The spontaneous combustion behavior of some low rank coals and a range of dried products

This paper compares the spontaneous combustion of two Victorian brown coals and a Pakistan lignite with that of products of the same coals which have been dried, mainly by the mechanical thermal expression technique (MTE) in which aqueous slurries of the materials are heated (125 or 200 °C) and pressed (5-15 MPa). Samples of briquettes obtained by steam heating and pressing from a similar Victorian coal have also been studied. The amount of residual moisture in the dried samples had the largest effect on T_crit values. The inorganic content of the samples varied and may also have contributed to the changes. T_crit increased with increasing particle size for the dried coals and MTE samples. There was no obvious correlation between T_crit and pore volume.     

干燥褐煤的自燃与爆炸特性试验研究

为抑制褐煤堆放时发生自燃与煤粉爆炸,利用自燃装置试验台和煤粉爆炸试验台,对褐煤及其干燥到不同水分的干燥煤进行试验研究,分析其自燃特性和爆炸特性。结果表明,对于自燃特性而言,褐煤水分减少10%,耗氧速度增加约0.17%/min,粒径减小一个等级,耗氧速度增加约0.11%/min;对于爆炸特性而言,褐煤水分减少5%,平均煤粉爆炸下限质量浓度约降低0.015 kg/m~3,温度每升高10℃,平均煤粉爆炸下限质量浓度降低约0.03 kg/m~3;总体而言,随着干燥程度的加深,褐煤自燃、爆炸特性均增强,危险性增加。     

不同干燥程度胜利褐煤燃烧与自燃特性

为了探究高水分褐煤干燥后的燃烧与自燃特性变化,采用一维火焰炉、煤粉着火炉以及自燃试验台对不同干燥程度的胜利褐煤进行了试验研究。试验结果表明,试验样品的着火温度随着干燥程度的增加而降低,随着风煤比的增加而增加,煤粉细度同样会对着火温度产生影响。在燃用干燥褐煤过程中宜采用较高一次风率以提高制粉系统的安全性。胜利褐煤及其干燥褐煤均属于极易燃尽煤种,燃尽率均在99.4%以上,水分的变化对燃尽率影响不大。建议胜利褐煤与20%水分干燥褐煤的运行氧含量控制在3.5%左右。随着干燥程度的加深和粒径的减小,褐煤越容易自燃。胜利褐煤干燥到20%以下可能有自燃的风险。     

褐煤低温干燥特性的实验研究

中国褐煤储量丰富,但较高水分极大地限制了其开采和利用,对其进行脱水提质是解决褐煤高效利用的关键。通过热重分析仪（TGA）对HL和YN的不同粒径褐煤,分别在50,80和110℃等温干燥2h。结果表明：在每个干燥温度下煤样都能达到恒重,随着温度的升高煤样总失重增加,说明煤中水分与煤表面之间具有不同的结合强度;随着水分的降低,水分蒸发所需能量增加,煤水之间的相互作用加强,其中包括氢键和微孔对水的束缚力。粒径0．250～0．150mm、水分26．61％的HL褐煤在50c（二干燥后,水分降至6．96％,此时水分以分子层水的形式存在;干燥温度升至110℃时,煤中水分并未明显降低,说明煤中官能团与水分子间形成的氢键对水分有强烈的吸附作用。HL褐煤50℃干燥后,0．150～0．074mm和0．074～0．038mm煤样残留水分分别为6．52％和3．93％,均低于0．250～0．150mm煤样的6．96％,说明0．250～0．150mm煤样中不能脱除的残留水是孔隙水,被禁锢在狭小空间内。     

Drying behavior of lignite under microwave heating

Because of lignite’s high moisture content, it must be dried before most applications. Microwave radiation may be suitable for efficient drying because of its special heating properties. This study investigated the drying behavior of lignite samples from eastern Inner Mongolia by microwave thermogravimetric analysis. Three stages of microwave drying were observed: preheating, fast weight loss, and falling rate drying periods. Samples’ surface temperatures increased dramatically during preheating, dropped slightly in the second period, and rose again in the final period. The measured surface temperature was <95°C during microwave heating. The overall moisture content decreased more rapidly under higher microwave power. Fine lignite particles (diameter <0.2?mm) and lump samples (particle size 10?mm) dried better than granular lignite (particle size 1–2?mm). The samples also underwent slight natural drying (1–2% point reduction in moisture content) after microwave treatment. The critical moisture content of lignite (11–15% under experimental conditions) was redefined. Energy consumption was analyzed to evaluate the feasibility of the proposed drying process.     

Investigation of combustion reactivity and NO emission characteristics of chars obtained from the devolatilization of raw and partially dried lignite

In an attempt to achieve the clean and efficient utilization of lignite, drying pre-treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%-35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%-19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from L_c1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion.     

Effect of Drying Conditions on Moisture Re-Adsorption Performance of Dewatered Lignite

In order to improve the drying efficiency of lignite and restrain the moisture re-adsorption of dewatered coal, the drying characteristics of typical Chinese lignite, the re-adsorption performances of dewatered samples and the change in pore structure throughout the entire processes were investigated in this study. Lignite samples with four different particle size fractions were dried in a fixed-bed reactor in the temperature range 60–160°C. The re-adsorbing moisture behaviors of dewatered coal samples containing different water contents were investigated at temperatures of 20–40°C and humidities of 55–95%. The changes in the pore structure of raw coal and different dried samples were measured by mercury intrusion porosimetry (MIP) and the relations between their re-adsorption performance and change in pore structure were explored. The moisture removal yields of lignite increased with an increase in drying time and temperature and was close to 100% above 120°C and over 100% after holding 40 and 15 min at temperatures of 140 and 160°C due to the release of CO₂ from the decomposing carboxyl group in the coal matrix. The re-adsorbed moisture content in dewatered coal was influenced by drying temperature and coal particle size through varying pore structure. The temperature and relative humidity in the re-adsorbing process were the main factors that influenced the moisture re-adsorption capacity of dewatered lignite, in which the re-adsorbing temperature mainly operated by varying the bonding ability of water on the surface of dewatered coal, and the relative humidity was connected with the pore structure as well. The mesopore was the main factor that influenced the re-adsorption of dewatered coal and the re-adsorption of moisture in dewatered coal at 100°C was highest due to the narrow range of the pore radius and because the relative volume ratio of 5 to 50 nm mesopore (above 91%) was high. The water loss yield of lignite with smaller particle size was higher due to its larger pore volume and surface area, but its re-adsorption capacity was lower because of lower volume ratio of 5 to 50 nm mesopore volume in dewatered coal obtained from the smaller size lignite.     

Minimization of Moisture Readsorption in Dried Coal Samples

Low-rank coals csonstitute a major energy source for the future as reserves of such high-moisture coals around the world are vast. Currently they are considered undesirable since high moisture content entails high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in combustion of such coals. Furthermore, low-moisture-content coal is needed for the various coal pyrolysis, gasification developed. Hence, various upgrading processes have been developed to reduce the moisture content. Moisture readsorption and spontaneous combustion are important issues in coal upgrading processes. This article discusses results of laboratory experiments conducted to study the options for minimization of readsorption of moisture after drying of selected coal samples. Results suggest that there is little benefit in drying low-rank coal at high temperatures. It was found that the higher the amount of bitumen used for coating, the lower is the readsorption of moisture by dried coal. Also, mixing high-temperature-dried coal with wet coal in appropriate proportion can yield reduced moisture content as the sensible heat in the hot coal is utilized for evaporation.     

The effect of moisture content and air-drying on spontaneous combustion characteristics of two Turkish lignites

The results of an experimental study aimed at evaluating the spontaneous combustion characteristics of two Turkish lignites moistured and air-dried at varying times are discussed. The spontaneous combustion characteristics of the coals were determined using Crossing Point Methods adapted to our laboratories conditions. The content of three predominant oxygen functional groups (carboxyl, hydroxyl, and carbonyl) of untreated, moisten and air-dried coal samples were also determined with wet chemical methods. The content of oxygen functional groups in moisten coal samples do not differ significantly that of untreated coal samples, for realized in vacuum desicator to moistured of coal samples. The liability of spontaneous combustion of the two coals was reduced when moisture content increased with increase in contacted time to water vapour. The moisten coal samples was dried in laboratory during 24 and 48 h time period. The concentration of oxygen contain functional groups of drying coal samples increased with increase of contact time with air and decrease of particle size. The liability of spontaneous combustion of the air-dried coal samples increased with increase of contacted time with air and with decrease of moisture content.     

Analysis of D2 law in case of Shengli lignite drying under inert and uninert environment

Shengli lignite coal, originated from inner Mongolia China, contains significantly high amount of moisture (more than 30%) which can cause spontaneous combustion or other application problems. Thus, it is of interest to understand the heat and mass transfer mechanism of the low-rank lignite drying under different drying environments such as N₂, CO₂, air, argon, and helium. In this study, fundamental drying experiments with different drying agents were performed on coal samples using thermogravimetric analysis (TGA) method. Lignites with size of 0.045–0.075?mm were heated up from ambient temperature to a target temperature of 175°C under different environments at heating rates of 5, 10, 20, 40, and 80 °C/min, respectively. It was found that thermal conductivity of drying media, heating rate, and initial moisture content are three most significant factors affecting lignite drying process. The highest moisture release rate and the lowest T_peak (when maximum moisture release rate occurred) were observed when drying with helium due to its highest evaporation constant (i.e., highest thermal conductivity). Moreover, higher heating rate and moisture content resulted in higher evaporation rate and T_peak. In the meantime, the classical D² law, which is used to simulate the liquid fuel droplet combustion, was further developed to describe the “group effect” of moisture evaporation process of solid fuel during drying. The D² law well explains the experiment results. Finally, the structures of the dried lignite samples under different drying mediums were investigated through scanning electron microscopy studies. It was found that lignite coals shrank and became more compact when dried out, especially with drying agent CO₂.     

Comparison of drying methods on physical and chemical properties of Shengli lignite

The physicochemical properties of Shengli lignite dewatered by three methods in hot air, microwave, and vacuum conditions were investigated. The effects of drying method on the coal pore structure, surface morphology, surface carbon-containing groups, and moisture readsorption performance of dewatered lignite were studied by the Brunauer-Emmett-Teller (BET) method, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and dynamic water vapor sorption analysis (DVS). The results indicate that a large quantity of semi-open micropores and open-ended mesopores with a slit-like or cylinder shape exist in lignite. The specific surface area and total pore volume value of dewatered lignite decrease as follows: microwave-dried lignite (MD) > vacuum-dried lignite (VD) > hot air–dried lignite (HAD), which is consistent with MIP results. SEM images show that lignite dried by different methods is characterized by varied surface morphology. The XPS results reveal that VD has the highest content of C–C/C–H and the lowest content of C–O, and HAD has a higher content of C–O and a higher surface oxidation degree (α) than MD. In addition, the moisture readsorption capability of the dewatered lignite follows the sequence MD > VD > HAD.     

褐煤干燥特性及产品特性研究

通过对内蒙古锡林浩特褐煤的热重分析可知,褐煤干燥过程约30 min,干馏过程约1.5 h。对不同粒径褐煤进行了干燥特性研究,说明+100,100～50,-50 mm原煤完全干燥分别需要4.0,1.0,0.5 h,粒径越小,煤样失重率越大,干燥速度越大,达到相同干燥效果所需的干燥时间也越短。最后分析了干燥温度对产品特性的影响,同时测试了干燥、干馏提质产品的自燃特性和复吸水特性,结果表明:与原煤相比,150,200℃产品挥发分分别提高了33.72%和31.13%,更易发生自燃,而550℃干馏过程中挥发分降低了30.89%,热稳定性大幅增加;150,200,550℃产品吸氧量分别提高了0.43,0.65,0.72 cm3/g;干燥产品燃点要低于原煤,而干馏产品燃点则高于原煤;干馏煤因改变了孔隙结构,最高内在水分降低,即复吸水的能力降低。因此,干燥提质产品与褐煤性质基本一致,而干馏提质产品性能则获得较大改善。     

Effect of mechanical thermal expression drying technology on lignite structure

Mechanical thermal expression (MTE) is a developing nonevaporative lignite dewatering technology. It has been proved to be effective to dewater high moisture content in low-rank coals via the application of mechanical force and thermal energy at elevated temperatures. In this paper, the dewatering behavior of the Xiaolongtang lignite in Yunnan province, China during the MTE process was studied with three process parameters: time, temperature, and pressure. Meanwhile, the mechanism was also explored of how variations in temperature and pressure during the MTE process affect the surface oxygen functional groups and pore structure, which was mainly conducted by means of Fourier transform-infrared spectrometer (FTIR) and mercury intrusion porosimetry (MIP). Increases in MTE temperature and pressure resulted in significant reductions in residual moisture content and moisture-holding capacity, along with the increase in fixed carbon content and content reductions of other elements, especially oxygen content, this could be largely attributed to the destruction of the surface oxygen functional groups and porosity in the lignite. Technologically, the optimal conditions for temperature and pressure are 150–220°C and 6–10?MPa. The residual moisture content of the lignite treated by MTE at 200°C, 10?MPa is lower than 8%; the dewatering rate reaches over 76% with the calorific value being approximately 22?MJ/kg. Carboxyl and hydroxyl groups break down at drying temperatures above 120°C and constant applied pressure 10?MPa; with the pore volume significantly reduced, only few pores (diameter?相似文献   

典型褐煤干燥技术比较

分析了国内外典型褐煤干燥技术的原理、关键参数、技术特点等,并从技术适应性、工艺条件、干燥效果、环境影响等方面进行综合评价比较。结果表明:美国K-Fuel工艺要求入炉粒度较其它技术大,适应性稍差;褐煤提质后均可作为锅炉燃料。干燥温度越高,褐煤干燥效果越好。德国泽玛克管式干燥成型技术、美国K-Fuel工艺为带压干燥,其他工艺为常压工艺。5种工艺可降低水分7.0%～25.0%,增加热值3.24～5.94kJ/g;澳大利亚BCB工艺、神华HPU-06工艺、德国泽玛克管式干燥成型技术产品为型煤,规格可调;其他工艺产品粒度变小。神州干燥-干选联合工艺回吸可能性大、自燃特性改善不明显,其余工艺深度脱水后可防止自燃,不易回吸。5种工艺污染物排放均较为简单,对环境影响较小。     

褐煤深度脱水的实验研究

针对褐煤低温烟气干燥技术脱水幅度难以提高,发热量增加不多等问题,提出了低温干燥与深度脱水的联合工艺。研究了不同干燥温度、干燥时间和煤样粒度对褐煤深度脱水效果的影响,结果表明:褐煤深度脱水适宜的干燥温度为500～800℃,较为适宜的干燥时间为80 s左右,当粒度小于50 mm时,干燥产物发热量随粒度变化不大。最后提出了褐煤深度脱水最佳工艺条件和参数,即当预干燥煤的Mar约为18%,煤样粒度为-50 mm,干燥温度为700℃,干燥时间为80 s时,深度脱水产物的Mar为8%,Vdaf在46%左右,折算Qnet,ar约为21 kJ/g。褐煤深度脱水促进了褐煤发热量的进一步提高,实现了褐煤资源的增值,研究结果为褐煤低温干燥与深度脱水联合工艺技术方案的确定奠定了基础。     

Moisture readsorption and low temperature oxidation characteristics of upgraded low rank coal

This study investigated the behavior of upgraded low rank coal produced by a coal-oil slurry dewatering process regarding moisture readsorption, low temperature oxidation, and spontaneous combustion. The upgraded low rank coal had higher heating values than raw coal. It also showed lower moisture readsorption than raw coal and had less susceptibility to low temperature oxidation and spontaneous combustion. This seemed to result from the coating of the asphalt on the surface of the coal, which covered fine pores and suppressed the active functional groups from reacting with moisture and oxygen in the air. The increasing upgrading pressure negatively affected the moisture readsorption, low temperature oxidation, and spontaneous combustion.     

EVOLUTION LIGNITE MESOPORE STRUCTURE DURING DRYING. EFFECT OF TEMPERATURE AND HEATING TIME

The knowledge of the intrinsic pore structure of coals is significant in elucidating the kinetics of mass transport and chemical reaction that leads to design of more efficient coal combustion and conversion equipment. The results of pore structure studies of Greek lignite are reported in this work. Isothermal drying of Greek lignite samples, under vacuum, caused mesopore structure evolution despite the severe (∼50%) particle size contraction due to heating. Mesopore volume and surface area were increased as the drying temperature was raised to 200°C while further drying up to 250°C caused a mesopore volume and surface area decrease. Lignite drying at 100°C for up to 3 h resulted in a monotonic increase of the mesopore structure properties while heating for a longer period i.e., 6 h, despite a slight increase of weight loss, caused pore volume and surface area reduction. Nitrogen sorption (77 K) hysteresis data obtained for partially dried samples have been processed to deduce BET surface area and pore size distributions (PSD) by using both the Roberts and a new method based on a Corrugated Pore Structure Model (CPSM-nitrogen) methods. The latter method was applied successfully in hysteresis loop simulations and predicted pore surface areas consistent with the respective BET values. Bimodal PSD have been detected with one peak at 3 nm and the second at 20 nm while surface area varied over the range 2.98-5.30 m²/g. Dry Greek lignite has shown a higher mesopore volume than that of several American and Canadian coals of varying rank. Mesopore volume distribution of dry Greek lignite, obtained from nitrogen sorption data, agree well with those deduced from mercury penetration data corrected for coal compressibility.     

DEACTIVATION OF DRIED SUBBITUMINOUS COAL

Deep drying of inherent moisture from subbituminous coal produces a dried product which is friable and very reactive to oxygen in air. The major problems in handling dried coal, which are unique to deep drying of low-rank coal, are: (1) avoidance of spontaneous combustion, (2) prevention of moisture reabsorption, and (3) control of dust. Atlantic Richfield has developed, and successfully demonstrated, technologies which solve all three problems. However, this paper will be confined to the spontaneous combustion problem

Low-temperature 'oxidation may result in spontaneous combustion' of the coal during storage or transit. The increased reactivity to low-temperature oxidation of dried coal is due to two factors: (1) the reduction in moisture content, and (2) the increase in temperature. A laboratory technique was developed to measure the coal reactivity to low-temperature oxidation at temperatures near ambient. Laboratory data was then used to develop a pseudo-first order kinetics model for dried subbituminous coal as a function of temperature and moisture content. The influence of particle size was also determined

A one-dimensional spontaneous heating model was formulated to evaluate the effects of wind velocity, coal pile porosity, thermal conductivity of crushed coal, and geometry of coal piles on the dried coal's tendency to spontaneous combustion. This model was validated against two large-scale spontaneous heating tests. Each test consumed about 800 pounds of dried coal.

Based on a selected combination of three deactivation methods and predictions of the computer model, a special product treatment procedure was developed and successfully demonstrated in pilot plant tests in 1983. Dried coal stockpiles (about 100 tons per pile) were monitored for four months under severe weather conditions with no evidence of either moisture reabsorption or spontaneous combustion.     

Combustion and Pollutant Emission Characteristics of Lignite Dried by Low Temperature Air

Lignite is a kind of coal that has high moisture content and needs to be dried before being utilized. In this article, a Chinese lignite was dried in air at 120–180°C and the changes in its physical and chemical structures after drying were investigated. The results showed that the pore volume and specific surface area of the lignite decreased after drying. Some of the methylene and methyl groups were oxidized by the oxygen in the drying air, resulting in an increase in oxygen functional groups. The combustion characteristics of the dried coals and parent coal (dry basis) were studied via thermogravimetric analysis. The total volatile yields of the dried coals increased compared to the parent coal. The burnout temperatures of the dried coals were higher than the parent coal, whereas the ignition temperatures stayed almost unchanged. An entrained flow system was set up to study the release of nitrogenous gas products during rapid pyrolysis and combustion. The HCN yields of the dried coals during pyrolysis were higher than that of the parent coal, and a similar trend was found for the NO yield during combustion. The mechanism changes of combustion and pollutant emission characteristics were discussed according to the results of the physical and chemical structure analyses.     

褐煤流化床提质性能研究

针对褐煤水分高、发热量低、易风化自燃等特点,以内蒙古褐煤为研究对象,进行了褐煤静态干燥实验和褐煤提质多因素实验。以O2体积分数10.5%的烟气为干燥介质,在分析褐煤流化床提质干燥机理的基础上,对褐煤进行流化床动态提质实验。结果表明:褐煤提质水分控制在5%左右为宜。褐煤粒级小于3 mm时,温度对煤样干燥速度影响最大,其次是煤样粒度,风速对干燥速度影响最小。确定全粒级、0.5~1.25、1.25~2、2~3 mm褐煤临界流化风速分别为38、20、40和50 m3/h。褐煤适宜提质温度和时间分别为200~240℃和5~8 min。最后建立了褐煤提质模型,说明褐煤提质规律与提质介质温度、风速密切相关,该模型对褐煤提质生产具有指导作用。