Waste to Energy (WTE) means many things to many people. Incineration dominates the industry today, which classically means the “mass burn” of unprocessed, unsegregated municipal solid waste (MSW). Simply defined, waste to energy is the process of generating electricity and/or heat from municipal solid waste.
The most common form of WTE is combustion, but there are other ways to turn waste into usable energy, namely anaerobic digestion (AD) and capture of landfill gas (LFG). AD is a process by which organic materials, such as livestock manure, food scraps and municipal or industrial wastewater are broken down by microorganisms in the absence of oxygen to produce biogas. LFG is a form of biogas produced naturally in all landfills by decomposition of organic waste. LFG can be captured and used for productive purposes instead of being vented or flared.
In fact, electricity generated from MSW emits half as many pounds of CO2 per MWh as coal plants. AD and LFG facilities also recover energy from waste that would otherwise be buried in landfills, thereby reducing the methane emissions that would be released in decomposition.
According to Robert Keough, vice president, communications at Advanced Energy Economy, the most common form of WTE is mass-burn combustion, in which MSW is burned “as is” to produce steam that spins a turbine attached to an electric generator.
“There is a small amount of ash – typically 5 percent to 15 percent of the volume of the processed trash – left over as a byproduct that is sent to a landfill,” Keough said. “Some WTE facilities sort out as many recyclable materials as possible prior to combustion, whereas others recover metals post combustion.
There are other WTE technologies as well, including modular systems, which are essentially the same as mass-burn combustion, only smaller, and refuse derived fuel systems. These systems shred the MSW prior to burning and remove non-combustible materials.
“Gasification is an alternative to direct combustion that offers some benefits with respect to emissions and efficiency,” Keough said. “The technology is well developed, but not in widespread use.”
Today, most biogas is burned to generate electricity onsite, but it can also be purified and made into a pipeline-quality substitute for natural gas. In the U.S., anaerobic digestion is most commonly used at municipal wastewater treatment facilities but is also found on farms, at industrial sites (e.g., food and beverage processing plants), and other locations.
As Bruce Rittmann, environmental engineering professor at Arizona State University, explained, the main WTE technologies are incineration with heat recovery and anaerobic digestion to make methane.
Quite simply, incineration is viable solid waste that has high organic content and low water content. This includes paper, cardboard, and plastic. While wet material becomes a net energy consumer to drive off the water, significant pollutant removal is needed for the off-gas.
Anaerobic digestion works for the solid waste that has high organic content and is wet. This includes garbage (food waste) and certain types of yard waste.
“The most convenient way to do this is to add the material (normally after grinding) into the anaerobic digester at a local wastewater treatment facility,” Rittman said. “This augments the usual methane production and offers valuable economy of scale. It is possible to digest solid waste on its own too.”
According to Ibrahim AlHusseini, founder and managing partner of the FullCycle Energy Fund, which funds companies with the goal of turning municipal solid waste into clean energy, some incinerators take their waste after recyclates have been harvested, often due to regulations or municipal contracts that require they do so.
“We work the next generation of WTE, systems which gasify the waste and remove the impurities from the waste stream while it is in gas form,” AlHusseini said. “Unlike incinerators, gasifiers aiming to produce a high quality of syngas require sorting and segregation of waste, providing a natural point from which to harvest recyclates. Our gasification systems do not require plastics be left in. We generally try to avoid disrupting existing recycling ecosystems or we can alternatively harvest the recyclates ourselves. Our systems aim to take the residual fraction that does not get recycled and convert it to power, chemicals, or a clean substitute for natural gas.”
The term waste to energy generally refers to incinerators or gasifiers, as described above. Pyrolysis is also sometimes lumped into the category, but it generally addresses only plastics and has modest yields.
“It could be perceived as a direct competitor of recycling,” AlHusseini said. “Similarly, anaerobic digestion is also in theory a source of energy from waste. Like pyrolysis, it too is a component solution, best adapted to homogenous and wet organics such as food processing or agricultural by-products. It is not a good fit for MSW generally, as the impurities in MSW frequently kill off the organisms, which produce the methane. Finally, there is landfill gas, which is just anaerobic digestion in situ at a landfill.
Impact on the Industry
So do WTE facilities adversely affect recycling rates? “They need not and should not,” AlHusseini said. As AlHusseini explained, ultimately, each component of the waste stream should go to its highest and best use, and the next generation allows for the harvesting of recyclates from the stream so that only the residual, which does not have a higher and better use, goes to energy.
“We think of this as recycling as well, as the hydrogen, carbon and oxygen elements within the waste stream are recycled into something useful, even if they do not retain the molecular form in which they were thrown away,” AlHusseni said. “Even countries that are the most advanced in recycling still have a substantial unrecycled residual, so both are needed.”
Keough agreed. “There’s no reason to think recycling rates are affected by WTE, in and of itself,” he says. “Recycling rates are influenced mainly by ease of recycling as well as rules and incentives. More materials get recycled under single-stream recycling than if recyclables need to be sorted and separated, and even more under pay-as-you-throw collection programs, which incentivize recycling to the maximum extent. No matter what, there are non-recyclable residuals that need to be disposed of, whether deposited in landfills or converted to electricity by WTE.”
How Gasification Works
In the early days, mass burn incinerator plants earned a poor reputation for emissions.
Today, these types of plants now bring this into levels compliant with EPA or EU standards by way of huge investments in their backend cleaning systems.
Conventional gasification or staged combustion is not much different, since it too relies on contaminant removal from the exhaust.
When you remove its inert material impurities, municipal solid waste consists of pure energy: hydrogen, carbon, and oxygen. Gas cleaning systems are now available to enable the true potential of gasification.
“While there has been a lot of focus on gasification as the future of WTE, efforts to do so have been held back by the lack of an efficient means to clean the gas,” AlHusseini says. “This has limited most conventional gasification plants to merely burning their syngas, often in a different stage of the same vessel. The net result is not different from simple combustion and thus nothing close to gasification’s true potential.”
Unlike plasma, oil scrubbing systems are basically passive and consume little energy, scale easily, and work well under pressure. They are proven at scale over many years in the refinery industry and are capable of removing 99.9 percent of the problematic tars. This produces a pure gas suitable as a substitute for natural gas in combined cycle gas turbines, fully realizing the potential of WTE gasification for producing power.
As AlHusseini explained, alternatively, this gas is suitable as a substitute for natural gas in industrial processes or chemical production.
“This transforms the WTE focus from waste destruction to maximizing the useful energy, and it does so with an emphasis on clean emissions,” AlHusseini said. “While the best incinerator operators have invested heavily to clean up the exhaust streams of their plants, the exhaust is an expensive place to try and remove impurities. It’s also after the money making process, so in countries with more relaxed regulations, a less scrupulous operator of an incineratoror conventional gasification plant actually makes more money if she or he does not install or maintain all of their exhaust clean-up equipment. In contrast, systems such as ours, which are based on purified syngas do not run—they lose money—if the impurities are not removed.”
Truly advanced gasification, which cleans the syngas to a stage that it can be used in an engine or turbine or for chemicals, has to remove the contaminants “before the money maker” or it will not make revenue.
These systems are very clean with an exhaust stream similar to natural gas, except unlike natural gas, roughly 60 percent is biogenic. Only the portion based on plastics is of a fossil fuel nature.
“Moreover, unlike natural gas, diversion from the landfill keeps the MSW from emitting methane to the atmosphere,” AlHusseini says. “Some of this methane might be captured in a sanitary, well-constructed landfill, but some of which is always fugitive and considered 21x more harmful than CO2 as a greenhouse gas.”
Advantages & Challenges
Traditional WTE produces heat and electricity while reducing waste. As of 2014, there were 84 WTE facilities in the United States. Located across 23 states, primarily in the Northeast, these facilities have a generating capacity of over 2.7 GW and process more than 30 million tons of municipal trash annually.
“Analysts estimate that WTE could replace nearly 5 percent of coal-fired generation capacity if all MSW were used for electricity generation,” Keough said.
Recent analysis estimates the total national economic impact of WTE to be $5.6 billion with nearly 14,000 jobs created by the industry. WTE facilities also can play a strong role in local community economic development by providing high-paying jobs, enabling long-term savings in disposal fees, and injecting money into local economies.
“According to our ‘Advanced Energy Now 2016 Market Report,’ In 2015, U.S. revenue from installation of waste-to-energy, anaerobic digestion, and landfill gas facilities totaled $524 million, up from $348 million in 2014,” Keough said.
“These effects are greatest in the developing world, where the baseline of existing sanitation is typically a very low standard,” AlHusseini said. “There are few disadvantages, as long as the incentives are aligned to keep the emissions of a plant clean. The technology we work with has such alignment and removes the impurities of the gasified waste stream before it is used.”
And while WTE is more expensive than open dumps, like those you find in many parts of the developing world, it is not inherently more expensive than new sanitary landfills of modern construction (e.g., compliant with the U.S. EPA).
“Once they become the new norm with the financial community and this is reflected in their cost of capital and plants’ scale, gasification-based WTE systems in the USA (or EU) should not require increases in tipping fees,” AlHusseini said.
Compared to a landfill, a WTE plant produces a valuable revenue stream from the sale of energy, in addition to providing a sanitary disposal of unrecycled waste. It creates more jobs and jobs of higher quality than a landfill, and it frees up land for higher and better uses.
“The advantages of WTE is that is allows us to gain the energy value of the organic material,” Rittman said. “And we reduce the amount of material for ultimate landfilling, while producing a soil amendment. The disadvantages include the capital cost required to start a WTE system and the ability to find a market for the energy products produced.”
On the Horizon
There is growing interest, at the state and municipal level, in source-separating organic food waste for composting and anaerobic digestion.
As Keough explained, a number of cities have begun voluntary or mandatory curbside compost collection, and Massachusetts has imposed a landfill ban on food waste, directed primarily at large wholesalers and retailers (i.e., supermarkets) and institutional food operations. Such policies will create market opportunities for developers and operators of anaerobic digestion facilities.
The next generation of WTE technologies have built demonstration plants and these have helped people focus on the technologies well-suited to delivering a cost-effective solution.
AlHusseini said the next step is building large-scale versions of this next generation and establishing it as the new norm.
“Waste haulers and recyclers have the opportunity to capitalize on this by helping to develop such plants after the reference plants are built,” AlHusseini said.
“Though providing electricity to the U.S. grid since the 1970s, WTE still has opportunities to expand,” Keough said. “With over 63 percent of waste still sent to landfills each year, MSW is an underused source of energy capable of producing baseload power to meet energy needs.”
Published in the June 2016 Edition of American Recycler News