GB2483985A - Biomass Gasification Plant comprising filter candles and steel fibre tubular filters - Google Patents

Abstract

A gasification plant (100) comprises means (7) to subject biomass to gasification in a downflow gasifier, means (11) to generate flow of gas through the gasifier, and a filter unit 8 to remove particulate material from the gas flow from the gasifier (7), wherein the filter unit 8 comprises a chamber divided into an inlet zone 23 and an outlet zone 24 by a divider plate 22, with a multiplicity of hollow ceramic filter candles 20 supported by the divider plate, and wherein each ceramic filter candle 20 is provided with a gas-permeable steel fibre filter pad 30 at its downstream side. If a ceramic filter candle 20 breaks, the corresponding steel fibre tubular filter 30 will trap particulate material and will become blocked; this will suppress unwanted gas flow the broken filter candle, allowing normal filtration to occur through the unbroken candles.

Description

Biomass Gasification Plant This invention relates to a gasification plant for processing biornass by means of a gasification process, so as to produce a fuel gas.

Gasification (which may also be referred to as pyrolysis) is a known process for converting biomass to a gas stream, and the biomass might for example be straw, bagasse, or other agricultural wastes, waste paper, or wood. Such a process may be applied to biomass grown specifically for the purpose, or to waste materials. In particular waste wood would be a suitable material for such a process. The gasification process involves heating the biomass to an elevated temperature, possibly in the presence of a restricted quantity of air, to break down organic materials and to generate carbon monoxide and hydrogen. This gas mixture may be used to generate power in an engine. Other products of the process are charcoal, particulate carbon, and tars. A gasification plant is described in GB 2 455 869 (0-Gen UK Ltd) in which wood is shredded and then subjected to downdraft gasification, and the resulting gases are purified before being used as a fuel. The purification of the gases makes use of a hot gas filter consisting of several hollow ceramic filter candles hanging from a top plate, the gas stream flowing upwardly through the filter candles (so the candles can be cleaned by back pulsing clean gas down the candles) These filter candles remove particulate matter down to about 5 microns. However, if a filter candle breaks during operation it is necessary to shut down the gas filter to prevent particulate material contaminating downstream components of the plant. This can be achieved by providing two filter units that operate in parallel, so one can be shut down for repair or treatment while the other filter unit continues to function, or to split a single vessel into a duty part and a standby part, but these are expensive solutions.

According to the present invention there is provided a gasification plant comprising means to subject biomass to gasification in a downflow gasifier, means to generate flow of gas through the gasifier, and a filter unit to remove particulate material from the gas flow from the gasifier, wherein the filter unit comprises a chamber divided into an inlet zone and an outlet zone by a divider plate1 with a multiplicity of hollow ceramic filter candles supported by the divider plate, wherein each ceramic filter candle is provided with a gas-permeable steel fibre filter pad at its downstream side.

Preferably the hollow ceramic filter candles extend downwardly from the divider plate, the outlet zone being above the divider plate, and each steel fibre filter pad is fixed to an annular metal plate that rests on the upper surface of the filter candle.

Preferably the steel fibre filter pad is of a knitted construotion. Preferably it is of total thickness between 25 mm and 100 mm, more preferably between 40 mm and 90 -, for example of thickness 60 mm or 75 mm.

In the event that a ceramic filter candle should break, the bulk of the gas flow will flow through the broken filter candle, because it imposes a much smaller pressure drop than the unbroken filter candles. The bulk of the gas flow is therefore flowing through the steel fibre filter pad associated with the broken filter candle, and at least initially the steel fibre filter pad imposes a very small pressure drop. The steel fibre filter pad traps the particulate material carried by the bulk of the gas flow, and consequently rapidly becomes blocked. This suppresses the gas flow through the broken filter candle, so normal filtration continues through the unbroken filter candles.

The use of such a steel fibre filter pad is compatible with filtering gases while they are hot, as is the use of the ceramic filter candles; and the gases are preferably at a temperature above 500°C when they are filtered, as such high temperature filtration ensures that tarry material is not deposited on the filter.

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 schematic diagram of a waste wood gasification plant of the invention; Figure 2 shows a longitudinal sectional view of a part of a filter unit of the plant of figure 1; and Figure 3 shows a modification to the filter unit of figure 2.

Referring to figure 1, a gasification plant 100 includes a reception building A where the organic feedstook (waste wood in this example) is of floaded from delivery vehicles. It is then loaded into a shredder 1.

In this example the shredder 1 contains four shafts carrying cutting disks. The shafts are rotated at a low speed, approximately 25 rpm, driven by electric motors through gearboxes, producing a substantially consistent product size with minimal production of dust. The shredded waste wood passes through a sizing screen (not shown) within the shredder 1 before falling on to a discharge conveyor which transfers it to an overhead magnet 2 arranged to extract any ferromagnetic material.

The shredded wood is then passed over an eddy current separator 3 to remove any non-ferrous metals, and then over a vibrating finger screen or a rotary screen 4 to separate fines. The cleaned and sized shredded wood then travels along an inclined conveyor to a storage container with a mechanism for feeding out the feedstock at a substantially constant rate.

The shredded wood from the storage container 5 is passed over a further magnetic separator 6 to ensure removal of any ferromagnetic materials, and then over an in-feed weighing device 6a, and so to one or more gasifiers 7 (only one is shown). By way of example the plant may include several such gasifiers 7, for example twelve or eighteen. These operate continuously, but are fed by a batch process through individual lock-off hopper systems. Each can process 270 kg of prepared organic feedstock per hour, but processes in batches of about 17 kg, and treating fifteen batches per hour. Any excess organic feedstock is returned tc the storage container 5 via another in-line weighing device 6b. Within each gasifier 7 there is a low oxygen atmosphere (typically about or less than 2%) at a slightly negative pressure (i.e. less than atmospheric pressure), the gas flowing downwardly through a bed of hot feedstock and charcoal reaching a temperature of for example above 800°C. The combination of temperature and residence time in the gasifier 7 not only brings about gasification of the waste wood, but also cracks most of the tar that is produced during the gasification, because the gases (and tar) pass down through the hottest region of the bed.

The gases produced by gasification are sucked out from the base of the unit 7 by an induced draft fan 11.

The hot gas stream consisting principally of hydrogen and carbon monoxide is then passed through a filter unit 8 consisting of several hollow ceramic filter candles 20 hanging from a top plate 22 (shown in figure 2), the gas stream flowing upwardly through the filter candles 20 (so the candles can be cleaned by back pulsing clean gas down the candles). These filter candles 20 remove particulate matter down to about 5 pm. The hot clean gas, typically at between about 450 and 500°C, is then passed through a heat exchanger 9 to cool the gas down to less than 50°C (this may be achieved using two heat exchangers 9 in series), for example down to 25°C.

This leads to condensation of moisture in the gas flow, and the resulting condensate is discharged (shown as H2C). The cool gas stream is then passed through a fluidic vortex scrubber 10 which consists of a cylindrical vortex chamber into which the gas flows through a tangential inlet port, and emerges from an axial outlet port, such that there is a vigorous vortex flow within the chamber; and a scrubbing liquid is sprayed through the gas in the vortex. The sorubbed gas stream then passes through the induced draft fan 11 into a buffer storage vessel 12, the induced draft fan 11 ensuring that the pressure in the storage vessel 12 is above atmospheric.

The clean dry gas in the buffer storage vessel 10 is used to provide fuel to a gas-fuelled internal combustion engine 13, and the exhaust gas from the engine 13, after treatment, is discharged through an exhaust stack 15.

The plant 100 may have several such engines 13, for example six engines, but sharing the exhaust stack 15.

Typically each gasifier 7 is associated with its own filter unit 8, heat exchanger 9, scrubber 10 and fan 11.

Typically two gasifiers 7 provide gas to one buffer storage vessel 10 which supplies gas to a single engine 13, as combining the outlets from two gasifiers 7 provides a more consistent composition of the gas mixture supplied to each engine 13. Each engine 13 may for example generate electricity. Alternatively gas from a number of gasifiers 7 may instead be connected to a boiler, to provide steam to drive a steam turbine and generator -Each gasifier 7 also produces charcoal, or carbon char, depending on the nature of the feedstcck, and this is removed from the base of the gasifier (shown as C) The filter unit 8 captures particulate carbon char, and this is released from the filter when it is back-pulsed; this separated carbon char is also shown as C. These supplies of carbon (charcoal or char) may be used to form granules or briquettes, as a by-product.

Referring now to figure 2, the filter unit 8 is divided by a top plate 22 into an inlet zone 23, to which the hot gases from the gasifier 7 are supplied, and an outlet zone 24 from which the filter gases are extracted by the fan 11. Typically the hot gases are at a temperature of between 500 and 600°C when they enter the inlet zone 23. The top plate 22 defines several circular apertures, and in each aperture is fixed a ceramic filter candle 20, which is tubular, closed at its lower end (as indicated by the chain dotted lines), and open at its upper end, and provided with a circumferential flange 25 around the upper end. Such ceramic filter candles 20 are known. They are made of porous material, so the hot gases flow through the filter candles 20 from the inlet zone 23 to the outlet zone 24, while the particulate material is trapped on the outer surface of the filter candles 20.

At intervals, a gas pressure pulse may be applied to the outlet zone 24, so that the particulate material at the outer surface of each filter candle 20 is blown off, providing the separated carbon char C mentioned above.

Within the upper part of each ceramic filter candle is a knitted steel mesh filter pad 30, typically of thickness between 40 mm and 90 mm, for example 65 mm thick, that fills a short tube 32 that fits within the bore of the ceramic filter candle 20 and is open at its lower end; the short tube 32 is integral with a thin annular metal plate 33 that fits on top of the flange 25 of the ceramic filter candle 20. The filter pad 30 may be secured into the short tube 32 by protruding spikes 34, which may be tabs that are cut out from the wall of the tube 32 and bent inward. The knitted steel mesh filter pad 30 is readily gas permeable, so it imposes negligible pressure drop on the gas flow during normal operation.

The knitted steel mesh filter pad 30 may for example be made of knitted 316 stainless-steel fibres. The thin annular metal plate 33 is sealed either to the top of the flange 25, or alternatively is sealed to a clamping plate (not shown) that is fixed above the top plate 22 to clamp both the metal plates 33 and also the filter candles 20 into position, the clamping plate having apertures that align with the apertures in the top plate 22, and so with the locations of the filter candles 20.

It has been found that during operation there is a risk that one of the ceramic filter candles 20 may break, and such a break typically takes place at an intermediate position along the length of the tubular part of the filter candle 20, so causing the closed end of the filter candle 20 to fall off. In Figure 2 the filter candle 20 is shown in this broken state, with only the upper part of the filter candle 20 present. If a ceramic filter oandle 20 breaks in this way, the bulk of the gas flow will flow through the bore of the broken filter candle 20, because it imposes a much smaller pressure drop than the unbroken filter candles 20. The bulk of the gas fiow therefore flows through the knitted steel mesh filter pad 30 mounted on the broken filter candle 20, and at least initially the knitted steel mesh filter pad 30 imposes a very small pressure drop. Typically the particulate concentration in the inflowing gases is approximately 1 g/m3, and since substantially the entire gas flow including all the partioulates flows into the broken filter candle 20, the knitted steel mesh filter pad 30 traps this particulate material, and consequently rapidly becomes blocked. This suppresses the gas flow through the broken filter candle 20, so normal filtration continues through the unbroken filter candles 20. Since particulate material is trapped throughout the thickness of the knitted steel mesh filter pad 30, it remains blocked even if the filter unit is subjected to a reverse pressure pulse to remove char C from the ceramic filter candles 20.

In a modification, the knitted steel mesh filter pad may be installed in the top of the bore of the filter candle 20, without the provision of the short tube 32 and the annular metal plate 33.

Referring now to figure 3 there is shown a filter unit 40 which is a modification to the filter unit 8 shown in figure 2. n most respects the features of the filter unit 40 are identical to those described above, and are referred to by the same reference numerals. In the filter unit 40 a separate gas line 42 is provided for each ceramic filter candle 20. The gas line 42 extends through the mesh filter pad 30, and has an open end a short distance below the bottom of the mesh filter pad 30. For example the gas line 42 may be a stainless-steel tube of external diameter 10 mm, and its open end may be mm below the bottom of the mesh filter pad 30.

The filter unit 40 operates as described above, except that the back-cleaning pressure pulses are applied at intervals to the gas lines 42, so that the pressure pulse is directed below the mesh filter pad 30, instead of being applied to the outlet zone 24. If the ceramic filter candles 20 are unbroken, the effect is substantially identical to that of applying the pressure pulse to the outlet zone 24, because the mesh filter pad 30, when clean, has little effect on the gas flow or the pressure wave. Particulate material at the outer surface of each filter candle 20 is blown off, providing the separated carbon char C mentioned above. If a candle 20 is broken, however, then the corresponding mesh filter pad 30 will become blocked with particulate material as described above, and in this case there may be a benefit from applying the pressure pulse below the mesh filter pad 30, through the gas line 42, as the pressure pulse does not dislodge the particulate material that blocks the pad 30. Whether or not this is advantageous depends on the nature of the particulate material.

It will be appreciated that the gas line 42 may be of a different diameter to the described above, and it may follow any convenient path through the filter pad 30 -it does not have to be concentric with the ceramic filter candle 20.

Claims (7)

1. Claims 1. A gasification plant comprising means to subject biomass to gasification in a downfiow gasifier, means to generate flow of gas through the gasifier, and a filter unit to remove particulate material from the gas flow from the gasifier, wherein the filter unit comprises a chamber divided into an inlet zone and an outlet zone by a divider plate, with a multiplicity of hollow ceramic filter candies supported by the divider plate, wherein each ceramic filter candle is provided with a gas-permeable steel fibre filter pad at its downstream side.
2. 2. A gasification plant as claimed in claim I wherein the hollow ceramic filter candles extend downwardly from the divider plate, the outlet zone being above the divider plate, and each steel fibre filter pad is fixed to an annular metal plate that rests on the upper surface of the filter candle.
3. 3. A gasification plant as claimed in claim 1 or claim 2 wherein the steel fibre filter pad is of a knitted construction.
4. 4. A gasification plant as claimed in any one of the preceding claims wherein the steel fibre filter pad is of thickness between 25 mm and 100 mm, preferably between 40 mm and 90 mm.
5. 5. A gasification plant as claimed in any one of the preceding claims provided with means for providing a reverse pressure pulse to the ceramic filter candles.
6. 6. A gasification plant as claimed in claim 5 comprising means for providing a reverse pressure pulse within each ceramic filter oandle at the upstream side of the steel fibre filter pad.
7. 7. A gasification plant substantially as hereinbefore described with reference to, and as shown in figures 1 and 2 of the accompanying drawings, or with the modification as shown in figure 3 of the accompanying drawings.