CN103314083B - Process for producing synthesis gas - Google Patents

Abstract

A method of controlling carbon conversion in a gasifier fueled by a carbonaceous feedstock by mixing the carbonaceous feedstock into biomass, the method comprising the steps of: (a) pressurizing the biomass and the carbonaceous feedstock; (b) introducing the biomass and carbonaceous feedstock into the gasification reactor vessel; (c) partially oxidizing the carbonaceous feedstock/biomass with a gas containing molecular oxygen to obtain a synthesis gas containing carbon monoxide and hydrogen; (d) measuring _CO content in the syngas and compared with a predetermined range of values; (e) adjusting the ratio of biomass/carbonaceous feedstock by changing the feed rate of biomass; wherein the biomass and carbonaceous feedstock comprise 10wt% to 50 wt% biomass and wherein the level of biomass is adjusted within this range to control carbon conversion.

Description

Methods of Producing Syngas

The present invention relates to a method for the production of a synthesis gas stream from a carbonaceous feedstock comprising controlled carbon conversion with biomass fuel during the gasification process at a given gasification temperature.

WO03/012013 describes a process for the incomplete combustion of domestic waste to produce synthesis gas. The described method also includes controlling the temperature of the oxygen-containing gas fed thereto, controlling the feed rate of waste and the resulting ratio of waste feed to oxygen-containing gas feed, and thermally insulating the reaction space in the The temperature is controlled in different parts of the reaction space.

JP2002194363 describes a method of pressurized entrained bed coal gasification, by which biomass can be effectively used to reduce the high heat generated by coal gasification, thereby reducing ash generated on the operation of pressurized entrained bed gasifier Problems with buildup and melting. The method is characterized in that coal particles are introduced into the lower part of the pressurized entrained entrained gasifier to produce high-temperature gas, and biomass fuel is introduced into the upper part to bring in contact with the high-temperature gas and gasify the biomass fuel. Therefore, the height of the waste heat recovery boiler in the gasification process can be controlled.

In the present invention, carbonaceous feedstock, such as coal, may be mixed with biomass prior to introducing the mixture into the combustor section of the gasifier. Such blending of biomass and coal helps to ensure a synergy between oxygen-rich biomass and oxygen-poor carbonaceous feedstock. Due to the high oxygen content in the biomass feedstock, the need for moderator gas to obtain sufficient conversion of the carbonaceous feedstock at a given gasification temperature is reduced, and this requirement is even high enough in the coal/biomass mixture The biomass fraction is eliminated. Typically steam or carbon dioxide or a combination thereof is used as moderator gas.

It is an object of the present invention to provide a simplified, energy efficient and renewable method for controlling the conversion of carbonaceous feedstocks by mixing them into biomass fuels. Biomass generally has a high oxygen content and can provide some of the oxygen required for the overall conversion of the carbonaceous feedstock, which in turn reduces the need for moderator gases such as steam. A certain minimum biomass fraction is required in the biomass/carbonaceous feedstock mixture to eliminate steam. This fraction is determined by the final composition of lignocellulosic biomass and carbonaceous feedstock. The usual minimum fraction to achieve complete carbon conversion is 10-30% by weight biomass in the carbonaceous feedstock/biomass mixture; however any fraction between 10% and 50% may be used.

Use the following methods to achieve the above purpose.

A method of controlling carbon conversion in a gasifier fueled by a carbonaceous feedstock by mixing the carbonaceous feedstock into biomass, the method comprising the steps of:

(a) pressurizing biomass and carbonaceous feedstock;

(b) introducing biomass and carbonaceous feedstock into a gasification reactor vessel;

(c) Partial oxidation of carbonaceous feedstock/biomass with a gas comprising molecular oxygen to obtain synthesis gas comprising carbon monoxide and hydrogen;

(d) measuring the _CO2 content in the syngas and comparing it with a predetermined range of values;

(e) adjusting the biomass/carbonaceous feedstock ratio by varying the biomass feed rate;

wherein the gasifier feed comprises a minimum of 10 wt% to 50 wt%, preferably 10 wt% to 30 wt% biomass, and wherein the level of biomass can be adjusted within this range as needed to control carbon conversion and reduce or eliminate the impact on Moderator Gas Requirements. Syngas produced by this method can also contain water, hydrogen sulfide and carbon dioxide.

Applicants have found that by mixing 10 to 50 wt%, preferably 10 to 30 wt%, of biomass with a carbonaceous feed, the temperature in the gasifier can be controlled using the oxygen/carbonaceous fuel ratio and the conversion of carbon can be controlled using Biomass/carbonaceous fuel ratio control, thereby simplifying process control. The addition of said biomass has the additional beneficial effect of saving steam and boiler feed water consumption and also reduces the overall carbon footprint of the gasifier.

The present invention will be described in more detail below.

Pressurization in step (a) can be achieved by a lock hopper system or a (solids blowing) pump. Lock hopper systems are generally used to pressurize dry and solid raw materials. Pumps can be used to pressurize liquid or slurry (a mixture of solid and liquid) feedstock. Solid blowing pumps can also be used to pressurize solid feedstock.

The comminuted or liquid slurry biomass in step (b) can be mixed with the carbonaceous feed or fed separately to the gasifier. A separate biomass feed delivery pipe feeding the gasification reactor is preferred as it will significantly reduce the response time on steps from the carbon conversion controller. The mixing of biomass with carbonaceous feedstock can occur at various locations in the fuel preparation and pressurization areas.

In step (c), any type of coal or oil burner configuration can be used. When the biomass is supplied separately into the gasification reactor, the biomass can also be subsequently introduced through nozzles into the burner region of the gasification reactor. The oxygen required for the partial oxidation of the biomass will then be supplied via a carbonaceous fuel burner.

In step (c), the carbonaceous/biomass feedstock is subjected to partial oxidation with a gas comprising molecular oxygen. The partial oxidation is preferably carried out at a temperature between 1000 and 1800°C, and more preferably at a temperature between 1200 and 1800°C. The pressure at which the partial oxidation is carried out is preferably between 0.3 and 12 MPa and preferably between 3 and 10 MPa. When using ash-comprising raw materials, the temperature conditions are chosen such that a slag layer will form inside the reactor vessel where partial oxidation takes place.

In step (d), the _CO2 content is preferably measured in the cleaned and cold syngas, eg downstream of a wet scrubber. Fast measurements are preferred to minimize the time between a _CO2 concentration step and control action. An infrared based analyzer is an example of such a fast measuring device.

In step (d), the CO ₂ content of the syngas is compared with a preset value. The controller in step (e) will then adjust the biomass/coal ratio, preferably by changing the set point of the biomass feed rate controller.

The carbonaceous feedstock is preferably coal, such as anthracite, lignite, bituminous and sub-bituminous coal. Examples of alternative carbonaceous raw materials are petroleum coke, peat and heavy residues extracted from tar sands or bituminous fractions separated from said residues during deasphalting. Residues from refineries (eg resid fractions boiling above 360°C) and derived directly from crude oil or from oil conversion processes (eg thermal cracking, catalytic cracking, hydrocracking, etc.) can also be used as carbonaceous feedstock.

Any biomass-derived feedstock comprising a low moisture content, typically less than 20 wt%, and which can be ground to particles having a size between 10 and 1000 microns is a suitable solid biomass feedstock comprising ) 10wt% to 50wt% biomass component of carbonaceous raw materials. Feedstocks obtained by roasting of biomass sources are very suitable and preferably have a higher biomass fraction in the mixture. Torrefaction is preferably combined with a pressing or pelletizing step in order to make the biomass feed more suitable for the gasification process, wherein the biomass feed is supplied in so called dry form or slurry when the torrefied pellets are mixed with the carbonaceous liquid. Roasting of biomass raw materials is well known and for example in M.Pach, R.Zanzi and E.Bjornbom, Torrefied Biomass a Substitute for Wood and Charcoal. 6th Asia-Pacific International Symposium on Combustion and Energy Utilization. May 2002, Kuala Lumpur and in Bergman , P.C.A., "Torrefaction in combination with pelletisation-the TOP process", ECN Report, ECN-C-05-073, Petten, 2005 described.

Another suitable solid biomass fuel in the process of the invention is obtained by drying and slow pyrolysis of a biomass source. During slow pyrolysis, a solid char feed component is usually obtained. Slow pyrolysis is well known and described for example in "Pyrolysis and Other Thermal Processing". US DOE14-08-2007.

Suitable liquid or solid biomass feedstocks for use in the process of the invention are obtained by drying and flash cracking of biomass sources. In the flash cracking process, solid char and liquid biomass feed components are generally obtained. Both can be used as feedstock for the gasification process. Flash thermal cracking is well known and for example in EP-A-904335; Dinesh Mohan, Charles U. Pittman, Jr., and Philip H. Steele. Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy & Fuels 2006, 20, 848-889; and in E.Henrich: Cleansyngas from biomass by pressurized entrained flow gasification of slurries from fast pyrolysis. In: Synbios, the syngas route to automotive biofuels, conference held from 18-20May2005, Stockholm, Sweden (2005). The invention is also directed to an embodiment in which a so-called biomass slurry is used as feedstock. The slurry can be obtained by mixing pyrolysis oil and charcoal.

Suitable biomass sources are weeds or agricultural residues. Examples of suitable residue products are streams generated in the palm oil industry, corn industry, biodiesel industry, forestry industry, wood processing industry and paper industry. Certain biomass is relatively expensive, such as wood pellets. Therefore, in order to keep operating costs as low as possible, lower cost biomass sources such as oil palm trunks and sawdust are preferred. However, the low amount of biomass required to replace moderator steam increases the opportunity to use biomass from waste streams, which are often inexpensively available in small quantities.

The present invention now provides a method by which the partial oxidation of a carbonaceous feedstock is carried out in an efficient manner, whereby a synthesis gas with a reduced carbon footprint suitable for catalytic conversion reactions is obtained. Catalytic conversion reactions of particular interest are hydrocarbon synthesis processes. In hydrocarbon synthesis processes, synthesis gas is catalytically converted into hydrocarbon compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms (or even higher in special cases). An example of a hydrocarbon synthesis process is the Fischer-Tropsch process as described eg in WO02/02489, WO01/76736, WO02/07882, EP510771 and EP450861.

Step (c) can be carried out by different gasification methods, such as the so-called moving bed method, fluidized bed gasification described in Gasification, by Christofer Higman and Maarten van der Burgt, 2003, Elsevier Science, Burlington MA, Pages 85-128 gasifier method or entrained bed gasifier method. The use of an entrained bed gasifier is preferred because the process can handle a wide variety of feedstocks and because a tar-free synthesis gas is produced. In this process, feedstock and oxygen are introduced into the reactor in cocurrent, preferably by means of suitable burners. Examples of suitable burners and their preferred applications are described in US-A-4510874 and US-A-4523529. The operating conditions are such that the process operates in slagging mode, which means that the operating temperature is above the melting point of the ash. Carbon-containing feedstock and molecular oxygen-containing gas are produced by providing said reactants at a pressure between 3 and 10 MPa, and preferably at a pressure between 4 and 8 MPa, into a burner present in the gasification reactor Suitable conversion to synthesis gas. The operating temperature is suitably between 1200 and 1800°C. The synthesis gas is preferably quenched directly with evaporated water, directly with a methanol-water mixture, indirectly by heat exchange with evaporated water or a combination of these cooling steps to a temperature below 1000°C, preferably below 500°C. At the bottom end of the reactor, slag and other molten solids are suitably discharged from the gasification reactor.

The solid carbonaceous feedstock/biomass feed mixture may be provided as a slurry in water to the combustor of the stream gasifier reactor. A coal slurry feeding process is described, for example, in EP-A-168128. The solid carbonaceous feedstock/biomass feedstock is preferably provided to the burner in a gas-solid mixture comprising the solid feed in powder form and a suitable carrier gas. Suitable carrier gases are nitrogen, carbon dioxide, natural gas or synthesis gas, i.e. a mixture comprising CO and _H2 . The carrier gas is preferably carbon dioxide. The use of such a carrier gas is described, for example, in WO-A-2007042562.

Figure 1 shows a preferred embodiment of the invention.

Figure 1 schematically shows a system for producing and cleaning syngas. In gasification reactor 9 are introduced carbonaceous feed, biomass feed and oxygen-containing feed. Oxygen and carbonaceous streams are fed via lines 19 and 8, respectively. Biomass can be fed separately into gasification reactor 9 via line 25 , but can also be mixed with a carbonaceous stream in feed and pressurization zone 7 via line 5 . The carbonaceous stream 8 and the biomass stream 25 are at least partially oxidized in the gasification reactor 9, whereby raw synthesis gas 10 and slag are obtained. For this purpose, usually a plurality of burners (not shown) are present in the gasification reactor 9 . The biomass stream 25 can also be introduced into the gasification reactor 9 via nozzles in the combustion zone of the gasification reactor, eg at the burner level. Oxygen for the partial oxidation of the biomass will then be added to the burner oxygen of the carbonaceous stream 8 .

The carbon conversion of the gasifier is controlled by a controller 18 . The input for the carbon conversion controller is the CO ₂ content of the syngas 16 downstream of the wet scrubber 15 measured using a CO ₂ analyzer 17 . As soon as the _CO2 content drops below a predetermined value, the biomass/coal ratio needs to be increased by changing the biomass feed 3 and increasing the flow in line 5 or 25. As soon as the CO ₂ concentration exceeds a predetermined maximum value, the biomass/coal ratio can be reduced by changing the biomass feed 3 and reducing the flow in line 4 or 25 . _CO2 analyzers, and the benefits of controlling the ratio of steam and oxygen to carbon (O/C), are described in WO2008125556A1 and WO200768684A2.

The advantage of using biomass to control carbon conversion is the use of a natural raw material of _CO2 , which reduces the _CO2 footprint of the plant. Additionally, this application reduces the need for high-pressure steam and high-quality boiler feed water for the gasification island.

The raw synthesis gas produced is fed via line 10 into cooling zone 11 ; here the raw synthesis gas is typically cooled to about 200-400°C. Cooling zone 11 may be an indirect heat exchanger or a quench vessel. In the case of a quench vessel, liquid water is preferably injected into the synthesis gas stream via line 23 . Liquid water is preferably injected in the form of a mist.

The cooling zone 11 can be integrated into the gasification reactor pressure vessel at the bottom of the reactor or installed as a separate vessel. When both syngas and slag come out at the bottom of the gasification reactor, a cooling zone 11 is integrated into the pressure vessel and the syngas and fly ash and slag will enter the cooling zone. When the syngas leaves the gasification reactor 9 at the top and the slag at the bottom, then only the syngas and fly ash will enter the cooling zone installed as a separate zone downstream of the reactor.

As shown in the embodiment of Figure 1, the raw synthesis gas leaving the cooling zone 10 is further processed. To this end, the raw synthesis gas is fed via line 12 to a dry or wet solids removal unit 13 in order to at least partially remove ash from the raw synthesis gas. Since the solids removal unit 13 is known to those of ordinary skill in the art, it is not discussed further here. Ash is removed from the solids removal unit via line 24 . After the solids removal unit 13 , the raw synthesis gas can be fed via line 14 into a wet gas scrubber 15 and subsequently via line 16 to the production area. After further processing of the syngas, the residual syngas can be used for various purposes such as power generation, production of _H2 , fertilizers, Fischer-Tropsch liquids and other chemicals.

Computer simulations model the reduction of moderator vapor with the addition of biomass to the coal. For the gasification of selected Drayton coal, approximately 76 kg of steam per ton of coal will be required to obtain complete carbon conversion at a gasification temperature of 1500°C. As shown in Table 1, the addition of coal with wood will gradually reduce the need for moderators. Moderator vapors can be eliminated by replacing >25% of coal with wood.

Table 1 - Typical moderator consumption for different biomass fractions in biomass/coal mixtures

Claims (14)

1. A method of controlling carbon conversion in a gasifier fueled by a carbonaceous feedstock by mixing the carbonaceous feedstock into biomass, the method comprising the steps of: (a) pressurizing biomass and carbonaceous feedstock; (b) introducing biomass and carbonaceous feedstock into a gasification reactor vessel; (c) Partial oxidation of carbonaceous feedstock/biomass with a gas comprising molecular oxygen to obtain synthesis gas comprising carbon monoxide and hydrogen; (d) measuring the _CO2 content in the syngas and comparing it with a predetermined range of values; (e) adjusting the biomass/carbonaceous feedstock ratio by varying the biomass feed rate; wherein the biomass and carbonaceous feedstock comprises 10 wt% to 50 wt% biomass and wherein the level of biomass is adjusted within this range to control carbon conversion. 2. The method according to claim 1, wherein the carbonaceous feedstock and the biomass are introduced separately into the gasification reactor. 3. The method according to claim 1, wherein the carbonaceous feedstock and biomass are introduced into the gasification reactor as a mixture. 4. The method according to claim 1, wherein the feedstock fed to the gasifier comprises 10 wt% to 30 wt% biomass fuel. 5. The method according to claim 1, wherein the biomass is solid biomass obtained by roasting. 6. The method according to claim 1, wherein the biomass is solid biomass obtained by slow pyrolysis of a biomass source. 7. The method according to claim 1, wherein the biomass is solid biomass obtained by flash thermal cracking of biomass sources. 8. The method according to claim 4, wherein the biomass is liquid biomass obtained by flash thermal cracking of biomass sources. 9. The method according to claim 4, wherein the biomass is slurry biomass consisting of a mixture of pyrolysis oil and char obtained by flash thermal cracking of biomass sources. 10. The method according to claim 1, wherein the carbonaceous feedstock is coal. 11. The method according to claim 1, wherein the carbonaceous feedstock is residual oil. 12. The method according to claim 1, wherein in step (c), the carbonaceous feedstock/biomass fuel mixture and the gas containing molecular oxygen are provided to a gasification reactor at a pressure of 1 to 10 MPa by providing said reactants Converted to syngas in the combustor. 13. The method according to claim 1, wherein the _CO2 content in step (d) is measured using an infrared based analyzer. 14. The method of claim 1, wherein in step (e) the controller will adjust the biomass/coal ratio by changing the set point of the biomass feed rate controller.