FR3110569A1 - Process for the production of an organic fertilizer and corresponding installation - Google Patents

Abstract

The invention relates to a process and an installation for producing an organic fertilizer from compost and / or macroalgae comprising the following steps: - obtaining sludge from sewage treatment plant - drying of said sludge from the purification station so as to obtain residual solid fuel having a dry matter content of at least 60%; - mixing of the residual solid fuel with organic matter having a dry matter content of at least 60%; - pyrogasification of said mixture at a temperature between 900 and 1200 ° C in the presence of a small amount of oxygen so as to obtain a synthesis gas; - autocombustion in a reactor of at least part of said synthesis gas obtained by pyrogasification of said mixture; - drying of said compost and / or of said macroalgae from the heat of the fumes and of the residual synthesis gas leaving said reactor. Figure to be published: Figure 1

Description

Process for the production of an organic fertilizer and corresponding installation

Field of invention

The field of the invention is that of the recovery of waste, and in particular of sewage treatment plant sludge.

More specifically, the invention relates to a process and an installation for producing an organic fertilizer from compost and/or macroalgae.

The invention finds particular application in the production of fertilizers which can be used in organic farming.

Prior art

In 2019, in France, approximately 70% of sludge from wastewater treatment plants was spread after treatment in the fields. The remaining sewage sludge was either incinerated, in the case of large sludge deposits, or landfilled.

To be able to spread sewage sludge, one or more treatments must first be applied. These treatments consist of dehydration, by centrifugation, by filtering or by adding chemical reagents, which makes it possible to reduce the water content of the sludge, conditioning, which makes it possible to stabilize the sludge by stopping the fermentation process, in order to limit the odours, and hygienisation, by liming, thermal drying, thermophilic anaerobic digestion or composting, which makes it possible to reduce biological risks during spreading.

Following the epidemic episode of SRARS-Cov-2, the National Health Security Agency (ANSES) now recommends not to spread sludge from wastewater treatment plants that have not undergone a prior hygienization treatment.

Furthermore, due to the presence in the sludge from the treatment plant of micropollutants, such as metals, plasticizers, for example bisphenol A, disinfectants, pesticides, polychlorinated biphenyls, drug residues, flame retardants , phenols and alkylphenols, perfluorinated compounds, etc., which are likely to seriously disrupt ecosystems, , or, for antibiotics and disinfectants, to play a major role in the dissemination of antibiotic resistance, there is a reluctance on the part of increasingly strong in spreading sewage sludge with relatively high amounts of micropollutants.

There is therefore a need to offer new outlets for sewage treatment plant sludge, and in particular for non-sanitized sewage treatment plant sludge.

Techniques have thus been proposed for the gasification of sewage station sludge, also called pyrogasification techniques, which make it possible to transform sewage station sludge into synthesis gas, also called syngas, which can be used for example in gas turbines. gas or in co-generation facilities, to produce electricity and heat.

A drawback of these known pyrogasification techniques is that it is complex to obtain a syngas with a high energy content from sludge from purification stations.

Another disadvantage of these known techniques is that the cost of the electricity produced is significant.

There are also known techniques for recovering sludge from wastewater treatment plants by methanation or by burning in cement works, for example. However, the capacity of methanization facilities or cement works is limited and does not allow the treatment of a large proportion of sludge from wastewater treatment plants, the quantity of which is increasing regularly with the growth of the population.

Objectives of the invention

The object of the invention is therefore in particular to overcome the drawbacks of the prior art mentioned above.

More specifically, the object of the invention is to provide a technique for the production of fertilizers which offers an outlet for sewage treatment plant sludge and in particular for non-sanitized sewage treatment plant sludge.

An object of the invention is also to provide such a technique which makes it possible to produce organic fertilizer with a reduced cost price.

Another object of the invention is to provide such a technique which is simple to implement.

Another object of the invention is to provide such a technique which is reliable.

These objectives, as well as others that will appear later, are achieved using a process for producing an organic fertilizer from compost and/or macroalgae comprising the following steps:

- obtaining sludge from the treatment plant

- drying of said sewage treatment plant sludge so as to obtain residual solid fuel having a mass content of dry matter at least equal to 60%;

- mixing of the residual solid fuel with organic matter having a mass content of dry matter at least equal to 60%;

- pyrogasification of said mixture at a temperature between 900 and 1200°C in the presence of a small quantity of oxygen so as to obtain a synthesis gas;

- self-combustion in a reactor of at least part of said synthesis gas obtained by pyrogasification of said mixture;

- drying of said compost and/or said macroalgae from the heat of the fumes and the residual synthesis gas leaving said reactor.

Thus, in a novel way, the invention proposes to produce heat from the pyrogasification of sewage plant sludge mixed with organic matter, in particular wood waste, followed by self-combustion of the synthesis gas for drying and thus recovering compost and/or macroalgae, in particular stranded macroalgae, in the form of fertilizer.

This process is particularly efficient from an energy point of view, because it avoids melting the ashes, resulting in the formation of clinker which obstructs the combustion hearths and leads to a deterioration in the burning of the sludge.

This process is also inexpensive because the main fuel used in this process is drawn from sewage treatment plant sludge, which is waste with no market value.

Advantageously, said step of drying said sewage sludge is implemented with heat produced by a wood-fired boiler.

This uses a renewable energy source.

In a preferred embodiment of the invention, said pyrogasification step is a fluidized bed pyrogasification step.

This may in particular involve pyrogasification on a circulating fluidized bed.

In variants of this embodiment of the invention, it may be envisaged to implement counter-current or co-current fixed bed pyrogasification or entrained bed pyrogasification.

According to a preferred aspect of the invention, said step of drying said compost and/or said macroalgae is implemented in a belt dryer.

The compost and the algae are thus dried at a low temperature, which preserves the nutritional qualities of the compost and the algae.

Advantageously, said residual solid fuel has a dry matter content greater than or equal to 85%, or in other words a dry matter content less than or equal to 15%.

In a particular embodiment of the invention, a method for producing an organic fertilizer as described above further comprises a step for recovering the ash from said wood-fired boiler and the ash produced during said pyrogasification step.

This ash can thus be used in particular in the form of potash and a source of calcium for soil amendment.

According to a particular aspect of the invention, said pyrogasification step is implemented with air at atmospheric pressure.

In variants of this embodiment of the invention, provision may be made to use a mixture of air and water vapor or of oxygen and water vapor as the gasifying agent.

According to a particular embodiment of the invention, a method for producing an organic fertilizer as described above comprises a step of preheating the air used for the pyrogasification from the heat of the fumes and the residual synthesis leaving said reactor.

This optimizes the recovery of heat from the fumes and the residual synthesis gas leaving the reactor.

Advantageously, said organic material comprises wood waste.

In variants of this embodiment of the invention, it can be envisaged that said organic material comprises green waste, and for example from agricultural products.

The invention also relates to an installation for producing an organic fertilizer from compost and/or macroalgae.

According to the invention, such an installation for producing an organic fertilizer comprises:

- means for drying sludge from the treatment plant so as to obtain residual solid fuel with a dry matter content of at least 60%;

- Means for mixing the residual solid fuel with organic matter having a dry matter content at least equal to 60%;

- a pyrogasification unit of said mixture at a temperature between 900 and 1200°C in the presence of a small quantity of oxygen so as to obtain a synthesis gas;

- a reactor configured for the self-combustion of at least part of said synthesis gas obtained by pyrogasification of said mixture;

- means for drying said compost and/or said macroalgae comprising means for exchanging heat with the fumes and the residual synthesis gas leaving said reactor.

List of Figures

Other characteristics and advantages of the invention will appear more clearly on reading the following description of an embodiment of the invention, given by way of a simple illustrative and non-limiting example, and the appended drawings, among which:

is a schematic representation of an exemplary embodiment of an installation for the production of an organic fertilizer according to the invention;

details the pyrogasification unit of the installation shown with reference to Figure 1;

is a representation, in the form of a block diagram, of the steps of an exemplary embodiment of a method for producing an organic fertilizer according to the invention implemented in the installation presented with reference to Figure 1.

Detailed description of the invention

Illustrated in schematic form in Figure 1 an exemplary embodiment of an installation 10 for the production of an organic fertilizer according to the invention from compost and stranded algae .

This installation 10 comprises a hopper 11 for receiving sewage treatment plant sludge transported by tank truck from a sewage treatment plant to the site of the installation 10. In this particular embodiment of the invention, the capacity of the hopper 11 is 180m ³ .

This hopper 11 feeds sludge from the treatment plant to a hot air dryer 12 via a conveyor belt 13.

Dryer 12 uses hot water at 90°C heated by a wood-fired boiler 14 capable of supplying 1.6MWh of energy to dry the sludge.

It will be noted that part of the hot air from the dryer 12 is transferred to the storage hopper 15 of woodchips intended to supply the boiler 14, in order to dry the woodchips below a humidity level of 20 to 30 % so as to improve the combustion efficiency of the boiler 14.

In addition, the flue gases from boiler 14 are treated using a cyclone filter and an electrofilter.

The ashes are sucked into the hearth of the boiler 14 and discharged at a storage platform 16, to be used as a soil amendment, due to their high potassium and calcium content.

In the dryer 12, the sludge from the treatment plant, the dryness of which, that is to say the percentage by mass of dry matter, when they are introduced into the dryer is 21%, is dried at a temperature of 70° C. for a period of more than 1 hour, until a dryness of 85% is reached in this particular embodiment of the invention.

It should be noted that to limit odor nuisance, the hot air discharged by the dryer is treated in a sulfuric acid scrubber (not shown in Figure 1).

The dried sludge forms a residual solid fuel of gross lower calorific value equal to 12M/kg, which is transferred by a conveyor 17 to a storage area 18, where it is mixed with wood waste having a mass rate of dry matter greater than 60%, in a tank equipped with a mixer 19.

This mixture is introduced by a screw conveyor 21 into a pyrogasification unit 110, shown in detail in Figure 2.

As can be seen in Figure 2, the mixture injected into the reactor 22 of the pyrogasification unit 110 is dispersed in a dense fluidized bed in the hearth 23 where the temperature is 950 ° C, where it undergoes pyrolysis and decomposes in the form of synthesis gas, or syngas.

In this particular embodiment of the invention, air preheated in the preheater exchanger 25 is injected in small quantities into the reactor 22. Furthermore, a burner 24 is provided for starting the pyrogasification unit 110 .

The reactor 22 also has a second chamber 26 where a substantial part of the syngas emitted in the hearth 23 undergoes spontaneous combustion at a temperature above 800°C. This self-combustion of the syngas advantageously makes it possible to recover the energy of the syngas at the level of the pyrogasification unit.

Approximately 95% of the combustion fumes leaving the reactor 22 are directed to a flue gas/water heat exchanger 27 intended to heat the air circulating in the belt dryer 111 to a temperature of 66° C., then discharged into the atmosphere after have circulated through a bag filter 28. The rest of the combustion fumes is sent to the preheater exchanger 25.

It should be noted that part of the fumes leaving the bag filter 28 is reinjected towards the hearth of the reactor 22 using the fan 29.

The belt dryer 111 is fed by a hopper 112 in which compost and stranding seaweed have been mixed, the humidity of which is around 80%. 1540 kg per hour of this mixture is continuously injected into the dryer 18, which makes it possible to obtain 360 kg/hour of a mixture of compost and seaweed whose humidity rate has been reduced to 15%, directly usable as fertilizer organic.

The stages of the process for the production of an organic fertilizer from compost and/or macroalgae implemented in the installation 10 have been summarized in Figure 3, in a synoptic manner.

In a step 32, the sewage sludge delivered in a step 31 by tank truck to the site of the installation 10 is dried in the dryer 12.

The residual solid fuel obtained after drying is mixed with wood waste in a step 33.

It follows in a step 34 a pyrogasification of the mixture which generates synthesis gas, part of which self-combusts in the second chamber 26 of the reactor 22.

The heat of the fumes and the heat of the residual synthesis gas leaving the reactor 22 are largely recovered in the heat exchanger 27 and transferred to a hot water circuit feeding the belt dryer 112 in which the compost and the macroalgae which are introduced are dried until they have a moisture content of less than 15%, in a step 35.

Claims (10)

Process for producing an organic fertilizer from compost and/or macroalgae comprising the following steps:
- obtaining sludge from the treatment plant;
- drying of said sewage treatment plant sludge so as to obtain residual solid fuel having a mass content of dry matter at least equal to 60%;
- mixing of the residual solid fuel with organic matter having a mass content of dry matter at least equal to 60%;
- pyrogasification of said mixture at a temperature between 900 and 1200° C. in the presence of a small quantity of oxygen so as to obtain a synthesis gas;
- self-combustion in a reactor of at least part of said synthesis gas obtained by pyrogasification of said mixture;
- drying of said compost and/or said macroalgae from the heat of the fumes and the residual synthesis gas leaving said reactor. Process for the production of an organic fertilizer according to Claim 1, characterized in that the said step of drying the said sludge from the treatment plant is implemented with heat produced by a wood-fired boiler. Process for the production of an organic fertilizer according to any one of claims 1 and 2, characterized in that said pyrogasification step is a fluidized bed pyrogasification step. Process for the production of an organic fertilizer according to any one of Claims 1 to 3, characterized in that the said step of drying the said compost and/or the said macroalgae is carried out in a belt dryer. Process for the production of an organic fertilizer according to any one of Claims 1 to 4, characterized in that the said residual solid fuel has a dry matter content greater than or equal to 85%. Process for the production of an organic fertilizer according to claim 2, characterized in that it further comprises a step for recovering the ashes from the said wood-fired boiler and the ashes produced during the said pyrogasification step. Process for the production of an organic fertilizer according to any one of Claims 1 to 6, characterized in that the said pyrogasification stage is carried out with air at atmospheric pressure. Process for the production of an organic fertilizer according to any one of Claims 1 to 7, characterized in that it comprises a step of preheating the air used for the pyrogasification from the heat of the fumes and the synthesis gas residual leaving said reactor. A method of producing an organic fertilizer according to any one of claims 1 to 8, characterized in that said organic material comprises wood waste. Plant for producing an organic fertilizer from compost and/or macroalgae, characterized in that it comprises:
- Means for drying sewage sludge so as to obtain residual solid fuel having a dry matter content at least equal to 60%;
- Means for mixing the residual solid fuel with organic matter having a mass content of dry matter at least equal to 60%;
- a pyrogasification unit of said mixture at a temperature between 900 and 1200° C. in the presence of a small quantity of oxygen so as to obtain a synthesis gas;
- a reactor configured for the self-combustion of at least part of said synthesis gas obtained by pyrogasification of said mixture;
- means for drying said compost and/or said macroalgae comprising means for exchanging heat with the fumes and the residual synthesis gas leaving said reactor.