WO2009145724A2

WO2009145724A2 - A two-stage high-temperature preheated steam gasifier

Info

Publication number: WO2009145724A2
Application number: PCT/SE2009/050630
Authority: WO
Inventors: Wlodzimierz Blasiak; Weihong Yang
Original assignee: Boson Energy Sa
Priority date: 2008-05-29
Filing date: 2009-05-29
Publication date: 2009-12-03
Also published as: AP2010005514A0; WO2009145724A3; SE1051371A1; ZA201009075B; AU2009251931A1; JP2011522084A; EA017739B1; SE534810C2; EA201001822A1; BRPI0912280A2; EP2300568A2; US20110078951A1; SE0801266A0; CA2726250A1; CN102076830A

Abstract

A gasifier is disclosed combing two reactors using externally generated preheated high temperature steam injection into the first reactor, where the heating demand for gasification is supplied by the sensible energy from the steam. The gasifier can produce a medium and higher LCV syngas. The first reactor is a fixed bed gasification section where the coarse feedstock is gasified, and the second reactor is an entrained-bed gasification section where the liquid and fine feedstock is gasified. Solid coarse feedstock is devolatilized in the first fixed bed reactor of the gasifier by means of high-temperature steam, and subsequently, in the second reactor subjected to a higher temperature sufficient to crack and destroy tars and oils. Activated carbon may be formed as co-product. The gasifier may be used with various solid and liquid feedstocks. The gasifier is capable of gasifying such different feedstocks simultaneously.

Description

A TWO-STAGE HIGH-TEMPERATURE PREHEATED STEAM GASIFIER

FIELD OF THE INVENTION

The present invention generally relates to a two-stage high-temperature steam gasifier for producing synthesis gas and, optionally, activated carbon, from a coarse carbonaceous feedstock, and more particularly to a gasifier capable of simultaneously gasifying a coarse solid carbonaceous feedstock and a fine solid carbonaceous feedstock or a liquid carbonaceous feedstock. The present invention also relates to a process of gasifying a coarse carbonaceous feedstock using a two-stage gasifier having two reactors in order to produce synthesis gas, optionally together with activated carbon, wherein no oxygen is fed to the first stage reactor, but only preheated steam having a temperature of at least 700⁰C.

BACKGROUND QF THE INVENTION

Gasification is a high-temperature thermal decomposition process of converting a solid feedstock, such as solid coal, petroleum coke, biomass, and/or solid waste, a liquid feedstock, such as black liquid oil, or a gaseous feedstock, into a fuel gas, consisting primarily of hydrogen (H₂) and carbon monoxide (CO), with lesser amounts of carbon dioxide (CO₂), water (H₂O), methane (CH₄), higher hydrocarbons, and nitrogen (N₂) using reactants such as air, steam, and oxygen, either alone or in any combination thereof.

The thermal gasification processes are highly endothermic chemical reactions. The general methods for supplying heat for the gasification use either of the following: a) an external source, e.g. sensible heat from hot char recirculation, and/or sensible heat from a heated gasification agent, b) reaction heat from oxidization of a part of the feedstock (incoming carbonaceous materials), and c) exothermal reaction heat from a non-carbonaceous material such as calcined lime and CO₂.

The application of the technology of partial combustion of incoming carbonaceous materials has been widely adopted. By means of the technology, the non-combustible gas, CO₂, is produced, and, as it is not removed, it leads to a diluted syngas, and the LCV (low caloric value, a measure for the burning value of the dry gas mass) of the produced syngas becomes limited. Moreover, the presence of CO₂ resulting from partial combustion (oxidation) leads to a small partial pressure of other gas species, which is not favorable for other valuable gasification reactions, such as for example, the water-gas shift reaction. Thus, the hydrogen content in the syngas will be negatively affected.

The idea of supplementing most of the energy required for the gasification process using sensible heat has recently been considered, and positive results have been shown. For example, US 2004/0060236 Al teaches an economic small scale gasification system for gasifying solid fuel into pyrolysis gas, wherein heated mixed gas of steam and air is introduced into a reformer along with the pyrolysis gas producing reformed high temperature crude gas. The mixed gas of air and steam is preferably heated to at least 300⁰C, and more preferably at least 400⁰C. Any type of heat exchanger or heater may be employed as the air/steam heating device for heating the mixed gas of air and steam.

US 6,837,910 teaches an apparatus and method for gasifying liquid or solid fuel, wherein a heated mixed gas of steam and air is introduced into at least one of the thermal decomposition area of the solid or liquid fuel and the reforming area of the thermal decomposed gas. The mixed gas of air and steam is heated to a temperature of at least 700⁰C, and more preferably higher than 800⁰C.

Other known systems using high- temperature air/steam/oxygen as high as 1000⁰C for a bio- mass/waste gasification process have also been applied (Lucas C₅ Szewczyk D., Blasiak W., Mochida S., High Temperature Air and Steam Gasification of Densified Biofuels, Biomass and Bioenergy, Vol. 27, No. 6, December 2004, pages 563-575). A char free hydrogen rich gas, where the process is performed with only steam at a temperature of 1000⁰C and at a conventional pressure of about 1 atm has been proposed by Ponzio Anna, Yang Weihong, Lucas, C, Blasiak W., in Development of a Thermal Homogenous Gasiiier System using High Temperature Agent, CLEAN AIR - International Journal on Energy for a Clean Environment., Vol. 7, No. 4., 2007. In US 2003/0233788 Al, a method for gasifying carbonaceous materials into fuel gases is disclosed. It involves the formation of an ultra- superheated steam (USS) composition substantially containing water vapor, carbon dioxide and highly reactive free radicals thereof, at a temperature of about 1316°C to about 2760⁰C. The USS composition comprising a high tem- perature flame is contacted with a carbonaceous material for rapid gasification/reforming thereof. The USS is formed by burning s substantially ash-free fuel with "artificial air" comprising an enhanced oxygen gas and water vapour, wherein the "artificial air" is at least about 60 mole percent. The oxygemfuel ratio will have to be controlled so that soot is not is not formed. The use of enhanced oxygen gas in the method will obviously increase the operation cost of the method.

According to US 2003/0233788 Al, steam-only gasification has been investigated and used commercially since about 1950-1960. However, because of the limited heat in the steam, the problems associated with steam-only gasification include low achievable reaction tempera- tures, i.e. typically less than about 815⁰C, where long residence times and high energy consumption prevail.

All the above prior art only use one-stage reactor, either a fixed bed, or a fluidized bed gasi- fier.

It is known that the thermal conversion of biomass/waste/coal can be understood as comprising two mainly highly endothermic stages: devolatilization of volatiles, and char conversion, respectively. As indicated by previous studies, 90% of the volatile content in the total weight of biomass will be released instantaneously if it would be heated above 600⁰C. The second stage is char conversion. In order to get char-free ash, i.e. 100% char conversion, a much higher temperature is needed for the thermal conversion of char. Generally, this temperature should be higher than 1000⁰C, depending on the ash melting point.

Fixed-bed gasifier types are widely used in small-scale energy production (<10 MWj,) due to its very simple construction and operation. It has been found that if the design of a gasification fixed-bed reactor follows the above two stages, it would be more efficient from many point of views. There are extensive works on this way of operation for the fixed bed gasifier. Secondary air injection to the gasifier is often used. For example, Pan et al.( Y. G. Pan, X. Roca, E. VeIo and L. Puigjaner, in Removal of tar by secondary air injection in fluidized bed gasification of re- sidual biomass and coal, Fuel 78 (1999) (14), pp. 1703-1709 ) reported 88.7 wt.% of tar reduction by injecting secondary air just above the biomass feeding point in the fluidized bed at a temperature of 840-880⁰C.

Narv et al. (Biomass gasification with air in an atmospheric bubbling fluidized bed. Effect of six operational variables on the quality of produced raw gas, Industrial and Engineering

Chemistry Research 35 (1996) (7), pp. 2110-2120) performed secondary air injection in the freeboard of a fluidized bed gasifier and observed a temperature rise of about 7O⁰C which resulted in a tar reduction from 28 to 16 g/Nm³.

The Asian Institute of Technology (AIT), Thailand, modified a biomass gasifier which resulted in a fuel gas with a tar production of about 50 mg/Nm³, which is about 40 times less than a single-stage reactor under similar operating conditions (T.A. Milne and RJ. Evans, Biomass Gasification "Tars": Their Nature, Formation and Conversion. NREL, Golden, CO, USA, Report No. NREL/TP-570-25357 (1998). This concept involves a downdraft gasifier with two levels of air intakes. The produced tar in the biomass pyrolysis process will pass through a high-temperature residue char bed at the bottom and will be decomposed at the elevated temperature.

Bhattacharya et al. in A study on wood gasification for low-tar gas production, Energy 24 (1999), pp. 285-296, reported a similar gasifier with char produced inside the gasifier itself to act as a filter to further reduce tar production considerably at 19 mg/Nm³ higher CO and H₂ concentration in fuel gas.

Cao et al. in A novel biomass air gasification process for producing tar-free higher heating value fuel gas, Fuel Processing Technology 87 (2006) 343-353, reported a work of two-region fluidized bed reactor. In this work, an assisting fuel gas and second air stream were injected into the upper region of the reactor in order to reduce the tar compositions. Experimental results showed a heating value of about 5MJ/Nm³.

US 6,960,234 discloses a multi-faceted gasifier and related methods. It is a gasifier combing a fixed bed gasification section and an entrained flow gasification section. Activated carbon may be formed in the upper fixed bed section and in the entrained flow section.

US 6,647,903 discloses a method and apparatus for generating and utilizing combustible gas using a gasifier comprising first and second reaction sections, wherein oxidizing gas is intro- duced into both sections. The invention operates in a manner that enhances tar destruction while forming output fuel gas products H₂ and CO. In addition some methane may also be formed. In a certain mode of operation, activated carbon may be generated.

JP 6256775 discloses two-stage complete gasification of organic matter for methane synthesis, wherein in a first stage gasification process organic matter is gasified in the presence of steam and oxygen, and, in a second stage gasification process gaseous un-reacted matter and tar gas are gasified at a higher temperature than in the first stage gasification process. A gasifier comprising two stages is also disclosed. In order to disturb solid carbonaceous material from passing from the first stage gasification process to the second stage gasification process, the passage between the two stages may be narrowed, or a filter may be set between the two stages. The gasifier includes two inlets for oxygen and steam, one in the first stage, and the other in the second stage.

The aim of secondary air/oxygen and/fuel injection in above works is increasing the tempera- ture in freeboard in order to decompose tar, and improve the steam-reform reaction. However, the injection of secondary air will not only increases the diluents contents, notably nitrogen, but will also reduce the combustible contents generated from gasification. This results in a decrease of LCV of the fuel gas produced. Furthermore, injection of secondary air makes it hard to control the composition of the produce gas.

US 6,960,234, mentioned above, also states that fixed-bed gasification requires coarse fuels (typically %" to 2" in diameter, and that limiting technical features of fixed-bed gasification include: tar and oil carry over with the syn gas; difficulty in using coal/fuel fines because they clog the void space between the coarse fuels in the fixed bed; and difficulty in using liquid hydrocarbon feedstocks.

In order to be able to produce medium and high low calorific value (LCV) combustible gases, and gasify both solid and liquid/fine feedstock simultaneously, and also produce other added- value materials, such as activated carbon, a novel fixed bed gasifier is proposed herein. Such gasifier is specified in claim 1. Also, a method of gasifying a coarse carbonaceous feedstock, using a two-staged gasifier having two reactors, in order to produce synthesis gas, optionally together with activated carbon, wherein no oxygen is fed to the first stage reactor, but only preheated steam having a temperature of at least 700⁰C is also claimed and disclosed. Such method is provided for in claim 4.

SUMMARY QF THE INVENTION

Thus, for a two-stage gasifier of the prior art, such as disclosed by JP 6256775 and specified by the preamble of claim 1 comprising: a first reactor provided with an inlet for a coarse carbonaceous feedstock, and a first inlet for steam; and a second reactor provided with a second inlet for steam, optionally together with air or oxygen; and an outlet for synthesis gas; wherein the first and second reactors are separated by a narrowed portion having a reduced cross-section for restricting passage from the first reactor to the second reactor of unreacted solid carbonaceous substance, wherein the first reactor is capable of being operated at a temperature of at least 600⁰C, and wherein the second reactor is capable of being operated at a higher temperature, the above object has been achieved by means of the technical features of the characterizing portion of said claim, according to which the second reactor is the lower reactor, the first reactor is the upper reactor, a grate is provided at the bottom end of the first reactor, said first inlet for steam is located adjacent to the bottom of the first rector, so as to enable preheated steam having a temperature of at least 700⁰C to be fed into the first reactor from below the grate via said inlet, said first reactor is provided with an outlet for synthesis gas, the second reactor is provided with an inlet for a fine solid carbonaceous feedstock and/or a liquid carbonaceous feedstock, said second inlet for steam is located adjacent to the bottom of the second rector, so as to enable preheated steam having a temperature of at least 700⁰C₅ optionally together with preheated air or oxygen of same temperature, to be fed to the second reactor from below via said inlet, and a second narrowed portion having a reduced cross- section is provided at the bottom end of the second reactor.

Accordingly, in one aspect the present invention relates to a two-stage gasifier as set out above.

In the gasifier of the invention simultaneous gasification of solid coarse material, on the one hand, and solid fine and/or liquid material, on the other, is enabled. Carbonaceous coarse ma- terial is fed to the first reactor and carbonaceous (waste) liquid and/or carbonaceous fine solid material is fed to the second reactor.

In a further preferred embodiment of the two-stage gasifier one or more, and preferably all of the inlets for steam, air, oxygen and carbonaceous (waste) liquid and/or carbonaceous fine solid material enter into the gasifier tangentially in corresponding portions of the gasifier, which portions have an inner, circular cross-sections.

In a further preferred embodiment of the two-stage gasifier the inlet for carbonaceous (waste) liquid and/or carbonaceous fine solid material comprises at least two inlets separated at a maximum distance from each other along the circumference of the circular cross-section.

In another aspect the present invention relates to a process of gasifying a coarse carbonaceous feedstock, using a two-stage gasifier having two reactors, a first and a second, respectively, in order to produce synthesis gas, optionally together with activated carbon. Such process is provided for in claim 4, and includes the following steps: (a) feeding a coarse carbonaceous feedstock to the first stage reactor of the gasifier; (b) subjecting the coarse carbonaceous feedstock to steam in the first stage reactor at an operational temperature of at least 600⁰C of the reactor, to effect gasification of the carbonaceous feedstock, in which process no oxygen is fed to the first stage reactor, but only preheated steam having a temperature of at least 700⁰C, and which process further includes a step (c), wherein any solid and/or liquid carbonaceous materials obtained from step (b) are subjected to preheated steam, optionally together with air or oxygen, in the second stage reactor operating at a temperature of at least 700⁰C to obtain any combination of the following products: activated carbon; CO; CO₂, and heat of combustion.

In a preferred embodiment, the process comprises a further step (d) wherein a fine solid car- bonaceous, and/or liquid carbonaceous feedstock, is/are being fed simultaneously into the second stage reactor of the gasifier. Accordingly, in this embodiment both a coarse feedstock and a fine solid and/or liquid carbonaceous feedstock may be fed simultaneously into the gasifier.

In another a preferred embodiment of the process externally generated preheated steam hav- ing a temperature of at least 700⁰C is also fed into the second stage reactor. With this embodiment internal combustion, also referred to as partial combustion or oxidation, in the gasifier can be kept to a minimum, since the required energy is provided externally. Consequently, supply of air or oxygen is not required for heat generation by internal combustion in this embodiment. Also, when air or oxygen is not being fed to the second reactor the yield of activated carbon can be maximised.

In a further preferred embodiment of the process, air is fed to the second reactor (i.e. in addition to the high temperature steam). With this embodiment especially high quality synthesis gas can be obtained, since carbon is converted also to CO, and not only to activated carbon. Also, depending on the ratio of steam/air, internal combustion can still be avoided (i.e. production of CO₂). At the same time the ratio of CO:activated carbon also can be controlled by controlling the ratio of steam/air.

In a further preferred embodiment of the process pure oxygen is used (instead of air). In this embodiment the process can be used for industrial purposes. Also, the need for separation of by-products is minimized, and undesired dilution of the gaseous product is kept to a minimum.

Further embodiments and advantages will become apparent from the detailed description and claims. The terms "internal combustion", "partial combustion" and "partial oxidation" have been used interchangeably to denote combustion occurring inside the gasifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a system flow diagram generally illustrating the inventive gasification process for biomass and solid waste.

Figure 2 illustrates a cross-sectional view of an embodiment of the gasifier 21.

Figure 3 is a plan view of the inventive gasifier showing the tangential liquid feedstock injection via inlets 19a and 19b.

DETAILED DESCRIPTION QF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The inventive gasifier combines two reactors using externally generated preheated high temperature steam injection into the first reactor, where the heating demand for gasification is supplied by the sensible energy from the steam. The gasifier can produce a medium and higher LCV syngas. The first reactor is a fixed bed gasification section where the coarse feedstock is gasified, and the second reactor is an entrained-bed gasification section where the liquid and fine feedstock is gasified. Solid coarse feedstock is devolatilized in the first fixed bed reactor of the gasifier by means of high-temperature steam, and subsequently, in the second reactor subjected to a higher temperature sufficient to crack and destroy tars and oils.

Activated carbon may be formed as co-product. The gasifier may be used with various solid and liquid feedstocks. The gasifier is capable of gasifying such different feedstocks simultaneously.

The idea behind the present invention is that the gasifier 21 is separated into two stages: a first upper stage 3 for devolatilization of volatiles, which first stage only uses externally generated high-temperature preheated pure steam (preferably 700⁰C-IOOO⁰C), and a second lower stage 4 for char thermal conversion, using a high-temperature (preferably 700-1600⁰C, more preferably 800-1200⁰C) preheated mixture of air and steam, oxygen and steam, or steam only as shown in Figure 1. The reactor 3 includes a fixed bed comprising grate 8.

In the first reactor 3, the energy used for the devolatilization process is supplied both by the sensible energy of steam fed into the first reactor via inlet 7, and by the hot stream coming from the second reactor through the narrowed portion 20. The temperature in the first reactor is controlled at the level of at least 600⁰C by the quantity and the temperature of the steam fed into said reactor.

In the first reactor 3, high-temperature steam is mixed with coarse feedstock (biomass) 1 entering via inlet 2. When the biomass is heated by the high-temperature steam, the devolatilization process occurs as:

Carhona_Ce_{O US} - feedst_OCk ^H^-™™^d ^^y_^h _lg ^h t^emperature _^sieam _> > VoIatiles (C_mH_n, CO, H₂, CO₂, O₂, etc.) + Char ⁽¹⁾

Simultaneously, due to the presence of steam, steam reacts with the volatiles: C_mH_n + H₂O <->■ CO +H₂ (2)

CO +H₂Q-* CO₂+ H₂ (3)

A little oxygen released from the pyrolysis (which occurs in the first reactor, and also in the second reactor when a liquid and/or solid fine feedstock is being injected) and from the second reactor 4 reacts according to the following: C_mU_n + (m/2 + n/4) O₂ → mCO + n/2 H₂O (4) CO + ¹Z₂ Q₂ -^ CO₂ (5)

H₂ + ¹Z₂ O₂ ^ H₂O (6)

CO +H₂O→ CO₂+ H₂ (7)

Since the reactor temperature in the first stage reactor 3 is controlled at the level of at least 600⁰C, and the residence time is controlled, and the gases in the first reactor are located in an environment which is very much in lack of oxygen, any solid and/or liquid char produced in the first reactor will not be reacted with any oxidizers in said reactor. Consequently, any solid and/or liquid char will instead fall into the second reactor 4 by the action of the gravity.

In the second reactor 4, the energy used for char the conversion process is preferably supplied by the sensible energy of mixture of steam and air, and from partial oxidization of char. In order to achieve char-free conversion, the temperature in the second reactor should be higher than that of the ash melting point, in order for the ash to form slag. Normally, for wood bio- mass the ash melting point can be 1300⁰C. The reactor 4 includes an entrained bed compris- ing grate 5.

The main reactions when there is no other feedstock (liquid and fine particle) injection are:

Gasification : C + O₂ => CO₂ - 393.5 kJ/mol (8)

C + H₂O => CO + H₂ +131.3 kJ/mol (9) C + 2H₂O => CO₂ +H₂ + 90.2 kJ/mol (10)

- Partial Oxidation:

C + 0.5O₂ => CO - 110.5 kJ/mol (11) - Boudouard reaction:

C + CO₂ =>2CO - 172.4 kJ/mol (12)

- Water Gas Shift:

CO + H₂O => CO₂ +H₂ - 41.1 kJ/mol (13)

- Methanation: CO + 3H₂ => CH₄ + H₂O - 206.1 kJ/mol (14)

- Hydrogenation:

C +2H₂ => CH₄ - 75 kJ/mol (15)

When a second feedstock (liquid and fine particles) is injected into the second reactor, all the reactions from (1) to (15) will occur. Many reactions occur simultaneously and it is difficult to control the process precisely as indicated here. Nevertheless, by careful selection of the process parameters (temperature, residence time and oxygen/steam ratios) in this invention, it is possible to maximize certain desired products, such as activated carbon and syngas.

Further on, the activated carbon can be treated as a co-production from thermal conversion of carbon-based materials through this invention. Generally, in the prior art the preparation of activated carbon involves two steps: carbonization of the raw material in absence of oxygen at high temperature (500-1000⁰C) in order to eliminate maximum amounts of oxygen and hy- drogen elements, and activation of the carbonized product at a higher temperature in the presence of oxidizing gas such as water, carbon dioxide or both. The activation shall be carried out under well controlled conditions in order to achieve a desired conversion.

In this invention, the feedstock is first gasified by high-temperature pure steam (at the level of at least 600⁰C) in the first reactor 3, then the carbon is preferably activated in the second reactor 4 by high-temperature steam.

In this invention, as generally illustrated by Figure 1, high temperature steam, and optionally air or oxygen (over 700⁰C), will be obtained mainly by use of a honeycomb regenerative heat exchanger as explained in, for example, EP 0 607 921, or in co-pending PCT/SE2009/050019, the relevant contents of which disclosures are incorporated herein by reference.

Figure 2 illustrates a cross-sectional view of the gasifier 21. Carbonaceous feedstock 1 enters at the top of the gasifier, through a feed inlet 2, and proceeds downward moving through the first reactor 3, then pass the grate 8, then enters second reactor 4, then pass the grate 5 until it becomes a molten ash at the bottom 6. The feedstock can include biomass, coal, municipal solid waste, or any combination thereof. The particle size of the coarse carbonaceous feedstock 1 is typically from 0.5 cm to 1,8 cm, and preferably from 0.5 to 1.2 cm.

In the first rector 3, the feedstock is heated by a combination of the sensible heat carried by the high-temperature steam (over 700⁰C), and the sensible heat carried by the flue gas produced by char oxidization and gasification in the second reactor 4. High-temperature steam carried by pipe 7 for the feedstock gasification in the first reactor enters a narrowed portion or throat 20 through a port (ports) 11. The amount of high-temperature steam added at port 7, is set to keep the temperature at point 3 (first reactor) between 600-900⁰C, and preferably above 700⁰C. At the point around 8 (grate), when air or oxygen is being fed into the second reactor a hot combustion flame may occur as the surplus oxygen burns with the pyrolysis gases released from feedstock 1 , and form any liquid and/or fine solid feedstock being fed into the second reactor.

The temperature in the reactor 3 is controlled by the temperature and flowrate of injection of steam from point 7, and the temperature and quantity of surplus oxygen from reactor 4. The residence time of the feedstock 1 inside reactor 3 is mainly controlled by the gap of the grate 8.

In order to accomplish a good mixing between the gasification agents (steam) with the feed- stock 1 , a throat 20 is provided. The diameter of the throat is generally smaller than that of the hearth of reactor 3. The inclination of the conical portion 14 should preferably be around 45- 60°. The diameter of the steam injection port 11 should preferably be 2-3 times smaller than that of the throat 20.

After the coarse carbonaceous feedstock has been devolatilized by high-temperature steam in the first reactor 3, the remaining fixed carbon has become activated carbon char and ash solids, which continue to move downward through the grate 8, then enter a throat 20, then enter into the second reactor 4, where they are oxidized and gasified by a mixture of high-temperature air (or oxygen) and steam. When no air or oxygen is being fed with the steam into reactor 4, no oxidation will occur in reactor 4, but only gasification. The temperature of the second reactor 4 is further increased to a temperature slightly above the ash softening point of the fuel at the grate 5. The pipe 9 carries the preheated high-temperature steam or mixture of high- temperature air (or oxygen) and steam to the port 10, which then enters into the second throat 18.

For wooden pellets produced from wood grown in Sweden, the ash softening point typically ranges from 1350-1400⁰C. If slag formation of the ash is to be avoided, the maximum peak temperature in the reactor 4 during operation is maintained at a temperature at least 50⁰C below the ash softening point, with 100⁰C below as the normal and thus preferred maximum condition.

The temperature in reactor 4 is controlled by the preheating temperature, flowrate and the ratio of steam to carbon, and, when air or oxygen also is being used with the steam, the ratio of steam to oxygen of the mixture.

The diameter of the second narrowed portion or throat 18 is generally smaller than that of the diameter of reactor 4, and preferably also smaller than that of the first narrowed portion or throat 20. The inclination of the conical portion 17 should preferably be around 45-60°. The diameter of the steam injection port 10 should preferably be 3-5 times smaller than that of the throat 18.

The ash is dropped into bottom 6 through throat 18, and may be taken out batch- wise from the reactor.

The syngas flows out through the exit pipe 12. Since the temperature in the first reactor 3 is high enough, and also steam is present, most of the tar is destroyed and converted to syngas. The main chemical constituents of syngas are hydrogen, carbon monoxide, and methane, and carbon dioxide.

The inventive design of the gasifier has the ability to advantageously control the ratio of hydrogen to carbon monoxide in the syngas, since the gasifier enables control within wide ranges of steam to oxygen ratio within the gasifier.

In one embodiment of operation of the reactor, by controlling the temperature in the second reactor 4 at 700⁰C, i.e. the same temperature as first reactor 3, and by only feeding steam to the second reactor, all tars and oils are consumed by the high-temperature steam. This con- verts most of the fixed-carbon to activated carbon within the gasifier. Therefore, the gasifier and process described herein can also effectively generate activated carbon. This mode of operation is very effective for generating activated carbon, and will also improve the quality of the activated carbon obtained. If gasification is to be maximized, on the other hand, the second reactor should be operated at a higher temperature than the first reactor.

The invention can consequently also be used to produce activated carbon. There are two methods in which activated carbon is created within the gasifier. In the first one, only the first reactor is used, i.e. only high-temperature steam is injected through pipe 7. The high- temperature mixture of steam and air from pipe 9 is closed. Another and more preferred method is to have both reactors running, but from pipe 9, only high-temperature steam is injected. In this case, the activated carbon is collected into dry directly. The second method has surprisingly been found to be apt to give higher quality of the activated carbon char. This is believed to be due to that the high-temperature steam injected from pipe 9 makes the pores of the activated carbon opening in the second reactor 4. Activated carbon having wider pores than in the prior art may thus be obtained by means of the method of the invention. The size (diameter of pores) can be controlled by the temperature of steam in the reactor 4. Generally, a higher temperature of steam increases the pore number of the activated carbon.

Thus, the present invention is capable of achieving a bi-generation (gas and activated carbon) from one and the same feedstock 1. The desired ratio of the products can be decided upon according to the type of feedstock available, price of the products, and so on.

Further on, this invention can be used to treat both coarse particles (diameter larger than 0.5 cm) of carbonaceous materials and fine particles and or liquid feedstock.

Figure 3 shows a cross-sectional view of the gasifier 21, which shows the tangential liq- uid/fine particles feedstock injection. Two injection lances 19 (19a and 19b) are shown connecting to the reactor 4. Liquid feedstock, such as the liquid residues collected after a micro- oven pyrolysis process of the Automotive Shredder Residue (ASR), and fine or pulverized feedstock can be injected into the reactor 4. The injected feedstock enters into the reactor 4 tangentially and mixes with the high-temperature air/steam coming from the grate 5. The tan- gential injection can increase the residence time of the liquid and/or fine feedstock. The entrained flow gases pass through the upper fixed bed grate 8, then enter reactor 3 before leaving the gasifier at the exit pipe 12. The injection port 19 should be located to the lower part of the hearth of the reactor 4 in order to increase the residence time. Generally, for a small-scale gasifier, the location of this injection port(s) is 10 cm above the inclination wall 17.

The residue time can be controlled by the injection velocity, and the angle of the injection lance to the gasifier.

In a preferred embodiment, the walls of the gasifier consist of two layers: an outer steel cover, preferably 5.0 mm thick, and an inner layer of fibrous ceramic insulation, preferably a high temperature resistant, high quality ceramic. The ceramic used at walls 13 and 14 can prefera- bly operate with, i.e. withstand, a maximum temperature of 1400⁰C. A suitable material may be composed of: Al₂O₃ 45%, SiO₂ 36%, Fe₂O₃, 0.9% and CaO 16%. The ceramic used for the walls at 15, 16 and 17 is preferably apt to operate at a higher temperature of 1400-1500⁰C. The maximum allowed working temperature of this wall material is 1600⁰C. A suitable material may have the following composition: Al₂O₃ 61%, SiO₂ 26%, Fe₂O₃, 0.5%, CaO 2.6%, ZrO2 2.95%, and BaO 3.3%. The ceramic materials are supported by a steel shell.

In a preferred embodiment refractive ceramic tubes are used as the grates 8 and 5. The composition of these ceramic tubes can for example be 97% ZrO₂, and 3 % of MgO.

A high temperature mixture of steam, optionally together with air or oxygen, being fed through pipe 9 enters the throat 18 which below the grate 5. This high temperature mixture of air and steam can keep the ash in a molten state in the throat 18, which ash finally drops to the bottom 6, and can be taken out in batches.

Example 1 :

97 kg/h of wood pellets 1 with a diameter around 8 mm is feed into the first reactor from the inlet 2 by weight at room temperature (15⁰C). The properties of the wood pellets are shown in Table 1. Table 1 Proximate and ultimate analysis of the feedstocks used

Proximate analysis

Wood Pellets (WP)

Total Moisture (SS 187170) 8 %

Ash content (SS-187171) 0.5-0.6 % (dry)

LHV (SS-ISO562) 17.76 MJ/kg (as received)

Volatile matter (SS-ISO) 84 % (dry)

Density 630-650 kg/m³

Ultimate analysis (dry compositions)

Wood Pellets

Sulphur (SS-187177) S 0.01-0.02 %

Carbon (Leco-600) C 50 %

Hydrogen (Leco-600) H 6.0-6.2 % Nitrogen (Leco-600) N <0.1 %

Oxygen (Calculated) O 43-44 %

Ash fusion temperatures (oxidizing conditions)

Wood Pellets

Initial deformation, IT 1350-1400⁰C

Softening, ST 1450-1500⁰C

Hemispherical, HT 1500⁰C

Fluid temperature, FT 1500-1550⁰C

Example 2:

A 60 kg/h of Refuse Derived Fuel (RDF), a pellet formed fuel made from paper fiber mixed with other substances such as fabric fiber, wood chips and plastics, was used as feedstock, with a diameter of about 8 mm, and was feed into the first reactor 3 from the top 1 by weight, i.e. by the action of gravity, at room temperature (15°C). The properties of the RDF pellets are shown in Table 2. Table 2 Proximate and ultimate analysis of the RDF feedstock used

Proximate analysis

Refused Derived Fuel (RDF)

Total Moisture (SS 187170) 2.9 %

Ash content (SS-187171) 6.0 % (dry)

LHV (SS-ISO562) 26.704 MJ/kg (as received)

Volatile matter (SS-ISO) 84.4 % (dry)

Density 472 kg/m³

Ultimate analysis (dry compositions)

RDF

Sulphur (SS- 187177) S 0.09 %

Carbon (Leco-600) C 63.3 %

Hydrogen (Leco-600) H 8.9 %

Nitrogen (Leco-600) N 0.3 %

Oxygen (Calculated) O 20.95 %

Ash fusion temperatures (oxidizing conditions)

RDF

Initial deformation, IT 1210⁰C

Softening, ST 1220⁰C

Hemispherical, HT 1230⁰C

Fluid temperature, FT 1240⁰C

Claims

1. A two-staged gasifier (21) for producing synthesis gas and, optionally, activated carbon, from a coarse carbonaceous feedstock, which gasifier comprises: - a first reactor (3) provided with an inlet (2) for a coarse carbonaceous feedstock (1), a first inlet (7) for steam; and

- a second reactor (4) provided with a second inlet (9) for steam, optionally together with air or oxygen;

- and an outlet (12) for synthesis gas; wherein the first and second reactors are separated by a narrowed portion (20) having a reduced cross-section for restricting passage from the first reactor to the second reactor of solid carbonaceous unreacted substance, wherein the first reactor is capable of being operated at a temperature of at least 600⁰C, and the second reactor is capable of being operated at a higher temperature, characterized in that the second reactor (4) is the lower reactor, and the first reactor (3) is the upper reactor, a grate (8) is provided at the bottom end of the first reactor, inlet (7) is located adjacent to the bottom of the first rector, so as to enable preheated steam having a temperature of at least 700⁰C to be fed into the first reactor from below grate (8) via inlet (7), said second reactor is provided with an inlet (19) for a fine solid carbonaceous feedstock and/or a liquid carbonaceous feedstock, inlet (9) is located adjacent to the bottom of the second rector so as to enable preheated steam having a temperature of at least 700⁰C, optionally together with preheated air or oxygen of same temperature, to be fed to the second reactor from below via inlet (9), and in that a second narrowed portion (18) having a reduced cross- section is provided at the bottom end of the second reactor (4).

2. The two-stage gasifier of claim 1, wherein one or more, and preferably all of the inlets (7, 9, 19) enter into the gasifier tangentially in corresponding portions (20, 18, 16) of the gasifier having inner, circular cross-sections.

3. The two-stage gasifier of claim 1 or 2, wherein the inlet (19) comprises at least two inlets (19a, 19b) separated at a maximum distance from each other along the circumference of the circular cross-section.

4. A process of gasifying a coarse carbonaceous feedstock using a two-stage gasifier having two reactors, a first and a second, respectively, in order to produce synthesis gas, optionally together with activated carbon, comprising the steps of:

(a) feeding a coarse carbonaceous feedstock to the first stage reactor of the gasifier; (b) subjecting the coarse carbonaceous feedstock to steam in the first stage reactor at an operational temperature of at least 600⁰C of the reactor, to effect gasification of the carbonaceous feedstock, characterized in that no oxygen is fed to the first stage reactor, but only preheated steam having a temperature of at least 700⁰C, and in a further step (c), wherein any solid and/or liquid carbonaceous materials obtained from step (b) are subjected to preheated steam, optionally together with air or oxygen, in the second stage reactor operating at a temperature of at least 700⁰C to obtain any combination of the following products: activated carbon; CO; CO₂, and heat of combustion.

5. The process of claim 4, comprising a further step (d) of simultaneous feeding of a fine solid carbonaceous, or liquid carbonaceous feedstock, into the second stage reactor of the gasifier.

6. The process of claim 4 or 5, wherein in step (c) the steam entering the second stage reactor is preheated to a temperature of 700-1600⁰C, and preferably 800-1200⁰C.

7. The process of any one of claims 4-6, wherein the ratio of steam/air or steam/oxygen used in step (c) is selected so at to minimize the internal combustion, and to maximize the yield of CO and/or activated carbon.

8. The process of any one of claims 4-7, wherein oxygen is used instead of air.

9. The process of any one of claims 4-6, wherein no air or oxygen is supplied to the gasifier.