| 1. |
- Johansson, Andreas, et al.
(författare)
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Dynamics of furnace processes in a CFB boiler
- 2007
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509 .- 1873-4405. ; 62:1-2, s. 550-560
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Tidskriftsartikel (refereegranskat)abstract
- With the aim of understanding the dynamics of combustion, this work examines simultaneous fluctuations in fluid dynamic parameters and gas composition measured in a CFB furnace operated with coal as a fuel. The fluid dynamic parameters investigated are pressure and air flow entering the furnace. Gas composition was recorded by a zirconia-cell probe and a gas suction probe connected to a mass spectrometer having a high time resolution (10 Hz). The principal fluctuations detected are around I Hz and below 0.3 Hz. The fluctuations below 0.3 Hz mostly originate from variations in the fuel-feed rate. These variations create periods of reducing conditions caused by a momentarily high fuel input accompanied by a pressure rise in the furnace and a reduction of the air feed, which occurs concurrent with the release of an enhanced quantity of volatiles. Modelled pressure fluctuations based on the relation between volatile release and pressure in the furnace give similar pressure fluctuations as the measured pressure fluctuations, with respect to amplitude and characteristic time scale of the fluctuations. There is also a correlation between reducing conditions and the concentration of hydrocarbons. (c) 2006 Elsevier Ltd. All rights reserved.
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| 2. |
- Nevalainen, H., et al.
(författare)
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Firing of coal and biomass and their mixtures in 50 kW and 12 MW circulating fluidized beds - Phenomenon study and comparison of scales
- 2007
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 86:14, s. 2043-2051
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Tidskriftsartikel (refereegranskat)abstract
- The combustion dynamics of coal, wood chips and their mixture is investigated. Load change capability and the effect of the individual control variables, for example the mixture ratio of different fuels, on pilot-scale CFB boiler dynamics were also studied. Disturbances in fuel feeding cause fluctuations in the flue gas concentrations. Changes in the heating value are possible due to varying moisture content of the fuel. Both these disturbances affect the instantaneous firing rate in a boiler. Also the characteristics of the fuels have to be taken into consideration when designing boiler control systems. When co-firing two fuels with clearly distinct combustion characteristics, direct assumptions based on each fuel's characteristies cannot be made about combustion behaviour of their mixture. Combustion experiments with coal and wood chips and their co-firing were carried out in a pilot-scale CFB reactor (VTT) and a large-scale CFB boiler (Chalmers). A comparison of the combustion in the two different size reactors, provides information about scaling. The combustion responses due to changes in the fuel feeding of the two circulating fluidized beds are analyzed by a dynamic model. (c) 2007 Elsevier Ltd. All rights reserved.
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| 3. |
- Niklasson, Fredrik, 1972, et al.
(författare)
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Biomas co-firing in a CFB boiler
- 2006
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Ingår i: Proceedings of the 19th FBC Conference. ; 1
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Konferensbidrag (refereegranskat)abstract
- The aim of present paper is to investigate the effects on gas distribution in a CFB furnace fromburning varied fractions of wood fuel with coal as a base fuel. The work is based on in-situmeasurements of gas concentrations at different locations in the furnace of the Chalmers12 MWth CFB boiler. The boiler was operated at two different pressure drops over the furnace,corresponding to different amounts of bed material. The latter was done since a reducedpressure drop over the furnace improves the overall boiler efficiency as long as the combustionperformance can be maintained. The gas-concentration measurements in the furnace arecompared to results from experiments in a cold laboratory scale fluidized bed unit in which therates of mixing of solids (fuel particles) are determined at varied bed heights and gas velocitiesby frame-by-frame digital image analysis of video recordings. Thus, the gas distribution in thefurnace is assumed to be related to the mixing of fuel particles.The cold bed experiments show a pronounced reduction in solids-mixing rate when lowering thebottom bed height. Since an increased fraction of wood in the fuel mixture leads to a largeramount of volatile gases emitted in the bottom region, a combination of a low bed height and ahigh biomass fraction in the fuel could result in significant maldistribution of combustible gasesover the furnace cross section. The conclusion from the present boiler measurements is that theperformance of an existing CFB boiler, operated satisfactorily with a low furnace pressure dropwhen firing coal, may deteriorate when adding wood to the fuel mixture due to lateralmaldistribution of combustible gases. The results show that this effect can, to some extent becounteracted by increasing the amount of bed material in the furnace, at the cost of higherpressure-drop losses.
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| 4. |
- Shen, L. H., et al.
(författare)
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Biomass mixing in a fluidized bed biomass gasifier for hydrogen production
- 2007
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 62:1-2, s. 636-643
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Tidskriftsartikel (refereegranskat)abstract
- To investigate the biomass mixing in the bubbling fluidized bed gasifier, a digital image processing based technique is developed in the paper. Red wooden balls are used as tracer particles to study biomass mixing in a two-dimensional fluidized bed gasifier. The experimental results show a high degree of biomass mixing in the vertical direction in the bed, whereas horizontal mixing in the bottom zone is found to be relatively limited. Along the vertical direction of the bed, a more uniform distribution of biomass particles is found at a low superficial velocity than at a high superficial velocity. With respect to lateral mixing, the biomass concentration in the center increases with the superficial velocity, whereas it decreases in the wall region. The biomass concentration in the bottom region decreases with the superficial velocity, but increases in the upper region of the bed. (c) 2006 Elsevier Ltd. All rights reserved.
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| 5. |
- Ström, Henrik, 1981, et al.
(författare)
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Experimental and Numerical Investigations of Ash Behaviour in Fixed-Bed Combustion of Woody Biomass Pellets
- 2018
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Ingår i: Industrial Combustion Journal. - 2075-3071.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Combustion of woody biomass in a fixed-bed configuration could play an important role in the development of small-scale combined-heat-and-power solutions, such as externally-fired micro-gas turbines. However, many challenges still remain with regard to the complexity of maintaining high availability with an acceptable fuel flexibility, as ash transformations in the fuel bed can cause serious problems for the robustness of the process as well as for downstream components. In this work, fixed-bed combustion of two woody biomasses of different ash contents is studied experimentally and numerically in an attempt to advance the understanding of ash-related disturbances. A mathematical model for fixed-bed conversion of biomass is extended to account for ash transformations on both the single-pellet level and the bed level, and the predictions from this model are assessed against the experimental results. The agreement between simulations and experiments is good for low air-flow rates, whereas additional considerations are needed for air-flow rates closer to the stoichiometric limit. Although there was no catastrophic slagging observed in the experiments, the capabilities of the model to describe effects of such slagging, down to only slightly reduced combustion rates due to milder ash transformations, are confirmed by the simulations.
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