Low-ash Coal Research Articles

Thermodynamic evaluation of integrated gasification combined cycle: Comparison between high-ash and low-ash coals

Summary In the present study, a coal-integrated gasification combined cycle power plant is simulated. A high-ash coal and low-ash coal are considered to compare the performance of the plant. The combined cycle is in typical commercial size with 450 MW capacity. The feeds are Tabas and Illinois #6 coals which approximately contain more than 30% and 10% ash and have higher heating values of 22.7 MJ/kg and 26.8 MJ/kg, respectively. Energy and exergy analyses are done by aspen plus® and ees, respectively. Energy analysis shows that the overall efficiencies of power plants using high-ash and low-ash coals are 33% and 28%, respectively. The result shows that in high-ash case, 52 kg/s coal, 10 kg/s water, and 1050 kg/s air and in low-ash case, 48 kg/s coal and 820 kg/s air are required for providing mentioned power, approximately. Exergy analysis shows that maximum exergy destruction is in heat recovery steam generator unit. Investigating the emissions shows that high percent of ash in the coal composition has slight effects on the IGCC pollution. Finally, from thermodynamic viewpoint, it is concluded that the high-ash coal, like the conventional one, can be used as thermally efficient and environmentally compatible feed of IGCCs. Copyright © 2016 John Wiley & Sons, Ltd.

Sorbent production from low-ash brown coal

The process for the production of a sorbent upon the carbonization of low-ash brown coal in a fluidized bed in a temperature range of 700–850°C was studied. The dependences of the main characteristics of the sorbent on temperature and the fraction of processed coal were found. The characteristics of the lignite sorbent obtained by the TERMOKOKS-KS technology are presented.

Numerical modeling of an E-Gas entrained flow gasifier to characterize a high-ash coal gasification

In this paper, a three-dimensional numerical simulation was carried out to characterize a high-ash coal gasification in a pressurized, oxygen-blown, two-stage E-Gas entrained flow gasifier. The Tabas coal was used as a high-ash feedstock, with 32wt% ash and was compared with the Illinois #6 low-ash coal, with 10wt% ash. The computational model was conducted in ANSYS FLUENT 14 based on the conservation equations described in the Eulerian–Lagrangian approach. Also, four heterogeneous combined with seven homogeneous reactions were solved via the finite rate and eddy-dissipation/finite rate model, respectively. Two different parametric analyses were conducted to characterize the gasification process in the E-Gas gasifier. These analyses cover the effects of the O2/Coal ratio and coal slurry water content on the gasifier performance characteristics such as the exit temperature, main species mole fractions, carbon conversion, high heating value, yield syngas fraction and cold gas efficiency. It was found that using the higher ash content coal decreases the cold gas efficiency about 5%, limits the gasifier economical operating range, and reduces the exit temperature, yield syngas and total higher heating value.

Fluidization characteristics and density-based separation of dense-medium gas–solid fluidized bed: An experimental and simulation study

Experimental and simulation approaches were combined to investigate the fluidization characteristics of a dense-medium gas–solid fluidized bed (DMGFB) for coal cleaning. Results indicated that the static bed height should be adjusted by maintaining a stable height of 150–300 mm to avoid its harmful effect on fluidization stability. Bed pressure drops presented favorable stability with slight fluctuations at a superficial gas velocity of 11.68 cm/s (1.6 Umf). The uniform density distributions with slight variations throughout the whole bed were verified by comparing the experimental, simulated and calculated results. A fluidization index, Umb/Umf, was used to conduct stability-region analysis for DMGFB. On the basis of fluidization stability, a successive beneficiation flowsheet via a DMGFB separator was used to perform density-based separation of 6–50 mm sized run-of-mine coal. Results demonstrated that the low-ash clean coal with a yield of 61.36% was efficiently obtained with the ash content reduced from 39.64% to 10.18% and sulfur content rejected from 1.36% to 0.85%. Probable error E values were 0.06 and 0.075 g/cm3 in high- and low-density separation stages, indicating efficient separation performance. The dry coal beneficiation technique has great potential for utilization in arid and water-deficient areas and countries.

Characterization of Some Nigerian Coals for Power Generation

Five coal samples from Odagbo (Kogi State), Owukpa (Benue State), Ezimo (Enugu State), Amansiodo (Enugu State), and Inyi (Enugu State) of Nigerian coal deposits were subjected to proximate analysis, ultimate analysis, calorific value determination, and petrographic and thermogravimetric analysis to determine their suitability for power generation. Based on results of tests carried out, Amansiodo coal is a bituminous, low sulphur, and medium ash coal, while Owukpa coal is a subbituminous A, low sulphur, low ash coal rich in huminites, Odagbo coal is a subbituminous B, medium sulphur, low ash coal rich in huminites, Ezimo coal is a subbituminous C, low sulphur, high ash coal, and Inyi coal is a subbituminous C, low sulphur, high ash coal. Between Odagbo and Owukpa subbituminous coals, Owukpa has a lower ignition temperature (283.63°C) due to its higher volatile matter content (39.1%). However, Ezimo subbituminous coal, which has a lower volatile matter (31.1%), unexpectedly has the same ignition temperature as Owukpa (283.63°C) due to its higher liptinite content (7.2%) when compared with that of Owukpa (2.9%). The ease of combustion of the coal samples in decreasing order is Odagbo < Owukpa < Inyi < Ezimo < Amansiodo.

Geological controls on mineralogy and geochemistry of the Late Permian coals in the Liulong Mine of the Liuzhi Coalfield, Guizhou Province, Southwest China

This paper investigates the mineralogical and geochemical compositions of the Late Permian coals in the Liulong Mine, Liuzhi Coalfield, Western Guizhou Province, China, by using XRD, SEM-EDX, ICP-AES and ICP-MS. Both the No.3 and No.7 Coals are classified as low volatile bituminous coal. However, the No.3 Coal is a medium-ash (29.8% on average) and high-sulfur (4.76% on average) coal while the No.7 Coal is a low-ash (12.4% on average) and medium-sulfur (2.16% on average) coal. Major minerals in both coal seams include quartz, pyrite, marcasite, kaolinite, calcite, and to a lesser extent, gypsum and anatase. Illite, chlorite, rectorite, albite, ankerite and siderotil are only present in the No.3 Coal. The No.3 Coal contains higher elemental concentrations than the No.7 Coal probably due to the higher mineral contents in the No.3 Coal, and is enriched in Nb and Ta, and to a lesser extent in K, Ti, Li, V, Co, Cu, Sr, Zr, Mo and Ba.The mineralogical and geochemical data presented in this study have shown that the No.3 Coal was formed under a higher degree of marine-influenced coal-forming environment than the No.7 Coal. However, the both Coals have the Kangdian Upland (made of mafic igneous rocks, such as basalt rocks) as sediment-source region like other coals from Guizhou Province. Three geological factors control the geochemical patterns of the No.3 and No.7 Coals, namely: (1) the high detrital input in the No.3 Coal causing high ash yields; (2) the sediment-source region (Kangdian Upland basalt) causing high Ti, Nb and Ta contents, and a minor proportion of Co, Cr, V and Cu, when combined with the high detrital input in the No.3 Coal; (3) the higher degree of marine influenced depositional environment in the No.3 Coal causing a high S, Sr and Ba contents.

Mineralogy of Furnace Deposits Produced by South African Coals during Pulverized-Fuel Combustion Tests

Pilot-scale combustion tests were carried out on coals with low (13.5%) and high (39.6%) ash percentages from the Highveld Coalfield of South Africa, using the ALS-ACIRL 100–200 kW Combustion Test Facility. The mineral matter in the feed coals and also the chemistry and mineralogy of the ash deposits from different parts of the test furnace were analyzed using quantitative X-ray diffraction and X-ray fluorescence techniques, and the results were used to develop an understanding of the mineralogical processes taking place in the different parts of the pulverized-fuel combustion system. Low-temperature oxygen-plasma ashing and quantitative X-ray diffractometry showed that the mineral matter of both coals contained abundant kaolinite, lesser but still very significant proportions of quartz and carbonates (ankerite and calcite), and minor proportions of pyrite and other minerals. A small but significant proportion of nonmineral Ca also occurs in the organic matter, especially of the low-ash coal. The chemical composition of the combustion products taken from the different sampling points in the test facility (slagging panels, furnace ash, and fouling probes, etc.) was overall very similar for each coal sample. However, the percentages of the different crystalline minerals in the combustion products showed a wide range of variation at the different sampling points. Anorthite, derived from interaction between Ca and the aluminosilicate components, was formed from both coals at temperatures above 1300 °C, mostly as part of sintered aggregates that built up and became detached from the slagging panel in the highest temperature part of the combustion system. Anhydrite was formed by interaction of Ca with SO2; lime and periclase were also formed from the low-ash coal, where insufficient sulfur was available for complete sulfate development.

Effect of fuel origin on synergy during co-gasification of biomass and coal in CO2

The effect of fuel origin on synergy in coal/biomass blends during co-gasification has been assessed using a congruent-mass thermogravimetry analysis (TGA) method. Results revealed that synergy occurs when ash residuals are formed, followed by an almost complete gasification of biomass. Potassium species in biomass ash play a catalytic role in promoting gasification reactivity of coal char, which is a direct consequence of synergy during co-gasification. The SEM–EDS spectra provided conclusive evidence that the transfer of potassium from biomass to the surface of coal char occurs during co-pyrolysis/gasification. Biomass ash rich in silica eliminated synergy in coal/biomass blends but not to the extent of inhibiting the reaction rate of the blended chars to make it slower than that of separated ones. The best result in terms of synergy was concluded to be the combination of low-ash coal and K-rich biomass.

From coal towards renewables: Catalytic/synergistic effects during steam co-gasification of switchgrass and coal in a pilot-scale bubbling fluidized bed

Recent environmental sharp curbs on fossil fuel energy systems such as coal power plants due to their greenhouse gas emissions have compelled industries to include renewable fuels. Biomass/coal co-gasification could provide a transition from energy production based on fossil fuels to renewables. A low-ash coal and switchgrass rich in potassium were selected on the basis of previous thermogravimetric studies to steam co-gasify 50:50 wt% coal:switchgrass mixtures in a pilot scale bubbling fluidized bed reactor with silica sand as the bed material at ∼800 and 860 °C and 1 atm. With the switchgrass added to coal, the hydrogen and cold gas efficiencies, gas yield and HHV of the product gas were enhanced remarkably relative to single-fuel gasification. The product gas tar yield also decreased considerably due to decomposition of tar catalyzed by switchgrass alkali and alkaline earth metals. Switchgrass ash therefore can act as inexpensive natural catalysts for steam gasification and assist in operating at lower temperatures without being penalized by an increase in product tar yield. An equilibrium model over-predicted hydrogen and under-predicted methane concentrations. However, an empirically kinetically-modified model was able to predict the product gas compositions accurately.

The geochemistry and mineralogy of high-S coals, with examples mainly from the East Pennines Coalfield in Yorkshire and Nottinghamshire, UK: an overview

Over a number of years, coals have been analysed in the geochemical laboratories of the University of Sheffield. Much of this work has been published and this paper presents an overview and, in some cases, a reinterpretation of the data. Most of the coal samples were obtained from seam sections in the Yorkshire and Nottinghamshire coalfields (now included in the East Pennines Coalfield), and from the Parkgate Coal in particular. Sulphur variations are essentially due to pyrite. In individual plies of the seam, lateral variation in total sulphur is thought to be due to the availability of sulphate at the time of deposition. Access of sulphate into the coal swamp environment can be directly related to river channels, many of which are known to have been long-lived features. The pyrite is of concern because of S emissions, but in addition it is the location in the coal for Hg, Tl, Pb, As and Se, all of which are of major environmental concern. Concentrations were determined by direct and indirect methods. The most common of the latter methods is statistical, and some of the potential problems are reviewed. Also contained within pyrite, but not exclusively so, are Cd, Ni and Sb. Trace element concentrations from different pyrite samples are similar, strongly suggesting a common source. Comparable trace element enrichment patterns for coals and marine shales provide evidence of a seawater origin. The chloride concentration in the coals is high and can also be linked to seawater. The two elements, S and Cl, differ in that Cl is conservative, is not involved in diagenetic reactions and is retained in the coal in residual pore fluids. Illite and kaolinite are the main clay minerals present, with subsidiary chlorite and mixed-layer illite-smectite. In most coals, the clay minerals are dominantly detrital, but in low ash coals, kaolinite of diagenetic origin can be detected. In some samples from the Barnsley Coal, the source of this kaolinite, based on the trace element geochemistry, is dispersed volcanic ash (basaltic). It is also highly likely that organic matter contributes small amounts of Si and Al in the coal, and phytoliths are one potential source. However, the detrital sediment is a major contributor to the major and trace element geochemistry of the coals. Trace elements with a dominantly detrital origin are Rb, Cs, Cr, Ga, Th, Nb and Li. Trace elements are also associated with the organic matter to a greater or lesser extent. Germanium is mainly associated with organic matter, but V is also present in the clays. These elements are mainly contained in vitrinite, although Ge concentrations are more variable, suggesting some mobility during diagenesis. This is borne out by Ge, V and Be showing enrichment at the margins of the the Parkgate Coal, following ‘Zilbermints Law’. Mobility during diagenesis is also demonstrated by the mineral infill of the cleat (joints). Stages of cleat mineralization have been recognized and related to diagenetic stages in siliciclastic sequences. Although there is evidence of element mobility after deposition in the coals, this is not thought to be related to the mineralizing fluids in the Pennine Orefields.

Water-Carrying Properties of Flotation Frothers and Its Effect on Fine Coal Flotation

The water-carrying property of three flotation frothers (1-Pentanol, Methyl iso-butyl carbinol (MIBC), and 2-Octanol) was investigated by water-carrying rate. In this work, the performance of frothers in a three-phase coal flotation was evaluated and related to the water-carrying property in a two-phase system. The frother having a high water-carrying rate, 2-Octanol, shows overall faster kinetics and less selectivity for each size fraction due to entrainment through water. The frothers having a low water-carrying rate were more selective for producing low-ash content coal, such as 1-Pentanol and MIBC. MIBC showed better flotation performance for all size fractions in terms of kinetics and selectivity. The three frothers present good correlation between the water-carrying property and fine coal flotation performance.

Paleoenvironment of the Eocene coal seam in the Fushun Basin (NE China): Implications from petrography and organic geochemistry

The Fushun Basin in northeast China is a strike–slip basin filled with coal-bearing Eocene sediments. The basin fill includes from base to top volcaniclastic rocks, a 20 to 145m thick coal seam (Lower Eocene Guchengzi Formation) and lacustrine oil shale, up to 300m thick (Jijuntun Formation). The sub-bituminous coal of the Guchengzi Formation, famous for the amber inclusions, is exploited in the largest opencast coal and oil shale mine in Asia. In the present contribution the depositional environment and the thermal history of the Guchengzi Formation are investigated based on macro- and micropetrographic data, bulk geochemical parameters, biomarker analysis, stable isotope geochemistry, and vitrinite reflectance measurements. According to our interpretation, accumulation of the low-sulfur coal commenced in a low-lying mire near a shallow lake. Subsequently low-ash coal was deposited either in a low-lying mire protected from clastic influx or in a raised mire. Finally, high subsidence rates, typical for strike–slip basins, caused the flooding of the mire and terminated peat accumulation. Pristane/phytane ratios suggest a gradual change from oxic to less oxic conditions during the drowning phase of the seam. Organic-rich shales accumulated in the newly formed freshwater lake and resulted in the deposition of the overlying oil shale-bearing Jijuntun Formation. The coal is rich in liptinite macerals, especially sporinite. Gymnosperms dominated the paleovegetation of the mire, something that is also reflected by the frequent occurrence of resinite and amber and their biomarker composition. The coal reached the sub-bituminous or even the high volatile bituminous stage, despite the limited burial depth. Simple 1D models suggest that this is a result of high heat flow, rather than of deep burial and erosion. The hydrogen-rich coal is oil-prone.

Utilizing an Air-Dense Medium Fluidized Bed Dry Separating System for Preparing a Low-Ash Coal

An industrial-scale air-dense medium fluidized bed was used to prepare a low-ash coal from a feed coal that was difficult to wash. The low-ash coal provided raw material for preparing low-ash activated carbon. A new fluidized bed density control method for preparing a low-ash coal was proposed, which effectively reduced bed density fluctuation, thus maintaining the uniformity and stability of the bed density. A separation density of 1.42 g/cm3 was maintained in the air-dense medium fluidized bed to produce a clean coal with an ash content of 3.71%, with an E P value of 0.055. Results showed that the air-dense medium fluidized bed system is an efficient dry separation method for difficult-to-wash coal.

Technology for Producing Crystalline Silicon with the Use of Non-Traditional Reducing Agents

The process of reducing silicon from SiO2 includes a stage that involves the formation of a gas phase which contains lower oxides of silicon and carbon. The primary product is silicon carbide, which is formed from the reaction of gaseous silicon oxide with carbon on the surface of the reducing agent. It is shown that it is possible to obtain crystalline silicon which meets the requirements of the standard by using quartz from the Karaganda deposit and employing a reducing agent comprised of special low-ash coke and long-flame coal. A distinguishing feature of the new technology is the replacement of costly charcoal and petroleum coke with new, unconventional reducing agents – the special low-ash coke and long-flame coal from the Shubarkol field in Kazakhstan.

SCADA-based control system of pilot plant for chemical leaching of high ash Indian coals

The chemical leaching process of coal opens up many opportunities as it can produce very low ash coal without deteriorating the yield and product quality. In this context, as the chemical leaching process is influenced by many factors such as mineralogy of feed coal, reaction temperature, reagent concentration, solid-to-liquid ratio and reaction time, etc., the maximum ash reduction with minimum operating cost is not practically possible without the assistance of efficient control system. In this paper, the various steps involved in the design and development of the SCADA-based control for chemical leaching process has been discussed. The control system implemented at pilot plant with the production capacity of 500 kg coal per batch is strong and efficient in regulating the process parameters, operating the plant and acquiring the valuable data for further scale-up. The ROCKWELL-based Control-Logix PLC and the state-of-art instruments have been used to accomplish the task.

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.

The Effect of Advanced Steam Parameter-Based Coal-Fired Power Plants With Co2 Capture on the Indian Energy Scenario

This study deals with the performance analysis of 500 MWe pulverized coal-fired power plants based on subcritical (SubC), supercritical (SupC), and ultrasupercritical (USC) steam parameters with postcombustion CO2 capture using high ash (HA) Indian coal. The performance of power plants is evaluated in terms of plant energy efficiency and CO2 avoided. Furthermore, the effect of an imported low ash (LA) coal on power plant performance parameters is also determined compared with the HA Indian coal. SupC and USC power plants result in an increase of about 0.8% and 6.4% points, respectively, in the plant energy efficiency compared with the SubC power plant. Consequently, a reduction of about 3% and 16.5% in CO2 emission is observed for the same plants using both HA and LA coals that, in turn, will aid in significant gain in carbon credits for a developing country such as India. However, incorporation of postcombustion CO2 capture unit in the proposed power plant configurations result in a significant drop of about 8%--13% points in the plant energy efficiency.

Study of Hydrodynamics and Dry Beneficiation Characteristics of Coal in the Riser of a Circulating Fluidized Bed Reactor

Dry beneficiation is an effective way of reducing the ash content of coal especially in the water scarce region. Fast fluidization region was used to carry out some exploratory investigations with raw coal to reduce the ash below a 12% level from a feed ash of 43%. As the coal was fluidized in the riser of a fast fluidized bed, the heavier and coarse particles generally settled at the bottom in a dense zone of the riser while the finer and lighter particles were entrained to the top dilute zone and, using a cyclone, were fed back to the bottom of the circulating fluidized bed (CFB) riser column. Most of the low-ash fine-size coals being lighter moved up to the riser, where most of the beneficiation occurred.

Evaluation of combustion characteristics in thermogravimetric analyzer and drop tube furnace for Indian coal blends

Availability of low ash coals is diminishing in India very fast. As such, trend of utilization of high-ash coals by blending with low ash coals is rapidly growing in Indian power plants with the growing demand of coal and with emerging environmental strictures to reduce green house gas emission and to restrict the ash limit to a targeted value. Quick decision on acceptance of certain blends in power stations very often leads to various operational problems/uncertainties. To address such uncertainties in coal combustion, three blend combinations of high-ash–low-ash coals have been examined in this study through lab- and bench-scale combustion experiments. Combustion parameters are found to be mostly non-additive. Both synergistic and anti-synergistic combustion characteristics were noticed at different combustion-stages. This study reveals necessity of pre-assessment of certain blends through lab/bench scale studies before those are adopted in power plant. This work also attempts estimation of low rank intertinite (i.e., reactive component of inertinite having Rr ≤ 1.30) followed by ‘total reactive content’ for prediction of burn out characteristics in TGA/DTF. Here, total reactive content is the sum of low rank inertinite and other traditional reactive macerals viz., vitrinite, semi-vitrinite and liptinite. Interestingly, satisfactory correlations between ‘combustion parameters’ and the ‘total reactive maceral content’ were found.

Chemical Leaching of High-Ash Indian Coals for Production of Low-Ash Clean Coal

It is difficult to reduce the ash content in Indian coals below a certain level by conventional physical beneficiation techniques due to their poor washability characteristics. In the present work, the effect of aqueous alkali leaching at elevated temperature, followed by acid washing on the removal of mineral matter, was evaluated for different captive coals of Tata Steel. The research study revealed that the ash content of all these coals could be reduced by more than 50% using this method. The degree of demineralization improved by increasing the reaction time, alkali concentration, and temperature, and by reducing the coal particle size. No significant change was observed in the alkali content, whereas silica, alumina, and phosphorous content reduced significantly after the final acid treatment. The mechanism of demineralization was evaluated by analyzing the coal samples before and after the alkali and acid treatments using X-ray diffraction (XRD) techniques. Overall, this study provides an insight into possible alternative methods of beneficiation for removal of ash from physically beneficiated high-ash Indian coals.