1,721,041 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
RELEVANCE OF ATTRITION TO BED SOLIDS INVENTORY AND PARTICLE SIZE DISTRIBUTION IN CIRCULATING FLUIDIZED BED COAL COMBUSTORS
No full textFluidized bed combustion of a biomass char (Robinia pseudoacacia)
No full textFluidized bed combustion of char from a biomass, Robinia pseudoacacia, was investigated in a bench scale combustor. Different experimental techniques have been adopted to characterize the combined role of combustion and comminution phenomena (primary, secondary, and percolative fragmentations, attrition by abrasion) in determining fixed carbon conversion and the rate of carbon elutriation. Comparison of experimental results obtained under steadily oxidizing conditions and under alternating oxidizing/inert conditions suggested mechanistic aspects of the fluidized bed combustion of biomass char. Fixed carbon combustion was almost always complete. Conversion occurred to a large extent via the generation of carbon fines followed by postcombustion during their residence time in the bed. Approximately half of the initial fixed carbon followed this pathway, the remainder being directly burnt as coarse char. The prevailing mechanism of carbon fines generation in the bed was percolative fragmentation rather than attrition by abrasion. In spite of the extensive generation of elutriable carbon fines, the combined effect of high fuel reactivity and of relatively long fines residence times in the reactor determined the large combustion efficiency. It is inferred from experimental results that char fines adhesion onto bed solids might be relevant to the observed phenomenology
The relevance of attrition to the fate of ashes during fluidized-bed combustion of a biomass
No full textThe fate of ashes during the fluidized-bed combustion of a biomass fuel (Robinia pseudoacavia) has been investigated both experimentally and theoretically. Fluidized-bed combustion experiments with Robinia have been carried out with a bed of pure quartz at temperatures ranging from 700 OC to 850 OC at different oxygen concentrations in order to investigate the tendency of the biomass ashes to deposit on the bed particles and to give rise to bed agglomeration, SEM/EDX analysis of the silica bed particles after the tests was carried out to determine the formation of alkali-rich layers on inert bed particles and possibly of silicate melts. Results indicated that large quantities of biomass ashes are retained on the bed particles under all operating conditions, but only at the higher temperatures could molten surface layers and agglomerated bed particles be noticed. Experimental results have been interpreted on the basis of a single-particle combustion model applied to both fine and coarse char particles' burnout in a fluidized bed. Calculations show that extremely high temperatures are rapidly reached by fine particles even at very low oxygen concentrations. These temperatures are well beyond typical potassium silicate melting temperatures. On the other hand, coarse particles burn at temperatures only slightly above bed temperature. Experimental and theoretical results indicate that the following mechanism is relevant to the fate of ashes during fluidized-bed combustion of biomass: ash is mostly detached from the coarse char as attrited fines whose temperature is raised significantly by carbon after burning. Fines can further adhere onto inert bed particles, with formation of alkali-rich surface layers. If the bed temperature is higher than the alkali-silicate eutectic, a melt forms that enhances bed particle stickiness and may ultimately lead to bed agglomeration
A thermogravimetric study of nonfossil solid fuels. 2. Oxidative pyrolysis and char combustion
No full textThe study addresses the competition between the course of purely pyrolytic processes and heterogeneous oxidation during oxidative pyrolysis of nonconventional fuels. This feature is one key, together with volatile matter flammability and effectiveness of oxygen transport to the particle, to establish whether flaming or glowing ignition of the fuel takes place. A selection of six nonconventional high volatile solid fuels is considered in the study, namely two plastics (polyethylene and poly(ethylene terephthalate)), two lignocellulosic materials (Robinia Pseudoacacia and waste wood), and two rubber-derived materials (scrap tires and ebonite). The analysis is based on the comparison of the behavior of fuels when heated in a thermogravimetric apparatus under inert and oxidizing atmospheres. Further analysis is directed to assess the heterogeneous combustion kinetics of chars obtained by pre-pyrolyzing in inert conditions samples of the fuels. The study is complementary to a companion paper (Senneca, O.; Chirone, R.; Masi, S.; Salatino, P. A Energy Fuels 2002, 16, 653) where pyrolysis in nitrogen of the same fuels has been addressed. Results indicate that heterogeneous oxidation and pyrolytic processes play different roles depending on the nature of the fuel. A general feature of all fuels tested is that burnoff cannot be simply described as sequential reaction paths corresponding to purely thermal degradation and heterogeneous oxidation. Synergistic effects between these processes are significant and need to be taken into account
Comminution of Carbons In Fluidized-bed Combustion
No full textThe study surveys fifteen years of research on carbon comminution in bubbling fluidized bed combustors. It is carried out on the premise that comminution can be seen as the result of at least four phenomena occurring in series-parallel with each other and with combustion, namely: the primary fragmentation of coals or other carbonaceous materials, the secondary fragmentation, the fragmentation by uniform percolation and the attrition of chars, cokes and graphite. In combination with combustion, these phenomena control size reduction of feed carbon particles into fines of elutriable size and, as a consequence, combustion efficiency and particulate emissions. Information on fragmentation and attrition behaviour is conveniently obtained by means of laboratory and pre-pilot scale combustors when carbon is charged batchwise to the bed. Some of the literatiure data separately reported in the paper for each of the four phenomena taken into consideration can be used, as they are, or after appropriate modification, in the design of larger units. To this end, submodels directed to extrapolate comminution data beyond the ranges of experimental conditions in which they have been determined are thoroughly examined. The paper also discusses how comminution parameters are embodied into the equation model of bubbling fluidized bed combustors. The starting point is the carbon particle population balance. Depending on whether secondary fragmentation is or is not relevant, the population balance is expressed by means of an integro-differential or an ordinary differential equation. Reference is also made to a simplified model which contains essential comminution effects and adequately describes the performance of combustors charged with coals of different rank
In-duct removal of mercury from coal-fired power plant flue gas by activated carbon: assessment of entrained flow versus wall surface contributions
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