Fuel gas and biogas boosting moves treated or process gas to engines, boilers or other users at a defined pressure and flow. Gas produced in digesters, wastewater treatment plants, landfills and industrial biological processes is frequently available at a pressure too low to overcome treatment equipment, piping losses and the minimum inlet-pressure requirement of the final consumer. A purpose-designed roots-type gas blower can provide the necessary pressure increase where the required differential pressure is within the operating range of a rotary-lobe machine.
This application must not be treated as an ordinary air-blowing duty. Biogas and fuel gas may be combustible, toxic, corrosive and saturated with water vapour. The blower package must therefore be designed specifically for the actual gas composition, hazardous-area classification and operating pressure. A conventional industrial air blower should not be installed in a fuel-gas line unless its construction and package design have been formally assessed and approved for that gas service.
The Function of a Roots Blower in a Fuel-Gas System
A roots blower is a positive-displacement machine. Two synchronized lobed rotors capture discrete volumes of gas at the inlet and transfer them to the discharge side without internal compression. The discharge pressure develops when the delivered gas encounters resistance in downstream piping and equipment.
In a power-generation installation, the blower may be required to overcome pressure losses through:
- Moisture separators and condensate knock-out vessels
- Gas coolers and heat exchangers
- Biological or chemical hydrogen sulphide removal units
- Activated-carbon filters
- Siloxane-removal systems
- Fine particulate filters
- Flowmeters, control valves and flame arresters
- Long gas pipelines and multiple fittings
- Gas-engine or burner inlet systems
- Gas holders operating at low positive pressure
The blower does not increase the heating value of the gas. Its function is to deliver the available fuel gas at the required flow rate and inlet pressure while maintaining stable operation of the downstream energy-conversion equipment.
Roots blowers are particularly suitable for low-to-moderate pressure boosting duties in which relatively constant volumetric flow is required despite changes in system resistance. If the required pressure ratio becomes too high for a single-stage rotary-lobe blower, a two-stage arrangement, a screw compressor or another gas-compression technology may be more appropriate.
Biogas Characteristics and Their Influence on Blower Selection
Biogas is not a single gas with fixed properties. Its composition depends on the feedstock, digestion process, landfill age, temperature, operating conditions and gas-treatment system. Typical constituents may include:
- Methane
- Carbon dioxide
- Water vapour
- Hydrogen sulphide
- Nitrogen
- Oxygen
- Ammonia
- Hydrogen
- Siloxanes
- Volatile organic compounds
- Fine liquid droplets and solid contaminants
Methane provides most of the usable energy, while carbon dioxide reduces the volumetric heating value. Hydrogen sulphide can cause corrosion and presents a serious toxicity hazard. Water vapour can condense as the gas cools, and siloxanes can form abrasive silica deposits during combustion.
Gas composition influences density, heat capacity ratio, compressibility, discharge temperature, power consumption and material compatibility. Blower selection should therefore be based on a representative gas analysis rather than on airflow data alone.
The gas mass flow at the blower inlet can be expressed as:
Gas Mass Flow Rate
- ṁgas = gas mass flow rate
- ρin = gas density at blower inlet conditions
- Qin = actual volumetric flow rate at blower inlet conditions
A flow rate stated in normal cubic metres per hour must not be used directly as the blower inlet volume. It must first be converted to the actual inlet temperature and absolute pressure.
For a real gas, the approximate conversion is:
Gas Flow Conversion to Actual Conditions
- Qactual = volumetric flow at actual inlet conditions
- Qnormal = volumetric flow at the specified normal conditions
- Tactual = actual absolute gas temperature
- Tnormal = normal reference temperature in kelvin
- Pactual = actual absolute inlet pressure
- Pnormal = normal reference pressure
- Zactual = compressibility factor at actual conditions
- Znormal = compressibility factor at normal conditions
The reference conditions associated with “normal” flow must always be stated. Different standards may use different reference temperatures, so an unspecified value in Nm³/h can create a significant sizing error.
Pressure Requirement and System Resistance
The required blower differential pressure must be calculated from the complete gas path. Considering only the pressure required by the engine or burner can result in an undersized machine.
A preliminary pressure balance may be written as:
Total Differential Pressure
- ΔPsuction = pressure loss on the suction side
- ΔPtreatment = loss through gas-cleaning and conditioning equipment
- ΔPpiping = friction and fitting losses in the pipeline
- ΔPequipment = pressure required at the engine, burner or other consumer
- ΔPcontrol = pressure allowance across control devices
A reasonable design allowance may be added for filter fouling, variations in gas flow, future pipeline modifications and measurement uncertainty. Excessive allowance, however, can lead to an unnecessarily large blower, higher energy consumption and poor turndown performance.
All compression ratios must be calculated with absolute pressures:
Pressure Ratio
Gauge pressures must not be used in this equation. This is particularly important in suction-boosting systems because a relatively small gauge-pressure difference can correspond to a meaningful absolute pressure ratio.
Suction and Pressure-Boosting Arrangements
A gas blower may operate in different positions within the process.
In a suction arrangement, the blower draws gas from a digester, landfill collection header or low-pressure gas holder. The blower inlet may operate below atmospheric pressure, while the discharge side supplies treatment equipment or a gas-storage system. Preventing uncontrolled air ingress is critical because oxygen entering a methane-rich system may create a flammable mixture.
In a pressure-boosting arrangement, treated gas enters the blower at a small positive pressure and is boosted to the pressure required by the engine, burner or distribution header. The gas is usually cleaner at this location, reducing the risk of corrosion, liquid carry-over and internal deposits.
Some installations use both functions: a gas extraction blower stabilizes collection pressure, and a separate booster delivers conditioned gas to the generating equipment. Whether one or two machines are required depends on the layout, pressure profile, control philosophy and gas-treatment sequence.
Glossary
Absolute pressure ratio: Absolute discharge pressure divided by absolute suction pressure.
Fuel-gas booster: Machine that raises gas pressure to meet a downstream user requirement.
Fuel-gas heating value: Energy released by complete combustion of a specified quantity of fuel gas.
Gas composition: Fractions of the individual components in a process-gas mixture.
Gas conditioning: Removal or control of moisture and contaminants before gas use.
Gas-tight sealing: Construction intended to limit leakage into or out of a gas machine.
Knockout vessel: Separator that removes entrained liquid before gas reaches downstream equipment.
Suction scrubber: Vessel that removes liquid droplets from gas before a machine inlet.