Method and system for recycling carbon dioxide from biomass gasification

Abstract

A biomass gasification system. The system includes: a) a gasifier; b) a waste heat exchanger; c) a waste heat boiler; d) a cyclone separator; e) a gas scrubber; f) a shift reactor; g) a desulfurizing tower; h) a first decarburizing tower; i) a synthesizing tower; and j) a second decarburizing tower. In the system, the gasifier, the waste heat exchanger, the cyclone separator, the gas scrubber, the shift reactor, the desulfurizing tower, the first decarburizing tower, the synthesizing tower, and the second decarburizing tower are connected sequentially. In addition, CO.sub.2 outlets of the first decarburizing tower and the second decarburizing tower are both connected to a cold medium inlet of the waste heat exchanger; and a cold medium outlet of the waste heat exchanger is connected to a gasifying agent entrance of the gasifier.

Claims

1. A biomass gasification system, the system comprising: a) a gasifier; b) a waste heat exchanger; c) a waste heat boiler; d) a cyclone separator; e) a gas scrubber; f) a shift reactor; g) a desulfurizing tower; h) a first decarburizing tower; i) a synthesizing tower; and j) a second decarburizing tower; wherein: a syngas outlet of the gasifier is connected to a heat medium inlet of the waste heat exchanger; a heat medium outlet of the waste heat exchanger is connected to a heat source inlet of the waste heat boiler; a heat source outlet of the waste heat boiler is connected to a gas inlet of the cyclone separator; a gas outlet of the cyclone separator is connected to an inlet of the gas scrubber; an outlet of the gas scrubber is connected to a gas inlet of the shift reactor via a compressor; a vapor outlet of the waste heat boiler is connected to a vapor inlet of the shift reactor; a vapor outlet of the shift reactor is connected to an inlet of the desulfurizing tower, and an outlet of the desulfurizing tower is connected to an inlet of the first decarburizing tower which is configured for the decarburizing of the syngas; an outlet of the first decarburizing tower is connected to an inlet of the synthesizing tower; an exhaust outlet of the synthesizing tower is connected to an exhaust inlet of the second decarburizing tower which is configured for the decarburizing of the exhaust; CO.sub.2 outlets of the first decarburizing tower and the second decarburizing tower are both connected to a cold medium inlet of the waste heat exchanger; and a cold medium outlet of the waste heat exchanger is connected to a gasifying agent entrance of the gasifier.

2. The system of claim 1, wherein the CO.sub.2 outlets of the first decarburizing tower and the second decarburizing tower are both connected to an inlet of a gas holder, and an outlet of the gas holder is connected to the cold medium inlet of the waste heat exchanger via a blower.

3. The system of claim 2, wherein the inlet of the gas holder is also connected to a CO.sub.2 outlet of a calcinator.

4. The system of claim 1, wherein an air distributor is disposed in a lower part of a chamber of the gasifier; a wall of the gasifier above the air distributor comprises a primary gasifying agent entrance; the wall of the gasifier below the air distributor comprises an auxiliary gasifying agent entrance; an external energy entrance is disposed on the wall of the gasifier above the auxiliary gasifying agent entrance; and a cold medium outlet of the waste heat exchanger is connected to both the primary gasifying agent entrance and the auxiliary gasifying agent entrance.

5. The system of claim 3, wherein an air distributor is disposed in a lower part of a chamber of the gasifier; a wall of the gasifier above the air distributor comprises a primary gasifying agent entrance; the wall of the gasifier below the air distributor comprises an auxiliary gasifying agent entrance; an external energy entrance is disposed on the wall of the gasifier above the auxiliary gasifying agent entrance; and a cold medium outlet of the waste heat exchanger is connected to both the primary gasifying agent entrance and the auxiliary gasifying agent entrance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a biomass gasification system according to one embodiment of the invention;

(2) FIG. 2 is a schematic diagram of a gasifier according to one embodiment of the invention; and

(3) FIG. 3 is a sectional view taken from line A-A in FIG. 2.

(4) In the drawings, the following reference numbers are used: 1. Gasifier; 2. Waste heat exchanger; 3. Waste heat boiler; 4. Cyclone separator; 5. Gas scrubber; 6. Compressor; 7. Shift reactor; 8. Desulfurizing tower; 9. First decarburizing tower; 10. Synthesizing tower; 11. Second decarburizing tower; 12. Gas holder; 13. Blower; 14. Fuel; 15. External energy entrance; 16. Auxiliary gasifying agent entrance; 17. Feed inlet; 18. Syngas outlet; 19. Air distributor; 20. Primary gasifying agent entrance; 21. Slag discharging outlet; 22. Feeding device; 23. Slag cooler; 24. Vapor; 25. Cooled slag; 26. Fly ash; 27. Oil product; 28. Exhaust gas; 29. CO.sub.2; 30. Syngas; 31. Effluent gas; 32. Calcinator; 33. Limestone.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) Technical solution of the invention is illustrated with detailed embodiments hereinbelow, the embodiments, however, should not be explained as limitation of the protection range of the invention.

(6) FIG. 1 is a schematic diagram of a biomass gasification system of the invention. The system comprises a gasifier 1, a waste heat exchanger 2, a waste heat boiler 3, a cyclone separator 4, a gas scrubber 5, a compressor 6, a shift reactor 7, a desulfurizing tower 8, a first decarburizing tower 9, a synthesizing tower 10, and a second decarburizing tower 11. The gasifier 1 comprises a syngas outlet 18 at the top, a slag discharging outlet 21 at the bottom, and a feed inlet 17 in the middle of the external wall of the gasifier. The feed inlet is connected to a feeding device 22. An air distributor 19 is disposed in a lower part of the chamber of the gasifier 1. The wall of the gasifier above the air distributor 19 comprises a primary gasifying agent entrance 20. The wall of the gasifier below the air distributor 19 comprises an auxiliary gasifying agent entrance 16. An external energy entrance 15 is disposed on the wall of the gasifier above the auxiliary gasifying agent entrance 16.

(7) The syngas outlet 18 of the gasifier 1 is connected to a heat medium inlet of the waste heat exchanger 2. A heat medium outlet of the waste heat exchanger 2 is connected to a heat source inlet of the waste heat boiler 3. A heat source outlet of the waste heat boiler 3 is connected to a gas inlet of the cyclone separator 4. A gas outlet of the cyclone separator 4 is connected to an inlet of the gas scrubber 5. An outlet of the gas scrubber 5 is connected to a gas inlet of the shift reactor 7 via a compressor 6. A vapor outlet of the waste heat boiler 3 is connected to a vapor inlet of the shift reactor 7.

(8) A vapor outlet of the shift reactor 7 is connected to an inlet of the desulfurizing tower 8, and an outlet of the desulfurizing tower 8 is connected to an inlet of the first decarburizing tower 9 which is configured for the decarburizing of the syngas. An outlet of the first decarburizing tower 9 is connected to an inlet of the synthesizing tower 10. An exhaust outlet of the synthesizing tower 10 is connected to an exhaust inlet of the second decarburizing tower 11 which is configured for the decarburizing of the exhaust. CO.sub.2 outlets of the first decarburizing tower 9 and the second decarburizing tower 11 are both connected to a cold medium inlet of the waste heat exchanger 2. A cold medium outlet of the waste heat exchanger 2 is connected to both the primary gasifying agent entrance 20 and the auxiliary gasifying agent entrance 16.

(9) The CO.sub.2 outlets of the first decarburizing tower 9 and the second decarburizing tower 11 are both connected to an inlet of the gas holder 12. An outlet of the gas holder 12 is connected to the cold medium inlet of the waste heat exchanger 2 via a blower 13. The inlet of the gas holder 12 is also connected to a CO.sub.2 outlet of a calcinator 32.

(10) In this example, solid fuel 14 such as biomass is introduced from the feeding device 22 to the gasifier 1 via the feed inlet 17. The recyclable gasifying agent CO.sub.2 is blown into the gasifier 1. There are two routes for the gasifying agent CO.sub.2 to enter the gasifier. One is introduced from the primary gasifying agent entrance 20, and then into the gasifier via the air distributor 19; the other is introduced into the gasifier 1 from the auxiliary gasifying agent entrance 16. Meanwhile, external heat energy is introduced to the gasifier via an external energy entrance 15. The biomass is gasified in the gasifier 1 under high temperature to yield CO, CO.sub.2, CH.sub.4, H.sub.2 and semicoke. The reaction temperature in the gasifier is controlled at between 600 and 1600 C., so that the semicoke reacts with CO.sub.2, the reaction equation is: C+CO.sub.2=2CO+Q, with high reaction rate.

(11) Take rice hull and 1 Nm.sup.3 of syngas as an example, the external energy accounts for 15-25% of the total energy of the fed fuel, the reaction temperature is 800 C., the circulating volume of CO.sub.2 is 0.51 Nm.sup.3, the biomass is 0.48 kg, and the syngas from the outlet of the gasifier comprises 0-55% by volume of CO, 22-28% of CO.sub.2, and 6-12% of H.sub.2.

(12) The high temperature syngas 30 is discharged from the syngas outlet of the gasifier 1. The cooled slag 25 is discharged from the slag discharging outlet 21 and cooled by a slag cooler 23.

(13) The reaction temperature in the gasifier is controlled at 600-1300 C., preferably at 850-1250 C. The outlet temperature of the syngas is controlled at 800-1100 C. In the gasifier, the carrier gas of the feedstock and the sweeping gas both employ the recyclable CO.sub.2. The external energy accounts for 15-30% of the total energy of the fed fuel. The external energy is any type of energy that can be transformed into heat energy, including but not limited to plasma torch, microwave energy, solar energy, laser energy, electric induction energy. The circulating volume of CO.sub.2 can be regulated according to the furnace temperature and fuel category. The flow rate of the gasifying agent passing through the air distributor in the gasifier can be regulated according to the particle size of the fuel, preferably, the particle size of the fuel is below 50 mm, and the flow rate is 30-60 m/s. When the gasifier is operating, the calcinator 32 is also started, and limestone 33 is calcined to yield CO.sub.2 to act as starting gas.

(14) To achieve the optimal working conditions and the overall performance of the method, the reaction bed temperature is accurately controlled, and the plasma power and the supplied CO.sub.2 are real-time regulated. The above key parameters can be monitored by a monitoring unit disposed at the syngas outlet of the gasifier, or by interlock control, to achieve full automatic operation thereby ensuring the stable running of the system.

(15) The high temperature syngas 30 is then introduced to the waste heat exchanger 2 and exchanges heat with the gasifying agent CO.sub.2 29. Thus, the gasifying agent is preheated by the syngas thereby improving the conversion efficiency of the gasifier. After the primary cooling, the high temperature syngas is further introduced to the waste heat boiler 3 and cooled therein, to yield vapor 24. After the two-stage cooling, the syngas flows into the cyclone separator 4 and the gas scrubber 5 for further cooling and dust removal. The resulting fly ash 26 is collected and discharged. The preheated CO.sub.2 has a temperature of 350-600 C.

(16) The cooled and scrubbed syngas is boosted by the compressor 6 and then introduced to the shift reactor 7, where a water gas reaction happens between the syngas and the vapor 24 which is originated from the waste heat boiler 3, thereby achieving the modifying treatment of the syngas and ensuring the full utilization of the reaction products in the whole process.

(17) The modified syngas is introduced to the desulfurizing tower 8 and the first decarburizing tower 9 for desulfurization and decarbonization. The purified syngas from the first decarburizing tower 9 flows into the synthesizing tower 10. CO.sub.2 from the first decarburizing tower 9 flows into the gas holder 12 by the help of the residual pressure.

(18) The purified syngas in the synthesizing tower 10 is transformed into oil produces 27 through a catalytic synthesis reaction, together with the generation of exhaust gas 28.

(19) The exhaust gas 28 is introduced to the second decarburizing tower 11 and CO.sub.2 29 is separated. The remained effluent gas 31 free of greenhouse gas is treated and discharged outside. Thus, the process of the invention achieves the zero emission of greenhouse gas.

(20) CO.sub.2 from the first decarburizing tower 9 and the second decarburizing tower 11 are both introduced to the gas holder 12, blown by the blower 13 and transported to the gasifier 1 via the primary gasifying agent entrance 20 and the auxiliary gasifying agent entrance 16, and then the next cycle of gasification starts.

(21) While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.