Turning Trash to Treasure (#3): The New Circular Economy of Waste

November 24, 2015

As Einstein once stated, "Energy cannot be created or destroyed, it can only be changed from one form to another". The Law of the Conservation of Energy is one of the most basic laws of science and it means a whole lot when we apply it to the things we throw away. See, energy is stored in everything from the foods we eat to the plastic toys our children play with. This means that when our trash gets taken to landfills, it becomes not only an environmental burden, but a wasted source of energy.

Trash is simply treasure that is yet to be transformed into something valuable. In our last posts, we explored how organic waste is being converted into products like compost and animal feed. This time, we'll explore how waste can be used to power our growing world, where traditional forms of energy (coal, oil, & natural gas) are quickly moving towards extinction and contributing to global concerns like climate change.

Waste-to-energy (WTE) technologies generate clean, renewable energy from trash which is discarded by the public, also known as municipal solid waste (MSW). According to the Energy Recovery Council, these technologies "recover valuable energy from trash after efforts to ‘reduce, reuse, and recycle' have [already] been implemented by households and local governments." What is generally left after these efforts are plant or animal-based products (paper and forest products, yard trimmings, food discards) that, when left in landfills, decompose and release toxic greenhouse gases like methane. The impact of methane (CH4) on climate change is 25 times greater than that of carbon dioxide (CO2) and 18% of the methane created in the United States comes from landfills. That represents enough methane gas to power 1.86 million homes!


Source: Environmental Protection Agency

Anaerobic Digestion Creates Methane Gas:

Capturing methane gas and using it to produce heat and electricity, a process called anaerobic digestion, is one of three ways in which to recover energy from waste. Anaerobic digestion is a biochemical conversion that takes place when bacteria break down organic waste in the absence of oxygen. In the United States, more than 500 landfills are currently preventing methane from escaping into the air by capturing and using it to power boilers, turbines for electricity or for natural gas to fuel trucks. Outside of landfills, anaerobic digester systems are popular at wastewater treatment plants and farms, where they process a range of things from wastewater sludge, to animal manure, food waste, and crop residues.

One of the key problems with anaerobic digestion is that only 5% of the material put into a digester is converted into energy, while the remaining 95% (called digestate) must be spread on land and composted. Not only is composting a lengthy process, but the digestate is a very smelly, bacteria-laden material that can run up against state regulations pertaining to greenhouse gas emissions and odors. At Regreen, we've created a machine that can solve both the front-end and back-end issues of anaerobic digestion. First, our Total Waste Processing System can boost the efficiency of AD systems by shredding and pressing organic waste to create a standardized mix of material with the right amount of liquid content required for rapid processing. Then after anaerobic digestion has occurred, our processor can convert any remaining liquids in the digestate to irrigation water and dry the solids to produce bacteria-free and odor-free compost in less than 30 minutes. Because technologies like the Total Waste Processing System more quickly and cleanly close the energy recovery loop, they set the foundation for waste-to-energy systems like anaerobic digestion to become economically and environmentally viable on a large scale.

Recycled Food Oils Make Biofuel:

The second way that waste can produce energy is through physiochemical conversion. Physiochemical conversion is fairly common practice for large food manufacturers, retailers and restaurants like McDonald's, who recycle used vegetable oils, animal fats, and greases by reprocessing them into biofuels. More recently, smaller players in the food industry have been experimenting with waste-to-energy. In Atlanta, an alliance of over 200 eco-friendly restaurateurs have joined together to open a biodiesel fueling station that provides fuel made from their discarded food oils.

Combustion Makes Heat & Electricity:

Thermochemical conversion, which involves the application of heat to waste, is the final and most common method of extracting energy from discarded materials. Thermochemical conversion processes include combustion, gasification, and pyrolysis. These processes are capable of extracting 500-700 kilowatt hours of power per ton of waste they process, compared to the 100 kilowatt hours per ton produced by landfill methane. And according to the US Environmental Protection Agency, by keeping municipal solid waste (MSW) out of landfills, these kinds of waste-to-energy facilities effectively reduce the amount of greenhouse gases in the atmosphere by 1 ton for every ton of waste used!

Combustion (aka incineration) facilities represent the majority of thermal conversion waste-to-energy facilities in the United States. In these facilities, trash is burned to generate heat, which produces steam that propels turbines and creates electricity. Filters trap ash and metals from the trash which are collected for recycling or used to make products like building materials and road base. Any gases produced by burned waste are filtered and cleaned before being emitted into the air.

Critics of this method of waste-to-energy conversion have worried most about the public health and environmental impacts of emissions from these facilities. While it is true that about one third of the MSW burned includes plastics or other fossil fuel-based materials that produce CO2 emissions, these emissions are miniscule when compared to the greenhouse gas reductions that combustion technologies achieve by 1) replacing fossil fuel-based electrical generation, 2) recovering metals for recycling, and 3) preventing methane emissions from landfills. In addition, today's incinerators must comply with the EPA's maximum available control technology (MACT) regulations, which are some of the strictest pollution standards in the world.

Most combustion facilities (76% in the US) employ mass burn technology, which allows MSW to be combusted without pre-processing. The rest burn pre-processed refuse-derived fuel (RDF). The difference lies at which point recyclables are recovered: either before or after combustion takes place. By sorting recyclables beforehand, RDF plants can remove potentially environmentally harmful materials prior to combustion. What is left typically consists of pelletized or fluff municipal solid waste (MSW), which has the advantage of being safe for burning outside of waste-to-energy plants in power boilers at factories or even apartment complexes.

The Power & Possibilities of Refuse-Derived Fuel:

At Regreen, we believe RDF is the future of fuel. As coal usage shrinks, RDF pellets offer a comparable alternative that is high in energy content, burns cleaner, can be easily transported and, best of all, removes the need to landfill waste. Regreen's Total Waste Processing System is designed with the capacity to turn anything from pure organic waste (food scraps, etc.) to mixed municipal solid waste with up to 30% plastics into RDF at the fast rate of 1-42 tons per hour. Our goal is to make it as easy as possible to cut landfills out of the equation and save the value of materials that would otherwise be lost if buried.

Pyrolysis & Gasification Create Synthetic Oil:

Outside of combustion facilities, the emerging thermochemical conversion technologies of pyrolysis and gasification are making innovative waves in the waste-to-energy sphere. Pyrolysis is the decomposition of organic material at high temperatures in the absence of oxygen. Gasification, on the other hand, takes place in the presence of a limited amount of oxygen. Unlike incineration, both of these processes limit the conversion of waste so that combustion does not take place. Instead of energy and ash, pyrolysis and gasifcation technologies produce energy-rich fuels like syngas (composed primarily of methane) and other valuable byproducts like charcoal. What makes these technologies unique is their ability to process a wide range of materials from biomass to medical waste and even the residues of recycling efforts (electronic scrap, mixed plastics, etc.). Recently, Dow Chemical Company experimented with pyrolysis in its Energy Bag Pilot program, which sought to demonstrate how non-recyclable plastics like candy wrappers can be diverted from landfills and converted into synthetic crude oil for fuel. The program resulted in 6,000 pounds of plastics in Citrus Heights, CA being turned into 512 gallons of fuel!

A key problem for pyrolysis and gasification technologies is that the efficiency and nature of these processes depend on the particle size and moisture content of their inputs (aka "feedstocks"). Regreen's Total Waste System boasts shredders that can effectively reduce difficult materials such as plastic bags and green palm to uniform sizes as small as ¼ inch in one pass. In addition, this machine will reduce the moisture content of waste from 30% to 2%, meeting the needs of waste-to-energy technologies that require a maximum of 5% moisture content to operate. As a result, Regreen systems allow for the creation of a dry, homogeneously-sized feedstock that boosts the efficiency of pyrolysis and gasification technologies.


Much work still needs to be done if waste-to-energy processes are to become widely-practiced in the United States. Currently, 63.5% of America's 390 million tons of annual waste is being landfilled and only 29% is being recycled. The remaining 7.6% is processed at 84 waste-to-energy facilities that recover upwards of 730,000 tons of recyclable metals and generate more than 14,500 kilowatt hours of energy each year. One of the main things impeding the construction of new facilities is the fact that many states have the physical space to landfill trash cheaply. But in large cities and densely-populated states, landfills are struggling to accommodate the trash produced by growing populations and are being forced to shut down. This is what happened to the Puente Hills Landfill in Southern California, which used to be the largest landfill in the country and accepted between one-third and one-half of Los Angeles County's waste.

Hauling trash to landfills—particularly when landfill closures increase the distances for waste disposal—is both financially and environmentally costly. Last year, New York City, paid more than $300 million to transport its trash to out-of-state landfills. The environmental footprint of trucking trash combined with the greenhouse gas emissions produced by decomposing waste will render landfilling an unsustainable practice in the long term. But the enormous problems presented by our waste, when viewed from a different perspective, provide equally enormous opportunities for clean energy production through waste-to-energy recovery. Municipal solid waste is an abundant and underutilized resource that, at bare minimum, can supplement intermittent renewable energy sources like wind and solar as we work to achieve a future free of coal, oil, and natural gas.

All of us must work together to push for nationwide standards that will mandate landfill diversion and waste-to-energy recovery. Meanwhile, we'll continue to develop systems at Regreen that make it easy to turn your trash into treasure.


Source: Environmental Protection Agency


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