WO2006004490A1

WO2006004490A1 - Gas purification

Info

Publication number: WO2006004490A1
Application number: PCT/SE2005/000927
Authority: WO
Inventors: Lennart Granstrand
Original assignee: Svensk Rökgasenergi Intressenter Ab
Priority date: 2004-07-05
Filing date: 2005-06-16
Publication date: 2006-01-12
Also published as: SE526864C2; EP1765505A1; SE0401751D0; SE0401751L

Abstract

Method and device for separation of pollutants, especially particles from a gas flow (2). The gas flow is led through a plurality of parallel tubes (42) which each comprise a substantially coaxial electrode (43) whereby a voltage is established between the electrodes and the tubes. The tubes are preferably maintained at earth potential and the electrodes (43) are supplied with a negative potential. The tubes (42) are disposed substantially vertically. The flue gas is introduced into the tubes in a substantially saturated state and is preferably led upwards through the tubes. The tubes are cooled externally and the condensate formed on the inside of the tubes flows down along the inside of the pipe walls, is gathered and is subjected to cleaning (Fig. 2).

Description

Gas purification

The invention relates to a method and a device for separation of particles from a gas flow, of the kind indicated by the preambles of the attached independent method claim and device claim respectively.

Process gas from a process industry or flue gas from a fuel boiler, e.g. a biofuel boiler, often contains pollutants which need separating and treating before the flue gas, after the extraction of heat from it, is released to the environment. Heat may for example be extracted primarily for some useful purpose, e.g. district heating, which entails the flue gas being cooled to, for example, 5O⁰C, e.g. by conventional heat exchange or by water sprayed directly into the flue gas flow and drawn off in a hot state at various points along the gas line for direct or indirect heat exchange against a district heating flow.

Although the result may be that certain types of particles are washed out of the flue gas flow, the flue gas still contains particles of various kinds which need substantially removing before the flue gas flow can or may be discharged directly to the environment. It is of course also desirable, at least as far as economically possible, to extract residual heat from the gas flow after the extraction of thermal energy for the primary purpose, before the gas flow is discharged to the environment.

Previous proposals have included the practice, after the actual extraction of heat from it, of leading the flue gas through beds of solid particles which have liquid flowing through them so that flue gas pollutants, e.g. fine particles in the flue gas, are deposited on bed particles and can be washed out of the. particle bed by the resulting liquid flowing through the bed. The liquid containing dissolved and associated solid pollutant particles can thereafter be cleaned and possibly be recirculated.

However, such previously known solutions are complex and expensive to set up and operate. One object of the invention is therefore to indicate a simple technique which at relatively low cost makes it possible to substantially clean flue gas and also makes it possible to recover heat from it easily and effectively in conjunction with said clea- ning. A particular object is to extract heat and clean flue gas at a low temperature in order to achieve particularly effective cleaning of it.

These objects are achieved in whole or in part by the invention.

The invention is defined in the attached independent claims.

Embodiments of the invention are indicated in the attached dependent claims.

In a particularly preferred embodiment of the invention, flue gas after primary heat extraction is at a relatively low temperature, e.g. 5O⁰C or lower. At the same time, care is taken to ensure that the flue gas is substantially saturated before it is led through a special heat exchanger comprising a plurality of parallel separate tubes which are cooled on the outside by a flow of a fluid such as ambient air, whereby the heat absorbed by the fluid, and possibly also the fluid itself, can be utilised, e.g. as preheated combustion air for a biofαel boiler which produces the flue gas flow. Alternatively, the tubes may be surrounded by a vessel with an inlet and an outlet for the cooling fluid, e.g. a liquid such as water, or ambient air, which is thus heated, making it possible for the heat content of the fluid to be utilised in a conventional manner.

The tubes are substantially vertical and each comprise a coaxial electrode supplied with an electrical potential which differs greatly from earth potential, while the tubes are maintained at earth potential. The electrode and its power supply are with advantage so disposed and designed as to cause corona arcs in the saturated gas flow which passes through axially, in order to charge and/or treat the particles therein, the gas flow being preferably led downwards so that it remains substantially saturated during its movement along the tubes, despite being progressively cooled.

As the tubes are cooled from the outside, water from the flue gas will condense on the inside of the tubes and ran down along their inside walls, thereby washing away flue gas particles deposited, inter alia electrostatically, on the inside of the tubes.

In a practical embodiment, the amount of particles in the flue gas may be reduced to, for example, 30 mg/m³ in cases where the gas flow is cooled to a temperature of about 3O⁰C, e.g. against ambient air. The invention thus makes it possible in practice, by cooling the flue gas flow to low temperature levels of, for example, about 3O⁰C, to free the flue gas from pollutants which are otherwise particularly difficult to deal with, such as certain types of salts. It certainly might be possible to remove pollutants of the dioxin type by the technique according to the invention. Further cooling of the gas flow, e.g. down to 1O⁰C or 5⁰C, makes it possible for the separation to be still more effective for certain pollutants and/or might enable further pollutants to be separated effectively. The condensate with associated pollutants from the inside walls of the tubes is produced in relatively small flows, and effective and relatively low cost techniques are now available for the separation and final treatment of pollutants from the condensate flow.

The invention is described below in the form of examples with reference to the attached drawings.

Fig. 1 depicts schematically a combustion installation comprising a particle- separating flue gas cooler according to the invention.

Fig. 2 depicts schematically a sectioned sideview through a tubular heat exchanger whose tubes are intended to have flue gas or process gas flowing through them with a view to separation of pollutants from the flue gas flow with simultaneous extraction of heat from it.

Fig. 3 depicts a sectioned view along the line III-III in Fig. 2.

Figs. 4 and 5 depict schematically a sideview and an end view respectively of a group of electrodes fitted in the heat exchanger tubes.

Fig. 6 depicts schematically a transverse section through the heat exchanger's tubes and electrodes and illustrates the positioning of liquid injection pipes in intermediate spaces between groups of mutually adjacent heat exchange tubes.

Fig. 7 illustrates schematically a section along the line VII-VII in Fig. 6. Fig. 1 illustrates a process in which flue gas from a biofuel boiler is led out along a flow path 2 along which it is cooled in a cooler 3 by local injection of water sprays at separate points along the flow path. The resulting flows of hot water and condensate are drawn off locally along the flue path, have correspondingly different temperature levels and are subjected to heat exchange at respective corresponding temperature levels against a flow path which has flowing through it a fluid, e.g. district heating water, which is to be heated as a primary purpose of the biofuel boiler. At the same time, coarse pollutants such as ash, soot particles and the like are washed out of the flue gas flow. After the primary energy extraction from the flue gas flow, the outgoing flue gas, which is in a saturated state, are at a temperature of about 6O⁰C and is then led into a heat exchanger 4.

The heat exchanger 4 comprises a bundle 41 of mutually separate axially parallel cylindrical tubes 42 made of thermally conductive material, e.g. acid-resistant stainless steel. The tubes are preferably of circular cross-section and are disposed substantially vertically, and the flue gas flow is distributed to the tubes 42 via an inlet box 45 and is led in at the lower ends of the tubes and led out at the upper ends of the tubes 42 to an outlet box 46, and thence to, for example, the environment. The tubes 42 are cooled externally by a forced flow 50 of a fluid such as, preferably, ambient air 58. Alternatively, the fluid may be a liquid flowing through a casing which surrounds the heat exchanger 4. The cooling air 58 is heated and its energy content is utilised in a heat consumer 56 and/or is introduced as combustion air into the boiler 1.

Each of the circular cylindrical tubes 42 has extending along its centre a respective electrode 43 which may be provided with protrusions distributed along the length of the electrode and around the circumference of the electrode. The electrodes 43 are supplied via a distribution system 44 with electric current which has a potential of, for example, 50,000 volts relative to earth potential. The tubes 42 are with advantage maintained at earth potential. The electrodes are with advantage negatively charged relative to earth potential. Each electrode is preferably so powered and designed as to generate corona effects in the gas flow along the respective tube. An electrical field is also established between the electrode and the tube wall. Particles in the gas flow passing through the respective tube will thus become charged and be deposited on the inside wall of the tube, on which water from the saturated gas condenses, l ne condensate washes down the pollutants deposited on the inside wall of the tube. The electrical field and/or the corona effects may possibly break down certain types of pollutants in the gas flow.

The condensate with accompanying solid and loose pollutants washed down in it accumulates in the box 45 and is led out to a cleaner 60 in which the condensate undergoes some form of conventional cleaning not described in more detail, after which the condensate can be recirculated to some part of the process concerned.

In a special embodiment of the invention, liquid tubes 70 with a diameter of, for example, 25 mm may be placed in respective intermediate spaces between groups of mutually adjacent condensation tubes 42 and extend parallel with the tubes 42. The liquid tubes 70 may be supplied via a water supply system and have small perforations 71 distributed along and around the tubes 70 and serving as spray nozzles for finely divided delivery of pressurised water/liquid. The resulting water sprays saturate the cooling air 58 and cool the condensation tubes 42, with the result that the cooling air acquires a power energy content at relatively low temperature and that the flue gas/process gas can be effectively cooled to a low temperature despite a relatively small heat exchange surface, while the condensate which forms on the inside of the tubes 42 is used for washing down solid pollutants which become deposited on the inside of the tubes 42. The flow direction selected for the process gas results in advantageous washdown conditions and limits recontamination of the process gas. The cooling air can be utilised, e.g. as combustion air for the fuel boiler. The liquid tubes 70 constitute a humidifier for the condenser tubes 42 and provide a high transfer rate and an advantageous large temperature difference across the wall of the condenser tubes 42.

The condenser tube heat exchanger 4 according to Figs. 1 and 2 has a heat exchange area of about 100 m² (the local wall area of the tubes 42) and the air velocity across the condenser tubes 42 is, for example, about 14 m/s.

Although the invention is described above in relation to flue gas from a biofuel boiler, it should be clear that similar process gas from a process installation can be cleaned in a manner corresponding to that described above. It should also be clear that cleaning according to the invention can be carried out on flue gas or process gas without any prior extraction of heat from the gas. For the reasons indicated above, a relatively low temperature is nevertheless necessary for the flue gas/process gas introduced into the rubes equipped with electrodes.

Claims

1. A method for separation of pollutants, especially particles, from a hot gas flow (2), whereby the gas flow is led through a plurality of parallel tubes (42) which each comprise a substantially coaxial electrode (43), a voltage is established between the electrodes and the tubes, the tubes (42) are disposed substantially vertically, the gas flow is introduced into the tubes in a substantially water-saturated state, the tubes are cooled externally by a fluid stream (58), the heat absorbed by the fluid (58) is utilised, the gas flow and the fluid stream are led through respective channels bounded by the tubes (42) in a heat exchanger (4) and are subjected to heat exchange via the walls of the tubes (42), and the condensate formed on the inside of the tubes flows down along the inside of the tube walls, is gathered and is subjected to cleaning, characterised in that the cooling fluid (58) is a gas, and that the cooling fluid (58) is moisture- saturated as it flows through channels of the heat exchanger (4) on the outside of the tubes (42) by injection of water sprays (71) in the spaces along and between groups of mutually adjacent tubes (42).

2. A method according to claim 1, characterised in that the tubes are maintained at earth potential and that the electrodes are preferably maintained at a negative potential relative to the tubes.

3. A method according to claim 1 or 2, characterised in that the heat absorbed by the fluid stream (58) is utilised in the process by which the hot gas flow (2) is generated.

4. A method according to any one of claims 1-3, characterised in that the gas flow (2) is a flue gas flow from a biofuel boiler (1) or is a hot process gas, that the gas flow is cooled, with heat extraction, to a temperature not exceeding about 7O⁰C, and that the cooled flue gas flow/process gas in a saturated state is led through the tubes.

5. A method according to any one of claims 1-4, characterised in that the saturated gas flow (2) is led upwards through the substantially vertically oriented tubes (42).

6. A method according to any one of claims 1-5, characterised in that the gas flow (2) led into the tubes is cooled to a temperature not exceeding 5O⁰C, preferably no higher than about 4O⁰C and most preferably no higher than about 3O⁰C.

7. A method according to any one of claims 1-6, characterised in that the cooling fluid comprises ambient air which is preferably utilised in the process by which the hot gas flow (2) is generated.

8. A device for separation of pollutants, especially particles from a gas flow (2), comprising a plurality of parallel pipelines (42) which each comprise a substantially coaxial electrode (43) along their length, and a voltage unit (70) by which a voltage difference is established between the tube wall and the electrode, whereby the tubes (42) are maintained at earth potential and corona effects are established at the electrodes, the tubes (42) are disposed substantially vertically, the flue gas (2) is adapted to being introduced in a saturated state at the lower portions of the tubes and being led upwards through the tubes, and means are provided for directing a cooling fluid (58) at the outside of the tubes in order to condense water on the inside of the tubes (42), with the result that pollutants dissolved in and/or accompanying the condensate are carried away with a view to separation of the pollutants from the condensate, characterised in that pipelines (70) extending parallel with and along the tubes (42) in spaces between groups of mutually adjacent tubes (42) are provided with spraying apertures (71) distributed along and round the pipelines (70), and that the pipelines (70) are connected to a source of pressurised water with a view to injection of water for spraying by water sprays in said spaces and towards the tubes in order to humidify a cooling fluid (58) which preferably comprises air.

9. A device according to claim 8, characterised by line means for transfer of the moisture-saturated cooling fluid to a heat consumer (56) or to the source of the gas flow (2), preferably a biofuel boiler (1).