Characterization of tar from sawdust gasified in the pressurized fluidized bed

The paper investigated the features of tar releasing from sawdust gasification in a new designed pressurized fluidized bed. Gel permeation chromatography (GPC) was used to determine the average molecular weight and the molecular weight distribution of sawdust gasified tar, also a photodiode array (PDA) detector was used in parallel to analyze the Ultraviolet (UV) spectra from 210 to 380 nm. At the pressure of 0.5–2.0 MPa and 800 °C, tar had the similar molecular weight distribution and the average molecular weight, the UV spectra absorbed at 254 nm revealed tar of sawdust air gasification at different pressures had aromatic character which increased with the increment of the pressure. By increasing the temperature from 700 to 900 °C, the relative amount of different molecular weight distribution fractions showed that tar contained higher fractions of small molecular weight components. It so gave an indication that tar progressively broke down with the increment of temperature under gasification conditions. The results from the UV absorbed in 254 nm showed a similar aromatic characteristic between 700 and 900 °C, which expressed the presence of larger polynuclear aromatic ring systems, unsaturated conjugated long chain alkyl, heterocyclic structures and alkyl of heteroatom substitutions.     

Activation of raw pitch coke with alkali hydroxide to prepare high performance carbon for electric double layer capacitor

Powder of raw pitch coke was activated with alkali hydroxides at 500–900 °C to prepare carbon electrode of high capacitance for electric double layer capacitor (EDLC). KOH provided very high surface area of 2320 m²/g at 800 °C, while NaOH did moderate surface area of 1000 m²/g at 650–750 °C. High surface area provided by KOH led to a high capacitance per weight of 39 F/g. However, its capacitance per volume was as low as 16 F/ml. Although the coke of moderate surface area activated with NaOH showed a similar capacitance per weight, its capacity per volume was as high as 28 F/ml because of its high density. Adequate porosity must be selectively introduced by NaOH activation to the coke to obtain moderate surface area. Much smaller expansion of layers in the present needle type coke activated by NaOH than that by KOH is indicative for the higher density of the former activated coke.     

Evaporation characteristics of kerosene droplets with dilute concentrations of ligand-protected aluminum nanoparticles at elevated temperatures

The evaporation characteristics of kerosene droplets containing dilute concentrations (0.1%, 0.5%, and 1.0% by weight) of ligand-protected aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber were studied experimentally at different ambient temperatures (400–800 °C) under normal gravity. The evaporation behavior of pure and stabilized kerosene droplets was also examined for comparison. The results show that at relatively low temperatures (400–600 °C), the evaporation behavior of suspended kerosene droplets containing dilute concentrations of Al NPs was similar to that of pure kerosene droplets and exhibited two-stage evaporation following the classical d²-law. However, at relatively high temperatures (700–800 °C), bubble formation and micro-explosions were observed, which were not detected in pure or stabilized kerosene droplets. For all Al NP suspensions, regardless of the concentration, the evaporation rate remained higher than that of pure and stabilized kerosene droplets in the range 400–800 °C. At relatively low temperatures, the evaporation rate increased slightly. However, at relatively high temperatures (700–800 °C), the melting of Al NPs led to substantial enhancement of evaporation. The maximum increase in the evaporation rate (56.7%) was observed for the 0.5% Al NP suspension at 800 °C.     

Hydropyrolysis of a Turkish lignite (Tunçbilek) and effect of temperature and pressure on product distribution

The hydropyrolysis of a Turkish lignite (Tunçbilek) in a swept fixed bed reactor connected with a thermo-balance was performed at a heating rate of 3 °C/min up to 950 °C under 0.5 MPa, 1 MPa and 10 MPa hydrogen pressures. The formation rates of gaseous hydrocarbons, carbon oxides, water and tar were determined. The difference between the weight loss due to tar formation of the non-condensable total and the weight loss of the sample recorded continuously with the thermo-balance showed the corresponding curve of tar formation rate as a function of temperature. It was shown that the total conversion and the formation rate of the products during hydropyrolysis could be influenced by varying the pressure. On the other hand, the volatile matter evolved during pyrolysis was substantially increased in the presence of hydrogen and especially when elevated hydrogen pressure was used.     

Heat transfer and single-phase flow in internally grooved tubes

This study investigates heat transfer and flow characteristics of water flowing through horizontal internally grooved tubes. The test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. All test tubes were made from type 304 stainless steel. The length and inner diameter of the test tube were 2 m and 7.1 mm, respectively. Water was used as working fluid, heated by DC power supply under constant heat flux condition. The test runs were performed at average fluid temperature of 25 °C, heat flux of 3.5 kW/m², and Reynolds number range from 4000 to 10,000. The effect of grooved pitch on heat transfer and pressure drop was also investigated. The performance of the grooved tubes was discussed in terms of thermal enhancement factor. The results showed that the thermal enhancement factor obtained from groove tubes is about 1.4 to 2.2 for a pitch of 0.5 in.; 1.1 to 1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove.     

Characterization of Manihot residues and preparation of activated carbon

Rind, vascular system and pith of raw Manihot stems were studied in this work. The characterization included proximate and ultimate analysis and the determination of cellulose, hemicellulose, lignin, functional groups, acidic–basic groups, the point of zero charge, the specific surface area and the thermal behavior of each stem part. Carbonization was performed at 500 °C and 800 °C. The results indicated that the raw residues exhibit mainly acidic character in the order: rind > vascular system ≈ pith with 1.80, 1.35 and 1.35 mol kg^?1 acidic groups, respectively. The carbon obtained at 500 °C of the different parts of the stem was mesoporous and the carbons obtained at 800 °C from the vascular system exhibited 61% microporosity. The raw residues presented smaller specific surface area than the carbonized samples. They removed however, 95% of the acid dye methyl orange and 50% of the basic dye methylene blue.     

In-tube passive heat transfer enhancement in the process industry

Enhanced heat transfer surfaces are used in heat exchangers to improve performance and to decrease system volume and cost. In-tube heat transfer enhancement usually takes the form of either micro-fin tubes (of the helical micro-fin or herringbone varieties), or of helical wire inserts. Despite a substantial increase in heat transfer, these devices also cause non-negligible pressure drops.By making use of well-proven flow pattern maps for smooth tubes and the new ones for smooth and enhanced tubes, it is shown from the refrigerant condensation data that flow patterns have a strong influence on heat transfer and pressure drop. This is done for data obtained from in-tube condensation experiments for mass fluxes ranging from 300 to 800 kg/m² s at a saturation temperature of 40 °C, for refrigerants R-22, R-134a, and R-407C. The flow regimes, pressure drops, heat transfer coefficients, and the overall performance of three different tubes, namely a smooth-, 18° helical micro-fin-, and a herringbone micro-fin tube (each having a nominal diameter of 9.51 mm), are presented and compared to the performance of smooth tubes with helical wire inserts (with pitches of 5 mm, 7.77 mm and 11 mm corresponding to helical angles of 78.2°, 72°, and 65.3°, respectively).     

Performance and ageing of an anode-supported SOFC operated in single-chamber conditions

Anode-supported cells made of conventional materials were tested in single-chamber conditions under various CH₄/air gas mixtures. Methane-to-oxygen ratio (R_MIX) and nominal temperature between 600 and 800 °C both affect the performance of the cell. At a flow rate of 350 sccm, maximum values of power density (260 mW cm⁻²) and cell voltage (1.05 V) were obtained for R_MIX = 2 at 800 °C. However, short term ageing experiments show that the stability of the cells depends on R_MIX as well as the flow of current. Scanning electron micrographs (SEM) reveal some important changes in anode microstructure close to the fuel inlet that may be, assign to the volatilization of the nickel contained in the Ni–YSZ cermet.     

Thermodynamic equilibrium calculations of hydrogen production from the combined processes of dimethyl ether steam reforming and partial oxidation

Thermodynamic analyses of producing a hydrogen-rich fuel-cell feed from the combined processes of dimethyl ether (DME) partial oxidation and steam reforming were investigated as a function of oxygen-to-carbon ratio (0.00–2.80), steam-to-carbon ratio (0.00–4.00), temperature (100 °C–600 °C), pressure (1–5 atm) and product species.Thermodynamically, dimethyl ether processed with air and steam generates hydrogen-rich fuel-cell feeds; however, the hydrogen concentration is less than that for pure DME steam reforming. Results of the thermodynamic processing of dimethyl ether indicate the complete conversion of dimethyl ether to hydrogen, carbon monoxide and carbon dioxide for temperatures greater than 200 °C, oxygen-to-carbon ratios greater than 0.00 and steam-to-carbon ratios greater than 1.25 at atmospheric pressure (P = 1 atm). Increasing the operating pressure has negligible effects on the hydrogen content. Thermodynamically, dimethyl ether can produce concentrations of hydrogen and carbon monoxide of 52% and 2.2%, respectively, at a temperature of 300 °C, and oxygen-to-carbon ratio of 0.40, a pressure of 1 atm and a steam-to-carbon ratio of 1.50. The order of thermodynamically stable products (excluding H₂, CO, CO₂, DME, NH₃ and H₂O) in decreasing mole fraction is methane, ethane, isopropyl alcohol, acetone, n-propanol, ethylene, ethanol and methyl-ethyl ether; trace amounts of formaldehyde, formic acid and methanol are observed.Ammonia and hydrogen cyanide are also thermodynamically favored products. Ammonia is favored at low temperatures in the range of oxygen-to-carbon ratios of 0.40–2.50 regardless of the steam-to-carbon ratio employed. The maximum ammonia content (i.e., 40%) occurs at an oxygen-to-carbon ratio of 0.40, a steam-to-carbon ratio of 1.00 and a temperature of 100 °C. Hydrogen cyanide is favored at high temperatures and low oxygen-to-carbon ratios with a maximum of 3.18% occurring at an oxygen-to-carbon ratio of 0.40 and a steam-to-carbon ratio of 0.00 in the temperature range of 400 °C–500 °C. Increasing the system pressure shifts the equilibrium toward ammonia and hydrogen cyanide.     

Creep behavior of Ni-12 wt.% Al anodes for molten carbonate fuel cells

Ni-12 wt.% Al anodes are fabricated for use in molten carbon fuel cells by tape casting and sintering. Sintering is performed in three steps, first at 1200 °C for 10 min in argon, then at 700 °C for 2.5 h in a partial oxidation atmosphere (P_H2/P_H2O=10⁻²), and finally at 950 °C for 5 min, 30 min or 1.5 h in hydrogen. Three anodes with different phases or microstructures are produced at different reduction times. One anode contains three phases, namely Ni–Al solid solution, Ni₃Al, and Al₂O₃. The amount of Al₂O₃ is extremely small at 5 min. A second anode also contains the three phases with the amount of Al₂O₃ comparable with that of Ni₃Al at 30 min. Third anode contains two phases, i.e. Ni–Al solid solution and Al₂O₃ formed at 1.5 h. The creep strains measured for the three anodes after a 100-h creep test are practically the same with an average value of 0.85%.     

Thin yttria-stabilized zirconia electrolyte and transition layers fabricated by particle suspension spray

In order to develop high performance intermediate temperature (<800 °C) solid oxide fuel cells (SOFCs) with a lower fabrication cost, a pressurized spray process of ceramic suspensions has been established to prepare both dense yttria-stabilized zirconia (YSZ) electrolyte membranes and transition anode layers on NiO + YSZ anode supports. A single cell with 10 μm thick YSZ electrolyte on a porous anode support and ∼20 μm thick cathode layer showed peak power densities of only 212 mW cm⁻² at 700 °C and 407 mW cm⁻² for 800 °C. While a cell with 10 μm thick YSZ electrolyte and a transition layer on the porous anode support using a ultra-fine NiO + YSZ powder showed peak power densities of 346 and 837 mW cm⁻² at 700 and 800 °C, respectively. The dramatic improvement of cell performance was attributed to the much improved anode microstructure that was confirmed by both scanning electron microscopes (SEM) observation and impedance spectroscopy. The results have demonstrated that a pressurized spray coating is a suitable technique to fabricate high performance SOFCs and at lower cost.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Cresol–formaldehyde based carbon aerogel as electrode material for electrochemical capacitor

Carbon aerogels have been prepared through a polycondensation of cresol (Cm) with formaldehyde (F) and an ambient pressure drying followed by carbonization at 900 °C. Modification of the porous structures of the carbon aerogel can be achieved by CO₂ activation at various temperatures (800, 850, 900 °C) for 1–3 h. This process could be considered as an alternative economic route to the classic RF gels synthesis. The obtained carbon aerogels have been attempted as electrode materials in electric double-layer capacitors. The relevant electrochemical behaviors were characterized by constant current charge–discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy in an electrolyte of 30% KOH aqueous solution. The results indicate that a mass specific capacitance of up to 78 F g⁻¹ for the non-activated aerogel can be obtained at current density 1 mA cm⁻². CO₂ activation can effectively improve the specific capacitance of the carbon aerogel. After CO₂ activation performed at 900 °C for 2 h, the specific capacitance increases to 146 F g⁻¹ at the same current. Only a slight decrease in capacitance, from 146 to 131 F g⁻¹, was observed when the current density increases from 1 to 20 mA cm⁻², indicating a stable electrochemical property of carbon aerogel electrodes in 30% KOH aqueous electrolyte with various currents.     

Effect of pore size and surface area of carbide derived carbons on specific capacitance

This work presents a systematic study on how pore size and specific surface area (SSA) of carbon effect specific capacitance and frequency response behavior. Carbide derived carbons (CDC) produced by leaching metals from TiC and ZrC at temperatures from 600 to 1200 °C have highly tailorable microstructure and porosity, allowing them to serve as excellent model systems for porous carbons in general. BET SSA and average pore size increased with synthesis temperature and was 600–2000 m² g⁻¹ and 0.7–1.85 nm, respectively. Maximum specific capacitance in 1 M H₂SO₄ was found to occur at an intermediate synthesis temperature, 800 °C, for both ZrC and TiC derived carbons and was 190 and 150 F g⁻¹, respectively. Volumetric capacitance for TiC and ZrC derived carbons was maximum at 140 and 110 F cm⁻³. These results contradict an oft-reported axiom that increasing pore size and SSA, all other things being held constant, increases specific capacitance. A correlation between specific capacitance and SSA of micropores (less than 2 nm in diameter) has been shown. As expected, increasing pore size was found to improve the frequency response. However, CDCs with similar pore size distributions but obtained from different starting materials showed noticeable differences in impedance behavior. This highlights the importance of not only the pore size and specific surface area measured using gas sorption techniques, but also the pore shape or tortuousity, which is non-trivial to characterize, on energy storage.     

Fabrication and characterization of a YSZ/YDC composite electrolyte by a sol–gel coating method

The compatibility of a composite electrolyte composed of a yttria stabilized zirconia (YSZ) film and a yttria-doped ceria (YDC) substrate in a solid oxide fuel cell (SOFC) that can be operated under 800 °C was evaluated. The YSZ film coated on a YDC substrate was derived from a polymeric YSZ sol using a sol–gel spin coating method followed by heat-treatment at 1400 °C for 2 h. The SEM and XRD analysis indicated that there were no cracks, pinholes, or byproducts. The composite electrolyte comprising a YSZ film of 2 μm thickness and a YDC substrate of 1.6 mm thickness was used in a single cell performance test. A 0.5 V higher value of open circuit voltage (OCV) was found for the composite electrolyte single cell compared with an uncoated YDC single cell between 700 and 1050 °C and confirmed that the YSZ film was an electron blocking layer. The maximum power density of the composite electrolyte single cell at 800 °C, 122 mW/cm² at 285 mA/cm², is comparable with that of a YSZ single cell with the same thickness at 1000 °C, namely 144 mW/cm² at 330 mA/cm². The hypothetical oxygen partial pressure at the interface between the YSZ film and the YDC substrate for the composite electrolyte with the same thickness ratio at 800 °C is 5.58×10⁻¹⁸ atm which is two orders of magnitude higher than the equilibrium oxygen partial pressure of Ce₂O₃/CeO₂, 2.5×10⁻²⁰ atm, at the same temperature.     

LiFePO4/carbon cathode materials prepared by ultrasonic spray pyrolysis

Small crystallites LiFePO₄ powder with conducting carbon coating can be synthesized by ultrasonic spray pyrolysis. Cheaper trivalent iron ion is used as the precursor. The pure olivine phase can be prepared with the duplex process of spray pyrolysis (synthesized at 450, 550 or 650 °C) and subsequent heat-treatment (at 650 °C for 4 h). The results indicate that the pyrolysis temperature of 450 °C is appropriate for best results. The carbon coating on the LiFePO₄ surface is critical to the electrochemical performance of LiFePO₄ cathode materials of the lithium secondary battery, since the carbon coating does not only increase the electronic conductivity via carbon on the surface of particles, but also enhance the ion mobility of lithium ion due to prohibiting the grain growth during post-heat-treatment. The carbon of 15 wt.% evenly distributed on the final LiFePO₄ powders can get the highest initial discharge capacity of 150 mA h g⁻¹ at C/10 and 50 °C.     

Nano-structured spherical porous SnO2 anodes for lithium-ion batteries

Spherical porous tin oxide was fabricated via a spray pyrolysis technique. TEM revealed that the primary SnO₂ crystals had an average size of about 5 nm. The electrochemical measurements showed that the spherical porous SnO₂ samples have excellent cyclability, which can deliver a reversible capacity of 410 mAh g⁻¹ up to 50 cycles as a negative electrode for lithium batteries. The second step of the study was to thermal treat the initial tin oxide for 3 h at 600, 800, 1000 and 1200 °C, respectively, in order to identify the particle size effect on the electrochemical performance toward lithium. It was found that the morphologies of these spherical clusters could be maintained even after thermal treatment at 1200 °C. It was also proved that finer the size of the tin oxide particles the better the electrochemical performance.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

Pressure drop and heat transfer of square pin-fin arrays in in-line and staggered arrangements

This work experimentally studied the pressure drop and heat transfer of a square pin-fin array in a rectangular channel by using the transient single-blow technique. The variable parameters are the relative longitudinal pitch (X_L = 1.5, 2, 2.8), the relative transverse pitch (X_T = 1.5, 2, 2.8) and the arrangement (in-line or staggered). Compared with the open articles, the present relative pitches are smaller and independently variable. The performance of the square pin-fins as the cooling devices is compared with that of the circular pin-fins. Besides, empirical formulas for the pressure loss and the heat transfer are suggested. Finally, the optimal inter-fin pitches are provided based on the largest Nusselt number under the same pumping power, while the optimal inter-fin pitches of square pin-fins are X_T = 2 and X_L = 1.5 for the arrays in in-line arrangements as well as X_T = 1.5 and X_L = 1.5 for the arrays in staggered arrangements.     

Investigating the potential for energy,fuel, materials and chemicals production from corn residues (cobs and stalks) by non-catalytic and catalytic pyrolysis in two reactor configurations

The results of thermogravimetric analysis (TGA), non-catalytic and catalytic pyrolysis of corn cobs and corn stalks are reported in this paper. Pyrolysis took place in two different reactor configurations for both feedstocks: (1) fast pyrolysis in a captive sample reactor; and (2) non-catalytic slow pyrolysis and catalytic pyrolysis in a fixed-bed reactor. Experiments were carried out in atmospheric pressure at three temperatures: low temperature (360–380 °C), medium temperature (500–600 °C) and at high temperature (600–700 °C). The results of the experimental study were compared with data reported in the literature. Investigating the potential of corn residues for energy, fuel, materials and chemicals production according to their thermochemical treatment products yields and quality, it can be stated that: (a) corn stalks could be suitable raw material for energy production via gasification at high temperature, due to their medium low heating value (LHV) of pyrolysis gas (13–15 MJ/m³); (b) corn cob could be a good solid biofuel, due to the high LHV (24–26 MJ/kg) of the produced char; (c) additionally, corn cobs could be a good material for activated carbon production after being activated or gasified with steam, due to its high fixed carbon content(～74 wt%); (d) liquid was the major pyrolysis product from catalytic pyrolysis (about 40–44 wt% on biomass) for both feedstocks; further analysis of the organic phase of the liquid products were hydrocarbons and phenols, which make them interesting for chemicals production.