Until recently, an incinerator’s primary function was to dispose of waste material by rendering it harmless and reducing its volume; the recovery of heat was always regarded as secondary. This philosophy is now changing, particularly with the advent of NFFO for electricity production making the economics of waste to energy far more attractive. Also, with the ever increasing cost of landfill and with some waste streams now attracting a gate fee, the benefits of heat recovery compared with a decade ago are far greater.
Better understanding of the behaviour of salts and heavy metals produced by waste incineration is necessary to keep up with the increasing complexity of fouling and corrosion problems, as more difficult waste streams are burnt.
Many would consider that burning waste effectively, and cleanly, is the difficult step of the incineration process, and heat recovery is simply a case of fitting a conventional boiler between the incinerator outlet and the chimney. Early designs assumed this, but it was soon realised that if a system was going to be operating continuously for more than a few days then conventional designs of boiler were inappropriate. The boilers were experiencing rapid fouling and corrosion and it was clear that these problems needed addressing at the design stage.
Design issues
When designing a heat recovery module, apart from having an appreciation of the basics of heat transfer (conduction, convection, radiation), a number of operational considerations should be taken into account:
•High gas inlet temperatures (l,000°C – 1,400°C)
•Variable excess air levels leading to oxidising/reducing zones promoting corrosion
•High concentrations of low boiling point vapour phase metals
•High particulate/flyash concentrations
•Variable duty due to inconsistent calorific value and feed rates of the waste fuel.
All waste streams exhibit the properties indicated above, to a greater or lesser extent, although where liquid and gaseous wastes are concerned, the problems are generally more easily overcome.
The choice of materials of construction is the key to overcoming the problems of temperature. Ceramics and high temperature stainless steels tend to be employed in particularly high temperature regimes. Mild steel is employed extensively in the lower temperature range to keep costs down. Selecting the appropriate firing system will minimise corrosion and the general problems of burning the waste completely, minimising dioxin formation and other VOCs and PCBs.
Low boiling point metals condensing on the heat exchanger surfaces is a problem which is not easy to resolve. Sulphates and chlorides, particularly sodium and calcium based, have a tendency to stick to surfaces acting as the glue for ash particles. These build up rapidly causing blockages/fouling in the gas passes. Designing wider gas passes and the employment of sootblowing techniques help reduce the problem but cannot be considered a total solution.
Blending of waste streams to provide a consistent calorific value fuel will considerably reduce the problems of unstable operation and the production of high levels of unburned hydrocarbon gases. Where this is not possible, the heat exchanger needs to be designed to prevent stagnant zones of flue gases which could otherwise cause corrosion.
Typical designs
Most wastes have varying properties which make different demands of the boiler. Site requirements also need to be taken into account. Is there a requirement for hot water or steam? Is the steam needed for process or power generation, or both? A range of designs is required to meet these needs. Typical designs are described below.
Shell and smoke tube boilers are examples of heat exchangers used throughout the small-scale industrial boiler market (generally less than 15MWth). The boiler comprises two tube plates, held apart by stay bars, which support the smoke tubes through which the products of combustion pass. The smoke tubes are surrounded by water and the entire assembly is encased in a pressure vessel (the shell). The shell may contain several flue gas passes, although two or three is typical. A water tube section can be installed in front of the inlet to the first pass to produce superheated steam for power generation.
The water tube design is effectively the reverse of the smoke tube heat exchanger; the combustion gases pass over banks of tubes containing water/steam. The advantage of this over the shell design is that higher steam pressures, and hence superheat temperatures, can be achieved typically 45 bar, 450°C. These are the conditions required to drive a conventional steam turbine/generator set for electricity production.
Both the above designs have the inherent problems of passing large volumes of combustion gases through confined spaces. Of the two, the smoke tube design is the more susceptible to fouling. The use of steam/air lances to disturb and remove deposits is commonplace (sootblowing). They are reasonably effective when the deposits are friable but less so when they are hard and firmly bonded to the tubes.
In both designs, it is inevitable that, in time, there will be a reduction in heat transfer, or an increase in pressure drop, which reaches a level where the boiler needs to be taken off-line for cleaning. The best designs are the ones where these interruptions are kept to a minimum. Some of these improvements are described as follows.
Heat exchanger variants
There are many instances when a reduction in the combustion gas temperature prior to entering the heat exchange passes of the boiler can reduce fouling and/or corrosion. Two such methods of achieving this are wall panel and fluidised bed heat exchangers.
Wall panel (also termed radiant chamber) designs are available where the heat transfer surface is arranged in the form of wall panels installed in such a manner as to minimise obstruction to the combustion gas flow. This has the benefit of minimising pressure drop and also allows for ease of cleaning, or even replacement, reducing down-time. A design exit temperature from the panel section should be below the melting point of the primary fouling components, so they enter the higher pressure drop heat exchanger section in the solid phase rather than in liquid form.
Combustion systems employing fluidised bed technology as a means of heat transfer tend to benefit from being self scouring or cleaning. Unfortunately, some designs of fluidised bed boilers have suffered from high rates of erosion, reducing the life of the boiler plant. The successful designs are those where erosion is minimised while the action of the inert material keeps the heat transfer surfaces clean.
Circulating fluidised bed (CFB) systems have been designed to minimise the problems of erosion. This has been achieved by removing the tubes that were immersed within the bed in the first combustion chamber and inserting them in a secondary chamber operating with a much reduced fluidising velocity. Mixing of combustible material and carbon burn-out is particularly high in these systems ensuring that corrosion and fouling are minimised.
Radiant superheaters are positioned in the high temperature region of the furnace upstream of the convective section of the boilers. In this position, and depending on the type of waste, they can be prone to fouling resulting from the high gas temperatures required for generating superheated steam. One way of avoiding this is to shield the superheater from the radiant combustion zone but, to maintain the same level of output, it must be made a lot larger. Sometimes this is not possible and if significant levels of superheated steam are required, the superheater can be located external to the boiler and heated via a separate oil or gas fired burner.
Future developments
Predictions are that there will be an increased requirement for combined heat and power systems so that heat recovery from the burning of waste streams or renewable fuel sources is here to stay. The main question is, can heat recovery units be improved to meet this demand? With the advent of new higher specification steels and the use of ceramics in difficult applications, there is plenty of scope to improve heat exchanger performance and hence efficiency.
New filtration technologies involving ceramic based filters capable of operating at temperatures of up to l,000°C offer the prospect of filtering the particles upstream of the heat exchanger hence avoiding fouling. If they could be made to work reliably, it would significantly improve plant availability. Additionally, introducing more sophisticated sorbents to remove gaseous pollutants would help to reduce corrosion.
Corrosion and fouling in coal fired applications is reasonably well understood but this cannot be said of the burning of waste streams. What is needed is a more complete understanding of the physical and chemical behaviour of the salts and heavy metals produced by the burning of these wastes. More fundamental research is necessary in both of these areas, and into high temperature filtration, to keep up with the increasing complexity of these problems as more difficult waste streams are burned. Enquiry 18
Environmental Development Technology (EDT) specialises in combustion and air pollution control within the industrial energy market. It aims to assist industry in improving energy efficiency and minimising the environmental impact of combustion equipment.
READER ENQUIRY NO 262