Lignite Particles Research Articles

Thermal fragmentation and pulverization behavior of lignite during the vibration drying process and its grey relational analysis of multi-factors

Lignite particles are easy to break into smaller particles in the process of thermal drying, which affect the heat and mass transfer inside the particles. Therefore, it is important to study the fragmentation behavior of lignite for optimizing the drying process. In this article, two concepts of degree of fragmentation (F′) and pulverization ratio (R p ) were put forward to describe the overall fragmentation property of lignite after vibration drying. The results showed that the higher the drying temperature, the more favorable it was for the product particles to fragment. The impact of particle size and feeding amount on F′ and R p was relatively small, but it was still not negligible. There was a critical vibration frequency during vibration drying. Once the vibration frequency exceeded critical value, the fragmentation of particles would be greatly promoted. The grey correlation model of lignite was established to quantitatively analyze the influence of five factors on the fragmentation and pulverization of products in the process of multi-factor coupling drying. Following the grey correlation model, five factors, such as the drying time, vibration frequency, drying temperature, feed particle size and feeding amount were ranked according to their influence on F′ and R p .

Chemical looping combustion of lignite using iron ore modified by foreign ions: Alkaline-earth and transition metal ions

In the chemical looping combustion (CLC) of lignite, oxygen carrier reactivity is the key to combustion performance. In this paper, the effects of particle size and temperature upon the CLC of lignite are investigated in a fixed bed reactor using iron ore as the oxygen carrier. Under optimized working conditions, the iron ores are loaded with alkaline-earth metals (K, Na and Ca) and transition metals (Ni, Mn and Cu) in a mass ratio of 10%, and their effect upon its CLC reactivity are studied. The carbon conversion rate, carbon capture rate, CO2 capture rate, CO2 selectivity, and other parameters are obtained. The size of lignite particles had a direct effect on the carbon capture rate of the fuel reactor. The particle size with the best effect on carbon and CO2 capture for both the Yunnan and Yizhong lignite is 80–200 mesh. The increase in temperature effectively promotes the reaction between oxygen carrier and volatiles, thereby resulting in an increase in CO2 concentration. The capacity of the oxygen carrier to transform the lignite is effectively enhanced by the modification with Cu, K, Na, Ca and Mn. In particular, the CO2 selectivity is greatly improved (up to 94.84%) for the Cu-modified oxygen carrier. These results provide important references for the CLC process, especially for the design of the oxygen carrier.

Dynamic experiments of acid mine drainage with Rhodopseudomonas spheroides activated lignite immobilized sulfate-reducing bacteria particles treatment

Aiming at the problem that the treatment of acid mine drainage (AMD) by sulfate-reducing bacteria (SRB) biological method is susceptible to pH, metal ions, sulfate and carbon source. Lignite immobilized SRB particles (SRB-LP) and Rhodopseudomonas spheroides (R. spheroides) activated lignite immobilized SRB particles (R-SRB-LP) were prepared using microbial immobilization technology with SRB, R. spheroides and lignite as the main substrates. The dynamic experimental columns 1# and 2# were constructed with SRB-LP and R-SRB-LP as fillers, respectively, to investigate the dynamic repair effect of SRB-LP and R-SRB-LP on AMD. The mechanism of AMD treated with R-L-SRB particles was analyzed by scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectrometer and low-temperature nitrogen adsorption. The result showed that the combination of R. spheroides and lignite could continuously provide carbon source for SRB, so that the highest removal rates of SO42−, Cu2+ and Zn2+ in AMD by R-SRB-LP were 93.97%, 98.52% and 94.42%, respectively, and the highest pH value was 7.60. The dynamic repair effect of R-SRB-LP on AMD was significantly better than that of SRB-LP. The characterization results indicated that after R-SRB-LP reaction, the functional groups of −OH and large benzene ring structure in lignite were broken, the lignite structure was destroyed, and the specific surface area was 1.58 times larger than before reaction. It illustrated that R. spheroides provided carbon source for SRB by degrading lignite. The strong SRB activity in R-SRB-LP, SRB can co-treat AMD with lignite, so that the dynamic treatment effect of R-SRB-LP on AMD is significantly better than that of SRB-LP.

Numerical simulation of the processes of burning lignite in a vortex furnace with swirling countercurrent flows

Abstract This work presents the results of a numerical study of the working processes of burning lignite in a vortex furnace with swirling countercurrent flows. The results of computer simulation of the processes of burning lignite with a moisture content of 30%, an ash content of 20% and 35% and a higher calorific value of Qрв = 13.9 MJ/kg and 9.7 MJ/kg, respectively are given. The fields of temperature distribution, gas velocity and particle trajectory in the volume and at the outlet of the furnace are determined. The values of the swirling flow velocity near the exit from the furnace reach 150-170 m/s. Mechanical underburning is 3.7% and 9.4% depending on the ash content. The results of a numerical study have showed that the diameter of lignite particles affects their combustion process: coke particles with an initial diameter from 25 microns to 250 microns burn out by 96%. The furnace provides a complete combustion of pulverized coal particles - 99.8% and of volatiles - 100% at volumetric heat stress in the 2500 kW/m3 furnace. The afterburning of fuel particles containing carbon is ensured by their circulation

Physicochemical properties and slurry ability changes of lignite after microwave upgrade with the assist of lignite semi-coke

Microwave pyrolysis is a promising technique for the clean and effective utilization of low-rank coal, which has aroused widespread concern during the past decades. This paper applied microwave energy with the assist of 10 wt% lignite semi-coke (LSC) to upgrade lignite and prepared the solid residue into lignite water slurry (LWS). The physicochemical property and slurry ability changes of upgraded lignite were analyzed. The micro-mechanisms on interactions between dispersant molecules and upgraded lignite particles were investigated through Molecular Dynamics (MD) simulations. It turns out that LSC could effectively prompt the rapid heating of lignite in the microwave field. And after microwave upgrade the coal quality of lignite was improved, the oxygen-containing functional groups was removed remarkably. The adsorption conformation of dispersant on the lignite-water interface exhibited a double adsorption, while that on upgraded lignite-water interface gradually showed single-layer and multi-point adsorptions. Besides, the physicochemical property changes varied most when lignite was upgraded under 800 W, which leads to the maximum solid concentration of LWS improved most, from 51.84 wt% to 61.45 wt%. It further increased to 62.43 wt% as the LWS was directly prepared from the solid residues after microwave irradiation. Morever, the pseudoplasticity of LWS became weaker, but the static stability became better. Therefore, LSC, a self-product of lignite, exerted an enormous function on improving the slurry ability of lignite.

Preparation of biologically activated lignite immobilized SRB particles and their AMD treatment characteristics

In response to the insufficient supply of carbon sources and the toxicity of heavy metal ions when using sulfate reducing bacteria (SRB) to treat acid mine wastewater (AMD), the immobilized particles are prepared with Rhodopseudomonas, SRB and lignite as the main raw materials. And based on single factor test and orthogonal test to determine the optimal ratio of biologically activated lignite fixed SRB particles. The adsorption characteristics of immobilized particles were studied under the optimal ratio, and the reaction kinetics and adsorption capacity of SRB particles immobilized on biologically activated lignite to different ions were analyzed. The results show that: lignite not only has good adsorption performance, but also can be used as the carbon source of SRB after being degraded by Rhodopseudomonas, solving the problems of low removal efficiency of SRB treatment of AMD and insufficient carbon source supply. When the dosage of lignite (particle size is 200 mesh), Rhodopseudomonas, and SRB are 3%, 10%, and 10% mesh, the prepared biologically activated lignite-immobilized SRB particles have the best effect on AMD treatment. The removal rates of SO42−, Zn2+, and Cu2+ were 83.21%, 99.59%, and 99.93%, respectively, the pH was increased to 7.43, the COD release was 523 mg/L, and the ORP value was − 134 mV. The reduction process of SO42− by the biologically activated lignite-immobilized SRB particles conforms to the pseudo-first-order kinetics, and the adsorption of Zn2+ is more in line with the Freundlich isotherm adsorption equation and the pseudo-second-order kinetic model. And it does not spread in a single form, both internal and external diffusion occur. SEM, FT-IR, and BET analysis of biologically activated lignite immobilized SRB particles showed that the pore structure is developed, has a large number of adsorption sites, and some activated groups participate in the reaction. The adsorption process of Zn2+ and Cu2+ in AMD meets the multi-layer adsorption theory.

Hot-air drying shrinkage process of lignite and its cracking mechanism

Lignite fragmentation during drying can significantly impair the operation of a drying system. Volume shrinkage is the main factor that causes lignite fragmentation during low-temperature drying. A two-dimensional lignite plate was prepared for this study, and its drying behavior was analyzed. Instead of volume shrinkage, two-dimensional area shrinkage was measured because of the ease of measurement. The shrinkage characteristics of a two-dimensional lignite plate were studied using the shrinkage percentage, shrinkage rate, shrinkage kinetic model, and shrinkage activation energy (Es). The development of surface cracks was quantitatively analyzed. The results reveal that the sample shrinks continuously during drying and that the shrinkage is severe at a high temperature within the selected range of experimental temperature. The Weibull distribution function better predicts the shrinkage process of the two-dimensional lignite plate. Volume shrinkage is caused by water removal. High-temperature drying leads to greater shrinkage owing to the fast water removal. The high pressure of water vapor generated within the lignite plate can lead to the formation of a dense crack network on the surface during drying at a high temperature. The direction of surface cracking is consistent with the direction of moisture removal. The difference in moisture content between the outer layer and the core of lignite particles leads to a difference in their shrinkage. The outer layer produces tensile stress under restriction by the core, which leads to cracking. In the range of 80–170 °C, high-temperature drying results in greater moisture and shrinkage differences, resulting in more severe cracking.

Assessment of lignite upgrade and hydrogen evolution via electrolysis

Lignite, considered as the lowest-rank coal, accounts for 45% of the total coal resources worldwide, and its low carbon and high ash contents limit its large-scale application. In order to upgrade lignite effectively, a modified coal electrolytic cell (MCEC) was designed, built, and then evaluated for lignite upgrade and hydrogen production through electrolysis, and the operating conditions were optimized. The results show that the performance of the MCEC outperformed the previous coal electrolytic cell (CEC). The operating temperature and iron concentration significantly impact the average current density per milligram of catalyst (ACDMC). The content of dry fixed carbon and heating value increased by 20.9% (wt%) and 8.15%, respectively, in electrolyzed lignite compared with raw lignite. Besides, 71.5% ash and 39.6% mercury were removed from raw lignite after electrolysis, which is the first report. The Faradaic efficiency (FE) for hydrogen evolution reached 99% using MCEC. In addition, the analysis of spectroscopy methods (SEM, EDX, FTIR, and XRD) revealed that the lignite particles were cracked; the oxygen-containing functional groups increased; the minerals of quartz and hematite formed on the surface of lignite particles after electrolysis. The facts indicate that electrolysis of lignite is a competitive approach for lignite upgrade and hydrogen production.

Moisture removal behaviour of single hard lignite particle during drying and quantitative characterization for its surface damage

AbstractIt is extremely meaningful to study the damage behaviour of lignite for safe operation during the drying process. In this study, a single hard lignite particle (SHLP) was prepared for drying. The drying characteristics of an SHLP were described, and its drying kinetics were studied from the perspective of the drying model and effective diffusion coefficient (Deff). The development of surface cracks in drying were obtained, and the changes in surface damage were quantitatively described by relative crack rate (RCR). The results showed that the drying process for an SHLP was divided into three stages following the slope of the drying curve. The rapid removal of a large proportion of moisture mainly occurred in stage I. The Page model is the best model for describing the drying of an SHLP. In addition, Deff increased with increasing drying time. High temperature promoted the moisture transfer from the inside to the surface of an SHLP. Finally, surface cracks on the SHLP developed rapidly in the early drying stage; specifically, the primary cracks widened and lengthened, and the small cracks mainly attached to the primary cracks. In the later drying stage, cracks shrank and cracks in the branch structure closed. High drying temperature made the RCR of an SHLP reach the maximum value more quickly in stage I.

Investigations on lignite use for lead removal from aqueous solutions under static and dynamic conditions: adsorption properties and mechanism exploration

Lignite, as an abundant and low-cost material, was tested for lead (Pb(II)) removal from aqueous solutions under various experimental conditions for both static (batch) and dynamic (column) experiments. Static assays showed that Pb(II) removal efficiency increases with rising in its initial concentration, aqueous pH, and adsorbent dosage values. Adsorption kinetic and isothermal data were well fitted with the pseudo-second-order and Freundlich models, respectively, suggesting that lead removal by lignite is mainly governed by chemical processes and occurs heterogeneously on multilayer surfaces. The maximum Langmuir’s adsorption capacity was equal to 61.4 mg·g -1 , which is high in comparison to various natural materials. The laboratory column experiments showed that Pb(II) breakthrough curves and subsequent lignite adsorption efficiency is highly dependent on the bed height. Due to the short time contact between Pb(II) and lignite particles inside the column, the highest adsorption capacity was about 21%, which is lower than the one found in the batch mode. Even under dynamic conditions, lignite exhibits a high adsorption capacity compared to other adsorbents, which promotes its use as a low-cost and efficient material for Pb(II) and the removal of other heavy metals from wastewaters.

The effect of torrefaction, slow, and fast pyrolysis on the single particle combustion of agricultural biomass and lignite coal at high heating rates

The present paper studies single particle combustion of olive residue (OR), almond shell (AS), and Tunçbilek lignite (TL) and their thermal pre-treated particles in a vertical wire mesh reactor (WMR) setup. Pre-treatment of the fuels was done via torrefaction, slow, and fast pyrolysis. The particles with different masses were heated to 1000 °C by radiation with a heating rate of 900 °C/s. The combustion process was recorded with a high-speed camera. The time analysis demonstrated that the particle mass and size did not affect ignition delay time; however, slow pyrolysis had a significant effect on the ignition delay time. The ignition delay time was significantly increased for the chars produced from slow pyrolysis compared to raw and other heat-treated particles for all fuels. Burnout times of all fuels increased proportionally with particle mass. Fast and slow pyrolysis generally increased the char combustion and burnout times. The average burnout times for fast and slow pyrolyzed biochars were ∼20 s and ∼29 s, respectively, while those of raw biomass were ∼9 s. The average burnout times for fast and slow pyrolyzed lignite particles was ∼20 s and ∼53 s, respectively, while those of raw lignite was ∼19 s, when 2 mg particles were compared. Fast pyrolyzed biomass particles had similar burnout times compared to those of raw TL particles. This depicts the potential of co-firing TL with biomass chars produced from fast pyrolysis.

Characteristics of Moisture Transfer and Surface Crack Development of a Single Lignite Particle Driven by Humidity Difference.

The research on moisture transfer characteristics and surface crack development of a single lignite particle (SLP) driven by humidity difference is helpful to achieve a better understanding of the fragmentation characteristics of lignite during the moisture transfer process. This is of great significance to the safe operation of a drying system. The characteristics of moisture transfer within SLP driven by humidity difference were studied in different stages. Six drying equations commonly used in the literature were selected to describe the moisture transfer behavior. The apparent diffusion coefficient (Deff) of moisture in each stage was calculated to compare the driving forces of moisture transfer in different stages. The surface crack rate (CR) was used to quantitatively analyze the fragmentation characteristics of SLP caused by moisture transfer. The results showed that the moisture transfer process of SLP driven by humidity difference can be divided into three stages, and stage I is the main moisture removal stage. The larger the particle size, the longer the stage I, while less moisture is removed in this stage. A logarithmic drying equation best simulates the moisture transfer process of SLP. The larger the particle size, the larger the Deff value in each stage. The driving force of moisture transfer in stage I is the largest, which is the opposite of a thermal drying process. CR for SLP has experienced a rapid increase – stable at the highest value – rapid decrease – stable during the moisture transfer process driven by the humidity difference.

Microwave upgradation of Yunnan lignite: dielectric properties, drying pattern and kinetics

The work attempts to investigate the dielectric properties and microwave drying characteristics of Yunnan lignite, so as to upgrade it. The effect of drying temperature, moisture content and microwave heating frequency on dielectric properties was measured using resonant cavity method. Results indicate that both the values of ε′ and ε″ presented two stage at the investigated temperatures. Bulk water was found to be the main factor contributing to an increase in the ε′ and ε″. Microwave drying kinetics of lignite were carried out. The Page model was the best model reflecting the drying kinetics, and the effective diffusion coefficient and active energy at different drying stages were obtained. In addition, the microwave drying mechanism has been proposed based on the study of dielectric and drying properties of lignite. The SEM images indicated that microchannels were formed within the lignite particle by the microwave pumping effect. FT-IR spectra showed that the oxygen-containing functional groups and hydrogen bonds between them were break by microwave drying. Based on the achieved results, the microwave treatment at 150 °C is helpful to upgrade the lignite.

Combustion of hazelnut shell-lignite blends in poly-particulate beds

In this study, combustion behaviours of blends prepared from 2.18 mm mean size lignite and hazelnut shell particles in specific proportions were investigated using a vertical pre-heated furnace system through which air was flowing upwards by natural convection. The results revealed that the ignition time of the blends approached those of hazelnut shell with temperature and showed certain interactions of the two materials during the volatiles combustion period. Combustion time generally complied with volatiles and carbon combustion rates of the two materials and their blends. Carbon combustion time decreased with temperature and hazelnut ratio, and implied some interactions of the materials. Although any effect of initial furnace temperature and hazelnut ratio of the blends on the NOx emissions was not clearly seen, SO2 emission levels depended on these factors.

Vibration drying of lignite based on the thermal fragmentation property and its prediction model

A lignite vibrating mixed-flow screening and drying system was implemented in this study. Vibration is conducive to heat and mass transfer between the hot carrier gas and lignite particles and prompts large particles into small particles based on their thermal fragmentation to improve the drying efficiency. During the vibration drying process, most or all of the large particles were broken into smaller particles, and there was always a clear moisture gradient with respect to the particle size fractions. The particle size–remaining moisture gradient (Ms) was defined to describe the nonuniformity of the dryness among different size particles based on their mass content. Ms increased first and then decreased with drying time, and a high drying rate usually resulted in larger differences in moisture among the products, whereas a lower residual moisture content reduced the differences. The Weibull model was suitable for describing and predicting the vibration drying kinetics of lignites. Based on the results of this study, a vibration drying pilot system will be designed and set up to obtain a new drying system.

Use of Mixed Fuel Based on Brown Coal and its Thermal Processing Products in Energy Boilers

The work investigated the thermal behavior of Borodino coal, brown coal coke and mixed fuel, consisting of 80 % Borodino brown coal and 20 % brown coal coke during the combustion process using thermogravimetric analysis. The chemical composition of the samples is presented. When analyzing the TG-DTG curves, combustion characteristics were determined, such as: ignition temperature (Ti) and coke burnout temperature (Te), flammability index, integral combustion parameter. To determine Ti and Te, the technique of crossing the TG and DTG curves was applied and justified. An electron microscopic study of the cross-section of a lignite coke particle has been carried out. The results of the study showed that the addition of lignite coke to brown coal has a positive effect on its combustion characteristics

Conversion characteristics of lignite and petroleum coke in chemical looping combustion coupled with an annular carbon stripper

Chemical looping combustion (CLC) of solid fuels was studied in a 30 kW dual fluidized-bed system coupled with an annular carbon stripper for char separation by using a kaolin-promoted manganese ore oxygen carrier. Two solid fuels, a lignite and a petroleum coke, were tested in the CLC unit. The CLC unit was operated continuously for more than 160 h. The effects of the fuel size and residence time in the fuel reactor on the carbon capture efficiency and oxygen demand were investigated. The char separation efficiency of the annular carbon stripper reached 90%, and a maximum carbon capture efficiency of 97% was achieved for the 106–212 μm lignite. The carbon capture efficiency was affected by the fuel particle size and residence time. When >212 μm lignite particles were used, there was a ~ 12% oxygen demand. The carbon capture efficiency in the experiment with petroleum coke was only 45% due to the slow gasification rate. A model was applied to analyze the influence of key factors on the carbon capture efficiency. The redox kinetics of the tested oxygen carrier were studied by a micro-fluidized bed thermogravimetric analysis method.

Theoretical Analysis and Optimization of Fine Lignite Drying and Separation with a Pulsed Fluidized Bed.

We established a pulsed fluidized bed system to dry and concurrently separate fine lignite (−6 + 3 and −3 + 1 mm lignite). The kinetics and evaporation of lignite moisture were investigated in the pulsed air flow. The variation in the evaporation rate was studied theoretically with respect to temperature, velocity of the pulsed air flow, and pulsed frequency. The rubbing effect between the air and lignite particle probably dominates the evaporation of water. The influence of temperature on the evaporation rate is more significant than that of air velocity by merely considering the effect of air entrainment of the evaporated moisture. Four operational parameters, including inlet temperature, air velocity, pulsating frequency, and bed height, were investigated and optimized through a response surface method to study the interactions between factors and determine the optimal separation conditions. Results indicate that the maximum standard deviation of the ash content of 23.74% was recorded under the optimal condition of the inlet temperature (80 °C), pulsating frequency (3.93 Hz), air velocity (1.09 m/s), and bed height (120 mm) for −6 + 3 mm lignite, and the maximum standard deviation of 24.99% was recorded for −3 + 1 mm lignite under the condition of the inlet temperature (100 °C), pulsating frequency (3.49 Hz), air velocity (0.55 m/s), and bed height (80 mm). The probable error values of separations of −6 + 3 mm lignite and −3 + 1 mm lignite with the pulsed fluidized bed were 0.12–0.16 and 0.10–0.16 g/cm3, respectively, which demonstrates that efficient drying and simultaneous separation of lignite can be achieved with the pulsed fluidized bed.

Enriched Charred Rice Husk to Improve the Nutrient Management in Rainfed Groundnut

Background: The uncertainty of groundnut productivity in the rainfed areas could be minimized by in situ conserving the soil moisture received through precipitation during the cropping period and improving the nutrient status. Methods: A field experiment was conducted at the Department of Agronomy, Agricultural College and Research Institute, Tamil Nadu Agricultural University, Madurai, Tamil Nadu during 2014-2015 rabi season to find out the effect of charred rice husk and other organic materials enriched with or without fertilizer nutrient on the moisture retention and nutrient supply during the deficit period of crop growth. The treatment consists of charred rice husk, biochar, lignite and farm yard manure alone and enriched with the recommended dose of nitrogen (10 kg ha-1) and phosphorus (10 kg ha-1) fertilizer and compared with the absolute control and recommended dose of fertilizer (10:10:45 kg NPK ha-1). Result: The structural morphology of organic materials was studied using Scanning Electron Microscope (SEM) showed that the particles of charred rice husk were uneven and the diameter varied from 3-4 ìm with clod and block appearance, the particles of biochar were irregular in shape, uneven sized and diameter varied from 2-3 ìm with block appearance and the lignite particles found spherical and aggregated. The chemical composition of organic manure with nutrient examined under SEM-EDAX revealed that the charred rice husk contains 64.31% carbon, 28.42% oxygen and 5.96% silica. Results of the study revealed that, application of enriched FYM at 5 t ha-1 recorded significantly higher pod yield (2190 kg ha-1). The enriched biochar and charred rice husk produced comparable pod yield of 2010 kg ha-1 and 1983 kg ha-1, respectively.

Effect of pH on surface hydration of coal particles and its attachment with oily bubbles in flotation

ABSTRACT The pH of flotation slurry directly affects the surface potentials of mineral particles and bubbles, and therefore the hydration of coal surface and the mineralization process of flotation. In this paper, the effect of pH on the surface potential, hydration degree, and floatability of lignite particles was studied, wherein Einstein’s dispersion viscosity theory was adopted to calculate the hydration degree, and DLVO theory was used to estimate the interaction between coal particles and oily bubbles. The effect of pH on the lignite floatability was evaluated in terms of induction time and oily-bubble flotation results. The results showed that with increasing pH, the zeta potentials of coal particles and oily bubbles decrease, causing a 37.89% increase in the coal surface hydration from pH 3 to pH 11. As a result, the energy barrier for attachment between the coal particles and oily bubbles reaches the maximum value of 2.669 × 10−16 J at pH 11, resulting in low attachment efficiency. Correspondingly, the combustible matter recovery of oily-bubble flotation is significantly decreased from 57.76% to 28.00%.