Paper Technology International 2020 - Journal - Page 90

PAPERTECHNOLOGYINTERNATIONAL
Renewable gas for tissue drying hoods enabled through collaboration
between Sofidel, Meva Energy and ANDRITZ
Thomas Bräck, MSc, Business Development Director, Meva Energy
INTRODUCTION:
Combustion of fossil gas to generate process heat for tissue drying is one of the few remaining unsolved sustainability challenges
within the tissue production industry. High quality demands on the hot air due to its direct contact with the tissue, combined with demands
for low cost and high availability, has rendered renewable energy unfeasible...until now! At Sofidel’s tissue mill in Sweden, a renewable gas
production unit from Meva Energy will be installed directly at the mill, supplying the tissue drying hoods with decentralized renewable gas
from biomass. This has been enabled through extensive research and verification with Sofidel, Meva Energy, ANDRITZ and the research
institute RISE.
Producing tissue involves drying the tissue to create the
softness and fluffy character required to get a high-quality product.
To achieve this, the best practice today is to let the paper travel over
a large heated steel cylinder, a yankee, at high speed. The yankee
cylinder is generally heated from the inside with steam and from
the outside with hot air. Direct combustion in gas burners is used
to generate this hot air, wherein the hot flue gases are blown into a
drying hood and used to dry the tissue paper.
As the hot air is in direct contact with the paper it needs to
be free from particles and other contaminants in order to ensure
high paper quality: black particles or a unpleasant odour cannot be
accepted. To achieve low cost, with stable and high quality process
heat, the best practice today is to combust fossil gas, i.e. natural
gas or LPG, in tissue drying hoods. This contrasts with low-carbon
sustainability targets within the tissue industry and it needs to be
resolved to enable fossil free tissue production.
The steam used to heat the inside of the yankee can be
generated through direct combustion of wood residues and other
biomass because the heat is not in contact with the paper product.
However, this principle cannot be applied to the production of the hot
air that is subjected directly on to the paper, as direct combusting of
solids is inherently associated with high emissions of particles.
One interim solution which has become more popular has
been to install co-generation, or even tri-generation, solutions where
fossil gas is combusted in a gas turbine which can generate both
steam for the inside of the yankee as well as power and heat for
tissue drying hoods. In this way a better usage of the fossil gas can
reduce the energy cost and the CO2 footprint, but as long as such
a solution is still based on fossil gas energy it can never be truly
sustainable. The problem is the source of energy: fossil gas needs
to be replaced with renewable gas.
Figure 1: Example of combustion chambers at a tissue mill, generating
heat for the drying hoods.
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