Multiphysics modeling of water transport in high-intensity lignite drying process on pore scale

The understanding of the moisture transfer process in the pore network is quite important to improve the lignite drying efficiency. Scanning electronic microscopy image was used for construction of pore topology closely approximating the true topology of the real lignite for the heat and mass transfer processes on pore scale by COMSOL simulation. Considering the gas?liquid phase coexistence of water, “Laminar Two-Phase Flow, Phase Field” module and “Liquid Heat Transfer” module were used. The pore size had significant effects on the flow velocity and the larger pores acted as the main pathway for the moisture transport, therefore affected the maximum drying rate. On the other hand, the connection of pores and the throats distribution in the pathway also had a significant effect on the flow velocity, and the moisture between the throats was hard to transfer as a flow, maybe by vapor diffusion. In high-intensity lignite drying process, the moisture vaporization quickly when heated up and vapor pressure was beneficial to keep the pore size and ensure the smooth of moisture flow pathway, thus improving the efficiency of the drying process.     

Dielectric properties of Australia lignite at radio frequency

Dielectric properties (dielectric constant and loss factor) of Australia lignite were determined using an impedance analyzer and other instruments over the moisture contents of 8–48%, temperature of 25–100°C, and frequency of 1–50 MHz. Both dielectric constant and loss factor of lignite samples increased monotonically with the increase of moisture content at all temperatures and frequencies. Dielectric constant increased with the increase of frequency, and loss factor changed quadratically: first decreased and then increased. Dielectric constant increased with the increase of temperature at low moisture contents (8–28%), but it decreased at high moisture contents (38–48%), and loss factor increased with the increase of temperature at all moisture contents. The penetration depth of radio frequency energy of Australia lignite was obtained through the equation. It decreased with the increase of moisture content, temperature, and frequency.     

Pore structure development in Xilingol lignite under microwave irradiation

The pore structure of Xilingol lignite irradiated by microwave was investigated to determine drying time, microwave power level, and mineral composition. Pore structure was also determined using N₂ adsorption/desorption as well as scanning electric microscopy. The results show that as microwave power increases from 400 W to 800 W, and irradiation time increases from 4 to 16 min, the specific surface area of lignite samples increases, average pore diameter and total pore volume decreases, and the percentage of mesopores increases. The pore volumes, average pore diameters, and special surface area in the center of the lignite sample were greater than those in the outer layers, while the percentage of mesopores decreased slightly. The cluster structure of the lignite samples is simpler and the surface neater, while flat and fibrous structures remain the same. Evolution mechanisms for pore structures during microwave drying were similar, and include structural collapse caused by shrinkage forces resulting from the removal of moisture, the opening and crosslinking of blind and closed pores, and the thermal decomposition of organic macromolecular structures under high temperatures.     

水分对褐煤燃烧特性及锅炉性能的影响

以660 MW褐煤锅炉为对象,研究了含水量升高对褐煤燃烧特性及锅炉性能的影响。模拟中采用一种特殊的方法来考虑褐煤中水分的存在,计算准确性高。研究结果表明:水分较高的褐煤炉膛温度相对较低,炉内受热面吸热量减少,但气体发射率变大使得屏区的辐射传热差异相比水冷壁减小。水分含量升高对炉内气化反应有促进作用,但总体影响小于温度降低带来的抑制作用。高含水率煤的未燃尽碳浓度升高,煤粉燃尽变差,锅炉整体效率降低。因此,实际锅炉运行时不宜燃用超过设计燃料水分太大的褐煤。     

600 MW超超临界塔式褐煤锅炉设计方案

主要介绍褐煤的特性,论述了褐煤锅炉的设计要点及塔式锅炉的特点,说明了对于高水分褐煤锅炉采用塔式炉配风扇磨的优势.风扇磨制粉系统具有很强的干燥能力,对原煤水分有较大的适应性,对于塔式锅炉,可以选取相对较大的容积和炉膛断面和低的屏底烟温,降低炉膛的结渣风险,并且没有转弯烟室,炉内烟温偏差小、磨损低、燃尽率高.最后详细介绍了600 MW超超临界塔式褐煤锅炉设计方案和能达到的关键性能指标.     

The relevance between water release behavior and pore evolution of hard lignite during the thermal-drying process

As known, the shrinkage of coal and the pore collapsed during the drying process due to water removal. The relevance between water release behavior and pore evolution of lignite during the thermal-drying process was studied in this work. From N₂ adsorption of traditional pretreated samples and freezing wet samples, most of moisture was in the pores in the range of 26–100 nm, and results from low-field nuclear magnetic resonance (LFNMR) confirmed this conclusion that the removed moisture was in the pores with pore size smaller than 200 nm, while from the mercury intrusion porosemitry (MIP) results, the most obvious reduction of macropores was in the range of pore dimeter larger than 1 μm and there was almost no change of pore volume in the range of pore diameter smaller than 200 nm, which was completely different from traditional ideas that the pore shrunk or collapsed due to water removed. The macropores were more prone to collapse than mesopores and micropores, maybe due to its structural mechanical characteristics. And the effect of pore evolution on moisture release behavior was related to the specific drying methods and mass transfer processes.     

Effect of hydrothermal dewatering on the moisture content of brown coal

Brown coal was dewatered under hydrothermal dewatering (HTD) conditions. The variation in moisture content and distribution of brown coal were studied. The results showed that equilibrium moisture content decreased from 28.2% to 9.6% after hydrothermally treated at 300 ºC. Three types of moisture existed in brown coal and its HTD products according to the coal-water binding energy (E_b): Surface adsorption water (E_b < 2000 kJ/kg), interparticle water (2000 < E_b < 9000 kJ/kg), and interior adsorption water (E_b > 9000 kJ/kg). Drying rates of these types of moisture? respectively followed logarithmic, linear, and indicial equations.     

High-pressure methane adsorption-induced coal swelling on equilibrium moisture coal samples

The methane adsorption at the surface of coal pores can induce coal swelling. Swelling strain is not only related to adsorption capacity but also equilibrium moisture content and coal ranks. The swelling strain in confined conditions will lead to the closure of coal reservoir cleat system, which reduces coal reservoir permeability, and further impedes free gas percolation. In this work, adsorption and strain experiments on two coal samples from China with vitrinite reflectance of about 1.64 and 3.68% were performed at 30, 40, and 50°C with the pressure of 15 MPa in the corresponding moisture state. The error is compared between measurement values and theoretical values. The results indicate that coal swelling is irreversible and anisotropic, and the methane adsorption capacity and swelling strain on two coal samples have a concave parabolic curve relationship with the increase in temperature. The effects of temperature on sorption and swelling on middle-rank coal is inferior to equilibrium moisture in the temperature range of 30–40°C, and superior to it in the range of 40–50°C, however, it is the opposite on high-rank coal. The swelling strain curve shapes of different rank coals are similar no matter what temperature, pressure, and moisture content are, and also are a function of time. The whole strain process can be divided into three phases: rapid growth, slow growth, and stability with the increase in pressure. These show that swelling strain of tested coal samples is synthetically affected by moisture, temperature, and pressure.     

Towards the improvement of thermal efficiency in lignite‐fired power generation: Concerning the utilization of Polish lignite deposits in state‐of‐the‐art IGCC technology

Integrated coal Gasification Combined Cycle (IGCC) is the most advanced technology for coal‐fired power generation. The two‐stage entrained flow gasification process allows for the use of a wide range of coal, as long as the gasification temperature is above the ash melting point of a used fuel. In this gasification technology, lignite, which often has a low ash melting point, can be preferably utilized. However, ash fluidity is also another importance, because the behaviour of molten slag can diminish a stable ash discharge from a gasifier. As the eligibility of coal ash properties is a considerable factor, water physically and chemically kept in lignite (30 – 60% in mass) attributes to deteriorating gasification efficiency, because it causes significant heat loss and increasing oxygen consumption. Developing a thermal evaporative lignite drying method will be a necessary attempt to apply lignite to the coal gasification process. For those preceded objectives, coal and ash properties and drying characteristics of several grades of Polish lignite, extracted from Belchatow and Turow deposits, have been experimentally investigated in a preliminary study evaluating the applicability and consideration for its utilization in state‐of‐the‐art clean coal technology, IGCC. This paper particularly discusses the eligibility of Polish lignite from the perspective of the fusibility and fluidity of ash melts and the fundamental drying kinetics of lignite in superheated steam in the light of water removal. The viscosity of ash melts is measured at high temperature up to 1700 °C. In the drying tests, the significant influence of structural issues, because of the provenance and origin of lignite on the drying characteristics, was found by applying the method of sensitivity analysis of physical propensity. This paper concludes that the investigated Polish lignite has characteristics favourable for utilization in IGCC technology, once the precautions related to its high moisture have been carefully addressed. Copyright © 2016 John Wiley & Sons, Ltd.     

A basic study on humidity recovery using micro‐porous media: General characteristics and effect of material properties on transport performance

As a first step toward evaluating factors that influence humidity and heat transfer from moist air to dry air through porous media having very small pores, the present paper attempts to clarify factors that influence moisture transport between constant‐temperature water and dry air through a porous media plate. The effect of the pore diameter of the porous plate on the humidity transport through a porous plate was found to depend strongly on the thickness and pore diameter of the porous plate. That is, the smaller the pore diameter and the thicker the plate, the greater the effect of the pore diameter on the humidity transfer. In addition, the performance of heat and mass transfer were confirmed to increase with respect to the increase of air velocity. In addition, a parameter called the humidity absorption rate was introduced to evaluate the utilization degree of the air capacity to absorb humidity. The humidity absorption rate decreased with increases in both air velocity and plate thickness. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(2): 137–151, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20102     

Lignite‐fired air‐blown IGCC systems with pre‐combustion CO2 capture

Detailed analyses based on mass and energy balances of lignite‐fired air‐blown gasification‐based combined cycles with CO₂ pre‐combustion capture are presented and discussed in this work. The thermodynamic assessment is carried out with a proprietary code integrated with Aspen Plus^® to carefully simulate the selective removal of both H₂S and CO₂ in the acid gas removal station. The work focuses on power plants with two combustion turbines, with lower and higher turbine inlet temperatures, respectively, as topping cycle. A high‐moisture lignite, partially dried before feeding the air‐blown gasification system, is used as fuel input. Because the raw lignite presents a very low amount of sulfur, a particular technique consisting of an acid gas recycle to the absorber, is adopted to fulfill the requirements related to the presence of H₂S in the stream to the Claus plant and in the CO₂‐rich stream to storage. Despite the operation of the H₂S removal section representing a significant issue, the impact on the performance of the power plant is limited. The calculations show that a significant lignite pre‐drying is necessary to achieve higher efficiency in case of CO₂ capture. In particular, considering a wide range (10–30 wt.%) of residual moisture in the dried lignite, higher heating value (HHV) efficiency presents a decreasing trend, with maximum values of 35.15% and 37.12% depending on the type of the combustion turbine, even though the higher the residual moisture in the dried coal, the lower the extraction of steam from the heat recovery steam cycle. On the other hand, introducing the specific primary energy consumption for CO₂ avoided (SPECCA) as a measure of the energy cost related to CO₂ capture, lower values were predicted when gasifying dried lignite with higher residual moisture content. In particular, a SPECCA value as low as 2.69 MJ/kg_CO2 was calculated when gasifying lignite with the highest (30 wt.%) residual moisture content in a power plant with the advanced combustion turbine. Ultimately, focusing on the power plants with the advanced combustion turbine, air‐blown gasification of lignite brings about a reduction in HHV efficiency equal to almost 1.5 to 2.8 percentage points, depending on the residual moisture in the dried lignite, if compared with similar cases where bituminous coal is used as fuel input. Copyright © 2016 John Wiley & Sons, Ltd.     

Liquefaction of Shengli lignite with methanol and CaO under low pressure

The behavior of liquefaction of Shengli (SL) lignite with methanol and CaO in the presence of FeS and tetralin (THN) was studied. The effects of reaction temperature and time on the liquefaction behavior of SL lignite with methanol and CaO were preliminarily investigated. The liquefaction products obtained were analyzed by FTIR spectrum and elemental analysis. The results show that SL lignite liquefaction with methanol and CaO in the presence of FeS is one of the feasible liquefaction methods. This method has some advantages such as the higher liquefied product yields and n-hexane soluble (HS) + gas yield compared to that of liquefaction in H₂ atmosphere; and carrying out at lower pressure, which was only one third of that of liquefaction in H₂ initial pressure of 5 MPa. The reaction temperature and time significantly affect the liquefied product yields and distributions. The lignite liquefied product yield and gas + HS yield reach to 81.8% and 62% respectively at 400 °C for 2 h, which are significantly higher than those of liquefaction in H₂ initial pressure 5 MPa.     

蒙东褐煤脱水改质的孔隙特性研究

对蒙东褐煤及其灰分在一定温度和压力下进行脱水,利用扫描电子显微镜、低温氮吸附法研究脱水前后褐煤的表面特性及煤样和灰样的孔隙特性.结果表明:褐煤的脱水率达到70%以上,脱水后煤样的空干基低位发热量增加10%以上,且热压后褐煤的外表面相对光滑.由吸附脱附等温线表明,30 MPa热压后褐煤的孔隙特性和回水性能无明显变化.50 MPa热压后褐煤的比表面积和总孔体积均减小,吸水回潮能力减弱,回水率仅为58.7%.与印尼褐煤相比,蒙东褐煤受热压压力的影响不明显,灰分会抑制热压过程中煤样孔隙结构的变化,影响热压脱水的效果.     

Emissions estimation for lignite-fired power plants in Turkey

The major gaseous emissions (e.g. sulfur dioxides, nitrogen oxides, carbon dioxide, and carbon monoxide), some various organic emissions (e.g. benzene, toluene and xylenes) and some trace metals (e.g. arsenic, cobalt, chromium, manganese and nickel) generated from lignite-fired power plants in Turkey are estimated. The estimations are made separately for each one of the thirteen plants that produced electricity in 2007, because the lignite-fired thermal plants in Turkey are installed near the regions where the lignite is mined, and characteristics and composition of lignite used in each power plant are quite different from a region to another. Emission factors methodology is used for the estimations. The emission factors obtained from well-known literature are then modified depending on local moisture content of lignite. Emission rates and specific emissions (per MWh) of the pollutants from the plants having no electrostatic precipitators and flue -gas desulfurization systems are found to be higher than emissions from the plants having electrostatic precipitators and flue -gas desulfurization systems. Finally a projection for the future emissions due to lignite-based power plants is given. Predicted demand for the increasing generation capacity based on the lignite-fired thermal power plant, from 2008 to 2017 is around 30%.     

褐煤干燥提质技术比较与水分回收及余热利用方案

针对褐煤的高水分、低热值、易风化自燃、难以洗选、储存和运输等缺点,总结研究目前褐煤干燥脱水技术,并提出一套针对燃煤电站的褐煤干燥及其高水分烟气水分回收与余热来利用的方案,从而提高褐煤使用范围和经济性以适应当前能源紧缺及节能环保的能源局面。     

The effect of air staged combustion on NOx emissions in dried lignite combustion

Experiments were carried out in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of BRXL lignite and its dried coals. The impact of moisture content, multiple air staging, pulverized coal fineness and burnout air position on NOx emissions under deep, middle and shallow air-staged combustion conditions. Moreover, the impact of blending coals on NOx emissions was investigated in this paper. The unburned carbon concentration in fly ash was also tested. Experimental results based on the combustion of BRXL lignite and its dried coals show that NOx emissions can be reduced drastically by air-staged combustion. NOx emissions reduce with the increase of the air that is staged and the distance between the burner and burnout air position. Dried coal of BRXL lignite emits a smaller amount of NOx than that of BRXL lignite. However, the dried degree of BRXL lignite is closely related to R₉₀ fineness. Dried coal with optimal moisture content yields least NOx emissions. When deep or middle staged combustion was adopted, the application of multi-staged combustion is conducive to NOx reduction. However, when shallow staged combustion was adopted, NOx emissions are higher in multi-staged combustion than that in single-staged combustion with M_S = 0.54. Thus, the existence of a certain concentration of O₂ in reduction zone would significantly reduce NOx emissions. The blending coals that dried coals of BRXL lignite were blended with bituminous coals emit a larger amount of NOx than that of the dried coal alone. NOx emissions decrease with the increase of the proportion of dried coal in the blending coal. Moreover, the unburned carbon concentration in fly ash of dried coal in staged combustion is lower than that of BRXL lignite in staged combustion. On the whole, the dried coal of BRXL lignite is conducive to NOx reduction in staged combustion.     

褐煤提质技术浅析

褐煤因其热值低、含水量高、易风化自燃等特点,给其燃烧、运输、储存等方面带来了很多困难,如何实现褐煤的高效利用已成为亟待解决的问题。褐煤提质是指褐煤通过干燥或热解工艺降低水分、转化成具有烟煤性质的提质煤。提质技术已成为近年来褐煤应用方面的主要研究方向之一。     

微波加热对褐煤干燥及成浆特性的影响

针对外水分和内水分含量都很高的褐煤,利用微波700 W加热10 min使其干燥,可有效地改善其成浆性能,使最大成浆浓度由原煤的45.67%增加到51.65%。利用六种薄层物料干燥数学模型对褐煤干燥过程进行计算,发现Page模型可很好地描述褐煤的微波干燥过程。褐煤脱水能耗随着微波功率增大而降低,当用大反应容器、大装样量时的微波能耗比小反应容器、小装样量时要低。经微波干燥后的褐煤若放置于空气中,发现微波改性使煤样二次吸水的负作用受到明显抑制。     

Solar cooling with aluminium pillared clays

Aluminium modified clays were prepared, characterised and tested for their potential application as solar coolers of roof surfaces. The water adsorption isotherm of the samples with restored cation exchange capacity was of hydrophilic type II, indicating multilayer adsorption with large heat of adsorption and pore condensation of water vapour at the pressures of the proposed application. In addition, high moisture adsorption capacity (more than 0.1 g of moisture per g of material at 60-70% of relative humidity) and fast kinetics for night sorption (comparable to silica gel) were determined for the freeze-dried pillared sample. In cyclic experiments with low irradiation during the day and night relative humidity of 55%, the maximum temperature inside the pillared montmorillonite was 6.5 °C lower than the corresponding temperature inside a typical soil sample. The primary mechanism for the reduced temperature elevation at aluminium modified clays was evaporative cooling and desorption with minor influence of solar reflection. These results indicate the suitability of aluminium pillared clays for lowering the roof surface temperatures.     

褐煤脱水提质技术的研究与进展

介绍了国内外学者关于褐煤中水分赋存形式及脱水机理的相关研究、几种褐煤脱水干燥工艺及各自特点,以期为进一步提高褐煤利用奠定基础。