By Siân Green

A new plant based on Mitsui Engineering & Shipbuilding Co’s latest waste processing technology has entered commercial operation in Japan. The technology is now bound for Europe, offering high levels of waste volume reduction, efficient energy recovery and low dioxins.

In 1997, Japan’s Mitsui Engineering & Shipbuilding Co (MES) received its first commercial order for its Recycling 21 (R21) technology, a pyrolysis, gasification and ash melting processing plant for municipal solid waste (MSW). The 70 000 t/a Yame Seibu plant is now operational, demonstrating the high environmental standards of this latest generation waste-to-energy technology.

The Yame Seibu plant is the first plant of its kind in Japan and exports around 1 MW of power to the local grid. Following its successful implementation, MES has taken a further five orders for R21-based plants in Japan, ranging from 40 000 t/a to 140 000 t/a in size.

Involving the low temperature pyrolysis of MSW and combustion of the pyrolysis products, R21 was developed during the 1990s in response to the needs of the Japanese waste market. MES subsidiary Mitsui Babcock has described the technology as a “very significant advance in the treatment of municipal waste”, and is now looking to market it as a waste-to-energy system in the UK and other parts of Europe.

A new solution

Because of scarce landfill resources in Japan, the country focussed on the use of traditional grate-type incinerators to handle mixed waste materials that could not be recycled. The high level of dioxins emitted from the incinerators led to restrictions on waste incineration, and the technology is no longer permitted in the country.

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A solution was therefore required that could process waste efficiently and to high environmental standards. MES developed R21 in response to this, and built and operated a 20 t/day pilot facility in Yokohama from 1994-1997 to gain experience with and refine the process. Construction then started on Yame Seibu.

The Yame Seibu plant has now been in operation for over a year. It consists of a waste reception unit, a pyrolysis unit, a combustor and ash melting furnace, and flue gas cooling and clean-up facilities. Waste is delivered to the plant’s refuse bunker and is fed by crane to a refuse shredder.

The shredded waste then undergoes low temperature drying and pyrolysis at 450°C in a pyrolysis drum in which it resides for around one hour. Pyrolysis produces a gas, which is fed directly to a high temperature combustor, and a solid residue, which comprises char, inert solids and metals.

The solid residue is cooled and sorted to extract ferrous and non-ferrous metals. Separation is achieved by the use of sieves, magnetic separators and aluminium separators, and due to the absence of oxygen in the pyrolysis process, the metals are recovered in their unfused form, making them more marketable. The remaining solids, which contain combustible char and inert material, are crushed and conveyed to the high temperature combustion chamber.

The solids and pyrolysis gas are co-fired in the combustor at 1300°C. Operating as a cyclone unit, the ash particles attach to the furnace walls, melt and flow down the furnace into the slag tap at the bottom. Combustion conditions are stable so that the production of dioxins is minimized.

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Melted ash in the slag tap is quenched with water to produce an inert vitreous material which can be marketed as a construction material and so does not need to be landfilled.

The exhaust gases from the combustor pass to a high temperature airheater where it heats the air used to heat the pyrolysis drum. The gases exit this unit at around 600°C and pass to a natural circulation heat recovery boiler which generates 400°C, 40 bar steam to generate electricity in the steam turbine. Electricity generated is used to operate the plant while the remainder (around 1 MW) is exported to the grid.

Flue gases are cooled to 170°C and fed to No. 1 bag filter where fly ash particles are caught, and then to No. 2 bag filter, where HCl and SOx are removed.

The dioxin issue

According to Bill Bryce of Mitsui Babcock, the principal benefits of R21 are its high levels of waste volume reduction and the low levels of dioxins that are produced.

Dioxins produced by waste processing facilities fall into two main categories: those that are emitted via the stack and those that exist in the material that goes to landfill. There is legislation for the former type, but not for the latter, and this is often of concern to the public.

In a conventional incineration plant, ash is produced that contains dioxins, and although it can be treated and used as a road construction material, it is often landfilled. The R21 technology melts all the ash produced and turns it into a vitrified, inert material that contains no carbon or dioxins. This can also be used for roadbuilding.

Overall with R21 technology, just two per cent of the original input volume needs to be landfilled. This compares to 28-30 per cent for conventional incinerators.

In addition, R21 allows metals such as aluminium and iron to be recycled; no additional power is required to run the process; and any excess power produced can be exported to the grid.

These benefits make the technology highly suitable not only for the Japanese market, but also the UK market, which Mitsui Babcock is now targeting. According to Bryce, the waste-to-energy market in the UK is large and it is now at a turning point because of the landfill directive, under which the amount of waste going to landfill must be significantly reduced.

Most of the plants that are at various stages of planning and pollution control approval in the UK are either fluidized bed combustion-based or grate-type incinerator plants. These types of plants have a poor perception with the UK general public largely due to their dioxin emissions, and they are therefore experiencing difficulties with planning.

“In the UK, 90 per cent of waste currently goes to landfill, and this will have to be reduced to 30 per cent under the directive,” comments Bryce. “There are 13 incinerators working and they’re getting bad publicity. The feeling in the industry is that perhaps traditional incinerators don’t have a very much longer life. Obviously the operators of the incinerators oppose this view.”

So with its low dioxin emission levels and volume reduction abilities, R21 is an obvious option for the UK, and Mitsui Babcock is currently in the process of modifying the technology for this market.

Making modifications

According to Bryce, the technology must be modified because in Japan, the main aim of R21 plants is the processing of waste rather than energy production. In the UK, however, energy production from the process will be an important consideration. The energy recovery process of the R21 system must therefore be enhanced and optimized.

In addition, municipal waste in Japan is highly sorted into its different constituents, e.g., glass, plastics, and paper. This is not the case in the UK, where waste is mixed. This must therefore be taken into account in the design of the technology, and in Japan MES has tested a number of wastes of UK composition in its pilot plant.

R21 technology is designed to reduce waste volume and produce power with low environmental impact
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“They have put a whole range of different types of waste through it and have gathered a lot of operational data from that pilot plant,” said Bryce, adding: “We are also looking at ways of optimizing the design in terms of the footprint and the costs.”

An important part of cost optimization will be the procurement of parts for the pyrolysis plant. Mitsui Babcock is aiming to procure most parts from the UK and other parts of Europe, rather than Japan. “This will be quite possible because the components are the sort of thing that we would build in our plant in Renfrew or that are available from the various specialist suppliers across Europe. For example, the equipment for shredding the waste, for separating out the metals, and for flue gas cleanup.”

And cost will be something that the potential buyers of R21 in the UK will be focussing on. While the running costs of an R21-based plant are projected to be similar to those of conventional incinerators, the capital cost of the technology will be higher.

“However,” argues Bryce, “if you add to incinerators the cost of the plant needed to bring them in line with the performance of R21, then the capital costs become similar.”

In the UK, Mitsui Babcock is working with the Environment Agency and various government departments to demonstrate the benefits of R21. It has made an offer for one project and hopes to bid in early 2002 for a number of other projects.