Gasification converts MSW to a usable synthesis gas, or syngas. It is the production of this intermediate product, syngas, which makes gasification so different from incineration. In the gasification process, the MSW is not a fuel, but a feedstock for a high temperature chemical conversion process.
Instead of producing just heat and electricity, as is done in a waste-to-energy plant using incineration, the syngas produced by gasification can be turned into higher value commercial products such as transportation fuels, chemicals, fertilizers, and even substitute natural gas. Incineration cannot achieve this.
One of the concerns with incineration of MSW is the formation and reformation of toxic dioxins and furans, especially from PVC-containing plastics.
These toxins end up in incinerator exhaust streams by three pathways:
Incineration does not allow control of these processes, and all clean-up occurs after combustion. 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 in incineration plants. The clean syngas can be used in reciprocating engines or turbines to generate electricity or further processed to produce hydrogen, substitute natural gas, chemicals, fertilizers or transportation fuels, such as methanol.
The ash produced from gasification is different from what is produced from an incinerator. While incinerator ash is considered safe for use as alternative daily cover on landfills, there are concerns with its use in commercial products.
In high-temperature gasification, the ash actually flows from the gasifier in a molten form, where it is quench-cooled, forming a glassy, non-leachable slag that can be used for making cement, roofing shingles, as an asphalt filler or for sandblasting. Some gasifiers are designed to recover melted metals in a separate stream, further taking advantage of the ability of gasification technology to enhance recycling.
There are many types of gasifiers for waste gasification. These gasifiers vary in size and the type of MSW that they can gasify. Some gasifiers are designed to gasify construction and demolition debris, others are for MSW.
Many gasifiers require some type of pre-processing of the MSW to remove the inorganic materials (such as metals and glass) that cannot be gasified. Some gasifiers require the shredding, drying and sizing of the feedstock before it is fed into the gasifier.
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 10,000ºF (5,500ºC). 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 safely decompose and destroy hazardous wastes, as well as to melt ash from mass-burn incinerators into a safe, non-leachable slag.
Use of plasma technology for gasification is much newer. While other gasifier feedstocks, such as coal and biomass, are relatively homogeneous and easily gasified, MSW and other wastes are typically eterogeneous and may be difficult to gasify. The high temperature of a plasma gasifier initiates and supplements the gasification reactions, and can even increase the rate of those reactions, making gasification more efficient.
The high energy input from the plasma arcs or torches can be easily increased or decreased as the quantity and quality of the MSW fed into the gasifier changes. The high temperature maintains the gasification reactions, which break apart the chemical bonds of the feedstock and converts them into syngas. Some MSW gasification systems use conventional gasification technology to convert the feedstock into syngas, and then treat the syngas stream with plasma arcs or torches to ensure the efficient breakdown of any unconverted compounds into 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 feedstocks reacting within the gasifier are converted into their basic elements, even hazardous wastes can be converted into a useful syngas. Inorganic materials in the feedstock are melted and fused into a glassy-like slag, which is nonhazardous and can be used in a variety of applications, such as roadbed construction and roofing materials.
Dozens more plasma gasification plants are being developed worldwide, particularly where landfill costs are high, and the renewable electricity generated from the gasification facility has a high value.