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Residues to Riches:A Case for Covering

This is the final column in a series on the importance of moisture content in forest-origin biomass, paying by energy content, and managing residues through the use of best practices to reduce moisture content in the field.

Heavy-duty paper wrap covering residue piles that are drying in the field is a common sight in Sweden and Finland. Photo: Feric

In my previous column, I discussed proper piling and handling methods. These methods can go a long way in reducing the moisture content of harvest residues and can provide direct financial benefits by increasing the energy content and facilitating a more efficient grinding operation.

One issue that I have not addressed is the timing between the harvest, the piling of residues at roadside, and the eventual comminution operation. To complicate matters, the material is usually needed in the winter, which is the worst timing for moisture content in forest residues because of fall wetness and the inclusion of snow in the pile and ice sticking to the branches. A complementary and beneficial practice to a proper piling technique and good timing is to use a covering material on the roadside piles.

There are a multitude of factors at play when it comes to drying residues in the field: for example, the season of harvest, sunny days, rainfall, snowfall, temperature, wind (airflow in the pile), location, species, proper pile formation and orientation, etc. FPInnovations has a number of trials on the go to examine some of these factors under local conditions, but it is too early yet to provide any results.

Nevertheless, a good example of the importance of covering and aging residues comes from a Finnish Forest Research Institute study. The trial included two treatment methods: fresh Norway spruce residues (Green) were immediately forwarded to roadside after harvest in early June; others (Brown) were left in the cutover to season and then forwarded to roadside in August. Half the piles in each treatment were covered at roadside with a special paper. Fresh residues at roadside piles and in the cutover dried rapidly to below 40%. By August, when the Brown residues were forwarded to roadside, they had started to reabsorb moisture. The roadside piles continued to lose moisture and actually dropped to 20%. In the fall, however, the piles that were not covered from both treatments regained moisture rapidly. In the end, the covered piles (Green and Brown sources) maintained their low moisture content through the winter and were 15% lower in moisture than the uncovered residues. Although the species and other factors will be different, we expect similar trends in Canada.

Residue covering is now a common sight in the Nordic countries, especially Sweden and Finland. The wrap is designed for covering roadside piles of harvest residues or small trees and is made of ex-terior layers of wet-strength paper and two middle layers to provide reinforcement, tear resistance, and waterproofing.

Covering piles adds costs related to the price of the wrap and its application. Laying paper over residue piles is a relatively simple procedure. After cut-to-length operations, the application cost is minimal. The wrap is laid on top of the roadside pile as it is being formed using a wide “paper” dispenser and the forwarder’s crane. Some loose residues are placed on top of the cover so that it does not blow off. In full-tree operations, we envision that an excavator could lay the paper during a pre-piling operation, or the paper could be laid on top of roadside piles formed during the conventional harvest in a well-integrated operation. In these cases, there is a more substantial application cost, as an additional machine may be needed. Shipping 4 m wide rolls to the field is a burden as well.

Despite these costs, we expect a positive return under Canadian conditions with only a 5% reduction in moisture content. A 15% reduction would result in a return of over 200%. Such a dramatic return will depend on the payment system and price for energy (power or heat), but is well within current energy pricing.

Using Mother Nature to dry harvest residues in the field with a little help by using a cover to reduce rewetting and ice forming within the pile will go a long way to increase the value of the feedstock and improve the overall efficiency of the supply chain.

Mark Ryans is with FPInnovations–Feric Division and can be reached at mark.ryans@fpinnovations.ca.