This article was published under the title “Move Your Biomass” as the cover story in the May – June 2010 issue of Canadian Biomass Magazine. Read it online at:

http://www.canadianbiomassmagazine.ca/index.php?option=com_content&task=view&id=1781&Itemid=132

Biomass is not an easy material to handle. It appears in a myriad of species, forms and sizes; it knits together, doesn’t flow well, consolidates and packs easily; it can have a wide range of moisture contents, basic and bulk densities and calorific values; it will freeze; it is very dusty, catches fire easily and is self-combustible; it can contain all manner of contaminants.  Conversely, wood pellets are uniform in size and moisture content, are very free flowing, but are quite fragile and easily degrade and require special handling.

Increasing numbers of power utilities are eyeing biomass as a source of fuel, primarily as a means of lowering CO₂ emissions.  New biomass boilers are being constructed but many coal-fired boilers are being converted for co-firing biomass or converted to 100% biomass.  Biomass can be introduced into combustors as `hog fuel’, wood pellets or injected as a powder.  Working in the forest and converting the wood into usable products is part of the Canadian heritage.   There are many people and companies who have extensive experience handling woody biomass in all its myriad of forms.  However, with the recent rush to `green energy’ and the identification of woody biomass as a green fuel, there are a lot of interested and well-intentioned but woefully inexperienced people vying for various grants and proposing new projects, but who have little or no experience with biomass.

Care must be taken in the design of your biomass handling system whatever the form.  The topic of design is as varied and complex as the material and the intent in this article is to cover woody biomass, in the form of hog fuel and pellets, which is utilized as fuel. I will discuss the subject in a broad-brush manner suitable for this publication.  Following is a list and short description of things to consider when designing your system.

A general tendency in most projects is to view the biomass handling system as the least glamorous part of the processing facility and it is the area where most people try to cut costs. In any project it is imperative to minimize costs, but the biomass processing system is not always the best place to do so.

The purpose of a biomass processing and handling system is to produce a boiler feedstock that is consistent in constituents, size and moisture content.  Depending upon the variety and form of the raw incoming material, processing systems can be quite complex.

There are many factors that affect the design of the biomass system, including:

Type of facility

The requirements for a facility that generates power for sale and must run 24 hours per day, 350 days per year at >95% uptime are much more rigorous than for a facility that utilizes the energy in-house and doesn’t suffer punitive damages from being off-line.  A power producer must guarantee reliability, therefore his fuel handling systems must be reliable and in some cases redundant.  Additionally, an independent cogeneration or combined heat and power facility must meet the requirements of both the host company and the power recipient.

Capital Cost

Generally, the cost of the biomass handling system is not a constant percentage of the cost of whole facility; the smaller the size of the whole facility, the greater the percentage of cost of the fuel handling system in relation to the cost of the whole facility. Hence, there is greater pressure to reduce costs and often this results in an inferior biomass handling system.

Location and space

A facility that is located within a city or town will likely be required to meet stricter environmental conditions than one isolated in the country, possibly including an enclosed biomass handling system and an odour control system.

Biomass handling and storage requires considerable space. However, biomass-fired facilities are often located adjacent to other existing facilities and space can be at a premium.  The available space has a tremendous impact on layout and layout affects process operability and capital cost.

Incoming material

For this article, it is assumed that the incoming material will be pre-processed hog fuel either as residuals from wood products plants or as hogged forest residues; or wood pellets.  If possible, establish a hog fuel quality specification with your biomass suppliers; one that will have material, size, moisture content, and contaminant specifications. Establish a fuel sampling process to ensure quality.  Wood pellets are usually manufactured to an industry standard. The handling systems for hog fuel and wood pellets would be quite different.

Required Quality of hog fuel

If the quality and form of the incoming material is quite different from that of the required combustor fuel, then suitable processing equipment will be required on site.  It is my experience that the quality of incoming material will not always be as specified and you must have the ability to deal with sub-quality material arriving on site, either with equipment or procedures.

Delivery method and timing

How the material is delivered to the plant site has a big impact on the biomass handling system.  The systems required for truck, rail and barge receiving and unloading are each quite different and affect the layout, cost and process.

Timing of deliveries does as well and the delivery hours per day and days per week must be considered; as do interruptions in supply.  If the fibre source is shut down for prolonged periods, you will need more storage.  If you have established supply contracts, and your plant is down for a prolonged period, you will need more storage or will need to find another place that can take the material temporarily.

Climate

The biomass handling system for a facility located in northern Canada is much different from one located on the west coast or from one in the southern USA.  So, make sure your system designer has the appropriate experience.

Type of project

The type of project can have an impact on the fuel handling system you end up with. An EPC or design / build contractor will want to minimize his costs and often does so in the biomass processing area.  To avoid this, hire a competent owner’s engineer with good biomass handling experience to do the conceptual design and write the design / build specification document for your biomass handling system.

Generally, a hog fuel handling system requires the following sub-systems:

Delivery / Receiving System

The design of a truck receiving system depends upon the type of truck being used to haul the biomass.  Trucks can be self-unloading or non-self unloading.  Trucks can vary in size from a single trailer to Super B-Trains. Additionally, trucks tend to come during the daylight hours, so the system must be designed to handle the peak flow.

A delivery receiving system utilizing self-unloading trucks would include a truck scale, receiving hopper(s) and conveyors.  Self-unloading trucks are available as end or side-dumping or with `shuffle floors’. If non-self unloading trucks are utilized, then a truck tipper would be required.  There are many configurations and sizes available for end-dumping tippers.  A new side-dumping tipper has shown to be very effective at dumping Super B-Trains in extreme cold climates.

Biomass delivery by rail requires a railcar scale, a railcar moving system, an unloading system, receiving hoppers and conveyors to storage.  Unloading systems can be as simple as mobile equipment, or can be an end-dumper or for high capacity a roll-over dumper.

Biomass delivery by barge is popular on the west coast and the equipment utilized varies greatly.  The best and most simple system utilizes mobile equipment that drives onto the barge via a ramp.  The ramp is equipped with a receiving hopper that the mobile equipment fills.  The hopper discharges into the conveyor system to storage.  Other systems utilize dockside knuckleboom unloaders or overhead cranes with clamshell buckets.  Delivery by barge requires a docking / mooring facility and possibly a warping system, that will accommodate water level changes.

Conveying Systems

Belt and chain conveyors are most common because of their reliability.  Generally, belt conveyors are less costly but require more real estate, due to their lower operating slope; however, belt conveyor slope is greatly reduced in cold climates.  Conveyor speed should be kept quite low, less than 400 fpm.  And belt loading should also be kept low to allow for oversized material.  A belt weigh scale is often included.

Chain conveyors can operate at steeper slopes and are suitable for shorter or complicated runs.  Avoid concave vertical curves.  Utilize abrasion resistant wear plates; however, rocks and grit tend to become embedded in UHMW and will wear out chain quickly, so take care where you use it. Chain conveyors are costly, so resist the urge to undersize components to save on cost.

Avoid chute valley angles less than 45°; with some materials and in some conditions, even this is not steep enough.

Dilute phase pneumatic conveyors have long been used for transporting hog fuel; they are less costly than belt or chain conveyor systems, but have high power requirements and can generate a lot of fugitive dusting.

Screening / Metal Removal System

A scalping screen is recommended for removing gross over-sized material, which you can expect to get in your system regardless of the assurances of your biomass supplier.  The disc type of screen is best suited to this tough application and is also quite good at removing stones larger than the screen opening.  Utilize abrasion resistant steels and don’t undersize the screen.

Depending upon the quantity of gross over-sized material rejected by the screen, there may or may not be a grinder / hog.  If one is required, it is recommended that a solid rotor, high inertia hog be utilized. Additionally, a metal detector belt should be installed before the hog to stop the feed should large pieces of tramp metal be detected.

Ferrous metal removal is essential and can be accomplished with a magnet.  There are several magnet types and arrangements available, but the primary magnet should be a self-cleaning one.  Avoid locating magnets in the cross-belt configuration if possible, as the magnet will have difficulty removing metal which is underneath the load on the belt.  Locate the magnet over a pulley transition, where the material is falling and the material is in suspension.  If possible locate the magnet after the scalping screen; doing so ensures that gross over-sized material will not become jammed between the magnet and the conveyor.

Storage and Reclaiming System

The amount of storage required depends upon: a) the minimum amount of material required to maintain reliable operation; b) the longest period of time that the flow of incoming material may be interrupted; and c) the longest amount of time that the plant must continue to receive biomass, when it is not consuming fuel.

Covered storage is costly and is only necessary in special circumstances such as in certain urban environments that mandate it; or where the biomass is quite dry (<25%) and the climate very wet; or conversely where the material is wet and there is a real possibility of the material freezing into lumps.  In most climates and locations, open storage piles are suitable.  Dusting issues can be minimized by the use of wind fences.

Pile building can be accomplished by mobile equipment, over-pile belt or chain conveyors, slewing / luffing stackers, or pneumatic conveyors.

There are many storage and reclaim systems available and each have their place.  Choosing the right system depends upon: the material being handled; the amount of storage required; the amount of live-storage required; the need or not for first-in / first-out flow; the ability to accurately meter; the need for mixing; and whether covered storage is required or not.

I’ve found that fully automatic systems with 100% live-storage are prohibitively costly and most projects that I have been involved in end up with something less, usually with enough live-storage for 2-4 hours of operation with no intervention.

Under-pile chain reclaimers are the lowest cost but have the smallest live-storage and tend to bridge.  Ladder-stoker reclaimers have been in use for decades, are quite reliable and have somewhat more live-storage.  Depending upon their size, over-pile, rotary chain reclaimers can have live-storage of 12-24 hours, but they are more costly.

Under-pile, rotary screw reclaimers work well and do a good job of mixing but are costly. Under-pile, linear screw reclaimers also work well, do a good job of mixing and can have the largest live-storage volume; unfortunately, they are very costly.

Live-bottom silos are suitable for storing some biomass such as wood chips, sawdust, pellets and shavings but are not recommended for hog fuel that contains a wide variety of sizes and stringy bark.

Conveying System to Combustor

The conveying system to the combustor should include a weigh scale and a tramp metal magnet.  Additionally, a final scalping screen is recommended.

The type, quantity and size of fuel feed bins is usually specified and supplied as part of the combustor package.  The conveying system to the combustor should be capable of distributing the hog fuel uniformly so that the fuel bins remain full.

The conveyor system from the reclaimer through to the combustor feed bins should be variable speed and should remain full so as to minimize the lag time between the reclaimer and the fuel feed bins, thereby keeping a near constant level in the fuel bins.

With today’s accurate bin level sensors controlling the feed conveyors to the combustor, it is no longer necessary to overfeed the bins and have a return conveyor system to the storage pile.

Dust Collection Systems

Hog fuel can be quite dusty and you should provide for dust containment, suppression and collection in your design.  Depending upon the locale, high efficiency cyclones may suffice, but in others, bag houses will be required.  If the dust is sufficiently fine, spark detection, deluge systems and abort gates may be required.

Rock / Sand Removal Systems

Rock and sand removal systems are costly to install and operate and are rarely used.  It is far better to prevent the rocks from entering the material flow than it is to remove them afterwards.  Consequently, outdoor storage piles should be paved.

Bark Presses / Dryers

In most greenfield cases, it is best to design the combustor to suit the moisture content of the material being utilized as fuel, rather than attempting to dry the biomass.  However in some brownfield installations, bark dryers are necessary to reduce the moisture content of the biomass to a range that is compatible with the combustor.

Sometimes on the west coast, biomass is so wet (>60%) that bark presses must be utilized to reduce the moisture content to 55%.

Wood pellets are quite dry (12-15%), are fragile and easily break-up every time they are handled; so a material handling system for wood pellets has special requirements, including keeping them dry.  The dust produced by degrading pellets is very fine and is combustible and explosive; therefore dust collection systems with spark detection, deluge systems and abort gates are essential.

Minimize the number of times the pellets are handled and utilize gentle handling equipment, including: self-unloading, `shuffle-floor’ trucks or bottom dump railcars; `shuffle-floor’ hoppers; conventional belt conveyors or steep angle belt conveyors; silos with gentle loading chute and sloped bottoms to variable opening discharge chutes; belt conveyor(s) to the fuel bins; screw distribution conveyor over the bins; and dust collection systems.

If the wood pellets are being co-fired with pulverized coal, they will also need grinding or pulverizing before injecting into the combustor.

About the Author

Paul Janzé has more than 30 years experience in engineering design, project management, equipment manufacturing and maintenance, primarily in the forest products and energy industries. His material handling experience includes: biomass handling and processing including forest residuals, logs, lumber, chips, woodwaste, straw and poultry litter, sludge and biosolids; municipal solid waste (MSW); and coal and ash handling.

He has a keen interest in technologies which recover and utilize waste materials and convert them into products such as wood pellets.  Paul’s specialties are fibre flow analysis and mass balances, process optimization and designing novel solutions to complex processing and handling problems.

For other articles related to biomass processing and handling, go to Paul’s blog at www.advancedbiomass.com

Paul can be reached by email at: pjanze@telus.net