ITMI20111599A1 - PROCEDURE AND PLANT FOR THE TREATMENT OF THE PERCOLATE FROM WASTE STORAGE - Google Patents

Description

Description of the patent for industrial invention entitled:

â € œPROCESSION AND PLANT FOR THE TREATMENT OF PERCOLATE FROM WASTE STORAGEâ €

Field of application

The present invention refers to plants connected with waste storage and more particularly with conventional and unconventional landfills of urban solid waste and agricultural residues.

Conventional landfills are those made using solid urban waste after separate collection, while non-conventional landfills are those obtained after a bio-drying process as described in EP70639.

The latter are notoriously inert but after abundant wetting they can be activated as described in EP1520634.

In these conditions they produce biogas which is usually used on site to produce electricity through internal combustion engines or biogas turbines coupled to an electric current generator and also produce a leachate that must be treated and disposed of.

The present invention relates to a plant and a process for the disposal of leachate in the purified gaseous phase, using for evaporation the residual heat coming from plants that use biogas for the production of electrical or mechanical energy, even when said heat is at a low energy level, below 100 ° C, as in the case of using endothermic engines, where most of the residual heat is found in the engine cooling water.

Previous art

A conventional landfill of municipal solid waste can produce through anaerobic fermentation up to 150 Nm <3> / t of biogas in 20-30 years which corresponds to the conversion into biogas of all the biodegradable portion of the waste while an unconventional landfill or better a bioreactor made with bio-dried waste it produces, after wetting, the same quantity of biogas in 6-7 years as described in patent EP1520634.

The biogas is transformed into electricity by means of internal combustion engines or biogas turbines with thermodynamic efficiencies normally between 30% and 41% depending on the type of engines or turbines and the quality of the biogas produced in the landfill or in the bioreactor, the residual energy is removed in the form of heat.

Depending on the performance of the motor, a 1000 kW electric motor dissipates from 660 to 1070 thermal kW with the cooling water leaving at 90 ° C which must be brought to 75 ° C and recirculated to the motor itself. they dissipate from 660 to 700 thermal kW through the combustion fumes that come out at 410-450 ° C. Therefore no less than 50% of this heat is at a low temperature level, below 100 ° C.

The landfill or the bioreactor also generates considerable quantities of leachate whose ammonia nitrogen concentration can reach values of 3-4 g / l which is harmful both for the anaerobic fermentation process and for the purification treatment of the leachate in phase liquid.

Water is normally evaporated by boiling, in one or more stages. This procedure allows to remove the water in the vapor phase at atmospheric pressure only if the temperature of the fluid used for heating is higher than 100 ° C. If the temperature of the heating fluid is lower, it is necessary to operate by condensing the steam at temperatures such as to keep the boiling temperatures of the leachate below 100 ° C, therefore the distilled leachate is compulsorily condensed and collected in the liquid phase. Since together with the leachate they also distil the most volatile components of water, including ammonium, said leachate must be purified before discharging with systems that are in some cases complex and expensive. The evaporation of the leachate in this case does not solve the problem of its disposal, but creates a new problem of purification and disposal of the distillate.

In the event that the temperatures of the heating fluid are greater than 100 ° C, as is the case with the use of biogas turbines, where the exhaust gases are significantly higher than 100 ° C, the leachate can be evaporated at ambient pressure, however the problem of its correct disposal involves the separation of the volatile components, including the ammonia contained and the consequent disposal of discharges or liquid waste if the purification is not carried out in the gas phase.

In the event that the available energy is at a level below 100 ° C, as occurs in the case of using the cooling water of endothermic engines for the production of energy from biogas, it is not possible, from the above described, operate by distillation under vacuum, without creating significant problems of purification of liquid waste.

A different system from boiling to evaporate water consists in the saturation of unsaturated air. This process is commonly used in cooling towers with natural or forced air convection. Their use is normally aimed at cooling the air in order to use it in cooling processes, such as steam condensation, not to evaporate fluids at a temperature below boiling point.

In the case of evaporation in air of hot fluids containing other volatile materials in addition to water, an air treatment process is necessary to eliminate these materials present as pollutants, which generally presents technical and economic aspects that are difficult to solve.

The current solutions adopted to evaporate the leachate in the air at pressures lower than the atmospheric one, do not solve these technical and economic problems.

Patent US0173031 proposes a unique cooling tower consisting of an evaporative vessel containing a head of water in which a flow of air is made to bubble through a long tube. The strong turbulence that occurs in the liquid is controlled by means of an appropriate gas distribution system, breakwaters, a large empty disengagement section in the tank and a septum for the abatement of the dragged or demister drops in the outlet duct. This equipment could be used to evaporate the leachate using the hot fumes of the engines, but it has too high pressure drops as the pipe is immersed under the head of liquid. Furthermore, the equipment is not designed to use the heat contained in the water. engine cooling, which contains most of the engine heat not used for mechanical energy production.

The patent FR2853308 uses for the evaporation of the leachate a part of the biogas produced by the landfill which is burned in a boiler and the heat of the fumes and the cooling water of the engines to preheat the fed leachate.

Patent DE102007018958 mixes ambient air and fumes at high temperature and sends the hot gas into the top of a tank equipped with a well in which a pump is located which, through a series of nozzles, sprays the leachate into the top of the tank.

Above the tank there are biofilters which have the task of degrading the substances present in the fumes and released by the leachate before being released into the environment.

In all the cases described above, the low thermal level heat is not exploited, in particular that contained in the engine cooling water, which is over 60% of the total, therefore it is not possible to remove the leachate in phase gaseous, after purification of the same, using only the thermal energy of the engines that use biogas for the production of electricity, but other energy at a high thermal level is required. Furthermore, as regards the purification of saturated air, the operating conditions of the fumes passing through the biofilter are not specified, which could function as a condenser, still creating a considerable amount of liquid fluid to be purified and lose efficiency, even in a way complete if the temperature is not that required by biological activity or the content of ammonia, a biotoxic compound, is too high.

Patent WO01 / 12548 presents a thermocompression and multiple effect system in which ammonia is separated by acid washing of the steam produced in a scrubber. Therefore, also in this case the use of other sources of energy is required and not only that contained in the cooling water of the engines, furthermore liquid waste is obtained.

All these processes do not fully exploit the residual heat made available by internal combustion engines, in particular that of cooling water, for the evaporation of the leachate, or they need additional energy.

Furthermore, the fundamental aspect for the evaporation of fluids in air without the production of other polluting liquid fluids is not considered, which concerns the possibility of purifying the gaseous phase without the production of other liquid phases to be discharged after purification or to be sent to plants purification.

The purpose of the present invention is to propose a new process that allows to achieve all the following objectives: a) to use all the low thermal level energy (below 100 ° C, such as the cooling water of endothermic engines) , obtained as a by-product of mechanical or electrical energy generators that use biogas, to evaporate the leachate from landfills by saturating the air, b) avoid the production of other liquids as waste necessarily associated with evaporation, to be purified before discharge or sent to purification, c) avoiding, during the purification of leachate in the gaseous phase, the formation of condensates to be purified or sent for purification, d) purifying the saturated air in the gas phase from the volatile compounds contained in the leachate, so that the pollutants are lower than the allowed limits.

This object is achieved by means of a method and a plant having the characteristics of the annexed independent claims 1 and 7, respectively.

Advantageous embodiments of the invention appear from the dependent claims.

Description of the invention

The process, object of the present invention, has as its main objective the evaporation of leachate from waste storage and in particular from landfills of both conventional and unconventional solid urban waste, using low-level heat, at a temperature below 100 ° C, such as that generated by internal combustion engines for the production of electricity from the biogas produced by the landfill itself, its purification and its removal in the vapor phase avoiding the pollution produced by the volatile components contained in the leachate.

No less important objective is to operate in conditions such as to make it possible to treat the evaporated material containing ammonia and other pollutants by biofiltration before release to the environment, without the production of liquid effluents.

The process according to the invention provides for the evaporation of the leachate in a closed tank, with recirculation of leachate and equipped with an exchanger on the recirculation, at atmospheric pressure, which uses the cooling fluids of the mechanical or electrical energy production plants that use the biogas from the landfill, when the average temperature of this energy source is below 100 ° C.

This procedure basically includes the following steps:

â € possible mixing of the hot combustion fumes with a high flow rate of primary ambient air to produce a hot gas, or to an exchanger on the recirculation of the leachate; however, the use of these hot combustion fumes is not essential to the process, which can only work with the energy referred to in the point below,

â € sending to the recirculation exchanger to heat the recirculated leachate of the cooling fluids, coming from generators that use biogas, at a temperature below 100 ° C, which generally contain the most prevalent quantity of thermal energy produced, as in the case of ™ cooling water for internal combustion engines,

â € introduction of the air, possibly mixed with combustion gas and of the recirculated leachate and heated in the tank through a pipe suspended in the top of the tank where they undergo an intimate contact at high speed in turbulent conditions causing evaporation of the water and of the volatile components of the leachate until saturation of the air introduced at the temperature required for the optimal purification of said air in a biofilter. The temperature of the saturated air at the outlet is regulated, according to the same fluids used for heating, by increasing or reducing the amount of incoming air,

â € possible addition of air to lower the temperature of the saturated air at the evaporator outlet, this operation is not essential for the operation of the process,

â € purification of the saturated air in a biofilter whose temperature, equal to that of the saturated air introduced, is such as to optimize the biological degradation of the volatile polluting components contained.

Further characteristics of the invention will appear clearer from the detailed description that follows, referring to a purely exemplary and therefore non-limiting form of realization, illustrated in the attached drawing, in which the single Figure is a simplified block diagram of the System according to the invention.

The system includes these essential components: 1) an evaporator A preferably consisting of a tank equipped with a long pipe D for the gas inlet and for mixing with the liquid, possibly connected to a central groove in order to reduce pressure drops , 2) an exchanger B for heating the leachate by means of the low thermal level fluids coming from the engines and 3) a biofilter C.

The leachate 1 can be fed directly to evaporator A or as indicated in the Figure on the recycling 11 of the evaporator itself, generating flow 6.

Said flow 6 exchanges heat in B with fluids at temperatures even lower than 100 ° C, for example the cooling water 3 of endothermic engines that use the biogas from the landfill, which enters the jacket of the exchanger at 90 ° C and exits at about 75 ° C, and heats up (flow 6) returning as flow 7 to evaporator A.

On tank A or on recycling 11 there is a purge 10 of concentrated leachate which is re-introduced to the landfill in order to avoid the accumulation of deposits in tank A. This recycling of leachate with a temperature of 30-35 ° C in landfill further favors / stimulates the degradation and stabilization process of the organic substance present in the landfill body.

The hot fumes 2 of the 390 ° C motors can be used, if available, to increase the ambient air temperature 4 before the air is fed into evaporator A where mixing takes place at high speed and in turbulent conditions with the recycling flow 7 coming out of the exchanger B. The mixing of the hot fumes with the air to be saturated also allows this energy flow to be used, if it has no other alternative uses. The use of hot fumes both mixed with air and to increase, by means of an additional exchanger, the temperature of the leachate sent to the evaporator, is not an essential condition for the operation of the process, and can be avoided if other uses for these high temperature fumes.

The gas is thus saturated and heated to the desired temperature by means of the high contact surface between the gas and the liquid drops in the high turbulence tube, the drops are separated by gravity during the rising of the gas in the surrounding tank and the gas exits from duct 8, a possible demister can be inserted before said duct to eliminate any water particles still present.

As will be better clarified in the examples, the high volumetric flow rate of gas with respect to the liquid, the equivalence of the momentum (mass x velocity) and the high speeds of the two fluids in turbulent conditions guarantee efficient mixing and the achievement of Thermodynamic equilibrium for water, ammonia and the other volatile components of the leachate.

At this point it should be noted that it is important that the flow 8 is at a temperature around 37 ° C so that the biological process of degradation of the pollutants in the biofilter C takes place in the most efficient way: if this were not the case, it would be necessary to increase or decrease the quantity of air contained in the flow 5 to decrease or increase the temperature of the flow 8. It is also possible, in case of too high temperature of the flow 8, to inject air into said flow to lower the temperature.

The amount of air used, as will be shown in the examples, is so high as to reduce the ammonia concentration to levels that can be tolerated and digested by the biofilter.

It should be noted that the lower the evaporation temperature and therefore the higher the air flow rate, the higher the evaporator volume is in order to maintain low rising gas speeds to allow the separation of water particles. plummeting into the empty section. It can be demonstrated how by operating at 50 ° C, instead of 37 ° C, it is possible to significantly reduce the flow of air introduced into the evaporator, while still guaranteeing the temperature of 37 ° C to the biofilter, by introducing air downstream of the evaporator in flow 8 before the biofilter, to ensure the optimum operating temperature of the same, and how the total flow rate is almost equivalent to that which would occur when operating in the evaporator at the same temperature as the biofilter. Even the quantity of evaporated water is, with the same heat input, it is almost the same as the temperature of the evaporator varies, with the same temperature as the biofilter.

The recycling flow rate 11 will be of the same order of magnitude as that of the flow 3 of the heating fluids coming from the engines, higher recycling flow rates will result in lower temperatures in flow 7 and reduced dimensions of the exchanger B.

It should be noted that the temperature of flow 7 is irrelevant for evaporation purposes, as the heat introduced into the evaporator is important, with flow 7 which is constant. The recycling rate can therefore be managed for the purpose of optimizing the mixing tube D.

As already pointed out, the mixer D is important for the process because it achieves the balance between a high flow rate of gas at high speed and a flow rate of liquid, where the flow rate of the liquid is about 1/10 by volume.

Typical speeds of the gas and liquid in the pipe are around 20 - 30 m / s and the pressure drops caused are substantially those of the inlet and outlet, since the pipe is not immersed in the bottom liquid of the tank. To reduce these losses, pipe D can preferably be made as a Venturi pipe, consisting of a converging and a diverging section with speeds in the throat between the two sections of up to 50 m / s with reduced pressure drops.

The system can operate under slight pressure with the fans placed on the air entering the tank (flow 4) or in slight depression with the fans positioned at the outlet (flow 8) not shown in the Figure.

The process object of the present invention has a high efficiency, is simple and therefore with low plant cost and low operating cost, because it uses waste heat in situ normally unused because the temperature is below 100 ° C and removes it in the air the leachate evaporated after purification of the volatile pollutants contained, not creating the problem downstream of the purification and removal of liquid discharges.

The process is perfectly integrated not only in a landfill, but in any plant where similar conditions are available.

Example 1

In an engine bank with a total power of 1 MWe It disposes of approximately 650 kWt in the fumes, corresponding to approximately 4450 Nm <3> / h of fumes at 390 ° C and in the engine cooling water 1070 kWt, corresponding to 61.5 m3 / h of water at 90 ° C to be cooled at 75 ° C, before recycling for cooling the motors themselves. Therefore, the low temperature energy disposed of in the cooling water, usable in a simple way only by means of the present invention, is about 150% of that disposed of in the fumes at high temperature.

With reference to Figure, the fumes 2 are added to a flow equal to 60000 Nm <3> / h of ambient air at a temperature of 20 ° C creating a flow of gas at about 50 ° C which is injected into pipe D. Therefore it is both the energy of the fumes and that of the cooling water are used.

The leachate 1, coming from the landfill, is introduced with a flow rate of 3 m <3> / h into recirculation 11. The recycling of the leachate is carried out with a recirculation pump (not shown in the Figure) with a flow rate of 64 m < 3> / h.

The heat exchanger B brings the flow 7 of the recycle leachate to the temperature of 50 ° C, this hot leachate is fed just above the throat of a suitably sized Venturi tube D.

A flow 10 of about 1m3 / h of leachate treated at a temperature of around 37 ° C and with an ammonia concentration of around 0.15 g / l and a flow 8 of 68500 Nm3 / h of saturated gas comes out of tank A 37 ° C. Part of the leachate leaving the evaporator is recycled in the landfill to avoid the precipitation, due to excessive concentration, of the non-volatile components contained.

For normal leachate from municipal waste landfills, the concentration in the air of ammonia, a pollutant that could limit the proper functioning of biofilters, results in these conditions around 70 mg / Nm3, a concentration that can be tolerated and digested by biofilters.

The quantity of evaporated leachate is around 2 m3 / h.

Example 2

We operate as in the previous example but with an air flow rate of 20150 Nm3 / h by bringing the evaporator to work at 50 ° C.

960 kg / h of leachate and 27600 Nm3 / h of humid fumes at 50 ° C are extracted from the evaporator; under these conditions 2040 kg / h of water evaporated.

In order to reach the temperature considered optimal for the biofilter and dilute the ammonia to avoid any toxicity for the same, the humid fumes are mixed with 36500 Nm3 / h of air at 20 ° C (flow 5) obtaining 64700 Nm3 / h of saturated gas at 37 ° C (flow 12) to be sent to the biofilter and 170 kg / h of condensate.

As can be seen, the flow of gas passing through the evaporator has been considerably reduced, the total air used has been reduced considerably and the evaporated leachate has only apparently increased because it is necessary to recycle the condensate to the evaporator (stream 15).

Of course, the invention is not limited to the particular embodiment previously described and illustrated, but numerous detailed modifications can be made to it within the reach of the person skilled in the art, without thereby departing from the scope of the invention itself. as defined by the appended claims.

Claims (8)

CLAIMS 1. Process for the treatment of leachate from waste storage and in particular from landfills that produce biogas and equipped with plants for the generation of mechanical or electrical energy, also using the heat disposed of by these plants at a temperature below 100 ° C, insufficient temperature to distill the leachate at atmospheric pressure, characterized by the fact that the leachate is removed in the gas phase by saturating the air with water and the other volatile components of the leachate, purifying the air from volatile components in a biofilter and removing the leachate water in purified saturated air saturated with humidity, using low temperature heat to heat the leachate, transferring heat, water and the volatile components of the leachate to the air in a pipe where the leachate is placed in contact with atmospheric air under turbulent conditions, separating the aqueous phase and the liquid phase into a volume in which the phase ga ssosa is made to rise with a lower speed than the precipitation of the drops of liquid, and operating with an air flow rate such as to obtain an optimal saturated air temperature for purification in the biofilter of the polluting components of the leachate, including ammonia . 2. Process according to claim 1, in which the hot combustion fumes are also used as a further energy source in order to increase the evaporative capacity, mixing said hot fumes with the incoming air and then proceeding according to the methods described in the claim previous one. 3. Process according to the preceding claims, in which the saturated air coming from the saturation phase is at a temperature higher than that required by the biofilter and is cooled by mixing with external air before the biofilter to bring it to optimum temperature. 4. Process according to claim 2 or 3, in which the evaporation of the leachate preferably takes place at 37 ° C by suitably regulating the quantity of primary ambient air (4), possibly mixed with hot fumes (2), or alternatively at temperatures higher, providing for cooling the gases coming out of the evaporator to the optimum temperature for purifying the pollutants in the biofilter before being introduced into it. 5. Process according to any one of the preceding claims, in which the saturation tube is inside a tank of sufficient diameter to ensure the separation of the drops of liquid that collect on the bottom for recycling and partial removal of the concentrated leachate in landfill in order to avoid encrustations due to the accumulation of the solid components of the same. Process according to any one of the preceding claims, in which the leachate (1) coming from waste storage is sent directly to the evaporation tank (A) or to the exchanger (C). 7. Plant for the elimination from landfill leachate of volatile components harmful to the production of biogas or its pollutants, in particular ammonia, with simultaneous acceleration of production obtained by heating the leachate itself and recycling it in rubbish dump. 8. Plant for evaporation and treatment of leachate from waste storage and in particular from landfills that produce biogas and equipped with plants for the production of energy that produce heat at a temperature below 100 ° C, such as that contained in the water of cooling of endothermic engines, which uses this heat at a low thermal level, exclusively or possibly in combination with that at a high level of hot fumes, characterized by the fact of providing: â € a leachate evaporator consisting of a closed tank (A), with recirculation of leachate with a pipe inside, possibly connected as a venturi in which there is the exchange of materials and energy between the leachate and the air; â € an exchanger (B) on the recirculation of leachate, in which energy is exchanged between the fluid at low thermal level and the leachate (11); â € possibility of adding hot combustion gases, unless otherwise used, to the air before entering the evaporator in order to increase the evaporative capacity; - possibility of lowering the air temperature, as an alternative to regulating the main air entering the evaporator, by adding air downstream, as this alternative procedure is essentially indifferent to the evaporative capacity of the system. Milan, 6 September 2011