Solid waste disposal has been an issue since the dawn of humanity. In the earliest times, land filling, or, more simply, land application, was the most likely scenario for disposal, as anything unusable or nonessential for immediate survival was discarded and left by the trail as the hunter-gatherers moved to follow life-sustaining herd migrations. As humans became more "civilized" and established permanent residences and villages and obtained an existence through means of agriculture, trade, etc., the accumulation of wastes increased due to the localized populations and increased permanence of those societies. Waste disposal practices included both landfill and combustion, as anything of fuel value was most likely burned for heat or cooking purposes. In all likelihood, landfill accounted for a minimal fraction of the waste in those early times, until the advent of the industrial revolution opened the way for greater leisure time and the economy to create and consume greater quantities of nonessentials, which, in turn, created a greater flow of disposable waste from the thriving communities.
Solid waste disposal has always consisted of two methods, burning or discarding. Requirements of communal living conditions and a greater understanding of the health and sanitary implications of haphazard waste disposal created the need to concentrate solid wastes into a landfill and bury the material. Convenience and availability of seemingly unlimited space favored land filling as the universal means of waste disposal until midway through the twentieth century. As populations increased, however, capacities in existing landfills were rapidly used up and sites for new "garbage" dumps were being pushed farther and farther from the population centers, leading to increased hauling and operating costs for the newer facilities. Since the early 1990s, a new environmental regulations continue to increase both the complexity and the costs for new landfill facilities in the United States and the other more developed nations worldwide.
Where any pressures arose concerning the siting or operating costs of land filling, consideration has been immediate for the option of combustion of the same waste stream. The earliest systems were designed to incinerate the incoming waste. With no energy recovery capabilities and only basic wet scrubber gas cleanup technology, this practice reduced the quantity of waste ultimately routed to the landfill by seventy-five to eighty-five percent. These early incinerators were typically small, only fifty to one-hundred tons per day capacity, mass burn, starved air, multistaged incinerators. During operation, the waste would be introduced into the furnace onto the first of a series of ram-activated cascading grates. In this primary zone, the fresh fuel contacted some combustion air and a stream of recycled gases from the exhaust header. As the waste began to volatilize and burn, the ram feeders on the grate pushed the pile out on the grate where it fell to the next step. In the process, the material was stirred and agitated to present a fresh combustion surface to the air stream. Likewise, the material from this second step was ram-fed inward until it "cascaded" onto the next grate. This process continued until the remaining material, primarily unburnable material plus some uncombusted carbon, fell into the water-cooled ash trough and was removed from the furnace in an ash conveyor. The combustion and volatile gases driven off from the fuel pile were mixed with combustion air in the primary chamber directly above the grate. Further combustion and quench occured in the secondary and tertiary chambers where additional ambient air and recycled flue gases were mixed into the combustion exhaust stream. All of the gases from the tertiary chamber were conveyed into the wet scrubber where particulate was removed.
In the early 1970s, the surge in energy costs, spurred by the oil embargo, created a demand for energy recovery in these waste disposal facilities. At that time, while still in its infancy stage, waste-to-energy was poised to take one of two paths to implementation—mass burn (MB) or process fuel, typically called refuse derived fuel (RDF). Like the predecessors, mass-burn technology opted to burn the waste virtually as it is received, eliminating preprocessing of the material prior to burning. The mass-burn units are typically very large furnaces, most often field erected, wherein the waste is literally plucked from a receiving floor with a grappling hook and ramfed into the furnace onto a cascading grate design. Following the success of other European designs beginning in the 1950s, these systems were typically of the waterwall furnace design, as opposed to a more "conventional" refractory furnace. One advantage of this was the direct removal of much of the furnace heat by the waterwalls, minimizing the need for significant volumes of excess combustion air or recycled flue gas to maintain acceptable operating temperatures in the furnace. These plants have also evolved into much larger capacity facilities. They were already field erected, so up-sizing the capacity did not represent any further economic disadvantage, and have been built to capacities as great as 2,500 tons per day.
Because the mass burn systems are designed to handle the fuel as it is received, implying the obvious variations in quality, sizing, and handling therein, the combustion systems must be extremely robust and conservative in design with regard to the quality of fuel being introduced. In addition, the ash removal system must be capable of handling the size and capacity of material coming out the furnace as is fed into the furnace on the front end. While this universal acceptability has proven to be one of the most important features of the mass burn success, it has also created two major liabilities in the performance of the furnace. First, extreme variations within the fuel pile on the grate can creat significant temperature variations across the grate which, in turn, can cause high temperature damage to the equipment or generate greater emission from products of incomplete combustion. Second, unpredictable fuel composition can create surges in ash quantities, resulting in slagging and fouling of the boiler surfaces and increased emission levels of unburned hydrocarbons, acid gases, and dioxins/furans in the outlet gases. In spite of these drawbacks, the mass burn technology has captured a majority of the waste-to-energy market in the United States in the last three decades of the twentieth century. Part of that success came as a direct result of the early failures of RDF in the industry.
Simplified in process terms, RDF involves processing the incoming municipal solid waste (MSW) stream to remove a substantial portion of the non-combustible components, namely aluminum, ferrous, glass and dirt. Various sources list these components in the range as follows:
A review of this composition would indicate the noncombustibles in the raw MSW range from twenty to forty percent. By removing as much of these fractions as possible from the fuel stream, the quality of fuel presented to the combustor is improved as well as the contaminants from the combustor being reduced.
Unfortunately for the RDF industry, the first attempts at implementing an RDF processing system met with disappointment and failure. With no European technology to draw from, RDF processing evolved from experience and inspiration gained from the U.S. applications. In the earliest processes, the design called for all of the incoming waste to be shredded as it entered the process. Conceptually, this was a good idea, providing a means to get all of the material sized to a maximum particle size, thereby enabling further separation based upon particle sizing and density; however, the actual results were less favorable. First, shredding all material as it was introduced into the process included numerous explosive items such as propane bottles, gas cans, etc. The damage caused to the equipment in these early processes was significant but the loss of life and limb from some of the early accidents was even more devastating. Aside from the catastrophic failures due to explosions, this initial shredding also contaminated the waste mixture and made further separation difficult. Shards of glass and shredded metal became imbedded in the paper or biomass fraction of the material in the shredder and were carried through the process in that fraction, thereby reducing the removal efficiencies of those noncombustible fractions. Although many of the earliest RDF processes are still in operation, numerous modifications have been made to improve their performance.
With such a rocky beginning, the RDF option soon fell out of favor and yielded the market to the mass burn technology. The complexity, added costs of material handling, and the poor operating history of the RDF processes proved to be sufficient negative factors for any significant consideration of RDF fired waste-to-energy facilities for the next ten to fifteen years. That situation did not turn around until the push toward more waste recycling activities in the late 1980s and early 1990s. With that new impetus, plus many years of experience in operating RDF processing systems, the new generation of automated waste processing plants gained favor from a political/social base as well as from a more proven technical design basis. In numerous instances, RDF systems were promoted strictly to comply with mandated recycling directives. These material recovery facilities, or MRFs, accomplished essentially the same function as the earlier RDF processes in separating combustible from noncombustible materials, but were now done in the name of "recycling" with the recovery of ferrous and aluminum metals, glass, newsprint and corrugated paper, and plastics being the primary objective. Although oftentimes reduced yields of RDF were achieved because of the higher removal of recyclable materials, the quality of the fuel stream was strongly enhanced by this approach. With a national mandate in the United States on recycling, the cost and need for preprocessing of the waste became a burden to be borne by all new waste management plans. The advantages of mass burn technology had just been eliminated via the legislated mandate for higher recycling.
With more proven methods for RDF processing being demonstrated, the increase in RDF combustion technology has followed. Some of the facilities burning RDF utilize similar grate and furnace technology as the mass burn, but others, most notably fluidized bed or circulating fluidized bed combustion, offer a new and enhanced means of combusting the waste. Fluidized bed technology refers to the concept of burning a fuel in a combustion zone comprised of small particles of sand suspended in a stream of upward flowing air. The air velocity provides a buoyancy force to lift and suspend the individual sand particles such that they are able to float throughout the mixture and move freely within the furnace. The sand bed displays the characteristics of a pot of boiling water, hence the term, fluid bed. In a circulating fluid bed, the air velocities are actually increased to the terminal velocity of the sand particle, thereby carrying the sand up and out of the furnace. The sand is mechanically separated from the air/gas stream at the outlet of the furnace, and the sand is recirculated back into the furnace at the bottom of the bed. Hence, the term circulating, or recirculated, fluid bed.
The advantages of fluid bed combustion over the more traditional technology arise from the increased turbulence provided by the bed particle action. This fluidization increases the interaction of the fuel particles with the combustion air and creates a very accelerated combustion environment for the incoming fuel. Additionally, the sand, initially heated to an ignition temperature for the incoming fuel, provides a means of heating and drying the new fuel introduced into the furnace and igniting that fuel as it heats up. The thermal stability provided by the hot sand in the bed plus the turbulence of the fluidization causes the fuel particles to be consumed in a very short period, typically a few minutes or less. Complete combustion is achieved by the uniform temperature zone within the furnace plus the enhanced intermixing of fuel and combustion air. Historically, fluidized bed combustion systems have been able to achieve better operating flexibility, lower emission levels, and improved combustion and boiler efficiencies.
All methods of combustion of the waste pose certain "problems." The fuel, or waste, stream is nonhomogeneous, so the likelihood of variances in operation and performance are typical. With mass burn systems, the inventory of fuel within the furnace is maintained fairly high, for up to thirty minutes, and the "management" of the fuel feeding system is responsible for selecting a feed blend that maintains some measure of constancy throughout the process. With RDF systems, the processing of the fuel and sizing/shredding has enhanced the fuel homogeneity significantly, but variations in quality, content, etc., can still be expected. The fuel is known to contain measurable levels of sulfur, chlorine and other contaminants that can generate acid gases in the combustion products. These acids can, and do, attack the boiler surfaces and other equipment in the gas train, causing corrosion and necessitating continuous maintenance. Most boilers establish steam conditions of 650 psi and 750°F (398.9°C) as the maximum design condition in order to minimize the corrosion losses in the boiler. Recent improvements in metallurgy and furnace designs have enabled these design limits to be pushed to 850 psi and 825°F (440.6°C) or higher.
As a fuel, municipal solid waste (MSW) does not compare too favorably with more traditional solid fuels, such as coal. MSW averages somewhere around 4500 Btu/lb, versus coal at 10,500–13,000 Btu/lb. However, given the current U.S. population of 250 million and the annual generation of waste per person of fifteen hundred pounds, the potential energy content in the annual waste generated in the U.S. alone is comparable to nearly seventy million tons of coal and has the potential to generate over 13,000 MW of electrical power. As of a published report in 1993, 128 facilities were actually in operation, with an additional forty-two planned or under construction. Of the existing plants, ninety-two produced electricity to some degree. The remaining thirty-six produced only steam or hot water. The number of plants in the various sizes range from twenty plants less than one-hundred tons per day to twenty plants greater than two-thousand tons per day. Roughly forty plants are in the capacity of one-hundred to five-hundred tons per day and about twenty-five plants are sized for each of the five-hundred to one-thousand tons per day and the one-thousand to two-thousand tons per day capacity. Most of the smaller plants were installed in the early period of waste to energy development. Most of the facilities installed in the last ten years represent sizes from five-hundred to one-thousand tons per day. As of 1993 the capacity of waste consumed in these facilities was approximately 103,000 tons per day, representing approximately 18 percent of the projected annual waste generation in the United States.
Although waste-to-energy probably will never supply more than one percent of U.S. electricity (U.S. electricity generation in 1997 was 3,533 billion kilowatts), it is still a very useful renewable energy source. A state-of-the-art RDF cogeneration plant (hot water and electricity) burns much cleaner than the majority of existing coal-fired plants. At the same time, it is an attractive option in communities that desire inexpensive electricity and predictable waste disposal costs ten to fifteen years into the future, and that lack the real estate for additional landfill space. The major obstacle facing new waste-to-energy facilities is the considerable resistance contingency of the community who voice the "not-in-my-backyard" objection to the smell, noise and emissions. New facilities are likely to be located in rural areas, or as supplemental energy suppliers for industrial complexes, or on or near existing landfill operations.
The potential economic benefit exists for many regions to elect to become "host" facilities for disposal of wastes shipped in from greater metropolitan areas, such as New York City, which is closing its Freshkill landfill and already ships thousands of tons to Ohio, Georgia, and elsewhere for disposal, mostly in landfills. As the cost for energy (oil and natural gas) increases, the energy value of the waste will escalate. Its value as an alternate fuel will compensate for some of the underdesirable attributes and create a greater demand for future waste-to-energy facilities.
Michael L. Murphy
See also: Efficiency of Energy Use; Environmental Economics; and Environmental Problems and Energy Use
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