PROCESSING OF LIGNOCELLULOSIC BIOMASS

Abstract

A plant and process for performing thermolysis of cellulose uses a thermolysis reactor which comprises a reactor duct having a longitudinal axis, and an eductor at one end of the reactor duct. The eductor has an entry chamber and a venturi-shaped exit channel, a nozzle, a sloping deflector plate above the nozzle, and an inlet port through which particulate material may be fed onto the deflector plate and into the entry chamber. The nozzle and the venturi-shaped exit channel are aligned with the longitudinal axis. A superheater provides superheated steam at a temperature above 450° C. to flow through the nozzle. The particulate matter is entrained in the flowing steam, and undergoes thermolysis. This may be combined with a pre-treatment to hydrolyse hemicellulose, so that lignocellulosic biomass can be processed.

Claims

1. A plant for performing thermolysis of cellulose comprises a thermolysis reactor, the thermolysis reactor comprising a reactor duct having a longitudinal axis, and an eductor at one end of the reactor duct, the eductor comprising an entry chamber and a venturi-shaped exit channel, a nozzle, a sloping deflector plate above the nozzle, and an inlet port through which particulate material may be fed onto the deflector plate and into the entry chamber, the nozzle and the venturi shaped exit channel both being aligned with the longitudinal axis of the reactor duct, and means for providing superheated steam at a temperature above 450° C. and for supplying the superheated steam to flow through the nozzle.

2. The plant as claimed in claim 1, also comprising a hopper with a twin screw conveyor whereby the particulate material is fed into the inlet port.

3. The plant as claimed in claim 1, wherein at least the downstream end of the reactor duct incorporates a screw mechanism having screw flights, the screw flights having a diameter less than 70% of the reactor duct.

4. The plant as claimed in claim 3, wherein the flights along part of the screw are left-handed while those along the remainder of the screw are right-handed, so rotation of the screw conveys the particulate material to an intermediate position along the length of the screw.

5. The plant as claimed in claim 1, also comprising a cyclone downstream of the reactor duct to remove and collect entrained particulate material.

6. The plant as claimed in claim 5, wherein at least the downstream end of the reactor duct incorporates a screw mechanism having screw flights, the screw flights having a diameter less than 70% of the reactor duct and wherein the downstream end of the reactor duct communicates with a Y-shaped junction, defining an outlet duct for the gases which is unobstructed and, at least initially, of the same cross-sectional area as the reactor duct.

7. The plant as claimed in claim 1, also comprising a two-stage condenser, each stage utilising direct contact between an aqueous liquid and the gas/vapour stream, and the first stage cooling the gas/vapour stream only sufficient to achieve saturation.

8. The plant as claimed in claim 7, wherein the second stage of the condenser incorporates a stack of trays onto which coolant liquid is fed, to cascade in counter current to the flow of gas/vapour.

9. A process for performing thermolysis of cellulose in particulate matter, using the plant as claimed in claim 1.

10. The process as claimed in claim 9, wherein in operation of the plant steam is arranged to flow through the reactor at above 10 m/s.

11. The process as claimed in claim 9, wherein the modulus, that is to say the ratio between the mass flow of steam and the mass flow of particulate matter, is at least 3.

12. The process as claimed in claim 11 wherein the length of the reactor duct and the steam velocity are such as to ensure that the particles of particulate material reach the end of the reactor duct in less than 1 s.

13. A plant to process particulate biomass, comprising a hemicellulose hydrolysis reactor, means to feed the particulate biomass into the hemicellulose hydrolysis reactor; means to separate water and water-soluble compounds from a resulting particulate material, and means to feed the resulting particulate material to the cellulose thermolysis plant as claimed in claim 1.

14. The process as claimed in claim 9, wherein the particulate material is obtained from particulate biomass, by subjecting the biomass to hemicellulose hydrolysis in a hemicellulose hydrolysis reactor, and then separating water and water-soluble compounds from resulting particulate material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:

[0025] FIG. 1 shows a flow diagram for a plant and process of treating lignocellulosic biomass, including a thermolysis reactor of the invention;

[0026] FIG. 2 shows a schematic longitudinal cross-sectional view of the thermolysis reactor of FIG. 1, showing the inlet end of the thermolysis reactor;

[0027] FIG. 3 shows a flow diagram of the steam supply system for the thermolysis reactor of FIG. 1;

[0028] FIG. 4 shows a perspective view of a hopper to feed particulate matter into the thermolysis reactor of FIG. 1;

[0029] FIG. 5 shows a longitudinal cross-sectional view of the downstream end of the thermolysis reactor;

[0030] FIG. 6 shows a side view of a screw of the downstream end of the thermolysis reactor;

[0031] FIG. 7 shows a longitudinal cross-sectional view of the condensers of FIG. 1, along with the associated flow diagram; and

[0032] FIG. 8 shows a perspective view of a tray forming part of one of the condensers of FIG. 7.

DETAILED DESCRIPTION

[0033] Referring to FIG. 1, the present invention provides a process and plant 10 for treating biomass 11 such as wood chips so as to obtain C5 and C6 sugars by breaking down the hemicellulose and cellulose polymers within the biomass. The process uses two different reaction steps that are performed at different temperatures, and may also be carried out at different pressures, but both use water as the reactant. However, prior to performing the reaction steps the biomass may be chopped into small pieces, and may be heated to evaporate vapours for example of naturally-occurring oils such as turpentine or eucalyptus oil if these are present in significant concentration.

[0034] After performing any such pre-treatment, the biomass is impregnated 12 with a strong acid, for example with dilute sulphuric acid (i.e. about 1 mole/L) typically at a rate of between 1-2 wt % of the dry biomass, before being introduced by a screw conveyor into a reactor 14 in which the biomass is contacted with steam/water at a temperature of between 150° and 180° C. and a pressure of between 6 bar and 10 bar, for example at 165° C. and a pressure of 6.5 bar (gauge); there is little air present. This may be a tube reactor along which the biomass travels along with the high-pressure steam/water. Under these reaction conditions the hemicellulose breaks down mainly to form C5 sugars, with also some C6 sugars, and organic acids, depending on the inherent composition of the biomass being processed.

[0035] At the operating pressure of 6.5 bar, the water at 165° C. is liquid as long as the pressure is maintained. As the material leaves the reactor 14 it may be cooled by depressurisation, exploding the biomass. A proportion of the water evaporates, typically about 10%, cooling the remaining biomass to below 100° C. For example the mixture of biomass and hot water may be released in slugs from the reactor. As mentioned above, it is necessary to remove the liquid phase from the treated biomass before it can be subjected to the second reaction step, both to remove sugars and to remove alkali material and any inorganic acid. Although some of the liquid may be removed by compression, this doesn't enable all the liquid to be expelled. It is therefore preferable to remove the liquid phase by a washing step.

[0036] The solid material that has been subjected to the hydrolysis step in the reactor 14 is therefore then washed at step 16, for example using an aqueous solution, and/or clean water, as indicated at 15.

[0037] The washed material is then dried at step 17 to evaporate excess moisture, and may be further comminuted (not shown) to ensure all the particles small enough to heat up quickly. The material is then introduced into a hopper 18 with a twin screw outlet, and thence into a thermolysis reactor 20 in which the solid material is entrained into a flow of superheated steam at a significantly higher temperature, for example 550° C. The particles of solid material cool the superheated steam while being heated up, and typically reach a final temperature of 370° C. to 450° C. The particles are effectively subjected to a temperature in the range for example 370° C. to 410° C. or 420° C. for a short period which may for example be between 0.4 seconds and 1 second. Under these reaction conditions the cellulose undergoes thermolysis, mainly producing C6 sugar derivatives which are volatile under these conditions. After passing through the thermolysis reactor 20 the particulate material, which at this stage is a solid lignin char 24, may be separated from the vapours and gases by passing through a cyclone 21, and the vapours then condensed in a condenser 22. A fan 23 ensures the through-flow of any uncondensed vapours or steam and any non-condensable gases, such as air entrained in the particulate material.

[0038] The condensed vapours create an output stream 25 which is an aqueous solution of the products of degradation or thermolysis of the cellulose, which will primarily consist of C6 sugars, typically in an anhydrous form. For example the products may be anhydroglucose (which is also called levoglucosan), but there may also be phenolic-substituted glucose, and phenolic-substituted anhydrosugars as well as oligomeric anhydrosugars and sugar oligomers.

[0039] This output stream 25 of condensed vapours from the cellulose thermolysis reactor 20 may optionally be fed back (not shown), and used in the washing step 16. In that case the resulting aqueous solution 26 contains the C5 sugars produced during hemicellulose hydrolysis, and also the C6 sugars produced during cellulose thermolysis, and at least the latter may be in an anhydrous form. The aqueous solution 26 also contains the bulk of the acid added at step 12 before the biomass was treated in the hemicellulose hydrolysis reactor 14. Alternatively, as indicated in FIG. 1, the aqueous solution 25 containing the products from cellulose thermolysis and the aqueous solution 26 containing the products of hemicellulose thermolysis may be treated as separate product streams.

[0040] Referring now to FIG. 2, which shows a longitudinal view of the thermolysis reactor 20, showing the inlet end of the reactor 20. The reactor 20 comprises a tubular reactor duct 30 with a longitudinal axis 31, which may for example comprise a stainless steel tube of diameter 0.26 m and of length 6.0 m. An eductor 32 is attached to one end of the duct 30. The eductor 32 comprises an entry chamber 33 with an inlet port 34, a nozzle 36 for inflow of steam, and a venturi-shaped exit duct 38 whose diameter narrows down and then broadens out; the end of the venturi-shaped exit duct 38 is attached to one end of the reactor duct 30. The nozzle 36 and the exit duct 38 are aligned with the longitudinal axis 31 of the reactor duct 30. A sloping deflector plate 40 is mounted above the open end of the nozzle 36.

[0041] In operation of the thermolysis reactor 20, particulate material from the hopper 18 is fed at a steady rate of 175 kg/h into the inlet port 34, and steam at 550° C. is supplied at 700 kg/h through the nozzle 36, corresponding to a modulus of 4. This ensures a steam flow at 12 m/s along the reactor duct 30. The steam is supplied to the nozzle at 130 kPa (1.3 bar(a)), generating a pressure reduction of about 2 kPa (20 mbar) below the inlet port 34. The deflector plate 40 ensures the particles do not contact the hot surfaces of the nozzle 36 itself, but fall down under the influence of the pressure reduction, into the jet of steam from the nozzle 36, and are entrained into the flowing steam. The steam pressure within the entry chamber may be 100 kPa (1.0 bar(a)), and at the other end of the reactor duct 30 the steam pressure drops to about 95 kPa (0.95 bar(a)). Arrow A indicates the steam flow, arrow B represents the flow of particulate material, and arrow C represents the resultant flow of these reagents and of thermolysis products.

[0042] Referring now to FIG. 3, the steam supply system 42 receives a supply of saturated steam, indicated at 43, at 1.0 MPa (10 bar(a)) from a conventional boiler (not shown). The saturated steam is passed through a pressure reducing valve 44 to throttle the steam down to about 150 kPa (1.5 bar(a)). The flow rate of the steam is primarily defined by the restriction imposed by the nozzle 36, but in addition, in this example, upstream of the pressure reducing valve 44 is a flow control valve 45 which operates in response to a flowmeter 46 to further ensure the required mass flow of steam. The steam is then passed through a superheater 47 to raise its temperature to 550° C., or slightly above (for example 570° C.) if it is necessary to compensate for any significant temperature drop between the outlet of the superheater 47 and the outlet of the nozzle 36. A pressure sensor 48 downstream of the superheater 47 works in conjunction with the pressure reducing valve 44 to ensure that the steam pressure supplied to the nozzle 36 of the eductor 32 is consistently at 130 kPa (1.3 bar(a)).

[0043] To monitor and control operation, the steam supply system 42 also includes temperature sensors 49a and 49b to measure the temperature of the steam before and after passage through the superheater 47; and there is also a pressure relief valve 49c which would release pressure if it exceeded a safe value; this may for example be set at 200 kPa (2 bar(a)).

[0044] Referring now to FIG. 4, the hopper 18 comprises a chamber 50 of tapered form, supported on weight sensors 52 (only two are shown) so the mass of dispensed material can be monitored and controlled. At the base, which is the narrower end of the chamber 50, is a twin screw outlet 54, arranged to feed the particulate material into the inlet port 34 of the eductor 32. The twin screw outlet 54 is enclosed, so minimal air is entrained in the particulate material fed into the inlet port 34 (this enclosure is not shown).

[0045] Referring now to FIG. 5, this shows the downstream end of the thermolysis reactor 20. The downstream end of the tubular reactor duct 30 is connected to an end fitting 60 which is generally Y -shaped, with a first portion 61 aligned with the longitudinal axis 31 of the tubular reactor duct 30, and with a diverging portion 62 inclined at 135° to the longitudinal axis 31. The cross-sectional area of the diverging portion 62 is substantially the same as that of the tubular reactor duct 30, and provides an unobstructed flow path. The end of the first portion 61 is closed, and within the first portion 61 is mounted a screw 64 driven by an external motor 65. There is an outlet port 66 at the midpoint of the first portion 61, in the bottom wall, and this outlet port 66 is closed by a rotary valve 68. Referring also to FIG. 6, the screw 64 has screw flights with a diameter 60% of the tubular reactor duct 30, the flights along the section of the first portion 61 closer to the tubular reactor duct 30 being of opposite hand to those closer to the closed end of the first portion 61, such that rotation of the screw 64 by the motor 65 displaces any particulate matter that settles on the lower walls of the first portion 61 towards the outlet port 66.

[0046] Referring again to FIG. 5, the diverging portion 62 is joined to a connecting duct 69 which curves back to horizontal and gradually tapers in width (not shown in the side view of the figure), and the cross-section gradually changes to rectangular, being connected at its other end to a rectangular inlet port 70 of the cyclone 21, being aligned so as to feed the gases, vapours and solid particles tangentially into the cyclone 21. Gases and vapours emerge through an outlet port 72 at the top of the cyclone 21, while particulate material collects at the tapered bottom portion of the cyclone 21, where there is an outlet 74 with a rotary valve 75.

[0047] The desired reaction of the thermolysis of cellulose to produce sugars takes place under these conditions very quickly, typically in less than 1 second. If the particulate material is exposed to the reaction conditions (i.e. the high temperature steam) for significantly longer, the lignin will degrade to produce unwanted organic compounds such as phenols, and may also produce tar. It is therefore advantageous to remove the particulate material from contact with the high temperature steam once the desired reaction has taken place. Hence during operation the screw 64 is rotated continuously to transfer any deposited particulate material to the outlet port 66, and the rotary valve 68 is operated continuously to remove any such particulate material and so to prevent further degradation; and similarly the rotary valve 75 at the base of the cyclone 21 is operated continuously to remove the particulate material from the cyclone 21. The rotary valves 68 and 75 significantly inhibit any flow of air into the thermolysis reactor 20 or the cyclone 21.

[0048] Operation of the thermolysis reactor 20 leads to the formation of particles of lignin char. Some of these particles may settle out at the end fitting 60, in which case they can be removed through the gate valve 66. Most of the particles are carried along with the high temperature gas and vapour stream along the diverging portion 62, and so are fed through the connecting duct 69 into the cyclone 21. Within the cyclone 21 all but the smallest of particles settle out, so the gas and vapour stream emerging from the cyclone 21 contains only very small particles, if any.

[0049] Referring now to FIG. 7, the condenser 22 in this case is a two-stage condenser using direct contact with cooling liquid, comprising a first condenser 80 and a second condenser 82 which are both mounted on a support frame 83. The first condenser 80 may act as a de-superheating condenser; it is a vertical cylindrical chamber, the flow of gases, vapours and any remaining particles (as indicated by arrows C) being fed to near the bottom of the chamber to emerge from the top. Water is sprayed into the gas stream from spray nozzles 84 near the top of the chamber; and an outlet 85 near the bottom allows water to be recycled as indicated by arrow R. The bottom portion 86 of the chamber tapers down to an outlet 87 which is usually closed by a valve 88.

[0050] The spray nozzles 84 may be fed either with mains water, indicated by arrow W or by recycled water indicated by arrow R. When operation starts, mains water is used, but during steady-state operation the water is primarily recycled water. The quantity of water sprayed is only enough to cool the steam so that it is saturated. This de-superheating condenser 80 will also tend to trap any particles, which will therefore collect in the bottom portion 86, and can occasionally be withdrawn through the valve 88.

[0051] The second condenser 82 is a significantly larger vertical cylindrical chamber, and the de superheated gases and vapours from the first condenser 80, indicated by arrow C, are fed into an inlet port 90 towards the bottom of the chamber, but above the water level 92 during operation. Remaining gases and vapours leave the second condenser 82 from the top, indicated by arrow G, and this flow is ensured by the fan 23 (shown in FIG. 1).

[0052] The second condenser 82 is provided with three inlet ports 94 spaced apart along its length, each port 94 being a short distance above a horizontal tray 96 within the chamber, each tray 96 being clamped between flanges of tubular elements that form the cylindrical chamber. Each port 94 is closed by an end plate 97 and is provided with a water inlet 98 to feed water onto the tray 96. In operation water is continuously cascading down from the trays 96, so the gas flow is counter current to the falling water. Initially, as indicated by arrow W, the water may be mains water, but during normal operation the water provided to each water inlet 98 is recycled water, indicated by arrow R, which is an aqueous solution of the products produced by the cellulose thermolysis.

[0053] During operation the quantity of water provided to the second condenser 82 is such as to ensure that the outgoing gases, indicated by arrow D, remain at a temperature of at least 60° C. Nevertheless the bulk of the steam provided to the thermolysis reactor 20 will be condensed during its passage through the second condenser 82.

[0054] An outlet duct 100 connects to the bottom of the chamber, and connects to a pressure sensor 102 and to an outlet valve 104 which is normally closed. The recycled water which is provided to both the spray nozzles 84 and the water inlets 98 is taken from outlet ports 106 near the bottom of the chamber, being recirculated for example by a pump 108. During operation the concentration of the products of thermolysis of cellulose in the aqueous solution will at least initially increase as this solution is recirculated, and the total quantity of water will gradually increase. When the liquid level 92 reaches a preset threshold, as sensed by the pressure sensor 102, a valve 109 is opened so that some of the aqueous solution is transferred to a product storage tank 110.

[0055] Referring now to FIG. 8, one of the trays 96 is shown in perspective view. It is of stainless-steel and consists of a circular plate 112 with a D-shaped aperture 114 on one side, and a rectangular open-topped box 116 alongside it. The rectangular box 116 projects above the plate 112 and slightly below the plate 112, and the box 116 has a removable perforated base plate 118 with multiple perforations; the perforated base plate 118 rests on an internal flange or lip around the bottom of the wall of the box 116. A short rod is welded to the centre of the perforated base plate 118, with a small flat circular disc 120 at its top. Referring also to FIG. 7, each water inlet 98 feeds water downward onto the circular disc 120 of the corresponding tray 96, so the water spreads out over the perforated base plate 118 to fall through the perforations. The second condenser 82 includes three trays 96, and these are arranged with the D-shaped apertures 114 on alternate sides. During operation, the gases and vapours flow upwards through these D-shaped apertures 114, and because successive trays 96 have their D-shaped apertures 114 on opposite sides the gases and vapours must follow a sinuous path, contacting the falling water.

[0056] It has been found that operating at a temperature no lower than 60° C. ensures that any tar that may be present, as a potential product of thermolysis, remains sufficiently fluid that it does not form blockages. It will therefore be carried through into the product storage tank 110 along with the aqueous solution, and can subsequently be separated. Nevertheless, if any blockage of the perforations of a tray 96 occurs, the perforated base plate 118 can be replaced during maintenance by disconnecting the end plate 97 from the port 94, lifting out the base plate 118 through the port 94, and replacing it with a clean base plate 118. It will also be appreciated that both the first condenser 80 and the second condenser 82 have a dead volume below the inlet for gases and vapours, to remove any tar or particulate char material, which ensures that such material does not cause blockages to pipework or pumps downstream.

[0057] It will be appreciated that the description above is given by way of example only, and that modifications may be made while remaining within the scope of the invention, which is as defined in the claims. By way of example, the flow diagram of FIG. 7 indicates that the condensate formed by the first condenser 80 is fed from the outlet 85 to combine with the condensate in the bottom of the second condenser 82. Under some circumstances substantially all the sugar vapour is condensed within the first condenser 80, whereas the bulk of the steam is condensed in the second condenser 82, and under these conditions it may be preferable to keep the condensate from the first condenser 80 separate from the condensate from the second condenser 82, so it may be appropriate to feed the liquid from the outlet 85 to a storage tank corresponding to the product storage tank 110.

[0058] As described above, the downstream end of the thermolysis reactor 20 is provided with the generally Y-shaped end fitting 60 in order to accommodate the screw 64, as shown in FIG. 5; however, if there is substantially complete entrainment of the particulate matter within the flow of steam, and so negligible deposition onto the walls, then both the screw 64 and the Y-shaped end fitting 60 can be omitted, being replaced by a straight duct leading from the end of the reactor tube 30 to the inlet port 70 of the cyclone 21.