Biogas digesters kept warm without burning gas
How a 50 m³-per-day compressed biogas plant can keep its digesters at 35–55 °C with heat pumps, and what it saves against a briquette-fired hot water generator.
▶Video: Process heat for engineering and utilities
V06 · 2–2.5 minPlating, phosphating, washers, curing and boiler feedwater
Video coming soon01Problem statement
- Gas yield falls when digesters cool. Methane-forming bacteria are sensitive to temperature. A drop of 2–3 K in winter cuts gas yield and can upset the digester for weeks.
- Cold feed is the largest load. Fresh slurry enters at ambient temperature and must be heated to digester temperature every day.
- Burning product to keep warm. Plants that heat with their own biogas sell less gas. Plants that use briquettes add fuel handling and ash at a site already busy with feedstock.
- Losses between fuel and process. Burning briquettes in a boiler or heater and distributing the heat loses a quarter or more of the fuel's energy before it reaches the process.
- Compliance, safety and handling. Fired equipment brings boiler inspections, fuel storage and unloading, flue emissions and an attendant on every shift.
02Use case
The reference plant digests 50 m³ a day of press mud and food waste slurry in two 2,000 m³ mesophilic digesters held at 38 °C. Biogas is upgraded and sold as compressed biogas, so every cubic metre burnt on site is lost revenue.
| Heat user | Process temp. | Basis | Heat, kWh/day |
|---|---|---|---|
| Feed slurry heating | 25 → 38 °C | 50,000 L/day slurry, 25 → 38 °C | 755 |
| Digester shell and roof losses | 38 °C held | 2 digesters × 450 m² × 0.6 W/m²K × 13 K × 24 h | 168 |
| Storage and pipework standing losses | – | about 5% | 46 |
| Total useful heat | 969 | ||
Averaged over 20 running hours, the heat pumps need about 48 kW.
03P&ID
Tags: HP heat pump, TK tank, SG steam generator, P pump, PHE plate heat exchanger, TCV temperature control valve, TIC temperature indicating controller (panel), TT temperature transmitter, MT moisture transmitter, PT pressure transmitter, LT level transmitter, FS flow switch, NC normally closed. Process fluids are heated through their own exchanger, coil or jacket.
04Solution
System description
- 1 × SE-SHT-50C (50 kW) (SE-SHT cascade heat pump) delivering 60 °C, covering the day's 969 kWh in about 19.4 running hours.
- A 1,500 L insulated storage tank decouples the heat pumps from the process peaks.
- The biomass-briquette fired hot water generator stays connected behind a normally closed valve as standby.
- Feed slurry is warmed in a tube-in-tube or spiral exchanger before it enters the digesters, which is where most of the heat goes.
- Existing wall or internal coils in each digester take 50–55 °C water from the storage tank to cover shell losses.
- The 60 °C supply also allows thermophilic operation at 50–55 °C in one digester if the plant chooses it later.
- Heat pumps run day and night, with the tank smoothing feeding batches.
Selected equipment
| Model | SE-SHT-50C × 1 |
|---|---|
| Heating capacity | 50 kW each, 50 kW total |
| COP | 2.6 at A20 °C / W90 °C (data sheet T1) |
| Power input | about 18.6 kW total at rated conditions |
| Rating | cascade R410A + R515B, rated 90 °C, 95 °C maximum |
| Supply temperature | 60 °C |
| Storage | 1,500 L insulated tank |
Ratings from the Sunniva TDS 2026 data sheets.
Control philosophy
TT-101 at the top of the tank starts the heat pumps below 52 °C and stops them at 58 °C. TIC-201 sets the feed slurry outlet at 38 °C. TIC-202 and TIC-203 hold each digester within ±0.5 K using the wall-coil valves. The existing hot water generator stays as standby.
05Energy saving and ROI
Change any input to match a customer's site. Defaults are typical Indian prices for October 2026. The installed cost is a placeholder; replace it with the actual quotation.
| Existing | Heat pump | |
|---|---|---|
| Daily consumption | – | – |
| Annual energy cost | – | – |
| Cost per 100 m³ biogas | – | – |
| CO₂ per year | – | – |
06Benefits
More gas to sell
No biogas is burnt for heating, so all of it goes to upgrading and sale.
Stable digesters all year
Constant digester temperature through winter keeps gas yield and quality steady.
Clean, low-temperature coils
Coil water at 50–55 °C reduces baking of slurry onto heating surfaces compared with hotter boiler water.
Lower energy cost
Each kWh of electricity delivers about 2.6 kWh of heat at the required temperature, against well under one kWh of useful heat per kWh of fuel burnt.
No flame on site
No burner, flue or fuel tank for this duty, and no fuel deliveries to schedule.
Solar-ready
Running hours can be placed in daylight so rooftop solar supplies much of the electricity, which also cuts CO₂ further.
07Engineering pointers
Site data to collect
- Daily feed volume, solids content and feed temperature by season
- Digester sizes, insulation and target temperature
- Existing heating coil details
- Current heating fuel use or biogas burnt on site
- Hazardous area drawing
- Present fuel or electricity use and monthly bill
- Sanctioned electrical load and spare capacity
- Space for the heat pumps and storage, with service access
- Winter minimum ambient at site
Design notes
- Slurry exchanger. Slurry with fibre and grit needs a wide-passage tube-in-tube or spiral exchanger. Do not use a gasketed PHE on the slurry side.
- H₂S atmosphere. Keep the heat pumps away from digester vents and gas holders. Hydrogen sulphide attacks copper and aluminium fins.
- Hazardous area. Check the plant's hazardous area classification and install heat pumps and panels outside the zoned areas.
- Electrical. About 19 kW of heat pump input at rated conditions, plus pumps and fans. Check spare sanctioned load before ordering.
- Winter output. The SE-SHT cascade gives about 80% of rated output at 7 °C ambient and still runs at −20 °C. Size on the coldest design day for northern sites.
- Water quality. Scale forms quickly above 60 °C. Use softened make-up and keep the heat pump loop closed, with process fluids on the far side of an exchanger or coil.
Service and warranty
Sunniva has supplied 5,000+ heat pumps since 2012, with nine service technicians across India, including remote sites such as Leh. All components carry a one-year warranty.
Talk to us
Biogas digesters kept warm without burning gas
How a 50 m³-per-day compressed biogas plant can keep its digesters at 35–55 °C with heat pumps, and what it saves against a briquette-fired hot water generator.
▶Video: Process heat for engineering and utilities
V06 · 2–2.5 minPlating, phosphating, washers, curing and boiler feedwater
Video coming soon01Problem statement
- Gas yield falls when digesters cool. Methane-forming bacteria are sensitive to temperature. A drop of 2–3 K in winter cuts gas yield and can upset the digester for weeks.
- Cold feed is the largest load. Fresh slurry enters at ambient temperature and must be heated to digester temperature every day.
- Burning product to keep warm. Plants that heat with their own biogas sell less gas. Plants that use briquettes add fuel handling and ash at a site already busy with feedstock.
- Losses between fuel and process. Burning briquettes in a boiler or heater and distributing the heat loses a quarter or more of the fuel's energy before it reaches the process.
- Compliance, safety and handling. Fired equipment brings boiler inspections, fuel storage and unloading, flue emissions and an attendant on every shift.
02Use case
The reference plant digests 50 m³ a day of press mud and food waste slurry in two 2,000 m³ mesophilic digesters held at 38 °C. Biogas is upgraded and sold as compressed biogas, so every cubic metre burnt on site is lost revenue.
| Heat user | Process temp. | Basis | Heat, kWh/day |
|---|---|---|---|
| Feed slurry heating | 25 → 38 °C | 50,000 L/day slurry, 25 → 38 °C | 755 |
| Digester shell and roof losses | 38 °C held | 2 digesters × 450 m² × 0.6 W/m²K × 13 K × 24 h | 168 |
| Storage and pipework standing losses | – | about 5% | 46 |
| Total useful heat | 969 | ||
Averaged over 20 running hours, the heat pumps need about 48 kW.
03P&ID
Tags: HP heat pump, TK tank, SG steam generator, P pump, PHE plate heat exchanger, TCV temperature control valve, TIC temperature indicating controller (panel), TT temperature transmitter, MT moisture transmitter, PT pressure transmitter, LT level transmitter, FS flow switch, NC normally closed. Process fluids are heated through their own exchanger, coil or jacket.
04Solution
System description
- 1 × SE-SHT-50C (50 kW) (SE-SHT cascade heat pump) delivering 60 °C, covering the day's 969 kWh in about 19.4 running hours.
- A 1,500 L insulated storage tank decouples the heat pumps from the process peaks.
- The biomass-briquette fired hot water generator stays connected behind a normally closed valve as standby.
- Feed slurry is warmed in a tube-in-tube or spiral exchanger before it enters the digesters, which is where most of the heat goes.
- Existing wall or internal coils in each digester take 50–55 °C water from the storage tank to cover shell losses.
- The 60 °C supply also allows thermophilic operation at 50–55 °C in one digester if the plant chooses it later.
- Heat pumps run day and night, with the tank smoothing feeding batches.
Selected equipment
| Model | SE-SHT-50C × 1 |
|---|---|
| Heating capacity | 50 kW each, 50 kW total |
| COP | 2.6 at A20 °C / W90 °C (data sheet T1) |
| Power input | about 18.6 kW total at rated conditions |
| Rating | cascade R410A + R515B, rated 90 °C, 95 °C maximum |
| Supply temperature | 60 °C |
| Storage | 1,500 L insulated tank |
Ratings from the Sunniva TDS 2026 data sheets.
Control philosophy
TT-101 at the top of the tank starts the heat pumps below 52 °C and stops them at 58 °C. TIC-201 sets the feed slurry outlet at 38 °C. TIC-202 and TIC-203 hold each digester within ±0.5 K using the wall-coil valves. The existing hot water generator stays as standby.
05Energy saving and ROI
Change any input to match a customer's site. Defaults are typical Indian prices for October 2026. The installed cost is a placeholder; replace it with the actual quotation.
| Existing | Heat pump | |
|---|---|---|
| Daily consumption | – | – |
| Annual energy cost | – | – |
| Cost per 100 m³ biogas | – | – |
| CO₂ per year | – | – |
06Benefits
More gas to sell
No biogas is burnt for heating, so all of it goes to upgrading and sale.
Stable digesters all year
Constant digester temperature through winter keeps gas yield and quality steady.
Clean, low-temperature coils
Coil water at 50–55 °C reduces baking of slurry onto heating surfaces compared with hotter boiler water.
Lower energy cost
Each kWh of electricity delivers about 2.6 kWh of heat at the required temperature, against well under one kWh of useful heat per kWh of fuel burnt.
No flame on site
No burner, flue or fuel tank for this duty, and no fuel deliveries to schedule.
Solar-ready
Running hours can be placed in daylight so rooftop solar supplies much of the electricity, which also cuts CO₂ further.
07Engineering pointers
Site data to collect
- Daily feed volume, solids content and feed temperature by season
- Digester sizes, insulation and target temperature
- Existing heating coil details
- Current heating fuel use or biogas burnt on site
- Hazardous area drawing
- Present fuel or electricity use and monthly bill
- Sanctioned electrical load and spare capacity
- Space for the heat pumps and storage, with service access
- Winter minimum ambient at site
Design notes
- Slurry exchanger. Slurry with fibre and grit needs a wide-passage tube-in-tube or spiral exchanger. Do not use a gasketed PHE on the slurry side.
- H₂S atmosphere. Keep the heat pumps away from digester vents and gas holders. Hydrogen sulphide attacks copper and aluminium fins.
- Hazardous area. Check the plant's hazardous area classification and install heat pumps and panels outside the zoned areas.
- Electrical. About 19 kW of heat pump input at rated conditions, plus pumps and fans. Check spare sanctioned load before ordering.
- Winter output. The SE-SHT cascade gives about 80% of rated output at 7 °C ambient and still runs at −20 °C. Size on the coldest design day for northern sites.
- Water quality. Scale forms quickly above 60 °C. Use softened make-up and keep the heat pump loop closed, with process fluids on the far side of an exchanger or coil.
Service and warranty
Sunniva has supplied 5,000+ heat pumps since 2012, with nine service technicians across India, including remote sites such as Leh. All components carry a one-year warranty.