Investors want to build a plant as large as possible because processing cost (per unit of product) typically goes down as plant size increases. Overend [4], Jenkins [5], and others more recently have shown how the average delivered cost of feedstock increases as plant size increases. The reason is that, as the feedstock consumption increases, the size of the production area increases, and hauling cost increases with average haul distance. Two factors are important:

1. Density of feedstock production — the percentage of total land area within a given radius of the plant that is attracted into feedstock production.

2. Feedstock yield — the tons of biomass harvested per unit of production land.

The two terms are sometimes lumped together into the term “feedstock density”. The influence of feedstock density is shown in Figure 13.4. Average hauling cost increases with plant size for all densities. Note, however, that the cost increase is much less for the higher density curves. A plant owner seeking to obtain an economy-of-scale benefit by building the largest plant possible will want to locate where the maximum number of surrounding land owners sign up to grow feedstock.

Figure 13.4 Influence of plant size on mean hauling cost. Production area percentage is the percentage of the total land area attracted into feedstock production.