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

Na2CO3 is the chemical that leaves the recovery boiler which will be recausticized to
form NaOH. Na2SO4 is active in the smelt, reacting with carbon atoms to form Na2S, the primary
active chemical in sodium carbonate formation. The following formula describes the main
reaction that drives the formation of Na2CO3:
Na2S + CO2
+ H 2O
→ Na2CO3
+ 2H
2S
For the sake of simplicity, it is assumed that all of the Na2SO4 is converted to Na2S which
is then fully converted into Na2CO3. Back calculations performed on the KAM2 mill [87] found
that 82.2% of Na2CO3 is converted into NaOH. Calculations show that 17,534 tonnes of sodium
sulfide must be added to continue the pulping process at a rate that sustains pulp production.
3.1.2.3 Pulp Mull Effluent
There is a high cost to producing pulp to the environment. It has the potential to generate
significant quantities of wastewater. The average pulp mill will produce 60 cubic meters of
wastewater per metric tonne of pulp produced. For the model purposes this would be the
equivalent of 120 cubic meters per day or about 45,000 cubic meters of wastewater per year.
For the current pulp mill, the treatment of the effluent will be through a three stage
process: primary clarification, secondary treatment and tertiary treatment. Current standards that
regulate current pulp mills require just two stages of effluent treatment. However, it is
anticipated that future legislation will require pulp mills to require further treatment.
The large, suspended solids are removed through dissolved air flotation which removes
80% of the impurities [89]. Simple chemical treatment of the remaining sludge will be the
aerobic process that uses chlorine and hydrogen peroxide to fill the filamentous bacteria in the
wastewater[90]. While it is an old technology, it is the most convenient because these chemicals
are the same ones which are being used in the bleaching process of the pulp downstream from
the energy flow.
The final stage is experimental because most mills do not have tertiary treatment in their
infrastructure since two stages meet the current federal regulations. In order to prepare for the
future, this already top-of-the-line pulp mill will again make further use of the ozone created for
the bleaching process to react with the pretreated wastewater to give final impurity of less than
100g/L [91]. This is an acceptable amount to dump into a waterway.
3.2 Black Liquor and Coal Gasification to Syngas
The general design of the coal-black liquor gasifier is based on a GE/Texaco design. The
Texaco gasifier design was chosen for black liquor-coal gasification because the technology is
“simpler and more reliable” than dry-fed units, generates a higher H2/CO ratio, and also “allows
for more flexibility in feedstock selection” [92]. Furthermore, if the feedstock blend is altered,
hardly any changes to the gasification design are necessary [92]. Because the Texaco gasifier
requires a slurry feed, and weak black liquor is an aqueous solution of about 15-17 % solids, the
significant energy requirement for evaporation of black liquor to 80% solids for dry-feed
gasification is avoided. The general operating conditions for a Texaco gasifier are summarized
in Table 16. The slurry should contain approximately 50-60 % solids [92, 93].
44