DYNAMIC SOLIDS MIXING BETWEEN COAL AND LIMESTONE IN A GAS-SOLID FLUIDIZED BED

Dynamic characteristics of solids mixing between coal and limestone were studied. Experiments were carried out in a 0.203 m diameter cold fluidized bed with coal and limestone of different sizes as the fluidized particles. Experimental parameters examined included operation time, air flow rate, bed height, initial bed setup, relative particle size and relative amount of the two particles. The coal concentration profile was analyzed through bed sectioning, and a mixing index was used to describe the profile. The observed results were presented graphically and in correlation forms. The steady state results were also compared with those available in the literature.     

HEAT TRANSFER BETWEEN HORIZONTAL TUBES AND FREEBOARD REGION OF A LOW-RANK COAL FLUIDIZED BED COMBUSTOR

Experiments are conducted to measure the heat transfer rates to air-cooled horizontal tubes of diameter 38.1mm in the freeboard region of a pilot-scale fluidized-bed combustor burning North Dakota lignite- Silica sand with diameter ranging from 883 to 973 μim is used as bed material. The tests are carried out without limestone addition and ash recycle, at average bed temperatures ranging from 698 to 1117K, freeboard temperatures of 700 to 1139 K and superficial velocities of 1.74 to 1.86 m/s. Among the existing correlations, those proposed by George and Grace (1982), and Byam et al. (1981) are found to predict the experimental results quite well and their root-mean-square deviations are found to be 15.8 and 18.4 percent respectively. @KEYWORDS: Heat transfer, Fluidized bed Coal combustor     

循环床锅炉燃烧份额分布的实验研究和理论分析

在循环流化床锅炉小型实验台上,研究了床温、过量空气系数、一二次风比例和煤种等因素对燃烧份额分布的影响,证实了循环流化床锅炉密相区处于欠氧燃烧状态,并且密相区产生的一氧化碳和部分挥发分被带到了稀相区进行燃烧。从流动和燃烧角度对实验结果进行了分析,并从密相区气固两相流行为出发,解释了循环流化床锅炉不同于鼓泡床的一些技术特点。     

INVESTIGATIONS OF THE BUBBLE PHASE BEHAVIOR IN A HIGH TEMPERATURE FLUIDIZED BED REACTOR WITH COALCOMBUSTION

The properties of bubbles in a bench-scale fluidized bed reactor, 30 cm in diameter, during coal combustion were determined by means of a newly developed cooled bubble probe and a data processing system at temperatures up to 850°C and fluidization indexes up to 10 in axial and radial positions in the bed. The fluidization index above 4 and the temperature have only slight effect on the bubble properties. Their variation along the height above the gas distributor is dominant.

Oxygen and Co₂-concentration profiles were measured in the bed and in the freeboard, and the o₂-profiles were calculated by means of measured bubble data and bubble models. A comparison of measured and calculated o₂-profiles indicates that the mass transfer rates between the emulsion and bubble phases are larger than the ones calculated by the models.     

COAL PROPERTY EFFECTS ON N2O AND NOx FORMATION FROM CIRCULATING FLUIDIZED BED COMBUSTION OF COAL

The effects of coal properties on N₂O and NO_x formation from circulating fluidized bed combustion of coal was examined through burning nine typical coals and a coal shale, widely used in China over a wide range of coal ranks, in a bench-scale circulating fluidized bed. It was found that N₂O and NO_x formation had similar dependence on coal rank. Fixed carbon content and nitrogen content were the most important coal properties to influence N₂O and NO_x emissions from circulating fluidized bed combustion of coal. A coal with high fixed carbon content had high conversion ratio of fuel-N into N₂O and NO_x. The conversion ratio of fuel-N into N₂O or NO_x increased with nitrogen content of coal, whereas it decreased with O/N ratio. No significant correlation between conversion ratio of fuel-N into N₂O or NO_x and C/N ratio was identified. To clarify the coal property effect, investigation of a wide range of coal rank, is important.     

Reductive pyrolysis study of sulfur compounds in different Tabas coal samples (Iran)

A comparative study of the effects of mineral and pyrite removal on the determination of organic sulfur functionalities in representative, as well as its low and high sulfur analogue samples of Tabas coal (C1 seam) were studied by atmospheric pressure-temperature-programmed reduction (AP-TPR) method. The AP-TPR investigation showed the presence of di-aryl sulfides, aryl-alkyl sulfides, simple and complex thiophenic structure in all representative, low and high sulfur samples. There is no indication of the presence of pure di-alkyl sulfides, disulfides or thiols, nor oxidised sulfur forms in the samples. Only minor differences in sulfur functionalities were detected qualitatively between the three analogue samples. Quantitatively differences in sulfur forms were found to be more pronounced. The presence of CH fragments on gases emitted during AP-TPR investigation of demineralised and demineralised pyrite-free samples, shows evidence for the formation of shorter CH chains and of polyaromatic compounds, and lower molecular weight material in the demineralization and pyritic sulfur removal stages.     

Characteristics of coal combustion products (CCP's) from Kentucky power plants, with emphasis on mercury content

The Center for Applied Energy Research conducts a survey of Kentucky's coal-fired power plants every five years. The last survey was conducted in 2002 and covered most units at all of the plants in Kentucky. Special emphasis was placed on the spatial distribution of ash products, with each row of ESP's or baghouses samples wherever possible. In this manner, we can track the change in concentration of trace elements with relative temperature of the flue gas. Certain elements, such as Zn, Pb, and As, are known to be temperature dependent. The behavior of Hg, while also temperature dependent, is more complex owing to the adsorption of Hg on fly ash carbon. The survey provides a wide array of coal sources, ESP/baghouse collection temperatures, and fly ash carbon composition, all important in determining the behavior of Hg in the flue gas. In addition, many plants have FGD systems, allowing an assessment of the efficiency of FGD in capturing Hg from the post-ESP flue gas.     

PREDICTION OF SOLIDS CIRCULATION RATE IN THE RISER OF AN INTERNALLY CIRCULATING FLUIDIZED BED (ICFB)

A predictive mathematical model based on pressure drop in the riser of an Internally Circulating Fluidized Bed (ICFB) is developed to predict solids circulation rate (Gs). The values of G_s predicted using the model agree well with the experimental data. The model also predicts well G_s at higher operating temperatures in the riser of an ICFB.     

PREDICTION OF SOLIDS CIRCULATION RATE IN THE RISER OF AN INTERNALLY CIRCULATING FLUIDIZED BED (ICFB)

A predictive mathematical model based on pressure drop in the riser of an Internally Circulating Fluidized Bed (ICFB) is developed to predict solids circulation rate (Gs). The values of G_s predicted using the model agree well with the experimental data. The model also predicts well G_s at higher operating temperatures in the riser of an ICFB.     

节涌流化床压力波动动力学研究(Ⅰ)波动机理与数学模型

分析了气固节涌流化床的非均匀特性及其特殊的波动结构.基于波动机理分析,提出了离散化的动力学分析方法,并建立了节涌流化床多自由度随机波动模型.结合频谱分析和统计分析方法,求得了模型矩阵解G_(△j△k)(ω)=H~*(ω)G_(δjδk)(ω)H~T(ω)此解可以表达节涌流化床的波动结构和压力波动功率谱.     

液固循环流化床的研究(Ⅰ)相含率及颗粒循环速率

在内径为140mm、高为3m的有机玻璃设备中对液固并流向上的循环流化床中两相流动特性进行了研究.相含率的研究表明,在循环流态化条件下,全床相含率轴向均匀分布;而径向具有显著的不均匀性,表现为中心区液相含率高,在r/R=0.7处液合率最低.颗粒循环速率的研究表明,在循环流态化区域,在一定的二次水流速下,颗粒循环速率不随总液速的改变而改变.由于在循环流态化下存在床层径向参数的不均匀性,因此床层的相合率与颗粒循环速率的关系与广义流态化方法预测的结果间存在差异,即在操作条件一定的情况下,床层内真实液合率比由广义流态化预测的低.     

Characterization and re-use potential of by-products generated from the Ohio State Carbonation and Ash Reactivation (OSCAR) process

Two novel sorbents (i.e. “regenerated sorbent” and “supersorbent”) for dry flue gas desulfurization were tested, and by-products characterized, using a pilot-scale version of the Ohio State Carbonation and Ash Reactivation (OSCAR) process. The main elements of the process consisted of sorbent production, a riser reactor, cyclone and baghouse. Trace elements, including As, Se and Hg, were found at higher levels in the OSCAR solid by-products (in both the cyclone and baghouse) compared to traditional lime spray dryer (LSD) ash. Polycyclic aromatic hydrocarbons (PAHs) detected on solid by-products were primarily small molecular weight compounds at low concentration (e.g., μg/kg). Small particulates (?3 μm) that escaped from the cyclone and were captured by the baghouse showed higher trace element concentrations, possibly due to the lower operating temperature and greater specific surface area of solids in the baghouse. Operating conditions including flue gas flow rate and sorbent injection rate influenced the levels of trace elements and PAHs in OSCAR by-product material. Capture of PAHs was observed to increase with Ca concentration in experiments using supersorbent injection. However, possible release of PAHs occurred with regenerated sorbent injection. The concentrations of trace elements in leachate for all OSCAR cyclone samples tested were below Resource Conservation and Recovery Act limits. The concentrations of most trace elements in OSCAR by-product were also below the limits regulated in the EPA 503 Rule except As and Se. The similarity in the physical and engineering properties of OSCAR cyclone samples to natural cohesive soils suggests that this material can be utilized in a variety of construction, reclamation, and agricultural applications.     

Reaction characteristics investigation of CeO2-enhanced CaSO4 oxygen carrier with lignite

Calcium sulfate (CaSO₄) has been verified as a promising oxygen carrier (OC) in the chemical looping combustion (CLC) for its high oxygen capacity, abundant reserve and low cost, but its low reactivity and deleterious sulfur species emission from the side reactions of CaSO₄ should be well considered for its wide application in CLC. In order to promote the reactivity of CaSO₄ and increase its potential to inhibit the gaseous sulfur emission, a CeO₂-enhanced CaSO₄ OC mixed OC of core-shell structure was prepared using the combined template synthesis method. Reaction characteristics of the prepared CaSO₄-CeO₂ mixed OC with a typical lignite was first conducted and systematically investigated, and an improved reactivity of the prepared CaSO₄-CeO₂ mixed OC was demonstrated than its single component CaSO₄ or CeO₂ due to the fast transfer and exchange of oxygen from the CaSO₄ substrate to coal via the doped CeO₂. Furthermore, the solid products formed from the mixed CaSO₄-CeO₂ OC with the selected coal were collected and analyzed. Especially, evolution and redistribution of the sulfur species of different forms were focused. At the latter reaction stage of YN reaction with the CaSO₄-CeO₂ mixed OC, the SO₂ emitted from the side reactions of CaSO₄ was greatly diminished and the doped CeO₂ was proven effective to directionally fix the SO₂ released to turn into different solid sulfur compounds, which were determined as Ce₂O₂S, Ce₂S₃ and Ce₂(SO₄)₃·5H₂O and formed through the different pathways. In addition, good regeneration of the reduced CaSO₄-CeO₂ mixed OC could be reached in spite of the unavoidable interaction between the included minerals in coal and the reduced mixed OC. Overall, the combined template method-made CaSO₄-CeO₂ mixed OC reported herein was not only endowed with enhanced reactivity for coal conversion, but also owned the potential to directionally fix the gaseous sulfur emission, which is quite applicable as OC for simultaneous decarbonatization and desulfurization in the real CLC process.     

Multiphase CFD Modeling for a Chemical Looping Combustion Process (Fuel Reactor)

There are growing concerns about increasing emissions of greenhouse gases and a looming global warming crisis. CO₂ is a greenhouse gas that affects the climate of the earth. Fossil fuel consumption is the major source of anthropogenic CO₂ emissions. Chemical looping combustion (CLC) has been suggested as an energy‐efficient method for the capture of carbon dioxide from combustion. A chemical‐looping combustion system consists of a fuel reactor and an air reactor. The air reactor consists of a conventional circulating fluidized bed and the fuel reactor is a bubbling fluidized bed. The basic principle involves avoiding direct contact of air and fuel during the combustion. The oxygen is transferred by the oxygen carrier from the air to the fuel. The water in combustion products can be easily removed by condensation and pure carbon dioxide is obtained without any loss of energy for separation. With the improvement of numerical methods and more advanced hardware technology, the time required to run CFD (computational fluid dynamic) codes is decreasing. Hence, multiphase CFD‐based models for dealing with complex gas‐solid hydrodynamics and chemical reactions are becoming more accessible. To date, there are no reports in the literature concerning mathematical modeling of chemical‐looping combustion using FLUENT. In this work, the reaction kinetics models of the (CaSO₄ + H₂) fuel reactor is developed by means of the commercial code FLUENT. The effects of particle diameter, gas flow rate and bed temperature on chemical looping combustion performance are also studied. The results show that the high bed temperature, low gas flow rate and small particle size could enhance the CLC performance.     

Reactivity and stability of Ni/Al2O3 oxygen carrier for chemical-looping combustion (CLC)

Chemical-looping combustion (CLC) is a technology that reduces the carbon dioxide emissions from fossil fuel power stations. A nickel supported on -alumina oxygen carrier is investigated in this study, for use in a CLC process. Oxygen carriers with various nickel loadings on alumina are prepared according to the incipient wetness technique. The reactivity and stability of the prepared oxygen carrier samples, during repeated reduction–oxidation cycles, is demonstrated using temperature programmed reduction and oxidation. Pulse chemisorption results show that the dispersion and active crystallite diameter of the nickel particles remain constant over multiple reduction–oxidation cycles, indicating that no agglomeration occurs up to a nickel loading of 20 wt% supported on alumina. The stability and reactivity of the oxygen carriers, under industrial relevant conditions, are also investigated using the CREC fluidized bed riser simulator. It is observed that a 20 wt% nickel supported on alumina oxygen carrier is stable under industrial relevant fluidized bed reaction conditions, converting 76% of methane to carbon dioxide and water vapor, the combustion products. The metal support interaction is assessed by H₂ temperature programmed desorption, which shows that the metal-support interaction decreases as more nickel is loaded onto the alumina support.     

(MnzFe1—z)yOx combined oxides as oxygen carrier for chemical‐looping with oxygen uncoupling

Oxygen carrier particles with the composition (Mn_0.8,Fe_0.2)₂O₃ were found to readily release gas phase oxygen at 850°C, and were capable to oxidize CH₄ completely and convert wood char rapidly to CO₂ during experiments in a batch fluidized bed reactor. The particles were able to release oxygen corresponding to more than 3% of their mass in less than 40 s. Because of the low price and favourable environmental properties of manganese and iron oxides, this finding could be of great importance for the development of chemical‐looping combustion with oxygen uncoupling. © 2012 American Institute of Chemical Engineers AIChE J, 59: 582–588, 2013     

On the high‐gasification rate of Brazilian manganese ore in chemical‐looping combustion (CLC) for solid fuels

The high rate of char gasification observed when using a Brazilian manganese ore as compared to ilmenite is investigated in a batch fluidized‐bed reactor. Experiments were carried out at 970°C using petroleum coke, coal and wood char as fuel with a 50% H₂O in N₂ as fluidizing gas. A manufactured manganese oxygen carrier was also used, however, which presented a slower char conversion rate than the manganese ore. It is concluded that decrease in H₂ inhibition and oxygen release are unlikely to be the main responsible mechanisms for the ore's unexpected gasification rate. The ore was also mixed in different ratios with ilmenite and it was observed that the presence of even small amounts of ore in the bed resulted in increased gasification rate. Thus, the high‐gasification rate for the manganese ore could be due to a contribution from the impurities in the ore by catalyzing the gasification reaction. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4346–4354, 2013     

Production and examination of oxygen‐carrier materials based on manganese ores and Ca(OH)2 in chemical looping with oxygen uncoupling

This study concerns production of oxygen‐carrier particles using six different manganese ores. The ores were made to react with Ca(OH)₂ at elevated temperature, forming calcium manganite. The method utilized to manufacture particles was extrusion. Methane and syngas conversion and oxygen release of the samples in inert atmosphere were investigated. The oxygen carrier based on South African (B) manganese ore, showed good methane conversion and was able to transfer oxygen corresponding to 1.5% of its mass during reduction with gaseous fuel. All examined oxygen carriers were capable of converting syngas completely. The ability to release gaseous oxygen was examined by adding wood char in a stream of nitrogen for four selected samples sintered at 1300°C/6 h. These samples released an amount of oxygen corresponding to 0.37–0.68% of their mass. The reactivity of all the ores was improved after the proposed treatments. Reactivity results of the oxygen carrier made from South African (B) ore and Ca(OH)₂, sintered at 1300°C for 6 h were the most promising. Attrition measurements with a jet cup of the oxygen carriers sintered at 1300°C/6 h showed that all the samples made from ores were at least three times more resistant to mechanical attrition compared to particles made from synthetic Mn₂O₃. Producing feasible oxygen carriers directly from ores could potentially cut the cost of chemical looping with oxygen uncoupling and have a significant impact on its competitiveness among other carbon capture technologies. © 2013 American Institute of Chemical Engineers AIChE J 60: 645–656, 2014     

Process development for the catalytic conversion of cyclohexene oxide and carbon dioxide into poly(cyclohexene carbonate)

The synthesis of polycarbonates from carbon dioxide is an alternative route to the use of phosgene for the introduction of the carbonate functionality in these polymers. In this work, a preliminary process design has been made for the catalytic conversion of cyclohexene oxide and carbon dioxide into poly(cyclohexene carbonate). For this purpose, the low- and high-pressure phase behaviour, reaction rate and downstream removal of residual cyclohexene oxide from the product stream are described. Based on these findings a design for such a process is suggested and evaluated from a technical and economic perspective. The economic evaluation indicates that the required selling price of PCHC is intermediate to the market price of engineering and specialty polymers.