Experimental study on cotton stalk combustion in a circulating fluidized bed

This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO₂ and NO) in flue (stack) gas were measured.     

Combustion characteristics of cotton stalk in FBC

The present work reports studies on the mixing and combustion characteristics of cotton stalk with 10–100 mm in length in FBC. Experiments on a cold model show that cotton stalk cannot fluidize, and adding bed material can improve the fluidization condition. Cotton stalk can mix well with 0.6–1 mm alumina at fluidization number N = 3–7. However, when the fluidization number is higher more than 7, the mixing bed will exist a little segregation comparing with N = 3–7. Thermogravimetric experiments show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick. Fluidized-bed combustion of cotton stalk was tested in a 0.2 MW_th test facility. According to the temperature distribution along the bed height, when the primary and secondary air is adapted cotton stalk can be burned stably in the fluidized bed. During pure cotton stalk combustion tests, silica sand and alumina are used as bed material to compare their agglomeration characteristics. SEM/EDX analysis on agglomerate samples after combustion about 38 h suggests that the high alkali metals content causes the formation of the coating around silica sand particles. The coating consists of compounds with low-melting temperature results in agglomeration of silica sand particles. By contrast, alumina is difficult to react with alkali metals from biomass ash, and the agglomeration of alumina was not found at 910 °C. It is found that alumina is more favorable than silica sand particle for use in a fluidized bed in cotton stalk combustion.     

Effects of atmospheres on bonding characteristics of silver and alumina

Joints prepared using the silver–copper oxide based reactive air brazing (RAB) technique are known to experience a significant decrease in joint strength when exposed to a high-temperature reducing environment. To investigate the effects of ambient atmosphere on the bonding characteristics of ceramic joints brazed with Ag–CuO filler metals, alumina joints prepared using a series of Ag–CuO compositions were exposed to a reducing hydrogen atmosphere and re-oxidized in air at 800 °C. As previously reported, joints exposed only to hydrogen revealed significant reduction in flexural strength and exhibited interfacial de-bonding between the filler metal and the alumina substrate. In the case of the joints brazed with a filler metal containing a high copper content, 8 mol% CuO, the formation of interfacial porosity caused by the reduction of interfacial oxide phases led to an extremely weak interface, which was not recovered after subsequent reoxidation in air at 800 °C. However, no such microstructural change or formation of interfacial porosity was observed in joints brazed with filler metals containing no or low copper content and the substrate/filler metal interface remained intact even though interfacial strength was found to be relatively weak. Subsequent reoxidation of these joints resulted in the recovery of interfacial strength and flexural strength. The results clearly indicate that interfacial adhesion in this type of braze filler metal is significantly influenced by the oxidizing/reducing characteristics of the surrounding atmosphere during high-temperature exposure. XPS analysis conducted on the in situ fractured surfaces of as-brazed and hydrogen-treated samples prepared using a filler metal with 2 mol% CuO indicates that it is the concentration of oxygen in the silver matrix which is critical to the level of bond strength between silver and alumina. This was confirmed by exposing an equivalent set of joining specimens to an inert atmosphere at high temperature, which subsequently also displayed low flexural strengths and de-bonding along the silver/alumina interface.     

Hydrogen-rich gas from catalytic steam gasification of biomass in a fixed bed reactor: Influence of particle size on gasification performance

The catalytic steam gasification of biomass was carried out in a lab-scale fixed bed reactor in order to evaluate the effects of particle size at different bed temperatures on the gasification performance. The bed temperature was varied from 600 to 900 °C and the biomass was separated into five different size fractions (below 0.075 mm, 0.075–0.15 mm, 0.15–0.3 mm, 0.3–0.6 mm and 0.6–1.2 mm). The results show that with decreasing particle size, the dry gas yield, carbon conversion efficiency and H₂ yield increased, and the content of char and tar decreased. And the differences due to particle sizes in gasification performance practically disappear as the higher temperature bound is approached. Hydrogen and carbon monoxide contents in the produced gas increase with decreasing particle size at 900 °C, reaching to 51.2% and 22.4%, respectively.     

Effect of particle size on pyrolysis of single-component municipal solid waste in fixed bed reactor

According to the differences in components, three representative components (plastic, kitchen garbage and wood) in municipal solid waste (MSW) were pyrolyzed in a fixed bed reactor to evaluate the influence of particle size on pyrolysis performance of single-component municipal solid waste (MSW). The bed temperature was set at 800°C and each sample was separated into three different size fractions (0–5 mm, 5–10 mm and 10–20 mm). The results show for all the samples particle size has an effect on pyrolysis product yields and composition: smaller particle size results in higher gas yield with less tar and char; the decrease of particle size can increase H₂ and CO contents of gas, as well as the ash and carbon element contents in the char. And the influence is the much more significant for sample with higher fixed carbon and ash contents, such as kitchen garbage, and less for sample with higher volatile content, plastic in the test.     

Effects of ultra-high injection pressure and micro-hole nozzle on flame structure and soot formation of impinging diesel spray

The effects of ultra-high injection pressure (P_inj = 300 MPa) and micro-hole nozzle (d = 0.08 mm) on flame structure and soot formation of impinging diesel spray were studied with a high speed video camera in a constant volume combustion vessel. Two-color pyrometry was used to measure the line-of-sight soot temperature and concentration with two wavelengths of 650 and 800 nm. A flat wall vertical to the injector axis is located 30 mm away from the injector nozzle tip to generate impinging spray flame. Three injection pressures of 100, 200 and 300 MPa and two injector nozzles with diameters of 0.16 and 0.08 mm were used. With the conventional injector nozzle (0.16 mm), ultra-high injection pressure generates appreciably lower soot formation. With the micro-hole nozzle (0.08 mm), impinging spray flame shows much smaller size and lower soot formation at the injection pressure of 100 MPa. The soot formation is too weak to be detected with the micro-hole nozzle at injection pressures of 200 and 300 MPa. With eliminating the impact of injection rate on soot level, both ultra-high injection pressure and micro-hole nozzle have an obvious effect on soot reduction. Soot formation characteristics of impinging spray flame were compared with those of free spray flame using both the conventional and micro-hole nozzles. With the conventional nozzle, flat wall impingement deteriorates soot formation significantly. While soot formation characteristics of free spray flame with the micro-hole nozzle are not altered obviously by flat wall. Liquid length of the 0.16 mm nozzle is longer than the impingement distance and liquid length of the 0.08 mm nozzle is shorter than the impingement distance. Liquid impingement upon the wall is responsible for the deteriorated soot level of impinging flame compared to that of free flame with the conventional nozzle.     

CFD modeling of heat transfer of CO2 at supercritical pressures flowing vertically in porous tubes

Computational fluid dynamics (CFD) tool has been used for investigation of convective heat transfer of CO₂ in two porous tubes. Effects of some important parameters such as pressure, inlet temperature, mass flow rate, wall heat flux and porosity on temperature distribution and local heat transfer coefficients have been studied numerically. Near the supercritical conditions, these parameters are very effective on temperature gradient and local heat transfer coefficients. For example at p = 9.5 MPa, under the same conditions, the heat transfer coefficient in a tube with particle diameters of 0.1–0.12 mm is about 20–30% higher than when the particle diameter of 0.2–0.28 mm were used. The heat transfer coefficient increases with decreasing pressure and increasing mass flow rate. Also the porosity of the bed has the important role on the heat transfer. The CFD predictions have been compared to the experimental data and showed pretty good agreement.     

Anode-supported micro tubular SOFCs for advanced ceramic reactor system

In this study, micro tubular SOFCs under 1 mm diameter have been fabricated and investigated at 450–550 °C operating temperature with H₂ fuel. The performance of the 0.8 mm diameter tubular SOFC was 110–350 mW cm⁻² at 450–550 °C operating temperatures. To maximize the performance of the cell as well as to optimize the geometry of tubular cells, a current collecting method used in the experiment was examined. A model was proposed to estimate the loss of performance for single cell due to the current collecting method as functions of anode tube length and thickness. The results showed that the losses of performance were calculated to be 0.8, 2.0, and 4.6% at 450, 500, and 550 °C operating temperatures, respectively, for the 0.8 mm diameter tubular SOFC with the length of 1.2 cm.     

EINOx scaling in a non-premixed turbulent hydrogen jet with swirled coaxial air

The effect of swirl flow on pollutant emission (nitrous oxide) was studied in a non-premixed turbulent hydrogen jet with coaxial air. A swirl vane was equipped in a coaxial air feeding line and the angle of the swirl vane was varied from 30 to 90 degrees. Under a fixed global equivalence ratio of φ_G = 0.5, fuel jet air velocity and coaxial air velocity were varied in an attached flame region as u_F = 85.7–160.2 m/s and u_A = 7.4–14.4 m/s. In the present study, two mixing variables of coaxial air and swirl flow were considered: the flame residence time and global strain rate. The objective of the current study was to analyze the flame length behavior, and the characteristics of nitrous oxide emissions under a swirl flow conditions, and to suggest a new parameter for EINOx (the emission index of nitrous oxide) scaling. From the experimental results, EINOx decreased with the swirl vane angle and increased with the flame length (L). We found the scaling variables for the flame length and EINOx using the effective diameter (d_F,eff) in a far-field concept. Normalized flame length (L divided by d_F,eff) fitted well with the theoretical expectations. EINOx increased in proportion to the flame residence time (∼τ_R^1/2.8) and the global strain rate (∼S_G^1/2.8).     

Glass fiber entrapped sorbent for reformates desulfurization for logistic PEM fuel cell power systems

Glass fiber entrapped ZnO/SiO₂ sorbent (GFES) was developed to remove sulfur species (mainly hydrogen sulfide, H₂S) from reformates for logistic PEM fuel cell power systems. Due to the use of microfibrous media and nanosized ZnO grains on highly porous SiO₂ support, GFES demonstrated excellent desulfurization performance and potential to miniaturize the desulfurization reactors. In the thin bed test, GFES (2.5 mm bed thickness) attained a breakthrough time of 540 min with up to 75% ZnO utilization at 1 ppm breakthrough. At equivalent ZnO loading, GFES yielded a breakthrough time twice as long as the ZnO/SiO₂ sorbent; at equivalent bed volume, GFES provided a three times longer breakthrough time (with 67% reduction in ZnO loading) than packed beds of 1–2 mm commercial extrudates. GFES is highly regenerable compared with the commercial extrudates, and can easily be regenerated in situ in air at 500 °C. During 50 regeneration/desulfurization cycles, GFES maintained its desulfurization performance and structural integrity. A composite bed consisting of a packed bed of large extrudates followed by a polishing layer of GFES demonstrated a great extension in gas life and overall bed utilization. This approach synergistically combines the high volume loading of packed beds with the overall contacting efficiency of small particulates.     

Thermal characteristics of turbulent rib-grooved channel flows

Experimental work has been performed to examine the combined effects of rib-grooved turbulators on the turbulent forced convection heat transfer and friction characteristics in a rectangular duct under a uniform heat flux boundary condition. In the experiments, three types of rib-groove arrangements: rectangular-rib and triangular-groove (RR–TG), triangular-rib and rectangular-groove (TR–RG) and triangular-rib with triangular-groove (TR–TG), were examined. Measurements were carried out for the duct of one aspect ratio, AR = W/H = 20 and duct height, H = 9 mm with rib height, e = 3 mm at three pitch ratios, PR = P/e = 6.6, 10 and 13.3. Experiments were conducted for the Reynolds number range of 3000 to 10,000. Influences of rib-groove arrangements on the Nusselt number and friction factor have been discussed and compared with smooth duct results under similar test conditions. Isothermal friction factors were also taken and presented. The obtained results of the smooth duct are in good agreement with the previous studies found in the literature. Experimental results also show that the duct with RR–TG arrangement provides maximum heat transfer rate and friction factor than others. On the other hand, the thermal enhancement index obtained at constant pumping power reveals that the TR–TG provides the highest values for all pitch ratios studied. Finally, correlations for the heat transfer (Nu), friction factor (f) and the enhancement index (η) have been developed as a function of pitch ratio (PR) and Reynolds number (Re).     

Effect of nickel loading on hydrogen production and chemical oxygen demand (COD) destruction from glucose oxidation and gasification in supercritical water

Gasification and partial oxidation of 0.25 molar glucose solution was conducted over different metallic nickel (Ni) loadings (7.5, 11, and 18 wt%) on different catalyst supports (θ-Al₂O₃ and γ-Al₂O₃) in supercritical water. Experiments were carried out at three different temperatures (T) of 400, 450, and 500 °C at constant pressure of 28 MPa and a 30 min reaction time (t). For comparison, some experiments were conducted using high loading commercial catalyst (65 wt% Ni on Silica–alumina). Hydrogen peroxide (H₂O₂) was used as a source of oxygen in the partial oxidation experiments. Oxygen to carbon molar ratios (MR) of 0.5–0.9 were examined to increase the hydrogen production via carbon monoxide (CO) production. Results showed that in the absence of the catalyst, the optimum molar ratio was 0.8 i.e. 80% of the amount of oxygen required for complete oxidation of glucose. At a molar ratio of 0.8, the hydrogen yield was 0.3 mol/mol, as compared to 0.2 mol/mol glucose at molar ratio of 0.5 and 0.9. This optimized oxygen dose was adopted as a base line for catalysts evaluation. The main gaseous products were carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). Results also showed that the presence of Ni increased the total gas yield increased in the 7.5–18 wt Ni/Al₂O₃ catalyst. An increase in MR from 0.55 to 0.8 increased the of carbon dioxide and hydrogen yields from 1.8 to 3.8 mol/mol glucose and from 0.9 to 1.1 mol/mol. The carbon monoxide and methane yields remain constant at 2 and 0.5 mol/mol glucose, respectively. The introduction of hydrogen peroxide (H₂O₂) prior to the feed injection inhibited the catalyst activity and did not increase the hydrogen yield whereas the introduction of H₂O₂ after 15 min of reaction time increased the hydrogen yield from 0.62 mol/mol to 1.5 mol/mol. This study showed that approximately the same hydrogen yield can be obtained from the synthesized low nickel alumina loading (18 wt%) catalyst as with the 65 wt% nickel on silica–alumina loading commercial catalyst. The highest H₂ yield of 1.5 mol/mol glucose was obtained with commercial Ni/silica–alumina with a BET surface area of 190 m²/g compared to 1.2 mol/mol with the synthesized Ni/θ alumina with a BET surface area of 46 m²/g.     

Advances in anodic alumina membranes thin film fuel cell: CsH2PO4 pore-filler as proton conductor at room temperature

Anodic alumina membranes (AAM) filled with cesium hydrogen phosphate proton conductor have been tested as inorganic composite electrolyte for hydrogen–oxygen thin film (≤50 μm) fuel cell (TFFC) working at low temperatures (25 °C), low humidity (T_gas = 25 °C) and low Pt loading (1 mg cm⁻²). Single module TFFC delivering a peak power of around 15–27 mW cm⁻², with open circuit voltage (OCV) of about 0.9 V and short circuit current density in the range 80–160 mA cm⁻² have been fabricated. At variance with pure solid acid electrolytes showing reproducibility problems due to the scarce mechanical resistance, the presence of porous alumina support allowed to replicate similar fuel cell performances over numerous AAM/CsH₂PO₄ assemblies. A scale-up process of the electrodic area has been optimized in order to increase the delivered peak power of AAM thin film fuel cell. Morphological, chemical and electrochemical studies on the alumina composite electrolyte have been carried out by means of scanning electron microscopy, X-ray diffractometry, Micro-Raman spectroscopy, DTA/DTG analysis, ac impedance spectroscopy and single fuel cell tests.     

Effects of temperature dependent thermal conductivity on Nu number behavior in micro-tubes

The numerical modeling of the conjugate heat transfer and fluid flow through the micro-tube was presented in the paper. Three different fluids with temperature dependent fluid properties are considered: water and two dielectric fluids, HFE-7600 and FC-70. The diameter ratio of the micro-tube was D_i/D_o = 0.1/03 mm with a tube length L = 70 mm, geometry used in [D. Lelea, Nishio S., Takano K., The experimental research on microtube heat transfer and fluid flow of distilled water, International Journal of Heat and Mass Transfer 47 (2004) 2817–2830]. The laminar fluid flow regime is analyzed. Two different heat transfer conditions are considered: heating and cooling. The influence of the temperature dependent thermal conductivity on Nu number is analyzed for these two cases and compared with k = const.     

The effects of different entrance sections lengths and heating on free and forced convective heat transfer inside a horizontal circular tube

An experimental study was done for hydrodynamically fully developed and thermally developing laminar air flows in a horizontal circular tube has a 30 mm inside diameter and 900 mm heated length (L/D = 30) under a constant wall heat flux boundary condition, with different aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths of 600 mm (L/D = 20), 1200 mm (L/D = 40), 1800 mm (L/D = 60), and 2400 mm (L/D = 80). The Reynolds number ranged from 400 to 1600 and the heat flux is varied from 60 W m^− 2 to 400 W m^− 2. This paper examines the effects of the entrance sections lengths and heating on the free and forced convection heat transfer process. The surface temperature data were measured and heat transfer rates at different heat flux levels as well as different Reynolds numbers were calculated and correlated in the form of relevant parameters. The buoyancy force has a significant effect on the heat transfer and the combined convection factor was approximately varied form 0.13 ≤ Gr/Re² ≤ 7.125. It was found that the surface temperature increases as the entrance section length increases. It was inferred that the heat transfer decreases as the entrance section length increases due to the flow resistance and the mass flow rate. The proposed correlation was compared with available literature and with laminar forced convection and showed satisfactory agreement.     

High-resolution simulations of magnetohydrodynamic free convection in an enclosure with a transverse magnetic field using a velocity–vorticity formulation

The differential quadrature method (DQ) is employed to simulate the effect of a transverse magnetic field on buoyancy-driven magnetohydrodynamic (MHD) flow in an enclosure. The DQ numerical procedure is adopted for solving the velocity–vorticity form of Navier–Stokes equations in two dimensions. These equations together with respective (appropriate) boundary conditions are solved numerically using a DQ method by a coupled algorithm for the velocity–vorticity–temperature coupled together with a bi-conjugate gradient iterative solver technique. The velocity–vorticity formation is properly utilized to obtain results in the range of Grashof numbers (10⁴ ≤ Gr ≤ 10⁵), Hartmann numbers (0 ≤ Ha ≤ 100), and Prandtl numbers (0.01 ≤ Pr ≤ 10), as well as aspect ratios A = L/H varying from 1 to 3 in a differentially heated cavity with a transverse magnetic field. The algorithm is then employed to compute the average Nusselt numbers and flow parameters for all the cases. With support from the present simulations of the heat transfer characteristics for a constant value of Gr within the cavity, the heat transfer rate is at its maximum for higher Pr and in the absence of MHD effects (Ha = 0), while it is lower with increase in external magnetic field strength in the lower region of the Prandtl number.     

Oil shale pyrolysis kinetics and variable activation energy principle

A modified first order kinetic equation with variable activation energy is employed to model the total weight loss of Ellajjun oil shale samples. Fixed bed retort with 400 g of oil shale sample size is used in this study in 350–550 °C temperature range. Variable heating rate, h, in the range 2.6–5 °C min⁻¹ are tested.     

Experimental and numerical study of the natural convection from a heated horizontal cylinder wrapped with a layer of textile material

Natural convection heat transfer enhancement from a horizontal cylinder with a textile coating is studied experimentally and numerically. The coating layer may be used for two purposes. According to the thickness of the coating it may be used as an insulating material or for surface augmentation. In the experimental study, two cylinders having different diameters of 4.8 mm and 9.45 mm are used. The bare cylinders having a radius smaller than a certain critical size were wrapped with a textile material. Wrapped cylinder diameters were increased to 9 and 12.8 mm respectively after coating and constant heat flux was applied to all bare and wrapped cylinders. Experimental study was carried out at different ambient temperatures in a conditioned room which can be maintained at a stable required value and inside a sufficiently designed test cabin. The ambient and cylinder surface temperatures (T_∞ and T_w) varied between 10 °C – 40 °C and 20 °C – 60 °C respectively. Heat transfer rates from bare and wrapped horizontal cylinders were compared and heat transfer enhancement was observed. On the basis of the experimental data average Nusselt numbers were calculated and compared with the well known correlations on natural convection heat transfer from a horizontal cylinder in the specified range of Rayleigh number, and it is seen that the results are in good agreement. The problem is also investigated numerically. Experimental and the numerical results fall in ± 30% band.     

Hybrid filtration combustion of natural gas and coal

Rich and ultrarich combustion of natural gas in a porous medium composed of aleatory coal particles and alumina spheres was studied experimentally to evaluate the suitability of the concept for hydrogen and syngas production. Temperature, velocity and chemical products of the combustion waves were recorded experimentally in two stages: (1) natural gas in an inert porous medium at filtration velocities of 12, 15 and 19 cm/s for equivalence ratios (φ) from φ = 1.0 to φ = 3.8; (2) natural gas in a porous medium composed of coal and alumina particles for a range of volume coal fractions from 0 to 75% at φ = 2.3, and a filtration velocity of 15 cm/s. It was observed that the flame temperatures and hydrogen yields were increased with the increase of filtration velocity in inert porous media. In hybrid porous media the flame temperature decreased with an increase of coal fraction, and hydrogen and carbon monoxide were dominant partial oxidation products. Syngas yield in hybrid filtration combustion was found to be essentially higher than for the inert porous medium case. The maximum hydrogen conversion for the hybrid coal and alumina bed was ∼55% for a volumetric coal content of 75%.     

Thermal performance of a meso-scale liquid-fuel combustor

Combustion in small scale devices poses significant challenges due to the quenching of reactions from wall heat losses as well as the significantly reduced time available for mixing and combustion. In the case of liquid fuels there are additional challenges related to atomization, vaporization and mixing with the oxidant in the very short time-scale liquid-fuel combustor. The liquid fuel employed here is methanol with air as the oxidizer. The combustor was designed based on the heat recirculating concept wherein the incoming reactants are preheated by the combustion products through heat exchange occurring via combustor walls. The combustor was fabricated from Zirconium phosphate, a ceramic with very low thermal conductivity (0.8 W m⁻¹ K⁻¹). The combustor had rectangular shaped double spiral geometry with combustion chamber in the center of the spiral formed by inlet and exhaust channels. Methanol and air were introduced immediately upstream at inlet of the combustor. The preheated walls of the inlet channel also act as a pre-vaporizer for liquid fuel which vaporizes the liquid fuel and then mixes with air prior to the fuel–air mixture reaching the combustion chamber. Rapid pre-vaporization of the liquid fuel by the hot narrow channel walls eliminated the necessity for a fuel atomizer. Self-sustained combustion of methanol–air was achieved in a chamber volume as small as 32.6 mm³. The results showed stable combustion under fuel-rich conditions. High reactant preheat temperatures (675 K–825 K) were obtained; however, the product temperatures measured at the exhaust were on the lower side (475 K–615 K). The estimated combustor heat load was in the range 50 W–280 W and maximum power density of about 8.5 GW/m³. This is very high when compared to macro-scale combustors. Overall energy efficiency of the combustor was estimated to be in the range of 12–20%. This suggests further scope of improvements in fuel–air mixing and mixture preparation.