The invention also concerns an apparatus for implementing the method.

Coal is one of the most common fuels used in large scale energy production. Coal as a fuel can be divided in two main categories, brown coal and bituminous coal. Brown coal has much higher ash and moisture content than bituminous coal, whereby burning brown coal is more demanding than burning bituminous coal. All brown coal firing units in Europe are equipped with tangential firing combustion system where flame is stabilized in the central vortex instead of individual burners. Bituminous coal is burned in modern facilities in wall fired furnaces where the burners are arranged on a vertical wall of the furnace. In wall fired furnaces the burners are situated on the walls of the furnace and directed perpendically away from the wall. In such a furnace no central vortex is formed and the combustion circumstances are radically different than those of a tangentially fired boiler. In a tangentially fired boiler the fuel burns mostly in the central vortex and in a wall fired boilers mostly in the flame of the burner.

In the tangentially fired furnaces, burners are located perpendicularly in each corner, and the flames and the combustion air are directed to the opposite corner to form a vortex in the center of the furnace. In case of the tangentially fired boilers, fuel and combustion air are injected axially into the boiler, and the final mixing

occurs in the central vortex (fireball). Central vortex compensates the lack of swirl of the combustion air and takes care of the flame stabilization. A tangential jet burner of the prior art typically comprises a fuel pipe, secondary air channel and sometimes intermediate air channel for cooling materials between fuel pipe and secondary air channel. Normally, using jet burners, the distance between the ignition point and the throat of the burner is 2-3 meters, and the burning of the fuel occurs mainly in the central vortex. The fuel travels over the distance between the corner of the furnace and the central vortex as stream that has not yet ignited. If the fuel is bituminous coal, the parallel streams of fuel and combustion air have been mixed together the ignition point causing combustion in oxidizing atmosphere and forming NOx emissions. In case of two-stage combustion an air-deficient reduction zone does not form until the central vortex, and no staging occurs in the fuel stream between the throat of the burner and the central vortex. The staging concerns only the central vortex flame, and as deep staging as in the modern wall mounted low-NOx burners can not be achieved by using jet burners. If the fuel is brown coal, the fuel contains more water and the oxygen content of the fuel/feeding gas stream is lower and the fuel does not ignite and burn an a considerably amount before the central vortex. Thereby the burning of the fuel occurs mostly in a single stage and control of NOx emissions is not effectively possible by using staged combustion.

Normally brown coal has high moisture content in a range 20 -50 m-% and high ash content. The moisture of the brown coal is dependent of the quality and source of the fuel and the combustion process has to be adapted to a particular type of fuel. Because of high moisture content, brown coal boilers have to be equipped with a drying system. This drying system is different than drying systems used in bituminous coal firing. The moisture content of bituminous

coal is normally 7-15 % and drying system consist of coal grinding mill which is normally ball or roller mill.

Heated air is fed into the mill during grinding and usually this is sufficient to lower the moisture content of the fuel to a desired level. The drying process uses hot (400° C) and cold air (120° C) obtained from the air preheating system of the boiler. The volume and temperature of the air is rather low, but since the moisture content of the fuel is also low, there is not need to use large amounts of heating energy for handling the fuel. The heating air acts at least partially as carrier air of the fuel feeding system but also further feeding air can be used.

Practically this means that bituminous coal is injected to the boiler only with air (°2 = 21 %) as the carrier gas so that the temperature of this primary air/coal mixture is normally 80°-90° C. The moisture content of coal after drying process is normally about 2-4 %. Because of low inherent moisture and high 02 content of primary air, it is easy to achieve good ignition even with moderate, simple burners.

In the case of brown coal firing, it is necessary to use large amount of extra drying energy and normally hot flue gases together with air is used for drying. Brown coal firing units use normally hammer mills for grinding and hot and cold air is taken from the air preheating system of the furnace for drying. However since the moisture content of the fuel may be quite high, this energy insufficient to bring the moisture and temperature of the fuel to a level that is suitable for combustion and ignition in present furnaces. In order to be able to direct sufficient drying energy into the fuel very hot (800° C) flue gas is taken from exit of the furnace as well as cold flue gas (150° C) after air preheater of the furnace. The amount of hot and cold flue gases needed is dependent on the moisture content of the brown coal. The volume of flue gases needed is big

since they must contain enough energy for drying the fuel, whereby a large amount of gas having a very low oxygen content is fed to the furnace with the fuel. Since large amounts of cold flue gases are introduced into the burner, the rate of efficiency of the burning process decreases, which decreases the amount of energy obtained from a unit of fuel. The flue gas contains practically no oxygen and because of the low oxygen content of the flue gas, it is mixed with primary air in the hammer mill, after which the final oxygen content of the fuel and carrier gas varies between 5-15 vol-%, being typically 7-12 vol-%, which is considerably lower than the oxygen contents of bituminous coal firing systems. Since the oxygen content for ignition of the fuel is 15%, brown coal does not ignite until it has reached the central vortex of the furnace where more oxygen is mixed into the fuel/air mixture and the heat of the burning fuel in the vortex ignited the entering fuel. The final moisture content of brown coal after drying is about 7-12 % which is also higher than that of bituminous coal firing systems. Especially low oxygen content and higher moisture content of the mixture of brown coal and primary gas has negative effect on coal ignition causing low combustion efficiency and high NOx emissions.

Existing brown coal firing units apply very simple burners which are actually simple coal injectors only. The coal feeding system in these burners is a simple straight pipe of desired cross section and it feeds the fuel/carrier gas mixture on a straight line parallel to the center axis of the pipe in a similar way that water exits from a garden hose. No or very little mixing with the surrounding gasses occurs. This means that brown coal ignites very far from the tip of the coal pipe, normally after 4-5 m from the tip in the central vortex where mixing of fuel and gases occur. Further, the lower the oxygen content in the brown

coal/primary gas mixture, the more difficult is the ignition and flame stabilization. This causes narrow operation range for boilers, typically 70-100 % and when operated on low loads, it is necessary to use oil for the flame stabilization and for safe boiler operation. Since the brown coal plants can not operate on low loads because of above mentioned reasons, they can not be used effectively for compensating the variations in the demand of energy and more flexible methods of energy production are needed to maintain desired flexibility and controllability of the energy production system.

As stated above, the need to use flue gases in combustion of brown coal causes several problems. Even though a large amount of gas having big energy capacity is readily available, the low oxygen content of the gas reduces considerably the ignitability of the fuel/gas mixture which again leads to poor adjustability of the power of the plant, reduced reliability on low loads and high emissions of NOx compositions as well as poor combustion efficiency.

The object of this invention is to provide a method for burning brown coal wherein the amount of flue gas used for drying the fuel can effectively be reduced whereby the rate of efficiency of the burning process is increased.

Another object of the invention is to provide a method wherein the emissions of NOx-compounds in burning of brown coal can be reduced.

Another object of this invention is to introduce a new method, wherein the moisture content of brown coal that is injected into a furnace may be kept higher than in present combustion methods without endangering the ignitability of the fuel/gas mixture.

The invention is based on feeding the mixture of brown coal fuel and carrier gas into a tangentially fired furnace through a burner comprising a stabilizer/ignition ring at the mouth of the fuel feeding pipe and mixing flue gas to the fuel stream in such proportion that the oxygen content of the fuel/gas mixture is at least 15%.

According to one embodiment of the invention, a stream of secondary combustion air is passed around the flame formed of the fuel to provide a separating blanket of air around the flame, and a stream of tertiary combustion air is directed towards the water walls and horizontally away from the flame in such a way that a return vortex is formed between the tip of the burner flame and the central vortex..

A burner according to this invention is called a RI-LIG burner in the following.

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

The invention provides essential benefits.

The main object and advantage of this invention is considerably better ignition of the fuel and more reliable combustion process that enables even control of the power of the plant over a range of 30-100% of the total capacity of the plant without endangering the stability of the process.

By using a modern type of burner comprising a stabilizing ring, water content of the fuel may be higher and the ignition can still be ensured, whereby the amount of flue gases mixed to the fuel stream can be lowered. By using less flue gas the oxygen content of the fuel/gas mixture increases and ignitability as well as stability are further enhanced. Thus, the use of a stabilized burner leads to a

effect chain that changes all characteristics of the brown coal combustion process and enables ignition and burning of brown coal in circumstances that have not been obtained by conventional processes. Conventionally it has been considered that it is not possible to burn brown coal without using large amounts of flue gases for drying the fuel. If the use of the flue gas can be minimized, the combustion efficiency increases.

The invention makes it possible to move the burning process to a completely new area wherein burning of the brown coal has traditionally been considered impossible. The invention makes it possible to ignite the fuel in much higher moisture content, whereby less flue gas is needed for drying the fuel. Together these features make it possible to provide a mixture of brown coal, air and flue gases having an oxygen content higher than 15% and ignite such mixture even if the initial moisture content of the fuel is high.

In this new application a new type RI-LIG burner is installed to the brown coal firing boilers and the burner makes use of a special flame stabilizing ring and guide sleeve for the tertiary air. This geometrical structure create a big recirculation zone causing better mixing of tertiary air and coal/carrier (primary) air mixture having low oxygen content. Because of better mixing of tertiary air to carrier gas, a stable flame is formed despite of the fact that oxygen content is too low in the carrier gas for stable combustion. This improved ignition mechanism was compared to a conventional design was proved by combustion simulation that is described below.

Because of high ignition intensity of the burner it is now possible by means of this new invention to ignite brown coal having higher moisture content. This makes possible the reduction of the amount of drying flue gases

(especially cold flue gases) giving higher oxygen content of the brown coal/carrier gas mixture. This further improves ignition and increases flame temperature, which means lower NOx emissions and better combustion efficiency.

With this invention it is possible to expand boiler operation load range from existing 70-100% of conventional boilers to 30-100% without using support oil to ensure the ignition and stable combustion. Also the whole drying system will become more simple and the reduction of the amount of drying gas also increases boiler efficiency due to lower flue gas temperature after teh electrostatic precipitator (which is the temperature measuring point for calculating efficiency).

With the new type RI-LIG burners it is possible to keep the oxygen content of brown coal/primary air mixture above the critical 15 % all the time. In the case of low moisture content brown coal (15-25 %) flue gas drying system is not needed at all and it is possible to perform drying using only hot and cold primary air from the air preheating system of the boiler, whereby the oxygen content of the drying gas is the same as that of the atmospheric air.

The slagging problem of tangentially fired boilers is avoided by directing air to the waterwalls and thereby providing an oxidizing atmosphere near the walls. The construction of the RI-LIG burner is relatively simple. One main application of the RI-LIG burner is retrofitting old tangentially fired brown coal boilers. When an old boiler is retrofitted with these burners, NOx emissions are reduced remarkably, and combustion efficiency is improved, too.

Below, the invention is explained in detail with references to the enclosed drawings.

Fig. 1 shows a burner that can be used for implementation of the invention.

Fig. 2 is a schematic drawing of a boiler arrangement according to the invention.

Fig. 3 is a schematic illustration of a comparative combustion simulation Fig. s 4a and 4b are a schematic illustrations on temperature distribution of a conventional burner and a RI-LIG burner.

In the figures, Io is the volatilization zone, I the primary recirculation zone, II the reducing zone, III the vigorous turbulent combustion zone, IV the tertiary recirculation zone, V the stagnation zone, VI the secondary recirculation zone and VII the main vortex.

Figure 1 shows one RI-LIG burner that comprises rectangular fuel pipe 1 having injection port 2. Around the fuel pipe there is concentrically arranged rectangular secondary air channel 7 forming a secondary air passageway around the outer periphery of fuel pipe 1, and injection port 8 of channel 7. In the uppermost and lowermost parts of the burner there is upper tertiary air channel 11 and lower tertiary air channel 13, and corresponding injection ports 12 and 14. Between tertiary air channel 11 and secondary air channel 7 there is upper spacer 16 and between lower tertiary air channel 13 and secondary air channel 7 there is lower spacer 15. The primary function of the spacers is to separate the secondary and the tertiary air streams in order to protect the formation of reduction zone II in front of the burner. The height (d3) of spacers 15,16 varies normally between 30 and 350 mm.

The RI-LIG burner is also equipped with flame holder 9, which comprises ring 9a inside coal pipe 1 and guide sleeve 9b in secondary air channel 7. Ring 9a has the same rectangular form as the cross section of injection port 2 of fuel pipe 1, and it extends perpendicularly towards the central axis of fuel pipe 1. The cross section of ring 9a may be a continuous ring, but in this construction ring 9a is provided with teeth, that extend into fuel pipe 1.

Flame holder 9 is a ring that surrounds the inner wall of fuel pipe 1, and it is made of, or coated by, a wear and heat-resistant material such as ceramics or heat-resistant steel. In this construction, flame holder 9 is a rectangular or cylindrical bluff body having a hole through which the pulverized coal stream is passed in the central part thereof, and it is arranged in the opening end of fuel pipe 1. The inner side of the flame holder, ring 9a, extends nearly perpendicularly to the axial direction of fuel pipe 1, and the secondary air guide sleeve 9b thereof being formed either in parallel to the axial direction of the pulverized coal pipe toward the combustion furnace or at such an angle that the diameter of the guide sleeve is enlarged to the radial direction of secondary air channel 7.

Furthermore, in order to enhance ignitability at the exit of the injection port of fuel pipe 1, and to generate the high temperature reducing flame at the exit end with certainty, ring 9a forms a toothed apron protruding at the inner peripheral surface of the fuel pipe 1 at the exit of injection port 2 thereof towards the center of fuel pipe 1 to ensure efficiency of the present invention. The apron may be a continuous ring, but in this embodiment it is serrated, i. e. provided with cut-away parts in it. The inner diameter or dimension dl of ring 9a of flame holder 9 and inner diameter d2 of fuel pipe 1 are preferably determined to satisfy a relation of 0.7 c (dl/d2) < 0. 98, and most prefe- rably determined so as to give a dl/d2 of about 0.9. The

ratio of dl/d2 is not limited to the above range, but if the ratio of dl/d2 is too low, the flame holder protrudes too much into fuel pipe 1, increasing the flow rate of the pulverized coal stream passing through the injection port, and hence increasing the pressure drop inside the fuel feeding pipe.

Both upper and lower tertiary air channels 11 and 13 are also equipped with guide sleeves 17 and 18 having vertical angle 63. Normally 03 is between 5 and 40°. In order to achieve sufficiently good funnel like effect for tertiary air in the tertiary air injection ports 12,14, the length of the sleeves should be designed so that the relation between the length of sleeve 1 and the height of the tertiary air passage hl is 1/h, 2 (figure 1.). It is possible to shorten the length of the sleeves by using intermediate guide sleeves 17a and 18a without losing the funnel like effect, but then the sleeves should be designed so that the relation between length 1 of the sleeves and height h2 of the channel formed between the intermediate guide sleeve and the wall of the air channel is 1/h2 2.

The above mentioned burner and other embodiments thereof are described in US 5,799,594 which is disclosed herein by reference.

Figure 2 is a process diagram of one embodiment of the invention. Brown coal is ground to a suitable particle size in a mill 19 wherefrom it is delivered through a feed line 20 to a dryer 28. Before the fuel flow enters the dryer, hot drying air is mixed to the feed line from hot air duct 22.

Cold feeding air is also mixed to the fuel flow through cold air duct 24. The amounts of fuel, hot and cold air are controlled by valves 21,23,25. In the dryer 28 moisture is removed from the fuel by the hot air and hot exhaust gases

from a furnace 32. The exhaust gases are led to the dryer through line 26 and the amount of exhaust gases is controlled by a valve 27. In the dryer the relative amounts of the air and exhaust gases are controlled in such a way that the oxygen content of the mixture of fuel, air and exhaust gas has an oxygen content of at least 15% whereby the ignition of the mixture can be guaranteed in a burner comprising a flame holder. From the dryer 28 the above mentioned mix is fed through a shut-off valve 31 to burner feeding lines 29, which include valves 30 for controlling the flow of the mix. The burners used herein are of above mentioned type.

Figure 3 shows a comparative simulation calculation of conventional brown coal combustion process and a process according to the invention. In conventional process, nominated"Conventional drying"the oxygen content is 7- 10% and in the process according to the invention the oxygen content in the calculation is 13-17%. The calculation shows two notable variables. First, the curve connected to the left-hand column of the illustration shows a dramatic decrease of NOx emmissions when the oxygen content of the fuel/primary air mix increases. On the typical operation area of the conventional process, the NOx emissions are about double compared to those achieved on oxygen content of 15%, that lies on the preferred operating area of the invention.

Generally speaking it has been found out that the critical oxygen content for stable ignition of the mixture of brown coal and primary gas mixture is 15 %. The upper limit of the oxygen content is 21% which is achieved when no flue gas is mixed into the fuel/primary gas mix. This can be achieved when the moisture content of the brown coal is low, near the 20% that may be the moisture content of some grades of brown coal.

In the conventional drying process using large amounts of flue gas it is impossible to increase the oxygen content much above 10%, whereby the NOx emissions of the conventional process can not be lowered by changing the process parameters. On the other hand, the invention provides excellent means for increasing the oxygen content and thereby decrease the NOx emissions effectively. Further, this leads simultaneously to increase in combustion efficiency, as can be seen from the curve connected to the right-hand column of the illustration.

The figures 4a and 4b show temperature fields of conventional brown coal burning burner and RI-LIG burner.

The level curves present temperatures in °K. As can be seen in the figures, the temperature in the central line of the conventional burner is very low and the area of low temperature is long and narrow. This shows that practically no ignition occurs before the central vortex shown in the right of the figure 4a. Similar calculations show that the oxygen content on the central area in front of the burner has first an area where the oxygen content is high right in front of the burner, whereafter it decreases rapidly and increases slowly towards the central vortex. This shows that very little combustion occurs and all combustion happens in relatively high oxygen content, whereby NOx compounds are formed. On the other hand, in a RI-LIG burner the temperature is high directly in front of the burner, which shows rapid ignition and burning of fuel. The fig. 4b shows also an enveloping area of low temperature that is caused by tertiary air flow. This air flow envelops the ignited flame and prevents deposition of impurities on the walls of the furnace. Also oxygen distribution shows rapid ignition and burning of fuel. The oxygen content decreases rapidly in front of the burner, which shows that the flame uses the available oxygen rapidly, whereafter an oxygen deficient zone extends to the central vortex preventing formation of

NOx compounds.

The flue gas used for drying the fuel has to be as hot as possible in order to keep its volume low. The best place to exhaust flue gas for drying is before the first radiant superheaters since the temperature there is the highest.

Conventional brown coal plants also comprise an exhaust on this area already used for exhausting flue gas whereby modification of such a plant to operate according to the invention is relatively easy. However, if the oxygen content of the fuel/gas mix can be maintained high enough, also cooler gas taken further on the exhaust line can be used.