MRFs in the Age of Green Energy
The era of green energy from municipal solid waste and its associated fuel sources is just beginning. Like landfills and waste-to-energy plants before, these facilities will require an assured flow of tonnage for consistent operations and profitability.
Unlike landfills and existing waste-to-energy plants, the newer forms of waste conversion facilities can be more sensitive to the makeup of the feedstock, both in composition and physical characteristics. How does this affect the material recovery facility (MRF) of tomorrow? Certain materials will have to be included or removed, sized properly (e.g., screened or shredded), dried, or densified per the requirements of the selected green energy technology. Other forms of green energy are also viable (e.g., solar heat and electric, wind, or wave) and can be used to supplement biofuels from waste. None, however, can be as easily operated for baseload power like biofuel facilities.
The future MRF will be required to function both as a materials recovery facility and as a fuels-processing facility, similar to previous refuse-derived fuel (RDF) plants. (Note that most past RDF plants focused primarily on fuel preparation at the expense of recycling.) Ideally, these functions could be activated on the fly, either with a flip of a switch or with operational changes, to seamlessly convert back and forth from recovered materials to renewable fuels/energy, based on the best revenue stream on a given day (or given hour, with electric rates).
In the far distant future, as green energy technologies evolve (or as someone invents a small, safe, fusion reactor) the MRF as we know it could actually become extinct. A utopian dream right now, but wouldn’t it be nice to have single-bin curbside collection of all MSW (including greenwaste, household hazardous waste, pharmaceuticals, and electronic waste), where you could adjust the system as desired for heat, electricity, syngas, propane, methane, ethanol, or biodiesel? Potentially any number of carbon-hydrogen-based molecules, or even pure metals and other elements off the periodic table, could be produced, given the right technology. (OK, maybe in 100 years or so for that long-awaited journey “back to the future!”)
Figure 1 shows the relation between today’s MRF and proposed fuel-preparation and green-energy technologies of the future. The boxes denote process trains and do not have to be separate facilities. In fact, collocating these facilities, either on the same site or even within one building, has advantages.
Future MRF operators will have to be satisfied with handling even more materials than they do now, and in more flexible ways. The coming push for green energy fuel sources will most certainly make the MRF designer’s life more interesting.
A Brief History in (MRF) Time
The basic MRF, along with the requisite collection system, has been rapidly changing over the last 20 to 30 years. It has evolved from the multi-bin/truckside sort method to dual-stream (termed commingled), to a brief look at wet/dry European systems, blue bag co-collection with MSW, to the current single-stream approach. Although it is considered a modern incarnation of a MRF, single-stream processing attempts were made more than 20 years ago. Unfortunately, it faced higher equipment costs for the paper/container auto-sort machines, contaminated materials (still a potential problem), and industry reluctance to completely convert collection methods (i.e., most places still had large investments in multibin or dual-stream dedicated recycling trucks).
Times change, and so do processing schemes and equipment. Single-stream MRFs are the pre-imminent technology of today. They can range from low technology with mostly manual sorts to extremely high technology with almost no human hands touching the material. Advances in paper screening from containers, acceptance of eddy current magnets, and the real stars of the process line—optical and near-infrared sorting units—are helping. Modern single-stream MRFs automatically sort by paper grade or container type/color/resin, with quality and speeds far greater than human sorters can achieve. Although these advanced sort units can be expensive, especially if used in tandem for multiple sorts, they are still cheaper per unit than the initial equipment used 20 years ago. Decades ago, a single paper-container screen could cost upwards of $400,000 to 500,000. Today, a modern disk-based paper/container screen can cost half as much or less—a real bargain. Figure 2 shows a state-of-the-art implementation of multiple automated sorting units in a single-stream MRF.
Throughout the last several decades, the dirty MRF (sorting through pure MSW) was slinking in the background. It is still utilized in several places, sometimes as secondary sorting in a “blue bag” MRF. Getting dirty is attractive in that no MSW collection changes needed to be made, but, on the processing side, things get a little rough. Dirty MRFs require a lot of equipment and sorters and result in a lot of contaminated materials and oversized conveyor systems to handle the volume, as well as attention to a host of other issues. The diversion rates for a dirty MRF can be impressive compared to clean MRFs (a clean MRF system is essentially one that handles recyclables only, no MSW). But the dirty MRF requires good equipment, skilled operators, and extensive maintenance programs, not to mention an understanding of the end markets for the lower-quality sorted materials. A saving grace for the dirty MRF is that the technology will most likely serve well in prepping material for several of the green-energy technologies.
A driving factor in MRF advancements was the realization that collection costs of any MSW/recyclable handling system are far greater than the processing or disposal costs (potentially by a factor of 3 or more). So the question quickly became “How do I minimize collection costs and maximize recovered tonnage?” instead of “How I minimize processing costs or maximize material quality?” If the latter question held sway, we never would have moved from multibin/truckside sorting, which has enormous associated collection costs. The evolving renewable fuel preparation facility may take this collection efficiency to the extreme by allowing other current separate collections to be combined, such as yardwaste and C&D.
Some areas are now looking at separate foodwaste collection, which is a seemingly backward step toward more bins in an already multibin collection system. While it might look environmentally friendly to remove readily compostable or fermentable materials from the MSW stream in the new green-energy era, we now have potentially more collection vehicles on the road. More vehicles increase overall collection costs, traffic congestion, and greenhouse gases (GHGs). To verify these assumptions, each proposed green-energy project should perform full life cycle analyses beginning with collection and not just the processing lines.
As an alternate to separate collection of various materials, why not throw it all in one bin and let the renewable fuel and green energy facilities take care of it? In this case, even the old wet/dry collection approach may make better sense by using a “thermal” process on the dry fractions and an anaerobic process on the wet fraction (i.e., two green-energy technologies in parallel).
It is easy to visualize such innovative processing systems with emerging green energy technologies in mind. However, since these facilities aren’t yet commercially proven in the US market, the separate foodwaste collections and similar projects still make sense, at least in the near term. This will immediately help limit the amount of GHG production that escapes to the atmosphere (i.e., get it out of the landfill before methane is formed) or to provide clean material streams for composting, codigestion, or existing green-energy facilities (e.g., hog fuel systems or paper/plastic-based RDF co-fired with coal).
Green Energy Conversion Technologies
This paper does not contain a technical treatise of all the existing and planned permutations of these technologies, for they are too numerous. Most of these technologies are still unproven at anything more than bench-top scale, so the actual feedstock and processing intricacies are still to be ferreted out. We will be looking at the general categories of the emerging green-energy technologies and their requirements for feedstock, with the resulting impact on the sorting and fuel-preparation facilities.
The most common existing green-energy conversion technologies are advanced combustion (e.g., mass burn or RDF), size reduction, and anaerobic digestion. Emerging technologies include thermal gasification and pyrolysis, biofuel conversion (e.g., thermochemical or biochemical), chemical, and/or combinations of the above. Many of the emerging green-energy technologies have long histories and have been tried in the past, but advancements and/or volatility of fuel prices have made them attractive again. State and federal legislation initiatives have also created significant revenue incentives for many green-energy processes. The actual resources that can be recovered from waste for production of green energy include MSW, RDF (processed MSW or source-separated high-Btu material), biomass (cellulosic materials and yardwaste, energy crops, or agricultural residues), organics (foodwaste as well as fats, oils, and grease, or FOG), and wastewater plant biosolids.
Figure 3 represents today’s relative risk (or maturity) in the technologies employed for converting the feedstock into green fuels/energy. Once these emerging technologies become commercially viable and accepted, they will start competing for the various waste streams and require the requisite changes in collection systems and/or fuel processing techniques.
Another issue with emerging green technologies is that within each major category or type of facility, specifically the thermal processes, there may be multiple subsets of processing requirements. Some thermal processes may be plasma-arc-based, whereas others use heat produced within the process, sometimes augmented with other fuels. Each of these processes will have specific requirements on how the fuel needs to be processed, typically maximum size requirements, composition, and moisture content. All of this needs to be carefully assessed so that the entire waste system, from collection through processing and conversion, can be designed properly. Some systems may have quick residence times in the “reaction” area, hence requiring smaller fuel-particle size for complete reaction or conversion. Others may have longer reaction times and/or employ higher temperatures and can process larger particle sizes. This information about the specific process is important for the fuel processing facility in order to design the equipment system to produce the proper renewable fuel.
Similar arguments about sizing can be made concerning the waste fuel composition for each type of conversion facility, even down to the subset of each vendor’s process. Some processes can take unprocessed MSW, whereas others may want plastics but not biomass, and still others may want biomass but not plastics. In addition to all those issues to worry about, the moisture content of the feedstock can also cause problems or inefficiencies with certain conversion technologies, although “too dry” in an anaerobic process is much easier to correct than being too wet in others.
As described and noted below, there are many potential pitfalls in producing the right feedstock for a specific green-energy fuel-conversion process. The work for any fuel-processing facility designer or planner is certainly “in the details.” As with most solid waste facilities, a cookie-cutter approach does not always work well, and each situation needs to be examined individually. Table 1 lists some of the feedstock requirements for each of the general green energy fuel technologies.
Why bother with green-energy technologies? Current waste-to-energy (WTE) facilities only capture about 13% of the total MSW wastestream in the United States. That means there is an immense untapped resource that can be converted to energy of some form (e.g., liquid fuel, heat, electricity, or gas). While some of this is currently being converted into methane in landfills, the production and capture of the methane has limited efficiency as much of the gas is still lost to atmosphere or flared with no beneficial use.
In electric terms alone, from only 13% of the total waste being captured for electricity, potentially more than 16,000 MW of power is being wasted by not using an advanced green-energy technology or existing WTE methods. Some of this energy could even be used in the form of syngas, biomethane, or steam to power existing or mothballed fossil-fuel power plant turbines. This type of energy use would further reduce dependence on coal and oil, also reducing air pollution, as compared with fossil fuels. Figure 4 represents some relative pollution amounts from existing WTE technology versus fossil fuels. The advanced green-energy conversion technologies should result in similar or better “low or no” pollution numbers, once proven at commercial scales.
Almost all solid waste master plans, and even projects currently in the pipeline for construction, include some form of “black box” for future technology. The challenge today is to design a proposed facility with some form of coherent integration with these future green-energy technologies. There are several ways to provide for this flexibility in both designing a specific site and the process line:
- Ignore the problems initially and assume the future project will be independent. This is the easiest and all-too-often path followed.
- Box out an area on the site plan (usually in the most awkward space on the property that no one wants for their current operations).
- Actually design the site and building(s) with the thought of future green-fuel processing in mind.
Naturally, option 3 is the best approach to be well positioned for the green-energy era. Without knowing which green-energy technology is most appropriate for the local markets, how do you even begin to design the required fuel processing flexibility into the MRF? Most of the existing technologies will require preparing the feedstock to a specification smaller than the average container size, something an average MRF does not do. With that requirement, some kind of shredding will be needed at the fuel processing facility, or at least planned for in the design. The shredding line could be a complete secondary process line, or it could be integrated into the various sorted streams of the current MRF. Ideally, the layout would provide for both, shredding directly from the tipping floor or handling a designed waste-to-fuel stream from the sorted MRF materials.
Once some of the fuel feedstock requirements are defined, the sorting/shredding function can be designed into the MRF and/or fuel processing facility. Again, one must keep in mind that no single system will be perfect for all scenarios. However, a well-designed sorting/processing system should function under most conditions, if it has enough flexibility in feed systems, and various options to recycle or divert to fuel the various sorted material streams.
Biomass and other organics are typically collected in a source-separated manner, but they don’t have to be. Dirty MRF screening and sorting technology can do an adequate job of separating these resources from the MSW feedstock if required by the selected green-energy process. In the case of wastewater biosolids and FOG, it is best to keep them separate until final stages of the fuel blending, or better, hauled directly to the green-energy facility if the process allows.
Raw MSW typically does not require sorting other than the possible upfront removal of such inerts as glass, ceramics, and metals. Several of the gasification processes, which also employ vitrification, do not require upfront sorting of these materials, as they may be vitrified into an inert slag and metal pellets that can be separated and marketed. Current advanced mass-burn combustion facilities also do not require presorting, as the process can liberate many of the metals and allow for removal from the residual ash in a post-combustion process.
RDF is basically MSW processed through a dirty-MRF-type system to minimize inerts and wet organics, sometimes with high-value recyclables “cherry-picked” out of the stream.
Once all the various feed streams are sorted, the primary means of producing process-ready renewable fuel is that of shredding (see Table 1). Shredding MSW and other organic feedstock can be a complex subject in itself and worthy of a separate article. Suffice it to say that shredding is a noisy, dusty, high-maintenance, costly operation. And with a potential for explosions, it can be dangerous. Proper selection of shredder capacity and type is critical. Naturally, shredding costs rise rapidly as throughput increases or shred size decreases (i.e., sizing at minus-one-half inch costs more than a rough 24-inch chop). Figure 5 represents a simplistic view of the throughput-versus-size issue (and each type or make of shredder will have its own unique curve). The trick is to select a shredding system with the throughput/size crossover point at the selected technology’s requirements and at a reasonable cost point.
Since some shredder types are better at rough chops versus fine sizing, and vice versa, higher throughput shredding lines normally utilize two to three shredders in tandem. MSW shredder lines can easily run into millions of dollars for even modest 5- to 15-tons-per-hour (tph) systems. With current single-stream processing lines topping 20–30 tph, and if portions of recyclables are diverted to fuel production along with raw MSW, several shredding lines will be required in even smaller-size green-fuel processing layouts.
After shredding and possibly mixing with source-separated biomass, the resulting fuel may require pelletizing and/or drying for storage prior to use in the green-energy process as a renewable fuel. As it’s best to keep the green-energy fuel processes running smoothly with a constant feed of materials, the produced fuel will need to be stored for possibly extended lengths of time. There are two factors to keep in mind when designing the storage area: moisture control and fire prevention. If the drying step mentioned above is not done properly, the material could begin to decompose, which can cause odors or even spontaneous combustion. Fire prevention is therefore a primary concern for this area, since you are basically storing fuel indoors.
Progressive Site Planning
With the site, building, and equipment laid out appropriately for the selected green-energy process requirements, the hardest decision will be whether to build the full material recovery and/or fuel processing facilities now, or leave a building expansion for later. This decision to build big or small naturally depends on the timeframe for implementing a green-energy facility and the renewable-fuel processing line. Recently, several projects have been rushing to build facilities larger than currently needed due to attractive construction pricing.
With the actual green-energy-processing site planned, the surrounding land use and potential energy users need to be under contract. This step should actually be developed parallel with, or even before, the actual fuel production facilities. Past successful WTE projects primarily sold electricity to local utilities or steam/biogas to industrial users within close proximity. Recent experience in Japan and Europe with syngas being sold to adjacent industrial facilities shows that projects can overcome the restriction on retail sale of electricity from nonregulated utilities. Syngas, steam, and hot-water users also used to have to be located fairly close to the production facility in order for there to be direct pipeline feeds (within several miles usually). However, recent spikes in fuel costs and efficiencies in raw gas treatment and handling facilities have increased the economical distance for a pipeline to much longer distances.
Another fairly common approach, but not widespread, is to actually collocate renewable energy users on the same property as the energy producer. Recently this form of site planning has been termed the eco-park or eco-campusapproach. Typically, other municipal facilities, such as water or wastewater treatment plants have been powered under such arrangements (see Figure 6 for a successful example).
A recent development of the green-energy movement has seen even nonindustrial corporations becoming interested in collocating offices in an eco-park setup in order to directly use green-produced electricity. Other users desiring collocation include university research departments (especially those in the environmental fields) and hence the term eco-campus.
The collocation arrangements can work out nicely for the green-energy producer as they can sometimes use the energy or electricity directly to the benefit of the energy host without having to connect to a power grid and sell their power at a discounted rate. In this scenario, the entity generating the green power will capture the full retail value of the energy, or agree to share the higher revenue stream with the participating energy host. As a word of caution, disconnecting from the bulk electric system will require the addition of a reliable backup power system to ensure continuous operation during planned outages or other downtimes.
There are many combinations of facilities that eco-parks can include and service, and as always, each project needs to be evaluated on its own merits. Figure 7 illustrates just a few ways several municipal utility facilities can coexist and produce valuable products and green energy with significant benefits to the host community.
One additional market for green-energy facilities could be at existing WTE facilities. Although on the outside they may appear to competing for the same waste, many facilities are at or over capacity, and portions of the local MSW tonnage now goes to landfills. Employing upfront sorting and green-fuel/energy production at these facilities would make sense for efficient waste deliveries and provide for a green renewable energy solution that, in all likelihood, will be less costly than a normal combustion facility expansion or building one from scratch. Figure 8 represents the potential core processing facilities for any future MRF complex.
How are all these green-energy facilities paid for? It is nice to be environmentally sound, but projects in today’s economy need to be financially viable as well. The initial push has been a credit trading system with the federal Renewable Fuel Standard, which are counted in RINs (one RIN = 1 gallon of fuel), and the state initiatives for Renewable Energy Credits, or RECs (one REC = 1 MWh of electric power). RINs pertain to liquid fuels and RECs to electricity generated from renewable fuel sources. Syngas and steam have to compete solely on their inherent fuel value, as no renewable credit system is currently in place.
The key issue to keep in mind with RECs and RINs is that these are additional revenue sources over and above the energy sale itself. Pending federal legislation may soon result in a national Renewable Portfolio Standard (RPS) that could result in significant opportunities for improved project revenues. The bottom line: Now is the time to start planning for the renewable era.
From an economics standpoint, the revenue from the high-value fuel or electricity, plus any REC and/or RIN credits, has to offset the costs of producing the fuel and the green-energy facility itself. With the complicated economics involved, it usually is best to perform life cycle analyses of any proposed solid waste management system, from collection through disposal and product sales.
Conclusion (or the Beginning?)
Both MRF processing and green-energy production have come a long way from the days of simple landfilling of MSW and WTE projects. While the latter two will continue to be mainstays of many solid waste management systems, the modern MRF has also gained prominence and equal partner status with the proven methods of the past. Most state and federal legislation also requires that a host community must employ a robust recycling program in order for MSW to qualify as a renewable fuel. Green energy from MSW will also gain equal footing in the waste management hierarchy as technologies mature and are proven at commercial scale. The humble MRF and its associated equipment have progressed to the point where efficient and flexible preprocessing of fuel can be a reality. Some careful forethought and proper equipment selection is all you really need to help you “sort” through all the possibilities.
Author's Bio: Russ Filtz, P.E. is a discipline leader within CDM, specializing in recycling, waste transfer, and alternative technologies.
Author's Bio: Paul Hauck, P.E., is senior engineer with CDM Smith in Tampa, FL.