Experimental studies on cotton stalk combustion in a fluidized bed

The present work reports studies on the mixing and combustion characteristics of cotton stalk (CS) with 10–100 mm in length in a fluidized bed. Effects of length and initial weight percentage of CS, diameter of alumina bed material as well as gas velocity on the mixing characteristics of CS with alumina were investigated. CS can mix well with 0.6–1 mm alumina at fluidization number N=3–8.     

Experimental study on cotton stalk combustion in a circulating fluidized bed

This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO₂ and NO) in flue (stack) gas were measured.     

Biomass gasification as the first hot step in clean syngas production process – gas quality optimization and primary tar reduction measures in a 100 kW thermal input steam–oxygen blown CFB gasifier

Syngas production based on biomass gasification is an attractive, feasible alternative to fossil fuel feedstock for the production of transportation fuels. However, the product gas from biomass gasification must be cleaned and tailored to comply with strict syngas quality requirements, as it consists of a wide variety of major and minor components and impurities. The characterization of such species is important to determine downstream gas treatment steps, and to assess the efficiency of the gasification process.This paper gives an overview of the results obtained during experiments on steam–oxygen gasification of biomass using 100 kW maximal thermal input circulating fluidized bed gasifier (CFBG) that have been performed at Delft University of Technology during the CHRISGAS project. The unit is also equipped with a high-temperature ceramic gas filter and downstream reactors for upgrading of the gas.In the experiments biomass types of both woody and agricultural origin have been used. They were represented by clean wood, demolition wood, an energy crop species (miscanthus) and a true residue (Dutch straw), respectively. Moreover, different bed materials have been applied, namely quartz sand, treated and untreated olivine and magnesite. During the experiments extensive measurements of gas composition have been carried out throughout the integrated test rig. The gas characterization included major gas components as well as certain minor species and tar.The results show that with the use of magnesite as bed material, remarkable increases of hydrogen yield were attained, as compared to sand or olivine; up to a volume fraction of almost 40% (dry, nitrogen-free basis). Also the H₂:CO ratio increased from values near or lower than 1 to 2.3–2.6. This is near the values needed, for e.g., Fischer–Tropsch diesel production, indicating a potential for simplification of the gas upgrading. Furthermore, by using magnesite tar content of the raw gas was reduced to values near 2 g m^?3 (STP). Moreover, magnesite complied with the expectation to have a positive impact on agglomeration prevention for the agricultural fuels containing alkali and chlorine in the ash. The kind of olivine applied during the experiments did not yield the expected tar reduction; the measured tar concentration was even higher than when quartz sand was used as bed material. Finally kaolin proved to be an effective additive to counteract the agglomeration when fuels with high alkali content in the ash are gasified using bed material that is rich in silica, as it is the case with quartz sand and olivine.     

Combustion of spent mushroom compost and coal tailing pellets in a fluidised-bed

Previous investigations found that fluidised-bed combustion of spent mushroom compost–coal tailing pellets was preferred for these high ash content fuels. This paper considers the combustion tests carried out on these wastes in a laboratory-scale fluidised-bed, where parameters, including the pellet feedrate, primary/fluidising air flowrate and bed depth, were investigated. Based on the minimum air ratio of 2.5 required to achieve high combustion efficiencies of around 97%, the optimum operating conditions for the combustor employed were a pellet feedrate of 3.25 kg/h (180 kg/m²h) and a total air flowrate of 650 kg/m²h. A lower sand bed depth of 0.22 m was also deemed beneficial, as deeper beds resulted in slugging and noticeable reductions in combustion efficiency. Acid gas emissions (NO_x, SO_x and HCl) were found in limited concentrations, as species remained primarily as inorganic compounds in the flyash. Some N₂O is thought to have formed, as fluidised-bed combustors are particularly prone to this. The alkali index of the ash suggests probable fouling/slagging in the system. For industrial-scale combustion of these wastes, the combustion efficiency could be improved by the presence of secondary air jets to aid turbulent mixing.     

Flame stabilization between two beds of alumina balls in a porous burner

Conditions for flame stabilization in a porous media combustor formed by two beds of different sizes of alumina balls were studied. Premixed combustion of lean methane–air mixtures were used as variables. Measurements performed included temperature profiles and chemical products compositions. Stabilized flames were observed in the range of volumetric flow rate from 7.01 l/min to 19.00 l/min at equivalence ratio of ? = 0.6 and ? = 0.7. Low pollutants emissions were found in the entire operation range.     

Experimental research on agglomeration in straw-fired fluidized beds

Agglomeration is a major problem in straw combustion in a fluidized bed. This paper presents the results of rice straw combustion experiments carried out under different operating conditions in a bubble-fluidized bed (BFB). The influences of bed material size and type, feeding mode, temperature, and fluidization number on agglomeration were discussed, and the mode of alkali accumulation in the bed was analyzed. The results indicated that low bed temperature, short fuel in-bed residence time, high fluidization number, and small bed material particles are conducive to agglomeration prevention. Successful extended combustion in a small BFB and in a bench-scale circulating fluidized bed was carried out without agglomeration under selected parameters. This work suggests the possibility for combusting high-alkali straw using fluidization technology.     

Modeling of the chemical-looping combustion of methane using a Cu-based oxygen-carrier

A mathematical model for a bubbling fluidized bed has been developed to simulate the performance of the fuel-reactor in chemical-looping combustion (CLC) systems. This model considers both the fluid dynamic of the fluidized bed and freeboard and the kinetics of reduction of the oxygen-carrier, here CuO impregnated on alumina. The main outputs of the model are the conversion of the carrier and the gas composition at the reactor exit, the axial profiles of gas concentrations and the fluid dynamical structure of the reactor. The model was validated using measurements when burning CH₄ in a 10 kW_th prototype using a Cu-based oxygen-carrier. The influence of the circulation rate of solids, the load of fuel gas, the reactor temperature and size of the oxygen-carrier particles were analyzed. Combustion efficiencies predicted by the model showed a good agreement with measurements. Having validated the model, the implications for designing and optimizing a fuel-reactor were as follows. The inventory of solids for a high conversion of the fuel was sensitive to the reactor’s temperature, the solids’ circulation rate and the extent to which the solids entering to the reactor had been regenerated. The optimal ratio of oxygen-carrier to fuel was found to be 1.7–4 for the Cu-based oxygen-carrier used here. In this range, the inventory of solids to obtain a combustion efficiency of 99.9% at 1073 K was less than 130 kg/MW_th. In addition, the model’s results were very sensitive to the resistance to gas diffusing between the emulsion and bubble phases in the bed, to the decay of solids’ concentration in the freeboard and to the efficiency contact between gas and solids in the freeboard. Thus, a simplified model, ignoring any restriction to gas and solids contacting each other, will under-predict the inventory of solids by a factor of 2–10.     

棉秆流化床燃烧的成灰特性和床料选择

介绍了山东某地棉秆的物理、化学特性,在0.2MWth试验装置上研究了棉秆流化床燃烧时不同床高的温度分布,利用ICP技术对灰的元素成分进行了分析,发现棉秆流化床燃烧时部分碱金属以挥发态析出,其中钾元素析出比例最大.飞灰中碱金属成分含量较高,对流受热面的沾灰可能性很大.该文中重点对棉秆流化床燃烧时两种床料:石英砂和高铝矾土的烧结特性进行了研究,通过使用SEM/EDX对两种床料燃烧前后微观结构和元素成分的变化进行了对比,结果发现石英砂床料颗粒表面形成了低温共熔混合物,烧结非常严重,并最终影响到正常流化,高铝矾土床料经长时间运行没有发现烧结现象,因此,棉秆流化床燃烧选择弱酸性的高铝矾土床料为宜.试验结果为棉秆流态化燃烧锅炉设计和运行提供了理论基础.     

An experimental investigation of the effect of longitudinal fin orientation on heat transfer in membrane water wall tubes in a circulating fluidized bed

Experiments were conducted in a cold model circulating fluidized bed having riser cross-sectional area of 100 mm × 100 mm, height of 4.8 m, bed temperature of 75 °C and superficial velocity of 8 m s^?1. Local sand having average diameter of 231 μm was used as bed material. The experiments were conducted for three tube configurations: membrane tube, membrane tube with a longitudinal fin at the tube crest and membrane tube with two longitudinal fins at 45° on both sides of the tube crest. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes and the heat is mainly transferred in the combination portion of tube and membrane fins. In addition, the membrane tube has the highest heat transfer coefficient.     

Combustion characteristics of rice-husk in a short-combustion-chamber fluidized-bed combustor (SFBC)

A short-combustion-chamber fluidized-bed combustor (SFBC), of 250 kW_th capacity, was developed and tested for combustion characteristics of rice-husk, i.e. combustion efficiency (E_c), heat rate intensity (I_c), temperature distribution, and gaseous pollutant emissions. The effects of fluidizing velocity, excess air, and combustor loading were analyzed. The results indicated that the system could operate without any secondary solid as bed material, and could achieve high combustion efficiency and high heat rate intensity. Solid recirculation within the bed, created by a solid recirculating ring and an air vortex, played an important role in efficient combustion, even in a relatively short-combustion-chamber. A maximum E_c of 99.8% and a maximum I_c of 1.54 MW_th m^?2 were realized. Increasing fluidizing velocity and excess air caused decreases in E_c. CO and NO_x emissions increased with increased excess air, and were in the range 50–550 ppm and 230–350 ppm, respectively.     

Models of fluidized bed drying for thin-layer chopped coconut

Methods for efficiently drying agricultural products are in ever-increasing demand. Due to its thorough mixing ability, a fluidized bed technique was employed to evaluate the drying kinetics of thin-layer chopped coconut. The experiments were conducted at drying temperatures of 60–120 °C and a constant velocity of 2.5 m/s. Chopped coconut was dried from about 105% d.b. to approximately 3% d.b. The moisture transport phenomenon in fluidized bed thin-layer drying is described by immense acceleration in MR diminution in the early stage of drying, followed by considerable deceleration. Falling-rate drying, an outgrowth of restraining moisture transfer via internal mass-diffusion mechanism, thoroughly characterized chopped coconut drying. Among the 10 selected models, statistic analysis inferred that the Modified Henderson and Pabis model could predict changes in moisture content most accurately. Compared with the values of D_eff derived from Fick’s law for other food and biological materials usually dried in conventional tray dryers, the current values (5.9902 × 10^?8–2.6616 × 10^?7 m²/s) were substantially high, principally attributable to the unique characteristic of fluidized bed drying, remarkably encouraging heat and mass transfer. Activation energy was also described.     

Turbulent forced convection effectiveness of alumina–water nanofluid in a circular tube with elevated inlet fluid temperatures: An experimental study

The present study aims to explore experimentally the influence of elevated inlet fluid temperature on the turbulent forced convective heat transfer effectiveness of using alumina–water nanofluid over pure water in an iso-flux heated horizontal circular tube at a fixed heating power. A copper circular pipe of inner diameter 3.4 mm was used in the forced convection experiments undertaken for the pertinent parameters in the following ranges: the inlet fluid temperature, T_in = 25 °C, 37 °C and 50 °C; the Reynolds number, Re_bf = 3000–13,000; the mass fraction of the alumina nanoparticles in the water-based nanofluid formulated, ω_np = 0, 2, 5, and 10 wt.%; and the heating flux, q_o^″ = 57.8–63.1 kW/m². The experimental results clearly indicate that the turbulent forced convection heat transfer effectiveness of the alumina–water nanofluid over that of the pure water can be further uplifted by elevating its inlet temperature entering the circular tube well above the ambient, thereby manifesting its potential as an effective warm functional coolant. Specifically, an increase in the averaged heat transfer enhancement of more than 44% arises for the nanofluid of ω_np = 2 wt.% as the inlet fluid temperature is increased from 25 °C to 50 °C.     

Effects of ambient pressure,gas temperature and combustion reaction on droplet evaporation

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.     

Influence of operational parameters and material properties on the contact heat transfer in rotary kilns

The heat transfer by contact between the covered inner wall surface and the solid bed in a rotary kiln was experimentally investigated. An indirect heated batch rotary drum with a diameter of D = 600 mm and a length of L = 450 mm has been used. Inside the drum, a rotating and stationary measuring rod was installed, each assembled with 16 k-type thermocouples to measure the temperatures. Thus, the radial and circumferential temperature gradients within the solid bed were measured, and the contact heat transfer was determined from the heating of the bed. The effect on the contact heat transfer coefficient was estimated for the operational parameters of the drum, rotational speed (1, 3, 6 rpm) and filling degree (10%, 15%, 20%); the solid bed parameters, particle diameter (0.7, 1.3, 2.0 mm) and thermo physical properties (conductivity, heat capacity, density). As test material, quartz sand, glass beads and copper beads were used. The measured heat transfer coefficients were compared with theoretical calculations of four model approaches from the recent literature. A good agreement with just one of these model approaches could be achieved.     

Coal-gasification kinetics derived from pyrolysis in a fluidized-bed reactor

Coal pyrolysis and gasification reactions were carried out in a fluidized-bed reactor (0.1 m i.d. by 1.6 m height) over a temperature range from 1023 to 1173 K at atmospheric pressure. The overall gasification kinetics for the steam–char and oxygen–char reactions were determined in a thermobalance reactor. The compositions of the product gases from the coal-gasification reactions are 30–40% H₂, 23–28% CO, 27–35% CO₂ and 6–9% CH₄ with heating values of 2000–3750 kJ m⁻³. The heating value increases with increasing temperature and steam/coal ratio but decreases with increasing air/coal ratio. Our kinetic data derived from the two-phase theory on coal gasification in a thermobalance reactor and coal pyrolysis in a fluidized bed may be used to predict the product-gas compositions.     

Unsteady fluid mechanics and heat transfer study in a double-tube air–combustor heat exchanger with porous medium

Fluid mechanics and heat transfer are studied in a double-tube heat exchanger that uses the combustion gases from natural gas in a porous medium located in a cylindrical tube to warm up air that flows through a cylindrical annular space. The mathematical model is constructed based on the equations of continuity, linear momentum, energy and chemical species. Unsteady fluid mechanics and heat transfer by forced gas convection in the porous media, with combustion in the inner tube, coupled to the forced convection of air in the annular cylindrical space are predicted by use of finite volumes method. Numerical simulations are made for four values of the annular air flow Reynolds number in the range 100 ? Re ? 2000, keeping constant the excess air ψ = 4.88, the porosity ε = 0.4, and the air–fuel mixture inlet speed Uo = 0.43 m/s. The results obtained allow the characterization of the velocity and temperature distributions in the inner tube and in the annular space, and at the same time to describe the displacement of the moving combustion zone and the annular porous media heat exchanger thermal efficiency. It is concluded that the temperature increase is directly related to the outer Reynolds number.     

Case studies––Problem solving in fluidized bed waste fuel incineration

Fluidized bed combustion technology has been widely used as the new, flexible, multi-fuel boiler for waste combustion and energy recovery from low grade fuels. However, problems such as low thermal efficiency, high emissions, bed agglomeration etc. are still encountered in the operation of fluidized beds. Valuable experiences were gained from two case studies recently conducted regarding wastes combustion in industrial scale fluidized beds.In the first case, the performance of a fluidized bed combustor for energy recovery from oil sludge was evaluated during the commissioning trials. Apart from the sludge characterization and bed material analysis, the combustion efficiency, solid flow balance and on stack emission of CO, SO_x and NO_x were investigated, as well as the fluidization quality. Although the system was operated with good combustion efficiency (>99.9%), sulfur dioxide emission (>1000 ppm) was found to be substantially higher than the allowable discharge limit. It was recommended to increase the limestone feed rate in order to meet the SO₂ emission standard, and subsequently, installation of a cyclone is suggested to remove the potentially significant increase in ash and fine particles.The second case study focused on the bed agglomeration observed in a fluidized bed incinerator where a burning blend of three wastes (i.e. carbon soot, biosludge and fuel oil) is involved. To understand the mechanisms and related chemistry, several analytical approaches are employed to identify the bed materials (fresh sand and degrader sand) and the clinkers formed from full scale incinerator tests. The formation of clinker is believed to follow the mechanism of partial melting and/or reactive liquid sintering. The effects of temperature and blending ratio are tested in a muffle furnace. Carbon soot is believed to be more susceptible than the other two fuels. Thermodynamic multi-phase multi-component equilibrium (TPCE) calculations predict that the main low melting point species are predominant under the oxidizing condition, suggesting that reducing conditions might be favorable to restrain bed agglomeration. This study provides valuable information for better understanding of the chemistry related to clinker formation; it also helps in developing methods for control and possible elimination of the bed agglomeration problem for the design fuels.     

Micro-alumina particle volatilization temperature measurements in a heterogeneous shock tube

Peak flame temperatures in aluminum particle combustion should approach the volatilization temperature of the product alumina. References are divided in assigning this temperature anywhere between 3200 and 4000 K, which can provide significant uncertainty not only in numerical models for combustion but also in the interpretation of flame structure from temperature measurements. We present results in the controlled conditions of the UIUC heterogeneous shock tube of volatilization temperature, made by measuring the extinction of light by nano- and micro-alumina particles at non-resonant wavelengths at different ambient temperatures. At 10 atm, there is a sharp cutoff at 3860 K beyond which nano-particles volatilize and stop extinguishing within the shock tube test time. Numerical modeling of the evaporation rate of these particles is used to assign a volatilization temperature of 4340 K at 10 atm. Similarly, a volatilization temperature of 4260 K at 3 atm is measured. From our analysis, the best estimate for the volatilization temperature at 1 atm was 4189 ± 200 K, which is consistent with the high range of volatilization temperature reported in the literature.     

Heavy metals distribution characteristics in different particle size of bottom ash after agglomeration/defluidization at various fluidization parameters

This study examined the effects of variations in alkali and alkaline earth metal content, bed material diameter, static bed height and gas velocity in a fluidized-bed combustion process to understand the distribution of heavy metals in bottom ash after agglomeration/defluidization. A smaller diameter bed material increased the relative abundance of small particle sizes in the bottom ash due to attrition and thermal impact at high temperature. The addition of Na led to an increase in the large particle abundance of the bottom ash, likely due to the formation of a eutectic with a low melting point, causing agglomeration. The addition of Ca inhibited the agglomeration/defluidization and increased the abundance of large particles in the bottom ash.In general, heavy metal concentrations increased when the bottom ash size was smaller than 0.59 mm and larger than 0.84 mm. Regarding the different fluidization parameters, the bottom ash had the lowest concentration of heavy metals at 1.5 U_mf, an H/D of 2.1 and a bed material (silica) particle size of 0.645 mm. The concentrations of heavy metals in the bottom ash after Ca addition were higher than of those without Na or with Na only. Addition of Ca prolonged the operation time of fluidization and increased the feed quantity of heavy metal, helping the bed material adsorb more heavy metal. Therefore, the addition of Ca not only prolonged the fluidization time, reducing agglomeration/defluidization, but also resulted in a higher adsorption of heavy metals by the bed material, reducing their emission.     

Numerical calculation of local entropy generation in a methane–air burner

This study considers numerical simulation of the combustion of methane with air including 21% oxygen and 79% nitrogen in a burner and the numerical solution of the local entropy generation rate due to the high temperature and velocity gradients in the combustion chamber for various fuel flow rates (from 5 to 10 lpm). Swirling air flow is also used to burn the methane more efficiently. The effects of equivalence ratio (ϕ) and swirl number (S) on the combustion and entropy generation rate are investigated for different (consecutive) equivalence ratios (from 0.5 to 1.0) and swirl numbers (from 0 to 0.3). The numerical calculation of combustion is performed individually for these cases with the help of the Fluent CFD code. The volumetric entropy generation rate distributions and the other thermodynamic parameters are calculated numerically by using the results of the combustion calculations. The maximum values of the rates of reaction-1 and -2 decrease with the increase of ϕ. In the case of ϕ < 1, complete combustion occurs, and the combustion in the case of ϕ = 1 is very close to the complete combustion state. In the case of no swirl, the entropy generation rate decreases exponentially with the increase of ϕ in the cases of high Q_f, whereas they have quadratic profiles having their minimum values in cases of low Q_f. In terms of the entropy generation rates, the optimum equivalence ratios for Q_f = 5, 6, 7, >7 lpm in the case of S = 0 and Q_f = 10 lpm in the case of S = 0.3 are obtained as ϕ = 0.66, 0.8, 0.86, 1.0 and 0.92, respectively.