J.L Miranda

Prevention of spontaneous combustion in coal stockpiles: Experimental results in coal storage yard

The spontaneous ignition of coal stockpiles is a serious economic and safety problem. This paper deals with oxidation and spontaneous combustion of coal piles laid in coal storage yard and the measures to avoid the heat losses produced. Investigations on self heating were carried out with five test piles (2000-3000 tons) built at the ENDESA power station in Teruel (Spain), and the results are here reported. The efficiency of several measures to reduce the heat losses were tested: periodic compaction, the use of a low angle slope, protection of the coal stockpiled with an artificial barrier and covering it with an ash-water slurry made with fly ash from the same power station. Wind tunnel tests were used to design the wind barrier which was showed to be very effective although the results indicated that the best way to avoid the heat losses is the use of an ash-water slurry to cover the coal pile. A direct method to determine the coefficient of total losses was developed and the coefficients of heat losses and total losses were determined. The agreement between the temperatures measured by infrared thermography and thermocouples leads to the conclusion that this technique is also a very effective method to quantify heat emissions from coal piles.

Effect of the pyrolysis process on the physicochemical and mechanical properties of smokeless fuel briquettes

A greater understanding of the physics and chemistry of lignite–biomass briquetting could lead to better briquette performances and cost-effectiveness making these fuels more attractive to both producers and consumers. With this aim, chars obtained from different low rank coals and biomasses (sawdust, straw, olive stone and almond shell) were used to prepare smokeless fuel briquettes and their physicochemical and mechanical properties were studied depending on the pyrolysis conditions. Coal was pyrolysed at temperatures between 500 °C and 700 °C and the temperature chosen to carry out pyrolysis was 600 °C due to the lowest content of sulphur per thermie in the pyrolized material. In order to study the influence of the pyrolysis process on the properties of the briquettes, biomasses were pyrolysed separately at 400 °C and 600 °C and together with the coal at 600 °C of temperature. The materials pyrolysed at 600 °C showed a lower content of volatile matter and a higher calorific value than the standard levels reported in the literature for materials to prepare smokeless briquettes. The briquettes were prepared by mixing the pyrolysed materials with humates as binder and Ca(OH)2 as sulphur sorbent. The briquetting process was followed by FT-IR, scanning electron microscopy (SEM), CO2 adsorption and the mechanical properties were tested evaluating their impact resistance, water resistance and compression strength. The best briquettes with respect to the mechanical properties were those prepared with coal and biomasses co-pyrolysed at 600 °C although some of them fixed a higher percentage of sulphur during pyrolysis due to the metal content of the biomasses.

Curing temperature effect on mechanical strength of smokeless fuel briquettes prepared with molasses

Environmental acceptable smokeless fuel briquettes have been prepared with a low-rank coal and olive stone as biomass. The binder chosen for this study was molasses which acts with different roles, as chemical and matrix type. The effect of the curing temperature on these briquettes has been studied by Fourier transform infrared spectroscopy, temperature programmed decomposition (TPD) followed on-line by mass spectrometry and optical microscopy. TPD experiments help to predict the final properties of the briquettes more clearly than infrared spectroscopy. The aliphatic structures and methoxy groups as well as the hydrogen bonds decrease during the curing. On the other hand, the carboxylic groups tend to be formed due to the oxidation produced by the effect of curing temperature. In addition, the briquettes cured at 200 °C, 2 h showed the highest mechanical strength. These curing conditions also produce waterproof briquettes due to the presence of carboxylic groups which contribute to the stabilisation of the briquettes because of the formation of hydrogen bonds.

Visbreaking of an asphaltenic coal residue

A residue from deasphalting of liquids obtained by direct liquefaction of a Spanish subbituminous coal was processed by thermal treatment. This residue is rich in asphaltenes which do not undergo cracking easily and are coke precursors in thermal cracking. The kinetics of the cracking and coke formation reactions were studied and the viscosity, coke content, boiling point distribution, elemental analysis and aromaticity of the reaction products were determined. The experimental data fit the first-order kinetic model proposed. The main effects produced by the thermal treatment were a large decrease in the viscosity from 4608 mm2 s−1 for the feedstock to 939 mm2 s−1 for the product. The conversion of the heavy fraction (b.p. > 350°C and soluble in toluene) increased with the temperature and residence time, the conversion to coke being higher than the conversion to light products (b.p. < 350°C and soluble in toluene).

Two-stage liquefaction of a Spanish subbituminous coal

Abstract A Spanish subbituminous coal has been processed in two-stage liquefaction in a non-integrated process. The first-stage coal liquefaction has been carried out in a continuous pilot plant in Germany at Clausthal Technical University at 400°C of temperature, 20 MPa hydrogen pressure and anthracene oil as solvent. The second-stage coal liquefaction has been performed in continuous operation in a hydroprocessing unit at the Instituto de Carboquimica at 450°C and 10 MPa hydrogen pressure, with two commercial catalysts: Harshaw HT-400E (Co-Mo/Al 2 O 3 ) and HT-500E (Ni-Mo/Al 2 O 3 ). The total conversion for the first-stage coal liquefaction was 75.41 wt% (coal d.a.f.), being 3.79 wt% gases, 2.58 wt% primary condensate and 69.04 w% heavy liquids. The heteroatoms removal for the second-stage liquefaction was 97–99 wt% of S, 85–87 wt% of N and 93–100 wt% of O. The hydroprocessed liquids have about 70% of compounds with boiling point below 350°C, and meet the sulphur and nitrogen specifications for refinery feedstocks. Liquids from two-stage coal liquefaction have been distilled, and the naphtha, kerosene and diesel fractions obtained have been characterized.

Curing temperature effect on smokeless fuel briquettes prepared with molasses and H3PO4

Abstract Chars from the co-carbonisation of a low-rank coal and olive stones have been used to prepare environmental acceptable smokeless fuel briquettes. The blend was a mixture of char, molasses and H3PO4. This acid was added to favour the polymerisation of the binder. The effect of the curing temperature on the physico-chemical features of the briquettes was studied by Fourier Transform Infrared Spectroscopy and Temperature Programmed Decomposition followed by Mass Spectrometry. The presence of H3PO4 as well as the curing process at 200 °C of temperature, contribute to the formation of carboxylic acids which lead to the production of briquettes with adequate mechanical properties.

Product distribution and sulfur forms in the low temperature pyrolysis of a Spanish subbituminous coal

Abstract The product distribution from the low temperature pyrolysis of a high sulfur Utrillas coal is studied in the 350–600°C interval. Runs were carried out under helium (0.1 MPa) in a horizontal fixed bed reactor with a heating rate of 7°C min −1 . Gases from pyrolysis were analysed by gas chromatography and low temperature chars were characterized in terms of proximate and ultimate analyses, heating value and sulfur forms distribution. At 500°C and with a pyrolysis time of 1 hour the transformation of pyrite into pyrrhotite is almost complete. Organic sulfur tends to accumulate in higher temperature chars. A structural characterization of tars from different temperatures was made by FT-i.r. and 1 H n.m.r. spectroscopy.

Micro-FTIR study of the blend of humates with calcium hydroxide used to prepare smokeless fuel briquettes

Abstract The blend of humates with calcium hydroxide and the briquettes prepared with this blend have been used by Fourier Transform Infrared Microscopy Spectrometers. An adequate preparation of the microsamples allows to obtain good quality spectra to elucidate the coal constituents and additives of the blend. It has been followed the molecular changes produced by the effect of the temperature. It has been observed the decrease of the hydroxyl and methoxy groups and the increase of the carboxylates. Fuel briquettes have been prepared with carbonaceous materials which require the use of a binder to link particles. Moreover, a calcium additive has been sometimes added to reduce sulphur emissions during combustion. As a consequence of this addition, a chemical reaction is produced. It is observed that these prepared briquettes are waterproof.

Liquids from coal. Catalytic hydrogenation of two Spanish subbituminous coals

Abstract Two samples of Teruel (Spain) subbituminous coals were liquefied in batch type autoclave experiments with reaction temperature in the range 400 to 440°C, initial hydrogen pressure 80 kg/cm 2 , reaction time 60 min, anthracene oil as slurry vehicle agent and pyrite, CoMo and NiCo catalysts. Experiments without catalysts and with demineralized coal have also been carried out. Products of each experiment were balanced and separated into solid residue, gases and liquids. These were fractionated into oils, asphaltenes and preasphaltenes. The best results, on the basis of the highest conversion, the highest oil content and lowest gas formation, have been obtained with CoMo catalyst.

Application of petroleum processing technology to the upgrading of coal syncrude

The feasibility of producing distillate fuels (gasoline, diesel and aviation fuels) from coal syncrude was determined. A syncrude from direct liquefaction of subbituminous coal was first deasphalted by ultrasonic disaggregation in n-hexane, to reduce the heteroatom content. The oil was then catalytically hydrogenated in a continuous hydroprocessing unit at 450°C and 10 MPa hydrogen pressure. The hydroprocessed oil was fractionated by vacuum distillation and the cuts obtained were characterized. The high aromaticity of the hydroprocessed oil necessitated a second hydrotreatment under conditions suitable for hydrogenation of aromatic structures. Deasphalting removed 39% of the N, 43% of the S and 47% of the O. The first hydrotreatment removed 99% of the S, 87% of the N and 37% of the O, and increased the HC ratio. The second hydrotreatment provided an important reduction in aromaticity, necessary to achieve certain fuel quality specifications.