WO2013068643A1

WO2013068643A1 - A method and a system for producing liquid fuel from biomass

Info

Publication number: WO2013068643A1
Application number: PCT/FI2012/051085
Authority: WO
Inventors: Jari Hangasluoma; Peter RÖGER; Pekka Jokela
Original assignee: Upm-Kymmene Corporation
Priority date: 2011-11-09
Filing date: 2012-11-06
Publication date: 2013-05-16
Also published as: FI20116107L

Abstract

A method for producing liquid biofuel from biomass. The method comprises gasifying the biomass (100) in an elevated temperature to produce synthesis gas (200) and post processing at least part of the synthesis gas (200) to the liquid biofuel. The post processing comprises cooling the synthesis gas (200) before filtering, and filtering the synthesis gas to produce filtered synthesis gas. Synthesis gas is cooled by recycling recycled gases (205) and/or by quenching. In addition, use of a cooling device (218) in the described method. The cooling device (218) comprises at least one of a quencher (210), and a gas mixer. Furthermore, a system for producing liquid biofuel from biomass (100). The system comprises a gasification reactor (150) for gasifying biomass (100) at an elevated temperature to produce synthesis gas (200), a cooling device (218) arranged to cool the synthesis gas (200), and a filtration unit (220) arranged to filter the cooled synthesis gas to produce filtered synthesis gas.

Description

A method and a system for producing liquid fuel from biomass

Field of the Invention The invention relates to a process for producing liquid biofuel from solid biomass. In the process synthesis gas is produced by gasifying the solid biomass and the synthesis gas is post processed to produce biofuel. In post processing the synthesis gas is reformed and before reforming the synthesis gas is cleaned e.g. by filtration.

Background of the Invention

Liquid fuels are commonly used in many areas of human life including heating, transportation and producing electricity. Liquid fuels are often made of fossile oil containing substances by distillation and cracking. However, sustainable energy sources have received a lot of interest in recent years due to environmental changes— observed or predicted— such as global warming. One possibility to use liquid fuels and answer the environmental issues is to produce liquid fuels from biomass.

Gasification is a promising process for converting solid biomass to gaseous or liquid fuel. In gasification, solid biomass is first converted to a synthesis gas, which can be further processed to produce liquid fuel or gaseous fuel or both liquid and gaseous fuels. The gasification is done at an elevated temperature, e.g. 850 Ό. The synthesis gas thus pr oduced typically comprises tars and methane. Both tars and methane influence the possibilities of utilizing the synthesis gas, and therefore, their content may be reduced by reforming. In reforming the synthesis gas, oxygen, steam, and synthesis gas are reacted at elevated temperature. In the end, the reformed synthesis gas is sent to a Fischer-Tropsch (FT) synthesis process to produce liquid biofuel. At some phase of the process, the synthesis gases or reformed synthesis gases need to be cleaned, since the FT -process is sensitive to impurities such as solid particles, e.g. ash. Even if some of the process steps are known in the art, efficient cleaning of synthesis gas and recycling of intermediate process products are still problematic in the process. The yield of the process depends on the order in which different process steps are performed. Moreover, the overall consumption of energy and raw materials depends on the recycling of intermediate process products. Still further, the investment costs for a system for producing liquid fuel from solid biomass depends on the complexity of the process equipment, and the complexity of the process equipment may depend on the process itself.

Summary of the Invention

A method for producing liquid fuel from solid biomass is disclosed. The biofuel yield of the process is relatively high, partly due to efficient recycling of intermediate process products and partly due to the order of different process steps. In the method, synthesis gas is produced from solid biomass in a gasification reactor. According to the invention, the synthesis gas is cleaned by filtration before a reforming process to protect reformer-reactor and reformer-catalyst from fouling (deposition of particulate matter such as ash, soot, dust), thus improving the yield and stability of the reforming process. However, the synthesis gas is hot and filters can only be operated up to a certain maximum temperature. According to the invention, the synthesis gas is cooled before filtration. The features of the method are described in the independent claim 1 .

In an embodiment, the synthesis gas is cooled by recycling gases in the process, and mixing recycled gases with synthesis gas. The recycled gases are cooler than the synthesis gas, and thereby mixing recycled gas with synthesis gas cools the synthesis gas.

In another embodiment, the synthesis gas is quenched by spraying liquid onto the synthesis gas before filtration. At least part of the liquid evaporates, thereby cooling the synthesis gas. In addition to quenching, recycled gases may be mixed with the synthesis gas to further cool the synthesis gas. Quenching reduces the amount of recycled gases in the process, and thus decreases the size and complexity of process equipment. These reductions show also as smaller investment costs for the system for producing liquid biofuel from biomass. Furthermore, if water is used for quenching, the generated additional steam in the synthesis gas has beneficial effects in the reforming process. Furthermore some of the heat picked up by the quench water can be recovered for drying the raw biomass.

These and other aspects of the invention are described in the independent claims 2-13.

According to another aspect of the invention, a cooling device can be used in the process, as described in the independent claim 13.

In addition, a system for producing liquid biofuel from solid biomass has been disclosed. The system has the features as described in the independent claim 16, and other aspects are described in the independent claims 17-26.

Description of the Drawings

Figure 1 shows a process for producing biofuel from biomass, Figure 2 shows more details of the process of Fig. 1 ,

Figure 3a shows a process of gasifying biomass to produce synthesis gas in a process for producing liquid biofuel from solid biomass, Figure 3b shows quenching, filtration, reforming, and cooling of synthesis gas in a process for producing liquid biofuel from solid biomass,

Figure 3c shows synthesis gas shifting, synthesis gas scrubbing, and subsequent process steps in a process for producing liquid biofuel from solid biomass,

Figure 4a shows recycling gases to cool synthesis gas in a process for producing liquid biofuel from solid biomass, Figure 4b shows quenching and recycling gases to cool synthesis gas in a process for producing liquid biofuel from solid biomass, Figure 4c shows quenching and recycling gases to cool synthesis gas in another process for producing liquid biofuel from solid biomass, Figure 4d shows optimizing the amount of quenching liquid ,

Figure 4e shows a process for producing liquid biofuel from solid biomass, wherein only the tail gases are recycled, Figure 5a shows an embodiment of a quencher 210,

Figure 5b shows another embodiment of a quencher 210, and

Figure 5c shows a third embodiment of a quencher 210.

Detailed Description of the Invention

Gasification is a promising process for converting solid biomass to fuel. In gasification solid biomass is converted to synthesis gas, which can be further processed to produce liquid fuel or gaseous fuel or both liquid and gaseous fuels. The present invention relates especially to a process for producing liquid fuels from wood-based solid biomass. Referring to Fig. 1 the process comprises gasifying solid biomass to synthesis gas and post processing the synthesis gas to biofuel. Referring to Fig. 2, in an embodiment the post processing comprises conditioning the synthesis gas to a product gas, Fischer-Tropsch processing the product gas to product fluid, and upgrading the product fluid to biofuel.

The conditioning comprises

- cleaning the synthesis gas by using at least one filter to produce filtered synthesis gas,

- cooling the synthesis gas prior to filtering, and

- reforming the filtered synthesis gas to reduce the amount of tar and methane in the synthesis gas. The conditioning may further comprise at least one of

- cleaning the synthesis gas e.g. by using a cyclone,

- recycling gas and mixing recycled gas with synthesis gas to cool the synthesis gas,

- using a quenching liquid to cool the synthesis gas by quenching,

- cooling the reformed synthesis gas to produce high pressure steam,

- adjusting the ratio of hydrogen (H₂) to carbon monoxide (CO) of the reformed synthesis gas,

- scrubbing the reformed synthesis gas to clean the synthesis gas, and

- conveying the scrubbed synthesis gas to compression and acid gas removal processes to produce the product gas.

The whole post processing may comprise

- the conditioning phase,

- Fischer-Tropsch (FT) processing of the product gas to produce a product fluid,

- recovering heat from a FT processing unit to produce steam,

- using the produced steam in at least one preceding process step, and - upgrading the product fluid to produce the biofuel, e.g. by means of at least one of hydroprocessing and fractionating.

In an embodiment, gasification is performed in a fluidized bed to increase the yield of synthesis gas from the gasification process. Moreover, the synthesis gas is cleaned before reforming to improve the yield and stability of the reforming process. However, gasification is typically done at an elevated temperature. Therefore, cleaning of synthesis gas poses a problem since e.g. there is no filter, which can withstand such high temperatures. Furthermore, excessive cooling of synthesis gases before reforming is not beneficial and should therefore be optimized, since the synthesis gas is reformed typically at an elevated temperature.

According to the invention, the synthesis gas is cleaned by filtration before reforming. In an embodiment, synthesis gas is cooled by quenching before filtration. Quenching is done by spraying quenching liquid onto the synthesis gas in a quencher before filtration. In another embodiment, synthesis gas is cooled by mixing recycled gas in the process. Recycled gas may comprise at least one of: scrubbed synthesis gas, tail gas 397 of an acid gas removal phase, FT tail gas 395, and off gas 410, as will be discussed in more detail later. In other embodiments, synthesis gas is cooled both by quenching and by gas mixing, in either order.

Synthesis gas is cooled in a cooling device 218. The cooling device 218 comprises at least one of a quencher and a gas mixer. If the cooling device comprises a quencher and a gas mixer, the cooling device comprises means for conveying synthesis gas from the quencher to the gas mixer or from the gas mixer to the quencher.

Figs. 3a-3c describe in more detail an embodiment of the invention. Referring to Fig. 3a, biomass 100, e.g. wood based biomass is stored in a container 102. The container is located in normal atmosphere and therefore the container 102 contains typically air and the biomass 100. Air on the other hand comprises nitrogen. From the container 102 biomass is fed using a lock hopper mechanism to another container 108. The lock hopper mechanism is used to remove nitrogen from the biomass-air mixture, since the presence of nitrogen has some disadvantages in the later process steps. The lock hopper mechanism is also used to recycle carbon containing gases, namely carbon dioxide CO2, available from later process steps. Recycling of CO2 is advantageous, as liquid biofuel comprises carbon.

The lock hopper mechanism comprises two air lock valves 104 and a third container 106. Biomass-air mixture is first fed to the container 106. At least some air is removed from the container by feeding carbon dioxide (CO2) 120 to the container 106. Carbon dioxide replaces at least some of the air in the container 106. The biomass-air-CO2 mixture is then fed to the container 108. Carbon dioxide 120 may be used also in the container 108 to remove air from the container 108. In addition to decreasing the nitrogen content, CO2 may be fed to the process also to increase the carbon content of the biomass-air- CO2 feed. CO2 may be available from an Acid Gas Removal -process. From the container 108 biomass 100 is fed with a feeder 122 through a channel 135 to a gasification reactor 150. The gasification reactor is a fluidized bed reactor. In addition to biomass 100, bed material 130 is fed to the gasification reactor 150. Bed material 130 is fed to the process from a container 132 through the channel 135. Preferably the bed material 130 comprises dolomite, since this reduces the tar content of the synthesis gas 200. More preferable the bed material consists essentially of dolomite.

The gasification reactor 150 is a fluidized bed reactor. The fluidized bed reactor comprises means for forming a fluidized bed of biomass 100 and bed material 130. The means may comprise nozzles through which fluidizing gases are fed to the reactor from below. Biomass 100 and bed material 130 are arranged in a fluidized state by feeding oxygen 140 and steam 142 to the gasification reactor 150. Oxygen is fed to the reactor in order to facilitate burning of some of the biomass. Burning releases energy from the biomass 100 and thus heats the gasification reactor 150 and the material in the reactor. The temperature in the gasification reactor 150 may be e.g. about 850 Ό. The gasification reactor may be pressurized . The (absolute) pressure in the gasification reactor may be e.g. 10 bar. Steam 142 (H₂O) is fed to the process to increase the hydrogen (H₂) content in the gasification reactor and for later process steps. As the synthesis gas 200 will be conveyed later to a reformer 240 and eventually fed into a FT -process, the hydrogen (H₂) and carbon monoxide (CO) content of the synthesis gas 200 is increased already in the gasification reactor 150 by fuidizing the bed material 130 and the biomass 100 in the reactor 150 using both steam 142 and oxygen 140 as the fluidizing medium. Only small amounts of oxygen is used on one hand to facilitate burning and on the other hand to facilitate the formation of synthesis gas. Bottom ash 155 is removed from the gasification reactor 150. Bottom ash 155 comprises ash (burned biomass), impurities (such as stones or metal pieces), and bed material 130. Synthesis gas 200 is led from the gasification reactor 150 through a cyclone 160. Cyclone 160 separates synthesis gas 200 from solid materials such as bed material, ash (burned biomass), and biomass. Solid material is fed back to the gasification reactor through the channel 165. Synthesis gas 200 is conveyed to subsequent process steps. Solid material is removed from the synthesis gas in the cyclone 160 to clean the synthesis gas 200 before filtration, at least to some degree.

Referring to Fig. 3b synthesis gas 200 is filtered in a filtration unit 220 and reformed in a reformer 240. It has been noticed that the yield and the stability of the reforming process step increase, if the synthesis gas is filtered before reforming. Moreover, since reforming requires a relatively high temperature, the synthesis gas is advantageously not cooled between the filtration unit 220 and the reformer 240. Therefore, the filtered synthesis gas may be conveyed to the reformer 240 such that the temperature of the filtered synthesis gas leaving the filtration unit 220 is essentially same as the temperature of the filtered synthesis gas entering the reformer 240, to minimize heating of the filtered synthesis gas in the reformer 240. The filtered synthesis gas may also be heated before the reformer 240. In that case the filtered synthesis gas is conveyed to the reformer 240 such that the temperature of the filtered synthesis gas leaving the filtration unit 200 is less than the temperature of the filtered synthesis gas entering the reformer 240, to minimize heating of the filtered synthesis gas in the reformer 240. In that case the system comprises a heater arranged to heat the filtered synthesis gas. The heater (not shown in Fig. 3b) is arranged to heat the filtered synthesis gas before the reformer. The heater may be e.g. a heat exchanger. The heater may be arranged to heat the filtered synthesis gas using the heat of at least one of: the reformer 240, the synthesis gas in the reformer, the reformed synthesis gas, the synthesis gas 200 before quenching, the material in the cyclone, the cyclone 160, the material in the gasifier, and the gasifier 150.

As the temperature of the synthesis gas 200 after the gasification reactor 150 is relatively high, e.g. 850 Ό, filtration of synt hesis gas poses a technical problem: the filtration unit 220 may not withstand such a high temperature. Referring to Fig. 3b, the filtration unit 220 may comprise filter plates 225 to filter the synthesis gas. The filter plates 225 may comprise metallic plates for filtering the synthesis gas. Such metallic plates are typically designed to operate at maximum temperatures of 600-800 Ό, dep ending on the metal used in the filter plates 225. Therefore, the synthesis gas needs to be cooled before filtration. In the embodiment shown in Fig. 3b, synthesis gas 200 is cooled by quenching the synthesis gas 200 in a cooling device 218. The cooling device 218 comprises a quencher 210. In the quencher 210, the synthesis gas is cooled by spraying quenching liquid 215 onto the synthesis gas. In the embodiment, synthesis gas 200 is cooled also by mixing (quenched) synthesis gas with recycled gas 205. The recycled gas 205 comprises only scrubbed synthesis gases or at least one of: a part of scrubbed synthesis gas, tail gas 397 of an acid gas removal phase, FT tail gas 395, and off gas 410, as will be discussed in more detail later. As will be discussed later, synthesis gas may be cooled only by mixing recycled gas, only by quenching, or both by quenching and mixing recycled gas, the last option shown in Fig. 3b.

The temperature of the recycled gas 205 is much lower than the temperature of the synthesis gas. E.g. the temperature of scrubbed synthesis gas may be in the range of 30 - 150 Ό. Even if not depicted i n Fig. 3b, recycled gas 205 may be mixed with synthesis 200 gas before or after the quencher 210. Recycled gas 205 may be obtained from later phases of the process, as will be discussed later. In one embodiment the recycled gas comprises only scrubbed synthesis gas and recycled FT tail gas 395 is introduced into the gasifier 150.

The mass ratio of recycled gas 205 to synthesis gas 200 depends on the temperature of the gases as well as the maximum allowed temperature of the gas filtration unit 220. If the ratio is high, i.e. a lot of gas needs to be recycled, piping for the recycled gas 205 needs a lot of space in the gasification system, and material costs for the piping become significant. Therefore the ratio of mass flow of the recycled gas 205 to mass flow of the synthesis gas 200 is preferably small. Most preferably the cooling is done only by quenching.

In Fig. 3b, the synthesis gas 200 is cooled by quenching in a quencher 210 to produce quenched synthesis gas. In the quenching step, quenching liquid 215 is sprayed onto the synthesis gas 200. Preferably the evaporation temperature of the quenching liquid 215 is so low that the quenching liquid 215 is evaporated when the quenching liquid 215 is sprayed onto the synthesis gas 200. Preferably the liquid 215 comprises water. The quenching liquid 215 may comprise at least 90 % of water. The quenching liquid may consist essentially of demineralized water. As the synthesis gas 200 is quenched in the quencher 210 by spraying quenching liquid 215 onto the synthesis gas 200, some of the quenching liquid 215 is evaporated. Preferably all the quenching liquid 215 is evaporated. Evaporation of the quenching liquid 215 effectively converts sensible heat into latent heat, and therefore, the temperature of the synthesis gas 200 decreases significantly in the quencher 210.

The quenched synthesis gas may be further cooled by mixing it with recycled gas 205, as depicted in Fig. 3b. Mixing with cool gas is referred to as gas cooling. However, in another embodiment (cf. Fig. 4c), the gas cooling is performed before quenching, The process comprises at least one of gas cooling and quenching. In case the process comprises both the steps, the steps may be in either order. The synthesis gas after these steps will be referred to as filterable synthesis gas. In an embodiment, scrubbed synthesis gas is not recycled. In this embodiment, other gases may be recycled in the process. If such gases are recycled, at least one of the tail gases from the FT process 395, the tail gases 397 from the acid gas removal phase, and the off gas 410 from the upgrading are recycled. If only the tail gases 395, 397 and off gases 410 are recycled, the ratio of mass flow of the recycled gas 205 to mass flow of the synthesis gas 200 may be less than 10 %, for example about 5 %. If only the tail gases and the off gases are recycled, the FT tail gases 395 and the off gases 410 may be recycled to the gasifier 150 and acid gas removal tail gases 397 may be recycled to a container 108, 106 for biomass.

In the filtration unit 220 solid particles 228, such as ash, are filtered from the filterable synthesis gas 200. These solid particles 228 are removed from the filtration unit. The filterable synthesis gas entering the filtration unit 220 is preferably dry, i.e. the filterable synthesis gas does not comprise liquid droplets of quenching liquid 215. In the filtration unit 220 the filterable synthesis gas is filtered to filtered synthesis gas. From the filtration unit 220 the filtered synthesis gas is led to a reformer 240. After the filtration unit 220, the temperature of the filtered synthesis gas may be below 800 Ό and as an example 700 Ό. However, reforming of synthesis gas is performed in a higher temperature. For example the temperature in the reformer 240 may be between 900 - 950Ό. To reduce the need for heating of synthesis gas before reformer 240, the synthesis gas is preferably not cooled further after the quencher 210 or the point 212 of recycle gas 205 cooling, whichever is the later in the process, and before the filtration unit 220. The system may comprise a gas mixer at the point 212 of recycle gas 205 cooling. The method may comprise using a gas mixer to cool the synthesis gas 200.

The filterable synthesis gas is conveyed to the filtration unit 220 such that the temperature of the filterable synthesis gas leaving the quencher 210 or the point 212 of recycle gas 205 cooling, whichever is the later in the process, is essentially the same as the temperature of the synthesis gas in the filtration unit 220. Moreover, as discussed above, the filtered synthesis gas may be conveyed from the filtration unit 220 to the reformer 240 such that the temperature of the filtered synthesis gas leaving the filtration unit 220 is essentially same as, or less than, the temperature of the filtered synthesis gas entering the reformer 240.

To increase the temperature of the filtered synthesis gas in the reformer 240, part of the synthesis gas is burned in the reformer 240. To facilitate burning, oxygen 230 (O₂), is fed to the reformer 240. Moreover, to increase the hydrogen content in the reformed synthesis gas and to prevent carbon formation on the reformer catalyst and thus facilitate the reforming process, steam 232 (H₂O) is fed to the reformer 240. Filtered synthesis gas is reformed in the reformer 240 to decrease the amount of tars and methane in the filtered synthesis gas. Furthermore, the filtered synthesis gas is reformed to reformed synthesis gas. It is noted, that at elevated temperatures steam (H₂O) and carbon monoxide (CO) may react to produce carbon dioxide (CO₂) and hydrogen (H₂). Still further, in the reformer 240, the tar and methane of the synthesis gas may react with steam to produce carbon monoxide and hydrogen. From the reformer 240 the reformed synthesis gas is led to a heat exchanger 250. In the heat exchanger 250, the reformed synthesis gas is cooled, and the heat of the reformed synthesis gas is recovered to a fluid. The fluid may comprise water. It may also comprise steam. As an example, in Fig. 3b water 252 is fed to the heat exchanger 250, and in the heat exchanger 250, the water 252 is evaporated to produce steam 254. Steam 254 is used for generating power, such as electricity. Steam 254 may be superheated i.e. heated above the saturation temperature. In the heat exchanger 250, the reformed synthesis gas is cooled to cooled synthesis gas. The temperature of the cooled synthesis gas may be e.g. 150-300 Ό.

Referring to Fig. 3c, the cooled synthesis gas 300 is led to a gas shifting process. The cooled synthesis gas flow 300 may be divided into a first part 302 and a second part 304. The first part 302 is led to a gas shifter 310. The second part 304 bypasses the gas shifter and is led to subsequent process steps without gas shifting. The mass ratio of the first part 302 and the second part 304 may be controlled e.g. with a valve 306. In the gas shifter 310, the ratio of hydrogen (H₂) to carbon monoxide (CO) is modified to optimize the performance of the subsequent FT process. In addition to the first part of the synthesis gas 302, steam 259 is also fed to the gas shifter 310. Steam 259 may comprise steam 399 obtained from a heat exchanger 386. After the gas shifting process, and after mixing the first part 302 and the second part 304 of the cooled synthesis gas, as denoted by 315 in the Fig. 3c, the molar ratio of H₂ to CO in the shifted synthesis gas 315 is between 2.5 to 1 and 1 .0 to 1 . Preferably the molar ratio of H₂ to CO in the shifted synthesis gas 315 is between 2.1 to 1 and 1 .8 to 1 . The mixture of the first part 302 after the gas shifter 310 and the second part 304 of the cooled synthesis gas will be called shifted synthesis gas.

The shifted synthesis gas 315 is conveyed to a scrubber 320. The scrubber comprises two scrubbing stages, a first stage 330 and a second stage 340. In the first stage 330, a (first) scrubbing solution (331 , 332) is circulated. The scrubbing solution 331 is led from the first stage 330 to a heat exchanger 335, after which the cooled scrubbing solution 332 is led to the first stage 330. The circulation of the scrubbing solution may be enabled with a pump (not shown in the figure). The cooled scrubbing solution 332 is sprayed onto the shifted synthesis gas to scrubb the shifted synthesis gas. In the scrubber 320, the scrubbing solution is essentially in liquid form to ensure scrubbing.

The (first) scrubbing solution (331 , 332) comprises water. The scrubbing solution may consist of water or it may consist essentially of water. As the scrubbing solution is cooled in the heat exchanger 335, heat is recovered from the scrubbing solution. A cool heat transfer medium 336 is fed to the heat exchanger 335 where it heats to a heated heat transfer medium 337. The heat transfer medium may comprise water. The recovered heat may be used to dry biomass 100. To dry biomass, the heated heat transfer medium may be conveyed to the container 102 (Fig. 3a), where the biomass is stored. Even if not shown in Fig. 3c, the system may comprise means for conveying the heated heat transfer medium 337 from the heat exchanger 335 to a dryer 450 (cf. Fig. 4a) to dry the biomass. The container 102 may be used as the dryer 450 for biomass. Alternatively, the dryer 450 may be located near the container 102. The system may comprise means for conveying the dried biomass from the dryer 450 to the container 102.

In the second stage 340 of the scrubber 320, a second scrubbing solution is circulated. The second scrubbing solution 341 is led from the second stage 340 to a heat exchanger 345, after which the cooled second scrubbing solution 342 is led to the second stage 340. The second scrubbing solution is sprayed onto the shifted synthesis gas to scrubb the shifted synthesis gas. In the scrubber 320, the second scrubbing solution is essentially in liquid form to ensure scrubbing. The circulation of the second scrubbing solution may be enabled with a pump (not shown in the figure). The second scrubbing solution comprises water. The second scrubbing solution may consist of water or it may consist essentially of water. The first scrubbing solution may be used as the second scrubbing solution. As the second scrubbing solution is cooled in the heat exchanger 345, heat is recovered from the second scrubbing solution. A cool heat transfer medium 346 is fed to the heat exchanger 345 where it heats to a heated heat transfer medium 347. The heat transfer medium may comprise water. The temperature of the heated heat transfer medium 347 may be e.g. 60 Ό. From the scrubber 320 condensed scrubbing solution, i.e. condensate 350, is led out. Condensate 350 is treated as liquid waste and fed to a waste water treatment plant. The condensate 350 comprises the quenching liquid 215. In the scrubber 320 shifted synthesis gas is scrubbed to scrubbed synthesis gas.

After the scrubber 320, at least some of the scrubbed synthesis gas is led to an acid gas removal step 385. In the acid gas removal step 385 the scrubbed synthesis gas is purified in an acid gas removal unit, which is a physical wash processing unit such as Rectisol®, Purisol®, or Selexol®. The acid gas removal step 385 produces product gas 400 and a tail gas 397. The tail gas 397 comprises CO₂. In an embodiment, the tail gas 397 consists essentially of CO₂, and the tail gas 397 is used as the carbon dioxide 120 (cf. Fig. 3a).

In the embodiment of Figs. 3b and 3c, a part 380 of the scrubbed synthesis gas is circulated in the process to cool the synthesis gas 200 or quenched synthesis gas before the filtration unit 220 to obtain the filterable synthesis gas. In this embodiment, the scrubbed synthesis gas is divided to a first part 390 and to a second part 380. The first part 390 is compressed and conveyed to the acid gas removal step to produce product gas 400. The second part 380 is recycled to the process and used as the recycled gas 205 (c.f. Fig. 3b). In addition, the tail gas 397 from the acid gas removal step 385 may be used as the recycled gas 205.

From the acid gas removal step 385 the product gas 400 is further conveyed to a Fischer-Tropsch synthesis process for producing product fluid 396 and FT tail gases 395 from the product gas 400. The Fischer-Tropsch synthesis process is denoted by "FT" in Fig. 3c. The product fluid 396 comprises liquid, and waxy compounds. Heat is recovered from FT processing unit with a heat exchanger 386. The heat exchanger 386 is arranged in connection with the FT processing unit, is arranged to cool the FT processing unit, and is arranged to recover heat from the FT processing unit. The heat is recovered to a heat transfer liquid, such as water 398, which is fed to the heat exchanger 386. As a result, the water evaporates to steam 399 in the heat exchanger 386. The steam 399 may be used in preceding process steps. The steam 399 may be used in at least one of following: the gasification reactor 150 as the steam 142, the reformer 240 as the steam 232, and the gas shifter 310 as steam 259. The product fluid 396 is further upgraded in an upgrading unit to produce biofuel. In addition to the product fluid 396, FT tail gas 395 is obtained from the FT process. The tail gas may comprise CO, H₂ and light hydrocarbons, e.g. methane. The FT tail gas 395 may be recycled to the process, as the recycled gas 205, or to the gasifier 150, as will be discussed later.

After FT, the product fluid 396 is upgraded to produce biofuel and off gas 410. The upgrading process comprises hydrocracking of product fluid 396 and fractionated distillation to obtain liquid biofuels such as at least one of biodiesel and kerosene. The formed waxy-compounds, i.e. FT-wax are hydrocracked into middle distillates. The hydrocracking process produces also light end hydrocarbons which are considered as the off gas 410, which also contains unconverted hydrogen. The off gas 410 may also be recycled to the process as the recycled gas 205 and /or to the gasifier 150. (not shown in the figure).

Fig. 4a shows some parts of a process for producing liquid fuel from biomass. In the process, the synthesis gas is cooled to filterable synthesis gas only by gas cooling using recycled gases 205. The recycled gases comprise gases from the scrubber 320. The FT tail gases 395 may be recycled to the gasifier 150. Alternatively or in addition, the FT tail gas 395 may be used for synthesis gas cooling. These possibilities are illustrated with the dotted lines near the symbol 395 in Fig. 4a. As shown in the figure, the tail gas 397 from the acid gas removal step 385 may be recycled as the CO2 used the containers 106 and 108 for the biomass. Even if not shown in the figure, the tail gas 397 may, alternatively or in addition, be used to cool the synthesis gas. Even if not shown in the figure, also the off gas 410 (cf. Fig. 3c) may be recycled to the gasifier 150, or used to cool the synthesis gas. If gases 395, 397, 410 are used to cool the synthesis gas, they may be mixed with the synthesis gas 200, or with other recycled gas 205, including the other of these gases 395, 397 and 410. Feeding of biomass and gasification are not shown in detail in Fig. 4a, as they are described in detail above. However, process steps between cooling of synthesis gases and FT, and recycling of different gases is shown in more detail.

In the embodiment shown in Fig. 4a, synthesis gas 200 is cooled at the point 212, where the synthesis gas is mixed with the recycled gas 205. In Fig. 4a, a part 380 of the scrubbed synthesis gas is used as the recycled gas 205. In addition all or part of the tail gases 395 and 397 and the off gas 410 may be used as the recycled gas 205 (not shown for the tail gas 397 or the off gas 410). Thus, the recycled gas 205 comprises scrubbed synthesis gas, and may further comprise at least one of the FT tail gas 395, the acid gas removal tail gas 397, and the off gas 410. The system of Fig. 4a comprises piping for conveying the recycled gas 205 from the scrubber 320 to the synthesis gas before it enters the filtering unit 220. The system may also comprise piping for conveying recycled gas 205 (i.e. the FT tail gas 395) from the FT processing unit to the synthesis gas. The system may also comprise piping for conveying recycled gas 205 (i.e. the acid gas removal tail gas 397) from the acid gas removal unit to the synthesis gas. The system may also comprise piping for conveying the recycled gas 205 (i.e. the off gas 410) from the upgrading unit to the synthesis gas. The circulation of recycled gas is ensured with at least one booster compressor (not shown in the figure).

High pressure steam is produced from water in the heat exchanger 250. This high pressure steam is used to generate power. In addition, steam 399 is produced in the heat exchanger 386 arranged to cool the FT reactor. Steam 399 may be used in at least one of: the gasification reactor 150, the reformer 240, and the gas shifter 310. Heat from the scrubber 320 is used to dry biomass in a dryer 450. Heat is transferred to the dryer 450 by conveying the heated heat transfer medium 337 to the dryer 450.

Fig. 4b shows some parts of an embodiment of the invention, as described above. Referring to Fig. 4b, synthesis gas 200 is quenched in the quencher 210 before the filtration unit 220. In the embodiment, demineralized water is used as the quenching liquid 215. Moreover, quenched synthesis gas is further cooled by mixing quenched synthesis gas with recycled gas 205 at the point 212. Recycled gas 205 comprises a part of scrubbed synthesis gas 380 and may comprise at least one of FT tail gas 395, acid gas removal tail gas 397, and off gas 410. Fig. 4c shows another embodiment of the invention. Compared to Fig. 4b, synthesis gas 200 and recycled gas 205 are mixed before the quencher 210 at the point 212. This may be done e.g. to reduce the temperature of the quencher 210. In addition, in this embodiment the gas flow in the quencher 210 is increased, as compared to the embodiment of Fig. 4b. Thus, the turbulence of the flow in the quencher 210 may be increased, and the quenching liquid 215 may be evaporated more rapidly in the quencher.

It is noted that the embodiments shown in Figs. 4a-4c may comprise multiple points 212 for mixing recycled gas and synthesis gas. The different components of the recycled gas 205: the scrubbed synthesis gas, the tail gases 395, 397, and the off gas 410; may be mixed with the synthesis gas 200 in the same or in different locations.

Fig. 4d shows another embodiment of the invention. In contrast to the embodiment of Fig. 4a, a quencher 210 is used to quench the synthesis gas. Also, compared to Fig. 4a, the scrubbed synthesis gas is not recycled in the process to cool the synthesis gas. Therefore, in this embodiment, essentially all scrubbed synthesis gas is conveyed to compression and acid gas removal processes to produce the product gas.

Furthermore, at least one of the tail gas 395 from the FT process, the tail gas 397 from the acid gas removal step, and the off gas 410 may be recycled as recycled gas 205. The amount of quenching water 215 is selected such that the synthesis gas cools enough for filtration purposes in the quencher 210. This has some beneficial effects in the process and the process equipment, as will be discussed below.

First, as water is used for quenching the synthesis gas 200 in the quencher 210, the amount of steam needed for reforming is decreased. Steam is needed in the reformer 240, since the reforming process consumes steam. In the reformer, steam (H₂O) is needed to reform methane as CH₄ + H₂O = CO + 3H₂. Furthermore, steam (H₂O) is needed in the reformer to reform tar, i.e. to react tar with steam to produce carbon monoxide and hydrogen. Steam is also used in some side reactions such as CO + H₂O = CO₂ + H₂ and C + H₂O = CO + H₂. It is also noted that carbon and carbon dioxide may react to produce carbon monoxide: C + CO₂ = 2CO. To illustrate the reduced amount of steam in the figure, the arrow from the heat exchanger 386 to the reformer 240 is changed to a dotted arrow (Figs. 4b and 4c). Even if some additional steam was needed in the reformer 240, the decreased amount of additional steam would enable the use of smaller piping for the additional steam, which would imply smaller investment costs. It may also be possible that no excess steam is needed in the reformer 240, as illustrated in Figs. 4d and 4e.

Second, in contrast to gas cooling by mixing recycled gas, water quenching utilizes the evaporation enthalpy of water. Therefore much smaller amounts of water compared to the amount of recycled gas in gas cooling have to be added (both molar and mass related) to cool the gas stream to the required filter temperature. As an example, the target temperature of the filterable synthesis gas may be 650 . To obtain this tempera ture by mixing recycled synthesis gas, the mass flow of the recycled synthesis gas, where the recycled synthesis gas consists of scrubbed synthesis gas with the temperature 20 should be approximately 30% of th e mass flow of the uncooled synthesis gas. Alternatively, the mass flow of the recycled synthesis gas, where the recycled synthesis gas consists of CO₂, e.g. the tail gas 397 from the acid gas removal step, with the temperature 20 "C should be approximately 50% of the mass flow of the uncooled synthesis gas. In contrast, to obtain this temperature using water quench, the mass flow of the water should be approximately only 8% of the mass flow of the uncooled synthesis gas. In the reformer, the gas has to be heated up to reformer operation temperature by partial combustion of synthesis gas with oxygen. Due to the effect described above, the heat duty to be fulfilled in the reformer in the case of a water quench is much lower compared to a gas cooled synthesis gas. The effect may even be larger, if the steam feed to the reformer can be decreased (since it is replaced by quench water steam). The result for the reformer is lower consumption of oxygen and savings in the combustion of synthesis gas, which finally results in a higher Diesel yield. The lower heat duty implies lower oxygen consumption. The lower oxygen consumption is indicated in Figs. 4b - 4e as "less O₂".

Third, compared to a gas quench the heat load to the heat exchanger 250 is smaller. For that less high pressure steam is produced. This is shown in the Figs. 4b - 4e as "less steam" near the heat exchanger 250. The advantage is that the size of the heat exchanger 250 can be smaller.

Fourth, since for the water quench a significant part of the heat absorbed during quenching is stored as latent heat (evaporation-enthalpy), and the water condenses only in the scrubber, this evaporation energy is released as condensation heat in the scrubber. Thus more low value heat is available at the scrubber 320 for drying the biomass 100 in the dryer 450 compared to gas cooling. This is shown in Figs. 4b-4e as "more heat" near the arrows describing circulation of heat transfer medium (336,337) from the scrubber 320 to the dryer 450 .

Fifth, if scrubbed synthesis gas is not recycled in the process, as depicted in Figs. 4d and 4e, piping from the scrubber 320 to the filtration unit 220 and the booster compressor for the recycled gas 205 are not needed. Essentially all scrubbed synthesis gas is conveyed to compression and acid gas removal processes to produce the product gas. Therefore, the system of Fig. 4d or 4e does not comprise means for recycling scrubbed synthesis gas in the process. Thus, the recycled gases 205 are free scrubbed synthesis gas. The recycled gas 205 consist at least one of the FT tail gas 396, the acid gas removal tail gas 397 and the off gas 410. Furthermore, the recycled gases are recycled to the process to or before the gasifier 150. The lack of piping, as discussed, simplifies the system for producing liquid fuel from solid biomass, reduces material costs, and thus reduces the overall investment costs.

Sixth, operation of a water quench using independent demineralized water supply is expected to be simpler, be more controllable and robust than a gas quench using a recycle gas stream. Fig. 4e shows the embodiment of Fig 4d, where the FT tail gas 395 is recycled to the gasifier 150, and the acid gas removal tail gas 397 is recycled to the process as the CO₂ used in the lock hopper. The off gas 410 may be recycled to the gasifier 150. The system comprises means for conveying FT tail gas from the Fischer-Tropsch processing unit to the gasifier 150. The system further comprises means for conveying the tail gases 397 from the acid gas removal step to a container containing the biomass. In addition to the container 106 in the lock hopper, the tail gas can be conveyed to another container. The tail gas 397 consists essentially of CO₂. The system comprises means for conveying the off gas 410 from the upgrading unit to the gasifier 150. Also in the other embodiments shown in Figs. 4a-4d, the FT tail gas and/or the off gas 410 may be recycled to the gasifier 150, and the tail gases 397 may be recycled to the lock hopper. This embodiment has essentially the same advantages as the embodiment of Fig. 4d.

Quenching produces steam that is comprised in the quenched synthesis gases. In the embodiments shown in Figs. 4d and 4e, all the steam needed in the reforming process is comprised in the quenched synthesis gases, and therefore also in the filtered synthesis gases. Thus, the filtered synthesis gases are reformed with only oxygen 230. All the needed steam is comprised in the filtered synthesis gas and no excess steam 232 (Fig. 3b and Figs. 4a- 4c) is needed. In other embodiments, the excess steam 232 can be used also to smoothen the hot spots in the reformer, as generated by the oxygen injection.

As for the technical details of the quencher 210, three embodiments are shown in Figs. 5a - 5c. For the filtration unit 220 and for the reforming process it may be preferable that all the quenching liquid 215 is evaporated in the quencher 210. Therefore, the size of the droplet in the quenching spray needs to be reasonably small. Moreover, the flow of the quenching liquid into the quencher should be small enough such that essentially all the liquid becomes evaporated. The flow of the quenching liquid should be kept small also in order not to excessively cool the synthesis gas before the reformer 240, as the temperature of the cooled synthesis gas in the reformer 240 should be high. However, since the synthesis gas needs to be cooled, the flow of the quenching liquid should be large enough to facilitate the required cooling. It is also noted that the pressure of the synthesis gas 200 arriving into the quencher 210 may be essentially the same as the pressure in the gasification reactor 150, i.e. for example approximately 10 bar(a). The volumetric flow of the quenching liquid is defined by the flow velocity of the quenching liquid, and the cross sectional area of the conduit wherein the quenching liquid is arranged to flow. The flow, i.e. mass flow, of the quenching liquid is defined by the density of the quenching liquid and the volumetric flow. The density is affected e.g. by the temperature of the quenching liquid. As the bulk modulus of a liquid is large (e.g. compared to the bulk modulus of a gas), density of the quenching liquid generally is practically independent on pressure.

In the embodiment shown in Fig. 5a, the flow velocity and pressure of the quenching liquid 215 are controlled with a pump 515. The quencher 210 comprises nozzles 510 for spraying the quenching liquid 215 to the quencher 210. The droplet size of the sprayed quenching liquid may depend on the quenching liquid 215, the pressure of the quenching liquid 215 and the nozzle 510. In one embodiment, water was used as the quenching liquid 215. In this embodiment, it was noticed that the pressure with which the liquid 215 is sprayed may be of the order of 20 - 40 bar. Moreover, it was noticed that a suitable amount of quenching liquid is about 5 m-% of the synthesis gas flow to the quencher 210. With these values the temperature of the synthesis gas was decreased from about 850 Ό to about 700 Ό. Fu rthermore, all the water used for quenching was evaporated in the quencher 210. A suitable ratio of the mass flow of the quenching liquid 215 to the mass flow of the synthesis gas before quenching depends on the amount of recycled gas in the process. A suitable ratio of the mass flow of the quenching liquid 215 to the mass flow of the synthesis gas before quenching may be in the range of 4 - 15 %, depending on the needed temperature decrease of the synthesis gas. In addition to quenching, the quenched synthesis gas may be cooled by mixing it with recycled gas. The temperature of the filterable synthesis gas depends on the filtering unit, and may be from 600 to 800 Ό. The temperature of the synthesis gas before quenching may be from 800 to 900 Ό. The temperature and mass flow of the synthesis gas 200 before the quencher 210 may vary significantly. The variation may be due to the variation in the amount of synthesis gas produced in the gasification reactor 150. Also, if recycled gas 205 is recycled in the process through the quencher 210, the flow of recycled gas 205 affects the temperature and mass flow of gases in the quencher 210. In addition, the amount of quenching liquid (water) 215 depends on the temperature requirement of the filtration unit 220, and on the amount of recycled gas 205, as mixed with the quenched synthesis gas. The amount of quenching liquid is defined, to a reasonable accuracy, by the flow velocity, as discussed above.

To comply with the temperature and flow variations of the synthesis gas, the system may comprise one or more sensors arranged to measure at least one value of at least one process variable. The process variable may be one of temperature, pressure, flow velocity, and composition. A value of the process variable may be measured from the synthesis gas. The value may be measured before or after the quencher 210. The value may be measured before or after the point 212, where the synthesis gas is mixed with the recycled gas 205. In case the synthesis gas is cooled both by quenching and by gas cooling, the value may be measured before or after both of these locations or in between these locations.

The system may comprise a temperature sensor 530 to measure the temperature of the synthesis gas entering the quencher 210. The system may comprise a flow velocity sensor 532 to measure the flow velocity of the synthesis gas entering the quencher 210. In addition, the system may comprise a pressure sensor to measure the pressure of the synthesis gas entering the quencher 210. Still further, the system may comprise a composition sensor 534 to analyze the composition of the synthesis gas entering the quencher 210. Information on the composition is needed, if accurate information on the heat capacity of the synthesis gas entering the quencher 210 is needed.

The sensor(s) is/are arranged to send the measured value(s) of at least one process variable to a controller 535. The value(s) may be used to deduce information indicative of the thermal flow and/or heat capacity flow of the synthesis gas in the quencher.

The controller 535 may be arranged to control the pump 515. Moreover, by controlling the pump 515, the controller may be arranged to control at least one process parameter. The process parameter may be the pressure of the quenching liquid 215 or the flow velocity of the quenching liquid 215. The controller 535 may be arranged to use the information on at least one process variable to control at least one of process parameter. The controller may e.g. calculate another flow velocity for the quenching liquid based on the information indicative of the thermal flow of the synthesis gas. The controller may use this calculated flow velocity to control the actual flow velocity of the quenching liquid. A reliable and accurate temperature control is crucial to protect the filtering unit 220.

It is also preferable that essentially all the quenching liquid 215 entering the quencher is sprayed onto the synthesis gas 200 and not to the walls of the quencher 210. In some cases, the shape of the quencher may comprise a cylindrical part. In such cases, the nozzles 510 may be arranged close to the central axis of the cylindrical part, as shown in Fig. 5a. In the embodiment shown in Fig. 5a, the nozzles are further arranged to an upper part of the quencher 210, and the nozzles spray the quenching liquid 215 downwards towards the lower part of the quencher. The nozzles 510 are thus arranged to spray the liquid 215 parallel to the central axis of the cylindrical part. The nozzles 510 may also be arranged to spray the liquid 215 essentially parallel to the direction of the flow of the synthesis gas 200 in the quencher 210. This direction is depicted with the arrow 520.

Referring to Fig. 5b, the quenching liquid 215 may also be sprayed essentially perpendicular to the direction of the synthesis gas flow 520. The nozzles 510 may be located near the walls of the quencher 210 and arranged to spray the liquid 215 towards the central axis of the quencher 210.

In the embodiment of Fig. 5c, at least one sensor is arranged to measure at least one value of at least one process variable form the synthesis gas flow leaving the quencher 210. This may simplify the control, since the synthesis gas may be quenched to reduce the temperature to a level that the filtering unit can withstand. Alternatively, the synthesis gas may be quenched to a higher temperature, and later cooled by mixing with recycled gas 205 to a level that the filtering unit can withstand.

In an embodiment, a sensor is arranged to measure the temperature of the synthesis gas flow leaving the quencher 210 and arranged to send the measured information to the controller 535. The controller 535 is arranged to use the information on the temperature to control the flow velocity of the quenching liquid 215. The controller 535 may e.g. increase the flow velocity of the quenching liquid 215 if the measured temperature of the quenched synthesis gas is too high. In a similar way, the controller may decrease the flow velocity of the quenching liquid 215 if the measured temperature of the quenched synthesis gas is too low. This control mechanism is independent of the composition and heat capacity of the synthesis gas entering the quencher 210. Additional online analysis as for the cases depicted in Fig. 5a and 5b is not required.

Even if not shown in the figures, a sensor/sensors may be arranged to measure at least one value of at least one process variable, wherein the process variable is one of the temperature, the flow velocity, the pressure, and the composition of the filterable synthesis gas, i.e. synthesis gas after mixing the recycled gas to the synthesis gas. The sensor/sensors may send the measured information to a controller 535 arranged to control at least one process parameter, wherein the process parameter is one of

- the pressure of the quenching liquid 215,

- the flow velocity of the quenching liquid 215,

- the pressure of the recycled gas 205, and

- the flow velocity of the recycled gas 205.

Especially in the embodiment shown in Fig. 4a, where a quencher 210 is not used, the system may comprise such sensors. In addition, in the embodiment shown in Fig. 4a, the temperature of the filterable synthesis gas may be controlled by controlling the mass flow of the recycled gas 205. Moreover, in the embodiments shown in Figs. 4b and 4c, the temperature of the filterable synthesis gas may be controlled by at least one of the mass flow of the recycled gas 205 and the flow velocity of the quenching liquid 215.

The embodiments described above may be used for producing liquid fuel from solid biomass. The overall process yield, stability and robustness is enhanced by filtering synthesis gas before reforming. Filtration is enabled by cooling the synthesis gas. Synthesis gas could be cooled by mixing with recycled gas and/or by quenching. The embodiments described in detail how the amount of energy required for heating the synthesis gas to achieve reformer operating temperature could be reduced. Furthermore, the embodiments described in detail what other surprising beneficial effect water quenching has in the overall process. In comparison to gas cooling, water quenching is characterized by lower oxygen consumption in the reformer, a smaller size of the heat exchanger 250, increased capacity for medium pressure steam production, increased capacity for heat production for drying biomass and increased liquid product yield. Still further, since water- quenching the synthesis gas leads to lower volumetric and mass flows, some of the piping and equipment may be decreased in size or totally omitted, which reduces the overall investment costs of such a system.

It is apparent to a person skilled in the art that the basic idea of the implementation can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.

Claims

Claims:

1 . A method for producing liquid biofuel from biomass, the method comprising

- gasifying the biomass (100) in a fluidized bed reactor at an elevated temperature to produce synthesis gas (200) and

- post processing at least part of the synthesis gas (200) to the liquid biofuel, wherein the post processing comprises

- cooling the synthesis gas (200) before filtering,

- filtering the synthesis gas to produce filtered synthesis gas, and

- reforming the filtered synthesis gas for decreasing the amount of tars and methane in the filtered synthesis gas.

2. The method of claim 1 , comprising

- recycling recycled gases (205) in the process, and

- cooling the synthesis gas (200) before filtering by mixing the synthesis gas (200) with the recycled gases (205).

3. The method of claim 1 or 2, comprising

- cooling the synthesis gas (200) before filtering by quenching.

4. The method of claim 3, comprising

- using a quenching liquid (215) to quench the synthesis gas (200), wherein

- the quenching liquid (215) comprises at least 90 % water,

- the temperature of the synthesis gas before quenching is from 800 Ό to 900 Ό,

- the ratio of the mass flow of the quenching liquid (215) to the mass flow of the synthesis gas before quenching is from 4m-% to 15 m-% depending on the needed temperature decrease of the synthesis gas; to decrease the temperature of the synthesis gas to the range from 600 Ό to 800 Ό.

5. The method of claim 3 or 4, comprising

- scrubbing the filtered synthesis gas to produce scrubbed synthesis gas, and

- conveying essentially all scrubbed synthesis gas to compression and acid gas removal processes.

6. The method of any of the claims 1 - 5, comprising

- conditioning the synthesis gas to product gas,

- Fischer-Tropsch processing the product gas in a Fischer-Tropsch processing unit to produce product fluid (396), and

- upgrading the product fluid (396) to the liquid biofuel.

7. The method of claim 6, comprising

- producing FT tail gas (395) in the Fischer-Tropsch process and

- recycling the FT tail gas (395) to a gasifier (150).

8. The method of claim 6 or 7, comprising

- producing steam (399) from water (398) with a heat exchanger (386) arranged in connection with the Fischer-Tropsch processing unit and

- using the steam (399) in at least one of

-- a reformer (240),

-- a gasification reactor (150), and

- a gas shifter (310) to modify the ratio of hydrogen to carbon monoxide.

9. The method of any of the claims 3 to 7, wherein

- the quenching produces steam that is comprised in the filtered synthesis gas, and

- the method comprises reforming the filtered synthesis gas with only oxygen (230, O2) in a temperature of 900 - 950 Ό to reformed synt hesis gas.

10. The method of any of the claims 1 to 8, wherein the post processing comprises reforming the filtered synthesis gas with steam (232, 399, H₂O) and oxygen (230, O2) in a temperature of 900 - 950 Ό to reformed synt hesis gas.

1 1 . The method of any of the claims 1 - 10, comprising

- measuring a value of a process variable, and

- controlling a process parameter using the measured value of the process variable.

12. The method of claim 1 1 , wherein

the process parameter is one of - the pressure of the quenching liquid (215),

- the flow velocity of the quenching liquid (215),

- the pressure of the recycled gas (205), and

- the flow velocity of the recycled gas (205), and

the process variable is one of

- the temperature of synthesis gas,

- the flow velocity of synthesis gas,

- the pressure of synthesis gas, and

- the composition of synthesis gas.

13. The method of any of the claims 1 - 12, comprising

- scrubbing the filtered synthesis gas in a scrubber (320) with a scrubbing solution (331 , 332),

- heating a heat transfer medium (336, 337) with the scrubbing solution (331 , 332) using a heat exchanger (335) to produce heated heat transfer medium

(337), and

- drying the biomass (100) using the heated heat transfer medium (336, 337).

14. Use of a cooling device (218) to cool synthesis gas (200) in a process for producing liquid biofuel from biomass (100), wherein in the process

- the biomass (100) is gasified in a fluidized bed reactor at an elevated temperature to produce the synthesis gas (200),

- the synthesis gas (200) is post processed to produce the liquid biofuel,

- the cooling device (218) is used in the post processing to cool the synthesis gas (200) to produce cooled synthesis gas,

- the cooled synthesis gas is filtered to produce filtered synthesis gas, and

- the filtered synthesis gas is reformed for decreasing the amount of tars and methane in the filtered synthesis gas.

15. The use of claim 14, characterized in that the cooling device (218) comprises at least one of

- a quencher (210), and

- a gas mixer.

16. A system for producing liquid biofuel from biomass (100), the system comprising - a fluidized bed gasification reactor (150) for gasifying biomass (100) at an elevated temperature to produce synthesis gas (200),

- a post processing system arranged to produce the liquid biofuel from the synthesis gas (200),

- a cooling device (218) arranged to cool the synthesis gas (200),

- a filtration unit (220) arranged to filter the cooled synthesis gas to produce filtered synthesis gas,

- a reformer (240) arranged to reform the filtered synthesis gas for decreasing the amount of tars and methane in the filtered synthesis gas, - means for conveying the synthesis gas (200) from the gasification reactor (150) to the cooling device (218),

- means for conveying the cooled synthesis gas from the cooling device (218) to the filtration unit (220), and

- means for conveying the filtered synthesis gas to the reformer (240).

17. The system of claim 16, wherein

- the cooling device (218) comprises a gas mixer, and

- the system comprises means for recycling gases to the gas mixer.

18. The system of claim 16 or 17, wherein

- the cooling device (218) comprises a quencher (210).

19. The system of claim 18, wherein the post processing system

- is arranged to use a quenching liquid (215) in the quencher (210) to quench the synthesis gas (200), wherein the quenching liquid (215) comprises at least 90 % water, and

- is arranged to decrease the temperature of the synthesis gas (200) in the quencher (210) from 800 Ό - 900 Ό to 600 - 800 ° C by using a ratio of the mass flow of the quenching liquid (215) to the mass flow of the synthesis gas before quenching, the ratio being from 4m-% to 15 m-% depending on the needed temperature decrease of the synthesis gas.

20. The system of any of the claims 16 to 19, comprising

- a Fischer-Tropsch processing unit for processing a product gas to produce product fluid (396), and

- an upgrading unit for upgrading the product fluid (396) to the biofuel.

21 . The system of claim 20, comprising

- means for conveying FT tail gas (395) from the Fischer-Tropsch processing unit to the gasification reactor (150).

22. The system of claim 20 or 21 , comprising

- a heat exchanger (386) arranged in connection with the Fischer-Tropsch processing unit for producing steam (399) from water (398), and

- means for conveying the steam (399) to least one of

-- the gasification reactor (150) to be used as fluidizing medium,

- a gas shifter (310) for modifying the ratio of hydrogen to carbon monoxide, and

- the reformer (240).

23. The system of any of the claims 16 to 22, comprising

- a sensor (530, 532, 534) arranged to measure a value of a process variable,

- a controller (535) arranged to control a process parameter using the measured value of the process variable, and

- means for sending the measured value of the process variable to the controller (535).

24. The system of claim 23, wherein

the process parameter is one of

- the pressure of the quenching liquid (215),

- the flow velocity of the quenching liquid (215),

- the pressure of the recycled gas (205), and

- the flow velocity of the recycled gas (205), and

the process variable is one of

- the temperature of synthesis gas,

- the flow velocity of synthesis gas,

- the pressure of synthesis gas, and

- the composition of synthesis gas.

25. The system of any of the claims 16 - 24, comprising - a scrubber (320) for scrubbing the filtered synthesis gas with a scrubbing solution (331 , 332),

- heat transfer medium (336, 337),

- a heat exchanger (335) to heat the heat transfer medium (336) to heated heat transfer medium (337) with said scrubbing solution (331 , 332),

- a dryer (450) for drying the biomass (100), and

- means for conveying the heated heat transfer medium (337) from the heat exchanger (335) to the dryer (450).