177,191 research outputs found
On the mechanism of coal-biomass slurry fluidized bed gasification
No full textTwo pumpable, water-based, coal-biomass slurries were prepared, bottom-injected and tested in an experimental program at pilot-scale for atmospheric bubbling bed gasification with air. The bed was made of either alumina particles or alumina-supported catalysts of the same size and operated at about 850°C. The equivalence ratio, which was selected as the main operating variable, was in the range =0.3-0.49. The fluidization velocity was U=0.4 m/s; the jetting ratio, which was the ratio of the air flow rate for slurry dispersion to the overall air flow rate, was J=0.25.
The complex slurry gasification mechanism determines poor gasification efficiency. The switch from a bed of non-catalytic solids to one of Ni-supporting -alumina particles is effective in converting within the bed up to half of the generated tars. However, this advantage is offset by an enhanced carbon loss from the bed of the gasifier
Fluidized Bed Combustion of Liquid Bio-Fuels: Application of Integrated Diagnostics for Micro-Explosions Characterization
No full textA novel integrated diagnostic technique has been developed for the analysis of the “regime with microexplosions” that may be established during the low-temperature (T < 800 °C) fluidized bed combustion of liquid fuels. It consists of the comparison among three analogue data series: (i) pressure signals measured in the freeboard and high-pass filtered, (ii) oxygen molar fractions measured by zirconia-based probes at two elevations in the bed and in the splash region, and (iii) video frames of the bed surface recorded and purposely worked out. The integrated technique has been applied to the combustion of biodiesel at minimum fluidization and has proven to be a valid tool to provide the fingerprints of the mechanism of the low-temperature fluidized combustion of liquid fuels. The time series generated from the measured data sets have been analyzed with the aid of the Hurst’s rescaled range analysis, the V-statistic, and the Lyapunov exponents’ evaluation. The issue of localizing micro-explosions throughout bed, bubbles, and splash zone has been tackled by the V-statistic analysis, which has proven that the location of micro-explosions is just at the bed surface when T = 650 °C and moves deeper and deeper into the bed when its temperature increased to about 800 °C. The values found
for the largest Lyapunov exponent in the time series demonstrate that the investigated system is not only
dynamic but also chaotic in its nature
Sewage sludge ashes as a primary catalyst for the abatement of tar in biomass gasification: Bubbling versus spouted-fluidized bed configuration
No full textSewage sludge (SS) ashes, rich in iron and calcium, have been tested as a primary catalyst during air gasification of commercial wood pellets in a pre-pilot scale fluidized bed (FB) reactor. The shifting from a conventional fluidized bed to a spouted-fluidized bed configuration has been assessed on the catalyst performance. Specifically, at constant total air inlet flow rate, two different values of the air flow rate in a central spouting nozzle have been adopted, which correspond to 20% and 37.5% of the total inlet gas flow rate. Under the conventional fluidized bed configuration (i.e., bubbling regime), SS ashes exhibit good performance in term of tar reduction (about 20% decrease compared to a bed of inert silica sand), without significantly affecting the syngas composition. Concerning the transition to the spouted-fluid bed configuration, the gas-solid contact efficiency is enhanced at lower air flow rates through the central nozzle, with respect to the FB regime, leading to better gasification performance in terms of tar reduction (around 40% less) and syngas quality. A slightly worse gasification performance is obtained at high values of air flow rate in the central nozzle, due to a progressive increase of gas bed bypassing. Furthermore, moving from the conventional FB configuration to the spouted-fluid bed one dramatically boosts the elutriation rate of carbon fines as well as the attrition of catalyst particles
Mass and energy balances for a stand-alone tomato peels torrefaction plant
No full textTorrefaction is an emerging thermal pretreatment of biomass, which produces a solid biofuel having superior handling, milling, storage and co-firing properties compared to raw biomass. During the process a combustible gas (‘torgas’) consisting of different organic compound is also produced in addition to the torrefied solid product. In a properly designed and operated torrefaction system the torgas may be combusted to generate heat for the drying and torrefaction steps, thus increasing the overall process efficiency. In this paper, a simple process simulation of a stand-alone torrefaction plant with internal heat integration was performed to assess whether autothermal operation is conceivable for high moisture tomato peel residues (TPs). Results show that for typical torrefaction conditions where about 20-30% of the dry mass is removed in the form of volatile gases (i.e., 285 °C and 30 min for TPs), the process cannot be autothermal and, consequently, an additional utility fuel is required. Under these conditions, in fact, the total thermal energy potentially available in the torgas was approximately 72% lower than the overall energy required for torrefying raw tomato peels, which have 80.5% initial moisture content. The net thermal efficiency of the whole conversion process was estimated to be approximately 70%, whereas the energy yield of the torrefaction unit was 85%. This suggests that for high moisture content agro-industrial residues the integration of torrefaction unit with another plant providing waste heat may be a better option compared to stand-alone plant with internal heat integration in order to save the overall energy efficiency
DUAL-FUEL FLUIDIZED BED COMBUSTOR PROTOTYPE FOR RESIDENTIAL HEATING: STEADY-STATE AND DYNAMIC BEHAVIOR
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