Wood Waste Recovery: Size Reduction Technology Study NIST MEP. Environmental Program

Transcription

1 Wood Waste Recovery: Size Reduction Technology Study NIST MEP Environmental Program

2 WOOD WASTE SIZE REDUCTION TECHNOLOGY STUDY FINAL REPORT Prepared for CWC A division of the Pacific NorthWest Economic Region (PNWER) 2200 Alaskan Way, Suite 460 Seattle, Washington December 1997 Prepared by Re-Sourcing Associates, Inc. CPM Consultants, Inc. in partnership with Boise Cascade Corporation This recycled paper is recyclable Copyright 1997 CWC. All rights reserved. Federal copyright laws prohibit reproduction, in whole or in part, in any printed, mechanical, electronic, film or other distribution and storage media, without the written consent of the CWC. To write or call for permission: CWC, Alaskan Way, Suite 460, Seattle, Washington 98121, (206) Disclaimer CWC disclaims all warranties to this report, including mechanics, data contained within and all other aspects, whether expressed or implied, without limitation on warranties or merchantability, fitness for a particular purpose, functionality, data integrity, or accuracy of results. This report was designed for a wide range of commercial, industrial and institutional facilities and a range of complexity and levels of data input. Carefully review the results of this report prior to using them as the basis for decisions or investment. Report No. CDL-97-3

3 TABLE OF CONTENTS EXECUTIVE SUMMARY SUPPLY OF SECONDARY WOOD FIBER MILL RESIDUALS Clean Manufacturing Residuals Remanufacturing Residuals URBAN WOOD (MANUFACTURED AND CONSTRUCTION WOOD) Pallets and Crates Construction and Demolition Wood GREEN WOOD Forestry Residuals Agricultural Wood Land Clearing Wood WOOD WASTE PROCESSING SYSTEMS HISTORICAL INFLUENCES ON WOOD WASTE PROCESSING SYSTEMS Influence of the Solid Waste Industry Influence of the Forest Products Industry CURRENT WOOD WASTE PROCESSING SYSTEMS NEW DEVELOPMENTS VALUE-ADDED MARKETS FOR SECONDARY WOOD FIBER MARKET DYNAMICS THE IMPORTANCE OF SIZE REDUCTION TECHNOLOGIES REVIEW OF VALUE-ADDED APPLICATIONS Pulp & Paper Reconstituted Panelboard Fiber and Composite Applications Mid-Value Applications Biomass Fuel Applications SIZE REDUCTION EQUIPMENT ASSESSMENT PRIMARY SIZE REDUCTION SECONDARY SIZE REDUCTION...20

4 4.2.1 Re-Chipping SIZE REDUCTION EQUIPMENT TYPES Chippers Hogs Hammermills Shredders Hybrid Size Reduction Equipment MATRIX OF EQUIPMENT TYPES SCREENING EQUIPMENT ASSESSMENT FLAT OSCILLATING (CIRCULAR MOTION) Free Suspended Screens Controlled Throw Screens SHAKER AND VIBRATORY SCREENS/CONVEYORS Screen Applications Vibratory Screening/Conveying Vibratory Taper-Slot & De-Stoning Screens DISC SCALPING SCREENS TROMMEL SCREENS Dry Trommel Screens Wet Trommel Screens VAT TYPE WASHING SYSTEMS HYBRID CLEANERS/CLASSIFIERS/DEBARKERS AIR CLASSIFICATION/SEPARATION CHIP SAMPLING & PRODUCTION TESTS CHIP SAMPLING PROCEDURES Sub-sample Collection Compositing SAMPLE ASSESSMENT TESTING OBJECTIVES TEST # 1 - EVALUATION OF HOGGED FIBER SAMPLE Test Objectives...48

5 6.4.2 Description Findings TEST # 2 - EVALUATION OF CHIPPING EQUIPMENT Test Objectives Description Findings TEST # 3 - EVALUATION OF HYBRID EQUIPMENT Test Objectives Description Findings TEST # 4-A EVALUATION OF RE-CHIPPING EQUIPMENT (SEPTEMBER 1996) Test Objectives Description Findings TEST # 4-B EVALUATION OF RE-CHIPPING EQUIPMENT (APRIL 1997) Test Objectives Description Findings TEST #4-C EVALUATION OF RE-CHIPPING EQUIPMENT (JUNE 97: NICHOLSON ROTODRUM NICHOLSON - SYDNEY, BC) Test Objectives Description Findings CONCLUSIONS AND RECOMMENDATIONS FOR SECONDARY WOOD FIBER RECOVERY TECHNICAL BARRIERS RECOMMENDATIONS TO OVERCOME TECHNICAL BARRIERS Modified Tooling Chipper Feed System Equipment Operation FUTURE DIRECTIONS...58

6 8.0 ACKNOWLEDGMENTS BIBLIOGRAPHY...60

7 EXECUTIVE SUMMARY The critical steps in processing wood waste for recovery are size reduction and contaminant removal. This study is focused on engineered size reduction strategies that may allow processed wood waste to penetrate feedstock markets for high value fiber products, and includes contextual references to interdependent contaminant removal technologies. A broad range of alternatives for wood waste size reduction are considered in this report, within the realm of processing to meet the requirements of various end-use applications. Fiber market applications for recycled wood include the manufacturing of various pulp and paper products as well as engineered wood panels and other products, discussed in subsequent sections. Size reduction technologies for processing recovered wood wastes for recycling use in fiber market applications must be designed to produce fiber particles that meet specific manufacturing requirements. In the forest products industry, most manufactured fiber applications, including both paper and wood, are designed to begin with a virgin wood chip. A virgin wood chip is a wafer-like particle of wood fiber which is produced through some type of whole-log chipping process. Size reduced chunks of recovered wood waste are commonly referred to as chips, but often have very different characteristics. Technologies conventionally applied to the size reduction of recovered wood wastes tend to sacrifice cutting surface tolerances for durability of equipment. This has been necessary because recycled wood typically contains embedded contamination in the form of metals or inorganic grit which must be freed by grinding. One of the objectives of this study has been to consider current alternatives which may allow processors to balance these attributes to achieve high quality reduction in an economically viable manner. Chippers, hogs and hybrid reduction equipment are reviewed and contrasted within this study as the principal reduction alternatives. This study places an emphasis on identifying appropriate size reduction technologies for handling postconsumer and post-commercial sources of wood waste. Examples of such wood waste sources of include remanufacturing trim, pallets or crates, and wood recovered from construction and demolition activities. These represent the wood waste types which are the most difficult to process such that they 1

8 approximate the attributes and quality of virgin chips, and represent a significant waste management concern. Accomplishments in the European markets, where industry has been working at the recovery of low value wood wastes longer than domestic markets, because of a more acute timber scarcity, have inspired research on this project. Certain German wood recycling facilities have been found to have developed direct chipping of contaminated wood wastes using specially designed chippers with modified tooling to produce recycled wood chips. While economic conditions in other parts of the world may warrant more or less investment in recycling solutions than that which is viable under domestic market conditions, the German example is one model which demonstrates the ability to address processing needs through engineering. In assessing the major categories and specific types of size reduction equipment currently available to processors, it is clear that ongoing innovation in equipment design is occurring. Given a long-term decline in the availability of timber resources, increased emphasis and allocation of capital toward residuals recovery is being made by the forest products industry. Equipment manufacturers are beginning to respond to the specific requirements of the wood waste recovery industry. As explained by this study, many of the common waste streams of recoverable wood have significantly different attributes from the whole log sources used in traditional chip production. Those differences include the presence of foreign or non-wood contaminants, and often substantially lower moisture content of recovered wood sources. The ramifications of such attributes and other considerations are discussed in this study in order to provide criteria for the design and selection of size reduction technologies. During an industry expert focus group meeting held by the Recycling Technology Assistance Program of the Clean Washington Center on December 9, 1996, fiber procurement managers pointed to quality concerns as the principal reason for their limited consumption of recovered wood fiber. Chief among quality concerns was the physical disparities of processed wood waste from the accustomed virgin wood chips. Many in the industry now recognize that quality of recycled wood fiber is a function of the 2

9 type of reduction and level of ancillary processing applied. These experts generally agreed that market opportunities for recovered wood fiber in high-value fiber products will continue to grow as long term supply tightens, but placed emphasis on the need to diminish the gap in geometry attributes between recovered and virgin wood chips, while maintaining a price discount relative to virgin chips. The study concludes that chipping and hybrid reduction technologies offer the greatest potential for processors to bring product closer in quality to that of virgin wood chips, thus developing a more stable market position within the framework of market demand for chips from residuals. Several technical barriers currently exist which have inhibited the use of such size reduction technologies in handling this waste stream. Those include: unsuitable chipper cutter (knife type) and feeding methodology to control the workpiece while it is being chipped. This study discusses ways in which effective chipping, which requires that wood be cut across the grain at a relatively shallow angle, might be achieved in processing the targeted urban wood wastes and other wood residuals. The authors of this study have several years of experience in the wood processing industry, and have sought to consolidate information from industry experts, however, it is recognized that there are great number of variations in size reduction applications, all of which may not be recognized by this study. This project has been conducted in conjunction with other industry efforts to compile information about wood waste processing. Information about size reduction technologies was provided to the Best Practices in Wood Waste Recycling Project completed for the Clean Washington Center, in May 1997, and preliminary findings of this study were presented at the subsequent Best Practices Workshops. The Best Practices in Wood Waste Recycling Manual, produced by International Resources Unlimited for the Clean Washington Center addresses reference information about size reduction and other wood recycling. 3

10 1.0 SUPPLY OF SECONDARY WOOD FIBER Waste wood is found in many places. This section provides a brief working context of the major supply streams of recoverable wood wastes. It is important to understand the differences in wood waste types and generation sources as they relate to size reduction. Each attribute of a given stream of recoverable fiber has some influence on how it reacts in the size reduction process. Important attributes to be considered include typical levels of contamination such as rock and grit, metals, and other foreign materials; as well as moisture content, and physical dimensions of the unprocessed material. One of the fundamental distinctions that can be made with regard to wood waste streams is between green wood and dry wood. Green wood refers to fiber that has not been dried, while dry wood refers to fiber that has been dried. Green wood typically has a moisture content by weight of 50 percent or more (wet basis), contrasted with dry wood which typically has a moisture content of 10 to 25 percent by weight (wet basis). Moisture content influences the way fiber fractures or cuts in the size reduction process. The natural resiliency of green wood fiber can facilitate a smooth slicing action when being chipped, but may inhibit fracturing when being hogged to produce a coarse and fibrous particle. Dry wood fiber, conversely, will tend to shatter even under sharp reduction workpieces such as the knives of a chipper, creating a different set of size reduction challenges. Moisture content is only one of the distinctions between wood waste streams. Dry wood includes two major categories: mill residuals and the broad urban wood waste stream. The chief distinction between these two categories is the level of contamination. Mill residuals generally are defined as postindustrial or post-commercial waste materials, while urban wood waste is typically defined as postconsumer waste material. Post-consumer wood waste has been recovered after completing its intended use. Green wood has three sub-categories: forestry residuals, agricultural wood, and land clearing debris. This category is more difficult to fit under a conventional definition of recycled material, but represents a large waste stream that has historically been burned or left behind to rot. This type of material is not altogether dissimilar from conventional whole log timber processed through debarking and chipping 4

11 equipment for pulp chips, but requires special handling and size reduction configurations to handle short or irregular sizes. 1.1 MILL RESIDUALS Mill residuals historically have been processed internally by waste wood generators to fuel biomass combustion power sources. As discussed in the next section, due to its high quality, much of this stream is increasingly being directed to value-added use in either the pulp and paper industry where it can be re-chipped for use as pulp furnish, or to the panelboard sector for use in products such as particleboard Clean Manufacturing Residuals Clean manufacturing residuals are those materials generated by the primary manufacturing of wood products, such as trim ends from dimensional lumber sawing. Typically, these residuals represent the cleanest stream of waste wood in terms of the presence of foreign contaminants. While broadly classified in this report as dry wood, this sub-category of mill residuals can include some green fiber sources. As a general rule, a very high fraction of such residuals are recovered either for energy use or fiber applications. Even bark residuals historically have been processed and marketed for landscaping material Remanufacturing Residuals Remanufacturing residuals are the materials that result from the assembly of cabinets, furniture, trusses, or other mostly wood products. This stream is very similar to the clean manufacturing residuals described above, but may also include some foreign materials such as plastic laminates or paints. Even as the use of manufacturing and remanufacturing residuals as new product raw material continues to grow, significant quantities continue to be used as biomass (hog) fuel. According to the 1994 Washington State Directory of Biomass Combustion Facilities, prepared by the Washington State Energy Office, more than 3.4 million bone dry tons of wood residuals were used as 5

12 biomass fuel in Washington State alone, during This number represents additional fiber supply which may become available as increased tonnage is diverted for use as a raw material. Of the top ten boilers in Washington consuming wood as hog fuel in 1993, nine were configured to use other fuel sources as well, six of which included natural gas. 1.2 URBAN WOOD (MANUFACTURED AND CONSTRUCTION WOOD) Urban wood fiber encompasses a broad range of wood types found throughout today s economic landscape. With the increased scarcity of timber resources, large timber beams and posts are increasingly salvaged from demolition projects for reuse. Despite this practice, substantial quantities of wood are discarded from both demolition and new construction projects. Salvage networks for pallets and crates have developed in most urban hubs. In total, however, the infrastructure for recovery and size reduction processing of urban wood waste is still largely undeveloped in most regions. Urban wood is typically dry and often hardened from age, presenting a special challenge for controlled size reduction. Moisture levels in recovered urban wood vary seasonally, depending on storage and climate. End users in the Pacific Northwest report testing loads as low as 7 percent wet basis in the summer and as high as 25 percent wet baiss in the winter, though those figures do not represent a controlled test of the same fiber source Pallets and Crates This category includes wood pallets, wood crates and packaging lumber. Nails or other metal fasteners are common contaminants. This stream varies in species composition across the country, with a majority comprised of hardwood species. Pallet recovery and repair is on the rise, but even these businesses generate extensive wood residuals from broken and non-standard pallets. 6

13 1.2.2 Construction and Demolition Wood Construction and demolition wood consists of mostly softwood species dimensional lumber and engineered wood from construction and demolition activity, including the following categories: New Dimensional Solid Lumber Scraps New dimensional solid lumber scraps includes trim ends, off-spec materials, and other solid wood residuals from framing and related construction activities. The material may include wood painted with latex or oil based paints. New Reconstituted or Otherwise Engineered Wood Products. New reconstituted or otherwise engineered wood products may contain cured adhesives as the product binder. This includes plywood, Oriented Strandboard (OSB), particleboard, Medium Density Fiberboard (MDF), waferboard, and other fiberboard or products thereof. The material may also include wood painted with latex or oil based paints. Sorted Demolition Wood Sorted demolition wood is wood generated by demolition or renovation activity, which has been segregated. This material may include incidental amounts (usually under five percent per load) treated wood waste or non-wood materials. It does not include lead-based painted or asbestos laden materials. High tipping fees at disposal sites and restrictions on open burning are driving building and demolition contractors to seek less expensive options, such as recycling, for their debris and construction scrap. Some regions have seen the development of new service vendors, such as on-site construction recycling sub-contractors, which make wood recycling increasingly convenient and attractive to the construction industry. Additionally, new front-end sorting and segregation practices and technologies are developing to allow increased recovery of wood fiber from the mixed demolition waste stream. 7

14 1.3 GREEN WOOD Forestry Residuals Forestry residuals consist of short log sections, tops, small diameter trees and other material typically left behind after a logging operation is completed. This includes not only the material left in the forest, but also wood residuals piled at interim staging areas. Whether this is a source of recycled wood is debatable, but this is clearly an area where large amounts of usable fiber are left behind and mechanisms for increased recovery are beginning to develop. Accurate supply tonnage data is not available, but information from industry sources indicates that the yield from traditional forestry clean-up practices has been as little as a third of the total volume of cellulose fiber available after whole logs are harvested. Management practices for increasing the recovery of forest residuals are now developing, and are expected to be implemented over the next 10 to 20 years Agricultural Wood Agricultural generators of wood residuals include orchard operators who routinely generate prunings and whole fruit trees when making species transitions. This material has traditionally been burned or buried on site. However, expanding environmental regulations such as Clean Air Act requirements are making those practices less feasible. With these changes, orchard residuals represent a new secondary wood supply available for mulch and other applications. The average rate of generation is estimated at approximately 3 tons/acre/year, according to industry representatives in Washington State Land Clearing Wood Land clearing wood consists of woody vegetation from land clearing activity, such as stumps, log sections, and other woody vegetation containing less than ten percent soils or rock. This is the L component of the CDL acronym used to refer to the construction and demolition (C&D) waste stream in some parts of the country. Such debris is limited to those areas where forested lands are used for new development, often near growing urban areas. 8

15 2.0 WOOD WASTE PROCESSING SYSTEMS Waste wood represents a tremendous volume of fiber which has yet to be fully realized as a viable secondary fiber source. Utilization of mill residuals has been increasing over the past several decades, to the point where they are now used in many value-added products. Recovery of forestry residuals is much less mature in its development, but remains a tremendous potential resource. Urban wood waste represents a large frontier of discarded wood fiber that has only recently begun to be tapped as a material source for industry. 2.1 HISTORICAL INFLUENCES ON WOOD WASTE PROCESSING SYSTEMS There are two major influences that have guided much of the development of the wood recycling industry: the solid waste handling sector and the forest products industry. Both sides approach wood recycling with different objectives, but together form a diverse industry Influence of the Solid Waste Industry The major objectives of solid waste handlers with regard to wood recycling are twofold: process wood to divert material from the waste stream and to achieve high volume, low cost size reduction for transportation efficiency. Indeed many companies ground C&D debris for years just to maximize the efficiency of trucking waste to the landfill. Much of the size reduction equipment used by the solid waste industry to handle wood was developed to handle size reduction of a broad range of waste materials, not limited to wood for waste handling. By dedicating grinding equipment to wood waste, and processing to meet end-use requirements (such as hog fuel), the waste industry has successfully achieved their goal of finding a low-cost disposal alternative Influence of the Forest Products Industry Conversely, the forest products industry approaches wood waste recovery from an entirely different perspective. While low-cost, high-volume size reduction is generally a shared objective, consideration 9

16 of wood waste recovery is a means to an end of developing diverse supply portfolios. Size reduction in the industry generally is determined from very specific chip requirements. The result is a thorough operating familiarity with a broad range of equipment types. One of the major wood processors in the Northwest is a family owned business. At one time, the business operated three whole log chipping facilities. Today, two of those facilities have been closed and replaced by the company with two urban wood waste recovery operations. Such processors are able to draw on a long history of size reduction experience, while adapting to an entirely different stream of raw materials. Forestry / Logging Companies Chippers Swing Hammer Hogs Mass Rotor Hogs Construction & Demolition Debris Handlers Wood Recycling Industry Size Reduction Technologies Hybrid Technologies Hoggers Tub Grinders Shredders 2.2 CURRENT WOOD WASTE PROCESSING SYSTEMS Wood waste processing consists primarily of sorting, size reduction, and then screening for contaminant removal and sizing. The elements must all work together in order to form a successful wood processing operation. The old adage garbage in, garbage out, generally holds true when describing size reduction, so receiving practices and sorting are important pre-cursors to the size reduction process. Innovative processing configurations employing size reduction equipment (usually hogs) in various ways with ancillary feeding, screening, and cleaning equipment have allowed dramatic improvements in the ability to remove contamination from wood waste. Contaminant removal continues to be key to the successful high grading of low value wood wastes. However, the next step in approximating the characteristics of virgin wood chips is narrowing the gap in chip consistency and quality. 10

17 The primary size reduction step (see Section 4) is generally designed to produce a distribution of materials without excess fines or overs which fall outside the desired size distribution of the end product. However, screening of material subsequent to the size reduction step is important in order to assure product quality. Oversized materials are typically re-directed through the primary reduction step or sent to a secondary reduction step. Besides engineering the size reduction step to produce the desired size of particles, it must also be designed to free embedded contaminants for removal (see Section 5). In the Pacific Northwest, a well capitalized wood waste recycling industry has developed. The largest and most sophisticated processors have begun to serve the pulp and paper and reconstituted panelboard markets with secondary wood fiber, gaining experience processing secondary wood to tight market specifications. Generation Sector Chip Cleaning: Screening, Separation Metal Recycle Collection System Receiving / Sorting Magnetic Separation Market Size Reduction Screening Two Northwest wood recyclers, in particular, operate sophisticated processing lines with extensive contaminant removal capabilities. These facilities have the ability to meet tight fiber market specifications for contamination. Processing activity goes well beyond size reduction to include manual sorting, multi-stage magnetic separation, air classification, screening for contaminant removal, water separation, and screening for accurate sizing. Even with the sophisticated levels of processing applied at these facilities, processed secondary wood fiber continues to exhibit a lack of uniform physical characteristics and as a result, is having difficulty 11

18 successfully penetrating high value fiber markets. The lack of uniform physical characteristics is a direct function of the size reduction process for recovered wood wastes. A focus group of industry experts on the utilization of secondary wood fiber held by the Recycling Technology Assistance Program (ReTAP) on December 9, 1996, reaffirmed that concerns over uniformity and consistency in secondary wood fiber supply continue to be of significance. 2.3 NEW DEVELOPMENTS While there are no turn-key systems currently available to process waste wood feedstocks to fiber market specifications, equipment manufacturers are moving in that direction. Existing processors serving fiber markets typically assemble systems using equipment from three or more vendors and experiment with a wide range of screening technologies to reach desired output specifications. Such technologies may include electro-magnetic separation, vibratory screens, trommels, air knives, air classifiers, water separation, disc screens or manual picking lines. The industry is now driving the development of new types of equipment. These technologies are discussed in greater detail in sections 4 and 5 of this report. Hogs are slowly evolving toward the ability to provide a more consistent shape of chip with cleaner edges. Chippers are concurrently evolving to where they can handle the divergent nature of wood waste feedstocks with greater capacity, reliability, and flexibility. Wood waste processors can now choose among a wide range of equipment types and sizes, including large stationary or smaller, mobile size reduction equipment. While equipment continues to improve, processing remains capital intensive. Obtaining attractive financing and operating at a high percentage of capacity remain key to long-term economic viability. Since end-users are increasing their processing requirements, costs must be carefully controlled to maintain processor economics. In addition, wood waste transportation costs are high, and while improving, new options need to be developed to expand the processing from a local to a regional basis. 12

19 3.0 VALUE-ADDED MARKETS FOR SECONDARY WOOD FIBER Traditionally, raw wood has been abundant in many parts of the country, particularly in the Northwest, and wood waste has not been considered valuable enough to process and recycle. Industrial wood waste was recovered and burned in boilers as a low value hog fuel. Most construction, demolition and land clearing debris was burned or buried on site, or hauled to a disposal site. Today, many of the major mills still operate boilers that are powered by biomass combustion energy from hog fuel made from mill residuals. However, high quality mill residuals and non-saw grade timber often go to pulp chip applications, and more recently, to a growing number of engineered wood applications. These newer mill residual applications, combined with the decreasing supply of virgin timber available for milling, have created a shortage of mill residuals. At the same time, demand for chips in engineered wood products and pulp and paper applications continues to grow. As a result, the industry has seen the value of chips increase markedly. The differential value of wood chips in production applications versus combustion has caused mills to largely divert their mill residuals from hog fuel uses to higher value uses. 3.1 MARKET DYNAMICS Like many domestic natural resources, cyclical market conditions periodically boost the short-term supply of virgin wood chips, however, the long-term trend is generally anticipated to reveal increased scarcity. In the Northwest, curtailment of harvests on federally-owned forests since the late 1980s has created a decrease in overall supply. As the demand for wood chips in this region has begun to exceed conventional supply, end-users have begun to consider alternative fiber sources. Many mills have begun taking sophisticated approaches toward utilizing wood chips from a variety of secondary sources for high value uses and to meet hog fuel needs to power their boilers. Figure 2 illustrates relative values of both wood waste generation streams and end market uses. While low virgin chip prices have currently depressed the use of recycled wood in fiber applications, the development of 13

20 appropriate procurement strategies for such sources could offer a number of end-users long-term advantages in cost, diversity of supply, and performance. The wood recycling industry in many parts of the country developed to meet the forest industry s need for wood chips to replace mill residuals in hog fuel applications. In producing hog fuel, the name of the game is high-volume, low-cost size reduction. Particle geometry is not particularly important in producing hog fuel, except that it must be sized appropriately for automated feed systems. Large scale and portable hogs fill this need well. Yet, wood recyclers now have the opportunity to add-value through size reduction to serve higher value uses. Many of these higher value market opportunities have greater requirements on particle geometry, in some cases three dimensional sizing specifications. In shifting focus toward such application, many wood recyclers are now looking beyond basic hogging size reduction scenarios, to consider how raw wood waste might be engineered through size reduction. Figure 2 Source: CWC, 1995, John Yeasting CLEAN WOOD WOOD WASTE TYPES Green Wood & Mill Residuals Mill Residuals Forestry Residuals Agricultural Wood Land Clearing Wood Brush & trimmings Manufactured & Construction Wood Pallets & Crates Lumber Engineered Wood Sorted Demolition Wood MIXED WOOD WASTE Unsorted Demolition Wood Waste Roofing & Siding Commingled Wood Waste Treated Wood Waste HIGH VALUE LOW VALUE END USE OPPORTUNITIES Re-Sale or Re-Use High Value Fiber Markets Pulp & Paper Applications Reconstituted Wood Products Value-Added Uses Composite Wood Fiber Uses Cellulose & Molded Pulp Mulch Products & Soil Amendment Animal Bedding Interim Road Bed Compost Bulking Biofuels Combustion Chemical derivatives / gasification Processed Fuels: pellets / blends Hog Fuel Disposal 3.2 THE IMPORTANCE OF SIZE REDUCTION TECHNOLOGIES 14

21 During an industry expert focus group meeting held by the Recycling Technology Assistance Partnership of the Clean Washington Center on December 9, 1996, fiber procurement managers pointed to quality concerns as the principal reason for their limited consumption of recovered wood fiber. Chief among quality concerns was the physical disparities of processed wood waste from virgin wood chips. Many in the industry now recognize that quality of recycled wood fiber is a function of the type of reduction and level of ancillary processing applied. Industry experts explain that the globalization of virgin wood chip markets and cyclical pricing patterns influence short-term chip procurement decisions. However, given the long-term pressures on the supply of virgin fiber, the general consensus of industry experts was that market opportunities for recovered wood fiber in high-value fiber products will continue to grow. Emphasis, however, was placed on the need to diminish the gap in geometry attributes between recovered and virgin wood chips. At the same time, the costs of size reduction and ancillary processing must be controlled to give recovered wood chips a price advantage over virgin chips, necessary to justify procurement. 3.3 REVIEW OF VALUE-ADDED APPLICATIONS Applications for fiber from secondary wood sources continue to grow. For example, the ability of the industry to fiberize wood for uses such as new composite products, cellulose insulation and molded pulp is improving. To better understand the specific quality requirements of wood chips as they relate to sizing for these new applications, as well as existing higher value applications, mill representatives were informally interviewed during this study. The geometry of recovered wood chips has ramifications that are specific to many of the fiber manufacturing applications, as well as those which are common among several applications. 15

22 3.3.1 Pulp & Paper There are several technologies that can be employed in the production of wood pulp for use in paper manufacturing. Two major processes represented by industry representatives contacted are: Thermo- Mechanical Pulping (TMP) and Chemical Thermo-Mechanical Pulping (CTMP). CTMP, also known as semi-chem digesting, is a newer and more prevalent process today. It employs chemicals that aid in the breakdown, or fiberization, of the wood chips. Each of these processes employ large refiner plates, where the fiber is compressed between large counter rotating discs operating at very close tolerances. These refiners have a very low tolerance to any non-wood materials that may cause damage to the plates, requiring that recyclers provide very clean material. The size of chips entering the pulping process has several ramifications. The length and orientation of cut largely determine the length of fibers in the finished pulp. The thickness, width and coarseness of the chip determine resident time or length of the pulping process. The width and coarseness also influence how well the chips perform in large automated storage, conveying and feeding mechanisms. Overly coarse chips, such as the typical hogged fiber chunk, are subject to bridging (fibers adhering to each other), which can cause costly clogging of such systems. Beyond the importance of chip uniformity for proper feeding, most mill representatives explain that the pulping process can accommodate a range of chip sizes. However, it is important that there is a level of consistency in the chip characteristics, so that pulping reactions can be predicted with confidence. Semi-chem digesters have a greater ability to handle coarse fiber particles since the greater surface area allows rapid chemical and moisture penetration, but still require consistency in size. Many of the pulp users that have successfully utilized chips from recovered wood fiber have revised sizing requirements for processors downward, to effectively minimize the variation between individual particles. This is balanced with sacrificing fiber length. Most pulp producers today utilize significant quantities of post-consumer paper fiber in some grades of pulp. Secondary wood fiber may serve as a source of post-consumer fiber offering longer and stronger fibers than those of shorter waste paper fibers. 16

23 3.3.2 Reconstituted Panelboard Reconstituted panelboard is a broad category of engineered composite wood products that includes particleboard, MDF, OSB, plywood, hardboard, and several others. Representatives of this industry explain that fiber geometry can be one of the greatest limiting factors in their consideration of chips from recovered wood fiber, depending on the application. Currently, the only panelboard producers able to utilize secondary wood fiber in manufacturing are those producing particleboard, MDF, or hardboard. These products employ finely sized fiber particles as a manufacturing input, and have a recent tradition of utilizing mill residuals as the source of this fiber. Such residuals, along with other lower value wood waste sources can be processed through either hogging or chipping equipment, along with subsequent secondary reduction steps such as flaking or fiberization to approximate the characteristics or feedstocks from virgin fiber sources. Other applications, such as OSB and oriented strand lumber (OSL), rely more heavily on tight chip sizing requirements to achieve the engineered attributes of the finished product. In the case of OSB and OSL, clean-edge, wafer-like chips of specified lengths are required. Since chips of such geometry cannot currently be produced from waste wood, these applications use only virgin wood chips from whole-log sources. This market generally pays less for feedstock material than pulp and paper producers because of slightly less stringent requirements on species type and other attributes Fiber and Composite Applications Cellulose insulation and molded pulp products are emerging markets for recovered wood. These uses employ a fiberization process. Additionally, several new composite wood products are on the horizon which may offer potential for utilization of recovered wood, including cement-fiber, thermoplastic-fiber and others. The specification for wood fiber in such uses will be dependent on the characteristics of the desired product. Due to secondary size reduction steps required for most of these applications, they will typically have the same low tolerance to metals, dirt, rock or sand as pulp applications, but may offer broader tolerances for trace amounts of materials such as plastics, laminates or fiberglass which are not acceptable in pulp processes. 17

24 According to one major project considering recovered wood fiber in a plastic-wood composite application, chip geometry is also important. The process involves flaking as a final size reduction step. Flaking takes the fiber particles down to roughly.015 inches thickness with a desired length of 1 inch. If poor quality (overly coarse or highly fractured) chips are fed to this process, an excessive amount of fines or sawdust size material will be produced, adding significantly to material costs Mid-Value Applications In addition to the high and low value applications for recycled wood waste, a number of mid-value applications exist. Among these are: animal bedding, interim road beds, mulch and soil amendments, bulking agents for compost production, chemical derivatives, and processed fuel pellets or blends. Each of these end uses may have value in particular situations. For instance, interim road bed applications may make sense for the utilization of rural land clearing wood debris or forestry residuals. Most of these applications have relatively limited requirements on chip geometry, with the exception of animal bedding, which is desired to have clean, smooth edges Biomass Fuel Applications Biomass combustion of wood for energy, also known as hog fuel, has been a common use of secondary wood due to a historical abundance of wood residuals. Industrial conversion to lower cost fuels such as natural gas and the increased stringency of air quality control requirements have significantly eroded long term demand for hog fuel in the Northwest. As this market declines, Washington processors serving this market will have to find other markets, such as compost, for low grade waste wood chips. The alternative for processors is to make the necessary capital investment to bring chips up to the specifications for higher value markets. 4.0 SIZE REDUCTION EQUIPMENT ASSESSMENT Size reduction equipment is perhaps the most important element of wood waste processing systems as defined in Section 2. Size reduction equipment determines the size and shape of the wood particles that are produced. The types of size reduction used depend on the end-use markets being served. Most 18

25 wood recovery operations use primary size reduction and cleaning, and many add secondary size reduction and additional cleaning stages. Some wood recovery operations are now using hybrid size reduction technology instead of, or in combination with, primary or secondary size reduction. Re-chipping technologies may be employed as a secondary size reduction stage in recycled wood processing. This technology may provide the ability to take recycled wood fiber which has been processed through primary reduction and screened for contamination, to a state of greater uniformity in sizing. Alternatively, a simplified process may be developed which directly chips contaminated wood waste. The types of equipment discussed in this section typically employ either very sharp tooling (knives) to slice the wood fiber (chippers) or more blunt tooling which reduces the wood particles through impact force (hogs, shredders, hammermills), or a combination of the tooling (hybrid equipment). Each type of size reduction equipment has variations in how material is fed through the equipment, speed of operation, and several other design characteristics. 4.1 PRIMARY SIZE REDUCTION The primary size reduction philosophy to date has been to select primary processing machinery that fits with the quantity, quality, and range of materials that are to be processed. This philosophy is still sound. However, processing techniques and developing technologies must be utilized to allow recyclers to better customize wood waste products to best serve the variety of end markets, with the additional objective of optimizing the market value of the products. Current practices for primary reduction of C&D wood materials employ the use of large hogs; generally swing hammer, fixed hammer, and mass-rotor types. Typically, primary reduction equipment and setup is selected so that it will maximize the amount of processed materials in the desired size range, while minimizing fines and overs. This is often difficult due to the extreme variation in raw materials processed. Seasoned recyclers have experimented with the type of hog, and the size of bottom grate employed, that will provide the highest percentage of accepts in the desired size range. The target size 19

26 range for primary reduction in the industry is generally < 3", with some restrictions to the amount of "fines" depending on the end user. Hogs and other reduction equipment which employ more blunt tooling have characteristically greater durability for handling feedstocks with contamination. However, this type of equipment produces wood particles which are less uniform and less consistent than actual chip specifications allow. These particles have very coarse geometry. Newer equipment that has design characteristics of both types of equipment, known as hybrid size reduction equipment, employs semi-sharp cutting surfaces with high durability to produce particles which are more uniform in nature and capable of handling today s broader range of feedstocks. Hybrid equipment is increasingly being used in place of traditional primary size reduction equipment. In general, reduction equipment such as chippers, which employ sharp tooling, have a greater sensitivity to contaminants which dull the cutting surface and reduce efficiency. For this reason, chippers are generally not used as primary size reduction equipment for recycled wood waste. 4.2 SECONDARY SIZE REDUCTION In-line secondary reduction (or provisions to recycle overs to the primary hog) is often essential to ensure compliance with product specification and to maximize wood fiber recovery. Following primary reduction, tramp metal removal equipment is usually employed to remove "free" tramp metals (metal that has been separated from the parent wood piece in the primary reduction process). These magnets (depending on the type and strength) may also remove some wood where metal remains lodged in the wood piece, however, this will generally be a very small percentage of the total material processed. Following first stage metal removal, the processed material is generally screened by variety of screen types (refer to section 5 for description of screen types). The primary screening process will remove the fines as well as separate the overs, as defined by the targeted product application. It is these overs that are then directed back to the primary hog for final reduction. The product from this secondary 20

27 process will again require de-metalling and screening before joining the accepts flow from the primary screen(s) Re-Chipping Re-chipping has been part of the forestry based industry almost since the inception of primary chipping, at least for the past 50 years. As the demand for chips started to grow along with the growth of chemical based pulping, it quickly became apparent that chip quality and consistency were major factors, not only in final pulp quality, but in overall fiber recovery (ratio of finished pulp tonnage to raw fiber into the process). Research has been completed over the years by research groups, suppliers, and end users on chipping theory, and equipment control parameters (angle of cut, knife angles and clearances, etc.). Refinements based on the research have focused on trying to optimize chipping technology to produce the preferred chip. The preferred chip, initially for chemical pulping, and later for mechanical refining, consists of a chip with smooth cut ends, clean shear surfaces, and within a very tight thickness tolerance range of 4mm to 7mm (TAPPI Journal Chip Thickness Control with a Conventional Screening System, pp , Sept. 1991). It was quickly found that a number of variables, both equipment related as well as raw material related, can have a significant impact on the so called "perfect" chip. Even under the best of conditions and chipper set-up, a fairly wide range of chip sizes are produced, from very fine material to large slivers (overs). Primary chip producers, whether an integral part of the primary process, or chipping plants producing on contract, screen the chips to remove the unwanted fines (generally less than 3/16") and also for the overs (generally greater than 2"). The re-chipper was developed in order to capture this > 2" fraction (along with the ability to process small broken log chunks/slivers or "fishtails" recovered from the sawdust waste conveyors). It was quickly found that chipping small chunks of wood was not only challenging, but the results were 21

28 generally very poor. Despite the poor performance, many of the re-chipper configurations that were developed are still in use in one form or another. Three principal types of re-chippers have been developed: the straight drum-type, "V"-drum type, and conventional disc. These machines, all of which use knives to chip virgin wood, utilize various numbers of knives and angles, and different feed geometries, to try and align and hold the small chunks of wood as they enter the knife cutting zone. V-drum chippers, used for a number of years, have fallen out of favor due to their longer set-up time and higher maintenance costs (more susceptible to tramp metal damage than the disc or straight drum chipper). The trend for a number of years was toward disc chippers with a secondary spout above the primary spout, designed specifically to handle overs. This configuration is usually used with closed loop chipping, screening, re-chipping systems, where the chips are pneumatically conveyed to the screen overhead and the overs fall directly back into the small overs chute. The disadvantage is that the chips have to be re-screened, a costly and time consuming step. The trend today is more to the straight drum chipper since it provides the capacity for a wider range of feedstock sizes, and can handle broken ends and trim falldown along with a substantial flow of overs. The key here is that a bottom screen is used, which with appropriate experimentation and selection of opening size, can ensure a high level of acceptable chips on a one-pass-through. Wood recyclers to date generally have not employed chipper technology in their operations for the purpose of primary reduction of CDL materials. More recently, however, recyclers are becoming increasingly aware of final product value ranges, which provides significant premiums for well formed and clean chips. Many recyclers have implemented sort tables and conveyors where clean materials are diverted (hand picked) to a separate line(s), allowing separate processing of this portion. The downside here is the significantly higher processing costs (primarily labor) allocated to a very small percentage of the overall material processed. This generally has not been found to be economical. 22

29 The challenge for the industry is to develop systems and equipment that enable a large component of the CDL waste stream to be chipped at the primary stage rather than ground by conventional hogs. This is particularly true if the desired product is chips that approximate the size, geometry, and value of typical virgin wood pulp chips. 4.3 SIZE REDUCTION EQUIPMENT TYPES There are three very basic types of size reduction equipment which are used to process wood residuals, and many new types emerging which include multiple characteristics. These are chippers, hogs, hammermills and shredders Chippers Chippers have a long history of use with clean traditional feedstocks such as whole logs and mill residuals. This type of equipment produces wood particles which we know as chips with two surfaces and clean edges of specified dimensions. Disc Type Chippers. Disc type chippers have been used for many decades for primary reduction of logs to quality chips for the production of pulp and paper products. These chippers range in size from the whole log chippers of 140" to 150" in diameter with 2500 hp direct coupled synchronous motors, to the 48" size, belt driven with 150 to 200 hp induction motors for chipping waste wood in the sawmill (trim ends, off-cuts and edgings for the production of residual chips). Disc chippers have been refined to some degree over the years, however, the basic principle of cutting chips of varying fiber length (depending on the end use) at an angle diagonally to the grain, has not changed significantly. Tooling has become more sophisticated with various "throw-away" knife systems, along with refinement of machine set-up and materials to reduce the maintenance costs in operating this type of equipment. Disc type chippers are supplied in two basic configurations, 1) Horizontal feed, and 2) Gravity or drop spout feed. 23

30 Horizontal Feed Disc Chippers. Horizontal feed chippers are generally employed where the material to be chipped is longer than 8 feet, particularly tree length chipping operations. However, there are hundreds of smaller horizontal feed waste wood chippers operating in sawmill plants which handle all lengths from short blocks to full length edgings up to 20 feet long. This is a matter of economics, and is a relatively efficient way of handling this type of clean mill residual material, unless it is chipped directly as part of the primary sawing and integral chipping process. Although this type of chipper is in extensive use, one of the drawbacks, due to the mix of size and shapes of the material going into it, is the variable geometry of the chips produced, and the amount of fines losses. There is substantial motion of the feed materials in the throat of this type of chipper, leading to inconsistent sizing of the resulting chip, versus a comparable chip from a single solid piece being chipped (in the same type of chipper). Oblique Shearing Cutting Knife Disc Chipper Profile Angled Infeed chute To try and better control this action at the chipper knife face, several chipper manufacturers have produced machines where the chipper disc is canted over on an angle towards the infeed conveyor. This configuration has a tendency to "hold" the wood chunks better against the disc face because of the compound chipping angle. The wood not only approaches the disc at an angle in the horizontal plane, but is also cut on an angle in the vertical plane by the cant angle of the disc (this angle can be as much as 15 to 20 degrees). This action is intended to approximate the action from a drop spout chipper for short wood. Drop Spout Disc Chipper Basic design of this chipper is the same as the horizontal feed chipper, except that the disc is fitted with a spout which approaches the disc at an angle of about 38 to 42 degrees (vertical plane). The disc is also maintained at a fairly standard angle in the horizontal plane, 24

31 which provides a compound cutting angle on the wood being chipped. The chip is cut diagonally across the grain in the flat direction (width of the chip) and again diagonally across the grain in the thickness direction. Drop spout chippers have been extensively used, to chip short pulp wood (8'), in the pulp and paper industry on a world wide basis for many decades because of the high chip quality and reduced fines produced with this configuration. Chip discharge from the chipper housing can be by direct gravity discharge to a collector conveyor under the chipper; directly behind the chipper through the back of the chipper housing; or pneumatically discharged from the top of the chipper by fitting the disc with fan blades to provide pneumatic conveyance. This latter method has been widely used due to the convenience, smaller foot print and lower capital cost, however, not without some sacrifice in delivered chip quality in the form of additional fines. Drum Type Chippers. Drum type chippers are far less prevalent than the disc, however, have been seeing increased use in processing waste virgin or residual wood in the last 20 years. The main reason for their increased use is that the size of the finished product, specifically the amount of oversized chips (or chunks) can be controlled by employing a basket screen on the bottom of the drum chipper. For the primary processor, this can eliminate the need to screen the chips before shipment to the end user. Generally all disc chipper-produced chips must be screened to remove slivers (or overs) before being acceptable to the end user. A drum chipper can generally produce a more consistent chip while dealing with a wide range of feed stock. Another advantage of the drum chipper is its ability to process a wider size range of raw material. A disc chipper capable of handling the same larger block size range would not be economically viable, and would not be a wise choice, given the geometry of short waste material. 25

32 Drum chippers can also be supplied in horizontal feed with a multiple feed roll system, or alternatively a gravity feed system. However, the gravity feed system on the drum chipper is generally perpendicular to the drum shaft axis. There is no advantage to skewing the infeed, in the horizontal plane, on a drum type chipper because of the way the material is delivered to the knife Drum Chipper Profile In Feed Cutting Knife Screen cutting circle (very randomly because of the shortness and randomness of the wood chunks) Hogs As a general note, "hogs" have been synonymous with the wood processing industry for many years, and relates to their ability to take a wide range of raw material, under very wide surge conditions and convert it into a product suitable for fuel to fire a power boiler (to a < 3" specification). Swing Hammer. This type of hog has been used and has been preferred in the forest products industry for many years because of the robust design, its ability to tolerate fairly large contaminants (both metal and mineral), its ease of maintenance and minimum field set-up time for maintenance personnel. These types of hogs can and do operate for extended periods of time with little maintenance attention, other than necessary lubrication requirements. In many cases it is only after the machine fails to meet the production demands and/or the quality of hog fuel falls below the minimum requirement that the machine is shut down for service. Fixed Hammer. The fixed hammer hog is a fairly recent aberration from the swing hammer machines, and has been an attempt by some manufacturers (some fairly new in the field) to get higher energy into 26

33 the workpiece. The result, in effect, models the performance of the mass rotor hog, discussed below. Use of this type of hog is very application specific, and is generally not used where a high percentage of large mineral contaminants are expected. A typical application is for the reduction of reject large cross section engineered wood product materials for recycling into the process or for fuel purposes. Fixed hammer hogs usually have either replaceable hammer face inserts, or the ability to have the whole hammer replaced when worn out. Punch & Die. Punch and die hogs are as the name implies; the rotor is fitted with close tolerance fixed cutters, while the anvil, in the work zone, is fitted with a die pattern that matches the fixed cutters. This type of hog is very effective and efficient, however is very sensitive to mineral contaminants. It also has a much longer set-up time and maintenance costs than a comparable-sized swing hammer hog. As such, this type of hog is no longer used as the primary hog in large wood processing facilities. Mass Rotor. The mass rotor hog is fairly new, was develop in the last 10 years, and now is finding wide use as the primary reduction hog in many operations. The mass rotor hog is as the name implies; the rotor is made up of a very heavy shaft, to which is fitted flame cut and machined solid steel plates (plate up to 12" thick). The plates are cut so as to provide a machined surface for mounting replaceable wear bars. There are now approximately a dozen manufacturers of this type of hog (half a dozen major manufacturers), each stressing the uniqueness of their specific design. These hogs are also available in a full range of capacities from a few tons to over 150 tons per hour (TPH). Knife Hogs. Knife hogs are also as the name implies, and employ knives for the reduction action rather than impact per the previous types. Knife hogs are not in wide use today, however, were used in specialized applications, such as the hogging of western red cedar bark, and some eucalyptus species in Australia. This type of hog, if used in a highly contaminated application, often results in prohibitively expensive operation and maintenance costs Hammermills 27

34 In the wood processing industry, hammermills are generally used in association with secondary manufacturing, or as a secondary process in a primary manufacturing process. An example would be the hammermilling of screened dry fines in a panelboard plant for use as fuel for a wood dust burner to fire the drying system. Although hammermills are available in some very large sizes, they are generally employed in size reduction processes where there are no significant contaminants, and where the Hammer size of the finished product is fairly small. Final size is controlled by the use of screens on the bottom of the machine. This type of equipment also generally requires pneumatic scavenging to provide air flow through the machine as well as ensure efficient throughput capacity. Impact Hammer Mill Profile Heavy Duty Grid Grinders or Pulverizers are sometimes used interchangeably with hammermill nomenclature, and is highly dependent on the type of industry (and history) in which the equipment has been employed. Hammermills are generally run at rotor speeds two to three times faster than typical hog applications (up to 3600 rpm). Hogs in the wood processing industry generally have a 1200 rpm limit, with many of the larger units running in the 700 to 900 rpm range Shredders Shredders are another class of reduction machine and the name covers a very wide range and type of machine. Low Speed High Torque. This type of machine has been available for the last 15 to 20 years, however, it has only been in the last 10 years that it is finding wider use. This increase has been mostly 28

35 attributable to environmental issues; landfill tipping fees, and restrictions on the types of materials that may be landfilled. One of the major applications for shredders is that of tire shredding. Low speed high-torque shredders can also be used for the reduction of waste or residual wood. They are limited severely, however, in throughput capacity (relative to a large hammer or mass rotor type hog) because of the slow rotor speeds. Typically these machines have two rotors as illustrated at right, with rotor speeds < 20 rpm. Infeed Shear Shredder Because of the design and accuracy of the intermeshing discs, these machines have the capability of applying very high shear loads to the material being reduced. The pullthrough tip forces of this type of shredder can reach up to 200,000 lbs. with the use of dual hydraulic systems, allowing reduction of a wide range of metal and synthetic products (recycled carpets for example). Accepts High Torque Counter-Rotating Discs 29

36 High Speed Shredders. There are a wide variety of machines in operation that have been tagged with the shredder name, however, most of these machine can fit into the "Hammermill" category, either in the vertical or horizontal configuration. Again the name given to any particular type of reduction machine is highly dependent on industry experience, the function required of the machine, and the product produced Hybrid Size Reduction Equipment Hybrid size reduction equipment refers to equipment which seeks to combine the durability of hogging equipment with the high-quality cutting action of chipping equipment. Principally, these types of equipment seek to employ workpiece cutting surfaces which are sharper than those of a hog, but capable of withstanding the wear experienced in high volume processing. The reduction action of this equipment is not designed to slice wood like a chipper, but rather to use a combination of impact force and cutting action to produce wood chunks with cleaner edges than the typical coarse shredded geometry. New equipment designs and modifications to conventional equipment continue to emerge and are being driven by the needs of a diverse wood waste processing industry. In processing various forms of wood wastes and residuals, operational efficiency must be balanced with desired quality output. Due to wear caused by the contaminants present in typical wood waste streams, the use of chipping equipment as used to produce chips from logs and clean residuals is seldom feasible with lower grade wood wastes. As a result, hogging equipment, which is more resistant to such wear, must be used in the majority of wood waste processing applications. This equipment achieves cost effective size reduction, but produces coarse wood particles rather than true chips. As the industry seeks to move these lower grade wood waste materials to product applications requiring specific chip characteristics, there is a need for equipment which can combine precision tooling without sacrificing durability required to resist wear. 30

37 Rotary Knife Hog Designs. Rotary knife hogs employ a design similar to that of a swing-hammer hog, only with knives mounted on the rotor rather than hammers. These knives are not as sharp as those found in a chipper design, but achieve a cleaner cutting action than that of a hog. The knives cut materials against an anvil or breaker bar and force material out through heavy steel grates. These machines can tolerate a broader spectrum of contaminants than chippers, but generally require relatively clean feed materials, as excessively contaminated feedstocks can result in prohibitively expensive operation and maintenance costs. Pan and Disc Design Another hybrid size reduction design, called pan & disc has been developed by a Washington manufacturer, Universal Refiner Corp. of Montesano, WA. This machine employs a cutting disc fitted with sharp cutting teeth, rather than conventional hammers. According to product literature, one of the design criteria was to address safety concerns associated with rotary hog mechanisms used in the common tub grinders. The Universal Refiner design, found in their Contender model grinders, is a mobile unit, similar in size to large tub grinders, but is hopper fed with the cutting action occurring deep within the machine. Discharge grates on the perimeter of the bottom pan control particle size and allow hard contaminants to pass through without being forced against the cutting disc. The machines employ a rotor speed lower than conventional hogs, geared for high torque. 31

38 4.4 MATRIX OF EQUIPMENT TYPES Equip. Type / Mfrs Reduction workpiece Speed (RPM) Typical Feedstock Sensitivity to Contaminants Geometry of Particle Produced Chippers Disc Chippers replaceable knives High high clean edge, twosided Slant disc / horiz. feed whole logs Drop spout / gravity feed clean residuals Drum Chippers replaceable knives High high clean edge, twosided Horizontal feed whole logs Drop feed / gravity feed clean residuals Hogs Swing Hammer swinging hammers Mod urban wood waste, stumps/lc debris Fixed Hammer fixed hammers Mod urban wood waste, stumps/lc debris Punch & Die fixed impact surfaces Mod urban wood waste, stumps/lc debris Mass Rotor rotating impact surface Mod urban wood waste, stumps/lc debris Shredders Low Speed, High Torque dual rotor cutting discs Low urban wood waste, stumps/lc debris low low moderate low low coarse, multisurface coarse, multisurface coarse, multisurface coarse, multisurface coarse, multisurface High Speed swinging hammers, shredder hooks, knives or shearing tools High urban wood waste, stumps/lc debris low coarse, multisurface Other / Hybrid Size Reduction Knife Hogs semi-sharp hammers Mod urban wood waste, stumps/lc debris moderate semi-coarse V-Rotor Hog offset rotor hammers Mod moderate semi-course Horizontal Feed Mill fixed tooth drum Mod moderate semi-course Pan & Disc Grinder cutting disc w/ blade hammers Low moderate semi-course 32

39 5.0 SCREENING EQUIPM ENT ASSESSMENT Screening and/or size classification systems are used in a broad range of industries as a means of arriving at specified particle size distributions for a variety of end use markets. One of the primary functions of the screening process is to remove material that meets specification before subjecting the total to further processing. Screening, therefore, minimizes the loss of material through the generation of fines, as well as optimizes the use of power for the particular process. The various screen types and typical applications are described below: 5.1 FLAT OSCILLATING (CIRCULAR MOTION) Free Suspended Screens These types of screens have been in wide use in the forest products industry for the past 20 years. The screen consists of a body that is hung by its four corners, either by wire cable and sockets, or by pipe and universal joints at both ends. The screen rotates in the horizontal plane, with an amplitude based on the throw of the eccentric drive shaft and the weight. The screen deck(s) are set at an angle, with anywhere from one to three or four decks, depending on the segregation of the raw materials required. The decks are generally mounted in the body of the screen on an angle in the direction of flow. This angle can vary from a few degrees to up about 15 degrees depending on the density and nature of the material being processed. For example, free flowing granular material Simplified side view of eccentric drive shaft. would flow much more easily than long stringy and lighter material, and hence could be effectively screened at a significantly lower angle. Free suspended screens have been used extensively in the forest products industry as a primary screening device. However, with the advent of chip thickness screening, many of these screens are now used as secondary screening devices. Typically, two or three size classifications are used, along with 33

40 fines discharge on the bottom tray. These screens come in a wide range of sizes; ranging from an approximately 25 square foot deck area to over 300 square feet Controlled Throw Screens Motion on controlled throw screens is much the same as the free suspended horizontal oscillating screen. With controlled throw screens, however, the screen body is point connected at the corners (or at a minimum of two corners) through a male/female type raceway that accurately controls the throw (circular motion) of the screen. Rotex, a very large manufacturer of this type of screen, uses a three suspension screen with the drive acting as one of the control points at the infeed end of the screen. One of the advantages claimed for this type of screen is that due to accurate control of the throw, the screen can provide higher dynamic action on the material and thereby improve the screening efficiency and the capacity for equivalently- sized screens. However, this type of screen generally requires a higher initial investment, and may be subject to higher maintenance requirements after extended use. 5.2 SHAKER AND VIBRATORY SCREENS/CONVEYORS Screen Applications These screens are characterized by a vertical and forward lifting action, with this motion being derived from eccentric shafts, mounted and driven in the horizontal plane. Alternatively, the driving action may be provided by an electric vibrator mounted at the appropriate angle. Drives can either be overhead, underslung, or sometimes separate units mounted on each side of the screen body. Typically these screens are mounted on coil springs or synthetic fiber leafs to provide resistance and spring-back to the driving force being applied. Shaker and vibratory screens have been more prominently used in the mining, metals, foundry, and construction industries than the forest products industry due to their much more aggressive action, and ability to act effectively as feeders as well as screens. However, these types of screens are now finding 34

41 use in the waste wood reclamation industry as a means of providing a very robust primary separation of highly contaminated virgin waste materials (tree stumps for example) Vibratory Screening/Conveying This technology is now many decades old, and is effectively used in most of the primary processing industries: forest products, pulp & paper, mining, metal products, coal production, foundries, construction materials, chemicals, power plants, sanitary processing systems, foods and general manufacturing, for example. The philosophy in employing this technology is to effect efficient classification while the raw material is being conveyed to the next step in the process, thereby minimizing additional handling, equipment, and costs. This type of system has been extensively used in the sawmill industry for waste wood chipper infeeds. Typically, waste wood collection conveyor systems in the lower floors of sawmills collect not only waste wood, but all the sawdust as well. This sawdust not only chokes the chipper, and adds unnecessary fines to the chips, but can cause accelerated wear on the knives and wear components in the chipper. A vibratory screener/conveyor, rather than a belt on the infeed to the chipper, discharges the sawdust before the chipper throat, thereby reducing the chipper loading and wear. Similarly, chips discharging from the bottom of the chipper may be collected on a vibratory screener/conveyor, whereby the fines are screened while the chips are conveyed to the takeaway belt to the storage bin or process. These systems have been fitted with fiberglass trough sections to facilitate the installation of metal detection devices to protect the process equipment from metal contamination. Again these systems come in a wide range of size/capacities, either in dynamically balanced construction for simple bolt-down, or unbalanced, requiring a mass foundation for the resistance loading Vibratory Taper-Slot & De-Stoning Screens 35

42 The action of these screens is the same vertical/forward action of those previously covered, however, additional features have been added to provide a wider range of material separation. These features include tapered slots, or finger screens that widen in the direction of material flow, thereby minimizing any plugging of the slots. This type of screen is generally used for heavy duty coarse, and widely variable raw materials, such as log yard waste, made up of rocks, gravel bark and wood chunks of various sizes. The first section of the de-stoner typically provides a coarse scalping and side shear to discharge log chunks ranging in size from less than a foot up to 10 or 12 feet long. Material continuing on the screen is then progressively screened by size, and also by weight, at the air-knife section once the material has been characterized by size. The air-knife section consists of an opening in the conveyor bottom with an adjustable air knife opening to blow the lighter materials downstream to continue on the conveyor. The heavier material, rocks, metals, and saturated wood/knots drop-out by gravity to a side discharge on the conveyor. These types of systems are available in a single or twin airknife configuration to provide for a wide range of classification and cleaning. It should be noted that this system, along with other screening systems, can only clean free contaminants from the parent material, or those contaminants that can be shaken loose in the vibratory action. In the forest products industry, wet materials are generally much more difficult to screen and clean than dry material. 5.3 DISC SCALPING SCREENS Disc scalping screens have been in use for approximately 50 years in some form or another. They consist of a series of parallel shafts, perpendicular to the material flow, with a series of offset interspacial discs mounted in overlapping rows on the shafts. Successive shafts, or groups of shafts, are generally run at higher speeds to provide a "pulling" or separation in the direction of material flow (material flows perpendicular to the shaft axis). 36

43 Depending on the desired results, the type, consistency, and contamination of the material being processed, the disc scalping screen can take on the necessary characteristics to meet the needs of a particular industry or materials to be processed. Shafts may be solid, tightly spaced, with small discs, or large pipe torque tubes with large discs and wide spacing to effect either very fine or alternatively coarse separation depending on requirements. Discs can also have a variety of contours on the perimeter, as well as different thicknesses, depending on the materials and the application. Adjacent discs can also be different diameters to facilitate a more turbulent action of the material on the discs. In the initial years of chip thickness screening and slicing in the pulp and paper industry, precision disc screens of various types were used as the primary screen to remove pulp chips greater than 6 to 7 mm thick (thick chips were then sliced to 7 mm nominally before rejoining the stream to the pulping process. Because of the tight tolerance of the shafts and disc spacing and high capacity continuous use, these screens experienced high maintenance due to the continuous scrubbing of wood between the discs. Although quite a number of these screens are still in use, the machinery suppliers to Disc Scalping Screen Infeed Rejects Accepts this industry have now developed new generation separation systems that are equally refined with regard to processing capabilities, but much lower in operational and maintenance costs. 37

44 Scalping-type screening systems lend themselves very well to the primary sorting requirements in the area of waste wood recovery as this is a relatively cost effective way to remove unwanted product size and contaminants, prior to primary processing. 5.4 TROMMEL SCREENS Dry Trommel Screens Trommel or revolving drum screens have been used for many years in many of the primary industries for cleaning and classification of a wide variety of materials. These screens can be supplied in a wide range of diameters and lengths, from 3' diameter by 8' long to 12' diameter by 60' long. As many of these types of screens are now mounted on mobile platforms (fifth wheel low beds), sizes are generally in the 6 to 8' diameter by 30 to 40' long. This type of screen can be fitted with a wide variety of screen opening sizes and types; either woven wire or punched plate. In the longer lengths, the screen can achieve size classification by using mesh or opening sizes that are graduated from small to large as the material moves toward the discharge end. When dividers or chutes are placed beneath each section of the trommel, wood particles passing through each section of the screen can be effectively segregated. This type of screen is now being utilized quite extensively for dry cleaning (removal of fines) as well as for classification of mulch products. Infeed Trommel Screen Rejects & contaminants Accepts One of the many advantages of the trommel screen is its simplicity; a drum supported on trunnions and driven at a variety of speeds. The screening action is also excellent because of the lifting and cascading action of the material. This feature is particularly useful when processing wet and hard to untangle 38

45 materials, as well as materials of very wide size range. Trommel screens are used almost exclusively in the Oriented Strand Board industry to separate fines from the long wood strands after the drying process. Other types of screens do not provide enough action to separate the fines from the surface of the wood strands Wet Trommel Screens Trommel screens, because of their action (lifting and cascading) make excellent washers to clean dirt contaminated materials and de-water them. The internal construction of the wet system is slightly different than the dry, in that the wet has an internal ribbon screw to control the forward flow of the wet material. The screen is also fitted with a water nozzle header to deluge the material as it cascades down the screen. The disadvantage of this type of system is that very significant water pickup (with wood fiber and bark) occurs in the process, and if the material is to be used in an application that requires dry material, this cleaning method may be cost prohibitive. 5.5 VAT TYPE WASHING SYSTEMS A number of vat-type wood debris washing systems have been developed in the past few years to address logyard debris clean-up problems. These systems are quite generic in that they consist of a water containment tub, an overhead means of agitating and moving the floating debris forward, and a grit collector conveyor that discharges the heavies and rocks; a chain conveyor extracts the cleaned material from the water bath. These systems can be very effective in removing rocks, as well as grit that is not imbedded in the surface of the wood. However, they are generally restricted to use in warmer weather due to freeze-up and conveyor icing problems in the colder northern climates. Dry systems are also generally preferred for mobile systems due to environmental considerations. 39

46 5.6 HYBRID CLEANERS/CLASSIFIERS/DEBARKERS In the past ten years a a significant amount of time and effort has been spent by those directly involved in the wood debris recovery industry and equipment suppliers to customize equipment to meet the demanding needs of this industry, and at the same time try to keep costs under control. A number of machines that have been developed, and are in use, draw from fairly old and well developed technology. These include heavy duty rotary drum systems that are a combination of drum debarker technology and heavy duty trommel screens; combination systems utilizing heavy duty disc screens followed by conventional drum debarkers; or fixed body debarkers with internal rotors. There is also another hybrid cleaner/classifier/debarker now finding its way into the market, the Deal Processor. This patented machine combines heavy duty scalping screen technology with the positive tumbling action found in drum debarkers. Its multishaft, disc rotors are configured with the rotor axis in the longitudinal direction, and the shafts increasing in elevation, in section across the machine in the direction of rotation of the rotors. Rotors also increase in speed in the direction of rotation and the higher elevation. Disc spacing is fixed at time of manufacture to Hybrid Screening Technologies: The Deal Processor TM Infeed Accepts Differential speed off-set rotor mounted discs Rock Drop-out effect the piece size classification desired. The operation of the Deal Processor is characterized by aggressive tumbling of the wood debris material providing quick separation of the waste materials that 40

47 fall between the overlapping discs. The woody material retained above the rotors is effectively cleaned and debarked as it tumbles, in drum operation fashion toward the discharge. Like a drum debarker, the discharge is also fitted with an adjustable gate to control the level of material in the machine and the dwell time to effect the level of cleaning and debarking desired. 5.7 AIR CLASSIFICATION/SEPARATION Air separation systems that use the density of the materials in a typical mixture have been used for many years. These systems have been used extensively in chip classification systems after sizing (screening for size distribution) to remove high density materials such as wet knots and rocks. These systems use a crude adjustable air nozzle system with an updraft air flow to lift the acceptable product and allow the "heavies" to drop out into a reject hopper. Air classification systems can be a very cost effective means of removing heavy contaminants from classified material in conjunction with pneumatic conveyance to the next stage in the process. 41

48 6.0 CHIP SAMPLING & PRODUCTION TESTS Following assembly of initial data on the types of size reduction equipment available, several samples of processed fiber were gathered. Production tests were designed and executed with the assistance of Northwest wood processors to assess individual operating parameters. The overriding objective of this research has been to identify those control parameters with the greater influence on the ability to high grade currently low value wood waste materials. This section describes the sampling procedures, overall testing objectives, and the details of each the six individual tests conducted. The methodology employed for each test is included in the appropriate test description. 6.1 CHIP SAMPLING PROCEDURES During the course of this study, chip samples were taken from several Northwest wood processing facilities, each representing different variations in primary reduction equipment used. With dissimilar facility access and configuration in each case, adaptive sampling procedures were necessary for the purpose of obtaining a representative sample from each location. The following basic sampling strategies were used at each facility. Sampling of processed wood waste chips was achieved directly from storage locations such as stockpiles, or from conveying units such as a conveyor belts. In each case, a set of sub-samples was collected (typically 5 gallon buckets) to form a composite sample Sub-sample Collection 1) Sampling from conveying units: During chip processing, material samples were taken from conveyer units at ten minute intervals to ensure that samples were representative of the range of incoming feedstock. The conveying unit was not stopped to allow sample collection. 2) Sampling from storage piles: Samples were collected at three to five locations from the chip piles in the storage areas, including the top third, mid third, and bottom third of the stored material. Material was also retrieved at different depths within the pile. 42

49 6.1.2 Compositing The collected sub-samples were combined into a composite sample. The composite sample was then sub-divided into smaller sample sizes using a quartering procedure. Quartering was conducted on a clean surface so that foreign material was not introduced. 6.2 SAMPLE ASSESSMENT Sample # 1: Fixed-Hammer Hog Nominal Size: 3-inch minus Produced Geometry: Coarse & fibrous Comments: Large distribution of small splinters 43

50 Sample # 2: Swing Hammer Hog Nominal Size: 3 ½ -inch minus Produced Geometry: Coarse & fibrous Comments: Large distribution of small splinters Sample # 3: Mass Rotor Hog Nominal Size: 3-inch minus Produced Geometry: Coarse Comments: less lengthy chunks than other hogs 44

51 Sample # 4: Pan & Disc Nominal Size: 4 ½ -inch minus Produced Geometry: Semi-coarse Comments: long particles, cleaner edges than hogged 45

52 Sample # 5: Disc Rechipper Nominal Size: 2-inch minus Produced Geometry: Two-sided Comments: overly fractured The samples shown above, along with several other samples collected, illustrate the range of physical characteristics that processed urban wood chips may have, depending on the reduction process used. Note the similarity of appearance between the various hogged fiber samples number 1, 2 and 3. The fiber particles seen in these samples can be broadly characterized as coarse and fibrous in nature. The coarse characteristic refers to the irregular surface geometry produced by the blunt force reduction action of the hogging process. The fibrous characteristic refers to the way the particles have separated with the longitudinal grain of the wood and the frayed ends of each particle. Sample number 4 shows particles produced in a hybrid reduction process termed pan & disc by the manufacturer. These particles are generally coarse and fibrous like samples 1-3, but show fewer frayed ends and cleaner edges of each particle. This is judged to be a result of the sharper tooling used in the reduction mechanism, creating more cutting or slicing action, rather than the ripping action of more blunt tooling (hammers). 46