GB2448547A

GB2448547A - Electrical power generation using biomass

Info

Publication number: GB2448547A
Application number: GB0707727A
Authority: GB
Inventors: James G Price
Original assignee: Drax Power Ltd
Current assignee: Drax Power Ltd
Priority date: 2007-04-21
Filing date: 2007-04-21
Publication date: 2008-10-22
Also published as: GB0707727D0

Abstract

A method of generating electrical power includes processing biomass material to produce a processed biomass material feed. The processing includes rolling the biomass material by pressing the biomass material between a roller surface (51a. figure 1) of a rotating roller (5a, figure 1) and an adjacent surface (51b, figure 1) and blowing the processed biomass material into a combustion chamber 11 together with a gas including oxygen via at least one burner nozzle. The processed biomass material is burned in at least one flame as it emerges from the nozzle to generate heat, which is used to generate electrical power. The rolling of the biomass may produces flakes, and may comprise hammermilling, chipping or grinding before the rolling, or hammermilling after the rolling. An additional fuel such as pulverized coal may also be used in the combustion chamber 11.

Description

1 2448547 Electrical Power Generation Method and Apparatus

Field of the Invention

The present invention relates to the generation of electrical power using biomass material as a fuel source, either on its own or in combination with another fuel such as a fossil fuel (e.g. coal).

Bac]çground to the Invention Pulverised fuel (PF) combustion is one of the most widely used methods of burning coal for power generation. In pulverised fuel combustion systems, coal is pulverised (i.e. it is processed into the form of a powder) and then blown into a combustion chamber together with combustion air. Commonly, the combustion chamber is a boiler and the heat generated by combustion of the pulverised fuel is used to raise steam. As the coal is injected in the form of a powder, it has a very large surface area to volume ratio and burns easily in a flame as it emerges from a suitable injection (or burner) nozzle. Commonly, the heat of combustion is used to produce super heated steam which is then arranged to drive a turbine and, in turn, a generator to produce electricity.

In more detail, in conventional pulverised fuel combustion systems the coal is ground (pulverised to a fine powder so that less than 2% of it has a dimension in excess of 300microm, and typically 70-75% of the coal has a particle size below 75microm. The pulverised coal is then blown with some combustion air into a boiler plant through a series of burner nozzles. The air with which the pulverised coal is entrained is typically not the only source of combustion air. Further air streams may be injected into the boiler system from separate inlets or nozzles, and these further sources of combustion air maybe referred to as secondary or tertiary air streams.

I

In pulverised techniques, the combustion typically takes place at temperatures from 1300 to 1700 C depending on the particular coal used. As mentioned above, steam is typically generated from the heat produced by combustion, and this steam is used to drive as a turbine and an electrical generator. In many systems, the coal particle residence time in the boiler is in a range of 2-5 seconds, and it will be appreciated that the coal particles must be small enough for complete burnout to have taken place during this time. Complete combustion is important to ensure that the ash produced is substantially free of carbon (in other words its carbon content should be below a predetermined threshold). The ash from the pulverised fuel combustion system is commonly referred to as pulverised fuel ash (also known as fly ash).

When the pulverised coal is burnt, approximately 20% of the fuel forms fine glass spheres, the lighter of which arc born aloft by the combustion process. They are typically extracted from the flue gasses by cyclones and electrostatic precipitation.

Heavier ash falls to the bottom of the combustion chamber (i.e. the bottom of the boiler structure) and is extracted. Provided that the combustion has been substantially complete, the resultant ash (both fly ash and bottom ash) has value, in that it can be used as engineering fill and also as a component for concrete. It is important that the carbon content of the ash does not exceed a predetermined level because that would render it unsuitable for these construction purposes, and hence the ash would simply have to be disposed of. At the present time, such disposal of the carbon-contaminated ash incurs costs because of the charges made for disposing of material in landfill sites.

In known pulverised fuel combustion systems, two general boiler designs are used.

One is a so-called two-pass layout where there is a furness chamber, topped by some heat transfer structure such as tubing. The flue gasses are then diverted to turn through 1800 then passed downwards through the main heat transfer section of the boiler. The other general design is a so-called tower boiler where the heat transfer sections are generally mounted vertically above one another, over the combustion chamber.

Pulverised fuel combustion systems contrast with fluidised bed combustion systems which are typically fed with relatively coarse particles (around 3mm in size) and where the particles are introduced into a circulating bed of non-combustible material. In fluidized bed combustion systems combustion typically takes place at temperatures from 800 to 900 C i.e. temperatures substantially lower than those used in pulverised fuel combustion systems.

It will be appreciated from the above that in conventional pulverised fuel combustion systems, processing of the fuel material into particles of sufficiently small size is important to ensure complete combustion as the particles are burnt in a flame as they emerge from a burner nozzle.

The general desirability to generate electrical power from biomass material is well known, especially as a means of reducing the use of conventional fossil fuels, and hence reducing the amount of carbon dioxide released into the atmosphere.

Generating electrical power from the combustion of a biomass material is, in itself, essentially a carbon-neutral technique.

Clearly, as pulverised fuel combustion systems are in such widespread use, it is desirable to have a system in which biomass material can be used as fuel with such system, ideally without requiring modification to the boiler systems themselves (i.e. without requiring modification to the burner nozzles, the boiler chambers, the associated steam-raising and heat exchange systems, and the ash separation and collection systems).

In view of this, attempts have been made to co-fire biomass material with coal. One disclosed technique has involved the mixing of biomass material with raw (i.e. un-pulverised) coal and then the conventional coal pulverising techniques (using grinding and hammer milling) have been used to pulverise the mixture. This has been used with biomass in the form of paper sludge and sewage sludge. However, a problem with this technique is that the co-processing of coal and biomass material using grinding and hammer milling may not be suitable for certain types of biornass material. For example, it is not suitable when the biomass material is in the form of large pieces or chips of wood, where the techniques which are suitable for pulverising the coal are not suitable for producing fine particles of wood for complete combustion in the boiler flames. Another technique that has been disclosed is the separate milling and firing of waste wood. In this technique, waste wood in the form of large pieces or particles has been turned into a wood powder by a combination of grinding and hammer milling. In other words, it has been processed into a form essentially the same as that of the pulverised coal using conventional pulverised fuel combustion techniques. The wood powder has then been directly injected (on its own, without coal powder) into a boiler and satisfactory combustion has being achieved. However, it will be appreciated that a problem with this known technique is that the processing of the wood to achieve the wood powder end product itself consumes energy, and this partially offsets the benefit obtained by generating electricity from the biomass source. Energy is consumed in this process at various stages, including drying of the biomass material, the powering of the grinders/chippers, and the powering of the hammer mills.

The problem of ensuring that the ash produced from the combustion of such biomass material has sufficiently low carbon content (in other words the problem of ensuring substantially complete combustion of the biomass material) has been appreciated, and this is why the disclosed techniques have taken pains to ensure that the grinding and hammer milling has been sufficient to produce wood powder of sufficiently small particle size.

Summary of the Invention

Embodiments of the present invention aim to overcome, at least partially, one or more of the above-mentioned problems associated with the prior art techniques.

Certain embodiments aim to provide electrical power generation methods in which the processing of the biomass material into a form suitable for substantially complete combustion in a pulverised fuel combustion chamber (e.g. boiler) can be achieved using substantially less energy then previous techniques. Embodiments of the invention also aim to provide power generation methods in which biomass can be processed efficiently into a form such that when burnt in a pulverised fuel combustion chamber, the resultant ash has sufficiently low carbon content to give it commercial value as an engineering material.

According to a first aspect of the invention, there is provided a method of generating electrical power comprising: processing biomass material to produce processed biomass material, said processing comprising rolling said biomass material by pressing said biomass material between a roller surface of a rotating roller and an adjacent surface; blowing the processed biomass material into a combustion chamber together with a gas comprising oxygen via at least one burner nozzle; burning the processed biomass material in at least one flame as it emerges from said at least one nozzle to generate heat; and using said generated heat to generate electrical power.

Thus, rather than simply subjecting the biomass to the same processing used previously on fossil fuels (to produce pulverised material suitable for PFC boilers), embodiments of the invention utilise rolling. Generally, incorporation of rolling in the pre-combustion processing has been found by the present inventors to enable the biomass material to be processed into a form suitable for combustion in a PFC combustion chamber (e.g. boiler) using substantially less energy than would be the case using the conventional processing techniques.

In certain embodiments the rolling comprises rolling the biomass material to produce flakes. Preferably, the rolling may be arranged such that the flakes have an average thickness of no more than 0.6mm. In certain embodiments, the method comprises blowing these flakes into the combustion chamber with said gas via said at least one burner nozzle and burning said flakes in at least one flame as they emerge from the at least one nozzle to generate said heat. Thus, in certain examples, the processed biomass material blown into the combustion chamber comprises flakes. Advantageously, by combusting the rolled flakes directly in certain embodiments, and so avoiding further processing to pulverise the flakes before combustion, the energy consumed by the pre-combustion processing as a whole is reduced. In certain other embodiments, however, the pre-combustion processing does comprise hammermilling of the rolled flakes, but advantageously this hammermilling has been found to consume relatively low power as the flaked structures are easily broken apart.

Thus, in certain embodiments, the processing of the biomass material also comprises hammermilling. In certain examples, the processing comprises hamrnermilling the biomass material before said rolling. Additionally, or alternatively, in certain examples the processing comprises hammermilling said biomass material after said rolling.

The processing may also comprise at least one of chipping or grinding said biomass material before said rolling.

In certain embodiments, said blowing of the processed biomass material into a combustion chamber comprises blowing the processed biomass material into a combustion chamber together with said gas and a further fuel material via said at least one burner nozzle. For example, the additional fuel material may comprise pulverized coal (coal powder).

Preferably, the adjacent surface is a roller surface of a second roller. Then, each roller may be rotated such that said rolling comprises pressing the biomass material between the adjacent rotating roller surfaces.

In certain examples this rolling comprises rotating the two rollers at different speeds and pressing the biomass material between the roller surfaces of the two rollers.

This provides the advantage of breaking the material up into a format suitable for PFC more easily (i.e. using reduced power).

In certain examples the rollers have substantially the same diameter, and although in some embodiments the roller surfaces are each substantially smooth, in other cases at least one of the roller surfaces is textured (e.g knurled).

In certain embodiments using said generated heat to generate electrical power comprises using said generated heat to generate steam, driving a steam turbine with the generated steam, as using the steam turbine to drive an electrical generator.

In certain embodiments the processing comprises grinding said biomass material, screening the products of said grinding to separate relatively large biomass particles from relatively small biomass particles, rolling said relatively small biomass particles, and said step of blowing the processed biomass material into the combustion chamber compnses blowing said rolled relatively small biomass particles into the combustion chamber. The method may additionally comprise hammermilling said relatively large biomass particles, and blowing said haminermilled relatively large biomass particles into said combustion chamber.

In certain embodiments the processing comprises grinding said biomass material, screening the products of the grinding to separate relatively small biomass particles from relatively large biomass particles, rolling said relatively large biomass particles and haniniermilling said rolled relatively large biomass particles to produce said processed biomass material. The method may then further comprise the step of blowing said relatively small biomass particles into said combustion chamber.

Another aspect of the present invention provides electrical power generating apparatus comprising: a combustion chamber; at least one burner nozzle for injecting fuel and combustion gas into the combustion chamber; storage means for storing a supply of biomass material; processing means for processing the biomass material to produce processed biomass material, the processing means comprising rolling means for rolling said biomass material by pressing said biomass material between a roller surface of a rotating roller and an adjacent surface; and means for blowing the processed biomass material into the combustion chamber via said at least one burner nozzle together with a gas comprising oxygen.

Brief description of the Drawings

Embodiments of the present invention will now be described with reference to the accompanying drawings, of which Fig. I is a schematic representation of a roller mill used in methods and apparatus embodying the present invention; Fig. 2 is a schematic representation of part of an electrical power generating system embodying the invention; Fig. 3 is a schematic representation of a fuel processing method (and apparatus) used in embodiments of the invention; Fig. 4 s a schematic representation of another fuel processing method (and apparatus) which may be used in embodiments of the invention; Fig. 5 is a schematic representation of part of the fuel processing apparatus of an embodiment of the invention; Fig. 6 is a schematic representation of part of another power generation method and apparatus in accordance with another embodiment of the invention; and Fig. 7 is a schematic representation of part of power generation apparatus in accordance with another embodiment of the invention.

Detailed Descripjon of the preferred Embodiments Certain embodiments of the invention are directed to the generation of electrical power from the burning of biomass of various types including wood, miscanthus, straw, olive cake, rape seed meal, Palm Kernel Expeller etc. Generally, to burn biomass in conventional pulverised fuel boilers, in embodiments of the invention it is micronised to -0.5 mm average particle size. In certain embodiments this means that at least one of the dimensions is arranged to be less than 0.6 mm. This may be done in several steps including: chipping, grinding, milling; and injection. These steps each require power that is proportional to the moisture content (typically, more moisture requires more power) and characteristic of the biomass material (dense strong fibres and hard particles take more power than lightweight weak fibres or soft particles). Embodiments aim to provide a plant that requires the least amount (or a reduced amount) of power to get to a point where the microni sing is sufficient to give complete combustion. The present inventors have determined that, surprisingly, rather large particles can be burned in the PF furnaces (boilers) provided that such particles are flattened out (like rolled oats'; relatively 2 dimensional). One machine suitable for producing such flattened particles is a roller press, also known as a flaker mill. In tests, a quantity of ground up biomass was specifically sized through screens, and the screened material processed through a flaker mill. This rolling produced a 2 dimensional particle that would fly' in the boiler furnace, and would undergo the desired, substantially complete combustion.

On closer examination, it was clear that the roller press had splayed out the previously bunched biomass fibres in such a way as to render them much easier to process. Subsequent tests of these flattened chips showed substantially lower power requirements for milling than usual. Thus rolling can produce flattened particles for direct injection, and also can reduce power requirements for subsequent hammer milling, and so reduce overall biomass fuel processing power requirements.

Thus, one aspect of the invention is the application of a Roller Press (Flaker Mill) to pre-processing of biomass before Final Milling, i.e. in the preparation of biomass into powder suitable for combustion in a Pulverised Fuel Fired Boiler.

Another aspect of the invention is the application of a Roller Press (Flaker Mill) as a Final Processor to small granules of biomass, i.e. in the processing of biomass material into flakes suitable for combustion in a Pulverised Fuel Fired Boiler.

It will be appreciated that, in certain embodiments of the invention, rolling of biomass material (using a flaker mill or other rolling apparatus) may be performed at different stages in the processing of that material, before combustion in a suitable combustion chamber.

It will also be appreciated that a variety of rollers and roller mill systems may be used in embodiments of the invention. For example, with reference to figure 1, certain embodiments comprise the following roller mill: the roller mill 50 incorporates one pair of Chilled Cast hon Rolls 5a, 5b 460mm diameter x 915 mm face length (actual diameters 450mm) matt-finished, scratched or fluted rolls. The rolls have steel spindles 52a,52b, which are fitted with spherical roller bearings and inter-roll transmission is by chain drive. A guard assembly covers the inter-roll chain drive. It is set to -3% differential speed. Application of roll nip force is by left & right hand pre-loaded disc spring pressure assemblies (adjustable to 30 tonnes). The roller mill is fitted with one hardened spring-loaded steel scraper 53a,53b per roll, with external hand wheel adjusters 54a,54b for left and right-hand sides. Gap adjustment may be carried out quickly and simply with a single micrometer-calibrated capstan hand wheel 55, which opens and closes the gap in parallel i.e. it is not necessary to adjust both ends separately. The centre section of the mill is provided with hinged stainless steel panels, which cover the rolls.

Biomass material M is fed between the contr-rotating rolls 5a,5b from above, and hopper boards 56a,56b direct the rolled biomass material RM such that it is discharged out of the bottom of the mill 50. l0

In tests with an existing process, with no rolling, a hammer mill used 30 kW.hr./tonne (net) to process biomass chips into powder suitable for combustion in a Pulverised Fuel Fired Boiler. With the application of Roller Presses in the process line, in accordance with embodiments of the invention, tests showed that this hammer miii power consumption could be cut in half to 15 kW.hrs./tonne. Since Power is the single biggest operational expenditure, this has the potential to give a significant savings.

Referring now to fig. 2, this shows part of a power generation system embodying the invention. The system comprises a pulverised fuel combustion boiler I comprising a combustion chamber 1 1, and pipe work 13 arranged to convey water into the volume inside the combustion chamber for conversion to super-heated steam, and then to convey the steam out of the boiler to steam turbine 20. A burner I S nozzle system is indicated generally by reference numeral 12, and this is the inlet by which processed biomass material is blown into the burner chamber together with a gas comprising oxygen (typically air). Inserting embodiments just processed biomass material maybe injected into the boiler via this nozzle or inlet structure 12.

In alternative embodiments, a mixture of processed biomass material and pulverised coal maybe blown in through inlet 12 together with combustion air. The illustrated boiler in this example is a tower structure. Other boiler structures may of course be employed in other embodiments. The processed biomass material and any accompanying pulverised coal have sufficiently small dimensions that they are fully combusted in the combustion chamber 11, with ash being produced with substantially no residual carbon content. The lighter portion of this ash (i.e. the fly ash) is extracted from the top of the tower structure, and the heavier ash (the bottom ash) is removed from the bottom of the structure. The steam turbine 20 is arranged to drive an electrical generator 30. The system further comprises a condenser 32 arranged to condense the steam emerging from the turbine 20. The condensed water is then returned to the boiler I. Referring now to fig 3, this illustrates part of a power generation method embodying the invention, and in particular shows various steps in the processing of the biomass material. One source of biomass material M provided in this example is a supply of logs, which are then ground using a horizontal grinder 61. The processed material from the horizontal grinder are then supplied to a screen 7, which separates out the relatively large wood chips from any relatively small particles that may have been produced in the grinding process. Those screened-out chips are then supphed to a so-called flaker mill 50 in which they are pressed between the rolling surfaces of two adjacent rollers. This rolling of the chips produces flakes of biomass material which are then supplied to a hammermili 8. This hamniermill 8 further breaks up these flakes into small particles which are then supplied to a direct injection system 9. The direct injection system has the capacity to hold a quantity of processed biomass material and supply that material to a pulverised fuel combustion boiler such as that shown in fig 2. In other words, the direct injection system is able to blow the particles of processed biomass material into the combustion chamber together with a quantity of combustion air. Fig. 3 also shows additional forms of biomass material M which may be used in electrical power generation methods embodying the invention. These examples are straw bales, willow billets, and wood chips. These three forms of biomass maybe processed in a tub grinder 62, producing a mixture of relatively large chips of biomass material and relatively small particles, commonly refened to as fines. The mixture of chips and fines is supplied to a screen 7, which separates them into three components. The first component comprises the nominally "oversize" material which is optionally returned to the tub grinder for further breaking up. The screen is also able to separate the relatively large chips from the fines. These chips are also supplied to a flaker mill 50 in which they are rolled to produce flakes which are again then supplied to the hammermill 8 for further processing before supply to the direct injection system 9. The fines separated out by the screen can be supplied directly to the direct injection system as they have particle size sufficiently small for complete combustion when injected into the combustion chamber.

Referring now to fig. 4, this shows part of another power generation method embodying the invention, again with emphasis on the steps involved in the processing of the biomass material. Again, the biomass material M may be suppliedlprovided in a form of logs which are processed first by a horizontal grinder 61 (which essentially chips the bulky supplied material using a moving tool surfaces). The products of the grinding process are again screened, with the relatively small particles in the form of granules then being supplied to a flaker mill 50. In this example, the flaker mill 50 rolls the granules to produce flakes of biomass material and those flakes are supplied directly to the direct injection system 9. Thus, with this technique the flakes themselves maybe blown directly into the combustion chamber. Sufficiently complete combustion is achieved because the flakes are thin enough; in example of the present invention the thickness in no more than 0.6mm. By avoiding the need for further processing of the flakes after the flaker mill has rolled the wood granules, power consumption by the processing technique as a whole has been reduced. Previously, it had not been appreciated that biomass material in the form of flakes was suitable for direct combustion in a pulverised fuel combustion boiler. Thus, in previous systems efforts had been made to pulverise the wood in a form essentially resembling the granules of coal typically used in pulverised coal combustion systems.

Returning to the other details of fig. 4, the relatively large chips from the screening process 7 preformed on the products of the horizontal grinder 61 are supplied to a hammermill 8, and the products of that hammermill (which are therefore further pulverised) are supplied to the direct injection system 9.

Again, further biomass fuel supplies M are shown, in a form of straw bales, willow billets and wood chips. Again these are supplied to a tub grinder 62, generally producing chips, fines and oversized material. The oversized material is optionally returned to the tub grinder after a screening process 7. The screening process separates relatively large chips from fines, with those chips also being supplied to the hammermill 8 for pulverisation before supplied to the direct injection system 9.

The fines are supplied directly to the direct injection system 9. Thus, in the system of fig 3, the direct injection system may be arranged to store a mixed supply of biomass material, that is a supply comprising flakes, fines, and wood "powder" form the hammermill. This mixture is suitable for co-firing in the combustion chamber as all three component particles have at least one dimension smaller than 0.6mm.

It will be appreciated from the above that rolling techniques may be used in the processing of the biomass material in power generation methods embodying the invention. Suitable rolling techniques for use in embodiments of the invention, will now be described in more detail.

Referring now to fig 5, this shows a roller system suitable for use in embodiments of the invention to process biomass material. The apparatus comprises a pair of rollers 5a, 5b arranged with their rotational axes horizontal and parallel such that there is a substantially constant gap g between them. A suitable drive means (not shown in the figure) is arranged to drive the rollers so that they each rotate. In this example the first roller 5a is arranged to rotate the first angular velocity Vi in a clockwise direction, and the second roller 5b is arranged to rotate at a second, different angular velocity V2 in an anti-clockwise direction. The rolls 5a,5b in this example have substantially the same diameter, but in other embodiments maybe different. The difference in angular velocities of the two rotating rollers assists in the breaking apart of the biomassmaterial M rolled through the gap g. In the examle shown in fig 5 the surfaces 51a and 51b of both rollers are textured (in particular, they have been knurled) and this further helps the rolling process break up the supplied biomass material M. In other embodiments, however, one or both of the roller surfaces 51a,51b may be smooth.

Referring now to fig 6, this is another schematic representation of part of another power generation method and apparatus embodying the invention, with particular emphasis on the fuel processing steps. A supply of raw material (biomass material) M is provided and stored in storage means 100. This raw material will typically have a moisture content and this can be anything in the range of 35% to 55%, depending on the particular biomass source. This raw material is then subjected to a grinding and screening process 6,7 and the product stored in a storage silo 101. A transfer conveyor system 102 then arranged to transport this granulated material to an optional drying stage 103 which is adapted to reduce the moisture content in the granulated material. The dried material is then provided to a hammermill system 8 which pounds the biomass material. The hammermilled product is then supplied to a rolling system 50 in which the material is pressed between surfaces, at least one of which is the rolling surface of a roller. The rolled material is supplied to another processed material silo 103 and a further conveyor system 104 is arranged to transport that material to a direct injection feed hopper 105. That feed hopper supplies a direct injection system 9 which is able to blow the processed biomass material into a boiler together with air or some other gas containing oxygen for combustion.

Referring now to fig 7, this shows part of an electrical power generation system (i.e. apparatus and method) in accordance with an embodiment of the invention. A quantity of biomass material M is stored in biomass storage means 200 (which may be a storage silo, storage hopper, or some other suitable storage arrangement).

Some of this material M is then supplied to biomass processing plant 600. This processing plant includes at least one roller system for rolling the biomass material.

In this example, a portion M3 of the biomass material has undergone at least rolling in the processing plant 600 (it may, in certain embodiments, have undergone other operations as well, such as grinding, screening, chipping etc.) and this portion M3 is fed to an air stream 91 from an air blower 90 which forms part of the direct injection system to the boiler furness 1. Thus, a stream of 92 of rolled biomass material and combustion air is supplied to the boiler furnace and is injected into the combustion chamber 11 through one or more inlets (apertures, nozzles, or other such inlet ports) where the biomass material is then burnt. In addition to the portion M3 supplied directly to the direct injection system, the processing plant 600 in this example supplies a second stream of processed biomass material M2 to a conveyor system 400. In certain examples, this portion M2 has undergone at least one rolling process. In other embodiments, the material M2 has not undergone rolling, although it has been processed in other manners (e.g. involving chipping, grinding, screening etc.). A further stream of biomass material Ml is supplied to the conveyor system 400 directly from the biomass storage means 200, and this first portion Ml has not undergone any processing in this example. The system also comprises fossil fuel storage means 300 containing a quantity of fossil fuel F which is also arranged to feed the conveyor system 400. Thus, the conveyor system 400 conveys a mixture of fossil fuel F, unprocessed biomass material MI and processed biomass material M2 to a pre-milling storage means or bunker 500. In this example a hammer mill 8 is fed from this bunker 500 and produces an output stream 80 of milled coal and biomass. This output stream 80 is also injected directly into the boiler furnace I with the combustion air and processed biomass M3 mixture for combustion.

It will be appreciated from the above that certain embodiments of the invention use roller systems to increase the output of high-speed hammermills. The theory behind the use of roller mills in the application of biomass processing for co-firing is that the crushing of fibres prior to finishing is able to provide significant gains in final milling (hanmiermill) throughput and quality of the final product. Additionally, a flattened particle can more closely resemble the 2-dimensional ideal' for complete bum-out, and be inherently more liable to remain in a boiler flame for longer due to more favourable aero-dynamic behaviour. A useful drying effect can also be provided from rolling material. This means that in certain embodiments, no additional drying stage is required, further reducing the power consumption of the biomass processing overall, prior to combustion.

There are several permutations of roller mill configuration (i.e. a number of factors may be adjusted, to optimise the process), including the following: Roller diameter; Roller gap; Roller speed and the effects of a differential in speeds of rollers; and Roller patterns (or alternatively the use of smooth rollers). Tests have been performed regarding these parameters.

A first test was carried Out to wood of 2 different particle sizes -achieved by passing material through 6mm and 10mm screens. The wood was pine from roundwood of 27.96% moisture content. The 6mm material was fed into a Flaking Mill with plain rollers (i.e. flat and with no pattern) of 550mm diameter. The rollers were set at two different gap distances -firstly at 0.127mm and then at 0.0254mm.

There was a I % speed differential between the rollers. Rollers are set horizontally (i.e. side by side), each weighing 1.5 tonnes and the drive motor is rated at 45Kw.

The effect nipping force' (between rollers) was 60 tonnes. Moisture content was measured on the input material and on the output of the roller mill (i.e. before hanimermilling). This process was repeated for the 10mm material, but the output from this was then fed into a test hammermill, set up with 4.5mm screens and then with 6mm screens.

A second test used the same roller configuration as above but the material used was chopped Miscanthus. There was no subsequent hamnierrnilling.

In a third test the roller mill was changed to one with 460mm diameter plain rollers and wood of 10mm and 6mm particle size was used. There was no subsequent hammermilling.

From photographs of the material produced in the tests it could be seen that, in all cases when wood was rolled, there was a splaying and separation of the fibre ends, in addition to the simple flattening and breaking of particles.

The effect on moisture content was also significant, at a reduction of almost 9% for a very low energy input -this type of reduction is costly with conventional drying methods and roughly equivalent to 6 months of air drying.

Hammermilling this material showed that throughput and efficiently of final milling equipment was much increased and the test ammeter did not show any increase in current drawn under load when compared to running empty.

It was clear from the results that the introduction of a rolling stage to the processing of wood and Miscanthus for direct injection gave significant benefits to the cost effectiveness of the operation.

The fact that the Amps drawn by the hamniennill did not fluctuate upon loading it would indicate that the final finishing was unusually easy for a wood product. This makes the job of hamniermilling stage much easier and can only increase throughput and reduce wear and associated maintenance cost. The effect of splaying the fibres will have contributed greatly to the ease with which the hamniermill dealt with the product and will also help in combustion, as it will present a greater surface area to the flame.

The results were so good that it brings into question the need for a hamrnermill at all. If, as the tests show, wood through a 55mm diameter roller with a 0.0254mm gap can produce a 0.32mm thick particle it may be that this is sufficient for the purpose of direct injection.

With regard to moisture content, the input moisture was relatively low, and rolling green timber is likely to give even greater benefits (possibly as much as 25%) for the same low energy input.

The result show that Miscanthus (and, possibly other straws) can also be reduced to a burnable specification in this way with particle thickness results even better than those for wood due to its tubular construction. The growth nodes (or knuckles') present in Miscanthus that was originally throughout to represent a difficulty to milling were dealt with very well by the rollers.

It is possible that straw or Miscanthus could be processed very cheaply using rollers, especially if it could be delivered chopped and baled. This would remove the need for on-site primary processing. if, in turn, the direct injection system can cope with short, flattened straw/miscantus particles, it would also obviate the need to hammermill. In an adaptation of silage making, these materials can be chopped baled and netted in the field, resulting in bales of up to 20% higher density than conventional bales, so significantly reducing transport costs.

Fo]lowing the tests it was recommended that a further investigation should be carried out in order to define the effect on final product (processed biomass material) when larger volumes are rolled. In the tests the input material was sieved but this may not be viable in high volume production so a batch of unscreened material should be tested to ensure that the finer (but still out of specification) particles are not allowed through the roller gap while thicker particles are being rolled -not likely, but worth confirming. The effects of using different roller speeds may well also be significant as a slower roller will have the effect of holding particles as the other roller works on them, increasing the effective time in the pinch area'. As was expected the larger diameter rollers also gave a better result because of the increased time in this pinch area. The effect of different roller surface patterns upon quality may also be significant -a knurled pattern may introduce a tearing effect upon the fibres, so allowing easier final milling, although in certain embodiments final milling is adequately easy (low power) after rolling with smooth rollers). The significance of roller patterning may depend on the particular biomass material being processed.

Claims

1. A method of generating electrical power comprising: processing bioniass material to produce processed biomass material, said processing comprising rolling said biomass material by pressing said biomass material between a roller surface of a rotating roller and an adjacent surface; blowing the processed biomass material into a combustion chamber together with a gas comprising oxygen via at least one burner nozzle; burning the processed biomass material in at least one flame as it emerges from said at least one nozzle to generate heat; and using said generated heat to generate electrical power.

2. A method in accordance with claim I wherein said rolling comprises rolling said biomass material to produce flakes of said biomass material.

3. A method in accordance with claim 2, wherein said flakes have an average thickness of no more than 0.6mm.

4. A method in accordance with any one of claims 2-3, further comprising blowing said flakes into the combustion chamber with said gas via said at least one burner nozzle and burning said flakes in at least one flame as they emerge from the at least one nozzle to generate said heat.

5. A method in accordance with any one of claims 2-4, wherein said processed biomass material blown into the combustion chamber comprises said flakes.

6. A method in accordance with any proceeding claim wherein said processing comprises hammermulling.

7. A method in accordance with claim 6, wherein said processing comprises hammermilling said biomass material before said rolling.

8. A method in accordance with any one of claims 1 to 7, wherein said processing comprises hammermilling said biomass material after said rolling.

9. A method in accordance with any proceeding claim, wherein said processing comprises at least one of chipping or grinding said biomass material before said rolling.

10. A method in accordance with any proceeding claim, wherein said blowing the processed biomass material into a combustion chamber comprises blowing the processed biomass material into a combustion chamber together with said gas and a further fuel material via said at least one burner nozzle.

II. A method in accordance with claim 10, wherein said additional fuel material comprises pulverized coal.

12. A method in accordance with any proceeding claim wherein said adjacent surface is a roller surface of a second roller.

13. A method in accordance with claim 12, wherein each said roller is rotated such that said rolling comprises pressing the biomass material between the adjacent rotating roller surfaces.

14. A method in accordance with claim 13, wherein said rolling comprises rotating the two rollers at different speeds and pressing the biomass material between the roller surfaces of the two rollers.

15. A method in accordance with any one of claims 12 to 13, wherein the rollers have substantially the same diameter.

16. A method in accordance with any one of claims 12 to 15, wherein at least one of said roller surfaces is textured.

1 7. A method in accordance with claim 16, wherein said textured surface is knurled.

18. A method in accordance with any proceeding claim wherein using said generated heat to generate electrical power comprises using said generated heat to generate steam, driving a steam turbine with the generated steam, as using the steam turbine to drive an electrical generator.

19. A method in accordance with any proceeding claim wherein said processing comprises grinding said biomass material, screening the products of said grinding to separate relatively large biomass particles from relatively small biornass particles, rolling said relatively small biomass particles, and said step of blowing the processed biomass material into the combustion chamber comprises blowing said rolled relatively small biomass particles into the combustion chamber.

20. A method in accordance with claim 19, further comprising hamniermilling said relatively large biomass particles, the method further comprising blowing said hammermilled relatively large biomass particles into said combustion chamber.

21. A method in accordance with any one of claims 1 to 18 claim, wherein said processing comprises grinding said biomass material, screening the products of the grinding to separate relatively small biomass particles from relatively large biomass particles, rolling said relatively large biomass particles and hamrnermilling said rolled relatively large biomass particles to produce said processed biomass material.

22. A method in accordance with claim 21, further comprising the step of blowing said relatively small biomass particles into said combustion chamber.

23. Electrical power generating apparatus comprising: a combustion chamber; at least one burner nozzle for injecting fuel and combustion gas into the combustion chamber; storage means for storing a supply of biomass material; processing means for processing the biomass material to produce processed biomass material, the processing means comprising rolling means for rolling said biomass material by pressing said biomass material between a roller surface of a rotating roller and an adjacent surface; and means for blowing the processed biomass material into the combustion chamber via said at least one burner nozzle together with a gas comprising oxygen.

24. A method of generating electrical power substantially as hereinbefore described with reference to the accompanying drawings.

25. Electrical power generating apparatus substantially as hereinbefore described with reference to the accompanying drawings.