As discussed earlier in this blog, a vitrified matrix or slag is the primary solid byproduct of plasma arc waste-to-energy processing. The vitrified matrix from plasma arc processing contains the mineral matter associated with the feed materials in a vitrified form – a hard, glassy-like substance. The amount of matrix produced is a function of how much non-combustible mineral matter is present in the feedstock.

This matrix is the result of operating temperatures within the plasma arc reactor above the melting temperature of the mineral matter. Under these conditions in the plasma arc reactor, non-volatile metals and metal oxides bind together in molten form until it is cooled via natural heat loss or via a pool of water, where it would fracture and granulate.

The compressive strength of a slag sample generated from fly ash from coal-fired power plant as well as some sodium carbonate (fluxing agent) was 480 kg/cm2, while its average mortar strength was tested at 169 kg/cm.

The vitrified matrix or slag generated by plasma arc treatment is primarily made up of silicon dioxide (SiO2), aluminum oxide (Al2O3) and calcium oxide (CaO). Toxicity Characteristic Leaching Procedure (TCLP) tests are designed to determine the mobility of both organic and inorganic analytes present in the slag. The most recent TCLP results on the vitrified matrix from the plasma arc waste-to-energy system located at a China refinery is presented in the below table.

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There are a number of perceptions within the marketplace regarding plasma arc treatment and plasma arc gasification processes. Two weeks ago, this blog addressed the smaller physical footprint with regards to plasma arc gasification waste-to-energy systems.

This week we discuss the idea/claim regarding the plasma arc technology’s ability to generate significant useable recyclable end-products and energy with no residual waste.

Certainly this depends on the waste feedstock; however it is worth noting that if metals and glass are processed simultaneously in a plasma arc system, additional processing would be required to separate out these products for any re-use potential. The re-use of the vitrified slag product generated from to plasma arc systems has been demonstrated commercially in France and Japan.

Depending on the feedstock and moisture content and plasma utilization (combustion vs. plasma gasification/ plasma pyrolysis), to plasma arc gasification waste-to-energy systems can require significant amounts of energy to operate as such the net energy claims made by some within the industry may be overly optimistic in theory and largely unproven at this point in time in commercial operations. Criticizing the electrical loads associated with processing municipal solid waste in a plasma gasification waste-to-energy system may be valid as the primary goal would be net energy production, however when it comes to industrial, hazardous and universal waste treatment, the primary goal is destruction efficiency and thus the electrical consumption/generation should be considered secondary. Energy balances associated with to plasma-arc gasification waste-to-energy systems should to be reviewed on a project-by-project basis, rather than at a macro level within the industry.

There are a number of perceptions within the marketplace regarding plasma-arc treatment and plasma-arc gasification processes. This stems from limited operating systems as well as “too-good-to-be-true” marketing claims made by some within the industry, some of these key points include:

o Ability to process a wider range of feedstocks that other thermal treatment technologies
o Smaller environmental and physical footprints
o Generate significant usable recyclable end-products and energy with no residual waste

Over the next few weeks, this blog will look to address each of these. With regards to the “Ability to process a wider range of feedstocks that other thermal treatment technologies”

As illustrated here , PEAT’s plasma-arc gasification waste-to-energy technology has been implemented treating a wide range of feedstocks. Further, the fact that the plasma-arc gasification heat source is independent of the waste being processed means that plasma-arc gasification systems can process waste with very low calorific values (i.e. high moisture and inorganics). Further, plasma-arc gasification systems can co-process a variety of waste streams simultaneously.

Following are the Peat’s Waste-To-Energy Systems / Waste Treatment Plants which has ability to process a wide range of feedstocks with Plasma Thermal Destruction Recovery (PTDR) Technology and convert waste streams into useful end-products (Syngas, Vitrified Glass, Concrete Aggregate, Road Bed/Fill, Sandblasting , and Recovered Metal Alloys).

PTDR-100
Waste Treatment Plant

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PTDR-500
Waste Treatment Plant

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PTDR-1000
Waste Treatment Plant

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Dioxins are an issue often cited in the marketing literature of many plasma gasification waste-to-energy technology suppliers as an area where plasma gasification may be superior to other thermal waste processing options. Studies have shown the majority of dioxins are formed within the cooler regions of processes via flyash catalyzed processes, involving chlorine and organic compounds (usually called products of incomplete combustion) in so called de-novo synthesis reactions.

It has been demonstrated (see below table) that the higher temperatures from PEAT’s plasma thermal destruction and recovery waste-to-energy process provides for substantial conversion of the organic constituents of the waste and therefore significantly reduces the likelihood of downstream dioxin formation. (There is some credence in the claims that the reducing conditions present in plasma gasification processes could minimize dioxins as the precursor formation reactions usually require excess oxygen).

Dioxins form when all of the following constituents present: carbon, hydrogen, chlorine, and oxygen in appropriate quantities. Once all these elements are present in sufficient quantities, the temperature must also be high enough to promote the formation of such a complex compound, and not so high that the molecules formed become unstable. This temperature zone has been widely estimated to be between 200°C and 450°C. However temperature is not the only mitigating factor as there could be dioxin precursors in the off-sygas/pre-cleaned syngas leaving the plasma gasification reactor thus PEAT’s plasma thermal destruction and recovery waste-to-energy systems provide for rapid quenching of the gas (i.e Venturi quench). This is to avoid the de-novo synthesis temperature window.

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A vitrified matrix or slag is the primary solid byproduct of plasma arc waste-to-energy processing. The vitrified matrix from plasma arc processing contains the mineral matter associated with the feed materials in a vitrified form – a hard, glassy-like substance. The amount of matrix produced is a function of how much non-combustible mineral matter is present in the feedstock.

This matrix is the result of operating temperatures above the melting temperature of the mineral matter. Under these conditions in the plasma arc reactor, non-volatile metals and metal oxides bind together in molten form until it is cooled via natural heat loss or via a pool of water, where it would fracture and granulate.

The compressive strength of a slag sample generated from fly ash from coal-fired power plant as well as some sodium carbonate (fluxing agent) was 480 kg/cm², while its average mortar strength was tested at 169 kg/cm. A report issued by Geo Engineering Services of Baroda, India, who was testing the slag for its physical suitability for use as coarse grained material in the sub-base layers of pavement, indicated that the “slag can be effectively used in the lean concrete (up to M10) as a sand replacement” Slag from plasma arc treatment is typically used in construction or road asphalt.

The vitrified matrix or slag generated by plasma arc treatment is primarily made up of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃) and calcium oxide (CaO). Toxicity Characteristic Leaching Procedure (TCLP) tests are designed to determine the mobility of both organic and inorganic analytes present in the slag. The results from various plasma arc waste-to-energy process testing is presented in the below table.

It is not unreasonable to presume that vitrified slag from the plasma arc treatment of hazardous waste will have far superior leaching characteristics that incinerator bottom ash in comparable leaching tests.

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