The Myth of Plasma Arc Technology in Regards to Optimally Processing MSW

I will shed some light to the actual description of Plasma Arc Technology, and its inter-related attributes, both positive and negative. Hereinder, I will give some insight to feedstock (Municipal Solid Waste (MSW) and Automobile Shredded Residue (ASR)) and Plasma Arc Technology:

 

Municipal Solid Waste (MSW)

 

Municipal Solid Waste can be defined as all solid waste generated in an area except industrial and agricultural wastes, typically from residences, commercial or retail establishments. Sometimes includes construction and demolition (C&D) debris and other special wastes that may enter the municipal waste stream. The EPA (1998c) defined municipal solid waste as "a subset of solid waste and as durable goods (e.g., appliances, tyres, and batteries), non-durable goods (e.g., newspapers, books, and magazines), containers and packaging, food wastes, yard trimmings, and miscellaneous organic wastes from residential, commercial and industrial non-process sources.

The MSW can be classified in the following categories:

a) Organics/Biomass is any organic matter that can be used as fuel to generate energy. Biomass, also known as biofuels or bio-energy, is obtained from organic matter either directly from plants or indirectly from industrial, commercial, domestic or agricultural products. The use of biomass is classed as a 'carbon neutral' process because the carbon dioxide released during the generation of energy from biomass is balanced by that absorbed by plants during their growth (35-40% of MSW).

b) Fuels include synthetic materials, generally consisting of a hydrocarbon-based polymers, and wood chips amongst others (25 – 30% of MSW).

c) Inerts-Recyclables are in general, those materials that can be recycled into the same or new products. These include metals, glass, and papers amongst others (10 – 15% of MSW).

d) Inerts-Construction & Demolition (C&D) Debris are wastes generated solely from construction, remodeling, repair, or demolition operations on pavement, buildings, or other structures (10 -15% of MSW).

e) Miscellaneous – batteries, and bones amongst others (1 – 2% of MSW).

Each category has its own distinct composite classification. To achieve an optimal Waste to Energy procurement, one has to analyze separately the inherent category contributions to energy yield and its correlated technological process of extraction in obtaining same in the most economical sense available; thus, the importance of segregating the MSW into the appropriate categories of distinct feedstock is of principal importance for optimal performance in the appropriate technological processes; thus, distinct processes (biomethanation and composting for organics and RDF for fuels) can be combined in a hybrid process system to yield optimum biofuels and other marketable by-product (high grade fertilizer) outputs.

 

Automobile Shredded Refuse (ASR)

 

Automobile shredded refuse includes the car interiors like seats, the dash board, roofing materials and other plastic and synthetic materials.

 

Plasma Arc Gasification

 

Plasma is a hot ionized gas resulting from an electrical discharge. Plasma technology uses an electrical discharge (some use AC, some DC, and some a combination) to heat gas, typically air, oxygen, nitrogen, hydrogen, or argon, or combinations of these gases, to temperatures above 7,000°F. The heated gas, or plasma, can then be used for welding, cutting, melting, or treating waste materials.

 

Most of the use of plasma arc technology has been for melting incinerator ash or for thermally decomposing hazardous or medical wastes and ASR. Only very recently has development occurred for using plasma technology integrated with gasification technologies to process MSW (only the FUELS fraction (approximately 25%) of the MSW; the organics are not ideal). This has great potential to convert MSW to electricity more efficiently than conventional pyrolysis and gasification systems, due to its high heat flux, high temperature, almost complete conversion of carbon-based materials to syngas, and conversion of inorganic materials to a glassy, non-hazardous slag.

 

There are two types of plasma torches, the transferred torch and the non-transferred torch. The transferred torch creates an electric arc between the tip of the torch and either a metal bath or the conductive lining of the reactor vessel wall. In a non-transferred torch, the arc is produced within the torch itself. Plasma gas is fed into the torch, heated, and then exits through the tip of the torch. There are several approaches to the design of plasma gasification reactors. In one approach, developed by Westinghouse Plasma Corporation (plasma torch manufacturer) and Hitachi Metals (plasma gasification system developer and user), a medium pressure gas (usually air or oxygen) flows through a water-cooled, non-transferred torch, outside of the reactor. The hot plasma gas then flows into the reactor to gasify the MSW and melt the inorganic materials.

 

Another design is an in-situ torch, where the plasma torch is placed inside the reactor. This torch can either be a transferred or non-transferred torch. When using a transferred torch, the electrode extends into the gasification reactor and the arc is generated between the tip of the torch and the molten metal and slag in the reactor bottom or a conducting wall. The low-pressure gas is heated in the external arc. Alternatively, a non-transferred torch can be used for creating plasma gas within the torch, which is injected into the reactor.

 

Several suppliers utilize a completely different approach. In these designs, the reactor is heated by electric induction coils or an electric arc produced by graphite rods, forming a molten metal and slag bath. The MSW enters the reactor, where it is subjected to high temperatures, resulting in partial gasification of the feedstock. From there, the syngas exits the reactor. The plasma torch is situated either in a secondary reactor or in a recycle line, which goes back to the first reactor, assuring complete gasification of the feedstock.

 

Proponents of the in-situ torch claim its advantages include better heat transfer to MSW and a hotter reactor temperature, resulting in more complete conversion to syngas. The main disadvantage is the potential corrosion of the torch from hot MSW and gases. An external torch is more protected from the corrosive effects, which can prolong the mechanical integrity. A disadvantage of an external torch is the possibility of a somewhat lower reactor temperature, resulting in lower conversion of the MSW. Electrodes in all designs experience some corrosion and must be replaced.

 

The first two approaches have been applied to small-scale commercial waste and medical waste processing units. The throughput of the largest external system is approximately four tons/hour and the throughput of the largest internal system is approximately 10 TPD. The Westinghouse/Hitachi design has been scaled up to 83 tpd per reactor at Utashinai, Japan, which treats a combination of MSW and auto shredder residue.

 

Plasma arc gasification typically occurs in a closed, pressurized reactor. The feedstock enters the reactor, where it comes into contact with the hot plasma gas. In some designs, several torches arranged circumferentially in the lower portion of the reactor help to provide a more homogeneous heat flux. When used for gasification, the amount of air or oxygen used in the torch is controlled to promote gasification reactions.

 

Syngas can either be burned immediately in a close-coupled combustion chamber or boiler, or cleaned of contaminants and used in a reciprocating engine or gas turbine. In the first approach, the exhaust gases are cleaned after combustion, in an emission control system. Hot gases flow through the boiler, creating steam used for power generation in a conventional steam turbine. In the second approach, the syngas is cleaned before it enters the engine or gas turbine.

 

As noted above, the primary solid output from plasma facilities is a glassy slag, the result of melting the inorganic fraction of the waste. Any waste processing facility generating an ash or slag is required by the United States Environmental Protection Agency (USEPA) to subject it to a Toxicity Characteristic Leaching Procedure (TCLP) test. The TCLP test is designed to measure the amount of eight elements that leach from the material being tested. Data from existing facilities, even those processing highly hazardous materials or medical waste, show results that are well below regulatory limits. While there are only a few plasma torch manufacturers, there are over a dozen companies that have taken the plasma technology and are developing it for use in MSW gasification. This has led to several suppliers claiming the same operational experience; i.e., several suppliers that incorporate Westinghouse plasma torches claim the experience in the Hitachi Metals plants as being their own or representative of how their system would perform.

 

By-products of Plasma Arc Gasification are similar to those produced in high-temperature Gasification. Due to the very high temperatures produced in Plasma Arc Gasification, carbon conversion nears 100%.

 

There is currently a 300 TPD Plasma Arc Gasification Plant operating in Yoshi, Japan. The plant took 3 years to construct and began operations in April, 2003. The plant uses 67% of ASR and 33% of MSW (the fuels fraction) as its feedstock. The Yoshi Plant claims that they generate approximately 7900 kW of power, out of which 4,700 kW is sold to the grid and 3,200 is used for self consumption, i.e. there is a 40% self consumption rate, whereby 80% of that 40% is energy consumed by the plasma torch. The plant derives approximately 90% of its revenue from Tipping fees from the Municipalities and 10% of its revenue comes from the resale of electricity. They also claim that the cost of implementation was approximately 10 Billion Yen (110 MUSD). Thorough due diligence should be implemented on the evaluation of optimal requisite feedstock, capital cost of implementation and actual operating costs (this is very critical) vis a vis claims.