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.

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.

Last week, we discussed and demonstrated how plasma-arc gasification nearly eliminates dioxin formation, this entry looks to address semi-volatile heavy metal compounds and other air emissions.

The high temperatures at which plasma gasification processes operate can result in the generation of volatile inorganic constituents (i.e. metals and metal oxides), sometimes at a higher level than compared to convention thermal treatment processes, particularly if the waste feedstock comes in direct contact with the very hot plasma gasification plume as these compounds may become volatized and carried downstream with the syngases generated. While many are removed by the gas cleaning and conditioning systems, in plasma gasification processes where the off gases are not cooled (i.e. plasma combustion, which is not utilized by PEAT) these heavy metal compounds could be carried out in the stack gases, increasing the levels of potential contaminants that are emitted.

Downstream of any quench system or syngas cooler, any entrained particulate matter and/or acid gases (H2S, HCl, etc.) are scrubbed with water typically using either a packed-bed tower/Venturi scrubber or through a dry filtration system. Additional equipment in the form of HEPA or baghouse filters may also be utilized.

The results presented in the below reflect emissions from PEAT International plasma gasification waste-to-energy systems where the syngas was not utilized and ultimately processed in a thermal oxidizer or secondary reaction chamber.

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