FR2810312A1 - System recovering heat and generating electricity from sludge recirculates charge with steel balls in screw conveyors equipped for drying, oxidation and heat exchange - Google Patents

Abstract

The system is charged with sludge and steel alloy balls (6), sized for recirculation under 200-400 deg C. A thermal fluid loop is pumped in under 150-300 deg C. This transfers and controls the thermal flux required by associated systems. Hot dried sludge is subjected to oxidation in air preheated to 180-300 deg C. Heat evolved from oxidation is recycled to heat the recirculating charge and thermal fluid. The organic material is at 250 deg C-300 deg C before oxidation. Air is injected into the system at 2-6 bar. The charge, comprising the steel balls and sludge, is transferred by screw conveyor. Heat exchange takes place into the shaft and flights of the screw. At the same time, friction is exerted on the charge by the rotating screw spirals. Heat is recovered by conduction and convection from the jacket (26) surrounding the oxidation reactor (20), into cold, pressurized air (25). This preheats the air, before it is injected into the reactor. A co-generation system receives heat from the heat exchanger in the thermal fluid circuit, and from a circulating charge cooler. The system produces steam under pressure, exploited in rotary machinery generating electricity. Further preferred details of the plant and its variants are provided. An Independent claim is included for the corresponding plant.

Description

The present invention relates to a thermal recovery process of sludge from wastewater treatment and device for its implementation.

It is well known that in recent years the sludge from wastewater treatment is the subject of renewed studies and research to define their elimination at the lowest cost while incorporating the precautionary principles relating to health hazards. and inherent toxicities.

The easy outlet of the landfill will be closed in 2002, at least for the countries of the European community.

 The evacuation of sludge towards the agricultural spreading is subject to polemics, principal arguments opposable to this practice being the unknown of the threshold of phytotoxic and ecotoxic saturation as well as the survival time in place pathogenic organisms thus interfering in the food chatne .

The transfer of these sludges to the current means of incineration of household waste requires prior conditioning generating costs passed on to the price of water distributed, an important economic factor being the transport of said sludge to the means of disposal.

Among the rational solutions, on-site combustion has certain advantages provided that an efficient process can replace incinerators as currently designed for solid waste.

The situation is even more problematic for small wastewater treatment plants whose sludge flow rate does not allow for the amortization and operation of individual incineration systems.

Some achievements specific to the on-site treatment of these sludges are experimentation, it is necessary to mention in particular a fluidized vertical sand bed process resulting from the technology of the recycling of foundry sands, several processes of oxidation in aqueous phase by injection of oxygen under pressure, and a multitude of methods and systems covering static or rotary dryers, with or without energy recovery.

All processes using a prior drying of said sludge are confronted with a sticking phenomenon that occurs when the dryness rate reaches about 50%, this bonding causes clogging which lead to shutdowns with breakage of equipment. This phenomenon of bonding is linked to the presence of phosphates whose origin is in detergents and feces, or even the fertilizers recovered in wastewater during thunderstorms.

The sludge also contains calcium oxides, iron oxides, aluminum oxides, silicon oxides, whose combined properties with each other and phosphate generate a creamy and sticky paste. At the end of drying, this paste hardens and remains attached in successive layers on the heating surfaces, hence the operating difficulties.

It is also practiced the dewatering of sludge by liming, but this method has the disadvantage of increasing the masses and volumes to be evacuated, which does not solve the problem.

To obtain optimum sludge combustion, the drying phase is essential, so it is important to improve it by using a system that thwarts the phenomenon of sticking.

To achieve the drying of sludge it is necessary to have enough heat to exceed threshold of latent energy vaporization of water constituting the largest part said sludge, so it is also important to exploit the combustible properties of materials organic contained in said sludge.

Another disadvantage of the present sludge combustion systems is the formation of a residual slag resulting from the melting of the mineral substances.

The method and the device according to the present invention are designed to avoid the above difficulties and to provide a new tool for the needs of elimination and thermal recovery of sludge from wastewater treatment while co-generating a self-consumable or exportable energy.

The method and the device according to the present invention are also transferable to the elimination and energetic recovery of any liquid or pasty effluent containing organic materials whose oxidation generates a controllable exothermic reaction. According to the present invention, the process is characterized by a combination of systems which is obtained a total drying of the sludge allowing its mass reduction in volume by oxidation of the organic part whose exothermic reaction provides the heat of dissociation necessary to drying, the combination comprising a circulating charging system associated with an Archimedean screw transport system associated with a heat transfer fluid system and an exothermic oxidative reaction system integrated in said Archimedes screw transport system, as well as a system of energy cogeneration, which hereinafter detailed a) a circulating load system consisting of spherical-shaped alloy steel bodies of dimensions compatible with the transport system, load being maintained at a variable temperature between 200 C and 400 C circulating endless loop.

b) a system of circulating heat transfer fluid configured in endless loop maintained at a variable temperature between 150 C and 300 C, which system is transmitted and regulated the heat flow necessary for the associated systems, c) an oxidizing and exothermic reaction system by combining the oxygen contained in a flow of hot air with the hot organic material of the dry sludge, said sludge being disseminated in the circulating charge, the heat resulting from the exothermic reaction being used to heat the circulating charge and the coolant and the air intended for oxidizing reaction, the temperature of the hot air being between 180 and 300 ° C., the temperature of the organic material being between 250 and 300 ° C. before the oxidation reaction, air being previously injected into the system in compressed form at a pressure of between 2 and 6 bar, d) a transport system called screw of Archimedes, the said transport system is also a heat exchange system by forced circulation of a heat transfer fluid inside the axial body and turns of said screw, the transport of the circulating charge being caused by the rotation of the screw, the sludge charge mixture being obtained by friction and rotation of the metal bodies between and friction on the turns of the screw. e) a heat recovery system by conduction and convection in a double jacket around the reactors of the exothermic oxidative reaction system, said double jacket receiving cold air under pressure to heat this before injection into the reactor; energy cogeneration by heat exchanger on the circuit of the coolant and the cooler of the circulating charge said system generating pressurized steam used in a rotating machine to produce electrical energy.

According to a variant of the process, it will be possible to perform the drying and volume and mass reduction of any aqueous liquid effluent charged with mineral matter by addition of an oxidizable organic material, preferably this additive material will be sawdust or coal of wood, crumb from the tire industry, or any other miscible organic matter in acquiescent medium.

The method according to the invention will be better understood with reference to Figure 1 by which is schematized a device by combination of known means for its implementation.

It is represented in 1 the dryer comprising one or more Archimedean screws 2 mounted in a trough 3 with a steam dome 4. The interior of screw 2 is traversed by a coolant 5, the circulating charge of metal bodies 6 is introduced into the dryer by a metering extractor 7, the sludge is introduced into the dryer by a pump 8, the water vapor resulting from drying is sucked by an extractor 9 and directed to a condenser-washer 10, the load circulating and the dried sludge are directed to the reactor 12 by means of an extractor 11 equipped with anti-return.

Advantageously for a good performance of the process, the filling factor of the dryer will be between 50 and 75%, the gross sludge volume ratio on the circulating charge at the inlet of the dryer will be 1 on 1. Advantageously for a good performance of the process, the raw sludge will be filtered beforehand or centrifuged until a solids content of between 35% is obtained.

Advantageously, for a good performance of the process, the metal bodies of the circulating charge 6 will be refractory steel balls with a diameter of between 8 and 12 mm.

It is represented at 12 the primary reactor comprising an Archimedean screw 13 mounted in a sealed tubular enclosure 14, the interior of the screw 13 is traversed by the coolant 5, the circulating charge of metal bodies 6 and the dried sludge from the dryer 1 are introduced into said reactor by means of the extractor 11 which feeds a metering chamber 15, which chamber supplied with compressed hot air 16 at a temperature of about 200 C, which hot obtained by heating of a cold compressed air 17 by means of the jacket 18, the adjustment of the flow of hot air being carried out to obtain the oxidation of about 20% of the organic matter of the sludge, which oxidation generates sufficient exothermic reaction to raise the temperature of the mixture circulating charge and dry sludge up to about 300 C, which temperature of 300 C allows the partial cracking of organic matter with formation of gas com busses, the circulating charge and the solid particles from the cracking of the sludge and the resulting primary gas stream are directed to the secondary reactor 20 by means of an extractor 19 equipped with anti-return. Advantageously for a good performance of the process, the filling coefficient of the reactor 12 will be between 85 and 95% calculated on the actual internal volume of said reactor.

Advantageously for a good performance of the process, the compressed cold air 17 will be injected at a pressure of between 2 and 6 bar.

It is represented in 20 the secondary reactor comprising an Archimedean screw 21 mounted in a sealed tubular enclosure 22, the interior of the screw 21 is traversed by the coolant 5, the circulating charge and the solid particles and the gas stream from the reactor 12 are introduced into the reactor 20 by means of the extractor 19 which feeds a metering chamber 23, which chamber is supplied with compressed hot air 24 at a temperature of about 300 ° C., which hot air is obtained by heating a cold compressed air by means of the double jacket <B> 26, </ B> the regulation of the flow of hot air being carried out to obtain the stoichiometric oxidation of the solid organic particles and the combustible gases the reaction resultant exothermic being controlled and regulated not to exceed a temperature of 450 C measured mid length said reactor, which exothermic reaction brings the heat sufficient for heating metal bodies of the circulating charge and heat transfer fluid to obtain the drying of the incoming sludge by evaporation of its free and bound in the dryer 1.

Advantageously, for a good performance of the process, the reactor filling coefficient will be between 85 and 95% calculated on the actual internal volume called reactor.

Advantageously for a good performance of the process, the compressed cold air 25 will be injected at a pressure of between 2 and 5 bar.

At the end of the stoichiometric oxidation phase, the circulating charge of metal bodies, the residual solid particles as well as the gaseous compounds resulting therefrom are separated by means of a separator 27, the gaseous flow charged with solid particles is directed towards a bag filter 28, the metal bodies the circulating charge are directed to a hollow screw heat exchanger 29 to give part of their heat to the cogeneration system, the temperature of the metal bodies at the outlet of the exchanger 29 being between 200 and 250 C, said metal bodies being taken up by an elevator 30 and directed to the dryer 1 for a new cycle.

After filtration at 28, the gases are sent to the condenser-scrubber 10 and the residual solid particles are recovered at 31 for recovery to the building materials industry.

The cogeneration is obtained by producing steam under pressure by means of the heat exchange circuits 32 and 33, a complementary heat recovery can be performed on the drying line 34 and the hot gas dewatering 35. The adaptation to the needs of small wastewater treatment plants being carried out by construction of all or part of the whole system of the process on a mobile platform and towable road trailer type. The initiation of the process is obtained by injection into the chamber 15 of a powdered solid fuel preheated to a temperature of 300 ° C., advantageously for the safety of the handling, this fuel will be charcoal, the oxidation being cold-started. under reduced pressure of 100 mbar with gradual rise in regime until thermal equilibrium different systems, the introduction of sludge in 8 intervening only after this balance.

According to a variant of the device, the hollow Archimedes screw is replaced by a so-called ribbon-less screw, in which case the heat transfer fluid will be heated by means of a primary double jacket mounted around the reactor and the compressed air heated by a double secondary envelope mounted around the double primary envelope.

According to another variant of the device, the compressed air is replaced by gaseous oxygen.

According to another variant of the device, the metal bodies constituting the circulating charge may be replaced by inorganic bodies of the ceramic type or other refractory oxides.

The device assembly is dimensioned, constructed, mechanized, insulated, instrumented automated and secured by ordinary industrial means and / or specialized fields of mechanical engineering and thermal engineering associated with chemical engineering, all within reach of the man of the 'art.

Claims (1)

Claims characterized by a combination of systems by which a total drying of the sludge is obtained allowing its reduction both in mass and in volume by oxidation of the organic part whose exothermic reaction furnishes the heat. dissociation apparatus required for drying, the combination comprising a circulating charge system associated with an Archimedean screw transport system associated with a heat transfer fluid system and an exothermic oxidative reaction system integrated in said screw conveyor system. Archimedes, as well as an energy cogeneration system, as hereinafter detailed a) a circulating charge system consisting of spherical-shaped alloy steel bodies of dimensions compatible with the transport system, the load being maintained at a variable temperature between 200 C and 400 C circulating in endless loop. b) a forced circulation heat transfer fluid system configured in an endless loop and maintained at a variable temperature between 150 ° C. and 300 ° C., a system by which the thermal flux required for the associated systems is transmitted and regulated, and (c) an oxidative reaction system. and exothermic by combining the oxygen contained in a hot air flow with the hot organic material of the dry sludge, said sludge being disseminated in the circulating charge, the heat resulting from the exothermic reaction being used to heat the charge circulating and the coolant and the air for the oxidizing reaction, the temperature of the hot air being between 180 300 C, the temperature of the organic material being between 250 300 C before the oxidation reaction, l air being previously injected into the system in compressed form at a pressure of between 2 and 6 bar, d) a transport system said to Archimedes e, the said transport system being also a heat exchange system by forced circulation of a heat transfer fluid inside the axial body and turns of said screw, the transport the circulating load being caused by the rotation of the screw the mixture charge and mud being obtained by friction and rotation of the metal bodies between them and friction on the turns of the screw. e) a heat recovery system by conduction and convection in a double jacket around the reactors of the exothermic oxidative reaction system, said double jacket receiving the cold air under pressure to heat this air before injection into the reactor, f) a system of cogeneration of energy by heat exchanger on the coolant circuit and on the cooler of the circulating charge, said system generating pressurized steam that can be used in a rotating machine to produce electrical energy. Claim <U> n 2 </ U> Device for the application of the method according to claim 1 characterized in that it consists of a dryer 1 comprising one or more Archimedean screws 2 mounted in a trough 3 with a steam dome 4. The interior of the screw 2 is traversed by a heat transfer fluid 5, the circulating charge of metal bodies 6 introduced into the dryer by a metering extractor 7, the sludge is introduced into the dryer by a pump 8, the water vapor resulting from drying is sucked by an extractor 9 and directed to a condenser-washer 10, the circulating charge and the dried sludge are directed to the reactor 12 by means of an extractor 11 equipped with -return. Advantageously for a good performance of the process, the filling factor of the dryer will be between 50 and 75%, the gross sludge volume ratio on the circulating charge at the inlet of the dryer will be 1 on 1. Advantageously for a good performance of the process, the raw sludge will be previously filtered or centrifuged until a solids content of between 25 and 35% is obtained. Advantageously, for a good performance of the process, the metatotic bodies of the circulating charge 6 will be refractory steel balls with a diameter of between 8 mm and 12 mm. Of a primary reactor 12 comprising an Archimedean screw 13 mounted in a sealed tubular enclosure 14, the interior of the screw 13 is traversed by the coolant 5, the circulating charge of the metal bodies 6 and the dried sludge from the dryer 1 are introduced into said reactor by means of the extractor 11 which feeds a metering chamber 15, which chamber is supplied with compressed hot air 16 at a temperature of approximately C, which hot air is obtained by heating an air cold compact 17 by means of the jacket 18, the adjustment of the flow of hot air being carried out to obtain the oxidation of about 20% of the organic matter of the sludge, which oxidation generates sufficient exothermic reaction to raise the temperature of the mixing circulating charge and dry sludge up to about 300 which temperature of 300 C allows partial cracking of the organic material with formation of combustible gases, the charge circulates Ulante and the solid particles resulting from the cracking of the sludge and the resulting primary gas stream are directed to the secondary reactor 20 by means of an extractor 19 equipped with anti-return. Advantageously, for a good performance of the process, the filling coefficient of the reactor 12 will be between 85 and 95% calculated on the actual internal volume of said reactor. Advantageously for a good performance of the process, the compressed cold air 17 will be injected at a pressure of between 2 and 6 bar. Of a secondary reactor 20 comprising an Archimedean screw 21 mounted in a sealed tubular enclosure 22, the interior of the screw 21 is traversed by the coolant 5, the circulating charge and the solid particles as well as the primary gas stream. from the reactor 12 are introduced into the reactor 20 by means of the extractor 19 which feeds a metering chamber 23, which chamber supplied with compressed hot air 24 at a temperature of about 300 ° C., which hot air is obtained by heating of a cold compressed air 25 by means of the jacket 26, the adjustment of the hot air flow rate being carried out in order to obtain the stoichiometric oxidation of the solid organic particles and the combustible gases, the resulting exothermic reaction being controlled and regulated so as not to exceed a temperature of 450 C measured at the length of said reactor, which exothermic reaction provides sufficient heat for heating the c metallic orps of the circulating charge and of the coolant for the purpose of obtaining the drying of the incoming sludge by evaporation of its free water and its bound water in the dryer 1. Advantageously for a good performance of the process, the reactor filling factor 20 will be between 85 and 95% calculated on the actual internal volume called reactor. Advantageously for a good performance of the process, the compressed cold air 25 will be injected at a pressure of between 2 and 6 bar. Of a separator 27 by which, at the end of the stoichiometric oxidation phase, the circulating charge of metal bodies and the residual solid particles as well as the gaseous compounds resulting therefrom are separated, the gaseous flow charged with solid particles is directed towards a bag filter 28, the metal bodies the circulating charge are directed to a hollow screw heat exchanger 29 to give part of their heat to the cogeneration system, the temperature of the metal bodies at the outlet of the exchanger 29 being between 200 and 250 C, said metal bodies being taken up by an elevator 30 and directed to the dryer 1 for a new cycle. Claim <U> n 3 </ U> Alternative process according to claim 1 characterized in that it is carried out the drying and the volume and mass reduction of any liquid aqueous effluent charged with mineral matter by addition of an oxidizable organic material advantageously, this additive material will be sawdust or charcoal, or powder from the tire industry, or any other miscible organic material in aqueous medium. <U> Claim 4: </ U> Device according to claims 1 to 3 by which the hollow Archimedean screw is replaced by a so-called ribbonless web screw, in which case the heat transfer fluid will be heated by means of a double jacket primary reactor mounted around the reactor and the compressed air will be heated by a secondary double envelope mounted around the primary double envelope. the compressed air is replaced by gaseous oxygen. the metal bodies constituting the circulating charge may be replaced by inorganic bodies of the ceramic type or other refractory oxides.