FR2536240A1 - Anaerobic digester for continuous feed manure heap - Google Patents

Abstract

The invention relates to a device capable of subjecting the manure heap to an anaerobic fermentation which has the effect of producing a combustible gas. This device comprises an air-tight body, fitted with two coffers which allow the insertion of the new manure and its withdrawal after fermentation. An internal system of compartments constituted for example by an endless screw allows the manure to be displaced between the two coffers as fermentation proceeds so that it is possible to feed the device continually, while the manure which it contains is at various stages of fermentation. This device is essentially intended to be used in agricultural installations for producing a gas which is an energy source and at the same time converting the manure into fertiliser while eliminating pollution.

Description

 Anaerobic fermentation, that is to say in the absence of air or oxygen, organic waste of various origin (plant or animal) but characterized by a dry matter rate higher than about 15% (manure by example) provides a gas with a high proportion of methane, which therefore has a certain energy value. The sludge collected after this fermentation can be used as fertilizer. In addition, the treatment is accompanied by a significant reduction in the pollution load (approximately 80%).

 This fermentation (or digestion) takes place through a set of bacterial populations which degrade complex organic molecules into simpler molecules, then into carbon dioxide (C02) and methane (CH4).

 This fermentation is carried out in an airtight enclosure called a "digester". It is loaded with fermentable materials and then sealed. It is discharged when the methanogenic potential of the materials is exhausted (around 6 weeks).

 The necessary conditions for initial anaerobiosis are obtained by the self-consumption of the introduced materials which have a high D.B.O.

They can be improved by initially vacuuming the digester.

 Installations of this type generally include several tanks so that an almost constant production of biogas is achieved.

But this method has some drawbacks - need to have several tanks (significant investment), - - long and painful operations of loading and unloading of quan
large quantities of manure, - need to store it for long periods before intro
duction in a free tank (2 weeks for a set of 3 tanks), this
which causes too much aerobic pre-fermentation, therefore a re-
duction of methanogenic power.

 The apparatus which will be described must make it possible to continuously process manure from various origins, avoiding the drawbacks which we have just listed and increasing the efficiency of the system.

It consists essentially of - a body equipped with two airlocks, one for introducing the waste
the other to remove them, - from a system for moving waste between the two airlocks.

A possible realization is as follows
The body is a cylinder with a horizontal or slightly inclined axis, in which turns an endless screw which can be actuated from the outside without affecting the tightness of the body.

 At one end of this cylinder is the entry airlock and at the other the exit airlock.

 Along the upper generatrix of the body is a chute which collects the gas produced. Similarly, along the lower generator is arranged a chute which collects the existing broth or that which has been produced.

A pump makes it possible to create a vacuum in each of the two airlocks. A third valve allows you to introduce either outside air or gas contained in the body of the device
Another pump makes it possible to transfer the broth contained in the lower chute to the entry airlock and thus to sow the anaerobic bacteria with freshly introduced waste.

The operation of the device is as follows
During the first loading, a certain amount of fresh material is introduced into the entry airlock. The air is extracted from the airlock and from the body of the device. The materials are sprinkled with broth obtained elsewhere. After a certain time, on the order of 24 hours, during which the materials have started to undergo a start of fermentation, the airlock and the body of the apparatus are put into co-operation. The fermented materials then fall into the body. We close the airlock while waiting for a new use.

 Subsequent loads are then carried out periodically, for example, every two days. A new quantity of fresh waste is introduced into the airlock. We create a vacuum in the airlock, and by the set of taps we put in communication the atmosphere of the body and that of the airlock which then contains the gas produced by the beginning of fermentation.

 Before opening the communication door between the body and the airlock, the worm is rotated so as to move the waste which is already in the body and to make available the space located in front of the airlock. We can then open the communication door if the waste falls into the body. We close the communication door. We can then sprinkle the new waste with broth.

 The body then contains two "doses" of waste at two stages of fermentation.

 The operation is repeated periodically until after approximately 40 days the initial dose has traveled the length of the cylinder and is opposite the exit airlock. It has then exhausted its methanogenic potential and can therefore be extracted from the digester.

 We then create a vacuum in the airlock, then put its atmosphere in communication with that of the body. We open the communication door to drop the waste in the airlock and close it. The gas contained in the airlock is transferred to the body of the digester and then the outside air is admitted into the airlock which can then be opened. The sludge is then recovered: the manure from the digester has lost more than 50% of its dry matter and has seen its polluting load reduced by approximately 80%, without losing any of its qualities as a fertilizer.

 We then advance all the doses in order to free the entry box and introduce a fresh dose.

 At this stage, the body of the device is filled with doses of organic waste at various stages of their digestion. So we made a continuous digester.

 A dose putting in theory 40 days to complete its fermentation cycle and the frequency of introduction of the doses being every n days, the number of doses during digestion is equal to 40 / n. This determines the number of "boxes" defined by the worm, therefore the pitch of the latter.

 Another exemplary embodiment consists of a body having the shape of a cylinder with a vertical axis and of small thickness (such as a bunch of pie). The organ ensuring the displacement of the doses of waste is a kind of star formed of radial partitions integral with a central hub.

the entry airlock and the exit airlock are arranged one above the other at any point on the circumference. The gas or broth recovery ducts can be replaced by double partitions.

Claims (2)

1. Apparatus intended for the anaerobic fermentation of organic waste of animal or vegetable origin containing at least 15% dry matter, characterized by its airtightness to the outside air and by the presence of the following elements  - an airlock allowing fresh waste to be introduced into the device without introducing air into it,  - an airlock for extracting completely fermented waste without introducing air into the device.  - a system for advancing waste, arrived at different stages of fermentation, between the two airlocks.  - a pneumatic system formed by a pump and an associated valve, which eliminates the air from each airlock and replaces it with the gas produced.  - a gas recovery manifold. 2. Apparatus according to claim 1 characterized by a pump which allows to spray the waste freshly introduced with culture broth in order to start anaerobic fermentation.