This is an old and polluting technology used to simply burn household trash and waste. Incineration produces a limited amount of heat that is normally used to heat boilers to produce steam to drive steam turbines to generate limited amounts of electricity. This technology is no longer considered a viable alternative. Many of these old facilities are being upgraded to gasification facilities.
This is clearly thought to be the way of the future in both terms of efficiency and the environment. Gasification is a flexible and clean energy technology that can turn a variety of feedstock into energy, helping to reduce dependence on carbon based energy sources providing a clean alternative source of electricity, fertilizers, fuels, and other useful by-products. Gasification converts almost any material into a useable and efficient gas (syngas). The syngas can be used to produce electricity directly, via gas turbines or used to produce liquid fuels, bio fuels, a substitute for natural gas (SNG), or hydrogen. There are more than 140 gasification plants operating worldwide. Nineteen of those plants are located in the United States. Worldwide gasification capacity is projected to grow 70% by 2015, with 80% of that growth occurring in Asia. There are many companies producing gasification technologies. There are two main types of gasification; Pyrolysis and Plasma Arc.
Gasification is an environmental friendly solution to an environmental problem
The world is facing rapid growth in energy demand, persistently high energy prices, and a challenge to reduce carbon dioxide emissions from power generation and manufacturing. No single technology or resource can solve the problem, but gasification can be part of the solution along with renewable power sources such as wind and energy efficiency programs.
Gasification can enhance the U.S. and world energy portfolio while creating fewer air emissions, using less water, and generating less waste than most traditional energy technologies. Whether used for power generation, for production of substitute natural gas, or for production of a large number of energy intensive products, gasification has significant environmental benefits over conventional technologies.
Gasification provides significant environmental benefits
- Gasification plants produce significantly lower quantities of air pollutants.
- Gasification can reduce the environmental impact of waste disposal because it can use waste products as feedstock, generating valuable products from materials that would otherwise be disposed as wastes.
- Gasification byproducts are non-hazardous and are readily marketable.
- Gasification plants use significantly less water than traditional coal-based power generation, and can be designed so they recycle their process water, discharging none into the surrounding environment.
- Carbon dioxide (CO2) can be captured from an industrial gasification plant using commercially proven technologies. In fact, since 2000, the Great Plains Substitute Natural Gas plant in North Dakota has been capturing the same amount of CO2 as a 400 MW coal power plant would produce and sending that CO2 via pipeline to Canada for Enhanced Oil Recovery.
- Gasification offers the cleanest, most efficient means of producing electricity from coal and the lowest cost option for capturing CO2 from power generation, according to the U.S. Department of Energy.
- Gasification can compete effectively in high-price energy environments to provide power and products.
- Gasification can be used to turn lower-priced feedstock, such as petcoke and coal, into very valuable products like electricity, substitute natural gas, fuels, chemicals, and fertilizers. For example, a chemical plant can gasify petcoke or high sulfur coal instead of using high-priced natural gas, thereby reducing its operating costs.
- While a gasification power plant is capital intensive (like any very large manufacturing plant), its operating costs are potentially lower than conventional processes or coal-fired plants because gasification plants are more efficient and require less back-end pollution control equipment. With continued research and development efforts and commercial operating experience, the cost of these units will continue to decrease.
- Gasification offers wide fuel flexibility. A gasification plant can vary the mix of solid feedstock, or run on gas or liquid feedstock—giving it more freedom to adjust to the price and availability of its feedstock.
- The ability to produce a number of high-value products at the same time (polygeneration) also helps a facility offset its capital and operating costs. In addition, the principal gasification byproducts (sulfur and slag) are readily marketable. For example, sulfur can be used as a fertilizer and slag can be used in roadbed construction or in roofing materials.
- A state-of-the-art gasification power plant with commercially available technology can perform with efficiency in a range of 38-41 percent. Technology improvements now in advanced testing will boost efficiency to significantly higher levels.
- Gasification can increase domestic investment and jobs in manufacturing industries that have recently been in decline because of high energy costs.
- Many predict that coal-based power plants and other manufacturing facilities will be required to capture and store CO2, or participate in a carbon cap and trade market. In this scenario, gasification projects will have a cost advantage over conventional technologies. While CO2 capture and sequestration will increase the cost of all forms of power generation, an IGCC plant can capture and compress CO2 at one-half the cost of a traditional pulverized coal plant. Other gasification-based options, including production of motor fuels, chemicals, fertilizers, or hydrogen, to name a few, have even lower carbon capture and compression costs. This will provide a significant economic and environmental benefit in a carbon-constrained world. (See Carbon Capture & Compression Costs.)
- Gasification can replace volatile natural gas as a fuel or a feedstock. Read more.
- Gasification is being used around the world. Read more about gasification economics in practice.
PRODUCTS AND APPLICATIONS
Chemicals and Fertilizers
Modern gasification has been used in the chemical industry since the 1950s. Typically, the chemical industry uses gasification to produce methanol as well as chemicals, such as ammonia and urea, which form the foundation of nitrogen-based fertilizers. The majority of the operating gasification plants worldwide produce chemicals and fertilizers. And, as natural gas and oil prices continue to increase, the chemical industry is developing additional coal gasification plants to generate these basic chemical building blocks.
Eastman Chemical Company helped advance the use of coal gasification technology for chemicals production in the U.S. Eastman’s coal-to-chemicals plant in Kingsport, Tennessee converts Appalachian coals to methanol and acetyl chemicals. The plant began operating in 1983 and has gasified approximately 10 million tons of coal with a 98 to 99 percent on-stream availability rate.
Power Generation with Gasification
Coal can be used as a feedstock to produce electricity via gasification, commonly referred to as Integrated Gasification Combined Cycle (IGCC). This particular coal-to-power technology allows the continued use of coal without the high level of air emissions associated with conventional coal-burning technologies. In gasification power plants, the pollutants in the syngas are removed before the syngas is combusted in the turbines. In contrast, conventional coal combustion technologies capture the pollutants after combustion, which requires cleaning a much larger volume of the exhaust gas. This increases costs, reduces reliability, and generates large volumes of sulfur-laden wastes that must be disposed of in landfills or lagoons.
Today, there are 15 gasification-based power plants operating successfully around the world. There are three such plants operating in the United States. Plants in Terre Haute, Indiana and Tampa, Florida provide baseload electric power, and the third, in Delaware City, Delaware provides electricity to a Valero refinery. (See World Gasification-Based Power Generating Capacity)
Substitute Natural Gas
Gasification can also be used to create substitute natural gas (SNG) from coal and other feedstocks, supplementing U.S. natural gas reserves. Using a “methanation” reaction, the coal-based syngas—chiefly carbon monoxide (CO) and hydrogen (H2)—can be profitably converted to methane (CH4). Nearly identical to conventional natural gas, the resulting SNG can be shipped in the U.S. natural gas pipeline system and used to generate electricity, produce chemicals/fertilizers, or heat homes and businesses. SNG will enhance domestic fuel security by displacing imported natural gas that is generally supplied in the form of Liquefied Natural Gas (LNG).
Hydrogen for Oil Refining
Hydrogen, one of the two major components of syngas, is used in the oil refining industry to strip impurities from gasoline, diesel fuel, and jet fuel, thereby producing the clean fuels required by state and federal clean air regulations. Hydrogen is also used to upgrade heavy crude oil. Historically, refineries have utilized natural gas to produce this hydrogen. Now, with the increasing price of natural gas, refineries are looking to alternative feedstocks to produce the needed hydrogen. Refineries can gasify low-value residuals, such as petroleum coke, asphalts, tars, and some oily wastes from the refining process, to generate both the required hydrogen and the power and steam needed to run the refinery.
Gasification can be used to produce transportation fuels from oil sands, coal and biomass. Read more about each of these technologies.
Gasification has been reliably used on a commercial scale worldwide for more than 50 years by the chemical, refining, and fertilizer industries and by the electric power industry for more than 35 years. Currently, there are more than 340 gasification plants—with more than 820 gasifiers—operating worldwide.
Nineteen of those gasification plants are located in the United States. (See Existing Gasification Plants in the U.S).
The Future of Gasification
Worldwide gasification capacity is projected to grow 70 percent by 2015, with 80 percent of the growth occurring in Asia. The prime movers behind this expected growth are the chemical, fertilizer, and coal-to-liquids industries in China, oil sands in Canada, polygeneration (hydrogen and power or chemicals) and substitute natural gas in the United States, and refining in Europe
The use of gasification is expanding. Several gasification projects are under development to provide steam and hydrogen to upgrade synthetic crude in the oil sands industry in Canada. In addition, the paper industry is exploring how gasification can be used to make their operations more efficient and reduce waste streams.
- A number of factors contribute to a growing interest in gasification, including volatile oil and natural gas prices, more stringent environmental regulations, and a growing consensus that CO2 management will likely be required in power generation and energy production. (See U.S. Energy Prices).
- China is expected to achieve the most rapid growth in gasification worldwide. Since 2004, 29 new gasification plants have been licensed and/or built in China. In contrast, no new gasification plants have begun operation in the United States since 2002.
- The gasification industry is expected to grow significantly in the United States despite a number of challenges, including rising construction costs and uncertainty about policy incentives and regulations.
Transportation Fuels from Oil Sands
The “oil sands” in Alberta, Canada are estimated to contain as much recoverable oil (in the form of bitumen) as the vast oil fields in Saudi Arabia. However, to convert this raw material to saleable products requires mining the oil sands and refining the resulting bitumen to transportation fuels. The mining process involves massive amounts of steam to separate the bitumen from the sands and the refining process demands large quantities of hydrogen to upgrade the “crude oil” to finished products. Residuals from the upgrading process include petcoke, deasphalted bottoms, vacuum residuals, and asphalt/asphaltenes – all of which contain unused energy.
Traditionally, oil sands operators have utilized natural gas to produce the steam and hydrogen needed for the mining, upgrading, and refining processes. However, a number of operators will soon gasify petcoke to supply the necessary steam and hydrogen. Not only will gasification displace expensive natural gas as a feedstock, it will also enable the extraction of useable energy from what is otherwise a very low-value product (petcoke). In addition, black water from the mining and refining processes can be recycled to the gasifiers using a wet feed system, reducing fresh water usage and waste water management costs. (This is not inconsequential, since traditional oil sand operations consume large volumes of water.)
Transportation Fuels from Coal
Gasification is the foundation for converting coal and other solid feedstocks and natural gas into transportation fuels such as gasoline, ultra-clean diesel fuel, jet fuel, naphtha, and synthetic oils. Two basic paths are employed in converting coal to motor fuels via gasification. In the first, the syngas undergoes an additional process, the Fischer-Tropsch (FT) reaction, to convert it to a liquid petroleum product. The FT process, with coal as a feedstock, was invented in the 1920s, was used by Germany during World War II, and has been utilized in South Africa since the 1950s. Today, it is also used in Malaysia and the Middle East with natural gas as the feedstock.
In the second process, so-called Methanol- to-Gasoline (MTG), the syngas is first converted to methanol (a commercially used process) and the methanol is converted to gasoline by reacting it over a bed of catalysts. A commercial MTG plant successfully operated in the 1980s and early 1990s in New Zealand and plants are under development in China and in the U.S.
Transportation Fuels from Biomass
Gasification is also being used as a basis for converting biomass to transportation fuels. Biomass, (such as agricultural waste, switch grass, or wood waste) is converted to a synthesis gas via gasification. The synthesis gas is then passed over various proprietary catalysts and converted to transportation fuels, such as cellulosic ethanol or bio-diesel. Several biomass-to-liquids plants are now under development.
Pyrolysis is a thermo chemical decomposition of organic material at elevated temperatures in the absence of oxygen. Pyrolysis typically occurs under pressure and at operating temperatures above 430 °C (800 °F). The word is coined from the Greek-derived elements pyr “fire” and lysis “separating”. Pyrolysis is a special case of thermolysis, and is most commonly used for organic materials. The Pyrolysis or gasification of wood, which starts at 200–300 °C (390–570 °F), and occurs naturally for example when vegetation comes into contact with lava in volcanic eruptions. In general, pyrolysis of organic substances produces gas and liquids leaving a solid residue richer in carbon content. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization.
Simplified depiction of pyrolysis chemistry.
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. Pyrolysis typically occurs under pressure and at operating temperatures above 430 °C (800 °F). The word is coined from the Greek-derived elements pyr “fire” and lysis “separating”.
Pyrolysis is a special case of thermolysis, and is most commonly used for organic materials, being then one of the processes involved in charring. The pyrolysis of wood, which starts at 200–300 °C (390–570 °F), occurs for example in fires or when vegetation comes into contact with lava in volcanic eruptions. In general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization.
The process is used heavily in the chemical industry, for example, to produce charcoal, activated carbon, methanol and other chemicals from wood, to convert ethylene dichloride into vinyl chloride to make PVC, to produce coke from coal, to convert biomass into syngas, to turn waste into safely disposable substances, and for transforming medium-weight hydrocarbons from oil into lighter ones like gasoline. These specialized uses of pyrolysis may be called various names, such as dry distillation, destructive distillation, or cracking.
Pyrolysis also plays an important role in several cooking procedures, such as baking, frying, grilling, and caramelizing. And it is a tool of chemical analysis, for example in mass spectrometry and in carbon-14 dating. Indeed, many important chemical substances, such as phosphorus and sulfuric acid, were first obtained by this process. Pyrolysis has been assumed to take place during catagenesis, the conversion of buried organic matter to fossil fuels. It is also the basis of pyrography.
In their embalming process, the ancient Egyptians used a mixture of substances, including methanol, which they obtained from the pyrolysis of wood.
Pyrolysis differs from other high-temperature processes like combustion and hydrolysis in that it does not involve reactions with oxygen, water, or any other reagents. In practice it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system, a small amount of oxidation occurs.
The term has also been applied to the decomposition of organic material in the presence of superheated water or steam (hydrous pyrolysis), for example in the steam cracking of oil.
Occurrence and uses
Pyrolysis is usually the first chemical reaction that occurs in the burning of many solid organic fuels, like wood, cloth, and paper, municipal waste and also of some kinds of plastic. In a wood fire, the visible flames are not due to combustion of the wood itself, but rather of the gases released by its pyrolysis; whereas the flame-less burning of embers is the combustion of the solid residue (charcoal) left behind by it. Thus, the pyrolysis of common materials like wood, plastic, and clothing is extremely important for fire safety and fire-fighting.
Pyrolysis occurs whenever food is exposed to high enough temperatures in a dry environment, such as roasting, baking, toasting, grilling, etc.. It is the chemical process responsible for the formation of the golden-brown crust in foods prepared by those methods.
In normal cooking, the main food components that suffer pyrolysis are carbohydrates (including sugars, starch, and fibre) and proteins. Pyrolysis of fats requires a much higher temperature, and since it produces toxic and flammable products (such as acrolein), it is generally avoided in normal cooking. It may occur, however, when barbecuing fatty meats over hot coals.
Even though cooking is normally carried out in air, the temperatures and environmental conditions are such that there is little or no combustion of the original substances or their decomposition products. In particular, the pyrolysis of proteins and carbohydrates begins at temperatures much lower than the ignition temperature of the solid residue, and the volatile sub-products are too diluted in air to ignite. (In flambé dishes, the flame is due mostly to combustion of the alcohol, while the crust is formed by pyrolysis as in baking.)
Pyrolysis of carbohydrates and proteins require temperatures substantially higher than 100 °C (212 °F), so pyrolysis does not occur as long as free water is present, e.g. in boiling food — not even in a pressure cooker. When heated in the presence of water, carbohydrates and proteins suffer gradual hydrolysis rather than pyrolysis. Indeed, for most foods, pyrolysis is usually confined to the outer layers of food, and only begins after those layers have dried out.
Food pyrolysis temperatures are however lower than the boiling point of lipids, so pyrolysis occurs when frying in vegetable oil or suet, or basting meat in its own fat.
Pyrolysis also plays an essential role in the production of barley tea, coffee, and roasted nuts such as peanuts and almonds. As these consist mostly of dry materials, the process of pyrolysis is not limited to the outermost layers but extends throughout the materials. In all these cases, pyrolysis creates or releases many of the substances that contribute to the flavor, color, and biological properties of the final product. It may also destroy some substances that are toxic, unpleasant in taste, or those that may contribute to spoilage.
Controlled pyrolysis of sugars starting at 170 °C (338 °F) produces caramel, a beige to brown water-soluble product which is widely used in confectionery and (in the form of caramel coloring) as a coloring agent for soft drinks and other industrialized food products.
Solid residue from the pyrolysis of spilled and splattered food creates the brown-black encrustation often seen on cooking vessels, stove tops, and the interior surfaces of ovens.
Pyrolysis has been used since ancient times for turning wood into charcoal in an industrial scale. Besides wood, the process can also use sawdust and other wood waste products.
Charcoal is obtained by heating wood until its complete pyrolysis (carbonization) occurs, leaving only carbon and inorganic ash. In many parts of the world, charcoal is still produced semi-industrially, by burning a pile of wood that has been mostly covered with mud or bricks. The heat generated by burning part of the wood and the volatile byproducts pyrolyzes the rest of the pile. The limited supply of oxygen prevents the charcoal from burning too. A more modern alternative is to heat the wood in an airtight metal vessel, which is much less polluting and allows the volatile products to be condensed.
The original vascular structure of the wood and the pores created by escaping gases combine to produce a light and porous material. By starting with dense wood-like material, such as nutshells or peach stones, one obtains a form of charcoal with particularly fine pores (and hence a much larger pore surface area), called activated carbon, which is used as an adsorbent for a wide range of chemical substances.
Residues of incomplete organic pyrolysis, e.g. from cooking fires, are thought to be the key component of the terra preta soils associated with ancient indigenous communities of the Amazon basin. Terra preta is much sought by local farmers for its superior fertility compared to the natural red soil of the region. Efforts are underway to recreate these soils through biochar, the solid residue of pyrolysis of various materials, mostly organic waste.
Biochar improves the soil texture and ecology, increasing its ability to retain fertilizers and release them slowly. It naturally contains many of the micronutrients needed by plants, such as selenium. It is also safer than other “natural” fertilizers such as manure or sewage since it has been disinfected at high temperature, and since it releases its nutrients at a slow rate, it greatly reduces the risk of water table contamination.
Biochar is also being considered for carbon sequestration, with the aim of mitigation of global warming. Because pyrolysis burns the volatile gases, biochar only emits water vapor. By burning the harmful gases, a stabile form of carbon can be sequestered into the ground where it will remain for thousands of years.
Pyrolysis is used on a massive scale to turn coal into coke for metallurgy, especially steelmaking. Coke can also be produced from the solid residue left from petroleum refining.
Those starting materials typically contain hydrogen, nitrogen or oxygen atoms combined with carbon into molecules of medium to high molecular weight. The coke-making or “coking” process consists in heating the material in closed vessels to very high temperatures (up to 2,000 °C or 3,600 °F), so that those molecules are broken down into lighter volatile substances, which leave the vessel, and a porous but hard residue that is mostly carbon and inorganic ash. The amount of volatiles varies with the source material, but is typically 25-30% of it by weight.
Carbon fibers are filaments of carbon that can be used to make very strong yarns and textiles. Carbon fiber items are often produced by spinning and weaving the desired item from fibers of a suitable polymer, and then pyrolyzing the material at a high temperature (from 1,500–3,000 °C or 2,730–5,430 °F).
The first carbon fibers were made from rayon, but polyacrylonitrile has become the most common starting material
For their first workable electric lamps, Joseph Wilson Swan and Thomas Edison used carbon filaments made by pyrolysis of cotton yarns and bamboo splinters, respectively.
Pyrolysis is the basis of several methods that are being developed for producing fuel from biomass, which may include either crops grown for the purpose or biological waste products from other industries.
Although synthetic diesel fuel cannot yet be produced directly by pyrolysis of organic materials, there is a way to produce similar liquid (“bio-oil”) that can be used as a fuel, after the removal of valuable bio-chemicals that can be used as food additives or pharmaceuticals. Higher efficiency is achieved by the so-called flash pyrolysis where finely divided feedstock is quickly heated to between 350 and 500 °C (660 and 930 °F) for less than 2 seconds.
Fuel bio-oil resembling light crude oil can also be produced by hydrous pyrolysis from many kinds of feedstock, including waste from pig and turkey farming, by a process called thermal depolymerization (which may however include other reactions besides pyrolysis).
Plastic waste disposal
Anhydrous pyrolysis can also be used to produce liquid fuel similar to diesel from plastic waste.
In many industrial applications, the process is done under pressure and at operating temperatures above 430 °C (806 °F). For agricultural waste, for example, typical temperatures are 450 to 550 °C (840 to 1,000 °F).
In vacuum pyrolysis, organic material is heated in a vacuum in order to decrease boiling point and avoid adverse chemical reactions. It is used in organic chemistry as a synthetic tool. In flash vacuum thermolysis or FVT, the residence time of the substrate at the working temperature is limited as much as possible, again in order to minimize secondary reactions.
Processes for biomass pyrolysis
Since pyrolysis is endothermic, various methods have been proposed to provide heat to the reacting biomass particles:
- Partial combustion of the biomass products through air injection. This results in poor-quality products.
- Direct heat transfer with a hot gas, ideally product gas that is reheated and recycled. The problem is to provide enough heat with reasonable gas flow-rates.
- Indirect heat transfer with exchange surfaces (wall, tubes). It is difficult to achieve good heat transfer on both sides of the heat exchange surface.
- Direct heat transfer with circulating solids: Solids transfer heat between a burner and a pyrolysis reactor. This is an effective but complex technology.
For flash pyrolysis the biomass must be ground into fine particles and the insulating char layer that forms at the surface of the reacting particles must be continuously removed. The following technologies have been proposed for biomass pyrolysis:
- Fixed beds were used for the traditional production of charcoal. Poor, slow heat transfer resulted in very low liquid yields.
- Augers: This technology is adapted from a Lurgi process for coal gasification. Hot sand and biomass particles are fed at one end of a screw. The screw mixes the sand and biomass and conveys them along. It provides a good control of the biomass residence time. It does not dilute the pyrolysis products with a carrier or fluidizing gas. However, sand must be reheated in a separate vessel, and mechanical reliability is a concern. There is no large-scale commercial implementation.
- Ablative processes: Biomass particles are moved at high speed against a hot metal surface. Ablation of any char forming at the particles surface maintains a high rate of heat transfer. This can be achieved by using a metal surface spinning at high speed within a bed of biomass particles, which may present mechanical reliability problems but prevents any dilution of the products. As an alternative, the particles may be suspended in a carrier gas and introduced at high speed through a cyclone whose wall is heated; the products are diluted with the carrier gas. A problem shared with all ablative processes is that scale-up is made difficult since the ratio of the wall surface to the reactor volume decreases as the reactor size is increased. There is no large-scale commercial implementation.
- Rotating cone: Pre-heated hot sand and biomass particles are introduced into a rotating cone. Due to the rotation of the cone, the mixture of sand and biomass is transported across the cone surface by centrifugal force. Like other shallow transported-bed reactors relatively fine particles are required to obtain a good liquid yield. There is no large scale commercial implementation.
- Fluidized beds: Biomass particles are introduced into a bed of hot sand fluidized by a gas, which is usually a recirculated product gas. High heat transfer rates from fluidized sand result in rapid heating of biomass particles. There is some ablation by attrition with the sand particles, but it is not as effective as in the ablative processes. Heat is usually provided by heat exchanger tubes through which hot combustion gas flows. There is some dilution of the products, which makes it more difficult to condense and then remove the bio-oil mist from the gas exiting the condensers. This process has been scaled up by companies such as Dynamotive and Agri-Therm. The main challenges are in improving the quality and consistency of the bio-oil.
- Circulating fluidized beds: Biomass particles are introduced into a circulating fluidized bed of hot sand. Gas, sand and biomass particles move together, with the transport gas usually being a recirculated product gas, although it may also be a combustion gas. High heat transfer rates from sand ensure rapid heating of biomass particles and ablation is stronger than with regular fluidized beds. A fast separator separates the product gases and vapors from the sand and char particles. The sand particles are reheated in fluidized burner vessel and recycled to the reactor. Although this process can be easily scaled up, it is rather complex and the products are much diluted, which greatly complicates the recovery of the liquid products.
Many sources of organic matter can be used as feedstock for pyrolosis. Suitable plant material includes: greenwaste,, sawdust, waste wood, woody weeds; and agricultural sources including: nut shells, straw, cotton trash, rice hulls, switch grass; and poultry litter, dairy manure and potentially other manures. Pyrolysis is used as a form of thermal treatment to reduce waste volumes of domestic refuse. Some industrial byproducts are also suitable feedstock including paper sludge and distillers grain
There is also the possibility of integrating with other processes such as mechanical biological treatment and anaerobic digestion.
- syngas (flammable mixture of carbon monoxide and hydrogen): can be produced in sufficient quantities to both provide the energy needed for pyrolysis and some excess production
- solid char that can either be burned for energy or recycled as a fertilizer (biochar).
Destructive fires in buildings will often burn with limited oxygen supply, resulting in pyrolysis reactions. Thus, pyrolysis reaction mechanisms and the pyrolysis properties of materials are important in fire protection engineering for passive fire protection. Pyrolytic carbon is also important to fire investigators as a tool for discovering origin and cause of fires.
PLASMA or PLASMA ARC
Plasma arc gasification is a waste treatment technology that uses very powerful electrical energy creating extremely high temperatures by an electric arc. This is like a continuous lightning bolt and instantly breaks down all material into elemental gas and limited solid waste (slag), in a device called a plasma converter.
The process has been intended to be a net generator of electricity, depending upon the composition of input wastes, and to dramatically reduce the volumes of waste sent to landfills. Relatively high voltage, high current electricity is passed between two electrodes, spaced apart, creating an electrical arc. Inert gas under pressure is passed through the arc into a sealed container of waste material, reaching temperatures as high as 25,000 °F (13,900 °C) in the arc column. The temperature a few feet from the torch can be as high as 5,000–8,000 °F (2,760–4,427 °C).
At these temperatures, most types of waste are broken into basic elemental components in a gaseous form, and complex molecules are separated into individual atoms. The reactor operates at a slightly negative pressure, meaning that the feed system is complemented by a gaseous removal system, and later a solid removal system. Depending on the input waste (plastics tend to be high in hydrogen and carbon), gas from the plasma containment can be removed as syngas.
PLASMA or PLASMA ARC GASIFICATION
Some types of gasification use plasma technology, which generates intense heat to initiate and supplement the gasification reactions. Plasma gasification or plasma-assisted gasification can be used to convert carbon-containing materials to synthesis gas that can be used to generate power and other useful products, such as transportation fuels. In an effort to reduce both the economic and environmental costs of managing municipal solid waste, (which includes construction and demolition wastes) a number of cities are working with plasma gasification companies to send their wastes to these facilities. One city in Japan gasifies its wastes to produce power. In addition, various industries that generate hazardous wastes as part of their manufacturing processes (such as the chemical and refining industries) are examining plasma gasification as a cost-effective means of managing those wastes streams.
Plasma is an ionized gas that is formed when an electrical discharge passes through a gas. The resultant flash from lightning is an example of plasma found in nature. Plasma torches and arcs convert electrical energy into intense thermal (heat) energy. Plasma torches and arcs can generate temperatures up to 10,000 degrees Fahrenheit. When used in a gasification plant, plasma torches and arcs generate this intense heat, which initiates and supplements the gasification reactions, and can even increase the rate of those reactions, making gasification more efficient.
Inside the gasifier, the hot gases from the plasma torch or arc contact the feedstock, such as municipal solid waste, auto shredder wastes, medical waste, biomass or hazardous waste, heating it to more than 3,000 degrees Fahrenheit. This extreme heat maintains the gasification reactions, which break apart the chemical bonds of the feedstock and converts them to a synthesis gas (syngas). The syngas consists primarily of carbon monoxide and hydrogen—the basic building blocks for chemicals, fertilizers, substitute natural gas, and liquid transportation fuels. The syngas can also be sent to gas turbines or reciprocating engines to produce electricity, or combusted to produce steam for a steam turbine-generator.
Because the feedstock reacting within the gasifier are converted into their basic elements, even hazardous waste becomes a useful syngas. Inorganic materials in the feedstock are melted and fused into a glassy-like slag, which is non-hazardous and can be used in a variety of applications, such as roadbed construction and roofing materials.
Plasma technologies have been used for over 30 years in a variety of industries, including the chemical and metals industries. Historically, the primary use of this technology has been to decompose and destroy hazardous wastes, as well as to melt ash from mass-burn incinerators into a safe, non-leachable slag. Use of the technology as part of the waste-to-energy industry is much newer.
There are currently plasma gasification plants operating in Japan, Canada and India. For example, a facility in Utashinai, Japan has been in commercial operation since 2001, gasifying municipal solid waste and auto shredder waste to produce electricity. There are a number of proposed plasma gasification plants in the United States.
Benefits of Plasma Gasification
Plasma gasification provides a number of key benefits:
- It unlocks the greatest amount of energy from waste
- Feedstock can be mixed, such as municipal solid waste, biomass, tires, hazardous waste, and auto shredder waste
- It does not generate methane, a potent greenhouse gas
- It is not incineration and therefore doesn’t produce leachable bottom ash or fly ash
- It reduces the need for landfilling of waste
- It produces syngas, which can be combusted in a gas turbine or reciprocating to produce electricity or further processed into chemicals, fertilizers, or transportation fuels—thereby reducing the need for virgin materials to produce these products
- It has exceptionally low environmental emissions
Biomass includes a wide range of materials, including energy crops such as switch grass and agricultural all sources such as corn husks, wood pellets, lumbering and timbering wastes, yard wastes, construction and demolition waste, and bio-solids such as sewage sludge. Gasification helps recover the energy locked in these materials and can convert biomass to electricity and products, such as ethanol, methanol, fuels, and fertilizers.
Biomass gasification plants differ somewhat from the large-scale gasification processes typically used in major industrial facilities such as power plants, refineries, and chemical plants, although the differing types of gasification can easily be combined.
Biomass usually contains a high percentage of moisture which can be 25% (By Weight) in some cases. The presence of high levels of moisture in the biomass reduces the temperature inside the gasifier, which then reduces the efficiency of the gasifier. Therefore, many biomass gasification technologies require that the biomass be dried to reduce the moisture content prior to feeding into the gasifier. This can be an added benefit as the moisture can be taken out and processed into large quantities of deionized (Distilled) water. Pure water.
Most biomass gasification systems use air instead of oxygen for the gasification reactions. Gasifiers that use oxygen require an air separation unit to provide the gaseous/liquid oxygen; this is usually not cost-effective at the smaller scales used in biomass gasification plants. Air-blown gasifiers use the oxygen in the air for the gasification reactions.
Scale of plants
In general, biomass gasification plants are much smaller than the typical coal or petroleum coke gasification plants used in the power, chemical, fertilizer and refining industries. As such, they are less expensive to build and have a smaller facility “footprint”. While a large industrial gasification plant may take up 150 acres of land and process 2,500-15,000 tons per day of feedstock (such as coal or petroleum coke), the smaller biomass plants typically process 25-200 tons of feedstock per day and take up less than 10 acres.
Biomass to Ethanol and Liquid Fuels
Currently, most ethanol is produced from the fermentation of corn. Vast amounts of corn and land, water and fertilizer are needed to produce the ethanol. As more corn is being used, there is an increasing concern about less corn being available for food. Gasifying biomass, such as corn stalks, husks, and cobs, and other agricultural waste products to produce ethanol and synthetic fuels such as diesel and jet fuel can help break this energy-food competition.
Biomass, such as wood pellets, yard and crop wastes, and “energy crops” such as switch grass and waste from pulp and paper mills can be used to produce ethanol and synthetic diesel. The biomass is first gasified to produce the synthetic gas (syngas), and then converted via catalytic processes to these downstream products.
Biomass to Power
Biomass can be used to produce electricity—either blended with traditional feedstocks, such as coal or by itself. Nuon’s IGCC plant in Buggenum, Netherlands blends about 30% biomass with coal in their gasification process to produce power.
Cutting Costs, Increasing Energy
Each year, municipalities spend millions of dollars collecting and disposing of wastes, such as yard wastes (grass clippings and leaves) and construction and demolition debris. While some municipalities compost yard wastes, this takes a separate collection by a city which is an expense many cities just can’t afford.
Yard waste and the construction and demolition debris can take up valuable landfill space, shortening the life of a landfill. Many cities face a shortage of landfill space. With gasification, this material is no longer a waste, but a feedstock for a biomass gasifier. Instead of paying to dispose of and manage a waste for years in a landfill, using it as a feedstock reduces disposal costs, landfill space and converts the waste to power and fuels.
Benefits of Biomass Gasification
- Converting waste product into high value energy & products
- Reduced need for landfill space for disposal of solid wastes
- Decreased methane emissions from landfills
- Reduced risk of groundwater contamination from landfills
- Production of ethanol from non-food sources
Throwing Away Energy
Gasification can convert materials normally considered waste into energy and valuable products. In the U.S. alone thousands of tons of a potential source of energy are collected and thrown away each week. Most of the waste that we discard from our homes and businesses every day – such as non-recyclable plastics, construction debris, used tires, household trash, and sewage – contains energy. Gasification can convert the energy in all of this waste into electric power, substitute natural gas, chemicals, transportation fuels, and fertilizers.
Gasification is Not Incineration
Gasification is not incineration. Incineration is the burning of fuels in an oxygen-rich environment, where the waste material combusts and produces heat and carbon dioxide, along with a variety of other pollutants. Gasification is the conversion of feedstock into their simplest molecules – carbon monoxide, hydrogen and methane forming a syngas which then can be used for generating electricity or producing valuable products.
250 Million Tons/Year of Municipal Solid Waste
According to the U.S. Environmental Protection Agency, each year Americans generate about 250 million tons of municipal solid waste (MSW) – about 4.5 pounds per person per day. This MSW includes a wide variety of wastes, including kitchen and yard waste, , electronics, light bulbs, plastics, used tires, and old paint. Despite significant increases in recycling and energy recovery, only about one-third of the total MSW is recovered – leaving the remaining two-thirds (or 135 million tons/year) to be dumped into landfills or incinerated. These figures do not include the 7.2 million dry tons of biosolids from wastewater treatment, much of which is also landfilled or incinerated.
Cities and towns spend millions of dollars per year to collect and dispose of MSW wastes in landfills – using thousands of acres of land. Many states have banned incinerators and a number of states, such as New York, New Jersey, Massachusetts, Connecticut, California and Florida are faced with limited landfill space, forcing them to transport their MSW hundreds of miles for disposal in other states.
In addition to consuming valuable land, the decomposing MSW generates methane, a greenhouse gas, and the leaching wastes may also pose a threat to the groundwater. However, there is an alternative to putting this waste in a landfill – it can be converted through gasification to useful products.
Billions of Tons of Industrial Waste Every Year
American industrial facilities dispose of 7.6 billion tons of industrial solid waste per year. This waste includes plastics and resins, chemicals, pulp and paper. In addition, the debris generated during construction, renovation and demolition of buildings, houses, roads and bridges adds another 136 million tons/year. (source: U.S. EPA)
Much of this industrial waste is also suitable for gasification. For example, the construction and demolition waste can be gasified to produce power and products. The non-recyclable industrial plastic wastes can also be gasified.
THE WASTE GASIFICATION PROCESS
From Waste to Energy and Valuable Products
All of this waste contains unused energy. Instead of discarding that energy source, gasification can convert it to electric power and other valuable products, such as chemicals, substitute natural gas, transportation fuels, and fertilizers. On average, waste-to-energy plants that use mass-burn incineration can convert one ton of MSW to about 550 kilowatt-hours of electricity. With gasification technology, one ton of MSW can be used to produce up to 1,000 kilowatt-hours of electricity, a much more efficient and cleaner way to utilize this source of energy. Industrial waste also contains a large source of untapped energy. For example, the energy content of wood construction and demolition waste is about 8,000 Btu/lb and about 10,000 Btu/lb for non-recyclable industrial plastics.
MSW gasification faces a number of challenges. Because MSW can contain such a wide variety of materials, the materials may need to be sorted to eliminate those items that cannot be readily gasified or that would harm the gasification equipment. In addition, the gasification system may need to be designed to handle a variety of different materials because these materials may be gasified at different rates.
Further, one of the important advantages of gasification is that the syngas can be cleaned of contaminants prior to its use, eliminating many of the types of after-the-fact (post-combustion) emission control systems required by incineration plants. Technologies used in waste gasification include conventional gasification systems, as well as plasma arc gasification. Whether generated from conventional gasification or from plasma gasification, the syngas can be used in reciprocating engines or turbines to generate electricity or further processed to produce substitute natural gas, chemicals, fertilizers or transportation fuels, such as ethanol. Read more about the products of gasification.
Gasification Does Not Reduce Recycling Rates
Gasification does not compete with recycling. In fact, it enhances recycling programs. Materials can and should be recycled and conservation should be encouraged. However, many materials, such as metals and glass, must be removed from the MSW stream before it is fed into the gasifier. Pre-gasification feedstock processing systems are added up-front to accomplish the extraction of metals, glass and inorganic materials, resulting in the increased recycling and utilization of materials. In addition, a wide range of plastics cannot be recycled or cannot be recycled any further, and would otherwise end up in a landfill. Such plastics are an excellent, high energy feedstock for gasification.
In addition, not all cities or towns are set up to collect and process recycled materials. And, as populations grow, the amount of waste generated grows. So even as recycling rates increase, the amount of waste is increasing at a greater rate. All of this waste represents lost energy and economic value – which gasification can capture.
- Gasifying waste has a number of significant environmental benefits:
- Reduces need for landfill space
- Decreases methane emissions
- Reduces risk of groundwater contamination from landfills
- Extracts useable energy from waste that can be used to produce high value products
- Enhances existing recycling programs
- Reduces use of virgin materials needed to produce these high value products
- Reduces transportation costs for waste that no longer needs to be shipped hundreds of miles for disposal
- Reduces use of fossil fuels