WO2025226173A1 - Method for heat treating sewage sludge - Google Patents

Abstract

The invention relates to the treatment of sewage sludge. Sewage sludge is dewatered, dried in two stages and fed to a pyrolysis reactor. In the first drying stage, the sludge is dried to a moisture content of 40-55% by direct contact with flue gases from a furnace, the drying being carried out by convection under mechanical activation of the sludge. In the second drying stage, the sludge is dried to a moisture content of 10-12% at a lower temperature difference. After the first drying stage, the flue gases are purified and sent to heat a drying agent of the second drying stage. After the second drying stage, heat from the drying agent is used to heat sludge after the mechanical dewatering stage, before being fed to the first drying stage. Pyrolysis is carried out at a temperature of 350-600°С. The solid pyrolysis products are cooled, with the heat being transferred to a heat transfer agent that is used to warm combustion air supplied for burning the vapour-gas mixture produced. The vapour-gas mixture is purified of solid product particles and unreacted sludge and fed to a furnace to produce thermal energy for providing heat for pyrolysis and for serving as a drying agent in the first drying stage. The flue gases from the furnace are used as thermal energy for pyrolysis and drying. The result is that of minimizing the amount of thermal energy lost with exhaust flows and reducing the pollution level of gas emissions.

Description

METHOD OF THERMAL PROCESSING OF SEWAGE SLUDGE

Field of technology

The invention relates to the thermochemical processing of wastewater sludge (WSS) to produce solid products. The method enables efficient processing of WSS while minimizing energy costs and harmful gaseous emissions. The method can be used in wastewater treatment for municipal and industrial enterprises.

Prior art

The main existing methods for processing irrigated wastewater treatment plants (IWTS) are: biogasification, on-site burial, thermal processing, and application to agricultural soils. Biogas processing converts only a small portion of the IWTS volume into methane. This technology also does not meet modern requirements for process speed, processing quality, or sanitary requirements. IWTS disposal requires large areas and has a negative impact on the environment. Application of IWTS to agricultural soils can lead to contamination with heavy metals, pathogens, helminth eggs, microplastics, antibiotics, viruses, etc.

Thermal processing practices utilize technologies such as direct and catalytic combustion, liquid-phase oxidation, gasification, and pyrolysis. Direct combustion is one of the most widely used practical methods for processing irrigated wastewater. The disadvantages of direct combustion include dioxin formation, large amounts of fly ash, high cost of irrigated wastewater incineration units, and stringent requirements for the inert heat carrier and the irrigated wastewater itself in terms of moisture, calorific value, and non-combustible particle content. Compared to direct combustion, catalytic combustion technology reduces acid gas emissions, atmospheric emissions, and heat loss by lowering the combustion temperature. A disadvantage of catalytic combustion of irrigated wastewater is the effect of high feedstock moisture and heavy metals on catalyst stability. A disadvantage of liquid-phase oxidation of irrigated wastewater is the use of high pressure and

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SUBSTITUTE SHEET (RULE 26) The impossibility of complete oxidation. The specific costs of gasification of the IOSW exceed those of combustion and pyrolysis, due to the high cost of equipment and the difficulty of maintaining the process. Furthermore, in the high-temperature process, exhaust gases containing heavy metals, phosphorus, and halogens require purification similar to that of direct combustion.

A known method for catalytic processing of wastewater treatment plants (RU No. 2568978, 17.10.2014) includes mechanical dehydration of wastewater treatment plants, drying the dehydrated wastewater treatment plants to a moisture content of 1-2% upon contact with a fluidized bed of a mixture of dispersed catalyst particles and an inert material, separating about 60% of the wastewater treatment plants from a vapor-gas mixture, processing this portion of the wastewater treatment plants at a temperature of 700-750°C in a fluidized bed of a mixture of dispersed catalyst particles and an inert material, heat treatment of the remaining portion of the wastewater treatment plants at a temperature of 500-750°C in a fluidized bed of a mixture of dispersed catalyst particles and an inert material organized by fixed packing, cooling and separating solid processing products from flue gases, treating the product with an aqueous solution of an inorganic acid, and using the suspension to purify the original wastewater.

The closest in technical essence and the achieved result is the method of thermal processing of organo-containing raw materials (application RU 2021104148, 02/18/2021), which includes

1. mechanical dehydration of wastewater treatment plants,

2. drying of the IOSV,

3. hermetically sealed supply of dried IOSV into the pyrolysis reactor,

4. pyrolysis of dried iodine wastewater with the formation of solid products and a steam-gas mixture,

5. unloading of the resulting solid products,

6. cleaning the steam-gas mixture from particles of solid product and unreacted iosine-containing wastewater,

7. feeding the steam-gas mixture into the furnace to obtain thermal energy,

8. condensation of the steam-gas mixture to obtain liquid pyrolysis products of the IOSV,

9. feeding non-condensable gaseous pyrolysis products of the IOSV into the furnace to obtain thermal energy,

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SUBSTITUTE SHEET (RULE 26) 10. supply of flue gases from the furnace to provide thermal energy for the pyrolysis and drying processes of the IOSV.

The disadvantages of this method include: significant heat losses with the exhaust drying agent and discharged solid product, heat losses associated with the supply of cold blast air to the combustion chamber, and the lack of exhaust gas purification. Drying costs account for up to 92% of the total energy consumption of the process.

Disclosure of invention

The aim of the proposed invention is to increase the efficiency of processing of waste water by a thermal method while minimizing energy costs and harmful gaseous emissions.

This objective is achieved by recovering thermal energy from spent drying agent streams during the purification of exhaust gases and solid pyrolysis product by heating the air supplied to the combustion chamber. The technical result of the invention is a method for processing ionized wastewater treatment plants (IWTS) while minimizing thermal energy losses with the exhaust streams and reducing the contamination of gas emissions.

The technical result is achieved by extracting thermal energy from the exhaust streams of drying agent and solid product, heating the air supplied to the combustion chamber, and organizing a drying agent purification stage. The extracted thermal energy is used in the processes of drying the raw materials, preheating the raw materials before pyrolysis, and preheating the blast air.

Two-stage drying allows the heat removed from the drying agent before it is released into the atmosphere to be used to dry the IOWS in the second stage. Separating the drying stages based on the raw material moisture content is due to the fact that IOWS with a moisture content of over 40-55% has high adhesive properties and adheres to the surfaces of the apparatus with which it comes into contact, while at lower moisture content, the IOWS reduces its adhesive properties and does not adhere to surfaces. Dividing the drying into two stages allows for different methods of heat supply to the IOWS in its two states, namely, mechanical activation in the first stage and granular bed drying in the second. Separating the drying stages also improves drying efficiency by using different temperature pressures (the difference in temperature between the raw material and the drying agent): a higher pressure in the first stage and a lower pressure in the second.

SUBSTITUTE SHEET (RULE 26) pressure on the second, as well as due to the use of direct contact between the raw material and the drying agent at the first stage of drying.

Almost any type of convective drying can be used at both stages, for example, in a belt dryer. In the first stage of drying, adhesion

5 is eliminated through mechanical activation, which includes, for example, mixing the IOWS and/or breaking up the top layer. Mechanical activation can be accomplished, for example, by using blades that, as they move, break up the surface of the IOWS particles, separating them from the apparatus surface and increasing the surface area for heat and mass transfer.

10 The second stage of drying can be carried out at temperatures no higher than 180-200°C, since above these temperatures the main processes of thermal decomposition begin, which can be seen from the results of thermal analysis of sludge sediments.

After the first drying stage, the spent drying agent (flue gases) is sent to the cleaning stage. This stage includes mechanical removal of solid particles and liquid treatment. Mechanical removal of solid particles allows the particles of the spent drying agent (DWA) emitted with the flue gases to be returned to the flow directed to the second drying stage, thereby reducing the particulate matter content of the flue gases emitted into the atmosphere. This mechanical removal of solid particles can be accomplished, for example, using cyclones.

20 In a particular case, it is possible to dilute the flue gases with air before feeding them to the first stage of drying (not shown in the diagram), while the flue gases do not enter the second stage of drying.

Flue gas treatment with liquids is designed to reduce the pollutant content of flue gases to values that do not exceed maximum permissible concentrations. This allows flue gases to be released into the atmosphere without harming the environment. Flue gas treatment with liquids involves wet scrubbing, absorption, and condensation by cooling the compounds contained in the flue gases. Flue gas treatment with liquids can involve chemical, physical, physicochemical, and biological treatments. Liquid treatment involves cooling the flue gases, transferring heat to the coolant, which then transfers this heat to the second-stage drying agent (air). Liquid treatment can be carried out, for example, in a scrubber. The second-stage drying agent is then used to heat the ionizing water treatment system (IWTS) after the mechanical dehydration stage before being fed to the first stage.

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SUBSTITUTE SHEET (RULE 26) Drying, which allows for more efficient use of the heat from the flue gases and reduces the cost of drying the wastewater. Mechanical dewatering can be carried out, for example, in a filter press.

The pyrolysis process is carried out at temperatures of at least 350°C and no more than 600°C. Conducting the pyrolysis process at temperatures of 350-600°C reduces heat consumption, since, according to thermal analysis data for various types of IOWW, the thermal decomposition process occurs with main peaks at 250-280°C and around 350°C and continues up to 550-600°C. At temperatures below 350°C, the main stage of thermal decomposition does not occur, and heating above 600°C does not lead to a significant increase in the degree of decomposition, but increases the costs of heating and maintaining the temperature.

The pyrolysis process produces solid products and a steam-gas mixture. The solid products are discharged as a marketable product, and the steam-gas mixture is sent for combustion to produce flue gases, the thermal energy of which is used in the process. To improve the thermal efficiency of the process, the solid products are cooled during discharge, transferring their heat to a coolant, which is then used to heat the blast air supplied for combustion of the steam-gas mixture.

The use of heat from the exhaust drying agent (flue gases), condensation of flue gas compounds after the first stage of drying and the resulting solid products to heat the raw materials and blast air reduces the need for heat input to close the heat balance and increases the energy efficiency of the process.

The resulting solid products can subsequently be used as commercial products for various applications. In particular, the solid products can be used to purify various exhaust gases, to treat wastewater, and in the production of building materials.

Brief description of the drawings

The essence of the invention is explained in Fig. 1. Technological scheme for processing wastewater sludge

Implementation of the invention

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SUBSTITUTE SHEET (RULE 26) The initial WSWS (stream 1) is fed to the mechanical dewatering stage. The dewatered WSWS (stream 2) is fed to the first drying stage via a heat exchanger. WSWS dried to a moisture content of 40-55% (stream 3) is fed to the second drying stage. WSWS dried to the final moisture content (stream 4) is fed to pyrolysis, where solid products (stream 6) and a steam-gas mixture (stream 5) are formed. The steam-gas mixture, after being purified from solid products (stream 7), is fed for incineration. The solid products (stream 6) are discharged and cooled, transferring heat to the coolant (stream 8) in the heat exchanger, which then transfers the heat to the air fed for combustion of the steam-gas mixture (stream 9). The heat transfer fluid (stream 8) may be either liquid (e.g., water) or gaseous (e.g., air). The combustion products of the steam-gas mixture (stream 10) are used to provide heat for pyrolysis and then as a drying agent in the 1st drying stage (stream 11). The spent drying agent from the 1st drying stage (stream 12) is sent to the mechanical cleaning stage (solid particles are returned to the dried sludge stream 3) and the liquid processing stage. At the liquid processing stage, it is mixed with the circulating coolant (stream 16), which can be water. The heated heat transfer fluid transfers heat to the air supplied as a drying agent to the 2nd drying stage (stream 17). Make-up of the circulating water is provided by stream 15. Condensed water (stream 18) is discharged to the sewer. The purified drying agent from the first stage of drying (stream 14) is released into the atmosphere. The drying agent from the second stage of drying (stream 19) is then used to heat the WSW after mechanical dewatering and before the first stage of drying (stream 1), combines with the drying agent after heating the WSW (stream 20), and then combines with the drying agent stream from the first stage of drying after mechanical cleaning (stream 13).

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SUBSTITUTE SHEET (RULE 26)

Claims

CLAUSES OF THE INVENTION 1. A method for thermal processing of wastewater sludge, including mechanical dewatering of wastewater sludge, drying, sealed feeding of dried wastewater sludge into a pyrolysis reactor, pyrolysis with formation of solid products and a steam-gas mixture, unloading of the resulting solid products, purification of the steam-gas mixture from particles of the solid product and unreacted wastewater sludge, feeding of the steam-gas mixture into a furnace to obtain thermal energy, feeding of flue gases from the furnace to provide thermal energy for the pyrolysis and drying processes, characterized in that the drying of the wastewater sludge is carried out in two stages, wherein in the first stage drying is carried out to a moisture content of 40-55% with direct contact of the wastewater sludge and flue gases from the furnace in a convective mode with simultaneous mechanical activation of the wastewater sludge, and the second stage is carried out to a moisture content of 10-12% at a lower temperature head, wherein the flue gases after the first drying stage are fed to the stage of cleaning and extraction of heat supplied to heat the drying agent of the second drying stage, wherein the stage of cleaning the steam-gas mixture includes mechanical cleaning from solid particles and liquid treatment of the flue gases, and the heat of the drying agent after the second drying stage is used to heat the sludge sediments of wastewater after the stage of mechanical dehydration before feeding to the first drying stage, the pyrolysis process is carried out at temperatures of not less than 350°C and not more than 600°C, the resulting solid products after unloading are cooled with the transfer of heat to the coolant used to heat the blast air supplied for burning the steam-gas mixture, and the combustion products of the steam-gas mixture are used to provide heat for pyrolysis and as a drying agent in the 1st drying stage. 2. The method according to paragraph 1, characterized in that air is used as the drying agent of the second stage. 3. The method according to paragraph 1, characterized in that the solid products are used for cleaning exhaust gases and cleaning wastewater. 4. The method according to paragraph 1, characterized in that the liquid treatment of flue gases includes means of chemical, physical, physicochemical, and biological action to reduce the content of pollutants to values not exceeding maximum permissible concentrations. 7 SUBSTITUTE SHEET (RULE 26) 5. The method according to paragraph 4, characterized in that the liquid treatment of flue gases includes wet cleaning, absorption and condensation as a result of cooling the compounds contained in the flue gases. 8 SUBSTITUTE SHEET (RULE 26)