JP7628671B2 - Method for controlling and inhibiting silica crystallization, and combustion device - Google Patents

Description

The present invention relates to a method for controlling and inhibiting silica crystallization, and a combustion device.

Biomass fuels such as rice husks are difficult to decompose and tend to become industrial waste. However, by burning biomass fuels, it is possible to extract both energy and silica, which can be used as an industrial raw material.

Pure silica does not crystallize unless it is at a temperature much higher than the combustion temperature of a normal boiler (cristobalite: 867°C to 1470°C, tridymite: 1470°C to 1727°C), but biomass fuel contains inorganic elements such as K and Na in addition to silica, and due to the eutectic point effect of these elements, the silica in rice husks crystallizes even at a relatively low temperature of 600°C to 900°C. Therefore, when biomass fuel is burned, energy can be obtained, but there is a risk that the silica contained in the biomass fuel will crystallize during the combustion process. Specifically, the silica (SiO ₂ ·nH ₂ O) contained in biomass fuel is amorphous, but when burned, it produces crystalline silica such as cristobalite and quartz. This crystalline silica is recognized as a substance that is as harmful to the human body as asbestos.

For this reason, a technology has been disclosed that, in order to suppress the eutectic point effect, involves washing inorganic elements other than silica in rice husks with an acidic aqueous solution, thereby shifting the crystallization temperature to a higher temperature (see, for example, Patent Document 1).

Patent No. 6618131

However, this technology is difficult to implement because it requires the addition of chemicals such as an acid solution, energy for drying, and the necessary equipment. Therefore, no technology has yet been developed that can easily control or suppress the crystallization of silica during combustion treatment.

The present invention has been made to solve the above problems. That is, the object is to provide a method for controlling silica crystallization, which can control silica crystallization during combustion treatment in a simple manner, and a combustion device. The object is also to provide a method for suppressing silica crystallization, which can suppress silica crystallization during combustion treatment in a simple manner.

[1] A method for controlling the crystallization of amorphous silica during the combustion process of biomass fuel containing the silica, comprising the steps of: determining an amorphous region in which the silica is in an amorphous state and a crystalline region in which the silica is in a crystalline state based on a combustion temperature of 600°C or higher and 1150°C or lower and a holding time at the combustion temperature; and controlling the combustion temperature of the biomass fuel, the total elapsed time from the start of the combustion of the biomass fuel, and the cooling rate of the ash generated by the combustion of the biomass fuel based on the amorphous region and the crystalline region.

[2] The determination of the amorphous region and the crystalline region includes the following steps: Step 1: Burn the biomass fuel at an arbitrary first temperature within a combustion temperature range of 600° C. to 1150° C., and hold the ash generated by the combustion of the biomass fuel at the first temperature for t ₁ minute, and then confirm whether the silica contained in the ash is in an amorphous state; Step 2: If it is confirmed that the silica is in an amorphous state as a result of Step 1, burn the biomass fuel again at the first temperature, and confirm again in a state in which the holding time of the ash generated by the combustion of the biomass fuel at the first temperature is increased by t ₂ minutes from the previous holding time, and this step is repeated until a holding time T _C1 (min) at which the silica is in a crystalline state is found, and a holding time T _C1 is represented by T _C1 -t ₂ , and a holding time T _A1 at which the silica is in an amorphous state is found. (min) at which the silica is in a crystalline state; if it is confirmed as a result of step 1, combust the biomass fuel again at the first temperature, and confirm again after reducing the holding time of the ash generated by the combustion of the biomass fuel at the first temperature by _t3 minutes from the previous holding time; repeat this step until a holding time T _A1 (min) at which the silica is in an amorphous state is found; find a holding time T _A1 and a holding time T _C1 (min) at which the silica is in a crystalline state, which is represented by T _A1 + _t3 ; and step 3: combust the biomass fuel at a second temperature lower than the first temperature within a combustion temperature range of 600° C. to 1150° C., and combust the ash generated by the combustion of the biomass fuel at the second temperature by t After holding the mixture for ₄ minutes, it is confirmed whether the silica contained in the ash is in an amorphous state. Step 4: If it is confirmed that the silica is in an amorphous state as a result of step 3, the biomass fuel is burned again at the second temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the second temperature is increased by t ₅ minutes from the previous holding time, and the confirmation is repeated until a holding time T _C2 (min) at which the silica is in a crystalline state is found, and a holding time T _A2 (min) represented by T _{C2 -t 5} and at which the silica is in an amorphous state is found. If it is confirmed that the silica is in a crystalline state as a result of step 3, the biomass fuel is burned again at the second temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the second temperature is decreased by t ₆ minutes from the previous holding time, and the confirmation is repeated until a holding time T _A2 (min) at which the silica is in an amorphous state is found. Step 5: in a graph with the horizontal axis being temperature (°C) and the vertical axis being retention time (min), T _A1 , T _C1 , T _A2 , and _{T C2} are plotted, connecting T _A1 and T _A2 with straight _{lines and connecting T C1 and T C2 with straight} lines; Step 6: in a graph with the horizontal axis being temperature (°C) and the vertical axis being retention time (min), determining an approximation function T _{A based on at least the retention times T A1 and T A2} , and determining an approximation function T _C based on at least the retention times T _C1 and T _C2 , and determining the lower region of _{the approximation function T A as} the amorphous region and the upper region of the approximation function T _C as the crystalline region.

[3] The following steps are included between step 5 and step 6. Step A: Based on the straight line connecting T _A1 and T _A2 and the straight line connecting T _C1 and T _C2 , a predicted retention time T' A3 (min), T' _C3 (min) is read by extrapolating to an arbitrary third temperature higher than the first temperature or lower than the second temperature within a combustion temperature range of 600° C. to 1150° _C .; Step B: Burn the biomass fuel at the third temperature, and confirm whether or not the silica contained in the ash generated by the combustion of the biomass fuel is in an amorphous state while the retention times at the third temperature of the ash generated by the combustion of the biomass fuel are set to the predicted retention times T' _A3 and T' _C3 , respectively; Step C: If it is confirmed that the silica is in an amorphous state at T' _A3 as a result of step B, T' _A3 is set to the retention time T _A3 during which the silica is in an amorphous state. (min), and if it is confirmed that silica is in a crystalline state, the biomass fuel is burned again at the third temperature, and the confirmation is repeated with the holding time of the ash generated by the combustion of the biomass fuel at the third temperature reduced by t ₇ minutes from the previous holding time, to find a holding time T _A3 when silica is in an amorphous state, and if it is confirmed that silica is present in a crystalline state at T _{' C3} , T' _C3 is set as the holding time T _C3 (min), and if it is confirmed that silica is in an amorphous state, the biomass fuel is burned again at the third temperature, and the confirmation is repeated with the holding time of the ash generated by the combustion of the biomass fuel increased by t ₈ minutes from the previous holding time, to find a holding time T _C3 when silica is in a crystalline state, and in step 6, an approximation function T A is obtained based on at least the holding times T _A1 , T _A2 , and T _A3 in a graph with the horizontal axis being temperature (° C. ₎ and the vertical axis being holding time (min), and The control method according to the above-mentioned [2], further comprising the step of determining an approximation function T _C based on _{T C1} , T _C2 , and T _C3 , and setting the lower region of the approximation function T _A as the amorphous region and the upper region of the approximation function T _C as the crystalline region.

[4] The control method described in [2] above, wherein the first temperature and the second temperature are each within a temperature range of 750°C or more and 950°C or less.

[5] The control method described in [4] above, in which the temperature difference between the first temperature and the second temperature is 50°C or more.

[6] The control method described in [3] above, wherein the first temperature, the second temperature, and the third temperature are each within a temperature range of 750°C or more and 950°C or less.

[7] The control method described in [6] above, in which the temperature difference between the first temperature and the second temperature is 50°C or more, and when the third temperature is higher than the first temperature, the temperature difference between the first temperature and the third temperature is 50°C or more, and when the third temperature is lower than the second temperature, the temperature difference between the second temperature and the third temperature is 50°C or more.

[8] The control method described in any one of [1] to [7] above, wherein the biomass fuel is grain husk.

[9] A method for suppressing crystallization of amorphous silica during the combustion process of biomass fuel containing the silica, comprising the steps of: determining an amorphous region in which the silica is in an amorphous state and a crystalline region in which the silica is in a crystalline state based on a combustion temperature of 600°C or higher and 1150°C or lower and a holding time at the combustion temperature; and controlling the combustion temperature of the biomass fuel, the total elapsed time from the start of combustion of the biomass fuel, and the cooling rate of the ash generated by the combustion of the biomass fuel based on the amorphous region and the crystalline region so that the silica does not deviate from the amorphous region.

[10] The suppression method described in [9] above, wherein the biomass fuel is grain husk.

[11] A combustion device comprising: a combustor for burning a biomass fuel containing amorphous silica; a cooler for cooling ash produced by the combustion of the biomass fuel; and a control unit for controlling the combustion temperature of the biomass fuel, the total elapsed time from the start of the combustion of the biomass fuel, and the rate of cooling of the ash produced by the combustion of the biomass fuel, based on an amorphous region in which the silica is in an amorphous state and a crystalline region in which the silica is in a crystalline state, which are determined in advance based on a combustion range of 600°C to 1150°C and a retention time at the combustion temperature.

[12] The amorphous region and the crystalline region are determined by the following steps: Step 1: Burn the biomass fuel at an arbitrary first temperature within the combustion temperature range of 600° C. to 1150° C., and hold the ash generated by the combustion of the biomass fuel at the first temperature for t ₁ minute, and then confirm whether the silica contained in the ash is in an amorphous state; Step 2: If it is confirmed that the silica is in an amorphous state as a result of Step 1, burn the biomass fuel again at the first temperature, and confirm again in a state in which the holding time of the ash generated by the combustion of the biomass fuel at the first temperature is increased by t ₂ minutes from the previous holding time, and this step is repeated until a holding time T _C1 (min) at which the silica is in a crystalline state is found, and a holding time T _A1 represented by T _C1 -t ₂ and at which the silica is in an amorphous state is found _. (min) at which the silica is in a crystalline state; if it is confirmed as a result of step 1, combust the biomass fuel again at the first temperature, and confirm again after reducing the holding time of the ash generated by the combustion of the biomass fuel at the first temperature by _t3 minutes from the previous holding time; repeat this step until a holding time T _A1 (min) at which the silica is in an amorphous state is found; find a holding time T _A1 and a holding time T _C1 (min) at which the silica is in a crystalline state, which is represented by T _A1 + _t3 ; and step 3: combust the biomass fuel at a second temperature lower than the first temperature within a combustion temperature range of 600° C. to 1150° C., and combust the ash generated by the combustion of the biomass fuel at the second temperature by t After holding the mixture for ₄ minutes, it is confirmed whether the silica contained in the ash is in an amorphous state. Step 4: If it is confirmed that the silica is in an amorphous state as a result of step 3, the biomass fuel is burned again at the second temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the second temperature is increased by t ₅ minutes from the previous holding time, and the confirmation is repeated until a holding time T _C2 (min) at which the silica is in a crystalline state is found, and a holding time T _A2 (min) represented by T _{C2 -t 5} and at which the silica is in an amorphous state is found. If it is confirmed that the silica is in a crystalline state as a result of step 3, the biomass fuel is burned again at the second temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the second temperature is decreased by t ₆ minutes from the previous holding time, and the confirmation is repeated until a holding time T _A2 (min) at which the silica is in an amorphous state is found. Step 5: in a graph having temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, T _A1 , T _C1 , T _A2 , and _{T C2} are plotted, connecting T _A1 and T _A2 with straight _{lines and connecting T C1 and T C2 with straight} lines; Step 6: in a graph having temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, determining an approximation function T _{A based on at least the retention times T A1 and T A2} , and determining an approximation function T _C based on at least the retention times T _C1 and T _C2 , and determining a lower region of the approximation function _{T A as} the amorphous region and an upper region of the approximation function T _C as the crystalline region.

[13] The following steps are included between step 5 and step 6. Step A: Based on the straight line connecting T _A1 and T _A2 and the straight line connecting T _C1 and T _C2 , a predicted retention time T' A3 (min), T' _C3 (min) is read by extrapolating to an arbitrary third temperature higher than the first temperature or lower than the second temperature within a combustion temperature range of 600° C. to 1150° _C .; Step B: Burn the biomass fuel at the third temperature, and confirm whether or not the silica contained in the ash generated by the combustion of the biomass fuel is in an amorphous state while the retention times at the third temperature of the ash generated by the combustion of the biomass fuel are set to the predicted retention times T' _A3 and T' _C3 , respectively; Step C: If it is confirmed that the silica is in an amorphous state at T' _A3 as a result of step B, T' _A3 is set to the retention time T _A3 during which the silica is in an amorphous state. (min), and if it is confirmed that silica is in a crystalline state, the biomass fuel is burned again at the third temperature, and the confirmation is repeated with the holding time of the ash generated by the combustion of the biomass fuel at the third temperature reduced by t ₇ minutes from the previous holding time, to find a holding time T _A3 when silica is in an amorphous state, and if it is confirmed that silica is present in a crystalline state at T _{' C3} , T' _C3 is set as the holding time T _C3 (min), and if it is confirmed that silica is in an amorphous state, the biomass fuel is burned again at the third temperature, and the confirmation is repeated with the holding time of the ash generated by the combustion of the biomass fuel increased by t ₈ minutes from the previous holding time, to find a holding time T _C3 when silica is in a crystalline state, and in step 6, an approximation function T A is obtained based on at least the holding times T _A1 , T _A2 , and T _A3 in a graph with the horizontal axis being temperature (° C. ₎ and the vertical axis being holding time (min), and The combustion apparatus according to the above-mentioned [12], further comprising a step of determining an approximation function T _C based on _{T C1} , T _C2 , and T _C3 , and determining a lower region of the approximation function T _A as the amorphous region and an upper region of the approximation function T _C as the crystalline region.

[14] The combustion device described in [12] above, wherein the first temperature and the second temperature are each within a temperature range of 750°C or more and 950°C or less.

[15] The combustion device described in [14] above, in which the temperature difference between the first temperature and the second temperature is 50°C or more.

[16] The combustion device described in [13] above, wherein the first temperature, the second temperature, and the third temperature are each within a temperature range of 750°C or more and 950°C or less.

[17] The combustion device described in [16] above, in which the temperature difference between the first temperature and the second temperature is 50°C or more, and when the third temperature is higher than the first temperature, the temperature difference between the first temperature and the third temperature is 50°C or more, and when the third temperature is lower than the second temperature, the temperature difference between the second temperature and the third temperature is 50°C or more.

[18] The combustion device described in any one of [11] to [17] above, wherein the biomass fuel is grain husk.

The silica crystallization control method and combustion device of the present invention can control the crystallization of silica during combustion processing. In addition, the method for suppressing silica crystallization can suppress the crystallization of silica during combustion processing.

FIG. 1 is a phase diagram showing the relationship between the combustion temperature and the retention time of biomass fuel. FIG. 2 is a block diagram illustrating the steps in determining amorphous and crystalline regions. FIG. 3 is a schematic configuration diagram of the combustion device according to the embodiment. FIG. 4 is a phase diagram showing the relationship between the combustion temperature of rice husks, the total elapsed time, and the temperature decreasing rate in Examples 1 to 10. FIG. 5 is an X-ray diffraction pattern of the silica according to Examples 3 and 8. FIG. 6 is a scanning electron micrograph of the silica according to Examples 3-6 and 8-10.

Hereinafter, the method for controlling and inhibiting silica crystallization and the combustion device according to the embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a phase diagram showing the relationship between the combustion temperature and the retention time of biomass fuel, FIG. 2 is a block diagram showing the steps in determining the amorphous and crystalline regions, and FIG. 3 is a schematic diagram of the combustion device according to the embodiment.

Methods for controlling and inhibiting silica crystallization
A biomass fuel containing amorphous silica is prepared. The biomass fuel is not particularly limited as long as it contains amorphous silica, and examples thereof include cereal husks such as rice husks, barley, wheat, rye, millet, foxtail millet, and millet.

After preparing a biomass fuel containing amorphous silica, the amorphous regions where the silica is in an amorphous state and the crystalline regions where the silica is in a crystalline state are determined based on the combustion temperature of 600°C to 1150°C and the holding time at the combustion temperature. Specifically, these regions can be determined from a phase diagram that shows the relationship between the combustion temperature of the biomass fuel and the holding time at the combustion temperature. The phase diagram is created as follows.

First, biomass fuel is prepared, lightly washed with water to remove dust, dried, and then pulverized at a specified rotation speed, for example, using a planetary ball mill.

The pulverized biomass fuel is fed into a combustion device, and then heated to an arbitrary first temperature within the combustion range of 600°C to 1150°C at a heating rate of 20°C/min under an air flow of 500mL/min in the combustion device, and combusted. The ash generated by the combustion of the biomass fuel is held at the first temperature for _t1 minute, and then immediately allowed to cool naturally, and the obtained ash is checked by X-ray diffraction to see whether silica is in an amorphous state (step 1: S1 in FIG. 2). In checking the amorphous state of silica by X-ray diffraction, if silica is crystallized, a peak or shoulder appears, so whether the silica is in a crystalline state or an amorphous state can be confirmed by the presence or absence of a peak or shoulder. The first temperature can be 750°C to 950°C, specifically, for example, 875°C to 925°C, and _t1 can be 10 minutes to 15 minutes.

If it is confirmed that silica is in an amorphous state as a result of step 1, the biomass fuel is burned again at the first temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the first temperature is increased by t ₂ minutes from the previous holding time, and the above confirmation is performed again. This step is repeated until the holding time T _C1 (min) at which silica is in a crystalline state is found, and the holding time T _A1 (min) represented by the holding time T _C1 and T _C1 -t ₂ and at which silica is in an amorphous state is found (step 2 (1): S2 (1) in FIG. 2). In this case, T _A1 (min) can be regarded as the longest time for silica to crystallize. In addition, for example, from the viewpoint of efficiently finding T _A1 close to the boundary between the amorphous region and the crystalline region, it is suitable to set t ₂ to about 1.6 to 8.7 minutes.

If it is confirmed that silica is in a crystalline state as a result of step 1, the biomass fuel is burned again at the first temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the first temperature is reduced by t ₃ minutes from the previous holding time, and the above confirmation is performed again. This step is repeated until the holding time T _A1 (min) at which silica is in an amorphous state is found, and the holding time T _C1 (min) represented by the holding time T _A1 and T _A1 +t ₃ and at which silica is in a crystalline state is found (step 2 (2): S2 (2) in FIG. 2). In this case, T _C1 (min) can be regarded as the shortest time for silica to crystallize. In addition, for example, from the viewpoint of efficiently finding T _C1 close to the boundary between the amorphous region and the crystalline region, it is suitable to set t ₃ to about 1.6 to 8.7 minutes.

Similarly, the biomass fuel is burned at a second temperature lower than the first temperature within the combustion temperature range of 600°C to 1150°C, and the ash generated by the combustion of the biomass fuel is held at the second temperature for _t4 minutes, and then the obtained ash is checked by X-ray diffraction method to see whether silica is in an amorphous state or not (step 3: S3 in FIG. 2). The second temperature can be 750°C to 950°C, specifically, for example, 775°C to 825°C, and _t4 may be about 480 to 800 minutes. From the viewpoint of improving accuracy, it is preferable that the temperature difference between the first temperature and the second temperature is 50°C or more.

If it is confirmed that silica is in an amorphous state as a result of step 3, the biomass fuel is burned again at the second temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the second temperature is increased by t ₅ minutes from the previous holding time, and the above confirmation is performed again. This step is repeated until a holding time T _C2 (min) at which silica is in a crystalline state is found, and the holding time T _C2 and T _A2 (min) represented by T _C2 -t ₅ and at which silica is in an amorphous state are found (step 4 (1): S4 (1) in FIG. 2). In this case, T _A2 (min) can be regarded as the longest time for silica to crystallize. In addition, for example, from the viewpoint of efficiently finding T _A2 close to the boundary between the amorphous region and the crystalline region, it is appropriate to set t ₅ to about 70 to 350 minutes.

If it is confirmed that silica is in a crystalline state as a result of step 3, the biomass fuel is burned again at the second temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the second temperature is reduced by t ₆ minutes from the previous holding time, and the above confirmation is performed again. This step is repeated until a holding time T _A2 (min) at which silica is in an amorphous state is found, and the holding time T _A2 and T _C2 (min) represented by T _A2 +t ₆ and at which silica is in a crystalline state are found (step 4 (2): S4 (2) in FIG. 2). In this case, T _C2 (min) can be regarded as the shortest time for silica to crystallize. In addition, for example, from the viewpoint of efficiently finding T _C2 close to the boundary between the amorphous region and the crystalline region, it is suitable to set t ₆ to about 70 to 350 minutes.

Then, T _A1 , T _C1 , T _A2 , and T _C2 are plotted on a semi-logarithmic graph (vertical axis is logarithmic) with the horizontal axis representing temperature (°C) and the vertical axis representing retention time (min), and T _A1 and T _A2 are connected with straight lines, and T _C1 and T _C2 are connected with straight lines (step 5: S5 in FIG. 2).

Then, in this graph, predicted holding times _T'A3 (min) and T'C3 (min) are read which are extrapolated to an arbitrary third temperature higher than the first temperature or lower than the second temperature within the combustion temperature range of 600°C to 1150 _°C (Step A: SA in FIG. 2). Here, when the third temperature is lower than the second temperature, it is preferable to read predicted holding times _T'A4 (min) and _T'C4 (min) which are extrapolated to an arbitrary fourth temperature higher than the first temperature within the combustion temperature range of 600°C to 1150°C in addition to _T'A3 (min) and _T'C3 (min). When the third temperature is higher than the first temperature, it is preferable to read predicted holding times _T'A4 (min) and _T'C4 (min) extrapolated to an arbitrary fourth temperature lower than the second temperature within the combustion temperature range of 600° C. to 1150° C., in addition to _T'A3 (min) and _T'C3 (min). By reading the predicted holding times _T'A4 (min) and _T'C4 (min) in this manner, it is possible to obtain approximate functions T _A and T _C with high accuracy.

Then, the biomass fuel is burned at a third temperature, and the retention times of the ash produced by the combustion of the biomass fuel at the third temperature are set as predicted retention times _T'A3 and _T'C3 . Then, the X-ray diffraction method is used to confirm whether or not the silica in each of the obtained ashes is in an amorphous state (Step B: SB in Figure 2).

If it is confirmed that silica is in an amorphous state at T' _A3 as a result of step B, T' _A3 is set as the holding time T _A3 (min) during which silica is in an amorphous state, and if it is confirmed that silica is in a crystalline state, the biomass fuel is burned again at the third temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the third temperature is reduced by t ₇ minutes from the previous holding time, and the confirmation is repeated to find the holding time T _A3 during which silica is in an amorphous state (step C(1): SC(1) in FIG. 2). The third temperature can be 750°C or higher and 950°C or lower, specifically, for example, 750°C or higher and 775°C or lower, or 925°C or higher and 950°C or lower. Here, from the viewpoint of increasing accuracy, when the third temperature is higher than the first temperature, the temperature difference between the first temperature and the third temperature is preferably 50°C or higher, and when the third temperature is lower than the second temperature, the temperature difference between the second temperature and the third temperature is preferably 50°C or higher. Also, for example, from the viewpoint of efficiently finding T _A3 close to the boundary between the amorphous region and the crystalline region, when the third temperature is in the range of 750° C. or more and 775° C. or less, it is suitable that t ₇ is about 1550 minutes to 3100 minutes, and when the third temperature is in the range of 925° C. or more and 950° C. or less, it is suitable that t ₇ is about 10 seconds to 20 seconds.

If it is confirmed that silica is in a crystalline state at _T'C3 as a result of step B, _T'C3 is set as the holding time T _C3 (min), and if it is confirmed that silica is in an amorphous state, the biomass fuel is burned again at the third temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the third temperature is increased by t ₈ minutes from the previous holding time, and the confirmation is repeated to find the holding time T _C3 when silica is in a crystalline state (step C(2): SC(2) in FIG. 2). From the viewpoint of efficiently finding T _C3 close to the boundary between the amorphous region and the crystalline region, when the third temperature is in the range of 750°C or more and 775°C or less, it is suitable to set t ₈ to about 1550 minutes to 3100 minutes, and when the third temperature is in the range of 925°C or more and 950°C or less, it is suitable to set t ₈ to about 10 seconds to 20 seconds.

In addition, when the predicted retention times _T'A4 (min) and _T'C4 (min) are also read, the biomass fuel is burned at the fourth temperature, and the retention times of the ash generated by the combustion of the biomass fuel at the fourth temperature are set as the predicted retention times _T'A4 and _T'C4 , and then, in each of the ashes obtained, it is confirmed by X-ray diffraction whether or not the silica is in an amorphous state. The subsequent steps are similar to steps C(1) and C(2), and therefore will not be described.

Then, using commercially available software such as EXCEL (Microsoft Corporation), an approximation function TA is calculated based on at least the retention times _TA1 , _TA2 , and _TA3 in a semi-logarithmic graph (vertical axis is logarithmic) with the horizontal axis being temperature (°C) and the vertical axis being retention time (min) (step 6: S6 in FIG. 2). If _TA4 is also found above, the approximation function _TA is calculated based on the retention times _TA1 , _TA2 , _TA3 , and _TA4 . At this time, it is desirable to calculate the approximation _{function TA based} on the retention times of four points, but depending on the precision of the device, it may be difficult to obtain _TA4 . In that case, the approximation function _TA may be calculated based on the retention times of three points. Similarly, the approximation function _TC is calculated based on at least the retention times _TC1 , _TC2 , and _TC3 (step 6). In this case, it is desirable to obtain the approximate function T _C based on the retention times of four points, but since it may be difficult to obtain T _C depending on the precision of the device, in that case, the approximate function T _C may be obtained based on the retention times of three points shown in FIG. 1. The lower region of the approximate function T _A is the amorphous region, and the upper region of the approximate function T _C is the crystalline region (step 6). The approximate functions T _A and T _C can be obtained, for example, by using the least squares approximation function of EXCEL (manufactured by Microsoft Corporation). The vertical axis is preferably logarithmic. The intermediate region between the approximate functions T _A and T _C is a gray zone, and the higher the measurement precision, the narrower this region becomes, and the closer the two become to one, and the higher the precision of the phase diagram.

In the above, steps A to C are included, but steps A to C may not be included. In this case, in step 5, a straight line connecting T _A1 and T _A2 and a straight line connecting T _C1 and T _C2 are drawn on the graph, and then an approximation function T _A is obtained based on the retention times T _A1 and T _A2 , and an approximation function T _C is obtained based on the retention times T _C1 and T _C2 . Then, the region below the approximation function T _A is regarded as an amorphous region, and the region above the approximation function T _C is regarded as a crystalline region.

After creating such a phase diagram, the combustion temperature of the biomass fuel, the total time elapsed since the start of the combustion of the biomass fuel, and the temperature drop rate of the ash generated by the combustion of the biomass fuel are controlled based on the amorphous and crystalline regions shown in this phase diagram. Here, when data on the ash are plotted on a semi-logarithmic graph (vertical axis is logarithmic) with the horizontal axis representing temperature (°C) and the vertical axis representing the total time elapsed (min), and the plotted data are connected with a curve to draw the temperature curve of the ash (see Figure 4 described later), if a part of the temperature curve of the ash is in the crystalline region, the silica will be in a crystalline state even if the other part of the temperature curve of the ash is in the amorphous region. Therefore, when suppressing the crystallization of silica, the combustion temperature, the total time elapsed, and the temperature drop rate are controlled so that all parts of the temperature curve of the ash are in the amorphous region.

The total elapsed time is not particularly limited, but can be adjusted within a range of, for example, 0 minutes to 13 days. However, although this depends on the combustion temperature, the shorter the total elapsed time, the lower the temperature curve of the ash will be, and the more likely it will remain in the amorphous region.

When the temperature of ash generated by burning biomass fuel is lowered, the temperature curve of the ash bends to the left (towards lower temperatures). The faster the temperature drop rate, the more the temperature curve of the ash bends to the left. For this reason, although it depends on the maximum temperature reached during the combustion process and the holding time, if the temperature drop rate is high, the ash is more likely to remain in the amorphous region. The temperature drop rate differs depending on the type of biomass fuel, but may be adjusted within the range of 0.05°C/min to 20°C/min, for example.

The approximation functions T _A and T _C vary depending on the type of biomass fuel, or even if the rice husk is used, the place of origin, etc., so it is necessary to find the approximation functions T _A and T _C according to the biomass fuel to be burned.

Such a method for controlling or suppressing silica crystallization can be realized, for example, by the following combustion device and program.

<<Combustion device>>
The combustion device 10 shown in Fig. 3 includes a combustor 11, a cooler 12, an ash collector 13, and a control unit 14. The biomass fuel supplied to the combustor 11 is preferably one that has been pulverized in advance by a pulverizer.

The combustor 11 is for burning biomass fuel to generate combustion gas 15 and ash 16. The combustor 11 is a burner, but instead of a burner, for example, a heater that applies heat to the biomass fuel to burn the biomass fuel may be used. The combustor 11 is equipped with a combustion chamber 11A, a supply pipe 11B that supplies biomass fuel to the combustion chamber 11A, an air supply pipe 11C that supplies air to the combustion chamber 11A, and an ignition means such as an ignition combustor (not shown). The biomass fuel supplied from the supply pipe 11B is mixed with air supplied from the air supply pipe 11C and ignited by the ignition means, so that the biomass fuel burns in the combustion chamber 11A. Combustion gas 15 and ash 16 generated mainly by burning the combustible gas are discharged from the outlet of the combustor 11.

The cooler 12 is for quickly cooling the ash generated in the combustor 11 to a predetermined temperature. As shown in FIG. 3, the cooler 12 is equipped with a can body 12B in which a cooling medium 12A is stored, a combustion gas inlet 12C arranged in the can body 12B, a plurality of smoke tubes 12D, and a gas collection section 12E. The cooler 12 receives the ash 16 discharged from the outlet of the combustor 11 together with the combustion gas, and can cool the temperature of the ash 16 together with the combustion gas 15 to a desired temperature by heat exchange with the cooling medium 12A. A temperature sensor (not shown) is arranged near the outlet of the cooler 12, and is configured to be able to measure the temperature of the ash.

The ash collector 13 is for separating and collecting the ash 16 after cooling treatment, which is introduced through the exhaust duct 17, from the combustion gas 15. The ash collector 13 is equipped with an outer cylinder 13A, which has a cylindrical upper part and a conical lower part that tapers downward, an inner cylinder 13B inserted concentrically into the center of the outer cylinder 13A, and a collection box 13C provided below the outer cylinder 13A. In the ash collector 13, the combustion gas 15 is taken out through the inner cylinder 13B, and the ash 16 separated from the combustion gas 15 is collected in the collection box 13C.

<Control Unit>
The control unit 14 is for controlling the combustion temperature, the total elapsed time from the start of the combustion of the biomass fuel, the temperature drop rate of the ash generated by the combustion of the biomass fuel, etc. The combustion temperature is a temperature measured by a temperature sensor arranged in the combustor 11, and the ash temperature is a temperature measured by a temperature sensor arranged near the outlet of the cooler 12. The control unit 14 is configured to adjust the flow rate of air input into the combustor 11 and the movement of the refrigerant in the cooler 12 based on these temperatures.

The control unit 14 is a computer. The control unit 14 is electrically connected to the combustor 11 (e.g., the air supply pipe 11C) and the cooler 12, and is capable of controlling the combustion temperature, the total elapsed time, and the rate at which the ash cools.

The control unit 14 controls the combustion temperature of the biomass fuel, the total time elapsed since the start of the combustion of the biomass fuel, and the temperature drop rate of the ash generated by the combustion of the biomass fuel, based on the amorphous region and the crystalline region that are determined in advance based on the combustion temperature of 600°C or more and 1150°C or less and the holding time of the combustion temperature. The amorphous region and the crystalline region are determined by the above-mentioned steps. The holding temperature is not particularly limited, but can be controlled within a range of, for example, 0 minutes to 13 days, and the temperature drop rate differs depending on the type of biomass fuel, but can be controlled within a range of, for example, 0.05°C/min to 20°C/min.

The above control in the control unit 14 is executed by a control program. The control program causes the computer to execute a step of accepting input of information regarding the amorphous regions and crystalline regions that has been determined in advance based on the combustion temperature of 600°C or more and 1150°C or less and the time for which the combustion temperature is maintained, and to execute a step of controlling the combustion temperature of the biomass fuel, the total elapsed time from the start of the combustion of the biomass fuel, and the rate of temperature drop of the ash generated by the combustion of the biomass fuel based on the above input information.

According to this embodiment, the crystallization of silica can be controlled simply by controlling the combustion temperature of the biomass fuel, the total time elapsed since the start of the combustion of the biomass fuel, and the rate of temperature drop of the ash generated by the combustion of the biomass fuel based on the amorphous region and the crystalline region, so that the crystallization of silica can be controlled in a simple manner.

According to this embodiment, silica crystallization can be suppressed simply by controlling the combustion temperature of the biomass fuel, the total elapsed time from the start of the combustion of the biomass fuel, and the rate of temperature drop of the ash generated by the combustion of the biomass fuel so as not to deviate from the amorphous region based on the amorphous region and the crystalline region, so that silica crystallization can be suppressed in a simple manner.

In addition, during the process of carbonizing biomass fuel, if the combustion temperature and holding time of the biomass charcoal are not maintained so that the silica remains in the amorphous region, the biomass charcoal will contain silica in the form of harmful crystalline silica. In contrast, biomass charcoal that contains amorphous silica can be used for soil improvement and organic cultivation.

In order to explain the present invention in detail, the following examples are given, but the present invention is not limited to these. Figure 4 is a phase diagram showing the relationship between the combustion temperature, total elapsed time, and temperature drop rate of silica in Examples 1 to 10, Figure 5 is an X-ray diffraction pattern of silica in Examples 3 and 8, and Figure 6 is a scanning electron microscope photograph of silica in Examples 3 to 6 and 8 to 10.

<Rice husks>
In the following, rice husks used were those of Oryza sativa, japonica harvested in the Kyoto region, lightly washed with water to remove dust, dried, and then pulverized in a planetary ball mill (model number "LA-PO.1", manufactured by Ito Seisakusho Co., Ltd.) at 400 rpm for 30 minutes.

<Combustion device>
In the following, a thermal analyzer (TG/DTA) (product name "Thermo plus TG8120", manufactured by Rigaku Corporation) was used as the combustion device in order to perform the combustion and subsequent cooling processes under as accurate temperature control as possible.

<X-ray Diffraction Apparatus>
In the following, a high-intensity X-ray generator (product name "MicroMax 007HF", manufactured by Rigaku Corporation) was used as the X-ray diffractometer. The X-ray diffraction image was recorded by filling a soft glass capillary (manufactured by Hilgenberg) with ash and placing it in a Laue camera, and irradiating it with CuKα rays generated at 40 kV and 30 mA for 60 minutes.

<Preparation of phase diagram>
10 mg of the crushed rice husks were placed in a platinum sample pan, and the rice husks were burned by heating the rice husks to a first temperature of 900° C. at a rate of 20° C./min under an air flow of 500 mL/min, and the ash generated by the combustion of the rice husks was kept at that temperature for 9 minutes as _t1 . Then, it was immediately allowed to cool naturally. Then, in the obtained ash, it was confirmed by X-ray diffraction method using an X-ray diffraction device whether silica was in an amorphous state.

If it was confirmed that the silica was in an amorphous state, the rice husks were burned at 900°C, and the holding time of the ash produced by the combustion of the rice husks at 900°C was increased by 2 minutes, _t2 , from the previous holding time, and the above confirmation was repeated.This step was repeated until the holding time T _C900°C (min) at which the silica was in a crystalline state was found, and the holding time T _C900°C (min) and the holding temperature T A900 _°C (min) at which the silica was in an amorphous state, expressed as T _C900°C -2 (min), were found.

Furthermore, if it was confirmed that the silica was in a crystalline state, the rice husks were burned at 900°C, and the above confirmation was repeated with the holding time of the ash produced by the combustion of the rice husks at 900°C reduced by 2 minutes, _t3 , from the previous holding time. This step was repeated until the holding time T _A900°C (min) at which the silica was in an amorphous state was found, and the holding time T _A900°C (min) and the holding temperature T _C900°C (min), which was expressed as T _A900°C +2 (min) and at which the silica was in a crystalline state, were found.

Similarly, the second temperature was raised to 800°C to combust the rice husks, and the ash generated by the combustion of the rice husks was kept at 800°C for 66 minutes as _t4 . Then, it was immediately allowed to cool naturally. Then, in the obtained ash, it was confirmed whether or not silica was in an amorphous state by X-ray diffraction method using an X-ray diffraction device.

As a result, if it was confirmed that the silica was in an amorphous state, the rice husks were burned at 800°C, and the holding time of the ash produced by the combustion of the rice husks at 800°C was increased by 150 minutes from the previous holding time, _t5, and the above confirmation was performed again.This step was repeated until the holding time T _C800°C (min) at which the silica was in a crystalline state was found, and the holding time T _C800°C and the holding temperature T A800°C (min) at which the silica was in a crystalline state, which was expressed as T _{C800°C -150} (min), were found.

Furthermore, if it was confirmed that the silica was in a crystalline state, the rice husks were burned at 800°C, and the holding time of the ash generated by the combustion of the rice husks at 800°C was reduced by 150 minutes, as _t6, from the previous holding time, and the above confirmation was performed again. This step was repeated until a holding time T _A800°C (min) at which the silica was in an amorphous state was found, and a holding time T _A800°C and a holding temperature T _C800°C (min) represented by T _A800°C +150 at which the silica was in a crystalline state were found.

Thereafter, T _C900°C , T _A900°C , T _C800°C , and T _A800°C were plotted on a semi-logarithmic graph where the horizontal axis is temperature (°C) and the vertical axis is holding time (min), and a straight line was connected between T _A900°C and T _A800°C , and between T _C900°C and T _C800°C , and the predicted holding times T' _A750°C (min) and T' _C750°C (min) extrapolated to 750°C were read, and the predicted holding times T' _A950°C (min) and T' _C950°C (min) extrapolated to 950°C were read.

Then, the rice husks were burned at 750°C, and after the predicted holding times _T'A750°C and _{T'C750°C of the ash produced by the combustion of the rice husks had passed, it was confirmed by X-ray diffraction method whether the silica was in an amorphous state in the ash obtained at each of the ash obtained at 750°C} . Similarly, the rice husks were burned at 950°C, and after the predicted holding time _T'A950°C had passed, it was confirmed by X-ray diffraction method using an X-ray diffractometer whether the silica was in an amorphous state in the ash obtained at each of the ash obtained at 750°C.

As a result, when it was confirmed that silica was in an amorphous state at _T'A750°C , T'A750 _°C was set as the holding time T _A750°C (min) during which silica was in an amorphous state, and when it was confirmed that silica was in a crystalline state, rice husks were burned at 750°C, and the holding time of the ash produced by the rice husk combustion at 750°C was reduced by 790 minutes from the previous holding time, _t7 , and the confirmation was repeated to find the holding time T _A750°C during which silica was in an amorphous state.

In addition, when it was confirmed that silica was in a crystalline state at T'C750 _°C , T'C750 _°C was set as the holding time T _C750°C (min), and when it was confirmed that silica was in an amorphous state, rice husks were burned at 750°C and the holding time of the ash produced by the combustion of the rice husks at 750°C was increased by 790 minutes from the previous holding time, _t8 , and the confirmation was repeated to find the holding time T _C750°C at which silica was in a crystalline state.

In addition, when it was confirmed that silica was in an amorphous state at T' _A950°C , T' _A950°C was set as the holding time T _A950°C (min) during which silica was in an amorphous state, and when it was confirmed that silica was in a crystalline state, rice husks were burned at 950°C, and the holding time of the ash produced by the rice husk combustion at 950°C was reduced by 1.5 minutes from the previous holding time, and the holding time T _A950°C during which silica was in an amorphous state was found.

Then, using EXCEL (Microsoft Corporation), an approximation function T was calculated based on the holding times T _A900 °C, T _{A800°C, T A750°C} , and T _A950 °C in a semi-logarithmic graph in which the horizontal axis is temperature (°C) and the vertical axis is holding time (min) (see FIG. 4). Similarly, an approximation function T _C was calculated based on holding times T _C900°C , T _{C800 °C} , and T _{C750°C . The lower region of the approximation function T A was} determined as the amorphous region, and the upper region of the approximation function T _C was determined as the crystalline region. Specifically, the T _A function was y = 4 × 10 ¹⁵ e ^-0.037 x (correlation coefficient R = 0.9974), and the T _C function was y = 10 ¹⁵ e ^-0.036 x (correlation coefficient R = 0.9987) (see Figure 4). The approximation functions T _A and T _C were determined using the least squares approximation function of EXCEL (Microsoft Corporation).

<Examples 1 to 10>
10 mg of the crushed rice husks were placed in a platinum sample pan and burned in a combustion device. Specifically, the rice husks were first heated to various maximum temperatures x _max °C at a heating rate of 20 °C/min under an air flow of 500 mL/min, and then burned. The ash generated by the combustion of the rice husks was then cooled to x _end °C at a constant controlled rate of r °C/min, and then allowed to cool naturally. x _max , r, and x _end are shown in Table 1, and the ash data are plotted on the phase diagram in Figure 4.

<Measurement by X-ray diffraction method>
In Examples 1 to 10, the powdered ash obtained after combustion was subjected to X-ray diffraction (XRD) images recorded by an X-ray diffractometer. In Examples 4, 5, 8 to 10, four peaks attributed to α-cristobalite and a shoulder attributed to α-quartz were observed (see FIG. 5). In contrast, no peaks derived from crystals were observed in Examples 1, 2, 3, 6, and 7 (see FIG. 5). This confirmed that silica was in a crystalline state in Examples 4, 5, 8 to 10, and that silica was in an amorphous state in Examples 1, 2, 3, 6, and 7. From these results, it was confirmed that silica was in an amorphous state when controlled to be present within the amorphous region below the approximation function T _A shown in FIG. 4, and that silica was in a crystalline state when outside the amorphous region below the approximation function T _A shown in FIG. 4.

<Scanning electron microscope photography>
In Examples 3 to 6 and 8 to 10, the powdered ash obtained after combustion was mounted on a sample stage using conductive tape and observed with a scanning electron microscope (SEM) (product name "S-4800", Hitachi High-Technologies Corporation) at an acceleration voltage of 1 kV and a magnification of 30,000 times, resulting in the photograph shown in Figure 6. It was confirmed from Figure 6 that silica was in a crystalline state in Examples 4, 5 and 8 to 10. It was also confirmed from Figure 6 that silica was in an amorphous state in Examples 3 and 6.

These results confirmed that it is possible to control or inhibit silica crystallization by controlling the combustion temperature, the total time elapsed from the start of combustion, and the rate of temperature reduction based on the amorphous and crystalline regions.

10... Combustion device 11... Combustor 12... Cooler 13... Ash collector 14... Control unit

Claims (10)

A method for controlling crystallization of amorphous silica in a combustion process of a biomass fuel containing said silica, comprising:
Determining an amorphous region in which the silica is in an amorphous state and a crystalline region in which the silica is in a crystalline state based on a combustion temperature of 600° C. or more and 1150° C. or less and a retention time at the combustion temperature;
and controlling the combustion temperature of the biomass fuel, the total time elapsed since the start of combustion of the biomass fuel, and the temperature drop rate of ash generated by the combustion of the biomass fuel based on the amorphous region and the crystalline region;
The determination of the amorphous regions and the crystalline regions comprises the steps of:
Step 1: Burning a biomass fuel at an arbitrary first temperature within a combustion temperature range of 600° C. to 1150° C., and holding the ash generated by the combustion of the biomass fuel at the first temperature for t ₁ minute, and then confirming whether or not the silica contained in the ash is in an amorphous state;
Step 2: If it is confirmed that the silica is in an amorphous state as a result of step 1, burn the biomass fuel again at the first temperature, and confirm again with the retention time of the ash generated by the combustion of the biomass fuel at the first temperature increased by t ₂ minutes from the previous retention time, and repeat this step until a retention time T _C1 (min) at which the silica is in a crystalline state is found, and find a retention time T _C1 and a retention time T _A1 (min) represented by T _C1 -t ₂ and at which the silica is in an amorphous state. If it is confirmed that the silica is in a crystalline state as a result of step 1, burn the biomass fuel again at the first temperature, and confirm again with the retention time of the ash generated by the combustion of the biomass fuel decreased by t ₃ minutes from the previous retention time, and repeat this step until a retention time T _A1 (min) at which the silica is in an amorphous state is found, and find a retention time T _A1 and a retention time T _A1 +t ₃ and the retention time T _C1 (min) at which the silica is in a crystalline state;
Step 3: Burning the biomass fuel at a second temperature lower than the first temperature within a combustion temperature range of 600° C. to 1150° C., and holding the ash generated by the combustion of the biomass fuel at the second temperature for _t4 minutes, and then confirming whether the silica contained in the ash is in an amorphous state;
Step 4: If it is confirmed that the silica is in an amorphous state as a result of step 3, burn the biomass fuel again at the second temperature, and confirm again with the holding time of the ash generated by the combustion of the biomass fuel at the second temperature increased by t ₅ minutes from the previous holding time, and repeat this step until a holding time T _C2 (min) at which the silica is in a crystalline state is found, and find a holding time T _C2 and a T _A2 (min) represented by T _C2 -t ₅ and at which the silica is in an amorphous state. If it is confirmed that the silica is in a crystalline state as a result of step 3, burn the biomass fuel again at the second temperature, and confirm again with the holding time of the ash generated by the combustion of the biomass fuel at the second temperature decreased by t ₆ minutes from the previous holding time, and repeat this step until a holding time T _A2 (min) at which the silica is in an amorphous state is found, and find a holding time T _A2 and a holding time T _A2 +t ₆ , and T _C2 (min) at which the silica is in a crystalline state;
Step 5: In a graph with temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, T _A1 , T _C1 , T _A2 , and T _C2 are plotted, and T _A1 and T _A2 are connected with straight lines, and T _C1 and T _C2 are connected with straight lines;
Step 6: In a graph having temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, an approximation function T _A is calculated based on at least retention times T _A1 and T _A2 , and an approximation function T _C is calculated based on at least retention times T _C1 and T _C2 , and a lower region of the approximation function T _A is defined as the amorphous region, and an upper region of the approximation function T _C is defined as the crystalline region. Between step 5 and step 6, the following steps are included:
Step A: based on the straight line connecting T _A1 and T _A2 and the straight line connecting T _C1 and T _C2 , predictive retention times T' _A3 (min) and T' C3 (min) are read out by extrapolating to an arbitrary third temperature higher than the first temperature or lower than the second temperature within a combustion temperature range of 600° _C. to 1150° C.;
Step B: Burning the biomass fuel at the third temperature, and while the retention times of the ash generated by the combustion of the biomass fuel at the third temperature are set to the predicted retention times _T'A3 and _T'C3 , it is confirmed whether or not the silica contained in the ash generated by the combustion of the biomass fuel is in an amorphous state;
Step C: If it is confirmed that silica is in an amorphous state at _T'A3 as a result of step B, set _T'A3 as the retention time T _A3 (min) during which silica is in an amorphous state; if it is confirmed that silica is in a crystalline state, burn the biomass fuel again at the third temperature, and repeat the confirmation in a state in which the retention time of the ash generated by the combustion of the biomass fuel at the third temperature is reduced by t ₇ minutes from the previous retention time, to find the retention time T _A3 during which silica is in an amorphous state; if it is confirmed that silica is present in a crystalline state at _T'C3 , set _T'C3 as the retention time T _C3 (min); if it is confirmed that silica is in an amorphous state, burn the biomass fuel again at the third temperature, and repeat the confirmation in a state in which the retention time of the ash generated by the combustion of the biomass fuel at the third temperature is increased by t ₈ minutes from the previous retention time, to find the retention time T _C3 during which silica is in a crystalline state;
The control method of claim 1, wherein step 6 is a step of determining an approximate function _TA based on at least retention times _TA1 , _TA2 , and _TA3 in a graph with temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, and determining an approximate function _TC based on at least retention times _TC1 , _TC2 , and _TC3 , and determining the region below the approximate function _TA as the amorphous region and the region above the approximate function _TC as the crystalline region. The control method according to claim 1, wherein the first temperature and the second temperature are each within a temperature range of 750°C or more and 950°C or less. The control method according to claim 3, wherein the temperature difference between the first temperature and the second temperature is 50°C or more. The control method according to claim 2, wherein the first temperature, the second temperature, and the third temperature are each within a temperature range of 750°C or more and 950°C or less. The control method according to claim 5, wherein the temperature difference between the first temperature and the second temperature is 50°C or more, and when the third temperature is higher than the first temperature, the temperature difference between the first temperature and the third temperature is 50°C or more, and when the third temperature is lower than the second temperature, the temperature difference between the second temperature and the third temperature is 50°C or more. 7. The method according to claim 1, wherein the biomass fuel is cereal husks. A method for suppressing crystallization of silica during combustion treatment of a biomass fuel containing silica, comprising:
Determining an amorphous region where silica is in an amorphous state and a crystalline region where silica is in a crystalline state based on a combustion temperature of 600° C. or more and 1150° C. or less and a retention time at the combustion temperature;
and controlling, based on the amorphous region and the crystalline region , the combustion temperature of the biomass fuel, the total elapsed time from the start of combustion of the biomass fuel, and the temperature drop rate of ash generated by the combustion of the biomass fuel so that the silica is present in the amorphous region ;
The determination of the amorphous regions and the crystalline regions comprises the steps of:
Step 1: Burning a biomass fuel at an arbitrary first temperature within a combustion temperature range of 600° C. to 1150° C., and holding the ash generated by the combustion of the biomass fuel at the first temperature for t ₁ minute, and then confirming whether or not the silica contained in the ash is in an amorphous state;
Step 2: If it is confirmed that the silica is in an amorphous state as a result of step 1, burn the biomass fuel again at the first temperature, and confirm again with the retention time of the ash generated by the combustion of the biomass fuel at the first temperature increased by t ₂ minutes from the previous retention time, and repeat this step until a retention time T _C1 (min) at which the silica is in a crystalline state is found, and find a retention time T _C1 and a retention time T _A1 (min) represented by T _C1 -t ₂ and at which the silica is in an amorphous state . If it is confirmed that the silica is in a crystalline state as a result of step 1, burn the biomass fuel again at the first temperature, and confirm again with the retention time of the ash generated by the combustion of the biomass fuel decreased by t 3 _minutes from the previous retention time, and repeat this step until a retention time T _A1 (min) at which the silica is in an amorphous state is found, and find a retention time T _A1 and a retention time T _A1 +t ₃ and the retention time T _C1 (min) at which the silica is in a crystalline state ;
Step 3: Burning the biomass fuel at a second temperature lower than the first temperature within a combustion temperature range of 600° C. to 1150° C., and holding the ash generated by the combustion of the biomass fuel at the second temperature for t4 _minutes , and then confirming whether the silica contained in the ash is in an amorphous state;
Step 4: If it is confirmed that the silica is in an amorphous state as a result of step 3, burn the biomass fuel again at the second temperature, and confirm again with the holding time of the ash generated by the combustion of the biomass fuel at the second temperature increased by t ₅ minutes from the previous holding time, and repeat this step until a holding time T _C2 (min) at which the silica is in a crystalline state is found, and find a holding time T _C2 and a T _A2 (min) represented by T _C2 -t ₅ and at which the silica is in an amorphous state. If it is confirmed that the silica is in a crystalline state as a result of step 3, burn the biomass fuel again at the second temperature, and confirm again with the holding time of the ash generated by the combustion of the biomass fuel at the second temperature decreased by t ₆ minutes from the previous holding time, and repeat this step until a holding time T _A2 (min) at which the silica is in an amorphous state is found, and find a holding time T _A2 and a holding time T _A2 +t ₆ , and T _C2 (min) at which the silica is in a crystalline state;
Step 5: In a graph with temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, T _A1 , T _C1 , T _A2 , and T _C2 are plotted, and T _A1 and T _A2 are connected with straight lines, and T _C1 and T _C2 are connected with straight lines;
Step 6: In a graph with the horizontal axis representing temperature (° C.) and the vertical axis representing retention time (min), an approximation function T _A is calculated based on at least retention times T _A1 and T _A2 , and an approximation function T _C is calculated based on at least retention times T _C1 and T _C2 , and the region below the approximation function T _A is defined as the amorphous region, and the region above the approximation function T _C is defined as the crystalline region;
The method of suppressing, wherein the biomass fuel is cereal husks. A combustor for burning a biomass fuel containing amorphous silica;
A cooler for cooling ash produced by the combustion of the biomass fuel;
and a control unit that controls the combustion temperature of the biomass fuel, the total elapsed time from the start of combustion of the biomass fuel, and the temperature drop rate of ash generated by the combustion of the biomass fuel based on an amorphous region in which the silica is in an amorphous state and a crystalline region in which the silica is in a crystalline state, which are determined in advance based on a combustion temperature of 600° C. or more and 1150° C. or less and a holding time at the combustion temperature,
The amorphous regions and the crystalline regions are determined by the following steps:
Step 1: Burning a biomass fuel at an arbitrary first temperature within a combustion temperature range of 600° C. to 1150° C., and holding the ash generated by the combustion of the biomass fuel at the first temperature for t ₁ minute, and then confirming whether or not the silica contained in the ash is in an amorphous state;
Step 2: If it is confirmed that the silica is in an amorphous state as a result of step 1, the biomass fuel is burned again at the first temperature, and the holding time of the ash generated by the combustion of the biomass fuel at the first temperature is increased by _t2 minutes from the previous holding time, and the confirmation is performed again in that state until a holding time T _C1 (min) at which the silica is in a crystalline state is found.
This step is repeated to find a retention time T _C1 and a retention time T _A1 (min) represented by T _C1 -t ₂ and at which the silica is in an amorphous state. If the result of step 1 indicates that the silica is in a crystalline state, the biomass fuel is burned again at the first temperature, and the retention time of the ash generated by the combustion of the biomass fuel at the first temperature is reduced by t ₃ minutes from the previous retention time, and the confirmation is performed again. This step is repeated until the retention time T _A1 (min) at which the silica is in an amorphous state is found, and a retention time T _A1 and a retention time T _C1 (min) represented by T _A1 +t ₃ and at which the silica is in a crystalline state are found.
Step 3: Burning the biomass fuel at a second temperature lower than the first temperature within a combustion temperature range of 600° C. to 1150° C., and holding the ash generated by the combustion of the biomass fuel at the second temperature for _t4 minutes, and then confirming whether the silica contained in the ash is in an amorphous state;
Step 4: If it is confirmed that the silica is in an amorphous state as a result of step 3, burn the biomass fuel again at the second temperature, and confirm again with the holding time of the ash generated by the combustion of the biomass fuel at the second temperature increased by t ₅ minutes from the previous holding time, and repeat this step until a holding time T _C2 (min) at which the silica is in a crystalline state is found, and find a holding time T _C2 and a T _A2 (min) represented by T _C2 -t ₅ and at which the silica is in an amorphous state. If it is confirmed that the silica is in a crystalline state as a result of step 3, burn the biomass fuel again at the second temperature, and confirm again with the holding time of the ash generated by the combustion of the biomass fuel at the second temperature decreased by t ₆ minutes from the previous holding time, and repeat this step until a holding time T _A2 (min) at which the silica is in an amorphous state is found, and find a holding time T _A2 and a holding time T _A2 +t ₆ , and T _C2 (min) at which the silica is in a crystalline state;
Step 5: In a graph with temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, T _A1 , T _C1 , T _A2 , and T _C2 are plotted, and T _A1 and T _A2 are connected with straight lines, and T _C1 and T _C2 are connected with straight lines;
Step 6: In a graph having temperature (°C) on the horizontal axis and retention time (min) on the vertical axis, an approximation function T _A is calculated based on at least retention times T _A1 and T _A2 , and an approximation function T _C is calculated based on at least retention times T _C1 and T _C2 , and a lower region of the approximation function T _A is defined as the amorphous region, and an upper region of the approximation function T _C is defined as the crystalline region, in the combustion device. 10. The combustion apparatus of claim 9 , wherein the biomass fuel is cereal husks.

Images