Don’t Pay for Water

One of the hidden enemies of an efficient supply chain is a simple
compound: water. Because of the importance of this subject, I will take
a couple of columns to cover the topic properly.

Water occurs primarily within the biomass itself, but in winter, it can
also show up as snow and ice clinging to tops and branches or on top of
uncovered storage piles of comminuted residues. Moisture content is the
single most important quality criterion for forest-origin biomass, yet
we rarely manage it. To put it frankly, moisture content is not managed
because we either don’t recognize its importance throughout the supply
chain or we don’t want to be bothered measuring it. Although attitudes
are changing, harvest residues are often taken for granted and not
treated as a product from the forest.

The most common scenario today is for feedstock payments to be made on
a green tonne basis at the mill gate, thus inciting the supplier to
deliver the material as moist as possible to maximize payload. This
results in paying for excess water, with a negative energy effect and a
less efficient bioenergy system than it should be. On larger-scale
operations, delivered biomass is often stored uncovered for months,
leading to increases in moisture content or losses in dry matter.

Before addressing the management of residues for the control of
moisture, I will introduce why moisture content is important, or more
precisely, why low moisture content should be an objective of any
biomass supply chain. Because most of the biomass used today is for
burning, whether it is for the generation of heat, CHP, or simple (and
inefficient) electrical power, we need to understand that water must be
“burned off” before the energy value of wood is realized. Another
emerging use of forest feedstocks is for pellets. Additional energy for
drying needs to be applied to the feedstock to reduce the moisture so
that the final product from the dies is less than 10% moisture. The
wetter the material, the more heat that is wasted.

Biomass burners are not all built the same, but they have more-or-less
similar burning efficiency curves (Fig. 1). The burning efficiency,
expressed as the available heat per unit weight at a given moisture
content decreases from a maximum of about 80% at 0% moisture to 40% at
70% moisture content. For example, burning fresh residues at 50%
moisture content would result in a burning efficiency of 61%. The heat
curve drops slowly until about 30% moisture content, rapidly between 35
and 50%, and then dramatically above 55%.

There is also a common misunderstanding that different types of woody
biomass differ in energy content. When expressed on a weight, rather
than volume basis, the energy value does not vary greatly between
species or parts of a tree and only varies by about 10%, from just less
than 19 to 20.5 megajoules/kilogram.

In a simple analogy, you wouldn’t use wet firewood in a wood-burning
stove if you want sufficient heat from it. You wouldn’t pay for green
firewood the same as you would for dry firewood, and if you do buy it
(at a lower price) you would season it before using it the next winter.

Small- and medium-scale boilers require more homogeneous feedstocks
with lower moisture content, so there is ongoing attention to feedstock
quality. Large modern boilers such as those used in Nordic countries
can handle various feedstocks at almost any moisture content, but that
does not mean that excessively wet biomass is acceptable. European mill
managers recognize the value of low moisture content and pay for
biomass on a megawatt-hour (MWh) basis. I am also amazed at how field
practitioners in those countries know the “volume” of a roadside pile
of harvest residues or stumps in MWh content. They don’t talk about
green tonnes. Deliveries are usually monitored for moisture content
using systematic sampling, or volume and weight conversions are used
when shipments from known providers are relatively stable. Under all
approaches, payments for harvest residues are based on energy content,
not green weight.

In my next column, I will show an example of the cost implications of
delivering wet biomass and discuss some approaches to manage the
moisture content of biomass.

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