Investments in reducing or recovering paper drying energy can yield major savings and a good ROI . Trimming a paper mill's carbon footprint goes hand in hand.
If you drive by any paper mill in Canada on a crisp winter day you will see swirling plumes of steam that some lay people mistake for smoke. We in the paper industry know it's not smoke; rather it's the vapour-filled exhaust from the paper machine dryer sections. But where there's steam of any type there's energy - and a lot of it flows through the paper drying process, either as active, energy-intensive steam in the drying cylinders or as passive, residual heat in the moist exhaust air.
The heat energy used in the dryer section accounts for up to 70% of the total energy required to make a sheet of paper, board or tissue. It's energy that costs quite a bit in fuel and related carbon emissions. To put the cost in perspective, a modest-sized 250,000 tpy paper machine consumes over $6 million per year in steam energy. If you could save 10 or 15 % of that, a lot of fuel, money and carbon emission could be conserved.
Nowadays, many mill investments are focused on reducing high energy costs; the environmental benefits come as a natural consequence. If a mill can save a million dollars per year or more, it makes good sense to pay attention to the dryer section to reduce and recover steam energy. There are proven technologies which reduce drying requirements, make the drying process more efficient, recover a lot of the exhausted heat energy, and generate a good ROI – often with less than one year payback on the investment. Let's have a look at a few possibilities which can squeeze the most energy from steam.
Less drying required
One way to reduce drying energy is simple - evaporate less moisture by increasing sheet dryness to the dryer section. An old rule of thumb – which still holds true – is that one percent increase in dryness saves four to five percent in drying costs or releases that amount of extra drying capacity. Furthermore, it is much less costly to remove water by pressing than by steam heated drying cylinders.
New shoe presses have made significant gains in this area, but that is a major investment which is justified by many other benefits. There are other, less costly ways to improve dryness. With an existing press section, many incremental improvements can be made by fabric and roll cover selection, press loading and vacuum management.
A significant step change in sheet dryness and drying energy is achieved by a new generation of profiling steam boxes which are more efficient than original units. Many have been installed on press section suction rolls with good results. A Pacific Northwest newsprint mill reports an 8% reduction in total steam consumption and an energy cost saving of over $600,000/year with a replacement steam box. Additionally, the moisture profiles are better, permitting high reel moisture levels. Board machine applications indicate dryness increases of 1 to 1.5% and some capacity increases up to 10%.
A Pacific Northwest newsprint mill reports a net steam energy saving over $600,000 per year with a replacement steam profiler in the press section. Data courtesy of Metso
Dryer section tune-up needed?
Next, the steam control, condensate removal, and drying efficiency in existing cylinder drying sections may be in need of a tune-up. While forming and press sections have been upgraded significantly over the years the dryer section may be lagging behind, trying to deal with condensing rates and multiple paper grades it was never designed to handle. If frequent flooding, steam venting and associated drive load instabilities are a problem, a thorough technical audit of the steam and condensate system may be in order. In one recent case at an Ontario containerboard mill, the rebuild resulting from one of these audits provided a steam cost saving of 16% or over $800,000 per year. The previous steam venting problem was eliminated and the drying efficiency improved considerably. Drive load stability and runnability were improved as well.
These rebuilds sometimes involve upgrading siphons to the most recent stationary design, new dryer bars, and reconfiguration of the steam and condensate handling system to match condensing rates and pressure drops. These solutions involve well proven technology and some good flow engineering design. For instance, more than ten years ago a Quebec newsprint mill reported a 10% energy savings with a steam and condensate revamp. Back then, the investment was focused on removing a bottleneck and improving production rates. Today, a mill will happily accept major energy savings instead.
In some cases, the steam and condensate piping and associated pressure drops can cause drying instabilities, dryer flooding and steam wastage. A Quebec tissue mill reports a five- month payback from an audit and steam and condensate system rebuild that corrected undersized piping and other control-related problems.
Steam consumption per ton has been reduced by 16% by a dryer section steam and condensate rebuild on an Ontario containerboard machine. The cost savings are over $800,000 per year. Data courtesy of Kadant.
Leftover heat is a source
Once the steam has given up its latent heat and done it's job of drying the paper sheet, the air supply and exhausat system takes over. One should not forget that air is a signifcant raw material for papermaking and the hot, moist air that carries the evaporated water away is really an energy source. The leftover heat content is quite substantial and the energy recovered can heat process water, hood supply and pocket ventilation air, and machine room ventilation air. When this passive exhaust heat is turned into active, useful heat less external energy is required. This is particularly relevant in the Canadian climate where a considerable amount of energy is required to just keep the machine room and processes at the right temperature.
Exhaust heat recovery has been done before, however old-style economizers, which have been around for many years, are often poorly maintained and inefficient, with stuck dampers, fouled heat transfer surfaces and sometimes gaping holes. Today, more heat can be transferred conistently, especially with high dewpoint hoods. Since the rewards are greater, there is a reason to maintain the equipment well.
Suppliers have re-invented and refined the old economizer concept and improved the heat transfer efficiency in modular, multi-stage heat recovery stacks that are engineered to provide specific energy recovery tasks from the otherwise wasted exhaust air flows . One supplier estimates that 50 to 60% of the waste heat in the exhaust can be recovered economically. A European mill reports the energy recovered for air and water heating adds up to a substantial 11 MW and provides a good ROI . In one central USA mill, a new heat recovery system involving air to glycol and air to water heat exchange saved about $1.6 million per year in heating costs.
The heat recovery potential in tissue mills is particularly relevant since tissue machines consume more energy per tonne than traditional paper machines. One Eastern Canadian tissue mill reports energy savings of 11% by exhaust heat recovery.
This heat recovery stack captures much of the formerly wasted heat in paper mill exhausts. Photo courtesy of Thermal Energy International.
Carbon footprint linked to ROI
Saving or recovering steam energy can produce a sizeable cost reduction on the bottom line, but let's not forget that reducing energy costs and trimming the need for fossil fuels often go hand in hand. A good energy-related ROI will reduce the carbon footprint of a mill. Paul Martin, former Prime Minister of Canada, underlined the connection between economics and the value of natural resources in a recent CBC Radio interview. "A good environmentalist is a good economist, and vice versa," he said. He was referring to the value that can be placed on the natural resources (or natural capital) that are essential for economic activity. With wise investments in energy efficiency, this natural capital can be preserved.
Mark Williamson, Journalist Engineer, Thornhill, Ontario