FI87834C - Suspension burning of wood waste, other biomass or peat - Google Patents

Description

1 87834

Suspension incineration of wood waste, other biomass or peat

BACKGROUND OF THE INVENTION This invention relates to heat recovery from moist wood waste or other biomass material and certain fuels such as peat. The target is in particular waste wood generated in the wood processing industry, commonly referred to as "wood chips".

As fossil fuel prices 10 have risen, the wood raw material processing industry, particularly sawmills and pulp and board plants, has shown a growing interest in recovering thermal energy from waste wood that cannot be used in the production of commercial products. In many companies, such waste is generated so much that it can cover a significant part of the energy needs. For others, peat is available which, if suitable means were available to recover the calorific value, would provide a cheap substitute for fuel oil or natural gas.

20 Many of the characteristics of waste wood from sawmilling and similar treatment of raw wood are such that they make efficient recovery of calorific value difficult. Wood chips are usually moist, i.e. their water content; is generally more than 50% by weight and often exceeds the self-combustion limit of 69%. Each mill's waste wood has its own ____ characteristic water content. A large part of the waste wood from sawmills and pulp mills is collected and stored outdoors, which absorbs rainwater during the rainy seasons.

An additional problem with wood chips is its considerable particle size of 30. Chip wood waste is generated in each wood treatment and processing operation. The size of wood waste varies from grinder dust with a particle size of 0.1-3 mm to bark and log garden waste, which can be more than 200 mm in diameter and more than a meter in length.

- - 35 In recent years, the common combustion method for wet wood chips has been grate burning with the oil-waste wood boiler combination 2 37334, oil is usually added to maintain combustion and improve the boiler's ability to meet the process's steam demand. Peat and other biomass material resemble wood chips in that they are moist and unsuitable in physical size. For this reason, these potential fuels have no significant use in many parts of the world. Although this is mainly a matter of moist waste wood or wood chips, the invention is applicable to all moist, organic plant materials.

10 Recent improvements to recover heat from wood chips require that the water content of the wood chips be reduced before it is fed to the boiler. Studies have shown that reducing the initial water content of the fuel improves steam production and reduces flue gas emissions from 15 boilers. All the latent heat generated in the woodchip combustion process is no longer needed to dry the fuel. The dry fuel requires less excess air and thus the heat loss of the boiler is reduced, which means an improved overall heat efficiency. At the high temperatures generated in the combustion zone, the particulate matter apparently burns for the most part before leaving through the chimney.

In U.S. Patent No. 4,235,174, Spurrell describes a prior art process by which wood chips can be successfully pre-dried and burned. In this process, some of the largest material taken from the wood chips is burned in a fluidized bed burner. The remainder of the wood chips are then dried with combustion products from the fluidized bed in a rotary dryer before being fed to a combined oil-waste wood boiler. Dried fuel. . sorted by size. The coarse fraction with a water content of about 35% is burned in a grate, and the fine fraction with a water content of 15% and a particle size of less than 3.2 mm is injected as an air suspension into the boiler.

35 However, in the Spurrell process, ignition oil must be added to the fines fed by fuel injection to maintain stable combustion. This ignition oil represents a significant amount of fossil fuel, up to 30% of the total burner input, expressed in BTU / h at full burner load. Such use of expensive fossil fuel is particularly unsatisfactory because it is not needed per se because of its energy content, but only serves as a source of ignition energy to achieve stable combustion of wood chips.

10 Attempts to burn the fines stream formed in the Spurrell process as an air suspension without ignition fuel or fire on the grate have generally failed. In test fires performed with this material, where n.

100% of the material had a particle size of less than 3.2 mm, stable combustion could not be maintained without the oil used as igniter. Even with the igniter, the overall conditions in the boiler were unstable and the boiler pressure varied greatly unless a fire was maintained on the grate.

Recently, it has been found that certain wood waste-20 roads can be burned in a boiler without the aid of oil or grate. Grinding machine dust, for example, which consists of very fine particles and has a water content of about 5%, has been successfully burned as an air suspension. In his informative review, "Power - 25 from Wood", Power, 124 (2), S1-S32 (1980), Schwieger describes the combustion of grinding machine dust containing about 12% water in a tube boiler. The average particle size of this material is reported as n.

0.8 mm. In this case, too, the use of ignition oil ... is recommended, which indicates unstable combustion conditions.

30 However, special materials such as grinder dust generally form only a very small part of the wood chips accumulated by a typical sawmill, in particular one which is used in the production of pulp. The amount of this dry, finely divided waste is not in most cases - ·. 35 not sufficient to cover a significant part of the energy needs of a typical plant. But in many companies where 4,878,834 wood wastes are generated, the woodchip heap would be able to fully cover this need.

Certain coarse and water-rich wood materials can be burned without the aid of oil in refractory boilers and kilns. The firebox of the refractory boiler is clad with a ceramic material whose temperature rises to approximately 800 ° C or higher. The hot gases then come into contact with the steam pipes. The heat retained by the ceramic material is continuously radiated back to the fuel and thus contributes to the maintenance of stable combustion in the furnace and thus it is possible to burn materials or wastes which are difficult to burn per se without the use of oil. The acquisition costs of refractory boilers 15 are high and high furnace temperatures mean high maintenance costs. The pipe area of such boilers must also normally be large, as the pipes are hardly exposed to the hottest part of the boiler.

20 Due to lower construction and maintenance costs, "water wall" boilers are preferred in the industry, where the flame is substantially completely surrounded by water pipes and the temperature in the pipes is generally at most about 320 ° C. The walls of these boiler structures are relatively cold compared to the flame and efficiently absorb heat. The ceramic cladding of the furnace radiates little heat and thus is not helpful in maintaining the ignition process. Due to this, dust combustion in water wall boilers. using conventionally available wood chips has generally required the use of fossil fuel to be able to continuously maintain heating energy sufficient to heat the fuel to the ignition temperature. Ignition occurs when sufficient volatile substances have pyrolytically formed from the fuel. 35 In the most recent attempt to burn a coarse fraction of wood chips, the wood chips have been finely ground. But due to its fibrous structure, grinding wood chips is typically more difficult than, for example, grinding coal. U.S. Patent 4,229,183 to Eneroth et al. show an improved method of burning wood chips in which the wood chips are simultaneously ground 5 and dried to a water content of 10-15%. The flow from the refiner is directed to a cyclone where the wood chips are separated from the air flow. The wood chips are then resuspended in air and the suspension is injected into the boiler. No grate is needed. Fagerlund, Tappi, 63 (3): 35-36 (1980) describes in more detail 10 Eneroth's method by mentioning that wood fuel is ground to a particle size of 1-3 mm. To adjust the flame, add 5 Ϊ ignition oil based on the bulb capacity. Fagerlund hopes that in the future, control systems will be developed that make the use of an oil additive unnecessary.

In another woodchip combustion system described by Baardson in U.S. Patent No. 3,831,535, the waste wood is dried and ground to a maximum particle size of 7.9 mm. This material is collected in a tank and incinerated by injection into a refractory lined chamber, where radiation from the refractory lining helps maintain stable combustion.

SUMMARY OF THE INVENTION The present invention converts whole wood chips or any other coarse or poorly sorted biomass or even peat into fuel that burns as an air suspension in a boiler without the aid of fossil fuels, refractory lining or grate, thus deviating from the prior art.

Fuel preparation and a method for burning the resulting fuel system can be used to heat incinerators, product dryers, and in particular water wall boilers or other "cold" wall type heat recovery processes.

The main object of the fuel preparation method of the present invention is to provide properly dried and reduced 6 87834 wood chips which, after leaving the refiner, can be fed to a vortex stabilized type dust burner and burned therein efficiently. The invention enables the steam boiler to be able to adapt to the varying energy needs of the process as efficiently as when burning oil or coal dust. In fact, the process of the invention is substantially more advantageous than coal compared to fuel oil No. 6, because the wood contains more volatile substances and the evaporation rate is higher. Slightly more ash is formed, but sulfur dioxide emissions are relatively insignificant, which is a considerable advantage for acid rain. NO emissions are also lower than Λ for coal or oil, which is an important consideration for plants and other boiler users in terms of environmental restrictions and requirements.

The main advantages of the invention are the elimination of the ignition oil, which is necessary to provide the ignition energy that maintains the stable combustion of wood waste in the water wall boiler. According to current commercial specifications for wood burning burners, 5-15% of the calculated BTU load on the burner must be covered with oil or other conventional fossil fuel to maintain flame stability.

Another important advantage of the invention is the elimination of the grate, which is avoidable in prior art boilers. incineration of such coarse-grained material that it cannot be burned as a suspension. The wood chips can be burned in their entirety as an air suspension. The efficiency of the system can very well be increased or decreased and thus adapted to the varying energy needs of the process. The burner power can be reduced by at least 2.5: 1.

It is an object of the present invention to apply the methods of the invention to existing woodchip boilers or. 35 coal-fired boilers so as to achieve fuel and capital cost savings. The combustion process 7 87834 of the present invention works equally well in a plant boiler burning lignite dust or oil.

Compared to conventional systems, the system of the present invention allows for substantial savings in operating costs by replacing oil or coal with wood. Elimination of the grate also eliminates the restriction on the maximum size of industrial boilers due to the size of the grate. The size of the boiler can also be reduced because the fuel is dried before combustion.

In the invention, wood chips with an initial water content of 50 S or more must be dried. Drying takes place mechanically or by heat to such an extent that the water content is less than about 30% by weight. A water content of up to about 15-20% by weight is preferred.

15 The grinding of the wood chips is then carried out in such a way that the particle size distribution is such that (a) even the largest particles burn substantially completely within the limits set by the heat recovery boiler, and (b) the fines are of such a particle size and quantity that they ignite and produce that the entire fuel flow burns evenly.

The drying is essentially completed before the grinding step, as it is easier to grind the dry wood to the fineness required at a later stage.

25 The upper limit for the particle size of wood powder depends on the boiler used to burn the prepared fuel and the prevailing emission limits. It has been found that a suitable upper particle size limit for wood chips to be burned in a grateless boiler is 65-100% less than 1 mm. When using a grate-equipped boiler, the upper size limit is less critical. In such a case, most of the coarse material drips onto the grate.

In pulverized fuel, the properties of the essential fines depend on the water content of the fuel and the type of burner with which the fuel is burned in the boiler. Higher water content requires a longer drying time 8 8 7 834 and this delays ignition. In such cases, the wetter wood chip must contain more fines if the limiting factor is the ignition time. The fuel of the invention is particularly intended for use in a vortex-stabilized dust burner known as commercial models, especially for coal dust combustion.

It was found that a fines component with at least 15% by weight of particles less than 150 was suitable for the burner shown in Figure 2. The particle size distribution 10 of the fuel is a critical factor for the invention. The preferred fuel specification is a distribution of about 100% less than 1 mm and 50 less than 150 mm when the burner is not optimized to run on wood.

In summary, balancing the water-15 content of the fuel, the particle size distribution and the way in which the fuel is mixed with the combustion air and injected into a boiler, e.g. a boiler with a burner, forms the basis for a fuel preparation and combustion process where stable fuel efficiency can be achieved. . Such an increase is therefore primarily a limiting factor in the prior art. The water content and the particle size distribution are not independent of each other, but these factors can be adjusted within the limits that allow reactive fuel to be obtained for the spent burner. The combustion of the fuel, which is entirely in the form of an air suspension, makes it possible to operate the boiler in such a way that it does not require a grate and can adapt well to variations in the boiler load. The method can be applied to all types of boilers, incinerators and the like, and the method is particularly well suited for water wall heating boilers and steam boilers, thus differing from prior art systems.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a method of the invention for burning ground wood chips in a water wall heat recovery apparatus.

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Figure 2 is a schematic drawing of a vortex stabilized bulb capable of burning dried, pulverized fuel.

Figure 3 shows a particle size distribution diagram characterizing a number of dried, pulverized fuels that can be used in the method of the present invention, wherein the burner is of the type shown in Figure 2. For comparison, there are a few prior art fuels.

Figure 4 schematically shows an embodiment of the invention in which the fuel is temporarily stored before combustion.

Figure 5 shows an alternative arrangement of the invention in which a separator is used to concentrate the fuel and part of the separated air is used as secondary filling air. 15 Description of the Preferred Embodiments

Referring to Figure 1, wood chips taken from a factory wood chipper, typically containing about 60% water and consisting of various wood materials from grinder dust to large log processing chips 20 and bark pieces, are fed to a drying and screening process 10. A drying process similar to the above can be used. in said and described U.S. Patent 4,235,174. The Spurrell patent is an example of a suitable drying process of this invention, but other methods are equally suitable. The spurrell process produces wood chips with a water content of less than about 30% as required by the present invention. The final water content depends on the operation of the dryer and the average particle size of the dry fuel formed. The spurrell process usually produces a material that varies from chips with dimensions of approx.

40 mm x 100 mm, for fines with a diameter of less than 3.2 mm. The water content of these particles can range from about 10 percent of finer material to about 30 percent of larger chips-35 pieces.

10 87834

The dried wood chips are conveyed by a belt to an intermediate leveling tank and a dosing unit 11, which may be similar to a coal dust feeder. The wood chips are initially kept in a tank 12, which serves to compensate for flow interruptions.

5 The wood chips are discharged from the tank 12 through the column 13 onto the weighing belt 14. The length of the column 13 is chosen so that the explosion protection of the tank system 12 becomes 550 kPa. In other words, the explosion in the refiner cannot proceed to the tank 12 due to the dimensioning of the column 13. The fuel 10 is fed through line 16 to refiner 15. Unlike volume-based systems, dosing system 14 feeds wood chips in a steady, continuous, weighed flow to refiner. The weight of the wood chips can be varied within wide limits.

15 The grinder 15 is a high-speed hammer mill. The machine is manufactured by the Pulverizing Machinery Division of Mikropul Corp., Summit, New Jersey, and is described by Duychinck et al. U.S. Patent 3,285,523.

The fuel preparation and combustion methods of this invention are designed to burn fuel as dust combustion using a vortex stabilized burner. In such a system, the amount of air required for grinding supplied by the blower 17 is preferably limited to such a small amount that the amount is just sufficient to transport the fuel to the combustion zone of the boiler. Thus, the preferred refiner produces ground fuel suspended in a minimum amount of air, i.e., about 0.6-2 kg of air per kg of fuel, according to the needs of the burner.

The transport or primary air transports the fuel 30 to the burner 18, from where it is injected into the combustion of the boiler 20. . to zone 21. In addition to fuel, the fan 19 supplies secondary air to the burner 18. Water-filled heat transfer pipes 22, which in fact substantially surround. '- completely burner flame 21, describe the load applied to the boiler, i.e. the need of the plant.

n 87S34

A key parameter of the method of the invention is a burner 18 which injects dried, ground wood chips into a boiler and mixes them with air so that the fuel burns substantially completely as a suspension. The vortex-stabilized burner, 5 of which are used to burn coal dust as an air suspension, was the starting point for designing a burner suitable for burning ground wood chips. Some routine changes had to be made to the geometry of the coal burner to provide the appropriate speeds, velocities, and trajectories to ensure complete suspension combustion of the substantially variable wood chip feed.

Figure 2 shows a vortex stabilized burner 18 of the type generally suitable for use with fuel prepared by the methods of this invention.

The burner 18 is mounted in an opening in the wall 23 of the boiler 20, the function of the igniter 24 in the form of an oil nozzle being to generate a flame and start combustion. Pipeline 25 surrounding the concentric oil pipeline 24 transports dried, ground wood chips and primary combustion air 20 from the refiner to the boiler. The primary vortex vanes 26 provide a moment of momentum in the fuel and primary air flow as the flow exits the burner 18 and the flow is injected into the boiler 20.

The secondary combustion air 25 generated by the fan 19 (see Fig. 1) enters the burner 18 via an air flap 27, with which the amount of supply air and the intensity of the vortex generated in the air can be controlled. The secondary vortex vanes 28 also provide momentum in the secondary air. The ratio of the area of the fuel pipe 25 leading to the burner to the area of the opening of the boiler inlet wall brackets 29, commonly referred to as a "choke", contributes to the flow characteristics of the secondary air entering the boiler.

The vortex and throttling effect of this secondary air flow creates a recirculation zone (see Figure 2) in which 35 combustion products and heat flow back into contact with fresh fuel discharged from fuel line 12 87834 in the air mass. and partially in secondary air to ignition temperature. The fines in the fuel ignite and release the ignition energy to the remainder of the fuel before the fuel can escape from the flame area.

The presence of fines as a source of ignition energy stabilizes the flame. The presence of fines therefore forms the core of the invention. The fines eliminate the need to continuously add an auxiliary oil to achieve burner stability.

The characteristics of the recommended burner are a high throttle ratio, i.e. the ratio of the primary burner area to the choke area, and low vortex. For purposes 15 herein, the term "high throttle ratio" means a ratio of burner area to choke area of 0.5 or greater. The main function of the combination of vortex and throttle is to create a recycling zone. Also, the mixing of the secondary air and the primary air takes place only at the rate required for combustion. With limited mixing of secondary air, there is no need to add excess "cold" air, which would retard ignition.

The main advantages of the method and apparatus of the present invention are the ability of the system to respond to the varying steam needs and other thermal energy needs of the plant. The power of the burners of the invention can be reduced to less than 100% capacity. The power of the system of the invention can be choked in a ratio of ·: at least 2.5: 1. This means that, depending on the load variations, the burner can be throttled in a ratio of 100 / 2.5, ie ... 30 to 40% of maximum power. Below the throttle level of 2.5, the burner usually operates unstably due to the recirculation zone. . whose activities cease.

To best burn dry, pulverized fuel in a boiler, it is necessary to use a ratio of primary air 35 to fuel of 0.6 to 2 kg of air per kg of fuel at a load of 100%, or 16 to 32% of the stoichiometric amount of air required for complete combustion. At low load or fuel flow, the air to fuel ratio increases to 3: 1. It is preferred to use a minimum amount of primary air to minimize the amount of "cold-5" air that must be heated with the fuel to reach the ignition temperature.

Prior to this invention, a tank system had to be installed between the refiner 15 and the burner 18 to provide the necessary primary air to fuel ratios. This was necessary because all existing refiner designs required an air to fuel ratio of approximately 3 kg air / kg fuel at high load and 8 kg air / kg fuel at low load, i.e. 50-150% of the stoichiometric amount. At such high air-fuel ratios, a burner connected directly to such a refiner could not be properly throttled.

A critical major aspect of the present invention is the particle size distribution of the dried wood chips fed by the burner.

Figure 3 shows a series of particle size distributions of ground wood chips consisting of a series of embodiment fuels of the present invention and three lettered prior art fuel distributions. According to a basic claim of the present invention, the size of the wood chips must be reduced to such an extent that it becomes a source of ignition energy that allows dust combustion without the addition of oil. A further observation based on the experiments is that all the dried, ground wood fuels described in the prior art are too coarse, which prevents them from burning in a water wall or cold boiler without the addition of fossil fuels.

Referring to Figure 3, curve A depicts the proportion of wood chips fines prepared in accordance with the drying and screening method 35 described in U.S. Patent 4,235,174 to Spurrell. Attempts to burn this fuel in water wall mode without some addition of oil fuel were not successful. Indeed, the fuel in Curve A is slightly finer than the pulverized fuel that, according to Baardson's description in U.S. Patent 3,831,535, was successfully burned in a refractory lined furnace. The Baardson 5 fuel is characterized by a maximum particle size of 7.9 mm and is ignited by the high wall surface temperature, which can be in the range of 1200-1315 ° C. If Baardson's fuel were plotted in Figure 3, it would be located slightly to the left of curve A, which is assumed to be typical of prior art fuels. These do not burn as dust combustion in a cold wall firebox without the addition of fossil fuel.

Curve B depicts another prior art fuel which, according to Fagerlund, cited above on page 4, 15 is a typical Eneroth (Flakt Inc.) and ASSI fuel. This fuel also burned unstable in the combustion tests because it was too coarse-grained.

Curve C depicts a prior art ground coal sample. This was substantially finer than wood-20 wood chips.

Curve 1 defines the maximum particle size of a suitable dried, ground wood chip in this invention. Curve 2 defines the minimum particle size of a normally usable wood chip. Most fuels fall between these 25-limits. The preferred fuel particle size range for a given boiler is easy to determine experimentally. The particle size distribution of the fuel shall be such that the fuel is completely self-igniting and burns in the cold wall firebox. Fuels located between lines 1 and 2 are in accordance with the invention. The distribution of the coarse and fine proportions of fuels used successfully must be substantially in accordance with Curves 1 and 2. This means that the slope of the acceptable fuel distribution must be approximately the same as in curves 35 1 and 2. The upper size limit of approx. 65-100% less than 1 mm ensures sufficient "burnout", i.e. the 87834 residence time available in the combustion boiler to meet the emission requirements. Stable combustion conditions require a lower limit, i.e. a fines content such that at least 15% is less than 150. However, fuels that are much finer than 85% below 5,150 are too "dusty", increasing the risk of dust explosion and consuming too much energy for grinding.

The particle size of the fine portion was measured with an Alpine Jet Sieve. Tyler standard screens are used in the device. There is a slight pressure difference between the screens. Device 10 is sold by Alpine American Corp., Natick, Massachusetts or Alpine Machines A.G., Augsburg, Germany. The fraction of 150yUm corresponds to the fraction passing through the standard sieve No. 100. When measured by other methods, slightly different results are obtained. For example, a fraction of -150 μm measured with an Alpine Jet 15 Sieve would have an average particle diameter of about 100 μm measured with a Malvern Laser Particle Analyzer (Malvern Instruments Ltd., Malvern, England).

Curves 3 and 4 illustrate the particle distributions of the selected fuels described in detail below.

In some cases, it is possible to burn fuels that only marginally meet the specifications set for fuels in this invention by adjusting the characteristics and operating conditions. For example, certain coarse-grained fuels may burn more stably without the addition of oil if the transport air is heated to a temperature of several hundred degrees. Experiments show that although the stability of marginal fuel is improved, the effect is not large enough to burn "as such" ground or coarsely ground fuels 30 in a stable manner. These fuels are generated, for example, in the Spurrell method as a fine fraction, which. . the particle size is about 3 μm. Heating the transport air improves combustibility (1) by reducing the water content of the fuel particles in the burner, (2) by raising the original:. · 35 temperatures in the fuel air injector and (3) enabling operation with reduced primary and secondary flow ratios.

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Reducing the water content of marginal fuels may help stabilize combustion, but reducing the water content to less than 10% by weight can be dangerous because it increases the risk of explosion.

5 Variations in fuel properties can affect grinding performance. A high ratio of wood to bark can substantially increase energy requirements.

Compared to coal wood, which has a fibrous structure, it is relatively difficult to grind. Wood grinding 10 requires high impact grinding as opposed to coal grinding, which normally uses a crusher type grinder. When grinding wood, the energy requirement is approximately 27.5 kWh / t when grinding bark and 15 55-90 kWh / t when grinding high-fiber fuels, but grinding coal requires only 11-16.5 kWh / t. Experiments have shown that wood is easiest to grind dry.

The experiments showed that the water content varied when different wood fibers and bark scok-20 were ground. The grinding efficiency was measured by a method used by the grinding industry to determine the area of the particles generated per unit of energy used, i.e. m / MJ. In order for this method to be able to determine the fuel distribution in practice, the fuel must first be dried and then ground.

The main advantages of the present invention are an arrangement in which the fuel is first dried to a water content of less than 30% by weight and then ground. The reverse arrangement, e.g., the Eneroth method 30 cited above, described by Fagerlund, requires twice the size of the device or twice the number of devices to achieve a given production rate, and more importantly, four times the amount of energy, which is the critical operating cost of grinding.

Figure 4 schematically shows an ongoing heat recovery process burning wood chips, in which dry, ground wood is temporarily stored before being fed to the boiler. The wood chips, which have been dried e.g. according to the Spurrell process, are collected in a first tank 30. The air from this tank 30 is mixed with air generated by the fan 31 5 and the mixture is conveyed via a pipe 32 jnuhimocn 33. The fan 37 sucks make-up air through The ground wood chips and air are discharged through a conveying line 35 to a hose dust separator 36. The carrier air is removed 10 by means of a fan 37. The ground, dry wood chips drop onto a conveyor 40 which transfers the fuel to a storage / intermediate compensation tank 38. The wood chips are then fed to a boiler 44 according to the heat demand of the plant process. If necessary, the fuel is mixed with air 41, which is supplied by a primary air blower 39.

The air-fuel mixture 42 is injected into the boiler 44 through a dust burner 45. This secondary air required for combustion is supplied by means of a conventional boiler air system 47.

In certain conversion situations, it may be necessary to use the interim storage tank process 20 shown in Figure 4, which may increase the cost of capital. The downside of a tank system is that it poses a much greater risk of dust explosion than immediate combustion. Thus, the use of a tank requires explosion detection and prevention instrumentation and equipment. The tank combustion system is, in fact, an intermediate form in the development of a system that allows the combustion of fuel dust taken directly from the refiner.

"'· Example ·: ·· * The following tables describe typical fuel, airflow, and certain other values that are characteristic of the operation of the process shown in Figure 4. The process operates completely without the addition of oil.

The drying process described by Spurrell in U.S. Patent 4,235,174 provided a dry, screened feed for this production run of the heat recovery process.

'' 35 The system provided fuel for two burners similar to burner 45 in Figure 4.

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The boiler was a water wall heating boiler, the heat recovery section of which had water-filled elements around the combustion zone for heat recovery.

The refiner had a built-in classifier

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5 equipped with a standard grinder Micro-ACM, manufactured by Pulverizing Machinery Division of MikroPul, U.S. Pat. Filter Corporation of Summit, New Jersey. The refiner was equipped with a 225 kW engine that provided an air-fuel ratio of 2.8: 1 with a high grinding load and a ratio of 8.1: 1 with a low load of 10. With these airflows, an intermediate tank was necessary to throttle the boiler output.

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* 2i 87 834

Figure 5 shows an alternative arrangement in which a high air flow refiner 59 discharges the fuel-air mixture 60 into a hose filter or cyclone 36 '. A portion of the air flow to the hose filter or cyclone 36 'is used as the secondary air 64 of the burner 45'. The fuel is discharged from the tank or cyclone 36 'and mixed with air from the primary air blower 63.

It is clear that several parallel dryers, grinders and burners are needed to satisfy the total load demand of the boiler energy recovery system. For example, it can be concluded that one refiner is needed per 100-200 million kJ / h of burnt wood chips.

Claims (8)

22 87 834 A method for burning moist organic fuel in a water wall or other cold wall boiler 5 (20; 44), characterized in that at least one vortex stabilized burner (18; 45; 45 ') modified for dust combustion of the fuel is used; drying the moist fuel to an average water content of less than about 30% such that at least a portion of the finely divided particles have a water content of not more than about 20%; grinding the dried fuel such that at least 60% by weight of the particles are less than about 1000 microns in size and at least about 15% of the particles are less than about 15,150 microns in size; adjusting the proportion of particles less than about 150 microns in the milled portion such that the fuel forms a self-igniting flame (21); transferring the dried and ground particles to the burner-20 (18; 45; 45 ') suspended in a primary air stream; and igniting the particles, thereby generating so much ignition energy from a portion of less than about 150 microns that the fuel as a whole burns continuously and stably. The method of claim 1, characterized in that substantially all of the particles are less than 1000 microns and at least 50% by weight is less than about 150 microns. A method according to claim 1, characterized in that the average water content of the fuel is less than about 20%. A method according to claim 1, characterized in that the burning ratio of the burner (18; 45; 45 ') is high in order to minimize the mixing of the secondary air with the primary air-fuel mixture. 35 23 87834 A method according to claim 1, characterized in that the organic fuel is waste wood consisting of wood and bark particles. Process according to Claim 1, characterized in that the organic fuel is peat. A method according to claim 1, characterized in that a weight ratio of primary air to fuel of about 1-3 kg of air per kilogram of fuel is used. A method according to claim 1, characterized in that the moist fuel is first screened before drying so that no dimension of the largest particles to be dried exceeds about 100 mm. 24 8 7 8 34