WO2010130988A1

WO2010130988A1 - Staged biomass pyrolysis process and apparatus

Info

Publication number: WO2010130988A1
Application number: PCT/GB2010/000939
Authority: WO
Inventors: Andreas Hornung; Andreas Apfelbacher
Original assignee: Aston University
Priority date: 2009-05-11
Filing date: 2010-05-11
Publication date: 2010-11-18
Also published as: EP2430121A1; GB0908082D0; US20110278149A1

Abstract

A biomass pyrolysis process in which biomass feedstock particles (having a d5o mean particle size of at least 1 mm) are thermally treated substantially in the absence of oxygen to pyrolyse the biomass feedstock particles. The thermal treatment of the biomass feedstock particles includes a heat treatment drying step at a temperature in the range 100°C to 250°C. Then, at a pre-pyrolysis heating location in a heat treatment system, the biomass feedstock particles are heated to a temperature in the range 280°C to 350°C, held within the same temperature range for a time of at least 5 seconds and then moved from the pre-pyrolysis heating location to a pyrolysis heating location by a conveyor system for heating to a temperature above 350°C for pyrolysis.

Description

STAGED BIOMASS PYROLYSIS PROCESS AND APPARATUS

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to biomass pyrolysis. It has particular, but not exclusive, application to biomass pyrolysis processes for the production of renewable liquid, gaseous and solid fuels.

Related art

Biomass pyrolysis is the thermal decomposition of biomass (e.g. plant material such as wood and wood bark) substantially in the absence of oxygen. Biomass is typically a mixture of hemicellulose, cellulose, lignin and small amounts of other organics. These components typically pyrolyse or degrade at different rates and by different mechanisms and pathways.

One traditional example of biomass pyrolysis is the production of charcoal, where the main product of the pyrolysis is char. Alternative biomass pyrolysis techniques provide a product which, after cooling, includes a substantial proportion of liquid. This liquid is typically a dark brown liquid having a heating value that is around one half the heating value of conventional fuel oil. The liquid is typically referred to as bio-oil. In many circumstances, it is the bio-oil which is the most valuable product of the pyrolysis reaction, since bio-oil can be easily stored for later use, e.g. for heat and/or electricity generation. Bio-oil typically is a homogenous hydrophilic mixture of polar organics and water.

The rate and extent of decomposition of the components of biomass depends on the process parameters of the pyrolysis reactor, e.g. the rate of heating of the biomass, the mode of heating of the biomass and the residence time of the subsequent products. In turn, these process parameters may also have an effect on the subsequent behaviour of the product, e.g. by secondary reactions such as cracking (of higher molecular mass products) or condensation reactions (of lower molecular mass products).

Biomass pyrolysis can be carried out using fast heating rates and short vapour residence times. Such "fast" pyrolysis processes are reviewed by Bridgwater et al (A. V. Bridgwater, D. Meierb and D. Radlein, "An overview of fast pyrolysis of biomass" Organic Geochemistry Volume 30, Issue 12, December 1999, Pages 1479-1493) . It is considered in that disclosure that optimum levels of organics in the bio- oil may be achieved by fast heating of the biomass to a reaction temperature of around 500⁰C and vapour residence times of less than around 1 second.

There are several different options for achieving heating of the biomass in a fast pyrolysis reactor. For example, ablative pyrolysis requires the biomass particles to be pressed against a heated surface and rapidly moved. This allows the use of relatively large biomass particles. Alternatively, fluid bed and circulating fluid bed pyrolysis reactors transfer heat from a heat source to the biomass particles by a mixture of convention and conduction. Since heat transfer must typically occur quickly, fluid bed pyrolysis reactor require the use of small biomass particles, e.g. not more than 3 mm. A further alternative is vacuum pyrolysis, in which heating rates may be relatively low, but the application of a vacuum quickly extracts the pyrolysis products and thus simulates some effects of fast pyrolysis.

Further, more recent, reviews of biomass pyrolysis have been conducted by A. V. Bridgwater ("Renewable fuels and chemicals by thermal processing of biomass" Chemical Engineering Journal Volume 91, Issues 2-3, 15 March 2003, pages 87-102; and "Biomass fast pyrolysis", Thermal Science Vol. 8 (2004), No. 2, pages 21-49) in which it is explained that lower process temperatures and longer residence times in the pyrolysis reactor favours the production of char. It is further explained that the critical issue in biomass pyrolysis is to bring the reacting biomass particle to the optimum process temperature and minimise its exposure to intermediate, lower temperatures that favour the formation of char. Typical product yields, stated as a weight percentage of dry wood feedstock, for fast pyrolysis are given as 75% liquid, 12% char and 13% gas.

SUMMARY OF THE INVENTION

The present inventors have realised that certain factors in the pyrolysis process may encourage the formation of high organics compounds such as tar, which is present in the resultant bio-oil. This is considered undesirable. Typical compounds that may be classified as tar in this technical field may be considered to be organic compounds of molecular weight 300 a.m.u. (atomic mass unit, 1 a.m.u. = 1.66 * 10^"27 kilograms) or greater. In particular, the present inventors consider that the rate of heating or, more generally, the heat treatment schedule, of the biomass feedstock particles is a critical parameter.

Accordingly, in a first aspect, the present invention provides a biomass pyrolysis process in which biomass feedstock particles are thermally treated substantially in the absence of oxygen to pyrolyse the biomass feedstock particles, wherein the thermal treatment of the biomass feedstock particles includes a step in which at least 50% of the mass of the biomass feedstock particles is heated to a temperature in the range 280°C to 350⁰C, held within the same temperature range for a time of at least 5 seconds and then heated to a temperature above 350°C for pyrolysis, the biomass feedstock particles having a d₅₀ mean particle size of at least 1 mm, wherein d₅o denotes a number length mean particle size such that 50% of particles have volume smaller than a sphere of diameter d₅₀ and 50% of particles have volume larger than a sphere of diameter dso.

In this way, the tar content of the pyrolysis products can be reduced, which is advantageous. Although not wishing to be bound by theory, the present inventors speculate that this is due to a modification of the complex reaction pathways that occur in the degradation of lignin in the biomass, favouring the production of lower organics compounds in the final pyrolysis products. It is considered that most tar formed in pyrolysis processes is derived, at least in part, from lignin, in particular during condensation reactions and uncontrolled fragmentation of large molecules derived from the degradation products of lignin.

In a second aspect, the present invention provides a biomass pyrolysis process in which biomass feedstock particles are thermally treated substantially in the absence of oxygen to pyrolyse the biomass feedstock particles, wherein the thermal treatment of the biomass feedstock particles includes a step in which at least a part of each of the biomass feedstock particles is heated to a temperature in the range 280⁰C to 350⁰C, held within the same temperature range for a time of at least 5 seconds and then heated to a temperature above 350⁰C for pyrolysis, the biomass feedstock particles having a dso mean particle size of at least 1 mm.

Preferably, in this second aspect, the part of each particle whose temperature is controlled in the range 280⁰C to 350⁰C is an outer part of each particle. The depth of this outer part of each particle will typically vary depending on the time for which the particle is held within the 280⁰C to 350⁰C temperature range. Preferably, this depth extends substantially to the centre of the particle, so that substantially the whole particle is within the required temperature range.

In a third aspect, the present invention provides a biomass pyrolysis apparatus for thermal treatment of biomass feedstock particles substantially in the absence of oxygen to pyrolyse the biomass feedstock particles, the apparatus providing a pre-pyrolysis heating location for heating the biomass feedstock particles to a temperature in the range 280⁰C to 350⁰C and for holding the temperature of the biomass feedstock particles in the range 280⁰C to 350⁰C for at least 5 seconds, the apparatus further providing a pyrolysis heating location for subsequently heating the biomass feedstock particles to a temperature above 350°C for pyrolysis .

Preferred and/or optional features of the invention will now be set out. These are applicable either singly or in any combination with any aspect of the invention, unless the context demands otherwise.

Preferably, the biomass feedstock particles are subjected to a heat treatment drying step at a temperature in the range

100⁰C to 250°C, before being heated to the temperature in the range 280⁰C to 350⁰C. Thus, it is preferred that there are at least three distinct stages to the heat treatment schedule: a first pre-pyrolysis heating stage, a second pre- pyrolysis heating stage and a pyrolysis heating stage. Preferably the biomass feedstock particles are subjected to a heat treatment drying step at a temperature in the range 100⁰C to 250⁰C for a time sufficient to allow at least 50% of the mass of the particles subjected to the heat treatment drying step (and preferably at least 60%, at least 70%, at least 80% or at least 90% of the mass of the particles subjected to the heat treatment drying step) to reach a temperature of at least 100⁰C (and more preferably at least 110⁰C, at least 120⁰C, at least 130⁰C, at least 140⁰C, at least 150⁰C, at least 160⁰C, at least 170⁰C, at least 180⁰C, at least 190°C, or at least 200⁰C) . This, of course, is dependent on the particle size. However, preferably this time is at least 1 second, more preferably at least 5 seconds, more preferably at least 10 seconds, at least 20 seconds or at least 30 seconds.

Whilst it is possible for the biomass feedstock particles to be temperature soaked at each, fixed temperature stage, it is also possible for the average temperature of the mass of the biomass to vary during each stage of heating. Thus, during the pre-pyrolysis heating stage (s), for example, the average temperature of the mass of the biomass may increase, provided that it remains within the required range for the required amount of time. However, it is preferred that between the heat treatment drying stage and the pre- pyrolysis heating stage the average temperature of the mass of the biomass has a faster rate of change of temperature than during either the heat treatment drying stage and the pre-pyrolysis heating stage. Similarly, it is preferred that between the pre-pyrolysis heating stage and the pyrolysis stage the average temperature of the mass of the biomass has a faster rate of change of temperature than during either the pre-pyrolysis heating stage and the pyrolysis stage. In this way, a stepped heat treatment schedule is provided for the biomass. This provides the benefit of relatively long residence times for the biomass in temperature ranges that provide technical benefits but relatively short residence times at temperatures that provide technical drawbacks, as identified by the present inventors .

Preferably, in the step in which at least 50% of the mass of the biomass feedstock particles is held within the same temperature range for a time of at least 5 seconds (i.e. the pre-pyrolysis heating stage) , the temperature range is 280°C to 320⁰C.

The biomass feedstock particles are typically pyrolysed at a temperature in the range 350⁰C to 550⁰C. Such temperatures are lower than temperatures typically used for gasification. Thus, it is preferred that the main product of the pyrolysis step is bio-oil. Preferably the pyrolysis step is for at least 1 second, more preferably at least 5 seconds, more preferably at least 10 seconds, at least 20 seconds or at least 30 seconds. As the skilled person will appreciate, the preferred time for the pyrolysis step depends on the properties of the biomass particles, and in particular on the dimensions of the biomass particles. For example, the time for the pyrolysis step may be at least one minute. The time for the pyrolysis step may preferably be at most 10 minutes. For smaller particles of dimensions of about 1 mm (e.g. rape seed), the time for the pyrolysis step may be relatively short. For larger particles of dimensions (in at least one direction) of up to about 4 cm (e.g. wood chips) the time for the pyrolysis step may be relatively longer. The d₅o particle size for the biomass feedstock may be, for example, at least 2 mm, at least 3 mm, at least 4 mm or at least 5 mm.

More generally, as the skilled person understands, the optimum time for each stage depends on the biomass particle size. Thus, in the step in which at least 50% of the mass of the biomass feedstock particles is heated to a temperature in the range 280⁰C to 350⁰C and held within the same temperature range for a time t2βo-₃5θ, it is preferred that the following relationship is satisfied:

^280-350 — ^-"50 in which d₅o is the number length mean particle size in metres and C is a constant and is at least 5000 seconds per metre. Preferably C is at least 6000 seconds per metre, and C may be at least 7000 seconds per metre. For example, for d₅₀ particle size of 4 cm, time t∑so-βso may be about 5 minutes (300 seconds), corresponding to C of 7500 seconds per metre. C therefore is in effect an empirical parameter based on the rate of increase of temperature of central regions of the biomass particle when the biomass particles are subjected to the heat^' treatment in the temperature range 280⁰C to 350⁰C.

Although not wishing to be bound by theory, the present inventors consider that the flow of heat energy per unit time to the biomass feedstock particles (and thus the rate of increase of temperature of the biomass feedstock particles) has an important effect on the products of each stage of the process. In particular, it is considered important that the required temperature for each stage is not overshot. In turn, this requires that the process is carried out relatively slowly in order to avoid such an overshooting effect.

It is therefore preferred that the heat carrier medium at each stage has a maximum temperature which is at most 200⁰C higher than the temperature of the biomass feedstock on entry into that stage of the process. This therefore provides a limit on the absolute increase in temperature for the biomass feedstock particles at that stage.

Similarly, it is preferred that the amount of heat carrier medium, in proportion to the amount of biomass, is limited at each stage. Preferably the amount of heat carrier medium is three to five times the amount of biomass feedstock, by mass.

Preferably, the heat carrier medium has a relatively high specific heat capacity, e.g. at least 0.4 kJ kg^"1 K^"1. Preferably, the heat carrier medium is a plurality of solid heat carrier particles or objects. For example, steel balls may be used as the heat carrier medium in at least one stage of the process. Alloy filled steel balls may have an even higher heat capacity, and therefore may be of particular utility.

Preferably, the biomass feedstock particles are heated to the temperature in the range 280⁰C to 350⁰C at a pre- pyrolysis heating location in a heat treatment system and subsequently the biomass feedstock particles are heated to the temperature above 350⁰C for pyrolysis at a pyrolysis heating location in the heat treatment system. The biomass feedstock particles may be moved between the pre-pyrolysis heating location and the pyrolysis heating location by a conveyor system. It is particularly preferred that the biomass feedstock particles are moved between the pre- pyrolysis heating location and the pyrolysis heating location substantially without reduction in temperature.

Preferably the conveyor system is a screw or auger. However, the conveyor system may be gravity-assisted, e.g. controlled by a valve or other closure means.

The process is preferably a continuous process. This is in preference to a batch process, for example. Thus, there is preferably a substantially continuous conveyance of biomass feedstock particles from the pre-pyrolysis heating stage to the pyrolysis heating stage. It is possible for the pre-pyrolysis heating location and the pyrolysis heating location to be parts of the same heat treatment kiln, at different temperatures.

Preferably, the conveyor system conveys the biomass feedstock particles at a substantially uniform rate. In this way, the residence time of the biomass feedstock particles in the pre-pyrolysis heating location and the pyrolysis heating location are typically determined by the spatial extent of the pre-pyrolysis heating location and the spatial extent of the pyrolysis heating location, and on the rate of conveyance of the biomass feedstock particles.

It is preferred that the heat treatment stages use a heat carrier to transfer heat to the biomass feedstock particles. For example, where a fluidized bed system is used, the heat carrier may be sand. Where a screw kiln is used, the heat carrier may be steel balls and/or ceramic balls. It is preferred that at least a portion of the heat carrier is recycled in the process. In particular, heat carrier used in a hotter stage of the process may be recycled for used in a lower temperature stage of the process, optionally without reheating. Where reheating is required in order to provide heat carrier at a suitable temperature, heating means may be provided. Optionally, one or more products of the overall process (e.g. char and/or waste gas) may be combusted in order to provide heat for the process. In the apparatus, it is preferred that the pre-pyrolysis heating location and the pyrolysis heating location are parts of a heat treatment kiln, at different temperatures. Similar preferences apply where there are first and second pre-pyrolysis heating stages and/or first and second pyrolysis heating stages.

The apparatus preferably has a conveyor system for moving the biomass feedstock particles between the pre-pyrolysis heating location and the pyrolysis heating location. The conveyor system may be a screw or auger, for example.

The apparatus may further provide a heat carrier recycling system for at least partial recycling of heat carrier material from the pre-pyrolysis heating stage and/or from the pyrolysis heating stage to one or more of the pre- pyrolysis heating stage and the pyrolysis heating stage.

The plateau in the heat treatment schedule, at an elevated temperature but below the temperature at which significant pyrolysis will occur, is used in order to control the steepness of the temperature gradient at the biomass feedstock particles during the subsequent transition into the pyrolysis regime. The inventors consider that the lower the heating rate in this transition, then the lower the concentration in the pyrolysis products of fragments from high tar lignin-based components.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows the schematic layout of a pyrolysis reactor apparatus according to one embodiment of the invention.

Fig. 2 shows the schematic layout of a pyrolysis reactor apparatus according to another embodiment of the invention.

Fig. 3 shows the schematic layout of a pyrolysis reactor apparatus according to another embodiment of the invention. Fig. 4 shows a modification to the embodiment of Fig. 3.

Fig. 5 shows the product distribution for pyrolysis of wheat straw, at different temperatures.

Fig. 6 shows the product distribution for pyrolysis of wheat straw pellets at different temperatures. Figs. 7a-7i illustrate the production of the main products released during pyrolysis of lignin, and provides corresponding numerical simulations.

Fig. 8 shows an enlarged annotated version of Fig. 7a. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, FURTHER OPTIONAL FEATURES

Fig. 1 shows the schematic layout of a pyrolysis reactor apparatus 10 according to one embodiment of the invention. In this apparatus, there is provided a first heating stage with a first fluidized bed reactor 12 with heat carrier (not shown) heated to 200⁰C. This heating stage amounts to a first pre-pyrolysis heating stage. Biomass feed material (not shown) is fed into first fluidized bed reactor 12 via inlet 14. Gaseous products (i.e. products, gaseous at or near 200°C, of heating the biomass feed material at 200⁰C) from the first fluidized bed reactor 12 are extracted at first reactor gas product outlet 16. The gas product of the first reactor may be water-rich, but may still have a useful heating value. In this case, the gas product of the first reactor may be used to heat the heat carrier used in one or more of the reactors.

At the base of the first reactor 12 is outlet 18, for conveying the processed biomass feed material from the first reactor 12 to a second reactor 20. The input of material into the second reactor 20 is controlled by second reactor inlet 22. The material may be conveyed from the first reactor 12 to the second reactor 20 by gravity alone, or may be conveyed additionally or alternatively by screw conveyor or auger. The heat carrier used in the first reactor is preferably recycled for further use in the apparatus, most preferably for further use in the first reactor in order that the amount of reheating required is minimised. However, it is possible for some of the heat carrier used in the first reactor to be conveyed into the second reactor.

The heat carrier may be, for example, char, sand and/or steel. Some or all of the heat carrier may be recycled or carried forward, depending on the effort needed in order to separate it from the feedstock material.

The second reactor 20 is also a fluidized bed reactor, but differs from the first reactor 12 in that it operates at 300°C. This therefore amounts to a second pre-pyrolysis heating stage. The biomass feed material is held in second reactor 20 for a time sufficient to allow at least 50% of the mass of the biomass feed material particles to reach about 300⁰C. The time required for this depends on the particle size of the feed material. Typically, the use of a fluidized bed reactor requires that the biomass feed material particle size is relatively small, e.g. at least 1 mm or about 2 mm. Thus, the biomass feedstock particles have a d₅₀ mean particle size of at least 1 mm or about 2 mm. As the skilled person understands, dso denotes a number length mean particle size such that 50% of particles have volume smaller than a sphere of diameter d₅o and 50% of particles have volume larger than a sphere of diameter dso- More preferably, the biomass feedstock particles have a dgo mean particle size of at least 1 mm, where dgo denotes a number length mean particle size such that 10% of particles have volume smaller than a sphere of diameter dgo and 90% of particles have volume larger than a sphere of diameter dgo. Typically the biomass feed particles are held in the second reactor for at least 5 seconds. For larger particle sizes, longer residence times are needed in order to ensure suitable uniformity in temperature through the diameter of the particles.

Gaseous products (i.e. products, gaseous at or near 300⁰C, of heating the biomass feed material at 300°C) from the second fluidized bed reactor 20 are extracted at second reactor gas product outlet 24. The gas product of the second reactor may be water-rich, but may still have a useful heating value. In this case, the gas product of the second reactor may be used to heat the heat carrier used in one or more of the reactors. Alternatively, this gas product may have a suitably high heating value to be used as fuel for alternative purposes (e.g. consumer heat generation, electricity generation, etc.).

At the base of the second reactor 20 is outlet 26, for conveying the processed biomass feed material from the second reactor 20 to a third reactor 28. The input of material into the third reactor 28 is controlled by third reactor inlet 30. The material may be conveyed from the second reactor 20 to the third reactor 28 by gravity alone, or may be conveyed additionally or alternatively by screw conveyor or auger.

The heat carrier used in the second reactor is preferably recycled for further use in the apparatus, most preferably for further use in the second reactor, in order that the amount of reheating required is minimised. However, it is possible for some of the heat carrier used in the second reactor to be conveyed into the third reactor.

The third reactor 28 is also a fluidized bed reactor, but differs from the first reactor 12 and second reactor 20 in that it operates at 400°C. This therefore amounts to a first pyrolysis heating stage. As discussed in more detail below, the result of heating at about 400°C is to promote pyrolysis, the products of pyrolysis depending on the temperature of the first pyrolysis heating stage (400⁰C in this case) and the nature of the heating schedule prior to the first pyrolysis heating stage. Since in the second reactor the biomass is heated substantially uniformly to about 300°C, the rate of heating as the biomass is transferred from the second pre-pyrolysis heating stage to the first pyrolysis heating stage is not particularly high. The effect of this is that the amount of tar in the products of the first pyrolysis heating stage is reduced. The gaseous products of the first pyrolysis heating stage are extracted from third reactor 28 along line 32. These gaseous products are condensed (not shown) to give bio-oil and permanent gases (e.g. syngas (mixture of H₂ and CO)). The gaseous products may be subjected to a char removal step (not shown), e.g. via electrostatic precipitation and/or cyclonic separation.

At the base of the third reactor 28 is outlet 34, for conveying partially pyrolysed biomass feed material from the third reactor 28 to a fourth reactor 36. The input of material into the fourth reactor 36 is controlled by fourth reactor inlet 38. The material may be conveyed from the third reactor 28 to the fourth reactor 36 by gravity alone, or may be conveyed additionally or alternatively by screw conveyor or auger.

The heat carrier used in the third reactor is preferably recycled for further use in the apparatus, most preferably for further use in the third reactor, in order that the amount of reheating required is minimised. However, it is possible for some of the heat carrier used in the third reactor to be conveyed into the fourth reactor. The fourth reactor 36 is also a fluidized bed reactor, but differs from the first reactor 12, second reactor 20 and third reactor 28 in that it operates at 500°C. This therefore amounts to a second pyrolysis heating stage. The result of heating partially pyrolysed biomass material at about 500°C is to further pyrolysis, in particular permanent gas formation, since oil production is concentrated in particular in the first pyrolysis stage.

The gaseous products of the second pyrolysis heating stage are extracted from fourth reactor 36 along line 40. In this embodiment, line 40 joins with line 32 at a manifold connection 42. Thus, the gaseous products of both the first and second pyrolysis stages are condensed together (not shown) to give bio-oil and permanent gases (e.g. syngas (mixture of H₂ and CO) ) . In view of other gas-borne products that may be entrained with the gaseous products, a char removal step (not shown) may be deployed, e.g. via electrostatic precipitation and/or cyclonic separation.

At the base of the fourth reactor 36 is outlet 44, for conveying the remaining solid material from the fourth reactor 36 to chamber 46. Within chamber 46, the solid char is separated from the heat carrier. The heat carrier may be recycled for use in the fourth reactor again, or it may be used to heat one or more of the other reactors. The collected char may be used as a fuel to provide heat where required in the apparatus, or may be used as a fuel elsewhere, e.g. for consumer heat generation, or for electricity generation.

Fig. 1 illustrates an embodiment of the invention in which a series of fluidized bed reactors are placed in series, with increasing reactor temperature, in order to provide to the biomass feed material a suitable heat treatment profile in order to obtain some of the advantages of the invention. However, as will be clear to the skilled person, other types of reactor may be used. For example, a mixing drum type of reactor may be used for each heating stage, with appropriate conveying means between them. These are particularly useful for biomass feed material that has a relatively large (e.g. up to about 4 cm dso) particle size, and may be suitable where the particle size is non-uniform. Alternatively, screw reactors may be used. Still further, steel ball reactors or cycled spheres reactors may be used. Furthermore, it is not necessary for each heating stage to be carried out using the same type of reactor. For example, one heating stage may be carried out in a screw reactor, whereas another stage may be carried out in a fluidized bed reactor.

Fig. 2 shows an alternative embodiment of the invention.

Here a mixing drum reactor apparatus 50 is provided. Mixing paddles 52, 54, 56, 58 provide mixing of the biomass feed material 53 and heat carrier 55. These are rotated about axis 51. In Fig. 2, the biomass feed material is conveyed in a direction generally left to right. This may be assisted by a slight declination of the axis of the reactor, but this is not essential. As can be seen, there are provided four heating stages, at 200⁰C, 300⁰C, 400⁰C and 500⁰C. These correspond to the first (200⁰C) pre-pyrolysis stage, the second (300⁰C) pre-pyrolysis stage, the first (400⁰C) pyrolysis stage and the second (500⁰C) pyrolysis stage described with respect to Fig. 1.

In cases where the speed of travel of the biomass along the apparatus 50 is fixed, the residence time of the biomass in each heating stage may be varied by varying the length of each stage. Conveniently, this may be done by varying the axial extent of the heating means 60, 62, 64, 66.

Gaseous product removal lines (not shown) and a char removal conduit (not shown) are also provided.

Fig. 3 shows an alternative embodiment of the invention. Here, two screw reactors 70, 72 are provided. Biomass feed material and heat carrier material at temperature tl are introduced into first screw reactor 70 at inlet 71. Thus, the temperature at the left hand end of the first screw reactor 70 is tl, as illustrated. The heat carrier is heated to temperature tl by heater 74. Thus, the left hand side of the first screw reactor 70 provides the first pre- pyrolysis heating stage, the biomass feed material and the heat carrier being conveyed by the screw and the residence time of the biomass in the first pre-pyrolysis heating stage being determined by the length of the first pre-pyrolysis heating stage and the speed of conveyance.

Heat carrier at temperature t2 is introduced at inlet 76. t2 is greater than tl. Therefore, the right hand side of the first screw reactor corresponds to the second pre- pyrolysis heating stage. By the time the biomass reaches the right hand end of the first screw reactor, it is at a temperature of t2*, where t2* is greater than tl, but less than t2. Accordingly, t2* is in the range 280-350°C, e.g. about 300-320⁰C. The pre-heated biomass, any char and the heat carrier is discharged at outlet 78 into separation chamber 80 where the heat carrier is at least partially separated from the feed, so that some heat carrier (and preferably no biomass feed) is recycled to inlet 71 at temperature tl via heater 74.

Typically, if the amount of heat carrier inserted at inlet 71 is 1 (arbitrary units) , then the amount of heat carrier inserted at inlet 76 at temperature t2 is 0.5. Thus, in separation chamber 80, 1 unit of heat carrier is recycled back to inlet 71 and 0.5 units is allowed to proceed into the second screw reactor 72. The pre-heated biomass feed particles are passed into the left hand end of the second screw reactor 72. Additional heated heat carrier (0.5 units) is supplied via inlet 82 at a temperature sufficient to provide a temperature t3 at the left hand side of the second screw reactor, where t3 is greater than t2. Thus, 1 unit of heat carrier in total is provided to the pre-heated biomass. At temperature t3, pyrolysis of the pre-heated biomass takes place, generating pyrolysis gas and vapours that are extracted from the second screw reaction 72 along an extraction line (not shown) . The left hand side of the second screw reactor therefore corresponds to the first pyrolysis heating stage.

Further additional heat carrier (0.5 units) is provided to the second screw reactor towards the right hand side of the second screw reactor. This heat carrier is at temperature t4, which is a higher temperature than t3, but is a temperature at which useful pyrolysis still occurs. The right hand side of the second screw reactor therefore corresponds to the second pyrolysis heating stage. Vapour and gaseous pyrolysis products from the second pyrolysis heating stage may be extracted along the same extraction line as for the first pyrolysis heating stage.

In an alternative embodiment, t3 may be lower than the temperature required for pyrolysis. In this case, there may be three pre-pyrolysis heating stages and only a single pyrolysis stage at temperature t4.

In either embodiment, the temperature of the combination of the remains of the biomass feed and the heat carrier may be reduced to about t2 at the right hand end of the second screw reactor. These materials exit the second screw reactor at outlet 86, being discharged into separation chamber 88. Here the char is separated from the heat carrier (1.5 units). 0.5 units of heat carrier are recycled to inlet 84 at temperature t4 via heater 90. 1 unit of heat carrier is recycled to inlet 82 at temperature t3, via conduit 92 and via additional heating means (not shown) if required.

Fig. 4 shows a modified embodiment compared with Fig. 3. Identical features are not described or numbered again. In Fig. 4, conduit 92 does not lead only to inlet 82, but also leads, via a manifold, to inlet 76.

The particle size distribution of the biomass feedstock can be determined, for example, by microscopic examination, based on a representative sample of the biomass feedstock particles.

The embodiments of the invention described here are principally concerned with intermediate pyrolysis processes. Such processes allow various types of feed material to be used, e.g. different types of biomass, different shapes of biomass particles and also allow for some inhomogeneity in the shapes and/or types of biomass particles. For this reason, intermediate pyrolysis is of particular interest for intermediate scale applications where the quality, type and pre-processing capability of biomass is limited.

Intermediate pyrolysis processes can produce high quality pyrolysis products. Variable relative yields of products (coke (char) , bio-oil and gas) are possible by varying the process parameters, as explained below.

It is considered that some intermediate pyrolysis processes become economically viable at biomass throughput rates of about 12,000 tonnes per year to about 20,000 tonnes per year (based on dry biomass) .

Fig. 5 shows the product distribution for pyrolysis of wheat straw at different temperatures, for a 500 kg/hour feedrate. Fig. 6 shows the corresponding product distribution for pyrolysis of wheat straw pellets (i.e. pressed particles of straw) at different temperatures. Note that the temperatures used in Fig. 6 are different to those used in Fig. 5. It is important to note that the dimensions and shape of the feedstock does not have a significant effect on the products of pyrolysis. Figs 5 and 6 show that similar good results can be achieved for both milled straw (Fig. 5) and for straw pellets (Fig. 6) , provided that the provision of heat is well-controlled.

Elemental analysis of the composition of the initial wheat straw and the char formed at 325°C, 350°C, 375°C, 400°C and 450⁰C shows the following trends with increasing temperature: N content increases from 0.350 wt% to 0.615 wt%; C content increases from 42.09 wt% to 63.02 wt%; H content decreases from 5.67 wt% to 3.26 wt%; S content increases from 0.105 wt% to 0.251 wt%; ash content increases from 5.60 wt% to 16.70 wt%; oxygen content decreases from 46.185 wt% to 16.152 wt% . The C:0 ratio increases from 0.9 to 3.9.

It is of interest to consider how tar forms during typical pyrolysis processes. Large amounts of tar in the bio-oil are considered to be undesirable. Typical high organics compounds that may be classified as tar in this technical field may be considered to be organic compounds of molecular weight 300 a.m.u. or greater.

The biomass feed material is in general composed of cellulose, glucan, xylan and lignin, in differing proportions depending on the nature of the feed material.

For lignin-rich biomass feed materials, it is of particular interest to consider the thermal decomposition of lignin and how the heat treatment schedule of the biomass may affect the formation of tar from lignin. Lignin encompasses a class of complex, high molecular weight polymers whose exact structure varies.

During thermal decomposition, lignin may decompose by a variety of mechanisms, such as decomposition into oligomers, the separation of side groups, and the formation of modified chains. Side groups in general may form permanent gases. The remaining components form useful bio-oil, tar and char.

In more detail, certain reactions can affect the principal polymer backbone to provide a molecular weight decrease in the sample and volatile products such as monomers, dimers, etc. Reactions may affect the principal polymer backbone to provide tars and waxes (typically from oligomers and polymer chain fragments) .

Furthermore, certain reactions can affect the polymer repeated units. For example, eliminations can provide volatile products and char residue. Cyclization can modify the chains, e.g. by unsaturation or cross linking, again producing volatile products and char residue. Cyclization can alternatively form volatile products by chain scission. Thus, the propagation of the reactions can affect both the molecular weight distribution and the chemical structure of the polymer and thus of the pyrolysis products.

Fig. 7 (Figs. 7a-7i) shows the main production of the products released during pyrolysis of lignin. Fig. 7 shows that with a higher heating rate, the amounts of fragments from high molecular weight species increases. In Fig. 7, the differential thermogravimetric analysis was carried out on fine powder biomass feedstock material.

In Figs. 7a, 7b and 7c, the graphs show an overall thermogravimetric envelope (OTE) for the analysis. The heating rate for each of Figs. 7a, 7b and 7c was 25°C/min. These graphs also show (using solid lines) numerical simulation results of the likely contribution to the overall thermogravimetric envelope from low temperature decomposition processes, medium temperature decomposition processes and high temperature decomposition processes. It is noted that the overall envelope and the numerical simulation results in Figs. 7a, 7b and 7c are identical. Figs. 7a, 7b and 7c further show individual curves (corresponding to mass spectrometry results) for individual species from the decomposition of lignin. In Fig. 7b, water has a molecular weight (MW) of 18 and carbon dioxide has a molecular weight of 44. In Fig. 7c, carbon monoxide has a molecular weight (MW) of 28 and methane has a molecular weight of 16.

Figs. 7d, 7e and 7f show the relative proportions of the production of water, carbon dioxide and carbon monoxide, respectively, at different heating rates.

Figs. 7g, 7h and 7i show the relative production of certain alcohols during the decomposition of lignin, based on the heating rate. These alcohols can be considered to be useful indicators of the likelihood of tar formation. Thus, as the heating rate increases, the amount of tar expected also increases. (Oxy-allyl) guaiacol has a molecular weight of 151. 4- (hydroxy-prop-2-enyl) guiaiacol has a molecular weight of 136. Syringol has a molecular weight of 155.

Fig. 8 shows a marked-up version of Fig. 7a. Here the main calculated differential thermogravimetric (DTG) curve is marked as 100 for the pyrolysis reaction at a heating rate of 25°C per minute. A low temperature step 102 peaks at about 300°C, corresponding to a low temperature decomposition step for lignin, produced by numerical simulation. The main decomposition step 104 peaks at about 410⁰C, corresponding to a medium temperature decomposition step for lignin, produced by numerical simulation. A high temperature step 106 has a much broader temperature distribution than either the low temperature step of the main decomposition step, but peaks at about 440⁰C, and corresponding to a high temperature decomposition step for lignin, produced by numerical simulation.

In Fig. 8, as the temperature increases, there is notable evolution of (oxy-allyl) guaiacol (curve 108), syringol (curve 110) and 4- (hydroxy-prop-2-enyl) guiaiacol (curve 112)

As illustrated in Figs. 7 and 8, the low temperature step 102 peaks at about 300⁰C, but the main decomposition step does not peak until 400⁰C. The embodiments of the present invention utilise this gap by heating the biomass feed material to a temperature in the range 280⁰C to 350⁰C and holding the biomass feed material within the same temperature range for a time of at least 5 seconds. As shown in Figs. 7 and 8, the result is that the low temperature step is allowed to start but the main decomposition step is not allowed to take place. The result is that the evolved products from the low temperature stage are water-rich. These can be extracted from the reactor if desired. The biomass feed material is then heated to a temperature above 350⁰C for pyrolysis.

Allowing the biomass feed material to be held at a temperature in the range 280⁰C to 350⁰C allows a large proportion of the material, by mass, to reach a temperature within this range. Since typically the feed material particles are relatively large (which is advantageous in that it reduces pre-processing requirements of the feed material) , the present inventors consider that not only the surface of the particles should be allowed to reach this temperature range, but also a significant proportion (and preferably all) of the internal mass of the feed particles should be allowed to reach this temperature range. However, this stage of heating does not result in substantial pyrolysis .

The subsequent stage of heating, to temperatures above 350°C for pyrolysis, requires only a small absolute increase in temperature of the feedstock particles. The present inventors consider that the result of a small increase in temperature to the pyrolysis stage is that the proportion of tar present in the pyrolysis products is reduced.

Test with oils from non woody biomass have been made on dual fuel engines working properly due to the low tar content. Normally from fast pyrolysis only woody material pyrolysis oil can be applied but with very high difficulties, everything else fails. In the present case, the inventors consider that the use of a staged intermediate pyrolysis process allows the production of useful fuels with relatively low tar content using, for example, woody biomass, which is far more abundant than biomass with an inherently low tar content. Preferred embodiments of the invention have been described by way of example. On reading this disclosure, modifications of these embodiments, further embodiments and modifications thereof will be apparent to the skilled person and as such are within the scope of the invention.

Claims

1. A biomass pyrolysis process in which biomass feedstock particles are thermally treated substantially in the absence of oxygen to pyrolyse the biomass feedstock particles, wherein the thermal treatment of the biomass feedstock particles includes a step in which at least 50% of the mass of the biomass feedstock particles is heated to a temperature in the range 280°C to 350°C, held within the same temperature range for a time of at least 5 seconds and then heated to a temperature above 350⁰C for pyrolysis, the biomass feedstock particles having a dso mean particle size of at least 1 mm, wherein dso denotes a number length mean particle size such that 50% of particles have volume smaller than a sphere of diameter d₅₀ and 50% of particles have volume larger than a sphere of diameter d₅o-

2. A biomass pyrolysis process according to claim 1 wherein the biomass feedstock particles are subjected to a heat treatment drying step at a temperature in the range 100⁰C to 250⁰C, before being heated to the temperature in the range 280⁰C to 350⁰C.

3. A biomass pyrolysis process according to claim 1 or claim 2 wherein when the biomass feedstock particles are subjected to pyrolysis at a temperature in the range 350⁰C to 550⁰C.

4. A biomass pyrolysis process according to any one of claims 1 to 3 wherein in the step in which at least 50% of the mass of the biomass feedstock particles is heated to a temperature in the range 280°C to 350°C and held within the same temperature range for a time t28o-35o_> the following relationship is satisfied:

^280-350 — ^"50 in which dso is the number length mean particle size in metres and C is a constant and is at least 5000 seconds per metre .

5. A biomass pyrolysis process according to any one of claims 1 to 4 wherein the biomass feedstock particles are heated to the temperature in the range 280°C to 350°C at a pre-pyrolysis heating location in a heat treatment system and subsequently the biomass feedstock particles are heated to the temperature above 350°C for pyrolysis at a pyrolysis heating location in the heat treatment system.

6. A biomass pyrolysis process according to claim 5 wherein the biomass feedstock particles are moved between the pre-pyrolysis heating location and the pyrolysis heating location by a conveyor system.

7. A biomass pyrolysis process according to claim 5 or claim 6 wherein the biomass feedstock particles are moved between the pre-pyrolysis heating location and the pyrolysis heating location substantially without reduction in temperature.

8. A biomass pyrolysis process according to any one of claims 5 to 7 wherein the conveyor system is a screw or auger.

9. A biomass pyrolysis process according to any one of claims 5 to 8 wherein the pre-pyrolysis heating location and the pyrolysis heating location are parts of a heat treatment kiln, at different temperatures.

10. A biomass pyrolysis process according to any one of claims 5 to 9 wherein the conveyor system conveys the biomass feedstock particles at a substantially uniform rate, the residence time of the biomass feedstock particles in the pre-pyrolysis heating location and the pyrolysis heating location being determined by the spatial extent of the pre- pyrolysis heating location and the spatial extent of the pyrolysis heating location, and on the rate of conveyance of the biomass feedstock particles.

11. A biomass pyrolysis process according to any one of claims 1 to 10 wherein in the step in which at least 50% of the mass of the biomass feedstock particles is held within the same temperature range for a time of at least 5 seconds, the temperature range is 280⁰C to 320°C.

12. A biomass pyrolysis apparatus for thermal treatment of biomass feedstock particles substantially in the absence of oxygen to pyrolyse the biomass feedstock particles, the apparatus providing a pre-pyrolysis heating location for heating the biomass feedstock particles to a temperature in the range 280°C to 350⁰C and for holding the temperature of the biomass feedstock particles in the range 280⁰C to 350⁰C for at least 5 seconds, the apparatus further providing a pyrolysis heating location for subsequently heating the biomass feedstock particles to a temperature above 350⁰C for pyrolysis.

13. A biomass pyrolysis apparatus according to claim 12 wherein the pre-pyrolysis heating location and the pyrolysis heating location are parts of a heat treatment kiln, at different temperatures.

14. A biomass pyrolysis apparatus according to claim 13 further providing a conveyor system for moving the biomass feedstock particles between the pre-pyrolysis heating location and the pyrolysis heating location.

15. A biomass pyrolysis apparatus according to claim 14 wherein the conveyor system is a screw or auger.

16. A biomass pyrolysis apparatus according to claim 14 or claim 15 wherein the biomass feedstock particles are moved between the pre-pyrolysis heating location and the pyrolysis heating location substantially without reduction in temperature .

17. A biomass pyrolysis apparatus according to any one of claims 14 to 16 wherein the conveyor system conveys the biomass feedstock particles at a substantially uniform rate, the residence time of the biomass feedstock particles in the pre-pyrolysis heating location and the pyrolysis heating location being determined by the spatial extent of the pre- pyrolysis heating location and the spatial extent of the pyrolysis heating location, and on the rate of conveyance of the biomass feedstock particles.

18. A biomass pyrolysis apparatus according to any one of claims 12 to 17 further providing a heat carrier recycling system for at least partial recycling of heat carrier material from the pre-pyrolysis heating stage and/or from the pyrolysis heating stage to one or more of the pre- pyrolysis heating stage and the pyrolysis heating stage.