Patent Application
20150315498
Coal gasification technologies are mainly classified into three types: fixed bed gasifiers such as those made available from Lurgi, fluidized bed gasifiers such as those from UCAS and entrained flow gasifiers like those from Shell and General Electric, The fixed bed gasifiers have disadvantages in terms of low throughput of single unit and costly syngas and water treatment systems. The fluidized bed gasifiers have problems with dry bottom ash discharging, high fly ash carried over and low carbon conversions, For the entrained flow gasifiers, the major defects are narrow range of suitable types of coal feedstock and the particle size of the coal fed to the gasifier must be kept below 75 micron. Additionally, large amounts of steam such as in TPRI gasifiers or other types of slurry gasifiers or cold syngas gas such as in Shell gasifiers must be injected for cooling purposes to ensure the temperature of the raw syngas outlet of the gasifier remains below 950 to 1000° C. due to the requirements of the downstream waste heat boiler. This can dramatically reduce the thermal efficiency of the gasification process.
The invention seeks to overcome these limitations by combining normal fluidized bed and entrained flow coal gasification technologies to improve their operation, efficiencies and reduce costs.
In one embodiment of the invention, there is disclosed a gasifier for producing syngas comprising an upper fluidized gasification zone in fluid communication through a Venturi throat with a lower entrained flow gasification zone.
The gasifier has refractory lining walls in the upper fluidized gasification zone and water membrane was in the lower entrained flow gasification zone. The two gasification zones have pipes and openings to avow for the injecting of materials for forming syngas, namely coal, coal fines and other reactants such as oxygen and steam.
The two gasification zones are connected by a Venturi throat which has a diameter that is narrower than both gasification zones. The entrained flow gasification zone has an opening at the bottom to avow for the removal of liquid slag while the fluidized gasification zone has an opening at the top to allow for recovery of the syngas produced as well as fines and other product gases produced.
In another embodiment of the invention, there is disclosed a method for producing syngas comprising the steps:
a) Feeding coal fines, oxygen and steam simultaneously to a fluidized gasification zone and an entrained flow gasification zone;
b) Reacting the coal fines in the presence of oxygen and steam to form syngas;
c) Feeding syngas from the entrained gasification zone to the fluidized gasification zone; and
d) Recovering syngas from the fluidized gasification zone.
The coal fines that are fed to the fluidized gasification zone are larger than those fed to the other zone and are greater than 75 micron in size. The coal fines fed to the entrained flow gasification zone are less than 75 micron in size.
The fluidized gasification zone is at a temperature of about 1000° C. or below the coal ash deformation temperature, while the entrained flow gasification zone is at a temperature of about 1500° C. or above the coal ash fluid temperature.
The syngas that is produced in both gasification zones is fed through an opening at the top of the fluidized gasification zone at a temperature of about 900 to 1000° C. to a first stage cyclone which will allow for recovery of the syngas as well as the recovery of unreacted coal fines which can be fed back to the fluidized gasification zone.
Steam is injected into said entrained flow gasification zone to drive the produced syngas into said fluidized gasification zone.
A second cyclone will capture smaller unreacted coal fines and these are recycled back to the entrained flow gasification zone.
The gasifier and methods of the invention can be used in a variety of applications such as integrated gas combined cycle, coal to chemicals, production of synthetic natural gas, etc.
The FIGURE is a schematic of a two-stage coal gasifier according to the invention.
As shown in the figure, the two-stage coal gasifier is shown. A fluidized gasification zone A sits above an entrained flow gasification zone B being connected by a Venturi throat C. The fluidized gasification zone has wails that comprise a refractory material D. The entrained flow gasification zone has wail that comprise by a water membrane wall E.
Gasification is occurring in both zones such that coal particles of various particle size distributions are contacted with oxygen and/or steam. The resulting products include syngas and a measured amount of carbon dioxide plus methane,
Coal having a particle size below 75 microns and fines recycled from a second stage cyclone are injected through line 9 into the entrained flow gasification zone B. Typically the coal is transported in nitrogen or carbon dioxide. The gasification temperature is controlled at around 1500 to 1600° C. above the coal ash fluid temperature. The operating pressure is about 3.0 to 4.0 MPa.
The steam that is injected into the entrained flow gasification zone B is used to control the gas temperature and flow rate entering the upper fluidized gasification zone A. The water membrane wall E is essentially high-pressure tubes containing water which helps cool the entrained flow gasification zone B. Steam is also generated inside the tubes. The tubes are provided with studs that act as anchors for a thin layer of castable refractory which is typically silicon carbide. During operation of the gasifier, the castable refractory will ideally be covered by a layer of solid slag, over which the liquid slag will run to the bottom 10 of the entrained flow gasification zone B.
The hot syngas that is generated enters the fluidized gasification zone A through a Venturi throat C where the local temperature above the Venturi throat C is maintained at about the coal ash softening temperature. The Venturi throat C also functions as a “classifier” in the sense that agglomerate char particles with higher density could drop into the section below the Venturi throat C and carry out secondary reactions thereby becoming melting slag. The melting slag will exit the entrained flow gasification zone B through opening 10 for quenching.
Coal having a larger particle size, above 75 microns and below 10 millimeter is fed into the dense phase of the fluidized bed section in the fluidized gasification zone A through line 1. The fluidizing gas is oxygen and steam, The fines collected from the first stage cyclone (not shown) are recycled and reinjected into the fluidized gasification zone A through line 2. The fluidizing oxygen and steam are injected through lines 3, 4, 5 and 6. The raw syngas produced which is primarily hydrogen and carbon monoxide with some methane present as well will exit the fluidized gasification zone A through opening 11 to the first stage cyclone. This syngas reaction temperature is at a temperature of about 900 to 1000° C. or below the coal ash deformation temperatures. The discharged rate of reacted residues or char is controlled by the Venturi throat C.
The fluidized gasification zone A is typically constructed with a normal refractory wail which consists of an outer layer of insulating bricks to protect the outer steel she of the reactor from high temperatures and an inner layer of more compact bricks that can better withstand the high temperatures and the erosive conditions inside the gasifier.
The cyclones that are used in the methods of the invention connect to the upper section of the gasifier, fluidized gasification zone through line 11 and the bottom part of the gasifier, entrained flow gasification zone B through line 9. The first cyclone captures 98 to 99% of the fines carried over the top of the gasifier through line 11. These fines contain significant amounts of carbon and are recycled to the fluidized gasification zone A through line 2 for secondary reactions in the gasifier. The second cyclone captures the remainder of fines having a smaller particle size which will lower gasification reactivity. These smaller fines are recycled in to the bottom of the entrained flow gasification zone B through line 9 where they are reacted under more severe conditions to burnout the extra amount of carbon present in the smaller fines.
Advantages of the invention include an ash slagging and quench procedure to alleviate the problem of dry ash discharging of fluidized bed gasifiers. The slag can simply be discharged through the bottom 10 of the entrained flow gasification zone B. By using a wider range of coal particle size distribution (PSD) of particles both smaller than 75 micron and larger than 75 micron, this will reduce the consumption of coal mills as well as allow for a wider range of coal feedstocks that can be employed in producing syngas. The carbon conversion of fluidized gasifiers can be improved by increasing the ash discharging temperature. Further, the thermal efficiency of entrained flow gasifiers can be improved by eliminating the recycle of cold syngas or extra steam. Also the amount of fly ash carried over from the fluidized bed gasifiers is reduced and with it there would be less disposal costs.
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2012/086808 | 12/18/2012 | WO | 00 |
