By Ulf Strenger, Senior Consultant Process Technology
Due to increasing
energy prices, energy has become an increasingly hot issue.
Significant efforts have been made to decrease energy costs
in tissue making by both the producers themselves and by the
machine manufacturers. In addition to increasing costs, energy
has become a sustainability issue and an environmental concern
- a factor that might soon affect our behaviour more than direct
cost issues. Carbon Footprint is expected to become an important
factor also for the end-user of tissue paper.
There
are several ways to attack energy costs; for the price levels
one cannot do much, but energy consumption can be impacted via
improved technology and efficiency. In this presentation, however,
we are focusing on the opportunity in energy supply systems and
leave the tissue technology to the rest of our own engineering,
to the machine manufacturers and to the producers that all have
already made a great job in this field.
The energy system chosen is to some extent dependant on concept
and technology, but also very much on prevailing price level,
availability of different fuels, environmental restrictions and
other circumstances. The price of gas (or some other fuel) and
electricity and their relation to each other can dictate what
system is suitable.
In this presentation we are also reviewing the different circumstances
in selected geographies.
Energy As % Of Costs
Manufacturing costs breakdown for major Western and Eastern
European tissue machines
shown in the pie chart in column 1 is extracted from Pöyry Forest
Industry Consulting's cost competitiveness analyses. Here the
costs refer to cash manufacturing costs for tissue mother reels;
all capital related costs, including depreciation, are excluded.
Energy costs refer only to cash costs of purchased fuel and electricity.
All labour costs are included in personnel costs and maintenance
materials and external services in 'other'.
The breakdowns can greatly vary in different locations, for
different type and standard of machines and also due to different
supply systems. Energy costs can account for one third of tissue
mother reel costs.
Energy cash costs in tissue making can vary from over €250/ton
to under €100/ton. A lot of this variation is
due to the concept or the technology. Important are also regional
differences in price levels.
Some examples of energy cost variations are presented in Figure
2. For the same process (normal dry creped) in the same location,
ie with same price level, the savings potential can be some €50/ton
just due to better technology and performance. Specific consumption
figures can havegreat variations as shown in Figures 3 and 4.
In addition to the consumption, another opportunity to reduce
energy cost is offered in the supply concept that we call here
the energy system. Cost savings here represent the same order
of magnitude potential as the energy consumption in the process.
Possible are also direct monetary impacts of subsidies to some
technologies and fuels although those are not considered in this
article.
Energy prices are lowest in regions that have their own fossil
fuels. Such low cost areas are for instance, MENA (Middle East,
North Africa), Russia and regions in the USA.
Certain geographies have low-cost coal available; some Eastern
European regions, eg Poland have this advantage and historically
also tissue mills had a coal based system instead of natural gas.
If natural gas is not available in pipe line, liquid gas can be
used in the hood. This is a somewhat more costly option, but allows
for a modern fast tissue machine. For instance in Sweden tissue
mills are based on LPG. When electricity generation is based on
nuclear power, like in France, electricity prices have typically
been clearly low. However, European prices have equalized in the
last about ten years.
Southern European energy prices are typically high. These areas
offer great potential for savings via optimizing the energy system;
also subsidies for improving energy efficiency are often available
The selection of the energy supply concept is strongly site
specific and is affected by tissue production data such as power
and heat consumption, production stability and mill lay-out. Important
non-technical site specific parameters include: availability of
different fuels and electric power; prices of fuels and electric
power; security of supply of fuels and electric power; governmental
support/subsidies for certain technologies/fuels; environmental
limitations and concerns; sustainability and carbon footprint.
Available options for energy systems are plenty and we are presenting
only some of those here in the following case studies.
For the same site various options might be feasible and should
be investigated thoroughly before the final decision for the energy
system of a new mill or change of system in an existing mill.
Natural Gas Available
There are of course a number of options for energy supply when
natural gas is available and some of those are described in the
subsequent text. We have chosen to compare the following: the
standard solution including gas boiler, direct gas burners for
hood and power from the grid; the advanced gas turbine cogeneration
configuration; gas turbine with exhaust sent to yankee hood and
to steam generator.
The standard solution: The most common solution for mills with
'normal' energy price levels includes direct burners in the hood
for hot air generation, steam boiler and electric power purchase
from a utility grid. Figure 3 is a schematic of the system.
In a gas turbine about 30% of the fuel energy input (LHV) results
in electric power
and the rest is wasted with the exhaust if there is no heat recovery.
The exhaust temperature is typically around 500°C. Compared
with combustion in a steam boiler, the combustion air and exhaust
flows are very large due to high excess oxygen.
In cogeneration of power and heat this large and hot exhaust
flow is used for heat generation. The most common way is probably
to connect a gas turbine exhaust to a heat recovery steam generator
(HRSG) in which steam is produced. This is usually denoted simple
cycle cogeneration. In larger applications more complicated systems
are common. The HRSG produces high-pressure steam and also steam
at different pressures for a steam turbine. Combine cycle cogeneration
is the term for this kind of systems.
For a tissue mill, the simple cycle cogeneration is often feasible.
To achieve high efficiency of the system, the gas turbine should
be selected so that the extractable exhaust heat is equal to the
mill's steam demand. Naturally, in a region with a very high electric
power price and low gas price also a less energy efficient system
can be more economically feasible than a highly efficient system.
This is because the less efficient system would involve a larger
gas turbine, higher power production and potentially higher power
income/savings.
Combined cycle cogeneration includes more equipment and is more
complicated. The specific investment cost for small units, sized
for a tissue mill, is very high. Steam turbines of this size are
also inefficient. According to our experience combined cycle cogeneration
is a less attractive option for the tissue mill.
In this article we are presenting a third cogeneration set up.
As stated above, high efficiency requests high utilization of
the exhaust heat and a good balance between power production and
heat consumption. To be able to produce more power than with a
simple cogeneration system for steam generation, still with high
efficiency, an additional heat consumer needs to be added.
Besides the steam to the yankee a tissue machine consumes hot
air and the solution is to use all or part of the exhaust as hot
air in the yankee hood.
In the studied system the energy in the gas turbine exhaust
is first used in the hood for drying the paper. The remaining
heat is then used to produce steam in a heat recovery steam generator
(HRSG). Burners for supplementary firing and back up are installed
in the hood air circulation system as well as in the HRSG. Supplementary
firing can be used when heat demand exceeds the available exhaust
heat. This is a less common solution, but there are a few installations
running. The system is suitable for a location with generally
high energy prices, restricted availability of electricity or
in a country where energy efficiency is subsidised.
A case study for comparison of the described cogeneration concept
was performed based on energy price data from Figure 2 for three
sample locations. The calculations were based on an average energy
consumption and economic life of 10 years and 5% interest rate.
Figure 5 shows the results.
For a tissue mill with one machine and no other heat consumers,
the heat demand is smaller than the heat available with the exhaust
gases if the selected gas turbine is based on the electric power
consumption. This means that the system efficiency is lower than
the potential. To increase efficiency, additional heat consumers
could be added, ie water heaters or heat driven cooling machines
of absorption type. Alternatively a smaller size gas turbine is
selected and the power deficit is purchased from the utility grid.
The present calculation is performed based on a gas turbine
size that matches
the power consumption.
It is further assumed that the machine is running at a constant
production level with constant energy consumption. In reality,
however, there might be large variations in products, production
level and energy consumption. These factors will have large impact
on the turbine selection and the feasibility.
Keeping the above in mind it can be seen that the extra investment
in cogeneration system of this kind would be less feasible in
the USA LA case and in the China Coast case, while it would be
feasible in Portugal where prices are higher. Note that the price
levels represent averages for large consumers and might not be
similar to what the individual tissue mill pays, and that subsidies
are not considered.
Natural Gas Is Not Available
LPG is used for hot air: Many mills are without access to natural
gas, simply because there is no pipe line in the vicinity. In
such cases liquid gas in tanks or kerosene commonly used solution
for hot air production in modern, fast tissue machines. The cost
is higher than for natural gas. This is the case in for example
Sweden.
Electric power is purchased from the grid and oil, liquefied
gas or kerosene are purchased for steam and hot air production.
Cogeneration with steam turbine: Instead of the simple system
described above, a steam turbine based cogeneration could be installed.
Heat would be generated in a high-pressure solid fuel boiler and
electric power in a steam turbine. The boiler could be fuelled
from biomass or coal and hot air produced in a flue gas heat exchanger.
The high pressure steam is used for electric power generation
in a steam turbine. Steam to the yankee is extracted from the
turbine and a small amount of steam is extracted for feed water
heating. In order to produce more power and to balance the heat
production with consumption, the turbine can be equipped with
a so-called condensing tail and a condenser. The steam is expanded
to below atmospheric pressure and the condensed at maybe 0.1 bar.
If biomass is readily available at a good price, it can be used
in this kind of boiler. Coal can also be used if inexpensive.
Figure 8 shows that cogeneration with solid fuel is not feasible
unless fuel prices are very low, mostly due to high investment;
however, subsidies can change the picture.. Generally the use
of back-pressure turbines designed to match the heat consumption
are more efficient and more economical, but the tissue machine
consumption is too low to allow this.
The other region in the graph is a region defined by lack of
gas, relatively high electric power price and very low price of
coal. However, this system is complicated and the investment is
high.
Future Possibilities
Current development in energy technology will open new opportunities,
such as:
- Gasification of biomass (or coal) to replace natural
gas, LPG or indirect
heating.
- Possibility to produce power with gasification and
gas turbine combined
cycle (IGCC).
Presently economically unfavourable at this scale
- Pyrolysis of biomass to produce liquid fuel to replace
LPG.
These technologies might be of greater interest in the future,
but for the time being it is probably not possible to justify
the investment and risks for real tissue machine projects.
Conclusions
To summarize the findings in previous sections we draw the following
conclusions: Energy costs depend on price levels, tissue manufacturing
process and energy supply system and can constitute up to one
third of the manufacturing cost for tissue paper. This means that
the energy supply system should be carefully considered to utilize
any possible energy cost savings potential, which might have a
significant effect on the total manufacturing cost and hence the
competitiveness.
Sustainability (is) will in addition to the cost be an important
factor for the end-user of tissue paper, e.g. carbon footprint
as a measure of the total CO2 emission production is already in
use for some products.
Energy cost comparisons between different energy systems show,
that there is no standard solution that fits all locations and
an open minded approach is necessary in order to find the most
suitable system.
We have presented a number of possibilities that can be considered
in the review of an energy system but there are plenty of others.
Remarkable is, that this is a complex issue with several parameters
to consider.
Optimizing the energy supply system is an opportunity often
forgotten. However, the investment can be fairly feasible.
Current development in energy technology will open new opportunities,
e.g. gasification,
pyrolysis. Also this development has to be followed for a continuous
improvement of the energy cost competitiveness of tissue products. TW
