Process Flow Diagram (PFD) for a Compressed Bio-Gas (CBG) plant using Napier Grass
- A Process Flow Diagram (PFD) for a Compressed Bio-Gas (CBG) plant using Napier Grass shows the complete conversion of fresh Napier grass into compressed biomethane (CBG) and bio-fertilizer.
- Modern CBG plants typically include feedstock preparation, anaerobic digestion, biogas cleaning, upgrading, compression, and digestate processing. (Biopower)
Typical Process Flow Diagram (PFD)
NAPIER GRASS
│
▼
Weighbridge & Sampling
│
▼
Feedstock Storage Yard
│
▼
Conveyor + Metal Separator
│
▼
Grass Chopper
(10–30 mm particle size)
│
▼
Slurry Preparation / Mixing Tank
(Water + Recycle Digestate Added)
│
▼
Feed Feed Pump
│
▼
┌─────────────────────────────────────┐
│ Anaerobic Digester (CSTR/PFR) │
│ Mesophilic: 35–38°C │
│ HRT: 45–60 Days │
└─────────────────────────────────────┘
│ │
│ │
▼ ▼
Raw Biogas Digestate
│ │
│ Screw Press
│ │
│ ┌─────────┴──────────┐
│ ▼ ▼
│ Solid Bio-manure Liquid Fertilizer
│
▼
Moisture Separator
│
▼
Biological H₂S Removal Tower
│
▼
Activated Carbon Polishing Filter
│
▼
Gas Chiller & Dryer
│
▼
Gas Booster Compressor
│
▼
Membrane / PSA / VPSA Upgrading
│
┌───────┴─────────┐
▼ ▼
Biomethane CO₂ Vent/Recovery
(97–99% CH₄)
│
▼
High Pressure Compressor
(250 bar)
│
▼
Cascade Storage Cylinders
│
▼
CBG Dispensing / Cylinder Filling
Major Process Sections
| Section | Main Equipment | Purpose |
|---|---|---|
| Feed handling | Conveyor, shredder, storage hopper | Reduce particle size and ensure uniform feeding |
| Slurry preparation | Mixing tank, agitator, feed pump | Prepare pumpable slurry |
| Anaerobic digestion | CSTR or Plug Flow Reactor | Convert biomass into biogas |
| Gas collection | Double-membrane gas holder | Buffer raw biogas production |
| H₂S removal | Biological scrubber, activated carbon | Protect downstream equipment |
| Moisture removal | Chiller, moisture separator, dryer | Reduce water content |
| Gas upgrading | Membrane, PSA, or VPSA | Remove CO₂ to produce biomethane |
| Compression | Multi-stage compressor | Compress gas to ~250 bar |
| Storage | Cascade cylinders | Store CBG before dispatch |
| Digestate treatment | Screw press, lagoons, fertilizer tanks | Produce solid and liquid bio-fertilizers |
| Parameter | Typical Value |
|---|---|
| Feedstock | Napier Grass |
| Total Solids (TS) | 12–15% |
| Hydraulic Retention Time (HRT) | 45–60 days |
| Digestion Temperature | 35–38°C (Mesophilic) |
| Methane Content (Raw Biogas) | 52–60% |
| Methane Content (Upgraded CBG) | ≥95% |
| Compression Pressure | 200–250 bar |
| H₂S after purification | <10 ppm |
| CO₂ after upgrading | <2–3% |
| Moisture Dew Point | Below specification for CBG |
Typical Material Flow (Example: 6 TPD CBG Plant)
| Stream | Approximate Quantity |
|---|---|
| Napier grass | 180–220 TPD |
| Slurry to digester | 350–450 m³/day |
| Raw biogas | 12,000–14,000 Nm³/day |
| Methane concentration | 52–58% |
| Upgraded biomethane | ~8,000–8,500 Nm³/day |
| CBG production | ~6 TPD |
| Digestate | 250–350 TPD |
These values are indicative and depend on the Napier grass dry matter, volatile solids content, operating temperature, and methane yield. (Biopower)
Design Recommendations for Napier Grass
- Feedstock: Chopped to 10–20 mm for better digestion.
- Digester type: Plug Flow Reactor (PFR) or Continuous Stirred Tank Reactor (CSTR), depending on solids content.
- Operating temperature: Mesophilic (35–38°C) is the most common for commercial plants.
- Hydraulic Retention Time (HRT): 45–60 days.
- Gas upgrading: Three-stage membrane systems or VPSA are commonly used for producing vehicle-grade CBG. (PNGRB)
For EPC engineering, this PFD is typically followed by:
- Process and Instrumentation Diagram (P&ID)
- Equipment sizing calculations
- Mass and energy balance
- Plant layout (GA drawing)
- Utility flow diagram
- Electrical single-line diagram (SLD)
- PLC/SCADA control philosophy
These documents together form the basis for detailed engineering, procurement, construction, and commissioning of a commercial CBG plant.
