Biogas boosting to CHP engines, boilers and flares requires defined gas composition, pressure conditions and downstream demand. The pressure naturally available from an anaerobic digester or flexible gas holder is often too low to overcome gas-treatment equipment, long pipelines, valves and the inlet requirements of a CHP engine, boiler burner or flare.
A gas-tight roots blower can provide the required pressure increase without introducing lubricating oil into the gas stream. Its positive-displacement operating principle is well suited to variable biogas production because it delivers a relatively stable volumetric flow and can be controlled by changing blower speed.
CHP engines, boilers and flares do not have identical fuel-gas requirements. Blower selection must reflect the pressure range, gas quality, turndown and operating philosophy of the actual downstream consumer.
Why Biogas Requires Pressure Boosting
Biogas pressure can fluctuate as production, storage volume and consumption change. Flexible gas holders operate at relatively low pressure, while treatment vessels and combustion equipment create additional downstream resistance.
The blower may need to overcome:
- Gas-holder outlet pressure variation
- Condensate separator resistance
- H₂S-removal equipment
- Activated-carbon filters
- Moisture-removal equipment
- Siloxane-treatment vessels
- Heat exchangers
- Gas-flow meters
- Control valves
- Flame-protection devices
- Pipeline friction
- Elevation and terminal-equipment losses
The required differential pressure can be expressed as:
Required Blower Differential Pressure
- Δpblower = required blower differential pressure
- prequired,consumer = pressure required at the engine, burner or flare inlet
- ΣΔpdownstream = combined losses between the blower and consumer
- pavailable,source = pressure available at the blower inlet
The maximum pressure loss normally occurs at peak gas flow and as filters approach their permitted operating resistance. Both clean and end-of-service filter conditions should be considered.
The blower must also be prevented from creating excessive suction at the digester or gas holder. Low inlet pressure can damage flexible storage membranes, admit air through leakage points or interrupt the biological gas-production process.
Independent underpressure and overpressure protection should therefore remain part of the gas system even when the blower has automatic speed control.
Different Requirements for CHP Engines, Boilers and Flares
CHP engine fuel supply
Combined heat and power engines convert biogas into electricity and usable heat. Stable gas pressure is important because variations can affect the engine’s fuel-control system, combustion stability and electrical output.
The gas flow required by a CHP engine can be estimated from:
Biogas Flow and Heating-Value Estimate
- Qgas,ref = biogas flow at the selected reference conditions
- Pelectric = required electrical output
- ηelectric = engine electrical efficiency expressed as a decimal
- LHVgas,vol = lower heating value of biogas per unit reference volume
- LHVbiogas = approximate lower heating value of biogas on the stated dry-gas volume basis
- yCH4 = methane volume fraction on the same basis
- LHVCH4 = lower heating value of methane on that basis
This approximation is suitable only for preliminary assessment. The engine supplier’s verified fuel-gas data and measured gas composition should be used for final design.
CHP engines can be sensitive to hydrogen sulphide, siloxanes, moisture and particulates. The acceptable concentrations depend on the engine manufacturer, lubricant, maintenance programme and emission-control system.
Hydrogen sulphide contributes to corrosion and sulphur emissions. Siloxanes can form hard silica deposits during combustion and damage valves, pistons, turbochargers and exhaust equipment. Blower pressure cannot compensate for inadequate gas treatment.
The booster should normally be integrated with the engine control system. If the engine load falls, blower speed should decrease or the excess gas should be directed safely to storage or flare. Continuously recycling a large volume through a bypass wastes energy and increases gas temperature.
Boiler and burner supply
Biogas boilers use the methane content of the gas to produce hot water, steam or process heat. Some boiler systems can tolerate a broader gas-quality range than high-performance engines, but this does not eliminate the need for moisture and H₂S management.
The fuel flow for a boiler may be estimated as:
Biogas Flow for Boiler Output
- Qgas,ref = biogas flow at the selected reference conditions
- Pthermal = required useful thermal output
- ηboiler = boiler efficiency expressed as a decimal
- LHVgas,vol = lower heating value of biogas per unit reference volume
The burner manufacturer should specify:
- Minimum and maximum inlet pressure
- Permitted gas-flow range
- Minimum methane concentration
- Acceptable pressure fluctuation
- Maximum H₂S and moisture
- Required flame-control equipment
- Start-up and shutdown sequence
The roots blower should provide sufficient pressure at maximum firing rate without exceeding the burner-train limit at minimum load.
Biogas heating value may change as methane concentration varies. A constant volumetric flow does not always represent a constant heat input. Gas analysis or energy-based control may therefore be required for installations with significant composition variation.
Flare gas supply
A flare provides safe destruction of biogas when the primary consumer is unavailable, gas production exceeds utilization capacity or the gas does not meet the required quality.
Flare duty can be highly variable. It may remain at low or zero flow during normal CHP operation and then receive the full plant production following an engine trip.
The blower and flare control system should therefore be designed for:
- Minimum stable flare flow
- Maximum emergency gas production
- Rapid change from CHP to flare service
- Gas-holder pressure control
- Safe ignition and flame supervision
- Prevention of air entry into the biogas line
- High- and low-pressure protection
- Compliance with applicable emission and safety requirements
Where one blower serves both the CHP and flare routes, valve sequencing must ensure that gas always has a safe destination. A control failure should not allow the blower to pressurize a closed line.
Some landfill-gas systems use the blower to create upstream collection vacuum and positive pressure toward the flare. In this configuration, suction performance, wellfield control and flare pressure must be evaluated together.
Gas Conditioning and Blower-Package Design
The correct position of the roots blower depends on the gas-treatment arrangement.
A blower installed before final treatment may handle:
- Saturated gas
- Condensate droplets
- High H₂S
- Particulates
- Siloxanes
- Variable gas composition
This service requires suitable materials, sealing and effective inlet liquid separation.
A blower installed after drying and contaminant removal handles cleaner gas, which can improve reliability. However, its pressure and temperature must remain compatible with the treatment equipment and end-use system.
Glossary
Boiler fuel demand: Gas flow required to meet a boiler’s firing load.
CHP: Combined heat and power generation using one fuel for electricity and useful heat.
Engine inlet pressure: Pressure required at the fuel connection of a gas engine.
Flare system: Equipment that safely combusts gas not sent to normal users.
Fuel-demand profile: Variation in gas flow required by an engine, boiler or flare over time.
Gas train: Valves, filters, regulators and instruments supplying fuel gas to a user.
Gas-train pressure drop: Pressure loss through filters, regulators, valves and piping in a fuel-gas train.
Water dew point: Temperature at which water vapor in gas begins to condense.