Effects of oxy-fuel condition on morphology and mineral composition of ash deposit during combustion of Zhundong high-alkali coal

Zhundong coalfield is a super-large coal reserve, with high-alkali feature exacerbating ash deposition. Oxy-fuel combustion technology could propel the clean utilization of Zhundong high-alkali coal. While the ash deposition behavior of high-alkali coal under oxy-fuel condition has yet to be sufficiently investigated. The present study compared the differences of ash deposits between oxy-fuel and air combustion, and also examined the effects of oxygen content on ash deposition mechanism, employing a drop-tube furnace equipped with a specially designed sampling probe and some analysis methods, such as X—ray diffraction equipment, simultaneous thermal analyzer, etc. Experimental results indicated that ash deposition was weaker, with fewer contents of sodium chloride, calcium sulphate and less agglomeration ash in oxy-fuel atmosphere compared to the air case with same oxygen content. The content of the ash particle distributed in the range of 0–40 μm was up to 60% under oxy-fuel condition. The first weight loss of ash deposits, around 850 °C, was put down to the decomposition of carbonate and the second one, about 1150 °C, was ascribed to the decomposition of the sulphate minerals in the thermal process. Ash deposition worsened with more large particles (>120 μm), as the oxygen content rose. Sodium chloride content reached 9.7% with 50% oxygen content. The present study not only focuses on the morphology and chemical components, but also probes into the thermal volatility of ash deposits, which benefits the further understanding of the ash deposition mechanism and utilization of Zhundong high-alkali coal during oxy-fuel combustion.     

Investigation on ash deposition characteristics during Zhundong coal combustion

The reserves of Zhundong (ZD) coal in China are huge. However, the high content of Na and Ca induces serious slagging and fouling problems. In this study, the ZD coal was burned in a DTF (drop tube furnace), and the ashes collected at different gas temperature with non-cooling probe were analyzed to obtain the ash particle properties and their combination mode. The results showed that Na, Ca and Fe are the main elements leading to slagging when the gas temperature is about 1000 °C during ZD coal combustion, but their mechanisms are quite different. Some sodium silicates and aluminosilicates and calcium sulfate keep molten state in the ashes collected at 1000 °C. These molten ash particles may impact and adhere on the bare tube surface, and then solidified quickly. With the growth of slag thickness, the depositing surface temperature is increased. The molten ash particles might form a layer of molten film, which could capture the other high fusion temperature particles. The Fe₂O₃ sphere were captured by the formed molten slag and then they blended together to form a new molten slag with lower melting temperature.     

Effects of leaching and additives on the ash fusion characteristics of high-Na/Ca Zhundong coal

Considering the intensive application of Zhundong coal and its serious slagging during combustion in pulverized coal fired boilers, the effects of water leaching and different dosages of additives (Na₂CO₃, CaO, SiO₂, and kaolin) into the leached coal on ash fusion characteristics were studied using sintering tester, X-ray Fluorescence, and X Ray Powder Diffraction. After leaching, the contents of Na₂O, K₂O, CaO, MgO, SO₃, and Cl decreased, whereas the contents of SiO₂ and Fe₂O₃ increased. As a result, the slagging and fouling index of Basic/Acid ratio decreased from 1.6 in the raw coal ash to 1.1 in the leached coal ash, and alkali-induced slagging was reduced. However, the decreased ash fusion temperatures (AFTs) of the leached coal worsen the melt-induced slagging. Detailed analysis on the effects of ash compositions on the AFTs by additions of Na₂CO₃, CaO, SiO₂, and kaolin additives into the leached coal showed that Na₂CO₃ and SiO₂ improved the formation of low-melting Na₂SO₄ and Na₂SiO₃, and thus decreased the AFTs; However, overdosed Na₂CO₃ (for instance 9 wt%) improved the formation of relative high-melting Na₄Al₂Si₂O₉ and thus increased the AFTs. Overdosed SiO₂ as a high-melting contaminant also increased the AFTs. Similar with the function of overdosed SiO₂, CaO in the ash of leached coal was excess, and the AFTs increased with increased CaO dosage. Kaolin, which decomposed as high-melting mullite and cristobalite, caused an elevation of the AFTs. Therefore, water leaching decreased the AFTs of Zhundong coal and consequently worsen melt-induced slagging, lime and kaolin additives increased the AFTs effectively, but Na₂CO₃ and SiO₂ additives showed ‘V’ shaped functions on the AFTs. These general results are useful for the pretreatment of Zhundong coal during combustion and alleviating slagging.     

Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace

Zhundong coalfield is one super-large coalfield recently discovered in China. However, the utilization of Zhundong coal in power plants has caused serious ash-related issues mainly due to its high-sodium feature. The ash deposition problem on convection heat exchanger surfaces is still particularly difficult to resolve and its mechanism has yet to be fully understood. This study deals with the ash deposition and alkali metal migration behaviors on convection heat exchanger surfaces between 400 and 800 °C during combustion of Zhundong coal using a lab-scale drop tube reactor. Experimental results show that the sodium content in ash deposit of Zhundong coals increases obviously as the deposition temperature decreases from 800 to 600 °C, while it is almost unchanged below 600 °C. The contents of iron and calcium in ash deposits exhibit nonmonotonic variations as the deposit probe temperature varies between 400 and 800 °C. Quartz and calcium sulfate are main crystalline phases in ash deposit of Zhundong coals. Calcium is inclined to present as calcite and lime at low deposition temperature, while high temperature facilitates calcium sulfation. Sodium of crystalline phase is found as albite and sodium sulfate at low deposition temperature. Both condensation of gaseous alkali metals and formation of low-melting minerals were responsible for the ash deposition phenomenon on convection heat exchanger surfaces involved in combustion of Zhundong coal. The sodium content in ash deposit decreases considerably with the increasing combustion temperature while the case of iron variation is opposite due to its low-volatility. In addition, the Na content in ash deposits increases obviously with the access air ratio reduced from 1.2 to 1.05, but the local weakly reducing atmosphere leads to less iron within ash deposits. Clarification of sodium migration and evaluation of ash deposition behaviors during combustion of Zhundong coal is helpful for a better exploration of the functional mechanism of ash deposit and then large-scale utilization of high-sodium coals.     

The correlations of chemical property,alkali metal distribution,and fouling evaluation of Zhundong coal

The utilization of Zhundong coal has encountered severe ash-related problems principally due to its high-alkali feature. However, the evaluation of fouling tendency and correlations of chemical properties of Zhundong coals based on a large database has been seldom performed. The present work aimed to reveal the correlations of chemical properties, distributions of alkali and alkaline earth metals (AAEMs) and iron, and the fouling propensity of Zhundong high-alkali coals based on datasets of >250 samples. The fuel ratio of the majority (∼95%) of Zhundong coals lay in the range of 1.0–2.45. A quantified correlation between fuel ratio and volatile matter content was obtained. The ash composition of Zhundong coal gathered in a small area of the ternary diagram of Fe₂O₃-alkali metal-alkaline earth metal oxide components. The principal sodium of water-soluble form in Zhundong coals varied notably from 45% to 95%, more than half of the potassium was insoluble form, the majority (∼90%) of alkaline earth metals were in forms of ammonium acetate-soluble and HCl-soluble, and ∼67% of iron was HCl-soluble form. The quantitative correlations based on plentiful datasets could improve the comprehension of correlations among the coal chemical properties, fouling behaviors and its reaction activity.     

Single particle ignition and combustion of anthracite,semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions

A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O₂/79%N₂) and four simulated dry oxy-fuel conditions: 21%O₂/79%CO₂, 30%O₂/70%CO₂, 35%O₂/65%CO₂ and 50%O₂/50%CO₂. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N₂ gas of air with CO₂ (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO₂ to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO₂. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.     

Air and oxy-fuel combustion kinetics of low rank lignites

The air and oxy-fuel combustion processes of two low-grade lignite coals were investigated by thermogravimetric analysis (TGA) method. Coals were provided from two different coal mines in the Aegean region of Turkey. Oxy-fuel combustion experiments were carried out with three different gas mixtures of 21% O₂–79% CO₂; 40% O₂–60% CO₂ and 50% O₂–50% CO₂ at 950 °C and heating rates of 10 °C/min, 20 °C/min and 40 °C/min. The kinetics of the oxy-fuel combustion of coals were studied by using four different methods namely, Coats-Redfern (model-fitting method), Friedman (FR), Flynn–Wall–Ozawa's (FWO) and Kissinger–Akahira–Sunose's (KAS) methods. The apparent activation energies of combustion process calculated by FWO method are slightly but systematically higher than that calculated by the KAS and FR methods for the oxy-fuel atmospheres. Combustion behavior of both coals in the oxy-fuel combustion environment could vary significantly, likely due to their characteristics such ash and volatile matter contents.     

Oxy-fuel combustion of solid fuels

Oxy-fuel combustion is suggested as one of the possible, promising technologies for capturing CO₂ from power plants. The concept of oxy-fuel combustion is removal of nitrogen from the oxidizer to carry out the combustion process in oxygen and, in most concepts, recycled flue gas to lower the flame temperature. The flue gas produced thus consists primarily of carbon dioxide and water. Much research on the different aspects of an oxy-fuel power plant has been performed during the last decade. Focus has mainly been on retrofits of existing pulverized-coal-fired power plant units. Green-field plants which provide additional options for improvement of process economics are however likewise investigated. Of particular interest is the change of the combustion process induced by the exchange of carbon dioxide and water vapor for nitrogen as diluent. This paper reviews the published knowledge on the oxy-fuel process and focuses particularly on the combustion fundamentals, i.e. flame temperatures and heat transfer, ignition and burnout, emissions, and fly ash characteristics. Knowledge is currently available regarding both an entire oxy-fuel power plant and the combustion fundamentals. However, several questions remain unanswered and more research and pilot plant testing of heat transfer profiles, emission levels, the optimum oxygen excess and inlet oxygen concentration levels, high and low-temperature fire-side corrosion, ash quality, plant operability, and models to predict NO_x and SO₃ formation is required.     

Transformation characteristics of Na and K in high alkali residual carbon during circulating fluidized bed combustion

In this study, the transformation characteristics of sodium (Na) and potassium (K) during combustion of Zhundong coal gasification fly ash in circulating fluidized bed (CFB) reactors were investigated by examining gasification fly ash (TCf) from a 0.1-MW CFB test system. Experimental results indicated that TCf was rich in Na and K, with water-soluble and insoluble Na the main Na forms. Insoluble K was the major K form in TCf, accounting for 70.6% of total K. Reactor bed temperature exerted important effects on Na release during combustion such that, as bed temperature increased, the proportions of Na in bottom and circulating ash decreased while the Na in fly ash increased. Hydrochloric acid-soluble and insoluble Na in ash accounted for a large fraction of total Na. However, insoluble K was the principle K form in ash and bed temperature showed little influence on K release and distribution in ash during combustion. With decreased flue gas temperature, the Na content in deposition ash initially increased, then decreased, and eventually stabilized, while the K content in deposition ash was basically unchanged. Agglomeration of ash particles occurred during combustion, being more apparent at higher gas temperatures, and the agglomerates were rich in Na, K, sulfur (S), chlorine (Cl), and calcium (Ca). Deposition ash Na was mainly contained NaCl and Ca/Na sulfates. The enrichment of these salts as well as of Ca sulfate in ash was the main cause of ash agglomeration and deposition.     

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO₂ and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO₂ and H₂O reaction rate coefficients for a 100 μm coal char particle reacting in environments of varying O₂, H₂O, and CO₂ concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO₂ gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O₂ concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.     

Combustion behavior of single particles from three different coal ranks and from sugar cane bagasse in O2/N2 and O2/CO2 atmospheres

The combustion behavior of single fuel particles was assessed in O₂/N₂ and O₂/CO₂ background gases, with oxygen mole fractions in the range of 20–100%. Fuels included four pulverized coals from different ranks (a high-volatile bituminous, a sub-bituminous and two lignites) as well as pulverized sugarcane-bagasse, a biomass residue. Particles of 75–90 μm were injected under laminar flow in a bench-scale, transparent drop-tube furnace (DTF), electrically-heated to 1400 K where, upon experiencing high heating rates, they ignited and burned. The combustion of individual particles was observed with three-color optical pyrometry and high-speed high-resolution cinematography to obtain temperature and burnout time histories. Based on combined observations from these techniques, a comprehensive understanding of the behaviors of these fuels was developed under a variety of conditions, including simulated oxy-fuel combustion. The fuels exhibited distinct combustion behaviors. In air, the bituminous coal particles burned in two distinctive modes; the volatiles burned in bright envelope flames surrounding the devolatilizing char particles followed by heterogeneous char combustion. The volatile matter of sub-bituminous coal particles burned either in subdued envelope flames, surrounding devolatilizing and occasionally fragmenting chars, or heterogeneously at the char surface. Lignite particles typically burned with extensive fragmentation, and their volatiles burned simultaneously with the char fragments. The volatiles of bagasse particles burned in spherical and transparent envelope flames. Increasing the oxygen mole fraction in N₂, increased flame and char surface temperatures, and decreased burnout times; particles of all fuels burned more intensely with an increasing tendency of the volatiles to burn closer to the char surface. When the background gas N₂ was substituted with CO₂, the combustion of all fuels was distinctly less intense; at moderate O₂ mole fractions (<30%) most particles did not ignite under active flow conditions in the furnace (they did ignite under quiescent gas flow conditions in the DTF). Increasing the oxygen mole fraction in CO₂ increased the likelihood of combustion and its intensity. Combustion of volatiles in envelope flames was suppressed in the presence of CO₂, particularly under active gas flow in the DTF.     

准东煤灰熔融特性试验研究

将神华准东煤（神华煤）和天池能源准东煤（天池煤）与碱沟煤按照不同质量掺混比进行混合并制得灰样,将NaCl、CaO、Al2O3和SiO2按不同添加比例加入神华煤和天池煤并制成灰样,对上述混合灰样的熔融特性进行研究.结果表明：碱沟煤掺混2种准东煤后,随着准东煤质量掺混比的增大,混合灰各个灰熔点特征温度先降低后升高;随着灰样中Na含量增加,准东煤灰样的变形温度显著降低,软化温度、半球温度和流动温度先降低后趋于不变;当灰样中Na含量达到一定比例后,NaCl对准东煤灰熔融特性的影响明显减弱;CaO对准东煤灰熔点的影响较复杂,可以降低也可以提高灰熔点;随着Al2O3添加比例的增加,准东煤灰熔点先升高后急剧降低;随着SiO2添加比例的增加,神华煤灰样的变形温度先升高后降低,而天池煤灰样的变形温度逐步升高,其他3个特征温度均逐渐降低.     

Effect of oxy-fuel combustion with steam addition on coal ignition and burnout in an entrained flow reactor

The ignition temperature and burnout of a semi-anthracite and a high-volatile bituminous coal were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under oxy-fuel atmospheres (21%O₂-79%CO₂, 30%O₂-70% O₂ and 35%O₂-65%CO₂) were compared with those attained in air. The replacement of CO₂ by 5, 10 and 20% of steam in the oxy-fuel combustion atmospheres was also evaluated in order to study the wet recirculation of flue gas. For the 21%O₂-79%CO₂ atmosphere, the results indicated that the ignition temperature was higher and the coal burnout was lower than in air. However, when the O₂ concentration was increased to 30 and 35% in the oxy-fuel combustion atmosphere, the ignition temperature was lower and coal burnout was improved in comparison with air conditions. On the other hand, an increase in ignition temperature and a worsening of the coal burnout was observed when steam was added to the oxy-fuel combustion atmospheres though no relevant differences between the different steam concentrations were detected.     

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO₂ is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO₂ capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH₄ concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO₂ capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO₂ capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.     

Characterization of ashes from a 100 kWth pilot-scale circulating fluidized bed with oxy-fuel combustion

Oxy-fuel combustion experiments have been carried out on an oxygen-fired 100 kW_th mini-circulating fluidized bed combustion (CFBC) facility. Coal and petroleum coke were used as fuel together with different limestones (and fixed Ca:S molar ratios) premixed with the fuel, for in situ SO₂ capture. The bed ash (BA) and fly ash (FA) samples produced from this unit were collected and characterized to obtain physical and chemical properties of the ash samples. The characterization methods used included X-ray fluorescence (XRF), X-ray diffraction (XRD), char carbon and free lime analysis, thermogravimetric analysis (TGA), and surface analysis. The main purpose of this work is to characterize the CFBC ashes from oxy-fuel firing to obtain a better understanding of the combustion process, and to identify any significant differences from the ash generated by a conventional air-fired CFBC. The primary difference in the sulfur capture mechanism between atmospheric air-fired and oxy-fuel FBC, at typical FBC temperatures (∼850 °C), is that, in the air-fired case the limestone sorbents calcine, whereas the partial pressure of CO₂ in oxy-fuel FBC is high enough to prevent calcination, and hence the sulfation process should mimic that seen in pressurized FBC (PFBC). Here, the char carbon content in the fly ash was much higher than that in the bed ash, and was also high by comparison with ash obtained from conventional commercial air-firing CFBC units. In addition, measurements of the free lime content in the bed and fly ash showed that the unreacted Ca sorbent was present primarily as CaCO₃, indicating that sulfur capture in the oxy-fuel combustor occurred via direct sulfation. Limestone utilization for oxy-fuel combustion in this unit was generally lower than that in industrial-scale air-firing CFBCs, with better limestone performance found during combustion of petcoke running at relatively higher temperatures. The Brunauer–Emmett–Teller (BET) surface area and also the pore volume in the fly ash were much higher than in the bed ash and smaller size pores predominated in the fly ash samples.     

Experimental study on effects of combustion atmosphere and coal char on NO2 reduction under oxy-fuel condition

Nitrogen oxides (NO_x) as the principal air pollutants are mainly from the combustion of fossil fuels. Oxy-fuel combustion is a promising clean coal technology, by which carbon dioxide (CO₂) can be captured in large-scale and NO_x emission can be reduced significantly. The formation of nitrogen dioxide (NO₂) in oxy-fuel combustion exceeds that under traditional air condition. However, the specific studies on NO₂ chemistry under oxy-fuel condition are still insufficient and the functional mechanisms of minerals and combustion atmosphere on NO₂ reduction have yet to be fully understood. The objective of present study is to experimentally clarify the effects of combustion atmosphere and coal char on NO₂ reduction in oxy-fuel combustion using a fixed-bed reactor. Experimental results showed that the decomposition of NO₂ had a strong temperature dependence and the NO₂ reduction rate showed a positive variation with temperature. The strength of catalytic activity in NO₂ reduction to nitric oxide (NO) was Fe₂O₃ > MgO > CaO > Al₂O₃ > Na₂CO₃ > K₂CO₃ > SiO₂. In addition, the increased concentrations of carbon monoxide (CO) and CO₂ could promote the reduction of NO₂, while the low content of CO₂ only established a slight impact on NO₂ reduction. However, the increase of oxygen (O₂) concentration displayed an inhibition effect on NO₂ reduction to a certain extent. The variation of atmosphere in oxy-fuel combustion generated a substantial influence on the creation and reduction of NO₂. The char prepared in lower temperature exhibited a higher promotion effect on the consumption of NO₂. Higher contents of fixed carbon and basic oxides had more obvious stimulation effects on NO₂ reduction. Fixed carbon had a superior activity in NO₂ reduction than ash. The kinetic analysis indicated that high content of CO and the presence of char could reduce the apparent activation energy of NO₂ reduction. The present study can be helpful to improve the understanding of NO₂ chemistry in oxy-fuel combustion.     

The influence of the addition of sodium on the transformation of alkali and alkaline-earth metals during oxy-fuel combustion

Alkali and alkaline-earth metals (AAEM) of coal directly affect the coal combustion properties and ash formation during coal oxy-fuel combustion. To further understand the influence of adding sodium on the transformation of AAEM, sodium chloride (NaCl) and sodium acetate (NaAc) were added to Shenmu coal in this study. A drop-tube reactor and ion chromatography were adopted in this study and a serial dissolution method was used to clarify the occurrence modes of the AAEM. The results showed that all types of AAEM can release and the release rates were increased with an increase in temperature during oxy-fuel combustion. Water-soluble (W-type) alkali metals react with SiO₂ and Al₂O₃ in coal and are converted into acid-soluble (H-type) silicate or acid-insoluble (I-type) aluminosilicate under certain experimental conditions. The addition of sodium can promote the release of AAEM via promoting coal combustion; the promotion effect was significant at 600 °C, and the effect of NaCl was more noticeable than that of NaAc. Furthermore, the promoting effect on alkali metals was more noticeable than that on alkali-earth metals. The added sodium can also react with SiO₂ and Al₂O₃ to form H-type sodium silicate or I-type sodium aluminosilicate.     

Effects of ash fouling on heat transfer during combustion of cattle biomass in a small‐scale boiler burner facility under unsteady transition conditions

Combustion of cattle biomass (CB) as a supplementary fuel has been proposed for reducing emissions of NO_x, Hg, SO₂, and nonrenewable CO₂ in large coal‐fired power plants; however, its high ash content resulted in fouling and slagging problems when the CB was co‐fired with coals during small‐ and pilot‐scale tests. Ash depositions during combustion of the CB as a reburn fuel were investigated using a 30 kW_t (100 000 Btu h^?1) boiler burner facility with water‐cooled heat exchangers (HEXs) under unsteady transition conditions and short‐term operations. Two parameters were used to characterize the effects of the ash deposition: (1) Overall heat transfer coefficient (U) and (2) Burnt fraction (BF). A methodology was presented and empirically demonstrated for the effects of ash deposition on heat transfer under unsteady transition conditions. Experiments involving ash deposition during reburning the CB with coals were compared with experiments involving only ash‐less natural gas. It was found that the growth of the ash layer during reburning the CB and coals lowered the heat transfer rate to water in the HEXs. In low‐temperature regions, the thin layer of the ash deposition promoted radiation effects, while the thick layer of the ash deposition promoted the thermal resistance of the ash layer. A chemical analysis of the heavy ash indicated that the BF increased when a larger fraction of the CB was used in the reburn fuels, indicating better performance compared with coal combustion alone. However, the results of ash fusion temperature indicated the ash deposited during combustion of the CB and coals was more difficult to remove than the ash deposited during coal combustion alone. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical simulation of pulverized coal MILD-oxy combustion under different oxygen concentrations

As an emerging clean coal combustion technology, Moderate or Intense Low-Oxygen Dilution (MILD) combustion or oxy-fuel combustion, compared with traditional coal combustion, has many advantages. However, compared with MILD combustion and oxy-fuel combustion, MILD-oxy combustion is believed more attractive. In this work, MILD-oxy combustion characteristics with oxygen concentrations from 10% to 50% are studied numerically. The results show that within a certain range, increasing the oxygen concentration is in favor of MILD-oxy combustion performance close to that of MILD-air combustion. When the oxygen concentration is higher enough, the momentum reduced by the increase of oxygen concentration has a great influence on the furnace temperature. With the increase of oxygen concentration, the radiation heat transfer is enhanced and the convective heat transfer is weakened. The increase of oxygen concentration can promote the occurrence of char gasification reaction with CO₂. In addition, MILD-oxy combustion has a large impact on CO emission.     

Characteristics of oxy-fuel combustion in gas turbines

This paper reports on a numerical study of the thermodynamic and basic combustion characteristics of oxy-fuel combustion in gas turbine related conditions using detailed chemical kinetic and thermodynamic calculations. The oxy-fuels considered are mixtures of CH₄, O₂, CO₂ and H₂O, representing natural gas combustion under nitrogen free gas turbine conditions. The GRI Mech 3.0 chemical kinetic mechanism, consisting of 53 species and 325 reactions, is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a high intensity turbulence mixing condition where the combustion chamber is assumed to be a well-stirred reactor, and a typical non-premixed flame condition where chemical reactions occur in thin flamelets. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. The numerical calculations suggest that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas.