compressor (compressor)

A compressor is a mechanical device that raises gas pressure by reducing its volume. In a CBG plant, the compressor is the single most capex-intensive item after the digester and the most energy-intensive item in operation, typically consuming 25–35% of total plant electricity. Its role is to take cleaned, upgraded biomethane from the upgrading-system discharge pressure (1–10 bar) up to the 200–250 bar required for cylinder filling, cascade loading, or pipeline injection.

Three compressor families are used in CBG service. Reciprocating piston compressors dominate the small and mid-scale market because of their pressure capability — they can deliver 250 bar in 3–5 stages with intercoolers between stages to keep gas temperature below 150°C. Typical specific energy consumption is 0.25–0.35 kWh per kg. Diaphragm compressors use a flexible metallic diaphragm separating the gas from the hydraulic drive fluid, eliminating any oil contamination of the product gas — they are preferred when the CBG is destined for ultra-high-purity uses or pipeline injection but cost 2–3 times more per equivalent throughput. Screw compressors are used for low-to-medium pressure duty (up to 25 bar) such as digester recirculation or membrane-system feed gas.

Design and material choices are dictated by gas composition and safety standards. Cylinders, valves, and piping that contact biomethane must use materials compatible with trace H₂S — typically SS 316 or higher. The complete compressor package — driver motor, cylinder, intercoolers, separators, and control panel — must be certified under the Gas Cylinder Rules and PESO requirements for explosive-atmosphere zones. Critical operating concerns include inlet gas quality (any liquid carryover causes immediate valve damage), discharge temperature control (sustained operation above 150°C causes oil coking on valves), and unloaded start-up sequence to protect the motor. VFDs are increasingly used to match compressor throughput to upstream gas availability, cutting energy use by 15–25% versus on/off operation. Maintenance intervals run every 4,000–8,000 hours for valve replacement and every 16,000–24,000 hours for piston ring overhaul.