PelIetized coals and coal blends: combustion efficiency in a fluidized bed

Single coals and coal mixtures were pelletized to give fuels of fixed size, density and carbon content for fluidized bed combustion experiments. Similar combustion efficiencies were generally obtained for pellets and whole coals of equal carbon content. The anomalously high carbon loss of a friable coal (British Columbia), caused by degradation and elutriation of fines, was not apparent when this coal was burned in a pelletized form. Pelletizing has no marked effect on the high carbon loss of high-rank coal.     

The characterization of weathered discard coals and their behaviour during combustion

With a finite amount of economically viable coal reserves in South Africa, it is becoming increasingly important to consider coal discards as vital energy resources. The energy content of discard coal is generally low and high in ash, but can be effectively utilised in power stations. However, coals weather during storage, and weathered coals behave differently to their fresh counterparts. During this investigation, discard coals from four stockpiles varying in age from 5 to 40 years were characterised chemically and petrographically, with a specific focus on the petrographic-based abnormal condition analysis to determine the degree of secondary weathering. Drop tube furnace tests were conducted to determine the combustibility of the weathered stockpiled or dumped coals relative to typical Eskom coals, and the resultant char forms were characterised. A direct correlation with the extent of weathering and combustion performance was determined, with the more weathered coals reporting enhanced combustion reactions at lower temperatures.     

Air and oxy-fuel combustion behaviour of petcoke/lignite blends

The pyrolysis and combustion behaviour of a petroleum coke (petcoke), an indigenous lignite and their 70/30 wt.% blend in air and oxy-fuel conditions were investigated by using non-isothermal thermo-gravimetric method (TGA) coupled with Fourier transform infrared (FTIR) spectrometer. Blend samples were prepared by mixing lignite, which has low calorific value, high ash and moisture contents with petcoke that has high calorific value, low ash and moisture content, in the proportion of 70:30. Pyrolysis tests were carried out in nitrogen and carbon dioxide environments which are the main diluting gases of air and oxy-fuel environments, respectively. Pyrolysis curves of parent fuels and their blend reveal close resemblance up to 700 °C in both N₂ and CO₂ environments. At higher temperatures, further weight loss taking place in N₂ and CO₂ atmospheres is attributed to calcite decomposition and CO₂-char gasification reaction, respectively. Gasification reaction leads to significant increase in CO and COS formation as observed in FTIR evolution profiles. Almost identical experimental and theoretical pyrolysis profiles of the blend samples show that there is no synergy between the parent fuels of the blend in both pyrolysis environments. Combustion experiments were carried out in four different atmospheres; air, oxygen-enriched air environment (30% O₂–70% N₂), oxy-fuel environment (21% O₂–79% CO₂) and oxygen-enriched oxy-fuel environment (30% O₂–70% CO₂). Combustion experiments show that replacing nitrogen in the gas mixture by the same concentration of CO₂ leads to delay in combustion (lower maximum rate of weight loss and higher burnout temperatures). Overall comparison of derivative thermogravimetry (DTG) profiles shows that effect of oxygen content on combustion characteristics is more significant than that of diluting gas in the combustion environment. At elevated oxygen levels, profiles shift through lower temperature zone, peak and burnout temperatures decrease, weight loss rate increases significantly and complete combustion is achieved at lower temperatures and shorter times. Theoretical and experimental combustion profiles of the blend mainly display different trends, which indicate synergistic interactions between lignite and petcoke during their combustion in different environments.     

餐厨垃圾热解实验研究

利用热重分析仪及管式炉反应器研究了餐厨垃圾的热解行为.餐厨垃圾的热解过程包括两个阶段,结合微分法与积分法对两阶段进行了动力学计算,确定管式炉热解实验反应温度.对热解产物进行了分析,结果表明,气体产物主要是CO和CH4;固体产物焦的恒容低位发热量为20.33 MJ·kg-1;液体产物焦油主要是醇和烃,占焦油总量的70.55％.通过分析不同热解温度焦的红外谱图、元素组成及热解过程的小分子气体释放规律,探究了餐厨垃圾的热解机理.290℃时,餐厨垃圾中肽键断裂,释放出大量CO2和CO:350℃时,脂类化合物已经完全分解或挥发;500℃时,焦中甲基及亚甲基峰消失,释放出大量烃类;未分解的物质主要是淀粉.最后考察了升温速率对热解产物分布的影响,发现提高升温速率,焦收率基本不变,焦油收率下降,气体产物收率增加.     

Studies of the effects of O-methylation on the pyrolysis behaviour of four coals

O-methylation of coal was found to exhibit a noticeable effect on the pyrolysis behaviour of the original coal. Significant increases in tar yields and a low-temperature fraction, 150–300 °C, were found for the low-rank O-methylated coals. It is proposed that methylation prevents formation of new ether cross linkages through water elimination reactions of hydroxyl functional groups thus allowing for the release of a low-temperature fraction as well as a higher temperature tar. KBr studies of CD₃ labelled O-methylated coals using CD₃I as the methylating agent have resulted in the identification of three distinct i.r. absorptions in the C-D stretching region which have been assigned to methylated phenolic, methylated carboxylic and methylated aliphatic hydroxyl groups. Pyrolysis studies of the O-methylated coals have resulted in better insight into the decomposition mechanisms of different methylated hydroxyl functional groups. Methanol or formaldehyde are seen to be the principle products from the pyrolysis of methylated carboxylic and aliphatic hydroxyls. Decomposition of methylated phenolic groups occurs at a slightly higher temperature and produces principally methane and carbon monoxide.     

Studies on dynamic behaviour and flow properties of HDPE/EPDM blends by torque rheometer

The effect of blend ratio on the crosslinking characteristics and dynamic elastic properties of HDPE/EPDM blends was studied by a torque rheometer. The crosslink density (a measure of tensile strength) first increases and then decrease with increase in EPDM content. The dynamic elastic property (a measure of impact strength) also showed a peculiar trend. Both studies showed that less than 40 parts loading of EPDM gave the best mechanical properties. In another study the flow activation energy was calculated by temperature programming, which gave the value of processing temperatures (cold mixing, hot mixing).     

Cleaning of Indian coals by agglomeration with xylene and hexane

A laboratory scale agglomeration process has been undertaken for cleaning Indian coals using oils namely, xylene and hexane. Maximum organic matter recovery for xylene has been found to be 91.9% whereas with hexane, the value is 54.7% on a dry basis. The highest ash rejection values with xylene (90.7%) and with hexane (89.7%) are almost same. Promising results for rejection of metals (Fe, Mg and Zn) have been observed. It has been found that xylene is more selective than hexane for the agglomeration process. Knowledge gained from this study will be helpful for technological advancement of this kind of work.     

Studies on thermal stability and behaviour of polyacetal and thermoplastic polyurethane elastomer blends

The influence of thermoplastic polyurethane (TPU) elastomer on the rigidity of polyacetal (polyoxymethylene, POM) was studied by determining heat deflection temperature (HDT). The higher the content of TPU in the POM/TPU blend, the lower the HDT of the blend, as would be expected. A comparative study of the recycle capability of POM and the blends was carried out by measuring melt flow index (MFI) on each successive extrusion. Stress-strain behaviour of the virgin material and that obtained after fourth-time extrusion was analysed for POM and the blends. The effect of γ-radiation on the mechanical behaviour of the blends was investigated. The kinetics of thermal degradation of POM, TPU and their blends was studied. The kinetic parameters, viz. activation energy and the order of reaction, were established. The values of the activation energy of the blends were found to be higher than those of the POM and TPU, indicating improved stability of the resultant blends.     

Flash pyrolysis of coals: influence of cations on the devolatilization behaviour of brown coals

The influence of cations on the pyrolysis behaviour of brown coals under flash heating conditions was investigated by means of a small fluidized-bed pyrolyser. A stream of coal particles in nitrogen was injected at rates of 1–3 g coal/h directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbons and total volatile matter from four Gelliondale brown coals and a Montana lignite were determined as a function of pyrolysis temperature. With all coals the maximum tar yield was obtained at 600 °C. Removal of cations present in the coals markedly increased the yields of tar and total volatile matter, with little effect on the yields of hydrocarbon gases. The converse was also observed in that the addition of Ca²⁺ to a cation-free coal decreased the yields of tar and total volatile matter. The extent of the reduction in tar yield at 600 °C in the presence of cations was found to be similar for all coals. After acid washing, tar yields appear to correlate with the atomic $\text{H}\text{C}$ ratios of the coals in a manner similar to that observed previously with bituminous coals.     

煤与生物质混合燃烧特性实验研究

生物质能是清洁的可再生能源.本文通过热分析技术,对无烟煤、玉米秸杆与废弃加工木屑两种生物质及其混合物的燃烧特性进行了实验研究.结果表明,纯煤与纯生物质在燃烧过程中表现出不同的特性,且随着生物质加入量的不同,使煤的着火性能得到不同程度的改善,并改善了煤燃烧放热的分布.混合燃烧对煤的燃烬性能几乎没有影响.     

DTGA combustion of coals in the Exxon coal library

Coal combustion experiments were carried out over the temperature range 25–900 °C using air at atmospheric pressure in a derivative thermogravimetric analysis system. Sixty-six coals high in vitrinite (> 80% mineral-matter-free basis) and low in inorganics (all but 12 samples < 10%) were examined as part of a coal characterization programme. The coals varied in rank from lignites (69% carbon on a dry, mineral-matter-free basis) to low-volatile bituminous (91% carbon). Combustion rates increased progressively with increasing temperature, passed through maxima and then declined. The rate data were fitted to an Arrhenius equation and plots showed four distinct regions of combustion. Apparent activation energies were calculated for each region and varied from ≈4 kJ mol^?1 in the high-temperature, diffusion-controlled region to 290 kJ mol^?1 in the chemical-reaction controlled, low-temperature region. The temperatures at which 50% of the sample had burned away ($\text{sol1}\text{2}\text{-}\text{life}$) were rectilinearly related to oxygen and carbon contents (correlation coefficient squared values of 0.88 and 0.86, respectively).     

Studies on the justification for microanalysis of coals

The possibilities of analysing coals on the microscale have been studied with statistical treatments. The effects, on the precision of microdetermination of ash, of particle size, sample weight, weighing error, vaporization of moisture from coal sample, and mixing of sample have been examined. A clear relation exists between theoretical considerations (e.g. calculation of numbers of particles needed for microanalysis from binomial distribution, separation of errors of taking increments, weighing and analytical procedure, etc.) and experimental results.     

Thermodynamic analysis of the formation of submicron particles on the combustion of coals

The thermodynamic analysis of the composition of the combustion products of 15 types of coals was carried out with consideration for the formation of potassium and sodium aluminosilicates and solid and liquid slag removal. Based on the results of the analysis, the approximating temperature dependences of the concentrations of condensed components (potassium and sodium sulfates) were obtained for the cases of two-phase and single-phase equilibriums; conclusions on the comparative influence of solid and liquid slag removal on the probability of the formation of submicron particles on the combustion of coals were made. The found dependences will make it possible to perform a numerical simulation of the bulk condensation of potassium and sodium sulfate vapors upon the cooling of coal combustion products in a process flow.     

NOx generation from the combustion of Australian brown and subbituminous coals

The formation of NO_x during the combustion of pulverized brown and subbituminous coals from Victoria and Queensland respectively was investigated in an entrainment reactor. As no NO₂ was detected, all the NO_x was present in the form of NO. The brown coals exhibited a significantly greater potential for NO emission under fuel-lean conditions than did the subbituminous coal, even though the latter coal had a higher nitrogen content. However, under fuel-rich conditions the conversion of coal nitrogen to NO for the subbituminous coal was higher than for the brown coals. The differences in conversion efficiency may have been related in part to the nature and reactivity of the volatile nitrogen species. Reactivity differences between the chars produced from the brown and subbituminous coals may also have accounted for different extents of removal of NO. There was a significant reduction in the amount of NO emitted when brown coal was added to a combustion gas stream containing an appreciable quantity of NO before coal injection.     

Thermal degradation and combustion of polymeric blends

The thermal stability of polymer blends was investigated by means of gas chromatography–mass spectroscopy (GC/MS) and thermal analysis. Evaluated changes in thermal stability can be attributed to blending. On the other hand, we were interested in whether blending may provide a method to control thermal stability and combustibility of polymeric materials. A new scheme of thermal degradation for polystyrene‐polydimethylsiloxane (PDMS) blend was suggested. In the case of polystyrene (PS) as a part of the blend, the products of degradation of PS diffuse through the phase boundary, which cause interaction with PDMS polymers. Apparently, PDMS acts as an inert component, slowing down the termination reaction by dilution of macroradicals formed in random scission degradation process of the PS component. On the other hand, it stabilizes the PS by means of interpolymer recombination, which leads to cross products of thermal degradation. Two of the degradation products: 2‐phenyl‐4(1′,3′,3′,5′,5′‐pentamethylcyclotrisiloxane)‐butane and 2‐phenyl‐4(1′,3′,3′,5′,5′,7′,7′‐heptamethylcyclotrisiloxane)‐butane were assigned to the products of cross‐interpolymer recombination which can accelerate the process of PDMS depolymerization by means of radical initiation of PS* fragments. The connection between a polymer thermal oxidative degradation and its combustion under diffusion flames condition was shown by using composition of polypropylene‐polypropylene‐co‐polyethylene (PP/PP‐co‐PE). In general, the solid‐phase polymer reaction can play a very important role in the reduction of polymer combustibility. It was shown that the composition of PP/PP‐co‐PE (62 : 38) has the highest induction period of autooxidation, which correlates with its combustibility. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3300–3311, 2002     

加热炉管的失效形式及分析

介绍了以乙烯裂解炉为主的加热炉炉管在工程上常见的13种失效形式，包括光管或翅片管基管的高温损伤、对焊接头组织恶化、热疲劳开裂、热应力开裂、奥氏体的氯离子晶间腐蚀、碱蚀、过热及其他失效，还包括翅片的高温磨损、应力腐蚀开裂及翅根积垢以及化学工业炉的介质腐蚀失效。对各种失效的表现特点及失效原因进行了简单的分析。     

浅析制氢转化炉炉管失效

在制氢装置停工检修中,发现转化炉炉管有部分炉管发生了失效。通过对失效炉管材料进行成分分析、力学性能分析、金相组织评价以及扫描电镜分析,结果表明:炉管材料成分合格,内壁金相组织基本正常;由于高温作用,外表面存在析出相、氧化层和金相组织转变;材质从内到外,局部区域产生高温蠕变,材质劣化,机械性能大幅度降低。     

用于混煤燃烧控制砷排放模型研究

混煤掺烧是控制燃煤砷排放的有效方式,但由于缺乏相关的配煤模型,限制了该技术的应用。根据燃煤过程中砷的挥发释放机制,提出一种采用砷的释放指数P表征煤燃烧过程砷的释放特性的配煤模型。该模型综合考虑煤的灰分、灰中主要矿物元素含量、各矿物元素对砷的固定系数以及煤中砷含量等因素。研究结果表明,随着煤灰固定系数由23.12增至50.90,煤灰的气相砷吸附量由3.39 mg/g增至6.14 mg/g;随着释放指数P增大,砷的固定率减小,且随着温度升高,两者相关性由900℃的0.67增至1 300℃的0.86。根据P值筛选煤种进行掺烧,当掺混煤种P值差异较大时,掺烧低P值煤种不仅可降低混煤中的砷含量,还能促进高P值煤的砷在灰中富集,促进率达77.14%;掺混煤种P值差异较小时,掺烧低P值煤种会促进砷的释放。本模型可较好地筛选煤种,为混煤掺烧控制砷等痕量元素的排放提供了新的思路。     

Pyrolysis by thermogravimetric analysis of blends of peat with coals of different characteristics and biomass

In this study, an investigation was carried out into the thermal behaviours of peat, reed, lignite, bituminous coal and blends of these with peat. The blends were prepared in 20:80, 40:60, 60:40, 80:20. The samples were pyrolysed in a TG analyzer in a nitrogen atmosphere (50 mL/min) at temperatures ranging from 25 to 900 °C. Using TG/DTG graphs, variations were investigated, which occurred in reaction intervals, percent of weight loss, peak temperatures and maximum devolatilization rate. The activation energy (E) and pre-exponential constant (A) were calculated using the Arrhenius type kinetic model.     

Studies on the transesterification of polycarbonate/poly(ethylene terephthalate) blends. II: Effect of transesterification on the compatibility and crystallization behaviour of the blends

The effect of transesterification on the compatibility of polycarbonate poly(ethylene terephthalate) (PC/PET) blends and the crystallization and melting behaviour of PET was studied. The results show that with increase of the reaction extent, the compatibility of the blends was enhanced during melt-processing, from completely incompatible to partially compatible, then to complete compatibility. In addition, with the increase of reaction extent, the crystallization ability and crystallinity of PET decreased. A new endotherm peak was discerned in DSC thermograms and its position and peak area were closely related to the reaction conditions, which was explained by the changes in the crystalline regions in the PC/PET blends. The effect of transesterification on the multiple melting behaviour of PET in PC/PET blends is also discussed.