Forest residue biomass bale weight tracking

Baling the small-diameter tree tops, branches, and brush fraction of forest residuals is an attractive alternative to in-woods collection and grinding. The Waste-to-Wisdom project is supporting development and testing of the Forest Concepts woody biomass baler.

The Forest Concepts baler was modified to increase bale density, reduce field crew requirements, and increase productivity. Beginning with field trials during August 2015, bales of woody biomass have been collected from each field trial for long-term stability assessments. Pallets with two bales each are being stored outdoors without cover at the Forest Concepts facility in Auburn, Washington. They are exposed to weather patterns typical of the Seattle area and are weighed at least monthly. Eventually, the bales will be chipped or ground into biomass fuel. After chipping, moisture content can be measured and back-calculated for previous weighings.

These bales address two important research questions posed by the Waste-to-Wisdom project. The first is how durable baled forest residuals are for initial transport, during long-term storage, and for after-storage transport if needed. As shown in the photographs, the baled material, some of which was baled a year ago, has changed little during storage and the bales show good integrity. A set of four pallets were taken 100 km (65 miles) after 6 months of storage to a public event to demonstrate transport integrity.

The second research question is how the moisture content of baled forest residuals changes with seasons and weather. Data from monthly weight measurements gives an indication of how baled forest residuals gain water content and dry down during seasonal weather.

The graphic below shows pallet weights (in pounds and kilograms) for bales of woody biomass made by Forest Concepts’ prototype baler from August 20, 2015 to-date. The start of new pallet numbers is an indication of the dates for subsequent field testing that produced new biomass bales that added to the study.

Pallets 1 and 2 were baled from very dry (about 23-25% moisture content) logging slash on Washington’s Snoqualmie Pass during a red-flag dry, hot weather event. Other materials were baled in the Seattle area in October 2015, and March, May and June 2016. Pallet 3 only had one bale on it until April 2016 when a fresh bale was added.

It is interesting to note that the pallets from August 2015 gained 453 kg (almost 1,000 lb) of water from rainfall during the winter per pallet. We do not know how much of the water gain was free water and how much was absorbed by the wood. They dried down almost back to their as-baled weight by the week of June 5, 2016 and below their as-baled weights as of early September 2016 The October 5 weights show the start of rehydration as we enter the fall.

The lessons from this side experiment are:

1. Bales of woody biomass store and transport well after at least 12 months.
2. Moisture (water) content gained dramatically during the winter months and then dried during the spring and summer dry season, even in the Pacific Northwest climate.
3. Drying and water loss during the spring is quite rapid with continuing dry-down during the summer.