Pyrolysis of biomass   

Abstract: A method of treating biomass material, particularly plant-derived biomass material, to produce pyrolysis thereof, comprising subjecting the biomass material to radio frequency electromagnetic radiation, e.g. microwave radiation, while the material is being agitated, under suitable conditions to produce a desired degree of pyrolysis.

FIELD OF THE INVENTION

This invention concerns pyrolysis of biomass. The term biomass is used herein to refer to any organic source of energy or chemicals that is renewable, including trees (wood) and other vegetation; agricultural products and wastes (corn, fruit, ensilage, etc.); algae and other marine plants; metabolic wastes (manure, sewage); and cellulosic urban waste. The invention is particularly concerned with plant-derived biomass, such as wood, straw, grass, etc.

Pyrolysis is the thermal transformation, typically degradation, of material by heating in the absence of oxygen or with limited oxygen (less than the amount required for stoichiometric reaction). Pyrolysis of biomass is a useful technique for the recovery of energy from biomass. Pyrolysis of biomass involves heating biomass, typically to a temperature in the range 400 to 1000° C., and results in a range of possible products including solid products such as char, charcoal, or coke, liquid products such as oils and tar, and low molecular weight gases such as carbon monoxide, carbon dioxide, hydrogen, steam, with the nature and relative amounts of the products depending on reaction conditions.

The present invention concerns an alternative approach to pyrolysis of biomass.

SUMMARY

The present invention provides a method of treating biomass material to produce pyrolysis thereof, comprising subjecting the biomass material to radio frequency electromagnetic radiation while the material is being agitated, under suitable conditions to produce a desired degree of pyrolysis.

The method results in production of a pyrolysed charred solid product.

The term radio frequency is used herein to refer to frequencies of about 10 kHz and higher. It is preferred to use a radio frequency in the range 300 MHz to 3 THz, more preferably 500 MHz to 300 GHz. It is particularly preferred to use microwave frequency electromagnetic radiation, e.g. having a frequency in the range 500 MHz to 100 GHz, preferably 800 MHz to 5 GHz. It is convenient to use microwave radiation from commercially available microwave generators, which commonly produce microwaves with a frequency of 895 MHz, 915 MHz, 922 MHz or 2450 MHz (2.45 GHz).

The biomass is preferably plant-derived biomass, particularly wood, straw, grass e.g. miscanthus, etc. Wood (containing about 15% by weight of water) typically has a calorific value of about 16 MJ/kg.

The biomass material is conveniently in the form of discrete solid pieces. The size and form of the pieces is not critical, but will affect processing to some extent. The pieces are ideally reasonably uniform in terms of size for uniformity of processing. For ease of handling, the pieces preferably have a maximum dimension of not more than about 100 mm, and are conveniently in the form of chunks, chips, pellets or granules. Each piece preferably has a volume that does not exceed about 30 cm3 as this has been found experimentally to give the best results. Where the biomass material comprises wood, the wood is conveniently in the form of wood chips, e.g. as obtained from a conventional wood chipping machine. Where the biomass material is in compressed form, e.g. briquettes, the pieces preferably have a maximum dimension of not more than about 25 mm, more preferably not more than about 10 mm, as heat is retained in the centre of the pieces to a greater extent than with uncompressed, less dense materials, resulting in preferential charring in the centre and hence a less uniform product for larger pieces.

The method is typically carried out in a processing vessel, e.g. a rotating drum as discussed below, having inlet means for supplying material to be treated to the vessel and outlet means for removing the resulting pyrolysed product. The vessel is closed or within a sealed enclosure to prevent unwanted ingress of air, enabling control of the gaseous environment within the vessel and so enabling processing to be carried out in the absence of oxygen or with limited oxygen (less than the amount required for stoichiometric reaction) as is required for pyrolysis to take place.

Processing should be carried out at a processing temperature (in the processing vessel) that is sufficiently high to ensure that volatile gaseous species (e.g. steam) released during pyrolysis do not condense in the processing vessel during processing. Processing is preferably carried out at a processing temperature of at least about 70° C., desirably at least 100° C., or at least 120° C., with the processing temperature preferably not exceeding about 175° C.

It is found that pyrolysis in accordance with the invention can be carried out at considerably lower temperatures than those required in conventional pyrolysis techniques, not using radio frequency electromagnetic radiation. This is because the mechanism of radio frequency-induced pyrolysis is such that heating is very limited in the biomass material, which accordingly has a relatively low bulk temperature. Less energy input is required with the method of the invention compared with conventional methods to produce the same degree of pyrolysis and charring. The method of the invention is thus more energy efficient. A further benefit is that the charred solid end product is at a lower temperature than with conventional techniques, reducing or eliminating the need for cooling, e.g. by water quenching. For example, using willow chips (having a calorific value of about 18 MJ/kg, at a water content of about 15% by wt) as the biomass material, treatment at a processing temperature of 120° C. (compared with about 250° C. in conventional thermal pyrolysis methods) produces a torrefied type of willow char (that is brown in colour and has a calorific value of about 22 MJ/kg), with the resulting char product having a temperature of about 70° C. on exiting the processing vessel. Similarly a willow charcoal (that is black in colour and has a calorific value of about 26 MJ/kg)) can be produced from the willow chips after rather longer processing at a processing temperature of about 175° C. (compared with about 600° C. in conventional thermal pyrolysis methods), with the resulting char product having a temperature of about 175° C. on exiting the processing vessel.

Reaction conditions, particularly reaction time, reaction temperatures and radio frequency, e.g. microwave, power, can be readily selected to produce desired results, having regard to factors including the type of biomass (material, moisture content, size of pieces etc) and desired degree of pyrolysis (and hence degree of charring and relative proportions of char and gas). In general, longer processing produces a solid product with a greater degree of charring and a higher calorific value. It is found that the degree of pyrolysis can be finely controlled by the processing time (typically up to about 45 minutes, and commonly in the range 5 to 30 minutes, typically 15 to 30 minutes, depending on the feedstock properties) and the power density of the radio frequency radiation. The processing temperature is typically in the range 120° C. to 175° C., with the resulting product temperature (on exiting the processing vessel) being in the range 70° C. to 175° C.

The biomass material is agitated, i.e. moved and mixed, during treatment, preferably on a continuous basis, to encourage uniformity of processing. This is conveniently achieved by performing the method in a rotating processing vessel e.g. a rotary drum. Additionally or alternatively, a gas may be passed through the biomass material being treated to cause agitation. The gas may be an inert gas, such as nitrogen. As a further possibility, the gaseous product of the pyrolysis reaction may be drawn off and recirculated through the biomass material, e.g. by being injected from beneath, providing a cheaper source of agitating gas.

The method is conveniently carried out in a rotating vessel such as a drum arranged for rotation about a generally horizontal axis, e.g. as disclosed in WO 2007/007068. The vessel is preferably rotated at a speed in the range 1 to 3 rpm, to cause gentle agitation of the biomass material. The axis of rotation is desirably slightly inclined to promote passage of the material along the vessel, with the angle of inclination being set to produce a desired residence time in the vessel, say about 15 minutes.

The moisture content of the biomass material as supplied for treatment is desirably at least about 10% (w/w), preferably about 15% (w/w) for energy-efficient processing. Material with a higher moisture content is preferably subjected to a pre-drying step to reduce the water content ideally to about 15% (w/w). It is important that the biomass material has some water content.

The method of the invention produces pyrolysis of the biomass material, resulting in production of a pyrolysed charred solid product of higher calorific value than the biomass material, and a gaseous product (pyrolysis gas) comprising low molecular weight gases such as carbon monoxide, carbon dioxide, hydrogen, steam, etc. The degree of pyrolysis, and hence charring, depends on treatment conditions, particularly treatment time and the power level of the radio frequency electromagnetic radiation, typically microwave radiation, with longer treatment times and/or higher power levels producing greater degrees of pyrolysis, with more charred products of higher calorific value. The relative proportions of solid and gaseous products also vary with the degree of pyrolysis, with increased gas being produced with increased pyrolysis levels.

The solid product is useful as a fuel (having an increased calorific value and greater friability compared with the biomass material from which it is derived), with other uses including as activated carbon (as the pyrolysed material has increased porosity), as a sequestering agent (having the benefit of being highly resistant to degradation), and as a soil improvement agent.

The solid product has particular use as a feedstock for coal-fired power stations, to replace or supplement coal. It is particularly preferred to use a char product, e.g. as discussed above, for this purpose. The material has a number of benefits and advantages: it has a calorific value comparable to that of coal (about 22 MJ/kg); it is brittle, and so can be fed directly to the power station without requiring any pre-treatment such as grinding; it has milling characteristics comparable to that of coal, as indicated by Hardgrove Grindability Index (HGI) values, and so can be processed through mills in a similar manner to coal with mills operating at full capacity. The char product thus provides a valuable supplementary plant-derived renewable energy source, which can be easily mixed with coal or utilized singly in existing coal-fired power stations.

The pyrolysis gas may be burnt to harness the energy therein, e.g. in the form of heat, electricity, etc.

The method may be carried out in the substantial absence of oxygen, with no introduced air or oxygen. Alternatively, a small, restricted, controlled amount of air/oxygen may be introduced (less than required for stoichiometric reaction with oxygen) during the pyrolysis reaction. It is found that this results in production of a greater proportion of gas, with a much reduced quantity of char and little tar, in a gasification process.

The method may be carried out batch-wise, but is preferably carried out on a continuous basis.

The invention also covers a charred solid product and/or pyrolysis gas produced by the method of the invention.

The invention also covers use of the charred solid product as a feedstock for a coal-fired power station.

The charred solid product of the invention can be readily processed to produce compacted forms such as pellets and briquettes, e.g. by use of well known processes and equipment. Processing typically involves combining the product in finely divided or particulate form with a suitable binding agent, and compressing the mixture. A range of binding agents may be used, e.g. starches, carboxy methyl cellulose (CMC), lignosulphonates, phenolic binders, etc. The resulting compressed products find use e.g. as domestic, commercial and industrial fuels.

It is surprisingly found that such compressed products prepared with starch-based binding agents are water-resistant and mould resistant. For example, pellets produced in this way and subjected to immersion in water for 14 days have not softened, broken down or deteriorated in any way. By contrast, pellets produced in similar manner from wood or from char manufactured by other torrefaction processes are not water-resistant; instead they break down within seconds on contact with water. Further, pellets produced from the solid product of the invention with starch-based binders have been held at high humidity for one month yet have shown no mould growth, despite having had no post processing heat treatment and despite not having had any fungicide added. Conventional pellets would be expected to exhibit mould growth in such circumstances.

Any starch can be used as the binding agent, with suitable starch binders being well known in the art and being readily available, including, e.g., potato starch, corn starch, wheat starch, etc.

The binding agent is conveniently used at a concentration of up to about 10% by weight, e.g. up to about 5% by weight, and at least about 1% by weight, e.g. in the range 1 to 4% by weight.

Moisture may be added, typically in an amount of up to about 10% by weight of water, if required, to ensure adequate lubrication and so reduce load on processing equipment.

The invention thus also includes within its scope a compressed product, such as a pellet or briquette, formed from the charred solid product of the invention, particularly a compressed product formed with a starch binding agent.

Also covered by the present invention is a water-resistant compressed char product, e.g. a pellet or briquette. The term “water-resistant” in this context means able to withstand immersion in water for at least 24 hours without losing structural integrity, e.g. breaking down. This can be achieved by use of the charred solid product of the invention processed with a starch binder, as described above, without requiring surface coating or any special treatment of the product.

The invention also covers a method of processing a charred solid product produced by the method of the invention, comprising mixing the product in finely divided form with a binding agent, particularly a starch-based binding agent, and compressing the mixture to produce a compressed solid product, particularly a pellet or briquette.

The invention also includes within its scope a method of treating biomass material, comprising subjecting the biomass material to radio frequency electromagnetic radiation while the material is being agitated, under suitable conditions to produce a pyrolysed solid product; mixing the product in finely divided form with a binding agent, particularly a starch-based binding agent; and compressing the mixture to produce a compressed solid product, particularly a pellet or briquette.

The invention will be further described, by way of illustration, in the following examples.

Experiments were carried out in a test rig comprising a rotary drum microwave processing vessel generally as described and illustrated in WO 2007/007068. The test rig comprises a louvered drum generally corresponding to vessel 10 of FIGS. 1 to 4 of WO 2007/007068, with an associated support cradle 60, feedstock input means 54 and discharge means 56, waveguide 58, enclosure 90 etc, as shown in FIGS. 5 to 15 of WO 2007/007068. Infrared heaters 84, 86, 88 are mounted inside the enclosure. The drum of the rest rig is of stainless steel and is 1500 mm long and 550 mm in diameter. The vessel of the test rig was modified in certain respects compared with vessel 10, lacking fins 40 which are not needed when processing biomass and which could be a source of undesirable hot spots, and lacking ceramic tiles 36 as biomass is not generally abrasive. The vessel ideally has weirs at the discharge end, sized to allow a feedstock bed depth of at least twice the penetration depth of the microwaves used. The vessel is mounted with the longitudinal axis at an angle of about 1° to horizontal, with the discharge end below the input end, so that in use the biomass material gradually travels along the vessel. The waveguide 58 is linked to a microwave generator operating at 895 MHz.

In use, a biomass feedstock is introduced to the vessel via the input means, with the vessel being continuously rotated at a speed in the range 1 to 3 rpm to cause gentle agitation. The feedstock travels along the vessel towards the discharge means, being continually gently moved and mixed. Residence time in the vessel is controlled by regulating factors including drum inclination and rotation speed. Microwave energy is introduced to the vessel via the waveguide, causing heating of the feedstock. This results in pyrolysis of the biomass, producing a charred solid product, low molecular weight gases and possibly also a liquid product such as oils or tar, with the exact products depending on the reaction conditions and the nature of the feedstock. The gaseous product and vapour is extracted from the vessel through the louvre vents in the upper quadrant of the drum, over the full length of the drum, by application of suction. This prevents pressure build up within the vessel, and also prevents possible undesirable secondary reactions. The extracted gas is recycled to the interior of the vessel via suitable pipework, being fed through louvre vents in the bottom quadrant of the drum to permeate up through the biomass bed. The gas assists in agitating the biomass and also helps flush away newly generated gaseous pyrolysis products from the bed from activation sites where they are produced, improving processing efficiency. Instead of recycling pyrolysis gas in this way, an inert gas such as nitrogen could be injected into the bottom of the biomass bed, but this is more expensive. The processing may be carried out on a continuous basis.

Processing is carried out in the substantial absence of oxygen. To this end, the processing vessel is located in a gas-tight enclosure to preventingress of air. The vessel may be initially flushed with an inert gas, e.g. nitrogen, but this is not essential and processing in the presence of air initially present in the vessel gives acceptable results.

In a variant process, processing is carried out with a restricted, controlled amount of oxygen (less than required for stoichiometric reaction) introduced to the vessel during pyrolysis. This results in production of a larger proportion of gas with a much reduced quality of char and little tar in a gasification reaction.

Experiments were carried out using willow chips having a maximum dimension of less than 40 mm as feedstock to the test rig described above. Analysis (in wt %) of the wood chips is as follows:

Moisture 14.4 Volatile matter 71.5 Fixed carbon 13.7 Ash 0.4 100.00 Calorific value 18.41 MJ/kg

The vessel was preheated by means of infrared heaters mounted inside the drum to raise the temperature of the vessel to about 120° C. to prevent condensation of steam. Feedstock comprising the willow chips was introduced to the drum via the input means, with the vessel rotated at a speed of 1 rpm, with microwave energy at 895 MHz introduced via the waveguide. Gas is recirculated as described above. The feedstock is gently agitated and exposed to microwave energy at a processing temperature of about 120° C., passing slowly along the vessel, with parameters being set to produce a residence time in the drum of approximately 15 minutes. The solid product leaving the drum by the discharge means was in the form of brittle, dark brown pieces of char that is at a temperature of about 70° C. on leaving the test rig. Analysis (in wt %) of the char product was as follows:

Moisture 5.4 Volatile matter 61.1 Fixed carbon 31.8 Ash 1.7 100.00 Calorific value 22.49 MJ/kg

The char product can be used as a fuel, with particular application as a feedstock for coal-fired power stations, without requiring further treatment, as discussed above.

In further similar experiments, a feedstock of willow chips with a size of less than 75 mm and a moisture content of about 14% was processed as described in Example 1 for 15 minutes at a processing temperature of 150° C., resulting in production of brittle, dark brown pieces of char.

The HGI value of the char product was measured as 54. HGI is a parameter specifically designed for coal and is a measure of the milling characteristics of coal. For coal, a HGI of 50 or above generally indicates that a mill can operate at full capacity. The willow char product can thus be processed through mills in a similar manner to coal.

The HGI test is unsuitable for the untreated willow chips (and other biomass materials) because of their fibrous nature. The fibrous nature of such materials means that they cannot be milled in the same way as coal, and would greatly reduce mill capacity.

In further experiments, char in accordance with the invention was produced from a mixture of soft woods by processing generally as described in Example 1. The char was then processed in a laboratory scale pellet mill to produce 10 mm diameter pellets, using potato starch as binding agent at a concentration of 2.4% by weight, at a pelleting temperature of 60° C. No drying or forced cooling of the pellets was undertaken. The pellets had a bulk density of about 600 kg/m3.

The resulting pellets exhibited water resistance and resistance to mould growth, despite having had no post processing heat treatment and no fungicide treatment. The pellets were totally immersed in water for 14 days, yet showed no softening or structural breakdown. The pellets were held in conditions of high humidity for one month, yet exhibited no mould growth.

Similar results were obtained using a variety of different starch binding agents.