WO2009019502A2

WO2009019502A2 - Hot gas supply

Info

Publication number: WO2009019502A2
Application number: PCT/GB2008/050629
Authority: WO
Inventors: Anthony Redman
Original assignee: Biojoule Limited
Priority date: 2007-08-08
Filing date: 2008-07-28
Publication date: 2009-02-12
Also published as: GB0715379D0; WO2009019502A3

Abstract

A burner unit (10) produces hot gas by burning a particulate biomass fuel and mixing the resulting hot combustion gas with an air flow, using a static mixing unit (16) containing baffles (42-48) to ensure thorough mixing. The combustion unit (10) comprises a first horizontal duct (18) of heat-resistant material above which is a second horizontal duct (20) of heat-resistant material, and these ducts communicate through a gas plenum (24) in which ash can collect. At the other end of the second horizontal duct (20) is the hot combustion gases outlet (34). The baffles (42-48) ensure mixing occurs in a plurality of stages.

Description

Hot Gas Supply

The invention relates to a burner unit for providing a supply of hot gas, particularly but not exclusively relating to a burner unit that can burn a biomass-derived fuel .

Biomass is one of the world's most widely available and green fuels. By-products from forestry and agricultural operations together with energy crops and wastes form a huge resource for sustainable energy. In their raw form these biomasses are often inconsistent fuels with high moisture (typically being between 40 and 70% by mass) and low calorific values, as much of the heat energy of combustion is taken up by the latent heat of this moisture. As a result, most burners designed for chipped or particulate biomass are relatively large fixed installations. It would be desirable to have a burner for such biomass with a much smaller footprint and installed in a container or on a skid so that it can easily be transported to and set up at different sites. Such a burner would be particularly suited to producing hot gas for drying applications.

According to the present invention there is provided a burner unit for burning a particulate biomass fuel and for obtaining a flow of hot gas, the burner unit comprising a combustion unit and a static mixing unit; the combustion unit comprising a first generally horizontal duct of heat-resistant material, a second generally horizontal duct of heat-resistant material above the first duct communicating with the first duct through a plenum and also communicating via a hot combustion gases outlet with the static mixing unit, and means to supply air and particulate biomass fuel to the end of the first duct remote from the plenum; the static mixing unit comprising an inlet for gas, and a plurality of baffles arranged to progressively mix hot combustion gases with gas from the gas inlet in a plurality of mixing stages.

The first horizontal duct may include a grate onto which particulate biomass fuel is introduced. Preferably each horizontal duct is sufficiently long that ash particles settle out in the ducts and in the plenum, and are not carried through the combustion gases outlet.

Similarly any red-hot soot or smoke particles complete the combustion process before reaching the combustion gases outlet. The ducts may for example be of diameter between 200 mm and 600 mm, preferably between 300 and 500 mm, and of length between 3.0 m and 6.0 m, preferably between 3.5 m and 4.5 m. However, for greater certainty, a non-combustible filter such as a wire mesh filter or a ceramic filter may be provided at the hot combustion gases outlet.

The burner is preferably transportable, so that it can be taken to different places where biomass is available and hot gas is required. For example it may be contained in a standard sized container such as an ISO container.

The burner allows a high heat output to be achieved from a compact unit. For example, between 250 kW and as much as 700 kW of heat can be produced from burning wood chips in a burner mounted in a 20' ISO container.

The hot combustion gases emerging through the combustion gases outlet will typically be at a temperature above 200⁰C, and may be too hot for many drying processes. For example, when drying wood chips using hot gas, the hot gas should not be so hot that the wood chips are charred; and for some purposes there is no benefit in reducing the moisture of wood chips to below 10% (weight) . In this case there is no benefit from using gas that is much above 100⁰C; the hot gas is preferably between about 7O⁰C and HO⁰C, more preferably between 85⁰C and 100⁰C, as such hot gas can rapidly evaporate moisture from wood chips. It will be appreciated that the burner is suitable for providing a supply of hot gas to a wide range of applications, for example drying other materials, such as minerals. As the hot combustion gases pass through the static mixing unit they are mixed with a flow of gas at a lower temperature, so as to provide an out-flowing hot gas stream at a desired temperature. This added gas is typically air, although it may instead be another gas (or vapour) such as steam, for example, or a mixture. The lower temperature gas flow may be significantly larger than the flow of the hot combustion gases, to achieve the desired temperature of the hot gas stream; for example the gas flow may be between 5 and 10 times the flow of the hot combustion gases (by mass) . This air flow may be brought about by a fan which is a component of the burner unit, or by an external fan.

Hence the present invention also provides a process for drying particulate biomass material using hot air from such a burner unit .

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a sectional view of a burner unit (on the line 1-1 of figure 2); and

Figure 2 shows a sectional view on the line 2-2 of the burner unit of figure 1 (to a slightly different scale) .

Referring to the drawings, a burner unit 10 for providing a supply of hot gas suitable for drying wood chips is shown. The hot gas might instead be used for a range of different applications. It comprises a combustion unit 14 and a static mixing unit 16. These are within a container defined by a steel casing 15.

The combustion unit 14 comprises two 4 m long solid cast refractories with central tubular chambers defining pipes 18 and 20, each of diameter 500 mm, extending horizontally and sited one above the other. In this example the pipes 18 and 20 are of fire cement, but they may be of any suitable refractory material. As shown in figure 2, at one end (the right-hand end in figure 2, as shown) the pipes 18 and 20 are closed by a refractory end panel 22 but communicate via a plenum 24; a ramp in the floor defines a gutter 25 in which ash tends to collect, and from which it is removed by a screw (not shown) . At the left-hand end the lower pipe 18 is provided with a grate 26, and communicates with an inlet chamber 28 in which is a mechanism (not shown) for feeding fuel onto the grate 26, and at least one fan 27 (represented diagrammatically ) for blowing air into the pipe 18 to provide primary, secondary and tertiary combustion air. At the left-hand end the upper pipe 20 is closed by a refractory end plate 30 and defines an outlet chamber 32; there is a wire mesh filter 34 at the top of the chamber 32 through which the hot combustion gases flow out into the mixing unit 16.

As shown particularly in figure 1, air is supplied as indicated by the arrow A through at least one inlet duct 40 just above the mesh filter 34 of the combustion unit 14, and flows above and below horizontal baffle plates 42 and 43 which are above the mesh filter 34. The upper baffle plate 43 extends the entire length of the container 15 and is L-shaped, having a vertical section 44 to divert the air flow downwardly, and a vertical baffle plate 45, which also extends the entire length of the container 15, diverts the air above the upper baffle plate 43 similarly. A short vertical baffle plate 46 similarly diverts some of the air flow from below the lower baffle plate 42. The plates 46, 44 and 45 extend progressively further down. The bottom end of the baffle section 44 is bent so the channel through which hot air flows becomes narrower; this enhances the turbulence below it, and also helps to distribute the hot air more uniformly along its length. Air flowing around the bottom of the vertical baffle plate 45 is then constrained to negotiate two layers of full-width horizontal baffle plates 47 and 48 with gaps between them, the gaps between the plates 47 being staggered relative to those between the plates 48, before emerging through an outlet duct 52.

Thus in operation of the burner unit 10 the fuel burns on the grate 26 in the air flow from the fan 30, and the resulting very hot combustion gases flow through the two pipes 18 and 20 in succession, and then emerge through the filter 34. As a first stage the hot combustion gases mix with the air flowing below the lower baffle plate 42; next they mix with the air flowing below the upper baffle plate 43; and then they mix with the air flowing above the upper baffle plate 43. The baffle plates 42-48 ensure that the hot combustion gases are thoroughly mixed with the air flow through the inlet duct 40, so as to produce a substantially uniform hot stream of gas through the outlet duct 52.

The burner 10 is transportable, so that it can be taken to different places where biomass is available and hot gas is required. It is of such a size that it may be contained in a standard sized container such as an ISO container .

It will be appreciated that the burner unit 10 described in relation to the drawings can be modified in various ways while remaining within the scope of the present invention. For example the air flow through the inlet duct 40 may be provided by a fan in the inlet duct, or indeed by a fan in the outlet duct 52. In another modification, airflow A may be replaced by a flow of steam so as to provide a stream of superheated steam through the outlet duct 52; such superheated steam may be used for steam drying applications.

Claims

Cl aims

1. A burner unit for burning a particulate biomass fuel and for obtaining a flow of hot gas, the burner unit comprising a combustion unit and a static mixing unit; the combustion unit comprising a first generally horizontal duct of heat-resistant material, a second generally horizontal duct of heat-resistant material above the first duct communicating with the first duct through a plenum and also communicating via a hot combustion gases outlet with the static mixing unit, and means to supply air and particulate biomass fuel to the end of the first duct remote from the plenum; the static mixing unit comprising an inlet for gas, and a plurality of baffles arranged to progressively mix hot combustion gases with gas from the gas inlet in a plurality of mixing stages.

2. A burner unit as claimed in claim 1 wherein the first horizontal duct includes a grate onto which particulate biomass fuel may be introduced.

3. A burner unit as claimed in claim 1 or claim 2 wherein each horizontal duct is sufficiently long that ash particles settle out and are not carried through the combustion gases outlet.

4. A burner unit as claimed in any one of the preceding claims wherein the ducts are of diameter between 300 and 500 mm, and of length between 3.5 m and 4.5 m.

5. A burner unit as claimed in any one of the preceding claims also comprising a mesh filter at the hot combustion gases outlet.

6. A burner unit as claimed in any one of the preceding claims which is mounted in a container or on a skid so as to make it transportable.

7. A burner unit as claimed in any one of the preceding claims where the hot combustion gases passing through the static mixing unit are mixed with a flow of steam so as to provide an out-flowing stream of steam at a desired temperature .

8. A process for drying particulate biomass by contacting the biomass with a stream of hot gas, wherein the hot gas is created by mixing combustion gases with an air stream using a static mixing unit comprising a plurality of baffles.

9. A process for drying particulate biomass by contacting the biomass with a stream of superheated steam, wherein the superheated steam is created by mixing combustion gases with a stream of steam using a static mixing unit comprising a plurality of baffles.