Low-temperature Ashing Research Articles

Low temperature combustion and ash deposition characteristics of palm oil and forest replanting waste

This study evaluation and analysis the low-temperature combustion characteristics and ash deposition of palm oil waste, including palm kernel shell (PKS) and empty fruit bunch (EFB), as well as forestry waste in the form of rubber wood (RBW) and replanting waste (RPW). The experiments were conducted using a drop tube furnace at 900 ᵒC, which is considered a low temperature compared to the operating temperature of pulverized coal. The thermogravimetric method (TGA) was applied to understand the combustion properties of the biomass, while the ash deposition tendency was assessed through empirical calculations, combustion experiments, and direct observation of the ash produced. The results showed that RBW had the best combustion performance compared to the other samples. Although RPW has the highest ash fusion temperature value among all samples, with a value above 1200 ᵒC, the low-melting-point mineral phase such as fairchildite, produced at low-temperature combustion at 24.74%, increases the risk of ash sticking to metal surfaces. PKS and EFB produce very high albite mineral content of more than 60%, which has a low melting point and could potentially cause ash deposition problems. On the other hand, RBW, with a dominant content of SiO2 (21.79%), Al2O3 (20.24%), and MgO (8.03%), produces minerals dominated by quartz at up to 78% and magnesioferrite at more than 5%, both of which have a high melting point that tends to produce harmless ash. These findings provide insight into the potential of utilizing biomass in low-temperature combustion processes, which is a way of reducing waste and lowering greenhouse gas emissions.

Trade-off between Pore-Throat Structure and Mineral Composition in Modulating the Stability of Soil Organic Carbon.

The preservation of soil organic carbon (OC) is an effective way to decelerate the emission of CO2 emission. However, the coregulation of pore structure and mineral composition in OC stabilization remains elusive. We employed the in situ nondestructive oxidation of OC by low-temperature ashing (LTA) combined with near edge X-ray absorption fine structure (NEXAFS), high-resolution microtomography (μ-CT), field emission electron probe microanalysis (FE-EPMA) with C-free embedding, and novel Cosine similarity measurement to investigate the C retention in different aggregate fractions of contrasting soils. Pore structure and minerals contributed equally (ca. 50%) to OC accumulation in macroaggregates, while chemical protection played a leading role in C retention with 53.4%-59.2% of residual C associated with minerals in microaggregates. Phyllosilicates were discovered to be more prominent than Fe (hydr)oxides in C stabilization. The proportion of phyllosilicates-associated C (52.0%-61.9%) was higher than that bound with Fe (hydr)oxides (45.6%-55.3%) in all aggregate fractions tested. This study disentangled quantitatively for the first time a trade-off between physical and chemical protection of OC varying with aggregate size and the different contributions of minerals to OC preservation. Incorporating pore structure and mineral composition into C modeling would optimize the C models and improve the soil C content prediction.

Clean and efficient separation of Pb-Sb alloy by vacuum gasification - Fractional condensation

The Lead (Pb) - Antimony (Sb) alloy derived from Jamesonite (Pb4FeSb6S14) using the "Roasting - Reduction Smelting" method is typically isolated through a separation method known as "High Temperature Ash Blowing - Low Temperature Ash Blowing - Electrolytic Refining". However, the "Low Temperature Ash Blowing" separation method encounters several challenges, including elevated production pressure of refined Pb, significant emissions of smoke and dust, and suboptimal separation efficiency. This study utilizes the "Vacuum Gasification - Fractional Condensation" method as an alternative to the "Low Temperature Ash Blowing" method. The experimental findings demonstrate that by implementing specific optimal experimental conditions, such as a condensation temperature range of 904–1054K, a vacuum degree of 5Pa, a gasification temperature of 1173K, a holding time of 7h, and a height-diameter ratio of 0.27, condensates with Sb content exceeding 99.5 wt% can be procured, resulting in a direct Sb yield of 33 %. The remaining residues, with an Sb content ranging from 14 to 15 wt%, serves as a valuable input for the "Electrolytic Refining" process. This utilization significantly alleviates the burden on refined Pb production during the "Electrolytic Refining" process. The treatment procedure does not require the utilization of chemical reagents, and it generates no wastewater or exhaust gas.

Characterisation of rare earth element-bearing mineral phases present in South African coal ash using Mineral Liberation analysis

Critical raw materials, including rare earth elements (REE), are the cornerstone of modern technologies. Due to their unique properties, REE are in high demand, leading to the exploration and research of alternative sources. Coal combustion by-products, specifically coal ash, has been identified as a potential alternative source of REE, but more research is required. This study aimed to identify REE’s occurring in power station and laboratory-derived coal ash. Results from Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Mineral Liberation Analysis are presented herein, to gain insight into rare-earth-bearing minerals occurring in the ash samples.REEs were determined in both sets of ash samples at values higher than 600 parts per million (ppm) using ICP-MS. Critical REE occur in ranges from 203 to 406 ppm; uncritical REE from 179 to 256 ppm; and excessive REE ranging from 197 to 348 ppm in these samples. Laboratory-derived coal ash and power station-generated ash samples were prepared as polished blocks and coated for examination using a Mineral Liberation Analyser (MLA). A bright phase search mode was employed to detect rare earth-bearing mineral phases using the back-scattered electron detector (BSE).Energy dispersive spectroscopy (EDS) was used to determine elemental compositions in these phases.Four rare earth-bearing mineral phases were determined, namely: monazite-REE, xenotime-REE, Ce-REErium rich, and REE-silicate. The laboratory-derived ash samples had a greater proportion of liberated rare earth-bearing minerals, related to the lower proportion of aluminosilicate glassy phase, related to the low temperature ashing procedure.The detailed mineralogy provides insight into the design of REE extraction processes, targeting specific REE’s hosted in the various minerals. This will be documented in a future paper.

Mineralogical Characteristics of No. 3 Coal in the Jurassic Coalfield of Northern Shaanxi

This study investigates the mineralogical characteristics of No. 3 coal seam in the Mahuangliang Coal Mine, located within the Jurassic coalfield of northern Shaanxi Province, China. Utilizing techniques such as low-temperature ashing (LTA), X-ray diffraction (XRD), scanning electron microscopy with energy- permissive spectroscopy (SEM-EDS), and Siroquant software, we examined the distribution and composition of the minerals present in the coal, partings, and roof and floor strata. The results reveal a rich assemblage of minerals, including quartz, kaolinite, and carbonates. The primary geological factors controlling the distribution of minerals and trace elements are the input of detrital materials from the depositional source area and the influence of hydrothermal fluids.

Occurrence and transformation behaviour of silicon-bearing minerals during high silicon coal combustion from Yunnan Province, China

To understand the mineralogical characteristics and transformation behaviours of silicon-bearing minerals during coal combustion, seven typical high silicon (high-Si) coals and two high-Si fly ashes generated from high-Si coal-fired power plants were chosen. The coals were first treated by a method of low temperature ashing and the float-sink test for heavy minerals separation. Subsequently, low temperature ashes (LTA), light mineral fractions (LM), heavy mineral fractions (HM), and fly ashes from coal-fired power plants were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) combined with energy spectrum analysis (EDX), X-ray fluorescence spectrometry (XRF), and simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC).The results show that The mineral compositions in the LTA and LM are almost the same and majorly composed of quartz, kaolinite, calcite, dolomite, daphnite, chlorite-serpentine, pyrite, and anatase. The mineral compositions identified in the HM are clearly different from those in the LTA and LM. Pyrite, fluorapatite, anatase, siderite, dolomite and a plenty of silicate minerals including quartz, clinochlore, daphnite, chlorite, and chamosite are found in the HM. Chlorite is a quite common mineral in the HM with different shapes, including schistose, long tetragonal prism, cylindrical, block-shaped, and near-spherical, can be identified. The minerals identified in the fly ashes from the high-Si coal-fired power plants include quartz, mullite, and hematite. The mullite in the fly ashes is derived from the transformations of quartz, kaolinite, and chlorite in the coals. The quartz in the fly ash is mainly derived from the unchanged quartz in coal and a secondary origin from the SiO2-Al2O3 phase transformation. The high-Si fly ash with the size fraction of 38.5–74 μm contains the highest SiO2 content.

Mineralogy and geochemistry of the Middle Jurassic coal from the Hexi Mine, Shenfu Mining Area, Ordos Basin: With an emphasis on genetic indications of siderite

Siderite is a common carbonate mineral in coal and usually contains geochemical indications (elemental associations and isotopic compositions) for sedimentary environments and regional geological history, but the origin and enrichment mechanism of geochemically-pure siderite in coal is not fully understood. A better understanding of siderite origin in coal can help to decipher the depositional environment of peat and related geological processes of rank advances and epigenesis. This paper analyzed the petrological, mineralogical, and geochemical characteristics, particularly the content, occurrence, and origins of siderite in the Middle Jurassic coal from the Hexi mine of the Shenfu mining area, Ordos Basin, using optical microscopy, field emission-scanning electron microscopy, X-ray fluorescence, X-ray diffraction, inductively coupled plasma mass spectrometry, and isotope ratio mass spectrometry. The results indicate that the No. 4 Coal is rich in inertinite and is defined as an ultra-low to low sulfur, high volatile bituminous coal. Siderite has contents varying from 5.8% to 54.4% (low temperature ash basis) and occurs in radial or concentric circle nodules in the coal and is mainly distributed within the clay matrix. Syngenetic siderite in the Hexi coals is relatively pure in chemical compositions, and was formed in a brackish water environment affected by salt lakes under arid conditions. The high content of inertinite, high Sr/Cu value, and positive Ce anomaly at the uppermost of the coal seam indicate that the escalating intensification of oxidation and evaporation. The Al2O3/TiO2 ratio and the diagrams of Al2O3/TiO2 vs. Zr/ TiO2 and Al2O3/TiO2 vs. Nb/Yb indicate that the sedimentary source rock in the study area is primarily intermediate-felsic. The δ13CPDB-δ18OSMOW diagram shows C and O for the siderite formation in this study were derived from marine carbonate rock alteration of the underlying Ordovician marine carbonate rock basement. The distribution patterns of rare earth elements and Y (Middle-type enrichment and Gd anomalies) in the coal further verify this hypothesis. Furthermore, siderite in the coal exhibits an enrichment in 13C suggest that bacterial fermentation of organic matter as another possible source of CO2. The iron of the siderite in the study coal was probably derived from the leaching of the magnetite-bearing granite.

Mineralogical Characteristics and Arsenic Release of High Arsenic Coals from Southwestern Guizhou, China during Pyrolysis Process

Coal is the primary energy source in China, and coal pyrolysis is considered an essential and efficient method for clean coal utilization. Three high arsenic coals collected from the southwestern Guizhou province of China were chosen in this study. Low-temperature ashing plus X-ray diffraction analysis (XRD) was used to identify the minerals in coals. The three coals were pyrolyzed in a tube furnace in an N2 atmosphere at 950 °C, 1200 °C, and 1400 °C, respectively. Environment scanning electron microscope (ESEM), XRD, X-ray fluorescence analysis (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS) were adopted to determine the morphology, mineral compositions, and element compositions and arsenic contents of the coal pyrolysis ashes, respectively. It can be found that minerals in coal are mainly composed of quartz, pyrite, muscovite, and rutile. The minerals in the ashes generated from coal pyrolysis mainly contain quartz, dehydroxylated muscovite, iron oxide minerals, mullite, and silicon nitride. Oldhamite and gupeite exist at 950 °C and 1400 °C, respectively. The morphologies of oldhamite and gupeite at these temperatures are irregular block-shaped particles and irregular spherical particles, respectively. The mineralogical transformations in the process of coal pyrolysis affect coal utilization. The arsenic release rate is higher than 87% during pyrolysis at 1400 °C. The arsenic in organic matter is more able to be volatilized than mineral components. The retention time can slightly influence the arsenic release rate, and the influence of temperature is much more significant than the influence of retention time. The understanding of mineral evolution and arsenic environmental emission is helpful for the safety of high-arsenic coal pyrolysis.

Physically protected organic matter drives soil carbon sequestration potential of a managed grassland ecosystem in Italian Alps

Mountain managed grasslands are characterized by a high biodiversity and are capable to store a considerable amount of carbon (C) in the soil. In this study, we evaluated the chemical characteristics of physically protected organic matter and its contribution to the soil C stock potential in a subalpine managed grassland ecosystem, combining low-temperature ashing (LTA), stable isotope ratio mass spectrometry (IRMS) and infrared photoacoustic spectroscopy (PAS). The physically protected C fraction, evaluated through LTA by oxygen plasma, amounted ∼74% of the whole soil organic C (SOC) in the first 15 cm depth. This C fraction was closely linked to mineral components in stable aggregates of ∼587 μm mean weight diameter. The increase of both C and nitrogen (N) stable isotope composition (δ13C; δ15N) in the physically protected organic matter, concomitant with a decrease of its C/N ratio, suggests higher microbial transformation degree, age and stability against decomposition of protected organic fraction compared to unprotected one. However, also the younger and less protected C fraction acted as cementing organic labile agent, creating larger but less stable soil aggregates. The infrared photoacoustic spectroscopy showed that the physically protected C fraction was characterized by a more marked presence of phenolic compounds and a reduced amount of proteinaceous and polysaccharides-like materials compared to the more labile and younger organic fraction. These results highlight that managed grasslands can effectively sequester large amount of long-residence C into the soil, promoting aggregate stability and soil fertility, and playing a key role for mitigation strategies against climate change.

Behavior of the components of carbon-containing ash from the combustion of power coals under the conditions of chlorination roasting

On the basis of thermodynamic calculations of interaction reactions between mullite, oxides of titanium, iron, and Cl2 (gas) in the presence of carbon, the values of the Gibbs free energy are determined in the current work. It has been established that the probability of reactions occurring in the temperature range 1073–1473 K is very high. The most negative value ΔG°Т = –1029.9 kJ/mol is typical for the titanium oxide reduction reaction at the roasting tem- perature (1100 °С). The value of the Gibbs free energy of the destruction reaction of mullite (Al2SiO5), which is important for practice, is ΔG°Т = –269.2 kJ/mol. This makes it possible to predict the complete decomposition of mullite from ash to easily soluble aluminum chloride under the conditions of chlorination roasting. The study of the behavior of carbon under the conditions of chlorination roasting of ash with CaCl2 in an oxidizing atmosphere is also important. The results of the differential thermal analysis of the non-magnetic fraction of ash, previously separated from the TPP ash by magnetic separation, showed clearly defined manifestations on the DTA, DTG and TG curves, caused by the presence of various types of reactions in the system. Low-temperature (20–115 °C) sample dehydration and ash dehydration were determined in the temperature range of 115–375 °C. In the range of further heating of the sample (375–685 °C), an exothermic projection was found on the DTA curve, which is characteristic of the oxidation of thermally active carbon present in the powder. The amount of carbon removed from the ash established as 6.75% by weight of the sample. The research was carried out within the framework of grant funding from the Committee of Science of the Ministry of Education and Science of the Republic of Kazakhstan for 2021–2023 in the priority area “Geology, mining and processing of mineral and hydrocarbon raw materials, new materials, technologies, safe products and structures” of project No. АР09259637 “Development of a highly efficient wastefree technologies for the utilization of ash from coal combustion with the production of marketable products.

Effect of semi-continuous water washing on the combustion behaviors of agricultural organic solid waste

Combustion of agricultural organic solid waste (AOSW) was an ideal solution for their resource utilization in view of their massive annual production and great potential for reduction of fossil fuel utilization. However, high alkali and alkaline earth metals (AAEMs) content in the feedstock can arose severe fouling and slagging issues and thus prohibiting its vast utilization. In this study, a semi-continuous water washing method was proposed to preliminarily remove AAEMs from agricultural organic solid waste and its effects on the combustion behaviors of washed solid product were investigated. Results showed that the combustion index S were improved to 2.63 × 10-6, over 68 % of the total ashes were removed from the cotton stalk, and 96.3 %, 89.0 % and 74.7 % of K, Na and Mg were effectively removed, respectively. Moreover, the softening temperature of low temperature ash from the washed sample was as high as 1450 °C, 538 °C higher than the low temperature ash from the original sample; the base acid ratio and fouling index were improved from high slagging and fouling risk (1.7 and 90.8) of the original organic solid waste to low and medium risk (0.4 and 3.5), respectively. All these results signified the contributing effect of proposed semi-continuous water washing method on the combustion of agricultural organic solid waste. In a word, this study provided a promising method for fouling and slagging inhibition during the agricultural organic solid waste combustion.

Investigation of corrosion and fouling resistance of Ni–P-nanoparticles composite coating using online monitoring technology

When the temperature drops, industrial flue gases (fly ash and corrosive gases) are condensed on the surfaces of heat exchangers and easily cause operational problems such as fouling and corrosion of heat exchangers. In the present paper, an online monitoring system is established to experimentally investigate the heat transfer, fouling resistance, and corrosion resistance of different materials in coal-fired boilers under low-temperature ash deposition and dew point corrosion. The materials include ND, 316L, 2205, Ni–P, Ni–P-PTFE, Ni–P–SiO2 (50 g/L silica sol), Ni–P–SiO2 (60 g/L silica sol), and Ni–P–SiO2 (70 g/L silica sol). It was found that Ni–P coating is excellent in anti-fouling performance, and Ni–P–SiO2 coating excels in corrosion resistance. Also, the coated tubes can significantly decrease the fouling phenomenon. Comparing the electrode potential, corrosion rate, and surface morphology reveals that the tested materials can be sorted in terms of the corrosion resistance as 2205 > 316L > Ni–P–SiO2 (50 g/L silica sol) > Ni–P–SiO2 (60 g/L silica sol) > Ni–P–SiO2 (70 g/L silica sol) > Ni–P > Ni–P-PTFE > ND. Note that the performance of Ni–P–SiO2 (50 g/L silica sol) and 316L is very close. Therefore, the application of coated tubes in heat exchangers helps recover waste heat from coal-fired boilers, improve heat transfer efficiency, extend the service life of heat exchangers, and save costs.

Optimization of Operating Parameters for Coal Low-Temperature Ashing: A Suitable and Efficient Treatment Method for Mineral Analysis in Coal

Low-temperature oxygen-plasma ashing plus X-ray diffraction analysis is one of the effective techniques to identify minerals in coal. However, previous publications have not provided any details of the exact low-temperature degrees and corresponding working conditions of ashers, and this could lead to two adverse effects without proper operating guidance: (1) a relatively high temperature (e.g., >150 °C) may cause alteration of minerals (particularly clay minerals), and (2) a relatively low temperature (e.g., <80 °C) may cause a long ashing time and incomplete ashing of organic matter. In this study, the authors introduced the most frequently used low-temperature plasma ashers (PVA TePla IoN 40 made in America and Quorum K1050X made in Britain) to reveal optional operating parameters for low-temperature ashing. The ashing effects were analyzed from the aspects of ash mass, X-ray diffraction patterns, and the qualitative and quantitative analysis of minerals. Considering all the factors above, it is concluded that the ashing is the best when the running power is 200 W for the IoN 40, at which the diffraction peaks of chlorite d(004) and kaolinite d(002) can be clearly distinguished by LTAs-XRD analysis. In addition, different low temperatures have certain influence on the crystal structure of minerals. When the power rises to above 300 W (about 150 °C), the crystal structure of minerals undergoes changes. The symmetry and integrity of the mineral peaks became worse, and destructive interference occurred between the spacing of reflection planes, resulting in significant decrease in diffraction peak intensity; thus, some trace minerals were unable to be identified. The study on the working parameters of the instrument would be helpful to ash coals more effectively and make qualitative and quantitative analysis of minerals more accurate.

Comparative observation of the flow behavior of low- and high-temperature ashes of biomass

Biomass ashes are frequently known for their limiting effects in thermochemical conversion processes. Thus, a comprehensive understanding and prediction of the temperature-depending behavior of the ashes is indispensable for practical applications. In the present study, the flow behavior of low-temperature ash (LTA) and high-temperature ash (HTA) of biomass is investigated in detail and monitored by in situ methods. For this purpose, the mineral matter formation and transformation of both ashes are studied by X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. A combination of heating-stage microscope and scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX) has enabled a systematic identification of the microstructures and observations of LTA and HTA. The results have illustrated an associated release of K and Cl and the formation of new mineral phases in the remaining material during ashing at increased temperature. Both ashes have exhibited flow temperatures below 1100 °C and a rarely observed behavior of higher characteristic temperatures for LTA than for HTA is detected. From the heating-stage microscopy, the ashes have given evidence to a significant formation of slag at the characteristic temperatures and a viscosity-driven behavior at the characteristic points of ash fusion. Based on these results, a conclusion can be drawn regarding the usability of this biomass for high-temperature conversion processes on the industrial scale.

Granite-bauxite provenance of abnormally enriched boehmite and critical elements (Nb, Ta, Zr, Hf and Ga) in coals from the Eastern Surface Mine, Ningwu Coalfield, Shanxi Province, China

Critical elements in coal have attracted much attention in recent years as some coals and coal combustion products may provide an alternative source of these. This paper provides mineralogical and geochemical data on the No. 4 coal seam, the major minable coal from the Eastern Surface Mine, Ningwu Coalfield, Shanxi Province, northern China. It also discusses the provenance of mineral matter, the origin of aluminum minerals (boehmite), carbonates (calcite and ankerite) and phosphates (goyazite) and the potential economic significance of critical elements (e.g., Al, Nb, Ta, Zr, Hf, and Ga). Mineralogically, the studied coal samples are characterized by high kaolinite (64.7% on average, low-temperature ash basis) and boehmite (24.1%) contents, followed by carbonates (calcite, ankerite, siderite, and dolomite), titanium oxides (anatase and rutile), goyazite, quartz, and apatite. In comparison with average values for Chinese coals, the studied coal samples are characterized by high Al2O3 contents (mean 11.82% on a whole-coal basis and mean 49.49% on an ash basis). Compared with world hard coals, the No. 4 Coal is enriched in Ga, Se, Zr, Nb, La, Ce, Hf, Ta and Pb. In addition to Al, critical elements including Nb, Ta, Zr, Hf, and Ga in this coal have an economic significance. The Al2O3/TiO2 vs. Nb/TiO2, Al2O3/TiO2 vs. Zr/TiO2, Zr vs. Nb, Hf vs. Ta, and REY distribution patterns (REY, rare earth elements plus Y), indicate that the provenance of mineral matter (e.g., boehmite and critical elements Nb, Ta, Zr, Hf and Ga) in the No. 4 Coal is a number of sediment-source regions including the Neoarchean granites of the Yinshan Oldland, and the Paleoproterozoic granites and Benxi Formation bauxites (Late Carboniferous, to the northeast of the coal basin) of the northern Trans North China Orogen. Carbonate minerals (calcite and ankerite) are of epigenetic origin while the phosphate mineral (goyazite) was syngenetically deposited in the peat swamp during a period of a low water table.

Nanopore Structure and Origin of Lower Permian Transitional Shale, Eastern Ordos Basin, China

Nanopores in the shale play a vital role in methane adsorption, and their structural characteristics and origins are of great significance for revealing the mechanism of methane adsorption, desorption, and diffusion. In this paper, through low-temperature ashing and low-pressure gas adsorption experiments, the nanopore structure of original shales and ashed shales was quantitatively characterized, and the nanopore origins in the transitional shale of lower Permian in eastern Ordos Basin were analyzed. The results show that the pore volume (PV) and specific surface area (SSA) of nanopores in transitional shale reservoirs are 0.0217–0.0449 cm3/g and 13.91–51.20 m2/g, respectively. The average contribution rates of micropores (<2 nm), mesopores (2–50 nm), and macropores (50–100 nm) to PV are 18.78, 72.26, and 8.96%, respectively, and the average contribution rates to SSA are 66.19, 33.10, and 0.71%, respectively. In addition, it is found that the average contribution rates of inorganic minerals and organic matter to the SSA of micropores are 55.9 and 44.1%, respectively, and the average contribution rates to the SSA of mesopores are 92.3 and 7.7%, respectively. Combining the adsorption properties of the main clay minerals and kerogen in shale, it is concluded that organic pores control the adsorption of methane with an absolute advantage in transitional shales. It is of great significance to understand the mechanism of methane occurrence, desorption, and diffusion in shales by clarifying the origins of multiscale pores.

Influence of water leaching on alkali-induced slagging properties of biomass straw

Deposition or slagging problem caused by the alkali metal species is one of the major obstacles to utilize biomass fuel in combustion and gasification plant. The paper mainly studies the effect of water leaching on alkali-induced slagging properties of corn straw and rice straw. The original mineral form of alkali elements in biomass straw was studied with comparison of low temperature ash of biomass before and after water leaching. The release and transformation of alkali compounds in biomass straw at different temperatures during heating were analyzed with combination of the XRD result of the heated biomass and the chemical composition of the digested samples. The result shows that the potassium in corn and rice straw mainly exists in the form of KNO3, KClO4, K2SO4 and KAlSi3O8, in which KNO3, K2SO4 and KClO4 is mostly removed in water leaching. The fusion temperatures increased after water leaching, especially for rice straw ash, which is a typical sample that the basic compounds are mainly potassium-containing compounds. The decrease of potassium content with temperature in the original corn and rice was because the potassium-containing minerals decomposed and escaped at 25−1000°C. The release of potassium in the range of 400−800°C significantly reduced for corn and rice straw after water leaching, but the potassium content will still decrease above 800°C. The decrease of magnesium content in corn with increasing temperature may be due to the volatilization of magnesium oxide under the action of carbon reduction. For fuels with high alkali metal content after water leaching, the residual alkali metal will still escape and cause deposition or slagging in the furnace, especially in the temperature range above 800°C.

Effects of atmosphere on mineral transformation of Zhundong coal during gasification in CO2/H2O conditions

Coal gasification has been regarded as a promising method for the thermochemical utilization of Zhundong (ZD) coals. However, ZD coals are characterized by the high content of alkali metal, which may cause a series of running problems such as ash accumulation, slag blocking and fouling on heating surface of the gasifier and waste heat boiler. To improve the efficiency of fuels utilization, the evolution behavior of minerals of ZD coals – Hongshaquan (HSQ) coal and Jiangjunmiao (JJM) coal was investigated by experiment and FactSage calculation in this work. The results gained by low temperature ashing method show that the minerals of both HSQ coal and JJM coal are basically similar mainly including quartz, kaolinite, calcite, halite and of pyrite. It was found that the minerals transformation of coal samples is greatly influenced by gasification atmosphere (CO2/H2O). For HSQ coal, the minerals of slag are mainly composed of quartz, nepheline, gehlenite and periclase under pure H2O or pure CO2 atmospheres, while the minerals of slag are mainly lime and periclase under the mixing CO2/H2O (CO2: H2O = 1:1) atmosphere. For JJM coal, the minerals of slag under the pure H2O atmosphere are obvious different from that under the pure CO2 atmosphere, and mainly include gehlenite, lime, magnesia, and nepheline in pure H2O atmosphere, and the pleonaste and merwinite minerals are detected under pure CO2 and CO2/H2O (CO2: H2O = 1:1) mixing atmosphere. Furthermore, the FTIR results for the two coal samples demonstrate that the more silicate minerals exist in coal slag, which might be caused by the factor that H2O promotes the melting and polycondensation of SiO2 and Al2O3 to increase the quantity of silicate. In addition, according to the calculation results of FactSage, iron is formed during the evolution of iron-containing minerals with water vapor in atmosphere which may be due to the promotion effect of water on the decomposition of pyrite in coal slag. Moreover, water vapor can promote the migration of the alkali metal sodium in the coal slag to the gas phase, reducing the sodium content in the slag, and thus results in a higher viscosity of the molten slag under the water vapor atmosphere than under the pure CO2 atmosphere. The order of slag viscosity is: η0(H2O) > η0(CO2/H2O) > η0(CO2). This work provides a theoretical information for the effect of CO2/H2O gasification conditions on the mineral’s evolution behavior of ZD coal and offers a guidance for the operation of gasifier.

Mineralogy and geochemistry of the Late Triassic coal from the Caotang mine, northeastern Sichuan Basin, China, with emphasis on the enrichment of the critical element lithium

Comprehensive research on the concentrations, modes of occurrence, and origin of critical elements in coal is significant from an economic point of view. Several previous investigations have attributed the host of critical elements such as Li, Ga, Nb, Ta, Zr, and Hf in coals to aluminosilicate minerals derived from terrigenous input, while Li of hydrothermal origin has rarely been found in coals. The Late Triassic coal of the Caotang mine, northeastern Sichuan Basin, China of this study is a high-ash, medium-sulfur, low volatile bituminous coal. The dominant minerals in these coals are illite, calcite, kaolinite, and quartz, with minor chlorite, sulfate minerals, paragonite, pyrite, and anatase, along with traces of phosphate minerals. In comparison with the world hard coals and Chinese common coals, the critical element Li is ∼20-times higher in the Caotang coal. The Li concentrations (range 89.1–520 μg/g, 291 μg/g on average) and the abundance of chlorites in coal low-temperature ashes (LTAs) show a strong correlation (r = 0.88), indicating that chlorites are the dominant host of Li in the coal. The chlorite minerals in this study belong to three types: cookeite, chamosite, and a chlorite mineral of intermediate composition between them. Both the kaolinite-cookeite assemblage with a vermicular texture and Li-bearing chlorite occurring as fracture-fills, indicate that they formed by interaction of kaolinite with Li-rich solutions or directly precipitated from Li-bearing solutions during diagenetic or epigenetic processes. Based on geochemical indicators such as Al2O3/TiO2 ratios, rare earth elements and yttrium (REY) distribution patterns, and the relationship between Al2O3/TiO2 and Zr/TiO2 and Nb/Yb ratios, the source-area compositions of Caotang coals are felsic-intermediate igneous rocks from the southeast Qinling thrust system, Longmenshan thrust belt, Hannan Upland, and Micangshan-Dabashan Uplift. The kaolinite-paragonite assemblage with authigenic minerals such as chamosite, anatase, calcite, barite, pyrite, quartz, and REY-rich minerals, as well as the REY enrichment patterns, suggest that the Caotang coals may have been subjected to a multi-stage injection of hydrothermal fluids. The Caotang coal is considered to be a promising source of the critical element Li. Furthermore, the concentrations of the critical elements Ga, Nb, and Ta in the coal increase its potential economic significance even further.

Comparative study on ash characteristics of various high-alkali fuels using different ashing methods and temperatures

The comparative study on ash characteristics of various high-alkali fuel helps to solve the ash-related problems. In this paper, the ashes of wheat straw (WS) and three high-alkali coals (LA, ZJ, and TC) were prepared using the plasma ashing method (at <200 °C) and the traditional ashing method (at 575 °C, 815 °C, and 1000 °C). The mineral matters close to the initial state in raw fuel could be elucidated by the plasma ashing method because it could remove organic material without converting the primary minerals. The traditional method was employed to study the ash characteristics at the following stages of fuel conversion. The ash samples, including plasma ash (PA), low-temperature ash (LTA), medium-temperature ash (MTA), and high-temperature ash (HTA), were subjected to a variety of analysis techniques. The results show that wheat straw has a large amount of potassium, while sodium is the main alkali in coals. The PA samples of wheat straw and coals contain little less sodium or potassium than the LTA samples, and the alkali contents decrease further in the MTA and HTA samples. The ashing procedure affects the phases of K-bearing mineral in WS ashes, Na-bearing and Ca-bearing minerals in LA coal ashes, Na-bearing, Ca-bearing, and Fe-bearing minerals in ZJ and TC coal ashes. During the thermal analysis, all PA samples exhibit significant weight losses due to the decomposition, volatilization, and oxidization of minerals. The MTA and HTA samples of WS present severe fused surfaces, while those of ZJ coal are slightly melted.