Landfill of opportunity3 June 2003
The power-from-waste business looks set to expand over the coming years, providing interesting possibilities for steam turbine designers. Raymond Bowell, Peter Brotherhood, Peterborough, UK
The EU Landfill Directive, which came into effect in 1999, is progressively increasing pressure on all European countries to tackle the issue of waste disposal as the gradual introduction of the Directive's targets begins to bite.
The aim of the Directive is to prevent or reduce the negative effects of landfill across Europe. It sets rigorous targets that countries need to meet within defined timescales and threatens tough sanctions and fines for countries that do not comply.
The Directive is being introduced in stages. It defines categories of waste substances for which the use of landfill (dumping waste in pits) needs to be reduced and dates by which this must be achieved. For example, the landfill of liquid waste, certain clinical and hazardous waste has already been banned. The landfill of whole tyres was banned this year (2003) and shredded tyres will be banned from 2006.
The rate at which countries have worked to comply with the legislation has generally been slow. Few alternatives to landfill have been put in place and international experience has shown that there are long lead times involved in developing new infrastructure.
What is becoming apparent is that there is a significant and rapidly increasing difference between the waste that is being produced and what is now allowed to be sent to a landfill site. In addition, governments are using the tax system to discourage landfill. For example, from 2005, landfill tax in the UK is being increased by £3 per year until it reaches £35 per tonne in 2012. The current level is £13, increasing at £1 per year.
For many countries waste is now a growing problem, with increasing interest in generating power from it.
Waste-to-energy systems are far from new but they are now being used in an increasingly diverse range of applications. There have also been technological improvements, resulting in higher efficiencies and enabling more electrical power to be generated from the waste than in the past.
For example, twisted and tapered turbine blade designs maximise the conversion of steam to energy.
Also, the increasing use of computer systems has significantly improved efficiency. Computational fluid dynamics allows models to be built to understand the behaviour of the steam during its passage through the turbine. Aerodynamics can now be optimised in real time so each individual machine can be tailored to its intended application.
Other techniques now allow the precise calculation of stress in critical areas and therefore the building in of safety factors that are invariably more accurate than the 'belt and braces' approach used in previous years. This has led to more economic use of materials and reductions in overall machine weight and cost.
CAD allows a virtual solid model of every component and the complete machine to be built. Problems of pipework clashes and difficult maintenance access, which a decade ago would only have become apparent on site, can now be detected and corrected at the design stage.
Turbine design models can be integrated with the customer's model of the entire plant, allowing such things as hazard analyses to be carried out before a single item is actually built.
The use of CAD has led to dramatic reductions in the time between receiving the order and delivering the product. It also radically reduces the possibility of human error.
New materials are also being introduced. For example, special inserts are welded onto the leading edge of turbine blades for extra strength and to improve erosion resistance. Ultra hard, ultra thin, coatings are also applied to the blades for a similar purpose and to improve vibration damping characteristics.
Typically, electricity generated by these systems has been used in the past for local plant use but now they are being used to supplement national power grids.
A good recent example of such a waste-to-energy system is a plant to be installed at a power station on the south coast of Spain designed to generate power from burning olive waste.
The condensing turbo alternator set (being supplied by Peter Brotherhood) will generate 8.5 MW of power from steam raised by burning waste stones, skins and fibre which remain after extracting oil from olives. Once the system has been installed and commissioned the electricity generated will be fed into the country's national grid.
In the UK, a new municipal waste treatment plant at Crymlyn Burrows near Neath, South Wales, has been designed specifically for reclaiming reusable materials and then converting the remaining waste into electrical power. It is capable of exceeding all government and EU targets on waste collection and disposal.
The plant is highly efficient. Over 70 percent (by weight) of the waste fed into it is recovered. The plant incorporates a fully integrated materials recovery and energy centre and reception area to receive all waste from mixed, pre-sorted and other sources. The materials recovery facility sorts and separate incoming waste. There is also a composting section and a fuel production section to produce refuse derived fuel (RDF) from combustible elements.
The waste-to-energy plant fired by the RDF is a £7 million development within the overall £32 million plant. It incorporates a steam turbine designed and built by Peter Brotherhood which is the result of two years dialogue to develop the most cost effective system to meet the power output requirements of the plant.
The turbine is a six stage condensing machine which uses steam at 18.25 bar at 250°C and exhausts at 0.1 bar absolute. It exhausts upwards to an air cooled condenser and runs at 6700 rpm which is geared to 1500 rpm for the generator. It produces 4.5 MW of electricity, half of which is used to run the plant itself with the remainder powering local council buildings and streetlights.
The development process for the turbine was a carefully balanced, iterative exercise which matched the boiler output with the condenser to provide the optimum, most efficient conversion between the two and the power output required.
There are potentially significant cost implications of changing the specification of any of the three components. A balance needed to be reached which would keep the initial development and production costs to a minimum and that would provide a system which is cost efficient in operation. A combination was achieved that would achieve the optimum results.
Elsewhere in the UK, Thames Water uses sewage gas to raise steam for a Peter Brotherhood turbine while North West Water burns sewage sludge thereby solving another environmental problem, as dumping sludge at sea is now illegal.
Hospital waste is also difficult to dispose of in an environmentally sensitive way but there are now a number of incineration facilities where the heat generated is being used to produce electricity.
Waste-to-energy-systems have also been designed and manufactured to recover energy from a meat and bone meal incinerator which was required to deal with the aftermath of the BSE crisis. Instead of the huge energy demands of keeping beef carcasses in cold storage, power is generated as a byproduct of their safe incineration.
At a UK coking works, the coke-oven gas is now recovered it had mostly been flared before and burned in boilers to raise steam for an extraction-condensing turbine. This produces up to 13MW of electricity as well as supplying low-grade steam to the coking process.
In Portugal the problem of disposing of waste vehicle tyres has already been tackled. A system installed there uses a Peter Brotherhood developed turbine to generate 3.6 MW of electricity from the incineration of 13 000 tonnes per year of scrap tyres.
Wood and sugar experience
The use of waste to generate electricity is well established in the wood and the sugar business and experience from these industries should be of benefit to the wider energy from waste sector. Peter Brotherhood alone has over 500 turbines operating in sugar mills across the world running on the fibrous waste known as 'bagasse' (which remains after sucrose extraction).
One of the biggest systems of this type is installed at Hippo Valley near Chirezdi, Zimbabwe, which generates 20 MW of power.
A similar approach to power generation applies to timber mills and processing plants around the world where waste wood is used to raise steam for turbines. From the manufacture of dowels in Maine and plywood in Ghana to the operation of sawmills from Finland to Fiji, and the burning of forest residue in the Newfoundland backwoods, steam turbines have been designed and manufactured for installation in power plants that are fuelled by many different forms of wood mass.
Meanwhile, in yet another variation on wood fuel, six Peter Brotherhood steam turbine-generators have been installed in a plant in the Philippines where the fuel is fast-growing trees, cultivated with the sole purpose of providing fuel. This kind of short-rotation coppicing has also been used to fuel a steam turbine on a tea plantation in Kenya, where the exhaust steam is used in the tea-drying process. While this is an example of using new rather than waste wood as fuel these applications are a further illustration of the range of fuels that has been used to power steam turbines around the world and the experience that can be brought to bear in tackling waste-to-energy projects in the future.