.Water soluble elements consist of Co, Ni and Mn, which are associated only with bituminous feed-coals. The removal of these elements by water is possibly due to an increased acidity of leaching solution and presence of soluble Cl and S.Beryllium and vanadium in low S coals are mostly removed after leaching by HCl, indicating a possible association with clay minerals. Limited Be and V is removed from lower clay mineral content coals; however, in high S content coals, more Be and V are removed.The speciation of As, Cr and Ni indicates that As is mostly present in the less-toxic form As⁺⁵ and Cr is present entirely as Cr³⁺, an essential human trace nutrient, found in both subbituminous and bituminous ranked coals. Nickel is present mostly as Ni²⁺ in oxygen coordination in these milled coals and carcinogenic compounds of nickel are not present in these feed-coals.     

煤在富氧流化床燃烧条件下汞的析出及形态分布

目前有关于富氧气氛下流化床燃烧汞的形态转化特性的报道还不是很多,因此本文开展了富氧气氛下煤种对汞形态转化特性的影响的实验研究。采用流化床作为实验设备,选用徐州烟煤和淮北烟煤作为实验燃料,研究了空气气氛下不同温度和不同煤种对汞析出规律的影响,富氧气氛下煤种对汞形态转化规律的影响,并深入分析了相应的汞氧化机理。研究结果表明：在空气气氛下,温度的增加会促进汞的氧化,煤中的含硫量对汞的氧化也有影响;在富氧气氛下,徐州烟煤燃烧产生的气态总汞浓度高于淮北烟煤燃烧产生的气态总汞浓度,徐州烟煤的Hg²⁺（g）的分布率也比淮北烟煤的Hg²⁺（g）的分布率高出16%左右,因为徐州烟煤中高含硫量会影响Hg²⁺（g）的分布率;富氧气氛下徐州烟煤的Hg²⁺的分布率低于空气气氛下的,而淮北烟煤的Hg²⁺（g）分布率则与之相反,这与两种煤中硫含量的不同有关。     

Mercury porosimetry and its application to the analysis of coal pore structure

Mercury porosimetry, gas adsorption, helium and mercury densities and small angle X-ray scattering were the methods used for studying porosity characteristics of a number of Czechoslovak coals ranging from lignite to semi-anthracite. Total pore volumes were measured in the pore radii range 0.1 – 7500 nm and then divided into the following groups: micropores (smaller than 1.8 nm), mesopores (1.8 – 30 nm) and macropores (over 30 nm). Coal porosity values ranged from 1.3 up to 43%. With low and medium-rank coals (carbon content under 76%), porosity is mostly due to mesopores. Micropores and mesopores account for a substantial share of the porosity of bituminous coals (carbon content over 80.6%). The distribution of mesopores was determined for an identical pore radii interval on the basis of data obtained independently by means of mercury penetration, gas adsorption and small angle X-ray scattering. High-pressure mercury penetration is an effective method of obtaining the coal porosity spectrum within a broad interval of pore radii, provided that a correction is made for the coal compressibility when evaluating the measured data. The recommended value of the mercury—coal contact angle is 1352 for brown coal and 130° for bituminous coal.     

Fractionation of coal by use of high temperature solvent extraction technique and characterization of the fractions

The authors have recently presented a new coal fractionation method that can separate a bituminous coal into several fractions, just like petroleum distillation, without decomposing coal. In this paper this method was applied to two bituminous coals and a brown coal. Sequential solvent extraction at different temperatures lower than 350 °C successfully separated the two bituminous coals into several fractions having different molecular mass compounds. Since all the extracted fractions are almost free from mineral matter, and some fractions were found to be fusible like a synthesized pitch when heated, the possibility of producing high performance carbon materials from the coal fractions was investigated. On the other hand, fractions obtained from the brown coal by the sequential solvent extraction were very close in both chemical composition and molecular mass, although the sequential extraction could greatly suppress the decomposition of the brown coal below 350 °C. The difference in the extraction behavior between the bituminous coals and the brown coal were attributed to the difference in their chemical structure.     

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.     

Classification of carbon in Canadian fly ashes and their implications in the capture of mercury

Fly ashes produced from Canadian power plants using pulverized coal and fluidized bed combustors were examined for their carbon content to determine their ability to capture mercury. The feed coal used in these power plants were lignite, subbituminous, high and medium volatile bituminous, their blends, and also blends of coal with petroleum coke (Petcoke). The carbon and mercury content of the coals and fly ashes were determined using the ASTM standard method and by the cold vapour atomic absorption spectrometry method. The carbon content of the fly ash was concentrated by strong acid digestion using HCl and HF. The quantitative and qualitative analyses of the carbon concentrate were made by using a reflected light microscope. The results show that the carbon content of fly ash appears to be partially related to depositional environment during coalification and to the rank of the coal. The Hg captured by the fly ash depends on the rank and blend of the feed coals and the type of carbon in the fly ash. The isotropic vitrinitic char is mostly responsible for the capture of most Hg in fly ash. The inadvertent increase in carbon content due to the blending of coal with petroleum coke did not increase the amount Hg captured by the fly ash. The fly ash collected by the hot side electrostatic precipitator has a low Hg content and no relation between the Hg and carbon content of the ash was observed. These results indicate that the quantity of carbon in the fly ash alone does not determine the amount Hg captured. The types of carbon present (isotropic and anisotropic vitrinitic, isotropic inertinitic and anisotropic Petcoke), the halogen content, the types of fly ash control devices, and the temperatures of the fly ash control devices all play major roles in the capture of Hg.     

The amenability of some Witbank bituminous ultra fine coals to binderless briquetting

Five ultra fine bituminous coals from the Witbank coalfield were studied. Selected chemical and physical properties, coal petrographic characteristics and mineralogical compositions of the coals were measured. The coals were compacted, using a briquetting press, at various feed moisture contents. The formed briquettes were then tested for compressive strength and water resistance and the values correlated with the coal characteristics and the briquetting conditions. The coals were found to be amenable to conventional binderless briquetting. The bonding in the briquettes was found to be due mainly due to the impurities in the coals, particularly kaolinite. This may add a new perspective to the fundamentals of the binderless briquetting of bituminous coals.     

Depolymerization of oxidized bituminous coals

Certain high-sulphur-containing, high-volatile bituminous (hvb) coals have been converted into products which are almost completely soluble in chloroform. These products were obtained by a series of reactions involving initial oxidation with 2N nitric acid, depolymerization with boron trifluoride-phenol, and acetylation. Solubility of the depolymerized coal decreased sharply when the nitric acid reaction was eliminated or was replaced with a hydrochloric acid treatment. The extent of oxidation by the nitric acid treatment is discussed. Also included in this investigation are a low-sulphur-containing hvb coal and a low-volatile bituminous coal. After depolymerization, the solubilities of these two coals in chloroform were comparatively low.     

The associative nature of lower rank coal

Initial volumetric swelling in tetrahydrofuran of pyridine-unextracted parts from subbituminous coal and lignite showed no dependence of their concentration, and was smaller than that of their pyridine extracts. These results are opposite to those obtained from high volatile bituminous coals and coincide with predictions for the cross-linked network model of coal. However, when ionic forces in these coals were reduced by acid washing or O-alkylation, these coals showed the same associative nature as did high volatile bituminous coals. Swelling kinetics were analysed on the basis of associative equilibria controlled by the ionic forces. It was concluded that solvation of the ionic forces was the rate determining step of volumetric swelling of lower rank coal rather than solvent diffusion into the coal, although diffusion has been proposed to be the most important factor in swelling.     

Emission characteristics of NOx and unburned carbon in fly ash on high-ash coal combustion

Three coal sources, two high-ash coals from South Africa and one bituminous coal from Australia, were evaluated at a pulverized coal combustion test furnace. These combustion trials were carried out on different blending ratios with the two South African coal sources. The NOx emission characteristics of these high-ash coals are similar with those of bituminous coals. The combustion efficiencies on the high-ash coals are lower than those of bituminous coals. The high-ash coals do not affect bituminous coal combustion on the blending combustion.     

Demineralization of a UK bituminous coal using HF and ferric ions

A UK bituminous coal has been used to study demineralization by two-stage chemical leaching. The first-stage uses hydrofluoric acid (HF) at 65 °C and reduces the ash content from 5.30 wt% to 1.37 wt% by mainly removing Al and Si containing minerals. Subsequent leaching by ferric ions decreases the ash content further to 990 ppm by removing most of the pyrite and fluorides formed during the HF leaching. Calorific value of the coal shows no change following leaching, which suggests no oxidation is occurring to the coal carbonaceous matrix. The mercury and sulfur contents after the two-stage leaching sequence decrease by 40% and 26%, respectively.     

The use of fractal analysis in the textural characterization of coals

The materials studied were four bituminous coals as well as the corresponding coal samples oxidized in air at 543 K for different periods of time. The method proposed by Friesen and Mikula, the procedure of Neimark and the methodology of Zhang and Li were successfully employed to analyze mercury porosimetry data from a fractal perspective. Fractal dimensions as well as fractal profiles are sensitive to oxidation treatment, being useful to follow the changes undergone by the coal samples. The evolution of the fractal dimension of coals with oxidation is determined by a balance between two main mechanisms of pore development: the oxidation of the pore surface itself that tends to lower the fractal dimension and the access of mercury to previously non-accessible regions that tends to increase this dimension.     

Model for coal oxidation kinetics. 2. An effectiveness factor interpretation

Rate expressions are developed to describe the oxidation of coal in a fixed-bed reactor under conditions of significant diffusional effects. The model defines individual effectiveness factors for carbonic gas formation, oxygen deposition, and water production, to modify a chemical model derived previously. Experiments carried out on seven coals ranging in rank from lignite to bituminous to anthracite. All the coals exhibited particle size effects and the results fit the model in most cases to within ±10% both in the rate of overall oxidation and with respect to the individual reaction rates for several products. Activation energies and effectiveness factors were obtained for each coal and for each reaction. The effluent gas $\text{CO}{}_2\text{CO}$ ratio was found to be virtually constant for all five bituminous coals, being independent of reaction temperature, coal particle size, and oxygen partial pressure.     

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment

Three kinds of subbituminous and bituminous coals with added potassium hydroxide were heated at 523 K in a nitrogen stream to transform thermally stable organic sulfur to reactive species. Extents of total sulfur removal were 27–52% during the course of alkali treatment, while weight loss was 8–13%. The extent of total sulfur removal was linearly proportional to the internal surface area of the parent coal. The parent coals and alkali-treated samples were pyrolysed rapidly in a free-fall reactor in a nitrogen stream at 1233 K. Under these conditions the alkali-treated samples lost more organic sulfur than did the parent coals. The observed changes in the content of sulfur forms were successfully simulated kinetically. The combined process of rapid pyrolysis with alkali leaching was effective for reduction of organic sulfur, except for a high-rank coal of small internal surface area.     

The removal of mercury from coal via sub-critical water extraction

The transformation behavior of mercury in two Chinese coals (WJP and HYS coal) during sub-critical water treatments was studied in a semi-continuous apparatus. Float–sink method, demineralization and sequential chemical extraction were used to study the occurrence mode of mercury in raw coals and extracted samples. The results show that with increasing temperature, pressure, water flow and extraction time, the removal of mercury increases. During sub-critical water treatment, the content of mercury associated with sulfates, monosulfides, disulfides, organic material, and insoluble forms decreases. The removal efficiency of mercury is almost 100% at 410 °C, 15 MPa, 0.58 l/h, and 60 min for HYS coal and 96.7% at 380 °C for WJP coal. Under the same temperature and pressure the mercury removal through pyrolysis is less than that through sub-critical water extraction which is an efficient method to remove most mercury from coal.     

An investigation of mercury distribution and speciation during coal combustion

A multi-field electrostatic precipitator (ESP) and a two-stage condensing heat exchanger (CHX^®) have been added to the pilot scale Vertical Combustion Research Facility (VCRF) in CETC-O to further research into integrated emissions control for coal fired power plants. A series of combustion trials were conducted on the VCRF with three different coals (bituminous, sub-bituminous and lignite) to study mercury distribution and speciation at various VCRF locations. Results showed that, with the bituminous coal, as the flue gas cools down from 700 to 200 °C, 80% of total mercury in the gas phase existed in oxidized form and 20% in elemental form. For sub-bituminous and lignite coals, elemental mercury was the dominant form throughout the system. Analysis of deposited ash samples showed that oxidized mercury can be absorbed on carbon-rich ash deposits, although overall only a very small percentage of total mercury was absorbed on the ash. The potential of the CHX^® at removing mercury from the flue gas was also explored. Results indicated that, using wet scrubbing, the CHX^® was able to remove 98% of oxidized mercury. Though elemental mercury went through the system unabated, it is suggested that, with appropriate agent to oxidize elemental mercury in the CHX^®, it is conceivable to use CHX^® to remove both oxidized and elemental mercury. Finally, mercury balance was performed and good mercury balance was obtained across the VCRF, validating our sampling procedures and analysis methods.     

钙基添加剂对不同煤种解耦燃烧下硫释放的影响

煤燃烧后排放的硫氧化物导致的环境污染问题已经引起人们的日益关注。基于一段式和两段式卧式炉,本文探究在传统燃烧和解耦燃烧条件下,温度、煤种以及CaO对燃煤释放SO₂规律的影响。试验结果表明：不同煤种的SO₂释放规律存在差异。随着温度的升高,不同煤种燃烧SO₂的释放量均不断增加,硫的动态析出曲线逐渐由单峰分布转化为双峰分布。传统燃烧模式下,添加的CaO对烟煤、无烟煤和褐煤脱硫效率可以达到70%以上,高氯煤脱硫效率则较低,仅有12.09%～20.45%;随温度升高,烟煤、褐煤和高氯煤的脱硫效率呈现先略微下降,后升高再下降的趋势,烟煤脱硫效率则逐渐降低。解耦燃烧模式下,CaO对烟煤、无烟煤脱硫效率在42.35%～76.23%,褐煤在21.35%～52.63%,高氯煤脱硫效率仍然较低,在8.93%～10.57%;随温度升高,烟煤、褐煤和高氯煤的脱硫效率呈现先增加后降低的趋势,烟煤脱硫效率逐渐降低。解耦燃烧模式下,不同煤种SO₂的总释放量大于传统燃烧模式,添加CaO后脱硫效率小于传统燃烧模式。     

Investigation into the fate of mercury in bituminous coal during mild pyrolysis

Two high-volatile bituminous coal (Lower Freeport #6A and Pittsburgh #8), used primarily for electricity production, were tested to determine the fate of their mercury content during mild pyrolysis. Mono-sized samples of the well characterized coals were tested under nitrogen in a horizontal tube furnace at different residence times at different temperatures throughout the range 275–600°C. The resulting char was analyzed for mercury, and compared to the original parent coal concentration. The percent Hg removal was found to be a function of both residence time and temperature. The data for both coals have shown two distinct regimes; a low temperature chemical evolution mechanism which follows an Arrhenius form (apparent activation energies for the Lower Freeport #6A and the Pittsburgh #8 coals are 25.6±1.5 and 21.7±1.9 kcal/mol respectively), and a higher temperature regime where the Hg evolution dramatically decreases. This can be attributed to the changing structure of the coal at these higher temperatures. The results of bomb calorimetry analysis performed on the Lower Freeport #6A coal samples verify that the overall heating value of the coal is essentially unaffected by mild pyrolysis at temperatures lower than 400°C.     

Influence of Preliminary Demineralization on the Alkylation of Brown Coal in a Microwave Field

The results of a study of the effect of preliminary demineralization in a microwave field on the interaction of brown coal with butanol in the presence of an acid catalyst are reported. The effectiveness of the preliminary demineralization of coal with a solution of hydrochloric acid before the extraction of bituminous substances from coals was demonstrated. The influence of microwave irradiation intensity on the processes of coal alkylation was considered. The coal matter alkylation process was represented as a set of consecutive and simultaneous reactions. The individual and group compositions of the modified products were analyzed. It was established that the alkylating activation of coal in a microwave field facilitated the modification of its organic components and the production of detarred waxes.