Now suppose the bulk density of the load is increased by 20%. The load is increased from 12 to 14.4 dry ton, and the cost per dry ton (10 min load, 10 min unload) is reduced from $9 to $7.50.

We are now ready to address a very important issue in logistics system design. It is obvious that increasing bulk density increases dry tons per load. (The volume of the truck is fixed by highway regulations.) But what is the cost of increasing the bulk density? In this example, suppose the cost of increasing the bulk density is $2.75/dry ton, and no cost benefit is assigned in loading or unloading operations for the higher bulk density material. The reduction in truck cost is only $9.00-7.50 = $2.50/dry ton. This means that $2.75 has been incurred in densification cost to save $2.50 in truck cost, thus the total delivered cost increased by $0.25/dry ton. It is better, just considering truck cost, to haul the raw biomass without densification.

13.6.2 24-h Hauling

Now suppose the same truck in the above example hauls the same dry ton per load over the same distance (10 min load, 10 min unload) and operates continuously 24 h/d. The truck can now haul 16.7 loads in a 24-h workday as compared to seven loads in a 10-h workday. (It is acceptable to use the 16.7 loads, since the truck is operating continuously. It can average 16.7 loads/d over some chosen time period.) The labor cost increases because operators must be hired for three 8-h shifts plus the maintenance cost per day increases due to more miles traveled per day. Use $800/d as the cost for the truck.

The truck hauls 16.7 loads per day, thus the fuel bill is:

$43.75/load x 16.7 loads/d = $731 /d

Total truck cost is:

$800/d(ownership + operating) + $731/d (fuel) = $1531 /d The cost per dry ton is:

$1531/d

16.7 loads/d x 12 dry ton/load

The truck cost has been reduced from $13 to $7.64/dry ton, or 41%, by operating 24 h/d rather than 10 h/d.

Why not design logistics systems to operate 24 h/d? The issue in feedstock logistics is not the unloading at the receiving facility — the plant operates continuously so the receiving facility can certainly operate continuously. The issue is loading the trucks. No one has devised a system to load trucks at night at some remote location. An example will be shown later for trailers loaded during the day and pulled during the night, so that the same truck tractor is used for 24-h hauling.

The issue of trucks operating on rural roads at night is unresolved. This may be accepted, or it may not; there is little experience to establish a precedent.