NO343011B1

NO343011B1 - Furnace for gasification and oxidation of solid fuel

Info

Publication number: NO343011B1
Application number: NO20170697A
Authority: NO
Inventors: Helge Rosvold
Original assignee: Aitos Gasification Tech As
Priority date: 2017-04-26
Filing date: 2017-04-26
Publication date: 2018-09-24
Also published as: GB2568985B; GB2568985A; GB202017530D0; NO20170697A1; GB201806785D0; GB2587276A; GB2587276B

Abstract

Combustion furnace and process for combustion of solid fuel comprises a primary chamber (100) with a drying zone (103), a gasification zone (104) and a fixed carbon zone (107) supplied with air through nozzles in a nozzle plate (109), and a grate. The furnace exhibits an inlet (101) for solid waste, a gas outlet (102) and a solids outlet (112). Solid fuel is conveyed by bar feeders (108a, 108b, 108c). Air is supplied to the drying zone (103) through nozzles provided with air deflectors (113) arranged to direct air into the solid fuel at an angle between horizontal and vertical. The solid fuel inlet is arranged at an elevation above the grate. The primary chamber (100) is in gasflowconnection with a reduction chamber (200) exhibiting two or more gas supply pairs, each par comprising numerous recycled flue gas supplies (203) and then numerous secondary air supplies (204). The reduction chamber is in gas-flow connection with an oxidation chamber (300). The reduction chamber (200) and oxidation chamber (300) exhibit numerous baffles (205; 305).Combustion furnace and process for combustion of solid fuel comprises a primary chamber (100) with a drying zone (103), a gasification zone (104) and a fixed carbon zone (107) supplied with air through nozzles in a nozzle plate (109) , and a grate. The furnace exhibits an inlet (101) for solid waste, a gas outlet (102) and a solids outlet (112). Solid fuel is conveyed by bar feeders (108a, 108b, 108c). Air is supplied to the drying zone (103) through nozzles provided with air deflectors (113) arranged to direct air into the solid fuel at an angle between horizontal and vertical. The solid fuel inlet is arranged at an elevation above the grid. The primary chamber (100) is in a gas flow connection with a reduction chamber (200) exhibiting two or more gas supply pairs, each comprising numerous recycled liquid gas supplies (203) and then numerous secondary air supplies (204). The reduction chamber is in a gas-flow connection with an oxidation chamber (300). The reduction chamber (200) and oxidation chamber (300) exhibited numerous baffles (205; 305).

Description

The present invention is related to a furnace for combustion of solid fuel, according to the preamble of claim 1.

Background

The present invention is related to a combustion furnace for combustion of solid material, such as different types of Refused Derived Fuel (RDF), for production of energy and to reduce pollution to a minimum. Other examples are bark, chips, sludge, and biomass. The raw solid material to be combusted is fed to a primary chamber provided with a conveyor, where the solid material being advanced on the conveyor is dried and partially gasified at under-stoichiometric conditions to produce ash, which is collected at the outlet of the primary chamber, and gas, which is fed to an oxidation chamber operated at super-stoichiometric conditions for further combustion. The solid material being conveyed in the primary chamber is supplied with air at elevated temperature injected into the gasification chamber from underneath through a nozzle arrangement.

NO 176455 B describes a grate furnace where the primary chamber and the oxidation chamber are separated by a partition plate and where individual zones in the primary chamber are individually controllable. The effect is described to be increased energy utilization and reduced discharge of pollutions.

NO 304450 B1 describes a device at a combustion furnace similar to the one described above, to obtain an improved distribution of solid material and air in the primary chamber. In the solution according to the claims, the nozzle openings for air supply arranged underneath the conveyor are delimited by spacers 13 carrying rails 14, which therebetween form slits 15 to eject the air substantially horizontally into the primary chamber. The publication also discloses a bar feeder comprising two separate bar feeders that operate in a reciprocal manner independently from each other.

A challenge with the air supply is to obtain a best possible penetration and distribution of air into the solid mass being conveyed in the primary chamber. Another challenge is to keep the concentration of NOxin the flue gas at acceptable levels.

Object

An object of the present invention is to improve the overall efficiency of the process. Another object of the present invention is to improve initial treatment of solid material being fed into the primary chamber and hence contribute to the overall process efficiency. Yet another object of the invention is to improve air supply into the primary chamber and hence contribute to the overall process efficiency. Yet another object of the invention is to provide improved control of retention time of solid material within the primary chamber. Finally, an object of the invention is to improve combustion efficiency in the oxidation chamber to extract more energy and reduce polluting effluents.

Invention

The objects above are solved by a furnace for combustion of solid fuel, according to the characterizing part of independent device claim 1. Additional advantageous features appear from the dependent claims.

General description

In a first embodiment the invention is directed to a furnace for combustion of solid fuel, including Refused Derived Fuel (RDF), municipal waste and wood chips, said furnace comprising:

a primary combustion chamber exhibiting a solid fuel inlet, a gas outlet, a solids outlet, a grate provided with one or more bar feeders arranged upon a nozzle plate exhibiting numerous nozzles arranged to inject air into solid fuel located upon the grate, wherein said primary chamber further comprises:

a drying zone, arranged to optionally dry the solid fuel by said air injected from the nozzles,

a gasification zone, arranged to gasify the solid fuel by means of hot air injected from the nozzles to produce CO and hydrocarbons, and

a fixed carbon zone, arranged to convert carbon to CO by means of hot air injected from the nozzles, and

an oxidation chamber, having an inlet in gas flow communication with the primary chamber, and an outlet for oxidized flue gas, wherein the improvement comprises:

an air nozzle deflector arranged between at least two adjacent nozzles, said air nozzle deflector exhibiting a substantially triangular cross-section with a lower end exhibiting a cross-section which is smaller than the distance between the adjacent nozzles, and with an upper end having a wider cross-section than the lower end, thus defining a first lateral deflector surface and a second lateral deflector surface which are inclined with respect to the vertical and horizontal directions.

The air nozzle deflector is preferably an elongate bar arranged between rows of nozzles. The elongate bar can with its lower narrow end be attached to the nozzle plate by welding.

Nozzle deflectors are preferably arranged only within the drying zone of the primary chamber, whereby the remaining zones of the primary chamber are provided with prior art nozzles.

In a second embodiment the invention is directed to a furnace for combustion of solid fuel, including Refused Derived Fuel (RDF), municipal waste and wood chips, said furnace comprising:

a fixed carbon zone, arranged to convert carbon to CO by means of hot air injected from the nozzles,

an air nozzle deflector arranged between at least two adjacent nozzles, said air nozzle deflector exhibiting a substantially triangular cross-section with a lower end exhibiting a crosssection which is smaller than the distance between the adjacent nozzles, and with an upper end having a wider cross-section than the lower end, thus defining a first lateral deflector surface and a second lateral deflector surface which are inclined with respect to the vertical and horizontal directions, and that

the solid fuel inlet into the primary chamber is arranged at an elevation above the grate.

The solid fuel inlet is advantageously arranged at least 30 cm above the grate.

A reciprocating feeder is in one embodiment provided at the solid fuel inlet, arranged to feed solid fuel into the solid fuel inlet, wherein a retractable table is provided underneath the reciprocating feeder, arranged to control the feeding point of solid fuel into the primary chamber.

In a third embodiment the invention is directed to a furnace for combustion of solid fuel, including Refused Derived Fuel (RDF), municipal waste and wood chips, said furnace comprising:

an air nozzle deflector arranged between at least two adjacent nozzles, said air nozzle deflector exhibiting a substantially triangular cross-section with a lower end exhibiting a cross-section which is smaller than the distance between the adjacent nozzles, and with an upper end having a wider cross-section than the lower end, thus defining a first lateral deflector surface and a second lateral deflector surface which are inclined with respect to the vertical and horizontal directions, wherein the furnace further comprises

a reduction chamber arranged in gas flow communication between the primary chamber and the oxidation chamber, said reduction chamber having a reduction chamber inlet in gas flow communication with a primary chamber gas outlet, and reduction chamber outlet in gas flow communication with an oxidation chamber inlet, wherein the reduction chamber further comprises

a first gas supply pair provided at the reduction chamber inlet, each gas supply pair comprising a recycled flue gas outlet and a secondary air outlet, said recycled flue gas outlet being arranged upstream of the secondary air outlet with respect to the gas flow direction through the furnace,

optionally one or more additional gas supply pairs arranged downstream of the first gas supply pair, and

at least one secondary air supply arranged at the reduction chamber outlet.

A process for combusting solid fuel in a combustion furnace described above, is set forth below, wherein solid fuel deposited upon a grate is subjected to drying, gasification and carbon conversion at sub-stoichiometric oxidative conditions in a primary chamber and supplied with oxygen from below the grate, and moved through the primary chamber by a conveyor means, whereupon solid inert material is removed from the primary chamber, and synthesis gas comprising CO, N2and hydrocarbons with varying length, is guided into an oxidation chamber to perform oxidation of substantially CO and hydrocarbons to form CO2and H2O.

The process further comprises the steps of:

controlling retention time and maintaining gas temperature above about 650°C in the primary chamber to favour ammonia formation, and further controlling air supply and retention time of solid fuel to obtain a gas temperature in the range of about 800-950°C at a gas outlet of the primary chamber,

guiding the synthesis gas from the primary chamber gas outlet into a reduction chamber,

supplying recycled flue gas and then secondary air at an inlet of the reduction chamber, at a rate, concentration and temperature that maintains super-stoichiometric oxidative conditions and a gas temperature within the range of 1000-1300°C throughout the reduction chamber,

supplying secondary air at an outlet of the reduction chamber.

and in the reduction chamber, supplying recycled flue gas and secondary air to the synthesis gas from one or more additional pairwise gas supply comprising recycled flue gas and secondary, said recycled flue gas being supplied upstream of the secondary air supply viewed in the main flow direction of gas through the combustion furnace, and then with secondary air,

viewed in the flow direction of the gas, wherein the gas is supplied with secondary air only before entering the secondary chamber, wherein the flow rates and temperatures of the recycled flue gas and secondary air being supplied to the reduction chamber are controlled to keep the temperature of the gas flowing there through within a range of about 850-950°C, particularly at about 900°C.

Air in the first part of the grate, viewed from the solid fuel feeding point, is preferably injected into solid fuel conveyed upon the grate at an angle between 40-65 degrees, particularly about 60 degrees, whereas air in the remaining part of the grate is injected at a smaller angle with the horizontal plane to direct air supply into a lower part of the solid fuel.

In an alternative embodiment of the furnace and the process, hydrogen gas is supplied before the solids outlet of the primary combustion chamber. In this connection hydrogen gas can also be supplied at the outlet of the reduction chamber instead of secondary air supply. This alternative embodiment turns the furnace into a hydrocarbon gas-producing furnace instead of an energyproducing furnace.

Detailed description

The invention is in the following described in further detail by reference to drawings, where:

Fig.1 illustrates a schematic process flow through a combustion furnace in accordance with the invention,

Fig.2 is a strongly simplified cross-section through the combustion furnace along the transport direction of the solid material, in a first operating mode,

Fig.3 is a drawing similar to Fig.2 but in a second operating mode,

Fig. 4 is a perspective view of the primary chamber in the combustion chamber, facing the feeding section of the furnace, and

Figs. 5a-d are cross-sections through deflectors in air nozzles in the primary chamber of the furnace.

In the following, the process flow and combustion progress is explained with reference to the drawings.

Primary chamber process

Now with reference to Fig.1 (and with reference to Figs.2 and 3 as well), the drawing illustrates a strongly schematic presentation of a furnace for the combustion of solid fuel. The furnace comprises a primary chamber 100 with an inlet 101 for solid material to be combusted, an outlet 102 for gaseous components, and a grate 150 to accommodate and convey the solid fuel deposited thereupon. The primary chamber also has an outlet for solid (inert material), indicated at reference numeral 112 in Figs.2 and 3. Downstream of the primary chamber 100 and its outlet 102, a reduction chamber 200 is in flow connection with the primary chamber, wherein the reduction chamber has an inlet 201 and an outlet 202. Downstream of the reduction chamber 200, an oxidation chamber 300 is arranged, with an inlet 301 forming a transition from the reduction chamber outlet 202, and with an outlet 302. One or more fans (not illustrated) are arranged to force gases through the furnace and exit the same at the oxidation chamber outlet 302 and further to filtering means and other purification means.

In general, solid fuel is fed into the furnace at the inlet 101 of the primary chamber. The primary chamber can be divided into three zones, a drying zone 103, a gasification zone 104, and finally a fixed carbon zone 107, with a gradual transition therebetween, enumerated in the flow direction of fuel and gas. The zones or compartments are imaginary in the sense that there are in general no flow restriction means between the compartments. In the drying zone 103, solid fuel fed into the furnace is flushed with air from underneath the base of the primary chamber and subjected to drying. Here there is no substantial gasification other than evaporation. Then, fully or partially dried fuel is conveyed by one or more bar feeders (indicated by reference numerals 108a and 108b in Figs. 2 and 3) into the gasification zone 104. In the gasification zone 104, the temperature is kept within the range of about 800 - 900 °C to promote formation of ammonia from nitrogen, primarily from supplied air, and hydrogen from decomposed solid fuel and water:

N2+ 3H2<-> 2NH3(1)

The reaction is exothermic and reversible, also known as the Haber-Bosch process. Formation of ammonia is favoured within the gasification zone 104 in order to keep the concentration of nitrogen low and hence reducing risk of forming NOxdownstream in the combustion furnace.

In addition to the formation of ammonia, solid fuel is decomposed in the gasification zone 104 to form synthesis gas comprising a wide numerous hydrocarbon compounds having varying chain length, including CO. The operating conditions such as amount of air supplied to the gasification zone 104, retention time of the fuel being gasified, the temperature, the gas flow rate are controlled to maintain a temperature of less than about 900 °C, typically within the range of about 800-900 °C. The oxygen concentration from air supplied to the grate 150 is controlled to maintain under-stoichiometric combustion in the primary chamber with respect to oxidation.

Downstream of the gasification zone 104, the gasification zone 104 gradually transitions into a socalled fixed carbon zone 107 where carbon in the solid fuel being forwarded on the grate 150 reacts with oxygen (still at under-stoichiometric conditions) to form CO. Here the temperature is kept above 650°C.

At the downstream end of the fixed carbon zone 107, substantially inert solid material leaves the fixed carbon zone 107 of the primary chamber 100 at the bottom right hand of the flow path (reference numeral 112 in Figs.2 and 3), whereas gaseous components leave the primary chamber outlet 102 and enters the reduction chamber 200 through its reduction chamber inlet 201. The temperature at the reduction chamber inlet 201 is typically within the range of about 800-950°C, particularly about 900-950°C.

Reduction chamber process

In order to keep formation of NOxand emission of ammonia in flue gas at a lowest possible level, the temperature, oxygen concentration and gas retention time in the reduction chamber 200 are controlled. In order to obtain this, the temperature is controlled by the supply of secondary air to keep the temperature far below the temperature for NOxformation, which is about 1300 °C. In order to keep the level of ammonia in flue gas at a lowest possible level, oxygen is supplied and temperature is controlled to force the nitrogen/hydrogen-ammonia equilibrium in the equation 1 above to the left to promote decomposition of ammonia back to nitrogen and oxygen, whereupon liberated and supplied oxygen, present in stoichiometric excess, reacts with hydrocarbons to form H2O and CO. Nitrogen leaves the reduction chamber 200 substantially as N2. This is obtained by providing a balanced supply of oxygen, combined with temperature control, to let oxygen participate in the reactions. The temperature throughout the reduction chamber 200 is kept within the range of about 1000-1300°C, particularly about 1000-1050°C, preferably about 1030°C.

The effects above are obtained by supplying the gaseous flow with at least one gas supply pair (203, 204 in Figs.2 and 3), where the first gas supply pair is located at the reduction chamber inlet 201. Each gas supply pair comprises, viewed in the flow direction, firstly a recycled flue gas supply (203 in Figs. 2 and 3) and secondly a secondary air supply (204 in Figs. 2 and 3). It should be emphasized that the sequence of the respective air supplies, i.e. first recycled flue gas and then secondary air, is important. Experiments has shown that recycled flue gas must be supplied first, followed by secondary air. The opposite sequence will not give the desired effect. The air supply within each pair and within the respective supplies in a pair, is preferably provided by numerous nozzles to provide a homogenous injection of gas into the main gas flow in the oxidation chamber.

The supply of recycled flue gas, comprising more CO/CO2and less O2at an elevated temperature, serves to control the temperature in the gas flowing through the reduction chamber 200 by supplying energy to the endothermic decomposition of ammonia to nitrogen and hydrogen. The injected recycled flue gas also serves to supress or quench any flash tongues that may form from injection of secondary air in the following supply point described below.

Adjacent to and downstream of the recycle flue gas supply, the secondary air supply feeds fresh air, at a known and controlled temperature and flow rate, into the gas flowing through the reduction chamber 200. The secondary air supply provides oxygen to promote formation of water and CO mentioned above.

Accordingly, the reduction chamber 200 serves to decompose ammonia back into nitrogen and hydrogen, operated at a temperature that prevents formation of NOx.

Oxidation chamber process

Gas, comprising substantially CO, residual hydrocarbons, H2, O2and some water, leaves the reduction chamber 200 at the reduction chamber outlet 202. There the gas is supplied with secondary air (secondary air supply 204c in Figs. 2 and 3) to complete the oxidation of CO and hydrocarbons to CO2and H2O in the oxidation chamber 300 operated at super-stoichiometric conditions (by the secondary air supply 204c). The gas flows through the oxidation chamber 300 and exits the combustion furnace at the oxidation chamber outlet 302. Then the flue gas is subjected to purification (not illustrated). The temperature in the oxidation chamber is typically maintained within the range of 900-950°C. The oxidation is typically not supplied with more external energy or gas.

In the following, essential components of the furnace according to the invention have been described.

Solid fuel feeder

Now with particular reference to Fig. 2, the drawing illustrates a cross-section through the combustion furnace along the direction of transportation of solid fuel. In the Figs. 2 and 3 some baffles have been omitted for the sake of simplicity. Solid fuel (omitted for simplicity) is fed into the furnace at the primary chamber inlet 101. A reciprocating feeder 106 retracts, to the left in the figure, to fetch solid fuel to be combusted. Then the feeder 106 pushes solid fuel, to the right in the drawing, to feed the solid fuel into the primary chamber 100 through the inlet 101. Solid fuel falls down onto a grate 150 provided with at least one reciprocating bar feeder described in further detail below. The reciprocating feeder 106 is arranged above a retractable table 105 that can serve as a feeding point extension. This is illustrated in Fig.3 where the retractable table 105 is inserted a distance into the primary extension chamber 100.

Accordingly, as illustrated in Figs. 2 and 3, the solid fuel is advantageously fed into the primary chamber 100 at an elevated position above the grate 150 at inlet 101. In this way, the solid waste to a certain degree disintegrates and becomes fluffier. As a result, the drying process and the subsequent gasification become more efficient. The feeding point can be arranged at varying elevations above the grate 150. The elevation vary with the thickness of solid fuel upon the grate 150, among other factors. Exemplary heights are from about 0.3 m to 2 m.

Primary chamber

As described above in connection with Fig. 1, the solid fuel (preferably disintegrated and fluffy as described immediately above), enters the drying zone 103 and is conveyed along the grate 150 by reciprocating bar feeders 108a and 108b working independently from each other. Further details of the bar feeders is omitted here, and reference is made to NO 304450 B1, for example. During its travel along the grate 150, the solid waste is supplied with air through nozzles (110 in Fig. 4) provided in a nozzle plate 109. As described above in connection with Fig.1, the solid waste is first subjected to a drying process during its travel through the drying zone 103, and then gradually subjected to gasification in the subsequent gasification zone 104, followed by formation of CO in the subsequent fixed carbon zone 107.

Air supply nozzles in primary chamber

Now reference is particularly made to Figs. 4 and 5. As illustrated in Fig. 4, the grate 150 in the primary chamber 100 is provided with a nozzle plate 109 having air ducts 111 arranged underneath. The nozzle plate 109 is provided with numerous rows of air nozzles 110 supplied with air from the air ducts 111.

Along the drying zone 103, an elongate air nozzle deflector 113 is arranged upon the nozzle plate 109 extending in the longitudinal extension of the nozzle rows and between adjacent nozzle rows. The air nozzle deflector 113 exhibits a lower end 117 which is attached to the nozzle plate 109, e.g.

by welding, between adjacent nozzles 110. The cross-section of the lower end 117 is smaller than the distance between the adjacent nozzles 110, thus allowing air to leave the nozzle holes without flow restriction.

The air nozzle deflector 113 further exhibits an upper end 118 having a wider cross-section than the lower end 117, thus defining a first lateral deflector surface 119 and a second lateral deflector surface 120 which are inclined with respect to the vertical direction.

Figs. 5a-c illustrates varying embodiments of the air nozzle deflector 113 in a cross-section taken perpendicular to the longitudinal extension of the nozzle rows. Fig. 5a shows a preferred embodiment of the air nozzle deflector 113, where the deflector is an elongate bar with a substantially triangular cross-section, attached to the nozzle plate 109 with one of the apexes of the bar, e.g. by welding. In this way, the air nozzle deflector defines inclined surfaces that during use deflect air ejected through the nozzles 110 in a direction upward. In this way, air is directed into the solid fuel at an angle, for example 30-70 degrees with the horizontal plane, particularly 40-65 degrees, preferably about 60 degrees. This air supply arrangement increases the drying effectivity in that more air can be forced into the solid fuel than with prior art nozzles, e.g. the prior art nozzle 114 illustrated in Fig.5d, where the nozzles are provided as “bell nozzles” with one air supply opening 110 in the nozzle plate and two air outlets 115 and 116 in the “bell”.

It should be noted that the cross-section of the air nozzle deflector 113 does not need to be exactly triangular, and deviating geometries will exhibit a competitive effect. Fig. 5b and 5c illustrates two examples of such alternative embodiments. In the embodiment shown in Fig. 5b, the air flow will be deflected in a more horizontal direction into the solid fuel than the embodiment shown in Fig. 5a. This works well in cases where the thickness of the solid fuel is relatively small, but can be less effective where the solid fuel layer thickness is relatively large since only the lowermost part of the solid fuel layer is supplied with drying air. In the embodiment shown in Fig. 5c, the air flow will be deflected in a more vertical direction into the solid fuel than the embodiment shown in Fig. 5a. This embodiment will eject drying air in a substantially vertical direction with a risk of leaving solid fuel located between nozzle rows unaffected by drying air. The substantially triangular cross-section illustrated in Fig.5a is therefore the preferred embodiment in that air to a higher degree is dispersed throughout the solid fuel.

As is apparent from Fig. 4, the air nozzle deflector 113 according to the invention can be an elongate member provided with its longitudinal extension along the longitudinal extension of the grate 150 between adjacent rows of nozzles 110, and hence in the transport direction of the solid fuel being conveyed upon the grate. In a preferred embodiment the air nozzle deflectors are arranged only in the first section of the grate 150, i.e. in the drying zone 103. In the gasification zone 104, the nozzle arrangement is preferably provided by traditional nozzles 114, e.g. as shown in Fig. 5d, or by a shape something like the embodiment shown in Fig. 5c. The reason is that it is desirable to inject and keep air in the solid waste being gasified to effect a gradual gasification throughout the whole mass of the solid fuel. If air was supplied directly upward, the air would flow directly unreacted through the bed and react with synthesis gas above the bed and increase temperature. As a result, sintering may occur and result in a lower degree of gasification and hence decreased total furnace efficiency.

Bar feeder

As indicated in Figs. 2 and 3, in the gasification zone 104 and fixed carbon zone 107, or alternatively in the fixed carbon zone 107 alone, the grate 150 can be provided with a separate (third) bar feeder 108c. Since the first and second bar feeders 108a and 108b can be operated at a first frequency and the third bar feeder 108c can be operated at a different frequency, the retention time of solid fuel upon the grate 150 in the gasification zone 104 and fixed carbon zone 107, or alternatively in the fixed carbon zone 107 alone, can be controlled. In this way, the retention time of solid fuel conveyed through the respective zones can be controlled. For example, the transport speed should be increased for solid waste being in an initial gasification state, and on the other hand the transport speed should be decreased for solid waste containing more carbon. In this way the gasification efficiency and CO formation can be increased in a controllable manner. Another effect of these independently operated bar feeders is increased temperature control, particularly in the gasification zone, to keep the temperature within the optimum range for the production of ammonia, as described above in connection with Fig.1 (the equilibrium in equation 1 above).

The separate (third) bar feeder 108c can be arranged at a lower level than the first and second bar feeders 108a, 108b.

Reduction chamber

The reduction chamber 200 is a gas flow duct having a reduction chamber inlet 201 and a reduction chamber outlet 202. One gas supply pair 203a, 204a is arranged at the reduction chamber inlet 201, arranged to inject a homogenous flow of recycled flue gas through recycled flue gas outlet 203a, and then downstream of the outlet 203a a homogenous flow of secondary air through secondary air outlet 204a. The respective outlets 203a, 204a are advantageously provided in the form of numerous nozzles arranged to perform a homogenous mixing of injected gas and gas in the main flow.

If required, one or more gas supply pairs can be arranged further downstream in the reduction chamber 200, indicated by reference numerals 203b, 204b in Figs.2 and 3.

The flow duct in the reduction chamber 200 preferably exhibits one or more baffles 205 to increase the flow path.

At least one secondary air outlet 204c is arranged at the reduction chamber outlet 202, arranged to provide a sufficient concentration of oxygen in the gas entering the oxidation chamber 300.

Oxidation chamber

The oxidation chamber 300 is in its simplest embodiment provided as a gas flow duct, preferably provided with one or more baffles 305 to increase the flow path. The oxidation chamber 300 is provided to complete the oxidation of CO and hydrocarbons to CO2and H2O.

Technical effect

The novel features described above do alone and in combination contribute to an increased efficiency of the solid waste combustion furnace. Hence, for a given rate of solid fuel, the combustion furnace can be build with a smaller volume, or the furnace can combust more solid fuel for a given furnace size than prior art furnaces. Moreover, by controlling the temperature in the respective chambers and the equilibrium between nitrogen/hydrogen and ammonia, and the stoichiometry with respect to oxygen, the combustion complete with a high energy yield and a low level of NOxin the effluent gas. The invention also improves combustion control from variations in fuel quality, especially variation/change in the calorific value.

Modifications

While the reduction chamber has been illustrated with two gas supply pairs, each comprising recycled flue gas supply and then secondary air supply, the reduction chamber may contain only one gas supply pair, or even three or more gas supply pairs, depending on the dimensions of the chambers.

Moreover, whereas the air nozzle deflector 113 has been illustrated as a substantially triangularly shaped elongate member arranged between a row of air nozzle holes 110, the invention is not limited to geometry like this. The nozzles 110 can for example be provided as continuous or discontinuous slits in the nozzle plate 109. On the other hand, the air nozzle deflector 113 can be provided as numerous discontinuous substantially triangular members, each defining deflector surfaces only in the area adjacent to the respective air nozzles 110.

Moreover, air nozzle deflectors 113 do necessarily not be attached to the nozzle plate 109. The air nozzle deflectors 113 can be suspended above adjacent nozzles 110, e.g. in the form of an elongate bar 113 with its ends suspended in respective walls in the primary chamber 100.

Claims

1. Furnace for combustion of solid fuel, including Refused Derived Fuel (RDF), municipal waste and wood chips, said furnace comprising:

a primary combustion chamber (100) exhibiting a solid fuel inlet (101), a gas outlet (102), a solids outlet (112), a grate (150) provided with one or more bar feeders (208) arranged upon a nozzle plate (109) exhibiting numerous nozzles (110) arranged to inject air into solid fuel located upon the grate (150), wherein said primary chamber (100) further comprises:

a drying zone (103), arranged to optionally dry the solid fuel by said air injected from the nozzles (110),

a gasification zone (104), arranged to gasify the solid fuel by means of hot air injected from the nozzles (110) to produce CO and hydrocarbons, and

a fixed carbon zone (107), arranged to convert carbon to CO by means of hot air injected from the nozzles (110), and

an oxidation chamber (300), having an inlet (301) in gas flow communication with the primary chamber (100), and an outlet (302) for oxidized flue gas,

characterized in that an air nozzle deflector (113) is arranged between at least two adjacent nozzles (110), said air nozzle deflector (113) exhibiting a substantially triangular cross-section with a lower end (117) exhibiting a cross-section which is smaller than the distance between the adjacent nozzles (110), and with an upper end (118) having a wider cross-section than the lower end (117), thus defining a first lateral deflector surface (119) and a second lateral deflector surface (120) which are inclined with respect to the vertical and horizontal directions.

2. The furnace of claim 1, wherein the air nozzle deflector (113) is an elongate bar arranged between rows of nozzles (110).

3. The furnace of claim 2, wherein the elongate bar (113) with its lower narrow end (117) is attached to the nozzle plate (109) by welding.

4. The furnace of any one of claim 1-3, wherein nozzle deflectors (113) are arranged only within the drying zone (103) of the primary chamber (100), whereby the remaining zones (104, 107) of the primary chamber (100) are provided with prior art nozzles (114).

5. The furnace of claim 1, wherein the solid fuel inlet (101) into the primary chamber (100) is arranged at an elevation above the grate (150).

6. The furnace of claim 5, wherein the solid fuel inlet (101) is arranged at least 30 cm above the grate (150).

7. The furnace of claim 5 or 6, wherein a reciprocating feeder (106) is provided at the solid fuel inlet (101), arranged to feed solid fuel into the solid fuel inlet (100), wherein a retractable table (105) is provided underneath the reciprocating feeder (106), arranged to control the feeding point of solid fuel into the primary chamber (100).

8. The furnace of claim 1, wherein the furnace further comprises a reduction chamber (200) arranged in gas flow communication between the primary chamber (100) and the oxidation chamber (300), said reduction chamber having a reduction chamber inlet (201) in gas flow communication with a primary chamber gas outlet (102), and reduction chamber outlet (202) in gas flow communication with an oxidation chamber inlet (301), wherein the reduction chamber (200) further comprises

a first gas supply pair (203a, 204a) provided at the reduction chamber inlet (201), each gas supply pair (203a, 204a) comprising a recycled flue gas outlet (203a) and a secondary air outlet, said recycled flue gas outlet (203a) being arranged upstream of the secondary air outlet (204a) with respect to the gas flow direction through the furnace,

optionally one or more additional gas supply pairs (203b, 204b) arranged downstream of the first gas supply pair (203a, 204a), and

at least one secondary air supply (204c) arranged at the reduction chamber outlet (202).