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.

Plasma pyrolysis and plasma-arc plasma gasification, like incineration, are options for recovering value from waste by thermal treatment. Both pyrolysis and plasma-arc plasma gasification convert feedstocks/wastes into energy by heating the waste under controlled conditions. Whereas incineration converts the input waste into a combusted flue-gas that can then be used to recover thermal energy (usually in the form of steam) and ash, pyrolysis and plasma-arc plasma gasification deliberately limits the conversion so that combustion does not take place directly. Instead, they convert the waste into potentially valuable intermediates that can be further processed for materials recycling or energy recovery. Pyrolysis and plasma-arc plasma gasification offer more scope for recovering products from waste than incineration.

One of the benefits associated with plasma-arc plasma gasification is that plasma-arc plasma gasification reactors do not require moving grates and the smaller volume of gases generated means that the plasma-arc plasma gasification reactors can accommodate the required minimum residence times in a smaller volume. Further, the smaller gas production and reducing environment within plasma-arc plasma gasification reactors does facilitate smaller sized air pollution control systems.

The composition of the end-products varies with the composition of the waste being processed in a PTDR plasma-arc plasma gasification system. For example, processing medical waste in PEAT’s plasma gasification system, with a relatively high percentage of paper and plastic or pharmaceutical manufacturing waste with high levels of carbon-based constituents would produce meaningful levels of syngas, and a lesser amount of recoverable metal and glass product. Conversely, processing ash from an incinerator via plasma-arc would produce lower amounts of syngas and relatively more vitrified product (containing metal oxides) and potentially recovered metal alloys.

Plasma can be described as an electrically-charged gas where a specific amount of energy is added to separate the molecules into a collection of ions, electrons and charge-neutral gas molecules. Plasma indicates a gas volume with sufficient energy supplied (electromagnetic, electric and/or thermal) so that electrons that normally exist in specific numbers and at distinct energy level orbiting around the nucleus are freed from their orbital bonds. This plasma, with its constituents of individual molecules and electrons acts as a conductor of electricity, the resistance of which converts electrical energy to heat.

Plasma-arc systems have been widely used for destruction of hazardous wastes. This extreme heat from the plasma-arc breaks down wastes, forming synthesis gas (hydrogen and carbon monoxide) and a rock-like solid byproduct called slag. The significant difference between pure plasma-arc plasma gasification systems (like PTDR systems) and other thermal waste processing technologies is that the heat required for waste degradation is generated by the plasma-arc itself and not via combustion of all or part of the waste.

PTDR plasma-gasification systems derive its energy from graphite plasma-arc electrodes thus wastes with little or no calorific value can be effectively and efficiently treated. Graphite plasma-arc electrodes are more effective than plasma-arc torches (typically marketed by other plasma-based companies) in that they reduce capital costs versus plasma-arc torches and have significantly higher electric-to-thermal energy conversion efficiencies (90-95% vs. 65-70%) thereby reducing operational costs when compared to plasma-arc torches

Plasma gasification is a phrase heard often when discussion plasma-arc treatment or waste-to-energy technologies, however this entry looks to give a closer look as to what plasma gasification is and its associated reactions. Plasma gasification is a thermal chemical conversion process designed to optimize the conversion of waste into the synthetic gas or (“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.

Equation 1. C (fuel) + O2 –> CO2 + heat (exothermic)
Equation 2. C + H2O (steam) –> CO + H2 (endothermic)
Equation 3. C + CO2 –> 2CO (endothermic)
Equation 4. C + 2H2 –> CH4 (exothermic)
Equation 5. CO + H2O –> CO2 + H2 (exothermic)
Equation 6. CO + 3H2 –> CH4 + H2O (exothermic)

Some of the waste undergoes partial oxidation by precisely controlling the amount of oxygen fed into the plasma reactor (see first reaction above). The heat released in the above exothermic reactions provide 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. Some plasma companies (not PEAT however) introduce supplementary fuels such as coal, petroleum coke or even other hot gases generated by their plasma torches (sometimes referred to as plasma-assisted gasification) to maintain the desired plasma gasification temperatures in the reactor.

The reducing atmosphere within the plasma gasification reactor avoids the formation of oxidized species such as sulfur dioxide (SO2) and nitrogen oxide (NOx). Instead, sulfur and nitrogen (organic-derived) in the feedstock are primarily converted to hydrogen sulfide (H2S) and nitrogen. Finally, typical halogens in the feedstock are converted to inorganic acid halides (HCl, HF, etc.)