The truth about plastic placed in recycling bins is that most ends up in landfills. Converting the energy stored in plastic into electricity via waste-to-energy power plants offers a beneficial option.
Plastics have become ubiquitous in our modern era. They have replaced numerous metal, glass, and paper products due to lower costs and weight, creating a positive impact in many sectors of manufacturing. According to one study, fuel consumption has been reduced by 20% as a result of a 30% weight decrease in automobiles by implementing plastic components.
Due to the many benefits of plastics, production has increased at an exponential rate. In 1950, 1.3 million tons of plastic were produced globally. By 2018, the production of plastics grew to 359 million tons with 19% coming from North America.
Plastic Waste Is a Problem
Over the last 60 years, the plastic in U.S. waste streams has grown to be the third-largest material category (behind paper and food waste). The Environmental Protection Agency (EPA) has reported that there are 35.7 million tons of plastic waste generated annually in the U.S., which comprises 12.2% of all municipal solid waste (MSW). Unfortunately, one means of recycling plastic waste—shipping it to China—has recently been disrupted. This has drawn renewed attention to the problem of plastic waste in the U.S.
As of Jan. 1, 2018, China updated its standards for the importation of recycled material in an effort to limit the amount of pollution in the country and to lessen its dependence on imports. Before the change, a massive 7 million tons of plastic were shipped annually to China with the U.S. responsible for 10% of it. The Chinese restrictions have caused a decrease in the value of recyclable commodities, but also, accordingly, a drop in the price of scrap waste, causing scrap polyethylene terephthalate (PET, or plastic resin #1) value to decrease 33%.
The U.S. currently has 633 materials recycling facilities. These facilities sort and bale plastics (Figure 1), leading to a national recycling rate of about 8.5%, according to the EPA. An estimated 940 million tons of plastic were produced in the U.S. between 1960 and 2017 with 83.6% ending up in landfills.
The Chinese restriction resulted in a buildup of plastic waste due to insufficient recycling infrastructure in place to collect it. For instance, Arizona landfills reported a 20% waste increase in 2020 with an increase in plastic waste from COVID-19 (personal protective equipment, packaging, etc.) cited as a driving factor.
The EPA’s waste hierarchy for sustainable management of MSW, in order of priority, is as follows: reduce, reuse, recycle, energy recovery, and disposal. Once reduction and reuse efforts are exhausted, if it is not possible to recycle the discarded material, then the next method to be considered is converting it to a useful form of energy.
Waste-to-Energy Plants Offer a Solution
Waste-to-energy (WTE) power plants are often met with opposition in the U.S.; however, their use is growing in Europe and China as a clean energy resource and solid-waste solution (Figure 2). The primary disposal option in the U.S. is landfilling, a practice that is far from ideal. In order to meet the need for the sustainable disposal of plastics, a deeper consideration for sending non-recycled plastics to WTE facilities is warranted.
WTE facilities are a large-scale solution for managing MSW. Most WTE facilities convert MSW via combustion and recover energy in the form of electricity or steam. A typical WTE facility can generate from 550 kWh to 700 kWh of electricity per ton of MSW. Through combustion of MSW, the volume of solid waste is reduced, resulting mainly in ash. Ferrous and nonferrous metals can be recovered from ash outputs. The ability to recover energy and metals from multi-layered non-recyclable plastics is one advantage WTE has over traditional plastic recycling. Recovery of these metals further reduces CO2 emissions in the production of new metal products.
Based on data from the EPA and the World Bank, there is an increasing trend of plastic composition in MSW over the last few years. Although many municipalities and local governments have robust systems for collecting plastics for recycling, the reality is nearly 25% of plastics are sent to WTE facilities due to market drivers.
Unlike recycling techniques, WTE facilities can process all forms of plastic resins (#1 through #7) and engineered resins including multi-layer plastics. Recycling facilities can only utilize a select few types of resins (usually #1 and #2). The WTE thermal conversion technique does not depend on the material properties or manufacturing additives (fillers, multilayers, colorants, and flame retardants). Therefore, WTE facilities can dispose of those non-recycled plastics that contain insufficient properties for reprocessing to produce virgin resins.
Several chemical recycling techniques of plastics have been proposed but none have successfully demonstrated commercial viability. The full-cost accounting for the collection of plastics from generators and processing the material (separately from MSW) plus the added energy to wash, clean, and chemically process presents significant economic drawbacks. Meanwhile, the alternative of disposing plastics in a landfill is not a good option, because those plastics could take up to 600 years to decompose—and that does not offer the benefit of energy or material recovery.
The enrichment of plastic in MSW streams increases the calorific value of the refuse. However, WTE facilities are not economically incentivized to source plastics as a feedstock for combustion. Taking additional or bulk quantities of high-heat-content materials, like paper and plastics, reduces the amount of waste that a typical WTE facility can process. Because most WTE revenues come from waste tip fees, revenues would decrease from taking in large amounts of paper and plastics.
According to research conducted by Demetra A. Tsiamis and Marco J. Castaldi at City College of New York, the average higher heating value of mixed plastics is 16,492 Btu/lb. Therefore, if all landfilled plastic in 2017 alone was combusted (at an MSW average of 20% efficiency), it would provide 1.77 x 108 MMBtu, if the proper combined heat and power production facilities were available to do so. This is the energy equivalent of the consumption of 32 million barrels of oil and a lot of energy to bury.
Modern WTE facilities utilize advanced emission controls. The EPA upholds specific WTE emission limits on particulate matter (PM), cadmium, carbon monoxide, dioxins/furans, hydrogen chloride, lead, mercury, nitrogen oxides, opacity, and sulfur dioxide. The typical U.S. WTE facility emits all monitored species at a rate far below the EPA limits. For example, Figure 3 shows 2020 emissions data compared to the EPA limits from one U.S. WTE facility. Notably, the PM, cadmium, and lead emissions were lower than allowable limits by factors of 57, 152, and 193, respectively.
Air pollution control systems reduce the emissions of acid gases and other byproducts through lime and activated carbon injection, while PM is usually separated through the use of bag filters and sometimes ceramic filter candles. Ammonia is often used to react with the NOx produced during the combustion process to greatly reduce the NOx and produce nitrogen and water. Otherwise, NOx emissions are managed via combustion control, such as combustion air staging. Additionally, efforts have been made in the area of CO2 capture through the approaches of post-combustion capture, pre-combustion capture, or oxyfuel combustion capture.
The combustion of MSW with higher plastic content has been shown to have no impact on emissions. Additionally, combustion of MSW emits about the same amount of CO2 compared to combustion of methane—1,294 lb/MWh vs. 1,176 lb/MWh, respectively—according to a paper published by the Energy Recovery Council.
There have been 17 closures of WTE facilities since 2001 in the U.S. due to policy obstructions. This is a shame considering the use of WTE capacities throughout the rest of the world are increasing. From 2011 to 2015, the WTE capacity of China has more than doubled to 339 plants with a total capacity of 7.3 GW. The amount of waste going to WTE plants also doubled in Europe between 1995 and 2015. The key reason for the U.S. closures is economics, mainly stemming from the depressed price of electricity in competition with natural gas.
The pronounced goal of instituting policies surrounding plastics is to develop a circular economy. It is clear that a significant portion of recyclable material remains in the waste stream. Efforts to extract as much recyclable material as is feasible to process must continue, but the upper limit with the current state of recycling technologies needs to be recognized. WTE facilities should be regarded as a method to deal with materials left after recycling, and does not take away from sustainability and increased recycling efforts.
—Devin Peck is a Chemical Engineering PhD candidate at University of Louisiana, Lafayette, and Jeff LeBlanc, PhD is a product design engineer with Advance Products and Systems. This article was reviewed by Nathiel Egosi, PE president of RRT Design & Construction, and Marco Castaldi, PhD associate professor at City College of New York.