Processes

Geothermal energy is a clean, renewable, and baseload-stable resource of strategic importance for carbon neutrality. Hot dry rock (HDR) reservoirs are characterized by high temperatures, great depths, and abundant reserves. However, their extremely low natural permeability requires artificial fracturing to establish effective heat exchange networks. Conventional hydraulic fracturing in enhanced geothermal systems (EGS) faces major challenges under HDR conditions, including excessive water consumption, strong water–rock interactions, and elevated induced seismicity risks, limiting its engineering applicability. Waterless or low-water fracturing technologies offer alternative stimulation pathways due to their distinctive physicochemical properties. Existing reviews have mainly addressed individual aspects, such as specific fracturing media or proppant transport, without systematically integrating recent advances in supercritical CO₂ fracturing, foam fracturing, liquid nitrogen fracturing, and hybrid-fluid fracturing technologies, or comprehensively evaluating their engineering implications. This review systematically analyzed the fracturing mechanisms, heat exchange performance, environmental risks, and HDR-specific engineering challenges of these technologies. Results indicate that waterless/low-water fracturing technologies enhance heat extraction efficiency by generating complex fracture networks while mitigating seismic and reservoir damage risks. However, large-scale application requires further advances in the high-temperature stability of fracturing media, material durability, multiphase flow control, and field validation.

Ozonation is widely applied for refractory wastewater treatment, but its practical engineering is often limited by poor ozone mass transfer and low ozone utilization. In this study, micro-nano bubbles (MNBs) technology was employed to improve ozone delivery, and the performance of an O₃-MNBs system for treating coking reverse osmosis concentrate (ROC) was systematically compared with the conventional millimeter-sized ozone bubbles (O₃-MBs) system. To further promote oxidation, hydrogen peroxide (H₂O₂) was introduced, forming an O₃-MNBs/H₂O₂ system. Results showed that O₃-MNBs (D₅₀ = 36 μm) achieved a volumetric mass transfer coefficient 2.5 times higher than O₃-MBs. Under optimized conditions (pH: 7–9, ozone dosage: 10 mg/(L·min), temperature: 20–30 °C), COD removal in the O₃-MNBs system reached 34.9 ± 1.2%, nearly twice that of the O₃-MBs system, while the O/C ratio decreased by approximately 50% (4.7 ± 0.2), indicating enhanced ozone utilization efficiency. The addition of H₂O₂ further increased COD removal to 52.1 ± 2.9% and reduced the O/C ratio to 2.9 ± 0.2, reflecting strong synergistic effects. Moreover, the integration of MNBs and H₂O₂ effectively reduced energy consumption per unit of pollutant removed. Overall, the O₃-MNBs-based technology enhances organic pollutant degradation, ozone utilization and energy efficiency, offering a promising strategy for high-salinity refractory wastewater treatment.

Thermochemical heat storage technology serves as an effective approach for efficient recovery and cross-seasonal storage of low-grade waste heat. However, traditional packed-bed heat exchange methods in industrial applications are prone to material contamination and performance degradation due to impurities in waste heat gases. To address this, this study proposes and constructs a thermochemical heat storage system based on moving-bed indirect heat exchange, using magnesium sulfate heptahydrate (MgSO₄·7H₂O) as the heat storage medium. The system investigates its desorption and heat storage characteristics within the moving bed. A small-scale moving-bed experimental platform was established, incorporating a vacuum-assisted system to promptly remove water vapor generated during desorption. The experimental system examines the effects of different operating parameters (e.g., inlet water temperature and flow rate) on particle temperature fields, desorption rates, and overall heat transfer performance. Results demonstrate that MgSO₄·7H₂O exhibits excellent heat storage stability and reaction controllability in the medium-low temperature range (60–95 °C). Increasing inlet water temperature and flow rate enhances desorption processes, but high temperatures also lead to increased temperature gradients, reducing waste heat recovery rates. Practical applications require optimizing the balance between heat transfer enhancement and desorption time. Compared to conventional heat storage particles, the moving-bed system using magnesium sulfate heptahydrate achieves approximately 30% higher overall heat transfer coefficient. Compared to traditional packed beds, the moving-bed heat exchange method demonstrates superior heat transfer uniformity and storage efficiency. This study validates the feasibility of the “moving-bed + thermochemical heat storage + vacuum desorption” technology under non-clean heat source conditions, providing experimental evidence and technical references for efficient industrial waste heat recovery and high-density storage.