Many areas in the United States have both carbonaceous resources, such as coal deposits, and renewable resources/wastes such as landfills, both of which are potential sources of energy. In the near future, more utilities will use these coal resources for gasification power plants utilizing integrated gasification combined cycle, or IGCC, technology to generate energy.

With new technology the wastes in the landfills can be gasified and converted to electrical/steam energy. Much of this potential energy source, oft stored in landfills, is in the form of municipal solid waste (MSW) and industrial waste. Whether coal or MSW is used as the raw material, new power plants will gasify these materials to predominantly CO and hydrogen, known as a synthesis gas, or “syngas.” This syngas can be converted to electrical/steam and/or liquid fuels/chemicals. Gasification power plants will have the ability to simultaneously produce some liquid fuels during off-peak periods. Such a synergy of processes in producing electricity and fuels should maximize the bottom line for a gasification facility.

Coal and MSW gasification have been evaluated on economic and commercials bases, but less attention has been paid to the commercial gasification economics of MSW to syngas for the production of liquid fuels such as ethanol and methanol.

Plasma arc gasification

Enlarge this picture Figure 1

Plasma arc gasification could be considered the most efficient gasification process. A conventional gasification plant produces about 685 kilowatt hours (kWh) per ton of MSW net energy to the grid, whereas a plasma arc gasification plant produces about 816 kWh per ton of solids. Plasma arc gasification can be used to produce a syngas, which can be converted to liquid fuels via the Fischer-Tropsch synthesis. A simplified schematic is shown for such a process in Figure 1.

Plasma arc gasification is a high-temperature pyrolysis process (7,200°F to 12,600°F) whereby the organics of waste solids are converted to a synthesis gas, and the inorganic materials and minerals of the waste solids produce a rock-like byproduct. The synthesis gas is predominantly CO and hydrogen, and the inorganic materials and minerals are converted to a vitrified slag, typically composed of metals and silica glass. This vitrified slag is basically non-leaching and exceeds EPA leach test standards. Metals can be recovered from the slag. The slag material can be used to produce other byproducts such as rock wool, floor tiles, roof tiles, insulation and landscaping blocks, or be recycled as a road aggregate material.

The synthesis type of gas can be used to produce electricity, steam and/or liquid fuels. The syngas can be converted into liquid fuels such as ethanol or methanol via the Fischer-Tropsch catalytic process. The syngas produces byproducts of HCl and sulfur via the cleanup step. The ethanol can be blended with gasoline to produce a motor fuel, while the methanol can be used in biodiesel manufacturing.

Plasma arc-onomics

Enlarge this picture Figure 2

The capital cost for using plasma arc gasification to produce electricity and/or syngas and a vitrified slag was discussed in the November 2006 issue of Pollution Engineering (pg. 26). Figure 2 provides a cost analysis of a plasma arc gasification facility with a 500-ton daily MSW capacity, showing a capital investment of about $67.7 million for the plasma arc processing plant and almost $27 million for the Fischer-Tropsch synthesis processing plant, including the distillation unit for the separation of the two alcohols.

The plasma plant supplies sufficient energy for the production of about 10.4 million gallons per year of alcohol mixture. The mixture is about 60-percent ethanol and 40-percent methanol. The mixture yield was taken at a conservative value of about 95 gallons per ton MSW (dry material). Capital cost of the overall plant is about $94.7 million. Capital financing for both plants was calculated at 6-percent interest for 20 years, making two payments per year. The byproduct rock material would be sold as a road construction material at $15 per ton. Cost considerations for operations & maintenance, a capital budget reserve, process water, and sewer have been considered in the economic evaluation. The estimate includes over 40 jobs created by the combined plasma and liquid fuel facility. Energy costs for the combined plant were calculated at zero, since the plant would produce all of its own energy.

From these initial economic analyses, one likely approach to a business plan could be a cooperative effort between a local utility, local industry and/or local governmental entities. As the local government participates in the MSW treatment part of the plasma arc facility, the local utility would produce the syngas and liquid fuels. Alternately, a local industrial/business entity could own/operate the liquid fuel facility, allowing the local government to remain in the MSW business while the utility handles the electrical, steam and liquid fuel energy businesses.

Case study

The preliminary economics of the combined plasma arc gasification and Fischer-Tropsch catalytic synthesis facility was considered for a Linn County/Cedar Rapids, Iowa, facility processing about 500 tons per day of MSW. A positive cash flow was obtained for a cooperative venture with a tipping fee for the solids of $35 per ton, and a selling price of ethanol at $2.09 per gallon and methanol at $0.93 per gallon. Annual net revenue – before taxes – from the combined facility operation was just under $6.13 million per year. If the alcohol mixture yield was taken at a value of about 113 gallons per ton MSW rather than the previous yield of 95 gallons per ton, that number would increase to almost $8.67 million per year. The alcohol mixture produced would be about 12.4 million gallons per year.

As expected, this economic evaluation demonstrates the commercial importance of utilizing the Fischer-Tropsch catalyst with the highest yield for the products desired. It is expected that such catalysts with higher yields and selectivity for desired products are most likely to be just over the horizon.

Logical approach for future progress

This economic model may not apply to every application. Industry and municipality groups interested in pursuing a plasma arc gasification investment are encouraged to conduct and periodically update their analyses. Cost is not the only obstacle; such an endeavor requires a cooperative effort between governmental body(s) and industrial entity(s), and both parties will require a fully transparent evolvement and trust in the final economic analysis.

The attractiveness of plasma arc gasification technology is the generation of renewable energy from a widely available resource, with useful byproducts. The process uses plasma technology to treat MSW, eliminating the need for a landfill, and can even be used to process existing landfill MSW sites. The energy generated from the plasma plant can supply energy that is self-sustaining, and additional syngas to be sold as liquid fuels. Other fuels can be produced using the Fischer-Tropsch synthesis.

A utility or industrial in partnership with a local government would likely be the most economical combination, and have the most positive benefit to the environment and financial reward to the local area. With some diligence, a viable business plan can be developed from the consideration of the many factors influencing the economics associated with a specific site selection, economy of scale and the surrounding community. PE

References are available upon written request.