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Residues to Riches: Pay for energy

In my last column, I introduced the importance of moisture content for biomass quality, as well as its effect on burning efficiency and the overall efficiency of the supply system.


April 22, 2010
By Mark Ryans

In my last column, I introduced the importance of moisture content for biomass quality, as well as its effect on burning efficiency and the overall efficiency of the supply system. Apart from the fact that some bioenergy plants will not accept biomass if the moisture content is higher than an acceptable limit, another underappreciated and negative effect of moisture content is on the delivered cost. There will always be some water in the delivered product, but the problem with biomass is its low bulk density and low energy density.  This problem is exacerbated by long transport distances and volume and weight limitations on trucks that travel on forest roads and highways.

residue

Download PDF of chart

The following is an example developed by my colleague, Denis Cormier of FPInnovations, to illustrate the effect of moisture content on costs.  The biomass used in the example is white birch, an underused species across Canada and a likely biomass source that could be delivered as a whole-tree chip. The effect will be similar for species that have a similar specific gravity, and slightly less for species with a lower specific gravity.  The truck and chip van (51-foot, four-axle) have a maximum gross vehicle weight of 57.5 tonnes, and the volume of the van is 113 m3. The bulking factor for the chipped birch is 37%.

Two moisture contents are used in the example. These levels represent a realistic range in managing the moisture content of biomass in the field, from little consideration of its importance (55% moisture content) to a supply system where moisture reduction is a goal (35%).

At legal loads (Fig. 1), the biomass at 35% moisture content (MC) fills the van with material; 37% of the volume is actually wood, 11% is water, and 52% is air.  However, at the higher 55% MC, the legal load is reached before the van is filled, resulting in biomass filling only 27% of the van volume. The corresponding loads of biomass are 22.6 oven-dry tonnes (ODt) at 35% MC and 16.3 ODt at 55% MC. Thus, there is a 28% reduction because of load limitations and the added weight of water.

If we bring in the burning efficiency of the biomass at the two moisture contents (Fig. 2), the calorific content of the two loads is 15.1 and 12.6 GJ/ODt at 35 and 55% MC, respectively.  When the total delivered loads are expressed in energy value (MWh), there is an even greater reduction in the “usable” portion of the load, from 95 to 57 MWh, with the higher moisture content. So the energy value of the load is reduced by 40%.

Now, if we pay on a green-tonne basis (Fig. 3), say $15 per green tonne for transportation only, the transport cost is $9.50/MWh at 55% MC, but only $5.50/MWh at 35%.  Thus, we have paid $4/MWh more for water, and there is a 73% cost increase using the same truck at legal loads when delivering wetter biomass.  Again, this is only an example to illustrate the importance of moisture content, but these levels are well within the realm of possibility for forest-
origin biomass when we manage for, or don’t care about, moisture content.

We could change the scenario using different truck-van combinations and legal loads as per provincial regulations, moisture content, bulking factors, species, and other factors, but the conclusion will be the same: If we continue to pay on a green weight basis, we create inefficiencies in the system.  Contractors are not rewarded for delivering the quality of biomass that we want.  Put simply, we pay for water, not energy, and the cost implications are dramatic.


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


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