Single-Stage Process

The single-stage process was developed and commercialized by Westing — house Corporation (Madison, Wisconsin, United States) and Europlasma Corporation (Mocenx and Cenon, France and Shimonoseki, Japan). The scale of the process varied from about 10 tons/day to about 42 tons/day in different locations [68, 69]. The largest commercial plant was established at Utashinai in Japan (180 tons/day) and a smaller (22 tons/day) at Mihama-Mikata in Japan both using Westinghouse plasma gasification technology [7]. Both of these plants are called Alter NRG/Westinghouse plasma gasification process [68-70]. In 2007, NRG acquired the Westinghouse Plasma Corporation and combined the Westinghouse updraft gasification reactor concept that uses plasma torches to provide part of the energy input, with synthesis gas clean­ing in order to convert the synthesis gas to heat and electricity and other value-added products. The process can use a variety of feedstock such as MSW, MSW plus tires, RDF, ASR, coal plus wood, petcoke, and other haz­ardous wastes. The ability to process heterogeneous, unsorted, or differ­ently sized feedstock reduces the cost required for feed handling prior to gasification.

The plasma gasification reactor (PGR) used by Alter NGR is graphically illustrated in Figure 6.7. The reactor has a refractory lining to withstand high temperatures and corrosive conditions. The reactor is first packed

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FIGURE 6.7

Alter NRG plasma gasification reactor. (Modified from Helsen and Bosmans, 2010. Waste to energy through thermochemical processes: Matching waste with process, Conference Proceedings on Enhanced Landfill Mining and Transition to Sustainable Materials Management, Molenheide, Houthalen-Heichteren, Belgium, October 4-6.)

with metallurgic coke which absorbs and retains the thermal energies from plasma torches and creates an environment to melt inorganic materials. The coke is consumed as the reaction proceeds. The temperature of the plasma plume varies from 5,000 to 7,000°C and the temperature at the bottom is about 2,000°C [7].

The reactor uses a mixture of oxygen and steam to improve hydrogen yield in the synthesis gas. As shown in Figure 6.7, whereas the synthesis gas leaves the reactor at the top at about 890-1,100°C, the molten slag at the bottom containing nocombustible inorganics and recoverable metals leaves the reactor at about 1,650°C. The molten slag then goes through a slag handling system for further processing. High residence time in the reactor assures the conversion of tar and it minimizes the particulate car­ryover. The electrical energy supplied by plasma torches counterbalances the heating value of the waste feed and thereby controls the temperature and the quality of the synthesis gas. The vitrified slag can be used for the construction industries, although the quality of this slag can be deterio­rated by the presence of ASR in the waste feed. The major problem with the single-stage process is the different types of harmful contaminants present in the synthesis gas which requires a series of downstream processes to remove them. These downstream processes add significantly to the overall cost of the gasification process [7].

6.5.4.2.1 Two-Stage Process

A two-stage Gasplasma™ process [70] was developed by Tetronics Corp. as well as Plasco Energy Group (Ottawa, Canada). Integrated Environmental Technologies of the United States and Pyrogenesis Corporation of Canada also use this process. The pilot-scale process is used by Advanced Plasma Power of Swindon, UK [72-74]. The process contains two stages, gasification followed by plasma conversion, which are followed by a number of processes to clean and cool the synthesis gas prior to delivering it to gas engines for conversion to mechanical/electrical energy.

The Gasplasma process converts waste feedstock into a clean, hydrogen — rich synthesis gas and a vitrified recyclate called Plasmarok™ that can be used for building material or replacement aggregate. The process is capable of producing synthesis gas which, after passing through further treatment, is suitable for use as fuel in a gas engine [75].

In the first stage, the pretreated waste stream (RDF, pretreated commer­cial and municipal waste and refined biomass) is gasified in a fluidized bed gasifier in the presence of oxygen and steam at a temperature around 800- 900°C. The reactor uses a portion of heat contained in the waste material. The synthesis gas generated in the reactor contains tar and soot and solid char and ash contained in the feed material are removed at the bottom of the reac­tor and processed in the plasma converter [7].

In the second stage, the plasma converter cracks tars and soot to synthesis gas to form a gas comprised primarily of hydrogen, carbon monoxide, car­bon dioxide, and nitrogen. The ash and inorganic fraction from the gasifier are vitrified to form Plasmarok™. The plasma converter is designed in such a way as to allow the maximum amount of residence time for the synthe­sis gas under most energy-intensive conditions. The synthesis gas leaves the reactor around 1,200°C, and it is then cooled to about 200°C [7]. The heat release during cooling is recovered for further use in the reactor via steam. The synthesis gas is further cooled and the acidic components in the gas are absorbed by the alkaline solutions. The final synthesis gas is introduced in the engines at constant pressure to generate electricity. The two-stage pro­cess shows great promise for the conversion of waste into heat, electricity, and other valuable products. It gives higher throughput rate, higher conver­sion efficiency to a clean high calorific syngas, and better control over VOCs/ tars compared to single-stage operation. Process control and engineering are critical in both single — and two-stage processes [7, 70].