Honghi Tran

Pretreatment of pulp mill secondary sludge for high-rate anaerobic conversion to biogas

Three pretreatment methods were compared based on their ability to increase the extent and rate of anaerobic bioconversion of pulp mill secondary sludge to biogas. The pretreatment technologies used in these experiments were: (i) thermal pretreatment performed at 170 degrees C; (ii) thermochemical (caustic) pretreatment performed at pH 12 and 140 degrees C; and (iii) sonication performed at 20 kHz and 1 W mL(-1). Sludge samples were obtained from a sulfite and a kraft pulp mill, and biochemical methane potential (BMP) assays were performed using microbial granules obtained from a high-rate anaerobic digester operating at a pulp mill. Biogas production from untreated sludge was 0.05 mL mg(-1) of measured chemical oxygen demand (COD) and 0.20 mL mg(-1) COD for kraft and sulfite sludge, respectively. Thermal pretreatment had the highest impact on sludge biodegradability. In this case, biogas yield and production rate from sulfite sludge increased by 50% and 10 times, respectively, while biogas yield and production rate from kraft sludge increased by 280% and 300 times, respectively. Biogas yield correlated to soluble carbohydrate content better than soluble COD.

Fouling Tendency of Ash Resulting From Burning Mixtures of Biofuels

Specified mixtures of peat with bark and peat with straw were burned in a lab-scale entrained flow reactor that simulates conditions in the superheater region of a biomass-fired boiler. Deposits were collected on an air-cooled probe that was inserted into the reactor at the outlet. For both mixtures, the deposition behaviour followed a non-linear pattern, which suggests that physico-chemical interaction between the ashes of the different fuels has taken place. The results indicate that it is possible to burn up to 30 wt-% bark (renewable biofuel and pulp mill waste) and up to 70 wt-% straw (renewable biofuel and agricultural waste) in mixtures with peat without encountering increased deposition rates in the reactor. The deposit composition was compared to the fuel ash composition using chemical fractionation analysis and SEM/EDX. While the composition of deposits obtained from pure fuels resembles the feed composition, a considerable change is observed in deposits obtained from mixtures. K and S compounds are attached to Si spheres and the substrate surface. The deposition rate is significantly lowered when removing K, S, Cl and Na in bark prior to burning by washing and mechanical/thermal dewatering.Copyright

Numerical simulation of supersonic jet flow using a modified k−ε model

Many papers have reported that the standard k − ϵ model fails to accurately predict the mean velocity profile of turbulent axisymmetric jets (Thies and Tam, Computation of turbulent axisymmetric and nonaxisymmetric jet flows using the K − ϵ model, AIAA J., 1996, 34(2), 309–316; Pope, Turbulent Flows, 2002 (Cambridge University press: Cambridge). As the jet velocity increases, the deviation of the model with respect to the experimental measurements also increases. This work is aimed at the development of a modified k − ϵ model that can be used to predict the mean properties of an axisymmetric jet as it (i) flows as a free jet, (ii) propagates between walls, and (iii) impinges on a solid object. Three additional terms are proposed to improve the standard k − ϵ model predictions. They are Durbin realizable, Heinz turbulence production and Sarkar compressibility correction terms. The performance of the modified model in predicting the velocity and the impact pressure profiles of a free jet with an exit Mach number range of 0.6–2.8 has been confirmed by its close agreement with the experimental measurements. In addition, the study suggests that the model is also capable of predicting the impact pressure of a supersonic jet propagating between smooth walls and impinging on the front edge of the wall in various degrees of intensity.

Application of Realizability and Shock Unsteadiness to k ε Simulations of Under-Expanded Axisymmetric Supersonic Free Jets

An explicit cell-centered finite volume solver coupled to a k turbulence model cor-rected for structural compressibility fails to satisfactorily predict the behavior of under-expanded supersonic jets exhausting into still air, because the model does not properlyaccount for the turbulence/shock wave interaction. Two approaches are examined: im-posing a realizability constraint and taking into account shock unsteadiness effects. Al-though both corrections yield better agreement with experimental data of under-expandedjets, the realizability constraint yields better results than the shock unsteadinesscorrection.

Failure of Cylindrical Brittle Deposits Impacted by a Supersonic Air Jet

The failure of a cylindrical brittle material impacted by a supersonic air jet is investigated. Gypsum was cast around steel tubes to simulate the deposit formed on tube surfaces in industrial boilers. The breakup behavior of two deposit sizes, positioned at several distances from the nozzle exit, was visualized and documented using a high-speed video camera. Three deposit failure behaviors were observed: (i) crack formation and propagation along the longitudinal axis of the cylinder, (ii) surface pitting followed by axial crack formation, and (iii) surface pitting followed by spalling. These types of failure depend on the ratio of jet diameter to deposit diameter, which affects the magnitudes of compressive, tensile, and shear forces induced within the material. By analyzing the breakup movies, characteristics of the broken deposits, such as the breakup duration and the amount of deposit removed, were measured. Also, the effects of deposit thickness and distance from the nozzle exit on these characteristics were investigated.

Butanal Condensation Chemistry Catalyzed by Brønsted Acid Sites on Polyoxometalate Clusters

The connection of active site structures and their catalytic chemistry during butanal deoxygenation on polyoxometalate clusters with varying H+ site densities and identity of central atoms [HxNa4−xSiW12O40 (x=0–4) and HyNa3−yPW12O40 (y=0–3)] was established with rate assessment, IR spectroscopic, and chemical titration methods. Butanal adsorbs on the H+ or Na+ ions on polyoxometalate clusters and forms RC=O⋅⋅⋅H+ or RC=O⋅⋅⋅Na+ complexes at 348 K. A portion of the adsorbed butanals on the H+ sites converts to surface acetates through their reactions with vicinal framework oxygen atoms, as confirmed from the detection of νas(OCO) band at approximately 1580 cm−1, and remains as the spectator species. Bimolecular reactions of butanals on the remaining H+ sites lead to 2‐ethyl‐2‐hexenal as the predominant products, within which a small fraction undergoes sequential cyclization–dehydration to produce aromatics. A trace amount of butanal converts through minor, competitive pathways that form light olefins and dienes. These findings on the connection between active sites and their catalytic chemistry provide mechanistic insights useful for tuning the rates of the various concomitant paths and thus yields towards the different products during deoxygenation reactions.

Bed Material Agglomeration Behavior in a Bubbling Fluidized Bed (BFB) at High Temperatures using KCl and K2SO4 as Simulated Molten Ash

Abstract The agglomeration of bed material is one of the most serious problems in combustion of biomass in fluidized-bed boilers, due to the presence of some inorganic alkali elements such as K and Na in the biomass ash, which form low-melting-point alkali compounds during the process. In this study, agglomeration behaviors of bed materials (silica sand particles) were investigated in a bench-scale bubbling fluidized-bed reactor operating at 800 °C using simulated biomass ash components: KCl, K2SO4, and a mixture of KCl and K2SO4 at eutectic composition (molar ratio K2SO4/(KCl+ K2SO4)=0.26). The signals of temperature and differential pressure across the bed were monitored while heating up the fluidized bed of silica sand particles premixed with various amounts of KCl, and the KCl-K2SO4 mixture in bubbling bed regime. A sharp decrease in temperature and differential pressure was observed around 750 °C and 690 °C for 0.4–0.6 wt% loading of the low melting-point KCl and KCl-K2SO4 mixture, respectively, suggesting the formation of bed material agglomeration and even de-fluidization of the bed. Moreover, this work demonstrated the effectiveness of kaolin and aluminum sulfate to minimize agglomeration. The results indicated that these additives could successfully prevent the formation of agglomerates by forming compounds with high melting points.

A Semianalytical Solution for a Compressible Turbulent Axisymmetric Jet

The change of variables (see [G. Kleinstein, Quart. J. Appl. Math., 20 (1962), pp. 49–54], and also see [G. Kleinstein, J. Spacecraft, 1 (1964), pp. 403–408]) applied to the mean flow equations for a free compressible axisymmetric shear layer is extended to the turbulence model equations. This reduces these nonlinear PDEs to the form of nonhomogeneous cylindrical heat transfer equations, which can be integrated using Greens functions. The integration results in a fixed point problem, the solution to which is obtained numerically. The method is validated against experimental data of free axisymmetric compressible turbulent jets, and good agreement is obtained.

Computational study of a supersonic free jet rotating perpendicular to the streamwise direction

We study the flow structure of supersonic jets rotating perpendicular to the streamwise direction using RANS simulations, and we assess the performance of different turbulence model rotation corrections. The Coriolis and centrifugal terms were added to the equations of motion to perform calculations in this non-inertial (rotating) frame of reference. An explicit, cell-centred, finite-volume numerical method, coupled to a k−ε turbulence model, was used for the computations. The turbulence model rotation corrections of Howard et al. (1980), Park and Chung (1999), and Cazalbou et al. (2005) were attempted. In the absence of experimental data for jets rotating perpendicular to the streamwise direction, the rotation corrections were examined against the available measurements of a swirling jet; the comparison of the numerical and experimental data indicates that the Cazalbou et al. (2005) and Park and Chung (1999) corrections improve the performace of the turbulence model. Simulations were then run of a supersonic jet rotating perpendicular to the stream direction at 0, 50, 100 and 150 rad/s, using no turbulence model rotation correction, and using the three rotation corrections. The results indicate that the Cazalbou et al. (2005) correction is more physical than the other two, as it yields results that are qualitatively consistent with the known effects of rotation: that turbulence is enhanced and suppressed on the concave and convex sides of a rotating jet centreline, respectively, and that the effect of rotation saturates as the rotation rate increases. The findings are in qualitative agreement with the available literature.

1D Heat Transfer Model for the Chain Section of a Lime Kiln

This work was aimed at gaining a better understanding of heat transfer within lime kilns, by both experiments and detailed modeling of heat transfer phenomena in the chain section. Experiments were conducted using a laboratory mockup of a rotating kiln to obtain convective heat transfer coefficient data for cooling of a steel rod in dry or wet lime mud. For moisture contents of 0% and 30%, the mud heat transfer coefficient was determined to be 170 and 320 W/m2 °C, respectively. A 1-D, unsteady heat conduction model was used to predict the temperature variations of all the chain rings in the chain system and calculate the amount of heat transferred by each chain ring to the lime mud. A thermal model was then developed to predict the steady axial temperature profiles of lime mud, gas and kiln wall throughout a rotating lime kiln equipped with a typical chain system.Copyright