M. Rosa Martínez-Tarazona

Phase-mineral and chemical composition of coal fly ashes as a basis for their multicomponent utilization. 1. Characterization of feed coals and fly ashes

Abstract The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulphide–sulphate authigenic mineral tendency. Their mineral composition includes quartz, kaolinite, illite–muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate composition with higher concentrations of alkaline and alkaline-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the respective mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral composition (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite–metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional separation procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere concentrate; (2) a water-soluble salt concentrate; (3) a magnetic concentrate; (4) a char concentrate; (5) a heavy concentrate; and finally (6) an improved FA residue.

The fate of trace elements and bulk minerals in pulverized coal combustion in a power station

The behaviour of 15 trace elements (As, Ba, Cr, Cu, Mn, Mo, Nb, Ni, Pb, Rb, Sr, V, Y, Zn and Zr) and 10 major and minor elements (Al, Ca, Fe, K, Mg, Na, P, S, Si and Ti) in coal during combustion in a power station has been studied. Synchronized sampling of pulverized coal, bottom ash and fly ash was undertaken over a limited time period. Fly ash morphology was studied by SEM and the mineral composition was studied by EDX and XRD. Major, minor and trace elements were determined by XRF and AAS. Differences between the composition of the ashes of pulverized coal, bottom ashes and fly ashes have been observed. As, Cu, Mo, Pb and Zn were concentrated in the fly ash. The relationship between the composition of the fly ashes and their particle size was studied. Enrichment factors were calculated for each element in different size fractions. As the particle sizes of fly ash decrease, the concentrations of As, Cu, Mo, Pb and Zn increase. From the different composition of bottom ashes and fly ashes (and relying on the results of the characterization of the feed coal carried out in previous work), it can be assumed that pyrite and carbonates make a greater contribution to the furnace bottom ashes. Quartz carries through into the fly ash. This mineral is almost absent in the finest fractions, reflecting the absence of small quartz particles in the feed coal.

Retention of mercury in activated carbons in coal combustion and gasification flue gases

To avoid the emission of toxic mercury compounds from coal combustion and gasification, efficient gas cleaning systems need to be developed. In this work, the effectiveness of activated carbons for retaining mercury in gases from coal gasification was evaluated and contrasted with the results obtained in a coal combustion atmosphere. The performance of a sulphur-loaded carbon (RBHG3) was compared with that of the same carbon without sulphur (RB3). Minor differences were observed in the two atmospheres studied. The retention of mercury at 120 °C was close to 30% in RB3 and up to 70% in RBHG3.

Cleaning of Spanish high-rank coals by agglomeration with vegetable oils

The aim of this work was to study the feasibility of using vegetable oils for coal agglomeration. Three Spanish anthracites were agglomerated with refined sunflower and soybean oils. The response of coals to agglomeration with these oils was evaluated by measuring the percentages of organic matter recovery and ash and pyritic sulfur rejections. The influence of oil type and concentration on agglomeration results was investigated. In addition, results were compared with those previously obtained in the agglomeration of these coals with n-heptane. It can be concluded that refined sunflower and soybean oils are suitable for use as agglomerants for these high-rank coals, especially to clean those coals with a high content of mineral matter.

Effect of oxy-combustion flue gas on mercury oxidation.

This study evaluates the effect of the gases present in a typical oxy-coal combustion atmosphere on mercury speciation and compares it with the mercury speciation produced in conventional air combustion atmospheres. The work was performed at laboratory scale at 150 °C. It was found that the minor constituents (SO2, NOx, and HCl) significantly modify the percentages of Hg(2+) in the gas. The influence of these species on mercury oxidation was demostrated when they were tested individually and also when they were blended in different gas compositions, although the effect was different to the sum of their individual effects. Of the minor constituents, NOx were the main species involved in oxidation of mercury. Moreover, it was found that a large concentration of H2O vapor also plays an important role in mercury oxidation. Around 50% of the total mercury was oxidized in atmospheres with H2O vapor concentrations typical of oxy-combustion conditions. When the atmospheres have similar concentrations of SO2, NO, NO2, HCl, and H2O, the proportion of Hg(0)/Hg(2+) is similar regardless of whether CO2 (oxy-fuel combustion) or N2 (air combustion) are the main components of the gas.

Regenerable sorbents for mercury capture in simulated coal combustion flue gas.

This work demonstrates that regenerable sorbents containing nano-particles of gold dispersed on an activated carbon are efficient and long-life materials for capturing mercury species from coal combustion flue gases. These sorbents can be used in such a way that the high investment entailed in their preparation will be compensated for by the recovery of all valuable materials. The characteristics of the support and dispersion of gold in the carbon surface influence the efficiency and lifetime of the sorbents. The main factor that determines the retention of mercury and the regeneration of the sorbent is the presence of reactive gases that enhance mercury retention capacity. The capture of mercury is a consequence of two mechanisms: (i) the retention of elemental mercury by amalgamation with gold and (ii) the retention of oxidized mercury on the activated carbon support. These sorbents were specifically designed for retaining the mercury remaining in gas phase after the desulfurization units in coal power plants.

Analytical methods for mercury analysis in coal and coal combustion by-products

The work was performed thanks to several financial sources. The project CTM2004-04252-CO2-02/TECHNO from the Spanish National Research Plan, the support of CSIC (Spanish Nacional Research Council) for providing M. Antonia Lopez-Anton with a postdoctoral position, and the help of the Asturias Research Programme for funding both a doctoral fellowship for Raquel Ochoa-Gonzalez and the project PC10-20.

Effects of oil concentration and particle size on the cleaning of Spanish high-rank coals by agglomeration with n-heptane

The response of three high-ash Spanish anthracites to agglomeration with n-heptane was evaluated by measuring the organic matter recovery, ash rejection and pyritic sulfur rejection. The influence of oil concentration and coal particle size on the agglomeration results was investigated. The combustible recoveries of these coals increased to a maximum as the n-heptane concentration is raised, and then decreased. Ash rejection fell as the coal recovery increased. The effect of oil concentration on pyritic sulfur rejection was comparable with that on ash rejection. Regarding ash, pyritic sulfur was removed from one coal with noticeable selectivity. Ash rejection and organic matter recovery were improved by grinding one coal to <45 μm. Coal particle size had a more pronounced effect than n-heptane concentration on selectivity of oil agglomeration for cleaning this coal.

Organic affinity of trace elements in Asturian bituminous coals

Abstract The distribution of a number of elements between organic and mineral parts was studied for three coals ranging from high-volatile to low-volatile bituminous from the Asturian Central basin (NW Spain). Nine major and minor elements (Al, Ca, Fe, K, Mg, Na, P, Si and Ti) and 13 trace elements (Ba, Cr, Cu, Mn, Nb, Ni, Pb, Rb, Sr, V, Y, Zn and Zr) were analysed by X-ray fluorescence spectrometry. Various calculation procedures, based either on the elemental concentration-ash content relation or on the detrital mineralogy, were used to establish the organic affinities of these elements. All the methods provided similar results. It was concluded that detrital and diagenetic minerals are the hosts for most of the trace elements. Traces concentrated in the lightest coal fractions cannot be regarded as bound to organic matter, but as forming part of fine-grained detrital minerals which are homogeneously distributed in the organic matter of coals.

On the role of oil wetting in the cleaning of high rank coals by agglomeration

Abstract The wettability of three Spanish anthracites with a large mineral matter content was studied by measuring the oil/water/coal contact angle with heptane and with crude and refined vegetable oils. The captive drop method was used for this purpose. The contact angle was found to depend on the coal, oil, and an interaction between coal and oil. Particle agglomeration tests were conducted to determine the maximum recovery of organic matter for each combination of coal and oil. The maximum recovery was observed to correlate well with the three-phase contact angle when heptane was employed as an agglomerant for the different anthracites. When vegetable oils were employed as agglomerants, the maximum recovery was also observed to correlate well with the contact angle in half the cases. For the other half the recovery exceeded what would have been predicted by the contact angle. Therefore, for these cases the oil wettability of the coal seemed greater under dynamic conditions than under static conditions.