EP1214549A1

EP1214549A1 - Method for burning biofuel in a furnace using fossil fuel

Info

Publication number: EP1214549A1
Application number: EP00960746A
Authority: EP
Inventors: Pauli Dernjatin; Kati Savolainen; Juha Kostamo
Original assignee: Fortum Engineering Oy
Current assignee: Fortum Engineering Oy
Priority date: 1999-09-23
Filing date: 2000-09-22
Publication date: 2002-06-19
Also published as: WO2001025689A1; PL353990A1; FI19992046A; FI119124B; PL195937B1; CZ2002999A3; HUP0203046A2; HU229926B1; AU7294400A; CZ303811B6

Abstract

The invention relates to a method of co-firing biofuel in a boiler designed for burning a fossil main fuel such as coal, brown coal or peat, the method comprising the steps of first grinding the main fuel for burning, then feeding the main fuel with the help of a carrier gas into at least one burner, and burning the main fuel in the boiler combustion chamber. After the grinding of the main fuel, biofuel is mixed into the flow of the pulverized main fuel prior to the feed of the fuel mixture into the burning flame, whereby the biofuel is co-fired with the main fuel in the same flame. The biofuel may be hog wood, straw, fuel separated from community waste or other available biomass.

Description

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The invention relates to a method according to the preamble of claim 1 for co-firing wood, waste wood or other type of biofuel along with conventional coal in a boiler designed for burning coal, brown coal, peat or other particulate fossil fuel as the main fuel.

Globally agreed limitations on carbon dioxide emissions work to reduce the interest of using fossil fuels in energy production. Substitution of these fuel types with renewable fuels give a possibility of lowering the overall rate of carbon dioxide emissions. However, renewable biofuels are still a rather expensive option and need investments in equipment inasmuch they are not suitable for combustion in fossil- fuel-fired power plants such as those burning coal or oil without undertaking costly modifications. As there are, however, a great number of coal-fired power plants still tagged with a long predicted life, it would be advantageous to burn a suitable biofuel in these plants if technical facilities thereto are provided.

US Pat. No. 5,609,1 13 describes one technique of burning wood fuel in energy pro- duction. In this arrangement, the boiler combustion chamber is fired with pulverized coal and wood-derived material, the latter being comminuted to a precisely determined particle size. Separate feed nozzles or burners are used for the pulverized coal and the wood particles, complemented with oil burners. This burning technique is hampered by a typical problem related to wood combustion. When wood material is burned in shred form without a supplemental fuel, the particle size of the wood fuel must be very small and the amount of large particulates in the infeed fuel relatively small in order to maintain a stable flame in the burner without using a pilot fuel such as gas or oil. Yet, there is a risk that large chips or other clumps may remain partially uncombusted in the burner. Furthermore, excessively large particles may adhere to the walls of the boiler combustion chamber thus causing fouling of the heat-exchanging surfaces. To achieve complete combustion of the wood fuel and minimum fouling of the combustion chamber walls, the material used as fuel must be milled carefully and its correct particle size distribution need to be secured by screening. The above-described combustion technique is intended for tangential firing only, which means that it is not necessarily applicable to other types of boilers or combustion methods. Moreover, as the maximum water content of wood fuel must herein be kept smaller than 20 %, at least scrap wood from harvesting and other water-containing material need to be dried prior to its use as a fuel.

In addition to the above example, other similar techniques are known for burning wood in coal-fired boilers. Also these methods require milling of the fuel to an extremely fine particle size, even smaller than 1 mm and then using separate burners for combustion. The small particle size is necessary, because the wood fuel cannot maintain combustion with a larger particle size. Moreover, the fuel must be sufficiently dry. Due to these reasons, the investment costs of power plants designed in a conventional manner for the use of wood as a secondary fuel become high. The coal burners of the firing system must be complemented with auxiliary burners for the wood fuel and, of course, infeed means for the wood fuel and its combustion and carrier air flows. Comminution of wood into a small particle size is complicated and expensive inasmuch wood being a fibrous material cannot be crushed as coal in a ball mill, but instead the fibers must be separated from each other by shredding. Milling becomes rapidly more complicated when the particle size is reduced. While cutting wood into chips can be made at a relatively low cost and appropriate equipment have been developed for paper and pulp industry, it is difficult to impose a high amount of energy on very small particles of wood and suitable comminution apparatuses are expensive. Inasmuch there is normally also needed a dryer system for the fuel, the investment cost of a wood-firing plant of the above-described kind becomes so high that its erection is justifiable only on the grounds that the fuel is available at zero cost or its burning is even subsidized or if it offers the only way to cut down the carbon dioxide emissions of the plant. The known methods outlined above, however, are capable of replacing just a small percentage of coal feed b> a biofuel, normally only about 5 %. Furthermore, the combustion efficiency of separate wood burners is low. It is an object of the invention to provide a method for burning a biofuel in a power plant boiler designed for firing with coal as the main fuel in a manner that requires only minor modifications and investments in the power plant and offers low operat- ing costs, whereby a substantially improved profitability in the utilization of the biofuel is attained.

The goεl of the invention is achieved according to the invention by virtue of mixing biofuel in particle form into the flow of the main fuel after the coal mill, but before the fossil fuel is fed into a flame.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention offers significant benefits.

The use of a biofuel counts as a reduction of carbon dioxide emissions from an energy production plant inasmuch the combustion of renewable biofuels is not considered to contribute to the total emissions of carbon dioxide. For instance, if wood is used to replace a portion of the fuel combusted in a plant, the carbon dioxide emissions of the plant are accredited as reduced by the amount of the renewable fuel used in the plant. Hence, the method according to the invention is well suited for use in conjunction with existing plants due to the small amount of modifications and investments required. Obviously, modification of an existing plant for partial co-firing with a biofuel is less costly than the erection of an entirely new plant and, moreover, the present method is an excellent technique of utilizing biofuels at a higher combustion efficiency thereof than what is offered by plants designed for using biofuel alone, such as those equipped with conventional fluidized-bed or stoker boilers or gasification equipment. An essential feature is that the biofuel combusted in accordance with the method need not be milled to a very small particle size, but instead, it can be fired in a reliable manner in a conventional burner designed for burning a fossil fuel. Also the requirements set on the biofuel moisture content are rather relaxed.

The biofuel can be made cost-efficiently, e.g., from waste wood or other RDF (refuse-derived fuel) material, whereby also such waste can be utilized that otherwise is difficult to put in use. The method according to the invention is even applicable to fuel mixtures containing plastic or other combustible material that is present in some amount in RDF fuels, for example. In the present context, biofuel refers to material that basically comprises substantial amounts of material of biological origin, but may also contain minor amounts of other combustible materials. A particularly suitable biofuel is waste wood scrapped from building sites. The use of refuse-derived fuel reduces the loading of dumping sites and, in some cases, even a compensation may be enjoyed from the incineration of such waste. Use of biofuels and refuse-derived fuels is often supported by tax exempts and other supportive actions of communities. Scrap wood collected from forest harvesting and waste materials from mechanical wood processing are also well suited for use in the method according to the invention and, since the biofuel fired in accordance with the method need not be dry, the quality of such fuel may vary within wide limits. In addition to wood-derived material, also the use of other waste for firing may be contemplated, such as straw, bagasses waste and the like waste remaining from a refining process, but for the time being, wood-derived fuels appear to be most suitable for use in the process according to the invention and, in practice, their availability in at least the Nordic countries is better than that of other biofuel materials. The particle size of material to be burned may be substantially larger than what is required in burners specifically designed for firing with particulate wood-derived material, because the wood-derived fuel need not sustain the flame, but rather, wood particles are ignited in the main fuel flame and subsequently serve to enhance the combustion of the main fuel. The moisture content of the particulate fuel may be much higher, which means that the quality of fuel to be burnt need not be as high as in conventional arrangements. Resultingly, wider resources of biofuels become available, whereupon the threshold limiting the use of biomaterials as fuel is lowered, since the common problem of insufficient volume and poor availability of biofuels close to the energy-generating plant as a factor in investment decision-making loses its prior weight. One essential factor in the evaluation of the profitability of the method is that it can burn a substantially higher percentage of biofuel in regard to the fossil fuel, whereby according to present experiences it is possible to substitute at least 30 % of fossil fuel by biofuel using the method in accordance with the invention. The arrangement according to the invention makes combustion of materials derived from municipal waste and the like refuse particularly advantageous inasmuch the waste being burned passes through the high-temperature fireball zone of the boiler, whereby organic hydrocarbon compounds are disintegrated.

Combustion of wood and possibly other biomaterials in combination with coal enhances the burning process of coal in multiple ways. Since wood is an extremely reactive fuel containing an abundance of volatile and readily reactive compounds, addition of wood particles, e.g., in the flow of coal particles improves the combustion efficiency of coal and reduces the amount of noncombusted components in fly ash. In fact, the quality of fly ash is an extremely important issue for energy-generating plants inasmuch high-quality fly ash of a low content noncombusted components can be sold as raw material to certain industries, while low-quality ash must instead be used as landfill or hauled to a suitable dump, whereby it becomes worthless or even invokes extra costs from its handling/dumping. Addition of wood also reduces the amount of oxides of sulfur and nitrogen occurring in the combustion of coal. Such improvement of the combustion process by co-firing with wood or other biofuel is a remarkable benefit by way of allowing a wider latitude in the selection of fuels. For instance, the reduced emissions of sulfur compounds permits firing with fuels of a higher sulfur content without resorting to pretreatment of the fuel or increase in the capacity of the flue gas cleaning system.

In the following, the invention is examined in detail by way of making reference to the attached drawings and preliminary firing test, in which drawings

FIG. 1 shows an embodiment of the biofuel feed arrangement; FIG. 2 shows an embodiment of the biofuel feed arrangement; and

FIG. 2 shows another embodiment of the biofuel feed arrangement.

While the invention appears to find its major applications in co-firing of wood- derived fuel with coal, the main fuel may as well be brown coal or peat, while the co- fired fuel can be selected from the group of other like biofuels already mentioned above, such as straw, bagasses refuse or the like material readily available in suffi- cient quantities, as well as municipal waste, particularly the combustible components thereof that may contain other than organic waste. Later in the text, also peat is counted as a fossil fuel, while in certain cases it is considered to be a renewable biofuel.

In a coal-fired energy-generating plant, coal is first fetched from a storage and fed into a coal mill, wherein it is comminuted into particles of a sufficiently small size for firing. All the different coal grades are generally fragile and, hence, easily grindable, for which purpose a ball mill is used. Each plant has typically a plurality of coal mills and feed lines for passing the milled coal into the combustion chamber of the boiler. Coal is fed into the combustion chamber pneumatically conveyed along a line exiting at a coal nozzle. The carrier gas of coal, conventionally air, forms a portion of the required combustion air, while the rest of combustion air is today generally fed stagewise into the burner flame and the combustion chamber. In the art is known a plurality of different burners and types of air feed, as well as boiler and combustion chamber constructions. The method according to the present invention is suited for use in such boiler plants where the fossil fuel is discharged with the help of a carrier gas from a burner nozzle into the combustion chamber of the boiler.

According to the invention, the biofuel is mixed with the fossil fuel prior to the igni- tion of the fossil main fuel, which in practice must take place prior to the exit of the burner's fuel feed line into the combustion chamber. An alternative arrangement might be contemplated such that has the biofuel feed is arranged to exit from a separate nozzle or other like feed means just in front of the main fuel nozzle tip, preceding the ignition point of the main fuel. To gain the benefits of method according to the invention, however, it is important to secure that the biofuel is combusted in the same flame with the main fuel. On the other hand, the mixing point of the biofuel in the upstream direction of the main fuel flow should advantageously take place not earlier than after the milling of the main fuel. This is because such devices as, e.g., coal mills used for pulverizing coal are ill suited for comminution of a fibrous bio- material such as wood and, in fact, feeding wood into a coal mill may even invoke the risk of a mill fire. Moreover, the grinding result obtainable from feeding a raw biofuel into a coal mill is so inferior that it is ill advised to attempt passing the biofuel via a coal mill. Hence, the milling of the biofuel is advantageously arranged to take place into a sufficiently small particle size in separate comminution devices.

Referring to FIG. 1 , therein is shown schematically the co-use of a biofuel in the firing of a coal-fired boiler 1. The invention is applicable to all types of boilers fired with particulate solid fuel. The biofuel is received at a hauling terminal 2, wherefrom the fuel is transferred by conveyors 3 in proper amounts to processing into fuel. First, the fuel is driven over a magnet drum 4, where possible magnetic tramp metal is separated from the fuel. From the magnet drum 4, the fuel is passed to a screen 5, where larger clumps are separated and conveyed to a precrusher 6. At this stage, the material may also be dried. As the end moisture content of the fuel fed into the combustion process may be as high as 40 %, this predrying stage is primarily helpful in making the grinding of the fuel easier. From the screen 5, the biomaterial is passed to a grinder that can be an impact mill 7 in an advantageous and simple arrangement. Boiler flue gases can be passed into the impact mill along a line 9 to help drying the fuel in the impact mill and the precrusher. When leaving the impact mill 7, the particle size of the crushed material should be smaller than 8 mm and the moisture content should advantageously be not greater than 25 %. The particle size may be allowed to be this large inasmuch the combustion process with the main fuel in a common flame is very effective. Even a higher moisture content can be allowed for the comminuted fuel, depending on the composition of the received material. Advantageously, the fuel moisture content is 10 % to 40 %. After grinding, the biofuel is passed along a line 8 to one of the burner groups 11 of a coal-fired boiler. Such a burner group 1 1 is formed by burners operating at the same height on the boiler wall. One possible location of the burners of a given burner group 1 1 is at the corners of the boiler, for instance. Pulverized coal serving as the main fuel is passed to the burners along lines 10. In the illustrated embodiment, the biofuel is fed to one coal burner group only, but with the provision of a sufficiently ample supply of biomaterial, it is possible to feed biofuel to plural burners, even to all of them.

FIG. 2 shows a possible arrangement for biofuel feed. In this embodiment, the biofuel is mixed with the main fuel flow prior to the burner by way of connecting the biofuel line 8 to the main fuel line 10. Herein, the biofuel particles undergo mixing with the pulverized main fuel, whereupon the ignition and burning of the fuel mix- ture take place in a conventional manner in the burner flame. Advantageously, the embodiment according to the invention uses the burner type which is schematically illustrated in the diagram and achieves ignition of the fuel flow by means of a stabilizing ring 12. With the help of such a stabilizing ring 12, the flame can be ignited efficiently even under air-deficient conditions so that the flame always ignites at the burner tip and burns under all conditions in a stable manner. The fuel flow contains less air than is needed for complete combustion of the fuel, whereby more combustion air is stagewise added to the flame via secondary-air and tertiary-air nozzles 13, 14. While this kind of fuel feed arrangement might be feasible as such, a problem may be encountered from sideflow of pulverized coal into the biofuel line or from backflow effects in the biofuel line due to pressure variations in the feed system. Hence, it has been found more advantageous to feed the biofuel in the fashion shown in FIG. 3 into the center of the main fuel flow at the stabilizing ring so that the mixing takes place just prior to the ignition point of the flame. Neither one of these feed arrangements needs a separate ignition system, and the completed combustion of coal is aided, among other factors, by the volatile components imported by the biofuel. Accordingly, the invention is particularly suited for use in conjunction with this kind of modern burners in which the combustion process can be readily controlled. Nevertheless, the use of the invention may also be contemplated in other types of burners.

Burners of the above kind are described, e.g., in US Pat. Nos. 5,799,594, 5,431 ,1 14, 4,907,962 and 5,263,426.

In the following is briefly outlined a co-firing test of wood and coal, wherein coarse wood fuel was mixed with coal prior to its milling. The mixing was carried out in the coal storage field and was fed through the coal mills. The biofuel used in the test was wet with a moisture content in the range of 38 - 55 % and a high quantity of volatiles. The biofuel was also characterized by a low content of ash, nitrogen, chlorine and sulfur, as well as by having a low heat content. The particle size of sawdust in the material was large and the number of large particles such as sticks was high. In screening, an 8 mm mesh was required to obtain 100 % classification. During the tests, the behavior of the fuel material was examined in the coal mill and the combustion result with different wood fuel mixing percentages was investigated in two different series of tests. Studies into the operation of the coal mills revealed that the wood-derived biofuel causes some problems in the coal mills. Particularly the larger particles of the biofuel, such as sticks and chips initiated accumulation of wood material in the mills, release of smoke, degraded grinding of coal and a possible risk of mill fire. Accordingly, it is plausible that softer and more fibrous biomaterials such as straw are entirely incompatible with co-feeding with coal into a coal mill, because coal mills are not designed for handling such material. Hence, the test results support the concept that the biofuel should be mixed with coal that is already pulverized, particularly when large amounts of biofuel or coarse biomaterial is being used. Hence, in accordance with the invention the biofuel is mixed with coal only after the milling of the coal. This arrangement gives the benefit that the expensive coal mill need not be used for processing the biofuel, but instead, its comminution may be carried out using less costly grinding equipment can be used such as an impact mill. In the first combustion test series, the volumetric amount of wood in the mixed fuel was 17 vol.-% which corresponds to approx. 4-5 wt.-% and approx. 1-2 % of the overall heat content of the mixed fuel. During the test sei . the amount of wood was raised to 20 vol.-% corresponding to approx. 6-7 wt.-%. Due to uneven mixing and changes in the heat content of the fuel components, the actual mixing percentage varied. In the second test series, the amount of wood in the mixed fuel was further raised to 25 vol.-% which corresponds to 9-10 wt.-% and 3-4 % of the overall heat content of the mixed fuel. In either one of the test series, the flame ignition and burning process was normal. Also the formation of fly ash and soot was normal.

Furthermore, boiler combustion chamber temperatures remained normal, as well as the overall operation of the boiler.

During the combustion tests, the O₂, CO, CO₂, NO, NO_x and SO₂ contents of the flue gases were measured. Since the sulfur content of the wood fuel used in the combustion tests was extremely low, its contribution to SO₂ emissions was favorable and, moreover, increasingly so with a higher proportion of the wood in the mixed fuel. Also NO_λ emissions were reduced with the use of wood-derived fuel. The impact on nitrogen oxide emission from co-firing with biofuel is a significant benefit inasmuch wood in particular is fully compatible with staged combustion owing to its high content of volatiles, which means that the operating conditions of a boiler can be modified in high-volume co-firing with wood such that produce lesser amounts of nitrogen oxide emissions. Hence, it appears that one route to reduced emissions o oxides of nitrogen can be found from combining the use of biofuel with proper changes in the combustion techniques.

The content of noncombusted components in fly ash was examined by daily sampling. It was originally assumed that the addition of wood, which caused reduced grinding efficiency of coal mills, would have resulted in a higher content of non- combusted components in the fly ash. Unexpectedly, however, a clear reduction was found from the very beginning of the co-firing tests in the content of noncombusted components in fly ash. This reduction can be attributed to the contribution of wood toward more complete combustion of coal inasmuch wood is a very reactive fuel containing an abundance of volatiles.

On the basis of these test series, it is evident that addition ot wood into the main tuel of a coal-fired boiler does not disturb the operation of the boiler, but rather, aids the combustion process and facilitates changing the combustion conditions into a direction of lesser emissions. In the spirit of the invention, it is essential that the wood and the coal are burned in the flame of the same burner, whereby the addition of wood contributes favorably to the completion degree of the combustion. Herein, the amount of added wood may be relatively high and its particle size may be coarse. As a final notice to the above-described test series, it must be borne in mind that the test arrangements were slightly offset from the teachings of the invention inasmuch the wood-derived fuel was mixed with the coal prior to the infeed of the coal mills.

Claims

What is claimed is:

1. Method of firing biofuel in a boiler designed for burning fossil fuel as its main fuel, the method comprising the steps of

- grinding the main fuel for burning,

- feeding the main fuel with the help of a carrier gas into at least one burner, and

- burning the main fuel in the boiler combustion chamber,

characterized in that

- after the grinding of the main fuel, biofuel is mixed into the flow of the pulverized main fuel prior to the ignition of the fuels, whereby the biofuel is co-fired with the main fuel in the same flame.

2. Method according to claim 1, characterized in that the biofuel is mixed into the flow of the main fuel just prior to the ignition of the latter.

3. Method according to claim 1, characterized in that the biofuel is mixed into the flow of the main fuel prior to the feed of the latter into the burner.

4. Method according to any one of claims 1 - 3, characterized in that the biofuel is wood, straw, bagasses waste or the like fibrous waste of biological origin or a combustible fraction separated from municipal waste.

5. Method according to claim 4, characterized in that the biofuel is wood-derived fuel.

6. Method according to claim 4 or 5, characterized in that a biofuel grade is used having a particle size not greater than 8 mm.

7. Method according to claim 1 , characterized in that the amount of biofuel used corresponds to at least 5 % of the overall heat content of the mixed fuel.

8. Method according to claim 1 , characterized in that the moisture content of biofuel used is in the range of 10 - 40 %.

9. Method according to claim 1 or 4, characterized in that the main fuel is coal.

10. Method according to any one of claims 2 or 3, characterized in that the main fuel and the biofuel are fed through a stabilizing ring serving to ignite and stabilize the burner flame.