Experimental study on ash deposition of Zhundong coal in oxy-fuel combustion

To facilitate the large-scale utilization of high-alkali and -alkaline earth metals (AAEMs) coals in power generation, the ash deposition behaviors of a typical Zhundong coal in oxy-fuel combustion were experimentally investigated using a drop tube furnace. A wall-temperature-controlled ash deposition probe by which the bulk gas temperature could be measured simultaneously was designed and employed in the experiments. The deposition tendencies, ash morphologies, chemical compositions of deposited ash particles were studied respectively under various oxygen concentrations, bulk gas temperatures, probe surface temperatures and probe exposure times. The experimental results revealed that the oxygen concentration had a significant influence on the deposition behavior during oxy-fuel combustion of high-alkali coal. Compared with air case, more fine ash particles were generated during the combustion of Zhundong coal in 21% O₂/79% CO₂ atmosphere but the deposition tendency was weaker. However, a higher oxygen concentration could aggravate the tendency of ash deposition. The high contents of iron (Fe), calcium (Ca), sulfur (S), and sodium (Na) in Zhundong coal could result in the generations of low-melting point compounds. Calcium in flue gas existed as CaO and was captured prior to SO₃ by the probe surface during the ash deposition process. At the initial 30 min of the ash deposition process, the dark spherical fine ash particles rich in Fe, Na, oxygen (O), and S were largely produced, while in the range of 60–90 min the light spherical fine ash particles with high contents of Ca, barium (Ba), O, and S were generated on the other hand. The deposition mechanisms at different stages were different and the melted CaO (BaO)/CaSO₄ (BaSO₄) would give rise to a fast growth rate of ash deposit.     

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.     

660 MW超临界直流锅炉大比例燃用准东煤燃烧调整试验分析

准东煤因其灰分低、易着火、燃尽性好等优点通常被作为优良的动力用煤,但因其煤灰中碱金属含量较高,容易出现受热面沾污、腐蚀、结渣等危害锅炉运行安全的问题.在大比例燃用准东煤的过程中,为了增大锅炉热效率、减小热偏差、降低污染物排放量、防止受热面结渣和提高锅炉燃烧经济性,首先通过摸底试验测定了某660 MW锅炉目前的运行情况及...     

Release and transformation characteristics of Na/Ca/S compounds of Zhundong coal during combustion/CO2 gasification

Zhundong (ZD) coal from northwest China is a high quality steam coal with reserves of more than 390 billion tons. However, the utilization of ZD coal is limited due to the high content of alkali and alkaline earth metals. This study aimed at revealing the release and transformation mechanism of Na/Ca/S compounds during combustion/gasification of ZD coal. The results demonstrate that Na was primarily influenced by temperature, mostly releases at 600–800 °C. The transformation of Ca compounds was affected by both temperature and atmosphere. The high temperature of the combustion process could accelerate the decomposition of CaCO₃ and CaSO₄, and the high content of CO₂ during gasification prolonged the decomposition of CaCO₃. The transformation of S was primarily influenced by atmosphere. SO₂ could react with CaO and form CaSO₄ during the combustion process. While S compounds were mainly released as S (g) and H₂S (g) during gasification process. There was a significant interaction among Na/Ca/S compounds during combustion, original CaSO₄ in coal could adsorb Na compounds with SO₂ at 600–800 °C and then reacted with aluminosilicates, by this reaction, Na could be fixed above 1000 °C.     

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.     

不同采矿深度准东煤物化特性和燃烧性能研究

选取天池能源浅层和深层煤样进行研究,以大同烟煤作为对比,了解采矿深度对煤质及燃烧特性的影响,为准东煤的勘探和燃烧提供必要的理论依据。研究发现,随着采矿深度的增加,煤中水分减少、灰分降低、发热量增加、含硫量减少,煤质更加接近烟煤;煤灰中Fe2O3明显减少,煤的结渣趋势减轻;煤的着火温度略有降低,燃烧速率提高,燃烧特性变好。与大同烟煤相比,准东煤高水分、低发热量、低氮含量,燃烧时着火温度显著降低且燃烧速率低。     

准东煤灰与液态排渣煤粉炉耐火材料的烧结特性

采用TGA-DSC分析确定了准东煤灰和其混合灰样(不同质量比的准东煤灰和耐火材料)燃烧过程中的特征温度,并分别采用XRD和FSEM-EDS对不同特征温度段灰样进行矿物识别和形貌、能谱分析,得到了原灰与混合灰的烧结温度、灰中主要矿物的转化和熔融过程,并对比了不同耐火材料含量的煤灰熔融温度;在此基础上提出了耐火材料构型的极...     

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.     

Influence of pyrolysis pressure on structure and combustion reactivity of Zhundong demineralized coal char

The investigation on evolution of coal char structure during pressurized pyrolysis can reveal the combustion reactivity of coal char in thermal utilization at elevated pressure. In this study, Zhundong subbituminous coal was demineralized and a pressurized drop tube reactor (PDTR) was used to prepare coal char under different temperature and pressure conditions. The physicochemical structures of raw and demineralized coal chars were characterized by the application of nitrogen adsorption analyzer, automatic mercury porosimeter, and Fourier transform infrared spectroscopy (FTIR). The change mechanism of char infrared structure with pyrolysis pressure is revealed on the molecular level in this paper. The results show that the N₂ adsorption quantity of raw coal char increases with the increase of pyrolysis temperature, while that of demineralized coal char decreases. Because of the difference in molecular volume and steric hindrance between aliphatic and aromatic structure in char, the increasing pressure has less inhibition effect on the escape of the former than the latter. With the increase of pyrolysis pressure, the combustion reactivity of char is related to the infrared structure at 700 and 800 °C while to macropore structure at 900 and 1000 °C.     

Effect of ash on dielectric properties and micro-structure of high alkali coal at different temperature pyrolysis

The effect of ash on dielectric properties and micro-structure of high alkali coal at different temperature pyrolysis was studied, so as to provide theoretical basis for coal deep processing by microwave. An acid-washed method was adopted to remove ash in Zhundong coal for preparing coal chars at 700 °C–1300 °C. X-ray diffraction analysis was used to characterize the microcrystal structure. The thermal stability was characterized by thermogravimetric analyzer, and the dielectric properties were measured by a vector network analyzer. The results showed that when the pyrolysis temperature was below 1100 °C, the presence of ash hindered the development of carbon structure in raw coal char. The main reason is that the alkali metal oxides (K₂O and Na₂O) in the ash promoted the solution loss reaction during pyrolysis. The structure of the original carbon layer was damaged, thereby the graphitization degree, thermal stability and dielectric properties of raw coal char were weaker than the ash-free coal char. When the pyrolysis temperature reached 1300 °C, the minerals were completely melted. The reaction of phase transition of SiO₂ in ash played a catalytic role on raw coal char structure, resulting in tighter arrangement of adjacent carbon layers. The raw coal char showed stronger dielectric properties and thermal stability.     

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.     

Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor

Growing concerns over greenhouse gas emissions have driven extensive research into new power generation cycles that enable carbon dioxide capture and sequestration. In this regard, oxy-fuel combustion is a promising new technology in which fuels are burned in an environment of oxygen and recycled combustion gases. In this paper, an oxy-fuel combustion power cycle that utilizes a pressurized coal combustor is analyzed. We show that this approach recovers more thermal energy from the flue gases because the elevated flue gas pressure raises the dew point and the available latent enthalpy in the flue gases. The high-pressure water-condensing flue gas thermal energy recovery system reduces steam bleeding which is typically used in conventional steam cycles and enables the cycle to achieve higher efficiency. The pressurized combustion process provides the purification and compression unit with a concentrated carbon dioxide stream. For the purpose of our analysis, a flue gas purification and compression process including de-SO_x, de-NO_x, and low temperature flash unit is examined. We compare a case in which the combustor operates at 1.1 bars with a base case in which the combustor operates at 10 bars. Results show nearly 3% point increase in the net efficiency for the latter case.     

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.     

微富氧条件下煤粉燃烧及NO生成特性的研究

利用热重分析技术对微富氧条件下煤粉的燃烧特性进行了研究,并与富氧条件下煤粉的燃烧特性进行了对比,利用固定床测定了燃煤NO的生成规律,分析了反应气氛和煤种的影响．结果表明：随着氧体积分数增加,微富氧条件下煤粉的燃烧向低温区移动,综合燃烧特性指数S逐渐增大;在相同的氧体积分数下,由于N2和CO2的物性差异,煤粉的微富氧燃烧特性优于富氧燃烧特性,但当氧体积分数升高到40％时,两种气氛的燃烧特性差别不大;反应气氛和煤种均对燃料氮的转化率影响显著;氧体积分数升高或N2的参与会使反应温度上升,影响燃料氮的转化率;煤的挥发分和元素氮的质量分数也会影响燃料氮的转化率．     

Combustion performance and slagging characteristics during co-combustion of Zhundong coal and sludge

Zhundong coal (ZDc) with a very large reserve is faced with severe problems of slagging and fouling during combustion in boilers because of the high-Na content. Sludge, the by-product of urban sewage treatment, is also faced with the problem in utilization. In this study, the co-combustion of ZDc and sludge was investigated in a laboratory-scale experimental apparatus before further studies in larger-scale setups. The experimental results confirm an interaction between ZDc and sludge during co-combustion, which was mainly caused by the Na catalytic action and improved the combustion performance of the co-fuels. The catalytic effect was particularly significant at low sludge mixing ratios. The reactions between Na-based compounds in ZDc and Si/Al/P-rich minerals in sludge, forming high-melting-point phosphates and aluminosilicates, not only increased Na retention in residual ash reducing the risk of fouling on tail-heating surfaces in boilers, but also raised the ash fusibility of the co-fuels avoiding low-temperature sintering. Even so, to prevent slagging, the high combustion temperature above 900 °C should be avoided during co-combustion because of the high Na retention in residual ash. Moreover, the high heavy metal retention in residual ash decreased the pollution caused by heavy metal volatilization during sludge combustion.     

The effect of wood biomass blending with pulverized coal on combustion characteristics under oxy-fuel condition

In this study, combustion from the co-firing of coal and wood biomass, and thermal characteristics such as ignition temperature, burn-out temperature, and activation energy were discussed using a thermogravimetric analyzer (TGA). We investigated the effects of biomass blending with two kinds of pulverized coal (bituminous Shenhua, and sub-bituminous Adaro) under air and oxy-fuel conditions. The coal fraction in the blended samples was set to 1, 0.8, and 0.5. The oxygen fraction in the oxidant was set to 0.21, 0.3, 0.5, and 0.8. The ignition temperature was governed by the fuel composition, particularly in the blended biomass which has a much higher content of volatile matter comparing to coal. However, the burnout temperature, which shows a strong relationship with char combustion, depended on the oxidant ingredients rather than on the fuel components. Thermal characteristics such as ignition, burnout temperature, reaction region, and heat flow were very similar between air and a 0.3 oxygen concentration under oxy-fuel conditions with Shenhua coal.     

Furnace water-wall slag deposition testing in a 0.5 MWt combustion pilot plant

This article presents combustion pilot-plant test results obtained using newly developed ash deposition probes (ADPs) designed to measure both the thermal and mechanical properties of furnace ash deposits. The ADP measurements are obtained at multiple elevations in the furnace to provide detailed heat absorption and ash deposition time constant profiles for 8 h soot-blowing cycles. Profiles of incident radiation are also presented. Results are presented for four different coal types representing major steam-coal-producing regions in the United States. The furnace heat absorption profiles are distinctly different for each coal type and also vary significantly with furnace elevation. Combined furnace heat absorption and time constant data are used to recommend optimal use of soot-blowing resources, or to specify soot blower and water-wall surface addition and location according to coal type.     

Thermal analysis and kinetics of coal during oxy-fuel combustion

The pyrolysis and oxy-fuel combustion characteristics of Polish bituminous coal were studied using non-isothermal thermogravimetric analysis. Pyrolysis tests showed that the mass loss profiles were almost similar up to 870℃ in both N_2 and CO_2 atmospheres, while further mass loss occurred in CO_2 atmosphere at higher temperatures due to char-CO_2 gasification. Replacement of N_2 in the combustion environment by CO_2 delayed the combustion of bituminous coal. At elevated oxygen levels, TG/DTG profiles shifted through lower temperature zone, ignition and burnout temperatures decreased and mass loss rate significantly increased and complete combustion was achieved at lower temperatures and shorter times. Kinetic analysis for the tested coal was performed using Kissinger-Akahira-Sunose(KAS) method. The activation energies of bituminous coal combustion at the similar oxygen content in oxy-fuel with that of air were higher than that in air atmosphere. The results indicated that, with O_2 concentration increasing, the activation energies decreased.     

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.