By A. Isiklar, L.Aydin, D. Mainardi and O. Lopez
At the beginning of 2006 PMT Italia and Hayat Group in
Izmit Turkey started up a new plant for the production
of high quality bulky tissue to be converted and sold
in the large demanding tissue area of the Turkish region.
The two companies jointly developed the project as the
greenfield Yenikoy mill over an 18-month period.
The
plant, which is considered one of the most modern in
Turkey, includes a Crescent Former PMT Italia tissue
machine for high-quality tissue paper, four unwind stations,
and one PMT Italia winding station for the production
of multi-layered types of tissue. It also includes two
PMT Italia stock preparation lines with the auxiliary
systems necessary for the operation of the machine (steam
system, high-performance cogeneration hood, lubrication
system, DCS, etc.).
The main technical parameters are:
• Production at converting : 60,000 air dry tons/yr
• Average daily production: 180 air dry tons
• Max. daily production: 230 air dry tons
• Design speed: 2200 m/min
• Max. speed: 2000 m/min
• Range of basis weights at reel: 14-28 g/m2
• The types of paper produced are mainly toilet
tissue and kitchen towel.
A key feature of the Hayat plant is that the machinery
has been designed to optimize the production cycle, the
quality of the tissue produced and the energy savings
factors. For this reason, the following key factors on
the equipment supplied by PMT have to be taken into account:
• The multilayered headbox (implementing production
of soft tissue) has been provided with a dilution system
that allows to reduce the 2
•'5f value of the basis
weight profile providing fiber savings and optimal winding
operation
• The felt run has been designed with one press
and with the possibility to be easily retrofitted with
a shoe press
• The large diameter press has been supplied with
no drive configuration
• The YD has been supplied as a high-load dryer
allowing nip pressure of up to 170 KN/m
• The creping doctor has been supplied as an adjustable
on-the-run creping doctor
• The reel has been provided with secondary-arm
center-wind assist in order to facilitate winding operations
maintaining bulk on the paper
• The Brunnschweiler hood has been designed to use
the residual energy in waste gases coming from the two
gas turbines that produce the electricity to run the
plant. This enables the mill to eliminate fuel consumption
in the Yankee hood burners in normal running conditions
• The
exhaust gases from the hood are used to produce the steam
needed to feed the YD and the mill’s other auxiliary
equipment (wet strength pulper, hall ventilation)
• The residual energy in exhaust gases from the
boiler are used to feed a chiller unit which operates
the air conditioning system of the electrical room.
These features have led to a highly efficient energy
cycle.
COGENERATION AT HAYAT
Drying tissue paper using high temperature and high drying
rates is a very inefficient process. Many attempts have
been made in tissue mills to reduce the cost of the drying
process. At Hayat, the mill decided to install a cogeneration
system to help reduce the cost of thermal energy.
The electrical
efficiency in Hayat Yenikoy plant is 33.7%; 66.3% of energy
input is released as residual heat in the waste gases coming
from the turbines. These waste gases are at about 500ºC
and, at high flow rates, can easily be used in a special-design
Yankee hood reducing and even eliminating the gas consumption
in their air systems depending on the production rate.
Thermal energy can be used in this way to dry the tissue,
then to generate steam and cold water at a constant temperature.
Electrical equipment is needed to distribute the electricity
or to produce it in parallel with the utility grid. Hydraulic
interconnections are needed to transport cold water or
steam wherever it is required.
The cogeneration system in Hayat’s plant consists
of six basic elements:
• Two 7.5 MW gas turbines, natural gas fired
• Brunnschweiler Yankee hood
• Two steam boilers
• Four duct burners (two reserve burners for the
hood systems and two are off)
• Absorption chillers
• New adapted control system in hood air circuits
for the perfect integration between the operation of
the hoods and the turbines.
The gas turbine is designed for continuous operation
from idle to full load. The turbines features a lean premix,
low emission combustion system for NOx control designed
to achieve low NOx and CO.
Selected turbine exhaust mass gas flows and temperatures
are suitable for the drying necessities of various tissue
grades. Full use of cogeneration gases is desired. Cogeneration
gases flow calculated have minimum gas consumption in
hood burners. Fans, burners, hood, boilers and chillers
are designed according to these new air flows and temperatures.
As a comparison with traditional systems involving a
conventional gasheated hood we can see the main difference
in the operation of a new system.
If we compare graphs 1 and 3, we can see that the waste
energy flows delivered to the atmosphere are reduced
with the complete integration of the three processes
(electricity generation, tissue drying and steam generation),
which means that the total energetic efficiency of the
system is improved. This improvement with the actual
efficiency parameters normally could be computed.
The improvement is much more evident if we consider that
the energy that we still deliver to the atmosphere is associated
with low-temperature waste gases that flow with very
low quality and little capacity for power generation or
any other useful effect. This can be easily seen in a Grassman
chart showing exergy flows instead of energy flows:
So, the main advantages of the system are:
• Better gas energy saving,
• Minimum gas energy loss through chimney because
of use of residual
thermal energy from cogeneration gases for drying.
RESULTS & CONCLUSIONS
In the Hayat installation there are two gas turbines,
each one producing 7,315 kW @ 15 C° and 97,000 kg/h
of waste gases at 490ºC. In Turkey, as in most countries,
electrical energy is more expensive than thermal energy
(natural gas, for instance). Some 3 kW of electrical
energy can be produced using 1 Nm3/h of natural gas.
The rest of the energy is used for steam generation and
chilled water production, apart from the use of the waste
gases in the Brunnschweiler Yankee hoods.
Because of the electrical balance of the Hayat site the
generation of electrical energy exceeds the requirements
for the tissue plant. This excess electrical energy could
be used in another Hayat plant. It can now be sold to the
grid.
Making some computation of the costs, using actual prices
for natural gas and electricity in Turkey, we get the following
results. These are divided according to two main operating
modes: full capacity of 230 tpd; and average capacity of
192 tpd. For each scenario results are given with and without
cogeneration.
In the first case (full capacity) there is a net cost
reduction of US$19 per ton of tissue on the reel when operating
in cogeneration-hood mode. Costs fall from US$145.5 to
US$126.5, equivalent to a 13% of reduction with cogeneration.
In the second case (average production) the reduction
in energy per ton of paper using cogeneration hood operation
mode is US$22.8. Cost per ton is reduced from US$166 to
US$143.2/ton, representing a 14% reduction with cogeneration.
In both cases we have left out of consideration the income
related to the excess of steam produced for ancillary
equipment.
Desired operation of the cogeneration system as explained
above is based on the simplified control of installations
which are listed below:
• Parallel arrangement to simplify controls and
enable easy integration
between gas turbines and Yankee hoods.
• Control of available pressure in the turbine exhaust.
• Control of balance in the hoods.
• Control of supply temperature.
• Control of p in combustion fans.
• No reason for moisture control in the exhaust
from the Yankee hood.
• Bypass arrangement.
• Control of pressure in waste heat boiler.
For the total efficiency point of view cogeneration type
drying is clearly the right choice, and it opens a new
field in the use of residual energies in the drying processes
where very modest attempts in this respect have been
carried out in the past. The main innovation of this
project (now a reality) lies in the fact that no one
has ever undertaken an integration to this extent.
TW
