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To understand the behavior of pyrite (a key slag-inducing coal mineral) in oxyfuel combustion (O2/CO2 combustion), our previous work firstly investigated the effects of pure CO2 on fixed bed reactors and at low temperatures (

The understanding of the pyrolysis behavior of petroleum coke (PC), a by-product of oil refining, is very critical to its energy utilization. Different from most previous work, this investigation particularly focused on PC pyrolysis at high temperatures > 1273 K. The CO2 gasification reactivity of in-situ char, rather than quenched char, was evaluated and correlated to PC pyrolysis behavior at different temperatures. A Chinese PC with sizes below 100 μm was tested on a high-temperature thermogravimetric analyser (TGA). Three sets of tests were carried out for different purposes. The first set was to simultaneously obtain sample mass loss and gas evolution data during pyrolysis through thermogravimetry-mass spectrometry (TG-MS). The second set aimed to collect char samples for subsequent analyses by scanning electron microscopy (SEM) and Raman spectrometry. The third set was to evaluate in-situ char-CO2 gasification reactivity through the TGA. The results showed that, in addition to the commonly-observed primary pyrolysis stage at low temperatures, there was a secondary PC pyrolysis stage at high temperatures > 1300 K. In this process, the gases such as HCN, CO2 and SO2 were significantly released. The observed changes of char morphology suggested a four-staged thermoplastic transformation of the PC during pyrolysis, which has little been discussed previously. At different stages, i.e. softening, plasticizing, resolidification and graphitization, the rate of carbon ordering was different. The in-situ char-CO2 gasification reactivity was found to first increase, then decrease and finally increase again with increasing temperature. Such changes coincided with the thermoplastic state of the pyrolyzed char, but not with the changes of char surface area or carbon ordering. The obtained knowledge is new and highlights the potentially important roles of char thermoplastic state in determining its reactivity towards CO2.

To mitigate the slagging, fouling and high-temperature corrosion problems caused by alkali metals during coal combustion process, measurement of time-resolved alkali metals release is very important. The paper proposed an in-situ approach for measuring sodium (Na) release in coal combustion by Flame Emission Spectroscopy (FES). Through the analysis of spontaneous emission spectra and a calibration procedure, the concentration of gas phase Na, temperature and thermal radiation can be obtained. Firstly, experimental measurement of Zhundong coal particles burning in a flat flame burner was done. Two kinds of Zhundong coal with similar proximate and ultimate analyses, but different ash composition were used. The Na-release history measured by FES was compared with that by LIBS. Results showed that the Na-release at the devolatilization, char, and ash stages can be distinguished by FES. The higher Si/Al content in ash can suppress the Na-release at the ash stage. Moreover, FES method was extended to the measurement of Na-release in four industrial boiler furnaces of two Zhundong coal-fired power plants. Results showed the Na-release measured by FES can reflect the change of fuel and load, and both temperature and thermal radiation play key roles in Na-release in coal combustion.

The chlorine evolution mechanism remains unclear during the thermal treatment of CaCl

Managing the risk of ash deposition is still a priority for oxy-fuel combustion of biomass–coal blends. This is especially a concern when high inlet O2 concentrations are adopted to reduce the amou...

Utilization of biomass when combined with carbon capture and sequestration (CCS) is one feasible solution for “bioenergy with carbon capture and storage” (BECCS), which can reduce the CO2 emissions in the atmosphere. Rice husk is a unique biomass resource because its ash content is typically higher than 15% and more than 88% of this mineral matter is composed of silica. Ash partitioning and deposition mechanisms in rice husk combustion are still under investigation and will be the focus of this paper. Two different rice husk resources were tested in this work: one is from China, and another is from the United States, which are abbreviated as RH_CN and RH_US, respectively. The experimental work was performed in a 100 kW down-fired oxy-fuel combustor (OFC), and both fuels were co-fired with natural gas to achieve combustion conditions that are comparable to coal combustion. Two conditions were tested: (1) air combustion and (2) oxy-combustion with 70% O2 and 30% CO2 in oxidant gas (denoted as OXY70). Result...

The aim of this study was to understand the kinetics of high-temperature tube corrosion and its rate-limiting step in oxy-fuel combustion mode. Four different types of ash deposits collected from low-rank coal combustion boilers were tested, with varying contents of sulfur and metallic species. A typical stainless steel, low chromium ferritic alloy, T23, was tested at a metal surface temperature of 650 °C for up to 200 h. Compared with air-fired flue gas which is lean in SO2 and steam, the aggressive flue gases formed under oxy-firing conditions enhanced T23 tube corrosion but to a marginal extent in a comparison with the S-rich ash deposit. The study found a positive correlation between the sulfur content within ash deposit and the thickness of the tube oxide layer. The sulfate in the ash deposit promoted the formation of molten and mobile sulfate complex and sulfides. The former species are predominant on the tube’s outer surface, while sulfides are preferentially formed inside the oxide layer where oxy...

Some silicon-rich mineral additives are known to be capable of capturing volatile elements evolved during combustion and, thus, may be used to mitigate ash-related problems (e.g., particulate matte...

In addition to sodium (Na), calcium (Ca) and magnesium (Mg) are also abundant in Zhundong coals and play important roles in ash deposition. This work investigated the occurrence of Ca and Mg in the...

Interactions between pyrite and silicates are very critical to ash slagging in coal-fired boilers. However, no work has been reported regarding the impacts on such interactions of CO2, the dominant component in the oxy–fuel combustion gas. This was investigated in the present work by using mixtures of pyrite and kaolinite, a prevailing silicate mineral in coal. Furthermore, the sintering strength of the generated products was also evaluated. The pyrite–kaolinite mixtures were treated on a fixed bed reactor in both N2 and CO2 for comparison. The treating temperature was 1050, 1150 and 1250 °C while the reaction time was 3, 6 and 12 min, respectively. The solid products were characterized by techniques such as X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Sintering tests of these products were carried out as well. It was found that some interactions between pyrite and kaolinite took place in the N2 atmosphere. This was evidenced by the formation of iron aluminosilicate and attributed to the effects of water vapor released from kaolinite dehydroxylation. Nevertheless, pyrite–kaolinite interactions in N2 were limited and had insignificant effects on product sintering strength development. In CO2, pyrite–kaolinite interactions were significantly enhanced, compared with those in N2. Although the kaolinite-derived water vapor had some effects, CO2 was found to play a dominant role. Enhanced pyrite–kaolinite interactions resulted in an increase of eutectic phases. Consequently, the product sintering strength development was greatly elevated. It was further found that, under the conditions investigated, the interactions between pyrite and kaolinite were actually through reactions between FeO, rather than other intermediates, and aluminosilicate. This new finding enabled us to develop the mechanisms for pyrite–kaolinite interactions in N2 and CO2.

Straw sample was torrefied at 260 °C and 300 °C in N2, respectively, to prepare torrefied straw named as T-260 and T-300, and the reduction effect of co-firing straw or torrefied straw and steam coal on PM1 is investigated. The combustion experiments were conducted in a high temperature drop tube furnace (DTF) at 1400 °C to collect the inorganic PM10 for further analysis. Combustion atmosphere was air for all cases and 50% O2/50% CO2 (OXY50) for coal, T-260 and their blends of 1:1 and 4:1. The results show that all three biomass fuels show obvious emission reduction of PM with aerodynamic diameters of ≤ 0.3 µm (PM0.3) under both mix ratios. Reduction ratios of co-firing are overall higher at mix ratio of 1:1 than 4:1, and co-firing of T-260 or T-300 with coal shows higher reduction ratio than co-firing of straw. The higher ash content in torrefied straw leads to higher contents of alkali and alkaline earth metals (AAEM), which will further react with both Si and S during co-firing and coagulate into particles of larger sizes, leading to higher reduction ratios of PM0.3 and unconspicuous reduction effects in PM0.3–1 emitted from co-firing. During co-firing in oxyfuel atmosphere, a higher combustion temperature compared to air leads to an intensitive gasification, may resulting in effective and even higher reduction ratio in PM0.3.

Torrefaction is a competitive biomass pretreatment technology. However, its impacts on particulate matter (PM) formation during biomass combustion and co-combustion with coal have little been investigated. This work provides new data on the formation of PM10 (particulate matter with aerodynamic diameters less than or equal to 10 µm) from combustion of raw (RH), torrefied rice husk (TRH) and their blends with a lignite (SZ). All combustion experiments were carried out on a drop-tube furnace at 1300 °C and in air. The combustion-generated PM10 was collected by a Dekati low pressure impactor and classified into 14 size fractions for further quantification and characterization. The results indicate that, compared with the RH, the TRH-derived PM10+ (particle size above 10 µm) contains more alkalis, leading to a decrease in the production of PM1 (particle size below 1 µm). During co-combustion, fuel interactions promote the transformation of alkali chlorides to aluminosilicates. A considerable amount of water-soluble Ca and P in PM1 transforms to PM1–10 (particle size between 1–10 µm). As a result, the production of PM1 (on an ash basis) decreases while that of PM1–10 increases. Co-combustion of coal with torrefied rice husk is found to generate less PM1 but more PM1–10 than that with raw rice husk.

This paper provides a new thought that a cost-effective chlorine-rich carbon-based sorbent could be prepared by directly pyrolyzing high chlorine coal in order to capture the elemental mercury, simultaneously realizing value-added utilization of the remained char instead of combustion during low rank coal pyrolysis poly-generation. In this work, the concentration and forms of chlorine in one kind of Turpan-Hami coal were determined quantitatively according to the proposed procedures. Then the release behavior of chlorine and the transformation of chlorine from inorganic state to organic were investigated during coal pyrolysis from 400 to 1000 °C. Moreover, the Hg0 removal performance was examined in a lab-scale fixed bed reactor by the char before and after CO2 activation. The results indicate that over 90 % of chlorine in raw coal is inorganic in the forms of ionic Cl and crystal NaCl, and ionic Cl can be transformed to inorganic NaCl and organic C-Cl species during coal pyrolysis, thus providing chlorine active adsorption sites for Hg0 chemisorption based on the results of temperature programmed desorption and adsorption kinetic analysis. Furthermore, CO2 activation can improve the char porosity and specific surface area, thus promoting the external mass transfer of Hg0 by increasing the opportunity of chlorine active sites being exposed to vapor Hg0. 100 % of Hg0 removal efficiency is achieved in the first hour and equilibrium adsorption amount is 400 ug/g for the activated char obtained by pyrolyzing coal at 600 °C combining with CO2 activation at 900 °C, indicating the good Hg0 removal performance of the as-obtained chlorinated activated char.

In this paper, special attention is paid to the scavenging of refractory elements (Ca, Mg, Fe) by kaolin during combustion of a low rank coal. Combustion experiments were carried out on an electrically-heated drop tube furnace at 1300 °C. The raw coal and its blends with kaolin additive (1–10 wt% of coal mass) were respectively burned in simulated air atmosphere. Combustion-generated ashes were collected and analyzed by Computer-Controlled Scanning Electron Microscopy (CCSEM). The data were used to elucidate the scavenging of Na/Ca/Mg/Fe by kaolin and the transformation of inorganic species after kaolin addition. Quantitative X-ray diffraction (XRD) analysis and ash fusion tests were carried out to characterize the amount of amorphous species and the fusion temperatures of bulk ashes after kaolin addition, respectively. Backscattered electron images (BSE) were collected on the raw coal and coal/kaolin mixtures to demonstrate the role of the dispersion state of kaolin in metal scavenging. It is found that apart from Na, the added kaolin can also strongly scavenge the refractory elements (Ca and Mg, except Fe) during low rank coal combustion. The scavenging of Na/Ca/Mg by kaolin promotes the formation of amorphous and molten alumino-silicate particles. The high dispersion of fine kaolin particles in coal is a key factor affecting the scavenging process, which provides sufficient opportunities for the contact and reaction between Ca/Mg species and kaolin. Two possible routes are proposed for the scavenging of Ca/Mg by kaolin. One is that kaolin firstly reacts with sodium vapors, forming melting sodium alumino-silicates that favor the capture of Ca/Mg containing species. Another possible route is that, kaolin directly reacts with active CaO/MgO fumes produced from coal combustion, forming Ca/Mg containing alumino-silicates. Kaolin dosage has significant impacts on the ash fusion temperatures. Which firstly decreased with the increase of kaolin dosage, reaching to the lowest at 4 wt%. Then it sharply increased with the increase of kaolin dosage.

Limited data from fluidized bed combustion tests have shown that rice husk, a silicon-rich residual biomass, has the potential to be cofired with coal while not inducing unacceptable ash-related problems. However, there is great concern regarding the behavior of rice husk ash under pulverized fuel combustion conditions, where the temperatures are much higher and expected to facilitate fuel interactions. This work, to the authors’ knowledge, is the first to investigate both ash formation and fouling behavior in rice husk firing and its cofiring with coal at a high temperature relevant to pulverized fuel combustion. A Chinese rice husk and a high alkali Xinjiang coal were selected. Combustion tests of individual fuels and their blends (with the share of rice husk being 10% and 20%, respectively) were performed at 1573 K on a laboratory drop tube furnace. Both bulk ash and fouling deposit samples were collected in each test. Techniques including X-ray diffraction (XRD), scanning electron microscopy with ener...

This study aims to clarify the roles of sulphur (S) in both lignite ash deposits and bulk flue gas on high-temperature corrosion. In particular, the potential synergism between these two different S sources during the oxy-fuel combustion was explored. The ash deposits were derived from the combustion of a Xinjiang lignite and its water and acid washed coal samples. It was found that the amount of S permeated inside the tube is predominantly limited by the availability of sulphates in the ash deposits, under both air-firing and oxy-firing combustion modes. However, the participation of flue gas in particular SO 2 and steam is essential, although their participation extent is very low. The lignite ash deposit functions as a medium which transfers the S from bulk gas to tube surface, via the formation of intermediate molten sulphates or sulphides. The formation of sulphides is highly dependent on ash properties (i.e. the content of sulphates) and tube composition. The high- S ash deposits with > 8 wt% sulphate have no propensity to cause the formation of sulphide. By contrast, for the low-S ash deposits with

This study aims to explore the effects of lignite ash deposit, in particular the Ca-bearing species in ash on the tube corrosion under the oxy-firing mode. Two tubes, low-alloy steel T23 and austenite SUS347 have been coated with two ash deposits and exposed to oxy-fuel flue gas at 650 °C for a total of 50 h. The ash deposits were collected from the combustion of a lignite mixed with and without silica additive in a 30 MWth pulverised coal-fired boiler. In this study, the roles of Ca-bearing species including sulphate, free oxide/calcite have been first time revealed by using synchrotron-based X-ray fluorescence (SXRF) and μ-XANES to detail the spatial distribution of individual elements, and in particular the speciation of Cr on the cross-section of tubes. Carburisation of Cr by CO2 is inhibited remarkably upon the co-existence of flue gas and ash deposit. However, the attack from the species in ash deposit other than sodium sulphate is more influential in accelerating the tube corrosion. Calcium sulphate is corrosive against the Cr-rich protective layer formed on the tube surface, resulting in the formation of Cr sulphide even in an exposure time of 50 h. Sulfidation of Cr by calcium sulphate is thermodynamically possible under the exposure conditions tested. Moreover, the free calcium oxide and even calcite in raw coal ash deposit are detrimental, accelerating the breakdown of the two tubes upon the formation of Ca chromite (Cr3 +) and even chromate (Cr6 +). The resultant chromite penetrated underneath the protective layer that is rich in Cr-rich oxide and Fe-Cr spinel, as well as merged intimately with ash deposits. The detrimental effect of free calcium oxide/calcite is more pronounced than calcium sulphate in the 50 h tested here. The use of silica additive to coal combustion immobilised the free Ca oxide, thereby forming an extra protective layer that minimised the oxidation of Cr. Consequently, the whole tube remained intact with little being damaged.

Knowledge is limited on the thermal behavior of sulfur during petroleum coke (PC) pyrolysis, a critical topic for its any thermal utilization processes. In this work, temperature-resolved evolution and speciation of sulfur in both gas and solid phases during pyrolysis of a high-sulfur petroleum coke were investigated, with simultaneous thermogravimetric and mass spectrometric analysis (TG-MS) and X-ray photoelectron spectroscopic analysis (XPS). The results show that the original PC contains dominantly thiophene and sulfoxide, while mercaptan, sulfate and sulfone are only present in minor amounts. The desulfurization ratio generally increases with increasing temperature or decreasing heating rate. Two slow desulfurization stages (Stage I (

To examine the real effects of typical inorganics on the emission behavior of PM10 during coal combustion and explore the interactions between the typical vaporized and refractory inorganics, Na-, Si-, and (Na+Si)-loaded coals prepared from the ash-removed PDS bituminous coal and ZD subbituminous coal were combusted in a high temperature drop-tube furnace (DTF) at 1500 °C in air. The produced inorganic PM10 and PM2.5 were collected and characterized. The experimentally measured PM emission results from the combustion of the (Na+Si)-loaded coals were compared with the calculated results that are the sum of PM emitted from the combustion of the Na- and Si-loaded coals. The calculated yields of PM0.3 of two studied coals are higher than the experimental ones, while the calculated yields of PM0.3-2.5 are lower. The experimental yield of PM0.3-2.5 is mainly enhanced by the sodium condensing heterogeneously on fine quartz particles, while the sodium would contribute to PM0.3 homogeneously, leading to the higher...

A novel compound demister that combines an upstream tube bank and downstream wave plates was proposed in this work for application in multistage flash (MSF) desalination process. Its performance was evaluated by experimental and numerical methods. Compared with the individual tube-bank and wave-plate demisters, the compound demister is found to have the highest separation efficiency (> 95%) with much less fluctuation for a wide range of gas velocities. At low gas velocities (≤ 4 m/s) or for removing small droplets ( 4 m/s), the compound demister shows higher resistance to droplet re-entrainment that occurs at inlet gas velocity of approximately 7 m/s compared with the tube-bank demister. This is due to the compensation from the wave plates in the compound demister that separate secondary droplets generated by tubes. The compound demister possesses higher dry pressure drops than either the tube-bank or wave-plate demister, but is acceptable for industrial application. All these advantages make the compound demister a promising candidate for droplet removal in the desalination process.

This contribution reports the real effects of coal particle combustion on the emission behavior of particulate matter (PM) with aerodynamic diameters of

The high levels of CO 2 and H 2 O in oxy-fuel/oxy-steam combustion are expected to have important effects on the oxidation of ferrous sulfide, which is critical to the understanding of ash deposition. In this work, the kinetics of ferrous sulfide oxidation in CO 2 , H 2 O, or the mixtures of CO 2 and H 2 O are explored on a purposely-designed thermo-gravimetric reactor (TGR). Based on the data obtained from TGR, XRD and SEM, the roles of CO 2 and H 2 O in the oxidation of ferrous sulfide are elucidated. It is found that pyrrhotite (FeS 1+ x ) transformation in CO 2 , H 2 O, or their mixtures is dominated by the oxidation of ferrous sulfide (FeS). The adsorption and desorption elemental reactions of CO 2 or H 2 O on ferrous sulfide are essentially the same. For all cases, the three-dimensional diffusion model can be used to account for the oxidation of ferrous sulfide. The reaction rate constants for the oxidation of ferrous sulfide increase with increasing the partial pressure of the oxidant. The dependence of the reaction rate constant on the partial pressure of CO 2 or H 2 O can be well explained by the Langmuir–Hinshelwood rate forms. In the mixtures of CO 2 and H 2 O, the active sites for the FeS–CO 2 reaction and the FeS–H 2 O reaction are partially independent and partially shared.

The high levels of CO 2 in oxy-fuel combustion are expected to have important effects on the transformation of pyrite, a major contributor to ash deposition. As a successive work to the previous study that explored the chemical role of CO 2 in pyrite decomposition, this paper is purposely designed to investigate the effects of CO 2 on the oxidation of pyrrhotite generated from pyrite decomposition. Pyrrhotite oxidation in pure CO 2 was respectively investigated at 900, 950 and 1000 °C on a well-designed thermo-gravimetric reactor (TGR). The time-resolved data of sample weight loss and gas evolution were collected online by a data collection module and a Horiba PG-350 gas analyzer, respectively. The solid products were characterized by X-ray diffraction (XRD). The results demonstrate that the influence of CO 2 on the oxidation of pyrrhotite is chemical in nature. The transformation of pyrrhotite in CO 2 consists of three stages, i.e., the fast weight loss stage, the slow weight loss stage and the slow weight gain stage. The mechanisms involving CO 2 at these three distinct stages are quite different. At the first stage, ferrous sulfide is formed by the decomposition reaction of pyrrhotite with CO 2 . At the second stage, the oxidation reactions of ferrous sulfide with CO 2 are responsible for the formation of magnetite or hematite. Hematite may also be formed through further oxidation of magnetite by CO 2 . The third stage is dominated by the oxidation of magnetite to form hematite with CO as the only gas product.

While co-firing Si-rich biomass showed the potential for mitigating ash-related issues in low-rank coal combustion, the underlying mechanisms need further investigation. This work aims to examine the impacts of co-firing rice husk and its torrefied product with a high-Na/Ca/Cl low-rank coal on PM10 (particle size below 10 μm) emission and elemental partitioning. Combustion tests were performed on a drop tube furnace (DTF) at 1300 °C. The bulk ashes and PM10 were collected by fiberglass filters and Dekati low pressure impactor respectively. The ash samples were carefully characterized by X-ray diffraction (XRD), scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS), ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). It is found that co-firing raw or torrefied rice husk reduces emissions of PM1 (particle size below 1 μm) together with the Na and Cl in it, mainly due to the capture of Na by coarse Si-rich particles from rice husk. The emissions of PM1-10 (particle size between 1 and 10 μm) are elevated after co-firing. This is because the formation of Ca-silicates is facilitated instead of K-silicates, thus weaken the agglomeration between the coarse particulates with melting surface from rice husk. Compared to raw rice husk, its torrefied product leads to a lower reduction ratio regarding PM1 from co-firing. Combined the results in this and previous investigations, for raw/torrefied rice husk and Xinjiang low-rank coals, the PM1 yields from both of the individual fuels and the blends almost linearly correlated with the mole ratio of (Na + K + Cl + 2S)/(Si + Al) in the ash.

Exchangeable calcium (Ca) in coals has significant catalysis during gasification. In previous work, coals were usually pyrolyzed for different time periods before char gasification. The extent of char aging during pyrolysis may change the state of Ca, and affect its catalytic behavior during gasification. This speculation is investigated by characterizing different aged chars and their in-situ gasification in a thermogravimetric reactor. Calcium-exchanged coal samples were first isothermally pyrolyzed at different pyrolysis time and then in-situ gasified in CO2 or steam at different temperatures. The pyrolyzed and partially gasified chars were collected and analyzed. The results show that char aging significantly changes the state and dispersion of Ca. While the nascent chars (product of 1-min pyrolysis) show no visible Ca-containing particles on the surfaces, the aged chars (product of 20-min pyrolysis) show evident CaO particles ranging from a few to tens of nanometers. During gasification, the extent of char aging does not change the catalytic action of Ca in either CO2 (surface recession mode) or steam (channeling mode). Increasing temperature favors the gasification reactivity of both nascent and aged chars in CO2, and that of aged chars in steam. However, the gasification reactivity of the nascent chars in steam does not seem to be considerably affected by increasing temperature. The increase of catalytic activity may be mostly offset by the decrease of Ca dispersion.

Fouling and slagging processes are important in oxy-coal combustion processes because they greatly influence heat transfer and boiler performance. Ash partitioning processes are precursors to deposition processes and so in order to predict the latter one must have a good understanding of the former. This work involves a 100 kW rated down-fired oxy-fuel combustor outfitted with sophisticated ash aerosol and ash deposit sampling equipment. The work differs from that previously published, in that here we focus on the effects of changes in the composition and amounts of actual recycled flue gas (RFG) streams used in oxy-coal combustion, not using once through CO2, as before. Conditions examined here included oxy-coal combustion with three different RFG cleanup options and two different RFG amounts, and results consist of size segregated compositions of the ash aerosol and spatially resolved compositions within the deposits. In general, results suggested that oxy-combustion conditions did not cause major changes in ash partitioning, fouling and slagging. There are, however, changes in both the compositions of the ultrafine particles and in that of the inner layer of the deposits, and these were due to higher flame temperatures, when these occurred. These results are discussed and interpreted in detail.

Zhundong coalfield is one of the core coalfields in Xinjiang, China. Due to its high alkali and alkaline earth metal (AAEM) contents, large amounts of PM were released during Zhundong coal combustion, causing serious environmental pollution. In this paper, a novel CO2-water leaching method was proposed to suppress the PM formation and release during Zhundong coal combustion. The results showed the CO2-water leaching effectively removed water insoluble minerals from coal, such as Na, Ca, Mg, Cl and S, and significantly reduced the PM formation and release during the Zhundong coal combustion, especially for submicron particle formation and release. The releases of PM1, PM2.5, PM10 were reduced by 29.09%, 22.68% and 18.48%, respectively. The detailed characterizations showed that the significant removal of the easily vaporized elements in the coal by CO2-water leaching, such as Na, Ca, Mg, S and Cl, was an main factor for the suppression of PM formation and release. Besides, after CO2-water leaching, the combustion characteristics of Zhundong coal were promoted. Thus, the CO2-water leaching was a feasible method to inhibit the formation and release of PM during Zhundong coal combustion. Furthermore, since the flue gas from coal-fired power plant can be used as a source of CO2, the CO2-water leaching method provided a new way for the efficient utilization of the coal-fired flue gas.

Exchangeable calcium (Ca) in coal has excellent catalysis on gasification reaction. Magnesium (Mg) may have effects on this process, but the previous researches are limited and the influence mechanism is still unclear. Thus, this work aims to investigate the influence of Mg on Ca catalytic gasification process. A low-rank coal was demineralized and then loaded with Ca and Mg, separately or jointly, by ion exchange. Subsequently, the samples were gasified in CO2 or steam at 800 ℃–900 ℃ in a specially designed thermogravimetric reactor. The partially-gasified chars were collected and analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD), N2 adsorption tests, and temperature-programmed desorption (TPD) with a mass spectrometer (MS). The results show that the exchangeable Ca and Mg in coal form Ca-containing particles (CaCO3 or CaO) and MgO particles, respectively. Moreover, Mg has different impacts on the catalytic activity of Ca during CO2 and steam gasification. In CO2, Mg has a promoting effect. It is rather evident at 800 ℃, resulting from that MgO inhibits the sintering of large CaCO3 particles. These particles are mainly on the char surface and the pore structure is less developed. On the contrary, in steam, Mg has an inhibiting effect. It reduces the calcium-carbon interface and inhibits the channeling reaction process.

This paper aims to understand the characteristics of iron (Fe) and sulfur (S) in a high-ash lignite, namely Pakistani lignite that is unique and notorious for its extraordinarily high contents of ash. To date, the properties of such a unique ash and its influence on pulverised-coal fired boiler remain blank. Both bench-scale facilities and synchrotron-based X-ray adsorption spectroscopy were used to characterise these two elements. Subsequently, the high-temperature tube corrosion tests on T23 alloy coated with the Pakistani ashes were conducted under an air-firing atmosphere at 650 °C for up to 200 h. For comparison, four super-heater ash deposits collected from a 30 MWth PC boiler burning Xinjiang (XJ) lignite were also selected for the corrosion tests. The results show that, the Pakistani lignite has very high contents of Fe (up to ~28 wt% in its most stable oxide form) and S (up to ~43 wt%) in its ash, where most of these two elements are organically bounds. However, such a high-S ash caused a less mass gain per unit area of the T23 specimen than the Xinjiang ash deposits did. Compared to S, the alkalis (sodium and potassium) that are rich in Xinjiang ash deposits are more influential on tube corrosion. For both two types of ash samples, the entire tube corrosion rate is controlled by both phase boundary reaction and diffusion, where the ash-induced corrosion starts with a phase boundary control in the interface at the initial stage. The presence of ash deposit on the tube surface promotes the diffusion when a parabolic law applies subsequently. Due to the lower contents of alkalis, the Pakistani ash is less corrosive with relatively smaller oxidation kinetic rate constants than the XJ ash deposits. Additionally, with regard to the ash deposit induced corrosion kinetic constants, and the magnitude is in proportion to the content of S with XJ ash deposits, whilst has a closer correlation with the content of Fe in Pakistani ashes, which could be due to the catalytic role of Fe on the oxidation of SO2 in flue gas into SO3 that is more corrosive than the solid sulfates.

Coal combustion and mineral particle heating experiments were carried out in a drop-tube furnace at 1373 and 1573 K, respectively, to investigate the temperature effect on central-mode particulate matter (PM) formation during the combustion of coals with different mineral compositions. Two bituminous coals, coal A and coal B, with similar organic properties but different Ca/Fe mineral contents were tested. Typical minerals in the two coals, calcite and kaolinite, were used in the mineral particle heating experiments. An air atmosphere, a sample-feeding rate of 0.3 g/min, and a particle residence time of about 2 s were adopted in these experiments. The PM and bulk ash samples were collected by a low-pressure impactor and fiber filters, respectively, through a water-cooled N2-quenched probe. The elemental compositions, mass concentrations of PMs, mineral compositions, and morphologies of bulk ashes were characterized. The results show that the mass fraction size distribution of aluminum (Al) can be used to ...

This work aims to investigate the melting potential of various inorganic ash components by CCSEM (computer-controlled scanning electron microscopy). Three pulverized coals (anthracite YQ, bituminous coal DT and lignite CF) were burned in a drop tube furnace (DTF) at 1300 °C in simulated air atmosphere to generate ashes. The contents, chemical compositions and particle circularity of each mineral species in coals and ashes were identified by CCSEM. On the basis of that, the spherical particle number ratio and the thermal behaviors of ash components and their original minerals were correlated to illustrate the melting potential of ash components. Typical results are presented as below: The slight increase of content of basic elements in mullite-like phase is the major reason for its partial melting. K Al-silicate in anthracite and lignite ashes is generated from illite and tends to be melted as the low melting point of illite, but K Al-silicate in bituminous ash is mainly formed from thermally stable quartz and is unable to be melted. Iron oxide derived from mainly excluded siderite in lignite CF is difficult to be melted but that from excluded pyrite in bituminous coal DT is melted moderately, that is because of the different melting behaviors of decomposition products of siderite and pyrite (FeO or FeO–FeS, respectively).

The knowledge of siderite transformation is critical to the understanding of ash deposition behavior in oxy-fuel combustion. This work aims to explore the effect of CO 2 on the transformation of siderite in different modes under low O 2 conditions, which are common in real boilers. Two samples, i.e. pure siderite and excluded siderite simulated by the addition of siderite into a demineralized coal sample, were combusted at 1350 °C in a drop tube furnace. Two conditions, i.e. O 2 /N 2 = 7/93 and O 2 /CO 2 = 7/93 were tested for elucidating the effect of CO 2 . The combustion products were analyzed by scanning electron microscopy and X-ray diffraction. The results show that, for pure siderite, the products generated in both conditions have similar morphology and mineralogy, suggesting negligible effects of CO 2 . They are dominated by irregular particles with sharp edges, and only magnetite is detected. However, for excluded siderite, the products generated in both conditions are distinctly different in morphology and mineralogy, which indicates significant effects of CO 2 on the transformation of excluded siderite. The product generated in O 2 /CO 2 = 7/93 is dominated by irregular particles with rounded edges, while that generated in O 2 /N 2 = 7/93 is dominated by spherical particles. Wustite and magnetite are detected in both products, but the production of magnetite is much higher in O 2 /CO 2 = 7/93 than that in O 2 /N 2 = 7/93, which is attributed to the presence of a high concentration of CO 2 . Compared with pure siderite, excluded siderite experiences more reducing conditions and higher temperatures due to coal combustion, which results in the survival of wustite and more extensive melting.

The transformation of pyrite (FeS2) is critical to ash slagging in oxy-fuel combustion but has little been explored. The chemical role of CO2, the dilution gas in oxy-fuel boilers, on pyrite decomposition was investigated in this work. Pyrite samples of 63–75 μm were decomposed at temperatures between 948 and 1073 K, and in high-purity CO2 or N2 in a novel thermo-gravimetric reactor (TGR) under isothermal conditions. Sample weight loss with respect to time was recorded online and time-resolved data on the evolution of gas products were obtained by Fourier transform infrared spectroscopy (FT-IR). The solid products were characterized by X-ray diffraction (XRD). The results show that pyrrhotite is the only iron product detected in either CO2 or N2, but the pyrrhotite from CO2 contains less sulfur than that from N2. Pyrite decomposition in CO2 is noticeably faster and has lower activation energies than that in N2. These effects of CO2 are chemical in nature and are evidenced by the evolution of SO2, CO and COS in addition to sulfur clusters. Based on the data of both solid and gas products, pyrite decomposition in either CO2 or N2 is divided into three distinct stages. However, different from N2, CO2 is found to participate in pyrite decomposition at each stage. A mechanism is proposed to account for the chemical role of CO2 in pyrite decomposition.