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This article investigates the performance of a novel hybrid heat pump system, which is developed to recover the waste heat from the flue gas of domestic heating boiler. The purpose of proposed system is: to increase the return water temperature, to heat the boiler supply air, and to provide the hot air for indoor heating. The developed system consists of an air heater (acts as a dedicated mechanical sub-cooler), a multistage condensing economizer and a vapor compression heat pump. A test case of common building is considered for the performance assessment of system, which is based on the 4E analysis. For this purpose, ten refrigerants are selected. The results show that about 25.58% increase in the coefficient of performance can be obtained for the proposed model as compared to simple vapor compression cycle if operated with R245fa, which also have a lowest exergy destruction rate (38.27%) as compared to other studied refrigerants. From the perspective of improvement in boiler’s energy and exergy efficiency, R114 is used to be the most effective among other refrigerant (10.33% increase in energy efficiency and 4.74% increase in exergy efficiency). Parametric study results have shown that the variation in ambient conditions should be considered carefully while selecting a refrigerant to significantly improve the system performance, exergy efficiency, and relevant economics on regional basis. From the obtained results, a novel yet simple refrigerant selection approach is proposed which can also be adopted for other thermodynamic cycles.

Oxygen-enriched and deep-staging (OE and DS) combustion has drawn increasing attention due to its superior nitrogen oxide reduction efficiency and combustion efficiency. To clarify the NOx generati...

The utilization of powdery semi-coke as a power fuel in pulverized coal-fired power plants has become a new and potential technique to consume the excess powdery semi-coke. The characteristic of low volatile results in poor combustion performance and high NOx emission, and to co-fire with bituminous coal is a practical strategy to address this problem. However, the co-combustion characteristics and the inherent interaction between semi-coke and coal remain insufficiently understood. In addition, the influences of secondary air arrangement, the boiler operation load, and the fuel type on co-combustion process are still unclear, which is urgent to be further explored. In the present study, experiments and numerical simulations were jointly utilized to inquire into the co-combustion behaviors and NOx emission features of semi-coke and coal. The results demonstrated that the “out-furnace method” was a suitable choice for small-capacity boiler when the proportion of semi-coke was 33%, due to the limited combinations of the semi-coke injection position. It was recommended that semi-coke was preferred to be injected from the middle layers of the furnace under the “in-furnace method” to improve the overall co-combustion performance. The critical value of the separated over fire air ratio in this study was 27.5%, over which a slight drop of carbon content in fly ash could come about. Moreover, the elevation in the proportion of separated over fire air gave rise to the significant decline of NOx concentration. The constricted secondary air arrangement was preferred to be employed due to the high boiler efficiency. The separated over fire air and the surrounding air needed to maintain a wide-open degree to prevent the increase of NOx emissions and the coking of nozzles. For the load reduction regulation method adopted in this study, the NOx concentration first rose and then dropped, while the burnout ratio decreased obviously as the operation load was reduced. Different combinations of coal and semi-coke generated significant influences on co-combustion behaviors within the furnace. The NOx generated by high-volatile fuel (bituminous coal) combustion was mainly affected by volatile-N, while the NOx generated by low-volatile fuel (semi-coke) was mainly impacted by char-N. This study is of guiding significance for the efficient and clean utilization and beneficial to the large-scale application of powder semi-coke in power plants.

Zhundong high-alkali coal has been studied a lot owing to its abundant reserve but serious ash-related issues. Oxy-fuel combustion profits the reduction of pollutant emissions and coal blending alleviates the ash-related problems practically. Nevertheless, little work if any has been carried out on oxy-fuel co-combustion of Zhundong coal, while the influences of blending on the migration of iron and alkali alkaline-earth metals have yet to be adequately understood. This study aims to illuminate the release and migration behaviors of alkali alkaline-earth metals, iron and aluminum in oxy-fuel co-combustion via a lab-scale experimental system. Experimental results showed that the yield, color, bulk, and composition of ash were greatly temperature-dependent and significantly affected by the blending ratio in oxy-fuel co-combustion. As the blending ratio was 50%, the transformation and release features of iron, sodium and calcium in the blend with combustion temperature were close to those of non-Zhundong coal. The blending of Lingxin coal into Zhundong coal could benefit the transformation from NH4Ac-soluble calcium to H2O-soluble form, and facilitate the transformation of HCl-soluble aluminum and iron to insoluble form. The occurrence modes of elements and the ash yield of individual coal both exerted profound impacts on the element release ratio and chemical forms within ash of the blends. The present research has potential to offer an improved knowledge on the functional mechanisms of alkali alkaline-earth metals, aluminum, and iron on ash-related issues during high-alkali coal oxy-fuel co-combustion.

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.

Co-combustion of semicoke and coal under oxy-fuel conditions is a promising approach for the utilization of semicoke with a low volatile fraction. The devolatilization and gasification of fuels at ...

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.

Removal of SO2 and NO emissions from coal-fired power plants have always been the focus in coal's utilization industries for which traditional wet desulfurization system hold the potential to achieve simultaneous removal of SO2/NOx. In this work, a novel liquid catalyst (tributyl phosphate, TBP) was investigated for the simultaneous removal of SO2 and NOx in a NO pre-oxidation (ozone oxidation) assisted process. The absorption process and reaction mechanism of SO2 and NO2 were studied in a small-scale experimental system, and the removal efficiency of pollutant and the key parameters were examined in a pilot-scale system. The results show that the removal efficiency of NO2 in traditional wet desulfurization system is only 20–40%; however greatly increases to >90% when TBP is added. Moreover, TBP can enhance the NO2 removal performance in the presence of SO2 due to the formation of TBP–NO2–SO32− complex. Considering the oily nature of TBP that can be easily separated, such addition strategy hold great potential for industrial SO2/NOx simultaneously removal.

Circulating fluidized bed (CFB) boilers has realized the clean and efficient utilization of inferior coal like gangue and coal slime, high sulfur coal, anthracite, petroleum coke, oil shale and other resources. As a country with the largest amount of CFB boilers and the largest installed capacity in the world, China has 440 100–600 MWe CFB boilers with a total capacity of 82.29 GWe, including 227 units of 135 MWe, 95 units of 300 MWe, and 24 supercritical units. The statistics of typical 100–300 MWe CFB boilers showed that the average number of unplanned shut-down was only 0.37 times per year, among which the 135 MWe was 0.26 times per year and 300 MWe was 0.46 times per year. The auxiliary power ratio of some 300 MWe CFB boilers based on flow-pattern reconstruction can be reduced to about 4%, which is closed to the same level of pulverized coal (PC) boilers. This paper summarizes the development process and application status of China’s large-scale CFB boilers, analyzes the characteristics and technical performance of the iconic units, and introduces solutions to the problems such as water wall wear and bottom ash cooling.

Coal O2/CO2 combustion is a promising carbon capture and storage technology for coal-fired power plant. Char consumption rate prediction is essential for the boiler design, but challenging for O2/CO2 combustion since char reaction behavior within dual reactive gas media is complex. There is also a doubt of whether or not the O2 reaction and CO2 reaction occur independently, whereby the overall rate would be simply additive, or the two reactions compete for the same char actives, in which case the rate prediction could be more complicated. Here, the competitive behavior for O2 and CO2 reactions of char active sites was investigated via thermogravimetric approach, with a focus on the effect of temperature and inherent mineral composition. For char combustion within O2/CO2 environment with molar fractions of 30/70, the contribution of O2 to char consumption is the same as the O2 reaction alone. However, as being out-competed by O2, only 37% CO2 could occupy char active sites and react. Consequently, the overall char reaction rate within O2/CO2 is only 83% of the sum of that for individual gases at 900 °C. With increasing temperature, this ratio becomes even smaller, as the contribution of CO2 that being out-competed (to the sum of individual gases) is larger. With increasing O2 fraction, it occupied more active sites, thus further inhibiting CO2 reaction. The inherent alkali and alkaline earth metal provides the char with more active sites, thus muting the competition and inhibition for CO2 gasification.

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.

Zhundong coal has attracted an ever-increasing concern due to its super-huge reserve but high content of alkali metals. Volatilized into the gaseous phase during the gasification process, alkali metals are extremely unfavorable for coal utilization. Gasification technology can promote the large-scale utilization of high-alkali coal. However, few efforts, if any, have been conducted on gasification of Zhundong coal. The present study aimed to elucidate the sodium migration and mineral transformation characteristics in coal ash deposition process under different gasification conditions, while the behavior of sodium migration and mineral transformation were further analyzed using Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-OES), X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques. In this paper, the sodium migration characteristics in gasification process under different conditions were investigated by adding three sodium salts (NaCl, Na2CO3 and Na2SO4), kaolin and diatomite into coal, respectively. The effects of ash deposition time on sodium migration and mineral transformation characteristics in ash were also explored. The experimental results showed that the addition of sodium salt could increase the sodium content in the ash to a large extent. Kaolin and diatomite could solidify alkali metals in the gaseous phase during gasification process. As the ash accumulation time increased, the minerals such as chlorine, sulphur, and iron in the gaseous phase were easily bonded to the surface of the ash and reacted to form other crystal phases. The present study can provide guidance for the utilization of Zhundong coal and benefit the development of clean coal technology.

The behavior of the slag layer is very important for the safe and efficient operation of the cyclone barrel of cyclone-fired boiler. Ash parameters are the key factors affecting the formation and evolution of the slag layer. In order to figure out whether a stable slag layer could be formed when the cyclone-fired boiler burns the coal with low ash content or burns the coal with high ash melting point mixed with various contents of fluxing agent, the effects of ash content and fluxing agent on slag layer behavior are numerically examined for a vertical cyclone barrel. The decrease in the ash content can significantly reduce the mass flux of the captured particles as well as the velocity of the liquid slag layer. However, a stable slag layer can still be formed on the internal surface of the cyclone barrel even when the coal with a low ash content of 6.77% is burned. When the mass ratio of the fluxing agent is increased, the area of the internal surface covered with slag layer expands, whereas the solid slag layer becomes thinner. The amount of the fluxing agent has an optimal value. For the studied coal, when the mass ratio of the fluxing agent is 2.8%, the area of the internal surface covered with the slag layer expanded by about 37%, compared with the case without fluxing agent.

Coal oxy-fuel combustion requires CO2 dilution (recycling) to reduce the furnace temperature for material protection. This enhanced CO2 concentration reduces the O2 reaction rate while CO2 gasification contributes to the char reactivity. These conflicting influences together with a competitive behavior between O2 and CO2 complicate char O2/CO2 combustion behavior. Individually quantifying the influences accompanying the replacement of N2 with CO2 and clarifying the competitive behavior are essential for predicting the char O2/CO2 combustion rate, but this is difficult to obtain via experimental approaches. Here, a simplistic atomistic simulation investigated the char oxy-fuel combustion behavior. In simulations, the reactants (O2 and CO2) were either permitted to both react with char or permitted to have only one reactive gas with the other being a passive (nonreactive) gas to explore reactivity for individual gases in a O2/CO2 atmosphere. Thus, gasification contribution to char reactivity was isolated an...

Nitrogen oxides (NOx) 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 NOx 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.

Experimental studies were conducted by thermogravimetric analyzer and drop tube furnace to clarify the co-combustion and NO formation characteristics of semi-coke and bituminous coal. The same experiments were carried out on the blend of anthracite and bituminous coal for comparison. Results indicated that the ignition and burnout temperatures of the blends decreased as the blending ratio of bituminous coal increased, and there existed significant interaction between semi-coke (or anthracite) and bituminous coal. The ignition index C and comprehensive combustion index S presented negative correlations with fuel ratio, and the relationships of C = 6.31–0.35FR and S = 3.87–0.26FR can be used to estimate the co-combustion characteristics of bituminous coal and semi-coke (or anthracite). The addition of bituminous coal gave rise to two competitive effects on semi-coke burnout and NO formation. When the bituminous coal accounted for 80%, the promotion on burnout was superior and the inhibition on NO formation was significant. Fuel ratio, rather than nitrogen content played more important role in the NO formation, and relationship of XNO = 40.96–4.24 N + 1.60FR was established to predict the NO formation during co-combustion of binary coal-based fuels. Besides, semi-coke had better combustion behavior and lower NO conversion ratio compared to anthracite, which mainly resulted from the more porous structure in semi-coke.

The greatly high contents of alkali and alkali earth metals in Zhundong coal have a negative impact on the combustion and utilization of the coal, which results in serious fouling and slagging problems. In the present study, the influences of different minerals on the oxy-fuel combustion of Zhundong coal via synthetic coal were investigated by thermal analysis, and two isoconversional methods were applied to conduct the kinetic analysis. Additive minerals with single metallic elements including sodium (NaCl, NaCO3, NaAlSi3O8), calcium (CaCl2, CaCO3, CaSO4), and iron (Fe2O3, Fe3O4 and FeS2) rather than complex mixtures were added into synthetic coal to avoid the combined catalysis among various minerals. The experimental results show that the thermogravimetric and derivative thermogravimetric curves of synthetic coals under oxy-fuel conditions are similar to those of Zhundong coals. Most of selected additive minerals affect the ignition temperature slightly (less than 10 °C) but tend to lower the burnout temperature (5–25 °C), which narrows the main combustion temperature zone. The addition of CaCl2 raises the burnout temperature from 645.4 to 689.5 °C and drives down the average combustion rate from 4.44 to 3.83% min−1, while other minerals exhibit negative correlation with the maximum and average combustion rates in most cases. The reaction kinetics analysis indicates that the apparent activation energy of coal combustion depends heavily on the category of additive minerals, which varies from 21.31 to 39.05 kJ mol−1 (average values calculated by KAS method). Besides, the apparent activation energy declines with an increase in the conversion rate of samples.

The injection of ammonia as a reducing agent into the selective catalytic reduction (SCR) system is usually excessive in order to achieve the strict NOx removal efficiency, which results in the formation of ammonium bisulfate on account of the reaction between the escaped ammonia and sulfur oxides in the flue gas. Ammonium bisulfate is viscous and corrosive and can seriously affect the rotary air preheater (RAPH) as well as other exhaust systems. The mitigation strategies require additional operating expenses of cleaning, ash-blowing. To address the problem, a split design of the rotary air preheater is provided in this paper. A traditional RAPH is split into two sub-preheaters, between which the recirculating flue gas extracted from the cold end of the second preheater is mixed with the main flue gas. After the mixing point, a reaction chamber is installed to induce the reaction between ammonia and sulfur trioxide to form as much (NH4)2SO4 as possible. Thus, the blockage and corrosion can be avoided both in the two preheaters by a proper split design and continuous adjustment of the recirculation flow rate. To realize the calculation of temperature distribution, a numerical finite difference method is adopted to model the preheaters. The economical analysis of the proposed system including break-even curves is also conducted, which is given as references for the measures to relieve the blockage problem in the RAPH.

Co-firing semicoke and coal in utility boilers is a promising approach to cleanly and efficiently utilize semicoke, while the co-combustion characteristics have yet to be fully understood, includin...

The Zhundong coalfield has attracted ever-increasing attention as a result of its super-large reserve of coal resources. However, the power plants encounter great challenges in the utilization of Z...

A reliable coal seam model is highly significant for mining design and resource assessment. However, due to the anisotropic nature of geological attributes, accurately modeling the surface using existing interpolation methods is difficult. Here, we propose a new method for coal seam surface modeling. First, we introduce a multiscale interpolation method using compactly supported radial basis functions (CSRBFs) and improve the modeling accuracy by anisotropy calculations using the raw mine data set. Then a fault modeling method is provided to simulate faults intersecting with coal seams. This method consists of three main parts: (1) anisotropy calculation to alleviate the effects of global anisotropy; (2) rapid coal seam surface modeling of large amounts of nonuniform data through an anisotropic multiscale CSRBF method and then visualization and organization of the surface into a triangulated irregular network (TIN); and (3) local reconstruction of the coal seam surface according to the faults. A prototype system was developed based on this method to build a coal seam model from the collected multisource coal seam data. A comparison with three existing interpolation methods shows that this method is feasible and time efficient and achieves higher accuracy than previous methods. We anticipate that the method can provide a reference for advances in digital and smart mines as well as 3D geological modeling.

Although much work has been done on the optimization of the recuperative heat exchangers, little attention has been paid to the optimization of rotary regenerative air preheater (RAPH) in coal-fired power plants. It is known that the ammonium bisulfate deposition (ABS) can adversely affect the safe operation of RAPH, so an improved Radian number was proposed to estimate the impact of ABS in the optimization of RAPH. The thermal hydraulic calculation program integrated with genetic algorithms was developed to conduct optimizations of RAPH. The design parameters included rotor radius, each fluid sector angle and each matrix layer height. The multi-objective optimization was carried out to maximize the effectiveness and to minimize the pressure drop from the perspective of users, while the single-objective optimization was performed to minimize the weight of the heat transfer elements from the perspective of manufacturers. The improved Radian number is more reasonable to reflect the ABS deposition compared with the original Radian number. The designer can estimate the ABS deposition status using the improved Radian number not only in the operating process but also in the design process of RAPH. The range of optimal solution becomes smaller but the rank of the Pareto solution stays unchanged with the decrease in the restriction of improved Radian number. The optimization with larger rotor radius restriction could obtain better Pareto results. The total weight of heat transfer elements could be reduced by at least 26.5% compared with that of the original design by using genetic algorithm.

Here, a large-scale char atomistic structure (41,438 carbon atoms) and simple simulation approaches explored char gasification behavior for H2O and CO2 — comparing their mixture with one gas being passive (non-reactive) and the dual reactive mixture. Reactivity was captured through simplistic atomistic simulations via an automated sequence of gas diffusion, close-contact determination for gas (es) to reactive edge sites, then “reaction” via atom deletion. The higher reactivity of H2O was captured by a reaction probability function. For the reactive mixture (where both gases are reactive), the char consumption rate was 14% higher than with H2O alone, but lower (∼80%) than the sum of the individual gases (where only one gas is reactive), demonstrating competitive behavior. The H2O out competed the CO2 molecules and contributed ∼83% to the char consumption — with the reaction rate being similar to that of H2O independently. The pore size development for individual gases also differed with H2O favoring development in the smaller pore sizes in comparison to CO2. With fewer gas molecules (using 10% to capture a lower pressure), the competitive behavior was muted and became much closer to additive behavior. These simple simulations are consistent with the emerging rationalization of contributing factors to char gasification.

Constructing high quality three-dimensional (3D) geological models of coal seams from multi-source data, such as boreholes and geological maps, is a crucial task in seam distribution analysis and production planning of coal mine. However, widely used interpolation methods in geology, such as inverse distance weighting (IDW) and ordinary kriging (OK), ignore the variety of uncertainties in multi-source data. In this paper, sample points extracted from the archival data of a coal mine were classified into two categories, the hard data and the soft data, according to their sources. Field boundaries and fault observations were also obtained from data processing. A 3D geological modeling workflow was developed to integrate these data, in which (1) the weighted kriging (WK) method was used to interpolate both hard data and soft data, and (2) a fault modeling method was proposed to build faults' geometry as well as their impact on coal seam surface model. The WK method improves the accuracy of interpolation compared with the OK and IDW methods, and the proposed fault modeling method can integrate expert interpretations into the final 3D geological model of the coal seams.

Previous studies have shown that predrying can improve lignite combustion performance and lignite-fired boiler efficiency. This paper numerically investigated the effect of the lignite pred...

Dust pollution is severe in some mining areas in China due to rapid industrial development. Dust deposited on the vegetation canopy may change its spectra. However, a relationship between canopy spectra and dust amount has not been quantitatively studied, and a pixel-scale condition for remote sensing application has not been considered yet. In this study, the dust dispersion characteristics in an iron mining area were investigated using the American Meteorological Society (AMS) and the U.S. Environmental Protection Agency (EPA) regulatory model (AERMOD). Further, based on the three-dimensional discrete anisotropic radiative transfer (DART) model, the spectral characteristics of vegetation canopy under the dusty condition were simulated, and the influence of dustfall on vegetation canopy spectra was studied. Finally, the dust effect on vegetation spectra at the canopy scale was extended to a pixel scale, and the response of dust effect on vegetation spectra at the pixel scale was determined under different fractional vegetation covers (FVCs). The experimental results show that the dust pollution along a haul road was more severe and extensive than that in a stope. Taking dust dispersion along the road as an example, the variation of vegetation canopy spectra increased with the height of dust deposited on the vegetation canopy. At the pixel scale, a lower vegetation FVC would weaken the influence of dust on the spectra. The results derived from simulation spectral data were tested using satellite remote sensing images. The tested result indicates that the influence of dust retention on the pixel spectra with different FVCs was consistent with that created with the simulated data. The finding could be beneficial for those making decisions on monitoring vegetation under dusty conditions and reducing dust pollution in mining areas using remote sensing technology.

The cyclone-fired boiler has become a very competitive boiler type for utilizing coal with a low ash melting point and high alkali. Because of the higher combustion intensity in the cyclone...

Lignite usually has a high moisture content and low calorific value, and should be predried before combustion to enhance power generation efficiency. Steam extracted from steam turbine and ...

Conventional natural gas-fired boiler exhaust flue gas contained large amount of waste heat and water vapor direct to the atmosphere at high temperature which results in relatively low thermal efficiency. In traditional ways only a part of sensible heat are recovered. The latent heat, contained in the water vapor, which is the majority of the waste heat in the flue gas, cannot be recovered. Using a heat pump system with a boiler, the temperature of flue gas of the boiler can be drooped to dew point and the waste heat in the flue gas and condensation heat can be recovered deeply. In this paper, a new type of system, absorption heat pump vs. boiler, is proposed in which the waste and vapor in flue gas can be recovered efficiently. In this system, not only the evaporator is used to absorb the waste heat from flue gas, but also the generator is used to get the heat from the flue gas at high temperature. At the same time, a heat exchanger is used to transfer the heat from the flue gas to the backwater. A collector is installed on the evaporator to collect the condensed water from flue gas which will be used to a feed-water for the boiler room after they are purified. The performance of system is evaluated basing on a practical project, the installed capacity of which is 31.5MW. Results show that with the system, 13793.84MJ of heat is recovered per hour, in a whole heating season, 1.14 million cubic meter of natural gas will be saved. The efficiency of boiler rises about 11.8%. The recycled condensed water is 1177.344m3. 227 tons of CO2 can be cut into atmosphere per year.

Oxy-fuel combustion of high-alkali coal is beneficial for near-zero emission of pollutants in power plants and has the potential for extensive, efficient, and safe utilization of Zhundong coal in future. The present work was performed on oxy-fuel combustion of Zhundong coal, while the effects of silicoaluminate oxide and coal blending on oxy-fuel combustion characteristics and kinetics of high-alkali coal were further studied using thermogravimetric analysis. The thermogravimetric curves of Zhundong coals present two obvious stages but the contrastive coals are different. The increase in oxygen content weakens the impact of coal property on oxy-fuel combustion behavior of high-alkali coal. The addition of Al2O3 and kaolin results in a slight decline of the peak combustion rate, while the influences of SiO2 and diatomite additives are negligible. The additive fraction of silicoaluminate oxide gives rise to a non-monotonic impact on combustion characteristics of Zhundong coal. The interaction effect within blended coal could cause a reduction in reaction rate during the intense combustion stage, while its influence on kinetics is intensified during the later stage of oxy-fuel combustion. The impact extent of silicoaluminate oxide and coal blending on oxy-fuel combustion kinetics of high-alkali coal is highly associated with additive species and individual coals.

Waste heat recovery power generation system (WHRPGS) plays a significant role in cement plants for its contribution to the electricity supplement. Optimization of WHRPGS is very challenging when the WHRPGS is coupled with the cement producing system (CPS). In this paper, a new simplified model was proposed to characterize the integrated system (involving WHRPGS and CPS). By combining pinch analysis and exergy analysis methods, the energy process of the integrated system was studied in details. In particular, pinch analysis was used to qualitatively evaluate the improvement potentiality of the integrated system, while exergy analysis was employed to quantitatively evaluate the power generation capacity of the WHRPGS. By reducing the heat transfer across pinch points and exergy destruction of the integrated system, an optimized mode of WHRPGS was identified and the power generation capacity of it can be increased by 7.559% theoretically. The optimized WHRPGS was applied in a 2500 t·d−1 clinker production line in Northwest China. Comparing the operational results of WHRPGS in original and optimized modes for 24 h respectively, the power generation capacity of the optimized system was increased by 4.96% on average.

In this paper, the effect of co-firing semi-coke in a 300 MW tangentially fired boiler was numerically investigated. The results indicate that the incomplete combustion heat loss and NO x emission both increase with semi-coke co-fired ratio. Semi-coke may be injected into the furnace at a different height, which can lead to different thermal efficiency and NO x emission. It is suggested that semi-coke should not be fed from the top or bottom layer burners, since this could give rise to high carbon content respectively in fly ash and bottom slag. In addition, injecting semi-coke from the top burners could significantly increase the NO x emission. Under 1/2 co-firing ratio, the optimal fuel allocation is that feeding semi-coke from the B, D, and E layer burners. The growth in semi-coke particle size could increase the unburned carbon loss and NO x emission. It is highly recommended to reduce the unburned carbon loss under semi-coke co-fired condition by increasing the stoichiometric ratio of primary air for semi-coke. As it is increased from 0.25 to 0.3, the combustion efficiency of the co-fired condition is 99.47%, the same as when only firing bituminous coal, and the NO x emission is about 30% higher.

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.

In this study, an improved configuration of lignite-fueled power plant integrated with a two-stage predrying system was proposed. The predrying system mainly consists of two fluidized-bed dryers an...

Zhundong coal has attracted an increasing concern due to its super-huge reserve but high content of alkali metals. Oxy-fuel combustion of Zhundong coal benefits the near-zero emission of pollutants in coal-fired power plants and promotes the large-scale utilization of high-alkali coal. However, few efforts if any have been conducted on oxy-fuel combustion of Zhundong coal. The previous studies related to Zhundong coal were mainly focused on sodium behaviors but little work has been performed on calcium and iron, while calcium and iron are very likely to generate significant influences on fouling problems in combustion of Zhundong coal. The present study aimed to elucidate the release and transformation behaviors of sodium, calcium, and iron in oxy-fuel combustion of Zhundong coal using a fixed-bed reactor. Experimental results indicated that calcium in Zhundong coal was mainly present as ammonium acetate-soluble form, while the iron existed in forms of hydrochloric acid-soluble and insoluble. With the inc...

The utilization of Zhundong coal has given rise to serious ash-related issues mainly because of its high-alkali characteristic, partly similar to biomass with high-potassium content. The chemical properties of such coals and biomass may have marked impacts on reactivity and ash fouling. However, the correlations of different chemical properties of high-alkali solid fuels have been seldom investigated, and then only on a few samples. Here, a comparative study of chemical properties of Zhundong coal and biomass was conducted based on a large database. Correlations between carbon content and ash yield and carbon content and fixed carbon were noted. The fuel ratios of nearly all types of agricultural biomass lay within a range of 0.15–0.30, approximately an order of magnitude lower than those of Zhundong coals, much lower than those of Zhundong coals. There were no significant associations between chemical property and wheat-growing region. In most cases, the ash composition, even for a single biomass type, showed considerable variation, with the K2O content of wheat and corn straw ash being particularly variable. Most Zhundong coals had medium and similar ash fusion temperatures regardless of the changing ash compositions. Contrary to biomass, the Na2O content in Zhundong coal ash was an order of magnitude higher than K2O content. The ash composition of a particular biomass type usually fell in a small region of the ternary diagram of acidic-alkali metal-alkaline earth metal oxide components.

The co-combustion of biomass and coal in a utility boiler could provide cleaner power production and ensure sustainable utilization of the solid fuels. This paper aims to numerically investigate complex processes in the tangentially-fired 900 MWth boiler furnace during direct co-firing of lignite and biomass with 10% thermal share of agricultural residues (wheat straw, corn straw and soybean straw) under variable boiler loads (100%, 85% and 70%). Simulations are conducted by means of in-house developed computer code, supported by the specially designed user-friendly graphical interface. Co-firing of agricultural residues provides lower pollutant emissions, somewhat higher furnace exit gas temperature and increase in unburnt carbon in bottom/fly ash, compared to the lignite combustion without biomass. Soybean is found to be the most suitable for co-firing regarding its ash melting point, however due to its abundance and availability the wheat straw is selected for this study. Co-combustion at partial boiler loads results in reduction of NOx and SOx up to 34% and 9.5%, respectively. Burners arrangement and furnace aerodynamics affect the abatement of pollutants. This study may help the global efforts in fighting the climate change, efficiently and cost-effectively, thus offering considerable economic and social benefits.

In order to develop suitable low-low temperature flue gas system for China and determine its operation parameters, the adsorption characteristics of ash particles for sulfuric acid mist need to be investigated, as well as the adhesion characteristics of ash particles after the adsorption. In this paper, experiments were conducted on a laboratory-scale fixed bed adsorption reaction system. The adsorption characteristic curves of different particle sizes and adsorption temperature were obtained. The results show that adsorption temperature has a larger influence on the adsorption process compared with particle size. And there exists an adsorption limit in the adsorption process. In addition, the ultimate D/S (Dp/Sp) of ash particle is defined, which means the ratio of ash particle weight (mg) to adsorbed H2SO4 weight (mg) when ash particle reaches the adsorption limit. It is an inherent attribute of ash particle, only influenced by the particle constituents and sizes, representing the ultimate adsorption capacity of ash particle for sulfuric acid. The experiment results show that the chemical constituents of ash are the main determinate factors of adsorption limit and the Dp/Sp. And the Dp/Sp could be used cooperatively with D/S when people design the parameters of low-low temperature flue gas system for different types of coal. Furthermore, there was a significant agglomeration phenomenon between ash particles after the adsorption reaction, and it can increase the average size of particles which is conducive to the removal of PM.

The ash-related issues due to the high-alkali characteristic are still unresolved during utilization of biomass and Zhundong coal. Here, the correlations of several chemical properties of high-alkali solid fuels were investigated. The distributions of ash compositions in various biomass exhibit obvious diversity, while the distribution of K 2 O content within wheat and corn straws are the most scattered. The ash content in biomass shows a decrease tendency with the increase of carbon content. Zhundong coals generally have medium but close ash fusion points despite of quite different ash compositions sometimes.

In this paper, a modified air staging technology for pulverized coal boilers was proposed, to notably reduce NO x emission by creating a zone with high temperature and strong reducing atmosphere (HT&SRA) in furnace. Here, the conventional air staging technology was employed in combination with increasing O 2 fraction of feeding gas in primary zone ( O 2 .p ). The new technology was numerically verified in a 600 MW wall-fired boiler and the temperature distribution, the char burnout and the effectiveness in NO x reduction were mainly examined. The results indicate that a sufficiently low stoichiometric O 2 ratio in primary zone ( SR p ) is essential to reduce NO x emission by increasing O 2 .p . The effectiveness in NO x reduction by increasing O 2 .p also depends greatly on the injection position of over fire air (OFA). Under the condition of 0.7 SR p and a relatively high OFA position, the NO x emission could be reduced by 27.6% when O 2 .p is increased from 21% to 33%, while the incomplete combustion heat loss only shows a slight rise. In addition, the NO x reduction ratio could reach to >60% through the proposed low-NO x combustion technology compared with the current operation condition.

The chemical properties of coal have conspicuous impacts on reactivity and thermal conversion of coal. Nevertheless, the quantitative investigation on correlations of coal chemical properties is still insufficient. The quantified correlations between fuel ratio and various indices of coal chemical property is yet lacking, and little research was conducted to quantitatively describe the proximate and ultimate analyses using ternary diagrams. Here, we investigated the correlations of coal chemical properties based on database of real-time (experimental or actual) data in China. Some noteworthy variations and quantified correlations among chemical properties of coal were firstly demonstrated. The majority of coals (∼87%) lie within a range defined by the fuel ratio of 1.0–10.0. Ternary correlations of ultimate and proximate analyses show different distributions with coal rank. The index of coal explosibility is slightly lowered with the increasing carbon content, while the increase of oxygen content leads to certain rise of coal explosibility. In addition, the majority of low-fusion coals have high levels of volatile matter. The occurrence and content of minerals both present considerable influences on ash fusion behaviors. The present study can provide useful information for better understanding the correlations between chemical properties and reactivities of coal.

Potassium removal from biomass combustion flue gas is important in power engineering because potassium may cause serious problems such as the agglomeration, contamination, corrosion and deposition of heating surfaces. It is feasible to capture potassium vapor in flue gas by solid adsorbents. Experiments were performed to study the reaction characteristics and mechanism of the potassium adsorption by coal ashes with KCl as the alkali model compound. The influences of adsorption temperature, alkali metal concentration and adsorption atmosphere on potassium removal were analyzed. The experimental results confirm that coal ash has a good ability of potassium adsorption. The potassium adsorption amount by coal ash in initial 2 h increases roughly linearly with the increase of alkali metal concentration. Potassium adsorption by Guizhou coal ash during the fixed adsorption period increases with the increasing temperature in the range of 650–900 °C; while the reaction temperature is over 900 °C, potassium adsorption by Guizhou coal ash begins to decline. The potassium adsorption under the conditions even with a little water vapor content is larger than that under the conditions without water vapor. However the effect of high water vapor content on the alkali adsorption capacity is not obvious. The alkali adsorption by coal ash sorbent at a given period is the highest in reducing atmosphere, the second in the conventional flue gas atmosphere and the lowest in the oxyfuel combustion flue gas. The present experimental results will provide the references for the development of economical efficient sorbent of alkali metal to reduce the harm of the alkali in the biomass combustion or gasification process.

Lignite has been extensively used for electricity generation in many regions worldwide. However, its high water content has obviously negative effect on plant thermal efficiency. Performance of lignite-fired power plant can be improved by predrying the lignite before combustion. In addition, recovery of waste heat from the dryer and the power generation system will enhance the plant thermal efficiency further. In the present study, a new lignite predrying power generation system integrated with an efficient waste heat recovery system was proposed. Both dryer exhaust waste heat and steam turbine exhaust latent heat were recovered to heat boiler feed water. Energy analysis indicates that system performance is improved significantly. The plant thermal efficiency increases linearly with drying degree and then increases at a lower rate. The generation of unused dryer exhaust changes the variation tendency of system performance with drying degree.

Gas explosion is one of the most serious events in coal mine. In consideration of the limitation of past research method about gas explosion, three-dimensional (3D) modeling and simulation are suggested. This paper analyzes the research achievements on 3D modeling, and studies the modeling method of strata and laneway based on geoscience modeling. After the 3D modeling of strata and laneways, the simulation of gas explosion (such as gas exploding, color, burning, virtual wandering and rock caving) is performed. The research method is meaningful for safety analysis and safety survey.

The influences of operating conditions, such as excess air ratio and SOFA arrangement, on temperature distributions, species distributions, NOx emissions and burnout of a tangentially-fired utility boiler using bituminous and blended coal were investigated by numerical approach in the present study. The simulation results show that the fuel type exhibits certain effect on the temperature distributions and the NOx emissions. Moreover, higher excess air ratio could restrain the CO formation but promote NOx emission. The temperature and the species distributions are highly related to the SOFA arrangement. In addition, SOFA technology is still an effective approach to control the NOx emissions during blended coal combustion, while the disadvantageous effect on char burnout should be considered.

Ash deposition is particularly serious during Zhundong coal combustion/gasification due to its high alkali metal content, which causes great troubles and limits its utilization. This paper focuses on the ash deposition characteristics and migration disciplinarian of akali metal during Zhundong coal gasification varing with different conditions, such as ash deposition temperature, gasification temperature, O 2 /coal, steam/coal, and deposition time. Experimental result indicates that gasification conditions play a significant role on ash deposition. The root cause of severe fouling is the high content of Na in Zhundong coal, most of which is water-soluble. Besides, the contents of Na, Fe, S, Cl in ash are obviously changed with flue gas temperature increasing, indicating these elements have great influence on ash deposition, and the migration of Na, Fe and Cl presents similar variation during deposition. In addition, Na deposits mainly above 873 K, and Fe gains enrichment in ash at various temperatures. With the increase of gasification temperature, the content of Na in the ash is increased, which could exacerbate ash deposition. Both O 2 /coal and steam/coal have a certain influence on migration of elements, and the increases of O 2 /coal and steam/coal can relieve the ash fouling phenomena. Deposited ash on heating surface will react with Na in flue gas at high temperature, leading to the increase of content of Na in the ash with deposition time increasing.

Co-firing biomass under oxy-fuel condition is one of the most attractive methods which is conducive to mitigating CO2 emissions by combining the advantages of these two respective technologies. The combustion characteristics of a wall-fired utility boiler operating in this mode have been seldom investigated. The burnout behavior of the blended fuel is still controversial. By using the newly proposed combustion mechanisms, a numerical study was carried out in a 600 MW wall-fired boiler to evaluate the influences of oxy-fuel working condition and biomass share on flow, temperature, O2 distributions, and burnout behavior in this combustion mode. Besides, the effect of biomass injection position was also explored, which has yet to be fully understood. The simulation results show that oxy-fuel working condition affected the combustion characteristics to some extent. The introduction of biomass led to a lower temperature but a better burnout within the furnace. O2 distribution was also correlated to the biomass...

In this paper, a new bell-type air nozzle, which overcomes the structural defects of traditional bell-type air nozzles, is proposed and validated by cold test and numerical simulation. The pressure drop characteristic of the new bell-type air nozzle is measured. Furthermore, the causes of cover outlet abrasion and blockage, inner tube fracture, and irregular resistance change in traditional bell-type air nozzles applied in circulating fluidized bed (CFB) boilers are analyzed. Then, the performance of the new bell-type air nozzle is evaluated in a real CFB boiler, which is operated under regular working conditions. The results show that the new bell-type air nozzle has stronger anti-wear ability, excellent resistance characteristics, longer service life, and easier maintenance.