Process for producing biogas from fibrous substrate

Abstract

The invention relates to a process for producing biogas from fibrous substrate by anaerobic fermentation. The process is characterised in that a) the fibrous substrate is fed together with process liquid to a fermenter containing anaerobic microorganisms depending on the TSS content ascertained in this fermenter, b) the fibrous substrate is subjected to wet fermentation in this fermenter to produce biogas c) the output containing fermented fibrous substrate is drawn off from the fermenter and the TSS content in the fermenter is ascertained, d) the TSS content ascertained is compared with a fixed target range and e) depending on the result from d), step a) is repeated with adjusted amounts in order to comply with the target range for the TSS in the fermenter.

Claims

1. A process for producing biogas from fibrous substrate by anaerobic fermentation, characterised in that a) the fibrous substrate is fed together with process liquid to a fermenter containing anaerobic microorganisms, the amount of the fibrous substrate and of the process liquid fed to the fermenter depending on a suspended dry substance (TSS content) ascertained in this fermenter, b) the fibrous substrate is subjected to wet fermentation in this fermenter to produce biogas, c) an output containing fermented fibrous substrate is drawn off from the fermenter and the TSS content in the fermenter is ascertained, d) the TSS content ascertained is compared with a fixed target range between 4% and 10%, e) the output of step c) is subjected to a solid-liquid separation, and in so doing moist fibrous fermentation residue and the process liquid are produced, and f) depending on the result from d), step a) is repeated with adjusted amounts in order to comply with the fixed target range for the TSS content in the fermenter.

2. The process according to claim 1, characterised in that the TSS content is a fibre content.

3. The process according to claim 1, characterised in that the fibrous substrate is fed together with the process liquid, such that the TSS content in a fermentation medium in the fermenter between 4% and 10% is given.

4. The process according to claim 1, characterised in that the fibrous substrate is fed together with the process liquid, such that a fibre content in a fermentation medium in the fermenter between 4% and 10% is given.

5. The process according to claim 1, characterised in that a content in the fermenter is stirred during the wet fermentation.

6. The process according to claim 1, characterised in that the process liquid produced in the solid-liquid separation is fed back to the fermenter.

7. A process for producing biogas from fibrous substrate by anaerobic fermentation, characterised in that a) the fibrous substrate is fed together with process liquid to a fermenter containing anaerobic microorganisms, the amount of the fibrous substrate and of the process liquid fed to the fermenter depending on a suspended dry substance (TSS content) ascertained in this fermenter, b) the fibrous substrate is subjected to wet fermentation in this fermenter to produce biogas, c) an output containing fermented fibrous substrate is drawn off from the fermenter and the TSS content in the fermenter is ascertained, d) the TSS content ascertained is compared with a fixed target range between 5% and 8%, e) the output of step c) is subjected to a solid-liquid separation, and in so doing moist fibrous fermentation residue and the process liquid are produced, and f) depending on the result from d), step a) is repeated with adjusted amounts in order to comply with the fixed target range for the TSS content in the fermenter.

8. A process for producing biogas from fibrous substrate by anaerobic fermentation, characterised in that a) the fibrous substrate is fed together with process liquid to a fermenter containing anaerobic microorganisms, the amount of the fibrous substrate and of the process liquid fed to the fermenter depending on a suspended dry substance (TSS content) ascertained in this fermenter, b) the fibrous substrate is subjected to wet fermentation in this fermenter to produce biogas, c) an output containing fermented fibrous substrate is drawn off from the fermenter and the TSS content in the fermenter is ascertained, d) the TSS content ascertained is compared with a fixed target range between 6% and 7%, e) the output of step c) is subjected to a solid-liquid separation, and in so doing moist fibrous fermentation residue and the process liquid are produced, and f) depending on the result from d), step a) is repeated with adjusted amounts in order to comply with the fixed target range for the TSS content in the fermenter.

9. The process according to claim 1, characterised in that the fibrous substrate contains a dry substance content (TS) within the range of 60%-95%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a flow diagram of the test structure of Example 3

DETAILED DESCRIPTION OF THE INVENTION

Example 1

(2) The determination of the TSS is described in detail hereinafter.

(3) Principle: The dry substance and the dissolved dry substance of a sample are determined by evaporating water of an undiluted sample or a diluted filtrate in a muffle furnace. The suspended dry substance (TSS) can be calculated using these values.

(4) Material: 50 ml centrifuge tube, disposable syringe 5 ml, syringe preliminary filter 0.2 μm, analytical balance, centrifuge, Nabertherm B180 muffle furnace, refractory glass dish, desiccator, crucible tongs, distilled water.

(5) Execution: The determinations of the dry substance and the dissolved dry substance are to be performed as double determinations. The sample to be examined is homogenised by shaking prior to its removal. For the TS determination, dishes are weighed and in each case the weight (m1) is noted. 10.00±1.0 g of the sample are filled into each dish and the end weight is likewise noted (m2). In order to determine the dissolved dry substance, 10.00±1.0 g of sample are weighed into a 50 ml centrifuge tube, and the exact masses noted (m4). Distilled water is added to 50.00 g±1.0 g and the total mass is noted (m5). The content of the tube is homogenised by shaking and centrifuged at 4,400 r for 5 min. Each dish is weighed and the weight is noted (m6). 10.00 g±1.0 g of the centrifuge is added to each dish via a 0.2 μm preliminary syringe filter. The end weight is noted (m7). All of the dishes are placed in the muffle furnace with the aid of crucible tongs in order to evaporate the water. The dishes are heated by 5° C./min to 105° C. and, once the temperature is reached, it is maintained for three hours. The samples are then placed in the desiccator for approximately 20-30 min for cooling. After the cooling the dishes are weighed and the mass noted (mass of the sample for the TS determination m3, mass of the sample for the dissolved TS determination m8). The dry substance TS and the dissolved dry substance TSgel. are calculated on the basis of the formulas stated below. The suspended dry substance TSS can be calculated from the difference between the value for the dry substance and the value for the dissolved dry substance. If the evaluation indicates a content of less than 1% for the diluted dissolved TS content TS*gel., the determination must be repeated with a smaller dilution.

(6) Calculation:

(7) Dilution factor DF for the sample weigh-in:
DF=m5/m4

(8) m4 mass of the sample

(9) m5 total mass after dilution

(10) The content of dry substance of the initial sample is calculated by:
TS=(m3−m1)/(m2−m1)

(11) m1 empty mass of the crucible of the TS determination

(12) m2 mass of the full crucible of the TS determination before 105° C.

(13) m3 mass of the crucible of the TS determination after 105° C.

(14) The dissolved dry substance of the diluted sample TS*gel. is calculated by:
TS*gel.=(m8−m6)/(m7−m6)

(15) m6 empty mass of the crucible of the TSgel. determination

(16) m7 mass of the full crucible of the TSgel. determination before 105° C.

(17) m8 mass of the crucible of the TSgel. determination after 105° C.

(18) The value for the dissolved dry substance TSgel. corrected by the suspended dry substance TSS can be determined by the following equation:
TSgel.=DF*(TS*gel.*(1−TS/DF))/(1−TS*gel.)

(19) TS*gel. determined dissolved dry substance in the diluted sample

(20) TS dry substance of the sample

(21) The difference between dry substance TS and dissolved dry substance TSgel. is the suspended dry substance TSS:
TSS=TS−TSgel.

Example 2

(22) A further method for determining the TSS content is described in detail hereinafter.

(23) The TSS is determined by vacuum filtration of a defined sample quantity through a cellulose acetate filter with a pore size of 0.45 μm.

(24) The 0.45 μm cellulose acetate filter can be prewashed with 150 ml water in order to remove the water-soluble contaminations. The filter is then dried at 105° C. for at least one hour to constant mass and stored in the desiccator until use. It should be noted that the filter is not contaminated by dust.

(25) The filter is weighed prior to the filtration (m1) and is then placed in the funnel of a filtering unit.

(26) The sample to be examined is homogenised by vigorous shaking, and then approximately 10 g (m2) of this sample are filled directly into a 25 ml measuring cylinder. The sample is filtered by means of vacuum, and the measuring cylinder is flushed twice with 25 ml water. The filter and the funnel are then flushed with a further 50 ml water, and the filter is sucked dry. The subsequent drying of the filter is performed at 105° C. in a drying cabinet to constant mass (m3). The TSS can be calculated from the end weight via the following equation.

(27) m1 mass of the unloaded filter

(28) m2 sample weigh-in

(29) m3 mass of the loaded filter after filtration
TSS=(m3−m1)/m2

Example 3

(30) The possibility of a technical execution of the process will be explained hereinafter on the basis of the fermentation of straw. Merely by way of example, straw represents the fibrous substrates considered in this application. The details are readily transferable by a person skilled in the art to the use of other suitable substrates.

(31) The example makes reference to FIG. 1, in which the reference signs have the following meanings: 100 straw 101 nutrients/trace elements 102 water 103 biogas 104 waste material 105 output 106 process liquid 1 fermentation 2 solid-liquid separation

(32) Straw, as an agricultural waste material, is very well suited as a substrate for biogas recovery. However, straw contains significant amounts of substances which are difficult to breakdown and which are hardly, or not at all, broken down within economically reasonable time periods. The substrate composition can vary greatly depending on the type of straw and the time of harvesting and also storage conditions. Rape straw in Germany, for example, has a moisture of approximately 30%, whereas wheat straw, if the weather is dry and if stored properly, usually contains only approximately 90% moisture. Rice straw by comparison contains a particularly large amount of mineral substances and can contain crude ash contents of more than 20% of the TS.

(33) Typical ranges of some of the contents of straw are shown in the following table:

(34) TABLE-US-00001 Min Max Moisture 5% of the OS 40% of the OS TS 60% of the OS  95% of the OS oTS 75% of the TS  97% of the TS Crude ash 3% of the TS 25% of the TS lignin (ADL) 3% of the TS 20% of the TS nitrogen (Kjeldahl) 0.3% of the OS   1.1% of the OS 

(35) Depending on the straw type and quality, the biogas potential can differ significantly. The dry substance remaining in the fermentation process following breakdown of the substance capable of being broken down can also differ significantly. In the case of rice straw for example a significant proportion of dry substance is retained after the fermentation due to the high mineral substance content. However, at the same time, a significant proportion of the mineral substances is converted into dissolved dry substance. This proportion is of subordinate importance for the mixing behaviour of the fermentation medium.

(36) Biogas tests were performed in a test facility consisting of a continuous stirrer tank (fermenter, 1). The schematic structure can be seen in FIG. 1. Once the system had been started with inoculum, water and wheat straw as substrate, with feed of nutrients and trace elements, quasi-continuous operation was provided thereafter with constant fill level with the same batch of wheat straw. Output (105) was drawn off daily from the system, and fresh, ground straw (100) and also process liquid (106) and water (102) were fed. The nutrient and trace element feed (101) was performed at regular intervals.

(37) A small part of the output (105) was analysed. The majority of the output (105) was subjected to a solid-liquid separation in a decanter (2). A pourable fibrous fermentation residue (104) and a process liquid (106) were thus obtained. The obtained process liquid (106) was passed back into the fermenter (1). In addition to the process liquid (106), 15% fresh water (102) was also used.

(38) In the stationary state, a TSS content of 6.5% was provided in the fermentation medium. The content of dissolved dry substance at this point in time was 2.0%.

(39) In further tests the TS content in the fermentation medium was increased by increased substrate feed and proportional reduction of the water feed. At TS contents above 8.5%, in particular above 9%, and an associated rise in the TSS content to values above approximately 6.8%, there was an accumulation of acetic acid in the fermentation medium, which was attributed to a significantly hindered mixing. It was possible to determine by means of inspection glasses that only local mixing was still provided in the immediate vicinity of the agitator at such a high TSS content, whereas, with a TSS content of 6.5%, mixing over a large volume was observed in the fermentation medium.

(40) By changing the substrate to rice straw with a crude ash content of approximately 20% of the TS (instead of 7% of the TS in the case of wheat straw), a significantly greater amount of TS not able to be broken down remained in the system. The fermentation process was initially performed such that a uniform TS content of approximately 8.5% was provided in the fermentation medium. This was possible only by a lower substrate feed in comparison to wheat straw.

(41) On the basis of measured values in the output, it was determined that the TSS content in the fermentation medium decreased continuously after the substrate change, whereas the content of dissolved TS increased. The feed of rice straw was then increased, and the feed of process liquid adapted such that a TSS of 6.5% was again produced in the fermentation medium. The proportion of dissolved TS increased accordingly, with the use of rice straw, to approximately 3.0%, and therefore the total content of TS in the fermentation medium increased to 9.5%. Stable operation alongside such a high TS was not possible in the case of wheat straw.

(42) Similarly to that experienced already in the previous test with wheat straw, a further increase of the contents of TSS or TS led to an accumulation of acetic acid as a result of a deteriorating mixing.

(43) in further tests, the control concept was also confirmed for other substrates.