CN101875863B - Waste heat-driven circulative heat-carrying gas methanation technology and device - Google Patents

Abstract

A coal gas methanation technology in which waste heat promotes steam cycle heat transfer. In the range of normal pressure and pressurization of 0.15 to 3.0 MPa, through an evaporation/condensation coupled heat exchanger, the product gas below 250 ° C is used to cool the shell side, The condensed waste heat drives part of the raw material gas to rise in temperature by falling film on the tube side while humidifying to a steam/gas ratio of 3.0 to 4.0. The heat released in the superheater is cooled and mixed with another part of the raw material gas at normal temperature, and the entire reaction is completed in the second-stage methanation adiabatic reactor and the outlet temperature does not exceed 680°C; the high temperature of the product gas is recovered through the high-pressure steam boiler and the superheater After the sensible heat, the low-temperature waste heat below 250°C is recovered through the heat exchanger coupled with evaporation/condensation, and the product gas is cooled by heating up the raw material gas and evaporating circulating water to precipitate condensed water; the condensed water is sent to the evaporation/condensing The tube side of the coupling heat exchanger is mixed with the raw material gas for falling film evaporation to form an evaporation/condensation cycle. The waste heat re-evaporated by the temperature rise can humidify, save power again, increase the by-product power output, and increase the energy utilization rate by at least 6%. The evaporation/condensation coupling heat exchanger is composed of multiple vertical sections. The raw material gas and circulating water descend from the top to the bottom of the tube, and the film evaporates to raise the temperature and increase humidity.

Description

Coal gas methanation technology and device with waste heat driving cycle heat transfer

technical field

The invention belongs to the field of coal-based natural gas production, and in particular relates to a coal gas methanation technology and device.

Background technique

Coal-to-methane (or substituted natural gas, SNG for short) is an important way to convert coal into high calorific value gas fuel, which is beneficial to pit mouth processing, long-distance transportation and on-site capture and emission reduction of CO ₂ .

The key step of coal-to-substitute natural gas is the catalytic methanation reaction of coal gas containing high concentrations of CO and _H2

ΔH ⁰ =-206.4kJ mol ^-1 (CH ₄ )

，From coalto substitute natural gas using TREMP^TM HALDOR 

recycle methanation process.[EB/OL][2009-03-28].http://www.topsoe.com.)。但是循环压缩机消耗动力，相当于产品热值的6％以上，直接降低了该工艺的能量转化效率和经济效益。For coal gas with H ₂ /CO = 3, every time 1% of CO is converted, the gas adiabatic temperature rises by 70-100°C (increases with the increase of reaction depth). In the industry, two processes, the tubular external cooling isothermal catalytic reactor and the inter-stage cooling multi-stage adiabatic catalytic reactor, are used to remove the heat of reaction (Wu Liandi, Chen Xingda, Wang Wenming, etc. The simulation and comparison of two gas methanation reactors, Coal Transformation, 2006, 29(2): 70-75). In the world, multi-stage adiabatic catalytic reactions are mainly used, and a large amount of product gas is circulated through a circulating compressor to dilute the reaction medium, remove the heat of reaction, control the temperature rise of the adiabatic reaction, and maintain the reactor temperature in the range of 250-700°C (Haldor

，From coalto substitute natural gas using TREMP ^TM HALDOR

recycle methanation process.[EB/OL][2009-03-28].http://www.topsoe.com.). However, the power consumed by the circulating compressor is equivalent to more than 6% of the calorific value of the product, which directly reduces the energy conversion efficiency and economic benefits of the process.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a compressor-free waste heat-propelled circulating heat-carrying gas methanation technology, which saves power, increases by-product power output, and improves energy utilization by at least 6%. One of the products of coal gas methanation reaction is water vapor, so the properties of water vapor as heat carrier are similar to those of product gas. In the range of reaction system pressure 0.15-3.0MPa, according to the thermodynamic properties of water vapor partial pressure changing with temperature in the gas-liquid two-phase system, the present invention utilizes low-temperature waste heat to promote water vapor evaporation-condensation cycle instead of mechanical compression cycle, and constitutes The process flow of gas methanation technology in which waste heat promotes circulating heat load is shown in the accompanying drawing.

The waste heat-propelled circulating heat-carrying coal gas methanation technology of the present invention, the process steps include: the coal gas is pretreated by CO conversion, desulfurization, decarburization, etc. to become a coal gas raw material gas with _H2 /CO=3 (molar ratio, the same below), According to the ratio of 1:1 to 1:2, it is divided into two paths and enters the first-stage methanation adiabatic reactor 1 and the second-stage methanation adiabatic reactor 2 respectively. The raw material gas entering the adiabatic reactor 1 of methanation is firstly heated and humidified on the falling film evaporation side of the evaporation/condensation coupled heat exchanger 3, and its heat comes from the cooling of the product gas on the other side of the dividing wall heat exchanger 3, Condensation exothermic. The raw material gas temperature rises from normal temperature to evaporation saturation temperature T1, and the moisture content rises simultaneously to steam/gas=3.0~4.0 (molar ratio, the same below); the product gas temperature drops from heat exchanger inlet temperature T8 to condensation saturation temperature T9, The moisture content drops to less than 0.02 steam/gas, and more than 95% of the contained water vapor is condensed and precipitated, and the condensed water returns to the falling film evaporation side of the heat exchanger, forming a water vapor heat transfer cycle of evaporation-condensation-evaporation. The feed gas with a steam/gas ratio of 3.0-4.0 leaves the heat exchanger 3 and enters the feed gas superheater 6, exchanges heat with the product gas partition wall whose inlet temperature is T7, and the feed gas temperature rises to T2 and enters the first-stage methanation adiabatic reactor 1, The heat-carrying capacity of the raw material gas is sufficient to control the gas outlet temperature not to exceed T3. After leaving the reactor 1, it enters the high-pressure steam superheater 7 to release heat and cool down to T4, which is 350-420°C. Mixing to temperature T5, the temperature of the product gas after the complete methanation reaction in the second-stage methanation adiabatic reactor 2 does not exceed T6, the reaction heat is introduced into the high-pressure boiler 8 and the by-product steam is lowered to T7, which is 320-420 °C, and passed The first-stage methanation raw material gas superheater 6 recovers the high-temperature waste heat of the product gas to lower its temperature to T8, and further recovers the low-temperature waste heat through the evaporation/condensation coupling heat exchanger 3 so that the terminal outlet temperature T9 is 20-30°C higher than the normal-temperature raw material gas inlet temperature The steam by-produced by the high-pressure boiler 8 is heated to 500-520° C. for output in the high-pressure steam superheater 7 after the mist is separated in the high-pressure steam drum 9 . The above steps form a closed cycle system to continuously produce methane gas and by-product high-pressure steam at 8-12MPa and 500-520°C. The time of each step is determined by the capacity of the selected equipment and the actual load.

In the above method, the raw material gas is heated and humidified on the falling film evaporation side of the partitioned wall evaporation/condensation coupling heat exchanger 3, and the raw material gas and circulating water are evenly distributed into the heat exchange tubes from the top of the heat exchanger, forming a vertical drop. The gas-liquid annular two-phase flow accepts the heat from the outside of the tube to heat up and evaporate to increase the steam/gas ratio of the raw material gas; the circulating water is the condensed water separated from the product gas, and the condensed water is collected from the condensed water by the circulating water pump 4 It is transported from the collection tank 5; the flow rate of the raw gas in the heat exchange tube is 2.0-10.0m/s; the ratio of the volume flow rate of the circulating water to the volume flow rate of the raw material gas is 1:20-1:60.

In the above method, the recovery of low-temperature waste heat through the evaporation/condensation coupling heat exchanger 3 means that the product gas with a temperature of T7 enters the shell side of the heat exchanger from the shell at the bottom of the evaporation/condensation coupling heat exchanger 3 . The pipes are baffled upward at an average speed of not less than 5.0m/s, and the heat is transferred to the two-phase flow of raw material gas and circulating water in the pipe through the pipe wall, and the product gas is cooled and condensed water is precipitated; the number of upward baffles is greater than 2. There is a condensed water discharge pipe connected to the low-level condensed water collection tank in each process; the condensed water collection tank has a discharge pipe to quantitatively discharge the water produced in the process.

In the above method, it is said: through the evaporation/condensation coupling heat exchanger 3, the raw material gas temperature rises from normal temperature to T1, T1 does not exceed 250°C, the product gas temperature drops from T8 to T9, T8 is 250-280°C, and T9 is 50 ~100°C; the inlet temperature T2 of the first-stage methanation adiabatic reactor 1 is 270-300°C, and the outlet temperature T3 is not higher than 680°C; the inlet temperature T5 of the second-stage methanation adiabatic reactor 2 is 300-320°C, and the outlet temperature T6 is not higher than higher than 680°C.

In the above method, the evaporation/condensation coupling heat exchanger 3 is composed of multi-stage fixed tube-sheet shell-and-tube heat exchangers, the number of which is at least two, stacked vertically; each fixed-tube-sheet shell-and-tube heat exchanger is Available on sale, the structure of each section is the same as the heat exchange tube specification, number of tubes and pipe layout method, and the length is at least 2m; the sections are sealed and connected by flanges; the centerlines of all heat exchange tubes in each section are arranged concentrically, The gap between the ends of the heat exchange tubes between the sections is 5-10m; the upper and lower ends of the shells of each section have product gas ports with the same structure and size, and the orientation is 180°symmetric. The product gas ports of the two adjacent sections are connected to form a shell-side baffle channel; each section of the shell is close to the lower tube plate to open a condensate discharge outlet, which is connected to the condensate discharge pipe of this section after installation; the uppermost and the lowermost two sections They are all connected to the conical head, and after installation, they are respectively connected to the inlet and outlet pipes of the raw material gas; the inner axis of the upper conical head is fixed at a height of 500-1200mm from the heat exchanger tube plate, and a spiral nozzle is fixed to distribute the circulating water downward evenly. Spray on the tube plate, and the circulating water connecting pipe extends horizontally from the upper conical head to connect with the spiral nozzle.

Description of drawings

Accompanying drawing 1 is the process flow chart of the coal gas methanation technology in which waste heat promotes steam cycle heat carrying according to the present invention.

In the figure, 1-one-stage methanation adiabatic reactor, 2-second-stage methanation adiabatic reactor, 3-evaporation/condensation coupling heat exchanger, 4-circulating water pump, 5-condensed water separator, 6-feed gas superheater , 7-high-pressure steam superheater, 8-high-pressure steam boiler, 9-high-pressure steam drum.

Detailed ways

The specific implementation is described in conjunction with Embodiment 1, and the accompanying drawing is the process flow of this embodiment.

Example 1 Coal gas methanation production device in which waste heat promotes steam cycle heat transfer. The production scale is 4,000 thousand moles of methane gas per hour, the product gas temperature is 70°C, and the moisture content is less than 2% (volume percent, the same below); by-product 10MPa, 510°C superheated steam is about 300 tons/hour. The raw material gas is coal gas with H ₂ /CO=3 16000 kmol/hour, pressure 2.5MPa, temperature 45°C, moisture content less than 1%. The evaporation/condensation coupling heat exchanger 3 used is composed of 4 sections of commercially available fixed tube-sheet shell-and-tube heat exchangers, each section has the same structure and size, and the number of heat exchange tubes is 1520, the length is 4 meters, and the tube The diameter is 25 mm, and the diameter of the shell side of the heat exchanger is 500 mm. The raw material gas enters the first-stage methanation adiabatic reactor 1 and the second-stage methanation adiabatic reactor 2 respectively in two paths at a ratio of 1:1.2. The raw material gas entering the adiabatic reactor 1 of methanation is first connected to the pipe side through the conical head raw material gas inlet on the top of the heat exchanger 3, and the circulating water pump 4 passes the circulating water 120 cubic meters per hour from the condensed water collection tank 5 The connecting pipe is sent to a spiral nozzle fixedly installed at a height of 1000mm from the heat exchanger tube plate on the inner axis of the head, sprays downward and mixes with the raw material gas evenly into the heat exchange tube, and flows vertically downward continuously through the heat exchanger 3 Each section of the tube side, the gas-liquid two-phase temperature rises and evaporates, the gas temperature reaches 230°C, and the steam/gas ratio is 3.8. It leaves the conical head at the bottom of the heat exchanger 3 and enters the feed gas superheater 6. It is heated to 280°C and enters After the first-stage methanation adiabatic reactor 1, the temperature rises to 600°C and leaves the reactor, enters the high-pressure steam superheater 7 to release heat and cools down to 400°C, then mixes with the normal temperature raw material gas entering the reactor 2 to a temperature of 310°C, and enters the second stage The temperature of the product methane gas after the methanation adiabatic reactor 2 completes the entire methanation reaction is about 650°C, the heat is released in the high-pressure boiler 8 to produce about 300 tons/hour of 10MPa steam by-product, the temperature of the product gas drops to about 350°C, and enters the feed gas The superheater 6 continues to cool down to 280°C, then connects from the bottom shell side of the evaporation/condensation coupling heat exchanger 3 and enters the tube of the heat exchanger to baffle for 4 times to cool down and precipitate condensed water, the product gas temperature drops to 70°C, and leaves the heat exchanger. The heat exchanger 3 enters the condensed water separator 5, and the gas-water separation finally achieves that the water content of the product methane gas is less than 2%, and the condensed water is discharged from the shell side of each section of the heat exchanger 3 to the condensed water separator 5 for collection. The heat exchanger 3 sends back 120 cubic meters per hour to re-evaporate, and the process produces about 70 cubic meters per hour of condensed water that is discharged from the bottom of the collection tank 5 out of the system. The boiler replenishment water enters the high-pressure boiler 8 to receive the second-stage methanation adiabatic reaction product gas heat evaporation, and the by-product of 10MPa steam is about 300 tons/hour. The high-pressure steam drum 9 separates the mist and receives the first-stage methanation adiabatic reaction gas in the high-pressure steam superheater 7 The heat superheated to 510°C.

Claims (2)

1. A coal gas methanation process in which waste heat promotes water vapor evaporation-condensation cycle heat-carrying, which is characterized in that there is no compression, and the coal gas raw material gas enters the first-stage methanation adiabatic reactor and the first-stage methanation adiabatic reactor and The second-stage methanation adiabatic reactor, the raw material gas entering the first-stage methanation adiabatic reactor, first evaporates in the tube of the evaporation/condensation coupled heat exchanger to raise the temperature and increase humidity, and the heat comes from the inter-tube of the partition wall heat exchanger The product gas is cooled and condensed to release heat, the temperature of the raw material gas rises from normal temperature to the evaporation saturation temperature of no more than 250°C, the moisture content rises simultaneously to steam/gas = 3.0-4.0 (molar ratio), and the temperature of the product gas decreases from 250-280°C When the condensation saturation temperature is 50-100°C, the moisture content drops to less than 0.02 steam/gas, and more than 95% of the contained water vapor is condensed and precipitated, and the condensed water returns to the falling film evaporation side of the heat exchanger, forming evaporation-condensation-evaporation steam heat transfer cycle; Coal gas with H ₂ /CO=3 (molar ratio) is evaporated and humidified to steam/gas=3.0-4.0 (molar ratio) through falling film heating, and its heat carrying capacity ensures that the gas outlet temperature of the methanation adiabatic reactor does not exceed 680°C; The condensed water cycle is to collect the condensed water precipitated by the product gas cooling in the shell side of each section of the evaporation/condensation coupling heat exchanger to the low-level condensate water collection tank, and send it back to the top of the evaporation/condensation coupling heat exchanger with a circulating water pump, evenly Distributed into the heat exchange tube, mixed with the raw material gas to form a vertically descending gas-liquid annular two-phase flow, heat up and evaporate, and increase the steam/gas ratio of the raw material gas. 2. The coal gas methanation process in which waste heat promotes water vapor evaporation-condensation cycle heat transfer according to claim 1, a multi-stage fixed tube-sheet shell-and-tube heat exchanger combination that realizes evaporation/condensation coupling, is characterized in that the number of stages is at least two Sections, stacked vertically; each section has the same structure, heat exchange tube specifications, number of tubes, and pipe layout method, and the length is at least 2m; the sections are connected with flange seals; the centerlines of all heat exchange tubes in each section are concentric Arrangement, the gap between the ends of the heat exchange tubes between the sections is 5-10m; the upper and lower ends of the shells of each section are provided with product gas ports of the same structure and size, the orientation is 180°symmetric, and the U-shaped joints are used to connect the two adjacent sections The product gas interface is connected to form a shell-side baffle channel; each section of the shell is close to the lower tube plate to open a condensate discharge outlet; the uppermost and the lowermost two sections are connected to the conical head, and the inner axis of the upper conical head is located A spiral nozzle is fixedly installed at a height of 500-1200mm from the heat exchanger tube plate, and the circulating water connecting pipe extends horizontally from the upper conical head to connect with the spiral nozzle.