Methane gas generator

Abstract

To provide a methane gas generator that can reduce power consumption as much as possible and utilizes recovered high-concentration methane as biofuel, thereby contributing to the prevention of global warming.SOLUTION: A methane gas generator comprises: a system for sorting and collecting food waste and plastics unsuitable for fermentation; a fermentation tank that uses food waste as a main raw material, adds sewage sludge and human waste to shorten the fermentation period, adds water as needed to form a liquid mixture, pressurizes this mixture to 5 to 9 atmospheres (preferably 7 to 9 atmospheres) using a water pump, and then performs methane fermentation; followed by a pre-treatment unit that supplies the mixture, after fermenting the same at 30 to 55°C for 15 to 20 days, to a gas separation membrane for pre-treatment such as drain removal, dust removal, and heating; and the gas separation membrane that purifies this mixture to supply high-concentration methane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a methane gas generator that uses food waste as a raw material.

[0002] According to Patent Document 1, the recovered high-concentration and low-concentration methane is used as fuel for the boiler and becomes a heat source for the methane fermentation tank, resulting in a problem of internal consumption. Furthermore, according to Patent Document 2, most of the recovered methane gas is used as fuel for the gas engine, but the power generation efficiency is low at 25%, and the heat generated by the boiler fuel and the heat of the gas engine cooling water are also used to heat the methane fermentation tank. Methane fermentation tanks have a long fermentation period of 12 to 18 days, a huge volume and surface area, and high internal tank temperatures (32 to 38°C), so a large amount of heat is dissipated between the tank and the outside air, resulting in a problem of internal consumption of most of the thermal energy of the recovered methane. [Prior art documents]

[0003] [Patent Document 1] Patent Publication No. 2025-085017 [Patent Document 2] Problems that Patent 3651836 aims to solve

[0004] According to Patent Document 1, methane gas can be purified without using a significant amount of power by pressurizing aqueous raw materials with a water supply pump and supplying them to a fermentation tank, pretreating the generated gas, and then purifying it with a gas separation membrane.However, if plastics, clothing, etc. that are not suitable for fermentation are mixed in during the food waste sorting stage, the crusher and sorting machines will be large and will require a lot of power.Furthermore, there is the problem that the methane that has been recovered and purified will be consumed in-house to heat the methane fermentation tank.

[0005] In Patent Document 2, the power generation efficiency is low at 25%, and the heating capacity of the methane fermentation tank is large, and the large amount of energy required to heat it is internally consumed but is not treated as a cost. Summary of the Invention

[0006] In this invention, the generated mixed gas is not compressed, but rather the mixed gas generated from the pressurized liquid is pretreated and purified using a gas separation membrane before use, thereby eliminating the need for electricity for the gas compressor; if the methane concentration or recovery rate is insufficient, part of the permeable gas is compressed using a gas compressor to be used as raw material gas, reducing power consumption, and the generated mixed gas is passed through the gas separation membrane and used as heating fuel, but the insufficient heat does not affect or cause global warming; and wood fuel, which has low fuel costs, is used as a fuel additive for the boiler, and the fermentation tank is heated with the heat of the generated steam hot water, thereby reducing costs. [Means for solving the problem]

[0007] The first problem to be solved by this invention is to use a garbage collection truck with a collection section divided into two sections, left and right or front and rear, to collect food waste. As shown in section C of Figure 1, the fermentable food waste is collected separately from non-fermentable waste, such as plastics and clothing. The raw material is then liquefied, pressurized with a water pump, and supplied to a fermentation tank. The resulting mixed gas, primarily composed of methane and carbon dioxide, is maintained under pressure. This gas pressure is then supplied to a gas separation membrane for gas separation, eliminating the need for a gas compressor to compress the mixed gas. The problem with gas compression is that in the case of methane fermentation, 1 cubic meter of liquid raw material expands to 100–150 cubic meters of mixed gas at atmospheric pressure. Because the raw material is an incompressible liquid, pressurization is achieved by a water pump, and the mixed gas by a gas compressor. Therefore, the power required for each pressurization can be calculated using physical and mechanical laws and methods. Assuming that the mechanical efficiencies of each are 80% and 70%, respectively, and 150m3 of mixed gas is produced from 1m3 of raw material, the power required for each will be 0.24 and 15kW, respectively. By pressurizing the raw material with a water pump and utilizing the pressure of the fermentation gas, it is possible to raise the pressure required for methane with little power. Furthermore, the hydrocarbon gas separation membrane used in this invention is modularized as shown in Figure 2, has no moving parts, and has been proven to last for more than 10 years in gas separation (methane / carbon dioxide) using city gas, a similar substitute for natural gas, provided there are no problems with pretreatment. This means that there is almost no maintenance required, labor savings, and cost reductions for the plant.

[0008] Furthermore, the permeable gas from the gas separation membrane can be effectively used as a fuel additive for the boiler of the fermenter, and the fermenter can be heated with the generated steam or hot water. If the insufficient heat is supplied by the steam or hot water from a boiler that uses wood chips or pellets as fuel, which do not cause global warming, the fuel cost can be reduced to about one-fourth of that of city gas 13A on a calorific value basis. This contributes to cost reduction and prevention of global warming, and allows methane fermentation to continue. [Effects of the Invention]

[0009] While global warming has become a serious problem in recent years, the majority of waste disposal plants have not implemented concrete measures to prevent global warming. This invention can contribute to preventing global warming by reducing power consumption as much as possible and utilizing the recovered high-concentration methane as biofuel. [Brief explanation of the drawings]

[0010] [Figure 1] shows the entire methane fermentation equipment. It shows the overall process and flow sheet of the methane fermentation equipment, including the separation and collection of food waste, processing, methane fermentation, exhaust gas treatment and gas purification by membrane separation, boiler combustion, and heating of the fermentation tank. Parts A and B in the figure are the gas separation section, and by adding a smaller capacity gas compressor 35 to Figure B compared to Figure A, a higher methane concentration and recovery rate can be achieved compared to Figure A. Garbage collection trucks 6-1, 2, and 3 are the same vehicle, collecting 6-1 and 6-2, delivering 6-3, and then loading unsuitable materials for fermentation onto the truck, which then heads to the existing waste treatment plant.

[0011] [Figure 2] shows a module diagram of a gas separation membrane. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0012] The actual configuration of this equipment is explained based on Figure 1. Raw material 1 is a constant supply of sewage sludge 1, while human waste 2 is intermittently supplied by a vacuum truck 3 and stored in temporary tank 4. The main raw material, food waste 5-2, is collected by a garbage truck 6-1, whose loading platform is divided into front and rear or left and right. Food waste suitable for fermentation and separated waste such as plastic and clothing 5-1 are collected and stored in food waste storage tank 7. After being finely crushed by crusher 8, the waste is separated by sorter 9 into food waste suitable for fermentation and waste unsuitable for fermentation. Since food waste accounts for 30-50% of the total waste, the reduction in volume and the lack of hard waste drastically reduce the load on crusher 8 and sorter 9 compared to when separate collection is not performed. The fermented material is supplied to secondary mixer 11 by conveyor 10, where it is mixed with the sewage sludge 1 and water is added if there is a moisture shortage. After thorough mixing, the water passes through filter 12, is pressurized by feedwater pump 13, and is filled into methane fermentation tank 14. Fermentation tank 14 is insulated with insulation 16 to prevent heat radiation from fermentation liquid 15. After fermentation, an appropriate amount of residue 20 is discharged from the bottom of fermentation tank 14. Meters 17 are attached to measure the temperature and pressure of the fermentation liquid and fermentation gas, as well as the temperature and pressure of the liquid, the temperature and pressure of the fermentation liquid, the methane concentration of the fermentation gas, the liquid level, pH, and ammonia in the liquid fermentation gas. Steam or hot water 32 supplied from boiler 31, described below, is supplied to heater 18 to maintain the heating of fermentation liquid 15. Methane fermentation generates mixed gas 21 containing carbon dioxide, sulfur dioxide, moisture, etc. in addition to methane.

[0013] The feedstock is primarily food waste 5. Adding a certain amount of the aforementioned sewage sludge 1 and human waste 2 provides nutrients for the fermentation bacteria, shortening the fermentation period and resulting in economic benefits. However, hydrogen sulfide is generated, reaching concentrations of several thousand ppm. This is absorbed and neutralized by alkaline solutions 23-1 and 23-2 in the desulfurizer 22, and the dissolved material is discharged from the system. The mixed gas 21 is then separated and discharged from the mist separator 22. After the mist is separated and discharged, the mixed gas 21 is heated to a temperature suitable for gas separation in a heater 25, dried, and dust-removed in a filter 26 to form treated gas 27, which is then supplied to a membrane separator 28. The non-permeated gas 29, primarily methane gas, is then supplied to a combustor 33 in a boiler 31. The permeated gas, primarily carbon dioxide, low-concentration permeated methane, and moisture, is then supplied to the combustor 33 of the boiler 31. The main fuel 34 for the boiler 31 is wood pellets and wood chips, which are supplied to the combustion device 33, and the methane component contained in the permeable gas 30 acts as a fuel, while the generated steam or hot water 32 heats the fermentation liquid 15 to sustain methane fermentation. High-concentration methane gas 29 is mainly sold as biofuel. [Example]

[0014] (Part 1: Gas separation membrane structure) Figure 2 shows gas separation membranes 28 (A) and 28 and 36 (B) in Figure 1, which have the same performance and are called a membrane module. This will be explained using gas separation membrane 28. Main bodies 28-2, 28-3, and 28-4 are housed in case 28-1. Treated gas 27, processed in the membrane pretreatment device, is supplied to the inside of hollow fibers 28-2, each with a diameter of 150 to 200 μm. Hollow fibers 28-2 are bonded at both ends with epoxy resin 28-3. The pressure difference between feed gas 27 and permeate gas 30 is supported by a core 28-4, both ends are sealed, and the module is independent of case 28-1 and can be replaced independently. This module is called a "membrane module." Permeate gas 30, which permeates hollow fibers 28-2, also contains low concentrations of methane and becomes permeate gas 30 at the outlet.

[0015] (Part 2: Overview of gas separation using gas separation membranes) Hollow fibers 28-1 are hollow fibers 28-2 with an outer diameter of 150 to 300 microns, both ends of which are bonded to epoxy 28-3, and are evenly packed at a packing ratio of approximately 40%. Treated gas 27 is supplied to the interior and first dissolves on the membrane surface. Depending on the partial pressure difference between the supply and permeate sides and the specific permeability (Nm3 / m2 / Hr / bar) of each gas separation membrane, individual gas components permeate at their specific speeds and partial pressure differences, resulting in permeate gas separation 30. The order of permeation speed is water vapor, carbon dioxide, and methane. In particular, water vapor and carbon dioxide permeate quickly, while methane, which permeates slowly, remains and is separated. Mainly water vapor and carbon dioxide permeate quickly, while the slower methane remains and becomes non-permeate gas 29. This gas separation using a gas separation membrane is highly reliable and requires little maintenance, unlike other gas separation methods, as there are no moving parts. The ratio of these permeation rates is called the separation factor. While a larger separation factor is desirable for separating carbon dioxide and methane, its value is currently roughly 20-30, resulting in a small amount of methane flowing downstream in the permeate gas 30. Figure 1 shows the gas flow in the membrane separation section in Diagrams A and B. In Diagram B, all or a portion of the permeate gas from membrane 28 is compressed by gas compressor 35 and supplied to membrane 36. This increases the gas flow rate and methane concentration of treated gas 27 to separation membrane 28. As a result, the feed gas flow rate and methane concentration are higher than in Diagram A. While this process allows for a higher methane recovery rate and methane concentration than in Diagram A, it requires more power to operate gas compressor 35. However, the gas flow rate is only a fraction of the flow rate required to compress the entire treated gas 27 generated from the feedstock, resulting in less power consumption. [Explanation of symbols]

[0016] 1. Sewage sludge 2 Human urine 3 Vacuum Truck 4 Temporary storage tank 5-1 Sorted garbage collection (garbage collection on-site) 5-2 Sorted food waste (inside the methane fermentation device) 6-1 Garbage collection vehicle (separate collection of food waste and non-fermented waste, on-site) 6-2 Garbage collection vehicle (food waste supply) 6-3 Garbage collection truck (non-fermentable materials such as plastics and cloth mixed in during sorting collection) 7. Food waste storage tank 8 Crusher 9. Sorting machine 10 Conveyor Belt 11 Extra mixer 12 Filter 13 Water supply pump 14 Methane fermentation tank 15 Fermentation liquid 16 Heat insulation material 17 Instruments 18 Fermenter heater 19 Stirrer 20 Residue 21 Mixed Gases 22 Desulfurizer 23-1 Absorbent liquid (inlet) 23-2 Absorbent (outlet) 24 Mist separator 25 Gas heater 26 Filter 27 Processed gas 28 Membrane separator 1 (The structure of membrane separator 2 of 36 is the same as 28.) 28-1 Outer cylinder 28-2 Hollow fiber 28-3 Epoxy resin 28-4 Mandrel 29 Non-permeable gas 30 Permeable gases (mainly carbon dioxide and water vapor) 31 Boiler 32 Steam or hot water for heating fermentation tank 33 Combustion Box 34 Boiler fuel (wood chips or wood pellets) 35 Gas compressor for boosting pressure of membrane separator 2 36 Membrane separator 2 37 Recycle gas from membrane separator 2 38 Confluence of treated gas 27 and recycled gas 37

Claims

1. The methane gas generator comprises a system for separating and collecting food waste from plastics and other materials unsuitable for fermentation, a fermenter that uses food waste as the main raw material, adds sewage sludge and human waste to shorten the fermentation period, and adds water as needed to pressurize the aqueous mixture to 5 to 9 atmospheres, preferably 7 to 9 atmospheres, using a water pump to perform methane fermentation, a pre-treatment unit that ferments the mixture at 30 to 55°C for 15 to 20 days and supplies the generated mixed gas to a gas separation membrane that undergoes pre-treatment such as drain removal, dust removal, and heating, and a gas separation membrane that purifies the mixed gas and supplies high-concentration methane.

2. A methane gas generator characterized in that, when a sufficient concentration and recovery rate of methane gas cannot be ensured using only the gas separation membrane of claim 1, a second separation membrane is provided which uses a gas compressor to pressurize all or part of the gas permeating through the separation membrane and return it to the raw material line.

3. A methane gas generator characterized in that the low concentration methane contained in the permeation gas of the gas separation membrane of claims 1 and 2 is supplied as a combustion aid to a wood biomass boiler that does not contribute to global warming, and the heat of the generated steam or hot water is used to heat a fermentation tank to continue methane fermentation.

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