WO2007059777A1

WO2007059777A1 - Biogas plant and process with immobilised bacteria

Info

Publication number: WO2007059777A1
Application number: PCT/DK2006/000657
Authority: WO
Inventors: Bjarne Uller; Gert Bjorn Petersen
Original assignee: Bioace I/S
Priority date: 2005-11-23
Filing date: 2006-11-22
Publication date: 2007-05-31
Also published as: EP2032685A1; DK200500270U3

Abstract

Device and method for biogas production by digestion of waste water or manure. The device consists of four compartments and the substrate is led to flow from the first via the second and third, to the fourth tank. The tanks contain a mesh where microorganisms effecting the methanation are immobilised. The method is caracterised by the start-up procedure during which the digester is fed to a certain extent with degraded substrate, ie methanation feed. In consequence the growth rate of methanation microorganisms is increased, ensuring a high concentration of methanation organisms in the digester.

Description

Biogas plant and process with immobilised bacteria

The BioACE BG separation and biogas plant is a device for converting residual organic matter like animal manure into combustible methane gas, a thin nutrient-rich liquid and sediment of high dry-matter concentration.

It is now for the first time possible to operate a rentable energy producing biogas unit on animal manure from a single farm without the addition of other high-quality organics.

The BioACE BG plant represents the first truly thorough process and design renewal since the biogas technology was introduced to the European marked during the 1970'ieth:

a. Fail proof pre-treatment b. Simplified flow-through without level control c. Buiit-in dry matter separation d. Fixed-filter process technology with balanced biology control e. Compact plant design with huge plant size reduction by comparison to existing plants , f. Simplified plant controls. The plant operation does not require special technical skills g_f Removable plant installation

Technical build-up: a. Pre-treatment of ingoing material ensuring that no particle over 10 mm enters the digester b. Four digesting chambers with gas producing and particle sedimenting zones c. Sediment transport from digester d. Biogas purification with biological desulphurisation e. Computer control with data logging

Disadvantages in the conventional biogas digester

The conventional biogas digester has several limitations that prevent the not-so- big producers of organic residual products from exploiting advantages of the biogas technology.

16 - 20 DAYS OF HYDRAULIC RETENTIONTIME

The digester dimensions are 16 - 20 times bigger than the daily feed volume making the plant footprint big in relation to the produced residual volume.

The retention time is only average due to the short-circuiting of some of the feed that travels directly from feed to bleed (discharge), not being subject to the biogas producing process.

Large digesters with permanent foundations are expensive and the installation of a biogas plant will only be rentable when the organic residual producer - e.g. an animal manure producer can supply organics to support digesters of more than app.10.000 m³. Smaller feed volumes can be rentable when high quality organics are added, but the cost price for such media is comparable to other fuel prices. MOBJLE PROCESS CARRIERS

In a conventional bio digester the microorganisms that perform the conversion from organic matter to biogas can move freely around the digester volume. Thus, the process is dependent on the different active bacteria to "find" their substrates - that is the substrate's diffusion rate in the digester medium.

The mobile process carriers are flushed out with the effluent and will be replaced by new bacteria that arrive with the feed and are produced during the digester process respectively. The slowest (rate-limiting) bacteria double their number once every 14'Th day. This sets a limit to the maximum allowable wash-out. When the limit is exceeded, the digester process will seize due to lack of active bacteria.

BIOLOGICAL BALANCE Under most natural circumstances, (organic material present together with the most widespread microbial activity), all the necessary conditions for making biogas are present - but not always optimal. The bacteria that degradate organic material to fatty acids and other metabolites multiply very quickly (app. 20 minutss). The bacteria that under anaerobic conditions utilises the residual products from these hydrolysing bacteria multiply very slowly (12 - 14 days) and are very sensitive to changes in their living conditions - e.g. temperature and pH. The latter strains of bacteria are the biogas forming agents.

In a biogas plant it is necessary to have a high concentration of both main groups of bacteria to obtain an efficient organics-to-biogas conversion. When the process is initiated with big numbers of the fast bacteria and smaller numbers of the slow bacteria the conversion-rate will not be limited by the number of slow-growing bacteria. Unfortunately, the balance between these active agents is very difficult to shift after running in the process, making it essential to facilitate the best starting-up conditions possible.

DEAD ZONES AND CHANNELING

The average digester retention time is determined from the assumption that the whole digester volume is well agitated. This is hardly ever the case. In the bottom of a conventional biogas digester, a sediment layer of 1 - 2 meters forms because the sedimenting particles have no way of escaping when they fall outside the effluent pipe opening zone. Because of this, dead-zones appear in the digester where bacteria and substrate cannot find each other. Consequently, the retention time and there through the bio-active volume will be reduced. Some bacteria secrete a slime-like substance (exo-polymer) when they die. After app. 6 months of digester operation, the digester liquid will be mixed with this natural polymer and will act as a thin gel when not stirred. In this situation the easiest way for new feed to pass through the digester is directly towards the 90 effluent pipe opening. This is called channelling and reduces the digesting ability of the digester biology because the bacteria never will be presented to a big part of the substrate.

When continuing the normal feed-rate in this situation, the bacteria that are in 95 the agitated zones and between feed-in and effluent zones risk overfeeding and the digester will gradually perform poorer. Unfortunately this is most often only discovered too late because it is not directly visible to the operator until it is too late.

Description of trie BioACE BG process

100 THE PROCESS

In the BioACE BG plant exactly the, same biological process as in the conventional digester goes on:

- Hydrolysing bacteria degradate complex organic molecules to free fatty acids, 105 alcohols, CO₂ and H₂.

- Methanogenic bacteria convert these products to methane (CH₄) and CO₂.

The process initiation must be done with respect to the different bacteria activities. In the Figure 1. graph on page 1/4 of the Graphs and Drawings 110 section the expected start-up and operation curves are shown.

IMMOBILISATION

In the BG the biological actors are fixed on filters allowing the different bacteria "families" to live together and to surround themselves with a substrate permeable slime layer - a bio film.

115 The Immobilisation technique is widely used in the chemical industry. It is very advantages to hold a catalyst fixed and let the reactant pass through or by. The active biology in the biogas process is not directly a catalytic process but the resemblance is evident.

120 By fixating the active biology the digester can be made much smaller because the nutrient-rich feed liquid just passed by a very high concentration of ready-to- concert-bacteria "families"

Because the bacteria are no longer flushed out with the digester effluent, The 125 balance ratio between slow and fast growing bacteria can be permanently changed in favour of the slow growing in order to speed up the overall conversion rate.

BIOLOGICAL BALANCE

130 If the BG process is initiated the same way the conventional digester processes are - by letting the bacteria themselves determine the balance between them, the BG will not be more efficient than the conventional digester - meaning 20 days of retention time to exploit the maximal biogas potential in the feed material.

135

Because of the bacteria fixation inside the digester the BG process is not dependent on a certain "sustainability-growth-rate" for the different bacteria. This allows us to "starve" one kind of bacteria while the other kind is optimally nourished. Once the wanted balance between the bacteria has been 140 established it will not change unless the substrate or the operating conditions changes,

TWO DAYS OF RETENTION TIME

Once the active biology is fixated, washout is no longer a problem. The maximum organic load acceptable by the newly balanced bacteria can then 145 determine the retention time in the digester.

In a conventional digester the organic load is normally around 5 gVS/L digester/day (gram organics per litre digester per day).

150 In the BG the organic load can be up to 45 gVS/L/day. This means that a BG with 60 m³ digester volume can produce as much biogas as a conventional digester of 600 m³.

FLOW THROUGH THE DIGESTER

155 The flow through the digester must be plug-flow. The bio-film in every of the 4 digesting chambers of the BG only sees the substrate that was left unprocessed by the previous chamber. Because of this, the bacteria in the different chambers can specialise in converting only the organics that was left unconverted by the previous chamber's bacteria.

160 SEPARATION

Sedimentation on the bottom of a biogas digester is very often seen. Normally, the digester must be cleaned regularly to remove the sediment.

The conical bottom of the BG digester collects the sediment as it forms and the 165 sediment is subsequently transported out of the plant by means of an auger. The auger operating frequency determines the dry matter content of the sediment up to the maximum dry matter content of app. 30% TS.

The sediment consists αf sand, bone fractions and other indigestible materials. The dry matter content and the consistency are similar to separation by means of a decanter centrifuge.

In the sedjment app. 20 % of the total feed material nitrogen can be found as well as most of the phosphor content in the feed i- depending on the pig feed management.

175

BUILD-UP

The BG consists of 4 separate digesting chambers with interconnection in top and bottom respectively, (see fig. 2 page)

180 Feed-in and effluent outlet occurs in wide slits (troughs) to distribute the incoming liquid and easing floating-layer discharge.

The effluent goes to a water seal that ensures the correct digester pressure.

The chambers are app. 2/3 filled PE-filters, housing the bio-film,

Incoming liquid and particles can only travel through the digesting areas in the 185 vertical direction. Thus, all parts of the substrate-liquid get in contact to all bio- film filters in every chamber.

To avoid clogging it is necessary that particles larger than app. 10mm are prevented to enter the digester.

The produced biogas collects in the head space over the digester liquid and can 190 flow freely through all cambers.

PLANT CONTROLS

The BioACE BG plant is a fully Integrated biogas plant. The plant controls are very simple and the forces of nature are widely used. Where possible, 195 gravitation is used instead of technical equipment.

Influent and effluent at the same level The feed is pumped in, and runs out by itself A simple water seal maintains the pressure 200 All available technical personnel can perform plant operation

The plant controls are doing the following:

a. Feed-in X times per day. b. Agitating pre-storage tank for Y seconds before the feed pump starts. c. Feed pump stops when Z m³ has been fed in.

205 d. Repetition of agitation and feed-in cycles if the feed flow meter does (not measure flow for XX seconds.) e. Feed-in sequence halts when temperature falls under XY ⁰C. f. Measurement and data logging of; Feed volume, gas production, power production, power consumption, and digester temperature are displayed on-

210 screen.

OPERATING CONDITIONS

a. Access to newly produced, pumpable bio-mass (residual organic products) b. Efficient downsizing of straw and other particles with diameters of more than 215 10mm.

c. Access to 400 V power supply.

d. Access to potable water.

e. Access to effluent receptive areas

220 LOCATION AND UTILISATION OF THE BioACE BG PLANT

The BioACE BG plant can be thought of as a part of the wastewater or manure treatment system. The plant only requires maintenance of devices similar to what is already on the farm/industry.

With its small size and the high performance, the BioACE BG plant can be 225 placed everywhere it is wished without a full concrete foundation. This also means that the plant can be relatively easily removed again.

The digester liquid can be removed from the digester without hurting the performance ability because of the bacteria immobilisation.

The BioACE BG system can be connected dfrectfy into the existing heating 230 system and deliver heat and power to the surrounding homes, farms and industries.

PERFORMANCE

When being fed with 36 m³ pig manure at app. 5% dry matter content, the

BioACE BG 36 plant with 72 m³ active digester volume will produce app.:

235 a. 33 m3 de-gassed biomass with app. 1 ,3 % dry matter content

b. 2 m3 sediment with app. 22% dry matter content

c. 900m3 cleaned biogas /day with app. 65% methane content

d. VS/COD reduction of app. 60%

e. 5800 kWh heat energy by burning all the gas in a biogas boiler.

240 The biogas plant itself consumes app. 18% of this energy.

Claims

Claims for the BioACE BG system

1. The total processing volume of the BioACE BG biogas system is 2 x daily 245 available volume of process feed media.

2. The total process volume shall be divided into three sections or zones:

a. Biogas producing zone where the biogas is formed by the methane and carbon dioxide producing bacteria.

b. Dry matter sedimenting zone where the particles heavier than 250 water will sediment without being disturbed by agitation devices.

c. Biogas collecting zone where the produced biogas will be collected. The biogas formation makes enough pressure to allow the produced biogas to flow out of the collecting zone to the subsequent biogas cleaning, storage and consummation.

255 3. The zones can be one integrated system or stand*alones.

4. The digester (biogas producing zone) consists of four consecutive tanks that allow for a liquid to flow from the first tank and to the fourth tank by flowing downwards through the first tank, upwards through the second tank, downwards through the third tank and upwards through the fourth

260 tank.

5. The digester liquid that flows through the four tanks must be moved by gravity - that is the in-flow in the first tank must be applied from a vertically higher point than the effluent leaves the fourth tank. Thus, the incoming feed pushes out the treated liquid at the flow rate specified in

265 claim 8.

6. The active biology shall maintain temperatures within the range of 25 degree Celsius to 54 degree Celsius.

7. The biogas producing zone shall contain an immobilisation media that can harness the process bacteria.

270 8. The active bacteria shall be immobilised inside the digester by means of growth on a polyethylene mesh media that allows bacteria growth attached to the immobilisation medium.

9. The immobilisation medium shall allow for the substrate liquid to flow past the immobilised bacteria at the maximum peak flow velocity of

275 0,000278 meters per second and the maximum average flow of

0,00004167 m/s

10. The active bacteria attach to the immobilisation medium. The biological degradation of the organic matter and the subsequent production of biogas ocόur through the metabolic pathways of several bacteria strains

280 that feed on each other's metabolic products. In the BioACE BG biogas process two main kinds of bacteria are managed during the process. One main bacteria group is the degradating the complex organic material into acetic acid, alcohols, carbon dioxide and hydrogen. The other main bacteria group consumes these products and produce the methane

285 biogas. Some of the alcohols, carbon dioxide and hydrogen are also converted into methane gas. The degradating bacteria are very robust, replicate every 40-120 minutes and operate perfectly under both cold (25) and hot (53) conditions and high (>9) pH and low (<5) pH. The methane forming bacteria operate only at very stable temperatures. Their

290 growth-rate and production-rate is greatly inflicted by the temperature. To perform well they need a very stable temperature that will not fluctuate very much over time.

11. Once the different bacteria "families" are formed and attached to the immobilisation medium as a bio-film the biological balancing can start.

295 This process invention exploits properties of the formed bio-film. The degradating bacteria are present in a larger number than the methane producing bacteria. The biologically balancing has the goal of equalising the production speed of the main bacteria groups in a way that allows the process to occur at the speed of the fastest bacteria in stead of -as in 300 the conventional biogas producing process- letting the slowest bacteria be rate-limiting for the whole process.

The means of balancing the active biology is to stop feeding the fastest bacteria and let that bacteria sit inactively in the bio-film. At the same 305 time, the slowest bacteria will be fed in a controlled way that allow for that bacteria to produce biogas and replicate at its highest pace.

During the biological balancing process the digester is fed with the metabolic products of the degradating bacteria. All four tanks have 310 feeding points for the dosing of the biological balancing medium. The digester liquid is recirculated while the dosage of the biological balancing medium is going on.

The dosing will begin at the rate of 1 gram of methane bacteria feed 315 per digester volume Litre per day. When the methane production occurs according to the dosed methane bacteria feed and the concentration of methane bacteria feed inside the digester does not increase the dosing will be increased every day and will be terminated when the methane bacteria accepts the load of 25 to 4δg methane 320 bacteria feed per digester volume Litre per day.

Towards the end of the biological balancing the actual digester feed will slowly replace the methane bacteria feed. When the digester is fed solely with the actual feed that can be manure, slaughterhouse waste, dairy 325 sludge, silage, etc. and the methane production and the digester concentration of degradating bacteria products is at an acceptable level the biological balancing j§ finalised.

12. The degradating bacteria have the capacity to degradate the incoming 330 organic matter within two days. In the normal biogas digester, the methane producing bacteria are not able to remove the products of the degradation fast enough and get inhibited. When the number of methane bacteria is high enough and they are immobilised along with the degradating bacteria, the methane bacteria can take in all the 335 degradating bacteria can produce. Thus, the fastest bacteria instead of the slowest set the degradation and biogas formation speed.

13. The imrriobilisation medium is a polyethylene mesh of extruded roughed- up polyethylene tubes. The immobilisation filters are placed with the tubes standing vertically. When the digester feed enters the first tank the

340 feed will enter the immobilisation tubes and flow vertically down through the filters at the flow velocity stated. The bacteria are housed on the mesh that makes up the walls of the immobilisation tubes. As the feed passes through the digester tanks the contact between active bacteria and feed substrate is very good and removes the need for in-digester

345 agitation. In the conventional biogas digester a thorough agitation is needed to provide contact between the active bacteria and the feed substrate. The volume flow to immobilisation media surface area shall be at a maximum of 0,8 L feed per m² immobilisation media per day.

14. As the bacteria are immobilised in the four tanks and the flow always 350 occurs from tank one to tank four, no bacteria is transferred from a downstream tank to an upstream tank. Thus, the active biology growing on the immobilisation medium in the tanks specialises in feeding on what always come from the upstream tank; Tank one receives the raw biomass, In the biomass some easy digestible fats and sugars will be. 355 The bacteria in tank one are used to this and quickly remove the easy digestibles - leaving the rest of the organic matters to the next tank. The bacteria in tank two are used to receive what is left from tank One, and specialises in converting that into biogas. The same action goes on in the remaining two tanks. Thus, by immobilising and balancing the digesting 360 biology the digester as a whole specialises in converting exactly the feed available into biogas and freed nutrients,

15. As no agitation occurs in the digester it is possible to create a "dead- zone" sedimentation in the bottom part of the digester. In the conventional digester, sedimentation also occurs but in a way that makes

365 the formed sediment difficult to remove. At the same time the agitation action stirs up many particles that are actually heavier than water and thus would sediment when given the chance. The BioACE BG biogas plant has a conical bottom that collects the sedimented particles that fall through the still standing liquid in the, sedimenting zone. When enough

370 particles have collected in the cone the built-in-auger will start and transport the sediment out of the digester as a sludge fraction with dry matter contents between 20 and 30% dry matter.

The sediment will contain much of the phosphor and nitrogen that the 375 current regulations do not want to be in the treated effluent. Thus, the produced sediment lives up to the EU legislation regarding the farmland area demand dispensation,

16. The produced biogas is water saturated and polluted with hydrogen sulphur (H2S). The gas is desulphurised biologically with the addition of

380 atmospheric air. After desulphurisation the bipgas is led to the condensation unit where the biogas is cooled to 5 - 10 degree Celsius. When cooled the water content of the gas decreases greatly and the water that can no longer be in the gas condenses to liquid water. The condenser section of the gas treatment is the coolest part of the whole

385 plant. When the gas is led from the condenser to the gasholder the gas is "dry" in the sense that water no longer can fall out of the gas as the temperature of the gas holder is higher than the dew-point.

17. The biogas production is allowed to pressurise the whole system to Ombar to 45mbar pressure. When the pressure rises over the

390 maximum allowable, or under the minimal allowable, four safety precautions come into action;

a. The gas flare-stack. When the pressure raises over the normal operation pressure the flare stack will burn the gas for as long as it is necessary to lower the gas pressure to an acceptable level.

395 b. When the gas pressure increases even though the gas flare-stack is activated the gas will bobble out through the two independent water-seals. One water-seal is the gas condenser water-seal and the other is the treated digester effluent water-seal.

c. When the gas pressure increases even though the gas flare-stack 400 is activated and the gas is bobbling out of the two water-seals the certified mechanical pressure/vacuum valve in the digester top will open and let out the overpressure.

d. When the gas pressure inside the gas system and/or the digester falls under an allowable level the mechanic pressure/vacuum

405 valve in the digester top will open and let in atmospheric air to equalise the gas pressure.

18. The de-gassed digester effluent is taken out from the digester through three wide troughs that span over the total digester width. When treating organic residual products the risk of foaming is always present. Foam

410 formation inside biogas digesters is a big problem in conventional biogas systems, as the foam has no way out of the digester. In some systems the foam is allowed to escape through a round pipe in the digester side. Foam does not enter round holes in a container wall but tends to fill the available gas-producing space and flow out through the gas piping. The troughs in the BioACE BG effluent section allow the foam to break off in the whole digester width. This feature takes the formed foam out of the digester at the same rate as the digester effluent.