converting waste leads to a sustainable future
ISE Magazine July 2019 Volume: 51 Number: 7
By Gurram Gopal and Shruthi Suresh
The enormous increase in quantity and diversity of waste materials generated by humans and the potentially harmful effects on the general environment and public health have led to increased awareness and the urgent need for safe disposal of waste.
Waste generation rates are affected by climate, socioeconomic development and degree of industrialization. Greater economic prosperity and urban migration results in greater quantities of solid waste. Reduction in the mass and volume of solid waste is a crucial issue, especially in the light of limited access to disposal sites in many parts of the world.
At the same time, the demand for energy is growing rap-idly, driven by the huge emerging middle class in developing nations and the power-driven gadgets used by consumers in these economies. This energy demand is met primarily by cheap fossil fuels like coal and petroleum and has caused global concerns about the associated greenhouse effect and global cli-mate change. This has driven the need to innovate and employ alternate or unconventional energy sources to ensure the future well-being of the planet, minimize waste from human activities and meet high pollution control standards.
Recent studies indicate that if we had the capacity to divert all of the solid waste that was landﬁlled in 2015 to waste-to-energy facilities, we could generate enough electricity to sup-ply about 13.8 million households, 12% of the United States.
Further, if just the non-recycled plastics in solid waste were to be source-separated and converted through today’s technologies into fuel oil, they could produce 135 million barrels of oil per year; that’s 5.7 billion gallons of gasoline, enough to fuel 8.9 million cars.
If we could convert our non-recycled waste to alternative energy instead of landﬁlling it, we could preserve more than 6,000 acres of open space every year otherwise used to store garbage.
Waste-to-energy (WtE) is the generation of energy in the form of heat or electricity from waste. The process is also called energy-from-waste (EfW) and includes a variety of proven and emerging methods aimed to compress and dispose waste while generating energy from them. Energy-from-waste is not just about waste management but it is a valuable domestic energy source contributing to energy security with an added advantage that it is non intermittent and can complement other renewable energy sources such as wind or solar. As a partially renewable energy source, it can also contribute to renewable energy targets aimed at decarbonizing energy generation. This article analyzes current WtE methods and discusses the potential uses for ﬂy ash, a key waste product of coal-ﬁred power plants.
Types of waste and appropriate WtE methods
Agricultural residues. Large quantities of harvest residues are produced annually worldwide and are excessively underutilized. A common agricultural residue is the rice husk, which makes up 25% of rice by mass. Other residues include sugar cane ﬁber (known as bagasse), groundnut shells, cereal straw, coconut husks and shells. Current farming practices are to constantly plow these residues back into the soil, burn them or use them for cattle grazing. These residues could be processed into liquid fuels or combusted/gasiﬁed to generate electricity and heat.
Animal waste. A wide range of animal wastes can be used as sources of biomass energy, the most common being animal and poultry manures. In the past, this waste was recovered and sold as a fertilizer or simply spread onto agricultural land. But the introduction of tighter environmental controls on odor and water pollution means some form of waste management is now required, which provides further incentives for waste-to-energy conversion. The most attractive method of converting these waste materials to useful form is anaerobic digestion, which gives off biogas used as fuel for internal combustion engines, to generate electricity from small gas turbines, burned directly for cooking or for space and water heating. Food processing and slaughterhouse wastes are also a potential anaerobic digestion feedstock.
Sugar industry wastes. The sugar cane industry produces large volumes of bagasse each year. Bagasse is a major source of biomass energy as it can be used as boiler feedstock to generate steam for process heat and electricity production. Most sugar cane mills utilize bagasse to produce electricity for their own needs but some are able to export a substantial amount of electricity to the grid.
Forestry residues. Forestry residues are generated by operations such as thinning of plantations, clearing for logging roads, extracting stem-wood for pulp and timber and natural attrition. Wood processing also generates signiﬁcant volumes of residues usually in the form of sawdust, off-cuts, bark and woodchip rejects. This waste material is often left to rot on-site. However, it can be collected and used in a biomass gasiﬁer to produce hot gases for generating steam.
Industrial wastes. The food industry produces a large number of residues and byproducts that can be used as bio-mass energy sources. These waste materials are generated from all sectors, meat production to confectionery. Solid waste includes peelings and scraps from fruit and vegetables, food that does not meet quality control standards, pulp and ﬁber from sugar and starch extraction, ﬁlter sludge and coffee grounds. These wastes are usually disposed of in landﬁll dumps.
Liquid wastes are generated by washing meat, fruit and vegetables, blanching fruit and vegetables, pre-cooking meats, poultry and ﬁsh, cleaning and processing operations and wine making. These waste waters contain sugars, starches and other dissolved and solid organic matter. The potential exists for these industrial wastes to be anaerobically digested to produce biogas or fermented to produce ethanol, and several commercial examples of waste-to-energy conversion already exist.
Municipal solid waste (MSW). Millions of tons of household waste are collected each year with the vast majority ending up in land-ﬁll dumps. In 2015, United States generated more than 262 million tons of MSW, with 52% of it ending up in landﬁlls. The percent of MSW that is recycled or composted grew from less than 10% in 1970 to nearly 35% by 2011 but it has remained at that level subsequently (see Figure 1).
The biomass resource in this waste includes the putrescible, paper and plastic, and averages 80% of the total MSW collected. Municipal solid waste can be converted into energy by direct combustion or by natural anaerobic digestion in the landﬁll. At the landﬁll sites, the gas produced by the natural decomposition of MSW (which is approximately 50% methane and 50% carbon dioxide) is collected, scrubbed and cleaned before being fed into internal combustion engines or gas turbines to generate heat and power. The organic fraction of the waste can be anaerobically stabilized in a high-rate digester to obtain biogas for electricity or steam generation.
Sewage. Sewage is a source of biomass energy similar to the other animal wastes. Energy can be extracted from sewage using anaerobic digestion to produce biogas. The sludge that remains can be incinerated or undergo pyrolysis (decomposition under high temperatures) to produce more biogas.
Black liquor. Considered to be one of the most highly pol-luting industries, pulp and paper consumes large amounts of energy and water in various unit operations. The wastewater discharged by this industry is highly heterogeneous as it contains compounds from wood or other raw materials, processed chemicals as well as a compound formed during processing. Black liquor can be judiciously used for production of biogas using anaerobic technology.
Creating a waste hierarchy, pathway to energy conversion
In an ideal world all waste would be avoided. In reality, this does not occur for a variety of social, ﬁnancial and practical reasons. As waste exists, it is usually best to reuse if feasible. What can’t be reused could either go toward energy recovery, and if all else fails, ends up being returned to the earth, usually in a landﬁll. This general approach to waste is referred to as resource waste hierarchy, shown in Figure 2 (Enright, 2017).
A host of technologies are available for realizing the potential of waste as an energy source, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste. Common WtE projects include thermochemical and biochemical processes.
Combustion of waste has been used for many years as a way of reducing waste volume and neutralizing many of its potentially harmful elements. Combustion can only be used as an energy source when heat recovery is included. Heat recovered from the combustion process with thermochemical conversion can then be used to either power turbines for electricity generation or provide direct space and water heating. Some waste streams are also suitable for fueling a combined heat and power system, although quality and reliability of supply are important considerations. Thermochemical conversion pathways include incineration, pyrolysis and gasiﬁcation, are characterized by higher temperature and conversion rates, and are best suited for lower moisture feedstock. Incineration is the controlled combustion of waste with the recovery of heat to produce steam, which in turn produces power through steam turbines.
Pyrolysis and gasiﬁcation represent reﬁned thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into a gas as an energy carrier for later combustion, typically in a boiler or a gas engine. The most common way to generate energy is to use hot gases from the thermal step to boil water to create steam. This is then fed into a steam turbine to generate electricity or used for heating. This is the only route for incineration.
Advanced thermal treatments create a mixture of products from the thermal step that still have a lot of chemical energy stored in them, such as gases and oils. These can be burned and used to raise steam as above. However, they also have the potential to be cleaned and burned directly in gas engines or gas turbines, or converted to transport fuels or synthetic natural gas. The latter routes have the potential to convert the energy from the waste more efﬁciently than through steam generation, which makes them attractive. However, they are technically difﬁcult, relatively unproven at a commercial scale and some of the generated energy is used to power the process, reducing the overall beneﬁts.
Biochemical conversion relies on biochemical transformation processes, including anaerobic saturation and fermentation. It is recommended for wastes having high percentage of biodegradable matter and valuable moisture content. Anaerobic consumption is a reliable technology for the treatment of wet biological waste.
Organic waste from various sources is composted in highly controlled oxygen-free conditions, resulting in the production of biogas that can be used to produce both electricity and heat. Anaerobic digestion also results in a residue called digestate, which can be used as a soil conditioner. Alcohol fermentation is the transformation of organic fraction of biomass to ethanol by a series of biochemical reactions using specialized micro-organisms.
Factors effecting energy recovery from waste
The two main factors that determine the potential for recovery of energy from waste are the quantity and quality (physio-chemical characteristics) of the waste. Some of these important parameters include size of constituents, density, moisture content, quantity of volatile solids and organic matter, ﬁxed carbon, total inserts and caloriﬁc value. In the event of anaerobic assimilation, important considerations are the C/N ratio, a measure of supplement focus accessible for bacterial development, and the poisonous quality, representing the presence of hazardous materials which hinder bacterial development.
Critics of WtE worry that promoting energy from waste discourages reducing, recycling and other higher levels in the waste hierarchy. However, evidence in Europe shows that cases where energy is recovered from waste also have high recycling rates. Waste infrastructure has a long lifetime and care needs to be taken at the start to ensure systems can adapt to potential long-term change and drive waste up the hierarchy, not constrain it.
While modern waste-to-energy plants are sometimes con-fused with incinerators of the past, the environmental performance of the industry has improved signiﬁcantly. Studies have shown that communities employing waste-to-energy technology have higher recycling rates than communities that do not. The recovery of ferrous and nonferrous metals from waste-to-energy plants for recycling is strong and growing each year. In addition, numerous studies have determined that waste-to-energy plants actually reduce the amount of greenhouse gases entering the atmosphere.
Today’s waste-to-energy plants based on combustion technologies are highly efﬁcient power plants that utilize municipal solid waste as fuel rather than coal, oil or natural gas. Waste-to-energy plants recover the thermal energy contained in trash using highly efﬁcient boilers to generate steam that can be sold directly to industrial customers or used on-site to drive turbines for electricity.
Some WtE plants are highly efﬁcient in harnessing the un-tapped energy potential of organic waste by converting the biodegradable fraction of the waste into high caloriﬁc value gases like methane. The digested portion of the waste is highly rich in nutrients and is widely used as biofertilizer in many parts of the world.
Major environmental concerns of WtE systems
Incinerators produce a variety of toxic discharges to the air, water and ground that are signiﬁcant sources of many powerful pollutants, including dioxin and other chlorinated organic compounds known for their toxic impacts on human health and the environment. Many of these toxins enter the food sup-ply and concentrate up through the food chain.
In addition to air and water emissions, incinerators create toxic ash or slag that must then be landﬁlled. This ash contains heavy metals, dioxins and other pollutants, making it too toxic to reuse, although industry often tries to do so.
Incinerators emit signiﬁcant quantities of direct greenhouse gases, including carbon dioxide and nitrous oxide that con-tribute to climate change. They are also large sources of indirect greenhouse gases including carbon monoxide, nitrogen oxide, nonmethane volatile organic compounds and sulfur dioxide. In fact, incinerators emit more CO2 per megawatt-hour than does any fossil fuel-based power source, including coal-ﬁred power plants.
But the overall contribution to greenhouse gas emissions is reduced as the WtE is using inputs that otherwise might end up in landﬁlls, and also reduces the use of fossil fuels for energy generation.
Converting coal ﬂy ash residue to concrete
Coal ﬂy ash, a residue of burning pulverized coal and lignite in thermal power stations, is the largest type of waste generated in the United States and in many other countries, with over 100 million tons produced in the USA every year. Chemically, ﬂy ash is classiﬁed as a “pozzolan,” a material when mixed with water and lime reacts to form cementitious compounds. It contains a toxic stew of chemicals including lead, arsenic, mercury and radioactive uranium.
Coal ﬂy ashes are lightweight particles captured in exhaust gas by electrostatic precipitators and bag houses of coal-ﬁred power plants. Fly ash is very ﬁne with cement-like proper-ties and has long been used as an additive in cement, though not without some controversy. Fly ash has two key advantages as an ingredient in concrete and other building materials: it improves the quality of the ﬁnished products and it creates signiﬁcant environmental beneﬁts. Fly ash has both mechanical and chemical properties that makes it a valuable ingredient in concrete and concrete-based products. Its spherical shape makes concrete easier to work with during mixing and placing; ﬂy ash acts like tiny ball bearings moving the aggregates and other components into voids.
When concrete hardens, the chemical properties of ﬂy ash provide greater strength, reduced permeability and improved resistance to several types of chemical attack. The result is a concrete product that lasts longer, a key sustainability consideration. Fly ash can be used to replace up to 40% of the cement in concrete, depending on mix requirements.
Conserving landﬁll space by utilizing ﬂy ash is a signiﬁcant environmental beneﬁt. In addition, using recovered ﬂy ash conserves natural resources by eliminating the need to mine new raw materials. Furthermore, concrete can be produced using much less water when ﬂy ash is in the mix. Fly ash use can also signiﬁcantly decrease greenhouse gas emissions because it reduces the need for cement production, an extremely energy-intensive practice that increases greenhouse gas production.
Experts estimate that cement production accounts for more than 5% of carbon dioxide emissions from human sources. Reducing cement production decreases greenhouse gas emissions on almost a ton-for-ton basis. By gradually doubling or tripling the design life of concrete with ﬂy ash, natural re-sources are preserved and the environmental footprint is dramatically reduced.
Building with concrete that contains ﬂy ash can contribute to earning points in the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) program, which recognizes sustainable use of materials, land, water and energy, as well as ergonomics and innovative design (see related article on page 38). Fly ash, in combination with other qualifying building materials, can contribute to points earned for recycled content, using regional materials and/or innovative design. The key to maximizing points is for the project team (owner, architect, engineer, contractor and concrete supplier) to work together early in the construction process.
Traditional masonry takes signiﬁcant volumes of energy to produce, and concrete and brick making are some of the big-gest sources of greenhouse gasses. Clay bricks are produced in a kiln and ﬁred at 2,000 Fahrenheit for three to ﬁve days. The kilns are generally left running continuously even when no bricks are being produced due to the difﬁculty in getting the temperatures up to optimum levels. According to the National Institute of Standards and Technology, the carbon foot-print for a cubic yard of ﬁred clay brick is 991 pounds and 572 pounds for concrete brick.
The leachability of toxins from ﬂy ash is a critical issue in determining whether it can be put to beneﬁcial use. It is well established that ﬂy ash on its own is highly toxic. It is also well established that those toxic chemicals can be safely contained in a crystalline matrix when the ﬂy ash is subjected to thermal or chemical treatments. When used to replace Portland cement, ﬂy ash reacts with lime to produce a glassy matrix that inhibits leaching. Firing of ﬂy ash bricks will also produce the requisite glassy matrix, rendering them inert to leaching.
Vitriﬁcation is a thermochemical process that occurs at high temperatures around 1,500 Celsius that melt the ash and turn it into slag, a glasslike substance similar in appearance to obsidian. Vitriﬁed slag has been subjected extensively to TCLP analysis (toxicity characteristic leaching procedure) and found to be very stable and reliable at containing all toxins in the glass crystalline matrix. Vitriﬁed slag has been approved for use as a construction aggregate and ﬁlling material.
The downside of this process is the amount of energy required to melt the ash. High temperature gasiﬁers, such as plasma gasiﬁers, will produce slag instead of ash and is more efﬁcient than treating the ash in a separate process.
Growing waste demands innovative solutions
Environmentally sound and economically viable methods to treat biodegradable waste are urgently needed in the world today. A transition from conventional energy systems to one based on renewable resources is necessary to serve the ever-increasing demand for energy, while managing environmental concerns and enhancing the overall quality of life
Waste-to-energy plants offer two signiﬁcant beneﬁts: environmentally safe waste management and disposal as well as the generation of clean electric power. WtE systems have already reduced environmental impacts of municipal solid waste management, including emissions of greenhouse gases, and will play a signiﬁcant role in sustainable waste management in the future.
During the past year new WtE systems have opened or have been commissioned in a number of countries including Can-ada, Australia, Denmark, Pakistan and Saudi Arabia. Some of these systems predominantly use combustion while others use biochemical conversion.
As an example, Bore Hill Farm Biodigester is a biothermal plant in Warminster, Wiltshire, England, that processes food waste and creates renewable electricity to power 2,500 houses, and biofertilizer as well. It diverts waste that would have been sent to landﬁlls. Waste Management World magazine reported it is the ﬁrst English anaerobic digestion facility to be certiﬁed for good operational, environmental, and health and safety performance.
Both business managers and civic leaders need to engage with WtE specialists and pursue solutions that improve the sustainability of the planet with economically feasible solutions.