Current Issue : July - September Volume : 2014 Issue Number : 3 Articles : 4 Articles
Theupdraft gasifier is a simple type of reactor for the gasification of biomass that is easy to operate and has high conversion efficiency,\nalthough it produces high levels of tar. This study attempts to observe the performance of a modified updraft gasifier. A modified\nupdraft gasifier that recirculates the pyrolysis gases fromdrying zone back to the combustion zone and gas outlet at reduction zone\nwas used. In this study, the level of pyrolysis gases that returned to the combustion zone was varied, and as well as measurements\nof gas composition, lower heating value and tar content. The results showed that an increase in the amount of pyrolysis gases that\nreturned to the combustion zone resulted in a decrease in the amount of tar produced. An increase in the amount of recirculated\ngases tended to increase the concentrations of H2 and CH4 and reduce the concentration of CO with the primary (gasification)\nair flow held constant. Increasing the primary air flow tended to increase the amount of CO and decrease the amount of H2. The\nmaximum of lower heating value was 4.9 MJ/m3....
The present study is devoted to verify current capabilities of Large Eddy Simulation (LES) methodology in the modeling of lean\npremixed flames in the typical turbulent combustion regime of Dry Low NOx gas turbine combustors. A relatively simple reactive\ntest case, presenting all main aspects of turbulent combustion interaction and flame stabilization of gas turbine lean premixed\ncombustors, was chosen as an affordable test to evaluate the feasibility of the technique also in more complex test cases. A\ncomparison between LES and RANS modeling approach is performed in order to discuss modeling requirements, possible gains,\nand computational overloads associated with the former. Such comparison comprehends a sensitivity study tomesh refinement and\ncombustion model characteristic constants, computational costs, and robustness of the approach. In order to expand the overview\non different methods simulations were performed with both commercial and open-source codes switching from quasi-2D to fully\n3D computations....
Thestatistical behaviours of the instantaneous scalar dissipation rateNc of reaction progress variable c in turbulent premixed flames\nhave been analysed based on three-dimensional direct numerical simulation data of freely propagating statistically planar flame and\nV-flame configurations with different turbulent Reynolds number Ret. The statistical behaviours of Nc and different terms of its\ntransport equation for planar and V-flames are found to be qualitatively similar. The mean contribution of the density-variation\nterm T1 is positive, whereas the molecular dissipation term (-D2) acts as a leading order sink. The mean contribution of the strain\nrate term T2 is predominantly negative for the cases considered here. The mean reaction rate contribution T3 is positive (negative)\ntowards the unburned (burned) gas side of the flame, whereas the mean contribution of the diffusivity gradient term (D) assumes\nnegative (positive) values towards the unburned (burned) gas side. The local statistical behaviours of Nc, T1, T2, T3, (-D2), and\nf(D) have been analysed in terms of their marginal probability density functions (pdfs) and their joint pdfs with local tangential\nstrain rate aT and curvature Km. Detailed physical explanations have been provided for the observed behaviour....
This paper presents process simulation of moving bed gasifier using low rank, subbituminous Usibelli coal from Alaska. All the\nprocesses occurring in a moving bed gasifier, drying, devolatilization, gasification, and combustion, are included in this model.\nThe model, developed in Aspen Plus, is used to predict the effect of various operating parameters including pressure, oxygen\nto coal, and steam to coal ratio on the product gas composition. The results obtained from the simulation were compared with\nexperimental data in the literature. The predicted composition of the product gas was in general agreement with the established\nresults. Carbon conversion increased with increasing oxygen-coal ratio and decreased with increasing steam-coal ratio. Steam\nto coal ratio and oxygen to coal ratios impacted produced syngas composition, while pressure did not have a large impact on the\nproduct syngas composition. A nonslagging moving bed gasifier would have to be limited to an oxygen-coal ratio of 0.26 to operate\nbelow the ash softening temperature. Slagging moving bed gasifiers, not limited by operating temperature, could achieve carbon\nconversion efficiency of 99.5% at oxygen-coal ratio of 0.33. The model is useful for predicting performance of the Usibelli coal in\na moving bed gasifier using different operating parameters....
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