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.

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 conducted in March 2013 on the vitrified matrix from the plasma arc waste-to-energy system located in Shanghai is presented in the below table along with previous results from processing refinery sludge.

Also, here are recent pictures of this system.

http://www.peat.com/ptdr_pictures.html

As you may know from reading this blog, in plasma arc, plasma gasification, waste is broken down at temperatures around 1,500°C. While this form of gasification can be energy intensive, it ensures that the plasma gasification syngas produced is cleaned of residue tar, which makes it a higher value product. Another major benefit of plasma arc, plasma gasification is that it creates little to no emissions. This blog entry attempts to take a quick look at some of the other waste to energy technologies available.

Conventional gasification slightly differs from plasma arc, plasma gasification in that the temperatures inside its primary reactor are lower and the fuel source may be natural gas. However conventional gasification is similar to plasma arc, plasma gasification in that both are thermo-chemical processes in which waste is heated in an oxygen deficient environment to produce syngas which contains hydrogen, carbon monoxide and sometimes methane. This gas can be used as fuel for electricity generation or to produce chemicals or biofuels.

Pyrolysis is sometimes linked to plasma arc, plasma gasification. Pyrolysis is also a thermo-chemical process, however here the waste is heated in the complete absence of oxygen. The products are olefin liquid, syngas-type product, and char. The liquid fuel can be used as an input to produce gasoline, while the char can be recovered or passed along to a gasification process.

Other waste to energy alternatives include landfill gas recovery that convert methane gas from decomposing trash in to power. Methane is collected through a series of pipes and compressed for electricity generation. There are also waste to biofuels and waste to chemical processes. These are similar to aforementioned waste to energy technologies as they may utilize gasification or other thermo-chemical processes. Appropriate waste feedstocks include soybean straw, wood waste and used oils/fats.

Finally, there is anaerobic digestion (AD). This differs greatly from the other waste to energy technologies mentioned here in that AD uses microorganisms to convert organic waste into a biogas (primarily methane) and digestate. This biogas can be used to generate electricity, while the digestate has applications as a fertilizer. Anaerobic digestion is limited to wet organic wastes (including manure and sewage) or food waste. As such it requires that these materials to be separated from regular waste.

Many contest that the primary goals for waste management; to reduce, reuse and recycle, and increasing waste conversion (i.e. waste-to-energy) rates are not compatible. However, in the United States, the states making the most use of waste-to-energy facilities are also those that recycle the most.

In addition, according to a recent study conducted by the EPA, increasing recycling wastes actually improves the efficiency of waste conversion (i.e. waste-to-energy).

Consumers are increasingly recycling more biogenic waste (paper and food) and throwing away more non-biogenic waste (rubber and plastics).

The higher energy content of non-biogenic waste makes it a more productive feedstock for generating electricity through a waste-to-energy technology such as the TVRC. Conversely to previously held views then, recycling is not just compatible with waste conversion, it actually improves the energy content of the leftover waste, boosting the potential of key waste-to-energy technologies, including plasma-arc plasma gasification.

According to the US EPA, in 2010 the US produced almost 250 million tons of municipal solid waste (MSW), of which only 12% was diverted towards waste-conversion (example: waste to energy) facilities. This generated approximately 14 million megawatt hours of electricity.

Landfilling is still the largest single means of trash disposal as more than half of all MSW produced in the US was sent to landfill in 2011 (The average American produces 4.4 pounds of waste per day with landfill diversion targets becoming more widespread and stringent). Diverting waste from a landfill to generate value from it is in itself a compelling reason to invest in waste conversion and/or waste to energy, however it also reduces greenhouse gas emissions.

In 2009, 17% of all human-related methane emissions in the US came from landfills. Further, the scarcity of land around urbanized areas means some municipalities are forced to transport waste long distances for disposal. For example, New York’s Department of Sanitation spends in excess of $300 million per year moving waste by truck to landfill and waste disposal facilities outside of the city.

PEAT’s TVRC is an innovative waste to energy technology that combines a thermal volume reduction (“TVR”) system on the front end with a core plasma-arc, plasma gasification PTDR system on the back-end for ash treatment. This combination maximizes electricity generation and minimizes residual by-product treatment.

Finally, in 2007, the EPA stated that waste to energy facilities comply with stringent air emissions standards and produce electricity with less environmental impact than almost any other source of electricity.