Method for the automatic combustion of solid granular fuels, particularly pellets, and burner for the boiler

The object of the invention is a method for the automatic combustion of solid granular fuels, particularly pellets, in a boiler having a water-heating heat exchanger, a burner, an air inlet and a flue gas outlet duct. The method comprises the steps of igniting and, utilizing ignition air flow, burning the fuel fed ito the combustion chamber. The formation of flame in the combustion chamber is monitored and, after flame formation has completed, the in-feed of fuel and, in specific cases, additives and the flow of primary air for sustaining the burning of fuel and also the flow of secondary air (having the purpose of retaining solid particles) is started, controlling the in-feed of fuel and, in specific cases, of additives and controlling the flow of primary and secondary air depending on the temperature of hot water generated in the boiler such that the in-feed of fuel and, in specific cases, additives, and the flow of primary and secondary air are stopped if the temperature of hot water exceeds a preset limit, and are restarted if the hot water temperature falls below a preset limit.

The further object of the invention is a boiler for the automatic combustion of solid granular fuels, particularly pellets having a boiler body including a flue gas duct and a water- heating heat exchanger disposed in the flue gas duct. A burner adapted for the automatic combustion of fuel is connected to the boiler body. The burner is fitted with an air inlet for the in-feed of combustion air, an is connected to a fuel container through a fuel transfer unit. The boiler further comprises a control unit adapted for controlling the flow of in-fed air, the amount of in-fed fuel, the ignition and stopping of the burner, and the automatic purging thereof depending on the temperature of water heated in the water-heating heat exchanger.

A further object of the invention is a burner comprising a combustion chamber that is surrounded by an outer jacket, is covered by a cover, bounded by an inner wall and terminated by a rear wall. An intermediate space if formed between the outer jacket and the inner wall. The intermediate space is interconnected with the combustion chamber such that air can flow

between them, with an opening being disposed on the rear wall, and a connection piece adapted for the in-feed of fuel being attached to the opening.

For the combustion of solid fuels in a boiler several solutions are known such a solution is described in Hungarian patent HU 195 573 which discusses a boiler, particularly a coal burning boiler. The boiler is suited for burning solid fuels of different particle size, such as wood, wood chips, or agricultural briquettes. The boiler comprises a feed spout for introducing fuel into the combustion chamber, a flue gas duct communicating with the combustion chamber, a grate disposed in the combustion chamber under the flue gas duct, and a system adapted for feeding air into the combustion chamber. A water-heating heat exchanger is disposed in the flue gas duct of the boiler. The boiler is suitable for combusting solid fuels economically, but is not capable of burning granular fuels such as pellets or sawdust.

An apparatus suitable for burning lump and granular fuel is disclosed by utility model description AT 003 798U. The boiler comprises a combustion chamber bounded by thermally insulated walls, with the combustion chamber being separated from the ash space disposed below by a grate. A burner is attached to the combustion chamber above the grate. The burner has a casing comprising slanting side walls, with a fuel feeding conveyor screw being introduced into the casing, and with primary air inlet openings being disposed at the bottom of the casing and secondary air inlet openings being disposed on the upper portion thereof. The introduction of secondary air from above makes it possible to achieve an explicitly horizontal flue gas outflow from the burner, the flow being directed upwards and getting mixed with flue gases from fuel burned on the grate only above the grate, which makes feasible independent firing through the grate and the burner.

Rising energy prices and the growing need for recycling rapidly accumulating waste has lead to the development of boilers suitable for the burning of combustible by-product materials such as coal fines or sawdust. Such a combustion apparatus, suitable for granular solid fuel such as wood pellets or granulated sawdust is disclosed by EP 1277012. The apparatus comprises a combustion chamber with a fan feeding air into it. A feed spout opens into the combustion chamber, with a feed screw transferring fuel from the feed spout to the combustion chamber. Air inlet openings adapted for the introduction of air into the combustion chamber are disposed on the bottom and side wall of the combustion chamber.

An apparatus for burning solid, granular fuel, for instance wood flour pellets, chippings and the like, is disclosed in the document published under publication number WO 9749951. The apparatus comprises a rotary burner with an air intake to the burner for the

introduction of combustion air, at least one fuel feed conduit and an outlet opening for the transfer of combustion gases to the water heater. The rotary burner comprises a vessel having a flue gas outlet opening and a jacket part disposed between the rear wall and the outlet opening. An air admission pipe surrounds the central fuel feed pipe at a distance from this so as to form an ring-shaped space between them. The apparatus comprises and air admission duct communicating with the ring-shaped space. The apparatus also comprises devices adapted for the in-feed of fuel and combustion air.

A method for the automated combustion of solid fuel is disclosed in the document WO 9928678. The object of the invention described in the document is the automated combustion of solid, granular fuels, primarily wood pellets and chippings. The inventive objective is realised by providing a combustion apparatus having a burner comprising a horizontally or diagoally oriented reactor drum mounted inside the boiler, and a stirring motor that rotates the reactor drum about the centre axis of the burner. A feeding-in opening for fuel is disposed in the rear end of the burner, and an outlet opening for introducing the flue gases into the combustion chamber is disposed in the burner's front end. A fan supplying combustion air is connected to the boiler. The boiler further comprises a water-heating heat exchanger having a hot water conduit where the temperature of water is measured and the measured value is transmitted to a control unit. The speed of the stirring motor, the fuel charge feeding motor and fan speed are adjusted on the command of the control unit depending on the water temperature measured in the hot water conduit, and on the heating effect that the burner generates according to various control programs.

Besides conventional fuels and combustion methods, devices utilizing renewable energy have been assuming an increasing role. Plants developed for energetic purposes have several advantages over conventional fuels. These advantages are becoming manifest on the one hand in replacing known (fossil) fuels that will inevitably be depleted, and on the other in observing stricter and stricter environmental regulations. Utilized as fuel, grass species that produce high amounts of dry matter and high quality raw cellulose can replace conventional fuels used for smaller-scale applications and in larger generating plants. The heating value of certain energy grass cultivars may reach or exceed the heating value of certain woods, for instance poplar, willow, and locust, and thus they are exceedingly suitable for generating electricity. These grass varieties can be grown on various soil types, are weather-hardy, disease-resistant and undemanding. The above advantages call for the development of combustion apparatus that can burn energy plants in an economical and environmentally friendly way.

Burning energy grass may be carried out by making uniform-sized pellets from the grass and utilizing it as fuel. Because of the high amount of harmful materials contained in their flue gases (way above environmental emission limits), energy grass pellets cannot be burned in an economical and environmentally friendly way in conventional combustion devices or boilers conventionally applied for burning wood pellets. A further difficulty is posed by combustion products getting deposited on the boiler wall during the burning of energy grass. Accumulated deposits form a non-removable layer which results in permanent damage to the boiler.

Several solutions are known for diminishing the amount of harmful substances contained in the flue gases. Patent description JP 2004286388 describes a cleaning system for fluidized bed combustion apparatuses. According to the invention, ceramic filters are utilized for diminishing the amount of harmful substances in the flue gases.

In US patent No. 4,828,573 a method of manufacturing pelletized fuel is disclosed. The invention utilizes coal fines or sawdust as fuel, mixing pulverized limestone and a binder material to it. By adding limestone as an additive, SC^-emission can be lowered significantly.

The above described, known solutions provide some kind of guidance with respect to the combustion of wood and grass pellets, but do not contain methods or apparatuses that could be utilized for burning such fuels in an economical and environmentally friendly way.

The object of the present invention is to provide a method, a boiler, and a burner to be used with the boiler, for burning such pellets in an easy and straightforward way. With the application of the inventive method the amount of harmful substances contained in flue gases emitted from the boiler does not approach emission limits, and no non-removable deposits or deposits removable with difficulty are left in the boiler.

The inventive method is essentially characterised by feeding primary air into the burner in such a way that particles of fuel and, in specific cases, additives are floated in the combustion chamber, feeding secondary air into the burner in a manner that it forms an air curtain closing off the open end of the combustion chamber, and also feeding purging air into the burner after the in-feed of fuel and, in specific cases, additives has been stopped, with the purging air passing along the inner wall of the combustion chamber following a spiral-shaped path and sweeping over the wall of the combustion chamber.

The method advantageously comprises the step of utilizing aluminium-oxide or kaolin as additive.

The method can be applied advantageously for burning energy grass pellets, in which case limestone is applied as an additive.

According to the invention an essential feature of the boiler detailed at the beginning of the present description is that a ceramic grate is disposed in the boiler body in the path of flue gases leaving the burner, with the ceramic grate being adapted for trapping harmful solid particles contained in the flue gases and for burning these harmful particles while it is acting as a catalyst, and/or an additive container is connected to the boiler, where said additive container contains an additive applied for ensuring perfect combustion and for preventing the deposition of harmful particles.

According to a preferred embodiment of the boiler the ceramic grate is implemented as a ceramic plate with holes. More than one ceramic grates can be disposed in the boiler.

The invention includes the configuration of the burner, which is essentially characterised by that ignition air is fed into the burner, with primary air being subsequently fed therein for sustaining the burning of fuel, and with secondary air and purging air being fed into the burner, with a through hole adapted for inserting an ignition assembly therein in a manner that the ignition assembly reaches into the combustion chamber and openings adapted for the introduction of ignition air and the passing through of primary air and secondary air being disposed on the rear wall of the burner. Holes connecting the intermediate space with the combustion chamber and adapted for the introduction of primary air are disposed on the bottom part of the inner wall, with holes adapted for directing secondary air towards the combustion chamber to form an air curtain therein, being disposed along the circumference of the inner wall at the end opposite the rear wall, and an inlet duct is passed into the combustion chamber in the vicinity of the rear wall, with the inlet duct being configured such that it imparts angular momentum to the purging air.

In a preferred embodiment of the burner holes adapted for the introduction of primary air are disposed at the bottom of the combustion chamber, arranged in rows parallel with the principal axis of the burner

According to an advantageous configuration the holes are disposed at the bottom of the combustion chamber in three rows.

In a further preferred embodiment of the burner the holes adapted for directing secondary air into the combustion chamber are distributed evenly along the circumference of the inner wall.

In a still further preferred embodiment the inlet duct is implemented as a pipe bent into a spiral shape, said pipe starting outside and reaching into the combustion chamber. The

inner end of the inlet duct is fitted against the inner wall of the combustion chamber. A fan is connected to the outside end of the duct.

In the following the invention is described in detail by referring to the accompanying drawings, where

Fig. 1 shows the schematic view of the inventive boiler configuration, Fig. 2 shows the boiler body with the burner installed, Fig. 3 is the longitudinal section of the burner,

Fig. 4 is a view of the burner of Fig. 3 taken from the direction of arrow A, Fig. 5 is a view of Fig. 3 taken from the direction of arrow B, Fig. 6 is a section of Fig. 3 taken in the plane VI-VI.

In Fig.l the schematic view of the inventive boiler is shown. Fig. 2 shows the boiler body 11 in more detail, together with a burner 2 improved according to the invention, and shows a ceramic grate 9. The starting point for the invention is a conventional boiler configuration, where the boiler may be hand-operated or automatic, using either coal or firewood as fuel. The boiler 1 comprises a water-heating heat exchanger through which flue gases are driven to heat water to the desired temperature. The boiler 1 is shown schematically in Fig. 1 having a water inlet 5 and a water outlet 6. According to the invention a burner 2 is connected to the combustion chamber of the conventional boiler. The flue gas outlet 7 is also indicated in the drawing. The desired level of air and flue gas circulation in boiler 1 is achieved by means of a fan not shown in the drawing. The inlet of combustion air is designated by reference numeral 8. The inventive improvements of the conventional burner made it possible to burn solid granular fuels, primarily wood pellet, but the inventive apparatus is also capable of burning pellets made from herbaceous plants. For instance, the apparatus can burn energy grass pellet in a way that is highly efficient and environmentally friendly.

The boiler becomes suited for burning pellets by means of a burner 2 configured according to the invention. The burner 2 can be mounted on the boiler wall 11, for example as a replacement for the ash door. The fuel container 3 is connected to the burner 2 through a fuel feed means not shown in the drawing. For trapping harmful substances generated by pellet burning an additive retrieved from additive container 4 is mixed with the fuel. The type and amount of additive is chosen with regard to the fuel to be burned. Pellets, especially pellets made from herbaceous plants, contain non-organic, non-combustible components, among others potassium, sodium and silicium in high percentage. If compounds of these

elements are not removed from flue gases, in addition to exceeding emission limits for deleterious substances they may cause deposits on the inner wall of the burner, which can lead to reduced efficiency, or in adverse cases even to explosion. By applying additives of different kind, harmful emissions can be reduced from dangerous levels and deposit build-up can be prevented. Aluminium-oxide, kaolin or pulverized limestone can be mixed with the fuel as additive.

In order to improve the degree of combustion and to reduce harmful emissions, a ceramic grate 9 is disposed in the path of flue gases. The ceramic grate 9 functions as a filter and catalyst. The ceramic grate 9 is installed in the path of flue gases in a manner that flue gases will pass through it in any case. The ceramic grate 9 is implemented as a ceramic plate with holes, which is either inserted between guides located in the flue gas duct or is placed on a guiding surface disposed therein. Due to its high surface-to-mass ratio the ceramic grate 9 is able to trap solid particles contained in the flue gases, and as it becomes heated it also facilitates the perfect combustion of combustible materials still present in the flue gases. If it is necessary, more than one ceramic grates 9 can be installed in the boiler 1.

The burner 2 is built in the boiler body 11 of the boiler 1. The burner 2 has an outer jacket 21 that reaches into the boiler body 11 and surrounds a combustion chamber 23, being attached in a sealed manner to the rear wall 24 of the burner 2. A cover 39 is attached to the other side of the rear wall 24 in such a way that it provides a sealed lock for the burner 2. The ignitor assembly 10 is disposed in the space bounded by the rear wall 24 and the cover 39. The ignitor assembly 10 is implemented as an electric heating wire connected to a power source, which ignites fuel by means of hot air generated by heating the air driven through it.

Fig. 3 shows the cross-section of the burner 2, for the sake of intelligibility with the cover 39 removed. The burner 2 has a rear wall 24 and an inner wall 22 attached to the rear wall 24 by welding. In the embodiment shown in the drawing the inner wall 22 is implemented as a cylindrical tube. The inner wall 22 surrounds a combustion chamber 23, with one end of the combustion chamber 23 being terminated by the rear wall 24 and the other end, directed towards the inner space of the boiler 1, remaining open. The inner wall 22 is surrounded by an outer jacket 21. The outer jacket 22 is also implemented as a cylindrical tube, being attached in a sealed manner at the end directed towards the inner space of the boiler 1 to the inner wall 22, and at the other end to the rear wall 24 such that an intermediate space 25 having an annular cross section is formed between the two attachment locations. Connection between the intermediate space 25 and the combustion chamber 23 is provided by holes 30 disposed on the inner wall 22, which also provide for the in-feed of primary air 33.

The direction of air flows is marked with double arrows in the drawings. In the embodiment shown in the drawing the holes 30 are arranged in three rows parallel with the axis of the cylindrical inner wall 22, one row being located along the bottommost linear generatrix of the inner wall 22 and two further rows disposed along generatrices located at an angular separation of 30-30 degrees with respect to the vertical axis of the burner 2.

In the vicinity of the open end of the inner wall 22 evenly distributed holes 31 are disposed along the circumference of the wall. Similarly to holes 30, these holes 31 also provide connection between the intermediate space 25 and the combustion chamber providing for the in-feed of secondary air 34. The intermediate space 25 communicates with the space terminated by the cover 39 through openings 38 disposed on the rear wall 24.

As it is shown in Fig. 5, multiple pass-through openings are disposed on the rear wall 24. A connection piece 27 adapted for the in-feed of fuel is attached to opening 26 by welding. Fuel, or, if necessary, fuel-additive mixture can be fed from the appropriate containers to the combustion chamber 23 through the connection piece. The casing of the ignitor assembly 10 is driven through a through hole 28 disposed on the rear wall 24. The through hole 28 is surrounded by openings 29, through which ignition air 32 can be fed into the combustion chamber 23.

A duct 36 bent into a spiral shape is connected to the rear wall 24. This inlet duct 36 leads from the space under the cover 39 to the combustion chamber 23. The orifice 37 of the inlet duct 36 is connected to the pressure pipe of a fan not shown in the drawing. The end piece 40 of the inlet duct 36 reaches into the combustion chamber fitting closely to the inner wall 22. The end piece 40 is configured in a manner that the purging air 35 delivered by it passes along the wall of the combustion chamber 23 along a spiral path.

The boiler 1 is equipped with sensors and a control unit that are not shown in the drawing. For control purposes commercially available automatic boiler control devices, for instance the likes made by Danfoss, can be applied. The sensors are monitoring the temperature of hot water generated in the boiler 1, reporting to a central processing unit (microprocessor) in case water temperature raises above a preset limit or falls below a preset value. The control unit starts and and stops the in-feed of fuel and air flows depending on the input signals received from the sensors. When hot water temperature is between the preset limits the amount of in-fed fuel and air can be freely adjusted on the command of the control unit.

A flame monitoring photocell, utilized for detecting the formation and disappearance of flame, is disposed in the combustion chamber 23. The control unit starts the in-feed of fuel and air triggered by the photocell signal.

The inventive pellet burning method is carried out according to the following detailed description. For ignition a small amount of pellet is fed from the fuel container 3 into the combustion chamber 23 of the burner 2 through connection piece 27. The air supply fan of the boiler 1 feeds air into the combustion chamber 23 through openings 29 of the rear wall 24 of the combustion chamber 23 and through the ignitor assembly 10. Once the ignitor assembly has been turned on, air flowing through is heated up and the hot air ignites the fuel batch. Ignition and flame formatioin is monitored by a photocell located in the combustion chamber 23. As soon as the flame has become continuous, triggered by the signal received from the photocell the central control unit starts the continuous in-feed of fuel and the primary and secondary air flows. The photocell sensor also performs flame safety functions, as it immediately stops fuel feeding in case the flame disappears for some reason.

The control unit gradually increases fuel feed rate and primary and secondary air flow rates until the desired water temperature is reached in the boiler 1. In case of lower water temperature demand the control unit decreases fuel feed and air flow rates. Simultaneously with fuel in-feed the control unit starts the feeding of a suitable additive (chosen with respect to the type of fuel being burned) from the additive container 4. Primary air 33 is fed into the bottom portion of the combustion chamber 23 through intermediate space 25 and holes 30. Air is fed into the combustion chamber at a speed sufficient for lifting pellet granules, such that granules stirred by the air flow in the combustion chamber 23 may "explode" and combust as perfectly as possible. During the burning process pellet granules not yet combusted should be kept from leaving the combustion chamber 23. This is achieved by means of a row of holes disposed on the inner wall 22. Secondary air 34 driven through the holes 31 of this row of holes enters the combustion chamber 23 in radial direction and prevents pellet granules from leaving the combustion chamber 23 through the free openings. In addition to performing its air curtain function, secondary air 32 improves the thoroughness of pellet combustion. In case the longitudinal axes of the holes 31 and the combustion chamber 23 are skew lines, the air flow driven through them receives angular momentum entering the combustion chamber 23, which contributes to the increase of turbulence inside the chamber, and improves combustion efficiency.

As soon as the temperature of hot water reaches the preset threshold, the control unit stops the feeding of fuel and air.

After fuel, additive and air feed has been stopped, the control unit issues a command for the fan generating purging air to start. The fan then starts feeding air into inlet duct 36. Pressurized air enters the combustion chamber 23 through the orifice of the inlet duct 36, removing potentially existing ash deposits from the inner wall 22 of the combustion chamber by sweeping over the wall along a spiral path.

Example 1

We burned wood pellets in a 40 kW boiler. The maximum temperature of produced hot water was 95 ° C. For ignition a 0.15 kg batch of pellets was fed into the combustion chamber. After the pellet has been ignited and flame has been formed, fuel feed rate was increased to 10 kg/hour, while air flow was adjusted between 0,1 m /hour and 3 m /hour. The purging air applied for cleaning the combustion chamber had a rate of 10 m /hour.

Example 2

Effluents from a 250 kW boiler running on wood pellet were measured. Comparisons were made using current official emission limits.

Example 3

Pellets made from "Szarvasi-1" energy grass were burned in the boiler of Example 1. The amount of fuel used and air flow parameters were the same as in the previous example.

Limestone was used as an additive for pellet burning in an amount equivalent to 2% of the mass of the pellet. Combustion was performed without deposits, with effluent emissions remaining under limits.

The main advantage of the inventive method and apparatuses is that they can be utilized for burning wood pellets as well as pellets made from herbaceous plants in an economical and environmentally friendly way.

List of reference numerals

boiler burner fuel container additive container water inlet water outlet flue gas outlet air inlet ceramic grate ignitor assembly boiler body outer jacket inner wall combustion chamber rear wall intermediate space opening connection piece through hole opening hole hole ignition air primary air secondary air purging air inlet duct orifice opening cover end piece