In the past year, energy prices have soared. In Europe, electricity now costs 25% more than a year ago, while gas prices have increased by as much as 50%. In the long term, prices seem set to continue on an upward trend, with the potential for mountainous spikes if conflict interferes with supply.

At 2006 prices, the energy cost per ton of paper is double the capital cost for a new tissue machine, according to Metso Paper Karlstad. President Anders Björn illustrates the point with reference to a new machine in Central Europe. In this example, capital cost is €65/ton, that of energy €125/ton.

Example:

•	Basis weight 18 g/m2, tissue machine design operating speed 2000 m/min, Yankee size 18ft
•	Investment cost €45 million, excluding civil and site costs. Financial cost based on 15 years payback at 7% interest rate with fixed annual instalments
•	Total jumbo roll production cost €850/ton, of which total energy is €125/ton
•	Jumbo roll sales price €925/ton
•	Existing margin €75/ton
•	30% energy saving (€37/ton) increases margin to €112/ton (+49%)

Traditional technology typically gives energy consumption of 2850 kWh/ton for a machine of this size and speed: 1800 kWh/ton in thermal energy (steam and gas); 1050 kWh/ton in electrical energy. A gas price of €0.030/kWh and an electricity price of €0.068/kWh will then give the total energy cost of €125/ton.

Clearly, there are many variables that could change this picture. However different they may be, though, it is clear that reducing the cost of energy can produce dramatic benefits to the bottom line. Metso is aiming for a 30% saving in energy consumption, which would amount to €37/ton in this example. And that would boost margins by 49% to €112/ton of tissue produced.

The full 30% saving is not available with current technology, the company admits. But the recently-launched DCT technology already allows about 20% energy savings compared to previous generations of machine and the remaining 10% could be available within 2-3 years.


How are these savings achieved? By a series of measures starting from stock prep and running the whole length of the tissue machine. The following details are based on a new Advantage™ DCTTM 200 machine designed to operate at 2000m/min and making 65,000 tons/yr of tissue.

For the OptiRefiner™ RF new refiner segments have been developed, MicroBar™. The new segments cut power consumption by 25% for softwood fibers and 30% for eucalyptus. This will reduce tissue line refining energy by 15-25 kWh/ton.

The OptiFlo™ II headbox is a new generation of headbox for the whole Mesto Paper group. The main process improvements are the result of a new design of turbulence generator and nozzle. The thin nozzle with thick wedges at the upstream end gives a headbox operating at high flow velocity and as a result an improved formation. The improved formation will allow about 20% reduction in the headbox flow, which in turn reduces the fan pump energy consumption by 20%. At high speeds, fan pump energy can be greater than 50% of the total machine drive load and the saving can thus be significant: 40-60 kWh/ton, worth €2-4/ton of tissue.

One of the key areas where energy can be saved is the press section. Today, post-press dryness of 39-40% can perhaps be taken as standard best practice using single suction press roll technology. The new ViscoNip™ press can operate over a range of linear loads, 70-160 kN/m, and with different pressure nip profiles, se graph above.


Metso expects its new ViscoNip™ press will boost dryness by 7% when operating at highest peak pressures, see graph above. This will reduce the drying load by 25%, which corresponds to an energy saving (steam and gas) of 450 kWh/ton. Payback time will be 1 to 2 years, depending if it is a new machine investment or a rebuild case.

The first ViscoNip™ press went into operation at InterTissue in the UK in September last year. The product quality is good and the machine has been operating with high machine efficiency. The Yankee dryer at InterTissue is designed for a linear load of maximum 120 kN/m. This means that it will not be possible to reach the very high dryness levels.

Another new product that can help save energy is the IQ SteamPro™. This is a steam profiler that maximises steam absorption to achieve the highest practical dryness after the press. It is expected to boost dryness by 1-2% with a consequent saving of 30-50 kWh/ton in drying energy.


Not all of the energy-saving technology on offer is brand new. An example of this is insulated covers for the Yankee cylinder heads, which were introduced already some 20 years ago but never became very popular.

However, rising energy prices have seen renewed interest in the approach.

Insulating the heads can reduce steam consumption by up to 5%, corresponding to energy savings of 35 kWh/ton.

Adding all of these savings together gives a reduction of about 10% in electricity and of about 28% in steam and gas consumption, as shown below.

This is still some way off Metso’s target savings of 30% but a number of improvements are in the pipeline which should enable mills to reach the final target. Ideas include:

•	The recovery of energy from the forming process by using a turbine installed after the headbox can recover half the energy used in the fan pump. This could amount to 100 kWh/ton.
•	Raising post-press dryness to 50% within a few years is realistic. It can be achived by higher nip pressure and improved felt design. It is of course also important to develop the creping process in order to maintain as much caliper as possible. Reaching 50% dryness means a drying energy saving of 30% compared to today.
•	Increasing the diameter of the Yankee cylinder. Electricity and gas consumption both decline if the diameter is increased, though steam consumption rises. With a bigger diameter Yankee, hood will operate at lower temperature and lower impingement air flow, which means lower gas and electricity consumption. Although the steam consumptionwill increase,the total energy for drying will still reduce because of the lower air temperature level in the hood. By installing an 18-foot Yankee in preference to a 15-foot unit the saving could be of the order of 120 kWh/ton at similar production capacity. With the same energy prices as in the example at the beginning of this article, a mill would save about €300,000/yr. The exact saving for a specific case will depend onthe ratio between steam and gas cost.
•	Reducing overdesign in new plants. It is important that the complete machine line is designed for what it is supposed to handle during most of the time. Today many lines are designed also for extreme situations, which are rare. This results in larger pumps, agitators, etc than needed for the normal operating situations.This overdesign easily corresponds to an energy saving potential of 5-10% in the stock prep area.
•	Speed-controlled vacuum pumps can offer savings, especially if load will vary over relatively short time cycles. Such a situation could be a machine where a wide range of grades are produced and changes are made several times a day.

Cogeneration can also lead to considerable cost savings in the right conditions. First of all this technology means a change in the mill’s electrical power supply. Own generated electricity will now replace part, or all, of what was earlier purchased.

Other changes more directly involve the tissue machine and include the option of a Yankee hood using cogeneration exhaust instead of traditional gas fired burners and the use of cogeneration exhaust to produce steam to the Yankee dryer. Potential cost savings depend highly on the ratio between gas and electricity prices. If this is favourable enough, cost savings can easily reach up to 30% or more. There is a growing interest for this technology and Metso's recommendation to anybody considering it is to thoroughly study the overall feasibility to fully understand all its implications for the mill.

Many of the products and technologies mentioned above can of course be of interest also for existing production lines, to improve efficency and energy consumption. In existing lines old and worn out Yankee hoods can be considerable production bottlenecks and also inefficient in terms of energy usage. Too long impingement distance due to structural deformation and internal leakages are examples of problems. Metso estimates 10-20% reductions in drying costs can be achieved at a given production capacity by replacing Yankee hood sections and optimizing air drying performance.

Finally, good maintenance practice can yield surprising results. Proper tuning of existing equipment can save 5-10% in the Yankee dryer/Yankee cylinder sections. Training of personnel to take advantage is of course a prerequisite.

Energy savings can clearly benefit the tissue mill by saving money.

There is another potential plus to energy reduction, though. Lower energy consumption equals lower environmental impact and that can be used as a sales tool in an increasingly environment-conscious market. In particular, Metso points out, in a market dominated by private labels, as Western Europe is, retailers exercise great power. And many retailers throughout the continent are strongly advocating ‘green’ products. Diminishing your environmental footprint can also help put you on the supermarket shelf. TW