Microwave Process for Asphalt Pavement Recycling by xeg10270

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      Published by EPRI     Program
                     Industrial                                - Environment & Energy
                                                                                   Management                                                     No. 2, 1992



      The Municipal                       The Background
                                                                                                         cause air emissions that exceed regulatory standards. To
                                                                                                         prevent coking, traditional RAP recycling methods use
                                                                                                         preheated virgin aggregate (over 300°F) to indirectly heat
                                                                                                         a mixture of RAP and fresh asphalt cement. Because the
                                          Los Angeles, California, has the largest municipal street      virgin aggregate is mixed in, RAP is usually only 10 to
. .   Save Money,                         paving operation in the world. Every year, its Bureau of
                                          Street Maintenance repaves and repairs approximately
                                                                                                         20% of the HMA composition and rarely more than 50%.


      Landfill Space, ’                   150miles of deteriorated roadways with 800,000 tons of
                                          hot mixed asphalt (HMA). The Bureau employs 4 full-
                                                                                                         Los Angeles has been manufacturing HMA with a 15%
                                                                                                         RAP composition for years. In 1987, thecity began
      md Natural                          time milling crews that remove up to250,000 tons of old
                                          asphalt yearly during. theirroadmaintenance          and
                                                                                                         seeking ways to recycle more RAP due toan impending
                                                                                                         landfill shortage. During that year, the city leamed of a
      Resources                           rehabilitation work. The removed material is called
                                          reclaimed asphalt pavement or RAP.
                                                                                                         process that uses direct microwave heating to produce
                                                                                                         high-quality HMA using little, if any, virgin aggregate.
                                                                                                         This process was developed by CYCLEAN, Inc. of Austin,
                                                                                                         Texas.
                                            TREATED EXHAUST                                  Q
                                                                                                         The Old Way
                                                                                         STORAGE
                                                                                            SILO         Prior to 1987,Los Angeles repaved its streets with HMA
                                                                                                         produced by two city-owned plants and outside sources.
                                                                                                         The city plants each used a mixture of about 85% virgin
                                                                                                         material and 15% RAP. When operatingat their combined
                                                                                                         maximum production rate of 600,000 tons per year, the
                                                                                                         plants consumed about 90,000 tons of RAP annually. The
                                                                                                         remaining RAP (at least 160,000tons) was disposed in a
                                                                                                         landfill. The HMA supplied by outside sources was
                                                                                                         produced from virgin ingredients.

                                                                                                         The New Way
                                                                                                         In 1987, the Bureau decided to increase the amount of RAP
      TheCI%LEANprocessproduces           Constructioncompaniesandroadcrewshavebeenreusing                recycles by using CYCLEANs process. Under a
                                                                                                         it
      hot mired u p h i t fHMAj from                                         ..
                                          RAPforover 15 years. RAPis approximately95%stone                                                               a
                                                                                                         contractual agreement, the city ships RAP to CYCLEAN
      reclaimed asphalt pavement          aggregate and 5% liquid    binder (called“aspha1tcement”).     asphalt reclamation plant installed at a city-owned facility
      (RAP). Majorsystemcomponents
                                          The aggregate does not decay and is a substitute for           in the northeast San Fernando Valley. The plant uses the
      include. (1) feedhoppers
      supplying RAP and      virgin       “fresh” stone. Although the cement      becomes brittle over   city’s RAP to produce HMA meeting pavement mix
      aggregate to the process, (21 a     time, it is reusable ifrestored with oil-based rejuvenating    specifications as required by the contract. This plant
      vibrating screen tosizefeed         agents.                                                        supplements the two city-owned plants that continue to
      material, {3)a rotary kiln heater                                                                  produce HMA using 15% RAP.
      thatpreheatsfeedmateriais, (4j a    HMA can be made withRAP as long as the R A P S asphalt
      mixerin which a proprietary         cement is notburned or “coked” during the heating              The CYCLEAN plant converts RAP directly into HMA at
      rejuvenating agent is added, {S)    process. Once coked, RAP is unusable because its
      enclosed microwave generators                                                                      production rates of up to 135 tons per hour. RAP entering
      that heat the mixture, and (6) a    cement cannot be rejuvenated. Coking also creates dust         the plant is combined with new aggregate (if required) and
      heated product storage silo.        that inhibits the cement’s binding properties and can          crushed and screened to 1.5 inches in diameter or less to
        .       assure uniform absorption of heat. The feed materials             Lowerproductionandroadmaintenance                costs.        City of Los Angeles employees
        '   .   are then routed to a rotary kiln heater in which they are         Substituting RAP for new aggregate and asphalt cement          pave city roads wirh HMA from
                blended and heated to over 220°F to remove moisture.              reduces material costs by over 30% and eliminates costs        rhe CYCLEANprocess.
                Resulting exhaust air is passed through a baghouse for            associated with RAP disposal. Consequently, roads are
                                              can
                particulate removal, and also be heated by a gas-fired            kept in good condition for less money.
                oxidizer to eliminate white plumes caused by water
                vapor exhaust. The preheated material is then conveyed            Ready supply.        High-quality aggregate is oken
                to a mixer where an oil-based rejuvenating agent is                                            to
                                                                                  unavailable locally and has be shipped agreatdistance.
                added torestore asphalt cement viscosity and flexibility.         Because RAP is generated as roads are prepared for
                This mixture is conveyed through an enclosed tunnel,              repaving, it is available at or near its point of use.
                where microwave generators heat the mixture to about
                300°F. Because the aggregate is polar and the asphalt             Minimalair emissions. Exhaust stack tests on the
                cement isnon-polar, only theaggregate isdirectly heated           CYCLEAN plant consistently show that emissions of
                by the microwave energy. After being heated, the                  nitrogen oxides, carbon monoxide, and volatile organic
                aggregate slowly transfers heat to the asphalt cement             compounds are below the stringent standards issued by
                without burning it. The resulting HMA is conveyed to              the South Coast Air Quality Management District.
                heated storage silos for city pickup and use.
                                                                                  Conserved resources. Each ton of RAP used to make
.   .           The Results                                                       HMA saves about 1,900 pounds of aggregate and 12.5
                                                                                  gallons of petroleum-based asphalt cement. Moreover,
                Los Angeles has foundacost-effective, environmentally             each ton of RAP saves about 14 cubic feet of landfill          Company
                responsible method to reduce its waste stream while               space, thus reducing the need for landfill space.              Profile
                simultaneously producing a high-quality product for its
                                                                                                                                                 Focnded In 1983, CYCLEAN
                repaving operations. From 1987 to 1992, the Bureau                The Production Costs                                           Inc   I S an   independent
                                           of
                divertedover660,000 tons RAP from being landfilled.                                                                              wmrthon wholly owned by its
                In addition, the Bureau's foresight enabled it to achieve         CYCLEAN owns and operates the asphalt reclamation                           and
                                                                                                                                                 mmnr~enen~ investors. Its
                early compliance with a city order prohibiting the                plant. In return, the city furnishes a loader and two          nmu:Jctunng facilities and
                                                                                                                                                 ccr.:rd offices are located in
                landfilling of road materials after August 11, 1989.              operating personnel, pays for the rejuvenating agent. and      .Aumc. T   ew
                                                                                  guarantees a minimum delivery of 120,000 tons of RAP
                The city currently is considering a transportable, 350            per year. The plant currently consumes over 200,000            CI'CL&AX currentlyoperates
                ton-per-hour plant. This CYCLEAN plant would enable               tons of city RAP. The total cost for HMA produced from         &spa!: rccycllng plants for the
                                                                                                                                                 T c x ~ \S u l c Department of
                RAP to be recycled closer to the job site.                        the CYCLEAN plant is about $17.50 per ton, including          T:~n\xm:~on.      GeorgiaState
                                                                                  labor, raw materials, and transportation. The city pays       D r w m c n : of Transportation,
                The Advantages                                                    about $21 per ton for HMA supplied by outside vendors.        a : t.?c C;I! of 10sAngeles.
                                                                                                                                                 -

                Excellent performance. Laboratory and field tests                 The Bottom Line                                             D3.jj~r~4~11:fKBureauofStr~t
                                                                                                                                              ~!%%c-L-x        l o r the City of Los
                show that the performance of RAP-based HMA is at                                                                              4n:c.c;    F ' d E Lorentzen of the
                least equal to that of HMA made from virgin materials.            TheCityofLosAngelesoperatesaninnovative,city-widr           k u - 7 ~ : : of Unrrer and Power
                Moreover, tests by the University of Washington indicate          asphalt recycling program that saves money and natural      f n ~ r it>' 1 4ngeles.Michael
                                                                                                                                                       ::,     -
                that the bond of asphaltcement to aggregateis                     resources. The city's increased reuse of RAP now saves      R lL?,u !'<~!K!K      AsphaltRecycling
                                                                                                                                                                               and
                                                                                                                                              d R ~ : . J , - : T F Atsociation.
                strengthened by microwave heating. As a result, asphalt           over $1.5 million per year in raw material and disposal     R -x- i! \co!CYCLEAN,Inc.
                mixtures heated by microwaves may have improved                   costs.                                                      m d c ~ J . ~ J ? ,.or:nbutionstothis
                                                                                                                                                                   C

                adhesion and water resistance properties.                                                                                     IS\bC




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