Municipal Waste Combustion Management for a Cleaner Environment
by Allen Williams
As government regulations continue to play an increased role in the nation's economy, there is a demand for cheaper energy sources to promote economic growth.
America is committing vast land resources for the storage of Industrial and residential waste. Municipalities must contract out or arrange for waste transportation to landfill sites which amount to vast quantities of energy buried.
Municipal waste is a growing problem in both rural and urban communities across the United States. Toxins are leached from the materials in the landfill over time that potentially threaten the water supply and many of today's modern components take centuries to decay in the earth. Landfills contain Combustible materials that could provide low cost energy for cities as well as improve the environment.
Photo: Land fill space a growing problem..
The average American now discards approximately 16-20 pounds of solid waste per day per person. This waste has traditionally been disposed of in landfills, which require huge tracts of land and have finite storage capacity. Many landfills will have to close by 2040, increasing the cost of trash disposal and preventing the land from more productive use.
The Energy Information Administration (EIA) has noted that the energy content of solid waste in landfills has been steadily rising over the last decade, hitting 11.73 million Btu/ton in 2005. The heat content of interred waste provides the basis for developing an engineered fuel supporting many industrial applications such as the production of hydrocarbons, solvents, motor fuels, and even electric power generation.
A 2010 study(3) has found that emissions from landfills versus municipal waste combustion using EPA's life cycle assessment (LCA) model for the range and scenarios evaluated, that waste combustion outperforms land filling in terms of Green House Gas emissions regardless of landfill gas management techniques.
Innovative technologies can use buried waste as energy to convert bio-waste into needed products. Recently, Sweden's 144 million Kristianstad biogas plant has successfully converted municipal bio-waste into methane for use in automobiles and heating, saving some $3.5 million per year. Biogas can be further processed to produce organic liquids and even motor grade fuels.
Municipal Waste can be converted into fuel pellets with combustion performance comparable to coal. The solid waste can be processed with an engineered heat content amenable to fluidized bed and other furnace combustion equipment. Optimally, the pellets could be manufactured to a specific heat content. This is accomplished by feeding shredded rubber from scrap tires into a Cuber machine to produce fuel cubes of very high heating value, approximately 10,000 to 12,000 Btu/lb, suitable for utility power generation.
Developing a plan
The rising level of municipal waste provides incentive to develop alternative fuels but municipal waste contains many non-combustible components, some of which possess considerable recycle value. Recovery of these materials help to defer the cost of producing an engineered heat content fuel. Figure 1 illustrates the potential economic return based on EPA waste content.
Figure 1 - Salvageable Materials And Byproducts Revenue
Locating a suitable waste transformation plant directly on a Landfill site saves added costs for property and waste transportation to a site as trucks are already servicing the facility. In natural gas producing landfills, a cheap supply of methane for hydrocarbon synthesis is readily available.
In cases where a bio-gas facility such as Johnson County Wastewater may be nearby, combustible waste sludge can be transported via pipeline for fueling a suitable fluidized bed boiler. The pipeline can pay for itself quickly as only the installation cost from the wastewater facility to the landfill need be considered. Additionally, Industrial solvents such as methanol and other light hydrocarbons can be produced from the readily available methane feedstock along with steam and or electricity.
Waste Site Considerations
An ideal plant site would be an 850-acre landfill of which approximately 770 acres are used for solid waste internment. Road infrastructure would already exist to handle the associated truck waste transport traffic. Only small infrastructure changes would be needed to support an onsite waste processing facility.
Waste transport vehicles would contain an average of 11 tons of municipal trash and make 1 to 3 trips per day to the site depending on weather and other factors. The landfill could receive as much as 5000 tons of trash per day, averaging nearly 19 trucks per hour.
Engineered fuels could be manufactured from this solid refuse on approximately 5 of the remaining 70 plus acres. The solid waste would be screened to remove various metals and other non-combustible materials before processing into specified heat content fuels.
Waste Separation process
Figure two illustrates dual waste handling units separating typical recyclable materials, shredding and blending recovered solids and vehicle tires from the municipal waste to produce a serviceable fuel pellet. It is a time and motion illustration of the effort required to produce a 24% blend of solid waste and rubber. The chart was developed from a real waste processing pilot operation by RCR Partners of Colorado during the early 1980's.
Since that time computer model studies have shown that a 50-50 waste blend of solids and scrap tires provided a better heat content fuel at 10,221 Btu/lb suitable for a small industrial boiler consuming approximately 30 tons of fuel pellets per hour. The 50-50 blend represents only an incremental change in processing times.
The 2010 RCR industries salvageable Materials and Byproducts chart documents recovery revenues that are used to offset the cost of manufacturing an engineered fuel which is the basis of the Figure 2 chart. From this study, recyclable materials savings, operating and labor costs can be estimated.
Figure 2 - Solids Separation and Fuel Pellet manufacture
RCR Material Flow
Bags enter the breaker machine from the truck where large boxes and bags are opened without damaging the contents. This permits the separation of light and heavy components. Food waste is removed prior to mechanical separation.
Figure 3 - Waste flow through mechanical and cyclone separators
The solid waste moves through three mechanical separators upon entering the waste handling facility to remove glass, metal cans and plastic. Separation efficiency is greater than 90%.
Material next enters the first of two identical separators, all components less than 1-½” size pass through the first separator and are collected together with any metals in a common bin. These materials require further processing to segregate glass and metal
Separator No 3 removes all fractions less than 3" x 8”. Plastics fall into a collection bin exiting the 3rd separator.
The remaining material enters the shredder and is now all light fraction material.
The shredder slices the solid waste into approximately a ¾” size. The material passes through a cyclone separator to remove any dust generated by the shredding operation. The material can then be moistened and compressed by a Cuber machine into approximately 1-1/2" x 2" size fuel pellets.
The pellets may then be conveyed to storage vessels.
Photo: Fuel Pellets conveyed to Storage
Company reports are often good sources of economic cost data. Our fuel processing cost is estimated from an RCR Partners pilot plant study for the year's 1982-'83. The ordinary expense average for these years was $2,308,500 per year and defines the fixed costs. The Jan. '84 - Mar. '11 inflation rate was 146.7%, adjusting the ordinary expenses to present costs gives $5,694,244 per year.
The following utility rates were used in the economic evaluation: Coal at $55.00/ton, electricity at 7.3 cents per kilowatt, plant water for 3.2 cents a gallon and engineered fuel expenses according to the following cost relation. Fuel Cost/(ton) = X% * fixed cost + (1-X)% * rubber + processing
The base rate is calculated from the total production cost minus the revenue from salvageable materials separated out during the manufacturing process, i.e. [$Cost - $Salvage]/Total Tons = $Cost/ton
The salvageable material quantities from waste separation that can be re-sold are indicated in figures 1and 2. Ferrous metal scrap pays a max of $250/ton, Aluminum $0.75/lb and plastics $150/ton. Salvageable tire steel belt is 2.5 lbs/tire:
Details of computer simulated quantities and expected margins in the production of an organic solvent using engineered fuel pellets may be found in the July 2012 issue of Chemical Engineering, Vol 119, No. 7.t
One of the most significant features of engineered fuels is the ability to reduce the quantity of sulfur that must be scrubbed out of atmospheric releases during combustion. In our simulation, the computer model predicted a 20.67% reduction in SO2 emissions..
Mercury is virtually eliminated from stack gas emissions and other airborne contaminants can be significantly reduced through controlled waste blending.
Combusting Municipal waste in a controlled environment not only alleviates the need for further land repositories but may also facilitate recovery of burnable materials from many existing landfills.
1. "Methodology for Allocating Municipal Solid Waste to Biogenic and Non-Biogenic Energy", Energy Information Administration, Office of Coal, Nuclear, Electric and Alternate Fuels U.S., May 2007 Report
2. “Evaluating Green Projects – Modeling Improves Economic Benefits”, A. Williams, K. Dunwoody, Chemical Engineering – 119, 7, July 2012
3. "Life-Cycle Assessment of Waste Management Greenhouse Gas Emissions Using Municipal Waste Combustor Data", J. Envir. Engr. 136, 749 (2010); doi:10.1061/(ASCE)EE.1943-7870.0000189 (7 pages),Brian Bahor, Michael Van Brunt, P.E., Keith Weitz, and Andrew Szurgot
4. "Multisolid Fluidized Bed Combustion", H. Nack, R.D. Litt, B.C. Kim, Chemical Engineering Progress, Jan 1984
5. "Energy Recovery from Fluidized Bed Combustion", Robert J. Sneyd, Chemical Engineering Progress, Jan 1984
6. "Methodology for Allocating Municipal Solid Waste to Biogenic and Non-Biogenic Energy", Energy Information Administration, Office of Coal, Nuclear, Electric and Alternate Fuels, May 2007 Report