Novel gasification furnace black water high-grade heat energy recovery process containing liquid-solid separation and scale inhibition treatment

Abstract

The invention discloses a novel gasification furnace black water high-grade heat energy recovery process containing liquid-solid separation and scale inhibition treatment. Aiming at the defects of heat energy grade reduction, equipment abrasion and low-temperature heat discharge caused by a traditional flash evaporation process, black water is pretreated through a liquid-solid separator (S-1800) and a scale inhibitor (T-1800), a multi-stage countercurrent heat exchange technology is combined, a flash evaporation step is omitted, and high-grade heat energy of the black water is used for heating process water and process condensate. The temperature of process condensate is increased to 190 DEG C or above from about 170 DEG C in the traditional process, the temperature of process water (grey water) is increased to 190 DEG C or above from about 140 DEG C in the traditional process, the temperature of coal gas discharged from a washing tower is increased to about 206 DEG C from about 192 DEG C, the water-gas molar ratio in the coal gas is increased to about 0.94 from about 0.58, and the amount of water vapor in saturated coal gas discharged from the tower is increased by 63% or above. According to the process, high-pressure steam does not need to be supplemented in a conversion working section, for example, a synthetic ammonia device with annual output of 180000 tons, 4.2 MPaA and 400 DEG C high-pressure superheated steam is saved and increased by 20.870 thousand tons per year, the energy-saving benefit reaches 31300000 yuan per year, and the energy-saving benefit is in direct proportion to the production scale of the device.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy conservation in coal chemical industry, and particularly relates to a new process for high-grade heat energy recovery of black water in a gasifier with liquid-solid separation and scale inhibition treatment. Aiming at the defects of the traditional process, such as the reduction of heat energy grade caused by flashing, equipment blockage and low-temperature heat discharge, the present invention realizes the efficient recovery and utilization of black water heat energy and the long-term stable operation of the system through the synergistic effect of liquid-solid separation, scale inhibition pretreatment and multi-stage countercurrent heat exchange technology for black water, and is particularly applicable to coal chemical plants such as synthetic ammonia and methanol with coal as raw materials. Background Art

[0002] In the coal-based synthetic ammonia process, the high-temperature black water (pressure 3.5 - 4.0 MPaA, temperature 190 - 210 °C) generated at the bottom of the gasifier (F-1701) and the gas scrubbing tower (C-1701) contains a large amount of fly ash particles (≥2.5 wt%) and scaling ions such as Ca 2+ and Mg 2+ etc. The traditional process (as shown in Figure 1 ) uses two-stage flashing to recover heat energy, and has the following core problems:

[0003] 1. Severe degradation of heat energy grade:

[0004] (1) After the high-temperature black water (190 - 210 °C) undergoes high-pressure flashing (0.6 MPaA) and low-pressure or vacuum flashing (0.08 - 0.12 MPaA), the flashed steam at about 158 °C and 100 °C is used to heat the incoming process water (ash water) in the stripping tower and heat exchanger. Since the black water flashes from 190 - 210 °C to a temperature below 158 °C before heating the process water (ash water), the temperature of the process water (ash water) after heating is very difficult to reach above 150 °C at most.

[0005] (2) The steam flashed out at low pressure or vacuum has low pressure and temperature and cannot be effectively utilized. Some enterprises just vent it. The black water at about 100 °C after vacuum flashing is directly sent to the black water treatment system, resulting in a large amount of heat energy loss.

[0006] 2. Complex process and high operation and maintenance cost:

[0007] The flashing process requires supporting equipment such as flash tanks and stripping towers, and the control instruments, pipelines, and valves are numerous and complex. Moreover, the black water directly drops from about 4.0 MPaA to 0.6 MPaA. Because the black water contains a large amount of solid particles, it causes serious wear to equipment, pipelines, instruments, control valves, etc., and the worn parts need to be frequently replaced, resulting in high equipment investment and operation and maintenance costs, and serious problems such as difficult operation and maintenance.

[0008] 3. Unstable system operation:

[0009] The flash steam in the black water directly enters the heat exchanger with coal ash, resulting in fouling and blockage of the pipe wall, a decrease in heat transfer efficiency, high cost of shutdown cleaning, and affecting the continuous operation of the device.

[0010] 4. The parent case of the present invention proposed to cancel the flash evaporation step and directly exchange heat between the black water and process water, solving the problem of reduction of high-grade heat energy in the black water. This divisional case further provides an optimized process to effectively solve the problems of solid particle wear in the black water and fouling of the black water affecting the heat transfer effect. Summary of the Invention

[0011] I. Key technical problems to be solved

[0012] The key technical problems solved by the present invention include:

[0013] 1. The problems of serious wear and blockage of equipment, pipelines, instruments, control valves, etc. caused by fly ash particles and fouling in the black water, and low heat transfer efficiency of the heat exchanger;

[0014] 2. The problem of degradation of high-grade heat energy in the black water to low-grade heat energy by flash evaporation in the traditional process;

[0015] 3. After vacuum or low-pressure flash evaporation, the black water at about 100 °C is discharged to the black water treatment system, resulting in waste of heat energy resources and environmental protection problems.

[0016] II. Technical solutions

[0017] The innovation of the present invention lies in the "synergy of pretreatment and heat transfer" technical system, specifically including:

[0018] 1. Liquid-solid separation and scale inhibition pretreatment:

[0019] (1) Liquid-solid separator (S-1800): Using hydrocyclone liquid-solid separation technology, the black water discharged from the bottom of the gasifier (F-1701) and the scrubbing tower (C-1701) directly enters the liquid-solid separator (S-1800). After liquid-solid hydrocyclone separation in the liquid-solid separator (S-1800), the removal rate of fly ash particles in the black water coming out from the upper part of the liquid-solid separator (S-1800) reaches more than 98%, and the solid content in the black water is reduced from about 25000 PPm to about 500 PPm, reducing the wear and blockage of particles on pipelines, valves, instruments, and heat exchange equipment from the source. The fly ash particles discharged from the bottom of the liquid-solid separator (S-1800) together with a part of water are sent back to the slag water settling chamber at the bottom of the gasifier (F-1701) by the slag water pump P-1800 for sedimentation separation. After the fly ash particles settle, they enter the slag crusher together with the slag coming down from the upper part of the gasifier (F-1701), and then enter the slag lock hopper and are discharged.

[0020] (2) Black water scale inhibitor (T-1800): The black water coming out from the upper part of the liquid-solid separator (S-1800) enters the heat exchange system after passing through the black water scale inhibitor (T-1800), inhibiting the deposition of scale on the heat exchange tube wall and ensuring that the heat exchange efficiency of the heat exchanger reaches more than 90% of the initial heat exchange efficiency within one year of operation of the heat exchanger.

[0021] 2. Cancel the high- and low-pressure flashing of the black water in the original process, adopt multi-stage countercurrent heat exchange, and make full use of and recover the high-grade heat energy of the black water:

[0022] (1) High-temperature section (207°C → 180°C):

[0023] The black water pretreated by the liquid-solid separator (S-1800) and the scale inhibitor (T-1800) is divided into two streams: one stream enters the heat exchanger group (E-1801) to exchange heat with the process condensate at about 170°C. The heat exchanger group (E-1801) can be composed of multiple heat exchangers, and the number of units needs to be designed and determined according to the production scale and process conditions of the specific device. The process condensate is heated to about 190°C and used as the washing water at the top of the washing tower; the other stream enters the heat exchanger group (E-1802) to exchange heat with the process water (ash water). The process water (ash water) is heated to about 190°C. The heat exchanger group (E-1802) is also composed of multiple heat exchangers, and the number of units also needs to be designed and determined according to the production scale and process conditions of the specific device.

[0024] (2) Medium-temperature to low-temperature section (180°C → 80°C):

[0025] The black water after heat exchange in the high-temperature section converges and enters the heat exchanger group (E-1803) to exchange heat with the process water (ash water) for the second time, and the process water (ash water) is heated to about 170°C.

[0026] (3) Low-temperature waste heat section (80°C → 40°C):

[0027] Some black water treatment systems need to perform operations such as sedimentation and flocculation at about 80°C. The treated water is used as the process water (ash water) and then returns to the washing tower and gasifier system after exchanging heat with the black water. Then the low-temperature waste heat section does not exist for the devices of some enterprises.

[0028] For the device that recycles the ash water obtained after the external treatment of the black water, the black water needs to be further cooled to recover its low-temperature waste heat. It enters the heat exchanger group (E-1804) to exchange heat with the process water (ash water) from outside the plant, and the black water is finally cooled to below 40°C and discharged to the black water treatment system.

[0029] The initial temperature of the process water (ash water) from outside the plant varies depending on different working conditions and seasons, and its initial temperature can vary from 2 to 70°C. After heat exchange in the low-temperature section, its outlet temperature should meet the heat transfer temperature difference with the black water ≥ 10°C.

[0030] 3. Process coupling and heat integration:

[0031] (1) Process water (ash water) at about 190 °C is divided into two streams: one stream is injected into the Venturi humidifier (Z-1702) to mix with the coal gas and increase its humidity; the other stream enters the bottom of the scrubber (C-1701) to mix with the liquid in the tower and raise the temperature of the quench water to above 190 °C.

[0032] (2) Process condensate at about 190 °C is used as the top scrubbing water of the scrubber (C-1701);

[0033] (3) Since the temperature of the process water (ash water) is raised from about 140 °C in the original process to above 190 °C, and the process condensate is raised from about 170 °C to above 190 °C and then enters the scrubber (C-1701) and the gasifier system (F-1701), the overall temperature of the system is increased. The outlet temperature of the coal gas from the top of the scrubber is raised from about 192 °C in the original process to about 206 °C. The amount of water vapor contained in the coal gas is increased by more than 65%. The water-gas molar ratio in the coal gas is raised from about 0.58 to about 0.94. The coal gas does not need to be supplemented with high-pressure steam to increase the water-gas ratio in the shift section, saving a large amount of high-temperature and high-pressure steam in the shift section; in the shift section, the heat energy carried by the coal gas and the heat energy released by the converter cooperate to produce excessive high-temperature and high-pressure superheated steam in the waste heat boiler.

[0034] 4. System simplification and reduction of system operation and maintenance costs:

[0035] (1) Since equipment such as the flash tank and stripping tower in the original process are cancelled, the corresponding control instruments, pipelines, and valves are also significantly reduced; the liquid-solid separation and scale inhibition pretreatment of the black water are adopted to reduce the abrasion and blockage of the coal ash particles in the black water to the equipment, pipelines, instruments, and valves, improve the heat transfer efficiency of the heat exchanger, and significantly reduce the equipment investment and annual maintenance cost. Brief description of the drawings

[0036] Appendix Figure 1 , the original black water heat energy recovery process flow chart of the gasification section:

[0037] Gasifier F-1701, Venturi humidifier Z-1702, Scrubber C-1701, Quench water pump P-1703, High-pressure flash tank V-1801, Normal (true) flash tank V-1804, High-pressure flash steam stripping tower C-1801, Scrubber feed pump P-1806, High-pressure flash steam condenser E-1804, High-pressure flash separation tank V-1803, Deaerator V-1808, Deaerator water pump P-1807, Normal (true) flash steam / process water (ash water) heat exchanger E-1805AB, Normal (true) flash separation tank V-1805, Black water / process water (ash water) heat exchanger E-1805C.

[0038] AppendixFigure 2 , New process flow chart for high-grade heat recovery of gasifier black water with liquid-solid separation and scale inhibition treatment:

[0039] Gasifier F-1701, Venturi humidifier Z-1702, Scrubber C-1701, Quench water pump P-1703, Liquid-solid separator S-1800, Scale inhibitor T-1800, Slurry pump P-1800, Scrubber ash water pump P-1806, High-temperature black water / Process condensate heat exchanger group E-1801, High-temperature black water / Process water (ash water) heat exchanger group E-1802, Medium-temperature black water / Process water (ash water) heat exchanger group E-1803, Low-temperature black water / Process water (ash water) heat exchanger group E-1803. Specific implementation method

[0040] Illustrated with examples as follows:

[0041] Taking the ammonia synthesis plant of a certain enterprise as an example: For the convenience of comparing the new process flow (attached Figure 2 ) with the original process flow (attached Figure 1 ), the dry gas flow rate and composition in the water gas leaving the gasifier in the original process flow and the new process flow of black water heat recovery in this example are the same. The dry gas flow rate is 68286.44 kg / h, among which the H2 concentration is 1.9884 wt%, the N2 concentration is 0.4457 wt%, the CO concentration is 74.8837 wt%, the CO2 concentration is 18.1461 wt%, and the CH4 concentration is 0.0042 wt%. The main logistics data of the original process flow and the new process flow of black water heat recovery in the gasification section for inlet and outlet of the process flow are listed in Table 1. It can be known from the data comparison in Table 1 that:

[0042] 1. The flow rate, composition, and pressure of the dry gas in the water gas leaving the gasifier in the new process flow and the original process flow are the same.

[0043] 2. The temperature of the saturated water gas leaving the gasifier in the new process flow is 209 °C, and the temperature of the original process flow is 204 °C. The amount of water vapor contained in the new process flow is 9200 kg / h more than that of the original process flow, and the heat carried by the gas leaving the furnace in the new process flow is also more.

[0044] 3. In the new process flow, the temperature of the black water after heat exchange directly drops to 36°C after exchanging heat with process water (ash water) and process condensate from 207°C and then is discharged; while in the original process flow, the black water undergoes two-stage flashing, and its temperature drops from 199°C to 158°C and 96°C respectively. The steam flashed out is used to heat the process water, and the black water after vacuum flashing is discharged at a temperature of 96°C. If the temperature of the black water in the original process flow drops from 96°C to the discharge temperature of 36°C in the new process flow, there is still 44.258Mkj / h of heat that can be recycled. However, due to the poor utilization of the high-temperature heat energy of the black water in the original process flow, flashing reduces the energy grade of the black water, resulting in an excess of low-temperature heat that cannot be recycled. Due to the unreasonable heat exchange and recovery of the black water in the original process, the final discharge temperature of the black water is high, taking away a large amount of heat; the low-temperature steam flashed out from the vacuum flashing in the original process flow cannot be used to heat the process water. Due to its low pressure and temperature, the excess low-temperature heat in the unit has nowhere to go and can only be discharged.

[0045] 4. In the new process flow, the process water from outside the plant exchanges heat with the black water at 2°C (the temperature of the process water in winter) and is heated to 196°C and then enters the Venturi humidifier and the gas scrubbing tower; the process condensate from outside the plant exchanges heat with the black water at 170°C and its temperature rises to 197°C and then enters the gas scrubbing tower; in the original process flow, the process water from outside the plant is heated by the flashing steam of the black water and undergoes contact heat exchange in the stripping tower, and its temperature rises from 2°C to 142°C and then enters the Venturi humidifier and the gas scrubbing tower; the process condensate from outside the plant in the original process flow is at 170°C and does not exchange heat with the black water and is directly used as the washing liquid in the gas scrubbing tower; the process water in the new process flow has a higher temperature after exchanging heat with the high-temperature black water than in the original process flow, which raises the temperatures of the material flows entering and leaving the gas scrubbing tower and the quench section of the gasifier. As a result, the amount of water vapor contained in the saturated water gas discharged from the gas scrubbing tower increases. The new process flow fully recycles the heat discharged from the black water in the original process flow and converts it into the heat carried by the high-temperature and high-pressure water vapor contained in the saturated water gas discharged from the gas scrubbing tower in the new process flow.

[0046] 5. In the new process flow, the temperature of the saturated water gas from the gas washing tower is 206°C, the pressure is 3.9 MPaA, and the flow rate is 116,494.86 kg / h, with the water vapor content being 48,425 kg / h. In the original process flow, the temperature of the saturated water gas from the gas washing tower is 192°C, the pressure is 3.9 MPaA, and the flow rate is 97,783 kg / h, with the water vapor content being 29,642 kg / h. The water-gas molar ratio of the saturated water gas from the gas washing tower in the original process flow is 0.5770, and in the new process flow is 0.9406. In the new process flow, no additional high-pressure steam needs to be added in the conversion section to meet the process requirements. The water vapor content in the saturated water gas from the gas washing tower in the new process flow is 18,793 kg / h more than that in the original process flow. The enthalpy value of the saturated water gas from the gas washing tower in the original process flow is 106.490 MKj / h, and in the new process flow is 161.386 MKj / h. The additional enthalpy value carried by the saturated water gas from the gas washing tower in the new process flow compared to the original process flow is 54.896 MKj / h. The additional thermal energy carried is converted into high-pressure superheated steam generated in the waste heat boiler of the converter in the conversion section, which is equivalent to the vaporization and superheating thermal energy of 26.091 t / h of 4.2 MPaA and 400°C high-pressure superheated steam. This is equivalent to the amount of high-pressure superheated steam that is less added and more generated in the conversion section of the new process being 26.091 t / h. Calculated based on 8,000 production hours per year and a unit price of 150 yuan / ton for high-temperature and high-pressure superheated steam, the additional thermal energy recovered in the new process flow is equivalent to saving 208,728 tons / year of high-pressure superheated steam, with an energy-saving benefit of 31.3092 million yuan / year.

[0047]

Claims

1. A new process for high-grade heat energy recovery of the black water in a gasifier with liquid-solid separation and scale inhibition treatment, characterized in that, It includes the following steps: (1) Liquid-solid separation pretreatment: The black water discharged from the bottom of the gasifier (F-1701) and the gas scrubber (C-1701) is input into the liquid-solid separator (S-1800). More than 98% of the fly ash particles in the black water are removed through hydrocyclone liquid-solid separation technology, reducing the solid content from 25000 ppm to less than 500 ppm. The fly ash particles discharged from the bottom of the liquid-solid separator (S-1800) together with a part of the water are sent back to the slag water settling chamber at the bottom of the gasifier (F-1701) by the slag water pump P-1800 for sedimentation separation. After the fly ash particles settle, they enter the slag crusher together with the slag coming down from the upper part of the gasifier (F-1701), and then enter the slag lock hopper and are discharged. (2) Scale inhibition pretreatment: The black water after liquid-solid separation enters the scale inhibitor (T-1800) to inhibit the deposition of scale on the heat exchanger tube wall. (3) Multi-stage countercurrent heat exchange: The traditional flashing step is cancelled, and the pretreated black water is directly heat-exchanged with process water and process condensate through a multi-stage countercurrent heat exchanger group (E-1801, E-1802, E-1803, E-1804). Specifically, it includes: High-temperature section: The black water is divided into two streams and heat-exchanged with process condensate and process water respectively, heating the process condensate and process water to above 190 °C. Medium-temperature section: The black water is heat-exchanged with the process water for the second time, and the process water is heated to about 170 °C. Low-temperature waste heat section: The black water is further heat-exchanged with low-temperature process water to below 40 °C and then discharged. (4) Process coupling: The high-temperature process water (above 190 °C) after heat exchange is divided into two streams. One stream is injected into the Venturi humidifier (Z-1702) to increase the humidity of the gas, and the other stream enters the bottom of the scrubber (C-1701) to increase the temperature of the quench water. The process condensate (above 190 °C) is used as the washing water at the top of the scrubber, raising the temperature of the gas leaving the tower to above 206 °C, increasing the water-gas molar ratio from about 0.58 to about 0.94, and the amount of water vapor in the saturated gas leaving the scrubber increases by more than 63% compared with the original process.

2. The process according to claim 1, characterized in that, The high-temperature and high-pressure black water coming out of the bottom of the gasifier and the bottom of the gas scrubber is subjected to symbolic flashing. After flashing, the pressure ≥ 1.0 MPaA, and the flashed black water is heat-exchanged with process water and / or process condensate.

3. The process according to claim 1, characterized in that, For the multi-stage countercurrent heat exchanger group (E-1801, E-1802, E-1803, E-1804), the number of heat exchanger units in each group needs to be designed and determined according to the production scale and process conditions to ensure that the average heat transfer temperature difference between the black water and the process water ≥ 10 °C.

4. The process according to claim 1, characterized in that, The scale inhibitor (T-1800) keeps the heat exchange efficiency of the heat exchanger above 90% of the initial efficiency within one year of operation through physical treatment of the black water.

5. The process according to claim 1, characterized in that, The process cancels the flash tank, stripping tower and related pipeline valves and instruments, reducing the equipment investment and the annual maintenance cost by more than 50%.

6. The process according to claim 1, characterized in that, The process enables the gas in the shift section not to require supplementary high-pressure steam. Taking a synthetic ammonia plant with an annual output of 180,000 tons as an example, it saves and produces 208,700 tons of 4.2 MPaA, 400 °C high-pressure superheated steam annually, and the energy-saving benefit reaches 31.3 million yuan / year; and the energy-saving benefit is proportional to the production scale of the plant.

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