| 1. |
- Bu, C. S., et al.
(författare)
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Iron based oxygen-carrier-aided oxy-fuel combustion of coal char: Reactivity and oxygen transfer mechanism
- 2023
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 333
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Tidskriftsartikel (refereegranskat)abstract
- Adding an active oxygen carrier to the inert bed material may favor the oxygen distribution in fluidized bed (FB) combustors. This so-called method “Oxygen-carrier-aided combustion, OCAC” has been applied recently to air combustion in FB boilers, but could also be potentially interesting in oxy-fuel combustion. Here we analyze OCAC in oxy-fuel combustion by experimental determination of the reactivity of char from three coals (lignite, bituminous and anthracite coal) using three oxygen carriers (OC, analytical reagent Fe2O3, hematite, and steel slag). The tests were conducted in a micro thermo-gravimetric analyzer (micro-TGA) and the combustion kinetics was determined for the nine combinations of char and OC. Additional tests were conducted in a macro-TGA i.e., much sample in a big ceramic crucible, to evaluate the phase composition and crystal quality using X-ray diffraction (XRD) to the combustion residues collected at various degrees of carbon conversion (0.2, 0.5, and 0.8). The results show that the burnout temperature decreases with OCs as the combustion rate increases, while the ignition temperature hardly changes. The enhancement of the rate of char combustion by OCs is in the following order: steel slag < hematite < AR-Fe2O3. The lower the fuel rank, the stronger the effect of the oxygen carrier. The combustion kinetics of chars using OC has a higher activation energy and pre-exponential factor than pure char. The enlarged lattice constant of Fe2O3 measured by XRD during the char combustion indicates that the redox reactions of Fe2O3 accelerate the oxidization of char by enhancing the rate of oxygen transport to the carbon surface.
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| 2. |
- Bu, C. S., et al.
(författare)
-
The effect of H2O on the oxy-fuel combustion of a bituminous coal char particle in a fluidized bed: Experiment and modeling
- 2020
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Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 218, s. 42-56
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Tidskriftsartikel (refereegranskat)abstract
- Oxy-fuel fluidized bed (FB) combustion is considered one of the promising ways to control CO2 emission from coal-fired power plants. The effect of H2O on the char conversion during wet flue-gas recycle, in which the H2O concentration could be around 40%, is still not well understood. To this end, experiments and modeling were performed in this work. Combustion tests with bituminous coal char were carried out in an electrically heated fluidized bed in O2/CO2, O2/H2O and O2/CO2/H2O for various O2, CO2 and H2O concentrations at the bed temperature of 850 °C. At the same time, the influence of the bed temperature and the char size on char combustion was investigated in O2/CO2/H2O atmosphere. A thermocouple was inserted into the center of the char particle to measure the particle temperature, from which the char combustion characteristics were determined and analyzed. The results indicate that the participation of H2O in the combustion atmosphere enhances the carbon conversion, and it also reduces the particle temperature. A transient char-particle conversion model, taking into account heat and mass transfer from the bed to the particle and heterogeneous combustion and gasification of char, was developed to quantitatively examine the role of H2O. The model shows a good ability to predict the measured char-temperature history. Simulations were carried out to establish the role of H2O in O2/H2O and O2/CO2/H2O as in the FB experiments. The model was used to analyze the peak temperature and the burnout time of a char particle, as well as the relative contributions to the consumption of the carbon in the char by O2 (combustion), and CO2 and H2O (gasification). The results indicate that the endothermic char-H2O reaction is the main reason for the prolongation of the burnout time of char and the decrease in the particle temperature in O2/CO2/H2O as compared in O2/CO2. During wet flue-gas recycle, char-O2 still accounts for a major part of the total carbon consumption, but the contribution of char-H2O to the overall carbon consumption increases with the H2O concentration and cannot be ignored (i.e. when the H2O concentration attains 30%, the contribution of the char-H2O reaction to the overall carbon consumption is 14%). However, the contribution of char-CO2 to the char conversion is limited.
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| 3. |
- Leckner, Bo G, 1936, et al.
(författare)
-
Change of existing circulating fluidized bed boilers to oxy-firing conditions for CO 2 capture
- 2021
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Ingår i: Applications in Energy and Combustion Science. - : Elsevier BV. - 2666-352X. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates a circulating fluidized bed boiler, originally designed for air-firing, retrofitted to oxy-firing with the purpose of removing the CO2 emission from coal combustion. Previous studies have shown that the heat balance on the gas-particle side can be satisfied without changes in the boiler, but then the volume flow of gas is reduced. To retain the operation like that during air-firing, the volume flow, that is the fluidization velocity, in oxy-firing should be equal to that in air-firing. It is the main purpose of this work to determine the conditions for the transition from air to oxy-firing, while the heat transfer conditions are maintained at a constant fluidization velocity. Measures to achieve this, such as adjusting the supply of additional gas and the heat transfer surface, are analysed. The fulfilment of the furnace's heat balance requires extra fuel or reduction of the heat-transfer surface in the furnace. These changes affect the performance of the back pass, which must be modified to accommodate the change in gas composition and the higher sensible heat content of the flue gas. Strategies to deal with these circumstances in CFB boilers are discussed.
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| 4. |
- Leckner, Bo G, 1936, et al.
(författare)
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Combustion of municipal solid waste in fluidized bed or on grate – A comparison
- 2020
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Ingår i: Waste Management. - : Elsevier BV. - 0956-053X .- 1879-2456. ; 109, s. 94-108
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Forskningsöversikt (refereegranskat)abstract
- Grate firing is the most common technology used for combustion of municipal solid waste. The more recently developed fluidized bed (FB) combustion is rarely employed for this purpose. The present work compares the technical properties of the two devices to find out why FB has not been more used, considering the recent importance of waste-to-energy. Several drawbacks of FB, the need for fuel preparation and bed material consumption, play a role, but these features also have advantages: combustion is improved by the sorted fuel and less ashes. Silica sand as a bed material has the positive property of being an alkali scavenger. If replaced by an oxygen carrier (e.g. ilmenite) the scavenging effect increases, and, in addition, oxygen carriers even out the non-combusted gaseous fields in the furnace, which improves combustion and allows higher steam data at a given corrosion level. There are other advantages of FB, such as end-superheaters in the circulation loop, heated by the bed material. However, also the environmental performance and energy efficiency of grate firing has been improved, and several advanced solutions have been proposed. In conclusion, it is not clear which of the devices that is the better one. An economic evaluation is made, based on available literature information, but still there is no clear winner.
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| 5. |
- Leckner, Bo G, 1936
(författare)
-
Fluid dynamic regimes in circulating fluidized bed boilers—A mini-review
- 2022
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 247
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Tidskriftsartikel (refereegranskat)abstract
- The fluid dynamics in the furnaces of large-scale circulating fluidized bed (CFB) boilers are surprisingly little known in contrast to the many laboratory studies made on conditions related to chemical reactors. Two areas are surveyed in the present work: the bottom bed and the upper dilute zone of a furnace. The bottom bed is considered bubbling, but the general opinion is that either it does not exist, or it is turbulent. The flow in the upper furnace is dilute phase transport, judging from regime maps, showing that the state of the flow is outside of the range of fast fluidization. However, this is also not generally accepted. Usually, the regime of fluidization in CFB boilers is said to be fast fluidization. In one work it is considered fast fluidization even though the authors agree that it is different from the general definition. In another investigation it is called entrained flow. Here, the conclusion is that the diversity of opinions should be resolved by further investigations with the aim of defining the conditions for the fluidized flow in furnaces, including the influence of particle size and density, fluidization velocity, gas properties, and effects from the furnace dimensions, if any.
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| 6. |
- Leckner, Bo G, 1936
(författare)
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Fluidization Characteristics of Circulating Fluidized Bed Boilers
- 2023
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Ingår i: Chemie-Ingenieur-Technik. - : Wiley. - 0009-286X .- 1522-2640. ; 95:1-2, s. 32-39
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Forskningsöversikt (refereegranskat)abstract
- The fluidization in the furnace of circulating fluidized bed (CFB) boilers is described. Data have been obtained from a 12 MWth CFB boiler and from literature. The bottom bed is bubbling rather than fast or turbulently fluidized as is often assumed. There is an extended splash zone above this bed, in which particles are thrown up by the bubbles and fall back onto the bottom bed. Other particles are carried further into the transport zone, initially as a saturated gas flow, but some particles move into the wall layers and the density decays towards the exit. The density of the transport zone is low, and the circulation rate is relatively seen small. Therefore, this flow is in a state of dilute-phase transport and not in the fast fluidization regime, such as is often claimed.
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| 7. |
- Leckner, Bo G, 1936
(författare)
-
From bubbling to circulating fluidized bed combustion—development and comparison
- 2024
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Ingår i: Heliyon. - 2405-8440. ; 10:13
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Tidskriftsartikel (refereegranskat)abstract
- The most significant introduction of fluidized bed combustion technology took place about 50 years ago. Initially the combustion beds were of the bubbling type. Once the designs had reached commercial application, several drawbacks were discovered: Erosion on in-bed heat-exchanger tubes, insufficient combustion and desulphurization efficiencies with coal, unfavourable scale-up to electric utility-size. The problems were solved by applying circulating systems. The present text compares these bubbling and circulating designs. It is concluded that the bubbling bed may not be suitable for coal combustion, but for biomass and organic waste most of the drawbacks disappear, and the bubbling bed, being simpler, may have an economic advantage over CFB that should be considered. In addition, combinations of CFB and BFB are quite favourable in many applications.
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| 8. |
- Leckner, Bo G, 1936
(författare)
-
Heat and mass transfer to/from active particles in a fluidized bed - An analysis of the Baskakov-Palchonok correlation
- 2021
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 168
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Tidskriftsartikel (refereegranskat)abstract
- Heat and mass transfer to or from single active particles surrounded by inert (passive) particles in a fluidized bed has been investigated based on published correlations. Special emphasis is on the application of a proposal by Baskakov, further developed by Palchonok. This representation describes heat and mass transfer as a function of the size ratio of inert to active particles. Two limits have been chosen: the limit of small active particles, where the active and the inert particles are equal, and the limit of large active particles, where the influence of the size of the active particle has vanished. The presentation aims at finding a suitable relationship, describing the size ratio of inert to active particles on heat and mass transfer to/from particles in fluidized beds and to critically evaluate its usefulness. It seems that the agreement between available correlations is qualitative and only approximate estimations can be made. A generalized scheme for calculations is presented. The formulation is made for bubbling fluidization. A discussion is presented on its use in circulating fluidized bed applications for fuel conversion as well.
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| 9. |
- Leckner, Bo G, 1936
(författare)
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Hundred years of fluidization for the conversion of solid fuels
- 2022
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 411
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Tidskriftsartikel (refereegranskat)abstract
- This is a summary of the development of conversion of solid fuels in fluidized bed during the hundred years that follow the first patent of Winkler in September 1922. The Winkler gasifiers and their followers are described first. Other fuel converters, such as boilers, appeared only in the 1960–70s and became of interest because of their expected environmental advantages. Initially, bubbling bed boilers were introduced, followed by circulating fluidized bed (CFB) boilers in the beginning of the 1980s. Now, CFB is the dominant technology. The entire development has not been conditioned by technological breakthroughs, but rather by the surrounding conditions: industrial demand, wars, environmental effects, availability and price of fuels. The recent development of the presently rather mature technology depends very much on the necessity to limit greenhouse gas accumulation in the atmosphere. Although fluidized bed technology offers solutions to reduce CO2 emissions, so far, no decisive line of application has been established for CO2 reduction, except for the use of biomass and waste.
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| 10. |
- Leckner, Bo G, 1936
(författare)
-
Negative CO 2 emission from oxy-fuel combustion in CFB boilers
- 2023
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 333
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Tidskriftsartikel (refereegranskat)abstract
- Oxy-combustion by flue-gas recirculation for CO2 capture is applied to an existing, already CO2-neutral, biomass-fired circulating fluidized bed (CFB) boiler, thus resulting in negative CO2 emissions. The required oxygen concentration is determined by a heat balance, but in an existing plant the volume flow is then reduced as well as the fluidization velocity, which affects the heat transfer. Methods to resolve this problem are investigated. The oxygen is usually proposed to be supplied by air separation; a method that consumes a considerable share of the energy produced in the plant. Here, it is instead suggested to use oxygen produced together with hydrogen in electrolysis by excess wind and solar power.
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