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 gasification 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 inplasma gasification processescould 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 gasification 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.

It has been discussed previously within this forum that plasma-arc, plasma gasification systems, such as PEAT’s plasma thermal destruction and recovery (PTDR) systems, have the ability to process a wider range of feedstocks that other thermal treatment technologies largely because the heat source is independent of the waste being processed. This means that plasma-arc plasma gasification systems can process industrial hazardous waste feedstocks with very low calorific values (i.e. high moisture and inorganics). Further, plasma systems can co-process a variety of waste streams simultaneously.

With this said when a feedstock has a higher moisture and/or inorganic material content, the required plasma heating power from the plasma arc increases. Simply put, waste streams with 30% moisture content vs. ones with 10% moisture require more plasma-arc power (i.e. more electricity) to ensure a complete destruction of the waste feedstock.

For example, here is a representative composition for medical waste treatment (infectious red bag) in Asia, reflecting a moisture content of 18%.

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Based on modeling exercises and experience, the plasma-arc power required in a PTDR-100 system is 80 kWe (based on 75% electrical-to-thermal efficiency). This plasma-arc power is required to vaporize the moisture required for medical waste treatment.

Now, here is a representative composition for medical waste treatment from the United States, reflecting a moisture content of 8.4% (we have been told the differences between US and Asia are related to how the medical waste is ultimately sorted).

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Based on modeling exercises and experience, the required plasma-arc power required in a PTDR-100 system is under 5 kWe (based on 75% electrical-to-thermal efficiency).

The medical waste treatment streams are largely the same except for the moisture content; however the required plasma-arc plasma gasification power is reduced by over 93%!

An important reminder, plasma gasification is a thermal chemical conversion process designed to optimize the conversion of waste into the syngas. The chemical reactions take place under oxygen starved conditions. The ratio of oxygen molecules to carbon molecules can be less than one in a plasma gasification reactor (sometimes a stoichiometric amount of oxygen to achieve pyrolysis).

The following simplified chemical conversion formulas describe some of the thermo-chemical processes that are typically occurring in gasification.

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Some of the waste undergoes partial oxidation by precisely controlling the amount of oxygen fed into the plasma-arc reactor (see first reaction above). The heat released in the above exothermic reactions provides additional thermal energy for the primary gasification reaction (endothermic formulas above) to proceed very rapidly.

At higher temperatures (around 3,600°F) the endothermic reactions are typically favored as such in a PTDR reactor only equations 1-3 are seen.

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. Last week, this blog addressed the idea/claim regarding the plasma arc technology’s ability to generate significant useable recyclable end-products and energy with no residual waste.

This week we address some aspects of the environmental footprint associated with plasma arc gasification processes

The high temperatures within plasma arc gasification reactors do facilitate higher thermal destruction with regards to volatizing organic material in the feedstock and breaking them down to simple molecules, however some plasma arc gasification systems require a secondary reactor or cracking stage to accomplish this breakdown indicating that not all reactions occur within the “blackbox.”

With regards to lower emissions claims, specifically dioxins for example, the high temperature within the plasma arc gasification reactors, while important, alone does not ensure little to no dioxin formation, the rapid cooling of the syngas as it leaves the plasma arc gasification reactor is equally important to ensure these complex compounds do not reform.

As discussed in this blog entry:

Plasma Arc Gasification and Wastewater Other Residuals

http://www.peat.com/blog/plasma-arc-gasification-and-wastewater-other-residuals/

other by-products generated during the gas cleaning and conditioning require proper handling. In some plasma arc gasification process configurations, it is feasible to re-inject the by-products collected/generated during this stage into the plasma arc gasification reactor to be vitrified, however this requires planning during the early design stages. If these by-products cannot be re-fed, then secondary treatment would be mandatory.

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.