Inexpensive natural gas, increased renewable energy utilization, and ever-tightening environmental regulations have limited the use of waste feedstocks for power generation. But while often dismissed out of hand, with proper planning and understanding of the market, power producers can realize benefits from waste-to-energy projects.
It was a tropically hot day at the plant, and it wasn’t even 9 a.m. As I rode up to the power building in an F-350 packed with five other engineers, the driver, sweating so much he looked like he’d just come back from a few laps across the Mississippi, told me, “You better drink a lot of water working around the boiler, ma’am, or you’re gonna die.” So, needless to say, I was dreading the day to come.
But upon entering the ice-box chill of the control room to start my data collection, I stopped and laughed out loud at what was on the wall before me. It was a novelty French-design menu—a blackboard with a wooden pig on one side and a caricature of a French chef with puffy hat on the other. Chalked in rather elegant script was the message shown in Figure 1.
1. Plat du jour. Source: POWER and Una Nowling
It was my first experience working at a plant that converted waste to energy (WTE), and during the project I learned much about the challenges of utilizing what are commonly called “waste fuels.” Definitions vary, but a waste fuel is typically a post-production or post-consumer substance that has no better purpose than to be used for energy production.
Waste fuels are sometimes called “opportunity fuels,” because they may offer a preferential price, give direct benefits to the power station, or provide indirect or co-benefits from their use. In the example I’ve given in Figure 1, the sawdust was a no-cost fuel that provided a small carbon emissions benefit, the sewage sludge reduced the landfill needs of the city’s treatment plant, the tires were cheap and high-energy, and the municipal solid waste (MSW) helped the city both utilize improved recycling and extend its landfill life.
Waste fuels (Figure 2) represent an energy source with perhaps the most highly variable quality of any fuel known. Some of the most common waste fuel types include the following:
2. Variety is the spice of life, and the bane of waste-to-energy (WTE) projects. Within each of these broad categories of municipal solid waste (MSW) lie innumerable unique subtypes of products. For example, “paper” includes everything from plain white bond paper to rough kraft packaging paper saturated with color inks. “Wood” could contain pine sawdust as well as construction debris featuring CCA-treated wood with galvanized nails. WTE facilities require either a highly flexible design or focused source fuel segregation and sorting to operate successfully. Source: U.S. Environmental Protection Agency
■ Municipal solid wasteis the most variable type of waste fuel, consisting of everything from banana peels to ball bearings. It may also contain varying amounts of industrial and even hazardous waste, depending upon the landfill location and local regulations. Waste screening and segregation is critical for the success of WTE projects using this fuel source (Figure 3).
3. Waste segregation is a key to success. Engineering efforts can be focused on improved waste processing before combustion, or the exploration of more fuel-flexible combustion technologies. At this Hartford, Conn., plant, substantial effort is spent separating MSW into recyclable components, inert waste, and combustible waste. Courtesy: Black & Veatch
■ Post-production waste often is more uniform in quality, due to more restrictive sourcing. Examples of this include sawdust and wood waste from a furniture factory, plastic fluff and trimmings from a consumer products manufacturer, or grain and plant waste from a cereal processor.
■ Sewage sludge is sometimes used for energy generation, especially in the European Union. Burned somewhat dried, or processed into pellets, sewage sludge can sometimes contain a surprising amount of chlorides and heavy metals, which can be an environmental concern.
■ Tires and similar rubber productsare often easy to separate, and thus amenable to being made into tire-derived fuel. Key concerns with burning these fuels include accommodation of the steel belts in the tread, and sulfur compounds in the rubber.
■ Landfill gasis formed from the anaerobic decomposition of organic materials in a landfill and is typically about 50% methane, with the balance being a mix of CO2, nitrogen, and other volatile compounds. Because methane is a much more powerful greenhouse gas than CO2, capturing and burning landfill gas provides direct environmental benefits.
■ Refuse-derived fuel is formed by heavily processing MSW to remove inert materials and moisture to produce a higher-quality fuel that can be used with conventional combustion technologies.
■ Biogas is typically formed by gasifying organic waste to produce a low-energy gas that can be directly used for heating or power production.
As a result, fuel flexibility has never had greater importance than for WTE facilities.
Three general methods for utilizing waste fuels are active biological treatment, passive biological utilization, and thermal treatment. Active biological treatment typically involves using an anaerobic digester to break down the waste into a biogas or (rarely) a bioliquid. Passive biological utilization is best exemplified by extracting landfill gas from existing waste storage piles or basins. Thermal treatment consists of many processes, which can include pyrolysis, gasification, and direct incineration.
Which utilization technique is selected depends upon many parameters, with fuel quality being the most important. Each of these techniques requires different levels of capital and operations and maintenance investment as well. For example, although mixing a modest amount of wood chips with coal might be a straightforward and low-cost utilization method, building a dedicated gasifier to convert those same wood chips to biogas can result in significant capital costs and may be a “first-of-a-kind” engineering project.
The end-use power conversion technology may also regulate the ability to utilize a waste fuel. For another example, though cofiring landfill gas in a standard boiler may be a low-technology modification, using landfill gas in a gas turbine may require significantly more plant modifications. As a result, each WTE project must be carefully assessed to determine the best solution for the situation.
WTE projects typically are considered to be greenhouse gas neutral through a variety of accounting factors. Although the combustion of biomass-derived waste is a net benefit, the combustion of plastics is a negative factor. However, if one considers the fossil fuel CO2avoidance, energy credits from increased metals recovery for recycling during the segregation process, and, most importantly, avoided landfill methane emissions, according to the U.S. Environmental Protection Agency, approximately 1 ton of CO2e emissions are avoided for every 1 ton of MSW combusted. (For more on this issue, see “Energy from Waste: Greenhouse Gas Winner or Pollution Loser?” in the July 2016 issue.)
Globally, more than 4 billion metric tons (mt) of solid waste are generated each year by humanity, of which between 1.6 billion and 2.0 billion mt is MSW. On a global scale, 70% of this waste is landfilled, 19% is recycled, and only 11% is utilized in WTE schemes. Due to many logistical and economic drivers—such as primary fossil energy scarcity, landfill volume restrictions, and high waste fuel availability—there is greater WTE project experience outside of the U.S. than within.
Many different factors determine whether a WTE project will be successful, so an engineer or plant owner who is interested in exploring the potential benefits of WTE should carefully examine not only the waste supply characteristics and availability but also how other WTE projects around the globe have encountered success—and failure. The following is a summary of several noteworthy or unique WTE lessons learned.
U.S. After removing recyclable and composted materials, according to the Department of Energy, more than 151 million mt of MSW are produced each year in the U.S. As of 2014, 32 million mt of MSW were burned each year in 84 WTE plants. With a total installed combined heat and power capacity of 2,769 MW, 14.5 TWh of electricity were generated from WTE in 2014.
Although greater utilization is possible, construction of new WTE plants in the country has lagged, despite most states considering WTE projects as being renewable energy. In 2015, the Palm Beach Renewable Energy Facility No. 2 commenced generation, utilizing three Babcock & Wilcox Stirling boilers with traveling grate combustors to generate 95 MW from processing 2,721 mt per day of MSW. This project is successful due to a well-engineered system, which includes comprehensive emissions controls: a spray-dryer absorber for SO2 control, pulse-jet fabric filter for particulate removal, and a cold-side selective catalytic reduction system for NOx control. Fly and bottom ash are processed to remove metals by rotary magnets and eddy current separators.
But other recent projects have been less fortunate. A proposed 160-MW MSW incinerator in Baltimore with a capacity of 3,628 mt per day was halted this May by the Maryland Department of the Environment over both health and environmental justice concerns. This is the second recent setback for WTE in the state; on March 17 of this year, Harford County’s WTE project of nearly 30 years was shut down after its main steam customer, the U.S. Army, decided to utilize a natural gas cogeneration project instead.
China. More than 160 million mt of MSW are produced each year in China, and facing a projected waste volume growth of 8% per year, China budgeted $42.5 billion for waste treatment facilities from 2011 to 2015, about half of which was directed for construction of new WTE incinerators.
In 2015, China employed 178 WTE incinerators with a gross installed capacity of 34.4 GW, processing in total 156,000 mt per day. China’s population-dense cities can yield concentrations of waste that lend themselves to creative use. One study found 956,300 mt per year of expired and waste food are generated from Beijing restaurants, which could be converted via anaerobic digestion into 300 million Nm3 of methane per year (about 3% to 4% of Beijing’s total natural gas consumption).
Despite both significant potential resources and experience with waste incineration, however, not all of China’s projects are successful. Public opposition to waste incinerators has resulted in several project cancellations. In 2013 a 12-MW incinerator in Huizhou was closed after local residents complained of noxious fumes and serious health problems from the facility. In 2014 violent protests against a proposed incinerator project broke out in Hangzhou, resulting in the project’s suspension.
Most MSW in China consists of food waste, which tends to be very damp and have low specific energy content. This results in a low combustion temperature, which can increase dioxin emissions in the flue gas. To prevent this from occurring, fuel oil may be cofired with the waste to maintain temperatures above 1,742F (950C). One of the problems at the aforementioned Huizhou station was an admitted reluctance to burn higher-cost fuel oil, resulting in elevated dioxin emissions. Undeterred by the problems, the local government is committed to developing a new, much cleaner incinerator to replace the canceled project.
Options for reducing dioxin emissions consist of combustion options and post-combustion options. Primary combustion options include higher combustion temperatures, separating out chloride-containing materials (mainly, reducing plastics such as PVC from the waste stream), reducing copper (which acts as a catalyst for dioxin formation), and rapid quenching of flue gas to avoid time spent at temperatures between 250C to 400C. Activated carbon injection can be used, with or without an activated carbon reactor. These are somewhat uncommon technologies. Conventional selective catalytic reduction systems that operate at cold temperatures (<200C) can reduce dioxins as well.
India. Although it is the second-most populous country on Earth, India’s per capita solid waste production is very low, resulting in an annual MSW generation of only 68 million mt per year. More than 90% of this waste is landfilled with almost no sanitary measures, and leaching of heavy metals from the waste is a problem in many coastal areas. Landfill fires and open burning of waste are a constant problem in the country, contributing to 20% of Mumbai’s particulate matter, carbon monoxide, and hydrocarbon emissions. What’s more, the growing population in India continues to reduce the land area available for landfill space, giving impetus to WTE projects as a means of reducing landfill use (Figure 4).
4. A growing fuel source. As developing economies like India transition to fully industrialized ones, their net waste generation could increase substantially. Courtesy: Una Nowling
However, Indian WTE projects face many challenges. Segregated waste collection efficiency is almost 0%, and nearly 50% of Indian MSW is neither digestible nor combustible, greatly limiting potential WTE generation. Several Indian WTE projects have failed due to lack of waste segregation, while others have failed due to financial and logistical problems.
Bureaucratic challenges often prevent WTE technologies and innovations from being implemented within the country, resulting in their export to countries such as Malaysia and China. A lack of consultants with professional expertise sometimes leads to tender documents that are incomplete, incorrect, or copied from prior documents without considering local waste supply characteristics and site environmental requirements. Finally, some WTE projects have failed because of delays in power payments from municipalities, which can commonly be delayed by three to six months.
One WTE plant in a Delhi suburb, which burned 1,900 mt per day of MSW, faced closure over its high stack emissions—dioxins and furans were found to be more than 120 times the permitted amount. The root cause of the problem was twofold. First, the plant utilized boiler technology designed for a more energy-dense fuel than Indian MSW. Second, the waste segregation entering the plant was especially poor, resulting in very poor combustion quality. In response to complaints and court actions against the plant, in November 2015 the National Green Tribunal ordered the plant to greatly improve its waste segregation prior to combustion.
Two government efforts are under way to improve the attractiveness of WTE projects. The first is a move to amend the Electricity Act 2003 to require state electricity distribution companies to purchase all power generated from MSW. The second is an effort by the Central Electricity Regulatory Commission to set a generic electricity price for WTE projects. Another effort is the Swachh Bharat Mission national cleanliness and waste reduction drive, which includes 74 MW of new WTE generation from six new plants by 2017.
Sweden. Sweden is often considered a WTE success story, as about 49% of its household waste is burned in WTE facilities. Driven by a desire to diversify its energy security, as a result of the oil shocks of the 1970s, Sweden now has a very diverse mix of nuclear, renewable, fossil, and unconventional energy sources.
Most of Sweden’s WTE output goes toward steam and hot water heating, with only a small amount used for direct electricity generation. As a result, the country’s energy utilization factor for MSW is quite high, with waste providing 15% of Sweden’s district heating needs in the most recent data available. Factors that have driven Sweden’s success include:
■ High landfill tipping fees
■ Bans on landfilling combustible waste
■ A carbon tax and an aggressive renewable portfolio standard
■ Comprehensive MSW recycling, resulting in improved segregation entering WTE plants
An extensive district heating network, expanded considerably after World War II, has indirectly helped WTE projects by allowing for easy use of generated steam. Perhaps most importantly, the Swedish public supports its WTE network, although cleaner renewable energy enjoys greater support.
Japan.WTE projects can sometimes yield unexpected benefits to a community. On March 11, 2011, a powerful earthquake hit the northeast coast of Japan, triggering a massive tsunami, which not only destroyed most of the Fukushima Daiichi Nuclear Power Plant but also caused extensive damage to or destroyed more than 300,000 buildings. Japan also has extensive experience with WTE projects and burns more than three-quarters of its MSW to produce energy. As a result, to help with the disposal of millions of tons of disaster debris and to provide much-needed power to replace that lost by the shutdown of Japan’s nuclear capacity, it was decided to construct 16 MW of WTE capacity to incinerate the combustible portion of the debris. When the debris waste has finally been exhausted, the power plant will continue generation with wood waste from local lumber and paper mills.
The plant was operational in 2013 and was located near Ishinomaki City. It was designed as a single plant for 1,500 mt per day capacity with multiple incinerator modules. However, a 2013 report said the incinerator operated way below capacity due to concerns with transporting waste to the site—both health concerns and concerns over spreading radiation. (POWER could not confirm whether or not this plant is still operating.)
Malta. Unlike the MSW “giants” of China, India, and the U.S., the tiny island nation of Malta faces a different problem with its waste. Although it has only 445,000 inhabitants, as a result of both its large tourist influx each year and its small land area, Malta has the highest ratio of MSW mass per square mile of land area. Because landfill space is at an absolute premium on the archipelago, one of the solutions to Malta’s waste problem is the construction of the Maghtab waste treatment plant.
Handling 66,000 mt of MSW, 47,000 mt of bulk waste, and 39,000 mt of animal manure per year, the Maghtab Environmental Complex uses both mechanical treatment and anaerobic digestion to generate almost 10 GWh of electricity.
Other WTE efforts are under way in Malta to further increase WTE generation and reduce landfilled waste, including engineered landfills to improve collection of gases and thermal treatment facilities to process both animal and hazardous waste.
Although WTE projects and the sector as a whole are unlikely to ever be a major player on the world energy stage, they can yield several co-benefits and play an important role in an energy business that is increasingly focused upon innovative methods of bringing electricity to all the peoples of the world. Even in the developed world, generating companies with a focus on the margins of resource utilization may find unique opportunities from utilizing waste as an energy source. ■
—Una Nowling, PE (email@example.com) is an adjunct professor of mechanical engineering at the University of Missouri-Kansas City and the technology lead for fuels at Black & Veatch.