Environmental Engineering Reference
In-Depth Information
The conversion of the old pulp mill to the biorefinery of today has been accomplished through
specialization and continuous innovation, as illustrated in the product mixture manufactured at
different time periods (Fig. 8.3). The innovative contribution is also seen in the development
of value-added applications and products and in the ability to find and explore niches in the
market place. In combination with an excellent mass balance, this has made Borregaard able to
achieve a sustainable biorefinery business.
The Borregaard Sarpsborg mill still produces sulfite pulps in batch digesters using calcium as
the cooking base. The calcium SSL has been transformed from a SHE problem to products of
high quality. The mill has been enlarged several times to a design capacity of 160,000 tonnes of
bleached market pulp per year (Bogetvedt and Hillstrom, 1996). In the following text, we will go
into more detail according to the main features of the schematic presentation of the Borregaard
biorefinery shown in Figure 8.2.
8.2.1
Lignocellulosic crops and residues
The Borregaard mill uses small dimension timber and wood chips of Norway spruce as raw
material. The wood chips are delivered from sawmills while the timber logs come from the
forests. The logs are debarked and chipped and are together with the sawmill chips fed to the mill
digesters.
Cooking liquor is added to the digester and the chips are delignified according to given target
specifications. The delignified chips are fibrillated at the end of the cooking process. The spent
sulfite cooking liquor containing lignosulfonates and soluble sugars is separated from the fiber
pulp by filtration, and is further processed to extract bioethanol, vanillin and lignosulfonate
chemicals.
8.2.2
Biomaterials, specialty celluloses
The fibrous pulp is treated in a hot alkali extraction stage and further bleached with an ECF
(Elementary Chlorine Free) sequence. This treatment removes hemicellulose, lignin and impu-
rities (metals, bark residues and extractives) and adjusts cellulose chain length. The resulting
pulp, known as specialty cellulose, is the raw material for production of cellulose ethers, cel-
lulose acetate and nitrocellulose. Each pulp grade has different requirements, and the specialty
celluloses are normally tailor-made for each individual customer. This requires a pulp production
facility with a high degree of flexibility.
The main parameter of ether pulps is the intrinsic viscosity. High viscosity (long cellulose
chains) or low viscosity pulps (short cellulose chains) are produced for cellulose ether customers.
Cellulose ethers find their use in construction, paint, food and pharmaceutical applications.
Acetate pulps do not vary much with regard to cellulose chain length but require a high degree
of purity (low hemicellulose content). Even small variations in purity can be crucial for the quality
of the final cellulose acetate product. Cellulose acetate is used in filters, plastics, textiles and
films (e.g. cellulose triacetate as a film layer in LCD screens).
8.2.3
Bioethanol
During the acid sulfite pulping process, the hemicelluloses and short chain celluloses are dissolved
and hydrolyzed to monomeric sugars as mentioned in section 8.2.1. In the spent sulfite liquor
from spruce, approximately 80% of the sugars are C6 sugars. The spent sulfite liquor is fermented
by saccharomyces yeast to convert the C6 sugars to ethanol. The ethanol is distilled off and the
desugared sulfite liquor is further processed to lignin chemicals.
Today Borregaard is the world's largest producer of second generation bioethanol produced
from lignocellulosics, with a capacity of 20,000m
3
per year. We produce both technical grades
(A-grade 95.8% and absolute technical 99.9%) and pharmaceutical grades (rectified 96.2% and
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