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

For our plant that produced 2000 air dried tones of pulp per day, the boiler production is given in
the chart below.
Table 12: Energy produced by KAM2 boiler.
KAM2 Steam (tonne/hr) Electricity (MWe)
Supplied by Boiler 463 54.6
Black liquor combustion in the recovery boiler provides electricity and steam to power
the KAM2 pulp mill. While this is ordinarily a sufficient process to meet the plant's energy
demand, the removal of the black liquor from the system requires energy input from external
sources. The chart below displays the calculated values of electricity and steam that are produced
through the recovery boiler and the associated back pressure steam turbines.
Table 13: Energy produced by KAM2 boiler.
Steam (tonne/hr) Electricity (MWe)
Supplied by boiler 336 39.42
From the calculations derived in Appendix A, the energy demand for the modified
pulping section of the plant is 56 MW of electricity and 267 tonnes per day of steam. The
demand for the steam in the plant is met by the recovery boiler alone, but there is a significant
amount of electricity which must be generated through the off gasses in the fuel process.
All pulping and integrated mills have a second, smaller recovery boiler that is used to
burn fuel oil, bark or natural gas to help meet auxiliary energy demands within the mill. In the
KAM2 mill, high pressure steam generated in the boilers is fed into back-pressure steam turbines
which generates electric power, and the heat demand is met using a steam of medium pressure
(10-12 bar) and low pressure (3-4 bar).
3.1.2 Group Design Modifications
3.1.2.1 Energy
Because of the design shift from a basic recovery boiler, there is an opportunity to save
energy due to a reduced load on the chemical recycle process. However, more chemicals will
need to be purchased which will offset the energy gains.
Weak black liquor is removed to contribute towards the gasification process, and 6333
tonnes per day of the weak wash is distributed to the gasifier. Because it is a weak mixture, only
950 tonnes of black liquor dry solids are present in this solution. This means that 2470 tonnes, or
72.2% of black liquor solids will go through the normal recovery cycle.
As a result, there will be a reduced steam and electricity load on the evaporators.
However, the plant will lose valuable chemicals that will need to be supplemented in order to
effectively pulp the next process.
The energy savings per day can be found in the chart below.
Because the energy and electricity values are based on air dried tonnes of pulp, the
assumption to determine energy saved is that there is a linear correlation between black liquor
42