Which CHP Engine Is Best for Biogas? Gas Engine Selection and Comparison
There is no single “best” biogas CHP engine—the right choice depends on plant scale, gas quality, and whether you value electrical efficiency, heat recovery, or lowest maintenance. For most commercial biogas plants, a spark-ignited lean-burn (Miller cycle) gas engine in the 200–1,500 kWe range is the proven default, delivering 40–43% electrical efficiency with 20,000+ operating hours between major overhauls.
The Four CHP Engine Families and Their Strengths
Family 1 — Standard Otto-cycle gas engines: the workhorse of small plants. Electrical efficiency 34–37%, robust to gas quality swings, lowest CAPEX per kW. Best for 50–500 kWe farm installations where capital discipline matters more than peak efficiency.
Family 2 — Lean-burn Miller-cycle engines: the 2026 commercial default. Electrical efficiency 40–43%, lower NOx emissions, knock-resistant with methane numbers above 80. Available from 400 to 4,000 kWe; premium over Otto engines is 10–20% CAPEX but pays back through 4–6% higher electricity yield.
Family 3 — Dual-fuel diesel-gas engines: ignite biogas with a small diesel or HVO pilot. High thermal efficiency and rapid load response, but higher NOx and diesel fuel cost. Suited to industrial cogeneration with variable loads and backup diesel availability.
Family 4 — Micro-turbines: 30–250 kW units with very low maintenance and vibration, accepting dirtier gas. Electrical efficiency is lower at 26–33%, but exhaust heat at 260–300°C enables efficient absorption chilling. Best where uptime and low-service environments (remote sites, rooftop CHP) outweigh efficiency.
Comparative Data Table: CHP Engine Types for Biogas
| Engine Type | Electrical Efficiency | Power Range | Maintenance Cost (EUR/kWh) | Best Suited For |
| Otto spark-ignited | 34–37% | 50–500 kWe | €0.015–0.025 | Small farms, tight CAPEX |
| Lean-burn Miller | 40–43% | 400–4,000 kWe | €0.012–0.020 | Most commercial biogas plants |
| Dual-fuel diesel-gas | 38–42% | 300–3,000 kWe | €0.015–0.022 + diesel | Variable loads, industrial CHP |
| Micro-turbine | 26–33% | 30–250 kWe | €0.008–0.012 | Remote sites, dirtier gas, low maintenance |
How to Match the Engine to Your Plant in Five Steps
Step 1 — Fix your electrical output target from digester gas production (rule of thumb: 1,000 Nm³/h raw biogas at 55% CH4 ≈ 2,100 kWe at 40% efficiency).
Step 2 — Measure or estimate your methane number: engines below MN 80 may knock on standard Otto engines; Miller engines tolerate MN 70–80 with derating, or you add upstream CO2 removal.
Step 3 — Decide heat utilization: if you can sell heat (district heating, drying, greenhouses), maximize total efficiency with engine-plus-exhaust economizers (85–90% total); if heat is a byproduct, optimize electrical efficiency.
Step 4 — Compare total lifecycle cost per kWh, not just engine price: include maintenance contracts, oil consumption, spark plug life, and overhaul interval. A €50,000 cheaper engine with 0.005 EUR/kWh higher maintenance loses on a 10-year horizon.
Step 5 — Check grid and permitting: feed-in tariffs, NOx limits, and noise regulations often decide between Miller engines (low NOx) and micro-turbines (very low noise) regardless of efficiency.
Frequently Asked Questions (FAQ)
Q1: Which CHP engine manufacturer is best for biogas?
A: For 2026, the market leaders are Jenbacher (INNIO) and MWM for Miller-cycle engines, Caterpillar and Cummins for Otto engines, and Capstone for micro-turbines. “Best” means best local support: compare spare-part lead times and service-contract response within your country before comparing brochures.
Q2: Is a micro-turbine better than a gas engine for small biogas plants?
A: Only if your priorities are maintenance, noise, and gas tolerance. Below 100 kWe, micro-turbines deliver fewer operating hours per euro at lower efficiency, so most farm plants still choose small Otto engines unless the site is remote or unmanned.
Q3: How often does a biogas CHP engine need major overhaul?
A: Lean-burn Miller engines typically run 20,000–40,000 hours between major overhauls with proper maintenance; Otto engines 15,000–25,000 hours. Oil and spark plugs are replaced every 500–1,000 hours depending on gas quality.
Q4: Can I run a CHP engine on upgraded biomethane instead of raw biogas?
A: Yes—engines run cleaner and longer on 96%+ biomethane, with 1–2% higher efficiency and lower maintenance. Many plants use a small upgrading skid to feed engines directly, though most still prefer selling biomethane at premium and using a separate CHP for heat.