Research & Innovation
Energy Costs: Where does your mill stand, and what can you do about it?
Why worry about energy costs?...
February 1, 2006 By Pulp & Paper Canada
Why worry about energy costs?
The pulp and paper industry has an impressive record of reducing energy use. However, rising energy prices are creating tremendous challenges for the industry. In spite of past successes, more needs to be done.
Reducing energy use first requires understanding where it is being used. To quote Lord William Thomson Kelvin (1824-1907), Scottish mathematician and physicist, “If you can’t measure it, you can’t improve it.” This article describes recently acquired energy benchmarking data, and shows how new understanding of energy use patterns in pulp and paper mills can lead to improved energy efficiency.
What’s new about this data?
Benchmarking compares the performance of a mill with its competitors, or with a model mill using best available technologies. It can be a valuable motivating force for change, since it allows a mill to assess its operating costs and environmental impact relative to its competitors and to practical minima.
Governments and associations, such as the Forest Products Association of Canada (FPAC), collect energy use and production data for economic policy and planning purposes. The energy intensity of a mill provides an initial assessment of the mill’s ranking within the industry without reference to its products or processes, and is defined as the energy consumption of the whole mill, divided by total saleable production at the site. It can identify the potential for energy savings, but not where they can be achieved. In order to identify specific opportunities for process improvements, benchmarking comparisons need to be made for individual process areas.
As part of ongoing efforts to help its member companies reduce energy costs and emissions levels, Paprican teamed up with the Office of Energy Efficiency (an arm of Natural Resources Canada) to develop and implement a method for benchmarking energy use by process areas in pulp and paper mills. Paprican experts visited almost 50 Canadian mills, representing over half of Canadian capacity, in a two-year period from October 2002 to October 2004. The data acquired during these mill visits, combined with descriptions of model or modern mills, will help mill staff target inefficient process areas, maximizing energy cost savings while minimizing the capital necessary to do so.
What does the data show?
The data set, presented to industry representatives during a roundtable session in Ottawa on June 16, 2005 (see sidebar #2), is extensive, and space prevents discussing more than an illustrative sample here. Figure 1 illustrates the range of thermal energy use, defined as steam used less condensate returned, in batch-type kraft pulping digesters. The range is from about 3 to over 9 GJ/ODt; depending on fuel prices, the value of 1 GJ of steam may be as high as $12. The theoretical value for a modern mill using conventional batch digesters is about 3.5 GJ/ODt; the best batch digesters appear to be operating at the level of best available technology.
Figure 2 shows thermal energy use in kraft continuous digesters. The range is from 1.5 to about 7 GJ/ODt, while the theoretical modern mill consumes 2.2 GJ/ODt. Different process boundary definitions explain the difference between the modern mill and Mill 1; nonetheless, the best mills again appear to be close to best available technology. Compared with Figure 1, it is also clear that while batch digesters are generally less efficient than continuous digesters, the best batch digesters are significantly better than the worst continuous digesters.
In mechanical pulping, electrical energy is used to convert wood to pulp. The thermo-mechanical pulping process (TMP), in particular, consumes large amounts of electricity, but has the ability to generate large amounts of excess steam for use elsewhere in the mill. Figure 3 shows the net steam generation from newsprint TMP plants across Canada. The best mills are generating up to 3.5 GJ/ODt of clean steam in the TMP plant for use in papermaking and elsewhere; the worst are consuming as much as 3 GJ/ODt of boiler-generated steam. The modern mill should be able to generate 5 GJ/ODt in the case of newsprint, and more in the case of a value-added grade such as light-weight coated (LWC) or supercalendered (SC) papers.
Similar patterns are evident when comparing paper machines. Figure 4 shows thermal energy use for newsprint machines. Ignoring the worst machine, the range is from 3.5 to about 8 GJ/ODt. A modern newsprint machine consumes about 4.4 GJ/ODt.
What does this mean for my mill?
The typical range of steam consumption values in Figures 1 to 4 varies by a factor of at least 2. It is also clear, for example from the comparison between batch and continuous digesters, that old mills are not necessarily less energy-efficient than new ones.
Reasons for the range of values will vary from mill to mill. First, the data collecting process took no account of where water is heated in the mill. A mill where steam is used for water heating in the paper machine area will report higher levels of steam use there, even though the hot water might be used elsewhere, than one where steam heating of water takes place in the pulp mill. In turn, this second mill will appear to be inefficient in the pulping area. Thus, high consumption in one area may be offset by low consumption elsewhere, due to reasons which have little to do with basic process requirements. Mill staff need to consider the entire site as well as each process area independently.
Similarly, while the best batch digesters are better than the worst continuous digesters, older equipment can limit energy efficiency. Open dryer hoods, older press sections or dryer siphon systems, atmospheric-discharge refiners, or TMP plants with no heat recovery equipment, to name a few, will limit the gains achievable without significant capital investment.
Some products require additional energy, and this will be part of the cost of doing business in that particular market niche. Mechanical grades for newsprint don’t need as much refining power as for LWC or SC grades. Once the base stock has been dried, coated grades require an extra drying stage for the coating formulation. These product-based constraints also impose limits on energy use reduction.
The energy cost structure in a mill will also affect efficiency. Mills with access to lower cost fuels will have less incentive to be efficient, as the return on investment for efficiency projects will be less attractive.
Finally, production rate and efficiency have an impact. Low paper machine efficiency leads to high energy use, as finished, dried paper is reslushed and dried again. Downtime for market reasons will also have an impact on energy use. Smaller lines will require more energy per tonne of product than larger mills where economies of scale rule.
Once these factors have been ruled out, there may remain opportunities for low-capital efficiency improvements.
Where should I begin?
The first step is to take a structured approach to setting up an energy management program. This involves identifying key staff and giving them the tools and authority to make changes.
Next, compare your mill to published data. Consider the reasons for high energy use listed above, and see which ones apply to your particular situation. Work through your mill to see where you stand relative to others. This will identify the biggest gaps. Consider water and energy use simultaneously; ask whether you really need that much water for a given operation, and whether it really needs to be that hot. Changing a temperature or flow setpoint can save a significant amount of energy without necessarily causing product quality or variability problems. Infiltration of cold tramp water through leaking packing seals or other areas also costs money.
Once the process areas have been evaluated and setpoints adjusted to their most energy-efficient levels, it is time to consid er larger process optimization studies. Do you use steam for heating water? If so, you are using the highest grade of heat available for the lowest heating duty. This is almost inevitably what is known as a cross-pinch violation; either water use is too high or there is a hot stream going to sewer with sufficient heat available to reduce or eliminate the steam heating. When direct steam injection is used, for instance into a wire pit or other tank around the paper machine, the situation is aggravated because condensate is not being returned to the boiler, as it is when indirect heating is used.
These and other opportunities will be identified by process integration methods such as pinch analysis, which identifies the theoretical minimum energy consumption for a given process. The cost to reach that theoretical minimum may be high, but it may be cost effective to move part way there. A pinch study provides a structured list of energy cost reduction projects leading towards that theoretical minimum, and allows mill staff to evaluate the cost penalties if projects are done in a sub-optimal sequence.
Boiler optimization studies can lead to improvements in steam generated from black liquor and wood waste, allowing a reduction in fossil fuel use, increased throughput and higher thermal efficiencies. Typically, optimizing air delivery and liquor spraying (in recovery boilers) or hog fuel supply and combustion air delivery (in power boilers) improves steam generation from biomass, reduces air emissions and reduces fossil fuel usage. While improvements can often be realized with little or no capital cost, upgrading air fans, changes to air dampers, measurement and control systems, or rebuilding the boiler air system may be necessary for the full benefit. However, the payback times on such investments are usually short.
Time to get started!
In spite of two decades of efficiency improvements, more is needed to offset rising energy prices. In many mills, easy improvements have been identified and implemented. Additional low-capital opportunities exist in many mills, but require effort to identify since many of the obvious ones have already been addressed. A structured approach, based on solid information and careful engineering, is the basis for a successful energy efficiency program leading to improved competitiveness and a stronger bottom line. It’s time to get started!
The financial support for this work by the Office of Energy Efficiency, Natural Resources Canada, is gratefully acknowledged.
For more information, see “Energy Cost Reduction in the Pulp and Paper Industry,” Browne, T.C and Williamson, P.N. ed., Pulp and Paper Research Institute of Canada, Montreal, 1999.
“Water Use Reduction in the Pulp and Paper Industry, 2nd ed,” Turner, P.A., Browne, T.C. and Williamson, P.N. ed., Pulp and Paper Research Institute of Canada, Montreal, 2001.
Dr. David W. (Bill) Francis is a senior research engineer, Fibre Supply and Quality.
Dr. Thomas (Tom) Browne is the program manager, Mechanical Pulping.
Mike Towers is a senior research engineer, Process Development.
A veritable powerhouse
The pulp and paper sector is the most energy intensive industrial sector in Canada, consuming 27% of all industrial energy consumption. However, it generates a majority of its own energy from renewable sources. Essentially all of the self-generated energy is from the use of carbon neutral biomass — either for thermal generation or cogeneration. A small portion is from small hydro installations. The sector has the largest industrial cogeneration capacity in Canada. In total, the sector generates 58% of its own energy consumption.
Energy efficiency improvements have reduced the sector’s energy intensity by 1% annually since 1990 — meeting its commitment under the Canadian Industrial Program for Energy Conservation. Combined with considerable fuel switching, the sector’s GHG emissions have dropped 28% while production increased 30%.
Since 1990, total pulp and paper production has increased while emissions of greenhouse gases (reported as carbon dioxide equivalent, or CO2e) have decreased.
The pulp and paper sector has reduced its energy intensity and increased the level of self-generated energy since 1990.
The Forest Products Association of Canada (FPAC), Paprican and Natural Resources Canada (NRCan) have forged an effective working relationship to steer Canada’s forest products industry towards better energy management practices. This team effort led to last June’s highly successful Energy Roundtable in Ottawa, ON. During the roundtable, PAPRICAN presented the results of the NRCan-funded study, and is now working to ensure opportunities identified in both studies are implemented.
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