Method and system for purifying a product gas formed from biomass

The invention relates to a method for purifying combustible gas, such as a product gas formed from biomass, which is contaminated with contaminants, such as tar and/or dust particles, comprising feeding oil to the contaminated gas, partially evaporating the oil through contact with the contaminated gas, and condensing said evaporated oil on a quantity of the contaminants in such a manner that said contaminants grow in size to form larger particles in the gas.

The environmentally friendly generation of energy from biomass is becoming increasingly important as a result of climate problems and the depletion of fossil energy sources. Biomass is present in biodegradable industrial and domestic waste, such as green waste and waste paper. Biodegradable products, such as cultivated plants, waste and residues from agriculture and other branches of industry, also contain biomass, for example mown grass, waste timber and prunings.

Heating biomass in a reactor to a temperature between 600-1300 ⁰ C with a substoichiometric quantity of oxygen leads to gasification of the biomass. This results in the formation of a combustible product gas or synthesis gas. The product gas is, for example, a gas mixture comprising CO, CO ₂ , H ₂ O, H _∑ , CH _t and if appropriate higher hydrocarbons. The solid carbon or charcoal which is present in the biomass is generally only gasified to a small extent. As a result, the product gas that is produced is contaminated with solid, ungasified carbon particles which take the form of dust in the product gas. In addition, ash is also formed in the product gas. The carbon and ash particles together form what is known as char. Moreover, gasification gives rise to contamination with tar in the product gas that is produced. The tars in the product gas are liquid at room temperature.

Moreover, the product gas can also be formed by pyrolysis of biomass. In the case of pyrolysis, the temperature is lower (400-700 ⁰ C) than in the case of gasification, and no oxygen whatsoever is supplied. Pyrolysis gives rise to greater levels of tar contamination in the product gas. In practice, when biomass is gasified, pyrolysis may simultaneously occur to some extent.

For downstream use of the combustible product gas, for example in a gas turbine for generating electricity, it is usually necessary to lower the temperature. However, the tars condense to form aerosols as a result of the contaminated product gas being cooled to below the tar and water dew point. The condensed tars and the dust in the cooled contaminated product gas then cause pipes and equipment to become dirty.

The water-soluble tars, such as phenol, can be washed out by feeding water to the contaminated product gas. However, this results in a tar-contaminated wastewater stream, which is expensive and difficult to purify.

The removal of in particular tar from a product gas is known from NLl 018 803. Here, the product gas is saturated, under substantially atmospheric conditions, with an oil that is supplied, after which said oil condenses with some of the tar. The oil and tar form droplets in the product gas, and these droplets grow in size as more oil condenses. The dust particles in the product gas likewise form growing droplets through condensation of oil.

Then, the contaminated product gas containing said particles of increased size flows to a gas/liquid separator, in which the liquid and the product gas are separated from one another. The product gas is then brought into contact with a washing oil, in which residual tar can be eliminated from the product gas. As a result, the product gas is substantially tar-free.

An object of the invention is to provide an improved method for purifying a combustible gas, such as a product gas formed from biomass.

According to the invention, this object is achieved in that the gas containing said particles of increased size is fed to a separation device which is provided with electrodes and at least one electrical leadthrough for leading through at least one electrical line to at least one electrode, and an electric field is applied between the electrodes, by means of which electric field said particles of increased size are electrically charged and removed from the gas, and the electrical leadthrough in the separation device is protected from becoming dirty.

According to the invention, the gas containing the particles of increased size is fed to a separation device which is designed for an electric field to be applied therein. It should be noted that evaporated oil, oil condensate, tar and/or dust can be entrained into the separation device with the gas and particles of increased size. Surprisingly, it has been found that the use of an electric field is particularly beneficial to the removal of the drops of dust and tar with oil vapour that are entrained with the gas. The electric field is applied, for example, in what is known as an electrostatic precipitator (ESP) or some other form of separation device.

The separation device, such as an ESP, has an electrical leadthrough for leading through at least one electrical line to the electrodes. The electrical line provides the power supply to the electrodes, so that a voltage difference can be applied between the electrodes. The mean voltage difference between the electrodes is, for example, 100-200 kV/m. By way of example, the absolute voltage at the electrode is 40-45 kV, while the distance between the electrodes is approximately 20 cm.

The electrical leadthrough in the separation device is located in a particularly aggressive environment - the presence of tars, dust and oil condensate can lead to the electrical leadthrough becoming dirty. This in turn has the risk of a leakage current to earth or a short circuit occurring. Therefore, according to the invention the electrical leadthrough is protected from becoming dirty, for example as a result of tar, dust and/or oil condensate, i.e. soiling of the electrical leadthrough is counteracted. Although the electrical leadthrough does to some extent become dirty during operation, this soiling is reduced by the protection provided according to the invention, with the result that the separation device is easy to maintain and has a relatively long service life.

The electrical leadthrough can be protected from becoming dirty in various ways.

By way of example, the condensation of the oil takes place at a temperature above the water dew point of the gas, with the temperature of the gas in the separation device being kept above said water dew point, and the electrical leadthrough being heated to a temperature that lies above the temperature of the gas in the separation device in order

to protect the electrical leadthrough in the separation device from becoming dirty.

The water dew point of the gas is the temperature at which condensation of water begins to occur as a result of cooling of the gas. The water dew point is dependent on the pressure. Under atmospheric conditions, the water dew point is for example between 50-100 ⁰ C, in particular between 50-80 ⁰ C, such as 60-70 ⁰ C. The process in the separation device is carried out at a temperature above the water dew point, i.e. without the presence of condensed water. This prevents the formation of a tar/water mixture.

The gas temperature in the separation device is above the water dew point. As a result of the electrical leadthrough temperature being heated to above the gas temperature, soiling of the electrical leadthrough with tar, oil condensate and/or dust is counteracted.

In this case, the electrical leadthrough is preferably heated to a temperature that is at least 1O ⁰ C, preferably 20 ⁰ C, above the temperature of the gas in the separation device. This leads to an effective reduction in the soiling of the electrical leadthrough. If the separation device is operated under atmospheric conditions, the electrical leadthrough is heated, for example, to a temperature between 100-180 ⁰ C, in particular 120-150 ⁰ C. This makes the structure of the separation device simple and reliable.

To protect the electrical leadthrough from becoming dirty, it is also possible for a tar- free gas stream to be passed over the electrical leadthrough in the separation device. The tar-free gas stream is preferably at a temperature that is higher than the temperature of the combustible gas. The optionally heated tar-free gas flow over the electrical leadthrough removes any precipitated tar condensate and/or condensed tar/oil from the electrical leadthrough. The tar-free gas flow may be formed by purified gas from the process itself or by a gas which is supplied from outside the process. The passing of a tar-free gas flow over the electrical leadthrough can be used separately from or in combination with the heating of the electrical leadthrough.

The condensing of the evaporated oil on a quantity of contaminants according to the invention takes place at a temperature that is above the water dew point of the gas. In addition, the electrodes of the separation device can be cleaned by the oil in the gas

substantially without the presence of water. The oil in the gas then also acts as a washing agent for washing the electrodes of the separation device.

The invention also relates to a system for purifying a combustible gas, such as a product gas which is formed from biomass, which is contaminated with contaminants, such as tar and/or dust particles, comprising an oil condenser device, which is provided with a first feed for the contaminated gas, a second feed for oil, a first discharge for the gas and a second discharge for oil, which oil condenser device is designed to partially evaporate the oil which is supplied through contact with the contaminated gas and to condense said evaporated oil on a quantity of the contaminants in such a manner that said contaminants grow in size to form particles of increased size. According to the invention, the first discharge of the oil condenser device is connected to a separation device that is provided with electrodes for applying an electric field, by means of which the particles of increased size are electrically charged and removed from the gas, and at least one electrical leadthrough for leading through at least one electrical line from a power supply located outside the separation device, to the electrodes in the separation device and the separation device comprises protection means for protecting the leadthrough from becoming dirty.

The electrical leadthrough can be protected in various ways. In one embodiment, the protection means comprise heating means which are designed to heat the electrical leadthrough to a temperature that is above the temperature of the gas in the separation device, for example to a temperature that is at least 10 ⁰ C, preferably 20 ⁰ C, above the temperature of the gas in the separation device.

As an alternative or in combination, the protection means may comprise one or more feed openings for feeding an optionally heated tar-free gas stream over the electrical leadthrough.

The invention also relates to a method for treating biomass, comprising gasifying said biomass in a reactor to form a product gas that is contaminated with contaminants, such as tar and/or dust particles, purifying the contaminated product gas, which purification comprises feeding oil to the contaminated product gas, partially evaporating the oil

through contact with the contaminated gas, and condensing said evaporated oil on a quantity of the contaminants in such a manner that said contaminants grow in size to form particles of increased size in the product gas, wherein the gas containing said particles of increased size is fed to a separation device that is provided with electrodes, and an electric field is applied between the electrodes, by means of which said particles of increased size are electrically charged and removed from the product gas. In this case, the separation device may have at least one electrical leadthrough for leading through at least one electrical line to at least one electrode, which electrical leadthrough in the separation device is protected from becoming dirty. By way of example, the electrical leadthrough is protected as described in Claims 2-12.

The invention will now be explained in more detail by way of an example that is illustrated in the drawing, in which:

Figure 1 shows a schematic process diagram of a method for treating biomass according to the invention.

Figure 2 shows a side view, in cross section, of an electrical leadthrough of a separation device according to the invention.

Figure 3 shows a cross-sectional view on IH-III in Figure 2. Figure 4 shows a side view, in cross section, of a second embodiment of a separation device according to the invention.

Figure 1 diagrammatically depicts an example of a method and system for treating biomass according to the invention. The method and system for purifying a product gas formed from biomass according to the invention form a part thereof.

Reference numeral 1 denotes a reactor. The reactor 1 has a first feed 2 and a second feed 3, which are diagrammatically indicated by arrows in Figure 1. A material that is to be gasified, for example biomass, is fed to the reactor 1 via the first feed 2. At the same time, a fluid comprising oxygen, for example air, flows into the reactor 1 via the second feed 3. The quantity of air supplied is such that a substoichiometric quantity of oxygen is present in the gasifier 1, i.e. the reactor 1 forms a low-oxygen environment. The biomass is heated in the reactor 1 to a temperature between 600-1300 ⁰ C, for example approximately 850 ⁰ C. This leads to gasification and possibly pyrolysis of the

biomass, forming a combustible product gas or synthesis gas. The product gas is a gas mixture comprising CO, CO ₂ , H ₂ O, H2, CH4 and possibly higher hydrocarbons.

The water dew point of this product gas is, for example, approximately 6O ⁰ C. However, the water dew point may be any temperature between 50-100 ⁰ C and in particular between 50-8O ⁰ C. The tar dew point of the product gas is considerably higher, for example between 120-400 ⁰ C. The tar dew point of the product gas is dependent on the gasification in the reactor 1. The tar dew point of the product gas is generally between

300-400 ⁰ C. The hot product gas contains contaminants, such as gaseous tar and dust particles. The dust particles comprise solid carbon and ash (char).

The reactor 1 has a discharge 5. The contaminated product gas flows via the discharge 5 to a first cyclone 6. In the cyclone 6, relatively large solid particles are separated out of the product gas. These particles comprise, for example, ungasified biomass and/or grains of sand originating from a fluidized bed in the reactor 1. The particles which are separated out are returned to the reactor 1 via a line 7. The product gas flows out of the cyclone 6 to a cooler 8, in which the product gas is cooled, for example to a temperature of 38O ⁰ C.

Then, the product gas flows to a second cyclone 10, in which particles smaller than those separated out in the first cyclone 8 are removed from the product gas. From the second cyclone 10, the product gas flows to an oil condenser device 12.

The oil condenser device 12 has a first feed 11 for the product gas. In the exemplary embodiment illustrated in Figure 1, this feed 11 is located at the bottom side of the oil condenser device 12. In addition to the tar contamination, the hot product gas which is supplied also still comprises fine solid particles, for example of a size of 0.1 μm.

The oil condenser device 12 has a second feed 14 for supplying oil at a temperature that is lower than that of the product gas. This feed 14 in Figure 1 is located at the top side of the oil condenser device 12. The temperature of the oil is higher than the water dew point of the product gas, for example approximately 70 ⁰ C. As a result, tar in the product gas cannot dissolve in water, which would result in a waste stream that is

difficult to purify. The oil supplied is preferably a tar oil, i.e. a mixture of aromatic compounds. In particular, the tar oil comprises tars corresponding to the tars which are present in the gas in the form of contaminants.

The product gas and the oil are in countercurrent with one another in the oil condenser device 12. While the product gas flows through the oil condenser device 12 from the bottom upwards, the oil that is supplied rains down on it. The product gas is saturated with the oil in the oil condenser device 12. As a result of the relatively cold oil coming into contact with the hot product gas, the oil is partially evaporated to form oil vapour. As the oil flows further downwards through the oil condenser device 12, the quantity of oil vapour increases. The temperature is then between the water dew point and the tar dew point of the product gas. As a result of supersaturation, said oil vapour condenses on the tar and dust particles in the product gas flowing upwards. This results in the formation of droplets which grow in size to form particles of increased size. These particles have a diameter of, for example, 1 -2 μm.

The oil condenser device 12 has a first discharge 15 for discharging the oil-saturated product gas containing the particles of increased size. The temperature of the product gas in the discharge 15 has dropped as a result of heat exchange with the oil, for example to 70 ⁰ C. The oil condenser device 12 therefore forms a cooling device. The oil condenser device 12 also has a second discharge 16 for discharging liquid oil.

Then, the oil-saturated product gas containing the particles of increased size flows to a separation device 18 for removing the particles of increased size from the product gas. The separation device 18 comprises two electrodes 20, 22 (see Figure 2 and 3). The electrode 20 comprises hexagonal tube parts which are joined to one another. The electrode 22 has a plurality of elongate rods which extend centrally inside the tube parts. The separation device 18 forms what is known as an electrostatic precipitator (ESP), in particular a wet ESP.

The separation device 18 has an electrical leadthrough 24 which is used to connect the electrodes 20, 22 to a power supply 21 located outside the separation device 18. In the present exemplary embodiment, the electrical leadthrough 24 is formed by dishes

stacked on top of one another. The electrodes 20, 22 form electrical lines which extend through the electrical leadthrough 24. The power supply 21 applies a voltage difference between the electrodes 20, 22. The particles of increased size - the drops - in the product gas are then charged and drawn out of the product gas under the influence of the voltage difference.

To protect the electrical leadthrough 24 from being soiled by tar and/or oil condensate, the separation device 18 has protection means. The protection means comprise one or more heating elements 23 for heating the electrical leadthrough 24 to a temperature that is at least 1O ⁰ C, preferably 20 ⁰ C above the temperature of the product gas. The temperature of the electrical leadthrough 24 in the present exemplary embodiment is between 120-150 ⁰ C.

The protection means also comprise feed openings 27 for feeding a tar-free gas stream over the electrical leadthrough 24. Further heating elements are preferably provided for heating the tar-free gas stream to a temperature that is higher than the temperature of the product gas (not shown). The tar-free gas stream is derived, for example, from the process itself, as will be explained below.

The separation device 18 comprises a first discharge 25 for discharging the drops of tar and/or dust with condensed oil that have been separated off. This stream is first of all combined with the liquid oil from the second discharge 16 from the oil condenser device 12. It then flows via a pump 28 and a cooler 29 to a filter 30. In the filter 30, the dust particles and liquid tar are separated from the oil. The dust particles and the tar form waste streams 17, 19. The oil can then be returned to the feed 14 of the oil condenser device 12. The oil that is fed to the oil condenser device 12 is therefore derived from the process itself, i.e. the oil is recirculated.

The separation device 18 has a second discharge 26 for discharging the product gas. This product gas is substantially dust-free. The temperature is substantially unchanged, in this exemplary embodiment about 70 ⁰ C. The product gas is then fed to an absorber device 32. On its bottom side, the absorber device 32 has a first feed 34, through which the product gas flows into the absorber device 32.

The absorber device 32, at its top side, comprises a second feed 35 for supplying fresh oil. The temperature of this oil is higher than the water dew point of the product gas, i.e. the conditions prevailing in the absorber device 32 are such that water does not condense. As a result, mixing of water and tar cannot occur. However, the temperature of the oil is lower than the tar dew point of the product gas. In this exemplary embodiment, the temperature of the oil supplied is approximately equal to the temperature of the product gas, i.e. approximately 70 ⁰ C.

The clean oil functions as a washing oil which moves through the absorber device 32 from the top downwards. The substantially dust-free product gas and the oil are in countercurrent contact with one another. The gaseous tar compounds in the product gas are absorbed as a result. Further tar is dissolved in the oil.

The absorber device 32 has a first discharge 37 for discharging the purified product gas. This combustible product gas is substantially dust-free and tar-free. Consequently, this product gas is suitable for use in, for example, a downstream gas turbine. It is possible for a quantity of this tar-free product gas to be returned to the feed openings of the separation device 18 for protecting the electrical leadthrough 24 from becoming dirty and/or for removing deposits on the electrical leadthrough 24.

The oil contaminated with tars flows out of the absorber device 32 via a second discharge 38. The second discharge is connected to an oil purification device 46 via a line 41. A pump 40 and a safety filter 42 for removing any remaining dust particles are incorporated in the line 41. The line 41 also comprises a heater 43 for heating the oil, for example to a temperature of approximately 180 ⁰ C.

The oil purification device 46 has a first feed 48 and a second feed 50. The contaminated hot oil flows into the oil purification device 46 via the first feed 48 at the top side. A fluid is fed to the bottom of the oil purification device 46 via the second feed 50. In the present exemplary embodiment, the fluid is air, but it may also, for example, be steam or another (rinsing) fluid.

The oil and the air are in countercurrent with one another. The air takes up tar from the oil as a result of the contact between the oil and the air. The air containing tar is discharged via a first discharge 51. This stream may also entrain a small quantity of oil. This stream is preferably returned to the reactor 1, in which the components can be gasified (not shown). The purified oil leaves the oil purification device 46 via a second discharge 52.

The clean oil discharged via the second discharge 52 is connected to the feed 35 of the absorber device 32 via a line 54. The line 54 comprises a pump 58 and a cooler 56, which lowers the temperature of the clean oil. The cooler 56 and the heater 43 are preferably in heat-exchanging contact with one another. This is of benefit to the overall efficiency.

Figure 4 shows a second embodiment of the separation device according to the invention, in which similar components are denoted by the same reference numerals.

The separation device 18 shown in Figure 4 likewise forms an electrostatic precipitator

(ESP), in particular a wet ESP. This separation device 18 differs from the separation device shown in Figure 2 in that two electrical leadthroughs 24 are provided. A supporting structure for electrical lines is provided between the electrical leadthroughs 24. Electric voltage is fed to the electrodes via these electrical lines.

In this exemplary embodiment, each electrical leadthrough 24 is formed by dishes stacked on top of one another. Each electrical leadthrough 24 is protected from becoming dirty as a result of tar and/or oil condensate by the use of one or more heating elements 23. The heating element 23 of each electrical leadthrough 24 heats said electrical leadthrough 24 to a temperature of, for example, 120-150 ⁰ C. At the same time, a tar-free gas stream is passed over the electrical leadthrough 24 via the feed openings 27.

In the exemplary embodiment illustrated in Figure 4, the dishes stacked on top of one another for each electrical leadthrough 24 are located at a relatively great distance from the gas flow which leaves the separation device 18 via the discharge 26. This delays soiling of the electrical leadthrough 24. Moreover, the supporting structure is

favourable from a structural perspective.

Of course, the invention is not restricted to the exemplary embodiments which have been described above for removing tar and/or dust from a contaminated product gas. Numerous variants are possible and certain parts and/or components of the system/process shown in the figures can be omitted or replaced with other separation and/or mixing devices which are known in the prior art. In addition, it is possible for some of the components which are described separately here to be integrated or equally to be detached into separate equipment. Also, the purification described above is suitable for any combustible gas, i.e. the purification according to the invention is not restricted to a product gas formed from biomass. This purification of gas and the use of an ESP during this purification can, for example, also be used in the coke oven industry and in petrochemistry.