Method and system for cogenerating gas-steam based on gasification and methanation of biomass

Abstract

A method for cogenerating gas-steam based on gasification and methanation of biomass. The method includes: 1) mixing oxygen and water vapor with biomass, transporting the resulting mixture via a nozzle to a gasifier, gasifying the biomass to yield crude gasified gas, and transporting superheated steam having a pressure of 5-6 MPa resulting from sensible heat recovery to a steam turbine; 2) adjusting the hydrogen/carbon ratio of the crude gasified gas generated from step 1) to 3:1, and eluting the crude gasified gas whereby yield purified syngas; 3) introducing the purified syngas from step 2) to a methanation unit and transporting intermediate pressure superheated steam generated in the methanation unit to the steam turbine; and 4) concentrating methane of synthetic natural gas containing trace nitrogen and water vapor obtained from step 3) through pressure swing adsorption.

Claims

1. A method for cogenerating gas-steam based on gasification and methanation of biomass, the method comprising: 1) mixing oxygen and water vapor produced from an air separation plant with biomass, transporting a resulting mixture via a nozzle to a gasifier, gasifying the biomass at a temperature of 1,500-1,800 C. and a pressure of 1-3 MPa to yield crude gasified gas, transporting the crude gasified gas to a waste heat boiler and heating water in the waste heat boiler by the crude gasified gas to yield a superheated steam having a pressure of 5-6 MPa, and transporting the superheated steam to a steam turbine; 2) according to demand for a methanation reaction, adjusting a hydrogen/carbon ratio of the crude gasified gas generated from step 1) to 3:1 using a shift reaction, and eluting the crude gasified gas using low-temperature methanol for desulfurization and decarburization, whereby yield purified syngas; 3) introducing the purified syngas from step 2) to a methanation unit comprising a primary methanation unit and a secondary methanation unit, the primary methanation unit comprising a first primary methanation reactor and a second primary methanation reactor connected in series; allowing part of process gas from the second primary methanation reactor to return to an inlet of the first primary methanation reactor to mix with the purified syngas, and then to enter the first primary methanation reactor, so that a concentration of reactants at the inlet of the first primary methanation reactor is reduced and a temperature of a catalytic bed of the first primary methanation reactor is controlled by the process gas; introducing syngas from the primary methanation unit to the secondary methanation unit comprising a first secondary methanation reactor and a second secondary methanation reactor connected in series, where a small amount of unreacted CO and a large amount of CO.sub.2 is converted into CH.sub.4; and transporting intermediate pressure superheated steam generated in the methanation unit to the steam turbine; and 4) concentrating methane of synthetic natural gas containing trace nitrogen and water vapor obtained from step 3) through pressure swing adsorption, so that a molar concentration of methane reaches 96%, and a calorific value of the synthetic natural gas is up to 8,256 kcal/Nm.sup.3.

2. The method of claim 1, wherein the mixture in step 1) is introduced to an entrained flow gasifier through the nozzle at a rate of 100-120 m/s for parallel flow gasification.

3. The method of claim 1, wherein the crude gasified gas from a side top of the gasifier in step 1) is controlled to have a temperature of 1,200-1,500 C.; waste heat of the crude gasified gas is recycled by the waste heat boiler to produce the intermediate pressure superheated steam; a part of the intermediate pressure superheated steam is mixed with oxygen as a gasification agent, and a remaining part of the intermediate pressure superheated steam is sent to the steam turbine; after recycling the waste heat of the crude gasified gas, the temperature of the crude gasified gas drops to 220-280 C.; the crude gasified gas is purified by a two-stage cyclone separator for removal of dust and is cooled by a washing tower, so as to reduce the temperature of the crude gasified gas to 50-55 C., reduce a dust content to below 50 mg/m.sup.3, and the crude gasified gas comprises CO, H.sub.2 and N.sub.2.

4. The method of claim 2, wherein the crude gasified gas from a side top of the gasifier in step 1) is controlled to have a temperature of 1,200-1,500 C.; waste heat of the crude gasified gas is recycled by the waste heat boiler to produce the intermediate pressure superheated steam; a part of the intermediate pressure superheated steam is mixed with the oxygen as a gasification agent, and a remaining part of the intermediate pressure superheated steam is sent to the steam turbine; after recycling the waste heat of the crude gasified gas, the temperature of the crude gasified gas drops to 220-280 C.; the crude gasified gas is purified by a two-stage cyclone separator for removal of dust and is cooled by a washing tower, so as to reduce the temperature of the crude gasified gas to 50-55 C., reduce a dust content to below 50 mg/m.sup.3, and the crude gasified gas comprises CO, H.sub.2 and N.sub.2.

5. The method of claim 1, wherein the purified syngas in step 3) is divided into two equal streams comprising a first feed gas and a second feed gas, which respectively enters the first primary methanation reactor and the second primary methanation reactor; the first feed gas is first mixed with the process gas to yield a mixed gas and then enters the catalytic bed of the first primary methanation reactor for the methanation reaction, a temperature thereof is controlled at 300-330 C.; an outlet temperature of the mixed gas is 600-630 C.; the intermediate pressure superheated steam is generated through a first waste heat boiler and a first steam superheater; the process gas from the first steam superheater is mixed with the second feed gas, and then enters the second primary methanation reactor for the methanation reaction at 300-330 C.; an outlet temperature of the process gas from the second primary methanation reactor is 600-630 C.; the process gas from the second primary methanation reactor passes through a second waste heat boiler and a second steam superheater to generate the intermediate pressure superheated steam; the process gas from the second primary methanation reactor is divided into two streams: one stream accounts for 30-40%, and flows to the first primary methanation reactor via a recycle compressor; another stream accounts for 60-70%, and enters the first secondary methanation reactor; the process gas is cooled to 270-290 C. and enters the first secondary methanation reactor for further methanation reaction; an outlet temperature of the mixed gas of the first secondary methanation reactor is 440-460 C., and an inlet temperature of a feed gas of the second secondary methanation reactor is 270-290 C.; and the intermediate pressure superheated steam generated from the methanation reaction is transported to the steam turbine.

6. The method of claim 2, wherein the purified syngas in step 3) is divided into two equal streams comprising a first feed gas and a second feed gas, which respectively enters the first primary methanation reactor and the second primary methanation reactor; the first feed gas is first mixed with the process gas to yield a mixed gas and then enters the catalytic bed of the first primary methanation reactor for the methanation reaction, a temperature thereof is controlled at 300-330 C.; an outlet temperature of the mixed gas is 600-630 C.; the intermediate pressure superheated steam is generated through a first waste heat boiler and a first steam superheater; the process gas from the first steam superheater is mixed with the second feed gas, and then enters the second primary methanation reactor for the methanation reaction at 300-330 C.; an outlet temperature of the process gas from the second primary methanation reactor is 600-630 C.; the process gas from the second primary methanation reactor passes through a second waste heat boiler and a second steam superheater to generate the intermediate pressure superheated steam; the process gas from the second primary methanation reactor is divided into two streams: one stream accounts for 30-40%, and flows to the first primary methanation reactor via a recycle compressor; another stream accounts for 60-70%, and enters the first secondary methanation reactor; the process gas is cooled to 270-290 C. and enters the first secondary methanation reactor for further methanation reaction; an outlet temperature of the mixed gas of the first secondary methanation reactor is 440-460 C., and an inlet temperature of a feed gas of the second secondary methanation reactor is 270-290 C.; and the intermediate pressure superheated steam generated from the methanation reaction is transported to the steam turbine.

7. The method of claim 5, wherein a pressure of the intermediate pressure superheated steam that is generated by the first/second waste heat boiler and the first/second steam superheater in the primary methanation unit is 4.5-5 MPa.

8. The method of claim 6, wherein a pressure of the intermediate pressure superheated steam that is generated by the first/second waste heat boiler and the first/second steam superheater in the primary methanation unit is 4.5-5 MPa.

9. The method of claim 1, wherein the methanation reaction in step 3) employs high-load nickel as a catalyst, with a reaction temperature of 270-630 C., a pressure of 1-3 MPa, and a reaction formula of CO+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O, H.sub.0.sup.=206 kJ/mol.

10. The method of claim 2, wherein the methanation reaction in step 3) according to formula CO+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O, H.sub.0.sup.=206 kJ/mol, employs high-load nickel as a catalyst, with a reaction temperature of 270-630 C., and a pressure of 1-3 MPa.

11. The method of claim 5, wherein after the process gas from the second primary methanation reactor is cooled through the second waste heat boiler and the second steam superheater, a first stream flowing to the first primary methanation reactor through the recycle compressor accounts for 60-70%; and a second stream entering the first secondary methanation reactor accounts for 30-40%.

12. The method of claim 6, wherein after the process gas from the second primary methanation reactor is cooled through the second waste heat boiler and the second steam superheater, a first stream flowing to the first primary methanation reactor through the recycle compressor accounts for 60-70%; and a second stream entering the first secondary methanation reactor accounts for 30-40%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flow diagram of a method for cogenerating gas-steam based on gasification and methanation of biomass of the invention;

(2) FIG. 2 is a schematic diagram of a gasification unit of the invention; and

(3) FIG. 3 is a schematic diagram of a methanation unit of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) The invention is further illustrated in more detail in the light of the drawings and embodiments, which are not intended to define the protection scope of the invention.

(5) As shown in FIG. 1, FIG. 2 and FIG. 3:

(6) Step 1): Rice straw, crop stalk and other biomass were dried, crushed and sorted. Biomass raw material with the diameter or maximum length below 2 mm was directly transported to the gasifier of the high-temperature airflow bed through a screw feeder. The pressure in the gasifier was 1-3 MPa. An external thermostatic heater was provided in the outlet segment of the gasified gas of the biomass gasifier to keep the gasification temperature within the gasifier at 1,500-1,800 C., and guarantee the gasification intensity, gasified gas quality and strengthened heat transfer process. As the gasification agent, superheated water vapor and oxygen were premixed, and then fully mixed with the treated biomass. The mixture was injected into the gasifier with a special nozzle at a rate of 100-120 m/s for gasification by way of parallel flow. Due to small particle size and strong heat transfer ability of the airflow bed, raw material was heated to the furnace temperature as soon as it entered the furnace. Almost at the same time, there was moisture evaporation, volatile matter decomposition, tar cracking, carbon combustion and gasification. Alkali-containing metal ash was converted to liquid slag, and then directly discharged. Small biomass particles were retained in the reaction zone for less than 3 s, rapidly gasified before melting, and respectively separated by the high-speed gas flow without the phenomenon of bonding and cohesion etc. Crude gasified gas from the side top of the gasifier was controlled at a temperature of 1,200-1,500 C. The upper part of the gasifier was provided with a waste heat boiler. The crude gasified gas from the gasifier entered the waste heat boiler to recover waste heat, and produced a lot of steam. After heat recovery, the gasified gas temperature dropped to 220-280 C. Then most dust and water vapor in the gasified gas were purified and cooled through two stage cyclone separators and a washing tower, so as to reduce the gasified gas temperature to about 50-55 C., reduce dust content to below 50 mg/m.sup.3, and generate the gasified gas mainly containing CO, H.sub.2 and N.sub.2.

(7) Step 2): After washing and dust removal, crude gasified gas entered the sulfur-tolerant shift process, the ratio of H.sub.2, CO and CO.sub.2 therein was adjusted to the hydrogen/carbon ratio of 3:1, and the vast majority of organic sulfur therein was converted into inorganic sulfur. Then the gas was purified, and washed with methanol at low temperature, so that methanol was used to remove unwanted CO.sub.2 and all sulfides from methanation reaction of the process gas, and the process gas composition achieved the methane production requirements. Methanol rich in CO.sub.2, H.sub.2S and COS was recycled by flash evaporation under reduced pressure and nitrogen stripping etc. and the cooling capacity therein was recovered for re-use.

(8) Step 3) methanation: The purified syngas in step 2) at adjusted hydrogen/carbon ratio can be divided into two approximately equal streams, which respectively entered the first primary methanation reactor and the second primary methanation reactor. Feed gas entering the first methanation reactor was first mixed with the recycle gas. The mixed gas at 300-330 C. entered the catalytic bed of the adiabatic reactor, wherein the exothermic methanation reaction occurred. Outlet temperature of the hot gas was about 600-630 C., and the hot gas was used to produce intermediate pressure superheated steam in the first waste heat boiler and superheat intermediate pressure superheated steam in the first superheater. The process gas from the first superheater was mixed with another stream of fresh feed gas. The mixed gas at 300-330 C. entered the second methanation reactor for further methanation reaction.

(9) Outlet temperature of the gas of the second primary methanation reactor was about 600-630 C., which was used to produce steam in the second waste heat boiler and preheat recycle gas in the second superheater. Hot process gas from the second waste heat boiler was divided into two streams: one stream accounted for 30-40%, and flowed to the first primary methanation reactor via a recycle compressor; another stream accounted for 60-70%, and entered the first secondary methanation reactor.

(10) Inlet temperature of the feed gas of the second secondary methanation reactor was 270-290 C., and further methanation reaction occurred in the second secondary methanation reactor, in order to achieve the specification of SNG product. Intermediate pressure superheated steam generated from the methanation reaction was transported to the steam turbine.

(11) Pressure of the intermediate pressure superheated steam that was generated by the waste heat boiler and steam superheater using the recycling reaction heat in the primary methanation reaction stage was 4.5-5 MPa.

(12) The methanation was carried out with high-load nickel as the catalyst, at the reaction temperature of 270-630 C., under the pressure of 1-3 MPa, and following
CO+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O H.sub.0.sup.=206 kJ/mol.

(13) Step 4) Methane concentration: After methane was concentrated through pressure swing adsorption of crude natural gas, substituent natural gas following national standard was obtained. In general, the natural gas enters the urban gas pipeline as domestic gas, and may also enter the gas turbine to generate electricity in case of power shortage.

(14) As shown in FIG. 1, a method and system for cogenerating gas-steam based on gasification and methanation of biomass comprises a gasification unit, a shift unit, a purification unit, a methanation unit and a methane concentration unit.

(15) As shown in FIG. 2, a waste heat boiler is provided in the upper part of the gasifier of the gasification unit. Intermediate pressure superheated steam is generated by the waste heat boiler using the recycling waste heat of the crude gasified gas from the gasifier, and is transported to the steam turbine. An external thermostatic heater is provided in the outlet segment of the gasified gas of the biomass gasifier to keep the gasification temperature within the gasifier at 1,500-1,800 C. Crude gasified gas from the gasifier is controlled at a temperature of 1,200-1,500 C.

(16) As shown in FIG. 3, the methanation unit comprises a primary methanation unit and a secondary methanation unit. The primary methanation unit is composed of two reactors connected in series with two reactors connected in parallel in either stage. The outlet of the second primary methanation reactor is provided with two bypasses: one bypass is connected to the inlet of the first primary methanation reactor, so that the process gas therein is mixed with fresh feed gas, and then enters the first primary methanation reactor; the other bypass is connected to the secondary methanation reactor, which comprises a first secondary methanation reactor and a second secondary methanation reactor connected in series. The second secondary methanation reactor is connected to the methane concentration unit. After the second stage primary methanation, a part of the process gas returns to the inlet of the first primary methanation reactor, is mixed with fresh feed gas, and then enters the first primary methanation reactor, in order to reduce the reactant concentration at the inlet of the primary methanation reactor. On the other hand, the recycle process gas is used for inert medium heating to control the catalyst bed temperature. The vast majority of methanation reaction is completed at the primary methanation stage. Reaction temperature of the secondary methanation is lower than that of the primary methanation. This stage is composed of two secondary methanation reactors connected in series, and converts a small amount of unreacted CO and most of the CO.sub.2 into CH.sub.4. Product gas from the methanation stage is transported to the methane concentration process.

(17) Reaction heat of the mixed gas at the outlet of the first primary methanation reactor is recycled by the first waste heat boiler and the steam superheater, and that of the mixed gas at the outlet of the second primary methanation reactor is recycled by the second waste heat boiler and the steam superheater. The intermediate pressure superheated steam generated therefrom is transported to the steam turbine.

EXAMPLE 1

(18) Calculation of the overall system performance under the basic load with 1,000 tons/day raw material biomass.

(19) Dry rice straw is used as the gasification biomass in Example 1. Its ingredients and calorific value are shown in Table 1.

(20) TABLE-US-00001 TABLE 1 Ingredients and calorific value of biomass Items Content Unit Rice straw calorific value Qar, net MJ/kg 11.346 Elements Carbon Car % 37.162 Hydrogen Har % 2.748 Oxygen Oar % 35.136 Nitrogen Nar % 0.905 Sulfur Sar % 0.029

(21) In example 1, biomass 1000 tons/day, and the gasification agent comprising 93 vol. % of oxygen is employed for gasification.

(22) TABLE-US-00002 TABLE 2 Material balance and performance parameters of gas product Example 1 Gasified gas Shift gas Syngas Gas product Rate of flow Nm.sup.3/h 43960 53320 29300 7350 Percentage vol. % CO 43.41% 12.03% 20.36% 0.01% H.sub.2 16.42% 37.30% 67.53% 0.30% N.sub.2 5.77% 4.76% 8.04% 1.17% CO.sub.2 21.19% 41.23% 1.88% 1.13% CH.sub.4 1.92% 1.59% 2.16% 96.13% H.sub.2O 9.29% 1.45% 0.03% 1.26% C.sub.2-C.sub.4 1.92% 1.59% 0.00% 0.00% Caloric value of syngas kcal/m.sup.3 2579 8227 Yield of syngas m.sup.3/d 176400 Chemical synthesis efficiency from biomass to syngas .sup.47% (chemical energy of SNG/biomass chemical energy of biomass) Steam from methanation 450 C., 4.7 MPa (t/h) 21.45

EXAMPLE 2

(23) Biomass 1000 tons/day, and the gasification agent comprising 98 vol. % of oxygen is employed for gasification.

(24) TABLE-US-00003 TABLE 3 Material balance and performance parameters of gas product Example 2 Gasified gas Shift gas Syngas Gas product Rate of flow Nm.sup.3/h 51140 64640 39670 10190 Percentage vol. % CO 52.10% 14.46% 21.91% 0.01% H.sub.2 22.81% 44.81% 72.66% 0.20% N.sub.2 2.02% 1.60% 2.48% 2.00% CO.sub.2 12.55% 36.70% 1.79% 0.28% CH.sub.4 1.17% 0.92% 1.13% 96.47% H.sub.2O 9.27% 1.45% 0.02% 1.03% C.sub.2-C.sub.4 0.02% 0.01% 0.00% 0.00% Caloric value of syngas kcal/m.sup.3 2672 8256 Yield of syngas m.sup.3/d 244560 Chemical synthesis efficiency from biomass to syngas .sup.64% (chemical energy of SNG/biomass chemical energy of biomass) Steam from methanation 450 C., 4.7 MPa (t/h) 30.90