In the last blog entry it was mentioned that plasma-arc plasma gasification is just one type of gasification. Other common forms include (1) updraft, (2) downdraft, (3) fixed bed and (4) fluidized bed. The first two are quick similar with exception of the gas flow. This entry looks to discuss in slightly more in depth the differences between these types.

In the updraft (sometimes referred to as “counter current”), air/oxidant is injected from the bottom and the material enters at the top. Following gravity, the material is dried then reduced to char (pyrolyis) and finally any ungasified solid remnant is burned. This type has a high energy efficiency because of the heat exchange between the rising gas and descending material. The main issue is the high concentration of oils and/or tars in the syngas, which must be cleaned prior to any utilization, which can decrease the overall efficiency. Updraft gasifier usage is generally focused to direct heating applications as little to no gas cleaning is necessary. PEAT’s plasma-arc plasma gasification process is configured similarly in that the feedstock material is fed from the top, however it also has similarities to a fluidized bed (see below) in that the vitrified material in PEAT’s plasma-arc plasma gasification is maintained in a somewhat molten form.

In a downdraft gasifier (or co current), the gas is drawn out from below through a combustion zone. The material and oxidant flow in the same direction. Compared to the updraft configuration, the gas tends to be cleaner with fewer tars – the reason being in a downdraft configuration the tars (product from any pyrolysis phase) have to pass through a higher-temperature oxidization zone. The key I in a downdraft design is temperature maintenance and if the feedstocks vary in composition and moisture content, this can be difficult to achieve. The gas must rapidly cooled prior to be used.

Finally, a fluidized bed, which is more common in large plants, has a hot bed of sand located at the bottom of the gasifier. Gas clean-up is key as many times tar and ash can be found in the syngas. Waste is typically shredded or pulverized prior to being fed from the top.

Plasma-arc plasma gasification is a phrase heard often when discussion hazardous waste treatment or waste-to-energy technologies, however this entry looks to give a closer look as to what plasma-arc plasma gasification is and its associated reactions. Plasma-arc 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-arc 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 plasma-arc plasma 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-arc plasma gasification reactor (see first reaction above). The heat released in the above exothermic reactions provide additional thermal energy for the primary plasma-arc plasma gasification reaction (endothermic formulas above) to proceed very rapidly.

At higher temperatures (around 3,600°F) the endothermic reactions are typically favored. Some plasma-arc companies (not PEAT however) introduce supplementary fuels such as coal, petroleum coke or even other hot gases generated by plasma torches (sometimes referred to as plasma-assisted gasification) to maintain the desired plasma-arc plasma gasification temperatures in the reactor.

Additionally, plasma-arc plasma gasification currently appears to be the option being promoted most widely for larger scale waste-to-energy applications mainly because of its ability to produce the syngas from which energy can be recovered in high efficiency recovery units so offsetting the high energy requirements of plasma-arc plasma gasification.

The reducing atmosphere within the plasma-arc 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.)

Northbrook, Illinois & Shanghai, China – October 10, 2013 – PEAT International, Inc., (“PEAT”) a leader in plasma-thermal waste destruction systems, announced the successful commissioning of a Plasma Thermal Destruction and Recovery (“PTDR”) system in Shanghai, China. The 60 kg/hr system – designed for medical waste and oil refinery sludge – was installed for Abada Plasma Technology Holdings, Ltd. – an Asian-based renewable energy project developer.

PEAT’s PTDR “single stage” plasma-thermal process transforms hazardous waste through molecular dissociation at 1,500°C (2,732°F) into recoverable, non-toxic end-products, synthetic gas and heat (sources for energy recovery), metals and a vitrified glass matrix. Emissions are below the most stringent environmental standards used anywhere.

“This is end-stage technology and sets the standard for clean hazardous waste remediation. Only with plasma can you achieve temperatures high enough for waste destruction in a single-staged process,” said Joseph Rosin, PEAT International Chairman. “It’s a 21st century solution that addresses three important needs: significant volume reduction, full pollution control and competitive pricing. We are currently preparing for other projects already under contract.”

PTDR systems are in operation in California, Taiwan and China. Go to http://www.peat.com/chinasystem.html for a video of operations and acceptance test run data.

About PEAT International

PEAT International, Inc., headquartered in Northbrook, Illinois, with offices in China, Taiwan and India, is a waste-to-energy (“WTE”) company with its two proprietary technologies – the Plasma Thermal Destruction and Recovery™ (“PTDR”) technology for the treatment and recycling of industrial, medical and other hazardous waste streams and the Thermal Volume Reduction & Conversion™ (“TVRC”) technology for municipal solid waste. For more information, contact Daniel Ripes, dripes@peat.com, at 847-559-8567 and visit www.peat.com.

As a pure waste-to-energy technology, the TVRC competes with mass burn incinerators, anaerobic digesters as well as traditional gasification systems within the MSW marketplace. While mass burn incinerators are the most popular still to this day, they generate significant amounts of fly ash, which in the near term could become even more expensive to treat as in June 2010, the US EPA started to consider classifying fly ash (CCRs) as a hazardous waste. Should this be the final directive, it will significantly impact how fly ash is ultimately handled.

While MSW applications have started to emerge using gasification, it is important to note that a significant amount of sorting and pre-treatment is required in order to make the feedstock more uniform in nature to maintain the steady flow and composition of the syngas generated. Additionally, the TVRC generates electricity at a much lower cost.

 
Finally, the TVRC generates significantly more electricity per ton of MSW versus some of the emerging technologies. Referencing back to one of the aforementioned proposed plasma gasification and plasma arc projects in Florida, the $120 million 600 TPD system was marketed to generate about 42 MW, of which only 18 MW would be sold to the grid.

A TVRC waste-to-energy & waste to resource system also represents the most efficient land usage when compared to other renewable energy options.

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

The high temperatures at which the plasma-arc 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-arc plasma gasification plume as these compounds may become volatilize and carried downstream with the syngas generated. While many are removed by the gas cleaning and conditioning systems, in plasma-arc 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-arc plasma gasification waste-to-energy systems where the syngas was not utilized and ultimately processed in a thermal oxidizer or secondary reaction chamber.