WO2003087276A1

WO2003087276A1 - Production of compact biomass fuel

Info

Publication number: WO2003087276A1
Application number: PCT/GB2003/001450
Authority: WO
Inventors: Michael Mason; Frederick Joseph Dumbleton
Original assignee: Biojoule Limited
Priority date: 2002-04-06
Filing date: 2003-04-03
Publication date: 2003-10-23
Also published as: AU2003226528A1; GB2389857A; GB2389857B; GB0307677D0

Abstract

A method is disclosed of producing compact biomass fuel comprising: (a) drying and grinding of biomass to reduce its moisture content to within a predetermined range and to reduce its particle size to within a predetermined range to form a substantially free-flowing particulate material; and (b) compression of the particulate material into a container by application of an applied pressure so as to substantially reduce the material's volume but without binding the particles thereof together. In a preferred embodiment, the moisture content, particle size and applied pressure are chosen to minimize the total cost of fuel preparation and transportation.

Description

Production of Compact Biomass Fuel

The present invention relates to a process for preparing densified biomass as fuel, and of the product produced by the process (hereinafter referred to as "compact biomass").

The process of the present invention involves preparing biomass from such materials as woody matter and grasses amongst others, increasing the energy density above that of the raw material, so enabling cost-effective transportation to sites where it can be burned.

The process involves several steps which, in concert, provide a novel combination of steps allowing the creation of a novel product. In addition, the equipment used to effect the process has all been conceived with a view to making it easily demountable and transportable.

There are several known alternatives to compact biomass. Thus biomass is currently sold in a number of forms. Firstly, it is sold as roundwood or log. This is used throughout the world as a fuel for fires, and for small domestic heating apparatus. It is also used in some commercial applications. It is unsuitable for combustion in pulverised fuel boilers, and its large size makes it difficult to dry at a high rate. Its size and wide variation in shape also make handling difficult.

A second form of known biomass is wood chip. Wood chip offers significant handling advantages over roundwood or logs. It is usually sold either wet, or at moisture contents down to 30% (by wet weight). It has the disadvantage that it is very bulky, with a typical bulk density of 150 Oven Dried Kg/m³, and an energy density of around 2.6GJ/m³. This limits its utility because of the high transport cost per unit of energy. A third known form are pellets. Pellets are significantly more dense than chips and are very easy to handle. Producing pellets requires high temperatures and very high pressures, so the cost of pelletising is high. Further, the raw material for making the pellets has to be carried to a central plant, which makes them uneconomic in most circumstances when competing with fossil fuels.

A fourth type of known biomass are briquettes. Briquettes are in effect logs made by compressing and binding sawdust. They are dense and have predictable handling characteristics. However they are expensive to produce, and thus only suitable for low volume production.

US4797135 discloses a process related to the drying (with the additional use of alkali) then grinding of wood particles. The document concentrates on the use of alkali to allow grinding of the particles to a fine size, but does not disclose any further steps to the process.

US4230459 discloses a process for conglomerating wood particles to form briquettes for use in heating barbecues and so forth. The process described therein involves taking wood chips of 10 to 40 per cent per dry weight moisture content, dry mixing such chips with lignosulfonate, and compressing the mixture under a relatively high pressure of 2,750k Pa upwards. However, no further steps of the process are disclosed, especially those used to reach the particular moisture content. Further, the pressure for compression required in the formation of briquettes is typically very high and thus the process has the disadvantage of being costly. The document concentrates on the use of sulfonate as part of the process. US 4935035 discloses the compaction and the fermentation of wood chips to form a dry compacted body. No further steps of the process are disclosed and fermentation appears to be essential to the process.

CH 677527 discloses a wood drying plant for chip board manufacture in which the wood source is pre-heated, compressed to remove moisture mechanically and then heat dried. Again, no further steps of the process are disclosed and the end product is not in the form of a powder comprising particles.

WO 89/00185, DE1143357, DE4336415, DE19539413 and DE4436208 all disclose various systems for merely drying loose materials such as wood shavings and chippings. The fate of such dried materials is not disclosed.

Although concepts related in some form or other to the individual steps of the process of the present invention are known, for example from the above mentioned prior art patent applications and so forth, the process of the present invention comprises a novel combination of steps providing significant advantages over prior art processes.

Thus, according to one aspect of the present invention, a method is provided of producing compact biomass fuel comprising:

(a) drying and grinding or milling biomass to reduce its moisture content to within a predetermined range and to reduce its particle size to within a predetermined range to form a substantially free- flowing particulate material; and

(b) compression of the particulate material into a container by application of an applied pressure so as to substantially reduce the material's volume but without binding the particles thereof together.

The raw material of the present process is usually wood chip, typically from forest products or purpose grown energy crops. The end product can be transported in a dispensing container as particulate fuel, with a bulk energy density significantly higher than the raw material in chipped form. The fuel can be fed without further preparation into a range of boilers from large pulverised fuel boilers, used in industrial power generation, to small heating boilers used for schools, swimming pools and industrial processes.

The heating and grinding or milling of the biomass provides a substantially free-flowing particulate material that can be easily handled both before and after compression, but is not in the form of non-free-flowing particles held within briquettes or pellets.

The process of the present invention provides a fuel having the following characteristics:

• a consistent particle size distribution, suitable for combustion in pulverised fuel (PF) boilers as well as in dust burning wood boilers

• a consistent moisture content or range

• a high energy density when compared with the raw material

• a low capital and operating cost of production

• produced with equipment that can easily be moved to remain close to the harvest site and avoid transporting the harvest long distances to a fixed processing plant. Further, a product is provided, according to a second aspect of the present invention, that is obtained by the above process, the product being particularly cost effective to transport, especially over a haulage distance of between 50 - 1000 km.

A use of the above-mentioned product is provided, in a third aspect of the present invention, as fuel in coal powered fuel stations or as a fuel or fuel additive for the reduction of nitrogen and/or sulphur oxide pollutants from emissions of fuel stations.

In addition, a fourth aspect of the invention provides a container holding the above-mentioned product, suitable for containing the product during transport, providing weather protection, dust control, and fire containment. The container also provides suitable on-site storage at the combustion site and a convenient means of discharging the product into the site feed mechanism.

Further preferred features are mentioned in the subsidiary claims accompanying this description.

The invention will now be described in further detail merely by way of example with reference to the accompanying drawings, in which;

Figure 1 is a graph showing the pressure required to produce a given bulk density in a powder under compression, such as the compression step of the present process; Figure 2 depicts a graph with three axes showing the relationship of delivery cost to compression factor and moisture content of wood particles as conceived by the present inventors;

Figure 3 is a graph showing the relationship of the cost of various biomass fuels per haulage distance as compared to the price of gas, including amongst the various biomass fuels a product according to one embodiment of the present invention;

Figure 4 represents a flow diagram showing the individual steps that are combined to produce the present process, according to one embodiment of the present invention; and

Figure 5 represents a second flow diagram showing the individual steps according to a further embodiment of the present invention.

Fuel Preparation

Known methods of producing biomass pellets involve compression of the biomass by a factor of 4 or more. However, the process of the present invention exploits the fact that most of the benefits of biomass in the form of pellets can be gained at a lower compression of typically 1.5-2 times (see Figure 2). Such processing requires only about 1% of the process pressure, and much less heat, than pelletising, so costs less and can be done with mobile equipment.

Figure 2 depicts the relationship of delivery cost to compression factor and moisture content. The graph shows how cost is governed by compression and moisture content for a 50 km haul of wood particles to a consumption site using a process according to the present invention. Use of pellets delivers an effective compression of about 4 times at 10% moisture content, which is more than is needed. Uncompressed chip, at unitary compression and at a moisture content of 30%, is expensive. The present process typically dries to 10%-15% and compresses about 1.5 - 2 times - this being enough to secure most of the benefits without incurring most of the costs.

Fuel preparation typically involves:

1. drying to about 12% moisture;

2. grinding or milling to a particulate material with particles having a volume each of around 70mm³ or less; and

3. compaction into standard sized containers.

The present process of "drying", "grinding" and "compaction" usually costs about a third less than known pelletising processes.

Typical steps in the process of the present invention are described below:

1. Forced drying. This is used to achieve a low moisture content, an increased calorific value for the fuel and a reduction in volume due to shrinkage. The forced drying is done at a temperature >= 100C, and heat is recovered in a condenser or other heat recovery device. Shrinkage of up to 15% can be achieved in the right conditions.

2. Grinding and screening. The material is ground and screened to produce a consistent fuel specification. The exact dimensions required vary according to national and regional markets, but typically are:

• Maximum particle size <10mm in any dimension

• Average particle volume of around 70 mm³ or less The sequence of drying and the grinding steps can be varied, depending on the characteristics of the feedstock. In essence the drier the material the easier it is to grind, and the smaller the material the faster the heat transfer rate and the easier it is to dry. This allows a relatively small (and thus more mobile) drier to handle high throughputs. The two sub-processes are, in general, integrated and optimised to achieve maximum plant throughput with minimum energy input.

4. Compression at low pressures into a closed container. Testing has shown that significant increases in density can be achieved at low pressures (for example preferably 0.1 to 5 Bar). Figure 1 shows the response of dry wood particles to compression. The relatively low pressure (typically up to 4 bar) used in the present invention is sufficient to secure a significant increase in density, without being sufficient to bind the product into a pellet or briquette. The use of such relatively low pressures means that smaller, less expensive equipment may be used than is used in pelletising processes, leading to a considerable reduction in the cost of producing the end product.

The fuel preparation equipment will preferably be mobile, allowing it to be taken from farm to farm and forest to forest and thus avoiding the high cost of transporting the wood from source to a processing plant. The drying equipment is sufficiently small to mount on a trailer or low-loader and is capable of being moved from site to site, as is the grinder, screening equipment, and the canister-loading device.

Alternatively, larger scale plants may not be fully mobile, but may be easily demounted and moved from time to time as resources in an area are consumed. The market for Compacted Biomass

Fuel in this form has a wide range of possible uses.

1. Coal fired power stations in conjunction with coal (co-firing). UK Government legislation has established an economic incentive for this.

2. Combustion in boilers for process heat, heating buildings and swimming pools, providing heat for absorption chilling plants, etc.

3. Combustion to supplement the steam cycle of conventional Combined Cycle Gas Turbine (CCGT) plant.

4. As a heat source for new designs of rotary steam engine being funded by the EU, (commercially available in 2004) which will have generating efficiencies close to those of a tiiesel engine (i.e. about 50%-100% higher than in a conventional small scale steam plant).

5. Alternatively, it may even be possible to burn directly in a turbine using a pressurised cyclone combustor, or similar, so avoiding gasification.

Containerisation of the fuel allows for economic transportation by road and/or rail, enabling links between distant fuel growing areas and industrial users. Reduced vehicle movements, as a result of the present compaction process, may also help with sensitive planning issues.

A typical 6m container holds about 10 tonnes of compacted biomass. Two are typically fitted onto a standard container lorry or three onto a rail wagon, and are transhippable between road and rail through existing transfer stations. Using two containers per lorry reduces costs but still allows the load to be split before final delivery. This approach allows maximum axle weights to be achieved, and full use to be made of the existing road and rail freight infrastructures. The Product

The product, compact biomass, is a ground, dry, particulate material delivered in a closed container, whose dimensions and fittings typically allow it to be handled by standard container handling equipment.

The energy density of the product in the container is around or in excess of 4GJ/m³, compared with a typical figure for wood chip of 2.6GJ/m³.

The container dimensions are usually a simple multiple or fraction of standard container sizes in the country of use. Thus, where 12m containers are widely used, the standard container sizes are preferably 3m, 6m or 12m long, and about 2.5m wide.

The Market Advantage of the Compact Biomass Product

Compact biomass competes favourably with pellets and woodchip in the renewable fuel market.

Figure 3 compares the delivered cost of compact biomass with the delivered costs of wood chip and pellets for different haulage distances. The price of gas is also shown for reference. (The haulage distances shown are based upon open road haulage - in urban areas the distance axis would be compressed by a factor of four or five.)

The graph shows that although wood chip is slightly cheaper than compact biomass over short haulage distances, compact biomass is the cheapest fuel for haulage distances from about 50 to 1000 km. Not only is compact biomass the cheapest option in this regard, but it also has other advantages over wood chip:

• Because it is typically twice the energy density of wood chip, compact biomass will require half as many transport movements. This both reduces logistics problems and reduces concerns about increasing road transport in urban areas and other areas of planning sensitivity.

• As mentioned above, the fuel preferably comes in closed containers that match standard container sizes. This means that they can be moved easily from road to rail and back through standard container transfer stations. This will allow the product to penetrate into highly populated areas (such as city centres) that chips cannot reach, and where lorry movements are restricted.

• The use of a standard fuel specification will allow it to be burned in all applications from power stations, co-fired with coal through to local school boilers, regardless of its source. This, coupled with the long economic haulage distances, will maximise flexibility in the market place and make it much easier to match supply and demand.

The energy density of compact biomass is usually up to 200% or more of that of wood chip. This allows it to compete economically over longer haulage distances than woodchip. This in turn allows the development of a regional marketplace for the fuel, rather than relying, as in the case of woodchip, on local markets close to where the fuel is grown. This ensures the presence of a more extensive market and improves the competitiveness of biomass over fossil fuels.

Compact biomass is typically prepared at pressures and temperatures much lower than pellets or briquettes. This allows it to be produced with much less expensive and lower energy-using equipment than pellets or briquettes. It also allows the equipment to be moved as needed. The process does not produce a self-binding block, as happens with pellets or briquettes. Therefore it usually requires a container to hold the product and prevent it from expanding after compression. The container also provides weather protection, transportability, dust control, fire containment, and easy storage, and acts as a fuel dispenser. The canister dimensions allow easy transport by road, rail or sea, and low cost transhipment between modes.

Two embodiments of the invention will now be described by way of the following non-limiting examples:-

Example 1

As shown in Figure 4, in one embodiment of the present invention, after harvesting and chipping the wood source on site, the wood chips are frequently air dried to 30% moisture content. After grinding, the particles are force dried to 10-12% moisture content in the form of about 3mm grindings. The present inventors have found surprisingly that such grindings can be adequately compressed at only relatively low levels of pressure to provide cost-effective transportation.

The present inventors have analysed how cost is governed by compression and moisture content for a given haul of hardwood particles. Previous methods relied on either pelletising at relatively high pressure of compression or transporting uncompressed wood chips. Pelletising at high pressures required delivery of effective compression of about 4 times at 10% moisture - i.e. more than was found to be needed by the present invention. Uncompressed wood chip at a moisture content of 30% was found to be expensive to transport. The present inventors found, by contrast, that drying to 10-15% and compressing by about 1.5-2 times is enough to secure most of the benefits without incurring most of the costs.

Example 2

As shown in Figure 5, in a further embodiment of the present invention after harvesting, the wood chips are placed in piles for air drying that may be augmented by for example, exhaust from chipping machines and the like. The partially dried chips are passed through a second stage of drying before being ground into more particulate material form. The second stage of drying also involves evaporative drying, but typically the wood chips are fed onto a conveyor belt which passes through a heated chamber so as to further reduce moisture. After grinding, the particulate material is compressed into containers ready for transport to power stations, industrial boilers and so forth.

As mentioned above, previously, harvested and chipped wood could be transported prior to processing at another site, which proved costly due to the bulky nature of the form in which the fuel was transported. The processing stages of the current concept are designed to be carried out prior to any significant transportation and preferably close to the site of initial harvesting.

Claims

1 A method of producing compact biomass fuel comprising :

(a) drying and grinding or milling of biomass to reduce its moisture content to within a predetermined range and to reduce its particle size to within a predetermined range to form a substantially free-flowing particulate material; and

2. A method as claimed in claim 1, wherein the moisture content, particle size and applied pressure are chosen to minimise the total cost of fuel preparation and transportation.

3. A method as claimed in either claim 1 or claim 2, wherein the drying of biomass is to between 20% or less, preferably 10-15%, more preferably 10-12% moisture content per wet weight.

4. A method as claimed in any preceding claim, wherein the compression pressure applied is of between 0.1 and 5 Bar, preferably 1 and 4 Bar, for example 2 and 3 Bar.

5. A method as claimed in any preceding claim, wherein the drying step comprises multiple stages.

6. A method as claimed in claim 5, wherein the method comprises an initial stage of drying the biomass to no more than 30 per cent moisture content.

7. A method as claimed in any preceding claim, wherein the biomass is in the form of grasses or chips, the chips being preferably derived from willow coppice, trees, and/or shrubs.

8. A method as claimed in either claim 6 or claim 7, wherein the grinding step occurs after at least one drying step but before the at least one other drying step, and/or the grinding step is succeeded by a screening step.

9. A method as claimed in any preceding claim, wherein the ground particulate material comprises particles of which 50%, preferably 75%, have a particle volume of less than 70mm³.

10. A product obtainable by the process of any preceding claim, the product being cost-effective to transport, particularly over a distance range of 50-1000 km.

11. A product as claimed in claim 10 having a bulk of energy density of 3 to 6 GJ/m³.

12. Use of a product as claimed in either claim 10 or 11 as a fuel in coal powered fuel stations or as a fuel or fuel additive for the reduction of nitrogen and/or sulphur oxide pollutants from emissions of fuel stations.

13. A container holding a product as claimed in either of claims 10 or 11.

14. A container as claimed in claim 13, wherein the container is of a size, or a simple fraction of a size, that is an industrially standard size as used in the haulage industry.

15 A method of producing compact biomass fuel, a product obtainable by such a method, a use of such a product or container holding such a product substantially as hereinbefore described with reference to, and/or as illustrated by, the accompanying drawings.