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Plasma gasification of waste is expensive and, to date, its commercial uptake has been limited. Now, in Canada, a technology is being developed that delivers high electricity output, incurs low capital cost, and is highly environmentally compliant. Could this be the long-awaited push for the plasma market?
by Rod Bryden
For decades, plasma has been recognized as an effective method of destroying hazardous waste. However, plasma generators use high flows of electricity. As a result, the increasing cost of electricity and pollution from coal-fired power plants have made plasma an expensive and environmentally questionable disposal method. (See article in Waste Management World March-April 2006 for an introduction to plasma gasification of waste.)
Prior to 2000, much of the research into plasma applications for waste processing was directed to achieving - as completely as possible - total pyrolytic decomposition. The more perfect the process, the greater the power consumed. Even with large volumes of waste destroyed, pyrolytic gasification demands much more energy than it produces. Only when the value of removing the hazard posed by the waste is very high does plasma become an economically viable method of waste destruction. The destruction of bovine brain and nerve tissue to eliminate possibility of BSE could be such an application.
Plasma treatment today
Currently the thermal treatment industry in Europe is looking closely at how to analyse and optimize the efficiency of plants (see article on p.19 by Ella Stengler). This offers a new framework for re-examining the historical issues of cost and power consumption, which previously limited the development of plasma treatment.
The challenge for those who would use plasma in waste processing is to achieve low power consumption, high energy value gas, low capital cost and high uptime. These challenges are not simple to meet.
MSW is a highly variable feedstock. Within any tonne of MSW will be a range of materials and compounds with a variety of pollutants and energy content. In addition, the composition of the waste is highly variable over time.
For optimum efficiency in engine operation, the syngas fuel generated from variable waste must be consistent in both flow and content. To deliver efficient conversion of MSW to gas, and the efficient conversion of that gas to electricity, the processing system must respond on a real-time basis to offset changes in the feedstock by adjustments in the operation of the process. Effective and timely system response requires real-time data reflecting the key factors to be stabilized, instantaneous communication of that information, and technical response capability in the relevant elements of the process.
One company which has been addressing these challenges is a Canadian-based company, Plasco Energy Group Inc.
Plasco’s technology
The Plasco gasification process uses plasma torches at strategic stages of the process to refine gases arising from low-temperature, non-plasma gasification of municipal solid waste (MSW) and to vitrify ash remaining after gasification.
Delivery of waste into the Plasco system in two streams, one with general MSW and one with a known waste component of the MSW, permits a degree of control of total energy content in the system. No fossil fuel is needed.
Gas production in the gasification system is controlled in two ways:
- the system analyses the gas produced and proprietary software feeds instructions back to a control system, delivering constant management of operating variables
- a surge and mixing tank blends the gas flow.
A proprietary design, utilizing standard equipment where possible, is used to remove pollutants from the gas stream, with all particulates collected and returned to the slag chamber in which residual solids are vitrified by a plasma torch.
The system is sealed from entry of the waste into the hopper that feeds the gasification chamber until the product syngas enters the GE Jenbacher engines.
Net power produced per tonne of waste will vary with the waste composition and energy content. For waste averaging 16.5 million BTU per tonne and 30% moisture, net saleable electrical output is more than 1.1 MWh per tonne of waste processed. When the system is operated using cogeneration and combined cycle to produce power from residual heat from the gasification process and engine exhausts, net power output increases by approximately 20%.
Modular design
Plasco’s plant design is comprised of manufactured, integrated subsystems that can be integrated on-site, to create a standard integrated plant with a capacity of 100 tonnes per day (tpd).
Large plants are built by combining the gas flow from the required number of standard modular units. Near optimal efficiency is achieved with two modules operating to supply a single power-generating block. Slight improvements in operating cost and efficiency are gained by adding additional units to support single larger power blocks. However, these efficiencies may be offset by greater transport costs to accumulate larger quantities of waste at a single site, and by transmission losses if power output exceeds demand where the plant is located. A 200-tpd plant operating in a mode that utilizes excess heat from gasification and power generation would supply about 12 MW of power, enough for about 10,000 households.
Environmental and economic performance
In terms of capital, the system costs less than CAN$600 (US$530) per tonne of annual throughput capacity, which is comparable to current waste-to-energy plants (WTE).
For this level of investment, the Plasco system delivers 82 kWh of net saleable electricity per million BTU of energy in waste processed. With MSW containing approximately 30% moisture and averaging 16.5 million BTU per tonne, the system uses about 20% of the electricity produced for operation of the gas production and power plants. The level of parasitic demand varies with the character of the material processed.
In terms of emissions, the absence of pollutants in the fuel gas driving the engines, together with high performance of the engines, delivers very low emissions per unit of power. A commercial (100 tonne per day) demonstration plant now nearing completion in Ottawa, Canada operates under a special regulation passed by the Province of Ontario. The emission standards set specifically for this demonstration by the regulator are significantly more stringent than standards applied generally in Ontario and more stringent than standards in the EU and US. Final integration and testing of this plant is planned for the first quarter of 2007, with waste processing and power production scheduled for March 2007.
And what happens to the residue?
Solid residue is vitrified in a separate plasma chamber, producing a glass-like material of a standard that is orders of magnitude better than established leachability standards. The material represents about 0.7% of original kerbside waste volume and about 15% of kerbside weight. The material produced by the demonstration plant is expected to be sold as aggregate for production of concrete or asphalt.
Less than 500 grams of particulate material containing heavy metals and other solid pollutants is collected from each tonne of waste and periodically removed for controlled disposal. Sulphur is removed in pure powder form, useable for soil enhancement, and represents about 500 grams per tonne of waste processed.
 Artist's impression of the commercial demonstration plant outside Ottawa, Canada |
Moisture from the waste is vapourized in the converter; the portion that is not required to support gasification is condensed in the gas quality control suite. This water dilutes the salts arising from the removal of all chlorine in the waste. The resulting slightly salty water is delivered to the sewer.
Final remarks
Plasma-based systems for general MSW gasification have had limited success in commericalization. If successful, the Plasco demonstration facility may invigorate further development in the use of this technology.
Roderick M. Bryden is President and CEO of Plasco Energy Group Inc., Canada.
e-mail: rbryden@plascoenergygroup.com
The evolution of plasma treatment with Plasco
For more than 20 years following the start of operations in 1974, Resorption Canada Ltd, a predecessor of today’s Plasco Energy Group Inc. (Plasco), destroyed hazardous and other wastes in the course of research and development in its facility near Ottawa, Canada. The company focused its research on proprietary methods of reducing energy consumption while achieving complete elimination of the hazard and vitrification of residual solids.
In the late 1990s, RCL Plasma Inc. (RCL), successor to Resorption Canada Ltd., achieved complete gasification and vitrification while recovering electrical energy substantially equivalent to the electricity consumed. In 2003, RCL formed a joint venture with Hera Holdings S.A. of Barcelona, Spain. The Ottawa plant was moved to Castelgalli, Spain, and applications research has produced patented and patent-pending designs for the conversion of non-hazardous or hazardous waste to synthetic gas, which is efficient as a fuel for internal combustion engines. Electricity produced from this gas is expected to be four to five times the amount of total electricity consumed to process the waste and operate the power plant.
