1. Introduction - Firenze University Press

ways. It could for example be used to cover the heat demand of production processes for
additional products (such as electricity production or capture of CO2)or it permits the
introduction of processes which reduces the steam production whilst producing new products
(such as extraction of lignin or black liquor gasification where the product gas is used to
produce motor fuels or electricity). Throughout this paper these two options are referred to as
“utilisation of a potential steam surplus”.In a previous study by the authors [18], it was shown
that making investments in steam saving measures and thereby enabling production of
additional products generally is very profitable and contributes to reduction of global CO2
emissions.
The trend in the pulp and paper industry is toward fewer mills with larger capacity. This
means that some mills will be closed down, while the remaining mills will increase their
production capacity [15]. For increased production capacity in pulp mills, the recovery boiler
is often a bottleneck. In a study by Axelsson et al.[16], lignin extraction was found to be an
economically attractive alternative for debottlenecking the recovery boiler, in comparison to
upgrading the recovery boiler (and steam turbines) at a kraft pulp mill. The economic
performance of lignin extraction compared to increased electricity generationfor utilization of
a potential steam surplus is better for mills investigating investment options in connection
with increased production capacity [8, 16]. Another approach to achieve debottlenecking of
the recovery boiler is to introduce a black liquor gasifier as a booster. This approach has been
investigated by for example Berglin and Andersson[17], who concluded that a black liquor
gasifier using the product gas for steam generation yields a better economic return than
investing in a new recovery boiler (it was assumed that the existing recovery boiler could not
be rebuilt).
2. Objective
This paper compares three different options for debottlenecking the recovery boiler and
utilizing a potential steam surplus at a typical Scandinavian kraft pulp mill when increasing
the production capacity by 25%:
1. Upgrading the recovery boiler
2. Lignin extraction
3. Black liquor gasification (as a booster)
For black liquor gasification two options for the product gas are considered; production of
electricity (black liquor gasification combined cycle, BLGCC) or DME motor fuel (black
liquor gasification with motor fuel production, BLGMF). Furthermore, both black liquor
gasification and upgrading of the recovery boiler are assumed to be possible to combine with
carbon capture and storage (CCS),where excess steam is used to cover the heat demand of the
capture process. The extracted lignin is assumed to either be valued as wood fuel or as oil.
The different options are evaluated and compared with respect to annual net profit and global
CO2 emissions for four different future energy market scenarios. A further analysis of how
different parameters such as policy instruments and investment costs affect the different
technologies is also included. The results are compared with the results from the previous
study by the authors where no production increase was considered; see [18].
3. Methodology
This work follows a methodology previously developed and described by one of the authors
[6, 9]. The methodology enables changes in a studied energy system to be analyzed in a
systematic way. The main steps in this methodology are briefly described below:
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