海藻生物质颗粒流化床燃烧试验研究

在小型流化床试验台上研究了海藻颗粒(条浒苔与马尾藻)的流化床燃烧。海藻在流化床内的挥发分析出燃烧时间都在1 min左右。条浒苔颗粒在流化床中燃烧先进行脱水和挥发分的燃烧,接着发生焦炭燃烧,其燃烧过程符合缩核模型,炭核由外向内逐层燃烧,而灰层半径几乎不变。但马尾藻颗粒由于挥发分的大量快速释放而迅速膨胀破碎成屑。另外通过对条浒苔颗粒及不同燃烧时间后收集的焦炭颗粒剖面的SEM扫描电镜观察,发现随着燃烧的进行,颗粒内孔隙增大,微孔表面粗糙。进一步详细研究了两种海藻颗粒(条浒苔与马尾藻)在流化床内单次投料下的燃烧。随着床温的升高,条浒苔释放NO_x相对浓度增加,CO相对浓度减少。而马尾藻释放气体中SO₂与NO_x含量相对条浒苔有所增加;随着床温的升高,CO相对浓度减少。床温的升高使得床内传热速率加快,两种海藻挥发分的析出提前,燃尽时间缩短。风速、床高的升高使得两种海藻燃烧容易,燃尽时间缩短。     

油页岩循环流化床燃烧室密相区物料颗粒与燃烧特性

在 65t/h油页岩循环流化床锅炉上进行了燃烧室密相区物料特性与燃烧特性的工业试验。得到了极具片状结构特点的油页岩颗粒特性 ,经燃烧破碎磨损后的密相区床料和循环物料的颗粒特性与燃烧特性及相应的锅炉运行参数。试验结果对油页岩循环流化床锅炉的放大设计与运行调节具有一定的参考价值。同时为进一步建立油页岩循环流化床燃烧室密相区的流动、燃烧、传热、磨损、扬析与夹带模型奠定了基础。     

Modelling of circulating fluidized bed combustion of a char

The combustion of a char in the 41 mm ID riser of a laboratory circulating fluidized bed combustor has been investigated at different air excesses and rates of solids (char and sand) circulating in the loop. Riser performance was characterized by an axial oxygen concentration profile as well as by the overall carbon content and particle size distribution. The proposed model accounts for carbon surface reaction, intraparticle and external diffusion, and attrition. External diffusion effects were relevant in the riser dense region where char was potentially entrapped in large clusters of inert solids. Experimental data and results of the model calculations are in satisfactory agreement.     

Limestone fragmentation and attrition during fluidized bed oxyfiring

Attrition/fragmentation of limestone under simulated fluidized bed oxyfiring conditions was investigated by means of an experimental protocol that had been previously developed for characterization of attrition/fragmentation of sorbents in air-blown atmospheric fluidized bed combustors. The protocol was based on the use of different and mutually complementary techniques. The extent and pattern of attrition by surface wear in the dense phase of a fluidized bed were assessed in experiments carried out with a bench scale fluidized bed combustor under simulated oxyfiring conditions. Sorbent samples generated during simulated oxyfiring tests were further characterized from the standpoint of fragmentation upon high velocity impact by means of a purposely designed particle impactor. Results showed that under calcination-hindered conditions attrition and fragmentation patterns are much different from those occurring under air-blown atmospheric combustion conditions. Noteworthy, attrition/fragmentation enhanced particle sulfation by continuously regenerating the exposed particle surface.     

Combustion characteristics of a two-stage swirl-flow fluidized bed combustor

In order to investigate the combustion characteristics of a two-stage swirl-flow fluidized bed combustor, combustion experiments of low-grade anthracite coal were performed. Experimental parameters were the fluidizing air velocity, coal feed rates, bed temperature, stoichiometric air ratio, swirl nozzle diameter and rotational diameter. The experimental results showed that, due to the swirl flow, the elutriation rates of fines were lower than those of the single-stage fluidized bed combustor. The combustible contents of the ash in the outflow streams were also reduced. Therefore, the combustion efficiency of the two-stage swirl-flow fluidized bed combustor was 20% greater than that of the single-stage fluidized bed combustor under the same operating conditions.     

Combustion of single char particles in a turbulent fluidized bed

The combustion of single bituminous char particles (4-12 mm diameter) was studied in a turbulent fluidized bed operated at 1098 K using air as the fluidising medium. Results indicated that particles burn with constant density following a shrinking sphere model. Burning rates are much higher than those observed in a bubbling fluidized bed. The rate of transfer of oxygen to the particle surface is also higher than that observed in bubbling beds. A model is proposed to calculate the Sherwood numbers of the burning carbon particles. Experimental values of the Sherwood numbers agree well with those predicted from the model.     

Comminution characteristics of Korean anthracite in a CFB reactor

Comminution characteristic of Korean anthracite has been determined with operation conditions in a laboratory scale circulating fluidized bed (CFB) combustor. The fragmentation of the anthracite occurs explosively, and generates lots of fine particles at an early stage of devolatilization. The fragmented particles continue to be reduced with generation of the fine particles during an attrition stage in the CFB combustor. With an increase of operation temperature, the coal shows a high degree of fragmentation and generation of fine particles in the CFB reactor. The particle fragmentation occurs actively as its size and Hard Grove Index (HGI) increase. The attrition is also affected with particle size and HGI of the coal. The initial surface crack and the fine clusters on the particle surface are found to be reasons for explosive fragmentation and for generation of fine particles during devolatilization and combustion in the CFB reactor.     

CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

Fluidized bed combustion of high ash anthracite: Analysis of combustion efficiency and particle size distribution

Fluidized bed combustion of high ash anthracite (HAA) was experimentally studied. The combustor consists of 0.25 m ID bed, and auxiliary equipments for coal feeding, ash removal, lemperature control, etc. Experimental results elucidate main cause of fuel loss to be elutriation of fines (i.e., flyash) containing unburned carbon. However, detailed balances of particle size distribution show majority of carbon in flyash comes from fines contained in the feed instead of attrition of coarse particles. The latter is the main source of flyash for conventional coal. The difference is due to much smaller attrition rate of HAA; feed HAA particles do not shrink much in size by combustion and attrition.     

High temperature desulphurization by fine limestone during staged fluidized‐bed combustion

This paper reviews the SO₂ emission from a 0.3 m² stainless‐steel fluidized‐bed combustor. Fine coal was premixed with fine limestone and fed pneumatically under the bed. The SO₂ emission was found to depend largely on air staging ratio and bed temperature, which agrees with previous observations. The SO₂ emission observed in sorbent‐free tests (reported earlier by Khan and Cibbs, 1995) was found to be proportional to the sulphur content of the fuel when limestone was added, the sulphur capture at a fixed Ca/S molar ratio was dependent on oxygen stoichiometry and bed temperature. Finely sized limestone enhanced the effectivity of the sorbent at low bed temperature and air staging ratio. During staged combustion, the combustion efficiency depended largely on primary air to coal ratio. Around 90% combustion efficiency was observed at 1 m/s fluidizing velocity which was reduced when fluidizing velocity was increased to 1.5 and 2 m/s. This reduction is due to increased elutriation of finer coal particles from the combustor.     

Elemental mercury vapor capture by powdered activated carbon in a fluidized bed reactor

A bubbling fluidized bed of inert material was used to increase the activated carbon residence time in the reaction zone and to improve its performance for mercury vapor capture. Elemental mercury capture experiments were conducted at 100 °C in a purposely designed 65 mm ID lab-scale pyrex reactor, that could be operated both in the fluidized bed and in the entrained bed configurations. Commercial powdered activated carbon was pneumatically injected in the reactor and mercury concentration at the outlet was monitored continuously. Experiments were carried out at different inert particle sizes, bed masses, fluidization velocities and carbon feed rates. Experimental results showed that the presence of a bubbling fluidized bed led to an increase of the mercury capture efficiency and, in turn, of the activated carbon utilization. This was explained by the enhanced activated carbon loading and gas-solid contact time that establishes in the reaction zone, because of the large surface area available for activated carbon adhesion/deposition in the fluidized bed. Transient mercury concentration profiles at the bed outlet during the runs were used to discriminate between the controlling phenomena in the process. Experimental data have been analyzed in the light of a phenomenological framework that takes into account the presence of both free and adhered carbon in the reactor as well as mercury saturation of the adsorbent.     

CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

Abstract

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

A novel dual circulating fluidized bed system for chemical looping processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion. Direct hydraulic communication of the two circulating fluidized bed reactors via a fluidized loop seal allows for high rates of global solids circulation and results in a stable solids distribution in the system. A 120 kW fuel power bench scale unit was designed, built, and operated. Experimental results are presented for natural gas as fuel using a nickel‐based oxygen carrier. No carbon was lost to the air reactor under any conditions operated. It is shown from fuel power variations that a turbulent/fast fluidized bed regime in the fuel reactor is advantageous. Despite the relatively low riser heights (air reactor: 4.1 m, fuel reactor: 3.0 m), high CH₄ conversion and CO₂ yield of up to 98% and 94%, respectively, can be reported for the material tested. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

流化床与固定床耦合反应器中的颗粒磨损

在直径50mm的冷模流化床与固定床耦合反应器中,考察了活性炭颗粒在130～150℃及不同气速下的磨损情况,得到了不同气速下固定床中圆柱状颗粒的磨损率随时间的变化关系,同时分析了滞留在流化床、固定床及袋滤器中的细颗粒在不同气速下的粒径分布与质量分布.结果表明,颗粒在该耦合反应器中磨损严重,在0.212～0.424m/s气速下,固定床中颗粒质量损失可达3%～4%,流化床中颗粒平均粒径由200μm降至100μm以下.     

Performance evaluation of a pilot scale vortexing fluidized bed combustor

To understand vortexing fluidized bed combustor (VFBC) performances, an investigation was carried out in a 0.45 m diameter and 4.45 m height pilot scale VFBC. Rice husks, corn, and soybean were used as the biomass feedstock and silica sand serving as the bed material. The bubbling bed temperature was controlled by using water injected into the bed. The experimental results show that the excess air ratio is the dominant factor for combustion efficiency. The in-bed combustion proportion increases with the primary air flow rate and bed temperature, and decreases with the volatile/fixed carbon ratio. The stability constant is proposed to describe the inertia characteristics of the vortexing fluidized bed combustor. The experimental results indicate that the stability of the VFBC increases with bed weight and primary air flow rate, but decreases with bed temperature.     

Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

The ignition and burnout of the volatiles in fluidized bed combustor are essential for its performance and emissions. NO_x are known to sensitize the oxidation of hydrocarbons, CO, and H₂. This effect is relevant especially for fluidized bed combustors, which are operated at relatively low temperatures (i.e. about 850 °C). Different reaction mechanisms and modifications to existing mechanisms have been proposed in the literature to account for these low temperature interactions of NO_x and hydrocarbons. In this work, an existing widely used reaction mechanism is adapted and tested for its capability to describe the NO sensitized oxidation of CH₄ under conditions relevant to fluidized bed combustion. NO lowers the ignition temperature to about 300 °C under the conditions investigated. Three different oxidation paths for the oxidation of CH₄ have been identified and discussed. Their relative importance strongly depends on combustion temperature, indicating that the presence of NO_x significantly affects the oxidation of the volatiles in fluidized bed combustion.     

Mixing and combustion in a shallow coal-limestone fluidized bed combustor

A model based on the Monte Carlo approach was developed to simulate the mixing and combustion behavior of a shallow coal-limestone fluidized bed combustor. The model involved the coupling of two sub-models: a combustion sub-model based on the two-phase concept of fluidization and a mixing sub-model based on our previously developed dynamic mixing model. The combustion sub-model considered both the volatile and char combustion. It assumed that the combustor consisted of three distinct phases, i.e., jet, bubble and emulsion, with combustion occurring only in the emulsion phase. The mixing sub-model considered the upward or downward movement of a coal particle in the bed as being governed by certain probability laws; these laws were, in turn, affected by the bubbling hydrodynamics. In all, the combustor simulation model took into consideration the effects of coal feed rate, coal size distribution, limestone size, air flow rate and combustor temperature on the combustor behavior. The simulation results included the dynamic response of coal concentration profile, coal size distribution, coal particle elutriation rate as well as the mixing status between the coal and limestone particles.     

Modelling fluidized bed combustion of high-volatile solid fuels

A model of an atmospheric bubbling fluidized bed combustor operated with high-volatile solid fuel feedings is presented. It aims at the assessment of axial burning profiles along the reactor and of the associated temperature profiles, relevant to combustor performance and operability. The combustor is divided into three sections: the dense bed, the splashing region and the freeboard. Three combustible phases are considered: volatile matter, relatively large non-elutriable char particles and fine char particles of elutriable size. The model takes into account phenomena that assume particular importance with high-volatile solid fuels, namely fuel particle fragmentation and attrition in the bed and volatile matter segregation and postcombustion above the bed. An energy balance on the splashing zone is set up, taking into account volatile matter and elutriated fines postcombustion and radiative and convective heat fluxes to the bed and the freeboard.Results from calculations with a high-volatile biomass fuel indicate that combustion occurs to comparable extents in the bed and in the splashing region of the combustor. Due to volatile matter segregation with respect to the bed, a significant fraction of the heat is released into the splashing region of the combustor and this results in an increase of the temperature in this region. Extensive bed solids recirculation associated to solids ejection/falling back due to bubbles bursting at bed surface promotes thermal feedback from this region to the bed of as much as 80-90% of the heat released by afterburning of volatile matter and elutriated fines. Depending on the operating conditions a significant fraction of the volatile matter may burn in the freeboard or in the cyclone.     

NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed – An experimental study

High temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed. Experiments on pulverized coal combustion in high temperature air from the circulating fluidized bed were carried out in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. The NO emission decreases with increasing the residence time of pulverized coal in the reducing zone, and the NO emission increases with excess air ratio, furnace temperature, coal mean size and oxygen concentration in high temperature air. The results also revealed that the co-existing of air-staging combustion with high temperature air is very effective to reduce nitrogen oxide emission for pulverized coal combustion in the down-fired combustor.