CA2789232A1 - Process and apparatus for gasification of refuse - Google Patents

Abstract

Described herein are improvements relating to a gasification reaction vessel for converting carbonaceous feedstock to synthetic gas. The reaction vessel being of the type having a lower section with a grid for retaining a bed of fluidizable and heat-retaining particulate material. The improvements generally consisting of an upper section of the reaction vessel having an enlarged average internal cross-sectional area compared to the average internal cross-sectional area of the lower section so as to facilitate disengagement and removal of said synthetic gas from the bed of particulate material. Also described in an improved gasification process using an oxygen-enriched fluidization agent wherein the oxygen content is about 25 to 35%, preferably 30 - 40% oxygen based on volume. Also described are improved conditioning processes for the hot synthetic gas exiting the reaction vessel. The conditioning process provides a clean, cold synthetic gas ready for use in various applications such as a fuel for boiler, furnace, internal combustion engine, etc. The process improvements consisting of subjecting the synthetic gas to a sequential treatment in at least one cyclone to remove particulate material therefrom, followed by a treatment in at least one counter-current wet-scrubber tower to cool said gas and solubilize or entrain further pollutants, followed by a treatment in at least one venturi scrubber to remove further pollutants, followed by treatment in a at least one demister to remove residual liquid droplets.
Optional additional conditioning treatments are also provided. Recycling steps are also described and are aimed at increasing energy efficiency and minimize emissions as well as by-product generation.

Description

PROCESS AND APPARATUS FOR GASIFICATION OF REFUSE
FIELD OF THE INVENTION

The present invention relates to synthetic fuel gas production from low cost carbonaceous materials and waste products such as biomass, municipal solid waste, plastic and rubber residues, wastewater treatment sludge, pulp and paper liquors, heavy petroleum residues.

More specifically, the present invention relates to an apparatus and method for the production of a synthetic gas sufficiently clean to comply with current environmental regulations.

BACKGROUND OF THE INVENTION
Synthetic gas has various uses such as:

= fuel for gas burners/boilers for hot water or steam generation = fuel for internal combustion engines (Diesel, Otto, Stirling) = fuel for gas turbines to generate electricity = feedstock for chemical synthesis feedstock for the production of hydrogen-rich gas by shifting the CO and reforming any hydrocarbons present in the synthetic gas.

Gasification is a known technique for converting carbonaceous materials to valuable combustible synthetic gas. In a gasification reactor, a fraction of the feedstock is oxidized thereby generating high temperatures inside the reactor (exothermic reaction).
The remainder of the feedstock decomposes at the high temperatures generated by the oxidation reactions generating hot combustible synthetic gas and small amounts of char (endothermic reaction). The reactor is constantly fed with carbonaceous material and oxygen-containing gas to keep the exothermic reaction going so as to maintain a high temperature inside the reactor. Typically, the oxygen supply is about 25-30%
of stochiometric values for total combustion. The partial oxidation regime present in the reactor renders the process self-sufficient in energy. Additional reaction processes taking place concurrently with thermal decomposition in the gasification reactor are steam reforming, the Boudouard reaction and the shift conversion reaction. The extent of these reactions determines, to a large degree, the composition of the synthetic gas produced.

At steady-state, a typical gasification reactor operates at temperatures above 700 C and pressures above 101 kPa.

Current gasification technology improvements focus on gasification reactor design and gas conditioning operations prior to its final use.

U.S. Patent 4,968,325 discloses a biomass gasification process and plant design. This process is commercially known as BIOSYN (trademark). The process uses a fluid bed gasification reactor containing fine sand fluidized by finely dispersed bubbles, introduced through a conveniently placed grid at the bottom of the reactor, of an oxygen-containing gas.
Technical viability of this process has been demonstrated for wood residues at capacities as high as 10 tones/h. Moreover, the synthetic gas resulting from wood residue gasification is not sufficiently "clean" to be used as a fuel in modern energy conversion devices. What is meant by "clean" is a synthetic gas free of harmful contaminants such as tar and particulate material including chemically aggressive species as defined by current environmental regulations.

U.S. Patent 4,448,588 relates to a synthetic gas conditioning process. What is meant by "conditioning" is the treatment of the synthetic gas prior to its use as a fuel or other intended use. The process shows the use of the carbon-rich solids, i.e. char, resulting from gasification to remove organic vapors from the synthetic gas.

There is still a need for a process and apparatus that overcomes the drawbacks of the prior art, more specifically, there is a need for a versatile and commercially valuable process and apparatus for gasification of various sources of refuse and industrial by-products and wastes.

There is also a need for synthetic gas conditioning steps and related apparatus for purifying the synthetic gas and minimizing the final effluents to be disposed of.

The scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.
SUMMARY OF THE INVENTION
In general terms, the present invention provides for improvements relating to a gasification reaction vessel for converting carbonaceous feedstock to synthetic gas. The reaction vessel being of the type having a bottom portion with a grid for retaining a bed of fluidizable and heat-retaining particulate material. The latter is most preferably silica sand of average diameter of 200 to 700 pm. The bed is fluidized by a stream of oxygen-containing gas fed through the grid tuyeres, the bottom portion also being adapted to receive, laterally, a feed of carbonaceous material to be gasified, the reaction vessel further having a top portion for recovering and evacuating the synthetic gas produced in the bottom portion.
The improvements generally consisting of: a top portion of the reaction vessel having an enlarged average internal cross-sectional area compared to the average cross-sectional area of the bottom portion so as to facilitate disengagement and removal of said synthetic gas from the bed of particulate material.

The present invention also provides an improved gasification process using a reaction vessel containing a fluidized bed for receiving a carbonaceous material feedstock for gasification and a fluidizing gas, the improvement in the gasification process consisting of:
providing as the fluidization gas an oxygen-enriched air containing up to 50%, preferably - 40%, oxygen to said reaction vessel (percentages based on volume).

More specifically, the present invention relates to a process for the gasification of a biomass 25 material, the process comprising:

a) selecting a biomass material derived from forest, agricultural or urban wastes, having a given moisture content;

b) pre-drying the biomass material to obtain a pre-dried biomass material having a moisture content below about 20% in weight;

30 c) converting the pre-dried biomass material to a synthetic gas in a gasification reaction vessel, the vessel having a lower section with a grid retaining a bed of granular material fluidized by a stream of gas fed through the grid, the lower section receiving the pre-3a dried biomass material to be gasified, and an upper-section for recovering and evacuating the synthetic gas produced in the lower section, the upper-section having an enlarged internal cross-sectional area compared to the lower section, and wherein a height of the bed at rest is comprised between 1.5 and 2 times an internal cross-section of the lower-section, the process also comprising providing to the feedstock a stream of oxygen-enriched air having an oxygen content comprised between 30% and 42% vol. oxygen.

The present invention also relates to a process for the gasification and conditioning of a carbonaceous material feedstock, the process comprising: achieving gasification of a feedstock of carbonaceous material in a reaction vessel comprising a fluidized gasification bed of particulate material, a feed port for feeding the carbonaceous material and a grid bearing tuyeres for introducing a fluidization gas to the gasification bed, the improvements in the gasification process consisting of: selecting as the carbonaceous material feedstock a forest, agricultural or urban waste biomass material, providing as the fluidization gas an oxygen-enriched air containing about 25 to about 40% oxygen to the reaction vessel, and conducting the gasification at temperatures ranging between about 680 and about 765 C and submitting the resulting synthetic gas to a conditioning process for removing impurities and pollutants, the improvement in the conditioning process consisting of: subjecting the synthetic gas to a sequential treatment in at least one cyclone to remove particulate material therefrom, followed by treatment in at least one counter-current wet-scrubber tower to cool the gas and solubilize or entrain further pollutants, followed by treatment in at least one venturi scrubber to remove further pollutants, followed by treatment in at least one demister to remove liquid droplets.

The present invention also provides for an improved conditioning process for the hot synthetic gas exiting the reaction vessel. The conditioning process provides a clean, cold synthetic gas ready for use in various applications such as a fuel for boiler furnaces, internal combustion engines, gas turbines, etc. The process improvements consisting of:
subjecting the synthetic gas to a sequential treatment in at least one cyclone to remove particulate material therefrom, followed by a treatment in at least one counter-current wet-scrubber tower to cool said gas and solubilize or entrain further pollutants, followed by a treatment in at least one venturi scrubber to remove further pollutants, followed by treatment in a at least one demister to remove residual liquid droplets. Optional additional conditioning treatments are also provided. Recycling steps are also described and are aimed at revealing means to increase energy efficiency and minimize by-product generation.
According to a first aspect, the invention relates to a process for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the process comprising:

= feeding the carbonaceous material into a reaction vessel containing a fluidized gasification bed of particulate material;

= introducing an oxygen-enriched fluidizing gas into the reaction vessel;

= conducting the gasification at a temperature between 680 and 765 C, thereby producing the synthetic gas;

= feeding the synthetic gas into at least one cyclone to yield a synthetic gas that is substantially free of particulate material; and = submitting the synthetic gas substantially free of particulate material to a three-stage wet-scrubbing treatment to yield a clean cold gas, or to a hot-gas conditioning treatment to yield a hot gas.

According to a second aspect, the invention relates to an apparatus for the gasification of carbonaceous material to yield a synthetic gas, comprising a reaction vessel having an upper section and a lower section, wherein:

= the upper section has an internal cross-section that is larger than an internal cross-section of the lower section;

= the lower section comprises means for introducing particulate material, means for introducing fluidized gas, means for feeding the carbonaceous material, and means for removing coarse solid materials; and = the upper section has means for removing the synthetic gas.

According to a third aspect, the invention relates to a plant system for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the system comprising:

= an apparatus for the gasification of the carbonaceous material, comprising a reaction vessel having an upper section and a lower section, wherein:

the upper section has an internal cross-section that is larger than an internal cross-section of the lower section, the lower section comprises means for introducing particulate material, means for introducing fluidized gas, means for feeding the carbonaceous material, and means for 5 removing coarse solid materials, and the upper section has means for removing the synthetic gas;

= at least one cyclone attached to the upper section of the reaction vessel;
= at least one water spray column attached to the at least one cyclone;

= at least one counter-current venture scrubber attached to the at least one water spray column; and = a cyclonic separator.

According to a fourth aspect, the invention relates to a plant system for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the system comprising:

= an apparatus for the gasification of the carbonaceous material, comprising a reaction vessel having an upper section and a lower section, wherein:

the upper section has an internal cross-section that is larger than an internal cross-section of the lower section, the lower section comprises means for introducing particulate material, means for introducing fluidized gas, means for feeding the carbonaceous material, and means for removing coarse solid materials, and the upper section has means for removing the synthetic gas;
= a mobile granular filter; and = a multitubular fixed-bed catalytic reforming reactor.

According to a fifth aspect, the invention relates to a plant system for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the system comprising:

= an apparatus for the gasification of the carbonaceous material, comprising a reaction vessel having an upper section and a lower section, wherein:

the upper section has an internal cross-section that is larger than an internal cross-section of the lower section, the lower section comprises means for introducing a fluidized gasified particulate material, means for introducing a fluidized gas, means for feeding the carbonaceous material, and means for removing coarse solid materials, and the upper section has means for removing the synthetic gas;

= at least one cyclone attached to the upper section of the reaction vessel;
= at least one water spray column attached to the at least one cyclone;

= at least one counter-current venturi scrubber attached to the at least one water spray column;

= a cyclonic separator;

= a mobile granular filter; and = a multitubular fixed-bed tar catalytic reforming reactor.

According to a sixth aspect, the invention relates to a process for the conditioning of synthetic gas obtained from gasification of carbonaceous material, the process comprising:

= feeding the synthetic gas into at least one cyclone to yield a synthetic gas that is substantially free of particulate material; and = submitting the synthetic gas substantially free of particulate material to a three-stage wet-scrubbing treatment to yield a clean cold gas, or to a hot-gas conditioning treatment to yield a hot gas.

BRIEF DESCRIPTION OF THE FIGURES

= Figure 1 represents the gasifier, in accordance with the invention;

= Figure 2 is a schematic representation of the process of the present invention;
= Figure 3 shows the cooling tower of the gas scrubbing system of this invention;

Figures 4.1-4.5 present the composition of the producer gas as function of the oxygen content of the gasifying agent;

= Figure 4.6 gives the producer gas flow rate as function of the oxygen content of the gasifying agent;

= Figure 4.7 gives the gasification temperature as function of the oxygen content of the gasifying agent;

= Figure 4.8 presents the higher heating value (HHV) of the gas as function of the gasifying agent;

= Figure 4.9 presents the cold gas efficiency as function of the percentage of oxygen content of the gasifying agent;

= Figures 5.1-5.5 present the composition of the producer gas as function of the oxygen content of the gasifying agent;

= Figure 5.6 gives the producer gas flow rate as function of the oxygen content of the gasifying agent;

Figure 5.7 gives the gasification temperature as function of the oxygen content of the gasifying agent;

= Figure 5.8 presents the HHV of the gas as function of the gasifying agent;

= Figure 5.9 presents the cold gas efficiency as function of the oxygen content of the gasifying agent;

Figure 5.10 presents the tar by-product generation as function of the oxygen content of the gasifying agent.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described herein below, by referring to specific embodiments and appended Figures.

The invention proposes a process to convert organic-rich wastes into a clean synthetic gas using an atmospheric or pressurized bubbling fluid-bed gasification reactor.
The gasification is followed by gas conditioning which will yield cold clean gas for use as a fuel in boilers, internal combustion engines, etc.

The present invention can advantageously be used to convert various forms of refuse rich in carbonaceous material. For example, wood residue, refuse derived fuel < RDF
from municipal solid waste, plastic and rubber residues, wastewater treatment sludge as well as residues from various industrial operations such as pulp and paper black liquors, petroleum heavy residues altogether with other non dangerous carbonaceous materials are examples of gasifiable residual streams.

These carbonaceous feedstocks may be predried or preheated in order to increase the efficiency of the overall process. Typically their moisture content does not exceed 20 wt% of the feedstock.

During gasification various chemical reactions take place. These chemical reactions are responsible for the conversion of the feedstock into 'synthetic gas'. The non-condensable portion of the synthetic gas is a mixture of CO, H2, CO2, N2, H2O and light hydrocarbons (HC) of up to 7 carbon atoms. The relative concentration of each constituent depends on the feedstock but it is a strong function of the gasification agent (percentage of oxygen in the oxygen-containing gas fed to the gasification reactor), the ratio of gasification agent to feedstock, and the extent of reactions taking place in the freeboard of the reactor. The synthetic gas entrains small particles, known as particulate matter, and contains low amounts of higher molecular weight hydrocarbons in the gaseous phase, known in the literature as tar. The particles come from two sources: the inorganic matter present in the feedstock and hydrocarbons condensation reactions responsible also for the formation of tar present in the synthetic gas. The latter reactions lower the carbon conversion of the process thus decreasing its total energetic efficiency. Appropriate conditions are used as function of the feedstock to minimize the condensation reactions.

In accordance with the present invention, the hot gas exiting the gasification reactor, after passing through a cyclone system (typically one or two cyclones) is sent to a wet scrubbing apparatus, generally multi-step, for gas cooling and conditioning. Generally speaking, the purpose of the wet scrubbing is to remove the particulate and the condensable species present in the synthetic gas.

Referring to FIG. 1, the gasification reactor of the present invention is illustrated. In its preferred embodiment, gasifier (10) is a cylindrical reaction vessel internally lined with appropriate insulation and refractory material layers, generally referred to as insulation (12).

Gasifier (10) may operate either under atmospheric or above atmospheric pressure. The lower section (18) of gasifier (10) is equipped with a channel-onlet (17) for entry of an oxidizing gas, and an oxygen-rich gas distribution grid (14). Above the grid (14) there is a section known as the fluidization bed (16) which is advantageously filled with silica sand or alumina sand. However, other granular material, for example magnesia, chromia, etc., are also acceptable. The granular material is introduced into the gasifier (10) via inlet (52).
Advantageously, the mean size of the granular material used in the bed will vary between 200 and 700 pm. One skilled in the art will readily appreciate that the final choice will depend on carbonaceous feedstock reactivity, oxygen content fed to the gasification reactor, operation pressure and reactor geometry (defining the fluid-dynamics and the residence-time distributions).

Preferably, the height of bed at rest (16) is between 1.5 and 2 times the internal cross-sectional area of the gasifier (10). Gasifier (10) is preferably cylindrical.
However, one skilled in the art will readily appreciate that gasifier (10) may be conical, pyramidal, square or rectangular or other suitable shapes. In all cases, the lower section (18) extending to an upper section (20) at a boundary (19) is designed to be filled almost entirely by the expanded fluid bed (16) during gasification. The upper section, i.e. the freeboard (20) will advantageously have an enlarged internal cross-sectional area when compared to the lower section (18), in the case of a cylindrical gasifier (10), it will have up to about 1.5 times the cross-sectional area of the lower section (18). In the case of conical or pyramidal gasifier (10), it would be installed with the wider portion on top. The larger cross-sectional area at the top favors an appropriate disengagement of the gas from the fluidized bed of granular solids and thus results in a shorter vessel than a reactor having an identical diameter in both the fluidization and the freeboard sections for the same levels of disengagement.

Once again, one skilled in the art will appreciate that the relative heights of the two sections (18) and (20) of the gasifier (10) will depend on feedstock physico-chemical properties and several other parameters among which gasification fluid-dynamics and kinetics as well as the desired synthetic gas composition.

Coarse solids not entrained by the synthetic gas exiting the gasifier (10) remain in the fluid bed and can be removed continuously from an outlet (50) at the bottom of the gasifier (10).
The separation of the coarse solids from the bed material (i.e. silica sand or alumina) is achieved naturally during fluidization due to density differences. In general, particulate matter of low inorganic content material has lower density than sand and tends to stay at the top of the expanded bed. This leads to higher attrition rates which gradually decrease the size of these particles and facilitate the 'washing' of these solids off the fluid bed. If high density inorganic material accompanies the feedstock it may be removed from the bottom of the bed through appropriately designed exit ports.

5 The total residence time of the synthetic gas evolved in gasifier (10) is about 10 to 20 seconds, usually half of it in the lower section (18) of the reactor.
Advantageously, the ratio of lower section (18) height to top section (20) height is between 2:1 and 3:1.

The gasifier (10) advantageously operates at gas velocities ranging between 5 and 20 times the minimum fluidization velocity (as defined in standard fluidization engineering literature).
10 Overall the gasification is "autothermal" meaning that once at steady state, gasifier (10) requires essentially no external heating or cooling. For the start-up, gasifier (10) will require preheating. This is preferably done with a gas burner to reach a temperature of about 500 C.
It is to be understood that steady state temperature inside gasifier (10) is set by a combination of the following parameters:

reactor geometry;

= operation pressure;

= insulation and refractory lining thermal properties;
= fluid-bed material and size;

= feedstock nature, physico-chemical properties (composition, humidity, size) and input rate;

= oxygen-containing gas composition, velocity and flow rate.

Due to the complexity of the numerous reactions taking place during gasification it is observed that for the same reactive mixture, different operating parameters can lead to different synthetic gas compositions. This means that some steady states are in fact metastable and slight parameter fluctuations can lead to other steady states.

The feedstock is introduced into gasifier (10) via an inlet (54) which comprises water cooler (not illustrated) and screw feeder. Tar may be injected with the gasifier (10) via an inlet (56) which comprises a valve and a nozzle (not illustrated).

One key and surprising feature of the present invention is the discovery that the use of an oxygen-enriched fluidizing gas vastly improves the properties of the resulting synthetic gas.
More specifically, it was discovered that using oxygen-enriched air, of about 30 to 50%vol of oxygen, preferably 30 - 40 % vol. oxygen as a fluidizing agent greatly improves the key gasification parameters: cold gas efficiency, lower tar production and higher calorific values of the synthetic gas. Supporting data is found in Figures 4.1 to 5.10 and in Tables 1 to 7 herein below.

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J Q m In The remaining apparatus and process steps relate to gas conditioning of the synthetic gas evolved from gasifier (10). These conditioning steps will now be described in greater detail.

Referring now to FIG. 2, the synthetic gas exiting through a channel-outlet (21) of gasifier (10) shown in FIG. 1 passes through cyclones (22) and (24) connected in series.
The number of cyclones require will depend on the cleaning strategy. In this preferred embodiment, cyclones (22) and (24) will generally retain between 90 and 95% of the total amount of particles entrained by the synthetic gas as fly ash. Modern cyclones are indeed able to cut out of the synthetic gas stream 99% of particles with mean particle size higher than 10 pm. This means that the remaining particles in the synthetic gas, estimated between 1000 and 2000 ppm/v, have a mean particle size of less than 10 pm.
The particles removed through cyclone (22) are split into two streams : C9 and C10. C9 is the solids purge of the gasification process while C10 is recycled back to the gasifier to increase the carbon conversion of the system. The ratio of these two streams (or expressed differently the recycle ratio) is a function of the carbon content of the particles removed by cyclone (22). These particles may eventually be mixed with any solids purged directly from the gasifier.
In an alternate embodiment, stream C9 may be partially directed to water treatment column (26) for use as an absorbent as described herein below. Indeed, the carbon in the particles has specific surface area and adsorption features resembling those of activated carbon. The proposed process utilizes this carbon-containing particle (also known as ash) together with activated carbon to treat, through adsorption, the wastewater purge. Upon saturation of the carbon by adsorption, part of the solids are recycled back to the gasifier to increase the carbon conversion while decreasing the amount of solids to be disposed of.

Thus the synthetic gas exiting cyclones (22) and (24) is subject to further conditioning prior to its use as fuel. At this point there are two main conditioning options:

= Wet scrubbing as described in the present patent application if clean cold gas is desired (i.e. the case for use in burners/boilers or internal combustion engines); and Hot gas conditioning if hot gas is required (i.e. the case for use in gas turbines or in integrated gasifier combined cycled: IGCC)_ The hot gas conditioning module consists of a mobile granular filter and a multitubular fixed-bed tar catalytic reforming reactor. The mobile granular filter and the catalysts are 5 proprietary inventions of the applicants and described in separate patents and/or applications.

Particles removed by cyclone (24) are fed to water treatment system (26) as shown by stream C36. These particles are characterized by sufficient surface area to be used as 10 adsorbent. Nevertheless, additional amounts of activated carbon, stream C37, are necessary for compliance with environmental regulations for disposal of treated water effluent, stream C38. As will be apparent from the following description, water treatment system (26) is used to treat the water used to scrub remaining pollutants from the synthesis gas exiting cyclone (24), stream C7.

To remove the remaining particles from the synthetic gas, stream C7, and to condense and remove the tarry products, which are in the vapor state at temperatures higher than 400 C, the process includes the use of a three stage wet (water) scrubbing module comprising water spray column (28), venturi scrubber (30) and cyclonic separator (32).

The synthetic gas stream C7 enters water spray column (28). Column (28) is illustrated in greater detail in Fig.3. Column (28) is a water-spray column preferably counter-current, cylindrical and with a conical bottom. Column (28) is used as a first stage scrubber for (a) cooling the synthetic gas down to about 90 C, b) removing about half of all remaining particles (targeting those comprised between 2 and 10 um) and c) condensing all tarry compounds having boiling points higher than 100 C (excluding some volatile organic compounds (VOCs)).

Referring to FIG.3, water stream W9 which is the scrubbing/quenching solution, enters from the top of column (28), under pressure, and is dispersed using screw-shaped nozzles (34) producting wide angle sprays (36). W2 represents the new synthetic gas. W3 represents the wastewater. And W4 represents the treated synthetic gas.

The pH of the scrubbing/quenching solution is adjusted in accordance with the carbonaceous feedstock and the pollutants contained in the synthetic gas.
There may be present: acid gases (HCI, HCN, SON, N,,Oy), alkaline gases such as ammonia as well as volatile metal and salts. Acid gases are removed through alkaline scrubbing.
Ammonia is removed through acidification of the scrubbing water. Volatile metals and salts are removed through quenching. Alkaline and acidic scrubbing produce environmentally neutral rejects. Condensed metals are subdivided into two categories : 1) alkali and alkaline earth metals giving non polluting salts and 2) heavy metals under their elemental form or as oxides if present in the gasified feedstock . The latter, being essentially insoluble in water, as shown in metal distribution studies, precipitate and can be recovered and removed from the process , streams C19 and C28.

Returning to FIG. 2, contaminated water stream C15 exists column (28) and is routed to heat exchanger (38) for further cooling to about 30 C. From there, stream C17 enters decanter/skimmer tank (40) to remove precipitated material. In tank (40), the heavier inorganics (and the organics sticking to the inorganics organics) decant and can be removed. Meanwhile the lighter organics float at the water surface and are removed by skimming.

The decanting and floating matter altogether with some entrained wastewater are pumped out , streams C18a, C18b, C19 and, after being mixed with other residual streams (defined later) or even separately, are sent back to gasifier (10). A purge, stream C32, equal to about 10% of the recirculating matter, is removed for final disposal to ensure the stability of the system.

The scrubbed synthetic gas, stream C16, still containing micronic particles, water and tar droplets, VOCs and some very volatile metals (if present in the initial solid feedstock of the gasifier) enters the second stage scrubbing process, namely venturi scrubber (30).

Venturi (30) is preferably designed to operate at gas velocity range of 80-100m/s and gives a total pressure drop between 0.07 and 0.15 atm. These conditions give a near micronic dispersion of the water stream entering the throat of the cyclone. This provides an appropriate removal of the remaining particles with an efficiency of near 99%
for particles of size as low as 0.01 pm. Ignoring the humidity of the synthetic gas, in this venturi scrubber, the ratio gas/water used is about 1 to 1 w/w. During the contact time (usually of less than 2s) the synthetic gas is cooled to about 35-40 C without significant temperature increase of the water stream.

From venturi (30), the synthetic gas enters the third and last stage of the scrubbing module, namely cyclonic separator (32). The water leaving the bottom of the gas-liquid cyclonic separator, stream C24 is fed into a second decanting/skimming tank (42) much like tank (40). Tank (42) is of significantly smaller size due to the lesser amounts of water involved in this recycle loop. Thus, streams C27 and C28 of Fig. 2 are pumped out to join the analogous streams C18 and C19 of tank (40). The cyclonic separator is a high velocity design unit able to remove droplets of mean size 5 /gym with an efficiency of about 80%.
This means that the global separation efficiency is higher than 95%.
Nevertheless smaller droplets are still present in the exiting gas stream (Fig. 2, stream C23) and require further removal.

Filter (44) is a high efficiency demister for completing droplet removal.
Commercially available viscous filters as well as corrugated plate coalescers can be used to accomplish this task. Filter (44) can efficiently remove residual aromatics present in minute quantities (ppm or ppb). Removal is generally performed with a bed of activated carbon which also removes residual organic vapors. Once saturated the spent activated carbon may be recycled back to gasifier (10).

The use of a dehumidification unit (46) depends on specifications imposed by the synthetic gas end-use devices. The synthetic gas leaving the filter/demister will have an average temperature of about 25 C and will be saturated with water. This means that it will contain between 2 and 3%w/w of water. Usually this level of humidity is not prohibitive for final synthetic gas use in commercial burners/boilers, internal combustion engines and gas turbines. Dehumidification units (46) work by adsorption, for example by passing over a bed of alumina, and can reduce the moisture content of the gas to the desired levels.

EXAMPLES OF GASIFICATION PERFORMANCE

Description of the unit The runs have taken place in a Process Development Unit (or pilot plant). The gasification reactor is a 4 m high refractory-lined cylinder having 60 cm as outside diameter in the lower section, expanded to 74 cm in the upper section. The inner diameters are 30 cm and 45 cm, respectively.

Fluidizing gas (air or oxygen-enriched air) enters the reactor through a distribution grid formed by an assembly of 9 tuyeres. The grid, of proprietary design, also serves as support for the sand bed.

The gasifier is fed from a 1.3 m3 live-bottom hopper by a system consisting of a transfer screw, a chute and an injection screw which can be located in either one of two ports situated at 48 cm or 29 cm above the grid.

Air (or oxygen-enriched air) is supplied by a compressor. The main flow (about 80%) enters the reactor through the grid. The rest (20%) is directed to the feeding system to prevent back flow of gases from the reactor to the hopper. The sand (SiO2) bed height (at rest) has been varied between 60 and 45 cm. The average diameter of the sand particles is 0.5 mm (2.6 kg/1 as density). Fluidizing velocities are comprised between 0.3 and 0.75 m/s. The biomass solids in the bed are estimated at less than 10% of the total bed solids.
The volumetric bed capacity is 1.12 tones of biomass (mbed3 - h).

The gas produced exits the gasifier and passes through two cyclones in series and the wet scrubbing module described in the present patent application. A bypass system permits to either send the entire gas produced to a granular filter and then to the flare, or go directly to the flare. Char and ash are collected in two reservoirs connected to the cyclones as well as in the filter media used.

The installation is equipped with more than 40 thermocouples and 20 pressure transducers.
Air and producer gas flows are measured by thermal dispersion and orifice flow meters, respectively.

The waste feed rate is monitored by a system of 3 load cells. The feed rate is adjusted by the rotation of the screws at the bottom of the hopper.

All measured variables are recorded by a Hewlett Packard data acquisition system taking samples at 10s intervals.
Feedstocks that have been successfully gasified (1) Woody biomass;
(2) Forest and agricultural residues;
(3) Biological treatment sludge;

(4) Refuse Derived Fuel (RDF) from urban wastes (MSW);
(5) Rubber residues containing 5-15% KEVLAR residues;
(6) Granular polyethylene and polypropylene residues.
Analytical strategies Sampling trains, designed following EPA's and Canadian standard methods, each consisting of a particulate material filter, a tar-water condenser, a demister and a drying/absorber allow collection and analysis of the gases.

When hot gas conditioning is applied three isokinetic samplers are used to determine the particulate, tar,VOCs and water contents in the off gases before and after gas conditioning.
Each isokinetic gas sampling probe has a heated filter, a water/tarNOCs condensing heat exchanger/collector, a gas impact demister, a drierite gas dryer, a regulated pump, a dry gas flow meter and an appropriate orifice/manometer arrangement insuring isokinetic conditions.

When wet scrubbing is applied, two isokinetic sampling trains are sufficient to measure the efficiency of the gas conditioning. An HP 5890 gas chromatograph analyses dry, tar free gases by separation of components on Porapak Q and Molecular Sieve 13X columns in series followed by TC detection. The sampling combined with the appropriate analysis methods allows to:

5 = Calculate the gross composition of the stack gas (the H2O, H2, CO, C02, N2, and 02 content) using Gas Chromatography (combination of Porapak Q & Molecular sieve column packings) as well as the particulate matter load of the stack gas.
= Detect and quantify toxic and acid components via colorimetric methods (Matheson-Kitagawa's Kits) in the stack gas (i.e. SOx, NOx, HCI) .

10 = Identify and quantify the main organic components dissolved in the scrubbing water by means of GC/MS analysis.

= Measure the inorganic anions retained by scrubbing water using ion chromatography.
= Identify and quantify the distribution of metals in 'the output streams of the gasifier and its modules using X-rays fluorescence, AES/ICP, SEM and AA analyses.

Other analyses have been used in order to gather additional information as follows:
= Calorimetry for the different feedstocks tested.

= TGA for the study of the thermal stability/instability of the different feedstocks tested.
= TOC, COD and BOD5 analyses for the scrubbing water before and after treatment = BET analysis for the porosity of the carbon-rich gasification ashes used for the adsorption tests.

= GC/MS analysis for tar and halogenated hydrocarbons.

Results with residual wood as gasification feedstock The four (4) runs reported herein below have the following common parameters/variables:

= Stoichiometric (or equivalence) ratio: A= 30% (refers to the stoichiometric amount of oxygen needed for complete combustion of the carbon and hydrogen fed to the gasifier);
= Mean Solids feed rate: 35 kg/h;

= Mean Solids humidity: 12%w/w;

= Mean Solids size: 1cm;
= Oxygen content of the fluidizing air was 20.9; 30; 40; and 50%vol. for the four runs respectively.

The results obtained are presented as follows:

= Figures 4.1-4.5 present the composition of the producer gas as function of the oxygen content of the gasifying agent. We can conclude that as oxygen content in the fluidizing gas increases the inert gases N2 and Ar are decreasing while all other gases coming from organic matter gasification increase.

= Figure 4.6 presents the synthetic gas flow rate as function of the oxygen content of the gasifying agent. As expected, the gas flow rate is decreasing. It is preferable for the commercialization of the technology to have flow rates as low as possible in order to decrease the cost associated with gas conditioning, piping and handling in general.

Moreover, for the same gasification reactor, lower gasifying agent and producer gas flow rates lead to lower linear velocity profiles and higher residence times in the fluid-bed gasification vessel. This in turn leads to lower particle entrainment (carry-over) and allows heavy tar gasification and CO shift reactions to proceed at higher conversion rates.

= Figure 4.7 presents the gasification temperature as function of the oxygen content of the gasifying agent. As the available heat is transferred to a smaller gas flow rate the temperature increases nearly linearly with the oxygen content.

= Figure 4.8 presents the HHV of the synthetic gas as function of the gasifying agent. It is also an increasing function. Values between 9 and 12 M)/Nm3 are obtained. Such values classify the synthetic gas as a medium calorific value gas and render its use in end-use devices easier and more efficient than low calorific value producer gas.

Results with RDF as gasification feedstock The runs reported herein below have the following common parameters/variables:
= Stoichiometric (or equivalence) ratio: I = 30%;

= Mean Solids feed rate: 35 kg/h;
= Mean Solids humidity: 12%w/w;
= Mean Solids size: 1cm;

= Oxygen content of the fluidizing air was 20.9; 30; and 40%v/v for the three runs respectively.

The following observations can be made from the results:

= Figures 5.1-5.5 present the composition of the producer gas as a function of the oxygen content of the gasifying agent. We can also conclude, as in the wood case, that as oxygen content increases in the fluidizing gas the inert gases N2 and Ar are decreasing while all other gases coming from organic matter gasification increase.

= Figure 5.6 gives the producer gas flow rate as a function of the oxygen content of the gasifying agent. The trend and the comments are the same as in the wood case.

= Figure 5.7 gives the gasification temperature as function of the oxygen content of the gasifying agent. The trend is the same as in the wood case. Here we observed however a levelling-off behavior around 35% of oxygen content. As the RDF calorific value is lower than that of wood it is probable that the balance between heat production over heat transfer and losses has attained its equilibrium faster than in the case of wood gasification.

= Figure 5.8 presents the HHV of the gas as function of the gasifying agent.
It is also an increasing function. As in the case of wood gasification, values between 9 and MJ/Nm3 are measured.

The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given broadest interpretation consistent with the description as a whole.

Claims (7)

1. A plant system for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the system comprising:

an apparatus for the gasification of the carbonaceous material, comprising a reaction vessel having an upper section and a lower section, wherein:

the upper section has an internal cross-section that is larger than an internal cross-section of the lower section, the lower section comprises means for introducing particulate material, means for introducing fluidized gas, means for feeding the carbonaceous material, and means for removing coarse solid materials, and the upper section has means for removing the synthetic gas;

at least one cyclone attached to the upper section of the reaction vessel;
at least one water spray column attached to the at least one cyclone;

at least one counter-current venture scrubber attached to the at least one water spray column; and a cyclonic separator.

2. A plant system for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the system comprising:

an apparatus for the gasification of the carbonaceous material, comprising a reaction vessel having an upper section and a lower section, wherein:

the upper section has an internal cross-section that is larger than an internal cross-section of the lower section, the lower section comprises means for introducing particulate material, means for introducing fluidized gas, means for feeding the carbonaceous material, and means for removing coarse solid materials, and the upper section has means for removing the synthetic gas;
a mobile granular filter; and a multitubular fixed-bed catalytic reforming reactor.

3. The system according to claim 1 or 2, wherein for the apparatus, the internal cross-section of the upper section is about 1.2 time the internal cross-section of the lower section.

4. The system according to any one of claims 1 to 3, wherein for the apparatus, a ratio of height of the lower section to height of the upper section is between 2:1 and 3:1.

5. The system according to any one of claims 1 to 4, wherein for the apparatus, the lower section further comprises means for supporting a bed of fluidized particulate material.

6. The system according to any one of claims 1 to 5, wherein for the apparatus, the reaction vessel is cylindrical, conical, pyramidal, or prismatic.

7. A plant system for the gasification of carbonaceous material to yield a synthetic gas and the conditioning of the synthetic gas, the system comprising:

an apparatus for the gasification of the carbonaceous material, comprising a reaction vessel having an upper section and a lower section, wherein:

the upper section has an internal cross-section that is larger than an internal cross-section of the lower section, the lower section comprises means for introducing a fluidized gasified particulate material, means for introducing a fluidized gas, means for feeding the carbonaceous material, and means for removing coarse solid materials, and the upper section has means for removing the synthetic gas;

at least one cyclone attached to the upper section of the reaction vessel;
at least one water spray column attached to the at least one cyclone;

at least one counter-current venture scrubber attached to the at least one water spray column;

a cyclonic separator;

a mobile granular filter; and a multitubular fixed-bed tar catalytic reforming reactor.