Rock Removal from Woody Biomass

The upsurge of interest in biomass-fired power plants usually is accompanied by an increase in curiosity regarding new methods for removing non-combustible, non-organics such as rocks, stones, sand and grit from woody biomass. It has long been a problem, and operators have tried a variety of ways to deal with it. The methods are diverse, usually depending upon what the biomass will be used for. There are some specific methods that are very successful; others more a measure of how best to accommodate the problem.

If not properly sorted, rocks of all sizes and shapes can wreak havoc on machinery and equipment.

Following is a brief description of the most common methods industry has utilized for removing such non-organics from woody biomass.

1. Sawmills
Primary log breakdown facilities, such as sawmills or pulpmill woodrooms, usually remove the bark before processing. Doing so also removes dirt and stones that have become embedded in the bark during the logging process or while being transported or stored on unpaved areas. The dirt and stones become concentrated in the waste stream, which is often utilized as fuel in biomass plants.

2. Panelboard Plants
In northern climates, other facilities such as panelboard plants utilize log ponds to thaw out or condition (soften) the logs prior to debarking, peeling, chipping or stranding. Most of the embedded dirt and stones falls off the logs in the ponds and the rest is removed during debarking.

3. Wood Pellet Plants
The standard for domestic wood pellets is <1% of ash, so the plants must be relatively contaminant free. Wood pellet plants have historically been fairly clean, so rock or sand contamination has not been much of an issue. 

If not properly cleaned, trucks can carry a large amount of non-organics into the biomass stream, which in turn can damage machines.

However, in the past few years, there has been a huge increase in the demand for wood pellets, primarily for the subsidized, domestic European market. North America is the primary source for wood pellets, although other regions are entering the pellet supply chain. As well, due to the economic crash of 2008-2009, there has been a decline in housing starts in the United States, thereby decreasing the requirement for lumber and panelboard and resulting in fewer residuals being produced. These two factors have resulted in a shortage of clean furnish for the manufacture of wood pellets. Therefore, pellet producers are utilizing other sources of wood fibre, including standing dead wood or other non-merchantable timber. In addition to the requirement for low-ash pellets and the usage of poorer wood fibre sources, pelletizers are susceptible to damage and rapid wear, so rock contamination has quickly become an issue and solutions are being sought for eliminating rocks from the furnish.

4. Pulp Mills
Pulp mills that utilize the Kraft pulping process have found that chips of a uniform length and thickness produce a better quality of pulp. Knots and over-thick wood chunks do not “cook” as quickly in the digesters and need to recirculate many times through the system, taking up valuable production space and time. Pin chips (long toothpick-shaped particles) can also plug screens, thereby reducing production, along with small particles or fines that require excess amounts of chemicals to process through the recovery boilers. Consequently, pulp mills have developed sophisticated multi-stage screening systems to remove gross oversized pieces of wood, reprocess over-thick pieces, regulate the amount of pins being utilized and remove the fines. The screening system also aids in the removal of some non-organic materials, such as rocks or sand.

Rocks larger than the primary screen openings are separated out of the main chip flow along with the over-thick chips, which before being reprocessed are passed through an air density separator (ADS) to remove the rocks, that can damage the chip slicers/conditioners. Chip fines passing through the tertiary screens are also removed from the main chip flow to the digester, along with sand and grit.

5. TMP Mills
Thermo-mechanical pulp (TMP) mills utilize mechanical refiners to reduce wood chips to basic wood fibres. The close-tolerance refiners cannot tolerate non-organics; so wood chips are first washed to remove stones and grit.

6. Biomass-fired Boilers
Mass-burn biomass boilers are fairly tolerant of non-combustible non-organics, most of which are removed with the leftover bottom ash. However, large rocks can cause downstream operating problems in the ash-handling systems. At the other end of the ash size scale, very fine particles of sand being carried by the flue gases can seriously abrade the pressure tubes in the boiler.

Bubbling fluidized bed (BFB) boilers utilize sand as an efficient medium for transferring heat to the biomass fuel, and are especially effective on fuels with high moisture content. Combustion air is blown through the sand bed, which is mixed with the burning fuel, which is then quickly dried, heated and combusted. Rocks entering with the biomass fuel do not fluidize and accumulate in the bottom of the boiler, so a fraction of the fluidizing sand containing the rocks is continuously removed from the boiler bottom. Some plants screen the rejected sand to remove the rocks, clinkers and fines, while others send it to landfill. Sand entering the boiler with the biomass makes up some of the sand loss, but new sand must also be purchased to replenish what is lost.

Generally, keeping rocks out of the biomass fuel supply reduces boiler operating downtime and lowers operating costs, particularly so for power plants, which have strict penalties for unplanned outages off the grid. The rock contamination problem is somewhat less problematic for industries that have their own cogeneration (cogen) or combined heat and power (CHP) plants and utilize the heat and steam internally.

Contamination
Contamination consists of varying amounts of sand, dirt, grit, and stones, and occurs generally as a result of bad handling practices, including:

1. Dragging felled wood along the ground where dirt and rocks can become embedded in the bark.
2. Allowing dirt, grit and stones to build up on transport trucks and, by not properly cleaning them off, allowing rocks to find their way into the biomass stream.
3. Storing woody biomass on unpaved ground. Even with a sacrificial layer of biomass present, rocks will work their way up from the underlying soil.
4. Picking up grit and stones when reclaiming roadside logging debris (RLD) or pre-processed “hog fuel.”
5. Not taking the requisite care with primary plant residuals, i.e., allowing clean sawdust or chips to be mixed with “dirty” bark.

The best method of minimizing rock contamination is to prevent rocks from entering the biomass flow in the first place. One pulp mill found that rocks were migrating from the unpaved chip storage yard into the chip supply, and once the area was paved, the problem went away. Another mill found that removing the sand, snow and stones that fell off trucks onto the truck dumper before they could get into the chip stream solved its contamination problem.

As it is not always possible to prevent rocks from entering the biomass flow, some form of rock removal may be necessary. However, trying to remove 100% of the non-organics from the material stream is costly. Therefore, before selecting a system, first determine what size of rock particle is causing the trouble and design a system to remove it.

Rock removal methods
Effectively removing rocks from biomass requires a lot of equipment that is costly to operate. There are a few methods of rock removal, including:

1. Water Bath
Most woody biomass is lighter than water and floats, whereas most sand and rocks are heavier and sink, which makes the
water bath effective for rock removal. However, the water bath has serious disadvantages, including:

• If you are using the biomass for fuel, it is best not to get it wetter than necessary.
• If you operate in a very cold climate, wet biomass can freeze into lumps.
• The cleaning water quickly becomes contaminated and must be continually refreshed. Treating the wastewater can be costly, unless the facility already has a plant with a large wastewater treatment system.

2. Air Density Separation
Whereas both rocks and biomass are heavier than air, it has been found that heavier particles will fall out of a moving air stream and lighter particles will be carried away. Air density separation (ADS) systems are effective at rock removal, but also have disadvantages, including:

• A large piece of wood can weigh more than a small pebble, so ADS systems effectively work only over a small range of densities, where the particles are similar in size.
• Generally, they are capacity limited with low throughputs, requiring multiple machines.
• Large volumes of air are required, resulting in the purchase and operation of large fans and motors.
• Effective air clean-up systems that will meet emission standards are required.

3. Screening for Size
If only one particular size of rock is causing problems, it is possible to screen-out that size of particle and discard it. However, similar sized particles of biomass will also be discarded, which may or may not be acceptable.

Generally, I have found that a combination of screening for size and air density separation is the best method of removing rock particles from biomass. First, screen the entire product flow into different size classifications, one or more of which will contain the offending rock particle size. Then process the selected contaminated stream through an appropriately designed air density separator to separate the biomass from the heavier non-organics.

In most cases, it is fairly easy and not too costly to retrofit a rock removal system into industrial processes that already utilize multiple levels of size screening, as many of the required components are already in place. 

One example is a pulp mill that discovered that rocks 1/4" to 3/8" were causing substantial wear to the pulp screens. This mill already had multiple levels of chip size screening in place for process purposes. It was found that most rocks larger than 3/8" were being removed by the primary screen and over-thick ADS system, whereas the tertiary screen removed sand and grit smaller than 1/8". But rocks between 1/8" and 3/8" were staying in the chip flow to the digester. It was found that these sizes of rocks were concentrated in the chip flow from the secondary to tertiary chip screens, and it was therefore recommended that this contaminated stream be diverted offline to an ADS system where the rocks could be removed.

Rock removal system for biomass fuel
In my experience, the simplest and lowest cost rock removal system for biomass being used as fuel would include the following equipment:

1. A heavy-duty disc scalping screen for removing large rocks, lumps of frozen biomass and large chunks of wood.
2. A robust trommel screen with one or two different screen sizes depending upon the rock size to be removed.
   • The grit-laden fines would fall out of the first screen section.
   • An intermediate particle size would fall out the second screen section (if installed).
   • The very large pieces of material would pass out the end of the trommel screen.
3. One or both of the larger-sized streams could be passed over a vibrating conveyor equipped with one or two air knives. Heavy particles would fall out the opening at the lip of the air knife and lighter particles would be blown over the gap.
4. If required, the grit-laden fines could be cleaned on a dedicated ADS system.
5. If there were an excessive amount of gross oversized material, it could be further processed on or off site in a grinder/hog.
6. A dust clean-up system would be required.

There are several types and makes of screens that are used for processing biomass.

• Scalping screens – the heavy-duty disc type of screen is best (Jeffrey-Rader, Acrowood, West Salem)
• Trommel screens – good for most biomass fuel (PowerScreen, McCloskey, WestPro)
• Gyratory screens – very good for chips, sawdust and shavings (BM&M, Acrowood, Bruks)
• Thickness screens – specific to pulp mills (Jeffrey-Rader, Acrowood)
• “Flip-flow” screens – perforated plastic mat screens; (Jeffrey-Rader, Hein-Lehman)

However, note that vibrating screens are not recommended for biomass. Air density separation equipment includes:

• Vibratory de-stoners with air knives (General Kinematics)
• ADS systems (Jeffrey-Rader,
• Acrowood)
• Sand/grit removal from small particles (Jeffrey-Rader, Clarke’s Industries).

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 (including forest residuals, logs, lumber, chips, wood waste, pellets, straw and poultry litter), animal tissue, sludge and biosolids, municipal solid waste (MSW), lime dust, coal and ash handling.  He has a keen interest in technologies that 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 .