EP0584327A1

EP0584327A1 - Process and installation for controlling the methane fermentation of organic materials

Info

Publication number: EP0584327A1
Application number: EP93905392A
Authority: EP
Inventors: Jean-François BETEAU; Philippe 6 Rue Du Bois-Taillis Graindorge
Original assignee: L&C Steinmueller GmbH; VALORGA PROCESS SA
Current assignee: Hitachi Zosen Inova Steinmueller GmbH
Priority date: 1992-02-12
Filing date: 1993-02-12
Publication date: 1994-03-02
Also published as: FR2687166A1; JPH06509718A; WO1993016169A1; US5470745A; FR2687166B1

Abstract

La présente invention concerne un procédé et une installation de contrôle de la fermentation méthanique de matières organiques. Cette installation comprend essentiellement un fermenteur (F1), des capteurs (10, 11) associés à ce fermenteur pour permettre des mesures physico-chimiques et un calculateur (C1) traitant ces mesures à l'aide d'un modèle de fermentation, tandis qu'un calculateur (C2) transforme les données de (C1) en données agissant sur le fermenteur (F1), et un dispositif (F2-C3) d'expérimentation à échelle réduite permet d'ajuster les paramètres caractéristiques du modèle de fermentation incorporé dans le calculateur (C1). Cette installation permet, avec un minimum de mesures, de maintenir un fermenteur dans un état biologique optimal.The present invention relates to a method and an installation for controlling the methane fermentation of organic materials. This installation essentially comprises a fermenter (F1), sensors (10, 11) associated with this fermenter to allow physico-chemical measurements and a computer (C1) processing these measurements using a fermentation model, while '' a computer (C2) transforms the data from (C1) into data acting on the fermenter (F1), and a device (F2-C3) for small-scale experimentation allows the parameters characteristic of the fermentation model incorporated in the computer (C1). This installation allows, with a minimum of measures, to maintain a fermenter in an optimal biological state.

Description

"Method for controlling the methane fermentation of organic materials and installation comprising application of this method".

The present invention essentially relates to a process for controlling the methane fermentation of various organic materials.

It also relates to an installation for the execution of this process.

It has already been proposed to control fermenters using mathematical models taking into account the diversity of the materials to be treated as well as the fluctuations in the catalytic behaviors of microorganisms.

It is also known, to control fermenters, to establish an anaerobic fermentation model for diluted or simple substrates, but not for concentrated or complex organic materials.

These models can integrate one or more populations of microorganisms. They may or may not take into account the physico-chemical balances, the pH-inhibiting effect or volatile fatty acids which are essential metabolic intermediates, such as acetic acid.

In addition, purely biological mathematical models are known for diluted substrates, in one step, based on the acetate concentration, or in two steps, namely acidogenesis and methanogenesis, from simple substrates, such as glucose. These models take into account an inhibition of methanogenic activity by volatile non-ionized fatty acids, but they do not integrate pH as a state variable, in relation to the physico-chemical equilibria. We also know of other biological mathematical models for diluted substrates of methanization taking into account the physico-chemical equilibria thus making it possible to integrate pH as a state variable and to consider its inhibitory role.

However, these latter models do not take into account the inhibitory effect of non-ionized volatile fatty acids, such as acetic acid.

However, none of the above models integrates the combination of physico-chemical equilibria, pH as a state variable, and the inhibitory effect of volatile non-ionized fatty acids, and / or does not take into account the concentrated or complex organic matter.

We also know that the management of industrial anaerobic digestion fermenters necessarily requires a large number of additional physico-chemical measurements and analyzes, and in particular the measurement of:

- the quantity of biogas produced and its quality (percentage of methane and percentage of carbon dioxide),

- pH and temperature,

- the quality of incoming and outgoing treated, and

- the content of volatile fatty acids (VFA) in the fermentation medium, and / or the percentage of hydrogen in the biogas produced.

In general, the control of fermenters on the basis of previously known models is not satisfactory, in the sense that the models are not adjusted to the specific operating conditions of the implantation site of the fermenters. The object of the present invention is therefore to remedy these drawbacks in particular by proposing a method which makes it possible to control the state of methane fermentation, and this on the basis of a special model.

To this end, the subject of the invention is a process for controlling the methane fermentation of organic matter in at least one fermenter, and of the type consisting in carrying out physicochemical measurements on the fermenter, in processing these measurements in at least one computer allowing using a fermentation model to obtain a variable characteristic of the biological state of the fermenter, and to process this variable to deduce an order from the fermenter, characterized in that the above-mentioned physicochemical measurements are limited to two measurements, such as for example pH and volatile fatty acid content, the variable characteristic of the abovementioned biological state is the methanogenic biological activity, and the abovementioned fermentation model is adjusted as a function of the biomass present in the fermenter and organic matter to be treated with it.

According to another characteristic of this process, the methanogenic biological activity is expressed by the calculator in the form of a specific growth rate (μ) of the methanogenic bacteria of the biomass of the fermenter.

It will also be specified here that the above-mentioned fermentation model is adjusted on the basis of an experiment on a reduced scale which is carried out on a sample of material taken from the fermenter, and which consists in automatically monitoring the evolution over time of the pH, and the concentration of volatile fatty acids in the fermenter as well as the quantities of methane and carbon dioxide produced.

It is therefore already understood that, under these conditions, a quick and easy adaptation of the model is obtained to the operating conditions of the fermenter on its implantation site.

This fermentation model integrates the combination of biological phenomena including the inhibitory effect of non volatile fatty acids ionized and physico-chemical equilibria in the fermenter with pH as state variable.

The invention also relates to an installation for the execution of the above process and of the type essentially comprising at least one fermenter, sensors associated with this fermenter to allow physico-chemical measurements and at least one computer processing these measurements using the using a fermentation model, this installation being characterized in that the calculator audit, which, thanks to the fermentation model, transforms the physicochemical measurements into a variable characteristic of the biological state, is added another calculator ensuring the control at least one of the fermenters and in that said model is adjusted to the biomass present in the fermenter and to the substrate to be treated by a reduced-scale experimentation device essentially comprising a fermenter containing a sample of material taken from the fermenter mentioned in first, sensors associated with said fermenter and connected to at least one computer supplied ant the values of the parameters used to calibrate the aforementioned fermentation model.

The functionalities of the abovementioned computers are combined in a single computer.

This experimentation device can advantageously be located in another place than the implantation site of the fermenter.

It will also be specified that the sensors of the experimentation device are capable of continuously measuring in pH, the quantities of CH-4 and of CO2 produced and the concentration of volatile fatty acids in the fermenter, while the parameters provided by the abovementioned calculator are :

- the maximum specific methanogenic growth rate,

- the inhibition constant of methanogenic bacteria, - the saturation constant of methanogenic bacteria, and

- CH4 / biomass, Cθ2 / biomass and substrate / biomass yields.

However, other characteristics and advantages of the invention will appear better in the detailed description which follows and refers to the appended drawings, given solely by way of example, and in which:

Figure 1 is a block diagram of an installation according to the principles of the invention.

Figure 2 is a surface illustrating the relationship between μ methanogenic activity - - - pH and acetate concentration. μ max

Figure 3 is a curve representing the cumulative production of methane as a function of time.

Referring to FIG. 1, we can see an installation for controlling the methane fermentation of organic materials, according to this invention, and which essentially comprises a fermenter FI with which sensors 10, 1 1 are associated to allow physico-chemical measurements, to know in particular a measurement of the pH and a measurement of the content of volatile fatty acids.

The sensors 10, 1 1 are connected to a computer C 1 processing these measurements using a fermentation model.

The computer C1, which processes the aforementioned measurements and which supplies a variable characteristic of the biological state of the fermenter FI, is connected to another computer C2 which processes this variable to calculate a command to be applied to the fermenter FI. We have shown in F2 a small laboratory fermenter which can be filled with organic matter from the fermenter FI, as shown by the dotted line between FI and F2. With this fermenter F2 are associated sensors 12, 13, 14 and 15 themselves connected to one or more computers C3 processing the information coming from the sensors and supplying to the computer C1 the adjusted values of the characteristic parameters of the fermentation model incorporated in said computer Cl .

The F2-C3 assembly constitutes a small-scale experimentation device which, as will be explained below in detail, makes it possible to automatically monitor the evolution over time of the pH and of the concentration of volatile fatty acids as well as the quantities of methane and CO2 produced.

The small fermenter F2 is equipped with the means necessary to carry out the aforementioned reduced-scale experiment. These means (not shown) are, in particular, a means for introducing a direct substrate for methanogenesis, such as acetic acid, conventional means for stirring, heating, etc.

The installation which has just been described advantageously makes it possible to follow the progress of the methane fermentation without the need to monitor the flow rate and the quality of the biogas produced by the fermenter F 1, and this by performing only measurements on this fermenter in limited number, namely essentially a measurement of pH and a measurement of the content of volatile fatty acids. In addition, thanks to the F2-C3 experimentation device, the parameters of the fermentation model can easily be adjusted to local conditions (substrate, bacteria, operating conditions) governing the fermentation in the fermenter FI. It should also be said that the information leaving the computer C1 is information which translates the overall state of the fermentation and which therefore makes it possible, via the computer C2, to obtain a control of the fermentation in the fermenter FI. This control could not be as effective if it were obtained directly from the physico-chemical measurements from the sensors 10, 11. Having described the installation of Figure 1, we will now explain its operation by first recalling some principles of methane fermentation.

Methane fermentation includes 4 stages:

- substrate hydrolysis, acidogenesis, acetogenesis and methanogenesis.

The hydrolysis step makes it possible to transform, if necessary, the complex molecules into simpler molecules.

Acidogenesis transforms these into fatty acids, alcohols, carbon dioxide and hydrogen.

Acetogenesis transforms the products of acidogenesis into immediate methane precursors such as acetic acid.

Methanogenesis mainly performs the synthesis of methane from acetic acid according to the simplified formula:

CH3COOH> CH + CH

According to the invention, the fermentation model incorporated into the computer Cl and making it possible to provide a variable characteristic of the biological state, takes account of the coupling of bacterial growth with the production of biogas, of the inhibition of the growth of methanogenic bacteria by an excess of non-ionized acetic acid, and acid-base equilibria of the liquid and gaseous phases.

There is a close relationship between instant methanogenic activity μ, maximum methanogenic activity μmax _? I ^e pH and acetate concentration.

This relationship is μ =

where μ is the specific growth rate of methanogenic bacteria consuming acetic acid,

μ _max is the specific maximum growth rate of these same bacteria,

Kj is the inhibition constant,

K _s is the saturation constant,

[HS] is the concentration (g / 1) of non-ionized acetic acid,

[S] is the concentration (g / 1) of total acetic acid,

[H ⁺ ] is the proton concentration

[S-] is the concentration (g / 1) of ionized acetic acid.

Consequently, the above equations will make it possible, by knowing the values measured online or offline of the pH and of the concentration of volatile fatty acids, such as acetate, to obtain the value of the methanogenic activity.

Thus, for given fermentation conditions, the μ fermentation model makes it possible to calculate the value - - - as a function of the pH and

H'ma-x of the content of volatile fatty acids or acetate. Figure 2 illustrates the relationship between these three parameters for a given substrate. Therefore, knowing the pH value and the fatty acid content volatile by means of sensors 10 and 1 1, the computer C l determines the

μ ratio

Then the information is transmitted to the computer C2 which contains operating instructions for the fermenter FI, so that the computer C2 can act on various organs of the fermenter FI, such as a supply valve for example.

When the adjustment of the fermentation model is necessary, for example due to the change of the substrate, the change in the operating conditions of the fermenter, or even when starting a new installation, the experimental device 7 will be used. reduced scale F2-C3 according to the following procedure.

This procedure for adjusting the parameters of the model requires at most four tests by performing a range of acetic acid concentration and pH ranging from conditions that are not very inhibiting to conditions that are strongly inhibiting methanogenic bacteria.

By way of example, these four tests are carried out by varying the initial pH value from 6.5 to 7 and the initial concentration of acetic acid, for example from 1 g / 1 to 15 g / 1. The variations in the initial pH are obtained by adding 10% hydrochloric acid to the fermenter while the variations in the initial acetate concentration are obtained by adding a solution of acetic acid or acetate of calcium.

Of course, these additions are made in the small fermenter F2 filled beforehand with material taken from the fermenter FI.

The table below summarizes for example the four tests carried out for a given substrate, for the experimental validation of the model:

For each of these batch experiments, we then have curves representing, as a function of time, the cumulative production of methane and carbon dioxide in the small fermenter F2, as well as the variations in pH and in acetate content.

Figure 3 shows by way of example one of the curves recorded during these experiments.

The dots represent the measured values. On the basis of the following equations, describing fermentation under experimental conditions, an adjustment method is used, which makes it possible to obtain the values of the parameters characteristic of this fermentation model by minimizing the difference between the measured values and the values calculated by the model. The solid line in Figure 3 represents the simulated evolution for the experiment after this adjustment of the model.

+

(1) H .S ^~ - K _a .HS = 0

(2) HS + S ^~ - S = 0

(3) H + B - K _a .CO _2d = 0

(4) B + CO ₂ d - IC = 0 (5) B + S ^~ - Z = 0

(6) - = μ.X dt

(9) ^ = R ₂ .R ₃ .μ.X

With the following intermediate equations:

P _C 02 = ^ -, 0 _2c um = ∞ _2g - C0 _2i ni;

^P C02

C0 _2g = (N _2in i + CH ₄ ), IC = ICP - C0 _2c um ^p t-PC02

with

H ⁺ : the concentration of H ⁺ ions,

S: the amount of substrate (acetate),

HS: the amount of non-ionized acetate,

S ": the amount of ionized acetate, B: the amount of bicarbonates,

K _a : the acid constant of acetic acid,

Kb: the dissociation constant of bicarbonates,

^Cθ2d: ^has the amount of dissolved carbon dioxide,

IC: the quantity of inorganic carbon, Z: the quantity of cations,

X: the quantity of biomass, u: the specific growth rate of the biomass, R3: the substrate / biomass yield,

CH-4 _cum : the quantity of methane product cumulated,

Ri: methane / biomass yield,

ICP: the quantity of inorganic carbon released, R-2: the yield of carbon dioxide / biomass,

? ^CP2: l ^a partial pressure of carbon dioxide in the gas volume,

Pt: the total pressure of the gas volume,

^Cθ2cum: l ^a cumulative amount of carbon dioxide, u _m: maximum specific growth rate,

K _s : the saturation constant

Kj: the inhibition constant

V: the volume of the experimental device fermenter,

KH: Henry's constant for carbon dioxide, Cθ2g: the amount of gaseous carbon dioxide,

Cθ2ini • the amount of initial carbon dioxide in the gas phase,

N2ini - the amount of initial nitrogen,

IC: the amount of inorganic carbon.

The first four algebraic equations translate the acid-base equilibria of the following two pairs of respective equilibrium constants K _a and Kb:

- acetic acid / acetate;

- dissolved carbon dioxide produced / bicarbonate.

The fifth algebraic equation represents the electroneutrality of the medium and introduces an additional variable Z, which is the set of cations present in the fermenter.

The state variables of the model are as follows: H ⁺ , HS, S-, B, C0 ₂ d, X, S, CH4 _cum > ICP and Z. The value of the R-2 yield of carbon dioxide on the biomass is fixed at 1. The initial values of the pH, of the quantities of substrate S, of methane and of carbon dioxide are measured.

A calculation from algebraic equations then makes it possible to obtain the initial values of the quantities of acetic acid HS, of acetate S ", of bicarbonate B, of dissolved carbon dioxide Cθ2d _> of inorganic carbon IC, as well as all the cations dissolved in the fermenter.

The acid constants of acetic acid K _a , dissolved CO2 K _D , and the Henry's constant KT-J are adjusted from the experimental data.

In the example presented here, the values of these constants are as follows:

Ka = l, 7.10-5M Kb = l, 7.10-7M KH = 0.065 M / atm

The biological parameters of the model are the maximum growth rate μ _max , the saturation constant Kg, the inhibition constant Ki, the methane yield compared to the biomass Rj, and the substrate yield compared to the biomass R3.

They are identified from experimental data. The estimation of the parameters requires the integration of the model and the knowledge of the initial values of the state variables. The unknown and non-measurable initial biomass is therefore considered to be a parameter that is also identified and expressed in arbitrary units (AU).

Still in the example presented here for a given substrate and in relation to FIG. 2, the values of the parameters characteristic of this fermentation model which gave the establishment of the surface of FIG. 2 are given below: u _max = 0.017 1 / h K _S = 2.18.10-5M KI = 8.22.10-4M R] = 1M / M R3 = 350 mM / UA

Of course, the invention is in no way limited to the embodiment described and illustrated, which has been given only by way of example.

Claims

claims

1. Method for controlling the methane fermentation of organic matter in at least one fermenter, and of the type consisting in carrying out physicochemical measurements on the fermenter, in processing these measurements in at least one calculator allowing using a fermentation model to obtain a variable characteristic of the biological state of the fermenter, and to process this variable to deduce an order from the fermenter, characterized in that the above-mentioned physicochemical measurements are limited to two measurements, such as for example , pH and volatile fatty acid content, the characteristic variable of the abovementioned biological state is the methanogenic biological activity and the abovementioned fermentation model is adjusted as a function of the biomass present in the fermenter and of the organic matter to be treated with it. this.

2. Method according to claim 1, characterized in that the methanogenic biological activity is expressed by the calculator in the form of a specific growth rate (μ) of the methanogenic bacteria of the fermenter biomass.

3. Method according to claim 2, characterized in that the aforementioned fermentation model is adjusted from an experiment on a reduced scale which is carried out on a sample of material taken from the fermenter, and which consists in automatically monitoring the evolution in the pH time of the volatile fatty acid concentration as well as the quantities of CH4 and CO2 produced.

4. Method according to one of claims 1 to 3, characterized in that the aforementioned fermentation model incorporates the combination of biological phenomena including the inhibitory effect of non-ionized volatile fatty acids and physicochemical equilibria in the fermenter with pH as a state variable.

5. Installation for implementing the method according to one of claims 1 to 4, and of the type comprising at least one fermenter (FI), sensors (10, 1 1) associated with the fermenter to allow physico-chemical measurements and at least one computer (Cl) processing these measurements using a fermentation model, characterized in that said computer (Cl), using the fermentation model, ensures the transformation of the physico-chemical measurements into a variable characteristic of the biological state, is added a calculator (C2) ensuring the control of the fermenter (FI) from this variable, and in that said model is adjusted according to the biomass present in the fermenter (FI) and substrate to be treated by a reduced-scale experimentation device essentially comprising a fermenter (F2) containing a sample of material taken from the first mentioned fermenter (FI), sensors (12 to 15) associated with this fermenter (F2) and connected to at least one computer (C3) providing the values of the parameters making it possible to calibrate the aforementioned fermentation model.

6. Installation according to claim 5, characterized in that the functionalities of the aforementioned computers (Cl and C2) are combined in one and the same computer.

7. Installation according to one of claims 5 and 6, characterized in that the aforementioned sensors (12 to 15) are capable of continuously measuring the pH, the quantities of CH4 and CO2 produced and the concentration of volatile fatty acids in the fermenter (F2), while the parameters supplied by the abovementioned computer (C3) are:

- the maximum methanogenic specific growth rate (u _max )

- the inhibition constant of methanogenic bacteria (K

- the saturation constant of methanogenic bacteria (Ks), and

- CH4 / biomass (Rn, Cθ2 / biomass (R2) and substrate / biomass (R3) yields.