PROCESS FOR PRODUCING SYNTHESIS GAS

The present invention relates to a process for producing synthesis gas stream from a carbonaceous feedstock, which process includes biomass fuel as a means to control the carbon conversion at a given gasification temperature in a gasification process.

WO 03/012013 describes a process for the incomplete combustion of domestic waste to produce syngas. The process as described may also include controlling the temperature in various parts of the reaction space by controlling the temperature of the oxygen-containing gas fed thereto, controlling the rate of feed of the waste and the resulting ratio of the waste feed to the oxygen- containing gas feed, and thermally insulating the reation space .

JP2002194363 describes a method for the pressurized entrained bed gasification of coal, by which biomass can be used to efficiently lower high heat generated by gasification of the coal, thereby reducing the occurrence of problems with the accumulation and fusion of ash on the operation of a pressurized entrained bed gasification furnace. The method is characterized by introducing coal particles into the lower portion of the pressurized entrained bed gasification furnace to produce the high temperature coal gas, introducing a biomass fuel into the upper portion to bring into contact with the high

temperature coal gas and gasifying the biomass fuel.

Thereby, the height of an exhaust heat recovery boiler of the gasification process can be controlled.

In the present invention a carbonaceous feedstock such as coal may be mixed with biomass prior to the mixture being introduced into the burner section of the - - gasifier. The mixing of biomass with the coal in such a manner helps to ensure synergy between the oxygen rich biomass and oxygen lean carbonaceous feedstocks. Due to the high oxygen content in the biomass feedstock, the requirement of a moderator gas to obtain full conversion of the carbonaceous feedstock at a given gasification temperature is reduced and can even be eliminated at sufficient high fractions of biomass in the coal/biomass mixture. As moderator gas steam or carbon dioxide or a combination thereof is typically used.

An object of the present invention is to provide a simplified, energy efficient and renewable means of controlling the carbonaceous feedstock conversion by mixing in a biomass fuel. Biomass in general has a high oxygen content and can provide part of the oxygen

required for full conversion of the carbonaceous

feedstock, which in turn reduces the requirement of a moderator gas, such as steam. A certain minimum biomass fraction in the biomass/carbonaceous feedstock mixture is required to eliminate the steam. This fraction is

determined by the ultimate composition of the

lignocellulosic biomass and carbonaceous feedstock. A typical minimum fraction to get complete carbon

conversion is 10-30% of biomass by weight in the

carbonaceous feedstock/biomass mixture; however, any fraction between 10% to 50% may be used.

The above object is achieved with the following process .

A process for controlling the carbon conversion of a gasifier fuelled with a carbonaceous feedstock by mixing in biomass, the process comprising the steps of

(a) pressurizing the biomass and carbonaceous feedstock; - -

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

(c) partially oxidizing the carbonaceous

feedstock/biomass with a molecular oxygen-comprising gas to obtain a synthesis gas comprising carbon monoxide and hydrogen;

(d) measuring the C02 content of the syngas and

comparing with a pre-determined value range;

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

wherein said gasifier feed comprises from a minimum of 10 wt% to 50 wt%, preferably 10 wt% to 30 wt% of biomass and wherein the level of biomass may adjusted within this range as needed to control the carbon conversion and reduce or eliminate the moderator gas demand. The

synthesis gas produced by this process may also contain water, hydrogen sulphide and carbon dioxide.

Applicant found that by mixing 10 wt% to 50 wt%, preferably 10 wt% to 30 wt% biomass with the carbonaceous feedstock, the temperature in the gasifier can be

controlled using the oxygen/carbonaceous fuel ratio and the carbon conversion can be controlled using the

biomass/carbonaceous fuel ratio, thereby simplifying process control. The incorporation of biomass as

described has the additional beneficial effects of saving on steam and boiler feed water consumption and also reduces the overall carbon footprint of the gasifier.

The invention will be described in more detail below .

In step (a) pressurization may be accomplished by a lock hopper system or (solids) pumps. A lock hopper system is generally used to pressurize dry and solid feedstocks. A pump can be used to pressurize liquid or - - slurry (mixture of solids and liquid) feedstocks. A solids pump can also be applied to pressurize solid feedstocks .

In step (b) pulverized or liquid/slurry biomass can be mixed with the carbonaceous feed or fed separately to the gasifier. A separate feedline for the biomass to the gasification reactor is preferred as this will reduce the response time on a step change from the carbon conversion controller significantly. Mixing of the biomass with the carbonaceous feed can be done at several locations in the fuel preparation and pressurization section.

In step (c) any type of coal or oil burner

configurations can be used. When the biomass is supplied separately to the gasification reactor then the biomass may also be introduced through a nozzle in the burner area of the gasification reactor. The oxygen for the partial oxidation of the biomass will then need to be supplied via the carbonaceous fuel burner.

In step (c) the carbonaceous/biomass feedstock is subjected to partial oxidation with a molecular oxygen comprising gas. The partial oxidation is preferably performed at a temperature of between 1000 and 1800 °C and more preferably at a temperature between 1200 and 1800 °C. The pressure at which partial oxidation is performed is preferably between 0.3 and 12 MPa and preferably between 3 and 10 MPa. When an ash containing feedstock is used the temperature conditions are so chosen that a slag layer will form on the interior of the reactor vessel in which the partial oxidation takes place.

In step (d) the C02 content is preferably measured in cleaned and cold syngas, for example downstream of the wet scrubber. A fast measurement is preferred to minimize - - the time between a step change in C02 concentration and the control action. An infrared based analyser is an example of such a fast measurement device.

In step (d) the C02 content of the syngas is

compared with a pre-defined value. In step (e) a

controller will subsequently adjust the biomass/coal ratio by preferably changing the set-point of the biomass feed rate controller.

The carbonaceous feedstock is preferably coal, as for example anthracite, brown coal, bitumous coal, and sub- bitumous coal. Examples of alternative carbonaceous feedstocks are petroleum coke, peat and heavy residues as extracted from tar sands or the asphalt fraction as separated from said residues in a de-asphalting process. Residues from refineries such as residual oil fractions boiling above 360 °C, directly derived from crude oil, or from oil conversion processes such as thermal cracking, catalytic cracking, hydrocracking etc may also be used as the carbonaceous feedstock.

Any biomass derived feedstocks containing low

moisture content of typically lower than 20 wt% and which can be ground to particles with sizes between 10 and 1000 micron are suitable solid biomass feedstocks comprising the 10 wt% to 50 wt% biomass component of the

carbonaceous feedstock. Feedstocks obtained by

torrefaction of a biomass source are suitable as well and are preferred at higher biomass fractions in the mixture. Torrefaction is preferably combined with a compression or pelletization step in order to make the biomass feed more suited for a gasification process wherein the biomass feed is supplied in a so-called dry form or slurry form when torrefied particles are mixed with a carbonaceous liquid. Torrefaction of biomass source material is well - - known and for example described 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.

Another suitable solid biomass fuel in the present process is obtained by drying and slow pyrolysis of a biomass source. In slow pyrolysis processes a solid char feed component is typically obtained. Slow pyrolysis is well known and for example described In: "Pyrolysis and Other Thermal Processing" . US DOE 14-08-2007.

A suitable liquid or solid biomass feedstock for use in the present process is obtained by drying and flash pyrolysis of a biomass source. In flash pyrolysis

processes a solid char and a liquid biomass feed

component are typically obtained. Both can be used as feedstock for the gasification process. Flash pyrolysis is well known and for example described in EP-A-904335; in 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: Clean syngas from biomass by pressurised

entrained flow gasification of slurries from fast

pyrolysis. In: Synbios, the syngas route to automotive biofuels, conference held from 18-20 May 2005, Stockholm, Sweden (2005) . The present invention is also directed to embodiments wherein a so-called biomass slurry is used as feedstock. The slurry can be obtained by mixing the pyrolysis oil and char.

Suitable biomass sources are weeds or residues of the agricultural industry. Examples of suitable residue - - products are streams generated in the palm oil industry, corn industry, bio-diesel industry, forestry industry, wood processing industry and paper industry. Certain biomass is relatively expensive—for example, wood

pellets. So, in order to keep operational costs as low as possible, less costly biomass sources are preferred, such as oil palm trunks and saw dust. However, the low

quantities of biomass needed for the replacement of the moderator steam increases the chance for the application of biomass from waste streams, which are normally cheaply available in small quantities.

The present invention now provides for a process whereby the partial oxidation of a carbonaceous feedstock is performed in an efficient manner thereby obtaining synthesis gas with a reduced carbon footprint suited for catalytic conversion reactions. An especially interesting catalytic conversion reaction is a hydrocarbon synthesis process. In a hydrocarbon synthesis process, synthesis gas is catalytically converted into hydrocarbon compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more. An example of a hydrocarbon synthesis process is the Fischer-Tropsch process, described in e.g. WO 02/02489, WO 01/76736, WO 02/07882, EP 510771 and EP 450861.

Step (c) may be performed by means of various

gasification processes, such as for example the so-called moving bed process, fluid bed gasifier process or the entrained-flow gasifier process as for example described in Gasification, by Christofer Higman and Maarten van der

Burgt, 2003, Elsevier Science, Burlington MA,

Pages 85-128. Preferably an entrained-flow gasifier is used because the process can handle a large variety of - - feedstock and because a tar-free synthesis gas is

prepared. In such a process the feedstock and oxygen are introduced into the reactor co-currently, preferably by means of a suitable burner. Examples of suitable burners and their preferred uses are described in US-A-4510874 and in US-A-4523529. The operating conditions are such that the process is operated in a slagging mode, which means that the operating temperature is above the ash melting point. Suitably the carbonaceous feedstock and the molecular oxygen comprising gas is converted to synthesis gas by providing said reactants to a burner as present in a gasification reactor at a pressure of between 3 and 10 MPa and preferably at a pressure between 4 and 8 MPa. The operating temperature is suitably between 1200 and 1800°C. The synthesis gas is preferably cooled to a temperature of below 1000°C, preferably below 500°C with either direct quenching with evaporating water, direct quenching with a methanol-water mixture, by indirect heat exchange against evaporating water or combination of such cooling steps. Slag and other molten solids are suitably discharged from the gasification reactor at the lower end of the said reactor.

A solid carbonaceous feedstock/biomass feed mixture may be provided to the burner of the entrained flow gasifier reactor as a slurry in water. Coal slurry feeding processes are for example described in EP-A- 168128. Preferably the solid carbonaceous

feedstock/biomass feedstock is provided to the burner in a gas-solids mixture comprising the solid feed in the form of a powder and a suitable carrier gas. Suitable carrier gasses are nitrogen, carbon dioxide, natural gas or synthesis gas, i.e. a mixture comprising of CO and ¾ .

The carrier gas is preferably carbon dioxide. The use of - - this carrier gas is for example described in WO-A- 2007042562.

Figure 1 shows a preferred embodiment of the present invention .

Figure 1 schematically shows a system for producing and cleaning synthesis gas. In a gasification reactor 9 a carbonaceous feed, a biomass feed and an oxygen

containing feed are introduced. The oxygen and

carbonaceous stream are fed via lines 19 and 8

respectively. The biomass can be fed separately via line

25 to the gasification reactor 9 but can also be mixed with the carbonaceous stream in the feeding and

pressurization section 7 via line 5. The carbonaceous stream 8 and biomass stream 25 are at least partially oxidised in the gasification reactor 9, thereby obtaining a raw synthesis gas 10 and a slag. To this end usually several burners (not shown) are present in the

gasification reactor 9. The biomass stream 25 can also be introduced in the gasification reactor 9 via nozzles in the burner area of gasification reactor, for example at burner level. Then the oxygen for the partial oxidation of the biomass will be added to the burner oxygen of the carbonaceous stream 8.

The carbon conversion of the gasifier is controlled with controller 18. Input for the carbon conversion controller is the measured C02 content of the syngas 16 downstream of the wet scrubber 15 with the C02 analyser 17. As soon as the C02 content drops below a pre ^¬ determined value than the biomass/coal ratio needs to be increased by changing the biomass feed 3 and increasing the flow rate of line 5 or 25. As soon as the C02

concentration exceeds a maximum pre-determined value, then the biomass/coal ratio can be reduced by changing - - the biomass feed 3 and decreasing the flow rate of line 4 or 25. The C02 analyser and benefits for controlling the steam and oxygen to carbon (O/C) ratio are described in WO2008125556A1 and W0200768684A2.

An advantage for the use of biomass to control the carbon conversion is that a C02 neutral feedstock is used, which reduces the C02 footprint of the plant. In addition this application reduces the high pressure steam and high quality boiler feed water requirements for the gasification island.

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

The cooling section 11 can be integrated in the gasification reactor pressure vessel at the bottom of the reactor or installed as a separate vessel. When the syngas and slag both exit at the bottom of the

gasification reactor then the cooling section 11 is integrated in the pressure vessel and both the syngas and fly ash and slag will enter the cooling section. When the syngas leaves the gasification reactor 9 at the top and slag at the bottom then only syngas with fly ash will enter the cooling section that is installed as separate section downstream of the reactor.

As shown in the embodiment of Figure 1, the raw synthesis gas leaving the cooling section 10 is further processed. To this end, it is fed via line 12 into a dry or wet solids removal unit 13 to at least partially remove ash in the raw synthesis gas. As the solids - - removal unit 13 is known to those of ordinary skill in the art, it is not further discussed here. The ash is removed from the solids removal unit via line 24. After the solids removal unit 13 the raw synthesis gas may be fed via line 14 to a wet gas scrubber 15 and subsequently via line 16 to battery limits. After further syngas treating, the residual syngas can be used for several applications like power generation, production of H2, fertilizer, Fischer Tropsch liquids and other chemicals.

The reduction of the moderator steam with the addition of biomass to coal was modelled using a computer simulation. For the gasification of a selected Drayton coal, approximately 76 kg steam per tonne coal will be required to obtain full carbon conversion at a

gasification temperature of 1500°C. The addition of coal with wood will gradually reduce the moderator requirement as shown in table 1. With the replacement of >25% of the coal with wood, moderator steam can be eliminated.

Table 1--Typical moderator consumption at different biomass fractions in biomass/coal mixture

Coal i n mixture 100% 90% 80% 70%

Biomass i n mixture 0% 10% 20% 30%

Gasification conditions

Tem perature °C 1500 1500 1500 1500

Pressure bara 41 41 41 41

GASIFIER INPUT

Carrier kg/s 2.7 2.7 2.7 2.7

Moderator steam kg/s 2 _* 1 1.1 0.2 0.0

02 kg/s 22.2 21.3 20.4 19.7

Fuel kg/s 27.6 27.6 27.6 27.6

GASIFIER OUTPUT

Syngas kg/s 49.9 48.5 47.1 46.6

Slag kg/s 6.0 5.5 4.9 4.4