The syngas leaving a coal-to-methanol plant reaches up to 280 ℃; the original design cooled it directly with water, wasting large amounts of low-grade heat every year. This project added a SAMIT fully-welded plate heat exchanger to recover syngas sensible heat for preheating boiler feed water, achieving cascaded energy utilization.
1. Project Background
Customer
A coal-chemical group in East China; 300 kt/a methanol capacity, with 2 × 220 t/h circulating fluidized-bed boilers.
Original Process
Syngas (280 ℃) was cooled to 40 ℃ in a shell-and-tube water cooler before entering the desulfurization tower, with the heat carried away and discharged by circulating water.
Core Pain Points
- Annual heat waste equivalent to 3500 t of standard coal
- High circulating-water pump energy
- Bulky shell-and-tube equipment, tight footprint
2. Technical Solution
A SAMIT WB25 fully-welded plate heat exchanger was inserted upstream in the syngas line, using syngas sensible heat to preheat deaerated boiler feed water (104 ℃ → 155 ℃). The fully-welded structure withstands high-temperature corrosion from the trace H₂S in the syngas and requires no gasket maintenance.
| Parameter | Syngas Side (Hot) | Boiler Feed-Water Side (Cold) |
|---|---|---|
| Medium | Syngas (CO+H₂+trace H₂S) | Deaerated feed water |
| Flow rate | 86000 Nm³/h | 180 t/h |
| Inlet temperature | 280 ℃ | 104 ℃ |
| Outlet temperature | 160 ℃ | 155 ℃ |
| Working pressure | 2.5 MPa | 5.4 MPa |
| Heat load | 13.5 MW | |
3. Process Flow
4. Implementation
- Phase 1 (Design & selection, 2 months): Site survey + operating-data collection + multi-option comparison (shell-and-tube / spiral / fully-welded); finally selected the SAMIT WB25 series, single-unit area 85 m².
- Phase 2 (Manufacturing & supply, 3 months): 254 SMO plate laser welding + third-party supervision + factory hydrostatic test at 1.5× working pressure.
- Phase 3 (Shutdown tie-in, 14 days): Used the annual overhaul window to remove the original water cooler and tie in the new PHE, with associated piping and control-system modifications.
- Phase 4 (Start-up & commissioning, 3 days): Grid-connected operation; performance test reached 102% of design.
Key technical decision: 254 SMO was chosen over 316L because, although the H₂S content of the syngas is only 200 ppm, it still causes stainless-steel pitting at 280 ℃. 254 SMO contains 6% Mo, with a pitting resistance equivalent (PREN) ≥ 43, guaranteeing the 15-year design life.
5. Operating Results
Energy Savings
Annual heat recovery of 85,000 GJ, equivalent to 2900 t of standard coal; saves about 7.8 million RMB/year in steam cost.
Environmental Benefits
CO₂ reduction 7800 t/year, SO₂ reduction 45 t/year, helping the enterprise pass environmental inspections.
Footprint Optimization
The fully-welded PHE occupies only 4.8 m², an 83% reduction versus the original shell-and-tube (28 m²), freeing up plant space.
6. Lessons Learned
Coal-chemical waste-heat recovery must focus on medium corrosivity (H₂S, Cl⁻) and the temperature/pressure rating. The keys in this case: ① selecting fully-welded over shell-and-tube, quadrupling the heat-transfer coefficient; ② selecting 254 SMO to handle high-temperature H₂S corrosion; ③ using the annual overhaul window for the shutdown tie-in so production was unaffected. This model is replicable for ammonia synthesis, ethylene glycol, coal-to-liquids, and similar coal-chemical units.