杜仲药渣与煤的混烧特性及灰熔融特性研究

本文采用热重分析仪分析了杜仲药渣和石下江煤的混合样品的燃烧特性,采用灰熔点测试仪、X射线荧光仪、X射线衍射仪及扫描电子显微镜对混合物灰样的灰熔融特性变化进行了分析.结果表明:药渣掺混比为40%时着火温度最低;加入90%药渣时,稳燃性指数、综合燃烧特性指数、最大燃烧速率和平均燃烧速率值均达到最大,燃尽温度最小;在杜仲叶渣和煤的共燃过程中,发生了协同作用,生成了一些新的矿物质,混合灰样的灰熔融温度随着药渣质量分数的增多而减小.     

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.     

Production of hydrogen in an autothermal fluidized gasifier

Production of hydrogen has gained increasing importance because of its use as a clean fuel and also for various industrial uses. For this purpose a laboratory scale fluidized bed autothermal gasifier has been designed to gasify the solid carbonaceous materials with steam. The heat required for the steam-carbon reaction is provided by the partial oxidation of the hydrocarbon gases. Highly reactive carbonaceous materials like charcoal and lignite have been gasified with steam in the presence of a high temperature flame produced by the partial oxidation of acetylene/natural gas with oxygen. About 95% carbon gasification and 40% steam decomposition have been achieved. The product gases contain about 80% synthesis gas (carbon monoxide and hydrogen). Further investigations with non-caking high ash Indian coals and agricultural wastes are planned to be carried out.     

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.     

玉米秆与玉米芯热重分析

用热重分析仪对玉米秆和玉米芯的热解和燃烧动力学特性进行了研究,通过热解试验发现玉米秆和玉米芯挥发分的析出基本在一个阶段内完成,而燃烧试验研究表明燃烧过程主要由挥发分的燃烧和焦碳及残余挥发分的燃烧这两个阶段组成。上述结果为进一步有效利用玉米秸秆提供了一定的理论基础。     

Effect of temperature on Lu’an bituminous char structure evolution in pyrolysis and combustion

In the process of pyrolysis and combustion of coal particles, coal structure evolution will be affected by the ash behavior, which will further affect the char reactivity, especially in the ash melting temperature zone. Lu’an bituminous char and ash samples were prepared at the N₂ and air atmospheres respectively across ash melting temperature. A scanning electron microscope (SEM) was used to observe the morphology of char and ash. The specific surface area (SSA) analyzer and thermogravimetric analyzer were respectively adopted to obtain the pore structure characteristics of the coal chars and combustion parameters. Besides, an X-ray diffractometer (XRD) was applied to investigate the graphitization degree of coal chars prepared at different pyrolysis temperatures. The SEM results indicated that the number density and physical dimension of ash spheres exuded from the char particles both gradually increased with the increasing temperature, thus the coalescence of ash spheres could be observed obviously above 1100°C. Some flocculent materials appeared on the surface of the char particles at 1300°C, and it could be speculated that β-Si₃N₄ was generated in the pyrolysis process under N₂. The SSA of the chars decreased with the increasing pyrolysis temperature. Inside the char particles, the micropore area and its proportion in the SSA also declined as the pyrolysis temperature increased. Furthermore, the constantly increasing pyrolysis temperature also caused the reactivity of char decrease, which is consistent with the results obtained by XRD. The higher combustion temperature resulted in the lower porosity and more fragments of the ash.     

Experimental study on ash deposition of Zhundong coal in oxy-fuel combustion

To facilitate the large-scale utilization of high-alkali and -alkaline earth metals (AAEMs) coals in power generation, the ash deposition behaviors of a typical Zhundong coal in oxy-fuel combustion were experimentally investigated using a drop tube furnace. A wall-temperature-controlled ash deposition probe by which the bulk gas temperature could be measured simultaneously was designed and employed in the experiments. The deposition tendencies, ash morphologies, chemical compositions of deposited ash particles were studied respectively under various oxygen concentrations, bulk gas temperatures, probe surface temperatures and probe exposure times. The experimental results revealed that the oxygen concentration had a significant influence on the deposition behavior during oxy-fuel combustion of high-alkali coal. Compared with air case, more fine ash particles were generated during the combustion of Zhundong coal in 21% O₂/79% CO₂ atmosphere but the deposition tendency was weaker. However, a higher oxygen concentration could aggravate the tendency of ash deposition. The high contents of iron (Fe), calcium (Ca), sulfur (S), and sodium (Na) in Zhundong coal could result in the generations of low-melting point compounds. Calcium in flue gas existed as CaO and was captured prior to SO₃ by the probe surface during the ash deposition process. At the initial 30 min of the ash deposition process, the dark spherical fine ash particles rich in Fe, Na, oxygen (O), and S were largely produced, while in the range of 60–90 min the light spherical fine ash particles with high contents of Ca, barium (Ba), O, and S were generated on the other hand. The deposition mechanisms at different stages were different and the melted CaO (BaO)/CaSO₄ (BaSO₄) would give rise to a fast growth rate of ash deposit.     

Waste wood characterization and combustion behaviour in pilot lab scale

This work aims at testing the suitability of waste wood as feedstock in an industrial combustion unit. Firstly, an extensive characterization of properties relevant for combustion was performed on two samples of waste wood and one sample of natural wood for comparison. Both waste wood samples were found to contain significantly higher amounts of metals such as iron and titanium than natural wood. However, one sample was much richer than the other one in all types of impurities, which resulted in a particularly high amount of ash (about 10 w %), and the sample was also found to contain many fine particles. These observations seemed to be explained by an external pollution of the sample by soil. Then, experiments were carried out in a bench-scale fixed-bed combustion unit on the different samples under variable conditions of air flow. All samples could be successfully burnt. However, the waste wood sample polluted with soil appeared to have a lower conversion rate due to its heterogeneity of particle size. A separation between fines and the rest of the sample seemed to solve this issue. Regarding gas emissions, as expected, both waste wood samples gave rise to higher amounts of NO_x emissions than natural wood due to their content.     

Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor

In this study, wheat straw pyrolysis was conducted in an entrained flow reactor at 900–1300 °C, and PM_2.5 were sampled from the flue gas through a heated sampling system. Multi-phase PM_2.5 including carbonaceous matter, potassium-containing particles, and ash particles, was separated and quantified using a thermogravimetric analyzer (TGA). The micro-morphologies and chemical compositions of these three phases were characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Results show that PM_2.5 yields during biomass pyrolysis are in the range of 7–34 g/kg (dry-basis biomass) and increase with the increase of pyrolysis temperature. At low pyrolysis temperatures (900–1000 °C), the carbonaceous matter is dominated by char-carbon. When the pyrolysis temperature increase from 1000 °C to 1100 °C, the production of soot is greatly enhanced and soot becomes dominant in PM_2.5, and the amorphous morphologies of soot are replaced by the concentric graphitic layers. With the further increasing in pyrolysis temperature, soot particles become more spherical and onion-like. Above 1100 °C, the KCl content in PM_2.5 declines, which is because of the capture of KCl and the formation of low-melting potassium aluminosilicates in large char particles. At 1300 °C, the fragmentation of char particles is significantly strengthened, resulting in more ash in PM_2.5.     

酒糟在流化床中燃烧特性的试验研究

利用X射线荧光光谱仪、灰熔点仪和热重分析仪等对酒糟的灰成分、灰熔点及着火温度进行了测试,并在小型流化床试验台上进行了酒糟燃烧试验,对不同含水率酒糟的着火特性、烧结特性和NOx排放质量浓度进行了研究.结果表明:酒糟灰中K2O和Na2O的含量很低,只有4.936%;酒糟灰的软化温度较高,高于1 290℃;干燥酒糟的挥发分较高,高于250℃就能够着火燃烧;当石英砂的平均粒径为300μm、流化床内的流化速度大于0.36m/s时,流化状况良好;在900℃下,酒糟在石英砂流化床中燃烧不会出现烧结现象;在不同的燃烧温度下,NOx排放质量浓度均较大,且随着燃烧温度的升高而增大;酒糟可以在石英砂流化床中燃烧,且燃尽效果良好.     

Influence of steel industrial wastes on burnout rate and NOx release during the pulverized coal catalytic combustion

In this work, 3 kinds of coal, 5 kinds of wastes from steel industry were chosen, orthogonal combustion experiments were carried out in a lab-scale drop tube furnace. Burnout rate and NO_x release were selected as evaluation indexes to evaluate the catalytic combustion and denitration effect. The catalytic combustion performance was discussed in term of the X-ray diffraction (XRD) and Scanning electron microscopy (SEM) of combustion residues. The results showed that NO_x content and burnout rate of different coal varied greatly; some additives could change the apparent form of combustion residues, which indicated that the additives played good roles in combustion process. This paper will not only achieve purpose of improving coal burning efficiency and energy saving, but also turning wastes into valuables and protect the environments.     

Electrochemical properties of Mg-based hydrogen storage materials modified with carbonaceous materials prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS + MM)

Mg₂Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subsequently modified with various carbonaceous materials including graphite, multi-walled carbon nanotubes (MWCNTs), carbon aerogels (CAs) and carbon nanofibers (CNFs) by mechanical milling (MM) for 5 h. The structural properties of the modified hydrides were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). All of the modified hydrides show amorphous or nanocrystalline-like phases. The hydride modified with graphite exhibits the most homogenous distribution of particles and the smallest particle size. The effects of the modifications on electrochemical properties of the hydride were investigated by galvanostatic charge/discharge, linear polarization, Tafel polarization, electrochemical impedance spectroscopy and potentiostatic discharge measurements. The results show that the maximum discharge capacity, the high rate dischargeability (HRD), the exchange current density and the hydrogen diffusion ability of the hydride modified with the carbonaceous materials are all increased. Especially, the hydride modified with graphite possesses the highest discharge capacity of 531 mAh/g and the best electrochemical kinetics property.     

不同粒径棉花秆粉末压制成型颗粒燃烧动力学特性

为掌握不同粒径棉花秆粉末压制的成型颗粒燃烧的动力学特性,开展不同粒径粉末的热重分析,并将粉末压制为质量相当的成型颗粒,开展成型颗粒燃烧失重实验,并采用等温热分析的双对数方法进行动力学分析。结果表明:棉花秆粉末粒径较大时,灰分也较高,在较低温度下燃烧可形成骨架,有利于挥发分和氧气的扩散,促进燃烧速率。压制成型颗粒时,棉花秆粉末粒径越小,粉末间结合越紧密,压制的成型颗粒密度越大,相同条件下的燃烧失重速率明显降低,表现活化能从77.71 kJ/mol逐渐降至69.44 kJ/mol。成型颗粒的高温燃烧过程可分为2个阶段,首先是水分和挥发分全部析出,而后为焦炭燃烧阶段。燃烧温度可显著加快水分和挥发分的析出速度。     

Effect of oil shale semi-coke on deposit mineralogy and morphology in the flue path of a CFB burning Zhundong lignite

The effect of oil shale semi-coke (SC) on the mineralogy and morphology of the ash deposited on probes situated in the flue path of a circulating fluidized bed (CFB) which burns Zhundong lignite (ZD) was investigated. 10 wt% or 20 wt% SC was added to ZD, which were then combusted in the CFB furnace at 950°C. Two probes with vertical and horizontal orientations were installed in the flue duct to simulate ash deposition. Both windward and leeward ash deposits on probes (P₁W, P₁L, P₂W and P₂L) were analyzed by using a scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), an inductively coupled plasma optical emission spectrometry ICP-OES, and a particle size analyzer. When ZD was burned alone, the P₁W deposit was comprised of agglomerates (<30 mm) enriched in CaSO₄ and Na₂SiO₃, incurring significant sintering. The P₁L and P₂W deposits, however, were of both discrete and agglomerated particles in similar mineral phases but with coarser sizes. The P₂L deposit was mainly fine ash particles where Na₂SiO₃ and Na₂SO₄ were absent. As SC was added, the agglomerates in both P₁W and P₁L decreased. Moreover, SiO₂ and Ca/Na aluminosilicates dominated the mineral phases whereas Na₂SiO₃ and Na₂SO₄ disappeared, showing a decrease in deposit stickiness. Likewise, the P₂W deposit was found less spread on the probe, decreasing its deposition propensity. Na-bearing minerals turned into (Na, K)(Si₃Al)O₈ and (Ca, Na)(Si, Al)₄O₈ in the P₂W deposit. Moreover, Na in the deposits decreased from 32 mg/g to less than 15 mg/g as SC presented. The addition of SC would therefore help alleviate the propensity of ash deposition in the flue path in the CFB combustion of ZD.     

Effects of oxy-fuel condition on morphology and mineral composition of ash deposit during combustion of Zhundong high-alkali coal

Zhundong coalfield is a super-large coal reserve, with high-alkali feature exacerbating ash deposition. Oxy-fuel combustion technology could propel the clean utilization of Zhundong high-alkali coal. While the ash deposition behavior of high-alkali coal under oxy-fuel condition has yet to be sufficiently investigated. The present study compared the differences of ash deposits between oxy-fuel and air combustion, and also examined the effects of oxygen content on ash deposition mechanism, employing a drop-tube furnace equipped with a specially designed sampling probe and some analysis methods, such as X—ray diffraction equipment, simultaneous thermal analyzer, etc. Experimental results indicated that ash deposition was weaker, with fewer contents of sodium chloride, calcium sulphate and less agglomeration ash in oxy-fuel atmosphere compared to the air case with same oxygen content. The content of the ash particle distributed in the range of 0–40 μm was up to 60% under oxy-fuel condition. The first weight loss of ash deposits, around 850 °C, was put down to the decomposition of carbonate and the second one, about 1150 °C, was ascribed to the decomposition of the sulphate minerals in the thermal process. Ash deposition worsened with more large particles (>120 μm), as the oxygen content rose. Sodium chloride content reached 9.7% with 50% oxygen content. The present study not only focuses on the morphology and chemical components, but also probes into the thermal volatility of ash deposits, which benefits the further understanding of the ash deposition mechanism and utilization of Zhundong high-alkali coal during oxy-fuel combustion.     

Optical and electrical characterization of carbon nanoparticles produced in laminar premixed flames

Optical and electrical properties of carbonaceous particles produced in laboratory scale, premixed ethylene/air flames are obtained. Light absorption and Raman spectroscopy show that the change in particle nanostructure follows a graphitization trajectory as the flame richness increases. The optical band gap decreases and the size of the aromatic network in the particle increases, while the interlayer spacing between parallel layers decreases. The electrical conductivity of the materials increases by increasing flame richness in agreement to the graphitization trajectory. A non-ohmic behavior has been found and explained in terms of electron tunneling in a percolative network. Our results show that the electrical properties of flame formed carbon nanoparticles are strongly dependent on their nanostructure, and hence they have to be used carefully for the determination of particle concentration with conductometric sensors. Moreover, the dependence of the electrical properties of combustion formed particles might be useful for the development of cheap sensors for the selective detection of different classes of combustion aerosols.     

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.     

Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion

Biomass is regarded as CO₂-neutral, while the high contents of potassium and chlorine in biomass induce severe particulate matter emission, ash deposition, and corrosion in combustion facilities. Co-firing biomass with coal in pulverized-combustion (PC) furnaces is able to solve these problems, as well as achieve a much higher generating efficiency than grate furnaces. In this work, the particulate matter (PM) emission from biomass co-firing with coal was studied in an entrained flow reactor at a temperature of 1623 K simulating PC furnace condition. PMs were sampled through a 13-stage impactor, and their morphology and elemental composition were characterized by scanning electron microscopy and electron dispersive X-ray spectroscopy. SO₂ emissions were measured to interpret the possibility of potassium sulfation during co-firing. Results show that PMs from the separated combustion of both biomass and coal present a bimodal particle size distribution (PSD). The concentration and size of fine-mode submicron particles (PM_1.0) from biomass combustion are much higher than those from coal combustion because of the high potassium content in biomass. For the co-firing cases, with the coal ratio increasing from 0% to 50%, the PM_1.0 yield is reduced by more than half and the PM_1.0 size becomes smaller, in contrast, the concentration of coarse-mode particles with the size of 1.0–10 μm (PM_1.0-10) increases. The measured PM_1.0 yields of co-firing are lower than the theoretically weight-averaged ones, which proves that during the biomass and coal co-firing in PC furnaces, the vaporized potassium from biomass can be efficiently captured by these silicon-aluminate oxides in coal ash. In the studied range of coal co-firing ratio (≤50 wt.%), the chlorides and sulfates of alkali metals from biomass burning are the dominating components in PM_1.0, and a certain amount of silicon is observed in PM_0.1-1. The analysis of chemical composition in PM_1.0, together with that of SO₂ emission, indicates a marginal sulfation of alkali metal chloride occurring at high temperatures in PC furnaces.     

Mineral behavior during coal combustion 2. Illite transformations

The physical and chemical transformation of excluded crystalline illite particles and of illite grains included within a carbon matrix were examined in a laboratory scale reactor. Scanning electron microscopy was used to determine the particle morphology, and energy dispersive X-ray analysis, Mössbauer spectroscopy, and XAFS expectroscopy were used to monitor the chemical changes. At temperatures above 1400 K, illite lost its crystalline structure and was transformed to a glass. Melting, pore generation, and cenosphere formation were observed. For both included and excluded illite particles, neither segregation of volatile components at the particle surface, nor vaporization of potassium species, was observed during combustion. Combustion of synthetic chars containing illite inclusions demonstrated coalescence of these inclusions to form larger ash agglomerates. Comparison of these results with ash particle compositional data obtained from the combustion of a bituminous coal containing illite showed intermediate compositions indicating interaction between the molten illite and quartz, kaolinite, and pyrite. Deposition experiments revealed a distinct temperature range above which the transformed illite particles had sufficiently low viscosity to deform and stick upon impaction.