ELECTRICITY EFFICIENCY

Increased Energy Efficiency Can Lead to Improved Competitive Advantage

The costs of fibre, energy and regulatory compliance are significant parts of the pulp and paper industry's operating costs.

The cost of energy use has been addressed [3,4], but not as extensively as the cost of water and fibre. Where energy costs have been addressed, the focus has often been on improving the efficiency of a particular unit operation, which can easily lead to increased energy use elsewhere in the mill.

Energy Efficiency -- a Competitive Advantage
Increasing energy efficiency can lead to cost reductions and thus to a competitive advantage. However, high capital cost solutions, such as major modifications or rebuilds of pulping or papermaking equipment, are often difficult to justify when energy costs are the only incentive. Projects of this scale require simultaneous improvements in production rate or product quality, or exploitation of new market opportunities, to be economically viable. While energy efficiency should be addressed early in the planning of major projects, it is but one consideration.

Low capital cost solutions begin with careful housekeeping and specifying efficient pumps, motors and other equipment during rebuilds. Further gains can be obtained by applying systems analysis tools to ensure that heat recovery is maximized and to avoid offsetting energy cost savings in one area of the mill with similar or larger expenditures elsewhere.

Deregulation of electricity and natural gas utilities is causing rapid changes in the cost of energy in North America. The effect that will have on mill operations is difficult to predict as different jurisdictions follow different deregulation paths. However, large swings in energy prices can be expected as utilities respond to meet demand. At the time of writing, industrial electricity rates in several North American jurisdictions vary on a daily basis, with high rates during peak daylight hours. In the future, seasonal variations can also be expected, with high rates during extremely cold winter weather in Canada or during extremely hot summer weather in US Northeast and Midwest. Deregulation of the natural gas industry is likely to have a similar impact on the cost and availability of energy.

It is clear that a mill capable of increased flexibility in the purchasing and use of energy will reduce its energy costs, as mill staff and their operating methods become more effective at responding to variable energy rates.

Improving Energy Efficiency
Energy efficiency and fresh water consumption are linked. Any effort to reduce fresh water use, if properly executed, can lead to energy savings. This is particularly the case in a northern climate where fresh water has to be heated from a typical winter temperature of 4°C to temperatures necessary for proper operation of effluent treatment systems, typically 35-40°C. Every cubic metre of water consumed in this way leaves the mill carrying away purchased energy in the form of waste heat. Heating water from 4°C to 35°C requires about 130 MJ/m3, or 2600 GJ in a 1000-t/d mill using 20 m3/t of water.

It makes a lot of sense to implement water conservation and energy efficiency projects simultaneously. If water conservation measures reduce the mass flow rate to the water treatment plant without a simultaneous reduction in the heat flow rate, the effluent temperature will increase. Eventually a cooling tower or other effluent cook system may be necessary. If energy use is reduced without simultaneous water conservation measures, effluent temperatures will drop; in extreme cases, the effluent may have to be heated.

While the heat capacity of dry air is about half that of water, the temperature difference between cool incoming air and hot dryer exhaust is often as high as 80°C. As well, humid dryer exhaust air contains additional energy in the form of the latent heat of evaporation of water. The amount of purchased energy leaving the mill as waste heat from a paper machine dryer section is at least 2.8 GJ/t of water evaporated, or about 2800 GJ/d in a l000-t/d paper machine. These numbers can be substantially higher when the dryer section is not operated properly.

Large Capital Expenditures
Model mills have been described by several authors [6,7]. The optimal kraft market pulp mill should be able to generate all of its steam and power requirements from bio-fuels such as black liquor or the wood wastes associated with its wood consumption, and should be able to export electricity to the grid. A TMP mill should generate enough clean steam to dry the equivalent tonnage of paper.

When a new boiler or cogeneration system is proposed for a large integrated mill, the cost of upgrading process equipment is an alternative well worth considering. For instance, replacing atmospheric refiners with pressurized units, improving product quality and production rate, may eliminate the need for new steam generating capacity due to increased heat recovery.

Older equipment frequently uses more water or more chemicals as well as more energy than necessary. Such equipment may also present a bottleneck to future production increases, or limit improvements to product quality or to the mill's ability to produce higher value-added grades. Energy efficiency should be integrated into overall capital improvement projects, where they typically add little to over-all project costs.

Low Capital Cost Options
Major capital expenditures generally occur when several goals justify them, for instance when improvements to product quality and production rate are possible. However, it is not necessary to wait for large projects to obtain cost-effective improvements in energy efficiency. There are a large number of low capital cost energy efficiency projects which can lead to substantial cost savings. In order to identify and implement these projects, a number of steps need to be taken.

Management Leadership
A corporate-level decision to reduce energy costs is essential, in order to provide mill staff with a strong mandate backed up with sufficient financial and human resources. A team made up of mill staff, possibly with help from outside consultants, should be assembled, with membership drawn from all areas of mill operations. The team must be given the time to analyze current energy consumption patterns in the mill, to optimize heat flows throughout the mill, to identify and implement economically-viable projects, and to continue monitoring energy costs once the projects are complete.

System Engineering
Implementing improvements in individual unit operations can often lead to equivalent or greater losses in other areas of the mill if the system is not considered as a whole. System engineering tools such as heat exchanger network analysis ensure that energy saved in one portion of the mill is not offset by increased inefficiencies elsewhere, and that the proposed investments are the most advantageous ones from an economic perspective.

The starting point for an effective energy efficiency program is an up-to-date, accurate computerized heat and mass balance of the mill. This will allow the full effect of future changes on the entire process to be evaluated at the design stage. The interactions between individual departments or unit operations are far too complex for this step to be neglected, especially in an older integrated mill where new processes have been installed at regular intervals. Typical examples would be multi-line kraft mills, or newsprint operations with several paper machines of varying vintages. Time and money spent in preparing and maintaining a simulation is an essential investment in a systematic plan to reduce energy costs.

Pinch analysis identifies heat recovery opportunities by determining the minimum heat input required by the process, and assists in designing the necessary heat exchanger network which yields the minimum heat input. This minimum, however, will generally require the largest heat exchanger surface area, and is thus the most capital intensive. Pinch analysis can also be used to identify the heat exchanger network leading to the best compromise between reduced operating costs and increased capital costs.

As an example, pinch analysis was used in a Paprican Member Company mill to more effectively recover heat from the blow recovery system, thus eliminating the necessity to heat wash water by live steam injection. Steam savings of over 25,000 kg/h resulted, allowing two older, inefficient boilers to be shut down, and leading to a significant reduction in purchased fuel costs.

It is important to optimize processes at the same time as heat exchanger networks. Pinch analysis does not address process issues, such as whether wash stages could be operated with less water, and results thus need to be considered in the light of in-depth process knowledge, and with the goal of simultaneously minimizing both energy and water use.

Pinch analysis, in combination with a heat and mass balance, will also point to cases where a cost saving in one area of the mill leads to unanticipated operating or capital expenses elsewhere. Together, these tools prevent energy efficiency projects from simply displacing energy consumption from one area to another, with no over-all benefit to the mill's bottom line. For example, projects to reduce live steam injection into process lines can have as a consequence reduced flows of cold make-up water to the boiler. Where this make-up water is preheated by recovering heat from an effluent stream, a reduction in the cold water flow will increase the temperature of the effluent stream. Cross-effects such as these need to be considered at an early stage.

Plant utilities
Improved boiler operation, increased use of biomass fuels when these are available, and careful attention to steam distribution and condensate return systems will lead to reduced wastage and lower costs.

Boiler efficiency is a strong function of the excess air supplied. A well-adjusted boiler combustion control system with properly calibrated sensors for carbon monoxide and oxygen should be able to keep the excess air level close to the optimum.

Grate type has an important effect on boiler efficiency. Modern grates use some type of automatic ash removal system, reducing down-time for ash removal and improving efficiency. Although materials engineering issues remain a concern, fluidized bed furnaces can respond more quickly to a change in steam demand, leading to improved efficiencies. They will tolerate large amounts of sand, gravel and other debris that would clog a grate-type furnace, allowing the mill to take advantage of inexpensive, locally available fuels such as a supply of old tires or other industrial wastes.

Water content in wood wastes leads to lower boiler efficiencies. At a moisture content of 60 per cent, 25 per cent of the energy available in hog fuel goes to evaporating water; this figure drops to 11 per cent when the moisture content is 40 per cent. Fuel drying equipment could be cost-effective if wood wastes commonly tend to be wet. If wet debarking equipment is the cause of excessively wet fuel, a dry debarking system could also be cost effective when the increased boiler efficiency and reduced effluent flow are considered together.

Boiler efficiency is generally improved if heat exchanger surfaces are kept clean, regardless of the type of fuel, as described in [8]. On the other hand, excessive soot blowing consumes steam needlessly. Cleanliness of the boiler side surfaces can be tracked by monitoring the boiler exit gas temperature; when it starts to rise, heat exchange effectiveness is reduced and soot blowing is necessary. On the water side, proper water treatment measures will minimize scale buildup.

Combustion air or feedwater preheating using flue gases can improve boiler efficiency if the flue gas temperatures are high enough. Combustion air supply systems can also be upgraded at reasonable cost, leading to better combustion efficiencies.

Steam traps can be a significant source of loss. A cold steam trap is not returning condensate, while an open trap may be allowing excessive steam to blow through. A regular maintenance program can yield useful savings.

Increased condensate recovery uses less cold feedwater. This requires less chemicals for water treatment and for fuel. Many condensate streams are sewered because they occasionally contain impurities; a control system based on a conductivity sensor can be used to safely return clean condensate. Condensate is also lost when process streams are heated using direct steam injection; any steam injection should be justified or replaced with heat recovery equipment.

Cogeneration consists of burning fossil fuels or biomass to produce both mechanical and thermal energy. Some energy is used to generate electricity, and the remainder is used in the process. Cogeneration is an increasingly common way of lowering energy costs. Installing generating equipment on-site rather than at a remote utility site means that exhaust heat can be used to generate process steam for the mill, improving the over-all efficiency of the generating plant. Electricity generated may be used at the mill, or sold into the grid. The steam produced must meet a mill demand; if the demand is not there, or if it is possible to reduce the demand by other energy efficiency measures, the economic justification becomes more difficult.

There are a number of other mill-wide systems which can yield substantial savings if optimized. High-efficiency pumps and motors equipped with variable speed drives can save energy wasted in throttling or control valves. Compressed air leaks are often ignored, although maintenance time spent tracing and fixing leaks can be very cost-effective. Mill heating and ventilating systems are often poorly designed or have been disabled, poorly maintained or improperly modified, leading to wide open windows and doors in mid-winter while office buildings consume purchased energy for space heating. A careful audit of these and other mill services can turn up cost-effective opportunities for energy efficiency.

Energy purchasing strategies
Energy purchasing in a deregulated environment can be a difficult task, and variable energy pricing can have a significant impact on mill operations. Lower industrial electricity rates in off-peak hours are already common in several jurisdictions. TMP mills in these areas often shut down a portion of the pulp mill when rates are high, relying on large pulp storage facilities to maintain paper machine production. However, a power boiler is then required to keep the paper machine supplied with steam: the relative cost of electricity and fuel dictates operation of the TMP plant. Operation of these mills is flexible, allowing them to switch energy sources at relatively short notice. As the energy industry is further deregulated, seasonal variations in energy prices will likely be superimposed on these daily variations.

Deregulation thus offers significant opportunities for cost reduction to mills having careful purchasing strategies and sufficient operating flexibility; however, the possibility of substantial additional costs also exists if these strategies are not planned or properly executed.

Maintaining a secure energy supply to the mill has traditionally been considered essential. However, potential advantages exist for mills which are free of inflexible, long-term supply contracts. The degree of flexibility versus minimum security of supply should be reviewed periodically. For instance, in periods of poor market conditions for pulp and paper products, it might be advantageous for mills to buy low-cost energy supplies under short-term contracts, and to shut down briefly when energy spot prices climb steeply. A more secure supply with steadier prices is more sensible in times of good market conditions, when lost production due to an unplanned shutdown is more costly than the higher price paid for energy. Another tactic is to schedule maintenance shutdowns for periods of known high energy prices. Each mill should develop its own mix of purchasing and operating strategies aimed at balancing security of supply with low cost. Most mills will want to purchase a portion of their energy requirements under long term contracts, buying swing fuels under shorter term contracts or on the spot market.

Energy supply contracts often specify average and peak demand, since the utility's supply and distribution network must be large enough to cope with the peak demand. When the peak demand is significantly higher than average, prices will generally be higher, since a larger, more expensive distribution network is required. Preventing peak demand from exceeding a given level can be an important part of managing energy purchases. The ability to quickly switch to alternate energy sources can be an advantage in this respect.

Energy out-sourcing, defined as the third-party ownership and operation of energy facilities such as steam, power, black liquor recovery and other utilities assets, is becoming more common. Typically, a mill will purchase steam from an adjacent independent power producer (IPP), allowing the mill to close inefficient power boilers. There may be tax advantages for the IPP which are unavailable to the mill, and the mill may be able to free up capital for investments in core business activities. Another advantage could be a guaranteed long-term pricing contract for steam or power. Third party owners may also possess specialized expertise in building and operating power plants, ensuring more efficient operation.

On the other hand, long-term outsourcing contracts can oblige the mill to buy a certain minimum amount of steam or power, reducing the mill's ability to reduce energy costs at a later date. Excess electricity can be sold into the grid, but it may be difficult to find buyers for steam within a short distance of the power plant. As well, the cost of risk premiums incorporated by the IPP in the price of power to cover items such as unexpected mill shutdowns or other operating, regulatory, environmental, or fuel price risks can actually increase energy costs.

Selecting projects
Careful identification, ranking and selection of projects is an essential step in a successful energy efficiency program.

Project selection has to be consistent with the mill's priority list, and the energy manager should be aware of difficulties foreseen by all personnel. Each project should also consider other mill priorities, such as future development plans, production or quality targets, the impact on effluent treatment systems, as well as standard measures of profitability such as net present value or internal rate of return. Finally, it is beneficial to consider advantages other than energy efficiency. Improved production rate or product quality, and reduced loading in the effluent treatment system can make an energy efficiency project more attractive, even if these benefits are not included in the investment recovery period.

The order of implementation of a series of projects is an important consideration. Process stability, production rate, product quality and financial or environmental constraints, as well as impacts on other short or medium-term projects, will all have to be considered. The effect of the first project on subsequent projects must also be kept in mind.

Continuous Monitoring and Improvement Programs
Finally, improvements in operating practices can disappear over time as operating conditions change and as new energy leaks arise. For example, energy cost savings in one mill disappeared six months after implementing a number of energy efficiency projects [5], and changing operating procedures. A production rate increase required higher pressures in the paper machine dryer section, reducing the efficiency of the thermo-compressors; paper machine operating temperatures had been increased by 70°C; a failed heat exchanger had been bypassed rather than repaired.

On-line energy monitoring systems allow daily consumption reports to be produced and scrutinized at the same time as daily production reports. Steps can then be taken to rectify losses in energy efficiency before they accumulate over a long period. If the monitoring system is tied to a real-time energy purchasing system, further advantages can be had by tailoring day-to-day energy usage to account for variations in utility pricing.

An essential part of continuous monitoring is ensuring that the process simulation is updated as equipment is added or decommissioned, or as piping systems are rerouted. This requires that updating the simulation be considered an integral part of any process change to the mill. Cost savings due to energy efficiency projects may seem to disappear over a period of months or years as equipment is modified for production rate or product quality improvements. An up-to-date simulation can be used to show that the savings are still there, and that apparent increases in costs are due to other factors.

LITERATURE

1. TURNER, PA. (ed). Water use reduction in the pulp and paper industry 1994. Montreal: CPPA and Paprican (1994).
2. PARIS,]. and LEROY, C. (eds.). System Closure in Pulp and Paper Mills: A Monograph based on the Symposium on System Closute. 84th Annual Meeting, TS, CPPA, Montreal (1998).
3. NYGAARD,J. Energy audits in the pulp and paper industry. Information No. 411-84, lEA. Stockholm, Sweden: National Swedish Board for Technical Development (1984).
4. LAHEPELTO,J. (ed). Sustainable Paper: Energy in Paper and Board Production. Research Programme Final Report 1993-1998. Helsinki: The Finnish Pulp and Paper Research Institute (1998).
5. BROWNE, T.C. (ed). Energy Cost reduction in the pulp and paper industry. Montreal: Paprican (1999).
6. CAULE, R.P., CLAYTON, D.W. and SIMONSON, H.I. Energy efficient process for the pulp and paper industry: analysis of selected technologies. Ottawa: Natural Resources Canada (1995).
7. Canada's Emissions Outlook: An Event-based Update for 2010. Ottawa: Natural Resources Canada (1999).
8. ULOTH, V.C., MARKOVIC, C.M., WEARING, J.T. and WALSH, A.R. Observations of the Dynamics and Efficiency of Sootblowing in Kraft Recovery Furnaces. Pulp Paper Can 97(6):T196-T202 (June 1996) and 97(7):T223-T226 (July 1996).