Method for utilization of low-concentration gas mixtures of combustible gas and air with stable heat energy recovery

Abstract

The invention refers to the method for the utilization of low-concentration mixtures of a combustible gas and air with the stable recovery of heat and the flow-reversal device for the embodiment of the method. The method consists in the combustion, with heat recovery, of the mixtures in the flow-reversal device having at least a single pair of combustion sections, each of which has the structural packing of monolith blocks with small channels characterized by low pressure drop, provided with an internal heating device, temperature and composition sensors and the elements of the automatic control system, supplied with the low-concentration mixture with the combustible component and connected with the heat recovery apparatus through the pipeline, wherein the quantity of energy transferred in the heat recovery apparatus (22) is stabilized by supplying additional fuel to the flow-reversal device, selecting the flow reversal moment, and selecting the flow rate for hot gas supplied by the pipeline to the heat recovery apparatus (22). Additional fuel in the form of highly concentrated fuel mixture is introduced as an admixture to the stream of low concentrated mixture containing the combustible component, supplied to the flow-reversal device or to the internal heating device (7). The device according to the invention, in its combustion sections (I, II) is provided with symmetrical temperature sensors (T.sub.I, T.sub.II) and an additional supply of highly concentrated combustible mixture (17) connected to the supply system for low-concentration mixture (15) with the combustible component or to the internal heating device (7). The combustion sections (I, II) are packed with heat-accumulating material (1,2) of small porosity of the specific surface area below 30 m.sup.2/g, and advantageously below 1 m.sup.2/g.

Claims

1. A method for utilization of low-concentration mixtures of combustible gas and air with a stable recovery of heat energy, the method comprising: providing a heat recovery apparatus and a flow reversal device that includes a pair of combustion sections that each have therein a structural packing of monolith blocks having small low flow resistance ducts, the pair of combustion sections having a first combustion section and a second combustion section, the flow reversal device further having temperature sensors, composition sensors, an internal heating device and an automatic control device, the temperature sensors being in the pair of combustion sections and are symmetrical temperature sensors, the flow reversal device being connected by a pipeline to the heat recovery apparatus; supplying the low-concentration mixtures to the flow reversal device; flowing a fluid, including the low-concentration mixtures, in the flow reversal device; combusting, with heat regeneration, the low-concentration mixtures in the flow reversal device; and stabilizing a quantity of energy given off in the heat recovery apparatus by the following steps: a. supplying additional fuel to the flow reversal device, b. selecting a flow reversal moment when the flow is reversed in the flow reversal device, a moment of the reversing the flow is selected in such a way that switching a flow direction of the flow in the reverse flow device is realized in the following way: in a constant switching half-cycle, reversal in the flow direction is made at equal time intervals, if an absolute difference between a temperature measured in the second combustion section at a selected distance from an outlet of the second combustion section and a temperature measured in the first combustion section at the same distance from an inlet to the first combustion section |T.sub.IIT.sub.I| does not exceed or does not reach a preset positive value of T.sub.zad,1, or if the first combustion section is an inlet combustion section that receives the low-concentration mixtures before the second combustion section, a reversal in the flow direction is made at a time when a temperature difference (T.sub.IIT.sub.I) between the temperature in the second combustion section and the temperature in the first combustion section reaches the preset positive value T.sub.zad,1, or if the second combustion section is the inlet combustion section that receives the low-concentration mixtures before the first combustion section, the flow is reversed once a temperature difference (T.sub.IT.sub.II) between the temperature in the first combustion section and the temperature in the second combustion section reaches the preset positive value T.sub.zad,1, and c. selecting a flow rate for gas, discharged from the flow reversal device, supplied by the pipeline to the heat recovery apparatus.

2. The method of claim 1, wherein the additional fuel is in the form of a highly concentrated fuel mixture, and said step a of supplying the additional fuel includes introducing the highly concentrated fuel mixture as an admixture to the low-concentration mixtures while the step of supplying the low-concentration mixtures to the flow reversal device occurs.

3. The method of claim 2, wherein said step a of supplying the additional fuel further comprises adjusting a flow rate of the highly concentrated fuel mixture, the flow rate of the highly concentrated fuel mixture being adjusted manually or automatically by a valve, depending on a value of a signal that indicates information on a current heat stream transfer in the heat recovery apparatus.

4. The method of claim 1, further comprising: selecting a flow rate of the low-concentration mixtures in the flow reversal device; and selecting a duration of a reversal half-cycle of the flow reversal device in a way so that in the fluid, which is flowing between the pair of combustion sections, in each half-cycle and in a stable period of device operation, a conversion of combustible components is at least over 70%, a concentration of carbon monoxide in the gas discharge from the flow reversal device is below 5 ppm.

5. The method of claim 1, wherein the fluid, which is flowing between the pair of combustion sections, is distributed in such a way in a space between the pair of combustion sections so that the heat recovery apparatus receives through a passage, no more than 50% of the fluid and a remaining part of the fluid flows from one of the pair of combustion sections to the other of the pair of combustion sections.

6. The method of claim 1, further comprising sending the low-concentration mixtures through the pair of combustion sections, wherein the pair of combustion sections have heat-accumulating material that includes porosity of a surface area that is below 30 m.sup.2/g.

7. The method of claim 1, further comprising selecting a duration of half-cycles of the flow reversal device in such a way that temperature fluctuations in a space between the pair of combustion sections is in a range from 750 to 1100 C.

8. The method of claim 1, wherein if the first combustion section is an inlet combustion section that receives the low-concentration mixtures before the second combustion section, the flow direction is reversed once a temperature of the second combustion section reaches a positive value (T.sub.zad) that is preset by a process operator, or if the second combustion section is the inlet combustion section that receives the low-concentration mixtures before the first combustion section, the flow direction of the flow is reversed once a temperature of the first combustion section reaches the preset positive value (T.sub.zad).

9. The method of claim 1, wherein in case of asymmetry in temperature profiles of the pair of combustion sections, which is indicated by the absolute temperature difference |T.sub.IIT.sub.I| between the temperature of the first combustion section and the temperature of the second combustion section exceeding a preset positive value of T.sub.zad,2, where T.sub.zad,2>T.sub.zad,1, a duration of a half-cycle in which fluid from one of the pair of combustion sections with higher average temperatures than the other of the pair of combustion sections flows into the other combustion section with lower average temperatures is extended, and a duration of a half-cycle in which the fluid flowing out of the other combustion section to the one combustion section is shortened.

10. The method of claim 9, further comprising signaling with an alarm a flow direction reversal that takes place irrespective of the temperature values (T.sub.I and T.sub.II) and their absolute difference in case a duration of a current half-cycle t.sub.c exceeds an allowable t.sub.c,max value.

11. The method of claim 10 wherein a duration of subsequent half-cycles in the reverse flow device is controlled remotely in a manual mode depending on a decision of the process operator, or automatically.

12. The method of claim 1, further comprising sending the low-concentration mixtures through the of combustion sections, wherein the pair of combustion sections have heat-accumulating material that includes porosity of a surface area that is below 1 m.sup.2/g.

13. The method of claim 4, wherein at the ends of the monolith blocks at the inlets of the pair of combustion sections, in each half-cycle and in the stable period of device operation a conversion of combustible components is over 95%.

14. A method for utilization of low-concentration mixtures of combustible gas and air with a stable recovery of heat energy, in a heat recovery apparatus and a thermal flow reversal device that includes a pair of combustion sections that each have therein a structural packing and the pair of combustion sections have a symmetrical pair of temperature sensors T.sub.I and T.sub.II, the method comprising: selecting a flow reversal moment when a flow is reversed in the flow reversal device in such a way that the switching a flow direction in the reverse flow device is realized in the following way: in a constant switching half-cycle, at regular time intervals, if an absolute difference between a temperature measured in a second of the combustion sections at a selected distance from an outlet of the second combustion section and a temperature measured in a first of the combustion sections at the same distance from an inlet to the first combustion section I |T.sub.IIT.sub.I| does not exceed or does not reach a preset positive value of T.sub.zad,1, or if the first combustion section I is an inlet combustion section that receives the low-concentration mixtures before the second combustion section, a reversal in the flow direction is made at a time when a temperature difference (T.sub.IIT.sub.I) between the temperature of the second combustion section and the temperature in the first combustion section reaches the preset positive value T.sub.zad,1, or if the second combustion section II is the inlet combustion section that receives the low-concentration mixtures before the first combustion section, the flow is reversed once a temperature difference (T.sub.IT.sub.II) between the temperature of the first combustion section and the temperature of the second combustion section reaches the preset positive value T.sub.zad,1.

15. A method for utilization of low-concentration mixtures of combustible gas and air with a stable recovery of heat energy, the method comprising: providing a heat recovery apparatus and a flow reversal device that includes a pair of combustion sections that each have therein a structural packing of monolith blocks having small low flow resistance ducts, the flow reversal device further having temperature sensors, composition sensors, an internal heating device and an automatic control device, the flow reversal device being connected by a pipeline to the heat recovery apparatus; flowing a fluid, including the low-concentration mixtures, in the flow reversal device; combusting, with heat regeneration, the low-concentration mixtures in the flow reversal device by sending the low-concentration mixtures through the combustion sections while limiting adsorption of gas components on a surface of heat-accumulating material of the combustion sections where a possible adsorption of uncombusted components on the surface of the heat accumulating material is limited by a specific surface area of the heat-accumulating material being below 30 m.sup.2/g; and stabilizing a quantity of energy given off in the heat recovery apparatus by the following steps: a. supplying additional fuel to the flow reversal device, b. selecting a flow reversal moment when the flow is reversed in the flow reversal device, and c. selecting a flow rate for gas, discharged from the flow reversal device, supplied by the pipeline to the heat recovery apparatus.

Description

(1) The flow reversal device according to the invention is shown in its embodiments in the drawing, where:

(2) FIG. 1 shows the flow reversal device with two combustion sections located horizontally against each other and with the use of preheating of the packing with electric heaters 7, and

(3) FIG. 2 shows the flow reversal device where both sections are located in a vertical way, with preheating using the gas burner,

(4) FIG. 3 shows the diagram of the representative installation with the flow reversal device being the subject of the invention, and

(5) FIG. 4 shows the profiles of temperature along the packing.

(6) The device according to the invention has the refractory body with external thermal insulation, inside which there are two combustion sections I, II, packed with ceramic blocks of monolith structural packing 1, 2, on the randomly packed bed made of ceramic elements 3, 4 which safeguard the regular distribution of gas in the device. The walls of the flow reversal device are lined with refractory lining 5, and from the outside they are insulated with thermal insulation 6. To initiate the combustion of the combustible component, both sections I, II of the packing are preheated with electric heaters 7 which are shut off once the packing temperature reaches the level enabling the ignition of the mixture with the combustible component. Alternatively, instead of electric heaters 7 gas or oil burners can be used. Heaters in the form of burners can also be used in the situations when the content of the combustible component in the stream fed to the device is too low to meet the requirements of the consumer of the recovered energy, or if due to the sharp decrease in the concentration of the combustible component in the supply stream there is a risk of the shut-down of the device according to the invention.

(7) The device operates with the flow direction changed periodically. If the flow is realized in such a way that the medium flows first to section I, and then to Section II, Section I is the inlet section, and Section II is the outlet section. For opposite direction (first Section II and then Section I) Section II is the inlet section and Section I is the outlet section.

(8) In the flow reversal device shown in FIG. 1 the mixture with the combustible component is fed to the device by the reverse valve 11 through the inlet pipe 8, if the stem of the reverse valves 13 is in the border left side position and then the main outlet of the mixture is through the pipe 9, and the mixture flows out through the right chamber of the valve 12. After some time, referred to as the reversal half-cycle, the stem of the valves 13 is switched to the opposite position and the mixture flows through the left chamber of the valve 12 and flows into the device through the inlet pipe 9 and then the main outlet is the pipe 8 and the left chamber of the valve 11.

(9) A version of the device is the structure shown in FIG. 2. In the solution shown in FIG. 2, the mixture with the combustible component is fed to the device through the pipe 8, if the valves 11 and 12 are open, and the valves 13 and 14 are closed. Then Section I (packing 1 and bed 3) is the inlet section, and Section II (packing 2 and bed 4) is the outlet section. In the opposite half-cycle of reversal, the mixture is fed through the pipe 9 as the valves 13 and 14 are open and the valves 11 and 12 are closed, and then Section II is the inlet section and Section I is the outlet one.

(10) In both versions of the design of the flow reversal device, shown in FIG. 1 and FIG. 2, for operation with heat recovery in both half-cycles of reversal part of the mixture leaves the device through the outlet pipe 10 and this part is directed to the heat recovery apparatus 22, such as a steam boiler.

(11) In the diagram of the installation with the device according to the invention shown in FIG. 3 the air with the low-concentration fuel mixture is supplied by the conduit 15, where, through the valve 16 and conduit 17, highly concentrated additional combustible component is fed. After mixing, the fan 19 pumps the mixture through the conduit 21 to the reverse valve 11 or 12 depending on the current reversal half-cycle. Part of hot gas collected between sections 1 and 2 is directed to the heat recovery apparatus 22, usually a steam boiler where it is cooled down, most often to approx. 200 C. and is directed to the atmosphere through the stack 23. The remaining part of gas flows through the next section of the device and depending on the current half-cycle of reversal through the flow reversal 12 or 11 is directed to the stack 23, and then to the atmosphere. The flow rate of gas directed to the heat recovery apparatus 22 is controlled by the throttle valve 25. As illustrated in FIG. 3, the flow reversal device further has temperature sensors 27, 28.

(12) The stream of gas collected by the pipeline 10 should be such that only little part of heat generated in the combustion process is directed to the stack with gas flowing through the pipeline 26. For this reason the flow reversal device according to the invention, if the heat recovery apparatus 22 collects heat, should operate all the time close to the extinguishing threshold, and its symptom is that in longer and stable periods of equipment operation the average temperature of gas in the pipeline 26 is only slightly higher than the average temperature of gas in the pipeline 21. However, when the addition of the highly concentrated fuel mixture leads to the stabilization of the heat recovered in the apparatus 22, then the flow collected by the pipeline 10 will be more or less constant if only the fluctuations in the temperature of gas taken by the pipeline 10 have more or less constant average value. It is possible to adjust this flow remotely or on site, not automatically but manually. The location of the throttle valve for the adjustment of the flow may be either in front of or behind the heat recovery apparatus. Due to the temperature, in which the throttle valve operates, it is more favorable to put it behind the apparatus 22, as shown in FIG. 3.

(13) The method for utilization of low-concentration mixtures of combustible component and air with the stable heat recovery according to the invention can be for example realized fully or partly automatically through the use of the controller 24.

(14) Stabilization of the quantity of energy recovered in apparatus 22 (e.g. steam boiler) can be obtained with the signal from the controller 24 in two ways: adding highly concentrated fuel mixture fed with the pipeline 17 to the gas flowing through the conduit 18 in the mixer, e.g. for VAM combustion-methane mixture obtained during demethanation of the coal seam, so that regulation with the valve 16 of the concentration of fuel fed to the device through the pipeline 21 stabilizes the generated combustion energy and the quantity of energy collected from the device in the heat recovery apparatus 22. Alternatively, a similar quantity of fuel can also be supplied directly to the burner 7 shown in FIG. 2, which in such a solution would serve not only for preheating of the bed but also for stabilization of the quantity of energy collected from the device. The quantity of energy transported to the apparatus 22 can be approximated as the product of flow rate of the medium transported by the pipeline 10 and its temperature, given that the temperature of gas after the apparatus 22 is more less constant. More accurate methods for determining the quantity of heat utilized in the apparatus 22 can also be used.

(15) When highly concentrated fuel mixture is fed to the mixer 18 by the valve 16, the concentration of the combustible component fed to the flow reversal device with the pipeline 21 is controlled by the analyzer or the fuel concentration sensor 20 provided with an alarm function. The alarm threshold is set to keep the concentration of the mixture fed to the device suitably below the preset mixture explosion threshold. When the threshold is exceeded, a risk of emergency occurs and therefore once the alarm system is activated, the valve supplying fuel through the pipeline 17 to the mixer is closed. The supply of highly concentrated fuel can be closed with the valve 16 or with another cut-off valve. In such situations, after the alarm and fuel cut off, one should switch to manual control, which consists mainly in the reduction of the preset heat transfer value in the apparatus 22 and reduction in the discharge of gases through the pipeline 10, controlled by the throttle valve 25. After adjustment of the discharge it is possible to restore the supply of highly concentrated fuel through the pipeline 17 to the mixer 18 and then to return to the automatic control mode. It is favorable to use a warning alarm after the preset alarm threshold is exceeded, which is a bit lower than the alarm causing the cut-off of the fuel supply of highly concentrated fuel to the mixer.

(16) In both alternatives of the flow reversal device, both from FIG. 1 and FIG. 2, at the same distances from the inlet to the inlet section of the packing 1, 2 and outlet from its outlet Section there are temperature sensors T.sub.I and T.sub.II, the readings of which are used for the selection of the moment when to reverse the flow.

(17) In known flow reversal devices, control of the reversal system is effected by the presetting of the constant value of the half-cycle duration, or by switching after the preset value of temperature differences (T.sub.IIT.sub.I) or (T.sub.IT.sub.II) is exceeded depending on the current flow direction. Both control methods do not make it possible to sufficiently avoid the asymmetric operation of the device, and hence to meet the requirement of the stable heat recovery for the purpose of its utilization.

(18) The moment of switching the flow direction, that is the reversal in the method according to the invention, is selected by using the information on the selected temperature values in the flow reversal device according to the invention, or on the values of the differences between the temperatures and the knowledge of the current direction of the flow of the gas mixture through the device:

(19) Example: the control system 24 in the automatic control mode selects the moment of the flow direction change in such a way that the change in the direction of flow through the flow reversal device is made in the constant switching half-cycle (at equal time intervals) if the absolute difference between the temperature measured in Section II at the selected distance from the outlet from the section and the temperature measured at the same distance from the inlet to Section I |T.sub.IIT.sub.I| does not exceed the preset value T.sub.zad,1, or the switching is made once the difference in temperatures (T.sub.IIT.sub.I) between the selected temperature in Section II and selected temperature in Section I exceeds the preset positive value T.sub.zad,1, if Section I is the inlet section or once the difference in temperatures (T.sub.IT.sub.II) reaches the preset positive value T.sub.zad,1, if Section II is the inlet section. Consequently, in the periods when the heating profile of Sections I and II is more less symmetric, the duration of both half-cycles is equal or approximately equal.

(20) If for any reason the temperature profiles of both beds become significantly asymmetric, which is indicated by the absolute temperature difference |T.sub.IIT.sub.I| exceeding the preset positive value of T.sub.zad,2, where T.sub.zad,2>T.sub.zad,1, the control system 24, in the automatic control mode extends the half-cycle duration, where the fluid from the Section of higher temperatures on average flows into the Section with temperatures lower on average, and shortens the half-cycle duration where the fluid flowing from the section of the temperatures lower on average flows into the Section of the temperatures higher on average and thus facilitates the restoration of the symmetric temperature profiles in the reactor.

(21) The control system 24 may also facilitate the restoration of the symmetric temperature profiles in the device by remote manual control, with setting up the predefined durations of the half-cycles, different for each direction of flow through the device.

(22) When, due to different disturbances, in the automatic control mode, e.g. when the control is in based on the difference of temperatures T.sub.I and T.sub.II, the duration of flow in one direction is excessively long which usually leads to the formation of asymmetric temperature profiles, then it is necessary to reverse after some maximum duration of a single half-cycle t.sub.c,max while flowing in one direction is exceeded, wherein said maximum value is determined by experiments for a given object. So if in the automatic control mode the duration of the current half-cycle t.sub.c exceeds the allowable duration t.sub.c,max, that is (t.sub.c>t.sub.c,max), then the control system 24 reverses the flow, irrespective of the temperature values T.sub.I and T.sub.II.

(23) TABLE-US-00001 TABLE 1 Research and demonstration plant (results for the flow rate of around 400 m.sup.3.sub.STP/h) Discharge of Heat recovery CH.sub.4 CH.sub.4 hot gas for Discharge per 100k concentration conversion utilization temperature m.sup.3.sub.STP/h % vol. % % C. MW.sub.t 0.1 reactor extinguishes 0.22 87 0 0 0.35 85 0 0 0.42 90 2.3 863 0.6 0.75 96 9.9 905 2.8 1.0 97 17.4 950 5.3

(24) The method according to the invention has been realized using the research and demonstration plant of the VAM flow rate of up to about 400 m.sup.3.sub.STP/h. The summary of the experimental results is shown in the Table 1, where heat recovery from the installation has been recalculated for the flow rate of 100 k m.sup.3.sub.STP/h of the VAM feed mixture.

(25) The studies revealed that the reasonable volumes of heat for utilization are obtained for the concentrations of over 0.4% vol. of CH.sub.4 in the stream supplying the TFRR. Therefore, from this perspective, the application of additional fuel mixture in the method according to the invention seems justified in cases when methane concentration in the inlet stream of the device is lower than 0.4% vol.

(26) During the experiments much attention was given to the formation of temperature asymmetries which can occur in the flow reversal device according to the invention. The charts shown in FIG. 4 show the actual examples of symmetric and asymmetric profiles measured during the operation of the flow reversal device in the experimental installation.

(27) The symmetric profile shown in FIG. 4 has been formed during the operation of the device supplied with the mixture of the concentration of 1% vol. of CH.sub.4, without any withdrawal of hot gas for utilization, whereas the profile shown next to it, clearly asymmetric, has been formed during the supply with the mixture of the similar concentration but in the situation when around 15% of the total gas quantity has been discharged by the vent from the connector between the sections of the device.

(28) A suitable operation of the process using the method according to the invention makes it possible to avoid the formation of asymmetry that can have a very unfavorable influence on the stability of heat recovery in the apparatus 22.

(29) The method according to the invention that can be realized in the flow reversal device according to the invention makes it possible to purify ventilation gases from underground mining, and to purify the off-gases produced in oil and coke industries, said gases containing undesirable combustible components, and also makes it possible to produce heat energy in a stable way and deliver it to consumers so that it can be utilized efficiently.