High Ash Indian Coal Research Articles

Understanding the unusual fluidity characteristics of high ash Indian bituminous coals

High-temperature rheometry and 1H nuclear magnetic resonance (NMR) are two complementary techniques that have been used to investigate fluidity development quantitatively in the <53 μm and 53–212 μm size fractions of high ash Indian non-coking coals and imported non-coking, medium coking and good coking coals. It was found for the 53–212 μm size fraction of the Indian bituminous coal with higher ash content (30 wt%) that, despite its high complex viscosity (>105 Pa·s), the maximum concentration of fluid H was quite similar to that of the good coking coal (40%). This Indian non-coking coal developed fluid H with the highest mobility (T2L > 150 μs) in the coal series, regardless of the particle size fraction studied. The probable explanation for this abnormal behavior is that the mineral matter prevents bulk movement in the sample but the local mobility of the fluid phase is still high on the nanometer scale. Blending the two Indian non-coking coals with the highly fluid medium coking coal gave higher viscosities (i.e. lower fluidity) than predicted by the polymer blend rule, probably again due to the high mineral matter restricting bulk flow. This negative effect was less pronounced with the higher ash coal suggesting that the high mobility of the fluid entities in this coal might prevent the destruction of fluid entities evolving from the medium coking coal. Partial demineralization of the high ash Indian non-coking coal to 17 wt% through a sink-float method did not decrease the complex viscosity of this coal but reduced the maximum mobility of the fluid H to levels observed with the lower ash content (20 wt%) Indian coal. Therefore, this reduction in mobility could be directly related to the mineral matter in the Indian non-coking coal.

Improved Slurryability and Rheological Characteristics of High Ash Indian Coal by Hydrothermal Treatment

ABSTRACT The crushed and sieved high ash coal samples are treated hydrothermally in a 15 l (liters) lab scale autoclave at a temperature of 394 K and residence times of 30, 45, 75 and 120 minutes. The hydrothermal treatment (HTT) of the coal samples leads to the increase in the carbon and fixed carbon fractions whereas a decrease in the oxygen and volatile fractions. A significant decrease in the maximum settled concentration is observed upon HTT and settling time increases by ~48%. All the suspensions behave like a Newtonian fluid up to a solid concentration of 30% (10–30%) and for the concentrations >40% (40–60%), the suspensions behave like a pseudoplastic or shear thinning fluid. The HTT leads to a significant improvement in the rheological characteristics of the CWS. The apparent viscosity shows an increase with solid concentration for raw as well as HTT coals. Whereas a decreasing trend is observed with treatment time and shear rate. The apparent viscosity shows a maximum improvement by 60.1, 50.71, 39.9, 50.55, 65.69 and 75.41% at 10, 20, 30, 40, 50 and 60% solid concentration, respectively. As similar characteristics are observed at residence times of 75 and 120 minutes, the optimum treatment time is observed to be 75 minutes.

Chemical Beneficiation of High-Ash Indian Noncoking Coal by Alkali Leaching under Low-Frequency Ultrasonication

Coal mined from the Talcher region of Odisha, India is known to be high-ash, surface-oxidized, and noncoking in nature. It is quite challenging to beneficiate such low-grade coal by physical or physicochemical processes due to its oxidized nature and the presence of complex ash forming mineral matter in its matrix. Chemical beneficiation is one of the alternative process to such problems. However, this chemical process consumes more chemicals, treatment time, and energy, which limits its application further. Therefore, an attempt has been made to chemically beneficiate this coal cost-effectively with optimum chemicals, treatment time, and energy. In the present study, an application of ultrasound at low frequency on alkali-acid leaching is employed to investigate on the demineralization of high-ash Indian noncoking coal. The raw coal properties such as fixed carbon content, CHNS content, Hardgrove grindability index, ash fusion temperature, and gross calorific value (GCV) were investigated before the expe...

CO2-O2 dry reforming based underground coal gasification using low and high ash Indian coals

CO2-O2 based underground coal gasification (UCG) is a promising technology for exploiting deep coal seams using a greenhouse gasification agent. High molecular weight hydrocarbons, aromatics, tar etc. in the form of volatile matters in a coal seam possess a significant calorific value. During UCG, these components get evolved as vapours and leave the coal seam without undergoing significant cracking and reforming reactions. As a consequence, a substantial amount of tar is produced along with syngas stream. Alternatively, a novel result of the present study shows that tar-CO2 based dry reforming reactions are catalysed by the pyrolysed carbon zones in the borehole at low temperatures and, these reactions enhanced the calorific value of syngas under a CO2 reactive atmosphere. In continuation of our earlier studies, a detailed experimental study is carried out under single and two stage gasification methods using a typical high ash (42% ash) Indian coal and a low ash (4% ash) North East Indian coal. The effect of feed gas flow rate, molar feed ratio and inherent ash content of the coals on the product gas composition is studied. The results show that two stage gasification of high ash coal produces a medium calorific syngas in the order of 189 kJ/mol, which is equivalent to the heating value of a steam-O2 based UCG syngas. Further, the present study shows that a syngas with a high calorific value of 265 kJ/mol can be produced using the low ash coal under two stage gasification mode. Also, the single stage gasification of low ash coal shows the feasibility of producing a syngas with a medium calorific value of 250 kJ/mol.

Gasification reactivity of high ash Indian coals in varying concentrations of CO&lt;SUB align="right"&gt;2

Optimisation of coal gasification processes requires a good understanding of coal reactivity of the CO2 and steam gasification reactions. Presently there are very few studies on gasification behaviour of high ash coals. Therefore, present paper investigates the gasification behaviour of char samples in 100, 70 and 30% concentrations of CO2 in the temperature range of 900–1050°C using thermogravimetric analyser. Char samples were prepared from high ash Indian coals in inert atmosphere at 800, 900 and 1,000°C. Effects of coal/char properties, char preparation temperatures, char gasification temperatures and varying concentrations of CO2 on gasification reactivity of high ash char samples have been reported. Higher pyrolysis temperature shows adverse effect on CO2 gasification, whereas higher gasification temperature, CO2 concentration, alkali index, surface area as well as porosity favour CO2 gasification reactions. [Received: February 27, 2017; Accepted: December 1, 2017]

Gasification reactivity of high ash Indian coals in varying concentrations of CO&lt;SUB align="right"&gt;2

Gasification reactivity of high ash Indian coals in varying concentrations of CO&lt;SUB align="right"&gt;2

CO2-air based two stage gasification of low ash and high ash Indian coals in the context of underground coal gasification

CO2-air is a potential gasifying-oxidizing medium for two stage mode of underground coal gasification (UCG). The existing literature deficits the UCG studies on the use of CO2-air as a gasifying medium especially for high ash Indian coals. Thus, the present study investigated the viability of utilizing the CO2-air as a gasifying medium for high ash Indian coals using a laboratory scale borehole gasification set-up in a two-stage gasification mode of operation. A typical Indian coal having 42% ash and a North East Indian coal having 4% ash are used for the borehole gasification experiments. The effect of the process parameters such as the molar ratio of O2/air and the molar ratio of CO2/oxidizing agent on the product gas composition is evaluated. The results show that the gasification of low ash coal and high ash coal produced product gas with a calorific value as high as 260 kJ/mol and 214 kJ/mol, respectively. CO2-air based gasification necessitates the presence of oxygen in the feed gas with O2/air ratio of 0.1 and 1 for the low and the high ash coal, respectively. Repeatability experiments show 7% and 14% error in the calorific values of syngas for low and high ash coals, respectively.

Analysis of integrated gasification combined cycle power plant incorporating chemical looping combustion for environment-friendly utilization of Indian coal

The concern on the effect of global warming due to greenhouse gases has been increasing. Carbon dioxide is the most powerful greenhouse gas and more than 40% of its emission is from fossil fuelled power plants. So the efficient use of fossil fuel with carbon dioxide capture is the best method for achieving the energy demands with less pollutant emissions. But the conventional capture technologies are more energy intensive and cause a drop in net power production. This energy penalty can be overcome by using Chemical looping combustion technique. In this technique, combustion occurs in two reactors – Air reactor and Fuel reactor. The former is fluidised by air and the later by fuel. Metal oxides called oxygen carriers loop in between two reactors carrying oxygen and heat required for combustion to the fuel reactor. So there is no direct contact between air and fuel and results in pure combustion products- carbon dioxide and water. Therefore a separate air separation or gas separation unit is not required which are energy eating units in conventional capture methods. This work presents steady state simulations of three systems: Integrated Gasification Combined Cycle power plant (i) without carbon dioxide capture, (ii) with pre-combustion capture and (iii) integrated with chemical looping combustion. All the simulations are carried out using Aspen plus simulation package. High ash Indian coal is used as the fuel. The performance of each case is compared in terms of overall energy efficiency and carbon dioxide capture rate. The efficiencies of the system without capture, with looping combustion and with pre-combustion capture are found to be 42.69%, 40.2% and 35.8% respectively. The respective capture efficiencies of looping combustion and pre-combustion capture are 99.97% and 94%. It can be seen that the reduction in the overall efficiency is marginal in the case of chemical looping combustion. This shows the superiority of chemical looping combustion technique over the pre-combustion capture technique in terms of higher capture efficiency and overall plant thermal efficiency. An exergy analysis is also carried out to identify the units having higher exergy destruction rates. Parametric studies are carried out on these units to observe their effect on the overall plant performance. Thus, a process for an energy efficient and environment friendly utilization of high ash Indian coal is presented in this paper.

Mathematical Modelling of Coal Gasification Processes

Coal is by far the most commonly employed fuel for electrical power generation around the world. While combustion could be the route for coal utilization for high grade coals, gasification becomes the preferred process for low grade coals having higher composition of volatiles or ash. Indian coals suffer from high ash content-nearly 50% by weight in some cases. Instead of transporting such high ash coals, it is more energy efficient to gasify the coal and transport the product syngas. Integrated Gasification Combined Cycle (IGCC) plants and Underground Gasification of coal have become attractive technologies for the best utilization of high ash coals. Gasification could be achieved in fixed beds, fluidized beds and entrained beds; faster rates of gasification are possible in fluidized beds and entrained flow systems, because of the small particle sizes and higher gas velocities. The media employed for gasification could involve air/oxygen and steam. Use of oxygen will yield relatively higher calorific value syngas because of the absence of nitrogen. Sequestration of the carbon dioxide after the combustion of the syngas is also easier, if oxygen is used for gasification. Addition of steam can increase hydrogen yield in the syngas and thereby increase the calorific value also. Gasification in the presence of suitable catalysts can increase the composition of methane in the product gas. Several competing heterogenous and homogenous reactions occur during coal major heterogenous reaction pathways, while interactions between carbon monoxide, oxygen, hydrogen, water vapour, methane and carbon dioxide result in several simultaneous gas-phase (homogenous) reactions. The overall product composition of the coal gasification process depends on the input reactant composition, particle size and type of gasifier, and pressure and temperature of the gasifier. The use of catalysts can also selectively change the product composition. At IIT Madras, over the last one decade, both experimental and modelling work has been undertaken to investigate the gasification characteristics of high ash Indian coals and compare the yield with those of high grade Australian and Japanese coals. A 20 kW capacity entrained flow gasifier has been constructed and the gasification characteristics have been studied for Indian coals for different particle sizes, system pressures and air flow rates. The theoretical model incorporates the effects of Knudsen diffusion, devolatilization and various heterogenous and homogenous kinetic steps as well as two-phase flow interactions involving the gaseous and particle phases. Output parameters such as carbon conversion, cold gas efficiency and syngas composition have been compared for different grades of coals under a wide range of operating conditions. The model developed for the entrained flow gasifier predicts the gasification characteristics of both Indian and foreign coals well. Apart from the entrained flow gasifier, a bubbling bed gasifier of 100 kW capacity has also been studied. A pilot plant for the gasification of Indian coals has been set up for this capacity and its performance has been investigated experimentally as well as theoretically at different air and steam flow rates. Carbon conversion efficiency of more than 80% has been achieved.

Evaluation of feasibility of pelletized wood co-firing with high ash Indian coals

To reduce anthropogenic CO2 emissions from power plants, biomass is an immediate alternative fuel which has similar properties as coal. In this regard, the present study discusses about pelletized wood (PW) co-firing with high ash Indian coal by conducting co-milling and co-firing trials in a 1000 kg/hr of pilot scale test facility. Indian coals are typically high ash content and low calorific value fuels, therefore, its interaction with coal during combustion and ash deposition have studied in detail. Based on co-milling trails of PW and coal, it was observed that as PW proportion in coal increases, the quantity of particles of size below 50 μm and as well above 500 μm were increased. From co-firing studies, it was observed that higher volatile content in PW helping in stabilizing flames while co-firing. At lower proportions, up to 10% weight PW co-firing with coal, the flame temperature and heat flux values are very close to base test of 100% coal firing. However, beyond 10% by weight of PW co-firing with coal, the flame temperature and heat flux values were increased significantly from 100% coal tests. This is because of higher calorific value of PW than coal. The CO emission was decreased with increase in PW proportion in coal but at 30% of PW in coal, CO emission was increased suddenly. However, NO and SO2 concentrations were decreased up to 8% and 16% respectively with increase in PW proportion in coal due to lower fuel nitrogen and sulphur content in PW than coal. Analytical analysis of slagging indices suggest that the slagging potential for PW co-firing with coal is increasing as the PW proportion in coal increases.

3-E analysis of a Pressurized Pulverized Combined Cycle (PPCC) power plant using high ash Indian coal

3-E (Energy, Exergy, and Environment) analysis of Pressurized Pulverized Combined Cycle (PPCC) SubCritical (SubC) and SuperCritical (SupC) power plants of net capacity 400 MWe using High Ash (HA) Indian coal under Indian ambient conditions is performed to assess the potential of PPCC power plant for electricity generation in Indian energy sector. The study also includes the comparative performance of the plant using an imported Low Ash (LA) coal and a parametric study is performed to understand the effect of various parameters affecting the energy and exergy efficiencies of the PPCC plant. It is found that the plant energy efficiency of PPCC SubC and SupC plants using HA coal are 42.44% and 43.46%, respectively. The exergy efficiency of the same plant using HA coal are 38.94% and 39.87%, respectively. The energy balance shows that the maximum energy loss is observed in cooling water (24%) followed by loss in stack (22%). The exergy balance shows that the maximum exergy destruction in combustor (32%) followed by stack (7%). The environmental analysis reveals that the CO2, NOx, and SOx emissions are 426, 3.54, and 3.20 g/kWh, respectively.

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.

Chemical demineralization of high ash Indian coal by using alkali and acid solutions

In the present work an attempt have been made to upgrade the low grade Indian coal to high grade coal by using caustic (NaOH) leaching followed by acid (H2SO4) treatment. The experimental works aimed at a selective attack on in situ MCL (Mahanadi Coal Limited) coal, Odisha, India with some fluxing agents like NaOH and H2SO4. The leaching experiments were carried out in a stainless steel batch reactor of 2.5L capacity with controlled heating facility. Initially, the coal sample with −16+100mesh particle size was leached with 50–150g/L of NaOH solution at 100°C for one hour and then subjected to leach further with 20% sulfuric acid. The maximum demineralization from the level of 26% ash with NaOH (100g/L) leaching 27.5% is achieved and with combined chemical treatment method (100g/L NaOH followed by 20% H2SO4) was obtained 48% effectively. In addition, the results showed that the percent of demineralization improved adequately by decreasing the particle size also with rise of the leaching time and temperature. Coal characterization of raw and treated coal was done by Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) analysis and also showed that the calorific value of coal was increased with increase the percent of demineralization of coal.

Gasification of high-ash Indian coal in bubbling fluidized bed using air and steam – An experimental study

In this experimental study, gasification characteristics of Indian coals having high ash content (>35%) in a bubbling fluidized bed reactor, using air and steam as the gasification agents under atmospheric pressure, are presented. The thermal power output of the gasification device has been varied in the range of 80–100kW. The ratio of the actual mass flow rate of air supplied to the stoichiometric air mass flow rate (required for complete combustion) has been varied in the range of 0.25–0.35. The ratio of mass flow rate of steam to that of coal has been varied from 0.15 to 0.25. Effects of air and steam flow rates on the calorific value of product gas, carbon conversion and cold gas efficiency have been studied in detail. For high-ash Indian coals, supply of steam produces positive effect as all the metrics such as calorific value of product gas, carbon conversion and cold gas efficiency increases with increase in the steam supply rate.

Artificial neural network approach for rheological characteristics of coal-water slurry using microwave pre-treatment

Detailed experimental investigations were carried out for microwave pre-treatment of high ash Indian coal at high power level (900W) in microwave oven. The microwave exposure times were fixed at 60s and 120s. A rheology characteristic for microwave pre-treatment of coal-water slurry (CWS) was performed in an online Bohlin viscometer. The non-Newtonian character of the slurry follows the rheological model of Ostwald de Waele. The values of n and k vary from 0.31 to 0.64 and 0.19 to 0.81Pa⋅sn, respectively. This paper presents an artificial neural network (ANN) model to predict the effects of operational parameters on apparent viscosity of CWS. A 4-2-1 topology with Levenberg-Marquardt training algorithm (trainlm) was selected as the controlled ANN. Mean squared error (MSE) of 0.002 and coefficient of multiple determinations (R2) of 0.99 were obtained for the outperforming model. The promising values of correlation coefficient further confirm the robustness and satisfactory performance of the proposed ANN model.

Effect of some operating variables on the performance of a 150 mm Heavy Medium Cyclone treating high ash Indian Coking Coal

Tests were carried out on a 150 mm heavy medium cyclone test rig treating coal in the size range - 13 +0.5 mm. A total of twenty seven experiments were carried out by varying the parameters like vortex finder diameter, spigot diameter and inlet feed pressure. The effect of these variables on the performance of the 150 mm Heavy Medium Cyclone was investigated. The results indicate that it is possible to achieve about 30.1% clean coal at an ash content of 18.5% at a feed pressure of 10 lb/in2, vortex diameter of 63.75 mm and spigot diameter of 31.5 mm. The tromp curves obtained from operating the cyclone gave a probable error of 0.04, indicating good separation efficiency.

Effect of some operating variables on the performance of a 150 mm Heavy Medium Cyclone treating high ash Indian Coking Coal

Tests were carried out on a 150 mm heavy medium cyclone test rig treating coal in the size range - 13 +0.5 mm. A total of twenty seven experiments were carried out by varying the parameters like vortex finder diameter, spigot diameter and inlet feed pressure. The effect of these variables on the performance of the 150 mm Heavy Medium Cyclone was investigated. The results indicate that it is possible to achieve about 30.1% clean coal at an ash content of 18.5% at a feed pressure of 10 lb/in2, vortex diameter of 63.75 mm and spigot diameter of 31.5 mm. The tromp curves obtained from operating the cyclone gave a probable error of 0.04, indicating good separation efficiency.

Evaluation of pneumatic inclined deck separator for high-ash Indian coals

Application of pneumatic separators in coal beneficiation is increasing rapidly over the last decade primarily due to their low capital and operating costs, and waste handling problems associated with traditional wet processing methods. Large amount of shale/rock that is extracted in coal production can be removed prior to transportation at the mine face by using this methodology. Due to the limited washing facilities in India, most of the thermal power plants burn raw coal from run-of-mine (ROM) to generate electricity. This practice causes poor utilization efficiency, high operating and maintenance costs, and high emission rates for the power plants. One potential method that can be utilized is the air-fluidized inclined vibrating deck technology. The technology was demonstrated on a pilot-scale at different coal washeries in India at a feed rate of 5-ton per hour. The pilot-scale evaluation showed that 20 %–25 % high-ash incombustible material can be eliminated from ROM feed with only minor losses in energy content (<10 %) from respective ROM coal. Furthermore, a feasibility analysis showed significant economic gains in terms of transportation cost, improving power-plant efficiency, and reducing emissions rates by using the technology.

Statistical optimization study of jigging process on beneficiation of fine size high ash Indian non-coking coal

Non-coking coal is used for metallurgical and cement industries apart from generating energy. Indian non-coking coals are high in ash content because of drift origin. These high ash coals require beneficiation before being used. Jigging is one of the unit operations used for beneficiation of coal. Beneficiation by jigging is carried out for the coarser size of particles. Jigging of fine size coal is limited. However, in present study two fine non-coking coal samples having sizes −3+2mm and −2+1mm were used for jigging study. In present study jigging experiments were performed using laboratory Denver mineral jig by varying feed size, water rate, and feed rate. 23 full factorial experimental design was used to study the performance of jigging operation. The performance of jigging was judged by statistical analysis; where ash content and combustible recovery of concentrate were considered to be responses. In addition to statistical analysis, optimization study was also carried out by Nelder–Mead multidimensional pattern search method. Statistical models developed in the present study could predict ash content and yield of the concentrate accurately. At optimized condition, it was possible to achieve 22.6% ash content with 64.15% combustible recovery.

Organo-refining of high-ash Indian coals at bench scale

Organo-refining is a process of solvent extraction through which organic matters are extracted from coal using organic solvents. Indian coals have high mineral matter content ranging from 25 to 45wt.% (dry basis). The ash in coal is highly disseminated, which makes it difficult to remove by physical beneficiation. Thus, chemical beneficiation is needed to remove the mineral matter. This study was conducted for high ash Indian coals at bench scale. The capacity of the bench scale plant was 40kg/batch feed basis. Coking coal, washery discards and thermal coals of bituminous origin were used as feed material. The mineral matter of these coals varied in the range of 27–36wt.% (db). The feed size of the particle was below −0.5mm. N-methyl-2-pyrrolidone (NMP) and ethylenediamine (EDA) were used as main solvent and co-solvent respectively. The extraction was done at 200°C with 1:6 coal to solvent ratio (by mass, wt./wt.) for 60min. Extraction was followed by solid–liquid separation, solvent recovery and washing of products. The feed and product materials were subjected to proximate analysis, ultimate analysis, and Free Swelling Index (FSI) tests. Organo-refining process produced clean coal yield in the range of 30–45% and clean coal mineral matter was less than 10% for coking as well as non-coking coals. Free Swelling Index (FSI) also improved from 1–5 range for feed coals to 7–8 for product clean coals. Two types of filter (basket and centrifuge) were also tried for separation of solid and liquid from different types of slurries. It was observed that cake moisture can be reduced by 6–12% points for these slurries using decanter centrifuge. The cake moisture reduced to 70% from 76% for clean coal slurry and 60% to 48% for reject slurry. The clean coal yield increased by 5 units (from 35% to 40%) with decanter centrifuge filter. In order to have better filtration and precipitation, the extraction was carried out by mixing of two different raw coals (TL and TC) in a certain ratio (1:1 to 1:3). Also, the extraction was done separately for TL and TC coals and then the clean coal extracts were mixed in 1:1 to 1:3 proportions. Mixing of two raw coals gives better results (36wt.% yield with 6.4% ash clean coal) than mixing of their extracts (30wt.% yield with 6.7% ash clean coal).