| 1. |
- Bu, Changsheng, 1987, et al.
(författare)
-
Devolatilization of a single fuel particle in a fluidized bed under oxy-combustion conditions. Part A: Experimental results
- 2015
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 162:3, s. 797-808
-
Tidskriftsartikel (refereegranskat)abstract
- Devolatilization of a single fuel particle and the related flame combustion were studied in a two-dimensionalfluidized bed with a quartz wall, allowing visual observation of the conversion process. The aimwas to evaluate the devolatilization behavior (ignition, flame temperature, flame life-time, devolatilizationtime) of different fuels (4 ranks of coal from lignite to anthracite and wood) when replacing O2/N2 byO2/CO2 in O2 volume concentrations from 0% to 40%, at a fixed bed temperature of 1088 K using 6 mmspherical fuel particles. The volatiles’ flame was recorded by a color video camera to analyze ignitionand extinction. The flame temperature was estimated by two-color pyrometry. Two thermocouples wereinserted in the fuel particle to measure the temperature at the center and near the surface. Homogeneousand heterogeneous ignition modes, times of devolatilization, and flame duration (flame-life) under differentgas atmospheres were analyzed. Results indicate that the mode of ignition of bituminous coal, lignitecoal and wood changes when N2 is replaced by CO2. The ignition-delay time is much longer, and the flametemperature is lower in the O2/CO2 atmosphere than in an O2/N2 atmosphere for all the tested fuels. Thedevolatilization time of the anthracite particle is almost unaffected by the surrounding atmosphere,while for the other fuels this time is generally longer in O2/CO2 than in O2/N2 at the same O2 concentration.The presence of a flame during the volatiles combustion did not accelerate the particle heating, noteven at the highest O2 concentration tested (40 vol%), however, after the extinction of the flame, the rateof particle heating is significantly affected by the oxygen concentration.
|
|
| 2. |
- Bu, Changsheng, 1987, et al.
(författare)
-
Devolatilization of a single fuel particle in a fluidized bed under oxy-combustion conditions. Part B: Modeling and comparison with measurements
- 2015
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 162:3, s. 809-818
-
Tidskriftsartikel (refereegranskat)abstract
- A detailed one-dimensional transient model is developed to describe the conversion of a single fuelparticle in O2/N2 and O2/CO2 atmospheres in a fluidized bed (FB). The model takes into account the mainrelevant phenomena occurring from the addition of a particle to the FB up to the instant when most of thevolatiles have been released. The model accounts for the rates of drying, fuel devolatilization, homogeneouscombustion of volatiles in a thin flame, heterogeneous combustion of char, and mass and heattransfer, the latter involving the heat transfer from the FB reactor and flame to the particle. The modelis used to simulate and explain the experiments given in Part A of the present work, which includes testswith four ranks of coal (from anthracite to lignite) and one type of wood in O2/N2 and O2/CO2 atmosphereswith the O2 volume concentration varying in the range of 0–40% at a fixed bed temperature of1088 K. The predicted history of the temperature of the fuel particle and of the volatiles flame agrees wellwith the measurements. The simulated results indicate that the heat transfer processes at the particlescale are similar in pure N2 and CO2. The model reveals that only a small amount of heat from the flameis transferred to the fuel particle, explaining why the rate of particle heating is hardly affected by theflame. The decrease in the devolatilization time measured at higher O2 concentration is explained by heterogeneous(char) combustion, which is seen to be significant during the last stages of devolatilization.The model shows that the char combustion is limited by the rate of diffusion of O2 to the particle andjustifies the lower heating rate observed in O2/CO2 compared to in O2/N2. A sensitivity analysis showsthat the thermal capacity and conductivity of the fuel, as well as the convective heat transfer coefficient,are the most influencing parameters affecting the time of devolatilization.
|
|
| 3. |
- Bu, Changshen, et al.
(författare)
-
Oxy-fuel conversion of sub-bituminous coal particles in fluidized bed and pulverized combustors
- 2017
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 3331-3339
-
Tidskriftsartikel (refereegranskat)abstract
- Oxy-fuel combustion in pulverized coal (PC) and fluidizedbed (FB) boilers is being increasingly investigated due toits potential use for carbon dioxide capture. The combustionconditions in the two types of unit differ significantlybecause of fuel size, furnace temperature, and fluid dynamics. These differences affect the change of combustioncharacteristics from air (O2/N2)to oxy-fuel (O2/CO2) conditions in PC and FB in different ways. In this paper, the oxy-fuel combustion behavior of a single sub-bituminouscoal particle in PC is compared with that in FB conditions.The FB data were measured in our bench-scale FB test rig,whereas the PC data were collected from literature. The FB tests were performed at 1088 K with 0%vol
|
|
| 4. |
- Bu, C. S., et al.
(författare)
-
Effect of CO2 on oxy-fuel combustion of coal-char particles in a fluidized bed: Modeling and comparison with the conventional mode of combustion
- 2016
-
Ingår i: Applied Energy. - : Elsevier BV. - 1872-9118 .- 0306-2619. ; 177, s. 247-259
-
Tidskriftsartikel (refereegranskat)abstract
- A char combustion model is developed to study the effect of CO2 on the combustion of coarse char particles under oxy-fuel conditions in a fluidized bed (FB). It is a transient one-dimensional model, taking into account the heat and mass transfer from the bed to the particle and the heterogeneous combustion and gasification of char. The model shows good ability to predict the char temperature history measured in our previous work for different combinations of O2/CO2 and O2/N2 with various coal types. Simulations are carried out to establish the role of CO2 in oxy-fuel conversion at different O2 levels, particle sizes, and bed temperatures. The model is used to analyze the relative contribution of carbon in the char consumed by CO2 (gasification) and O2 (combustion), as well as the differences of the peak temperatures and the burnout times in O2/CO2 and O2/N2 for char particles in a commercial FB combustor. The results indicate that the conversion of coarse (mm size) char particles in an oxy-FB is controlled by the diffusion of O2 both in the O2/CO2 and O2/N2 case. The burn-out time decreases with the bed temperature also in both cases. The lower O2 diffusion rate in CO2 compared to N2, is the main reason for the longer burnout time and lower peak temperature found using O2/CO2 at bed temperatures of 1073–1173 K. In that temperature window, the contribution of the CO2-char gasification is limited, being notable only at high bed temperature in O2/CO2, e.g. 1223 K. In such high temperature conditions (rarely expected to be found in commercial coal FBC) the predicted burnout time of a lignite char-particle becomes shorter in O2/CO2 than in O2/N2.
|
|
| 5. |
- Bu, C. S., et al.
(författare)
-
Oxy-fuel combustion of a single fuel particle in a fluidized bed: Char combustion characteristics, an experimental study
- 2016
-
Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 287, s. 649-656
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of an oxy-fuel atmosphere on char conversion in a fluidized bed (FB) was examined by comparingmeasurements of single fuel particles exposed to O2/N2 and O2/CO2 atmospheres. The experimentswere carried out in a transparent and electrically heated FB at 1088 K and five O2 inlet concentrations(ranging from 0 up to 40%vol) using four ranks of coal (from anthracite to lignite) and one type of wood,all with 6 mm (spherical) diameter. The evolution of temperature with time of the various fuels at differentgas atmospheres and the microstructure of the char resulting after devolatilization are presented,from which the effect of the oxy-fuel atmosphere (changing CO2 by N2 at different fixed O2 concentrations)on char combustion characteristics is determined and analyzed. Results show that the apparentaverage combustion rate of a fuel particle decreases (and consequently the burnout time increases) whenchanging O2/N2 by O2/CO2. This effect was more significant at high O2 concentrations and most notablefor anthracite. Consistently, the time to reach the peak temperature was longer when shifting from O2/N2to O2/CO2. However, the char specific surface area, pore volume, and average pore diameter of char werenot significantly modified by replacing N2 with CO2.
|
|
| 6. |
- Bäfver, Linda S, 1975, et al.
(författare)
-
Particle emissions from pellets stoves and modern and old-type wood stoves
- 2011
-
Ingår i: Biomass and Bioenergy. - : Elsevier BV. - 1873-2909 .- 0961-9534. ; 35:8, s. 3 6 4 8-3 6 5 5
-
Tidskriftsartikel (refereegranskat)abstract
- The purpose of this work is to characterise particle emissions from pellets stoves and modern and old-type residential wood stoves. The mass concentration of particulate material in the hot flue gas was 19e82 mg/MJ, roughly the same for wood stoves and pellets stoves, but the old-type wood stoves tended to emit even higher quantities. Furthermore, during combustion of wood logs the considerably higher emission of organic gaseous carbon indicates an additional contribution to the emission from secondarily formed condensable organic particles.The particle mass emitted was dominated by fine particles (
|
|
| 7. |
- Fleig, Daniel, 1980, et al.
(författare)
-
Conversion of Sulfur during Pulverized Oxy-coal Combustion
- 2011
-
Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 25:2, s. 647-655
-
Tidskriftsartikel (refereegranskat)abstract
- On the basis of experiments in the Chalmers 100 kW(th) oxy-fuel test facility, this study presents an analysis of sulfur chemistry of pulverized lignite combustion, comparing oxy-fuel and air-fired conditions. Four test cases were investigated: an air-fired case, two oxy-fuel cases with dry recycling (30 and 35 vol % O-2), and one oxy-fuel case with wet recycling (43 vol % O-2 on a dry basis). The amounts of sulfur in the flue-gas, ashes, and condensed water from the condenser were quantified, and a sulfur mass balance was established. The composition of the ashes and the ash-forming matter in the fuel was analyzed. The ashes were investigated by X-ray diffraction, while the size of fuel and ash particles was determined by laser diffraction. In general, the results show that the lignite has a high sulfur self-retention by ash, especially in oxy-fuel combustion. The experiments also show that the conversion of fuel S to SO2 from oxy-fuel combustion is around 35% lower compared to air-fired conditions, whereas the flue-gas concentration of SO2 is higher in oxy-fuel combustion because of the absence of air-borne nitrogen.
|
|
| 8. |
- Gómez-Barea, Alberto, et al.
(författare)
-
Analytical solutions of sharp interface models with nth order kinetics.
- 2012
-
Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 183, s. 408-421
-
Tidskriftsartikel (refereegranskat)abstract
- When a reaction occurs in a narrow layer of a porous particle, it is practical to simplify the calculation ofthe reaction rate by assuming global kinetic parameters based on the surface of the reaction front. Theassumption of such simplified situation allows formulation of a sharp interface model (SIM). Though,there are three difficulties for the application of a SIM in reactor simulations: (1) No analytical solutionis available for general SIM with nth order kinetics with respect to gas reactant. For reactor model simulation,with a variety of particles and operating conditions, a numerical model is still necessary to beapplied, leading to numerical difficulties and time consumption; (2) The global surface kinetic coefficientis not a priori known. The reason is that this coefficient depends not only on the intrinsic reactivity, butalso on physical factors such as the size and density of the solid, as well as operation conditions of thereactor like gas reactant concentration and temperature, varying during conversion of the particle; (3)The SIM is applicable when chemical reaction is rapid compared to intraparticle diffusion because in thissituation the reaction occurs within a narrow region compared to the size of the particle. However, noquantitative criteria has been developed to delimit the conditions for application of SIM. In the presentwork these questions are answered. Char conversion (combustion and gasification) is taken as reference,but most conclusions are applicable to isothermal non-catalytic gas–solid irreversible reactions with asingle gaseous reactant.b
|
|
| 9. |
- Gómez-Barea, Alberto, et al.
(författare)
-
Biomass oxy-gasification integrated into a coal oxy-boiler.
- 2011
-
Ingår i: Proceedings of 19th European Biomass Conference, 06-09 June 2011, Berlin (Germany)..
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- ABSTRACT: A biomass gasifier coupled to an oxy-combustion boiler is an attractive concept, enabling simultaneous production of syngas of medium heating value and carbon “capture” from concentrated CO2 taken from the flue-gas of an oxyboiler. Pure oxygen is available for the gasifier, because it is produced for the oxyboiler, taking advantage of the economy of scale of the coal plant. A model of the gasifier is used to study the effect of process conditions on the gas composition and efficiency. The potential use of the gas produced for various applications (co-combustion in the oxyboiler, electricity production in gas engines and synthesis of chemicals with and without CO2 separation are discussed. The capability of sequestration of green CO2 (from biomass) is discussed. A 3 MWth bubbling fluidized bed gasifier in León (Spain) has been erected to demonstrate the viability of this gasification process.
|
|
| 10. |
- Gómez-Barea, Alberto, et al.
(författare)
-
Estimation of gas composition and char conversion in a fluidized bed
- 2013
-
Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 107, s. 419-431
-
Tidskriftsartikel (refereegranskat)abstract
- A method is presented to predict the conversion of biomass in a fluidized bed gasifier. The model calculatesthe yields of CO, H2, CO2, N2, H2O, CH4, tar (represented by one single lump), and char, from fuelproperties, reactor geometry and some kinetic data. The equilibrium approach is taken as a frame forthe gas-phase calculation, corrected by kinetic models to estimate the deviation of the conversion processesfrom equilibrium. The yields of char, methane, and other gas species are estimated using devolatilizationdata from literature. The secondary conversion of methane and tar, as well as the approach toequilibrium of the water–gas-shift reaction, are taken into account by simple kinetic models. Char conversionis calculated accounting for chemical reaction, attrition and elutriation. The model is comparedwith measurements from a 100 kWth bubbling fluidized bed gasifier, operating with different gasificationagents. A sensitivity analysis is conducted to establish the applicability of the model and to underline itsadvantages compared to existing quasi-equilibrium models.
|
|
