Conquering Cofiring Flow Challenges

Cofiring biomass with coal is an attractive method of meeting renewables goals, but several material flow-related issues may occur during handling and storage.
By Jayant Khambekar and Roger A. Barnum | August 29, 2012

State renewable portfolio standards, limited ability to increase solar, wind or hydro power, and broad public support for renewable initiatives have sparked strong interest amongst U.S. utility companies to utilize biomass fuel. While cofiring biomass may be an appealing way to reduce coal consumption, there are challenges associated with handling and storing biomass.

Generally speaking, biomass storage systems can be divided into two groups: gravity-discharge systems and mechanical-reclaim systems.  As the name suggests, gravity-discharge systems rely on the force of gravity to promote the flow of material, whereas mechanical-reclaim systems use a mechanism to assist material discharge from storage facility. Depending upon the plant’s needs, this storage facility can be a stockpile, a storage silo or both. Stockpiles have surface reclaimers or gravity reclaim systems, which typically use a bottom screw reclaimer or reclaim hoppers with belt feeders.

Biomass storage silos often have a bottom reclaimer for discharging material. Once reclaimed, biomass is conveyed from the storage facility to the boiler-feed bins, which are usually gravity-discharge and meter biomass into the boiler using screw feeders.

Several material handling steps are involved in a biomass cofiring process. If the material storage and handling equipment is not designed properly, material flow issues can occur, some more common than others.

Common Flow Problems

A no-flow condition can result either from arching—also known as bridging—or ratholing. Arching occurs when an obstruction in the shape of an arch or bridge forms over the hopper outlet, due to the cohesive strength of the material or the mechanical interlocking of large particles. When material forms a stable arch above an outlet, discharge is prevented and a no-flow condition results.

For materials such as milled biomass and sawdust, ratholing may occur as a result of flow channeling. During this condition, material moves toward the outlet through a steep, funnel-shaped flow channel surrounded by stagnant material. Hopefully, as the level of material in the flow channel drops, layers of material from the top surface of the stagnant region will slide into this active channel. If this fails to occur, the flow channel empties and a stable rathole forms, resulting in a no-flow condition. 

Whenever stagnant material is present inside a bin or silo, it will result in limited live storage capacity during flow. Such stagnant material can be a result of ratholing or steep drawdown angles; this material will not discharge by gravity, thus occupying valuable storage space. 

Another common flow problem is the tripping of feeder and bottom reclaim drive motors.  When tripping occurs, the device cannot operate, resulting in the inability to discharge material from storage facility. The only way to address this problem, which typically occurs if the material consolidation pressures acting on the feeder or reclaimer are not properly calculated, may be to vacuum out the entire contents of the silo.

Other flow problems that could occur include transfer chute pluggages, attrition and dusting of pellets, and particle segregation. Equipment wear can also be an important issue due to abrasive nature of woody biomass, particularly if it contains bark.

In general, when flow problems such as the aforementioned occur, valuable generation capacity and production time are lost, excessive maintenance and housekeeping costs are incurred, and health and safety issues arise. Flow problems and their solutions can be best understood by first learning how bulk solids flow. 

Flow Patterns, Properties

As bulk solids discharge by gravity from a storage system, two types of flow patterns can develop: funnel flow and mass flow. In funnel flow, only a portion of the material is in motion during discharge, which flows toward the outlet through a channel that forms within the stagnant material. Funnel flow occurs when a hopper is not sufficiently steep and smooth to ensure flow along the walls, or when the hopper outlet is not fully activated.

In mass flow, all of the material is in motion whenever any is discharged; there is no stagnant material. Mass flow occurs when the hopper is sufficiently steep and smooth to ensure flow along the hopper walls. Shallow valleys—at the intersection of adjacent hopper sidewalls—cannot be present, and the outlet must be fully active.

The flow pattern in which a bulk solid discharges from a storage system strongly influences the flow problems that may occur.

For the reliable storage and feeding of biomass, the type of handling system used— including its geometry and materials of construction— must be designed to suit the flowability of the material.  Whether it is mechanical-reclaim or gravity-reclaim, characterization of flow behavior is necessary in order to design features of the system. Various flow property tests can be performed to determine flowability, which is influenced by moisture content, fines content and storage time at rest. Flow property tests must be run at representative handling conditions and include the following tests: cohesive strength, interlocking strength, wall friction, compressibility, permeability, chute angle, angle of repose, and drawdown angle (see sidebar).

Achieving Reliable Material Flow

When properly designed, the components of a handling system will be able to store and reliably feed difficult-to-flow materials, whereas poorly designed equipment may not be able to do the same, even with free-flowing materials.

For gravity discharge systems such as boiler-feed bins, selecting the appropriate flow pattern is critical for reliable performance. Mass flow is recommended for fine, cohesive materials such as sawdust and milled woodchips; funnel flow is suitable for coarse, free-flowing, non-interlocking, non-degrading materials in cases where particle segregation is not important. 

For mechanical reclaim systems such as silos using bottom screw reclaimers, the discharge pattern is almost always funnel flow. The key for designing a mechanical reclaim system is selecting the right type of mechanism that will work reliably for the particular application. There are various types of configurations available for mechanical reclaim systems, such as a flat bottom silo with a rotating screw or sweep arm reclaimer, a revolving screw reclaimer within a conical bottom silo, a flat bottom silo with a top screw reclaimer, etc. Using flow property test results, the right type of mechanism can be selected so that the system is not over- or under-designed for the application.  The test results can also aid in the design of mechanism features.

The storage system should be designed so as to minimize the consolidation pressures acting on the feeder or reclaimer. The compressibility and wall friction information obtained from flow properties testing can be used in this regard. Additionally, for gravity discharge systems where mass flow is required, the hopper angles must be steep enough to promote flow along the walls.   

Material handling is a key component to successful biomass cofiring. Technology is available for analyzing storage and handling systems to eliminate or minimize flow problems in existing facilities, as well as for designing new installations to avoid such problems in the first place. Flow property test data is a key component in the process, as it will ensure the reliable storage and flow of biomass materials.

Authors: Jayant Khambekar
Project engineer, Jenike & Johanson Inc., (978) 649-3300

Roger A. Barnum
Senior consultant, Jenike & Johanson Inc., 978-649-3300


Flow Property Tests
Cohesive strength: Used to calculate opening sizes to avoid flow stoppages due to cohesive arching and ratholing. Measured as a function of consolidating pressure in accordance with ASTM Standard D6128.
Interlocking strength:  Used to calculate opening sizes to avoid flow stoppages due to mechanical interlocking of its particles.  Characterized by particle size, particle shape and elasticity behaviors.
Wall friction: Measured as a function of consolidating pressure in accordance with ASTM Standard D 6128.  Information obtained from this test is used to determine critical hopper angles for achieving mass flow. The wall material of construction and surface finish must match what will be used for design or analysis purposes.
Compressibility: Measures the change in its bulk density as a function of consolidating pressure.  Used to determine storage capacity of equipment and to calculate material-induced material induced loads; measured in accordance with ASTM Standard D 6683.
Permeability: Measure of material resistance to gas flow through it, particularly important when material contains a significant portion of fines.  Data obtained is used to calculate the critical, steady-state, flow rate of the material that can occur during discharge as a function of outlet size and consolidating pressure. 
Chute angle: Determines the minimum (shallowest) required chute angle to maintain flow after impact of a material stream with its surface.  Angles are measured as a function of impact pressure.
Angle of repose and drawdown angle: Measured to help determine the total and live storage capacities of a system; these tests provide no further insight regarding material flowability for design purposes.