Figure 1: Typical Updraft Gasification Reactor

The need to manage solid waste is a fundamental problem facing any organized society. Regulatory programs and technologies assist solid waste managers. A promising technology that appears to be market-driven, as opposed to technology-driven, is plasma gasification. Neither plasma nor gasification are new technologies.

Plasma gasification process

Gasification provides a means by which municipal solid wastes (MSW) can be transformed into a fuel gas and a vitreous slag. The gas and slag have industrial value, therefore the MSW becomes a raw material to the process as opposed to waste material. The fuel gas produced, called syngas, primarily consists of a mixture of H₂ and CO. The syngas can also be used as raw materials for the production of methanol, ethanol or other industrial chemicals.

The syngas contains contaminants such as particulate matter, HCl, dioxins, furans, sulfur oxides and others. The gasification process is similar to pyrolysis except that limited quantities of O₂ are introduced to the reactor, and given the proper process controls, the system is self-sustaining. The gasification process requires a source of heat to drive the reaction. While there are various methods to provide heat energy, this article focuses on plasma torches.

Plasma gasification technology has been used since the late 1800s in the metal industry, and expanded into the chemical industry in the 20th century. Plasma, often referred to as the fourth state of matter, exists as an ionized gas capable of conducting electric current. The plasma is generated when a gas is exposed to a high-energy field that occurs between two electrodes within the torch. Being conductive, it generates an arc of light between the two electrodes and reaches temperatures of 9,000 to 12,000°F.

Plasma is generated and directed into the reactor, where it directly or indirectly heats the MSW and drives the gasification reaction. MSW is continuously fed into the reactor. The syngas exits via a vent at the top of the reactor at approximately 1,650 to 2,012°F, and the liquid vitreous slag exits via a tap at the bottom of the reactor at approximately 3,000°F. Figure 1 illustrates a typical updraft gasification reactor.

Four gasification processes exist commercially: 1) counter-current fixed bed (updraft), 2) co-current fixed bed (downdraft), 3) fluidized bed and 4) entrained flow. Counter-current fixed bed gasification, as depicted in Figure 1, is characterized by a fixed bed of MSW through which the plasma flows counter-current to the feed. This type of reactor has a relatively low throughput and a high thermal efficiency. Co-current gasification is characterized by the plasma flow co-current with the MSW downward towards the bottom of the reactor. The syngas from a downdraft gasifier leaves at high temperatures and is used to preheat the incoming gases, thus resulting in high thermal efficiencies. Fluidized-bed gasifiers and entrained-flow gasifiers are generally reserved for gasification of coal due the requirements for particle size and consistency of the fuel. The few MSW gasification plants that are currently in operation utilize the updraft configuration.

Once the MSW has been converted to syngas, it can be piped to a variety of unit operations depending upon the process design and goals. One configuration routes syngas to units designed to remove contaminants such as hydrochloric acid, particulate matter, SOX, and trace metals. The cleaned syngas can then be used in a boiler to produce steam and electricity, or the syngas can be routed to a gas turbine and used in an integrated gasification combined cycle (IGCC).

Potential benefits and drawbacks

The benefits of MSW plasma gasification are straightforward: value products (energy, industrial chemicals and construction material), a management solution for MSW, and potential environmental impact and risk reduction when compared to other management alternatives.

Due to the newness of this process, operational data is limited. Two MSW plasma gasification plants operating in Japan, Utashinai and Mihama-Mikata, serve as reference plants for operational, economic, and environmental data and analysis. While two reference plants are not ideal for comprehensive process analysis, they are better than engineering estimates based upon operational assumptions.

The Utashinai plant started up in 2003 and was designed to process 180 metric tonnes per day of a 50:50 blend of MSW and auto-shredder residue (ASR). There are two process lines, one of which leads to two updraft reactors to produce syngas and electricity for operation of the plant and sale to the grid. The MSW and ASR require shredding prior to a feed hopper blending operation and subsequent feed to the gasification reactors. Each reactor is lined with refractory and heated by four flush-mounted plasma torches rated at 80 to 300 kW each.

The reactors require a metallurgical coke and limestone feed. The metallurgical coke provides a bed for absorbing heat energy from the plasma torches and maintains a consistent temperature in the reactor's gasification zone as it slowly combusts. The limestone acts as a fluxing agent to control the melting and flow properties of the slag. The plant operates 300 days a year with two 30-day outages for maintenance and repairs.

According to plant managers, the performance of the reactors and plasma torches has been very good. Operationally, the process is stable due to the metallurgical grade coke and limestone feeds. However, this benefit is not without economic cost. The greatest operational benefit of this type of process is the use of plasma torches as the heat source for gasification. The plant design allows each reactor to operate with three torches so that one can be removed for maintenance without requiring a shutdown. Torch maintenance is performed by the plant operators after every 500 hours of operation.

Shredding prior to gasification presents two significant challenges. Due to the variability of MSW composition, shredder jamming can occur if large metal objects are present, causing process downtime and costly repairs. Gasification of ASR results in a sticky slag buildup on the refractory of the boiler. As the slag builds, it forms stalactites that eventually break off and damage the refractory at the bottom of the combustion chamber.

The Mihama-Mikata plant also started in 2003 and was designed to process 22 metric tonnes per day of solid waste: 17.2 tonnes per day of MSW and 4.8 tonnes per day of sewage sludge. The plant has a single process line, which includes one updraft reactor to produce syngas that is immediately combusted in an afterburner. No electricity is produced at the plant. Similar to the Utashinai plant, MSW requires shredding prior to being fed to the gasification reactor. The reactor requires a metallurgical coke and limestone feed. The plant's operating cycle is 2.5 months of operation followed by a two-week shutdown for maintenance. This translates into 10 months of planned operations annually.

The operational benefits are similar to the other plant. However, this plant has not been plagued with the same operational challenges due to the absence of ASR in the feed. The slag has a different consistency and composition than the Utashinai slag and does not require unplugging the slag tap. The same shredder-jamming problem exists if large metal objects are present in MSW.

An added benefit is the potential to add revenue from the production and sale of the vitreous slag as a construction material (aggregate). This benefit is not seen in all processes. The Utashinai plant was unable to produce a saleable construction aggregate due to processing ASR with MSW, whereas the Mihama-Mikata plant produces 1.5 tonnes per day of slag that was used in local road construction projects. Another potential economic benefit is the ability to produce industrial chemicals from the syngas. The industry is too new at this time to determine how much benefit this is.

Table 1: MSW to Electricity Thermal Process Comparisons

Table 1 illustrates some potential economic benefits. It is apparent that the efficiencies gained through the use of plasma technology allow more net electricity to be sold to the grid per mass of MSW gasified than other thermal treatment technologies. Tipping fees and construction aggregate revenues being equal, the energy production advantage is the most significant economic advantage of the technology.

One drawback, or at least potential barrier, is the high capital costs required for plant construction. Capital costs, like many industrial projects, are directly proportional to the capacity of the facility. In the United States, the largest project under consideration is a 3,000 ton per day facility in St. Lucie, Fla., with an estimated cost of $425 million.

Given the fact that no commercial plasma gasification facility exists in the United States and the application of this technology to process MSW is new, acquiring the necessary financing to complete a project may be difficult. A possible solution to this problem would be to seek a joint effort between a local government and the gasification project developer. This way the local government would be included in the waste management efforts and share in the costs and revenue of the facility, while the project developer would manage the business aspects of operations and product sales. A project that could not acquire the necessary financing through traditional means might be completed.

Figure 2: Potential MSW Gasification Process Block Flow Diagram

Community considerations

Overall, the economic outlook for plasma gasification technology seems favorable. A study conducted by Juniper Consultancy Services Ltd. concluded that the economic model used by Alter NRG (formerly Westinghouse Plasma Corp.) is robust, projecting a pre-tax return on equity of 18 percent for IGCC and 13 percent for traditional steam cycle energy production. Another study conducted for a proposed 100 ton-per-day plasma gasification facility in International Falls, Minn., projected an annual operating income of $1.9 million from the combination of tipping fees, electricity generation and sale of construction aggregate.

Gasification of MSW, like all industrial processes, produces environmental contaminants that must be managed. Wastewater production from a gasification process is highly dependent upon process design and was not considered in this article. The gasification products, syngas and vitreous slag, are common to all gasification process designs, and their environmental impacts will be the focus of this section.

The syngas from the process could contain a variety of pollutants, which includes but is not limited to: particulate matter, liquid tar droplets, metal carbonyls, gas-phase halogenated species (HCl, HF or HBr), sulfur species (H₂S, COS or SO₂), nitrogen species (NH₃ or HCN), dioxins, furans and greenhouse gases. Air pollution controls currently employed by the Japanese facilities include bag filters to remove particulates and activated carbon filters to remove trace metals and VOCs. These facilities have tailored their air pollution controls to meet Japanese regulatory requirements. The much larger facility planned in St. Lucie provides a better understanding of the air pollution controls necessary to meet regulatory requirements in the United States. That facility will employ a thermal oxidizer to minimize the formation of NO_X, an electrostatic precipitator to control particulate matter, selective catalytic reduction to further reduce NO_X, activated-carbon filters to reduce mercury and trace elements not captured by the precipitator, and flue gas desulfurization to control acid gasses (SO₂ or HCl). Figure 3 shows a process flow diagram of the St. Lucie facility's plasma gasification process and depicts the proposed air pollution control equipment.

Figure 3: St. Lucie Florida Process Flow Diagram

The need for extensive air pollution control and monitoring that will be required by federal regulations (NESHAP, MACT and NSPS) and construction permit conditions will adversely affect the economics and overall profitability of a plasma gasification facility. However, this will be necessary to mitigate the environmental impacts of the facility's emissions.

A drawback of some air pollution control equipment is the subsequent generation of solid and potentially hazardous wastes. Another aspect of the plasma gasification process with respect to emissions is the emission of greenhouse gases (GHGs). All carbon-based combustion processes emit GHGs. The plasma gasification process, followed by the combustion of syngas to produce electricity, compares favorably with the combustion of other carbon-based fuels with respect to GHG emissions. Figure 4 illustrates the pounds of CO₂ emissions per megawatt-hour of electricity produced for different power generation processes. PE

References are available by written requests.

Figure 4: Pounds of CO2 Emissions per Megawatt-hour of Electricity Generated