Flash of water-coal slurry gasification unit
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The gasification unit in Shenmu, Shaanxi, seems to use two-stage flashing. What are the advantages of three-stage flashing? There are also systems that use four-stage flashing – with two additional vacuum flashes – is this done to recover more heat? What are the flashing pressures for each stage of the three-stage flash distillation? If an evaporative hot water tower is used, what is its pressure, and what are the advantages? Is it just to avoid Texaco’s technology?
1. For Texaco 6.5 MPa and 8.9 MPa systems, level 4 flashing is generally used, with two stages: high pressure, low pressure, and vacuum； Texaco typically uses two-stage flashing at 4.5 MPa and 2.8 MPa, for high pressure and vacuum conditions ； The 6.5 MPa multi-material slurry gasification process at the Northwest Research Institute uses three-stage flashing: high pressure, low pressure, and vacuum ； The 4.0 MPa multi-material slurry gasification process at the Northwest Research Institute uses two-stage flashing, under high pressure and vacuum conditions ； 2. The multi-feed slurry gasification process of the Northwest Research Institute uses a hot water tower. I don’t think adding this ingredient offers any significant advantage over Texaco’s version ； 3. The flash pressures at stage 4 are: 0.8 MPa, 0.06 MPa, -488 mmHg, -688 mmHg ； The tertiary flash pressures are: 0.8 MPa, 0.05 MPa, -48 kPa respectively ； The two-stage flash pressures are 0.3 MPa and -48 kPa, respectively. 4. The fourth-stage flash has a better effect; it is not for recovering heat but for reducing the temperature of the graywater.
The system has a high pressure rating, and the number of flash stages is generally high as well. Staged flashing enables stable operation and prevents overheating and overpressure. For example, if it is high-pressure flash evaporation at 2.1 MPa in Nanhua, and it goes directly to vacuum flash evaporation at -0.06 MPa, the pressure difference is too large, which can easily cause the vacuum flash system to operate under overload.
It mainly depends on the pressure level; generally, for pressures of 6.5 MPa, high-flash evaporation is used, while for lower pressures, low-flash, atmospheric-pressure flash, and vacuum flash methods are employed
The Texaco system in our plant operates at a pressure of 6.5 MPa. The treatment of black water involves three stages of flashing, with the pressures for high flashing, low flashing, and true flashing being 0.8, 0.15, and 0.03 respectively.