Patent Publication Number: US-2007094929-A1

Title: Apparatus of catalytic gasification for refined biomass fuel at low temperature and the method thereof

Description:
TECHNICAL FIELD  
      The present invention relates, in general, to gasification techniques for using biomass having a low inorganic ash content and a high nitrogen content as clean fuel in a local heating system of a big city.  
      In particular, the present invention relates to an apparatus and method for manufacturing a gas fuel via clean gasification of a selectively refined mixture (SOCA: Sludge-Oil-Coal Agglomerates) comprising biomass organic waste, heavy oil, and coal.  
      As such, the gas fuel obtained after clean gasification is a clean gas fuel usable in gas combustors, such as gas engines, gas turbines, vapor turbine generators, fuel cells, boilers, etc., or in heating devices. In addition, the biomass is organic solid materials, which include industrial waste such as sewage sludge, pulp sludge, etc., living waste such as home waste, excretions, etc., agricultural waste, livestock excretions, or wood chips.  
     BACKGROUND ART  
      Generally, gasification techniques have begun to be used to easily prepare gas fuel or synthetic gas from coal in the absence of a catalyst. In recent years, the above techniques have been developed toward entrained gasification and catalytic gasification for fine coal.  
      While fluidized-bed catalytic gasification, which is combined with fluidized-bed combustion techniques, has been applied to heavy oil reforming, and has been attempted for use in gasification of coal and biomass, methods for high-temperature and clean gasification to produce a minimum of tar are being devised.  
      Solid fuel is gasified along with reactive materials, such as air, oxygen, or steam, and thus, is converted into a flammable gas, condensed liquid/tar, and solid residue. Generally, although gasification is used to maximally convert solid fuel into gas fuel, it may be limitedly applied in a partial gasification process. In addition, pyrolysis, which is different from gasification, means thermal decomposition of solids in an inert atmosphere. However, the initial state of gasification is assumed to be devolatilization of pyrolysis. At this time, the fuel is decomposed into char and volatile components. After the devolatilization, a final component distribution of product gas is determined through the secondary reaction between char and volatile gas. In practice, the distribution of the product varies greatly with the gasification methods and the process conditions.  
      In high-temperature gasification, almost all inorganic material contained in coal or sludge is converted into ash or slag. In addition, iron or sodium contained in coal or sludge is volatilized at 900° C. or higher, and then sticks on the wall of a heat exchanger. In addition, fuel-nitrogen (fuel-N) is converted into NH 3 , HCN, or N 2 , via gasification, in which the conversion depends on the type of gasification reactor, fuel properties and operation conditions.  
      Since the gasification of coal is typically performed at a high temperature, it requires a lot of energy. The gas thus obtained has a low caloric value, and ashes enter a molten state. Consequently, the system is too large or is complicated. However, when a catalyst able to improve gas composition and operation conditions is used, a gas composition the same as is obtained upon high-temperature gasification in the absence of a catalyst is obtained even at a relatively low temperature, and the conversion of fuel-N is easier. According to some reports, the conversion of fuel-N into NH 3  and HCN is affected by the catalyst and reaction temperature. That is, fuel-N is readily converted into NH 3  in the presence of Fe and Ni catalysts at 900° C. or more, while HCN is readily obtained in the presence of a dolomite catalyst at 800° C. or more.  
      A gas resulting from gasification typically has a low caloric value. For example, compared to LNG having a caloric value of about 10,000 kcal/Nm 3 , a gas having a low caloric value of 1,100-1,450 kcal/Nm 3  is obtained when gasifying coal having a caloric value of 6,850 kcal/kg.  
      The low-temperature non-catalytic gasification of coal is, in practice, performed at a temperature of not less than a melting point of ash, due to a low conversion rate. However, since the gasification of biomass having lower ash content adopts catalytic gasification, fuel having high quality may be obtained while decreasing the conversion of fuel-N into NO at a low temperature at which ash slagging does not occur.  
      Further, the above process is applied to gasification of waste having a high caloric value or heavy oil, or to a fuel mixture containing coal. Particularly, waste containing chlorine ion is designed to have a retention time of 2 sec or more at 1200° C. or higher to remove it or directly burn it after gasification. In addition, in the case of polymer waste having high quality, a specific gasification process, which produces a fuel having high quality, such as hydrogen, may be proposed.  
      However, a specific gasification apparatus requires a means for removing and refining large amounts of impurities, such as ash included in a raw material, and generally, also requires a quenching system to produce ash, which is melted by increasing the temperature to achieve high gasification conversion in the absence of a catalyst, into fine slag. Further, a pure oxygen or air separating device for the production of a gas having high caloric value is used, thereby increasing the driving cost or mounting cost. For gasification of a raw material having a low caloric value, a system is installed for indirect heating particularly by an external heat source and pyrolysis through supplying only steam to the system, and is thus used for specific purposes when economy is unimportant.  
      Hence, the partial oxidation in the absence of a catalyst is disadvantageous because high-temperature air or enriched oxygen must be used to achieve high-temperature gasification. Also, additional fuel is consumed to obtain a product gas (CO and H 2 ) having high quality. Further, expensive heat resistant material suitable for high-temperature reactions should be used, and also, the reactor has a short service life. Furthermore, about 2˜5% of the free carbon that is produced by high-temperature partial combustion using a fixed-bed reactor is deposited, and the reaction efficiency is gradually decreased, thus requiring additional cost for removing the deposited material.  
      In the clean solid fuel having less tar or char, circulating low-temperature catalytic gasification, in which organic hydrocarbon and water vapor are converted into a product gas in the presence of an oxide catalyst (MO), may be conducted. Therein, the catalyst is reduced and converted into a pure metal (M). A metal (M) having decreased catalytic activity is reproduced into metal oxide (MO) in a combustor. In addition, the catalytic reaction is conducted at a low temperature of 400-600° C., and a liquid product may be produced in a very small amount. However, the above gasification is limited to use for waste containing a higher ash content or catalytic poison.  
      The circulating reforming catalyst generally includes Ni and Co, and preferably, V, Cr, Fe, Cu, Mo, Ag, Cd, La, Ce, or perovskite catalysts, and more preferably, a precious metal having high catalytic efficiency, such as Rh or Ru. In addition, such a catalyst may be affixed to a support formed of oxides of at least two metals selected from among Mg, Ca, Sr, Ba, Al, Ce, Si, Ti, and Zr. However, since these catalysts have low activities due to catalytic poison at a low temperature, they may need to undergo high-temperature reaction or reproduction to be stably used. Consequently, free carbon is deposited on the catalyst, or reacts with the support to form another product. For example, an Ni catalyst reacts at high temperature with alumina, to produce NiAl 2 O 4 , resulting in decreased catalytic activity. To avoid such a reaction, hexaaluminate (MeO.6Al 2 O 3 ), which has high temperature resistance, may be used as a supporter.  
      Moreover, liquid waste containing large amounts of solid impurities, such as heavy metals, may be gasified in a supercritical state using a reactor into which a catalyst is loaded. As such, the catalyst, such as Ru, Pd, R, Pt, Au, Ir, Os, Fe, Ni, Ce, or Mn, may be impregnated into titania or zirconia having high temperature resistance, and be driven at 250˜600° C. under 5˜130 MPa. The catalyst used is expensive precious metal, and is thus recovered using a gas-liquid separator to be reused.  
      A solid-solid catalytic reaction is difficult to perform in practice. Hence, the catalytic gasification of coal, in which an alkali catalytic component is impregnated into coal or a large amount of alkali metal in ash is used, has been initially developed. However, in recent years, as the char gasification reaction of coal was proven usually to be caused by fine volatilization on the surface of particles, attempts have been made to mix a solid catalyst with coal. In the case of sub-bituminous coal having a relatively large amount of volatile material, potassium carbonate is used as a catalyst. As such, the solid material contained in the ash affects the gasification properties. On the whole, a potassium catalyst has a large difference in activity depending on the type of anion combined therewith. In addition, the activity of an iron ion is easily decreased by sulfur, and a nickel ion having low catalytic activity due to catalytic poison restores the activity at a high temperature at which the poisonous material is desorbed.  
      Using the catalytic properties, the gasification may be rapidly conducted at a relatively low temperature of 700˜850° C. in the presence of an optimal catalytic composition, such as K 2 SO 4 +FeSO 4 , K 2 SO 4 +Ni(NO 3 ) 2 , or K 2 SO 4 +CaCO 3 . However, a complicated apparatus for melting the remaining ash is necessary due to its low conversion.  
      Of two-stage gasification methods, air gasification in a cylindrical reactor and water vapor gasification outside of the reactor are conducted at about 850° C., thus obtaining a gas product having a mean caloric value. Limestone is used as a catalyst, in consideration of sulfur poisoning in coal. As such, the reaction is as follows: 
 
H 2 S+CaO→CaS+H 2 O 
 
      Then, the solid materials, for example, CaS, CaO and limestone, may be separated from each other by use of the difference in density in the reactor. The ash and limestone may be separated from each other in the lower portion of the reactor. To this end, however, accurate driving is required due to a complicated reactor and process.  
       FIG. 1  is a view showing a conventional apparatus for two-stage gasification of biomass at a high temperature in the absence of a catalyst. In a conventional two-stage pyrolysis apparatus of biomass shown in this drawing, a biomass fuel is supplied to a circulating fluidized-bed heating furnace  102  from a fuel hopper  101 , and then sequentially passed through a cyclone  103 , a char separator  104  and a gas reforming furnace  105 , to achieve two-stage pyrolysis. Subsequently, the fuel gas is passed through a pre-heater  106  and then a gas quencher  107 , whereby fly ash is collected in a collector  108  and a gas is refined in a refiner  109 .  
      The above apparatus is disadvantageous because first-stage pyrolysis is conducted at 450˜850° C., which is not high, in the absence of a catalyst, in consideration of high-temperature volatilization of heavy metal, thus obtaining a low gasification yield and generating excess tar. Hence, a process of reforming tar should be necessary to increase the gasification yield, which is conducted at 1000˜1200° C. in the absence of a catalyst. Although exhaust flue gas desulfurization is considered for biomass containing a low sulfur content, pollution attributed to high content of phosphorous or fuel-N is not considered, causing secondary environmental pollution. In particular, the above apparatus has the gas quencher  107  for inhibiting the dioxin conversion by chlorine ions present in the raw material.  
       FIG. 2  is a view showing a conventional apparatus for two-stage catalytic gasification of waste having high quality. Since the waste having low impurities with a high caloric value has a small amount of poisonous material, as shown in  FIG. 2 , a raw material undergoes first stage partial oxidation and pyrolysis using a fluidized-bed in the absence of a catalyst at about 700˜800° C. in a fluidized-bed gasifier  110 , after which the temperature of the produced flammable gas is decreased to about 300° C. Then, slaked lime is added to fix Cl and S, which are then collected in a cyclone  103  to remove them. The temperature of the flammable gas is increased again after passing through a gas mixer  111  and a combustor  112 , and thereafter, second stage tar catalytic reforming is conducted in a gas reformer  113 . It is known that an NiO/MoO catalyst is active at 400˜500° C., and a catalyst obtained by supporting Ni, Cr and Fe to alumina is active at 800˜1000° C. The reference numeral  114  designates a boiler, and the reference numeral  115  designates a gas-holder. Also, parts common to the apparatus shown in  FIG. 1  have the same reference numerals.  
     DISCLOSURE OF THE INVENTION  
      Technical Tasks To Be Solved By The Invention  
      Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a catalytic gasification technique of poison-resistance in a first stage gasification process using a further refined fuel to increase the gasification yield at a low temperature, and then in the gasification of tar and conversion of tar-N and HCN present in a flammable gas into NH 3  in a second stage catalytic reforming process.  
      Another object of the present invention is to provide a gasification technique, using a compact apparatus without the need for a molten ash quenching system, in which the unit calories of the produced gas are increased and ash is formed not in a molten state but as fly ash, by minimizing the content of CO 2  in the gas while decreasing the energy consumption of a reaction system by decreasing the temperature of a total process.  
      Technical Solution  
      In order to accomplish the above objects, the present invention provides an apparatus for low-temperature catalytic gasification of a refined biomass fuel, comprising a fuel hopper to momentarily receive refined fuel, and including a screw feeder to quantitatively feed the fuel, provided at the lower portion thereof; a catalytic circulating fluidized-bed gasifier provided in the rear of the fuel hopper, and including a shutter connected to the screw feeder, provided at a middle portion of the gasifier, and a hot air pipe and a steam pipe, provided at a lower portion thereof; a dust collector connected to the catalytic circulating fluidized-bed gasifier via a pipe extending from the upper portion of the catalytic circulating fluidized-bed gasifier to the side wall of the upper portion of the dust collector, to collect fly ash; a catalyst reformer connected to the dust collector via a pipe extending from the upper portion of the dust collector to the lower portion of the catalyst reformer, and including a lower layer of fixed filter adsorbent bed and an upper layer of fluidized catalyst bed; a heat exchanger connected to the catalyst reformer via a pipe extending from the upper portion of the catalyst reformer to the middle portion of the heat exchanger; a tar scrubber disposed in the rear of the heat exchanger, and including a tar scrubbing chamber, a tar-storing bath, and a circulation pump for circulating tar; and a gas-holder disposed in the rear of the tar scrubber.  
      In addition, the present invention provides a method of low-temperature catalytic gasification of a refined biomass fuel, comprising a fuel supplying step of supplying a refined mixture including biomass organic waste, coal and heavy oil to the middle portion of a gasifier through screw feeder; a catalytic circulating fluidized-bed gasification step of drying, volatilizing, low-temperature catalytic gasifying, and partially burning the fuel using hot air and steam in the presence of a catalyst; a collecting step of collecting fly ash contained in the gas in the previous step; a catalyst reforming step of reforming the gas through a lower layer of filter and reforming tar-nitrogen, aromatic-nitrogen, phosphorous and sulfur through an upper layer; a heat exchanging step of cooling the gas to 200° C. or less and transferring condensed liquid to a tar-storing bath; a tar scrubbing step of condensing non-converted tar or non-condensed liquid to be recovered, and gas stripping the condensed liquid; and a gas-holding step of compressing the gas to be stored temporarily.  
      Advantageous Effects  
      According to the present invention, the gasification of a fuel which is initiated at a temperature lower than that of a single fuel material may be conducted at a temperature which is further decreased by using a catalyst. Thereby, oxygen consumption required to maintain the operation temperature is decreased, thus a desired fuel may be inexpensively produced. In addition, since the operation temperature of a gasifier is low, little heat is released and a slagging treatment system is not needed, thereby realizing a compact apparatus. In addition, a gas product obtained by using a smaller amount of air in the present invention has the same caloric value as a gas product resulting from conventional gasification using oxygen, therefore generating economic benefits.  
      The present invention pertains to clean energy producing techniques for converting a highly refined mixture comprising sludge and coal into an inexpensive gas fuel having a high caloric value.  
      The gasification of the refined mixture comprising sludge/coal/oil is initiated, along with a material having a high initiation temperature of gasification, at a temperature lower than that of a single component. Thus, gasification may be performed for a short time, hence achieving rapid gasification. A low ash content and easy control of fly ash at a low temperature make it easier to decrease the size of an apparatus, leading to saved energy and efficient operation. Moreover, since a heavy metal and a salt are present in a very small amount, a combustion post-treatment system is not needed.  
      The gasification adopts a fluidized-bed type which may be driven at a relatively low temperature. Thus, even if the gasification is conducted at 850° C. using inexpensive coarse limestone powders or particles which enable gasification at a low temperature, it may exhibit the same effects as conventional gasification at 1100° C. or more in the absence of a catalyst.  
      Conventionally, the reformation temperature of a tar reformer is in the range of 1200° C. or more, which is higher than a gasification temperature. However, in the present apparatus, the reformation temperature is decreased to 650° C. or less, and thus, the reformer of the present invention does not need an additional heat source. Before the catalytic reformation, hydrogen sulfide and phosphorous pentoxide gas acting as a catalyst poison component are fixed to be removed by the use of caustic lime, thus increasing the reformation of tar and the conversion of fuel-N into NH 3  in the presence of a catalyst.  
      Although tar, regarded as an unnecessary material, is re-circulated or wasted in a conventional process, in the present invention, unreacted tar, tar generated from the catalytic reformation, and a liquid product formed upon cooling the gas are recovered and then used for other purposes. Typically, additional devices or usage methods are required to re-introduce an undesired liquid component such as tar caused by a conventional coal gasification process into the gasification process or to use it as a liquid fuel. However, in the present invention, since such a component may be used as an agglomerating agent to form an agglomerate, it is not problematic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view showing a conventional apparatus for two-stage gasification of biomass at a high temperature in the absence of a catalyst;  
       FIG. 2  is a schematic view showing a conventional apparatus for two-stage catalytic gasification of waste having high quality;  
       FIG. 3  is a view showing an apparatus for two-stage catalytic gasification of a refined biomass fuel, according to the present invention;  
       FIG. 4  is a view showing the gasification properties of refined sewage sludge fuel in the absence of a catalyst and in the presence of a catalyst; and  
       FIG. 5  is a view showing the two-stage catalytic gasification characteristics of refined sewage sludge fuel. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.  
       FIG. 3  is a view showing a catalytic gasification apparatus for use in recovering an energy source in the form of gas from a refined biomass fuel, according to the present invention.  
      The refined biomass fuel, which is a flammable material obtained by selectively separating and recovering an organic solid component from biomass and coal, along with oil, using an oil agglomeration or floating process, has a non-flammable inorganic material (hereinafter, referred to as ‘ash’) present in an amount less than 6% based on dried material, and is a solid fuel having high quality with a caloric value of 7,000 kcal/kg or more.  
      The apparatus of the present invention comprises a fuel hopper  10  for receiving fuel, a screw feeder  11  to supply the received fuel to a subsequent device, and a catalytic circulating fluidized-bed gasifier  20  provided in the rear of the fuel hopper  10 .  
      The catalytic circulating fluidized-bed gasifier  20  includes a shutter provided at a middle portion thereof and through which the fuel is supplied from the screw feeder  11 .  
      In addition, the catalytic circulating fluidized-bed gasifier  20  has a hot air pipe  21  and a steam pipe  22  disposed in a conical lower portion thereof. As such, the hot air pipe  21  is positioned at the same level as that of the conical lower portion, and the steam pipe  22  is positioned such that its end is protruded to a height of 15˜30 cm apart from the lower portion.  
      At the upper portion of the catalytic circulating fluidized-bed gasifier  20 , a small cyclone  23  may be further provided.  
      A dust collector  30  is provided in the rear of the catalytic circulating fluidized-bed gasifier  20 , and a pipe extending from the upper portion of the catalytic circulating fluidized-bed gasifier  20  is connected to the side wall of the upper portion of the dust collector  30 . The dust collector  30  functions to collect fly ash in the gas in the lower portion thereof. In addition, a catalyst reformer  40  is provided in the rear of the dust collector  30 , and a pipe  31  extending from the upper portion of the dust collector  30  is connected to the lower portion of the catalyst reformer  40 .  
      The catalyst reformer  40  has a fixed filter adsorbent bed  41  disposed in the lower portion thereof, and a fluidized catalyst bed  42  formed on the fixed filter adsorbent bed  41 .  
      The fixed filter adsorbent bed  41 , which is a cartridge type, may include a mixture comprising an asbestos filter, particles of alkali earth metal oxide and powdery particles of alkali metal salt.  
      The pipe  31  extending from the upper portion of the dust collector  30  is provided with a valve  32  at an intermediate position thereof that communicates with a steam pipe  33 , and the pipe  31  communicates with a steam sprayer  43  disposed at the lower portion of the fixed filter adsorbent bed  41  of the catalyst reformer  40 .  
      In order to prevent liquid from condensing in the gas pipe extending from the cyclone  30  to the catalyst reformer  40 , super-heated steam is fed into the gas pipe through the steam pipe  33  so that the gas pipe discharges the super-heated steam into the catalyst reformer  40  along with the gas to cause the remaining tar to be further gasification reformed.  
      In addition, a heat exchanger  50  is provided in the rear of the catalyst reformer  40 . Also, a tar scrubber  60  is provided after the heat exchanger  50 , and includes a tar scrubbing chamber  61 , a tar-storing bath  62  disposed at the lower portion thereof, and a circulation pump  63  for circulating tar. The tar-storing bath  62  communicates with a lower pipe of each of the catalyst reformer  40  and the heat exchanger  50  via individual tar valves  64  to collect the generated tar from the catalyst  40  and the heat exchanger  50 .  
      Further, a gas-holder  70  is provided in the rear of the tar scrubber  60 , and a fuel gas-storing pump  71  is disposed between the tar scrubber  60  and the gas-holder  70 .  
      Hereinafter, a method of manufacturing clean gas using the apparatus of the present invention is described.  
      A refined fuel mixture, which is supplied through a screw feeder  11  from a fuel hopper  10 , undergoes drying, volatilization, low-temperature catalytic gasification, pyrolysis gasification and partial burning, by air or oxygen and water vapor fed via a hot air pipe  21  and a steam pipe  22  in a catalytic circulating fluidized-bed gasifier  20 . Unreacted fuel comes into contact with air or oxygen at the conical lower end of the gasifier, and thus, is completely burned.  
      The ratio of air or oxygen supplied into the catalytic circulating fluidized-bed gasifier  20  is about 0.3˜0.7 based on the amount of air required for theoretical complete combustion of the refined fuel mixture, and water vapor is fed at a volume ratio of 0.5˜10 times that of air. The fluidized catalyst for gasification in the catalytic circulating fluidized-bed gasifier  20  includes natural limestone, lime magnesite, or caustic lime, an alkali earth metal such as calcium, magnesium or barium and oxides thereof, an alkali metal such as potassium and oxides thereof, alumina, or mixtures thereof, each of which is provided in the form of particles or coarse powders suitable for fluidization. By using the catalyst, the gasification is conducted at a maximal temperature of 900° C. or less by high-speed operation, for example, for a gas retention time of 2˜4 sec.  
      Preferably, partial oxidation and low-temperature catalytic pyrolysis are simultaneously conducted at 850° C. or less. Upon partial oxidation for feeding a heat source to a system, most gasification uses oxygen at a high temperature. However, in a novel process, even if a fuel having high quality reacts with air serving as a fuel oxidant at a relatively low temperature, the gas may be produced to have the same caloric value as that of a conventional process using oxygen. As such, air is sprayed from the lowermost portion of the gasifier, whereby the lowermost portion of the gasifier has excessive oxygen for complete combustion of unreacted flammable material.  
      In addition, a small cyclone  23  is provided to the upper portion of the catalytic circulating fluidized-bed gasifier  20  to efficiently collect scattered catalyst or fuel agglomerate such as unreacted material and heavy tar, which is then re-circulated to the catalytic circulating fluidized-bed gasifier  20 , thus completing the gasification.  
      In the cyclone  30 , a small amount of fly ash may be efficiently collected and removed.  
      The catalyst reformer  40  has a two-layered structure, including a lower layer of cartridge type fixed filter adsorbent bed  41  and an upper layer of fluidized catalyst bed  42 .  
      In the fixed filter adsorbent bed  41 , fine fly ash is removed via an asbestos filter, and sulfur and phosphorous poisoning are chemically adsorbed and removed using potassium oxide and sodium carbonate as an adsorbent. Also, a detoxification filter may be recycled or replaced after being used for a predetermined period. For example, hydrogen sulfide (H 2 S) generated upon gasification reacts with CaO and is converted into CaS, which is then adsorbed. Also, a vapor compound such as PH 4 -halogen reacts with Na 2 CO 3  to produce NaPO 3 , which is then chemically adsorbed. A phosphorous component is converted into P α H β S γ Halogen δ (α=1-7, β=0-5, γ=0-7, δ=0-7) to be chemically adsorbed. Each of P x S y  compounds, resulting from the gasification, reacts with a chemical adsorbent suitable therefor or a calcium salt to be chemically adsorbed.  
      The fluidized catalyst of the fluidized catalyst bed  42  functions to decompose tar by gasification and convert aromatic nitrogen or HCN into an alkane or alkene compound and NH 3 . The usable reformation catalyst includes a single metal, such as Ni, Fe, Co, Mo, Mn, Zr, Ti, Ce, Ru, Rh or Pt, and oxides thereof, or mixtures thereof. Such a catalyst is preferably used at 650° C. or less.  
      The heat of reformed gas is exchanged by using the heat exchanger  50  to cool the gas to 200° C. or less, and the condensed liquid is transferred in the tar-storing bath  62 . As such, air or oxygen and water used in the gasification may serve as a cooling medium for heat exchange, and then be converted into hot air and steam. Moreover, the heat exchanger  50  may increase the energy efficiency if a metal heat exchanger made of a high-temperature material is used.  
      The non-converted tar or non-condensed liquid in the catalyst reformer  40  is condensed in the tar scrubber  60 , and then recovered in the tar-storing bath  62 . In this case, to increase the recovery efficiency of dust and tar, condensed liquid at 150° C. or less is re-transferred to the upper portion of the tar scrubber  60  through the tar circulating pump  63 , thus conducting gas stripping.  
      The resultant clean gas fuel is compressed and then stored temporarily in the gas-holder  70 .  
      A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.  
     Example 1  
     Improvement Of Gas Generation Efficiency BY Catalytic Gasification  
      For gasification of SOCA (Sludge-Oil-Coal Agglomerates) in the presence of a catalyst mixture comprising Fe 2 O 3  and CaO, the catalyst mixture and SOCA were uniformly mixed at a weight ratio of 3.4:1 under operation conditions similar to gasification in the absence of a catalyst, thus obtaining a gas product. The state of the product is shown in  FIG. 4 . The gasification in the presence of the catalyst mixture was initiated at 230° C., which was considerably lower than 560° C. required for gasification in the absence of a catalyst. In addition, compared to gasification in the absence of a catalyst, CO conversion was lower, and more hydrocarbons were generated. In particular, almost all hydrocarbons generated were confirmed to be methane gas. In the gasification in the absence of a catalyst, hydrocarbons were generated at 850° C. or more, and CO was generated at 1050° C. or more. However, when using a catalyst mixture, CO and hydrocarbon were mainly generated at about 500° C. Further, CO was maximally generated at 850° C. Thereby, gasification was completed in a short time. After the gasification in the presence of the catalyst mixture, unreacted char was present in an amount of about 0.35%, which was superior to the about 11.31% char remaining after conventional gasification at 1050° C. for 2 hr in the absence of a catalyst.  
     Example 2  
     Less Generation Of Tar and Fuel-N Pollutant By Two-Stage Catalytic Gasification  
      As shown in  FIG. 5 , when a CaO catalyst, which is an oxide of an alkali earth metal, was used in the first stage gasification and an NiO catalyst was used in the second stage catalytic reformation, CO was similarly generated but a slightly larger amount of hydrocarbon was generated, compared to gasification using only CaO as the first stage catalyst Thereby, the reaction was completed in a shorter time. However, as is apparent from Table 1 below, in the case where calcium oxide was used as a first stage catalyst and NiO or MnO 2  was used as a second stage catalyst, the generation of tar, NH 3  and HCN was remarkably lower than when using only a first stage catalyst. It was considered that tar was reformed and fuel-N was converted into N 2 . In addition, the MnO 2  catalyst was inferior in tar reformation to the NiO catalyst, by which fuel-N was converted not into ammonia but into HCN. Thus, the NiO catalyst exhibited superior fuel-N reformation performance as a second catalyst.  
               TABLE 1                          Conversion Of Tar and Fuel-N By Using Two-Stage Catalysts                                                         Flammable Gas Composition After           Conversion   Tar   NH 3     HCN   Gasification for 45 min (ppm)                                                         Catalyst   (%)   (%)   (%)   (%)   CO   CH 4     C 2 H 4     C 2 H 6     C 3 H 8     C 4 H 10                                                                   1 st : CaO   99.08   3.385   0.889   0.656   14,805   209   12   76   223   4,670       2 nd : x       1 st : CaO   99.90   0.123   0.278   0.045   14,429   305   4,974   6,223   4,228   367       2 nd : NiO       1 st : CaO   99.84   0.703   0.465   0.164   15,766   12,134   46   408   2,102   1,263       2 nd : MnO 2