EP0676023B1

EP0676023B1 - Grate furnace

Info

Publication number: EP0676023B1
Application number: EP94904017A
Authority: EP
Inventors: Helge Rosvold; Erling A. ÖSTERBÖ
Original assignee: Sinvent AS
Current assignee: ENER-G HOLDINGS PLC
Priority date: 1992-12-28
Filing date: 1993-12-14
Publication date: 1998-07-15
Anticipated expiration: 2013-12-14
Also published as: WO1994015148A1; ATE168459T1; NO176455C; DE69319741T2; DE69319741D1; AU5823894A; NO925023A; NO176455B1; EP0676023A1; DK0676023T3; NO925023D0; NO176455B

Abstract

A grate furnace for different kind of waste materials such as biomass, mud, and derived fuel, comprising a primary combustion chamber (2) and a secondary combustion chamber (3). A cooled grate (6) for fuel is arranged in the lower edge of the primary combustion chamber (2), and below the grate is provided air supply (5), arranged in a plurality of zones. Furthermore there are provided nozzles (15, 16) for the supply of air to the secondary chamber (3). The primary and the secondary combustion chambers (2, 3) are at least partly separated by a separation plate (10). The zones (5) are individually controllable and are supplied with air and recirculated flue gas, preferably with a high temperature. At least one of the nozzles (15) is adjustable and directed towards the primary air chamber (2), in order to ensure an annealing on the grate (6).

Description

The present invention concerns a grate furnace for alternative fuel, as stated in the introductory part of claim 1.

The so called "Refuse Derived Fuel", RDF, is the result of sorting out non-combustible elements from household waste, etc. This is mixed with cortex and wood chips in order to achieve a better calorific value. This fuel is combusted in special furnaces to extract the optimum amount of energy with pollution as low as possible. Such furnaces should also be able to take other solid materials, such as all kinds of biomass, sludge, and certain types of hazardous waste.

In principle, a furnace for combustion of RDF operates in a manner where the fuel is supplied at a grate which is located over a primary air chamber, the chamber often being divided in sections. The combustion furnace is divided into a primary chamber and a secondary chamber, where the fuel is combusted in two steps.

US-A-3 808 619 discloses an incinerator having a first combustion chamber which receives the waste to be incinerated. An ignition-burner is provided in the first chamber to ignite the waste; the burner automatically turns off at waste incineration temperature.

A second combustion chamber is spaced above and communicates with the first chamber. Separators are provided in each chamber for separating partially burned particles from the fully burned combustion gases and preventing the particles from rising beyond the respective separators. An after-burner is arranged to project a substantially horizontal flame into the second combustion chamber; the flame produced by the after-burner substantially filling the cylindrical second chamber and reducing any microscopic particles or unburned gases which have passed through the separators.

Disadvantages with known grate furnaces of this type is that it is difficult to achieve an optimum combustion, both for utilization of energy and reduction of pollution. This is substantially due to the furnaces not being flexible, and not including control options for adjustment of fuel of different consistency and content.

It is thus an object of the present invention to provide a grate furnace for refuse derived fuel, which can be adjusted to the kind of fuel present, in order to achieve a combustion process as optimal as possible, with respect to energy utilization and reduction of pollution.

The object of the invention is achieved with a device having features as stated in the characterizing part of claim 1. Further features are clear from the dependent claims.

In the following, the invention will be described with reference to a preferred embodiment, and with reference to the accompanying drawings, in which

Fig. 1 discloses the principles of a grate furnace according to present invention, and

Fig. 2 discloses in more details a grate furnace according to present invention.

Referring firstly to Fig. 1, there is shown a grate furnace generally denoted 1, which is divided into a primary chamber 2 and a secondary chamber 3. The primary and

secondary chambers

2, 3 are protected by a surrounding insulation 4. Below the primary chamber 2 is located a primary air chamber 5 which is divided into a plurality of sections for flexible supply of primary air and recirculated flue gas. Above the primary air chamber 5 is located a grate 6 adapted for energy-rich fuel (e.g. RDF). The grate 6 is cooled, e.g. by water. The grate is constructed to give a high pressure drop. Above the grate 6 is located a feeder 7, supplying fuel to the grate 6. A guillotine 8 controls the amount of fuel supplied by the feeder 7. The guillotine 8 ensures a good control option for the fuel height above the grate 6, and a stable and even fuel distribution over the entire width of the grate 6. The fuel is supplied from a fuel stock (not shown), by means of a feed screw 9, to a fuel container 18 in the area outside of the guillotine 8. The fuel container 18 is preferably insulated/cooled, so that it does not cause exhaust gas from the fuel to occur.

The primary chamber 2 and the secondary chamber 3 are separated by a separating plate 10. The separating plate 10 is movable both in height and lateral direction, in order to vary the volume of the two

chambers

2, 3 and furthermore, to direct the gas flow in the desired direction. By moving the separating plate 10 in forward and backward directions, respectively, the flow pattern can be influenced, and it can be determined whether the flue gas is to leave the primary chamber 2 at the leading edge, the trailing edge, or both. Preferably baffles 11 are arranged on the separating plate 10, to ensure good mixing and turbulence in the secondary chamber 3. The baffles are also movable to a desired position and may optionally be removed or interchanged with baffles having a different geometry. Also in the top wall is arranged a baffle 12.

An ash hopper 13 is arranged away from and below the grate 6 on the opposite side of the feeding area. In the top wall of the combustion chamber is arranged at least a flue outlet 14.

In the wall 4 of the combustion chamber are arranged nozzles 16a for secondary air. Preferably further nozzles for tertiary air are also arranged in the area 16b.

The grate furnace 1 operates by dehydration, degassing and pyrolysis being carried out in the primary chamber 2. In the secondary chamber 3 combustion of gases is performed. The temperature in the primary chamber 2 is preferably in the range 500-700 °C, while the secondary chamber has a temperature of approximately 1000 °C. The temperature in the primary chamber 2 is set to impede gasification of heavy metal, and the formation of cinders. The temperature in the secondary chamber 3 is set to ensure a good combustion of organic and chlororganic compounds. Low air velocity in the primary chamber is meant to restrict the transporting of dust particles. In addition, regulation of the temperature is determined by restricting the air supply to the primary chamber. This is performed by a plurality of the zones in the primary air chamber 5 being individually controlable, i.a. to avoid cooling in areas where the temperature is under control. By regulating the temperature control through air supply in the primary chamber, it is possible to stay below the critical temperatures at which cinder form.

At the end of the grate 6 combustion of solid carbon is ensured by a combination of controlled glowing, increased resistance at the end of the grate and screening of the ash from the remainder of the combustion chamber. Glowing is achieved by leading air and recirculated flue gas combining with flue gas and secondary air through a nozzle 15 located in the wall of the combustion chamber, so that it screens the zone for combustion of the ash from the remaining combustion chamber. This air is simultaneously meant to establish a pressure pattern where heat from the glowing zone is moved to the supply zone and ensures an even pyrolysis activity. Dimensioning and geometry of the nozzles 15 should be adjusted to the amount of recirculated gas and the flow pattern desirable in the primary air chamber 2. Output of flue gas at the flue gas outlets 14 should be varied dependent upon what flow pattern is desirable in the secondary chamber. To ensure flexibility in the flow pattern, a flue outlet is preferably situated both in the leading and the trailing edge of the secondary chamber 3.

Secondary air nozzles

16a, 16b are dimensioned for the actual amount of air and the flow pattern desirable in the secondary chamber 3. The

secondary air nozzles

16a, 16b are formed such that each can vary output velocity and output angle of the air. The location of the secondary air nozzles as shown is meant as an example. The nozzles 16a in the walls of the secondary air chamber are dimensioned on the basis of the volume of the secondary chamber in order to achieve velocity and a direction leading to good mixing. The air pre-heating in the wall between the

combustion chamber

2, 3 and the fuel container 18 also provide insulation to the fuel container and prevent high temperatures in the fuel container. The heat output should, however, not be so great that the temperatures in the primary and

secondary chamber

2, 3 are influenced to a negative degree.

In Fig. 2 there is shown a more detailed example of an embodiment of present invention. Broadly the construction and operational mode is similar to what is described above with reference to Fig. 1. The combustion chamber is divided into a primary chamber 2 and a secondary chamber 3 which is surrounded by an insulated steel mantel 4. The two

chambers

2, 3 are separated by horizontal separating plate 10, which at its ends merges into vertical baffles 11. Turbulence in the secondary chamber is secured by vertical baffle 12. Below the grate 6 is arranged five primary air boxes 5 for distribution of primary air. These are furnished with supply pipes 21 for recirculated flue gas, and supply pipes 22 for primary air which is heated by a heat element 25.

The guillotine 8 adjusts the height of the fuel being supplied to the grate 6. The fuel emerges from the cell feeder 9, which simultanously shuts off air to the fuel container 18 and provides for closing off the fuel feed. Before the ash is transported from the grate 6 to the ash hopper 13, pre-combustion and glowing by means of adjusted air supply and resistance is performed in the ash combustion chamber 19.

Secondary air is supplied through

nozzles

15, 16a and 16c, and tertiary air is supplied through nozzles 16b.

The grate 6 is cooled by the supply of cooling medium through the supply pipes 20. A cooling element 23 is arranged on the flue gas duct 14, so as to cool the flue gas before moving through the pipe 24. Furthermore, in Fig. 2 there is shown an observation glass through which the combustion process can be observed.

Claims

Grate furnace for different kind of waste material such as biomass, sludge, and derived fuel; comprising a primary combustion chamber (2) and a secondary combustion chamber (3), wherein a cooled grate (6) for fuel is arranged in the lower side of the primary combustion chamber (2), and air supply (5), arranged in a plurality of zones, is provided below the grate, furthermore are provided nozzles (15, 16) for supply of air to the primary and secondary combustion chambers (2, 3), the primary and the secondary combustion chambers (2, 3) being at least partly separated by a separation plate (10), characterized by the zones (5) being individually controllable and supplied with air and recirculated flue gas, preferably with a high temperature, and that at least one of the nozzles (15) is adjustable to screen the zone for the combustion of ash from the remaining combustion chamber and directed towards the grate (6), in order to ensure a glowing on the grate (6).
Furnace according to claim 1,
characterized by the separation plate (10) being adjustable in lateral direction.
Furnace according to claim 1 or 2,
characterized by the separation plate (10) being adjustable in height direction.
Furnace according to any one of claims 1-3,
characterized by the separation plate (10) being provided with baffles (11), which can be moved on the separation plate, and can have different geometry.
Furnace according to any one of claims 1-4,
characterized by further baffles (12) being provided in the top wall of the combustion chamber.
Furnace according to any one of claims 1-5,
characterized by the mixing of air and flue gas to the zones (5) being controlled by measured values of the temperature in the primary and secondary combustion chambers (2, 3), respectively, and the composition of the exhaust through flue outlets (14).