Performance Despite Pests

The performance of a pellet plant greatly depends on feedstock quality and pretreatment.
By Holger Streetz | December 02, 2018

The performance of a pellet plant greatly depends on feedstock quality and pretreatment. The usual difficulties in operation increase when the wood is infected with pests and varmints. Due to a dry and hot summer in Germany, we are expecting a lot more infected wood than usual. Here are some thoughts and clues to be better prepared for what is coming.

Twenty to 30 percent of woody plants’ dry mass consists of lignin. While biological and chemical processes can break down lignin, higher-evolved organisms cannot, thus infestations by insects such as longhorn beetles do not decrease lignin in wood. Pelleting high ratios of infested wood in the feedstock mix, however, will cause major issues with stability and abrasion resistance.

Plant cell walls are made of cellulose fibrils set in a matrix of pectins, hemicellulose, proteins and lignin. Cellulose is the backbone of the plant cell wall, characterized by its tensile strength, whereas lignin gives pressure strength, comparable to steel beams in a concrete ceiling.

A healthy tree has multiple defense mechanisms against varmints. However, as soon as the bark is bruised, weakened by drought, or the tree is cut, varmints come along to lay eggs below the bark, in the wood, and the emerging larvae eat their way through. The most common green wood varmints are bark beetles, wood wasps, carpenter millers or longhorn beetles. There is also a variety of wet wood insects that need a fairly high moisture content to thrive. Often, the infected wood is not debarked and is already seized by fungi.

Longhorn beetles are the most common green wood pest. About 26,000 species are known worldwide, with 200 in central Europe. Larvae are laid in the wood, where they develop and pupate. The active life of the adult is usually a maximum of 90 days, but in many species, 30 or less days. Adult beetles pose no threat to the wood, and some beetles are so rare that they are under species protection. Europe is mostly suffering from the red long horn beetle and the violet tanbark beetle.

The biggest problem of varmints in green wood is larvae eating through in a hook-shaped path. Before pupating, larvae eat up to 1.5 inches horizontally into the heartwood, and gnaw a 1 to 1.5-inch-long vertical chrysalis chamber. These eating paths lead to a larger waste.

A fungal infection is characterized by different destructive phenomena. White rot manifests itself in attacking the white matter, or lignin. The local lignin degradation creates honeycomb holes in the wood and turns it white. Brown rot mainly occurs on coniferous wood. It destroys the cellulose of the wood, leaving the lignin, which then turns the wood dark. The wood is cracked, crumbly and eventually decays.

The questions at hand now are: How should infected wood be dealt with? Is it still pelletizable? Does it affect the pelleting process and the quality?

The unsightly truth is yes, varmints have a great negative effect on the quality of pellets, especially the structural strength. Pellets with a high content of infected wood tend to break much faster and have higher abrasion. The infestation often comes multilateral, affecting both structural components of the wood. A fungal infestation destroys lignin; insects digest the cellulose. Even with only one infestation, the woody material is weakened, and the ratio between cellulose and lignin is misbalanced.

Many pellet producers in Europe add starch as a binder, but there are certain limits to its addition (1 percent), and it does not substitute for the natural balance of cellulose and lignin. Eventually, it works more as a lubricant than an adhesive. A longer, effective press channel only pretends to have an effect. The pellets look shiny and good, but this is only on the surface, as they will still suffer from the weak structure of the wood. Also, the effective press channel is longer in North America than in Europe, so any prolongation possibilities are already exhausted.  It is common to add water in the pelleting process, which helps release lignin and lowers stress on the equipment. Increasing the moisture content, however, has no effect on the binding characteristics if the wood is already damaged. The only solution is to mix the damaged wood with healthy, green wood, fresh wood chips and clean sawdust to lower the structure weakening effects of infested wood.

In portfolio theory, risk mitigation is achieved by diversification. The same principle can be applied to feedstock management: By sourcing from many suppliers and different woody biomass types, a plant can process a more homogenous feedstock than it would relying on fewer sources. This applies not only to already-infested or overaged wood, because the risk of a quality change is given at any time. In the case of infested wood, we found that the share should not exceed 15 percent, and must be mixed thoroughly with healthy wood to avoid feedstock imbalances.

Author: Holger Streetz
International Operations Manager, Bathan AG