GB2455869A - Gasification of biomass - Google Patents

Abstract

Energy is obtained from biomass by subjecting it to gasification in a down-flow gasification reactor, treating the gas stream by passage through a filter and a heat exchanger, and using the resulting gas stream as fuel for an engine. The filtered gas stream is also subjected to a scrubbing stage using an aqueous or organic liquid in a fluidic vortex gas/liquid contactor without moving parts, and this preferably uses a hydrocarbon of biological origin (e.g. biodiesel) . The biomass may also be combined with an alkaline material or a precursor therefor (e.g. chalk), prior to the gasification, to suppress formation of acid gases. The combustion characteristics of the fuel gas may be enhanced by the injection of biodiesel or bioethanol. The biomass may be waste wood that is contaminated with plastic or paint.

Description

Gasification of Biomass This invention relates to a process and a plant for obtaining energy from biomass, by means of a gasification process.

Gasification (which may also be referred to as pyrolysis) is a known process for converting biomass to a gas stream, and the biomass might for example be straw, bagasse, or other agricultural wastes, waste paper, or wood. Such a process may be applied to biomass grown specifically for the purpose, or to waste materials. In particular waste wood would be a suitable material for such a process. The gasification process involves heating the biomass to an elevated temperature, possibly in the presence of a restricted quantity of air, to break down organic materials and to generate carbon monoxide and hydrogen. This gas mixture may be used to generate power in an engine. Other products of the process are charcoal, particulate carbon, and tars. The particulate carbon and tars can cause problems in the operation of the power-generating engine. Particularly where the wood is waste material (for example used constructional materials) it may be contaminated with materials such as plastic or paint, and consequently the gas mixture may contain acidic or alkaline components, such as HC1.

According to the present invention there is provided a process for obtaining energy from biomass, the process comprising subjecting the biomass to gasification in a gasification reactor, passing the resulting gas stream through a filter to remove particulate material, passing the filtered gas stream through a heat exchanger to cool the gas stream, and using the cooled gas stream as fuel for an engine, wherein the filtered gas stream is also subjected to a scrubbing stage using an aqueous or organic liquid in a fluidic vortex gas/liquid contactor without moving parts, in which the gas flows along a vortex path in a generally cylindrical chamber between a tangential inlet and an axial outlet and the aqueous or organic liquid is sprayed outwardly through the gas vortex path.

A preferred aspect of the present invention is that the filtering is performed on hot gases. The gases emerge from the gasification reactor at a temperature above 700°C; and preferably the gases are at above 500°C when they are filtered. This high temperature filtration ensures that tarry material is not deposited on the filter. Preferably the gases are cooled before being subjected to the scrubbing stage.

Such a fluidic vortex gas/liquid contactor is known for example from EP 0 642 836 and from WO 2002/100507.

Where the liquid used in the scrubbing stage is an organic liquid, it is preferably a hydrocarbon of biological origin, for example a plant-derived oil, for example a biodiesel, although other organic liquids may be used instead. It is preferably a non-polar liquid, to ensure it is a good solvent for any tarry material.

Preferably the gasification is carried out in a down-flow gasification reactor, as this reduces the quantity of tar in the gas stream. And preferably the gasification is carried out in such a way that the gas within the gasification reactor has an oxygen concentration no more than 3% and more preferably no more than 2.2%. Preferably the gasification is carried out at a pressure no higher than atmospheric pressure, and preferably slightly below atmospheric pressure, the pressure in the gasification reactor being reduced by extracting gases from it.

Preferably a gasification process comprises combining the biomass with an alkaline material or a precursor therefor, prior to the gasification. For example the biomass may be combined with caustic soda (NaOH), quicklime (CaO), or a carbonate such as calcium carbonate. The calcium carbonate breaks down to provide calcium oxide during the high-temperature associated with gasification. The alkaline material reacts with any acidic gases produced during the gasification, such as any sulphur oxides or hydrogen chloride, to form insoluble particulate material which is trapped on the filter, so that the resulting gas stream does not contain acidic gases. Where the biomass undergoes shredding, it may be combined with the alkaline material or its precursor either before or after undergoing shredding.

Alternatively or additionally suitable pH-changing liquids may be sprayed into the gas stream before the heat exchanger, to control the pH of aqueous material condensing within the heat exchanger. For example if the gas stream contains acidic gases, the pH-changing liquid The processes of the invention are particularly suitable for treating waste wood that may be contaminated with glue, laminate, plastic or paint, and so enabling such waste wood to be used as a source of energy, for example to generate electricity.

The invention also provides plant suitable for performing such processes.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a flow chart of the plant and the processes of the invention; and Figure 2 is a transverse axial sectional view through a fluidic scrubber for use in the plant and process of figure 1.

Referring to figure 1, the plant 100 includes a reception building 101 where the organic feedstock (wood or waste wood in this example) is offloaded from delivery vehicles on to the floor of the building 101. A materials handling machine feeds the offloaded material into a shredder 1. In this example the shredder 1.

contains four shafts carrying cutting disks. The shafts are rotated at a low speed, approximately 25 rpm, driven by electric motors through gearboxes, producing a substantially consistent product size with minimal production of dust. Because of the low speed operation it generates little heat, and is quiet. The organic feedstock then passes through a sizing screen (not shown) within the shredder 1 before falling on to a discharge conveyor which transfers the material to an overhead magnet 2. The magnet 2 is arranged above the conveyor carrying the organic feedstock, and extracts any ferromagnetic material; any extracted material may be passed to a skip for recycling. The organic feedstock is then passed over an eddy current separator 3 to remove any non-ferrous metals, and then over a vibrating finger screen 4 to separate fines. The cleaned and sized organic feedstock then travels along an inclined conveyor to a storage container 5 with a mechanism for feeding out the feedstock at a substantially constant rate. The container 5 is a bunker that is capable of holding about 60 tonnes of prepared organic feedstock, to allow for about 20 hours of plant operation without the need for shredding further organic feedstock. This ensures that the plant 100 can operate unsupervised overnight. The feeding mechanism may be a walking floor with movable slats that are hydraulically operated, moved to and fro to ensure that the output conveyor carries a substantially constant quantity of material (e.g. 3 tonnes per hour), and the end portion of the floor may be perforated to allow through flow of warm air, to enhance drying of the organic feedstock.

The prepared organic feedstock output from the storage container 5 is passed over a further magnetic separator 6 to ensure removal of any ferromagnetic materials, and then over an in-feed weighing device 6a, and so to one or more thermal treatment units 7 (only one is shown) . By way of example the plant may include twelve such thermal treatment units 7. These operate continuously, but are fed by a batch process through individual lock-off hopper systems with knife valves at top and bottom. Each can process 250 kg of prepared organic feedstock per hour, but processes in batches of about 17 kg, and treating fifteen batches per hour. Any excess organic feedstock is returned to the storage container 5 via another in-line weighing device 6b.

Jithin each thermal treatment unit 7 there is a low oxygen atmosphere (typically about 2%) at a slightly negative pressure (i.e. less than atmospheric pressure), the gas flowing downwardly through a bed of hot feedstock and charcoal reaching a temperature of for example >800°C. The combination of temperature and residence time in the thermal treatment unit 7 not only brings about gasification of the feedstock, but also cracks most of the tar that is produced during the gasification, because the gases (and tar) pass down through the hottest region of the bed. The gases produced by gasification are sucked out from the base of the unit 7 (by an induced draft fan 110) The hot gas stream consisting principally of hydrogen and carbon monoxide is then passed through a gas filter 8 consisting of several hollow ceramic filter candles hanging from a top plate, the gas stream flowing upwardly through the filter candles (so the candles can be cleaned by back pulsing clean gas down the candles).

These filter candles remove particulate matter down to about 5 pm. The hot clean gas, typically at between about 450 and 500°C, is then passed through a tube heat exchanger 9 to cool the gas down to less than 50°C (this may be achieved using two heat exchangers 9 in series), for example down to 25°C. This leads to condensation of moisture in the gas flow, and the resulting condensate is discharged to a water treatment plant 21. The cool gas stream is then passed through the induced draft fan 110 into a buffer storage vessel 10, the induced draft fan ensuring that the pressure in the storage vessel 10 is above atmospheric. The clean dry gas in the buffer storage vessel 10 is used to provide fuel to a gas-fuelled internal combustion engine 11. The exhaust gas from the engine 11 is treated in unit 12 to remove NOx, then through a catalytic converter 13 to remove CO, and via a silencer 14 to an exhaust stack 15. The plant 100 may have several such engines 11, for example six engines, each being provided with its own gas treatment units 12 and 13 and its own silencer 14, but sharing the exhaust stack 15. Typically each thermal treatment unit 7 is associated with its own filter 8, heat exchanger 9 and fan 110. Typically two thermal treatment units 7 provide gas to one buffer storage vessel 10 which supplies gas to a single engine 11, as combining the outlets from two thermal treatment units 7 provides a more consistent composition of the gas mixture supplied to each engine 11. Alternatively a single buffer storage vessel 10 might be supplied by gas from a single thermal treatment unit 7, or from more than two such units; and might itself supply gas to more than one engine 11. Each engine 11 drives an alternator 18, for example producing electricity at 400 V1 50 Hz, three-phase, and each capable of producing approximately 600 kWe. The output from the alternator 18 is supplied through a transformer and switchgear 19 to the electrical grid 20.

Each thermal treatment unit 7 also produces charcoal, or carbon char, depending on the nature of the feedstock, and this is removed by a screw conveyor to pass by another magnetic separator 16 to a storage silo 17. The filter 8 captures particulate carbon char, and this is released from the filter when it is back pulsed; this separated carbon char is dropped on to a screw conveyor and passed to a second storage silo 17. These silos 17 can discharge directly to a road vehicle 25, or discharge into a macerator 23 which blends the chars with water to make a granule of approximately 2-4 mm in size.

These granules can be discharged to a road vehicle 25, or passed to a briquette machine 24 which makes briquettes to be loaded in a road vehicle 25.

As previously mentioned, each heat exchanger 9 discharges condensate to a water treatment plant 21. The plant 21 incorporates a carbon filtration system to remove phenols and metals. The coolant water used in the heat exchanger 9 is circulated through an induced draft cooling system 22 so it can be reused as cooling water.

Make-up water for the coolant may be provided from mains water, or rainwater, or the cleaned condensate.

Alternatively the cleaned condensate may be discharged from the plant 100.

Operation of the plant 100 as described above is improved by certain additional features. Firstly a fluidic scrubber 112 is provided to treat the filtered gas. As indicated in figure 1 this may be installed downstream of the heat exchanger 9. Referring now to figure 2, this scrubber consists of a cylindrical vortex chamber 120 into which the gas flows through a tangential inlet port 122, the gas then flowing out through an axial outlet duct 124. The gas flow rate is such that the gases form a vigorous vortex flow within the chamber 120.

A scrubbing liquid 125 is fed through a pipe 126 which divides into two branches which enter the cylindrical vortex chamber 120 to terminate at opposed nozzles 128 near the middle of the chamber 120. Below the chamber 120, and separated from it by a horizontal baffle 130, is a sump 132 for the used scrubbing liquid 125, there being tubes 133 to enable liquid to drain back into the sump 132.

A pump 134 recirculates scrubbing liquid 125 from the sump 132 at a Reynolds number greater than 12 000 through the pipe 126 to the nozzles 128, so that the flow is at least partially turbulent, and that the opposed jets of liquid from the nozzles 128 form a spray 136 of droplets moving generally radially outward. Hence the gas stream is brought into contact with the droplets of the spray 136, and any organic vapours (such as any tar) or any particulate material (small carbon particles that have passed through the filter 8) are washed out of the gas phase. Preferably the scrubbing liquid 125 is a hydrocarbon liquid of biological origin, for example biodiesel, or it may be an aqueous liquid. The vigorous vortex flow ensures that any particles and droplets are effectively and efficiently removed from the gas stream during its passage through the scrubber 112, so a further demisting stage is not required.

The condensate from the heat exchanger 9, as mentioned previously, contains some organic compounds.

Rather than being treated by the water treatment system 21 the condensate may, at least in part, be injected into the thermal treatment units 7. This is particularly advantageous if the condensate contains tarry residues.

The organic materials in the condensate will undergo gasification, so reducing the volume of waste organic liquid. A second advantage is that the addition of some water into the thermal treatment units 7 leads to some production of water gas (i.e. carbon monoxide and hydrogen) by the reaction of steam with red hot carbon, and some reduction in the quantity of charcoal produced.

This improves the quality of the resulting fuel gas.

Since the water gas synthesis reaction is endothermic, amount of water introduced into the thermal treatment units 7 must not be so high as to significantly reduce the operating temperature.

Similarly at least some of the used scrubber liquid may also be injected into the thermal treatment units 7. Where the scrubber liquid 125 is an organic or hydrocarbon liquid, this enhances the quality of the resulting fuel gas, and avoids the need to treat or dispose of used scrubber liquid 125.

Secondly, a feeder 116 is provided to introduce small pieces of chalk (similar in size to the chopped pieces of organic feedstock) into the storage container 5, so that the material fed into the thermal treatment unit 7 consists of a mixture of treated organic feedstock with chalk. The feeder 116 may for example be a hopper with a rotary feed valve. During the passage through the thermal treatment unit 7 the chalk breaks down to provide calcium oxide, which reacts with any acidic gases that may be generated during gasification of the organic -10 -feedstock. This is a particular issue when treating waste material such as waste wood that may be contaminated with paint, stains, preservatives, or plastic.

Thirdly, a spray nozzle 114 is provided to spray a small quantity of sodium hydroxide solution into the gas flow immediately upstream of the heat exchanger 9 -assuming that the gas stream contains a small quantity of acidic gases. If the gas stream were to contain small quantities of alkaline gases, then the spray would be of an acid. This ensures that the water that condenses on the pipes of the heat exchanger 9 is not significantly acid (or alkaline), so reducing the risk of corrosion of the structure of the heat exchanger 9.

In a modification, three fluidic scrubbers 112 are installed in series to treat the filtered gas, with or without the heat exchanger 9. The first such scrubber 112 might use an acidic solution to remove metals; the second fluidic scrubber 112 might use an alkaline solution to neutralise any acid exhaust gas; the third uses a bio-diesel for the removal of tars. In some cases the third scrubber 112 may be omitted; and in some cases both the second and third scrubbers 112 may be omitted.

Benefits of using the acidic, alkaline and bio-diesel scrubbing or injection are a reduction in the need for maintenance of the engine 11 and in the frequency of oil changes, and an increase in the longevity of the catalysts 13, improving availability of the engine 11, and furthermore reducing the exhaust stack 15 emission levels.

A further benefit of the use of biodiesel in the fluidic scrubber 112 is that some of the biodiesel evaporates, so that the gas mixture supplied to the -11 -engine 11 contains a proportion of biodiesel. This may be sufficient to increase the "octane" rating of the resultant gas mixture, so that the engine can run with a somewhat higher compression ratio. This may also be achieved by introducing bioethariol vapour into the gas mixture supplied to the engine 11. This may be achieved in the plant 100 by using a second fluidic scrubber arranged either before or after the induced draft fan 110, identical to the scrubber 112 but using bioethanol as the liquid; this primarily acts as a gas/liquid contactor (as scrubbing has already been achieved), optimising the proportion of bioethanol in the gas mixture.

Alternatively bioethanol or biodiesel may be introduced, in addition to the gas stream from the buffer storage vessel 10, to provide fuel to the engine 11, using a fuel injector or a carburettor. As indicated above, this may increase the effective octane rating, so the engine can run at a higher compression ratio and with greater efficiency.

The description above was of steady-state operation of the plant 100. When the plant 100 is first started up the thermal treatment units 7 are first fed with charcoal, or a mixture of charcoal with prepared organic feedstock (chopped wood) from the storage unit 5, to form a bed. Hot air blowers, which heat the air electrically, are used to blow hot air through the bed, until the material of the bed becomes sufficiently hot that combustion is initiated. Once the bed is sufficiently hot, the flow of hot air is decreased to zero, and normal operation can be initiated. The output gas stream during the initial phase of operation is preferably passed directly from the output of the thermal treatment unit 7 through a thermal oxidiser unit (not shown) to the -12 -exhaust stack 15.

It will be appreciated that the plant 100 may, as described, incorporate all three additional features, that is to say the fluidic scrubber 112, the feeder 116 and the spray nozzle 114, or may include only one or two of them. It will also be appreciated that where a fluidic scrubber 112 is provided, it may be installed either upstream or (preferably) downstream of the heat exchanger 9.

Claims (14)

1. -13 -Claims 1. A process for obtaining energy from biomass, the process comprising subjecting the biomass to gasification in a gasification reactor, passing the resulting gas stream through a filter to remove particulate material, passing the filtered gas stream through a heat exchanger to cool the gas stream, and using the cooled gas stream as fuel for an engine, wherein the filtered gas stream is also subjected to a scrubbing stage using an aqueous or organic liquid in a fluidic vortex gas/liquid contactor without moving parts, in which the gas flows along a vortex path in a generally cylindrical chamber between a tangential inlet and an axial outlet and the aqueous or organic liquid is sprayed outwardly through the gas vortex path.
2. 2. A process as claimed in claim 1 wherein the filtering is carried out on a hot gas stream.
3. 3. A process as claimed in claim 1 or claim 2 wherein the liquid used in the scrubbing stage is an organic liquid which is a hydrocarbon of biological origin.
4. 4. A process as claimed in claim 3 wherein the organic liquid is a biodiesel.
5. 5. A process as claimed in any one of the preceding claims wherein the filtered gas stream is passed through a fluidic vortex gas/liquid contactor without moving parts, in which the gas flows along a vortex path in a generally cylindrical chamber between a tangential inlet and an axial outlet and a liquid is sprayed outwardly through the gas vortex path, wherein the sprayed liquid forms a vapour which changes the combustion properties of the gas mixture.

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6. 6. A process as claimed in any one of the preceding claims wherein the engine is also provided with bioethanol or biodiesel to change the combustion properties of the fuel mixture.
7. 7. A process as claimed in any one of the preceding claims wherein the biomass is combined with an alkaline material or a precursor therefor, prior to the gasification.
8. 8. A process as claimed in claim 7 wherein the biomass is combined with chalk prior to the gasification.
9. 9. A process as claimed in any one of the preceding claims wherein a pH-changing liquid is sprayed into the gas stream before the heat exchanger, to control the pH of aqueous material condensing within the heat exchanger.
10. 10. A process as claimed in any one of the preceding claims wherein the filtered gas stream is passed successively through a plurality of successive fluidic vortex gas/liquid contactors without moving parts arranged in series, in each of which the gas flows along a vortex path in a generally cylindrical chamber between a tangential inlet and an axial outlet and a liquid is sprayed outwardly through the gas vortex path, wherein in at least one contactor the liquid is selected from aqueous alkali and aqueous acid.
11. 11. A process as claimed in claim 10 wherein at least one of the fluidic vortex gas/liquid contactors
12. 12. A process for obtaining energy from biomass, the process comprising subjecting the biomass to gasification in a gasification reactor, passing the resulting gas -15 -stream through a filter to remove particulate material, passing the filtered gas stream through a heat exchanger to cool the gas stream, and using the cooled gas stream as fuel for an engine, wherein the biomass is combined with an alkaline material or a precursor therefor, prior to the gasification.
13. 13. A process for obtaining energy from biomass substantially as hereiribefore described with reference to, and as shown in, the accompanying drawings.
14. 14. A plant for obtaining energy from biomass by a process as claimed in any one of the preceding claims.

    OGEN/POOl (16119 MdSd) P.T. Mansfield

    Chartered Patent Attorney Agent for the Applicants