It is necessary to give the reader some additional perspective on SSL operations. If the yield of switchgrass in a commercial-scale operation averages 4 ton/acre, this equates to approximately 9 bales/acre. It is specified for this example that the minimum size SSL is 60 acres = 540 bales, and the maximum size SSL stores 1200 acres = 10 800 bales. If 70% of the theoretical loading of 30 racks/d is achieved, the contactor will load 21 racks x 16 bales/rack = 336 bales/d. The minimum size SSL will be loaded out in about 540/336 = 1.6 days. The maximum size SSL will be loaded out in /10 800/336 = 32 days.

Cost of the SSL operations at the smaller SSLs will be higher because of the mobilization charge to move equipment in for a relatively few days of operation. It is probable that the load-haul contract offered by the plant for each individual SSL will consider both the haul distance and SSL size, thus the per-ton payment will be different for each SSL.

B.3 Total Trucks Required — 24-h Hauling

To achieve 24-h hauling, the truck drivers will work 8-h shifts and the trucks will run continuously from 0600 Monday to 0600 Sunday, a total of 144 h/wk. The total racks processed each week is 630, equal to 315 truckloads. If a uniform delivery is assumed, the average truck unload time is

315 trucks/144 h = 2.2 trucks/h

or about one truck every 27 min. This productivity is well within the Rack System design goal of a 10-min unload time.

As previously stated, the 24-h hauling concept envisions that the loading crew will leave a supply of loaded racks on trailers at the SSL when they finish their 10-h workday. These racks will be hauled during the night. The next morning the loading crew will go to work on the trailers with empty racks delivered during the night and fill them during their workday.

The key variable in hauling is the truck cycle time. To calculate cycle time for this example, we need an average haul distance. An actual database was developed for a proposed bioenergy plant location at Gretna, Virginia and was used to calculate an average haul distance.

An analysis was done for a 30-mile radius around Gretna to identify potential production fields based on current land use determined from current aerial photography. Using a conservative assumption, about 5% of the total land base could be attracted into switchgrass production. SSLs were established at 199 locations (Figure 13.11), and the existing road network was used to determine the travel distance from each SSL to the proposed plant location at Gretna. Some loads were hauled two miles and some were hauled over 40 miles. A weighted ton-mile parameter was computed and found to be 25.4 miles. This means that, averaged across all 199 SSLs, each ton travels 25.4 miles to get to the plant.

Truck cycle time is calculated using the 25.4-mile average haul distance, a 45 mile/h average speed, 10-min load time, and 10-min unload time. Theoretical cycle time is 1.46 h. In 24 hours of operation, one truck can haul

16.4 loads per truck per day

Assuming that a truck can average 70 % of the theoretical capacity, the analysis uses 0.7 x 16.4 = 11.5 loads per truck per day. Remember, since the trucks run continuously, a decimal number of loads can be used as the average achieved per-day productivity.

It is not practical to use the each-contractor-runs-their-own-trucks assumption for 24-h hauling. The way to maximize truck productivity is to have the Feedstock Manager be able to send any truck to any SSL where a trailer with full racks is available. This greatly facilitates the hauling at both day-haul and night-haul SSLs.

Total trucks being controlled by the Feedstock Manager are:

53 loads/d required at the plant/ 11.5 loads per truck = 4.6 trucks

= 5 trucks

B.4 Total Racks Required — 24-h Hauling

Since the only time deliveries are not being made is the 24-h period, 0600 Sunday to 0600 Monday, the amount in at-plant storage can be reduced. It was decided to use 1.5 days as the minimum at-plant storage, so the total hours of capacity required in at-plant storage at 0600 Sunday, when deliveries are ended for the week, is:

24 h (actual) + 1.5 d x 24 h/d (at-plant storage) = 60 h

3.75 racks/h x 60 h = 225 racks

Total trailers are calculated as follows. Each truck has one trailer connected, two parked at a “day-haul” SSL and nine parked at a “night-haul” SSL for a total of 12 trailers. The total racks on trailers is calculated as follows:

5 trucks x 12 trailers per truck x 2 racks/trailer = 120 racks

Ostensibly, total racks required is calculated as follows:

At-plant + On 60 trailers + Reserve = Total 225 + 120 + 5 = 350

The actual number of racks required is calculated by subtracting the racks on parked trailers from the rack total (empty + loaded) at the plant. Potentially, 60 loaded trailers can be parked at the plant when hauling ends for the week at 0600 Sunday. In order for this procedure to work, the racks on most of these 60 trailers have to be returned to SSLs during the period 0600 Sunday to 0600 Monday so that they will be in position for operations to begin at each SSL at 0600 Monday. This requires some empty back hauls. Cost for these empty back hauls is a level of detail that must wait for a more sophisticated analysis.

When racks on trailers are counted as part of the at-plant storage, the minimum number of racks is:

At-plant + On 60 trailers + Reserve = Total (225 — 120) + 120 + 5 = 230 Average number of cycles per rack is

29 610 racks processed per year/230 = 129 cycles/y,

or about 2.7 cycles per week for 47 weeks of annual operation.