In 2007, the City of Sacramento generated an average of 750 tons/day of solid waste. About 348 tons/day (46%) of this was recycled, leaving 402 tons/day to be trucked across the Sierra Nevada mountains and buried in the desert at the Lockwood Regional Landfill in Sparks, Nevada. The City, consistent with its sustainability initiative, has been exploring more environmentally sound alternatives for the unrecycled portion of the waste, including the prospect of converting it into energy and other useful products. On August 24, 2007, the City issued a Request for Qualifications (RFQ) to attract a development partner to build, own, and operate a resource recovery and waste-to-energy facility in Sacramento that would meet five specific goals: (1) be environmentally friendly and reduce greenhouse gas emissions; (2) be economically viable and cost-neutral to rate-payers; (3) leave little or no residuals requiring treatment or landfill disposal; (4) continue the City's existing recycling program; and (5) utilize a proven technology at a commercial scale.

Eleven companies responded to the RFQ, proposing a total of five different technologies. The City then formed an independent, cross-functional committee to review the RFQ responses and recommend a development partner to move forward with. The committee was comprised of staff from the City Manager's Office, the Solid Waste Division, the City Attorney's Office, the Sacramento County Department of Waste Management and Recycling, and professors from the California State University College of Engineering. In November 2007, the committee determined that the plasma gasification technology proposed by US Science & Technology (USST) would best meet the City's goals indicated above. On February 26, 2008, following further research and due diligence, the City staff recommended and the City Council unanimously approved the project concept and awarded USST an exclusive right to negotiate Principles of Agreement with the City for the development of a waste-to-energy project. The City Staff's Report to Council provides more detail on the project and the rationale for selecting USST. This article describes the technology proposed and its environmental impacts and benefits.

The specifics of the technology, and how it's implemented, are incredibly important. Beneficial outcomes and environmental challenges are entirely dependent on the composition of the feedstock (garbage, in this case) and the specific chemical processes employed. Some comparisons have been made between our process and other technologies (e.g. plasma gasification and incineration) that are completely baseless and without fact; the processes employ different chemistry and yield very different results. Even technology comparisons within the field of plasma gasification can yield very different results, depending on the design of the process equipment and the specific chemical process steps employed. This article intends to make our proposed process more clear, so that useful comparisons can be made.

The "How Stuff Works" web site has an introductory article on plasma gasification that can be helpful for understanding the technology in general. The most detailed public description of the process USST is proposing for Sacramento is provided in an independent analytical report by Juniper Consultancy Services, Ltd. It provides a candid, in-depth examination of the technology provided by Alter NRG and Westinghouse Plasma Corporation, including emissions data from existing facilities. It is important to note, however, that the system design proposed by USST (and reviewed in the analytical report) makes significant improvements compared to the design of the existing facilities.

The waste-to-energy facility proposed for Sacramento includes three major sections: (1) gasification, (2) syngas cleaning, and (3) power generation. The plasma gasifier uses plasma torches with an external energy source (electricity) to heat the waste to very high temperatures in an oxygen-starved atmosphere (less oxygen than is needed for complete combustion). The chemical bonds in the waste material are broken down and very simple molecules are formed: carbon monoxide (CO), hydrogen (H2), nitrogen (N2), water (H2O), hydrochloric acid (HCl), etc. This mixture of primarily carbon monoxide and hydrogen gases is often called synthetic gas, or "syngas", and it exits the reactor at about 1600 degrees Fahrenheit. The inorganic materials in the waste flow out the bottom of the gasifier as an inert volcanic glass and liquid metal, in two separate phases, at about 3000 degrees Fahrenheit. However, some of the more volatile metals, such as mercury (Hg), lead (Pb), cadmium (Cd), and zinc (Zn), are vaporized and exit the gasifier with the syngas. These metals are removed in the syngas cleaning stage, along with hydrochloric acid (HCl) and sulfur compounds, namely carbonyl sulfide (COS) and hydrogen sulfide (H2S).

The syngas cleaning section of the facility is focused on cleaning the syngas so that it has a purity about like that of natural gas. This is done to protect human health, the environment, and the most expensive piece of equipment in the facility: the gas turbine. The syngas cleaning section has multiple operations, each designed for a specific purpose. The first step is a venturi scrubber, which ensures thorough mixing of process water and syngas. This step, along with the following step, a spray tower, are focused on rapidly cooling the syngas and capturing particulate matter and hydrochloric acid (HCl). Sodium hydroxide (NaOH) is added in the spray tower to neutralize the acid and form sodium chloride (NaCl), also known as regular table salt. The syngas is then sent to a wet electrostatic precipitator as a "polishing" step for further removal of sub-micron particles. At this point, the syngas is fully saturated with moisture (100% relative humidity). It is sent to a condenser, where the syngas temperature is further reduced to about 68 degrees Fahrenheit, and on to a compressor, which raises the pressure to about 350 psig and condenses even more moisture out. Process water from these steps is sent to an on-site water treatment plant, where the water is purified and reused in the facility (and incidentally, the facility is slightly net-positive in water production; it comes in with the solid waste and is formed in some of the chemical reactions). The cooled, compressed syngas is then sent through a pair of activated carbon beds for 99.75% removal of any mercury (Hg) that may be present in the syngas. The final step in the syngas cleaning section of the facility is focused on removing sulfur. This technology is commonly used to remove sulfur from natural gas, and it involves both a carbonyl sulfide (COS) hydrolysis reaction to convert COS to hydrogen sulfide (H2S), and a subsequent step wherein the H2S is removed from the syngas and converted into elemental sulfur crystals. At this point, the syngas has been thoroughly cleaned and is ready for use as a fuel in the power generation section of the facility.

The power generation section of the facility is based on a conventional combined-cycle power plant. These are typically fueled with natural gas, and are some of the cleanest and most efficient power plants in existence. Just such a facility was recently built in Roseville; it began operation in 2004. Our facility would be fueled primarily with clean syngas; natural gas would be used as a supplemental fuel for start-up and shut-down of the gas turbine and to even out natural variations in the real-time production of syngas which would be expected with the variability in municipal solid waste. The fuel gas is burned in a gas turbine, where the expanding gases spin the turbine and power an electric generator. The hot exhaust from the gas turbine is directed to a steam generator, which produces steam that drives a steam turbine and another electric generator. An oxidation catalyst would be used to remove any trace amounts of carbon monoxide (CO) that may remain in the exhaust, and Selective Catalytic Reduction (SCR) would be used to remove nitrogen oxides (NOx, or smog) before sending the exhaust up a stack. In a generic reference design for a 750 ton/day facility similar to the one described here, the gas turbine generator produces 44 MWe, the steam turbine generator produces 22 MWe, and the facility uses 13 MWe to run the plasma torches, motors, etc. In that design, 80 percent of the electric power produced (53 MWe) is available to be put on the electric grid, and 20 percent (13 MWe) is used internally to run the plant. The total capacity of the facility proposed for Sacramento has yet to be determined. The emissions would be about what you would expect for a natural gas-fired combined-cycle power plant, except that most of the carbon dioxide (CO2) would come from a renewable energy source: municipal solid waste.

The solid residuals from the plasma gasifier (metal and glass) would be recycled. The glass portion is particularly interesting, in that it can be used to manufacture higher-valued products, such as rock wool, stone tile, and other construction materials. The solid residuals from plasma gasification of municipal solid waste have been proven to be non-toxic; they easily pass the EPA's Toxicity Characteristic Leaching Procedure (TCLP). In fact, plasma vitrification is sometimes used specifically to render ash from municipal solid waste incinerators non-toxic.

Sometimes innovative, game-changing technologies seem too good to be true, but a review of recent history proves that innovative technologies can and do change our lives for the better. The primary challenge for a facility like this is the capital cost to build it. It's very expensive, and investors need assurance that there will always be a waste stream for the facility so that the long-term investment can pay for itself. That's why a long-term waste supply agreement is so important; it makes the funding possible. And we're finding that there is a tremendous level of interest in the financial community for funding renewable energy projects. If the City will supply the waste, we can make it happen.

About the author: David Prinzing is Vice President and Chief Engineer for US Science & Technology. He is a registered Professional Engineer (Chemical Engineer) in the States of California and Washington, and has more than 20 years of diverse technology experience, including combustion, chemical, nuclear, and environmental engineering.