GB2339576A

GB2339576A - A method for the production of charcoaland the generation of power via the pyrolysis of biomass material

Info

Publication number: GB2339576A
Application number: GB9916480A
Authority: GB
Inventors: David Beedie
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-07-15
Filing date: 1999-07-15
Publication date: 2000-02-02
Anticipated expiration: 2019-07-15
Also published as: GB9916480D0; GB2339576B; GB9815312D0

Description

Biomass Treatment This invention relates to methods and apparatus for the treatment of biomass and in particular, but not exclusively, to such apparatus and methods intended to produce charcoal as a primary product, with the generation of power and heat as co-products.

Charcoal manufacture on a small-scale has traditionally mainly been done with kilns, which are simple but inefficient and environmentally damaging; retorts offer increased charcoal yield and reduced emissions but have seen relatively little take-up. Before petrochemicals displaced wood-based chemicals in industry, the organic compounds in kiln efflux were more highly valued and large scale manufacture of charcoal was usually accompanied by large scale production of these compounds. The charcoal industry mainly used batch processes. Since the invention of continuous retorts such as the Lambiotte retort recent charcoal manufacture on an industrial scale has more been able to resemble modern chemical industry. This large-scale device produced charcoal and chemicals efficiently. Other continuous processes have since been devised, usually with the production of chemical by-products as a contribution to their economics. Immediate energy recovery from the effluent stream has not been done by using it directly as

fuel for a pressure-charged internal-combustion heat engine, if any heat engine at all.

Several charcoal manufacturing techniques are long established, and developments continue. For example Antal et al (WO 96/29378) describe a pressurised pyrolysis process, with the aim of maximising the process speed and yield of the charcoal, at the expense of the co-product substances. A process developed by CSIRO of Australia uses effluent from atmospheric pressure carbonisation as fuel for low efficiency steam cycle power-generating process.

Pyrolysis may be defined as the thermal breakdown of substances in the absence of oxidant. Pyrolysis processes typically convert solid organic materials into a mixture of solid, liquid and gas, the proportions being dependent on the feedstock and the exact pyrolysis techniques. Pyrolysis processes can be categorised according to duration as carbonisation, conventional, fast, flash or ultrapyrolysis. The carbonisation (or'slow pyrolysis') process has been known longest and it has been the basis of charcoal manufacture for millennia. The timescale of the process is hours, days or weeks. More recently other pyrolysis processes have been developed utilising various techniques of inducing more rapid heating in the substance to be pyrolysed, variously conventional (5-30 minutes), fast (0.5-5 seconds), flash ( < 1 second) or ultra-pyrolysis ( < 0.5 seconds). The principal reason for these developments has been to alter the balance of the products

of the process. One of the features of slow pyrolysis is the low temperature experienced by the charge relative to the faster processes. This has particular advantage in the context of the invention.

In the charcoal-making kiln process, the heat required to sustain carbonisation is supplied by partial oxidation of the charge inside the kiln vessel (sometimes these are called Partial Combustion Kilns). The oxidant is a limited air supply admitted though controlled ports. Even though a limited amount of air is admitted charcoal manufacture in this process is regarded as a form of slow pyrolysis because the air admitted supports, and is used up by, localised combustion reactions only the hot products of which circulate around the vessel to pyrolyse the majority of the charge. The effluent gases contain the volatilised components of the wood diluted with combustion products and modified by secondary pyrolysis reactions. The energy content of the effluent varies during the carbonisation cycle as a result of the changing condition of the charge.

During the first stage of the kiln cycle the effluent carries much water vapour, for a period dependent on the starting moisture content of the wood, and has a low or negative net calorific value. Nitrogen is always present in the effluent as it is the major constituent of the combustion products. Nitrogen is effectively inert and thus dilutes the energy content of the effluent, which does not exceed'Low Calorific Value' ( < 10 MJ/nm3). The

effluent gases may thus be difficult or impossible to combust cleanly without support fuel.

In the charcoal-making retort process the heat necessary to sustain carbonisation is generated externally rather than by partial combustion of the charge within the retort vessel, and reaches the charge by conduction (for example through the retort vessel wall) and/or convection (for example if forced circulation is employed). A simple form of retort, a batch-loaded externally heated vessel, is used for illustration in the following brief description: when the loading door is closed the vessel is sealed apart from the effluent outlet-it has no air inlets; all or a part of the gaseous effluent evolved from the charge may be lead from the gas outlet to a combustion chamber; this chamber heats the underside of the retort vessel; this drives the pyrolysis process inside the retort.

The retort process is considered as pure slow pyrolysis because air involvement in the reactions inside the vessel is negligible, being only due to the initial air present with the charge. Nitrogen is not present in the effluent gases, which can have an energy content as high as 'Medium Calorific Value' (10-20 MJ/nm3).

We have developed biomass treatment methods and apparatus which may be operated on a small yet efficient scale to produce relatively large amounts of charcoal with power and heat as co-products, whilst maintaining an

acceptably low level of emissions of noxious substances such as organic compounds, particulates, or heavy metals.

The term"biomass"is used herein to mean woody and herbaceous material largely composed of or derived from trees or plants, for example wood, logs, chips, corn cobs, forestry and agricultural residue, processed cellulosic materials such as pulp, paper, paper board, rope, bagasse, and the organic fraction of solid municipal waste, and other biomass of plant or animal origin.

Accordingly, in one aspect, this invention provides a method of treating biomass (as herein defined) to produce charcoal, which comprises the steps of:- (i) loading said biomass into a retort means; (ii) heating said biomass to effect low temperature pyrolysis thereof thereby to produce fluid pyrolysis effluent; (iii)maintaining the interior of said retort means at a pressure above atmospheric pressure; (iv) supplying at least part of said fluid pyrolysis effluent at above atmospheric pressure to a pressure charged heat engine, to generate power.

The effluent gas from a slow pyrolysis retort is generally considered to be very dirty and a disposal problem, often being heavily loaded with tars, oils and/or ash and steam. It would not generally be considered a good candidate for gas turbine fuel, gas turbines being extremely sensitive to fuel quality. However I have found

that it is possible using embodiments of this invention to achieve a substantially completely combustible effluent, so that the harmful residues passing to the turbine are very low.

Preferably the said retort means is self-pressurised by generation of said fluid pyrolysis effluent.

Preferably said pyrolysis is effected at a fluid

temperature of between about 150 C and 900 C, more preferably between 200 C and 700 C and ideally between 250 C and 500 C. Preferably the fluid temperature is caused to increase during said pyrolysis. Pyrolysis is initiated at a material temperature of about 150 C, with peak mass loss rate occurring at a material temperature of about 300 C.

Adjustment of the end-point temperature of the process effects control over the volatile content of the charcoal produced. Typically it is around 500 C for a 35% charcoal yield of about 90% carbon. It might be desirable to be able to raise the temperature as high as about 9000C to obtain charcoal of very high carbon content.

The process may be carried out as a continuous process or as a batch process.

Advantageously, the pressure within said retort means is maintained below a predetermined threshold, e.g. by venting.

Preferably, said biomass is heated to effect drying prior to said low temperature pyrolysis. We have found that this two stage process provides several benefits. The

drying effluent stream will be substantially pure steam, and the latent heat energy contained in it will be easily recovered and used for external purposes. The pyrolysis effluent stream contains less water vapour and so is more useful as an energy source. The rate of pyrolysis is not constrained by moisture evaporation. Drying by superheated steam tends to crack larger pieces, facilitating progress of the pyrolysis front and reducing explosive cracking and particulate production during the subsequent pyrolysis phase and facilitating comminution of the charcoal product.

The drying is preferably done at a temperature below that at which low temperature pyrolysis occurs, e.g. below about 150 C.

The drying step preferably includes initially venting air from said retort means, thereafter effecting drying within said retort means by superheated steam, and venting steam produced during said drying step. Alternatively, the drying step could be carried out in a different vessel.

Preferably, heat is recovered from the steam vented during said drying step, for example by means of a heat exchanger. Alternatively or additionally, where the heat engine is a gas turbine engine, some or all of the steam expelled from the retort during the drying phase, if the retort is pressurised, may be injected into the combustion chamber of said heat engine, to provide an increased power output.

Preferably, forced circulation is applied to the atmosphere within said retort to enhance heat exchange with said biomass.

Preferably, said forced circulation receives heat for said retort by transfer of the heat through a heat exchanger connected in series with the circulation device.

The invention conveniently includes the step of recovering waste heat from said pressure charged heat engine. In a preferred arrangement, at least part of the heat for said retort means is obtained by recovery of waste heat from said pressure charged heat engine.

Alternatively, or in addition, a portion of the fluid pyrolysis effluent may be combusted to generate at least part of the heat for heating said retort means.

For start-up, an external heat source may be used to provide at least initial heat for said retort.

Where cooling of a charge of charcoal is required or cooling of the retort is required for process control purposes, and the heat engine comprises a gas turbine engine, having a pressurised combustion chamber, a heat exchanger may transfer heat from said retort means and/or its contents to the fluid inlet stream to the air inlet to the pressurised combustion chamber of said gas turbine engine, thereby to effect cooling of said retort means and its contents, and making effective use of the heat so removed.

In order to provide a substantially continuous supply of fluid pyrolysis effluent, a plurality of retort means may be connected to supply fluid pyrolysis effluent to a common heat engine.

Said heat engine may be any one of a number of different types of pressure-charged energy recovery system, for example turbo-or super-charged internal combustion engines, but a gas turbine engine is preferred.

In another aspect, this invention provides apparatus for the treatment of biomass, to provide charcoal and power, said apparatus comprising:- a retort means for receiving biomass and generating fluid pyrolysis efflux at above ambient pressure; a pressure charged heat engine; means for supplying said fluid pyrolysis efflux to said heat engine, and heat exchange means for transferring heat from a heat source to said retort.

Preferably, said heat source derives from the combustion of part of said fluid pyrolysis effluent.

Alternatively and additionally, said heat source may comprise waste heat from said heat engine.

Whilst the invention has been described above it extends to any inventive combination of the features set out above or in the following description.

The invention may be performed in various ways and, by way of example only, an embodiment thereof, and certain variations thereof, will be described, with reference to the accompanying Figure which is a schematic block diagram of a preferred biomass treatment system in accordance with this invention.

Referring initially to the Figure, the illustrated embodiment comprises a carbonisation retort 10 having an access door 12 for the introduction and removal of biomass material, and an effluent vent 14 which is connected to the combustion chamber 16 of a pressure charged heat engine 18.

In this embodiment the heat engine 18 is in the form of a gas turbine engine, although an internal combustion reciprocating engine could be used. The gas turbine engine comprises a compressor 20 and a turbine 22 on a common shaft 24, and a combustion chamber 26 between the compressor outlet 28 and the turbine inlet 30.

In this embodiment, heat is transferred to the carbonisation retort 10 by a primary heat exchanger 32, whose secondary circuit receives atmosphere from the carbonisation retort 10 and passes it back, at an elevated temperature, to the retort 10. Although this flow could be achieved by natural convection, it is preferred for the convection to be forced by means of a fan 34 in the flow path between the retort 10 and the heat exchanger 32. The forced circulation creates a consistent quality of product and also allows process control.

In broad outline, in operation a supply of suitable biomass material is introduced into the retort 10 on a continuous or batch basis. The retort 10 is then closed and a drying stage is carried out (if required) followed by a slow pyrolysis stage. In the drying/pyrolysis stages heat is applied via the heat exchanger 32 and the circulating atmosphere from the retort.

During at least the pyrolysis stage, the retort 10 is self pressurising, and the pressurised pyrolysis products pass via the effluent vent 14 to the combustion chamber 26 of the gas turbine engine 18. At the gas turbine engine, the pyrolysis products are mixed in the combustion chamber 26 with the compressed air delivered from the compressor 20, via the heat exchangers 44 and 46. The pyrolysis products burn substantially completely, leaving little or no residue, and expand through the turbine 22 resulting in a net mechanical work output on the shaft 24. The mechanical work could be used, for example, to drive an electrical generator.

The primary side of the heat exchanger 32 receives heat from one or more sources, for the purpose of drying the charge (when drying is not done externally) and later adding heat into the pyrolysis process whenever that process is endothermic.

The heat exchanger 44 removes heat from the retort for the purposes of process control (whenever the process is exothermic) and later cooling of the char product, and

transfers the heat into the gas turbine combustion chamber air inlet stream.

The heat exchanger 32 receives heat from one or more of the following sources: the exhaust from the gas turbine; a burner 35 burning a proportion of the pyrolysis products tapped from the retort 10, and a support fuel burner 48.

If and when the temperature of the heat transferred from the gas turbine exhaust to the primary side of the heat exchanger 32 is not sufficiently high, it could be augmented by additional combustion of support fuel.

The support fuel burner may at times constitute the entire heat sources for the primary heat exchanger, for example for drying a charge when the heat engine is not in operation.

Alternatively, support fuel combustion for the purpose of supplying heat to the primary heat exchanger may be achieved by running the heat engine on the support fuel, the heat engine exhaust stream then being the primary heat source.

Thus the primary heat exchanger transfers heat from the primary heat source to the carbonisation retort.

Where the nature of the biomass charge requires, the system may be operated to provide an initial drying stage. Again this is achieved by means of heat transferred from the primary heat exchanger 32 to the carbonisation retort 10. During the drying stage, the carbonisation retort is vented via steam vent 36. During the drying phase the

atmosphere of the retort is circulated through the primary heat exchanger but, as the temperature in the retort increases, the air is expelled by the steam evolved from the biomass charge.

Alternatively, if a supply of steam is readily available, a quantity of steam may be injected into the system during the early stages of drying, in order to accelerate the process. In the next stage of the drying phase, the fluid in the retort is mainly superheated steam, at a temperature of up to about 150 C (the lower limit of pyrolysis). In a system in which a gas turbine is operated at the same time as a charge of biomass is being dried, the steam produced during the drying phase may be supplied via line 40 to the combustion chamber outlet 30 of the gas turbine to give increased power output.

During the pyrolysis phase, the temperature of the fluid recirculating between the retort and the heat exchanger increases from the lower pyrolysis temperature of

150 C up to typically 500 C with a maximum limit being set at 900 C. During the pyrolysis stage, the retort is self-pressurising and the pressure within the retort may be limited to a pressure at the range of from 300 to 2000 kPA, by means of a suitable pressure relief valve 40. The typical carbonisation mid-point temperature is about 300 C.

During the pyrolysis phase, an effluent stream of pyrolysis products is supplied to the combustion chamber. Because the pyrolysis is carried out at a slow rate and at

relatively low temperatures, the release of alkalis (which can cause fouling and corrosion of turbine blades) is reduced. Likewise the release of particulates which can cause fouling, corrosion and erosion of the gas turbine surfaces is reduced. In addition, the structural integrity of any woody biomass is retained during slow pyrolysis, so ash tends to be retained in the char matrix.

The compressed air inlet to the gas turbine combustion chamber may be pre-heated by passing it through the secondary side of a further heat exchanger 44, through the primary side of which is circulated the hot atmosphere of the retort chamber. The heat exchanger 44 is operated either for the purpose of control of the pyrolysis process or to cool the charcoal product.

Likewise, if exothermic reactions occur in the retort, the further heat exchanger may be used to control or reduce the temperature in the retort.

The thermal efficiency of the gas turbine cycle may be increased by the known technique of recuperation, which involves recovering heat energy from the exhaust stream and transferring it to the compressor air output stream. Less fuel is burned, and an extra heat exchanger 46 is required.

In the context of the present invention, the recuperation heat exchanger, if implemented, is in series with the retort cooling heat exchanger 44 if this is implemented.

Thus as shown in the Figure, the primary side of the further heat exchanger 46 is supplied with a portion of the

hot exhaust from the turbine 22. The secondary side of the heat exchanger receives air, optionally heated by the heat exchanger 44. Alternatively, the primary side of the further heat exchanger 46 may be located downstream of the heat exchanger 32. Instead of passing in series to the heat exchanger 44 and the further heat exchanger 46, the compressor outlet flow may be split, to supply the heat exchangers in parallel.

The above system provides a method and apparatus for generating charcoal from biomass with recovery of the energy contained in the effluent stream. The embodiment links a pressure-charged energy recovery system to a selfpressurising biomass converter. The self-pressurising capability of the biomass converter allows it to directly deliver the effluent stream (typically composed of a mixture of gases bearing water vapour, oil and/or tar droplets in suspension) to any pressure-charged energy recovery system.

The system may be enhanced by providing several retorts each feeding a common gas turbine engine so that the gas turbine may be operated substantially continuously.

Claims

CLAIMS 1. A method of treating biomass (as herein defined) to produce charcoal, which comprises the steps of:- (i) loading said biomass into a retort means; (ii) heating said biomass to effect low temperature pyrolysis thereof thereby to produce fluid pyrolysis effluent; (iii) maintaining the interior of said retort means at a pressure above atmospheric pressure; (iv) supplying at least part of said fluid pyrolysis effluent at above atmospheric pressure to a pressure charged heat engine, to generate power.
2. A method according to Claim 1, wherein said retort means is self-pressurised by generation of said fluid pyrolysis effluent.
3. A method according to Claim 1 or Claim 2, wherein said pyrolysis is effected at a fluid temperature of between about 150 C and 900 C.
4. A method according to Claim 3, wherein said pyrolysis is effected at a fluid temperature of between 200 and 7000C.
5. A method according to Claim 4, wherein said pyrolysis is effect at a fluid temperature of between 2500C and 5000C.

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6. A method according to any of the preceding Claims, wherein the fluid temperature is increased during said pyrolysis.
7. A method according to Claim 6, wherein the end point fluid temperature is about 500 C.
8. A method according to any of the preceding Claims, wherein said biomass is loaded into said retort means in batches.
9. A method according to any of Claims 1 to 7, wherein said biomass is loaded into said retort means on a continuous basis.
10. A method according to any preceding Claim, further including the step of maintaining the pressure within said retort means below a predetermined threshold.
11. A method according to any preceding Claim, which includes the step of heating said biomass to effect drying prior to said low temperature pyrolysis.
12. A method according to Claim 11, wherein said drying is done at a temperature below that at which low temperature pyrolysis occurs.
13. A method according to Claim 11 or Claim 12, wherein said drying step includes initially venting air from said retort means and effecting drying within said retort means by superheated steam, and venting steam produced during said drying step.

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14. A method according to Claim 13, including the step of recovering heat from steam vented during said drying step.
15. A method according to Claim 13, or Claim 14, which includes the step of supplying at least a portion of the steam vented during said drying step to said heat engine, for increasing the power output thereof.
16. A method according to any preceding Claim, which further includes the step of circulating the atmosphere within said retort to enhance heat exchange with said biomass.
17. A method according to Claim 16, which includes passing said atmosphere through a heat exchanger, to transfer heat to said atmosphere.
18. A method according to any preceding Claim, which includes the step of recovering waste heat from said pressure charged heat engine.
19. A method according to any preceding Claim, wherein at least part of the heat for said retort means is obtained by recovery of waste heat from said pressure charged heat engine.
20. A method according to any preceding Claim, which includes the step of combusting a portion of the fluid pyrolysis effluent to generate at least part of the heat for heating said retort means.

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21. A method according to any preceding Claim, which includes the step of using an external heat source to provide at least initial heat for said retort means.
22. A method according to any preceding Claim, which includes the step of transferring heat from said retort means to the fluid inlet stream to said pressure charged heat engine, thereby to effect cooling of said retort means and its contents, and heating of said inlet stream.
23. A method according to any preceding Claim, which includes the use of a plurality of retort means each providing a supply of fluid pyrolysis effluent to a common heat engine.
24. A method according to any preceding Claim, wherein said heat engine is a gas turbine engine.
25. Apparatus for the treatment of biomass, to provide charcoal and power, said apparatus comprising:- a retort means for receiving biomass and generating a fluid pyrolysis efflux at above ambient pressure; a pressure charged heat engine; means for supplying said fluid pyrolysis efflux to said heat engine, and a heat exchanger for transferring heat from a heat source to said retort.
26. Apparatus according to Claim 25, wherein said heat source includes the combustion of part of said fluid pyrolysis effluent.
27. Apparatus according to Claim 25, wherein said heat engine comprises a gas turbine engine which further

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includes a heat exchanger for transferring heat from said retort to the combustion chamber of the heat engine.
28. Apparatus according to Claim 26, wherein said heat source includes the waste heat from the said heat engine.
29. A method of treating biomass substantially as hereinbefore described with reference to the accompanying drawing.
30. Apparatus for the treatment of biomass, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.