Fuel Processing Technology (v.86, #2)

Behavior of a high-capacity steam boiler using heavy fuel oil by Jorge Barroso; Félix Barreras; Javier Ballester (89-105).
Problems related to the combustion of heavy oils with high vanadium contents in a high-capacity steam boiler have been analyzed. In this study, two types of additives have been tested both to diminish high-temperature corrosion and to obtain more brittle deposits. Results show that heavy fuel oil can be suitably burned in this type of boilers if it is chemically treated with an anticorrosive additive. In this research, the best results have been obtained when the heavy fuel oil was mixed with an organometallic additive. The influence of fouling on the behavior of some operational parameters such as the pressure in furnace and pressure drop in superheaters and pipe metal temperature, among others, has been verified. It is concluded that monitoring of these parameters can provide direct information on the degree of fouling, as well as of the effectiveness of the treatment during normal boiler operational conditions. Some other improvements in the atomization and combustion processes due to the use of the organometallic additive have also been observed.
Keywords: High-temperature corrosion; Vanadium; Magnesium; Additive; Steam boilers;

An experimental study has been performed on a high-capacity steam boiler burning heavy fuel oil to assess cold-end corrosion damages. In this second part of the research, acid corrosion in the rotary continuous-regenerative air heaters (CRAHs) has been analyzed. Corrosion potentiality has been evaluated from both qualitative and quantitative viewpoints. Results have shown that acid corrosion in the CRAHs is reduced when the low-quality heavy fuel oil is mixed with a magnesium-based additive. In this research, two commercial additives have been tested: a magnesium oxide-based slurry and an organometallic one. The best results have been obtained when the organometallic additive was used in the treatment of the heavy fuel. For this experimental condition, an increase in the useful lifetime of the pie-shaped baskets, as well as a decrease in both the acid dew point temperature (ADT) at the stack gases and the pressure drop (fouling) on the CRAHs have been confirmed.
Keywords: Acid corrosion; Vanadium; Magnesium; Additive; Steam boilers;

Three supported nickel catalysts were prepared by conventional impregnation (IMP), sol–gel and water-in-oil microemulsion (ME) methods, respectively. Their catalytic activities and coking resistant for partial oxidation of methane (POM) to syngas were studied in a fixed-bed reactor under atmospheric pressure. The crystalline structure and catalytic properties for three catalysts were investigated by using X-ray diffraction (XRD), transmission election microscopy (TEM) and temperature-programmed reduction (TPR). For partial oxidation of methane to syngas, although three catalysts had comparable activity, they showed a great difference in the way of coking resistant. The catalyst prepared by microemulsion method had excellent coking resistant with no obvious coke observed even after 5 h of reaction on stream, under thermodynamically severe conditions (O2/CH4=0.53) at 923 K. Due to nickel strong interaction with supported Al2O3 and high BET surface area, and small metallic Ni particles, the catalyst prepared by water-in-oil microemulsion has excellent coking resistant and stability.
Keywords: Partial oxidation of methane; Syngas; Water-in-oil microemulsion; Coking resistant;

Textural characterization of chars using fractal analysis by Manuel Mahamud; Óscar López; José Juan Pis; Jesús Alberto Pajares (135-149).
The aim of this study is to explore the potential of fractal analysis in helping to understand the textural changes of materials during the manufacture of active carbons. Textural characterization of the chars is carried out in order to obtain a better understanding of the phenomena underlying char formation. The materials selected for study were a series of chars obtained from coals oxidized in air at various temperatures for different periods of time. The data from mercury porosimetry were analyzed using fractal models. The average fractal dimensions for the chars were calculated by using the methods proposed by Friesen and Mikula and that of Zhang and Li. Fractal profiles of the chars obtained by the method of Neimark were compared with the corresponding fractal profiles of the precursor coals. Pore development during carbonization depends—among other factors that are kept constant in this study—on the textural properties of the precursor coal, the devolatilization process and the plastic properties of coals. The evolution of the fractal characteristics of the chars is also studied. At the same time pore volume development is analyzed. These analyses help to clarify the role that various phenomena occurring during carbonization have on the textural properties of the chars.
Keywords: Mercury porosimetry; Fractal analysis; Char texture; Coal oxidation;

Naphthene is generally considered difficult to convert in traditional pyrolysis, but the ring rupture becomes fairly easy with the presence of oxygen in the gas phase oxidative cracking of the model compound, cyclohexane. About 86.8% conversion of cyclohexane, 43.7% yield of light alkenes, 6.6% yield of benzene and 14.3% yield of CO could be obtained at 750 °C, at which temperature the pyrolysis of cyclohexane was negligible, while at 850 °C, the total yield of alkenes, benzene and CO was as high as 80% (50%, 12% and 18%, respectively) with 98% conversion of cyclohexane. The gas phase oxidative cracking process could be run in an autothermal way (cyclohexane/O2 mole ratio of 0.69–0.8 in theory), which would minimize energy consumption and capital costs of the whole process. CO prevailed in the produced CO x and the yield of CO2 was always below 1%, which means about 90% of CO2 emission by fuel burning in pyrolysis would be saved. The gas phase oxidative cracking process appears to be an environmentally benign and efficient route for light alkene production with naphthene rich feedstocks.
Keywords: Gas phase oxidative cracking; Cyclohexane; Light alkenes; Low CO2 emission;

Sulfurization remarkably affects the physical and chemical properties of dispersed catalysts. In this study, molybdate is taken as catalyst precursor for slurry-bed hydroprocessing of residuum to study the mechanics of sulfurization. Several conditions are used to evaluate the activities of the sulfurized catalysts, and XRD, TEM and BET are employed to characterize the sulfurized catalysts. The results show that molybdate is firstly transferred into some kind of crystallized oxothiomolybdate and then mainly into micro-crystallized MoS2 in the end. The dispersivity of the MoS2 granules is greatly enhanced compared to the oxothiomolybdate. XPS analysis shows the portion of MoIV in molybdenum species on the surface of the catalysts is enhanced by the addition of NH4Cl during sulfurization. Therefore, the catalytic activity, measured using anthracene as chemical probe in the practical reaction system of hydroprocessing for residue, is also enhanced.
Keywords: Slurry-bed; Hydroprocessing; Water-soluble; Catalyst; Sulfurization;

A technique for selectively separating approximately 65 wt.% of the Si(IV) in coal has been developed. The technique first uses aqueous hydrofluoric acid (HF) to react with aluminosilicates and quartz to form fluoride complexed Al and Si species in solution. Aluminium cations, in the form of Al(NO3)3, are then added to the solution to complex fluoride as AlF2 + and hydrolyse the silicon fluoride species to silicon hydroxide, which precipitates as a gel and is removed by filtration. The solution is then distilled to recover a water stream, a nitric acid stream and a solid residue. The water stream is used to pyrohydrolyse the solid residue at temperatures in excess of 500 °C to liberate HF for recycling. To complete the circuit, the solid remaining after pyrohydrolysis is treated with the nitric acid stream to produce Al(NO3)3 for recycling. The technique satisfies the objective of not requiring on-going purchase of chemicals. The application of this work is primarily as part of a process for producing ultra-clean coal. As it is a technique for the selective separation of Al and Si from aluminosilicates, it may have application in other areas of mineral processing.
Keywords: Silica; Selective separation; Aluminosilicates; Ultra-clean coal; Hydrofluoric acid; Re-generation;

The paper explores several alternative ways for improving the performance of zinc ferrites, which are common high-throughput sorbents used in hot gas desulphurisation processes (IGCC). The catalysts have been prepared by supporting the pure oxides (zinc and iron oxides) in porous supports (sepiolite, ZrO2 and zeolite) that provides the necessary strength. The active phase (zinc ferrite) is “in situ” synthesized as combined mixed oxides with a spinel structure, produced after calcination at high temperature. It has been found that the preparation method has a strong influence on the spatial distribution of the chemical species inside the pellet. In some cases, it is even possible to produce a profile where metallic oxides concentrate near the outer edge of the particle. According to XRD, it is found a preferential formation of enstatite phase rather than franklinite, due to substitution of interstitial ions in the framework of supports for Fe2+ and Zn2+. These conclusions set up strong requirements for the synthesis of specific sorbents in FGD reactions. This can be achieved by means of a combination of different preparation methodologies, which allow selecting the metallic ions and the supports in order to build the active spinels required for the FGD reaction.
Keywords: Zinc ferrites; Supported sorbents characterisation; Hot coal gas desulphurisation;

Supercritical gas extracts from low-quality coals: on the search of new precursors for carbon materials by Roberto Garcı́a; Ana Arenillas; Fernando Rubiera; Sabino R. Moinelo (205-222).
This paper studies the chemical composition of several supercritical gas (SCG) extracts and its influence on the thermal behaviour under carbonisation conditions. The extracts were obtained from a Spanish lignite (Mequinenza), a low-quality coal from the point of view of energy applications. The lignite was treated with toluene, ethanol (EtOH) and tetrahydrofuran (THF) as solvents under different supercritical temperature and pressure conditions. The extracts display high aliphatic nature and enhanced concentrations of oxygen functional groups, aided by the contribution of hydrogenation and oxygen incorporation reactions occurring in the SCG extraction with EtOH and THF. Thiophenic compounds are also present in great concentrations derived from the exceptionally high organic sulphur content of the parent coal. The carbonisation of the extracts renders anisotropic material with fine mosaic texture, as a consequence of the significant thermal reactivity inferred by the aliphatic and oxygenated groups. The size of the mosaic increases with the temperature of the SCG extraction and varies with the supercritical solvent in the order: toluene<EtOH<THF.
Keywords: Supercritical gas extracts; Carbonisation; Thermal reactivity; Mesophase;

Characterization of alkylated aromatic sulphur compounds in light cycle oil from hydrotreated vacuum gas oil using GC-SCD by Ulf Nylén; Juana Frontela Delgado; Sven Järås; Magali Boutonnet (223-234).
A light cycle oil (LCO) fraction in the boiling point range 220–337 °C was successfully characterized in detail with respect to 29 individual aromatic sulphur compounds and total sulphur content using high-resolution gas chromatography with sulphur chemiluminescence detection (HRGC-SCD). The resulting chromatogram, although very complex with hundreds of different alkyl-substituted aromatic sulphur derivatives, was well resolved owing to the high performance Equity-1 column and the optimised GC method. A shift in the sulphur distribution towards the heavier homologues was observed due to the origin of the LCO, derived from hydrotreated vacuum gas oil.
Keywords: Aromatic sulphur compounds; Light cycle oil (LCO); Analysis; High-resolution gas chromatography (HR-GC); Sulphur chemiluminescence detector (SCD);

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