Novel Design of an Integrated Pulp Mill Biorefinery for the ...

2 Background
2.1 Pulp Mill Background
2.1.1 Harvesting and Chipping
The pulping process begins at the site where trees are harvested. When all factors are
taken into account, the most important idea behind cost minimization is that “optimizing forest
fuel supply essentially means minimizing transport costs” [6]. Two main options are available
for the transpiration of wood to the mill, one as solid logs and one as wood chips, where the
wood is chipped in the forest. Chipping is advantageous because it increases the bulk volume
which can be transported. The main disadvantages of chipping in the forest are the decreased
length of time for which chips can be stored. After their size reduction, microbial activity in the
chips increases, releasing poisonous spores, and energy is lost within the wood increasing the
risk of self ignition [7].
Recently the idea of storing the wood as bundles has arisen as a viable option to improve
forest-fuel logistics. Large eight cylinder machines are used to drive two compression arms
which bundle the wood similar to the way a person rolls a cigarette. The figure below shows the
difference between shipping loose residuals on the same size truck as a bundle [7]. This new
technology reduces the impact of transporting forest-fuel matter across larger distances.
Figure 1: Price of wood as a function of transportation distance.
There are a number of available technologies for debarking wood entering the plant.
Three main technologies at the head of the industry are ring style debarkers, cradle debarkers,
and enzyme assisted debarking [8].
Ring style debarkers fall into two categories, wet and the more common dry debarkers.
Wet debarkers remove bark by rotating logs in a pool of water and knocking the logs against the
drum. Dry debarkers eliminate the use of about 7-11 tons of water per ton of wood, thus reducing
water and energy use [9]. Wet debarkers need 0.04 GJ per ton of debarked logs of energy, while
ring style debarkers use approx. 0.025 GJ per ton of debarked logs [10]. A Cradle Debarker has
an electricity demand of 90 kWh and can debark 120 cords an hour [11]. An Enzyme assisted
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