Thinning and harvesting forests in the Pacific Northwest generate lots of residual woody biomass. Currently, much of that biomass is left in slash piles while a small percentage is used to heat buildings or generate electricity. By law, those slash piles left unused are burned to reduce forest fire hazard. This process contributes to smoke and air pollution. NARA is helping to develop an industry that converts this unused wood material into chemicals like isobutanol.  As this new industry develops, demand for forest residuals as a feedstock should increase.

As an industrial feedstock, forest residues are quite variable. Factors that influence the quality of this potential feedstock are the species mix, residue age, moisture content, harvest method, as well as the amount of dirt, rocks and trash mixed with the wood.  As demand for forest residuals increase, the market will require new grade classifications and standards for these forest residues. In fact, according to a recently published paper “Characteristics of Forest-Derived Woody Biomass Collected and Processed in Oregon” draft standards for solid biofuels are being developed by the International Organization of Standardization (ISO) and the American Society of Agricultural and Biological Engineers (ASABE) to serve the future markets.

NARA researcher and Feedstocks Logistics Team Leader John Sessions is a co-author of this research aimed at assisting both the ISO and ASABE efforts to classify solid biomass fuels. This NARA effort measures vital quality characteristics of forest- residuals, such as moisture content, particle size distribution, bulk and energy density, and noncombustible ash content. In this study, 55 forest residual biomass samples, processed in the field for energy production, from 34 harvest sites in Oregon were collected over a two-year period. Roughly two-thirds of the samples were collected from western Oregon and contain predominantly Douglas-fir residuals.  A third of the samples (19 of 55) were collected in eastern Oregon containing predominately pine and juniper species. The information obtained from this study suggests that characteristics of forest wood residuals remaining after commercial harvest differ widely.  Here is a summary of their important findings.

Moisture content

Moisture content is important because it can have a significant effect on residue transport costs due to weight considerations. In addition, wood biomass with high moisture content does not burn efficiently and is less attractive for boiler use than dry biomass. In this study, moisture content (wet basis) for the 55 samples averaged 40% but was found to range from 12 to 66%. This range is strongly influenced by the collection period; dry or rainy.

Particle distribution

Particle size distribution within each sample was determined by sorting through a two of screen sizes; 50 mm (2-in.) and 10 mm (3/8-in) round hole sizes. Screened samples were separated into three fractions: large particles retained on the top screen (overs), material in between screens (mids) and those that fell through the lower screen (fines). On average, 5% of the particles ended up as overs and 25% percent were fines; thus leaving 70% as mids.

Particle variation among the samples was significant. The maximum percentage of overs, mids and fines was 12, 98 and 94 respectively; whereas the minimum was 0, 6 and 2. In general, fines contain much of the bark, needles and contaminants (dirt and rocks). For conversion to isobutanol, fines are not an attractive feedstock due to their relatively high ash and low carbohydrate content (see earlier newsletter story). In addition, the authors point out that the particle size distribution can impact processing and handling characteristics at the point of use particularly for energy producers.

Non-combustibles (ash)

The samples contained noncombustible (ash) content as high as 26.7 (% of dry weight). The lowest ash content detected was 0.3% with an average ash content of 6%. Ash content in the fines was also assessed. On average, ash content in the fines was 14.1 %. Since the ash content of softwoods is typically under 1%, these findings indicate that the majority of samples contained contaminants such as dirt and rocks.

Heating value and bulk density

Energy density values for the samples were determined under two classifications: low heating value (LHV) was determined from green samples and high heating value (HHV) was from oven-dried samples. Energy density values varied with a maximum of 16.8 (MJ/kg) and 21.6 and a minimum of 5.0 and 11.5 for the LHV and HHV samples respectively. Lower values associated with the green samples can be attributed to moisture content.

Bulk density was measured for wet and dried samples. Maximum range for wet samples was 432 (kg/m3) and the minimum was 108. Maximum range for dried samples was 224 and the minimum was 72. Bulk density can have a substantial effect on transport costs as described in an earlier paper co-authored by Dr. Sessions and summarized here.

Management implications

The variability shown in the measurements reflect the variability of woody biomass feedstock from forest operations available in today’s marketplace.  The authors offer some options forest contractors can consider to reduce feedstock variability. For instance, an economical method to reduce wood biomass moisture is to field dry the residues. Field drying also allows needles to drop, which would lower ash and fines content.

The authors observed that residual samples contained far less ash if they were pre sorted and kept relatively clean by chipping operators. In the paper, they state:

“…noncombustible contamination content can be managed by avoiding pushing piles together with tractors, running over piles with skidders, or trying to fully recover piles by scratching in the dirt. (p. 526)”

In addition, the type of machinery used can improve feedstock quality. The authors point out that chippers or mill hogs produced a lower percentage of “overs” and consequently a feedstock with higher bulk density than feedstock produced from horizontal and tub grinders.

In conclusion

Offering solutions, however, was not the paper’s intent. Below are concluding remarks in the text:

“The reader should note that this study was not designed to test interactions with process variables that might influence these quality characteristics. Our purpose was simply to generate information on what is currently occurring in the commercial marketplace. We note that the large variations observed in particle size distribution, bulk density, moisture content, and noncombustible ash are signs of an evolving industry. Development of residue classification systems, expanding markets, and continuing equipment development may markedly change the characteristics of tomorrow’s woody biomass derived from western forests. Future steps including structured tests of interactions between process variables and quality characteristics will be useful to inform managers on the costs and methods for reaching solid biomass fuel specifications.”