Tissue360 - Fall/Winter 2017 - 17
Reducing CO2 Emissions
on Tissue Machines
An Air Side Control System that can be retrofitted to
existing machines is a key element
BEN THORP, FRANK D. SORRELLS
Conventional Tissue Machine
Drying (CTMD) technology was developed
to reduce low carbon emissions by reducing
the amount of natural gas burned, a significant
reduction in energy costs. An Air Side Control
System (ASCS) with the CTMD software are the
key elements of the technology and the ASCS can
be retrofitted to existing machines. In a recent
computer study, a machine with hoods arranged
in cascade was compared with a machine with
hoods in parallel. The machines are typical two
wide tissue machines with gas-fired hoods operating at 1,500 m/min producing 14.5-g/m2 tissue.
* The parallel machine reduced CO2 emissions by
33 percent when the moisture of the drying air
was increased from a humidity ratio of 0.1 to 0.4.
* The parallel machine saved €479,000
($564,700) per year in energy costs over the
The authors of this paper are Ben Thorp, who
retired from Georgia Pacific where he served in
top management positions including director,
pulp and paper engineering; and, Frank Sorrells,
who retired from Valmet Enerdry where he
served as manager of technology transfer.
Some years ago, Thorp's engineers attempted
to optimize the moisture in the drying air on
one of its machines. They had some success, but
operators could not restore high humidity after
machine changes. Thorp concluded that a control
system would be required, but this was beyond
the scope of work. Sorrells has spent more than 20
years developing the CTMD software programs
and the ASCS for controlling the various air side
devices. Ten copyrights have been issued to him.
It is easy to state how the energy is saved.
Adjusting the exhaust air damper to decrease
the amount of air leaving the system causes the
moisture in the air to increase if the makeup
air damper is adjusted to decrease the amount
of fresh air entering the system. An air balance
must be maintained. When those changes are
made, there is less makeup air to heat to the
blowing air temperature, so less burner energy
is required. It is easy to state, but complex to do.
The CTMD software program includes an optimization routine where the entire drying spectrum is
displayed on a graph for each hood section. The three
controllable variables on the air side are the blowing
air temperature, velocity, and humidity. An optimize
graph (Fig. 1) is shown with all three variables plotted
along with energy cost. Every potential operating set
point for a machine is displayed in the graph.
These graphs permit mill management to select
their desired operation. In most EU countries the
cost of electricity is about twice the cost of natural
gas when compared on an equal energybasis. That is
reflected in the slope of the humidity lines.
In a 31-page paper that Thorp and Sorrells wrote,
this graph is discussed in depth, including important
information about how moisture influences the tissue drying rate.
The longer document also describes the computer study and includes 12 pages of computer
printouts; verification using theoretical equations
to calculate the energy to heat the makeup air and
that differs by 1.9 percent from the program's
CTMD, Version 6.0
WE SecƟon "OpƟmize" Data, RN 4
Copyright [c] 1966 - 2013 by Frank D. Sorrells
All rights reserved..
$ / Hr
Color - Hum.
Blue - 0.25
Green - 0.35
Cost = 202
The red double
ring is present
[cost vs vel].
Nozzle Velocity - thousands of fpm
Yellow - 0.25
Magenta - 0.45
Dry End Curve
Fig. 1. Optimize Graph
The cost savings of the parallel arrangement over the cascade arrangement is:
Savings = (€108.3 - €49.5)/hr * 24 hrs/day
* 365 days/yr * 0.93 eff = €479,000/yr
Tissue360º FALL/WINTER 2017