Bringing the R21 process to Europe1 January 2002
The Mitsui Recycling R21 process for pyrolysis of municipal solid waste is notable for its very low dioxin levels. It also promises a very large reduction in the volumes of material needing to go to landfill - very important in the light of the recent EC Landfill Directive - and it achieves high levels of recycling for metals and other materials. Operational experience with the first commercial plant in Japan, at the Yame Seibu Clean Centre, has been good. R21 was originally developed in response to the Japanese waste situation - limited landfill, widespread incineration, need to reduce dioxins - but is now being introduced into the European market. W B Bryce, Mitsui Babcock Energy Ltd, Renfrew, Scotland
It is clear that the nature of waste management in Britain, and elsewhere in Europe, is changing, and will continue to change over the next 10-20 years, largely in response to the provisions of the EC Landfill Directive. Local authorities, and other bodies responsible for waste disposal, have been re-examining their forward strategies and are seeking a more diverse mix of non-landfill options, including thermal processing.
In this context, there has been increasing interest in the more advanced thermal processes for the treatment of waste materials, based on pyrolysis or gasification of the waste, as an alternative to combustion-based incineration technologies. These advanced technologies are increasingly perceived as providing a better option for the thermal processing of wastes than conventional incineration, because of their environmental performance.
Mitsui Engineering and Shipbuilding (MES), the parent company of Mitsui Babcock Energy Ltd (MBEL), the British based international energy engineering company, has been involved in the development and commercialisation of an advanced thermal process for municipal solid waste (MSW) since 1991. This process is known as the Mitsui Recycling R21 process.
In Japan, the R21 process is now fully commercial, with one plant, having a capacity of 70 000 tonnes of MSW per annum, in commercial operation since March 2000. A photograph of this plant, at the Yame Seibu Clean Centre, is shown above.
The Centre is located in a rich fruit and agricultural area and is strikingly clean and quiet in operation.
The actual building contains local authority offices plus conference rooms and viewing galleries for the general public to see and learn about the process.
A second plant, with a design capacity of 150 000 tonnes of MSW per annum, is due for hand over to commercial operation in March 2002. In addition, MES is currently constructing a further four R21 plants in Japan, with capacities ranging from 40 000 to 150 000 tonnes of MSW per annum.
The R21 process
The R21 process involves the low temperature pyrolysis of coarsely shredded MSW, to produce syngas and crushed char which are then combusted under well controlled low excess air conditions in a high temperature slagging furnace.
MSW reception and handling
The R21 plants in Japan handle both MSW and items of bulky waste. The MSW and the bulky waste residue, after recovery of recyclable materials, are delivered to the refuse pit. The waste is recovered from the pit by crane, and fed to a biaxial shearing shredder unit. This reduces the waste to a top size of about 200 mm. The shredded waste is then fed, via the waste conveyor, to the inlet hopper of the screw feeder to the pyrolysis drum.
The pyrolysis drum
The shredded waste undergoes drying and low temperature pyrolysis, at temperatures up to 450°C, in a rotary drum reactor. The pyrolysis reactor is heated indirectly by hot air, which passes through a number of heat transfer tubes, arranged along the length of the drum. The pyrolysis drum is very effectively sealed against air ingress. The residence time of the waste in the pyrolysis drum is of the order of one hour, meaning that there is a large inventory of material in the pyrolysis drum. This has the effect of damping the impact of variations in the quality of the MSW feed, permitting stable operation of the pyrolysis unit and of downstream processes, ie, the high temperature combustor, the heat recovery boiler and the flue gas cleaning equipment.
The ferrous and non-ferrous metals in the MSW remain in a non-oxidised and unfused condition, and can be recovered in a clean form for recycling.
The pyrolysis process converts the MSW, a very poor and inconsistent fuel, into two higher quality fuels, namely:
• syngas, which is carried forward directly to the high temperature combustor; and
• a solid residue, which comprises char, inert solids and metals and which, after metal recovery, is also carried forward to the high temperature combustor.
The pyrolysis solids handling system
The hot solids are separated from the syngas in a purpose-designed unit, and are passed to the solids handling system. The solids are cooled, and the metals are recovered for recycling. The remaining solid residue, comprising both combustible char and inert material, is crushed to a top size of 1 mm and then conveyed pneumatically to the high temperature combustor.
High temperature combustion
The pyrolysis gas and the crushed material from the solids handling facility are co-fired in suspension in the high temperature combustor. The furnace is operated as a down-fired cyclone combustor, with the ash particles being encouraged to attach to the furnace walls. The combustor operates at temperatures in excess of 1300°C, to ensure complete melting of the ash, and a continuous flow of molten ash down the furnace walls into the slag tap unit at the bottom of the furnace.
The pyrolysis gas and the crushed solids are reasonably good quality and consistent fuels. Very good, stable combustion conditions are achieved without support fuel and with very low CO concentrations at O2 levels around 3.5 per cent (excess air ratio around 1.2). The introduction of the combustion air into the furnace is staged to allow control over the NOx emissions. The very good combustion conditions ensure that the levels of dioxin precursor compounds in the combustion gases are minimised.
The fused ash flows down the furnace walls and through a slag tap at the bottom, where it is immediately water quenched to produce an inert granular glassy material. Since metallic items have been removed with high efficiency in the pyrolysis solids handling facility, the slag is largely free of metals and is of high quality. The very good combustion results in zero carbon in the slag.
At the Yame Seibu Clean Centre, all of the slag is sold as a road construction material.
The high temperature airheater
The flue gases pass from the furnace exit to the high temperature airheater. This unit generates hot air at around 520°C, which is used for the indirect heating of the pyrolysis drum. The air is circulated in a closed loop system by the hot air circulation fan. The air temperature in the return from the pyrolysis drum is around 300°C.
The waste heat boiler
The combustion flue gases pass from the exit of the high temperature airheater at around 600°C to the waste heat boiler. This is a natural circulation, tail end boiler unit, which generates steam at 400°C and 40 bar, for supply to the turbo-generator. The electricity produced is used to operate the plant, and the excess can be exported.
The boiler has design features intended to minimise high temperature corrosion and ash deposition, and has effective on-line cleaning systems for both the radiant and convective heat transfer surfaces.
The flue gas cleaning system
The flue gases at the exit of the boiler are cooled to temperatures below 170°C, and pass to bag filter No 1, where the entrained fly ash particles are collected. All of the ash materials collected in bag filter No 1 and in the airheater and boiler hoppers are recycled to the high temperature combustor. This means that there are no fly ash discard streams which require to be sent for landfill disposal.
Bag filter No 2 is fitted with a lime injection system for acid gas emission abatement. The material collected in bag filter No 2 comprises largely a mixture of unreacted lime, calcium sulphate/sulphite and calcium chloride. This is the only solid discard stream from the R21 system that has to be sent for landfill disposal.
Performance at Yame Seibu
The successful handover and commercial operation of the Yame Seibu Clean Centre has demonstrated a number of the key features of the R21 process:
• Thermal processing of the MSW and bulky waste residues, with ash melting and with no requirement for a supplementary fuel.
• Very stable operation of the process at low excess air levels, with all of the gaseous and gas-borne emission levels below the consent limits.
• Very high landfill volume reduction levels. All of the recovered metals and the slag are sold as recycled materials. Only the acid gas clean-up residues have to be sent for landfill disposal, and this represents a landfill volume requirement of only 1 per cent, approximately, of that of the MSW.
The R21 process has been designed for minimum generation and release of dioxins and furans, and the measured performance of the plant at Yame Seibu indicates exceptional performance in this regard:
• The measured values of the dioxin concentration in the flue gases emitted from the chimney are less than 20 per cent of the consent limit of 0.1 ng TEQ
Nm-3 (dry, at 12 per cent O2).
• The dioxin content of the slag is well below the soil environmental standard of 1 ng TEQ g-1 and is actually below the detection limit of 0.0018 ng TEQ g-1.
• The dioxin content of the acid gas clean-up residues collected in bag filter No 2 is 0.0053 ng TEQ g-1.
• The total dioxin releases from the Yame Seibu plant to all media are approximately 0.26 µg TEQ per tonne of MSW processed. This is less than 10 per cent of the proposed Japanese government target value of 4.25 µg per tonne of MSW processed.
• The R21 process is one of the very few novel thermal treatment processes for MSW that are capable of achieving this level of dioxin destruction performance.
The process involves a high degree of recovery of energy and useable materials. The illustrations below shows the ferritic and the aluminium recovered in a clean, unfused condition plus the glassy slag and examples of roads built with it.
Bringing R21 to Britain
The UK represents one of the most buoyant markets for energy from waste projects over the next 10-20 years, driven largely by the impact of the EU Landfill Directive. It is also clear that there will be a significant role for the more advanced thermal processes for MSW within this growing market, and particularly for plants processing 50 000-150 000 tonnes of MSW per annum.
The R21 process is probably the leading advanced thermal process for MSW worldwide, and this success has been based on many years of experience with the handling and thermal processing of MSW within MES. The good performance of the R21 plant at the Yame Seibu Clean Centre is an important step forward in the commercialisation of the technology.
The key attractions of the R21 process in this context are:
• Efficient thermal processing of MSW, with energy recovery and power production, and with no requirement for an oxygen supply or a support fuel.
• Very low levels of dioxins, both in terms of the levels emitted from the chimney and of the total releases from the process to all media.
• High levels of recycling of metals and solid residues. And
• Very low volumes of solid materials sent for landfill disposal.
Mitsui Babcock is currently involved in refining the design and operation of the R21 process to meet the specific requirements of the British market.
It is considered that the R21 process can play a significant role in the implementation of waste management strategies in Britain over the short to medium term future. The technology represents a step change improvement over conventional incineration, in terms of:
• the high level of recycling of metals and other materials;
• the very low total dioxin releases from the process; and
• the large reduction in volume of material that must go to landfill.
The recent performance record of the R21 plant at Yame Seibu and the current Mitsui Engineering and Shipbuilding order book in Japan is helping to establish R21 as the leading advanced thermal process for MSW worldwide, and will help to reassure the local authorities and waste management companies of the environmental and commercial viability of this advanced pyrolysis process.