Roots Blowers for Coal, Biomass and Auxiliary Fuel Pneumatic Conveying

Auxiliary fuel pneumatic conveying transfers prepared coal, biomass and other fuel fractions through enclosed routes to storage or dosing points. Pneumatic conveying can transfer these materials through enclosed pipelines between unloading stations, storage silos, preparation systems, dosing units and combustion equipment.

roots blowers are well suited to many low- and moderate-pressure pneumatic conveying duties because they deliver an approximately constant volume of conveying gas over a practical range of system resistance. Their suitability, however, depends on the fuel properties, required conveying pressure, transport distance, pipeline geometry and applicable fire and explosion protection measures.

A roots blower should not automatically be specified for every fuel-handling function in a power plant. The primary air supplied to a large pulverizer or ball-and-race coal mill, for example, is normally provided by a primary-air fan. High-pressure dense-phase injection into a pressurized reactor may require a compressor or another specialized conveying system. roots blowers are most relevant where clean conveying air or a compatible inert gas must be supplied to an enclosed low- or moderate-pressure transfer system.

Solid Fuels Conveyed in Power-Generation Facilities

Pneumatic fuel-handling systems may be required to transport:

• Pulverized or finely milled coal
• Coal dust recovered from collection systems
• Petroleum coke
• Wood flour and fine sawdust
• Wood chips and screened biomass particles
• Agricultural residues
• Ground straw and husk materials
• Biomass pellets and pellet fragments
• Refuse-derived fuel fractions
• Dried sewage-sludge granules
• Combustible process residues
• Sorbents and additives mixed with the fuel
• Dust collected from fuel-preparation equipment

These materials do not behave in the same way. Coal powder can be relatively dense, abrasive and combustible. Sawdust is lighter, more compressible and frequently fibrous. Biomass pellets can break down under impact and generate increasing quantities of fines. Refuse-derived fuels may contain irregular particles with wide variations in shape, density and moisture content.

The conveying system must therefore be designed for the actual material rather than selected from the fuel name alone.

Where Pneumatic Conveying Is Used

A pneumatic fuel-transfer system may connect several stages of the power-generation process, including:

• Truck, railcar or ship unloading to storage
• Storage silo to preparation equipment
• Crusher or mill discharge to an intermediate bin
• Prepared fuel storage to a dosing system
• Fuel dosing equipment to a burner or furnace
• Dust-collector hoppers to a fuel-recovery silo
• Biomass receiving systems to day bins
• Alternative-fuel preparation lines to combustion units
• Fuel sampling or recirculation systems

Pneumatic conveying is particularly valuable when the route must be fully enclosed, mechanically simple or flexible enough to pass around existing equipment. It can also reduce spillage and uncontrolled dust release compared with open mechanical handling.

Enclosure alone does not make the process safe. The system must control combustible dust, ignition sources, static electricity, oxygen concentration where applicable and the consequences of an abnormal pressure event.

Positive-Pressure Conveying

In a positive-pressure system, the roots blower is installed upstream of the solids feeding point. Clean conveying gas passes through the blower and the fuel is introduced into the pressurized pipeline by a rotary airlock, pressure vessel, screw feeder or another suitable feeding device.

The blower normally handles only clean gas. Solid fuel should not pass through the blower casing.

Positive-pressure conveying is commonly considered when:

• Material must be delivered over a significant distance
• One source feeds one or several destinations
• The discharge point operates above atmospheric pressure
• Higher conveying capacity is required
• The destination is equipped with an adequate gas-solid separator

A correctly selected feeder must introduce the material while limiting reverse gas leakage. In rotary-valve systems, leakage from the pipeline into the feed hopper can represent an important part of the required blower flow.

The preliminary blower-flow balance may be expressed as:

All terms must be converted to the same pressure and temperature reference conditions before they are added.

Vacuum Conveying

In a vacuum conveying system, the blower or exhauster is positioned downstream of the receiving vessel and filtration equipment. The pressure inside the conveying pipeline remains below atmospheric pressure, drawing material from the pickup point toward the receiver.

Vacuum systems can be advantageous when:

• Material must be collected from several pickup points
• Dust leakage into the surrounding workplace must be minimized
• Fuel is unloaded through suction nozzles
• The pickup point cannot easily be pressurized
• A centralized receiving station is preferred

The conveying gas must be separated from the fuel before it reaches the blower. A damaged receiver filter or ineffective secondary filter could allow combustible and abrasive particles to enter the machine. Differential-pressure monitoring and a high-level or filter-failure interlock are therefore important protective measures.

Air ingress through seals, flexible connections, rotary valves and pipeline joints increases the total gas flow handled by the vacuum blower. Leakage must be included in the capacity calculation.

Dilute-Phase and Dense-Phase Conveying

In dilute-phase conveying, the particles are generally suspended in a relatively high-velocity gas stream. The system is comparatively simple and can handle a wide range of fine fuels, but high velocity increases pipeline wear, particle degradation and energy consumption.

Dense-phase conveying operates at a higher solids loading and lower gas velocity. Material may move as plugs, dunes, pulses or a fluidized bed, depending on its physical properties and the equipment configuration. Dense-phase operation can reduce particle impact and pipeline wear, but it cannot be achieved merely by increasing pressure.

The material must possess suitable permeability, deaeration and flow characteristics. Many fibrous or cohesive biomass materials do not form stable dense-phase flow without specialized conditioning or conveying methods.

High-pressure dense-phase fuel injection may fall outside the practical range of a single-stage roots blower. In such cases, the system may require a pressure vessel and compressor, staged compression or another purpose-designed technology.

Conveying Velocity

The superficial conveying-gas velocity is calculated from the actual volumetric flow and the internal cross-sectional area of the pipeline:

The required velocity must be high enough to prevent unstable deposition and pipeline blockage, but it should not be unnecessarily high. Excess velocity can produce:

Equipment Scope and Selection Limits

Power-plant duties differ by medium and process. Confirm flow, pressure, temperature, solids exposure, gas quality and operating hours for each service before selecting equipment.

Glossary

Biomass fuel: Organic material prepared for use as a boiler or process fuel.

Conveying velocity: Gas velocity used to keep solid fuel moving through the pipe.

Dust explosibility: Ability of suspended combustible dust to ignite and propagate an explosion.

Explosible dust concentration: Dust concentration range in air within which a dust cloud can propagate an explosion.

Fuel-feed point: Location where metered solid fuel enters the conveying route.

Minimum ignition energy: Lowest spark energy capable of igniting a specified dust cloud under test conditions.

Pulverized coal: Finely ground coal prepared for combustion or pneumatic transfer.

Route pressure loss: Combined pressure requirement of the line, bends, feeder and receiver.