Grindability Index Research Articles

Kinetics and sub sieve morphology of ball mill grinding for different grades of Indian coals

The interactions of various factors affecting the ball mill grinding such as diameter of the ball, particle size and grinding time were studied. The kinetics of grinding on 80% passing size of product d80 was studied for two grades of coal namely lignite and bituminous using Box-Behnken method of surface response experiment. The bond grindability index and work index of the ball mill were determined. The variation in specific rate of breakage with ball diameter and number of balls was studied. The study on power consumption was done by varying the number of balls, ball diameter and grinding time. It was observed that the breakage was better for more balls with increase in ball diameter. The sub sieve morphology of particles in sieve was analysed using scanning electron microscopic observations to determine the breakage characteristics. [Received: March 8, 2016; Accepted: July 17, 2016]

Prediction of Hardgrove Grindability Index of Afsin-Elbistan (Turkey) Low-grade Coals Based on Proximate Analysis and Ash Chemical Composition by Neural Networks

ABSTRACT The aim of the study is to provide insights about the Hardgrove grindability index (HGI) value of Piocene aged Afsin-Elbistan low-grade coal determination without human or experimental errors, timely and low cost. For this reason, the HGI for Afsin-Elbistan low-grade coal based on twelve coal parameters was predicted through linear regression (LR), feed forward neural network (ANN), generalized regression neural network (GRNN) and elman network (EN) approaches in this study.. The obtained results show that LR and EN approaches were unsatisfactory because of high differences determined between actual and predicted HGI. However, ANN and GRNN approaches make quite reliable predictions on HGI with a high accuracy. The R-value of the models was 0.93 for ANN and 0.98 for GRNN approaches. The percentage of predicted HGI within ±3 deviation through ANN and GRNN approaches was found to be 89.53 and 94.19, respectively. The sensitivity of approaches to predictors was evaluated by an enterremove selection method to determine the individual effect of each parameter on the prediction of HGI. The best prediction approach for the Afsin-Elbistan Pliocene aged coal was GRNN that can be applied to predict HGI for the similar aged coals.

Investigation of forestry wastes: Torrefaction and their thermal properties for use in energy recovery schemes

ABSTRACTThree biomasses like Eucalyptus, Azadirachata, and Ficus religiosa were torrefied to investigate the effect of temperature and residence time was investigated on torrefied biomasses for yield, volatile matter, fixed carbon, ash, and Gross calorific value (GCV). Thermogravimetric analysis and Hardgrove grindability index (HGI) of three torrefied samples were studied at optimum conditions of temperature and residence time. According to the results, an optimum temperature was found to be 260°C at 45 min residence time. A maximum GCV of torrefied biomasses of Eucalyptus, Azadirachata, and F. religiosa were found to be 4,301, 3,190, and 3,278 kcal/kg, respectively. According to thermogravimetric analyzer results, the Azadirachata has shown higher weight loss compared to Eucalyptus and F. religiosa during thermogravimetirc study. The weight loss rate for Azadirachata was maximum to nearly 12.8%/min compared to 6.11 and 5.12%/min for Eucalyptus and F. religiosa, respectively. The order of reactivity based on mean reactivity and combustion characterization factor was found to be Azadirachata indica > Eucalyptus > F. religiosa. According to HGI results, Eucalyptus, Azadirachata, and F. religiosa have shown HGI values of 71, 60.7, and 81.7, respectively. The results of this study could be useful for the energy recovery schemes in the country.

Alternative Evaluation of the grindability of Pozzolanic Materials for Cement Production

An ecological way of reducing CO2 emissions in cement production is the usage of blended cement with an active pozzolan. A part of this issue is the adjustment of granularity of raw materials. This is often achieved by the grinding of the input components. All conventional methods for the evaluation of grindability require a specific grinding instrument. These grinding instruments do not have any other practical use and thus are not very common. This paper focuses on the alternative evaluation of grindability. Inspired by the VTI method used in the coal industry, which uses porcelain laboratory ball mill and compares material based of oversize particles, a new method was created. The first modification in methodology was the use of a planetary mill instead of a porcelain drum mill. Another modification was in the measurement of undersize by means of laser granulometry. This method was then tested on clinker, slag, and recycled glass, which can also be used as an active pozzolan in blended cement. Also, co-milling measurements were made on clinker-pozzolan combinations. These results were then used in the calculation of the grindability index, which can be used for the comparison of properties. The modification of the VTI methodology has a positive impact on the evaluation of grindability, especially with regard to fine particles, thanks to the use of laser granulometry and at the same time it makes use of more a commonly available grinding apparatus

Physicochemical Characterization of Torrefied Wood Biomass under Air as Oxidizing Atmosphere

The effects of torrefaction under an oxidizing atmosphere on the physicochemical properties of patula pine wood chips were studied. Raw and torrefied pine were characterized to evaluate the effect of temperature and residence time on biofuel properties, such as bulk density, equivalent Hardgrove grindability index (HGIeq), ultimate and proximate analyses, heating value, and fuel value index (FVI). In contrast, the torrefaction process was characterized by mass and energy yields, and by the energy-mass co-benefit index (EMCI). Torrefaction was performed in a rotary kiln at temperatures between 180 °C and 240 °C during residence times between 30 min and 120 min. The torrefaction process under an oxidizing atmosphere tended to increase the fixed carbon/volatile matter ratio (from 0.19 to 2.5), while the H/C and O/C atomic ratios decreased 73% and 55%, respectively. The best properties of wood reached in the experimental plan were obtained at 210 °C during 75 min. For this torrefaction condition, energy yield, FVI, and EMCI were 85.91%, 1.91 MJ/cm3, and 4.41%, respectively. Additionally, the lower heating value for torrefied pine (18.65 MJ/kg) was higher than for the raw material (17.76 MJ/kg), and the HGIeq was 17% greater, which resulted in a better grindability behavior.

Prediction of grinding behavior of low-grade coal based on its moisture loss by neural networks

ABSTRACTIn this article, it was aimed to determine the influence of moisture amount on the grindability of Afsin-Elbistan low-grade coal using impact strength index (ISI) and hardgrove grindability index (HGI) tests. For this purpose, the sample was dried at a temperature range of 60°C–150°C for various times (80–240 min). A drying rate was further determined for each experiment. ISI and HGI values of each sample varied between 25.56–90 and 25–120, respectively. The obtained results show that a decrease of moisture in each sample led to an increase its grindability. In addition, artificial neural network (ANN) approach with two different learning techniques (Levenberg–Marquardt “LM” and Bayesian regularization “BR”) was carried out to predict the HGI of Afsin–Elbistan coal. Three input parameters (moisture amount, ISI, and drying rate) obtained from the experiments were used for predicting HGI. LM learning algorithm gave a more satisfactory prediction (R2 = 0.92, overall) compared to another technique.

다양한 바이오매스의 분쇄도 실험을 통한 미분탄 화력발전 적용가능성 연구

기후 변화 대응을 위한 온실가스 감축 측면에서, 석탄화력발전소에서 바이오매스 사용량은 계속하여 증가되어 왔다. 파리 협정 이후 온실가스 감축 목표치가 더욱 구체화되면서 바이오매스 사용은 급격히 더 많아질 것으로 예상된다. 미분탄 석탄 화력발전에서 바이오매스 혼소시 가장 큰 문제점 중 하나는 바이오매스의 미분성이 석탄에 비해 훨씬 낮다는 것으로, 이를 해결하기 위해 가장 먼저 바이오매스의 미분성 측정 방법을 확립하는 작업이 필요하다. 석탄의 경우 HGI (hardgrove grindability index)측정 장치를 통해 분쇄도 측정이 가능하여 이를 표준으로 삼고 있지만, 바이오매스의 경우 표준 측정 방법이 확립되어있지 않다. 본 연구에서는 볼 밀과 입자 크기별 분포량을 이용한 석탄과 바이오매스의 분쇄 실험을 진행하였다. 실험에는 석탄 1종과 바이오매스 6종을 사용하였다. 분쇄시간에 따른 입자 분포량을 비교하고, <TEX>$75{\mu}m$</TEX> 이하 입자 분포량으로 분쇄도를 평가하였다. 실험결과 반탄화 바이오매스 TBC (torrefied biomass chip)와 TWP (torrefied wood chip)는 발전용 사용적합 기준에 대해 대략적으로 70%의 값을 나타냈다. 다른 바이오매스들의 경우 반탄화 바이오매스와 비교했을 때 분쇄성이 훨씬 더 낮은 결과를 보였다. TBC와 TWP는 수분이 감소하고 섬유질 구조가 분해되는 반탄화 과정을 통해 분쇄가 향상되었다. 또한 분쇄도가 높은 반탄화 바이오매스가 소모전력이 낮게 측정되었다. 본 연구를 통해 바이오매스의 석탄화력발전 적용을 위한 표준화 작업의 기초 자료들을 확보할 수 있다. Recently usage of biomass is increased in pulverized coal power plants for reduction of <TEX>$CO_2$</TEX> emission. Many problems arise when thermal share of the biomass is increased, and milling of the biomasses is one of the most important problems due to their low grindability when existing coal pulverizer is used. Grindability of coal can be measured through the HGI (Hardgrove grindability index) equipment as a standard, but method of measuring biomass grindability has not been established yet. In this study, grinding experiment of coal and biomass was performed using a lab-scale ball mill. One type of coal (Adaro coal) and six biomasses (wood pellet (WP), empty fruit bunch (EFB), palm kernel shell (PKS), walnut shell (WS), torrefied wood chip (TBC) and torrefied wood pellet (TWP)) were used in the experiment. Particle size distributions of the fuels were measured after being milled in various pulverization times. Pulverization characteristics were evaluated by portion of particles under the diameter of <TEX>$75{\mu}m$</TEX>. As a result, about 70% of the TBC and TWP were observed to be pulverized to sizes of under <TEX>$75{\mu}m$</TEX>, which implies that they can be used as alternative biomass fuels without modification of the existing mill. Other biomass was observed to have low grindability compared with torrefied biomass. Power consumption of the mill for various fuels was measured as well, and the results show that lower power was consumed for torrefied biomasses. This result can be used for characterization of biomass as an alternative fuel for pulverized coal power plants.

Grindability Studies on Blended Coals of High-Ash Indian Coals with Low-Ash Imported Coals

ABSTRACTThe study investigates the Hardgrove Grindability Index (HGI) values for binary blends of high-ash Indian coals and low-ash imported coals. The coals were selected in such a way that their grindability values were not closely spaced for studying the effect of blending in the HGI of coal blends. In addition, the coals were selected with the proximate analysis values being close to each other, where the proximate-based predictions of grindability values differ greatly for individual coal samples. It was observed that when the difference between the HGI values of the parent coals was less than 25% from the softer coal, then the grindability of coal blends closely followed the linear weighted average of the parent coals; otherwise a nonlinear relationship was observed. The nonlinear relationship between blending proportion and HGI was due to the preferential grinding of softer coal, which was confirmed by the increased ash content in the fines corresponding to a different blending ratio. A practical approach has been discussed to arrive at a suitable blending ratio to achieve the desired coal properties in the final product during the comminution process in pulverized fuel-fired boilers.

Performance evaluation of co-firing various kinds of biomass with low rank coals in a 500 MWe coal-fired power plant

Recently in Korea, the co-firing of biomass in existing pulverized coal power plants has become an important mean to comply with the nation's renewable portfolio standard (RPS). This study investigated boiler efficiency, net plant efficiency, and combustion characteristics from co-firing various biomasses along with two coal blends and the combustion of low rank coals through process simulation based on an existing 500 MWe coal fired power plant using a commercial process simulator (gCCS). Five sources of biomass - wood pellet, empty fruit bunch pellet, palm kernel shell, walnut shell, and torrefied biomass were selected as renewable fuels for co-firing. In addition, hardgrove grindability index tests were conducted for a blend of coal (90%) and each selected biomass (10%) based on thermal share input to investigate the milling power consumption of each blended fuel for a more rigorous simulation. The results show quantitatively that when biomass is co-fired the plant efficiency is decreased due to its lower heating value and more power consumption in mills. The plant efficiency of torrefied biomass co-firing was the highest among all biomass co-firing and combustion of low rank coals due to higher energy density and enhanced grindability of torrefied biomass after torrefaction.

Explaining relationships among various coal analyses with coal grindability index by Random Forest

Application of Random Forest (RF) via variable importance measurements (VIMs) and prediction is a new data mining model, not yet wide spread in the applied science and engineering fields. In this study, the VIMs (proximate and ultimate analysis, petrography) processed by RF models were used for the prediction of Hardgrove Grindability Index (HGI) based on a wide range of Kentucky coal samples. VIMs, coupled with Pearson correlation, through various analyses indicated that total sulfur, liptinite, and vitrinite maximum reflectance (Rmax) are the most importance variables for the prediction of HGI. These effective predictors have been used as inputs for the prediction of HGI by a RF model. Results indicated that the RF model can model HGI quite satisfactorily when the R2=0.90 and 99% of predicted HGIs had less than 4 HGI unit error in the testing stage. According to the result, by providing nonlinear VIMs as well as an accurate prediction model, RF can be further employed as a reliable and accurate technique for the evaluation of complex relationships in coal processing investigations.

Grindability of Coal Changing with Burial Depth

ABSTRACTGrindability of a coal is generally represented in terms of the Hard Groove Grindability Index (HGI); a coal of lower HGI needs more energy to be ground. Samples of this study were taken from the Amasra coalfield of the Zonguldak Hard Coal Basin to determine if coal grindability varies with the burial depth of coals. Samples from different burial depths were analyzed and corresponding HGI values were obtained. HGI values of the original samples were found between 51.43 and 79.26. A float-and-sink analysis was performed for some of the original samples and it was found that the HGI values of floated coal samples change between 49.04 and 73.52.A new method, which employs proximate and petrographic analysis data and the burial depth, was proposed for the prediction of HGI. Correlation of HGI with burial depth or the discrimination of floated and original coal currently exists. Correlations of the proposed methods have satisfactory coefficient of determination values (R2 =.75, R2 = .86) for the original and floated coal samples studied. This new proposed method was tested on the data and the previously studied original and floated coal samples of the Zonguldak coalfield and yielded a good correlation (R2 = .87, R2 = .90). The proposed method can be considered to yield reasonable predictions for the Amasra coalfield.

Fundamental evaluation of petrographic effects on coal grindability by seasonal autoregressive integrated moving average (SARIMA)

A series of coal Hardgrove grindability index (HGI) tests were performed to determine each iteration's content of the monomacerite microlithotypes vitrite (Vt) and inertite (In), the bimacerite microlithotypes clarite (Cl) and durite (Du), and the trimacerite microlithotypes duroclarite (Dc) and clarodurite (Cd). In HGI tests, larger particles are broken in grinding, the resulting daughter particles can be composed of different microlithotypes than the parent particle. Therefore, predicting the overall composition of the later iterations is complicated. A time series of the compositions values was constructed by sequencing the contents of all iteration (in different mesh sizes) by the order of iterations. Seasonal autoregressive integrated moving average (SARIMA), a widely used method of time series analysis, was employed for forecasting the final iteration's content of Vt, In, Cl, Du, Dc, and Cd. The proposed methodology was able to forecast the last iteration's composition, comparable to the actual observation.

Coal Grindability and Breakage Parameters

ABSTRACTAn empirical relationship of the Hardgrove Grindability Index (HGI) was established in terms of breakage parameters. In order to model particle-breakage behavior in a crusher, breakage parameters and HGI of material should be taken into consideration. Coal grindability has long been studied and empirical equations evaluating coal characteristics data have been proposed. Basically knowing the size distribution of the feed coal to the crusher and obtaining an experimental product size distribution after processing could help to model the operation conditions of the device. Breakage processes for coal have been modelled namely as π-breakage process, π, κ-breakage process and π, κ, ω-breakage process (π: proportion selected for breakage in an assembly, κ: number of breakage in a breakage operation ω: coefficient of size dependence). Having evaluated the breakage model for any crusher, the size distribution of crushed coal and its HGI could be related to each other. An empirical method between HGI and π proposed in this study was in good agreement with the experimental results. Experimental analysis of the crusher feed coal yielded an average HGI of 66 and the predicted HGI values were in the range of ± 10% of this average by employing the proposed method. As a second contribution of the proposed method, the stabilization of the size distribution of the crushed coal can be achieved with a decision mechanism for the plant operators about communition process.

Effect of undersize misplacement on product size distribution of Bond’s ball mill test

The standard Bond ball mill grindability test determines the ore grindability (g rev−1) and work index on an ore which gives the ball mill power consumption. The test is carried out on a standard sample and the number of revolutions per minute (rev min−1) required for the next cycle is determined after screening out and weighing the undersize fraction from a screen of a mesh the same size as the intended product passing size. The standard procedure assumes 100% efficiency in screening out the undersize fraction. However in plant grinding and classification circuits especially those based on dry grinding and classification, the efficiency of classifiers will be less than 100%. The misplacement of undersize particles in the oversize recirculating load is a practical reality which cannot be avoided. The aim of this study was to examine the effect of misplaced undersize particles on the P80 of product. The P80 values obtained from the standard Bond ball mill procedure and those of an inefficient ball mill grinding – classifier circuit were compared. The P80 values obtained with an inefficient classification were lower than the values obtained from the standard Bond ball mill procedure. The relationship between standard P80 values and those obtained with inefficient classification can be used as a guide to adjust operating parameters such as mill load and mill speed to produce the desired product size.

Maceral Liberation and Distribution of Bituminous Coal for Predicting Maceral-Separation Performance

ABSTRACTThe success of gravitational separation of coal macerals for a given coal is dependent on the liberation and distribution of macerals. On a basis of four coals of varying ranks from high-volatile bituminous coal to low-volatile bituminous coal, this article presents the maceral liberation and distribution under various crushing degrees and predicts the maceral separation performance with various separation techniques. The determination of the Hardgrove Grindability Index of maceral concentrates was also performed. The liberation and distribution results show that vitrinite tends to be concentrated in particles with lower density and finer size, and good liberation performance of high-volatile bituminous coal is achieved by crushing to -3 mm. The medium-volatile bituminous coal behaved differently than the other three coals, which is confirmed by the relationship between HGI and coal rank as well as maceral composition. The prediction results show that there is good performance for high-volatile and low-volatile bituminous coals using the technique of crushing to -3 mm + density-based separation while good separation performance of medium-volatile bituminous could be achieved using a technique of 50 x 0.5 mm + density-based separation, not necessarily crushing. This study provides theoretical support of rational utilization of the coking coals using maceral separation.

Calculation model of coal comminution energy consumption

Experiments on comminution energy of different coals under conditions for CFB (circulating fluidized bed) boilers were carried out in a jaw crusher and a parallel roller crusher. The R–R (Rosin–Rammler) distribution was used to describe the size distribution of the crushing product. The size distribution of the product has a significant influence on comminution energy. With lower values of xP=63.2% (size modulus) or α (distribution modulus), the comminution energy increases. With a fixed particle size distribution of the product, the comminution energy increases gradually with the increasing of feed coal size, while, when the feed coal size is larger than 15mm, the effect can be neglected. With a fixed outlet width of the crusher, the average product size tends to be finer when smaller sized particles or coal with higher HGI (Hardgrove grindability index) are fed into the crusher. Therefore, in coal preparation process, the adjustment of the crusher should be performed considering coal grindability or crushability and the feed coal size distribution. At last, an empirical model of comminution energy was proposed and validated by the measured values with ±25% of accuracy.

Investigation into the applicability of Bond Work Index (BWI) and Hardgrove Grindability Index (HGI) tests for several biomasses compared to Colombian La Loma coal

With increasing quantities of biomass being combusted in coal fired power stations, there is an urgent need to be able to predict the grindability of biomass in existing coal mills, but currently no standard biomass grindability test exists. In this study, the applicability of the Hardgrove Grindability Index (HGI) and Bond Work Index (BWI) as standard grindability tests for biomass were investigated for commercially sourced wood pellets, steam exploded pellets, torrefied pellets, sunflower pellets, eucalyptus pellets, miscanthus pellets, olive cake and Colombian La Loma coal. HGI predicts the behaviour of fuels in vertical spindle mills and BWI for tube and ball mills. Compared to La Loma (HGI of 46), all biomasses tested performed poorly with low HGI values (14–29). Miscanthus pellets had the highest BWI or Wi at 426 kW h/t. Despite similar HGI values, some untreated biomasses showed lower BWI values (Eucalyptus pellets Wi 87 kW h/t, HGI 22) compared to others (sunflower pellets Wi 366 kW h/t, HGI 20). Torrefied pellets had the lowest Wi (16 kW h/t), with La Loma coal at 23 kW h/t. Wood, miscanthus and sunflower pellets exhibited mill choking during the BWI test, as the amount of fines produced did not increase with an increasing revolution count. An approximate correlation between HGI and BWI was found for the biomass samples which did not experience mill choking in the BWI test. Milling results in this paper suggest that biomass pellets should be composed of pre-densified particles close to the target size in order to minimise the energy use in mills and possibility of mill choking. Our findings would also suggest that the BWI is a valid test for predicting the potential for mill choking of biomass in a tube and ball mill. HGI, however, appears to be a poor method of predicting the grindability of biomass in vertical spindle mills. A new standard grindability test is required to test the grindability of biomasses in such mills.

Grinding kinetics, modeling, and subsieve morphology of ball mill grinding for cement industry ingredients

ABSTRACTThe dependence and interactions of various factors affecting the ball mill grinding like diameter of ball, particle size, and time of grinding on 80% passing size of product d80 was studied for cement industry raw materials of lime stone and sand using Box–Behnken design of response surface experiments. The bond grindability index and work index of ball mill were determined for different feed characteristics by varying the number of balls involved in grinding operation in the ball mill. The higher specific rate of breakage was obtained for new or smooth surfaced balls than old or rough surfaced balls. The reduction in size due to ball mill grinding was found to be better for wet grinding than dry grinding based on specific rate of breakages obtained. The subsieve morphology of particles in sieve pan for sand for both dry and wet grinding were analyzed using scanning electron microscopic observations to determine the breakage characteristics.

Reprint of “Petrological and organic geochemical properties of lignite from the Kolubara and Kostolac basins, Serbia: Implication on Grindability Index”

The influence of different coal lithotypes on grindability has been investigated using lignite from two of the most important Upper Miocene lignite basins in Serbia (Kolubara and Kostolac). Yellow xylite-rich types demonstrated the most negative impact on Hardgrove Grindability Index (HGI). All different types of xylite-rich coal, as well as total xylite-rich coal from the Kolubara basin have a negative influence on the grindability properties, while only the yellow type of xylite-rich coal from the Kostolac showed a negative impact on HGI. Matrix coal does not show a clear effect on HGI.A negative correlation between textinite content and HGI is observed in both basins, whereas contents of other macerals do not show influence on grindability properties.Content of total organic carbon demonstrated the negative impact on HGI. Correlation analysis indicates that the negative impact of the yellow type of xylite-rich coal and the sum of total xylite-rich coal on the grindability properties partly can be related to content of total organic carbon and high amount of soluble organic matter. Matrix lithotype does not show any significant correlation with bulk geochemical parameters in both basins.The peat-forming vegetation of all samples from both basins were dominated by decay-resistant gymnosperm (coniferous) plants, belonging to one or several of the families Taxodiaceae, Podocarpaceae, Cupressaceae, Araucariaceae, Phyllocladaceae and Pinaceae. Lignite from the Kolubara basin is characterized by a higher contribution of angiosperm vegetation than coal from the Kostolac basin. Peatification of the Kolubara coal occurred under more oxic conditions than the Kostolac one.Analysis of biomarkers indicated that the negative impact of all types of xylite-rich coal from the Kolubara on HGI can be related to the higher proportion of angiosperms, abundance of mid-chain n-alkanes and sesquiterpenoids, aromatization of non-hopanoid triterpenoids and hopanoids, and intense degradation of wood tissues in a more oxic environment. The positive impact of matrix coal on HGI in the Kolubara samples can be attributed to elevated content of non-aromatic hopanoids and low amounts of aromatic non-hopanoid triterpenoids and sesquiterpenoids, which seems to hinder the grindability properties.

An Assessment of Chemical Properties and Hardgrove Grindability Index of Punjab Coal

An Assessment of Chemical Properties and Hardgrove Grindability Index of Punjab Coal