Energy Implications of Environmental and Technological Transition

Kiln Drying

A significant amount of energy is consumed by industrial operations such as wood pulping and drying. Kilns are enclosures or large machines used to dry products like lumber, poles, and raw materials such as the veneered wood and core fiber used in plywood panels. Large quantities of poles are manufactured for use in telephone signal and electricity distribution. Kiln drying is an energy-intensive process that is essential for imparting desirable properties to wood, including dimensional stability, workability, and hardening (e.g., as is required for tools), and promoting better absorption of treatments or adhesives. The United States Department of Agriculture's Forest Product Laboratory research indicates that drying operations more commonly burn wood wastes rather than fossil fuels for their energy source.31

Frequently, rail-mounted platforms carry the wood material in and out of a kiln. The kiln chamber is then sealed and heat is applied by steam or direct-fired air. Sometimes pressure or a vacuum is introduced into the chamber, depending on the product. Typical kiln temperatures range between 200 and 230 degrees F.32 While absolute estimates of the energy used in kiln drying are highly specific to the conditions of a given operation, engineering data indicate that steam applied and maintained at a temperature of near the 230-degree-F limit permitted by the American National Standards Institute standard will apply heat to a product surface at a potential rate of roughly 22,000 Btu per square inch. Drying times generally vary from 1 to 6 days. Longer drying times are required for wood that receives oilborne or preservative treatments. Subjective anecdotal information indicates that the energy required to dry about 500 cubic feet of lumber from an as-received condition to a 20-percent wet basis moisture content is approximately 10 million Btu.

Poles were air dried before the late 1960's, but the majority are now kiln dried, due to the shorter residence time involved. Research on air circulation and optimum temperature and residence schedules have resulted in technologies which have reduced original kiln drying energy by as much as half of previous requirements.33 In addition, some electricity is used as motive force for fans and product repositioning during drying. Environmental concerns involve emissions from kilns,34 combustion systems, and treating agents.35 Waste heat from kilns can be recovered by means of heat exchangers. Wood-drying kilns have been suggested as a candidate technology using ground-source heat pumps for supplemental energy.

Waste-to-Energy

Sawmills convert timber to dressed logs and lumber, some of which are then kiln dried, as just described. The wood waste produced by sawmills is frequently used as fuel. In fact, a typical modern sawmill produces enough waste to exceed its own energy requirement of 113 kilowatthours per ton of wood processed (equivalent to 2.25 million Btu) by 10-30 percent.36 In some cases, waste wood in excess of requirements is used for a variety of products or for other fuel purposes (e.g., as a raw material for charcoal). Environmental concerns with sawmills are mainly focused on alternatives to stockpiling excess sawdust and finding product uses for waste to avoid the use of landfills. According to the APA-The Engineered Wood Product Association37-85-90 percent of the log is typically used. The bark, sawtrim, and remaining sawdust are used for energy or pulpchips. Production of additional electricity and steam for sale are also energy products. Sales of electricity to the grid, of electricity and steam to industrial customers for process energy, and of steam for district heating fall into this category.

Improvements in resins and epoxies permit clamping to replace thermosetting for some products in the engineered wood products industry with a resultant savings in energy. However, use of phenolic resin, which requires thermosetting and has some adverse environmental characteristics, is still common. Plywood and oriented strandboard markets accounted for more than half of the total demand for phenolic resin.

Bleaching

Paper companies make a host of products requiring the use of technically complex chemical, thermal, and thermo chemical processes. These processes involve numerous stages and combinations of stages. Each major process is defined by distinct energy and environmental characteristics. Delig nification of pulpwood and bleaching of wood pulp involve the most environmentally sensitive group of processes, due to the by-products that result in mill effluents. The most prevalent bleaching technology currently used involves some form of chlorine. Using chlorine is economical and results in high process efficiencies. One reason chlorine is economical is that it is co-produced with sodium hydroxide, an agent required in large quantities during another stage of papermaking. The chlorinated organic compounds generated during chlorine processes are serious toxins and are a primary focus of regulation in the United States, Canada, and Europe.

Mill effluents currently require treatment by one of several methods, depending on the particular mill.38 A variety of new technological strategies to reduce chlorinated organics are now being employed, or considered, to achieve compliance with pending regulation. These pollution reduction methods can be categorized in three ways: (1) substitution of other chemical agents for chlorine, (2) recovery of some of the chlorine used and incineration or secondary treatment of the remainder, or (3) use of closed-cycle technology in new or reconstructed mills. All these options involve increases in capital and operating costs. However, each has a different energy profile. Overall, the paper and pulpboard manufacturing industry consumed an average 26 million Btu per ton of output in 1994, but the trend in energy use in this sector is downward.39

The Canada Centre for Mineral and Energy Technology (CANMET) has completed a definitive study of the pulp and paper industry. Several reports produced from this study form the informational basis on which the following discussion is based.

No increase in steam consumption is required to implement the first alternative to chlorine bleaching-chemical substitution. Oxygen delignification, for example, does not require as great a degree of pulp and water heating. However, this process requires more electricity to bleach paper than if chlorine dioxide were used. As mentioned previously, chlorine dioxide is essentially co-produced "free" at the bleaching plant. The second option, recovery or treatment of chlorine, increases primary energy consumption and in some cases doubles it. An increase in total primary energy is also associated with the third option, closed-cycle processing, although it has other advantages previously mentioned. This option, however, is not expected to be prevalent before the year 2010.

In 1993, CANMET established energy and material baselines to characterize papermaking methods. Energy and material use have subsequently been projected to future years. As a result of all process changes, total electricity consumption for bleaching is expected to increase 7 percent between 1993 and 1997.40 Electrical energy costs represent 8 percent and steam represents 17 percent of bleaching expenses in Canadian mills.41

Closed-cycle processing requires extensive reconstruction or total facility replacement and is currently employed in only a few mills. However, closed-cycle and minimized effluent designs are likely to become more common in the next few years. State and Federal regulatory agencies are granting more latitude to mills that incorporate such improvements. This factor is critical in the highly competitive paper industry, where profitability frequently hinges on the speed with which these immense plants can diversify products and redirect mill output from poor to favorable markets. Such latitude may be especially attractive to the paper industry because production flexibility by means of computer integration has not been completely successful.42 Nearly one-half the mills in operation after 2010 may be closed-cycle facilities (Table FE1).

Table FE1. Selected Papermaking Technologies Ranked by Industry-Wide Energy, Economic,
and Environmental Benefits and Predicted Extent of Use in Canada

Other Innovations

Other recent technological innovations do not replace old processes, but represent variations of established methods of dealing with by-products or using chemical agents in various process stages. These innovations include extended delignification, biomass dewatering and combustion, de watering of pulping liquor and sludge, deinking of newsprint (from recovered paper), medium consistency processing, and high-intensity refining.

Extended delignification involves longer residence time for wood chips in digesters and it is characterized by both higher steam and electricity consumption rates.43 Biomass dewatering and combustion by suspension firing are similar. Typically, mill sludge is dewatered before it is used for fuel. In suspension firing, sludge and hog fuel are dewatered in mechanical presses, further dried  by  use  of  hot  flue  gas,  hammermilled to a fine form, and fired in a boiler. Biomass dewatering and suspension firing offer several benefits, including substantial savings in primary energy, significant reduction in combustion emissions, and favorable process economics.

Fluidized-bed boilers have the capability to burn unde watered sludge, which can be an important capability to newsprint mills as use of recovered paper continues to increase and deinking results in increased quantities of sludge. Fluidized-bed boilers also contribute to reductions in fossil fuel emissions. However, they require new construction, whereas suspension boilers can be retrofitted.

Medium-consistency processing involves the use of higher concentration ratios of fiber to process water. This type of processing can claim only modest energy and environmental impact, and its commercial use may not occur until after the year 2000.

High-intensity refining involves changes in the operational parameters of the machinery used to break down fiber for pulping. Its use causes very little change in total primary energy consumption (large savings in electricity are offset by higher direct heat requirements) and has minimal environ mental impact.

Table FE1 ranks these technologies and predicts their acceptance by industry, based on a survey of Canadian mills. The significance of environmental impact can be seen in the table's rankings for Secondary Treatment of Effluents. Although this technology is the least favorable from an economic standpoint and ranks among the lowest in terms of energy consumption, it is the most environmentally favorable technology, and it has the highest predicted extent of use by the years 2000 and 2010.

Contact:

Fred Mayes

fred.mayes@eia.doe.gov

Phone: (202) 287-1750