A municipal sludge mechanical dewatering, drying and carbonization process whole-process economic optimization method

Abstract

The application discloses a kind of municipal sludge mechanical dewatering, drying and carbonization process whole-process economic optimization method to improve the efficiency and economy of municipal sludge dewatering, drying and carbonization system.The whole-process optimization strategy in the application mainly shows in two aspects: one is the heat tail gas reuse of sludge thermal drying stage and the reuse strategy of pyrolysis oil in sludge carbonization stage, which realizes efficient use and alternative use of energy;Two is that, aiming at the key factor of sludge moisture content affecting drying and carbonization, the moisture content conditions of mechanical dewatering, thermal drying, carbonization stages are accurately adapted to determine the optimal economic benefit operation parameters.The application realizes the recycling of waste heat and pyrolysis oil gas, and explores the most economic operating point of sludge dewatering, drying and carbonization, which greatly reduces the system energy consumption, greatly improves the sludge drying and carbonization efficiency, and significantly reduces the system operation cost.

Description

Technical Field

[0001] This invention relates to the field of sludge drying and carbonization technology, and more specifically, to an economically optimized method for the entire process of mechanical dewatering, drying and carbonization of municipal sludge. Background Technology

[0002] Rapid economic and social development has led to a rapid increase in industrialization, resulting in a surge in the number and scale of wastewater treatment plants. This necessitates the addition of at least 20,000 tons of new wet sludge (80% moisture content) harmless treatment facilities nationwide by 2025. Centralized sludge stockpiling not only occupies significant land but also easily causes secondary environmental pollution and resource waste. Sludge has a high moisture content, complex composition, and contains various organic and inorganic substances, heavy metals, salts, and small amounts of pathogens and parasites, leading to high treatment and disposal costs. Sludge pyrolysis and carbonization, comprising two stages—sludge drying and carbonization—uses a heat source to evaporate internal moisture, significantly reducing sludge volume. This is a primary technology for urban sludge treatment and a crucial step in the process. However, sludge drying often involves high energy consumption, resulting in high operating costs, contradicting recent strategies and hindering its development and promotion. Therefore, research on energy conservation and consumption reduction in the sludge drying process is of practical significance. During the sludge pyrolysis and carbonization process, exhaust gas is generated at the outlet of the drying furnace along with the dried sludge. This exhaust gas contains a large amount of heat energy, which can be recovered and used as a heat source for the sludge drying stage. Of the pyrolysis oil and gas produced during sludge pyrolysis, a portion is recycled back to sludge pyrolysis, and the remaining portion is allocated to the drying stage, providing additional energy for sludge drying, provided that the pyrolysis requirements are met. Precise control of waste heat recovery involves accurately allocating the drying exhaust gas from the drying stage with the pyrolysis oil and gas from the pyrolysis stage, using excess heat energy for the sludge drying stage, reducing energy consumption, and is one of the important measures to reduce energy consumption in sludge pyrolysis and carbonization.

[0003] Existing technologies have studied waste heat recovery systems for the hydrothermal carbonization process of sludge, and confirmed the effectiveness of total heat exchange technology in improving the calorific value of sludge and system efficiency through design and thermodynamic analysis. Simultaneously, the thermodynamic characteristics of the sludge drying-incineration system were analyzed, indicating that the sludge's self-heating is insufficient to support system operation, leading to an increase in auxiliary fuel demand as the sludge moisture content decreases. Regarding the pyrolysis characteristics of municipal sludge, energy balance analysis revealed that pyrolysis oil and gas recycling for heating can not only be self-sufficient but also save energy during the drying stage. Existing technologies have also studied the optimal operating conditions for a combined sludge drying and incineration system, determining the optimal operating conditions for energy saving and consumption reduction. However, precise control of waste heat recovery from sludge pyrolysis carbonization, a crucial aspect of energy saving and consumption reduction, is rarely reported. Summary of the Invention

[0004] The purpose of this invention is to improve the current process system by addressing the problems of significant heat loss and insufficient waste heat utilization. Based on actual operating parameters, an energy balance equation for the sludge pyrolysis and carbonization system is constructed to calculate the heat consumption of each process step. The energy-saving potential of each part is evaluated and analyzed to determine a reasonable waste heat recovery ratio, fuel supply rate, and air matching amount, accurately matching the energy requirements of sludge drying and carbonization, and improving the overall energy utilization efficiency of the system, thereby solving the problems mentioned in the background art.

[0005] To achieve the above objectives, according to Figure 11 As shown, the present invention provides an economic optimization method for the entire process of mechanical dewatering, drying and carbonization of municipal sludge, including the mechanical dewatering stage, the thermal drying stage and the carbonization stage of sludge.

[0006] The thermal drying stage employs an internally heated rotary drying furnace, and a heat exchanger is added at the furnace outlet to recover residual heat from the drying exhaust gas.

[0007] Part of the pyrolysis oil and gas generated in the carbonization stage is recycled in the drying stage to achieve energy recycling.

[0008] An energy balance equation for the sludge pyrolysis and carbonization system was constructed, the heat consumption of each process was assessed, and a reasonable waste heat recovery ratio, fuel supply rate, and air matching amount were determined to accurately match the energy requirements of sludge drying and carbonization.

[0009] As a further improvement to this technical solution, the heat exchanger includes a precooler, a condenser, and a heat exchanger, used to gradually reduce the temperature of the dried exhaust gas and recover the heat therein.

[0010] After being treated by the precooler, part of the exhaust gas is discharged, while the other part is heated to the drying stage after the moisture is removed by the condenser and then heated by the heat exchanger.

[0011] As a further improvement to this technical solution, the drying tail gas is mixed with high-temperature flue gas to form the inlet flue gas of the drying furnace, realizing the waste heat recovery of the tail gas and pyrolysis oil gas. The high-temperature flue gas comes from the complete combustion of pyrolysis oil gas generated by the pyrolysis of dry sludge in the carbonization combustion chamber.

[0012] As a further improvement to this technical solution, under different temperatures during the drying stage, the evaporation efficiency of sludge with varying moisture content is improved as the drying temperature increases from 105℃ to 150℃.

[0013] As a further improvement to this technical solution, the energy balance relationship of the drying stage includes input energy (heat Q provided by fuel combustion). f Heat recovery from the dried exhaust gas (Q) re and the heat Q distributed from the combustion of pyrolysis oil and gas to the drying section gas) and output energy (energy consumed by sludge heating Q) s The residual water in the sludge absorbs heat when heated (Q) w Energy Q required for water evaporation e The energy Q carried away by the exhaust gas at the drying outlet out and heat dissipation energy loss Q c1 ).

[0014] As a further improvement to this technical solution, the energy distribution and reuse of drying tail gas and pyrolysis oil gas under different moisture content conditions were analyzed. It was found that when the moisture content of sludge decreases, the energy required for water evaporation decreases accordingly, and the distribution ratio of pyrolysis oil gas reuse in the drying and carbonization stages is 19:6, which improves the overall energy utilization efficiency.

[0015] As a further improvement to this technical solution, when the moisture content after the mechanical dehydration stage is 62%, the drying stage uses a drying temperature of 150°C to dry to a moisture content of 20%, and then proceeds to the carbonization stage.

[0016] As a further improvement to this technical solution, the pyrolysis oil and gas generated in the carbonization stage are transported to the carbonization combustion chamber via a separator, where they are fully combusted to generate high-temperature flue gas. A portion of the high-temperature flue gas is recycled back to the carbonization furnace to meet the operation of the pyrolysis system, and the remaining high-temperature flue gas is mixed with the tail gas recycled from the drying furnace and the high-temperature flue gas provided by the raw materials burned in the drying combustion chamber to form the inlet flue gas of the drying furnace, thereby realizing the recovery and utilization of waste heat from the drying tail gas and the pyrolysis oil and gas.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] The economic optimization method for the entire process of municipal sludge mechanical dewatering, drying, and carbonization mainly focuses on two aspects: First, the strategy of reusing the waste gas during the sludge thermal drying stage and the pyrolysis oil during the sludge carbonization stage achieves efficient energy utilization and alternative use. Second, recognizing that sludge moisture content is a key factor affecting drying and carbonization, the optimal operating parameters for economic benefits are determined by precisely adapting the moisture content conditions at each stage of mechanical dewatering, thermal drying, and carbonization. Taking municipal sludge produced by an A2O process wastewater treatment plant as an example, when the sludge moisture content after mechanical dewatering is 62%, and the drying temperature is 150℃ to 20% after drying, the operating cost of the entire system is lowest, with a cost of 257.47 yuan per ton of sludge treated. This invention achieves the recovery and utilization of waste heat and pyrolysis oil and gas while exploring the most economical operating points for sludge dewatering, drying, and carbonization, significantly reducing system energy consumption and greatly improving sludge drying and carbonization efficiency, resulting in a significant reduction in system operating costs. Attached Figure Description

[0019] Figure 1A graph showing the energy consumption potential analysis of the drying stage (A) and carbonization stage (B) under different initial moisture contents;

[0020] Figure 2 This is a diagram showing the energy balance relationship during sludge drying.

[0021] Figure 3 This is a diagram showing the energy balance relationship in sludge carbonization.

[0022] Figure 4 The diagram shows the energy balance between the waste heat recovery of the dried tail gas (A) and the pyrolysis oil and gas (B).

[0023] Figure 5 Figures showing the analysis of drying efficiency and exhaust gas reuse rate at different drying temperatures;

[0024] Figure 6 Graph showing the flow rate analysis of high-temperature drying flue gas at different drying temperatures;

[0025] Figure 7 This is a schematic diagram showing the distribution of energy output (A) and energy input (B) during the drying stage at different drying temperatures;

[0026] Figure 8 A schematic diagram comparing fuel consumption under different moisture contents of sludge before and after improvement;

[0027] Figure 9 This is a schematic diagram illustrating the waste heat recovery process under different moisture contents of sludge.

[0028] Figure 10 A schematic diagram illustrating the disposal costs of sludge dewatering and drying throughout its entire life cycle under different moisture contents using mechanical dewatering.

[0029] Figure 11 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] (1) Using a certain A 2Taking the sludge treatment at the O wastewater treatment plant as an example, this plant is designed to treat 150 tons of sludge per day, employing a pyrolysis carbonization process to achieve sludge resource recovery and volume reduction. The main processes include mechanical dewatering, drying, carbonization, and flue gas treatment. After dewatering, the sludge discharged from the wastewater treatment plant is transported to a sludge cake silo. Inside the silo, the wet sludge is crushed and then transferred to an internally heated rotary dryer. There, it comes into contact with high-temperature flue gas provided by the drying heating system, exchanging heat and raising its temperature. The moisture in the wet sludge evaporates, reducing the moisture content to approximately 20%. The flue gas from the dryer's outlet contains water vapor, volatiles, and other substances. These are separated by the flue gas treatment system. The water vapor condenses into wastewater and is returned to the wastewater treatment plant for secondary treatment. The remaining flue gas is treated to meet emission standards before being discharged. The dried sludge is then transported to an externally heated carbonization furnace to absorb heat conducted from the high-temperature flue gas to the furnace's inner wall, raising its temperature to approximately 500°C and undergoing pyrolysis. The pyrolysis products are collected and disposed of in a carbon storage silo. The pyrolysis oil and gas produced by the pyrolysis of dry sludge contains a small amount of gaseous tar and flows back to the carbonization heating system. It reacts with air and burns completely to produce high-temperature flue gas, which provides a heat source for the carbonization furnace and maintains the operation of the pyrolysis system.

[0032] (2) The sludge drying stage in the case uses an internally heated rotary dryer. Figure 1 This paper analyzes the energy-saving potential of the sludge pyrolysis carbonization stage under different sludge moisture contents. The results show that when the initial sludge moisture content exceeds 60%, energy consumption increases significantly due to the need to evaporate a large amount of water. Some of the pyrolysis exhaust gas is directly emitted without utilization, becoming a major source of energy loss in the system. Conversely, when the initial sludge moisture content is below 60%, the dry weight increases, the proportion of organic components rises, and the yield of pyrolysis oil and gas is higher, becoming a highly valuable energy source. Therefore, the process improvement should focus on improving the distribution of drying exhaust gas and pyrolysis oil and gas, as well as waste heat recovery.

[0033] (3) The drying tail gas directly emitted in the original process is utilized for energy by adding a heat exchanger at the outlet of the drying furnace, which includes a precooler, a condenser, and a heat exchanger. The drying tail gas is heated by the circulating water in the precooler, and the tail gas is cooled down. After cooling, part of the tail gas is discharged to carry away the ash produced by sludge drying. The other part is heated by contacting the hot circulating water in the heat exchanger after the condenser removes the moisture carried in during the heat exchange process and is reused in the sludge drying stage. The pyrolysis oil and gas produced by sludge pyrolysis is transported to the carbonization combustion chamber through a separator, where it is fully combusted to produce high-temperature flue gas. Part of the high-temperature flue gas is reused in the carbonization furnace to meet the operation of the pyrolysis system. The remaining high-temperature flue gas is mixed with the tail gas reused in the drying furnace and the high-temperature flue gas provided by the raw materials in the drying combustion chamber to form the inlet flue gas of the drying furnace, thereby realizing the recovery of waste heat from the tail gas and pyrolysis oil and gas.

[0034] (4) During the sludge drying stage, the wet sludge exchanges heat with the inlet flue gas. In the system energy balance, the heat Q is provided by fuel combustion as the input energy. f(563000 kJ / T), heat Q from waste heat recovery of drying tail gas re (557000kJ / T) and the heat Q distributed from the combustion of pyrolysis oil and gas to the drying section. gas (1842000kJ / T) in three parts. The output energy is mainly the energy consumed by sludge heating Q. s (23940kJ / T), the residual water in the sludge absorbs heat upon heating Q w (29800kJ / T), Energy required for water evaporation Q e (1349000kJ / T), energy Q carried away by the dried exhaust gas. out (1559000kJ / T) and heat loss energy Q c1 (98000kJ / T) in five parts. The energy balance relationship is as follows: Figure 2 As shown.

[0035] (5) In the energy balance of the sludge carbonization stage, the input is only the energy Q of drying sludge. d (2842400kJ / T), the output is the energy Q of the pyrolysis products. p (422980kJ / T), the energy Q allocated to the drying stage after the combustion of pyrolysis oil and gas to maintain the operation of the system. gas (1842000kJ / T) and heat dissipation loss energy consumption Q c2 (588300kJ / T). The energy balance relationship is as follows: Figure 3 As shown.

[0036] (6) The waste heat recovery system for the drying outlet exhaust gas only uses the energy Q of the drying exhaust gas. out (1559000kJ / T) is the input, and the output is the energy Q removed by the flue gas cooling. c (582800kJ / T), Energy loss due to exhaust gas heat dissipation Q out2 (409000kJ / T) and the reuse energy Q of the recycled flue gas re (557000 kJ / T). Regarding the waste heat recovery system for pyrolysis oil and gas, its energy input depends on the complete combustion Q of the pyrolysis oil and gas. g (2417000kJ / T), the output energy is divided into the energy Q that sustains the self-sustaining operation of the pyrolysis system. g1 (575000 kJ / T) and the energy Q allocated to the drying stage for reuse. gas (1842000kJ / T). The energy balance relationship in the waste heat recovery stage is as follows: Figure 4 As shown.

[0037] (7) Figure 5The drying efficiency of sludge and the heat recovery rate of exhaust gas were analyzed at different drying temperatures. As shown in the figure, the evaporation efficiency of sludge with varying moisture contents improved as the drying temperature increased from 105℃ to 150℃. Particularly at a sludge moisture content of 55%, the combustion efficiency improved most significantly, with fuel consumption decreasing from 105.89 kg / T to 26.42 kg / T, a reduction of 75.05%. However, this upward trend gradually leveled off as the temperature increased. The proportion of heat recovered from the drying exhaust gas decreased by 2.5% during this process.

[0038] (8) Figure 6 As shown, the rate of decrease in flue gas flow gradually slows down with increasing drying temperature. This indicates that while increasing temperature helps improve drying efficiency, the reduction in flow rate offsets this effect to some extent, causing the increase to eventually plateau.

[0039] (9) When the amount of water evaporated during the drying process remains constant, the required drying energy, the amount of ash removed, and the total amount of volatile gases remain constant. When the drying flue gas changes from low temperature and high flow rate to high temperature and low flow rate, the proportion of energy required for exhaust gas purification increases, resulting in a decrease in the heat recovery rate. Figure 7 The figure shows the energy distribution during the drying stage at different drying temperatures. It indicates that when the wastewater moisture content is 65%, although the increased temperature reduces the proportion of energy reused from the exhaust gas, the reduced flow rate decreases the heat loss carried away by the exhaust gas, thus reducing fuel demand. Within the temperature range of 105℃ to 150℃, the higher the drying temperature, the greater the efficiency improvement in the drying stage.

[0040] (10) The moisture content of sludge is closely related to the energy consumption of evaporating water, the flow rate of hot flue gas during the drying stage, and the dry composition of sludge during the carbonization stage. Figure 8 The results show that at a drying temperature of 150℃, when the sludge moisture content decreases from 85% to 55%, the original process reduces biomass fuel consumption by 106 kg / t, while the improved process reduces it by 173 kg / t. At a moisture content of 85%, the fuel consumption saving rate is 19.31%, while at 55%, the saving rate reaches as high as 83.34%. Therefore, after the process improvement, fuel savings can be achieved by using waste heat recovery to treat sludge with different moisture contents.

[0041] (11) Figure 9Energy balance analysis was conducted to investigate the energy distribution and reuse of drying exhaust gas and pyrolysis oil gas at different sludge moisture contents. As the sludge moisture content decreases, the energy required for water evaporation decreases accordingly. When the sludge moisture content is 55%, approximately 19% of the total heat from the drying exhaust gas can be reused, creating conditions for its reuse. Since the sludge carbonization stage is a pyrolysis reaction, it also produces pyrolysis oil gas. Generally, as the moisture content decreases, the dry basis mass ratio of the sludge increases, thereby increasing the yield of pyrolysis oil gas in the carbonization stage, creating conditions for its reuse. The ratio of pyrolysis oil gas reused in the drying and carbonization stages is 19:6. Furthermore, [the remaining text appears to be incomplete and requires further context]. Figure 9 It can be seen that as the moisture content decreases, the total recoverable calorific value of the drying tail gas and pyrolysis oil and gas increases. This is due to two reasons: First, the recoverable heat in the drying tail gas decreases at a rate of 14970 kJ per 1% decrease in moisture content. Second, the recoverable energy in the pyrolysis oil and gas used in the drying stage increases at a rate of 48475 kJ per 1% decrease in moisture content.

[0042] (12) In the economic benefit analysis of the entire life cycle of sludge dewatering and drying, considering the level of sludge dewatering technology, the moisture content of sludge is a key factor affecting economic benefits and energy utilization. In the mechanical dewatering stage of sludge, assuming that the wet sludge is dewatered to a certain moisture content, the hourly power consumption for deep sludge dewatering can be calculated by formula. When the sludge is dewatered to a certain moisture content, it enters the sludge drying stage. In this process, energy consumption is mainly concentrated in the water evaporation and air supply of the mixing fan. During the drying process, high-temperature flue gas is formed by mixing biomass fuel with recycled flue gas, and water evaporation is achieved by heat exchange with the dewatered sludge. In this step, the cost mainly comes from the consumption of biomass fuel. At the same time, the mixing fan delivers mixed flue gas to the drying furnace. The hourly fan power is obtained according to the fan model of the research project. The price of biomass fuel is calculated at 1.2 yuan / kg, and the average price of industrial electricity is 0.59 yuan / kWh. The total cost of the system can be obtained through calculation. After mechanical dewatering, the sludge enters the drying stage at 150℃. The cost is examined when the sludge moisture content during mechanical dewatering is between 55% and 85%. The cost gradually increases as the moisture content decreases; for example, it is 73.49 yuan / ton at 85% moisture content and 270.22 yuan / ton at 55%. Conversely, the lower the moisture content, the lower the cost for the carbonization stage. For example, it is 213.66 yuan / ton at 85% moisture content and 2.90 yuan / ton at 55%. Since the cost of the carbonization stage is offset by the reuse of pyrolysis oil and gas, the cost is calculated as 0 yuan. Figure 10 It can be seen that the operating cost of the entire system is lowest when the sludge moisture content after mechanical dewatering is 62%. At this point, the cost of mechanical dewatering is 205.39 yuan / ton, and the cost of drying is 50.08 yuan / ton.

[0043] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. An economically optimized method for the entire process of mechanical dewatering, drying, and carbonization of municipal sludge, characterized in that, This includes the mechanical dewatering stage, the thermal drying stage, and the carbonization stage of sludge. The thermal drying stage employs an internally heated rotary drying furnace, and a heat exchanger is added at the furnace outlet to recover residual heat from the drying exhaust gas. Part of the pyrolysis oil and gas generated in the carbonization stage is recycled in the drying stage to achieve energy recycling. The heat exchanger includes a precooler, a condenser, and a heat exchanger, used to gradually reduce the temperature of the dried exhaust gas and recover the heat therein; After being treated by the precooler, part of the exhaust gas is discharged, and the other part is heated to the drying stage by the heat exchanger after the moisture is removed by the condenser. It also includes constructing the energy balance equation of the sludge pyrolysis and carbonization system, assessing the heat consumption of each process, and determining a reasonable waste heat recovery ratio, fuel supply rate and air matching amount to accurately match the energy requirements of sludge drying and carbonization. Analysis of the energy distribution and reuse of drying tail gas and pyrolysis oil gas under different moisture contents revealed that when the moisture content of sludge decreases, the energy required for water evaporation decreases accordingly, while the distribution ratio of pyrolysis oil gas reuse in the drying and carbonization stages is 19:6, which improves the overall energy utilization efficiency. When the moisture content after the mechanical dehydration stage is 62%, the drying stage uses a drying temperature of 150℃ to dry to a moisture content of 20%, and then proceeds to the carbonization stage. During the carbonization stage, the pyrolysis oil and gas generated are transported to the carbonization combustion chamber via a separator, where they are fully combusted to produce high-temperature flue gas. A portion of the high-temperature flue gas is recycled back to the carbonization furnace to meet the operation of the pyrolysis system. The remaining high-temperature flue gas is mixed with the tail gas recycled from the drying furnace and the high-temperature flue gas provided by the raw materials burned in the drying combustion chamber to form the inlet flue gas of the drying furnace, thus realizing the recovery and utilization of waste heat from the drying tail gas and the pyrolysis oil and gas.

2. The method for optimizing the entire process of mechanical dewatering, drying, and carbonization of municipal sludge according to claim 1, characterized in that, The drying tail gas and high-temperature flue gas are mixed to form the inlet flue gas of the drying furnace, realizing the waste heat recovery of the tail gas and pyrolysis oil gas. The high-temperature flue gas comes from the complete combustion of pyrolysis oil gas generated by the pyrolysis of dry sludge in the carbonization combustion chamber.

3. The method for optimizing the entire process of municipal sludge mechanical dewatering, drying, and carbonization according to claim 1, characterized in that, At different temperatures during the drying stage, the evaporation efficiency of sludge with varying moisture content increased as the drying temperature rose from 105℃ to 150℃.

4. The method for optimizing the entire process of municipal sludge mechanical dewatering, drying, and carbonization according to claim 1, characterized in that, The energy balance relationship during the drying stage includes input energy and output energy; Input energy includes the heat provided by fuel combustion. Q f Waste heat recovery from drying exhaust gas Q re and the heat distributed to the drying section from the combustion of pyrolysis oil and gas. Q gas ; Energy output includes energy consumed in sludge heating. Q s The remaining water in the sludge absorbs heat as it heats up. Q w Energy required for water evaporation Q e The dried exhaust gas carries away energy. Q out and heat dissipation and energy loss Q c1 .

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