Water Content of Biomass (BEF, 2010) [WGT10140]

[This page is adapted from the former Biomass Energy Foundation website.]

The water content of biomass can be a major problem if it isn’t recognized, and a wet log looks exactly like a dry one, so one can’t judge “by eye”.

Biomass is a complex fuel, composed of the volatile components (typically 70-90%), charcoal that results on heating (10-30%), some mineral/ash content (1-20%) and varying amounts of moisture. For this reason most analyses are given on a “dry ash free” (DAF) basis.

However, wood and other biomass are almost NEVER bone dry, and go up and down with the seasons. Worse yet, sometimes moisture content is reported on a wet basis (MCWB) and sometimes on a dry basis. When a tree is cut down in summer it can contain 50% moisture (wet basis) or 100% moisture (!) dry basis. These two standards can cause a lot of confusion unless understood.

However, it is easy to measure water content. Weigh a sample, then heat to 105 C for 1-4 hours, depending on sample size to find bone dry weight. (I use my wife’s oven.) The moisture content wet basis (MCWB) is given by:

MCWB = 100 X (Initial Weight – Dry Weight)/(Initial Weight)

However, since we are interested in the dry use of biomass and don’t want to pay for the water, biomass moisture content is sometimes reported on a “dry” basis (MCDB) where

MCDB = 100 X (Initial Weight – Dry Weight)/(Dry Weight)

MCWB would be the basis usually assumed, but if one is interested in the fuel or lumber properties, one is most interested in the MCDB.