Thomas Stringfellow and Robert Witherell, CH2M HILL Engineers, Inc.
January 13, 2014 | 11 Comments
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Conventional gasification is defined as the thermal conversion of organic materials at temperature of 1,000 °F - 2,800 °F (540 °C – 1,540 °C), with a limited supply of air or oxygen (sub-stoichiometric atmosphere). This is not combustion and therefore there is no burning. Gasification uses a fraction of the air/oxygen that is generally needed to combust a given material and thus creates a low to medium Btu syn-gas. Although more mature than other processes, it does require complex systems, such as gas clean up equipment.
The U.S. Department of Energy’s (DOE) Worldwide Gasification Database shows that the current gasification capacity has grown to 70,817 megawatts thermal (MWth) of syn-gas output at 144 operating plants with a total of 412 gasifiers. The database also shows that 11 plants, with 17 gasifiers, are presently under construction, and an additional 37 plants, with 76 gasifiers, are in the planning stages to become operational between 2011 and 2016. The majority of these plants—40 of 48—will use coal as the feedstock. If this growth is realized, worldwide capacity by 2016 will be 122,106 MWth of syn-gas capacity, from 192 plants and 505 gasifiers. This data base does show that there are gasifiers operating on both biomass and waste. Figures 6 and 7 are two basic types of gasifiers, Figure 6 is fluidized bed gasifier and char combustor and Figure 7 is a typical slagging gasifier.
Plasma Arch Gasification
Plasma Arc gasification is the process of that utilizes a plasma torch or plasma arc using carbon electrodes, copper, tungsten, hafnium, or zirconium to initiate the temperature resulting in the gasification reaction. Plasma temperature temperatures range from 4,000 °F – 20,000 °F (2,200 °C – 11,000 °C), creating not only a high value syn-gas but also high value sensible heat. The technology has been used for decades to destroy wastes that may be hazardous. The resulting ash is similar to glass that encapsulates the hazardous compounds.
The first Plasma Arc unit began operation in 1985 at Anniston, Alabama. The unit used a catalytic converter system to improve gas quality and the gasifier was designed to destroy munitions. The second system began operation in 1995 in Japan followed by the third system in Bordeaux, France, both design for MSW. There are other operating systems in Sweden, Norway, the UK, Canada, Taiwan and the U.S., Japan has added nine more since 1995. All of these are small in size but have the ability to scale up, using multiple units. Figure 8 and Figure 9 show a couple of current systems available on the market and both can be employed to reduce waste and generate clean electric energy.
The advantage of the Plasma gasification is the high temperature that minimizes air pollutants well below those of traditional WTE facilities. At the elevated temperatures, there is no odor, and the cooled off gas has lower NOX, SO2 and CO2 emissions. The solid residue resembles glass beads.
In order to fairly evaluate each of these technologies we assessed the overall technology capabilities, commercial viability and associated costs, while asking the following questions:
Note: If it doesn’t work technically then it doesn’t work and if it doesn’t work economically, then it doesn’t work. (Both are needed to be viable)
Ranges for Capital Costs for each of the Thermal Technologies assumes a 15 MW output for a:
Costs vary from technology to technology due to each having unique design characteristics, variations in equipment costs, site specific waste characteristics and site space requirements. There are significant other factors that can negatively affect the costs of construction.
If the site is located at an intercity location several issues can occur:
If the site is located in a unionized craft location several issues can occur:
In our opinion, all of the technologies presented provide the end user with different results. Although mass burn and RDF have the most units installed around the world, the lesser used technologies (Pyrolysis, Gasification and Plasma Arc) all have the capability of changing the landscape of the WTE arena. All three of these technologies provide systems with lower emissions than the mass burn and RDF system simply due to their process characteristics. The Plasma Arc has proven that it has the lowest emissions of all the technologies presented, but does not have a track record of multiple units around the world. That said, it is gaining in acceptance and increasing the number of installations due to its complete elimination of the waste stream. Although there are few Pyrolysis systems installed around the world it appears as though this technology will not be used to produce electrical energy rather it will be used to produce bio-fuels for the transportation industry. We opine that even though it could make electrical energy the likelihood will be rare.
Although the capital costs are conservative and high compared to other energy technologies, we need to look at the possible revenue streams. The revenue for all of the technologies is; Electric Energy Sales, Government Subsidies, Renewable Energy Credits, Sale of any Recyclables and tipping fees. Although some are more valuable than others, WTE technologies have more ways to generate revenue that any other power generation technology. The only revenue stream that may not be included with the Plasma Arc, as it may not have a recyclable revenue stream due to its complete consumption of waste and this will depend on the ultimate final design.
We would like to thank the companies who have provided their technology, the Department of Energy, the EPA and other sources we have reviewed prior to writing this paper.