ADVANCED THERMAL PROCESSING
Waste has been thermally processed since earliest man discovered fire. Burning waste reduces its volume and converts the organic portion which would otherwise rot, leaving a relatively inert, sterile ash. It was once common practice to set fire to the town rubbish tip to reduce the size of the waste heap and extend the life of the tip, as well as controlling vermin and protecting groundwater from contamination. However, while that practice survives in a few places, its potential to pollute is now recognised and it is generally prohibited.
Thermal processing can be achieved in several different ways. The most commonly used technique for thermally processing municipal solid waste (MSW) is mass burn incineration. Mass burn technologies involve the delivery of unsorted waste into a storage pit or bunker, from where it is moved by conveyor into the furnace. The waste is burned as delivered, apart from the removal of bulky items like mattresses and bicycles. To aid combustion, there is also likely to be some mixing of the waste. In the past incineration plants were mainly designed purely to process waste, but today’s plants are usually designed to recover energy (as steam, hot water or electricity) from the waste. Municipal waste typically has an energy value around half that of high grade coal.
Mass burn techniques continue to be developed and improved, particularly with regard to air emissions controls, and modern plants are very much more advanced than those of even ten years ago. There are however, circumstances in which the mass burning of waste is not appropriate, perhaps because the volumes are too small (modern mass burn incineration plants typically process 200,000 to 400,000 tonnes of waste a year), or perhaps because of the particular character of the wastes. These differing needs have driven technology developments for some alternative thermal processing techniques.
One alternative means of thermally processing mixed MSW is to make a fuel from it. The manufacture of refuse-derived fuel (RDF) is not new. It was originally devised as a means of avoiding the need to burn MSW immediately, and instead to turn it into a transportable, storable fuel. Although not a thermal process RDF production enables the subsequent thermal conversion of combustible portions of waste. While mass burning involves little or no sorting or processing of the waste, in RDF production the waste may undergo a number of processing stages.
In the USA, THE American Society for Testing Materials Committee’s work on resource recovery defines seven different categories of RDF, according to the amount of processing involved: from simple removal of over-sized material to screening, shredding, densifying and pelletising through to conversion into liquid or gaseous fuels. These last two categories were originally used to cover treatment of petroleum wastes, but a number of new facilities are now using the techniques to treat other wastes including MSW (mostly at pilot demonstration scale).
At its simplest, RDF may be a coarse, fluff-like material produced from mixed MSW by a series of screening stages, plus magnetic removal of ferrous metals. The fluff RDF is partly compacted to simplify handling, but would not normally be transported off-site.
Alternatively additional processing may turn it into a densified, pelletised (or cubed) fuel, for ease of transport and storage. Turning waste into coarse or pelletised RDF differs from mass burn in being two-stage can be conducted completely separately from the second burning stage, and may be at a different site and time.
With increasing attention being paid world-wide to separating waste for material recycling, RDF production techniques may provide a valuable complement, and since pre-processed waste needs different combustion conditions, the use of alternative and more advanced combustion systems is also likely to develop. One such system is fluidised bed combustion.
Fluidised bed combustion
Fluidised bed combustion technology is based on a system where, instead of the waste being burned on a grate (as in mass burn processes), the fire bed is composed of inert particles such as sand or ash. When combustion air is blown up through the bed, the bed material becomes fluidised and the fuel (in this case waste) is mixed with the hot bed material (usually sand) to ensure it is completely burned. There are several different designs of fluidised bed (FB) combustors, for example circulating and bubbling beds. All have in common the need for the fuel (i.e. waste) to be of uniform size when fed into the system.
Compared to mss burn, fluidised bed combustion systems have reduced emissions, partly because of the process itself and partly because it is possible to add lime to the bed. Since as much as a third of the cost of mass burn plants is absorbed by the air pollution control (APC) system there are savings to be made by the FB combustors’ smaller APC needs. On the other hand, mass burn plants have no need for front-end processing of the waste. Also, as they are typically larger, and so enjoy economies of scale, the cost per tonne of waste processing in the two systems may not be markedly different.
Because FB systems are typically smaller, they can be more appropriate for smaller communities. The need to pre-process waste prior to combustion in an FB combustion plant, in order to reduce its size and make it uniform, provides an opportunity to maximise materials recycling. However, while metals can be separated from the waste when it is being shredded and reduced in size, for successful recycling of most materials they must be kept clean and this requires them to be separated at source, not mixed with other wastes.
While FBs have been used in industrial applications for a number of years, and to burn wood chips and similar single-material fuels, their use for mixed waste is more recent. Mixed MSW is not an easy fuel to burn, because of its variability, and maintenance costs of FB combustors used for MSW are likely to be much higher than those for a single, predictable waste stream like wood chips.
A £42 million bubbling fluidised bed combustion plant with a capacity of 120,000 tonnes per year (in two boilers) was scheduled to start operation in Dundee, Scotland in 1999. The plant can generate around 8 MW of electricity for export to the grid, in addition to meeting its own needs. Metals recovery is included in the waste processing stages.
Exothermic: a process which gives off heat
Endothermic:a process which absorbs heat
Incineration:an exothermic process in the presence of oxygen in which carbon-based matter is decomposed, leaving an ash residue
Pyrolysis:an endothermic process in the absence of oxygen in which carbon-based matter is decomposed into gaseous or liquid fuels and a solid char
Gasification:a process similar to pyrolysis but with the addition of oxygen, for the production of a gaseous fuel
Some Emerging Technologies
Swedish company TPS Termiska Processer AB has developed a starved-air gasification system for RDF, based on a combination of bubbling and circulating fluidised bed reactors. A gasification facility was constructed by TPS in Grève-en-Chianti, Italy en 1992. Designed to handle pelletised RDF, the gas was used in a nearby cement kiln. Two additional wood-fuelled circulating FB gasifier plants are currently under construction, one of which, in Brazil, will generate 30 MW of electricity.
The Proler SynGas process, originally designed to gasify car shredder residue, has also been tested with MSW. Consisting of a rotating kiln-like reactor, it produces a fuel gas that is suitable for power generation. The waste is processed at 650 to 850ºC under a reducing atmosphere. This technology is not yet at commercial stage.
A Battelle commercial scale demonstration gasification plant is operating in Vermont, USA using wood chips as fuel. The process consists of two indirectly heated circulating fluidised bed reactors.
A recent and innovative use of FB systems is the development of a gasification mode: gasification is the process of reacting carbon-based material with oxygen or steam to produce a gaseous fuel. The gas can then either be cleaned and burned in a gas engine or turbine, or used in boilers to generate steam that in turn can be used in a turbine to generate electricity.
The gasification process itself is not new, and it has most commonly been used for the manufacture of town gas. Biomass and sewage sludge, as well as wood, are the most common fuels processed in this way. Gasification of chipped wood and tyres has been successfully demonstrated in the USA and Scandinavia, among other places. Emissions from such systems have been shown to be low in substances such as dioxins, acid gases and other contaminants, as well as having low amounts of solid residues (around 10 per cent).
However there is little practical experience of MSW gasification as yet and further investigation is under way. In addition to the recent use of FB combustors, specially designed gasifiers are also in use.
Both pyrolysis and gasification involve the heating of organic material in an oxygen-controlled environment. Where gasification depends on the addition of oxygen (sometimes in the form of steam), pyrolysis takes place in the absence of oxygen. Both technologies have primarily been used for specific and generally single, unmixed waste streams such as tyres and plastics, or to process RDF.
However, one German pyrolysis plant has been processing MSW since 1985. German waste company Deutsche Babcock Anlagen GmbH began trials on the pyrolysis of MSW in the 1970s and a full scale six tonnes/hour plant was commissioned in Günzburg, Bavaria in 1983. Since 1985 the plant has been in permanent operation. The shredded waste is fed into a gas-fired rotary drum where it is pyrolysed at temperatures of 400 a 500ºC. The pyrolysis gas passes through a cyclone for the removal of coarse particulates and is then directly burnt in a post-combustions chamber at temperatures of around 1200ºC.
Despite the above example, neither pyrolysis nor gasification is generally considered suitable for handling mixed, untreated MSW in large volumes at present, althroug a number of developments world-wide may change that within the next five to ten years.
Two stage processes
There are a number of recent two-stage thermals processing projects. Among those are several which have been used to handle MSW, sewage sludge and commercial waste in addition to bulky wastes like tyres, which need to be shredded first.
One example is the company Thermoselect, which has a patented two-stage process for waste gasification followed by vitrification of the solid residues. The controlled combustion of parts of the material supplies the energy for the overall process. Around 1,000 cubic metres of gas are produced per tonne of gasified waste. Thermoselect operates a 100 tonnes per day pilot plant in Northern Italy and is building a full scale plant in Karlsruhe, Germany, with an annual capacity of 225,000 tonnes.
Another example is German company, Siemens AG’s thermal waste recycling process, a two-stage thermal treatment process for municipal solid waste, including the pyrolysis of shredded and homogenized waste. The process has been tested in a 200 kg/hour pilot plant in Ulm-Wiblingen and in a one tonne/hour demonstration plant in Japan. I February 1997, a full scale plant started test operation in the city of Fürth in Germany, although it later experienced some difficulties.
Thermal conversion of MSW is an important component of an integrated solid waste management strategy which has both environmental and economic benefits. The technology chosen for any specific situation should reflect the volumes and types of waste.
The most well-established and proven of the thermal processes for MSW is mass burn incineration and development continues to improve the technology. Fluidised bed combustion technologies are increasingly being considered for MSW, and their smaller scale may make them suitable for applications where a large scale mass burn plant would not be viable.
To date, the less widely used technologies of pyrolysis and gasification have valuable specific applications, particularly for single waste streams. While they may not yet be appropriate for MSW as an general rule, that situation may change when current demonstration projects have sufficient operating experience.
However, it is important to note that many of the alternative thermals processing techniques rely on a uniform, consistent fuel, and MSW is very far from that. Mass burn systems which have been specifically developed to deal with the vagaries of varying wastes, and which are robust and reliable, will continue to be relied upon and improved.
Source: Warmer Bulletin September 1999