NO302422B1 - Process for refining raw gas produced from a carbonaceous material by means of a gasification process - Google Patents

Abstract

The invention relates to a process for the refining of a raw gas produced from a carbonaceous material by means of a gasification process, refining taking place in a secondary stage separated from the gasifier. In order to reduce the gas contents of tar in the form of organic compounds condensible at lower temperatures, such as ambient temperatures, and of ammonia, the refining is carried out in a secondary stage being a fast circulating fluidized bed, the bed material of which at least mainly being an active material in the form of a material that is catalytic for tar and ammonia conversion, whereby a catalytic conversion of tar and ammonia contained in the raw gas is obtained. In order to decrease the content of hydrogen chloride in the gas, an active material that also can absorb chloride is used. Fresh catalytic and absorbing material is supplied in an amount sufficient to have the hydrogen chloride present in the raw gas absorbed on the material, a corresponding amount of the material containing absorbed chloride being discharged from the secondary stage.

Description

This invention relates to a method for refining a raw gas produced from a carbonaceous material by means of a gasification process whereby the refining takes place in a secondary step separate from the gasifier in the gasification process.

A raw gas produced from different types of biological fuels and used as a fuel gas is a valuable oil substitute for demanding purposes where the process requirements make direct firing with solid fuel impossible, e.g. firing of lime kilns or conversion of existing oil-fired boilers.

For other types of purposes, e.g. so-called cogeneration (of electrical energy and heat) using diesel engines, very high demands are placed on the purity of the gas with regard to tars and dust. Furthermore, environmental aspects often lead to requirements for low concentrations of compounds which, when burned, form harmful emissions such as N0X, S0X and various chlorinated compounds. The latter applies in particular to a gas produced from fuels based on waste (RDF = refuse derived fuel). These requirements for gas purity can be satisfied by refining the raw gas using an appropriate method.

Gasification of RDF with subsequent refining of the raw gas means an environmentally favorable method for energy recovery from waste by using refined gas in existing boilers or for cogeneration in diesel engines and/or boilers.

In addition, the use of raw gas often causes other technical problems.

At temperatures below 1200°C, tar is always present in a raw gas produced by gasification of a carbonaceous material, e.g. coal, peat, bark, wood or RDF, which limits the utilization to the combustion of hot gas in direct or close connection with the gasifier. Operational disruptions caused by tar coatings on equipment and fixtures are a major problem that limits applicability. During combustion of hot gas, nitrogen and in certain cases also sulfur (e.g. from peat) bound in tars as well as ammonia, H2S (peat) or HC1 (from RDF) further give rise to emissions that are harmful to the environment (N0X, S0X and respectively Hel, and chlorinated hydrocarbons, including dioxins).

Despite extensive research into tar and ammonia conversion, no process that can provide sufficient far-reaching gas refining on an industrial scale has yet been developed. The traditional way to reduce tar content in a raw gas is by means of wet scrubbing, but aerosol formation in the scrubber makes tar removal ineffective. Furthermore, a process water with a high content of organic compounds and ammonia is obtained. Consequently, this water must again be purified before it is emptied into a sewage system. When gasifying RDF, the process water also contains high concentrations of dissolved hydrochloric acid and/or ammonium chloride. When gasifying more sulphur-rich fuel, e.g. peat or coal, the raw gas must also be cleaned to remove hydrogen sulphide.

The aim of the present invention is to provide a raw gas refining process, by means of which the above-mentioned problems will be solved to a large extent.

This goal is achieved by a method according to the invention with the features defined below and in the accompanying claims.

The invention thus relates to a method for refining a raw gas to be used as fuel gas and which is produced from a carbonaceous material by means of a gasification process, where the refining takes place in a secondary step separate from the gasification process, deactivated active material being discharged from the secondary step is optionally activated by treatment with an oxidizing gas, characterized in that in order to reduce the gas's content of organic compounds in the form of tar that is condensable at low temperatures such as ambient temperatures, and of ammonia, the refining is carried out in a secondary step in the form of a fast-circulating, fluidized bed, with standing reactor shaft, as the bed material mainly consists of an active material which is catalytic for tar and ammonia conversion and which consists of a magnesium/calcium carbonate-containing material, preferably dolomite and/or the corresponding calcined (burnt) product, where the material's particle size else is less than 2 mm, preferably less than 1 mm, the operating temperature in the secondary stage being kept within the range 600 - 1000 °C, preferably within the range 700 -

900°C, the average suspension density in the reactor

the shaft is kept in the range 80 - 250 kg/m<3>, the gas velocity in the reactor shaft in the secondary stage, calculated for an empty reactor shaft, is kept below 10 m/s, preferably below 6 m/s,

and the residence time of the gas, calculated for an empty reactor shaft,

is held in the range 0.2 - 20 seconds, preferably in the range 0.5-7 seconds, so that a contact is provided between the passing gas and the active material for catalytic conversion of tar and ammonia present in the gas, preferably to concentrations in the refined gas of less than 500 and 300 mg/Nm<3> respectively.

In special cases, the raw gas may also contain significant amounts of hydrogen chloride, the gas being produced by means of any gasification process from a carbonaceous material, e.g. bark, wood, peat or "refused derived fuel", RDF. In special cases, absorb-

tion of hydrogen chloride to near thermodynamic equilibrium at the same time place. The secondary stage consists of a circulating fast fluidized bed (CFB = Circulating Fast Fluidized Bed)

with a layer material at least partly in the form of an active material, e.g. dolomite. With this device could also

the secondary stage integrates with any CFB carburettor, which only has a primary particle separator in front of it, or another type of carburettor.

We have found that sufficient conversion of tars and ammonia, and in special cases simultaneous absorption of hydrogen chloride, can be achieved by first separating the tar-

containing gas from larger fuel pyrolysis particles in the gasification stage, and then in a separate, secondary stage in the form of a circulating, rapidly fluidized bed and contacting the gas with a suitable active material such as dolomite at suitable process parameters.

If the carbonaceous material also contains sulphur

in significant quantities, such as is the case for peat, the absorption of hydrogen sulphide on the catalytic and absorbent material will of course also take place.

The amount of active material required in connection with the raw gas content is determined by the required space velocity for catalytic conversion of tars and ammonia and depends on several parameters such as the temperature, the residence time of the gas, the particle size of the active material, the partial pressure of the reactants and the degree of deactivation of the active material. Too low a temperature and/or CO 2 partial pressure can lead to tar conversion which causes carbon deposition on the active surface, leading to deactivation. If this occurs, the material can be activated by treatment with an oxidizing gas, e.g. air and/or steam. Absorption of HC1 (and/or H2S) takes place so quickly at the relevant temperatures that these reactions are almost determined by the equilibrium and lead to a consumption of active material corresponding to the formed solid chloride (and respectively sulphide).

We have thus found that absorption of chloride (and in certain cases also of hydrogen sulphide) on an active material such as dolomite, is a fast reaction and requires the presence of significantly smaller amounts of active material in relation to the gas stream than catalytic conversion of tars and ammonia.

Use of a secondary step in the form of a rapidly circulating fluidized bed (CFB) means significant advantages.

Such a layer is able to handle entrained dust from the carburettor, provides very even temperatures in the reaction zone and also provides a homogeneous contact between gas and layer material, i.e. little risk of variations in conversion/absorption rate. Furthermore, the particle size can be varied downwards to a large extent for such cases where this is required to provide increased conversion at a certain temperature and space velocity. Significant erosion of the layer material also leads to increased available active surface. A secondary stage designed as a CFB can also be advantageously integrated with any CFB carburettor which only has a primary particle separator, or other type of carburettor. Relatively small diameters are also achieved by increasing the dimensions, as the gas velocities can be kept relatively high, up to 10 m/second, preferably up to 6 m/second.

In case the gasifier consists of a CFB gasifier, a connection directly after the primary dust separation can thus be made. If an active material is used as a bed material in the CFB gasifier, the secondary stage can advantageously be integrated with the gasifier, e.g. so that dust from a secondary particle separator after the secondary stage is totally or partially recycled to the gasifier. In this way, the total losses of layer material are also lower, and you also gain the advantage of only using one type of layer material.

The required amount of active material in the reactor shaft in the secondary stage for sufficient catalytic conversion of tar and ammonia is regulated by the total added amount and by controlled recycling of layer material. The required conversion determines the suitable combination of temperature, particle size and amount of active material. Due to wear, deactivation and/or absorption of HC1 (and possibly H2S), consumed active material is replaced by the addition of corresponding amounts of fresh, active material and/or such activated material. The residence time of the gas can be regulated by the combination of diameter/height above the gas inlet.

In such special cases when HC1 is present in the raw gas in significant amounts, the active material entrained by the secondary stage off-gas means that the HCl absorption is enhanced, as it thermodynamically becomes more extensive at lower temperatures under the assumption that the refined gas is passed down to a significantly lower temperature before final dust removal.

In the following, the invention will be described using an embodiment that refers to the attached drawing, which schematically shows a gasification and gas refining system that illustrates the present invention.

In the system shown in the drawing, carbonaceous material 1 is transported to a gasifier 3, which consists of a circulating, rapidly fluidized bed (CFB). This comprises a reactor 51, a primary separator 52 and the recycling device 53 for the bed material separated from in the primary separator. The layer material consists of an active catalytic and absorbent material, preferably in the form of dolomite, mixed with non-gasified carbonaceous material, tar. The primary separator 52 is a non-centrifugal type mechanical separator, suitably a U-beam separator, as described in our European patent EP 0.103.613 relating to a CFB boiler and referred to herein.

The hot raw gas 2 which is produced in the gasifier 3 is taken out directly from the primary separator 52 and is fed directly to a secondary gas cleaning stage 25 without any further dust removal. The secondary stage 25 is constructed as a circulating, rapidly fluidized bed (CFB) 26 and has the same type of active bed material as the gasifier 3.

The raw gas is fed to the secondary stage 25 so that it constitutes a fluidizing gas.

The secondary stage 25 is constructed with a long and narrow reactor shaft of any cross-section (eg circular or square). The bed material that accompanies the gas stream from the top of the reactor shaft is mainly separated in a primary particle separator 27, preferably a U-beam separator of the same type as the U-beam separator in the gasifier followed by a secondary separator 28, preferably a cyclone. The material 30 which is separated in the primary particle separator is recycled to the lower part of the circulating layer 26 through a recycling plant. The material 29 that is separated in the secondary particle separator 28 is mainly fed to the lower part of the gasifier 3, stream 31. If necessary, part of the material stream 29 can also be fed to the lower part of the circulating layer 26, stream 34, and/or is drained out of the system, stream 43.

For the supply of fresh, catalytic and absorbent material 14 to the secondary stage 25, a side supply device 15 is used which is located at a suitable height. Spent and/or deactivated layer material 35 is emptied by means of an emptying device 3 6 which is located in connection with the bottom of the secondary stage 25.

The active material used in the secondary stage in this example consists of a calcium-magnesium carbonate-containing material, preferably dolomite, with a particle size of less than 2 mm, preferably less than 1 mm, which in combination with the flow of the gas forms the rapidly circulating fluidized layer 26.

The gas velocity in the upper part of the reactor shaft, calculated from the free cross-section, is adjusted so that it is below 10 m/sec., preferably not above 6 m/sec.

The fluidizing gas in the rapidly circulating layer 26 consists of the raw gas 2 and added oxidizing gas 13, e.g. air. If necessary, additional oxidizing gas 33 can be added to the secondary stage 25 at one or more other suitable, higher localized levels.

Conversion of tar and ammonia in the raw gas 2 and absorption of chloride in the raw gas takes place by means of contact with the catalytic and absorbent material in the circulating layer 26 in a temperature range of 600 - 1000°C, preferably 700 - 900°C or preferably 850 - 950°C. The necessary temperature level is maintained by burning combustible gas components inside the secondary stage 25, which is controlled by adjusting the amount of added oxidizing gas, streams 13 and 33.

The average suspension density in the reactor shaft in the secondary stage 25 is kept in a range of 20 - 300 kg/m<3>, preferably in a range of 80 - 250 kg/m<3>, so that the necessary contact between the passing gas and the active material is obtained. This is achieved by adjusting the total amount of circulating material in combination with regulation of the flow rate of recycled material 30 and 34.

The residence time of the gas in the reactor shaft calculated for an empty reactor shaft is kept in a range of 0.2 - 20 seconds, preferably in a range of 0.5 - 7 seconds.

If necessary, activation of deactivated catalytic and absorbent material can be carried out by adding oxidizing gas 32, e.g. air, to the material that is recycled to the lower part of the circulating layer, flows 30 and 34. The amount of added oxidizing gas 32 is regulated so that the activation takes place within a temperature range of

600 - 1000°C, preferably within a range of 750 - 900°C.

Before the operation of the process starts, heating of the secondary stage 25 which contains its layer material takes place by means of combustion of LP gas 24 therein.

The refined gas stream 4 leaving the secondary separator 28 of the secondary stage 25 is freed from entrained, finely divided bed material and steam in subsequent gas treatment stages.

The gas passes through two heat exchangers. In the first heat exchanger 37, heat exchange takes place with oxidizing gas, stream 10, intended for both the gasifier 3 and the secondary stage 25, so that preheated oxidizing gas 11 at the outlet from the heat exchanger 37 has a suitable temperature, preferably around 400°C. The pre-heated oxidizing gas 11 is used both in the gasifier 3 (among other things as fluidizing gas), stream 12, and in the secondary stage 25, streams 13, 32 and 33.

In the subsequent secondary heat exchanger 38, the temperature of the gas 5 is lowered to a level which allows the outlet gas 6 to be further purified by using e.g. standard textile fibers or a cyclone for further dust removal at 39, i.e. preferably down to 150 - 300°C. The removed dust 18 is taken out from the dust removal step 39.

As previously mentioned, the gas stream 4 contains entrained, finely divided active material which accompanies the gas stream out of the secondary separator 28. In special cases, e.g. in connection with the gasification of RDF, the raw gas 2 from the gasifier contains significant amounts of HC1. As absorption of HC1 on calcareous materials such as dolomite is favored by decreasing temperature, the gas cooling in the heat exchangers 37 and 38 helps to increase the degree of absorption of remaining HC1 on the entrained material.

The almost dust-free gas 7 that leaves the dust removal step 39 is led to a washer 40, in which it is freed from moisture and other water-soluble components. In the washer 40, both moistening of the gas stream 7 and condensation of steam take place. Under normal conditions, precipitation of almost all remaining fines and absorption of water-soluble gas components, e.g. NH3, HC1 and/or NH4C1 place.

The water stream 20 that leaves the washer 40 is recycled with a pump 41, whereby it is cooled in a heat exchanger 42, so that the temperature of the water 19 that is recycled to the washer 40 is kept in the range 15 - 20°C. Excess water 21 is drained from the water circuit.

The gas 8 leaving the scrubber can be considered clean for industrial applications, i.e. it is almost free of tars, ammonia, dust, HC1 and H2S. At the current outlet temperatures (approx. 30°C), however, it is saturated with steam; depending on the purpose, in order to reduce the relative humidity, the gas stream 8 can be preheated or passed through a further drying step to reduce its moisture content. The clean gas satisfies the requirements for engine operation, e.g. using turbocharged diesel engines, and can be burned without any subsequent exhaust gas cleaning. For simpler applications, e.g. heat generation in boilers, the washer 40 can be omitted, so that the refined gas can be used either directly after the heat exchanger 37, stream 22, or after the dust separator 39, stream 23.

In the described example, the secondary stage 25 has been integrated with a carburettor 3 based on CFB technology. The gasifier 3 can produce the raw gas 2 from several different types of fuel, e.g. coarse bark, peat or fuels based on waste (RDF). As layer material in the circulating layer in the carburettor 3, it is, as mentioned, good to use a catalytic and absorbent material of the same type as in the secondary stage 25.

The total pressure drop in the added oxidizing gas, e.g. air, when passing through the production loop, is just over 1 bar. This requires the use of a compressor 16, which increases the pressure of the oxidizing gas in stream 9 to the required pressure level in stream 10.

Claims (15)

1. Process for refining a raw gas to be used as fuel gas and which is produced from a carbonaceous material by means of a gasification process, where the refining takes place in a secondary stage separate from the gasification process, deactivated active material being discharged from the secondary stage possibly activated by treatment with an oxidizing gas, characterized by- in order to reduce the gas's content of organic compounds in the form of tar which is condensable at low temperatures such as ambient temperatures, and of ammonia, the refining is carried out in a secondary stage in the form of a fast-circulating, fluidized bed, with a standing reactor shaft, the bed material mainly consists of an active material which is catalytic for tar and ammonia conversion and which consists of a magnesium/calcium carbonate-containing material, preferably dolomite and/or the corresponding calcined (burnt) product, where the material's particle size is less than 2 mm . in the reactor shaft in the secondary stage, calculated for an empty reactor shaft, is kept below 10 m/s, preferably below 6 m/s, and the residence time of the gas, calculated for an empty reactor shaft, is kept in the range 0.2 - 20 seconds, preferably in the range 0.5-7 seconds, so that a contact is provided between the passing gas and the active material for catalytic conversion of tar and ammonia present in the gas, preferably to concentrations in the refined gas of below 500 and 300 mg/Nm<3> respectively. 2. Method according to claim 1, where the raw gas comprises hydrogen chloride, characterized in that active material containing absorbed chloride is periodically or continuously emptied from the secondary stage and that a corresponding amount of fresh, active material is periodically or continuously supplied to the secondary stage. 3. Method according to claim 1 or 2, characterized by the simultaneous supply of oxidizing gas, preferably an oxygen-containing gas and/or steam, to the reactor in the secondary stage. 4. Method according to any one of claims 1, 2 or 3, characterized in that the operating temperature in the secondary stage is regulated with added quantities of oxygen-containing gas. 5. Method according to any one of the preceding claims, characterized in that active, material which is deactivated as a result of carbon deposition or for another reason, is periodically or continuously emptied from the secondary stage and replaced with equivalent quantities of fresh and/or activated material. 6. Method according to claim 5, characterized in that the deactivated active material which is discharged from the secondary stage is activated by treatment with an oxidizing gas, preferably an oxygen-containing gas and/or steam, in a separate activation step, and that the thus activated material is returned to the secondary stage. 7. Method according to any one of the preceding claims, characterized in that the active material which is deactivated as a result of carbon deposition or for another reason, is activated by treatment with an oxidizing gas, preferably an oxygen-containing gas and/or steam in the system which recycles separated bed material from the second stage. 8. Method according to claim 6 or 7, characterized in that the activation takes place at an operating temperature in the range 600 - 1000 °C, preferably within the range 750 - 900 °C. 9. Method according to any one of claims 6, 7 or 8, characterized in that the operating temperature in the activation is controlled by means of added amounts of oxygen-containing gas. 10. Method according to any one of the preceding claims, characterized in that the raw gas is fed to the secondary stage directly from the gasifier without any intermediate dust removal. 11. Method according to claim 10, in which the gasifier comprises a rapidly circulating, fluidized layer, characterized in that the raw gas is fed to the secondary stage directly from the primary separator for the gasifier. 12. Method according to any one of the preceding claims, characterized in that the fluidizing gas from the secondary stage comprises the raw gas and possibly an oxidizing gas. 13. Method according to claim 12, characterized in that the oxidizing gas which does not constitute a fluidizing gas is fed to the reactor in the secondary stage at one or more heights above the fluidizing gas supply. 14. Method according to any one of the preceding claims, characterized in that the hydrogen chloride content in the refined gas leaving the secondary stage is further lowered by means of absorption on the catalytic and absorbent material that remains in the gas after the particle separation in the secondary stage, the gas after the secondary stage first being cooled to a significant lower temperature, preferably to a temperature between 150 and 300°C, and then undergoes a further dust separation. 15. Method according to any one of the preceding claims, characterized in that the raw gas feed to the secondary stage is directly connected to the primary particle separator of a gasifier with a rapidly circulating, fluidized bed, in which the circulating bed material consists of an active material of the same type as in the secondary stage, in that dust separated in the secondary stage , preferably in a secondary particle separator, is at least partially recycled to the lower part of the gasification reactor, and in that the bed material that is torn with the raw gas from the gasifier and the material that is possibly emptied out from the bottom of the gasifier is replaced with material that has been recycled to the gasification reactor from the secondary stage in combination with fresh, catalytic and absorbent material which is added to the gasifier periodically or continuously.