Comparisons of In-Woods Densification Options in the Western Gulf

Wood Energy July 31, 2013 Print Friendly and PDF
Wood Densification eXtension's team of wood energy experts walk you through the various processing activities to forward woody biomass feed stocks to centralized processing stations in the Western Gulf region. Take a look!

by Eric L. Taylor, A. Gordon Holley, and Mike Blazier

Forest resources can be considered a “scattered biomass stock” as they are spread throughout large geographic areas. Also, on average, forests produce relatively few annually available tons per acre, per year compared to annually harvested agricultural crops. Instead, woody biomass sources produce many tons per acre on an infrequent basis (10 – 30 year rotations). Even if conversion technology had no more limitations and could convert woody biomass into products and fuels economically, a major hurdle would still exist with transporting and delivering the woody biomass feedstock to a centralized biorefinery (Badger and Fransham 2005) (Badger 2002). Scattered, low-bulk density feed stocks and infrequent access results in high handling and transportation costs. Handling includes not only in-woods activities such as harvesting, skidding or forwarding, chipping and loading, but also includes operations at the end-use point such as weighing, dumping, screening, storage and grinding.

Possible harvest chains for woody biomass systems in the Western Gulf states

Economies of scale dictate that biorefineries be large and centralized. As a result, several methods exist to comminute biomass into a form that can be economically transported to a centralized location. These included the common woody biomass pre-processing operations of chipping, grinding or { shredding. Woody biomass may also be compacted into cylindrical bales or bundles that may be handled similarly to roundwood. In-woods prefining through mobile pyrolysis units can also densify bulky, low-value forest biomass into higher-value, low bulk feedstocks for the energy and chemical industry. The objective of this work is to provide a theoretical feasibility comparison of processing activities to forward woody biomass feed stocks to centralized processing stations in the Western Gulf region.

The Resource

Logging residue consists of limbs, tops, and unutilized cull trees that remain on site once roundwood has been merchandized for conventional products. Harvest residues also include trees that do not meet mill specification (oversized logs, undesirable species, deformed trees, small diameter trees). A recent study by estimates that logging residues in East Texas could generate 1.5 dry Mt per yr of woody biomass. An additional 1.6 dry Mt per yr of woody biomass can be harvest by conventional methods from Central Texas from non-traditional forest stands. (Xu et al. 2008). Logging residues on Louisiana forestland in Louisiana can produce 3 Mt per yr (Hubbard et al. 2007). Together, the total yields 6.1 Mt per yr which is equal to 120 PJ (assuming 19.6 GJ per ton) that can be supplied year-round.

Unfortunately, energy density of loose packed biomass from logging residue is generally too low for it to be economically transported over long distances (bulk density of logging slash here and how, because of the bulk, cannot be loaded to maximize the carrying load of trailers of approximately 24 tons). This results in approximately 15% of the total softwood and 25% of the total hardwood resources being abandoned that could otherwise be utilized for energy. It has been estimated that an average of 3.6 million metric tons (37 trillion BTUs) of woody biomass is allowed to rot in place (Polagye et al. 2007) (Bentley and Johnson, 2004).

Whole-tree Harvest (Felling, Skidding, Delimbing, Merchandizing)

Massive amounts of conventional harvest, handling, and transport infrastructure is already in place the across the Western Gulf region. The preferred harvesting method is the whole-tree, one-pass timber harvesting system in which the roundwood and biomass is harvested simultaneously. This system have proven to be the most cost effective extraction method when terrain is an issue. It uses conventional timber harvesting equipment for the region including a feller/buncher, skidder, delimber, and loader.

At least in the near term, a biomass harvesting system should be built-around this current system. The desirable woody bioenergy system would be one in which the conventional harvest operations are modified in such a way that it capture residues without reducing production (or value) of conventional products. The system must be cost competitive, and the system must produce a desirable feedstock. For this study, we assume that a one-pass, whole tree method of harveting is employed, whereby the entire tree is felled and skidded back to the logging deck. With this system, the energy wood is cut and skidded at the same time as the other products. However, the feller-buncher simply pile the energy wood separately from the roundwood for the skidder At the logging deck, whole trees are delimbed, de-topped and merchandized for roundwood. The remaining limbs, tops, and boles as well as the energy wood serve as the input for the bio-fuel production in this study. When combined with round wood harvest, forest residues have very low costs to bring it to the landing (logging deck). But there is a cost. If only conventional harvest for roundwood, The cost of felling, whole-tree skidding, delimbing, merchandize and loading is currently $14 (US) t-1 in East Texas/Louisiana. Collecting smaller material and conventionally cull trees adds may add an additional 15% ($2 t-1) to deliver it to and sort it at the logging deck.

Transportation of Roundwood

Trucking for round wood or chips, within 60 miles of mill costs $7.50/ton. Total cost for logging and shipping round wood to mills within 60 miles is $21.50 t-1.

Chipping

Chipping provides a way of processing woody material into an acceptable fuel for some applications and simultaneousely improving bulk density, homogeneity and handling characteristics. Chipping independently or along side of a round wood operation, utilizing the non-merchantable debris (tops and slash) for chips, involves a chipper, another loader, another man, and trailers and costs $18 t-1 plus the $14 t-1 for felling, delimbing and loading. Total cost for logging and transporting (within 60 miles, 111 kilometers) non-merchantable chips from a tandem merchantable/non-merchantable logging operation is $32 t-1. The biggest reason for the increased cost of chipping is cost of equipment. A chipper costs $500,000.000, chip vans cost $30,000.00 each (verses pole trailers at $2,000.00 each, another loader cost $125,000.00. Chipper maintenance is higher than regular logging, as the knives or blades have to be changed out pretty frequently. Additionally, most wood burning electric providers require a moisture content of 50% or less, which reduces net payload weights, even with a full load of chips. If these bio-energy plants do start paying the $32.00 t-1, it is obvious they will get a lot of chipped merch wood because OSB and paper mills are only paying $22-25 t-1 delivered.

If chipping is performed in the at or near the landing, there are a number of negatives. Storage is negatively affective. Independent studies of in-woods chipping in East Texas suggest that maximum storage time is approximately 60 days. Once material is chipped, it is important to use the fuel as soon as possible to prevent excessive energy value loss from microbial activity, and risk of self ignition from storing green chips (Johansson et al, 2006). However depending upon time of year the material is harvested. Past that, energy value is significantly reduced. Chipping costs for loose residue at or near the harvest site are higher than chipping bundles at either a terminal or power plant. Chipping/grinding costs are reduced by 65% at a centralized facility with parasitic load.

Grinding

Pyrolysis requires additional grinding to reduce chips to 3 mm or less. In-woods grinding cost $14 - $18 greent-1 and may constitute the greatest cost of the overall process. However, advanced pyrolysis reactors technology currently

being researched (e.g., ablative) may tolerate feedstocks at 6 mm. This can be obtained with chipping and would eliminate the need for grinding. However, additional drying is required to reduce the moisture content to 10%. The energy for drying may be obtained from heat recovered from the pyrolysis reactor or by burning a portion of the woody biomass. Rotary drum dryers are commonly 

used for drying chips. Drying cost 1.5 – 3 greent-1 .

Bundling

Bundling is a recently introduced technology to the Western Gulf State. Bundling creates a compressed and uniform composite residue log (CRL) from harvest residues and other small dimensional wood (Johansson et al, 2006). CRLs are approximately 24 inches in diameter (61cm) and 10 feet (3m) long. The John Deere 1490D Slash Bundler is currently being used in the Western Gulf region of the US. Retail price is $400,000. Bundles are efficiently handled and transported with conventional equipment used for roundwood. Simple modification to the trailer may be required depending upon CRL length and makeup. The 1490D Slash Bundler can produce 18 – 26 bundles per hour. Cost to bundle at the logging deck is approximately $11 – 14 greent-1. Total cost of operations to gather, bundle, and deliver = $21 - $25 green t-1. A blend of pine and hardwood offers best economic advantages due to the amount of available material on ground, relative ease of handling and energy content. The calorific heat content has been measured at 10 MJ green kg-1 (4300 Btu green lb-1) and an energy density of 4.5 GJ m3. By comparison, calorific heat content of oven dry bundles yield 19.7 MJ ovendry kg-1 (8500 Btu lb-1).

Terminal

Slash Bundling offers superior storing characteristics. Private data shows that seasoning bundles (storing) for 11 months reduces moisture content to 25 -30% and increase energy value to 17.2 MJ kg-1 (7400 Btu lb-1). The post-seasoned delivered cost is 

14.50 t-1. To date, approximately 67,280 m3 have been bundled from 15 sites in East Texas. The best commintion option appears to be horizontal grinders at a terminal. Grinders accepts bundles easily and costs are minimized. Independent data show a 65% reduction in grinding costs using a 1500 horsepower electric motor from parasitic load of the electric power plant for which the bundles are serving.

Pyrolysis

In-woods pyrolysis can convert forest biomass into liquid form, which simplifies and reduces transportation costs. Pyrolysis oil (bio-oil) has comparatively higher energy density than raw forms of biomass, 6 to 7 times that of green whole tree chips (Badger and Fransham 2006). Produced under the right technology, Bio-oil can be used as a substitute or as a blend with No. 2 fuel oil for heating, power plant fuel, or for use in the chemical industry. A pyrolysis facility can be mobile, transportable, relocatable, or stationary. The cost and production capacities for the two facilities of interest are outlined in Table 1. In-woods, mobile pyrolysis costs $159 t-1 compared to $73 t-1 for a transportable facility at a more central location.

Table 1. Projected mobile and transportable pyrolysis facility costs and production capacity.


Transportation costs are a significant determinant of where bio-oil should be produced. Transportation costs can be minimized if bio-oil is produced with an in-woods system at the logging deck. Though current technology of mobile pyrolysis units have relatively low throughputs and are considerably more costly than centralized facilities, transportable pyrolysis may become cost effective as the technology matures. Though transportation of bio-oil is limited by weight versus volume, bio-oil tankers can carry roughly twice the energy potential of chip vans. Once delivered to a centralized power plant a bio-oil handling system requires less land area (1.8 ha) than a wood chip or bundling facility (1.9 and 4.2 ha, respectively), though all cost roughly the same to construct.

Conclusions

Table 2. Comparison of deliver cost to a electrical power plant and anticipated value of various biomass forms.


The only densifying method that showed to be profitable at current market conditions is the transportable pyrolysis unit with a net value of $1.21 GJ-1 (Table 2). Green and seasoned composite residue logs from bundling came the next closest to profitability with a loss of only $0.09 and $0.05 GJ-1, respectively. If oil prices increase in the future as expected and as the biofuels market and technology improve, biomass market prices should increase. Even with elevated pricing levels power plants likely need to pay an additional $3-5 t-1 to cover cost and to encourage new investments in processing facilities and equipment.

Additional Notes

  • Please note that all of these costs are what loggers would like to receive, but because of the economy, they are now willing to produce roundwood at lower rates. They are not willing to make the investment that in-woods chipping requires for delivering chips at even a lower value.
  • Currently, the production of bio-fuels using mobile pyrolysis facilities is significantly more costly than production at centralized facility, and is more costly than production and transportation of bundles for the same end use. Fast pyrolysis becomes significantly competitive when is can utilize larger feedstock sizes produced from densification processes which do not require energy intensive pretreatment of feedstock (such as bundling). However, the substitution of bio-oil for heavy fuel or chemical building blocks must be reliable demonstrated in order to encourage adoption by industrial users and gain market share (Polagye et al. 2007).
  • Mobile and transportable Pyrolysis may become cost effective as the technology becomes more mature, widely adopted, and as the value of bio-oil is appreciated.
  • Current fast pyrolysis processes require pretreatment of feedstock. Chips are dried to 10% moisture and then grinding to 2-3 mm. Future fast pyrolysis may eliminate the grinding requirement allowing larger, dried wood chips to be used in an efficient fast pyrolysis conversion. This is termed “advanced case pyrolysis” …reactor capable of converting forestry chips without size reduction.

Units of Measure

  • 1 cubic meter of solid wood = 2.5 cubic meters of wood chips
  • A chip van carries approximately 25 tons of green wood chips = 10 cubic meters
  • Wood is 45 – 55% moisture
  • One bone dry pound of wood = 6.3 million Joules of energy = 6,000 Btu (depends upon species) others have it listed as 7600 – 9600 Btu/lb.
  • 1 Btu = 1,055 Joules
  • 1 bone-dry tone of wood chips = 1,000 kilowatt-hours (kWh) = 1 Megawatt hour (MWh) - the average American home uses 888 kWh per month and 10,656 kWh per year
  • 1 MWh = 3,600 MJ = 3,412,141 Btus
  • 1 bone-dry tone = 0.58 barrels of oil = 1 MWh = more than one month’s worth of home electricity in US.
  • 1 cord = 1.2 US ton = 2400 lbs = 1089 kg = 1.08 metric tons
  • 1 metric tonne wood = 1.4 cubic meters (solid wood, not stacked)
  • Energy content of wood fuel (HHV, bone dry) = 18-22 GJ/t (7,600-9,600 Btu/lb)
  • Energy content of wood fuel (air dry, 20% moisture) = about 15 GJ/t (6,400 Btu/lb)

References

  • Badger PC, Fransham P. 2006. Use of mobile fast pyrolysis plants to densify biomass and reduce biomass handling costs—A preliminary assessment. Biomass & Bioeenrgy. 30:321-325.
  • Bentley JW, Johnson TG. 2004. Eastern Texas harvest and utilization study, 2003. Resource Bulletin, SRS-97. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 28p
  • Hubbard W, Biles L, Mayfield C, Ashton S. 2007. Sustainable Forestry For Bioenergy and Bio-based Products: Trainers Curriculum Notebook. Athens, GA: Southern Forest Research Partnership, Inc. http://www.forestbioenergy.net
  • Johansson J, Liss J, Gullberg T, Bjorheden R. 2006. Transport and handling of forest energy bundles – advantages and problems. Biomass and Bioenergy 30:334-341.
  • Polagye BL, Hodgson KT, Malte PC. 2007. An economic analysis of bio-energy options using thinnings from overstocked forests. Biomass and Bioenergy 31:105-125.

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There are many factors that help determine the use woody biomass for energy production.  Below we consider the decision-making points involved in the process.  

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.