There is growing political concern about the negative environmental effects of burning fossil fuels for power generation. Coupled with similar concern about the environmental costs of disposing of municipal solid waste (MSW) in landfill, this has led to a growing interest in generating useful heat and electrical power by incinerating MSW.
In the UK alone, some 34 million tonnes of MSW are produced annually. This could theoretically produce more than 200 million GJ of thermal energy.
Over the past decade the proportion of MSW in the UK from which energy is recovered by incineration has doubled from the level of around 6 per cent recorded in 1991. At that time there were five MSW incinerators operating, in Sheffield, Coventry, Nottingham, Edmonton and Jersey – with an installed capacity of 42 MW of electricity plus district heating. Since then the SELCHP plant in east London has come on stream, in December 1993, producing 32 MWe, and the Tyseley plant came on stream in July 1996, with a capacity of 30 MWe. Others followed shortly afterwards, such as Stoke and Wolverhampton.
There has been a similar growth in the number of such plants in France (see panel right for a recent example) and Germany. But the pattern has not been the same in Scandinavia during the last decade because energy from waste had already been maximised there.
A further trend throughout Europe has been the move away from very large plants, handling upwards of 250 000 tonnes of MSW per year. This size had been deemed necessary to gain economies of scale. However, recently the tendency has been instead to build plants of around half that capacity for towns of 100 000-200 000 population. This has been driven to a large degree by public opposition to the transport of waste (which causes more pollution than its incineration) and the so-called ‘proximity principle’ that waste should bedealt with close to where it is generated.
Special problems
The combination of cooling flue gas from MSW incineration and generating steam makes special demands on equipment which cannot be satisfied from conventional boiler making experience alone. The special problems which arise for a boiler cooling the flue gas from a waste incinerator include:
• high dust content in the flue gas which leads to fouling and erosion;
• corrosive components in the flue gas;
• the need to prevent the formation of dioxins as the gas cools; and
• variations in the heat load arising from the heterogeneous nature of the fuel and the batch feeding of it.
Despite all the difficulties, however, waste incineration plants can be planned, designed, built and operated to the same high standards as other process plants.
Boiler design
The absence of any direct cooling of the flue gas by air injection or water quenching results in high boiler efficiency (typically 85 per cent) for an energy-recovering MSW incinerator and a minimum plant operating cost. However, the boiler should be designed for maximum reliability, not for maximum heat recovery. It must also be extremely flexible to cope with the inherent load changes which result from the heterogeneous fuel.
There is not sufficient space within this article to cover the details of boiler design necessary to minimise the problems of erosion, corrosion and emissions production referred to above. A great deal of practical experience has been gained in this field, however, and boiler designs capable of meeting these objectives have been tried and proven.
The steam cycle concept and all its components must be considered together with the boiler if energy recovery is to be optimised.
To avoid metal temperature levels at which accelerated high temperature corrosion takes place in the boiler, the selection of the steam conditions is critical. The optimum conditions are a maximum of 45 bar steam pressure and 420°C live steam temperature.
Providing increased corrosion allowance by the selection of tubing with thicker walls than the design code requires is a practical approach to extending component life. In the MSW incineration plant at Würzburg, Germany, for example, where the steam conditions are 42 bar and 415°C, signs of corrosive attack in the superheater did not become apparent until the plant had logged 25 000 hours. The complete replacement of the superheater elements was not required until 35 000 operating hours. This contrasts favourably with the histories of other steam generators operating with higher steam conditions, whether built to similar or different designs, and where comparable problems have sometimes developed after as few as 3 000 to 5 000 operating hours.
Steam turbine design
Electricity generation is by far the most common major use of the steam produced by MSW incineration but this does not preclude some of the energy being available for other uses.
For example, it would be possible to export all the steam produced to a process plant. This would eliminate the need for a turbine-generator, with consequent savings in capital costs. However, it is rare for any process plant operator to be willing to be dependent on a waste incineration facility for its steam. In addition, the value of steam is low compared with the value of electricity. The difference usually justifies the cost of a turbine-generator and its ancillaries.
When the aim is solely to produce electricity, steam is expanded through a condensing steam turbine to as low a vacuum as is practical, in order to convert as much as possible of the energy available in the steam into electricity.
For example, in the Tynes Bay energy-from-waste plant on the island of Bermuda, steam is supplied to the stop valve of the 3.8 MWe turbine at 43 bar and 380°C, and is exhausted to a water-cooled condenser at a pressure of 0.11 bara. The turbine has seven stages and runs at 7000 r/min, geared down to a four-pole speed of 1800 r/min.
Whilst the steam turbine for this type of application is a condensing unit, it is usual to have one or two bleeds for feed-heating. Condensate leaves the condenser hot-well at a temperature corresponding to the exhaust pressure saturation temperature and is raised to the required feedwater temperature by using heat from the bled steam in either a feedwater heater or the deaerator.
Condensers can be either water- or air-cooled. A water-cooled condenser integral with the turbine’s bedplate – which is feasible up to a power output of about 5.0 MWe – provides savings in the cost of civil works and installation.
At many energy-from-waste plants the income from disposal of waste is as important as the power generated and it is not possible or practical to shut down the incinerator/boiler when the turbine-generator is out of operation. In such a situation it is necessary to isolate the condenser from the turbine and to dump the steam into the condenser at atmospheric pressure through a pressure reducing and de-superheating station.
It is not possible to provide an exhaust isolating valve with an integral condenser, so a separate condenser must be used. However, for capacities up to 3 MW it is always worth considering using a small separate atmospheric dump condenser because the additional cost of this may be offset by the saving in civil costs associated with integral condensers.
It is not acceptable to dump steam into a condenser without isolating it from the turbine, as this will subject the exhaust end of the turbine to prolonged temperature increases which may cause damage. All steam turbines are designed such that the correct running clearances are achieved when operating at the design temperature – typically 45ºC to 60º C at the exhaust of a condensing machine. By subjecting the casing to a prolonged temperature of 100ºC, which will occur when dumping steam at atmospheric pressure, there is the danger of blades rubbing on the casing or even of casing distortion.
The choice between air cooling and water cooling for the condenser depends on a number of considerations, the most obvious being water availability. It is not cost effective to use town water for cooling tower make-up and it is usually difficult to get permission to extract either ground water or river water. Planning considerations must also to be taken into account as many authorities object to cooling tower plumes.
It is important in this type of application to exhaust to as great a vacuum as possible to present the turbine with the maximum enthalpy drop. This vacuum is limited by two considerations, however:
• the temperature difference between the cooling medium leaving the condenser and the saturation temperature at the turbine’s exhaust pressure, which is rarely less than 3°C; and
• the exhaust wetness, which can be up to 15 per cent.
The importance of the exhaust vacuum can be demonstrated by comparing the enthalpy drop (ie the useful energy available in the steam) for a typical inlet condition of 40 bar and 400ºC at different exhaust pressures. At an exhaust pressure of 0.1 bara the enthalpy drop is 1070 kJ/kg, whereas at 0.05 bara it is 1150 kJ/kg – an increase of almost 8 per cent in potential output.
Clearly, increasing the exhaust vacuum is very desirable. In recent years Peter Brotherhood Ltd, for example, has developed blade and casings technology to handle the large volumetric flows associated with such low vacuums. The only disadvantages of an increased vacuum are that the condenser increases in size and cost and the c ooling water quantity is increased. But these additional costs are rapidly repaid by the increased output at the generator terminals.
Combined heat and power
In a condensing turbine as described above, as much as two thirds of the energy in the steam is lost to the cooling medium. It is possible to increase the plant efficiency considerably by raising the exhaust pressure so that the steam leaving the turbine can be used to produce hot water for district heating.
A good example is the turbine-generator set installed in the district heating plant at Thisted, Denmark. This is a relatively small plant which takes all the MSW from the local community and produces electricity, which is fed to the grid, and hot water, which is piped round the city.
The principal difference between this scheme and a pure condensing arrangement is the exhaust pressure – in this case 0.46 bara compared with the usual 0.05 to 0.15 bara. The corresponding exhaust temperature is higher, enabling the district heating return water to be raised from 45°C to 75°C. Of the 15.7 MW entering the system, 13.3 MW is available either as electricity (3.2 MW) or district heating (10.1 MW).
An interesting variation on the district heating principle can be found at the Becton sewage works on the banks of the Thames, east of London, UK. One of Becton’s energy-from-waste plants burns sewage gas in a gas turbine. Exhaust gases from the gas turbine are passed through a heat recovery steam generator to raise steam for a 2.6 MWe condensing steam turbine. Steam is supplied to the turbine at 39 bar and 400°C and exhausted into a district heating type condenser at 0.35 bara to produce hot water. The heat in the water is used to assist digestion of the sewage which produces the gas for the gas turbine. Electrical power from generators connected to both the gas turbine and the steam turbine is exported to the grid.
Extraction steam turbines are available with either uncontrolled or controlled extractions.
Uncontrolled extractions are a simple connection in between two stages on the turbine casing at the appropriate pressure for the steam to be used elsewhere. Steam for deaerators and feed-heaters is usually provided from an uncontrolled bleed. The disadvantage of an uncontrolled bleed is that the bleed pressure will vary in proportion to the downstream flow which varies as the load fluctuates.
Controlled extraction turbines provide steam at a constant pressure by having a valve or number of valves part way down the machine to control the flow to the low-pressure (LP) section of the turbine. The governing system keeps the required process steam pressure constant by controlling the valves either to increase or to decrease the flow to the LP section of the machine.
Extraction condensing machines can usefully be incorporated into an energy-from-waste plant when the waste to be burned is relatively wet and the plant efficiency can be increased by drying it prior to feeding it into the combustor. The heat in the extraction steam can be used for this process.
Efficiency and availability
It is very easy to follow the route to maximum efficiency at the expense of both practicality and overall cost. A designer of an energy-from-waste plant should involve both the incinerator/boiler designer and the steam turbine designer at a very early stage. The cost of steam turbines and their ancillaries varies with power, inlet conditions, exhaust conditions and efficiency. Any changes to the steam turbine will have an impact on the boiler and vice versa.
Whilst efficiency is always important, on occasions it may be appropriate to use an inefficient turbine. For example, if the electrical power demand is finite and the operator has more waste to dispose of than needed for efficient power generation, it may be advantageous to use a lower efficiency, less costly turbine, thus allowing the operator both to burn more waste and to reduce capital cost.
Turbines are designed with certain basic frame sizes. If steam or power parameters require a turbine frame to be used which is at the bottom of its range, the equipment will have a considerably higher specific cost than if the top end of a frame size can be used. If power is needed for the plant site alone, it may be more economical to generate at 415 V rather than at 11 kV. Only by a specific study can optimisation and therefore economic viability be determined.
Viability
It is necessary to consider fully all aspects of equipment design, plant operation and potential revenue earners at the earliest stage of a project in order to maximise viability.
The optimum operating parameters for the steam generator are a maximum of 45 bar steam pressure and 420°C live steam temperature. Whilst higher pressures and temperatures improve system efficiency the anticipated economic benefits must be evaluated against the increase in maintenance costs.
It is essential to ensure that the design and quality of all steam/water cycle components to be incorporated are optimised both individually and as a whole system and that their design is appropriate for an energy-from-waste facility. If they are not, the life, reliability and/or availability may be disappointingly poor. If they are appropriately designed and optimised, the plant availability need not be detrimentally affected by the inclusion of energy recovery.
General guidelines for the economic viability of MSW incineration plant with power generation can be developed. For example, each 1 MWe at 0.06 £/kWh for 7 500 h/year will support a capital expenditure of £1.8 million on a four year payback. More accurate figures can be produced taking into account the capital cost, the maintenance costs, the extra personnel needed and the potential effect on plant availability of having energy recovery. The current total construction cost is likely to be £3 000/kWe in the UK – clearly the figures will be different (although perhaps not by very much) in other countries.
Each case must be decided on its own merits and a feasibility study is necessary before any meaningful conclusions can be reached. Expert engineering contractors, as well as consultants, often conduct such studies for their clients. They are in a good position to do so because they have detailed technical, capital cost and operating information to hand on all the components which need to be considered.
