VILJAKAINEN, Olli-Pekka (Palokalliontie 12, Vantaa, FI-01490, FI)
TIMONEN, Mika (Pesäpolku 6, Jyväskylä, FI-40420, FI)
KAUTTO, Jorma (Harjutie 28 B, Vantaa, FI-01390, FI)
VILJAKAINEN, Olli-Pekka (Palokalliontie 12, Vantaa, FI-01490, FI)
TIMONEN, Mika (Pesäpolku 6, Jyväskylä, FI-40420, FI)
|What is claimed is:
1. A method for utilizing thermal energy of gases generated in a BtL plant, characterized in that the thermal energy of gases generated in the production process of the BtL plant is utilized in superheating steam for driving the turbine machineries of the BtL plant.
2. The method of claim 1 , characterized in that in the method the compressors and/or electric generators of the plant process stages are driven by steam turbines using the steam streams generated in the BtL plant processes that are superheated by the flue gases of a steam reformer, that is, an SMR reactor (10) integrated with the BtL plant equipment and, additionally, to maximize the yield of end products of the BtL plant, hydrogen (43) is recovered with the help of a PSA unit (42).
3. The method of claim 1 or 2, characterized in that in the method superheating the saturated steam streams generated in the BtL processes substantially increases the self-supported electrical balance of the BtL and facilitates self-contained operation of the BtL plant as a stand-alone facility independent from another industrial plant or power utility, while simultaneously the yield of end product is maximized.
4. The method of any of claims 1 - 3, characterized in that a BtL plant has integrated thereto a method for utilizing the thermal energy of the flue gases (18) and/or reformed gas (17) of a steam reformer (10) for superheating (39) the steam that is used in the BtL plant for driving a syngas turbocompressor (23, 24) and/or producing electricity as well as improving the hydrogen yield (43), whereby a WGS process is included having a steam reformer (10) connected to a PSA unit (42).
5. The method of any of claims 1 - 4, characterized in that a BtL plant generates in its FT process (8) different FT tail gases, that is, process reject gases (16), which are passed to a steam reformer (10), wherein the FT tail gases are reformed (17) in such a way that the hydrocarbons of gases (17) are reformed into hydrogen (43) and substantially into carbon monoxide and therefrom further to carbon dioxide (44), from which gas stream after cooling (33) in a PSA unit (42) hydrogen (43) is recovered, whereupon the remaining gases (44) are recirculated to a steam reformer ( 0) for heating the steam reformer (10) to a correct temperature of about 800- 100 °C.
6. The method of any of claims 1 - 5, characterized in that gases (17) reformed in steam reformer (10) and flue gases (18) exiting the steam reformer (10) are passed for cooling to a heat exchanger (33), whereby the thermal energy of the gas streams is used for superheating the saturated high-pressure steam (26c) exiting the gasifier.
7. The method of any of claims 1 - 6, characterized in that heat exchanger (33) comprises superheaters (33a) and (33b), wherein saturated high- pressure steam (26c) is superheated (39) for compressor/turbine (24) and (23) and that boilers (46a) and a superheater (46b) are employed for producing steam required in steam reforming (10).
8. The method of any of claims 1 - 7, characterized in that to the compressor/turbine combination (23) and (24a) operating as a turbocompressor is connected a steam condensation turbine (24b), whereto are passed the low- pressure and middle-high-pressure steam streams generated in the BtL plant and a generator (34) is provided for electricity generation.
9. Use of thermal energy of gases generated in a BtL plant for superheating steam streams driving the turbine machineries of the BtL plant and postprocessing gas streams for hydrogen recovery therefrom.
10. The use according to claim 9 for utilizing saturated steam streams generated in a BtL plant, wherein superheating increases the self-supported electrical balance of the BtL and facilitates self-contained operation of the BtL plant as a stand-alone facility independent from another industrial plant or power utility, while simultaneously the yield of end product is maximized.
11. The use according to claims 9 - 10 of a steam reformer such as an SMR reactor (10) integrated with the BtL plant equipment for superheating the discharge gas streams generated in the BtL plant to drive the plant's compressors and/or electricity generation units and, additionally, to maximize the yield of end products of the BtL plant, whereby hydrogen (43) is recovered with the help of a PSA unit (42). 12. The use according to claims 9 - 11 for utilizing the thermal energy of the flue gases (18) and/or reformed gas (17) for superheating (39) the steam that is used in the BtL plant for driving the syngas turbocompressor (23, 24) and/or producing electricity.
13. The use according to claims 9 - 12 for improving the yield of the BtL plant, whereby a WGS process is used having a steam reformer (10) and a PSA unit (42) integrated with the BtL plant equipment.
The invention relates to a method in accordance with the preamble of claim 1 for utilizing thermal energy of gases generated in a BtL plant. The invention also relates to a use in accordance with claim 9.
In a BtL plant using state-of-the-art technology, solid biomass is gasified in a high-temperature or a low-temperature gasifier. The function of a BtL factory is to convert biomass into liquid fuels (Biomass to Liquid) from syngas generally through Fischer-Tropsch synthesis. In high-temperature gasification the gasifier operates at a temperature higher than the ash melt temperature, more specifically at about 1200-1400 °C. Depending on the technology used, gasification takes place at a pressure of 1-40 bar. Recently a technology has been developed particularly suited for high-temperature gasification of biomass at a gasifier pressure of about 5 bar.
The gas generated in gasification and further subjected to purification is generally called syngas, since it is subsequently used in preparation of other products such as ammonia or long-chain aromatic hydrocarbons.
In the manufacture of synthetic biofuels, the raw syngas generated in gasification must be cooled and purified free from dust, whereupon other components except hydrogen and carbon monoxide need be separated from the gas stream. The resulting pure syngas, i.e., hydrogen and carbon monoxide is passed to a Fischer-Tropsch reactor (FT reactor), wherein paraffinic hydrocarbons are generated in the presence of a catalyst. The FT process is typically carried out at a pressure of 20-40 bar and at a temperature of about 200 °C. The wax-like product thus obtained is known as biowax.
The biowax taken out from the FT process requires further refining to produce therefrom fuels suited for engine use by way of, e.g., hydrogenation, cracking and distillation. Also these processes are carried out under elevated pressure (30-80 bar). Hydrogenation refers to processing in a hydrogen atmosphere, wherein double bonds between carbons are saturated. Cracking in turn refers to breaking excessively long hydrocarbon chains in a reactor. Distillation finally separates the fuel fractions from each other thus resulting in diesel fuel, naphtha, kerosene, liquefied petroleum gas, etc.
If gasification is carried out at a pressure lower than that of the FT and refining processes, the syngas pressure must be elevated. Conventionally this step takes place after the syngas is cooled and filtered free from solid impurities. Pressure elevation is accomplished by gas compressors that are available in plural different types. Typically they can be categorized as axial, radial, piston and screw compressors. Most generally, syngas compression has been performed using axial and radial compressors. A suitable
compressor type is selected based on the required pressure elevation, gas composition and volume.
A common feature of compressors is that they are rotary equipment. The mechanical energy required for compression is typically derived from an electric motor or, alternatively, from a steam or gas turbine. E.g., syngas compression in a BtL plant having a gasification fuel power of 300-500 MW at a pressure of about 5 to 35 bar, requires an input power of about 10-17 MW.
Instead of air, gasification is carried out using oxygen that must be pressurized for the gasification process. Oxygen can be prepared from air by first cooling it into liquid form and then distilling the air gases apart from each other. The compressor power of an air gas plant is 10 - 15 MW for a syngas plant of 300-500 MW gasification fuel power. At an oxygen plant,
compressors are needed in the cooling process and oxygen pressure elevation to the gasification process pressure.
In the BtL process, the pressure decreases after the pressurization step downstream toward the process exit end. This is due to the pressure losses occurring in the different process stages in a cumulative manner. If gas streams are desired to be fed backward in the process, the pressure levels of such streams must be elevated by compression. These compressors, however, are relatively low-powered with regard to the compression power required to move the main gas stream, typically in the order of 200-700 kW per compressor. Moreover, if the process includes liquefaction of carbon dioxide for its capture, the pressure of gaseous carbon dioxide must be elevated prior to its cooling to about 20 bar followed by cooling to -50 °C with the help of heat exchangers and an expansion valve. The input power requirements of carbon dioxide compression and compressors of the cooling equipment in a BtL plant of 300-500 MW gasification fuel power is in the order of 10-15 MW, whereby the liquefied amount of carbon dioxide is about 50-75 t/h.
As is obvious from the above discussion, the number of various compressors consume a major portion of the electrical energy required by a BtL plant. Hence, notwithstanding the possible in-plant electric energy generation of the BtL plant based on its process steam resources, the plant is dependent on external electricity.
The invention is primarily directed to syngas compression, but more generally the invention may be applied to other use, for example to the process steps mentioned above that require compression.
Herein it must be noted that a BtL process generates saturated steams at different pressure levels, especially at a high pressure, from the cooling of raw syngas and the gasifier itself. More particularly, controlled cooling of the FT synthesis releases a large volume of saturated steam at a middle-high pressure. Besides the relatively small own-use requirements of the BtL process itself, the dominatingly largest consumer of backpressure steam in the plant is biomass drying. Nevertheless, the BtL plant has plentiful inherent supply of low-pressure steam.
Typically, due to the above-mentioned reasons, the goal is to integrate a BtL plant with another industrial plant capable of using its excess steam. Advantageously steam is used, e.g., in drying paper, pulp and cardboard as well as in district heating and electricity generation. Steam available from a BtL plant reduces the fuel consumption of the factory to be integrated therewith.
A problem herein, however, arises therefrom that finding a suitable factory for integration poses a major limitation to the viable number of potential locations for erecting a BtL plant. As the BtL plant must be located on the remaining area of the factory's real estate having no earlier reservations, the layout of the BtL plant often becomes less optimal. Neither will a logistically optimally integrated plant location necessarily be the most advantageous site for the BtL plant.
Further noteworthy is that the BtL plant and the factory integrated therewith must be constructed to cope with such constraints that either one of the plants/factories is not running continuously, which means that both of them need be equipped with stand-alone facilities. Hence, integration does not necessarily result in a cost reduction of overall investment, but on the contrary, requires erection of integration facilities and processes for standalone operation.
The excess steam generated in a stand-alone situation should be utilized maximally effectively - which situation, in the lack of steam consumers, requires electricity generation with the help of a steam turbine. However, use of saturated steam in turbines is impossible unless it is superheated by way of burning gaseous fuel, e.g., in a separate superheater boiler that otherwise is a redundant unit in the process.
Now the present invention provides an arrangement capable of overcoming the above-discussed problems. This object of the invention relates to a situation, wherein there is a deficit of electricity and plentiful excess of saturated steam simultaneously in the BtL process. Electricity is required to energize various compressors driven by electric motors. Simultaneously, steam is delivered to a process or power plant of the integrated factory wherein the steam drives turbines producing electricity. A portion of this electricity can be returned for use in the BtL process.
In the embodiment of the invention, the essential aim is to utilize the excess steam on-site and thereby reduce the quantity of purchased electricity. These two goals can be combined by driving the compressors with the steam generated in the BtL plant processes. Thereby the conversion losses of mechanical energy into electricity and back into mechanical energy are minimized. For using steam in turbines, it must first be superheated. Next will be described a method, wherein superheating the steam is accomplished with the help of process equipment known as steam reformer.
Steam reformer is a unit generally used oil refining industry for producing hydrogen from methane and heavier hydrocarbon fractions for use in oil refinement. Reforming is accomplished by feeding steam into the gas being reformed with the help of a suitable catalyst and under high temperature. The process is also known by English terms Steam Methane Reformer (SMR) or Steam Reformer Unit (SRU). Next is described a method, wherein superheating of steam is accomplished using the SMR technique. The raw syngas resulting from the gasification stage of the BtL process contains an insufficient amount of hydrogen for the FT process. Therefore, addition of hydrogen is necessary by means of a process based on a water gas transfer reaction known as the WGS technique (WGS = Water Gas Shift). In this process carbon monoxide (CO) is separated from syngas generated in gasification, into the gas stream is injected steam and, in a subsequent catalyzed reaction, hydrogen is generated as follows: CO + H 2 O -> CO 2 + H 2
The FT process of a BtL plant and different stages of oil refinement generate various tail gases, wherefrom hydrogen can be recovered using the SMR technique. Thereby the yield of the BtL process is improved and the thus produced hydrogen is more ecological, i.e., derived from a biomass as compared to a situation, wherein the hydrogen source is a fossil resource such as methane.
Tail gases, whose free translation into Finnish is "rear-end gases" and which are generated in the Fischer-Tropsch process and subsequent postprocessing stages, stem from the biomass-based raw material of the plant and contain different kinds of light hydrocarbons. Conventionally, hydrogen is produced from the methane of natural gas using the SMR technique: CH 4 + H 2 O -> CO + 3H 2 Now this technique is applied to the tail gases generated in the FT process. Thus, gas molecules with a longer chain such as propane are reformed as follows:
C 3 H 8 + 3H 2 O -> 3CO + 7H 2 wherefrom carbon monoxide can be further utilized in the process via the WGS reaction:
CO + H 2 O -> CO 2 + H 2
Accordingly, the BtL process can be complemented with WGS and/or SMR processes for improved yield and adjustment of its hydrogen-to-carbon monoxide ratio. In regard to methane, for instance, the overall reaction is:
CH 4 + 2H 2 O -> CO 2 + 4H 2
For utilizing in the high-pressure turbine the high-pressure saturated steam resulting from cooling the raw syngas of the gasifier, the steam must be superheated, since no water condensed during the expansion of steam may be passed to the turbine. Superheating can be accomplished in a separate superheater boiler or by the hot flue gases of the steam reformer which is more appropriately adaptable to the BtL process. In the reformer takes place the reforming the tail gas of the FT process that contains a mixture of different hydrocarbons. The temperature of the gas being reformed is typically elevated to about 900-1100 °C by burning a portion of the gas or some other fuel in the boiler.
The process temperature is so high that the exhausted flue gases may still be used for superheating steam, which means that superheating the steam can be accomplished with a minimal extra investment to the SMR technique without the need for installing a boiler. The flue gases are clean as they originate from ash-free tail gases, that is, from exhaust and surplus gases. Obviously, the SMR unit may also be heated using externally fed fuels such as natural gas or other combustible gases resulting from the BtL process. The excess backpressure steam may be converted into electricity in a steam condensation turbine. The condensing turbine may be a separate piece of equipment or integral with the syngas turbocompressor. Also the backpressure turbine can be separate from the compressor thus facilitating entirely independent operation of the compressors and turbines.
This arrangement allows more efficient utilization of the entire process chain. The essential features of the invention are the crucial factors in the method and its use. More specifically, the invention is characterized by what is stated in the claims.
In the following, the invention is described in more detail with the help of a preferred embodiment by making reference to appended Figs. 1 - 3, in which drawings:
Figs. 1 - 3 show process flow schematics of arrangements for implementing the method according to the invention.
Fig. 1 shows a flow schematic for producing biofuels from solid biomass. The biomass 12 being fed into the process is dried and its particle size is homogenized in raw material preprocessing step 1 suitable for feeding to the gasifier. The preprocessed biomass is fed to oxygen gasification 3 having such a high temperature that makes the tar components of gas to
decompose entirely. Decomposition of tar components is essential to prevent them from condensing in the process equipment downstream of the gasification step. The process oxygen is prepared in oxygen plant 2.
Raw syngas 28 is cooled in step 4 and is filtered free from dust in process 5. Subsequently, gas pressure can be elevated by compressor 24 to the level required by FT reactor 8. Prior to the feed to the FT reactor, the carbon monoxide-hydrogen ratio of the gas is adjusted in WGS reactor 6 and from the syngas are separated other gaseous components and catalyst poisons 7 that are derouted to stream 22. The biowax resulting from the FT process is postprocessed in a refinery plant 9 into fractions 15 suitable for different uses such as biodiesel. Cooling of raw syngas 28 is carried out with the help of a heat exchanger in process 4, whereto high-pressure infeed water 20 is passed. In the heat exchanger the water is evaporated into steam and removed in saturated state. In the beginning of plant start-up, gasification 3 is functional but not the downstream stages 6-9 of the process. This means that the saturated steam must be passed via a pressure reduction valve 25 to backpressure network 38 as shown in alternative 26a of Fig. 1. This operational state must be continued so long until pure syngas is received at compressor 24. Hereinafter, the compressor can be started to output compressed syngas 27.
If the compressor and turbines are permanently connected to each other by a common shaft, that is, are forced to rotate with each other either at the same speed or, via a gearbox, at different speeds, a small amount of cooling steam must be passed to the turbines when the machineries are driven by an electric motor.
If turbine 23 is designed to be driven by saturated steam, the steam can be routed to the turbine via the path shown in block 26b of Fig. 1. However, as high-pressure turbines are not generally designed to receive saturated steam, in this case the steam is passed to the backpressure network via path 26a so long until steam reformer 10 becomes operational up to which moment the compressor has been driven by an electric motor. Subsequently, the saturated steam stream is directed to the path shown in block 26c of Fig. 1.
In Fig. 2 is shown the layout of a steam flow network of a process related to the present invention. Therein the saturated steam stream 26c of Fig. 1 is passed via superheater 33, whereby a superheated steam stream 39 is generated suitable for feeding to turbine 23.
While a saturated-steam condensation turbine 24b is connected on the same shaft with a high-pressure turbine 24a and compressor 23, it can also operate as a separate machinery. Operating the machineries separate from each other allows better runnability of the plant in special situations and startup occasions. The efficacy and investment cost of a system of a fixed configuration will be more advantageous in a process running extended periods of stable operation.
A BtL process is substantially self-sufficient as regards to backpressure steam 38. The excess steam may be utilized, e.g., in condensation-steam generation of electricity or use in heat-intensive processes such as those of a paper mill or chemical plant. When the steam condensation turbine 24b is mounted on the same shaft with the high-pressure turbine 24a, the synchronous motor 34 mounted on the same shaft can also perform as a generator when the combined power of turbines 24a and 24b exceeds the power demand of compressor 23.
The excess amount of backpressure steam 38 varies from winter to summer inasmuch as biomass drying 37 consumes in the winter even three times more steam than in the summer due to the higher moisture content of the biomass in the winter and lower temperature of drying air taken from outdoors. The output of saturated high-pressure steam 26 from the gasifier and other steam generation from the BtL process 35 remain unchanged irrespective of the season. When necessary, to the backpressure steam network of the BtL plant can be fed also other excess steam streams, e.g., from the backpressure steam network 36 of a pulp mill, for instance.
In an equilibrium state, all high-pressure steam 26 available is passed via a high-pressure turbine 24a to the backpressure steam network 38. The excess steam is passed to a steam condensation turbine 24b and
subsequently condensed in a condenser 30. If the cooling capacity provided by the steam condensation turbine is insufficient, the excess steam can be cooled, e.g., in an auxiliary cooler 31 connected to a waterway. The condensates 29 are returned to the process as infeed water 20.
In Fig. 2 is also shown an intermediate cooling unit 32 for cooling the syngas 28 being compressed. Depending on the technology used, the number of intermediate cooling stages can be plural , e.g., 4-6. The warm exit water of the intermediate cooler can be utilized, e.g., for drying the biomass raw material of the BtL plant provided that the water temperature is sufficiently high, advantageously about +50 °C or higher. In Fig. 3 is shown a flow schematic for an SMR process according to the invention. Together with steam 40b, combustible gases 16a not utilized in the process are fed to the SMR reactor 10, which is heated by burning a portion of the reject gases 16b and purge gases 44b of the PSA unit with the help of combustion air 13. The process generates reformed gas 17, wherefrom hydrogen is separated after cooling in PSA unit 42.
The PSA exit or purge gases 44 also contain combustible gases that are routed to serve as fueling the SMR unit. The PSA, that is, the Pressure Swing Absorption reactor is a unit capable of separating gases of different molecular weight from each other, e.g., generally hydrogen 43 from a gas mixture of carbon dioxide and hydrogen.
The process may also be implemented without a PSA unit by way of feeding the reformed gas stream with the help of a recirculation compressor 11 to the compressed syngas stream 27 as shown in Fig. 1.
From SMR 0 the exiting flue gas 8 and reformed gas 17 are hot and thus contain enough energy at a high temperature sufficient for superheating in superheaters 33a and 33b the saturated steam 26c exiting the gasifier. For instance, saturated steam at a pressure of 90 bar has a temperature of about 305 °C. For feeding to the turbine, the temperature must be further elevated to about 500 °C. When necessary, the temperature of superheated steam may be adjusted by spraying 41 feed water 20 into the steam stream.
After exiting the high-pressure superheaters 33a and 33b, the temperature of flue gases 18 is still quite high allowing the flue gas to be utilized for producing steam 40 at a lower pressure on a boiler 46a and a superheater 46b, heating waters for drying biomass or, alternatively, preheating 47 the combustion air of the SMR unit in a heat exchanger 45. The cooled flue gases can be discharged to chimney 19.
In the above-described fashion, the present invention is directed to a novel method and use capable of utilizing the thermal energy of gases formed in a BtL plant for in-plant use. The method offers significant benefits, more specifically by utilizing the thermal energy of gas streams generated in a BtL plant for superheating steam for driving the turbine machineries of the BtL plant and postprocessing the tail gases in order to maximize the yield of the plant's end product.
This goal is achieved by driving the compressors and/or electric generators of the BtL plant process stages by steam turbines using the steam streams generated in the BtL plant processes that are principally superheated by the flue gases of a steam reformer, that is, an SMR reactor 10, integrated with the BtL plant equipment. Additionally, the plant yield is maximized by recovering hydrogen 43 at a PSA unit 42.
In this fashion, supercharging the saturated steam streams exiting BtL processes significantly improves the self-supported electrical balance of the BtL and facilitates self-contained operation of the BtL plant as a stand-alone facility independent from another industrial plant or power utility.
According to the invention, a BtL plant has integrated thereto a method for utilizing the thermal energy of the flue gases 18 and/or reformed gas 17 of a steam reformer 10 for superheating 39 the steam that is used in the BtL plant for driving the syngas turbocom pressor 23, 24 and/or producing electricity as well as improving hydrogen yield 43, whereby a WGS process is included having a steam reformer 10 connected to a PSA unit 42.
A BtL plant generates in its FT process 8 different FT tail gases, that is, process reject gases 6, that are passed to a steam reformer 10, wherein the FT tail gases are reformed 7 in such a way that the hydrocarbons of gases 17 are reformed into hydrogen 43 and substantially into carbon monoxide and therefrom further to carbon dioxide 44, from which gas stream after cooling 33, 46 in a PSA unit 42 is recovered hydrogen 43, whereupon the remaining gases 44 are recirculated to a steam reformer 10 for heating the steam reformer 10 to a correct temperature of about 800- 100 °C. Gases 17 reformed in steam reformer 10 and flue gases 18 exiting the steam reformer 10 are passed for cooling to a heat exchanger 33, whereby the thermal energy of the gas streams is used for superheating the saturated high- pressure steam 26c exiting the gasifier. In this fashion the thermal energy of gases generated in a BtL plant is utilized for superheating gases used for driving the turbine machineries of the BtL plant.
As described above, a process generates a large volume of saturated steam streams, e.g., those exiting from the cooling of the gasifier vessel envelope or the syngas stream. In accordance with the present invention, superheating these gases can be accomplished with the help of the hot gases generated in the process such as the flue gas of the steam reformer. Superheating is absolutely necessary to make steam usable in turbines and further for driving compressors. Resultingly, the present invention makes it possible avoid the need for acquisition of a separate superheater boiler.
Another significant benefit is that the plant can be erected without having an another plant located nearby such as a paper mill, for instance, that is capable of using saturated steam. Now the method according to the invention permits integration of a power plant with the process. Moreover, the saturated steam can be used in other processes such as drying paper and pulp or in the production of district heating energy.
To a person skilled in the art it is obvious that the invention is not limited by the above-described exemplary embodiments, but rather may be varied within the inventive spirit and scope of the appended claims.