B. Rubio

DIFFERENT APPROACHES TO PROXIMATE ANALYSIS BY THERMOGRAVIMETRY ANALYSIS

Abstract The experimental optimization by the simplex method of the proximate analysis of coal and biomass by thermogravimetry analysis (TGA) is reported. Heating rate, final temperature, holding time, Ar flow rate and sample size were the control variables. The response function used was chosen to minimize the difference in percentage of volatile matter with the ASTM characterization. The relative accuracy of the method was demonstrated by determination of the volatile matter contents of a number of coals in parallel with the ASTM certified method. The method is successfully used with biomass samples.Authors acknowledge the European Commission for the financial support by contract No. ECSC 7220-PR/048. M.C.M. and M.T.I. are thankful to the Ministry of Science and Technology.

Influence of low-rank coal char properties on their SO2 removal capacity from flue gases: I. Non-activated chars

Abstract The SO2 adsorption capacity of a series of chars obtained by N2 pyrolysis at different temperatures from a low-rank coal with high ash content was assessed. The adsorption process was carried out at representative flue gas conditions (100 °C adsorption temperature, 1 second residence time, 10% H2O v/v, 6% O2 v v , 1000 ppmv SO2). Chars obtained by nitrogen pyrolysis at 500, 600, 700, 800 and 900 °C were characterized by the following techniques: XRD, AA, FTIR, Hg porosimetry, N2 and CO2 surface areas, and acid-base titration. Correlations between the performance of these chars and their physicochemical features were investigated. The SO2 adsorption capacity of pyrolized chars depends mainly on their surface area, but the content and type of functional groups and their accessibility are also important parameters to be considered.

Low cost adsorbents for low temperature cleaning of flue gases

The adsorption capacity with regard to SO2 of chars prepared from a low-grade coal has been evaluated at typical conditions of flue gas (presence of oxygen and water vapour and linear gas velocity of 0.12 m s−1) at an inlet concentration of 2 g SO2 m−3. The flue gases have been simulated, varying the water vapour and oxygen contents as well as the adsorption temperature which was varied between 80°C and 150°C. The study has focused on the influence of several process variables as well as the intrinsic features of the chars, such as their porous texture, on the SO2 adsorption capacity of the chars. Under the conditions studied, the positive effect of water vapour in the absence of oxygen has been observed. This effect can be explained by the intervention of oxygen functionalities in chars. Hero two effects overlap: the oxygen present in chars and their surface chemistry as well as the porous structure available. Temperature is also an important parameter in the SO2 adsorption efficiency. A minimum surface area is required for the temperature effect to be observed; otherwise the catalytic oxidation of SO2 to SO3 and subsequent storage of H2SO4 takes place to a very small extent and the negative effect of temperature increase to 150°C is not appreciable.

Effect of binder addition on the mechanical and physicochemical properties of low rank coal char briquettes

A series of chars obtained from a low rank coal by N2 pyrolysis at different temperatures was blended in different proportions with a commercial binder pitch and pressed at 125 MPa in a mould-and-plunger press. The briquettes obtained (1.5 cm long, 1 cm diameter) were subjected to a heat treatment consisting of a curing stage at 200°C in air followed by a carbonization stage in N2, at 500°C or 700°C depending on the starting char. A further stage of activation in steam was performed on the briquettes as well as on the chars. The mechanical properties of the briquettes were assessed by means of their impact resistance index and compressive strength, and their surface properties were determined by N2 B.E.T. surface area and pore volume measurements, as well as by acidic–basic titration. An increase in pitch content within the range tested (10–25% by weight) improves significantly the impact and compressive resistance of the briquettes. Whatever the amount of pitch used, the mechanical resistance of the briquettes decreases after activation. In both cases (before and after activation) the mechanical resistance of the briquettes depends on the starting char and the pitch content used. The changes, with respect to parent chars, found in surface properties of the briquettes were explained in relation to textural and acidic–basic properties of the pitch added, as well as to the changes undergone by the char itself during heat treatment. The effect of steam activation on these properties of the briquettes depends on the parent char, pitch content and the temperature of activation.

Aluminosilicates transformations in combustion followed by DSC

Abstract The behavior of quartz, kaolinite and calcite, as main components of coal mineral matter, has a direct impact on slags formations and development in pulverized coal combustion. The mineral transformations along temperature can be followed by differential scanning calorimetry (DSC), provided that the formation and crystallization of mullite (3Al2O3·2SiO2) are exothermic phenomena. Peak heights and areas allow to quantify the occurrence and extent of those mineral transformations for pure kaolinite and its mixture with calcite as fluxing agent. On the other hand, sinterization time at temperatures bellow mullite formation has a clear impact on the onset and the shape of the peaks. In this way, it is demonstrated that mineral transformations at solid state have a direct effect on the final structure of mineral species generated by slagging.

Influence of temperature and regeneration cycles on Hg capture and efficiency by structured Au/C regenerable sorbents.

The objective of this work is to evaluate a novel regenerable sorbent for mercury capture based on gold nanoparticles supported on a honeycomb structured carbon monolith. A new methodology for gold nanoparticles deposition onto carbon monolith support has been developed to obtain an Au sorbent based on the direct reduction of a gold salt onto the carbon material. For comparison purposes, colloidal gold method was also used to obtain Au/C sorbents. Both types of sorbents were characterized by different techniques in order to obtain the bulk gold content, the particle size distribution and the chemical states of gold after deposition. The mercury capture capacity and mercury capture efficiency of sorbents were tested in a bench scale facility at different experimental conditions. The regenerability of the sorbents was tested along several cycles of Hg capture-regeneration. High retention efficiencies are found for both types of sorbents comparing their gold content. Moreover, the high retention efficiency is maintained along several cycles of Hg capture-regeneration. The study of the fresh sorbent, the sorbent after Hg exposition and after regeneration by XPS and XRD gives insight to explain those results.

Mechanism of interaction of pyrite with hematite as simulation of slagging and fireside tube wastage in coal combustion

Abstract Iron-bearing minerals have been recognised as a mayor source of fire-side wall slagging in pulverised fuel boilers, which not only reduces the thermal efficiency of heat transfer of the exchangers surface, but also affects its integrity as a result of corrosion and erosion. Nevertheless, the root cause of adhesion and growing of deposits has not been clearly addressed. Our approach suggests that differential scanning calorimetry (DSC) combined with simultaneous thermogravimetry can follow the chemical reaction between pyrite and the outer layer of iron oxide on tubewalls. The changes in composition are followed by X-ray diffraction (XRD). The results indicated that the mechanism of wetting and adherence of molten pyrite over iron oxides is chemically induced: both di and mono iron sulphides interact with the oxide layer, changing the oxidation state of iron in oxide scale, from hematite to magnetite. This would imply a change in the protective ability of the scale as well as a great increment in corrosion tendency.

Tail-end Hg capture on Au/carbon-monolith regenerable sorbents

In this work, a regenerable sorbent for Hg retention based on carbon supported Au nanoparticles has been developed and tested. Honeycomb structures were chosen in order to avoid pressure drop and particle entrainment in a fixed bed. Carbon-based supports were selected in order to easily modify the surface chemistry to favour the Au dispersion. Results of Hg retention and regeneration were obtained in a bench scale experimental installation working at high space velocities (for sorbent, 53,000 h(-1); for active phase, 2.6 × 10(8) h(-1)), 120 °C for retention temperature and Hg inlet concentration of 23 ppbv. Gold nanoparticles were shown to be the active phase for mercury capture through an amalgamating mechanism. The mercury captured by the spent sorbent can be easily released to be disposed or reused. Mercury evolution from spent sorbents was followed by TPD experiments showing that the sorbent can be regenerated at temperatures as low as 220 °C.

Analytical Determination of Polycyclic Aromatic Hydrocarbons in Gases from Coal Conversion by Synchronous Fluorescence Spectrometry.

Abstract A method to analyze the most hazardous Polyclyclic Aromatic Hydrocarbons (PAH) (acenaphtene, anthracene, benzo(a) anthracene, benzo(a)pyrene, biphenyl, coronene, chrysene, dibenzo (a,h) anthracene, phenantrene, fluoranthene, fluorene, naphtalene and pyrene), by using excitation and energy constant synchronic fluorescence has been researched in depth in this study. Spectral studies carried out allow characteristic peaks to be obtained for the qualitative identification of 8 from 11 PAH tested. From the quantitative and interference studies, the most important analytical characteristics (linear range, detection limit and reproduciblity) for the determination of ten of these compounds have been obtained. The method was applied to the PAH determination in two different samples: 1) air filter samples for urban pollution control and 2) air samples from a laboratory scale coal carbonization oven. Fluorene (in the first type) and benzo (a) pyrene (in the second type), were identified and quantified.

DSC study of curing in smokeless briquetting

Abstract Briquettes potentially suited for smokeless fuel were successfully prepared from mixtures of previously pyrolyzed coal and biomass, and mixed with sugar cane molasses as binder. Briquettes were cured at 200°C in air for different times, and the physical performance of the cured briquettes were studied using the water and impact resistance tests (WRI and IRI). Since curing was an exothermic process, the residual heat of reaction evolved was considered to be an indication of the degree of curing. In this work, the degree of curing as a function of time was monitored quantitatively, measuring the residual heat of reaction at each condition studied by differential scanning calorimetry (DSC), and compared with the physical characterization. The results implied that the technique did not give a fast alternative to WRI and IRI tests, but was very useful in describing the mechanism by which the molasses behaved as binders.Authors acknowledge the European Commission for the financial support by Contract No. ECSC 7220-EA/133. M.C.M. and M.T.I. are thankful to the Ministry of Science and Technology.