DESC:Field of the Invention
The present invention relates to sludge treatment and more particularly to a system and method for sludge-volume reduction and environment friendly disposal thereof.
Background of the invention
Chemical and Bio-sludge generated from Effluent Treatment Plants, Sewage Treatment Plants, Desalination plants, and manufacturing industries is generally dewatered up to 50%-80% moisture content and disposed-off to the respective type of landfills. The moisture present in the sludge contributes to the landfill leachate that, if not collected and treated, causes land,surface water, and ground water contamination. The problem is compounded by ever increasing need for the landfilling space across the world. Prior arts have figured different ways of reducing the volume of this sludge through high efficiency dewatering equipment, green house drying, sludge dryers of various types, dry sludge incinerators, and so on.
However, these methods either aren’t able to sizably reduce the water content or have a huge footprint or are ineffective during monsoon or involve high operating costs making their application commercially unviable in the areas where landfilling charges are low and where landfilling site is located close to the sludge generating plant. As a result, as a widely prevalent practice across the globe, the sludge containing 50% - 80% moisture is still disposed-off by landfilling. This landfilled (or as-is dumped) sludge upon digestion over time releases GHGs (Greenhouse Gases) i.e. primarily methane, with 25 times CO2 (eqv) global warming potential, exacerbating the Global Warming Challenge we’re struggling to tackle
In light of the above, there exists a need to provide a low operating cost, low foot-print, and non-seasonal sludge volume reduction solution to minimize sludge landfilling space requirement and leachate generation as well as GHG generation post landfilling to safeguard our environment.
Summary of the Invention-
The present invention provides a system for sludge volume reduction and environment friendly disposal thereof. The system comprises of a compressor, a condenser, an expansion valve, an evaporator, a low grade heat extraction heat exchanger, a high grade heat extraction heat exchanger and an air to air heat exchanger that forms a closed loop heat recovery and waste heat recovery system adapted to assist sludge volume reduction in efficient and effective manner. The compressor compresses a depressurized refrigerant having temperature above 100C to provide the compressed refrigerant having temperature in a range of 40 0C to 90 0C. The condenser facilitates indirect heat transfer between the compressed refrigerant at a temperature of 40 0C to 90 0C and an unsaturated air at 25 0C to 50 0C thereby raising the temperature of the unsaturated air in the range of 35 0C to 75 0C and lowering the temperature of the refrigerant in the range of 20 0C to 70 0C. The expansion valve receives the refrigerant at a temperature of 20 0C to 70 0C for reducing temperature of said refrigerant in the range of -35 0C to 0 0C. The evaporator facilitates heat transfer between the warm humid saturated air at a temperature above 30 0C and the depressurized refrigerant entering at a temperature of -350C to 0 0C thereby raising the refrigerant temperature to greater than 10 0C and lowering the saturated air temperature below 25 0C (or local dew point) for condensing out the moisture. The low grade heat extraction heat exchanger facilitates heat transfer between low temperature unsaturated air at a temperature of 200C to 40 0C and a waste heat source at a temperature greater than 40 0C thereby increasing a temperature of the unsaturated air in the range of 25 0C to 50 0C and reducing temperature of the waste heat source to a temperature closer to but above ambient temperature. The high grade heat extraction heat exchanger facilitates heat transfer between a thermic fluid or steam at a temperature above 100 0C and unsaturated warm air at a temperature of about 35 0C to 75 0C thereby raising the temperature of the unsaturated air in the range of 40 0C to 90 0C and lowering the temperature of the thermic fluid (or steam condensate) below 90 0C. The air-to-air heat exchanger facilitates heat transfer between warm humid saturated air at a temperature above 30 0C and a mixture of ambient air at a temperature greater than 15 0C and saturated air cooled down below 25 0C or local dew point for heating the aforementioned mixture to a temperature between 200C -40 0C.
The system includes a sludge dryer body that connects to an extruder at a top end thereof. The extruder facilitates entry of wet extruded sludge in the sludge dryer body. The sludge dryer body houses a variable speed controlled multi-level mesh belt assembly that provides residence time to extrude wet sludge which is spread thereon by a sludge spreader having a zigzag profile. The sludge dryer body has a hopper shaped exit a bottom end and a bottom distributor at bottom of the sludge dryer body. The bottom distributor facilitates supply of hot unsaturated air over the multi-level mesh belt assembly for obtaining the extruded dry sludge.
The system includes a pulveriser that crushes the extruded dry sludge to form fluidizable particles having particle size less than 1 mm size. The system includes a solid fuel boiler that receives the fluidized particles for combustion at a temperature above 600 0C in presence of stoichiometrically excess oxygen supply controlled by a blower to liberate the calorific value in dried sludge for heating thermic fluid or generating steam and obtaining flue gases and ash. The system includes a wet scrubber that receives the flue gases through an induced draft fan to facilitate scrubbing thereof. The dry ash discarded from the solid fuel boiler is disposed for landfilling.
Brief Description of Drawings
FIG. 1 is a schematic flow diagram of a system adapted for sludge treatment in accordance with the present invention; and
FIG, 2 is a schematic flow diagram of the system of FIG.1 showing various processing parameters thereof.
Detailed Description of the invention
The foregoing objectives of the present invention are accomplished, and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
Although specific terms are used in the following description for sake of clarity, these terms are intended to refer only to particular structure of the invention selected for illustration in the drawings and are not intended to define or limit the scope of the invention. References in the specification to “preferred embodiment” means that a particular feature, structure, characteristic or function described in detail thereby omitting known constructions and functions for clear description of the present invention.
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention.
The present invention provides a system and method for sludge volume reduction and environment friendly disposal thereof which employs pumping of heat to the ambient air and back from it by heat exchange with a suitable refrigerant which in turn is made to gain and lose temperature using electricity powered compressor and using one or more expansion valves respectively.
In particular, the present invention provides a power operated low operating cost sludge dryer system that works on heat pump principle and equipped with closed loop heat recovery and waste heat recovery system to further dehumidify the dewatered biological and chemical STP (Sewage Treatment Plant) / ETP (Effluent Treatment Plant) sludge to minimize sludge volume and hence landfilling space requirement as well as minimizing leachate generation and GHG generation resulting from sludge landfilling
Referring to FIG. 1, a schematic diagram of a system 100 adapted for sludge treatment according to an embodiment of the present invention is shown. The system 100 includes a refrigerant line (107), a compressor (101) , an expansion valve (102), a condenser (103), an evaporator (104), a low grade heat extraction heat exchanger (105), a high grade heat extraction heat exchanger (106), an air-to-air heat exchanger (108), a pulveriser (301), a solid fuel boiler (302), a steam or thermic fluid flow control valve, a transmitter (303), an induced draft Fan (304), an oxygen auto-control system (305), a wet scrubber (306), a sludge dryer body (201), a slitter/extruder (202), a multi-level mesh belt assembly (203) with speed control, an extruded sludge spreader (204), a bottom distributor (205) with distributor fans, an induced draft fan (206), a top exit of warm saturated air, filter(s) (207), a sludge feed conveyor (208), a sludge outlet conveyor (209), and a sticky sludge scrapper (210) .
Referring to FIG. 2, a schematic diagram of a system (100) adapted for sludge treatment showing various flow lines, temperature parameters.
Referring now to FIGS. 1 and 2, the refrigerant line (107) circulates refrigerant into the system (100). The refrigerant used in the system has a super critical temperature above 800C and a boiling temperature less than 00c. The refrigerants utilized in accordance with the present invention include but not limited to R134a, R123, R600, R601, R601a and the like.
The compressor (101) is a combination of one or more electrical compressor(s) (101) that operates with an evaporating temperature above 10 0C and a condensing temperature above 40 0C. The compressor (101) receives depressurized refrigerant at a temperature greater than 10 0C along the line (502) and compresses the same to provide compressed refrigerant having temperature of about 40 0C to 90 0C along the line (501).
The condenser (103) is configured to transfer the heat energy from compressed refrigerant at the temperature of 40 0C- 90 0C along the inlet line (501) to low temperature unsaturated air entering the condenser heat exchanger at 25 0C to 50 0C along the inlet line (505) in order to raise the unsaturated air temperature in the range of 35 0C to 75 0C along the exit line (506) to enhance the moisture bearing capacity thereof. The refrigerant leaves the condenser (103) along the exit line (503) at a temperature of 20 0C to 70 0C.
The expansion valve (102) has an operating pressure drop in the range of 8 bar to 40 bar. The expansion valve (102) receives the refrigerant at a temperature of 20 0C to 70 0C along the line (503) and reduces its temperature at about -35 0C to 0 0C along the line (504) depending upon the type of refrigerant utilized in the system (100).
The evaporator (104) receives warm saturated air, typically less than 60% (v/v), along the line (507) at a temperature above 30 0C that transfers heat to a depressurized refrigerant entering along the line (504) at a temperature of -350C to 0 0C in order to raise the refrigerant temperature to greater than 10 0C along the line (502). It is understood here that, the saturated air temperature is brought below its dew point, typically at a temperature below 25 0C, which leaves the evaporator (104) along the line (508) thereby condensing out the moisture absorbed from the wet sludge fed to the dryer.
In an embodiment, the low grade heat extraction heat exchanger (105) is an air-to-liquid type heat exchanger (105) adapted for utilizing low grade heat from the waste heat sources such as process condensate or flash steam that enters the heat exchanger (105) via entry point (907) along the line (602) at a temperature greater than 40 0C and leaves the heat exchanger (105) via exit point (908) along the line (603) at a temperature closer to but above ambient temperature. On the other side, the low grade heat extraction heat exchanger (105) receives low temperature unsaturated air along the line (601) at a temperature of 20 to 40 0C which is increased to a temperature of 25-50 0C while leaving the low grade heat extraction heat exchanger (105) along the line (505) in order to reduce the heat requirements from the compressor (101). In an optional embodiment, the low grade heat extraction heat exchanger (105) can be an water-to-air type or air-to-air type (105) for utilizing low grade heat from the waste heat sources in appropriate phases.
In an embodiment, the high grade heat extraction heat exchanger (106) is an air-to-liquid or air-to-steam type heat exchanger (106) adapted for utilizing high grade sensible or latent heat of the thermic fluid or of steam respectively that enters the heat exchanger (106) along the line (702) at a temperature greater than 100 0C and exits from the heat exchanger (106) along the line (703) at a temperature below 90 0C. On the other side, the warm unsaturated air enters the heat exchanger (106) along the line (506) at a temperature of about 35 0C to 75 0C such that said temperature is raised to 40 0C to 90 0C while leaving the heat exchanger (106) along the line (701) in order to further increase moisture bearing capacity of the unsaturated air.
The air-to-air heat exchanger (108) receives the warm saturated air, typically greater than 40% (v/v), along the line (802) at a temperature above 30 0C and transfers heat to a mixture of unsaturated ambient air, which is typically above 40% (v/v), entering the heat exchanger (108) via entry point (903) and along the line (801) at a temperature greater than 15 0C and , saturated air, typically less than 60% (v/v), already cooled down below its dew point in the evaporator (104), along the line (508) at a temperature less than 25 0C. The process in turn would heat the aforementioned mixture to a temperature of 200C to 400C before leaving from the heat exchanger (108) along the line (601).
The sludge dryer body (201) has a nearly leak proof cuboidal chamber configuration adapted for circulation of unsaturated air and dehumidification of sludge. The sludge dryer body (201) has an opening on the top end that connects to the extruder (202) for the entry of wet extruded sludge. The sludge dryer body (201) has a hopper shaped exit at the bottom end for dry sludge collection and transfer. The sludge dryer body (201) includes a bottom distributor (205) which is a hot unsaturated air distribution assembly located at the bottom or at the sludge exiting side of the sludge dryer body (201). The sludge dryer body (201) has a warm saturated air exit at the top or at sludge entering end thereof. The sludge dryer body (201) houses a variable speed controlled multi-level mesh belt assembly (203) that carries extruded wet sludge, uniformly spread over belt width with a extruded sludge spreader (204), from top to bottom of the chamber through the neighboring rotors fixed in zigzag positions and rotating in opposite directions to give the wet sludge a long residence time for releasing moisture. In an embodiment, the system (100) may have a provision of an array of circulation fans on side walls of the sludge dryer body (201) to prevent formation of air dead zones therein. The bottom distributor (205) is connected to an induced draft fan (206) for generating adequate flow rate of the air through the sludge dryer body (201) ensuring sludge drying using low temperature air at 400C to 900C in combination with condenser (103), evaporator (104), low grade heat extraction heat exchanger (105), high grade heat extraction heat exchanger (106) and an air-to-air heat exchanger (108). In an embodiment, the sludge dryer body (201) is made of material selected from Aluminium, Epoxy Coated/ rubber lined MS, or SS. The sludge dryer body (201) has quickly openable air tight windows to comply with maintenance requirements.
The extruder (202) is configured to extrude the wet sludge lumps having dry solids greater than 15% (w/w) into any convex or concave polygon cross-section, including but not limited to a circle, with the circumscribing circle of diameter between 1 mm to 12 mm. The extruder (202) slits/extrudes the wet sludge into pieces of fixed extrusion length between 2 mm to 25 mm to increase the surface to mass ratio of the wet sludge. The sludge feed conveyor (208) is a screw or belt type sludge conveyer (208) that feeds the sludge lumps to the hopper on top of extruder (202) at an entry point (901) from an upstream sludge dewatering equipment such as volute, belt, or filter press and the like.
The multi-level mesh belt assembly (203) consists of a mesh belt made of contact layer only or two layers (contact and support) with the contact layer mesh opening of 0.5- 8 mm and the support layer mesh opening of 5-25 mm. In one of the embodiments, the contact layer of mesh belt is made up of high tensile strength, high abrasion resistance, corrosion resistant, and high temperature tolerant materials such as glass-fibre reinforced polymers including but not limited to polyamide, Polyethylene, Polypropylene; the supporting layer mesh is made of high durability materials including but not limited to SS. The multi-level mesh belt assembly (203) consists of 1 to 10 levels of mesh belt, wherein each level comprises of the mesh belt stretched over a bed of rotors having two at extreme ends and intermittently placed supporting rotors in between. One of the two end rotors of each of the consecutive belt layers is coupled with a VFD controlled motor through a gear box or a belt coupler such that the top part of consecutive belt layers move in opposite directions at same speed to prevent sludge accumulation. The consecutive mesh belt layers are offset such that the sludge from the end of the upper layer falls on the layer just beneath preceding layer. The sticky sludge scrapper (210) involves a belt scrapper mechanism grazing the bottom layer of each belt level such that the wet sludge sticking to the belt surface after passing over one of the end rotors is scrapped off the belt surface and lands on the level underneath the preceding layer. The extruded sludge spreader (204) has a zigzag profile and uniform clearance, between one to one and a half times the extrusion cross-section circumscribing circle diameter, between the spreader (204) and the top layer of the top-most belt level, to space out the overlaying extruded segments and spread them over the width of the belt so as to maximize the contact area available on the extruded sludge. In this embodiment, the filters (207) are quickly replaceable filter(s) (207) which are positioned in the path of exiting warm saturated air to trap any large particles that might have gotten carried away from the sludge along with its moisture. In one of the embodiments, the filter is made up of polymer / metal / alloy material and has an opening size in a range of 5 µm to 100 µm. However, it is understood that the material of construction and opening size of the filter may vary in other alternative embodiments of the system (100).
The pulveriser (301) is designed to receive the extruded dry sludge (705) through a belt or screw type conveyer (209). The dry sludge typically has contents of dry solids above 80% w/w. The pulveriser (301) crushes the dry extruded and cut sludge into easily fluidizable particles having particle size less than 1 mm size for downstream colorific value extraction in the solid fuel boiler (302). The solid fuel boiler (302) combusts the pulverized dry sludge at a temperature above 600 0C in presence of stoichiometrically excess oxygen supply controlled by the blower (305) in order to liberate the calorific value, typically between 1000 – 4500 kCal/kg dry sludge, contained in the dried sludge (705). It is understood here that the complete combustion results into flue gases containing CO2, H2O along with unwanted contaminants that are transported to the wet scrubber (306) for scrubbing through an induced draft fan (304). It is further understood here that ash (902) left after combustion of sludge (705) in the solid fuel boiler (302) is disposed for landfilling. The liberated calorific value due to combustion of the sludge (705) in turn heats the incoming thermic fluid or steam condensate along the line (703) at a temperature less than 90 0C and increases its temperature above 100 0C along the line (702) or generating low pressure steam with temperature above 100 0C before being fed to the heat exchanger (106). The flow and the temperature of thermic fluid / steam are controlled with the help of the flow control valve and transmitter (303).
The wet scrubbing system (306) is provided with packing media that provides contact sites for mixing of water, condensed out from a part of saturated air along line (507) in the evaporator (104). The wet scrubbing system (306) is optionally dosed with dilute NaOH solution or other basic solution via a dosing point (905) that reacts with the contaminants and forms stable salts. The wet scrubbing system (306) is fed with the mixture of the flue gas coming from the induced draft fan (304) and excess saturated air coming via line (803). The stable salts formed in the wet scrubbing system (306) do not contribute to GHGs, absorbs the particulate matter, and is transferred to an ETP equalization tank for standard treatment along the line (906). The wet scrubbing system (306) includes a stack and chimney (904) that safely releases decontaminated and neutralized mixture of flue gas and saturated air to the environment.
The operational description of the system (100) is described hereinafter. In operation, the wet sludge enters the system (100) via the sludge feed conveyor (208) which is subsequently extruded by the extruder (202) and wet extruded sludge is fed to the sludge dryer body (201). The sludge spreader (204) and the variable speed controlled multi-level mesh belt assembly (203) contained within the sludge dryer body (201) carry extruded wet sludge, uniformly spread the same over belt to give the wet sludge a long residence time for releasing moisture contained therein. The sludge scrapper (210) assists in removal or scrapping of the wet sludge sticking to the belt surface of the multi-level mesh belt assembly (203). The bottom distributor (205) connected to sludge dryer body (201) is fed with the high temperature unsaturated air having high moisture bearing capacity along the line (701) via induced draft fan (206). The bottom distributor (205) evenly distributes said hot unsaturated air having high moisture bearing capacity at an adequate flow rate over the multi-level mesh belt assembly (203) such that said unsaturated air absorbs the moisture contained in the sludge and moisture saturated warm air exits from the sludge dryer body (102) via top exit (704) after being passed through air filter (207) thereby obtaining extruded dry sludge (705). The air filter (207) traps any large particles that might have gotten carried away from the sludge. The extruded dry sludge (705) is fed to the pulveriser (301) via the belt or screw type conveyer (209). The pulveriser (301) crushes the dry extruded and cut sludge into easily fluidizable particles having particle size less than 1 mm size for downstream heat recovery in the solid fuel boiler (302). The solid fuel boiler (302) combusts the pulverized dry sludge in presence of stoichiometrically excess oxygen supply controlled by the blower (305) in order to liberate the calorific value contained in the dried sludge (705). It is understood here that the complete combustion results into flue gases containing CO2, H2O along with unwanted contaminants that are transported to the wet scrubber (306) for scrubbing through an induced draft fan (304). It is further understood here that an ash (902) left after combustion of sludge (705) is disposed for landfilling. The liberated calorific value due to combustion of the sludge (705) in turn heats the incoming thermic fluid or condensate along the line (703) and increases its temperature along the line (702) or generates low pressure steam temperature above 100 0C before being fed to the high grade heat extraction heat exchanger (106). The moisture saturated warm air obtained via top exit (704) is optionally fed either to the evaporator (104) along the line (507) or to the air to air heat exchanger (108) along the line (802).
In operation, the compressor (101) receives depressurized refrigerant at a temperature greater than 10 0C along the line (502) and compresses the same to provide compressed refrigerant having temperature of about 40 0C to 90 0C along the line (501). The expansion valve (102) receives the refrigerant at a temperature of 20 0C to 70 0C along the line (503) and reduces its temperature at about -35 0C to 0 0C along the line (504) depending upon the type of refrigerant utilized in the system (100). The condenser (103) is configured to transfer the heat energy from compressed refrigerant at the temperature of 40 0C- 90 0C along the inlet line (501) to low temperature unsaturated air entering the condenser heat exchanger at 25 0C to 50 0C along the inlet line (505) in order to raise the unsaturated air temperature in the range of 35 0C to 75 0C along the exit line (506) to enhance moisture bearing capacity thereof. The refrigerant leaves the condenser (103) along the exit line (503) at a temperature of 20 0C to 70 0C. The evaporator (104) receives warm saturated air along the line (507) at a temperature above 30 0C that transfers heat to a depressurized refrigerant entering along the line (504) at a temperature of -350C to 0 0C in order to raise the refrigerant temperature to greater than 10 0C along the line (502). It is understood here that, the saturated air temperature is brought below its dew point, typically at a temperature below 25 0C, which leaves the evaporator (104) along the line (508) thereby condensing out the moisture absorbed from the wet sludge fed to the dryer. The low grade heat extraction heat exchanger (105) utilizes low grade sensible or latent heat from the waste heat sources such as process condensate or flash steam entering the heat exchanger (105) via entry point (907) along the line (602) at a temperature greater than 40 0C and leaving the heat exchanger (105) via exit point (908) along the line (603) at a temperature close to but above ambient temperature. On the other side, the low grade heat extraction heat exchanger (105) receives low temperature unsaturated air along the line (601) at a temperature of 20 to 40 0C which is increased to a temperature of 25-50 0C while leaving the low grade heat extraction heat exchanger (105) along the line (505) in order to reduce the heat requirements from the compressor (101). The high grade heat extraction heat exchanger (106) utilizes high grade sensible or latent heat of the thermic fluid or of steam that enters the heat exchanger (106) along the line (702) at a temperature greater than 100 0C and exits from the heat exchanger (106) along the line (703) at a temperature below 90 0C. On the other side, the high temperature unsaturated air enters the heat exchanger (106) along the line (506) at a temperature of about 35 0C to 75 0C such that said temperature is raised to 40 0C to 90 0C while leaving the heat exchanger (106) along the line (701) in order to further increase moisture bearing capacity of the unsaturated air. The air-to-air heat exchanger (108) receives the warm saturated air, along the line (802) at a temperature above 30 0C and transfers heat to a mixture of unsaturated ambient air entering the heat exchanger (108) via entry point (903) and along the line (801) at ambient temperature typically greater than 15 0C and saturated air already cooled down below its dew point in the evaporator (104) along the line (508) at a temperature typically less than 25 0C. The process in turn would heat the aforementioned mixture to a temperature of 20 0C – 40 0C before leaving from the heat exchanger (108) along the line (601).
Accordingly, the compressor (101), the expansion valve (102), the condenser (103), the evaporator (104), the low grade heat extraction heat exchanger (105), the high grade heat extraction heat exchanger (106), and the air-to-air heat exchanger (108) mutually form a closed loop assembly in order to facilitate the cycle of heating and cooling in a closed loop by adjusting the process variables such as air pressure, air path, air flow rate and sludge flow rate and the like such that dried sludge having moisture content less than 30% (w/w) is obtained along a dried sludge line 705.
The system of the present invention facilitates extraction of calorific value (CV, hereinafter) content of dried sludge, or any solid fuel for that matter, having less than 30% moisture to the maximum extent possible, typically in a range of 50 – 80% of intrinsic CV and employs said energy to reduce moisture of the sludge in an energy efficient manner thereby ensuring that the additional external energy required to take the sludge to a dry form to be able to meaningfully extract the CV of the sludge, is minimized or, in case the sludge CV is really high, is negative (i.e. generate surplus energy).
In an embodiment, a method for sludge treatment utilizing the system (100) of the present invention comprising the steps of:
a) feeding wet sludge to a sludge dryer body (201) through the sludge feed conveyor (208) followed by extruding through the extruder (202);
b) spreading extruded wet sludge over multi-level belt assembly (203) through the sludge spreader (204);
c) contacting the extruded wet sludge on the multi-level belt assembly with an unsaturated (or dehumidified) hot air obtained from the high grade heat exchanger (106) and supplied through the bottom distributor (205) thereby obtaining humidified saturated hot air at the top exit (704) and extruded dried sludge (705);
d) pulverizing the dried sludge (705) in the pulveriser (301) followed by combustion in a solid fuel boiler (302) for obtaining an ash (902) and flue gases containing CO2 and H2O at high temperature and moderate pressure;
e) utilizing liberated calorific value due to combustion of the sludge (705) to heat incoming thermic fluid or steam condensate along the line (703) at a temperature less than 90 0C for increasing temperature thereof above 100 0C along the line (702) or generating low pressure steam temperature above 100 0C;
f) scrubbing the flue gases with unwanted contaminants from the solid fuel boiler (302) in the wet scrubber (306) thereby treating with NaOH and transferring to an ETP equalization tank;
g) cooling the humidified saturated hot air in step c) by contacting with refrigerant and as a result obtaining condensed moisture from the humidified hot air thereby obtaining saturated cold air and water for scrubbing as described in step f);
h) heating cold air in step g) to obtain unsaturated (or dehumidified) hot air; and
i) repeating cycle of heating and cooling in steps g) and h) in a closed loop assembly formed by the compressor (101), the expansion valve (102), the condenser (103), the evaporator (104), the low grade heat extraction heat exchanger (105), the high grade heat extraction heat exchanger (106), and the air-to-air heat exchanger (108) thereby adjusting the process variables until obtaining dried sludge having moisture content less than 30% (w/w) along the sludge line 705.
In the context of the present invention, the high temperature of gases obtained at the top exit (704) by the above process isothermally increase the temperature and hence pressure of the depressurized refrigerant at a temperature of -35 0C to 0 0C in order to raise its temperature to greater than 10 0C through an indirect heat exchange in the evaporator (104). The condensed H2O is drained out as condensate and used as a utility in the wet scrubber (306) or transferred to ETP. The heat exchange process through low grade heat exchanger (105) increases the temperature of the unsaturated air before entering condenser (103) and hence reduces the amount of energy requirements from the sets of compressor(s) for temperature increase upto a target level. This eventually allows one or more of the compressor(s) in the set of compressor(s) (101) to remain off whenever low grade waste heat is available in the plant. The heat exchange process through high grade heat exchanger (106) increases the temperature upto the target level of the unsaturated air post temperature increase in the condenser (103). This contributes to reduction in the amount of energy requirements from the sets of compressor(s). This eventually allows one or more of the compressor(s) in the set of compressor(s) (101) to remain off whenever high grade heat recovery is employed in the close loop. Either of the cases help reduce the overall work to be done by the compressor set and hence the operating power requirement. The low operating cost is a major driver for adoption of the process of the present invention that makes its use highly viable.
In an embodiment, the complete combustion of the sludge in the solid fuel boiler (302) leads to conversion of sludge contents into their oxides such as CO2, SOx and NOx which are, subsequently, dissolved into water during wet scrubbing in the wet scrubber (306). The water used for wet scrubbing is nothing but the sludge moisture condensed away from the hot saturated air in the evaporator (104). In this embodiment, the dissolution of these oxides in water converts them to acidic solutions that are treated with NaOH to form stable salts that can be directly taken to ETP for standard treatment. This process ensures that the portion of sludge that can be oxidized ends up into a stable salt form that will not generate GHGs unlike its usual path wherein the sludge goes to landfill and then disintegrating into methane that has high GWP.
In another embodiment, the leftover sludge after oxidation or gasification is not only significantly low in but also has extremely low disintegration potential into GHGs such as CH4 and CO2 and consists largely of very stable compounds such as for example ash.. Hence, this method of sludge treatment in accordance with the present invention not only saves landfill space but also minimizes leachate related contamination and global warming by reducing GHGs generation.
In the context of the present invention, high efficiency and lower operating cost sludge volume reduction ensures economic viability even in areas with lower sludge disposal costs.
The foregoing description of specific embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others, skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated.
It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.
,CLAIMS:

1) A system for sludge volume reduction and environment friendly disposal thereof, said system (100) comprising:
a set of compressor(s) (101) configured to compress a depressurized refrigerant having temperature above 100C to provide the compressed refrigerant having temperature in a range of 50 0C to 90 0C;
a condenser (103) configured to facilitate indirect heat transfer between the compressed refrigerant at a temperature of 40 0C to 90 0C and an unsaturated air at 25 0C to 50 0C thereby raising the temperature of the unsaturated air in the range of 35 0C to 75 0C and lowering the temperature of the refrigerant in the range of 20 0C to 70 0C;
an expansion valve (102) receiving the refrigerant at a temperature of 20 0C to 70 0C for reducing temperature of said refrigerant in the range of -35 0C to 0 0C;
an evaporator (104) configured to facilitate heat transfer between the warm saturated air at a temperature above 40 0C and the depressurized refrigerant entering at a temperature of -350C to 0 0C thereby raising the refrigerant temperature to greater than 10 0C and lowering the saturated air temperature below 25 0C for condensing out the moisture;
an optional low grade heat extraction heat exchanger (105) configured to facilitate heat transfer between low temperature unsaturated air at a temperature of 200C to 40 0C and low grade sensible heat from a waste heat source at a temperature greater than 40 0C thereby increasing a temperature of the unsaturated air in the range of 25 0C to 50 0C and reducing temperature of the waste heat source close to but above ambient temperature;
an optional high grade heat extraction heat exchanger (106) configured to facilitate heat transfer between a thermic fluid at a temperature above 100 0C and unsaturated air at a temperature of about 35 0C to 75 0C thereby raising the temperature of the unsaturated air in the range of 40 0C to 90 0C and lowering the temperature of the thermic fluid below 90 0C;
an optional air-to-air heat exchanger (108) facilitating heat transfer between warm saturated air at a temperature above 30 0C and a mixture of ambient air at a temperature greater than 15 0C and saturated air cooled down below its dew point of 25 0C for heating the aforementioned mixture of unsaturated ambient air to a temperature of 200C - 400C. 0C;
a sludge dryer body (201) connecting to the extruder (202) at a top end thereof, the extruder facilitating entry of wet extruded sludge, the sludge dryer body (201) housing a variable speed controlled multi-level mesh belt assembly (203) carrying extruded wet sludge which is spread thereon by a sludge spreader (204) with zigzag profile, the sludge dryer body (201) having a hopper shaped exit at a bottom end and a bottom distributor (205), the bottom distributor (202) facilitating supply of hot unsaturated air over the multi-level mesh belt assembly (203) for obtaining the extruded dry sludge (705);
a pulveriser (301) crushing the extruded dry sludge (705) to form fluidizable particles having particle size less than 1 mm size;
a solid fuel boiler (302) receiving the fluidized particles for combustion at a temperature above 600 0C in presence of stoichiometrically excess oxygen supply controlled by a blower (305) to liberate the calorific value for heating thermic fluid thereby obtaining flue gases and ash; and
a wet scrubber (306) receiving the flue gases through an induced draft fan (304) to facilitate scrubbing thereof.
2) The system (100) as claimed in claim 1, wherein said refrigerant has a super critical temperature above 800C and a boiling temperature less than 0 0C.
3) The system (100) as claimed in claim 1, wherein said refrigerant is selected from R134a, R123, R600, R601, R601a and the like.
4) The system (100) as claimed in claim 1, wherein the expansion valve (102) has an operating pressure drop in the range of 8 bar to 40 bar.
5) The system (100) as claimed in claim 1, wherein low grade heat extraction heat exchanger (105) is selected from the air-to-liquid type heat exchanger or air-to-air type heat exchanger.
6) The system (100) as claimed in claim 1, wherein the low grade heat extraction heat exchanger (105) reduces the energy requirements from the sets of compressor(s) (101).
7) The system (100) as claimed in claim 1, wherein high grade heat extraction heat exchanger (106) is selected from the air-to-liquid type heat exchanger or air-to-steam type heat exchanger.
8) The system (100) as claimed in claim 1, wherein high grade heat extraction heat exchanger (106) reduces the energy requirements from the sets of compressor(s) (101).
9) The system (100) as claimed in claim 1, wherein the waste heat source is process condensate or flash steam.
10) The system (100) as claimed in claim 1, wherein the high grade heat is conducted through thermic fluid or steam.
11) The system (100) as claimed in claim 1, wherein the sludge body (201) has a cuboidal chamber configuration to facilitate dehumidification of sludge and circulation of unsaturated air.
12) The system (100) as claimed in claim 1, wherein the multi-level mesh belt assembly (203) has a zigzag configuration to facilitate a long residence time of the wet sludge within chamber to facilitate release of moisture contained therein.
13) The system (100) as claimed in claim 1, wherein the sludge dryer body (201) includes an array of circulation fans on side walls thereof.
14) The system (100) as claimed in claim 1, wherein the sludge dryer body (201) is made of material selected from Aluminium, Epoxy Coated/rubber lined MS, or SS.
15) The system (100) as claimed in claim 1, wherein the sludge dryer body (201) includes an extruder (202) configured to extrude the wet sludge lumps into a convex or concave polygon cross-section with circumscribing circle of diameter between 1 mm to 12 mm.
16) The system (100) as claimed in claim 14, wherein the extruded wet sludge is slitted at regular intervals to have a fixed extrusion length between 2 mm to 25 mm.
17) The system (100) as claimed in claim 1, wherein the multi-level mesh belt assembly (203) includes a mesh belt having a contact layer mesh opening of 0.5 mm to 8 mm and a support layer mesh opening of 5 mm to 25 mm.
18) The system (100) as claimed in claim 1, wherein the sludge dryer body (201) includes a sticky sludge scrapper (210) that involves a belt scrapper mechanism that scraps wet sludge sticking to the belt surface.
19) The system (100) as claimed in claim 1, wherein the filter (207) is made up of a material selected from polymer, metal or alloy.
20) The system (100) as claimed in claim 1, wherein the filter (207) has an opening size in a range of 5 µm to 100 µm.
21) The system (100) as claimed in claim 1, wherein the sludge dryer body (201) has a top exit (704) that facilitates passage of moisture saturated warm air.
22) The system (100) as claimed in claim 1, wherein the bottom distributor (205) is connected to an induced draft fan (206) to facilitate adequate flow rate of the air through the sludge dryer body (201) ensuring sludge drying using low temperature air at 400C to 900C.
23) The system (100) as claimed in claim 1, wherein the dry sludge (705) has dry solid contents above 70% w/w.
24) The system (100) as claimed in claim 1, wherein the solid fuel boiler (302) receives the thermic fluid/ steam at a temperature less than 90 0C and increases its temperature above 100 0C for generating low pressure steam before being fed to the heat exchanger (106).
25) The system (100) as claimed in claim 1, wherein the wet scrubber (306) is dosed with dilute NaOH solution or other basic solution that reacts with the contaminants and forms stable salts.