煤粉与钙基复合添加剂混烧的灰分矿相特性

在自行设计的小型高温试验炉上对煤中掺入钙基复合添加剂进行了静态烧成试验,并对所烧成的灰样进行了XRD分析,着重讨论了温度和时间以及添加剂对所烧成灰分的矿相特性的影响.试验结果表明,当温度在1000℃时,灰分中的矿物主要以游离态的氧化物形式存在,部分矿物生成钙铝黄长石（C₂AS）、硅酸二钙（C₂S）等;当温度高于1200℃时,灰分中的主要矿物是C₂AS、C₂S.同时在高温下有硫铝酸钙（C₄A₃S -）生成,该矿物对固硫有促进作用.     

钙基复合添加剂与煤粉混烧的燃烧特性研究

采用热重法对煤粉中掺入钙基复合添加剂的燃烧特性进行了试验研究，同时，在单角炉上研究了钙基复合添加剂对煤粉燃烧的影响，试验结果表明，以煤粉单位重量计，当添加剂掺入量在30％以下时，对着火燃烧稳定性几乎无影响，当添加剂的掺入量在30％－50％之间时，着火延迟，火焰滞后，但基本上能稳定燃烧，当添加量达到50％以上，时，会造成燃烧不稳定甚至灭火现象，还对其各种影响因素进行了探讨。     

中国动力煤的着火温度与着火热的分布规律

为了寻求动力煤成分分布与煤质着火及稳燃特性之间的函数关系,根据燃烧学的热力着火理论,对国内大型电站煤粉锅炉燃用的166个典型的煤质分析数据进行了数学分析。分析了固态碳(C_gt)和当量气态碳(C_qt 3.67H_ar)比值随V_daf的分布规律及其对煤质着火的影响;根据文献中77个煤质的一维沉降炉实验数据,拟合出煤粉的着火温度与V_ad及A_ad的函数关系,计算分析了本文大数据样本煤质的着火温度及着火热的分布规律,提出将着火热和着火温度作为煤质着火特性的图像分辨新方法。据此分析了各类煤质的着火与稳燃特性。结果表明,本方法能明确地定量区分不同类煤质和同类煤质的着火特性差异,且在理论上给出符合宏观规律的合理解释。     

Inert coal macerals in combustion

Pilot-scale pulverized-coal combustion experiments with two western Canadian coals have shown that the combustion efficiencies of the coals were inversely related to the inert contents of the coals, i.e. semi-fusinite, fusinite and oxidized vitrinite. These macerals were found to persist through the flame in a partially reacted form. The burn-out of a coal with an inert maceral content over 50% was unaffected by normal changes in combustion conditions; the same changes were successful in improving the burn-out of a coal containing less than 40% inert macerals. Coals with high inert maceral contents produce fly-ash with high levels of unburnt carbon and will require a combination of higher temperatures and higher excess-air levels and longer residence times to achieve combustion efficiencies similar to those of coals with higher reactive-maceral contents. The association of resinitic and bituminous materials with the mineral matter in the Canadian sub-bituminous coal indicated that macerals which contribute significantly to ignition and flame stability are discarded as a washery reject.     

Physical structure and combustion properties of super fine pulverized coal particle

Physical structures and combustion properties of super fine pulverized coal particles of eight Chinese coals, Heshan subbituminous coals and Jincheng lean coals from two areas of China, have been investigated using accelerated surface area and porosimetry, thermobalance (TGA), and Fourier transform infrared spectrometer. Results showed that the particle specific surface area and pore volume increased greatly when the coal particle size was reduced. The higher the carbon content on a dry ash-free basis is, the larger the particle specific surface area and pore volume are. When the coal particle size decreases, the combustion process can be largely improved, ignition temperature is reduced, and SO₂ emission from coal combustion is also lower.     

Pulverized coal combustion characteristics of high-fuel-ratio coals

It is strongly desired for coal-fired power plants in Japan to utilize not only low-rank coals with high moisture and high ash contents, but also high-rank coals with high fuel ratio for diversifying fuel sources and lowering cost. In this study, pulverized coal combustion characteristics of high-fuel-ratio coals are experimentally investigated using an approximately 100 kg-coal/h pulverized coal combustion test furnace. The combustion characteristics are compared to those for bituminous coal. The coals tested are six kinds of coal with fuel ratios ranging from 1.46 to 7.10. The results show that under the non-staged combustion condition, the minimum burner load for stable combustion rises as fuel ratio increases. To improve the stability, it is effective to lengthen the residence time of coal particles in the high gas temperature region close to the burner outlet by using a recirculation flow. The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increases with increasing the fuel ratio. In addition, as the fuel ratio increases, NOx reduction owing to the staged combustion becomes small, and unburned carbon fraction increment becomes significant. The numerical simulations conducted under the staged combustion condition show that although the numerical results are in general agreement with the experimental ones, there remains room for improvement in NOx reduction model for high-fuel-ratio coals.     

Factors affecting coal particle ignition under oxyfuel combustion atmospheres

A set of 13 coals of different rank has been tested for ignition propensity in a 20-L explosion chamber simulating oxyfuel combustion gas conditions. Their char residues were also analysed thermogravimetrically. The effects of coal type, coal concentration (from 100 to 600 g/m³), O₂ in CO₂ atmospheres (up to 40% v/v) and particle size were investigated.The higher rank coals were significantly more difficult to ignite and mostly required higher energy chemical igniters (1000 or 2500 J) whereas the lower rank coals could be ignited with a 500 J igniter even at low coal dust concentrations.The minimum explosibility limit/ignition concentration in air varied slightly around a value of 200 g/m³, a little higher for low volatile coals and a little lower for high volatile coals.The ignition limit changed significantly, however, with O₂ concentration in CO₂, where coals required more oxygen to ignite. Most coals failed to ignite at all in 21% v/v O₂ in CO₂, but an increase to 30 or 35% v/v O₂ gave ignition patterns similar to those in air. In addition, the minimum ignition concentration decreased with increase in O₂. However, a further increase to 40% v/v O₂ did not generally affect the minimum ignition concentration.Particle size had a non-linear effect on coal ignition. The fine particles (<53 μm) behaved almost identical to the whole coal. However, the larger size fraction (>53 μm) was generally more difficult to ignite and exhibited a much lower weight loss.     

Vacuum pyrolysis of Prince Mine coal, Nova Scotia, Canada

Preliminary results are given on thermal decomposition characteristics of a high volatile A bituminous coal from Eastern Canada using vacuum pyrolysis experiments (pressure 2–200 mm Hg) over the temperature range 322–1000 °C. The objectives of the study were to determine the optimum temperature range for the formation of coal tar and to study the influence of reaction temperature on the nature of the solid residues. Significant decomposition reactions start at 300–400 °C and the optimum temperature range for the production of coal tar was 450–600 °C. The major gaseous products H₂S, CO₂ and CH₄ are formed up to 600 °C. Above 600 °C the coal decomposes mainly into CO and H₂. The solid residues are characterized by volatile matter content, calorific values and elemental analysis. The volatile matter content decreases rapidly from 322 °C and stabilizes at reaction temperatures > 750 °C. The 15% VM level, a minimum requirement in coal combustion processes, was reached at 500 °C. The changes in calorific values do not show any significant trend up to 600 °C, but decrease markedly above 600 °C. From the preliminary results vacuum pyrolysis is regarded as an effective process in which valuable coal tar by-products can be obtained from coal prior to its combustion.     

烟煤与无烟煤混合煤粉燃烧过程的动力学研究

利用热重分析(TGA)方法系统研究了配加烟煤对无烟煤燃烧特性的影响。结果表明:煤粉燃烧主要由3个阶段组成,烟煤配加量和升温速率对燃烧过程有重要影响,随着烟煤配加量的增加,燃烧DTG曲线向低温区移动;实验所用煤种中的主要矿物元素是Si,Ca,Al,Fe,S,P,其中Mg,Zn,K,Na,Cl等含量较低,这些元素主要以氧化物、硅酸盐、碳酸盐、硫酸盐、氯化物和磷酸盐等形态存在,在低温燃烧过程中,矿物质挥发损失量对煤粉燃烧率的影响较小。采用非等温模型Flynn-Wall-Ozawa(FWO)对主要燃烧过程进行动力学分析,当烟煤配加量从0增加到100%时,煤粉燃烧活化能从133.94 kJ/mol降低到78.03 kJ/mol,且烟煤的配加量低于60%时,能够显著降低煤粉燃烧的活化能。     

Ignition of droplets of liquid fuels solvent extracted from coal

A single suspended-droplet technique has been used to study droplet combustion of a range of solvent-refined coal extracts in a furnace at 850 °C. The small particles of extract are solid at room temperature but they rapidly liquefy on exposure to the hot furnace environment, permitting the size and mass-time combustion histories to be measured. Their combustion behaviour is discussed in terms of their high aromaticity and comparison with the single droplet combustion of fuel oils of different asphaltene content was used to highlight potential solids emissions problems which may arise when firing in large systems.     

Investigations into the ignition behaviors of pulverized coals and coal blends in a drop tube furnace using flame monitoring techniques

This paper presents experimental investigations into the ignition behaviors of pulverized coals and coal blends in a drop tube furnace using a flame monitoring system. Seven different ranks of coals and coal blends of different mixing proportion were tested. Characteristic parameters including relative ignition temperature, maximum ignition points, oscillation frequency, fluctuation ratio and combustion dynamic energy were determined from the flame monitoring system. The ignition behaviors of the coals are established by combining the parameters. Results demonstrate that the parameters are suitable for distinguishing ignition behaviors from homogeneous, hetero-homogeneous to heterogeneous in the ignition section of a drop tube furnace. The ignition behaviors of a coal blend are found to have similar characteristics as the coal of higher volatile matter in the blend and depend on its proportion in the blend. The results from this study are used to predict the operation of a coal fired power plant in terms of fuel selection, fuel blending, and flame stability.     

Change of combustion characteristics of Indonesian low rank coal due to upgraded brown coal process

Upgraded brown coal (UBC) process has been developed to produce a dry upgraded coal which is comparable with a bituminous coal. Differential thermal and thermogravimetry analyses (DTA–TG) have been applied to study the combustion characteristics of some Indonesian raw and upgraded coals produced by the UBC process. DTA–TG profiles of the entire combustion process can be used to predict the combustion performance of the pulverized coal in industrial furnaces. By using three samples of Indonesian low rank coals coming from Taban, Berau and Samaranggau, East Kalimantan, which have been upgraded by the UBC process, the ignition temperature (Tig) of the upgraded coals increased. Whereas the temperature, at which the maximum combustion rate occurs (Tmax), is not so changed. The DTA combustion peaks, which reflect the specific energy of coals, and the upgraded coals are significantly increased compared to those of raw coals.     

Releases of NO and its precursors from coal combustion in a fixed bed

Experiments have been carried out to investigate the emissions of nitrogen species including NO and its precursors during temperature-programmed coal combustion by TG/EGA method. Experimental results show that the conversion ratio of fuel nitrogen to NO is the highest, followed by that of fuel nitrogen to HCN and the conversion ratio to NH₃ is negligibly small. Nitrogen is retained in the char and released mainly as NO at the later stages of coal combustion. HCN and NO are both primary products from coal char oxidation. Coal rank, heating rate, indigenous minerals and external additives are the major influential factors of the nitrogen species release. Higher rank coals with higher fuel ratio have higher NO releases. HCN release decreases as fuel ratio increases for most coals. The fuel nitrogen conversion to NO increases and the fuel nitrogen conversion to HCN decreases with the increase of heating rate, which may imply that the char nitrogen prefers to react with oxygen to form NO instead of HCN while coal char is combusted at higher temperatures. Different metallic additives show different effects on nitrogen species emission and the effects of indigenous minerals on nitrogen release can be qualitatively estimated by ash analyses.     

Firing a sub-bituminous coal in pulverized coal boilers configured for bituminous coals

It is important to adapt utility boilers to sub-bituminous coals to take advantage of their environmental benefits while limiting operation risks. We discuss the performance impact that Adaro, an Indonesian sub-bituminous coal with high moisture content, has on opposite-wall and tangentially-fired utility boilers which were designed for bituminous coals. Numerical simulations were made with GLACIER, a computational-fluid-dynamic code, to depict combustion behavior. The predictions were verified with full-scale test results. For analysis of the operational parameters for firing Adaro coal in both boilers, we used EXPERT system, an on-line supervision system developed by Israel Electric Corporation. It was concluded that firing Adaro coal, compared to a typical bituminous coal, lowers NO_x and SO₂ emissions, lowers LOI content and improves fouling behavior but can cause load limitation which impacts flexible operation.     

Upgrading of low rank coal with solvent

In this study, thermal upgrading of low-rank coal with solvent at 380–440 °C under an initial nitrogen pressure of 2 MPa was studied as a possible method for producing clean solid fuel with a high heating value and less spontaneous ignition behavior. Upgrading of Buckskin coal (USA, subbituminous coal) in the presence of t-decalin (non hydrogen-donor solvent) at 440 °C gave 11.4 wt.% of gas, 5.3 wt.% of oil and 74.1 wt.% of upgraded solid product with a small amount of water. The gaseous product consisted mainly of carbon dioxide (67 wt.%), methane, carbon monoxide, hydrogen and a trace of C₂ and C₃ hydrocarbon gases. The oil product from coal contained BTX, phenol, and their alkyl-derivatives. The heating value of the upgraded solid product from the Buckskin coal increased to 31.0 MJ/kg in dry base as compared to the heating value of wet base of the untreated raw coal, which was 19.3 MJ/kg. Spontaneous ignition behavior was greatly reduced by the upgrading. The effect of catalyst and additives on the upgrading was investigated in terms of product distribution and the quality of the solid product. Taiheiyo (Japan, subbituminous) and Yallourn (Australia, brown) coals were also studied.     

Influence of additives of various origins on thermoplastic properties of coal

Two coking coals of different rank were chosen in order to assess the influence of various additives on their thermoplastic properties. Six additives of different origin and characteristics were selected: two non-coking coals, together with a commercial coal tar pitch, a residue from the bottom of the benzol distillation tower and two residues from the tyre recycling industry. The effect of the additives on coal thermoplastic properties was assessed by means of the Gieseler test. The additives were pyrolyzed to a final temperature of 550 °C and the tar characterized by means of Fourier transform infrared spectroscopy (FTIR) and gas chromatography (GC). The influence of the additives on coal thermoplasticity is related to the volatile matter content of the additive, its evolution profile with temperature and the composition of the tar obtained during the pyrolysis of the additives.     

Formation processes and main properties of hollow aluminosilicate microspheres in fly ash from thermal power stations

The main parameters of aluminosilicate microspheres formed at thermal power stations in Russia were studied; these parameters are responsible for the prospective industrial application of these microspheres. A comparative analysis of the properties of mineral coal components, the conditions of coal combustion, and the effects of chemical and phase-mineralogical compositions of mineral impurities in coals from almost all of the main coal deposits on the formation of microspheres was performed. The effects of thermal treatment conditions on gas evolution processes in mineral particles and on the fraction of aluminosilicate microspheres in fly ash were considered. It was found that the yield of microspheres was higher in pulverized coal combustion in furnaces with liquid slag removal, all other factors being equal. The regularities of microsphere formation were analyzed, and the mechanism of microsphere formation in fly ash during the combustion of solid fuels was considered.     

Differences in the ignition characteristics of coal–water slurries and composite liquid fuel

The processes of the inert heating, ignition, and combustion of the drops of typical coal–water slurries and promising composite liquid fuel were experimentally studied with the use of high-speed (to 10⁵ frame/s) video recording facilities. The particles of brown and black coals with of sizes 80–100 μm were used as the basic components of the coal–water slurries and composite liquid fuel. Spent automobile oil (from an internal combustion engine) was also added to the composite liquid fuel (relative mass concentration, 0–15%). The characteristic stages of the processes of the inert heating and evaporating of liquid components and the ignition and combustion of coal–water slurries and composite liquid fuel (the initial radii of drops varied from 0.5 to 2 mm) were established. The ignition delay and complete combustion times of the drops of fuel compositions were determined under changes of the temperature of an oxidizing agent (air) in a range from 600 to 900 K at fluid velocities from 0.5 to 5 m/s. Representative temperatures at the centers of coal–water slurry and composite liquid fuel drops were measured at all of the established stages of the combustion initiation process. The necessary and sufficient conditions for the steady ignition of the drops of the test fuel compositions were recognized.     

Emission characteristics of NOx and unburned carbon in fly ash on high-ash coal combustion

Three coal sources, two high-ash coals from South Africa and one bituminous coal from Australia, were evaluated at a pulverized coal combustion test furnace. These combustion trials were carried out on different blending ratios with the two South African coal sources. The NOx emission characteristics of these high-ash coals are similar with those of bituminous coals. The combustion efficiencies on the high-ash coals are lower than those of bituminous coals. The high-ash coals do not affect bituminous coal combustion on the blending combustion.     

Effect of H/C ratio on coal ignition

The Cahn balance technique was found to be suitable for estimating ignition temperature and its dependence on the H/C ratio of the coal. This temperature decreased with increasing H/C ratio of coals. For coals a linear correlation between H/C ratio and the temperature was established. Chars derived from the coals deviated from the linear correlation established on coals.