Subcritical water treatment of municipal solid waste coupled with molten salt energy storage system and method

Abstract

The application discloses a kind of subcritical water processing urban and rural garbage systems and methods coupled with fused salt energy storage, belong to organic solid waste resource and energy storage technical field.The system includes subcritical water processing unit, fused salt heat storage energy supply unit, heat coupling unit, product separation and energy unit and intelligent control unit.The core innovation is to provide stable, renewable main heat source for subcritical water reactor through fused salt heat storage system, and intelligently integrate product waste heat recovery, self-produced fuel recycling and multi-mode scheduling strategy based on real-time electricity price and renewable energy output.The method is particularly suitable for the collaborative processing of urban and rural household garbage and agricultural straw in cold regions of the north, can efficiently utilize off-peak electricity or surplus green electricity, significantly reduce processing energy cost, and realize the transformation of garbage processing facilities into regional integrated energy stations with combined heat and power supply and grid peak shaving functions.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization and comprehensive energy utilization technology. Specifically, it relates to an innovative system and method that is particularly suitable for cold regions in northern my country, capable of co-processing urban and rural domestic waste and agricultural straw (such as corn straw), and deeply integrating large-scale thermal storage and intelligent energy management. Background Technology

[0002] In cold northern regions like Northeast my country, comprehensive urban and rural waste management faces unique challenges: on the one hand, household waste has a high moisture content and low seasonal (winter) calorific value, resulting in poor efficiency and economic benefits from traditional incineration; on the other hand, there is seasonal pressure to collect and process large amounts of agricultural straw, leading to persistent illegal open burning and severe air pollution. Subcritical water treatment technology, as an advanced organic waste resource utilization technology, can efficiently convert complex organic matter into high-value-added products such as bio-oil and biochar in water, achieving thorough waste reduction and resource recovery, and the treatment process is environmentally friendly. However, the large-scale application of this technology is limited by its high operating energy consumption—maintaining the subcritical water reaction (typically requiring 250-350℃) requires a continuous and stable high-grade heat source. Existing projects mostly rely on gas or direct electric heating, resulting in high treatment costs and considerable carbon emissions.

[0003] Meanwhile, northern regions are rich in wind and solar energy resources, and the installed capacity of renewable energy is growing rapidly. However, the intermittency and volatility of power generation put pressure on the stable operation of the power grid and also generate a large amount of "curtailed wind and solar" resources. Molten salt thermal energy storage technology, as a large-capacity, long-cycle, and low-cost physical energy storage method, is already very mature, but it is currently mainly used in large-scale solar thermal power plants and has not yet been effectively developed and applied in distributed and modular urban and rural environmental governance scenarios.

[0004] Therefore, a significant gap exists in the current technological field: the lack of an innovative solution that can systematically couple the needs for distributed organic waste treatment, intermittent renewable energy consumption, grid peak shaving, and regional clean heating. This invention aims to fill this gap. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a subcritical water treatment system and method for urban and rural waste coupled with molten salt energy storage. This system can not only efficiently and cleanly co-process domestic waste and agricultural straw, but also achieve optimized energy allocation in time and space through molten salt energy storage, transforming waste treatment facilities from simple "energy consumption units" into "integrated energy supply nodes" capable of participating in regional energy regulation, thereby maximizing environmental and economic benefits.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, this invention provides a subcritical water treatment system coupled with molten salt energy storage. This system is a highly integrated physical-digital system, mainly comprising the following five functional units: Subcritical water treatment unit: This unit follows a mature subcritical water treatment process and includes pretreatment devices for crushing and homogenization, high-pressure feed pumps for pressurization, tubular or batch reactors for the core hydrolysis and liquefaction reactions, heat exchangers for energy recovery and product cooling, and back pressure valves for maintaining pressure. Its core function is to convert organic waste into homogeneous fluid products under subcritical conditions.

[0007] Molten salt thermal energy storage unit: This unit constitutes the system's "energy bank," mainly including high-temperature / low-temperature molten salt storage tanks, electric heaters, molten salt pumps, and related piping and insulation systems. Its core function is to convert electrical energy into high-temperature thermal energy and store it in the molten salt medium when electricity costs are low or renewable energy is abundant; and to stably release the thermal energy when needed.

[0008] Thermal Coupling Unit: This is the crucial bridge connecting the "energy bank" and the "processing plant." Its core equipment is the molten salt-thermal oil heat exchanger. Here, the high-temperature molten salt transfers its heat energy to the thermal oil, which then flows through a closed loop through the external jacket or internal coils of the subcritical water reactor, providing a precise and stable primary heat source for the reaction. This indirect heat exchange method is safe, reliable, and facilitates precise temperature control.

[0009] Product Separation and Energy Conversion Unit: This unit is responsible for the resource-based separation of the post-reaction mixture and typically includes gas-liquid separators, solid-liquid separators (such as centrifuges), and storage tanks. Its innovative design features a backup heater, to which the separated bio-oil and non-condensable combustible gas are connected via pipelines. This allows the system to utilize its own generated fuel for heating when molten salt heat is insufficient or external energy prices are extremely high, forming an internal energy security buffer and cost control mechanism.

[0010] Intelligent Control Unit: As the "brain" of the system, this unit is built upon industrial computers, programmable logic controllers, and advanced algorithms. It collects real-time data on grid electricity prices, local wind / solar power output forecasts, waste feed volume, and temperature and pressure at various nodes. Through built-in optimization algorithms, it automatically generates and executes optimal scheduling strategies to achieve the lowest overall system operating cost or maximum carbon emission reduction, dynamically managing the storage / release of molten salt, reactor load, waste heat recovery paths, and the start-up and shutdown of backup fuel.

[0011] Secondly, this invention provides a method for the resource-based treatment of urban and rural waste based on the aforementioned system. This method deeply integrates the waste treatment process with the dynamics of the energy market and changes in natural conditions, and includes the following steps: S1: Material preparation. The organic components of urban and rural domestic waste that have been sorted to remove inert materials are mixed with agricultural straw (such as corn stalks) that has been harvested and crushed to a suitable size. A typical mixing ratio is 7:3, and water is added to make a uniform slurry.

[0012] S2: Smart Energy Storage. The intelligent control unit formulates an energy storage plan based on the received electricity price curve for the next 24 hours and renewable energy generation forecasts. Typically, during the deep off-peak electricity period from midnight to 8 a.m., or during the daytime when wind and solar power output exceeds the local base load, electric heaters are activated to convert cheap or surplus electricity into thermal energy stored in high-temperature molten salt.

[0013] S3: High-efficiency processing and energy supply. During peak or off-peak hours when electricity prices are higher, or during fixed waste treatment operation periods, the system initiates the processing flow. The molten salt thermal storage system releases heat as needed, providing all or most of the required thermal energy to the subcritical water reactor through a thermal coupling unit, driving the waste slurry to complete the resource-based conversion reaction.

[0014] S4: Multi-stage energy synergy. The high-temperature products (approximately 300-320℃) generated by the reaction first flow through the primary heat exchanger, exchanging heat with the cold slurry (approximately 20℃) entering the reactor, preheating the cold slurry to above 180℃. This process saves approximately 35% of the main heat source energy consumption. Subsequently, the cooled products (approximately 200℃) enter the secondary heat exchanger, where their residual heat is further transferred to the plant's heating water circuit (in winter) or used to heat some of the low-temperature molten salt (in other seasons), achieving complete energy utilization.

[0015] S5: Resource Output and Closed Loop. The final cooled product enters the separation unit to obtain bio-oil that can be sold directly or used as high-quality fuel, biochar that can be used as a soil conditioner or solid fuel, and high-efficiency liquid fertilizer rich in humic acid. A portion of the separated bio-oil is transported to a backup fuel tank, ready to participate in the energy supply closed loop under the command of the intelligent system.

[0016] Beneficial effects Compared with existing single waste treatment technologies or independent energy storage technologies, this invention achieves a systemic innovation of "1+1>2", and has the following outstanding advantages: Disruptive economics: Successfully transforming the main cost item in waste treatment—thermal energy costs—into a potential profit point. Through the "low storage, high utilization" energy arbitrage model and the internal circulation of self-produced fuel, the cost per ton of waste treatment is significantly reduced, and the investment payback period can be shortened by 1-2 years compared to traditional projects, greatly improving the commercial viability of the project.

[0017] Ultimate energy efficiency: Through a three-stage thermal energy integration design of "main heat source coupling + feed preheating + waste heat storage / heating", the energy barrier of traditional processing technology is broken, realizing the gradient utilization and maximum recovery of thermal energy within the system. The overall energy efficiency of the system can be increased from about 50% in the traditional method to more than 85%.

[0018] Deep environmental protection and system friendliness: The waste treatment process can rely entirely on renewable energy to achieve near-zero carbon treatment. More importantly, as a large, flexible, and adjustable "virtual power plant" unit, the system can provide ancillary services such as peak shaving and valley filling, frequency regulation, etc., to the power grid, effectively smoothing out the fluctuations of renewable energy and helping to build a new power system based on new energy sources, with enormous social value.

[0019] Excellent regional adaptability: Perfectly meets the comprehensive needs of cold northern regions. The system can flexibly adjust its operation strategy according to the season: generate more electricity and feed it into the grid in summer, make full use of straw in spring and autumn, and participate in heating as a stable clean heat source in winter, truly realizing "one plant, multiple functions, and multiple benefits", making it an ideal core technology for building "zero-waste cities" and "smart energy parks". Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall system process and heat flow provided in an embodiment of the present invention.

[0021] Figure 2 Detailed diagram of thermal coupling and energy cascade utilization provided for embodiments of the present invention.

[0022] Figure 3 This is a schematic diagram of the intelligent control logic and operation mode provided in the embodiments of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is based on a demonstration project in a county in Northeast my country that processes 200 tons of mixed organic waste per day; however, the invention is not limited to this scale or application scenario.

[0024] Example: County-level co-processing energy station for waste and straw in Northeast China System configuration and parameters: Targets for processing: 140 tons / day of urban and rural domestic waste and 60 tons / day of corn stalks.

[0025] Subcritical water treatment unit: Employs a continuous tubular reactor with a designed processing capacity of 8.5 tons / hour. The core reaction temperature is set at 320℃, the pressure at 12MPa, and the material residence time at approximately 30 minutes.

[0026] Molten salt thermal energy storage unit: The thermal storage medium is commercially available dinitrate (Solar Salt). The total designed thermal storage capacity is 50MWh (equivalent to meeting the thermal energy demand of the system operating at full load for approximately 6 hours). The total power of the electric heater is 8MW, and the molten salt operating temperature range is 290℃ (cold tank) - 380℃ (hot tank).

[0027] Thermal coupling unit: The main heat exchanger is a molten salt-thermal oil heat exchanger, with a designed thermal oil outlet temperature of 340℃. It is precisely controlled by a PID control system to provide a stable heat source for the reactor.

[0028] Intelligent Control Unit: Developed based on an industrial IoT platform, the hardware utilizes a high-performance PLC and server, while the software integrates real-time data acquisition, multi-objective optimization algorithms, and a human-machine interface. This platform has been connected to the provincial power dispatch center's data interface via a secure gateway, enabling it to obtain real-time electricity prices and ancillary service market information.

[0029] Detailed Explanation of Operation Method (with accompanying diagram): Reference Figure 1 To understand the overall system, the material flow (solid arrow) starts from the garbage and straw on the left, goes through pretreatment, reaction, and separation, and finally yields various products. The heat flow (dashed arrow) clearly shows three main paths: (1) the molten salt thermal storage unit supplies heat to the reactor as the main heat source; (2) the high-temperature products preheat the feed through a heat exchanger; and (3) the self-generated bio-oil is fed back to the system as a backup heat source. The intelligent control center coordinates all flows.

[0030] Reference Figure 2 Understanding the core of efficiency: The thermal energy utilization of this system is not a one-time event, but rather a stepped, waterfall-like process. The high-temperature molten salt heat (380℃) is first used to drive the most energy-intensive reaction itself (320℃). The heat carried by the reaction products (300℃) is "intercepted" by the primary heat exchanger to significantly preheat the cold materials. The remaining medium- and low-temperature heat (200℃) is then recovered by the secondary heat exchanger for heating or to "replenish heat" the molten salt system itself. This design is key to achieving an overall energy efficiency of over 85%.

[0031] Reference Figure 3Experience intelligent scheduling. The system's operation is not static, but rather acts like a "smart energy robot," automatically selecting the optimal behavior mode based on external signals. For example, on a typical winter workday: Mode A (2:00 AM): The controller detects off-peak electricity with a real-time price of 0.28 yuan / kWh and predicts high wind power output, and automatically starts the "off-peak electricity thermal storage-green electricity consumption" hybrid mode to heat molten salt at maximum power.

[0032] Mode switching (10:00 AM): Electricity prices return to normal (0.65 yuan / kWh), and garbage trucks begin feeding. The controller switches to "processing mode," the molten salt releases heat stably, and the system operates at full load. Simultaneously, primary waste heat recovery automatically starts.

[0033] Mode C (7:00 PM, cold wave): Electricity prices reach their peak (1.20 yuan / kWh), and regional heating demand surges. After evaluation, the controller decides to activate the "high-demand supplement mode": In addition to molten salt heating, a backup heater is ignited, and some self-generated bio-oil is mixed in to transfer more heat into the heating network. This meets social demand while avoiding the need to purchase grid electricity at extremely high prices, achieving a win-win situation for both economic and social benefits.

[0034] Expected results: The following benefits are expected to be achieved through the implementation of this embodiment system: Environmental benefits: It processes 73,000 tons of waste and disposes of 22,000 tons of straw annually, completely solving the problem of open burning of straw; it replaces approximately 11,000 tons of standard coal annually, reducing carbon dioxide emissions by approximately 28,000 tons.

[0035] Economic benefits: After taking into account energy storage arbitrage, bio-oil sales, heating revenue and possible grid ancillary service revenue, the project’s internal rate of return (IRR) is expected to increase from about 8% for a single treatment project to 12-15%, with an investment payback period of about 5-7 years.

[0036] Energy system benefits: It provides the local power grid with a maximum adjustable load / power of about 8MW, and can absorb about 5 million kWh of curtailed wind and solar power annually.

[0037] In summary, the system and method provided by this invention offer an innovative, operable, and economically beneficial comprehensive solution to the interdisciplinary challenges currently faced in urban and rural waste management and energy system transformation. It has significant practical value for promoting regional circular economy development, ensuring energy security, and achieving dual-carbon goals.

Claims

1. A subcritical water treatment system coupled with molten salt energy storage, characterized in that, include: The subcritical water treatment unit is used to carry out resource-based transformation reactions of mixed organic waste of urban and rural domestic waste and agricultural straw in a subcritical water state. The molten salt thermal energy storage unit is used to store thermal energy during periods of low electricity prices or when renewable energy is abundant, and to supply heat to the subcritical water treatment unit. A thermal coupling unit connects the molten salt thermal energy storage unit and the subcritical water treatment unit, and is used to transfer the thermal energy stored in the molten salt to the subcritical water reactor. The product separation and energy conversion unit is used to separate the products generated by the subcritical water treatment unit, and can return some of the combustible products to the molten salt thermal energy storage unit as a supplementary or emergency heat source. The intelligent control unit is used to dynamically schedule the operation mode and energy flow of the entire system based on real-time electricity prices, renewable energy output forecasts, and waste feeding load signals.

2. The system according to claim 1, characterized in that, The thermal coupling unit includes a molten salt-thermal oil heat exchanger and a thermal oil circuit connecting the heat exchanger to the subcritical water reactor, forming a closed main heating circuit that indirectly and controllably supplies molten salt heat to the reactor.

3. The system according to claim 2, characterized in that, It also includes a multi-stage waste heat recovery circuit, wherein the waste heat recovery circuit includes at least: A primary heat exchanger is used to preheat the material slurry entering the reactor using the high-temperature products generated by the subcritical water treatment unit. The secondary heat exchanger is used to recover the low- and medium-temperature waste heat of the preheated product and store it in the molten salt thermal energy storage unit or for heating the plant area.

4. The system according to claim 1, characterized in that, The intelligent control unit is configured to intelligently switch between at least the following operating modes: Off-peak electricity thermal storage-peak electricity processing mode: During periods of low electricity prices, electric heaters are activated to store thermal energy, and during periods of high electricity prices, molten salt heat is released for waste processing; Green electricity priority consumption mode: When the output of local renewable energy exceeds the system base load, real-time green electricity is used first for processing, and the excess electricity is stored in molten salt; High-demand replenishment mode: When the system's heat demand surges or there is a shortage of external energy supply, the backup heater is activated to burn self-generated bio-oil or combustible gas to replenish the system's heat energy.

5. The system according to claim 1, characterized in that, The subcritical water treatment unit has a tubular reactor with a reaction temperature range of 250-350℃ and a pressure range of 4-22MPa; the molten salt thermal energy storage unit has an operating temperature range of 280-400℃.

6. A method for the resource-based treatment of urban and rural waste using the system described in any one of claims 1-2, characterized in that, Includes the following steps: S1: Material preparation, mixing the sorted organic components of urban and rural domestic waste with crushed agricultural straw in a certain proportion to make pulp; S2: Energy storage, under the command of the intelligent control unit, uses electricity to heat molten salt to store thermal energy during periods of low electricity prices or surplus green electricity. S3: Processing and energy supply, releasing the thermal energy stored in molten salt to heat the subcritical water reactor and treat organic slurry with subcritical water. S4: Energy synergy, recovering the waste heat of high-temperature products generated during the process for preheating feed and / or storage; and selectively reusing self-produced combustible products for heating according to energy supply and demand status; S5: Product output, separating and obtaining bio-oil, biochar and liquid fertilizer resource products.

7. The method according to claim 2, characterized in that, During winter operation, the waste heat recovered by the secondary heat exchanger and part of the heat energy stored in the molten salt are directly used for heating the plant area or surrounding buildings, realizing combined heat and power.

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