coal mill 80%

the data related to the sieve analysis of the study

The data related to the sieve analysis of the study, in which it is aimed to grind 100 g of 3.15/2 mm coal sample in a ball mill with 80% of the particle size smaller than 1000 , are given in the table below. Using derivative sieve analysis, find the particle number, specific surface area and volume surface average diameter of the product.

vertical coal mill

Vertical Coal Mill, also known as coal mill, for superfine grinding of raw coal, soft coal specifically for cement, power plant, steel mill and other fields. Its special design structure and production process creates the higher production efficiency with lower energy consumption, less dust, lower noise than ball mill, which meets national standard of energy saving industry. Chaengs Vertical Coal Mill technology reaches the international advanced level, and its manufacturing quality reaches the leading domestic level, and is the preferred equipment for grinding raw coal and soft coal.

In general, coal pulverisers are designed to achieve the maximum rated capacity grinding a design coal with a grindability of 55 HGI and 8-12 per cent moisture and achieving a discharge fineness of 70 per cent passing a 200 mesh screen (74 micron) and 99.5 per cent passing a 50 mesh screen. Variation of the coal hardness and/or moisture content will effect, up or down, the discharge capacity or the discharge fineness.

2. After grinding system has started, please turn on the rod valve in the bottom of raw coal storage, and start sealing measuring belt to coal mill, then raw coal will be dryed and ground in the vertical coal mill.

3. The mixture of hot air or waste gas of cement production line are sucked into Vertical Coal Mill, under blowers function. After complete heat exchange with ground coal, the coal powder are brought to separator for separation. The unqualified coarse coal powder falls to the grinding table for regrinding. The qualified coal powder will be collected with the airflow into the anti-explosion impulsive dust collector, through the grid wheel are discharged into pulverized coal storage.

Pre-sales Service: The wide product range enables us to provide our customers with stand-alone machines or complete processing plants. Based on our customers request and budget, our experts make efficient, reliable solutions. Following customers order we produce strictly, whats more, before placing the order every customer has the chance to visit XinXiang Great Wall (Chaeng) working machines or complete plant in the site. To ease the trip for every visitor to China, in particular the first-time visitor, we provide FOR FREE all relevant visitor-friendly services including invitation letter preparation, hotel reservation, airport pick-up, incity transportation, and sightseeing guide, etc. After-sales Service: Experienced technicians guidance is available on the phone, and on the internet. One or more engineers will be dispatched to the quarry site to help install the customers plants. Necessary training about machine daily maintenance to local workers is provided also. After-Sales department is made of well-trained employees and installation engineers, the installation engineers are special and professional members of XinXiang Great Wall (Chaeng), they are now strategically located home and abroad, working for our customers.

Pre-sales Service: The wide product range enables us to provide our customers with stand-alone machines or complete processing plants. Based on our customers request and budget, our experts make efficient, reliable solutions. Following customers order we produce strictly, whats more, before placing the order every customer has the chance to visit XinXiang Great Wall (Chaeng) working machines or complete plant in the site.

To ease the trip for every visitor to China, in particular the first-time visitor, we provide FOR FREE all relevant visitor-friendly services including invitation letter preparation, hotel reservation, airport pick-up, incity transportation, and sightseeing guide, etc.

After-sales Service: Experienced technicians guidance is available on the phone, and on the internet. One or more engineers will be dispatched to the quarry site to help install the customers plants. Necessary training about machine daily maintenance to local workers is provided also.

After-Sales department is made of well-trained employees and installation engineers, the installation engineers are special and professional members of XinXiang Great Wall (Chaeng), they are now strategically located home and abroad, working for our customers.

fossil fuel share in global energy mix flat from decade ago

The share of fossil fuels in the global energy mix remains almost flat as it was in a decade ago, even when renewables cost declines and governments put efforts to reduce carbon emissions, Reuters reported, citing a report from a green energy policy network REN 21.

Growing energy consumption, continued construction and investment in new fossil fuel plants, and reduced use in biomass energy in heating and cooking supported the high proportion of fossil fuels in energy mix.

Fossil fuels accounted for 80.2% of the global energy mix in 2019, down slightly from 80.3% in 2009. Renewable energy sources, such as solar and wind, accounted for 11.2%, up from 8.7% in 2009, the report said.

Many countries' COVID-19 economic recovery plans have identified further incentives for investment in renewable energy, but the investment in renewable energy is only about one-sixth of investment in fossil fuels, even though it is now cheaper to build new wind or solar power plants than to operate exiting coal-fired power plants in many regions.

(Writing by Emma YangEditing by Harry Huo) For any questions, please contact us by [email protected] or . Share this article Connect with us

Chinese domestic thermal coal market is gradually edging up as supply tightens at northern transfer ports, although buyers expected regulative measures to cool down the market and are still holding of

Indonesian thermal coal offer-bid spread widens as outlook views varied Offer-bid spread widened for Indonesian thermal coal on July 8, as buyers and sellers held different views about the ma

China's watchdog warns of coal mine safety hazards after Jul 1 event China's National Mine Safety Administration in a video meeting on July 5 warned of possible safety hazards in coal product

Coal mining operations in Indonesia has not been much affected by the recent spike in coronavirus cases across the country, although supply remained tight amid weather-related disruptions.

Chinese market participants were increasingly bearish over the near-term metallurgical coke market, as steelmakers started to cut coke purchases and prices in anticipation of demand downturn.

Chinese coking coal market is feeling the cooling effect of the latest steel production curb policy which has received active response from several provinces, with coal buying appetite temporarily res

Thermal coal Production area Thermal coal production and sales resumed at main production regions. Rail station and traders slowed down purchase amid wait-and-see sentiment. Coal sales de

Russia's coking coal exports in May was the second high this year, mainly bolstered by stronger demand from Ukraine and China, customs data showed. The country's coking coal exports incre

Chinese steelmakers posted strong profit in the first half of the year, according to multiple financial reports seen by Sxcoal recently, as continued surge of steel product prices significantly booste

Total profit (including subsidy) of 4,276 major coal enterprises reached 161.44 billion yuan ($21.92 billion) in the first five months, up 109.4% from the year-ago level, showed data released by China

Indonesia's Ministry of Energy and Mineral Resources set its July thermal coal reference price, which is also known Harga Batubara Acuan (HBA), at $115.35t, jumping 121.15% from a year ago.

China's top coal province Shanxi pledged to add coal reserve capacity of more than 3 million tonnes for government dispatch when necessary in 2021, said the Shanxi Energy Bureau in a notice recently.

Indonesia announced a lockdown on its main island Java and the holiday resort of Bali to cope with the startling spike in COVID-19 cases, but the impact on its coal production could be limited, accord

Australian metallurgical coal exports are forecast to increase to 186 million tonnes in the fiscal year 2022-23 (July 2022- June 2023) from 171 million tonnes in the FY 2020-21, according to an offici

Thermal coal stockpiles were falling quickly at northern China's transfer ports, indicating supply remained under pressure in the domestic market, which may dent the government's expectation of a "rel

China's largest inland port Ganqimaodu in Inner Mongolia resumed coking coal imports from the neighboring Mongolia in midweek after three weeks of closure due to deteriorating COVID-19 pandemic in the

China's metallurgical coke market saw a continued impasse between coke producers and steel mills over the first 120 yuan/t coke price hike, but participants foresaw a downside risk in mid and late Jul

Mongolia extended lockdown measures at Tsagaan Khad, a customs stockyard for coal exports to China through Ganqimaodu border crossing, to July 20 from the original July 6, because of untamed infection

Spot thermal coal prices softened on June 6 at northern China transfer ports after miners in key production areas slashed prices to align with the government directive, market sources said.

China's coastal coal freight rates tumbled last week following mild declines for over two weeks, as shipping demand was obviously reduced by loading suspension at Bohai-rim ports due to multiple facto

synthesis and characterization of metallic iron reduced from low-grade coal in rajasthan | springerlink

Todays industrial technology offers to optimize and reutilize the resources available in terms of scum that remains after hot rolling in steel rerolling mills in Rajasthan, India. Vast reserves of low-grade coal exist and tons of mill scale are produced everyday after hot rolling in steel rerolling mills in Rajasthan, India. The present research work aimed to optimize the various parameters of solid-state reduction of mill scale using low-grade coal from Rajasthan, India. This study investigates the effect of reduction temperature (850950C in steps of 50C), reduction time (60 to 180min, in step of 60min), mill scale size (800300m), and mill scale to coal ratio (1:1, 1:2 and 1:3) on achievable metallic iron percentage and its metallization. A higher percentage of iron content is obtained at a lower particle size of 300m. The structural analysis is carried out on 300m mill scales using XRD analysis, which shows the percentage of iron is approximately 63%, and metallization above 80% can be obtained under the reduction temperature at 900C for 180min, for 1:3 ratio (mill scale to coal ratio). The chemical composition of the mill scale is obtained using X-ray fluorescence (XRF), and particle sizes as well as distribution of mill scale are determined by scanning electron microscopy (SEM). The elements present in the mill scale are also verified using EDS, which indicates the presence of a major portion of Fe, followed by Fe2O3, Si, and C, respectively.

Sen R, Dehiya S, Pandel U, Banerjee MK (2015) Utilization of low grade coal for direct reduction of mill scale to obtain sponge Iron: effect of reduction time and particle size. Procedia Earth Planetary Sci 11:814

Ye Q, Zhu H, Zhang L, Ma J, Zhou L, Liu P, Chen J, Chen G, Peng J (2014) Preparation of reduced iron powder using combined distribution of wood-charcoal by microwave heating. J Alloys Compd 613:102106

Sen R, Dehiya S, Pandel U, Banerjee MK (2015) Utilization of low grade coal for direct reduction of mill scale to obtain sponge Iron: effect of reduction time and particle size. Procedia Earth Planetary Sci 11:814

Sen R, Maan A, Goyal U, Birdhaniya A, Pandel U (2018) In crucible reduction of mill scale by lean grade coal: study of time, temperature and arrangement for optimum reduction conditions. Materials Today Proc 5(2):72567263

The authors would like to give sincere thanks to the Ministry of Steel, Government of India (F. No 11(11)/GBS/2013-TW DATED 28.09.2015) for financial support. Acknowledgement is also due to the Advanced research lab for Tribology, Mechanical Engineering Dept., MNIT, Jaipur, for providing the solid-state reduction facility and the Material Research Centre, MNIT Jaipur, for providing the characterization facility.

Kumar, A., Kiragi, V.R., Kumar, S. et al. Synthesis and Characterization of Metallic Iron Reduced from Low-grade Coal in Rajasthan. Mining, Metallurgy & Exploration 37, 17411751 (2020). https://doi.org/10.1007/s42461-020-00281-y

sustainability of coal mines: separation of clean coal from the fine-coal rejects by ultra-fine grinding and density-gradient-centrifugation - sciencedirect

Ultrafine grinding and decanter centrifugation to recover clean coal from rejects.Centrifugation of coal-water-slurry (50% pulp density) produces 19% ash clean coal.Extra dilution results 15% ash clean coal (from 35% ash feed coal) with 58% yield.Vitrinites macerals enriched by removing silica and 82% carbon values recovered.Validated the experimental with breakage kinetics and separation models.

Due to the availability of high ash feedstocks and drift origin, Indian coal washeries face tremendous pressure in maintaining the clean coal yield and handling huge quantities of rejects. Since flotation rejects (tailings) discharged in the form of fine-sized (500) coal-water-slurry, handling and disposal is an environmental concern and it impacts mine sustainability. As conventional methods for extracting clean coal from tailings reported poor yields, a two-stage process i.e. wet-grinding followed by density-gradient-centrifugal separation was explored and optimized for recovering >80% of the carbon values. Clean coal of 19.2%21.4% ash with >40% yield was obtained by density-gradient-centrifugation of different ultrafine size (75) coal-water-slurries at 50% pulp density. However, further reduction in clean coal ash by maintaining the yield was limited due to the hindrance in the movement of the high-ash coal particle caused by particle-particle interactions in a thick slurry. Hence, the pulp density of coal-water-slurry was optimized with the addition of extra water, which resulted in 15.1%17.3% ash clean coal at an average yield of 5458% from coal tailings having 34.9%39.3% ash content. Optimization and kinetics of grinding and density-gradient-centrifugation were studied to check the effect of pulp density and other operating conditions. Ash reduction was mainly due to the removal of silica-based materials, where alumina to silica ratio was increased from 0.394 to 0.475. The petrographic analysis showed that the product coal was rich in macerals with ~60% of vitrinites and~30% of inertinites.

the coal industry has lost almost one thousand jobs since trump became president

President Trump tweeted about the coal industry years before he announced his candidacy for the nations top office. He complained about President Obamas policies and coal plants closing, even though market forces such as natural gas fracking were making coal less and less economically viable. Trump has embraced the coal industry and even appointed an ex-coal lobbyist to head the EPA.

Coal miners may be the hardest working Americans who do a very dangerous job in unhealthy conditions. Unfortunately for them, market forces, along with continued concerns about climate change, have pummeled the industry. Trumps desire and actions to help miners have at best kept the industry from declining further, while not investing in renewable energy, where there is progress, is hampering job growth and negatively impacting the environment.

In an economy with over 129 million people employed by businesses, there are only just over 50,000 coal miners in the U.S. or 0.04% of the total number of people working. While there was a small uptick in coal miners after Trump was elected by a few thousand, the latest jobs report shows that there are fewer coal miners now working vs. three years ago. This compares to over 6.4 million jobs being added in the past three years.

The Dow Jones U.S. Coal Index tracks the coal industry subsector. When Trump was elected in November 2016 the index was around 45 and closed on Friday at 8.57, down approximately 80%. There have been numerous mine closures and bankruptcies during his time in office, with little positive outlook for the industry.

The EIA or Energy Information Administration estimates that coal exports were less than 14% of total coal production in 2019. Thermal coal exports, used for power plants, was less than 6% of total production, while metallurgical coal exports, used for steelmaking, was 8%.

There is also a downward trend in coal exports this year as reported by S&P Global Market Intelligence by the Institute for Energy Economics and Financial Analysis. The data shows that through February 22 exports via the top five coal-export ports have declined from 13% to 69% year over year.

For the second month over the past year, renewable energy has generated more electricity than coal in the U.S. While the numbers are still being confirmed, having this occur during the winter season when coal generated electricity is in high demand highlights the continued decline in coal and the slow but gradual increase in renewables.

Even in Oklahoma, wind provided over 40% of the states electricity generation last year. This is an increase from 33% in 2017 and 36% in 2018. It has also been estimated that over $20 billion has been invested in renewable projects in the state, which I suspect is multiples of what has been invested in the entire U.S. coal industry over the same timeframe.

In Germany the Fraunhofer Institute for Solar Energy Systems estimates that, renewable energy sources provided a record 61.2% of Germanys net public electricity generation in February and that wind energy provided nearly half of the countrys electricity during the month. It added, Throughout the month, Germanys renewable energy sector regularly provided around 60% or above of the countrys electricity production including over a dozen days around or above 70%. Wind power generated almost 46% of the countrys total electricity for the month.

I provide independent research of technology companies and was previously one of two analysts that determined the technology holdings for Atlantic Trust (Invesco's high net worth group), a firm with $15 billion under management. Before joining Atlantic Trust I was the Internet Security Software analyst for Smith Barney (where I authored the most comprehensive industry report Internet Security Software: The Ultimate Internet Infrastructure) and an Enterprise Server Hardware analyst at Salomon Brothers. Prior to becoming an equity analyst, I spent 16 years at IBM in a variety of sales and manufacturing positions. I have a B.S. in Industrial Engineering from Stanford University and a Postgraduate Diploma in Economics from the University of Sussex, England.

I provide independent research of technology companies and was previously one of two analysts that determined the technology holdings for Atlantic Trust (Invesco's high net worth group), a firm with $15 billion under management. Before joining Atlantic Trust I was the Internet Security Software analyst for Smith Barney (where I authored the most comprehensive industry report Internet Security Software: The Ultimate Internet Infrastructure) and an Enterprise Server Hardware analyst at Salomon Brothers. Prior to becoming an equity analyst, I spent 16 years at IBM in a variety of sales and manufacturing positions. I have a B.S. in Industrial Engineering from Stanford University and a Postgraduate Diploma in Economics from the University of Sussex, England.

coal calculations | sgs

Accurate and precise calculations are vital to the success of your coal operation. These calculations are used to calculate various skeleton parameters including ash and calorific value that let you determine the grades of your coal.

SGS global teams of chemists and experts use a range of coal analytical calculations and indexes to arrive at the calorific values, total hydrogen, Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR) of your coal sample. Our third party test results and calculations provide you with reliable data that complies with international standards.

The following are some of the interesting protocols and calculations regularly used by SGS coal and coke specialists. These calculations are provided for your information, but SGS cannot guarantee that all data or formulas match the most current standards. Clients trading coal must familiarise themselves with the current standards being used in contracts.

Extract from COAL - D.W. Krevelen. (page 529) "All the empirical equations are modifications of the original Dulong equation with "some theoretical foundation", and are, by adaption to empirical CV data of coal, de facto empirical relationships. The correlations given by GIVEN (1986) and NEAVEL (1986) are the most reliable."

Net Calorific Value (NCV) Calculations and Conversion Factors Ref: Net Calorific Value (ASTM D5865-12) The heat produced by combustion of a substance at a constant pressure of 0.1 Mpa (1 Atm), with any water formed remaining as vapour. ASTM D5865-12 / D3180 at constant pressure Qv-p= 0.01 * RT * (Had / (2*2.016)) - Oad / 31.9988 - Nad / 28.0134) Qh = 0.01 * Hvap * (Had / 2.016) Qmad = 0.01 * Hvap * (Mad / 18.0154) Qmar = 0.01 *Hvap * (Mar / 18.0154) Qvar = Qvad *((100-Mar) / (100-Mad)) Qpad(net) = Qvad(gross) + Qv-p - Qh Qmad Qpd(net) = (Qvad(gross) + Qv-p - Qh) * (100/(100-Mad) Qpar(net) = ( Qvad(gross) + Qv-p - Qh) * (100 - Mar) / (100 - Mad) Qmar Where: Qv-p = The energy associated with this change in the volume of the gaseous phase for the combustion reaction R = the universal gas constant [8.3143 J/(mol *K)] T = the standard thermochemical reference temperature (298.15 K) Had = Had,m 0.1119 * Mad (total Hydrogen H in moisture) Oad = Oad,m 0.8881 * Mad (total Oxygen O in moisture) Hvap = heat of vaporization of water at constant pressure (43985 J/mol) Qh = heat of vaporization of hydrogen content in the sample Qmad = heat of vaporization of water content in the analysis sample Qmar = heat of vaporization of total moisture content in the sample Atomic Weights: O2 = 31.998 / N2 = 28.0134 / H2 2.016 / H2O = 18.0154 ISO 1928-2009 at constant volume Qv, net,m,J/g =( Q gr,v,d - 206.0 [ wHd ] ) x (1-0.01xMT) - (23.05x MT) Qv, net,m,kcal/kg = ( Q gr,v,d - 49.20 [ wHd ] ) x (1-0.01xMT) - (5.51x MT)

ISO 1928-2009 at constant pressure Qp, net,m,J/g = { Q gr,v,d - 212.2 [ wHd ] - 0.8 x [wOd + wNd] } x (1- 0.01MT) - 24.43 x MT Qp, net,m,kcal/kg = { Q gr,v,d - 50.68 [ wHd ] - 0.191 x [wOd + wNd] } x (1- 0.01MT) - 5.84 x MT

Seylers Formula Various parameters of coal can be estimated from the Ultimate Analysis and Calorific Value determinations, using Seyler's formula, and other similar calculations (e.g. Dulong's formula). ISO 1928 2009 Determination of Gross Calorific Value The ISO standard is the only international standard that allows for the estimation of hydrogen content to be calculated using Seylers Formula. Seylers calculation is only valid for most bituminous coals. Note 1. NOT valid when the estimated Hdb is less than 3% Note 2. NOT valid when the Odaf content is greater than 15% Note 3. NOT valid for estimation of H if coal shipments are a blend of low rank coal, or anthracite, or petcoke, and bituminous coals Note 4. NOT valid for low rank coal, anthracite, petcoke, or coke ISO 1928 2009 Part E.3.3 wH = 0.07 x w(V) + 0.000165 x qv,gr,m - 0.0285 x [ 100 - MT - w(A) ] w(H) - is the H content of sample less H contained in moisture, as % mass w(V) - is the VM content of sample with moisture content MT, as % mass w(A) - is the ash content of sample with moisture content MT, as % mass qv,gr,m - is the gross CV of sample with moisture content MT, in joules/g

Where: a,b,c,dh,j,k are the hole sizes, in mm., of successive sieves; 'A,B,C,DH,J,K are the cumulative percentage oversizes for each of the sieves. Note: The sieve with hole size 'a' is the smallest size through which all coke passes (i.e. A = 0%). The sieve with hole size "k" is the hypothetical sieve through which no coke will pass (k=0mm, K=100%).

When coke descends in the blast furnace, it is subjected to reaction with countercurrent CO2 and abrasion. These concurrent processes weaken the coke and chemically react with it to produce excess fines that can decrease the permeability of the blast furnace burden. SGS conducts CRI and CSR testing to provide high accuracy results with good turnaround times. CRI and CSR tests determine how much energy your coal will produce when being burned in the furnace.

The CRI/CSR test measures coke reactively in carbon dioxide at elevated temperatures and its strength after reaction by tumbling. In the test, duplicate 200g samples of x (19 x 22 mm) sized coke are extracted from a minimum 1kg parent sample and reacted in a vessel with CO2 gas for two hours at 1100C. The weight loss after the reaction equals the CRI. The reacted coke is then tumbled in an I-shaped tumbler for 600 revolutions at 20 rpm and is then weighed. The weight percent of the + coke equals the CSR. Most blast furnaces will require a coke with a CSR greater than 60 and CRI less than 25.SGS is committed to providing accurate, cost effective blast furnace coke analysis for your operation.

deposit formation and emissions from co-firing miscanthus with daw mill coal: pilot plant experiments - sciencedirect

An investigation has been carried out into the deposits generated on cooled probes and the gas compositions produced from co-firing miscanthus mixed with Daw Mill coal. The biomass additions to the coal were at 0, 20, 40, 60, 80, 100wt.% in a 100kWth pilot-scale pulverised fuel (PF) combustion test rig operated with a feed rate of 715kg/h. Online monitoring of the combustion gas stream was carried out for CO2, O2, H2O, SO2, CO, NO, NO2, N2O, HCl and HF using a high resolution multi-component Fourier Transform Infra-Red (FTIR) gas analyser. The deposits were collected from the upstream, side and downstream surfaces of three air-cooled probes that were exposed in the flue gas path and operated at surface temperatures of 500, 600 and 700C (to simulate heat exchanger tubes). The compositions of these deposits were determined using a range of analytical techniques including Environmental Scanning Electron Microscopy (ESEM) with Energy Dispersive X-ray (EDX) and X-ray Diffraction (XRD). The composition of the combustion gas streams and deposits changed as a function of the fuel mixtures used in the combustion process. The increasing miscanthus share in combination with the Daw Mill coal resulted in a reduction of SOx, NOx and HCl levels. The deposition on the upstream surfaces of the probes decreased with increasing biomass percentage, but increased on the downstream surfaces. The concentration of K and S in the deposits increased with increasing miscanthus share in the fuel up to 80wt.%, particularly on the coolest probe (500C surface temperature). In addition, Cl was detected in the deposit on the two lower temperature probes only when 100% miscanthus was used in the combustion process. Thermodynamic modelling of the combustion/deposition process carried out in parallel also suggested formation of KCl only at the highest levels of biomass co-firing for miscanthus/Daw Mill coal, with K2SO4 being predicted at lower co-firing levels for these specific fuels.

coal grinding - cement plant optimization

To achieve good combustion and satisfactory flame formation, coal needs to be dried and ground to a proper degree of dryness and fineness. Drying of moisture in coal is achieved normally by ducting part of the kiln exhaust gas through the mill with inlet temperatures of up to 300C. Inert kiln exhaust gases with oxygen content of about 3-5% are most suitable for the intended purpose due to high risk of fire/explosion in fine coal. However, the provision for inertization of coal mill circuit and fine coal bins (with CO2, N2 or CO2+N2 to replace O2 which promotes spontaneous ignition of fine coal) is strongly recommended to be available. Gas analyzers and explosion vents are essentially provided in mill circuits to monitor the potential of fire/explosion and mitigate fire/explosion incident respectively. Drying and grinding are generally being done in either air swept ball mill or a vertical mill. The selection of mill system will depend mostly on the factors like initial capital cost, drying and grinding capacity required, cost of energy (power) etc.

Coal Fineness: It is understood generally that the finer we grind, the easy it is for burning. However, the fineness required will always be directed by where to fire and what type of coal it is and lastly the fineness will be dictated by the risk factor involved in finer grinding high volatile coals. The recommended fineness for coal verses volatile matter percentage is depicted in graph beside.

As understood from the above graph, the relationship between 90-micron and 200-micron residue is quite important as well. So, it is generally recommended to have 200-micron residue as low as possible, because coarse particles delays ignition, gives long flames in kiln (coating & ring issues), CO formation at kiln inlet, higher preheater exit temperatures (EGT). As a rule of thumb, the residue on 90-micron sieve should not be less than half of the volatile content for safety purpose. ie. R90>=1/2. Volatile Content %.

Coal Moisture: The degree of drying, and therefore the required mill outlet temperature (from 65-80 0C) will depend upon the type of coal ground. Some residual moisture in fine coal is recommended (Graph below) to minimize the potential of spontaneous ignition of fine coal, which will again vary for different coal types as below:

While considering the safe mill outlet temperature, care should be taken to avoid the temperatures below dew point of mill outlet gases, so that the condensation inside the bag filter and consequent material jamming problems can be avoided.

Important Note: If you chose to use different types of coal (having different rank) simultaneously or use coal and Petcoke, remember to grind them separately as per above guidelines and feed them from different fine coal bins in required proportion to kiln and pre-calciner as required.

Coal Grinding Operation Objectives and KPIs: Highly energy intensive unit operation of coal grinding is intended to provide a fine coal as a fuel for calcination and clinkerization. Coal grinding operation is monitored for following parameters to ensure objectivity and economy of operation.

Note: Proximate and ultimate analysis are generally provided by coal supplier. However, Proximate analysis, Determination of calorific value, Ash analysis, Hardgrove analysis and Abrasion analysis are done as and when required in plant laboratory or by a third party agency.

Mill Load (Kw or Amps). Mill sound/filling % (in ball mills). Mill Inlet Temperature (0C). Mill Outlet Temperature (0C). Gas flow through mill (m3/h) or mill fan power (kw) Mill DP, or inlet/outlet draft (mmH2O). Separator DP (mmH2O, mbar) and temperatures (0C). Bag filter DP (mmH2O, mbar), Temperature (0C).

Position of Explosion vents. Operational readiness of quick shutoff dampers. Inertization section readiness (N2, CO2 pressure in bars) Mill Inlet Temperature (0C). Mill Outlet, bag filter outlet Temperature (0C). O2 + CO Percent at bag filter outlet and in fine coal bins. Bag filter hopper and fine coal bin temperature (0C).

Mill Feeding: Consists of following activities Coal Crusher: Generally, require when ball mill is used for grinding and raw coal size is on higher side(>25mm). Conveying to Hoppers: Covered belt conveyors, horizontal or inclined are most suitable and commonly used for conveying. Metal Detector and Magnetic Separator Arrangement of metal detector and magnetic separator is integral part of feeding system in vertical roller mills and roller presses. Both are installed on mill feeding belt conveyor. Magnetic separator, separates out small metallic impurities from mill feed. While as metal detector signals the presence of metallic debris, which can damage the grinding path and give rise vibrations issues. Mill Feeding Hoppers Hoppers for coal, petcoke serve the purpose of providing a buffer storage for mill feed and a convenient arrangement for feeding to weigh feeders. Hoppers are generally designed to hold the requirements of one shift or more. Coal hoppers are generally steel hoppers with conical steep bottom (inclination >700), wide opening for discharge as possible to ensure mass flow of coal, mounted on load cells and/or equipped with level sensors to guide filling in auto mode. De-dusting bag filters needs to be installed at the top to vent air when material is fed to a hopper. Mill Feeders: Feeders for coal mill are generally installed directly under hoppers with rod gate in between. The feeders are generally 2 to 3 m long and discharge on to conveyor or feeding chute to mill. For coal mill feeding, table feeders, belt feeders, chain feeders and weigh feeders have been used. However, weigh feeders are the most commonly preferred to feed and report production counters. Metering on Feeders: Metering can be either direct (gravimetric) or indirect (Volumetric). In direct method of metering the material passes over a load cell installed in weigh feeder/apron feeder and the travel speed is monitored with installed tachometer. Weight and speed together determines the mass flow rate of material in metric tons per hour (t/hr). Alternatively, in some of the arrangements direct system consists of weigh feeder and its pre-feeder. Feed rate is generally controlled with the speed of weigh feeder belt, which is driven by a variable speed drive. Feed rate is monitored and controlled by a control panel generally supplied with weigh feeder. Set points are passed to control panel from CCR by Operator. In indirect metering system feed rate is determined by measuring cross section of material and rate at which it flows and taking into account bulk density of material.

Calibration and Drop Test Facilities: Provisions for drop test for calibration of weigh feeders are commonly available in cement plants to validate production figures. Although the weigh feeder calibration is not required frequently unless there is a disturbance in mechanical system due to various reason including maintenance. However, it is a common practice to validate feed rate through drop test periodically. Weigh feeders generally come with self-calibration devices. A simple way of 'self-calibration' is to have the hopper mounted on load cells, so that a weight loss for predefined time will be used for calibration purpose, and in this case calibration of hopper load cells needs to done at least once a year with standard weights.

Cold Air Locking at Mill Inlet: This is very important for coal mill, as ambient air throttling the drying capacity of mill as well as increases the oxygen content of kiln gases making it riskier. Oxygen percentage of more than 12-14% is considered catalyst for fire/explosion risk. Rotary feeders (gravel gate), double flap valve are used to stop/minimize cold air leakage into mill system. Cold air leakage percentage can be determined by measuring oxygen percentage at inlet and outlet of the circuit element.

Ball Mill: Single chamber ball mills (with classifying liners and dam ring) with drying chamber and static or dynamic separator are commonly existing for coal grinding in cement plant for one or more kilns as per capacity. Ball mill is a cylinder rotating at about 70-80% of critical speed on two trunnions in white metal bearings or slide shoe bearings for large capacity mills. Grinding media consists balls of 3-4 sizes (60mm-30mm) in designed proportions with large sizes in feed end and small sizes in discharge end. About 27 to 35 % volume of mill is filled with grinding media. Equilibrium charge is that charge where compensation for wear can be done by balls of one size only usually the largest size in the compartment. Grinding media could be made of forged steel, cast steel or even cast iron. To economize grinding media consumption, presently grinding media used are high chrome steel balls. Mill shell is lined with lining plates to protect it from wear, high chrome steel liners are now commonly preferred to give longer life. Ground material is swept out of the mill by hot air /gas of significant velocity (5-6 m/s), through separator and coarse fraction is returned to the system for regrinding and fine material passed to bag filter for collection.

Vertical Roller Mills: In Vertical Roller mill 2 - 4 rollers (lined with replaceable liners) turning on their axles press on a rotating grinding table (lined with replaceable liners) mounted on the yoke of a gear box. Pressure is exerted hydraulically. This mill also has a built in high efficiency separator above the rollers to reduce circulation loads and consequently reducing differential pressure across the mill. Feed material is directed onto the center of the table and is thrown outward by rotation under the rollers by centrifugal action. Material gets partially ground and as it falls over the edge of the table, where it is picked up by hot gases, and is separated into coarse fraction falling back on grinding table and fine fraction is carried with hot gases to product collector. The mill is started either with the rollers in lifted-up position, or with the hydro-pneumatic system at low pressure. In grinding mode, actual metal to metal contact should be prevented by limit switches or a mechanical stop and by consistent feed. In VRMs the material cycle time is usually less than a minute against several minutes for a ball mill or tube mill. Thus, control response should be accordingly faster. In case mill feed fails action should be taken within no more than 45 seconds or excessive vibration will cause mill shut-down. Moreover, the vertical mills are subject to vibrations if material is too dry to form a stable bed. Therefore, provision is made for controlled spray water inside the mill During mill operation magnetic separator and metal detector should be always functional to ensure to exclude tramp metal which can damage the grinding surfaces.