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Agglomeration is one of the main challenges for combustion and gasification of biomass in fluidized beds. Bed agglomeration is related to K species present in biomass. Understanding the role of different types of K species on formation of agglomerates at different conditions can reveal the mechanisms of biomass ash induced agglomeration. Extensive experiments are conducted in a laboratory scale fluidized bed reactor, using mixtures of quartz sand and K species, including KCl, K2SO4 and K2CO3, to study the agglomeration mechanisms. The effects of gas composition, including air, H2– and H2O– containing gas are investigated. The morphology and elemental analyses of the agglomerate samples are examined by SEM/EDS analysis. Thermodynamic equilibrium calculations are performed for verifying the proposed mechanisms. The results show that the role of various forms of potassium salts on agglomeration in fluidized beds is different. Gas composition also has strong impact on the agglomeration tendency. In the air and H2-containing gas, defluidization of KCl in a sand bed is caused by the melt of KCl. However, KCl reacts with SiO2 to form K-silicates in the H2O-containing gas, which results in a lower defluidization temperature. No defluidization is observed for K2SO4 in the presence of oxygen or water. However, K2SO4 decomposes and reacts with SiO2 to form K-silicates, causing defluidization at 850 °C in the H2-containing gas. In the air, H2– or H2O-containing gas, K2CO3 would react with SiO2 to form potassium silicates and KOH may be additionally formed in the H2– and H2O-containing gas. The possible mechanisms of agglomeration of various potassium salts at different conditions are discussed.

Char and soot characterizationwas performed on samples obtainedfrom pilot-scale entrained flow gasification of coal bio-oil slurry(CBS) and coal/water slurry (CWS) at different operating conditions.The compositions and gasification reactivity of char and soot havebeen investigated to reveal the difference of CBS and CWS slurries.The results show that both char and soot obtained from CBS gasificationcontain alkali and alkaline earth metals which come from the bio-oil.These CBS char and soot alkali-containing samples exhibit higher gasificationreactivity than those from CWS gasification. For CBS gasification,soot has higher gasification reactivity compared with char. The gasificationreactivity may be caused by the catalytic effect of KCl and K2SO4 existing in char and soot. The reactivity ofchar and soot is also influenced by the gasification conditions (suchas H2O/C ratio), and high CO2 gasification reactivityis observed for chars exposed to high H2O/C ratios in theentrained flow reactor.

Fluidized beds have been widely applied to gasification and combustion of biomass. During gasification, a high temperature is preferable to increase the carbon conversion and to reduce the undesirable tar. However, the high temperature may lead to a severe agglomeration problem in a fluidized bed. Understanding of the agglomeration in various atmospheres is crucial to optimize the design and operation conditions. This study focuses on the effects of gases on agglomeration tendency with different types of biomass, including corn straw, rice straw, and wheat straw. The biomass ash samples are mixed with quartz sand and fluidized by the gas mixtures of N2/CO2, N2/H2, and N2/steam or by air. At 550 °C, the bed temperature is increased at the rate of 3 °C/min until defluidization occurs. In this way, the defluidization temperature can be determined, which represents the agglomeration tendency. The agglomerates are analyzed by scanning electron microscopy–energy-dispersive X-ray spectrometry (SEM–EDS) for morph...

The combustion characteristics of Huadian oil shale and its semicoke are comparatively studied using a thermobalance (TB) and a drop-tube furnace (DTF). It is found that the ignition mechanism of oil shale and semicoke is hetero-homogeneous and heterogeneous, respectively. Drop-tube furnace experiments with both oil shale and semicoke show that the carbon (C) conversion proceeds almost simultaneously with the particle burnout, while the hydrogen (H) conversion is faster and that of nitrogen (N) and sulphur (S) slower than the particle burnout. The kinetic behavior of semicoke combustion is analyzed by drop-tube furnace experiments based on the first-order reaction model, and the kinetic model with known pre-exponential factor Aa 26.3 g.cm-2 S-1.atm-1 and apparent active energy E-a (65.8 kJ/mol) is obtained.

High activity catalyst system of K2CO3 supported on perovskite and its reaction mechanism for carbon materials were studied. It showed higher catalytic activity at lower temperature region than the case using gamma-alumina supporter or K2CO3 alone. And also, the catalyst supported perovskites showed one third smaller of catalyst loss after the reaction than that with the alumina supporter or K2CO3 alone. Catalytic activation of K2CO3 supported LaMn(1-x)CuxO(3) system was considered to be progress by two factors. First, K2CO3 has to be decomposed into K+ and CO32- ions. Second, decomposed K+ ions are activated with oxygen at the lower temperature than K2CO3 activation temperature. On oxide surface of support, K2CO3 was found to exist in partially dissociated form. In addition, perovskites had the active oxygen at the surface. K+ ions were able to be activated with the active oxygen on the surface of perovskites at the low temperature. The releasing temperature of the active oxygen could be controlled by Cu substitution in LaMn(1-x)CuxO(3) system. Using these properties, we synthesized a high activity K2CO3 supported on perovskite catalyst for carbon combustion. Oxygen contents of perovskite could be reduced easily. And also, the reduced state could be re-oxidized to the original oxygen level. This reversible property of oxygen contents of perovskites was considered to be the reason of high recovery of catalyst (K+). (C) 2011 Elsevier Ltd. All rights reserved.