WO2010067302A2 - Apparatus and method for waste biomass pre-digestion - Google Patents

Apparatus and method for waste biomass pre-digestion Download PDF

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Publication number
WO2010067302A2
WO2010067302A2 PCT/IB2009/055550 IB2009055550W WO2010067302A2 WO 2010067302 A2 WO2010067302 A2 WO 2010067302A2 IB 2009055550 W IB2009055550 W IB 2009055550W WO 2010067302 A2 WO2010067302 A2 WO 2010067302A2
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WO
WIPO (PCT)
Prior art keywords
container
slurry
waste biomass
waste
biomass
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PCT/IB2009/055550
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French (fr)
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WO2010067302A3 (en
Inventor
Barry John Taber
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Blue Horizons Consulting Ltd
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Publication of WO2010067302A2 publication Critical patent/WO2010067302A2/en
Publication of WO2010067302A3 publication Critical patent/WO2010067302A3/en

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    • C—CHEMISTRY; METALLURGY
    • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M45/00—Means for pre-treatment of biological substances
    • C12M45/20—Heating; Cooling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
    • B01F27/60—Mixers with rotary stirring devices in fixed receptacles; Kneaders with stirrers rotating about a horizontal or inclined axis
    • B01F27/72—Mixers with rotary stirring devices in fixed receptacles; Kneaders with stirrers rotating about a horizontal or inclined axis with helices or sections of helices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
    • B01J8/08—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with moving particles
    • B01J8/10—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with moving particles moved by stirrers or by rotary drums or rotary receptacles or endless belts
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/34—Treatment of water, waste water, or sewage with mechanical oscillations
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F11/00—Treatment of sludge; Devices therefor
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F3/00—Biological treatment of water, waste water, or sewage
    • C02F3/02—Aerobic processes
    • C02F3/12—Activated sludge processes
    • C02F3/1205—Particular type of activated sludge processes
    • C02F3/1221—Particular type of activated sludge processes comprising treatment of the recirculated sludge
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
    • C08B1/00—Preparatory treatment of cellulose for making derivatives thereof, e.g. pre-treatment, pre-soaking, activation
    • C—CHEMISTRY; METALLURGY
    • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M1/00—Apparatus for enzymology or microbiology
    • C12M1/02—Apparatus for enzymology or microbiology with agitation means; with heat exchange means
    • C—CHEMISTRY; METALLURGY
    • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M1/00—Apparatus for enzymology or microbiology
    • C12M1/16—Apparatus for enzymology or microbiology containing, or adapted to contain, solid media
    • C—CHEMISTRY; METALLURGY
    • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
    • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
    • C12M45/00—Means for pre-treatment of biological substances
    • C12M45/02—Means for pre-treatment of biological substances by mechanical forces; Stirring; Trituration; Comminuting
    • C—CHEMISTRY; METALLURGY
    • C13—SUGAR INDUSTRY
    • C13K—SACCHARIDES OBTAINED FROM NATURAL SOURCES OR BY HYDROLYSIS OF NATURALLY OCCURRING DISACCHARIDES, OLIGOSACCHARIDES OR POLYSACCHARIDES
    • C13K1/00—Glucose; Glucose-containing syrups
    • C13K1/02—Glucose; Glucose-containing syrups obtained by saccharification of cellulosic materials

Definitions

  • the present invention relates to the bioconversion of biomass into useful chemicals, for example, chemicals such as alcohols (middle distillates) for the production of liquid biofuels, and in particular to an apparatus and method for waste digestion of biomass.
  • useful chemicals for example, chemicals such as alcohols (middle distillates) for the production of liquid biofuels, and in particular to an apparatus and method for waste digestion of biomass.
  • biomass recovered from waste into useful chemicals such as such as carboxylic acids (e.g., acetic, propionic, butyric acid), ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone) and biofuels, such as a mixture of primary alcohols (e.g., ethanol, propanol, butanol) and/or a mixture of secondary alcohols (e.g., isopropanol, 2-butanol, 3-pentanol).
  • carboxylic acids e.g., acetic, propionic, butyric acid
  • ketones e.g., acetone, methyl ethyl ketone, diethyl ketone
  • biofuels such as a mixture of primary alcohols (e.g., ethanol, propanol, butanol) and/or a mixture of secondary alcohols (e.g., isopropanol, 2-butanol, 3-p
  • biomass feedstock is converted into organic chemicals and alcohols in a multi-stage process that includes lime pre-treatment, non-sterile acidogenic digestion, product concentration, thermal conversion to ketones and their subsequent hydrogenation to create mixed alcohol end products.
  • An advantage of this process is that more energy from the biomass will end up as liquid fuels than in converting biomass to ethanol by yeast fermentation.
  • AII such biological/chemical processes require a liquefied biomass feedstock having a certain consistency in terms of its water content and the range of solid particulate sizes.
  • This biomass slurry feedstock will normally be produced from domestic or industrial biomass waste, and so may be composed of a wide range of possible materials having differing physical sizes and properties. The process for producing this slurry is referred to in this description as "pre-digestion”.
  • the predigestion process is but the first stage in a multistage biochemical/chemical plant.
  • the total throughput of the plant can be limited by any one stage, and so it is sometimes the case that there may be insufficient time to generate a sufficiently consistent and smooth biomass slurry. If particulate matter in the slurry is not sufficiently fine grained, then either the yield or the conversion time of the next stage, for example enzyme conversion into simple sugars, may be adversely affected.
  • a waste biomass pre-digester for producing a waste biomass slurry from waste biomass
  • the pre-digester comprising: a container for containing waste biomass, the container having an internal volume, said volume being bounded by a floor and surrounding sides and being accessible by means of an opening by which waste biomass may be introduced to the container volume; - a screw conveyor within the container for macerating waste biomass; a slurry circulation system comprising at least one slurry outlet from the container volume, at least one slurry inlet to the container volume, and a pump for circulating biomass slurry from said outlet(s) to said inlet(s); and the screw conveyor is arranged to convey biomass waste towards said slurry inlet(s).
  • the screw conveyor is an open helical screw, so that waste biomass can come into contact with the screw conveyor around substantially the complete circumference of the screw conveyor.
  • the combined action of the screw conveyor and pump circulation helps to homogenise waste biomass within the container.
  • the screw conveyor moves waste biomass around inside the container, with the turning action of the screw acting to macerate waste biomass, while the circulation pump redistributes biomass slurry between the inlet and outlet.
  • any solids in the biomass slurry drawn into the circulation system pump will be further macerated by the movement of the slurry between the inlet and outlet, and preferably also by the pumping action of the pump.
  • the screw conveyor is arranged to convey biomass waste towards said slurry inlet(s), and also the screw conveyor is arranged to convey biomass waste away from said slurry outlet(s). This helps keep the level of biomass slurry within the container relatively constant.
  • the circulation system can also be used to help redistribute biomass slurry between upper and lower portions of the container.
  • the (or each) slurry outlet is at a higher level than the (or each) slurry inlet, relative to the floor of the container volume or relative to the level of slurry within the container.
  • the container volume is preferably elongate, with the screw conveyor being oriented with the length of the container volume.
  • the screw conveyor is open on all sides and is aligned along a central long axis of the container volume, and separated from lateral sides of the container by a distance at least as great as the diameter of the screw conveyor. Waste biomass within the container that is pushed to one end of the container volume can then recirculate back to the other end of the container volume in the space between the screw conveyor and the lateral sides of the container.
  • the screw conveyor is also in a lower portion of the conveyor proximate the container floor so that, in use, with waste biomass completely covering the screw conveyor, waste biomass within the container that is pushed to one end of the container volume can then recirculate back to the other end of the container volume in the space between the screw conveyor and the top of the waste biomass within the container.
  • a method of producing a waste biomass slurry from waste biomass using a waste biomass pre-digester comprising a container, a screw conveyor within the container, and a slurry circulation system including a circulating pump, the method comprising the steps of: introducing waste biomass and liquid water into the container; using the screw conveyor to macerate said waste biomass within the container; and - using the slurry circulation system to withdraw said macerated waste biomass from a first portion of the container proximate an upstream end of the screw conveyor and to reintroduce said macerated waste into a second portion of the container proximate a downstream end of the screw conveyor.
  • the circulation pump further macerates said macerated waste biomass during said circulation.
  • the screw conveyor is oriented to convey macerated waste biomass towards the first portion of the container.
  • the slurry circulation system may include a diverter valve upstream of the circulation pump.
  • the diverter valve is then initially set in a first setting to direct circulated macerated waste biomass towards the second portion of the container until said macerated waste biomass achieves a desired consistency.
  • the diverter valve is then set in a second setting to direct circulated macerated waste biomass towards an outlet to the waste biomass pre-digester, for subsequent chemical or biochemical processing to produce useful chemicals.
  • Figure 1 is a block schematic view of a pre-digestion stage of a chemical plant, having a plurality of waste biomass pre-digesters, each one of which feeds a continuously stirred tank reactor (CSTR);
  • CSTR continuously stirred tank reactor
  • Figure 2 is a perspective view of one of the pre-digesters of Figure 1 ;
  • Figures 3 and 4 are side views of the CSTR of Figure 1 ;
  • Figure 5 is a perspective view of the CSTR showing external circulation pipe work.
  • Figure 1 shows a pre-digestion stage of a chemical plant 1 , having a plurality of waste biomass pre-digesters 2, each one of which feeds a continuously stirred tank reactor (CSTR) 4.
  • the plant 1 has a water inlet pipe 6 that is used to feed water as required into the pre-digesters 2 and CSTRs 4 by means of inlet control valves 8, one for each pre-digester 2, each protected by its own non-return valve 10. Water may also be fed as required into the CSTRs by means of inlet control valves 12, one for each CSTR, all protected by a common non-return valve 14.
  • each pre-digester 2 is run to produce a water-based slurry of finely macerated waste biomass.
  • Each pre-digester is connected to a corresponding CSTR by a slurry feed pipe 16 by which the waste biomass slurry is fed into the CSTR.
  • FIG. 2 shows the pre-digester 2 in more detail.
  • the pre-digester 2 has a container 18 that is used to contain a water-based waste biomass slurry.
  • the slurry may be formed using any type of useful waste biomass, for example biomass that contains cellulose or carbohydrates, for example waste wood, plant matter, vegetable matter, cardboard or paper.
  • the container 18 is elongate, in this example having a rectangular outline that is 5 metres long (L), 2.5 metres wide (W) and 1.6 metres high (H), formed from steel plates along each of the two long sides 21 , 22 and two short sides 23, 24 and base 25.
  • the container has an opening in the form of an open top 26 by which waste biomass may be dumped into the container, for example using a fork lift truck or belt conveyor (not shown).
  • the total volume of the enclosure formed by the sides 21-24 and base 25 is about 20,000 litres.
  • the container 18 is drawn partly cut away in order to show inside the container 2 a screw conveyor 28 for circulating and macerating waste biomass within the container.
  • the screw conveyor 28 has an open helical screw 29 that extends from a central shaft 30.
  • the shaft 30 extends along a central longitudinal axis of the container 2 proximate the container base 25.
  • the diameter of the helical screw may be in the region of about 200 to 500 mm, so that there is free space within the container between the opposite long side walls 21 , 22 and the screw conveyor 28.
  • the shaft 30 extends through apertures (not shown) in the opposite short end walls 23, 24 to be supported at bearings 31 , 32 that permit the shaft 28 to rotate freely.
  • One end 33 of the shaft is engaged by an electric motor 34 that turns the shaft.
  • the sense of rotation with respect to the helical screw 29 is such that waste biomass in proximity with the screw conveyor 28 is conveyed towards the short end 23 of the container from which the shaft 30 is driven by the motor 34.
  • Water from the water inlet pipe 6 may be introduced into the container using any convenient means, for example at an inlet port 36 that is preferably in an upper portion of the container.
  • the pre-digester 2 also includes a slurry circulation system 40 comprising a slurry outlet 41 from the container 18, a slurry inlet 42 to the container and a pump 43 external to the container for circulating biomass slurry from the outlet to the inlet.
  • the circulated slurry passes from the outlet to the pump through a downstream conduit pipe 44 and from the pump to the inlet 42 through an upstream conduit pipe 45.
  • the upstream conduit 45 includes a diverter valve 46 that is initially set to send circulated slurry from the pump 43 to the inlet.
  • the diverter valve is set to divert slurry flow from the pump 43 to the CSTR slurry feed pipe 16 in order to discharge slurry from the container volume.
  • the recirculation system outlet 41 and inlet are provided at opposite ends of the elongate container, referred to as an upstream end 3 and a downstream end 5 relative to the flow driven by the screw conveyor, so that circulated slurry discharged by the slurry inlet 42 replenishes waste biomass slurry within the contained that has been moved generally towards the slurry outlet by the action of the conveyor.
  • Waste biomass slurry may also be redistributed by a flow of slurry opposite to the flow driven by the screw conveyor 28 in the space between the helical screw 29 and the lateral side walls 21 , 22 or in the volume of slurry that lies above the helical screw 29.
  • the pump is preferably a waste water pump capable of passing biomass solids drawn into the container outlet 41. Moving parts within the pumping mechanism (not shown) help to macerate any such recirculated solids, which speeds up the process of reducing the waste biomass slurry to a desired consistency and a desired maximum particulate size.
  • the slurry is heated up inside the CSTR 4 by an internal heating coil (not shown) fed by a pair of ports 47, 47' in the vessel 48 until this reaches 80 0 C while maintaining a pressure inside the CSTR of 2.5 bar, in order to sterilize the slurry.
  • the slurry is circulated within the CSTR 4 by means of an internal paddle (not shown) near the bottom of the cylinder 48, and a bottom to top circulation by means of an external arrangement of pipes 50.
  • the pipe arrangement is shown most clearly in the schematic representation of the CSTR 4 shown in Figure 5, and includes a plurality of outlets 51 evenly spaced around an upper circumference of the cylinder 48, and a corresponding plurality of inlets 52 spaced around an upper circumference of the cylinder.
  • the outlets and inlets 51 , 52 are each connected together by a ring manifold which is itself connected at opposite connection points 55, 55', 56, 56' of the ring to a pair of upper and lower connecting pipes 53, 54. This is done to even out any pressure differences at the four inlets and outlets, and so these connection points 55, 55', 56, 56' are equidistant between a pair of adjacent inlets 52 or outlets 51.
  • This external circulation is driven by an external electric pump 58 in line with a pipe 59 that connects the upper and lower connecting pipes 53, 54.
  • the tank After the contents of the vessel 48 have reached a stable 80 0 C, the tank is allowed to cool. Cooling may be speeded up by passing a coolant though the internal coil. The contents of the vessel 48 are then allowed to cool to 35 0 C after which one or more type of enzyme is added and the temperature is maintained for up to 6 to 8 hours.
  • the slurry is continuously mixed during this time by the internal rotary paddle and external circulation network. During this time, the enzymes break down the cellulose or starch molecules in the slurry to release simple C5 and C6 sugars into the slurry.
  • sugar slurry is then pumped out of the tank through a drain pipe 60 into a buffer tank 62. If necessary, sugar slurry from the buffer tank 62 can be returned to any one of the CSTRs by means of a return sugar pipe 65, 64 and associated control valves 66.
  • the invention When the invention is used in a pre-digestion stage of a chemical plant as described above, it is possible to achieve near continuous processing of waste biomass and to significantly reduce the time needed to pre-digest the waste biomass.
  • the invention helps to achieve a low overall particulate size which greatly helps to speed up subsequent chemical or biochemical processing of the waste biomass slurry.
  • the invention provides a more reliable and consistent method and apparatus for waste biomass pre-digestion.

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Abstract

The present invention relates to the bioconversion of biomass into useful chemicals, for example, chemicals such as alcohols for the production of liquid fuels, and in particular to an apparatus and method for waste digestion of biomass. A waste biomass pre-digester (2) for producing a waste biomass slurry from waste biomass, the pre-digester comprises a container(18) for containing waste biomass that is bounded by a floor (25) and surrounding sides (21-24) and which is accessible by means of an opening (26) by which waste biomass may be introduced to the container. The pre-digester also comprises a screw conveyor (28) within the container (18) for macerating waste biomass, and a slurry circulation system (40) comprising at least one slurry outlet (41) from the container volume, at least one slurry inlet (42) to the container volume, and a pump (40) for circulating biomass slurry from said outlet(s) to said inlet(s).

Description

Apparatus and Method for Waste Biomass Pre-Digestion
BACKGROUND
a. Field of the Invention
The present invention relates to the bioconversion of biomass into useful chemicals, for example, chemicals such as alcohols (middle distillates) for the production of liquid biofuels, and in particular to an apparatus and method for waste digestion of biomass.
b. Related Art
There is increasing interest in the conversion of biomass recovered from waste into useful chemicals, such as such as carboxylic acids (e.g., acetic, propionic, butyric acid), ketones (e.g., acetone, methyl ethyl ketone, diethyl ketone) and biofuels, such as a mixture of primary alcohols (e.g., ethanol, propanol, butanol) and/or a mixture of secondary alcohols (e.g., isopropanol, 2-butanol, 3-pentanol).
In view of the increasing scarcity of crude-oil and natural gas, there is a particular interest in various processes for converting biomass waste into mixed alcohol fuels.
In a fermentation process, enzyme digestion of starch or cellulose in biomass feedstock produces simple sugars which are then fermented by yeast to produce ethanol or other simple alcohols.
In another process, biomass feedstock is converted into organic chemicals and alcohols in a multi-stage process that includes lime pre-treatment, non-sterile acidogenic digestion, product concentration, thermal conversion to ketones and their subsequent hydrogenation to create mixed alcohol end products. An advantage of this process is that more energy from the biomass will end up as liquid fuels than in converting biomass to ethanol by yeast fermentation. AII such biological/chemical processes require a liquefied biomass feedstock having a certain consistency in terms of its water content and the range of solid particulate sizes. This biomass slurry feedstock will normally be produced from domestic or industrial biomass waste, and so may be composed of a wide range of possible materials having differing physical sizes and properties. The process for producing this slurry is referred to in this description as "pre-digestion". A number of problems have been noted with known methods and equipment for the predigestion of biomass waste. The predigestion process is but the first stage in a multistage biochemical/chemical plant. The total throughput of the plant can be limited by any one stage, and so it is sometimes the case that there may be insufficient time to generate a sufficiently consistent and smooth biomass slurry. If particulate matter in the slurry is not sufficiently fine grained, then either the yield or the conversion time of the next stage, for example enzyme conversion into simple sugars, may be adversely affected.
It is an object of the present invention to provide a more reliable and consistent method and apparatus for waste biomass pre-digestion.
SUMMARY OF THE INVENTION
According to the invention, there is provided a waste biomass pre-digester for producing a waste biomass slurry from waste biomass, the pre-digester comprising: a container for containing waste biomass, the container having an internal volume, said volume being bounded by a floor and surrounding sides and being accessible by means of an opening by which waste biomass may be introduced to the container volume; - a screw conveyor within the container for macerating waste biomass; a slurry circulation system comprising at least one slurry outlet from the container volume, at least one slurry inlet to the container volume, and a pump for circulating biomass slurry from said outlet(s) to said inlet(s); and the screw conveyor is arranged to convey biomass waste towards said slurry inlet(s).
In a preferred embodiment of the invention, the screw conveyor is an open helical screw, so that waste biomass can come into contact with the screw conveyor around substantially the complete circumference of the screw conveyor.
The combined action of the screw conveyor and pump circulation helps to homogenise waste biomass within the container. The screw conveyor moves waste biomass around inside the container, with the turning action of the screw acting to macerate waste biomass, while the circulation pump redistributes biomass slurry between the inlet and outlet. In addition, any solids in the biomass slurry drawn into the circulation system pump will be further macerated by the movement of the slurry between the inlet and outlet, and preferably also by the pumping action of the pump.
The screw conveyor is arranged to convey biomass waste towards said slurry inlet(s), and also the screw conveyor is arranged to convey biomass waste away from said slurry outlet(s). This helps keep the level of biomass slurry within the container relatively constant.
The circulation system can also be used to help redistribute biomass slurry between upper and lower portions of the container. Preferably, the (or each) slurry outlet is at a higher level than the (or each) slurry inlet, relative to the floor of the container volume or relative to the level of slurry within the container.
The container volume is preferably elongate, with the screw conveyor being oriented with the length of the container volume. In a preferred embodiment of the invention, the screw conveyor is open on all sides and is aligned along a central long axis of the container volume, and separated from lateral sides of the container by a distance at least as great as the diameter of the screw conveyor. Waste biomass within the container that is pushed to one end of the container volume can then recirculate back to the other end of the container volume in the space between the screw conveyor and the lateral sides of the container.
Preferably, the screw conveyor is also in a lower portion of the conveyor proximate the container floor so that, in use, with waste biomass completely covering the screw conveyor, waste biomass within the container that is pushed to one end of the container volume can then recirculate back to the other end of the container volume in the space between the screw conveyor and the top of the waste biomass within the container.
Also according to the invention, there is provided a method of producing a waste biomass slurry from waste biomass using a waste biomass pre-digester comprising a container, a screw conveyor within the container, and a slurry circulation system including a circulating pump, the method comprising the steps of: introducing waste biomass and liquid water into the container; using the screw conveyor to macerate said waste biomass within the container; and - using the slurry circulation system to withdraw said macerated waste biomass from a first portion of the container proximate an upstream end of the screw conveyor and to reintroduce said macerated waste into a second portion of the container proximate a downstream end of the screw conveyor.
Preferably, the circulation pump further macerates said macerated waste biomass during said circulation.
The screw conveyor is oriented to convey macerated waste biomass towards the first portion of the container.
The slurry circulation system may include a diverter valve upstream of the circulation pump. The diverter valve is then initially set in a first setting to direct circulated macerated waste biomass towards the second portion of the container until said macerated waste biomass achieves a desired consistency. The diverter valve is then set in a second setting to direct circulated macerated waste biomass towards an outlet to the waste biomass pre-digester, for subsequent chemical or biochemical processing to produce useful chemicals.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which:
Figure 1 is a block schematic view of a pre-digestion stage of a chemical plant, having a plurality of waste biomass pre-digesters, each one of which feeds a continuously stirred tank reactor (CSTR);
Figure 2 is a perspective view of one of the pre-digesters of Figure 1 ;
Figures 3 and 4 are side views of the CSTR of Figure 1 ; and
Figure 5 is a perspective view of the CSTR showing external circulation pipe work.
DETAILED DESCRIPTION
Figure 1 shows a pre-digestion stage of a chemical plant 1 , having a plurality of waste biomass pre-digesters 2, each one of which feeds a continuously stirred tank reactor (CSTR) 4. The plant 1 has a water inlet pipe 6 that is used to feed water as required into the pre-digesters 2 and CSTRs 4 by means of inlet control valves 8, one for each pre-digester 2, each protected by its own non-return valve 10. Water may also be fed as required into the CSTRs by means of inlet control valves 12, one for each CSTR, all protected by a common non-return valve 14.
As will be explained in more detail below, each pre-digester 2 is run to produce a water-based slurry of finely macerated waste biomass. Each pre-digester is connected to a corresponding CSTR by a slurry feed pipe 16 by which the waste biomass slurry is fed into the CSTR.
Figure 2 shows the pre-digester 2 in more detail. The pre-digester 2 has a container 18 that is used to contain a water-based waste biomass slurry. Although not illustrated, the slurry may be formed using any type of useful waste biomass, for example biomass that contains cellulose or carbohydrates, for example waste wood, plant matter, vegetable matter, cardboard or paper.
The container 18 is elongate, in this example having a rectangular outline that is 5 metres long (L), 2.5 metres wide (W) and 1.6 metres high (H), formed from steel plates along each of the two long sides 21 , 22 and two short sides 23, 24 and base 25. The container has an opening in the form of an open top 26 by which waste biomass may be dumped into the container, for example using a fork lift truck or belt conveyor (not shown). The total volume of the enclosure formed by the sides 21-24 and base 25 is about 20,000 litres.
The container 18 is drawn partly cut away in order to show inside the container 2 a screw conveyor 28 for circulating and macerating waste biomass within the container. The screw conveyor 28 has an open helical screw 29 that extends from a central shaft 30. The shaft 30 extends along a central longitudinal axis of the container 2 proximate the container base 25. The diameter of the helical screw may be in the region of about 200 to 500 mm, so that there is free space within the container between the opposite long side walls 21 , 22 and the screw conveyor 28.
The shaft 30 extends through apertures (not shown) in the opposite short end walls 23, 24 to be supported at bearings 31 , 32 that permit the shaft 28 to rotate freely. One end 33 of the shaft is engaged by an electric motor 34 that turns the shaft. The sense of rotation with respect to the helical screw 29 is such that waste biomass in proximity with the screw conveyor 28 is conveyed towards the short end 23 of the container from which the shaft 30 is driven by the motor 34.
Water from the water inlet pipe 6 may be introduced into the container using any convenient means, for example at an inlet port 36 that is preferably in an upper portion of the container.
The pre-digester 2 also includes a slurry circulation system 40 comprising a slurry outlet 41 from the container 18, a slurry inlet 42 to the container and a pump 43 external to the container for circulating biomass slurry from the outlet to the inlet. The circulated slurry passes from the outlet to the pump through a downstream conduit pipe 44 and from the pump to the inlet 42 through an upstream conduit pipe 45. The upstream conduit 45 includes a diverter valve 46 that is initially set to send circulated slurry from the pump 43 to the inlet. Once the waste biomass slurry in the container 18 achieves the desired degree of consistency and maximum particulate size, the diverter valve is set to divert slurry flow from the pump 43 to the CSTR slurry feed pipe 16 in order to discharge slurry from the container volume.
The recirculation system outlet 41 and inlet are provided at opposite ends of the elongate container, referred to as an upstream end 3 and a downstream end 5 relative to the flow driven by the screw conveyor, so that circulated slurry discharged by the slurry inlet 42 replenishes waste biomass slurry within the contained that has been moved generally towards the slurry outlet by the action of the conveyor.
Waste biomass slurry may also be redistributed by a flow of slurry opposite to the flow driven by the screw conveyor 28 in the space between the helical screw 29 and the lateral side walls 21 , 22 or in the volume of slurry that lies above the helical screw 29. The pump is preferably a waste water pump capable of passing biomass solids drawn into the container outlet 41. Moving parts within the pumping mechanism (not shown) help to macerate any such recirculated solids, which speeds up the process of reducing the waste biomass slurry to a desired consistency and a desired maximum particulate size.
Reference is now made also to Figures 3, 4 and 5. When the waste biomass slurry is ready to be pumped into the adjacent CSTR 4, the screw conveyor is switched off, and the diverter valve 46 set to cause the pump 43 to empty the container 18 into the CSTR 4, which includes a generally cylindrical stainless steel pressure vessel 48. The waste biomass slurry may be introduced into the vessel 48 by any convenient means, and in this example a hatch 16' in the top of the vessel is opened to permit the vessel to be filled using a hose (not shown) that leads from the pre-digester discharge outlet 16.
When this is done, the slurry is heated up inside the CSTR 4 by an internal heating coil (not shown) fed by a pair of ports 47, 47' in the vessel 48 until this reaches 80 0C while maintaining a pressure inside the CSTR of 2.5 bar, in order to sterilize the slurry. During this process, the slurry is circulated within the CSTR 4 by means of an internal paddle (not shown) near the bottom of the cylinder 48, and a bottom to top circulation by means of an external arrangement of pipes 50.
The pipe arrangement is shown most clearly in the schematic representation of the CSTR 4 shown in Figure 5, and includes a plurality of outlets 51 evenly spaced around an upper circumference of the cylinder 48, and a corresponding plurality of inlets 52 spaced around an upper circumference of the cylinder. In this example there are four such inlets and outlets. The outlets and inlets 51 , 52 are each connected together by a ring manifold which is itself connected at opposite connection points 55, 55', 56, 56' of the ring to a pair of upper and lower connecting pipes 53, 54. This is done to even out any pressure differences at the four inlets and outlets, and so these connection points 55, 55', 56, 56' are equidistant between a pair of adjacent inlets 52 or outlets 51. This external circulation is driven by an external electric pump 58 in line with a pipe 59 that connects the upper and lower connecting pipes 53, 54.
After the contents of the vessel 48 have reached a stable 80 0C, the tank is allowed to cool. Cooling may be speeded up by passing a coolant though the internal coil. The contents of the vessel 48 are then allowed to cool to 35 0C after which one or more type of enzyme is added and the temperature is maintained for up to 6 to 8 hours. The slurry is continuously mixed during this time by the internal rotary paddle and external circulation network. During this time, the enzymes break down the cellulose or starch molecules in the slurry to release simple C5 and C6 sugars into the slurry.
The sugar slurry is then pumped out of the tank through a drain pipe 60 into a buffer tank 62. If necessary, sugar slurry from the buffer tank 62 can be returned to any one of the CSTRs by means of a return sugar pipe 65, 64 and associated control valves 66.
When the invention is used in a pre-digestion stage of a chemical plant as described above, it is possible to achieve near continuous processing of waste biomass and to significantly reduce the time needed to pre-digest the waste biomass. The invention helps to achieve a low overall particulate size which greatly helps to speed up subsequent chemical or biochemical processing of the waste biomass slurry. The invention provides a more reliable and consistent method and apparatus for waste biomass pre-digestion.

Claims

1. A waste biomass pre-digester for producing a waste biomass slurry from waste biomass, the pre-digester comprising: - a container for containing waste biomass, the container having an internal volume, said volume being bounded by a floor and surrounding sides and being accessible by means of an opening by which waste biomass may be introduced to the container volume; a screw conveyor within the container for macerating waste biomass; - a slurry circulation system comprising at least one slurry outlet from the container volume, at least one slurry inlet to the container volume, and a pump for circulating biomass slurry from said outlet(s) to said inlet(s); and the screw conveyor is arranged to convey biomass waste towards said slurry inlet(s).
2. A waste biomass pre-digester as claimed in Claim 1 , in which the screw conveyor is arranged to convey biomass waste away from said slurry outlet(s).
3. A waste biomass pre-digester as claimed in any preceding claim, in which the (or each) slurry outlet is at a higher level than the (or each) slurry inlet.
4. A waste biomass pre-digester as claimed in any preceding claim, in which the container volume is elongate, and the screw conveyor is oriented with the length of the container volume.
5. A waste biomass pre-digester as claimed in any preceding claim, in which the container volume is elongate, and said slurry outlet(s) and said slurry inlet(s) are provided at opposite ends of the container volume.
6. A waste b iomass pre-digester as claimed in any preced ing claim, comprising additionally a motor external to the container volume for turning the screw conveyor.
7. A waste biomass pre-digester as claimed in any preceding claim, in which the screw conveyor is provided in a lower portion of the container volume proximate the container floor.
8. A waste biomass pre-digester as claimed in any preceding claim, in which the screw conveyor includes a helical screw to which, in use, waste biomass introduced to the container volume is fully exposed.
9. A waste biomass pre-digester as claimed in any preceding claim, in which the slurry circulation system includes a diverter valve between the circulation pump and the slurry inlet(s) to the container by which slurry circulated through the pump may be pumped away from the waste biomass pre-digester in order to discharge slurry from the container volume.
10. A waste biomass pre-digester as claimed in any preceding claim, in which the opening to the container volume is an open top to the container volume.
11. A method of producing a waste biomass slurry from waste biomass using a waste biomass pre-digester comprising a container, a screw conveyor within the container, and a slurry circulation system including a circulating pump, the method comprising the steps of: introducing waste biomass and liquid water into the container; using the screw conveyor to macerate said waste biomass within the container; and using the slurry circulation system to withdraw said macerated waste biomass from a first portion of the container proximate an upstream end of the screw conveyor and to reintroduce said macerated waste into a second portion of the container proximate a downstream end of the screw conveyor.
12. A method as claimed in Claim 11 , in which the circulation pump further macerates said macerated waste biomass during said circulation.
13. A method as claimed in Claim 11 or Claim 12, in which the slurry circulation system includes a diverter valve upstream of the circulation pump, the method comprising the steps of: - setting the diverter valve in a first setting to direct circulated macerated waste biomass towards the second portion of the container until said macerated waste biomass achieves a desired consistency; and then setting the diverter valve in a second setting to direct circulated macerated waste biomass towards an outlet to the waste biomass pre-digester.
PCT/IB2009/055550 2008-12-10 2009-12-07 Apparatus and method for waste biomass pre-digestion WO2010067302A2 (en)

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GB2466048B (en) 2012-03-21

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