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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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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Through PEAT’s India subsidiary, a Special Purpose Vehicle has been formed to support a PTDR-1000 plasma gasification waste-to-energy project within the Ankleshwar Industries Association to process a range of industrial waste and hazardous waste. A project site has been set aside by the local industrial development corporation and the official application to secure the land is expected later this year for the plasma gasification waste-to-energy project.

Further, the Environmental Clearance (EC) for the plasma gasification waste-to-energy project was granted in November 2009 and now the focus is on the process of securing the No Objection Certificate (NOC) from the Government of Gujarat.

The plasma gasification waste-to-energy project includes the utilization of a portion of the syngas produced to dewater an additional 15 metric tons of aqueous organic wastes (brining gate capacity to 45 metric tons per day).

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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As mentioned earlier in this blog, PEAT advancing a project for a client in China to treat refinery sludge and petrochemical waste. The system construction is finished and the system is currently en route to China. This plasma gasification system is expected to be commissioned during the later part of July.

Unlike incineration, where combustion gases are treated at atmospheric pressure, the volume of syngas that is generated in a plasma gasification system is significantly reduced. The smaller flue gas volumes also bring benefits in terms of the scale of downstream air pollution control equipment.

Here is a representative syngas composition, composition and heat value for processing 500 lbs/hr of petroleum sludge and fluxing agents with the following chemical composition (Carbon: 30.85%, Hydrogen: 3.18%, Chlorine: 0.65%, Oxygen: 0.41%, Sulfur 0.31%, water 23.82%, metals 0.17%, inorganics 40.54%) in a plasma thermal destruction and recovery system.

The syngas generated from a plasma gasification system could be converted into a variety of different fuels such as ethanol, natural gas (methane) and hydrogen or can be used to generate electricity directly similar to gasification. Some companies are trialing systems for upgrading syngas from their plasma gasification processes to be used as a chemical feedstock for products such as hydrogen recovery. However, the commercial status of such projects indicates that these are still in the development stage.