How do you measure biomass moisture? The short answer: with a portable moisture meter such as the M75 or the M75D, or with an inline meter. But it is worth going deeper. Materials called biomass are of plant origin (firewood, wood chips, sawdust, sugarcane bagasse or straw, rice husk, coconut husk, etc.) and are made up mainly of carbon (49% to 50%), oxygen (44% to 45%), hydrogen (5% to 7%), nitrogen (0.1% to 1%) and small amounts of other elements. All of them also contain a certain amount of water, and that amount is what we call moisture content.

What wet-basis moisture content means

Wet-basis moisture content is the percentage of the material’s total mass that is water. Example: 100 kg of biomass at 35% moisture contains 35 kg of water.

Moisture and energy: HHV, LHV and useful calorific value

For combustion to take place, the water in the fuel has to be evaporated. Part of the fuel’s energy is therefore spent evaporating the water in the material (as indicated by its moisture content), and another part evaporating the water formed when the hydrogen in the material combines with oxygen. The energy left after that is what is actually used when the biomass is burned.

To characterize a material by its ability to generate energy, three terms are used: HHV, LHV and useful calorific value, especially the last. What do they mean?

• HHV (higher heating value, PCS in Portuguese): the energy produced by a fuel at 0% moisture content. This value does NOT deduct the energy spent evaporating the water formed during combustion.
• LHV (lower heating value, PCI): the energy produced by a fuel at 0% moisture content, minus the energy spent evaporating the water formed during combustion.
• Useful (net) calorific value (PCU): the energy produced by burning the biomass, minus the energy spent evaporating both the water in the material (moisture content) and the water formed from its hydrogen.

In other words, always HHV > LHV > useful calorific value.

So how do you measure biomass moisture?

One way is to oven-dry a sample of the material until it reaches constant mass. The drawbacks are the time it takes, the small sample size and the equipment required. The method is described in the Brazilian standard ABNT NBR 14929:2017.

Another way is to use electronic meters. They give instant readings and, thanks to their simplicity and sample size, allow large volumes of material to be analysed, which makes the assessment far more representative (see the article “Using the dielectric (capacitive) method to measure wood chip moisture”). Results can also be stored in the meter’s memory for reports and statistics. Some meters also show the useful calorific value of the material, so it can be assessed directly by its ability to produce useful energy.

Author: Celso Martini, Eng., Sales Director at Marrari Automação

Want to apply this in your plant?

Talk to a Marrari specialist on WhatsApp or see the right solution for your process.