Enhancing the recovery of sensible and latent heat from flue gas provides a direct and effective approach to improving the energy efficiency of gas boiler systems. To address challenges associated with low-temperature corrosion and insufficient heat recovery efficiency, a practical deep waste heat recovery system integrating a corrosion-resistant wall-type flue gas condensing heat exchanger (FGCHE) with an absorption heat pump (AHP) has been developed. Field measurements were conducted to evaluate its thermodynamic performance, and the heat and mass transfer mechanisms governing flue gas-side convective condensation were systematically analyzed. The flue gas exhaust temperature was reduced from 59.7-70.4 degrees C to 23.8-34.1 degrees C, achieving a waste heat utilization ratio of 72.1-94.2% and an overall energy-saving efficiency of 10.1-12.8%. Latent heat transfer via condensation played a dominant role, contributing 5.9-8.4 times more than sensible heat transfer, and was significantly influenced by flue gas velocity and water vapor content. Based on field data, empirical correlations for flue gas condensation heat transfer and pressure drop within the FGCHE were established, exhibiting reliable engineering applicability. The system achieved a payback period of 2.61 years, along with significant reductions in natural gas consumption and pollutant emissions, demonstrating strong potential for widespread application in high-efficiency, low-carbon heating systems.

Efficient recovery of waste heat from industrial corrosive flue gas is hindered by severe challenges in corrosion resistance, wear durability, and heat transfer efficiency of heat exchangers. Here, we develop a novel sandwichstructured multifunctional composite coating comprising a wear-resistant silicon carbide (SiC)/ perfluoroalkoxy alkane (PFA) surface layer, a corrosion-resistant fluorographene (FG)/PFA interlayer, and a polyetheretherketone (PEEK) bonding layer for enhanced adhesion to metal substrates. The FG/SiC/PFA coating significantly improves both protective performance and thermal transport. The optimized formulation (PGPSi10), containing 0.75 wt% FG and 10 wt% SiC, retained structural integrity after 60 days immersion in 90 wt% H2SO4 at 140 degrees C, exhibiting over two orders of magnitude enhancement in corrosion resistance. Thermal conductivity increased by 13.07-16.25 %, while the convective condensation heat transfer coefficient on PGPSi10 was 1.7-1.9 times higher than filmwise condensation. This multifunctional coating offers an effective strategy for extending service life and boosting thermal efficiency of flue gas condensing heat exchangers under harsh industrial conditions.

Heat exchangers of thermoelectric generators (TEGs) need parallel connection for absorbing more waste heat and maintaining low pressure drop. The effect of flow maldistribution is inevitable due to hydraulic imbalance in parallel connection, which affects temperature distribution and output performance of TEG. Previous studies have not quantitatively defined the value of maldistribution factors (MF) that can affect the output performance of TEG under varying operating and structural parameters. These mechanisms are crucial to designing TEG for large-scale applications. Thus, a mathematical model is established to simulate the output performance of TEG under varying operating parameters and thermoelectric leg geometry conditions in different MFs. A test rig is designed to validate model accuracy. The results show that the output performance of TEG decreases with an increase in MF. When MF exceeds 44.1 %, the output power and efficiency decline by 10 % and 5.1 %, respectively. The air-cooled TEG exhibits inferior output performance to the water-cooled TEG at the same MF. As the flow rate augments or the number of serially connected thermoelectric modules (TEMs) declines, the effect of flow maldistribution on the output performance of TEG diminishes. The higher thermal resistance of the thermoelectric leg can mitigate the effect of flow maldistribution on the output performance at the same MF. This study provides a theoretical foundation for the integration and optimized design when TEG is used in large scale.

Dropwise condensation (DWC) is among the most efficient phase-change heat-transfer modes, and hydrophobic surface engineering enables flue gas condensing heat exchangers (FGCHEs) to maintain stable DWC with significantly enhanced condensate removal and heat-transfer performance. This study develops a comprehensive theoretical framework for flue gas DWC on multifunctional finned-tube surfaces. Extending the classical Le Fevre-Rose and Kim-Kim models, the formulation incorporates the coupled effects of gas-liquid interfacial shear, Knudsen-layer vapor transport, liquid-solid interfacial dynamics, and substrate conduction resistance. A microdroplet size-distribution function is established, together with closed-form expressions describing droplet growth kinetics, shedding radius, and circumferential variation along the finned tube. The model resolves the evolution of droplet-scale heat flux and decomposes the total thermal resistance into heat flow through droplets and heat flow through exposed hydrophobic areas, enabling the dominant interfacial pathways governing flue gas DWC and condensate generation to be rigorously identified. The finned-tube FGCHE measurements match the model predictions with high fidelity, supporting the validity of the developed formulation. Parametric analysis reveals that flue gas velocity is the primary governing factor, amplifying condensation heat transfer and condensate production through strengthened vapor-phase transport and accelerated droplet renewal. Higher vapor content further strengthens interfacial mass transfer and reduces droplet residence time, collectively improving overall DWC performance. This study delivers a rigorous mechanistic basis for advancing the design and performance optimization of high-efficiency FGCHEs with multifunctional hydrophobic finned-tube surfaces.

Lake sediment is a high moisture solid waste that carries a large amount of water pollutants, significantly impacting the environment and urban landscape. The efficient management of lake sediment has emerged as a critical challenge requiring immediate attention. This paper focuses on the characteristics of co-combustion and its pollutant emissions. The combustion characteristics of mixed combustion of lake sediment and coal in different proportions were obtained by the thermogravimetric method. Experiments elucidated the influence of diverse factors on the variability of NO and SO2 concentrations. The remaining bottom residue of the reaction and the reasons for the changes in emission concentration are analysed from a microscopic perspective. Results indicate that the appropriate proportion (<20 %) of mixed lake sediment promotes coal combustion. NO and SO2 decrease with increased bed temperature, lake sediment mixing ratio (5 %-15 %), and particle size. As the bed temperature increases, the particle surface melts, reducing the pore structure and reaction sites, thereby reducing the generation of pollutants. When the mixing ratio exceeds 15 %, NO increases while SO2 decreases. Calcium based substances have little catalytic effect on NO, but are still effective for SO2. Larger particles will promote the decomposition of NO on their surface.

Smart heating is important to build a resilient smart city. Power supply in the smart heating system is the key to guarantee operating safety. The lithium battery is a convenient way to power devices in the smart heating system, but it needs to be replaced periodically and cannot continuously supply power. In this paper, the thermoelectric generation (TEG) system is applied to smart heating system by using the temperature difference between the supply and return water of the heating network, which can achieve self-powered. This paper establishes a mathematical model of TEG system for self-powered devices and builds an experimental rig to verify the accuracy of the numerical model. The effects of load resistance/internal resistance, heat exchanger surface area/module area, and geometry of thermoelectric leg on the power generation performance of TEG system are investigated. The results show that when the load resistance/internal resistance (RL/Rin) exceeds 1, the output performance of TEG system is optimal. Meanwhile, optimal RL/Rin at maximum output power increases as the contact thermal resistance rises. Increasing the heat exchanger surface area/module area (Ahx/Amo) has no impact on improving the output power of TEG system. As the height of thermoelectric leg increases, there exists an optimal height to maximize the output power and output efficiency.

Using thermoelectric generation to recover low-temperature thermal energy for power generation is an important topic of thermal optimization. In this study, energy, economic, and environmental analysis of a lowtemperature thermoelectric generator system (LTTEG) is analyzed. A numerical model considering variable material properties, Thomson heat, and air gap heat dissipation effects is established for LTTEG. A genetic algorithm is applied for the multi-objective optimization of LTTEG. Key operating conditions and geometrical parameters such as the inlet flow rate, thermoelectric leg geometry, and fin geometry are selected as decision variables. Net output power, net efficiency, savings-to-investment ratio (SIR), and the year achieving zero carbon emissions (YCO2) are considered as optimization objectives (objective functions). The linear weighted evaluation method is proposed for the objective function. The results show a contradictory relationship among the objective functions. The multi-objective optimization of LTTEG is performed when the hot source is within 50-100oC, and the optimal design is obtained. Compared with the previous design, the net output power, net power generation efficiency, and SIR performance can be improved by 18.1-159.5 %, 0.4-95.8 %, and 6.0-145.5 %, respectively. Meanwhile, YCO2 decreased by 21.0-66.5 %. The multi-objective optimization method based on the genetic algorithm is beneficial to improving the comprehensive performance of LTTEG.

Hydrogen-enriched natural gas (HENG) is one of the significant ways to store, transport, and utilize hydrogen energy. This paper presents the characteristics of HENG and flue gas, energy saving, carbon reduction, and condensate recovery potential of flue gas waste heat. Based on the theoretical and experimental study of the existing flue gas condensing heat exchanger (FGCHE) added at the rail of gas-fired boiler, the performance of the FGCHE under different hydrogen blending ratios is studied. The results show that augmenting the hydrogen blending ratio decreases the volume heating value of HENG, and CO2 content of flue gas. Meanwhile, the mass heating value of the HENG, the water vapor content, and dew point temperature of flue gas increase as the hydrogen blending ratio increases. The flue gas waste heat recovery utilization ratio and energy-saving efficiency of the FGCHE can be promoted. Moreover, the heat transfer capacity of the FGCHE using HENG is enhanced, while the flue gas flow pressure drop is reduced. Compared with the FGCHE used in pure natural gas systems, the all-hydrogen system increases the flue gas waste heat recovery amount by 62.4-120.4 %, reduces the flue gas flow pressure drop by 33.9 %, and reduces the carbon emission by 100 %.

Lake sediment,as a solid waste,can be disposed harmlessly through combustion.The ash melting temperature is an important factor affecting boiler coking and restricting its safe and stable operation.Determining the ash melting tem-perature after mixing lake sediment with coal is of great significance for achieving the resource utilization of lake sedi-ment.This paper takes the Yunnan Xiaolongtan lignite and the Dianchi Lake sediment as the research objects.The Dian-chi Lake sediment is a high moisture solid waste with high nitrogen content and rich heavy metal elements.The Yunnan Dianchi Lake sediment has a large residual stock,mainly generated by a large amount of sediment entering the Dianchi Lake with river water and rainwater.It is the main source of pollution in the Dianchi Lake.However,the Xiaolongtan lig-nite has problems such as high production,poor coal quality,and high transportation costs,which seriously affect its utiliz-ation rate,if the local conditions can be adapted and the problems of high production,difficult treatment,and insufficient efficiency of local sediment can be solved through mixed combustion,in addition to achieving an efficient utilization of lignite,it can also alleviate the local disposal pressure.However,there is currently little research on the blending of lignite with sediment.Through experiments on the ash melting characteristics,the deformation,softening,hemisphere,and flow temperature of ash at a 5％-15％mixing ratio were determined,and the variation law of ash melting point was derived.The mineral composition,oxide,and elemental composition were obtained through XRD,XRF,and SEM-EDS detection and analysis.The crystal structure and mineral composition in the ash were confirmed,and a ternary phase diagram was constructed for thermochemical calculations.The results show that when 5％sediment is mixed,the melting temperature of the ash decreases sharply,and then the downward trend tends to be gentle.The acid-base ratio and silicon aluminum ra-tio are positively correlated with the downward trend.The oxides of Fe+2,Fe+3,and Ca+2 play a main role in reducing the melting point of the ash,and these oxides are prone to react with refractory minerals to form fluxing minerals such as anorthite and calcium iron pyroxene.During the low temperature stage,Ca oxides mainly promote melting,while during the high temperature stage,Fe oxides act.

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For urban heating pipeline network, the intelligent and precise detection is an important guarantee for urban heating infrastructure of the safe and low-carbon operation. Heating pipe network was directly buried in the soil and difficult to detect the leakage point due to lack of non-excavation detection method for rapid repair. An improved noise reduction algorithm of the wavelet threshold function coupled with the acoustic method was used for the directly buried pipe leakage detection. Large-scale leakage experimental tests were used to validate the reliability. The influences of temperature, pressure, flowrates and leakage distance on the frequency and time domains were then investigated. Results show that the improved threshold function could well detect the leakage for directly buried hot water heating pipes. When the temperature increases, the leakage frequency domain spreads from 200 - 800 Hz to 50-1500 Hz. The pressure, flow rate, and leak point location could affect the amplitude feature. The accuracy ranges of the improved method in the different location, temperature and pressure and flowrates are separately 0.11-2.36%, 0.11-1.96%,0.11-3.49% and 0.16-1.55%, respectively.

Low-temperature thermal energy (<100 degrees C) is widely present in energy systems and nature, and its utilization is of great significance for energy conservation and carbon reduction. Thermoelectric generator (TEG), without refrigerant and moving parts, can directly convert low-temperature thermal energy into high-grade electricity through the Seebeck effect. One of the biggest defects of TEG is the low efficiency from thermal to electricity. To improve the efficiency of TEG, a staggered fin was arranged in the main flow area for the heat exchanger. Meanwhile, a header with pin-finned and variable triangle geometry is applied in the inlet and outlet of heat exchange to uniform fluid flow. A TEG experimental system containing the heat exchanger whose header with pin-finned and variable triangle geometry was built to evaluate the output performance at variable load resistance, hot and cold source temperature, flow rate, flow direction, and electrically connected array configurations. Moreover, experimental results were compared with other literature. When the inlet hot and cold source temperatures are 40.8-81.0 degrees C and 20.0-30.0 degrees C, the TEG system could generate around 0.6-18.8 W with a flow rate of 5 L/min. The TEG system has relative high output performance and a low flow resistance compared to other literatures, and it is the potential to recover low-grade thermal energy recovery.

The corrosion and abrasion performance of flue gas heat exchangers is a prominent research focus in the field of industrial flue gas waste heat recovery. In this study, we have developed a multi-layered composite coating containing fluorographene and carborundum fillers, which exhibits excellent anti-corrosion and wear-resistant properties. Electrochemical and abrasion experiments demonstrate that the multi-layered composite coating effectively improves both the corrosion resistance and the mechanical properties of the material. The structure and preparation of this multi-layered composite coating provide a novel approach for design and application of advanced coatings for highly corrosive and dusty flue gas waste heat recovery.

The refinery heating furnace has high energy consuming and high carbon emission, and the flue gas condensing waste heat recovery can achieve prominent results in saving energy and reducing emissions of petrochemical industry. Based on adding a flue gas condensing heat exchanger (FGCHE) at the rail of heating furnace, this paper investigates the effects of flue gas waste heat recovery and pressure drop on the energy saving and carbon reduction through theoretical and on-site testing methods. Flue gas heat recovery and pressure-drop charac-teristics of the FGCHE are analyzed to achieve maximum energy saving and emissions reduction under the normal operating conditions of heating furnace. Meanwhile, the effects of flue gas temperature and pressure drop on the energy saving and carbon reduction are derived under different combustion air temperature, humidity and excess air coefficient after fuel gas complete combustion. The results show that when the flue gas temperature is reduced from 180 degrees C to 20-40 degrees C, the energy saving efficiency, utilization ratio of flue gas waste heat, condensate recovery efficiency and carbon emission reduction ratio reach 12.5-16.9%, 74.4-98.1%, 58 similar to 89.3% and 13.7-18.3%, respectively. Reducing the flue gas pressure drop can significantly improve the flue gas waste heat utilization ratio and carbon reduction.

Hydrogen production reformer affects the thermal and productive efficiency of the hydrogen production through the petroleum refining. Amounts of heat are wasted with the exhausted corrosive gas of the reformer within 150~250 °C and difficult to be recovered. To recover the sensible and latent heat from the exhausted gas, a new anti-corrosion, high-efficiency and low-pressure-drop flue gas condensing heat exchanger (FGCHE) with low consumption and pressure drop was developed. The energy-saving performance is evaluated through on-site measurements and theoretical analysis. The results show that the exhausted gas temperature is reduced from 161.3~175.9 °C to 33.9~38.9 °C after using the new FGCHE to recover waste heat. The energy saving efficiency and the utilization ratio of flue gas waste heat are 12~16.1% and 74~81.9%, respectively. The latent heat accounts for 41.3~48.1% of the total recovered heat. The exergy efficiency and the total thermal efficiency of reformer reach 73~86.8% and 95.2~96.6%, respectively. The condensation in the flue gas reduces pollutant emissions (SO2 and NOx). It can provide a practical application reference for the heat recovery equipment and operation for petrochemical heating furnaces.

为提升内燃机与燃煤机组耦合的高灵活性发电系统的能量利用效率,提出了内燃机与燃煤机组相复合的新型热力系统.通过将内燃机烟气和冷却水余热输入燃煤机组热力系统,从而降低燃煤机组发电煤耗率.采用EBSILON软件对复合系统进行建模,以燃煤机组汽耗率和热耗率为评价指标,分析了不同复合方案下燃煤机组的热经济性.结果表明:通过将内燃机余热复合进燃煤机组热力系统,可显著降低燃煤机组的热耗率和汽耗率;内燃机烟气余热复合位置越靠近锅炉,参与烟气换热的给水和凝结水比例越小,机组热耗率、汽耗率越低;当烟气余热一部分加热高压加热器给水,另一部分加热低压加热器凝结水时,分配到给水的余热越多,机组热耗率、汽耗率越低;优化后的内燃机与燃煤机组复合热力系统,热耗率最多可降低6.62％.

The liquid film thickness seriously affects the distribution of temperature, velocity, and components near the interface for steam condensation in the presence of small concentration noncondensable gas. The liquid film thickness for H2O/CO2 forced convection condensation separation on vertical plate has not been studied numerically. Thus, volume of fluid model and a phase change model were used to study steam condensation in the presence of noncondensable gases. The calculations studied the effects of velocity, surface subcooling, and noncondensable gas mole fraction on the heat transfer for H2O/air or H2O/CO2 mixtures. The results show that the predicted heat transfer coefficients agree well with previous experimental data. The gas and liquid film thicknesses controlling the condensation heat transfer in the presence of a noncondensable gas become thicker as the mixture flows along the cooled wall. The condensate mass flow rate and the heat transfer coefficient are both seriously reduced by the noncondensable gas. The condensate heat transfer in the presence of noncondensable gases is mainly determined by the diffusion coefficient and the thermal conductivities of the components in the gas film layer.