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• An analytical solution of heat transfer in karst conduit was proposed. • Conduit structure and flow properties determine thermal equilibrium depth. • Insufficiently heat exchange results in temperature fluctuations in karst springs. • Event thermal response was verified by heat exchange and mixing function. Thermal responses of groundwater are useful signatures to reveal the recharge-discharge process and characterize aquifer structure in karst groundwater. Four karst springs with varying circulation depths ranging from ∼ 60 to 820 m in South China, were used to decipher the mechanism of heat exchange between recharge water and surrounding rock formations. An analytical solution of heat transfer in karst conduit was proposed to simulate the peak or trough temperatures of pulse flow. Hydraulic diameter, flow velocity, input temperature and circulation depth in karst aquifer, mainly depending on the recharge water volume after rainfall events, are important transport factors to control the thermal equilibrium depth and the efficiency of heat exchange that directly controls the behavior of warm peaks or cold troughs in groundwater temperature. In shallow karst aquifers wherein the circulation depth is less than its theoretical thermal equilibrium depth, the recharge water does not sufficiently exchange heat with the surrounding rocks, resulting in clear event-scale warm peaks in summer and cold troughs in winter. Otherwise, the recharge water will firstly reach a thermal equilibrium state and then mix with the warmer base flow at deeper location, which is verified in a karst spring with anomalous event-scale temperature drops after rainfall events all year around. The thermal response simulations obtained from this work provide new ways and insights to characterize geographic structure of karst aquifer and elucidate heat transfer mechanism in karst aquifer systems. [ABSTRACT FROM AUTHOR]

The permeability of the tight sandstone reservoirs will change when the thermal equilibrium in geological formations is disturbed. Understanding the permeability evolution under different temperature levels is thus crucial to reservoir evaluation and production prediction. In this study, a series of coupled thermal-hydraulic-mechanical triaxial flow-through tests are conducted to investigate the effect of the elevated temperatures on the permeability evolution and stress sensitivity of a typical argillaceous sandstone from Zhongjiang Gas field in the Sichuan Basin, China. Experimental results show that the initial permeability gradually decreases with the increase of the temperature ranging from 20 °C to 100 °C, with a decreasing magnitude of up to 1.5 orders. Stress sensitivity also indicates a negative correlation with the temperature levels. Whereas the mechanical parameters are less sensitive to elevated temperatures. According to the Scanning Electron Microscopy (SEM) analysis, the mechanism of the permeability reduction is mainly caused by the thermal expansion of the illite, as well as the pore-filling and throat-clogging effects induced by the newly released particles. Moreover, a modified permeability model with the exponential decay form is proposed and validated. This model considers the thermal effect and permeability recovery during the dilation regime and fits well with the experimental data. • A series of coupled thermal-hydraulic-mechanical triaxial flow-through tests were conducted on an argillaceous sandstone. • The permeability and stress sensitivity both show a negative correlation with elevated temperatures. • The permeability reduction is caused by the thermal expansion of the illite and throat-clogging. • A modified permeability model considers the thermal effect and permeability recovery was proposed and validated. [ABSTRACT FROM AUTHOR]

• The alkaline deposition through a PC boiler were analysed. • Na 2 SO 4 and CaSO 4 (l/slag) promote deposition at medium temperature. • Co-firing WD and WCW coals can reduce the amount of sulphate (l/slag). • Sulphurisation of Na on heating surface is important for deposition. The huge reserve of Zhundong (ZD) coal makes it important for energy utilization of China. However, big concerns on fouling and slagging in pulverized coal-firing boilers were risen due to its high alkali content. It's important to investigate deposition in different heating surfaces of a boiler burning ZD coal. In this paper, XRF, XRD, ICP-OES were conducted to analyze the properties of ash deposits collected from different heating surfaces in a 350 MW boiler burning 30% Wudong (WD) and 70% Wucaiwan (WCW) coals. Thermodynamics calculations were applied to study the mineral composition of coal ash in the range of 200–1600 °C under different coal co-firing ratio. The analytical results of the ash deposits showed that in medium-temperature (800–1100 °C) flue gas zone (FR and FS), ash deposits have higher sulfate content. While in high temperature flue gas zone (WW and PS), ash deposits have higher Si/Al content and lower S content. The Na/S contents in FR (~1000 °C) are much higher than in other deposits. The thermal equilibrium calculation results showed that a large amount of liquid Na 2 SO 4 and CaSO 4 will be generated when burning WCW coal. Co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect. It can be speculated that not all the sulfates in the ash deposits are formed in flue gas. The sulfation of the Na-containing minerals condensed on the heating surfaces will promote the ash deposition. [ABSTRACT FROM AUTHOR]