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Biomass has a great potential for production of syngas and chemicals; however, it has stood in the shadow of natural gas (NG) and coal due to technical problems and issues such as tar formation. In this paper, syngas production from catalytic air–steam gasification of biomass in a fluidized bed was investigated. To enhance the yield of produced syngas and reduce its tar content by cracking, limestone, calcined dolomite, and olivine were used as catalyst. The maximum mole fraction of H2 was found to be 49.1 vol% at 1000 °C and a steam/biomass ratio (S/B) of 1.0 with dolomite present. Compared to olivine and dolomite, calcined dolomite was proved to be more effective for gas production and tar destruction. The results also showed that the particle size has a weak influence on gasifier performance, with only a slight decrease in tar content with decreasing biomass particle size.

In the small-scale industrial CFB(SICFB) boiler, the furnace height is limited, the fuel particles can't be burned out in the furnace completely. So the circulating solid particles would combust in the external loop including the cyclone and the external fluidized bed heat exchanger(FBHE). Moreover, there's a macroscopic horizontal movement of circulating solid particles exists in the FBHE from its inlet to outlet. Under the impacts of combustion, heat transfer and particles attrition, parameters such as the bed temperature, combustion products concentrations, solids physical properties and heat transfer coefficient are changed along the solids movement direction. In this research, a one-dimensional cell model along the solids horizontal movement direction was established in the FBHE of a industrial CFB test-rig. The distributions, as well as the variations of the above parameters were calculated and analyzed based on the energy balance and material balance in each cell. The calculation results were compared with the experimental results to be validated. The energy balance and material balance characteristics of the FBHE were obtained accordingly. These results can provide referential materials for design and operation of the FBHE in small-scale industrial CFB system.

This paper was aimed at the thermal decomposition and kinetics of the byproducts from MgO flue gas desulfurization. The effects of particle size and charcoal on thermal decomposition of the byproducts were investigated by the thermogravimetric analysis method, and the non-isothermal kinetic parameters of the byproducts were evaluated with the Coats–Redfern method. The results showed that the mass loss increased with the decrement of the particle size. However, the effect of the particle size on the mass loss of each thermal decomposition stage was different. Increasing the amount of charcoal powder in the byproducts had an obvious effect on the temperature characteristics of the thermal decomposition of magnesium sulfate, resulting in a reduced thermal decomposition temperature which could facilitate the thermal decomposition process. The equations G ( α ) = 1−(1− α ) 1/2 , G ( α ) = [−ln(1− α )] 2 , and G ( α ) = 1−(1− α ) 1/3 were the quite appropriate kinetic mechanisms describing the thermal decomposition process of MgCO 3 , MgSO 3 , and MgSO 4 , respectively.

In this paper, the pressure fluctuation in a fluidized bed was measured and processed via standard deviation and power spectrum analysis to investigate the dynamic behavior of the transition from the bubbling to turbulent regime. Two types (Geldart B and D) of non-spherical particles, screened from real bed materials, and their mixture were used as the bed materials. The experiments were conducted in a semi-industrial testing apparatus. The experimental results indicated that the fluidization characteristics of the non-spherical Geldart D particles differed from that of the spherical particles at gas velocities beyond the transition velocity U c . The standard deviation of the pressure fluctuation measured in the bed increased with the gas velocity, while that measured in the plenum remained constant. Compared to the coarse particles, the fine particles exerted a stronger influence on the dynamic behavior of the fluidized bed and promoted the fluidization regime transition from bubbling toward turbulent. The power spectrum of the pressure fluctuation was calculated using the auto-regressive (AR) model; the hydrodynamics of the fluidized bed were characterized by the major frequency of the power spectrum of the pressure fluctuation. By combining the standard deviation analysis, a new method was proposed to determine the transition velocity U k via the analysis of the change in the major frequency. The first major frequency was observed to vary within the range of 1.5 to 3 Hz.

Conventional recycle valve employed in the external circulation loop of CFB boiler was modified in the study for adapting the combustion and heat transfer characteristics of the small scale industrial CFB boiler. The evaporating immersed tubes were proposed to be set in the modified recycle valve which was treated as an evaporating recycle valve with integrated functions of solids recycle and heat transfer. The gas–solid flow and heat-transfer characteristics of the evaporating recycle valve were systematically investigated on a cold test rig and an industrial CFB boiler respectively. Based on the results, the low load performance of the industrial CFB boiler was discussed.

Most of the small scale industrial CFB (SICFB) boilers in China work at low operating pressure. Compared with the big scale CFB boiler in power plant, the heat ratio of water evaporation is higher in the SICFB boiler. Consequently, evaporating submerged tubes are always installed in the furnace emulsion zone in order to avoid overheating of flue gas at the furnace outlet. The boiler metal consumption could also be reduced because of the higher heat transfer coefficient of submerged tubes. But serious erosions are brought to the submerged tubes as the furnace emulsion zone always works at turbulent fluidized bed state with higher fluidization velocity. In the study, for alleviating the erosion to the submerged tubes, all the evaporating submerged tubes are set in the loop seal instead of in the furnace emulsion zone, and the loop seal is enlarged accordingly to be an evaporating loop seal (ELS). An 11t/h SICFB boiler with an ELS has been put into industrial application, the design consideration and operating characteristics of the boiler are systematically discussed in the paper.

Contributed by excellent features of fuel flexibility, high combustion efficiency and controllable emission, circulating fluidized bed (CFB) combustion technology has been found wide application in burning coal, it is the only clean coal combustion technology of being commercialized in the world currently [1]. However, with regard to the industrial area, the existing CFB boiler technologies are fail to meet the challenges of small scale industrial boiler, such as small capacity, low parameters of steam, artificial ignition, high combustion and desulfurization efficiency. Thereby, in this work, a new type of industrial CFB (ICFB) with a circulating ash heat exchanger has been proposed and developed. In this kind of ICFB, the immersed tube heat absorbing surfaces are located in the circulating ash heat exchanger (CAHE), instead of the furnace emulsion zone, so as to reduce the erosion of immersed tubes and extend the time of burning and desulfurization. The absence of immersed tubes in the bottom of the furnace facilitates the artificial ignition, and the fluidization air can be operated at a relative high velocity as well, which will contribute to keeping the upper region of the furnace in a fast fluidization state. This article summarizes some experiments and industrial tests conducted on a heat test rig and an ICFB respectively, the results demonstrate that this new type of ICFB can satisfy the industrial demands mentioned above.Copyright © 2012 by ASME

Using the external heat exchanger in large-scale CFB boilers can control combustion and heat transfer separately, make the adjustments of bed temperature and steam temperature convenient. The state of gas-solid two phase flow in the external heat exchanger is bubbling fluidized bed, but differs from the regular one as there is a directional flow in it. Consequently, the temperature distribution changes along the flow direction. In order to study the heat transfer characteristics of the water cooled tubes in the bubbling fluidized bed and ensure the uniformity of heat transfer in the external heat exchanger, a physical model was set up according to the similarity principle and at the geometric ratio of 1∶28 to an external heat exchanger of a 300MW CFB boiler. The model was connected with an electrically heated CFB test-bed which provides the circulating particles. The influencing factors and the distribution rule of the particles’ heat transfer coefficient in the external heat exchanger were assessed by measuring the temperature changes of the water in the tubes and different parts of particles flow along the flow direction. At the end, an empirical correlation of particles’ heat transfer coefficient in external heat exchanger was given by modifying the Veedendery empirical correlation.