When Should I Choose Biogas over Combustion?
Choose biogas when feedstock is wet above 60% moisture, putrescible, or nutrient-rich—manure, food waste, sewage sludge, agro-industrial effluent—or when untreated disposal would emit methane. Choose combustion when feedstock is dry below 50% moisture, lignocellulosic, storable, and available at scale. Moisture is the deciding variable in most real projects.
A Feedstock-First Decision Rule
Start with moisture, then composition, then scale. Above 60% moisture, digestion is almost always correct because combustion would spend its output on evaporation. Between 40 and 60%, both routes are technically viable and the choice turns on energy price and ash handling. Below 40% with lignin-rich material, digestion leaves most of the energy locked in fibre, so combustion or gasification recovers more.
Comparative Data Table: Decision Factors
| Decision factor | Choose biogas (AD) | Choose combustion |
| Feedstock moisture | above 60% | below 50% |
| Typical feedstock | manure, food waste, sludge, effluent | wood, straw, shells, RDF |
| By-product value | digestate fertiliser, N-P-K | ash, low value |
| Methane avoidance credit | primary driver | not applicable |
| Scale sweet spot | 0.2–5 MW | 1–100 MW |
| Time to stable output | 20–40 days | immediate |
| CAPEX per kW installed | $2,500–7,000 | $1,500–4,000 |
Four Situations Where Biogas Is Clearly Right
Manure and slurry you already handle, where storage emissions are a liability. Food waste with a gate fee, where tipping revenue and energy revenue stack. Effluent with high COD, where treatment cost avoidance pays part of the plant. And any project selling renewable gas certificates or grid biomethane, where combustion has no route to that product.
Frequently Asked Questions (FAQ)
Q1: When should I choose biogas over combustion?
A: When your feedstock is wetter than 60%, putrescible, or nutrient-rich; when you want a fertiliser by-product; or when avoiding methane from existing storage or landfill is part of the business case.
Q2: What feedstock suits anaerobic digestion best?
A: Material with high volatile solids and high moisture: cattle and pig manure, source-separated food waste, sewage sludge, and food-processing effluent. Dry lignocellulosic material such as straw or wood digests slowly and yields little.
Q3: When does combustion make more sense?
A: When fuel is dry, storable, and delivered in volume year-round; when the product is mainly heat or process steam; and when capital per kW matters more than by-product value.
Q4: Can I combine both routes?
A: Yes, and many sites do. A common layout digests the wet fraction and combusts the dry rejects or screening oversize, sharing the same steam or power infrastructure. The rule is to split by moisture, not to force one technology across the whole stream.