FI119124B - Procedure for combustion of biofuel in fossil fuel boiler - Google Patents

Description

119124

A method for burning biofuel in a fossil fuel boiler

The invention relates to a method for burning wood, waste wood or other biowaste according to claim 1 in a boiler using coal, lignite, peat or other 5 particulate fossil fuels as the main fuel in connection with conventional coal combustion.

Restrictions on carbon dioxide emissions worldwide will make fossil fuels more attractive for power generation. Replacing these fuels 10 with renewable fuels is one way of reducing total CO2 emissions. However, renewable biofuels are still relatively expensive and require investment in equipment as they cannot be used in plants designed for fossil fuels such as coal or oil without costly conversion work. Given the large number of coal-fired plants, 1S with a long-term future life would be advantageous to use suitable biofuel in these plants, if technically possible.

U.S. Patent 5,609,113 describes one way of using wood fuel for energy production. In this solution, the coal furnace furnace is fed with 20 carbon dusts and wood material, which are ground to a specified grain size. Carbon dust and • · · wood have their own nozzles or bulbs, and ·· ♦ · ;;; Oil Burners. This type of incineration has the typical problem of burning wood. When wood • ♦ * * '. combusted as granular material without support fuel, the grain size of the fuel must be • · · • ·· very small and the proportion of large particles in the fuel small enough for the flame to sustain combustion in the burner without support fuel such as gas or «Φ * oil. In addition, large chips or other pieces may be partially unburned. ·; ·. in the burner. In addition, too large particles may end up on the boiler furnace walls • ♦ ·. *: *. causing fouling of the heat exchange surfaces. To ensure that wood fuel combustion is as complete and dirty as possible, the material to be used is carefully ground * and screened to ensure correct grain size distribution. This combustion method is mainly * * · * intended for tangential combustion only and may not be suitable for other • «·: boilers and combustion methods. In addition, the maximum moisture content of wood fuel is * * · • 9 2 119124 less than 20%, so at least when using logging waste and other damp material, the fuel must be dried.

In addition to the above example, there are other similar ways of burning wood 5 in coal boilers. In these methods, too, the fuel is ground very finely, up to less than 1 mm in grain size, and burned in separate specimens. A small fuel grain size is necessary because the fuel cannot sustain combustion as the grain size increases. Similarly, the fuel must be sufficiently dry. For these reasons, the investment costs of existing wood-fired plants 10 are high. In addition to carbon burners, the system requires other burners for wood fuel and, of course, fuel and combustion and transport air feeders. Grinding the wood to a small grain size is difficult and expensive because it is not crushed as a fibrous material, for example in a ball mill such as carbon, but the fibers have to be removed by tearing. Grinding becomes more difficult as the grain size decreases.

1S Chopping wood into chips is reasonably inexpensive and equipment has been developed for the paper industry, but targeting the required amount of energy to small pieces of wood is difficult and suitable equipment is expensive. Because the fuel drier system is usually still needed, the investment required for an incinerator such as the one described above is so great that it is only justified when the fuel is 20 free or paid for or if it is the only way to reduce the fuel. carbon dioxide emissions. However, these known methods are capable of replacing • · · · ;;; only a small percentage of carbon with biomaterial, usually only about 5%. For discrete • · '; 3. the efficiency of wood burners is also poor.

It is an object of the present invention to provide a process for the use of biofuel in a boiler using coal as its main fuel, which enables the plant to operate. ”···, modifications and investments low and low running costs, • · ·. ·: ·, The profitability of using biofuel is substantially improved.

The invention is based on the blending of granular biofuel with the main fuel stream! 1 before the fossil fuel is fed into the flame and after the coal mill.

In particular, the process according to the invention is characterized in what is set forth in the characterizing part of claim 1.

The invention provides considerable advantages.

5

The use of biofuel can reduce the CO2 emissions of an energy production plant, since the use of renewable biofuels is not considered to increase the total amount of carbon dioxide. Thus, for example, if wood replaces part of the fuel in a coal-fired power plant, the carbon dioxide emissions are reduced by an amount equivalent to the amount of substitute fuel 10. The method according to the invention is well suited for use with old plants and the amount of modifications and investments required is small. Conversion of an old plant to a partially biofuel plant is more cost effective than the establishment of an entirely new plant, and this process can efficiently utilize biofuels and has a higher efficiency than 15 biofuel only plants, ie better efficiency than known fluidized bed or grate boilers or gasification. It is essential that the biofuel used in this process does not need to be finely milled, but can be burned in a conventional fossil fuel burner. Also, high humidity requirements are not set for the fuel.

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· ;;; The fuel used can be e.g. inexpensive waste wood or other RDF material (RDF

* · P * - refuse derived fuel), which can at the same time be used for waste that is difficult to reuse. The process according to the invention is also applicable to S * 25 fuel blends containing plastics or other combustible materials, for example, to some extent in RDF fuel. Thus, in this specification, biofuel is understood to mean a material comprising a significant amount of • 1 »biomaterial, but may also contain a smaller amount of other combustible material. Construction waste wood is particularly suitable as a fuel. By using waste material, the load on the landfill is reduced and fuel consumption can be up to e · * · *. The use of biofuels and waste fuels is also often subject to # *** · · _ · * tax breaks or other community support. Harvesting waste or mechanical • wood waste is also well suited for use with the process of invention 4 119124, and since the biofuel need not be dry in the process of the invention, the quality of the fuel may vary considerably.

In addition to wood-based material, it is conceivable that agro-food waste such as straw, sugar cane waste or other waste material from the S processing process will be used as fuel, but currently wood-based materials appear to be best suited to the process of the invention. The grain size of the material to be burned can be significantly larger than using separate pellet wood burners, since the wood fuel itself does not have to maintain 10 flames, but the wood ignites in the main fuel flame and further contributes to the combustion of the main fuel. The humidity of the fuel can also be considerably higher, so the quality of the fuel used does not have to be as demanding as in the known solutions. This improves fuel availability, which lowers the threshold for biomaterial use, as often the problem of low fuel availability and poor availability close to the power plant is not as strongly a constraint on the investment decision as before. One of the key factors in assessing the viability of the process is that it can achieve a much higher biofuel ratio than fossil fuels, at least 30% at present. ·. 20 of the fuels can be replaced by the process of the invention. biofuel. From municipal or other waste · »» «. · · ·. the incineration of the material is advantageous in the manner of the invention because the waste passes · · ·. *. ^: through the high-temperature zone of the boiler, whereby the organic hydrocarbon compounds are destroyed.

• * · 25 • · ·

Combining at least wood and possibly other biomaterials with carbon: V improves the flow of the coal burning process in many ways. Because wood is a highly reactive fuel and contains high levels of volatile, highly reactive compounds, wood. for example, adding to a stream of coal improves the efficiency of coal burning and reduces • · · ·. · · ·. 30 proportion of non-combustible ash. The quality of fly ash is a very important factor * * *., * For power generating plants, as good quality low ash ash • · · can be sold as an industrial raw material but poor quality ash must be used · for land improvement or disposed of in a suitable location. you may even have to pay compensation or handling it. Addition of wood also reduces the amount of sulfur and nitrogen oxides produced by burning coal. Improving the combustion process by using wood or other biofuels is a major advantage as it allows for a wider range of fuels.

5 For example, lower sulfur emissions allow the use of higher-sulfur fuel without pre-treatment of the fuel or raising the capacity of the purification plant.

The invention will now be considered with reference to the accompanying drawings and the preliminary use tests carried out in this field.

10

Figure 1 schematically illustrates one embodiment of the invention for using biofuel in coal combustion.

Figure 2 illustrates one known biofuel delivery method.

15

Figure 3 illustrates one mode of biofuel feeding according to the invention.

The invention seems to be best suited for the combined use of wood and coal, but lignite or peat can also be used as the main fuel, and other biofuels such as the abovementioned straw, cane or other similarly available material other than municipal waste may be used as by-product. organic • · · • φ · * 1; waste. In the following, fossil fuel also refers to peat, although it · · · *! 1.1. in some cases, they are classified as renewable fuels.

V ·: 25 • · · • · · *. / In a coal-fired power plant, the carbon stored in the plant is fed first to · · · \. · # Coal mill where it is ground finely for combustion. Different types of carbon * · 1 1 are usually easily grindable and brittle and can be treated with, for example; ·, a ball mill. The plants usually have several coal mills and feed lines for powdered coal • · ♦. ···. 30 for feeding into the furnace furnace. Carbon is fed into the furnace by air through a pipeline • · 1 *. through a carbon nozzle. The carbon carrier gas, usually air, forms a part of the required • 1 combustion air, and the rest of the combustion air is currently fed to the burner • · * · 1 flame and furnace. There are many ways to think about and supply air, such as boiler and fireplace types. The process of the present invention is suitable for use in boiler plants in which fossil fuel is fed by carrier gas through the burner nozzle into the boiler furnace.

According to the invention, the biofuel is blended with the fossil fuel prior to ignition of the main fossil fuel, i.e. practically before the burner fuel nozzle tip. It is conceivable that the biofuel is also fed by a separate nozzle or other feeder just to the mouth of the main fuel nozzle prior to ignition of the main fuel. In the process 10 of the invention, the biofuel must be burned in the same flame as the main fuel in order to achieve the advantages of the invention. On the other hand, it is most advantageous to blend the biofuel with the main fuel after grinding the main fuel. For example, coal mills used to grind coal are poorly suited to grinding fibrous biomaterial such as wood, and feeding wood to a coal mill may cause fire. The grinding result of the biofuel 15 in the coal mill is poor, so that feeding it through the coal mill is not advantageous for the grinding result. Thus, it is advantageous to grind the biofuel into sufficiently small particles in separate devices.

Fig. 1 is a diagram showing the use of biofuel in a coal boiler 1. 20 connection. The invention is applicable to all types of dust incinerators. The biofuel • · · is brought to reception station 2, where it is transported by conveyor 3 as needed. * * *. fuel. First, the fuel is treated with a magnetic roller 4 to separate • · ·; \: Magnetic metals possibly present in the fuel. From the magnetic roller 4 • ·: * · *: The material is passed to a screen 5, where the largest particles are separated for pre-crushing: *: ': 25 6. At this stage, the material can also be dried. Since the humidity of the fuel used can be as high as 40% for the combustion process, drying is needed primarily to: facilitate the grinding of the fuel. From the screen 5, the biomaterial is fed to ··· grinding, which can be done in an inexpensive and simple hammer mill 7.

. ·. Burner gases from the boiler are fed to the hammer mill along line 9 to fuel • · · φ · ***; 30 for drying in a hammer mill and pre-crushing step. Hammer Mill 7 Outbound • · ·; The material should have a particle size of less than 8 mm and preferably less than 25% humidity.

• ··, ···, Particle size can be kept so large because combustion with the main fuel • ♦ • · · in the same flame is efficient. The humidity of the fuel can be higher and depends on the material used. Preferably, the humidity of the fuel is between 10 and 40%.

The milled biofuel is led along line 8 to one of the burner levels 11 of the coal boiler. The burner level 11 consists of reflections which are at the same elevation level in the boiler. Burners on the burner level 11 may be, for example, in the corners of the boiler.

5 Carbon dust used as the main fuel is fed to the reflectors on lines 10. In this solution, only one carbon level is fed into the biofuel, but if sufficient biomaterial is available, it can be led to more and even to all the reflectors.

Figure 2 shows one known method for feeding biofuel. In this solution, the biofuel is blended into the main fuel stream before the burner by connecting the biofuel line 8 to the main fuel line 10. The biofuel particles are then mixed with the main fuel dust and the mixture is ignited and burned normally. In the solution of the invention, the pliers illustrated schematically in Figure 15 are preferably used, in which the fuel flow is ignited by means of a stabilizing ring 12. By means of the stabilizing ring 12, the flame can be ignited efficiently even at a submerged level and the flame always ignites at the mouth of the burner and stays stable under all conditions. The fuel stream contains significantly less air than is needed to burn the fuel and the combustion air is added to the flame in stages via secondary and a tertiary air connections 13,14. This fuel injection solution is »· * functional in itself, but the problem may be the carbon flow into the biofuel line or • M ((i.e., ···, due to pressure fluctuations in the biofuel line backflow and the system.) ·; *. In the center of the stabilizer ring so that it mixes with the main fuel just before the flame ignition point. * '*: In a context where combustion can be well controlled, however, it is conceivable that the invention will also be used with another type of burner.

Issues such as those described above are described, for example, in U.S. Patent Nos. 5,799,594, 5,431,114, 4,907,962, and 5,263,426.

• * • «·« «8 119124

The following is a brief test of the co-combustion of wood and coal, in which coarse wood fuel was mixed before grinding. The mixture was prepared on a carbon field and fed through a coal mill. The fuel used was moist, the moisture content was 38-55% and the volatile matter was high. Low ash, nitrogen, chlorine, sulfur and low calorific values were also typical. The sawdust used had a large particle size and a large particle size, such as sticks. For screening, 100% penetration was obtained with an 8 mm screen. The experiments looked at the behavior of the fuel in the coal mill and in two sets of tests on different combustion results with different amounts of wood fuel. When examining the behavior of coal mills, it was found that wood causes some problems in the coal mills. In particular, the large particles in the fuel, such as sticks and chips, caused wood to accumulate in the mills, to burn, to degrade the carbon milling result, and to create a potential mill fire risk. It is expected that softer and more fibrous materials such as straw cannot be fed at all through the coal mill because the carbon mill is not capable of handling such material. These experimental results support the notion that biofuel should be blended with pulverized coal, especially when large amounts of biofuel or coarse biofuel are used. Thus, according to the invention, the biofuel is blended with the fossil fuel after milling. This also achieves the advantage that the expensive coal mill does not need to be used for biofuel processing, but that it can be milled 20 using more advanced milling equipment, for example a hammer mill.

In the first set of combustion tests, the volume of wood in the fuel was 171%, which corresponds to about 4-5p% and 1 -2% of the total fuel efficiency. During the test series, the amount of wood was increased by 201%, which corresponds to 6-7% by weight. Due to uneven mixing and changes in the calorific values of the fuels, the actual mixture ratio varied. corresponds to 9-10% by weight and 3-4% by: *; *: total fuel efficiency In both sets of flames, the flame ignited and was • normal, as well as slagging and fouling were also normal, as were the furnace I temperatures.

···· 30 «· • · · ..] During the combustion tests, the concentrations of O2, CO, CO2, NO, ΝΟχ • · ·" ... and SO2 in the flue gases were continuously measured. The sulfur content of the wood fuel used in the combustion tests was very low. The effect of biofuel addition on nitrogen oxide emissions is significant, because wood is well suited for phased combustion due to high volatile matter content, thus increasing the amount of wood in the boiler, It seems that combining the use of biofuel with changes in combustion technology can lead to lower nitrogen oxide emissions.

The amount of non-flammable constituents of fly ash was examined with daily samples. It was expected that a decrease in the grinding result of coal mills due to the addition of wood would increase the number of unburned fly ash. However, at the start of the co-incineration experiments, a clear reduction in the amount of fly ash unburned was observed. This reduction is due to the fact that wood helps to burn off carbon because wood is a highly reactive fuel with a high content of volatiles.

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Based on the test results, adding wood to the fuel of the coal boiler does not hinder the operation of the boiler, but rather aids the combustion process and allows the combustion conditions to be changed so as to achieve a lower emission level. It is essential that wood and coal are burned in the same burner, with the effect of adding wood,. 20 positively to burn-out. The amount of wood can be quite large and the grain size • · · Λ rough. It should also be noted that the experimental arrangement was not entirely an invention

• • M

. * * *. because the wood fuel was blended with the coal before the coal mills.

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Claims (8)

119124 1. A process for combusting biofuel in a fossil fuel-fired boiler, comprising: - comminuting the main fossil fuel for combustion; ignition of fuels, whereby the biofuel burns together with the main fuel in the same flame. characterized in that the biofuel is blended into the main fuel stream in the burner just before it is ignited in the flame. Method according to claim 1, characterized in that wood, straw, sugar cane waste or other fibrous biological: (waste or incineration fraction separated from municipal waste) is used as biofuel. A method according to claim 2, characterized in that: • e:; 25 of wood-based fuels. • · · • · e • · «• · · V 1 Method according to claim 2 or 3, characterized in that biofuel with a particle size of less than 8 mm is used. • · · ···· • · · • 1 * ··· 1 30 Method according to Claim 1, characterized in that the amount of biofuel * ϊ 1 "is at least 5% of the total fuel efficiency. 119124 n A process according to claim 1, characterized in that the biofuel has a moisture content of 10 to 40%. Process according to one of Claims 1 to 6, characterized in that the main fuel is carbon. Method according to one of Claims 1 to 7, characterized in that the main fuel and the biofuel are passed through a stabilizing ring for igniting and stabilizing the flame. e • · M M M M M 1 1 1 1 1 1 1 «1 et et et et et et et · · · · ··· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11