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 Publication: Pulp & Paper Magazine WOODYARDEffective Chip Pile Storage Design Reduces Pulp Variation, Improves Mill ProfitsBY STEVE QUILLINProper Management Of Chip Handling Area Can Help Mills Increase Yield And Enhance Pulp Quality, Strength, And Brightness.THE TYPICAL KRAFT PULP MILL has limited control over many of the critical factors associated with raw fiber supply, including delivery schedule, wood harvest upsets, and as-received quality. Consistent chip quality has been proven to have a measurable effect on pulping quality and economics measured as a function of reduced pulp mill rejects, increased yield, and lower chemical usage. Control of the woodchip storage, chip reclaim, and chip pile management systems has a dramatic effect on mill profitability. ECONOMICS OF CHIP STORAGE. In a typical kraft pulp and paper facility, the largest single component of production cost is the cost of raw fiber. In general, raw fiber accounts for more than one-third of the total manufacturing cost for a ton of unbleached paper. In fact, the basic cost of raw fiber often is greater than the combined costs of operating labor, maintenance labor, and maintenance materials. fig 1Figure 1 illustrates the relative production costs for a typical unbleached kraft pulp and paper facility. Most chip screening in the U.S. is performed after the storage pile. This practice is common because the screening system is generally perceived to be a "cure-all" for bad practices elsewhere in the woodyard. It is certain that a properly sized screening system can improve the quality and consistency of the fiber supply. However, proper screening will improve quality only relative to chip size. No screening system can correct thermal or biological degradation, staining, fungal growth, or variations in age of the woodchip supply. These factors directly affect mill profitability and are measured as pulp yield, quality, strength, and brightness. In optimization of overall wood system performance, chip handling and rotation techniques should be an integral part of the manufacturing process design, and not considered merely an inventory buffer against uncertainties of weather or inconsistent wood delivery. Maintaining a large chip inventory in the woodyard represents a substantial operating cost. Chip inventory adds approximately $2/b.d. ton to the cost of producing kraft pulp for each 30 days of chip storage. In addition to the raw wood inventory cost, fiber deterioration during extended storage is well documented. Raw woodfiber deterioration is measured as a lower yield of byproducts (such as tall oil and turpentine), lower pulping yields, and lower pulp strength. Woodfiber deterioration in a chip storage pile is principally attributable to the following:
Biological and biochemical losses result from elevated temperatures in the chip pile. Chip pile temperature is influenced by many factors, including climatic conditions, pile size, and pile geometry. Wood substance losses that directly affect pulping yield accelerate as a consequence of microbiological activity up to about 125´F internal chip pile temperature. At pile temperatures above 125´F, microbiological degradation decreases significantly. Unfortunately, thermal degradation commences at pile temperatures above 125´F. Consequently, the optimum chip pile temperature should not exceed 125´F. If spot temperatures within the chip pile reach 160´F to 165´F, chips in the affected pile area should be reclaimed immediately. Concerns resulting from elevated chip pile temperatures include both spontaneous combustion and thermal degradation issues. fig 2Figure 2 illustrates byproduct losses resulting from extended chip pile storage. Woodchips consume oxygen in storage and release heat. Elevated temperatures in the pile promote the growth of bacteria that feed on wood starches in the chips. At approximately 140´F, acetic acid forms as a result of the thermal degradation. The acetic acid product further degrades the woodfiber strength and imparts a dark appearance to the chips. Acetic acid formation can be tracked by testing chip pile runoff for pH. fig 3Figure 3 illustrates the various mechanisms of chip deterioration as a function of pile temperature and storage time in months. Physical damage or contamination in the chip pile is usually attributed to techniques utilized for wood delivery to the storage area and reclaim and transport of chips to the pulp mill. The chip pile is not the only factor affecting chip quality. Many other factors affect physical chip deterioration, often to a greater degree than the processes used for chip stackout and reclaim. Size, fines content, bark content, and contamination of the chip flow to the digester are critical factors to be considered in the design and selection of process equipment and pile management techniques. Control of fines content and generation in the chip pile is the most serious quality problem in woodyard operation and represents the greatest economic penalty. If conveying methods or pile management techniques permit extensive size segregation in the pile, pockets or blankets of fines may be formed. If blankets of fines accumulate in the pile, heat dissipation from the pile is severely retarded, and pile temperature will increase. Ultimately, pile temperature can elevate to the point of creating a fire hazard. Chip piles with a high fines content are susceptible to higher compaction rates, which exacerbates temperature increase and thermal degradation of the chips. Economically, reduced pulping yield is the most significant consequence of woodchip deterioration from outside chip storage. Overall pulping losses are generally expected to be about 1% of o.d. chip weight per month. At current worldwide production levels, this pulping loss represents the equivalent to a loss of a forest the size of Holland each year. It is apparent that mill economics cannot tolerate excessive pulping losses resulting from chip quality issues. Outside chip storage must be an integral part of the total management of forest resources. WHY OUTSIDE CHIP STORAGE? As illustrated in fig 4Figure 4, a 30-day chip inventory for a 1,000-b.d.-tpd kraft mill represents an investment of approximately $6 million. At an average 1% pulping yield loss per month, an annual penalty of $720,000 is attributable to chip storage. This loss represents a production cost penalty of about $2/ton of paper production. fig 5Figure 5 shows a calculation of overall production cost penalty associated with chip storage time. Chip storage in the kraft mill is necessary for two reasons:
Log and chip pile inventories represent protection against wood shortages or fluctuating supply. Wood inventory management is usually considered a tradeoff between risk avoidance and inventory costs. This narrow view of wood inventory management does not consider several other positive factors associated with outside chip storage. There are many financial benefits associated with an outside chip storage operation. A chip pile affords wood procurement personnel some advantage to purchase on a spot-market basis, or to buffer differences in fiber supply and mill demand. Additional financial reasons for storing chips include protection against long-term wood price excursions, use of wood inventory as depreciation, collection of capital, and manipulation of a favorable income over a specific calendar period. The pulping process benefit of a properly designed and managed chip storage and reclaim system is the ability to provide consistent chip quality to the pulp mill. A properly designed wood handling system provides routine chip turnover and tends to dilute the effects of periodic chip problems. A consistent, high-quality chip supply to the digester will reduce pulp kappa number variation. This benefit results from the uniformity of the chips and superior impregnation of the cooking chemicals into the chip. This benefit is not achievable with a small chip silo storage unit or direct feed from the chipper to the pulp mill. HOW TO STORE CHIPS. All outside chip storage systems fall into one of four categories:
The design of the chip storage system represents an ultimate compromise between process and financial requirements. A mill must balance the demands of initial capital investment, operating costs, wood storage requirements, and pulping requirements. Chip storage design is site-specific, and no standard rules apply. The proper wood processing design for a particular mill is the one that will result in the lowest capital investment and production cost and the longest equipment life. Any of the alternative chip handling design configurations can be operated successfully, despite the inevitable chip deterioration and byproduct losses. Woodchip quality is generally acceptable for up to 60 or 65 days of storage. Most system design studies favor chip blending systems, though these systems generally require a dozer to intensively tailor chip quality to the digester. Alternatively, an automated blending system is capital-intensive and may limit operating options, though less physical chip deterioration will result. The initial capital cost of the automated system is often offset by low operating costs. Again, the best chip handling system for a particular mill is one that best balances the design, operating, and financial criteria for the specific mill site. RECOMMENDATIONS FOR A CHIP PILE.fl Thirty years of chip storage research have resulted in consistent recommendations for chip system design and chip pile management. Adherence to the following goals should ensure maximum profitability from the woodyard operation:
REFERENCES 1. R. Schmidt, The Effect of Woodchip Inventory Policies on Storage Costs, Chip Quality, and Chip Variability, TAPPI Journal, 1990, Vol. 73. 2. W. Fuller, Chip Pile Storage--A Review of Practices to Avoid Deterioration and Economic Losses, TAPPI Journal, 1985, Vol. 68. 3. B. O'Connor, Chip Preparation Methods and Rt's Application on Frozen Chips, presentation at the 1990 TAPPI Pulping Conference. 4. J. Hatton, Chip Quality Monograph No. 5, Joint Textbook Committee of the Paper Industry.
  
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