Abstract:
The present invention provides a method for making an external circulation fluid bed furnace which does not require a chamber for retaining heat transporting medium communicating with the down comer. According to the method of the present invention, said chamber is eliminated by properly determining the dimension of the down comer etc. According to the present invention, the ratio of the diameter of the down comer to the diameter of the riser so that the ratio (X) thereof falls in an area between two lines described as follows, in a Ws-X plane: 
     
       Ws=12500X.sup.5 -12080X.sup.4 +4370X.sup.3 -600X.sup.2 +36X 
     
     and 
     
       Ws=5800X.sup.4 +1600X.sup.3 -580X.sup.2 +44X 
     
     wherein, Ws is flow rate of solid heat transport medium in the riser at each unit area the cross section (kg/m 2  sec); X is a ratio of the diameter of down comer (d) to the diameter of riser (D).

Description:
This is a continuation of Ser. No. 07/966,938 filed Oct. 27, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for making a fluid bed furnace having an external circulating system for use in various facilities used for purposes such as incinerating or burning, drying or vaporization, or heat decomposition. 
     2. Prior Art 
     A fluid bed furnace having an external circulation system (called as &#34;a fluid bed type furnace&#34; hereafter) comprises a riser or a heating vessel of a cylindrical shape in which a fluid bed is formed by installing solid particles therein as heat transporting material, for heating material thereby for the purpose of burning, drying, vaporizing or decompositing the material. The heat transporting material are drawn out from an outlet pipe equipped at the top of the riser, and sent to a cyclone separator equipped at the end of the outlet pipe to be returned the lower portion of the riser by way of a downcomer. In this type of furnace, a smooth operation can be done such as heating of materials, reaction between materials, or drafting of the products by circulating the heat transporting material as described above. 
     However, the fluid bed furnace has a common problem to be solved in general in obtaining a smooth circulation of the heat transporting material as well as in controlling the quantity thereof. It is essential to obtain a smooth circulation of the heat transporting material for realization of the quantity control thereof. This realization of smooth circulation of solid particles mostly depends on the design of the downcomer. Namely, it is important to correctly select the height level of an end connection which opens at the lower part of a riser, and then it is important to determine the size of the downcomer in relation to the selected height level. 
     In the conventional technology, such determination of height level of the end connection and determination of the size of the downcomer were performed under totally different ideas from each other. That is, the former is determined from the total amount of the heat transporting material installed in the furnace, and the latter is determined from the calculated amount of heat transporting material circulating in the furnace. Whether the height level of an end connection is proper or not is judged from density of heat transporting material at a determined location when stirring the heat transporting material in the riser. However, since the density of heat transporting material at the location alters according to the parameters such as grain size or specific gravity of the heat transporting material, or velocity of the gas in the riser, it is impossible to evaluate the height level univocally according to the density of heat transporting material. That is, the pressure or quantity of pressure drop at the present location can be used for the justification of the height level of an end connection. 
     On the other hand, the judgment of the size of the downcomer is done according to the velocity of the heat transporting material in the downcomer as well as above mentioned calculated amount of the heat transporting material in circulation. Since these values also alter according to the parameters such as grain size or specific gravity of the heat transporting material, the optimization of the operation includes much difficulty. 
     In the conventional technology, in order to avoid such intricacy, a pooling device is provided at the middle of the downcomer for pooling the heat transporting material therein, to which a means for blowing compressed gas into the pooling device for sending the heat transporting material into the riser. By this method, it is necessary to put gas energy at an exalted state since the gas is blown into the furnace to raise the velocity of the heat transporting material which at first is zero or very small. It is also necessary by this method to distend the size of the downcomer since the total volume of flow increases due to the gas blow. 
     As described above, in the conventional method of making the fluid bed type furnace, there are some difficulties as follows. In determining the height of the connecting end from the total amount of the heat transporting material, or in determining the size of the downcomer from the calculated amount of heat transporting material in circulation, it is difficult to obtain correct values since these values cannot be determined univacally as described above. Thus, in the furnace designed after the conventional process, many problems will occur relating to the downcomer, such as blocking of the heat transporting material when the size of the downcomer is small, or decrease of the heat transporting material in circulation due to the generation of gas flow in a direction from the lower part of the riser to a cyclone separator by way of the downcomer (called as a &#34;reverse gas flow&#34; hereafter) when the size of the downcomer is large. 
     Otherwise, in the method of equipping a pooling device at the middle of the downcomer, not only is the size of the downcomer necessarily large due to increase of volume of flow occurring from the gas blow, but also a large energy is necessary for returning the heat transporting material by bringing them at high speed from stationary state or a state of very low speed. 
     SUMMARY OF THE INVENTION 
     The present invention was made in view of the above background, and is aimed at presenting a method for making a fluid bed type furnace having an external circulation system, in which generation of blocking of the heat transporting material or reverse gas flow can be prevented by properly selecting the size of the downcomer or so in accordance with required amount of heat transporting material in circulation or characteristics of material charged to the furnace. 
     The present invention is to accomplish the objectives mentioned above, and thus presents a method for making a fluid bed type furnace comprising a riser for containing a mixture of primary air and solid heat transport medium, a cyclone for separating said solid heat transport medium from gas, and a down comer disposed outside of the riser for returning said solid heat transport medium from said cyclone to said riser, the method comprising the step of determining the diameter of the down comer and the diameter of the riser so that the ratio (X) thereof falls in an area between two lines described as follows, in a Ws-X plane: 
     
         Ws=12500X.sup.5 -12080X.sup.4 +4370X.sup.3 -600X.sup.2 +36X 
    
     and 
     
         Ws=5800X.sup.4 +1600X.sup.3 -580X.sup.2 +44X 
    
     wherein, Ws is flow rate of solid heat transport medium in the riser at each unit area the cross section (kg/m 2  sec); X is a ratio of the diameter of down comer (d) to the diameter of riser (D). 
     According to another aspect of the present invention there is also provided a method for making an external circulation fluid bed furnace according to, wherein the fluid bed furnace further comprises a supplemental air supply means for blowing hot air in one of said cyclone and said down comer, the method-further comprising the step of determining the diameter of the down comer so that ratio (r) of the diameter of down comer (da) and the diameter of down comer without supplemental air supply means (dO) falls in an area between first and second lines described as follows, in a r-F plane: 
     First line being defined as: 
     
         ______________________________________r = -2.8F + 1       for 0 ≦ F &lt; 0.1r = -0.7F + 0.79    for 0.1 ≦ F______________________________________ 
    
     Second line being defined as: 
     
         ______________________________________r = -3F + 0.877      0 ≦ F &lt; 0.02r = -0.27F + 0.871   0.02 ≦ F &lt; 0.1r = -33F + 0.663     0.1 ≦ F______________________________________ 
    
     wherein r=da/d0; F=Fa/Ft; Fa is the volume of supplemental air, and Ft is the volume of total air. 
     Another aspect of the present invention provides a method for making an external circulation fluid bed furnace according to, wherein said down comer has an aperture opening to a lower part of said riser for returning said solid heat transporting medium, and the method comprises a step of determining the height (H) of said aperture measured from a standard level, and a ratio (L) of said height (H) to a radius (D/2) of said down comer so that the height (H) and ratio fall within the area between first and second lines as defined as follows in a y-L plane: 
     First line: 
     
         ______________________________________y = -0.5L + 0.75  for -1.3 ≦ L &lt; -0.715y = -0.15L + 1.0  for -0.715 ≦ L &lt; 0y = -0.3L + 1.0   for 0 ≦ L &lt; 0.5y = -0.05L + 0.875             for 0.5 ≦ L &lt; 1.5______________________________________ 
    
     Second line: 
     
         ______________________________________y = -0.4L + 1.0    for -1.3 ≦ L &lt; 0y = -0.05L + 1.0   for 0 ≦ L ≦ 1.5______________________________________ 
    
     wherein L=H/D/2=2H/D 
     Still further aspect of the invention provides a method for making an external circulation fluid bed furnace according to, wherein at least one of fuel and combustible material supplied to said furnace contains alkali element, and the method comprises a step for determining the diameter of the down comer so that the diameter falls within the area between first line and second line defined as follows in an A-X plane: 
     First line: 
     
         A=714X.sup.3 -2256X.sup.2 +2424X-882 
    
     Second line: 
     
         A=169.7X.sup.2 -346.8X+1777.1 
    
     wherein X is a deviation of the diameter of down comer from that of standard diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic drawing showing an embodiment of the present invention. 
     FIG. 2 is a cross-sectional drawing of the riser. 
     FIG. 3 is a graph showing relationship between a ratio x of diameter d of the downcomer to the diameter D of the riser and the amount of solid particles in circulation Ws. 
     FIG. 4 is a graph showing relationship between a ratio r of diameter da of the downcomer when subsidiary air is supplied, to diameter do of the downcomer when no subsidiary air is supplied, and a ratio F of the amount of the subsidiary air to the total amount of supplied air. 
     FIG. 5 is a graph showing relationship between ratio of height of the end connection of the downcomer from a standard level to radius of riser and deviation y of diameter of the downcomer. 
     FIG. 6 is a graph showing relationship between deviation x of the diameter of the downcomer from its standard value and density A of alkari salt produced through reaction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to FIGS. 1 through 6 and Table 1 through 4 in the following sections. 
     FIGS. 1 and 2 show an embodiment wherein the furnace according to the present invention is utilized to burn industrial liquid wastes and sludges. 
     As shown in FIGS. 1 and 2, the furnace is provided with a riser 1, a cyclone 5, an outlet conduit 2 communicating the riser to the cyclone, a down comer 4 provided underneath the cyclone. The riser 1 defines a cylindrical internal space for combustion reaction and having an inner lining made of a heat resistant material. The riser is provided with first and second primary air inlet conduits 11, 12 at a lower part thereof and at different heights. The down comer 4 has a vertical upper portion and an inclined lower portion which is connected to the lower portion of the internal space of the riser through an aperture 3a. The aperture 3a is located at a higher location compared to the secondary air inlet conduit 11 and their vertical distance is denoted by H. 
     Two supplementary air inlet nozzles 6, 8 are provided to cyclone 5 and outlet conduit 2 at locations closer to riser 1 and supply supplemental air to cyclone 5 and outlet conduit 2 through flow control valves 7, 9. First and second primary air inlet conduits 11, 12 supply primary and secondary air to the riser through flow control valves 13, 14. 
     Riser 1 is provided with a combustible material inlet port 15 and an outlet valve 18 at a middle portion and a bottom portion thereof, respectively. Cyclone 5 is provided with an outlet line 16 and an outlet port 16. 
     Heat transporting medium is provided in the riser when the furnace is operated and air is blown from the air inlet conduits 11, 12 for combustion and fluidization of the heat transporting medium. The combustible medium is supplied to the furnace through combustible medium inlet port 15. The total amount of the air supplied to the riser, which is a sum of the air supplied through primary air inlet conduits 11, 12 and secondary air inlet conduits 6, 8, is determined so that the oxygen enough to react all the combustible material is burned. When the air is not supplied through secondary air conduits 6, 8, the flow rate of the lower primary air inlet 12 remains constant while that of the upper primary air inlet 11 varies. The total air flow rate approximately determines the rate of flow of the heat transporting medium in the riser per unit time and per unit cross sectional area. 
     The heat transporting medium, which is a form of small particles, is first mixed with the primary air supplied from the primary air inlet conduits, blown in a form of a gas-solid particle mixture. The combustible material is also mixed with the air and the heat transporting material, and receives heat from therefrom consequently losing humidity contained in it. The combustible material is crushed into small pieces by the numerous collisions with the heat transporting material. The combustible material crushed into small pieces are blown upward with the heat transport medium by the circulating air as burning. 
     The combustible material blow upward with the heat transporting medium is accelerated by the secondary air blow through secondary air inlet conduit 11 and continues burning. The combustible material completes burning approximately when it reaches the top portion of the riser, and the leavings of the combustion, that is mainly ash, is lead to cyclone 5 through outlet conduit 2 with the heat transporting medium. 
     The heat transporting medium lead to the cyclone is separated from the gas mixture by the centrifugal force in the cyclone, and exited from a lower part of the cyclone to enter the down comer. The air and the combustion leavings separated from the heat transporting medium is lead to outlet conduit 16 so as to be laid outside the system described above. 
     In the case wherein either the combustible material or the fuel for combustion contains a compound containing potassium or sodium, a salt of these elements are formed while the combustible material is burned in the riser. Most portion of the salt formed as described above is in a form of small particles or formed on the surface of the heat transporting material. However, some portion of the salt further reacts with the heat transporting medium to produce reaction products. Such reaction products have lower melting temperatures, and tend to melt and stick to the internal surface of the system, such as inside the down comer and obstacles a smooth flow of the medium in the system. The flow condition in the system is affected seriously when such reaction products stick to the inner surface and hinders the flow depending on the dimensions of down comer 4 and inlet conduits 3. 
     It has been found by experiments that is supplemental air is provided by supply air conduits 6, 8 provided with flow control valve 7, 9, the hindrance due to the sticking reaction products is substantially reduced especially for small down comer sizes. When the diameter of the down comer is relatively small, the effective diameter of the down comer tends to be reduced due to the sticking reaction products. The flow rate in the down comer is consequently reduced and the separation ability of the cyclone is degraded, and the amount of the heat transport medium effective operating in the system is reduced. The operation of the system thus becomes unstable and difficult. 
     When the diameter of down comer 4 is larger than an appropriate size, an upward air flow occurs in the down comer from the aperture of the down comer opening to the lower part of riser 1 toward cyclone 5. Due to the upward air flow, a portion of the heat transporting medium is blown upward and exited to the outside of the system through cyclone 5 and outlet conduit 16. This upward flow can also be reduced or stopped by providing a downward stream by supplementary air through supplementary air conduits 6, 8. 
     A number of cold experiments have been performed without heating the medium in order to evaluate the effects of the parameters except for the effects of alkali salts. Hot experiments with heating the mediums have also been performed in order to evaluate the effects of the alkali elements contained in the combustible materials. The results of the experiments are described as follows in Tables 1 through 4. 
     In the experiments, diameters of the riser was selected from 0.3 m and 0.32 m, diameters of the down comer was selected from various values between 0.033 m and 0.1 m, total air supply was varied between 480 and 1720 Nm 3  /h, and supplementary air which was included in the total air supply was varied between 0 and 350 Nm 3  /h. Height of inlet conduit 3 measured from the center of secondary air inlet conduit 11 to the center of the inlet conduit 3 was varied from 0.15 to 0.525 m. 
     The operation of the furnace is evaluated according to the existence of congestion in down comer 4 or inlet conduit 3. 
     Effects of the variables on the congestion obtained from the experiments are described in FIGS. 3 to 6. 
     Table 1 shows the effects of the diameter d of down comer and diameter D of the riser on the upward stream in the down comer. The flow was measured in terms of the amount of solid particles flowing upwards in the down comer. In the comparison, diameter D of the riser and the height H of inlet conduit 3 measured from secondary air inlet conduit 11 are maintained unchanged, and total air circulation Ws and diameter d of down comer 4 were varied without supplying supplemental air. FIG. 3 shows the conditions wherein no congestion occurred in the down comer. FIG. 3 shows that by properly choosing the variable, it is possible to create a regular circulation of the air and heat transporting medium, that is, unidirectional flow without reversal flow in the down comer. The conditions is that the ratio of the diameter d of the down comer to the diameter D of the riser (X) falls in an area between two lines described as follows, in a Ws-X plane: 
     
         Ws=12500X.sup.5 -12080X.sup.4 +4370X.sup.3 -600X.sup.2 +36X 
    
     and 
     
         Ws=5800X.sup.4 +1600X.sup.3 -580X.sup.2 +44X 
    
     wherein, Ws is flow rate of solid heat transport medium in the riser at each unit area the cross section (kg/m 2  sec). The parameter Ws was between 0 and 50. 
     When X is larger than the above area, an upward stream occurs in the down comer. When X is smaller than the above area, a congestion occurs in the down comer. 
     It becomes possible to determine the diameter of the 
     
                                           TABLE 1__________________________________________________________________________            weight                 height    dia-     supple            of   ofdia-    meter    total        mental            circu-                 down-meter    of   air air lating                 comerof  down-    volume        volume            media                 aper-                     blownriser    comer    (Ft)        (Fa)            (Ws) ture                     amount                         conges   F =            r =(D)m    (d)m Nm.sup.3 /h        Nm.sup.3 /h            kg/m.sup.2 · s                 (H) (Wc)                         tion                             X = d/D                                  Fa/Ft                                      L = 2H/D                                            y = d/do                                                 da/do                                                     x__________________________________________________________________________                                                     = d/do0.3 0.04 1200        0   4.5  0.3 1.3 X   0.133                                  0   2     --   --  --0.3 0.04 1300        0   5.2  0.3 1.5 X   0.133                                  0   2     --   --  --0.3 0.04 1350        0   5.4  0.3 --  ◯                             0.133                                  0   2     --   --  --0.3 0.05 1700        0   19.1 0.3 --  ◯                             0.167                                  0   2     --   --  --0.3 0.06 1750        0   20.0 0.3 --  ◯                             0.2  0   2     --   --  --0.3 0.06 1700        0   19.2 0.3 1.6 X   0.2  0   2     --   --  --0.3 0.07 1000        0   3.2  0.3 6.3 X   0.233                                  0   2     --   --  --0.3 0.07 1100        0   3.5  0.3 1.2 X   0.233                                  0   2     --   --  --0.3 0.07 1200        0   36   0.3 1.7 X   0.233                                  0   2     --   --  --0.3 0.07 2200        0   38   0.3 --  ◯                             0.233                                  0   2     --   --  --0.3 0.1  1500        0   9.4  0.3 8.9 X   0.333                                  0   2     --   --  --0.3 0.1  1560        0   9.9  0.3 1.1 X   0.333                                  0   2     --   --  --0.3 0.1  1700        0   19.8 0.3 1.6 X   0.333                                  0   2     --   --  --0.3 0.1  2200        0   49.0 0.3 1.7 X   0.333                                  0   2     --   --  --0.3 0.125    1750        0   18.8 0.3 5.7 X   0.417                                  0   2     --   --  --0.3 0.125    1850        0   20.6 0.3 1.8 X   0.417                                  0   2     --   --  --0.3 0.125    1950        0   26   0.3 1.5 X   0.417                                  0   2     --   --  --0.3 0.125    2200        0   46   0.3 1.9 X   0.417                                  0   2     --   --  --0.3 0.14 2000        0   35   0.3 4.9 X   0.467                                  0   2     --   --  --0.3 0.14 2100        0   41   0.3 1.0 X   0.467                                  0   2     --   --  --0.3 0.14 2300        0   50   0.3 1.3 X   0.467                                  0   2     --   --  --0.3 0.15 2000        0   41.5 0.3 8.9 X   0.5  0   2     --   --  --0.3 0.15 2300        0   50   0.3 7.0 X   0.5  0   2     --   --  --__________________________________________________________________________ 
    
     down comer according to the conditions described in FIG. 3 as follows. First, the total amount of air circulation is determined for burning total amount of combustible material in the furnace. Then the rate of circulation of the heat transporting medium is determined. Then, according to FIG. 3, a variable X is determined, and the diameter d of the down comer is determined by using the diameter D of the riser and variable X. 
     Table 2 shows the data obtained from experiments wherein the diameter d of the down comer was varied around 0.06 m while maintaining the diameter D of the riser and total air circulation Ws constant. Supplemental air flow was supplied through supplemental air conduits 6, 8, and the proportion F of the supplemental air Fa to the total air flow Ft was varied. FIG. 4 shows the effects of the diameter of the down comer in terms of the deviation from its mean value, that is, 0.06 m. 
     The condition wherein no congestion occurs is when the ratio r of the diameter da of down comer and the diameter of down comer do without supplemental air supply means falls in an area between first and second lines described as follows, in a r-F plane: 
     First line being defined as: 
     
         ______________________________________r = -2.8F + 1       for 0 ≦ F &lt; 0.1r = -0.7F + 0.79    for 0.1 ≦ F______________________________________ 
    
     Second line being defined as: 
     
         ______________________________________r = -3F + 0.877      0 ≦ F &lt; 0.02______________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________            weight                 height    dia-     supple            of   ofdia-    meter    total        mental            circu-                 down-meter    of   air air lating                 comerof  down-    volume        volume            media                 aper-                     blownriser    comer    (Ft)        (Fa)            (Ws) ture                     amount                         conges   F =            r =(D)m    (d)m Nm.sup.3 /h        Nm.sup.3 /h            kg/m.sup.2 · s                 (H) (Wc)                         tion                             X = d/D                                  Fa/Ft                                      L = 2H/D                                            y = d/do                                                 da/do                                                     x__________________________________________________________________________                                                     = d/do0.3 0.06 1700         0  19.2 0.3 1.6 X   0.2  0   2     --   --  --0.3 0.058    1700         20 19.1 0.3 5.4 X   0.193                                  0.012                                      2     --   0.967                                                     --0.3 0.057    1700         20 19.4 0.3 1.2 X   0.190                                  0.012                                      2     --   0.95                                                     --0.3 0.052    1700         20 19.2 0.3 1.4 X   0.173                                  0.012                                      2     --   0.867                                                     --0.3 0.050    1700         20 19.5 0.3 --  ◯                             0.167                                  0.012                                      2     --   0.833                                                     --0.3 0.050    1705         35 19.4 0.3 1.6 X   0.167                                  0.021                                      2     --   0.833                                                     --0.3 0.048    1705         35 19.6 0.3 --  ◯                             0.160                                  0.021                                      2     --   0.800                                                     --0.3 0.051    1700         80 19.2 0.3 6.3 X   0.170                                  0.047                                      2     --   0.850                                                     --0.3 0.050    1700         80 19.0 0.3 1.1 X   0.167                                  0.047                                      2     --   0.833                                                     --0.3 0.042    1700        150 18.7 0.3 1.4 X   0.140                                  0.088                                      2     --   0.700                                                     --0.3 0.044    1720        170 19.8 0.3 5.1 X   0.147                                  0.099                                      2     --   0.733                                                     --0.3 0.043    1720        170 18.7 0.3 1.3 X   0.143                                  0.102                                      2     --   0.717                                                     --0.3 0.036    1675        175 18.9 0.3 1.5 X   0.120                                  0.104                                      2     --   0.600                                                     --0.3 0.035    1675        175 19.4 0.3 --  ◯                             0.117                                  0.104                                      2     --   0.583                                                     --0.3 0.042    1700        250 19.4 0.3 8.1 X   0.14 0.147                                      --    --   0.700                                                     --0.3 0.041    1700        250 20.1 0.3 1.9 X   0.137                                  0.147                                      --    --   0.683                                                     --0.3 0.038    1750        250 19.9 0.3 1.2 X   0.127                                  0.143                                      --    --   0.633                                                     --0.3 0.035    1700        250 19.1 0.3 1.1 X   0.117                                  0.147                                      --    --   0.583                                                     --0.3 0.034    1700        250 18.8 0.3 --  ◯                             0.113                                  0.147                                      --    --   0.567                                                     --0.3 0.040    1700        350 19.3 0.3 6.0 X   0.133                                  0.206                                      --    --   0.667                                                     --0.3 0.039    1700        350 18.9 0.3 1.3 X   0.130                                  0.206                                      --    --   0.65                                                     --0.3 0.034    1700        350 19.2 0.3 1.7 X   0.113                                  0.206                                      --    --   0.567                                                     --0.3 0.033    1700        350 18.6 0.3 --  ◯                             0.110                                  0.206                                      --    --   0.550                                                     --__________________________________________________________________________ 
    
     
         ______________________________________r = -0.27F + 0.871   0.02 ≦ F &lt; 0.1r = -33F + 0.663     0.1 ≦ F______________________________________ 
    
     wherein r=da/d0; F=Fa/Ft; Fa is the volume of supplemental air, and Ft is the volume of total air. 
     The figure tells that when the supplemental air supply is high, the diameter of the down comer can be small. FIG. 4 provides information how the diameter of the down comer must be modified by taking into account the supplemental air flow. 
     Table 3 shows the effects of the height 3a of the aperture through which the down comer is connected to the riser. The parameters H and d as described above were varied in order to obtain this information. 
     FIG. 5 shows the relationship between the diameter of the down comer and the height of the aperture 3a. According to FIG. 5, it is understood that the down comer must have a large diameter so that the heat transporting medium is returned to the lower part of the riser where the air-solid mixture has a relatively high density. The relationship is when the ratio (L) of said height (H) to a radius (D/2) of said down comer falls within the area between first and second lines as defined as follows in a y-L plane (wherein L=H/D/2=2H/D): 
     First line: 
     
         ______________________________________y = -0.5L + 0.75  for -1.3 ≦ L &lt; -0.715y = -0.15L + 1.0  for -0.715 ≦ L &lt; 0y = -0.3L + 1.0   for 0 ≦ L &lt; 0.5y = -0.05L + 0.875             for 0.5 ≦ L ≦ 1.5______________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________            weight                 height    dia-     supple            of   ofdia-    meter    total        mental            circu-                 down-meter    of   air air lating                 comerof  down-    volume        volume            media                 aper-                     blownriser    comer    (Ft)        (Fa)            (Ws) ture                     amount                         conges   F =            r =(D)m    (d)m Nm.sup.3 /h        Nm.sup.3 /h            kg/m.sup.2 · s                 (H) (Wc)                         tion                             X = d/D                                  Fa/Ft                                      L = 2H/D                                            y = d/do                                                 da/do                                                     x__________________________________________________________________________                                                     = d/do0.3 0.085    1700        0   17.9 0.15                     9.3 X   0.283                                  0   1.0   1.417                                                 --  --0.3 0.083    1700        0   18.6 0.15                     1.9 X   0.280                                  0   1.0   1.400                                                 --  --0.3 0.077    1700        0   18.8 0.15                     1.7 X   0.257                                  0   1.0   1.283                                                 --  --0.3 0.075    1700        0   18.1 0.15                     --  ◯                             0.250                                  0   1.0   1.250                                                 --  --0.3 0.073    1700        0   18.9 0.225                     7.8 X   0.243                                  0   1.5   1.217                                                 --  --0.3 0.072    1700        0   19.3 0.225                     2.1 X   0.240                                  0   1.5   1.200                                                 --  --0.3 0.064    1700        0   19.6 0.225                     1.8 X   0.213                                  0   1.5   1.067                                                 --  --0.3 0.063    1700        0   19.1 0.225                     --  ◯                             0.210                                  0   1.5   1.050                                                 --  --0.3 0.060    1700        0   19.2 0.3 1.6 X   0.2  0   2.0   1.000                                                 --  --0.3 0.058    1700        0   19.6 0.375                     12.6                         X   0.193                                  0   2.5   0.967                                                 --  --0.3 0.057    1700        0   20.5 0.375                     2.3 X   0.190                                  0   2.5   0.950                                                 --  --0.3 0.053    1700        0   20.0 0.375                     1.5 X   0.177                                  0   2.5   0.883                                                 --  --0.3 0.051    1700        0   20.3 0.375                     --  ◯                             0.170                                  0   2.5   0.850                                                 --  --0.3 0.058    1700        0   19.1 0.45                     7.3 X   0.193                                  0   3.0   0.967                                                 --  --0.3 0.057    1700        0   18.6 0.45                     1.8 X   0.190                                  0   3.0   0.950                                                 --  --0.3 0.053    1700        0   19.4 0.45                     1.4 X   0.177                                  0   3.0   0.883                                                 --  --0.3 0.051    1700        0   17.9 0.45                     --  ◯                             0.170                                  0   3.0   0.850                                                 --  --0.3 0.056    1700        0   18.6 0.525                     9.2 X   0.187                                  0   3.5   0.933                                                 --  --0.3 0.054    1700        0   19.5 0.525                     1.3 X   0.180                                  0   3.5   0.900                                                 --  --0.3 0.050    1700        0   18.1 0.525                     1.6 X   0.167                                  0   3.5   0.833                                                 --  --0.3 0.048    1700        0   18.6 0.525                     --  ◯                             0.160                                  0   3.5   0.800                                                 --  --__________________________________________________________________________ 
    
     
         ______________________________________y = -0.4L + 1.0    for -1.3 ≦ L &lt; 0y = -0.05L + 1.0   for 0 ≦ L ≦ 1.5______________________________________ 
    
     The Figure provides a method for adjusting the diameter of the down comer for the deviations of H from its standard value that is 0.3 m. 
     Table 4 shows the results of the experiments which was performed at 800° C. for a combustible material containing alkali elements. 
     FIG. 6 shows the relationship between the concentration of alkali salts in the combustible material and the diameter of the down comer. The optimal conditions are when the diameter falls within the area between first line and second line defined as follows in an A-X plane: 
     First line: 
     
         A=714X.sup.3 -2256X.sup.2 +2424X-882 
    
     Second line: 
     
         A=169.7X.sup.2 -346.8X+177.1 
    
     wherein X is a deviation of the diameter of down comer from that of standard diameter. Parameter A is between 0 and 20. The parameter A is the weight ratio of Na 2  CO 3  and K 2  CO 3  derived from Na and K contained in the combustible material to the total dry weight of the combustible material. The relationship is described as follows: 
     A={(total weight of Na 2  CO 3  derived from Na content 
     
                                           TABLE 4__________________________________________________________________________            weight                 height                     density    dia-     supple            of   of  ofdia-    meter    total        mental            circu-                 down-                     alkarimeter    of   air air lating                 comer                     saltof  down-    volume        volume            media                 aper-                     in ma-                         blownriser    comer    (Ft)        (Fa)            (Ws) ture                     terial                         amount                             conges                                 X =                                    F = L =      r =(D)m    (d)m Nm.sup.3 /h        Nm.sup.3 /h            kg/m.sup.2 · s                 (H) (A) (Wc)                             tion                                 d/D                                    Fa/Ft                                        2H/D                                            y = d/do                                                 da/do                                                     x__________________________________________________________________________                                                     = d/do0.32    0.062    480 0   5.5  0.3 0   1.4 X   0.194                                    0   2.0 --   --  1.0000.32    0.062    480 0   5.2  0.3 3.6 --  ◯                                 0.194                                    0   2.0 --   --  1.0000.32    0.065    480 0   5.6  0.3 3.6 --  ◯                                 0.203                                    0   2.0 --   --  1.0480.32    0.067    480 0   5.1  0.3 3.6 1.7 X   0.209                                    0   2.0 --   --  1.0810.32    0.072    480 0   5.5  0.3 3.6 1.4 X   0.225                                    0   2.0 --   --  1.1610.32    0.074    480 0   5.6  0.3 3.6 7.1 X   0.231                                    0   2.0 --   --  1.1940.32    0.070    480 0   5.4  0.3 7.0 --  ◯                                 0.219                                    0   2.0 --   --  1.1290.32    0.074    480 0   5.3  0.3 7.0 1.1 X   0.231                                    0   2.0 --   --  1.1940.32    0.076    480 0   5.1  0.3 7.0 1.4 X   0.238                                    0   2.0 --   --  1.2260.32    0.078    480 0   4.8  0.3 7.0 7.9 X   0.244                                    0   2.0 --   --  1.2580.32    0.076    480 0   5.2  0.3 12.0                         --  ◯                                 0.238                                    0   2.0 --   --  1.2260.32    0.078    480 0   5.6  0.3 12.0                         1.6 X   0.244                                    0   2.0 --   --  1.2580.32    0.080    480 0   4.6  0.3 12.0                         1.2 X   0.250                                    0   2.0 --   --  1.2900.32    0.082    480 0   6.0  0.3 12.0                         9.9 X   0.256                                    0   2.0 --   --  1.3230.32    0.076    480 0   5.6  0.3 17.0                         --  ◯                                 0.238                                    0   2.0 --   --  1.2260.32    0.078    480 0   4.8  0.3 17.0                         1.3 X   0.244                                    0   2.0 --   --  1.2580.32    0.080    480 0   5.2  0.3 17.0                         1.0 X   0.250                                    0   2.0 --   --  1.2900.32    0.082    480 0   5.6  0.3 17.0                         1.7 X   0.256                                    0   2.0 --   --  1.3230.32    0.083    480 0   5.5  0.3 17.0                         14.5                             X   0.259                                    0   2.0 --   --  1.339__________________________________________________________________________ supposing that all the Na content in the combustible material reacted to make it)+(total weight of K.sub.2 CO.sub.3 derived from K content supposing that all the K content in the combustible material reacted to make it)}/(dry weight of the combustible material) 
    
     For example, if the combustible material contains 50% by weight of water which contains 2 wt % percent Na and 0.5 wt % of K, the variable A is calculated as follows: 
     
         ______________________________________A = 0.02 × (molecular weight of Na.sub.2 CO.sub.3)/2(molecularweight of Na) + 0.005 × (molecular weight ofK.sub.2 CO.sub.3)/2(molecular weight of K) =0.02 × 2.304 + 0.005 × 1.767 =0.055______________________________________ 
    
     The above relationship provides the method determining the diameter of down comer when the concentration A of the alkali salt is provided.