Coal Grinding Research Articles

Application of Microwaves for Obtaining Ground Solid Fuel

This paper gives an analytical overview of theoretical and experimental data on the microwave grinding of coal. As a result of the work done, we have substantiated the urgency of microwave treatment (MWT) of coal for obtaining ground coal, which in turn has a positive effect on the operation efficiency of TPPs. Moreover, we have considered the positive effects of microwave grinding as applied to coke.

Numerical simulation of multiphase flow around suction plates of ventilation mill in the function of extending its remaining working life

This paper presents results of the analyses obtained by numerical simulation for the possibilities of increasing the wear resistance of the ventilation mill working parts for coal grinding in the Kostolac B power plant. The possibilities for modification are based on 3D numerical simulation of multiphase flow of ventilation mill. Mineral materials during the process of milling in the ventilation mill cause strong wear of the suction plates. The multiphase flow simulations are performed in order to obtain the mineral materials paths and velocity vectors. The mixture model of the Euler-Euler approach is used. The results obtained in the numerical simulation serve for the selection of an optimal redesign of the suction plates. The application of this approach can reduce the number of possible repairs and extends the period between them, resulting in significant economic effects.

Evaluation of the effects of coal grinding in terms of coal water slurry preparation

Coal Water Slurry (CWS) is a specific form of solid fuel. It occurs in the form of finely ground coal particles and water. Depending on the use, the content of combustible matter is from 40 to 70% by weight. The attractiveness of the fuel is primarily its properties, i.e. liquid form, high energy efficiency (for water evaporation 4% energy is used – for CSW with 70% concentration of coal), decreased environmental impurities (lower NOx emission) and reduced risk of explosion. The advantages of CWS fuels, the possibility of independence from petrochemical fuels, wide availability of coal and emphasis on the use of cleaner technologies are the driving force for development of slurry fuel technologies. The major parameters characterizing the fuel suspension are: solid phase concentration (share of coal in the slurry expressed as either weight or volume fraction of dry coal), time stability (resistance to delamination and separation of the dispersed phase from the continuous phase) and viscosity, determining the flow of suspension. The mentioned parameters are dependent on the susceptibility of coal for production of aqueous suspensions (slurrability), conditioned by natural properties of coal, such as: coalification degree, petrographic composition and surface properties. They are also dependent on the slurry fuel preparation process: particle size, solid phase concentration, used additives (stabilizing and dispersion agents) and modification of primary coal properties (ash removal, change of surface properties). Preparation of sustainable, high concentrated CWS fuel coal is promoted by the hydrophobic nature of the coal surface, characteristic for coals of higher coalification. A great technological problem is to obtain a highly concentrated coal slurry fuel from less coalified hydrophilic steam coals. The paper presents the results of lab scale research on the CWS prepared from Polish steam coal by wet grinding in mill drum and vibrating. The milling process was carried out in a batch system by varying the composition of the grinding media and varying degrees of working space filling. The feed was pre-ground steam coal mixed with water at the ratio of 1: 1. The changes of particle size, viscosity and stability of suspensions in time were assessed. The milling effectiveness was higher for the vibrating mill. After the same milling time the quality of the obtained product was higher. The obtained milling degree was depended on the share and composition of grinding aids. In the investigated range (10-30% share mill working space), the use of higher share of grinding aids caused higher milling degree of coal. A trend was observed that stronger fragmentation of coal was achieved with the use of polydisperse composition of aids distinguished by smaller diameters. The changes of granulation influenced the viscosity and stability of the obtained product. The lower the mean particle size, the higher the viscosity and stability of the obtained product was particularly noted for the vibrating mill.

Wear resistance of high chromium white cast iron for coal grinding rolls

Las fundiciones blancas al alto cromo utilizadas en componentes demolinos de carbon son materiales que presentan diferentes resistencias al desgaste,dependiendo del tratamiento termico usado en su fabricacion. Las propiedades mecanicas,microestructurales y tribologicas de la aleacion con 22% de cromo y 3% de carbono (II DASTM A532-10) y su infl uencia en la resistencia al desgaste fueron investigadas. Para lacaracterizacion de la aleacion fueron realizados diversos ensayos, utilizando los siguientesequipos de laboratorio: durometro, microdurometro, microscopio optico, microscopioelectronico de barrido, difraccion de rayos X, espectrofotometro de emision optica, yequipo para ensayo de desgaste abrasivo de acuerdo con la norma ASTM G65-04 (2010).Los resultados encontrados mostraron que la resistencia al desgaste incrementa cuandola matriz tiene una baja fraccion volumetrica de austenita retenida y una alta fraccion decarburos secundarios. Tambien indican que no es necesario usar largos tiempos durante ladesestabilizacion en la aleacion II D para obtener una optima resistencia al desgaste, cuandola aleacion fue previamente recocida. Estos resultados indican que la industria puede usartiempos cortos durante la desestabilizacion de los rodillos de molinos de carbon.

The effects of particle grinding on the burnout and surface chemistry of coals in a drop tube furnace

Grinding coals to a pulverised coal specification for blast furnace injection can be costly, which is why some iron manufacturers choose a larger granulated coal size specification. However, there is a concern that these coals may have lower burnout in the raceway region so there is a technical and economic balance with coal grinding. This paper investigates how the process of grinding alters the physical properties, plus the surface chemistry, of coals and their chars formed in a drop tube furnace; it was found that in many cases the larger particle size coals gave improved combustion burnout compared to smaller sizes.The physical properties of the chars, formed from grinding coals to different sizes, resulted in char swelling in the smaller particle sizes, compared to char fragmentation for the larger size classifications. Minerals phases associated with better coal reactivity were found to undergo higher conversion to other chemical forms with the larger size coals, suggesting a potential catalytic or synergistic contribution to their burnout. A closer look at the surface chemistry suggests that the action of grinding coals has an important effect on the surface chemistry. The XPS spectra of the chars, formed in a drop tube furnace, indicated that grinding the coals to a smaller particle size reduced the carbon–oxygen and carbon–mineral interactions compared to the larger sizes and correlated with the higher burnouts. An increasing trend was identified for the carbon sp2 bonding with larger size and higher rank coals which correlated with their burnout at low carbon conversions; however, this did not hold at higher conversions, suggesting other factors were more dominant.

Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere – Experimental and numerical study

The article presents the results of a comprehensive research on the impact of O2/CO2/H2O atmosphere and coal milling quality on pulverized coal combustion process. In particular the content of combustibles in fly ash and heat transfer parameters were investigated. Experiments were conducted in two research stands: in a laboratory rig for single particle combustion investigations and in a semi-industrial 0.5 MW combustion chamber for pulverized fuel burner studies. It was shown that because of faster coal conversion in the oxy-fuel atmosphere it is possible to use lower quality coal grinding, which will save energy required for milling. Experimental results confirmed also the beneficial effect of the coarse coal milling that resulted in a decrease of necessary flue gas recirculation rate, which also means lowering the energy demand of flue gas fans. The impact of oxycombustion optimization on net efficiency of electric energy production was presented on the basis of 220 MW unit with OP-650 steam boiler.

Influence of Pulverized Coal on Characteristics of Fly Ash in SCGP

The chemical composition, microstructure and size distribution of pulverized coal and fly ash samples from SCGP(Shell Coal Gasification Process) devices of four different regions were measured using an energy dispersive spectrometer, a scanning electron microscope, and Coulter particle analyzer. The paper aims to analyze the influence of pulverized coal on characteristics of fly ash. The energy dispersive analysis showed the main elements of chemical composition of fly ash and pulverized coal were C, O, Si, Al, and small amounts of metal elements. The difference of chemical composition of pulverized coal samples illustrates the types of coal were different. The similarity in chemical composition of four types of fly ash was due to the same technical process. Scanning electron microscope measurements showed particulate microstructures of four samples of pulverized coal were irregular, however, particulate microstructures of samples of fly ash were all spherical. The surface of most spherical particles had microspheres that were often agglomerated together. This phenomenon indicated that particulate microstructures of pulverized coal had no influence on particulate microstructures of fly ash. The size distribution showed median size of four pulverized coal samples were 31.59μm, 28.85μm, 25.57μm and 20.62μm. Median size of four fly ash samples were 4.07μm, 3.89μm, 3.45μm and 3.04μm. The particle size of fly ash increases with the growth of particle size of pulverized coal. The crests of particle size distribution curves of pulverized coal were not obvious due to the grinding evenness of coal were low. Particle size distribution curves of fly ash were opposite. This phenomenon indicates that uniform degree of fly ash is not affected by coal grinding evenness. In conclusion, the particle size of pulverized coal is the main influencing factor on the characteristics of fly ash while the influences of type of coal, particulate microstructures and grinding evenness of pulverized coal are not so much. Particle size of fly ash has a vital influence on high-temperature ceramic filters in SCGP. Controlling particle size of pulverized coal has a certain guiding significance to reducing the working load and extending the life of filter elements.

Application ofT2Control Charts and Hidden Markov Models in Condition-Based Maintenance at Thermoelectric Power Plants

An innovative approach to condition-based maintenance of coal grinding subsystems at thermoelectric power plants is proposed in the paper. Coal mill grinding tables become worn over time and need to be replaced through time-based maintenance, after a certain number of service hours. At times such replacement is necessary earlier or later than prescribed, depending on the quality of the coal and of the grinding table itself. Considerable financial losses are incurred when the entire coal grinding subsystem is shut down and the grinding table found to not actually require replacement. The only way to determine whether replacement is necessary is to shut down and open the entire subsystem for visual inspection. The proposed algorithm supports condition-based maintenance and involves the application ofT2control charts to distinct acoustic signal parameters in the frequency domain and the construction of Hidden Markov Models whose observations are coded samples from the control charts. In the present research, the acoustic signals were collected by coal mill monitoring at the thermoelectric power plant “Kostolac” in Serbia. The proposed approach provides information about the current condition of the grinding table.

河曲电厂磨煤机差压料位的改造及优化

河曲电厂一期磨煤机料位采用的是差压测量方式，机组运行期间，经常出现差压料位测量不准确、料位波动大、给煤机料位自动无法投入的情况。通过对原有料位计设备进行改造，减少故障的发生，保证了自动投入率，提高了公司节能减排效益。 The first phase of Hequ power plant coal grinding machine level uses the differential pressure measurement. During the operation of the unit, the differential pressure level measurement is of-ten not accurate, the level fluctuation is major, and the coal feeder automatically disjoints situation. The transformation of the original material level meter equipment reduces the occurrence of the fault, ensures the automatic input rate, and makes contribution to the company’s energy-sav- ing emission reduction.

Analyzing Properties of New Hard Coating Technologies for Increasing the Wear Resistance

The aim of this paper is, improving the characteristic of Ventilation mill for grinding coal which is one of the main part of steam power plants in the system that makes a significant influence on the level of energy efficiency with its work. Working parts of the mil during exploitation are dominantly exposed to intensive abrasive and erosive wear and also to impact loading at elevated temperatures, which can lead to damage and fracture of homogeneous materials, thus shortening their working life. For solving the problem of increasing the wear resistance of the parts for transport and coal grinding in power plants, to investigate the optimum application of technology in hard coatings by different procedures, and filer materials. Hard coatings were carried out with cold (plasma and arc treatment) and hot metallization (with concurrent and subsequent drowning). It has been used the additional materials of different chemical composition and properties based on Ni (NiCrBSi; NiCrBSiFe; NiCrBSi / WC) and Fe (FeCrCSiMo; FeCrCBSiTi; FeCrCBNiSi). The main goal of this paper is to make the selection of optimum hardfacing procedures, group of filler materials and hardfacing technology definition based on the results of structural and mechanical properties of samples, experimental model hardfacing. These were investigation of its structure, measurement and distribution of hardness in the samples, tribology characteristic samples with hard coatings. Based on the results of these tests there were selected the additional materials and procedures for applying hard coatings, which would be apply in the revitalization of parts in transport at and coal grinding.

The use of Bayesian Networks in Detecting the States of Ventilation Mills in Power Plants

 Abstract—The main objective of this paper is to present a new method of predictive maintenance which can detect the states of coal grinding mills in thermal power plants using Bayesian networks. Several possible structures of Bayesian networks are proposed for solving this problem and one of them is implemented and tested on an actual system. This method uses acoustic signals and statistical signal pre-processing tools to compute the inputs of the Bayesian network. After that the network is trained and tested using signals measured in the vicinity of the mill in the period of 2 months. The goal of this algorithm is to increase the efficiency of the coal grinding process and reduce the maintenance cost by eliminating the unnecessary maintenance checks of the system

Modeling and Simulation of Motor Discrete Variable Frequency Soft Starting Based on the Variable Reactance

The started motors are characterized by the low staring torque and large currents when the motors draw the heavy-loads. Therefore, a method of motor discrete variable frequency soft starting based on the variable reactance is presented in this paper [1,2]. According to the topological structure and working principle of motor discrete variable frequency soft starting based on the variable reactance, the simulation model of the system has been set up by the Matlab/Simulink tool. Simulation results show that the motor starters developed with this method have been successfully applied to the loads of the drive equipment in the kiln, coal grinding mill or the mine. It can improve the starting performance when the motors draw the heavy-loads., save energy, stabilize the voltage of the power supply network.

Exploring ball size distribution in coal grinding mills

Tube mills use steel balls as grinding media. Due to wear in the abrasive environment it is necessary to charge new balls periodically to maintain a steady balanced ball charge in the mill. The amount and ball size distribution in this charge, as well as the frequency with which new balls are added to the mill, have significant effects on the mill capacity and the milling efficiency. Small balls are effective in grinding fine particles in the load, whereas large balls are required to deal with large particles of coal or stone contaminant. The steady state ball size distribution in the mill depends on the top-up policy.The effect of the ball size distribution on the milling rate of coal has been measured as a function of ball size distribution. The change in ball size distribution as affected by wear and ball top-up policy has been modelled. From this a best ball top-up policy can be recommended that will ensure a close approximation to the desired steady-state ball size distribution that gives the required PF size distribution for the selected mill demand.

Substitution of Coke and Energy Saving In Blast Furnaces. Part 5. Problems and Prospects of Low-Coke Blast-Furnace Technology

Two most promising known non-traditional blast furnace technologies can be used to minimize the coke rate: 1) the tuyere injection of the hot reducing gases with removal of CO2 from the top gas and 100% oxygen blast; 2) injection of 300-400 kg pulverized coal/thm (PCI-technology). The injection of hot reducing gases (HRG) is limited by the heat balance of the entire Iron & Steel Works since the blast furnace gas will not be supplied to other works in sufficient quantity. The combustion and gasification of significant amount of coal and liquation of coal ash in the tuyere’s raceway are the limitations for the PCI-technology. Lack of resources, low-ash coal for pulverized coal injection—PCI (pulverized coal full—PCF) requires solutions to technical problems of the use of high-ash coals, in particular, partial and full gasification of fuel before entering the tuyere area of BF. Injection of the products of coal gasification (PCG) instead of PCI into the blast furnace tuyeres eliminates these limitations. The advantages of this technology in comparison with traditional PCI injection are as follows: increase in coal rate and decrease in coke consumption; possibility of low grade coals usage; elimination of fine coal grinding; gas desulphurization in the course of coal gasification and the possibility of coal ash fluxing and being removed from the process. The special compact coal gasifiers attached to the blast furnaces tuyeres were developed and tested at industrial scale. The estimated decrease in coke consumption from 565 kg/thm to 305 kg/thm with injection of 300 kg/thm of coal gasification products and 105 kg/thm of oxygen was specified with respect to operation of blast furnaces at Zaporozgstal Iron & Steel Works (90 years of the last century). The study by using a multi-zone mathematical model showed that the temperature-concentration and phase fields of the charge and the gas flow in the furnace change under the influence of the same tendencies that are seen with the injection of pulverized-coal fuel (PCF). The fact that the amount of coal which can be injected could be increased significantly by subjecting it to preliminary gasification and fluidizing the ash in tuyere-mounted gasifiers means that the targeted savings of coke could be realized by replacing coke with either high-grade coals (in the form of PCF) or low-grade coals (in the form of CGPs). In this case, for the best variants of the technology the ratio of the equivalents for the replacement of coke by coal is close to the ratio of the contents of nonvolatile carbon in the high- and low-grade coals (0.65 in the present case). The application of the developed technology for blast furnaces with burden composition of BF-5 “Severstal” and BF-9 AMKR will allow further decrease in coke rate to the minimal level of 180-200 kg/thm. The payback period of capital investments for this technology is estimated in the range of one year.

Enhancement in Flotation of Oxidized Coal by Oxidized Diesel Oil and Grinding Pretreatment

In this investigation, a comparison was done between the use of dodecane and oxidized diesel oil as collectors to float oxidized coal. FTIR was used to determine the chemical properties of oxidized diesel oil and oxidized coal, which showed that oxidized coal has less hydrophobic functional groups while oxidized diesel oil has high hydrophobic groups that will enhance oxidized coal flotation. Oxidized coal was dry ground for different times, namely, 0 min, 40 min, and 80 min and the flotation results showed that, compared to the dodecane as the collector, the oxidized diesel provides much higher (40% to 70%) combustible recovery. Grinding of coal and the use of oxidized diesel oil improved the flotation of oxidized coal due to the hydrogen bonding with the polar part of oxidized coal surface and oxidized diesel oil.

A Novel Integrated Method of Motor Discrete Variable Frequency Starting and Harmonic Filtering

The started motors with the discrete frequency converter are characterized by high staring torque and a large number of harmonics when the motors draw heavy-loads. It may pull down the voltage of the power supply network; it will severely influence the other electrical devices in the same power network. Furthermore, it increases the motor temperature, which may lead to motor damage. Therefore, a novel integrated method of motor discrete variable frequency starting and harmonic filtering is presented in this paper, to achieve quick and smooth motor starting, to produce high staring torque, to keep the harmonics in permitted ranges. The following works have been done in the study: analyze the principle, design the structure of the integrated device, and present the implementation steps. The motor starter developed with this method has been successfully applied to the loads of the drive equipments in the kiln, coal grinding mill or the mine. It can improve the starting performance when the motors draw the heavy-loads, save energy, stabilize the voltage of the power supply network.

Effect of metamorphism on the grindability of coals under impact action

The grindability of coals in impact mills was studied depending on coal rank. With the use of coals of grades D, K, and T as an example, the efficiency of dispersion to a size of 20 μm was evaluated, and it was found that grindability increased with the degree of metamorphism to reach the greatest value for coals of grade T.

LCI Study of SCR DeNOx Technology for Cement Industry

LCI study is short for life cycle inventory study. In the situation of more strict regulation limit of 500 mg NOx /m3 a demonstration project to reduce NOx emissions with clinker production capacity of 2000 t/d in Beijing is supported by the 'Five-twelfth' National Science and Technology Support Program of China Science and Technology Department. A selective catalytic reduction denitration (SCR DeNOx) technology is expected to be applied in this project. LCA models the life cycle of cement manufacturing with SCR DeNOx technology applied as its SCR system. The life cycle is from the coal and raw materials transportation, through the coal and raw meal grinding, to the clinker incineration, and finally the flue gas including NOx to the SCR reactor. The functional unit is 1 ton clinker. Data for cement produced in LNB technology as bench mark is analyzed and the SCR scenarios are to show that the SCR reactor can be established because the additional environmental impact is small due to small consumptions of reducing agent and electricity. SCR technology enable the deNOx efficiency much higher with small environmental impact.

LCA Method of MSC and Low-NOx Burner Technology in Cement Manufacturing

LCA method was used to model the life cycle of cement manufacturing with multi-stage combustion and low-NOx Burner technology applied as its low-NOx system. The life cycle is from the coal and raw materials transportation, through the coal and raw meal grinding, to the clinker incineration, and finally the flue gas including NOx to the air atmosphere. The functional unit is 1 tonne clinker. Data for cement produced in MSC and LNB technology is analyzed. The data is collected from the real clinker production situation and the measurement is taken in 12 hours continuously.

Study of a Multivariable Coordinate Control for a Supercritical Power Plant Process

the paper presents our recent research work in study of a novel multivariable coordinate control for a 600MW supercritical (SC) power plant process. The nonlinear mathematical model of the plant is briefly described in the paper. Then, a control strategy is designed which is based on Generalized Model Predictive Control (MPC) theory and tested within small load changes. But the MPC alone performs well only within small load changes even with consideration of constant disturbance and measurement noises in prediction algorithm. We then propose a dynamic compensator in parallel with the MPC to track large load changes. Because the model has been identified with on-site closed loop data, the multivariable optimal control signals have been used as a correction to the reference of the plant local controls instead of direct control signal applications. The simulation results have shown good performance of the controller in response to some load changes. Furthermore, it has been proved that the plant dynamic response can be improved by increasing the coal grinding capability and pulverized coal discharging through the implemented coal mill controllers. The benefit from this study is to provide convinced methodology to improve the SC power plant dynamic response for fulfilment of National Grid Code (NGC) requirements.