稻壳在循环流化床中燃烧现象的分析

为了研究稻壳在循环流化床中的燃烧特性，在生物质循环流化床试验台上对稻壳进行了燃烧试验。通过对试验过程中各测点温度及压力变化的分析，探讨了二次风和循环回料对稻壳在循环流化床燃烧炉内燃烧过程的影响。试验结果表明：二次风可以促进挥发分在稀相区的燃烧，对提升稀相区的温度作用明显；正常循环回料使得温度沿炉膛高度均匀分布。所得结论对生物质循环流化床的试验研究及实际运行有一定的参考意义。     

粒度对循环流化床锅炉燃烧影响的试验研究

循环流化床燃烧技术具有高效,低污染,燃料适应性广的优点,燃用福建无烟煤的循环流化床锅炉在福建沿海地区得到了广泛开展,取得了许多的成果,但是普遍存在的问题是飞灰含炭量太高,热效率不高。在一台75 t.h-1的循环流化床锅炉上对三种不同粒度的福建无烟煤进行了燃烧试验,测量了炉膛各段的温度水平,获得了粒度对底渣、飞灰的粒径分布及其含炭量和热效率等的影响特性,并针对燃用福建无烟煤的循环流化床锅炉的优化燃烧提出了一些建议,对锅炉的运行具有一定的指导意义。     

Numerical simulation on pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed

High temperature air combustion is a prospecting technology in energy saving and pollutants reduction. Numerical simulation on pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed was presented. The down-fired combustor, taken as the calculation domain, has the diameter of 220 mm and the height of 3000 mm. 2 cases with air staging combustion are simulated. Compared the simulation results with experimental data, there is a good agreement. It is found that the combustion model and NOx formation model are applicable to simulate the pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed. The results show that there is a uniform temperature profile along the axis of the down-fired combustor. The NOx emissions are lower than those of ordinary pulverized coal combustion, and the NOx emissions are 390 mg/m3 and 352 mg/m3 in Case 1 and Case 2, respectively. At the range of 300-600 mm below the nozzle, the NO concentration decreases, mainly resulting from some homogeneous reactions and heterogeneous reaction. NO concentration has a little increase at the position of 800 mm below the nozzle as the tertiary air supplied to the combustor at the position of 600 mm below the nozzle.     

Characterization of ashes from a 100 kWth pilot-scale circulating fluidized bed with oxy-fuel combustion

Oxy-fuel combustion experiments have been carried out on an oxygen-fired 100 kW_th mini-circulating fluidized bed combustion (CFBC) facility. Coal and petroleum coke were used as fuel together with different limestones (and fixed Ca:S molar ratios) premixed with the fuel, for in situ SO₂ capture. The bed ash (BA) and fly ash (FA) samples produced from this unit were collected and characterized to obtain physical and chemical properties of the ash samples. The characterization methods used included X-ray fluorescence (XRF), X-ray diffraction (XRD), char carbon and free lime analysis, thermogravimetric analysis (TGA), and surface analysis. The main purpose of this work is to characterize the CFBC ashes from oxy-fuel firing to obtain a better understanding of the combustion process, and to identify any significant differences from the ash generated by a conventional air-fired CFBC. The primary difference in the sulfur capture mechanism between atmospheric air-fired and oxy-fuel FBC, at typical FBC temperatures (∼850 °C), is that, in the air-fired case the limestone sorbents calcine, whereas the partial pressure of CO₂ in oxy-fuel FBC is high enough to prevent calcination, and hence the sulfation process should mimic that seen in pressurized FBC (PFBC). Here, the char carbon content in the fly ash was much higher than that in the bed ash, and was also high by comparison with ash obtained from conventional commercial air-firing CFBC units. In addition, measurements of the free lime content in the bed and fly ash showed that the unreacted Ca sorbent was present primarily as CaCO₃, indicating that sulfur capture in the oxy-fuel combustor occurred via direct sulfation. Limestone utilization for oxy-fuel combustion in this unit was generally lower than that in industrial-scale air-firing CFBCs, with better limestone performance found during combustion of petcoke running at relatively higher temperatures. The Brunauer–Emmett–Teller (BET) surface area and also the pore volume in the fly ash were much higher than in the bed ash and smaller size pores predominated in the fly ash samples.     

Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion

Fluidized bed combustion has been proven to be an attractive method for the conversion of agroresidues to energy offering economical and environmental benefits. The low melting point ash of agroresidues cause a number of problems e.g., sintering, agglomeration, deposition, etc., which consist the main obstacles for economical and viable application of this conversion method. Leaching that is considered to be a low cost pretreatment technique for the elimination of ash related problems in biomass boilers studied here. The produced results clearly demonstrate that leaching could help significantly to reduce the ash related problems caused during the operation of fluidized bed combustors with biomass.     

不同采矿深度准东煤物化特性和燃烧性能研究

选取天池能源浅层和深层煤样进行研究,以大同烟煤作为对比,了解采矿深度对煤质及燃烧特性的影响,为准东煤的勘探和燃烧提供必要的理论依据。研究发现,随着采矿深度的增加,煤中水分减少、灰分降低、发热量增加、含硫量减少,煤质更加接近烟煤;煤灰中Fe2O3明显减少,煤的结渣趋势减轻;煤的着火温度略有降低,燃烧速率提高,燃烧特性变好。与大同烟煤相比,准东煤高水分、低发热量、低氮含量,燃烧时着火温度显著降低且燃烧速率低。     

The simulation of influence of different coals on the circulating fluidized bed Boiler’s combustion performance

The combustion performance of the boiler largely depends on the coal type. Lots of experimental research shows that different fuels have different combustion characteristics. It is obvious that fuel will change the whole operating performance of Circulating Fluidized Bed Combustion (CFBC). We know even in a pilot-scale running boiler, the measurement of some parameters is difficult and costly. Therefore, we developed the way of simulation to evaluate the combustion performance of Chinese coals in CFB. The simulation results show that, different coals will result in different coal particle diameter and comminution depending on their mineral component and the change will affect the distribution of ash in CFBC system. In a word, the computational results are in accordance with experimental results qualitatively but there are some differences quantitatively.     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

Effects of temperature and limestone on sulfur release behaviors during fluidized bed gasification

Gaseous sulfur is released during fluidized bed coal gasification, and control the yield of gaseous sulfur or the conversion between gaseous organic sulfur and inorganic sulfur at source is necessary, because it can economically satisfy the requirements of industrial production and protect the environments. In this study, sulfur release behaviors of a middle-sulfur coal called Guizhou coal were quantitatively determined through controlled experiments in a lab-scale fluidized bed during oxygen rich-steam gasification. The measured gaseous sulfur species were H₂S, SO₂, COS and CS₂. The effects of temperature (850^OC-950^OC) and limestone (Ca/S = 2) on the sulfur release behaviors were investigated. Among the above four gaseous sulfur, the yield of H₂S is the highest, followed by COS, while only less than 1.5% of sulfur in coal is released as SO₂ and CS₂. With the increase in temperature, the yield of H₂S increases while that of SO₂ decreases, and the change of COS yield and CS₂ yield is not obvious. The molar ratio of H₂S/COS increases with increasing temperature, which is qualitatively matched by thermodynamic analysis. The addition of limestone reduces the released sulfur but not change the distribution of gaseous sulfur forms. Meanwhile, the molar ratio of H₂S/COS increases after adding limestone, while the trend with temperature of H₂S/COS does not change. The removal rate of H₂S is between 23% and 28%, which increases with temperature. The distributions of sulfur in bottom char and fly ash are similar. The main sulfur species in the bottom char is organic sulfur, and thiophene dominates the organic sulfur. The increase of temperature and the addition of limestone will both promote the increase of inorganic sulfur content, and the decrease of organic sulfur content.     

Release and transformation characteristics of Na/Ca/S compounds of Zhundong coal during combustion/CO2 gasification

Zhundong (ZD) coal from northwest China is a high quality steam coal with reserves of more than 390 billion tons. However, the utilization of ZD coal is limited due to the high content of alkali and alkaline earth metals. This study aimed at revealing the release and transformation mechanism of Na/Ca/S compounds during combustion/gasification of ZD coal. The results demonstrate that Na was primarily influenced by temperature, mostly releases at 600–800 °C. The transformation of Ca compounds was affected by both temperature and atmosphere. The high temperature of the combustion process could accelerate the decomposition of CaCO₃ and CaSO₄, and the high content of CO₂ during gasification prolonged the decomposition of CaCO₃. The transformation of S was primarily influenced by atmosphere. SO₂ could react with CaO and form CaSO₄ during the combustion process. While S compounds were mainly released as S (g) and H₂S (g) during gasification process. There was a significant interaction among Na/Ca/S compounds during combustion, original CaSO₄ in coal could adsorb Na compounds with SO₂ at 600–800 °C and then reacted with aluminosilicates, by this reaction, Na could be fixed above 1000 °C.     

Overview of combustion and gasification of rice husk in fluidized bed reactors

Rice is cultivated in more than 75 countries in the world. The rice husk is the outer cover of the rice and on average it accounts for 20% of the paddy produced, on weight basis. The worldwide annual husk output is about 80 million tonnes with an annual energy potential of 1.2 × 10⁹ GJ corresponding to a heating value of 15 MJ/kg. India alone generates about 22 million tonnes of rice husk per year. If an efficient method is available, the husk can be converted to a useful form of energy to meet the thermal and mechanical energy requirements of the rice mills themselves. This paper provides an overview of previous works on combustion and gasification of rice husk in atmospheric bubbling fluidized bed reactors and summarizes the state of the art knowledge. As the high ash content, low bulk density, poor flow characteristics and low ash melting point makes the other types of reactors like grate furnaces and downdraft gasifiers either inefficient or unsuitable for rice husk conversion to energy, the fluidized bed reactor seems to be the promising choice. The overview shows that the reported results are from only small bench or lab scale units. Although a combustion efficiency of about 80% can normally be attained; the reported values in the literature, which are more than 95%, seem to be in higher order. Combustion intensity of about 530 kg/h/m² is reported. It is also technically feasible to gasify rice husk in a fluidized bed reactor to yield combustible producer gas, even with sufficient heating value for application in internal combustion engines. A combustible gas with heating value of 4-6 MJ/Nm³ at a rate of 2.8-4.6 MW_th/m² seems to be possible. Only very little information is available on the pollutant emissions in combustion and tar emissions from gasification. The major conclusion is that the results reported in the literature are limited and vary widely, emphasizing the need for further research to establish suitable and optimum operating conditions for commercial implementations.     

Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace

Zhundong coalfield is one super-large coalfield recently discovered in China. However, the utilization of Zhundong coal in power plants has caused serious ash-related issues mainly due to its high-sodium feature. The ash deposition problem on convection heat exchanger surfaces is still particularly difficult to resolve and its mechanism has yet to be fully understood. This study deals with the ash deposition and alkali metal migration behaviors on convection heat exchanger surfaces between 400 and 800 °C during combustion of Zhundong coal using a lab-scale drop tube reactor. Experimental results show that the sodium content in ash deposit of Zhundong coals increases obviously as the deposition temperature decreases from 800 to 600 °C, while it is almost unchanged below 600 °C. The contents of iron and calcium in ash deposits exhibit nonmonotonic variations as the deposit probe temperature varies between 400 and 800 °C. Quartz and calcium sulfate are main crystalline phases in ash deposit of Zhundong coals. Calcium is inclined to present as calcite and lime at low deposition temperature, while high temperature facilitates calcium sulfation. Sodium of crystalline phase is found as albite and sodium sulfate at low deposition temperature. Both condensation of gaseous alkali metals and formation of low-melting minerals were responsible for the ash deposition phenomenon on convection heat exchanger surfaces involved in combustion of Zhundong coal. The sodium content in ash deposit decreases considerably with the increasing combustion temperature while the case of iron variation is opposite due to its low-volatility. In addition, the Na content in ash deposits increases obviously with the access air ratio reduced from 1.2 to 1.05, but the local weakly reducing atmosphere leads to less iron within ash deposits. Clarification of sodium migration and evaluation of ash deposition behaviors during combustion of Zhundong coal is helpful for a better exploration of the functional mechanism of ash deposit and then large-scale utilization of high-sodium coals.     

准东煤与福建无烟煤燃烧特性的对比实验研究

通过对比准东煤与福建无烟煤的煤质分析、燃烧特性指数、燃烧动力学参数等,分析了准东煤与福建无烟煤的燃烧特性差异。结果表明：准东煤的挥发分明显高于福建无烟煤,属于极易着火的煤种;福建无烟煤与准东煤硫分低,燃烧中对环境污染小,但灰熔点都相对较低,容易结渣;准东煤的热重曲线较福建无烟煤向低温区偏移,峰区较宽,燃烧失重峰为双峰,着火方式由非均相着火向均相着火转变。准东煤的综合燃烧特性指数较大,表观活化能小;准东煤的煤反应活性和燃烧特性明显优于福建无烟煤。依热重分析,准东煤与福建无烟煤的着火温度分别为673.17 ℃和397.49 ℃,燃尽温度分别为803.05 ℃和741.18 ℃,活化能分别为191.39 kJ/mol和26.32 kJ/mol,综合燃烧特性指数分别为3.162×10-5和2.775×10-3。     

Investigation on PM formation from combustion of lignite with high contents of AAEMs (alkali and alkaline earth metals) and Fe: Effect of the reaction temperature

Coal-fired power plants require higher flexibility and a broader range of the operating temperature than before to accommodate the load regulation of the power grid. The relationship between the reaction temperature and the characteristics of particulate matter (PM) need to be better understood. In this study, Zhundong coal combustion was conducted in a drop tube furnace at different reaction temperatures in air. The PM characteristics and elemental contributions are investigated in detail. The experimental results show that the mass yields of PM_0.4 and PM_0.4-10 are non-monotonic with the reaction temperature. The competition between the generation of inorganic fumes and the removal of inorganic fumes by Si–Al-bearing minerals governs the mass yield of PM_0.4. At higher reaction temperature, generation of Ca, Mg, Fe-containing fumes increases, contributing most to the increment of PM_0.4; while the sulfation of chlorides is inhibited, resulting in more Cl in PM_0.4. The S content in PM_0.4 is mainly affected by the sulfation of AAEMs (alkali and alkaline earth metals) oxides. The mass yield of PM_0.4-10 is controlled by the competition between the fragmentation of char or mineral particles and the coalescence of mineral particles. For Zhundong coal combustion, the reaction temperature is recommended to be 1273K–1373K to control PM emission.     

Migration and transformation of sodium and chlorine in high-sodium high-chlorine Xinjiang lignite during circulating fluidized bed combustion

The Xinjiang lignite mined from Shaerhu coalfield (SEHc) easily causes severe fouling and corrosion because of its high sodium and chlorine contents. Therefore, it is necessary to study the migration and transformation behavior of sodium and chlorine during combustion in order to reveal the mechanisms of fouling and corrosion, and propose the effective solutions of above problems. In this study, based on the 0.4 T/D circulating fluidized bed (CFB) test system, the migration and transformation behavior of sodium and chlorine in SEHc during combustion at 950 °C was explored. The migration and transformation paths of sodium and chlorine were proposed through the chemical characterization of ash samples along the flue gas flow direction, as well as the thermodynamic equilibrium calculation by the software of Factsage 6.1. The experimental studies show the sodium and chlorine mainly in the form of NaCl crystal in raw coal underwent a series of physical and chemical changes during combustion, and subsequently distributed in bottom ash/circulating ash, fly ash and gas phase in various forms including sodium aluminosilicates, chlorides and sodium oxides. Sodium was more inclined to be resided in ash in the form of aluminosilicates through the reactions with other minerals (SiO₂ and Al₂O₃), while chlorine was easily released into the flue gas in forms of HCl, Cl₂, NaCl, etc. The Cl-based species might result in the corrosion of metal heating surfaces because of the presence of corrosion products (metal chlorides) in fly ash. As temperature decreased, the sodium or chlorine vapors would successively deposit in fly ash via physical condensation or chemical reaction. At 840～570 °C, the sodium-based species (Na₂O and NaCl) would first deposit in fly ash, then gaseous chlorine species (NaCl, FeCl₃ and so on) primarily deposited at 570～180 °C.     

Experimental studies of ash film fractions based on measurement of cenospheres geometry in pulverized coal combustion

In pulverized coal particle combustion, part of the ash forms the ash film and exerts an inhibitory influence on combustion by impeding the diffusion of oxygen to the encapsulated char core, while part of the ash diffuses toward the char core. Despite the considerable ash effects on combustion, the fraction of ash film still remains unclear. However, the research of the properties of cenospheres can be an appropriate choice for the fraction determination, being aware that the formation of cenospheres is based on the model of coal particles with the visco-plastic ash film and a solid core. The fraction of ash film X is the ratio of the measuring mass of ash film and the total ash in coal particle. In this paper, the Huangling bituminous coal with different sizes was burnt in a drop-tube furnace at 1273, 1473, and 1673 K with air as oxidizer. A scanning electron microscope (SEM) and cross-section analysis have been used to study the geometry of the collected cenospheres and the effects of combustion parameters on the fraction of ash film. The results show that the ash film fraction increases with increasing temperature and carbon conversion ratio but decreases with larger sizes of coal particles. The high fraction of ash film provides a reasonable explanation for the extinction event at the late burnout stage. The varied values of ash film fractions under different conditions during the dynamic combustion process are necessary for further development of kinetic models.     

A cold model experimental study on the flow characteristics of bed material in a fluidized bed bottom ash cooler in a CFB boiler

IntroductionA fluidized bed bottom ash cooler is often used totreat high temperature bottom ash to reclaim heat andfine particles from the ash, and to have the ash easilytransported. Among the large CFB boilers in operation inthe world, there are many ash coolers that often workabnormally['-','].There are six fluidized bed bottom ash coolers in theimported 410im CFB boiler that was built and operatedin Gaoba power plant, Sichuan province, China in 1996N].High temperature slag-bond and jam …     

Optimization of emissions from fluidized bed combustion of coal,biofuel and waste

Several types of fuel can be burned in a fluidized bed. This paper discusses the impact of low‐ or medium‐volatile fuels (coal) and high‐volatile fuels (biomass and waste) on the emissions of NO, N₂O and other pollutants. It is found that high‐ and low‐volatile fuels behave in different ways, and measures to reduce emissions from coal combustion are not necessarily effective for high‐volatile fuels. This forms a basis for various design options. Copyright © 2002 John Wiley & Sons, Ltd.     

Experimental Study on an On—Line Measurement of High Temperature Circulating Ash Flux in a Circulating Fluidized Bed Boiler

A new kind of measuring method that may be used to measure high temperature circulating solid particles flux in a circulating fluidized bed boiler is studied in this paper. The measuring method is founded on the principle of thermal equilibrium. A series of cold tests and hot tests were carried to optimize the structure and collocation of water-cooling tubes and showed that the method had the advantage of simple, accurate, reliable and good applicability for on-line usage in a circulating fluidized bed boiler.     

Particle-metal interactions during combustion of pulp and paper biomass in a fluidized bed combustor

We compare interactions between metals and solid particles during the classic fluidized bed combustion (FBC) and a new low-high-low temperature (LHL) combustion of selected biomass. The biomass was a mixture of bark and pine wood residues typically used by a paper mill as a source of energy. Experiments, conducted on a pilot scale, reveal a clear pattern of surface predominance of light metals (Ca, Na, K) and core predominance of heavy metals (Cd, Cr) within the LHL-generated particles. No such behavior was induced by the FBC. Metal migration is linked to the evolution of inorganic particles. A composite picture of the metal rearrangements in the particles was obtained by a combination of independent analytical techniques including electron probe microanalysis, field emission scanning electron microscopy, inductively coupled plasma spectrometry, and X-ray diffractometry. It is suggested that the combination of (1) the high-temperature region in the LHL and (2) changes in the surface free energy of the particles is the driving force for the metal-particle behavior. Important practical implications of the observed phenomena are proposed, including removal of hazardous submicron particulate and reduction in fouling/slagging during biomass combustion. These findings may contribute to redesigning of currently operating FBC units to generate nonhazardous, nonleachable, reusable particles where heavy metals are immobilized while environmental and technological problems reduced.