Search

Abstract Segregation exists extensively in the circulating fluidized bed (CFB) system, with solid particles of different sizes and densities. This paper studied the segregation of binary particles in a CFB system. Experimental results showed that segregation in the riser became weaker with increasing fluidizing gas velocity, u g , and solid circulating rate, G s. In the case of high u g (6.0 m/s) and high G s , segregation almost disappeared and the binary particles in the whole system became complete mixing. The average jetsam mass fraction along the riser height showed different relations with the initial jetsam mass fraction in the literature. In order to explain the discrepancy in the literature, this paper indicated the exit geometry influenced the segregation in the whole CFB loop, i.e. not only in the riser, but also in the external loop. For the abrupt exit, the internal refluxing of solids always existed in the sharp bend for the inertial separation of particles, and more coarse particles rebounded at the riser exit than the fine particles. Thus more coarse particles stayed in the riser than in the standpipe. Graphical abstract Unlabelled Image Highlights • Influencing factors on segregation of binary particles were studied. • Three exit structures were studied in a cold CFB test rig. • Separation effect of the T-shape exit was discovered. • The controversy in literature was explained by the impact of the exit. [ABSTRACT FROM AUTHOR]

Highlights • The CPFD method was used to simulate the gas-solid flow in a cyclone and standpipe. • Gas leakage reduced the swirling intensity in the cyclone. • Gas leakage made the downward vortex turn upward in advance. • Gas leakage increased the pressure in the standpipe and the cyclone cone. • The particle circulations in the cyclone might be enhanced by gas leakage. Abstract This paper, the second part of a two-paper series, discusses the computational particle fluid dynamics (CPFD) simulation results to clarify the effects of gas leakage on the separation performance of a cyclone. Compared with the experimental results in Part 1 of this two-paper series, the CPFD simulation results showed good agreements on the pressure drop, the overall separation efficiency, and the grade efficiency. Gas leakage would reduce the swirling intensity in the cyclone, make the downward vortex turn upward in advance, and increase the pressure in the standpipe and the cyclone cone. The particle circulations in the cyclone might be enhanced by gas leakage. [ABSTRACT FROM AUTHOR]

Highlights • Experiments were carried out in a cold model of a cyclone. • Gas leakage fraction less than 8% did not affect the pressure drop over a cyclone. • Gas leakage fraction above 4% decreased the separation efficiency of a cyclone. • Gas leakage worsened the separation efficiency on fine particles of diameter less than 200 μm. Abstract This paper, first of a two-paper series, analyzes the experimental results to clarify the effects of gas leakage on the separation performance of a cyclone. Experiments were carried out in a cold model of a cyclone. Varied parameters were inlet gas velocity, inlet solid concentration, and gas leakage velocity. In the experimental range, the pressure drops over a cyclone showed small variation with the increase in gas leakage velocity. When the gas leakage fraction exceeded 4%, experiments showed that the separation performance deteriorated significantly. Gas leakage re-entrained separated fine particles in the standpipe into the cyclone, indicated by a significant decrease of the separation efficiency for fine particles. [ABSTRACT FROM AUTHOR]

In this paper, experiments were carried out to obtain the actual operation characteristics of external heat exchangers (EHEs) in the first 600 MW supercritical CFB boiler. The furnace temperatures and some operational parameters in EHEs were measured and the heat transfer coefficients of heating surfaces in EHEs were calculated. The results showed that by controlling the solid flow rates to the EHEs, the furnace temperature were adjusted for good performance at various boiler loads and the bed temperature were maintained stable at full load. The heat transfer coefficients in EHEs were mostly over 200 W/(m 2 ∙ K), which contributed to the flexible temperature regulation. The tube panel wall temperatures were found to be a saddle-shaped distribution, which was high in the bed center but low near the wall. Packet renewal theory was used to explain such uneven temperature distribution. These results confirm the importance of EHEs to the furnace temperature control of the supercritical CFB boilers, but the uneven tube wall temperatures need to be well solved in the future. [ABSTRACT FROM AUTHOR]

Abstract Circulating fluidized bed (CFB) boilers face sever challenges in China due to the new ultra-low emission rules, more specifically, the NOx emission must be lower than 50 mg/m3. In this paper, NOx evolution inside CFB furnaces and the gas-solid flow characteristics were reviewed. It is found that NOx emission can be reduced through re-specification of fluidization state, namely, improving the bed quality. A low NOx emission (de-NOx) technology applied to CFB boilers was thus proposed. The keys of the technology are decreasing the average size of bed materials, limestone included, and increasing the solid circulating rate by improving the performance of circulating loop including improving cyclone efficiency, controlling slag discharge, etc. Some industrial practices showed that when the average size of bed inventories is markedly reduced, and when using ultrafine limestone, NO x emission can be significantly decreased without undermining high efficiency desulfurization. This paper provides a new solution to low-cost pollution control of CFB boilers. Highlights • NOx emission can be reduced through re-specification of fluidization state, namely, improving the bed quality. • Using ultrafine limestone can relieve the contradiction between high efficiency desulfurization and de-NOx combustion. • A low NOx emission technology applied to CFB boilers was proposed. • Some industrial practices showed that the ultra-low NO x emission can be achieved by applying this technology. [ABSTRACT FROM AUTHOR]

Coal type adaptability for CFB boilers is of great importance because it influences whether the unit can operate safely, economically and environmentally when the coal type is changed. Research is carried out on a 300 MW CFB boiler located in Longyan power plant, Fujian province by burning three types of coal, i.e. Fujian anthracite, Shenhua bituminous coal and Indonesia lignite. The results show that all the three coals are capable to meet the basic requirement of bed temperature. But the fly ash carbon content and exhaust gas temperature depend on the coal type. Compared with Fujian anthracite, Shenhua bituminous coal and Indonesia lignite tend to possess lower fly ash carbon content and lower exhaust gas temperature under high boiler load, which helps to increase the boiler efficiency. The auxiliary power consumption rates under all the three types of coal are all lower than 4.3% with the successful utilization of lower energy consumption technology based on state specification design theory. Besides, when the fuel is changed from Fujian anthracite to Shenhua bituminous coal and Indonesia lignite, SO 2 emission and dust emission decreases, while the NO x emission increases. [ABSTRACT FROM AUTHOR]

• The effects of particle size on sulfation was studied under various conditions. • Fine limestones are less sensitive to the variation in SO 2 concentrations. • A model was established to predict the effects of particle size on sulfation. • The effects of particle size mainly depend on the reaction on external surface. Limestone particle size has a significant influence on the desulfurization effect in CFB boilers. However, few studies have focused on the effects of particle sizes under different conditions. As a result, many existing sulfation models have deficiencies in predicting the sulfation process of limestones with a broad range of particle sizes. In this paper, the sulfation process of limestone with a broad range of particle sizes was investigated under different SO 2 concentrations, CO 2 concentrations, and temperatures by using a large-capacity TGA. A semi-empirical model was established based on the experimental results. It is found that the fine limestone exhibited the superior desulfurization performance compared with the coarse limestone, and it is less sensitive to the variation in SO 2 concentrations. The highest ultimate conversion achieved at 850 °C–900 °C, and showed little difference between different particle sizes. Besides, the conversion and initial reaction rate increase slightly with the CO 2 concentrations. The established model divided the sulfation process of limestone into the direct contact reaction on the external surface and the diffusion-controlled reaction on the inner pore surface, which can well predict the particle sizes under different SO 2 concentrations. [ABSTRACT FROM AUTHOR]

• Fe 2 O 3 composition is prone to be removed due to attrition. • The fly ash make-up is mainly from attrition of coal ash. • Chemical composition deviation is enhanced due to attrition. The Samcheok CFBC (circulating fluidized bed combustion) power generation plant, the world largest, consists of 4 units of 550 MWe USC (ultra-supercritical) CFB boilers, started operation since the end of 2016. The years' operation proves that there lacks circulating material and low solid hold-up in the upper furnace when burning Indonesia coal with low ash content of 4.5%. To solve this problem, the present study studied the mass balance of the Samcheok CFB boiler by investigating the ash formation and attrition characteristics of the feed coal in the lab. The ash formation PSD (particle size distribution) proves that the burnout-produced fly ash and fragmentation-produced fly ash are not the main sources of the production of fly ash. The heterogeneity of specific chemical compositions of particles in terms of particle sizes is proved to be one of the key factors for determining the attrition rate. For large particles, SiO 2 and Al 2 O 3 compositions dominate and results in low attrition rate, while for small particles, Fe 2 O 3 and CaO compositions dominate and results in high attrition rate. The fine particles produced due to attrition have very high Fe 2 O 3 contents, indicating that the heterogeneity of chemical composition is enhanced due to attrition. These findings show good agreement with the phenomenon of the high content of Fe 2 O 3 in fly ash in the CFB boiler. [ABSTRACT FROM AUTHOR]

Abstract: The Computational Particle Fluid Dynamics (CPFD) numerical method was used to study the gas solid flow characteristics in a circulating fluidized bed (CFB) with a loop seal in this work. The influences of operating parameters, including loop seal aeration rate (Q l), fluidized air velocity in the riser (U r) and total bed inventory (M p) on the solid circulation characteristics were mainly investigated in this part. The solid circulating rate (G s), total pressure drop in the riser (ΔP r), particle packed height (H s), pressure gradient (ΔP s/H s) and gas flow rate (G g) in the standpipe, as well as the particle concentration and pressure distributions in the loop were carefully analyzed under different operating conditions. The simulation results were compared with the experimental results also reported in this work and good agreements were obtained. The results showed that the increase of Q l resulted in a decrease of H s and increases of G s, ΔP r and ΔP s/H s. By increasing U r, the increase of G s was limited and the maximum value was approaching. An increase of M p also had a positive effect on G s while an elevated H s was obtained as a result of mass and pressure balance. No gas bypass flow phenomenon occurred in the operating cases of this work from the simulation results. The pressure drop in the riser could be properly predicted while the gas–solid interactions in the standpipe and loop seal were slightly underestimated by the CPFD method, where a relatively lower ΔP s/H s was obtained for CFB modeling. [Copyright &y& Elsevier]

Abstract: The Computational Particle Fluid Dynamics (CPFD) method was applied in the simulation of the gas solid flow in a circulating fluidized bed (CFB) with a loop seal in this work, and the modeling parameters were mainly investigated in this part. The influences of some crucial modeling parameters in CPFD simulation, such as mesh size, particle close pack volume fraction, interphase drag model and particle size distribution (PSD) on the flow behaviors in the whole loop of CFB were carefully analyzed with the help of experimental data conducted on the same CFB test rig. A medium mesh size was proved to be appropriate and used for the CFB modeling in this work. The particle close pack volume fraction largely affected the particle packed condition in the dense loop seal and standpipe. A value of 0.58 was proved to be suitable to describe the particle circulation and pressure distribution characteristics. Different gas–solid drag models showed certain influence on the particle circulation behaviors, and the combination of Wen-Yu and standard Ergun model was determined as the optimum choice. The consideration of the real PSD in CPFD scheme would better predict the overall flow characteristics for CFB system than the mono-size simplifications. The determination of modeling parameters in this work provides the base for simulating the gas–solid flow in the whole loop of CFB reactors by CPFD method, especially for the dense flow behavior in the standpipe and loop seal. [Copyright &y& Elsevier]

Oxygen-enriched combustion in fluidized bed is a promising clean coal combustion technology which can efficiently control the emission of coal-fired pollutants, especially CO 2. In this paper, by using several judging methods, the combustion characteristics of five kinds of coal under four different oxygen concentrations (20.9%, 27.8%, 40.7% and 52.7%, CO 2 as equilibrium gas) were measured in the small-scale fluidized bed reactor with electric heating in detail. The oxygen consumption and volatile release characteristics were explored online. The influence of coal types, volatile matter and oxygen concentration on coal combustion characteristics was analysed. It's found that the higher the bed temperature was, the faster the reaction rate was and the more oxygen consumption was. At a certain oxygen concentration and proper bed temperature, the separated combustion of volatile and char were observed. When the oxygen concentration was high, the separation phenomenon was more obvious. High-volatile, a certain bed temperature and oxygen concentration were the basic factors for the formation of separated combustion. The total volatile matter is not comprehensive as the index of reaction combustion characteristics and the effect of combustible components should be specifically considered. It's found that for the coal with high combustible components, the separated combustion is more obvious. Volatile matter plays a dual role in the ignition and combustion of coal. The volatile matter (lower volatile matter) of some coals has a great promoting effect on coal combustion at higher temperature, while that (higher volatile matter) of other coals is not, which is closely related to the properties of char and the diffusion of volatile matter in pores. • The combustion characteristics of five kinds of coal were measured. • Four different oxygen concentrations were studied. • The separated combustion of volatile and char was described. • High-volatile was important for separated combustion. • The effect of volatile components on separated combustion was confirmed. [ABSTRACT FROM AUTHOR]