Gasification can convert materials, normally considered waste, into energy and valuable products. Every city produces thousands of tons of solid waste per day. For example; a small South African town like Krugersdorp produces 500 tons of Municipal Solid Waste per day while a larger city would produce on average over 7000 tons of MSW per day. 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. 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 can then be used for generating electricity or producing 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 18600 kJ/kg and about 23000 kJ/kg 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.
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 kilometers/miles for disposal
- Reduces use of fossil fuels
Information with thanks to the “Gasification Technologies Council”
AfriPlasma Plasma Gasification System (below), exposes “feedstocks” such as municipal solid waste, industrial waste or biomass to temperatures over 5,000°C in the presence of controlled amounts of steam, air and oxygen.
The feedstock reacts in the gasifier with the steam, air and oxygen to produce a synthesis gas (syngas) and slag. Syngas, composed primarily of carbon monoxide and hydrogen, can be used for industrial purposes as a substitute for natural gas.
The system uses plasma torch technology, where a plasma stream is created by the interaction between air and an electric arc created between two electrodes. The interaction of the gas with the electric arc dissociates the gas into electrons and ions enabling the gas to become electrically and thermally conductive.
Feedstock materials enter the gasifier through a feed port which can be located on the side of the reactor or at the top. The feedstock either gasifies immediately upon entering the reactor or falls on to the coke bed where complete gasification occurs.
Non-gaseous, inorganic components in the gasified feedstock, such as rocks, dirt and other impurities, separate and leave through the bottom of the gasifier as a glass-like slag. This slag, which is environmentally benign, can be sold as an aggregate to the construction industry.
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 (2 kg) 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.